On certain geological effects of the cooling of the earth

Abstract

The former molten state of the earth has been widely accepted as an important and fruitful hypothesis in physical geology ever since the enunciation of Laplace’s theory of the origin, of the solar system. Elie de Beaumont, followed by other investigators, suggested that the cooling and consequent contraction of the crust was a potent cause, and probably the chief cause, of the horizontal compression that led to mountain building. The theory was put into quantitative form by C. Davison and Sir G. H. Darwin, their numerical data being taken from Lord Kelvin’s theory of the cooling of the earth. Osmond Fisher objected to this that the compression they found was inadequate to account for existing mountain chains, but neither he nor those geologists who have followed him appear to have given an estimate of any significance of the amount of compression actually needed. Such an estimate, admittedly rough and provisional, but probably correct in order of magnitude, was made by the writer in a paper in the ‘Philosophical Magazine ’ for 1916. At the same time, I used Holmes’s data about the age and thermal properties of the earth in preference to Kelvin’s, and found that with these the available compression appeared to be sufficient. According to the form of the tidal theory of the origin of the solar system, developed by Chamberlin and Moulton in their Planetesimal Theory, the earth became solidified by adiabatic expansion immediately on its ejection from the sun, and in its subsequent growth by accretion never attained fusion temperature; so that we have to contemplate an earth that has always been solid, and cannot have cooled to anything like the extent that was implied by the older theory. I believe, however, that this supposition is erroneous. It is at least probable that most of the matter that went to form the earth came from the superficial regions of the sun (if the encounter with the passing star was “slow” in Jeans’s sense, the whole of it would), and that, when it gathered together, the relative increase in the depth was greater than the relative diminution in gravity. If this were so, the average pressure inside this matter would be greater than before instead of less, and the temperature would rise on ejection instead of falling, even if the change was adiabatic. Again, even if it were shown that the mean pressure would fall, adiabatic cooling below the boiling point could be caused only by evaporation, and therefore would not lower the temperature below a point at which the vapour pressure was insignificant. Thus the temperature could never be reduced in this way by more than 200°C. at most below the boiling point. But the difference between the melting and boiling points of the substances concerned is at least several hundred degrees. Hence the primeval earth, if adiabatic cooling took place at all, could at most have cooled to the liquid state and not to the solid state. Solidification must have taken place later and more gradually in consequence of radiation.