CONTINENTAL DRIFT
The shapes of the continents suggest that they could be joined like pieces of a jigsaw puzzle. This observation led to the suggestion, made in 1924, that in the distant past there had been one super-continent (pangaea) that broke up, with the various sections drifting apart to form the present-day continents. This concept, called continental drift is supported by the theory of plate tectonics6 - a theory that offers a comprehensive explanation of the distribution of continents, mountain chains, volcanoes, earthquake sites, and ocean trenches.
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PLATE TECTONIC THEORY
Earth is the only planet that has plate tectonics. Models of the Earth have shown that the lithosphere (crust + mantle) is too thick for smaller planet, while the gravitational force for larger planet would squeeze any plates together. Even when the size criterion is met, it needs a way to crack the lithosphere. Numerous computer models fail to simulate conditions in which a break in the crust would spontaneously occur. It is suggested that perhaps asteroid or comet strikes may have led to the creation of the subduction process as shown in Figure 09-06b.
TYPES OF PLATES
There are four types of plate boundaries as shown in Figure 09-06a:
Divergent boundaries - where new crust is generated as the plates pull away from each other. In mid-ocean, this movement results in seafloor spreading and the formation of ocean ridges; on continents, crustal spreading can form rift valleys.
Convergent boundaries - where crust is destroyed as one plate dives under another. In mid-ocean, this causes ocean trenches, seismic activity, and arcs of volcanic islands. Where oceanic crust is subducted beneath continental crust or when continents collide, land may be uplifted and mountains formed.
Transform boundaries - where crust is neither produced nor destroyed as the plates slide horizontally past each other such as the San Andreas fault (Figure 09-06c). Such movement produces earthquakes.
Plate boundary zones - broad belts in which boundaries are not well defined and the effects of plate interaction are unclear. Because plate-boundary zones involve at least two large plates and one or more micro-plates caught up between them, they tend to have complicated geological structures and earthquake patterns.
The movement of the Earth is induced by the convection currents of molten magma deep down in a zone called the mantle. These currents rise, then turn sideways below the solid crust. The crust is divided into nine major plates in the lithosphere (Figure 09-06e, the 9th one is uncertain). Slowly, at rates of a few centimeters per year, the rising current moves these plates. If the plate moves over a localized hot spot (Figures 09-06a and c) in the mantle, volcano will form until the plate carries it away from this source of magma.
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sanandreas fault
For example, the Hawaiian group of volcanic islands, which lie in the middle of the Pacific plate, has been built up while the plate has been drifting over a hot spot (Figure 09-06c). But volcanoes occur most commonly along the boundaries of crustal plates (Figure 09-06f). Crustal movement on continents may result in earthquakes, while movement under the sea bed can
lead to tidal waves (tsunami).