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Much of our understanding of earthquakes is based upon a revolutionary theory of earth dynamics, called PLATE TECTONICS.
2. Earth Structure
According to plate tectonics the earth can be divided into three layers:1. Core: An inner, solid core and an outer liquid core.
2. Mantle: Consists of rock that behaves like plastic. Near the crust there are convection currents.
3. Crust: Solid outer layer 0-100 km thick and consists of dense oceanic crust and lighter continental crust.
The lithosphere is the name given to the crust and the solid upper part of the mantle to which it is attached. Beneath the lithosphere is the asthenosphere where molten (liquid) rock heated from below rises in convection currents (similar to the currents in water when it heated from below in a saucepan). The lithosphere is broken into immense sections, called plates, and the cracks between the plates are called faults. From the map below you can see that Europe and Asia sit on a different plate from North America - the separating fault runs through the middle of the Atlantic Ocean. These plates may have both oceanic and continental crust upon them. Notice that the Pacific Plate consists almost entirely of ocean apart from a sliver of land, California.
Plate Movement: On either side of an active fault the plates are moving (see arrows on the map). The faults can be classified according to the type of movement:
1. Divergent: the plates on either side are moving away from each other.
2. Convergent: the plates on either side are moving toward each other.
3. Transform: the plates on either side are moving alongside each other in opposite directions (neither toward nor away)
At divergent faults material from the inside the molten earth (Asthenosphere) 'wells up' to form new lithosphere on the ocean floor. This has the effect of pushing the plates on either side apart. In the south Pacific sea floor spreading is pushing the ocean floor northward. There is one large piece of land attached to this plate, California, and when the plate moves so does California. In the middle of the Atlantic a long range of mountains (mid-oceanic ridge) is found where this spreading is occurring. The Atlantic Ocean, as a result is getting larger over time. This spreading is slow, perhaps just a couple of centimetres per year.
At convergent boundaries two plates collide. In some cases the heavier plate goes under the other, lighter, one. This process of subduction forms deep oceanic trenches. At its northwestern end the Pacific Plate is disappearing beneath the EuroAsian Plate producing the Mariana Trench (if Everest were dropped into this hole it would disappear). Just as the Atlantic Ocean is growing, the Pacific Ocean is closing. At other converging boundaries the two plates are the same weight and, rather than sinking, the convergence causes the earth to crumple. For example, the Himalayan Mountains are growing mountains created because the Austral-Indian plate is 'crashing' into the Euro-Asian plate. Finally, at transform faults the plates move alongside each other. While the Pacific Plate is moving northward California is moving with it. The rest of the North American continent by comparison is moving southward.
