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Earth's crust

Earth's crust is its thick outer shell of rock, referring to less than 1% of the planet's radius and volume. It is the top component of the lithosphere, a division of Earth's layers that includes the crust and the upper part of the mantle.[1] The lithosphere is broken into tectonic plates whose motion allows heat to escape the interior of the Earth into space.

Plates in the crust of Earth

The crust lies on top of the mantle, a configuration that is stable because the upper mantle is made of peridotite and is therefore significantly denser than the crust. The boundary between the crust and mantle is conventionally placed at the Mohorovičić discontinuity, a boundary defined by a contrast in seismic velocity.

Geologic provinces of the world (USGS)

The temperature of the crust increases with depth,[2] reaching values typically in the range from about 100 °C (212 °F) to 600 °C (1,112 °F) at the boundary with the underlying mantle. The temperature increases by as much as 30 °C (54 °F) for every kilometer locally in the upper part of the crust.[3]

Composition edit

 
Thickness of Earth's crust (km)

The crust of Earth is of two distinct types:

  1. Oceanic: 5 km (3 mi) to 10 km (6 mi) thick[4] and composed primarily of denser, more mafic rocks, such as basalt, diabase, and gabbro.
  2. Continental: 30 km (20 mi) to 50 km (30 mi) thick and mostly composed of less dense, more felsic rocks, such as granite. In a few places, such as the Tibetan Plateau, the Altiplano, and the eastern Baltic Shield, the continental crust is thicker (50 km (30 mi) to 80 km (50 mi)).

The average thickness of the crust is about 15 km (9 mi) to 20 km (12 mi).[citation needed]

Because both the continental and oceanic crust are less dense than the mantle below, both types of crust "float" on the mantle. The surface of the continental crust is significantly higher than the surface of the oceanic crust, due to the greater buoyancy of the thicker, less dense continental crust (an example of isostasy). As a result, the continents form high ground surrounded by deep ocean basins.[5]

The continental crust has an average composition similar to that of andesite,[6] though the composition is not uniform, with the upper crust averaging a more felsic composition similar to that of dacite, while the lower crust averages a more mafic composition resembling basalt.[7] The most abundant minerals in Earth's continental crust are feldspars, which make up about 41% of the crust by weight, followed by quartz at 12%, and pyroxenes at 11%.[8]

Most Abundant Elements of Earth's Crust Approximate % by weight Oxide Approximate % oxide by weight
O 46.6
Si 27.7 SiO2 60.6
Al 8.1 Al2O3 15.9
Fe 5.0 Fe as FeO 6.7
Ca 3.7 CaO 6.4
Na 2.7 Na2O 3.1
K 2.6 K2O 1.8
Mg 1.5 MgO 4.7
Ti 0.44 TiO2 0.7
P 0.10 P2O5 0.1

All the other constituents except water occur only in very small quantities and total less than 1%.[9]

Continental crust is enriched in incompatible elements compared to the basaltic ocean crust and much enriched compared to the underlying mantle. The most incompatible elements are enriched by a factor of 50 to 100 in the continental crust relative to primitive mantle rock, while oceanic crust is enriched with incompatible elements by a factor of about 10.[10]

The estimated average density of the continental crust is 2.835 g/cm3, with density increasing with depth from an average of 2.66 g/cm3 in the uppermost crust to 3.1 g/cm3 at the base of the crust.[11]

In contrast to the continental crust, the oceanic crust is composed predominantly of pillow lava and sheeted dikes with the composition of mid-ocean ridge basalt, with a thin upper layer of sediments and a lower layer of gabbro.[12]

Formation and evolution edit

Earth formed approximately 4.6 billion years ago from a disk of dust and gas orbiting the newly formed Sun. It formed via accretion, where planetesimals and other smaller rocky bodies collided and stuck, gradually growing into a planet. This process generated an enormous amount of heat, which caused early Earth to melt completely. As planetary accretion slowed, Earth began to cool, forming its first crust, called a primary or primordial crust.[13] This crust was likely repeatedly destroyed by large impacts, then reformed from the magma ocean left by the impact. None of Earth's primary crust has survived to today; all was destroyed by erosion, impacts, and plate tectonics over the past several billion years.[14]

Since then, Earth has been forming a secondary and tertiary crust, which correspond to oceanic and continental crust, respectively. Secondary crust forms at mid-ocean spreading centers, where partial-melting of the underlying mantle yields basaltic magmas and new ocean crust forms. This "ridge push" is one of the driving forces of plate tectonics, and it is constantly creating new ocean crust. Consequently, old crust must be destroyed, so opposite a spreading center, there is usually a subduction zone: a trench where an ocean plate is sinking back into the mantle. This constant process of creating a new ocean crust and destroying the old ocean crust means that the oldest ocean crust on Earth today is only about 200 million years old.[15]

In contrast, the bulk of the continental crust is much older. The oldest continental crustal rocks on Earth have ages in the range from about 3.7 to 4.28 billion years [16][17] and have been found in the Narryer Gneiss Terrane in Western Australia, in the Acasta Gneiss in the Northwest Territories on the Canadian Shield, and on other cratonic regions such as those on the Fennoscandian Shield. Some zircon with age as great as 4.3 billion years has been found in the Narryer Gneiss Terrane. Continental crust is a tertiary crust, formed at subduction zones through recycling of subducted secondary (oceanic) crust.[15]

The average age of Earth's current continental crust has been estimated to be about 2.0 billion years.[18] Most crustal rocks formed before 2.5 billion years ago are located in cratons. Such an old continental crust and the underlying mantle asthenosphere are less dense than elsewhere on Earth and so are not readily destroyed by subduction. Formation of new continental crust is linked to periods of intense orogeny, which coincide with the formation of the supercontinents such as Rodinia, Pangaea and Gondwana. The crust forms in part by aggregation of island arcs including granite and metamorphic fold belts, and it is preserved in part by depletion of the underlying mantle to form buoyant lithospheric mantle. Crustal movement on continents may result in earthquakes, while movement under the seabed can lead to tidal waves.

See also edit

References edit

  1. ^ Robinson, Eugene C. (January 14, 2011). "The Interior of the Earth". U.S. Geological Survey. Retrieved August 30, 2013.
  2. ^ Peele, Robert (1911). "Boring" . In Chisholm, Hugh (ed.). Encyclopædia Britannica. Vol. 4 (11th ed.). Cambridge University Press. p. 251.
  3. ^ Philpotts, Anthony R.; Ague, Jay J. (2009). Principles of igneous and metamorphic petrology (2nd ed.). Cambridge, UK: Cambridge University Press. p. 14. ISBN 978-0-521-88006-0.
  4. ^ Structure of the Earth. The Encyclopedia of Earth. March 3, 2010
  5. ^ Levin, Harold L. (2010). The earth through time (9th ed.). Hoboken, N.J.: J. Wiley. pp. 173–174. ISBN 978-0-470-38774-0.
  6. ^ R. L. Rudnick and S. Gao, 2003, Composition of the Continental Crust. In The Crust (ed. R. L. Rudnick) volume 3, pp. 1–64 of Treatise on Geochemistry (eds. H. D. Holland and K. K. Turekian), Elsevier-Pergamon, Oxford ISBN 0-08-043751-6
  7. ^ Philpotts & Ague 2009, p. 2.
  8. ^ Anderson, Robert S.; Anderson, Suzanne P. (2010). Geomorphology: The Mechanics and Chemistry of Landscapes. Cambridge University Press. p. 187. ISBN 978-1-139-78870-0.
  9. ^ Klein, Cornelis; Hurlbut, Cornelius S. Jr. (1993). Manual of mineralogy : (after James D. Dana) (21st ed.). New York: Wiley. pp. 221–224. ISBN 0-471-57452-X.
  10. ^ Hofmann, Albrecht W. (November 1988). "Chemical differentiation of the Earth: the relationship between mantle, continental crust, and oceanic crust". Earth and Planetary Science Letters. 90 (3): 297–314. Bibcode:1988E&PSL..90..297H. CiteSeerX 10.1.1.464.5698. doi:10.1016/0012-821X(88)90132-X. S2CID 3211879.
  11. ^ Christensen, Nikolas I.; Mooney, Walter D. (June 10, 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.
  12. ^ Philpotts & Ague 2009, pp. 370–371.
  13. ^ Erickson, Jon (2014). Historical Geology: Understanding Our Planet's Past. Infobase Publishing. p. 8. ISBN 978-1-4381-0964-0. Retrieved September 28, 2017.
  14. ^ Taylor, S. Ross; McLennan, Scott M. (1996). "The Evolution of Continental Crust". Scientific American. 274 (1): 76–81. Bibcode:1996SciAm.274a..76T. doi:10.1038/scientificamerican0196-76. JSTOR 24989358.
  15. ^ a b Taylor & McLennan 1996.
  16. ^ "Team finds Earth's 'oldest rocks'". BBC News. September 26, 2008. Retrieved March 27, 2010.
  17. ^ P. J. Patchett and S. D. Samson, 2003, Ages and Growth of the Continental Crust from Radiogenic Isotopes. In The Crust (ed. R. L. Rudnick) volume 3, pp. 321–348 of Treatise on Geochemistry (eds. H. D. Holland and K. K. Turekian), Elsevier-Pergamon, Oxford ISBN 0-08-043751-6
  18. ^ A. I. S. Kemp and C. J. Hawkesworth, 2003, Granitic Perspectives on the Generation and Secular Evolution of the Continental Crust. In The Crust (ed. R. L. Rudnick) volume 3, pp. 349–410 of Treatise on Geochemistry (eds. H. D. Holland and K. K. Turekian), Elsevier-Pergamon, Oxford ISBN 0-08-043751-6

External links edit

earth, crust, thick, outer, shell, rock, referring, less, than, planet, radius, volume, component, lithosphere, division, earth, layers, that, includes, crust, upper, part, mantle, lithosphere, broken, into, tectonic, plates, whose, motion, allows, heat, escap. Earth s crust is its thick outer shell of rock referring to less than 1 of the planet s radius and volume It is the top component of the lithosphere a division of Earth s layers that includes the crust and the upper part of the mantle 1 The lithosphere is broken into tectonic plates whose motion allows heat to escape the interior of the Earth into space Plates in the crust of EarthThe crust lies on top of the mantle a configuration that is stable because the upper mantle is made of peridotite and is therefore significantly denser than the crust The boundary between the crust and mantle is conventionally placed at the Mohorovicic discontinuity a boundary defined by a contrast in seismic velocity Geologic provinces of the world USGS Shield Platform Orogen Basin Large igneous province Extended crust Oceanic crust 0 20 Ma 20 65 Ma gt 65 MaThe temperature of the crust increases with depth 2 reaching values typically in the range from about 100 C 212 F to 600 C 1 112 F at the boundary with the underlying mantle The temperature increases by as much as 30 C 54 F for every kilometer locally in the upper part of the crust 3 Contents 1 Composition 2 Formation and evolution 3 See also 4 References 5 External linksComposition editMain articles Abundance of elements in Earth s crust and Goldschmidt classification nbsp Thickness of Earth s crust km nbsp Abundance atom fraction of the chemical elements in Earth s upper continental crust as a function of the atomic number The rarest elements in the crust shown in yellow are not the heaviest but are rather the siderophile iron loving elements in the Goldschmidt classification of elements These have been depleted by being relocated deeper into Earth s core Their abundance in meteoroid materials is higher Additionally tellurium and selenium have been depleted from the crust due to formation of volatile hydrides The crust of Earth is of two distinct types Oceanic 5 km 3 mi to 10 km 6 mi thick 4 and composed primarily of denser more mafic rocks such as basalt diabase and gabbro Continental 30 km 20 mi to 50 km 30 mi thick and mostly composed of less dense more felsic rocks such as granite In a few places such as the Tibetan Plateau the Altiplano and the eastern Baltic Shield the continental crust is thicker 50 km 30 mi to 80 km 50 mi The average thickness of the crust is about 15 km 9 mi to 20 km 12 mi citation needed Because both the continental and oceanic crust are less dense than the mantle below both types of crust float on the mantle The surface of the continental crust is significantly higher than the surface of the oceanic crust due to the greater buoyancy of the thicker less dense continental crust an example of isostasy As a result the continents form high ground surrounded by deep ocean basins 5 The continental crust has an average composition similar to that of andesite 6 though the composition is not uniform with the upper crust averaging a more felsic composition similar to that of dacite while the lower crust averages a more mafic composition resembling basalt 7 The most abundant minerals in Earth s continental crust are feldspars which make up about 41 of the crust by weight followed by quartz at 12 and pyroxenes at 11 8 Most Abundant Elements of Earth s Crust Approximate by weight Oxide Approximate oxide by weightO 46 6Si 27 7 SiO2 60 6Al 8 1 Al2O3 15 9Fe 5 0 Fe as FeO 6 7Ca 3 7 CaO 6 4Na 2 7 Na2O 3 1K 2 6 K2O 1 8Mg 1 5 MgO 4 7Ti 0 44 TiO2 0 7P 0 10 P2O5 0 1All the other constituents except water occur only in very small quantities and total less than 1 9 Continental crust is enriched in incompatible elements compared to the basaltic ocean crust and much enriched compared to the underlying mantle The most incompatible elements are enriched by a factor of 50 to 100 in the continental crust relative to primitive mantle rock while oceanic crust is enriched with incompatible elements by a factor of about 10 10 The estimated average density of the continental crust is 2 835 g cm3 with density increasing with depth from an average of 2 66 g cm3 in the uppermost crust to 3 1 g cm3 at the base of the crust 11 In contrast to the continental crust the oceanic crust is composed predominantly of pillow lava and sheeted dikes with the composition of mid ocean ridge basalt with a thin upper layer of sediments and a lower layer of gabbro 12 Formation and evolution editMain article Earth s crustal evolution Further information Formation of the Earth Earth formed approximately 4 6 billion years ago from a disk of dust and gas orbiting the newly formed Sun It formed via accretion where planetesimals and other smaller rocky bodies collided and stuck gradually growing into a planet This process generated an enormous amount of heat which caused early Earth to melt completely As planetary accretion slowed Earth began to cool forming its first crust called a primary or primordial crust 13 This crust was likely repeatedly destroyed by large impacts then reformed from the magma ocean left by the impact None of Earth s primary crust has survived to today all was destroyed by erosion impacts and plate tectonics over the past several billion years 14 Since then Earth has been forming a secondary and tertiary crust which correspond to oceanic and continental crust respectively Secondary crust forms at mid ocean spreading centers where partial melting of the underlying mantle yields basaltic magmas and new ocean crust forms This ridge push is one of the driving forces of plate tectonics and it is constantly creating new ocean crust Consequently old crust must be destroyed so opposite a spreading center there is usually a subduction zone a trench where an ocean plate is sinking back into the mantle This constant process of creating a new ocean crust and destroying the old ocean crust means that the oldest ocean crust on Earth today is only about 200 million years old 15 In contrast the bulk of the continental crust is much older The oldest continental crustal rocks on Earth have ages in the range from about 3 7 to 4 28 billion years 16 17 and have been found in the Narryer Gneiss Terrane in Western Australia in the Acasta Gneiss in the Northwest Territories on the Canadian Shield and on other cratonic regions such as those on the Fennoscandian Shield Some zircon with age as great as 4 3 billion years has been found in the Narryer Gneiss Terrane Continental crust is a tertiary crust formed at subduction zones through recycling of subducted secondary oceanic crust 15 The average age of Earth s current continental crust has been estimated to be about 2 0 billion years 18 Most crustal rocks formed before 2 5 billion years ago are located in cratons Such an old continental crust and the underlying mantle asthenosphere are less dense than elsewhere on Earth and so are not readily destroyed by subduction Formation of new continental crust is linked to periods of intense orogeny which coincide with the formation of the supercontinents such as Rodinia Pangaea and Gondwana The crust forms in part by aggregation of island arcs including granite and metamorphic fold belts and it is preserved in part by depletion of the underlying mantle to form buoyant lithospheric mantle Crustal movement on continents may result in earthquakes while movement under the seabed can lead to tidal waves See also edit nbsp Earth sciences portal nbsp World portalBrittle ductile transition zone Internal structure of EarthReferences edit Robinson Eugene C January 14 2011 The Interior of the Earth U S Geological Survey Retrieved August 30 2013 Peele Robert 1911 Boring In Chisholm Hugh ed Encyclopaedia Britannica Vol 4 11th ed Cambridge University Press p 251 Philpotts Anthony R Ague Jay J 2009 Principles of igneous and metamorphic petrology 2nd ed Cambridge UK Cambridge University Press p 14 ISBN 978 0 521 88006 0 Structure of the Earth The Encyclopedia of Earth March 3 2010 Levin Harold L 2010 The earth through time 9th ed Hoboken N J J Wiley pp 173 174 ISBN 978 0 470 38774 0 R L Rudnick and S Gao 2003 Composition of the Continental Crust In The Crust ed R L Rudnick volume 3 pp 1 64 of Treatise on Geochemistry eds H D Holland and K K Turekian Elsevier Pergamon Oxford ISBN 0 08 043751 6 Philpotts amp Ague 2009 p 2 Anderson Robert S Anderson Suzanne P 2010 Geomorphology The Mechanics and Chemistry of Landscapes Cambridge University Press p 187 ISBN 978 1 139 78870 0 Klein Cornelis Hurlbut Cornelius S Jr 1993 Manual of mineralogy after James D Dana 21st ed New York Wiley pp 221 224 ISBN 0 471 57452 X Hofmann Albrecht W November 1988 Chemical differentiation of the Earth the relationship between mantle continental crust and oceanic crust Earth and Planetary Science Letters 90 3 297 314 Bibcode 1988E amp PSL 90 297H CiteSeerX 10 1 1 464 5698 doi 10 1016 0012 821X 88 90132 X S2CID 3211879 Christensen Nikolas I Mooney Walter D June 10 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 Philpotts amp Ague 2009 pp 370 371 Erickson Jon 2014 Historical Geology Understanding Our Planet s Past Infobase Publishing p 8 ISBN 978 1 4381 0964 0 Retrieved September 28 2017 Taylor S Ross McLennan Scott M 1996 The Evolution of Continental Crust Scientific American 274 1 76 81 Bibcode 1996SciAm 274a 76T doi 10 1038 scientificamerican0196 76 JSTOR 24989358 a b Taylor amp McLennan 1996 Team finds Earth s oldest rocks BBC News September 26 2008 Retrieved March 27 2010 P J Patchett and S D Samson 2003 Ages and Growth of the Continental Crust from Radiogenic Isotopes In The Crust ed R L Rudnick volume 3 pp 321 348 of Treatise on Geochemistry eds H D Holland and K K Turekian Elsevier Pergamon Oxford ISBN 0 08 043751 6 A I S Kemp and C J Hawkesworth 2003 Granitic Perspectives on the Generation and Secular Evolution of the Continental Crust In The Crust ed R L Rudnick volume 3 pp 349 410 of Treatise on Geochemistry eds H D Holland and K K Turekian Elsevier Pergamon Oxford ISBN 0 08 043751 6External links edit nbsp The Wikibook Historical Geology has a page on the topic of Structure of the Earth Crust of the Earth Encyclopedia Americana 1920 Retrieved from https en wikipedia org w index php title Earth 27s crust amp oldid 1205540131, wikipedia, wiki, book, books, library,

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