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Plate tectonics

The new science of plate tectonics is developed in the 1960s. For the first time there is a convincing explanation for the process of continental drift.

The solid surface of the earth (the lithosphere) is now known to be divided into about nine large slabs, referred to as 'plates', and a number of smaller ones. With the infinite slowness of geological time, these plates move in relation to each other. And they move in various ways.

If plates move apart under the ocean, new ocean bed is created in seafloor spreading. If one plate slides under another (usually at a coast line), the continental plate may be pushed up into a ridge of mountains; the seafloor plate will tuck down under the other and its material will return to the molten levels below. And if two plates are on a long-term collision course, as India has been towards central Asia, the result will be the thrusting up of a large mass of material - in this case the Himalayas.

Where two plates are grinding past each other, the result will be periodic earthquakes as they judder into a new position. The San Andreas Fault in California is the best known example of this kind of uneasy relationship.

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