Here's why:

* Plate Boundaries: Earth's outer layer is made up of large pieces called tectonic plates. These plates constantly move and interact.

* Convergent Boundaries: When plates collide (converge), one plate can slide beneath the other (subduction). This process pushes rock down into the Earth's mantle, where it melts. The molten rock, called magma, then rises to the surface, often erupting as volcanoes.

* Divergent Boundaries: When plates move apart (diverge), it creates gaps that allow magma to rise from the mantle. This magma solidifies, forming new crust and often resulting in volcanic activity along mid-ocean ridges.

Essentially, the movement of tectonic plates provides the energy and mechanism for magma to rise and erupt, leading to volcanic activity and the formation of igneous rocks (rocks formed from cooled magma or lava).