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A volcano is a mountain or hill, existing on land or beneath the ocean, built up of ejected material. Most volcanoes are located close to existing or old plate boundaries. Volcanoes usually occur in groups know as volcanic chains or volcanic fields.
Eruptions take place from the sides and/or the top. Eruptions may be due to the production of steam, formed when water percolating through the ground meets hot magma; dense clouds of steam generally accompany such an eruption. Eruptions can also occur due to earth movements. In addition, gases dissolved in the magma can build up enough pressure to force an eruption.
A volcano is regarded as active, extinct, or dormant. A volcano is considered active, if an eruption has occurred and there is very little alteration due to weathering. If a volcano has not erupted in historical times and exhibits only minor alterations due to erosion, it is considered dormant. When a volcano has erupted in historical times, has been seriously altered by erosion, and shows no signs of physical activity, it is considered extinct.
Volcanoes are classified by their disgorged material that has accumulated around their vents. A volcano is either classified as shield, composite, plug, or cinder cone. During a volcano's lifetime, it may begin as a shield volcano and turn into a composite volcano later on.
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