The Theory of Plate Tectonics
List the major differences between Earth’s lithosphere and asthenosphere and explain the importance of each in the plate tectonics theory.
Following World War II, oceanographers equipped with new marine tools and ample funding from the U.S. Office of Naval Research embarked on an unprecedented period of oceanographic exploration. Over the next two decades, a much better picture of large expanses of the seafloor slowly and painstakingly began to emerge. From this work came the discovery of a global oceanic ridge system that winds through all the major oceans.
In other parts of the ocean, more new discoveries were being made. Studies conducted in the western Pacific demonstrated that earthquakes were occurring at great depths beneath deep-ocean trenches. Of equal importance was the fact that dredging of the seafloor did not bring up any oceanic crust that was older than 180 million years. Further, sediment accumulations in the deepocean basins were found to be thin, not the thousands of meters that were predicted. By 1968 these developments, among others, had led to the unfolding of a far more encompassing theory than continental drift, known as the theory of plate tectonics (tekto = to build).
Rigid Lithosphere Overlies Weak Asthenosphere
According to the plate tectonics model, the crust and the uppermost, and therefore coolest, part of the mantle constitute Earth’s strong outer layer, the lithosphere (lithos = stone). The lithosphere varies in both thickness and density, depending on whether it is oceanic or continental lithosphere (Figure 2.9).
Oceanic lithosphere is about 100 kilometers (60 miles) thick in the deep ocean basins but is considerably thinner along the crest of the oceanic ridge system—a topic we will consider later. By contrast, continental lithosphere averages about 150 kilometers (90 miles) thick but may extend to depths of 200 kilometers (125 miles) or more beneath the stable interiors of the continents.
Further, the composition of both the oceanic and continental crusts affects their respective densities.
Oceanic crust is composed of basalt, rich in dense iron and magnesium, whereas continental crust is composed largely of less dense granitic rocks. Because of these differences, the overall density of oceanic lithosphere (crust and upper mantle) is greater than the overall density of continental lithosphere. This important difference will be considered in greater detail later in this chapter.
The asthenosphere (asthenos = weak) is a hotter, weaker region in the mantle that lies below the lithosphere (see Figure 1). The temperatures and pressures in the upper asthenosphere (100 to 200 kilometers [60 to 125 miles] in depth) are such that rocks at this depth are very near their melting temperatures and, hence, respond to forces by flowing, similar to the way a thick liquid would flow. By contrast, the relatively cool and rigid lithosphere tends to respond to forces acting on it by bending or breaking but not flowing. Because of these differences, Earth’s rigid outer shell is effectively detached from the asthenosphere, which allows these layers to move independently.
Earth’s Major Plates
The lithosphere is broken into about two dozen segments of irregular size and shape called lithospheric plates, or simply plates, that are in constant motion with respect to one another (Figure 2).
Seven major lithospheric plates are recognized and account for 94 percent of Earth’s surface area: the North American, South American, Pacific, African, Eurasian, Australian-Indian, and Antarctic plates. The largest is the Pacific plate, which encompasses a significant portion of the Pacific basin. Each of the six other large plates consists of an entire continent, as well as a significant amount of oceanic crust. Notice in Figure 3 that the South American plate encompasses almost all of South America and about one half of the floor of the South Atlantic.
Note also that none of the plates are defined entirely by the margins of a single continent. This is a major departure from Wegener’s continental drift hypothesis, which proposed that the continents move through the ocean floor, not with it.
Intermediate-sized plates include the Caribbean, Nazca, Philippine, Arabian, Cocos, Scotia, and Juan de Fuca plates. These plates, with the exception of the Arabian plate, are composed mostly of oceanic lithosphere.
In addition, several smaller plates (microplates) have been identified but are not shown in Figure 3.
One of the main tenets of the plate tectonics theory is that plates move as somewhat rigid units relative to all other plates. As plates move, the distance between two locations on different plates, such as New York and London, gradually changes, whereas the distance between sites on the same plate—New York and Denver, for example— remains relatively constant. However, parts of some plates are comparatively “weak and can become fragmented” such as southern China, which is literally being squeezed as the Indian subcontinent rams into Asia.
Because plates are in constant motion relative to each other, most major interactions among them (and, therefore, most deformation) occur along their boundaries. In fact, plate boundaries were first established by plotting the locations of earthquakes and volcanoes. Plates are bounded by three distinct types of boundaries, which are differentiated by the type of movement they exhibit. These boundaries are depicted in Figure 3 and are briefly described here:
• Divergent plate boundaries—where two plates move apart, resulting in upwelling and partial melting of hot material from the mantle to create new seafloor (see Figure 3A)
• Convergent plate boundaries—where two plates move together, resulting either in oceanic lithosphere descending beneath an overriding plate, eventually to be reabsorbed into the mantle, or possibly in the collision of two continental blocks to create a mountain belt (see Figure 3B)
• Transform plate boundaries—where two plates grind past each other without the production or destruction of lithosphere (see Figure 3C)
Divergent and convergent plate boundaries each account for about 40 percent of all plate boundaries.
Transform boundaries, or faults account for the remaining 20 percent. In the following sections we will summarize the nature of the three types of plate boundaries.