Plate Tectonics and Continents Essay

Continental Drift and Plate Tectonics

Background
Continental Drift is the theory that continents move slowly about the Earth's surface, changing their positions relative to one another and to the poles of the earth. In the past the theory has been discussed but not generally accepted, most geologists believing the continents to be fixed in place and subject only to vertical movements, such as those observed during mountain uplift. In recent years, however, a sound body of evidence in support of a modified form of the drift theory has been found. Ideas are becoming precise and unified, with emphasis on a moving, evolving ocean floor. The new theory is called plate tectonics (Wilson, 1996).
Theory of Continental Drift
Alfred Wegener proposed that continents float on denser underlying interior of the Earth and periodically break up and drift apart. He proposed that all the continents were once joined together 200 million years ago. This supercontinent was called Pangaea meaning “all lands.” Pangaea covered at least 40% of the Earth’s surface and the majority of it was in the Southern Hemisphere. To show how the continents were arranged, New York City would have been on or near the equator and would have been covered in lush green vegetation. Africa, Australia, and India would have been covered under glacial ice near the South Pole. Around 180 million year ago the continents started to drift apart. This continental drift formed a number of continents. The shapes of the continents, patterns of present day life, similarities in fossils and rocks, and evidence of past climate at odds with climate at present locations support the theory of continental drift. Sir Francis Bacon was the first to note outlines of the continents and saw that if arranged in a certain way they would fit together like a jig-saw puzzle. For example, the west coast of Africa and the east coast of South America have a cut-out that would allow the continents to fit together. Wegener found evidence of fossils of Mesosaurus. They were found only in Southern Africa and Brazil. These two areas are about 3,000 miles apart. The only explanation for the fossils to be in both locations is that the two continents were once connects.
Paleomagnetic Studies Paleomagnetism is a permanent record of past changes in the Earth’s magnetic field. It is formed as mafic lava cools which turns into small crystals of magnetite (Geology; pg. 377). We can use Paleomagnetism to estimate how far a rock (and therefore its plate) has moved with respect to latitude (Geology; pg. 378-80). By studying rocks of Wrangell Mountains in Alaska, geologists have been able to determine that these rocks probably formed near the warm equator and have moved to their present location by plate motion (Geology; pg 380).
Earth’s Magnetic Field Earth’s magnetic field penetrates and surrounds the planet. The field extends into space 37,000 miles. It is perpendicular to the Earth’s surface at the planets magnetic poles and parallel to the surface at the magnetic equator. The North and South poles can move and even switch. However, this only applies to the poles. When the poles switch, the Earth does not flip only the poles do. The magnetic poles change overtime keeping with the geographic poles. At the present time, the magnetic poles move two degrees westward every year and at intervals averaging half-million years the magnetic field reverses. The magnetic

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