glaciers and glaciology

glaciers and glaciology A glacier will form whenever a body of snow accumulates, compacts, and turns to ice. If large enough, this body of ice will flow, thus forming a glacier. Glaciology is the study of glaciers. Glaciers range in size from small valley glaciers, such as those in the European Alps, to large ice sheets, such as that in Antarctica which today contains 91 per cent of all the glacier ice on Earth. In the past, during the Quaternary Ice Age, large ice sheets covered much of northern Europe and North America and the landscape of these regions is dominated by glacial landforms and glacial sediments.

Glaciers can be classified in terms of a combination of topography and size. There are two broad categories: those glaciers that are constrained by topography, and those that are not. Ice sheets and ice caps are unconstrained by topography and are sufficiently large to drown much, if not all, of the landscape over which they flow. An ice sheet is simply a large ice cap (>50 000 km²). Ice sheets are composed of several smaller elements. In the centre there may be one or more domes forming the central topography of the ice sheet. Ice flows from these domes (ice divides) towards the ice-sheet margin, either as a broad sheet or in well-defined outlet glaciers or ice streams. Where ice sheets terminate in water they may form ice shelves, which are floating slabs of glacier ice. In Antarctica 7 per cent of the ice volume is due to ice shelves, the largest of which is the Ross Ice Shelf over which polar explorers such as Shackleton, Scott, and Amundsen travelled to the South Pole.

Three main types of glacier are constrained by topography: the ice fields, valley glaciers, and cirque glaciers. An ice field is an interconnected network of valley glaciers and snow fields, while a cirque glacier is a small, discrete body of ice within an armchair-shaped depression.

Glacier mass balance and flow

Glacier formation and size is controlled by the balance between the accumulation of snow or ice and its loss through ablation. Ablation includes losses due to evaporation (sublimation), melting, and the breaking off (calving) of icebergs where glaciers terminate in water. A glacier forms whenever the accumulation of snow/ice exceeds ablation over a sustained period of time. Once a glacier is established, this balance will also determine glacier size and is known as a glacier's mass balance. If glacier mass balance is positive, that is, if there is more accumulation than ablation, then the glacier will grow in size to cover a greater area. If, however, the mass balance is negative (that is, if there is more ablation than accumulation), then the glacier will shrink and ultimately disappear. Changes in mass balance need to be sustained over a number of years to have an impact on the size of a glacier.

Accumulation and ablation do not occur uniformly over the surface of the glacier, and it is this that causes glaciers to flow. Snowfall, and therefore accumulation, increases with altitude and is greatest high up on the glacier. In contrast, ablation decreases with altitude, as it gets colder and less snow/ice melts. A glacier can be divided, therefore, into an upper accumulation zone, where accumulation exceeds ablation, and a lower ablation zone, where ablation is greater than accumulation (Fig. 1a). Where these two zones meet there is a balance between ablation and accumulation, a point on the glacier surface known as the equilibrium or firn line. This imbalance in accumulation and ablation over the surface of a glacier drives glacier flow. The addition of snow and ice in the accumulation zone and its loss in the ablation zone cause the glacier's downslope or long profile to steepen, increasing the stress on the ice crystals within it (Fig. 1b). Above a critical level of stress, ice crystals will deform or creep past one another. In this way, ice is transferred from the accumulation zone to the ablation zone to counteract the increasing glacier slope and associated build-up of shear stress.

The degree of imbalance between accumulation and ablation across the glacier determines the velocity of ice flow. The greater the difference between accumulation and ablation across the glacier, the greater the amount of ice that must be transferred to maintain an equilibrium glacier slope, and consequently the faster the glacier will flow. The difference in accumulation and ablation across a glacier is known as the mass balance gradient. A glacier with a high mass balance gradient (that is, a large amount of both accumulation and ablation) will flow faster than a glacier with a low mass balance gradient. Warm, wet, maritime climates give high mass balance gradients and have fast-flowing glaciers. The Franz Josef glacier in New Zealand is a good example, with a velocity of 300 m year⁻¹. Dry, continental climates, in contrast, are associated with slower-moving glaciers; for example the Meserve Glacier in Antarctica flows at only 3 m year⁻¹.

Glacier flow is not restricted to ice deformation or creep, but may also occur through internal deformation, basal sliding, and subglacial deformation (Fig. 2). Internal deformation is the folding, faulting, and thrusting of ice which causes displacement and flow. Basal sliding may occur by one of two processes: enhanced basal creep and regelation slip. At a glacier bed the pressure of ice pushing against basal obstacles cause the ice to deform or creep at a faster rate (enhanced basal creep). Similarly, this pressure against obstacles may cause ice close to its melting point to melt. The resulting melt water flows round the obstacle to freeze again as regelation ice downstream of the obstacle where the pressure is lower (regelation slip). As a result of both these processes, ice may slide or move rapidly over its bed. Glaciers can also flow by subglacial deformation. When glaciers flow over unfrozen sediment they may cause this sediment to deform beneath the weight of the ice. The deformation occurs when the water pressure in the pores or spaces between the sediment grains increases sufficiently to reduce the internal friction of the sediment. This allows the grains to move or flow relative to one another as a slurry-like mass. In response to the shearing force imposed by the overriding glacier, this slurry forms a continuously deforming layer on which the glacier moves. This process may be dramatic. In the case of the Icelandic glacier BreiDhamerkurjökull, 90 per cent of the glacier flow is due to subglacial deformation. The principal factor controlling which combination of flow processes—creep, basal sliding, or subglacial deformation—operates beneath a given glacier is the temperature of the basal ice, its basal thermal regime.

Basal thermal regime

The basal thermal regime is the temperature of the ice at the glacier bed. Some glaciers are frozen to their beds. No melt water is present at the ice–bed interface and such processes as basal sliding and subglacial deformation cannot therefore operate. Such glaciers are known as cold glaciers. In contrast, other glaciers are composed of warm ice, where basal ice is constantly melting and the ice–bed interface is therefore lubricated with melt water. Basal sliding and subglacial deformation may therefore both operate. A glacier composed of warm ice has a much greater potential for fast flow and therefore greater potential to modify its bed by erosion, producing glacial landforms, than one which is frozen to it. Basal thermal regime is therefore one of the most important controls on the geomorphological impact of a glacier, since it controls the processes of erosion, transport, and deposition.

Not only does basal thermal regime vary between glaciers, but it may also vary within a particular ice body. In some cases the glacier margin may be cold and frozen to the bed while the middle of the glacier is warm and melting, such glaciers are referred to as polythermal.

The temperature at the base of a glacier is determined by the balance between (1) the heat generated at the base of the glacier by friction and from geothermal sources and (2) the temperature gradient within the overlying the ice. The temperature gradient within the overlying ice is critical; if it is positive, heat is conducted away from the glacier bed and it may be frozen. If the gradient is negative, heat is not conducted away and may build up to cause basal melting. This temperature gradient is determined by ice thickness, climate, mass balance, and the flux of cold and warm ice through the glacier.

In warm temperature regions most glaciers are warm-based, while in cold, continental regions, such as Antarctica, they may be completely cold-based. In practice, the pattern of basal thermal regime within a glacier is complex and varies through time with changes in glacier geometry. The pattern of basal thermal regime will control the patterns of erosion and deposition within a glacier and therefore the regional character of the glacial landscape produced.

Matthew R. Bennett

Bibliography

Bennett, M. R. and and Glasser, N. F. (1996). Glacial geology: ice sheets and landforms. John Wiley and Sons, Chichester.