GEO 111 Notes Ch 12


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CHAPTER 12 PLATE TECTONICS: A UNIFYING THEORY CHAPTER OVERVIEW The concept of continental movement was first suggested when it was noticed that Africa and South America had coastlines which appeared to be counterparts of one another, suggesting they may once have been joined and drifted apart. The hypothesis of continental drift is generally credited to Alfred Wegener. His treatment and hypothesis were the most extensive and well developed. Wegener presented paleontological and geological evidence that the continents were once united into one supercontinent which he named Pangaea. Wegener could not provide a convincing mechanism to demonstrate how the continents could have moved, and his ideas were largely ignored. Paleomagnetic studies during the 1950's revived interest in continental drift because they indicated that either the magnetic poles had wandered and each continent had its own pole (an impossibility), or the continents had moved over time. If the continents were moved into different positions relative to each other, the separate poles could be resolved into one. Magnetic surveys revealed that oceanic crust exhibited magnetic anomalies — reversals of magnetic polarity— and that these anomalies were disposed in symmetrical stripes paralleling the oceanic ridges. This indicates that new oceanic crust must be formed along the spreading ridges. Sea floor spreading is confirmed by the ages of fossils in sediments overlying oceanic crust of various ages, and radiometric dating of rocks on oceanic islands. These indicate that oceanic crust is youngest at the spreading ridges and oldest at the farthest points from the ridges. Overwhelming evidence in support of plate tectonics led to its rapid acceptance and elaboration since the early 1970's. The theory is widely accepted because it explains so many geologic phenomena, including volcanism, seismicity, mountain building, climatic changes, animal and plant distributions in the past and present, and the distributions of natural resources. For these reasons, it is known as a unifying theory. Supercontinents like Pangea form, break up, and re-form in a cycle spanning
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This note was uploaded on 06/05/2011 for the course GEO 111 taught by Professor Craig during the Spring '11 term at Owens.

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