Caldera

A caldera is a cauldron-like volcanic feature formed by the collapse of land following a volcanic eruption. They are sometimes confused with volcanic craters. The word comes from Latin "caldarium", meaning cauldron (hot bath). In some texts the English term cauldron is also used.

In 1815, the German geologist Leopold von Buch visited the Las Cañadas Caldera Teide, Tenerife and the Caldera de Taburiente, La Palma, both in the Canary Islands. When he published his memoirs he introduced the term "caldera" into the geological vocabulary.

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" above it. A roughly circular fracture - the "Ring Fault" develops around the edge of the chamber. These "ring fractures" serve as feeders for fault intrusions which are also known as ring dykes. Secondary volcanic vents may form above the ring fracture. As the magma chamber empties, the center of the volcano within the ring fracture begins to collapse. The collapse may occur as the result of a single cataclysmic 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.

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 does not flow like basalt due to having a high viscosity. As a result, gases tend to become trapped at high pressure within the magma. When the magma approaches the surface of the Earth, the gases decompress rapidly, causing explosive destruction of the magma and spreading volcanic ash over wide areas. Further lava flows may be erupted.

If volcanic activity continues the centre of the caldera may be uplifted in the form of a "resurgent dome" such as is seen seen at Cerro Galán, Toba, Yellowstone etc; 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 some 640,000 years ago, it released about 1,000 km³ of dense rock equivalent (DRE) material, covering a substantial part of North America in up to two metres of debris. By comparison, when Mount St. Helens erupted in 1980, it released ~1.2 km³ (DRE) 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.

Toba

About 75,000 years ago, this Indonesian volcano released about 2,800 km³ DRE of ejecta, the largest known eruption within the Quaternary Period (last 1.8 million years). In the late 1990s, anthropologist Stanley Ambrose [ [http://www.anthro.uiuc.edu/faculty/ambrose/ Stanley Ambrose ] ] proposed that a volcanic winter induced by this eruption reduced the human population to about 2,000 - 20,000 individuals, resulting in a population bottleneck ("see" Toba catastrophe theory). More recently several geneticists, including Lynn Jorde and Henry Harpending have proposed that the human race was reduced to approximately five to ten thousand people. [ [http://www.bbc.co.uk/science/horizon/1999/supervolcanoes_script.shtml BBC - Science & Nature - Horizon - Supervolcanoes ] ] Whichever figure is right, the fact remains that the human race seemingly came close to extinction about 75,000 years ago.

Eruptions forming even larger calderas are known, especially La Garita Caldera in the San Juan Mountains of Colorado, where the 5,000 km³ Fish Canyon Tuff was blasted out in a major single eruption about 27.8 million years ago.

At some points in geological 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).

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 basalt which is silica poor. As a result, the magma is much less 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. [ [http://www.volcano.si.edu/world/volcano.cfm?vnum=1503-01=&VErupt=Y&VSources=Y&VRep=Y&VWeekly=Y&volpage=photos&photo=062078 Global Volcanism Program | Fernandina | Photo ] ] Kilauea Caldera has an inner crater known as Halema‘uma‘u, which has often been filled by a lava lake. At the summit of largest volcano on Earth, Mauna Loa is a subsidence caldera called Moku‘āweoweo Caldera.

It is very frequent for a caldera to become emptied by drainage of melted lava throughout a breach on the caldera's rim. The Caldera de Taburiente and the Caldereta, both in the island of La Palma (Canary Islands) are calderas emptied by a river of lava some 500.000 years ago.

Mineralization

Some calderas are known to support rich mineralogy. One of the world's best preserved mineralized calderas is the Neoarchean Sturgeon Lake Caldera in northeastern Ontario, Canada. [ [http://www.d.umn.edu/prc/workshops/S08workshop.html UMD: Precambrian Research Center] ]

Notable calderas

"See also
*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)
*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 Volcano (Oregon, US)
***Mount Okmok (Alaska, US)
***Valles Caldera (New Mexico, US)
***Yellowstone Caldera (Wyoming, US)
**Canada
***Silverthrone Caldera (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/Ontario, Canada)
****New Senator Caldera (Quebec, Canada)
****Misema Caldera (Ontario/Quebec, Canada)
****Noranda Caldera (Quebec, Canada)
**Ecuador
***Pululahua Geobotanical Reserve
**El Salvador
***Lake Ilopango
***Lake Coatepeque
**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)
**Glen Coe (Scotland)
*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

ee also

* Supervolcano
* Volcanic Explosivity Index
* Somma volcano
* Complex volcano

External links

* [http://volcanoes.usgs.gov/Products/Pglossary/caldera.html USGS page on calderas]
* [http://volcanodb.com/search.php?type=Caldera List of Caldera Volcanoes]
* [http://eis.bris.ac.uk/~gljhg/Workgroup/Workgroup_files/Edited-list-publications_calderas-71206.pdf Collection of references on collapse calderas] (43 pages)
* [http://www.bigvolcano.com.au/natural/wollum.htm The Caldera of the Tweed Volcano - Australia]
* [http://host.uniroma3.it/progetti/cev/Web%20CEV%20folder/lagarita.html Largest Explosive Eruptions: New results for the 27.8 Ma Fish Canyon Tuff and the La Garita caldera, San Juan volcanic field, Colorado]
* [http://www.bbc.co.uk/science/horizon/1999/supervolcanoes_script.shtml]

Notes

References

* Clough, C. T; Maufe, H. B. & Bailey, E. B; 1909. The cauldron subsidence of Glen Coe, and the Associated Igneous Phenomena. Quart. Journ. Geol. Soc. 65, 611-678.
* Kokelaar, B. P; and Moore, I. D; 2006. Glencoe caldera volcano, Scotland. ISBN. 0852725252. Pub. British Geological Survey, Keyworth, Nottinghamshire. There is an associated 1:25000 solid geology map.
* Lipman, P; 1999. "Caldera". "In" Haraldur Sigurdsson, ed. "Encyclopedia of Volcanoes". Academic Press. ISBN 0-12-643140-X
* Williams, H; 1941. Calderas and their origin. California University Publ. Geol. Sci. 25, 239-346.