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:
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Not only was the apparent fit of the continents real, he maintained, but
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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.
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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.
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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. |