Plate Tectonics

It seems strange at first to think that the continents, which we think of as huge, stable, and still blocks of the earth's crust, have been moving over the face of the Earth, breaking up and reforming, coming together and moving apart, ever since they formed. It seemed downright preposterous in the 1930s, when the German Alfred Wegener first proposed the theory. . . How do we know that the continents have drifted?

• Geographical evidence.You can see on a world map that the outlines of Africa and South America fit together fairly well. In fact, if you were to fit them together using, not their present coastlines, but their contours at 300 meters below sea level, the fit would be nearly perfect. Continent shape is one line of evidence for drift. Large geologic features on several continents may indicate that these continents were once joined. For example, the Appalachian Mountains of the eastern United States and the highland hills of Scotland were formed at the same time and contain similar rocks...

  Large belts of rocks that have been folded provide evidence for continent collisions in the past. The best example of this is the Himalayas, which arose when India, which had been a separate island, collided with Asia.

• Biological evidence. Continents that were once connected may share distinctive groups of organisms that are not found anywhere else. This may be due either to dispersal -- i.e. the organisms have migrated from one continent to the other -- or vicariance -- the organisms have not migrated, but have been separated by the breakup of the continents. If the organisms shared by two continents are known not to be able to disperse easily, and if many different kinds of organism are shared between two continents, the hypothesis that the two continents were once one becomes more probable.

  Example: Australia and South America share southern beech trees (genus Nothofagus), araucaria "pine" trees (family Araucariaceae), marsupial mammals. . . Other taxa are common to Australia, South America, and Africa, such as ratite birds (ostrich, moa, kiwi, rhea, and their relatives), lungfish, peripatopsid onychophorans, . . . Taken all together, this data suggests a pattern that can be explained most simply by the hypothesis that these continents were once joined and separated after their shared taxa had evolved.

• Geological evidence. The formation of certain types is known to be restricted to certain latitudes. Finding such rocks at unusual latitudes may provide evidence that the continents on which they are found have moved since the rocks were laid down.

  One example is carbonate. Extensive deposits of calcium carbonate are being laid down today in tropical areas, such as coral reefs and carbonate muds. Finding carbonate deposits of late Precambrian and Cambrianage in northern Sibera suggests that Siberia was much closer to the equator at this time.