DEFINITION OF CALDERA

Huge basin-shaped semicircular depressions,

calderas, like Crater Lake in Oregon, are often many kilometers

in diameter, produced when deep magma chambers

under a volcano empty out (during an eruption), and the

overlying land collapses inward producing a topographic

depression. Yellowstone Valley occupies one of the largest

calderas in the United States. Many geysers, hot springs, and

fumaroles in the valley are related to groundwater circulating

to depths, being heated by shallow magma, and mixing with

volcanic gases that escape through minor cracks in the crust

of the Earth.

Calderas may be relatively stable for tens or hundreds

of thousands of years, but often magma reenters the collapsed

magma chamber and causes it to rise up or inflate.

This forms a resurgent caldera, which can produce a catastrophic

volcanic eruption. Some of the largest volcanic eruptions

in history have come from resurgent calderas. For

instance, 600,000 years ago Yellowstone caldera experienced

a resurgent eruption that spewed more than 240

cubic miles (1,000 km3) of volcanic material into the air,

covering much of the United States with thick volcanic

debris. This is more than 1,000 times the amount of material

erupted by Mount Saint Helens in 1980. Other dormant

calderas in the United States include Long Valley in California

and Crater Lake in Oregon. One of the greatest volcanic

eruptions in history was the 1883 explosion from the

caldera of Krakatau in the Java-Sumatra Straits. On August

27, 1883, the four-mile (6-km) wide caldera of Krakatoa

exploded with a force 5,000 times stronger than the atomic

bomb dropped on Hiroshima. Water surged into the collapsed

caldera and exploded with a sonic boom that was

heard 1,240 miles (2,000 km) away in Australia. Ash covered

270,000 square miles (700,000 km2), and 13 percent

of the global sunlight was blocked, lowering global temperatures

by several degrees for more than a year. The explosion

also generated a huge 130-foot (40-m) high tsunami

that killed approximately 36,000 people.

See also VOLCANO.

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