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II.—Radio-activity and the Earth's Thermal History

Published online by Cambridge University Press:  01 May 2009

Arthur Holmes
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Two years ago, in discussing the thermal energy of the earth, I suggested that while it had become impossible to deduce the earth's age from its thermal condition alone, Kelvin's problem might profitably be reversed by accepting the earth's age as a known factor, and deducing with its help the thermal history of the earth. This paper is a first attempt to attack the new problem then suggested. For geological purposes, one of the most fundamental aspects of the problem is that relating to the depth within the earth's crust at which temperatures are attained such that, under suitable conditions of pressure, molten rock magmas may exist. The determination of the minimum depth of possible rock fusion is a first essential to any adequate theory of vulcanism, and indeed of igneous activity in general. It is not sufficient, however, to ascertain that depth for present conditions alone, its variation during the earth's geological history must also be investigated ; for if, as is generally believed, the earth is a cooling body, the depth must be slowly increasing, and in former periods it must necessarily have been nearer the surface than it is now. In the limiting conditions both of position and time, tlie depth of fusion may have been at, or so near as to be for all practical purposes at, the surface itself. That is to say, at the beginning of geological history the earth may have been in a molten condition at, or immediately below, the then existing surface

Type
Original Articles
Information
Geological Magazine , Volume 2 , Issue 2 , February 1915 , pp. 60 - 71
Copyright
Copyright © Cambridge University Press 1915

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References

page note 1 Strutt, Proc. Roy. Soc., A, vol. lxxvii, p. 475, 1906Google Scholar; , Holmes, Science Progress, No. 33, p. 15, 1914.Google Scholar

page note 2 , Holmes, Science Progress, No. 33, p. 21, 1914.Google Scholar

page note 3 My Science Progress paper (No. 33, 1914) deals fully with this question.

page note 1 Eve, & Adams, , Nature, p. 209, 07, 1907.Google Scholar

page note 2 One part of radium in equilibrium with its parent uranium implies the presence of 3,000,000 parts of the latter element.

page note 3 Proc. Roy. Soc., A, vol. lxxvii, p. 472, 1906.Google Scholar

page note 4 See , Joly, Phil. Mag., vol. xxiv, p. 694, 1906Google Scholar; Holmes, , loc. cit., p. 15, 1914, for references to literature.Google Scholar

page note 5 The unusually high radium content of recent Vesuvian lavas found by Joly is probably due to—

  1. (1)

    (1) Gravitational differentiation.

  2. (2)

    (2) Concentration by volatile fluxes.

  3. (3)

    (3) Association of radium, unaccompanied by its parent uranium, with potash minerals. It is well known that barium is commonly associated with magmas rich in potash, and it is therefore suggested that free radium may be similarly concentrated. In old lavas any such free radium would almost entirely have disintegrated, leaving to be measured only the radium which is directly related to the uranium present in the rock.

page note 1 , Joly, Phil. Mag., vol. xx, pp. 125 and 353, 1910Google Scholar, and Cong. Intern. de Radiologie et d'Electricité, p. 876, 1911Google Scholar; , Fletcher, Phil. Mag., vol. xx, pp. 102 and 770, 1910Google Scholar. See also , Poole, Phil. Mag., vol. xxix, 1915.Google Scholar

page note 2 Results for richly alkaline rocks, which are abnormally high in their radium and thorium contents, are omitted from this Table.

page note 3 Implying the series Uranium to Lead.

page note 4 , Suess, The Face of the Earth (Eng. trans.), vol. iv, p.544Google Scholar; , Farrington, Journ. Geol., vol. ix, p. 623, 1901Google Scholar; , Merrill, Am. J. Sci., vol. xxvii, p. 469, 1909Google Scholar; vol. xxxv, p. 509, 1913Google Scholar; Holmes, , loc. cit., pp. 30–5, 1914.Google Scholar

page note 5 , Von Cotta, Geologische Fragen, Frieburg, 1858, pp. 76–8Google Scholar; , Green, Vestiges of the Molten Globe, pt. ii, p. 61, 1887Google Scholar; , Daly, U.S.G.S. Bull. 209, p. 110, 1903Google Scholar; Igneous Rocks and their Origin, p. 162, 1914.Google Scholar

page note 1 , Milne, Proc. Roy. Soc., A, vol. lxxvii, p. 365, 1906Google Scholar; Oldham, R. D., Q.J.G.S., vol. lxii, p. 456, 1906; vol. lxiii, p. 344, 1907CrossRefGoogle Scholar; Nature, August 21, p. 635, 1913.Google Scholar

page note 2 In spite of their very high atomic weights, the radio-elements tend to be carried upwards on account of their association with the lighter components of rock-magmas.Google Scholar

page note 2 If it is thought that this value of k (0·006) combined with a temperature gradient of 0·00032° C. per cm. gives an unfairly high result, it should be remembered (1) that the tendency in measuring temperature gradients is always to under- rather than over-estimate, and (2) that the heat lost from the earth by igneous activity is not included in the product k . dθ/dx.Google Scholar

page note 3 See , Rutherford, Radio-active Substances and their Radiations, pp. 583, 650, 1913.Google Scholar

page note 1 Considered not only superficially but in depth also.Google Scholar

page note 2 , Joly, Radio-activity and Geology, p. 272, 1909.Google Scholar