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Plate Tectonics |
Destructive plate movement - This occurs when plates move towards each other. Different types of collision may take place at these boundaries. Oceanic and Continental plates or two continental plates can collide with each other.
Oceanic and Continental plates colliding: Since the oceanic plate is more dense, it is always forced underneath the continental plate. This is called subduction (see animation below). As the oceanic plate is pushed down a deep trench is formed. The plate melts and creates pressure in the surrounding area due to all the melting rock. The resulting molten rock finds its way to the surface and volcanoes form. Earthquakes also happen as the two plates slowly grind past each other. The continental crust is not destroyed. It is simply compressed, folded into anticlines and synclines and thickened to form a fold mountain range similar to the Andes in South America.
Two continental plates colliding: When two continental plates collide head on, neither of them is subducted. Instead the sediment layers laying between the two continent land masses get squeezed. The effect is to form fold mountains similar to the Himalayas. This type of fold mountain range has no volcanoes or deep focus earthquakes. India, in fact detached it self from Africa and piled into the bottom of Asia. It is still doing so, pushing the Himalayas up and up. This means Mount Everest is getting taller by a few centimeters every year as India continues to push up into the continent of Asia.
Conservative plate movement: When plates slide past each other this type of movement results. Here, material is neither created or destroyed. The best known example of this is the San Andres Fault in California (see animation below). It marks the boundary between the Pacific plate and the North American plate. These plate of rock don't glide smoothly past each other. They catch on each other and as the forces build up they suddenly jerk. This sudden jerking only lasts a few seconds, but brings devastation in a built and heavily populated area. Building come tumbling down and many people get killed. The city of San Francisco sits along side this fault line. It was completely destroyed in 1906 and was again hit by a powerful tremor in 1991. This could happen again any time.
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These days in earthquake zones, developers try to build earthquake-proof buildings which are designed to withstand small amount of shaking. In poorer countries, earthquakes usually cause much devastation where they have badly constructed properties, overcrowding and inadequate rescue services.
| Useful Websites | |
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BBC Earthquakes Page |
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WikiPedia Earthquakes Page |
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