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large oval-shaped or round depressions of volcanic origin, with steep sides that are frequently stepped. Calderas are up to 10-20 km across and several hundred meters deep. A distinction is made between explosion calderas, which are formed during powerful explosions of gases escaping from volcanic vents, and collapse calderas, which occur when the roof of an underground volcanic focus sinks along circular fractures because material has been discharged from the focus during volcanic eruptions. Collapse calderas are more common and larger than explosion calderas.