Here's why:

* Subduction Zones: These occur where two tectonic plates collide, and one plate (the denser one) is forced beneath the other. The descending plate melts as it enters the hot mantle, creating magma that can rise to the surface and cause volcanoes.

* Molten Rock: The process of the plate melting as it descends is what creates the molten rock, or magma.

Let me know if you'd like more details on subduction zones!