Teachers Guide to Stratovolcanoes of the World
Eruption Feature

Veniaminof, Alaska, USA:
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Caldera formation

Calderas are formed by collapse of the volcano usually after much of the material in the magma chamber has been blown out (see animation courtesy of Exploring the Environment). Calderas formed at the summit of stratovolcanoes may range in depth from a few hundred to several thousand feet. Formation of a caldera usually takes place late in the history of the volcano and often follows a long pause in activity during which the cone may become deeply eroded. A long quiet period preceding the eruption allows the magma mass to separate. The caldera results not from explosive decapitation of the mountain, but from subsidence of the summit along ring shaped fractures. Partial drainage of an underlying magma chamber removes support from beneath the top of the mountain, causing it to collapse.

Large calderas mark the surfaces of Mars, Venus, and Jupiter's moon, Io. When Mt. Mazama, in southern Oregon, exploded 6,500 years ago it lost two miles of its height and formed the caldera that now holds Crater Lake. Kilauea, in Hawaii, and Katmai in Alaska's Aleutian Islands, are calderas resulting from the collapse of volcanoes. The 80 km (50 mi) long Lake Balaton in Hungary is a water-filled caldera formed when a magma chamber collapsed.