A caldera is a large, usually circular depression at the summit of a volcano
formed when magma is withdrawn or erupted from a shallow underground magma
reservoir. The removal of large volumes of magma may result in loss of
structural support for the overlying rock, thereby leading to collapse of the
ground and formation of a large depression. Calderas are different from
craters, which are smaller, circular depressions created primarily by explosive
excavation of rock during eruptions.
Left: Mount Aso vents a large cloud of steam at Japan's Aso
National Park. The mountain's caldera is one of the world's largest.