Tectonic plate margins

The outer 70 to 200 kilometers of the earth comprises a number of rigid plates that move about independently of one another. Each plate interacts with the adjacent plate along one of three types of plate margin: convergent, divergent, or conservative. The interactions of the moving plates along these margins cause most earthquakes as well as much of the volcanic activity on the surface of the earth.

Plate Tectonics

The surface of the earth is a mosaic of several large and more numerous smaller plates that move about laterally. The boundaries, or margins, of these plates are the sites of most of the volcanic and earthquake activity on the earth. The geological concept of the plate and plate margins ultimately has its origins in the theory of continental drift. The first comprehensive theories of continental drift were independently proposed around 1910 by the American geologists F. B. Taylor and H. H. Baker and the German meteorologist Alfred Wegener. Wegener's work was particularly thorough, and he is generally considered to be the person who first made the theory of continental drift an important scientific issue. Wegener devoted much time and effort to matching geological features and fossil types on both sides of the Atlantic Ocean. He argued, based on this work, that approximately 230 million years ago, all the continents were joined as one supercontinent that he named Pangaea. Furthermore, Wegener suggested that Pangaea broke apart approximately 170 million years ago. Since then, the various parts of the supercontinent—the modern continents—have moved to their present positions. Wegener also postulated that as the continents move through the oceans, their leading edges become crumpled and often collide with other continents, thereby forming the mountain belts.

In the period after World War II, several important discoveries made by oceanographers studying the ocean basins lent support to Wegener's scoffed-at theory. This postwar research ultimately led to the formulation of the theory of seafloor spreading proposed by Harry Hammond Hess of Princeton University. According to Hess's theory, new ocean floor is continuously forming at the ocean ridges, or the large subsea mountain ranges that traverse the earth, in a conveyor belt fashion. As this process continues, the newly formed sea floor moves laterally away from the ocean ridge on both sides of the ridge. The opening in the earth's surface created by the spreading of the sea floor at the ridge is filled with magma, or molten rock, from the mantle, which cools to form new sea floor. Hess and other scientists suggested that if the sea floor is continuously moving, the continents must also be moving with it. Thus, the concepts of continental drift and seafloor spreading were combined into the more comprehensive theory of plate tectonics. This revolutionary theory in the earth sciences describes the movement of rock in the earth's 70 to 200 kilometer-thick outer brittle shell, the lithosphere, as it moves over the deeper, more ductile, partially molten asthenosphere. The lithosphere, which includes continental and oceanic crust and the upper part of the mantle, comprises a number of large and small, rigid lithospheric plates that move independently of one another. As these plates move, they interact along one of three types of plate margin: divergent or accreting margins, convergent or destructive margins, and conservative or neutral margins.

Divergent Plate Margins

Divergent or accreting plate margins are tensional plate boundaries that correspond to the ocean ridges. According to plate tectonics theory, ocean ridges, also referred to as spreading centers, are sites on the earth's surface where new oceanic lithosphere is formed by the process of rifting, or the tensional separation of plates. Rifting can occur in the oceans as well as on the continents, as in the case of the East African rift zone. If rifting begins on a continent and continues for an extended period of time, a new ocean basin will ultimately form—as is now occurring in the Middle East, where the Red Sea rift is creating a new ocean between Saudi Arabia and northeastern Africa.

The crests of ocean ridges are characterized by deep valleys believed to have been caused by tensional faulting in response to oppositely directed lateral movement of the plates bound by the ocean ridge. This valley is referred to as the rift valley and is marked by a high degree of earthquake activity. The rift valley is also the site of voluminous outpourings of basaltic magma, or molten rock enriched in iron and magnesium.

Some of the most dramatic evidence of the processes occurring at divergent plate margins has come from a series of observations made from submersibles on the Galápagos spreading ridge near the equator, just west of South America and that part of the East Pacific Rise south of the Gulf of California. Researchers observed undersea hot springs and mounds of iron-rich clay minerals and manganese dioxide precipitated from the hot ore-carrying springs. Similar submersible dives to the Mid-Atlantic Ridge southwest of the Azores permitted observation of submarine volcanism and yielded abundant evidence of tensional faulting within the narrow rift valley.

Convergent Plate Margins

Convergent or destructive plate margins are those areas of the earth's surface where the lithospheric plates grind together head-on and are then recycled back into the asthenosphere. Thus, these margins are characterized by compressive tectonic forces. Submarine features typical of convergent plate margins are long, narrow troughs on the sea floor referred to as trenches. Hess postulated that the trenches mark the positions where ocean lithosphere created at ocean ridges is drawn down or sinks into the mantle. The trenches, which are the deepest points of the oceans, are closely associated with volcanic island arcs, or linear or arcuate groups of volcanic islands such as Japan or the Aleutian Islands.

Convergent margins are characterized by a high incidence of earthquakes, many of which originate at depths greater than 600 kilometers within the earth. Scientists have demonstrated that the focus points of these earthquakes (those points within the earth from where the seismic energy is first generated) are generally found within a band inclined from the trench toward the volcanic island arc or continent called the Wadati-Benioff zone (better known as the Benioff zone). Realization of this distribution of earthquakes at convergent margins led scientists to speculate that the Wadati-Benioff zone delineates a slab of dense lithosphere formed at an ocean ridge that, according to plate tectonics theory, is sinking at the trench into the mantle. This process of lithospheric sinking at convergent margins is referred to as subduction. In general, the most powerful earthquakes at convergent margins are the shallow-focus earthquakes generated close to the trenches. These earthquakes occur when a sinking lithospheric plate moves beneath an island arc or a continent and drags the overriding plate down a bit. Eventually, this process reaches a critical point, and sudden slip occurs along the boundary of the sinking plate and the plate beneath which it is moving, thereby creating the earthquake.

Subduction

The volcanic island arcs, like the distribution of earthquake focus points, can be considered in terms of the process of subduction. The basic question concerns how the magma that spewed from the volcanic islands forms beneath the island arcs. It is generally agreed that island arc volcanism is caused by melting of the subducting plate as it descends into the hot mantle. Generation of magma may also be assisted by frictional melting along the upper surface of the subducting plate as it moves beneath the island arc.

A final point regarding convergent margin processes concerns the fate of marine sediments on subducting oceanic lithosphere. Initially, it was postulated that all the marine sediments on descending oceanic lithosphere should be piled up and folded on the bottom of the trench. Studies of modern trenches, however, indicate that trenches generally contain only minor amounts of sediment. A more complete understanding of this problem was gained when more sophisticated geophysical techniques were applied to the study of convergent margins. Results of these investigations suggest that marine sediments carried on a subducting plate are stripped off the plate at the trench. These sediments are attached to the leading edge of the overriding plate to form a complexly deformed sequence of sedimentary rocks called an accretionary prism that builds the landward wall of the trench.

In summary, convergent plate margins display a series of features that have been interpreted in terms of subduction, the dominant plate tectonic process occurring at these margins. Trenches mark the locations on the earth where oceanic lithosphere, formed at a divergent plate margin, sinks into the mantle. At least a portion of the deep marine sediment carried atop this plate is mechanically transferred to the leading edge of the overriding plate to form an accretionary prism. As the plate moves deeper into the mantle, it fractures and generates earthquakes along its length. Additionally, it begins to melt, thereby producing magma that rises to the surface of the earth to form a volcanic island arc. If the oceanic plate is attached to a continent, that continent eventually reaches the trench. Because continental lithosphere is less dense and therefore more buoyant than oceanic lithosphere, however, the continent cannot be subducted, and instead it collides with the volcanic island arc or the overriding continent. This collision results in the formation of a mountain belt, as in the case of the collision of the Indian subcontinent and the Tibetan plateau to the north, which is still forming the Himalaya belt.

Conservative Plate Margins

The third type of plate margin is the conservative, or neutral, margin. These margins, along which lithosphere is neither created nor destroyed, are characterized by oppositely directed horizontal movement of adjacent plates. The actual boundaries of the moving plates are marked by transform faults, or faults along which plates slide horizontally past one another. The San Andreas fault of California, perhaps the best-known example of a transform fault, marks the boundary between the northwest-moving Pacific plate and the North American plate. This plate boundary is characterized by contrasting geology on both sides of the fault, by little if any volcanic activity, and by powerful shallow-focus earthquakes such as the kind that devastated San Francisco in 1906.

Transform faults separate offset segments of ocean ridges. Although ocean ridges extend continuously for thousands of kilometers, they are actually broken into much smaller segments separated by transform faults that are oriented at nearly right angles to the ridge segments. The relative movement of the two plates along a transform fault is caused by creation of new sea floor at the two offset ridge segments. As ocean lithosphere forms at and moves away from one ridge segment, it slides in the opposite direction past lithosphere forming at the other ridge segment. The transform fault, therefore, marks the contact of the oppositely moving plates and is situated between the ridge segments.

Triple Junctions

Plate tectonics theory requires that there be single points, called triple junctions, at which three lithospheric plates meet. In the Middle East, for example, three divergent plate margins—the Gulf of Aden, the East African rift, and the Red Sea rift—meet at what is referred to as a ridge-ridge-ridge triple junction. Almost any combination of the three plate margins—ridge, trench, and transform fault—can form triple junctions. Some types of triple junctions move with the plates, and they may even be subducted.

Echo Sounding and Seismic Reflection Profiling

Much of what is known about plate margins, particularly convergent and divergent margins, has come about through detailed study of the surface and interior structure of the ocean floor. One of the most important techniques developed in this regard is echo sounding. In echo sounding, a sound pulse generator-receiver system mounted on the hull of a ship emits sound pulses at regular intervals. Each pulse travels to the ocean floor at a known velocity and echoes back to the ship, where its return is detected by the pressure-sensitive receiver. The recording apparatus, a precision depth recorder, indicates the travel time of the sound pulse to and from the ocean bottom on an advancing paper chart. As the ship moves across the ocean, travel time marks for a succession of pulses detected by the receiver are displayed on the chart profile. The depth to the sea bottom is then calculated by multiplying the velocity of the sound pulse by one-half its travel time. This method was most instrumental in defining the ocean ridges and trenches on the ocean floor.

A somewhat more sophisticated approach using seismic waves allows scientists to study the internal structure of the upper part of oceanic lithosphere. In this approach, referred to as seismic reflection profiling, a ship emits sound waves powerful enough to penetrate the bottom of the ocean and then to reflect back to the ship. More specifically, these sound waves, which may be generated either by undersea explosions or by compressed air, reflect from the surface of the ocean floor and from internal sediment layers and faults back to the ship, where they are picked up by a receiver, or hydrophone, towed behind the ship. The travel times of the waves reflected off and from within the sea floor are recorded on charts by a sparker-profiler. Seismic reflection profiling has helped scientists to understand better the interior structure of the ocean bottom. For example, the faults bounding the slivers of marine sediments in accretionary prisms at convergent margins were recognized through the use of this technique.

Multinational Research Projects

The Deep Sea Drilling Project (DSDP), a multinational research program initiated in 1968, attempted to understand better the evolution and geologic history of the modern oceans by drilling through the deep-sea sediments into the underlying igneous floor of the ocean. A specially designed ship, the Glomar Challenger, was used in the drilling. Among other things, results of the DSDP indicated that the age of the igneous ocean floor increases away from ocean ridges, thereby substantiating the major tenet of seafloor spreading: that oceanic lithosphere is produced at, and moves away from, the ridges. The Deep Sea Drilling Project was superseded by a new program of research with many of the same goals, the Ocean Drilling Program (ODP).

Many details of plate margins, particularly divergent ocean ridges, have been revealed through direct observation of the sea floor. The French-American Mid-Ocean Undersea Study (project FAMOUS) of 1973 and 1974, for example, concentrated on a small area of the Mid-Atlantic Ridge southwest of the Azores. Several deep-sea submersible submarines were used to dive to the ridge to map the shape of the rift valley and to collect samples of the ocean floor. This project permitted observation of the tensional faults that formed the rift valley and extrusion of basaltic magma. In several submersible dives to transform faults, scientists recovered igneous rock samples that showed evidence of the shearing associated with horizontal plate movement along the transform faults. A number of submersible dives to the East Pacific rise in the eastern Pacific Ocean have allowed marine geologists to observe submarine hot springs associated with lava extrusion at a divergent margin.

Heat Flow and Rock Sequence Studies

Measurement of terrestrial heat flow (the amount of heat that escapes from the earth's interior through the sea floor) yielded particularly valuable information regarding the nature of plate margins. Results of heat-flow studies indicated that the ocean ridges, once thought to be dormant submarine mountain ranges, are actually sites where large amounts of heat from the interior of the earth reach the earth's surface. This finding fit in well with Hess's convection-cell interpretation of ocean ridges. In addition, heat-flow studies demonstrated extremely low heat-flow values in the trenches, an observation consistent with Hess's proposal that convection cells sink into the mantle at convergent margins.

Finally, studies of modern plate margins have been supplemented by investigations of rock sequences exposed on land and interpreted to have formed at ancient plate margins. This approach is particularly useful to the study of convergent plate margins. For example, highly deformed or chaotic rock units, referred to as mélanges and exposed in the Appalachian belt, along coastal California, and elsewhere, have been interpreted as marine sediments that were incorporated into ancient accretionary prisms. By studying these exposed sedimentary rocks, geologists can understand better the processes occurring at modern convergent plate margins.

Dangers and Benefits

Plate margins are generally not a major concern of most people unless they happen to live or work near one. Nevertheless, one has only to pick up a newspaper to see the effects of plate margins and their attendant processes on human life. Convergent margins, for example, are characterized by frequent earthquakes and are generally prone to volcanic activity, some of which may be violent. The powerful earthquakes and deadly volcanoes of the Aleutian Islands, Central and South America, Japan, and Indonesia attest the potentially dangerous conditions of convergent plate margins. Conservative plate margins, like the San Andreas fault, are susceptible to powerful earthquakes, although volcanic activity is not likely. The instability of plate margins must be kept in mind by community planners so that proper building codes can be created and followed to reduce the potential for catastrophe in these areas.

Despite the obvious dangers of living close to or along plate margins, there can be some benefits. In Iceland, for example, the heat emanating from the Mid-Atlantic Ridge is used as geothermal energy. Indeed, Reykjavík, the capital of Iceland, is heated entirely by geothermal energy.

Understanding the relation of plate tectonics and metal deposits is of paramount importance given the growing global need for various metals. At ocean ridges, for example, marine geologists and oceanographers have observed the formation of metallic sulfide ores. These deposits, which form in association with the basalt magma extruded at the ridge, precipitate out of the hot water that circulates through the newly erupted basalt. Convergent plate margins are characterized by various types of metal deposits formed in association with magma generated during subduction. In the Andes belt of South America, iron, copper, and gold ores accumulated in response to subduction of the Pacific Ocean floor beneath the western coast of South America.

Principal Terms

accretionary prism: a complex structure composed of fault-bounded sequences of deep-sea sediments mechanically transferred from subducting oceanic lithosphere to the overriding plate; it forms the wall on the landward side of a trench

asthenosphere: the soft, partially molten layer below the lithosphere

convection cell: a single circular path of rising warm material and sinking cold material

lithosphere: the outer, rigid shell of the earth that contains the oceanic and continental crust and the upper part of the mantle

rifting: the process whereby lithospheric plates break apart by tensional forces

seafloor spreading: the concept that new ocean floor is created at the ocean ridges and moves toward the volcanic island arcs, where it descends into the mantle

subduction zone: a region where a plate, generally oceanic lithosphere, sinks beneath another plate into the mantle

transform fault: a fault connecting offset segments of an ocean ridge along which two plates slide past each other

volcanic island arc: a curving or linear group of volcanic islands associated with a subduction zone

Wadati-Benioff zone: the inclined band of earthquake focus points interpreted to delineate the subducting oceanic lithosphere; better known as the Benioff zone

Bibliography

Bonatti, E., and K. Crane. “Ocean Fracture Zones.” Scientific American 250 (May 1984): 40. Excellent discussion of transform faults and associated oceanic fractures. Suitable for the college-level reader.

Condie, Kent C. Plate Tectonics and Crustal Evolution. 4th ed. Oxford: Butterworth Heinemann, 1997. An excellent overview of modern plate tectonics theory that synthesizes data from geology, geochemistry, geophysics, and oceanography. A very helpful tectonic map of the world is enclosed. The book is nontechnical and suitable for a college-level reader. Useful “suggestions for further reading” follow each chapter.

Dewey, J. F. “Plate Tectonics.” Scientific American 226 (May 1972): 56. A good overview of the theory of plate tectonics and plate margins. Can be read by the high school or college student.

Heirtzler, J. R., “Seafloor Spreading.” Scientific American 219 (December 1968): 60. This article provides an excellent discussion of the theory of seafloor spreading. Suitable for high school students.

Heirtzler, J. R., and W. B. Bryan. “The Floor and the Mid-Atlantic Rift.” Scientific American 233 (August 1975): 78. Excellent discussion of divergent plate margins, with the Mid-Atlantic Ridge as the example. Can be read by high school and college students.

Kearey, Philip, Keith A. Klepeis, and Frederick J. Vine. Global Tectonics. 3rd ed. Cambridge, Mass.: Wiley-Blackwell, 2009. This college text gives the reader a solid understanding of the history of global tectonics, along with current processes and activities. The book is filled with colorful illustrations and maps.

Kious, Jacquelyne W. This Dynamic Earth: The Story of Plate Tectonics. Washington, D.C.: U.S. Department of the Interior, United States Geological Survey, 1996. Kious is able to explain plate tectonics in a way suitable for the layperson. The book deals with both historic and current theory. Illustrations and maps are plentiful.

Ladd, J. W., et al. “Caribbean Marine Geology: Active Margins of the Plate Boundary.” In The Caribbean Region. Geological Society of America, edited by G. Dengo and J. E. Case. The Geology of North America, Vol. H (1990): 261-290.

Marsh, B. D. “Island-Arc Volcanism.” American Scientist 67 (March/April 1979): 161. A detailed discussion of volcanic activity at convergent margins. Suitable for college students.

Ogawa, Yujiro, Ryo Anma, and Yildirim Dilek. Accretionary Prisms and Convergent Margin Tectonics in the Northwest Pacific Basin. New York: Springer Science+Business Media, 2011. Discusses new techniques in plate tectonics studies. One volume of the series Modern Approaches in Solid Earth Sciences. Accretionary Prisms, Tectonics, and Pacific Ocean Events.

Sutherland, Lin. The Volcanic Earth: Volcanoes and Plate Tectonics, Past, Present, and Future. Sydney, Australia: University of New South Wales Press, 1995. Although Sutherland focuses on volcanic activity in Australia, the book provides an easily understood overview of volcanic and tectonic processes, including the role of igneous rocks. Includes color maps and illustrations, as well as a bibliography.

Tokosoz, M. N. “The Subduction of the Lithosphere.” Scientific American 233 (November 1975): 88. This article describes the process of subduction at convergent margins and can be read by high school and college students.

Uyeda, Seiya. The New View of the Earth: Moving Continents and Moving Oceans. San Francisco: W. H. Freeman, 1971. An excellent presentation of the evolution of the theory of plate tectonics from continental drift. Convergent and divergent plate margins are particularly well discussed, with numerous examples from the Pacific Ocean. Probably most suitable for college-level readers.