Continental Drift
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Continental drift:-
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Continental drift:The concept that the world's continents once formed part of a single mass and have since drifted into their present positions. Although it was outlined by Alfred Wegener in 1912, the idea was not particularly new. Paleontological studies had already demonstrated such strong similarities between the flora and fauna of the southern continents between 300,000,000 and 150,000,000 years ago that a huge supercontinent, Gondwana, containing South America, Africa, India, Australia, and Antarctica, had been proposed. However, Gondwana was thought to be the southern continents linked by land bridges, rather than contiguous units. Wegener's ideas were almost universely rejected in 1928; the fundamental objection was the lack of a suitable mechanism. Almost simultaneously with the temporary eclipse of Wegener's theory, Arthur Holmes was considering a mechanism that is still widely accepted. Holmes conceived the idea of convective currents within the Earth's mantle which were driven by the radiogenic heat produced by radioactive minerals within the mantle. At that time, Holmes's ideas, like those of Wegener, were largely ignored. Nonetheless, several geologists, particularly those living in the Southern Hemisphere, continued to believe the theory and accumulate more data in its support. By the 1950s, convincing evidence had accumulated, with studies of the magnetization of rocks, paleomagnetism, beginning to provide numerical parameters on the past latitude and orientation of the continental blocks. Early work in North America and Europe clearly indicated how these continents had once been contiguous and had since separated. The discovery of the midoceanic ridge system also provided further evidence for the geometric matching of continental edges, but the discovery of magnetic anomalies parallel to these ridges and their interpretation in terms of sea-floor spreading finally led to almost universal acceptance of continental drift as a reality. In the 1970s and 1980s, the interest changed from proving the reality of the concept to applying it to the geologic record, leading to a greater understanding of how the Earth has evolved through time. The fundamental change in concept was that not only have the continents drifted, but the continents are merely parts of thicker tectonic plates comprising both oceanic and continental crust, with 50–300 km (31–186 mi) of the Earth's mantle moving along with them. OR Large-scale movements of continents over the course of geologic time. The first complete theory of continental drift was proposed in 1912 by Alfred Wegener, who postulated that a single supercontinent, which he called Pangea, fragmented late in the Triassic Period (248–206 million years ago) and that the parts began to move away from one another. He pointed to the similarity of rock strata in the Americas and Africa as evidence to support his hypothesis. Wegener's ideas received support from the concepts of seafloor spreading and plate tectonics beginning in the 1960s. The modern theory states that the Americas were joined with Europe and Africa until c. 190 million years ago, when they split apart along what is now the Mid-Atlantic Ridge. Subsequent tectonic plate movements brought the continents to their present positions. OR Geological theory that the relative positions of the continents on the earth's surface have changed considerably through geologic time. Though first proposed by American geologist Frank Bursley Taylor in a lecture in 1908, the first detailed theory of continental drift was put forth by German meteorologist and geophysicist Alfred Wegener in 1912. On the basis of geology, biology, climatology, and the alignment of the continental shelf rather than the coastline, he believed that during the late Paleozoic and early Mesozoic eras, about 275 to 175 million years ago, all the continents were united into a vast supercontinent, which he called Pangaea. Later, Pangaea broke into two supercontinental masses—
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Continental drift:-
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Laurasia to the north, and Gondwanaland to the south. The present continents began to split apart in the latter Mesozoic era about 100 million years ago, drifting to their present positions. As additional evidence Wegener cited the unusual presence of coal deposits in the South Polar regions, glacial features in present-day equatorial regions, and the jigsaw fit of the opposing Atlantic continental shelves. He also pointed out that a plastic layer in the earth's interior must exist to accommodate vertical adjustments caused by the creation of new mountains and by the wearing down of old mountains by erosion (see continent). He postulated that the earth's rotation caused horizontal adjustment of rock in this plastic layer, which caused the continents to drift. The frictional drag along the leading edges of the drifting continents results in mountain building. Wegener's theory stirred considerable controversy during the 1920s. South African geologist A. L. Dutoit, in 1921, strengthened the argument by adding more exacting details that correlated geological and paleontological similarities on both sides of the Atlantic. In 1928, Scottish geologist Arthur Holmes suggested that thermal convection in the mantle was the mechanism that drove the continental movements. American geologist David Griggs performed scale model experiments to show the mantle movements. The theory of continental drift was not generally accepted, particularly by American geologists, until the 1950s and 60s, when a group of British geophysicists reported on magnetic studies of rocks from many places and from each major division of geologic time. They found that for each continent, the magnetic pole had apparently changed position through geologic time, forming a smooth curve, or pole path, particular to that continent. The pole paths for Europe and North America could be made to coincide by bringing the continents together. OR The concept of continental drift was first proposed by Alfred Wegener. In 1912 he noticed that the shapes of continents on either side of the Atlantic Ocean seem to fit together (for example, Africa and South America). Francis Bacon, Antonio Snider-Pellegrini, Benjamin Franklin, and others had noted much the same thing earlier. The similarity of southern continent fossil faunae and some geological formations had led a relatively small number of Southern hemisphere geologists to conjecture as early as 1900 that all the continents had once been joined into a supercontinent known as Pangaea. The concept was initially ridiculed by most geologists, who felt that an explanation of how a continent drifted was a prerequisite and that the lack of one made the idea of drifting continents wholly unreasonable. The theory received support through the controversial years from South African geologist Alexander Du Toit as well as from Arthur Holmes. The idea of continental drift did not become widely accepted as theory until the 1950s in Europe. By the 1960s, geological research conducted by Robert S. Dietz, Bruce Heezen, and Harry Hess along with a rekindling of the theory including a mechanism by J. Tuzo Wilson led to acceptance among North American geologists. The hypothesis of continental drift became part of the larger theory of plate tectonics. This article deals mainly with the historical development of the continental drift hypothesis before 1950. See: plate tectonics for information on current ideas underlying concepts of continental drift.
Various data South America and Africa are moving apart at an average of 5.7 cm per year, due to the seafloor spreading along the Mid-Atlantic Ridge. This is comparable to the growth speed of a fingernail.
file://C:\Documents and Settings\s h a f i q\Desktop\Continental drift.htm
3/22/2007
Continental drift:-
Page 3 of 4
The fastest recorded seafloor spreading takes place along the East Pacific Rise at 17.2 cm per year. OR 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, in the form of plant and animal fossils of the same age found around different continent shores, suggesting that these shores were once joined. For example the fossils of the freshwater crocodile found in Brazil and South Africa. Another illustrative example is the discovery of fossils of the aquatic 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 Alfred Wegener to study what he defined as continental drift.
Permo-Carboniferous Permo-Carboniferous was a period of great glaciation that occurred about 250 million years ago. It is one of the many ice ages that has occurred on this Earth. This is also an era that has been used to submit proof that the continents were once a large land mass called pangaea. Permo-Carboniferous rocks are widely distributed in pangaea. The 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. Glacial activity spanned virtually the whole of Carboniferous and Early Permian time (A.G. Smith 1997). Toward the end of the Carboniferous, and around 290 million years ago, Gondwanda hovered over the south polar regions, where glacial centres expanded across the continents, as evidenced by glacial deposits of tillites along with striations in ancient rocks. Those heavily grooved by the advancing glaciers showed lines of ice flow away from the equator and toward the poles, which is the opposite direction if the continents were situated where they are today. Overall, the southern continents drifted together over the South Pole, and massive ice sheets radiating outward from a central point crossed the present continental boundaries. The PermoCarboniferous ice sheet is so extensive that it can fit within a latitude circle of 50 degrees (A.G.Smith 1997) (Rahul Megharaj 1985).
The debate over continental drift Before geophysical evidence started accumulating after World War II, the idea of continental drift caused sharp disagreement among geologists. Wegener had introduced his theory in 1912 at a meeting of the German Geological Association. His paper was published that year and expanded into a book in 1915. In 1921 the Berlin Geological Society held a symposium on the theory. In 1922 Wegener's book was translated into English and then it received a wider audience. In 1923 the theory was discussed at
file://C:\Documents and Settings\s h a f i q\Desktop\Continental drift.htm
3/22/2007
Continental drift:-
Page 4 of 4
conferences by Geological Society of France, the Geological Section of the British Association for the Advancement of Science, and the Royal Geological Society. The theory was carefully but critically reviewed in the journal Nature by Philip Lake. On November 15, 1926, the American Association of Petroleum Geologists (AAPG) held a symposium at which the continental drift hypothesis was vigorously debated. The resulting papers were published in 1928 under the title Theory of continental drift. Wegener himself contributed a paper to this volume (Friedlander 1995:21-27). One of the main problems with Wegener's theory was that he believed that the continents "plowed" through the rocks of the ocean basins. Most geologists did not believe that this could be possible. In addition, Wegener did not have an acceptable theory of the forces that caused the continents to drift. He also ignored counter-arguments and evidence contrary to his theory and seemed too willing to interpret ambiguous evidence as being favorable to his theory (Williams 2000:59). For their part, the geologists ignored Wegener's copious body of evidence, allowing their adherence to a theory to override the actual data, when the scientific method would seem to demand the reverse approach - a common obstacle to the advancement of knowledge (see paradigm shift and belief perseverance). Plate tectonics, a modern update of the old ideas of Wegener about "plowing" continents, accommodates continental motion through the mechanism of seafloor spreading. New rock is created by volcanism at mid-ocean ridges and returned to the Earth's mantle at ocean trenches. Remarkably, in the 1928 AAPG volume, G. A. F. Molengraaf of the Delft Institute (now University) of Technology proposed a recognizable form of seafloor spreading in order to account for the opening of the Atlantic Ocean as well as the East Africa Rift. Arthur Holmes (an early supporter of Wegener) suggested that the movement of continents was the result of convection currents driven by the heat of the interior of the Earth, rather than the continents floating on the mantle. In the words of Carl Sagan (1995:302-03), it is more like the continents are being carried on a conveyor belt than floating or drifting. The ideas of Molengraaf and of Holmes led to the theory of plate tectonics, which replaced the theory of continental drift, and became the accepted theory in the 1960s (based on data that started to accumulate in the late 1950s). However, acceptance was gradual. Nowadays it is universally supported; but even in 1977 a textbook could write the relatively weak: "a poll of geologists now would probably show a substantial majority who favor the idea of drift" and devote a section to a serious consideration of the objections to the theory (Davis, 1977).
file://C:\Documents and Settings\s h a f i q\Desktop\Continental drift.htm
3/22/2007
Page 1 of 4
Continental drift:The concept that the world's continents once formed part of a single mass and have since drifted into their present positions. Although it was outlined by Alfred Wegener in 1912, the idea was not particularly new. Paleontological studies had already demonstrated such strong similarities between the flora and fauna of the southern continents between 300,000,000 and 150,000,000 years ago that a huge supercontinent, Gondwana, containing South America, Africa, India, Australia, and Antarctica, had been proposed. However, Gondwana was thought to be the southern continents linked by land bridges, rather than contiguous units. Wegener's ideas were almost universely rejected in 1928; the fundamental objection was the lack of a suitable mechanism. Almost simultaneously with the temporary eclipse of Wegener's theory, Arthur Holmes was considering a mechanism that is still widely accepted. Holmes conceived the idea of convective currents within the Earth's mantle which were driven by the radiogenic heat produced by radioactive minerals within the mantle. At that time, Holmes's ideas, like those of Wegener, were largely ignored. Nonetheless, several geologists, particularly those living in the Southern Hemisphere, continued to believe the theory and accumulate more data in its support. By the 1950s, convincing evidence had accumulated, with studies of the magnetization of rocks, paleomagnetism, beginning to provide numerical parameters on the past latitude and orientation of the continental blocks. Early work in North America and Europe clearly indicated how these continents had once been contiguous and had since separated. The discovery of the midoceanic ridge system also provided further evidence for the geometric matching of continental edges, but the discovery of magnetic anomalies parallel to these ridges and their interpretation in terms of sea-floor spreading finally led to almost universal acceptance of continental drift as a reality. In the 1970s and 1980s, the interest changed from proving the reality of the concept to applying it to the geologic record, leading to a greater understanding of how the Earth has evolved through time. The fundamental change in concept was that not only have the continents drifted, but the continents are merely parts of thicker tectonic plates comprising both oceanic and continental crust, with 50–300 km (31–186 mi) of the Earth's mantle moving along with them. OR Large-scale movements of continents over the course of geologic time. The first complete theory of continental drift was proposed in 1912 by Alfred Wegener, who postulated that a single supercontinent, which he called Pangea, fragmented late in the Triassic Period (248–206 million years ago) and that the parts began to move away from one another. He pointed to the similarity of rock strata in the Americas and Africa as evidence to support his hypothesis. Wegener's ideas received support from the concepts of seafloor spreading and plate tectonics beginning in the 1960s. The modern theory states that the Americas were joined with Europe and Africa until c. 190 million years ago, when they split apart along what is now the Mid-Atlantic Ridge. Subsequent tectonic plate movements brought the continents to their present positions. OR Geological theory that the relative positions of the continents on the earth's surface have changed considerably through geologic time. Though first proposed by American geologist Frank Bursley Taylor in a lecture in 1908, the first detailed theory of continental drift was put forth by German meteorologist and geophysicist Alfred Wegener in 1912. On the basis of geology, biology, climatology, and the alignment of the continental shelf rather than the coastline, he believed that during the late Paleozoic and early Mesozoic eras, about 275 to 175 million years ago, all the continents were united into a vast supercontinent, which he called Pangaea. Later, Pangaea broke into two supercontinental masses—
file://C:\Documents and Settings\s h a f i q\Desktop\Continental drift.htm
3/22/2007
Continental drift:-
Page 2 of 4
Laurasia to the north, and Gondwanaland to the south. The present continents began to split apart in the latter Mesozoic era about 100 million years ago, drifting to their present positions. As additional evidence Wegener cited the unusual presence of coal deposits in the South Polar regions, glacial features in present-day equatorial regions, and the jigsaw fit of the opposing Atlantic continental shelves. He also pointed out that a plastic layer in the earth's interior must exist to accommodate vertical adjustments caused by the creation of new mountains and by the wearing down of old mountains by erosion (see continent). He postulated that the earth's rotation caused horizontal adjustment of rock in this plastic layer, which caused the continents to drift. The frictional drag along the leading edges of the drifting continents results in mountain building. Wegener's theory stirred considerable controversy during the 1920s. South African geologist A. L. Dutoit, in 1921, strengthened the argument by adding more exacting details that correlated geological and paleontological similarities on both sides of the Atlantic. In 1928, Scottish geologist Arthur Holmes suggested that thermal convection in the mantle was the mechanism that drove the continental movements. American geologist David Griggs performed scale model experiments to show the mantle movements. The theory of continental drift was not generally accepted, particularly by American geologists, until the 1950s and 60s, when a group of British geophysicists reported on magnetic studies of rocks from many places and from each major division of geologic time. They found that for each continent, the magnetic pole had apparently changed position through geologic time, forming a smooth curve, or pole path, particular to that continent. The pole paths for Europe and North America could be made to coincide by bringing the continents together. OR The concept of continental drift was first proposed by Alfred Wegener. In 1912 he noticed that the shapes of continents on either side of the Atlantic Ocean seem to fit together (for example, Africa and South America). Francis Bacon, Antonio Snider-Pellegrini, Benjamin Franklin, and others had noted much the same thing earlier. The similarity of southern continent fossil faunae and some geological formations had led a relatively small number of Southern hemisphere geologists to conjecture as early as 1900 that all the continents had once been joined into a supercontinent known as Pangaea. The concept was initially ridiculed by most geologists, who felt that an explanation of how a continent drifted was a prerequisite and that the lack of one made the idea of drifting continents wholly unreasonable. The theory received support through the controversial years from South African geologist Alexander Du Toit as well as from Arthur Holmes. The idea of continental drift did not become widely accepted as theory until the 1950s in Europe. By the 1960s, geological research conducted by Robert S. Dietz, Bruce Heezen, and Harry Hess along with a rekindling of the theory including a mechanism by J. Tuzo Wilson led to acceptance among North American geologists. The hypothesis of continental drift became part of the larger theory of plate tectonics. This article deals mainly with the historical development of the continental drift hypothesis before 1950. See: plate tectonics for information on current ideas underlying concepts of continental drift.
Various data South America and Africa are moving apart at an average of 5.7 cm per year, due to the seafloor spreading along the Mid-Atlantic Ridge. This is comparable to the growth speed of a fingernail.
file://C:\Documents and Settings\s h a f i q\Desktop\Continental drift.htm
3/22/2007
Continental drift:-
Page 3 of 4
The fastest recorded seafloor spreading takes place along the East Pacific Rise at 17.2 cm per year. OR 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, in the form of plant and animal fossils of the same age found around different continent shores, suggesting that these shores were once joined. For example the fossils of the freshwater crocodile found in Brazil and South Africa. Another illustrative example is the discovery of fossils of the aquatic 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 Alfred Wegener to study what he defined as continental drift.
Permo-Carboniferous Permo-Carboniferous was a period of great glaciation that occurred about 250 million years ago. It is one of the many ice ages that has occurred on this Earth. This is also an era that has been used to submit proof that the continents were once a large land mass called pangaea. Permo-Carboniferous rocks are widely distributed in pangaea. The 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. Glacial activity spanned virtually the whole of Carboniferous and Early Permian time (A.G. Smith 1997). Toward the end of the Carboniferous, and around 290 million years ago, Gondwanda hovered over the south polar regions, where glacial centres expanded across the continents, as evidenced by glacial deposits of tillites along with striations in ancient rocks. Those heavily grooved by the advancing glaciers showed lines of ice flow away from the equator and toward the poles, which is the opposite direction if the continents were situated where they are today. Overall, the southern continents drifted together over the South Pole, and massive ice sheets radiating outward from a central point crossed the present continental boundaries. The PermoCarboniferous ice sheet is so extensive that it can fit within a latitude circle of 50 degrees (A.G.Smith 1997) (Rahul Megharaj 1985).
The debate over continental drift Before geophysical evidence started accumulating after World War II, the idea of continental drift caused sharp disagreement among geologists. Wegener had introduced his theory in 1912 at a meeting of the German Geological Association. His paper was published that year and expanded into a book in 1915. In 1921 the Berlin Geological Society held a symposium on the theory. In 1922 Wegener's book was translated into English and then it received a wider audience. In 1923 the theory was discussed at
file://C:\Documents and Settings\s h a f i q\Desktop\Continental drift.htm
3/22/2007
Continental drift:-
Page 4 of 4
conferences by Geological Society of France, the Geological Section of the British Association for the Advancement of Science, and the Royal Geological Society. The theory was carefully but critically reviewed in the journal Nature by Philip Lake. On November 15, 1926, the American Association of Petroleum Geologists (AAPG) held a symposium at which the continental drift hypothesis was vigorously debated. The resulting papers were published in 1928 under the title Theory of continental drift. Wegener himself contributed a paper to this volume (Friedlander 1995:21-27). One of the main problems with Wegener's theory was that he believed that the continents "plowed" through the rocks of the ocean basins. Most geologists did not believe that this could be possible. In addition, Wegener did not have an acceptable theory of the forces that caused the continents to drift. He also ignored counter-arguments and evidence contrary to his theory and seemed too willing to interpret ambiguous evidence as being favorable to his theory (Williams 2000:59). For their part, the geologists ignored Wegener's copious body of evidence, allowing their adherence to a theory to override the actual data, when the scientific method would seem to demand the reverse approach - a common obstacle to the advancement of knowledge (see paradigm shift and belief perseverance). Plate tectonics, a modern update of the old ideas of Wegener about "plowing" continents, accommodates continental motion through the mechanism of seafloor spreading. New rock is created by volcanism at mid-ocean ridges and returned to the Earth's mantle at ocean trenches. Remarkably, in the 1928 AAPG volume, G. A. F. Molengraaf of the Delft Institute (now University) of Technology proposed a recognizable form of seafloor spreading in order to account for the opening of the Atlantic Ocean as well as the East Africa Rift. Arthur Holmes (an early supporter of Wegener) suggested that the movement of continents was the result of convection currents driven by the heat of the interior of the Earth, rather than the continents floating on the mantle. In the words of Carl Sagan (1995:302-03), it is more like the continents are being carried on a conveyor belt than floating or drifting. The ideas of Molengraaf and of Holmes led to the theory of plate tectonics, which replaced the theory of continental drift, and became the accepted theory in the 1960s (based on data that started to accumulate in the late 1950s). However, acceptance was gradual. Nowadays it is universally supported; but even in 1977 a textbook could write the relatively weak: "a poll of geologists now would probably show a substantial majority who favor the idea of drift" and devote a section to a serious consideration of the objections to the theory (Davis, 1977).
file://C:\Documents and Settings\s h a f i q\Desktop\Continental drift.htm
3/22/2007
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