Volcanoes form at vents or openings in the Earth's crust which allow hot, molten rock (magma) and gases from the interior of the Earth to reach the surface. Most of the time, the solid parts pile up around the vent to form a steep-sided mountain such as the Soufrière Hills Volcano in Montserrat. But there are some volcanoes that are broad mountains with gentle slopes and even some which are simply holes or slits in the ground.

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