Pangaea
From Wikipedia, the free encyclopedia
Animation of the break-up of Pangaea and the formation of modern
continents.
Pangaea,
Pangæa, or
Pangea (pronounced
/pænˈdʒiːə/,
pan-JEE-ə,
[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.
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]
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
- ^ OED
- ^ Plate Tectonics and Crustal Evolution, Third Ed., 1989, by Kent C. Condie, Pergamon Press
- ^ 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.
- ^ 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.
- ^ 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.
- ^ Stanley, Steven (1998). Earth System History. USA. pp. 355–359.
- ^ Stanley, Steven (1998). Earth System History. USA. pp. 386–392.
- ^ Benton, M.J. Vertebrate Palaeontology. Third edition (Oxford 2005), 25.
- ^ Barbara W. Murck, Brian J. Skinner, Geology Today: Understanding Our Planet, Study Guide, Wiley, ISBN 978-0-471-32323-5
- ^ Philip Kearey, Keith A. Klepeis, Frederick J. Vine (2009). Global Tectonics (3rd. ed), p.66-67. Chichester:Wiley. ISBN 978-1-4051-0777-8
- ^ Zeeya Merali, Brian J. Skinner, Visualizing Earth Science, Wiley, ISBN 978-0470-41847-5
- ^ a b Zeeya Merali, Brian J. Skinner, Visualizing Earth Science, Wiley, ISBN 978-0470-41847-5
- ^ Nature 421, pp245-249 (16 January 2003) http://www.nature.com/nature/journal/v421/n6920/abs/nature01290.html
+Tri%C3%A1ssico+Superior+%28220%29.png)
(
(
(
(
