Weathering & Soil Formation

This is the personal website of
Mayen Davis
created for The Young Women's Leadership School - Queens

Weathering and Soil Formation

Weathering is a mechanical or chemical process that breaks rocks into smaller pieces.  Freezing, thawing, oxygen in the air, plants, and animals can affect the stability of rocks. 
Mechanical Weathering - breaks rocks into smaller pieces without changing the chemical composition of the rock.  Two causes of mechanical weathering are ice wedging and living organisms.
  1. Ice Wedging - rain or melting snow seeps into cracks in rocks.  When the temperature drops the water freezes (ice) and expands.  The expansion exerts pressure on the rock forcing it to crack or break apart.  In the same manner, ice wedging also causes potholes.
  2. Plants & Animals (living organisms) - plant roots grow in cracks in rocks where water collects.  As they grow roots become thicker and longer.  They exert pressure and wedge rocks apart.  Gophers and prairie dogs also weather rock.  They burrow through sediment or soft sedimentary rock, and break the rock apart.  They also push some rock to the surface.

Chemical Weathering - occurs when the chemical composition of rock changes.  Chemical weathering occurs more rapidly in tropical areas.  Two causes are natural acids and oxygen.   See chemical weathering
  1. Natural Acids - some acids, like carbonic acid, change the chemical composition of the minerals of the rocks. (Like the vinegar changed the organic rock (chalk).
  2. Plant Acids - plant roots also produce an acid called tannin that reacts with rocks.  Tannin dissolves some minerals in rocks which weakens the rock and causes it to break Ex// (moss)
  3. Oxygen - chemical changes due to oxygen is called oxidation.  Iron oxidizes and becomes rust.  Rocks that conatain the mineral iron break, when the iron in the rock oxidizes and changes to a rust like substance
Soil - mixture of weathered rock, organic matter, water, and air that supports the growth of plant life.  Organic matter (decomposed leaves, twigs, roots, and other material)     
Factors Affecting Soil Formation
  1. Parent rock being weathered.  When limestone is chemically weathered it leaves behind clay.  This produces clayey soil.  When sandstone is chemically weathered, it leaves behind sand which produces sandy soil.
  2. Topography (surface features) - on steep hillsides, soil rarely develops because rock fragments are constantly moving down the hillside.  In lowlands (where land in flat), wind and water deposits fine sediments that form thick soils.
  3. Climate - if rocks weather quickly, deep soil develops rapidly.  This is more likely to happen in tropical areas where the weather is warm and moist.  Climate also affects the amount of organic material in the soil.  Soils in desert climates have little organic material because there are few plants and animals to decompose.  In humid areas, where vegetation (plants) is plentiful, the soil has more organic material.  When plants and animals decompose, a dark colored material forms called humus.  Most of the organic material in soil is humus.  The humus helps soil hold water and provides plants with nutrients to grow.
  4. Time - It takes time for rocks to weather and soil to form.  young soil resembles its parent rock while older soil looks less like the parent rock.
  5. Organisms - organisms can add or take away from the nutrients in soil.  Lichens are small organisms that consists of alga and fungus.  Lichens can grow on rock and take nutrients from the rock.  The rock begins to break down forming a thin soil.  After the soil has formed, many types of plants can grow.
Erosion of Earth's Surface

Erosion - the wearing away and removal of rock or sediment.  Erosion occurs because gravity, ice, wind, and water sculpt Earth's surface. 

Gravity is a force that pulls everything on Earth towards its center.  This causes rocks to fall down slopes and water to flow downhill.  When gravity alone causes rock or sediments to move down a slope it's called mass movement.  One place this often occurs is near volcanoes.  Four types of mass movement are creep, slump, rock slides, and mudflows.
  1. Creep - when soil and sediment move slowly downhill.  Creep is common where freezing and thawing occur.  As ice expands in soil it pushes sediment up.  As the soil thaws, the sediment move further down slope.
  2. Slump - when a mass of rock or sediment moves downhill, leaving a curved scar.  A slump usually occurs after an earthquake or a heavy rain.

   3. Rock Slides - when rock breaks free from the side of a cliff or mountain and slide or crash to the bottom.

  •     4. Mudflows - a mass of wet sediment that flows downhill over the ground surface.  Mudflows can be thick and flow slowly or thin a slow as fast as 160km/h (99.4 miles/hour).


    Ice - When ice in a glacier becomes thick enough its weight causes it to flow downhill; it becomes an agent of erosion.  As glaciers move over the Earth's surface they erode material in some places and deposit sediment in others.  There are two kinds of glaciers: continental glaciers and valley glaciers.

    Glacial Erosion - glaciers can erode rock in two different ways.  If the underlying rock has crack in it the ice is able to pull out pieces of rock.  This causes slow erosion in the rock.  The looses pieces of rock freeze into the bottom of the glacier and are dragged over the Earth's as the glacier moves.  They scratch the Earth (rock) below like giant pieces of sandpaper.  This scratching is another way that glaciers can erode rock.  Scratching makes large grooves or small scratches in the rock beneath.  It can also grind the rock into a fine powder called rock flour.

    Effects of Glacial Erosion - Glaciers are capable of reshaping Earth's surface.  Valley galciers are able to remove rock from mountain tops. This leaves behind large bowls, called cirques and steep peaks.  When a glacier moves into a stream valley it erodes rock along the valley sides making a wider U shaped valley.  Continental glaciers can move through large lakes and completely remove rock layers from the land's surface.
    Glacial Deposition - glaciers can also deposit sediment.  When glaciers stop moving and begins to melt or when ice melts at the bottom of a flowing glacier, the sediment the ice was carrying along the bottom of the ice gets left behind on the Earth's surface.  Sediment deposited directly from glacier ice is called till.  Till is a mixture of of different sized particles, ranging from clay to large boulders.  Glaciers increase melting during summer months.  So much water can melt that streams can form that flow away from the glacier.  These streams deposit sediment.  Sand and gravel deposits from these streams are called outwash.  The outwash consists of particles that are all about the same size.


    Wind blows across looses sediment like silt and sand and lifts and carries it.  The erosion of land by wind is called deflation.  Deflation can lower the land’s surface by several meters.

    Water            erosion animation

    Water that flows over Earth’s surface is known as runoff.  Runoff causes erosion; especially when the water is moving fast.  The more speed water has the more sediment it can carry. 

    1.   Sheet Flow - As raindrops hit the Earth, they break up clumps of soil and loosen small grains of sediment.  On a sloped land surface, a thin sheet of water may begin to move downhill.  Water flowing downhill in a thin sheet is known as a sheet flow.  Sheet flows can carry loose sediment grains causing erosion of the land.   Erosion due to sheet flow is called sheet erosion.

    2.   Rills & Gullies – When sheets of water flow around obstacles and become deeper rills can form.  Rills are small channels cut into the sediment Earth’s surface.  These channels can carry more sediment than sheet flow.  As runoff flows through the rills, the sediment erodes the channel even more and causes it to widen and deepen.  When the channels gets to be about 0.5 meters across they are called gullies.

    3.   Streams – Gullies usually connect to stream channels.  Streams can be small enough for you to step over or large enough for barges to transport products along their course.  In mountainous / hilly areas streams have quite a bit of energy and the rushing water can cut into the rock beneath the stream.  As streams flow back and forth through the valley, they erode and deposit sediment along the way.  Streams shape more of the Earth’s surface than ice, wind, or gravity.  Streams can have enough power to cut large canyons into the Earth.  When rivers / streams enter an ocean or lake, large amounts of sediment are deposited from the slowing water.  This accumulation at the mouth of the lake / ocean is called a delta.

    Evidence of Glacial Erosion 

    Effects of Erosion (summary)

    Erosion is caused by gravity, ice, wind, and water.  All of these help shape the surface of the Earth.  Rock and sediment are removed from one area and deposited somewhere else.  When this happens canyons, valleys, and cirques can form.  When sediment accumulates in one area many things can form like deltas, sandbars, and sand dunes.

    Fossils        Organism Burial  Fossil Animation


    Paleontologists are scientists who study fossils.  They are able to learn about extinct animals from their fossil remains.  Fossils are the remains, imprints, or traces of prehistoric organisms.  Fossils help scientist determine:

    1.       When life first appeared

    2.       When plants and animals first lived on land

    3.       When organisms became extinct

    Fossil Formation      animation here

    When an organism dies, sediment can quickly cover it, preserving it from scavengers and physical destruction(sediment buries a dead organism).  Over time, the sediment, plus pressure and heat will fossilize the remains in the sedimentary rock.  Organisms have a better chance at being fossilized if they have hard parts like bones, shells, or teeth because scavengers are less likely to eat these parts and they decay slower than soft parts. 

    Types of Preservation

    1.       Mineral Replacement – Upon death most hard parts of organisms, like teeth, bones, and shells, have tiny spaces in them from decay.  Groundwater can fill these spaces and deposit minerals.  Permineralized remains are fossils that have the spaces inside them filled with mineral deposits from groundwater.  DNA can sometimes be recovered here.  Minerals can also replace the hard parts of the fossil organisms.  If a mineral inside of water dissolves a shell, it will leave the mineral in its place. 

    2.       Carbon Films – organisms’ tissues are made of compounds that contain carbon.  When sediment covers a dead organism and the result of the added pressure and heat will force gas and liquids from the organism leaving behind a thin film of carbon residue that forms a silhouette of the organism called a carbon film. 

    3.       Coal – in swampy areas, there are large amounts of plant matter.  Over millions of years this accumulation of matter carbonizes and becomes coal.  We use coal for fuel.  Coal is not a good source to learn about the past because the structure of the original organism is not usually found.

    4.       Molds & Casts – impressions are formed when hard parts of organisms fall into soft sediment and then buried by more sediment.  Compaction and cementation turn the sediment into a rock (sedimentary).  Open pores in the rock allow water and air into to rock and this can decay the hard part or the organism leaving behind a cavity in the rock called a mold.  As time passes, more water and sediment can enter the cavity and form new rock which is a copy of the original organism.  This is called a cast.  It is not the original.