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This indicates a tropical setting. Furthermore, unlike trees in colder climates, the tree trunks lacked growth rings. Growth rings do not form in tropical plants, because there are minimal seasonal fluctuations in temperature. Wegener believed that a better explanation for the paleoclimatic regimes he observed is provided by fitting together the landmasses as a supercontinent, with South Africa centered over the South Pole (Figure 7.7). This would account for the conditions necessary to generate extensive expanses of glacial ice over much of the Southern Hemisphere. At the same time, this geogra- North America Appalachian Mountains South America British Isles Caledonian Mountains Scandinavia Africa Chapter 7 Plate Tectonics 197 Figure 7.5 These sketches by John Holden illustrate various explanations for the occurrence of similar species on landmasses that are presently separated by vast oceans. (Reprinted with permission of John Holden) phy would place the northern landmasses nearer the tropics and account for their vast coal deposits. Wegener was so convinced that his explanation was correct that he wrote, “This evidence is so compelling that by comparison all other criteria must take a back seat.” How does a glacier develop in hot, arid Australia? How do land animals migrate across wide expanses of open water? As compelling as this evidence may have been, 50 years passed before most of the scientific community would accept it and the logical conclusions to which it led. North America Greenland South America Europe A. B. Figure 7.6 Matching mountain ranges across the North Atlantic. A. The Appalachian Mountains trend along the eastern flank of North America and disappear off the coast of Newfoundland. Mountains of comparable age and structure are found in the British Isles and Scandinavia. B. When these landmasses are placed in their predrift locations, these ancient mountain chains form a nearly continuous belt. These folded mountain belts formed roughly 300 million years ago as the landmasses collided during the formation of the supercontinent of Pangaea. Africa
198 Unit Three Forces Within ?S TUDENTS SOMETIMES ASK... Someday will the continents come back together and form a single landmass? Yes. It is very likely that the continents will come back together, but not anytime soon. Since all of the continents are on the same planetary body, there is only so far a continent can travel before it collides with other continents. Based on current plate motions, it appears that the continents may meet up again in the Pacific Ocean. Recent research suggests that the continents may form a supercontinent about once every 500 million years or so. Since it has been about 200 million years since Pangaea broke up, we have only about 300 million years before the next supercontinent is completed. The Great Debate: Rejecting an Hypothesis Wegener’s proposal did not attract much open criticism until 1924 when his book was translated into English. From this time on, until his death in 1930, his drift hypothesis encountered a great deal of hostile criticism. To quote the respected American geologist T. C. Cham- Figure 7.7 A. The supercontinent Pangaea showing the area covered by glacial ice 300 million years ago. B. The continents as they are today. The shading outlines areas where evidence of the old ice sheets exists. A. B. Equator Equator Ice mass berlin, Wegener’s hypothesis takes considerable liberty with our globe, and is less bound by restrictions or tied down by awkward, ugly facts than most of its rival theories. Its appeal seems to lie in the fact that it plays a game in which there are few restrictive rules and no sharply drawn code of conduct. One of the main objections to Wegener’s hypothesis stemmed from his inability to provide a mechanism that was capable of moving the continents across the globe. Wegener proposed that the tidal influence of the Moon was strong enough to give the continents a westward motion. However, the prominent physicist Harold Jeffreys quickly countered with the argument that tidal friction of the magnitude that’s needed to displace the continents would bring Earth’s rotation to a halt in a matter of a few years. Wegener also proposed that the larger and sturdier continents broke through the oceanic crust, much like ice breakers cut through ice. However, no evidence existed to suggest that the ocean floor was weak enough to permit passage of the continents without themselves being appreciably deformed in the process. Although most of Wegener’s contemporaries opposed his views, even to the point of open ridicule, a few considered his ideas plausible. For these few geologists who continued the search for additional evidence, North America South America Africa India Antarctica Eurasia Tethys Sea Australia
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