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Nature's Conveyor Belt 6 6 Nature’s conveyor belt Few natural processes on the Earth’s surface can match glaciers in transporting debris long distances away from their source. Indeed, one of the most immediately obvious features of mountain glaciers is the amount of rubble and the huge blocks of rock that litter their surface. Commonly, the entire ablation area of a mountain glacier is completely debris-covered, although the melting of ice beneath the debris layer creates a very uneven and unstable topography that is arduous to walk over. Thus, a glacier may be considered as a sort of conveyor belt for rock debris, transporting material from all points along its length towards the snout. Typically, this material is carried on the surface (where it is referred to as supraglacial debris), or near its bed in basal ice (where it is described as basal debris). Large amounts of debris may also be ingested from both the surface and bed as englacial debris, giving the glacier a very dirty appearance. In addition to debris carried within the ice, debris is also transported within the deforming bed zone. Surface debris For a glacier to be laden with supraglacial debris, it normally requires rocks to be exposed above its flanks. Thus, ice sheets and ice caps, which submerge mountains, carry almost no supraglacial Huge amounts of debris are transported debris, whereas the lower reaches of mountain valley glaciers and deposited by valley glaciers. This carry many boulders. The main sources of this supraglacial photograph shows Breithorngletscher debris are rock-fall from frost shattering, aided by the unstable (right) and Schwarzgletscher (centre) flowing off Breithorn (4164 metres) near nature of the hillsides over-steepened by the glacier. Most rock- Zermatt in Switzerland to join falls are small, although occasionally entire landslides cover broad Gornergletscher flowing from left to right. expanses of ice, especially in regions prone to earthquakes, such Note how lateral moraines become medial moraines where glaciers flow as Alaska, the Andes, the Himalaya or the Southern Alps of New together. Zealand. Downloaded from Cambridge Books Online by IP 130.226.229.16 on Wed May 21 09:43:42 BST 2014. http://dx.doi.org/10.1017/CBO9780511807602.007 Cambridge Books Online © Cambridge University Press, 2014 Supraglacial (or surface) debris is A well-publicized large-scale rock-fall was the collapse of the normally the result of rock-falls, but summit of Mount Cook (Aoraki), New Zealand’s highest mountain. occasionally larger landslides occur, as in the accumulation area of Glacier Pré du Just after midnight on 14 December 1991, climbers, in preparing for Bar in the Mont Blanc area, French Alps. the ascent of the mountain at Plateau Hut, heard a loud rumble and felt their accommodation shake. Looking out into the darkness, they saw bright orange flashes before the hut was enveloped in a cloud of dust. Deciding that it was unwise to climb the mountain, they found next morning that falling rock and ice had come within 300 metres of the hut, and that the entire glacier below the summit was obliter- ated. In fact, the rock-fall swept across the ice terrace of the Grand Plateau, down the Hochstetter Icefall, fanned out across the two- kilometre-wide Tasman Glacier below, and up the other side of the glacier for 70 metres – a 2.7-kilometre vertical fall and a horizontal distance of seven and a half kilometres. New Zealand Government scientists subsequently calculated that 29 million cubic metres of Downloaded from Cambridge Books Online by IP 130.226.229.16 on Wed May 21 09:43:42 BST 2014. http://dx.doi.org/10.1017/CBO9780511807602.007 Cambridge Books Online © Cambridge University Press, 2014 Nature’s conveyor belt 103 The parallel ‘tramlines’ on the Grosser snow, ice and rock had been involved, largely derived from the Aletschgletscher, Berner Oberland, eastern face of Mount Cook, but taking with it the summit, thereby Switzerland are medial moraines, each formed where two ice streams combine. reducing the mountain’s height by 10 metres to 3754 metres. The arcuate structure between the Although there were no fatalities or damage, the event made the moraines is foliation, a layered news throughout the world, and was even recorded by a seismo- deformation structure in the ice. The three peaks in the background, from left to graph in Wellington, 500 kilometres away. Although Mount Cook right, are Jungfrau (4158 metres), Mönch lies near a fault, the underlying cause appears to have been the (4099 metres) and Eiger (3970 metres). undermining of highly fractured rock by freeze–thaw processes. Earthquakes frequently cause spectacular rock-falls onto glaciers in Alaska. An earthquake in 1964 led to the covering of the lower reaches of Sherman Glacier with debris, slowing ablation so much that the glacier started to advance. More recently, the Denali Fault earthquake of 3 November 2002, which measured 7.9 on the Richter Downloaded from Cambridge Books Online by IP 130.226.229.16 on Wed May 21 09:43:42 BST 2014. http://dx.doi.org/10.1017/CBO9780511807602.007 Cambridge Books Online © Cambridge University Press, 2014 104 Glaciers Opposite. Supraglacial debris may be scale, resulted in spectacular changes to the landscape. Of these more evenly scattered over the glacier changes, huge rockslides that crossed the Black Rapids Glacier are surface if rock-falls are intermittent. This shows a view down the Gornergletscher, especially impressive. These rockslides are so large that reduced looking towards the Matterhorn (4477 melting in the ablation area may lead to an advance like that of metres). Sherman Glacier. The size of the rocks carried by the glacier depends on the rock type: cli¬s of resistant, unbedded rocks like granite produce huge blocks, often the size of small houses, but softer rocks like shale or limestone invariably break down into small boulders. Generally, supraglacial debris contains relatively small amounts of sand and Where the ice slows down and becomes crevassed, medial moraines merge into finer material. one another, as on Hubbard Glacier, Most debris falling down mountainsides becomes caught up Alaska. Despite now merging into a with the ice as it slides past the valley sides. The lines of debris on continuous cover of debris, the individual moraines may be distinguished on the the surface at the edge of the glacier and the ridges of debris left basis of the different-coloured rock types. behind as the glacier recedes are known as lateral moraines. Where Downloaded from Cambridge Books Online by IP 130.226.229.16 on Wed May 21 09:43:42 BST 2014. http://dx.doi.org/10.1017/CBO9780511807602.007 Cambridge Books Online © Cambridge University Press, 2014 Nature’s conveyor belt 105 Downloaded from Cambridge Books Online by IP 130.226.229.16 on Wed May 21 09:43:42 BST 2014. http://dx.doi.org/10.1017/CBO9780511807602.007 Cambridge Books Online © Cambridge University Press, 2014 106 Glaciers Opposite. Supraglacial streams often two streams of ice join, the two lateral moraines combine to form a wash out the finer material (sand and single medial moraine, which appears as a line of debris extending pebbles) from the moraine cover and transport it down-glacier. If it collects in towards the snout, down the middle of the glacier. Commonly, hollows and the stream is rerouted, the moraines comprise a variety of rock types that reflect the di¬erent debris protects the ice from subsequent source areas. When ablation near the snout brings medial ablation, so that ultimately it stands proud of the ice surface as a dirt cone. This moraines of di¬erent composition into close proximity, a glacier three-metre-high cone on Glacier de may take on a multicoloured striped appearance, as in the justly Tsijiore Nouve, below Pigne d’Arolla (3796 named Variegated Glacier in Alaska or the Unteraargletscher in the metres), Switzerland, is typical in comprising a thin cover of debris over ice. Alps. Dark debris on the surface of a glacier absorbs the Sun’s radiation better than the surrounding ice. The debris cover becomes relatively warm, and increases the melting of the ice, particularly if the debris Isolated blocks on the glacier surface is no more than a few centimetres thick, with the result that the often protect the ice from melting, moraines occupy depressions. In contrast, a thick continuous cover especially when solar radiation is strong, forming glacier tables. This example is on of debris normally slows down ablation, so the debris stands proud Vadret Pers, southeastern Switzerland. above the general glacier surface as a distinct ridge. Downloaded from Cambridge Books Online by IP 130.226.229.16 on Wed May 21 09:43:42 BST 2014. http://dx.doi.org/10.1017/CBO9780511807602.007 Cambridge Books Online © Cambridge University Press, 2014 Nature’s conveyor belt 107 Downloaded from Cambridge Books Online by IP 130.226.229.16 on Wed May 21 09:43:42 BST 2014. http://dx.doi.org/10.1017/CBO9780511807602.007 Cambridge Books Online © Cambridge University Press, 2014 108 Glaciers Isolated boulders also protect the ice from melting, so they may end up sitting on perches of ice. These glacier tables often tilt towards the Sun as time goes by, until they slide o¬, and the process is repeated. Streams on the surface may recirculate much of the finer debris, some of which collects in hollows. Once a stream course has been The tongue of Chola Glacier in the abandoned, the surrounding surface continues to ablate. However, Khumbu Himal of Nepal is completely the debris in the depression may slow down the melting of the debris-mantled. The thick debris-cover has prevented the glacier in the current underlying ice, so that sand or gravel eventually rests on small rises.
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