Plate Tectonics and the Earth’s Geography Lecture 11
You look at a map of the world, and what you see is what looks like a jigsaw puzzle whose pieces have been slowly moved apart.
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n the previous lecture, we saw that the early Earth was very different from today’s Earth, and distinctly less friendly to life. How did the Earth acquire today’s geography, a geography that has profoundly shaped the course of human history? Two ¿gures, one German and one American, will play vital roles in our understanding of how the geography of today’s Earth was constructed. What they showed was that the Earth’s surface also has a history and has changed profoundly over time. To understand modern ideas about the history of the Earth’s surface, it will help to contrast them with more traditional ideas. Traditionally, geologists assumed that, though mountains might rise (through processes such as earthquakes or volcanic activity) and fall (by erosion), the basic geography of the Earth’s surface was ¿xed. The idea that the Earth’s surface had changed was ¿rst proposed seriously by a German meteorologist, Alfred Wegener (1880–1930). In 1912, Wegener published a book called The Origins of Continents and Oceans, in which he proposed a theory that would come to be known as “continental drift.” Wegener argued that the Earth’s continents had once been joined in a single supercontinent called Pangaea. What evidence did he offer for this revolutionary idea, which contradicted most of the basic assumptions of contemporary geologists? The ¿rst modern world maps, created early in the 16th century, showed that the continents seemed to ¿t together like pieces of a jigsaw puzzle, particularly across the Atlantic Ocean. Wegener identi¿ed geological formations of a similar date and composition in West Norway, East Greenland, much of Britain, Northwest Africa, and the eastern seaboard of the U.S. This made sense only if all these regions had once been joined together. An Austrian geologist, Eduard Suess (1831–1914), had already proposed that the southern 49
