Year 10 Geography Christmas Exam

TECTONICS, EARTHQUAKES AND VOLCANOES

The earths structure

Plate Boundaries

There are two theories as to why these plates move. The first, and most common, theory is that the plates move due to convection currents in the Earth’s mantle. This is where the heat from the Earth’s core causes magma to rise. As it nears the Earth’s surface, it then cools and sinks. This circular motion causes the plates in the crust to move.

The second theory is known as ‘slab pull’ where it is thought that the movement is caused by the weight of heavier denser plates sinking into the mantle and dragging other sections of the plate with it.

Types of volcanoes – shield, composite and supervolcanoes

Composite volcanoes

Composite volcanoes are found on destructive plate margins, where the oceanic crust sinks beneath the continental crust. Composite volcanoes have the following characteristics:

Acidic lava, which is very viscous (sticky).
Steep sides as the lava doesn't flow very far before it solidifies.
Alternate layers of ash and lava. For this reason, they're also known as stratovolcanoes. Strato means layers.
Violent eruptions.
Longer periods between eruptions.

An example of a composite volcano is Mount Pinatubo in the Philippines.

Shield volcanoes

Shield volcanoes are found on constructive plate margins, where two plates move away from one another. Shield volcanoes have the following characteristics:

basic lava, which is non-acidic and very runny
gentle sides as the lava flows for long distances before it solidifies
no layers, as the volcano just consists of lava
less violent eruptions
shorter periods between eruptions

Mauna Loa is a shield volcano, but was formed over a hot spot, rather than at a constructive plate margin, like other volcanoes.

Supervolcanoes

A supervolcano is a volcano on a massive scale. It is different from a volcano because:

it erupts at least 1,000 km3 of material (a large volcano erupts around 1 km3);
it forms a depression, called a caldera (a volcano forms a cone shape);
often has a ridge of higher land around it;
erupts less frequently - eruptions are hundreds of thousands of years apart.

The stages in the creation of a supervolcano caldera

Step 1

Step 2

Step 3

Step 4

Case study: Yellowstone

Yellowstone is one example of a supervolcano. Three huge eruptions have happened in the last 3 million years.

The last eruption was 630,000 years ago, and was 1,000 times bigger than the Mount St Helens eruption in 1980.

The large volume of material from the last Yellowstone eruption caused the ground to collapse, creating a depression called a caldera. The caldera is 55 km by 80 km wide.

The next eruption is predicted to have catastrophic worldwide effects.

The supervolcano at Yellowstone is formed because of a volcanic hotspot.

Every year millions of visitors come to see the related features, such as geysers and hot springs. Old Faithful is one example of a geyser.

Diagram of a volcano

Why people live near volcanoes

People choose to live in volcanic areas despite the risks of an eruption. Volcanoes can provide people with many benefits such as:

volcanic rock and ash provide fertile land which results in a higher crop yield for farmers
tourists are attracted to the volcano, which increases money to the local economy
geothermal energy can be harnessed, which provides cheaper electricity for locals
minerals are contained in lava, eg diamonds - these can be mined to make money

The causes of earthquakes

An earthquake is the shaking and vibration of the Earth's crust due to movement of the Earth's plates (plate tectonics). Most earthquakes will happen along any type of plate boundary. Big earthquakes usually only occur at these plate boundaries. There is an "earthquake belt" around the world where plate activity gives rise to eathquakes.

Earthquakes occur when tension/energy is released from inside the crust. Plates do not always move smoothly alongside each other and sometimes get stuck. When this happens pressure builds up. When this pressure is eventually released, an earthquake tends to occur.

Focus and epicentre

The point inside the crust where the pressure is released is called the focus. The point on the Earth's surface above the focus is called the epicentre. Earthquake energy is released in seismic waves. These waves spread out from the focus. The waves are felt most strongly at the epicentre, becoming less strong as they travel further away. The most severe damage caused by an earthquake will happen close to the epicentre.

How earthquakes are measured

The power of an earthquake is measured using a seismometer. A seismometer detects the vibrations caused by an earthquake. It plots these vibrations on a seismograph. The strength, or magnitude, of an earthquake is measured using the Richter scale. The Richter scale is numbered 0-10 with 10 being the greatest strength or magnitude.

Damage caused by earthquakes

Liquefaction is when an earthquake takes place and shakes the wet soil. The energy released by the earthquake causes the water within the soil (and rock) to rise up to the surface and can turn the soil into a liquid mud. As a result of this, foundations will be compromised and buildings will either sink or tip over.

The cause of a tsunami

A tsunami is a large wave that is created due to an underwater earthquake sending its shockwaves into the water which causes a surge of water to build up and make its way towards the coastline.

The energy transferred into a tsunami wave can often be transported for thousands of miles across an ocean.

Case Study

Know One case study (an earthquake, volcano or tsunami)

· The cause

· The effects

· The responses

Indian Ocean Tsunami.pptx

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