Magma: the Engine Below

A volcano begins with magma — molten rock generated deep in the Earth's mantle or lower crust. Rock melts when temperature rises, pressure drops, or water lowers the melting point of surrounding minerals. All three mechanisms operate at tectonic plate boundaries. Once formed, magma is less dense than solid rock, so it rises.

It collects in underground reservoirs called magma chambers, which can sit 1 to 10 kilometers below the surface. When pressure exceeds the strength of the overlying rock, the magma forces its way upward through cracks and fissures — and a volcano is born.