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Plate Tectonics

  By the 1950s, the debate about continental drift had generated a good deal of evidence that the continents might have moved. This evidence included the fit of coastlines, structures, rocks and fossils and the palaeomagnetic deposits with non N-S orientation. But most geologists remained sceptical because of the absence of a mechanism. Even evidence of high heat-flows at the mid-ocean ridges and of gravity anomalies along the ocean trenches, which seemed to some consistent with the presence of convection cells, was not regarded as sufficiently persuasive. But improved ability to investigate the sea floor brought in dramatic new evidence.

In 1960, an American geologist, Harry Hess, circulated a draft paper suggesting a number of lines of evidence which were consistent with the idea that the oceanic crust might be spreading away from the ocean ridges. In 1961, the Scripps Institute of Oceanography published the results of a magnetic survey which showed the existence of narrow bands of rock with alternating magnetic polarity, the bands being parallel to the mid-Atlantic ridge. In 1963, Vine and Matthews connected the argument to the evidence and presented more detailed evidence to show that the pattern of magnetic reversals in the ocean floor, parallel to the Juan de Fuca ridge in the north Pacific, was exactly consistent with the known timetable of reversals in the Earth\'s magnetic field which would have been fixed into basalts as they rose, solidified and then spread three cm per year. Over the next few years several similar studies were published, and in 1969 drilling of the ocean floor provided strong support by showing that the age of sediments resting on the basalt layer increased away from the ridges. It was also significant that no ocean-floor basalt had been found to exceed 190 million years in age: this suggested that the oceanic crust must be being destroyed as well as created. Indeed, as South America has ocean ridges on both sides, with crust in the Atlantic moving west, and crust in the Pacific moving east, it is clear that there must be a zone of crust destruction. The volcanically and seismically active range of the Andes and the parallel ocean trench suggested that the Atlantic plate might be being pushed up by the Pacific plate, but the fate of the Pacific plate required another step.

By the mid-1960s the evidence for ocean floor spreading was so strong that the idea of horizontal movement of the crust was becoming generally accepted. The scene was set for the statement of a theory which would account for all the available evidence. This was first attempted by W.J. Morgan of Princeton in 1968 and many other geologists were quick to present similar schemes.

The basic ideas are as follows: the Earth\'s surface is made up of seven major plates, five minor ones and a small number of smaller ones. Most large plates include both continental and oceanic crust, but the Pacific plate is almost entirely oceanic. These plates are being simultaneously created and destroyed and the crust, including the continents, is moved horizontally in the process. Plates have three kinds of boundary:

Constructive These are mainly the ocean ridges and have the features already described. One such boundary is in the Red Sea, where the Arabian plate is moving away from the African plate. This is a situation similar to that in the South Atlantic 100 million years ago.

Destructive Once the idea of plate movement was accepted, it was realized that it made sense of earlier information, including observations by Wadati and Benioff that near ocean trenches there are zones where earthquakes occur at progressively greater depths. These are now thought to indicate subduction of ocean crust—that is, it dips down diagonally below the adjacent plate, which is usually continental, and returns to the mantle. There is also frictional heating causing volcanic activity: thus subduction is the explanation of a phenomenon known for centuries, the ‘fiery ring of the Pacific’, the concentration of most of the world\'s great volcanoes in northwest and southeast coastal zones of the Pacific.

Conservative In some parts of the world, adjacent plates are moving past each other without either creation or destruction of crust. These are called conservative because material is conserved, but they are seismically active because friction stops movement for a time until sufficient stress builds up to cause sudden movement and release of shock waves experienced as earthquakes. The San Andreas fault, which parallels the coast of California, is the best-known example of such a plate boundary. As the plates move past each other, rocks from one may be scraped off on to the other to form very complex areas known as ‘terranes’: this concept explained a number of areas where traditional geology had failed to explain the observed structures.

The theory of plate tectonics is a major revolution in the way we see our world. The resistance to the idea of continental drift is a measure both of its importance and its improbability. The story of its acceptance is a good example of the way science works, seeking new evidence and then generating new theories to make sense of the new knowledge as well as of earlier knowledge. But no theory is perfect and a great deal of work is going on to fill in the detail and to account for evidence which does not seem to fit in. In turn, these exceptions may provoke some future theorist to propose new interpretations. PS



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