Lava moves across the ground as a pahoehoe flow, Kilauea Volcano, Hawaii. Credit: J. D. Griggs / U.S. Geological Survey.
Lava is molten rock that erupts onto the surface of a planet or moon, from a volcano or fissure, and is hot enough to flow (before it reaches the surface it is called magma); also, the rock formed by solidification of this material. Lava flows are described as pillow lava, pahoehoe, and a'a. See also scoria.
Silica-rich lava hardens before flowing far, forming a dense-texture rock of tiny crystals or glass. Basic lava flows further before it solidifies, giving rise to coarse-grained igneous rock, such as granite or gabbro. In many eruptions, lava is ejected with such force that it fragments in the atmosphere, hardens while airborne, and lands to form thick layers of volcanic tuff and related pyroclastic rock.
The word lava is from the Italian word for stream, which is derived from the Latin verb lavare – to wash.