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Pangaea

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Map of Pangaea
Animation of the break-up of Pangaea and the formation of modern continents.
Pangaea, Pangæa, or Pangea (pronounced /pænˈdʒiːə/, pan-JEE,[1] from Ancient Greek πᾶν pan "entire", and Γαῖα Gaia "Earth", Latinized as Gæa) was the supercontinent that existed during the Paleozoic and Mesozoic eras about 250 million years ago, before the component continents were separated into their current configuration.[2]
The name was coined during a 1926 symposium discussing Alfred Wegener's theory of continental drift. In his book The Origin of Continents and Oceans (Die Entstehung der Kontinente und Ozeane) first published in 1915, he postulated that all the continents had at one time formed a single supercontinent which he called the "Urkontinent", before later breaking up and drifting to their present locations.[3]
The single enormous ocean which surrounded Pangaea was accordingly named Panthalassa.

Contents


Formation

The breaking up and formation of supercontinents appear to be cyclical through Earth's 4.6 billion year history. There may have been several others before Pangaea. The next-to-last one, Pannotia, formed about 600 million years ago (Ma) during the Proterozoic eon, and lasted until 540 Ma. Before Pannotia, there was Rodinia, which lasted from about 1.1 billion years ago (Ga) until about 750 million years ago. Rodinia formed by the accretion and assembly of fragments produced by breakup of an older supercontinent, called Columbia or Nuna that was assembled in the period 2.0-1.8 Ga.[4][5] The exact configuration and geodynamic history of Rodinia are not nearly as well understood as Pannotia and Pangaea. When Rodinia broke up, it split into three pieces: the supercontinent of Proto-Laurasia and the supercontinent of Proto-Gondwana, and the smaller Congo craton. Proto-Laurasia and Proto-Gondwanaland were separated by the Proto-Tethys Ocean. Soon thereafter Proto-Laurasia itself split apart to form the continents of Laurentia, Siberia and Baltica. The rifting also spawned two new oceans, the Iapetus Ocean and Paleoasian Ocean. Baltica was situated east of Laurentia, and Siberia northeast of Laurentia.
Around 600 Ma, most of these masses came back together to form the relatively short-lived supercontinent of Pannotia, which included large amounts of land near the poles and only a relatively small strip near the equator connecting the polar masses.
Only 60 million years after its formation, about 540 Ma, near the beginning of the Cambrian epoch, Pannotia in turn broke up, giving rise to the continents of Laurentia, Baltica, and the southern supercontinent of Gondwana.
In the Cambrian period, the independent continent of Laurentia, which would 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 that would become the northeastern United States, Nova Scotia and England, broke free from Gondwana and began its journey to Laurentia.[6]
Euramerica's formation
Appalachian orogeny
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.[7]
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 to form Euramerica. Avalonia had not collided with Laurentia yet, and a seaway between them, a remnant of the Iapetus Ocean, was still shrinking as Avalonia slowly inched towards Laurentia.
Meanwhile, southern Europe fragmented from Gondwana and started to head towards Euramerica across the newly formed Rheic Ocean and collided with southern Baltica in the Devonian, though this microcontinent was an underwater plate. The Iapetus Ocean's sister ocean, the Khanty Ocean, was also shrinking 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 rifted away from Gondwana and started to head northward across the shrinking Proto-Tethys Ocean, and on its southern end the new Paleo-Tethys Ocean was opening. In the Devonian Period, Gondwana itself headed towards Euramerica, which caused 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 towards the South Pole from the equator.
North China 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 the Ural Mountains, and the formation of 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 rifted away from Gondwana and headed towards Laurasia, with a new ocean forming in its southern end, the Tethys Ocean, and the closure of the Paleo-Tethys Ocean. Most of the landmasses were all in one. By the Triassic Period, Pangaea rotated a little, in a southwest direction. 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 looked like a C, with an ocean inside the C, the new Tethys Ocean. Pangaea had rifted by the Middle Jurassic, and its deformation is explained below.

Evidence of existence

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 therapsid Lystrosaurus 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 only been found in localized regions of the coasts of Brazil and West Africa.[8]
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.[9]
Paleomagnetic study of apparent polar wandering paths also support the theory of a super-continent. 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 typically are an average of these "secular variations" in the orientation of Magnetic North because their magnetic fields are 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. This leaves geologists with the portion of this motion that shows continental drift, and can be used to help reconstruct earlier continental positions.[10]
The continuity of mountain chains also provide evidence for Pangea. One example of this is the Appalachian Mountains chain which extends from the northeastern United States to the Caledonides of Ireland, Britain, Greenland, and Scandinavia.[11]

Rifting and break-up

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 and 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.[12]
The Atlantic Ocean did not open uniformly; rifting began in the north-central Atlantic. The South Atlantic did not open until the Cretaceous. Laurasia started to rotate clockwise and moved northward with North America to the north, and Eurasia to the south. The clockwise motion of Laurasia also led to the closing of the Tethys Ocean. Meanwhile, on the other side of Africa, new rifts were also forming along the adjacent margins of east Africa, Antarctica and Madagascar that would lead to the formation of the southwestern Indian Ocean that would 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.[12]
This subduction zone was 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) per year (a plate tectonic record), closing the Tethys Ocean, while Madagascar stopped and became locked to the African Plate. New Zealand, New Caledonia and the rest of Zealandia began to separate from Australia, moving eastward towards 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 did more than 40 million years earlier, and is currently on a collision course with eastern Asia. Both Australia and India are currently moving in a northeastern direction at 5–6 centimeters (2–3 in) per 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. The latter of which, together with decreasing atmospheric carbon dioxide concentrations caused a rapid cooling of Antarctica and allowed glaciers to form, which eventually coalesced into the kilometers thick ice sheets we see today.[13] 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 Great Rift Valley.

See also

References

  1. ^ OED
  2. ^ Plate Tectonics and Crustal Evolution, Third Ed., 1989, by Kent C. Condie, Pergamon Press
  3. ^ cf. Willem A. J. M. van Waterschoot van der Gracht (and 13 other authors): Theory of Continental Drift: a Symposium of the Origin and Movements of Land-masses of both Inter-Continental and Intra-Continental, as proposed by Alfred Wegener. X + 240 S., Tulsa, Oklahoma, USA, The American Association of Petroleum Geologists & London, Thomas Murby & Co, 1928.
  4. ^ Zhao, Guochun; Cawood, Peter A.; Wilde, Simon A.; Sun, M. (2002). Review of global 2.1–1.8 Ga orogens: implications for a pre-Rodinia supercontinent. Earth-Science Reviews, v. 59, p. 125-162. 
  5. ^ Zhao, Guochun; Sun, M.; Wilde, Simon A.; Li, S.Z. (2004). A Paleo-Mesoproterozoic supercontinent: assembly, growth and breakup. Earth-Science Reviews, v. 67, p. 91-123. 
  6. ^ Stanley, Steven (1998). Earth System History. USA. pp. 355–359. 
  7. ^ Stanley, Steven (1998). Earth System History. USA. pp. 386–392. 
  8. ^ Benton, M.J. Vertebrate Palaeontology. Third edition (Oxford 2005), 25.
  9. ^ Barbara W. Murck, Brian J. Skinner, Geology Today: Understanding Our Planet, Study Guide, Wiley, ISBN 978-0-471-32323-5
  10. ^ Philip Kearey, Keith A. Klepeis, Frederick J. Vine (2009). Global Tectonics (3rd. ed), p.66-67. Chichester:Wiley. ISBN 978-1-4051-0777-8
  11. ^ Zeeya Merali, Brian J. Skinner, Visualizing Earth Science, Wiley, ISBN 978-0470-41847-5
  12. ^ a b Zeeya Merali, Brian J. Skinner, Visualizing Earth Science, Wiley, ISBN 978-0470-41847-5
  13. ^ Nature 421, pp245-249 (16 January 2003) http://www.nature.com/nature/journal/v421/n6920/abs/nature01290.html

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