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 pull apart or come together. 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.
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