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Volcano, fissure or vent in a planet's crust through which magma and associated material may be extruded onto the surface. This may occur with explosive force. The extruded magma, or lava, solidifies in various forms soon after exposure to the atmosphere. In particular it does so around the vent, building up the characteristic volcanic cone, at the top of which is a crater containing the main vent. There may be secondary vents forming “parasitic cones” in the slopes of the main cone. If the volcano is dormant or extinct, the vents may be blocked with a plug (or neck) of solidified lava. Classifying volcanoes in order of increasing violence, the main types are Hawaiian, Strombolian, Vulcanian, Vesuvian, and Peléan. Volcanoes are generally restricted to belts of seismic activity, particularly active plate margins, but some intraplate volcanic activity is also known, as in the case of Hawaii. At midocean ridges magma rises from deep in the mantle and is added to the receding edges of the plates. In mountain ranges, where plates are in collision, volatile matter ascends from the subducted edge of a plate, perhaps many kilometers below the surface, bursting through the overlying plate in a series of volcanoes. Around 500 active volcanoes are known of on earth; active volcanoes have also been found on Mars and on Jupiter's satellite Io.

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