Continental drift

Continental drift Continental drift describes the process whereby continents move laterally over the surface of Earth in the course of geological time. Early versions of the theory, current during the first half of the twentieth century, generally viewed the continents as somehow moving through a sea of mantle. This concept never found widespread acceptance, and in particular was opposed by geophysicists who considered the mantle, on seismic evidence, to be too strong for such motion to occur. This problem was resolved with the discovery of thermally activated solid-state creep and the development of plate tectonics in the 1960s. In consequence, the phenomenon of continental movement is now widely accepted. However, because of its association with a discredited mechanism, the term ‘continental drift’ is now eschewed by some in favour of ‘plate tectonics’. It is nevertheless still useful as a term for the phenomenon of continental movement as opposed to the theory of its mechanism.

Although a number of early scholars had noted the matching shapes of the coastline flanking the Atlantic Ocean and had pondered on the origins of their congruence, it appears to have been the American glaciologist Frank Taylor who first suggested, in 1910, that the continents move about on the surface of the Earth. In 1912 the German meteorologist Alfred Wegener proposed his hypothesis of continental drift, and produced a variety of evidence in its favour. Wegener's theory received its greatest support in the southern hemisphere, where some of the consequences of continental drift after the break-up of Gondwanaland are best displayed. The South African geologist Alexander du Toit was one of its leading proponents.

There is a wide variety of geological evidence for continental drift. Perhaps the most direct is evidence of climatic variation with time, especially where it can be shown that it was not part of a global change but that different patterns of variation occurred in different continents.

A number of lines of evidence suggest that continents that are now widely separated were once joined. These include the matching of rock types, climatic regions, ancient mountain belts, and fossils across continental margins that were once contiguous. An important example is a Permo-Carboniferous glaciation which occurred approximately 300 million years (Ma) ago and affected parts of all the southern continents, leaving characteristic deposits such as tillites. When these continents are reconstructed in their pre-drift configuration, not only do the boundaries of the glaciation match across continental boundaries, but also the total extent of the glaciation forms a roughly circular area, approximately centred on the ancient pole position.

Further important evidence for continental drift comes from palaeontology. For example, some species in now separated land masses can be seen to have followed identical evolutionary paths until the time of continental break-up, and have subsequently diverged. In many instances species, particularly terrestrial ones, inhabit a restricted region bounded by climatic or geographic barriers such as seas or mountains, and one can track the break-up or collision of continents by the fossil evidence of species either becoming isolated by the creation of new barriers or freed to expand by the removal of barriers. Many good examples of the latter effect are provided by the migration of species between Asia and Australia as these two continents gradually collided.

Despite the now compelling nature of much of the geological evidence for continental drift, the most unequivocal proof of the movement of continents now surely comes from palaeomagnetism. This branch of geophysics studies the magnitude and direction of the ancient magnetic field. Several natural mechanisms can permanently magnetize rocks with a magnetic field parallel to the Earth's field at the time. The direction of this natural remanent magnetism can be determined from oriented samples, and the position of the ancient magnetic pole can be inferred.

When these ancient pole positions are plotted, it is found that those for different continents are often widely separated. The only way of explaining such results is by assuming that the continents have moved in relation to each other and in relation to the Earth's magnetic field. Moreover, if pole positions from samples in the same continent but for different epochs are plotted, it is found that they do not coincide but seem to move in what is known as an apparent polar wander (APW) curve. APW curves from different continents are generally different, although some may share matching sections (indicating that the continents concerned were joined and moved together for that period) separated by non-matching ones (indicating periods of drift).

Finally, for about the past 200 Ma there is widespread evidence of plate movements preserved in the sea-floor. The linear magnetic anomalies produced by sea-floor spreading are especially important. These mark the ancient positions of plate boundaries, and from them detailed reconstructions of plate (and hence continental) motions can be inferred.

A combination of palaeomagnetic and geological evidence has made it possible to trace the history of continental drift back to the Precambrian, although the degree of detail diminishes for earlier times (Fig. 1). Wegener himself suggested that in Permo-Carboniferous time all the continents were joined in one supercontinent called Pangaea, although du Toit recognized a northern supercontinent of Laurasia (North America, Greenland, Europe, and Asia) and a southern supercontinent of Gondwanaland (South America, Africa, India, Australia, and Antarctica). Pangaea began to break apart during the Mesozoic, and this break-up continues today. Pangaea itself was, however, formed from the earlier convergence of Laurasia and Gondwanaland in the Silurian, and Laurasia itself was formed from a number of earlier continental blocks. The recurring episodes of continental break-up, drift, and collision have been called the Wilson cycle after J. Tuzo Wilson, one of the pioneers of plate tectonics. It is likely that there have been a number of such cycles, each lasting a few hundred million years, in the history of the Earth.

Roger Searle

Bibliography

Kearey, P. and and Vine, F. J. (1996) Global tectonics. Blackwell Science, Oxford.