A cirque (French, from the Latin word circus) is a theatre-like valley formed by glacial erosion. Alternative names for this landform are corrie (from Scottish Gaelic coire meaning a pot or cauldron) and cwm (Welsh for "valley", pronounced coom). A cirque may also be a similarly shaped landform arising from fluvial erosion.
The concave theatre shape is open on the downhill side corresponding to the flatter area of the stage, while the cupped seating section is generally steep, cliff-like slopes down which ice and glaciated debris combine and converge from the three or more higher sides. The floor of the cirque ends up bowl-shaped as it is the complex convergence zone of combining ice flows from multiple directions and their accompanying rock burdens, hence it experiences somewhat greater erosion forces, and is most often overdeepened below the level of the cirque's low-side outlet (stage) and its down slope (backstage) valley. If the cirque is subject to seasonal melting, the floor of the cirque most often forms a tarn (small lake) behind the moraine, glacial till or bedrock lip marking the downstream limit of glacial overdeepening of the basin, which serves as a dam at the outlet.
Another form of cirque or makhtesh, found in karst landscapes, is formed by intermittent river flow cutting through layers of limestone and chalk leaving sheer cliffs. A common feature for all fluvial-erosion cirques is a terrain which includes erosion resistant upper structures overlying materials which are more easily eroded.
Glacial-erosion cirque formation
Glacial cirques are found amongst mountain ranges throughout the world; 'classic' cirques are typically about one kilometer long and one kilometer wide. Situated high on a mountainside near the firn line, they are typically partially surrounded on three sides by steep cliffs. The highest cliff often is called a headwall. The fourth side forms the lip, threshold or sill, the side at which the glacier flowed away from the cirque. Many glacial cirques contain tarns dammed by either till (debris) or a bedrock threshold. When enough snow accumulates it can flow out the opening of the bowl and form valley glaciers which may be several kilometers long.
Cirques form in conditions which are favorable; in the northern hemisphere the conditions include the north-east slope where they are protected from the majority of the sun’s energy and from the prevailing winds. These areas are sheltered from heat, encouraging the accumulation of snow; if the accumulation of snow increases, the snow turns into glacial ice. The process of nivation follows, whereby a hollow in a slope may be enlarged by ice segregation weathering and glacial erosion. Ice segregation erodes the rock vertical rock face and causes it to disintegrate, which may result in an avalanche bringing down more snow and rock to add to the growing glacier. Eventually, this hollow may become large enough that glacial erosion intensifies. The enlarging of this open ended concavity creates a larger leeward deposition zone, furthering the process of glaciation. Debris (or till) in the ice also may abrade (glacial abrasion) the bed surface; should ice move down a slope it would have a ‘sandpaper effect’ on the bedrock beneath, on which it scrapes.
Eventually, the hollow may become a large bowl shape in the side of the mountain, with the headwall being weathered by ice segregation, and as well as being eroded by plucking. The basin will become deeper as it continues to be eroded by ice segregation and abrasion. Should ice segregation, plucking and abrasion continue, the dimensions of the cirque will increase, but the proportion of the landform would remain roughly the same. A bergschrund forms when the movement of the glacier separates the moving ice from the stationary ice forming a crevasse. The method of erosion of the headwall lying between the surface of the glacier and the cirque’s floor has been attributed to freeze-thaw mechanisms. The temperature within the bergschrund changes very little, however, studies have shown that ice segregation (frost shattering) may happen with only small changes in temperature. Water that flows into the bergschrund can be cooled to freezing temperatures by the surrounding ice allowing freeze-thaw mechanisms to occur.
If two adjacent cirques erode toward one another, an arête, or steep sided ridge, forms. When three or more cirques erode toward one another, a pyramidal peak is created. In some cases, this peak will be made accessible by one or more arêtes. The Matterhorn in the European Alps is an example of such a peak.
As glaciers can only originate above the snowline, studying the location of present-day cirques provides information on past glaciation patterns and on climate change.
Fluvial-erosion cirque formation
Although a less common usage,[nb 1] the term cirque is also used for theatre-shaped, fluvial-erosion features. For example, an approximately 200 square kilometres (77 sq mi) anticlinal erosion cirque is at on the southern boundary of the Negev highlands. This erosional cirque or makhtesh was formed by intermittent river flow in the Makhtesh Ramon cutting through layers of limestone and chalk, resulting in cirque walls with a sheer 200 metres (660 ft) drop. The Cirque du Bout du Monde is another such a feature, created in karst terraine in the Burgundy region of the department of Côte-d'Or in France.
Yet another type of fluvial erosion formed cirque is found on Réunion island, which includes the tallest volcanic structure in the Indian Ocean. The island consists of an active shield-volcano (Piton de la Fournaise) and an extinct, deeply eroded volcano (Piton des Neiges). Three cirques have eroded there in a sequence of agglomerated, fragmented rock and volcanic breccia associated with pillow-lavas overlain by more coherent, solid lavas.
A common feature for all fluvial-erosion cirques is a terrain which includes erosion resistant upper structures overlying materials which are more easily eroded.
- Europe (glacial)
- Circo de Gredos, Sierra de Gredos, Spain
- Cirque de Gavarnie, Pyrenees, France
- Cirque d'Estaubé, Pyrenees, France
- Cirque du Fer à Cheval, Savoy Alps, France
- Coire an t-Sneachda, Grampian Mountains in the Scottish Highlands
- Śnieżne Kotły, Karkonosze, Poland
- Coumshingaun Lake, County Waterford, Ireland 
- Europe (fluvial)
- North America
- Cirque of the Towers, Wyoming, United States
- Iceberg Cirque, Montana, United States
- Summit Lake cirque, and others on Mount Evans, Colorado, United States
- Great Basin and others on Mount Katahdin, Maine, United States
- Great Gulf, New Hampshire, United States
- Tuckerman Ravine, New Hampshire, United States
- This concern is not new, see Evans, I.S. & N. Cox, 1974: Geomorphometry and the operational definition of cirques, Area. Institute of British Geographers, 6: 150–153 regarding term usage.
- Knight, Peter G. (2009). Encyclopedia of Earth Sciences Series: Encyclopedia of Paleoclimatology and Ancient Environments. Cirques 1358. Springer Netherlands: [[Springer-Verlag|]]. pp. 155–156. doi:10.1007/978-1-4020-4411-3_37.
- Evans, I.S. (1971). "8.11(i) The geomorphology and Morphometry of Glacial and Nival Areas". In Chorley R.J. & Carson M.A. Introduction to fluvial processes. University paperbacks 407. Routledge. p. 218. ISBN 978-0-416-68820-7. Retrieved 2010-01-24.
- Johnny W. Sanders; Kurt M. Cuffey; Jeffrey R. Moore; Kelly R. MacGregor; Jeffrey L. Kavanaugh (2012). "Periglacial weathering and headwall erosion in cirque glacier bergschrunds". Geology. Bibcode:2012Geo....40..779S. doi:10.1130/G33330.1.
- Rempel, A.W.; Wettlaufer, J.S.; Worster, M.G. (2001). "Interfacial Premelting and the Thermomolecular Force: Thermodynamic Buoyancy". Physical Review Letters 87 (8): 088501. Bibcode:2001PhRvL..87h8501R. doi:10.1103/PhysRevLett.87.088501. PMID 11497990.
- "Mt Field National Park: Landforms, Flora and Fauna". Parks and Wildlife Service Tasmania. Retrieved 2009-05-12.
- Barr, I.D.; Spagnolo, M. (2015). "Glacial cirques as palaeoenvironmental indicators: Their potential and limitations". Earth-Science Reviews 151: 48. doi:10.1016/j.earscirev.2015.10.004.
- Distinguishing signal from noise: Long-term studies of vegetation in Makhtesh Ramon erosion cirque, Negev desert, Israel ; David Ward, David Saltz and Linda Olsvig-Whittaker; Plant Ecology, 2000, Volume 150, Numbers 1-2, Pages 27-36
- Early volcanic rocks of réunion and their tectonic significance; B. G. J. Upton and W. J. Wadsworth; Bulletin of Volcanology, 1969, Volume 33, Number 4, Pages 1246-1268
- Irish Times
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