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Continental Drift

  Anyone with an atlas can see the coincidence between the shapes of South America and Africa, indeed scholars first speculated about it as early as the 17th century. Early explanations tended to involve floods or the drowning of continents, but these were rejected by the middle of the 19th century. At that time, the Earth was thought to be cooling and shrinking, and the idea that the continents could move was unthinkable, as it remained for most geologists for centuries. The problem with the idea of continental drift was that the mounting evidence that the continents had changed their positions remained unconvincing in the absence of a mechanism which seemed adequate to move them.

The main advocate of the idea of continental drift was a German meteorologist, Alfred Wegener, who first published in 1915. He recognized the difference between continental and oceanic crust and believed that the continents were floating on the ocean crust. He noted that isostatic adjustment (for example, after a period of glaciation) required vertical movement and argued that this implied that horizontal movement must be possible. His main contribution was in going beyond the fit of the coastlines to present evidence of other kinds of fit.

Three of his lines of argument are still regarded as sound. First, he showed that mountain ranges of the same age existed in North America and the British Isles, as they did in South Africa and Argentina, and that when the coastlines were brought together the alignment fitted perfectly. Similarly, there were rocks on each side of the Atlantic with identical fossils, including those of freshwater fish. Third, he suggested that geological features distinctive of particular climatic conditions, such as desert sandstones, showed that all the continents had moved relative to each other and the world\'s climatic zones. He suggested that the existence of glaciated areas of similar age in South America, South Africa, India and Australia could only be explained as the result of their combination into a huge supercontinent. Indeed, he went further, suggesting that all the continents had once formed a single land mass, which he called Pangaea.

Wegener\'s ideas were slow to impress most geologists; for example, his work was not translated into English until 1925. Most preferred to regard the distributions as coincidental or as the result of earlier connections by ‘land-bridges’ rather than accepting the movement of continents. But Wegener did have some supporters, most notably du Toit, a South African who not only assembled further evidence in the southern continents, but also suggested that the mechanism must be convection currents in the mantle. In this he was following a British expert on radioactive dating, Arthur Holmes, who had also suggested that convection currents could have pulled apart the supercontinents. Even without such a mechanism, studies of mountain ranges were showing that very large horizontal movements were required to account for the observed pattern of faulting and folding.

From the 1950s onward, new evidence from several areas of research revived the debate. Studies of palaeomagnetism—the magnetic field locked into certain rocks at the time of their formation—showed that these rocks had changed their orientation to the magnetic poles and to each other. To account for the changes required accepting that the continents must have moved. Measurements of heat flow through the Earth\'s crust showed that the flow was highest at mid-ocean ridges and lower in the ocean trenches, where there were also negative gravity anomalies. These observations seemed consistent with the existence of convection currents in the mantle. The scene was set for a revolution. The final irony was that when the revolution did occur, it did not do so under the old banner of continental drift but under a new banner plate tectonics. PS



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