Comparison of the hypotheses.—By comparing the three hypotheses of the early states of the earth’s temperature, it will be seen that there is a radical difference, thermally, between the first and the last two. The first assumes a nearly uniform distribution of internal temperature, and hence, owing to the exceedingly slow rate of conduction, limits the movements and deformations of the crust, so far as dependent on heat, to very superficial horizons. The second and third views agree in postulating changes of temperature in the deep portions, as well as in the superficial, and hence involve the central portion of the earth in the great movements and deformations. It is not to be supposed that this of itself necessarily increases the sum-total of the effects of contraction, for, given a certain loss of heat from the surface, it may be relatively immaterial whether this loss arose from a large reduction of temperature in a shallow zone, or a small reduction of temperature in a deep zone, for, except as the coefficient of expansion varies, the total shrinkage would be the same. But the difference in distribution makes a radical difference in the resulting movements, for, in the first case, the movements are in a weak superficial shell that cannot accumulate great stresses, and hence must yield practically as fast as the stresses arise, while, in the second case, the stress-accumulating power of the thick segments may be great, and the stresses may gather for long periods and give rise to great cumulative results at long intervals. In this respect the last two views have much in common, though they differ in other important particulars.
With this general background of hypothesis, we may now turn to the direct evidences of the distribution of internal temperature which observations near the surface afford. Unfortunately they are limited to a mere film, as it were, little more than ¹⁄₄₀₀₀ of the radius of the earth.
OBSERVED TEMPERATURES IN EXCAVATIONS.
As the earth is penetrated below the zone of seasonal changes by wells, mines, tunnels, and other excavations, the temperature is almost invariably found to rise. The rate of rise, however, is far from uniform. If we set aside as exceptional the unusually rapid rise near volcanoes and in other localities of obvious igneous influence, the highest rates are still six times the lowest. A large number of records have been collated by the Committee on Underground Temperatures, of the British Association for the Advancement of Science. These range from 1° F. in less than 20 feet to 1° F. in 130 feet, with an average of 1° F. in 50 to 60 feet, which has usually been taken as representative. The more recent deep borings that have been carefully measured with due regard to sources of error indicate a slower rate of rise. Some of the more notable records are as follows:[257]
| Depth. | Rate of rise. | |
|---|---|---|
| Sperenberg bore (Germany) | 3492 feet. | 1° F. in 51.5 feet. |
| Schladeback bore (Germany) | 5630 “ | 1° F. in 67.1 “ |
| Cremorne bore (N. S. Wales) | 2929 “ | 1° F. in 80 “ |
| Paruschowitz bore (Upper Silesia) | 6408 “ | 1° F. in 62.2 “ |
| Wheeling well (W. Va.) | 4462 “ | 1° F. in 74.1 “ |
| St. Gothard tunnel (Italy-Switzerland) | 5578 “ | 1° F. in 82 “ |
| Mt. Cenis tunnel (France-Italy) | 5280 “ | 1° F. in 79 “ |
| Tamarack mine (N. Mich.) | 4450 “ | 1° F. in 100 “[257] |
| Calumet and Hecla mine (N. Mich.) | 4939 “ | 1° F. in 103 “[257] |
| Ditto, between 3324 feet and 4837 feet | 1° F. in 93.4 “ | |
It is to be noted that even these selected records vary a hundred per cent. Very notable variations are found in the same mine or well, and often much difference is found in adjacent records, especially those of artesian wells. Some of these are explainable, but the full meaning of other variations is yet to be found.
Explanations of varying increment.—Certain apparent variations are merely due to inequalities of topography. The isogeotherms, or planes of equal underground temperature, do not normally rise and fall with every local irregularity of the surface, but more nearly strike an average. A well on a bluff 500 feet high would probably reach nearly the same temperature at 1000 feet, as a well 500 feet deep in the adjacent valley, giving a gradient twice as great in the one case as in the other.
In interpreting the temperatures of artesian flows, regard must be had to the depths of rock under which the waters have passed, as well as the depths at the location of the wells. Darton has found[258] unusually high and varying temperatures in the artesian wells of the Dakotas, some part of which may be due to this cause, though a full explanation of their singular variations is not yet reached.
The permeation and circulation of water affect the temperature in two important ways: (1) wet rocks are better conductors than dry ones, and (2) the convective movement of water is a means of conveying heat from lower to higher horizons. As the circulation of underground water is very unequal, much irregularity of thermal distribution in the upper zones probably arises from this source. The general effect of water circulation is to reduce the thermal gradient where the circulation is relatively rapid, as it is near the surface and in the main thoroughfares of circulation, and hence to cause a relatively rapid rise in the gradient just below the zone of effective water influence. Some records conform to this theoretical deduction, but in general it is masked by other influences.
Chemical action, especially oxidation, carbonation, hydration, solution, and precipitation, modify the normal temperature gradient, but how effectively is not well determined. With little doubt the first three mentioned above raise the temperature, while solution and precipitation in some large measure offset each other.[259] The sum-total is probably an appreciable rise in temperature. It has even been conjectured that the heat of volcanic action is due to chemical combination in the lower reaches of water circulation, but this is obviously an over-estimate.