A volcano is an opening, or rupture, in the Earth's surface or crust, which allows hot, molten rock, ash and gases to escape from deep below the surface. Volcanic activity involving the extrusion of rock tends to form mountains or features like mountains over a period of time.
Volcanoes are generally found where tectonic plates pull apart or are coming together. A mid-oceanic ridge, like the Mid-Atlantic Ridge, has examples of volcanoes caused by "divergent tectonic plates" pulling apart; the Pacific Ring of Fire has examples of volcanoes caused by "convergent tectonic plates" coming together. By contrast, volcanoes are usually not created where two tectonic plates slide past one another. Volcanoes can also form where there is stretching of the Earth's crust and where the crust grows thin (called "non-hotspot intraplate volcanism"), such as in the African Rift Valley, the European Rhine Graben with its Eifel volcanoes, the Wells Gray-Clearwater Volcanic Field and the Rio Grande Rift in North America.
Finally, volcanoes can be caused by "mantle plumes", so-called "hotspots"; these hotspots can occur far from plate boundaries, such as the Hawaiian Islands. Interestingly, hotspot volcanoes are also found elsewhere in the solar system, especially on rocky planets and moons.