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The Continental Drift Theory
Examine original sources related to Alfred Wegener's Continental Drift Theory. Why was this theory rejected by the majority of earth scientists in the 1920's? Compare and contrast the major features of the Continental Drift Theory and the Plate Tectonics Theory. Almost everyone who has looked at a map of the world has noticed that the continents of South America and Africa seem to fit together fairly well. An even better fit is obtained if continental margins (the underwater edge of the continent) are used. Other "fits" of this sort can be assembled as well. It is difficult to believe that this is purely accidental, yet it is equally difficult to imagine and support a theory that explains it. It is one thing to notice a possibility and another to take it seriously enough to seek evidence to support the theory, but Alfred Wegener, a German meteorologist, decided to look for such evidence. Wegener carried out several successful expeditions. He found fossil distribution patterns that were consistant between continents. He found that the species dispersed when the continents were connected and later carried to their present positions as the continents drifted apart. For example, Glossopteris, a fern, was found on the continents of South America, Africa, India, and Austrailia. If the continents are reassembled into the supercontinent Pangea, the distribution of Glossopteris can be accounted for over a much smaller contiguous geographic area. Also, Wegener theorized that if South America and Africa had been joined at some earlier time, there would be similar geological formations such as mountain ranges which would extend from one continent to another across the same boundry. Rock sequences in South America, Africa, India, Antartica, and Austailia show remarkable similarities. Wegener proposed that three distinct layers occur at each of these localities. The bottom, also the oldest, layer is called tillite and is thought to be a glacial deposit. The middle layer is composed of sandstone, shale, and coal beds. Glossopteris fossils are in the bottom and middle layers. The top layer is the youngest and is made up of lava flows. These three layers occur in the same order in areas now separated by great distances of ocean. Wegener proposed that these rock layers were made when all the continents were part of Pangea. Thus, they formed in a small contiguous area that was later broken and drifted apart. Wegener also used the distribution of specific rock types to determine climactic zones in the geological past. For example, glacial till and striations (scratches on the rocks), sand dunes, and coral reefs, indicate polar, desert, and tropical climates, respectively. He used this information to find that, unlike the present distribution, in which zones parallel the equator, the past zones occupied very different positions. This implies that the rotational pole was in a different location relative to today. Wegener offered an alternative explanation. He believed that the climate zones remained stationary and the continents drifted to different locations. The drift of the continents caused the apparent movement of the climate zones. Wegener's model was not accepted by all geologists. Some thought that dispersion by winds or ocean currents could explain the fossil species being traced to where they were found. Other geologists thought the poles might wander and the continents remained stationary. Many people thought that Wegener's evidence was insufficient. But the greatest shortcoming, at least in the eyes of American geologists, was the lack of an adequate mechanism for moving the continents. Wegener proposed that the Earth's spin caused the continents to move, plowing through the oceanic plate and producing mountains on their leading edge. Geologists at that time understood enough about the strength of rocks to know that this was highly unlikely which caused many people to find a different solution for the moving of the continents. These shortcomings became the driving force behind the introduction of the plate techtonics theory. The lack of a mechanism for moving the large continents was evident in this new theory. Deep inside the Earth's crust, at extremely high temperatures, the iron core heats the bottom of the rocky mantle. The hottest rocks near the bottom of the mantle become less dence than the cooler rocks above them, so buoyancy forces try to push the hootest rocks upward. Although the rocks in the mantle are solid, the pressures and heat are so great that the rocks can deform slowly, like hot wax. So the hot rocks begin to move upward while the cooler rocks begin to sink to the bottom. Most of the rock in the mantle moves in this broad cyclic flow. This zone where rocks are soft enough to flow is called the astenosphere. The movement of heat by convection in the astenosphere causes the rocks of the mantle to slowly move in large streams. The solid, but brittle, rocks of the lithosphere are resting directly on top of the rocks of the astenosphere. As the rocks of the astenosphere move in different directions, they carry parts of the lithosphere along. The lithosphere rocks can not stretch, so they break into pieces forming the plates. Once the plates form, they begin to act independantly of the convection flow because their cold edges tend to sink into the mantle wherever they happen to be. It is this model of movement that Alfred Wegener did not initiate that caused scientists to form a new opinion of the way continents drift.


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