caldera

caldera A roughly circular topographic and structural depression, varying in diameter from about 1 to 100 km (but up to 70 km in some martian examples), and 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 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.