Here's why:

* Subduction is the process where one tectonic plate slides beneath another. This is exactly what happens in the scenario you described. The molten rock (magma) that emanates from the edge of the plate is a result of the subducting plate melting as it descends. The other side of the plate is pushed into a trench by the force of the subducting plate.

Here are some other related terms:

* Trench: The deep depression formed at the point where the plates meet.

* Volcanism: The process of magma rising to the surface and erupting as lava. This is often associated with subduction zones.

* Convergent Plate Boundary: The type of plate boundary where two plates collide. Subduction occurs at convergent boundaries.

Let me know if you have any other questions!