The evolution of a shrinking earth

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Abstract
Summary: Fold and thrust mountain structures are the consequence of compression of the earth's crust, but beyond that simple fact there has been wide divergence of opinion on the causal mechanism. The seen geological evidence of the compression zones which weave intricate patterns through all continental areas, and the presumptive evidence that they continue into the oceans, have been undervalued and, instead, deductions drawn from geophysical observations—seismic, gravity and magnetic— have unduly influenced geological opinion. The Mohorovičić seismic discontinuity at about 35 km. depth, where there is a velocity increase to 8-1 km./sec, has been interpreted as a change in composition from a “granitic” layer to a “peridotitic” layer because at the surface these rocks give corresponding velocities. The continents are said to float on this mobile layer and hence the assumption of the isostatic principle. Geological evidence shows the extreme improbability that there can be a layered arrangement of this type and that the observed compression structures can terminate downwards within such a thin crust. An alternative explanation of the seismic facts, more acceptable geologically, is that the discontinuities result from phase modifications at critical pressure levels with consequent marked changes in density and elastic characteristics. The compressed rocky crust with the same pattern as seen at the surface, though with increasing metamorphism downwards, may have a thickness of many hundreds of kilometres; and there is now no reason to assume any important change in chemical composition even at deeper levels. The old theory of a nickel-iron core is no longer necessary to explain the physical data. Contraction of the earth's surface due to shrinkage of the interior is the only adequate explanation of the compression zones, but the amount attributable to cooling is thought to be inadequate. Estimates of the extent of compression during the Alpine, Hercynian, Caledonian and at least three Pre-Cambrian phases give a contraction of the earth's circumference of at least 3000 km. and perhaps much more. The processes of compression, mountain-forming and erosion were already an established regime at the time of the oldest exposed Archaean rocks, and in consequence there is no geological evidence on the nature of the primordial crust and certainly no hint that it was at one time molten. Assuming no important change in the mass of the earth since the Archaean, the average density at that, already advanced, stage of compression may have been about 4–5 g/cm 3 compared with the present value of 5–517. Adiabatic compression may have generated more heat than has been lost by radiation and hence the basis of the assumption of a cooling earth is very insecure. The planetesimal theory of Chamberlin and Moulton for the formation of the earth is the most acceptable geologically. The present-day moon may illustrate the appearance of the earth at some pre-Archaean stage, the moon's craters being interpreted as infall craters of meteorites.

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