Plate Tectonics(Part-III)
The Birth of Plate Tectonics
The new hypotheses of the early 1960s explained several puzzling sets of observations. All that remained was a synthesis of these hypotheses.
The synthesis began in 1965 when Tuzo Wilson introduced the term plate for the broken pieces of the Earth's lithosphere. In 1967, Jason Morgan proposed that the Earth's surface consists of 12 rigid plates that move relative to each other. Two months later, Xavier Le Pichon published a synthesis showing the location and type of plate boundaries and their direction of movement.
Since the mid-1960s, the plate tectonic model has been rigorously tested. Because the model has been successfully tested by numerous methods, it is now called the plate tectonic theory and is accepted by almost all geologists.
Types of Plate Motion
The ways that plates interact depend on their relative motion and whether oceanic or continental crust is at the edge of the lithospheric plate. Plates move away from, toward, or slide past each other. Geologists call these divergent, convergent, and transform plate boundaries.
At a divergent plate boundary lithospheric plates move away from each other. The mid-Atlantic Ridge, a topographically high area near the middle of the Atlantic Ocean, is an example of a divergent plate boundary.
At a convergent plate boundary, lithospheric plates move toward each other. The west margin of the South American continent, where the oceanic Nazca Plate is pushed toward and beneath the continental portion of the South American Plate, is an example of a convergent plate boundary.
At a transform plate boundary, plates slide past each other. The San Andreas fault in California is an example of a transform plate boundary, where the Pacific Plate slides past the North American Plate. Location of Plate Boundaries
Earthquakes and volcanoes, evidence of unrest in the Earth, help locate the edges of plates. Earthquakes are distributed in narrow, linear belts that circle the Earth
Some of these belts have only shallow (0-20 miles; 0-35 km) earthquakes, like the mid-Atlantic and east Pacific ridges. In contrast, earthquakes in other belts, like western South America and south-central Asia, are at shallow, intermediate (20-45 miles; 30-70 km), and deep (45-450 miles; 70-700 km) levels.
Volcanoes are also distributed in long belts that circle the Earth. A dramatic example is the line of volcanoes that circles most of the Pacific Ocean. This belt is known as the "Ring of Fire" because it is the site of frequent volcanic eruptions. 
The distribution of earthquakes and volcanoes coincides at most locations. The Ring of Fire is an excellent example. Geologists believe that areas of intense geologic activity, indicated by earthquakes, volcanoes, and/or mountain building, mark the boundaries between lithospheric plates. The distribution of earthquakes, volcanoes, and mountain ranges define 7 large plates and 20 smaller plates. The Nazca and Juan de Fuca Plates consist of only oceanic lithosphere. The Pacific Plate is mostly oceanic lithosphere only a small slice of continental lithosphere in southern California and Baja Mexico. Most of the other plates consist of both oceanic and continental lithosphere.
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