The feature that provides evidence that a volcano existed in the past is a caldera. A caldera is a large, basin-shaped depression that forms when the summit of a volcano collapses after a massive eruption or the draining of a magma chamber. The caldera is generally surrounded by steep walls or cliffs, and may be filled with water to form a lake. By examining the geological characteristics and composition of a caldera, geologists can identify it as evidence of a former volcano.
