Here's why:

* Magma: Volcanoes are formed when molten rock, called magma, rises to the surface. Magma originates in the Earth's mantle, a layer of hot, semi-solid rock.

* Plate Tectonics: The Earth's crust is broken into large plates that move and interact with each other. When these plates collide, one plate can slide under the other, called subduction. This process melts the rock in the mantle, creating magma.

* Volcanic Activity: The magma rises through the crust, eventually erupting onto the surface as lava, ash, and gases.

It's important to note: While volcanoes occur at the boundary between the crust and mantle, the actual process of magma formation and eruption involves both layers.