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GEOLOGY INDEX
 STUDY QUESTIONS
PLATE TECTONICS 

Today, it is clear that it is the interaction of lithospheric plates that shape the major features and influence the processes of the upper earth. And yet, the concept of plate tectonics, the ultimate acceptance of which marked perhaps the greatest revolution in the geological sciences, and which seems so obvious today, was rather slow in unfolding. This concept, that the upper part of the earth is active and mobile, first came from the notion that continents had once been joined into a single landmass; and that, over geologic time, they had drifted across the face of the earth like giant rafts to achieve today's configuration. 

CONTINENTAL DRIFT

First ideas: As a result of the voyages of discovery and the associated advances in cartography, several people noted the resemblance between the shapes of the coastlines of various continents and speculated as to why they would match so well. Francis Bacon (ca. 1620) was the first to suggest that the continents had once been joined. Buffon (ca. 1750), Snider (ca. 1858), and Suess (ca. 1890) all repeated the concept that the various continents had been united in the past. In fact Suess went on to name this hypothetical supercontinent: Gondwanaland. 

A Pioneer: Alfred Wegener 

Cohesive attempts to suggest that this match was more than coincidental were independently proposed between 1910 and 1915, first by two americans, F.B. Taylor and H.H. Baker, and then by a German meteorologist, Alfred Wegener. Both the Americans and Wegener suggested that over time, continents had moved, drifted over the surface of our globe. The American proposal had few evidential data associated with it. In contrast, Wegener supported his proposal with several lines of carefully gathered and argued evidence: 

  • Not only was the apparent fit of the continents real, he maintained, but 
  • the fossil evidence, both floral (plants) and faunal (animal) also shows that similar organisms had once existed on continents, now so far apart as to make the idea of their dispersal by migration highly improbable. 
  • Similarities in rock layers and their structures (folds, faults) on opposite sides of oceans also supported the idea that continents had been joined at one time and subsequently ripped apart. 
  • Indications of glaciations in locations now near the Equator also led him to conclude that it was the continents that had moved, and not climates that had changed.
His proposal, published in German, was largely ignored until the twenties when his book was translated into English. Criticism was instant and virulent. In geological circles, this radical theory raised a storm of controversy, the likes of which had not been seen since Darwin’s theory of evolution. Most of the arguments against his idea centered around the fact that the mechanisms he proposed as causes for the motion of continents were either inadequate or impossible. Because the rocks of the ocean floor were thought to be static, it stood to reason that if continents had moved over time, they would have had to plough through the rigid ocean floor, much like a rock forcing its way through solid concrete. There was no known mechanism that could generate such forces, and moreover there was no evidence on the ocean floor that such ploughing had actually taken place. 

Needless to say, his ideas were not accepted. Still, around 1930, a British geologist, Holmes, suggested that, conceivably, convection currents in the mantle might be a driving force capable of moving continents. Nevertheless, most geologists dismissed Wegener’s ideas as too fanciful, and his theory was generally ignored during the thirties and forties. Despite this generally negative attitude, some geologists such as DuToit, a South African, continued to accumulate evidence for continental drift.