Trenches, more formally called subduction zones, form when older, denser oceanic plates, the lithosphere, collide with newer, lighter continental plates in a process called subduction. The heavier, denser lithosphere is pushed down into the mantle, under the lighter continental plates. In the process, some of the lithosphere melts, forms magma and rises to the crust and helps form the trenches that are already developing where the oceanic crust has been pushed under. In this process, old crust is destroyed and formed into new crust in the formation of trenches.
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