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. 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 are diverging or converging. A mid-oceanic ridge, for example 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 and thinning of
the Earth's crust (called "non-hotspot intraplate volcanism"), such as in the African Rift Valley, the Wells Gray-Clearwater
volcanic field and the Rio Grande Rift in North America and the European Rhine Graben with its Eifel volcanoes.
Volcanoes can be caused by "mantle plumes". These so-called "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.
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