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Continental drift

Movements of the tectonic plates

Throughout geological time, continents have drifted over the Earth's surface as rigid plates. Periodically, they have collided and gathered together to form large supercontinents, and large continents have disintegrated to form smaller plates. Before 180 million years ago, Antarctica formed the centre of a large land-mass called Gondwana which also included South America, Africa, India, Australia and New Zealand. Geologists are uncertain as to why very large continents like Gondwana disintegrate, but the break-up processes seem to involve thermal disturbances or hot spots that originate deep below the Earth's surface in the layer called the mantle. One effect of a hot spot beneath Gondwana was to produce a very large volcanic province just before Gondwana started to disintegrate 180 million years ago. Remnants of this province can be seen today in southern Africa, Patagonia, Antarctica and in Tasmania.

There were three main episodes in the disintegration of Gondwana. The initial rifting stage led to a seaway forming between West (South America and Africa) and East (Antarctica, Australia, India and New Zealand) 150 million years ago. The second stage occurred about 130 million years ago with South America separating from an African-Indian plate by opening of the South Atlantic Ocean, and the African-Indian plate from Antarctica by seafloor spreading in the Indian Ocean. Finally, about 100 million years ago, the break-up of that once large continent was completed when Australia and New Zealand separated from the Antarctic core. One interesting aspect of Gondwana history, is that in the South Atlantic region, Gondwana broke up into at least six and probably more microplates. Most of these can be seen today in West Antarctica, but one, which originated off southeast Africa, rotated approximately 180 degrees to form the Falkland Islands close to South America. The formation of these small microplates may also be related to the hot spot that existed beneath the original Gondwana continent.

Antarctica was not always the cold polar desert it is today. Geological evidence tells us that luxuriant vegetation covered the continent in the Cretaceous, 100 million years ago, and that glaciation seems to have started in Antarctica about 35 million years ago. The break-up and dispersal of Gondwana resulted in the formation of new oceans and oceanic currents significantly changing the Earth's climate. Oceanic currents act as a major mechanism for transporting heat and the development of new currents resulted in profound changes to the Earth's climate. The best example of how oceanic currents can affect climate can be seen today in Britain and Norway where the warm waters of the Gulf Stream flow northwards along the coast resulting in milder winter temperatures. In contrast, similar latitudes in North America have winter temperatures as low as -40°C. It appears that the development of the Antarctic ice sheet corresponds to the final stages in the isolation of the continent with separation of South America from Antarctica, and to the development of the circumpolar oceanic current.

Nothofagus (Southern Beech) is a good example. Seeds from this group of plants do not appear to be able to cross oceanic barriers to colonise and so their presence in both South America and Australia/New Zealand implies these regions were once connected. The presence of fossil Nothofagus leaves in the Antarctic confirms the close relationship of these regions, and incidentally implies that the Antarctic was warm enough millions of years ago for sustained tree growth. In a similar way to the plants dinosaurs were stranded on the continental fragments and these also confirm that the South America/Antarctica and the other southern continents were linked. We can see that the process of continental drift plays an important role not only in the evolution of the global climate, but also in the formation of the distinct flora and fauna of each region of the Earth.