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C H A P T E R7 Plate Tectonics 191
192 Unit Three Forces Within Will California eventually slide into the ocean as some predict? Have continents really drifted apart over the centuries? Answers to these questions and many others that have intrigued geologists for decades are now being provided by an exciting theory on large-scale movements taking place within Earth. This theory, called plate tectonics, represents the real frontier of the Earth sciences, and its implications are so far-reaching that it can be considered the framework from which most other geological processes should be viewed. Early in this century, most geologists thought that the geographic positions of the ocean basins and continents were fixed. During the last few decades, however, vast amounts of new data have dramatically changed our understanding of the nature and workings of our planet. Earth scientists now realize that the continents gradually migrate across the globe. Where landmasses split apart, new ocean basins are created between the diverging blocks. Meanwhile, older portions of the seafloor are carried back into the mantle in regions where trenches occur in the deep-ocean floor. Because of these movements, blocks of continental material eventually collide and form Earth’s great moun- Figure 7.1 Waputik Range, Banff National Park, Alberta, Canada. (Photo © by Carr Clifton) tain ranges (Figure 7.1). In short, a revolutionary new model of Earth’s tectonic* processes has emerged. This profound reversal of scientific understanding has been appropriately described as a scientific revolution. Like other scientific revolutions, considerable time elapsed between the idea’s inception and its general acceptance. The revolution began early in the twentieth century as a relatively straightforward proposal that the continents drift about the face of Earth. After many years of heated debate, the idea of drifting continents was rejected by the vast majority of Earth scientists. However, during the 1950s and 1960s, new evidence rekindled interest in this proposal. By 1968, these new developments led to the unfolding of a far more encompassing theory than continental drift—a theory known as plate tectonics. Continental Drift: An Idea Before Its Time The idea that continents, particularly South America and Africa, fit together like pieces of a jigsaw puzzle originated with improved world maps. However, little *Tectonics refers to the deformation of Earth’s crust and results in the formation of structural features such as mountains.
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