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Roci dating

These units, called in their molten form, constitute major crustal additions.

By contrast, crustal destruction occurs at the margins of two colliding continents, as, for example, where the subcontinent of India is moving north over Asia.

It is only by correlations that the conditions on different parts of Earth at any particular stage in its history can be deduced.

Continents move, carried on huge slabs, or plates, of dense rock about 100 km (62 miles) thick over a low-friction, partially melted zone (the (i.e., long, narrow belts at which one plate descends beneath another).

Where this occurs at the edge of a continent, as along the west coast of North and South America, large mountain chains develop with abundant volcanoes and their subvolcanic equivalents.

Unlike ages derived from fossils, which occur only in sedimentary rocks, absolute ages are obtained from minerals that grow as liquid rock bodies cool at or below the surface.

When rocks are subjected to high temperatures and pressures in collide, certain datable minerals grow and even regrow to record the timing of such geologic events.

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Radiometric dating and certain other approaches are used to provide absolute chronologies in terms of years before the present.

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