SNOW- AND ICE-FIELDS.
Snow-fields.—Mountain heights and polar lands are the most common habitats of snow-fields, though they are not confined to these situations. In North America there are numerous small snow-fields in the western mountains, from Mexico on the south to Alaska on the north, their number and size increasing in the latter direction. In the United States there are few snow-fields south of the parallel of 36° 30′, and most of the many hundreds north of that latitude (excluding Alaska) are small ([Pl. XVIII, Fig. 1], Washington, Lat. 48° 5′, Long. 121° 5′; [Fig. 2], Lat. 41° 25′, Long. 122° 12′. From Glacier Peak and Shasta Special Quadrangles, U. S. Geol. Surv.). Farther north, especially in Alaska, the snow-fields of the western mountains attain much greater size. In Europe snow-fields comparable to those of the northwestern part of the United States and British Columbia occur in the Alps ([Fig. 221]), the Pyrenees, the Caucasus, and the Scandinavian mountains. In Asia snow-fields occur in the Himalayas and in many of the high mountains farther north, from Turkestan on the southwest nearly to the coast on the northeast. In South America there are snow-fields of small size even in equatorial latitudes, and farther south in the Chilean Andes there are some of considerable size. Small snow-fields occur on the highest peaks of tropical Africa, and in the mountains of New Zealand. For reasons which will appear later, much of every considerable snow-field is really ice.
In addition to these limited fields of snow in mountain regions, there are fields of much greater extent covering wide expanses of plain and plateau in the polar regions. The greater part of the island of Greenland is covered with a single field of ice and snow, the size of which is variously estimated at 300,000 to 400,000 square miles ([Fig. 222])—an area 400 to 600 times as large as the snow-and-ice-covered area of Switzerland. Numerous islands to the west of North Greenland are also partly covered with snow, the areas of the snow-fields far exceeding those of most mountain regions. In Antarctica there is believed to be a still larger field, the largest of the earth. Its area is not even approximately known, but such data as are at hand indicate that it may have an extent of 3,000,000 or 4,000,000 square miles.
Fig. 221.—An alpine snow-field.
Fig. 222.—Map of Greenland. The borders only are free from ice. (Stieler.)
The only condition necessary for a snow-field is an excess of snowfall over snow waste. The lower edge of a snow-field, the snow-line, is dependent chiefly on temperature and snowfall. In general it does not depart much from the summer isotherm of 32°, though it may be well above this isotherm where the snowfall is light. That the snow-line is not a function of temperature only is shown by its position in various places. In the equatorial portion of the Andes, for example, the snow-line has an altitude of about 16,000 feet on the east side of the mountains, where the precipitation is heavier, and of about 18,500 feet on the west side, where it is lighter. For the same reason the snow-line in the Himalayas is 3000 or 4000 feet lower on the south side than on the north.
While in equatorial regions the snow-line has an altitude of 15,000 to 18,000 feet, it approaches or even reaches sea-level in the latitude of Antarctica and North Greenland. In intermediate latitudes it has an intermediate position.
While temperature and snowfall are the most important factors controlling the position of the snow-line, both humidity and the movements of the air are of some importance, since both affect the rate of evaporation of snow and ice.
The passage of snow into névé and ice.—The snow does not lie on the surface long before it undergoes obvious change. The light flakes soon begin to be transformed into granules, and the snow becomes “coarse-grained.” The granular character, so pronounced in the snow of the last banks which remain in the spring in temperate latitudes, is even more distinct in perennial snow-fields, either at the surface or just beneath it. This granular snow is called névé, or firn. Still deeper beneath the surface, where the thickness of the snow is great, the névé becomes more compact and finally coherent, and grades into porous ice. This gradation is accomplished at no great depth, though the thicknesses of snow and névé are by no means constant.
Structure of the ice.—Ice formed beneath a snow-field is in some sense stratified. It is made up of successive falls of snow which tend to retain the form of layers. This follows from two or three conditions. The snow of one season, or of one period of precipitation, may have been considerably changed before the succeeding fall of snow. So also the surface of the snow-field at the end of the melting season is often covered with a visible amount of earthy matter, some of which was blown up and dropped on the surface during the melting season, and some of which was concentrated in that position by the melting of the snow in which it was originally imbedded. The amount of earthy matter is often sufficient to define snows of successive years, or perhaps of minor periods of precipitation, and makes distinct the stratification which would otherwise remain obscure. The snowfall of successive years has been estimated by this means[122] where the snow is exposed in crevices in the snow-field.
In addition to its rude stratification, the ice of the deeper portions often acquires a stratiform structure which may perhaps best be called foliation to distinguish it from the stratification which arises from deposition. The foliation appears to result mainly from the shearing of one part over another in the course of the movements to which the ice is subjected, as will be illustrated presently.
Texture.—The ice derived from the snow is formed of interlocking crystalline grains. The crystalline character is present from the beginning, for it is assumed by the snowflakes when they form, and the subsequent changes seem only to modify the original crystals by building up some and destroying others. By the time the snow is converted into névé, the granules have become coarse, and wherever the ice derived from the névé has been examined, the granular crystalline texture is present. The individual crystals in the ice are usually larger than those of the névé, and more closely grown together. In the fresh unexposed ice the crystals are so intimately interlocked that they are not readily seen except under a polarizing microscope, but when the ice has been honeycombed by partial melting, the granules become partially separated and may be easily seen. [Fig. 223] shows quantities of them which have been washed down from the surface, and disposed as cones at its base. While a given mass of snow in a great snow- and ice-field cannot be followed consecutively through its whole history, yet since (1) the granular texture is pronounced in the névé stage where the granules show evidences of growth, and since (2) the same texture is also pronounced in the last stages of the ice when it is undergoing dissolution, as well as at all observed intermediate stages, and since (3) the crystals are, on the average, larger in proportion to the lateness of the stage of their history, while (4) experiment has shown that granules grow under the conditions which exist in snow-fields, and (5) that they persist under very considerable pressure, it is legitimate to assume that a granular crystalline condition persists throughout all stages, and is a feature of progressive growth.
Fig. 223.—Figure showing cones of granules of ice which have been washed down the front of the glacier by streamlets, and accumulated after the manner of talus or alluvial cones. North Greenland.
Inauguration of movement.—Eventually the increase in depth of snow and ice in a snow-field gives rise to motion. The exact nature of the motion has not yet been demonstrated to the satisfaction of all investigators. Brittle and resistant as ice seems, it exhibits, under proper conditions, some of the outward characteristics of a plastic substance. Thus it may be made to change its form, and may even be moulded into almost any desired shape if carefully subjected to sufficient pressure, steadily applied through long intervals of time.[123] These changes may be brought about without visible fracture, and have been thought to point to a viscous condition of the ice. There is much reason, however, as will be seen later, to question this interpretation of the ultimate nature of the movement. Whatever this may be, the mass result of the movement in a field of ice is comparable, in a superficial way at least, to that which would be brought about if the ice were capable of moving like a viscous liquid, the motion taking place with extreme slowness. This slow motion of ice in an ice-field is glacier motion, and ice thus moving is glacier ice.
Fig. 224.—Ice-caps of small size. The figure also shows some valley glaciers extending out from the main ice-sheet and from the local ice-caps. A portion of the North Greenland coast, north of Inglefield Gulf. Lat. about 78°. (Peary.)
Fig. 225.—Small ice-caps in the northwestern part of Iceland. (Thoraddsen’s geological map of Iceland.)
Fig. 226.—A glacial lobe, midway between an ice-cap and a valley glacier. A protrusion from a local ice-cap east of Cape York, Greenland.
If both the surface on which the ice-sheet develops and its surroundings be essentially plane, as may happen in high latitudes, and if the snow- and ice-field be symmetrical in shape, the outward movement will be approximately equal in all directions, and the area covered by the spreading ice-field will remain more or less circular. If the ice-field rests on a steeply inclined surface, like a mountain slope, the movement becomes one-sided in conformity to the slope. If the surface, otherwise plane, be affected by valleys parallel to the direction of movement, the ice in the valleys will be deeper than that on the divides between them, and its movement stronger. In the valleys, therefore, the ice will advance farther than elsewhere before being melted, and the outline of the ice will become lobate, the lobes occupying the depressions. These general relations are shown in Figs. [224] and [225]. If the depressions be wide and shallow, the lobes will be broad and short ([Fig. 226]); if the depressions be narrow and deep, the lobes will be relatively narrow and Long. If the snow and ice rest on a surface consisting chiefly of steep valleys and sharp ridges, as is common on mountain slopes, the snow and ice are chiefly gathered in the valleys, and take a linear form.