Gondwana

The southern supercontinent Gondwana (originally Gondwanaland) 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 India.

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 Orogen, the East African Orogen, and Kuungan Orogen. 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.

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

550 Ma reconstruction showing final stages of assembly of Gondwana

Gondwana began to break up in the mid- to late Jurassic (about 167 million years ago) when East Gondwana, comprising Antarctica-Madagascar-India-Australia, began to separate from Africa during the Middle Jurassic. South America began to drift slowly westward from Africa as the South Atlantic Ocean opened, beginning about 130 million years ago (Early Cretaceous) and resulting in open marine conditions by 110 million years ago. East Gondwana itself began to be dismembered as India began to move northward, in the Early Cretaceous (about 120 million years ago).

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 to gradually 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 acted 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 which 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 (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'), which the Gondwana sedimentary sequences (Permian-Triassic) are also described from.

References

Cattermole, Peter John. Building Planet Earth: Five Billion Years of Earth History. Cambridge, Cambridge University Press, 2000.
Cowen, Richard. History of Life. London, Blackwell Publishing, 2000.
Lowrie, William. Fundamentals of Geophysics. Cambridge, Cambridge University Press, 1997.

External links

Animation showing the dispersal of Gondwanaland
Another animation
Graphical subjects dealing with Tectonics and Paleontology, here: http://www.scotese.com/earth.htm
DeWit, M., and others, 1999, Gondwana Reconstruction and Dispersion: American Assoc. of Petroleum Geologists, Search and Discovery [1]
International Polar year [2]
König, M., Jokat, W., 2006, The Mesozoic breakup of the Weddell Sea: Journal of geophysical research [3]
Meert, J.G., 2003, A synopsis of events related to the assembly of eastern Gondwana, Tectonphysics, 362, 1-40.
Turner, Brian, Tectono-stratigraphic modelling of the Upper Karoo foreland basin: orogenic unloading versus thermally-induced Gondwana rifting [4]
Scheffler, K., and others, 2003, Global changes during Carboniferous–Permian glaciation of Gondwana: Linking polar and equatorial climate evolution by geochemical proxies [5]