Chapter 2: The Way the Earth Works: Plate Tectonics
Study Plan
This photo from space shows the Sinai Peninsula, separated from Egypt to the west and the Arabian Peninsula to the East by rifts, narrow belts where the crust has stretched and broken apart.
Credit: NASA
Guide to Reading
This chapter introduces plate tectonics. In a sense, this chapter also sets the stage for the rest of the text, because plate tectonics theory, a relative newcomer to geologic thought, supplies the fundamental explanation for so many geologic processes that it has become a unifying principle in the modern study of geology.
The author begins by drawing a picture of Alfred Wegener who, in the early 1900s, proposed the idea of continental drift, the idea that the continents have moved around in relation to one another. His arguments in support of continental drift included:
- the fit of the continents
- paleoclimatic studies that showed evidence of past glaciation, coal deposits, ancient reef deposits, ancient sand dunes, and salt beds that make no sense in today's world climate belts
- the occurrence of the same fossils on lands now separated by oceans
- the matching of geologic units (distinct assemblages of rocks) on lands now separated by oceans
- changes in the earth's magnetic field over time (paleomagnetism)
- changes in the sea floor-the shape of its surface, types and ages of its rock, heat flow within it, and sea-floor earthquakes-all of which support the idea of sea-floor spreading
- working through the reasons for accepting new theories, such as the meaning and significance of paleomagnetism, provides practice in good scientific thinking
- the gradual acceptance of plate tectonics theory provides an excellent example of the process by which scientific knowledge advances as new evidence and better instruments and techniques are introduced
- plate tectonics was a revolutionary idea that caused profound changes in the study of geology and thus merits your thoughtful study and understanding
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. This chapter will introduce you to or let you take a closer look at these details.
The chapter includes 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
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.
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