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A volcano is a landform that develops around a weakness in the Earth's crust from which molten magma, disrupted pre‐existing volcanic rock, and gases are ejected or extruded. Volcanoes may be of the central‐vent type, where eruption takes place from a single pipe, or the fissure type, where magma is extruded along a linear fracture. They may also form large, low‐lying craters known as calderas, created as a result of subsidence or collapse following an eruption. Central‐vent volcanoes typically form cones or domes, while eruptions from fissures build plains or plateaux. Volcanoes can be classified as active, dormant, or extinct; but this loose, generic description of a volcano's status should be used with caution.
Magma forms in the hot plastic layer of the Earth's upper mantle known as the asthenosphere. At deeper levels in the Earth, temperatures are higher but so too are pressures, thereby inhibiting the melting of rock. Magma originates beneath constructive plate margins, where the convection processes bring hot, mantle rock into lower‐pressure regimes. Once the magma reaches the brittle layer (the lithosphere) that makes up the plates, it makes room for itself by exploiting channels of weakness or breaking the overlying rock.
How violently the magma eventually breaches the surface depends largely on the behaviour of the gas it contains. If the eruption is violent, the magma will be torn into fragments, collectively known as tephra, ranging in size from blocks the size of automobiles to fine ash. In contrast, the effusion of lava, perhaps combined with spectacular lava fountaining, characterizes quieter eruptions. All volcanic eruptions are driven by rapidly expanding gases within the magma, so the two factors that determine the violence of an eruption are the amount of dissolved gases and how easily it can escape.
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