Thermal Gradients: Solar energy reaching the surface of the glacial ice will not result in uniform changes in temperature through the thickness. Ice is a poor conductor of heat and the snow covering often cuts-down the radiant energy reaching the lower sections of ice. However, it is well known that ice sheets display significant seasonal temperature variations (see for example Burgess et al, 2001) that can penetrate well into the body of the glacier. Inevitably the temperature fluctuations will show considerable attenuation with depth, giving rise to thermal variations like those shown in Figure 1(a). For this reason there is likely to be considerably greater thermally driven ratchet actions occurring over the upper layers of the glacial ice as compared with the ice at lower levels. This will be manifest by the increased fracturing, in the form of cracking and crevasses, experienced in the upper layers and of course greater than average annual forward motion.
Surface melt water is known to play an important part in the motion of glaciers. This is sometimes explained in terms of the water reaching the lower levels and acting as a form of lubricant to the frictional resistance at the rock interface. Why this explanation is less than convincing for all glacial motions is that if a lowering of the boundary shear failure stress were to always occur it would be extremely difficult to reconcile this with the extremely high shear forces that are clearly being generated at these boundaries That high shear forces are present is evidenced by the huge forces that are obviously needed to grind-away at the rock valleys and gouge out massive chunks of rock to form the glacial base and walls. Melt water could however, be having another very important action. The high volumes generated are probably an important component in the closing-up of the tension cracks and fissures. The latent heat stored in this runoff water will be considerable. When it is turned back into ice within the body of the fissured glacial ice it will release this latent heat energy into the lower regions of the underlying glacier. This form of latent heat flow from the surface into the body of the glacier could help to explain how the thermal expansion effects are more uniformly distributed through the thickness of the glacier. Increased forward motion experienced during periods of high rates of surface water melt might be expected to result from this most effective form of heat transfer. Such an alternative explanation would overcome one of the serious problems associated with the current models that ascribe this increased summer flow rate to just the effects of a form of bed fluidisation process.
Elastic-visco-plastic Distortions: The greater thermal activity in the upper layers of the glacier, together with the nature of the stress distributions through its thickness, as suggested in Figure 1(c), will increase the levels of shear with depth. Both elastic and visco-plastic creep shear straining will also increase with depth explaining why the rates of “flow” display typical vertical profiles like that shown in Figure 1(b). Relatively constant flow might be anticipated over the upper layers where the shear stresses will be low. Flow gradients will increase with depth. These gradients will be associated with the increasing shear strains occurring at depths approaching the lower layers. The high shear strains will accompany the build-up of shear stresses to levels required to fracture the rock and cause frictional failure at the ice-rock boundaries.
High edge shear stresses required to induce slip failures against the glacial valley walls will likewise result in shear stress gradients across the width of the glacier as suggested in Figure 2(b). Just as for the vertical variations in ice motion a shear strain, flow related, profile like that shown in Figure 2(a) would be expected.
Discontinuous Boundary Shear: While the basic ratchet motions were established in relation to idealised, uniform, kinematic bed friction failure, the same model would apply for ice-rock interfaces that display discontinuous failure properties. An extreme case could be that shown in Figure 3(a). Imagine that the section B is firmly bonded to a massive protruding rock, either on the glacial wall or bed. As the temperature is increased the compression force developed by the now restrained expansion relative to the fixed section A, will also build-up. Eventually a temperature might reach a level when the rock is fractured and gouged out from its surrounding rock face. This fracture will be associated with the sudden release of a huge amount of thermally induced strain energy, with a rapid (surge) motion of the glacial ice likely to follow. Similar discontinuous forms of forward behaviour could be experienced over the entire length of the glacier. The effects will be essentially similar to that described above for the smooth frictional behaviour. During the subsequent cooling period the tensions resulting from the restrained contraction will be most unlikely to cause a reversal of the bed friction motion for the simple reason that the ice is likely to fracture in tension well before it reaches the levels required to induce bed shear failure.
Burgess, M and Smith, S. (2001) Climate change indicators – permafrost. WG1, IPCC Third Assessment Report, Climate Change 2001: The Scientific Basis, Chapter 2.2.5.3 Permafrost. See also: http://sts.gsc.nrcan.gc.ca/permafrost.
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