Geology, Vol. 1 [of 3]

by Thomas C. Chamberlin; Rollin D. Salisbury

The special geological action of the atmosphere will be discussed in the next chapter.

II. The Hydrosphere.

About 1300 quadrillion tons of water lie upon the surface of the solid earth. This equals about ¹⁄₄₅₄₀ part of the earth’s mass. Were the surface of the solid earth perfectly spheroidal, this would constitute a universal ocean somewhat less than two miles deep. Owing to the inequalities of the rock surface, the water is chiefly gathered into a series of great basins or troughs occupying about three-fourths (72%) of the earth’s surface. These basins are all connected with each other and act as a unit, so that anything which changes the level of the water in one changes the level of all. This helps to make a common record of all great movements of the earth’s body, for the level of the ocean determines where the detritus from the land shall lodge, and hence where the edge of the marine beds shall be formed. This will appear more clearly when the formation of marine strata is discussed.

Oceanic dimensions.—The surface area of the ocean is estimated by Murray at 143,259,300 square miles. Of this, somewhat more than 10,000,000 square miles lie on the continental shelf, i.e., lap up on the borders of the continental platforms. This shows that the great basins are somewhat more than full. If about 600 feet of the upper part of the ocean were removed, the true ocean basins would be just full, and the surfaces of the true continental platforms would be dry land. The area of the true oceanic basins is about 133,000,000 square miles, and that of the true continental platforms about 64,000,000 square miles. Under about 20% of the ocean area, the bottom sinks to depths between 6000 and 12,000 feet; under about 53% it sinks to depths between 12,000 and 18,000 feet; and under the remaining 4% it ranges from 18,000 feet down to about 30,000. The last includes those singular sunken areas known as “deeps,” and sometimes called anti-plateaus, as they extend downward from the general ocean bottom much as the plateaus protrude upwards from the general land surface.

Besides the ocean, the hydrosphere includes all the water which constitutes the surface streams and lakes, together with that which permeates the pores and fissures of the outer part of the solid earth; but altogether these are small in amount compared with the great ocean mass.

Geologic activity.—Of all geological agencies water is the most obvious and apparently the greatest, though its efficiency is conditioned upon the presence of the atmosphere, upon the relief of the land, and upon the radiant energy of the sun. Through the agency of rainfall, of surface streams, of underground waters, and of wave action, the hydrosphere is constantly modifying the surface of the lithosphere, while at the same time it is bearing into the various basins the wash of the land and depositing it in stratified beds. It thereby becomes the great agency for the degradation of the land and the building up of the basin bottoms. It works upon the land partly by dissolving soluble portions of the rock substance, and partly by mechanical action. The solution of the soluble part usually loosens the insoluble, and renders it an easy prey of the surface waters. These transport the loosened material to the valleys and at length to the great basins, meanwhile rolling and grinding it and thus reducing it to rounder forms and a finer state, until at length it reaches the still waters or the low gradients of the basins and comes to rest. The hydrosphere is therefore both destructive and constructive in its action. As the beds of sediment which it lays down follow one another in orderly succession, each later one lying above each earlier one, they form a time record. And as relics of the life of each age become more or less imbedded in these sediments, they furnish the means of following the history of life from age to age. The historical record of geology is therefore very largely dependent upon the fact that the waters have thus buried in systematic order the successive life of the ages. Aside from this, the means of determining the order of events of the earth’s history are limited and more or less uncertain.

The special processes of the hydrosphere in its various phases will be the subject of discussion hereafter (Chaps. [III], [IV], [VI]). Suffice it here to recognize its great function in the constant degradation of the land, and in the deposition of the derived material in orderly succession in the basins.

Chief horizons of activity.—The great horizons of geological activity are (1) the contact zone between the atmosphere and the hydrosphere, chiefly the surface of the ocean, (2) the contact zone between the hydrosphere and the lithosphere, chiefly the shore belts, and (3) the contact zone of the atmosphere and surface waters, with the face of the continents. It is in these three zones that the greatest external work is being done and has been done in all the known ages.

III. The Lithosphere.

The atmosphere and hydrosphere are rather envelopes or shells than true spheres, though in some degree both penetrate the lithosphere. The lithosphere, on the other hand, is a nearly perfect oblate spheroid with a polar diameter of 7899.7 miles, and an equatorial diameter of about 26.8 miles more. Its equatorial circumference is 24,902 miles, its meridional circumference 24,860 miles, its surface area 196,940,700 square miles, its volume 260,000,000,000 cubic miles, and its average specific gravity about 5.57. The oblateness of the spheroid is an accommodation to the rotation of the earth, the centrifugal force at the equator being sufficient to cause the specified amount of bulging there. Computations seem to indicate that the accommodation is very nearly what would take place if the earth were in a liquid condition, from which the inference has been drawn that it must have been in that condition when it assumed this form, and must have continued essentially liquid until it attained its present rate of rotation, since, if the earth once rotated at a much higher speed, the flattening at the poles and the bulging at the equator must have been correspondingly greater. It is thought by others, however, that the plasticity of the earth is such that it would at all times assume a close degree of approximation to the demands of rotation, even if the interior were in a solid condition. By still others it is thought that the contraction of the earth has tended to accelerate the rotation about as much as the tides have tended to retard it, and that it has undergone little change of form.