Geography- Tectonics

About the Tectonics section of Geography A on AQA exam board

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  • Created by: Sam
  • Created on: 05-06-12 12:33

Why is the earth's crust unstable?

Structure- The outer layer of crust is relatively thin, made up many different pieces called plates which can move because they are floating on the semi-solid mantle. Convection currents within the mantle determine the direction of movement.


Destructive - plates move together. If one plate is oceanic and the other continental then the denser oceanic sinks under the continental in a process know as subduction. If two continental meet they collide. This can form fold mountains.

Constructive- plates move apart. This usually happens in oceans. When the plates move apart cracks form between them. Magma forces it's way through the crust to form volcanoes and new land is formed as they gradually pull apart.

Conservative- Plates sliding past each other. Either in opposite directions or in the same direction at different speeds. Friction causes earthquakes. Crust not destroyed or made.

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What landforms are found at different plate margin

Fold mountains (where rock layers have been crumpled as they were forced together) and Ocean trenches (deep sections of the ocean usually where oceanic plate is sinking below a continental plate) both form at destructive plate margins. If both are in the same area it is related to subduction but if only fold mountain occur it is at a collision boundary.

There are two different types of volcanoes:

Composite- occur at destructive plate margins, steep slopes and
narrow base, layers of ash and lava, secondary cones, eruptions
infrequent but often violent.

Shield- occur at constructive plate margins, gentle slopes and
narrow base, low rounded peak, layers of runny lava with little
ash, eruptions frequent and non-violent.

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How do volcanoes effect people?

  • Volcanoes are a Natural Hazard that cause damage and destruction. An area especially prone to volcanoes is the 'pacific ring of fire'. Most volcanoes are found at plate margins.
  • The primary effects of a volcanoes are what happens immediatly after an eruption e.g number of deaths or destruction of houses.
  • The secondary effects are what happens as a result of the eruption e.g. people fleeing or more fertile soil.
  • Aid is usually sent to help those who have suffered e.g immediate like food or long-term like money for the rebuild.
  • The more volcanoes are monitiored and eruptions predicted the less damage they will cause in terms of deaths and injuries as more people can be evacuated.
  • Volcanoes can be monitored in many ways such as 'swelling' as the magma rises, changes in levels of sulfur dioxide and looking at frequency of eruptions in the past.
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Supervolcanoes are on a much bigger scale than normal volcanoes as they emit at least 1,000 km cubed of material compared to about 1km cubed from a normal volcano. They do not have a cone shape but are instead flat. They bulge before erupting and after an eruption the land usually dips to form a caldera.

Geothermic features top the volcano as heat from the hot spot boils the water underneath. The hotspots that cause supervolcanoes do not move but the crust does move over them.

If Yellowstone were to erupt it would likely destroy 10,000km squared of land, kill 87,000 people and the ash would spread world wide causing crops to fail and also would cause a mini ice age as the ash reflected heat back out of the atmosphere before it could reach earth.

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  • The place where an earthquake orginates from is called the focus and the point above it on the surface called the epicentre.
  • Shock waves radiate out from the focus; it is these that cause the damage. Primary waves travel fastest causing up/down movement, Secondary waves then arrive causing side-to-side movement which cause the most damage.
  • The Richter scale is used to measure earthquakes. It is logarithmic meaning that for  each increase of 1 the earthquake is 10 times stronger. The mercalli scale measures the effect of an earthquake on a scale of 1-12 (by how much damage was caused)
  • Earthquakes occur at plate boundaries due to build up of pressure. They are usually strongest at conservative margins because the plates can stick for long periods of time and weakest at constructive because friction and pressure cause by plates moving apart os less intense.
  • Preperation is the most important part to reducing effects of earthquakes. building can be build with shock absorbers and people can be educated.
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Tsunamis are usually triggered by earthquakes. The crust moves displacing the water above which causes a wave/waves to move outwards from the epicentre.

At sea a tsunami can be 200km in length and about 1m high. They move at speeds of around 800kph rapidly approaching the coast almost unnoticed (even a boat at sea would unlikely notice it pass beneath). As the sea shallows the wave slows down reduces in lenght but increases in height up to waves as tall as 30m.

Tsunamis can surge far inland and often secondary waves are much more powerful than the inital one.

They can effect a large area as they move out to different coastlines such as the boxing day tsunami that effected the area around the Indian Ocean.

They are hard to detect so evacuation can sometime only have minutes of warning.

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