Cirque Glaciers. Synonyms. Definition. Introduction. Morphology and size. Mountain glacier

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1 Synonyms Mountain glacier Definition Cirque glacier. Small glacier found in half-open, semicircular shaped niches, or hollows located on mountainsides or in upper part of valleys. The French term cirque originates from the Latin word circus meaning circle. Introduction Cirque glaciers are among the most frequent types of glacier found on Earth, and typically observed in any Alpine landscape where climate condition allows glacier formation. Cirque glacier is a true hybrid concept referring both to the landform it creates through erosion and evacuation of rocks and sediments (Glacial Erosion: Glaciological Variables Controlling Glacial Erosion; Glacial Geomorphology and Landforms Evolution), and to the ice body occupying it in the first place. In certain areas of the world such as the Himalayas and the Andes, cirque glaciers represent vital water reservoirs that are critical for agriculture and human living conditions. Because of their small size and volume they are extremely sensitive to changing climate conditions, which is emphasized by the fact that cirque glaciers worldwide are receding at an unprecedented rate (e.g., Oerlemans, 005; Haeberlie et al., 007), suggesting that they are among the most vulnerable parts of the now ever-diminishing Cryosphere (Cryosphere). Morphology and size The shape of any cirque glacier is to a large extent determined by the surrounding topography, which not only constrains the morphology, but also directs the flow and movement of the ice mass (cf. Lewis, 1960). Because cirques (i.e., the landform) come in such a wide variety of forms, the glaciers hosting these hollows also exist in many shapes. Evans ( 007) offers a rigid classification of different types of cirques, divided into five grades (ranging from premature to mature). He also demonstrates that the length and the width of cirques in Wales and the English Lake District are positively correlated to the overall size of the cirque, which suggests that the glaciers tend to increase in size and volume with cirque enlargement. As the cirque evolves, the more sheltered the glacier occupying it will be by steepening sidewalls and headwall, which outline its basin. Through this slow process, certain positive feedbacks will be amplified such as added accumulation through wind-drifted snow, snow avalanching, and net shading. An example of a cirque glacier is shown in Figure 1. 3 May 01 16:13 1

2 , Figure 1 A small cirque glacier in the mountain massif called Snøhetta (,86 m) located in central Norway. The glacier, which appears to be of a polythermal character, has been steadily retreating for the last 10 years. The ice-cored moraine damming up a small proglacial lake has yet to be breached, but erosion along proximal sides of the moraines indicates that this is bound to happen in near future. The age of the moraine is uncertain, but it shows that the cirque glacier has been considerably larger than at present. The author took the picture in 007. Another aspect that influences the morphology of a cirque glacier is its thermal regime, which to some extent also controls its erosional capacity. Glaciers are by convention defined as being either temperate, polythermal, or polar (the old classification would be temperate, subpolar, and polar (Ahlmann, 1935). The thermal regime of an ice body is usually different in the accumulation area (usually the upper part, but not exclusively) and the ablation area ( Glaciology, Glacier Mass Balance ). In polythermal glaciers, which usually are warm based in the upper part and cold based in the lower part, the erosional efficiency would be highest in the upper part (e.g., Richardson and Holmlund, 1996). The frequently observed low-relief base of cirques agrees well with such an erosional system. Since cirque glaciers are found within all three classes of thermal regimes, so their shapes also vary significantly, which simply adds to the complexity of the glacier morphology ( Polythermal Glaciers). It should be kept in mind when relating glacier physics to cirque geometry that the development of a mature cirque arguably takes several 100,000 years (e.g., Benn and Evans, 1998). This implies that reoccurring cirque glaciers of different thermal regimes have eroded the same cirque over the period in question, including non-glacial processes. In a slightly outdated, but comprehensive glacier inventory from 1973, covering Northern Norway and Sweden, most of the 1,491 registered glaciers are categorized as cirque glaciers (Østrem et al., 1973). They are often smaller than 0.5 km and certainly smaller than 3 km. A more up-to-date survey carried out for southwestern Greenland shows similar numbers (Weidicke et al., 199). Out of 1,473 mapped mountain glaciers, the majority is labeled as cirque glaciers. They 3 are on average 1 km, with a mass volume of around 0.1 km and a corresponding thickness of 0-50 m (see also Østrem et al., 1988). Inception and decay of cirque glaciers A cirque glacier may develop from a snowfield,, or that is already established in concave, perennial ice apron glacieret often steep, mountainsides. Leeward sides and shaded areas are favored places for cirque glacier inception. Separating perennial snowfields from cirque glaciers are, however, not straightforward, although in theory it is simple, that 3 May 01 16:13

3 is, when the ice mass starts to deform it is per se a glacier. Benn and Ballantyne ( 1994) suggest, for instance, that the threshold boundary separating a snowfield from becoming a glacier is related to the length measured from the headwall to the terminus of the snowfield, which is set to be m. Whenever exceeding this threshold limit, the overlying pressure will be big enough to trigger the existing ice mass to start flowing. The average temperature of the snowfield is critical to this approach because colder temperatures will slow the annual process of transforming snow to ice, and it will also increase the pressure required to instigate and sustain deformation (cf. Paterson, 1994). Once a glacier is established, it will quickly start to erode the substratum ( Formation and Deformation of Basal Ice). A cirque glacier that expands beyond the outer limit of the rock basin can develop, often by coalescing with other advancing cirque glaciers, into a valley glacier - a process that can be reversed. How fast can cirque glaciers develop? In theory, this will depend solely on climate conditions promoting growth, that is, temperature, precipitation, and prevailing wind direction. Few empirical studies currently exist that addresses this question, perhaps because most glaciers currently are receding. One option for obtaining estimates on cirque glacier development is by reconstructing the behavior of paleoglaciers. A thoroughly studied cirque glacier system, once located on a small island in Western Norway, might shed light on this issue. At this site (called Kråkenes) a small, but temperate glacier (0.14 km ) formed due to the transient cooling of the North Atlantic realm referred to as the Younger Dryas, which existed from 1,900 to 11,700 calendar years BP (cf. Rasmussen et al., 006). The reconstruction of this glacier is based on exceptionally well-dated downstream lake sediments that efficiently track its inception and subsequent decay. According to a novel study carried out by Jostein Bakke and coworkers the glacier formed within 90 years and disappeared in 55 years (Bakke et al., 009). Modern observations of receding cirque glaciers indicate that the recession rate can be even faster than what the paleodata predicts for the Lateglacial-Holocene transition. At South Georgia (54 S, 34 W), Hodges Glacier was still present in 198 with a size of 0.19 km, even producing a terminal moraine at the time (Gordon and Timmis, 199). By 008, it had completely disappeared (author's observation, unpublished). The initial size of the glacier, as it starts to retreat will obviously impact the time it takes to melt it completely. A massive loss of cirque glaciers is nevertheless to be expected for the coming century, as global temperatures keep getting warmer. However, as valley glaciers diminish and tributary glaciers become separated from their main glacier trunk, a significant number of glaciers will be reclassified as cirque glaciers. A molder of landscapes Glaciers erode through physical processes such as abrasion, abrading, and plucking, and chemically through meltwater interaction ( Subglacial Weathering; Mechanical Weathering; Hydrochemical Characteristics of Snow, Ice, and Glaciers). The erosional capacity of cirque glaciers is obvious from the many empty cirques, Alpine horns, and arêtes, but they are also responsible for shaping mountain ranges with peaks rising above the regional equilibrium line altitude (ELA) ( Equilibrium-Line Altitude (ELA)). The influence and significance of mountain glaciers, and particularly cirque glacier, as chief landscape molders led Mitchell and Montgomery ( 006) to launch the concept of "glacial buzzsaw," arguing that the formation of glaciers on mountain ranges that gradually elevates, due to tectonic movements, will constantly lower the range as long as it keeps rising. Their case study area was the Cascade Range in central Washington, USA. In a long-term perspective with climates significantly warmer than present day, this theory might be less valid, but for average Quaternary (<3 million years) conditions (cf. Porter, 1989) there is abundant empirical work to support it (Oskin and Burbank, 005; Mitchell and Montgomery, 006 and references therein; Foster et al., 008). Climatic indicator Of all glaciers used as indicators of climate change, cirque glaciers are probably the optimal ones to use. Due to their small size and volume, their response time to positive or negative changes in mass balance is rapidly manifested, often within a couple of years. This implies that a sustained negative mass balance, lasting no less than a decade, will rapidly lead to a reduction in size and vice versa. In the European Alps, cirque glaciers (including other types of mountain glaciers) have since 1850 lost more than 50% of their volume, and the rate of melting keeps increasing toward present 1 with an ongoing loss of -3% a (Haeberlie et al., 007). If moraines have been deposited, that later can be mapped, the accumulation area ratio (AAR) can be invoked in order to estimate the corresponding ELA (Andrews, 1975; Osmaston, 005). Given that the moraines can be properly dated, 3 May 01 16:13 3

4 either directly or indirectly, they can be used as proxies for past climate change. Because downstream lakes are extremely efficient in trapping suspended sediments ( Suspended Sediment Dynamics), they currently represent the only possibility to produce continuous glacier reconstructions. The fact that most cirque glaciers have such defined catchments is an advantage for reconstructions utilizing lake sediments precisely because it reduces the risk of unwanted "sediment pollution" by extraglacial sediment sources (cf. Paasche et al., 007). Another advantage with using cirque glaciers for the purpose of reconstructing past climate conditions is that they very rarely surges, which otherwise would seriously hamper the quality of the reconstruction. Summary Cirque glaciers are small and extremely responsive to changing climate conditions. Currently, such glaciers are in a global state of demise. Because of their swift response to changing climate conditions, they are preferred for continuous reconstructions of past glacier activity. Cirque glaciers erode the hollows they occupy, which tends to be enlarged through time. In a long perspective, they are probably among the most important molders of mountain ranges such as the Alps, the Himalayas, and the Andes. Cross-references Equilibrium-Line Altitude (ELA) Formation and Deformation of Basal Ice Glacial Erosion Glacial Geomorphology and Landforms Evolution Glacier Mass Balance Glaciology Hydrochemical Characteristics of Snow, Ice, and Glaciers Ice Mechanical Weathering Polythermal Glaciers Subglacial Weathering Bibliography Ahlmann, H. W., Contribution to the physics of glaciers. Geographical Journal, 86, Andrews, J. T., Glacial systems: an approach to glaciers and their environments. Duxbury Press, 191 pp. Bakke, J., Lie, Ø., Heegaard, E., Dokken, T., Haug, G. H., Birks, H. H., Dulski, P., and Nilsen, T., 009. Rapid oceanic and atmospheric changes during the Younger Dryas cold period. Nature Geoscience,, Ballantyne, C. K., and Benn, D. I., Glaciological constraints on protalus rampart development. Permafrost and Periglacial Processes, 5, Benn, D. I., and Evans, D. J. A., Glaciers and Glaciation. London: Edward Arnold, p Evans, I. S., 007. Glacial landforms, erosional features: major scale forms. In Elias, S. A. (ed.), Encyclopedia of Quaternary Science. Amsterdam: Elsevier, pp Foster, D., Brocklehurst, S. H., and Gawthorpe, R. L. (008). Small valley glaciers and the effectiveness of the glacial buzzsaw in the northern Basin and Range, USA. Geomorphology, 10, Gordon, J. E., and Timmis, R. J., 199. Glacier fluctuations on South Georgia during the 1970s and early 1980s. Antarctic Science, 4, Haeberli, W., Hoelzle, M., Paul, F., and Zemp, M., 007. Integrated monitoring of mountain glaciers as key indicators of global climate change: the European Alps. Annals of Glaciology, 46, Lewis, W. V. (ed.), Norwegian, R.G.S. Research Series. London: Royal Geographical Society, Vol. 4, 104 p. Mitchell, S. G., and Montgomery, D. R., 006. Influence of a glacial buzzsaw on the height and morphology of the Cascade Range in central Washington State, USA. Quaternary Research, 65, Oerlemans, J., 005. Extracting a climate signal from 169 glacier records. Science, 308, May 01 16:13 4

5 Oskin, M., and Burbank, D. W., 005. Alpine landscape evolution dominated by cirque retreat. Geology, 33, Osmaston, H., 005. Estimates of glacier equilibrium line altitudes by the Area x Altitude, the Area x Altitude Balance Ratio and the Area x Altitude Balance Index methods and their validation. Quaternary International, 138, -31. Østrem, G., Haakensen, M., and Melander, O., Atlas over breer i Nord-Skandinavia. Hydrologisk avdeling, Norges Vassdrags-og Energiverk, Meddelelse, Vol., 315 pp. Østrem, G., Selvig, K. D., and Tandberg, K., Atlas over Breer i Sør-Norge. Norges Vassdrags-og Energiverk: Oslo. Paasche, O., Dahl, S. O., Bakke, J., Løvlie, R., and Nesje, A., 007. Cirque glacier activity in arctic Norway during the last deglaciation. Quaternary Research, 68, Paterson, W. S. B., The Physics of Glaciers. Oxford: Butterworth-Heinemann, 481 pp. Porter, S. C., Some geological implications of average quaternary glacial conditions. Quaternary Research, 3, Richardson, C., and Holmlund, P., Glacial cirque formation in northern Scandinavia. Annals of Glaciology,, Rasmussen, S. O., et al., 006. A new Greenland ice core chronology for the last glacial termination. Journal of Geophysical Research-Atmospheres, 111. Weidick, A., Bøggild, C. E., Knudsen, N. T., 199. Glacier inventory and atlas of west Greenland. Rap-port Grønlands Geologiske Undersøgelse, p Dr. Øyvind Paasche Department of Research Management, University of Bergen, Bergen, Norway DOI: URL: Part of: /_ Encyclopedia of Snow, Ice and Glaciers Editors: - PDF created on: May, 3, 01 16:13 3 May 01 16:13 5