caldera

[kal-der-uh, kawl-]

caldera: see crater.

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Large, bowl-shaped volcanic depression that forms when the top of a volcanic cone collapses into the space left after magma is ejected during a violent volcanic eruption. The term is Spanish for “caldron.” Subsequent minor eruptions may build small cones on the floor of the caldera which may still later fill up with water; an example is Crater Lake in Oregon.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.

Kahlo (y Calderón de Rivera), (Magdalena Carmen) Frida

“Diego and I,” oil on masonite, self-portrait (with forehead portrait of Diego Rivera) elipsis

(born July 6, 1907, Coyoacán, Mex.—died July 13, 1954, Coyoacán) Mexican painter. The daughter of a German Jewish photographer, she had polio as a child and at 18 suffered a serious bus accident. She subsequently underwent some 35 operations; during her recovery, she taught herself to paint. She is noted for her intense self-portraits, many reflecting her physical ordeal. Like many artists working in post-revolutionary Mexico, Kahlo was influenced by Mexican folk art; this is apparent in her use of fantastical elements and bold use of colour, and in her depictions of herself wearing traditional Mexican, rather than European-style, dress. Her marriage to painter Diego Rivera (from 1929) was tumultuous but artistically rewarding. The Surrealists André Breton and Marcel Duchamp helped arrange exhibits of her work in the U.S. and Europe, and though she denied the connection, the dreamlike quality of her work has often led historians to identify her as a Surrealist. She died at 47. Her house in Coyoacán is now the Frida Kahlo Museum.

Learn more about Kahlo (y Calderón de Rivera), (Magdalena Carmen) Frida with a free trial on Britannica.com.

Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.

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 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. 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. 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.

Notable calderas

External links

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.