Glacier

Morland and Morris (1977) used a mathematical model to study the potential for bedrock crushing by a glacier. Their aim was to see whether the stress field produced in bedrock by an overriding glacier was sufficient to cause failure in the bedrock. They calculated the likelihood of bedrock failure for different object shapes and different bedrock lithologies. The results of one experiment designed to predict the region of a bedrock hump where bedrock failure is most likely is shown below. The maximum stress generated by the glacier moving over this bump is located deep within the rock mass on the downstream flank of the bump. The value of this failure stress is less than the coherent strength of the bedrock itself, which led Morland and Morris (1977) to conclude that failure will not occur if the rock is coherent. In this situation the profile of a bedrock hummock, such as that illustrated below, will remain stable unless bedrock joints or other internal weaknesses are present within the rock. Morland and Morris (1977) concluded that these weaknesses therefore must be present to allow bedrock failure to occur and to facilitate the development of a typical roche moutonneee profile.

Source: Morland, L.W. and Morris, E.M. (1977) Stress in an elastic bedrock hump due to glacier flow. Journal of Glaciology, 18, 67-75. [Modified from: Morland and Morris (1977) Journal of Glaciology, 18, figure 7, p. 74]

contains joints, bedding planes and other lines of weakness, which may be exploited and expanded by the stress field generated by the flow of ice over them. The importance of these weaknesses is reflected in the fact that the morphology of many glacial erosional landforms is controlled by the pattern of discontinuities, joints and bedding planes within the parent rock mass (Figure 5.5). In general, the faster the rate of ice flow the more pronounced are the variations in basal ice stress and therefore the stress field generated within the underlying bedrock. The pattern of effective normal pressure over obstacles may fluctuate with time, causing the stress fields within the bedrock obstacle to vary. Dramatic changes or repeated changes in the stress fields may be particularly important in propagating fractures along lines of weakness within the rock mass.

Figure 5.5 (A) Ice margin of the Greenland Ice Sheet showing blocks of bedrock removed by glacial quarrying. (B) Blocks of bedrock removed from a rock face by glacial quarrying, Norway. Former ice flow was from right to left. In both cases the size of the quarried blocks is controlled by the joint spacing in the bedrock. [Photographs: N.F. Glasser]

Figure 5.5 (A) Ice margin of the Greenland Ice Sheet showing blocks of bedrock removed by glacial quarrying. (B) Blocks of bedrock removed from a rock face by glacial quarrying, Norway. Former ice flow was from right to left. In both cases the size of the quarried blocks is controlled by the joint spacing in the bedrock. [Photographs: N.F. Glasser]

Temporal fluctuations in the pattern of ice pressure may also cause variation in the temperature of basal ice and in some cases may generate small cold-based patches within an otherwise warm-based glacier. This process is known as the heat-pump effect. At its simplest this process involves the melting of ice in areas of high basal ice pressure, for example on the upstream side of an obstacle (see Figure 4.6). The high pressure reduces the freezing point, allowing melting to occur. Melting of ice consumes thermal energy, latent heat, and will cause the ice mass to cool. Some or all of the meltwater generated will move under the glacier to areas of lower basal ice, pressure where it refreezes. If the basal ice pressure then falls over the original obstacle, refreezing of the available meltwater will occur around this obstacle. On freezing, latent heat is given off and will warm the basal ice. However, because some of the meltwater has now been lost the temperature of the basal ice cannot regain its former level and a cold patch will form. The same amount of water would need to refreeze as was melted to return the basal ice to its original temperature, but since meltwater has flowed away this cannot occur. Consequently temporal pressure variations beneath an ice sheet, associated for example with diurnal fluctuations in ice velocity or meltwater discharge, may generate cold patches on the glacier bed. Beneath these cold patches ice will be frozen to the bedrock, creating sticky spots at the bed in these places. Lumps of rock beneath cold patches may therefore be entrained by freezing to the glacier as the ice flows forward (see Section

Fluctuations in basal water pressure may also help to propagate bedrock fractures beneath a glacier (Figure 5.6). Basal water pressure influences fracturing in two ways: (i) it affects the distribution and magnitude of the stress fields set up by ice in bedrock surfaces; and (ii) its presence within fractures and microscopic cracks is important to the process of fracture propagation. As we saw in Section 4.6, basal water pressure helps determine the presence or absence of basal cavities beneath a glacier. The presence or absence of these basal cavities has an important influence of the distribution of stresses imposed on a bedrock obstacle by the glacier. Changes in the basal water pressure within lee-side cavities causes them to vary in size and may cause cavity closure, and thereby alter the stress field within the bedrock

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Bedrock

Cracks may exploit an existing subhorizontal joint as block rotates away from cliff

Bedrock

Figure 5.6 Schematic diagram of the processes involved in glacial quarrying. Fluctuations in subglacial water pressure are important in the development of bedrock fractures, and blocks are rotated out of the rock face by the passage of ice.

obstacle. For example, the sudden drainage of meltwater from within a cavity may cause it to close even if the ice velocity remains unchanged, due to increase in the effective normal pressure caused by the fall in the water pressure (see Section 4.6). Rapid or repeated changes of this sort may help to widen or propagate fractures within the bedrock mass (Figure 5.6). As a cavity forms due to an increase in water pressure this hydrostatic pressure can effectively lift up the base of the glacier. If rock fragments are frozen to the glacier bed due to the heat-pump effect during this process they will be lifted up and moved forward as the base of the glacier rises (Figure 5.7). This process is known as the hydraulic-jack effect.

Figure 5.7 The role of the hydraulic-jack and heat-pump effects in glacial quarrying. Fragments of bedrock are frozen to the glacier bed by the heat-pump effect and then lifted from it as increasing water pressure opens basal cavities.

Where fluctuations of basal water pressure are combined with the heat-pump effect a cycle of erosion may occur. As basal water pressure falls the resulting increase in effective normal pressure over an obstacle may cause a period of fracture propagation. As basal water pressure rises basal cavities may open and rock may be entrained by the hydraulic-jack effect and the heat-pump process. This would imply that glacial quarrying will be most effective under a glacier where there are regular fluctuations in basal water pressure.

Debris entrainment includes the processes by which bedrock or sediment is detached from the glacier bed and incorporated into the basal ice. The evacuation and entrainment of a rock fragment from the glacier bed is governed by the balance between the tractive force exerted on it by the overriding ice and the frictional forces which act to hold the rock in place. High basal water pressures may help to reduce the frictional forces holding debris in place. Entrainment can occur in the following ways.

1. By the heat-pump effect, causing local patches of basal ice to freeze to the bed and therefore detaching debris as the ice flows forward.

2. The drag between ice and bedrock may be sufficient to detach very loose particles, particularly if they become surrounded by ice.

Figure 5.7 The role of the hydraulic-jack and heat-pump effects in glacial quarrying. Fragments of bedrock are frozen to the glacier bed by the heat-pump effect and then lifted from it as increasing water pressure opens basal cavities.

5.2.2 Rock and Debris Entrainment

3. Loose debris collected within basal cavities may become surrounded by ice and simply swept away if the cavity closes.

4. Freezing-on of material may occur in the lee of obstacles as meltwater generated on their upstream faces refreezes in the low-pressure zone in their lee to form regelation ice. In warm-based glaciers the debris layer produced by the freezing of regelation ice is usually thin because debris is also released by melting on the upstream side of obstacles. However, if freezing-on dominates over melting, perhaps at boundary between warm and cold ice, where there is a constant flux of meltwater freezing onto the glacier, a large thickness of debris-rich regelation ice may develop (Figure 5.8).

Heat flow

Inflow meltwater warm-based and cold-based

Heat flow

Transition to cold based ice

Figure 5.8 Debris incorporated into regelation ice in the lee of bedrock bumps. (A) In a warm-based glacier regelation ice and debris layers tend to be destroyed by pressure melting associated with other bumps and the debris layer will be thin. (B) In a zone of transition from warm-based to cold-based ice, meltwater and debris can be frozen into the basal ice layer and significant thicknesses of debris can form. [Modified from: Boulton (1972) in: Polar Geomorphology (eds R.J. Price and D.E. Sugden), Institute of British Geographers, Special

Publication 4, figure 5, p. 101]

5. Through refreezing associated with glaciohydraulic supercooling, a process where subglacial meltwater flowing in a distributed drainage system can 'supercool' (i.e. exist as liquid water at a temperature below its freezing point) as it moves up an adverse bed slope of an overdeepening. At contemporary glaciers (e.g., in Alaska and Iceland) this process can create thick zones of debris-laden basal ice in areas where the bed slope exceeds the ice-surface slope by more than 1.2-1.7 times. It has also been suggested that this process was widespread beneath the former Laurentide and Scandinavian Ice Sheets, for example in areas with overdeepenings close to the former ice margin, but this has yet to be substantiated.

6. Beneath glaciers with a mixed thermal regime, fluctuation of the thermal boundary between warm and cold ice may lead to the freezing-on of large rafts of sediment or rock. For example, if an area of previously warm basal ice was to turn cold, perhaps during deglaciation, large rafts of previously saturated sediment and bedrock may become frozen to the glacier bed and entrained as it flows forward.

7. Debris may also be incorporated into the ice along thrust planes. In glaciers with a mixed thermal regime in which there is a cold ice margin and warmer interior, compressive flow is common because the cold ice moves less quickly than the warm ice within the glacier interior. This compression may lead to the development of thrusts within both the ice and underlying sediment along which debris may be incorporated (Figures 7.11 and 7.12: see Section 7.3).

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