FIGURE 2.12 Map of the age of the ocean floor, with age in million years before present (Ma). Solid black lines are midocean ridges. SOURCE: Müller et al. (1997). Copyright 1997 American Geophysical Union. Reproduced with permission.

are significantly hotter and more deformable than rocks at a comparable depth under the oceans.

The greater thickness, lower density, and more deformable character of continental crust cause it to behave differently than oceanic crust. The different behaviors of oceanic and continental crust influence the nature of plate boundaries. Boundaries that are within oceanic crust tend to be narrow, except in cases where the relative motion between plates is very slow (Royer and Gordon, 1997; Zatman et al., 2001). Boundaries that are between continents tend to be broad because the continental crust is more deformable and much more difficult to subduct, although there is a large variation of deformation styles within the continental crust. Similarly, boundaries that juxtapose oceanic and continental crust exhibit a wide range of deformation styles, from wide to narrow, sometimes changing from one to the other over time. Approximately 10 to 15 percent of Earth’s surface is made of broadly deforming regions, while the rest is comprised of the rigid plates characteristic of the plate tectonics model.

The plate tectonics model provides a coherent and simple explanation for many important features of Earth’s surface that are not predicted simply by mantle convection. Perhaps its most elegant achievement is to explain the relative youth and other major features of the oceanic crust, such as why the oceanic crust becomes older, and the ocean deeper, with distance from midocean ridges (Figures 2.11a and 2.12). This age-depth correlation is almost entirely explained by the aging and cooling of the plate as it moves away from the ridge. The oceanic crust is relatively young because it sinks back into the mantle via subduction zones. Plate tectonics also accounts for “continental drift” and allows us to reconstruct where continents were in the past and where they will go in the future. But it still leaves us with significant puzzles about fundamental large-scale features of Earth’s crust: the occurrence of hot spots (Question 4), the existence and durability of continental crust, and the complex structure of large mountain ranges where continents collide. It also leaves open the question of why some areas have suffered broadly distributed deformation (e.g., in the Basin and Range Province the distance between what is now Salt Lake City and the West Coast has doubled in about 30 million years), rather than behaving rigidly as is common of plate interiors.

Why Plate Tectonics?

Plate tectonics is a kinematic notion—a description of how things move. Although thermal convection in the

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