Abstract
A DISCUSSION on the origin of igneous rocks was held in Section C (Geology) of the British Association at York on September 7. The discussion was opened by Prof. Arthur Holmes, who sketched out a general scheme of petrogenesis in the light of our present knowledge concerning the geological history, structure and thermal condition of the earth's crust. In certain continental regions the sedimentary and granitic layers appear to be separated from a deeper ‘basaltic’ layer by one which is probably composed of both acid and basic materials. The ‘basaltic’ layer is itself composite, a tentative interpretation being that amphibolite (a potential source of over-saturated basalts) is succeeded in depth by more basic granulite (a potential source of undersaturated basalts). Beneath these layers peridotite probably comes into place, merging within a few tens of kilometres into the glassy state regarded as characteristic of the substratum. Given some such setting of materials, igneous activity could arise, either by the ascent into the crust of heat from the feebly radioactive substratum, or by the accumulation of heat in specially thickened belts of the more strongly radioactive crustal rocks. The latter process fails, however, to account for the flooding by plateau basalts of regions where the crust has been thinned by denudation, and also for the ascent of granitic magma during orogenesis instead of long afterwards. The hypothesis of refusion of the crustal layers by heat from the substratum, with successive production of peridotitic, basaltic and granitic magmas, therefore seems to be worthy of special consideration.
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The Origin of Igneous Rocks. Nature 130, 745–746 (1932). https://doi.org/10.1038/130745a0
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DOI: https://doi.org/10.1038/130745a0