Here's a breakdown:

* Oceanic crust is the relatively thin, dense layer of rock that makes up the ocean floor.

* Mantle is the thick layer of mostly solid rock that lies beneath the Earth's crust.

* Subduction is the process where one tectonic plate slides beneath another. In subduction zones, denser oceanic crust is forced beneath less dense continental crust or another oceanic plate.

At subduction zones, the descending oceanic crust is heated and eventually melts, contributing to volcanic activity and mountain building.