Diagram of colliding oceanic tectonic plates. The tectonic plates are large-scale units of crust which move relative to one another in response to the thermal activity of the core and mantle. The plate on the right is moving leftwards. As it does so, it moves under the adjacent plate (arrow) - a process called subduction. An oceanic trench forms at the subduction zone, as well as episodes of volcanism, as the sinking layer melts and rises as magma to form islands.