Lava lakes are large volumes of molten lava, usually basaltic, contained in a crater or broad depression. The term is used to
describe both lava lakes that are molten and those that are partly or completely solidified.
Facts and Figures: Active lava lakes typically consist of a partially solidified shiny gray crust because its surface is constantly cooled by the atmosphere. The crust is seldom more than 5-30 cm thick, or more than a few minutes or hours old, because it continually circulates, breaks, and sinks into the moving molten lava below. The pattern of movement on the surface of lava lakes is often compared to the type of large-scale movement that occurs between the huge plates that make up the Earth's crust, including subduction, spreading, and strike-slip movement. Lava lakes occur at relatively few volcanoes in the world. For example, since 1980, lava lakes have formed at Kilauea Volcano (more images here) in Hawaii, Mount Erebus in Antarctica (involving rare phonolitic lava), Erta Ale in Ethiopia ( a lot of material here) and Nyiragongo in Zaire.
Erta Ale lava lake, Ethiopia, 2002. Photo: M. Fulle