Seeing The Past In Present Events

Even though the theory of plate tectonics had many supporters when it was finally introduced, there were still a great many scientists who

Figure 6.1 Examples of divergent (top), convergent (middle), and transform (bottom) plate boundaries

J. Tuzo wilson: Champion of the Theory

John Tuzo Wilson was a Canadian geologist who graduated from the University of Toronto in 1930 with a degree in geophysical studies. He received his Ph.D. from Princeton University in 1936 and went to work for the Geological Survey of Canada. During World War II, he served with the Royal Canadian Engineers. When the war ended, he returned to the University of Toronto, where he became a professor of geophysics.

In the late 1950s and early 1960s, Wilson followed the research of Hess, Heezen, and others and became an outspoken supporter of seafloor spreading. One of the questions that interested him the most concerned the unusual faults that cut across the mid-ocean ridges and trenches. Most geologists believed that these "transform faults" happened after the ridges had formed, but Wilson disagreed. He believed that the transform faults had always been part of the ocean crust and that the crust itself simply moved as a solid, unbroken piece. He used the term "plate" to describe large sections of Earth's crust and suggested that the entire surface of Earth was made up of about two dozen plates of different sizes.

Within a few years, earthquake data collected along these areas showed that Wilson was right. The center areas of plates had very few earthquakes, while plate boundaries were extremely active. His work on transform faults provided the missing piece in Hess's theory of seafloor spreading, and it directly led to the development of the theory of plate tectonics.

were skeptical. These doubters were eager to put the theory to the test. One of the first questions they had concerned the age of the crust. Radioactive age dates from the continents showed that Earth was at least 4 billion years old. The oldest crust in the ocean does not appear to be more than 200 million years old. Were there any oceans before the breakup of Pangaea? If so, what happened to the missing crust?

According to plate tectonic theory, any ocean crust that existed before the breakup of Pangaea would have either been recycled back into the mantle or have been plastered onto the side of a continent. The evidence to support this can be seen in the presence of the many numerous marine fossils dating back over 500 million years ago that can be found high in the mountains along the edge of continents, right where they should be if two plates had collided.

The question is, if ocean crust keeps getting recycled, why does the same process not happen with continental crust? The answer is related to the relative density of the two types of crust. Since ocean crust is made of heavier elements (or sima), it is denser and will easily subduct under the less dense continental crust (or sial). Also, the oceanic crust is thinner, so it offers less resistance to subduction than the continental crust. When two plates containing continental crust collide, instead of one subducting under the other, they both rise up. This is exactly what can be seen happening in the Himalayan mountains today. In fact, based on recent surveys, it appears that many of the mountains of the Himalayas are still getting taller. It also explains how many inland mountain chains such as the Appalachians and the Alps were created. Both of these mountain chains were located in areas that were caught between two pieces of continental crust that had collided in the distant past.

One of the most important features of plate tectonic theory is that the continents have not always looked the way they do today. Based on the reconstructions done by Wegener and others, most geologists agree that all the continents were joined about 200 million years ago in a single mass called Pangaea. The best reconstruction of this great continent was published in 1965 by Edward Bullard of Cambridge University in England.

Using a computer (which was a very new device at the time) and data supplied by undersea maps, Bullard did something very clever: Rather than fit the continents back together along their coastlines, he fit them together along the continental shelves. Continental shelves are really extensions of the continents into the oceans that lie under shallow water. This new "Bullard fit" removed many of the gaps that were present in Wegener's original map of Pangaea. It left little doubt that the continents had been joined together in a single landmass.

Before there was a Pangaea, however, the continents were again separated. The shapes of those earlier continents did not look like the present-day continents though. Geologists are still trying to work out the continental reconstructions from these earlier times, but there does seem to be some type of cycle at work. Every 250 million years or so, for some reason, it appears that the continents separate and then rejoin in a different pattern. One thing that is known for sure is that the process of continents joining together and separating is happening today, because we can see it in action. One area that has geologists' attention is the Red Sea and the Gulf of Aden. This area is known as a rift zone, and it is an active spreading center. Given enough time, the Red Sea may become an ocean as wide as the Atlantic.

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