thermal ratchetting of restrained ice-sheets

My first posting on this blog discussed some observations of curious ice-rings that had appeared on lake ice in New York's Central park reservoir. My attention had been drawn to these features as a result of my earlier attempts to explain the initiation and growth of pingos as a form of thermal uplift buckling, similar to that described in my most recent posting. Some of these ice rings seemed to have evolved from the collapse of uplift buckles caused by the restraint of the expansion that would have otherwise occurred when the ice was subjected to a diurnal increased temperature. This put me in mind of some work I had read while trying to find evidence in support of the hypothesis that pingos might also be a form of thermal upheaval of the frozen ground caused by seasonally induced temperature changes through the thickness of the pernennially frozen ground - referred to as permafrost.  

In-plane Forces Developed by Restrained Thermal Expansion of Ice-sheets: When a floating ice sheet undergoes an increase in temperature it will either expand freely if unrestrained around the shore line or it will develop massive in-plane forces when restrained. The extent of these deformations and/or the levels of force that can be developed have been described by Frellson (1963). Based upon his extensive experience as a Director of the Division of Waters at the Minnesota Department of Conservation, he describes how in one case at Cotton Lake near Detroit daily temperature fluctuations of 25 to 30oF were enough to cause shoreline expansion as much as 18 feet. Where they are restrained these expansions developed compressive forces sufficient to buckle the ice into ridges up to 3.5 feet in height. Where they were in the path of the expanding ice, lakeside cottages were severely damaged and in some instance their foundations were “so badly crushed that it was evident the entire ground on which they stood had been removed”. Other instances cited include cases where trees were overturned, massive masonry bridge piers moved out-of-line, retaining walls tipped over, and huge boulders moved considerable distances. Further evidence of the extreme levels of force generated by restrained thermal expansion of ice is that even very thick ice sheets can experience buckling and crushing at the edges. With coefficients of linear expansion some eight times higher than steel, and estimated, Frellson (1963) at 90x10-6 / oC, the behaviours of ice sheets are clearly extremely sensitive to changes in temperature.

Frellson (1963) also describes the “ratchet” or “jack” actions that can be caused by cyclic variations in temperature. Where the ice sheet has been attached around the shoreline a decrease in temperature by night can result in cracking before any appreciable tension force is generated, on account of ice being relatively weak in tension. Water penetrating these cracks soon freezes so that the next time there is an increase in temperature the now integral ice sheet will undergo a further expansion related deformation. Repetition of this cycle can result in the edges being gradually jacked out further and further. Similar behaviour occurs in the development of ice wedges and ice polygons within permafrost. Indeed, it is well recognised that decreased temperatures during the late summer to midwinter cooling period are sufficient to cause major tension cracking in permafrost. In this regard the long term field studies undertaken by Mackay and others, see for example Makay et al (2002), Burn (2002), are particularly illuminating. It is also well documented that these cracks attract water which subsequently turns to ice, see for example Washburn (1979), French (1996), Mackay (2002) and Burn (2002), forming well defined ice wedges that over time and in a cyclic annual pattern can grow up to significant widths and depths. It is also seems fairly self evident that if tension stresses are sufficient to cause cracking during the cooling, contraction, phase of thermal loading, that major levels of compression will similarly be developed during the warming, expansion phase of the thermal cycles. In the case of ice polygon formations these high compressions would appear to be the cause of the material shoving that is responsible for the gradual growth of the ramparts either side of the ice wedges around the periphery of the ice polygons, Burn (2004). There would even appear to be evidence that in high centred polygons, Washburn (1979), a form of mini-pingo is sometimes developed possibly as a result of the compression stress causing a form of general uplift, doming, rather than the local, edge rampart, form of buckling failure. There seem to be good reasons for supposing that the ratchet process responsible for the development of ice polygons, and particularly some examples of high centred ice polygons, might be a close analogy for the proposed model for the development of pingos.