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Convergent boundary

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Simplified diagram of a convergent boundary

A convergent boundary (also known as a destructive boundary) is an area on Earth where two or more lithospheric plates collide. One plate eventually slides beneath the other, a process known as subduction. The subduction zone can be defined by a plane where many earthquakes occur, called the Wadati–Benioff zone.[1] These collisions happen on scales of millions to tens of millions of years and can lead to volcanism, earthquakes, orogenesis, destruction of lithosphere, and deformation. Convergent boundaries occur between oceanic-oceanic lithosphere, oceanic-continental lithosphere, and continental-continental lithosphere. The geologic features related to convergent boundaries vary depending on crust types.

Plate tectonics is driven by convection cells in the mantle. Convection cells are the result of heat generated by the radioactive decay of elements in the mantle escaping to the surface and the return of cool materials from the surface to the mantle.[2] These convection cells bring hot mantle material to the surface along spreading centers creating new crust. As this new crust is pushed away from the spreading center by the formation of newer crust, it cools, thins, and becomes denser. Subduction begins when this dense crust converges with a less dense crust. The force of gravity helps drive the subducting slab into the mantle.[3] As the relatively cool subducting slab sinks deeper into the mantle, it is heated, causing hydrous minerals to break down. This releases water into the hotter asthenosphere, which leads to partial melting of the asthenosphere and volcanism. Both dehydration and partial melting occur along the 1,000 °C (1,830 °F) isotherm, generally at depths of 65 to 130 km (40 to 81 mi).[4][5]

Some lithospheric plates consist of both continental and oceanic lithosphere. In some instances, initial convergence with another plate will destroy oceanic lithosphere, leading to convergence of two continental plates. Neither continental plate will subduct. It is likely that the plate may break along the boundary of continental and oceanic crust. Seismic tomography reveals pieces of lithosphere that have broken off during convergence.

Subduction zones

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Main article: Subduction
See also: Forearc and Wadati–Benioff zone

Subduction zones are areas where one lithospheric plate slides beneath another at a convergent boundary due to lithospheric density differences. These plates dip at an average of 45° but can vary. Subduction zones are often marked by an abundance of earthquakes, the result of internal deformation of the plate, convergence with the opposing plate, and bending at the oceanic trench. Earthquakes have been detected to a depth of 670 km (416 mi). The relatively cold and dense subducting plates are pulled into the mantle and help drive mantle convection.[6]

Oceanic – oceanic convergence

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In collisions between two oceanic plates, the cooler, denser oceanic lithosphere sinks beneath the warmer, less dense oceanic lithosphere. As the slab sinks deeper into the mantle, it releases water from dehydration of hydrous minerals in the oceanic crust. This water reduces the melting temperature of rocks in the asthenosphere and causes partial melting. Partial melt will travel up through the asthenosphere, eventually, reach the surface, and form volcanic island arcs.[citation needed]

Continental – oceanic convergence

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When oceanic lithosphere and continental lithosphere collide, the dense oceanic lithosphere subducts beneath the less dense continental lithosphere. An accretionary wedge forms on the continental crust as deep-sea sediments and oceanic crust are scraped from the oceanic plate. Volcanic arcs form on continental lithosphere as the result of partial melting due to dehydration of the hydrous minerals of the subducting slab.[citation needed]

Continental – continental convergence

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See also: Continental collision

Some lithospheric plates consist of both continental and oceanic crust. Subduction initiates as oceanic lithosphere slides beneath continental crust. As the oceanic lithosphere subducts to greater depths, the attached continental crust is pulled closer to the subduction zone. Once the continental lithosphere reaches the subduction zone, subduction processes are altered, since continental lithosphere is more buoyant and resists subduction beneath other continental lithosphere. A small portion of the continental crust may be subducted until the slab breaks, allowing the oceanic lithosphere to continue subducting, hot asthenosphere to rise and fill the void, and the continental lithosphere to rebound.[7] Evidence of this continental rebound includes ultrahigh pressure metamorphic rocks, which form at depths of 90 to 125 km (56 to 78 mi), that are exposed at the surface.[8] Seismic records have been used to map the torn slabs beneath the Caucasus continental – continental convergence zone,[9] and seismic tomography has mapped detached slabs beneath the Tethyan suture zone (the Alps – Zagros – Himalaya mountain belt).[10]

Volcanism and volcanic arcs

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See also: Volcanic arc

The oceanic crust contains hydrated minerals such as the amphibole and mica groups. During subduction, oceanic lithosphere is heated and metamorphosed, causing breakdown of these hydrous minerals, which releases water into the asthenosphere. The release of water into the asthenosphere leads to partial melting. Partial melting allows the rise of more buoyant, hot material and can lead to volcanism at the surface and emplacement of plutons in the subsurface.[11] These processes which generate magma are not entirely understood.[12]

Where these magmas reach the surface they create volcanic arcs. Volcanic arcs can form as island arc chains or as arcs on continental crust. Three magma series of volcanic rocks are found in association with arcs. The chemically reduced tholeiitic magma series is most characteristic of oceanic volcanic arcs, though this is also found in continental volcanic arcs above rapid subduction (>7 cm/year). This series is relatively low in potassium. The more oxidized calc-alkaline series, which is moderately enriched in potassium and incompatible elements, is characteristic of continental volcanic arcs. The alkaline magma series (highly enriched in potassium) is sometimes present in the deeper continental interior. The shoshonite series, which is extremely high in potassium, is rare but sometimes is found in volcanic arcs.[5] The andesite member of each series is typically most abundant,[13] and the transition from basaltic volcanism of the deep Pacific basin to andesitic volcanism in the surrounding volcanic arcs has been called the andesite line.[14][15]

Back-arc basins

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Main article: Back-arc basin
See also: Back-arc region

Back-arc basins form behind a volcanic arc and are associated with extensional tectonics and high heat flow, often being home to seafloor spreading centers. These spreading centers are like mid-ocean ridges, though the magma composition of back-arc basins is generally more varied and contains a higher water content than mid-ocean ridge magmas.[16] Back-arc basins are often characterized by thin, hot lithosphere. Opening of back-arc basins may arise from movement of hot asthenosphere into lithosphere, causing extension.[17]

Oceanic trenches

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See also: Oceanic trench

Oceanic trenches are narrow topographic lows that mark convergent boundaries or subduction zones. Oceanic trenches average 50 to 100 km (31 to 62 mi) wide and can be several thousand kilometers long. Oceanic trenches form as a result of bending of the subducting slab. Depth of oceanic trenches seems to be controlled by age of the oceanic lithosphere being subducted.[5] Sediment fill in oceanic trenches varies and generally depends on abundance of sediment input from surrounding areas. An oceanic trench, the Mariana Trench, is the deepest point of the ocean at a depth of approximately 11,000 m (36,089 ft).[citation needed]

Earthquakes and tsunamis

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See also: Earthquake

Earthquakes are common along convergent boundaries. A region of high earthquake activity, the Wadati–Benioff zone, generally dips 45° and marks the subducting plate. Earthquakes will occur to a depth of 670 km (416 mi) along the Wadati-Benioff margin.[citation needed]

Both compressional and extensional forces act along convergent boundaries. On the inner walls of trenches, compressional faulting or reverse faulting occurs due to the relative motion of the two plates. Reverse faulting scrapes off ocean sediment and leads to the formation of an accretionary wedge. Reverse faulting can lead to megathrust earthquakes. Tensional or normal faulting occurs on the outer wall of the trench, likely due to bending of the downgoing slab.[18]

A megathrust earthquake can produce sudden vertical displacement of a large area of ocean floor. This in turn generates a tsunami.[19]

Some of the deadliest natural disasters have occurred due to convergent boundary processes. The 2004 Indian Ocean earthquake and tsunami was triggered by a megathrust earthquake along the convergent boundary of the Indian plate and Burma microplate and killed over 200,000 people. The 2011 tsunami off the coast of Japan, which caused 16,000 deaths and did US$360 billion in damage, was caused by a magnitude 9 megathrust earthquake along the convergent boundary of the Eurasian plate and Pacific plate.

Accretionary wedge

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See also: Accretionary wedge

Accretionary wedges (also called accretionary prisms) form as sediment is scraped from the subducting lithosphere and emplaced against the overriding lithosphere. These sediments include igneous crust, turbidite sediments, and pelagic sediments. Imbricate thrust faulting along a basal decollement surface occurs in accretionary wedges as forces continue to compress and fault these newly added sediments.[5] The continued faulting of the accretionary wedge leads to overall thickening of the wedge.[20] Seafloor topography plays some role in accretion, especially emplacement of igneous crust.[21]

Examples

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Map of Earth's principal plates (convergent boundaries shown as blue or mauve lines)

See also

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References

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  1. ^ Wicander, Reed; Monroe, James S. (2016). Geol (2nd ed.). Belmont, CA: Cengage Learning. ISBN 978-1133108696. OCLC 795757302.
  2. ^ Tackley, Paul J. (2000-06-16). "Mantle Convection and Plate Tectonics: Toward an Integrated Physical and Chemical Theory". Science. 288 (5473): 2002–2007. Bibcode:2000Sci...288.2002T. doi:10.1126/science.288.5473.2002. ISSN 1095-9203. PMID 10856206.
  3. ^ Conrad, Clinton P.; Lithgow‐Bertelloni, Carolina (2004-10-01). "The temporal evolution of plate driving forces: Importance of "slab suction" versus "slab pull" during the Cenozoic". Journal of Geophysical Research: Solid Earth. 109 (B10): B10407. Bibcode:2004JGRB..10910407C. doi:10.1029/2004JB002991. hdl:2027.42/95131. ISSN 2156-2202.
  4. ^ Bourdon, Bernard; Turner, Simon; Dosseto, Anthony (2003-06-01). "Dehydration and partial melting in subduction zones: Constraints from U-series disequilibria". Journal of Geophysical Research: Solid Earth. 108 (B6): 2291. Bibcode:2003JGRB..108.2291B. doi:10.1029/2002JB001839. ISSN 2156-2202. Archived from the original on 2019-12-31. Retrieved 2019-09-01.
  5. ^ a b c d P., Kearey (2009). Global tectonics. Klepeis, Keith A., Vine, F. J. (3rd ed.). Oxford: Wiley-Blackwell. ISBN 9781405107778. OCLC 132681514.
  6. ^ Widiyantoro, Sri; Hilst, Rob D. Van Der; Grand, Stephen P. (1997-12-01). "Global seismic tomography: A snapshot of convection in the earth". GSA Today. 7 (4). ISSN 1052-5173. Archived from the original on 2018-12-06. Retrieved 2018-12-06.
  7. ^ Condie, Kent C. (2016-01-01). "Crustal and Mantle Evolution". Earth as an Evolving Planetary System. Academic Press. pp. 147–199. doi:10.1016/b978-0-12-803689-1.00006-7. ISBN 9780128036891.
  8. ^ Ernst, W. G.; Maruyama, S.; Wallis, S. (1997-09-02). "Buoyancy-driven, rapid exhumation of ultrahigh-pressure metamorphosed continental crust". Proceedings of the National Academy of Sciences of the United States of America. 94 (18): 9532–9537. Bibcode:1997PNAS...94.9532E. doi:10.1073/pnas.94.18.9532. ISSN 0027-8424. PMC 23212. PMID 11038569.
  9. ^ Mumladze, Tea; Forte, Adam M.; Cowgill, Eric S.; Trexler, Charles C.; Niemi, Nathan A.; Burak Yıkılmaz, M.; Kellogg, Louise H. (March 2015). "Subducted, detached, and torn slabs beneath the Greater Caucasus". GeoResJ. 5: 36–46. doi:10.1016/j.grj.2014.09.004. S2CID 56219404.
  10. ^ Hafkenscheid, E.; Wortel, M. J. R.; Spakman, W. (2006). "Subduction history of the Tethyan region derived from seismic tomography and tectonic reconstructions". Journal of Geophysical Research. 111 (B8): B08401. Bibcode:2006JGRB..111.8401H. doi:10.1029/2005JB003791.
  11. ^ Philpotts, Anthony R.; Ague, Jay J. (2009). Principles of igneous and metamorphic petrology (2nd ed.). Cambridge, UK: Cambridge University Press. pp. 604–612. ISBN 9780521880060.
  12. ^ Castro, Antonio (January 2014). "The off-crust origin of granite batholiths". Geoscience Frontiers. 5 (1): 63–75. doi:10.1016/j.gsf.2013.06.006.
  13. ^ Philpotts & Ague 2009, p. 375.
  14. ^ Watters, W. A. (7 April 2006). "Marshall, Patrick 1869 – 1950". Marshall, Patrick. Dictionary of New Zealand Biography. Archived from the original on 24 May 2010. Retrieved 26 November 2020.
  15. ^ White, A. J. R. (1989). "Andesite line". Petrology. Encyclopedia of Earth Science: 22–24. doi:10.1007/0-387-30845-8_12. ISBN 0-442-20623-2.
  16. ^ Taylor, Brian; Martinez, Fernando (March 2002). "Mantle wedge control on back-arc crustal accretion". Nature. 416 (6879): 417–420. Bibcode:2002Natur.416..417M. doi:10.1038/416417a. ISSN 1476-4687. PMID 11919628. S2CID 4341911.
  17. ^ Tatsumi, Yoshiyuki; Otofuji, Yo-Ichiro; Matsuda, Takaaki; Nohda, Susumu (1989-09-10). "Opening of the Sea of Japan back-arc basin by asthenospheric injection". Tectonophysics. 166 (4): 317–329. Bibcode:1989Tectp.166..317T. doi:10.1016/0040-1951(89)90283-7. ISSN 0040-1951.
  18. ^ Oliver, J.; Sykes, L.; Isacks, B. (1969-06-01). "Seismology and the new global tectonics". Tectonophysics. 7 (5–6): 527–541. Bibcode:1969Tectp...7..527O. doi:10.1016/0040-1951(69)90024-9. ISSN 0040-1951.
  19. ^ "Questions and Answers on Megathrust Earthquakes". Natural Resources Canada. 19 October 2018. Archived from the original on 27 October 2020. Retrieved 23 September 2020.
  20. ^ Konstantinovskaia, Elena; Malavieille, Jacques (2005-02-01). "Erosion and exhumation in accretionary orogens: Experimental and geological approaches" (PDF). Geochemistry, Geophysics, Geosystems. 6 (2): Q02006. Bibcode:2005GGG.....6.2006K. doi:10.1029/2004GC000794. ISSN 1525-2027. S2CID 128854343.
  21. ^ Sharman, George F.; Karig, Daniel E. (1975-03-01). "Subduction and Accretion in Trenches". GSA Bulletin. 86 (3): 377–389. Bibcode:1975GSAB...86..377K. doi:10.1130/0016-7606(1975)86<377:SAAIT>2.0.CO;2. ISSN 0016-7606.
  22. ^ Carr, Steve (March 31, 2022). "UNM continental-scale helium study probes the deep structure of the Tibetan Plateau and the Himalayan plate collision". UNM Newsroom. Retrieved 5 July 2022.
  23. ^ Stanford University (March 14, 2022). "Hot springs reveal where continental plates collide beneath Tibet". ScienceDaily. Retrieved 5 July 2022.
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