Mechanism

Most ablation will occur at the snout or terminus of a glacier, which is usually its point of lowest elevation, where air temperatures will be highest and where iceberg calving may occur. The rate of mass loss (ablation) will decrease with elevation, as atmospheric air temperature falls with altitude. Accumulation may be more uniform over the surface of a glacier, but will tend to increase with elevation. A glacier or ice sheet can, therefore, be divided into two areas: (i) an accumulation zone, where accumulation exceeds ablation; and (ii) an ablation zone, where ablation exceeds accumulation (Figure 3.3). The line between the accumulation zone and ablation zone is known as the equilibrium line; along this line accumulation is balanced by ablation.

On a valley glacier or ice sheet, mass is added at the top in the accumulation zone and taken away through melting and calving at the terminus in the ablation zone. Consequently the surface profile of the glacier will steepen with increasing accumulation (Figure 3.4). In this way the surface slope will steepen until sufficient stress builds up within the ice to cause it to flow. Ice flow transfers mass from

3.2 The Mass Balance Gradient: The Glacial Driving Mechanism 45

Glacial Erosion Diagram

Figure 3.3 Schematic diagram of an ice sheet and valley glacier showing the location of the accumulation zone, the ablation zone and the equilibrium line (the line where accumulation and ablation are equal in any given year). Principal flow paths are also shown. [Modified from: Sugden and John (1976) Glaciers and Landscape, Edward Arnold, figure 4.8, p. 63]

Figure 3.3 Schematic diagram of an ice sheet and valley glacier showing the location of the accumulation zone, the ablation zone and the equilibrium line (the line where accumulation and ablation are equal in any given year). Principal flow paths are also shown. [Modified from: Sugden and John (1976) Glaciers and Landscape, Edward Arnold, figure 4.8, p. 63]

Boeing Folding Wing Tip

Figure 3.4 Idealised glacier with net accumulation or input 'wedge' and net ablation or output 'wedge'. Glacier flow from the accumulation zone to the ablation zone is necessary if the glacier is to maintain a constant slope. [Modified from: Sugden and John (1976) Glaciers and Landscape,

Figure 3.4 Idealised glacier with net accumulation or input 'wedge' and net ablation or output 'wedge'. Glacier flow from the accumulation zone to the ablation zone is necessary if the glacier is to maintain a constant slope. [Modified from: Sugden and John (1976) Glaciers and Landscape,

the accumulation zone to the ablation zone, thereby reducing the surface slope of the glacier and therefore the stress imposed on the ice. This transfer maintains the glacier slope at a constant or equilibrium angle. It is the gradient therefore between accumulation and ablation across a glacier that causes it to flow. The larger this gradient the greater the glacier flow required to maintain an equilibrium slope. This gradient is known as the net balance gradient and is defined as the increase in net balance (accumulation minus ablation) with altitude; that is the sum of the rate of increase in accumulation and the rate of decrease in ablation with altitude. The higher the net balance gradient, the thicker the wedges of accumulation and ablation in Figure 3.4 will be, and as a result the more rapid the glacier flow must be to maintain a constant or equilibrium slope. The net balance gradient will be high on glaciers that have high rates of accumulation and large amounts of ablation. It will therefore be steepest on glaciers that experience warm damp maritime climates and lowest for those in cold dry continental areas (Figure 3.5). Consequently glaciers located in continental areas will flow more slowly than those in warm maritime areas. The rate of glacier flow varies from one glacier to the next and much of this variation is due to differences in the net balance gradient between different glaciers.

Glacial Erosion Diagram

Figure 3.5 The relationship between climate and net mass balance gradient. Two glaciers with net accumulation and ablation 'wedges' are shown: one in a continental climate (A) and one in a maritime climate (B). Although the equilibrium line altitude (ELA) is the same for both glaciers, the maritime glacier has a much steeper mass balance gradient, causing it to flower faster than the continental glacier. [Modified from: Kerr (1993) Terra Nova, 5, figure 3.2, p. 333]

Figure 3.5 The relationship between climate and net mass balance gradient. Two glaciers with net accumulation and ablation 'wedges' are shown: one in a continental climate (A) and one in a maritime climate (B). Although the equilibrium line altitude (ELA) is the same for both glaciers, the maritime glacier has a much steeper mass balance gradient, causing it to flower faster than the continental glacier. [Modified from: Kerr (1993) Terra Nova, 5, figure 3.2, p. 333]

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  • lisa
    What is mechanism of glacial erossion..?
    5 months ago

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