Did the five continents once fit together like a gigantic jigsaw? The answer to this puzzle is now causing a revolution in the geological sciences comparable with the effect of Darwin on the biological sciences a century ago. Our increased knowledge of the surface geology of the world, combined with the recent great advances in the investigation of the interior of the Earth, make it possible to say that not only must the continents have once fitted together and have since drifted apart, but the solid floors of the oceans themselves take part in this movement.

The idea of continental drift is, in fact, very old and the correspondence between the western coastline of Africa and the eastern coastline of South America was known to most of the explorers of the Age of Discovery. In Novanum Organum, published in 1620, Sir Francis Bacon commented that such similarities could hardly be accidental, but he did not offer any interpretation of the observation. Shortly afterwards, in 1658, the Frenchman, R.P. François Placet, wrote a memoir (la corruption du grand et petit monde, où il est montré que devant le déluge, l'Amerique n'était point separée des autres parties du monde) in which he suggested that the Old and New Worlds became separated following the Flood. This concept was common knowledge during the seventeenth and eighteenth centuries and, in 1800, Alexander von Humboldt, the pioneer explorer of Mexico and the United States, retained the idea that the Atlantic was essentially a huge river valley whose sides had been separated by the great volume of water over which Noah's Ark had sailed.

The first observation of the geological, as opposed to the geometrical, similarities of the continents on either side of the Atlantic was made by Antonio Snider Pelligrini, in 1858. He described in his work, La Création et ses mystères dévoilés, the fitting together of the continents bordering the Atlantic, in order to explain the occurrence of identical fossil plants in the coal deposits of both Europe and North America. In this work, Snider gave the first diagram showing the continents bordering the Atlantic before and after their separation (see Figure).

The nineteenth century, of course, saw many fruitful ideas emerging on a vast range of subjects including continental drift. Charles Darwin, on his epic-making voyages, found convincing signs of vertical movements of land masses, but did not see evidence for large-scale horizontal movement. One of his sons, George Darwin, suggested, in collaboration with Osmond Fisher, that the Moon originated by being thrown off from the Pacific area of the spinning Earth or being drawn from it by the gravitational attraction of a passing star. This idea was extended by Fisher in 1882 when be suggested that the continents, which broke up at the time of the Moon's separation, would subsequently readjust their positions to the new shape of the Earth. This association of continental drift with the origin of the Moon dominated many of the later ideas of the twentieth century.

Early in the twentieth century two Americans, Frank B. Taylor and Howard B. Baker, independently and almost simultaneously outlined their ideas on continental drift. Taylor, in 1908, suggested the phenomenon in order to explain the origin of our modern mountains (Figure 2), while Baker, in a series of articles between 1911 and 1928 used the match of these mountains on opposite sides of the Atlantic, although in his later articles he expanded his ideas to include other criteria (Figure).

Taylor, in particular, gave an elegant statement of the argument for large-scale continental movements, but most scientists regard the German astronomer, geophysicist, and meteorologist Alfred Wegener as the real pioneer of the modern theory of continental drift. He was originally drawn to the idea to explain the ancient climates of the past (Chapter 5). Why should tropical ferns have grown in London, Paris, and even Greenland, yet glaciers have covered Brazil and the Congo at the same time? However, in his book Die Entstehung der Kontinente und Ozeane, published in 1915 (although the idea was presented in 1912), he drew evidence not just from the study of ancient climates, but from the whole range of the sciences. It was unfortunate that, in so doing, he cited several examples that were incorrect or capable of quite logical explanation without recourse to a theory of continental movements. Nonetheless, he invoked a tremendous body of evidence that formed the basis of extremely heated discussions, particularly during the 1920s.

Most of these debates centred on the 1924 English translation of Wegener's 1922 German edition, despite a further edition before his death while exploring Greenland in 1930. Many of the objectors merely expressed their complete and utter disbelief, with little attempt to justify their rejection of the concepts, but the majority of the opposition arose in one of two ways. Experts in one scientific field would either demonstrate errors in the details of evidence that Wegener had quoted and therefore concluded that the whole basis was incorrect, or they were even less logical and accepted the majority of Wegener's evidence from their own speciality but then disproved continental drift by quoting evidence from other sciences.

Typical of the latter was the way in which many bio geographers accepted Wegener's statements that ancient plant and animal life clearly demonstrated ancient land connections between the continents, and then said that as geologists did not believe in continental drift these former land connections (continents or huge land bridges) must have sunk beneath the waves of the oceans, although the geologists and geophysicists had proved almost 100 years before that this was impossible. This piecemeal approach by specialists gave the impression that the vast majority of the evidence presented by Wegener was wrong and it was very difficult to see that the undoubted errors in detail did not prevent the overall argument being highly significant, if only because it was drawn from so many independent scientific fields. However, the main reason for disbelief, or at least skepticism, arose because there was no known mechanism at that time which was capable of separating the continents in the manner that Wegener, or his supporters and predecessors, had suggested. After all, the continents and ocean floors are extremely solid rocks and tremendous forces must be envisaged to move even the smallest continent, Australia, which weighs some 5oo million million million kilograms, through the oceanic floor. The most accepted mechanism at that time was still thought to be the effect of the separation of the Moon from the Pacific, but we now know that this is not the way in which the Moon originated as it was formed at the same time as the Earth, 4550 million years ago.

With hindsight it is easy to envisage these early opponents of continental drift as ultra-conservative diehards, but it is not really surprising that the majority of scientific opinion swung against continental drift during the two decades between the two World Wars and, indeed, there are still some observations which are not explained by large-scale continental movements alone. There were, however, some notable supporters. In particular the South African geologist Alexander du Toit undertook pioneer work in both Africa and Brazil. This work, extended later by Henri Martin, forms the basis of some of the best examples of the geological arguments for continental drift (Chapter 3). Another of the main proponents was a Scottish geologist, Arthur Holmes, who pioneered the dating of rocks by radioactive methods and also propounded, in 1927 and more extensively in 1929, the mechanism of convection currents driven by radioactive heat within the Earth. He suggested that areas of the interior of the Earth became hot from the heat which Madame Curie found was released during the breakdown of the small amounts of radioactive elements contained in all rocks. The hotter areas would eventually rise, spread out near the Earth's surface, where the hot rocks would cool, and eventually sink back as cold, dense material into the interior of the Earth. Holmes suggested that these movements within the Earth could give rise to continental movements, the continents being carried across the face of the Earth like gigantic icebergs. Similar sorts of movements were almost simultaneously invoked by the famous Dutch geophysicist F. A. Vening Meinesz to explain his geophysical observations in the East and West Indies. Our modern ideas of these convection currents differ in some very important details from these original suggestions, but the basic driving force is essentially the same (see Chapter 9).

By the 1940s, therefore, there was a possible mechanism to account for continental drift and as more and more geological evidence accumulated, more and more scientists became convinced of the reality of drift. This was particularly true of geologists in the southern hemisphere. The South African Lester C. King and the Australian, S. Warren Carey were particularly ardent proponents of drift, and they obtained much new vital evidence. However, the main impetus to the wider acceptance of drift came in the 195os with the rapid development of the investigation of palaeomagnetism (the ancient magnetism of rocks) following the suggestion by the British physicist Lord Blacked of the application of new, extremely sensitive instruments capable of measuring this very weak residual magnetism. Using these instruments, the British geophysicist, S. Keith Runcom, found that he could only explain his observations of the palaeomagnetism in rocks from Europe and North America in terms of continental movements.

With the extension of this work to other continents, mainly under the lead of British researchers such as Kenneth M. Creer in South America and Edward Irving in Australia, more and more people became convinced of the reality of large scale continental movements. It was, however, the application of palaeomagnetic studies to the rocks of the ocean floor in the 1960s by Frederick J. Vine that finally led to a complete swing of scientific opinion towards the acceptance of the theory of continental drift.

The fact of continental drift is not merely an interesting, though purely academic observation, but is an extremely important economic factor to be considered in terms of mineral exploitation. The occurrence of valuable deposits on one continent can lead to the assumption that they might also occur on another several thousand kilometres away. An example is the occurrence of diamond fields in West Africa and their counterparts in northeastern South America. The oil man is also concerned; although he is primarily interested in finding the geological structures that may contain oil, be has to bear in mind that economic quantities of oil, or natural gas, can be produced only if the rocks he is looking at were once in the right latitudes to allow the formation of oil in these quantities. The oil and natural gas fields of Europe and North Africa are in existence only because this part of the world was once lying much closer to the Equator.

Potentially more important is the development of the floors of the oceans which our knowledge of continental drift helps us to understand. With the mineral resources of the continents rapidly diminishing (on current estimates the world's present copper mines will be exhausted within the next twenty years), it will become increasingly necessary to tap the mineral resources of the ocean floors. This can only be done economically if we understand their geological history. However, before the start of economic exploitation, it is necessary in this technological age, to have an inventory of the total resources of this planet, and yet two-thirds of the world's surface is hidden by the oceans whose potential can only be estimated if we know the story of their development.

Today, the understanding of the movements of the continents and ocean floors has lead to better understanding the formation of mountains and the mechanisms of earthquakes and volcanoes. Such an understanding is essential to the prediction of natural disasters and possible methods of controlling such tremendous forces. Perhaps the most important aspect of all, however, is that the final acceptance of continental drift is resulting in an entirely new scientific outlook on the evolution of the surface of our planet which will lead us to a better understanding of the origin of the Earth.