The Principles Of Basal Thermal Regime

The principal control on which combination of flow processes operate beneath a given glacier - creep or basal sliding - is the temperature of the basal ice. Some glaciers are frozen to their beds and no meltwater is present at the ice-bed interface and basal sliding does not occur (Figure 3.9). Such glaciers are composed of cold ice. In contrast, other glaciers are composed of warm ice, where basal ice is constantly melting and the ice-bed interface is therefore lubricated with meltwater. In such situations basal sliding is an important component of flow (Figure 3.9). An ice sheet with a warm base therefore has a much greater potential for fast flow and therefore to modify its bed by erosion than one which is frozen to it. Basal ice temperature (basal thermal regime) is, therefore, one of the most important controls on the geomorphological impact of a glacier because it controls the pattern of erosion and deposition within it. Not only does basal thermal regime vary between glaciers, but it also varies within a particular ice body.

The temperature at the base of a glacier is determined by the balance between: (i) the heat generated at the base of the glacier; and (ii) the temperature gradient within the overlying ice, which determines the rate at which the basal heat is

A: Warm-based glacier resting on bedrock

B: Cold-based glacier resting on bedrock

Figure 3.9 The velocity distribution within three glaciers of different basal thermal regimes resting on different substrate. A vertical line a-b inserted in the glacier would be displaced as follows: (A) Warm-based glacier resting on hard bedrock: the vertical line a-b is displaced to c-d by basal sliding and deformed to line c-e by internal deformation. (B) Cold-based glacier resting on hard bedrock: movement is by internal deformation alone. (C) Warm-based glacier resting on deformable sediment: the line a-b is displaced to c-d by subglacial sediment deformation, to e-f by basal sliding and e-g by internal deformation. [Modified from: Boulton (1993) in Holmes' Principles of Physical Geology (ed. P.McL.D. Duff), Chapman and Hall, figure 20.20, p. 416]

Figure 3.9 The velocity distribution within three glaciers of different basal thermal regimes resting on different substrate. A vertical line a-b inserted in the glacier would be displaced as follows: (A) Warm-based glacier resting on hard bedrock: the vertical line a-b is displaced to c-d by basal sliding and deformed to line c-e by internal deformation. (B) Cold-based glacier resting on hard bedrock: movement is by internal deformation alone. (C) Warm-based glacier resting on deformable sediment: the line a-b is displaced to c-d by subglacial sediment deformation, to e-f by basal sliding and e-g by internal deformation. [Modified from: Boulton (1993) in Holmes' Principles of Physical Geology (ed. P.McL.D. Duff), Chapman and Hall, figure 20.20, p. 416]

drawn away by conduction from the ice-bed interface. Heat is generated at the base of a glacier in three ways: (i) by geothermal heat entering the basal ice from the Earth's crust; (ii) by frictional heat produced by sliding at the base of the glacier; and (iii) by frictional heat produced by the internal deformation of the glacier. These three heat sources combine to warm the base of the glacier. The rate at which this heat is conducted away from the base of a glacier depends upon the temperature gradient within the overlying ice. This temperature gradient depends on: (i) the temperature at the base of the ice; (ii) the temperature of the glacier surface; (iii) the thickness of the ice; and (iv) the thermal conductivity of the ice. The temperature of the basal ice is therefore a function of the amount of heat generated at the base of the glacier and the rate at which it is conducted away along the thermal gradient within the overlying ice. Three basal ice conditions can be defined.

1. Boundary Condition A. Net basal melting (warm ice). In this case more heat is generated at the base of the glacier than can be removed by conduction in the direction of the temperature gradient.

Basal heat generated > Heat conducted away Heat input > Heat output

2. Boundary Condition B. Equilibrium between melting and freezing. In this case the heat generated at the base of the glacier is equal to that conducted away along the direction of the temperature gradient.

Basal heat generated = Heat conducted away Heat input = Heat output

3. Boundary Condition C. Net basal freezing (cold ice). In this case all the heat generated at the base of the glacier is quickly removed from the bed along the direction of the temperature gradient and the ice remains frozen to its bed.

Basal heat generated < Heat conducted away Heat input < Heat output

These three boundary conditions or basal temperature states can be thought of as separate thermal zones, each of which has its own attributes in terms of the types of basal flow (Figure 3.9). Each is also dominated by different processes and geomor-phological activity as we will see in later chapters.

In the discussion so far heat transfer within the glacier has been assumed simply to be a function of conduction along the temperature gradient within the ice. In practice heat is also transferred through advection. Advection is the transfer of heat energy in a horizontal or vertical direction by the movement of ice or snow. For example, the downward movement of cold snow or firn within the accumulation area of a glacier will lead to glacier cooling. The rate of advection is therefore partly a function of accumulation rates; high accumulation rates result in strong heat fluxes due to the passage of cold ice through the glacier system. Listed below are the principle variables that help determine the basal ice temperature of a glacier.

1. Ice thickness. Increasing ice thickness will have the effect of increasing the basal ice temperature. This is due to the insulating effect of ice: more ice equals more insulation.

2. Accumulation rate. Basal ice temperature is also affected by accumulation rates through the process of advection. In a glacier advection occurs as the result of the accumulation of fresh snow on the surface gradually moving down through the glacier to the base. If this snow is cold it will cause the glacier to cool and conversely if it is warm it may cause an increase in temperature. For example, in the interior of an ice sheet, where accumulation rates may be very low and where snow accumulates at low temperatures, advection will favour basal freezing. Towards the margin of this ice sheet, where accumulation rates may be higher and where the snow accumulates at higher temperatures, advection may favour basal melting. The incorporation of large amounts of cold dry snow also has the effect of improving the temperature gradient within the ice and therefore the rate of heat conduction, which also helps reduce basal ice temperatures. Advection may also warm a glacier through the upward movement of warm ice in the ablation zone.

3. Ice surface temperature. An increase in the surface temperature of a glacier will reduce the temperature gradient within the ice and is therefore likely to increase basal ice temperature. There is a direct relationship between surface temperature and basal temperature: a fall of 1°C in surface temperature causes a fall of 1°C in basal temperature. This may be achieved by incorporating large amounts of wet snow and through summer melting. The percolation of meltwater through an ice body will have the effect of raising its temperature, because as it refreezes latent heat is liberated. For every gram of meltwater that freezes the temperature of 160 g of ice is raised by 1°C.

4. Geothermal heat. An increase in the flux of geothermal heat will increase basal ice temperature.

5. Frictional heat. An increase in ice velocity will increase the amount of frictional heat generated and in turn increase the basal ice temperature. A glacier flow of 20 m per year produces the same amount of heat as that produced by the average geothermal heat flux. As we will see in Section 3.5 ice flow within a glacier varies spatially, increasing from zero beneath the ice divide of an ice sheet towards the equilibrium line before it decreases towards the ice margin. Consequently the heat generated by friction will also increase to a maximum close to the equilibrium line. This variation in the amount of heat generated by friction is particularly important in determining the spatial variation of ice temperature within a glacier.

The above allows us to predict the situations likely to produce cold- and warm-based ice. Cold-based glaciers are likely to occur where the glacier is thin, slow moving and where there is little or no surface melting in summer and the surface layers of ice are cooled severely each winter. A typical temperature profile through a cold glacier is shown in Figure 3.10A. The increase in temperature with depth is due to the insulating effect of the overlying layer of ice and the increase in pressure with depth. The temperature gradient is positive - warmer at the base than at the surface - and therefore the heat will flow from the glacier bed to the surface. Heat will only flow along a negative gradient: from warm to cold. Any heat generated at the base of a glacier will in this case be conducted quickly away from the glacier base through the ice. Boundary Condition C will therefore prevail.

In contrast, warm-based glaciers are likely to occur where the ice is thick, fast moving and where summer melting is high. The percolation of meltwater through the glacier body will warm the ice as it refreezes through the release of latent heat and ice temperature may be close to its melting point. The melting point of ice varies with depth due to the change in pressure. Beneath 2000 m of ice, within an ice sheet, melting point will be -1.6°C instead of 0°C. This is termed the pressure melting point.

A: Cold Ice

B: Warm Ice

Colder

Temperature profile

Colder

Glacier surface

Pressure melting point

Glacier bed

Glacier surface

C: Cold ice overlying warm ice Colder

Ice at pressure melting point throughout Glacier bed

Glacier surface

Pressure melting point

Glacier bed

Figure 3.10 Idealised temperature profile through three glaciers. (A) Temperature profile within a cold-based glacier. (B) Temperature profile within a warm-based glacier. (C) Temperature profile where cold ice overlies warm ice at depth. [Modified from: Chorley et al. (1984) Geomorphology, Methuen, figure 17.4, p. 435]

Within an ice mass close to its melting point the temperature profile will look like that in Figure 3.10B. In this case the profile is positive - colder at the base than at the surface - and consequently any basal heat generated will not be able to escape and Boundary Condition A will therefore apply. The cases outlined above represent two extremes of a continuum. Glaciers in rare instances may be entirely warm- or cold-based, but in practice basal boundary conditions vary both in space and in time within a single glacier. For example, Figure 3.10C shows a situation where cold ice overlies warm ice at the glacier bed.

3.4.1 Spatial Variation of Basal Thermal Regime within a Glacier

Figure 3.11 shows just one way in which the three boundary conditions or thermal zones can be combined within a single glacier. At its simplest this can be thought of as a continuum between cold and warm ice (Figures 3.11A-E). Boundary Condition B has been split into two zones, one in which there is slight net freezing (B1) and one in which there is slightly more melting (B2). This reflects the position of this thermal zone within the transition between the two extreme types of basal boundary condition. In Figure 3.11A Boundary Condition C occurs throughout the glacier and the underlying ground is frozen (permafrost); because the glacier is frozen to its bed no basal sliding occurs. In the next diagram the central part of the glacier lies in thermal balance and is neither predominantly melting nor freezing (Figure 3.11B). In this central zone basal sliding will occur. This will cause compression toward the

Figure 3.11 The different patterns of basal thermal regime that can exist in a glacier: Condition A, warm; Condition B, thermal equilibrium; Condition C, cold. See text for full explanation of each thermal condition. [Modified from: Boulton (1972) in: Polar Geomorphology (eds Price and Sugden), Institute of British Geographers, special publication 4, figure 3.1, p. 4]

Figure 3.11 The different patterns of basal thermal regime that can exist in a glacier: Condition A, warm; Condition B, thermal equilibrium; Condition C, cold. See text for full explanation of each thermal condition. [Modified from: Boulton (1972) in: Polar Geomorphology (eds Price and Sugden), Institute of British Geographers, special publication 4, figure 3.1, p. 4]

transition into the outer zone of the glacier, which is still frozen to its bed and therefore will not experience basal sliding. Situations like this result in large compressive zones near glacier margins in which basal ice is often thrust toward the glacier surface (see Section 7.5). Figure 3.11C shows a more complex pattern of thermal zones, in which the centre of the ice sheet or glacier experiences basal melting. Meltwater passes out from this zone under hydrostatic (water) pressure and then freezes to the bed beneath the cold ice in the intermediate zone (B1) and new basal ice is formed. Beyond this the glacier is frozen to its bed. Figure 3.11D and E shows the other end of the continuum, moving toward a glacier that is melting at its base and one which therefore will experience basal sliding throughout.

In practice, the pattern or order of these thermal boundaries within a glacier may vary dramatically. For example, irregular basal topography or climatic variation across a glacier produce more complex localised patterns. Figure 3.12A shows a cross-section through a large mid-latitude ice sheet. The pattern of basal thermal regime within it is one idea of what the pattern may have looked like in one of the mid-latitude ice sheets of the Cenozoic Ice Age. This pattern of basal thermal regime

C

Figure 3.12 Cross-sections through three ice sheets showing: (A) the pattern of basal thermal regime in a schematic ice sheet; (B) the role of the underlying lithology in modifyingthis pattern; and (C) the role of the underlying topography in modifying this pattern. Condition A, warm; Condition B, thermal equilibrium; Condition C, cold. See text for full explanation of each thermal condition

Figure 3.12 Cross-sections through three ice sheets showing: (A) the pattern of basal thermal regime in a schematic ice sheet; (B) the role of the underlying lithology in modifyingthis pattern; and (C) the role of the underlying topography in modifying this pattern. Condition A, warm; Condition B, thermal equilibrium; Condition C, cold. See text for full explanation of each thermal condition is based primarily upon the effects of ice thickness and climate and ignores the effects of advection. Climatic variation across the ice sheet results in the contrast between the northern and southern margins: a contrast between a cold continental and a warm maritime climate. Figure 3.12B shows how the presence of a deformable bed can be exploited only where the glacier is warm-based. The pattern of basal thermal regime can be further complicated by introducing basal topography (Figure 3.12C). A deep trench beneath a zone of the ice sheet that would normally be cold at the base may induce melting at the glacier sole. Equally a raised mountain area may experience basal freezing in what would otherwise be a zone of melting (Figure 3.12C).

An alternative view of the basal temperature distribution within the former mid-latitude ice sheets of the Cenozoic Ice Age is obtained when the role of advection is emphasised. In this model the cooling effect of incorporating cold snow, advection cooling, in the ice-sheet centre causes it to be cold-based in the middle. While the increase in frictional heating towards the ice margin and the upward transfer of warm ice by compressive flow (advective warming) in the ablation zone cause the ice sheet to be warm-based towards its margin (Figure 3.13).

Whatever the pattern of basal thermal regime within an ice sheet, it has a profound effect upon the work done by the ice sheet or the geomorphology produced. This point will be returned to in later chapters but is well illustrated

Figure 3.13 Schematic time-distance diagram through an ice sheet showing the growth and decay of the ice sheet and the evolution of its basal thermal regime. As the ice sheet grows and decays, a zone of warm-based ice migrates across the landscape. The pattern of basal thermal regime at the ice-sheet maximum can be envisaged by taking a time-slice through the ice sheet at approximately 20 000 years

Figure 3.13 Schematic time-distance diagram through an ice sheet showing the growth and decay of the ice sheet and the evolution of its basal thermal regime. As the ice sheet grows and decays, a zone of warm-based ice migrates across the landscape. The pattern of basal thermal regime at the ice-sheet maximum can be envisaged by taking a time-slice through the ice sheet at approximately 20 000 years by the control of basal regime on the mechanisms of ice flow (Figure 3.9). Where the ice is frozen to its bed, irrespective of the nature of the substrate, flow may occur only by internal deformation and movement is concentrated above the bed. As a consequence the potential to modify the bed is small. In contrast, in zones of thermal melting flow may occur by basal sliding and by subglacial deformation where the substrate is appropriate (Figure 3.9) and the ice sheet has a more profound effect on its bed.

3.4.2 Temporal Variation in Basal Thermal Regime within a Glacier

Just as the basal ice temperature may vary within a glacier it may also vary through time. In particular the pattern of basal thermal regime within an ice sheet is not static. As an ice sheet grows and decays the pattern of basal ice temperature within it will also evolve. Consequently the pattern of processes controlled by it will also vary through time as the pattern of thermal regime changes. Figure 3.13 shows a hypothetical cross-section through a mid-latitude ice sheet, showing the evolution of the pattern of basal thermal regime within it. This diagram is highly schematic and in reality the patterns are likely to have been much more complex. It serves to illustrate, however, that at any location the temperature of the ice above may change as the ice sheet grows and decays and consequently the processes operating upon it will also change. This pattern results primarily from two main factors: (i) the increase in fric-tional heating towards the equilibrium line maintains a warm outer ring to the ice sheet; and (ii) cooling by advection causes the central part of the ice sheet to remain cold.

At a much smaller scale the pattern of basal thermal regime, and in particular the boundaries between one thermal zone and another, may change as a result of local or regional fluctuations in: (i) ice velocity; (ii) accumulation; (iii) geothermal heat flux; and (iv) ice thickness. On a longer time scale glacial erosion may modify basal topography, causing variations in the thermal regime. Any of these variables may change the temperature of the basal ice and therefore the dynamic nature of the processes operating at the ice-bed interface.

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