When a tectonic plate sinks, the process is called subduction. This happens when one plate slides underneath another and both plates are moved towards the Earth's mantle. The mantle is made up of hot rock that slowly moves and this helps to push the plates around on the surface. The sunken plate becomes heated and starts to melt which forms magma. Magma is less dense than solid rock so it rises up through cracks in the overlying plate. When it reaches the surface, it can erupt as a volcano