Gondwana

• 1. Footprints Across An Ocean

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  Dinosaur footprints in Brazil and in Cameroon record identical species assemblages, even though the South Atlantic now lies between them. Those tracks are a clue to Gondwana, a vast southern landmass that is sometimes called a supercontinent. They date to around the early Aptian, which suggests that Africa and South America kept some form of land connection very late in the story. Gondwana no longer exists as one piece. Even so, its remnants make up around two-thirds of today's continental area. South America, Africa, Antarctica, Australia, Zealandia, Arabia and the Indian subcontinent are all pieces of it. How did a landmass of that size come together? Which fragments joined it, and which wandered off to join other worlds? What forced it apart, and how did its slow collapse change oceans, climates and forests?

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• 2. A Name From India

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  The Austrian scientist Eduard Suess named the continent after a region of India, whose own name comes from the Sanskrit goṇḍavana, meaning forest of the Gonds. Suess was not the first to use the word in geology. H. B. Medlicott had applied it in 1872 to the Gondwana sedimentary sequences, rock layers that date to the Permian and Triassic. Because one name now covers a region and an ancient landmass, some scientists prefer Gondwanaland for the landmass. That longer form is also common in everyday use. The word supercontinent fits uneasily as well. Under the earliest definition, Gondwana did not qualify, because Baltica, Laurentia and Siberia stayed separate from it. Before any of those landmasses mattered, Gondwana first had to be assembled from older fragments.

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• 3. Assembly By Collision

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  Oil and mining companies compiled the data sets that, during the 1990s, let geologists recognise three great collisions behind Gondwana's construction. Two of them were the East African Orogeny and the Kuunga orogeny, in which East Gondwana struck East Africa in two steps. The third was the Brasiliano orogeny, the successive impact of the South American and African cratons. Together with other events known as the Pan-African orogeny, they fused fragments of a much older supercontinent called Rodinia. The whole process was slow, and it remains incompletely understood because paleomagnetic data are scarce. Earlier researchers read the Mozambique Belt as the seam between East Gondwana and West Gondwana. The Mozambique Ocean once separated central Africa from Neoproterozoic India, and an island called Azania sat in its waters. Azania took in much of central Madagascar, the Horn of Africa and parts of Yemen and Arabia. By about 550 million years ago, India had reached its Gondwanan position, which started the Kuunga orogeny. On the other side of the forming Africa, Kalahari struck Congo and Rio de la Plata and closed the Adamastor Ocean. Between 540 and 530 million years ago, the Mozambique Ocean shut and brought India beside Australia and East Antarctica. The Malagasy orogeny, at about 550-515 million years ago, then sutured India to Azania and the Congo-Tanzania-Bangweulu Block. Around the western, southern and eastern margins, the Terra Australis Orogen grew to 18000 km. Its Cambrian arc belts survive in eastern Australia, Tasmania, New Zealand and Antarctica. The final stages overlapped with the opening of the Iapetus Ocean, and with the Cambrian explosion. For a brief spell near the Precambrian-Cambrian boundary, Laurentia docked against western Gondwana. That union formed Pannotia, a short-lived supercontinent whose existence is still disputed. When assembly ended, Gondwana was the largest piece of continental crust of the Paleozoic Era, covering some 100,000,000 square kilometres, about one-fifth of the Earth's surface.

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• 4. Strays At The Edge

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  The Armorican terrane, which now forms large parts of France, belonged to Peri-Gondwana in the Early Paleozoic. The Rheic Ocean closed in front of it while the Paleo-Tethys Ocean opened behind it. Precambrian rocks of the Iberian Peninsula hint that Iberia was part of core Gondwana before it detached close to the Carboniferous-Permian boundary. In the south-west, Gondwana traded pieces with other landmasses. The Cuyania, or Precordillera, terrane of north-west Argentina came from Laurentia, when Gondwana's western edge scraped against south-east Laurentia in the Ordovician. It may have continued the line of the Appalachians southward. The Patagonian terrane struck south-western Gondwana in the late Paleozoic. Igneous rocks beneath the North Patagonian Massif are 320-330 million years old, so subduction there began in the early Carboniferous. It lasted only about 20 million years. In the Devonian, an island arc named Chaitenia joined Patagonia in what is now south-central Chile. Gondwana's northern margin produced a different cast of travellers. When the Neo-Tethys Ocean opened along it in the Early Permian, it released a chain of terranes stretching from Turkey to north-eastern India. They include the Taurides in southern Turkey, the Alborz and Lut terranes of Iran, the Afghan Terrane and the Karakorum Terrane in northern Pakistan. The Lhasa and Qiangtang terranes of Tibet belong to the same chain. As the ocean widened in the Permian and Triassic, it pushed all of them across the Equator toward Eurasia. Many are still being deformed by the Himalayan orogeny. South-east Asia was assembled from Gondwanan and Cathaysian fragments in three phases of rifting. The first began in the Devonian, when North and South China, Tarim and Quidam rifted away. Whether the Kazakh and Mongolian terranes of Central Asia also began on Gondwana's shores is not known.

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• 5. Pangaea's Heavy Hand

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  The Variscan-Appalachian Mountains once stretched from present-day Mexico to southern Europe, raised when Gondwana and Laurasia collided during the Carboniferous. That collision closed the Rheic and Paleo-Tethys oceans, and the closure was oblique. Some northern terranes docked in the Marathon, Ouachita, Alleghanian and Variscan orogenies. Southern terranes such as Chortis and Oaxaca were largely unaffected. Yucatán and Florida were shielded by large promontories, while Carolina and Meguma were directly involved. Meanwhile Baltica struck Siberia and Kazakhstania, which produced the Uralian orogeny and completed Laurasia. Pangaea was fully assembled in the Late Carboniferous and Early Permian. In the east, collisions came slightly later. North China docked with Mongolia and Siberia during the Carboniferous and Permian, and South China followed. Western Pangaea had already begun to rift while its eastern end was still being built. The new mountains reshaped global climate and sea level, bringing glaciations and widespread sedimentation. In North America, the base of the Absaroka sequence coincides with the Alleghanian and Ouachita orogenies. These upheavals helped drive the Permian-Triassic extinction event. They also left large deposits of hydrocarbons, coal, evaporite and metals. Pangaea's breakup began with the Central Atlantic magmatic province, which covered more than seven million square kilometres over a few million years. It coincided with the Triassic-Jurassic extinction event. The Gondwana that re-emerged afterwards was not the same one that had existed before Pangaea. Most of Florida and southern Georgia and Alabama sits on rock that was originally Gondwanan, yet it stayed with North America.

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• 6. Cracks Around Antarctica

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  Antarctica sat at the centre of the supercontinent and shared a boundary with every other Gondwanan continent, so the break-up spread clockwise around it. Fragmentation began in the Early Jurassic, around 180 million years ago, with the fast emplacement of the Karoo-Ferrar flood basalts. That eruption ranks among the most extensive large igneous provinces on Earth. Before the Karoo plume split Africa from Antarctica, it peeled small blocks from the southern margin: the Antarctic Peninsula, Marie Byrd Land, Zealandia and Thurston Island. The Falkland Islands and the Ellsworth-Whitmore Mountains were rotated 90 degrees in opposite directions. Patagonia was pushed westward. The earliest break-up between South America, Africa and Antarctica is recorded in the southern Weddell Sea. Later, the Madagascar block and the Mascarene Plateau broke away from India. The split seems to match the eruption of the Deccan basalts, and its timing nearly coincides with the Cretaceous-Paleogene extinction event. The Seychelles and the Maldives are now divided by the Central Indian Ridge. East Gondwana, made of Antarctica, Madagascar, India and Australia, then separated from Africa. India later moved northwest away from Australia and Antarctica. During this opening, the Kerguelen hotspot built the Kerguelen Plateau and then the Ninety East Ridge. Zealandia, including New Zealand, New Caledonia, the Campbell Plateau and the Lord Howe Rise, pulled away from West Antarctica. The South Atlantic opened last, and the exact timing is still debated. Rifting ran from south to north, and the Paraná and Etendeka magmatics drove further seafloor spreading. Insular India eventually struck Asia, and more than 1400 km of crust has since been absorbed by the Himalayan-Tibetan orogen. That crumpling built the Tibetan Plateau between the Tethyan Himalayas and the Kunlun and Qilian mountains. Even now the break-up continues at the Afar triple junction, where the Arabian, African and Somali plates diverge, opening the Red Sea and the East African Rift.

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• 7. The Cold Gateway

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  In the Early Cenozoic, Australia was still joined to Antarctica and lay roughly 35-40 degrees of latitude south of its present position. Both continents were largely free of ice. Antarctica formed a land bridge, with Australia at one end and South America at the other. A rift opened between Australia and Antarctica but stayed a shallow embayment until the Eocene-Oligocene boundary. Then the Circumpolar Current developed and Antarctica began to glaciate. The opening of the Tasman Gateway at that boundary brought abrupt cooling to Australia, which had been warm, wet and dominated by rainforest in the Paleocene. The Oligocene was rainy, with swamps in the south-east. In the Miocene, rainforest persisted in pockets even in central Australia, until a colder, drier climate sharply reduced it. A brief wetter spell in the Pliocene gave way to dry conditions that favoured grassland. Swings between wet interglacial and dry glacial periods then produced the arid regime of today. Across 15 million years, Australia thus saw a gradual fall in rainfall. The Antarctic Circumpolar Current was established in the Late Oligocene, with the full opening of the Drake Passage and the deepening of the Tasman Gateway. The oldest oceanic crust in the Drake Passage hints that spreading between the Antarctic and South American plates started near the Eocene-Oligocene boundary. Deep-sea settings in Tierra del Fuego and along the North Scotia Ridge point to an earlier Proto-ACC. Later, shallowing along the North Scotia Ridge, closure of the Fuegan Seaway and uplift of the Patagonian Cordillera restricted it. With a reactivated Iceland plume, that restriction contributed to global warming. When the Drake Passage widened again in the Miocene, the renewed current cooled the planet. Meanwhile the northward drift of the Australian Plate drove an arc-continent collision with the Philippine and Caroline plates, which raised the New Guinea Highlands. Far to the north-west, the Isthmus of Panama joined South America to North America. It cut off a flow of warm water, made the Arctic colder and allowed the Great American Interchange.

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• 8. Forests Of The South

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  Baragwanathia, a primitive land plant, was among the first to colonise Gondwana, by the early Silurian. Biogeographers use the adjective Gondwanan for living organisms found only in two or more of the now-separated regions that once formed the landmass. In the Late Devonian, large Archaeopteris trees colonised its coasts. During the Permian, after the fusion with Pangaea, glossopterid trees, especially Glossopteris, dominated large parts of southern Gondwana. Those ecosystems collapsed in the end-Permian mass extinction. In the Triassic, trees of the genus Dicroidium, which had fern-like leaves, took over, until the end-Triassic extinction brought them down too. Jurassic ecosystems were ruled by conifers and by Bennettitales, an extinct order of seed plants. The Cretaceous brought flowering plants across Gondwana and a decline of the gymnosperms, just as the landmass was breaking apart. Toward the end of that period, the angiosperm tree Nothofagus emerged in the southern parts of former Gondwana, including Antarctica, and rose to prominence in temperate forests. Outside Antarctica, Nothofagus still holds a prominent place in those forests today, as part of what botanists call the Antarctic flora.

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