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SNIPPETS FROM THE ARCHIVES
"Powerful currents generated in the Underworld"
BBC 2 recently ran three programmes presented by Professor Ian Stewart called 'Men of Rock'. They featured major contributions to World geology by Scottish geologists. These included the explanation in 1928 by Professor Arthur Holmes of how continents move around the surface of the planet.
His paper on radioactivity and continental drift was published in the Geological Society of Glasgow's 1931 Transactions and was based on a lecture first delivered to members of the Geological Society of Glasgow at a meeting on 12 January 1928. The following is an extract from the orginal Minute book held in our Archives.
"The hypocentric curve was interpreted by Wegener to indicate the existence of two dominant kinds of materials in the earth's crust, sial* corresponding to the lighter continental areas, and sima* to the denser oceanic floors. This view has been confirmed by Miller's study of surface seismic waves. The properties of the sima of the Pacific floor correspond to those of gabbro. The effect of compression on gabbro would be to transform the material into eclogite. Such change of density and the simultaneous action of isostacy would lead to subsidence of the compressed belt and therefore to the formation of oceanic seeps. Applying the principle of isostacy, it is easily shown that the average thickeness of the sial should be about 30 km. This conclusion is confirmed by the recent work of Jeffreyson near earthquakes, which reveals the existence of an upper layer 10 km thick (identified with granitic rocks) ; an intermediate layer 20 kms. thick (supposed by Jeffreys to be tachylite, but identified by Holmes with diorite and quartz-diorite on petrological and thermal grounds), and a lower layer which may be eclogite or peridotite or both. The continents are thus thin slabs of sial, ranging in composition from granite to diorite, averaging 30 km thick by about 3000 km across, and embedded in material which on any interpretation is much more dense than sial.
Thus for physical reasons it becomes as impossible to "sink" a continent as to sink an iceberg. Considerable areas of the Atlantic and Indian Oceans were formerly occupied by continental masses, and since these ancient lands are no longer there we are driven to believe that their material has been moved away sideways. Evidence of lateral movement is also forthcoming from tear faults; overthrust structures of the Alpine type; the geological history of geosynclines; the echelon structures of the Asiatic Island festoons; and by the opening of the Urals geosyncline at the same geological moment as the compression of the Caledonian Mountains of Britain and Scandinavia. Moving the continental regions back in the directions indicated by the evidence leads to a Permo-Carboniferous reconstruction similar to that of Wegener's diagrams, a reconstruction that is independently called for on palaeo-climatic grounds. Wegener's deduction that the equator of the time ran through the coalfield belt of N America to China is supported by the distribution of Permo-Carboniferous laterites and bauxites. It is concluded that there is now evidence pointing to the former occurrence of continental drift on a scale of the same order as that advocated by Wegner.
The dominant forces available to move the continental slabs in the required directions(outwards from Africa towards the Pacific) tend to set up a westerly drift (tidal action) and a drift from the poles towards the equator (due to the departure of a polar section through the earth from a circle). Since our actual geography is totally different from the picture thus visualised, we have an indication that some other agency must have been at work to move the continents into the positions they now occupy. There seems no escape from the deduction that slow but overwhelmingly powerful currents must have been generated in the underworld at various times in the earth's history. These, as suggested by A.J. Bull, are probably convection currents set up in the lower layer as a result of differential heating by radioactivity. In place of the mobile basaltic magma of Joly, one imagines a highly viscous sima heated unequally to very great depths. A sheet-like upward current would develop beneath the region of greatest heat output. In turning over at or near the base of the sial it would exercise a powerful drag on the under-surface in two opposed directions, leading to the formation of a geosyncline. The return downward current would be looked for just beyond the continental edges. A continental mass would move forward by stoping of a heavy ocean floor just in front. When this ceased, mountain building would set in, and ultimately the direction of the currents would be reversed. convection currents which themselves move their boundaries and the sources of much of the heat responsible for their existence can clearly lead to periodic alternations of heating and cooling in any one region"
* In his 'Principles of Physical Geology', first published in 1944, Professor Holmes uses the term sima for a dense unltra basic rock with silica, iron and magnesia, and sial for the less dense rock with silica and alumina.