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.

10-metre-high (33 ft) lava fountain in Hawaii, United States
Lava flow during a rift eruption at Krafla, Iceland in 1984

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 lava can be up 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.