Continental drift is the movement of the Earth's
continents relative to each other, thus appearing to "drift" across the ocean bed.
[2] The speculation that continents might have 'drifted' was first put forward by
Abraham Ortelius in 1596. The concept was independently and more fully developed by
Alfred Wegener in 1912, but his theory was rejected by some for lack of a mechanism (though this was supplied later by Holmes) and others because of prior theoretical commitments. The idea of continental drift has been subsumed by the theory of
plate tectonics, which explains how the continents move.
[3]
In 1858 Antonio Snider-Pellegrini created two maps demonstrating how America and Africa continents may have once fit together, then separated.
History[edit]
Early history[edit]
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]."
Writing in 1889, Alfred Russel Wallace remarks "It was formerly a very general belief, even amongst geologists, that the great features of the earth's surface, no less than the smaller ones, were subject to continual mutations, and that during the course of known geological time the continents and great oceans had again and again changed places with each other."[8] He quotes Charles Lyell as saying "Continents, therefore, although permanent for whole geological epochs, shift their positions entirely in the course of ages"[9] and claims that the first to throw doubt on this was James D. Dana in 1849.
In his Manual of Geology, 1863, Dana says "The continents and oceans had their general outline or form defined in earliest time. This has been proved with respect to North America from the position and distribution of the first beds of the Silurian - those of the Potsdam epoch. … and this will probably prove to the case in Primordial time with the other continents also".[10] Dana was enormously influential in America - his Manual of Mineralogy is still in print in revised form - and the theory became known as Permanence theory.[11]
This appeared to be confirmed by the exploration of the deep sea beds conducted by the
Challenger expedition, 1872-6, which showed that contrary to expectation, land debris brought down by rivers to the ocean is deposited comparatively close to the shore in what is now known as the
continental shelf. This suggested that the oceans were a permanent feature of the earth's surface, and did not change places with the continents.
[12]
Wegener and his predecessors[edit]
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.
[22][23][24]
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.
[25][clarification needed]
Wegener was the first to use the phrase "continental drift" (1912, 1915)
[13][14] (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.
[26] The
Polfluchthypothesis was also studied by
Paul Sophus Epstein in 1920 and found to be implausible.
Evidence of continental 'drift'[edit]
Mesosaurus skeleton, MacGregor, 1908.
Evidence for the movement of continents on tectonic plates is now extensive. Similar plant and animal fossils are found around the shores of different continents, 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 in rocks of the same age at locations in Africa, India, and Antarctica.[27] There is also living evidence—the same animals being found on two continents. Someearthworm 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 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, based on continents' current positions and orientations, and supported the idea that the southern continents had previously been in dramatically different locations, as well as being contiguous with each other.
[14]
Rejection of Wegener's theory[edit]
The theory of continental drift was not accepted for many years. One problem was that a plausible driving force was missing.
[2] And it did not help that Wegener was not a geologist. Other geologists also believed that the evidence that Wegener had provided was not sufficient. It is now accepted that the plates carrying the continents do move across the Earth's surface; ironically one of the chief outstanding questions is the one Wegener failed to resolve: what is the nature of the forces propelling the plates?
[2]
The British geologist
Arthur Holmes championed the theory of continental drift at a time when it was deeply unfashionable. He proposed in 1931 that the Earth's mantle contained convection cells that dissipated radioactive heat and moved the crust at the surface.
[28] His
Principles of Physical Geology, ending with a chapter on continental drift, was published in 1944.
[29]
David Attenborough, who attended university in the second half of the 1940s, recounted an incident illustrating its lack of acceptance then: "I once asked one of my lecturers why he was not talking to us about continental drift and I was told, sneeringly, that if I could prove there was a force that could move continents, then he might think about it. The idea was
moonshine, I was informed."
[30]
Geological maps of the time showed huge land bridges spanning the Atlantic and Indian oceans to account for the similarities of fauna and flora and the divisions of the Asian continent in the Permian era but failing to account for glaciation in India, Australia and South Africa.
[31]
As late as 1953 – just five years before
Carey[32] introduced the theory of
plate tectonics – the theory of continental drift was rejected by the physicist Scheidegger on the following grounds.
[33]
- 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 (in which the forces of gravity and buoyancy are in balance). But gravitational measurements showed 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 solidified while other parts were still fluid. Various attempts to explain this foundered on other difficulties.
Geophysicist
Jack Oliver is credited with providing seismologic evidence supporting plate tectonics which encompassed and superseded continental drift with the article "Seismology and the New Global Tectonics", published in 1968, using data collected from seismologic stations, including those he set up in the South Pacific.
[34][35]
It is now known that there are two kinds of crust:
continental crust and
oceanic crust. Continental crust is inherently lighter and its composition is different from oceanic crust, but both kinds reside above a much deeper "
plastic" mantle. Oceanic crust is 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. The theory of
plate tectonics explains all this, including the movement of the continents, better than Wegener's theory.
Pangaea or
Pangea (
//[1]) was a
supercontinent that existed during the late
Paleozoic and early
Mesozoic eras.
[2] It formed approximately 300 million years ago and then began to break apart after about 100 million years.
[3] Unlike the present
Earth, much of the land mass was in the
southern hemisphere. Pangaea was the first reconstructed supercontinent and it was surrounded by a super ocean, known as
Panthalassa.
Origin of the concept[edit]
The name is derived from
Ancient Greek pan (
πᾶν, "all, entire, whole") and
Gaia (
Γαῖα, "
Mother Earth, land").
[4][9] The concept was first proposed by
Alfred Wegener, the originator of the theory of
continental drift, in his 1912 publication
The Origin of Continents (
Die Entstehung der Kontinente).
[10] He expanded this hypothesis into his theory in his book
The Origin of Continents and Oceans (
Die Entstehung der Kontinente und Ozeane), first published in 1915, in which he postulated that (before breaking up and drifting to their present locations) all the continents had formed a single
supercontinent that he called the "
Urkontinent". The name first occurs in the 1920 edition of
Die Entstehung der Kontinente und Ozeane, but only once, when Wegener refers to the ancient supercontinent as "the Pangaea of the Carboniferous".
[11] Wegener used the Germanized form "Pangäa", but the term entered German and English scientific literature (in 1922
[12] and 1926, respectively) in the Latinized form "Pangaea" (of the Greek "Pangaia"), especially due to a symposium of the American Association of Petroleum Geologists in November 1926.
[13]
Formation of Pangaea[edit]
The forming of supercontinents and their breaking up appears to have been
cyclical through Earth's history. There may have been
many others before Pangaea. The fourth-last supercontinent, called
Columbia or Nuna, appears to have assembled in the period 2.0–1.8
Ga.[14][15] Columbia/Nuna broke up and the next supercontinent,
Rodinia, formed from the
accretion and assembly of its fragments. Rodinia lasted from about 1.1 billion years ago (Ga) until about 750 million years ago, but its exact configuration and geodynamic history are not nearly as well understood as those of the later supercontinents, Pannotia and Pangaea.
When Rodinia broke up, it split into three pieces: the supercontinent of
Proto-Laurasia, the supercontinent of
Proto-Gondwana, and the smaller
Congo craton. Proto-Laurasia and Proto-Gondwana were separated by the
Proto-Tethys Ocean. Next
Proto-Laurasia itself split apart to form the continents of
Laurentia,
Siberia and
Baltica. Baltica moved to the east of Laurentia, and Siberia moved northeast of Laurentia. The splitting also created two new oceans, the
Iapetus Ocean and
Paleoasian Ocean. Most of the above masses coalesced again to form the relatively short-lived supercontinent of
Pannotia. This supercontinent included large amounts of land near the poles and, near the equator, only a relatively small strip connecting the polar masses. Pannotia lasted until 540 Ma, near the beginning of the
Cambrian period and then broke up, giving rise to the continents of
Laurentia,
Baltica, and the southern supercontinent of
Gondwana.
In the
Cambrian period, the continent of
Laurentia, which would later become
North America, sat on the
equator, with three bordering oceans: the
Panthalassic Ocean to the north and west, the
Iapetus Ocean to the south and the
Khanty Ocean to the east. In the Earliest
Ordovician, around 480 Ma, the microcontinent of
Avalonia – a landmass incorporating fragments of what would become eastern
Newfoundland, the southern
British Isles, and parts of
Belgium, northern
France,
Nova Scotia,
New England,
Iberia and northwest Africa – broke free from Gondwana and began its journey to
Laurentia.
[16] Baltica, Laurentia, and Avalonia all came together by the end of the Ordovician to form a minor supercontinent called
Euramerica or Laurussia, closing the Iapetus Ocean. The collision also resulted in the formation of the northern
Appalachians.
Siberia sat near Euramerica, with the
Khanty Ocean between the two continents. While all this was happening, Gondwana drifted slowly towards the South Pole. This was the first step of the formation of Pangaea.
[17]
The second step in the formation of Pangaea was the collision of Gondwana with
Euramerica. By
Silurian time, 440 Ma, Baltica had already collided with Laurentia, forming Euramerica.
Avalonia had not yet collided with
Laurentia, but as Avalonia inched towards Laurentia, the seaway between them, a remnant of the
Iapetus Ocean, was slowly shrinking. Meanwhile,
southern Europe broke off from Gondwana and began to move towards Euramerica across the newly formed
Rheic Ocean. It collided with southern
Baltica in the
Devonian, though this microcontinent was an underwater plate. The Iapetus Ocean's sister ocean, the Khanty Ocean, shrank as an island arc from Siberia collided with eastern Baltica (now part of Euramerica). Behind this
island arc was a new ocean, the
Ural Ocean.
By late Silurian time,
North and
South China split from Gondwana and started to head northward, shrinking the Proto-Tethys Ocean in their path and opening the new
Paleo-Tethys Ocean to their south. In the Devonian Period, Gondwana itself headed towards Euramerica, causing the Rheic Ocean to shrink. In the Early
Carboniferous, northwest
Africa had touched the southeastern coast of
Euramerica, creating the southern portion of the
Appalachian Mountains, and the
Meseta Mountains.
South America moved northward to southern Euramerica, while the eastern portion of Gondwana (
India,
Antarctica and
Australia) headed toward the South Pole from the
equator. North and South China were on independent continents. The
Kazakhstania microcontinent had collided with
Siberia. (Siberia had been a separate continent for millions of years since the deformation of the supercontinent
Pannotia in the Middle Carboniferous.)
Western
Kazakhstania collided with
Baltica in the Late Carboniferous, closing the
Ural Ocean between them and the western Proto-Tethys in them (
Uralian orogeny), causing the formation of not only the
Ural Mountains but also the supercontinent of Laurasia. This was the last step of the formation of Pangaea. Meanwhile, South America had collided with southern
Laurentia, closing the
Rheic Ocean and forming the southernmost part of the
Appalachians and
Ouachita Mountains. By this time, Gondwana was positioned near the South Pole and glaciers were forming in Antarctica, India, Australia, southern Africa and South America. The
North China block collided with
Siberia by Late Carboniferous time, completely closing the Proto-Tethys Ocean.
By Early
Permian time, the
Cimmerian plate split from
Gondwana and headed towards Laurasia, thus closing the
Paleo-Tethys Ocean, but forming a new ocean, the
Tethys Ocean, in its southern end. Most of the landmasses were all in one. By the
Triassic Period, Pangaea rotated a little and the Cimmerian plate was still travelling across the shrinking Paleo-Tethys, until the
Middle Jurassic time. The Paleo-Tethys had closed from west to east, creating the
Cimmerian Orogeny. Pangaea, which looked like a
C, with the new Tethys Ocean inside the
C, had rifted by the Middle Jurassic, and its deformation is explained below.
Evidence of existence[edit]
The distribution of fossils across the continents is one line of evidence pointing to the existence of Pangaea.
Fossil evidence for Pangaea includes the presence of similar and identical species on continents that are now great distances apart. For example, fossils of the
therapsidLystrosaurus have been found in
South Africa,
India and
Australia, alongside members of the
Glossopteris flora, whose distribution would have ranged from the polar circle to the equator if the continents had been in their present position; similarly, the freshwater reptile
Mesosaurus has been found in only localized regions of the coasts of
Brazil and
West Africa.
[18]
Additional evidence for Pangaea is found in the
geology of adjacent continents, including matching geological trends between the eastern coast of
South America and the western coast of
Africa. The
polar ice cap of the
Carboniferous Period covered the southern end of Pangaea. Glacial deposits, specifically
till, of the same age and structure are found on many separate continents which would have been together in the continent of Pangaea.
[19]
Paleomagnetic study of apparent polar wandering paths also support the theory of a supercontinent. Geologists can determine the movement of continental plates by examining the orientation of magnetic minerals in rocks; when rocks are formed, they take on the magnetic properties of the Earth and indicate in which direction the poles lie relative to the rock. Since the magnetic poles
drift about the rotational pole with a period of only a few thousand years, measurements from numerous lavas spanning several thousand years are averaged to give an apparent mean polar position. Samples of
sedimentary rock and
intrusive igneous rock have magnetic orientations that are typically an average of these "secular variations" in the orientation of
Magnetic North because their magnetic fields were not formed in an instant, as is the case in a cooling lava. Magnetic differences between sample groups whose age varies by millions of years is due to a combination of
true polar wander and the drifting of continents. The true polar wander component is identical for all samples, and can be removed, leaving geologists with the portion of this motion that shows continental drift and can be used to help reconstruct earlier continental positions.
[20]
The continuity of mountain chains provides further evidence for Pangaea. One example of this is the
Appalachian Mountains chain which extends from the southeastern
United States to the
Caledonides of Ireland, Britain, Greenland, and Scandinavia.
[21]
Rifting and break-up[edit]
Animation of the rifting of Pangaea
There were three major phases in the break-up of Pangaea. The first phase began in the
Early-
Middle Jurassic (about 175 Ma), when Pangaea began to rift from the Tethys Ocean in the east to the
Pacific in the west, ultimately giving rise to the supercontinents
Laurasia and
Gondwana. The rifting that took place between North America and Africa produced multiple
failed rifts. One rift resulted in a new ocean, the North
Atlantic Ocean.
[22]
The Atlantic Ocean did not open uniformly; rifting began in the north-central Atlantic. The
South Atlantic did not open until the
Cretaceous when Laurasia started to rotate clockwise and moved northward with North America to the north, and
Eurasia to the south. The clockwise motion of Laurasia led to the closing of the Tethys Ocean. Meanwhile, on the other side of Africa and along the adjacent margins of east Africa, Antarctica and
Madagascar, new rifts were forming that would not only lead to the formation of the southwestern
Indian Ocean but also open up in the Cretaceous.
The second major phase in the break-up of Pangaea began in the
Early Cretaceous (150–140 Ma), when the minor supercontinent of Gondwana separated into multiple continents (Africa, South America, India, Antarctica, and Australia). About 200 Ma, the continent of
Cimmeria, as mentioned above (see "
Formation of Pangaea"), collided with Eurasia. However, a subduction zone was forming, as soon as Cimmeria collided.
[22]
This subduction zone is called the
Tethyan Trench. This trench might have subducted what is called the Tethyan
mid-ocean ridge, a ridge responsible for the Tethys Ocean's expansion. It probably caused Africa, India and Australia to move northward. In the Early Cretaceous,
Atlantica, today's South America and Africa, finally separated from eastern Gondwana (Antarctica, India and Australia), causing the opening of a "South Indian Ocean". In the Middle Cretaceous, Gondwana fragmented to open up the South Atlantic Ocean as South America started to move westward away from Africa. The South Atlantic did not develop uniformly; rather, it rifted from south to north.
Also, at the same time,
Madagascar and India began to separate from Antarctica and moved northward, opening up the Indian Ocean. Madagascar and India separated from each other 100–90 Ma in the Late Cretaceous. India continued to move northward toward Eurasia at 15 centimeters (6 in) a year (a plate tectonic record), closing the Tethys Ocean, while Madagascar stopped and became locked to the
African Plate.
New Zealand,
New Caledoniaand the rest of
Zealandia began to separate from Australia, moving eastward toward the
Pacific and opening the
Coral Sea and
Tasman Sea.
The third major and final phase of the break-up of Pangaea occurred in the early
Cenozoic (
Paleocene to
Oligocene).
Laurasia split when North America/Greenland (also called
Laurentia) broke free from Eurasia, opening the
Norwegian Sea about 60–55 Ma. The Atlantic and Indian Oceans continued to expand, closing the Tethys Ocean.
Meanwhile, Australia split from Antarctica and moved rapidly northward, just as India had done more than 40 million years before. It is currently on a collision course with
eastern Asia. Both Australia and India are currently moving northeast at 5–6 centimeters (2–3 in) a year. Antarctica has been near or at the South Pole since the formation of Pangaea about 280 Ma. India started to collide with
Asia beginning about 35 Ma, forming the
Himalayan orogeny, and also finally closing the
Tethys Seaway; this collision continues today. The African Plate started to change directions, from west to northwest toward
Europe, and South America began to move in a northward direction, separating it from Antarctica and allowing complete oceanic circulation around Antarctica for the first time. This motion, together with decreasing atmospheric
carbon dioxide concentrations, caused a rapid cooling of Antarctica and allowed
glaciers to form. This glaciation eventually coalesced into the kilometers-thick ice sheets seen today.
[23] Other major events took place during the
Cenozoic, including the opening of the
Gulf of California, the uplift of the
Alps, and the opening of the
Sea of Japan. The break-up of Pangaea continues today in the
Red Sea Rift and
East African Rift.
