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Roger Weller, geology instructor regional geology planetary gems
by Theresa Tate
Continental Drift: Predecessor to Plate Tectonics
The shape of the Earth as we see it today is far from the form it held in the
past; it is, in fact, constantly changing due to the process of plate tectonics,
in which fragments of the planet’s crust slowly flow over the asthenosphere,
altering the contours of the continents and oceans over time. The theory of
plate tectonics, however, is a young one which has only been fully formed in the
last few decades. Although individual researchers had long suspected the
continents were capable of lateral movement, the overriding scientific consensus
was that they could rise or sink, but were otherwise stationary.
Tectonic plates, courtesy of the U.S. Geological Survey
Until quite recently, scientists who suggested the continents were mobile rather than static were ridiculed by the rest of the scientific community; the most notable example of this is Alfred Wegener, the founder of the theory of continental drift. Wegener was already an accomplished meteorologist and astronomer when he began to notice geologic features which seemed to contradict their traditionally held scientific explanations, thus prompting him to seek alternate possibilities for their existence. When he presented his theory of continental drift to better explain these structures in 1912, not only was his theory almost universally criticized and dismissed, but he lost much of this previous acclaim, damaging his reputation until long after his death.
Although Wegener’s theory was rejected, his research was sound and his
evidentiary support was overwhelming. The first indication of a changing
planet he observed was the occurrence of similar fossils on far separated
continents; scientists at the time attributed this to the past presence of wide
land bridges which had connected the continents but had since disappeared into
the ocean, but Wegener asserted that the distance would have been too great for
either plant and animal migration or for transportation by wind and water to
properly account for the distribution seen. Noting that the edges of the
continents, particularly those in the southern hemisphere, seemed to fit
together, he suggested that these distant locations had been connected directly
as part of a supercontinent he called Pangaea (or “all earth”) rather than by
Map of Pangaea, courtesy of Kieff
In addition to fossil evidence, Wegener observed patterns in rock structures
that seemed to suggest the continents had once been parts of a single landmass.
He found that rocks in India and Africa, both in extremely warm climates today,
bore distinct characteristics that could only have been formed by the presence
of glaciers. Declaring that widespread glaciation so near the equator was
unlikely, he concluded this must have occurred when both were nearer the present
location of Antarctica.
Glacial Striations, courtesy of W. Siegmund
Wegener also cited coinciding sedimentary layers on opposite sides of the
Atlantic Ocean as evidence that the rock must have been deposited at the same
time and in the same location but then subsequently separated. When the
continents were placed together, one area of particular importance, the coal
layers in England and in the Northeastern United States, formed a single,
continuous mass; also aligning in this manner were several mountain ranges, ore
and mineral deposits, and areas of ancient rock known as continental shields.
In spite of what he considered indisputable evidence, however, Wegener’s theory
failed to gain acceptance by the majority of scientists.
Rock layers at Bryce Canyon, courtesy of R. Weller
Though still unable to attain full approval, some recognition was gained when
Wegener reversed his research procedures, using his theory to find supporting
evidence. Given the known presence of a tropical plant known as
glossopteris in Africa, South America, Australia, and India, Wegener
hypothesized that he should be able to find fossils in a particular region of
Antarctica to which he believed these had once been connected. If the
continents had been stationary, he suggested, it would have been impossible for
a tropical plant to have ever grown in the vicinity, but, on an expedition to
the area, he found fossil evidence of the plant as he had expected.
Glossopteris, courtesy of R. Weller
The primary source of skepticism toward Wegener’s theory was his inability to provide a reasonable mechanism by which the continents might move. He suggested both centrifugal force caused by the Earth’s rotation and tidal forces as possible means of mobility, but his fellow scientists rightly dismissed these possibilities because they would destroy the planet were they strong enough to move large bodies of land. Also hurting Wegener’s argument was his belief that the continents were moving through the ocean crust like large ice bergs, an action they lacked the structural integrity to accomplish.
With Wegener’s death in 1930, his theories, still opposed by most scientists, faded into obscurity until technological developments during the 1950s brought renewed interest in the subject. New methods of mapping the ocean floor allowed scientists to see the patterns of ridges and troughs, the edges of the Earth’s tectonic plates, along which rock material is created and destroyed, respectively. It was by this manner, they discovered, that the continents moved, not through the ocean floor but along with it. This new theory of plate tectonics is more complete than that of continental drift, but it confirms most of Wegener’s theory.