A caldera is a large, roughly circular or elliptical volcanic depression formed when the ground above a magma reservoir collapses after a major volcanic eruption or after substantial withdrawal of magma from beneath the surface. The word comes from the Spanish caldera, meaning cauldron, reflecting the broad basin like shape of many examples. A caldera is substantially larger than an ordinary volcanic crater, although the boundary between the two can sometimes be gradual rather than absolute.
Calderas may range from less than a kilometre to several tens of kilometres across, depending on the size and geometry of the underlying volcanic system. Their formation is fundamentally a structural response to loss of support beneath the ground. If a large volume of magma is evacuated during an eruption, the overlying rocks may become unable to support themselves and collapse along fractures or ring faults. The resulting depression can subsequently become partly filled by lava, ash, sediments, water or new volcanic structures.
Caldera formation is commonly associated with very large explosive eruptions, particularly at volcanoes where viscous, silica rich magma can retain dissolved gases and fragment violently. Before a major eruption, magma may accumulate in a shallow crustal reservoir or interconnected magma storage system. Pressure, magma ascent and the opening of fractures can lead to an eruption through vents and fissures. As magma is withdrawn, the roof above the reservoir may fracture along approximately circular or arcuate faults.
Large blocks of crust can then move downward into the partially emptied reservoir. This process is known as subsidence. The collapse may occur progressively during the eruption or in a relatively rapid episode. Importantly, the caldera is produced principally by collapse rather than simply by explosive excavation of a hole in the ground. The volume and geometry of the collapsed region depend on the amount of magma removed, the strength and structure of the surrounding rocks and the shape of the magma storage system.
There are several broad types and formation mechanisms of caldera. Collapse calderas associated with large explosive eruptions are particularly well known, but caldera like depressions can also develop through subsidence caused by magma withdrawal without a single enormous explosive event. Some volcanic systems experience repeated episodes of inflation, eruption and subsidence, producing complex nested or overlapping calderas.
Calderas can also form in association with basaltic volcanic systems where large quantities of relatively fluid magma drain away, although their appearance and scale may differ substantially from those of silicic explosive systems. Nested calderas occur when a younger collapse develops inside an older one. Multiple eruptions and collapses can therefore create complicated geological histories in which several generations of faults, lava flows and pyroclastic deposits overlap. Scientists reconstruct these histories through field mapping, geochronology, geochemistry, geophysical measurements and analysis of volcanic deposits.
The landscape inside a caldera can change dramatically after collapse. A depression may become occupied by a lake if precipitation, groundwater or rivers supply water faster than it drains. Examples include Crater Lake in Oregon, formed within the collapsed Mount Mazama volcanic system, and numerous other volcanic lakes worldwide. Post caldera volcanism can construct lava domes, cinder cones, lava flows or new stratovolcanoes on the caldera floor.
Magma may also intrude into the crust without reaching the surface, producing renewed uplift and deformation. Some calderas therefore remain volcanically active for thousands or millions of years after their initial collapse. The presence of a large caldera does not automatically mean that another giant eruption is imminent, because volcanic systems evolve and may become inactive. Nevertheless, active calderas are important areas for geological and volcanic monitoring.
Caldera systems can contain extensive hydrothermal systems because heat from magma remains beneath the volcanic complex after an eruption. Groundwater circulating through hot rocks can become heated and chemically altered, producing hot springs, fumaroles, geysers and mineral deposits. Hydrothermal activity can also weaken rocks and contribute to hazards such as steam explosions, toxic gas emissions, ground instability and sudden changes in geothermal activity. Calderas are consequently important not only to volcanology but also to geothermal energy research and mineral exploration. Hydrothermal fluids can transport and concentrate elements, sometimes producing economically valuable ore deposits. The relationship between magma, heat, groundwater and fractures makes a caldera a complex geological system rather than simply an old crater.
Calderas can produce several hazards depending on whether the underlying volcanic system remains active. Future eruptions may generate lava flows, ashfall, pyroclastic density currents, volcanic gases and lahars. Caldera lakes can create additional hazards because volcanic gases entering deep or poorly mixed water can accumulate beneath the surface and potentially be released suddenly under unusual circumstances.
Ground deformation and earthquake swarms may indicate movement of magma or hydrothermal fluids, although such signals do not by themselves prove that an eruption will occur. Large caldera eruptions are rare compared with ordinary volcanic eruptions, and the term supervolcano is an informal term rather than a precise geological classification. Risk assessment therefore requires examination of the particular volcanic system rather than assuming that every caldera has the same behaviour.
Calderas are also important records of Earth's geological history. Their volcanic deposits can cover enormous areas and preserve information about past climates, atmospheric chemistry, ecosystems and volcanic processes. Layers of ash produced by major eruptions can sometimes be identified over great distances and used as chronological markers called tephra horizons. Minerals in volcanic rocks can be dated using radiometric techniques, allowing scientists to establish eruption ages and reconstruct the recurrence of major events. The chemical composition of erupted material can reveal information about magma sources, crustal assimilation, storage conditions and eruption evolution. Caldera research therefore contributes to studies of crustal evolution, tectonics, climate change, biodiversity and natural hazards.
A common misconception is that a caldera is simply an unusually large volcanic crater. Both are volcanic depressions, but a classic caldera is principally associated with structural collapse of a substantial volcanic area, whereas a crater is generally a smaller depression around a vent or other volcanic feature. Calderas can contain craters, cones, lava domes, lakes and younger volcanic structures, making their modern appearance highly variable. Another misconception is that every caldera was formed by one instantaneous catastrophic explosion.
Some develop through prolonged or multi stage magma withdrawal and subsidence. Their importance comes from the interaction of volcanism, crustal deformation, hydrothermal activity, erosion, sedimentation and subsequent volcanic renewal. A caldera is therefore best understood as a large, evolving volcanic structural system whose surface depression is only one part of a much deeper magmatic process.
