The earth's magma, a siliceous melt which is believed to underlie the entire solid crust of the earth, is, in all probability, a highly viscous liquid. Wherever molten lava is observed in motion, either in craters of volcanoes (Kilauea, Mauna Loa, Hawaii), or as disastrous flows which descend volcanic mountains (Vesuvius, Mount Aetna, etc.), or in the vicinity of volcanic fissures (Laki, Eldgja rift, Iceland) it behaves like a liquid of appreciable viscosity; the degree of viscosity, however, varies, depending on the proportion of admixed gases and water vapor, and the chemical composition of the lava. Lavas rich in silica are highly viscous (rhyolite, dacite), while those rich in iron and magnesium (basalt, andesite) are more fluid. The viscosity of every lava flow increases until the melt slowly freezes, because of the chilling effect of the surface temperatures on the earth.

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