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Continental Drift Johnson Space Center Collection/NASAJohnson Space Center Collection/NASA Over long periods of time, the continents drift into new configurations, as the plates carrying them move across Earth’s surface. The plates—and thus the continents— move at a rate of about 0–5 inches (0–13 centimeters) per year. North America, for example, is currently moving northwest at a little over an inch (three centimeters) per year. On the other hand, Antarctica is hardly moving. About 270 million years ago, nearly all the land on Earthformed one enormous continent called Pangea. This “supercontinent” began to break apart about 200 million years ago during the early Jurassic period, fracturing into two large pieces that drifted away from each other. These continents were Laurasia in the north and Gondwana in the south. Laurasia consisted of the land that now makes up North America, Europe, and Asia (except India and Arabia). Gondwana included what are now South America, Africa, Australia, Antarctica, India, and Arabia. Later, Laurasia and Gondwana fractured into pieces. Gondwana began to break apart about 180 million years ago, and Laurasia about 66 million years ago. The fragments of these continents drifted over time to form the present continents. Evidence suggests that this process has probably occurred several times during Earth’s long history, as continents came together to form supercontinents, which later broke apart. The idea that the continents move about the surface of the planet is an old one. It was formulated initially to explain the striking parallels of the shape of the Atlantic coast of Africa with that of South America. The shape of these continents suggests that they would fit together well. However, the close geometric fit of the continental margins was not universally accepted as evidence that they were once joined together and that the Atlantic Ocean was once closed. Major Tectonic Plates, Encyclopedia Britannica, Inc In the early 20th century the German meteorologist Alfred L. Wegener and others developed a comprehensive theory of continental drift. In 1912 Wegener proposed that all the land was once joined into the supercontinent Pangea, which broke up over time into the present continents. In 1937 the South African geologist Alexander Du Toit modified Wegener’s hypothesis by suggesting that Pangea first fractured into Laurasia and Gondwana. In addition to the geometrical fit of the continents, there is much evidence supporting the idea that the continents were once united. The main lines of evidence are based on the study of ancientclimates and the matching of fossils, rocks, and geological structures on land on opposite sides of oceans. A belt of ancient rocks along the Brazilian coast, for example, matches one in West Africa. This matching suggests that these pieces of land were once joined together. As the continents drift into new positions, their climates change. There is evidence that Antarctica, southern South America, southern Africa, India, and Australia were extensively covered by glaciers from 380 to 250 million years ago. The pattern of the ancient glaciers suggests that these landmasses once formed a single large continent near the south polar region. At first, continental drift theory met with strong opposition. Many scientists, especially those in North America, rejected the theory. They opposed it partly because it could not adequately explain what causes the continents to move. They were also unwilling to scrap their existing theories that involved unmoving landmasses. Starting in the 1950s, new data about the magnetic patterns of rocks provided strong support for continental drift. When certain rocks form, their particles are aligned according to the direction of Earth’s magnetic poles. This magnetization helps scientists determine how the rocks were positioned when they first formed. Evidence from such rocks suggests that the continents were indeed once joined together. Scientists discovered major features of the seafloor in the early 1960s. A new theory to explain these features proposed that the seafloor spreads at oceanic ridges. Molten rock material from the mantle wells up at the ridges and pushes aside the existing oceanic crust, thereby causing the seafloor to spread. By the late 1960s the satisfactory parts of the theories of continental drift and seafloor spreading were combined to form the theory of plate tectonics. According to this revolutionary theory (as mentioned above), Earth’s outer shell is divided into rigid plates that carry the continents and the ocean basins. Oceanic crust descending at subduction zones pulls the plates along. Seafloor spreading pushes plates apart. Many scientists think that the motion of the plates may be driven by the circulation of heated material in the mantle. Plate tectonics has very successfully explained many phenomena. After decades of controversy, the vast majority of scientists came to accept the theory of continental drift as explained by plate tectonics. See table: The continents compared Modern technology has provided tools that can actually measure the slow relative motion of the continents. Extensive networks of radio telescopes, the Global Positioning System (GPS) of satellites, and a technique known as satellite laser ranging (SLR) have all successfully been used to detect continental drift. Charles W. Finkl, Jr. The continents compared region area (sq mi) area (sq km) population, 2008 Africa 11,678,801 30,247,722 955,761,100 Antarctica 5,500,000 14,200,000 — Asia 12,239,721 31,700,654 4,018,522,000 Europe 8,896,305 23,041,330 735,213,700 North America 9,418,467 24,393,718 526,827,700 Australia & Oceania 3,287,718 8,515,146 35,120,640 South America 6,882,027 17,824,370 378,448,500 World 57,903,039 149,922,940 6,649,893,640 region literacy (%) (male) literacy (%) (female) life expectancy (male), 2005 Africa 69.6 52.0 51.8 Antarctica — — — Asia 82.5 65.2 67.2 Europe 99.4 98.5 71.0 North America 89.2 87.1 71.9 Australia & Oceania 94.7 91.9 74.5 South America 89.7 88.6 68.9 World 84.0 70.8 http://school.eb.com/levels/middle/article/273790#199150.toc 66.0