background to analysis of thermal ratchetting in periglacial morphologies

Previous posts have hinted that thermal ratchet processes may be much more prevalent than previously thought in the formation of some of the curious patterns observed in glacial and periglacial environments. Before I move on to more recent musings it might be helpful to consider how fluctuations in heating and cooling, and its accompanying expansion and contraction, could be playing a role in many more fascinating morphologic forms observed in these regions. The following few posts have been extracted from  a paper entitled "Thermal Ratchetting and Periglacial Morphologies" presented at a conference in Lanzhou, China, in 200? to coincide with the opening of the new railway linking Golmud in Western China with Lhasa in Tibet. [The permafrost institute in Lanzhou was heavily involved with research activity required to construct a railway over what is extremely difficult permafrost terrain on the Qinghai Plateau. Following the conference a number of the participants were able to visit the research facilities on the Qinghai Plateau and subsequently take one of the first trips on this new train to Lhasa - a fascinating experience which perhaps I will expand upon in a future post]. Some of what follows will inevitably repeat discussions from earlier posts. But in the interests of continuity these repetitions may be helpful to place the extensions of the ideas in their mechanical and historical context.

While fluctuations in solar energy are considered to provide the driving force for these ratchet processes, the initiation and growth of the various periglacial features are suggested to be made possible by some form of differential mechanical property exhibited by the systems during the cooling and warming cycles. Restrained contraction during cooling can produce tension cracking that will not be recovered during the restrained expansion accompanying warming. The result can be a number of non-recoverable cumulative forms of compression failures. The following posts explore whether these processes may be at work in a representative range of periglacial features. They will also attempt to provide an alternative classification framework for these and other periglacial features in accordance with the nature of the thermo-mechanical process that might be involved in their formation. Furthermore, it will be suggested that various forms of thermo-mechanical ratchet may be of considerably greater significance in shaping periglacial environments than has hitherto been acknowledged.     

Many curious geomorphic features of periglacial environments have been the subject of long standing fascination. From the formation of geometrically patterned ground, to the sometimes distinctive mounds developing out of otherwise flat areas of permafrost, to the humps and other regular geometric patterns observed over ground subject to seasonal freeze-thaw cycles, to the relic geomorphic features remaining long after previous permafrost has receded, the processes that might be at work have occupied considerable research attention. In many cases there is widespread agreement on the processes responsible for these formations while in others there remains considerable uncertainty. The following posts approach the subject of the mechanical causes of these various features from a somewhat different perspective. They will postulate that the dynamic mechanisms responsible for their formation are all in some form the result of alternations of solar energy reaching the ground surface. They will suggest that these alternations of solar energy produce warming periods when the frost layer, or permafrost where it is present, is subject to compressive forces arising from the restrained expansion. During subsequent cooling tensions will accompany the restrained contraction. Because of differential mechanical properties exhibited at high and low temperatures, or under compression and tension, certain forms of deformation will accumulate to form what might be referred to as thermal ratchets.

Some years ago I became involved with the modelling and understanding of the behaviour of pipelines when subject to elevated temperatures associated with the transport of hot oil and gas (Croll 1997, 1998). For both sub-sea and land based pipelines it is possible that unwanted buckles can develop (Palmer et al 1974; Hobbs 1984; Taylor et al 1986, 1996; Maltby et al 1995). Longitudinal restraint of the expansion otherwise occurring as temperatures are elevated can result in the sudden and at times violent formation of buckles. Similar forms of thermal buckling are also an unwelcome feature in railtracks (Martinet 1936; Kerr 1974; Croll 2005b), and rigid road pavements (Kerr et al 1984; Croll 2005a). When a few years ago some equally unwanted upward blisters, appeared within the thin asphalt covering that had recently been laid on the footpaths outside UCL, in London, it seemed clear that they too were the result of the unusually hot weather experienced that summer (Figure 1). Although it was some years before the local authority responsible for these pavements accepted this explanation, it is now fairly widely acknowledged that a form of thermal uplift buckling is the cause of what are for pedestrians hazardous bulges (Croll 2005a, 2005c). Earlier analysis of the behaviour of heavy sheets subject to in-plane compressive stress (Hobbs 1989, 1990, 2004) had been partly inspired by the observation that parquet flooring can, when subject to compression resulting from constraints to either thermal or moisture induced expansions, exhibit a similar form of upheaval buckling. Indeed, it was a recent conversation with Professor Roger Hobbs that first alerted me to the possibility that “pingos” forming in permafrost could also be the result of a similar thermally induced upheaval buckling process.

Over the academic year 2003-2004 a pair of UCL undergraduate students were asked to investigate as part of their final year research thesis what exactly are pingos, since I had not previously come across them, and what are the current explanations for their growth? Their investigation was presented as part of their final assessment and subsequently written-up as a still unpublished report (Croll et alia 2004), which in slightly modified form (Croll 2004a) was presented at the IUTAM (2004) Congress in Warsaw (Croll 2004b). This thermal ratchet process will be briefly described below. It was while preparing material putting forward this alternative explanation for the initiation and growth of pingos that it became evident that many other periglacial features might also involve the working of related forms of thermal ratchet processes. At first it was the possibility that a very similar thermal ratchet mechanism could be contributing to the motions of glacial ice (Croll 2004c). In this work attention was particularly focused on glacial ice for which the gravity forces would be very low. This might for example, be on account of the lower interface with the bedrock having very low gradients, such as in the upper reaches of some alpine glaciers, cirques, contemporary continental ice sheets or those that existed at even larger scales during the ice ages. In these situations simple calculations suggested that the stress levels being induced in the body of the ice by gravity alone seemed too low to account for the motions being observed. Or, even if they were not too low, the considerably higher stresses that would be induced by relatively moderate fluctuations in ice temperature could, it was suggested, be even more capable of accounting for observed motions of the glacial ice. Such an explanation based upon short and long term fluctuations in temperature regime within the glacial ice would help to explain inter alia why movement of glacial ice is so much greater during the warm than the cold season. Current explanations for these seasonal variations based upon loss of frictional resistance at the glacier’s base, due to surface melt waters reaching the glacial base, seemed for at least some glaciers somewhat less than convincing. If glacial flow required such low bed friction what could possibly account for the massive frictional forces clearly being mobilised to inflict the immense erosion damage to the rock surfaces over which the glacier slides? It was while looking to see if anyone else shared these concerns that I stumbled upon the work of Henry Moseley (Moseley 1855, 1869). Moseley showed by observations of the motions of lead sheet and through supporting analysis, that fluctuations of temperature are capable of moving solid sheets down inclined surfaces for which gravity alone would be insufficient. With the exception of reference to Moseley in relation to “insolation creep” (Davison 1888; Strathan 1977) there appears to be very little other acknowledgement of this essentially gravity ratchet process (Croll 2004c, 2005d) which would appear to be such a major factor in the mass movements of certain surface solids. But it would seem that Moseley’s views that a similar form of gravity ratchet might be a major causal factor in the movement of glaciers has been even less widely acknowledged. Indeed, at the time of its publication (Moseley 1869) his work on glacial motion received what can only be regarded as a hostile reception. It was James Croll (Croll 1869, 1870) who is said to have “completely demolished” Moseley’s ratchet theory, at the time referred to as the “crawling theory”, for glacial motion (Benn 2002). Whatever might be the relevance of Moseley’s gravity ratchet to the motion of glacial ice it seems clear that it has considerable importance in many surface transport processes. It is mentioned only briefly in what follows because present interest centres on a family of other forms of thermal ratchet process. In my next post I will explore the nature of the present thermal ratchet processes and how they might be at work in the development of so many familiar periglacial geomorphic features?