Chapter 2

The importance of rigidity


At the same time as modelling heat flow and gravitational effects around ridges, McKenzie was absorbing the new wave of more precise seismological data that showed earthquakes confined to narrow belts. Few or no quakes took place across large areas of the Earth, particularly across the oceans. He saw that these regions could be treated as rigid and undeforming, subject to simple geometrical analysis. But the scale of these low seismicity regions on the Earth was such that the geometry needed to be that of a sphere: Euler’s Theorem, used in the Bullard fit, proved to be the key.

“Computation of Best Fit” by Jim E Everett which has been annotated by Edward ‘Teddy’ Bullard* [likely to have been used for Everett’s PhD thesis ‘The Fit of the Continents around the Atlantic’ (1965) and the paper Bullard, E and J E Everett & A Gilbert Smith, “The Fit of the Continents around the Atlantic”, Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences, 258: 41-51 (1965), see also LDGSL/1107/A/7, May 1969]