Background (Encyclopedia of Global Resources)
Plate tectonic theory is based on a concept of the Earth in which a rigid, outer shell, the lithosphere, lies above a hotter, weaker, partially molten part of the mantle known as the asthenosphere. The thickness of the lithosphere varies between 50 and 150 kilometers, and it consists of crust and the underlying upper mantle. The asthenosphere extends from the base of the lithosphere to a depth of about 700 kilometers. The brittle lithosphere is broken into a pattern of internally rigid plates that move horizontally across the Earth’s surface relative to each other. Seven major plates and a number of smaller ones have been distinguished, and they grind and scrape against one another as they move independently, similar to chunks of ice on water. Most of the Earth’s dynamic activity, including earthquakes and volcanism, occurs along plate boundaries, and the global distribution of these tectonic phenomena delineates the boundaries of the plates.
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Plate Boundaries and Motion (Encyclopedia of Global Resources)
Geophysical data, geological observations, and theoretical deductions support the existence of three basic types of plate boundaries: divergent boundaries, where adjacent plates move apart (diverge) from each other; convergent boundaries, where adjacent plates move toward each other; and transform boundaries, where plates slip past one another in a direction parallel to their common boundary. The velocity with which plates move varies from plate to plate and within portions of the same plate, ranging from two to twenty centimeters per year. This rate is determined from radioactive dating estimates of the age of the seafloor as a function of distance from mid-oceanic ridge crests (seafloor spreading ridges).
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Divergent Plate Boundaries (Encyclopedia of Global Resources)
At mid-oceanic ridges, or divergent plate boundaries, new seafloor is created from molten basalt (magma) rising from the asthenosphere. A great deal of volcanic activity thus occurs at divergent boundaries. Because of the pulling apart (rifting) of the plates of lithosphere, earthquake activity will also occur along divergent boundaries, and since the rift is caused by magma rising from the mantle, the earthquakes will be frequent, shallow, and mild.
An example of continental rifting (divergence) in its embryonic stage is seen in the Red Sea, where the Arabian plate has separated from the African plate, creating a new oceanic ridge. Another modern-day example is the East African Rift system, which is the site of active rifting. If it continues, it will eventually fragment Africa, and an ocean will separate the resulting pieces. Through divergence, or rifting, large plates are broken up into smaller ones.
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Convergent Plate Boundaries (Encyclopedia of Global Resources)
Because the Earth is neither expanding nor contracting, the increase in lithosphere created along divergent boundaries must be compensated for by the destruction of lithosphere elsewhere. Otherwise the radius of the Earth would change. At convergent plate boundaries, plates are moving together, and three scenarios are possible depending on whether the crust of the lithosphere is oceanic or continental.
If both converging plates are made ofoceanic crust, one will inevitably be older, and thus cooler and denser than the other plate. The denser plate will plunge (subduct) below the less-dense plate and descend down into the asthenosphere. This type of plate boundary is called a subduction zone, and the boundary along the two interacting plates forms a trench. The subducted plate is heated by the hot asthenosphere and, in time, becomes hot enough to melt. Some of the melted material rises buoyantly through fissures and cracks to form volcanoes on the overlying plate, whereas other parts of the melted material will eventually migrate to and rise again at a divergent boundary (spreading ridge). Thus the oceanic lithosphere is constantly being recycled. The volcanoes along the overriding plate may form a string of islands called island arcs. Japan, the Aleutians, and the Marianas are good examples of island arcs resulting from subduction of two plates consisting of oceanic lithosphere.
If the leading edge of one...
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Transform Plate Boundaries (Encyclopedia of Global Resources)
The actual structure of a seafloor spreading ridge is more complex than a single straight crack. Rather, ridges consist of many short segments slightly offset from one another. The offsets are a special kind of fault, or break in the lithosphere, known as a transform fault, and their function is to connect segments of a spreading ridge. The opposite sides of a transform fault belong to two different plates, and these are moving apart in opposite directions. The transform faults are just boundaries along which the plates move past one another. The classic transform boundary is the San Andreas fault that slices off a sliver of western California that rides on the Pacific plate from the rest of the state, which is on the North American plate. As the two plates scrape past each other, stress builds up and is released in earthquakes.
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Why Plates Move (Encyclopedia of Global Resources)
One mechanism that creates energy to move the huge plates is convection currents that are driven by heat from radioactive decay in the mantle. These convection currents in the Earth’s mantle carry magma up from the asthenosphere. Some of this magma escapes to form new lithosphere, but the rest spreads out sideways beneath the lithosphere, slowly cooling in the process. As it flows outward, it drags the overlying lithosphere outward with it, thus continuing to open the ridges. When it cools, the flowing material becomes dense enough to sink back deeper into the mantle at convergent boundaries. A second plate-driving mechanism is the pull of the dense, cold, downward-moving slab of lithosphere in a subduction zone on the rest of the trailing plate, opening up the spreading ridges so magma can move upward.
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Mineral Deposits (Encyclopedia of Global Resources)
The theory of plate tectonics has greatly enhanced understanding of why many mineral deposits form where they do and has thus made mineral exploration more efficient. During the evolution of new oceanic plates and mountain belts by plate tectonics, a large number of mineral deposits form, particularly in association with the plate boundaries.
Hot fluids(hydrothermal fluids) circulate at spreading ridges (divergent boundaries) and deposit minerals. For example, niobium deposits are found in the intrusions in the East African Rift zone, and iron and manganese are found in the sediments of the Red Sea. hydrothermal fluids also flow through the cracks and pores in rock along convergent boundaries and deposit metals along these boundaries as they cool. Good examples are the copper ore deposits associated with the collisional boundary of the Himalayas and tin ores in southwestern England. A general sequence of minerals found when passing inland from a trench associated with subduction is iron, gold, copper, molybdenum, gold, lead, zinc, tin, tungsten, antimony, and mercury.
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Further Reading (Encyclopedia of Global Resources)
Brown, G. C., and A. E. Mussett. The Inaccessible Earth: An Integrated Approach to Geophysics and Geochemistry. 2d ed. New York: Chapman & Hall, 1993.
Cox, Allan, and Robert Brian Hart. Plate Tectonics: How It Works. Palo Alto, Calif.: Blackwell Scientific, 1986.
Erickson, Jon. Plate Tectonics: Unraveling the Mysteries of the Earth. Rev. ed. New York: Facts On File, 2001.
Hamblin, W. Kenneth, and Eric H. Christiansen. Earth’s Dynamic Systems. 10th ed. Upper Saddle River, N.J.: Pearson/Prentice Hall, 2004.
Kearey, Philip, Keith A. Klepeis, and Frederick J. Vine. Global Tectonics. 3d ed. Hoboken, N.J.: Wiley-Blackwell, 2009.
Keller, Edward A., and Nicholas Pinter. Active Tectonics: Earthquakes, Uplift, and Landscape. 2d ed. Upper Saddle River, N.J.: Prentice Hall, 2002.
Kusky, Timothy. Earthquakes: Plate Tectonics and Earthquake Hazards. New York: Facts On File, 2008.
Montgomery, Carla W. Fundamentals of Geology. 3d ed. Dubuque, Iowa: Wm. C. Brown, 1997.
Oreskes, Naomi, ed. Plate Tectonics: An Insider’s History of the Modern Theory of the Earth. Boulder, Colo.: Westview Press, 2001.
Van der Pluijm, Ben A., and Stephen Marshak. Earth Structure: An Introduction to Structural Geology and Tectonics. 2d ed. New York: W. W. Norton, 2003.
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Background (Encyclopedia of Global Warming)
The atmosphere of the Earth has varied through geologic time, from times of global warming with no glaciers to times with abundant glaciers. For instance, there was significant cooling and glaciation from about 1 billion to 550 million years ago, from 300 to 260 million years ago, and from 35 million years ago to nearly the present. At other times, significant global warming occurred, especially in the interval from about 145 to 55 million years ago. Thus, global warming has occurred in the past and is not just a recent event. Reasons for these large global changes in temperature must be explained by natural processes such as plate tectonics.
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Earth’s Tectonic Activity (Encyclopedia of Global Warming)
The lithosphere of the Earth is composed of about twenty plates of varied sizes in which igneous rocks are being formed by volcanism or crystallization of magma along oceanic rises. In addition, volcanic gases are continually being evolved along the oceanic rises.
The oceanic lithosphere moves laterally away from oceanic rises, usually at a rate of less than 10 centimeters per year. A portion of the oceanic lithosphere eventually moves below another oceanic lithospheric plate or below continental lithosphere at a subduction zone. For instance, the portion of the lithosphere beneath the Pacific Ocean continuously moves below the portion of the lithosphere on which South America occurs. In addition, abundant volcanic activity, evolution of volcanic gases, and earthquakes occur above the subducted plate in South America. Also, continents resting on these lithospheric plates move laterally and vertically through geologic time.
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Greenhouse Gases (Encyclopedia of Global Warming)
The amount of greenhouse gases (GHGs) in the atmosphere can affect the amount of energy given off from the Earth into space. carbon dioxide (CO2) is believed to be one of the most important of these gases that affect the atmospheric temperature. Most notable is the direct correlation of the amount of CO2 in atmosphere trapped in ice cores in Antarctica over the last 400,000 years to that of the inferred temperature of the atmosphere.
Thus, understanding how and why the CO2 content of the atmosphere changes is critical to understanding why the temperature of the atmosphere changes with time. Increased volcanic activity, for instance, will increase the rate of CO2 production into the atmosphere, thus favoring temperature rise. Weathering of rocks uses up CO2 and moves it as dissolved bicarbonate into the oceans, where it is used by organisms to create shells and skeletons of calcium carbonate, thereby sequestering the carbon from the atmosphere and favoring a decrease in atmospheric temperature. Movement of continents that allows for increased plant growth should also remove CO2 from the atmosphere and favor a decrease in atmospheric temperature.
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Warming Climate (Encyclopedia of Global Warming)
Global temperatures from about 250 million years to 55 million years ago were much warmer than those today. During this time, the temperatures at the equator are estimated to have been 2° to 6° Celsius warmer and those at the north and south poles on the order of 20° to 40° Celsius warmer than those today. Thus, there were no ice fields at the poles and many plants and animals could live further north and south than they can today. For example, dinosaurs could live at high latitudes until they disappeared about 65 million years ago.
The continents formed a supercontinent called Pangaea from 250 to 180 million years ago that extended from the north to the south poles. Then, the continental landmass began to rupture along an oceanic rise, so that what is now North and South America moved to the west and what is now Europe, Asia, and Africa moved to the east. Pangaea was likely dry in the interior, since rain near the coast would have dried the air before it reached the interior. This could have resulted in significant temperature variation in the interior of Pangaea from hot, moist summers to cold, dry winters. The lack of moisture could have kept glaciers from forming in the winter, and the hot summers could have melted most ice at the poles.
Spreading rates from the oceanic rift around 100 million years ago could have moved the continents at more rapid rates than do current plate tectonics. The rapid spreading...
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Cooling Climate (Encyclopedia of Global Warming)
The climate began to cool about 55 million years ago. Large continental glaciers formed during some of this time that covered, for instance, what is now Canada and the northern United States. The reasons for this drastic cooling are a matter of debate, but there are several possibilities. Ocean spreading rates may have decreased slowly to the current spreading rates, reducing the amount of CO2 emitted into the atmosphere and favoring cooling. Another possibility is that the continents may have been uplifted, making more moisture available to produce more rapid chemical weathering of the rocks. This increased weathering would have used more CO2, as would the consequent increase in carbonate organisms in the oceans, sequestering carbon from the atmosphere and decreasing the greenhouse effect.
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Context (Encyclopedia of Global Warming)
Plate tectonics are a function of geological and geothermal processes that bring heat to Earth’s surface and that directly and indirectly influence the level of GHGs in the atmosphere. As such, they contribute to increases and decreases in global temperature, although they are not generally thought to be determining factors in climate change, either recent or on geologic timescales.
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Further Reading (Encyclopedia of Global Warming)
Kump, Lee R., James F. Kasting, and Robert G. Crane. The Earth System. Upper Saddle River, N.J.: Prentice Hall, 2003. Describes the changes in the Earth as they affect the environment. Includes global warming, the atmosphere, and plate tectonics, among other topics. Appendixes, tables, figures, glossary.
Marshak, Stephen. Earth: Portrait of a Planet. New York: W. W. Norton, 2005. Gives a reasonably detailed description of plate tectonics and geology and touches on the evolution of climates over time. Figures, glossary, index.
Ruddiman, William F. Earth’s Climate: Past and Future. 2d ed. New York: W. H. Freeman, 2008. Describes the Earth’s climate changes through geologic time and the possible reasons for these changes. Figures and tables, glossary, index.
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Plate Tectonics (Encyclopedia of Science)
Plate tectonics is the geologic theory that Earth's crust is made up of rigid plates that "float" on the surface of the planet. Tectonics comes from the Greek word meaning "builder." The movement of the plates toward or away from each other either directly or indirectly creates the major geologic features at Earth's surface.
Plate tectonics revolutionized the way geologists view Earth. Like the theory of evolution in biology, plate tectonics is the unifying concept of geology. It explains nearly all of Earth's major surface features and activities. These include faults and earthquakes, volcanoes and volcanism, mountains and mountain building, and even the origin of the continents and ocean basins.
Plate tectonics is a comparatively new idea. The theory of plate tectonics gained widespread acceptance only in the 1960s. About 50 years earlier, German geophysicist Alfred Wegener (1880930) developed a related theory known as continental drift. Wegener contended that the positions of Earth's continents are not fixed. He believed instead that they are mobile and over time drift about on Earth's surfaceence the name continental drift.
Wegener's most obvious evidence for his theory was the fact that several of the world's continents fit together like pieces in a jig-saw puzzle. Based on this, he proposed that...
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Plate Tectonics (World of Earth Science)
Plate tectonics is the theory explaining geologic changes that result from the movement of lithospheric plates over the asthenosphere (the molten, ductile, upper portion of the earth's mantle). The visible continents, a part of the lithospheric plates upon which they ride, shift slowly over time as a result of the forces driving plate tectonics. Moreover, plate tectonic theory is so robust in its ability to explain and predict geological processes that it is equivalent in many regards to the fundamental and unifying principles of evolution in biology, and nucleosynthesis in physics and chemistry.
Based upon centuries of cartographic depictions that allowed a good fit between the western coast of Africa and the eastern coast of South America, in 1858, French geographer Antonio Snider-Pellegrini, published a work asserting that the two continents had once been part of larger single continent ruptured by the creation and intervention of the Atlantic Ocean. In the 1920s, German geophysicist Alfred Wegener's writings advanced the hypothesis of continental drift depicting the movement of continents through an underlying oceanic crust. Wegner's hypothesis met with wide skepticism but found support and development in the work and writings of South African geologist Alexander Du Toit, who discovered a similarity in the fossils found on the coasts of Africa and South Americas that derived from a common source.
The technological advances necessitated by the Second World War made possible the accumulation of significant evidence now underlying modern plate tectonic theory.
Plate tectonic theory asserts that Earth is divided into core, mantle, and crust. The crust is subdivided into oceanic and continental crust. The oceanic crust is thin (3.3 mi [5 km]), basaltic (2), dense, and young (2), light, and old (250,700 million years old). The outer crust is further subdivided by the subdivision of the lithospheric plates, of which it is a part, into 13 major plates. These lithospheric plates, composed of crust and the outer layer of the mantle, contain a varying combination of oceanic and continental crust. The lithospheric plates move on top of mantle's asthenosphere.
Boundaries are adjacent areas where plates meet. Divergent boundaries are areas under tension where plates are pushed apart by magma upwelling from the mantle. Collision boundaries are sites of compression either resulting in subduction (where lithospheric plates are driven down and destroyed in the molten mantle) or in crustal uplifting that results in orogeny (mountain building). At transform boundaries, exemplified by the San Andreas fault, the continents create a shearing force as they move laterally past one another.
New oceanic crust is created at divergent boundaries that are sites of sea-floor spreading. Because Earth remains roughly the same size, there must be a concurrent destruction or uplifting of crust so that the net area of crust remains the same. Accordingly, as crust is created at divergent boundaries, oceanic crust must be destroyed in areas of subduction under-neath the lighter continental crust. The net area is also preserved by continental crust uplift that occurs when less dense continental crusts collide. Because both continental crusts resist subduction, the momentum of collision causes an uplift of crust, forming mountain chains. A vivid example of this type of collision is found in the ongoing collision of India with Asia that has resulted in the Himalayan Mountains that continue to increase in height each year. This dynamic theory of plate tectonics also explained the formation of island arcs formed by rising material at sites where oceanic crust subducts under oceanic crust, the formation of mountain chains where oceanic crust subducts under continental crust (e.g., Andes mountains), and volcanic arcs in the Pacific. The evidence for deep, hot, convective currents combined with plate movement (and concurrent continental drift) also explained the mid-plate "hot spot" formation of volcanic island chains (e.g., Hawaiian Islands) and the formation of rift valleys (e.g., Rift Valley of Africa). Mid-plate earthquakes, such as the powerful New Madrid earthquake in the United States in 1811, are explained by interplate pressures that bend plates much like a piece of sheet metal pressed from opposite sides.
As with continental drift theory two of the proofs of plate tectonics are based upon the geometric fit of the displaced continents and the similarity of rock ages and Paleozoic fossils in corresponding bands or zones in adjacent or corresponding geographic areas (e.g., between West Africa and the eastern coast of South America).
Modern understanding of the structure of Earth is derived in large part from the interpretation of seismic studies that measure the reflection of seismic waves off features in Earth's interior. Different materials transmit and reflect seismic shock waves in different ways, and of particular importance to the theory of plate tectonics is the fact that liquid does not transmit a particular form of seismic wave known as an S-wave. Because the mantle transmits S-waves, it was long thought to be a cooling solid mass. Geologists later discovered that radioactive decay provided a heat source within Earth's interior that made the asthenosphere plasticine (semi-solid). Although solid-like with regard to transmission of seismic S-waves, the asthenosphere contains very low velocity (inches per year) currents of mafic (magma-like) molten materials.
Another line of evidence in support of plate tectonics came from the long-known existence of ophiolte suites (slivers of oceanic floor with fossils) found in upper levels of mountain chains. The existence of ophiolte suites are consistent with the uplift of crust in collision zones predicted by plate tectonic theory.
As methods of dating improved, one of the most conclusive lines of evidence in support of plate tectonics derived from the dating of rock samples. Highly supportive of the theory of sea floor spreading (the creation of oceanic crust at a divergent plate boundary (e.g., Mid-Atlantic Ridge) was evidence that rock ages are similar in equidistant bands symmetrically centered on the divergent boundary. More importantly, dating studies show that the age of the rocks increases as their distance from the divergent boundary increases. Accordingly, rocks of similar ages are found at similar distances from divergent boundaries, and the rocks near the divergent boundary where crust is being created are younger than the rocks more distant from the boundary. Eventually, radioisotope studies offering improved accuracy and precision in rock dating also showed that rock specimens taken from geographically corresponding areas of South America and Africa showed a very high degree of correspondence, providing strong evidence that at one time these rock formations had once coexisted in an area subsequently separated by movement of lithospheric plates.
Similar to the age of rocks, studies of fossils found in once adjacent geological formations showed a high degree of correspondence. Identical fossils are found in bands and zones equidistant from divergent boundaries. Accordingly, the fossil record provides evidence that a particular band of crust shared a similar history as its corresponding band of crust located on the other side of the divergent boundary.
The line of evidence, however, that firmly convinced modern geologists to accept the arguments in support of plate tectonics derived from studies of the magnetic signatures or magnetic orientations of rocks found on either side of divergent boundaries. Just as similar age and fossil bands exist on either side of a divergent boundary, studies of the magnetic orientations of rocks reveal bands of similar magnetic orientation that were equidistant and on both sides of divergent boundaries. Tremendously persuasive evidence of plate tectonics is also derived from correlation of studies of the magnetic orientation
of the rocks to known changes in Earth's magnetic field as predicted by electromagnetic theory. Paleomagnetic studies and discovery of polar wandering, a magnetic orientation of rocks to the historical location and polarity of the magnetic poles as opposed to the present location and polarity, provided a coherent map of continental movement that fit well with the present distribution of the continents.
Paleomagnetic studies are based upon the fact that some hot igneous rocks (formed from volcanic magma) contain varying amounts of ferromagnetic minerals (e.g., Fe3O4) that magnetically orient to the prevailing magnetic field of Earth at the time they cool. Geophysical and electromagnetic theory provides clear and convincing evidence of multiple polar reversals or polar flips throughout the course of Earth's history. Where rock formations are uniform.e., not grossly disrupted by other geological processeshe magnetic orientation of magnetite-bearing rocks can also be used to determine the approximate latitude the rocks were at when they cooled and took on their particular magnetic orientation. Rocks with a different orientation to the current orientation of Earth's magnetic field also produce disturbances or unexpected readings (anomalies) when scientists attempt to measure the magnetic field over a particular area.
This overwhelming support for plate tectonics came in the 1960s in the wake of the demonstration of the existence of symmetrical, equidistant magnetic anomalies centered on the Mid-Atlantic Ridge. Geologists were comfortable in accepting these magnetic anomalies located on the sea floor as evidence of sea floor spreading because they were able to correlate these anomalies with equidistant radially distributed magnetic anomalies associated with outflows of lava from land-based volcanoes.
Additional evidence continued to support a growing acceptance of tectonic theory. In addition to increased energy demands requiring enhanced exploration, during the 1950s there was an extensive effort, partly for military reasons related to what was to become an increasing reliance on submarines as a nuclear deterrent force, to map the ocean floor. These studies revealed the prominent undersea ridges with undersea rift valleys that ultimately were understood to be divergent plate boundaries. An ever-growing network of seismic reporting stations, also spurred by the Cold War need to monitor atomic testing, provided substantial data that these areas of divergence were tectonically active sites highly prone to earthquakes. Maps of the global distribution of earthquakes readily identified stressed plate boundaries. Improved mapping also made it possible to view the retrofit of continents in terms of the fit between the true extent of the continental crust instead of the current coastlines that are much variable to influences of weather and ocean levels.
In his important 1960 publication, History of Ocean Basins, geologist and U.S. Navy Admiral Harry Hess (1906969) provided the missing explanatory mechanism for plate tectonic theory by suggesting that the thermal convection currents in the asthenosphere provided the driving force behind plate movements. Subsequent to Hess's book, geologists Drummond Matthews (1931997) and Fred Vine (1939988) at Cambridge University used magnetometer readings previously collected to correlate the paired bands of varying magnetism and anomalies located on either side of divergent boundaries. Vine and Matthews realized that magnetic data revealing strips of polar reversals symmetrically displaced about a divergent boundary confirmed Hess's assertions regarding seafloor spreading.
See also Dating methods; Earth, interior structure; Fossil record; Fossils and fossilization; Geologic time; Hawaiian Island formation; Lithospheric plates; Mantle plumes; Mapping techniques; Mid-ocean ridges and rifts; Mohorovicic discontinuity (Moho); Ocean trenches; Rifting and rift valleys; Subduction zone