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Illustrated Volcano Glossary
Large crater, usually several kilometers across, formed by the collapse of the roof of a magma chamber emptied by large explosive eruptions.
A caldera is a large, usually circular depression at the summit of a volcano formed when magma is withdrawn or erupted from a shallow underground magma reservoir. The removal of large volumes of magma may result in loss of structural support for the overlying rock, thereby leading to collapse of the ground and formation of a large depression. Calderas are different from craters, which are smaller, circular depressions created primarily by explosive excavation of rock during eruptions. (From USGS Photoglossary).
Actually, calderas are a very common feature with most larger volcanoes. During the lifetime of a typical volcano, larger caldera-forming eruptions (that typically discharge around 1 km3 of magma or more) tend to ocur at typical time intervals of a few 1000 years. Subsequent smaller eruptions often rebuid the cone of the volcano, so that earlier calderas often are not easily visible.
Famous calderas include Santorini in Greece, the Campi Flegrei caldera, Italy, Crater Lake caldera in Oregon (USA), Yellowstone Caldera, Wyoming (USA), Long Valley Caldera in east California (USA), Batur, Tengger and Krakatoa volcanoes (Indonesia) and many more.
-- The largest eruption of the 20th century from the Novarupta vent in the Valley of 10,000 Smokes of Alaska, ejected about 12 km3 of magma and resulted in the formation of a caldera 3 km across. Amazingly, the caldera collapse didn't occur at the eruption vent, but 10 km away at Katmai, a stratovolcano! Apparently magma drained away from Katmai's magma reservoir to Novarupta's erupting vent.
-- Yellowstone National Park consists of three enormous calderas that erupted about 2, 1.2, and 0.6 million years ago. The most recent caldera is 45 km across and 75 km long!
-- Caldera-forming eruptions are the largest eruptions on Earth. For example, the Fish Canyon eruption in southwestern Colorado (United States) about 28 million years ago erupted more than 5,000 km3 of magma from La Garita caldera. That's enough magma to bury the entire state of California to a depth of nearly 12 m!
(from: USGS Photoglossary)
Related keywords (2): crater
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: A third tectonic setting where volcanism occurs is believed to be the result of mantle plumes and not directly related to plate boundaries. So called hot spot volcanoes fall into this category.