Here's why:

* Heat and Pressure: The Earth's interior is incredibly hot due to the decay of radioactive elements and the residual heat from the planet's formation. This intense heat, coupled with immense pressure, causes rocks to melt, forming magma.

* Plate Tectonics: The movement of tectonic plates plays a crucial role in magma formation. Where plates collide (convergent boundaries), one plate can subduct (slide) under the other, carrying oceanic crust down into the mantle. This process generates heat and melts the surrounding rock, creating magma.

* Mantle Plumes: Hot, buoyant plumes of rock rise from deep within the mantle. These plumes can melt the overlying crust, creating magma.

Lava is the term for magma that has erupted onto the Earth's surface.