Caldera

A caldera is a large, usually circular depression at the summit of a volcano . Most calderas are formed by subsidence or sinking of the central part of the volcano; a rare few are excavated by violent explosions.

Craters and calderas are distinct structures. Both are circular depressions at the tops of volcanoes, but a crater is much smaller than a caldera and is formed by the building up of material around a vent rather than by the subsidence of material below a cone.

A volcano's summit may subside in two ways. First, eruptions of large volumes of pumice or magma , or subterranean drainage of the latter to other areas, may empty a chamber beneath the volcano into which a portion of the cone collapses. Second, the summit of the volcano may act as a thin roof over a large magma chamber that breaks under its own weight and sinks, partly or wholly, into the magma beneath. The term cauldron is sometimes reserved for calderas formed by the foundering of a cone summit in underlying magma.

The largest volcanic structures in the world are resurgent calderas. Resurgent calderas form following intense volcanic eruptions comparable in violence to asteroid impacts. (None has occurred during historical times.) During such an eruption, vast ejections of volcanic material—in some cases, thousands of cubic miles of pumice and ash—excavate very wide underground chambers, much wider than the volcano itself. Large calderas, up to hundreds of square miles in extent, collapse into these chambers. After settling, the caldera floor resurges or bulges up again, lifted by the refilling magma chamber below. Is in the case of the 22 mile (35 km) wide Cerro Galan caldera in Argentina, which is visible as a whole only from orbit, resurgence has raised the center of the caldera to almost a mile (1500 m) above the point of lowest subsidence.

Caldera complexes—overlapping calderas, some swallowing parts of others—are sometimes formed by repeated episodes of partial subsidence. Calderas and caldera complexes are common not only on Earth but on other bodies in the Solar System where volcanoes have erupted in the past or are presently erupting, including Mars, Venus, and Io.

See also Silicic

caldera A roughly circular topographic and structural depression, varying in diameter from about 1 to 100 km (but up to 70 km in some martian examples), and formed by the foundering and collapse of a magma chamber roof into its underlying magma body (e.g. Crater Lake, Oregon, formed by the eruption of Mt Mazama about 6000 years ago). Caldera collapse is commonly preceded or accompanied by rapid explosive evacuation of magma from the chamber in the form of surface pyroclastic flows. This leaves the chamber roof unsupported by magma pressure and collapse follows. Slumping and erosion of the caldera walls may enlarge the topographic rim of the depression well beyond the structural rim. Later injection of magma into the chamber can cause doming of the caldera floor to create a resurgent caldera. Tobu caldera, Sumatra, almost 100 km in longest dimension, is the largest terrestrial caldera.

caldera Large, shallow crater formed when a volcano collapses and the magma migrates under the Earth's crust. The caldera of an extinct volcano, if fed by floodwater, rain or springs, can become a crater lake.

cal·de·ra / kalˈderə; kôl-; -ˈdi(ə)rə/ • n. a large volcanic crater, typically one formed by a major eruption leading to the collapse of the mouth of the volcano.