The Pressure Melting Point of Ice and the Excavation of Cirques and Valley Steps by Glaciers
Joel E. Fisher
Recent studies1 have developed a now well-established principle that outside the arctic the temperature of all glaciers 10 or 20 meters below the surface (and deeper) tends to be at all times at the local pressure melting point; that is, the temperature is continually very slightly below 0° Centigrade2. The upper 10 or 20 meters of ice, affected by colder winter temperatures, may cool down somewhat lower, seasonally, each winter.
This principle requires that the temperature at the very bed of any glacier, as one follows that bed up-glacier, shall gradually drop from exactly 0° Centigrade (at the snout) to temperatures fractionally lower, up-glacier, as the pressure of overlying ice and névé irregularly increases. This “pressure” is primarily that of the static load of vertically overlying ice and névé; but it may include some measure of the hydraulic head of the glacier upstream, because, under the extrusion flow processes of glaciers, the brittle surface ice, joining in with the rock walls and bed, provides a sort of pipe, within which the flowing ice behaves to a degree like a liquid, and therefore exerts a pressure (measured by its up-glacier head) in all directions, including vertically downwards.
The temperature of the bed ice, under 500 meters of overlying névé, alone with a static pressure of some 700 pounds per square inch, must evidently be lower than the temperature of any ice near the snout — else, if the temperature of the ice everywhere on its bed were exactly 0° Centigrade, melting would be greater, under the heavier pressure of thicker beds of ice, up-glacier than at the snout. Actually, the melting point of ice is lowered .0075° Centigrade per additional atmosphere of pressure.
Successively colder temperatures below the surface up-glacier are altogether logical, as (except for localized precipitation) all glacier ice and névé derive originally from the snows falling above the bergschrunds, and the upper snows and ice originally possessed a temperature well below 0°, the year round.3 Witness the aprons of ice, frozen solid to the surrounding walls of rock, above the bergschrunds on any hardy Alpine peak. Noontime melt water observed on such upper snowfields above bergschrunds is only a superficial surface feature and should not be viewed as indicative of temperatures, below the immediate surface, warm enough to permit melting. If this were more than superficial, the bergschrunds would close up! In fact, it is possible to say that bergschrunds mark the boundary between snow or ice whose temperature is below the pressure melting point (above the bergschrunds) and the névé or ice whose temperature4 is at the local pressure melting point (below the bergschrunds). This tie-in of any bergschrunds with climatic altitudes explains the much discussed “Cirque-Niveau” in any given district.
From these propositions, it is reasonable that the temperature of the ice (névé) on the bed of any glacier, up close to the berg- schrund, must be just at the local pressure melting point5; but, because that very same ice (névé) was only very recently part of the upper regions of snows (above the bergschrund) which are definitely below their local pressure melting point, such névé has no past history of existence at the pressure melting point. It can not yet have accumulated, within its capillary passages, any substantial amount of melt water — no more than just enough to assure that its contact with the bed rock will be a moderately wet, lubricated contact as opposed to any frozen “adherence” to the rock, as is found above bergschrunds. Thus, any rocks held within that recently born névé ice will be held in a matrix which carries so little water within its voids as to minimize any retreat under pressure, within its own icy medium, of those tools which it carries, even though they may be held against the bed rock under the heavy pressure of the weight of thick overlying névé. (Attention is invited to the particularly greater thickness of overlying snow at the uppermost limits of any névé field, due to aprons of snow reaching steeply up the cirque slopes to the bergschrunds.)
Thus the temperature of the basal layer of névé, up close to the bergschrund, however heavy a static load it may carry, is just edging above the local pressure melting point; just able to provide a wet, lubricated bed; just able to cause the névé to yield, minutely, to pressure—not, however, sufficiently over the line of the pressure melting point to permit any rocks held in that névé to be pushed back readily into its icy medium. It will be recognized that the latent heat required for any minute local “melting” will always delay any quick transition from “hard” to “soft” ice, as a result of quickly increased pressure, so that for some time any ice, although under increasing pressure, it is still very hard rigid ice. There is thus a triple combination which exists only immediately below the bergschrund: (a) presence of vigorous cutting tools freshly fallen from the heights through the open bergschrund to the bed of the glaciers; (b) extreme static pressure thereon, which generates, right up at the foot of the headwall of a cirque, a particularly vigorous cutting action; (c) ice temperature barely reaching the local pressure melting point and therefore holding such tools in an extremely firm grip against the bed. It is this triple combination which is responsible for the excavation of cirques; and of these three factors it is the third, the relationship of ice temperature to local pressure melting point, which is a feature unique to the physics of glaciers. Against these three factors, velocity of motion is insignificant, provided only that there is some motion. We are too prone to impute to faster motion of ice a correspondingly faster supply of cutting tools. That is not so. The number of cutting edges carried across the bed by a glacier depends altogether on the rate of supply of cutting tools, not on the velocity of the glacier itself.
Consider next what changes occur in the characteristics of the ice as it ages and moves downstream from the cirque wall. Except for a thin veneer of surface ice, the glacier can never become any colder, for winter temperatures do not penetrate below 20 meters and mean summer temperatures are well above freezing. On the other hand, the ice can pick up some heat by conduction from melt water seeping down and (very slowly) from bed rocks which conduct heat from within the earth, and also by friction. If heat from these sources is insufficient to warm the ice enough to keep pace with a rising pressure melting point (rising because of the gradual reduction in pressures as the ice thins out down-valley), then flowage slows down, erosion of the bed increases, and the névé reaccumulates deeper and deeper until its thickness is again sufficient to build up a static pressure to place it once again on the warm side of the curve of pressure melting points. Once this equilibrium is regained, the ice in the glacier’s bed will begin to accumulate ever so little more water in its capillaries and to accumulate heat sufficient to place it a little further over the equilibrium of local pressure melting points. When this begins to happen, while the ice is still far from any spongy, cheesy condition, it no longer has quite the vise-like characteristics that it formerly had; the cutting tools in its bed no longer are quite so insistent on gouging out the bed rock; and, also, those glacier-worn cutting tools are no longer so sharp. Here, then, in this slight relaxation of pristine erosional vigor, we can see just how the downstream lips of alpine tarns are first induced into existence.6
This very same process may repeat itself any place farther down the glacier’s course, wherever initiated by a steep drop in the grade of the bed (due, perhaps, originally to structure). Against the back wall of such an incipient step, the same three factors thus exist which accounted for cirque excavation — or for tarn excavation: unworn cutting tools (derived now from the back wall of the step itself), locally increased static pressure of ice on its bed, and (to a lesser degree, but still there) temperatures of ice colder locally than the local pressure melting point.
It will be noted that a rise in the pressure melting point occurs first at the top of the step. This ice, now hardened, as it goes over the step, is soon subjected to greater static pressure; but this greater static pressure can not instantly resoften the ice because the latent heat absorbed in minute local melting operations creates a lag. Pressure can thus rejuvenate cutting power — appearing as excavation at the foot of the step, just as at the headwall of a cirque. Eventually, this greater pressure overcomes this lag in softening the physical state of the ice: the ice softens again, and the extreme vigor of its erosion ends once more.
In short, while temperature of the ice near the bed of a glacier follows a nearly straight line graph, rising slowly as one goes down-glacier, pressure at selected points may vary greatly, depending on special local conditions. The temperature of any particular portion of the ice will therefore vary, often quite quickly, from one side of the curve of equilibrium of pressure melting points to the other, with resulting changes in the “state” of the ice and in the cutting power of tools embedded therein, as the glacier moves through regions of varying pressures.
The processes of cirque excavation, of tarn excavation and of step cutting by glaciers, with their apparent anomalies of reverse slopes persisting in profiles, are thus seen to stem from the irregular curve of the local pressure melting point of the ice, locally superimposed on a more nearly straight line gradual rise in the temperature of the bed ice, the latter starting from a temperature definitely below any possible pressure melting point (above the bergschrund) and rising all the way to a point (at the snout) which is exactly the pressure melting point of ice at atmospheric pressure.
1 T. P. Hughes and G. Seligman, “The temperature, melt water movement, and density increase in the névé of an Alpine glacier,” Monthly Notices of the Royal Astronomical Society, Geophysical Supplement, IV (1939), 615-47 [Publication No. 2 of the Jungfraujoch Research Party, 1938].
2 W. V. Lewis, “Formation of Roches Moutonnées: Comments on Dr. Carol’s Article,” Journal of Glaciology, I (1947), 60-63. See also G. Seligman, “The Structure of a Temperate Glacier,” Geographical Journal, XCVII
(1941), 295-317 [Publication No. 4 of the Jungfraujoch Research Party, 1938].
3 Hughes and Seligman, op. cit., p. 631.
4 This statement refers, of course, to the main body of névé, not to the superficial upper crust where temperatures during winter may fall below the pressure melting point.
5 In the oft-reported classic exploration of a bergschrund on Mt. Lyell by the late W. D. Johnson, in which he told of water permeating its up-hill face, that water is to be thought of as merely surface melt water flooding down into the bergschrund from above, and not for a moment as water exuded from capillary passages within the bed of the supra-bergschrund ice slope, from regions 20 meters or more below its surface—regions therefore unaffected by surface air temperatures.
6 The reader at this point is referred to the interesting studies of Hans Carol. See “The Formation of Roches Moutonnées,” Journal of Glaciology, I (1947), 57-59.