Geology, Vol. 1 [of 3]

by Thomas C. Chamberlin; Rollin D. Salisbury

Another view of the globe’s origin is that the earth was built up gradually by the infall of matter, bit by bit, at such a rate that though each little mass became hot as a result of its fall, it cooled off before others fell on the same spot, the rain of matter not being fast enough to heat up the whole mass to the melting-point. Under this view, the internal heat arose chiefly from compression due to the earth’s gravity.

A clear conception of the three hypotheses of thermal distribution which rest on these two views of the origin of the earth is important to the further discussion.

1. Thermal distribution on the convection hypothesis.—It was formerly the prevailing opinion that the molten condition of the earth persisted in the interior until after the crust had formed, and that solidification proceeded from the surface downwards. It was a natural corollary of this view that, previous to the beginning of solidification, convection stirred the liquid mass from center to circumference and equalized the temperature so that the whole mass cooled down equably until it approached the point of solidification and became too viscous for ready convection. The temperature should, therefore, have been nearly the same from center to surface at the stage just preceding incipient solidification. This conception forms the basis of most discussions involving internal temperatures.[248] The famous studies of Lord Kelvin are based on the assumption of a uniform initial temperature of 7000° Fahr.[249] Other temperatures have been assumed in similar studies by others, but the results do not differ materially. On this hypothesis there would be no deep-seated change of temperature until a temperature-gradient, extending to the deeper horizons, had been developed by surface cooling. In the earliest eras, the loss of heat would be felt solely in the outer zone. By surface cooling, a temperature gradient would be slowly developed, and gradually changed from age to age, as shown by the curved lines in [Fig. 450], each of which shows the temperature at the successive stages stated in the legend. The computations for these curves were based on the methods and assumptions of Lord Kelvin. The two lower curves represent greater periods than those usually assigned by geologists to the whole history of the earth. It will be seen that the modification of the original temperature line extends only about 160 miles below the surface for the 100,000,000-year period, only about 240 miles for the 237,000,000-year period, and only about 320 miles for the excessive period of 600,000,000 years. The superficial nature of the whole thermal problem under this hypothesis is thus made clear and impressive.

Fig. 450.—Diagram showing the original distribution of heat assumed by the convection hypothesis and the modifications of this distribution near the surface in successive long periods. The base-line of the figure represents divisions of the earth-radius with center at the left and surface at the right. The vertical lines represent temperatures ranging from 0° C. to 5000° C. The assumed initial temperature 3900° C. (7000° F.) is represented by the horizontal line TC, full at the left and dotted at the right to indicate the original extension of the initial temperature to the surface. The upper curve at the right shows how much the temperature will have been modified at the end of 100,000,000 years, computed according to the method of Lord Kelvin. The middle curve shows the change at the end of 237,000,000 years, and the lower curve the change at the end of 600,000,000 years. Similar curves may be found in an article by Clarence King, Am. Jour. of Sci., XLV, 1893, p. 16.

After the outer shell had cooled so as to be in approximate equilibrium with the environment of the earth, it suffered practically no contraction.

So also it appears from the diagram that there was practically no contraction below 160 miles up to the end of the 100,000,000-year period, because cooling had not yet reached that depth. Between these two non-contracting horizons the greatest rate of contraction at the close of the 100,000,000-year period lay about 60 miles below the surface. The contraction of this middle zone, while the outermost shell and the interior body remained constant, is held to have developed a state of horizontal thrust in the outer shell, because this shell, being too large for the shrinking subcrust, tended to settle, and to crowd upon itself horizontally. The wrinkling and other modes of deformation of the outer part of the earth are referred, under this view, to the thrust so developed. This is the view which has been most generally accepted.

Level of no stress.—As the outer shell is thus held to be in a state of thrust while the zone below is in a state of shrinkage, there must be, between these two zones, a level of no stress, where there is neither compression nor stretching. Above this level, the thrust increases to the surface, and below it, the stretching increases to the depth of most rapid change of temperature, below which it decreases and finally vanishes at the lower limit of temperature change. In the earliest stages of cooling, the level of no stress must have been near the surface, and must have descended gradually as the cooling proceeded. The depth of this level has been repeatedly computed on the basis of assumed times and rates of cooling. Fisher, assuming the temperature of solidification to have been 4000° Fahr. and the period of cooling 33,000,000 years, computed its depth at only ⁷⁄₁₀ of a mile below the surface.[250] T. Mellard Reade, with somewhat different assumptions, placed it at 2 miles after 100,000,000 years of cooling.[251] Davison (1897) placed it at 2.17 miles,[252] and G. H. Darwin at 2 miles after the same period.[252] In a later computation, based on the assumption that the coefficient of dilatation increases with the temperature, Davison placed the level of no stress at 7.79 miles, and stated that if the coefficient of conductivity and the initial heat also increased down wards, the zone would lie still deeper. To suppose the initial heat to increase downwards, however, is to abandon the hypothesis we are now considering. These computations seem to show that, at the very utmost, the level of no stress, under this hypothesis, lies at a very slight depth, and that the thrust zone above is, therefore, very shallow. This should be kept constantly in mind in all deductions drawn from this hypothesis. If the thickness of the thrust zone be taken at 8 or 10 miles, it will apparently be conceding to the view all that can legitimately be claimed for it.

Fig. 451.—Diagram illustrating the internal temperatures of the earth when it first became solid, under the hypothesis that it solidified from the center outward, and assuming that the fusing-point rose directly as the pressure, in accordance with Barus’ experiments with diabase. The divisions of the base-line represent fractions of the earth’s radius. The divisions of the vertical lines represent pressures in atmospheres at the left, and temperatures in degrees C. at the right. The lower curve, PC, represents the interior pressures, ranging from one atmosphere at the surface to 3,000,000 atmospheres at the center, derived from Laplace’s law of density. The upper curve, FC, represents the fusion-points of diabase at the various depths and pressures, and hence the temperatures at which the interior would become solid at the various depths, or, in other words, the initial temperatures of the solid earth. The lower curve is derived from Slichter; the upper is formed by directly plotting the temperatures given by Barus (Am. Jour. Sci., 1893, p. 7).