Much of the surface of the caldera is lined with a white crust of carbonate salts such as sodium carbonite and natrolite. This substance is also known as natron, leading to the French name for the site. This crust is sometimes known as the Tibesti Soda Lake. The crusts are formed when mineral-rich steam is emitted from small vents on the crater's floor. When this water evaporates in the desert heat, the minerals remain behind as part of the crust.