Plate tectonics became a central organizing paradigm for geology over 30 years ago. The tenets of plate tectonics theory have been so thoroughly assimilated by the scientific population, and their implications so extensively pursued, that in some ways this report could be considered a description of Earth science in the “post-plate tectonics era.” The questions regarding plate tectonics that have now come to the fore have less to do with the soundness of the theory than with the even more basic questions of why Earth has plate tectonics in the first place and how closely it is related to other unique aspects of Earth—the abundant water, the continent-ocean elevation dichotomy, the existence of life. We do not know whether it is possible to have one aspect without the others or how exactly they are interdependent. Can these questions help us understand why Earth is different from the other terrestrial planets?

What Is Plate Tectonics?

Plate tectonics is the representation of Earth’s outermost rock layers in terms of a small number of rigid spherical caps or plates. These plates are in relative motion, and their boundaries form the seismic (earthquake-producing) and tectonic (volcanic and mountain-building) belts of the world. The plates interact at three types of boundaries—divergent, convergent, and transform—all marked by the occurrence of earthquakes (Figure 2.10). At divergent boundaries, plates move away from one another as new crust forms between them. The most common type of divergent boundary occurs at the midocean ridge system, which takes the expression of a 40,000-km-long submarine mountain range that rises about 2.5 km above the average ocean floor (Figure 2.11a). At convergent boundaries of oceanic plates, one oceanic plate bends and subducts into the mantle. Convergent boundaries are the loci of the major deep earthquakes (>100 km below the surface); the principal volcanic belts, notably the “ring of fire” around the Pa-

FIGURE 2.10 Locations of earthquakes of Richter magnitude 5 or greater for the period 1991 to 1997. These earthquakes mark the edges of Earth’s tectonic plates. SOURCE: Romanowicz (2008). Reprinted by permission from Macmillan Publishers Ltd.: Nature, copyright 2008.

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