Snow, ice, avalanches and glaciers
Features Resulting from Glacial Erosion
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Features Resulting from Glacial ErosionThe growth and decline of mountain glaciers leads to a predictable pattern of landform development (Figure 45). Upon initial accumulation, the snow and ice adapt to the pre-existing topography. If the snow accumulation is sufficient, the mountains may be totally covered by glacial ice. When this happens the landscape is actually somewhat protected, since temperatures under the ice remain near the freezing level and surfaces are not subject to intense frost-shattering. The rugged topography revealed in Figures 45 b and c, however, is believed to develop under a partial ice cover; frost and mass-wasting processes attack the exposed surfaces while glaciers occupy the valleys and slope depressions to carve, deepen, and sculpture the topography into a distinctive landscape (Cotton 1942, 1968, Flint 1971, Embleton and King 1975a). The dominant features of this type of landscape are cirques, glacial troughs, horns, arêtes, (sawtooth ridges), tarns and pater-noster lakes (rock-basined lakes), and hanging valleys. < Cirques and glacial lakes 5.16 Fig 46 near here> “Few landforms have caught the imagination of geomorphologists more than the glacial cirque (corrie)” (Sugden and John 1976). A cirque is a semicircular bowl-like depression carved into the side of a mountain where a small glacier has existed (Figure 45 and Figure 46). Cirques are typically located at the heads of valleys, but they may develop anywhere along a mountain slope. They vary in size from shallow basins a few meters in diameter to the huge excavations several kilometers deep and wide found from Antarctica to the Himalayas. A well-developed cirque usually contains a headwall, basin, and threshold. The headwall is the steep and smoothed bedrock surface at the back of the cirque, extending concavely upward to the ridge. The basin is a circular or elongated depression at the base of the headwall, and the threshold is a lip or slightly elevated rampart at the outlet end of the basin. The threshold, composed of bedrock or depositional material, results from the decreasing glacial mass and rate of movement at the periphery, so that the intensity of erosion is less and deposition occurs. If the cirque was occupied by a valley glacier with ice flowing downward and away into the valley, a threshold may not form. Thresholds are typical of cirques occupied by small glaciers (either during their formation, or afterward, when a large glacier has shrunk). The presence of a threshold produces an enclosed basin where water collects, often forming a tarn (lake) as described later. The origin of cirques involves at least two distinct processes: frost-shattering and glacial erosion. At the turn of the twentieth century there was a major controversy over the relative importance of these processes. Some subscribed to the "bergschrund theory:” which attributed great efficacy to frost processes caused by the freezing and thawing of water in rock joints near the base of the highest crevasse (the bergschrund) or the gap between the rock and ice (the moat or randkluft). Others objected to this theory because many glaciers do not have bergschrunds, because the temperature fluctuations in these crevasses and gaps are not great (Battle and Lewis 1951, Gardner 1987), and because many cirque headwalls far exceed the height of bergschrund depths. They argued that plucking and abrasion by the moving ice alone was sufficient to create cirques (Embleton and King 1975a, pp. 205-38). Today, most glaciologists support the nivation theory which ascribes initial cirque development to freeze-thaw processes at the base of small snowfields. As full glaciers develop, cirques become enlarged by plucking. (Gordon, 1977, Thorn 1979, Thorn and Hall 1980, Sharp 1988, Benn and Evans 1998, Hooke 1998). The distribution, orientation, and elevation of cirques can reveal a great deal about their development (Derbyshire and Evans 1976, Graf 1976). Cirques exist at lower elevations and are best developed on the windward side of mountains where precipitation is relatively heavy. As the snowline rises toward the interior or toward more continental conditions, so do the elevations at which cirques develop. Within this general pattern, however, cirques have preferred orientations. In the northern hemisphere they are found primarily on slopes facing north and northeast, while in the southern hemisphere they are found on south and southeast-facing slopes. This is largely in response to wind direction and shade. The prevailing wind in middle latitudes is westerly, so exposed west-facing slopes are typically blown free of snow, that is then re-deposited on east-facing slopes (especially in continental climates with dry, powdery snow). Shade is also important, since protection from the direct rays of the sun allows the snow to persist in areas where it otherwise might melt (Alford, 1980). This is the case even in mountains with oceanic climates receiving heavy amounts of snowfall. For example, the present distribution of glaciers in the Cascades is largely restricted to north-facing slopes, and cirque development follows the same pattern. Since cirques require a glacier for their formation, the presence of cirques in areas not now glaciated indicates the former existence of glacial ice. It is generally estimated that the level of cirque floors roughly approximates the annual snowline that existed when the cirques were made. Plots of the elevation and orientation of cirques in different regions have provided a great deal of information about past climatic conditions. As with all natural phenomena, however, caution must be used in their interpretation. For example, the formation of cirques may take a long time, and most areas have experienced more than one glaciation; also cirques may have been occupied and reoccupied during several glacial periods. Once a depression is formed, it provides a greater reservoir for snow collection and greater protection from melting, so that more snow will accumulate and less will melt than on surrounding slopes. When one cirque sits above the other, the two may coalesce to form one large cirque. This may be the reason why a single cirque exists at the head of many mountain valleys. In other cases, cirques are simply enlarged with each glaciation, so their present level is not a true representation of the most recent snowline but is, instead, a composite feature resulting from a combination of events. Despite these problems, cirques can provide excellent information about past conditions if care is used in their interpretation (Flint 1971, p. 138; Embleton and King 1975a, p. 223). The headward erosion of cirque glaciers (along with frost processes and avalanching) is largely responsible for the rugged topography of glaciated mountains. When cirque glaciers develop on opposite sides of a ridge, they erode headward, and eventually meet to create a saddle or notch in the ridge crest (col) (Figures 45c, 48). This also tends to reduce the thickness of the ridge, making it narrow and knife-like. The continuation of this process along the ridge creates sawtoothed arete ridges (Figures 45c, 48). The headward erosion of cirque glaciers on all sides of a summit may result in a pyramidal peak called a horn. The Matterhorn in the Swiss Alps is the classic example, but such features are common in most glaciated mountains (Figures 45c, 40). Although cirque erosion is the dominant glacial process operating on upper slopes and depressions, the larger glaciers may overflow the cirque basins to form valley glaciers. The ice commonly inherits a preexisting drainage system, and the former stream channels are soon transformed into glacial troughs (Figure 45a, 45b; cf. Figures 34, 6.39 FIX). Most stream-cut valleys in mountain regions have roughly V-shaped cross-profiles, while a glaciated valley is typically U-shaped. Streams are (see pp. 189- 210) limited to channel cutting along their beds, while other processes (especially mass-wasting) erode the valley slopes and transport material to the stream. A glacier, on the other hand, occupies the entire valley and its much greater mass and erosive capacity soon widen and deepen the valley into a semicircular or elliptical cross-section with steep rock walls (Hooke 1998). The valley floor may be bare rock, or it may be back-filled with glacial meltwater deposits, resulting in flat valley bottoms. In longitudinal profile, glacial valleys have a more irregular surface than stream valleys and often display a series of steps and risers. Various origins have been postulated for the stepped nature of glacial valleys, including differential rates of erosion controlled by valley width, different rock types, more intensely fractured zones within the same rock type, greater erosion occurring at the base of deep crevasses, and association with places where tributary glaciers join the main stream (Thornbury 1969, Flint 1971, Embleton and King 1975a). Massive erosion and excavation of material by the ice deepens, widens, and straightens the former stream valley along its axis so that the lower reaches of tributary streams and their interfluves are cut off, leaving them truncated at some height above the main valley. After the glacier melts, the water of these streams cascades down as waterfalls over the trough sidewall. Such tributary valleys with floors higher than the floor of the trunk valley, known as hanging valleys, are a scenic feature of glaciated mountains (Figure 45c). Due to the rough and irregular terrain left behind by glaciers, lakes are common in such landscapes. Tarns are lakes commonly found in cirques (a.k.a. cirque-lakes). Tarns are characteristically clear and blue, since the glacier has removed most of the loose debris, leaving a smoothed bedrock depression. Lakes frequently form in the depressions behind the treads down valley from the cirque lakes. These lakes often occur in “chains” along glacial valleys, and are called paternoster lakes because of their resemblance to beads on a rosary, (Figure 6.39 FIX). Depending on their age, size, and history, glacial lakes may or may not contain fish. Generally, they have to be at least several hundred years old before they support fish. The question of how fish get to the high mountain lakes has always been puzzling (aquatic birds are probably most important in transporting the fish and their eggs). Nowadays, of course, the fish population of high mountain lakes is largely maintained through intensive fish-stocking programs.
The larger rock debris can only be transported directly by the glacier or by ice rafting (chunks of ice floating in water), but the smaller material may be carried considerable distances by wind and glacial meltwater streams. The winds that blow off the glacier in summer (see p. 113FIX) are often very effective at picking up and transporting the finely ground rock particles produced by grinding and scraping during glacial transport (glacial flour). In some valleys where glaciers exist, the development of such winds is an almost daily occurrence during clear weather in summer. Larry Price (1981), in his original edition of this book recalls spending several weeks camped in one such valley in Yukon Territory, and the presence of afternoon dust storms made working conditions truly miserable. Dust and grit coated his hair, clothes, cooking utensils, and food. Ecologically, however, the deposition of this silt (called loess) is beneficial as it expedites soil development and greatly improves local productivity. Several major agricultural regions have developed on loess soils (e.g. the Palouse region of eastern Washington). Glacial melt-streams are the main mechanism for transport of the smaller material. The amount a stream can carry depends primarily upon the stream's velocity, which in turn depends, among other factors, upon the volume. Glacial streams, of course, display great fluctuations in flow, between winter and summer as well as between day and night (Figure 6.35 FIX). If you have ever hiked in glacierized mountains during the summer, you know that the best time to cross the meltwater fed streams is in early morning, since by late afternoon they may become raging torrents following daytime melting. Such volume fluctuations produce an irregular pattern of erosion and re-deposition; during periods of high velocity, the stream erodes and carries a large load of material, only to drop it again as the water volume subsides and the velocity decreases (R. J. Price 1973). Glacial streams are characteristically choked with sediment, much of which is eventually deposited near the glacial terminus. Such deposits, called valley train, create flat-floored valleys and may reach considerable depths and extend for several kilometers beyond the glacial terminus (Figures 49, 50). An extreme example is the Yosemite Valley of California, where seismic investigations reveal that over 600 m (2,000 ft.) of deposits cover the original bedrock floor excavated by the glacier (Gutenberg et. al. 1956). Glacial and glaciofluvial deposits are important ecologically because soil and vegetation develop much more rapidly on aggregate material than on bare rock. Such areas frequently become locally important agricultural regions. The contribution of glacial meltwater streams to the runoff of watersheds in many cases amounts to millions of liters annually. On the negative side, glacial streams are commonly so choked with sediment that the water is not immediately usable by human populations. Very little life exists in the headwaters of these streams. The sediments can be transported long distances and provide increased deposition and infilling of the stream or lake into which they empty. A good example of this is Kluane Lake along the Alaska Highway in Yukon Territory. The melt-stream of the Kaskawulsh Glacier (Figure 50), located about 24 km (15 mi.) away in the Saint Elias Mountains, is building a delta into the lake (Figure 51). The quality of the lake water is affected in several ways, but the most obvious is that the normal crystal-blue of the lake is transformed to a murky gray around the mouth of the stream and that the fishing near this end of the lake is very poor (Bryan 1974a, b). Citations to add: Alford, D. L. 1980. The orientation gradient: regional variations of accumulation and ablation in alpine basins. In Ives, J. D. (ed.), Geoecology of the Front Range: a study of alpine and subalpine environments. Westview Press, Boulder, Colorado, p. 214-223. Armstrong, B., and Williams, K. 1986. The Avalanche book. Fulcrum Inc. Golden, Colorado, 231 pp. Avery, C. C. and Dexter, L. R. 1993. Where has all the snow gone? Proc. 61st Western Snow Conference, Jackson, Wyoming. Bader, H. P., Haefeli, R., Bucher, E., Neher, J., Eckel, O., Tharms, C. and Niggle, P. 1939. Snow and its metamorphism. U. S. Army Corps of Engineers Snow, Ice, and Permafrost Research Establishment Translation Number 14, 313 pp. Download 197.56 Kb. Share with your friends:
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