Shield volcanoes are large volcanoes that are built almost entirely of fluid lava flows. It has broad sloping sides and is usually surrounded by gently sloping hills in a circular or fan shaped pattern, that looks like a warrior's shield.
The volcano is produced by the action of the gas (steam or water vapor) with heat from the earth's core. This action melts rock turning it into magma. The pressure from the heat of the gas pushes the magma upwards till it explodes. Molten magma shoots upward from deep below the ocean floor and breaks through the drifting plates to form shield volcanoes. Lava flows gently and continuously out of the central volcanic vent or group of vents. This lava is very runny, and can't be piled up into steep mounds. It gradually accumulates and cools around the volcano. The eruptions are characterized by low explosivity lava-fountaining that forms cinder cones and spatter cones at the vent. The volcanoes are built up slowly by the accretion of thousands of highly fluid lava flows called basalt lava. The lava spread widely over great distances, then cools as thin gently dipping sheets. Lavas also erupt from vents along fractures (rift zones) that form on the flanks of the cone. Some of the largest volcanoes in the world are Shield volcanoes.
Shield volcanoes may be produced by hot spots which lay far away from the edges of tectonic plates. Shields also occur along the mid-oceanic ridge, where sea floor spreading is in progress and along subduction related volcanic arcs.
In northern California and Oregon, many shield volcanoes have diameters of three or four miles and heights of 1,500 to 2,000 feet. A good example of a shield volcano is the Island of Hawaii. The Big Island is formed of five coalesced volcanoes of successively younger ages. The Hawaiian Islands are composed of linear chains of volcanoes including Kilauea and Mauna Loa on the island of Hawaii.
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