Thermal Ratchetting in Other Permafrost Features:

Descriptions of periglacial geomorphic features suggest the possible role of thermal ratchet processes like those described in the most recent posts are ubiquitous. Stone pavements and string bogs appear to have many features in common with the process described above for the formation ice polygons. As do of course the features referred to as stripes and steps. High centred polygons seem to share a close resemblance to and possible similarity in origin to the pingo. As observed at p135 (Washburn 1979) “some ice wedge polygons enclose small pingo-like ice cored mounds”. One also wonders whether for some classes of high centred ice-wedge polygon it is necessarily gravity that is responsible for moving the stones down into the peripheral troughs. Also, for anyone familiar with the ways in which timber, and certain laminated composite materials, suffer a form of delamination when buckling under compression, cannot but help wonder to what extent similar thermally induced buckling might not be at work in the development of earth hummocks, as discussed above. 

There is a further form of thermal ratchet that might be of significance greater than that for which it is currently credited. In reopening consideration of Moseley’s gravity ratchet as a possible contributory cause of some glacial ice motions (Croll. 2005d) it has been suggested that a thermal ratchet like that described above for pingos and ice polygons, etc, based upon the differential properties of ice failures in tension and compression, may also be a significant factor in the motion of certain forms of glacier (Croll 2004c). In being able to account for motions of glacial ice on essentially flat surfaces, and even those with negative gradient, this material ratchet would seem worthy of consideration. The forces developed are consistent with both the observed motions and the immense erosive power of glacial ice in motion. It is possible that a related mechanism could be powering the movements of certain rock glaciers and contributing to such features as sheets, benches, lobes and streams.

Mention should also be made of frost-creep in the context of ratchetting processes. In this case the ratchet mechanism within currently accepted models is somewhat different, and thought to involve a combination of frost-heave occurring normal to the slope, followed by a thaw that results a stone or other object settling vertically under the action of gravity. The result is a gradual downward motion accompanying each freeze-thaw cycle. It is possible that the thermal ratchet similar to that possibly occurring in the movement of glacial ice could also be contributing to the phenomenon of frost-creep. For the temperature fluctuations taking place above freezing it is again possible that some of the down-slope motion classified as frost-creep could also be the result of the Moseley-Davison insolation-creep process. Space does not permit consideration of the extent to which those phenomena explained in terms of solifluction and gelifluction might also have contributions from the thermo-mechanical ratchet processes outlined above. But in all of these surface motions it is possible to construct models that could be complementing the processes currently considered to provide the driving force.

Some Closing Remarks

One of the purposes of the past few posts has been to suggest that thermo-mechanical ratchet processes, derived from fluctuations in the levels of solar radiation reaching the Earth’s surface, may be contributing to rather more periglacial processes than is currently accepted. These ratchets could be classified in terms of the thermo-mechanical process responsible, so that for example in:

·         up-freezing it is thought to be the combination of frost heave followed by soil failure in the form of slumping, together with the differential bond in frozen and unfrozen ground;

·         the development of ice-wedges and ice-wedge polygons, side-shift to form stone circles, polygons, etc, possibly the outward movements of ice-sheets, and even rock glaciers, it would appear to be the differences in tensile and compressive material failures of the frost layer or the permafrost that give rise to the one-way ratchet actions. This process could also be a factor in the movement of glacial ice;

·         the growth of pingos, palsas, frost-mounds and possibly hummocks, it could be the differences in tension material failures and the compressive geometric, buckling, structural failures that provide the driving force;

·         the down-slope motion referred to as insolation creep, and possible also as a contribution to what is referred to as frost creep and gelifluction, it might also be the effects of gravity acting differently during the warming, expansion, phase than during cooling, contraction, phase that provides the basis for the ratchet action. This process might also contribute to the movement of glacial ice.

In each of these processes it is fluctuations in temperature caused by changes in levels of solar radiation reaching the ground surface that provides the energy source. A further sub-classification might be possible in terms of the time period over which these temperature fluctuations occur. Some will be driven by circadian or in the case of Moseley’s crawling theory by even shorter time-scales. Others may be influenced by weather changes occurring over days or weeks, and others by the annual seasonal cycle. Yet others may depend upon changes that occur over many years while some may have their dynamic processes taking place over hundreds, thousands, inter-glacial or even longer time periodicity. At each increase or decrease in order of magnitude of the periodicity of the thermal cycle it is likely that the spatial scale of the processes will also be changed by similar orders of magnitude. The spatial scales will be related to the lengths over which thermal waves will be conducted into the ice or the Earth’s crust. As is becoming clear from evidence from recent planetary probes, many of the surface features being observed on the other planets and their satellites (see for example Kuzmin 2002; Yoshikawa 2002) would appear to have close relationships with similar features on Earth. Improved understanding of the mechanics responsible for developing these features will be essential if these observations are to allow better understanding the origins of the solar system, including a more complete appreciation of the nature and influences of present and past climatic conditions.

It is also becoming clear that closely related morphological features exist at very small scale in the behaviour of asphalt (Croll 2005a,c, 2006a,b). In these analogous cases similar forms of thermo-mechanical ratchet processes would appear to be responsible. Again, better understanding of the mechanics will be essential if future design and maintenance strategies for asphalt pavements are to allow reductions from their currently very high costs. To capture all these effects it is clear that account must be taken of the relationships linking the thermal periodicities and the spatial scales. With a key factor in this inter-linkage being related to the depth of propagation of the thermal wave, it seems clear that one-dimensional, surface only, models like those employed (Plug and Werner 2001, 2002) will be unlikely to capture this crucial spatial-temporal link. It also seems clear that fluctuations in levels of solar energy and the operation of various forms of thermo-mechanical ratchet process are rather more common in the shaping of periglacial environments than is currently recognised. This paper represents a small start towards suggesting a new classification of many periglacial features based upon the form of the thermo-mechanical ratchet processes that might be at work in their formation.