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SHIELD VOLCANO
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Shield Volcanoes
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.