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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.

Contents

History

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Early history

In the 11th century, the Persian geologist, Abu Rayhan Biruni (973-1048), observed the geology of India and discovered that the Indian subcontinent was once a sea and later became land. He hypothesized that this was due to the drifting of alluvium, but didn't realize it was in fact due to continental drift.[1]

Abraham Ortelius (1597), Francis Bacon (1625), Benjamin Franklin, Antonio 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. W. J. Kious described Ortelius' thoughts in this way:[2]

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.[3] 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:[4][5] Franklin Coxworthy (between 1848 and 1890),[6] Roberto Mantovani (between 1889 and 1909), William Henry Pickering (1907)[7] 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.[8][9][10]

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.[11]

Wegener was the first to use the phrase "continental drift" (1912, 1915)[3][4] (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 of the Earth's rotation was rejected as calculations showed that the force was not sufficient.[12]

Evidence that continents 'drift'

Fossil patterns across continents.

The notion that continents have not always been at their present positions was suggested as early as 1596 by the Dutch map maker Abraham Ortelius in the third edition of his work Thesaurus Geographicus. Ortelius suggested that the Americas, Eurasia and Africa were once joined and have since drifted apart "by earthquakes and floods", creating the modern Atlantic Ocean. For evidence, he wrote: "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." Francis Bacon commented on Ortelius' idea in 1620, as did Benjamin Franklin and Alexander von Humboldt in later centuries.

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. There is also living evidence — the same animals being found on two continents. An example of this is a particular earthworm found in South America and South Africa.

The complementary arrangement of the facing sides of South America and Africa is obvious, but is a temporary coincidence. In millions of years, seafloor spreading, continental drift, 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.

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. As late as 1953 – just five years before Carey[13] introduced the theory of plate tectonics – the theory of continental drift was rejected by the physicist Scheiddiger on the following grounds.[14]

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 are 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 resulting 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.

References

  1. ^ A. Salam (1984), "Islam and Science", in C. H. Lai (1987), Ideals and Realities: Selected Essays of Abdus Salam, 2nd ed., World Scientific, Singapore, pp. 179-213:
    "But if you see the soil of India with your own eyes and meditate on its nature, if you consider the rounded stones found in earth however deeply you dig, stones that are huge near the mountains and where the rivers have a violent current: stones that are of smaller size at a greater distance from the mountains and where the streams flow more slowly: stones that appear pulverised in the shape of sand where the streams begin to stagnate near their mouths and near the sea—if you consider all this you can scarcely help thinking that India was once a sea, which by degrees has been filled up by the alluvium of the streams."
  2. ^ Kious, W.J.; Tilling, R.I. (2001) [1996]. "Historical perspective". This Dynamic Earth: the Story of Plate Tectonics (Online ed.). U.S. Geological Survey. ISBN 0160482208. http://pubs.usgs.gov/gip/dynamic/historical.html. Retrieved 2008-01-29. 
  3. ^ a b Wegener, A. (1912), "Die Entstehung der Kontinente", Peterm. Mitt.: 185–195, 253–256, 305–309 
  4. ^ a b Wegener, A. (1929/1966), The Origin of Continents and Oceans, Courier Dover Publications, ISBN 0486617084 
  5. ^ Wegener, A. (1929), Die Entstehung der Kontinente und Ozeane, 4. Auflage, Braunschweig: Friedrich Vieweg & Sohn Akt. Ges. 
  6. ^ Coxworthy, F. (1848/1924), Electrical Condition or How and Where our Earth was created, London: W. J. S. Phillips 
  7. ^ Pickering, W.H (1907), "The Place of Origin of the Moon - The Volcani Problems", Popular Astronomy: 274–287 
  8. ^ Mantovani, R. (1889), "Les fractures de l’écorce terrestre et la théorie de Laplace", Bull. Soc. Sc. Et Arts Réunion: 41–53 
  9. ^ Mantovani, R. (1909), "L’Antarctide", Je m’instruis. La science pour tous 38: 595–597 
  10. ^ Scalera, G. (2003), "Roberto Mantovani an Italian defender of the continental drift and planetary expansion", in Scalera, G. and Jacob, K.-H., Why expanding Earth? – A book in honour of O.C. Hilgenberg, Rome: Istituto Nazionale di Geofisica e Vulcanologia, pp. 71–74, http://hdl.handle.net/2122/2017 
  11. ^ Taylor, F.B. (1910), "Bearing of the tertiary mountain belt on the origin of the earth's plan", GSA Bulletin 21 (2): 179–226, doi:10.1130/1052-5173(2005)015[29b:WTCCA2.0.CO;2] 
  12. ^ Plate Tectonics: The Rocky History of an Idea
  13. ^ Carey, S. W. (1958), "The tectonic approach to continental dirft", in Carey, S. W., Continental Drift—A symposium, Univ. of Tasmania, pp. 177—355 
  14. ^ Scheidigger (1953), "Examination of the physics of theories of orogenesis", GSA Bulletin 64: 127—150 
  • Le Grand, H. E. (1988). Drifting Continents and Shifting Theories. Cambridge University. ISBN 0-521-31105-5. 

External links


File:Antonio Snider-Pellegrini Opening of the
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.

Contents

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'

File:Snider-Pellegrini Wegener fossil map.gif
Fossil patterns across continents.

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:

  1. ^ Romm, James (February 3, 1994). [Expression error: Unexpected < operator "A New Forerunner for Continental Drift"]. Nature 367: 407–408. doi:10.1038/367407a0. 
  2. ^ a b Schmeling, Harro (2004). "Geodynamik" (in german). Univesity of Frankfurt. http://www.geophysik.uni-frankfurt.de/~schmelin/skripte/Geodyn1-kap1-2-S1-S22-2004.pdf. 
  3. ^ Brusatte, Stephen, Continents Adrift and Sea-Floors Spreading: The Revolution of Plate Tectonics, http://www.lib.uchicago.edu/e/crerar/crerar-prize/2003%2004%20Brusatte.pdf 
  4. ^ Kious, W.J.; Tilling, R.I. (February 2001) [1996], "Historical perspective", This Dynamic Earth: the Story of Plate Tectonics (Online ed.), U.S. Geological Survey, ISBN 0160482208, http://pubs.usgs.gov/gip/dynamic/historical.html, retrieved 2008-01-29 
  5. ^ a b Wegener, Alfred (6 January 1912). "Die Herausbildung der Grossformen der Erdrinde (Kontinente und Ozeane), auf geophysikalischer Grundlage". Petermanns Geographische Mitteilungen 63: 185–195, 253–256, 305–309. http://epic.awi.de/Publications/Polarforsch2005_1_3.pdf. 
  6. ^ a b c Wegener, A. (1929/1966), [Expression error: Unexpected < operator The Origin of Continents and Oceans], Courier Dover Publications, ISBN 0486617084 
  7. ^ Wegener, A. (1929), [Expression error: Unexpected < operator Die Entstehung der Kontinente und Ozeane] (4 ed.), Braunschweig: Friedrich Vieweg & Sohn Akt. Ges. 
  8. ^ Coxworthy, F. (1848/1924), [Expression error: Unexpected < operator Electrical Condition or How and Where our Earth was created], London: W. J. S. Phillips 
  9. ^ Pickering, W.H (1907), [Expression error: Unexpected < operator "The Place of Origin of the Moon - The Volcani Problems"], Popular Astronomy: 274–287 
  10. ^ Mantovani, R. (1889), [Expression error: Unexpected < operator "Les fractures de l’écorce terrestre et la théorie de Laplace"], Bull. Soc. Sc. Et Arts Réunion: 41–53 
  11. ^ Mantovani, R. (1909), [Expression error: Unexpected < operator "L’Antarctide"], Je m’instruis. La science pour tous 38: 595–597 
  12. ^ Scalera, G. (2003), "Roberto Mantovani an Italian defender of the continental drift and planetary expansion", in Scalera, G. and Jacob, K.-H., [Expression error: Unexpected < operator Why expanding Earth? – A book in honour of O.C. Hilgenberg], Rome: Istituto Nazionale di Geofisica e Vulcanologia, pp. 71–74, http://hdl.handle.net/2122/2017 
  13. ^ Taylor, F.B. (1910), [Expression error: Unexpected < operator "Bearing of the tertiary mountain belt on the origin of the earth's plan"], GSA Bulletin 21 (2): 179–226, doi:10.1130/1052-5173(2005)015[29b:WTCCA]2.0.CO;2 
  14. ^ "Plate Tectonics: The Rocky History of an Idea". http://www.ucmp.berkeley.edu/geology/techist.html. "Wegener's inability to provide an adequate explanation of the forces responsible for continental drift and the prevailing belief that the earth was solid and immovable resulted in the scientific dismissal of his theories." 
  15. ^ "Rejoined continents [This Dynamic Earth, USGS"]. USGS. http://pubs.usgs.gov/gip/dynamic/continents.html. 
  16. ^ Carey, S. W. (1958). "The tectonic approach to continental drift". In Carey, S. W.. Continental Drift—A symposium. Hobart: Univ. of Tasmania. pp. 177–363. 
  17. ^ Scheidegger, Adrian E. (1953), [Expression error: Unexpected < operator "Examination of the physics of theories of orogenesis"], GSA Bulletin 64: 127–150, doi:10.1130/0016-7606(1953)64[127:EOTPOT]2.0.CO;2 

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


Simple English

File:Gondwana fossil map
Fossil records suggesting that continents now separated were once together: see Pangaea
File:Antonio Snider-Pellegrini Opening of the
First known illustration of the opening of the Atlantic Ocean, by Antonio Snider-Pellegrini, 1858.

Continental drift is a historical theory. The theory was first proposed by Abraham Ortelius in 1596. It was fully developed by the German geologist and meteorologist Alfred Wegener in 1915.

The theory said that parts of the Earth's crust moves slowly on top of a liquid core. The theory has now been included in the wider theory of plate tectonics.

Evidence

The theory was supported by finding the same rocks and fossils in western Europe and the eastern North America. There are also similar fossils on the western coast of Africa and the eastern South America. The shapes of these continents nearly fits together. The theory was plausible (believable), but there was no known mechanism to drive these great movements. This problem was later solved by plate tectonics.

The pattern of volcanic action, mountain building (orogeny) and earthquakes is explained by continental drift.

The existence of identical rocks widely apart supports the theory. Examples (there are hundreds):

  1. Table Mountain at the Cape of Good Hope, South Africa, and mountains south of Rio de Janeiro are made of identical rocks. This corresponds to the fit of Africa with South America in Pangaea.
  2. The Giant's Causeway in Northern Ireland, and Fingal's Cave on Staffa, Inner Hebrides, Scotland is the same rock formation.

Mechanism

The basic machine driving continental movement is heat moving from the Earth's mantle through the crust and out of the planet.[1] The effect of this it to cause convection, and plate movement.

Two events in particular are of huge importance:

  1. The production of new crust at places such as the Mid-Atlantic Ridge.
  2. The removal of crust by subduction (moving under) at the points where plates collide.

Footnote

  1. The heat is caused by the disintegration of radioactive isotopes in the Earth. See discussion in Age of the Earth.


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