Sites of converging lithospheric plates. Since the earth remains virtually constant in diameter, addition of new material to lithospheric plates along spreading centers must be balanced by the loss of material from lithospheric plates where they collide with one another. These plate collisions result in subduction zones. Collisions result in a compression force, pushing two plates together. It also causes the denser of the two plates to be subducted, or plunged downward into the mantle, where the lithospheric plate is melted and minerals are recycled.
Most subduction zones occur as collisions between an oceanic and continental plate.
Note depth of trenches along Pacific Coast of the Americas. Some subduction zones form where two oceanic plates collide. Rarely, two continental plates will collide along a subduction zone. Since continental plates are thick, their collision unleashes tremendous mountain building forces, in the form of folding and faulting. This is only occurring in one area today.