Lava

Lava, any rock ejected from a volcanic orifice in a state of fusion. Lavas differ much in liquidity at the time of eruption—the basic lavas being more fluid generally than those that contain a high percentage of silica. The surface of a lava-stream, which speedily cools and hardens, is generally more or less porous and vesicular, from the escape of the confined gases; but, as rock is always a bad conductor of heat, the interior often remains long in a liquid condition, permitting the continued flow of the stream sometimes to a very great distance from the orifice from which it has been discharged, notwithstanding its indurated covering. The end of the stream is a slowly-moving mass of loose porous blocks, rolling and tumbling over each other with a loud rattling noise, being pushed forward in fits and starts by the viscid lava, when it bursts the hardened crust and rushes on. The structure of the interior of a solid lava-stream shows a compact and homogeneous rock, assuming a more or less crystalline structure as the cooling has been the work of a longer or shorter period of time. Caverns are sometimes formed in lava-streams by the escape of the molten mass below, leaving the cooled crust standing like the roof of a tunnel.