Dictionary:

caldera

  (kăl-dâr'ə, -dîr'ə, käl-)

A large crater formed by volcanic explosion or by collapse of a volcanic cone.

[Spanish, cauldron, caldera, from Late Latin caldāria. See cauldron.]

A large volcanic collapse depression, typically circular to slightly elongate in shape, the dimensions of which are many times greater than any included vent. Calderas range from a few miles to 37 mi (60 km) in diameter. A caldera may resemble a volcanic crater in form, but differs genetically in that it is a collapse rather than a constructional feature. The topographic depression resulting from collapse is commonly widened by slumping of the sides along concentric faults, so that the topographic crater wall lies outside the caldera wall. As originally defined, the term caldron referred to volcanic subsidence structures, and caldera referred only to the topographic depression formed at the surface by collapse. However, the term caldera is now common as a synonym for caldron, denoting all features of collapse, both topographic and structural. See also Petrology.

Calderas occur primarily in three different volcanic settings, each of which affects their shape and evolution: basaltic shield cones, stratovolcanoes, and volcanic centers consisting of preexisting clusters of volcanoes. These last calderas, associated with broad, large-volume andesitic to rhyolitic ignimbrite sheets, are generally the largest and most impressive, and are those generally denoted by the term. Calderas have been formed throughout much of the Earth's history, ranging in age from Precambrian (greater than 1.4 billion years old) to Holocene (for example, Krakatau in Indonesia, which erupted in 1883). See also Rhyolite; Volcano.

In addition to Earth, large calderas occur on Mars, Venus, and Jupiter's moon Io. The presence of calderas on four solar system bodies indicates that the underlying mechanisms of shallow intrusion and caldera collapse are basic processes in planetary geology. See also Magma.

Collapse occurs because of withdrawal of magma from an underlying chamber some 2.4–3.6 mi (4–6 km) beneath the surface, resulting in foundering of the roof into the chamber. Withdrawal of magma may occur either by relatively passive eruption of lavas, as in the case of calderas formed on basaltic shield cones, or by catastrophic eruption of pyroclastic material, as accompanies formation of the largest calderas.

Caldera-forming eruptions probably last only a few hours or days. Eruption of pyroclastic material begins as gases (predominantly water) that are dissolved in the magma come out of solution at shallow depths. Magma is explosively fragmented into particles ranging in size from micrometers to meters. An eruption column develops, rising several miles into the atmosphere. This first and most explosive phase of the eruption, known as the Plinian phase, covers the area around the vent with pumice. Caldera subsidence occurs during eruption. As caldera subsidence proceeds and eruption becomes less explosive, the Plinian eruption column collapses. This collapse produces hot, ground-hugging pyroclastic flows that can travel as far as 93 mi (150 km) outward from the vent at speeds of 330 ft/s (100 m/s). Successive collapses of the column produce multiple flow units with an aggregate thickness that may be several hundreds of feet thick near the caldera.

The floors of many of the largest calderas (typically those with diameters exceeding 6 mi or 10 km) have been domed upward, resulting in a central massif or resurgent dome. Resurgence results from the continued or renewed buoyant rise of magma after collapse.

Calderas typically contain or are associated with extensive hydrothermal systems, because of two factors: (1) the shallow magma chambers that underlie them provide a readily available source of heat: and (2) the floors of calderas may be extensively fractured, which, along with the main ring faults, allows meteoric water to penetrate deeply into the crust beneath calderas. Hydrothermal activity related to a caldera system can occur any time after magmas rise to shallow crustal levels, but it is dominant late in caldera evolution.

Many metals, including such base and precious metals as molybdenum, copper, lead, zinc, silver, gold, mercury, uranium, tungsten, and antimony, are mobile in hydrothermal circulation systems driven by the shallow intrusions which underlie and give rise to large calderas. Many economically important ore deposits in the western United States lie within calderas. See also Ore and mineral deposits.

A sunken crater at the centre of a volcano, formed as a result of subsidence. As the magma founders, so the centre of the volcano collapses. The Plateau of Giant Craters in Tanzania contains many impressive calderas, including Ngorongoro, which attains a diameter of 22 km.

For more information on caldera, visit Britannica.com.

A large, basin-shaped volcanic depression caused by collapse.

Satellite image of Santorini's caldera, formed in the Minoan eruption around 1600 BCE. Clockwise from center: Nea Kameni; Palea Kameni; Aspronisi; Therasia; Thera

A caldera is a volcanic feature formed by the collapse of land following a volcanic eruption. They are often confused with volcanic craters. The word 'caldera' comes from the Spanish language, meaning "cauldron".

Caldera formation

A collapse is triggered by the emptying of the magma chamber beneath the volcano, usually as the result of a large volcanic eruption. If enough magma is erupted, the emptied chamber will not be able to support the weight of the volcanic edifice (the mountain) above. Fractures will form around the edge of the chamber, usually in a roughly circular shape. These ring fractures may in fact serve as volcanic vents. As the magma chamber empties, the center of the volcano within the ring fractures begins to collapse. The collapse may occur as the result of a single massive eruption, or it may occur in stages as the result of a series of eruptions. The total area that collapses may be hundreds or thousands of square kilometers.

Landsat image of Lake Toba. A resurgent dome formed the island of Samosir.

Explosive calderas

If the magma is rich in silica, the caldera is often filled in with ignimbrite, tuff, rhyolite, and other igneous rocks. Silica-rich magma is slow flowing or has high viscosity. As a result, gases tend to become trapped at high pressure within the magma. When the magma gets near the surface of the Earth, the gas expands quickly, causing explosions and spreading volcanic ash over wide areas. Further lava flows may be erupted, and the center of the caldera is often uplifted in the form of a resurgent dome by subsequent intrusion of magma. A silicic or rhyolitic caldera may erupt hundreds or even thousands of cubic kilometers of material in a single event. Even small caldera-forming eruptions, such as Krakatoa in 1883 or Mount Pinatubo in 1991, may result in significant local destruction and a noticeable drop in temperature around the world. Large calderas may have even greater effects.

When Yellowstone Caldera (last) erupted 640,000 years ago it released 1,000 cubic kilometers of material, covering all of North America in up to two meters of debris. By comparison, when Mount St. Helens erupted in 1980, it released 1.2 cubic kilometers of ejecta. The ecological effects of the eruption of a large caldera can be seen in the record of the Lake Toba eruption in Indonesia. About 75,000 years ago, this volcano released 2,800 cubic kilometers of ejecta, the largest known eruption within the Quaternary Period (last 1.8 million years). In the late 1990s, archeologist Stanley Ambrose [1] proposed that a volcanic winter induced by this eruption reduced the human population to a few thousand individuals, resulting in a population bottleneck (see Toba catastrophe theory). Even larger caldera-forming eruptions are known, especially La Garita Caldera in the San Juan Mountains of Colorado, where the 5,000 cubic kilometer Fish Canyon Tuff was blasted out in a truly major single eruption 27.8 million years ago.

At some points in geologic time, rhyolitic calderas have appeared in distinct clusters. The remnants of such clusters may be found in places such as the San Juan Mountains of Colorado (erupted during the Tertiary Period) or the Saint Francois Mountain Range of Missouri (erupted during the Proterozoic).

Satellite photograph of Fernandina Island's summit caldera

Non-explosive calderas

Some volcanoes, such as Kīlauea on the island of Hawaii, form calderas in a different fashion. In the case of Kilauea, the magma feeding the volcano is relatively silica poor. As a result, the magma is much more viscous than the magma of a rhyolitic volcano, and the magma chamber is drained by large lava flows rather than by explosive events. The resulting calderas are also known as subsidence calderas, and can form more gradually than explosive calderas. For instance, the caldera atop Fernandina Island underwent a collapse in 1968, when parts of the caldera floor dropped 350 meters. [2] Kilauea Caldera has an inner crater known as Halema‘uma‘u, which has often been filled by a lava lake. The largest volcano on Earth, Mauna Loa is also capped by a subsidence caldera called Moku‘āweoweo Caldera.

Non-volcanic calderas

It is possible, although rare, for a caldera-like formation to be created by erosion rather than volcanism. The Caldera de Taburiente on La Palma in the Canary Islands may be example of this.^{[citation needed]}

Notable calderas

See also Category:Volcanic calderas

• Africa
  • Ngorongoro Crater (Tanzania, Africa)
  • Mount Elgon (Uganda/Kenya)
  • Chã das Caldeiras, Cape Verde
  • See Europe for calderas in the Canary Islands
• Asia
  • Aira Caldera (Kagoshima Prefecture, Japan)
  • Aso (Kumamoto Prefecture, Japan)
  • Mount Halla (Jeju-do, South Korea)
  • Kikai Caldera (Kagoshima Prefecture, Japan)
  • Krakatoa, Indonesia
  • Mount Pinatubo (Luzon, Philippines)
  • Taal Volcano (Luzon, Philippines)
  • Lake Toba (Sumatra, Indonesia)
  • Mount Tambora (Sumbawa, Indonesia)
  • Tao-Rusyr Caldera (Onekotan, Russia)
  • Towada (Aomori Prefecture, Japan)
  • Tazawa (Akita Prefecture, Japan)
  • Ashi (Kanagawa Prefecture, Japan)

Crater Lake, Oregon, formed around 5,680 BCE

Massive Island Park Caldera, Idaho easily recognized from space. Yellowstone Park to east, Teton Range to southwest.

• Americas
  • USA
    • Battle Ground Lake State Park (Washington, US)
    • Mount Aniakchak (Alaska, US)
    • Crater Lake on Mount Mazama (Crater Lake National Park, Oregon, US)
    • Mount Katmai (Alaska, US)
    • La Garita Caldera (Colorado, US)
    • Long Valley (California, US)
    • Island Park Caldera (Idaho, US)
    • Newberry Caldera (Oregon, US)
    • Mount Okmok (Alaska, US)
    • Valle Grande (New Mexico, US)
    • Yellowstone Caldera (Wyoming, US)
  • Canada
    • Mount Silverthrone (British Columbia, Canada)
    • Mount Edziza (British Columbia, Canada)
    • Bennett Lake Volcanic Complex (British Columbia/Yukon, Canada)
    • The Ash Pit (British Columbia, Canada)
    • Mount Pleasant Caldera (New Brunswick, Canada)
    • Sturgeon Lake Caldera (Ontario, Canada)
    • Mount Skukum Volcanic Complex (Yukon, Canada)
    • Blake River Megacaldera Complex (Quebec, Canada)
  • Other
    • Masaya, Nicaragua
    • Lake Atitlan, Guatemala
    • Fernandina Island, Galapagos Islands, Ecuador
    • Galán, Argentina
• Europe
  • Santorini (Greece)
  • Askja (Iceland)
  • Campi Flegrei (Italy)
  • Lake Bracciano (Italy)
  • Caldera de Taburiente (Spain)
  • Las Cañadas on Teide (Spain)
  • Ardnamurchan (Scotland)

Satellite photo of Lake Taupo

• Oceania
  • Lake Taupo (New Zealand)
  • Mount Warning (Australia)
  • Blue Lake, South Australia (Mt Gambier)
  • Kilauea (Hawaii, US)
  • Moku‘āweoweo Caldera on Mauna Loa (Hawaii, US)
• Antarctica
  • Deception Island
• Indian Ocean
  • Cirque de Mafate, Cirque de Salazie, and Cirque de Cilaos on Réunion

• Mars
  • Olympus Mons Caldera
• Venus
  • Maat Mons Caldera

See also

• Supervolcano
• Volcanic Explosivity Index
• Somma volcano
• Complex volcano

External links

Wikimedia Commons has media related to:

Caldera

• USGS page on calderas
• List of Caldera Volcanoes
• Collection of references on collapse calderas (43 pages)
• The Caldera of the Tweed Volcano - Australia
• Largest Explosive Eruptions: New results for the 27.8 Ma Fish Canyon Tuff and the La Garita caldera, San Juan volcanic field, Colorado

References

• Peter Lipman (1999). "Caldera". In Haraldur Sigurdsson, ed. Encyclopedia of Volcanoes. Academic Press. ISBN 0-12-643140-X

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