A cave or tunnel formed from a lava flow. As the flow advances, the sides and roof cool and congeal. The outer crust hardens, while the molten lava flows like a river underneath (much like an iced-over river in winter). When the source of the lava subsides, the lava river empties out of the tube, leaving a tunnel which is often big enough for a human to walk or crawl through.

Log in or register to write something here or to contact authors.