Avalanches are great masses of snow which descend from the mountain tops at a very high velocity, and are frequent in all high mountains with heavy snowfall, and occur, though less commonly, on mountains of medium height. Winter avalanches of dry and powdery snow do comparatively little destructive work, but in thawing weather, when the snow is heavily charged with water, great masses of earth and rock are brought down in the avalanche, which sweeps everything before it. Though acting only occasionally, avalanches are efficient agents in the removal of material from higher to lower levels.
Fig. 60. - Summit of Mt. Blanc, Switzerland, showing the great accumulations of snow.
On a small scale, snow-slides remove unprotected soil from slopes. In the bad lands (see p. 109), where the rain wash produces comparatively little effect upon the debris-covered buttes, sliding masses of snow strip off the covering of soil and expose fresh surfaces of rock to the destructive action of the water.
Glaciers are much the most important form of ice as a geological agent. A glacier is a stream of ice which flows as if it were a very tough and viscous fluid, and does not merely glide down a slope, as snow slides from the roof of a house. Glaciers play a very important part in keeping up the circulation of the atmospheric waters, and produce geological results of an extremely characteristic kind. Their contribution to the sum total of rock destruction and reconstruction is, it is true, relatively small, but it often becomes important to trace the former extension of glaciers, which, in its turn, has a wide bearing upon some of the most far-reaching of cosmical problems.
As we ascend into the atmosphere from any point on the earth's surface, we find that it becomes continually colder with increasing height. In this ascent a level is eventually reached where the temperature of the air never rises for any length of time above the freezing-point, and above this level no rain, but only snow, falls. This level is called the limit of perpetual snow, or simply the snowline. While the height of the snow-line above the sea-level is, like climate in general, much affected by local factors, yet, speaking broadly, its elevation is determined by latitude. In the tropics the snow-line is 15,000 or 16,000 feet above the sea, - descending more and more, as we go toward the poles, and coming down nearly to sea-level within the polar circles, but does not actually reach that level at any known point in the northern hemisphere.
Were there no means of bringing the snow which accumulates above the snow-line to some place where it may melt, it would evidently gather indefinitely, and at last nearly all the moisture of the earth would be thus locked up. As a matter of fact, there is no such indefinite accumulation. In very dry regions the excess of snow is disposed of by direct evaporation, and on high mountains avalanches carry the snow down to lower levels, where it melts.
Fig. 61. - Two valley glaciers descending Mt. Blanc, showing the terminal moraine at the foot of each. On account of the foreshortening the glaciers appear to be unduly steep.
In places where the excess of snow cannot be disposed of in either of these ways, glaciers are fcrmed and thus keep up the circulation of the waters, by carrying the surplus snow down to lower levels at which it can melt, or by entering the sea and in the shape of icebergs (which are fragments of glaciers) being floated to warmer latitudes.
Though even at the present time there are in various parts of the world great tracts of glacier ice, they cannot be called common and are found only where certain conditions concur. The nature of these conditions will be best understood by examining the process of glacier formation.
Snow is made up of minute, hexagonal crystals of ice, which are intimately mixed with air and thus separated from one another. Though the individual crystals are transparent, snow is white and opaque, as always results when a transparent body is intimately mixed with a gas, as in the foam on water, or in powdered glass. Ice is composed of the same kind of crystals as is snow, but they are in contact with one another, not separated by air. To convert snow into ice, therefore, it is only necessary to expel the air and bring the crystals into contact, for which pressure alone is not ordinarily sufficient.
The first step in the transformation is the partial melting of the upper layers of snow, for which a change of temperature is necessary, though the change need not warm the air, but may be due to the direct rays of the sun. Glaciers are rare in the tropics because of the constancy of the temperature, and the small area which extends above the snow-line, which seldom permits the formation of extensive snow-fields. Sometimes, however, the conditions of glacier formation are fulfilled even in the equatorial zone; for example, there is a glacier on one of the peaks of Ecuador.
When the surface layers of snow have been partially melted, the water thus formed trickles down into the snow beneath, expelling much of the air. This underlying snow has still a temperature much below the freezing-point, and the percolating water is soon refrozen into little spherules of ice. This substance, midway between snow and ice, is called neve, and may be seen every winter wherever the snow lies for any length of time. The hardened "crust" which forms by the refreezing of partly melted snow is neve. The air, which is now in the form of discrete bubbles, is largely expelled by the increasing pressure of the overlying snow masses, which are continually added to by renewed falls, and the neve is thus converted into ice.