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Evidence of the Continental Drift

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    Introduction

                Continental drift is defined as the motion of the earth’s continents over a defined period of time. The theory of continental drift was first proposed by Alfred Wegener in 1912. It is thought that all the continents were once joined together in one super-continent, named Pangaea. The rest of the earth was covered with one huge ocean. Due to the inadequately explained geologic forces causing upwelling of molten material, Pangaea split apart into the major continents we have today. The upwelling material was thought to be a result of convection forces within the earth. This material reached the surface at oceanic ridges and returned to the earth at subducting oceanic trenches. Up until recently this theory was thought to be very controversial and was largely rejected because of an inadequate explanation for the forces required to move the continents. But the overwhelming evidence indicates the continents are moving and were at one time joined together in some fashion. It is the objective of this paper to prove this theory.

    Existing Theory of Continental Drift

                Wegener’s theory was based in part on what appeared to him to be the seemingly near-perfect remarkable fit of the South American and African continents, first noted by Abraham Ortelius three centuries earlier (Le Grand, HE. 1988). Wegener was also intrigued by the occurrences of unusual geologic structures and of plant and animal fossils found on the matching coastlines of South America and Africa, which at present is widely separated by the Atlantic Ocean. He reasoned that it was physically impossible for most of these organisms to have swum or have been transported across the vast oceans in the prehistoric eras. But to him, it was the presence of identical fossil species along the coastal parts of Africa and South America which to him was the most compelling evidence that the two continents were once joined.

    In Wegener’s mind, the drifting of continents after the break-up of Pangaea (Figure 1) explained not only the matching fossil occurrences but also the evidence of dramatic climate changes on some continents. Other mismatches of geology and climate included distinctive fossil ferns discovered in now-polar regions, and the occurrence of glacial deposits in present-day arid Africa.

                Known as the “theory of continental drift,” it would spark and ignite a new way of viewing the Earth. But at the time Wegener introduced his theory, the scientific community firmly believed the continents and oceans to be permanent features on the Earth’s surface. As he expected, his proposal was not well received, even though it seemed to agree with the scientific information available at the time (Oreskes, N 1999). A loophole in Wegener’s theory was that it could not satisfactorily answer the most fundamental question raised by his critics: What kind of forces could be strong enough to move such large masses of solid rock over such great distances? Wegener suggested that the continents simply “plowed” through the ocean floor, but Harold Jeffreys, a noted English geophysicist, argued correctly that it was physically impossible for a large mass of solid rock to plow through the ocean floor without breaking up (Frankel H, 1985). Wegener’s hypothesis received support through the controversial years from South African geologist Alexander Du Toit as well as from Arthur Holmes. The idea of continental drift did not become widely accepted even as theory until the late 1950s. By the 1960s, geological research conducted by Robert S. Dietz, Bruce Heezen, and Harry Hess, along with a rekindling of the theory including a mechanism by J. Tuzo Wilson led to widespread acceptance of the theory among geologists. Recent evidence from ocean floor exploration and other studies has rekindled interest in Wegener’s theory, and lead to the development of the theory of plate tectonics (Le Grand HE, 1988).

    Evidences of the Continental Drift

    Pieces of the Jigsaw Puzzle

                Like a jigsaw puzzle: The continents look as if they were pieces of a giant jigsaw puzzle that could fit together to make one giant super-continent. For example; The bulge of Africa fits the shape of the coast of North America while Brazil fits along the coast of Africa beneath the bulge. Figure 1 shown the earth at approximately 225 million years ago, the Permian period. Take note that the continents fit like a jigsaw puzzle.

    Figure 1. The Permian Period

                Figure 2 shows the earth at approximately 150 million years ago, the Jurassic period. Note that the continents have started to seemingly drift apart still revealing the areas where the jigsaw puzzle used to snugly fit.

    Figure 2. The Jurassic Period

                Figure 3 is the earth at the present day period. Still oceans apart, one may still see the shapes of the continents resembling the original jigsaw pieces, only this time, distorted.

    Figure 3. Present day Earth

    More Concrete Evidences

                Similar flora and fauna: Wegener noted that plant fossils of late Paleozoic age found on several different continents were quite similar. This suggests that they evolved together on a single large land mass. He was intrigued by the occurrences of plant and animal fossils found on the matching coastlines of South America and Africa, which are now widely separated by the Atlantic Ocean. He reasoned that it was physically impossible for most of these organisms to have traveled or have been transported across the vast ocean. To him, the presence of identical fossil species along the coastal parts of Africa and South America was the most compelling evidence that the two continents were once joined.

    Ice and Rocks Match: The findings of rock belts in Africa and South America are the same type. These broad rock belts found at the end of these continents are matched when joined together. Wegener was aware that a continental ice sheet covered parts of South America, southern Africa, India, and southern Australia about 300 million years ago. Glacial striations on rocks show that glaciers moved from Africa toward the Atlantic Ocean and from the Atlantic Ocean onto South America. Such glaciation’s is most likely if the Atlantic Ocean was missing and the continents joined. If the continents were cold enough so that ice covered the southern continents, why is no evidence found for ice in the northern continents? Simple! The present northern continents were at the equator at 300 million years ago. The discovery of fossils of tropical plants, in the form of coal deposits in Antarctica (Stewart, JA. 1990), led to the conclusion that this frozen land previously must have been situated closer to the equator, in a more temperate climate where lush, swampy vegetation could grow.

    The Theory of Plate tectonics

    A modern update of the old ideas of Wegener about “plowing” continents, called Plate tectonics, accommodates continental motion through the mechanism of seafloor spreading. It is a relatively new scientific concept, introduced some 30 years ago, but it has revolutionized our understanding of the dynamic planet upon which we live. The theory has unified the study of the earth by drawing together many branches of the earth sciences, from paleontology (the study of fossils) to seismology (the study of earthquakes). It has provided explanations to questions that scientists had speculated upon for centuries — such as why earthquakes and volcanic eruptions occur in very specific areas around the world, and how and why great mountain ranges like the Alps and Himalayas formed.

    The explanation is that new rock is created by volcanism at mid-ocean ridges and returned to the Earth’s mantle at ocean trenches. Molengraaf of the Delft Institute of Technology (Davis RA, 1977) proposed a recognizable form of seafloor spreading in order to account for the opening of the Atlantic Ocean as well as the East Africa Rift. Arthur Holmes, who was an early supporter of Wegener, suggested that the movement of continents was the result of convection currents driven by the heat of the interior of the Earth, rather than the continents floating on the mantle. In the words of Carl Sagan, “it is more like the continents are being carried on a conveyor belt than floating or drifting” (Sagan C, 1997). The ideas of Molengraaf and of Holmes led to the theory of plate tectonics, which replaced the theory of continental drift, and became the accepted theory in the 1960’s.

    However, acceptance was gradual. Nowadays it is universally supported; but even in 1977 a textbook could write the relatively weak: “a poll of geologists now would probably show a substantial majority who favor the idea of drift” and devote a section to a serious consideration of the objections to the theory (Davis RA, 1977).

    References

    Davis, RA. (1977) Principles of Oceanography, 2nd edition, Addison-Wesley

    Frankel, H (1985). “The continental drift debate,” in Resolution of Scientific Controversies:

    Theoretical Perspectives on Closure, edited by A. Caplan and A. T. Engleheart. Cambridge University Press: Cambridge.

    Le Grand, HE. (1988). Drifting Continents and Shifting Theories. Cambridge University Press:

    Cambridge.

    Oreskes, N (1999). The Rejection of Continental Drift: Theory and Method in American Earth Science. Oxford University Press: New York.

    Sagan, Carl. (1997) The Demon-Haunted World, Science As a Candle in the Dark, Ballantine Books.

    Stewart, J.A. (1990). Drifting Continents and Colliding Paradigms: Perspectives on the Geoscience Revolution. Indiana University Press: Bloomington.

     

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