Continental Drift: Not Even Wrong - Four Revolutions in the Earth Sciences - page 80

drift not from such facts as the jigsaw-puzzle fit of the continents or the distribu-
tion of Glossopteris but from chemistry and physics. “All that is necessary to start
[convection] currents,” he wrote, “is that the adjacent regions of the substratum
should be unequally heated . . . and that more heat should be generated at some
depth beneath the continents than can escape through the overlying rocks by con-
duction.” 6 Yet, he said, the attendees at the AAPG symposium had “entirely over-
looked” the possibilities of convection currents. 7
In a report on his address to the Glasgow Geological Society, Holmes's logic
was unrelenting: “For sound physical reasons it becomes as impossible to 'sink'
a continent as to sink an iceberg. We know that the sites of 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 ma-
terial has moved away sideways.” But sideways movement pointed to “continental
drift on a scale of the same order of that advocated by Wegener.” Understating the
opposition, Holmes noted that “many geologists have hesitated to accept this con-
sistent and straightforward reading of the rocks” because of the lack of a driving
mechanism. 8 But the mechanism could be the “slow but overwhelmingly powerful
currents [that] must have been generated in the underworld at various times in the
earth's history.” 9
In his TGSG paper, Holmes described in detail how convection might cause
drift, illustrating his model with a diagram that has become a classic. 10 He ex-
plained that the different temperatures under continents and ocean basins would
produce “a system of ascending currents somewhere within a continental region,
spreading out of the top in all directions towards the cooler peripheral regions. The
downward currents would become strongest beyond the continental edges where
weaker currents from the oceanic regions would be encountered.” 11
FIGURE 8 . The seafloor spreads. Source : A. Holmes, “Radioac? vity and Earth Movements,”
Transac? ons of the Geological Society of Glasgow 18 (1929). With permission of the Geologic-
al Society of Glasgow.