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Structured Geological Glossary: Plate Tectonics

plate tectonics The theory that the Earth's crust and upper mantle (the lithosphere) is broken into a number of more or less rigid, but constantly moving, segments or plate which move in relation to one another, shifting continent, forming new ocean crust, and causing volcanic eruption
tectonically active A term used to describe regions that are strongly affected by movement of Earth's tectonic plate. Earthquakes and volcanoes are common features in these regions.
plate The Earth is made up of a dozen or so major plate and several minor plates. Tectonic plates are constantly on the move. The fastest plate races along at 15 centimeters (6 inches) per year while the slowest plates crawl at less than 2.5 centimeters (1 inch) per year.
continental drift The theory that horizontal movement of the earth's surface causes slow relative movements of the continent toward or away from one another.
active margin The edge of a plate bordered by a trench.
volcanic arc Arcuate chain of volcanoes formed above a subducting plate. The arc forms where the downgoing descending plate becomes hot enough to release water and gases that rise into the overlying mantle and cause it to melt. Arc rocks are mostly volcanic rock from the volcanoes and sedimentary rocks made up of eroded debris from the volcanoes. Melted rock in the deeper plumbing of the arc which may crystallizes at depth to become an arc root pluton.
plates Pacific plate, North American plate, Juan de Fuca plate, Cocos plate, Nazca plate, South American plate, Scotia Plate, Caribbean plate, African plate, Eurasian plate, Arabian plate, Australian-Indian plate, China subplate, Philippine plate, Somalian subplate, Antartic plate
passive margin A tectonically inactive continental margin characterized by a lack of earthquake and volcanic activity.
plate boundary The place where two or more plate in the Earth's crust meet.
divergent plate boundary A boundary in which two tectonic plate move apart.
triple junction A point that is common to three plate and which must also be the meeting place of three boundary features, such as divergence zones, convergence zones, or transform fault.
island arc An arc-shaped chain of volcanic island produced where an oceanic plate is sinking (subducting) beneath another
island arc A group of island having a curving, arclike pattern. Most island arc lie near the continental masses, but inasmuch as they rise from the deep ocean floor, they are not a part of the continents proper
volcanic front (arc) Elongate zone of maximum volcanic activity that defines the main volcanic (or magmatic) arc.
spreading center An elongated region where two plate are being pulled away from each other. New crust is formed as molten rock is forced upward into the gap. Examples of spreading center include the Mid-Atlantic Ridge and the East African Rift.
sea-floor spreading The mechanism by which new sea floor crust is created at ridges in divergence zones and adjacent plate are moved apart to make room. This process may continue at 0.5 to 10 centimeters/year through many geologic periods.
subduction zone An elongated region along which a plate descends relative to another plate, for example, the descent of the Nazca plate beneath the South American plate along the Peru-Chile Trench.
transform fault A class of vertical fault with a horizontal displacement vector that does not link two or more plate tectonic scale boundaries.
spreading axes An imaginary axis through the Earth about which a set of tectonic plate move. The movement of a diverging plate can be decribed as rotation around a spreading axis.
subduction Process of one crustal plate sliding down and below another crustal plate as the two converge. The subduction zone is the area between the two plates, somewhat like a giant reverse fault.
subduction Process of one crustal plate sliding down and below another crustal plate as the two converge. The subduction zone is the area between the two plates, somewhat like a giant reverse fault.
subduction zone An elongated region along which a plate descends relative to another plate, for example, the descent of the Nazca plate beneath the South American plate along the Peru-Chile Trench.
Benioff zone A narrow zone defined by earthquake foci and tens of kilometers thick dipping from the surface under the Earth's crust. Zone of earthquake foci produced near the surface of and within the subducted lithosphere.
ring of fire A zone of volcanoes, earthquake, and mountain-building encircling the Pacific Ocean formed where plate collide.
convergent plate boundary A boundary in which two plate collide. The collision can be between two continent (continental collision), an relatively dense oceanic plate and a more buoyant continental plate (subduction zone) or two oceanic plates (subduction zone).
paleogeography Working out by means of geologic criteria (chiefly chemically sensitive sedimentary rocks and environmentally sensitive rocks and structure, such as nonmarine shoreline or deep-water types) and structural geologic evidence for the displacement of rock bodies that originally were continuous geophysical criteria (chiefly the evidence of remanent magnetism, which provides data regarding the original latitudinal position of rock bodies, although now whether that latitude is north or south) and paleontologic criteria (chiefly the evidence of historical biogeography that helps to establish which rock bodies contain fossil suggesting original reproductive communication between same) of the ever-changing geographies of the past.
paleogeographic map A map showing the surface landforms and coastline of an area at some time in the geologic past.
Gondwana A continent formed in the Southern Hemisphere during the Late Paleozoic. It included most of South America, Africa, India, Austrailia, and Antarctica.
Pangaea The supercontinent which formed at the end of the Paleozoic Era and began breaking up about 200 million years ago to form today’s continents.
accreted terrane Small crustal fragments, island arc, or seamount which are transported by the moving oceanic plate and are added to a continental mass at the subduction zone.
Panthalassa A hypothetical primaeval ocean covering two-thirds of the world except for the continent of Pangaea.
continental accretion The marginal growth of a continent along a subduction zone by mountain building or by addition of a microplate.
accretion A process that adds part of one tectonic plate to a larger plate along a convergent (collisional) plate boundary.
terrane A rock formation or assemblage of rock formations that share a common geologic history. A geologic terrane is distinguished from neighboring terranes by its different history, either in its formation or in its subsequent deformation and/or metamorphism. Terranes are separated by fault. An exotic terrane is one that has been transported into its present setting from some distance.
exotic terrane See terrane.
accretionary wedge Accumulation on the ocean floor of sedimentary material scraped off the subducted plate by the upper plate.
suture zone Lineation in continental lithosphere marking the location of an earlier plate collision that led to the amalgamation of formnerly separate continental masses.
stitching pluton Plutons of roughly the same age which that intruded several tectonic terrane after the terranes were faulted together. The pluton do not really sew the terranes together, but they help record when terranes were assembled.
polar motion Path of the earth's spin axis relative to an earth fixed-coordinate system. Movement of the rotation axis relative to geographic (earth-fixed) coordinates, consisting mainly of a slow drift, a 14-month Chandler wobble and a 12-month annual wobble.
Euler pole Pole passing through the center of the earth about which a plate can rotate as a rigid body from any initial to any final position on the earth's surface.
hotspot An area of concentrated heat in the mantle that produces magma that rises to the Earth’s surface to form volcanic island. The volcanic activity of the Hawaiian Islands is one example. Hot spots generally persist for millions of years.
continental collision Convergence of two continental plate. Such a convergence between the Indian and Eurasian plates is responsible for producing the Himalayas.
thermoremanent magnetization A permanent magnetization acquired by igneous rocks in the presence of the Earth's magnetic field as they cool through the Curie point.
oceanic magnetic anomalies Systematic fluctuations in magnetic field intensity that are a result of the juxtaposition of ocean floor formed (and magnetized) during intervals of normal and reversed geomagnetic polarity.
normal polarity Geomagnetic polarity that is the same as the present polarity.
mélange Mixture of rocks formed by tectonic disruption, such as multiple faulting, which brings disparate rock types together. Usually consists of a matrix of weak material, like shale, with hard pieces of exotic rocks, such as gneiss or igneous rocks.
magnetic stratigraphy The study and correlation of polarity epoch and events in the history of the Earth's magnetic field as contained in magnetic rocks.
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