Caldera. Caldera is a large roughly circular volcanic depressions often several hundred square kilometre's in area. Caldera usually has a number of smaller vents and can also contain a large crater lake. The distinction between craters and calderas is essentially one of size, one to kilometres being the lower limit for the diameter of a caldera. Calderas are formed in a variety of ways, but most accepted mechanism is the collapse or subsidence which may relate to explosive eruptions. There are numerous ex of caldera in Alaska and Aleutian Islands.