Ice as an Agent of Sculpting Land

By: Rasik Ravindra
The glaciers have sculpted various landforms transforming the geomorphology of earth. These landforms are the result of processes of weathering, erosion and deposition under harsh climatic regimes.
Climate Change Environment

Glaciers are formed when snow, compressed into large thickness of crystalline ice mass, develops an ability to move due to sheer mass. These glaciers together with snow and ice constitute the cryosphere which covers nearly 10 per cent of the surface of the earth (NSIDC, undated). The major part of ice and glaciers is found as ice caps and ice sheets in Arctic, Antarctic and Greenland, apart from the Himalaya, Andes and Europe. The glacierised area of the earth is spread over nearly 15 million sq km of the land surface and account for approximately 75 per cent of the freshwater resources of the world (ibid). During the Quaternary period that lasted for 2.6 million years, the earth saw many changes in its climate, forcing repeated glacial and interglacial cycles, caused by the earth’s orbital changes including the tilt of its axis (Milankovitch cycles). As evidenced by the ice cores drilled from various locations of Antarctica, such as Vostok and Dome C, the earth witnessed eight cycles of ice ages each separated by an interglacial period in its history of past 750,000 years (EPICA, 2004). During the immediate past ice maxima period (~20,000 years before present), the glaciers covered 32 per cent of the land surface and sculpted various landforms transforming the geomorphology of earth into a shape that we see today. Not far back, between 17th and late 19th century—during the period called ‘Little Ice Age’—the world saw consistently cooler temperatures that helped the glaciers advance. The advance or retreat of a glacier is seen at its snout, which may be defined as the terminus, toe, or the end of a glacier at any given point in time (Figs. 1 and 2).

There are several types of glaciers, such as mountain or valley glaciers which originate in higher altitudes of mountains and slide down the slopes occupying valleys over large distances for example the Himalayan or Alpine glaciers (Fig. 3). A tributary glacier may look like a hanging glacier if its link with the main trunk glacier is broken and it is left alone at the higher reaches of the mountains. Tide water glacier (Fig. 4) on the other hand extends up to sea and may give rise to icebergs as its tongues calve into the ocean. Large concentration of continental masses of glacial ice spread over land in the form of ice sheets are found in Antarctica (Ravindra  and Chaturvedi, 2011) and Greenland, the former being the largest accumulation of ice on this planet (Fig. 5).

Landform development

The landscape in glacial and periglacial environments is sculpted by the processes of weathering, erosion and deposition under harsh climatic regimes—caused by the agents of weathering and deposition such as strong winds, intense solar radiation, extreme cold surface temperatures and the movement of glaciers. The diurnal variation causes freezing and thawing in the rocks, resulting in their shattering, producing fields of broken rocks strewn in the form of block fields. On the higher reaches of the mountain, the glacial regime exhibit alpine topography that comprise serrated ridge tops, jagged peaks, arêtes, tors and/or horns (Fig. 6 a and b). The chemical leaching due to capillary action and salt formation imprint their marks on the landscape. The glacial and fluvio-glacial action of the ice (with melt water) together with the scouring action of the ice act upon comparatively weaker lithology to give rise to a number of depressions that gradually become the loci of accumulation of melt water, giving rise to melt water lakes (Fig. 7).
The advancing and retreating glaciers are powerful agents of erosion and deposition. The more prominent landforms under the influence of these factors are described below:

Glacial Striation and Polishing: As the glaciers move over the hard rocks, they carve out groves and striations on the rocks below caused by the boulders or fragments of the rocks carried by it on the under side. The accompanying polishing gives a look of varnishing to the rocks that can be seen when the glaciers retreat and rocks become exposed (Fig. 8 a and b). The presence of coarse sand and pebbles on the top surfaces of the outcrops as well as the spread of boulders on the hill tops speak of an extensive coverage of glaciers over a terrain. The boulders, up to 2 m×1.5 m size, of completely different composition than the rocks of the hills on which they are found, occur as glacial erratic or perched boulders on the hill tops vacated by glacier in Schirmacher hills of east Antarctica (Fig. 9) indicating long distance transportation.

Roche-Moutonees: These are glaciated bedrock surfaces, usually in the form of rounded knobs, the upstream side of which has been subjected to glacial scouring that has produced a gentle, polished and striated slope. The downstream side is subjected to glacial plucking that result in a steep and irregular slope. The ridges dividing the upstream and downstream slopes are therefore perpendicular to the general flow direction of the former ice mass. The modified roche-moutonees structures (Fig. 10), typical of a periglacial environment, are displayed along the northern margin of Schirmacher hills, Antractica, where one side of the hill is striated, rounded or flat with minor gradient towards the upstream direction, while the other side (lee side) has steeper gradient in the opposite direction.

Glacial valleys: Glacial landscapes show ‘U’ shaped glacial valleys as against ‘V’ shaped valleys in a fluvial domain (Fig. 11). This is so because a glacier cuts through the sides of the valley with equal force as it cuts downwards while making its way down the slope. A combined or post glacial fluvial action on the valleys may exhibit downward cutting of valley base, modifying the ‘U’ shape.

Patterned Ground: These micro-relief structures are found near flat or moderately sloping ground in a glacial environment and are formed due to sorting of soil material under the influence of the frost action in the upper layer of active zone of the permafrost (Ravindra, 2001). The response of the moisture present in the soil and freezing and thawing causes heaving of the soil resulting in the sorting of debris in the forms of strips, polygons, circles etc. The sorted polygons (Fig. 12) display a core area, comprising medium to coarse sand with cobbles and pebbles, while the outer rim shows concentration of larger sized boulders.

Moraines: These are depositional landforms that are seen as ridges, mounds or irregular mass of unstratified drift left behind by a retreating glacier. It comprises chiefly boulders, gravel, sand and clay material. Moraines are the most dominant landforms in a glaciated terrain that also play an important role in unearthing the movement pattern of the glaciers. Various parameters such as different levels and morphology of the moraines, degree of surface weathering, wind polishing, growth of lichens, development of cryogenic and honey comb-structures, etc., have been used to establish the chronology of moraines (Bardin, 1971).

Moraines are defined as terminal, push, medial or ablation moraines depending upon their location and mode of deposition. While the end moraine constitutes the debris dumped at the terminus of retreating glacier, lateral moraines are deposited on sides and often occur as long, flat or low gradient high ridges of boulders of diverse size and composition, loosely held together by sand and clay (Fig. 13). Medial moraines are present on the central part of the trunk glacier especially where two tributary glaciers coalesce to enact the union of two lateral moraines (Fig. 14). Ablation moraines are mainly concentrated along the margin of the continental ice, especially at locations where the gradient of ice sheet is moderate to low. The areas southwest of the  Indian Antarctic station, Maitri shows many such trails at different altitudes. There is marked parallelism between the curvature of these moraines and that of the margin of the continental ice.

Two other structures associated with glaciers are crevasses and moulins. A crevasse is a fissure or deep, wedge-shaped cleft or an opening in a moving mass of ice such as a glacier or an ice sheet (Fig. 15). Crevasses usually form in the top 50 m of a glacier, where the ice is brittle and the strain is accumulated due to differential movement. A moulin, on the other hand, is a near circular, vertical to near vertical well-like opening within a glacier or an ice sheet, into which the melt water enters from the surface and may lubricate the base of the glacier accelerating the speed of the movement of the glacier, as shown in figure 16  (NASA, 2006).

Way forward

Ice plays a critical role and is an active agent in sculpting landforms such as block fields, melt water lakes, roche-moutonees, glacial valleys, patterned ground, moraines etc.  With rising temperatures, many such landforms will transform again, with a greater fluvial influence.

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