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 pulled 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. Volcanoes can be caus