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Waves

Plate Tectonics.
An Oceanic Plate Colliding with a Continental plate.

In sea floor spreading, an oceanic plate moves away from
a mid-ocean ridge, and collides with a continental plate.
The oceanic plate (made from basalt) is thinner and denser
than the continental plate (which is made from granite).

What happens when Two Plates Collide?

When the two plates collide the oceanic plate is forced
underneath
the continental plate, down towards the hot mantle.
This "forcing underneath" is called "subduction".
The place where it happens is called a "subduction zone". 

As you can imagine, two huge pieces of the Earth's crust
crashing into each other generates an enormous amount of
energy. Rocks are heated, crushed, deformed, folded,
and some are melted and recycled (see the rock cycle).
During the process, metamorphic rocks are formed and
mountains are built. Volcanoes and earthquakes are common.

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