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the exciting concept of continents in motion held their interest. Others viewed continental drift as a solution to previously unexplainable observations. Plate Tectonics: The New Paradigm EEA R T H S C I E N C E Forces Within Plate Tectonics During the years that followed Wegener’s proposal, major strides in technology permitted mapping of the ocean floor. Moreover, extensive data on seismic activity and Earth’s magnetic field became available. By 1968, these developments led to the unfolding of a far more encompassing theory than continental drift, known as plate tectonics. The implications of plate tectonics are so far-reaching that this theory is today the framework within which to view most geologic processes. Earth’s Major Plates According to the plate tectonics model, the uppermost mantle, along with the overlying crust, behaves as a strong, rigid layer, known as the lithosphere. This outermost shell overlies a weaker region in the mantle known as the asthenosphere. Further, the lithosphere is broken into numerous segments called plates, which are in motion and are continually changing in shape and size. As shown in Figure 7.8, seven major lithospheric plates are recognized. They are the North American, South American, Pacific, African, Eurasian, Australian-Indian, and Antarctic plates. The largest is the Pacific plate, which is located mostly within the Pacific Ocean. Notice from Figure 7.8 that several of the large plates include an entire continent plus a large area of seafloor (for example, the South American plate). This is a major departure from Wegener’s continental drift hypothesis, which proposed that the continents moved through the ocean floor, not with it. Note also that none of the plates are defined entirely by the margins of a continent. Intermediate-sized plates include the Caribbean, Nazca, Philippine, Arabian, Cocos, and Scotia plates. In addition, there are over a dozen smaller plates that have been identified but are not shown in Figure 7.8. The lithospheric plates move relative to each other at a very slow but continuous rate that averages about 5 centimeters (2 inches) per year. This movement is ultimately driven by the unequal distribution of heat within Earth. Hot material found deep in the mantle moves slowly upward and serves as one part of our planet’s internal convection system. Concurrently, cooler, denser slabs of oceanic lithosphere descend into the mantle, setting Earth’s rigid outer shell into motion. Ultimately, the titanic, grinding movements of Earth’s lithospheric plates generate earthquakes, cre- Chapter 7 Plate Tectonics 199 ate volcanoes, and deform large masses of rock into mountains. Plate Boundaries Lithospheric plates move as coherent units relative to all other plates. Although the interiors of plates may experience some deformation, all major interactions among individual plates (and therefore most deformation) occur along their boundaries. In fact, the first attempts to outline plate boundaries were made using locations of earthquakes. Later work showed that plates are bounded by three distinct types of boundaries, which are differentiated by the type of movement they exhibit. These boundaries are depicted at the bottom of Figure 7.8 and are briefly described here: 1. Divergent plate boundaries (constructive margins)—where two plates move apart, resulting in upwelling of material from the mantle to create new seafloor (Figure 7.8A). 2. Convergent plate boundaries (destructive margins)—where two plates move together, resulting in oceanic lithosphere being thrust beneath an overriding plate, eventually to be reabsorbed into the mantle (Figure 7.8B). Convergence can also result in the collision of two continental plates to create a mountain system. 3. Transform fault boundaries (conservative margins)—where two plates grind past each other without the production or destruction of lithosphere (Figure 7.8C). Each plate is bounded by a combination of these three types of boundaries. For example, as shown in Figure 7.8, the Nazca plate has a divergent zone on the west, a convergent boundary on the east, and numerous transform faults, which offset segments of the divergent boundary. Although the total surface area of Earth does not change, individual plates may diminish or grow in area depending on the distribution of convergent and divergent boundaries. The Antarctic and African plates are almost entirely bounded by spreading centers and hence are growing larger. By contrast, the Pacific plate is being consumed into the mantle along its northern and western flanks and is therefore diminishing in size. Furthermore, new plate boundaries can be created in response to changes in the forces acting on these rigid slabs. For example, a relatively new divergent boundary is located in Africa, in a region known as the East African Rift Valleys. If spreading continues there, the African plate will split into two plates separated by a new ocean basin. At other locations, plates carrying continental crust are presently moving toward each other. Eventually, these continents may collide and be sutured together. Thus, the boundary that once separated two plates disappears as the plates become one. The result

200 Unit Three Forces Within ▲ ▲ ▲ East African Rift Urals ▲ ▲ ▲ Arabian plate African plate ▲ ▲ ▲ Mid-Indian Ridge Southwest Indian Ridge ▲ Eurasian plate ▲ Himalayas India ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ Australian-Indian plate Southeast Indian Ridge Antarctic plate Figure 7.8 A mosaic of rigid plates constitutes Earth’s outer shell. Baikal Rift ▲ ▲ ▲ ▲ Japan Arc ▲ ▲ ▲ Philippine plate ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ North American plate Mariana Arc ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ Kermadec Arc ▲ ▲ ▲ ▲ ▲ Pacific plate Alpine Fault Aleutian Arc Emperor-Hawaiian Chain ▲ ▲ ▲ ▲ Tonga Arc ▲ ▲ ▲ ▲ ▲ ▲

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