caldera

Caldera comes directly from Spanish caldera (a cauldron, a large pot, a kettle), from Late Latin caldaria (a vessel for boiling), from Latin caldus or calidus (hot), from calere (to be hot). The same Latin root gives English cauldron, and ultimately the chemical element calcium (named by Humphry Davy in 1808 from Latin calx, lime, because lime was produced by heating limestone). The geological use of caldera enters scientific English from the Canary Islands: the island of La Palma has a large volcanic depression called the Caldera de Taburiente, and Spanish explorers and naturalists used the local name. Nineteenth-century volcanologists adopted the Spanish term to distinguish the large collapse depressions of major volcanic systems from the smaller crater at a volcano's summit — a crucial distinction that the existing vocabulary of crater and vent did not adequately capture.

A caldera forms not by explosion but by collapse. When a volcanic system erupts a large volume of magma rapidly — either explosively as ash and pyroclastic flows, or effusively as lava floods — the magma chamber beneath the volcano is emptied or partially drained. The overlying rock, now unsupported, collapses inward under its own weight, creating a circular or elliptical depression that can be tens of kilometers across and hundreds of meters deep. This is the caldera. The eruption of Krakatau in 1883 and the subsequent collapse created a caldera largely below sea level. The eruption of Pinatubo in 1991 produced a caldera approximately 2.5 kilometers in diameter. The caldera of Yellowstone — the system that last erupted catastrophically 640,000 years ago — is approximately 55 by 72 kilometers, so large that it was not recognized as a volcanic caldera until aerial photography made its outline visible.

Calderas are not dead features. The collapsed floors of many calderas continue to be the sites of volcanic activity: lava lakes, fumarolic fields, geothermal springs, and smaller eruptions within the caldera. Crater Lake in Oregon fills a caldera created by the collapse of the ancient volcano Mount Mazama approximately 7,700 years ago; the blue lake that occupies the caldera is one of the deepest in North America. The Campi Flegrei (Phlegraean Fields) near Naples, Italy, is an active caldera system in which the ground periodically rises and falls by meters — a phenomenon called bradyseism — as the magma system beneath it inflates and deflates. Hundreds of thousands of people live within and around this caldera, making it one of the most closely monitored volcanic hazards in the world.

Supervolcanoes — a term popularized in the early 2000s for volcanic systems capable of eruptions orders of magnitude larger than recorded historical eruptions — are defined by their calderas. Yellowstone, Toba (Indonesia), Long Valley (California), Taupo (New Zealand), and the Aira caldera in Japan are all supervolcanic calderas formed by eruptions that ejected more than 1,000 cubic kilometers of material. The Toba eruption approximately 74,000 years ago — so large it may have created a volcanic winter lasting years — is implicated in genetic evidence for a severe human population bottleneck in late Pleistocene Africa. The caldera it left is now Lake Toba, a lake 100 kilometers long and 30 kilometers wide. The cooking pot that the Spanish saw in the Canary Islands turns out to be the correct metaphor at every scale: the planet's largest geological depressions are the pots where the greatest fires burned.