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Calderas and Caldera Formation From: Brantley, 1994, Volcanoes of the United States: USGS General Interest Publication The largest and most explosive volcanic eruptions eject tens to hundreds of cubic kilometers of magma onto the Earth's surface. When such a large volume of magma is removed from beneath a volcano, the ground subsides or collapses into the emptied space, to form a huge depression called a caldera. Some calderas are more than 25 kilometers in diameter and several kilometers deep. Calderas are among the most spectacular and active volcanic features on Earth. Earthquakes, ground cracks, uplift or subsidence of the ground, and thermal activity such as hot springs, geysers, and boiling mud pots are common at many calderas. Such activity is caused by complex interactions among magma stored beneath a caldera, ground water, and the regional buildup of stress in the large plates of the Earth's crust. Significant changes in the level of activity at some calderas are common; these new activity levels can be intermittent, lasting for months to years, or persistent over decades to centuries. Although most caldera unrest does not lead to an eruption, the possibility of violent explosive eruptions warrants detailed scientific study and monitoring of some active calderas. Recently, scientists have recognized volcanic unrest at two calderas in the United States, Long Valley Caldera in eastern California and Yellowstone National Park, Wyoming. Whether unrest at these calderas simply punctuates long periods of quiet or is the early warning sign of future eruptions is an important but still unanswered question. From: Wright and Pierson, 1992, Living With Volcanoes, The U. S. Geological Survey's Volcano Hazards Program: U. S. Geological Survey Circular 1973 Caldera: Large depression commonly formed by collapse of the ground following explosive eruption of a large body of stored magma. Calderas at Yellowstone and Long Valley are associated with eruption of silicic magma as pyroclastic flows that covered large areas around and within the caldera. Kilauea caldera, by contrast, is thought to be associated with draining of basaltic magma from beneath Kilauea's summit. The caldera now filled by Oregon's Crater Lake was produced by an eruption that destroyed a volcano the size of Mount St. Helens and sent volcanic ash as far east as Nebraska. From: Christopher G. Newhall and Daniel Dzurisin, 1988, Historical Unrest at Large Calderas of the World: U. S. Geological Survey Bulletin 1855, 2 volumes Processes of Caldera Unrest: Caldera unrest reflects tectonic, magmatic, and hydrologic processes. For the purposes of this discussion, we define tectonic processes as those that occur in country rock and dominantly involve changes in mechanical energy with little or no movement of mass into or out of the subcaldera environment. Magmatic processes are those that occur within a magma reservoir, and in which thermal energy, magma, and magmatic volatiles can (though need not) move into or out of the subcaldera environment. Hydrologic processes are those involving movement of subcaldera ground water or in which the physical or chemical state of subcaldera ground water is changed. Probably no episode of unrest is purely tectonic, purely magmatic, or purely hydrologic, because tectonic and magmatic changes invariably influence a ground water system and vice versa, and magma (if present) invariably interacts with the local tectonic stress field.