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Continental crust

April 10, 2023

Continental crust is the layer of igneous, metamorphic, and sedimentary rocks that forms the geological continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial because its bulk composition is richer in aluminium silicates (Al-Si) and has a lower density compared to the oceanic crust,[1][2] called sima which is richer in magnesium silicate (Mg-Si) minerals. Changes in seismic wave velocities have shown that at a certain depth (the Conrad discontinuity), there is a reasonably sharp contrast between the more felsic upper continental crust and the lower continental crust, which is more mafic in character.[3]

The thickness of Earth's crust (km)
Continental and oceanic crust on the upper earth mantle

The continental crust consists of various layers, with a bulk composition that is intermediate (SiO2 wt% = 60.6).[4] The average density of the continental crust is about, 2.83 g/cm3 (0.102 lb/cu in),[5] less dense than the ultramafic material that makes up the mantle, which has a density of around 3.3 g/cm3 (0.12 lb/cu in). Continental crust is also less dense than oceanic crust, whose density is about 2.9 g/cm3 (0.10 lb/cu in). At 25 to 70 km (16 to 43 mi) in thickness, continental crust is considerably thicker than oceanic crust, which has an average thickness of around 7 to 10 km (4.3 to 6.2 mi). Approx. 41% of Earth's surface area[6][7] and about 70% of the volume of Earth's crust are continental crust.[8]

Most continental crust is dry land above sea level. However, 94% of the Zealandia continental crust region is submerged beneath the Pacific Ocean,[9] with New Zealand constituting 93% of the above-water portion.

Importance

Because the surface of continental crust mainly lies above sea level, its existence allowed land life to evolve from marine life. Its existence also provides broad expanses of shallow water known as epeiric seas and continental shelves where complex metazoan life could become established during early Paleozoic time, in what is now called the Cambrian explosion.[10]

Origin

All continental crust is ultimately derived from mantle-derived melts (mainly basalt) through fractional differentiation of basaltic melt and the assimilation (remelting) of pre-existing continental crust. The relative contributions of these two processes in creating continental crust are debated, but fractional differentiation is thought to play the dominant role.[11] These processes occur primarily at magmatic arcs associated with subduction.

There is little evidence of continental crust prior to 3.5 Ga.[12] About 20% of the continental crust's current volume was formed by 3.0 Ga.[13] There was relatively rapid development on shield areas consisting of continental crust between 3.0 and 2.5 Ga.[12] During this time interval, about 60% of the continental crust's current volume was formed.[13] The remaining 20% has formed during the last 2.5 Ga.

Proponents of a steady-state hypothesis argue that the total volume of continental crust has remained more or less the same after early rapid planetary differentiation of Earth and that presently found age distribution is just the result of the processes leading to the formation of cratons (the parts of the crust clustered in cratons being less likely to be reworked by plate tectonics).[14] However, this is not generally accepted.[15]

Forces at work

In contrast to the persistence of continental crust, the size, shape, and number of continents are constantly changing through geologic time. Different tracts rift apart, collide and recoalesce as part of a grand supercontinent cycle.[16]

There are currently about 7 billion cubic kilometres (1.7 billion cubic miles) of continental crust, but this quantity varies because of the nature of the forces involved. The relative permanence of continental crust contrasts with the short life of oceanic crust. Because continental crust is less dense than oceanic crust, when active margins of the two meet in subduction zones, the oceanic crust is typically subducted back into the mantle. Continental crust is rarely subducted (this may occur where continental crustal blocks collide and overthicken, causing deep melting under mountain belts such as the Himalayas or the Alps). For this reason the oldest rocks on Earth are within the cratons or cores of the continents, rather than in repeatedly recycled oceanic crust; the oldest intact crustal fragment is the Acasta Gneiss at 4.01 Ga, whereas the oldest large-scale oceanic crust (located on the Pacific Plate offshore of the Kamchatka Peninsula) is from the Jurassic (≈180 Ma), although there might be small older remnants in the Mediterranean Sea at about 340 Ma.[17] Continental crust and the rock layers that lie on and within it are thus the best archive of Earth's history.[7][18]

The height of mountain ranges is usually related to the thickness of crust. This results from the isostasy associated with orogeny (mountain formation). The crust is thickened by the compressive forces related to subduction or continental collision. The buoyancy of the crust forces it upwards, the forces of the collisional stress balanced by gravity and erosion. This forms a keel or mountain root beneath the mountain range, which is where the thickest crust is found.[19] The thinnest continental crust is found in rift zones, where the crust is thinned by detachment faulting and eventually severed, replaced by oceanic crust. The edges of continental fragments formed this way (both sides of the Atlantic Ocean, for example) are termed passive margins.

The high temperatures and pressures at depth, often combined with a long history of complex distortion, cause much of the lower continental crust to be metamorphic – the main exception to this being recent igneous intrusions. Igneous rock may also be "underplated" to the underside of the crust, i.e. adding to the crust by forming a layer immediately beneath it.

Continental crust is produced and (far less often) destroyed mostly by plate tectonic processes, especially at convergent plate boundaries. Additionally, continental crustal material is transferred to oceanic crust by sedimentation. New material can be added to the continents by the partial melting of oceanic crust at subduction zones, causing the lighter material to rise as magma, forming volcanoes. Also, material can be accreted horizontally when volcanic island arcs, seamounts or similar structures collide with the side of the continent as a result of plate tectonic movements. Continental crust is also lost through erosion and sediment subduction, tectonic erosion of forearcs, delamination, and deep subduction of continental crust in collision zones.[20] Many theories of crustal growth are controversial, including rates of crustal growth and recycling, whether the lower crust is recycled differently from the upper crust, and over how much of Earth history plate tectonics has operated and so could be the dominant mode of continental crust formation and destruction.[14]

It is a matter of debate whether the amount of continental crust has been increasing, decreasing, or remaining constant over geological time. One model indicates that at prior to 3.7 Ga ago continental crust constituted less than 10% of the present amount.[21] By 3.0 Ga ago the amount was about 25%, and following a period of rapid crustal evolution it was about 60% of the current amount by 2.6 Ga ago.[22] The growth of continental crust appears to have occurred in spurts of increased activity corresponding to five episodes of increased production through geologic time.[23]

See also

References

  1. ^ Fairbridge, Rhodes W., ed. (1967). The Encyclopedia of Atmospheric Sciences and Astrogeology. New York: Reinhold Publishing. p. 323. OCLC 430153.
  2. ^ Davis, George H.; Reynolds, Stephen J.; Kluth, Charles F. (2012). "Nature of Structural Geology". Structural Geology of Rocks and Regions (3rd ed.). John Wiley & Sons. p. 18. ISBN 978-0-471-15231-6.
  3. ^ McGuire, Thomas (2005). "Earthquakes and Earth's Interior". Earth Science: The Physical Setting. AMSCO School Publications Inc. pp. 182–184. ISBN 978-0-87720-196-0.
  4. ^ Rudnick, R.L.; Gao, S. (1 January 2014). "Composition of the Continental Crust". Treatise on Geochemistry. pp. 1–51. doi:10.1016/B978-0-08-095975-7.00301-6. ISBN 9780080983004.
  5. ^ Christensen, Nikolas I.; Mooney, Walter D. (1995). "Seismic velocity structure and composition of the continental crust: A global view". Journal of Geophysical Research: Solid Earth. 100 (B6): 9761–9788. Bibcode:1995JGR...100.9761C. doi:10.1029/95JB00259. ISSN 2156-2202.
  6. ^ Mechanism of Continental Crustal Growth
  7. ^ a b Cogley 1984.
  8. ^ Hawkesworth et al. 2010.
  9. ^ Mortimer, Nick; Campbell, Hamish J. (2017). "Zealandia: Earth's Hidden Continent". GSA Today. 27: 27–35. doi:10.1130/GSATG321A.1. from the original on 17 February 2017.
  10. ^ Waggoner, Ben; Collins, Allen. "The Cambrian Period". University of California Museum of Paleontology. Retrieved 30 November 2013.
  11. ^ Klein, Benjamin; Jagoutz, Oliver (1 January 2018). "On the importance of crystallization-differentiation for the generation of SiO2-rich melts and the compositional build-up of arc (and continental) crust". American Journal of Science. 318 (1): 29–63. Bibcode:2018AmJS..318...29J. doi:10.2475/01.2018.03. ISSN 1945-452X. S2CID 134674805.
  12. ^ a b Hart, P. J. (1969). Earth's Crust and Upper Mantle. American Geophysical Union. pp. 13–15. ISBN 978-0-87590-013-1.
  13. ^ a b McCann, T. (2008). The Geology of Central Europe: Volume 1: Precambrian and Palaeozoic. London: The Geological Society. p. 22. ISBN 978-1-86239-245-8.
  14. ^ a b Armstrong 1991.
  15. ^ Taylor & McLennan 2009.
  16. ^ Condie 2002.
  17. ^ "World's oldest ocean crust dates back to ancient supercontinent".
  18. ^ Bowring & Williams 1999.
  19. ^ Saal et al. 1998.
  20. ^ Clift & Vannuchi 2004.
  21. ^ von Huene & Scholl 1991.
  22. ^ Taylor & McLennan 1995.
  23. ^ Butler 2011, See graphic.

Bibliography

  • Armstrong, R.L. (1991). "The Persistent Myth of Crustal Growth" (PDF). Australian Journal of Earth Sciences. 38 (5): 613–630. Bibcode:1991AuJES..38..613A. CiteSeerX 10.1.1.527.9577. doi:10.1080/08120099108727995.
  • Bowring, S. A.; Williams, I. S. (1999). "Priscoan (4.00–4.03 Ga) orthogneisses from northwestern Canada". Contributions to Mineralogy and Petrology. 134 (134): 3–16. Bibcode:1999CoMP..134....3B. doi:10.1007/s004100050465. S2CID 128376754.
  • Butler, Rob (2011). . Archived from the original on 1 March 2006. Retrieved 29 January 2006.
  • Cogley, J. Graham (1984). "Continental Margins and the Extent and Number of Continents". Reviews of Geophysics. 22 (2): 101–122. Bibcode:1984RvGSP..22..101C. doi:10.1029/RG022i002p00101.
  • Condie, Kent C. (2002). "The supercontinent cycle: are there two patterns of cyclicity?". Journal of African Earth Sciences. 35 (2): 179–183. Bibcode:2002JAfES..35..179C. doi:10.1016/S0899-5362(02)00005-2.
  • Clift, P.; Vannuchi, P. (2004). "Controls on Tectonic Accretion versus Erosion in Subduction Zones: Implications for the Origin and Recycling of the Continental Crust". Reviews of Geophysics. 42 (RG2001): RG2001. Bibcode:2004RvGeo..42.2001C. doi:10.1029/2003RG000127. hdl:1912/3466. S2CID 19916396.
  • Hawkesworth, C.J.; Dhuime, B.; Pietranik, A.B.; Cawood, P.A.; Kemp, A.I.S.; Storey, C.D. (2010). "The generation and evolution of the continental crust". Journal of the Geological Society. 167 (2): 229–248. Bibcode:2010JGSoc.167..229H. doi:10.1144/0016-76492009-072. S2CID 131052922.
  • Saal, A.L.; Rudnick, R.L.; Ravizza, G.E.; Hart, S.R. (1998). "Re–Os isotope evidence for the composition, formation and age of the lower continental crust". Nature. 393 (6680): 58–61. Bibcode:1998Natur.393...58S. doi:10.1038/29966. S2CID 4327383.
  • Walther, John Victor (2005). Essentials of Geochemistry. Jones & Bartlett. p. 35. ISBN 978-0-7637-2642-3 – via Google Books. (Diagram entitled "Model of growth of continental crust through time" by Taylor, S.R.; McLennan, S.M. (1995). "The geochemical evolution of the continental crust". Reviews of Geophysics. 33 (2): 241–265. Bibcode:1995RvGeo..33..241T. doi:10.1029/95RG00262.)
  • Taylor, S. Ross; McLennan, Scott (2009). Planetary Crusts: Their Composition, Origin and Evolution. Cambridge University Press. ISBN 9780521841863. Retrieved 28 June 2022 – via Google Books.
  • von Huene, Roland; Scholl, David W. (1991). "Observations at convergent margins concerning sediment subduction, subduction erosion, and the growth of continental crust". Reviews of Geophysics. 29 (3): 279–316. Bibcode:1991RvGeo..29..279V. doi:10.1029/91RG00969.

External links

  • Average composition of continental crust
  • Evolution of the continental crust
  • Continental crust world map

April 10, 2023
continental, crust, layer, igneous, metamorphic, sedimentary, rocks, that, forms, geological, continents, areas, shallow, seabed, close, their, shores, known, continental, shelves, this, layer, sometimes, called, sial, because, bulk, composition, richer, alumi. Continental crust is the layer of igneous metamorphic and sedimentary rocks that forms the geological continents and the areas of shallow seabed close to their shores known as continental shelves This layer is sometimes called sial because its bulk composition is richer in aluminium silicates Al Si and has a lower density compared to the oceanic crust 1 2 called sima which is richer in magnesium silicate Mg Si minerals Changes in seismic wave velocities have shown that at a certain depth the Conrad discontinuity there is a reasonably sharp contrast between the more felsic upper continental crust and the lower continental crust which is more mafic in character 3 The thickness of Earth s crust km Continental and oceanic crust on the upper earth mantle The continental crust consists of various layers with a bulk composition that is intermediate SiO2 wt 60 6 4 The average density of the continental crust is about 2 83 g cm3 0 102 lb cu in 5 less dense than the ultramafic material that makes up the mantle which has a density of around 3 3 g cm3 0 12 lb cu in Continental crust is also less dense than oceanic crust whose density is about 2 9 g cm3 0 10 lb cu in At 25 to 70 km 16 to 43 mi in thickness continental crust is considerably thicker than oceanic crust which has an average thickness of around 7 to 10 km 4 3 to 6 2 mi Approx 41 of Earth s surface area 6 7 and about 70 of the volume of Earth s crust are continental crust 8 Most continental crust is dry land above sea level However 94 of the Zealandia continental crust region is submerged beneath the Pacific Ocean 9 with New Zealand constituting 93 of the above water portion Contents 1 Importance 2 Origin 3 Forces at work 4 See also 5 References 5 1 Bibliography 6 External linksImportance EditBecause the surface of continental crust mainly lies above sea level its existence allowed land life to evolve from marine life Its existence also provides broad expanses of shallow water known as epeiric seas and continental shelves where complex metazoan life could become established during early Paleozoic time in what is now called the Cambrian explosion 10 Origin EditAll continental crust is ultimately derived from mantle derived melts mainly basalt through fractional differentiation of basaltic melt and the assimilation remelting of pre existing continental crust The relative contributions of these two processes in creating continental crust are debated but fractional differentiation is thought to play the dominant role 11 These processes occur primarily at magmatic arcs associated with subduction There is little evidence of continental crust prior to 3 5 Ga 12 About 20 of the continental crust s current volume was formed by 3 0 Ga 13 There was relatively rapid development on shield areas consisting of continental crust between 3 0 and 2 5 Ga 12 During this time interval about 60 of the continental crust s current volume was formed 13 The remaining 20 has formed during the last 2 5 Ga Proponents of a steady state hypothesis argue that the total volume of continental crust has remained more or less the same after early rapid planetary differentiation of Earth and that presently found age distribution is just the result of the processes leading to the formation of cratons the parts of the crust clustered in cratons being less likely to be reworked by plate tectonics 14 However this is not generally accepted 15 Forces at work EditIn contrast to the persistence of continental crust the size shape and number of continents are constantly changing through geologic time Different tracts rift apart collide and recoalesce as part of a grand supercontinent cycle 16 There are currently about 7 billion cubic kilometres 1 7 billion cubic miles of continental crust but this quantity varies because of the nature of the forces involved The relative permanence of continental crust contrasts with the short life of oceanic crust Because continental crust is less dense than oceanic crust when active margins of the two meet in subduction zones the oceanic crust is typically subducted back into the mantle Continental crust is rarely subducted this may occur where continental crustal blocks collide and overthicken causing deep melting under mountain belts such as the Himalayas or the Alps For this reason the oldest rocks on Earth are within the cratons or cores of the continents rather than in repeatedly recycled oceanic crust the oldest intact crustal fragment is the Acasta Gneiss at 4 01 Ga whereas the oldest large scale oceanic crust located on the Pacific Plate offshore of the Kamchatka Peninsula is from the Jurassic 180 Ma although there might be small older remnants in the Mediterranean Sea at about 340 Ma 17 Continental crust and the rock layers that lie on and within it are thus the best archive of Earth s history 7 18 The height of mountain ranges is usually related to the thickness of crust This results from the isostasy associated with orogeny mountain formation The crust is thickened by the compressive forces related to subduction or continental collision The buoyancy of the crust forces it upwards the forces of the collisional stress balanced by gravity and erosion This forms a keel or mountain root beneath the mountain range which is where the thickest crust is found 19 The thinnest continental crust is found in rift zones where the crust is thinned by detachment faulting and eventually severed replaced by oceanic crust The edges of continental fragments formed this way both sides of the Atlantic Ocean for example are termed passive margins The high temperatures and pressures at depth often combined with a long history of complex distortion cause much of the lower continental crust to be metamorphic the main exception to this being recent igneous intrusions Igneous rock may also be underplated to the underside of the crust i e adding to the crust by forming a layer immediately beneath it Continental crust is produced and far less often destroyed mostly by plate tectonic processes especially at convergent plate boundaries Additionally continental crustal material is transferred to oceanic crust by sedimentation New material can be added to the continents by the partial melting of oceanic crust at subduction zones causing the lighter material to rise as magma forming volcanoes Also material can be accreted horizontally when volcanic island arcs seamounts or similar structures collide with the side of the continent as a result of plate tectonic movements Continental crust is also lost through erosion and sediment subduction tectonic erosion of forearcs delamination and deep subduction of continental crust in collision zones 20 Many theories of crustal growth are controversial including rates of crustal growth and recycling whether the lower crust is recycled differently from the upper crust and over how much of Earth history plate tectonics has operated and so could be the dominant mode of continental crust formation and destruction 14 It is a matter of debate whether the amount of continental crust has been increasing decreasing or remaining constant over geological time One model indicates that at prior to 3 7 Ga ago continental crust constituted less than 10 of the present amount 21 By 3 0 Ga ago the amount was about 25 and following a period of rapid crustal evolution it was about 60 of the current amount by 2 6 Ga ago 22 The growth of continental crust appears to have occurred in spurts of increased activity corresponding to five episodes of increased production through geologic time 23 See also EditDigital geologic mapping Geologic map Geologic province Lithosphere Mohorovicic discontinuity Oceanic crust OneGeology Plate tectonics TerraneReferences Edit Fairbridge Rhodes W ed 1967 The Encyclopedia of Atmospheric Sciences and Astrogeology New York Reinhold Publishing p 323 OCLC 430153 Davis George H Reynolds Stephen J Kluth Charles F 2012 Nature of Structural Geology Structural Geology of Rocks and Regions 3rd ed John Wiley amp Sons p 18 ISBN 978 0 471 15231 6 McGuire Thomas 2005 Earthquakes and Earth s Interior Earth Science The Physical Setting AMSCO School Publications Inc pp 182 184 ISBN 978 0 87720 196 0 Rudnick R L Gao S 1 January 2014 Composition of the Continental Crust Treatise on Geochemistry pp 1 51 doi 10 1016 B978 0 08 095975 7 00301 6 ISBN 9780080983004 Christensen Nikolas I Mooney Walter D 1995 Seismic velocity structure and composition of the continental crust A global view Journal of Geophysical Research Solid Earth 100 B6 9761 9788 Bibcode 1995JGR 100 9761C doi 10 1029 95JB00259 ISSN 2156 2202 Mechanism of Continental Crustal Growth a b Cogley 1984 Hawkesworth et al 2010 Mortimer Nick Campbell Hamish J 2017 Zealandia Earth s Hidden Continent GSA Today 27 27 35 doi 10 1130 GSATG321A 1 Archived from the original on 17 February 2017 Waggoner Ben Collins Allen The Cambrian Period University of California Museum of Paleontology Retrieved 30 November 2013 Klein Benjamin Jagoutz Oliver 1 January 2018 On the importance of crystallization differentiation for the generation of SiO2 rich melts and the compositional build up of arc and continental crust American Journal of Science 318 1 29 63 Bibcode 2018AmJS 318 29J doi 10 2475 01 2018 03 ISSN 1945 452X S2CID 134674805 a b Hart P J 1969 Earth s Crust and Upper Mantle American Geophysical Union pp 13 15 ISBN 978 0 87590 013 1 a b McCann T 2008 The Geology of Central Europe Volume 1 Precambrian and Palaeozoic London The Geological Society p 22 ISBN 978 1 86239 245 8 a b Armstrong 1991 Taylor amp McLennan 2009 Condie 2002 World s oldest ocean crust dates back to ancient supercontinent Bowring amp Williams 1999 Saal et al 1998 Clift amp Vannuchi 2004 von Huene amp Scholl 1991 Taylor amp McLennan 1995 Butler 2011 See graphic Bibliography Edit Armstrong R L 1991 The Persistent Myth of Crustal Growth PDF Australian Journal of Earth Sciences 38 5 613 630 Bibcode 1991AuJES 38 613A CiteSeerX 10 1 1 527 9577 doi 10 1080 08120099108727995 Bowring S A Williams I S 1999 Priscoan 4 00 4 03 Ga orthogneisses from northwestern Canada Contributions to Mineralogy and Petrology 134 134 3 16 Bibcode 1999CoMP 134 3B doi 10 1007 s004100050465 S2CID 128376754 Butler Rob 2011 Making new continents Archived from the original on 1 March 2006 Retrieved 29 January 2006 Cogley J Graham 1984 Continental Margins and the Extent and Number of Continents Reviews of Geophysics 22 2 101 122 Bibcode 1984RvGSP 22 101C doi 10 1029 RG022i002p00101 Condie Kent C 2002 The supercontinent cycle are there two patterns of cyclicity Journal of African Earth Sciences 35 2 179 183 Bibcode 2002JAfES 35 179C doi 10 1016 S0899 5362 02 00005 2 Clift P Vannuchi P 2004 Controls on Tectonic Accretion versus Erosion in Subduction Zones Implications for the Origin and Recycling of the Continental Crust Reviews of Geophysics 42 RG2001 RG2001 Bibcode 2004RvGeo 42 2001C doi 10 1029 2003RG000127 hdl 1912 3466 S2CID 19916396 Hawkesworth C J Dhuime B Pietranik A B Cawood P A Kemp A I S Storey C D 2010 The generation and evolution of the continental crust Journal of the Geological Society 167 2 229 248 Bibcode 2010JGSoc 167 229H doi 10 1144 0016 76492009 072 S2CID 131052922 Saal A L Rudnick R L Ravizza G E Hart S R 1998 Re Os isotope evidence for the composition formation and age of the lower continental crust Nature 393 6680 58 61 Bibcode 1998Natur 393 58S doi 10 1038 29966 S2CID 4327383 Walther John Victor 2005 Essentials of Geochemistry Jones amp Bartlett p 35 ISBN 978 0 7637 2642 3 via Google Books Diagram entitled Model of growth of continental crust through time by Taylor S R McLennan S M 1995 The geochemical evolution of the continental crust Reviews of Geophysics 33 2 241 265 Bibcode 1995RvGeo 33 241T doi 10 1029 95RG00262 Taylor S Ross McLennan Scott 2009 Planetary Crusts Their Composition Origin and Evolution Cambridge University Press ISBN 9780521841863 Retrieved 28 June 2022 via Google Books von Huene Roland Scholl David W 1991 Observations at convergent margins concerning sediment subduction subduction erosion and the growth of continental crust Reviews of Geophysics 29 3 279 316 Bibcode 1991RvGeo 29 279V doi 10 1029 91RG00969 External links EditAverage composition of continental crust Crust 5 1 Evolution of the continental crust Continental crust world map Retrieved from https en wikipedia org w index php title Continental crust amp oldid 1148507077, wikipedia, wiki, book, books, library,

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