View source for Earth:Cirque

{{Short description|An amphitheatre-like valley formed by glacial erosion}}
[[File:Circosabisko.jpg|thumb|Two cirques with semi-permanent snowpatches near Abisko National Park, Sweden]]
[[File:Thornton Lakes 25929.JPG|thumb|Upper Thornton Lake Cirque in North Cascades National Park, U.S.]]

A '''{{Lang|fr|cirque|italic=no}}''' ({{IPA-fr|siʁk|lang}}; from the Latin word {{Lang|la|circus}}) is an amphitheatre-like [[Earth:Valley|valley]] formed by [[Earth:Glacier|glacial]] [[Earth:Erosion|erosion]]. Alternative names for this landform are '''corrie''' (from {{lang-gd|coire}}, meaning a pot or [[Engineering:Cauldron|cauldron]])<ref> </ref> and {{langnf|cy|'''cwm'''|italic=no|valley|paren=left}}; {{IPA-cy|kʊm|pron}}). A cirque may also be a similarly shaped landform arising from fluvial erosion.

The concave shape of a glacial cirque is open on the downhill side, while the cupped section is generally steep. Cliff-like slopes, down which ice and glaciated debris combine and converge, form the three or more higher sides. The floor of the cirque ends up bowl-shaped, as it is the complex [[Earth:Convergence zone|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 [[Earth:Overdeepening|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 [[Earth:Tarn (lake)|tarn]] (small lake) behind a dam, which marks the downstream limit of the glacial overdeepening. The dam itself can be composed of [[Earth:Moraine|moraine]], glacial till, or a lip of the underlying [[Earth:Bedrock|bedrock]].<ref name=Knight2009 />

The fluvial cirque or {{Transliteration|he|[[Earth:Makhtesh|makhtesh]]}}, found in [[Earth:Karst|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.

==Formation==
[[File:Glacial Tarn Formation EN.svg|upright=1.2|thumb|Formation of cirque and resulting [[Earth:Tarn (lake)|tarn]]]]
[[File:Marichin_cirkus_IMG_1452.jpg|thumb|Maritsa cirque in Rila Mountain, Bulgaria]]

===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 [[Earth:Cliff|cliff]]s. The highest cliff is often called a [[Earth:Headwall|headwall]]. The fourth side forms the ''lip'', ''threshold'' or ''sill'',<ref name="Chorley">{{cite book|last=Evans|first=I.S.|title=Introduction to fluvial processes|editor=Chorley R.J. & Carson M.A.|publisher=Routledge|year=1971|series=University paperbacks|volume=407|page=218|chapter=8.11(i) The geomorphology and Morphometry of Glacial and Nival Areas|isbn=978-0-416-68820-7 |chapter-url=https://books.google.com/books?id=X_ENAAAAQAAJ&q=cirque+geomorphology&pg=PA157|access-date=2010-01-24}}</ref> the side at which the glacier flowed away from the cirque. Many glacial cirques contain [[Earth:Tarn (lake)|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 [[Earth:Nivation|nivation]] follows, whereby a hollow in a slope may be enlarged by [[Earth:Ice segregation|ice segregation]] weathering and glacial erosion. Ice segregation erodes the 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.<ref name=Sanders2012 /> 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 [[Earth:Abrasion (geology)|abrade]] the bed surface; should ice move down a slope it would have a 'sandpaper effect' on the [[Earth:Bedrock|bedrock]] beneath, on which it scrapes.

[[File:Lowercurtis.jpg|thumb|The Lower Curtis Glacier in North Cascades National Park is  a well-developed cirque glacier; if the glacier continues to retreat and melt away, a lake may form in the basin]]

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 [[Earth:Plucking (glaciation)|plucking]]. The basin will become deeper as it continues to be eroded by ice segregation and abrasion.<ref name=Sanders2012 /><ref name=Rempel2001 /> 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 [[Earth:Bergschrund|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.

[[File:Lake Seal Mt Field NP edit.jpg|thumb|left|Lake Seal, Mt. Field National Park, Tasmania – a cirque formed from a glacier is visible in the walls around Lake Seal<ref>{{cite web|url=http://www.parks.tas.gov.au/file.aspx?id=6757|title=Mt Field National Park: Landforms, Flora and Fauna|publisher=Parks and Wildlife Service Tasmania|access-date=2009-05-12|archive-url=https://web.archive.org/web/20110609013854/http://www.parks.tas.gov.au/file.aspx?id=6757|archive-date=2011-06-09}}</ref>]]

If two adjacent cirques erode toward one another, an [[Earth:Arête|arête]], or steep sided ridge, forms. When three or more cirques erode toward one another, a [[Earth:Pyramidal peak|pyramidal peak]] is created. In some cases, this peak will be made accessible by one or more arêtes. The Matterhorn in the European [[Place:Alps|Alps]] is an example of such a peak.

Where cirques form one behind the other, a [[Earth:Cirque stairway|cirque stairway]] results, as at the Zastler Loch in the Black Forest.

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.<ref name=Barr2015 />

===Fluvial-erosion cirque formation===
{{Further|Earth:Steephead valley|Earth:Makhtesh}}
[[File:BoutDuMonde2.jpg|thumb|The Cirque du Bout du Monde]]
Although a less common usage,{{refn|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: 150–53 regarding term usage.|group="nb"}} the term cirque is also used for amphitheatre-shaped, fluvial-erosion features. For example, an approximately {{convert|200|km2|mi2}} anticlinal erosion cirque is at {{coord|30|35|N|34|45|E|name=Negev anticlinal erosion cirque}} on the southern boundary of the Negev highlands. This erosional cirque or ''{{Transliteration|he|[[Earth:Makhtesh|makhtesh]]}}'' was formed by intermittent river flow in the [[Earth:Makhtesh Ramon|Makhtesh Ramon]] cutting through layers of limestone and chalk, resulting in cirque walls with a sheer {{convert|200|m|ft}} drop.<ref name=Ward2000 /> The Cirque du Bout du Monde is another such feature, created in [[Earth:Karst|karst]] terraine in the Burgundy region of the department of {{Lang|fr|Côte-d'Or|italic=no}} in {{wipe|France}}.

Yet another type of fluvial erosion-formed cirque is found on Réunion island, which includes the tallest volcanic structure in the [[Earth:Indian Ocean|Indian Ocean]]. The island consists of an active shield-volcano ({{Lang|fr|[[Earth:Piton de la Fournaise|Piton de la Fournaise]]|italic=no}}) and an extinct, deeply eroded volcano ([[Earth:Piton des Neiges|Piton des Neiges]]). Three cirques have eroded there in a sequence of agglomerated, fragmented rock and volcanic breccia associated with [[Earth:Pillow lava|pillow lavas]] overlain by more coherent, solid lavas.<ref name=Upton1969 />

A common feature for all fluvial-erosion cirques is a terrain which includes erosion resistant upper structures overlying materials which are more easily eroded.
[[File:Western Cwm and Lhotse.jpg|thumb|The Western Cwm with the Lhotse face of Mount Everest in the background]]

==Notable cirques==
[[File:TuckermanRavineSkiing.JPG|thumb|Tuckerman Ravine cirque, headwall and spring skiers, New Hampshire]]
[[File:Gavarnie-Cirque.JPG|thumb|Cirque de Gavarnie, French Pyrenees]]
* Australia
** Blue Lake Cirque, New South Wales, Australia
* Asia
**[[Earth:Chandra Taal|Chandra Taal]], Himachal Pradesh, India
** Cirque Valley, Hindu Kush, Pakistan
** [[Earth:Makhtesh Ramon|Makhtesh Ramon]], Negev desert, Israel
** [[Earth:Western Cwm|Western Cwm]], Khumbu Himal, Nepal
* Europe (glacial)
**Cadair Idris, Wales
**Circo de Gredos, Sierra de Gredos, Spain
** Cirque de Gavarnie, Pyrenees, France
** Cirque d'Estaubé, Pyrenees, France
** Maritsa cirque, Rila Mountain, Bulgaria
** Malyovitsa cirque, Rila Mountain, Bulgaria
** Seven Rila Lakes cirques, Rila Mountain, Bulgaria
** Banderishki cirque, Pirin Mountain, Bulgaria
** {{Lang|gd|Coire an t-Sneachda|italic=no}}, Grampian Mountains, [[Place:Scottish Highlands|Scottish Highlands]]
** Śnieżne Kotły, Karkonosze, Poland
** Coumshingaun Lake, County Waterford, Ireland <ref>{{cite news|title=Go Walk: Coumshingaun, Co Waterford|url=http://www.irishtimes.com/life-and-style/travel/go-walk-coumshingaun-co-waterford-1.1990772|author=John O'Dwyer|newspaper=The Irish Times}}</ref>
* Europe (fluvial)
** Cirque de Navacelles, Grands Causses, France
** Cirque du Bout du Monde, Grands Causses, France
** Cirque du Bout du Monde, Burgundy, France
* North America
**Cirque of the Towers, Wyoming, United States
** Iceberg Cirque, Montana, US
** Summit Lake cirque, and others on Mount Blue Sky, Colorado, US
** Great Basin and others on [[Earth:Mount Katahdin|Mount Katahdin]], Maine, US
** Great Gulf, New Hampshire, US
** Tuckerman Ravine, New Hampshire, US

==See also==
* {{annotated link|Earth:Cirque stairway}}
* {{annotated link|Earth:Glacial landform}}

==References==
'''Notes'''
{{Reflist|group="nb"}}

'''Footnotes'''
{{Reflist|colwidth=25em|
refs=

<ref name=Barr2015>{{cite journal|author1=Barr, I.D. |author2=Spagnolo, M. |year=2015|journal=[[Earth:Earth-Science Reviews|Earth-Science Reviews]]|doi=10.1016/j.earscirev.2015.10.004|volume=151|title=Glacial cirques as palaeoenvironmental indicators: Their potential and limitations|page= 48|bibcode=2015ESRv..151...48B|s2cid=54921081 |url=https://pure.qub.ac.uk/portal/en/publications/glacial-cirques-as-palaeoenvironmental-indicators-their-potential-and-limitations(fccac20a-f112-4e7e-81dd-0a81604d0018).html }}</ref>

<ref name="Chorley">{{cite book|last=Evans|first=I.S.|title=Introduction to fluvial processes|editor=Chorley R.J. & Carson M.A.|publisher=Routledge|year=1971|series=University paperbacks|volume=407|page=218|chapter=8.11(i) The geomorphology and Morphometry of Glacial and Nival Areas|isbn=978-0-416-68820-7 |chapter-url=https://books.google.com/books?id=X_ENAAAAQAAJ&q=cirque+geomorphology&pg=PA157|access-date=2010-01-24}}</ref>

<ref name=Knight2009>{{cite book | last = Knight | first = Peter G. | title = Encyclopedia of Earth Sciences Series: Encyclopedia of Paleoclimatology and Ancient Environments | publisher = [[Physics:Springer-Verlag|]] | series = Cirques | volume = 1358  | location = Springer Netherlands | year = 2009  | pages = 155–56  | doi = 10.1007/978-1-4020-4411-3_37  | isbn =  978-1-4020-4551-6| chapter = Cirques }}</ref>

<ref name=Rempel2001>{{cite journal|author1=Rempel, A.W. |author2=Wettlaufer, J.S. |author3=Worster, M.G. |s2cid=10308635 |year=2001|journal=[[Physics:Physical Review Letters|Physical Review Letters]]|doi=10.1103/PhysRevLett.87.088501|volume=87|issue=8|title=Interfacial Premelting and the Thermomolecular Force: Thermodynamic Buoyancy|page= 088501|pmid=11497990|bibcode=2001PhRvL..87h8501R}}</ref>

<ref name=Sanders2012>{{cite journal|author1=Johnny W. Sanders |author2=Kurt M. Cuffey |author3=Jeffrey R. Moore |author4=Kelly R. MacGregor |author5=Jeffrey L. Kavanaugh |s2cid=128580365 |year=2012|journal= Geology |doi=10.1130/G33330.1   |volume=40|issue=9|title=Periglacial weathering and headwall erosion in cirque glacier bergschrunds |pages= 779–782|bibcode= 2012Geo....40..779S}}</ref>

<ref name=Upton1969>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, pp. 1246–68</ref>

<ref name=Ward2000>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, pp. 27–36</ref>

}}

==External links==
{{Commons and category}}
* [http://www.geography-site.co.uk/pages/physical/glaciers/corrie.html Photographs and case study of corrie glaciers]

{{Glaciers}}

[[Category:Cirques| ]]
[[Category:Glaciology]]
[[Category:Glacial landforms]]

es:Glaciar#Clasificación

{{Sourceattribution|Cirque}}