Gondwana

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A global paleogeographic reconstruction of the Earth in the later Triassic, 220 million years ago.

Gondwana,[1][2] formerly Gondwanaland, is the name given to a southern supercontinent which was formed when Pangaea broke up, starting 180 million years ago (mya), in the early middle Jurassic.[3]

The Pangaean global supercontinent completed its formation 250 million years ago (mya). After the split, Gondwana separated from Laurasia, which was the northern part of Pangaea. There were then these two large segments, nearly equal in area.[4]

While the northern hemisphere continent Laurasia moved further north, Gondwana drifted south. It included most of the landmasses in today's southern hemisphere, including Antarctica, South America, Africa, Madagascar, AustraliaNew Guinea, and New Zealand, as well as Arabia and the Indian subcontinent, which have now moved entirely into the Northern Hemisphere.

Gondwana itself began to break up in the mid-Jurassic, about 170 mya. It split into the present-day continents listed above.

History of the name[change | change source]

Nothofagus is a plant genus that illustrates Gondwanan distribution. It originated in the supercontinent and still exists in parts of Australia, New Zealand, New Caledonia and Chile. Fossils have also recently been found in Antarctica.[5]

The continent of Gondwana was named by Austrian scientist, Eduard Suess, after the Gondwana region of central northern India (from Sanskrit gondavana 'forest of the Gonds'), from which the Gondwana sedimentary sequences (PermianTriassic) are also found.

The adjective Gondwanan is in common use in biogeography when referring to patterns of distribution of living organisms, typically when the organisms are restricted to two or more of the now-discontinuous regions that were once part of Gondwana, including the Antarctic flora. For example, the Proteaceae, a family of plants that is known only from southern South America, South Africa, and Australia are considered to have a "Gondwanan distribution". This pattern shows an ancient lineage.

It was evidence of plant and animal distribution which lent weight to the ideas of Alfred Russel Wallace and Alfred Wegener. Wallace explained geographical distribution as the result of evolution, and Wegener used it as evidence for continental drift.

Gondwana breakup (160–23 mya)[change | change source]

Africa separated from Antarctica around 160 mya, followed by the Indian subcontinent, in the early Cretaceous (about 125 mya). About 65 mya, Antarctica (then connected to Australia) still had a tropical to subtropical climate, complete with a marsupial fauna. About 40 mya Australia-New Guinea separated from Antarctica, so that latitudinal currents could isolate Antarctica from Australia, and the first ice began to appear. During the Eocene-Oligocene extinction event about 34 million years ago, CO2 levels have been found to be about 760 ppm[6] and had been decreasing from earlier levels in the thousands of ppm. Around 23 mya, the Drake Passage opened between Antarctica and South America, resulting in the Antarctic Circumpolar Current that completely isolated the continent. Models of the changes suggest that declining CO2 levels became more important.[7] The ice began to spread, replacing the forests that then covered the continent. Since about 15 mya, the continent has been mostly covered with ice,[8] with the Antarctic ice cap reaching its present extension around 6 mya.

Submerged former lands[change | change source]

There are several submarine lands in the Indian Ocean off the west of Australia. They are under more than 1.5 kilometres of water. Evidence that they are former parts of Gondwana is given by their rocks, which are not typical oceanic rocks like basalt. They are typical land rocks such as granite, sandstone, and gneiss, and they have fossils of a kind now found on continental areas. Recently two of these sunken islands were found to the west of Perth.[9] These islands are almost the size of Tasmania, and have flat tops. This shows they were once at sea level before being submerged. The conclusion is that when India began to break away from Australasia in the early Cretaceous, the islands formed part of the last link between the two present-day continents.[10]

Naturaliste Plateau[change | change source]

Already known is Naturaliste Plateau, a submerged land off Western Australia.[11] It has an area of 90,000 square kilometres. Interest has been shown in its potential oil deposits. When above land in the Mesozoic, it had a tropical climate which might have been ideal for producing coal, oil and natural gas.

Kerguelen microcontinent[change | change source]

The Kerguelen Plateau is a submerged microcontinent in the southern Indian Ocean. It is about 3,000 km to the southwest of Australia and extends for more than 2,200 km in a northwest-southeast direction. It is in deep water, but there is a small part of the plateau that breaks sea level, forming the French Kerguelen Islands, and the Australian Heard Island and McDonald Islands.

The islands are part of a large igneous province (LIP) which started when Gondwana started to break up, 130 million years ago in the Lower Cretaceous.

Volcanic activity occurs sometimes on Heard and McDonald islands.[12]

References[change | change source]

  1. "Gondwana". Dictionary.com. Lexico Publishing Group. http://dictionary.reference.com/browse/gondwana. Retrieved 2010-01-18.
  2. "Gondwanaland". Merriam-Webster Online Dictionary. http://www.merriam-webster.com/dictionary/gondwana. Retrieved 2010-01-18.
  3. Buchan, Craig (2004). "Paper #207-8 - Linking subduction initiation, accretionary orogenesis and Supercontinent assembly" in 2004 Denver Annual Meeting. Geological Society of America. 
  4. Houseman, Greg. "Dispersal of Gondwanaland". University of Leeds. http://homepages.see.leeds.ac.uk/~eargah/Gond.html. Retrieved 21 Oct 2008.
  5. H.M. Li and Z.K. Zhou 2007. Fossil nothofagaceous leaves from the Eocene of western Antarctica and their bearing on the origin, dispersal and systematics of Nothofagus. Science in China. 50: 1525-1535.
  6. "New CO2 data helps unlock the secrets of Antarctic formation". Physorg.com. http://www.physorg.com/news172072921.html. Retrieved 2011-07-26.
  7. DeConto1, Robert M.; Pollard, David (2003). "Rapid Cenozoic glaciation of Antarctica induced by declining atmospheric CO2". Nature 421 (6920): 245–9. doi:10.1038/nature01290 . PMID 12529638 . http://www.nature.com/nature/journal/v421/n6920/abs/nature01290.html. Retrieved 19 October 2009.
  8. Trewby, Mary, ed. (2002). Antarctica: an encyclopedia from Abbott ice shelf to zooplankton. Firefly Books. ISBN 1-55297-590-8 .
  9. Sunken islands could cause tectonic shift in Gondwana story. University of Sydney [1]
  10. Scientists find two 'sunken islands' off WA – part of Gondwana land link. Perth Now [2]
  11. Naturaliste Plateau – regional setting. Geoscience Australia
  12. UT Austin scientist plays major role in study of underwater “micro-continent”. University of Texas. [3]