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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—lava 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. Temperature and composition of the magma determine its viscosity. Shield volcanoes in Hawai‘i erupt magma as hot as 2,200 °F, compare with 1,560 °F for most continental volcanoes. Because of the fluidity of the lava, major explosive eruptions do not occur. The most severe explosions occur if water enters a vent, although expanding gases in the magma can produce spectacular fountaining of the low viscosity lava.
Many of the largest volcanoes on Earth are shield volcanoes. The largest is Mauna Loa on the Big Island of Hawai‘i, but there are many other shield volcanoes, for example in Washington State, in Oregon, and in the Galapagos Islands. Also, the largest mountain in the solar system, Olympus Mons on Mars, is believed to be an extinct shield volcano. The Piton de la Fournaise, on Reunion Island, is the most active volcano on earth (1 eruption per year on the average); it is under constant monitoring.
Most volcanic activity is associated with tectonic activity (the movement of the Earth's 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 hotspot 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 those on Earth. On Earth, as hotspot volcanoes are moving away from the source of their magma, the volcanoes grow less massive than would otherwise be the case.
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