Volcanic craters and calderas differ mainly in the way they are formed. Craters tend to be more nearly circular than calderas, and they are often, though not always, smaller than calderas. Craters are the vents through which gas, magma and tephra are ejected, while calderas form as a result of local collapse of the surrounding rock.
Volcanic craters tend to be relatively small and circular, while calderas can be almost any shape and tend to be large. Craters form when volcanic ejecta emerges from an active site and recedes into the subterranean magma chamber. Calderas can begin as craters, but they widen and deepen when their underground magma chamber empties. This can happen explosively, with a sudden eruption that blasts away the surface strata, or it can happen gradually as the chamber drains. Once the underground chamber is empty, the ceiling above it is largely unsupported and prone to collapse. It is this process of collapse that gives calderas their distinctive size and irregular shapes. Calderas usually form at the end of a volcano's activity, as the magma chamber beneath is exhausted. Craters, however, can remain active for many years as multiple eruptions pass through them. Collapse sites that remain active often form new craters inside the older caldera.