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Perspectives on Plate Tectonics

Don Tarling* takes issue with the historical account of Plate Tectonics publicised by the Society in its promotion of the McKenzie Archive.

kljhI am sure everyone applauds the Society's recognition of the viability of Plate Tectonics, as marked by its recent William Smith meeting and acceptance of the McKenzie archive.  However, I do feel some concerns about a lack of perspective in Sue Bowler's interview with Dan McKenzie (picture, left), in particular the assertion that Plate Tectonics was brought about in 1967 by a "handful of people worldwide". 


The concept goes back many decades and involves the shoulders of many giants.  In 1855 Airy and Pratt separately proposed an 80 mile thick "rigid" outer layer to the Earth that overlaid a 'soft' mantle.  These were named as "lithosphere" and "asthenosphere" by Barrell in 1910; the lithosphere being either 'continental' or 'oceanic' according to their crustal composition.  Geologists in the early 20th century abbreviated these to "sial" and "sima" (based on their characteristic elements).  Most considered that such units were fixed in their current position, relative to each other and to the Earth's axis of rotation, since primordial times. 

Testing this view was difficult and generally considered unnecessary.  The mobility of the oceanic lithosphere was particularly difficult to investigate as most of its main features were unknown until the 1940s when shipboard acoustic sounding and seismic arrays began to enable relatively easy access.  Hence most early evidence for lateral motions was confined to the"sial", thereby "Continental Drift" was just becoming seriously considered when the basic properties of the ocean basins were beginning to emerge. 

Even a casual glance at Henry Frankel's intensely researched tomes on 'The Continental Drift Controversy' (2012) shows the many scores of individuals involved in establishing that large lateral motions of continental plates had occurred.  As in most sciences in the 21st Century, we stand on the shoulders of giants, although it is often timely "small" contribution that began to set the scene.  Jan Hospers defined the stratigraphy of a polarity sequence in Icelandic lavas in 1953.   Ron Mason's 1958 enthusiasm for studying 'uninteresting'  marine magnetic anomalies (and Roger Revelle's sponsorship) turned out to be vital and, in the same year, Ron Girdler identified the central crest magnetic anomaly in the Gulf of Aden as being due to a linear vertical dyke bounded by reversely magnetised dykes. A major trigger was Marie Tharp's recognition of the mid Atlantic Rift Valley in 1952 but only de-classified in 1962. 

Tharp afg

This was fundamental to Hess's (picture, right) conception of the current sea floor spreading model.  (The full Marie Tharp story is very revealing about the then prevailing attitude to both "mobilism" and feminism!)  Evidence from other disciplines was also vital.  The Vine-Matthews marine anomaly patterns were matched to the radiometrically dated global polarity time scales, but such dating was only possible thanks to John Reynolds in 1960.  Paul Roberts established that geomagnetic reversals could occur in a core convective system - and so on - but the contributions of "Fixist" geoscientists should not be denigrated.  Our knowledge of the Earth's interior owes much to Harold Jefferies;  Maurice Ewing determined much of the crustal structures of the oceans;  Ted Ringwood was vital to understanding of the chemical nature and properties of the mantle.  Elevation changes remain important for climate change and land bridges remain of major importance for migration routes. 

Thus the studies into the kinematics of plate motions in 1967 were only possible because of the work of numerous geoscientists in a range of fields and often with differing views.  Obviously this does not detract from the work being undertaken then or later, but we still know little about the mechanisms involved.  Plate tectonics is commonly said to be "due to mantle convection", but this gives no clear understanding of the actual mechanisms involved.  Nor does the acceptance of Plate Tectonics necessarily validate the assumptions that were made when promoting it.  To take just one example, Teddy Bullard, in determining total oceanic heat flow, had to extrapolate downwards from temperature measurements obtained using short surface probes. 

Picture: Bullard's Fit.  See article by Douglas Palmer, 2015.

lkjTo do this, he assumed that the mantle temperate at the base of the lithosphere was close to the solidus of olivine, ~1200°C, based on the decrease in seismic S-wave velocities these depths.  As a first approximation, such a temperature is understandable but the asthenospheric creep rate is only consistent with much lower temperatures, possibly as low as 750°C  (Tozer, 1973), suggesting that S-wave seismic velocity decrease is more likely to be due to dehydration.  Even a temperature of 850-950°C would mean that the heat loss from the oceanic mantle would be lower than originally evaluated, making it the same as for the sub-continental mantle.  In turn, the total amount of radiogenic heat producing elements in the oceanic mantle would be the same as beneath the continents, i.e., about one third less than current estimates.  This would also make the oceanic "helium deficiency" (Anderson 2007) normal, with no need for unknown reservoirs. 


However, while this trail of possibilities needs verification, it illustrates the importance of such checks.  This would include the effect of volatiles escaping from the outer core, in addition to the heat, as the inner core solidifies.  Similarly, our understanding of subduction processes is still in its infancy.  To quote Henry Frankel, "Plate tectonics is a kinematic theory that says nothing about mechanism; it is the geometrification of geology and blind to mechanisms." (IV 61-62).  The new name does not yet make it a paradigm shift as defined by Thomas Khun.  It is no time to sit down on any laurels, they may be far too mobile!

I should end here, but I am also concerned about the lack of recognition for one of the great giants.  Apart from his established contribution to radiometric dating, Arthur Holmes also evaluated that the evidence for continental motion had potential credibility as early as 1917.  On January 28th 1928 he presented a model of the lateral motion of the continents, and ocean basins, on the back of mantle-wide convective cells with new ocean floor being generated and old floor being digested.  His model is almost exactly the same as the one proposed by Menard in 1962 -  34 years later. 

Picture, right: Hank Frankel receives the Sue Tyler Friedman Medal 2013 for his work on the history of Plate Tectonics.  David Shilston, President.  Photo: Ted Nield

dghjBoth models differ from that of Hess, the current model, because he was the only one to create new ocean crust entirely at the crests of the Mid Ocean Rises.  When Holmes made his suggestion there was no knowledge of this 80,000 km feature.  In 1944, he provided a synthesis of the geological data then in supporting what has become Plate Tectonics and duplicated his model, requesting it be tested as new information became available.  No one bothered to test the model of an elderly "mobilist".  He should now be recognized for his incredible insight. 

Don Tarling

Author’s note

The references are too voluminous to be given here, but can be found in H R Frankel, The Continental Drift Controversy, I-IV, 2012, Cambridge Univ. Press; except for Tozer, D.C. The Concept of a Lithosphere, Geofiz. Int., 13, 363-388, 1973, and Anderson, D.L. New Theory of the Earth, Cambridge Univ. Press, 2007


* Don Tarling is Emeritus Prof Geophysics in the School Geography, Earth & Environmental Sciences, University of Plymouth, PL4 8AA, UK.