Lava

Lava is magma once it has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or undersea, usually at temperatures from 800 to 1,200 °C (1,470 to 2,190 °F). The volcanic rock resulting from subsequent cooling is also often called lava.

A lava flow is an outpouring of lava during an effusive eruption. On the other hand, an explosive eruption produces a mixture of volcanic ash and other fragments called tephra, rather than lava flows. Although most molten lava is about 10,000 to 100,000 times more viscous than water, with a viscosity roughly similar to ketchup, lava can flow great distances before cooling and solidifying because lava exposed to air quickly develops a solid crust that insulates the remaining liquid lava, helping keep it hot and inviscid enough to continue flowing.^[1]

The word lava comes from Italian and is probably derived from the Latin word labes, which means a fall or slide.^[2][3] Its first known use in connection with extrusion of magma from below the surface was in a short account of the 1737 eruption of Vesuvius written by Francesco Serao.^[4] Serao described "a flow of fiery lava" as an analogy to the flow of water and mud down the flanks of the volcano (a lahar) following heavy rain.

Lava on the Earth's crust is dominantly composed of silicate minerals: mostly feldspars, feldspathoids, olivine, pyroxenes, amphiboles, micas and quartz.^[5] Rare nonsilicate lavas can be formed by local melting of nonsilicate mineral deposits^[6] or by separation of a magma into immiscible silicate and nonsilicate liquid phases.^[7]

Silicate lavas are molten mixtures dominated by oxygen and silicon, the Earth's most abundant chemical elements, with smaller quantities of aluminium, calcium, magnesium, iron, sodium, and potassium, and minor amounts of many other elements.^[5] Petrologists routinely express the composition of a silicate lava in terms of the weight or molar mass fraction of the oxides of the major elements (other than oxygen) present in the lava.^[8]

The physical behavior of silicate magmas is dominated by the silica component. Silicon ions in lava strongly bind to four oxygen ions in a tetrahedral arrangement. If an oxygen ion is bound to two silicon ions in the melt, it is described as a bridging oxygen, and lava with many clumps or chains of silicon ions connected by bridging oxygen ions is described as partially polymerized. Aluminium in combination with alkali metal oxides (sodium and potassium) also tends to polymerize the lava.^[9] Other cations, such as ferrous iron, calcium, and magnesium, bond much more weakly to oxygen and reduce the tendency to polymerize.^[10] Partial polymerization makes the lava viscous, so lava high in silica is much more viscous than lava low in silica.^[9]