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 crust can be forced back down into the deeper regions of the Earth, as shown in this diagram. This process is called subduction. The slab that is forced back into the Earth usually undergoes melting when the edges get to a depth which is hot enough. (A temperature hot enough to melt lithosphere is about a thousand degrees!). This process is called "subduction".
Melted crust rises back towards the surface where it helps make volcanoes and islands. Thus the formation of some volcanoes, mountains, and islands is connected to the process of subduction and continental drift.
The melted lithosphere also releases gases of the atmosphere which had become trapped in the ground. Thus subduction of the lithosphere contributes to recycling of the atmosphere!
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