Variable glacier flow
Although all glaciers flow, the way in which they flow is highly variable. We already know that the thermal regime can impact on glacier flow, with cold-based glaciers being frozen to their bed and flowing very slowly, and with warm-based glaciers having lots of water at their bed, which can reduce friction, enhance ice deformation and result in faster flowing glaciers. We also know that glaciers range from temperate valley glaciers to ice streams.
However, some glaciers do not flow at a constant speed; instead, they are subjected to cyclical flow instabilities. These glaciers have been called ‘surging glaciers’ or ‘surge-type’ glaciers, and they typically have long periods of quiescence, with thinning and downwasting and little forward movement in the ablation zone and thickening in the accumulation zone, and short semi-periodical periods of rapid velocity, where they can advance dramatically. On Svalbard, in the high Arctic, the quiescent periods can last ~100 years and the fast-flow phase 1-5 years, although it varies regionally and by glacier[2-4].
Downwasting glaciers between surges
The lower reaches of stagnant quiescent-phase glaciers often downwaste and thin in situ, which can result in thick accumulations of debris on their snouts. During this time, surge-type glaciers thicken and build up a reservoir of ice in their upper reaches during their quiescent phases. This reservoir is then depleted during the surge phase.
Surges apparently arise from a combination of the deformable properties of the subglacial sediment, combined with changes in glacier thermal regime and feedbacks that disturb subglacial hydrology[5-7]. The surge starts when a critical threshold is passed, resulting in a critical basal shear stress (see Glacier Flow). This threshold is determined by a number of external factors, including the accumulation rate and glacier thickness and steepness. When the gravitational driving force of the glacier exceeds the friction at the base, changes occur in the subglacial system. Friction is decreased, subglacial deformation is enabled, and a surge occurs. The surge ends once either the reservoir is depleted, or a change occurs in the hydrological system.
Surges may also occur when meltwater suddenly becomes avaible at the base of the glacier. The meltwater lubricates the bed and helps the glacier flow faster.
The occurrence of surging is relatively unpredictable, and our understanding of surging glaciers is limited. This is a significant impediment to our understanding of melting of high Arctic glaciers, and makes it difficult to predict future sea level rise.
Surging glacier features
Surging glaciers typically produce a characteristic range of landforms[7-14]. Landforms characteristic of surges include:
- Concertina eskers or zig-zag eskers
- Thrust moraine complexes
- Crevasse-squeeze ridges
- Multiple stacked diamictons
- Tectonised sediments
- Hummocky moraine
Actively surging glaciers often have a shear line between fast- and slow-moving ice, and there may be ‘strandlines’ on the valley sides, where the glacier has rapidly thinned. There may be numerous crevasses, folding[16, 17], deformed longitudinal flow stripes, looped medial moraines[15, 18] and fragmented, digitate tidewater glacier termini.
Quiescent-phase glaciers, on the otherhand, are characterised by a pitted surface with lots of lakes, a dark surface with abundant debris, relict looped moraines and a slow velocity.
Surging glaciers in the Karakorum
Glaciers surging in the Karakorum have been visualised from Landsat imagery by Paul, 2015. Click on the image below to see the glaciers surging.
Other kinds of flow instability
In general, surge-type glaciers have not been recognised in Antarctica. However, there are many reasons why a glacier may experience flow instabilities. For example, around the Antarctic Peninsula, glaciers were observed to accelerate following the abrupt removal of an ice shelf.
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