Geology, Vol. 1 [of 3]

by Thomas C. Chamberlin; Rollin D. Salisbury

TEMPERATURE, WASTE, AND DRAINAGE.

The temperature of glacier ice may range downward from the freezing point of water much as other solid portions of the earth’s surface, but it has a fixed upper limit at 32° Fahr. (0° C.) because all the heat it receives tending to raise its temperature above that point, is converted into the latent form by the melting of the ice. The range of temperature is greatest at the surface, where it varies from 32° in the summer, to the coldest temperature of the region where the ice occurs. Beneath the surface the range of temperature is more restricted, and increasingly so with increasing depth.

Fig. 248.—Side view of end of glacier. Southeast side of McCormick Bay, North Greenland. Shows structure of ice as well as position of débris.

The variation of temperature at the surface is due primarily to the varying temperature of the air. During the cold season, a wave of low temperature (the winter wave), starting at the surface, penetrates the ice, and during the warm season a wave of higher temperature (the summer wave) takes the same course. The day and night waves and other minor variables are, for present purposes, negligible.

Fig. 249.—Side view of a North Greenland glacier (East glacier), showing position of débris and structure of the ice.

The winter wave.—There are but few observations on the internal temperatures of glaciers during the winter season, but it seems certain that the winter wave diminishes rapidly downward and dies out below, much as does the winter wave which affects land surfaces not covered with ice. Conduction alone considered, the temperature of the ice where the cold wave dies out, should correspond, approximately, to the mean annual temperature of the region, provided that temperature is below the melting point of ice.

Assuming that in the high altitudes and high latitudes where glaciers abound, the temperature of the surface may average about −12° Fahr. (about −25° C.) for the winter half of the year, which is about the case for north Greenland, Spitzbergen, and Franz Josef Land, and that the conductivity of the ice in the C. G. S.[130] system is .005, the temperature would be lowered appreciably only about 40 feet below the surface at the close of the winter period, conduction only being considered. How far the internal temperature may be influenced by air forced through the ice by winds and by variations of the barometer is not known and cannot well be estimated. The wave of low temperature descending from the surface in winter would probably become inappreciable before reaching a depth of 60 feet. At this depth the temperature should be about 15° Fahr.—the mean annual temperature of the region.

Fig. 250.—Contorted lamination shown at the surface. A small glacier south of Forno hut, Engadine, Switzerland. (Reid.)

The summer wave.—The warm wave follows the analogy of the summer wave of ice-free land surfaces much less closely. This is because of the low melting temperature of ice as compared with other forms of solid earth-matter. On this account the summer wave is bi-fold. The one part travels downward by conduction, the other by the descent of water; the one has to do primarily with the temperature before the melting-point of ice is reached; the other, with the temperature after that point is reached; the first conforms measurably to the warm wave affecting other solid earth-matter, while the second is governed by special laws. After the surface portion of the ice is brought to the melting temperature, the additional heat which it receives melts the ice and is transformed from sensible into potential heat. Ice charged with water is potentially, but not sensibly, warmer than ice which has just reached the melting temperature.

The warm wave of conduction dies out below like the cold wave. The warm wave descending by the flow of water stops where the freezing temperature of water is reached. In regions where the average temperature is below freezing, the water-wave does not descend so far as the wave of conduction, since the latter descends below the zone where the melting temperature is found.

The foregoing considerations warrant the generalization that glaciers normally consist of two zones (1) an outer or upper zone of fluctuating temperature, and (2) an under zone of nearly constant temperature. The under zone obviously does not exist where the thickness of the ice is less than the thickness of the zone of fluctuating temperature. This may be the case in very thin glaciers in very cold regions, and in the thin ends and edges of all glaciers.

The temperature of the bottom.—The internal heat of the earth is slowly conducted to the base of a glacier where it melts the ice at the estimated average rate of about one-fourth of an inch per year. The temperature of melting is a little below 32° Fahr. since pressure lowers the melting-point at the rate of .0133° Fahr. (.0075° C.) for one atmosphere of pressure. At the bottom of a mile of ice therefore the melting temperature is about 30.2° Fahr. (−1° C.) It is probable that in all thick glaciers the temperature of the bottom is constantly maintained at the melting-point. This may be indicated by the streams which issue from beneath glaciers during the winter, though this criterion is hardly decisive since the issuing waters may be derived partly or wholly from the rock beneath. In glaciers or in parts of glaciers so thin as to lie wholly within the zone of fluctuating temperature, the temperature of the bottom is obviously not constant.

Temperature of the interior of the ice.—The variation of temperature of the surface of a glacier has already been shown to lie between a maximum of 32° Fahr. and the minimum temperature of the region where the glacier occurs. Lower, in the zone of fluctuating temperature, the variation is less, and where the zone of fluctuating temperature passes into the zone of constant temperature, variation ceases. The thickness of the zone of fluctuating temperature varies with the temperature of the region where the glacier occurs, being greatest where the winters are coldest. In the case of all glaciers except thin ones in very cold regions, the temperatures within the zone of constant temperature range from the mean annual temperature of the region at the top of the zone (provided this is not above the melting-point of ice at this depth) to the melting temperature of the ice at the bottom. Within these limits the range may be great or slight.

If we consider only the effects of the external seasonal temperatures and the internal heat of the earth, it appears that all the ice in the zone of constant temperature in the lower end of a typical alpine glacier should have a constant melting temperature, for the average temperature of regions where the ends of such glaciers occur is usually above 32° Fahr., and this determines a temperature of 32° Fahr. (or a little less) at the top of the zone, while a melting temperature is maintained at the bottom by the earth’s interior heat. In thin glaciers of very cold regions, where the zone of constant temperature has relatively slight thickness, the low temperature descending from the surface may so far overcome the effect of internal heat as to keep the bottom of the ice at a freezing temperature. In all other cases, the ice at the bottom of the under zone has a melting temperature, while that above is probably colder.

In the higher altitudes and in the polar latitudes where glaciers are chiefly generated, the mean annual temperature of the surface is usually below the melting-point of ice. Here the temperature of the ice between the top and bottom of the zone of constant temperature must, on the average, be below the melting-point, unless heat enough is generated in the interior of the ice to offset the effect of the temperature above. For example, where the mean annual temperature is 20° Fahr. or lower, as in middle Greenland, Spitzbergen, and Franz Josef Land, and at certain high altitudes in more southerly latitudes, the mean temperature in the zone of constant temperature should range from 20° Fahr. at the top to 32° Fahr. (or a little less) below; i.e., it should average about 6° below the melting-point. Under these conditions, all the ice in the zone of constant temperature, except that at its bottom, must be permanently below the melting-point, but it is perhaps worthy of especial note that much of it is but little below. In alpine glaciers the part of the ice affected by this constant low temperature (below freezing) is presumed to be chiefly that which lies beneath the snow-fields. In polar glaciers the low temperature probably prevails beneath the surface, not only throughout the great ice-caps, but also in the marginal glaciers which descend from them.

From these theoretical considerations we may deduce the generalization that in the zone of constant temperature within the area of glacial growth, the temperature of the ice is generally below the melting-point, while within the area of wastage, the temperature of the corresponding zone is generally at the melting-point.

Compression and friction as causes of heat.—The foregoing conclusions are somewhat modified by dynamic sources of heat. The compression arising from gravity, and the friction developed where there is motion, are causes of heat. Since friction occurs only when motion takes place, the heat which it generates is secondary and may, for present purposes, be neglected. Compression not only lowers the melting-point slightly, but it produces heat at the point of compression. Where the ice is granular, the compression, due to weight, takes place at the contacts of the grains. At intermediate points the pressure tends to cause them to bulge, and this has the effect of lowering the temperature of the bulging points. If therefore the compression be considerable, the granules may be warmed to the melting-point where they press each other, while at other points their temperature may be lower. In this case melting will take place at the points of compression, and the moisture so produced will be transferred to the adjacent parts of the granule and immediately refrozen. Melting at the points of compression would result in some yielding of the mass, and in some shifting of the pressure to new points where compression and melting would again take place. Thus the melting, the refreezing, and the attendant movement might go on until the limits of the power of gravity in this direction were reached. From considerations already adduced, it appears that the temperature in some parts of every considerable body of ice must be such as to permit these changes. The heat due to depression and friction may modify the theoretical deductions drawn above from atmospheric and internal influences.

Summary.—If the foregoing generalizations be correct, (1) the surface of a glacier is likely to be melted during the summer, (2) its immediate bottom is slowly melting all the time (unless the thickness of the ice be less than the thickness of the zone of annual variation or of permanent freezing temperature); (3) its subsurface portion in the zone of waste is generally melting, owing to descending water, compression, and friction; while (4) its subsurface portion in the zone of growth is probably below the melting-point except as locally brought to that temperature by compression, friction, and descending water, and at the bottom by conduction from the rock beneath.

Movement under low temperature.—Glacier motion will not be discussed at this point, but one of the bearings of the preceding conclusions on glacier motion may be pointed out. Since there must be motion in the area of growth to supply the loss in the area of waste, the fundamental cause of motion must be operative in bodies of ice the mean temperature of which is below the melting-point, unless the dynamic sources of heat are considerable. This fundamental cause does not exclude the coöperation of causes that work only (1) at the melting temperature, or (2) where the ice is bathed with water, or (3) in the plane of contact between wet ice above and dry ice below. These may be auxiliary causes which abet the fundamental one in producing the more rapid movement of warm seasons, or in bringing about the especially rapid movement in situations where there is abundant water, or in inducing the shearing which is such a remarkable feature of arctic glaciers.

Evaporation.—The ice wastes by evaporation as well as by melting, and while the former process is far less important than the latter, its results are probably larger than is commonly apprehended. One of the most remarkable features of some of the deposits of ancient glaciers is the slight evidence they afford of escaping waters. The most plausible explanation seems to lie in the supposition that the ice was largely wasted by evaporation. This conclusion finds support in many places in the presence of a mantle of fine silt over the drift, the silt being apparently composed of dust blown upon the ice. It is supposed to imply aridity in the region about the ice. If a sufficient mantle of dust were spread over the border zone of the ice, and if the air were very dry, nearly all the water melted on the surface of the ice might be held back by the dust-wells until the water was evaporated or absorbed.

Fig. 251.—Spouting stream. Glacier south side of Olriks Bay, North Greenland.

Drainage.—Some of the water produced by surface melting forms little streams on the ice. Sooner or later they plunge into crevasses or over the sides and ends of the glacier. In the former case, they may melt or wear out well-like passages (moulins) in the ice, and even in the rock beneath. Much of the surface water sinks into the ice. Its ready penetration is aided by the “dust-wells” which mark the surface of many glaciers. In north Greenland wells which contain six or eight inches of water at the end of a warm day are often dry in the morning. The water has leaked out and passed to lower levels. From these and other harmonious observations it is inferred that the superficial part of a glacier at least is readily penetrated by water. The depth to which surface water penetrates is undetermined, but it doubtless varies greatly, not only in different glaciers, but in different parts of the same glacier, and in the same part at different times. Above the line of perennial snow there is little water either from melting or from rain, and hence relatively slight penetration. Below the line of perennial snow there is much melting and much rain, and here it is probable that the water sometimes, perhaps usually, penetrates to the bottom of the ice during the melting season, even independently of crevasses.

Once within the glacier, the course of the water is variable. Exceptionally it follows definite englacial channels, as shown by springs or streams issuing from the ice at some point above its bottom ([Fig. 251]). Oftener it descends or moves forward through the irregular openings which the accidents of motion have developed. If it reaches a level where the temperature is below its freezing-point, it congeals. Otherwise it remains in cavities or descends to the bottom. The water produced by melting within the glacier probably follows a similar course. So far as these waters descend to the bottom, they join those produced by basal melting and issue from the glacier with them. In alpine glaciers the waters beneath the ice often unite in a common stream in the axis of the valley, and hollow out a tunnel. Thus the Rhone is already a considerable stream where it issues from beneath the Rhone glacier. In the glaciers of high latitudes, subglacial tunnels are less common and the drainage is in streams along the sides of the glaciers or through the débris beneath and about them.

At the end of the glacier, all waters, whether they have been superglacial, englacial or subglacial, unite to bear away the silt, sand, gravel, and even small bowlders set free from the ice, and to spread them in belts along the border of the ice or in trains stretching down the valleys below. These are the most common of the glacio-fluvial deposits.