A large, bowl-shaped volcanic depression leading to the expulsion of a large quantity of molten rock (magma). The depression is more than one kilometer in diameter and surrounded by faults with instabilities that can bring about a renewal of volcanic activity. Calderas usually, if not always, form by the collapse of the top of a volcanic cone or group of cones because of removal of the underlying body of magma. Subsequent minor eruptions may build small cones on the floor of the caldera. These may later fill with water, as did Crater Lake in Oregon.