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 two to three tectonic plates pull apart or come 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 (like the San Andreas fault).
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 or the European Rhine Graben with its Eifel volcanoes). 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. The most common perception of a volcano is of a conical mountain, spewing lava and poisonous gases from a crater in its top.
This describes just one of many types of volcano and the features of volcanoes are much more complicated. Supervolcano is the popular term for a large volcano that usually has a large caldera and can potentially produce devastation on an enormous, sometimes continental, scale.
Such eruptions would be able to cause severe cooling of global temperatures for many years afterwards because of the huge volumes of sulfur and ash erupted.
They are the most dangerous type of volcano.
Examples include Yellowstone Caldera in Yellowstone National Park of western USA, Lake Taupo in New Zealand and Lake Toba in Sumatra, Indonesia.
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