Introduction Continental Drift Seafloor Spreading Plate Tectonics Divergent Plate Boundaries Convergent Plate Boundaries Transform Plate Boundaries Summary
This curious world we inhabit is more wonderful than convenient; more beautiful than it is useful; it is more to be admired and enjoyed than used. Henry David Thoreau
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Earth's lithosphere is divided into mobile plates. Plate tectonics describes the distribution and motion of the plates. The theory of plate tectonics grew out of earlier hypotheses and observations collected during exploration of the rocks of the ocean floor.
You will recall from a previous chapter that there are three major layers (crust, mantle, core) within the earth that are identified on the basis of their different compositions (Fig. 1). The uppermost mantle and crust can be subdivided vertically into two layers with contrasting mechanical (physical) properties. The outer layer, the lithosphere, is composed of the crust and uppermost mantle and forms a rigid outer shell down to a depth of approximately 100 km (63 miles). The underlying asthenosphere is composed of partially melted rocks in the upper mantle that acts in a plastic manner on long time scales. The asthenosphere extends from about 100 to 300 km (63-189 miles) depth. The theory of plate tectonics proposes that the lithosphere is divided into a series of plates that fit together like the pieces of a jigsaw puzzle. Although plate tectonics is a relatively young idea in comparison with unifying theories from other sciences (e.g., law of gravity, theory of evolution), some of the basic observations that represent the foundation of the theory were made many centuries ago when the first maps of the Atlantic Ocean were drawn. Geographer Abraham Ortellus noted the similarity between the coastlines of Africa, Europe and the Americas in the third edition of his Thesaurus Geographicus, published in 1596. Ortellus, adapting Plato's story of the demise of Atlantis, suggested that America was “torn away” from Europe and Africa and that the “projecting parts of Europe and Africa” would fit the “recesses” of America. Such observations were little more than idle speculation until Austrian climatologist Alfred Wegener used the fit of opposing coastlines as one of the pieces of evidence to support his hypothesis of continental drift. Continental drift proposed that the continents were once assembled together as a single supercontinent Wegener named Pangaea. Wegener was unable to suggest a suitable mechanism to explain the motion of the continents across Earth's surface and his hypothesis received 2
Figure 1. The outermost part of Earth is divided into two mechanical layers, the lithosphere and asthenosphere.
relatively little support until technology revealed the secrets of the ocean floor. Scientists gradually amassed additional data that would resurrect Wegener's hypothesis over 30 years after his death. By the 1960s the building blocks were in place to support a new hypothesis, Seafloor spreading, that would provide the mechanism for continental drift. Together these concepts would become the theory of plate tectonics. The theory of plate tectonics provides an example of the evolution of scientific thought. The first two sections of the chapter reveal the basic observations that were used to make predictions on the geologic processes that shaped the face of Earth. The theory of plate tectonics links Earth’s internal processes to the distribution of continents and oceans; it is the big picture view of how the earth works. Plate tectonics reveals that the lithosphere is divided into eight major pieces ("plates") with several smaller plates (Fig. 2). The plates are mobile, moving in constant, slow motion measured in rates of centimeters per year. The movements of plates over millions of years resulted in the opening and closure of oceans and the formation and disassembly of continents. Plates interact...