Continental drift

Antonio Snider-Pellegrini's Illustration of the closed and opened Atlantic Ocean (1858).

Continental drift is the movement of the Earth's continents relative to each other. The hypothesis that continents 'drift' was first put forward by Abraham Ortelius in 1596 and was fully developed by Alfred Wegener in 1912. However, it was not until the development of the theory of plate tectonics in the 1960s, that a sufficient geological explanation of that movement was found.

History

Early history

Abraham Ortelius (Ortelius 1596),^[1] Theodor Christoph Lilienthal (1756),^[2] Alexander von Humboldt (1801 and 1845),^[2] Antonio Snider-Pellegrini (Snider-Pellegrini 1858), and others had noted earlier that the shapes of continents on opposite sides of the Atlantic Ocean (most notably, Africa and South America) seem to fit together.^[3] W. J. Kious described Ortelius' thoughts in this way:^[4]

Abraham Ortelius in his work Thesaurus Geographicus ... suggested that the Americas were "torn away from Europe and Africa ... by earthquakes and floods" and went on to say: "The vestiges of the rupture reveal themselves, if someone brings forward a map of the world and considers carefully the coasts of the three [continents].

Wegener and his predecessors

The hypothesis that the continents had once formed a single landmass before breaking apart and drifting to their present locations was fully formulated by Alfred Wegener in 1912.^[5] Although Wegener's theory was formed independently and was more complete than those of his predecessors, Wegener later credited a number of past authors with similar ideas:^[6][7] Franklin Coxworthy (between 1848 and 1890),^[8] Roberto Mantovani (between 1889 and 1909), William Henry Pickering (1907)^[9] and Frank Bursley Taylor (1908).

For example: the similarity of southern continent geological formations had led Roberto Mantovani to conjecture in 1889 and 1909 that all the continents had once been joined into a supercontinent (now known as Pangaea); Wegener noted the similarity of Mantovani's and his own maps of the former positions of the southern continents. Through volcanic activity due to thermal expansion this continent broke and the new continents drifted away from each other because of further expansion of the rip-zones, where the oceans now lie. This led Mantovani to propose an Expanding Earth theory which has since been shown to be incorrect.^[10][11][12]

Some sort of continental drift without expansion was proposed by Frank Bursley Taylor, who suggested in 1908 (published in 1910) that the continents were dragged towards the equator by increased lunar gravity during the Cretaceous, thus forming the Himalayas and Alps on the southern faces. Wegener said that of all those theories, Taylor's, although not fully developed, had the most similarities to his own.^[13]

Wegener was the first to use the phrase "continental drift" (1912, 1915)^[5][6] (in German "die Verschiebung der Kontinente" – translated into English in 1922) and formally publish the hypothesis that the continents had somehow "drifted" apart. Although he presented much evidence for continental drift, he was unable to provide a convincing explanation for the physical processes which might have caused this drift. His suggestion that the continents had been pulled apart by the centrifugal pseudoforce (Polflucht) of the Earth's rotation or by a small component of astronomical precession was rejected as calculations showed that the force was not sufficient.^[14] The Polflucht hypothesis was also studied by Paul Sophus Epstein in 1920 and found to be implausible.

Evidence that continents 'drift'

Fossil patterns across continents.

Mesosaurus skeleton, MacGregor, 1908.

Evidence for continental drift is now extensive. Similar plant and animal fossils are found around different continent shores, suggesting that they were once joined. The fossils of Mesosaurus, a freshwater reptile rather like a small crocodile, found both in Brazil and South Africa, are one example; another is the discovery of fossils of the land reptile Lystrosaurus from rocks of the same age from locations in South America, Africa, and Antarctica.^[15] There is also living evidence — the same animals being found on two continents. Some earthworm families (e.g.: Ocnerodrilidae, Acanthodrilidae, Octochaetidae) are found in South America and Africa, for instance.

The complementary arrangement of the facing sides of South America and Africa is obvious, but is a temporary coincidence. In millions of years, slab pull and ridge-push, and other forces of tectonophysics will further separate and rotate those two continents. It was this temporary feature which inspired Wegener to study what he defined as continental drift, although he did not live to see his hypothesis become generally accepted.

Widespread distribution of Permo-Carboniferous glacial sediments in South America, Africa, Madagascar, Arabia, India, Antarctica and Australia was one of the major pieces of evidence for the theory of continental drift. The continuity of glaciers, inferred from oriented glacial striations and deposits called tillites, suggested the existence of the supercontinent of Gondwana, which became a central element of the concept of continental drift. Striations indicated glacial flow away from the equator and toward the poles, in modern coordinates, and supported the idea that the southern continents had previously been in dramatically different locations, as well as contiguous with each other.^[6]

Rejection of Wegener's theory

While it is now accepted that the continents do move across the Earth's surface – though more in a driven mode than the aimlessness suggested by "drift" – as a theory, continental drift was not accepted for many years. One problem was that a plausible driving force was missing. And it did not help that Wegener was not a geologist.

As late as 1953 – just five years before Carey^[16] introduced the theory of plate tectonics – the theory of continental drift was rejected by the physicist Scheiddiger on the following grounds.^[17]

• First, it had been shown that floating masses on a rotating geoid would collect at the equator, and stay there. This would explain one, but only one, mountain building episode between any pair of continents; it failed to account for earlier orogenic episodes.
• Second, masses floating freely in a fluid substratum, like icebergs in the ocean, should be in isostatic equilibrium (where the forces of gravity and buoyancy are in balance). Gravitational measurements were showing that many areas are not in isostatic equilibrium.
• Third, there was the problem of why some parts of the Earth's surface (crust) should have solidifed while other parts were still fluid. Various attempts to explain this foundered on other difficulties.

It is now known that there are two kinds of crust, continental crust and oceanic crust, with the former of a different composition and inherently lighter, and both kinds residing above a much deeper fluid mantle. Also, oceanic crust is still being created at spreading centers, and this, along with subduction, drives the system of plates in a chaotic manner, resulting in continuous orogeny and areas of isostatic imbalance. All this, as well as the motion of the continents, is better explained by the theory of plate tectonics.

Bibliography

Notes:

References:

• Le Grand, H. E. (1988). Drifting Continents and Shifting Theories. Cambridge University. ISBN 0-521-31105-5. 
• Ortelius, Abraham (1596). Thesaurus Geographicus (3 ed.). Antwerp: Plantin. 
• Snider-Pellegrini, Antonio (1858). La Création et ses mystères dévoilés. Paris: Frank and Dentu. 

External links

• A brief introduction to Plate Tectonics, based on the work of Alfred Wegener.
• Maps of continental drift, from the Precambrian to the future
• Four main evidences of the Continental Drift theory
• Wegener and his proofs