Gondwana
2008/9 Schools Wikipedia Selection. Related subjects: Geology and geophysics
Gondwana (IPA: /ɡɒnˈdwɑːnə/, originally Gondwanaland) was a southern supercontinent that existed about 500 to 200 Ma ago. Gondwana included most of the landmasses in today's southern hemisphere, including Antarctica, South America, Africa, Madagascar, Australia-New Guinea, and New Zealand, as well as Arabia and the Indian subcontinent, which are in the Northern Hemisphere. The name is derived from the Gondwana region of central northern India (from Sanskrit gondavana "forest of Gond").
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; e.g., the Proteaceae, a family of plants that is known only from Chile, South Africa, and Australia are considered to have a "Gondwanan distribution". This pattern is often considered to indicate an archaic, or relict lineage.
Formation
The assembly of Gondwana was a protracted process. Several orogenies led to its final amalgamation 550–500 million years ago in the Cambrian. These include the Brasiliano Orogeny, the East African Orogeny, the Malagasy Orogeny, and the Kuunga Orogeny. The final stages of Gondwana assembly overlapped with the opening of the Iapetus Ocean between Laurentia and western Gondwana. During this interval the Cambrian Explosion occurred.
Gondwanaland was formed by these earlier continents and microcontinents, and others, colliding in these orogenies:
- Azania: much of central Madagascar, the Horn of Africa and parts of Yemen and Arabia. (Named by Collins and Pisarevsky (2005): "Azania" was a Greek name for the East African coast.)
- The Congo–Tanzania– Bangweulu Block of central Africa.
- Neoproterozoic India: India, the Antongil Block in far eastern Madagascar, the Seychelles, and the Napier and Rayner Complexes in East Antarctica.
- The Australia/ Mawson continent: Australia west of Adelaide and a large extension into East Antarctica.
- Other blocks which helped to form Argentina and around, including a piece transferred from Laurentia when the west edge of Gondwana scraped against southeast Laurentia in the Ordovician.
One of the major sites of Gondwana amalgamation was the East African Orogen (Stern, 1994), where these two major orogenies are superimposed on each other:
The East African Orogeny (as later defined) at ~650–630 Ma ago affected a large part of Arabia, north-eastern Africa, East Africa and Madagascar. Collins and Windley (2002) propose that in this orogeny Azania collided with the Congo–Tanzania– Bangweulu Block.
The later Malagasy orogeny at ~550–515 Ma ago affected Madagascar, eastern East Africa and southern India. In it Neoproterozoic India collided with the already combined Azania and Congo–Tanzania–Bangweulu Block.
At the same time, in the Kuunga Orogeny Neoproterozoic India collided with the Australia/Mawson continent.
Pangaea
Other large continental masses, including the cores of North America ( Canadian Shield or Laurentia), Europe ( Baltica), and Siberia were added over time to form the supercontinent Pangaea by Permian time. When Pangaea broke up (mostly during the Jurassic), two large masses, Gondwana and Laurasia, were formed.
When Pangaea broke up, the re-formed Gondwana continent was not precisely the same as before Pangaea formed; for example, most of Florida and southern Georgia and Alabama are underlain by rocks that were originally part of Gondwana but that were left attached to North America when Pangaea broke apart.
Climate
During the late Paleozoic, Gondwana extended from a point at or near the south pole to near the equator. Across much of Gondwana, the climate was mild. India contains about 3% of the world's coal reserves and much of the mined coal is derived from the late Paleozoic Gondwana sedimentary sequence. During the Mesozoic, the world was on average considerably warmer than today. Gondwana was then host to a huge variety of flora and fauna for many millions of years. But there is strong evidence of glaciation during Carboniferous to Permian time, especially in South Africa.
Breakup
Mesozoic
Gondwana began to break up in the mid-Jurassic (about 167 million years ago), when East Gondwana, comprising Antarctica, Madagascar, India and Australia, began to separate from Africa. South America began to drift slowly westward from Africa as the South Atlantic Ocean opened, beginning about 130 million years ago during the Early Cretaceous, and resulting in open marine conditions by 110 million years ago. East Gondwana itself began to be dismembered about 120 million years ago as India began to move northward.
The Madagascar block, and a narrow remnant microcontinent presently occupied by the Seychelles Islands, were broken off India; elements of this breakup nearly coincide with the Cretaceous-Tertiary extinction event. The India–Madagascar–Seychelles separations appear to coincide with the eruption of the Deccan basalts, whose eruption site may survive as the Réunion hotspot.
Australia began to separate from Antarctica perhaps 80 million years ago (Late Cretaceous), but sea-floor spreading between them became most active about 40 million years ago during the Eocene epoch of the Tertiary Period.
New Zealand probably separated from Antarctica between 130 and 85 million years ago.
Cenozoic
As the age of mammals got underway, the continent of Australia-New Guinea began gradually to separate and move north (55 million years ago), rotating about its axis to begin with, and thus retaining some connection with the remainder of Gondwana for about 10 million years.
About 45 million years ago, the Indian Plate collided with Asia, buckling the crust and forming the Himalayas. At about the same time, the southernmost part of Australia (modern Tasmania) finally separated from Antarctica, letting ocean currents flow between the two continents for the first time. Cooler and drier climates developed on both continents because ocean currents enveloping Antarctica were no longer directed into the subtropics, where they would have flowed around northern Australia.
Another significant world climatic event was South America separating from West Antarctica some time during the Oligocene, perhaps 30 million years ago. Immediately before this, South America and East Antarctica were not connected directly, but the many microplates of the Antarctic Peninsula remained near southern South America acting as " stepping stones" allowing continued biological interchange and stopped oceanic current circulation. But when Drake Passage opened, there was now no barrier to force the cold waters of the Southern Ocean north, to be exchanged with warmer tropical water. Instead, a cold circumpolar current developed and Antarctica became what it is today: a frigid continent that locks up much of the world's fresh water as ice. Sea temperatures dropped by almost 10°C, and the global climate became much colder.
By about 15 million years ago, the collision between New Guinea (on the leading edge of the Australian Plate) and the southwestern part of the Pacific Plate pushed up the New Guinea highlands, causing a rain shadow effect which drastically changed weather patterns in Australia, drying it out.
Later, South America was connected to North America via the Isthmus of Panama, cutting off a circulation of warm water and thereby creating the Arctic.
The Red Sea and East African Rift are modern examples of the continuing dismemberment of Gondwana.
The continent of Gondwana was named by Eduard Suess after an area of India called Gondwana (meaning "Land of the Gonds"), from which the Gondwana sedimentary sequences (Permian-Triassic) are also described.