One of the most impressive features of the surfaces, soles and margins of temperature glaciers is the volume of water present in thin films, streams and ponds. Meltwater may be present in appreciable amounts in the upper parts of some subpolar glacier system. On polar glaciers, meltwater may be present at the base only where the ice is of great thickness. Surface meltwater streams are rare on high-polar glaciers, except where re-radiation from rock surfaces concentrates melting. The production of water through melting of ice that undergoes erosional process like loosening, dissolving, and removing action on debris or rock material in a glacial environment is called as meltwater erosion.
Phenomenon of meltwater erosion
The presence of meltwater is important because of its influence on mode and rate of glacier flow. It is produced beneath and within glacier by the flow of earth heat from the rock head. Heat from the meltwater production is also released during recrystallization associated with plastic creep and in overcoming the basal sliding friction. Another source of basal meltwater is groundwater moving out of subglacial aquifers in to basal zones.
Meltwater produced by the surface melting of ice and snow is added to that derived from glacier flow especially in summer. The volume of seasonal meltwater varies along climatological gradients (both in latitude any altitude), being high in the lower reaches of maritime temperature glaciers, but declining systematically towards continental interiors. Summer precipitation in the form of rain enhances the super glacial and englacial discharge of meltwater and, together with the runoff from adjacent unglaciated rock slopes, it may cause rapid peaking of discharge in proglacial streams fed from subglacial and marginal sources.
Summer melting of previous winter’s snow cover, and ultimately of bare surfaces of glacial ice, produces runoff of glacial surface. Water sometimes moves as thin films on smooth ice but flow in channels ranging in scale and form from rills to well-developed meandering channel systems is more common. Meanders may become incised largely as a result of thermal rather than mechanical erosion.
Water moves through and beneath glaciers in several ways. Several discontinuities such as crevasses produced by tensional stress in the ice commonly provide sinks for super glacial stream discharge. Where meltwater maintains a vertical avenue into the glacier in conditions favoring crevasse closure, a glacier mill is formed. There is a strong correlation between mill distribution and crevasse location on many glaciers. The water stream may advent warm air into the mill which enhances melting of ice walls and so helps maintain the passage. There is a lack of precise knowledge about the lower sections of mills. While some are known to extend over 200 metre to the ice rock interface, others may not penetrate more than a few tens of metres into the glaciers at such a steep angle. Low angle tunnels or conduits have been observed within and beneath relatively thin glacier ice and it is assumed that these represent the lower reaches of englacial streams which begin at the mills. In the vadose zone above the piezometric surface, conduits maybe relatively steeply inclined, water moving under gravity. Below the piezometric surface, meltwater pressures increase with depth and locally high pressures may be expressed by water spouts or turbulent stream flow issuing from beneath glacier margins.
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The geo-physics behind meltwater erosion
While the precise mode of initiation of water-carrying tubes within glaciers is not clear, it appears likely that they are supplied by water moving down a network of intergranular veins which become progressively enlarged preferential growth of larger once being favored by exponential increase in the ratio between the heat in the meltwater and the area of the tube walls. Englacial water pressures are commonly greater than approximation of the yield stress for the glacier ice (~1 bar) so that, once initiated, conduits may be expected to be maintained with actively moving glaciers. Enlargement may also occur by thermal erosion. The circular cross section of conduits in the zone of saturation becomes modified in vadose zone as open channel flow within the tubes becomes dominant and a variety of entrenched and flat-flowed tube forms developed. One result of this is the development and the perpetuation and meandering englacial tubes and their eventual intersection with the glacial bed. In the zone of saturation, englacial water flows in conduits down a pressure gradient which broadly parallels the eye surface slops, so that the melt water roots tend to be convergent where glacier surfaces are concave upward (as in the accumulation zone) and divergent beneath the convex ice profiles so that the ablation zone of the valley glacier may be drained by two essentially discrete systems. The great bulk of englacial water in a glacier ultimately flows out in one or a series of sub glacial tubes at ice berg interface.
Subglacial water in thin films, sheets and streams varies in volume and pressure with variation in glacier thickness and velocity, particularly deep depressions beneath very thick polar glaciers may give rise to thick water bodies called sub glacier lakes which are contained by thinner, cold polar ice up and down glacier. This situation may also occur in subpolar glaciers where the cold ice of the upper surface and thinner snout zone prevents meltwater issuing sub glacially at the snout. Basal meltwater bodies are characteristic of depressions within the undulating rock bed beneath thick and temperate glaciers. At relatively low water pressures such water bodies may be isolated from water in adjacent cavities but as pressure rises or as the glacier moves, the cavity may develop an outlet to other cavities and water will flow down to the pressure gradient.
The thin film of water at the base of a temperate glacier plays a critical role in the overcoming of small-scale obstacles in the bed, water produced at higher pressure (up-glacier) locations being transferred to low pressure sites in the process of regelation. In addition, meltwater may be transferred along the base of a glacier as a thin sheet. In the regelation process, the thickness is of the order of one micro meter but for down glacier flow of basal meltwater by sheet flow, very much greater water thickness (of, say, 1 mm) are required which would interfere with the regelation process. The melt water exists in separate systems contained in channels. These may be cut in the ice, in which case they will be likely to suffer and periodic closure, or in the underlying rock head. Such rock cut channels provide a more permanent avenue for meltwater and may provide for the dominant mode of subglacial meltwater flow.
The periodically very high discharge and velocity of subglacial meltwater flowing within channels and as thick sheets are a source of energy which is expended in the transport of huge quantities of sediments and in locally severe erosion of bedrock surfaces and previously deposited proglacial sediment. The saltation and bed loads of glacial rivers play a major role in abrasion of rock surfaces known as corrosion. Indirect evidence of this is provided by the high degree of edge rounding of particles transported. Rock surfaces swept by meltwaters show a high degree of smoothening and edge rounding. Some surfaces have been traced beneath modern glaciers.
Water flow within and beneath a glacier is controlled by a pressure gradient which broadly conforms to the ice surface gradient. While a great deal of water flow beneath temperate glacier occurs as very thin films and sheets, small amplitude depressions in an irregular sub glacial surface provide conditions in which concentrated (channel) flow may develop and persist. Incision of meltwater streams into the rock head may be essential for the evacuation of the great volume of meltwater produced by some glaciers. Evacuation of such high discharges by basal ice walled tubes or by means of basal films and sheet flow would interfere with the regelation process and produce very high, perhaps surging velocities.
Resultant meltwater-flow channels
The presence of rock cut channels beneath present-day glaciers is difficult to demonstrate, but many thousands of channels Pleistocene age ascribed to meltwater erosional processes given that sub glacial meltwater flow is controlled by hydrostatic rather than morphological gradients, channels eroded by sub glacial waters are aligned parallel to the ice surface slope. Channels cut into ridge rests sometimes by many tens of meters show a consistent trend. Such channels are steep sided and, when bedrock structure is conducive and subsequent slope modification slight, flat floored so that channels of Pleistocene age appear as discordant elements in the landscape. The importance of hydrostatic pressure gradients in their origin may be seen in some channels cut across spurs, cols and ridges which were disposed transversally to form gradient to form ice sheet. Many channels cut in such condition have a humped longitudinal profile, the bed rising sometimes for some hundreds of meters before declining in the direction of meltwater flow. Such ‘spur end’ and ‘col’ channels are often the largest members of the meltwater channel population. The great size of some subglacial meltwater channels may have several explanations. In some cases, there is little doubts that the channel, the cross-sectional areas reflect the peak discharges.
Other factors must be considered, however. These include the probability of prolonged incision once a trunk meltwater channel is established, the periodic occupation of part of the channel by basal ice and the use of the channels during more than one phrase of glaciation. Meltwater channels draped with till and having striated walls may be used in support of these interferences. Some group of channels have consistent gradients and great continuity suggesting that their location was determined by the presence of an ice margin. Indeed, sometimes the features are replaced by sinuous rock cut benches, the missing channel wall having been the glacier margin. Complex ice marginal channel morphology may reflect permeable, and therefore temperate, glacier conditions while simple channel morphology, and especially the presence of half- channels or benches, may be the product of impermeable polar ice. To some extent, both may be present in the same region either because of seasonal changes in meltwater- glacier relationships or because the regime of the ice sheet changed from polar in the early part of deglaciation to temperature as ice wasted toward the valley bottoms.
Meltwater channels are also cut across ridges and spurs and along hillsides as a result of over spilling of lake waters impounded in side valleys and re-entrants by glacier ice. For many years, it was believed that all meltwater channels were the product of ice-marginal lake overflow and so were subaerial. Detailed studies in the period 1950-1970 demonstrated that a great many meltwater channels of Pleistocene age are the product of subglacial erosion. Nevertheless, ponding of the lake waters by glaciers is widespread today and was probably so during the Pleistocene glaciations. While marginal and col type channels occasionally serve to drain glacial lake waters, the commonest mechanism of lake drainage is probably by subglacial tube and channel flow and by periodic flotation of the ice-edge, producing a very high discharge of subglacial waters known as glacier flood. Where the slope of the ground is in the same sense as the surface slope of the glacier, meltwater of subaerial type may incise channels into valley floor.
Geo-physical implications of erosion
The regional pattern of meltwater channels is often strikingly consistent, providing in outline, a record of dwindling ice volume, detachment of ice bodies and general ice wastage. However the interpretation of precise origin of channels produced by glacial meltwater is made because of many variables influencing their form and because some of these variables are indeterminable in many cases. Glacial meltwater channels remain, therefore, meltwater erosion remains a challenging subject of study.
Small irregularities produced on a rock surface beneath flowing water by cavitation, impact or detachment rapidly become smoothed and rounded along their rims by the process of abrasion. Within the cavity, abrasion may be concentrated as local vortexes develop in fluid and particles enlarge the cavity by attrition. Once entrenched, these potholes may be eroded into sound rock by the constant rotation of pebbles, cobbles or boulders trapped within them. Some potholes in glacial meltwater stream bed reach many meters in depth and diameter.
Frequently associated with potholes is a group of bedrock erosional features known as plastically molded surfaces because of their smooth, stream lined shapes. They include sichelwannen, grooves and cavettos. Sichelwannen are sickle shaped troughs disposed transversally into glacier flow with the horns pointing down glacier. Grooves and cavettos are channel forms disposed parallel to the ice flow lines, the former having rounded edges and latter sharp edges. Unlike sichelwannen and grooves which are round on flat, gently sloping surfaces, cavettos are cut into steep rock faces so that they often form an overhang. Crescentic gauges and striae have been seen in them. There is no agreement as to their precise origin. However, their frequent association with bedrock potholes has led to a strong body of opinion favoring the view that they are the result of the flow of the subglacial water, or subglacial slurry of water, debris and ice crystals moving under high pressure. Sichelwannen have been likened to certain flute and ripple marks made by running water nonresistant beds and, on the basis of this analogy, it has been suggested that they may result from differential corrosion associated with the areas of separated flow in subglacial stream channels.
The presence of fine striations in some of these plastically molded forms suggest that debris rich ice plays an essential role at least in final stages of their formation. The result from streaming of basal ice into ice bands rich in debris and bands poor in debris as a result of plastic deformation around bedrock obstacle or large boulders. As a groove develops, normal stress will decrease as the ice tends to bridge the deepening grooves, so that there may be an optimal or equilibrium depth for given conditions of ice thickness, debris concentration, water pressure and rock resistance.
Landscapes that experience glaciations undergo considerable modification as a result of the erosion caused by the moving masses of ice. The direct glacial erosion is accomplished by three processes, namely, abrasion, crushing, and plucking. Another important, but indirect erosion process is meltwater erosion. The production of water through melting of ice that undergoes erosional process like loosening, dissolving, and removing action on debris or rock material in a glacial environment is called as meltwater erosion. Glacial meltwater is a very effective agent of mechanical erosion as well as chemical erosion because of the high water pressure that flows through tunnels in the ice and emerges at the end as a meltwater stream. The meltwater inside the glacier may be under hundreds of feet of ice and is under great pressure. The release of this water under high pressure mixed with the sediment carves rock on the way like a saw and enhances the rate of erosion.