What Do We Mean by Plate Tectonics?
The paleomagnetic proof of continental drift (plate tectonics) and the discovery of sea-floor spreading set off a scientific revolution in geology in the 1960s and 1970s. Geologists realised that many of their existing interpretations of global geology, based on the premise that the positions of continents and oceans remain fixed in position through time, were simply wrong! Researchers dropped what they were doing and turned their attention to studying the broader implications of continental drift and sea-floor spreading. It became clear that these phenomena required that the outer shell of the Earth was divided into rigid plates that moved relative to each other. New studies clarified the meaning of a plate, defined the types of plate boundaries, constrained plate motions, related plate motions to earthquakes and volcanoes, showed how plate interactions can explain mountain belts and seamount chains, and outlined the history of past plate motions. From these, the modern theory of plate tectonics evolved. Below, we first describe lithosphere plates and their boundaries, and then outline the basic principles of plate tectonics theory.
The Concept of a Lithosphere Plate
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| Nature of the lithosphere and its behaviour. |
We learned earlier that geoscientists divide the outer part of the Earth into two layers. The lithosphere consists of the crust plus the top (cooler) part of the upper mantle. It behaves relatively rigidly, meaning that when a force pushes or pulls on it, it does not flow but rather bends or breaks (figure above a). The lithosphere floats on a relatively soft, or “plastic,” layer called the asthenosphere, composed of warmer ( 1280°C) mantle that can flow slowly when acted on by a force. As a result, the asthenosphere convects, like water in a pot, though much more slowly.
Continental lithosphere and oceanic lithosphere differ markedly in their thicknesses. On average, continental lithosphere has a thickness of 150 km, whereas old oceanic lithosphere has a thickness of about 100 km (figure above b). (For reasons discussed later in this chapter, new oceanic lithosphere at a mid-ocean ridge is much thinner.) Recall that the crustal part of continental lithosphere ranges from 25 to 70 km thick and consists largely of low-density felsic and intermediate rock. In contrast, the crustal part of oceanic lithosphere is only 7 to 10 km thick and consists largely of relatively high-density mafic rock (basalt and gabbro). The mantle part of both continental and oceanic lithosphere consists of very high-density ultramafic rock (peridotite). Because of these differences, the continental lithosphere “floats” at a higher level than does the oceanic lithosphere.
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| The location of plate boundaries and the distribution of earthquakes. |
The lithosphere forms the Earth’s relatively rigid shell. But unlike the shell of a hen’s egg, the lithospheric shell contains a number of major breaks, which separate it into distinct pieces. As noted earlier, we call the pieces lithosphere plates, or simply plates. The breaks between plates are known as plate boundaries (figure above a). Geoscientists distinguish twelve major plates and several microplates.
The Basic Principles of Plate Tectonics
With the background provided above, we can restate plate tectonics theory concisely as follows. The Earth’s lithosphere is divided into plates that move relative to each other. As a plate moves, its internal area remains mostly, but not perfectly, rigid and intact. But rock along plate boundaries undergoes intense deformation (cracking, sliding, bending, stretching, and squashing) as the plate grinds or scrapes against its neighbours or pulls away from its neighbours. As plates move, so do the continents that form part of the plates. Because of plate tectonics, the map of Earth’s surface constantly changes.
Identifying Plate Boundaries
How do we recognize the location of a plate boundary? The answer becomes clear from looking at a map showing the locations of earthquakes (figure above b). Recall from Chapter 1 that earthquakes are vibrations caused by shock waves that are generated where rock breaks and suddenly slips along a fault. The epicentre marks the point on the Earth’s surface directly above the earthquake. Earthquake epicentres do not speckle the globe randomly, like buckshot on a target. Rather, the majority occur in relatively narrow, distinct belts. These earthquake belts define the position of plate boundaries because the fracturing and slipping that occurs along plate boundaries generates earthquakes. Plate interiors, regions away from the plate boundaries, remain relatively earthquake-free because they do not accommodate as much movement. While earthquakes serve as the most definitive indicator of a plate boundary, other prominent geologic features also develop along plate boundaries.
Note that some plates consist entirely of oceanic lithosphere, whereas some plates consist of both oceanic and continental lithosphere. Also, note that not all plates are the same size (figure above c). Some plate boundaries follow continental margins, the boundary between a continent and an ocean, but others do not. For this reason, we distinguish between active margins, which are plate boundaries, and passive margins, which are not plate boundaries. Earthquakes are common at active margins, but not at passive margins. Along passive margins, continental crust is thinner than in continental interiors. Thick (10 to 15 km) accumulations of sediment cover this thinned crust. The surface of this sediment layer is a broad, shallow (less than 500 m deep) region called the continental shelf, home to the major fisheries of the world.
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| The three types of plate boundaries differ based on the nature of relative movement. |
Geologists define three types of plate boundaries, based simply on the relative motions of the plates on either side of the boundary (figure above a–c). A boundary at which two plates move apart from each other is a divergent boundary. A boundary at which two plates move toward each other so that one plate sinks beneath the other is a convergent boundary. And a boundary at which two plates slide sideways past each other is a transform boundary.
Credits: Stephen Marshak (Essentials of Geology)
