Obsidian-rich lava flow on the wall of a caldera (volcanic crater). Obsidian is a natural glass formed by the rapid cooling of lava (molten rock). The flow has similar properties to a glacier, and is evidence for recent vulcanism in the area. Photographed in the Canadas caldera on Tenerife, one of the Canary Islands. These are volcanic islands formed entirely from the eruptions of undersea volcanoes.