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Shield volcano

A shield volcano is a volcano that, because of its domical shape, resembles a warrior's shield. Shield volcanoes are formed by lava of low viscosity that flows easily; consequently, the volcanic dome is built up over time by flow after flow of fluid basaltic lava that issues from vents or fissures on the surface of the volcano. Because of the fluidity of the lava, major explosive eruptions do not occur. The most severe explosions occur if water enters a vent.

Many of the largest volcanoes on earth are shield volcanoes. The largest in the world is Mauna Loa on the Big Island of Hawaii, but there are many other shield volcanoes, for example in Washington State, in Oregon, and in the Galapagos Islands.

Most volcanic activity is associated with tectonic activity (the movement of the earths plates), with the majority of the activity occurring underwater in the mid-oceanic subduction zones. The Hawaiian volcanoes, e.g. Mauna Loa, are an exception to that rule occurring as they do in a mid-plate location where there is a hot spot in the Earth's crust.

There are shield volcanoes on other planets. For example, the shield volcanoes on Mars are higher and more massive than their equivalents on earth. On Earth, as most volcanoes are moving away from the source of their magma, the volcanoes grow less massive than would otherwise be the case. A suitable example would be the Hawaiian volcanoes where the Pacific Plate, which is slowly moving to the northwest, moves the volcanoes over the hot spot.