Acirque (French:[siʁk]; from the Latin wordcircus) is anamphitheatre-likevalley formed byglacial erosion. Alternative names for this landform arecorrie (fromScottish Gaelic:coire, meaning a pot orcauldron)[1] andcwm (Welsh for 'valley';pronounced[kʊm]). 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 complexconvergence zone of combining ice flows from multiple directions and their accompanying rock burdens. Hence, it experiences somewhat greater erosion forces and is most oftenoverdeepened 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 atarn (small lake) behind a dam, which marks the downstream limit of the glacial overdeepening. The dam itself can be composed ofmoraine,glacial till, or a lip of the underlyingbedrock.[2]
The fluvial cirque ormakhtesh, found inkarst landscapes, is formed by intermittent river flow cutting through layers of limestone and chalk leaving sheer cliffs. A common feature for allfluvial-erosion cirques is a terrain which includes erosion resistant upper structures overlying materials which are more easily eroded.
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 thefirn line, they are typically partially surrounded on three sides by steepcliffs. The highest cliff is often called aheadwall. The fourth side forms thelip,threshold orsill,[3] the side at which the glacier flowed away from the cirque. Many glacial cirques containtarns 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 ofnivation follows, whereby a hollow in a slope may be enlarged byice 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.[4] 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 mayabrade the bed surface; should ice move down a slope it would have a 'sandpaper effect' on thebedrock beneath, on which it scrapes.
Eventually, the hollow may become a largebowl shape in the side of the mountain, with the headwall being weathered by ice segregation, and as well as being eroded byplucking. The basin will become deeper as it continues to be eroded by ice segregation and abrasion.[4][5] 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. Abergschrund 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, allowingfreeze-thaw free mechanisms to occur.
If two adjacent cirques erode toward one another, anarête, or steep sided ridge, forms. When three or more cirques erode toward one another, apyramidal peak is created. In some cases, this peak will be made accessible by one or more arêtes. TheMatterhorn in the EuropeanAlps is an example of such a peak.
Where cirques form one behind the other, acirque stairway results, as at theZastler Loch in theBlack 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.[7]
Although a less common usage,[nb 1] the term cirque is also used for amphitheatre-shaped, fluvial-erosion features. For example, an approximately 200 square kilometres (77 sq mi) anticlinal erosion cirque is at30°35′N34°45′E / 30.583°N 34.750°E /30.583; 34.750 (Negev anticlinal erosion cirque) on the southern boundary of theNegev highlands. This erosional cirque ormakhtesh was formed by intermittent river flow in theMakhtesh Ramon cutting through layers of limestone and chalk, resulting in cirque walls with a sheer 200 metres (660 ft) drop.[8] TheCirque du Bout du Monde is another such feature, created inkarst terraine in theBurgundy region of the department ofCôte-d'Or inFrance.
Yet another type of fluvial erosion-formed cirque is found onRéunion island, which includes the tallest volcanic structure in theIndian 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 volcanicbreccia associated withpillow lavas overlain by more coherent, solid lavas.[9]
A common feature for all fluvial-erosion cirques is a terrain which includes erosion resistant upper structures overlying materials which are more easily eroded.
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