Geology, Vol. 1 [of 3]

by Thomas C. Chamberlin; Rollin D. Salisbury

Unless there is a very serious error in the estimated rate of thermal loss, or in the coefficients of expansion, cooling would seem to be a very inadequate cause for the shrinkage which the mountain foldings, overthrust faults, and other deformations imply. This inadequacy has been strongly urged by Fisher[264] and by Dutton.[265] In view of the apparent incompetency of external loss of heat, the possibilities of distortion from other causes invite consideration.

OTHER SOURCES OF DEFORMATION.

Transfer of internal heat.—It is theoretically possible that deformation of the subcrust may result from the internal transfer of heat without regard to external loss. It has already been shown (p. 539) that under certain possible conditions more heat would flow from the inner parts to higher horizons than would be conveyed through these latter to the surface and there lost, and that, as a result, the temperatures of the inner parts might be falling, while those of the outer parts (except the surface) might be rising. With the more conservative coefficient of expansion previously given, a lowering of the average temperature of the inner half of the earth 500° C. and the raising, by transfer, of the outer half to an equal amount would give a lateral thrust of about 83 miles, which is about the order of magnitude thought to be needed. It is not affirmed that this takes place, but some transfer of this kind is among the theoretical possibilities under the accretion hypothesis. The process could not continue indefinitely; but, for aught that can now be affirmed, it may still be in progress.

Denser aggregation of matter.—As already noted, matter under intense pressure tends to aggregate itself in the forms that give the greatest density. If the earth were built up of heterogeneous matter arranged at haphazard, the material would probably readjust itself more or less, as time went on, into combinations of greater and greater density. This process may be one of the important sources of shrinkage, for an average change of density of 1 percent., affecting the matter of the whole globe, would probably meet all the demands of deformation since the beginning of the Paleozoic period.

Extravasation of lavas.—It is obvious that if lavas are forced out from beneath the crust and spread upon it, a compensating sinking of the crust will follow. This, however, is rather a mode than an ulterior cause, for a cause must be found for the extrusion of the lavas, and this cause may be one of the other agencies recognized, such as a transfer of heat, a reorganization of matter, or a change of pressure. The more practical question, however, relates to its competency. Can the amount of lava that has been extruded have had any very appreciable effect on the descent of the crust? The great Deccan flow is credited with an area of 200,000 square miles, and a thickness of 4000 to 6000 feet. Vast as this is for a lava-flow, it would form a layer only about 5 feet thick when spread over the whole surface of the globe, and hence the sinking to replace it would cause a lateral thrust, on any great circle, of about 31 feet only. It requires a very generous estimate of the lavas poured out between any two great mountain-making periods since the beginning of the well-known stratigraphic series to cause a horizontal thrust of any appreciable part of that involved in mountain-making. The case is different, however, if we go back to the Archean era, in which the proportion of extrusive and intrusive rocks is very high. Very notable distortion may then be assigned to the extravasation of lavas. The outward movement of lava must also be credited with some transfer of heat from lower to higher horizons, and this is probably one of the agencies that have produced the relatively high underground temperatures in the outer part of the earth.

If lavas are thrust into crevices of the crust they contribute to its extension, but causes for the crevices and for the intrusion must be found, and these are probably only expressions of one or another of the more general agencies.

Change in the rate of rotation.—As previously noted, the tide acts as a brake on the rotation of the earth. The oblateness of the present earth is accommodated to its present rate of rotation. It is assumed that such accommodation has always obtained, and that if the rotation has changed, the form of the earth has changed also. Now, the more oblate the spheroid, the larger its surface shell and the less the total force of gravity. Hence if the earth’s rotation has diminished, its crust must have shrunk, because the form of the spheroid has become more compact, and the increase of gravity has increased its density. There is at present a water-tide chiefly generated in the southern ocean, and irregularly distributed to more northerly waters. This irregularity interferes with its systematic action as a brake, and its average effects are difficult of estimation. The water-tides of past ages are still more uncertain, as they must have depended on the configuration and continuity of the oceans. There are geological grounds for the belief that the southern ocean was interrupted by land during portions of the past at least, and it is unknown whether there were elsewhere ocean-belts well suited to the generation of large tides. The ocean-tide, therefore, furnishes a very uncertain basis for estimating the retardation of rotation.[266] The theoretical case rests largely on the assumption of an effective body-tide. The earth doubtless has some body-tide, but whether it is sufficiently great to be effective, and whether its position, which depends on its promptness in yielding and in resilience, is favorable to the retardation of rotation, are yet open questions. The existence of an appreciable body-tide has not yet been proved by observation.

G. H. Darwin, assuming that the earth is viscous enough to give a body-tide of appreciable value and of effective position, has deduced a series of former rates of rotation of the earth and has computed the corresponding distances of the moon.[267] C. S. Slichter has shown that the lessening of the area of the surface and the increase of the force of gravity corresponding to these assigned changes of rotation are large, and that if the changes were actually experienced they must have involved much distortion of the crust.[268] These distortions would, however, be of a peculiar nature, and should thereby be detectible, if they were realized; for in passing from a more oblate to a less oblate spheroid, the equatorial belt shrinks, and the polar tracts rise and become more convex. Wrinkles should, therefore, mark the equatorial belts, and tension the high latitudes. Slichter has computed that in a change from a rotation period of 3.82 hours to the present one, the equatorial belt must shorten 1131 miles and the meridional circles lengthen 495 miles. If we take Heim’s estimate of the crust-shortening involved in forming the Alps—74 miles—as a standard, the 1131 miles of equatorial shortening would be sufficient for the formation of 15 mountain ranges of Alpine magnitude. If, as some geologists urge, the estimate of mountain folding is too great, the quotient would be still larger. These ranges should run across the equator and be limited to about 33° N. and S. latitude. The high-latitude tension would be sufficient to cause the earth to gape more than two hundred miles at the poles, if there were simple ideal shrinkage. The amounts and the distribution of thrust and shrinkage are shown in [Fig. 453]. If the change of rotation were no more than from 14 hours to the present rate, there would still be 52 miles of thrust in the equatorial belt, and 40 miles of shortage in the meridional circles. There are no clear signs of such a remarkable distribution of thrust and tension as this hypothesis requires. Mountains are about as abundant and as strong north of 33°, the neutral line, as south of it, and they extend to high latitudes. The Archean rocks, in which this agency should have been most effective because of their early formation, are crumpled and crushed in the high latitudes much the same as in low latitudes. Furthermore, if there had been appreciable change in the form of the earth to accommodate itself to a slower rotation, the water on the surface, being the most mobile element, should have gathered toward the poles, and the less mobile solid earth should have protruded about the equator, but the distribution of land and water, present and past, gives no clear evidence of this. The equatorial belt contains a less percentage of land than the area north of it and more than that south of it. It varies but slightly from the average for the whole globe.

Fig. 453.—Polar projection of the earth’s hemisphere showing the theoretical high-latitude tension and low-latitude compression involved in a change of rotation from 3.82 hours to the present rate. The figure is drawn to true scale as seen from a point above the pole, and in consequence the equatorial tract is foreshortened. The black triangles show compression reduced in length by foreshortening; the white show tension in essentially true proportions to the high-latitude areas. The neutral line between the areas of compression and of stretching lies at 33° 20′ latitude.