Plate Tectonics and Landforms (A level Geography)
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Tectonic plates
The Earth's surface is made up of a series of large plates (like pieces of a giant jigsaw puzzle). These plates are in constant motion travelling at a few centimetres per year. The ocean floors are continually moving, spreading from the centre and sinking at the edges. Convection currents beneath the plates move the plates in different directions. The source of heat driving the convection currents is radioactive decay which is happening deep in the Earth.
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The edges of these plates, where they move against each other, are sites of intense geologic activity, such as earthquakes, volcanoes, and mountain building.
Constructive plate movement: In constructive plate movement continental crust is begins to separate creating a diverging plate boundary. When a divergence occurs within a continent it is called rifting. A plume of hot magma rises from deep within the mantle pushing up the crust and causing pressure forcing the continent to break and separate. Lava flows and earthquakes would be seen. A good example is the mid-Atlantic ridge.
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This is an example of a divergent plate boundary (where the plates move away from each other). The Atlantic Ocean was created by this process. The mid-Atlantic Ridge is an area where new sea floor is being created. ?
As the rift valley expands two continental plates have been constructed from the original one. The molten rock continues to push the crust apart creating new crust as it does.
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As the rift valley expands, water collects forming a sea. The Mid-Atlantic Ridge is now 2,000 metres above the adjacent sea floor, which is at a depth of about 6,000 metres below sea level.
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The sea floor continues to spread and the plates get bigger and bigger. This process can be seen all over the world and produces about 17 square kilometres of new plate every year.
Destructive plate movement: This is a convergent plate boundary, the plates move towards each other. The amount of crust on the surface of the earth remains relatively constant. Therefore, when plates diverge (separate) and form new crust in one area, the plates must converge (come together) in another area and be destroyed. An example of this is the Nazca plate being subducted under the South American plate to form the Andes Mountain Chain.
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Here we can see the oceanic plate moving from left to right. The top layer of the mantle and the crust (all called the lithosphere) sinks beneath the continent. A deep ocean trench is formed. Water gets carried down with the oceanic crust and the rocks begin to heat up as they travel slowly into the earth. Water is then driven off triggering the formation of pools of molten rock which slowly rises. The plate moves downwards at a rate of a few centimetres per year. The molten rock can take tens of thousands of years to then either:
Solidify slowly underground as intrusive igneous rock such as granite. Or Reach the surface and erupt as lava flows. Cooling rapidly to form extrusive igneous rock such as basalt.
Conservative plate movement: Conservative margins occur where two plates slide horizontally past each other. There is no subduction zone with one plate being destroyed beneath another nor is there a constructive zone as at the Mid-Atlantic ridge. As the two plates slide past each other, the high levels of friction create large strains along the slippage zone faults. Eventually the strain energy is released in an earthquake. When this happens there is likely to be large scale movement along the transform fault with substantial damage to nearby buildings.
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The best known example of a conservative plate margin can be found along the west coast of the USA where the Nazca plate is moving in a North Westerly direction and sliding past the North American Plate. The North American plate is also moving in a North-Westerly direction but not so fast as the Nazca plate so the two appear to be moving in opposite directions.
