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Guide to Reading
In Chapter 3 the author argued that there are good reasons to believe continents move; in this chapter he explains how and why they move. Great scientific minds found plate tectonics mind boggling at first—the acceptance of plate tectonics theory required an intellectual revolution—so try not to feel intimidated by the material. In fact, you may even find this chapter easier than the preceding one. Why? Because understanding why something is believed (Chapter 3) can be more demanding than understanding what is believed (Chapter 4). Wegener and the other pioneers of plate tectonics did the hard work: they developed a truly new idea. To do this required creativity, insight, and some moments of brilliance. Then they offered it to their peers. This took nerve. Compiling the proof needed to persuade a skeptical public took hard work and perseverance, with some anguish thrown in. But understanding plate tectonics is beautifully simple. Stripped to its barest essentials, the theory contends that Earth’s outer surface is split into pieces, or plates, that slowly shift around in relation to each other. Plates can move away from each other (divergence), toward each other (convergence), or past each other (transform motion). With all these huge masses shifting around, it’s not surprising that there are huge consequences, and these usually occur at plate boundaries, where Earth’s surface may be deformed, built up, or destroyed—all on a grand geologic scale.
Naturally there are details to be studied as well. Some of them weren’t recognized and studied until after the theory was accepted; some were part of the evidence used to develop the theory. (You’ll recognize the latter because they were introduced in Chapter 3.) This chapter will introduce you to or let you take a closer look at these details.
The chapter begins with a discussion of the types of plate boundaries and the features associated with them, including:
- earthquake belts
- mid-ocean ridges
- trenches and the subduction that takes place there
- strange and numerous offset segments of the midocean ridge and the transform faults that bracket them
- the far fewer transform faults that cross land, such as the infamous San Andreas Fault
Next the author calls attention to two special types of boundaries, where the action is not caused by the usual plate-against-plate motion. The first of these is the triple junction, where three plates meet in a point. The second is the hotspot phenomenon, illustrated by many exotic places, like Yellowstone National Park and the Hawaiian Islands.
This chapter explains that plate boundaries do not remain unchanged forever. Instead, old boundaries disappear, as illustrated by India’s collision with Asia to produce the Himalayas, and new ones appear, as illustrated by the rifting that produced both the East African Rift Valley and the Basin and Range Province of the U.S. West. Exactly how does all of this happen? The author concludes the chapter by discussing the probable explanations for actual mechanisms of plate motion and by explaining how we are able to determine the velocity of plates’ motions.
In his closing remarks for this chapter, the author reminds us that, directly or indirectly, plate tectonics is the key to understanding just about everything geologic. |
