 Lava. Lava is molten rock generated by geothermal energy and expelled through fractures in planetary crust or in an eruption, usually at temperatures from 700 to 1200 degrees C – 1292 to 2192 degrees F. The structures resulting from subsequent solidification and cooling are also sometimes described as lava. The molten rock is formed in the interior of some planets, including Earth, and some of their satellites, though such material located below the crust is referred to by other terms. The 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 thick isotropic and shear opening 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 lebs which means the fall or slide. The first use in connection with extruded magma molten rock below the Earth's surface was apparently in the short account written by Francesco Sarrao on the eruption of Vosuvius between May 14 and June 4, 1737. Sarrao described the flow of fiery lava as an analogy to the flow of water and munt down the flanks of the volcano following heavy rain.