Fig. 295.—Profile across the largest of the Icelandic ice caps, with the vertical scale greatly exaggerated (after Thoroddsen and Spethmann).
When the quantity of accumulated snow is so great that all hollows of the rock surface are filled, its own surface is no longer controlled by retaining rock walls, and it now assumes a form largely independent of the irregularities in the upland. Experience shows that this surface is approximately that of a flat dome or shield, and as it covers all the upland, save where the ice thins upon its margins, this type of glacier is called an ice cap ([Fig. 295]). All types of glacier in which rock projects above the highest levels of the ice and snow are known as mountain glaciers.
Fig. 296.—Ideal section across a continental glacier, with the vertical scale and the projecting rock masses of the marginal zone greatly magnified.
The flat domes of ice which mantle the continents of Greenland and Antarctica, though resembling in form the smaller ice cap, are yet because of their vast size so distinct from them, particularly in the manner of their nourishment, that they belong in a separate class described as inland ice or continental glaciers. Though they have some affinities with ice caps, they are most sharply differentiated from all types of mountain glaciers. Of them it is true that the lithosphere projects through them only in the neighborhood of their margins ([Fig. 296]), whereas in the case of mountain glaciers rock may project at any level but always above the highest snow surface. Ice caps may be regarded as intermediate between the two main classes of mountain and continental glaciers ([Fig. 297]). Because of the large rôle which continental glaciers have played in geological history, it is thought best to consider them first, leaving for later discussion the no less interesting but less important mountain glaciers.
Fig. 297.—View of the Eyriks-Jökull, an ice-cap of Iceland (after Grossman).
The nourishment of glaciers.—The life of a glacier is dependent upon the continued deposition of snow in aggregate amount in excess of that which is lost by melting or by other depleting processes. Whenever, on the other hand, the waste exceeds the precipitation, the glacier is in a receding condition and must eventually disappear, if such conditions are sufficiently long continued. The source of the snow is the water of the ocean evaporated into the atmosphere and transported over the land in the form of clouds. We are to learn that the changes which this moisture undergoes before its delivery to the glacier are notably different for the classes of continental and mountain glacier.
The upper and lower cloud zones of the atmosphere.—Before we can comprehend the nature of the processes by which glaciers are nourished, it will be necessary to review the results of recent studies made upon the earth’s atmospheric envelope. It must be kept in mind that the sun’s rays are chiefly effective in warming the atmosphere through being first absorbed by some solid body such as rock or water and their heat then communicated by contact to the immediately adjacent air layers. The layers thus warmed being now lighter than before, they rise and are replaced by colder air, which in its turn is warmed and likewise set in upward motion. Such currents developed in the air by contact with warmer solid bodies constitute the process known as convection.