Abstract
In the 1960s, geology was transformed by the paradigm of plate tectonics. The 1965 paper of Bullard, Everett and Smith was a linking transition between the theories of continental drift and plate tectonics. They showed, conclusively, that the continents around the Atlantic were once contiguous and that the Atlantic Ocean had grown at rates of a few centimetres per year since the Early Jurassic, about 160 Ma. They achieved fits of the continental margins at the 500 fathom line (approx. 900 m), not the shorelines, by minimizing misfits between conjugate margins and finding axes, poles and angles of rotation, using Euler's theorem, that defined the unique single finite difference rotation that carried congruent continents from contiguity to their present positions, recognizing that the real motion may have been more complex around a number of finite motion poles. Critically, they were concerned only with kinematic reality and were not restricted by considerations of the mechanism by which continents split and oceans grow. Many of the defining features of plate tectonics were explicit or implicit in their reconstructions, such as the torsional rigidity of continents, Euler's theorem, closure of the Tethyan ocean(s), major continental margin shear zones, the rapid rotation of small continental blocks (Iberia) around nearby poles, the consequent opening of small wedge-shaped oceans (Bay of Biscay), and misfit overlaps (deltas and volcanic piles) and underlaps (stretched continental edges). This commentary was written to celebrate the 350th anniversary of the journal Philosophical Transactions of the Royal Society.
Highlights
The pre-1965 harbingersThe general similarity of the shapes of the eastern and western shorelines of South America and Africa, respectively, was noticed and recorded by Francis Bacon at a time when the detailed geography of the shorelines was ill-determined and by Antonin Snider [1] when the existence of the continental shelf and its margin were unknown
A harbinger of plate tectonics: a commentary on Bullard, Everett and Smith (1965) ‘The fit of the continents around the Atlantic’
In the 1960s, geology was transformed by the paradigm of plate tectonics
Summary
The general similarity of the shapes of the eastern and western shorelines of South America and Africa, respectively, was noticed and recorded by Francis Bacon at a time when the detailed geography of the shorelines was ill-determined and by Antonin Snider [1] when the existence of the continental shelf and its margin were unknown. Implicit but not explicit was that the two continental margins and, by inference, the continents must have been torsionally rigid to maintain the fitting edge. (all continents fitted together), there appears the huge, westward-narrowing and terminating gap of the Tethyan ocean complex, which closed as a result of the Central and North Atlantic opening at different times around different poles of rotation. Some small continental blocks, such as Iberia, have rotated by up to 40◦ during periods as short as 10 Myr around nearby rotation poles to open small, wedge-shaped oceans (Carey’s sphenochasms) such as the Bay of Biscay These were profoundly important reconstructions, results and conclusions that led directly to the formulation of the theory of plate tectonics shortly afterwards, and to its geological implications and corollaries. It led to similar studies using the same techniques (e.g. [26,27,28])
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