How do volcanoes form?

Deep beneath the surface, heat melts rock into magma, which is lighter than the solid rock around it and rises toward the surface. Where it breaks through, it erupts as lava, ash, and gas, building up a volcano over time. Many volcanoes form at the edges of tectonic plates, either where plates pull apart or where one plate dives beneath another and melts. Others form over hotspots, plumes of hot material rising from deep in the mantle, such as those beneath Hawaii. Repeated eruptions gradually build the mountain, and its shape depends on the type of lava and eruption.