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Two cirques with semi-permanent snowpatches in Abisko National Park, Sweden.
Upper Thornton Lake Cirque in North Cascades National Park

A cirque (French for "circus") is an amphitheatre-like valley head, formed at the head of a valley glacier by erosion. The concave amphitheatre 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 experiences somewhat greater erosion forces, and is most often scooped out somewhat below the level of 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 and glacial till damming the outlet.

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Formation

Formation of cirque and resulting tarn

A cirque is a landform found among mountains as a result of alpine glaciers. They may be up to a square kilometre in size, situated high on a mountainside near the firn line, and typically are partially surrounded on three sides by steep cliffs. The highest cliff often is called a headwall. The fourth side is the lip, threshold or sill,[1] 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 freeze-thaw weathering and glacial erosion. The freeze-thaw cycle erodes at the lower rocks 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. 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.

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 constant freezing and thawing, and as well as being eroded by plucking. The basin will become deeper if it continues to become eroded by abrasion. Should 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 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.

Lake Seal, Mt. Field National Park, Tasmania - a cirque formed from a glacier is visible in the walls around Lake Seal [2]

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 glaciations patterns and climate change.



GERB U ALL

Notable cirques

See also

References

  1. ^ 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. pp. 218. ISBN 0416688209, 9780416688207. http://books.google.co.uk/books?id=X_ENAAAAQAAJ&pg=PA157&dq=cirque+geomorphology&ei=yCxcS5HRE4iGygTv7sWCCQ&cd=4#v=onepage&q=cirque%20geomorphology&f=false. Retrieved 2010-01-24. 
  2. ^ "Mt Field National Park: Landforms, Flora and Fauna". Parks and Wildlife Service Tasmania. http://www.parks.tas.gov.au/file.aspx?id=6757. Retrieved 2009-05-12. 

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1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

Additions, corrections and discussions on this subject by users of the Classic Encyclopedia can be found on the discussion page

COIRE (Ger. Chur or Cur, Ital. Coira, Lat. Curia Raetorum, Romonsch Cuera), the capital of the Swiss canton of the Grisons. It is built, at a height of 1949 ft. above the sea-level, on the right bank of the Plessur torrent, just as it issues from the Schanfigg valley, and about a mile above its junction with the Rhine. It is overshadowed by the Mittenberg (east) and Pizokel (south), hills that guard the entrance to the deep-cut Schanfigg valley. In 1900 it contained 11,532 inhabitants, of whom 9288 were German-speaking, 1466 Romonsch-speaking, and 677 Italianspeaking; while 7561 were Protestants, 3962 Romanists and one a Jew. The modern part of the city is to the west, but the old portion, with all the historical buildings, is to the east. Here is the cathedral church of St Lucius (who is the patron of Coire, and is supposed to be a 2nd-century British king, though really the name has probably arisen from a confusion between Lucius of Cyrene - miswritten "curiensis" - with the Roman general Lucius Munatius Plancus, who conquered Raetia). Built between 1178 and 1282, on the site of an older church, it contains many curious medieval antiquities (especially in the sacristy), as well as a picture by Angelica Kaufmann, and the tomb of the great Grisons political leader (d. 1637) Jenatsch (q.v.). Opposite is the Bishop's Palace, and not far off is the Episcopal Seminary (built on the ruins of a 6th-century monastic foundation). Not far from these ancient monuments is the new Raetian Museum, which contains a great collection of objects relating to Raetia (including the geological collections of the Benedictine monk of Disentis, Placidus a Spescha (1752-1833), who explored the high snowy regions around the sources of the Rhine). One of the hospitals was founded by the famous Capuchin philanthropist, Father Theodosius Florentini (1808-1865), who was long the Romanist cure of Coire, and whose remains were in 1906 transferred from the cathedral here to Ingenbohl (near Schwyz), his chief foundation. Coire is 74 m. by rail from Zurich, and is the meeting-point of the routes from Italy over many Alpine passes (the Lukmanier, the Splugen, the San Bernardino) as well as from the Engadine (Albula, Julier), so that it is the centre of an active trade (particularly in wine from the Valtelline), though it possesses also a few local factories.

The episcopal see is first mentioned in 452, but probably existed a century earlier. The bishop soon acquired great temporal powers, especially after his dominions were made, in 831, dependent on the Empire alone, of which he became a prince in 1170. In 1392 he became head of the league of God's House (originally formed against him in 1367), one of the three Raetian leagues, but, in 1526, after the Reformation, lost his temporal powers, having fulfilled his historical mission (see GxrsoNs). The bishopric still exists, with jurisdiction over the Cantons of the Grisons, Glarus, Zurich, and the three Forest Cantons, as well as the Austrian principality of Liechtenstein. The gild constitution of the city of Chur lasted from 1465 to 1839, while in 1874 the Burgergemeinde was replaced by an Einwohnergemeinde. Authorities. - A. Eichhorn, Episcopatus Curiensis (St Blasien, 1797); W. von Juvalt, Forschungen uber die Feudalzeit im Curischen Raetien, 2 parts (Zurich, 1871); C. Kind, Die Reformation in den Bisthumern Chur and Como (Coire, 1858); Conradin von Moor, Geschichte von Curraetien (2 vols., Coire, 1870-1874); P. C. von Planta, Das alte Raetien (Berlin, 1872); Idem, Die Curraetischen Herrschaften in der Feudalzeit (Bern, 1881); Idem, Verfassungsgeschichte der Stadt Cur im Mittelalter (Coire, 1879); Idem, Geschichte von Graubunden (Bern, 1892). (W. A. B. C.)

Additions, corrections and discussions on this subject by users of the Classic Encyclopedia can be found on the discussion page

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