Info on Craters & Calderas
Craters and Calderas
Craters are circular depressions, usually less than 1 km in diameter, that form as a result of explosions that emit gases and tephra.
Calderas are much larger depressions, circular to elliptical in shape, with diameters ranging from 1 km to 50 km. Calderas form as a result of
collapse of a volcanic structure. The collapse results from evacuation of the underlying magma chamber.
In shield volcanoes, like in Hawaii, the evacuation of the magma chamber is a slow drawn out processes, wherein magma is withdrawn to erupt on from
the rift zones on the flanks.
In stratovolcanoes the collapse and formation of a caldera results from rapid evacuation of the underlying magma chamber by voluminous explosive
eruptions that form extensive fall deposits and pyroclastic flows.
Calderas are often enclosed depressions that collect rain water and snow melt, and thus lakes often form within a caldera.
Crater Lake Caldera in southern Oregon is an 8 km diameter caldera containing a lake The caldera formed about 6800 years ago as a result of the
eruption of about 75 km3 of rhyolite magma in the form of tephra, found as far away as Canada, accompanied by pyroclastic flows that left thick
deposits of tuff on the flanks of the volcano. Subsequent eruptions have built a cinder cone on the floor of the caldera, which now forms an island
called Wizard Island.
Larger calderas have formed within the past million years in the western United States. These include Yellowstone Caldera in Wyoming, Long Valley
Caldera in eastern California, and Valles Caldera in New Mexico.
The Yellowstone caldera is an important example, as it illustrates the amount of repose time that might be expected from large rhyolitic systems, and
the devastating effect caldera forming eruptions can have on widespread areas.
Yellowstone Caldera which occupies most of Yellowstone National Park, is actually the third caldera to form in the area within the past 2 million
years. The three calderas formed at 2.0 million years ago, 1.3 million years ago, and the latest at 600,000 years ago. Thus the repose time is on
the average about 650,000 years.
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