A crater is a steep-sided roughly circular to elliptical depression in the earth caused either by volcanic activity or by the impact of an extraterrestrial body. Volcanic craters are formed by explosive events, and/or by the collapse of part of a volcano following withdrawal of magma. Impact craters are the result of collisions between Earth and extraterrestrial bodies such as meteors or comets.

Large volcanic craters are known as calderas among vulcanologists. There are two often-complementary processes involved in their formation; violent eruptions of ash and magma, and/or the collapse of a volcanic surface following withdrawal of a large body of magma from the subsurface. An example of the first type may be Crater Lake in Oregon, thought to have been produced by a violent explosion that destroyed a volcano the size of Mount St. Helens. The caldera at Kilauea, in contrast, is thought...