The Pacific Plate is almost entirely oceanic, but it does include the part of California west of the San Andreas Fault. Similarly the South American Plate extends across the western part of the southern Atlantic Ocean, while the European and African plates each include part of the eastern Atlantic Ocean. The North American Plate includes most of North America, plus half of the northern Atlantic Ocean. The fact that plates include both crustal material and lithospheric mantle material makes it possible for a single plate to be include both oceanic and continental crust. The upward convective motion of hot mantle material generates temperatures that are too high for the existence of a significant thickness of rigid lithosphere at the same time that the plates are falling away from each other (Figure 4.17). Image source.Īt spreading centres, the lithospheric mantle is relatively thin. Tectonic plates consist of lithosphere, which includes the crust and the lithospheric (rigid) part of the mantle. It deforms as the plates move, rather than locking them in place. Plates move along the lithosphere-asthenosphere boundary, because the asthenosphere is relatively weak. Even though the plates are in constant motion, and move in different directions, there is never a significant amount of space between them. The plates are made up of crust and lithospheric mantle (Figure 4.24). The explanation is that plates rotate as they move the North American Plate, for example, rotates counter-clockwise, while the Eurasian Plate rotates clockwise.īoundaries between the plates are of three types: divergent (moving apart), convergent (moving together), and transform (moving side by side). Plates move as rigid bodies, so it may seem surprising that the North American Plate can be moving at different rates in different places. The North American Plate is one of the slowest, averaging ~1 cm/y in the south up to almost 4 cm/y in the north. The Pacific Plate is the fastest, moving at more than 10 cm/y in some areas, followed by the Australian and Nazca Plates. Rates of motions of the major plates range from less than 1 cm/y to more than 10 cm/y. Plate motions can be tracked using Global Positioning System (GPS) data from different locations on Earth’s surface. Figure 4.23 A detailed map of Earth’s tectonic plates. ![]() ![]() The Juan de Fuca Plate is actually three separate plates (Gorda, Juan de Fuca, and Explorer), all moving in the same general direction but at slightly different rates. There are also numerous small plates (e.g., Juan de Fuca, Nazca, Scotia, Philippine, Caribbean), and many very small plates or sub-plates. The major plates are Eurasian, Pacific, Indian, Australian, North American, South American, African, and Antarctic plates. By the end of 1967, Earth’s surface had been mapped into a series of plates (Figure 4.23). The ideas of continental drift and sea-floor spreading became widely accepted by 1965, and more geologists started thinking in these terms. 4.4 Plates, Plate Motions, and Plate-Boundary Processes
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