Pangaea
Pangaea or Pangea (
Origin of the concept
The name "Pangaea" is derived from
Wegener used the name "Pangaea" once in the 1920 edition of his book, referring to the ancient supercontinent as "the Pangaea of the Carboniferous".[12] He used the Germanized form Pangäa, but the name entered German and English scientific literature (in 1922[13] and 1926, respectively) in the Latinized form Pangaea, especially during a symposium of the American Association of Petroleum Geologists in November 1926.[14]
Wegener originally proposed that the breakup of Pangaea was caused by
Evidence of existence
The geography of the continents bordering the Atlantic Ocean was the first evidence suggesting the existence of Pangaea. The seemingly close fit of the coastlines of North and South America with Europe and Africa was remarked on almost as soon as these coasts were charted. Careful reconstructions showed that the mismatch at the 500 fathoms (3,000 feet; 910 meters) contour was less than 130 km (81 mi), and it was argued that this was much too similar to be attributed to coincidence.[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 covered the southern end of Pangaea. Glacial deposits, specifically till, of the same age and structure are found on many separate continents that would have been together in the continent of Pangaea.[19] The continuity of mountain chains provides further evidence, such as the Appalachian Mountains chain extending from the southeastern United States to the Scandinavian Caledonides of Europe;[20] these are now believed to have formed a single chain, the Central Pangean Mountains.
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 Antarctica, 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.[21]
Geologists can also determine
Formation
Pangaea is the most recent supercontinent reconstructed from the geologic record and therefore is by far the best understood. The formation of supercontinents and their breakup appears to be cyclical through Earth's history. There may have been several others before Pangaea.
Paleomagnetic measurements help geologists determine the latitude and orientation of ancient continental blocks, and newer techniques may help determine longitudes.[23] Paleontology helps determine ancient climates, confirming latitude estimates from paleomagnetic measurements, and the distribution of ancient forms of life provides clues on which continental blocks were close to each other at particular geological moments.[24] However, reconstructions of continents prior to Pangaea, including the ones in this section, remain partially speculative, and different reconstructions will differ in some details.[25]
Previous supercontinents
The fourth-last supercontinent, called
According to one reconstruction,[29] when Rodinia broke up, it split into three pieces: proto-Laurasia, proto-Gondwana, and the smaller Congo Craton. Proto-Laurasia and proto-Gondwana were separated by the Proto-Tethys Ocean. Proto-Laurasia 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 split created two oceans, the Iapetus Ocean and Paleoasian Ocean.[30]
Most of these landmasses coalesced again to form the relatively short-lived supercontinent Pannotia, which included large areas of land near the poles and a small strip connecting the polar masses near the equator. Pannotia lasted until 540
Formation of Euramerica (Laurussia)
In the Cambrian, Laurentia—which would later become
Collision of Gondwana with Euramerica
The second step in the formation of Pangaea was the collision of Gondwana with Euramerica. By the middle of the Silurian, 430 Ma, Baltica had already collided with Laurentia, forming Euramerica, an event called the Caledonian orogeny. 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 Rheic Ocean. It collided with southern Baltica in the Devonian.[34]
By the late Silurian, Annamia (
The Variscan orogeny raised the Central Pangaean Mountains, which were comparable to the modern Himalayas in scale. With Pangaea stretching from the South Pole across the equator and well into the Northern Hemisphere, an intense megamonsoon climate was established, except for a perpetually wet zone immediately around the central mountains.[37]
Formation of Laurasia
Western Kazakhstania collided with Baltica in the late Carboniferous, closing the Ural Ocean and the western Proto-Tethys (Uralian orogeny), causing the formation of the Ural Mountains and Laurasia. This was the last step of the formation of Pangaea. Meanwhile, South America had collided with southern Laurentia, closing the Rheic Ocean and completing the Variscian orogeny with the formation the southernmost part of the Appalachians and Ouachita Mountains. By this time, Gondwana was positioned near the South Pole, and glaciers formed in Antarctica, India, Australia, southern Africa, and South America. The North China Craton collided with Siberia by the Jurassic, completely closing the Proto-Tethys Ocean.[38]
By the Early Permian, the Cimmerian plate split from Gondwana and moved towards Laurasia, thus closing the Paleo-Tethys Ocean and forming the Tethys Ocean in its southern end. Most of the landmasses were all in one. By the Triassic, Pangaea rotated a little, and the Cimmerian plate was still travelling across the shrinking Paleo-Tethys until the Middle Jurassic. By the Late Triassic, the Paleo-Tethys had closed from west to east, creating the Cimmerian Orogeny. Pangaea, which looked like a C, with the Tethys Ocean inside the C, had rifted by the Middle Jurassic.[39]
Life
Pangaea existed as a supercontinent for 160 million years, from its assembly around 335 Ma (Early Carboniferous) to its breakup 175 Ma (Middle Jurassic).
The evolution of life in this time reflected the conditions created by the assembly of Pangaea. The union of most of the
The lack of oceanic barriers is thought to have favored cosmopolitanism, in which successful species attain wide geographical distribution. Cosmopolitanism was also driven by
Mass extinctions
The tectonics and geography of Pangaea may have worsened the Permian–Triassic extinction event or other mass extinctions. For example, the reduced area of continental shelf environments may have left marine species vulnerable to extinction.[50] However, no evidence for a species-area effect has been found in more recent and better characterized portions of the geologic record.[51][52] Another possibility is that reduced seafloor spreading associated with the formation of Pangaea, and the resulting cooling and subsidence of oceanic crust, may have reduced the number of islands that could have served as refugia for marine species. Species diversity may have already been reduced prior to mass extinction events due to mingling of species possible when formerly separate continents were merged. However, there is strong evidence that climate barriers continued to separate ecological communities in different parts of Pangaea. The eruptions of the Emeishan Traps may have eliminated South China, one of the few continental areas not merged with Pangaea, as a refugium.[53]
Rifting and break-up
There were three major phases in the break-up of Pangaea.
Opening of the Atlantic
The Atlantic Ocean did not open uniformly; rifting began in the north-central Atlantic. The first breakup of Pangaea is proposed for the late Ladinian (230 Ma) with initial spreading in the opening central Atlantic. Then the rifting proceeded along the eastern margin of North America, the northwest African margin and the High, Saharan and Tunisian Atlas Mountains.[54]
Another phase began in the Early-Middle Jurassic (about 175 Ma), when Pangaea began to rift from the Tethys Ocean in the east to the
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 much later to the closing of the Tethys Ocean and the widening of the "Sinus Borealis", which later became the Arctic Ocean. Meanwhile, on the other side of Africa and along the adjacent margins of east Africa, Antarctica and Madagascar, rifts formed that led to the formation of the southwestern Indian Ocean in the Cretaceous.
Break-up of Gondwana
The second major phase in the break-up of Pangaea began in the Early Cretaceous (150–140 Ma), when Gondwana separated into multiple continents (Africa, South America, India, Antarctica, and Australia). The subduction at Tethyan Trench probably caused Africa, India and Australia to move northward, causing the opening of a "South Indian Ocean". In the Early Cretaceous, Atlantica, today's South America and Africa, separated from eastern Gondwana. Then 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 Insular 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 eastern 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 toward the Pacific and opening the Coral Sea and Tasman Sea.
Opening of the Norwegian Sea and break-up of Australia and Antarctica
The third major and final phase of the break-up of Pangaea occurred in the early Cenozoic (Paleocene to Oligocene). Laurasia split when Laurentia broke 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 quickly northward, just as India had done more than 40 million years before. Australia is currently on a collision course with
Climate change after Pangaea
The breakup of Pangaea was accompanied by outgassing of large quantities of carbon dioxide from continental rifts. This produced a Mesozoic CO2 high that contributed to the very warm climate of the
The expansion of the temperate climate zones that accompanied the breakup of Pangaea may have contributed to the diversification of the angiosperms.[59]
See also
- History of Earth
- Potential future supercontinents: Amasia
- Supercontinent cycle
- Wilson Cycle
References
- ^ "Pangaea". Lexico UK English Dictionary. Oxford University Press. Archived from the original on October 25, 2020.
- ^ "Pangea". Encyclopædia Britannica Inc. 2015.
- ^ ISBN 978-0-19-516589-0
- ^ "Pangaea". Online Etymology Dictionary.
- ^ Vergilius Mario, Publius. Georgicon, IV.462
- ^ Lucan. Pharsalia, I.679
- ^ Lewis, C.T. & al. "Pangaeus" in A Latin Dictionary. (New York), 1879.
- ^ Usener, H. Scholia in Lucani Bellum Civile, Vol. I. (Leipzig), 1869.
- scholiast on Lucan glossed Pangaea id est totum terra—"Pangaea: that is, all land"—as having received its name on account of its smooth terrain and unexpected fertility.[8]
- ^ Kearey, Klepeis & Vine 2009, p. 2.
- ^ Alfred Wegener: Die Entstehung der Kontinente. Dr. A. Petermann's Mitteilungen aus Justus Perthes' Geographischer Anstalt, 58(1): Gotha 1912
- ^ See:
- Wegener, Alfred, Die Entstehung der Kontinente und Ozeane, 2nd ed. (Braunschweig, Germany: F. Vieweg, 1920), p. 120: "Schon die Pangäa der Karbonzeit hatte so einen Vorderrand ... " [Already the Pangaea of the Carboniferous era had such a leading edge ...] (In the 1922 edition, see p. 130.)
- Wegener, A.; Krause, R.; Thiede, J. (2005). "Kontinental-Verschiebungen: Originalnotizen und Literaturauszüge"(Continental drift: the original notes and quotations). Berichte zur Polar- und Meeresforschung (Reports on Polar and Marine Research) 516. Alfred-Wegener-Institut: Bremerhaven, p. 4, n. 2
- S2CID 131160418.
- ^ 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, United States, The American Association of Petroleum Geologists & London, Thomas Murby & Co.
- ISBN 978-1-4051-0777-8.
- S2CID 122872384.
- ^ Kearey, Klepeis & Vine 2009, pp. 5–8.
- S2CID 27169876.
- ISBN 978-0-471-32323-5
- ^ ISBN 047174705X
- ^ Benton, M.J. (2005) Vertebrate Palaeontology. Third edition, Oxford, p. 25.
- ^ Kearey, Klepeis & Vine 2009, pp. 66–67.
- S2CID 135171534.
- PMID 24951557.
- ISBN 9781107105324.
- .
- .
- .
- S2CID 129275224.
- ^ Torsvik & Cocks 2017, pp. 78–83.
- S2CID 134018369.
- ISBN 0-7167-2882-6.
- ^ Stanley 1999, pp. 386–392.
- ^ Torsvik & Cocks 2017, pp. 125, 153.
- .
- ^ Torsvik & Cocks 2017, pp. 140, 161.
- .
- ^ Torsvik & Cocks 2017, pp. 161, 171–172, 237.
- ^ Torsvik & Cocks 2017, pp. 180–181, 198.
- ^ a b "Life of the Carboniferous". UC Museum of Paleontology. UC Berkeley. Retrieved 19 February 2021.
- JSTOR 2097019.
- ^ "Jurassic Period: Life". UC Museum of Paleontology. UC Berkeley. Retrieved 19 February 2021.
- ISBN 978-0470387740.
- S2CID 58572291.
- .
- .
- PMID 18198148.
- PMID 29018290.
- ^ Erwin 1990, p. 75.
- S2CID 128878541.
- JSTOR 3514573.
- ^ Erwin 1990, p. 83.
- ^ Erwin 1990, pp. 83–84.
- ^ Antonio Schettino, Eugenio Turco: Breakup of Pangaea and plate kinematics of the central Atlantic and Atlas regions. In: Geophysical Journal International, Band 178, Ausgabe 2, August 2009, S. 1078–1097.
- S2CID 4326971.
- S2CID 135097410.
- ^ Stanley 1999, pp. 480–482.
- ISBN 9780760719572.
- PMID 25225405.