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Encyclopedia > Continental collision

Continental collision is a phenomenon of the plate tectonics of our solid Earth. Continental collision is a variation on the fundamental process of subduction, whereby the subduction zone is destroyed, mountains produced, and two continents sutured together. Continental collision is known only from this planet and is an interesting example of how our different crusts, oceanic and continental, behave during subduction. Plate tectonics (from the Greek word for one who constructs, τεκτων, tekton) is a theory of geology developed to explain the phenomenon of continental drift, and is currently the theory accepted by the vast majority of scientists working in this area. ... Earth, also known as the Earth, Terra, and (mostly in the 19th century) Tellus, is the third-closest planet to the Sun. ... Subduction zones mark sites of convective downwelling of the Earths lithosphere. ... Mount Cook, a mountain in New Zealand A mountain is a landform that extends above the surrounding terrain in a limited area. ... Dymaxion map by Buckminster Fuller shows land mass with minimal distortion as only one continuous continent A continent (Latin continere, to hold together) is a large continuous mass of land on the planet Earth. ...


Continental collision is not an instantaneous event, like a car crash, but may take several tens of millions of years before the faulting and folding caused by collision stop. Collision between India and Asia has been going on for about 50 million years already and shows no signs of abating. Collision between East and West Gondwana to form the East African Orogen took about 100 million years from beginning (610 Ma) to end (510 Ma). Collision between Gondwana and Laurasia to form Pangea occurred in a relatively brief interval, about 50 million years long. A car accident in Yate, near Bristol, England, in July 2004. ... Old fault exposed by roadcut near Harrisburg, Pennsylvania. ... Fold or folding may refer to: fold (geology) folding, in poker, is the act of withdrawing from a hand rather than meeting the bet folding ingredients together is a cooking technique protein folding origami, the art of paper folding pattern welding, the folding of metal confining animals in a fold... [HELP! Needs re-writing. ... Laurasia was a supercontinent that broke off from the Pangaean supercontinent in the late Mesozoic era. ... Map of Pangæa Pangaea (Greek for all lands) is the name Alfred Wegener used to refer to the supercontinent that existed during the Mesozoic era, before the process of plate tectonics separated the component continents. ...

Contents


Subduction zone: the collision site

The process begins as two continents (different bits of continental crust), separated across a tract of ocean (and oceanic crust), approach each other, while the oceanic crust is slowly consumed at a subduction zone. The subduction zone runs along the edge of one of the continents and dips under it, raising volcanic mountain chains at some distance behind it, such as the Andes of South America today. Subduction involves the whole lithosphere, the density of which is largely controlled by the nature of the crust it carries. Oceanic crust is thin (~6 km thick) and dense (about 3.3 g/cm3), consisting of basalt, gabbro, and peridotite. Consequently, most oceanic crust is subducted easily at an oceanic trench. In contrast, continental crust is thick (~45 km thick) and buoyant, composed mostly of granitic rocks (average density about 2.5 g/cm3). Continental crust is subducted with difficulty, if at all. Normal subduction continues as long as the ocean exists, but the subduction system is disrupted as the continent carried by the downgoing plate enters the trench. Because it contains thick continental crust, this lithosphere is less dense than the underlying asthenospheric mantle and normal subduction is disrupted. The volcanic arc on the upper plate is slowly extinguished. Resisting subduction, the crust buckles up and under, raising mountains where a trench used to be. The position of the trench becomes a zone that marks the suture between the two continental terranes. Suture zones are often marked by fragments of the pre-existing oceanic crust and mantle rocks, known as ophiolites. Dymaxion map by Buckminster Fuller shows land mass with minimal distortion as only one continuous continent A continent (Latin continere, to hold together) is a large continuous land mass. ... The continental crust is the layer of granitic and sedimentary rock which forms the continents and the areas of shallow seabed close to their shores, known as continental shelves. ... Age of oceanic crust Oceanic crust is the part of Earths lithosphere which underlies the ocean basins. ... Categories: Geology stubs | Plate tectonics ... This article needs to be cleaned up to conform to a higher standard of quality. ... South America South America is a continent crossed by the equator, with most of its area in the Southern Hemisphere. ... The lithosphere (from the Greek for rocky sphere) is the solid outermost shell of a rocky planet. ... Basalt Basalt is an extrusive igneous rock, sometimes porphyritic, and is often both fine-grained and dense. ... Gabbro Gabbro is a dark, coarse-grained, intrusive igneous rock that is chemichly equivalent to basalt. ... Peridotite Peridotite is a dense, coarse grained ultrabasic rock, consisting mainly of the minerals olivine and pyroxene. ... The oceanic trenches are hemispheric-scale long but narrow topographic depressions of the sea floor. ... Granite is a common and widely-occurring group of intrusive felsic igneous rocks that form at great depths and pressures under continents. ... The asthenosphere (from an invented Greek a + sthenos without strength) is the region of the Earth between 100-200 km below the surface—but perhaps extending as deep as 400 km—that is is the weak or soft zone in the upper mantle. ... A volcanic arc is a chain of volcanic islands or mountains located near the edge of continents that are formed as the result of tectonic plate subduction. ... A terrane in paleogeography is an accretion that has collided with a continental nucleus, or craton but can be recognized by the foreign origin of its rock strata. ... Ophiolites are sections of the oceanic crust and the subjacent upper mantle that have been uplifted or emplaced to be exposed within continental crustal rocks. ...


Orogeny and Collapse

An orogeny is underway when mountains begin to grow in the collision zone. There are other modes of mountain formation and orogeny but certainly continental collision is one of the most important. Rainfall and snowfall increase on the mountains as these rise, perhaps at a rate of a few millimeters per year (at a growth rate of 1 mm/year, a 5,000m tall mountain can form in 5 million years, a time period that is less than 10% of the life of a typical collision zone). River systems form, and glaciers may grow on the highest peaks. Erosion accelerates as the mountains rise, and great volumes of sediment are shed into the rivers, which carry sediment away from the mountains to be deposited in sedimentary basins in the surrounding lowlands. Crustal rocks are thrust faulted over the sediments and the mountain belt broadens as it rises in height. A crustal root also develops, as required by isostasy; mountains can be high if underlain by thicker crust. Crustal thickening may happen as a result of crustal shortening or when one crust overthrusts the other. Thickening is accompanied by heating, so the crust becomes weaker as it thickens. The lower crust begins to flow and collapse under the growing mountain mass, formng rifts near the crest of the mountain range. The lower crust may partially melt, forming anatectic granites which then rise into the overlying units, forming granite intrusions. Crustal thickening provides one of two negative feedbacks on mountain growth in collision zones, the other being erosion. The popular notion that erosion is responsible for destroying mountains is only half correct - viscous flow of weak lower mantle also reduces relief with time, especially once the collision is complete and the two continents are completely sutured. Orogeny is the process of mountain building. ... Rain falling For other uses see Rain (disambiguation). ... A fresh snowfall in Colorados (USA) high forests. ... The Murray River in Australia. ... Austrias longest glacier, the Pasterze, winds its 8 km (5 mile) route at the foot of Austrias highest mountain, the Grossglockner A glacier is a large, long-lasting river of ice that is formed on land and moves in response to gravity. ... Severe soil erosion in a wheat field near Washington State University, USA. Erosion is the displacement of solids (soil, mud, rock, and so forth) by the agents of wind, water, ice, movement in response to gravity, or living organisms (in the case of bioerosion). ... Sediment is any particulate matter that can be transported by fluid flow and which eventually is deposited as a layer of solid particles on the bed or bottom of a body of water or other liquid. ... The term sedimentary basin is used to refer to any geographical feature exhibiting subsidence and consequent infilling by sedimentation. ... A thrust fault is a particular type of fault, or break in the fabric of the Earths crust with resulting movement of each side against the other, in which one side is pushed up relative to the other and somewhat over it. ... Isostasy is a term used in Geology to refer to the state of gravitational equilibrium between the Earths lithosphere and asthenosphere such that the tectonic plates float at an elevation which depends on their thickness and density. ... USGS image In geology, a rift is a place where the Earths crust and lithosphere are being pulled apart. ... Physics In physics, melting is the process of heating a solid substance to a point (called melting point) where it turns liquid. ... Quarrying granite for the Mormon Temple, Utah Territory. ... Pluton redirects here. ...


Convergence between the continents continues because the crust is still being pulled down by oceanic lithosphere sinking in the subduction zone to either side of the collision as well as beneath the impinging continent. Some pieces of continental crust may be subducted down to 150 km or more and then returned to the surface. These can be recognized by unusually high-pressure metamorphic assemblages including coesite and even diamond. Such units are known as "Ultra-High Pressure Metamorphic Terranes" and are known from the Alps and Himalaya as well as from older collision zones in China, Kazakstan, and Mali. Metamorphism can be defined as the mineralogical, chemical and crystallographic changes in a solid-state rock, i. ... Coesite is a form of silicon dioxide that is formed when very high pressure (2–3 gigapascals) and moderately high temperature (700 °C) are applied to quartz. ... A scattering of round-brilliant cut diamonds shows off the many reflecting facets. ... The West face of the Petit Dru above the Chamonix valley near the Mer de Glace. ... The Himalaya is a mountain range in Asia, separating the Indian subcontinent from the Tibetan Plateau. ...


The pace of mountain building associated with the collision is measured by radiometric dating of igneous rocks or units that have been metamorphosed during the collision and by examining the record of sediments shed from the rising mountains into the surrounding basins. The pace of ancient convergence can be determined with paleomagnetic measurements, while the present rate of convergence can be measured with GPS. Radiometric dating is a technique used to date materials based on a knowledge of the decay rates of naturally occurring isotopes, and the current abundances. ... Igneous rocks are formed when molten rock (magma) cools and solidifies, with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. ... Paleomagnetism refers to the orientation of the Earths magnetic field as it is preserved in various magnetic iron bearing minerals throughout time. ... Over fifty GPS satellites such as this NAVSTAR have been launched since 1978. ...


Far-field Effects

The effects of the collision are felt far beyond the immediate site of collision and mountain-building. As convergence between the two continents continues, the region of crustal thickening and elevation will become broader. If there is an oceanic free face, the adjacent crustal blocks may move towards it. As an example of this, the collision of India with Asia forced large regions of crust to move south to form modern Southeast Asia. Another example is the collision of Arabia with Asia, which is squeezing the Anatolian Plate (present day Turkey). As a result, Turkey is moving west and south into the Mediterranean Sea and away from the collision zone. These far-field effects may result in the formation of rifts, such as that occuppied by Lake Baikal, the deepest lake on Earth. Location of Southeast Asia Southeast Asia is a subregion of Asia. ... The Arabian Peninsula The Arabian Peninsula is a mainly desert peninsula in Southwest Asia at the junction of Africa and Asia and an important part of the greater Middle East. ... World map showing Asia (geographically) Asia is the central and eastern part of Eurasia and worlds largest continent. ... The Anatolian Plate is a continential tectonic plate consisting primarily of the country of Turkey. ... Satellite image The Mediterranean Sea is a part of the Atlantic Ocean almost completely enclosed by land, on the north by Europe, on the south by Africa, and on the east by Asia. ... Lake Baikal The Yenisei River basin, Lake Baikal, and the cities of Dikson, Dudinka, Turukhansk, Krasnoyarsk, Irkutsk Lake Baikal (Russian: О́зеро Байка́л (Ozero Baykal)), a lake in southern Siberia, Russia, between Irkutsk Oblast on the northwest and Buryatia on the southeast, near Irkutsk. ...


Fossil Collision Zones

Continental collisions are a critical part of the Supercontinent cycle and have happened many times in the past. Ancient collision zones are deeply eroded but may still be recognized because these mark sites of intense deformation, metamorphism, and plutonic activity that separate tracts of continental crust having different geologic histories prior to the collision. Old collision zones are commonly called "suture zones" by geologists, because this is where two previous continents are joined or sutured together. The Supercontinent Cycle describes the quasi-periodic aggregration and dispersal of Earths continental crust. ...


References

  • O'Brien, P.J. 2001. "Subduction followed by collision; Alpine and Himalayan examples." Physics of the Earth and Planetary Interiors, Vol. 127, Issue 1-4, pages 277-291
  • Toussaint, G., Burov, E., Avouac, J.-P. 2004."Tectonic evolution of a continental collision zone: A thermomechanical numerical model". Tectonics, Vol. 23, No. 6, TC6003 10.1029/2003TC001604

External links

  • Where Continents Collide
  • Dynamics of Continental Collision Zones
  • The Wilson Cycle

  Results from FactBites:
 
Continental collision - Wikipedia, the free encyclopedia (1135 words)
Continental collision is a phenomenon of the plate tectonics of Earth.
Continental collision is a variation on the fundamental process of subduction, whereby the subduction zone is destroyed, mountains produced, and two continents sutured together.
Continental collision is not an instantaneous event, like a car crash, but may take several tens of millions of years before the faulting and folding caused by collision stop.
continental crust: Definition and Much More from Answers.com (618 words)
The continental crust is the layer of granitic, sedimentary and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as continental shelves.
Continental crust is also less dense than oceanic crust, though it is considerably thicker; mostly 35 to 40 km versus the average oceanic thickness of around 7-10 km.
The thinnest continental crust is found in rift zones, where the crust is thinned by detachment faulting and eventually severed, replaced by oceanic crust.
  More results at FactBites »

 
 

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