Calderas (caldaria = a cooking pot) are large depressions with diameters that exceed one kilometer and have a somewhat circular form. (those less than a kilometer across are called collapse pits or craters.) most calderas are formed by one of the following processes: (1) the collapse of the summit of a large composite volcano following an explosive eruption of silica-rich pumice and ash fragments (crater lake-type calderas); (2) the collapse of the top of a shield volcano caused by subterranean drainage from a central magma chamber (Hawaiian-type calderas); and (3) the collapse of a large area, caused by the discharge of colossal volumes of silica-rich pumice and ash along ring fractures (Yellowstone-type calderas).