Lava is molten rock (magma) that has been expelled from the interior of some planets (including Earth) and some of their moons. Magma is generated by the internal heat of the planet or moon and it is erupted as lava at volcanoes or through fractures in the crust, usually at temperatures from 700 to 1,200 °C (1,292 to 2,192 °F). The solid rock resulting from subsequent cooling is also often described as lava.
A lava flow is a moving outpouring of lava created during a non-explosive effusive eruption. When it has stopped moving, lava solidifies to form igneous rock. The term lava flow is commonly shortened to lava. Although lava can be up to 100,000 times more viscous than water, lava can flow great distances before cooling and solidifying because of its thixotropic and shear thinning properties.
Explosive eruptions produce a mixture of volcanic ash and other fragments called tephra, rather than lava flows. The word lava comes from Italian, and is probably derived from the Latin word labes which means a fall or slide. The first use in connection with extruded magma (molten rock below the Earth's surface) was apparently in a short account written by Francesco Serao on the eruption of Vesuvius in 1737. Serao described "a flow of fiery lava" as an analogy to the flow of water and mud down the flanks of the volcano following heavy rain.
The biggest hazard of lava flows is that they destroy property. In the late 1980's, the town of Kalapana in Hawai'i was destroyed by lava flows. Lava flows buried cars and burnt homes, buildings, and vegetation. Electric power, water, and communications were cut off from the community.