Introduction

Geography is the study of places and the relationships between people and their environments. Geographers explore both the physical properties of Earth‘s surface and the human societies spread across it. They also examine how human culture interacts with the natural environment and the way those locations and places can have an impact on people. Geography seeks to understand where things are found, why they are there, and how they develop and change over time.

Physical Geography

The natural environment is the primary concern of physical geographers, although many physical geographers also look at how humans have altered natural systems. Physical geographers study Earth‟s seasons, climate, atmosphere, soil, streams, landforms, and oceans.

Geomorphology is the study of landforms and the processes that shape them. Geomorphologists investigate the nature and impact of wind, ice, rivers, erosion, earthquakes, volcanoes, living things, and other forces that shape and change the surface of the Earth.

Climatologists study Earth‘s climate system and its impact on Earth‘s surface. For example, climatologists make predictions about El Nino, a cyclical weather phenomenon of warm surface temperatures in the Pacific Ocean.

Biogeographers study the impact of the environment on the distribution of plants and animals. For example, a biogeographer might document all the places in the world inhabited by a certain spider species, and what those places have in common.

Oceanography, a related discipline of physical geography, focuses on the creatures and environments of the world‘s oceans. Observation of ocean tides and currents constituted some of the first oceanographic investigations. Today, oceanographers conduct research on the impacts of water pollution, track tsunamis, design offshore oil rigs, investigate underwater eruptions of lava, and study all types of marine organisms from toxic algae to friendly dolphins.

Human Geography

Human geography is concerned with the distribution and networks of people and cultures on Earth‘s surface. Human geographers also study how people use and alter their environments. When, for example, people allow their animals to overgraze a region, the soil erodes and grassland is transformed into desert. The impact of overgrazing on the landscape as well as agricultural production is an area of study for human geographers.

Finally, human geographers study how political, social, and economic systems are organized across geographical space. These include governments, religious organizations, and trade partnerships. The boundaries of these groups constantly change.

The main divisions within human geography reflect a concern with different types of human activities or ways of living. Some examples of human geography include urban geography, economic geography, cultural geography, political geography, social geography, and population geography.

Geomorphology

Geomorphology is the study of landforms, their processes, form and sediments at the surface of the Earth (and sometimes on other planets). Study includes looking at landscapes to work out how the earth surface processes, such as air, water and ice, can mould the landscape.

Landforms are produced by erosion or deposition, as rock and sediment is worn away by these earth-surface processes and transported and deposited to different localities. The different climatic environments produce different suites of landforms. The landforms of deserts, such as sand dunes and ergs, are a world apart from the glacial and periglacial features found in polar and sub-polar regions. Geomorphologists map the distribution of these landforms so as to understand better their occurrence.

Earth-surface processes are forming landforms today, changing the landscape, albeit often very slowly. Most geomorphic processes operate at a slow rate, but sometimes a large event, such as a landslide or flood, occurs causing rapid change to the environment, and sometimes threatening humans. Geological hazards, like volcanic eruptions, earthquakes, tsunamis and landslides, fall within the interests of geomorphologists.

I. The Origin and Evolution of the Earth

A large number of hypotheses were put forth by different philosophers and scientists regarding the origin of the earth. One of the earlier and popular arguments was by German philosopher Immanuel Kant. Mathematician Laplace revised it in 1796. It is known as Nebular Hypothesis. The hypothesis considered that the planets were formed out of a cloud of material associated with a youthful sun, which was slowly rotating.

Later in 1900, Chamberlain and Moulton considered that a wandering star approached the sun. As a result, a cigar-shaped extension of material was separated from thesolar surface. As the passing star moved away, the material separated from the solar surface continued to revolve around the sun and it slowly condensed into planets.

In 1950, Otto Schmidt in Russia and Carl Weizascar in Germany somewhat revised the

‗nebular hypothesis‘, though differing in details. They considered that the sun was surrounded by solar nebula containing mostly the hydrogen and helium along with what may be termed as dust. The friction and collision of particles led to formation of a disk-shaped cloud and the planets were formed through the process of accretion.

However, scientists in later period took up the problems of origin of universe rather than that of just the earth or the planets.

Origin of the Universe

The most popular argument regarding the origin of the universe is the Big Bang Theory. It is also called expanding universe hypothesis. Edwin Hubble, in 1920, provided evidence that the universe is expanding. As time passes, galaxies move further and further apart.

The Big Bang Theory considers the following stages in the development of the universe.

(i) In the beginning, all matter forming the universe existed in one place in the form of a ―tiny ball‖ (singular atom) with an unimaginably small volume, infinite temperature and infinite density.

(ii) At the Big Bang the ―tiny ball‖ exploded violently. This led to a huge expansion. It is now generally accepted that the event of big bang took place 13.7 billion years before the present. The expansion continues even to the present day. As it grew, some energy was converted into matter. There was particularly rapid expansion within fractions of a second after the bang. Thereafter, the expansion has slowed down. Within first three minutes from the Big Bang event, the first atom began to form.

(iii) Within 300,000 years from the Big Bang, temperature dropped to 4,500K (Kelvin) and gave rise to atomic matter. The universe became transparent.

The expansion of universe means increase in space between the galaxies. An alternative to this was Hoyle‘s concept of steady state. It considered the universe to be roughly the same at any point of time. However, with greater evidence becoming available about the expanding universe, scientific community at present favours argument of expanding universe.

Our Solar System

Our Solar system consists of eight planets. Out of the eight planets, Mercury, Venus, Earth and Mars are called as the inner planets as they lie between the sun and the belt of asteroids the other four planets are called the outerplanets. Alternatively, the first four are called Terrestrial, meaning earth-like as they are made-up of rock and metals, and have relatively high densities. The rest four are called Jovian or Gas Giant planets. Jovian means Jupiter-like. Most of them are much larger than the terrestrial planets and have thick atmosphere, mostly of helium and hydrogen. All the planets were formed in the same period sometime about 4.6 billion years ago.

The difference between terrestrial and Jovian planets can be attributed to the following conditions:

(i) The terrestrial planets were formed in the close vicinity of the parent star where it was too warm for gases to condense to solid particles. Jovian planets were formed at quite a distant location.

(ii) The solar wind was most intense nearer the sun; so, it blew off lots of gas and dust from the terrestrial planets. The solar winds were not all that intense to cause similar removal of gases from the Jovian planets.

(iii) The terrestrial planets are smaller and their lower gravity could not hold the escaping gases.

Evolution of the Earth

The planet earth initially was a barren, rocky and hot object with a thin atmosphere of hydrogen and helium. This is far from the present day picture of the earth. Hence, there must have been some events–processes, which may have caused this change from rocky, barren and hot earth to a beautiful planet with ample amount of water and conducive atmosphere favoring the existence of life. In the following section, you will find out how the period, between the 4,600 million years and the present, led to the evolution of life on the surface of the planet.

The earth has a layered structure. From the outermost end of the atmosphere to the centre of the earth, the material that exists is not uniform. The atmospheric matter has the least density. From the surface to deeper depths, the earth‘s interior has different zones and each of these contains materials with different characteristics.

Evolution of Lithosphere

The earth was mostly in a volatile state during its primordial stage. Due to gradual increase in density the temperature inside has increased. As a result the material inside started getting separated depending on their densities. This allowed heavier materials (like iron) to sink towards the centre of the earth and the lighter ones to move towards the surface. With passage of time it cooled further and solidified and condensed into a smaller size. This later led to the development of the outer surface in the form of a crust. During the formation of the moon, due to the giant impact, the earth was further heated up. It is through the process of differentiation that the earth forming material got separated into different layers. Starting from the surface to the central parts, we have layers like the crust, mantle, outer core and inner core. From the crust to the core, the density of the material increases.

Evolution of Atmosphere and Hydrosphere

The present composition of earth‘s atmosphere is chiefly contributed by nitrogen and oxygen. There are three stages in the evolution of the present atmosphere. The first stage ismarked by the loss of primordial atmosphere. In the second stage, the hot interior of the earthcontributed to the evolution of the atmosphere. Finally, the composition of the atmosphere wasmodified by the living world through the process of photosynthesis.

The early atmosphere, with hydrogen and helium, is supposed to have been stripped off as a result of the solar winds. This happened not only in case of the earth, but also in all the terrestrial planets, which were supposed to have lost their primordial atmosphere through the impact of solar winds.

During the cooling of the earth, gases and water vapor were released from the interior solid earth. This started the evolution of the present atmosphere. The early atmosphere largely contained water vapor, nitrogen, carbon dioxide, methane, ammonia and very little of free oxygen. The process through which the gases were outpoured from the interior is called degassing. Continuous volcanic eruptions contributed water vapor and gases to the atmosphere. As the earth cooled, the water vapor released started getting condensed. The carbon dioxide in the atmosphere got dissolved in rainwater and the temperature further decreased causing more condensation and more rains. The rainwater falling onto the surface got collected in the depressions to give rise to oceans. The earth‘s oceans were formed within 500 million years from the formation of the earth. This tells us that the oceans are as old as 4,000 million years. Sometime around 3,800 million years ago, life began to evolve. However, around 2,500- 3,000 million years before the present, the process of photosynthesis got evolved. Life was confined to the oceans for a long time. Oceans began to have the contribution of oxygen through the process of photosynthesis. Eventually, oceans were saturated with oxygen, and 2,000 million years ago, oxygen began to flood the atmosphere.

Origin of Life

The last phase in the evolution of the earth relates to the origin and evolution of life. It is undoubtedly clear that initially the earth or even the atmosphere of the earth was not conducive for the development of life. Modern scientists refer to the origin of life as a kind of chemical reaction, which first generated complex organic molecules and assembled them. This assemblage was such that they could duplicate themselves converting inanimate matter into living substance. The record of life that existed on this planet indifferent periods is found in rocks in the form of fossils. The microscopic structures closely related to the present form of blue algae have been

found in geological formations much older than some 3,000 million years. It can be assumed that life began to evolve sometime3,800 million years ago.

II. Internal Structure of The Earth

Understanding the structure of the earth‘s interior (crust, mantle, core) and various forces (heat, seismic waves) emanating from it is essential to understand the evolution of the earth‘s surface, its current shape and its future. The study of the earth‘s interior is essential

to understand the earth‘s surface

to understand the geophysical phenomenon like volcanism, earthquakes, etc. to understand the earth‘s magnetic field

to understand the internal structure of various solar system objects

to understand the evolution and present composition of the atmosphere for mineral exploration

Sources of information about the interior

The earth‘s radius is 6,370 km. reaching the centre of the earth and make observations or collect samples of the materials is almost impossible. Under such conditions, most of our knowledge about the interior of the earth is largely based on analogies and inferences. Yet, a part of the information is obtained through direct observations and analysis of materials.

Direct Sources

The readily available solid earth material is surface rock we get from mining areas. Besides mining, scientists world over are working on two major projects such as ―Deep Ocean Drilling Project‖ and ―Integrated Ocean Drilling Project‖. The deepest drill at Kola, in Arctic Ocean, has so far reached a depth of 12 km. These drilling projects have provided large volume of information through the analysis of materials collected at different depths. Volcanic eruption forms another source of obtaining direct information. As and when the magma comes out to the surface of the earth during volcanic eruption it becomes available for laboratory analysis.

Indirect Sources

Analysis of properties of rocks and magma indirectly provides information about the interior. Through mining we know that temperature and pressure increase with the increasing depth. It is also known that the density of the material also increases with depth. Scientists have estimated the values of temperature, pressure and the density of materials at different depths.

Meteor is another source of information about the interior of the earth. However, the material, that becomes available for analysis from meteors, is not from the interior of the earth. It is only similar to that of the earth. Meteors are solid bodies developed out of materials same as, or similar to, earth. So, by analogy meteors provide valuable information about the earth‘s interior.

Other indirect sources include gravitation, magnetic field and seismic activity. The gravitational force is greater near the poles and less at the equator. It also differs according to the mass of material. Thus the uneven distribution of material within the earth influences its value. The readings of the gravity, may, at places differ from the expected values. Such a difference is called gravity anomaly. Gravity anomalies give us information about the distribution of mass of the material in the crust of the earth.

Seismic/Earthquake Waves

The study of seismic waves provides a complete picture of the layered interior. An earthquake in simple words is shaking of the earth. It is a natural event. It is caused due to

release of energy, which generates waves that travel in all directions. The energy waves travelling in different directions reach the surface.

Earthquake waves are basically of two types-body waves and surface waves. Body waves are generated due to the release of energy at the focus and move in all directions travelling through the body of the earth. They interact with the surface rocks and generate new set of waves called surface waves. These waves move along the surface. The velocity of waves changes as they travel through materials with different densities. Denser the material, higher is the velocity.

There are two types of body waves. They are called P and S-waves. P-waves move faster and are the first to arrive at the surface. These are also called ‗primary waves‘. The P-waves are similar to sound waves. They travel through all materials gaseous, liquid and solid. S-waves arrive at the surface with some time lag. These are called secondary waves. S-waves can travel only through solid materials. This characteristic of the S-waves has helped scientists to understand the structure of the interior of the earth.

Different waves travel in different manners. P-waves vibrate parallel to the direction of the wave. This exerts pressure on the material in the direction of the propagation. As a result, it creates density differences in the material leading to stretching and squeezing of the material. The direction of vibrations of S-waves is perpendicular to the wave direction in the vertical plane. Hence, they create troughs and crests in the material medium through which they pass. Surface waves are considered to be the most damaging waves.

Shadow Zone

Earthquake waves are recorded in seismographs located at far off locations. However, there are certain areas where the waves are not reported. Such a zone, where the waves are not recorded, is called the ‗shadow zone‘. The study reveals that for each earthquake, there exists an altogether different shadow zone. Given figure shows the shadow zones of P and S-waves.

It was observed that seismographs, located within 105° from the epicenter, recorded the arrival of both P and S-waves. But, beyond 140°from epicenter, they record the arrival of P- waves, but not that of S-waves. Thus, a zone between 105° and 140° from epicenter is identified as the shadow zone of P-waves. However, the entire zone beyond 105° does not receive S-waves. Thus shadow zone of S-wave is much larger than that of the P-waves. The shadow zone of P- waves appears as a band around the earth between 105° and 140° away from the epicenter whereas that of S-wave is a continuous zone.

The interior of the earth is not composed of homogeneous or uniform material. The structure of Earth can be visualized in two ways: chemically or by material properties. By comparing material strength, the layering of Earth is categorized as lithosphere, asthenosphere, upper mantle, lower mantle, outer core, and the inner core.

Chemically, a tripartite arrangement for the earth has been suggested by Austrian geologist, Suess. The three concentric shells advocated by him on the basis of their density are outer Sial layer, inner Sima layer and the innermost nickel-iron core of Nife. The scientific study and analysis of various seismic waves of natural and man induced earthquakes unrevealed the mystery of earth's interior and divided the earth in three different layers as explained below.

Thus, the earth's interior has three different layers; they are (i) the crust (ii) mantle and (iii) the core. These layers are distinguished on the basis of their (i) Physical and chemical properties,

(ii) Thickness, (iii) density (iv) Temperature, (v) Metallic content and (vi) rocks. The geologic component layers of Earth are at the following depths below the surface:

a) Earth's Crust: All of the Earth's landforms (mountains, plains, and plateaus) are contained within it, along with the oceans, seas, lakes and rivers. There are two different types of crust: thin oceanic crust that underlies the ocean basins and thicker continental crust that underlies the continents. These two different types of crust are made up of different types of rock. The thin oceanic crust is composed of primarily of basalt and the thicker continental crust is composed primarily of granite. The low density of the thick continental crust allows it to "float" in high relief on the much higher density mantle below. The boundary between the crust and the mantle is Mohorovicic discontinuity.

While the crust appears to be solid, it is subjected to repeated movements including bending, folding, and breaking associated with the movement of material in the mantle below. The processes of weathering and erosion are continually wearing the high points of crust away. The low points are being filled in with the debris generated by these destructive processes.

b) Earth's Mantle: It is the thick, dense rocky matter that surrounds the core with a radius of about 2885 km. The mantle covers the majority of the Earth's volume. This is basically composed of silicate rock rich in iron and magnesium. The mantle is less dense than the core but denser than the outer crust layer.

It has different temperatures at different depths. The temperature is lowest immediately beneath the crust and increases with depth. The highest temperatures occur where the mantle material is in contact with the heat-producing core. This steady increase of temperature with depth is known as the geothermal gradient.

The geothermal gradient is responsible for different rock behaviors which divide the mantle into two different zones. Rocks in the upper mantle are cool and brittle, while rocks in the lower mantle are hot and soft (but not molten). Rocks in the upper mantle are brittle enough to break under stress and produces earthquakes. However, rocks in the lower mantle are soft and flow when subjected to forces instead of breaking. The lower limit of brittle behavior is the boundary between the upper and lower mantle. This layer is separated from the core by Gutenberg-Wiechert Discontinuity. The outer and the inner mantle are separated by another discontinuity named Repetti discontinuity.

c) Earth's Core: Earth's Core is thought to be composed mainly of an iron and nickel alloy. This composition is based on calculations of its density. The core is earth's source of internal heat because it contains radioactive materials which release heat as they break down into more stable substances.

The core is divided into two different zones. The outer core is a liquid because the temperatures there are adequate to melt the iron-nickel alloy. However, the inner core is a solid even though its temperature is higher than the outer core. Here, tremendous pressure, produced by the weight of the overlying rocks is strong enough to crowd the atoms tightly together and prevents changing it to the liquid state.

Temperature, Pressure and Density of the Earth‟s interior:

Temperature

Rise in temperature with increase in depth is observed in mines and deep wells. These evidences along with molten lava erupted from the earth‘s interior, support that temperature increases towards the centre of the earth. The different observations show that the rate of increase of temperature is not uniform from the surface towards the earth‘s centre.

It is faster at some places than at others. In the beginning this increase is at an average rate of 1^oC for every 32 metres increase in depth. At such a constant rate of increase in temperature, at 10 km depth, the temperature will be approximately300^oC and at 40 km depth it will be 1200^oC. At this rate, earth‘s interior should be in a molten state. Yet it is not so because the rocks buried under the pressure of several km thickness of overlying rocks melt at higher temperature than similar rocks at the surface. A basaltic lava rock which melts at 1250^oC at the surface will melt at 1400^oC at 32 km depth. The extra heat required for melting is produced by radioactivity. It is the result of breakdown of atomic nuclei of minerals emitting radiant energy in the form of heat from the rocks. .

The behaviour of earthquake waves is another evidence for this phenomenon. They further confirm that the composition of different layers is as variable as is therate of change of temperature. While in the upper 100 km, the increase in temperature is at the rate of 12°C per km, in the next 300 km it is 20°C per km but is only 10^oC per km below it. Thus the rate of increase of temperature beneath the surface decreases towards the centre. The temperature at the centre is estimated to lie somewhere between 3000°C and 5000^oC.Such a high temperature inside the earth may be due to chemical reactions under high pressure conditions and disintegration of radioactive elements.

Pressure

The pressure also increases from the surface towards the centre of the earth due to huge weight of the overlying rocks. Therefore in deeper portions, the pressure is tremendously high. The pressure near the centre is considered to be 3 to 4million times the pressure of atmosphere at sea level. At high temperature, the material beneath will melt towards the central part of the earth. This molten material under tremendous pressure conditions acquires the property of a solid and is probably in a plastic state.

Density

Due to increase in pressure and presence of heavier materials towards the earth‘s centers, the density of earth‘s layers also goes on increasing. Obviously the materials of the innermost part of the earth are very dense as already stated.

Composition of the Earth

Iron -35%, Oxygen -30%, Silicon -15%, Magnesium -13%, Nickel -2.4%, Sulphur -1.9%,

Calcium -1.1%, Aluminum -1.1%, etc.

III. Formation and Distribution of Oceans and Continents

Continents cover 29 per cent of the surface of the earth and the remainder is under oceanic waters. The positions of the continents and the ocean bodies, as we see them in the map, have not been the same in the past. Moreover, it is now a well-accepted fact that oceans and continents will not continue to enjoy their present positions in times to come. Following are the few major views about distribution of Continents and Oceans, and their present and past positions.

Continental Drift Theory

In 1912, the German geologist and meteorologist Alfred Wegener first proposed the theory of continental drift, which states that parts of the Earth's crust slowly drift atop a liquid core.

Wegener hypothesized that there was a gigantic supercontinent 200 million years ago, which he named Pangaea, meaning "All-earth". Pangaea started to break up into two smaller supercontinents, called Laurasia and Gondwanaland, during the Jurassic period. By the end of

the Cretaceous period, the continents were separating into land masses that look like our modern- day continents.

Evidences Supporting the Theory of Continental Drift

Wegener gave a number of evidences in support of the unification of landmass in geologic past. They are such which cannot be negated even today.

a. Jig-saw-fit: - Eastern coast of South America is identical to Western coast of Africa which fits to a certain depth in the ocean. To a certain extent coastal areas and continental shelves have been modified by oceanic waves through denudation.

b. Geological similarities: - The mountain systems of Southern Atlantic coast in South America and Africa show the similarity of the extension in both continents.

c. Coal and Vegetation evidences: -The distribution of coal and vegetation over South America, Africa, India and Australia proves that they were together in geological past. The classical glacial deposits during carboniferous period over these landmasses resemble each other which tells the story of togetherness. Today they lie in different climatic zones.

Apart from above evidences put forward by Wegener, other evidences(known later) are also there which support the idea of continental drift.

d. Evidences from Pale magnetism: – Pale magnetism is the study of the direction of pole through ages. Magnetically susceptible minerals like hematite, pyrhotite magnetite etc. get aligned with the magnetic pole of the earth and recorded in the solidification of magma during that time. It is found that periodic changes have occurred and poles have wandered which is not possible for the entire earth. Hence, it is the twist and turn of the land block and not for the entire earth which has again explained that the continents have shifted their positions.

e. Sea floor spreading: – Along the mid-Atlantic ridge, magma comes out at the sea bed and gets solidified. A new zone is formed and this process is continuing since millions of years. It is leading for diversion of continental block, and hence the size of the Atlantic Ocean is increasing which is termed as sea floor spreading. It is the classical example of the shifting of continents.

Sea-Floor Spreading

The concept of sea floor spreading was first propounded by Harry Hess. According to him the mid oceanic ridges were situated on the rising thermal convection currents coming up from the mantle. The oceanic crust moves in opposite directions from mid oceanic ridges and thus there is continuous upwelling of lavas along the ridge. These molten lavas cool down and solidify to form new crust along the trailing ends of divergent boundary. Thus there is a continuous creation of new crust.

Sea floor spreading and magnetic bands

In 1963, Fred Vine, Drummond Matthews, and others found that the crust surrounding the mid ocean ridges showed alternating bands --each band magnetized with a polarity opposite to the surrounding bands. They suggested that as new sea-floor crust was formed around the rift in the mid ocean ridge, it magnetized differently, depending upon the polarity of the planet at that time. This supported the theory that Harry Hess had put forth, that the ocean progressively widens as new sea floor is created along a crack that follows the crest of mid-ocean ridges.

Sea Floor spreading is still active in many parts of the world's oceans. This can be observed from under sea vents and trenches. As the sea floor pushes the continents apart the rest of the plates it sinks beneath continental plates. This creates deep sea trenches.

Plate Tectonics

The theory states that Earth's outermost layer, the lithosphere, is broken into 7 large, rigid pieces called plates: the African, North American, South American, Eurasian, Australian, Antarctic, and Pacific plates. Several minor plates also exist, including the Arabian, Nazca, and Philippines plates.

The plates are all moving in different directions and at different speeds (from 2 cm to 10 cm per year--about the speed at which our fingernails grow) in relationship to each other. The place where the two plates meet is called a plate boundary. Boundaries have different names depending on how the two plates are moving in relationship to each other. These plates lie atop a layer of partly molten rock called the asthenosphere. The plates can carry both continents and oceans, or exclusively one or the other. The Pacific Plate, for example, is entirely oceanic.

Continental plates are composed mainly of granite, while oceanic plates are mostly basalt, which is considerably heavier. Essentially, the continents are lighter and more buoyant; hence, they float higher on the earth's mantle than the ocean's crust does.

A. Convergent boundary: Convergent boundaries are the places where plates crash or crunch together. When plates converge, one slips under the other and is said to be sub ducted. At depths from 185 to 435 miles beneath the earth's surface, the sub ducted parts of the plate melt and become part of the molten mantle. As new plate material is being formed continuously, and the excess is melted into magma, the earth's rocky crust is constantly recycled.

Oceanic Plate vs. Oceanic Plate Convergence

The older of the two plates descends into the subduction zone when plates of oceanic lithosphere collide along a trench. The descending plate carries water-filled sediments from the ocean floor downward into the mantle. The presence of water alters the physical and chemical conditions necessary for melting and causes magma to form. The magma rises up through the overriding oceanic plate, reaching the surface as a volcano. As the volcano grows, it may rise above sea level to form an island.

Oceanic Plate vs. Continental Plate Convergence

When oceanic lithosphere collides with continental lithosphere, the oceanic plate will descend into the subduction zone. Oceanic lithosphere is denser than continental lithosphere and is therefore consumed preferentially. Continental lithosphere is almost never destroyed in subduction zones. The Nazca Plate dives below South America in a subduction zone that lies along the western margin of the continent.

Continental Plate vs. Continental Plate Convergence

The tallest mountains in the world Himalayas were formed (and continue to grow) as a result of continental collision. Continental lithosphere is relatively light and is deformed adjacent to subduction zones rather than consumed.

B. Divergent boundary: Places where plates are moving apart are called divergent boundaries also called as spreading centers. When the Earth's brittle surface layer (the lithosphere) is pulled apart, it typically breaks along parallel faults that tilt slightly outward from each other. As the plates separate along the boundary, the block between the faults, cracks and drops down into the soft, plastic interior (the asthenosphere). The sinking of the block forms a central valley called a rift. Magma (liquid rock) seeps upward to fill the cracks. In this way, new crust is formed along the boundary. Earthquakes occur along the faults, and volcanoes are formed where the magma reaches the surface.

Where a divergent boundary crosses the land, the rift valleys get formed which are typically 30 to 50 kilometers wide. Examples include the East Africa rift in Kenya and Ethiopia, and the Rio Grande rift in New Mexico. Oceanic ridges rise a kilometer or so

above the ocean floor and form a global network tens of thousands of miles long. Examples include the Mid-Atlantic ridge and the East Pacific Rise. Plate separation is a slow process, e.g. Divergence along the Mid Atlantic ridge causes the Atlantic Ocean to widen at only about 2 centimeters per year.

C. Transform boundary: Places where plates slide past each other are called transform boundaries. The plates on either side of a transform boundary are merely sliding past each other and not tearing or crunching each other. The most famous transform boundary in the world is the San Andreas Fault. Although transform boundaries are not marked by spectacular surface features, their sliding motion causes lots of earthquakes. The strongest and most famous earthquake along the San Andreas Fault hit San Francisco in 1906. Many buildings were finished by the quake, and maximum of the city was destroyed by the fires that followed.

Salient points:

· The plate boundaries are mainly represented by oceanic ridges and trenches.

· Interactions at plate boundaries cause volcanic activity and earthquakes

· The plates are in motion, moving away from ridges and toward trenches.

· Plates descend into the mantle below trenches in subduction zones.

· Plates typically contain both oceanic and continental lithosphere.

· Oceanic lithosphere is continually created and destroyed.

· Continental lithosphere cannot be destroyed but continents can be subdivided and assembled into supercontinents.

IV. Materials of the Earth‟s Crust

The earth is composed of various kinds of elements. These elements are in solid form in the outer layer of the earth and in hot and molten form in the interior. About 98 per cent of the total crust of the earth is composed of eight elements like oxygen, silicon, aluminum, iron, calcium,

sodium, potassium and magnesium and the rest is constituted by titanium,hydrogen, phosphorous, manganese, sulphur, carbon, nickel and other elements

The Major Elements of the Earth‟sCrust

Sl. No.

Elements

By Weight (%)

1.

2.

3.

4.

5.

6.

7.

8.

9.

Oxygen Silicon Aluminum Iron Calcium Sodium Potassium Magnesium Others

46.60

27.72

8.13

5.00

3.63

2.83

2.59

2.09

1.41

Minerals

The elements in the earth‘s crust are rarely found exclusively but are usually combined withother elements to make various substances. These substances are recognized as minerals.

Thus, a mineral is a naturally occurring organic and inorganic substance, having an orderly atomic structure and a definite chemical composition and physical properties. A mineral is composed of two or more elements. But, sometimes single element minerals like sulphur, copper, silver, gold, graphite etc. are found.

Though the number of elements making up the lithosphere is limited they are combined in many different ways to make up many varieties of minerals. There are at least 2,000 minerals that have been named and identified in the earth crust; but almost all the commonly occurring ones are related to six major mineral groups that are known as major rock forming minerals.

The basic source of all minerals is the hot magma in the interior of the earth. When magma cools, crystals of minerals appear and a systematic series of minerals are formed in sequence to solidify so as to form rocks. Minerals such as coal, petroleum and natural gas are organic substances found in solid, liquid and gaseous forms respectively.

Some major minerals and their characteristics Feldspar

Silicon and oxygen are common elements in all types of feldspar and sodium, potassium, calcium, aluminum etc. are found in specific feldspar variety. Half of the earth‘s crust is composed of feldspar. It has light cream to salmon pink colour. It is used in ceramics and glass making.

Quartz

It is one of the most important components of sand and granite. It consists of silica. It is a hard mineral virtually insoluble in water. It is white or colorless and used in radio and radar. It is one of the most important components of granite.

Pyroxene

Pyroxene consists of calcium, aluminum, magnesium, iron and silica. Pyroxene forms 10 per cent of the earth‘s crust. It is commonly found in meteorites. It is in green or black colour.

Amphibole

Aluminum, calcium, silica, iron, magnesium are the major elements of amphiboles. They form 7 per cent of the earth‘s crust. It is in green or black colour and is used in asbestos industry. Hornblende is another form of amphiboles.

Mica

It comprises of potassium, aluminium, magnesium, iron, silica etc. It forms 4 per cent of the earth‘s crust. It is commonly found in igneous and metamorphic rocks. It is used in electrical instruments.

Olivine

Magnesium, iron and silica are major elements of olivine. It is used in jewellery. It is usually a greenish crystal, often found in basaltic rocks. Besides these main minerals, other minerals like chlorite, calcite, magnetite, hematite, bauxite and barite are also present in some quantities in the rocks.

Metallic Minerals

These minerals contain metal content and can be sub-divided into three types:

I. Precious metals: gold, silver, platinum etc.

II. Ferrous metals: iron and other metals often mixed with iron to form various kinds of steel.

III. Non-ferrous metals: include metals like copper, lead, zinc, tin, aluminium etc.

Non-Metallic Minerals

These minerals do not contain metal content. Sulphur, phosphates and nitrates are examplesof non-metallic minerals. Cement is a mixture of non-metallic minerals

Rocks

The outermost part of lithosphere is called crust. This is the most significant part of the earth because it is occupied by humans. The material of the crust is made up of rocks. The rocks are of different types. They are hard like granite, soft like clay and loose like gravel. Rocks have a great variety of colour, weight and hardness.

Rocks are composed of minerals. They are aggregates or physical mixture of one or more minerals. Minerals on the other hand are made up of two or more elements in a definite ratio. They have a definite chemical composition. Crust is made up of more than 2000 minerals, but out of these, 6 are the most abundant and contribute the maximum to this uppermost part of the earth. These are feldspar, quartz, pyroxenes, amphiboles, mica and olivine.

Granite is a rock and its constituent minerals bound together are quartz, feldspar and mica which make it a hard rock. Change in the ratio of these minerals gives rise to granites of different colours and hardness. The minerals containing metals are called metallic minerals. Hematite, a major iron ore is a metallic mineral. Ores are metallic minerals which can be profitably mined. Rocks are of immense economic importance to us.

Types of rocks

Rocks differ in their properties, size of particles and mode of formation. On the basis of mode of formation rocks may be grouped into three types:

a. Igneous

b. Sedimentary and

c. Metamorphic

Igneous Rocks

The word igneous is derived from the Latin word ‗ignis‘ meaning fire. Igneous rocks are formed by the cooling of highly heated molten fluid material, known as magma. The word magma is derived from a Greek word which means ‗dough‘. It requires a greater quantity of heat to melt the rocks under overlying pressure than at the surface. We do not know the exact depths at which magma forms but probably it is formed at different depths not exceeding 40 km. Molten rocks produce an increase in volume which is responsible for causing fractures or cracks in the crust. The overlying pressure gets weakened along these openings, thus forcing out the magma through them. Otherwise it can‘t escape due to great overlying pressure.

When magma is ejected to the surface, it is called lava. Igneous rocks are formed from solidified molten magma below or on the earth‘s surface. As they comprise the earth‘s first crust and all other rocks are derived from them, these are called the parent of all rocks or the ‗primary rocks‘. In simple words, all rocks can be described as of igneous origin because at one time or another, they were erupted to the surface: A younger series of igneous rocks is still being formed. About 95%of the volume of outermost 16 km of the earth is composed of them. These are largely hard and massive because of their magmatic origin and are crystalline in appearances.

On the basis of their mode of occurrence, igneous rocks can be classified as: extrusive or volcanic rocks and intrusive rocks.

(i) Extrusive igneous rocks are formed by cooling of lava on the earth‘s surface. As lava cools very rapidly on coming out of the hot interior of the earth, the mineral crystals forming these rocks are very fine. These rocks are also called volcanic rocks. Gabbro and basalt are very common examples of such rocks. These rocks are found in volcanic areas. Deccan plateau‘s regur soil in India is derived from lava.

(ii) Intrusive igneous rocks are formed when magma solidifies below the earth‘s surface. The rate of cooling below the earth‘s surface is very slow which gives rise to formation of large crystals in the rocks. Deep seated intrusive rocks are termed as plutonic rocks and shallow depth intrusive rocks are termed as hypabyssal. Granite and dolerite are common examples of intrusive rocks. From this point of view, therefore, igneous rocks can, in accordance with their mode of formation, be classified as (a) Plutonic, (b) Hypabyssal and (c) Volcanic rock masses. The huge blocks of coarse granitic rocks are found both in the Himalaya and the Deccan Plateau.

Magma on cooling produces rocks of different shapes and sizes, depending on the space available after it forces itself into the crust. Common forms of intrusive igneous rocks are batholiths, sills and dykes etc. Batholiths are huge masses of solidified magma. They vary in size; some are as much as several hundred kilometers across and thousands of kilometers thick. They generally form the core of the major mountains, as shown in this diagram. Their irregular dome shaped roofs sometimes appear on the surface after erosion of millions of years. Sill is the horizontal intrusion of solidified magma between the layers of pre-existing rocks. Dyke is similarly a more or less vertical formation from few metres to several kilometers in length and from few centimeters to hundreds of metres in thickness.

On the basis of chemical properties, igneous rocks are classified into acidic and basic rocks. These are formed as a result of solidification of acidic or basic lava. Acidic igneous rocks are composed of 65% or more of silica. These rocks are light coloured, hard and very strong. Granite is an example of an acidic rock. Basic igneous rocks contain less than 55% of silica and have more of iron and magnesium. These rocks are dark coloured and weak enough for weathering. Gabbro, basalt and dolerite are examples of basic rocks.

Sedimentary Rocks

These rocks are formed by successive deposition of sediments. These sediments may be the debris eroded from any previously existing rock which may be igneous rock, metamorphic or old sedimentary rock. Sedimentary rocks have layered or stratified structure. The thickness of strata varies from few millimeters to several metres. So these rocks are also called stratified rocks. Generally, these rocks have some type of fossil between their strata. Fossil is the solid part or an impression of a prehistoric animal or plant embedded in strata of sedimentary rocks. Sedimentary rocks are widely spread on the earth surface but to a shallow depth.

The individual rock particles are first broken from rocks and then transported by running water, ocean currents, and glaciers or even by wind from one place to another. The process by which rock forming material is laid down is called sedimentation or deposition. It may settle in calmer waters of lakes or oceans or at places where the transporting agent has no longer enough energy to carry them farther. These are identified as riverine, lacustrine (formed by lake), glacial

or Aeolian (formed bywind) sedimentary rocks with reference to their deposition near rivers, lakes, glacier or deserts respectively.

The sediments are often loose, unconsolidated, soft rock material, in the beginning like sand and clay, but in course of time they get hardened to a compact material by excessive pressure and cementation to form sedimentary rocks. The deposition of sediments in the beginning is generally horizontal but it may get tilted afterwards due to movements in the earth‘s crust. Sandstone, shale, limestone and dolomite are examples of sedimentary rocks.

Sediments get sorted by the transporting agents. Sediments of different sizes mayget bound by cementing material under suitable conditions. Conglomerate is an example of such a sedimentary rock. This type of formation of consolidated material is termed as mechanically formed sedimentary rock. The consolidation of organic matter derived from plants and animals forms sedimentary rocks of organic origin. Coal and limestone are organic sedimentary rocks. The sediments may also result from chemical reaction. Direct precipitation of minerals from their solution in water may give rise to sedimentary rocks of chemical origin. Gypsum, rock salt and nitre are examples of such sedimentary rocks.

Huge folded mountains of the world like Himalayas, Andes etc. are made up ofsedimentary rocks. All the alluvial deposits of the world are also due to sedimentary accumulations. All river basins, particularly their plains and deltas, e.g. Indo Gangetic plain and Ganga-Brahmaputra delta are good examples of sedimentary accumulations.

Metamorphic Rocks

Most rocks in mountainous regions show an evidence of change. All these in course of time become metamorphic or changed forms of rocks. Metamorphic rocks are formed under the influence of heat or pressure on sedimentary or igneous rocks. Tremendous pressure and high temperature change the colour, hardness, structure and composition of all types of pre-existing rocks. The process which brings about the change is known as Metamorphism and the ultimate products, formed due to operation of such processes are defined as the metamorphic rocks.

Temperature, pressure stress and access of chemically reactive substances are themain agents, which are responsible for metamorphism. Heat causes the minerals to recrystallize in the rock. The process of change by heat is called thermal or contact metamorphism. When molten magma or lava comes in contact with surrounding rocks, it bakes them and changes them into metamorphic rocks. Similarly the formation of metamorphic rocks due to tremendous pressure is known as dynamic or regional metamorphism. Slate, gneiss, schist, marble and diamond are good examples of metamorphic rocks. Metamorphic rocks are hard and tough in comparison to the parent rocks from which they are formed.

Parent Rock and its Metamorphic Changed Form

NAME OF THE ROCK

TYPE OF ROCK

NAME OF THE METAMORPHIC ROCK

Limestone

Sedimentary Rock

Marble

Dolomite

Sedimentary Rock

Marble

Sandstone

Sedimentary Rock

Quartzite

Shale

Sedimentary Rock

Slate

Slate

Metamorphic Rock

Phyllite/Schist

Coal

Sedimentary Rock

Graphite/Diamond

Granite

Igneous Rock

Gneiss

Phyllite

Metamorphic Rock

Schist

The Rock Cycle

The rock components of the crust are slowly but constantly being changed from one form to another and the processes involved are summarized in the rock cycle. The rock cycle is driven by two forces: (1) Earth‘s internal heat engine, which moves material around in the core and the mantle and leads to slow but significant changes within the crust, and (2) the hydrological cycle, which is the movement of water, ice, and air at the surface, and is powered by the sun.

The rock cycle is still active on Earth because our core is hot enough to keep the mantle moving, our atmosphere is relatively thick, and we have liquid water. On some other planets or their satellites, such as the Moon, the rock cycle is virtually dead because the core is no longer hot enough to drive mantle convection and there is no atmosphere or liquid water.

In describing the rock cycle, we can start anywhere we like, although it‘s convenient to start with magma. As we‘ll see in more detail below, magma is rock that is hot to the point of being entirely molten. This happens at between about 800° and 1300°C, depending on the composition and the pressure, onto the surface and cool quickly (within seconds to years) forming extrusive igneous rock.

Magma can either cool slowly within the crust (over centuries to millions of years) — forming intrusive igneous rock, or erupt onto the surface and cool quickly (within seconds to years) — forming extrusive igneous rock. Intrusive igneous rock typically crystallizes at depths of hundreds of metres to tens of kilometers below the surface. To change its position in the rock cycle, intrusive igneous rock has to be uplifted and exposed by the erosion of the overlying rocks.

Through the various plate-tectonics-related processes of mountain building, all types of rocks are uplifted and exposed at the surface. Once exposed, they are weathered, both physically (by mechanical breaking of the rock) and chemically (by weathering of the minerals), and the weathering products — mostly small rock and mineral fragments — are eroded, transported, and then deposited as sediments. Transportation and deposition occur through the action of glaciers, streams, waves, wind, and other agents, and sediments are deposited in rivers, lakes, deserts, and the ocean.

Unless they are re-eroded and moved along, sediments will eventually be buried by more sediment. At depths of hundreds of metres or more, they become compressed and cemented

into sedimentary rock. Again through various means, largely resulting from plate tectonic forces, different kinds of rocks are either uplifted, to be re-eroded, or buried deeper within the crust where they are heated up, squeezed, and changed into metamorphic rock.

Economic Significance of Rocks

Man has been interacting with the surface of the earth since long. With time and advancement in technology he is making different uses of rocks and minerals. The importance of rocks is given below:

(a) Soils: Soils are derived from rocks. Soils provide suitability for those agricultural products that provide food and provide raw material for many industries.

(b) Building Material: Rocks are the source of types of building material directly or indirectly. Granite, gneiss, sandstone, marble and slates are extensively used in the construction of buildings. Taj Mahal is made of white marble, Red Forts of Delhi and Agra, are made of red sandstone. Slates are used for roof purposes in different parts of India.

(c) Mineral Source: Minerals are the foundation of the modern civilization. Metallic minerals provide all metals ranging from very precious gold, platinum, silver, copper to aluminum and iron. These metals are obtained from different rocks.

(d) Raw Material: Certain rocks and minerals are used as raw material for many industries. In cement industry and limestone kilns different type of rocks and minerals are used for production of finished goods. Graphite is used in crucible and pencil manufacturing as raw materials.

(e) Precious Stones: Precious stones and metals are obtained from different metamorphic or igneous rocks. Diamond is a precious stone used in jewelry and is a metamorphic rock. Similarly other precious stones like gems, rubies and sapphires are obtained from different type of rocks.

(f) Fuel: Fuel in the form of coal, petroleum, natural gas and nuclear minerals are derived from different rocks.

(g) Fertilizer: Fertilizers are also derived from some rocks. Phosphatic fertilizers are obtained from phosphorite mineral found in abundance in some parts of the world.

V. Endogenetic and Exogenetic Processes

Earth‘s surface is not stable but is changing constantly. None of the landforms found on the earth surface are forever. There are various geomorphic processes working on the landforms. These processes were working in the past and also working presently though with varying degree and intensity. The earth‘s crust is influenced by both internal and external forces, which may affect the earth on both major and minor scale. Internal forces may include upliftment, contraction, expansion, disruption, distortion and outpouring. External forces would include forces of denudation like weathering and erosion. The description of the landforms cannot be explained only through these forces since these changes are so slow that they go unnoticed. But when these changes are sudden like volcanic eruptions, landslide and earthquakes we can observe and notice these changes. The present landforms are a manifestation of complex and intricate interaction of earth‘s material facing resistance on one hand from and tectonically- and climatically-derived forces.

As we already know that earth‘s crust is constantly affected by the internal forces. The main causes of such internal forces can be the radio activity originating within the deep interiors of the earth‘s crust or substratum and the resultant convection currents caused by it. These

convection currents cause the crust to move which further causes sudden and rapid movements like earthquakes or extremely slow movement like mountain building and continental building which may take millions of years. These movements can cause upliftment, warping, turning, twisting, tilting, fracturing, subsidence and distortion of crust and some may even cause, squeezing of the rocks which would give rise to high mountain ranges.

There are broadly two kind forces that cause earth movements. They are as follows

Exogenetic forces/Exogenic/Epigene: Exogenetic is mostly destructive forces, which through various processes would try to change the various relief features on the earth surface by smoothening, carving and molding these features.

Endogenetic/Endogenic/Hypogene: (In Greek ‗endo‘ means within and ‗genera‘-origin) Endogenetic forces can give rise to various structural features on earth‘s surface, which are related to upliftment and subsidence, folding, faulting, fracturing and volcanic eruption. All the landforms and relief features found on the earth are formed of earth‘s materials. The landforms are formed because of various geomorphic processes operating on and beneath the earth‘s surface at differential rate. The exogenetic processes derive their energy from Earth's internal (endogenetic) which gives mobility to the earth‘s crust (tectonism) and from climate which takes further help from the sun which is its driving force.

Geomorphic Process

The formation and deformation of landforms on the surface of the earth are a continuous process which is due to the continuous influence of external (exogenic) and internal (endogenic) forces. The internal and external forces causing stresses and chemical action on earth materials and bringing about changes in the configuration of the surface of the earth are known as geomorphic processes. Diastrophism and volcanism are endogenic geomorphic processes. Weathering, mass wasting, erosion and deposition are exogenic geomorphic processes.

Geomorphic Agents

Any exogenic element of nature (like water, ice, wind, etc.,) capable of acquiring and transporting earth materials can be called a geomorphic agent. When these elements of nature become mobile due to gradients, they remove the materials and transport them over slopes and deposit them at lower level. Geomorphic processes and geomorphic agents especially exogenic, unless stated separately, are one and the same

Geomorphic Processes vs. Geomorphic Agents

A process is a force applied on earth materials affecting the same. An agent is a mobile medium (like running water, moving ice masses, the wind, waves, and currents etc.) which removes, transports and deposits earth materials.

Endogenic Forces

The forces which originate from within the earth‘s crust or inside the earth are called internal or endogenetic forces. The sources providing them energy are the internal heat,chemical reactions taking place within the earth, and the transfer of rock materials on the earth‘s surface by external forces.

The forces working from inside the earth in turn cause movements in its crust. These movements are called earth movements. Since, these movements pertain to or rise from, the movements of the actual structure of the earth‘s crust; they are also called tectonic movements.

On the basis of time taken by such movements, they are divided into:

v Slow movement (diastrophic) and

v Sudden movement.

Slow movements (diastrophic):

The movement which bring about changes on the Earth‘s crust very gradually or slowly taking hundreds or thousands of years and which cover a period much longer than a human life span are called slow movements. These movements act on the earth‘s crust either vertically or horizontally. Acting vertically, they cause uplift or subsidence of a part of the crust.

1. Vertical movements (Epeirogenetic/continental building)

Vertical movements originate from the centre of the earth and affect its surface. Consequently large scale uplift or subsidence of a part of the earth‘s surface takes place. These movements are slow and wide spread and do not bring changes in the horizontal rock strata. These movements are mainly associated with the formations of continents and plateaus, hence these are also known as continent building or plateau building movements.

In Greek ‗epeiroes‘ means a continent, these are very wide and large scale movements, spreading over the continental platform or the stable block of land. Therefore these processes are also known as continental formation. They characterizes by broad, gentle and wide spread warping, subsidence, upliftment, emergence, submergence, of large land areas. These movements are so slow and widespread that no obvious folding and fracturing can be seen in the rocks.

This broad regional tectonic movement with no local deformation can be either positive movement like upliftment or negative movement like subsidence. Upliftment movement causes the upliftment of the continental masses, either whole continent or the part of it. It also causes upliftment of the coastal land of the continents. Such type of upliftment is called emergence. Subsidence of the continental masses again happens in two ways – one is subsidence of the land area called subsidence, alternately the land near the sea coast is moved downward or is subsided below sea level and is thus submerged under sea water. Such movements are called submergence.

Generally sedimentary rocks are deposited and formed in the oceans and seas. The presence of these sedimentary rocks is wide-spread in continents. This clearly shows that these were uplifted or raised to form continents. Contrary to the above, there are countless evidences of submerged buildings, river -valleys and cities due to subsidence into the sea. Some of such examples include the submerged ancient buildings in Mediterranean in its Crete Island and the ancient city of Dwaraka in Saurashtra, India. These changes clearly point out the downward movement of the Earth‘s surface.

2. Horizontal Movements (Orogenetic/ mountain building)

There are forces which act on the earth‘s crust from side to side i.e. horizontally or tangentially. Naturally, they cause a lot of disruption in the horizontal layer of strata as they do involve a good deal of compression and tension of the preexisting rocks since these forces act horizontally or tangentially to the earth‘s spherical surface.

We can divide them into two types:

i) Forces of compression, and

ii) Forces of tension.

(i) Forces of compression: When crustal rocks are subjected to stress, particularly lateral compression, they are often deformed by being bent in a process called folding. In this way, the compression forces lead to the bending of rock layers and thus lead to the formation of Fold Mountains. In them the rock strata primarily of sedimentary rocks get folded, into wave like structure. This process of bending, sometimes warping and twisting of rock strata is referred to as their folding. The upfolds are called anticlines and down fold are called synclines.

Types of Folds

Structural geologists recognize many kinds of folds. A monocline is a one-sided fold—a slope connecting two horizontal or gently inclined strata. A simple symmetrical up fold is an anticline, and a simple down fold is a syncline. Also relatively common is an up fold that has been pushed so extensively from one side that it becomes over steepened enough to have a reverse orientation on the other side; such a structure is referred to as an overturned fold. If the pressure is enough to break the over steepened fold and cause a shearing movement, the result is an over thrust fold, which causes older rock to ride above younger rock.

Different kind of folds depends upon different factors like nature of the rocks involved (elasticity and rigidity), intensity and duration of the compressive forces (magnitude). Difference in the elasticity and the magnitude give rise to leads to difference in the inclinations of the limbs.

1. Symmetrical folds: both limbs inclined uniformly

2. Asymmetrical folds: both limbs inclined at different angles.

3. Monocline folds: one limb inclined moderately and one steeply inclined.

4. Isocline folds: both limbs become parallel to each other due to immense force but are not horizontal.

5. Overturned fold: limbs folded beyond vertical and turn so much that both the limbs bend in same direction.

6. Recumbent fold: The compressive force is so strong that both the limbs bed and become parallel to each other.

7. Nappes: When the pressure of the compressive force continuous and as a result the root of one of the limb is uprooted and thrust on the opposite limb

When folding takes place on a gigantic scale, it represents the mountain building process. Most of the great mountain chains of the world viz, the Himalaya, the Rockies, the Andes, the Alps and others of this sort have been formed by compressional forces resulting in mountain building on a large scale. These are also called Orogenetic Movements.

(ii) Forces of tension: are produced when these forces are working horizontally in opposite directions i.e., away from a given plane or point. Under the operation of intense tensional forces, the rock strata is broken or fractured. As a result cracks and fractures develop. The displacement of rocks upward or downward from their original position along such a fracture is termed as faulting. The line along which displacement of the fractured rock strata takes place is called the fault line. Likewise the plane along which displacement of rock strata takes place is known as fault plane.

Faulting results in the formation of well-known relief features such as rift valleys and the block mountains.

A rift valley is formed by sinking of rock strata lying between two almost parallel faults. A rift valley is a trough with steep parallel walls along the fault lines. Such a valley is also called a graben. Examples of rift valleys in the world include the Midland Valley of Scotland, the Rhine Valley, the Valley of Nile, the Dead Sea basin and the Great Rift Valley of East Africa comprising few lakes of this region.

A rift valley may also be formed by upliftment of two blocks along the fault line. These uplifted blocks are called horsts or block mountains. The well-known examples of horsts are the Vosges and the Black forest mountains on both sides of Rhine rift valley and the Plateaus of Palestine and Trans Jordan.

Types of Faults

Although structural geologists recognize more than a dozen kinds of faults, they can be generalized into four principal types on the basis of direction and angle of movement. Two types involve displacement that is mostly vertical, a third encompasses only horizontal movement, and the fourth includes both horizontal and vertical offsets.

2. Sudden Movements:

These movements cause considerable deformation over a short spans of time, and may be of two types:

(i) Earthquake:

It occurs when the surplus accumulated stress in rocks in the earth‘s interior is relieved through the weak zones over the earth‘s surface in form of kinetic energy of wave motion

causing vibrations (at times devastating) on the earth‘s surface. Such movements may result in uplift in coastal areas.

Another earthquake in Chile (1822) caused one-meter uplift in coastal areas. Another earthquake in New Zealand (1885) caused an uplift of upto 3 meters in some areas while some areas in Japan (1891) subsided by 6 meters after an earthquake.

Earthquakes may cause change in contours, change in river courses, ‗tsunamis‘ (seismic waves created in sea by an earthquake, as they are called in Japan) which may cause shoreline changes, spectacular glacial surges (as in Alaska), landslides, soil creeps, mass wasting etc.

(ii) Volcanoes

A volcano is formed when the molten magma in the earth‘s interior escapes through the crust by vents and fissures in the crust, accompanied by steam, gases (hydrogen sulphide, sulphur dioxide, hydrogen chloride, carbon dioxide etc.) and pyroclastic material. Depending on chemical composition and viscosity of the lava, a volcano may take various forms.

(a) Conical or Central Type:

When cooled lava particles from successive volcanic eruptions form a cone around the vent, a conical mountain shape emerges. This is a central type of volcano. Examples: Fujiyama (Japan) and Mount Vesuvius (Italy). The magma in such volcanoes is viscous, acidic and silicate.

(b) Shield Type:

The less viscous, less acidic and less silicate magma flows out slowly and quietly and gives rise to a wide-based plateau like formation with a gentle slope. Thus, a ‗shield shaped‘ volcano with thin horizontal sheets emerges. Example: Mauna Loa (Hawaii).

(c) Fissure Type:

Sometimes, a very thin magma escapes through cracks and fissures in the earth‘s surface and flows after intervals for a long time, spreading over a vast area, finally producing a layered, undulating, flat surface. Example: Deccan traps (peninsular India)

(d) Caldera Lake:

After the eruption of magma has ceased, the crater frequently turns into a lake at a later time. This lake is called a ‗caldera‘. Examples: Lonar in Maharashtra and Krakatao in Indonesia.

Volcanic Landforms:

The intrusive activity of volcanoes gives rise to various forms.

· Batholiths: These are large rock masses formed due to cooling down and solidification of hot magma inside the earth. Batholiths form the core of huge mountains and may be exposed on surface after erosion.

· Laccoliths: These are basically intrusive counterparts of an exposed domelike batholith.

· Dykes: These are solidified vertical lava layers inside the earth.

· Sills: These are solidified horizontal lava layers inside the earth.

· Other features created by volcanoes include hot water springs, geysers etc.

Exogenetic Forces

The exogenetic processes derive their energy from atmosphere determined by the ultimate energy from the sun and also the gradients created by tectonic factors. All the exogenic geomorphic processes are covered under a general term, denudation. The word ‗denude‘ means to strip off or to uncover. Weathering, mass wasting/movements, erosion and transportation are included in denudation.

The flow chart gives the denudation processes and their respective driving forces. It should become clear from this chart that for each process there exist a distinct driving force or energy. As there are different climatic regions on the earth‘s surface the exogenetic geomorphic processes vary from region to region. Temperature and precipitation are the two important climatic elements that control various processes.

Climatic factors being equal, the intensity of action of exogenic geomorphic processes depends upon type and structure of rocks. Different types of rocks with differences in their structure offer varying resistances to various geomorphic processes. And, under varying climatic conditions, particular rocks may exhibit different degrees of resistance to geomorphic processes and hence they operate at differential rates and give rise to differences in topography. The effects of most of the exogenic geomorphic processes are small and slow and may be imperceptible in a short time span, but will in the long run affect the rocks severely due to continued fatigue.

Weathering:

Weathering may be defined as the disintegration or decomposition of rock in place. It is really a name for a group of processes which act collectively at and near the earth’s surface and reduce rock masses to the clasticstate. It is a static process and does not involve the seizure and removal of material by a transporting agency. -Thornbury

Weathering is the first stage in the denudation (wearing away) of the exposed landscape. It is a process by which surface and subsurface rocks break up, dissolve, and decompose. All weathering activity occurs on the spot (in situ). Since Weathering disintegrates, dissolves, decomposes the rock, it facilities the movement of rock debris by mass wasting. It is a part of Exogenic process because it directly and indirectly derives required energy from insolation, which guides systems above the surface of the earth, including the atmosphere. Weathering is also an important process of earth system because it facilitates in the formation of mineral and soil development. 60 per cent population of our country is directly and indirectly dependent on agriculture which is to a great deal dependent on soil.

Weathering Grades:

We can also identify the different weathering grades of rock. Stage wise there are six grades of weathering. “Weathering grades of rock‖ ranges between fresh weathering grade to residual soil

.The fresh grade exhibits no visible sign of rock material. The slightly weathered stage is identified by discoloration of rock. When less than 50 per cent rock material is disintegrated or decomposed in a soil it is called moderately weathered stage in relation to present grade. In the highly weathered category more than 50 per cent rock material is disintegrated or decomposed in a soil. In the last and final stage all the rock material is converted to soil which is known as residual soil.

It is important to note that the rate of weathering depends on property of parent rock (mineral solubility and rock structure), climatic conditions (frequency of frost, rainfall, temperature and length of exposure to sunlight). The presence of organic activity in the soil also influences the rate of weathering. Weathering processes are largely determined by the climate and vegetation of a place. Dry locations are largely influenced by physical weathering and moist places by chemical weathering.

In general there are three types of weathering: mechanical, chemical and biological weathering. Mechanical weathering denotes the breakdown of rock without any conspicuous degree of chemical change in the minerals of the rock. Chemical weathering involves the decomposition of rocks in which decay of minerals are recorded. It is important to note that in real world mechanical and chemical weathering process may operate together though in differing

proportions. Biological processes also contribute to weathering. The biological weathering is any type of weathering that is caused by living organisms.

Physical Weathering Processes:

The process by which a rock or mineral is broken down into smaller fragments without apparently altering its chemical composition is called physical weathering. It transforms rock by breaking it into smaller fragments. This transformation takes place through mechanical methods such as freeze-thaw, salt weathering and thermal cracking. In the process of physical weathering the forces which break the rock may also originate within the rock or mineral, while others may begin from outside the rock or mineral. Both of these stresses lead to strain and the rupture in the rock but without any conspicuous change in the chemistry of the rock material. The activity of physical weathering breaks large pieces into smaller ones. The broken or detached fragments of rock are called clasts. The action of physical Weathering is more apparent in deserts, high mountains and arctic regions (Waugh, 1990).

There are several types of Physical Weathering Processes. For example, Frost wedging is more noticed when accumulated water in a fractured area freezes and creates pressure in the rock. Diurnal Temperature changes in places like hot deserts may cause rocks to break apart. Tree-root wedging may also enlarge cracks in rock. Similarly in dry climatic region, salt crystallization aids to split the rock. The cracks may also develop in the rock mass due to unloading of deeply buried rock by erosion of the overlying layers and consequent isostatic expansion of rock mass.

Freezing, Thawing and Frost shattering:

Generally most substance expands when they are heated and they also contract when they are cooled down. But water is an exceptional wonder molecule unlike any other substance which expands when it freezes. The total expansion of water is about 9 per cent in volume at 0°C, and further to 13.5 per cent at -22°C. It is also noteworthy that this expansion is also responsible for the bursting of water pipes in the polar and sub-polar regions. The water in the form of ice crystals can generate very strong cryostatic pressure (Outcome of weight and thickness of the ice) of 2100 kg cm-2 against the hollow cracks, crevices and joints in the rock. In cold climatic region rocks contain small cracks, which are sometimes filled with water during the day time or in summer season. Water in these cracks expands as it freezes. If cryostatic pressure exceeds the tensile strength of the rock, the rock breaks. This entire mechanical process is known as frost action or frost shattering.

Frost shattering is one of the most effective processes in the polar and sub polar region. Frost shattering creates widespread disintegration of the rock along its prearranged structural weakness present in the rock. Frost shattering is most effective in rocks containing numerous fractures or bedding planes (Thornbury, 1993). The block disintegration on steep hilly slope ultimately produces scree and talus at the foot of the hill. In areas having gentle slope the frost shattering produces bulky blockfiled which is also known felsenmeer (German meaning 'sea of rock') and everest. Blockfileds look like sea or streams of angular rocks. But they are different from screes and talus.

Blockfields

Expansion resulting from Unloading

The release in pressure in the rock can cause physical weathering. It happens due to removal of overlying material by erosion. This process is known as unloading. In this process the rocks isostatically and elastically compensate for the erosional loss of mass by expanding perpendicular to the direction of pressure release. This unloading of pressure causes the rocks to fracture horizontally. The number of fractures increases as the rock approaches the Earth's surface due to unloading process. The horizontally arranged fractures are also called sheet fracture or sheeting. In Italian Alps region many sheet fractures have developed after Pleistocene glaciations which are attributed to isostatic rebound. The weathering of granitic monolithic rock due to Unloading and Expansion in Bhuvangiri town (Andhra Pradesh) has also formed rounded dome shaped masses called exfoliation domes or onion peeled domes. Many such domes are also found in the Sierra Nevada range in California in United States, Stone Mountain in Georgia, and Half Dome in Yosemite National Park.

Thermal Expansion and contraction or Insolation Weathering

In desert regions of the world the large diurnal temperature changes are mainly responsible for mechanical weathering. We know that the rocks are composed of many mineral suites having varying physical property in terms of absorption, conductivity, and retention of heat. These unique properties of rock result in differential rates of expansion and contraction of rock mass. The surface of the rock expands more than its interior part, and this expansive stress ruptures the rock. The variation in the colours of mineral grains present in rock may also cause differential expansion and contraction of rock mass in terms of its volume. It is also important to note that the dark coloured grains, because of their absorptive properties, will expand much more than light coloured grains. Therefore ultimately the rock disintegrates along the boundary of mineral grains. This process is also known as insolation weathering. In desert environment the

moisture in form of dew creates cycle of hydration and dehydration of minerals which ultimately weakens the rock fractures for insolation weathering.

Crystallization of Salts:

Salt weathering represents mechanical weathering of rock because no conspicuous chemical alteration of rock constituents is involved in this process. The saline water found in pore spaces evaporates during the high temperature i.e., during the day time or in dry conditions. It leaves behind the salt crystals in the rock‘s small opening. The expanding salt crystals exert a pressure on the walls of the host rock pores that exceeds the tensile strength (The ability of a material to withstand tension) of the rock. This process of falling off of individual grains may result in granular disintegration or granular foliation of rock generally effective in coarse crystalline igneous rocks. The magnitude of this stress, however, varies with the composition and concentration of ionic species, and the manner of crystallization at and within the impacted surfaces (Smith, 1994). Sodium chloride and gypsum crystals in desert areas lift overlying layers of materials which results in cracks in host rock. With salt crystal growth, chalk breaks down most readily, followed by limestone, sandstone, shale, gneiss and granite. For example, the capillary salts, including nitrates and nitrites, have caused extensive scaling, crumbling, and discoloration of the outer and inner building materials of the Islamic monuments of Khiva and Bukhara in Uzbekistan.

Chemical weathering Processes:

Chemical weathering decomposes the rock and creates a new chemically different material that has undergone weathering. The tendency of the minerals to strive for equilibrium with the environment is called chemical weathering (Sharma, 2010). In general, it is probably true that chemical weathering is more important than physical weathering ((Thornbury, 1993).) Several chemical reactions occur in the Earth-surface environment. In order to understand chemical weathering we need to know the 'raw materials' for chemical reaction found in the rocks found on the earth and Earth-surface environment. The main agents involved in chemical weathering are as follows:

· Oxygen, carbon dioxide and other gases from the atmosphere;

· Water from precipitation in the form of rain and snow; and

· Minerals present in the rock composition.

Rocks can be composed of a single type of element, or more than one element. These composite substances are known as minerals. Minerals are chemical substances found in the rock. It is important to note that, internal structure of minerals is the main factor, which controls all sorts of chemical and physical properties of any rock. The strength of the bond largely determines chemical alteration of minerals through chemical reaction.

During chemical weathering, the internal molecular structure of a mineral is altered by reaction with other elements. For example, dissolved oxygen in water has potential to oxidize minerals that contain iron; this process is known as oxidation. Similarly, Carbon dioxide dissolved in water forms creates carbonic acid that can effectively dilute limestone or change the feldspar in granite into clay minerals. This process is known as carbonation. In hydrolysis process pure water can also combines with some minerals to form new by-products. Water may also soften some minerals through its dissolving capability (solution).

Salient types of chemical weathering are as follows:

Oxidation and Reduction

In chemistry oxidation does not necessarily imply oxygen, but simply the loss of electrons. It involves a chemical reaction with oxygen. Conspicuous example can be given from rusting of a mineral. In Earth-surface environment, the element that is most prone to oxidation is iron (Fe). Most of the minerals found in igneous and metamorphic rocks contain Fe2+ ions which react with the oxygen. The signs of oxidation can be noticed in shallow water bodies and aquifers carrying organic matter.

Solution

Solution is generally the first stage of chemical weathering. It can take place in a thin film of water around a solid particle or in running water. A solution is a homogeneous mixture in which one substance called the solute is dispersed in another substance called the solvent. The solutions are everywhere around us. The Ocean is also a solution, consisting of many salts such as sodium chloride dissolved in water. The Pure water is also an effective agent in dissolving some minerals. The water can dissolve soluble rock-forming minerals and break down the molecular structure of certain minerals. The dissolved salt may also form salt crystals to facilitate physical withering mechanism indirectly. Solution tends to be most effective in areas that have hot humid climates.

Soil Collides may have the power to loosen or pull off small bits of rock from the surfaces with which they come in contact. This weathering process of uncertain importance is called collide plucking. (Thornbury, 1993)

Carbonation

Carbonation is the reaction of carbonate and bicarbonate with minerals. The formation of carbonates usually takes place as a result of other chemical processes. The atmosphere contains carbon dioxide (CO2). When rain falls, water and carbon dioxide combine (CO2 + H2O = H2CO3) and forms carbonic acid that is H2CO3, which in turn helps in dissolving minerals, particularly those susceptible to weak acids, such as calcite, dolomite, and marble. Carbonation is especially active when the reaction environment is abundant with carbon dioxide. The carbonation process is especially active in karst regions of the world. The solubility of karst relies on the volume of carbon dioxide stored in karst systems.

Hydration

Hydration actually denotes addition of water to a mineral. It denotes chemical combination of water molecule with other mineral substances. According to Petersen et.al (2011) in weathering by hydration, water molecules attach to the crystalline structure of a mineral without causing a permanent change in that mineral‘s composition. The water molecules are able to join and leave the ―host‖ mineral during hydration and dehydration, respectively. In many situations the H+ and OH ions (An atom, molecule or compound that carries either a positive or negative electrical charge) become a structural part of the crystal lattice or network of the mineral. The hydrated minerals expand many times the volume of their original state which creates mechanical stress in rock to disintegrate. The cycle of hydration and dehydration in minerals further boosts the stress mechanism eventually breaks the rock. Hydration also accelerates decomposition reactions by expanding the crystal lattice (network), which in turns creates more surface area for further reaction and extra surface area for physical weathering. Iron oxide, for example can absorb water and turn into hydrated iron oxides or iron hydroxide.

Biological Activity and Weathering:

The word ‗bio‘ means life. Thus biological weathering is any type of weathering that is caused by living organisms. The biological weathering has common characteristics of both physical and chemical groups of activities in disintegration and decomposition of rock.

Role of plants and animals:

For example, Plant roots penetrate into cracks in rock and slowly break the rocks apart mechanically. The roots of the plant also secrete certain chemicals to extract minerals, thereby causing chemical weathering. The major weathering agents produced by plants include CO2, organic acids, and ligands. The chelation is an important biochemical process generally occurs

in the roots of the plant. The chelationbreaks down the mineral structure by dissolving and dislocating metallic and non-metallic minerals which in turn reduces the toxicity in natural ecosystem. The specific process or set of plant related weathering positive feedback processes may increase weathering, whereas negative (-) feedbacks are likely to decrease weathering. The growth of plants on the upper portion of the historical Bharateswara temple has deteriorated the condition of the temple.

Animals and insects that den/nest/cave into soft rocks are also mechanical weathering agents. The earthworms are found in most of the moist parts of the world with the exception of extreme temperatures. They till, aerate and decompose the soil minerals. Similarly, termites and ants found in loose minerals of the soil are responsible for underground constructions. Many animals in the world dig up the earth for a variety of reasons: to dig up food, either floral or faunal; to cache provisions; and to excavate habitations. All above mentioned activities help in exposing the new surfaces to chemical attack.

Anthropogenic activity:

One of the most effective agents of biological weathering in the present time is the human beings. Mining and quarrying, highway construction projects and excavations for building foundations and basements all lead to significant weathering of the surface of the earth.

Significance of Weathering

Weathering is an important mechanism which weakens surface materials. This process is in turn is helpful for their eventual removal of material by erosion processes. It contributes to the formation of soil by providing mineral particles like sand, silt, and clay. The weathered rock fragments in the form of deposits make fertile plains, beaches and sand dunes. The weathering of minerals (from rocks) is of great importance for the economy of any country. The minerals created by weathering processes supply nutrients for plant uptake. Weathering liberates chemical compounds which are directly and indirectly helpful to suppress toxics created in the biosphere. Therefore, weathering is a process that is essential to many other aspects of the hydrosphere, lithosphere, and biosphere.

Mass Wasting:

The surface of the earth is everywhere attacked by forces that lead to decomposition and disintegration of rocks. The layer of waste this produces is called regolith. It is unconsolidated matter, and a mixture of rock pieces and fine soil. Lying on a sloping surface, regolith is pulled by the earth‘s gravitational force and moves down. This down-slope movement of weathered material under the influence of gravitational influence is defined as ‗Mass wasting‘, also termed

‗Mass movement‘. Sometimes mass wasting may also involve intact rock beds. The process has a variety of rates and mechanics of movement; involves particles ranging from minute fine clay

to massive rock beds; leads to impacts ranging from insignificant events to large scale disaster, and results in creation of various landforms.

According to A. S. Goudie – ―A mass movement is the downward and outward movement of slope-forming material under the influence of gravity.

Mechanics of Mass wasting:

Basically it is gravity, which is analyzed into two main components that together control movement of regolith on all slopes. These are:

· Slide component, also called stress;

·

Stick component, also called friction or shear resistance.

1. Slide component works in the down slope direction and pulls the rock mass towards foot of a slope, this leads to ‗stress‘ or tension between the solid unweathered rock surface and the unconsolidated regolith lying on it. It also exerts stress along bedding planes, joints, crevasses and fractures within a solid rock body. Even the mineral grains within a rock respond to this stress, and internal stress is experienced by the matter.

2.

Stick component works in a direction perpendicular to the slope and creates friction between regolith and slope and at all the other sites that are under stress, mentioned above. The friction created by stick component counteracts the downward pull. Whether a mass would move or not is decided by the critical balance between these forces, which in turn depends primarily on the steepness of slope.

Types of Mass wasting:

Solifluction

It can be translated as ‗soil flow‘. As the name indicates it is a sub-type of flow movement. In this type surface is covered with water saturated regolith. As water content increases soil cover changes to a soggy matter and loses cohesive strength. This weakens friction or stick component, enabling gravity to move the weathered layer on the slope.

Solifluction/gelifluction is a slow process as compared to the other types. Mostly it covers extensive areas. The whole of Siberian tundra and Greenland hillsides are an evidence of its regional scale. Ground influenced by this type of mass wasting is broken into gently sloping, terrace-like features, which are sometimes bound by vegetative growth.

Soil creep:

This type of mass wasting is present almost everywhere on the earth‘s surface, because first, it requires minimal support from steepness of surface and second, it can involve any type of debris. Third, it also does not need moisture, hence can happen in all types of climates. It follows mechanics of heave. Areas experiencing soil creep show several symptoms that are easy to identify trees with downward bending lower trunks, tilt in exposed rock beds, broken fences and tilted electricity poles are some examples.

In mountainous areas, heaps of weathered matter collects at the base of slopes as talus and scree cones due to soil creep. In these cones, the frontal margin of cone migrates forward as soil creep continues and cones keep growing in size. Similar to soil creep is rock creep, which involves large boulders and rock chunks.

Mud flow and earth flow:

The mechanics of earth flow and mud flow are similar to any other flow type of mass weathering; however, both have certain distinguishing features. Mud flow is large-scale, channelized movement while earth flow may happen on any surface in a localized manner.

Mud flow is confined to valleys and is a rapid movement. Arid regions have the most ideal conditions for it. Here seasonal streams cut valleys in mountainous areas. For most of the year, or sometimes for several years, the valleys stay dry and receive large amount of weathered debris from slopes. When eventually rains come, they are usually torrential and last for a short while (a general characteristic of precipitation in hot arid areas). As water is channelized and moves down the valleys, it keeps on incorporating accumulated rock debris from the valley floor. Absence of vegetative cover in these regions further supports this process. This mass has great velocity, erosive and transporting capacity. Movement stops under two conditions only – one, the matter becomes thick in consistency and is no longer fluid; two, flow reaches flat foothill area. At the terminal point all material brought down is left as a mound or in a fan shaped feature, called alluvial fan. Eliot Blackwelder (1928) considered mud flow an important geomorphic agent in

arid and semi-arid regions. He explained long distance transportation of huge rocks in these regions as the work of mud flow.

Slide and slump:

Both these types follow slide mechanics and their rate of movement is fast. For initial movement presence of water or ice as lubricant is not needed but rate of movement accelerates if regolith is wet. The distinction between these two is marked by difference in their shear planes. Slide has a rectilinear shear plane while slump has a concave upward, or spoon-shaped shear plane. Slide is again classified according to nature of weathered material. Debris slide, rock slide, mud slide and landslide are some examples.

Landslide is the most devastating, sudden and rapid slide movement. It is capable of transporting and burying settlements and destroying the ecosystem for a long time. It happens on slopes that have a combination of all positive stress and negative resistance factors, but the actual movement begins when it is triggered by some sudden change like heavy rain or earthquake. Mountains experience most landslides during rainy seasons. Rainwater increases weight of regolith and also lubricates it. In cold regions snow plays a similar role. Sometimes frozen rock mass may begin to slide, but as the heat of friction melts ice crystals and water content increases, the movement transforms into flow type.

Sub marine landslides

® Slump, mud flow and landslides keep happening all along shorelines.

® They carve network of canyons in off shore belts.

® Undersea cables are damaged as sediment travels with great velocity here.

® 1929 Grand Banks Earthquake (North East America) caused submarine landslide displacing 200 km3 sediment and snapped 12 submarine transatlantic telegraph cables

Debris fall and rock fall:

Velocity in fall type ranks highest among all mass wasting types. Movement in fall type is vertical or almost vertical, and long cliff faces are an ideal site for it. The motion may start as slide, but may change to topple and terminate as fall. Slopes that have experienced glaciation are susceptible to rock falls (Arthur Bloom 1998), because unloading and frost action separate large rocks from the main slope; later, these blocks may be shaken off by triggers like earthquake tremors. Falling rock pieces pose danger to lower slopes, especially transport links and settlements. To manage this risk, rocks have been covered by steel net along the Sion-Panvel highway.

Subsidence:

This type of mass wasting does not involve a shear plane or horizontal displacement. Surface layers move almost vertically downwards when they lose support from beneath. Sub-surface weakness may be caused by several factors, for example:

® Mining,

® Excess withdrawal of underground water,

® Removal of rocks by carbonation in Karst regions,

® Removal of sub-surface fluid lava.

Other types:

Besides the above mentioned types of mass wasting, there are other examples with minor variations in characteristics. Some of these are:

· Avalanche – Any sudden and disastrous landslide may be called avalanche (Bloom 2003). It is a phenomenon of humid regions (Thornbury, 1969). It involves weathered rock mass,

(that may have ice), both slide and flow mechanics, and rapid movement. If a slope experiences repeated debris avalanche, clear channels are cut, called ‗debris chute‘.

· Rockglide and topple – Large rock blocks are moved if the surface beneath them is lubricated or is softened to move plastically. The surface blocks simply ride over the mobile underlying material. Destabilized while gliding, these blocks tend to fall, called toppling.

· Spreading – This involves multiple blocks. The mechanism of this movement is similar to rock glide, but it is a lateral movement. Cambering is an example of this type. Cambering takes place in glacial environment, where sediment moves from hill sides towards valley, and carries along sedimentary blocks.

· Liquefaction – Solid surface (like sand and clay) is shaken during earthquake tremors and grain compaction within rock is loosened. This allows clay-rich rocks to behave like plastic matter, causing a typical spread movement known as ‗liquefaction‘. It can uproot buildings from their foundations and move them.

Erosion

Erosion is the process of eroding or being eroded by wind, water, or other natural agents.

Water Erosion

Water is the most important erosional agent and erodes most commonly as running water in streams. However, water in all its forms is erosional. Raindrops (especially in dry environments) create splash erosion that moves tiny particles of soil. Water collecting on the surface of the soil collects as it moves towards tiny rivulets and streams and creates sheet erosion. In streams, water is a very powerful erosional agent. The faster water moves in streams the larger objects it can pick up and transport. This is known as critical erosion velocity. Fine sand can be moved by streams flowing as slowly as three-quarters of a mile per hour.

Streams erode their banks in three different ways: 1) the hydraulic action of the water itself moves the sediments, 2) water acts to corrode sediments by removing ions and dissolving them, and 3) particles in the water strike bedrock and erode it.

The water of streams can erode in three different places: 1) lateral erosion erodes the sediment on the sides of the stream channel, 2) down cutting erodes the stream bed deeper, and

3) headward erosion erodes the channel upslope.

Wind Erosion

Erosion by wind is known as Aeolian (or eolian) erosion (named after Aeolus, the Greek god of winds) and occurs almost always in deserts. Aeolian erosion of sand in the desert is partially responsible for the formation of sand dunes. The power of the wind erodes rock and sand.

Ice Erosion

The erosive power of moving ice is actually a bit greater than the power of water but since water is much more common, it is responsible for a greater amount of erosion on the earth's surface.

Glaciers can perform to erosive functions -they pluck and abrade. Plucking takes place by water entering cracks under the glacier, freezing, and breaking off pieces of rock that are then transported by the glacier. Abrasion cuts into the rock under the glacier, scooping rock up like a bulldozer and smoothing and polishing the rock surface.

Wave Erosion

Waves in oceans and other large bodies of water produce coastal erosion. The power of oceanic waves is awesome, large storm waves can produce 2000 pounds of pressure per square foot. The pure energy of waves along with the chemical content of the water is what erodes the rock of the coastline. Erosion of sand is much easier for the waves and sometimes, there's an annual cycle where sand is removed from a beach during one season, only to be returned by waves in another.

VI. Earthquakes, Volcanoes and Tsunami

Earthquakes

An earthquake is a sudden movement of the Earth, caused by the abrupt release of strain that has accumulated over a long time. For hundreds of millions of years, the forces of plate tectonics have shaped the Earth as the huge plates that form the Earth's surface slowly move over, under, and past each other. Sometimes the movement is gradual. At other times, the plates are locked together, unable to release the accumulating energy. When the accumulated energy grows strong enough, the plates break free.

The magnitude or intensity of energy released by an earthquake is measured on the Richter scale. The place of the origin of an earthquake is called focus which is hidden inside the earth. The place on the ground surface which is perpendicular to the buried focus is called 'epicentre'. Seismic waves are recorded by an instrument called 'seismograph'.

Earthquakes can be caused by a variety of things, including meteor impacts and volcanic eruptions, and even sometimes man-made events like mine collapse and underground nuclear tests. But most naturally occurring earthquakes are caused by movement of pieces of the earth's surface, which are called tectonic plates.

Different theories that explain the origin of earthquakes

a) Faulting and Elastic Rebound Theory:

The horizontal and vertical movements caused by endogenetic forces result in the formation of faults and fold which in turn cause isostatic disequilibrium in the crustal rocks which ultimately causes earthquakes of varying magnitudes depending on the nature and magnitude of dislocation of rock blocks caused by faulting and folding. In fact, sudden dislocation of rock blocks caused by both tensile and compressive forces trigger immediate earth tremor due to sudden maladjustment of rock blocks.

A fault is a fracture in the Earth's crust along which two blocks of the crust have slipped with respect to each other. Faults are divided into three main groups, depending on how they move. Normal faults occur in response to pulling or tension; the overlying block moves down the dip of the fault plane. Thrust (reverse) faults occur in response to squeezing or compression; the overlying block moves up the dip of the fault plane. Strike-slip (lateral) faults occur in response to either type of stress; the blocks move horizontally past one another. Most faulting along spreading zones is normal, along subduction zones is thrust, and along transform faults is strike- slip.

According to the theory the underground rocks are elastic like rubber and expand when stretched and pulled. The stretching and pulling of crustal rocks due to tensile forces is slow process. The rocks continue to be stretched so long as the tensile forces do not exceed the elasticity of the rocks but as the tensile forces exceed the rocks elasticity they are broken and broken rock blocks try immediately to occupy their previous positions so that they may adjust

themselves. All these processes occur so rapidly that the equilibrium of the concerned crustal surface is suddenly disturbed and hence earth tremors are caused.

b) Hydrostatic Pressure and Anthropogenic Causes:

Certain human activities such as pumping of ground water and oil deep underground mining, blasting of rocks, nuclear explosion, storage of huge volume of water causes tremors. The introduction of additional artificial, superincumbent load through the construction of large dams and impounding of enormous volume of water cause disequilibrium of already fragile structures due to faults and fractures.

c) Volcanic Eruptions:

When volcanoes erupt it is because the molten magma under the crust of the earth is under enormous pressure and to release that pressure it looks for an opening and exerts pressure on the earth's crust and the plate in turn. A place, which is the seat of an active volcano, is often prone to earthquakes as well. Earthquakes are also caused after a volcanic eruption since the eruption also leads to a disturbance in the position of plates, which either move further or resettle and can result into severe or light tremors.

d) Plate Tectonic Theory

Finally, in the mid-1960s, researchers in the United States and Great Britain came up with a theory that explained why the Earth shook. The theory, called plate tectonics, is that the Earth's crust, or lithosphere, is comprised of many plates that slide over a lubricating as the no sphere layer.

Plate tectonics confirms that there are four types of seismic zones. The first follows the line of mid-ocean ridges. Activity is low, and it occurs at very shallow depths. The point is that the lithosphere is very thin and weak at these boundaries, so the strain cannot build up enough to cause large earthquakes. Associated with this type of seismicity is the volcanic activity along the axis of the ridges (for example, Iceland, Azores, Tristan da Cunha).

The second type of earthquake associated with plate tectonics is the shallow-focus event unaccompanied by volcanic activity. The San Andreas fault is a good example of this, so is the Anatolian fault in Northern Turkey. In these faults, two mature plates are scraping by one another. The friction between the plates can be so great that very large strains can build up before they are periodically relieved by large earthquakes. Nevertheless, activity does not always occur along the entire length of the fault during any one earthquake. For instance, the 1906 San Francisco event was caused by breakage only along the northern end of the San Andreas fault.

The third type of earthquake is related to the collision of oceanic and continental plates. One plate is thrust or subducted under the other plate so that a deep ocean trench is produced. In the Philippines, ocean trenches are associated with curved volcanic island arcs on the landward plate, for example the Java trench. Along the Peru -Chile trench, the Nazca plate is being subducted under the South American plate which responds by crumpling to form the Andes. This type of earthquake can be shallow, intermediate, or deep, according to its location on the down going lithospheric slab. Such inclined planes of earthquakes are known as Benioff zones.

The fourth type of seismic zone occurs along the boundaries of continental plates. Typical of this is the broad swath of seismicity from Burmato the Mediterranean, crossing the Himalayas, Iran, and Turkey, to Gibraltar. Within this zone, shallow earthquakes are associated with high mountain ranges where intense compression is taking place. Intermediate- and deep-focus earthquakes also occur and are known in the Himalayas and in the Caucasus. The interiors of

continental plates are very complex, much more so than island arcs. For instance, we do not yet know the full relationship of the Alps or the East African rift system to the broad picture of plate tectonics.

Effects

· Landslides and damming of the rivers in highland regions.

· Causes depression forming lakes. May cause faults, thrusts, folds, etc.

· Formation of cracks or fissures in the epicenter region and sometimes water, mud, gas are ejected from it.

· Causes the raising or lowering of parts of the sea floor e.g. "Sangami bay" in 1923. This causes "tsunamis" or tidal waves.

· May change surface drainage & underground circulation of water like the sudden disappearance of springs in some places.

· Rising and lowering of crustal regions for example in Alaska in 1899-16 m upliftment.

· Devastation of cities, fires, diseases, etc.

Distribution of Earthquakes

The world maps of the distribution of earthquakes prepared by the seismologists on the basis of computer analysis and simulation of 30,000 earthquakes have identified the three main zones of earthquake.

1. Circum Pacific Belt or Ring of Fire surrounding the Pacific Ocean. It is a junction of continental and oceanic margins; it is a zone of young folded mountains; it is a zone of active volcanoes thus this belt accounts for the 65 per cent of the total earthquakes of the world.

2. Mid-Continental Belt representing Alpine-Himalayan chains of Eurasia and northern Africa and epicenters of east African faultzone. This belt represents the collision or subduction zones of continental plates. About 21 per cent of the total seismic events occur in this belt.

3. Mid-Atlantic Belt representing the earthquakes located along the mid-Atlantic Ridge and its offshoots. This belt records moderate and shallow focus earthquakes which are essentially caused due to creation of transform faults and fractures because of divergent movement of plates.

Volcanoes

A volcano is an opening or rupture in the Earth's surface or crust, which allows hot molten magma, ash and gases to escape from deep below the surface. The term volcano in general refers to the landform built up from the accumulation of lava and pyroclastic debris. Therefore they are very different from other mountains; since they are not formed by folding and crumpling or by uplift and erosion.

Volcanoes are spectacular windows to the Earth's internal parts. These natural windows allow us to catch a glimpse of the inaccessible depths of the surface of the Earth. Volcanic activity directly provides an explanation that the earth‘s interior part must be experiencing high pressure and temperature. There are more than 500 active volcanoes on Earth‘s surface, including well-known examples such as Mt. Fuji, Mt. St. Helens, and Mauna Loa.

W.M. Davis (1905) treated volcanism as an ―accident‖ that occurs so arbitrarily in time and place and is so disruptive to the erosional development of landscapes that the landforms cannot be treated in a systematic manner.

On the basis of frequency of eruption, there are active, dormant and extinct or ancient volcanoes. The volcanoes which erupt fairly frequently as compared to others are active. Only a few volcanoes remain more or less continually in eruption for long periods, but intermittent activity is more common. The dormant (from Latin word dromir, meaning, ‗to sleep‘) volcanoes are those in which eruption has not occurred regularly recently. These volcanoes undergo long intervals of repose during which all external signs of activity cease. Those volcanoes in which no eruption has been recorded in historic times are said to be extinct. Before a volcano becomes extinct, it passes through a waning stage during which steam and other hot gases and vapours are exhaled. These are known as fumaroles or solfataras.

Causes of Volcanic Activity:

For volcanologist it is still a puzzling question that why do volcanoes erupts. Various hypotheses have been proposed. It is assumed that in the beginning Earth was in molten state. Then gradually it radiated its heat and cooled down. Earthquake waves suggest that the earth‘s mantle and core is hot which melts the rocks and creates the magma. The weaker areas within the

crust allow magma and gases to penetrate the rocks and may come out in the form volcanic eruption.

The radioactive substances inside the earth keep generating a lot of heat through decomposition and chemical reactions. As a result the material in the earth‘s interior is in constant flux. This molten, semi-molten and sometimes gaseous material appears on earth at the first available opportunity. This opportunity is provided by weak zones along the earth‘s surface. The earthquakes, for instance, may expose fault zones through which magma may escape. Because of high pressure in the earth‘s interior, the magma and gases escape with great velocity as the pressure is released through eruptions.

Before the eruption volcanoes may also give early warning signals by producing a rushing, a hissing and a coughing and puffing sounds. However it is still difficult to predict the volcanic eruption. Nevertheless scientists in order to predict the volcanic eruption monitor seismic activity in volcanic are, the geochemical conditions, change in temperature, topographical change and change in gas composition. The major human adjustment to volcanic activity is evacuation. Education and awareness plays an important role in informing people about the hazards of volcanoes

Structure of a Volcano:

Commonly a cone shaped volcano constitutes a vent, a pipe, a crater, and a cone. The volcanic vent is an opening at the Earth's surface through which volcanic materials are erupted at the surface of the earth. The shape of the vent may be roughly circular conduits or elongated fissures. The pipe is a passage way in the volcano in which the magma rises through to the surface during an eruption. The crater is a steep bowl-shaped depression at the top of the volcano out of which volcanic materials like, ash, lava, and other pyroclastic materials are released. The molten material beneath the surface of the Earth is called magma. Once it starts moving towards the crust or it reaches the surface, here it is referred to as lava.

Distribution of volcanoes and volcanic activity

Most of the world‘s volcanoes and volcanic activity can be sighted along the plate boundaries. The distribution can be classified into one of the following tectonic settings:

(1) Subduction Zones in the Circum Pacific Belt

The zones where one plate goes down under the other due to density difference are the sites of most of the world‘s active and explosive volcanoes. The oceanic plate having higher density is subducted under the continental crust. The subducted slab melts under the increasing pressure and temperature to produce magma which comes out through andesitic chain of volcanoes. The volcanoes are mainly situated on the continental side of the trenches. The figure portrays that the so called “Pacific Ring of Fire” is the collection of volcanoes bordering the Pacific Ocean. This zone is in fact a ring of subduction zones.

It includes some of the deadliest volcanoes known, such as Pinatubo and Mt. St. Helens. The figure demonstrates that it starts from the Andean region of South America and extends northwards through Central America, Mexico, Western U.S.A. and Canada to Alaska. From Alaska it extends through Aleutian Islands towards the islands off the eastern coast of Asia and passes through Kamchatka, the Kurile Islands, Japan, the Philippines and further south to the New Guinea, Solomon Islands, New Zealand and Antarctica. The volcanic belt of the Indian Ocean which passes through Andamans, Sumatra, Java, Bali, Sunda and Burma meets the Pacific belt near the Malacca Island.

(2) Divergence zones: volcanoes of the Mid Atlantic Ridge and over the Continents

In plate tectonics, a divergent boundary is a linear feature that exists between two tectonic plates that are moving away from each other. For example the Mid-Atlantic Ridge separates the North and South American Plate from the Eurasian and African Plate.

The figure demonstrates that, this pulling apart is causing "sea-floor spreading" as new volcanic material is added to the oceanic plates. The spreading sites are the common sites of basaltic lava eruption. On the whole, sea-floor spreading is basically volcanic, but it is a very slow and regular process, without the explosive outbursts of the volcanoes on land. Magma rises through the cracks and leaks out onto the ocean floor like a long, thin, undersea volcano. As magma meets the water, it cools and solidifies, adding to the edges of the sideways-moving plates. This process along the divergent boundary has created longest topographic feature in the form of Mid oceanic ridges under the Oceans of the world Most of this activity is out of sight under the oceans which is less hazardous to people.

The map shows that over the continents, the divergence zones with fissure type of volcanic eruptions are represented by the East African Rift Valley Zone. This belt extends from Ethiopia to Tanzania. The Kilimanjaro in Tanzania is a well-known example of this belt.

(3) Intra-plate oceanic volcanism (Hawaiian chain and other oceanic volcanic seamounts)

Intra-plate oceanic volcanism can be represented by a single oceanic volcano, or lines of volcanoes such as the Hawaiian-Emperor seamount chains. They are also popular as hotspots and are located within the tectonic plates instead of plate margins. The map demonstrate that the Hawaiian volcanoes are located well within the Pacific plate rather than near a plate boundary.

(4) Mid Continental belt and Volcanoes in the Mediterranean region:

This belt is extended from the Mediterranean Alps to the Himalayan Region. Most often visited active volcanoes are found in this belt. Vesuvius and Stromboli are well known example of this belt. Mount Etna in Sicily is Europe‘s largest volcano. Its frequent eruptions often attract visitors.

Types of volcanoes

Volcanologists have developed many schemes to categorize volcanoes into various types. On the basis of Periodicity of Eruption, Nature of Eruption and shape and morphology of volcanoes, the volcanoes can be classified into various types. The most popular classification is based on periodicity of Eruption i.e. active, dormant and dead volcano.

(A) Basis: Periodicity of Eruption

(i) Active volcano- A volcano is generally considered active if it has erupted within recent history. It is generally regarded as active so long as the magma reservoir is present. The map shows location of the recently active volcanoes of the world. It can be observed from the map that most of them are located in the so called ―Pacific Ring of Fire belt”. St. Mount Helens in Cascade Range of United States is a well-known example of this category. The catastrophic eruption of Mount St. Helens on May 18, 1980, and related mudflows and flooding caused significant loss of life and property damage (USGS).

(ii) Dormant Volcano- When the volcano has not erupted in recent times but is fresh looking is regarded as dormant. A dormant volcano exhibits no indication for future eruption but they may erupt suddenly and violently causing enormous damage to life and property. Vesuvius volcano is a classic example of this category which erupted in 79 A.D and destroyed the Roman cities of Pompeii and Herculaneum. It again erupted in 1631 A.D. the frequency of eruption again increased in 19th century.

(iii) Extinct- A volcano is considered extinct when it has no recent eruptive history. Impact of erosion can be seen on this type of volcano. They are unlikely to erupt again, for example Arthur‘s Seat in Scotland.

(B) Basis: Nature of Eruption

The most commonly used classification by volcanologist is that originally proposed by Lacroix in 1908. There are four principle types of eruptions i.e., Hawaiian, Strombolian, Vulcanian and Plinian. The figure demonstrates that degree of explosiveness, height of eruption and viscosity of magma increases from Hawaiian to Plinian type of volcano.

(C) Basis: The Shape and Morphology of Volcanoes

On the basis of shape and morphology volcanoes can be classified into shield, Cone and Composite cones, or strato volcanoes.

(i) Shield volcanoes:

The basaltic lavas are comparatively fluid and spread quietly, easily and widely. The figure shows that the gradual build-up of thousands of these flows slowly constructs broad flat shield volcano. They are named as shield volcanoes because they are shaped like a warrior‘s shield. The classic example of a shield volcano is Mauna Loa on Hawaii.

(ii) Cone volcanoes or Cinder Cones

Cinder cones are the most abundant of all volcanoes. Small cones consisting mostly of pyroclastic debris each having a single vent are called cinder cones. When pyroclastic fragments fall and accumulate close to the vent, they may pile up to form a very symmetric cinder cone. The figure shows a classic example of Cinder cones on Lanzarote Island of Canary Islands.

(iii) Composite cones or Stratovolcanoes

Sometimes pyroclastic material either flows through break in the crater wall or comes out from edges of the cone which forms composite volcanoes or alternatively, stratovolcanoes. The figure shows that they are known as composite volcanoes because they are built up of layers of more than one kind of material. The mix of lava and pyroclastic allows them to grow larger than either cinder cones or volcanic domes. The composite volcanoes have a crater at the summit which generally contains a central vent or a clustered group of vents. Some of the most beautiful volcanic mountains in the world are composite volcanoes, including Mount Cotopaxi in Ecuador and Mount St. Helens in U.S.A.

Features formed by Volcanoes

Volcanoes have shaped the earth‘s crust since its existence, producing dramatic volcanic cones, volcanic domes, volcanic plateaus, fertile farmlands, islands, Craters and collapsing Calderas on the surface of the Earth. The volcanic activity also creates unique intrusive landforms in the form of Batholiths, Dyke, and sills, Laccolith, Lapolith and Phacolith. Some minor features such as Hot Spring, Geysers and Fumaroles are also sites of tourist‘s attraction. The basic extrusive and Intrusive volcanic features are as follows:

(A) Extrusive Features

(i) Craters and Calderas

The crater is a bowl or funnel-shaped depression or cavity usually of volcanic origin. It is usually more or less circular in the plan at the summit of the volcanic mountain. The diameter of crater is commonly less than 1.6 kilometers. Craters may result from either explosive activity or from subsidence. It should be noted that craters may also form due to impact of meteorites and mining process but in this case it will not be associated with volcanic activity.

The huge carter-like depression is called Caldera. The diameter of a caldera is usually several times that of a crater. The figure demonstrates the formation of caldera due repeated volcanic eruption. The caldera may also form due to coalescence of several small craters.

(ii) Volcanic Plateaus and Plains

On the surface of the Earth many extensive fairly leveled lava plateaus have been built by fissure eruption. They have completely covered up the surrounding region. The figure shows that the Deccan lava plateau covers almost 6, 50,000 square kilometers of India‘s geographical area.

The basaltic plains in comparison to lava plateau contain much thinner and less extensive accumulation of lava sheets. They are common throughout the world.

(B) Intrusive Features

Intrusive igneous landforms are formed due to cooling and crystallization of magmas beneath the surface. The intrusive landforms can be exposed at the earth‘s surface by erosion of overlying rock. Therefore they provide important information about internal structure earth. Some of the important intrusive volcanic landforms forms are as follows:

(i) Batholiths

A batholith (from Greek bathos, depth + lithos, rock) represents huge mass of intrusive (plutonic) igneous rock which accumulates in the crust. It forms the root or heart of the volcanic mountain. The batholiths may be up to 100 kilometers wide and are exposed at the surface only after considerable denudation of the overlying mass. The figure shows that similar to batholiths intrusions but smaller in size are bosses or stocks. The northern part of isle of Arran (UK) contains the conspicuous example of stock.

(ii) Dyke

The figure demonstrates that dyke is a igneous intrusion that cuts across the bedding of the country rock through near vertical fissures. Hundreds of parallel dykes can be traced in North- Western Scotland, especially in the Islands of Mull and Arran. The denudation processes can expose the comparatively harder Dyke to from ‗walls‘ or Cliff across the beaches. Sometimes a zone of dykes may surround a circular or dome-shaped intrusion in more or less arcuate from; these are known as ring-dykes.

(iii) Sills

The figure shows that sill is the tabular or sheet-like intrusive body formed when magma is injected along sedimentary bedding surfaces. They are usually formed from low viscosity magma. The Great Whin Sill situated in Great Britain is a well-known example.

(iv) Laccoliths

The figure shows that in this type magma collects as a mushroom-shaped mass that arches the overlying strata upward. Laccoliths are formed due to injection of magma along the bedding planes of the horizontally bedded sedimentary rocks. They cover large areas, The Karnataka plateau is spotted with domal hills of granite rocks. Most of these, now exfoliated, are examples of laccoliths or batholiths.

(c) Minor Volcanic Features Geysers and Hot Spring

Sometimes water at depth is heated beyond 100° C or superheated by a body of magma or hot rock. The water may come out with great force along with the mixture of steam and diluted minerals. Such sites in the volcanic region are called geysers. The word geyser comes from the Icelandic word meaning ―to gush‖. The Figure shows that the „Old Faithful‟ geyser in Yellowstone National Park, U.S.A is one of the most famous geysers in the world which erupts

faithfully at regular interval of about 65 minutes or so. This geyser shoots water between 30 to 50 meters into the air. Minerals dissolved in the hot water are often precipitated around the vents because temperature and pressure drop suddenly as water or steam enters the atmosphere. This produces spectacular deposits of travertine (chemically precipitated calcium carbonate and other minerals). The heated water may also flow quietly generally along the fault zones in the form of hot spring. Such hot springs are quite common in volcanic region.

Pseudo-Volcanic Features

Certain topographic features of non-volcanic origin resemble volcanic forms and are therefore known as pseudo volcanic-features. They include the following:

(i) Meteorite Craters

These are the depressions formed due to the impact of falling materials of large dimensions. The Lonar Lake in Buldhana district of Maharashtra was originally thought to be a crater lake, but was later confirmed to have been formed by the impact of a giant meteorite.

Lonar Lake

(ii) Salt-plugs or Salt Domes

A domelike geologic formation formed when an underground salt deposit rises toward the surface, pushing up and curving what lies above it. As we know,

under high pressure salt deforms plastically and behaves like an intrusive, deforming and piercing the overlying sediments.

Salt extrusions may take the form of salt hills which exhibits many features of plug domes or lava cones with peaks and small sink-holes, which look like craters produced due to subsidence.

(iii) Mud-volcanoes

Some of the mud-volcanoes are of non- volcanic origin. As for example, the volatile hydrocarbons given off from the petroleum-bearing beds beneath cause mud-eruptions, as in case of the mud-volcanoes at Baku on the Caspian, in southern Baluchistan, in Burma etc.

Apart from what has been described above, William D. Thornbury (Principles of Geomorphology .Wiley International Edition, Second Edition, New York 1985) has mentioned that craters formed by bombs and mine blast (Bomb and mine crater.) have features of craters developed due to volcanic explosion.

Tsunamis

Tsunami is a Japanese word which means ‗harbour wave‘. It is a series of traveling ocean wavesof extremely long length generated by disturbances associated primarily with earthquakesoccurring below or near the ocean floor. Underwater volcanic eruptions and landslides can alsogenerate tsunamis. Tsunamis are a threat to life and property to anyone living near theocean. Large tsunamis have been known to rise over 100 feet, while tsunamis 10 to 20 feethigh can be very destructive and cause many deaths and injuries.

Causes

Tsunamis generally are caused by earthquakes. Not all earthquakes generate tsunamis. Togenerate tsunamis, earthquakes must occur underneath or near the ocean, be large and createmovements in the sea floor. All oceanic regions of the world can experience tsunamis, but inthe Pacific Ocean there is a much more frequent occurrence of large, destructive tsunamisbecause of the many large earthquakes along the margins of the Pacific Ocean.

Other less common causes of earthquakes are submarine landslides, submarine volcanic eruptions and very rarely a large meteorite impact in the ocean.

Propagation

In the open ocean a tsunami is less than a few feet high at the surface, but its wave heightincreases rapidly in shallow water. Tsunamis wave energy extends from the surface to thebottom in the deepest waters. As the tsunami attacks the coastline, the wave energy iscompressed into a much shorter distance creating destructive, life-threatening waves.Where the ocean is over 20,000 feet deep, unnoticed tsunami waves can travel at the speed ofa commercial jet plane, nearly 600 miles per hour. They can move from one side of the PacificOcean to the other in less than a day. This great speed makes it important to be aware of thetsunami as soon as it is generated. Scientists can predict when a tsunami will arrive since thespeed of the waves varies with the square root of the water depth. Tsunamis travel muchslower in shallower coastal waters where their wave heights begin to increase dramatically.

Offshore and coastal features can determine the size and impact of tsunami waves. Reefs, bays,entrances to rivers, under sea features and the slop of the beach all help to modify the tsunamias it attacks the coastline. When the tsunami reaches the coast and moves inland, the waterlevel can rise many feet. In extreme cases, water level has risen to more than 50 feet fortsunamis of distant origin and over 100 feet for tsunami waves generated near theearthquake's epicentre.

Consequences

The consequences vary from loss of livelihood for fishermen to unknown damages to coralreefs and flora and fauna. It may take years for the coral reefs to get back the balance andmangrove stands and coastal tree plantations get destroyed or severely affected.With so much sea water coming inland, salination is another effect that not only makes the soilless fertile to support vegetation but also increases vulnerability to erosion, the impacts ofclimate change and food insecurity. For humans, on the other hand, fisheries, housing andinfrastructure are the worst affected.

Early Warning and Mitigation

Major tsunami warning centres are:

1. Pacific Tsunami Warning Center (PTWC): The Tsunami Warning System (TWS) in thePacific, comprised of 26 participating international Member States, has the functions ofmonitoring seismological and tidal stations throughout the Pacific Basin to

evaluatepotentially tsunami genic earthquakes and disseminating tsunami warning information. ThePacific Tsunami Warning Center is the operational center of the Pacific TWS. Located nearHonolulu, Hawaii, PTWC provides tsunami warning information to national authorities inthe Pacific Basin.

2. The Alaska Tsunami Warning Center (ATWC): in Palmer, Alaska, serves as the regionalTsunami Warning Center for Alaska, British Columbia, Washington, Oregon, and California.

3. Indian Tsunami Early Warning System (ITEWS): The Indian Tsunami Early Warning Systemhas the responsibility to provide tsunami advisories to Indian Mainland and the Islandregions. Acting as one of the Regional Tsunami Advisory service Providers (RTSPs) for theIndian Ocean Region, ITEWS also provide tsunami advisories to the Indian Ocean Rimcountries along with Australia and Indonesia.

In order to confirm whether the earthquake has actually triggered a tsunami, it is essential tomeasure the change in water level as near to the fault zone with high accuracy. There are twobasic types of sea level gages: coastal tide gages and open ocean buoys.Tide gages are generally located at the land-sea interface, usually in locations somewhatprotected from the heavy seas that are occasionally created by storm systems. Tide gages thatinitially detect tsunami waves provide little advance warning at the actual location of the gage,but can provide coastal residents where the waves have not yet reached an indication that atsunami does exist, its speed, and its approximate strength.

Open ocean tsunami buoy systems equipped with bottom pressure sensors are now a reliabletechnology that can provide advance warning to coastal areas that will be first impacted by atsunami, before the waves reach them and nearby tide gages. Open Ocean buoys oftenprovide a better forecast of the tsunami strength than tide gages at distant locations.

Apart from technology, we can also use natural barriers to mitigate the effect of tsunamis.Coral reefs act as natural breakwaters, providing a physical barrier that reduces the force of awave before it reaches the shore, while mangrove forests act as natural shock absorbers, alsosoaking up destructive wave energy and buffering against coastal erosion.

VII. Erosional and Depositional Landforms

Fluvial Landforms

Moving water is as widespread and effective as an agent of erosion and deposition that its influence on the landscape is usually the prominent—and in many cases, the dominant—process at work. The shapes of most valleys are strongly influenced by the water that runs through them—especially through the transportation and deposition of sediment. Areas above the valleys are less affected by running water, but even there flowing water may significantly influence the shape of the land. The basic landscape-sculpting effect of running water is to smooth irregularities—in simplest terms, to wear down the topography by erosion and transportation of debris, and fill up the valleys by deposition.

Erosion and deposition by streams sculpt Earth‘s landscapes into an array of features. Flowing water in streams picks up sediment, transports it to lower elevations, and deposits it. Flooding rivers and smaller streams deposit sediment and nutrients critical to agriculture, but they can also inundate cities and destroy structures built too close to the riverbank. The landforms created as a result of degradational action (erosion) or aggradation work (deposition) of running water is called fluvial landforms.

These landforms result from the action of surface flow/run-off or stream flow (water flowing through a channel under the influence of gravity). The creative work of fluvial processes may be divided into three physical phases—erosion, transportation and deposition.

Fluvial Processes Erosion

Erosion is a process in which the surface of the earth is worn away by various agents of erosion like wind, water or Glacier. The removed material is carried away and deposited elsewhere. Erosion by streams occurs through several processes going together. These processes are Abrasion, Hydraulic action, Solution and Attrition.

Abrasion: Running water when armed with sand, silt and other sediments acts as very effective means of erosion of the river bed. In the upper reaches the river with a steep gradient carries a lot of such materials which act as a tool and are effective in carving huge valleys, gorges and canyons. In contrast if the river is free of these sediments the erosion by abrasion is minimal and the river would take much longer time to erode its bed.

Solution: Pure water seldom exists in natural conditions. It contains various gases and acids in them, the water is charged with the acids and act as an important solvent. It may not be effective on all the rocks but when it comes in contact with limestone, dolomite or chalk it becomes most powerful agent of erosion as it dissolves them much faster. So solution also forms an equally important process of erosion.

Attrition: When the stream loads (pebbles, sand, silt etc.) move together, they cause their own wear and tear by colliding with one another. This rounding and shaping of these pebbles and boulders amongst themselves is called attrition.

Hydraulic action (pressure): Running water has enormous force in it, when it strikes the stream bed on its banks it may loosen the rocks, lift it and easily transport it. It acts as a wedge when strikes the crack or fractures in the rock bed by widening and loosening it. Thus, Hydraulic pressure is also reckoned as an important means of erosion. The above discussed processes coupled with other factors give rise to diverse landforms occurring in different parts of the stream course.

Transportation

The stream is an important means of transportation of sediments of varying size and shape to distant lands much away from their source of origin. The sediments are transported in various ways depending upon the size of the material; their composition and volume of water.

Movement in solution: Materials like sulphate, carbonate, and chloride are dissolved in water and carried till the end of the stream. Rivers flowing in arid and semi-arid regions show more material in solution.

Movement in suspension: Very fine particles consisting of sand, silt and mud are transported in huge quantities by the rivers. These particles are kept in suspension by the river as the volume of water and its speed does not allow particles to settle. They are always kept in suspension while traveling and therefore are carried for a fairly longer distance than the larger boulder or pebbles which roll along the stream bed.

Movement by traction: Large boulders and pebbles which are heavier for the stream to carry them by means of suspension or saltation move by rolling along the stream bed. This rolling causes abrasion on the bed and for they are in constant contact with the bed. They emerge as one of the chief erosive agents of stream bed.

Movement by saltation: The particles jump and bounce when up rushing water comes with a force lifting them from the bed to some distance. They cannot be carried long as their weight is more and therefore cannot be kept in suspension. This type of movement is called saltation.

Deposition

A stream carries its load downslope toward a valley, a lake, or an ocean. Along that journey, whenever a stream‘s load exceeds its capacity, it deposits some of its load. This deposited sediment is called alluvium.

Deposition typically occurs where the velocity of streamflow decreases. For example, deposition occurs along stream banks when the streamflow slows down on the inside of a bend in the channel. During flooding, fast-moving floodwaters slow down and spread out over the valley floor, depositing alluvium in layers. Fine sediment, rich in organic matter, can improve soil fertility—although, sometimes, flooding can also leave behind sterile layers of sand or gravel.

Dams represent a special case for sediment deposition and can have a variety of negative influences on fluvial systems. Where earthen or concrete barriers block the streamflow, transported sediment quickly settles at the base of the dam. Sediment that would otherwise continue downstream and settle out during floods continuously fills in the reservoir bottom. Eventually, the sediment may displace enough water to render the original dam inoperable for water storage or electrical generation. Costly dredging is the only remedy for removing depositional sediment.

Erosional Landforms

River Valleys:

The extended depression on ground through which a stream flows throughout its course is called a river valley. At different stages of the erosional cycle the valley acquires different profiles. At a young stage, the valley is deep, narrow with steep wall-like sides and a convex slope. The erosional action here is characterised by predominantly vertical downcutting nature. The profile of valley here is typically ‗V‘ shaped. As the cycle attains maturity, the lateral erosion becomes prominent and the valley floor flattens out. The valley profile now becomes typically ‗U‘ shaped with a broad base and a concave slope.

A deep and narrow V shaped valley is also referred to as gorge and may result due to downcutting erosion and because of recession of a waterfall. Most Himalayan Rivers pass through deep gorges (at times more than 500 metres deep) before they descend to the plains. An extended form of gorge is called a canyon. The Grand Canyon of the Colorado River in Arizona (USA) runs for 483 km and has a depth of 2.88 km.

A tributary valley lies above the main valley and is separated from it by a steep slope down which the stream may flow as a waterfall or a series of rapids.

2. Waterfalls:

A waterfall is simply the fall of an enormous volume of water from a great height, because of a variety of factors such as variation in the relative resistance of rocks, relative difference in topographic reliefs; fall in the sea level and related rejuvenation, earth movements etc. For example, Jog or Gersoppa falls on Sharavati (a tributary of Cauveri) has a fall of 260 metres.

A rapid, on the other hand, is a sudden change in gradient of a river and resultant fall of water

3. Pot Holes:

The kettle-like small depressions in the rocky beds of the river valleys are called pot holes which are usually cylindrical in shape. Pot holes are generally formed in coarse-grained rocks such as sandstones and granites. Potholing or pothole-drilling is the mechanism through which the grinding tools (fragments of rocks, e.g. boulders and angular rock fragments) when caught in the water eddies or swirling water start dancing in a circular manner and grind and drill the rock beds of the valleys like a drilling machine. They thus form small holes which are gradually enlarged by the repetition of the said mechanism. The potholes go on increasing in both diameter and depth.

4. Terraces:

Stepped benches along the river course in a flood plain are called terraces. Terraces represent the level of former valley floors and remnants of former (older) flood plains.

5. Gulleys/Rills:

Gulley is an incised water- worn channel, which is particularly common in semi-arid areas. It is formed when water from overland-flows down a slope, especially following heavy rainfall, is concentrated into rills, which merge and enlarge into a gulley. The ravines of Chambal Valley in Central India and the Chos of Hoshiarpur in Punjab are examples of gulleys.

6. Meanders:

A meander is defined as a pronounced curve or loop in the course of a river channel. The outer bend of the loop in a meander is characterised by intensive erosion and vertical cliffs and is called the cliff-slope side. This side has a concave slope. The inner side of the loop is characterised by deposition, a gentle convex slope, arid is called the slip-off side. Morphologically, the meanders may be wavy, horse-shoe type or ox-bow/ bracelet type.

7. Ox-Bow Lake:

Sometimes, because of intensive erosion action, the outer curve of a meander gets accentuated to such an extent that the inner ends of the loop come close enough to get disconnected from the main channel and exist as independent water bodies. These water bodies are converted into swamps in due course of time. In the Indo-Gangetic plains, southwards shifting of Ganga has left many ox-bow lakes to the north of the present course of the Ganga.

8. Peneplane (Or peneplain):

This refers to an undulating featureless plain punctuated with low- lying residual hills of resistant rocks. According to W.M. Davis, it is the end product of an erosional cycle.

Depositional Landforms

The depositional action of a stream is influenced by stream velocity and the volume of river load. The decrease in stream velocity reduces the transporting power of the streams which are forced to leave additional load to settle down. Increase in river load is effected through (i) accelerated rate of erosion in the source catchment areas consequent upon deforestation and hence increase in the sediment load in the downstream sections of the rivers; (ii) supply of glacio-fluvial materials; (iii) supply of additional sediment load by tributary streams; (iv) gradual increase in the sediment load of the streams due to rill and gully erosion.

Various landforms resulting from fluvial deposition are as follows:

1. Alluvial Fans and Cones:

When a stream leaves the mountains and comes down to the plains, its velocity decreases due to a lower gradient. As a result, it sheds a lot of material, which it had been carrying from the mountains, at the foothills. This deposited material acquires a conical shape and appears as a series of continuous fans. These are called alluvial fans. Such fans appear throughout the Himalayan foothills in the north Indian plains.

2. Natural Levees:

These are narrow ridges of low height on both sides of a river, formed due to deposition action of the stream, appearing as natural embankments. These act as a natural protection against floods but a breach in a levee causes sudden floods in adjoining areas, as it happens in the case of the Hwang Ho river of China.

3. Delta:

A delta is a tract of alluvium usually fan-shaped, at the mouth of a river where it deposits more material than can be carried away. The river gets divided into two or more channels (distributaries) which may further divide and re-join to form a network of channels.

Conditions for Delta Formation:

The ideal favourable conditions for the formation and growth of delta include:

(1) Suitable place in the form of shallow sea and lake shores.

(2) Long courses of the rivers (i.e. long rivers so that they bring enough amounts of sediments).

(3) Medium size of sediments (because if the sediments are very fine, they would be carried in the sea in suspension for longer distances and if they are very coarse, they would soon settle down at the sea bottom, and hence no delta would be formed).

(4) Relatively calm or sheltered sea at the mouths of the rivers (so that ocean currents, strong waves or high tidal waves do not interfere with the natural process of gradual sedimentation and delta formation).

(5) Large amount of sediment supply.

(6) Accelerated rate of erosion in the catchment area of the concerned river.

(7) Almost stable condition of sea coast and oceanic bottom (because sea coast subjected to frequent emergence or submergence caused by tectonic movements does not allow regular sedimentation and thus disfavours delta formation) etc.

A delta is formed by a combination of two processes:

(i) Sediment is deposited when the load-bearing capacity of a river is reduced as a result of the check to its speed as it enters a sea or lake, and

(ii)

At the same time fine clay particles carried in suspension in the river coagulate in the presence of salt water and are deposited. The finest particles are carried farthest to accumulate as bottom-set beds; coarser material is deposited in a series of steeply sloping wedges forming the forest beds; and the coarsest material is deposited on the braided surface of the delta as topset beds.

Deltas undergo subsidence because of:

(1) Gradual sedimentation and consequent increase in the weight of delta materials,

(2) Compaction of sediments caused by load of sediments,

(3) Enormous thickness of sediments, and

(4) Isostatic adjustment etc.

Growth of Delta:

No doubt, there is growth in all types of delta towards the sea but the rate of growth varies considerably from one situation to the other.

The nature and rate of delta growth depends on a variety of factors e.g.:

· Velocity of the stream flow,

· Nature of sea waves,

· Supply of sediments,

· Oceanic currents, and

· Slope and height of deltas etc.

Most of the sediments are unloaded at the mouths of the rivers if their velocity is extremely low and thus the growth of deltas toward the sea becomes sluggish. On the other hand, streams with greater velocity transport their load far greater distance in the sea and thus allow faster rate of delta growth, but deltas formed in such situation are narrow and long. Strong sea waves and oceanic currents retard the growth of deltas because they erode and remove the sediments away. The sliding of materials from higher deltas towards the sea also encourages the seaward growth of deltas.

Depending on the conditions under which they are formed, deltas can be of many types.

Arcuate or Fan-shaped: This type of delta results when light depositions give rise to shallow, shifting distributaries and a general fan-shaped profile. Examples: Nile, Ganga, Indus.

Bird‟s Foot Delta: This type of delta emerges when limestone sediment deposits do not allow downward seepage of water. The distributaries seem to be flowing over projections of these deposits which appear as a bird‘s foot. The currents and tides are weak in such areas and the number of distributaries lesser as compared to an arcuate delta. Example: Mississippi river.

Estuarine delta: Sometimes the mouth of the river appears to be submerged. This may be due to a drowned valley because of a rise in sea level. Here fresh water and the saline water get mixed. When the river starts ‗filling its mouth‘ with sediments, mud bars, marshes and plains seem to be developing in it. These are ideal sites for fisheries, ports and industries because estuaries provide access to deep water, especially if protected from currents and tides. Example: Hudson.

Cuspate Delta: This is a pointed delta formed generally along strong coasts and is subjected to strong wave action. There are very few or no distributaries in a cuspate delta. It has curved sides because of an even deposition of material on either side of the mouth. Example: Tiber River on west coast of Italy.

Truncated Delta: Sea waves and ocean currents modify and even destroy deltas deposited by the river through their erosional work. Thus, eroded and dissected deltas are called truncated deltas.

Blocked delta: Blocked deltas are those whose seaward growth is blocked by sea waves and ocean currents through their erosional activities. The progradation of deltas may also be hampered due to sudden decrease in the supply of sediments consequent upon climatic change or management of catchment areas of concerned rivers.

Abandoned Delta: When the rivers shift their mouths in the seas and oceans, new deltas are formed, while the previous deltas are left unnourished. Such deltas are called abandoned deltas. The Yellow (formerly Hwang Ho) river of China has changed its mouths several times and thus has formed several deltas. For example, the present delta of the Yellow river is to the north of Shantung Peninsula while the previous delta was deposited to the south of the peninsula. The western part of the Ganga delta, which is drained by the Hoogli River is an example of abandoned delta.

4.

Braided streams: The streams when get overloaded with sediments do not carry them along and leave the excess material on the river floor in the form of sand bars. These deposits cause the stream to split into several channels. The braided stream is a common occurrence in the region which are relatively dry and arid and where the supply of water is not steady.

Arid Landforms

Arid lands are in many ways distinctive from humid ones, but there are no obvious boundaries to separate the two. It also important to understand that some of today‘s deserts had quite different climates in the geologic past. Parts of today‘s Sahara Desert, for example, were much wetter just a few thousand years ago than they are today. Thus, in addition to processes operating today, some desert landscapes we see have also been shaped by a different set of processes that were at work in the past.

Desert terrain is usually stark and abrupt, unsoftened by regolith, soil, or vegetation. Despite the great difference in appearance between arid lands and humid, most of the terrain-

forming processes active in humid areas are also at work in desert areas. There are, however, special conditions found in deserts that do significantly influence landform development.

Special Conditions in Deserts

Desert landforms are often conspicuously different from those found in wetter locations. These differences are largely the result of a variety of factors and special conditions found in arid regions. The most important of these special conditions include the following:

Weathering: Because moisture is required for nearly all kinds of chemical weathering, in many desert regions mechanical weathering is dominant—although chemical weathering is likely to be absent in only the driest of deserts. Mechanical weathering processes such as salt wedging are more common in arid regions than in humid ones. This predominance of mechanical weathering results not only in a generally slower rate of total weathering in deserts, but also in the production of more angular particles of weathered rock.

Soil and Regolith: In deserts, the covering of soil and regolith is either thin or absent in most places, a condition that exposes the bedrock to weathering and erosion, and contributes to the stark, rugged, rocky terrain.

Soil Creep: Soil creep is a relatively minor phenomenon on most desert slopes. This is due partly to the lack of soil but primarily to the lack of the lubricating effects of water. Creep is a smoothing phenomenon in more humid climates, and its lack in deserts accounts in part for the angularity of desert slopes.

Impermeable Surfaces: A relatively large proportion of the desert surface is impermeable to percolating water, permitting little moisture to seep into the ground. Caprocks (resistant bedrock surfaces) and hardpans (hardened and generally water-impermeable subsurface soil layers) of various types are widespread, and what soil has formed is usually thoroughly compacted and often does not readily absorb water. Such impermeable surfaces lead to high runoff when it rains.

Sand: Some deserts have an abundance of sand in comparison with other parts of the world. This is not to say, however, that deserts are mostly sand covered. Indeed, the notion that all deserts consist of great seas of sand is incorrect. Nevertheless, the relatively high proportion of sand in some deserts has three important influences on topographic development: (1) A sandy cover allows water to infiltrate the ground and inhibits drainage via streams and overland flow, (2) sand is readily moved by heavy rains, and (3) it can be transported and redeposited by the wind.

Rainfall: Although rainfall is limited in desert areas, much of the rain that does fall comes from intense convective thunderstorms—which result in very high and rapid runoff. Floods, although often brief and covering only a limited area, are the rule rather than the exception in deserts. Thus, fluvial erosion and deposition, however sporadic and rare, are remarkably effective and conspicuous.

Fluvial Deposition: Almost all streams in desert areas are ephemeral, flowing only during and immediately after a rain. Such streams are effective agents of erosion, shifting enormous amounts of material in a short time. This is mostly short-distance transportation, however. A large volume of unconsolidated debris is moved to a nearby location, and as the stream dries up, the debris is dumped on slopes or in valleys, where it is readily available for the next rain. As a consequence, depositional features of alluvium are unusually common in desert areas.

Wind: Another fallacy associated with deserts is that their landforms are produced largely by wind action. This is not true, even though high winds are characteristic of most deserts and even though sand and dust particles are easily shifted.

Basins of Interior Drainage: Desert areas contain many watersheds that do not drain ultimately into any ocean. For most continental surfaces, rainfall has the potential of flowing all the way to the sea. In dry lands, however, drainage networks are frequently underdeveloped, and the terminus of a drainage system is often a basin or valley with no external outlet.

Vegetation: All the previous environmental factors have important effects on topographic development, but perhaps the single most obvious feature of dry lands is the lack of a continuous cover of vegetation. The plant cover consists mostly of widely spaced shrubs or sparse grass, which provide little protection from the force of raindrops and function inadequately to bind the surface material with roots.

Fluvial Desert Landforms

Probably the most fundamental fact of desert geomorphology is that running water is by far the most important external agent of landform development. The erosional and depositional work of running water influences the shape of the terrain surface almost everywhere outside areas of extensive sand accumulation. The lightly vegetated ground is defenseless to whatever rainfall may occur, and erosion by rain splash, sheetwash, rilling, and streamflow is enormously effective. Despite the rarity of precipitation, its intensity and the presence of impermeable surfaces produce abrupt runoff, and great volumes of sediment can be moved in a very short time.

The steeper gradients of mountain streams increase the capacity of these streams for carrying large loads, of course, but the sporadic flow of mountain streams in arid lands results in an unpredictable imbalance between erosion and deposition. At any given time, therefore, much transportable rock debris and alluvium sit at rest in the dry stream bed of a desert mountain, awaiting the next flow. Loose surface material is either thin or absent on the slopes, and bedrock is often clearly exposed, with the more resistant strata standing out as Caprocks and cliff faces.

Where slopes are gentle in an arid land, the streams rapidly become choked with sediment as a brief flood subsides. Here stream channels are readily subdivided by braiding, and main channels often break up into distributaries in the basins. Much silt and sand are thus left on the surface for the next flood to move, unless wind moves them first.

Badland, pediments, bajadas and playas are special types of landforms of arid and semi- arid regions caused by mechanical disintegration of rocks and water action.

1. Badland Topography:

The regions of weak sedimentary formations are extensively eroded by numerous rills and channels which are occasionally developed due to occasional rainstorms. The linear fluvial erosion results in the formation of ravines and gullies which are divided by numerous undulating narrow ridges. Thus, the ground surface becomes so uneven and corrugated due to numerous gullies and ravines that it becomes entirely difficult to walk on it. Such type of landscape is called badland topography.

2. Bolsons and Playas:

The intermountain basins in arid or semiarid areas are generally called Bolsons. Such basins are characterized by 3 unique landforms which from the mountain front downward are pediments, bajadas and playas. Numerous ephemeral streams after originating from the surrounding mountain fronts drain into the Bolsons. Some water collects in the centre of is a playa in the centre of the Tarim Basin.

The playas range in areal extent from a few square metres to tens of square kilometres. These are called

‗khabari‘ and ‗mamlaha‘ in Arabian deserts while they are known as ‗shafts‘ in Sahara. Playa lakes may last for days, weeks or even longer before they are completely dried up by evaporation. Evaporation of playa lake water results in the formation of encrustation of alkaline materials (sodium bicarbonate or sodium carbonate), salt deposits such as borax. Such salt-covered playa beds are called salinas.

3. Bajada:

Gently sloping depositional plain between pediments and playa is called bajada. Bajada is formed due to coalescence of several alluvial fans. Thus, bajada is a wholly depositional feature. The slope in its upper part ranges between 8° and 10° but it becomes 1° to zero in its lowermost portion touching the playa.

4. Pediments:

Pediments situated between mountain front and bajada in intermontane basin are broad, extensive, and gently sloping areas of rock cut surfaces which spread as aprons around the bases of mountains. The term (pediment) was first used by G.K. Gilbert (1882) for the surfaces of the alluvial fans that encircle mountains in deserts.

Aeolian Landforms

The irrepressible winds of the desert create spectacular sand and dust storms and continuously reshape minor details of the landscape. However, the effect of wind as a sculptor of terrain is very limited, with the important exception of such relatively impermanent features as sand dunes.

Aeolian processes are those related to wind action (Aeolus was the Greek god of the winds). They are most pronounced, widespread, and effective wherever fine-grained unconsolidated sedimentary material is exposed to the atmosphere, without benefit of vegetation, moisture, or some other form of protection—in other words, in deserts and along sandy beaches.

Types of Aeolian land features

On the basis of the two major actions of wind, aeolian landforms are classified into two broad categories — erosional and depositional aeolian features

Aeolian erosional land features

Wind erodes in two ways, one: it picks up lose particles and removes them to create depressions. Secondly, wind attacks rocks with sand particles and destroys weak rock beds. The following are the features formed by these actions.

1. Lag Deposits:

While blowing over a surface, wind removes all unconsolidated fine particles. Those with less than 100 micrometres diameter are suspended and are taken to long distances. Those particles that are of 100 micrometres diameter, like sand, are removed gradually to short distances. The larger ones are left at their place of origin and keep rolling and shifting their place till they are tightly packed by this random jostling. These surfaces are called ‗lag deposits‘, because the surface is made of particles that could not keep pace with the rest of smaller ones moving out and ‗lagged behind‘. They are also known as ‗desert pavement‘ as the grains are fitted tightly, just like any man-made tiled pavement surface. The top of these desert pavements are polished by wind abrasion and have a thin shiny layer of oxides of iron and manganese, called desert varnish. These lag deposits have different names, e.g., desert armour in North America, serir, reg and hammada in the Arab world and gibber in Australia.

2. Deflation Hollow:

As the name suggests, these are low-lying surfaces which have been cleared of all loose particles and converted into hollows. The size of these depressions may range from a few metres in diameters and depth, to several kilometres. The dimension, especially the depth, is controlled by the underground water table. As deepening reaches humid layers close to the water table, wind fails to move the moist particles and no further hollowing is allowed. These are also known as blowouts. E.g. Quattara Depression in North Egypt, which has its deepest part 134 metres below the sea level.

3. Pans

Pans are closed depressions that are common in many dryland areas and that seem to be at least partly formed by deflation. In size, they range from a few metres wide and only centimetres

deep, to kilometres across and tens of metres deep. The largest known pan, which was discovered in eastern Australia, is 45 km wide.

4. Yardang and Zuegen:

Yardangs are elongated grooves, first described by Hedin in Turkestan (Thornbury p.299). Eliot Blackwelder used the term ‗Yardang‘ for these grooves in 1934. These are parallel ridges separated by parallel ‗u‘-shaped grooves, both developed in the direction of dominating wind flow in the region. They are called Mega-Yardang when they are large in scale. In central Sahara and Egypt Yardangs are 100 metres long and 1000 metres wide.

Zuegen (singular Zuege) – These are similar to Yardang, except they are smaller in scale and grooving is related to softer material alternated with more resistant rock beds.

5. Ventifacts:

Ventifacts are rock pieces with smooth, sand-blasted facets pointing to the direction of dominant wind flow. Sometimes there may be several such facets developed on a rock, indicating varying wind direction. All facets in this type of case intersect along sharp and angular edges. The number of edges or keels they carry is sometimes connoted by the German terms Einkanter(one-sided), Zweikanter(two-sided), and Dreikanter(three-sided). The pyramid-shaped Dreikanter are particularly common.

Besides the above main landforms other features that result from near-surface abrasion by sand laden winds are mesa, butte and mushroom rocks. These are formed when the base of a rock projection is eroded while the top is untouched by wind action, or is protected by some harder rock. Weak rock beds are eroded while harder beds stand out.

Aeolian depositional land features

All particles transported by wind are ultimately dropped under two conditions – one, if the velocity of wind drops; or two, if the wind meets an obstacle in its path. Different circumstances lead to formation of different landforms. Most important of these are sand dunes. Besides these,sand ripples and sand ridges cover vast areas. Bagnold has classified all depositional features into two broad classes on the basis of scale

1. Sand dunes:

Sand dunes are defined as hills and mounds of sand. They have a large variety and are classified on different bases. Bagnold defines dunes as ―mobile heap of sand whose existence is independent of either ground form or fixed wind obstruction‖. His classification mentioned only two types – barchans (transverse dune) and seif (longitudinal dune).

Barchan – Barchans are crescent shaped dunes. They are either single or may form groups. They migrate in the downwind direction, but maintain their shape as they move.

Seif – Seif is a linear dune with its axis aligned parallel to the prevailing strong winds. Its crest runs along its length and is marked by a sharp edge, hence it is also called sword dune.

2. Sand ripples

Ripples are small-scale aeolian features. They are 1 to 30 centimetres high and a few centimetres to some metres apart. The develop perpendicular to the wind direction. Their shape changes very quickly.

3. Sand ridges

Ridges are long, undulating aeolian features parallel to the wind direction. The main process responsible for their formation is saltation. In the beginning there are windward and slip- face activities, just like in the formation of sand dunes. Particles on the leeward side are protected from impact of the wind. The depression continually gets deeper as particles are removed from here rapidly. Due to saltation, large grains are pushed up along the windward slope to the crest of feature. The crest receives grains faster than it loses them. On the other hand, depressions lose grains faster than they receive, and hence get hollowed.

4. Whalebacks, Dunefields and Sand Sea:

Whalebacks are ―Coarse grained residues or platforms built up and left behind by the passage of a long-continued succession of seif dunes along the same path‖ – Bagnold Chains of transverse and seif dunes, barchans and other small scale features develop on these whalebacks to make dunefields.

Dunefields:

Dunefields are accumulations of sand, occupying areas of less than 30,000 km2 with at least ten individual dunes spaced at distances exceeding the dune wavelength (Cooke et al. 1993, 403). They contain relatively small and simple dunes. They may occur anywhere that loose sand is blown by the wind, even at high latitudes, and there are thousands of them. In North America, dunefields occur in the south-western region, and in intermontane basins such as Kelso and Death Valley, California.

Sand seas differ from dunefields in covering areas exceeding 30,000 km2 and in bearing more complex and bigger dunes. In both sand seas and dunefields, ridges or mounds of sand may be repeated in rows, giving the surface a wavy appearance.

5.

Sand Shadow – Formation of this feature dependents on the presence of an obstacle in the path of the wind. The velocity of wind dips in the lee of such an obstacle, while the flow circumventing the obstacle maintains its force. As a result, weak flow fails to remove any sand particle that arrives in the leeward side of the obstacle; this allows sand to collect and form a depositional feature called Sand Shadow of the obstacle. It is formed close to the obstacle in its shelter.

6. Sand Drift – This feature is related to presence of gaps in landforms that allow wind to blow as a channelized strong stream. In such cases, the rest of the landform obstructs wind and transportation of sand while the gap allows unobstructed flow. All sand accumulating against such obstacles is directed to the gap and moves forward through it. Close to the gap there is no deposition because here the force of channelized wind is strong and transports its entire load. As wind moves farther from the gap and loses its force, it drops the sand it is carrying. Right in the line of the gap a mound builds up. Later this mound forces the wind to slow down and deposit more sand here.

7. Loess

Loess is very fine soil that wind has transported and deposited in thick layers far away from the place of its origin. Loess is terrestrial sediment composed largely of windblown silt particles made of quartz. It covers some 5–10 per cent of the Earth‘s land surface, much of it forming a blanket over pre-existing topography that may be up to 400 m thick. On the Chinese loess plateau, thicknesses of 100 m are common, with 330 m recorded near Lanzhou.

Coastal Landforms

Coastlines are among the most varied of all landscapes on the planet. This diversity of form and appearance is a result of the great variety of local geology, climate, and geomorphic processes at work along the coastlines of the world. The coastal environment is usually dynamic, often showing changes in appearance and process from day to day or even hour to hour. Part of this dynamism is a consequence of shorelines acting as an interface of the lithosphere, hydrosphere, and atmosphere—and quite often the cryosphere and biosphere as well.

Coastal processes affect only a tiny fraction of the total area of Earth‘s surface, but they create a landscape that is almost totally different from any other on the planet. Generally along shorelines, waves are agents of erosion, and currents are agents of transportation and deposition. The most notable land features created by wave erosion are rocky cliffs and headlands. Depositional features along coastlines are diverse in form, but by far the most common are beaches and sandbars. Beaches along the shorelines of both oceans and lakes are sometimes the most distinctive aspect of coastal landscapes. They develop as a transition from land to water, and are usually impermanent features of the landscape— building up during times of ―normal‖ weather and eroding or completely disappearing during storms.

Coastal Erosional Landforms

Erosional landforms dominate rocky coasts, but are also found in association with predominantly depositional landforms. Tidal creeks, for instance, occur within salt marshes.

Shore platforms and plunging cliffs

Rocky coasts fall into three chief types – two varieties of shore platform (sloping shore platform and horizontal shore platform) and plunging cliff. Variants of these basic types reflect rock types and geological structures, weathering properties of rocks, tides, exposure to wave attack, and the inheritance of minor changes in relative sea level.

Horizontal platforms are flat or almost so. They go by a host of names: abrasion or denuded benches, coastal platforms, low-rock terraces or platforms, marine benches, rock platforms, shore benches, shore platforms, storm wave platforms, storm terraces, wave-cut benches, and wave-cut platforms. Some of these terms indicate causal agents, e.g. ‗wave-cut‘ and

‗abrasion‘. Because the processes involved in platform evolution are not fully known, the purely descriptive term ‗shore platform‘ is preferable to any others from the wide choice available.

Sloping platforms are eye-catching features of rocky coasts. As their name intimates, they slope gently between about 1° and 5°. They are variously styled abrasion platforms, beach platforms, benches, coastal platforms, shore platforms, submarine platforms, wave-cut benches, wave-cut platforms, wave-cut terraces, and wave ramps.

Plunging-cliff coasts lack any development of shore platforms. Most plunging cliffs are formed by the drowning of pre-existing, wave-formed cliffs resulting from a fall of land level or a rise of sea level.

Cliffs, notches, ramps, ramparts, and potholes

Cliffs are steep or vertical slopes that rise precipitously from the sea or from a basal platform. About 80 per cent of the world‘s oceanic coasts are edged with cliffs (Emery and Kuhn 1982).

Cliff-base notches are sure signs of cliff erosion. Shallow notches are sometimes called nips. The rate at which notches grow depends upon the strength of the rocks in which the cliff is formed, the energy of the waves arriving at the cliff base, and the amount of abrasive material churned up at the cliff–beach junction.

Ramps occur at cliff bases and slope more steeply than the rest of

the shore platform. They occur on sloping and horizontal shore platforms. Horizontal shore platforms may carry ridges or ramparts, perhaps a metre or so high, at their seaward margins.

Marine potholes are roughly cylindrical or bowl-shaped depressions in shore platforms that the swirling action of sand, gravel, pebbles, and boulders associated with wave action grind out.

Caves, arches, stacks, and related landforms

Small bays, narrow inlets, sea caves, arches, stacks, and allied features usually result from enhanced erosion along lines of structural weakness in rocks.Bedding planes, joints, and fault planes are all vulnerable to attack by erosive agents. Although the lines of weakness are eroded out, the rock body still has sufficient strength to stand as high, almost perpendicular slopes, and as cave, tunnel, and arch roofs.

A gorge is a narrow, steep-sided, and often spectacular cleft, usually developed by erosion along vertical fault planes or joints in rock with a low dip. They may also form by the erosion of dykes, the collapse of lava tunnels in igneous rock, and the collapse of mining tunnels. In Scotland, and sometimes elsewhere, gorges are known as geos or yawns, and on the granitic peninsula of Land‘s End in Cornwall, south-west England, as zawns.

A sea cave is a hollow excavated by waves in a zone of weakness on a cliff. The cave depth is greater than the entrance width. Sea caves tend to form at points of geological weakness,

such as bedding planes, joints, and faults. Fingal‘s Cave, Isle of Staffa, Scotland, which is formed in columnar basalt, is a prime example. It is 20 m high and 70 m long.

A blowhole may form in the roof of a sea cave by the hydraulic and pneumatic action of waves, with fountains of spray emerging from the top. If blowholes become enlarged, they may collapse. An example of this is the Lion‘s Den on the Lizard Peninsula of Cornwall, England.

Natural Arches

Natural Arch is formed when powerful wave action excavates cave along joints and weak fractures on a cliffed coast on a headland. The wave action cuts through the caves on both sides of the headland producing this spectacular geomorphic feature. On the Normandy coast of France Porte d‘ Aval is a natural arch on cliff of Heart chalk. Sometimes elongated natural arches may form tunnels. One such example is the tunnel on the north coast of Cornwall Merlin‘s Cave.

Stacks/Skarries/ Chimney Rock:

When a portion of the sea arch collapses, the remaining column-like structure is called a stack, skarry or chimney rock.

Hanging Valleys:

If the fluvial erosion by streams flowing down the coast is not able to keep pace with the retreat of the cliff, the rivers appear to be hanging over the sea. These river valleys are called hanging valleys.

Coastal Depositional Landforms Beaches

Beaches are the most significant accumulations of sediments along coasts. They form in the zone where wave processes affect coastal sediments. In composition, they consist of a range of organic and inorganic particles, mostly sands or shingle or pebbles. Pebble beaches are more common at middle and high latitudes, where pebbles are supplied by coarse glacial and periglacial debris.

Sand beaches are prevalent along tropical coasts; probably because rivers carry predominantly fine sediments and cliff erosion donates little to littoral deposits in the tropics. Under some conditions, and notably in the tropics, beach sediments may, through the precipitation of calcium carbonate, form beachrock.

Beach cusps and crescentic bars

Beach cusps are crescent-shaped scallops lying parallel to the shore on the upper beach face and along the seaward margins of the berm with a spacing of less than about 25 m. Most researchers believe that they form when waves approach at right-angles to the shore, although a few think that oblique waves cause them. Their mode of formation is disputed, and they have been variously regarded as depositional features, erosional features, or features resulting from a combination of erosion and deposition.

Inner and outer crescentic bars are sometimes called rhythmic topography. They have wave - lengths of 100–2,000 m, although the majority are somewhere between 200 and 500 m. Inner bars are short-lived and associated with rip currents and cell circulations. Their horns often extend across surf-zone shoals into very large shoreline cusps known as sand waves, which lie parallel to the shore and have wavelengths of about 200–300 m. Outer crescentic bars, may be detached from the shore and are more stable than inner crescentic bars.

Spits, barriers, and related forms

Accumulation landforms occur where the deposition of sediment is favoured. Suitable sites include places where obstructions interrupt longshore flow, where the coast abruptly changes direction, and in sheltered zones (‗wave shadows‘) between islands and the mainland.

Accumulation landforms are multifarious. They may be simply classified by their degree of attachment to the land. Beaches attached to the land at one end are spits of different types and forelands. Spits are longer than they are wide, while forelands are wider than they are long. Beaches that are attached to the land at two ends are looped barriers and cuspate barriers, tombolos, and barrier beaches. Beaches detached from the land are barrier islands.

Spits and forelands

Barrier spits often form at the mouths of estuaries and other places where the coast suddenly changes direction. Sediment moving along the shore is laid down and tends to extend along the original line of the coast. Some spits project into the ocean and then curve round to run parallel to the coast. An example is Orfordness on the east coast of England, where the River Alde has been deflected some 18 km to the south.

Recurved spits have their ends curving sharply away from incoming waves towards the land, and compound recurved spits have a series of landward-turning recurved sections along their inner side. Blakeney Point, which lies in north Norfolk, England, is a famous recurved spit. Spits that have twisting axes, created in response to shifting currents, are called ‗serpentines‟.

Comet-tail spits form where long - shore movement of material down each side of an island leads to accumulation in the island‘s lee, as has happened at the Plage de Grands Sables on the eastern end of the Île de Groix, which lies off the coast of Brittany, France.

Arrows are spit like forms that grow seawards from a coast as they are nourished by longshore movement on both sides. Sometimes spits grow towards one another owing to the configuration of the coast. Such paired spits are found at the entrance to Poole Harbour, in Dorset, England, where the northern spit, the Sandbanks peninsula, has grown towards the southern spit, the South Haven peninsula.

Forelands or cuspate spits tend to be less protuberant than spits. They grow out from coasts, making them more irregular.

Tombolos

Tombolos are wave-built ridges of beach material connecting islands to the mainland or islands to islands. They come in single and double varieties. Chesil Beach in Dorset, England, is part of a double tombolo that attaches the Isle of Purbeck to the Dorset mainland. Tombolos tend to grow in the lee of islands, where a protection is afforded from strong wave action and where waves arerefracted and convergent. Y-shaped tombolos develop where comet-tail spits merge with cuspate forms projecting from the mainland or where a cuspate barrier extends landwards or seawards. A tombolino or tie-bar is a tombolo that is partly or completely submerged by the sea at high tide.

Barriers and barrier beaches

Coastal barriers and barrier islands form on beach material deposited offshore, or across the mouths of inlets and embayments. They extend above the level of the highest tides, in part or in whole, and enclose lagoons or swamps. They differ from bars, which are submerged during at least part of the tidal cycle.

Coastal barriers are built of sand or gravel. Looped barriers and cuspate barriers result from growing spits touching an opposite shore, another spit, or an island. Looped barriers grow in the lee of an island when two comet-tail spits join. Cuspate barriers (cuspate forelands) resemble forelands except that the building of beach ridges parallel to their shores has enlarged them and they contain lagoons or swampy areas. An example is Dungeness in Kent, England,

which is backed by marshland. If the lagoons or swamps drain and fill with sediment, cuspate barriers become forelands. Cuspate barriers form by a spit curving back to the land (a looped spit), or else by two spits or tombolos becoming joined to an island, which then vanishes (double-fringing spit).

Barrier beaches seal off or almost seal off the fronts, middles, or heads of bays and inlets. They are the product of single spits growing across bays or from pairs of converging spits built out by opposing longshore currents. They may also possibly form by sediment carried into bays by wave action independently of longshore movement.

Barrier islands

Barrier islands are elongated offshore ridges of sand paralleling the mainland coast and separated for almost their entire length by lagoons,swamps, or other shallow-water bodies, which are connected to the sea by channels or tidal inlets between islands. They are also called barrier beaches, barrier bars, and offshore bars. Sections of long barrier-island chains may be large spits or barrier beaches still attached to land at one end.

Beach ridges and cheniers

Sandy beach ridges mark the position of former shorelines, forming where sand or shingle have been stacked up by wave action along a prograding coast. They may be tens of metres wide, a few metres high, and several kilometres long. Beach ridge plains may consist of 200 individual ridges and intervening swales.

Cheniers are low and long ridges of sand, shelly sand, and gravel surrounded by low-lying mudflats or marshes. These ridges bear rich vegetation and are settled by people. The word

‗chenier‘ is from a Cajun expression originating from the French word for oak (chêne), which species dominates the crests of the higher ridges. Cheniers can be up to 1 km wide, 100 km long and 6 m high.

Chenier plains consist of two or more ridges with marshy or muddy sediments between. Most Cheniers are found in tropical and subtropical regions, but they can occur in a wide range of climates. They cannot form in coasts with high wave energy as the fine grained sediments needed for their growth are carried offshore.

Estuaries

Estuaries are tidal inlets, often long and narrow; those stretch across a coastal alluvial plain or run inwards along a river to the highest point reached by the tide. They are partially enclosed but connected to the open sea. They are transition zones between rivers and the sea in which fresh river water mixes with salty ocean water.

Coastal Dunes

Where ample sand is available, a narrow belt of dunes, called foredunes, often occurs in the region landward ofbeaches. These dunes are usually held in place by a cover of beach grass. Although these dunes are typically built up by wind, they play an important role in maintaining a stable coastline by trapping sand blown landward from the adjacent beach. As sand from the beach collects along the foredunes, the dune ridge grows upward, becoming a barrier several meters above high-tide level. This forms a protective barrier for tidal lands on the landward side of a beach ridge or barrier island.

Tidal flats

Tidal flats are banks of mud or sand that are exposed at low tide. They are not actually flat but slope very gently towards the sea from the high-tide level down to a little below the low-tide level. Three basic units may be identified in tidal flats: the high-tide flat (a gently sloping surface that is partly submerged at high tide); the intertidal slope (a steeper but still gently inclined zone lying between the high-tide flat and the lower tidal limit); and the subtidal slope, which is submerged even at low tide

Marine deltas

Marine deltas are formed by deposition where rivers run into the sea. So long as the deposition rate surpasses the erosion rate, a delta will grow. Deltas are found in a range of coastal environments. Some deltas form along low-energy coasts with low tidal ranges and weak waves. Others form in high-energy coasts with large tidal ranges and powerful waves. The

trailing-edge coasts of continents (passive margins) and coasts facing marginal seas appear to favour the growth of large deltas.

Salt marshes

Salt marshes are widespread in temperate regions, and are not uncommon in the tropics. They start to form when tidal flats are high enough to permit colonization by salt tolerant terrestrial plants. Depending on their degree of exposure, salt marshes stretch from around the mean high-water, neap-tide level to a point between the mean and extreme high-water, spring- tide levels. Their seaward edge abuts bare intertidal flats, and their landward edge sits where salt- tolerant plants fail to compete with terrestrial plants. Salt marsh sediments are typically heavy or sandy clay, silty sand, or silty peat. Many salt marshes contain numerous shallow depressions,or pans, that are devoid of vegetation and fill with water at high spring tides.

Mangals

‗Mangrove‘ is a general term for a variety of mainly tropical and subtropical salt-tolerant trees and shrubs inhabiting low inter-tidal areas. Mangals are communities of mangroves – shrubs and long-lived trees and with associated lianas, palms, and ferns – that colonize tidal flats in the tropics, and occur in river-dominated, tide dominated, and wave-dominated coastal environments. They specifically favour tidal shorelines with low wave energy, and in particular brackish waters of estuaries and deltas. Some mangrove species are tolerant of more frequent flooding than salt marsh species, and so mangals extend from around the high spring-tide level to a little above mean sealevel. They often contain lagoons and pools, but not the pans of salt marshes. Like salt marshes, mangals have creek systems, although their banks are often formed of tree roots.

Coral reefs and atolls

A coral reef is a ridge or mound built of the skeletal remains of generations of coral animals, upon which grow living coral polyps. Reefs typically grow in shallow, clear waters of tropical oceans. The Great Barrier Reef, in the Coral Sea off the north-east Australian coast is, at over 2,600 km long, the world‘s largest living reef, and indeed the largest living organic feature. It comprises more than 3,000 individual reefs and hundreds of small coral islands, ranging in size from about 10 ha to 10,000 ha, formed along the edge of the continental shelf.

An atoll is a ring of coral reef and small sandy islands that encircles a shallow lagoon. Atolls are common in the tropical Pacific Ocean, where such groups of islands as the Marshall Islands andKiribati are chains of atolls. They form when volcanic islands move away from the heating anomaly that creates them, as in the Hawaiian island hot-spot trace, and they begin to subside beneath sea level. Reefs initially form as a fringe in the shallow waters around a volcanic island, as in Tahiti. With time, the island erodes and subsides. However, the reef continues growing upwards to create an offshore barrier reef separated from the main island by a lagoon, as in the case of Bora Bora in the Society Islands. The lifting of a reef above sea level creates a raised atoll. These often have spectacular cave landscapes.

Glacial and Periglacial Landforms

Glaciers are a powerful force in high-latitude and high-elevation areas, but in the past they also covered most of Canada, the northern parts of the U.S., and large areas of Europe and Asia. Erosion and deposition associated with glaciers reshape the underlying landscape and form many distinctive landforms, some of which indicate that ice once covered an area, even if none remains today.

Wherever glaciers have developed, they have had a significant impact on the landscape simply because moving ice grinds away almost anything in its path: most soil is carried away, and bedrock is polished, scraped, gouged, plucked, and abraded. Moreover, the rock that is picked up is eventually deposited in a new location, further changing the shape of the terrain. In short, preglacial topography is significantly reshaped.

Perhaps 7 per cent of all contemporary erosion and transportation of rock debris on the continents is accomplished by glaciers. This is a small amount in comparison with fluvial erosion, to be sure, but considering how small a land area is covered by glacial ice today, it is clear that glaciers make a significant contribution to continental denudation.

Glaciation modifies flat landscapes greatly, with the result that postglacial slope, drainage, and surficial material are likely to be totally different from what they were before the glacier passed by. In mountainous areas, the metamorphosis of the landscape may be less complete, but the topography is often deepened and steepened, and in many places rounded, by glacial action.

Glaciers are masses of ice that persist from year to year and tend to flow downhill under their own weight. They range in size from ice sheets that cover a large region —continental glaciers— to much smaller glaciers that are restricted to a single mountain or valley — alpine glaciers. Most glaciers are primarily ice and snow, but glaciers typically contain significant amounts of rock and finer sediment that are incorporated into the glacier as it flows from higher to lower elevations.

Erosional Landforms

A number of landforms are created due to erosion by glaciers. Important among then are the followings:

Cirque: The shape of the cirque is just like an armchair and it is being made by glacial erosion. When the glacier descends from the mountain slope, it moves faster due to very steep slope. Later when the slope lowers, the excessive accumulation of ice exerts enormous pressure and the base is eroded by ice rotation, plucking and abrasion. In due course of time the base is further eroded and a depression is created. It is called cirque which is a French term and meant for a circle or bowl shaped. It is also known as Corrie in Scottish.

Aretes: When several cirques or corries are developed on different sides of a mountain peak, their separating boundaries are further narrowed and sharpened. The sharp knife or blade

edged boundaries are called arêtes. Mountain climbers generally follow aretes for scaling the peak.

Col: Col is a depression like feature made between two peaks. Generally, col is made by

the

erosive action of glacier. This depression is created by the moving glacier. It helps in passing the mountain from one side to another. Col is also known as pass or saddle. Saddle is a seat fastened on the back of a horse for sitting and riding the horse. It is depressed at the center but raised in the front and rear and hence, it is called so.

Horn or Pyramidal Peak: When the glaciers are developed from different sides of a peak, they erode the pre-existing surface of the peak successively and make it a sharp and pointed peak with very steep slope. This gives a rise of a pyramidal peak which is also termed as horn as it is very pointed.

Hanging Valley: Hanging valley is a tributary glacial valley of a main glacier. Main glacier has huge amount of ice, hence, erode the base greater. Therefore, the main glacial valley is much deeper. The tributary glacier has lesser amount of ice, hence, less erosion. Therefore, the base of the tributary glacier remains above the main glacier valley. When the glaciation is over, the tributary valley is seen above the main valley. That is why, it is called hanging valley.

Truncated Spur: Spur is a long tongue shaped ridge dropping down in an inverted ‗V‘ separating the two adjacent valleys. The valleys confluence with another main river.When the same area comes under the influence of glacier (due to climate change), the main river turns into main glacier. Volume of ice in the main glacier is very large, it erodes its valley greater. Since, the glacier is the moving solid ice, it erodes not only the bed but side also. The dropped down spur is eroded and the slope of the spur is interrupted. This interrupted/ broken spur is called truncated spur.

„U‟ Shaped Valley: Glacial ice movement erodes its sides and make the trough much wider. The bed of the glacier is flat and both sides are almost vertical. This elongated wider trough is called ‗U‘ shaped valley. When the glaciation period is over and ice is melted, the valley is exposed.

Fjord: Fjords are submerged glacial valleys under the sea water along the coast. The bottom of the valley is cut down because of heavy accumulated ice movement even below the sea level. With the culmination of ice age, the sea level also rises and down cut glaciated valleys along the sea coast is drowned under the sea water. Because of this process, the coast is greatly dissected. These sorts of coasts are found in the high latitudes and near polar areas.

Roche Moutonnée: Hills are generally sheared off and rounded by the moving ice. A characteristic shape produced by both continental ice sheets and mountain glaciers is the roche moutonnée, which is often produced when a bedrock hill is overridden by moving ice. The stoss side (facing in the direction from which the ice came) of a roche moutonnée is smoothly rounded and streamlined by grinding abrasion as the ice rides up the slope, but the lee side (facing away from the direction from which the ice came) is shaped largely by plucking, which produces a steeper and more irregular slope.

Glacial Steps: As might be expected, a glacier grinding along the floor of a glacial trough does not produce a very smooth surface. Valley glaciers do not erode a continuously sloping channel because differential erosion works with ice as well as with water. Therefore, resistant rock on the valley floor is gouged less deeply than weaker or more fractured rock. As a result, the down-valley profile of a glacial trough is often marked by an irregular series of rock steps or benches, separated by steep (although usually short) cliffs on the down-valley side. Such landforms are known as glacial steps.

Depositional Landforms

Numerous landforms are created due to deposition of glacial loads when its capacity to carry is reduced. Important among then are the followings:

Moraine: The materials carried by the glacial ice are deposited in an ideal condition when the carrying capacity is reduced. These deposits are generally unsorted and un-stratified as it is deposited as and when the material reaches and dropped. Most of them are boulders of different sizes. Depending upon the place of deposits, the moraines are classed into different groups. They are:

Lateral Moraine: The two sides of the glacier is generally have exposed rocks. The weathered materials are dropped at the side of the glacier and are transported along. Materials are also supplied due to landslide as well. Carried materials also erode the side of the glacier and further more loads are generated. These materials are deposited along the side of the glacier and hence, they are called lateral moraine.

Medial Moraine: When tributary glacier or two glaciers meet with each other, the meeting side‘s lateral moraines also joins together and moves further downward. Slowly and slowly the

joined lateral moraine reaches towards the middle/ near middle. When the ice melts, the joined lateral moraine is deposited and is called medial moraine.

Terminal Moraine: The deposits which occur at the down end of the glacier in a ridge-like accumulation. This debris is pushed forward by advancing glacial tongue and dropped at the end. The terminal moraine spreads from one side or bank of the glacier to another covering the entire width. It appears like a belt of small hilly ground with knobs and joints.

Recessional Moraine: Recessional moraine is a sort of terminal moraine. After the formation of terminal moraine, when the glacier retreats, a new terminal moraine is formed at upstream. The previous terminal is, later, called as recessional (down) moraine.

Ground Moraine: Irregular blanket of glacial loads deposited on the floor in a rolling to flat landscape after glacial melt/ retreat is called ground moraine. As the term itself explains, it is the whole of the ground down side the existing glacier where the glacial load is deposited. It is formed in both the cases – continental glacier and valley glacier. Ground moraine is also known as till plain.

Glacial Till and Till Plain: The glacier carries huge amount of loads with it. When the glacier melts/ retreats, the transported loads are deposited. This deposition is undulating to flat in look. The deposition of such materials at a certain location at small scale is till and the widespread area containing till is termed as till plain. This type of till plain is very extensive in continental glacier but confined in the valley glacier.

Till Plain

Outwash Plain: Down side from the glacial melt, water associated with smaller pieces of ice floats below. Gravels, sands, silts and clays are carried downward by the melted water and

deposited far away. It forms a sloppy but relative flat plain known as outwash (material washed/ carried by water) plain. It is different from till plain. Till plain has unsorted materials directly deposited by melt of ice and, hence, dropped without ordering of the size of materials. Outwash plain has sorted deposits as it is deposited by glacio-fluvial (small ice pieces of ice and flowing water) action.

Esker: Esker is a long, narrow and zigzag ridge like structure generally made up of stratified sediments deposited by subglacial meltwater. Its height varies from 5 meter to 50 meter and width is 50 meter to 500 meter whereas the length many as 500 meter to many kilometers. Esker is not necessarily a continuous feature, but it may be a detached or broken. Literally, it is deposited by the meltwater of the glacier and hence, they are sorted.

Kettle: A kettle is a shallow and small pothole or depression area in which glacial brought sediment is deposited and remaining hollow part covers the water. It is formed and appears on the surface by retreating glacier. Probably, this depression is created by the plucking, but later deposition of sediment and water accumulation give the birth of kettle. This water-filled body is known as Kettle Lake.

Kame: Kame is the accumulation in a mound like shape made up of poorly sorted sand, boulders and gravels brought by glacier at its terminus. A number of kames a formed when the glacier melts and terminates or recedes. A group of closely formed kames are termed as kame complex. Several kettle lakes are seen in a kame complex.

Drumlin (Basket of Egg Topography): Drumlin is an elongated oval-shaped deposited mound of small hill-like feature formed by glacier. It is made up of glacial till containing boulder, gravels and sand transported by glacier and finally deposited in the above mentioned shape. They are seen in the plain and are many in number. They vary in dimension like one to two kilometres in length, 400 to 600 meter in width and 15 to 30 meter in height.

Karst Landforms

Karst is a landscape which is underlain by limestone and has been eroded by dissolution, producing towers, fissures, sinkholes, etc. It is so named after a province of earlier Yugoslavia on the Adriatic Sea coast where such formations are most noticeable. The development of all karst landforms requires the presence of rocks such as limestone, dolomite, and gypsum which is capable of being dissolved by surface water or ground water.

Erosional Landforms

The landforms in the Karst landscape develop on three scales ranging from a scale of less than 10 m – these include varieties of solution pits, pans or grooves collectively known as

―Karren‖; to intermediate scale features ranging approximately from 1 to 1000 m namely dolines or sinkholes to large scale forms where landforms are normally greater than 1 km in length e.g. poljes, dry valleys or gorges. Most of the karst landscapes are formed by the combination of all these landforms and sometimes may extend for thousands of square kilometres.

A great diversity of forms and combinations occur in the karst terrains found in different parts of the world. This diversity in the landforms is the product of physical and chemical variations in the rocks themselves; geologic structure; tectonic and geomorphic history; regional topography; and past and present climatic conditions.

Karren/Lapies:

Karrens are highly corrugated and rough surface of limestone lithology with low ridges and pinnacles. These are formed when rain falls onto bare limestone or waves break into it. Therefore, falling droplets, sheet and channelled runoff, film flow and ponded water all create small scale solution forms also termed as lapies. The most commonly form found are circular pits with rounded bottoms and pans with flat bottoms, sinuous channels and descending slopes where joints or dipping bedding planes have opened up.

Terra Rosa

These are red clay stones up to several meters thick and kilometres across that occur at the earth‘s surface. These are thought to be formed by residual dissolution of limestones and/or by accumulation of detrital mud, ash or dust on pre-existing karst terrain.

Grikes

Grikes are vertical or near-vertical fissures in limestone pavement. In the initial stage, the cracks and fissures are only microscopic in size but as rainwater seeps in and dissolves the limestone, the cracks become wider.

Limestone Pavement

These are only found in places that were covered by ice during the last ice age. They are formed in some beds of limestones where following characteristics are found – i) beds that do not contain many fractures ii) beds that are more resistant to dissolution by rainwater iii) beds that are mechanically stronger

Solution Holes

Holes produced by dissolution of lime stones by chemically active standing water. These are small, shallow, round and flat bottomed depressions or pools on the limestone pavement. They are usually 5 – 30 cm wide.

Cavern

This is an underground cave formed by water action by various methods in a limestone or chalk area. There are differing views on the mode of formation of these caverns. The Mechanical Action School represented by Penck, Weller and Dane considers mechanical action by rock debris and pebbles to be responsible for cavern excavation. This school argues that the water table is too low to have a solution effect. The Chemical Action School, on the other hand, considers the solution action of water to be mainly responsible for cavern excavation. This school is represented by Davis and Piper. The largest cavern in Kentucky (USA) is 48 km long and 25 metres high. In India, such caves can be seen in Bastar, Dehradun, and Shillong plateau.

Arch/Natural Bridge

These are formed due to collapse of the roofs of caves or due to disappearance of surface streams and their reappearance; which keeps standing forming an arch.

Sink Hole/Swallow Hole:

Sink holes are funnel-shaped depressions having an average depth of three to nine metres and, in area, may vary from one square metre to more. These holes are developed by enlargement of the cracks found in such rocks, as a result of continuous solvent action of the rainwater. The swallow holes are cylindrical tunnel-like holes lying underneath the sink hole at some depth. The surface streams which sink disappear underground through swallow holes because these are linked with underground caves through vertical shafts.

Karst Window: When a number of adjoining sink holes collapse, they form an open, broad area called a karst window.

Karst Plain: Upper surface having several sink holes is called karst plain.

Sinking Creek:

Wherenumerous sink holes located in a line the water often gets lost through cracks and fissures in the bed. These are called sinking creeks, and if their tops are open, they are called bogas.

Dolines:

Dolines are bowl shaped enclosed depressions in the Karst terrain that can be several metres to several hundred metres in range. The formation of dolines is associated with four distinct processes that usually operate alone. These are –

1. Solution, acting downward

2. Mechanical collapse

3. Subsidence without rupture into an inter-strata solution cavity and

4. Sapping or seepage erosion into caves or adjoining dolines.

Mostly, two or three of the above mentioned processes operate together to develop or enlarge a depression. These range from cylindrical shafts to shallow saucers although intermediate funnel and bowl shapes are more common.

Karst Lakes: When Dolines plugged by clay are filled with water karst lakes are formed.

Uvala: A number of adjoining dolines may come together to form a large depression called uvala.

Polje: A number of uvalas may coalesce to create a valley called polje which is actually a flat floored depression. An ideal polje is an elongated, flat-floored, closed depression surrounded by limestone hills that are well karstified.

Dry Valley/Hanging Valley/Bourne

Sometimes, a stream cuts through an impermeable layer to reach a limestone bed. It erodes so much that it goes very deep. The water table is also lowered. Now the tributaries start serving the subterranean drainage and get dried up. These are dry valleys or bournes. Lack of adequate quantities of water and reduced erosion leaves them hanging at a height from the main valley. Thus, they are also referred to as hanging valleys.

Blind Valleys:

If the streams lose themselves in these valleys, then these are called blind valley. These valleys may have surface streams and may be used for agriculture. In other worrds when streams become deeply entrenched creating valleys within the paleo valley a blind valley is formed.

Depositional Landforms

Water is also responsible for the formation of a large number of depositional landforms in a karst landscape. Chemically saturated water degasses as it drips, pools, flows and percolates in the below ground karst environment. In the following section, some of the landforms formed due to deposition process have been discussed.

Speleothems: Deposits in the karstic caves are collectively called ‗Speleothems‘; these are crystalline deposits of calcium carbonate as a result precipitation from dilute aqueous solutions entering the cave, typically as drips from the roof.

Travertines: Banded calcareous deposits are called Travertines

Drip Stones:

These are calcareous deposits formed by dripping-of water in dry caves or caverns. These can only form when a cave is at or above the water table where water can evaporate. When groundwater drips from the roof of a limestone cave, it slowly deposits calcite. Several types of drip stones are found in karstic caves. Stalactitis, stalagmites, cave pillars, drapes, helictites and heligmites are such drip stones found in a karst cavern.

Stalactites and Stalagmites:

The water containing limestone in solution seeps through the roof of caverns in the form of a continuous chain of drops. A portion of the roof hangs on the roof and on evaporation of water; a small deposit of limestone is left behind contributing to the formation of a stalactite, growing downwards from the roof.

The remaining portion of the drop falls to the floor of the cavern. This also evaporates, leaving behind a small deposit of limestone aiding the formation of a stalagmite, thicker and flatter, rising upwards from the floor.

Cave Pillars:Sometimes, stalactite and stalagmite join together to form a complete pillar known as the column.

Drapes or curtains:Numerous needle shaped dripstones hanging from the cave ceiling.

Helictites:Sideward growth from stalactites.

Helgmites: Sideward growth from stalagmites.

VIII. Drainage Systems and Patterns

Drainage system refers to the origin and development of streams through time while drainage pattern means spatial arrangement and form of drainage system in terms of geometrical shapes in the areas of different rock types; geological structure, climatic conditions and denudational history e.g. trellis pattern, dendritic pattern, parallel pattern, etc. The examples of drainage system are consequent streams, subsequent streams, obsequent streams, etc.

The origin and subsequent evolution of any drainage system in a region are determined and controlled by two main factors viz. (1) nature of initial surface and slope (2) geological structure (e.g. folds, faults, joints, dips and strikes, etc.)

Drainage systems are broadly divided into two categories on the basis of adjustment of the streams to the initial surface and geological structures.

Sequent Streams: Which follow the regional slope and are well adjusted to geological structures, e.g. consequent streams, obsequent, subsequent streams and resequent streams.

Insequent Streams: Which do not follow the regional slope and are not adjusted to geological structures, e.g. antecedent streams, and superimposed streams.

Sequent Drainage Systems:

Consequent streams:

Consequent streams are the first streams to be originated in a particular region. These streams have their courses in accordance with the initial slope of land surface. In other words, the consequent streams follow the regional slope. These are also called dip streams.

In a region of folded structure (when the crustal rocks are folded due to lateral compressive forces into parallel anticlines and synclines) consequent streams are formed in the synclinal troughs. Such consequent streams are called synclinal consequent streams, which become the master consequent streams of trellis drainage pattern at much later date.

The first streams to be initiated on a newly emerged coastal plain are consequent streams which are parallel to each other and thus form parallel drainage pattern. The longest stream of the whole system of consequent streams is called master consequent. Most of the streams

draining the coastal plains of India are the examples of consequent streams. The most ideal landscapes for the origin and development of consequent drainage system are domes and volcanic cones.

Subsequent streams:

The streams originated after the master consequent stream and following the axis of the anticlines or ridges and the strikes of beds are called subsequent streams. Some of the scientists have opined that the subsequent streams originate on the flanks of the anticlines and join the master consequent at almost right angle (which means that the lateral consequents, as referred to above, are the subsequents) while others maintain that the subsequents are parallel to the master consequent.

The Asan River, a tributary of the Yamuna River and the Song River, a tributary of the Ganga River in the Dehra Dun valley (in filled alluvial plain) are the examples of subsequent streams while the Yamuna and the Ganga are the master consequents.

Obsequent streams:

The streams flowing in opposite direction to the master consequent are called obsequent streams. In fact, obsequent streams are also consequents because they also follow the slopes of the ranges. The streams originating from the northern slopes of the west-east stretching ranges of the Himalayas flow northward to meet the east-west flowing tributaries (subsequent streams) of the southward draining master consequent streams.

For example several streams originating from the northern slopes of the Siwalik Range drain due northward to join east-west subsequent streams of the southward flowing master consequents of the Ganga and the Yamuna, north of the Someshwar Range of Dundwa Range (of the Siwalik Range). These northward flowing tributaries are the examples of obsequent streams.

Resequent streams:

The tributary streams flowing in the direction of the master consequents are called resequents. These are originated at much later date in comparison to the master consequents. Since they are of recent origin, and hence they are called resequent.

The resequent streams are originated during the initiation of second cycle of erosion in a folded structure. The gradual denudation of folded mountains during the first cycle of erosion results into inversion of relief with the passage of time wherein anticlinal ridges and synclinal valleys are converted into anticlinal valleys and synclinal ridges respectively.

Insequent Drainage System:

The streams which do not follow the regional slopes and drain across the geological structures are called Insequent or inconsequent streams.Antecedent and superimposed streams are the best representative examples of insequent drainage systems.

Antecedent drainage system:

Antecedent streams are those which are originated prior to the upliftment of land surface. In other words, antecedent streams antedate the upliftment of an upland or mountain across which they have maintained their present courses through continuous down-cutting of their valleys. It is, thus, obvious that if a river has developed in a particular region and if the subsequent uplift or up-warping of the land area across the pre-existing river does not divert or deflect the course of the river and the river maintains its previous course through down-cutting its valley at the rate equivalent to the rate of uplift of the land area, the river is called antecedent or anti-consequent. Examples of antecedent streams are found in almost all of the folded

mountains of the world. Many of the major Himalayan rivers are the examples of antecedent streams e.g., the Indus, the Sutlej, the Ganga, the Ghaghra, the Kali, the Gandak, the Kosi, the Brahmaputra etc.

Superimposed drainage system:

Like antecedent streams superimposed streams are also not adjusted to regional geological structures and slope and thus are insequent or anti-consequent streams. Superimposed stream means a river which, flowing on a definite geological formation and structure, has inherited the characteristics of its previous form developed on upper geological formation of entirely different structural characteristics.

In other words, a superimposed drainage is formed when the nature and characteristics of the valleys and flow direction of a consequent stream developed on the upper geological formation and structure are superim•posed on the lower geological formation of entirely different characteristics. It is not necessary that the upper geological formation is conformal to the lower geological formation, rather it happens that sometimes the upper rock cover is entirely different from the underlying geological formation viz., the upper cover may consist of horizontally bedded sedimentary rocks while the lower cover may be composed of folded sedimentary beds, or domed structure or batholithic intrusives.

The consequent streams are developed on almost flat ground surface of the horizontally bedded sedimentary rocks. These streams develop their valleys through vertical erosion (down cutting). With the passage of time the lower structure is exposed to the river which continues its down cutting and extends its valley downward on the lower geological structure (say anticlinal folds, domes or batholiths) and thus the valley developed on the upper structure is superimposed on the lower structure.

The lower structure has to accept the form of the valley already developed on the entirely different upper structure. Thus, the river maintains the form of its valley, the flow direction and its drainage patterns as usual. Such rivers are called superimposed rivers. Had the lower structure been present on the ground, the nature of drainage system would have certainly been entirely different but the buried different geological structure has no alternative other than to adapt the nature of valley and flow direction developed on the upper structure. Such streams are unconformable to the local geological structure and slopes. Such unconformal valleys have been named as ‗superimposed‘ by J.W. Powell (1857) and D. Maw (1886) and ‗superposed‘ by W.J. McGee (1888).

The fundamental difference between superimposed and antecedent drainage systems lies in the fact that the former represents the superimposition of the valley developed in the upper geological formation and structure on the lower structure irrespective of its types and complexities but without any upliftment of the land area while the latter represents the mainte•nance of former course of the river and its valley inspite of upliftment of the land area across the river course due to concomitant valley deepening and up warping of the land.

Most of the rivers of the Deccan trap (lava) region of Peninsular India are superimposed because new drainage system was evolved on the new surface formed due to cooling and solidification of lavas erupted during late Cretaceous and early Tertiary periods and these rivers were superimposed on the lower formation after the removal of lava covers. For example, the Subarnarekha River is superimposed on Dalma and Phyllite hills to the west of Chandil in the south-eastern Chotanagpur plateau region of Jharkhand.

Drainage Patterns

Drainage pattern means the form (geometrical forms) of the drainage systems and the spatial arrangements of streams in a particular locality or region. Generally the drainage patterns are divided into the following types

a. Trellised Drainage Pattern

b. Dendritic Drainage Pattern

c. Rectangular Drainage Pattern

d. Radial Drainage Pattern

e. Centripetal Drainage Pattern

f. Annular Drainage Pattern

g.

Parallel Drainage Pattern.

I. Atmosphere

CHAPTER-2 CLIMATOLOGY

Composition of the Atmosphere:

The Earth is surrounded by air, a mixture of various gases that reaches up to a height of many kilometers. This envelope of air makes up our atmosphere.It is held in place by the Earth‘s gravity. Almost all the atmosphere (97 percent) lies within 30 km (19 mi) of the Earth‘s surface. The upper limit of the atmosphere is at a height of approximately 10,000 km (about 6000 mi) above the Earth ‘s surface—a distance that is nearly as large as Earth ‘s diameter.

About 99 percent of pure, dry air is nitrogen (about 78 percent by volume) and oxygen (about 21 percent).

Nitrogen gas is a molecule consisting of two nitrogen atoms (N2). It does not easily react with other substances. Soil bacteria do take up very small amounts of nitrogen, which can be used by plants, but otherwise, nitrogen is largely―filler,‖ adding inert bulk to the atmosphere.

In contrast, oxygen gas (O2) is chemically very active, combining readily with other elements in the process of oxidation. Fuel combustion is a rapid form of oxidation, while certain types of rock decay (weathering) are very slow forms of oxidation. Living tissues require oxygen to convert foods into energy.

The remaining 1 percent of dry air is mostly argon, an inactive gas of little importance in natural processes, with a very small amount of carbon dioxide (CO2), amounting to about 0.0385 percent.

Although the amount of CO2 is small, it is a very important atmospheric gas because it absorbs much of the incoming shortwave radiation from the Sun and outgoing long wave radiation from the Earth. This contributes to the greenhouse effect. Carbon dioxide is also used by green plants, which convert it to its chemical compounds to build up their tissues, organs, and supporting structures during photosynthesis.

Water vapor is another important atmospheric gas. Water vapor usually makes up less than 1 percent of the atmosphere, but under very warm, moist conditions, as much as 2 percent of the air can be water vapor. Since it is a good absorber of heat radiation, like carbon dioxide, it plays a major role in warming the lower atmosphere and enhancing the greenhouse effect.

Another small, but important, constituent of the atmosphere is ozone;Ozone in the upper atmosphere is beneficial because it shields life at the Earth‘s surface from harmful solar ultraviolet radiation. But in the lowest layers of the atmosphere, ozone is an air pollutant that damages lung tissue and aggravates bronchitis, emphysema, and asthma.

Dust Particles

Dust particles are generally found in the lower layers of the atmosphere. These particles are found in the form of sand, smoke and oceanic salt. Sand particle have important place in the atmosphere. These dust particles help in the condensation of water vapour. During condensation water vapour gets condensed in the form of droplets around these dust particles. Due to this process the clouds are formed and precipitation is made possible.

Structure Of the Atmosphere:

The atmosphere extends from the surface of Earth upward for more than 100 km, with some characteristics of the atmosphere going out to thousands of kilometers. The atmosphere is not homogeneous in any of its attributes, but instead has different layers that vary in temperature, air pressure, and the amount and composition of gases. Each layer has the term ―sphere‖ as part of its name, referring to the way each layer successively wraps around the Earth with a roughly spherical shape.97 percent of the total amount of weight of the atmosphere is limited upto the height of about 30 kilometers.

1. Troposphere

Ø This is the lowest layer of the atmosphere. Its average height is 13 km and extends roughly to a height of 8 km near the poles and about 18 km at the equator. It contains the air we breathe, clouds, wind, rain, and other aspects of weather.

Ø This is the most important layer of the atmosphere because all kinds of weather changes take place only in this layer. This is the most important layer for all biological activity.

Ø

The air never remains static in this layer. Therefore this layer is called changing sphere or troposphere.

Ø The temperature in this layer decreases at the rate of 1°C for every 165m of height.

The zone separating the troposphere from stratosphere is known as the tropopause. The air temperature at the tropopause is about minus 80^oC over the equator and about minus 45^oC over the poles. The temperature here is nearly constant, and hence, it is called the tropopause.

2. Stratosphere

Ø The stratosphere is found above the tropopause and extends up to a height of 50 km.

Ø One important feature of the stratosphere is that it contains the ozone layer. This layer absorbs ultra-violet radiation and shields life on the earth from intense, harmful form of energy.

Ø The temperature remains almost the same in the lower parts of this layer upto the height of 20 kms. After this the temperature increases slowly with the increase in the height. The temperature increases due to the presence of ozone gas in the upper part of this layer.

Ø Weather related incidents do not take place in this layer. The air blows horizontally here. Therefore this layer is considered ideal for flying of aircrafts.

3. Mesosphere

Ø The mesosphere lies above the stratosphere, which extends up to a height of 80 km.

Ø In this layer, once again, temperature starts decreasing with the increase in altitude and reaches up to minus 100°C at the height of 80 km.

Ø ‗Meteors‘ or falling stars occur in this layer.

Ø The upper limit of mesosphere is known as the mesopause.

4. Ionosphere or Thermosphere

Ø The ionosphere is located between 80 and 400 km above the mesopause.

Ø It contains electrically charged particles known as ions, and hence, it is known as ionosphere. Radio waves transmitted from the earth are reflected back to the earth by this layerand due to this radio broadcasting has become possible.

Ø Temperature here starts increasing with height.

5. Exosphere

Ø This is the last layer of the atmosphere located above ionosphere and extends to beyond 400 km above the earth.

Ø Gases are very sparse in this sphere due to the lack of gravitational force. Therefore, the density of air is very less here.

Except for the extremely variable water vapour in the troposphere and ozone in stratosphere, the atmosphere is well mixed and has a constant composition up to about 80 km. The atmosphere up to 80 km is called the Homosphere and the atmosphere aloft is Heterosphere. At about 20-25 km, however, there is high concentration of ozone.

Heterosphere is the upper portion of a two-part division of the atmosphere according to the general homogeneity of atmospheric composition; the layer above the Homosphere. The hetero- sphere is characterized by variation in the composition and the mean molecular weight of constituent gases. This region starts at 50 to 60 miles (80–100 km) above the earth and, therefore, closely coincides with the ionosphere and the thermosphere.

The Elements of Weather and Climate:

The condition of the atmosphere at any time or place, i.e., the weather, is expressed by a combination of several elements, primarily (a) temperature and (b) precipitation and humidity but to a lesser degree by (c) winds and (d) air pressure as well.These four are called the elements of weather and climate because they are the ingredients out of which various weather and climatic types are compounded.

v The weather of any place is the sum total of its atmospheric conditions (temperature, pressure, winds, moisture, and precipitation) for a short period of time. It is the momentary state of the atmosphere.

v Climate, on the other hand, is a composite or generalization of the variety of day-today weather conditions.

THE CONTROLS OF WEATHER AND CLIMATE

Weather variesfrom day to day, and climate differs from place to place, because ofvariations in the amount, intensity, and areal distribution of these several weather and climatic elements, more particularlytemperature and precipitation.

Climatic controlsthat cause these several climatic elements to vary from place toplace and season to season on the earth, resulting in some placesand some seasons being hot and others cold, some wet and othersdry.These are

· latitude or sun,

· distribution of land and water,

· winds,

· altitude,

· mountain barriers,

· the great semipermanent high and low-pressure centers,

· ocean currents,

· Storms of various kinds, and a number of other more minor ones.

It is these controls,acting with various intensities and in different combinations thatproduce the changes in temperature and precipitation, which in turngive rise to varieties of weather and climate.

II. Atmospheric Temperature Insolation (Solar Radiation):

The sun is the primary source of energy on the earth. This energy is radiated in all directions into space through short waves.The energy transmitted from the Sun to Earth, called incoming solar radiation, or insolation.

The amount of insolation received on the earth‘s surface is far less than that is radiated from the sun because of the small size of the earth and its distance from the sun.Only two billionths or (two units of energy out of 1,00,00,00,000 units of energy radiated by the sun) of the total solar radiation reaches the earth‘s surface. Moreover water vapour, dust particles, ozone and other gases present in the atmosphere absorb a small amount of insolation.

Factors influencing Insolation

The amount of insolation received on the earth‘s surface is not uniformeverywhere. It varies from place to place and from time to time. The tropicalzone receives the maximum annual insolation. It gradually decreases towardsthe poles. Insolation is more in summers and less in winters.The following factors influence the amount of insolation received.

(a) The Angle of Incidence:The inclination of the sun's rays to the horizontal varies with the latitude, the time of year, and the time of day. Within the tropics the sun is overhead at noon on one or two days each year and is comparatively high at all seasons. Except within the tropics, the sun is never vertically overhead; its noon elevation decreases with increasing latitude and changes greatly with the seasons.

The higherthesun, the greater the amount of heat a given horizontal surfacewill receive; for, as the rays become more oblique, they are spreadout over a larger area.

(b) Duration of the day:Duration of the day varies from place to place and season to season. It decides the amount of insolation received on earth‘s surface. The longer the duration of the day, the greater is the amount of insolation received. Conversely shorter the duration of the day leads to receipt of less insolation.

(c) Transparency of the atmosphere:Transparency of the atmosphere also determines the amount of insolation reaching the earth‘s surface. The transparency depends upon cloud cover, its thickness, dust particles and water vapour, as they reflect, absorb or transmit insolation. Thick clouds hinder the insolation to reach the earth while clear sky helps it to reach the surface. Water vapour absorbs insolation, resulting in less amount of insolation reaching the surface.

Spatial Distribution of Insolation at the Earth‟s Surface

The insolation received at the surface varies from about 320 Watt/m2 in the tropics to about 70 Watt/m2 in the poles. Maximum insolation is received over the subtropical deserts, where the cloudiness is the least. Equator receives comparatively less insolation than the tropics. Generally,

at the same latitude the insolation is more over the continent than over the oceans. In winter, the middle and higher latitudes receive less radiation than in summer.

Heating and cooling of the Atmosphere

There are different ways of heating and coolingof the atmosphere.

Conduction:

Conduction takes place when two bodies of unequal temperature are in contact with one another; there is a flow of energy from the warmer to cooler body. The transfer of heat continues until both the bodies attain the same temperature or the contact is broken.

The earth after being heated by insolation transmits the heat to the atmospheric layers near to the earth in long wave form. The air in contact with the land gets heated slowly and the upper layers in contact with the lower layers also get heated. This process is called conduction. Conduction is important in heating the lower layers of the atmosphere.

Convection

Transfer of heat by movement of a mass or substance from one place to another, generally vertical, is called convection. The air of the lower layers of the atmosphere gets heated either by the earth‘s radiation or by conduction. The heating of the air leads to its expansion. Its density decreases and it moves upwards. Continuous ascent of heated air creates vacuum in the lower layers of the atmosphere. As a consequence, cooler air comes down to fill the vacuum, leading to convection.

The cyclic movement associated with the convectional process in the atmosphere transfer heat from the lower layer to the upper layer and heats up the atmosphere. The convective transfer of energy is confined only to the troposphere

Advection

The transfer of heat through horizontal movement of air is called advection. Horizontal movement of the air is relatively more importantthan the vertical movement. The temperature of a place will rise if it lies on the path of winds coming from warmer regions. The temperature will fall if the place lies on the path of the winds blowing from cold regions. In tropical regions particularly in northern India during summer season local winds called ‗loo‘ is the outcome of advection process.

Terrestrial Radiation

The insolation received by the earth is in short wave‘s forms and heats up its surface. The earth after being heated itself becomes a radiating body and it radiates energy to the atmosphere in long wave form. This energy heats up the atmosphere from below. This process is known as terrestrial radiation.

The long wave radiation is absorbed by the atmospheric gases particularly by carbon dioxide and the other green house gases. Thus, the atmosphere is indirectly heated by the earth‘s radiation.

The atmosphere in turn radiates and transmits heat to the space. Finally the amount of heat received from the sun is returned to space, thereby maintaining constant temperature at the earth‘s surface and in the atmosphere.

Heat Budget:

The earth as a whole does not accumulate or loose heat. It maintains its temperature. This can happen only if the amount of heat received in the form of insolation equals

the amount lost by the earth through terrestrial radiation. This balance is termed as a heat budget of the earth.

Let us suppose that the total heat (incoming solar radiation) received at the top of the atmosphere is 100 units. Roughly 35 units of it are reflected back into space even before reaching the surface of the earth. Out of these 35 units, 6 units are reflected back to space from the top of the atmosphere, 27 units reflected by clouds and 2 units from the snow and ice covered surfaces.

Out of the remaining 65 units (100-35), only 51 units reach the earth‘s surface and 14 units are absorbed by the various gases, dust particles and water vapour of the atmosphere.

The earth in turn radiates back 51 units in the form of terrestrial radiation. Out of these 51 units of terrestrial radiation, 34 units are absorbed by the atmosphere and the remaining 17 units directly go to space. The atmosphere also radiates 48 units (14 units of incoming radiation and 34 units of outgoing radiation absorbed by it) back to space.

Thus 65 units of solar radiation entering the atmosphere are reflected back into the space. This account of incoming and outgoing radiation always maintains the balance of heat on the surface of the earth.

Latitudinal Heat Balance.

v Although the earth as a whole, maintains balance between incoming solar radiation and outgoing terrestrial radiation. There are variations in the amount of radiation received at the earth‘s surface. Some part of the earth has surplus radiation balance while the other part has deficit.

v

In the tropical region the amount of insolation is higher than the amount of terrestrial radiation. Hence it is a region of surplus heat. In the Polar Regions the heat gain is less than the heat loss. Hence it is a region of deficit heat. Thus the insolation creates an imbalance of heat at different latitudes. This is being nullified to some extent by winds and ocean currents, which transfer heat from surplus heat regions to deficit heat regions. This is commonly known as latitudinal heat balance.

Temperature:

It is a measure of the level of kinetic energy of the atoms in a substance, whether it is a gas, liquid, or solid. When a substance receives a flow of radiant energy, such as sunlight, the kinetic energy level increases, and its temperature rises. Similarly, if a substance loses energy by radiation, its temperature falls.

The interaction of insolation with the atmosphere and the earth‘s surface creates heat which is measured in terms of temperature. While heat represents the molecular movement of particles comprising a substance, the temperature is the measurement in degrees of how hot (or cold) a

thing (or a place) is .Air temperature, is measured at a standard height of 1.2 m (4.0 ft) above the ground surface.

Importance of Temperature:

v Temperature influences the actual amount of water vapour present in the air and thus decides the moisture carrying capacity of the air.

v It decides the rate of evaporation and condensation, and therefore governs the degree of stability of the atmosphere.

v As relative humidity is directly related to the temperature of the air, it affects the nature and types of cloud formation and precipitation.

Five important factors influence air temperature:

1. Latitude.

Daily and annual cycles of insolation varysystematically with latitude, causing air temperaturesand air temperature cycles to vary as well. Yearlyinsolation decreases toward the poles, so less energyis available to heat the air. But because the seasonalcycle of insolation becomes more intense with latitude, high latitudes experience a much greaterrange in air temperatures throughout the year.

2. Surface type.

Urban air temperatures are generallyhigher than rural temperatures. City surface materials—asphalt, roofing shingles, stone, brick—hold littlewater, compared to the moist soil surfaces of ruralareas and forests, so there is little cooling throughevaporation. Urban materials are also darker andabsorb a greater portion of the Sun‘s energy thanvegetation- covered surfaces.

3. Coastal or interior location.

Locations near the oceanexperience a narrower range of air temperatures thanlocations in continental interiors. Because water heatsand cools more slowly than land, air temperaturesover water are less extreme than temperatures overland. When air flows from water to land, a coastal location will feel the influence of the adjacent water.

4. Elevation.

Temperature decreases with elevation. Athigh elevation there is less atmosphere above thesurface, and greenhouse gases provide a less effective insulating blanket. More surface heat is lost tospace. On high peaks, snow accumulates and remainslonger. The reduced greenhouse effect also results ingreater daily temperature variation.

5. Atmospheric and oceanic circulations.

Local temperatures can rise or fall rapidly when air from one region isbrought into another. Temperatures of coastal regionscan be influenced by warm or cold coastal currents.

Distribution of Temperature

The global distribution of temperature can wellbe understood by studying the temperaturedistribution in January and July,since the seasonal extremes ofhigh and low temperature are most obvious in both northern and southernhemispheres during these months . Thetemperature distribution is generally shownon the map with the help of isotherms.

The Isotherms are lines joining places having equaltemperature.In general the effect of the latitude ontemperature is well pronounced on the map,as the isotherms are generally parallel to thelatitude.

A. Horizontal Distribution of Temperature in January

In January, the sun shines vertically overhead near the Tropic of Capricorn.Hence it is summer in southern hemisphere and winter in northern hemisphere.High temperature is found over the landmasses mainly in three regions ofthe southern hemisphere. These regions are North- west Argentina, East, Central Africa, and, Central Australia. Isotherm of 30°C closes them. Innorthern hemisphere landmass arc cooler than oceans. During this time Northeast Asia experiences lowest temperatures.

As the air is warmer over oceans than over landmasses in the northernhemisphere, the Isotherms bend towards poles when they cross the oceans.In southern hemisphere, the position of the isotherms is just reverse. Theybend towards poles when they cross the landmasses and towards equator when they cross oceans.

Large expanse of water exists in southern hemisphere. Hence, isotherms areregular and widely spaced in the southern hemisphere. While they are irregularand closely spaced in northern hemisphere due to large expanse of landmasses.For these reasons no extreme seasonal contrasts between land and water arefound in middle and higher latitudes in the southern hemisphere as they existnorth of equator.

B. Horizontal Distribution of Temperature in July

During this period the sun shines vertically overhead near the Tropic of Cancer.Hence, high temperatures are found in the entire northern hemisphere.Isotherm of 30°C passes between 10° N and 40° N latitudes. The regionshaving this temperature include South Western USA, the Sahara, the Arabia,Iraq, Iran, Afghanistan, desert region of India and China. However, lowesttemperature of 0°C is also noticed in the Northern Hemisphere during summer in the central part of Greenland.

During summer in the northern hemisphere, isotherms bend equatorwardwhile crossing oceans and polewards while crossing landmasses. In southernHemisphere the position of isotherms is just opposite.Isotherms are wide spaced over oceans while they are closely spaced over landmasses.

Vertical Distribution of Temperature

The permanent snow on high mountains, even in the tropics, indicates the decrease of temperature with altitude. Observations reveal that there is afairly regular decrease in temperature with an increase in altitude. The averagerate of temperature decrease upward in the troposphere is about 60C per km,extending to the tropopause.

This vertical gradient of temperature is commonlyreferred to as the standard atmosphere or normal lapse rate, but is varies withheight, season, latitude and other factors.The normal, lapse rate is uniform at agiven level at all latitudes within thetroposphere.At the Tropopause, the lapse rate stops atzero i.e. there is no change in temperaturethere.In the lower stratosphere, the lapse rateremainsconstant for some height, whilehigher temperatures exist over the polesbecause this layer is closer to earth at thepoles.

Temperature Anomaly

The difference between the meantemperature of a place and the meantemperature of its parallel (latitude) iscalled the temperature anomaly orthermal anomaly. It expresses deviation from the normal. The temperature variation along latitude varies on account of altitude, land- water contrasts, prevailing winds and ocean currents.

The largest anomalies occur in the northern hemisphere and the smallest in the southern hemisphere. The anomaly is said to be negative when the temperature at a place is less than the expected temperature of the latitude. Conversely the anomaly is said to be positive when the temperature at a place is more than the expected temperature of the latitude.

For a year as a whole, the anomalies are negative over the continents from about 40⁰ latitude towards the poles and positive towards the equator. Over the oceans the anomalies are positive poleward from about 40^o latitude and negative towards the equator.

Temperature Inversion

Normally, temperature decreases with increasein elevation. It is called normal lapse rate. Attimes, the situation is reversed and thenormal lapse rate is inverted. It is calledInversion of

temperature.Inversion is usuallyof short duration but quite commonnonetheless.A long winter night with clearskies and still air is ideal situation forinversion.

Inversions play an important role in determining cloud forms, precipitation, and visibility. An inversion acts as a cap on the upward movement of air from the layers below. As a result, convection produced by the heating of air from below is limited to levels below the inversion. Diffusion of dust, smoke, and other air pollutants is likewise limited. In regions where a pronounced low-level inversion is present, convective clouds cannot grow high enough to produce showers and, at the same time, visibility may be greatly reduced below the inversion, even in the absence of clouds, by the accumulation of dust and smoke particles. Because air near the base of an inversion tends to be cool, fog is frequently present there.

Inversions also affect diurnal variations in air temperature. The principal heating of air during the day is produced by its contact with a land surface that has been heated by the Sun‘s radiation. Heat from the ground is communicated to the air by conduction and convection. Since an inversion will usually control the upper level to which heat is carried by convection, only a shallow layer of air will be heated if the inversion is low and large, and the rise in temperature will be great.

There are four kinds of inversions: ground, turbulence, subsidence, and frontal.

1. A ground inversion develops when air is cooled by contact with a colder surface until it becomes cooler than the overlying atmosphere; this occurs most often on clear nights, when the ground cools off rapidly by radiation. If the temperature of surface air drops below its dew point, fog may result. Topography greatly affects the magnitude of ground inversions. If the land is rolling or hilly, the cold air formed on the higher land surfaces tends to drain into the hollows, producing a larger and thicker inversion above low ground and little or none above higher elevations.

2. A turbulence inversion often forms when quiescent air overlies turbulent air. Within the turbulent layer, vertical mixing carries heat downward and cools the upper part of the layer. The unmixed air above is not cooled and eventually is warmer than the air below; an inversion then exists.

3. A subsidence inversion develops when a widespread layer of air descends. The layer is compressed and heated by the resulting increase in atmospheric pressure, and as a result the lapse rate of temperature is reduced. If the air mass sinks low enough, the air at higher altitudes becomes warmer than at lower altitudes, producing a temperature inversion. Subsidence inversions are common over the northern continents in winter and over the subtropical oceans; these regions generally have subsiding air because they are located under large high-pressure centres.

4. A frontal inversion occurs when a cold air mass undercuts a warm air mass and lifts it aloft; the front between the two air masses then has warm air above and cold air below. This kind of inversion has considerable slope, whereas other inversions are nearly horizontal. In addition, humidity may be high, and clouds may be present immediately above it.

Implications of TemperatureInversion

· Sometimes, the temperature of the air at the valley bottom reaches below freezing point, whereas the air at higher altitude remains comparatively warm. As a result, the trees along the lower slopes are bitten by frost, whereas those at higher levels are free from it.

· Due to inversion of temperature, air pollutants such as dust particles and smoke do not disperse in the valley bottoms. Because of these factors, houses and farms in intermontane valleys are usually situated along the upper slopes, avoiding the cold and foggy valley bottoms. For instance, coffee growers of Brazil and apple growers and hoteliers of mountain states of Himalayas in India avoid lower slopes.

· Fog lowers visibility affecting vegetation and human settlements.

· Less rainfall due to stable conditions.

III. Atmospheric Pressure

The weight of a column of air contained in a unit area from the mean sea level to the top of the atmosphere is called the atmospheric pressure. The atmospheric pressure is expressed in units of milibar. Atmospheric pressure is measured by an instrument called barometer.

At sea level the average atmospheric pressure is 1,013.2 milibar. Due to gravity the air at the surface is denser and hence has higher pressure. The pressure decreases with height. At any elevation it varies from place to place and its variation is the primary cause of air motion, i.e. wind which moves from high pressure areas to low pressure areas.

Distribution of Air Pressure

Distribution of atmospheric pressure on the surface of the earth is not uniform.It varies both vertically and horizontally.

Vertical Distribution

Air pressure in the atmosphere is notconstant. The largest variation is vertically, withan abrupt decrease in pressure upward fromnear the surface. Air pressuredecreases with increase in altitude but it does not always decrease at the same rate.

Temperature of the air, amount of water vapourpresent in the air and gravitational pull of the earth determine the air pressureof a given place and at a given time. Since these factors are variable withchange in height, there is a variation in the rate of decrease in air pressurewith increase in altitude. The normal rate of decrease in air pressure is 34millibars per every 300 metres increase in altitude.

The vertical pressure gradient force is much larger than that of the horizontal pressure gradient. But, it is generally balanced by a nearly equal but opposite gravitational force. Hence, we do not experience strong upward winds.Theeffects of low pressure are more clearly experienced by the people living inthe hilly areas as compared to those who live in plains.

Horizontal Distribution of Pressure

Just astemperature distribution is represented by isotherms, so atmosphericpressure distribution is represented by isobars, i.e., lines connectingplaces having the same pressure.Small differences in pressure are highlysignificant in terms of the wind direction andvelocity.

The spacing of isobars expresses the rateand direction of pressure changes and isreferred to as pressure gradient. Close spacing of isobars indicates a steepor strong pressure gradient, while widespacing suggests weak gradient. Thepressure gradient may thus be defined asthe decrease in pressure per unit distancein the direction in which the pressuredecreases most rapidly.

The distribution of atmospheric pressure across the latitudes is termed as global horizontal distribution. This distribution is characterised by presence of distinctly identifiable zones of homogeneous pressure regimes or pressure belts.On the earth‘ssurface, there are in all seven pressurebelts.The seven pressure belts are:1. equatorial low,2. the sub-tropical highs,3. the sub- polar lows and4. The polar highs.Except the equatorial low, all others formmatching pairs in the northern andsouthern hemispheres.

The factors responsible for variation in thehorizontal distribution of pressure are as follows:

Air Temperature:

Generally there is an inverse relationship between air temperature andair pressure. The higher the air temperature, the lower is the air pressure. The fundamental rule about gases is that when they are heated, they become less dense and expand in volume and rise. Hence, air pressureis low in equatorial regions and it is higher in Polar Regions. Along theequator lies a belt of low pressure known as the ―equatorial low ordoldrums‖. In polar region, cold air is very densehence it descends and pressure increases.

The Earth‟s Rotation:

The earth‘s rotation generates centrifugal force.This results in the deflection of air from its original place, causingdecrease of pressure. It is believed that the low pressure belts of the

subPolar Regions and the high pressure belts of the sub-tropical regions arecreated as a result of the earth‘s rotation. The earth‘s rotation also causesconvergence and divergence of moving air. Areas of convergenceexperience low pressure while those of divergence have high pressure

Pressure of Water Vapour:

Air with higher quantity of water vapourhas lower pressure and that with lower quantity of water vapour hashigher pressure. In winter the continents are relatively cool and tend todevelop high pressure centres; in summer they stay warmer than the oceans and tend to be dominated by low pressure, conversely, the oceansare associated with low

pressure in winter and high pressure in summer.

Pressure Belts Of The World

The horizontal distribution of air pressure across the latitudes is characterisedby high or low pressure belts. This is however, a theoretical model because pressure belts are not always found as such on the earth.

These pressure belts are:

(i) The Equatorial Low Pressure Belt

The sun shines almost vertically on the equator throughout the year. As aresult the air gets warm and rises over the equatorial region and produceequatorial low

pressure. This belt extends from equator to 100N and 100Slatitudes. Due to excessive heating horizontal movement of air is absent hereand only conventional currents are there. Therefore this belt is called doldrums(the zone of calm) due to virtual absence of surface winds. These are the regions of convergence because the winds flowing from sub-tropical highpressure belts converge here. This belt is also known as-Inter TropicalConvergence Zone (ITCZ).

(ii) The Sub-tropical High Pressure Belts

The sub-tropical high pressure belts extend from the tropics to about 35^olatitudes in both the Hemispheres. In the northern hemisphere it is called asthe North sub-tropical high pressure belt and in the southern hemisphere it isknown as the South sub-tropical high pressure belt. The existence of thesepressure belts is due to the fact that the uprising air of the equatorial regionis deflected towards poles due to the earth‘s rotation. After becoming cold and heavy, it descends in these regions and gets piled up. This results in highpressure. Calm conditions with feeble and variable winds are found here.

Inolden days vessels with cargo of horses passing through these belts founddifficulty in sailing under these calm conditions. They used to throw thehorses in the sea in order to make the vessels lighter. Henceforth these beltsor latitudes are also called ‗horse latitudes‘. These are the regions ofdivergence because winds from these areas blow towards equatorial and sub polar low pressure belts.

(iii) The Sub-polar low Pressure Belts

The sub-polar low pressure belts extend between 45^oN and the Arctic Circlein the northern hemisphere and between 45°S and the Antarctic Circle in thesouthern hemisphere. They are known as the North sub-polar low and theSouth sub-polar low pressure belts respectively. Winds coming from thesub-tropical and the polar high belts converge here to produce cyclonic stormsor low pressure conditions. This zone of convergence is also known as polarfront.

(iv) The Polar High Pressure Belts

In Polar Regions, sun never shines vertically. Sun rays are always slantinghere resulting in low temperatures. Because of low temperature, aircompresses and its density increases. Hence, high pressure is found here. Innorthern hemisphere the belt is called the North polar high pressure beltwhile it is known as the South polar high pressure belt in the southernhemisphere. Winds from these belts blow towards sub-polar low pressurebelts.

In reality, the location of these pressure belts is not permanent. They shiftnorthward in July and southward in January, following the changing positionof the sun‘s direct rays as they migrate between the Tropics of Cancer andCapricorn. The thermal equator (commonly known as the belt of highesttemperature) also shifts northwards and southwards of the equator. With theshifting of thermal equator northwards in summer and southwards in winter,there is also a slight shift in pressure belts towards north and south of theirannual average location.

Seasonal Distribution of Pressure

The variation of pressure from place to place and from season to season over the earth plays an important role in affecting the weather and climate.

January Conditions

In January, with the south-ward apparent movement of the Sun, the equatoriallow pressure belt shifts a little south of the mean equatorial position. Areas of lowest pressure occur in South America, Southern Africaand Australia. This is because the land tends to get hotter rapidly than water.Sub-tropical high pressure cells are centered over the ocean in the southernhemisphere. The belt of high pressure is interrupted by the continental land masses where the temperature is much higher. They are well developed ineastern part of the ocean where cold ocean currents dominate.

In the northern hemisphere, ridges of high pressure occur in the sub-tropical latitudes over the continent. A well-developed high pressure cell occurs inthe interior parts of Eurasia. This is

due to the fact that land cools morerapidly than oceans. Its temperatures are lower in winter than the surroundingseas. In the southern hemisphere, the sub-polar low pressure belt circles theearth as a real belt of low pressure and is not divided into cells, becausethere is virtually no landmass. In northern hemisphere two cells of lowpressure namely Iceland low and Aleutian low develop over the NorthAtlantic and the North Pacific oceans respectively.

July Conditions

In July, the equatorial low pressure belt shifts a little north of the meanequatorial position because of the northward apparent movement of the Sun.All the pressure belts shift northwards in July.

The Aleutian and Icelandic lows disappear from the oceans while thelandmasses, which developed high pressure during winter months, have extensive low pressure cells now. In Asia, a low pressure develops. The subtropical highs of the northern hemisphere are more developed over the oceans- Pacific and Atlantic. In the southern hemisphere, the sub-tropical highpressure belt is continuous. Sub-polar low forms a continuous belt in thesouthern hemisphere while in northern hemisphere; there is only a faintoceanic low.

IV. Winds

Air attemptsto balance the uneven distribution of pressure. Hence, it moves from highpressure areas to low pressure areas. Horizontal movement of air in responseto difference in pressure is termed as wind while vertical or nearly verticalmoving air is called air current. Both winds and air currents form the systemof circulation in the atmosphere.

Pressure Gradient and Winds

There is a close relationship between the pressure and the wind speed. Thegreater the difference in air pressure between the two points, the steeper isthe pressure gradient and greater is the speed of the wind. The gentler thepressure gradient slower is the speed of the wind.

The Coriolis Effect and Wind

Winds do not cross the isobars at right angles as the pressure gradient directsthem. They get deflected from their original paths. One of the most potentinfluences on wind direction is the deflection caused by the earth‘s rotationon its axis. Demonstrated by Gaspaved de Coriolis in 1844 and known as theCoriolis Effect or Coriolis force. Coriolis force tends to deflect the winds fromtheir original direction. In northern hemisphere winds are deflected towardstheir right, and in the southern hemisphere towards their left. This

is known as Ferrel‘s law. The Coriolis force is absent along the equatorbut increases progressively towards the poles.

TYPE OF WINDS

For ages man has observed that in some areas of the earth the winds blowpredominantly from one direction throughout the year; in other areas thewind direction changes with the season and in still others the winds are sovariable that no pattern is discernible. Despite these differences, the windsare generalized under three categories.

planetary winds or permanent winds periodic winds and

local winds

Planetary Winds

Planetary or permanent winds blow from high pressure belts to low pressurebelts in the same direction throughout the year. They blow over vast area ofcontinents and oceans. They are easterly and Westerlies and polar easterlies.

(i) The Easterlies: The winds that blow from sub-tropical high pressure areas towards equatorial low pressure areas called trade or easterly winds. The word trade has been derived from the German word ‗trade‘ which means track. To blow trade means ‗to blow steadily and constantly in the same direction‘. Because of the Coriolis Effect the northern trade winds move away from the subtropical high in north-east direction. In southern hemisphere the trade winds diverge out of the sub-tropical high towards the equatorial low from the southeast direction As the trade winds tend to blow mainly from the east, they are also known as the Tropical easterlies.

(ii) The Westerlies: The winds that move poleward from the sub-tropical high pressure in the northern hemisphere are detected to the right and thus blow from the south west. These in the southern hemisphere are deflected to the left and blow from the north-west. Thus, these winds are called Westerlies

(iii) Polar Easterlies Polar easterlies blow from Polar Regions towards sub-polar low pressureregions. Their direction in the northern hemisphere is from north-east to southwest and from south-east to north-west in the southern hemisphere.

Periodic Winds

The direction of these winds changes with the change of seasons. Monsoonwinds are the most important periodic winds.

Monsoon Winds

The word ‗Monsoon‘ has been derived from the Arabic word ‗Mausim‘meaning season. The winds that reverse their direction with the change ofseasons are called monsoon winds. During summer the monsoon winds blowfrom sea towards land and during winter from land

towards seas. Traditionallythese winds were explained as land and sea breezes on a large scale. But thisexplanation does not hold good now. Now a days the monsoon is generallyaccepted as seasonal modification of the general planetary wind system. TheAsiatic monsoon is the result of interaction of both planetary wind system and regional factors, both at the surface and in the upper troposphere.

India, Pakistan, Bangladesh, Myanmar(Burma), Sri Lanka, the Arabian Sea,the Bay of Bengal, South-east Asia, North Australia, China and Japan areimportant regions where monsoon winds are prevalent.

Local Winds

Till now we were discussing the major winds of the earth‘s surface, whichare vital for understanding the climatic regions. But we are all aware thatthere are winds that affect local weather. Local winds usually affect smallareas and are confined to the lower levels of the troposphere. Some of thelocal winds are given below:

i. Land and Sea Breezes

Land and sea breezes are prevalent on the narrow strips along the coasts or alake. It is a diurnal (daily) cycle, in which the differential heating of land andwater produces low and high pressures. During the day when landmass getsheated more quickly than the adjoining sea or large lake; air expands andrises. This process produces a local low pressure area on land. Sea breezethen develops, blowing from the water (high pressure) towards the land (lowpressure). The sea breeze begins to develop shortly before noon and generallyreaches its greatest intensity during mid-day to late afternoon. These coolwinds have a significant moderating influence in coastal area.

At night, the land and the air above it cools more quickly than the nearbywater body. As a result, land has high pressure while the sea has

comparativelya low pressure area. Gentle wind begins to blow from land (high pressure)towards sea (low pressure). This is known as land breeze.

ii. The Mountain and Valley Breezes

Another combination of local winds that undergoes a daily reversal consistsof the mountain and valley breezes. On a warm sunny day the mountainslopes are heated more than the valley floor.Hence, the pressure is low over the slopes while it is comparatively high inthe valleys below. As a result gentle wind begins to blow from valley towardsslopes and it assumes the name of valley breeze.After sunset, the rapid radiation takes place on the mountain slopes. Here,high pressure develops more rapidly than on the valley floor. Cold arid heavyair of mountain slopes starts moving down towards the valley floor. This is known as the mountain breeze.

iii. Hot Winds

Loo, Foehn and Chinook are important hot winds of local category.

(1) Loo: Loo are hot and dry winds, which blow very strongly over the northern plains of India and Pakistan in the months of May and June. Their direction is from west to east and they are usually experienced in the afternoons. Their temperature varies between 45°C to 50°C.

(2) Foehn: Foehn is strong, dusty, dry and warm local wind which develops on the leeward side of the Alps mountain ranges. Regional pressure gradient forces the air to ascend and cross the barrier. Ascending air sometimes causes precipitation on the windward side of the mountains. After crossing the mountain crest, the Foehn winds starts descending on the leeward side or northern slopes of the mountain as warm and dry wind. The temperature of the winds varies from 15°C to 20°C which help in melting snow. Thus making pasture land ready for animal grazing and help the grapes to ripe early.

(3) Chinook: Chinook is the name of hot and dry local wind which moves down the eastern slopes of the Rockies in U.S.A. and Canada. The literal meaning of Chinook is ‗snow eater‘ as they help in melting the snow earlier. They keep the grasslands clear of snow. Hence they are very helpful to ranchers.

(4) Harmattan: The warm and dry windsblowing from north-east and east to west in theeastern parts of Sahara desert are calledHarmattan. Similar winds are called

‗brickfielder‘ in Australia, ‗blackroller‟ in USA,‗Shamal‟ in Mesopotamia and Persian Gulf and‗Norwesters‟ in New Zealand.

(5) Sirocco: It is a warm, dry and dusty windwhich blows in northward direction fromSahara desert and after crossing MediterraneanSea reaches Italy, Spain etc. Similar winds areknown as ‗Khamsin‘ in Egypt, ‗Gibli‘ in Libya,‗Chilli‘ in Tunisia, and ‗Simoom‘ in Arabia

(iv) Cold Winds

The local cold winds originate in the snow-capped mountains during winterand move down the slopes towards the valleys. They are known by differentnames in different areas.

(1) Mistral: Mistrals are most common local cold winds. They originate on the Alps andmove over France towards the Mediterranean Sea through the Rhone valley.They are very cold, dry and high velocity winds. They bring down temperaturebelow freezing point in areas of their influence. People in these areas protecttheir orchards and gardens by growing thick hedges and build their housesfacing the Mediterranean Sea.

V. Air Masses and Fronts

Air Mass is an extremely large body of air whose properties of temperature and moisture content (humidity), at any given altitude, are fairly similar in any horizontal direction.

· Can cover hundreds of thousands of square miles.

· There can be small variations

When this air moves out of its region of origin it will carry these temperature and moisture conditions elsewhere, eventually affecting a large portion of a continent. In this process, they not only modify the weather of the area then they occupy but they are also modifying to a certain extent by the surface over which they are moving. The horizontal uniformity of an air mass is not complete because it may extend through 20 degrees or more of latitude and cover hundreds of thousands to millions of square kilo-meters. Consequently, small differences in temperature and humidity from one point to another at the same level are to be expected. Still, the differences observed within an air mass are small in comparison to the rapid rates of change experienced across air-mass boundaries.

Because it may take several days for an air mass to traverse an area, the region under its influence will probably experience generally constant weather conditions, a situation called air-

mass weather. Certainly, some day-today variations may exist, but the events will be very unlike those in an adjacent air mass.

Many significant middle-latitude disturbances originate along the boundary zones that separate different air masses.

Sources Region

The areas of the globe where air masses from are called source regions. A source region must have certain temperature and humidity properties that can remain fixed for a substantial length of time to affect air massesabove it. Air mass source regions occur only in the high or low latitudes; middle latitudes are toovariable.

· Where do air masses form?

· What factors determine the nature and degree of uniformity of an air mass?

These two basic questions are closely related, because the site where an air mass forms vitally affects theproperties that characterize it.

Areas in which air masses originate are called source regions. Because the atmosphere is heated chiefly frombelow and gains its moisture by evaporation from Earth‘s surface, the nature of the source region largelydetermines the initial characteristics of an air mass.

An ideal source region must meet two essential criteria. First, it must be an extensive and physically uniformarea. A region having highly irregular topography or one that has a surface consisting of both waterand land isnot satisfactory.

The Second criterion is that the area be characterized by a general stagnation of atmospheric circulation so thatair will stay over the region long enough to come to some measure of equilibrium with the surface. In general,it means regions dominated by stationary or slow- moving anticyclones with their extensive areas of calms orlight winds.

Regions under the influence of cyclones are not likely to produce air masses because such systems arecharacterized by converging surface winds. The winds in lows are constantly bringing air with unliketemperature and humidity properties into the area. Because the time involved is not long enough to eliminatethese differences, steep temperature gradients result, and air-mass formation cannot take place.

Modification ofan Air Mass

In its source region an air mass gains properties which are characteristic of the underlying surface. It may becold or warm and it may be dry or moist. As air moves from its source region the air is modified due tovariations in the nature of the underlying surface. Two processes, acting either independently or together, maymodify an air mass.

An air mass moving over the sea is said to have a maritime track. This air mass will typically increase itsmoisture content, particularly in its lowest layers, by evaporation of water from the sea surface (as seen in theabove diagram). On the other hand, and air mass moving over the land (with a continental track) willremainrelatively dry.

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A cold air mass flowing away from its source region over a warmer surface will be warmed from belowmakingthe air more unstable in its lowest layers. A warm air mass flowing over a colder surface is cooledfrom belowand becomes

stable in its lowest layers. In its source region an air mass gains properties which arecharacteristic of the underlying surface. It may be cold or warm and it may be dry or moist.

The air masses are classified according tothe source regions. There are five major sourceregions. These are: (i) Warm tropical andsubtropical oceans; (ii) The subtropical hotdeserts; (iii) The relatively cold high latitudeoceans; (iv) The very cold snow coveredcontinents in high latitudes; (v) Permanently ice covered continents in the Arctic andAntarctica. Accordingly, following types of air masses are recognised: (i) Maritime tropical(mT); (ii) Continental tropical (cT); (iii) Maritimepolar (mP); (iv) Continental polar (cP);(v) Continental arctic (cA); (vi) Maritime Equatorial (mE). Tropical air massesare warm and polar air masses are cold.

Fronts

When two different air masses meet, theboundary zone between them is called afront. The process of formation of thefronts is known as frontogenesis.

Thereare four types of fronts: (a) Cold; (b) Warm;(c) Stationary; (d) Occluded.

Cold Front: Cold fronts occur when a rapidly moving cold air mass runs into a slowly moving warm air mass, and the denser cold air slides under the lighter warm air. The warmer air pushes upward, along the colder air. Cold fronts can cause heavy rain or snowfall, depending on the amount of water vapor the warm air mass holds. The warm air holds more water vapor than the cold air, since warm air has more spaces between its molecules. Cold fronts can cause abrupt weather changes, like thunderstorms, because they tend to move quickly.

Warm Front: Warm fronts occur when a fast-moving warm air mass meets a slowly moving cold air mass, and the warm, less dense air moves over the cold air, as shown in the picture to the left. Light rain or snow may fall if the warm air is humid. If the warm air is dry, scattered clouds can form. After a warm front passes through an area,

the weather might be warm and humid.

Stationary Front:Stationary fronts occur when cold and warm air masses meet, but neither air mass can move the other. Where the warm and cold air meet, water vapor in the warm air condenses into rain, snow, fog, or clouds. If a stationary front stays over an area, it may bring many days of clouds and precipitation.

Occluded Front:Occluded fronts occur when a warm air mass is caught between two cooler air masses. The denser cool air mass moves underneath the less denser warm air mass, and push the warm air upwards. The two cooler air masses can mix when they meet in the middle. The warm air mass then becomes cut off from the ground, so the temperature near the ground becomes cooler. As the warm air cools and its water vapor condenses, the weather might turn cloudy and rain or snow may fall.

The fronts occur in middlelatitudes and are characterised by steepgradientin temperature and pressure. They bringabrupt changes in temperature and cause theair to rise to form clouds and cause precipitation.

VI. Tropical and Temperate ( Extra Tropical) Cyclones

The wind direction inthe cyclones is anti-clockwise in the northern hemisphere and clockwise in the southern hemisphere. Cyclones are of two types - the temperate or midlatitude cyclones and the tropical or low latitude cyclones.

Tropical Cyclones

Tropical cyclones are violent storms thatoriginate over oceans in tropical areas andmove over to the coastal areas bringing aboutlarge scale destruction caused by violentwinds, very heavy rainfall and storm surges.This is one of the most devastating naturalcalamities. They are known as Cyclones in theIndian Ocean, Hurricanes in the Atlantic,Typhoons in the Western Pacific and SouthChina Sea, and Willy-willies in the WesternAustralia.

Tropical cyclones originate and intensifyover warm tropical oceans. The conditionsfavourable for the formation and intensificationof tropical storms are:

Ø Large sea surface with temperature higher than 27° C;

Ø Presence of the Coriolis force;

Ø Small variations in the vertical wind speed;

Ø A pre-existing weak low-pressure area or low-level-cyclonic circulation;

Ø Upper divergence above the sea level system.

The energy that intensifies the storm comesfrom the condensation process in the toweringcumulonimbus clouds, surrounding thecentre of the storm. With continuous supplyof moisture from the sea, the storm is furtherstrengthened. On reaching the land themoisture supply is cut off and the stormdissipates. The place where a tropical cyclonecrosses the coast is called the landfall of thecyclone. The cyclones, which cross 20^oNlatitude generally, recurve and they are moredestructive.

A mature tropical cyclone is characterisedby the strong spirally circulating wind aroundthe centre, called the eye. The diameter of thecirculating system can vary between 150 and 250 km.

The eye is a region of calm with subsidingair. Around the eye is the eye wall, where thereis a strong spiraling ascent of air to greaterheight reaching the tropopause. The windreaches maximum velocity in this region,reaching as high as 250 km per hour.Torrential rain occurs here. From the eye wallrain bands may radiate and trains of cumulusand cumulonimbus clouds may drift into theouter region. The diameter of the storm overthe Bay of Bengal, Arabian Sea and IndianOcean is between 600 - 1200 km. The systemmoves slowly about 300 - 500 km per day.The cyclone creates storm surges and theyinundate the coastal low lands.

Temperate or Extra Tropical Cyclones

The systems developing in the mid and highlatitude, beyond the tropics are called themiddle latitude or extra tropical cyclones. Thepassage of front causes abrupt changes in theweather conditions over the area in the middleand high latitudes.

Extra tropical cyclones form along the polarfront. Initially, the front is stationary. In thenorthern hemisphere, warm air blows from thesouth and cold air from the north of the front.When the pressure drops along the front, thewarm air moves northwards and the cold airmove towards, south setting in motion ananticlockwise cyclonic circulation. The cycloniccirculation leads to a well-developed extratropical cyclone, with a warm front and a cold front.

The plan and cross section of a well-developed cyclone is given in Figure.

There are pockets of warm air or warm sectorwedged between the forward and the rear coldair or cold sector. The warm air glides over thecold air and a sequence of clouds appear overthe sky ahead of the warm front and causeprecipitation. The cold front approaches thewarm air from behind and pushes the warmair up. As a result, cumulus clouds developalong the cold front. The cold front moves fasterthan the warm front ultimately overtaking thewarm front. The warm air is completely liftedup and the front is occluded and the cyclonedissipates.

The processes of wind circulation both atthe surface and aloft are closely interlinked.The extra tropical cyclone differs from thetropical cyclone in number of ways. The extratropical cyclones

have a clear frontal systemwhich is not present in the tropical cyclones.They cover a larger area and can originate overthe land and sea. Whereas the tropical cyclonesoriginate only over the seas and on reachingthe land they dissipate. The extra tropicalcyclone affects a much larger area ascompared to the tropical cyclone. The windvelocity in a tropical cyclone is much higherand it is more destructive. The extra tropicalcyclones move from west to east but tropicalcyclones, move from east to west.

Tropical Cyclones and Temperate Cyclones ― Comparison

Tropical Cyclone

Temperate Cyclone

Origin

· Thermal Origin.

· Dynamic Origin: Coriolis Force, Movement of air masses.

Latitude

· Confined to 10-30º N and S of equator.

· Confined to 35-65º N and S of equator.

More pronounced in Northern hemispheredue to greater

temperature contrast.

Frontal system

· Absent.

· The very cyclone formation is due to frontogenesis. (Occluded

Front).

Formation

· They form only on seas with temperaturemore than 26-270 C. They dissipate on reaching the

land.

· Can form both on land as well as seas.

Season

· Seasonal: Late summers (Aug- Nov).

· Irregular. But few in summers and more inwinters.

Size

· Limited to small area. Typical size: 100 – 500 kms in diameter.

Varies with the strength of the cyclone.

· They cover a larger area. Typical size: 300 – 2000 kms in diameter.Varies from region to region.

Shape

· Elliptical

· Inverted ‗V‘

Rainfall

· Heavy but does not last beyond a few hours.

· If the cyclone stays at a place, the rainfallmay continue for a few days.

· In a temperate cyclone, rainfall is slow andcontinues for many days, sometimes evenweeks.

Wind Velocity and destruction

· Much greater. 100 – 250 kmph

200–1200 kmph in upper troposphere)

· Greater destruction due to winds, stormsurges and torrential rains.

· Comparatively low.

Typical range: 30-150 kmph.

· Less destruction due to winds but moredestruction due to flooding.

Isobars

· Complete circles and the pressure gradient is steep

· Isobars are usually ‗V‘ shaped and thepressure gradient is low.

Lifetime

· Doesn‘t last for more than a week

· Lasts for 2-3 weeks.

Path

· East – West. Turn North at 200 latitude andwest at 300 latitude.

· Move away from equator.

· The movement of Cyclones in Arabian Seaand Bay of Bengal is a little different.

· Here, these storms are superimposed uponthe monsoon circulation of the summermonths, and they move in northerly direction along with the monsoon

currents.

· West – East (Westerlies; Jet Streams).

Move away from equator.

Temperature distribution

· The temperature at the centre is almostequally distributed.

· All the sectors of the cyclone have different temperatures

Calm region

· The centre of a tropical cyclone is known asthe eye. The wind is calm

at the centre withno rainfall.

· In a temperate cyclone, there is not a single place where winds

and rains are inactive.

Driving force

· The tropical cyclone derives its energy fromthe latent heat of condensation, and the difference in densities of the air masses doesnot contribute to the energy of the

cyclone.

· The energy of a temperate cyclone depends on the temperature, humidity anddensity differences of air masses.

Influence of Jet streams

· The relationship between tropical cyclonesand the upper level air- flow is not very clear.

· The temperate cyclones, in contrast, have adistinct relationship with upper level airflow (jet streams, Rossby

waves etc.)

Clouds

· The tropical cyclones exhibit fewer varieties of clouds –

cumulonimbus, nimbostratus,etc.

· The temperate cyclones show a variety of cloud development at

various elevations.

Surface anti cyclones

· The tropical cyclones are not associated withsurface anticyclones and they have a greaterdestructive capacity.

· The temperate cyclones are associatedwith anticyclones which precede and succeed a cyclone.

· These cyclones are not very destructive.

Influence on India

· Both coasts affected. But east coast is thehot spot.

· Bring rains to North-West India.

The associated instability is called ‗Western Disturbances‘.

Weather Prediction

· Tough as the movement can be erratic dueto a lot of factors.

· Easy because of the general westerly pathof the cyclone, less variable jet stream pathand

simple frontal system.

Thunderstorms and Tornadoes

Other severe local storms are thunderstormsand tornadoes. They are of short duration,occurring over a small area but are violent.Thunderstorms are caused by intenseconvection on moist hot days. A thunderstormis a well-grown cumulonimbus cloudproducing thunder and lightening. When theclouds extend to heights where sub- zerotemperature prevails, hails are formed and theycome down as hailstorm. If there is insufficientmoisture, a thunderstorm can generate duststorms. A thunderstorm is characterised byintense updraft of rising warm air, whichcauses the clouds to grow bigger and rise togreater height. This causes precipitation. Later,downdraft brings down to earth the cool airand the rain. From severe thunderstormssometimes spiraling wind descends like atrunk of an elephant with great force, with verylow pressure at the centre, causing massivedestruction on its way. Such a phenomenon iscalled a tornado. Tornadoes generally occurin middle latitudes. The tornado over the seais called water spouts.

These violent storms are the manifestationof the atmosphere‘s adjustments to varyingenergy distribution. The potential and heatenergies are converted into kinetic energy inthese storms and the restless atmosphere againreturns to its stable state.

VII. Humidity and Precipitation Water Vapour in the Atmosphere

Water vapour is a highly variable component of the atmosphere. Its proportion varies from zero to four percent by volume of the atmosphere. Water can exist in the air in all the three states of matter i.e. solid (ice-crystals), liquid (droplets of water) and gaseous (water vapour). Most commonly water exists in air as tasteless, colourless, transparent gas known as water vapour. The presence of water in the atmosphere has made life possible on the earth.

Humidity

The invisible water vapour presentin gaseous form in the atmosphere at any time and place is termed as humidity.In other words, we can say that the term humidity refers to the amount ofwater vapour present in a given air. It indicates the degree of dampness orwetness of the air. Humidity of the air is mainly expressed in the followingtwo ways:

(i) Absolute humidity(ii) Relative humidity

(i) Absolute HumidityAbsolute humidity is the ratio of the mass of water vapour actually inthe air to a unit mass of air, including the water vapour. It is expressedin gram per cubic metre of air. For example, if the absolute humidity ofair is 10 grams it means that one cubic metre of that air holds 10 gramsof moisture in the form of water vapour. Absolute humidity is variableand changes from place to place and with change in time.

The ability of an air to hold water vapour depends entirely on itstemperature. The capacity of holding water vapour of an air increaseswith the increase in its temperature. For example, at 10°C, one cubicmetre of an air can hold 11.4 grams of water vapour. If the temperatureof the same air increases to 21°C, the same volume of air can hold 22.2grams of water vapour. Change in temperature and pressure conditions of an airresults in the change of its volume and consequently there is change inits absolute humidity. Hence, there is a need of some more reliablemeasure of humidity.

(ii) Relative Humidity

Relative humidity is the most important and reliable measure of atmosphericmoisture. It is the ratio of the amount of water vapor actually in a volumeoccupied by air to the amount the space could contain at saturation.

Relative Humidity =

Air can hold a definite maximumquantity of water vapour at a given temperature. When this situation isattained, we say the air is fully saturated. The temperature at which agiven sample of air becomes fully saturated is called the dew point orsaturation point The relative humidity of an air at saturation point ishundred percent.If the relative humidity of air is less than 100percent, the air is said to be unsaturated.

Evaporation

Evaporation is the process of which water changes from its liquid state to gaseous form. This process takes place at all places, at all times and at all temperatures except at dew point or when the air is saturated. The rate of evaporation is affectedby several factors. Important among them are as under:

(i) Accessibility of water bodies: The rate of evaporation is higher over the oceans than on the continents.

(ii) Temperature: We know that hot air holds more moisture than cold air. So, when the temperature of an air is high, it is capable of holding more moisture in its body than at a low temperature. It is because of this that the rate of evaporation is more in summers than in winters. That is why wet clothes dry faster in summers than in winters.

(iii) Air moisture: If the relative humidity of a sample of air is high, it is capable of holding less moisture. On the other hand if the relative humidity is less, it can take more moisture. Hence, the rate of evaporation will be high. Aridity or dryness of the air also increases the rate of evaporation. During rainy days, wet clothes take more time to dry owing to the high percentage of moisture content in the air, than on dry days.

(iv) Wind: Wind also affects the rate of evaporation. If there is no wind, the air which overlies a water surface will get saturated through evaporation. This evaporation will cease once saturation point is reached. However, if there is wind, it will blow that saturated or nearly saturated air away from the evaporating surface and replace it with air of lower humidity. This allows evaporation to continue as long as the wind keep blowing saturated air away and bring drier air.

(v) Cloud cover: The cloud cover prevents solar radiation and thus influences the air temperatures at a place. This way, it indirectly controls the process of evaporation.

The heat energy usedfor changing the state of water or a body from liquid to gaseous state or fromsolid (ice) to liquid (water) state without changing its temperature is calledlatent heat. It is a sort of hidden heat. The effect of which is not seen on thethermometer. The latent heat consumed in changing water into gaseous formis released when water vapour changes into water or ice. The release of latentheat in the air is an important source of energy for causing changes in weather.

A special case of evaporation is transpiration, which entails a loss of waterfrom leaf and stem tissues of growing vegetation. The combined losses ofmoisture by evaporation and transpiration from a given areas are termedevapo-transpiration.

Condensation

Condensation is the process by which atmospheric water vapour changes into water or ice crystals. It is just reverse of the process of evaporation. Whenthe temperature of saturated air falls below dew point, the air cannot hold theamount of humidity which it was holding earlier at a higher temperature. This extraamount of humidity changes into water droplets or crystals of ice depending uponthe temperature at which condensation takes place.

(a) Process of condensation

The temperature of the air falls in two ways. Firstly, cooling occursaround very small particles of freely floating air when it comes incontact with some colder object. Secondly, loss in air temperature takesplace on a massive scale due to rising of air to higher altitudes. Thecondensation takes place around the smoke, salt and dust particles whichattract water vapour to condense around them. They are calledhygroscopic nuclei. When the relative humidity of an air is high, aslight cooling is required to bring the temperature down below dewpoint. But when the relative humidity is low and the temperature ofthe air is high, a lot of cooling of the air will be necessary to bring thetemperature down below dew point. Thus, condensation is directlyrelated to the relative humidity and the rate of cooling.

(b) Forms of condensation

Condensation takes place in two situations, firstly, when dew point is below freezing point or below 0° C and secondly, when it is above freezing point. In this way, the forms of condensation may be classified into two groups:

(i) Frost, snow and some clouds are formed when dew point is below freezing point.

(ii) Dew, mist, fog, smog and some clouds are formed when dew point is above freezing point.

The forms of condensation may also be classified on the basis of place where it is occurring, for example, on the ground or natural objects such as grass blades and leaves of the plants or trees, in the air close to the earth‘s surface or at some height in the troposphere.

(i) Dew: When the atmospheric moisture is condensed and deposited in the form of water droplets on cooler surface of solid objects such as grassblades, leaves of plants and trees and stones, it is termed as dew.Condensation in dew form occurs when there is clear sky, little or nowind, high relative humidity and cold long nights. These conditionslead to greater terrestrial radiation and the solid objects become coldenough to bring the temperature of air down below dew point. In thisprocess the extra moisture of the air gets deposited on these objects.Dew is formed when dew point is above freezing point. Dew formationcan be seen if the water is poured into a glass from the bottle kept in arefrigerator. The outer cold surface of the glass brings the temperatureof the air in contact with the surface down below dew point and extramoisture gets deposited on the outer wall of the glass.

(ii) Frost: When the dew point is below freezing point, under above mentioned conditions, the condensation of extra moisture takes place in the form of very minute particles of ice crystals. It is called frost. In this process, the air moisture condenses directly in the form of tiny crystal of ice. This form of condensation is disastrous for standing crops such as potato, peas, pulses, grams, etc. It also creates problems for road transport system.

(iii) Mist and Fog: When condensation takes place in the air near the earth‘s surface in the form of tiny droplets of water hanging and floating in the air, it is called mist. In mist the visibility is more than one kilometer and less than two kilometers. But when the visibility is

reduced to less than one kilometer, it is called fog. Ideal conditions for the formation of mist and fog are clear sky, calm and cold winter nights.

(iv) Smog: Smog is a fog that has been polluted and discoloured by smoke, dust, carbon monoxide, sulphur dioxide and other fumes. Smog frequently occurs in large cities and industrial centres. It causes respiratory illness.

(v) Cloud: Clouds are visible aggregates of water droplets, ice particles, or a mixture of both along with varying amounts of dust particles. A typical cloud contains billions of droplets having diameters on the order 060.01 to 0.02 mm; yet liquid or solid water accounts for less than 10 parts per million of the cloud volume. Clouds are generally classified on the basis of their general form or appearance and altitude. Combining both these characteristics, clouds may be grouped as under.

Low clouds: The base level of low clouds varies from very near theground to about 2000m. The basic type of this family is the Stratus, alow, uniform layer resembling fog but not resting on the ground.

Stratocumulus clouds form a low, gray layer composed of globular massesor rolls which are usually arranged in groups, lines, or waves.

Clouds with vertical development fall into two principalCategories: cumulusand cumulonimbus.

Cumulus clouds are dense, dome-shaped and have flat bases. They may grow to become cumulonimbus, the extent of verticaldevelopment depending upon the force of vertical currents below the cloudsas well as upon the amount of latent heat of condensation liberated in the clouds asthey form.To an observer directly beneath, a cumulonimbus cloud may cover the whole sky and have the appearance of Nimbostratus, The word nimbus (or prefix nimbo)applies to a cloud from which rain is falling. It derives from the Latin for ―violentrain‖.

Medium clouds: These clouds are formed at altitudes between 2000 to 6000metres. This group of clouds includealtocumulus and altostratus.

High clouds: These clouds are formed above the altitude of 6000 metresand include cirrus, cirrostratus and cirrocumulus.

Precipitation

Precipitation is defined as water in liquid or solid forms falling to the earth.It happens when continuous condensation in the body of air helps the waterdroplets or ice crystals to grow in size

and weight that the air cannot holdthem and as a result these starts falling on the ground under the force ofgravity.

Forms of Precipitation

The precipitation falls on the earth in various forms of droplets of water, ice flakes and solid ice balls or hail and at times droplets of water and hail together. The form that precipitation takes is largely dependent upon the method of formation and temperature during the formation. The forms of precipitation are as follows:

ü Drizzle and Rainfall: Drizzle is a fairly uniform precipitation composed exclusively of fine drops of water with diameter less than 0.5 mm. Only when droplets of this size are widely spaced are called rain.

ü Snowfall: When condensation takes place below freezing point (-0° C), the water vapour changes into tiny ice crystals. These tiny ice crystals grow in size and form ice flakes which become big and heavy and start falling on the ground. This form of precipitation is called snowfall. Snowfall is very common in Western Himalaya and mid and high latitude regions in winter.

ü Sleet: Sleet is frozen rain, formed when rain before falling on the earth, passes through a cold layer of air and freezes. The result is the creation of solid particles of clear ice. It‘s usually a combination of small ice balls and rime.

ü Hail: Hail is precipitation of small balls or pieces of ice (hail stones) with diameters ranging from 5 to 50mm, falling either separately or agglomerated into irregular lumps. Hailstones are comprised of a series of alternating layers of transparent and translucent ice.

Types of Rainfall

When a mass of moist air ascends to high altitudes it cools downto lower temperatures. In doing so it attains dew point which leads tocondensation and precipitation. Thus the cooling of air occurs mainly whenit rises. There are three important ways in which a mass of air can be forcedto rise and each of these ways produces its own characteristic precipitationor rainfall.

(a) Convectional Rainfall:

Excessive heating of the earth‘s surface in tropical region results in the vertical air currents. These currents lift the warm moist air to higher strata of atmosphere. When-the temperature of such a humid air starts falling below dew point continuously, clouds are formed. These clouds cause heavy rainfall which is associated with lightning and thunder. This type of rainfall is called conventional rainfall. It is very common in equatorial region where it is a daily phenomenon in the afternoon.

(b) Orographic or Relief Rainfall

Orographic rainfall on formed where air rises and cools because of atopographic barrier. When their temperature falls below dew point,clouds are formed. These clouds cause widespread rain on the windwardslopes of the mountain range. This type of rain is called orographicrainfall. However when these winds cross over the mountain rangeand descend along the leeward slopes, they get warm and cause littlerain. Region lying on the leeward side of the mountain receiving littlerain is called rain shadow area. A famous example oforographic rainfall is Cherrapunji on the southern margin of the KhasiHills in Meghalaya India.

(c) Convergence or Cyclonic Rainfall

Convergence rainfall, produced where air currents converge and rise. Intropical regions where opposing air currents have comparable temperatures, the lifting is more or less vertical and is usually accompanied by convention. Convectional activity frequently occurs along fronts where the temperature of the air

masses concerned are quite different. Mixing of air alongthe front also probably contributes to condensation and therefore to thefrontal rainfall. When two large air masses of different densities and temperature meet, the warmer moist air mass is lifted above the colder one.When this happens, the rising warm air mass condenses to form cloudswhich cause extensive down pour. This rainfall is associated with thunderand lightning. ‗This type of rainfall is also called frontal rainfall. This type ofrainfall is associated with both warm and cold fronts; it is generally steady and may persist for a whole day or even longer.

In all these types, the cooling of large masses of humid air is essential toproduce

rainfall. In conventional rainfall, after rising of air, the subsequentprocesses are similar, to those of relief rainfall. In nature, these three methods work together and infact most of the earth‘s precipitation or rainfall is the result of two or more causes of lifting ofair rather than of anyone.

Distribution of Precipitation

The spatial distribution of precipitation is not uniform all over the world. The averageannual precipitation for the world as a whole is about 97.5 centimeters but theland receives lesser amount or rainfall than the oceans. The annual precipitationshows marked difference on the land. Different places of the earth‘s surface receivedifferent amount of annual precipitation and that too in different seasons.

The main features of the distribution of precipitation can be explained with the help of global pressure and wind belts, distribution of land and water bodiesand the nature of relief features. Before arriving at any conclusion regarding thecauses for regional and seasonal variation, let us first see regional and seasonaldistribution patterns of precipitation.

(a) Regional Variations

On the basis of average amount of annual precipitation,we can recognize thefollowing precipitation regions in the world.

(i) Regions of Heavy Precipitation: The regions which receive over 200centimeters of annual precipitation are included in this category. Theseregions include equatorial coastal areas of tropical zone and west-coastalregions of temperate zone.

(ii) Regions of Moderate Precipitation: The regions which receive 100to 200 centimeters of annual precipitation are included in this category.These regions lie adjacent to the regions of heavy precipitation. Easterncoastal regions of subtropical zone and coastal regions of the warmtemperate zone are included in this category.

(iii) Regions of Less Precipitation: This category includes regions whichreceive precipitation between 50 to 100 centimeters. These regions liein the interior parts of tropical zone and eastern interior parts oftemperate zone.

(iv)

Regions of Scanty Precipitation: The areas lying in the rain shadows(leeward) side of the mountain ranges, the interior parts of continents,the western margins of continents along tropics and high latitudesreceive precipitation less than 50 centimeters. These regions includetropical, temperate and cold deserts of the world.

ü Precipitation is greatest in the equatorial region and decreases towards the poles.

ü Precipitation is heaviest in the coastal regions and decreases towards the interior of the continents.

ü Eastern coastal areas of tropical lands and western coastal areas of temperate lands receive heavy precipitation including equatorial regions.

ü Precipitation is very heavy on the windward side of highlands; very dry condition prevails on the leeward side.

ü Coastal areas adjacent to cold currents are drier than coastal areas near warm currents.

ü The western margin of tropical land and polar region receive scanty rainfall. The main reason being that easterlies become dry winds and polar winds are cold and dry.

(b) Seasonal Variations

The regional variations in the distribution of precipitation in different parts of the world are based on average annual precipitation which do not give us any correct picture of the nature of precipitation specially of those regions where seasonal fluctuations in the amount of precipitation are very common, for example arid, semi-arid or sub-humid regions. Therefore, it is important to study seasonal variations of precipitation in the world. The facts related tothis are as follows:

(i) The equatorial regions and the western parts of temperate lands receive precipitation throughout the year. The former receive conventional type of rain while the later gets cyclonic cum orographic type through Westerlies.

(ii) About 2 per cent land areas of the world receive precipitation only in winter. These include Mediterranean regions of the world and Coromandel Coast of India. Due to the seasonal shift in pressure and planetary wind systems, these regions (Mediterranean) do not get precipitation in summer as they come under sub-tropical high pressure belts and trade winds which become dry while reaching to the western margins of continents.

(iii) The remaining parts of the world receive precipitation only in summer. It makes us clear that most parts of the world experience marked seasonal variation in precipitation. Seasonal distribution of precipitationprovides us idea to judge its effectiveness. For example, the scantyprecipitation during short growing season in high latitudes is moreeffective than that of heavy precipitation in lower latitudes. Likewise,precipitation in the form of dew, fog and mist in some parts like CentralIndia and Kalahari Desert has an appreciable effect on standing cropsand natural vegetation.

Factors Affecting Rainfall Distribution

ü Moisture supply to the atmosphere is the main factor in determining the amount of rainfall in any region. Equatorial and rest of the tropical region have highest evaporation and hence highest supply of moisture. Coastal areas have more moisture than interior parts of continents. Frigid regions have very low evaporation hence very scanty precipitation.

ü Wind direction in the belts of trades and Westerlies winds is very important. Winds blowing from sea to land cause rainfall. Land bearing winds are dry. Winds blowing from higher to lower latitudes will get heated and give no rain while those blowing from lower to higher latitudes will get cooled and cause rainfall. Sub-tropical deserts have very little rainfall because they have off-shore winds.

ü Ocean currents: Warm current are associated with warm moist winds which cause rainfal1, cold current have cold dry wind and hence no rainfall.

ü Presence of mountain across the direction of wind causes more rainfall on the windward side and creates rain shadow on the leeward side.

ü Pressure belts are closely related with wind direction and rainfall. Areas of low pressure attract rain bearing winds while areas of high pressure do not.

VIII. WEATHER AND CLIMATE

Weather

Temperature, pressure, wind, humidity and precipitation, interact witheach other. They influence the atmospheric conditions like the direction and velocity of wind, amount of insolation, cloud-cover and the amount of precipitation. These are known as the elements of both weather and climate. The influence of these elements differs from place to placeand time to time. It may be restricted to a small area and for a shortduration of time. We very often describe this influence in the name ofweather as sunny, hot, warm, cold, fine, etc. depending upon thedominant element of weather at a place and at a point of time. Therefore,weather is the atmospheric condition of a place for a short durationwith respect to its one or more elements. Two places even a shortdistance apart may have different kind of weather at one and the sametime.

Climate

The average weather condition, prevalent from one season to another in the course of a year, over a large area is known as climate. Theaverage of these weather conditions is calculated from the data collectedfor several years (about 35 years) for a larger area. Rajasthan, forexample, experiences hot and arid climate, Kerala has tropical rainyclimate, Greenland has cold desert climate and the climate of CentralAsia is temperate continental. Climate of a region is considered moreor less permanent.

Factors Affecting Climate

Different regions of the world have differences in temperature, humidityand precipitation. To understand differentclimatic conditions, let us discuss the factors which cause the variations inthe climate of a place or a region.

1. Latitude or Distance from the Equator: The places near the equator are warmer than the places which are far awayfrom it. This is because the rays of the sun fall vertical on the equator andslanting in the temperate and polar regions. As we have discussed earlierthe vertical rays are concentrated over a small area than the slanting one.Again, the vertical rays pass through a shorter distance in the atmospherebefore reaching the earth‘s surface. Therefore, lower the latitude higher isthe temperature and vice versa. Malaysia which is near the equator is warmer than England which is far away from the equator.

2. Altitude or the Height from the mean sea level: We know that mountains are cooler than the plains. Shimla situated on ahigher altitude is cooler than Jalandhar, although both are almost on thesame latitude. The temperature decreases with the height of a place. For avertical rise of 165 metres there is an average decrease in temperature atthe rate of 1°C. Thus the temperature decreases with increase in height.

3. Continentally or the Distance from the Sea: The water is a bad conductor of heat i.e. it takes longer time to heat andlonger time to cool. Due to this moderating effect of the sea, places near thecoast have low range of temperature and high humidity. The places in theinterior of the continent do not experience moderating effect of the sea.These places

have extreme temperatures. The places far from the sea havehigher range of diurnal (daily) and annual temperatures. Mumbai has relativelylower temperature and higher rainfall than Nagpur, although both are almostsituated on the same latitude.

4. Nature of the Prevailing Winds: The on-shore winds bring the moisture from the sea and cause rainfall onthe area through which they pass. The off-shore winds coming from theland are dry and help in evaporation. In India, the on-shore summer monsoonwinds bring rains while off-shore winter monsoon winds are generally dry.

5. Cloud Cover: In areas generally of cloudless sky as in deserts, temperature even undershade is very high because of the hot day time sunshine. At night thisheat radiates back from the ground very rapidly. It results in a largediurnal range in temperature. On the other hand under cloudy sky and heavy rainfall at Thiruvananthapuram the range of temperature is verysmall.

6. Ocean Currents: Ocean waters move from one place to another partly as an attempt toequalize temperature and density of water. Ocean currents are largemovements of water usually from a place of warm temperature to one ofcooler temperature or vice-versa. The warm ocean currents raise thetemperature of the coast and sometimes bring rainfall, while the cold currentslower the temperature and create fog near the coast. Port Bergen in Norwayis free from ice even in winter due to warm North Atlantic Drift while PortQuebec in Canada remains frozen during winter months due to chilling effectof the Cold Labrador Current in spite of the fact that Port Quebec is situatedin much lower latitude than Port Bergen. The on-shore winds passing overa warm current carry warm air to the interior and raise the temperature ofthe inland areas. Similarly, the winds blowing over cold current carry coldair to the interior and create fog and mist.

7. Direction of Mountain Chains: The mountain chains act as natural barrier for the wind. The on-shore moistureladen winds are forced to rise after striking against the mountain; and giveheavy rainfall on the windward side. These winds descending on the leewardside cause very low rainfall. The great Himalayas check the moisture ladenmonsoon winds from crossing over to Tibet. This mountain chain also checksbiting polar cold winds from entering into India. This is the reason for whichnorthern plains of India get rains while Tibet remains a perpetual rain shadowarea with lesser amount of rainfall.

8. Slope and the Aspect: The concentration of heat being more on the gentler slope raises thetemperature of air above them. Its lesser concentration along steeperslopes lowers the temperature. At the same time, mountain slopes facingthe sun are warmer than the slopes which are away from the sun‘srays. The southern slopes of Himalaya are warmer than the northernslopes.

9. The Nature of the Soil and Vegetation Cover: The nature of soil depends upon its texture, structure and composition.These, qualities vary from soil to soil. Stony or sandy soils are goodconductor of heat while black clay soils absorb the heat of the sun‘srays quickly. The bare surface reradiates the heat easily. The desertsare hot in the day and cold in the night. The forest areas have lowerrange of temperature throughout the year in contrast to non-forestedareas.

Climatic Classification

The varied effect of the major weather elements in different parts of the world and also the varied nature of the earth‘s surface give every location a distinct climate. Hence, the number of different climate is large. In order to easily understand and comprehend this large variety, the

climate of the world have been classified into a few major groups, each having certain common important characteristics. Although several attempts have been made by scholars to classify the climate of the world for the proper understanding of major climate types no single classification is perfect, as climate stands for the generalized and composite weather conditions. However, the Greeks, perhaps, made the first attempt to classify the world climates on the basis of the distribution of temperature and insolation. They divided the world into five latitudinal thermal zones, the boundary of these zones are fixed on the basis of the angle at which the sun‘s rays strike the earth. The following are the five thermal zones:

Thermal Zone Classification

(i) Torrid Zone: It is the largest of the thermal zones. It covers almost half the area of the earth‘s surface. It is situated

between the Tropic of Cancer (23½°N) and Tropic of Capricorn (23½^oSouth). The sun‘s rays are almost vertical throughout the year in this zone. The mid-day sun isoverhead at equator on equinoxes,

i.e. on 21st March and 23rd September.It is also overhead at Tropic of Cancer on 21st June and at Tropic ofCapricorn on 22nd December. The duration‘s of day and night are alwaysequal i.e. 12 hours each on the equator and they increase to 13 hours 27minutes at tropics. The range of temperature is

lowest at the equator and itincreases towards the tropics.

(ii) Temperate Zone: The temperate zones are on either side of the TorridZone. The North Temperate Zone lies between Tropic of Cancer (23½^oNorth) and Arctic Circle (66½^o North) The South Temperate Zone liesbetween Tropic of Capricorn (23½° South) and Antarctic Circle (66½° South). The sun is never overhead in this zone in winterseason, the nights are longer and days are shorter and vice versa insummer. The difference between the duration of the day and nightincreases towards the poles. The maximum duration of day in summerand that of night in winter in the polar circles is 24 hours. When it is summer in the northern hemisphere it is winter in the southernhemisphere and vice versa,

(iii) Frigid Zones: Like the temperate zone, Frigid Zone is also found inboth the hemispheres. The North Frigid Zone lies between Arctic Circle(66½^oN) and North Pole (90° North). The South Frigid Zone liesbetween Antarctic Circle (66½^o South) and South Pole (90^oSouth).During winter season, the sun does not rise above the horizon for almostsix months. These are the coldest regions of the world. The surfaceremains permanently frozen under thick snow.

Koeppen‟s Scheme of Classification of Climate

The most widely used classification of climateis the empirical climate classification schemedeveloped by V. Koeppen. Koeppen identifieda close relationship between the distributionof vegetation and climate. He selected certainvalues of temperature and precipitation

andrelated them to the distribution of vegetationand used these values for classifying theclimates. It is an empirical classification basedon mean annual and mean monthlytemperature and precipitation data. Heintroduced the use of capital and small lettersto designate climatic groups and types.Although developed in 1918 and modified over a period of time, Koeppen‘s scheme is stillpopular and in use.

Koeppen recognised five major climaticgroups; four of them are based on temperatureand one on precipitation. Table lists the climatic groups and their characteristicsaccording to Koeppen. The capital letters: A, C,D and E delineate humid climates and B dryclimates.

The climatic groups are subdivided intotypes, designated by small letters, based onseasonality of precipitation and temperaturecharacteristics. The seasons of dryness areindicated by the small letters: f, m, w and s,where f corresponds to no dry season,m - monsoon climate, w- winter dry season ands - summer dry season. The small letters a, b,c and d refer to the degree of severity oftemperature. The B- Dry Climates aresubdivided using the capital letters S for steppeor semi- arid and W for deserts.

Tropical Humid ClimatesA

Tropical humid climates exist between Tropicof Cancer and Tropic of Capricorn. The sunbeing overhead throughout the year and thepresence of Inter Tropical Convergence Zone(ITCZ) make the climate hot and humid.Annual range of temperature is very low andannual rainfall is high. The tropical group isdivided into three types, namely (i) Af- Tropicalwet climate;

(ii) Am - Tropical monsoon climate;(iii) Aw- Tropical wet and dry climate.

Tropical Wet Climate (Af)

Tropical wet climate is found near the equator.The major areas are the Amazon Basin in SouthAmerica, western equatorial Africa and theislands of East Indies. Significant amount ofrainfall occurs in every month of the year asthunder showers in the afternoon. Thetemperature is uniformly high and the annualrange of temperature is negligible. Themaximum temperature on any day is around30°C while the minimum temperature isaround 20°C. Tropical evergreen forests withdense canopy cover and large biodiversity arefound in this climate.

Tropical Monsoon Climate (Am)

Tropical monsoon climate (Am) is found overthe Indian sub-continent, North Eastern partof South America and Northern Australia.Heavy rainfall occurs mostly in summer. Winteris dry.

Tropical Wet and Dry Climate (Aw)

Tropical wet and dry climate occurs north andsouth of Af type climate regions. It borders withdry climate on the western part of the continentand Cf or Cw on the eastern part. ExtensiveAw climate is found to the north and south ofthe Amazon forest in Brazil and adjoining partsof Bolivia and Paraguay in South America,Sudan and south of Central Africa. The annualrainfall in this climate is considerably less thanthat in Af and Am climate types and is variablealso. The wet season is shorter and the dryseason is longer with the drought being moresevere. Temperature is high throughout theyear and diurnal ranges of temperature are thegreatest in the dry season. Deciduous forest andtree-shredded grasslands occur in this climate.

Dry Climates: B

Dry climates are characterised by very low rainfall that is not adequate for the growth of plants. These climates cover a very large area of the planet extending over large latitudes from 15° - 60° north and south of the equator. At low latitudes, from 15° - 30°, they occur in the area of subtropical high where subsidence and inversion of temperature do not produce rainfall. On the western margin of the continents, adjoining the cold current, particularly over the west coast of South America, they extend more equatorwards and occur on the coast land. In middle latitudes, from 35° - 60° north and south of equator, they are confined to the interior of continents where maritime-humid winds do not reach and to areas often surrounded by mountains.Dry climates are divided into steppe orsemi-arid climate (BS) and desert climate (BW).They are further subdivided as subtropicalsteppe (BSh) and subtropical desert (BWh) atlatitudes from 15° - 35° and mid-latitudesteppe (BSk) and mid-latitude desert (BWk) atlatitudes between 35° - 60°.

Subtropical Steppe (BSh) and SubtropicalDesert (BWh) Climates

Subtropical steppe (BSh) and subtropicaldesert (BWh) have common precipitation andtemperature characteristics. Located in thetransition zone between humid and dryclimates, subtropical steppe receives slightlymore rainfall than the desert, adequate enoughfor the growth of sparse grasslands. The rainfallin both the climates is highly variable. Thevariability in the

rainfall affects the life in thesteppe much more than in the desert, moreoftencausing famine. Rain occurs in shortintense thundershowers in deserts and isineffective in building soil moisture. Fog iscommon in coastal deserts bordering coldcurrents. Maximum temperature in the summeris very high. The highest shade temperature of58° C was recorded at Al Aziziyah, Libya on13 September 1922. The annual and diurnalranges of temperature are also high.

Warm Temperate (Mid-Latitude) Climates-C

Warm temperate (mid-latitude) climates extendfrom 30° - 50° of latitude mainly on the easternand western margins of continents. Theseclimates generally have warm summers withmild winters. They are grouped into four types:(i) Humid subtropical, i.e. dry in winter andhot in summer (Cwa); (ii) Mediterranean (Cs);(iii) Humid subtropical, i.e. no dry season andmild winter (Cfa); (iv) Marine west coast climate(Cfb).

Humid Subtropical Climate (Cwa)

Humid subtropical climate occurs poleward ofTropic of Cancer and Capricorn, mainly inNorth Indian plains and South China interiorplains. The climate is similar to Aw climateexcept that the temperature in winter is warm.

Mediterranean Climate (Cs)

As the name suggests, Mediterranean climateoccurs around Mediterranean sea, along thewest coast of continents in subtropical latitudesbetween 30° - 40° latitudes e.g. — CentralCalifornia, Central Chile, along the coast insouth eastern and south western Australia.These areas come under the influence of subtropical high in summer and westerly wind inwinter. Hence, the climate is characterised byhot, dry summer and mild, rainy winter. Monthlyaverage temperature in summer is around25° C and in winter below 10°C. The annualprecipitation ranges between 35 - 90 cm.

Humid Subtropical (Cfa) Climate

Humid subtropical climate lies on the easternparts of the continent in subtropical latitudes. In this region the air masses are generallyunstable and cause rainfall throughout theyear. They occur in eastern United States ofAmerica, southern and eastern China,southern Japan, northeastern Argentina,coastal south Africa and eastern coast ofAustralia. The annual averages of precipitationvary from 75-150 cm. Thunderstorms insummer and frontal precipitation in winter iscommon. Mean monthly temperature insummer is around 27°C, and in winter it variesfrom 5°-12° C. The daily range of temperatureis small.

Marine West Coast Climate (Cfb)

Marine west coast climate is located polewardfrom the Mediterranean climate on the westcoast of the continents. The main areas are:Northwestern Europe, west coast of NorthAmerica, north of California, southern Chile,southeastern Australia and New Zealand. Dueto marine influence, the temperature ismoderate and in winter, it is warmer than forits latitude. The mean temperature in summermonths ranges from 15°-20°C and in winter4°-10°C. The annual and daily ranges oftemperature are small. Precipitation occursthroughout the year. Precipitation variesgreatly from 50-250cm.

Cold Snow Forest Climates (D)

Cold snow forest climates occur in the largecontinental area in the northern hemispherebetween 40°-70° north latitudes in Europe,Asia and North America. Cold snow forestclimates are divided into two types: (i) Df- coldclimate with humid winter; (ii) Dw- cold climatewith dry winter. The severity of winter is morepronounced in higher latitudes.

Cold Climate with Humid Winters (Df)

Cold climate with humid winter occurspoleward of marine west coast climate and midlatitude steppe. The winters are cold andsnowy. The frost free season is short. Theannual ranges of temperature are large. Theweather changes are abrupt and short.Poleward, the winters are more severe.

Cold Climate with Dry Winters (Dw)

Cold climate with dry winter occurs mainlyover Northeastern Asia. The development ofpronounced winter anti cyclone and itsweakening in summer sets in monsoon likereversal of wind in this region. Polewardsummer temperatures are lower and wintertemperatures are extremely low with manylocations experiencing below freezing pointtemperatures for up to seven months in a year.Precipitation occurs in summer. The annualprecipitation is low from 12- 15 cm.

Polar Climates (E)

Polar climates exist poleward beyond 70°latitude. Polar climates consist of two types:(i) Tundra (ET); (ii) Ice Cap (EF).

Tundra Climate (ET)

The tundra climate (ET) is so called after thetypes of vegetation, like low growing mosses,lichens and flowering plants. This is the regionof permafrost where the sub soil is permanentlyfrozen. The short growing season and waterlogging support only low growing plants.During summer, the tundra regions have verylong duration of day light.

Ice Cap Climate (EF)

The ice cap climate (EF) occurs over interiorGreenland and Antarctica. Even in summer, thetemperature is below freezing point. This areareceives very little precipitation. The snow andice get accumulated and the mounting pressurecauses the deformation of the ice sheets andthey break. They move as icebergs that float inthe Arctic and Antarctic waters. Plateau Station, Antarctica ,79°S, portray this climate.

Highland Climates (H)

Highland climates are governed by topography.In high mountains, large changes in meantemperature occur over short distances.Precipitation types and intensity also varyspatially across high lands. There is verticalzonation of layering of climatic types withelevation in the mountain environment.

IX. Hydrological Cycle or Water Cycle

Water is essential for life. No organism can survive without water. Precipitation (rain, snow, slush dew etc.) is the only source of water on the earth. Water received from the atmosphere on the earth returns back to the atmosphere as water vapour resulting from direct evaporation and through evapotranspiration the continuous movement of water in the biosphere is called water cycle (hydrological cycle).

Water is not evenly distributed throughout the surface of the earth. Almost 95 % of the total water on the earth is chemically bound to rocks and does not cycle. Out of the remaining 5%, nearly 97.3% is in the oceans and 2.1% exists as polar ice caps. Thus only 0.6% is present as fresh water in the form of atmospheric water vapours, ground and soil water.

The driving forces for water cycle are 1) solar radiation 2) gravity.

Evaporation and precipitation are two main processes involved in water cycle. These two processes alternate with each other Water from oceans, lakes, ponds, rivers and streams evaporates by sun‘s heat energy. Plants also transpire huge amounts of water. Water remains in the vapour state in air and forms clouds which drift with wind. Clouds meet with the cold air in the mountainous regions above the forests and condense to form rain precipitate which comes down due to gravity.

On an average 84% of the water is lost from the surface of the through oceans by evaporation. While 77% is gained by it from precipitation. Water runoff from lands through rivers to oceans makes up 7% which balances the evaporation deficit of the ocean. On land, evaporation is 16% and precipitation is 23%.

CHAPTER – 3 OCEANOGRAPHY

Hydrosphere is one of five major components of the planet earth, namely, lithosphere, atmosphere, hydrosphere, biosphere, and cryosphere. About three-fourth of the globe (70.8 percent) is covered by hydrosphere.

The oceans and seas covering largest surface area of the globe are of paramount significance to all of the living organisms including man of the biosphere because they help in the functioning of global hydrological cycle through atmospheric-oceanic circulation system ; they are significant sink of carbon dioxide and thus help in reducing the greenhouse effect caused by human activities; they help in the dispersal of seeds and small animals through ocean currents; they provide vital mineral and biological resources, they help in the trade and commerce; they provide varying marine habitats for the evolution, and development of marine organisms etc. This is why the study of various aspects of oceans and seas under the banner of oceanography has always been at the center stage of the development of human culture and civilization.

The science of oceanography consists of two Greek words e.g. Okeanos or Oceanus, meaning thereby oceans, and graphia, meaning thereby description. Thus, based on literal meaning of oceanography, it may be defined as follows:

“Oceanography is a science that investigates and interprets the characteristics and origin of ocean basins and reliefs thereof, physical and chemical properties of sea water (temperature, salinity and density), ocean dynamics (tides, sea waves, ocean currents, and tidal surges including tsunamis), coastal processes and coastal scenery, marine sediments and ocean deposits, coastal habitats and marine ecology, marine resources, marine organisms and biological productivity, and man and marine environment.”

I. Bottom Topography of the Oceans

Ocean basins have a great deal of featuresthat match those on the surface (i.e. continents)such as, volcanoes, deep canyons, rift valleys,and large submarine plateaus.

The study of sea floor by echo method ofsound waves reveals that the sea floor is not aflat area. It consists of mountains, plateaus,plains and trenches etc. Some major submarinefeatures are described below.

a) Continental Shelf

The portion of the land which is submerged under sea water constitutes continental shelf. The continental shelf is shallow and its depth is not more than 200 metres. Its slope from land to the sea is about 2 metres per km. The average width is 70-75 km, though it varies from a few metres to hundreds of kms. In all about 7.5 percent of total area of the oceans is covered by the continental shelves. The continental shelves may be of different types e.g. Glaciated shelves (sea near New England); Broad river shelves (yellow sea and the gulf of Siam) and coral shelves (those of Eastern Australia).

The shelves are of great use to man because:

· Marine food comes almost entirely from them.

· About 20 percent of oil and gas of the world is extracted from them.

·

They are large stores of sand and gravel.

· They are the sites of productive fishing grounds.

b) Continental Slope

It is an area of steep slope extending just after the continental shelf up to a considerable depth from where a gentle sea plain takes its form. The extent of the slope area is usually between 200-2000 m. But sometimes it may extend to 3660

metres from the mean sea level. The continental slope along many coasts of the world is followed by deep canyon like trenches terminating as fan shaped deposits at the base. Continental slope covers 8.5 percent of the total ocean area.

c) Continental Rise

The gently sloping surface at the base of the continental slope is called continental rise. It may extend to hundreds of km into the deep-ocean basin. It consists of a thick accumulation of sediment that move downslope from the continental shelf to the deep-ocean floor. Turbidity currents deliver these sediments to the base of the continental slope, and when these muddy currents emerge from the mouth of a canyon onto the flat ocean floor, they drop their load in a form of deep-sea fan.

d) Deep Ocean Basins

It is the portion of sea floor that lies between the continental margin and the oceanic ridge system. It contains deep-ocean trenches, abyssal plains, and broad volcanic peaks called seamounts.

· Deep-Ocean Trenches:These are long, narrow features that form the deepest parts of the ocean. Most trenches are located in the Pacific Ocean. They may reach 10,000 m deep (Mariana trench is 11,000 m below sea level). They are the sites where lithospheric plates plunge back into the mantle, and are usually associated with volcanic activity.

· Abyssal Plains:These are the most level places on Earth. The abyssal plains may have less than 3 m of relief over a distance that may exceed 1300 km. Scientists determined that abyssal plains low relief is due to the fact that thick accumulations of sediment, transported by turbidity currents, have buried rugged ocean floor. Abyssal plains are more extensive in the Atlantic Ocean than in the Pacific. That is because the Atlantic Ocean has fewer trenches to trap sediments carried down the continental slope.

· Seamounts:It is an isolated volcanic peak that rises at least 1000 m (3300 ft) above the deep-ocean floor. They are more extensive in the Pacific, where subduction zones are common. These undersea volcanoes form near oceanic ridges (regions of seafloor spreading). Some of these volcanoes may emerge as an island. Erosion by running water and wave action may erode these features to sea level. Over a long time, the islands gradually sink.

e) Submarine Canyons

These are depressions with walls of steep slopes and have a V shape. They exist on the continental slopes and the shelves. They are found to have a length of 16 km at the maximum and have a dendritic pattern. The canyons are found to be close to river mouths

such as those of the Congo, the Sindh, the Hudson, the Delaware, the Columbia, etc. The depth varies from 1,800 m. to 2,800 m. Coarse deposits are found in the canyons.

Bank, Shoal and Reef:

These marine features are formed as a result of erosional, depositional and biological activity. Also, these are produced upon features of diastrophic origin. Therefore, they are located on upper parts of elevations.

A bank is a flat topped elevation located in the continental margins. The depth of water here is shallow but enough for navigational purposes. The Dogger Bank in the North Sea and Grand Bank in the north-western Atlantic off Newfoundland are famous examples. The banks are sites of some of the most productive fisheries of the world.

A shoal is a detached elevation with shallow depths, since they project out of water with moderate heights, they are dangerous for navigation.

A reef is a predominantly organic deposit made by living or dead organisms that forms a mound or rocky elevation like a ridge. Coral reefs are a characteristic feature of the Pacific Ocean where they are associated with seamounts and guyots. The largest reef in the world is found off the Queensland coast of Australia. Since the reefs may extend above the surface, they are generality dangerous for navigation.

The Mid-Ocean Ridges

Mid-ocean ridges, of volcanic origin, are the most extensive relief features not only of the ocean basins but of the entire globe. Not all the mid-ocean ridges are centered in the ocean basins. The mid-Atlantic Ridge, and the mid-Indian Ocean Ridge are examples of central locations in the ocean basins, but the East Pacific Rise is certainly of non-central location. The following are the characteristic common features of mid ocean ridges:

v Mid-ocean ridges are the longest mountain chains of the globe running for a distance of 60,000 to 65,000 km across deep ocean basin. They occupy about one-third of the ocean floor.

v Not all of the mid-ocean ridges occupy central locations in the deep ocean basins. For example, the East Pacific Rise is far away from the central axis of the deep ocean basin of the Pacific Ocean.

v All of the mid-ocean ridges are of volcanic origin and consist of basaltic pillow lava.

v They are always associated with divergent plate margins and sea floor spreading.

v The crestal parts of mid-ocean ridges are either dome-shaped with rounded top, or are characterized by rift valleys, which are the creation of sea floor spreading and associated faulting.

v Though the width of mid-ocean ridge considerably varies but on an average it is 1,000 km. The average height of these ridges from the deep sea plains is about 2,500 m.

v Mid-ocean ridges are characterized by active volcanisms and seismic events.

II. Temperature of the Ocean Water

The temperature of the world's ocean is highly variable over the surface of the oceans, ranging from less than 0°C (32°F) near the poles to more than 29°C (84°F) in the tropics. It is heated from the surface downward by sunlight, but at depth most of the ocean is very cold. Seventy five percent of the water in the ocean falls within the temperature range of 1° to +6°C (30 to 43°F) and the salinity range of 34% to 35%. The top part of the ocean water is called the surface layer. Then there is a boundary layer called the thermocline layer. The thermoclinelayer separates the surface layers and the deep water of the ocean.

The sun hits the surface layer of the ocean, heating the water up. Wind and waves mix this layer up from top to bottom, so the heat gets mixed downward too. The temperature ofthe surface waters varies mainly with latitude. The polar seas (high latitude) can be as cold as -2 ºCelsius (28.4º F) while the Persian Gulf (low latitude) can be as warm

as 36 º Celsius (96.8ºF).

The average temperature of the ocean surface waters is about 17° Celsius (62.6° Fahrenheit). 90 % of the total volume of ocean water is found below the thermocline layer in the deep ocean. Water in the deep ocean is not well mixed. The deep ocean is made up of horizontal layers of equal density. The temperature of much of this deep ocean water is between 0°C and 3°C (32°F - 37.5 °F).

Variations in total salinity and in temperature cause variations in the density of sea water. Cold sea water is denser than warm sea water. There are several areas at the ocean surface where surface water becomes very cold. In these locations, surface sea water becomes denser than the surrounding water and sinks to begin the formation of slow thermohaline currents, which move deep ocean water.

At depth, pressure from the overlying ocean water becomes very high (pressure at 4,000 meters is about 400 atmospheres), but water is only slightly compressible, so that there is only a minor pressure effect on density. At a depth of 4,000 meters, water decreases in volume only by

1.8 percent.

The horizontal distribution of the temperatureof ocean water depends upon the followingfactors:

1. Latitudinal Distance

Temperature of the ocean water decreases from equator towards the poles. The average temperature of ocean water is 26°C in open seas at equator but the temperature decreases to 23°C at 20° North and South latitudes. Temperature further decreases to 14°C at 40° latitudes and to 1°C at 60° latitudes.

2. Unequal distribution of land and water

The temperature of ocean water varies in the northern and southern hemisphere because of dominance of land in the former and water in the latter. The oceans in the northern hemisphere receive more heat due to their contact with larger extent of land, thus temperature is comparatively high.

3. The Effect of Ocean Currents

The temperature of warm current is higher than that of the surrounding areas. The warm currents keep the coastal lands warmer. For example, the Gulf Stream does not allow the Norway Coast to freeze even in winter and thus helps the development of trade and commerce in that country. The temperature between Davis Strait and New Foundland drops down because of cold Labrador Current washing the coasts.

4. Prevailing Winds

The prevailing winds deflect the warm and cold currents and causes change in temperature of ocean water. For example, the currents on the east coast in the Trade Wind Belt shift away from the coast. Hence, the warm currents flowing along the coast moves away from it which leads to the upwelling of cold water from below near the coast. Hence the temperature remains low in spite of the passage of warm currents. This is why the temperature remains lower on the eastern than on western parts of the oceans.

5. Iceberg

Icebergs are found near polar areas and can be seen to be floating up to 50° latitudes. One part of iceberg is above sea and eight parts remain submerged under sea water. Many icebergs have a height of hundreds of metres above sea level. Thousands of icebergs can be seen moving away from North Atlantic. The Falkland and Benguela currents carry them to far off places. It lowers the temperature of the water to a great depth.

6. Local Weather

Local weather comprises different types of storms, cloudiness, precipitation and other weather conditions. In the equatorial regions,despite the vertical rays of the sun, large amount of cloudiness obstructs the solar radiation from reaching the earth surface.

It is due to the clear sky that near the Tropic of Cancer and the Tropic of Capricorn the amount of solar radiation incident on the earth exceeds that reaching the equatorial regions.

Thus, in the subtropical high pressure belt the surface water temperature in the oceans is a little higher. Besides, the incidence of daily afternoon rains in the equatorial regions doesnot allow the temperatures to rise further, whereas the extremely dry weather and cloudless skies prove helpful in raising the temperatures in the subtropical regions. In the same way in regions of stormy weather the ocean water temperatures are relatively lower.

7. Minor factors

Other minor factors which also control the sea surface temperatures are: Location of the seas and their shapes, submarine ridges, the rates of evaporation and condensation, etc.

Isotherms

Isotherms are the imaginary lines drawn on the maps joining places of equal temperature reduced to sea level.

III. Salinity of the Ocean Water

Simply put, salinity means the amount of dissolved salts-per unit mass of water. Salinity is expressed as the number of grams of dissolved salts in 1,000 grams of sea water.

The average salinity of the sea water is 35 parts per thousand. It means that in one kilogram of sea water, there are 35 grams of dissolved salts.

Composition of Salts In every cubic kilometer of sea water

· Sodium chloride — 77.7%

· Magnesium chloride — 10.9%

· Magnesium sulphate — 4.7%

· Calcium sulphate — 3.6%

· Potassium sulphate — 2.5%

Sources of ocean salinity

Basically, the most significant source of seawater salinity is the chemical weathering of continental rocks and transport of weathered materials by the rivers to the oceans but there are also a few minor sources. Thus, ocean salinity is derived from the following three sources and processes:

· Chemical weathering of continental rocks and their transport by the rivers to the oceans.

· Degassing by the earth i.e. undersea volcanic eruption.

· Atmosphere and biological interactions.

River runoff is the most significant contributor of seawater salinity. The continental rocks are subjected to chemical weathering through different processes, namely carbonation, oxidation, solution, hydration, hydrolysis, chelation etc. and weathered materials containing different dissolved substances are carried by surface runoff and overland flow and are brought to the rivers which finally dump these dissolved materials into the oceans. Besides, rivers also erode rocks of their valleys and thus carry ions of salts to the oceans.

The second source of seawater salinity is vulcanicity in the oceans. It may be remembered that there is frequent volcanic activity along the divergent plate boundaries representing divergence zone of sea floor spreading and convergent plate boundaries representing subduction zone. These under sea volcanic eruptions spew chloride and sulphate which are added to the ocean water.

The other insignificant sources of seawater salinity include atmospheric and biological sources. Certain gases from the atmosphere are dissolved in ocean water and contribute to the increase in the ocean salinity. Certain biological interactions also add some sort of salt in the oceans.

Controlling Factors of Salinity

There is a wide range of variation in the spatial distribution of salinity with in the oceans and the seas. The factors affecting the amount of salt in different oceans and seas are called as controlling factors of oceanic salinity. Evaporation, precipitation, influx of river water, prevailing winds, ocean currents and sea waves, melting of ice etc. are significant controlling factors.

The factors and processes which affect spatial distribution of ocean salinity are grouped into the following two categories:

Ø Factors that increase ocean salinity, examples: evaporation, formation of ice.

Ø Factors that decrease ocean salinity, examples: precipitation, river runoff, melting of ice.

Evaporation

There is direct positive relationship between the rate of evaporation and salinity e.g. greater the evaporation, higher the salinity and vice versa. In fact, salt concentration increases with rapid rate of evaporation. Evaporation due to high temperature with low humidity (dry condition) causes more concentration of salt and overall salinity becomes higher. For example, salinity is higher near the tropics than at the equator because both the areas record high rate of evaporation but with dry air over the tropics of Cancer and Capricorn.

Ice Formation

Formation of ice in the high latitude areas of the oceans increases seawater salinity. It may be noted that the formation of ice in the oceans requires extraction of seawater and there after freezing of such water. Whenever temperature of seawater becomes at or below freezing point, water molecules are removed from seawater and are frozen to form sea ice. Thus, sea ice contains fresh water and only less than 30 percent of seawater salinity where water freezes to form sea ice. For example, if the salinity of seawater of a part of an ocean is 33%o, and if the seawater freezes and is changed to sea ice, it contains only 30 percent of seawater salinity of 33%o, i.e. about 10%o only. It appears that the sea ice contains mostly fresh water. This results in the reduction of volume of fresh water in the oceans. This situation causes increase in

seawater salinity. The opposite process of sea ice formation is melting of sea ice, which increases volume of fresh water and hence the salinity of seawater is reduced.

Precipitation

Precipitation is inversely related to salinity e.g. higher the precipitation, lower the salinity and vice versa. This is why the regions of high rainfall (equatorial zone) record comparatively lower salinity than the regions of low rainfall (sub-tropical high pressure belts). The extra water in the temperate regions supplied by melt-water of ice coming from the polar areas increases the volume of water and therefore reduces salinity. It may be simply stated that the volume of fresh water in the oceans is increased due to heavy rainfall and thus the ratio of salt to the total volume of water is reduced.

Influx of River Water

Though the rivers bring salt from the land to the oceans but big and voluminous rivers pour down immense volume of water into the oceans and thus salinity is reduced at their mouths. For example, comparatively low salinity is found near the mouths of the Ganga, the Congo, and the Amazon etc. The effect of influx of river water is more pronounced in the enclosed seas e.g. the Danube, the Dniester, the Dnieper etc. reduce the salinity in the Black Sea (18%o). Salinity is reduced to 5%o in the Gulf of Bothnia due to influx of immense volume of water brought by the rivers. On the other hand, where evaporation exceeds the influx of fresh river waters, there is increase in salinity (Mediterranean Sea records 40%o). There is seasonal variation of surface salinity with maximum and minimum run off from the land i.e. salinity decreases with maximum runoff during rainy season and increases in the season of minimum runoff.

Atmospheric Pressure and Wind Direction

Anticyclonic conditions with stable air and high temperature increase salinity of the surface water of the oceans. Sub-tropical high pressure belts represent such conditions to cause high salinity. Winds also help in the redistribution of salt in the oceans and the seas as winds drive away more saline water to less saline areas resulting into decrease of salinity in the former and increase in the latter. In other words, in the areas of upwelling of water less saline water moves up from below (and hence low salinity) whereas the areas where water is piled up, salinity is increased. For example, trade winds drive away saline waters from the western coasts of the continents (or eastern margins of the oceans) and pile them up near the eastern coasts (or western margins of the oceans) causing low salinity in the former area and high salinity in the latter. This is why the Gulf of Mexico records 36%o to 37%o salinity whereas it is only 34%o in the Gulf of California.

Westerlies increase the salinity along the western coasts of the continents whereas they lower the salinity along the eastern coast. Sometimes, winds minimize the spatial variation in salinity.

Circulation of Ocean Water

Ocean currents affect the spatial distribution of salinity by mixing seawaters. Equatorial warm currents drive away salts from the western coastal areas of the continents and accumulate them along the eastern coastal areas. The high salinity of the Mexican Gulf is partly due to this factor. The North Atlantic Drift, the extension of the Gulf Stream increases salinity, along the north-western coasts of Europe. Similarly, salinity is reduced along the north -eastern coasts of

N. America due to cool Labrador Current. Ocean currents have least influence on salinity in the enclosed seas but those marginal seas which have communication with open seas through wide

openings are certainly affected by currents in terms of salinity. For example, the North Atlantic Drift raises the salinity of the Norwegian and the North Seas.

Distribution of Salinity

The average salinity in the oceans and the seas is 35%o but it spatially and temporally varies in different oceans, seas, and lakes. The variation in salinity is both horizontal and vertical (with depth). Salinity also varies from enclosed seas through partially closed seas to open seas.

Thus, the spatial distribution of salinity is studied in two ways e.g.:

(1) Horizontal distribution and

(2) Vertical distribution.

1. Horizontal Distribution:

Horizontal distribution of oceanic salinity is studied in relation to latitudes but regional distribution is also considered wherein each ocean is separately described. Similarly, the pattern of spatial distribution of salinity in enclosed seas, partially enclosed seas and open seas is also considered.

(i) Latitudinal distribution:

On an average, salinity decreases from equator towards the poles. It may be mentioned that the highest salinity is seldom recorded near the equator though this zone records high temperature and evaporation but high rainfall reduces the relative proportion of salt. Thus, the equator accounts for only 35^o/_o salinity.

The highest salinity is observed between 20^o-40^oN (36^o/_o) because this zone is characterized by high temperature, high evaporation but significantly low rainfall. The average salinity of 35^o/_o is recorded between 10⁰-30⁰ latitudes in the southern hemisphere. The zone between 40⁰-60⁰ latitudes in both the hemispheres records low salinity where it is 31^o/_o and 33^o/_o in the northern and the southern hemispheres respectively.

Salinity further decreases in the polar zones because of influx of melt water. On an average, the northern and the southern hemispheres record average salinity of 34^o/_o and 35^o/_o respectively.

On the basis of latitudinal distribution of salinity, four zones of oceanic salinity may be identified e.g.:

1. Equatorial zones of relatively low salinity (due to excessive rainfall),

2. Tropical zone (20⁰-30⁰) of maximum salinity (due to low rainfall and high evapora•tion),

3. Temperate zone of low salinity, and

4. Sub polar and polar zone of minimum salinity.

It may be pointed out that the marginal areas of the oceans bordering the continents have lower salinity than their central parts because freshwater is added to the marginal areas through the rivers. The salinity varies in the open seas according to the latitudes though it depends on the ocean currents but there is no control of latitudes on the distribution of salinity in the inland seas. Salinity of partially enclosed seas in the higher latitudes is seldom controlled by latitudes rather it depends on influx of melt-water.

This is why the Baltic Sea records comparatively lower salinity than the North Sea though the latitudinal extent of both the seas is the same. Table presents latitude- wise distribution of oceanic salinity in both the hemi•spheres.

2. Vertical Distribution:

No definite trend of distribution of salinity with depth can be spelt out because both the trends of increase and decrease of salinity with increasing depths have been observed. For example, salinity at the southern boundary of the Atlantic is 33% at the surface but it increases to 34.5% at the depth of 200 fathoms (1200 feet). It further increases to 34.7% at the depth of 600 fathoms. On the other hand, surface salinity is 37% at 20°S latitude but it decreases to 35% at greater depth.

The following characteristics of vertical distribution of salinity may be stated:

1. Salinity increases with increasing depth in high latitudes i.e. there is positive relationship between the amount of salinity and depth because of denser water below.

2. The trend of increase of salinity with increasing depths is confined to 200 fathoms from the surface in middle latitudes beyond which it decreases with increasing depths. Salinity is low at the surface at the equator due to high rainfall and transfer of water through equatorial currents but higher salinity is noted below the water surface. It again becomes low at the bottom.

3. It may be mentioned that the depth zone of oceans between 300m and 1000m is characterized by varying trends of vertical distribution of temperature, density of seawater, and salinity of ocean water.

Significance of Salinity:

The ocean salinity has significant effects on physical property of seawater and other aspects of the oceans as follows:

1. The freezing and boiling points are greatly affected and controlled by addition or subtraction of salts in seawater. The saline water freezes slowly in comparison to fresh water. It is known to all that pure water freezes at the temperature of 0⁰ C freezing point. If the salinity of seawater becomes 35%_o then it would freeze at the temperature of – 1.91⁰ C. On the other hand, the boiling point of saline water (seawater) is higher than fresh water.

2. Salinity and density of seawater are positively correlated i.e. the salinity of seawater increases its density because solutes (here salts) in water have greater atomic weight than the molecules of fresh water. This is why man is seldom drowned in the seawater with very high salinity.

3. Evaporation is controlled by salinity of the oceans. In fact, solutes (salts) in water lower the rate of evaporation in the oceans. Thus more saline water is less evaporated than less saline water. It may be mentioned that evapo•ration also controls salinity of seawater. More evaporation reduces the volume of seawater and hence the concentration of salts increases (i.e., seawater salinity increases).

4. Spatial variation in seawater salinity becomes potent factor in the origin of ocean currents.

5. The ocean salinity affects the marine organisms and plant community.

IV. Density of the Ocean Water

Density refers to the amount of mass per unit volume of substance. It is usually measured in gram (amount of mass) per cubic centimeter of volume and is expressed g/cm³. The density of pure (distilled) water is 1.00 g/cm³ at the temperature of 4°C. The density of pure water is taken as standard for the measurement of density of other substances. Since the seawater carries a few dissolved substances such as salt in it, its density is slightly higher than that of pure water.

In fact, the average density of seawater is 1.0278 g/cm3 (1.02677 g/cm³) which is 2 to 3 percent higher than the density of pure water (1.00 g/cm³) at 4°C temperature. The density of seawater gradually increases with decreasing temperature and highest density is recorded at the temperature of -1.3°C.

The density is very important physical property of seawater because it determines the dynamics of ocean water i. e. whether the seawater will sink (subsidence and hence downward vertical movement of seawater), or will float (expansion and hence horizontal movement) depends upon its density. As per rule, relatively lighter seawater (less dense seawater) floats and moves horizontally, whereas heavier seawater (more dense water) sinks (downward movement). This is the reason that a person floats over seawater having high salinity because salinity increases density of seawater.

Controlling Factors of Density of Seawater

The density of seawater is related to the following 3 factors in one way or the other:

Ø temperature: thermal expansion

Ø pressure: compressive effects

Ø salinity: addition of dissolved substances

(1) Temperature is the most significant controlling factor of density of seawater. Temperature and density of seawater are, on an average, inversely related i.e. higher the temperature, lower the density, and lower the temperature, higher the density.

In fact, seawater is heated through insolation when more insolation is received on the sea surface and hence seawater expands. This phenomenon is called thermal expansion due to insolation heating resulting into low density. On the other hand, low temperature causes cooling of seawater and hence thermal contraction resulting into decrease in volume and increase in density of seawater.

It is also important to note that temperature of seawater below freezing point cannot increase seawater density because at 0°C temperature water starts freezing with the formation of ice crystals which do not allow the water molecules to come closer and coalesce rather they are kept apart and hence few water molecules are present in per unit volume (one cubic centimeter) of seawater. Thus, the seawater becomes less dense. This is why ice floats in water.

It is thus apparent that cooling effect on increase in the seawater density continues upto 4°C temperature only. Since there is less variation in temperature of seawater in polar areas, and hence the role of temperature as controlling factor of seawater density is minimized.

(2) Salinity is directly positively related to seawater density i.e. on an average, seawater density increases with increasing salinity and decreases with decrease in salinity. This is because of the fact that dissolved salt in the seawater becomes denser than pure water. It is

also important to note that salinity factor is sometimes offset by temperature factor. Similarly, sometimes temperature factor is suppressed by salinity variable.

(3) Pressure is directly positively related to ocean water density through its compressive effects, seawater density increases with increasing pressure, and decreases with decrease in pressure of seawater. It may be mentioned that unlike air seawater (even water) is not much compressible, rather ‗it is nearly incompressible‘, and hence it exerts negligible control over seawater density. Thus, pressure is considered as minor factor of seawater density.

V. Ocean Currents

Ocean currents are the vertical or horizontal movement of both surface and deep water throughout the world's oceans. Currents normally move in a specific direction and aid significantly in the circulation of the Earth's moisture, the resultant weather, and water pollution.

Oceanic currents are found all over the globe and vary in size, importance, and strength. Some of the more prominent currents include the California and Humboldt Currents in the Pacific, the Gulf Stream and Labrador Current in the Atlantic, and the Indian Monsoon Current in the Indian Ocean.

Surface currents are those found in the upper 400 meters (1,300 feet) of the ocean and make up about 10% of all the water in the ocean. Surface currents are mostly caused by the wind because it creates friction as it moves over the water. This friction then forces the water to move in a spiral pattern, creating gyres. In the northern hemisphere, gyres move clockwise and in the southern they spin counterclockwise. The speed of surface currents is greatest closer to the ocean's surface and decreases at about 100 meters (328 ft) below the surface.

As surface currents travel over long distances, the Coriolis force also plays a role in their movement and deflects them, further aiding in the creation of their circular pattern.

Finally, gravity plays a role in the movement of surface currents because the top of the ocean is uneven. Mounds in the water form in areas where the water meets land, where water is warmer, or where two currents converge. Gravity then pushes this water down slope on the mounds and creates currents.

Deep water currents, also called thermohaline circulation, are found below 400 meters and make up about 90% of the ocean. Like surface currents, gravity plays a role in the creation of deep water currents but these are mainly caused by density differences in the water.

Causes of Ocean Currents

There are several causes for ocean currents, including:

a) Solar Activity

This is the single most important cause. The Sun provides the bulk of the energy which drives the circulation of water in the oceans, either directly or indirectly (through winds). The uneven distribution of solar energy across the globe (highest at the equator, decreasing towards the poles) produces an uneven heating of water in the ocean. Like air, hot water expands. The differential heating is so pronounced that sea level at the equator is about 8 cm (3.15 inches) higher than at temperate latitudes.

b) Gravity

The equatorial bulge of the oceans, caused by the expansion of water under equatorial heat creates a slope, and water tends to run downhill under the force of gravity. This is one of the major reasons for surface water flow from the equator towards higher latitudes.

c)Winds

Winds produce a flow of water at the ocean surface due to frictional coupling between the wind and the surface of the oceans. Since the oceans are largely flat unobstructed by topography, winds can blow for long distances, for prolonged periods of time. Friction between the air and the surface of the water is sufficiently high, that a wind blowing for about 10 hours can produce a surface current in the water at about 2% of the wind velocity. So a steady wind blowing in a certain direction at 20 miles per hour for about 10 hours will produce a surface water current at about 0.4 miles per hour. The direction of the water current is not the same as that of the wind flow. The direction of the water current is affected by a phenomenon known as Eckman Transport. Briefly, a column of water can be thought of as consisting of many layers. Wind friction affects the topmost layer, pulling the water in the direction of wind flow. This top layer of water tends to pull layers of water beneath, but because of the Coriolis force thewater actually moves at an angle to the side. In progressively deeper layers, the sideways movement is enhanced, so the entire water column can been thought of as moving in a spiral. The net flow of water is almost at right angles to the direction of the wind.

The duration of the wind is very significant. Since water is much heavier than air, it also has much more inertia. Short duration winds only produce turbulence at the water's surface. It takes winds blowing over a longer duration to produce a sustained movement of water in the wind's direction.

d) Coriolis force and Ocean Gyres

This is a pseudo force resulting from the Earth's rotation from west to east about its axis. Because of the Earth's rotation, any movement away from the equator (in both the northern and southern hemispheres) is deflected eastwards, while movements towards the equator are deflected westwards. This effect is very pronounced in movements that happen within a fluid medium (atmosphere and oceans), and over long distances. Because of the Coriolis Effect, currents tend to flow in curves rather than in straight lines. When the space for movement is restricted (such as by land bounding the oceans), these curves can close in on themselves, and cause a circular flow of water around a center. Such circular flows are called oceanic gyres.

Gyres are usually bounded by the shallow waters of continental shelves. There are five major gyres in the world's oceans, which are delimited by the continents around them.These gyres are responsible for much of the world's surface currents. As you can see in the map above,

much of the eastern coast of Africa has a current going from north to south, partof the Indian Ocean Gyre. This current was a great problem to early European navigators, trying to go around the Cape of Good Hope (the southern tip of Africa) to find a trade route to India. Early sailing

ships tended to hug the coast, where the currents are strongest, and they didn't have a lot of motive power in the days of sail. Even today, ships use these currents to save fuel, since making way against the current is costly. Debris floating in the ocean also tends to converge in certain zones because of these currents. The North Atlantic Garbage Patch and the Great Pacific Garbage Patch are places where a lot of trash dumped into the oceans has aggregated.

Ocean currents in Atlantic Ocean

Circulation of surface water is generally clockwise in the North Atlantic and counterclockwise in the South Atlantic. There are, however, many exceptions to the general circulation, particularly along the coasts.

In the North Atlantic, the strongest current is the warm Gulf Stream, which forms in the Gulf of Mexico. It flows north-eastward off the United States coast, encounters the cold Labrador Current from the north, and continues across the Atlantic as the North Atlantic Current, or Drift. It continues northward as the Irminger and Norwegian curr ents.

West of Spain, part of the North Atlantic Current turns southward, flows along the "bulge" of Africa as the Canaries Current, then turns westward as the North Equatorial Current. This current crosses the ocean; part of its water reaches the Gulf of Mexico. South of the North Equatorial Current, running in the opposite direction, is the Equatorial Counter-current. In the western North Atlantic, around Bermuda, is the Sargasso Sea, a calm area named for its masses of floating sargassum weed.

In the South Atlantic, the cold Benguela Current flows northward up to the coast of Africa, turns west near the Equator, and flows westward as the warm South Equatorial Current. Near the South American coast, part of the current swings southward to become the Brazil Current, which runs down the coast of South America to about 40° South latitude. Here, it meets the cold Falkland Current. Flowingeastward across the South Atlantic in a broad belt around 50° S. is the West Wind Drift.

Ocean Currents in Pacific Ocean

General circulation of the currents is clockwise in the North Pacific, counterclockwise in the South Pacific. It is somewhat like the rotations of two giant disks turning in opposite directions. As a result, warm equatorial water is constantly being carried poleward along the Asian and Australian coasts, while cold water is flowing toward the equator along the coasts of North and South America. Thus, the currents have distinct climatic effects on nearby lands. In some areas, such as the coast of Chile and Peru, currents have cooling effects; in others, such as southern Japan, they are warm. In regions where warm and cold currents meet, notably northern Japan to southern Alaska, dense fogs frequently occur.

The chief currents of the North Pacific, beginning at the equator, are the North Equatorial Current, Kuroshio (Japan Current), North Pacific Current, or Drift, and California Current. Entering this circulation from the Arctic is the cold Oyashio (Okhotsk, or Kamchatka, Current). The Alaska Current starts off the coast of Oregon and flows northward along the coast of North America as far as Kodiak Island, bringing warmth tothe coastal areas in winter. In the South Pacific are the South Equatorial Current, East Australian Current, West Wind Drift, and Peru (Humboldt) Current. Separating the two systems in the vicinity of the equator is the Equatorial Counter-current, which flows eastward from Indonesia and the Philippines to the South American coast.

There are also deep, underwater currents, but little is known of them. They tend to form separate layers flowing in different directions, each layer being denser than the layer above. For example, Antarctic water is known to subside suddenly beneath warmer water at a line called the Antarctic Convergence. From here it flows northward far below the surface of the ocean.

Ocean currents in Indian Ocean

The Asiatic Monsoon influences the currents of the North Indian Ocean, while the currents of the South Indian Ocean are influenced by the atmosphere's Anticyclonic circulation.

During the northwest monsoon (February and March) the wind blows from the continent and aids in the development of the North Equatorial Current. The current flows from east to west; and upon reaching the east coast of Africa, a good portion turns southward, crosses the equator, and becomes the Mozambique Current. A strong counter-current exists south of the North Equatorial Current at this same time of year. In August and September, during the southwest monsoon, the North EquatorialCurrent reverses and flows west to east as the Monsoon Current. At the same time, the countercurrent seems to disappear.

The Mozambique Current flows south along the east coast of Africa from the vicinity of the equator to about 35°S, where it is known as the Agulhas Stream. The Agulhas Stream flows westward along the southern coast of Madagascar and joins the Mozambique Current along the east African coast. From there, it flows south to the southern tip of Africa (the Cape of Good Hope), where a good portion joins up with the West Wind Drift Current.

The West Wind Drift Current flows across the Indian Ocean to the waters southwest ofAustralia. Here it splits; one branch continues east along the southern coast, while the other flows northward along the western

coast. Thisbranch brings relatively cool waters to the western Australian coast and contributes to the formation of fog and low stratus clouds over the region.

The Importance of Ocean Currents

Because ocean currents circulate water worldwide, they have a significant impact on the movement of energy and moisture between the oceans and the atmosphere. As a result, they are important to the world's weather. The Gulf Stream, for example, is a warm current that originates in the Gulf of Mexico and moves north toward Europe. Since it is full of warm water, the sea surface temperatures are warm, which keeps places like Europe warmer than other areas at similar latitudes.

The Humboldt Current is another example of a current that affects weather. When this cold current is normally present off the coast of Chile and Peru, it creates extremely productive waters and keeps the coast cool and northern Chile arid. However, when it becomes disrupted, Chile's climate is altered and it is believed that El Niño plays a role in its disturbance.

Like the movement of energy and moisture, debris can also get trapped and moved around the world via currents. This can be man-made which is significant to the formation of trash islands or natural such as icebergs. The Labrador Current, which flows south out of the Arctic Ocean along the coasts of Newfoundland and Nova Scotia, is famous for moving icebergs into shipping lanes in the North Atlantic.

Currents play an important role in navigation as well. In addition to being able to avoid trash and icebergs, knowledge of currents is essential to the reduction of shipping costs and fuel consumption. Today, shipping companies and even sailing races often use currents to reduce time spent at sea.

Finally, ocean currents are important to the distribution of the world's sea life. Many species rely on currents to move them from one location to another whether it is for breeding or just simple movement over large areas.

VI. Waves and Tides

Waves

Sea waves are defined as undulation of seawater characterized by well-developed crests and troughs. Besides geomorphic importance, sea waves are now also considered as a source of

non-conventional energy. Thus, sea waves have great energy potential for future generations. This is why H. V. Thurman and A.P. Trujillo (1999) have defined sea waves in terms of energy level as follows:

“Waves are moving energy travelling along the interface between ocean and atmosphere, often transferring energy from a storm far out at sea over distances of several thousand kilometers” (Thurman and Trujillo, 1999).

The mechanism of the origin of sea waves is not precisely known but it is commonly believed that waves are generated due to friction on ocean water surface caused by blowing winds.

The undulations of seawater at the place of their origin are called swells which are low, broad, regular and rounded ridges and troughs of water.

The sea waves are characterized by the following components:

Wavecrest: The successive higher parts of progressive sea waves are called wave crests which are the highest parts of the waves.

Wave troughs: These are successive lowest parts of progressive sea waves which are alternated by wave crests. It is, thus, clear that a wave trough is located between two successive wave crests, or a wave crest is located between two successive wave troughs.

Still water level, also known as zero energy level represents the water zone halfway between the wave crests and the wave troughs.

Wave height is a vertical distance between the crest and horizontal straight distance between two successive troughs of progressive sea waves or between the trough and horizontal straight distance between two successive wave crests.

Wavelength is the straight horizontal distance between two successive wave crests or wave troughs, which is expressed in terms of length unit of meters in the case of sea waves.

Wave period: The time taken by a progressive sea wave to cover the distance of one wave length or one wave cycle is called wave period, which is usually expressed in the time unit of seconds.

Wave frequency : The number of sea waves (one wave is equal to one wavelength)passing through a certain point per unit time (usually one second or one minute) is called wave frequency, which varies according to the wavelengths of waves. There is inverse relationship

between the wavelength and wave frequency i.e. shorter the wavelength, higher the wave frequency, and longer the wavelengths, lower the wave frequency.

Generation of Sea Waves

The mechanism of the origin of sea waves is not precisely known but it is commonly agreed that the sea waves are generated by some sort of energy release. The following are the probable causes of generation of waves in the oceans and seas:

· atmospheric circulation and wind,

· movement of fluids of two contrasting densities (air and seawater) along the interfaces of two masses of fluids of varying densities,

· movement of water masses of varying densities in the oceans such as turbidity currents,

·

mass movement into the oceans such as landslides in the coastal areas,

· Tectonic activities on the sea floor such as faulting, thrusting etc.

· occurrence of undersea earthquakes, known as tsunami genic quakes,

· undersea volcanic eruptions, gravitational forces of the sun and the moon (tidal waves),

· atmospheric storms such as tropical cyclones (storm waves or storm surges),

· Anthropogenic activities, namely plying of large commercial ships, undersea nuclear tests and explosions etc.

Tides

The tide is the periodic rise and fall of the sea levels caused by the combined effects of the gravitational forces exerted by the Moon and Sun and rotation of the earth. Most places in the oceanusually experience two high tides and two low tides each day (semidiurnal tide), but some locations experience only one high and one low tide each day (diurnal tide).

The times andamplitude of the tides at the coast are

influenced by the alignment of the Sun and Moon, by the depth of the ocean, and by the shape of the coastline and near-shore bathymetry.

The moon, which is approximately 240,000 miles (386,240 km) from the earth, exerts a greater influence on the tides than does the sun, which is 93 million miles (150 million km) from the earth. The strength of the sun's gravity is 179 times that of the moon's but the moon is responsible for 56% of the earth's tidal energy while the sun claims responsibility for a mere 44%.

When the moon exerts gravitational force on the earth the tidal bulge moves out and causes high tide. Simultaneously on the side opposite to that place on the earth i.e. just at 180° to it, also experiences the tidal bulge due to reactionary force (centrifugal) of the gravitational (centripetal) force. Thus two tides are experienced twice at every place on the earth's water surface within 24

hours.Due to the cyclic rotation of the earth and moon, the tidal cycle is 24 hours and 52 minutes long.

Causes of Tides

· Gravitational attraction between moon andthe earth.

· Gravitational attraction between sun and theearth.

· Attraction force of the earth towards earthcentre.

· Moon is mainly responsible for the tides.

Types of Tides

· Semi diurnal tides - Recur at the intervals of12½ hours.

· Diurnal Tides - Recur at the intervals of 24½hours.

· Spring Tides - once a fortnight, due to therevolution of the moon and its declination.

· Neap tides - Once a fortnight due to therevolution and declination of moon.

· Monthly tides - Due to the revolution of themoon and its position at perigee and apogee.

Spring Tides

Spring tides are especially strong tides or high tides. They occur when the Earth, the Sun,and the Moon are in a line.The gravitational forces of the Moon and the Sun both contribute to the tides. Spring tides occur during the full moon and the new moon.

Neap Tides

Neap tides are especially weak tides. They occur when the gravitational forces of the Moon and the Sun are perpendicular to one another (with

respect to the Earth). Neap tides occur during quarter moons.

The Bay of Fundy between Nova Scotia andNew Brunswick in Canada experiences the world'sgreatest tidal range of 50 feet (15.25 meters)

Significance of Tides:

1. Tides generally help in making some of the rivers navigable for ocean-going vessels.

London and Calcutta have become important ports owing to the tidal nature of the mouths of the

Thames and Hooghly respectively.

2. Tides also clear away the sediments brought by the rivers and, thus, retard the process of delta formation.

3. The tidal force may also be used as a source for generating electricity. France and Japan, for example, have power stations which convert tidal energy into electricity.

VII. Ocean Deposits

The unconsolidated materials, derived from various sources and deposited at the ocean floors are called marine sediments, which include weathered and eroded particles of rocks, fragments of dirt, dust, volcanic ashes, remains of marine organisms, fragments of meteorites etc. The settling of marine sediments on the ocean floors is called ‗marine snow fall‟.

The unconsolidated marine sediments are lithified due to tectonic activities and thus we find layered consolidated materials on the deep ocean floors. Such consolidated marine sediments are called ocean deposits.

Sources of Marine Sediments

The marine sediments are derived and supplied from 4 major sources as follows:

· Terrigenous or lithogenous source,

· Biogenous source or organic source,

· Hydrogenous source, and

· Cosmogenous source.

The above mentioned sources of marine sediments may be alternatively grouped into the following 3 categories:

· external source (terrigenous source)

· internal source (biogenous and hydrogenous sources)

· cosmogenous source

The terrigenous or lithogenous source of marine sediments includes weathering and erosion of continental rocks and transport of eroded materials by the rivers; coastal erosion by sea waves; and glacial erosion of continental rocks and their transport by glaciers to the seas in high latitudes. The winds also transport dusts and sands from the hinterlands of the coasts to the oceans. The terrigenous source contributes rock fragments of varying sizes such as boulders, pebbles, cobbles, gravels etc., quartz sands, quartz silt, clay, dusts etc.

The biogenous source of marine sediments comprises the processes of decay and decomposition of shells and skeletons of marine organisms in situ. Such sediments are grouped in two broad categories of (1) calcium carbonate (calcareousoozes) and shells of marine organisms and fragments of corals, and (2) silica (siliceousoozes). The biogenous sediments of calcium carbonate are produced in warm sea surface water while those of silica are generated in cold sea surface water.

The hydrogenous source of marine sediments includes the sediments derived from precipitation of dissolved substances due to chemical reactions such as phosphorites (phosphorous), oolites (calcium carbonate), metal sulfides (copper, silver, zinc, iron, nickel etc.), evaporites (such as gypsum and some salts).

The cosmogenous source of marine sediments includes the sediments produced from the collision of meteorite s in the space and thus the space dusts so produced directly fall into the oceans.

Besides, the volcanic dusts and ashes, which are ejected through continental volcanic eruptions, are carried away by the atmospheric circulation and fin ally they fall down through precipitation into the oceans.

1. Terrigenous (Lithogenic) Sediments

The lithogenic sediments are derived from the weathering and erosion of rocks whether on land (lithosphere) or in the oceans (weathering and erosion of sea volcanic islands), whereas terrigenous sediments include only those sediments of various sizes (ranging from boulders to clay particles) which are produced by weathering and erosion of only continental rocks.

The continental rocks are disintegrated and decomposed due to various types of weathering and thus fine to coarse sediments are produced. Besides, rocks are also eroded by surface runoff and streams through the processes of surface wash, splash erosion, sheet wash, rain-wash, rill and gully erosion, lateral and vertical erosion of valleys by rivers. The weathered and eroded materials are carried by the rivers and are ultimately unloaded into the oceans and seas.The terrigenous sediments brought by the rivers to the oceans are also carried away and reworked by sea waves and currents. The turbidity currents carry sediments to deep ocean basins.

Most of terrigenous materials are deposited in the areas of continental margins and inner continental shelves but high buildup of terrigenous sediments on continental shelves forms heaps (mounds) of sediments with steep slope. Thus the sediments slide down en mass under the impact of gravity along the continental slope. The turbidity currents disperse these sediments on deep sea floor. The slumping of sediments is called bulk emplacement.

Terrigenous sediments have been found almost in all parts of the oceans but most concentration is found in the continental margins and continental shelves. Only traces of terrigenous sediments have been found on deep sea plains.

The fine particles are picked up by prevailing winds from tropical and subtropical deserts and are carried far away from the continents to the deep ocean where these particles fall down and settle down on deep sea plains particle by particle, this process of sedimentation in called retail deposition.

The terrigenous sediments are composed mostly of quartz mineral. The texture of terrigenous sediments is determined on the basis of grain size for which the following Wentworth scale is used.

· Gravels: The diameter of gravels ranges from 2 mm to 256 mm. There is marked gradation in the size of gravels. The following are sub-types of gravels on the basis of diameter of particles (figures in the brackets indicate diameter) boulders (> 256 mm), cobbles (65 - 256 mm), pebbles(4 to 64 mm), granules (2 to 4) mm) etc. Since these sediments are very large in size, these are deposited near the coast on the continent shelves by high energy currents. These sediments are further reduced in size due to further disintegration caused by sea waves. Gravels brought to the oceans by the rivers.

· Sands: The sediments varying in diameter from 2 mm to 1/16 mm are termed sands. The disintegration and communition of continental rock fragments into fine sediments produces sands which are deposited in the oceans by rivers, surface wash and winds. There is marked gradation of sand deposits in the oceans i.e. coarser sands are deposited close to the coast while fine sands are deposited away from the coast.

· Silt, Clay and Mud: The finer sediments ranging in diameter from 1/32 mm to 1/8192 mm are grouped under the category silt, clay and mud (silt = 1/32 mm to 1/256 mm, clay = 1/256 mm to l/8192m m ). Mud is still finer than clay. Clay is significant cementing element. These materials are brought from the continents by the rivers. Clay and mud are deposited in calm seawater by low energy currents. Generally, these deposits are found at the depth of 100 to 1000 fathom s (600 to 6000 feet).

Murray has divided mud into three types on the basis of colour.

i. Blue mud includes the materials derived through the disintegration of rocks rich in iron sulphide and organic elements. These are generally found at greater depth of the continental shelves. The original colour of blue mud is bluish black and it contains 35 per

cent of calcium carbonate. Blue mud predominates in the Atlantic Ocean, Mediterranean Sea, Arctic Sea and enclosed seas.

ii. Red mud: The sediments derived through the communition of rocks rich in iron oxides (FeO) form red mud. The reddish colour is mainly due to the dominance of iron content. It contains 32 per cent of calcium carbonate. The deposit of red mud is confined mostly to the Yellow Sea, Brazilian coast, and the floors of the Atlantic Ocean.

iii. Greenmud is formed due to chemical weathering wherein the colour of blue mud is changed to green mud due to reaction of seawater. It contains green silicates of potassium and glauconite (form of iron) which constitutes 7 - 8 per cent of total mineral com position whereas calcium carbonate ranges from 0 to 56 per cent. The deposits of green mud are found along the Atlantic and Pacific coasts of N. America, off the coasts of Japan, Australia and Africa. These are generally found at the depth of 100 to 900 fathoms (600 to 5,400 feet).

2. Volcanogenic Sediments

Volcanic materials deposited in the marine environment are derived from two sources, (i) Volcanic eruptions on the land- the volcanic materials through violent central eruptions become very fine due to collision among themselves and due to further disintegration. Fine volcanic materials nearer to the coastal lands are blown by wind and are carried to the oceans while volcanic materials of distant places are brought by the rivers via overland flow, rain-wash, rills and small rivulets, (ii) Volcanic eruption in the oceans and the seas-in such cases volcanic materials are directly deposited. Volcanic materials resemble blue mud and are grey to black in colour.

3. Biogenic Sediments

Biogenous (bio = life, genere = to produce), also known as organic marine sediments, are the decay and disintegration of hard parts (skeletons) of marine organisms. Thus, the source of biogenic sediments is sea itself. The process of formation of biogenous marine sediments includes the disintegration of hard parts of marine animals and plants such as their bones, shells, teeth etc. after their death. Such materials fall down one after another and are deposited on sea floors of varying locations. Primarily biogenous marine sedimentsare divided into the following two categories:

· Macroscopic biogenic sediments, and

· Microscopic biogenic sediments.

Macroscopic biogenic sediments include shells, bones, and teeth of large marine animals which are not widespread sea living organisms. Such sediments are found on continental shelves and very rare on deep sea plains because deep sea floors are dominated by very small organisms. On the other hand, microscopic biogenous sediments are very small particles of very small sea organisms. They are so small and minute that they cannot be seen without the aid of powerful microscope. There are countless tiny microscopic marine organisms. The tiny shells of these organisms are called tests which continuously fall down on the sea bed after death.

The biogenic sediments are composed of two main chemical constituents, namely calcium carbonate and silica. Diatoms and radiolarians contribute most of silica to the microscopic biogenic sediments, while foraminifers contribute most of calcium carbonate. The microscopic algae, which is called as cocolithophores also contribute calcium carbonate. The biogenous marine sediments, after mixed with terrigenous clays and their accumulation , form different

types of oozes which are named after the name of microscopic marine organisms such as diatoms, pteropods, radiolaria etc.

The biogenic sediments are also divided into the following two broad categories:

· Neretic biogenic sediments, and

· Pelagic biogenic sediments.

The neretic matter includes skeletons of marine organisms and plant remains while pelagic matter consists of remains of different types of algae. Neretic matter is deposited mostly on the continental shelves and is generally covered by terrigenous materials. These include shells of molluscs and their fragments, skeletons of radiolaria, calcareous and siliceous plant remains.

Pelagic sediments consist of matter derived from algae and are mostly in the form of liquid mud, generally known as ooze. Pelagic materials are oozes which are divided into two groups on the basis of lime and silica contents as follows.

(i) Calcareous oozes contain lime content in abundance and are seldom found at greater depth because of their high degree of solubility. They are generally found at the sea floor between the depths ranging from 1000 fathoms (6000 feet) to 2000 fathoms (12000 feet). On the basis of principal organisms calcareous oozes are further divided into two sub-types viz, (a) pteropod ooze, and (b) globigerina ooze.

a. PteropodOoze: Most of the pteropod oozes are formed of floating pteropod molluscs having thin shells of generally conical shape with average diameter of half inch. It contains 80 per cent calcium carbonate and is mostly found in the tropical oceans and seas at the depth of 300 -1000 fathoms. It decreases with greater depths and practically disappears beyond 2000 fathom depth. It is found mostly in the regions of corals. The main location of pteropod ooze includes the western and eastern parts of the Pacific Ocean, surroundings of Azores, Canary Island, Antilles, mid-Mediterranean sub marine ridge and Indian Ocean.

b. GlobigerinaOoze: Though this ooze is formed from the shells of a variety of foraminifera but most of such oozes are formed of germs called globigerina. When this deposit is dried up it becomes dirty white powder. Besides milky white colour, it is also blue, grey, yellow and green in colour. The chemical composition reveals 64.46 percent of calcium, 1.64 percent of silica and 3.33 percent of minerals. Globigerina is found mostly in the tropical and temperate zones of the Atlantic Ocean, on the eastern and western continental shelves of the Indian Ocean and in the eastern Pacific Ocean. It is generally found between the depths of 2000 to 4000 fathoms and becomes absent at greater depths.

(ii) Siliceous Ooze: When silica content dominates, the ooze becomes siliceous in nature. Silica is derived from a group of protozoa or radiolarians and benthic animals mainly sponges. This ooze does not dissolve as compared to calcareous ooze because of less calcium carbonate and dominance of silica. Thus, such oozes are found in both w arm and cold water at greater depths. This group is further divided into two subtypes on the basis of dominance of a particular organism.

(a) Radiolarian ooze is formed by the shells of radiolaria and foraminifera. It changes to dirty grey powder when dried. Silica predominates but calcium carbonate is also present (5 to 20 percent). Lime content decreases with increasing depth and it absolutely disappears at greater depth. This ooze is found upto the depth of 2000 to 5000 fathoms in the tropical oceans and seas. It covers the largest areas in the Pacific Ocean.

(b) Diatomooze is formed of the shells of very microscopic plants containing silica in abundance. It also contains some clay. Calcium content varies from 3 to 30 percent. It is blue near the land and the colour changes yellow or cream away from the land. It becomes fine coherent white powder when dried. Diatom ooze is very frequently found at greater depth in high latitudes. Significant area of this deposit includes the zone around Antarctica and a belt from Alaska to Japan in the N. Pacific at the depth of 600-2000 fathoms.

4. Hydrogenic Marine Sediments

Hydrogenous sediments are also inorganic matter and involve precipitates of dissolved substances from water both on land and in oceans.

Majority of inorganic elements are basically precipitates which fall down from above. These elements fall on the land as well as in the oceans. Some of the inorganic elements are transported from the land to the oceans by various agencies. The inorganic precipitates include dolomite, amorphous silica, iron, manganese oxide, phosphate, barite etc. Besides, glauconite, phosphorite, feldspar, phillipsite and clay minerals are also found. The organic and inorganic materials are so mixed together due to chemical processes that it becomes very difficult to isolate them from each other.

The significant hydrogenous marine deposition includes manganese nodules, phosphates, carbonates, metal sulfides, evaporites etc. These hydrogenous marine sediments have great economic significance.

· Manganese nodules have round shape and consist of manganese, iron and some metals. These are formed around nuclei of coral, volcanic rock, bones of fishes or fish teeth‘s. Manganese nodules are primarily composed of manganese dioxide and iron oxide which constitute together 50 percent by weight. The other constituents of manganese nodules include copper, nickel, cobalt etc.

· Phosphates are infact compounds of phosphorous which are precipitated as coatings around rocks. They are also found in the form of nodules. Phosphates are used for making fertilizers.

· Carbonates include two significant minerals i.e. aragonite and calcite which are com posed of calcium carbonate (limestone).

· Metalsulfides are generally found along mid-oceanic ridges and include iron, copper, silver, nickel, zinc etc.

· Evaporites, as the word implies, result from excessive evaporation of seawater. They are basically salts (halite). The other evaporite minerals are gypsum and calcite.

Red clay, previously considered to be of organic origin, is the most significant inorganic matter and very important member of pelagic deposits. It covers the largest area of deep sea deposits. Silicates of alumina (85.35 percent) and oxides of iron are the chief constituents of red clay. Besides, calcium (6.7 percent), siliceous organisms (2.39 percent) and a few minerals are also present. It also contains decomposed volcanic material. It may be pointed out that red clay contains more radioactive substances than anyother marine deposit. It is soft, plastic and greasy in character. It becomes reddish brown powder when dried. Red clay is widely distributed at the greatest depth in all the oceans. Its locations include the zone between 40°N and 40^oSin the Atlantic Ocean, eastern part of the IndianOcean and the North Pacific Ocean covering 129million km² of area.

5. Cosmogenic Sediments

Cosmogenous sediments are extraterrestrial materials which are produced due to collision of meteors in space. This is why cosmogenic sediments are called space dusts which regularly fall down on the earth‘s surface (both on the lands and in the oceans). Cosmogeneous sediments comprise (1) microscopic spherules, and (2) macroscopic debris of meteors.

Authigenic deposits: The materials derived through biochemical precipitation and deposited on sea floors in situ are called authigenic deposits.

VIII. Coral Reefs

Coral Reefs referred to as the "rainforests of the oceans" are underwater structures made from secreted by corals. The reef is made up of thousands of corals. When coral dies, it leaves its skeleton of calcium carbonate behind. Young corals attach themselves to the old skeleton and the cycle starts again. Each new generation is built upon the remains of the previous generation.

The sun is the source of energy for the coral reef ecosystem. Plant plankton, called phytoplankton, algae and other plants convert light energy into chemical energy through photosynthesis. As animals eat the plants and other animals, energy is passed through the food chain. Reef building corals work together with microscopic algae, called zooxanthellae that live in their tissue. The zooxanthellae provide oxygen and food to the coral through photosynthesis. The coral polyp gives the algae a home, and the carbon dioxide it needs through respiration.

Coral bleaching occurs when corals lose their zooxanthellae, exposing the white calcium carbonate skeletons of the coral colony. There are a number of stresses or environmental changes that may cause bleaching including disease, excess shade, increased levels of ultraviolet radiation, sedimentation, pollution, salinity changes, and increased temperatures.

Suitable conditions for the growth of Coral Reef:

Coral reefs develop in shallow, warm water, usually near land, and mostly in the tropics; coral prefer temperatures between 70 and 85 ° F (21 - 30 °C). Coral reefs are generally found in clear, tropical oceans. Coral reefs form in waters from the surface to about 150 feet (45 meters) deep because they need sunlight to survive.

Reefs usually develop in areas that have a lot of wave action because the waves bring in food, nutrients and oxygen to the reef. Waves also prevent sediment from falling on the reef. Reefs need calcium from the water to grow, which is more often available in shallow warm waters.

Corals can live only in saline water, and for their proper growth the average salinity should be between 27 to 40%.

Types of Coral Reefs:

Reefs are generally classified in three types:

v Fringing reefs are reefs that form along acoastline. They grow on the continental shelfin shallow water.

v Barrier reefs grow parallel to shorelines, butfarther out, usually separated from the landby a deep lagoon. They are called barrier reefsbecause they form a barrier between the lagoonand the seas, impeding navigation.

v Coral Atolls are rings of coral that grow ontop of old, sunken volcanoes in the ocean. Theybegin as fringe reefs surrounding a volcanicisland; then, as the volcano sinks, the reefcontinues to grow, and eventually only thereef remains.

Coral reefs provide habitats for a large variety of organisms. These organisms rely on corals as a source of food and shelter. Besides the corals themselves and their symbiotic algae, other creatures that call coral reefs home include various sponges; molluscs such as sea slugs, nudibranchs, oysters, and clams; crustaceans like crabs and shrimp; many kinds of sea worms; echinoderms like star fish andsea urchins; other cnidarians such as jellyfish and sea anemones; various types of fungi; sea turtles; and many species of fish.

Coral reefs are dying around the world. In particular, coral mining, agricultural and urban runoff, pollution (organic and inorganic), overfishing, blast fishing, disease, and the digging of canals and access into islands and bays are localized threats to coral ecosystems. Broader threats are sea temperature rise, sea level rise and pH changes from ocean acidification, all associated with greenhouse gas emissions.

Coral Bleaching:

Warmer water temperatures can result in coral bleaching. When water is too warm, corals will expel the algae (zooxanthellae) living in their tissues causing the coral to turn completely white. This is called coral bleaching. When a coral bleaches, it is not dead. Corals can survive a bleaching event, but they are under more stress and are subject to mortality.

In 2005, the U.S. lost half of its coral reefs in the Caribbean in one year due to a massive bleaching event. Not all bleaching events are due to warm water.

In some instances corals can recover from bleaching. If conditions return to normal, and stay that way corals can regain their algae, return to their bright colours and survive. However prolonged warmer temperatures and other stressors, like poor water quality, can leave the living coral in a weakened state. It can struggle to regrow, reproduce and resist disease – so is very vulnerable to coral diseases and mortality.

It can take decades for coral reefs to fully recover from a bleaching event, so it is vital that these events do not occur frequently. If we continue burning fossil fuels at our current rate then

severe bleaching events are likely to hit reefs annually by the middle of the century. This would be devastating for coral reefs as they would have no chance to recover.

Carbon pollution is causing unprecedented damage to Great Barrier Reef. In recent years, Reef has suffered severe mass coral bleaching, faster and more severe than scientists predicted. If we don‘t act to halt this pollution, we risk the future of our precious Reef.

Over 2016 and 2017, our Reef suffered back-to-back bleaching, leaving half of the shallow water corals dead. Results from extensive aerial and underwater surveys showed that 29% of corals died from the 2016 event alone – with most perishing in the northern section, where waters are warmest.

IX. Maritime Zones

The United Nations Convention on the Law of the Sea (UNCLOS), also called the Law of the Sea Convention or the Law of the Sea treaty has divided the maritime area as internal waters, the territorial sea, the contiguous zone, the exclusive economic zone, the continental shelf, the high seas and the Area in 1982.

Maritime zones delineate the sovereignty or sovereign rights a coastal state has over the offshore environment. The maritime areas are divided as:

Baseline:

A baseline is the line from which the seaward limits of a State's territorial sea and certain other maritime zones of jurisdiction are measured. Normally, a sea baseline follows the low- water line of a coastal State. When the coastline is deeply indented, has fringing islands or is highly unstable, straight baselines may be used.

Internal Waters:

Internal waters are the waters (for example,bays and rivers) on the landward side of the baseline. Each coastal State has full sovereignty over its internal waters as if they were part of its

land territory. The right of innocent passage does not apply in internal waters. The coastal state is free to set laws, regulate use, and use any resource.

Territorial waters:

Territorial Water is a belt of coastal waters extending at most 12 nautical miles (22 km; 14 mi) from the baseline of a coastal state. The territorial sea is regarded as the sovereign territory of the state, although foreign ships (both military and civilian) are allowed innocent passage through it; this sovereignty also extends to the airspace over and seabed below.

"Innocent passage" is defined by the convention as passing through waters in an expeditious and continuous manner, which is not "prejudicial to the peace, good order or the security" of the coastal state. Fishing, polluting, weapons practice, and spying are not "innocent", and submarines and otherunderwater vehicles are required to navigate on the surface and to show their flag. Nations can also temporarily suspend innocent passage in specific areas of their territorial seas, if doing so, is essential for the protection of its security.

Contiguous Zone:

The contiguous zone is a band of water extending from the outer edge of the territorial sea to up to 24 nautical miles (44 km; 28 mi) from the baseline. In its contiguous zone, a coastal State may exercise the control necessary to prevent the infringement of its customs, fiscal, immigration or sanitary laws and regulations within its territory or territorial sea, and punish infringement of those laws and regulations committed within its territory or territorial sea. Additionally, in order to control traffic in archaeological and historical objects found at sea, a coastal State may presume that their removal from the seabed of the contiguous zone without its consent is unlawful.

Exclusive Economic Zone:

An exclusive economic zone extends from the outer limit of the territorial sea to a maximum of 200 nautical miles (370.4 km) from the territorial sea baseline, thus it includes the contiguous zone.

In the exclusive economic zone, the Union has- sovereign rights for the purpose of exploration, exploitation, conservation and management of the natural resources, both living and non-living as well as for producingenergy from tides, winds and currents; exclusive rights and jurisdiction for the construction, maintenance or operation of artificial islands, off-shore terminals, installations and other structures and devices necessary for the exploration and exploitation of the resources of the zone or for the convenience of shipping or for any other purpose; Exclusive jurisdiction to authorize, regulate and control scientific research; Exclusive jurisdiction to preserve and protect the marine environment and to prevent and control marine pollution.

No person (including a foreign Government) shall, except under, and in accordance with, the terms of any agreement with the Central Government or of a license or a letter of authority granted by the Central Government, explore or exploit any resources of the exclusive economic zone or carry out any search or excavation or conduct any research within the exclusive economic zone or drill therein or construct, maintain or operate any artificial island, off-shore terminal, installation or other structure or device therein for any purpose whatsoever. But it is not applicable to the citizens of the nation.

Continental Shelf:

Each coastal State has a continental shelf that is comprised of the seabed and subsoil of the submarine areas that extend beyond its territorial sea throughout the natural prolongation of its land territory to the outer edge of the continental margin, or to a distanceof 200 nm from its baselines where the outer edge of the continental margin does not extend up to that distance (or out to a maritime boundary with another coastal State).

Wherever the outer edge of a coastal State's continental margin extends beyond 200 nm from its baselines, it may establish the outer limit of its continental shelf in accordance with the UN Convention on the Law of the Sea. The portion of a coastal State's continental shelf that lies beyond the 200 nm limit is often called the extended continental shelf.

A coastal State has sovereign rights and exclusive jurisdiction over its continental shelf for the purpose of exploring it and exploiting its natural resources. The natural resources of the continental shelf consist of the mineral and other non-living resources of the seabed and subsoil together with living organisms belonging to sedentary species, that is to say, organisms which, at the harvestable stage, either are immobile on or under the seabed or are unable to move except in constant physical contact with the seabed or subsoil.

High Seas:

The high seas are comprised of all parts of the sea that are not included in the exclusive economic zone, in the territorial sea or in the internal waters of a State, or in the archipelagic waters of an archipelagic State.

CHAPTER – 4 SOILS

Soils constitute a major element in the natural environment, linking climate and vegetation, and they have a profound effect on man's activities through their relative fertility. The scientific study of soils is known as pedology; the process of soil formation is referred to as pedogenesis (soil genesis)

Soil is the upper weathered layer of the Earth's crust affected by plants and animals. A vertical section through this zone constitutes a soil profile; in each soil profile there are usually several distinguishable layers or horizons, which enable different types of soil to be recognized.

Soil contains matter in all three states: solid, liquid and gaseous. The solid portion is partly organic and partly inorganic. The inorganic, or mineral, part of the soil is made up of particles derived from the parent material, the rocks which weather to form the soil. The organic portion consists of living and decayed plant and animal materials such as roots and worms. The end- product of decay is humus, black amorphous organic matter. Soil water is a dilute but complex chemical solution derived from direct precipitation and from run-off, seepage, and groundwater. The soil atmosphere fills the pore spaces of the soil when these are not occupied by water.

The texture of a soil refers to the sizes of the solid particles composing the soil. The sizes range from gravel to clay. The proportions of the different sizes present vary from soil to soil and from layer to layer. Texture largely determines the water-retention properties of the soil. In a sandy soil, pore spaces are large and water drains rapidly: in a clay soil, the individual pore spaces are too small for adequate drainage. Generally speaking, loam textures are best for plant growth.

Soil acidity is a property related to the proportion of exchangeable hydrogen ion present in the soil in relation to other elements. The degree of acidity is measured on the logarithmic pH scale which ranges from 0 (extreme acidity) to 14 (extreme alkalinity). Few soils reach these limits; a pH value of about 6.5 is normally regarded as the most favorable for the growth of cereal crops.

Colour varies considerably in soils and can tell us much about how a soil is formed and what it is made up of. In recently formed soils, the colour will largely reflect that of the parent material, but in many other cases, the colour is different from the underlying rock. Soils can range from white to black, usually depending on the amount of humus. In cool humid areas, most soils contain relatively high humus content and are generally black or dark brown, whereas in desert or semi-desert areas, little humus is present and soils are light brown or grey. Reddish colors in soils are associated with the presence of ferric compounds, particularly the oxides and hydroxides, and usually indicate that the soil is well drained, although locally the colour may be derived from a red-coloured parent material.

I. Soil Horizons (Soil Profile)

Most soils have distinctive horizontal layers that differ in physical and chemical composition, organic content, or structure. We call these layers as soil horizons.

They develop through interactions among climate, living organisms, and the land surface, over time. Horizons usually develop either by selective removal or accumulation of certain ions, colloids, and chemical compounds. This removal or accumulation is normally produced by water seeping through the soil profile from the surface to deeper layers. Horizons often have different soil textures and colors .A soil profile, as shown in Figure, displays the horizons on a cross section through the soil.

There are two types of soil horizons: organic and mineral.

Organic horizons, marked with the capital letter O, lie over mineral horizons and are formed from plant and animal matter. The upper O_i horizon contains decomposing organic matter that you can easily recognize by eye, such as leaves or twigs. The lower O_a horizon contains humus, which has broken down beyond recognition.

There are four main mineral horizons. The A horizonis enriched with organic matter, washed downward from the organic horizons. Next is the E horizon. Clay particles and oxides of aluminum and iron are removed from the E horizonby downward-percolating water, leaving behind pure grains of sand or coarse silt.

The B horizonreceives the clay particles, aluminum, and iron oxides, as well as organic matter washed down from the A and E horizons. It‘s dense and tough because its natural spaces are filled with clays and oxides.

Beneath the B horizon is the C horizon. It consists of the parent mineral matter of the soil. Below this regolith lies bedrock or sediments of much older age than the soil. Soil scientists limit the term soil to the A, E, and B horizons, which plant roots can readily reach.

II. Soil Forming Processes

There are four classes of soil-forming processes: soil enrichment, removal, translocation, and transformation.

Soil Enrichment

In soil enrichment, matter—organic or inorganic—is added to the soil. Surface mineral enrichment of silt by river floods or as wind-blown dust is an example. Organic enrichment occurs as water carries humus from the O horizon into the A horizon below.

Removal Process

In removalprocesses, material is removed from the soil body. This occurs when erosion carries soil particles into streams and rivers. Leaching, the loss of soil compounds and minerals by solution in water flowing to lower levels is another important removal process.

Cheluviation is downward movement of materials in the soil which is very similar to leaching. However cheluviation occur through the influence of organic agents which are also referred to as chelating agents. The process involves plant acids rather than mere water as the case with leaching.

Translocation Process

Translocationdescribes the movement of materials upward or downward within the soil.

Downward Translocation: Fine particles particularly clays and colloids are translocated downward, a process called eluviation. This leaves behind grains of sand or coarse silt, forming the E horizon. Material brought downward from the E horizon—clay particles, humus, or sesquioxides of iron and aluminum—accumulates in the B horizon, a process called illuviation.

The topmost layer of the soil is a thin deposit of wind-blown silt and dunesand, which has augmented the soil profile. Humus, moving downward from decaying organic matter inthe O horizon, has enriched the A horizon, giving ita brownish color. Eluviation has removed colloids andsesquioxides from the whitened E horizon, and illuviation has added them to the B horizon, which displaysthe orange-red colors of iron sesquioxide.

The translocation of calcium carbonate is anotherimportant process. In moist climates, a large amount ofsurplus soil water moves downward to the groundwater zone. This water movement leaches calcium carbonatefrom the entire soil in a process called decalcification.Soils that have lost most of their calcium are also usually acidic, and so they are low in bases. Adding limeor pulverized limestone will not only correct the acidcondition, but will also restore the missing calcium, animportant plant nutrient.

In dry climates, annual precipitation is not sufficient toleach the carbonate out of the soil and into the groundwater below. Instead, it is carried down to the B horizon,where it is deposited as white grains, plates, or nodules, ina process called calcification. Calcification can produce acemented layer, known as a hard pan that interferes withboth eluviation and illuviation. This renders the soil lessfertile by preventing the exchange of nutrients.

In colder climates, a pan can also form from theaccumulation of oxides of iron and aluminum by illuviation. This type of pan can block drainage and keep thesoil saturated for long periods, resulting in chemicalreducing conditions.

Upward translocation can also occur in desert climates. In some low areas, a layer of groundwater lies close to the surface, producing a flat, poorly drained area. As water at or near the soil surface evaporates, groundwater is drawn upward to replace it by capillary tension, much like a cotton wick draws oil upward in an oil lamp. This groundwater is often rich in dissolved salts. When the salt-rich water evaporates, the salts are deposited and build up. This process is called salinization. Large amounts of these salts are toxic to many kinds of plants. When salinization occurs in irrigated lands in a desert climate, the soil can be ruined, with little hope of revival.

Transformation Process

The last class of soil-forming processes involves thetransformationof material within the soil body. An example is the conversion of minerals from primary to secondary types,

anotherexample is decomposition of organic matter by microorganisms to produce humus, a process termed ashumifcation. In warm moist climates, transformation of organicmatter to carbon dioxide and water can be nearly complete, leaving virtually no organic matter in the soil.

Factors Controlling Soil Formation

Climate

Climate, measured by precipitation and temperature, isan important determinant of soil properties. As we haveseen, precipitation controls the downward movementof nutrients and other chemical compounds in soils bytranslocation. If precipitation is high, water will washnutrients deeper into the soil and out of reach of plantroots. If precipitation is low, salts will build up in the soiland restrict fertility.

Soil temperature affects the chemical development ofsoils and the formation of horizons. Below 10°C,biological activities are slowed; and at or below the freezing point (0°C; 32°F), biological activity stops and chemical processes affecting minerals are inactive. Thus, decomposition is slow in cold climates, and so organic matter accumulates to form a thick O horizon. This materialbecomes humus, which is carried downward to enrich theA horizon. Incontrast, bacteria rapidly decompose plantmaterial in the warm, moistclimates of low latitudes. O horizons are lacking, and theentire soil profile will containvery little organic matter.

Organisms

Living plants and animals, as well as their nonlivingorganic products, have an important effect on soil.Plant roots, by their growth,mix and disturb the soil and provide organic material directly to upper soil horizons.

Organisms living in the soil include many species,from bacteria to burrowing mammals. Earthworms continually rework the soil not only byburrowing, but also by passing soil through their intestinal tracts. And moles, gophers,rabbits, badgers, prairie dogs, and other burrowing animals make larger, tube like openings.

Relief

The configuration, or shape, of the ground surface,known as relief, also influences soil formation. Generallyspeaking, soil horizons are thick on gentle slopes and thinon steep slopes. This is because the soil is more rapidlyremoved by erosion on the steeper slopes. In addition,slopes facing away from the Sun are sheltered from directinsolation and so tend to have cooler, moister soils. Slopesfacing toward the Sun are exposed to direct solar rays, raising soil temperatures and increasing evapotranspiration.

Parent Material

Soil chemistry is influenced by the original source ofparent material. For example, iron- rich bedrock produces soils rich in iron oxides, whereas limestone formscalcium-rich soils. Some types of secondary minerals,weathered from particular primary minerals, can produce soils with unique properties. Also, soil texture islargely determined by the size of mineral grains withinthe parent material.

Time

The characteristics and properties of soils requiretime for development. For example, a fresh deposit ofmineral matter, like the clean, sorted sand of a dune, may require hundreds to thousands of years to acquirethe structure and properties of a sandy soil. A soil scientist‘s rule of thumb is that it takes about 500years to form 2.5 cm (1 in.) of topsoil.

Human Activity

Human activity also influences the physical and chemical nature of the soil. Clearing of native vegetation forcrops can induce erosion, removing upper layers thatare rich in organic matter. Large areas of agriculturalsoils have been plowed and planted for centuries. As a result, both the structure and composition of theseagricultural soils have undergone great changes. Thesealtered soils are often recognized as distinct soil classesthat are just as important as natural soils.

III. THE CLASSIFICATION & DISTRIBUTION OF SOILS:

It is very difficult to achieve a classification of soils that is both meaningful to the geographer and at the same time an accurate reflection of all soil types and gradations.Two main types of classification used today may be recognized as those based on the assumed origins of the soil; and those based on the observable properties of the profile. Examples of each are given below.

ZONAL SYSTEM:

One of the most popular classifications of soils has been the zonal system.This was proposed many years ago by Russian pedologists (Dukuchaiev, Glinka) who recognized thestrong relationship between climate, vegetation and soil zones throughout theworld. Three main classes of soil are recognized.

1. Zonal soils are those that are well developed and reflect the influence of climate as the major soil-forming factor.

2. Intrazonal types are well-developed soils formed where some local factor is dominant.

3. Azonal soils are those that are immature or poorly developed.

World Pattern of Soil Zonal Types:

Podzols (ash-soil):

The effect of the cheluviation process is toproduce soils with a characteristic bleached E horizon. In some profiles, the humus is washed down the profile and accumulates as a humus- enriched B horizon, forming a humus podzol.In others, there is a marked concentration of iron oxide at this level, formingan iron podzol. Sometimes this takes the form of an iron-pan, impeding drainage, and resulting in a gley podzol. Podzols of these three types are most wide- spread in the cool climates immediately south of the tundra region, and arefound typically in association with coniferous forest.

Brown Earth:

These soils are found equator ward of the main podzol zonein milder climates supporting a deciduous forest cover. The soils still exhibitleaching, but of a far less intense nature than podzols. Although free calciumis absent from the upper part of the profile, there is no downward movementof sesquioxides, and their dispersed distribution gives rise to the overall brown colour of the soil. In addition, humus is well distributed throughout the profile and is less acidic than in podzols. Brown earths are widespread in Britain,except in the highland areas

Tundra Soils:

The great variations that exist in the patterns of ground ice inthe tundra cause equally complex variations in soils. Where slope conditionsare fairly stable, the slow rate of plant decomposition usually results in thepresence of a peaty layer at the soil surface. In areas of active

slope movement,soils are inevitably thin. In the most extreme conditions where there is noplant growth, the soils are ahumic. The brown polar desert soils of theAntarcticareof this nature. Bywayofcontrast, the birch-forested tundra margins in the northern hemisphere possess Arctic brown forest soils, characterisedby a thick dark organic A horizon.

Sierozems:

Sierozems of desertic and semi-desertic areas can be regarded as extreme forms of chestnut soils in which lime and gypsum come even nearer to the surface because of upward capillary attraction. Since most of the plants are adapted to arid conditions, there is little leaf fall, and organic matter in these soils is low. However, when irrigated, Sierozems can be very fertile, because of their high base status.

Chernozem, Chestnut and Prairie Soils:

The best examples of chernozemsand their variants are found in association with steppe or prairie vegetation. The light rainfall of these areas leads to incomplete leaching and the formation of a calcium-rich horizon deep in the profile. Above this is a deep dark layer of soil which can be up to a meter thick. The humus content of this layer is surprisingly often no more than ten per cent, the dark colour being associated with the base( alkaline )-rich mineral matrix. Chernozems have a well-developed crumb structure. The ideal parent material for this soil seems to be loess, which iswidespread in the mid-west of North America, Russia and northern China.

Chestnutsoils occur on the arid side of the Chernozem belt under a naturalvegetation of low grass-steppe. The illuvial carbonate layer is closer to thesurface than in chernozems and they have a lower organic content.

Prairie soils occupy the transition zone of increasing wetness between chernozems andforest brown earths

Grumusols:

These are dark clayey soils of savanna or grass-covered areas whichhave a warm climate with wet and dry seasons. There are no eluvial or illuvialhorizons, but the whole solum is rich in bases, especially calcium, and henceits dark colour. These soils are characterised by a high degree of dry-seasoncracking

Ferralsols:

Soils of intertropical areas are often referred to as lateritic, but strictly speaking, laterite is a weathering product and not a soil type.Most tropical soils are, however, rich in ferric oxide and are collectively knownas Ferralsols. The abundance of sesquioxides of iron and aluminium accountsfor the red, brown or occasionally yellow colour of the soil. The A horizon makesup the first meter of a typical profile, and is usually acidic with a low humuscontent. The B horizon commonly extends to fifteen meters or more and is predominantly clayey. Ferralsolic soils are low in fertility because of the lackof humus and bases.

Intrazonal Types:

Hydromorphic soils are those which have undergone gleying and are associated with marshes, swamps or poorly drained upland. Two main types can berecognized, according to the position of the water-table in the profile: groundwater gleys, where ground water is below the surface; and surface-water gleys.

Gleying is essentially the process of waterlogging and reduction in soils. In waterlogged soils where water replaces air in pores, oxygen is quickly used up by microbes feeding on soil organic matter.

Calcimorphic soils develop on calcareous parent material. Rendzinas aredark, organic rich, and are associated with chalk rock in Britain. AnotherCalcimorphic soil is terra rossa, which by contrast is a predominantly mineralsoil and is found mainly in the Mediterranean region. The upper horizons arerich in clay and reddish in colour, sharply contrasting with the parent material.

Halomorphic (saline) soils are mostly found in deserts. There are threecommon types in this group.

· Solanchak (white alkali soils) develop in depressions and exhibit white salt crusts in dry periods.

· Solonetz (black alkali soils) are the product of intense alkalinisation and are characterised by the presence of sodium carbonate.

· Solodic soils develop when leaching in the presence of excess sodium causes the loss of clays and sesquioxides, forming a bleached, eluviated horizon looking rather like a podzol.

Azonal Soils:

Immature soils may exist because of the characteristics of the parent materialor the nature of the terrain, or simply the lack of time for development. Suchsituations typically occur in areas where fresh parent material is being deposited or exposed.

For example, on active flood-plains, alluvial soils have little orno profile development, because of their frequent burial under new sediments;Regosols are composed of dry and loose dune sands or loess. Lithosols areaccumulations of imperfectly weathered rock fragments on steep slopes whereerosion rates remove soil almost as fast as it is formed.

A number of criticisms have been leveled against the zonal concept.

· One is that the zonal soil type of one climate may well be found in another. For example, podzols, normally recognized as the zonal soil type of cool continental climates, also occur in maritime areas and in the tropics.

· Another difficulty concerns the azonal class: Azonal soils are not necessarily a reflection of the lack of time for development, but may be a result of local factors that have arrested soil development over a long period.

· A third point is that soil profiles do not always reflect the prevailing climate, and may have characteristics inherited from previous climates

USDA SOIL TAXONOMY:

In recent years, the US Department of Agriculture has adopted a system of soil classification based on observed soil properties rather than genetic considerations.For thisanalysis, we will group thesoil orders based on fourfactors that can characterizea particular order: maturity, climate, parent material,and high organic matter.

Soils Characterized By Maturity:

Where materials have been recently deposited soils have no horizons or poorly developed horizons and are capable of further mineral alteration.

Entisols and Inceptisols

1. Entisolsare mineral soils without distinct horizons. They are soils in the sense that they support plants, and they may be found in any climate and under any vegetation.Entisols lack horizons,often because they areonly recently deposited.They may occur in anyclimate or region.

2. Inceptisolsare soils with weakly developed horizons, usually because the soil is quite young.Inceptisols have onlyweakly developedhorizons. Inceptisols ofriver floodplains and deltas are often veryproductive.

Entisolsand Inceptisols can be found anywhere from equatorial to arctic latitude zones. Entisols and Inceptisols of floodplains and deltaplains in warm and moist climates are among the most highly productive agricultural soils in the worldbecause of their favorable texture, ample nutrientcontent, and large soil-water storage.

Alfisols and Spodosols

1. The Alfisolsare soils characterized by a clay-rich horizon produced by illuviation and a high base status. The world distribution of Alfisols is extremely wide in latitude, ranging from as high as 60° N in North America and Eurasia to the equatorial zone in South America and AfricaBecause the Alfisols span an enormous range in climate types, four important suborders of Alf sols, each with its own climate affiliation

· Boralfsare Alfisols of cold (boreal) forest lands of North America and Eurasia. They have a gray surface horizon and brownish subsoil.

· Udalfsare brownish Alfisols of the mid latitude zone.

· Ustalfsare brownish to reddish Alfisols of the warmer climates.

· Xeralfsare Alfisols of the Mediterranean climate, with its cool moist winter and dry summer. The Xeralfs are typically brownish or reddish in color.

2. Spodosols have a light-colored albic horizon of eluviation, and a dense spodic horizon of illuviation. They develop under cold needle leaf forests and are quite acidic.Spodosols are closely associated with regions recently covered by the great ice sheets of the Late Cenozoic Ice Age.Spodosols are naturally poor soils in terms ofagricultural productivity. Because they are acidic, limeapplication is essential.

Oxisols and Ultisols

· Oxisolshave developed in the moist climates of the equatorial, tropical, and subtropical zones on land surfaces that have been stable over long periods of time.We find these soils over vast areas of South America and Africa in the wet equatorial climate,where the native vegetation is rainforest.

· Ultisolsare similar to the Oxisols, but have a subsurface clay horizon. They originate in closely related environments.We find Ultisols throughout Southeast Asia and the East Indies. Other important areas are in eastern Australia, Central America, South America, and the southeastern United States.Ultisols are also vulnerable to devastating soil erosion, particularly on steep hill slopes.

Soils Characterized By Climate

Mollisols:

Mollisols are soils of grasslands in sub humid to semiarid climates. They have a thick, dark brown surface layer, termed a mollic epipedon. Because of their loose texture and high base status, they are highly productive. In North America, Mollisols dominate the Great Plains region, the Columbia Plateau, and the northern Great Basin. In South America, a large area of Mollisols covers the Pampa region of Argentina and Uruguay. In Eurasia, a great belt of Mollisols stretches from Romania eastward across the steppes of Russia, Siberia, and Mongolia.

Aridisols:

Aridisols are desert soils with weakly developed horizons. They often exhibit subsurface layers composed of an accumulation of calcium carbonate or soluble salts. With irrigation and proper management, they are quite fertile.The Aridisols are closely correlated with the arid subtypesof the dry tropical climate, dry subtropical climate, and dry mid-latitude climate.

Gelisols:

Gelisols are soils of permafrost regions that are churned by freeze/thaw ice action.They usually consist of very recent parent material, left behind by glacial activity during the Ice Age, along with organic matter that decays slowly at low temperatures.

Soils Characterized By Parent Materials

Vertisols:

Vertisols develop on certain types of volcanic rock in wet-dry climates under grassland and savanna vegetation. They expand and contract with wetting and drying, creating deep cracks in the soil.They are black in color and have a high content of the clay mineral montmorillonite, which is formed from the weathering of particular volcanic rocks.An important region of Vertisols is the Deccan Plateau of western India, where basalt, a dark variety of igneous rock, supplies the silicate minerals that are altered into the necessary clay minerals.

Andisols:

Andisols are unique soils that form on volcanic ash of relatively recent origin. They are dark in color and typically fertile.In moist climates they support a dense natural vegetation cover; they form over a wide range of latitudes and climates.

Soils High In Organic Matter

Histosols:

Histosols are organic soils, often termed peats or mucks. They are typically formed in cool or cold climates in areas of poor drainage. Throughout the northern regions of Spodosols are countless patches of Histosols. This unique soil order has a very high content of organic matter in a thick, dark upper layer.

IV. Importance of Soils

Soils are essential for life, in the sense that they provide the medium for plant growth, habitat for many insects and other organisms, act as a filtration system for surface water, carbon store and maintenance of atmospheric gases. Let us take a closer look at each of these:

Medium for plant growth:

v Soils support roots and keep them upright for growth.

v Soils provide plants with essential minerals and nutrients.

v Soils provide air for gaseous exchange between roots and atmosphere.

v Soils protect plants from erosion and other destructive physical, biological and chemical activity.

v Soils hold water (moisture) and maintain adequate aeration.

Habitat for many insects and other organisms:

v soils are habitat for animals

v Insects and microbes (very tiny single-cell organisms) live in the soils and depend on soils for food and air.

v Soils are homes to a diverse range of organisms such as worms and termites. They provide the needed moisture and air for the breakdown of organic matter. (learn more about soil ecosystem)

v They provide a home for many organisms such as insects to lay and hatch eggs and rodents to give birth to new offsprings.

A Filtration system for surface water:

After rainfall and snowmelts, water flows on the earth‘s surface to water bodies, but much of it soaks and gets infiltrated into the ground. As it continues its way downwards through the many layers in the ground, it is filtered from dust, chemicals and other contaminants. This is why aquifers (underground water) are one of the purest sources of water. Filtered water also provides plants with clean, unpolluted water needed for growth.

Carbon store and maintenance of atmospheric gases:

Soils help regulate atmospheric Carbon dioxide (co2) by acting as a carbon store. During humification (a process where soil organisms form complex and stable organic matter) some organic matter breakdown does not occur completely, especially in soils like peat, owing to its high acid and water content.

On a global scale, soils contain about twice as much carbon as the atmosphere and about three times as much as vegetation. This results in the accumulation of organic matter in the soil which is high in carbon content. Nitrogen, phosphorus, and many other nutrients are stored, transformed, and cycled in the soil.

CHAPTER – 5 NATURAL VEGETATION

Natural vegetation means the plants that have not been grown by humans. It doesn‘t need help from humans and gets whatever it needs from its natural environment. There is a close relationship between height of land and the character of vegetations. With the change in height, the climate changes and that changes natural vegetation. The growth of vegetation depends on temperature and moisture. It also depends on factors like slope and thickness of soil.

I. Tropical Rain/ Evergreen Forest

The tropical rain-forest occupies low-altitude areas near the equator in South America, Central and West Africa, and in the Indo-Malay peninsula and New Guinea regions. Although these areas are physically isolated, the forest growing in them shows great similarity of structure and function.These are found in the high rain fall areas on either side of the equator, having high temperature and high humidity and receive above 200 cm of rainfall per year. Soil is rich in humus.

It is a broad-leaved evergreen forest of dense, prolific growth and an extremely diverse fauna and flora. The hot, wet tropical climate is highly conducive to plant growth and there is very little seasonality which means that the growing period extends throughout the year.

All green plants strive to reach the light so that they either become very tall, or adopt a climbing habit or live as epiphytes (plants living on other plants but not deriving food from them). The dominant trees are extremely varied in species but have similar appearances, typically characterised by buttress roots, dark leaves and a thin bark. The leaves possess thick cuticles for protection against the strong sunlight, and drip tips whose probable function is to shed water rapidly, thereby aiding transpiration.

These forests have a very rich biodiversity e.g. Brazilian tropical rain forests have more than 300 species of trees in an area of 200 square kilometer. Trees are tall growing up to 50 to 60 m. These forests also support epiphytes, like vines, creepers, woody creepers and orchid etc. These forests are rich in tree dwelling animals such as monkeys, flying squirrels, snails, centipedes, millipedes, and many insect species are common on the forest floor.Many snakes and mammals are adapted to live in the trees because this is where the bulk of the foliage exists.

Providing the tropical rain-forest is undisturbed it is the most diverse and productive type of forest ecosystem, but if the canopy is depleted the soils soon become infertile. Nutrient cycling is rapid, as the vegetation is demanding, and decomposition is accomplished quickly by bacterial action.

II. Temperate Deciduous Forests

This type of forest, dominated by broad-leaved deciduous trees, had a great extent in the past when it covered most of the temperate areas of Europe, eastern North America, eastern Asia and small parts of South America and Australia.The temperate deciduous forest has probably been more modified by human activity than any other type of ecosystem.

Temperate deciduous forest consists largely of trees that drop their leaves during the cold season. It is characteristic of the marine west-coast and moist continental climates.

There is a longer growing season, higher light intensity and a moderate amount of precipitation of between 50 and 150 cm per annum. The temperature regime is also characterised by lack of extremes but there is still a marked cold season which plants and animals must endure.The climatic zone it occupies is less extreme than that of the boreal forest.

Trees common to the deciduous forest of eastern North America, southeastern Europe, and eastern Asia are oak, beech, birch, hickory, walnut, maple, elm, and ash. Where the deciduous forests have been cleared by lumbering, pines readily develop as second-growth forest.

In Western Europe, the mid-latitude deciduous forest is associated with the marine west- coast climate. Here, the dominant trees are mostly oak and ash, with beech found in cooler and moister areas. In Asia, the mid-latitude deciduous forest occurs as a belt between the boreal forest to the north and steppe lands to the south. A small area of deciduous forest is found in Patagonia, near the southern tip of South America.

Larger amounts of nutrients are used and their movement is more rapid. There is a bulk return of nutrients from the trees with the leaf fall of autumn. Characteristically the leaf litter is nutrient-rich and decays by the action of bacteria to form mull humus. The soils associated with the temperate deciduous forest are varied but on the whole they are brown earths.

III. Boreal Forestor North Coniferous forests:

Boreal forest is the cold-climate needle leaf forest of high latitudes .It occurs in two great continental belts, one in North America and one in Eurasia. These belts span their land masses from west to east in latitudes 45° N to 75° N and they closely correspond to the region of boreal forest climate.

The area occupied by this formation has been subjected to severe glacial or periglacial activity and has much subdued relief and surface water. The conditions for life are harsh

because of the adverse climate. The growing season is only of three or four months' duration and even during this time; the energy input from solar radiation is small because of the high latitude. Temperatures are low throughout the year, although the average temperature of the warmest month of the year is higher than 10° C. In the winter the temperatures fall too many degrees below freezing and permafrost frequently extends into the northern edge of the forest. Precipitation ranges from 40 to 70 cm per annum, mostly falling as snow, the weight of which may cause mechanical damage to the trees.

Despite the climate, coniferous trees forms dense canopies which intercept a great amount of light and precipitation so that conditions beneath are dark and dry. Consequently there is little opportunity for undergrowth to develop and very few other plants are associated with the coniferous trees.

The boreal forest of North America, Europe, and western Siberia is composed of such evergreen conifers as spruce and fir, while the boreal forest of north-central and eastern Siberia is dominated by larch. The larch tree sheds its needles in winter and is thus a deciduous needle leaf tree.

The combination of coniferous dominants which are low in nutrient demand, the lack of diversity, and the climatic conditions, results in slow, impoverished nutrient cycles.Most decomposition is fungal since bacterial activity will be slow in these conditions, and the resulting humus is the mor type. Characteristically the boreal forest is found growing on podzols which tend to become highly acidic.

IV. Tropical Deciduous or Monsoon Forest:

Monsoon forest, also called dry forest or tropical deciduous forest. It is typically open, but grades into woodland, with open areas occupied by shrubs and grasses .Monsoon forest of the tropical latitude zone differs from tropical rainforest in that it is deciduous; that is, most of the trees of the monsoon forest shed theirleaves due to stress during the long dry season, which occurs at the time of low Sun and cool temperatures.

This forest develops in the wet-dry tropical climate, where a long rainy season alternates with a dry, rather cool season. They are located in the monsoon climate beyond the equatorial region between 10◦ and 25 ◦ and North and South of the equator. The countries are along the coastal regions of southwest India, Sri Lanka, Bangladesh, Myanmar, Thailand, and Cambodia, South western Africa, French Guiana, and northeast and south- eastern Brazil.

In the monsoon forest of southern Asia, the teakwood tree was once abundant, but it was cut down and the wood widely exported to the Western world to make furniture, paneling, and decking.

V. Temperate Grasslands

These include the prairies of North America, the steppes of Eurasia, the pampas of South America, and the veldt of South Africa. Smaller tracts occur in Australia and New Zealand. Precipitation in these areas ranges from 25 to 100 Cm per annum, and the grasslands extend over a wide range of soil conditions. Trees only occur on steep slopes or near water. The geographical isolation of these areas from each other has led to some species differentiation, but most other features are similar.

The animals of the grassland are distinctive, and feature many grazing mammals. The grassland ecosystem supports some rather unique adaptations to life .Animals such as jackrabbits and jumping mice have learned to jump or leap, to gain an unimpeded view of their surroundings.

Tall grass prairie is a ground cover of tall grasses along with some broad-leafed herbs, named forbs. Steppe, or short-grass prairie, consists of sparse clumps of short grasses. Steppe grades into semi desert in dry environments and into prairie where rainfall is higher.

Steppe grassland is concentrated largely in the mid-latitude areas of North America and Eurasia.

Prairie grasslands are associated with the drier areas of moist continental climate, and steppe grasslands correspond well with the semiarid subtype of the dry continental climate.The Pampa region falls into the moist subtropical climate with mild winters and abundant precipitation.

This grassland biome includes tall-grass and short-grass prairie (steppe). Tall-grass prairie provides rich agricultural land suited to cultivation and cropping. Short-grass prairie occupies vast regions of semi desert and is suited to grazing.

VI. Tropical Grasslands (Savannas):

Tropical grasslands are commonly called Savannas. They occur in eastern Africa, South America, Australia and India. Savannas form a complex ecosystem with scattered medium size trees in grass lands.

The savanna biome is usually associated with the tropical wet-dry climate of Africa and South America. Its vegetation ranges from woodland to grassland. In savanna woodland, the trees are spaced rather widely apart because there is not enough soil moisture during the dry season to support a full tree cover.The woodland has an open, park like appearance. Savanna woodland usually lies in a broad belt adjacent to equatorial rainforest.

Savanna biome vegetation is described as rain-green.Fires occur frequently in the savanna woodland during the dry season, but the tree species are particularly resistant to fire. The much greater diversity of tropical as opposed to temperate grasslands is often a function of the added variety afforded by wooded plants.In some cases the tree cover may be as much as 50 per cent; in others it may be nil.Marked contrasts exist in the appearance of the savanna during the year: the brown and withered short grasses of the dry season give way rapidly to tall lush growth with the arrival of the summer rains.The Ferralsolic soils of savanna areas frequently include near-surface lateritic crusts, creating an impermeable surface soil layer in which nutrients, especially phosphates and nitrates, are markedly lacking.

As in the case of prairies, tropical grasslands tend to show little ecotone development, especially on margins adjacent to tropical rain-forest. Overall, savanna boundaries on all continents reveal only poor correlation with precipitation amounts or the duration of the rainy season.

The African savanna is widely known for the diversity of its large grazing mammals. With these grazers come a large variety of predators—lions, leopards, cheetahs, hyenas, and jackals. Elephants are the largest animals of the savanna and adjacent woodland regions.

VII. Deserts

The desert is a highly evolved ecosystem that supports a multitude of plants and animals.The desert biome includes semi desert and dry desert and occupies the tropical, subtropical, and mid- latitude dry climates. Desert plants vary widely in appearance and in adaptation to the dry environment.Deserts are hot and low rain areas suffering from water shortage and high wind velocity.Annual rain fall is very little. It may be less than 25 cm per annum. At some places if it is high it is unevenly distributed. They show extremes of temperature. Globally deserts occupy about 1/7thof the earth‘s surface.

The desert biome includes several formation classes that are transitional from grassland and savanna biomes intovegetation of the arid desert.

Tropical Desert

v These are mostly situated between 15º -30º N and S on the western sides of land masses.

v The chief regions are: Sahara (North Africa), Arabia, parts of Iran, Iraq, Syria, Jordan and Israel, parts of Pakistan, Central Australia, Namib Desert (South West Africa), Atacama (coastal Peru and North Chile).

v The most common plants are cacti, thorn bushes and coarse grasses.

Mid-latitude Deserts

v These are situated in the interior of Asia and North America between 30⁰ and 45⁰ latitudes.

v Aridity and a great annual temperature with extremes of winter cold mark the region.

v In North America these deserts are found in basins surrounded by the Rockies.

v In South America the Patagonia desert lying to the east of the Andes is a typical example.

Desert plants around the world look very different from each other. In the Mojave and Sonoran deserts of the southwestern United States, for example, plants are often large, giving the appearance of woodland.

Desert animals are insects, reptiles, and burrowing rodents. Desert shrew, fox, kangaroo, wood rat, rabbit; armadillo are common mammals in desert. Camel is known as the ship of the desert as it can travel long distances without drinking water for several days.

Adaptations: Desert plants are hot and dry conditions.

(i) These plants conserve water by following methods:

· They are mostly shrubs.

· Leaves absent or reduced in size.

· Leaves and stem are succulent and water storing.

· In some plants even the stem contains chlorophyll for photosynthesis.

· Root system well developed spread over large area.

(ii) The animals are physiologically and behaviorally adapted to desert conditions.

· They are fast runners.

· They are nocturnal in habit to avoid the sun‘s heat during day time.

· They conserve water by excreting concentrated urine.

· Animals and birds usually have long legs to keep the body away from the hot ground.

· Lizards are mostly insectivorous and can live without drinking water for several days.

· Herbivorous animals get sufficient water from the seeds which they eat.

VIII. Tundra

The word tundra means a ―barren land‖ since they are found in those regions of the world where environmental conditions are very severe. There are two types of tundra- arctic and alpine.

ü Arctic tundra extends as a continuous belt below the polar ice cap and above the tree line on the northern hemisphere. It occupies the northern fringe of Canada Alaska, European Russia, and Siberia and island group of Arctic Ocean.

ü Alpine tundra occurs at high mountain peaks above the tree line. Since mountains are found at all latitudes therefore alpine tundra show day and night temperature variations

Permanently frozen subsoil called permafrost is found in the arctic and Antarctic tundra. The summer temperature may be around 15°C and in winter it may be as low as –57°C in arctic

tundra .A very low precipitation of less than 400 mm per year .A short vegetation period of generally less than 50 days between spring and autumn frost. Productivity is low.

Typical vegetation of arctic tundra is cotton grass, sedges, dwarf heath, willows birches, and lichens. Animals of tundra are hurepian reindeer, musk ox, arctic hare, caribous, lemmings and squirrel. Their body is covered with fur for insulation; Insects have short life cycles which are completed during favourable period of the year.

Most of them have long life e.g. Salix arctica that is arctic willow has a life span of 150 to 300 years. They are protected from chill by the presence of thick cuticle and epidermal hair. Mammals of the tundra region have large body size and small tail and ear to avoid the loss of heat from the surface.

CHAPTER – 6 POPULATIONS

The people of a country are its real wealth. Itis they who make use of the country‘s resourcesand decide its policies. Ultimately a country isknown by its people.

I. World‟s Population Growth Pattern

200 years ago there were less than one billion humans living on earth. Today, according to UN calculations there are over 7 billion of us. This is the most conspicuous fact about world population growth: for thousands of years, the population grew only slowly but in recent centuries, it has jumped dramatically. Between 1900 and 2000, the increase in world population was three times greater than during the entire previous history of humanity—an increase from

1.5 to 6.1 billion in just 100 years.

A Picture of the world population in the very long-run shows extremely rapid growth,

Indeed, for a long time the world population grew at an increasing rate. However, if we focus on the last couple of decades, we see that this pattern no longer holds, as the annual rate of population growth has been recently going down. 1962 saw the growth rate peak at 2.1%, and it has since fallen to almost half. A long historical period of accelerated growth has thus come to an end.

Based on observations, world history can be divided into three periods marked by distinct trends in population growth.

· The first period, pre-modernity, was a very long age of very slow population growth.

· The second period, beginning with the onset of modernity—which was characterized by rising standards of living and improving health—had an increasing growth rate that continued to rise through 1962.

· Today, the second period is over, and the third period is unfolding; the population growth rate is falling and will likely continue to fall, leading to an end of population growth towards the end of this century. The world's current (overall as well as natural) growth rate is about 1.14%, representing a doubling time of 61 years.

The chart below shows the increasing number of people living on our planet over the last 12,000 years. A mind boggling change: The world population today that is 1,860-times the size of what it was 12 millennia ago when the world population was around 4 million or half of the current population of London.

What is striking about this chart is of course that almost the entire action happened just very recently. Historical demographers estimate that around the year 1800 the world population was only around 1 billion people. This implies that on average the population grew very slowly over this long time from 10,000 BCE to 1700 (by 0.04% annually).

After 1800 this changed fundamentally: The world population was around 1 billion in the year 1800 and increased 7-fold since then. For the long period from the appearance of modern Homo sapiens up to the starting point of this chart in 10,000 BCE it is estimated that the total

world population was often well under one million. In this period our species was often seriously threatened by extinction.

In the past the population grew slowly: it took nearly seven centuries for the population to double from 0.25 billion (in the early 9th century) to 0.5 billion in the middle of the 16th century. As the growth rate slowly climbed, the population doubling time fell but remained in the order of centuries into the first half of the 20th century. Things sped up considerably in the middle of the 20th century. The fastest doubling of the world population happened between 1950 and 1987: a doubling from 2.5 to 5 billion people in just 37 years — the population doubled within a little more than one generation. This period was marked by a peak population growth of 2.1% in 1962.

Since then, population growth has been slowing, and along with it the doubling time. In this visualization we have used the UN projections to show how the doubling time is projected to change until the end of this century. By 2088, it will once again have taken nearly 100 years for the population to double to a predicted 11 billion.

Negative Growth Rates

Most European countries have low growth rates. In the United Kingdom, the rate is 0.2%, in Germany, its 0.0%, and in France, 0.4%. Germany's zero rate of growth includes a natural increase of -0.2%. Without immigration, Germany would be shrinking, like the Czech Republic.

The Czech Republic and some other European countries' growth rate is actually negative (on average, women in the Czech Republic give birth to 1.2 children, which is below the 2.1 needed to yield zero population growth). The Czech Republic's natural growth rate of -0.1 cannot be used to determine doubling time because the population is actually shrinking in size.

High Growth Rates

Many Asian and African countries have high growth rates. Afghanistan has a current growth rate of 4.8%, representing a doubling time of 14.5 years. If Afghanistan's growth rate remains the same (which is very unlikely and the country's projected growth rate for 2025 is a mere 2.3%), then the population of 30 million would become 60 million in 2020, 120 million in 2035, 280 million in 2049, 560 million in 2064, and 1.12 billion in 2078. This is a ridiculous expectation. Increased population growth generally represents problems for a country - it means increased need for food, infrastructure, and services. These are expenses that most high-growth countries have little ability to provide today, let alone if the population rises dramatically.

II. Factors Affecting Population Growth

Birth Rate

It is the average number of the children born in a country compared to the rest of the population. In other words, it is the number of births for every 1000 people in the country. Increase in population is directly related to birth rate of the country.

Factors affecting birth rate of a country

a) Existing age-sex structure – More young people starting married life.

b) Availability and awareness among people about family planning.

c) Religious beliefs – there are many aspects to it. Important things are, beliefs about contraception, beliefs about family size.

d) Female employment – We can see that more and more women are opting to work for some time before getting married and even after getting married, their career commitments may reduce the number of children they may want to have.

e) Economic prosperity – Even though richer families can afford to have more children, their style of life may actually reduce the number of children they want to have.

f) Poverty Levels – Even though bringing up children could cost more, a poor family may opt to have more children as they it could give more working hands for the family later.

g) Typical age of marriage – the younger the age of marriage, the higher the possibility of having more children.

h) Infant mortality rate – if IMR is high, birth rate will be high as some children are expected to die young.

Death Rate

Death rate is the number of people who die every year compared to 1000 people in the population.

Factors affecting death rate of a country

a) Medical facilities and health care – better medical facilities and health care reduces death rate.

b) Nutrition levels

c) Living standard

d) Access to clean drinking water

e) Hygiene levels

f) Levels of infectious diseases

g) Social factors such as conflicts and levels of violent crime

Fertility Rate

Fertility rate of a population is the average number of children born to a woman in her life time. If fertility is high, population will increase.

Net Migration

Migration is the movement of people in to and out of the country. Net migration is the difference between the immigration (people moving in to the country) and emigration (people moving out of the country). A positive net immigration will increase the population, while the opposite decreases the population.

Why do some countries have higher population growth than others?

Given above are the main factors that affect population growth. However, there are specific factors that we can discuss.

· Most of the countries restrict immigration and emigration: Immigration is restricted by countries for many reasons. The most important reason is that countries want to encourage skilled workers rather than unskilled workers. It is often important to restrict immigration

to protect national social balance. Countries may also want to restrict emigration to prevent brain-drain (a situation of skilled workers leaving the country).

· Some countries discourage child birth: The most noticeable example of this is the one-child policy in China. Even though this could help reduce population, it also created many undesirable results.

III. Impacts of Population Growth

Although it may be difficult to measure the carrying capacity for humans on earth scientists have estimated the carrying capacity at around 7.7 billion people. It is now estimated that the world population will be around 9.1 billion by the year 2050. The very high population growth has raised concerns that the planet may not be able to sustain such population in the long run. Increasing population will mean increased demand for food, water, and other resources such as fossil fuel. The impact of population growth can be seen by everyone who cares for the world that we live in. Over the last few decades there has been large scale destruction of the tropical forests mainly to make land available for agriculture and for urbanization. In order to produce enough food to meet the demand of growing population, forests have been cleared to undertake farming. Due to increased industrialization and urbanization, there has been great increase in the pollution of air, water and the environment of the planet. Growing population will result in the depletion of natural resources such as water, fossil fuels; deforestation and loss of ecosystems; and emergence of new diseases. It will also lead to more starvation, hunger and unhygienic living conditions in poor countries.

Relationship between Environment and Population growth

Humans are an integral part of the eco-system of nature and there is close interconnection between human beings and environment. Ever since life existed humans have been depending on their environment for food, shelter, and other necessities. There is an inverse relationship existing between population growth and environment as overpopulation will lead to adverse effect on the environment. As human population increase, there is also increase in the demand for food and other energy sources. It is essential that the population is maintained at a level so that the natural resources are sufficient to meet the requirement for survival of all living beings.

IV. Density and Distribution of Population

Population distribution refers to the way in which the members of a population or of a specified subgroup of a population (for example, defined by age, sex, or ethnic status) are dispersed physically in a specific area. Population density provides a comparative measure of distribution with respect to a geographic area that usually is expressed as persons per square kilometer (or per square mile) of land.

The Distribution of the World's Population

Population distribution on a global scale is highly uneven, with the greater part of the world's population living in the northern hemisphere and in countries in the less developed world. Less than 10 percent of the world's population lives in the southern hemisphere, and 80 percent lives between 20 degrees and 60 degrees north latitude.

Sixty percent of the world‘s people live in Asia (4.5 billion), 17 per cent in Africa (1.3 billion), 10 per cent in Europe (742 million), 9 per cent in Latin America and the Caribbean (646 million), and the remaining 6 per cent in Northern America (361 million) and Oceania (41 million).

China (1.4 billion) and India (1.3 billion) remain the two most populous countries of the world, comprising 19 and 18 per cent of the global total, respectively.

The main factors responsible for unequal distribution of the world population are as under:

In many parts of the world, the density of population is high while in others it is very low.

1. Fertile Land: Human settlements on large scale are at places which are more fertile and where with less effort the agricultural yield is more. On the other hand, places which are less fertile have less density of population. As such, hilly and rocky regions are less densely populated. In India, since ancient times, the fertile Indo-Gangetic plain is one of the most densely populated regions in the world.

2. Availability of Mineral Wealth: Every nation possesses some type of mineral wealth. As a matter of fact, we can gauge the inequality between nations on the basis of their mineral wealth. The regions of mineral wealth become a major source of industrialization and employment of a country.They attract people from far and near. At such places, the density of population is high due to centralization of industries. The main reason for high density of Europe is the availability of mineral wealth, because chances of employment increase, more industries are established and cities develop.

3. Availability of Water Supply: Water is essential for life. The places which have abundant water for daily consumption and for agricultural purposes are moderately to densely populated. In the present era, even industries are established at places which have sufficient supply of water.In the past, human civilization originated on the banks of large rivers. The Indus and Nile valley civilizations originated and developed along the banks of mighty rivers. Ancient cities originated and developed along river banks. On the other hand, due to scarcity of water and harsh climate, there are less human dwellings in Arabia.

4. Increased facility of Transportation and Communication: Since ancient times areas with proper transportation and communication facilities have enjoyed large population. Means of transportation and communication make the economy dynamic and increase the exchange of commodities between peoples and regions. Goods produced in one region can be made available to the whole country in a very short period of time due to fast means of transportation.

At places where transportation costs are low, we find localization and centralization of industries and concentration of population. Consequently, the density of population increases. In ancient times, the main means of transportation were waterways.

Hence human population was centred at the banks of large rivers where cities developed. On the other hand, in hilly areas and desert regions, where transportation is difficult, human dwellings are few and far in between.

5. Availability of Electric Supply: Today, nearly all the countries are industrialized. Agriculture-based economy has given way to industrial economy. Ready and sufficient supply of electricity is an essential feature in industrial development. Thus, places where industries are centralised develop into industrial regions with high density of population, and the demand for electric power increases.

Many cities in India are familiar with this process. Due to industrial development, the previously small regions like Durgapur, Jamshedpur and Bhillai, etc. have become large industrial areas, where the density of population keeps on increasing. This has become possible due to the availability of power supply among other factors.

6. Favourable Climate: From the beginning of human civilization, favourable climate is considered as an important factor responsible for human habitation. The density is nearly zero in places which are extremely hot or extremely cold. The parts which have moderate climate are inhabited on a large scale.

7. Social Factors: Social factors also affect the distribution and concentration of population. Where social customs and taboos are extreme, people tend to migrate to another place, so the density of population of that area changes. If the cultural milieu is the same, people like to live permanently at that place. The African population is increasing in America because Negroes were settled there since the slavery era.

8. Migration of Population: Every time migration of population takes place, it has an important effect on the distribution of population. The places with more opportunities of employment or possibilities of high income generation attract people from different areas. In the past, people migrated in large numbers to America, and the present day migration to Middle Eastern countries are good examples of migration due to better employment and income generation opportunities.

9. Educational Factors: In each part of the country, there remains a class which has more curiosity to gain knowledge. This class moves from one country to another according to its own economic conditions and circumstances. If the environment of the new country is adaptable to them, they tend to settle there permanently. At present, lakhs of youths from Asia and Africa have migrated to America, Canada and European countries for higher studies and after getting employment, they have settled there permanently.

10. Political Factors: For changes in the world‘s population distribution, political factors are also responsible. In modern times, these factors have become more important. If the people feel that their expectations are not fulfilled by the government, or if the public is dissatisfied with the political system, then they leave that country and settle in another. In the past, such people have migrated to USA from Cuba. Similarly, the Tibetans with their head Dalai Lama migrated to India and settled here permanently.

11. Historical Factors: It is a well-known fact that humans never migrate easily to a new place. Once a person settles well at a certain place, he will never leave that area to settle in a new area. People like to settle at places where their ancestors have lived in the past. Sons and grandsons from generation to generation live at a place which they do not leave so easily.

12. Economic Factors: This has been considered an important factor affecting the distribution and concentration of population at one place. The places which have good opportunities for employment and possibilities of trade in industrial products attract migrants from different areas.

As the migration takes place on a large scale, the density of population increases at the migrated place. Generally, during the process of economic development, migration of population takes place from the villages to the cities. People are attracted by employment opportunities in cities. Thus the population of cities increases rapidly, and so does the density of population.

V. Demographic Attributes

Population attributes are characteristics of the population such as sex composition, age structure, rural urban composition, literacy, occupational structure, etc.

Sex Composition

The number of women and men in a country is an important demographic characteristic. The ratio between the number of women and men in the population is called the Sex Ratio. In some countries it is calculated by using the formula:

Or the number of males per thousand females

In India, the sex ratio is worked out using the formula: or the number

of females per thousand males.

The sex ratio is important information about the status of women in a country.

In regions where gender discrimination is rampant, the sex ratio is bound to be unfavourable to women. Such areas are those where the practice of female foeticide, female infanticide and domestic violence against women are prevalent. One of the reasons could be lower socio-economic status of women in these areas. You must remember that more women in the population does not mean they have a better status. It could be that the men might have migrated to other areas for employment.

The world pattern of sex ratio does not exhibit variations in the developed regions of the world. The sex ratio is favourable for females in 139 countries of the world and unfavourable for them in the remaining countries listed by the United Nations.

In general, Asia has a low sex ratio. Countries like China, India, Saudi Arabia, Pakistan, and Afghanistan have a lower sex ratio. On the other extreme is greater part of Europe (including Russia) where males are in minority. A deficit of males in the populations of many European countries is attributed to better status of women, and an excessively male-dominated out- migration to different parts of the world in the past.

Age Structure

Age structure represents the number of people of different age groups. This is an important indicator of population composition, since a large size of population in the age group of 15- 59 indicates a large working population. A greater proportion of population above 60 years represents an ageing population which requires more expenditure on health care facilities. Similarly high proportion of young population would mean that the region has a high birth rate and the population is youthful.

Age-Sex Pyramid

The age-sex structure of a population refers to the number of females and males in different age groups. A population pyramid is used to show the age-sex structure of the population.

The shape of the population pyramid reflects the characteristics of the population. The left side shows the percentage of males while the right side shows the percentage of women in each age group.

Expanding Populations

The age-sex pyramid of Afghanistan as you can see is a triangular shaped pyramid with a wide base and is typical of less developed countries. These have larger populations in lower age groups due to high birth rates.

Constant Population

United States of America‘s age-sex pyramid is bell shaped and tapered towards the top. This shows birth and death rates are almost equal leading to a near constant population.

Declining Populations

The Japan pyramid has a narrow base and a tapered top showing low birth and death rates. The population growth in developed countries is usually zero or negative.

Rural Urban Composition

The division of population into rural and urban is based on the residence. This division is necessary because rural and urban life styles differ from each other in terms of their livelihood and social conditions. The age-sex-occupational structure, density of population and level of development vary between rural and urban areas.

The criteria for differentiating rural and

urban population vary from country to country. In general terms rural areas are those where people are engaged in primary activities and urban areas are those when majority of the working population is engaged in non-primary activities.

Literacy

Proportion of literate population of a country in an indicator of its socio-economic development as it reveals the standard of living, social status of females, availability of educational facilities and policies of government. Level of economic development is both a cause and consequence of literacy. In India – literacy rate denotes the percentage of population above 7 years of age, who is able to read, write and have the ability to do arithmetic calculations with understanding.

Occupational Structure

The working population (i.e. women and men of the age group – 15 to 59) take part in various occupations ranging from agriculture, forestry, fishing, manufacturing construction, commercial transport, services, communication and other unclassified services.

Agriculture, forestry, fishing and mining are classified as primary activities manufacturing as secondary, transport, communication and other services as tertiary and the jobs related to research and developing ideas as quaternary activities. The proportion of working population engaged in these four sectors is a good indicator of the levels of economic development of a nation. This is because only a developed economy with industries and infrastructure can accommodate more workers in the secondary, tertiary and quaternary sector. If the economy is still in the primitive stages, then the proportion of people engaged in primary activities world be high as it involves extraction of natural resources.