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The viscosity of magma as it approaches the surface is a function of temperature and composition. Shield volcanoes in Hawai‘i erupt magma as hot as 2,200 °F, compared 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. Dark profile showing typical shape of a shield volcano ( Huallai) Shield volcanoes are known from other planets. The largest known mountain in the solar system, Olympus Mons on Mars, is thought to be an extinct shield volcano. Shield volcanoes on Mars are higher and much more massive than those on Earth. On Earth, because of plate tectonics, hotspot volcanoesIn geology, a hotspot is a location on the Earth's surface that has expreienced active vulcanism for a long period of time. Tuzo Wilson came up with the idea in 1963 that volcanic chains like the Hawaiian Islands result from the slow movement of a tectoni eventually move away from the source of their magma and the volcanoes are individually less massive than might otherwise be the case.