A volcano is an opening, or rupture, in a planet's surface or crust, which allows hot, molten rock, ash, and gases to escape from below the surface. Violent explosive eruptions from such vents often produce craters or calderas and coat extensive areas in volcanic ash, while the lava from comparatively gentle effusive eruptions may eventually form large plains, cones or mountains.

Volcanoes are generally found where tectonic plates are pulled apart or come together. A mid-oceanic ridge, such as the Mid-Atlantic Ridge, hosts volcanoes caused by divergent tectonic plates pulling apart; the Pacific Ring of Fire contains many volcanoes caused by convergent tectonic plates coming together. By contrast, volcanoes are not commonly created at transform boundaries, where two tectonic plates slide past one another.

Volcanoes can be caused by mantle plumes. The resulting hotspots, for example at Hawaii, can occur far from plate boundaries. Hotspot volcanoes are also found elsewhere in the Solar System, especially on rocky planets and moons. Intraplate volcanoes can also form where there is stretching and thinning of the Earth's crust, as in the Wells Gray-Clearwater volcanic field and the Rio Grande Rift in North America and the European Rhine Graben with its Eifel volcanoes.