A volcano is formed when hot molten rock, ash and gases escape from an opening in the Earth's surface. The molten rock and ash solidify as they cool, forming the distinctive volcano shape shown here. As a volcano erupts, it spills lava that flows downslope. Hot ash and gases are thrown into the air.

Why does a volcano form?

On land, volcanoes form when one tectonic plate moves under another. Usually a thin, heavy oceanic plate subducts, or moves under, a thicker continental plate. ... When enough magma builds up in the magma chamber, it forces its way up to the surface and erupts, often causing volcanic eruptions.

Where does a volcano form?

Volcanoes form at the edges of Earth's tectonic plates. These huge slabs of Earth's crust travel atop the partly molten mantle, the layer beneath the crust.