A large volcanic depression, usually several kilometres in diameter, formed primarily by the collapse of a magma chamber roof or, less frequently, by a volcanic explosion. A caldera often contains smaller craters, cones, and other types of volcanic vent. Small collapse craters less than 1 km in diameter are known as collapse pits. Calderas are often non-circular in shape. Many of the small number of volcanic craters on the Moon appear to be calderas, some of them at the heads of sinuous rilles. There are also huge calderas at the summits of many of the giant volcanoes of Mars. Olympus Mons has a complex of summit calderas more than 60 km in diameter, apparently the result of several collapse events. Similar features are found on the volcanoes of Venus.
http://www.esa.int/spaceinimages/Images/2004/02/Detail_of_the_complex_caldera_of_Olympus_Mons
A roughly circular topographic and structural depression, on Earth varying in diameter from about 1 to 100 km and averaging 68 km on Venus, 40 km on Io, and 48 km on Mars, formed by the foundering and collapse of a magma chamber roof into its underlying magma body (e.g. Crater Lake, Oregon, formed by the eruption of Mt Mazama about 6000 years ago). Caldera is Spanish for cauldron or cooking pot. Caldera collapse is commonly preceded or accompanied by rapid explosive evacuation of magma from the chamber in the form of surface pyroclastic flows. This leaves the chamber roof unsupported by magma pressure and collapse follows. Slumping and erosion of the caldera walls may enlarge the topographic rim of the depression well beyond the structural rim. Later injection of magma into the chamber can cause doming of the caldera floor to create a resurgent caldera. Tobu caldera, Sumatra, almost 100 km in longest dimension, is the largest terrestrial caldera.
A sunken crater at the centre of a volcano, formed when the magma founders, so that the centre of the volcano collapses. Kennedy and Stix (2007) GSA Bull. 119 examine magmatic processes and caldera collapse in New Hampshire; and Furuya (2003) Earth Planets Space 55 constructs an optimum source model of caldera formation.
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