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Flashcards in Plate Tectonics Key Terms Deck (40)
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Accretionary Wedge (prism)

A zone of deformed sediment made up of thrust slices scraped off a subducting oceanic plate and added onto the over-riding plate. Occur at oceanic-oceanic and oceanic-continental destructive plate boundaries



A fine grained volcanic rock formed by a complex series of processes at destructive plate margins. in mantle - richer in silica and more viscous than basalt. Typically produces explosive volcanic eruptions e.g.Andes mountain chain



Layer of the earth immediately below the lithosphere. Hotter and weaker than lithosphere



A fine grained volcanic rock formed by the partial melting of the mantle, typically at constructive plate margins. submarine eruptions of basalt lava produce pillow lavas - form upper part of oceanic crust


Benioff Zone

An inclined zone of earthquake foci in the upper part of a subducting oceanic plate at a destructive plate margin. AKA Benioff Wadati Zone


Continental Crust

The crust that forms the continents. average thickness of 35km, can be upto 70km in mountain ranges. Granite rock, density of 2.7 - doesnt subduct


Continental Drift

Theory proposed by Alfred Wegener in 1915 to support the notion that the continents had changed position through geological time. Unable to propose a mechanism to explain how drift occurred - theory ignored


Convection Currents

Heat generated by the breakdown of radioactive materials in the mantle is redistributed by currents that rise at the mid ocean ridges and descend at the ocean trenches


Conservative Margin

A plate margin where two plates slide past each other along a transform fault. Shallow focus earthquakes but no volcanoes. e.g. San Andreas Fault - North American and Pacific plates


Convergent (destructive) Oceanic-Oceanic Margin

A plate margin where two oceanic plates collide and one is subducted to produce a volcanic island arc oceanic trench system. Andesitic volcanoes and shallow, intermediate and deep focus earthquakes. e.g. Caribbean Islands - South American plate subducted down the Puerto Rican Trench


Convergent (destructive) Oceanic-Continental Margin

A plate margin where an oceanic plate is subducted beneath a continental plate - produces an ocean trench mountain belt system. Andesitic and rhyolitic volcanoes and shallow, intermediate and deep focus earthquakes. as oceanic plate subducts, accretionary wedge forme. e.g Andes fold mountain


Convergent (destructive) Continental-Continental Margin

A plate margin wherean ocean closesand two continental plates collide.They are to bouyant to subduct so they form a mountain belt with thick continental crust. shallow focus earthquakes, folding, faulting and regional metamorphism. e.g. Himalayas


Divergent (constructive) Margin

A plate margin where two oceanic plates are moving away from each other. responsible for the formation of ocean basins - start out as rift valleys e.g. African Rift
Plates move apart - mantle melts and forms basalt magma. New oceanic crust formed at ridge. shalow focus earthquakes and basaltic volcanoes. e.g. Mid Atlantic Ridge



Harry Hess proposed the idea of Sea Floor Spreading, following echo sounding work to reveal the topography of the ocean basins



An area of abnormally intense active volcanism thought to be underlain by a mantle plume. e.g. Hawaii, ocated in the middle of lithospheric plate
e.g. Iceland - located on constructive plate margins



Igneous rocks that crystallise beneath the earths surface. smaller scale intrusions - dykes/sills, larger - plutons/batholiths



the large land mass of the northern hemisphere that consisted of North America, Greenland, Europe and Asia all joined as a single continent



The outer cool, rigid and brittle layer of the earth. comprises of the crust and part of the upper mantle. average thickness of 100km. divided up into lithospheric plates


Lithospheric/Tectonic Plate

A segment of the lithosphere which has earthquake activity along its margins. can consist of just oceanic ares e.g. Nazca plate, or a combination of oceanic and continental e.g. Eurasian. 7 major, 8 minor and numerous microplates


Magnetic Stripes

Linear magnetic stripes that run paralel to mid ocean ridges. reflect the repeated magnetic reversal of the earths magnetic field. the magnetic anomalies are symmetrical either side of mid ocean ridges - crucial evidence for sea floor spreading



The zone lying between the earths crust and core. includes the lower part of the lithosphere and all of the asthenosphere


Mid Ocean Ridge

The junction between two oceanic plates along a constructive plate margin. the ridge comprises a submarine mountain chain of basaltic volcanoes. central part of ridge has a rift valley running through it and the ridge is offset sideways by transform faults e.g. Mid ocean Ridge


Mohorovicic discontinuity (Moho)

The boundary between the crust and the mantle. exists at an average depth of 7km beneath the ocean basin . discovered in 1909


Oceanic Crust

The crust that forms the ocean basins. madeu of basalt, has an upper layer of pillow lava, a middle layerof vertical dykes and a lower layer of gabbro.average thickness - 7km, formed at mid ocean ridges and subducted at ocean trenches. oldest is less than 200 million years old



The name given by Wegener to the supercontinent . a single land mass comprising of most of the worlds land areas joined as a single unit. broke up into laurasia and gondwana



The study of the fossil magnetism locked in rocks which record the inclination and direction of the Earths magnetic field at the time of their formation. Such data is used to determine the past arrangement of the continents over time and support theories of continental drift and sea floor spreading



A fine grained igneous rock with the same mineral composition as granite. commonly associated with volcanoes at destructive oceanic-continental plate margins. rich in silica, viscous, felsic and is characterised by infrequent but violent eruptions


Rift Valley

A central block downthrown between two normal faults that face one another. continental rift valleys represent areas where continents are being stretched e.g. East African Rift Valley. may extend to the point where new ocean floor is created e.g. Red Sea. slowly spreading mid ocean ridges e.g. Mid Atlantic Ridge - may have rift valleys but fast spreading ones e.g. East Pacific Rise


Sea Floor Spreading

The process by which oceans are formed at constructive plate margins. new oceanic crust is formed as two oceanic plates move apart. harry hess


Slab Pull

As oceanic lithosphere cools, becomes denser and thicker.At a convergent boundary the oceanic lithosphere sinks beneath the adjacent plate forming an ocean trench and subduction zone. as a result of its own weight the descending plate is pulled by gravity through the mantle asthenosphere. believed to be the major force driving plate motions


Subduction Zone

A zone where oceanic lithosphere is recycled back into the the surface the subduction zone generally coincides with the bottom of oceanic trenches. marked by earthquakes


Tethys Ocean

The ocean that opened between the continents of Gondwana and Laurasia during the breakup of Pangea


Transform Fault

A transform fault marks a conservative plate margin. e.g.San Andreas Fault- seperates pacific plate from north american plate.



Fred Vine- Cambridge student who correctly interpreted the pattern of normal and reversed magnetic anomalies in ocean floor basalts as a result of sea floor spreading


Volcanic Island Arc

A chainofvolcanoes generally with an arc shape, which runs parallel to an oceanic trench at a destructive oceanic-oceanic plate margin. e.g. caribbean islands



proposed the theory of continental drift in 1915



A coarse grained intrusive igneous rock, similar composition to basalt. cools very slowly to produce large crystals.



the name given to the large land mass of the southern hemisphere that consisted of South America, Africa, Arabia, Madagascar, India, Sri Lanka, Australia, New Zealand and Antarctica as a single continent



A coarse grained igneous rock comprising of the minerals quartz, feldspar and mica. similar composition to continental crust. formed at destructive margins, in fold mountain belts


Mantle Plume

Hot bouyant rock rising through the mantle from the core mantle boundary. Cylindrical shape, seems to be fixed in position and have a radius of around 150km