This is a drawing of a portion of the Earth's crust undergoing subduction.
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Image copyright 1997 by the American Geophysical Union. Further electronic distribution is not allowed.
When two sections of the Earth's crust collide, one slab of lithosphere can be forced back down into the deeper regions of the Earth, as shown in this picture. The slab that is forced back into the Earth usually becomes melted when the edges reach a depth which is hot enough. This process is called "subduction".
Melted crust rises back towards the surface where it helps make volcanoes and islands.
The melted crust also releases gases of the atmosphere which had become trapped in the ground. Thus subduction of the crust helps to recycle the atmosphere!
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