CONTINENTAL DRIFT

the large-scale horizontal movements of continents relative to one another and to the ocean basins during one or more episodes of geologic time. The idea of a large-scale displacement of continents has a long history. Noting the apparent fit of the bulge of eastern South America into the bight of Africa, the German naturalist Alexander von Humboldt theorized about 1800 that the lands bordering the Atlantic Ocean had once been joined. Some 50 years later Antonio Snider-Pellegrini, a French scientist, argued that the presence of identical fossil plants in both North American and European coal deposits could be explained if the two continents were formerly connected but was difficult to account for otherwise. In 1908 Frank B. Taylor of the United States invoked the notion of continental collision to explain the formation of some of the world's mountain ranges. The first truly detailed and comprehensive theory of continental drift was proposed in 1912 by Alfred Wegener, a German meteorologist. Bringing together a large mass of geologic and paleontological data, Wegener postulated that throughout most of geologic time there was only one continent, which he called Pangaea. Late in the Triassic Period (which lasted from 245 to 208 million years ago), Pangaea fragmented and the parts began to move away from one another. Westward drift of the Americas opened the Atlantic Ocean, and the Indian block drifted across the Equator to merge with Asia. In 1937 Alexander L. Du Toit, a South African geologist, modified Wegener's hypothesis by suggesting two primordial continents: Laurasia in the north and Gondwanaland (or Gondwana) in the south. Aside from the congruency of continental shelf margins across the Atlantic, modern proponents of continental drift have amassed impressive geologic evidence to support their views. Indications of widespread glaciation from 380 to 250 million years ago are evident in Antarctica, southern South America, southern Africa, India, and Australia. If these continents were once united around the south polar region, this glaciation would become explicable as a unified sequence of events in time and space. Also, fitting the Americas with the continents across the Atlantic brings together similar kinds of rocks and geologic structures. A belt of ancient rocks along the Brazilian coast, for example, matches one in West Africa. Moreover, the earliest marine deposits along the Atlantic coastlines of either South America or Africa are Jurassic in age (208 to 144 million years old), suggesting that the ocean did not exist before that time. Interest in continental drift increased in the 1950s as knowledge of the Earth's magnetic field during the geologic past developed from the studies of the British geophysicists Stanley K. Runcorn, P.M.S. Blackett, and others. Ferromagnetic minerals such as magnetite acquire a permanent magnetization when they crystallize as constituents of igneous rock. The direction of their magnetization is the same as the direction of the Earth's magnetic field at the time and place of crystallization. Particles of magnetized minerals released from their parent igneous rocks by weathering may later realign themselves with the existing magnetic field at the time these particles are incorporated into sedimentary deposits. Studies of the remanent magnetism in suitable rocks of different ages from all over the world indicate that the magnetic poles were in different places at different times. The polar wandering curves are different for the various continents, but in important instances such differences are reconciled on the assumption that continents now separated were formerly joined. The curves for Europe and North America, for example, are reconciled by the assumption that the latter has drifted about 30 westward relative to Europe since the Triassic Period (245 to 208 million years ago). Increased knowledge about the configuration of the ocean floor and the subsequent formulation of the concepts of seafloor spreading and plate tectonics provided further support for continental drift. During the early 1960s the American geophysicist Harry H. Hess proposed that new oceanic crust is continually generated by igneous activity at the crests of midocean ridgessubmarine mountains that follow a sinuous course of about 60,000 km (37,000 miles) along the bottom of the major ocean basins. Molten rock material from the Earth's mantle rises upward to the crests, cools, and is later pushed aside by new intrusions. The ocean floor is thus pushed at right angles and in opposite directions away from the crests. By the late 1960s several American investigators, among them Jack E. Oliver and Bryan L. Isacks, had integrated this notion of seafloor spreading with that of drifting continents and formulated the basis of plate tectonic theory. According to the latter hypothesis, the Earth's surface, or lithosphere, is composed of a number of large, rigid plates that float on a soft (presumably partially molten) layer of the mantle known as the asthenosphere. The midocean ridges occur along some of the plate margins. Where this is the case, the lithospheric plates separate and the upwelling mantle material forms new ocean floor along the trailing edges. As the plates move away from the flanks of the ridges, they carry the continents with them. On the basis of all these factors, it may be assumed that the Americas were joined with Europe and Africa until approximately 190 million years ago, when a rift split them apart along what is now the crest of the Mid-Atlantic Ridge. Subsequent plate movements averaging about 2 cm (0.8 inch) per year have brought the continents to their present position. It seems likely, though still unproven, that this breakup of a single landmass and the drifting of its fragments is merely the latest in a series of similar occurrences throughout geologic time.