some remaining glacial paradox

Some of the issues relating to the question of what constitutes the greater part of glacial motion, basal slip or shear distortion of the ice body, would seem to be as alive today as they were in the mid 19th C. Of the many glaciers classified as cold based the contribution to the total motion from base slip is evidently small – as it was in the glaciers observed by Tyndall and Forbes. It remains far from clear that the forces derived from gravity alone are sufficient to induce the shear distortions in the ice body. And if they are sufficient then when supplemented by the potentially very considerable forces derived from the conversion of solar radiation to mechanical energy they would be even more so. It is even less clear how the glacial ice subject to just gravity force could inflict the massive erosion damage to the bed and walls of the glacier whilst transporting huge quantities of debris and often massive boulders along its path. This must be especially so for continental glacial sheets and those that occurred in even greater extent during the periodic ice ages. Again, an additional source of primary power seems to be required.

It is furthermore difficult to reconcile current explanations of glacial motion with the clear evidence that the forces being exerted have a strong cyclical component. There are clearly periods when the motion is being pushed by massive compressive action, and other periods when the ice is being pulled by a dominantly tensile action. This pulsating characteristic of the motion of glaciers is accompanied by failure patterns that reflect this push–pull nature of the motion. It is also the reason why at certain times and at certain locations there will be a piling-up of the ice while at other periods at possibly different locations various forms of tensile tearing to form fissures and crevices will be more prevalent. The extruding of ice from a cirque over a rock shoulder or through a constricted portion of the valley is clearly associated with compression. The opening-up of transverse fissures at these same locations at different times of the annual cycle is symptomatic of tensile action. The differences in the ice properties that give rise to the regular ogives, or “Forbes bands”, would appear to reflect this cyclic form of motion. That the fractures caused by these cyclic forms of the motion, and indeed the motion itself, are more in evidence at the surface than at the base would support the view that the additional source of motivating energy is also in greater abundance near the surface.

There appear to be grounds for doubting whether current explanations are able to adequately account for the well known seasonal variations in glacial motion. While the explanation for the greater motion of glaciers during the spring to summer period might be partially explained by the greater abundance of surface melt water making its way to the base of the glacier, this cannot adequately account for the greater levels of shear deformation within the body of the ice that also occur during this period. This is especially true for cold based glaciers. Again, the seasonal variations in motion are much more in evidence over the top 10-20 m of the glacier. This is once more strongly suggestive of an additional energy source being in greater abundance near the surface of glaciers.

If a glacier is more appropriately thought of as a series of “more or less imperfectly welded separate portions, traversed by fissures” (15) it is far from clear how a model treating the ice as a continuous flowing, visco-plastic, fluid can fully capture the response. A similar argument was used to undermine Moseley’s implicit suggestion that the crawling lead sheet could help to explain glacial motion. In the same way that if some visco-plastic flow could occur within these more or less continuous blocks of ice so too could the thermal ratchet described by Moseley operate on these largely discrete blocks. As is shown in reference (17) the stresses derived from gravity on these lengths of more or less continuous ice would be relatively minor compared with those that might arise from what appear to be practical fluctuations in ice temperature.

References used here

(15) Ball, J. “On the Cause of the Descent of Glaciers”, Phil. Mag., July, 1870, 1-10.
(17) Croll, James G A. “Comparisons of the Stresses in Glaciers when subject to Gravity and Thermal Loading”, to be submitted to J of Glaciology, 2005.