The theory of plate tectonics
- Key People:
- Arnold Henry Guyot
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Plate tectonics has revolutionized virtually every discipline of the Earth sciences since the late 1960s and early 1970s. It has served as a unifying model or paradigm for explaining geologic phenomena that were formerly considered in unrelated fashion. Plate tectonics describes seismic activity, volcanism, mountain building, and various other Earth processes in terms of the structure and mechanical behaviour of a small number of enormous rigid plates thought to constitute the outer part of the planet (i.e., the lithosphere). This all-encompassing theory grew out of observations and ideas about continental drift and seafloor spreading.
In 1912 the German meteorologist Alfred Wegener proposed that throughout most of geologic time there was only one continental mass, which he named Pangaea. At some time during the Mesozoic Era, Pangaea fragmented and the parts began to drift apart. Westward drift of the Americas opened the Atlantic Ocean, and the Indian block drifted across the Equator to join with Asia. In 1937 the South African Alexander Du Toit modified Wegener’s hypothesis by suggesting the existence of two primordial continents: Laurasia in the north and Gondwanaland in the south. Aside from the congruency of continental shelf margins across the Atlantic, proponents of continental drift have amassed impressive geologic evidence to support their views. Similarities in fossil terrestrial organisms in pre-Cretaceous (older than about 146 million years) strata of Africa and South America and in pre-Jurassic rocks (older than about 200 million years) of Australia, India, Madagascar, and Africa are explained if these continents were formerly connected but difficult to account for otherwise. Fitting the Americas with the continents across the Atlantic brings together similar kinds of rocks and structures. Evidence of widespread glaciation during the late Paleozoic is found in Antarctica, southern South America, southern Africa, India, and Australia. If these continents were formerly united around the South Polar region, this glaciation becomes explicable as a unified sequence of events in time and space.
Interest in continental drift heightened during the 1950s as knowledge of Earth’s magnetic field during the geologic past developed from the studies of Stanley K. Runcorn, Patrick M.S. Blackett, and others. Ferromagnetic minerals such as magnetite acquire a permanent magnetization when they crystallize as components of igneous rock. The direction of their magnetization is the same as the direction of Earth’s magnetic field at the place and time 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 over the world indicate that the magnetic poles were in different places at different times. The polar wandering curves are different for the several continents, but in important instances these 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 America has drifted about 30° westward relative to Europe since the Triassic Period (approximately 251 million to 200 million years ago).
In the early 1960s a major breakthrough in understanding the way the modern Earth works came from two studies of the ocean floor. First, the American geophysicists Harry H. Hess and Robert S. Dietz suggested that new ocean crust was formed along mid-oceanic ridges between separating continents; and second, Drummond H. Matthews and Frederick J. Vine of Britain proposed that the new oceanic crust acted like a magnetic tape recorder insofar as magnetic anomaly strips parallel to the ridge had been magnetized alternately in normal and reversed order, reflecting the changes in polarity of Earth’s magnetic field. This theory of seafloor spreading then needed testing, and the opportunity arose from major advances in deepwater drilling technology. The Joint Oceanographic Institutions Deep Earth Sampling (JOIDES) project began in 1969, continued with the Deep Sea Drilling Project (DSDP), and, since 1976, with the International Phase of Ocean Drilling (IPOD) project. These projects have produced more than 500 boreholes in the floor of the world’s oceans, and the results have been as outstanding as the plate-tectonic theory itself. They confirm that the oceanic crust is everywhere younger than about 200 million years and that the stratigraphic age determined by micropaleontology of the overlying oceanic sediments is close to the age of the oceanic crust calculated from the magnetic anomalies.
The plate-tectonic theory, which embraces both continental drift and seafloor spreading, was formulated in the mid-1960s by the Canadian geologist J. Tuzo Wilson, who described the network of mid-oceanic ridges, transform faults, and subduction zones as boundaries separating an evolving mosaic of enormous plates, and who proposed the idea of the opening and closing of oceans and eventual production of an orogenic belt by the collision of two continents.
Up to this point, no one had considered in any detail the implications of the plate-tectonic theory for the evolution of continental orogenic belts; most thought had been devoted to the oceans. In 1969 John Frederick Dewey of the University of Cambridge outlined an analysis of the Caledonian-Appalachian orogenic belts in terms of a complete plate-tectonic cycle of events, and this provided a model for the interpretation of other pre-Mesozoic (Paleozoic and Precambrian) belts. Some of the oldest volcano-sedimentary rocks on Earth, in the 3.8 billion-year-old Isua belt in West Greenland, have been shown by geologists from the Tokyo Institute of Technology to have formed in a plate-tectonic setting—i.e., in a trench or mouth of a subduction zone—and it is plausible that the oldest known rocks, the 4.28 billion-year-old Nuvvuagittuq greenstone belt in Quebec, Canada, formed in a similar setting. For a detailed discussion of plate-tectonic theory and its far-reaching effects, see plate tectonics.
Hydrologic sciences
Water resources and seawater chemistry
Quantitative studies of the distribution of water have revealed that an astonishingly small part of Earth’s water is contained in lakes and rivers. Ninety-seven percent of all the water is in the oceans, and, of the fresh water constituting the remainder, three-fourths is locked up in glacial ice and most of the rest is in the ground. Approximate figures are also now available for the amounts of water involved in the different stages of the hydrologic cycle. Of the 859 millimetres of annual global precipitation, 23 percent falls on the lands; but only about a third of the precipitation on the lands runs directly back to the sea, the remainder being recycled through the atmosphere by evaporation and transpiration. Subsurface groundwater accumulates by infiltration of rainwater into soil and bedrock. Some may run off into rivers and lakes, and some may reemerge as springs or aquifers. Advanced techniques are used extensively in groundwater studies nowadays. The rate of groundwater flow, for example, can be calculated from the breakdown of radioactive carbon-14 by measuring the time it takes for rainwater to pass through the ground, while numerical modeling is used to study heat and mass transfer in groundwater. High-precision equipment is used for measuring down-hole temperature, pressure, flow rate, and water level. Groundwater hydrology is important in studies of fractured reservoirs, subsidence resulting from fluid withdrawal, geothermal resource exploration, radioactive waste disposal, and aquifer thermal-energy storage.
Chemical analyses of trace elements and isotopes of seawater are conducted as part of the Geochemical Ocean Sections (Geosecs) program. Of the 92 naturally occurring elements, nearly 80 have been detected in seawater or in the organisms that inhabit it, and it is thought to be only a matter of time until traces of the others are detected. Contrary to the idea widely circulated in the older literature of oceanography, that the relative proportions of the oceans’ dissolved constituents are constant, investigations since 1962 have revealed statistically significant variations in the ratios of calcium and strontium to chlorinity. The role of organisms as influences on the composition of seawater has become better understood with advances in marine biology. It is now known that plants and animals may collect certain elements to concentrations as much as 100,000 times their normal amounts in seawater. Abnormally high concentrations of beryllium, scandium, chromium, and iodine have been found in algae; of copper and arsenic in both the soft and skeletal parts of invertebrate animals; and of zirconium and cerium in plankton.
Desalinization, tidal power, and minerals from the sea
For ages a source of food and common salt, the sea is increasingly becoming a source of water, chemicals, and energy. In 1967 Key West, Fla., became the first U.S. city to be supplied solely by water from the sea, drawing its supplies from a plant that produces more than 2 million gallons of refined water daily. Magnesia was extracted from the Mediterranean in the late 19th century; at present nearly all the magnesium metal used in the United States is mined from the sea at Freeport, Texas. Many ambitious schemes for using tidal power have been devised, but the first major hydrographic project of this kind was not completed until 1967, when a dam and electrical generating equipment were installed across the Rance River in Brittany. The seafloor and the strata below the continental shelves are also sources of mineral wealth. Concretions of manganese oxide, evidently formed in the process of subaqueous weathering of volcanic rocks, have been found in dense concentrations with a total abundance of 1011 tons. In addition to the manganese, these concretions contain copper, nickel, cobalt, zinc, and molybdenum. To date, oil and gas have been the most valuable products to be produced from beneath the sea.
Ocean bathymetry
Modern bathymetric charts show that about 20 percent of the surfaces of the continents are submerged to form continental shelves. Altogether the shelves form an area about the size of Africa. Continental slopes, which slant down from the outer edges of the shelves to the abyssal plains of the seafloor, are nearly everywhere furrowed by submarine canyons. The depths to which these canyons have been cut below sea level seem to rule out the possibility that they are drowned valleys cut by ordinary streams. More likely, the canyons were eroded by turbidity currents, dense mixtures of mud and water that originate as mudslides in the heads of the canyons and pour down their bottoms.
Profiling of the Pacific basin prior to and during World War II resulted in the discovery of hundreds of isolated eminences rising 1,000 or more metres above the floor. Of particular interest were seamounts in the shape of truncated cones, whose flat tops rise to between 1.6 kilometres and a few hundred metres below the surface. Harry H. Hess interpreted the flat-topped seamounts (guyots) as volcanic mountains planed off by action of waves before they subsided to their present depths. Subsequent drilling in guyots west of Hawaii confirmed this view; samples of rocks from the tops contained fossils of Cretaceous age representing reef-building organisms of the kind that inhabit shallow water.
Ocean circulation, currents, and waves
Early in the 20th century Vilhelm Bjerknes, a Norwegian meteorologist, and V. Walfrid Ekman, a Swedish physical oceanographer, investigated the dynamics of ocean circulation and developed theoretical principles that influenced subsequent studies of currents in the sea. Bjerknes showed that very small forces resulting from pressure differences caused by nonuniform density of seawater can initiate and maintain fluid motion. Ekman analyzed the influence of winds and Earth’s rotation on currents. He theorized that in a homogeneous medium the frictional effects of winds blowing across the surface would cause movement of successively lower layers of water, the deeper the currents so produced the less their velocity and the greater their deflection by the Coriolis effect (an apparent force due to Earth’s rotation that causes deflection of a moving body to the right in the Northern Hemisphere and to the left in the Southern Hemisphere), until at some critical depth an induced current would move in a direction opposite to that of the wind.
Results of many investigations suggest that the forces that drive the ocean currents originate at the interface between water and air. The direct transfer of momentum from the atmosphere to the sea is doubtless the most important driving force for currents in the upper parts of the ocean. Next in importance are differential heating, evaporation, and precipitation across the air-sea boundary, altering the density of seawater and thus initiating movement of water masses with different densities. Studies of the properties and motion of water at depth have shown that strong currents also exist in the deep sea and that distinct types of water travel far from their geographic sources. For example, the highly saline water of the Mediterranean that flows through the Strait of Gibraltar has been traced over a large part of the Atlantic, where it forms a deepwater stratum that is circulated far beyond that ocean in currents around Antarctica.
Improvements in devices for determining the motion of seawater in three dimensions have led to the discovery of new currents and to the disclosure of unexpected complexities in the circulation of the oceans generally. In 1951 a huge countercurrent moving eastward across the Pacific was found below depths as shallow as 20 metres, and in the following year an analogous equatorial undercurrent was discovered in the Atlantic. In 1957 a deep countercurrent was detected beneath the Gulf Stream with the aid of subsurface floats emitting acoustic signals.
Since the 1970s Earth-orbiting satellites have yielded much information on the temperature distribution and thermal energy of ocean currents such as the Gulf Stream. Chemical analyses from Geosecs makes possible the determination of circulation paths, speeds, and mixing rates of ocean currents.
Surface waves of the ocean are also exceedingly complex, at most places and times reflecting the coexistence and interferences of several independent wave systems. During World War II, interest in forecasting wave characteristics was stimulated by the need for this critical information in the planning of amphibious operations. The oceanographers H.U. Sverdrup and Walter Heinrich Munk combined theory and empirical relationships in developing a method of forecasting “significant wave height”—the average height of the highest third of the waves in a wave train. Subsequently this method was improved to permit wave forecasters to predict optimal routes for mariners. Forecasting of the most destructive of all waves, tsunamis, or “tidal waves,” caused by submarine quakes and volcanic eruptions, is another recent development. Soon after 159 persons were killed in Hawaii by the tsunami of 1946, the U.S. Coast and Geodetic Survey established a seismic sea-wave warning system. Using a seismic network to locate epicentres of submarine quakes, the installation predicts the arrival of tsunamis at points around the Pacific basin often hours before the arrival of the waves.
Glacier motion and the high-latitude ice sheets
Beginning around 1948, principles and techniques in metallurgy and solid-state physics were brought to bear on the mechanics of glacial movements. Laboratory studies showed that glacial ice deforms like other crystalline solids (such as metals) at temperatures near the melting point. Continued stress produces permanent deformation. In addition to plastic deformation within a moving glacier, the glacier itself may slide over its bed by mechanisms involving pressure melting and refreezing and accelerated plastic flow around obstacles. The causes underlying changes in rate of glacial movement, in particular spectacular accelerations called surges, require further study. Surges involve massive transfer of ice from the upper to the lower parts of glaciers at rates of as much as 20 metres a day, in comparison with normal advances of a few metres a year.
As a result of numerous scientific expeditions into Greenland and Antarctica, the dimensions of the remaining great ice sheets are fairly well known from gravimetric and seismic surveys. In parts of both continents it has been determined that the base of the ice is below sea level, probably due at least in part to subsidence of the crust under the weight of the caps. In 1966 a borehole was drilled 1,390 metres to bedrock on the North Greenland ice sheet, and two years later a similar boring of 2,162 metres was cut through the Antarctic ice at Byrd Station. From the study of annual incremental layers and analyses of oxygen isotopes, the bottom layers of ice cored in Greenland were estimated to be more than 150,000 years old, compared with 100,000 years for the Antarctic core. With the advent of geochemical dating of rocks it has become evident that the Ice Age, which in the earlier part of the century was considered to have transpired during the Quaternary Period, actually began much earlier. In Antarctica, for example, potassium-argon age determinations of lava overlying glaciated surfaces and sedimentary deposits of glacial origin show that glaciers existed on this continent at least 10 million years ago.
The study of ice sheets has benefited much from data produced by advanced instruments, computers, and orbiting satellites. The shape of ice sheets can be determined by numerical modeling, their heat budget from thermodynamic calculations, and their thickness with radar techniques. Colour images from satellites show the temperature distribution across the polar regions, which can be compared with the distribution of land and sea ice.
