A caldera volcano's circular to oval crater exceeds 1 mile in diameter. These form when so much lava is erupted (blown out) so rapidly it partially empties the underlying magma chamber. When this happens the summit of the volcanic structure collapses into the emptied magma chamber. Typically the erupted material occurs as airfall or pyroclastic flows.This is one of several dormant volcanos with it's top blown out on the Alaska Peninsula. Its sides are covered with lava, a cloud is hovering above, and the Chigmit Mountains, part of the Aleutian Range, are in the distance. Photo © copyright by Robert Faucher.