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The term volcano refers to the opening or vent through which the molten rock and associated gases are expelled. Volcanoes are mountains, but they are very different from other mountains; they are not formed by folding and crumpling or by uplift and erosion. Instead, volcanoes are built by the accumulation of their own eruptive products. A volcano can also be defined as a liquid rock plumbing system which extends from several 10's of kilometers depth to the earth's surface, and includes the near vent deposits of eruptions. Geologists have generally grouped volcanoes into four main kinds: cinder cones, composite volcanoes, shield volcanoes, and lava domes. Deep inside Earth, between the molten iron core