Continental drift, large-scale horizontal movements of continents relative to one another and to the ocean basins during one or more episodes of geologic time. This concept was an important precursor to the development of the theory of plate tectonics, which incorporates it.
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 Humboldttheorized 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 had formerly been connected, a relationship otherwise difficult to account for. 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 Pangea. Late in the Triassic Period (which lasted from approximately 251 million to 199.6 million years ago), Pangea 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 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, fossils, 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 (approximately 199.6 million to 145.5 million years old), which suggests that the ocean did not exist before that time.
plate tectonics: Alfred Wegener and the concept of continental drift
Interest in continental drift increased in the 1950s as knowledge of Earth’s geomagnetic field during the geologic past developed from the studies of the British geophysicists Stanley K. Runcorn, Patrick 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 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 by Runcorn of the remanent magnetism in suitable rocks of different ages from Europe produced a “polar wandering curve” indicating that the magnetic poles were in different places at different times. This could be explained either by the migration of the magnetic pole itself (that is, polar wandering) or by the migration of Europe relative to a fixed pole (that is, continental drift).
However, further work showed that the polar wandering curves are different for the various continents. The possibility that they might reflect true wander of the poles was discarded, because it implies separate wanderings of many magnetic poles over the same period. However, these different paths are reconciled by joining the continents in the manner proposed by Wegener. 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.
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 oceanic ridges—submarine mountains that follow a sinuous course of about 65,000 km (40,000 miles) along the bottom of the major ocean basins. Molten rock material from 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, 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. Oceanic 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 taken the continents to their present position. It seems likely, though it is 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.
Sea Floor spreading
Seafloor spreading hypothesis, theory that oceanic crust forms along submarine mountain zones, known collectively as the mid-ocean ridge system, and spreads out laterally away from them. This idea played a pivotal role in the development of plate tectonics, a theory that revolutionized geologic thought during the last quarter of the 20th century.
The seafloor spreading hypothesis was proposed by the American geophysicist Harry H. Hess in 1960. On the basis of new discoveries about the deep-ocean floor, Hess postulated that molten material from the Earth’s mantle continuously wells up along the crests of the mid-ocean ridges that wind for nearly 80,000 km (50,000 miles) through all the world’s oceans. As the magma cools, it is pushed away from the flanks of the ridges. This spreading creates a successively younger ocean floor, and the flow of material is thought to bring about the migration, or drifting apart, of the continents. The continents bordering the Atlantic Ocean, for example, are believed to be moving away from the Mid-Atlantic Ridgeat a rate of 1–2 cm (0.4–0.8 inch) per year, thus increasing the breadth of the ocean basin by twice that amount. Wherever continents are bordered by deep-sea trench systems, as in the Pacific Ocean, the ocean floor is plunged downward, underthrusting the continents and ultimately reentering and dissolving in the Earth’s mantle from which it originated.
A veritable legion of evidence supports the seafloor spreading hypothesis. Studies conducted with thermal probes, for example, indicate that the heat flow through bottom sediments is generally comparable to that through the continents except over the mid-ocean ridges, where at some sites the heat flow measures three to four times the normal value. The anomalously high values are considered to reflect the intrusion of molten material near the crests of the ridges. Research has also revealed that the ridge crests are characterized by anomalously low seismic wave velocities, which can be attributed to thermal expansion and microfracturing associated with the upwelling magma.
Investigations of oceanic magnetic anomalies have further corroborated the seafloor spreading hypothesis. Such studies have shown that the strength of the geomagnetic field is alternately anomalously high and low with increasing distance away from the axis of the mid-ocean ridge system. The anomalous features are nearly symmetrically arranged on both sides of the axis and parallel the axis, creating bands of parallel anomalies.
Measurements of the thickness of marine sediments and absolute age determinations of such bottom material have provided additional evidence for seafloor spreading. The oldest sediments so far recovered by a variety of methods—including coring, dredging, and deep-sea drilling—date only to the Jurassic Period; that is, they do not exceed 200 million years in age. Such findings are incompatible with the doctrine of the permanency of the ocean basins that had prevailed among earth scientists for so many years.
Fossil Evidence
Alfred Wegener collected diverse pieces of evidence to support his theory, including geological “fit” and fossil evidence. It is important to know that the following specific fossil evidence was not brought up by Wegener to support his theory. Wegener himself did not collect the fossils but he called attention to the idea of using these scientific doc uments stating there were fossils of species present in separate continents in order to support his claim.
Geological “fit” evidence is the matching of large-scale geological features on different continents. It has been noted that the coastlines of South America and West Africa seem to match up, however more particularly the terrains of separate continents conform as well. Examples include: the Appalachian Mountains of eastern North America linked with the Scottish Highlands, the familiar rock strata of the Karroo system of South Africa matched correctly with the Santa Catarina system in Brazil, and the Brazil and Ghana mountain ranges agreeing over the Atlantic Ocean.
Another important piece of evidence in the Continental Drift theory is the fossil relevance. There are various examples of fossils found on separate continents and in no other regions. This indicates that these continents had to be once joined together because the extensive oceans between these land masses act as a type of barrier for fossil transfer. Four fossil examples include: the Mesosaurus, Cynognathus, Lystrosaurus, and Glossopteris.
The Mesosaurus is known to have been a type of reptile, similar to the modern crocodile, which propelled itself through water with its long hind legs and limber tail. It lived during the early Permian period (286 to 258 million years ago) and its remains are found solely in South Africa and Eastern South America. Now if the continents were in still their present positions, there is no possibility that the Mesosaurus would have the capability to swim across such a large body of ocean as the Atlantic because it was a coastal animal.
The now extinct Cynognathus, which translates to “dog jaw”, was a mammal- like reptile. Roaming the terrains during the Triassic period (250 to 240 million years ago), the Cynognathus was as large as a modern wolf. Its fossils are found only in South Africa and South America. As a land dominant species, the Cynognathus would not have been capable of migrating across the Atlantic.
The Lystrosaurus, which translates to “shovel reptile,” is thought to have been an herbivore with a stout build like a pig. It is approximated that it grew up to one meter in length and was relatively dominant on land during the early Triassic period (250 million years ago). Lystrosaurus fossils are only found in Antarctica, India, and South Africa. Similar to the land dwelling Cynognathus, the Lystrosaurus would have not had the swimming capability to traverse any ocean.
Possibly the most important fossil evidence found is the plant, Glossopteris. Known as a woody, seed bearing tree, the Glossopteris is named after the Greek description for tongue due to its tongue shaped leaves and is the largest genus of the extinct descendant of seed ferns. Reaching as tall as 30 meters, the Glossopteris emerged during the early Permian period (299 million years ago) and became the dominant land plant species until the end of the Permian. The Glossopteris fossil is found in Australia, Antarctica, India, South Africa, and South America—all the southern continents. Now, the Glossopteris seed is known to be large and bulky and therefore could not have drifted or flown across the oceans to a separate continent. Therefore, the continents must have been joined at least one point in time in order to maintain the Glossopteris’ wide range across the southern continents.
If the continents of the Southern Hemisphere are put together, the distribution of these four fossil types form continuous patterns across continental boundaries. Of course, possible explanations are brought to attention. One explanation is the species could have migrated via a land bridge or swam to the other continents. However, a land bridge is not applicable due to the differences in densities between the continents and oceans floor and violation of the isostasy concept. Moreover, swimming as a possibility is foolish due to the lack of formidable swimming capabilities to travel across such an extensive body of water like the Atlantic. An additional resolution is that the species could have merely evolved separately on the other continents. Undoubtedly, this interpretation is in complete disagreement with Darwin’s evolution theory.