Plate Tectonics
Earth's outermost sphere, called the lithosphere, rests on the plastic, movable and moving asthenosphere. Movements in the asthenosphere provide the energy for moving segments of the lithosphere, called plates. The plates interact by:

1. colliding (converging)
2. separating (diverging), or
3. siding past one another (transform movements).

There are three types of convergence:

1. continent-to-continent (India and Asia, making the Himalayas)
2. ocean crust-to-ocean crust, making arcs of volcanic islands and deep trenches
3. ocean crust-to-continent, making volcanic and complex mountains.

Subduction is a process through which oceanic crust is destroyed by going back into the mantle. The word subduction comes from the Latin "sub," which means "under," and "ducere," which means "to lead." Typically, subduction leads to the formation of a trench.Oceanic crust, though thinner than continental crust, is denser (i.e., heavier per unit volume). Oceanic crust is mostly composed of a lava called basalt. The continents, thicker than the ocean crust, are dominated by the igneous rock granite. Granite is less dense than basalt. It is also lighter in color, usually indicating that it has less iron and magnesium in its constituent minerals.

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