{{Short description|Uppermost layer of the oceanic portion of a tectonic plate}}
[[File:Global Map of Oceanic Plate Age by Fabio Crameri.png|thumb|Map of the Earth's oceanic crust, with colours indicating the age of the crust. Lighter shades indicate younger age, and darker shades indicate older age. The lines represent tectonic plate boundaries.]] [[File:Continental and oceanic crust.png|thumb|Continental and oceanic crust on the Earth's upper mantle]]
'''Oceanic crust''' is the uppermost layer of the oceanic portion of the [[Plate tectonics|tectonic plates]]. It is composed of the upper oceanic crust, with [[pillow lava]]s and a [[dike (geology)|dike]] complex, and the [[lower oceanic crust]], composed of [[troctolite]], [[gabbro]] and [[ultramafic rock|ultramafic]] [[cumulate rock|cumulates]].<ref>Gillis et al. (2014). Primitive layered gabbros from fast-spreading lower oceanic crust. Nature 505, 204-208</ref><ref name="Pirajno_2013">{{Cite book |url=https://books.google.com/books?id=PrXyCAAAQBAJ&q=ultramafic+cumulates+layer+3&pg=PA11 |title=Ore Deposits and Mantle Plumes |last=Pirajno F. |publisher=Springer |year=2013 |isbn=9789401725026 |pages=11}}</ref> The crust lies above the rigid uppermost layer of the [[mantle (geology)|mantle]]. The crust and the rigid upper mantle layer together constitute oceanic [[lithosphere]].
Oceanic crust is primarily composed of [[mafic]] rocks, or [[sima (geology)|sima]], which is rich in iron and magnesium. It is thinner than [[continental crust]], or [[sial]], generally less than 10 kilometers thick; however, it is denser, having a mean density of about 3.0 [[gram]]s per cubic centimeter as opposed to continental crust which has a density of about 2.7 grams per cubic centimeter.<ref name="Rogers">{{cite book | editor-first=N. | editor-last=Rogers | url=https://books.google.com/books?id=WA9ST5S_2v0C&pg=PA44 | title=An Introduction to Our Dynamic Planet | publisher=[[Cambridge University Press]] and [[The Open University]] | date=2008 | page=19 | isbn=978-0-521-49424-3}}</ref>
The uppermost crust is the result of the cooling of magma derived from [[Earth's mantle|mantle]] material below the plate. The magma is injected into the spreading center, which consists mainly of a partly solidified [[crystal mush]] derived from earlier injections, forming magma lenses that are the source of the [[Sheeted dyke complex|sheeted dikes]] that feed the overlying pillow lavas.<ref name="Sinton & Detrick">{{Cite journal |last1=Sinton J.M. |last2=Detrick R.S. |year=1992 |title=Mid-ocean ridge magma chambers |url=https://www.researchgate.net/publication/248793835 |journal=Journal of Geophysical Research |volume=97 |issue=B1 |pages=197–216 |doi=10.1029/91JB02508|bibcode=1992JGR....97..197S }}</ref> As the lavas cool they are, in most instances, modified chemically by seawater.<ref>H. Elderfield (2006). The Oceans and Marine Geochemistry. Elsevier. pp. 182–. {{ISBN|978-0-08-045101-5}}.</ref> These eruptions occur mostly at mid-ocean ridges, but also at scattered hotspots, and also in rare but powerful occurrences known as [[flood basalt]] eruptions. But most [[magma]] [[Crystallization|crystallises]] at depth, within the [[lower oceanic crust]]. There, newly intruded magma can mix and react with pre-existing crystal mush and rocks.<ref>Lissenberg, C. J., MacLeod, C. J., Horward, K. A., and Godard, M. (2013). Pervasive reactive melt migration through fast-spreading lower oceanic crust (Hess Deep, equatorial Pacific Ocean). Earth Planet. Sci. Lett. 361, 436–447. {{doi|10.1016/j.epsl.2012.11.012}}</ref>
==Composition== {{See also|Lithosphere#Oceanic lithosphere}}
Although a complete section of oceanic crust has not yet been drilled, geologists have several pieces of evidence that help them understand the ocean floor. Estimations of composition are based on analyses of [[ophiolite]]s (sections of oceanic crust that are thrust onto and preserved on the continents), comparisons of the [[Seismology|seismic structure]] of the oceanic crust with laboratory determinations of seismic velocities in known rock types, and samples recovered from the ocean floor by [[submersible]]s, dredging (especially from [[mid-ocean ridge|ridge]] crests and [[fracture zone]]s) and drilling.<ref>Kodaira, S., Noguchi, N., Takahashi, N., Ishizuka, O., & Kaneda, Y. (2010). Evolution from fore‐arc oceanic crust to island arc crust: A seismic study along the Izu‐Bonin fore arc. ''Journal of Geophysical Research: Solid Earth,'' ''115''(B9), N/a.</ref> Oceanic crust is significantly simpler than continental crust and generally can be divided in three layers.<ref>{{Cite journal|last1=Hansteen|first1=Thor H|last2=Troll|first2=Valentin R|date=2003-02-14|title=Oxygen isotope composition of xenoliths from the oceanic crust and volcanic edifice beneath Gran Canaria (Canary Islands): consequences for crustal contamination of ascending magmas|url=http://www.sciencedirect.com/science/article/pii/S000925410200325X|journal=Chemical Geology|language=en|volume=193|issue=3|pages=181–193|doi=10.1016/S0009-2541(02)00325-X|bibcode=2003ChGeo.193..181H|issn=0009-2541|url-access=subscription}}</ref> According to [[mineral physics]] experiments, at lower mantle pressures, oceanic crust becomes denser than the surrounding mantle.<ref>Li, M., & McNamara, A. (2013). The difficulty for subducted oceanic crust to accumulate at the Earth's core‐mantle boundary. ''Journal of Geophysical Research: Solid Earth,'' ''118''(4), 1807-1816.</ref> * Layer 1 is on an average 0.4 km thick. It consists of unconsolidated or semiconsolidated [[sediment]]s, usually thin or even not present near the [[mid-ocean ridge]]s but thicker farther away from the ridge.<ref>Peter Laznicka (2 September 2010). Giant Metallic Deposits: Future Sources of Industrial Metals. Springer Science & Business Media. pp. 82–. {{ISBN|978-3-642-12405-1}}.</ref> Near the continental margins sediment is [[Terrigenous sediment|terrigenous]], meaning derived from the land, unlike deep sea sediments which are made of tiny [[Exoskeleton|shells]] of marine organisms, usually calcareous and siliceous, or it can be made of volcanic ash and terrigenous [[sediment transport|sediments transported]] by [[turbidity current]]s.<ref>D. R. Bowes (1989) ''The Encyclopedia of Igneous and Metamorphic Petrology'', Van Nostrand Reinhold {{ISBN|0-442-20623-2}}</ref> * Layer 2 could be divided into two parts: Layer 2A is a 0.5 km thick uppermost volcanic layer of glassy to finely crystalline [[basalt]], usually in the form of [[Pillow lava|pillow basalt]]. Layer 2B is a 1.5 km thick layer composed of [[diabase]] [[Dike (geology)|dikes]].<ref>Yildirim Dilek (1 January 2000). Ophiolites and Oceanic Crust: New Insights from Field Studies and the Ocean Drilling Program. Geological Society of America. pp. 506–. {{ISBN|978-0-8137-2349-5}}.</ref> * Layer 3 is formed by slow cooling of [[magma]] beneath the surface and consists of coarse grained [[gabbro]] and [[cumulate rock|cumulate]] [[ultramafic rock]]s.<ref>Gillis et al (2014). Primitive layered gabbros from fast-spreading lower oceanic crust. Nature 505, 204-208</ref> It constitutes over two-thirds of oceanic crust volume with almost 5 km thickness.<ref>Jon Erickson (14 May 2014). Plate Tectonics: Unraveling the Mysteries of the Earth. Infobase Publishing. pp. 83–. {{ISBN|978-1-4381-0968-8}}.</ref>
===Geochemistry<span class="anchor" id="marine_geochemistry"></span>=== The most voluminous [[volcanic rock]]s of the ocean floor are the mid-oceanic ridge basalts, which are derived from low-[[potassium]] [[Tholeiitic magma series|tholeiitic magmas]]. These rocks have low concentrations of large ion [[lithophile]] elements (LILE), light rare earth elements (LREE), volatile elements and other highly [[incompatible element]]s. There can be found basalts enriched with incompatible elements, but they are rare and associated with mid-ocean ridge [[Hotspot (geology)|hot spot]]s such as surroundings of [[Galapagos Islands]], the [[Azores]] and [[Iceland]].<ref>Clare P. Marshall, Rhodes W. Fairbridge (1999) ''Encyclopedia of Geochemistry'', Kluwer Academic Publishers {{ISBN|0-412-75500-9}}</ref>
Prior to the [[Neoproterozoic|Neoproterozoic Era]] 1000 [[million years ago]], the world's oceanic crust was more [[mafic]] than the current crust. The more mafic nature of the crust meant that higher amounts of water molecules ([[Hydroxy group|OH]]) could be stored in the [[alteration (geology)|altered]] parts of the crust. At [[subduction]] zones this mafic crust was prone to metamorphose into [[greenschist]] instead of [[blueschist]] at ordinary [[metamorphic facies|blueschist facies]].<ref name=PaWh2016>{{cite journal |last1=Palin |first1=Richard M. |last2=White |first2=Richard W.|year=2016 |title=Emergence of blueschists on Earth linked to secular changes in oceanic crust composition |url= https://ora.ox.ac.uk/objects/uuid:48630722-57a2-4dd7-8101-92ea1c8df8a1|journal=Nature Geoscience |volume=9 |issue=1 |pages= 60|doi= 10.1038/ngeo2605|bibcode=2016NatGe...9...60P |s2cid=130847333 }}</ref>
===Life cycle=== Oceanic crust is continuously being created at mid-ocean ridges. As [[Plate tectonics|continental plates]] diverge at these ridges, magma rises into the upper mantle and crust. As the continental plates move away from the ridge, the newly formed rocks cool and start to erode with sediment gradually building up on top of them. The youngest oceanic rocks are at the oceanic ridges, and they get progressively older away from the ridges.<ref>{{Cite web|url=https://pubs.usgs.gov/gip/dynamic/understanding.html|title=Understanding plate motions [This Dynamic Earth, USGS]|publisher=United States Geological Survey|access-date=2017-04-16}}</ref>
As the mantle rises it cools and melts, as the pressure decreases and it crosses the [[Solidus (chemistry)|solidus]]. The amount of melt produced depends only on the temperature of the mantle as it rises. Hence most oceanic crust is the same thickness (7±1 km). Very slow spreading ridges (<1 cm·yr<sup>−1</sup> half-rate) produce thinner crust (4–5 km thick) as the mantle has a chance to cool on upwelling and so it crosses the solidus and melts at lesser depth, thereby producing less melt and thinner crust. An example of this is the [[Gakkel Ridge]] under the [[Arctic Ocean]]. Thicker than average crust is found above [[Mantle plume|plumes]] as the mantle is hotter and hence it crosses the solidus and melts at a greater depth, creating more melt and a thicker crust. An example of this is [[Iceland]] which has crust of thickness ~20 km.<ref>[[C.M.R. Fowler]] (2005) ''The Solid Earth (2nd Ed.)'', Cambridge University Press {{ISBN|0-521-89307-0}}</ref>
The age of the oceanic crust can be used to estimate the (thermal) thickness of the lithosphere, where young oceanic crust has not had enough time to cool the mantle beneath it, while older oceanic crust has thicker mantle lithosphere beneath it.<ref>{{cite journal|volume=233|date=May 2005|journal=Earth and Planetary Science Letters|issue=3–4|pages = 337–349|title=Thermal structure of oceanic and continental lithosphere|last1=McKenzie|first1=Dan|last2=Jackson|first2=James|last3=Priestley|first3=Keith|doi = 10.1016/j.epsl.2005.02.005}}</ref> The oceanic lithosphere [[subduction|subducts]] at what are known as [[convergent boundary|convergent boundaries]]. These boundaries can exist between oceanic lithosphere on one plate and oceanic lithosphere on another, or between oceanic lithosphere on one plate and continental lithosphere on another. In the second situation, the oceanic lithosphere always subducts because the continental lithosphere is less dense. The subduction process consumes older oceanic lithosphere, so oceanic crust is seldom more than 200 million years old.<ref>Condie, K.C. 1997. Plate Tectonics and Crustal Evolution (4th Edition). 288 page, Butterworth-Heinemann Ltd.</ref> The process of super-continent formation and destruction via repeated cycles of creation and destruction of oceanic crust is known as the [[Wilson Cycle]].
The oldest large-scale oceanic crust is in the west [[Pacific Ocean|Pacific]] and north-west [[Atlantic Ocean|Atlantic]] — both are about up to 180-200 million years old. However, parts of the eastern [[Mediterranean Sea]] could be remnants of the much older [[Tethys Ocean]], at about 270 and up to 340 million years old.<ref>{{cite journal|url=https://www.newscientist.com/article/2100988-worlds-oldest-ocean-crust-dates-back-to-ancient-supercontinent/|volume=9|date=April 2008|journal=Geochemistry, Geophysics, Geosystems|issue=4|pages=Q04006|title=Age, spreading rates, and spreading asymmetry of the world's ocean crust|last=Müller|first=R. Dietmar|bibcode=2008GGG.....9.4006M|doi=10.1029/2007GC001743|s2cid=15960331|doi-access=free}}</ref><ref>{{cite web |url=https://www.newscientist.com/article/2100988-worlds-oldest-ocean-crust-dates-back-to-ancient-supercontinent/ |title=World's oldest ocean crust dates back to ancient supercontinent |last1=Benson |first1=Emily |date=15 August 2016 |work= [[New Scientist]]|access-date=11 September 2016 }}</ref><ref>{{cite web |url=https://www.sciencedaily.com/releases/2016/08/160815114933.htm |title=Researcher uncovers 340 million year-old oceanic crust in the Mediterranean Sea using magnetic data |date=15 August 2016 |work= [[Science Daily]]|access-date=11 September 2016 }}</ref>
==Magnetic anomalies== {{Main|Seafloor spreading}}
The oceanic crust displays a pattern of magnetic lines, parallel to the ocean ridges, frozen in the [[basalt]]. A symmetrical pattern of positive and negative magnetic lines emanates from the mid-ocean ridge.<ref>{{Cite journal|last1=Pitman|first1=W. C.|last2=Herron|first2=E. M.|last3=Heirtzler|first3=J. R.|date=1968-03-15|title=Magnetic anomalies in the Pacific and sea floor spreading|journal=Journal of Geophysical Research|language=en|volume=73|issue=6|pages=2069–2085|doi=10.1029/JB073i006p02069|issn=2156-2202|bibcode=1968JGR....73.2069P}}</ref> New rock is formed by magma at the mid-ocean ridges, and the ocean floor spreads out from this point. When the magma cools to form rock, [[Geomagnetic reversal|its magnetic polarity]] is aligned with the then-current positions of the magnetic poles of the Earth. New magma then forces the older cooled magma away from the ridge. This process results in parallel sections of oceanic crust of alternating magnetic polarity.
==See also== {{Portal|Oceans}}
* [[Continental crust]] * [[Lithosphere]] * [[Mohorovičić discontinuity]] * [[Plate tectonics]] * [[General Bathymetric Chart of the Oceans#Seabed 2030 Project|Seabed 2030]] * [[Seafloor depth versus age]]
==Notes== {{reflist}}
==References== * {{cite book |last=Marshak |first=Stephen |year=2005 |title=Earth: Portrait of a Planet |pages=41–42 }} * {{cite web |url=http://www2.ocean.washington.edu/oc540/lec01-1/ |title=Ocean 540: Oceanic Lithosphere; Plate Tectonics; Seafloor Topography |publisher=School of Oceanography, University of Washington |access-date=9 August 2009 |last1=McDuff |first1=Russell E. |last2=Heath |first2=G. Ross |archive-date=2 March 2009 |archive-url=https://web.archive.org/web/20090302101820/http://www2.ocean.washington.edu/oc540/lec01-1/ |url-status=dead }}
{{Earthsinterior}} {{Physical oceanography|expanded=other}}
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[[Category:Plate tectonics]] [[Category:Structure of the Earth]] [[Category:Oceanographical terminology]] [[Category:Earth's crust]]