# Oceanic crust

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Uppermost layer of the oceanic portion of a tectonic plate

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.

Continental and oceanic crust on the Earth's upper mantle

**Oceanic crust** is the uppermost layer of the oceanic portion of the [tectonic plates](/source/Plate_tectonics). It is composed of the upper oceanic crust, with [pillow lavas](/source/Pillow_lava) and a [dike](/source/Dike_(geology)) complex, and the [lower oceanic crust](/source/Lower_oceanic_crust), composed of [troctolite](/source/Troctolite), [gabbro](/source/Gabbro) and [ultramafic](/source/Ultramafic_rock) [cumulates](/source/Cumulate_rock).[1][2] The crust lies above the rigid uppermost layer of the [mantle](/source/Mantle_(geology)). The crust and the rigid upper mantle layer together constitute oceanic [lithosphere](/source/Lithosphere).

Oceanic crust is primarily composed of [mafic](/source/Mafic) rocks, or [sima](/source/Sima_(geology)), which is rich in iron and magnesium. It is thinner than [continental crust](/source/Continental_crust), or [sial](/source/Sial), generally less than 10 kilometers thick; however, it is denser, having a mean density of about 3.0 [grams](/source/Gram) per cubic centimeter as opposed to continental crust which has a density of about 2.7 grams per cubic centimeter.[3]

The uppermost crust is the result of the cooling of magma derived from [mantle](/source/Earth's_mantle) material below the plate. The magma is injected into the spreading center, which consists mainly of a partly solidified [crystal mush](/source/Crystal_mush) derived from earlier injections, forming magma lenses that are the source of the [sheeted dikes](/source/Sheeted_dyke_complex) that feed the overlying pillow lavas.[4] As the lavas cool they are, in most instances, modified chemically by seawater.[5] These eruptions occur mostly at mid-ocean ridges, but also at scattered hotspots, and also in rare but powerful occurrences known as [flood basalt](/source/Flood_basalt) eruptions. But most [magma](/source/Magma) [crystallises](/source/Crystallization) at depth, within the [lower oceanic crust](/source/Lower_oceanic_crust). There, newly intruded magma can mix and react with pre-existing crystal mush and rocks.[6]

## Composition

See also: [Lithosphere § Oceanic lithosphere](/source/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 [ophiolites](/source/Ophiolite) (sections of oceanic crust that are thrust onto and preserved on the continents), comparisons of the [seismic structure](/source/Seismology) of the oceanic crust with laboratory determinations of seismic velocities in known rock types, and samples recovered from the ocean floor by [submersibles](/source/Submersible), dredging (especially from [ridge](/source/Mid-ocean_ridge) crests and [fracture zones](/source/Fracture_zone)) and drilling.[7] Oceanic crust is significantly simpler than continental crust and generally can be divided in three layers.[8] According to [mineral physics](/source/Mineral_physics) experiments, at lower mantle pressures, oceanic crust becomes denser than the surrounding mantle.[9]

- Layer 1 is on an average 0.4 km thick. It consists of unconsolidated or semiconsolidated [sediments](/source/Sediment), usually thin or even not present near the [mid-ocean ridges](/source/Mid-ocean_ridge) but thicker farther away from the ridge.[10] Near the continental margins sediment is [terrigenous](/source/Terrigenous_sediment), meaning derived from the land, unlike deep sea sediments which are made of tiny [shells](/source/Exoskeleton) of marine organisms, usually calcareous and siliceous, or it can be made of volcanic ash and terrigenous [sediments transported](/source/Sediment_transport) by [turbidity currents](/source/Turbidity_current).[11]

- 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](/source/Basalt), usually in the form of [pillow basalt](/source/Pillow_lava). Layer 2B is a 1.5 km thick layer composed of [diabase](/source/Diabase) [dikes](/source/Dike_(geology)).[12]

- Layer 3 is formed by slow cooling of [magma](/source/Magma) beneath the surface and consists of coarse grained [gabbro](/source/Gabbro) and [cumulate](/source/Cumulate_rock) [ultramafic rocks](/source/Ultramafic_rock).[13] It constitutes over two-thirds of oceanic crust volume with almost 5 km thickness.[14]

### Geochemistry

The most voluminous [volcanic rocks](/source/Volcanic_rock) of the ocean floor are the mid-oceanic ridge basalts, which are derived from low-[potassium](/source/Potassium) [tholeiitic magmas](/source/Tholeiitic_magma_series). These rocks have low concentrations of large ion [lithophile](/source/Lithophile) elements (LILE), light rare earth elements (LREE), volatile elements and other highly [incompatible elements](/source/Incompatible_element). There can be found basalts enriched with incompatible elements, but they are rare and associated with mid-ocean ridge [hot spots](/source/Hotspot_(geology)) such as surroundings of [Galapagos Islands](/source/Galapagos_Islands), the [Azores](/source/Azores) and [Iceland](/source/Iceland).[15]

Prior to the [Neoproterozoic Era](/source/Neoproterozoic) 1000 [million years ago](/source/Million_years_ago), the world's oceanic crust was more [mafic](/source/Mafic) than the current crust. The more mafic nature of the crust meant that higher amounts of water molecules ([OH](/source/Hydroxy_group)) could be stored in the [altered](https://en.wikipedia.org/w/index.php?title=Alteration_(geology)&action=edit&redlink=1) parts of the crust. At [subduction](/source/Subduction) zones this mafic crust was prone to metamorphose into [greenschist](/source/Greenschist) instead of [blueschist](/source/Blueschist) at ordinary [blueschist facies](/source/Metamorphic_facies).[16]

### Life cycle

Oceanic crust is continuously being created at mid-ocean ridges. As [continental plates](/source/Plate_tectonics) 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.[17]

As the mantle rises it cools and melts, as the pressure decreases and it crosses the [solidus](/source/Solidus_(chemistry)). 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−1 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](/source/Gakkel_Ridge) under the [Arctic Ocean](/source/Arctic_Ocean). Thicker than average crust is found above [plumes](/source/Mantle_plume) 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](/source/Iceland) which has crust of thickness ~20 km.[18]

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.[19] The oceanic lithosphere [subducts](/source/Subduction) at what are known as [convergent boundaries](/source/Convergent_boundary). 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.[20] The process of super-continent formation and destruction via repeated cycles of creation and destruction of oceanic crust is known as the [Wilson Cycle](/source/Wilson_Cycle).

The oldest large-scale oceanic crust is in the west [Pacific](/source/Pacific_Ocean) and north-west [Atlantic](/source/Atlantic_Ocean) — both are about up to 180-200 million years old. However, parts of the eastern [Mediterranean Sea](/source/Mediterranean_Sea) could be remnants of the much older [Tethys Ocean](/source/Tethys_Ocean), at about 270 and up to 340 million years old.[21][22][23]

## Magnetic anomalies

Main article: [Seafloor spreading](/source/Seafloor_spreading)

The oceanic crust displays a pattern of magnetic lines, parallel to the ocean ridges, frozen in the [basalt](/source/Basalt). A symmetrical pattern of positive and negative magnetic lines emanates from the mid-ocean ridge.[24] 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, [its magnetic polarity](/source/Geomagnetic_reversal) 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

- [Oceans portal](https://en.wikipedia.org/wiki/Portal:Oceans)

- [Continental crust](/source/Continental_crust)

- [Lithosphere](/source/Lithosphere)

- [Mohorovičić discontinuity](/source/Mohorovi%C4%8Di%C4%87_discontinuity)

- [Plate tectonics](/source/Plate_tectonics)

- [Seabed 2030](/source/General_Bathymetric_Chart_of_the_Oceans#Seabed_2030_Project)

- [Seafloor depth versus age](/source/Seafloor_depth_versus_age)

## Notes

1. **[^](#cite_ref-1)** Gillis et al. (2014). Primitive layered gabbros from fast-spreading lower oceanic crust. Nature 505, 204-208

1. **[^](#cite_ref-Pirajno_2013_2-0)** Pirajno F. (2013). [*Ore Deposits and Mantle Plumes*](https://books.google.com/books?id=PrXyCAAAQBAJ&q=ultramafic+cumulates+layer+3&pg=PA11). Springer. p. 11. [ISBN](/source/ISBN_(identifier)) [9789401725026](https://en.wikipedia.org/wiki/Special:BookSources/9789401725026).

1. **[^](#cite_ref-Rogers_3-0)** Rogers, N., ed. (2008). [*An Introduction to Our Dynamic Planet*](https://books.google.com/books?id=WA9ST5S_2v0C&pg=PA44). [Cambridge University Press](/source/Cambridge_University_Press) and [The Open University](/source/The_Open_University). p. 19. [ISBN](/source/ISBN_(identifier)) [978-0-521-49424-3](https://en.wikipedia.org/wiki/Special:BookSources/978-0-521-49424-3).

1. **[^](#cite_ref-Sinton_&_Detrick_4-0)** Sinton J.M.; Detrick R.S. (1992). ["Mid-ocean ridge magma chambers"](https://www.researchgate.net/publication/248793835). *Journal of Geophysical Research*. **97** (B1): 197–216. [Bibcode](/source/Bibcode_(identifier)):[1992JGR....97..197S](https://ui.adsabs.harvard.edu/abs/1992JGR....97..197S). [doi](/source/Doi_(identifier)):[10.1029/91JB02508](https://doi.org/10.1029%2F91JB02508).

1. **[^](#cite_ref-5)** H. Elderfield (2006). The Oceans and Marine Geochemistry. Elsevier. pp. 182–. [ISBN](/source/ISBN_(identifier)) [978-0-08-045101-5](https://en.wikipedia.org/wiki/Special:BookSources/978-0-08-045101-5).

1. **[^](#cite_ref-6)** 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](/source/Doi_(identifier)):[10.1016/j.epsl.2012.11.012](https://doi.org/10.1016%2Fj.epsl.2012.11.012)

1. **[^](#cite_ref-7)** 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.

1. **[^](#cite_ref-8)** Hansteen, Thor H; Troll, Valentin R (2003-02-14). ["Oxygen isotope composition of xenoliths from the oceanic crust and volcanic edifice beneath Gran Canaria (Canary Islands): consequences for crustal contamination of ascending magmas"](http://www.sciencedirect.com/science/article/pii/S000925410200325X). *Chemical Geology*. **193** (3): 181–193. [Bibcode](/source/Bibcode_(identifier)):[2003ChGeo.193..181H](https://ui.adsabs.harvard.edu/abs/2003ChGeo.193..181H). [doi](/source/Doi_(identifier)):[10.1016/S0009-2541(02)00325-X](https://doi.org/10.1016%2FS0009-2541%2802%2900325-X). [ISSN](/source/ISSN_(identifier)) [0009-2541](https://search.worldcat.org/issn/0009-2541).

1. **[^](#cite_ref-9)** 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.

1. **[^](#cite_ref-10)** Peter Laznicka (2 September 2010). Giant Metallic Deposits: Future Sources of Industrial Metals. Springer Science & Business Media. pp. 82–. [ISBN](/source/ISBN_(identifier)) [978-3-642-12405-1](https://en.wikipedia.org/wiki/Special:BookSources/978-3-642-12405-1).

1. **[^](#cite_ref-11)** D. R. Bowes (1989) *The Encyclopedia of Igneous and Metamorphic Petrology*, Van Nostrand Reinhold [ISBN](/source/ISBN_(identifier)) [0-442-20623-2](https://en.wikipedia.org/wiki/Special:BookSources/0-442-20623-2)

1. **[^](#cite_ref-12)** 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](/source/ISBN_(identifier)) [978-0-8137-2349-5](https://en.wikipedia.org/wiki/Special:BookSources/978-0-8137-2349-5).

1. **[^](#cite_ref-13)** Gillis et al (2014). Primitive layered gabbros from fast-spreading lower oceanic crust. Nature 505, 204-208

1. **[^](#cite_ref-14)** Jon Erickson (14 May 2014). Plate Tectonics: Unraveling the Mysteries of the Earth. Infobase Publishing. pp. 83–. [ISBN](/source/ISBN_(identifier)) [978-1-4381-0968-8](https://en.wikipedia.org/wiki/Special:BookSources/978-1-4381-0968-8).

1. **[^](#cite_ref-15)** Clare P. Marshall, Rhodes W. Fairbridge (1999) *Encyclopedia of Geochemistry*, Kluwer Academic Publishers [ISBN](/source/ISBN_(identifier)) [0-412-75500-9](https://en.wikipedia.org/wiki/Special:BookSources/0-412-75500-9)

1. **[^](#cite_ref-PaWh2016_16-0)** Palin, Richard M.; White, Richard W. (2016). ["Emergence of blueschists on Earth linked to secular changes in oceanic crust composition"](https://ora.ox.ac.uk/objects/uuid:48630722-57a2-4dd7-8101-92ea1c8df8a1). *Nature Geoscience*. **9** (1): 60. [Bibcode](/source/Bibcode_(identifier)):[2016NatGe...9...60P](https://ui.adsabs.harvard.edu/abs/2016NatGe...9...60P). [doi](/source/Doi_(identifier)):[10.1038/ngeo2605](https://doi.org/10.1038%2Fngeo2605). [S2CID](/source/S2CID_(identifier)) [130847333](https://api.semanticscholar.org/CorpusID:130847333).

1. **[^](#cite_ref-17)** ["Understanding plate motions \[This Dynamic Earth, USGS\]"](https://pubs.usgs.gov/gip/dynamic/understanding.html). United States Geological Survey. Retrieved 2017-04-16.

1. **[^](#cite_ref-18)** [C.M.R. Fowler](/source/C.M.R._Fowler) (2005) *The Solid Earth (2nd Ed.)*, Cambridge University Press [ISBN](/source/ISBN_(identifier)) [0-521-89307-0](https://en.wikipedia.org/wiki/Special:BookSources/0-521-89307-0)

1. **[^](#cite_ref-19)** McKenzie, Dan; Jackson, James; Priestley, Keith (May 2005). "Thermal structure of oceanic and continental lithosphere". *Earth and Planetary Science Letters*. **233** (3–4): 337–349. [doi](/source/Doi_(identifier)):[10.1016/j.epsl.2005.02.005](https://doi.org/10.1016%2Fj.epsl.2005.02.005).

1. **[^](#cite_ref-20)** Condie, K.C. 1997. Plate Tectonics and Crustal Evolution (4th Edition). 288 page, Butterworth-Heinemann Ltd.

1. **[^](#cite_ref-21)** Müller, R. Dietmar (April 2008). ["Age, spreading rates, and spreading asymmetry of the world's ocean crust"](https://www.newscientist.com/article/2100988-worlds-oldest-ocean-crust-dates-back-to-ancient-supercontinent/). *Geochemistry, Geophysics, Geosystems*. **9** (4): Q04006. [Bibcode](/source/Bibcode_(identifier)):[2008GGG.....9.4006M](https://ui.adsabs.harvard.edu/abs/2008GGG.....9.4006M). [doi](/source/Doi_(identifier)):[10.1029/2007GC001743](https://doi.org/10.1029%2F2007GC001743). [S2CID](/source/S2CID_(identifier)) [15960331](https://api.semanticscholar.org/CorpusID:15960331).

1. **[^](#cite_ref-22)** Benson, Emily (15 August 2016). ["World's oldest ocean crust dates back to ancient supercontinent"](https://www.newscientist.com/article/2100988-worlds-oldest-ocean-crust-dates-back-to-ancient-supercontinent/). *[New Scientist](/source/New_Scientist)*. Retrieved 11 September 2016.

1. **[^](#cite_ref-23)** ["Researcher uncovers 340 million year-old oceanic crust in the Mediterranean Sea using magnetic data"](https://www.sciencedaily.com/releases/2016/08/160815114933.htm). *[Science Daily](/source/Science_Daily)*. 15 August 2016. Retrieved 11 September 2016.

1. **[^](#cite_ref-24)** Pitman, W. C.; Herron, E. M.; Heirtzler, J. R. (1968-03-15). "Magnetic anomalies in the Pacific and sea floor spreading". *Journal of Geophysical Research*. **73** (6): 2069–2085. [Bibcode](/source/Bibcode_(identifier)):[1968JGR....73.2069P](https://ui.adsabs.harvard.edu/abs/1968JGR....73.2069P). [doi](/source/Doi_(identifier)):[10.1029/JB073i006p02069](https://doi.org/10.1029%2FJB073i006p02069). [ISSN](/source/ISSN_(identifier)) [2156-2202](https://search.worldcat.org/issn/2156-2202).

## References

- Marshak, Stephen (2005). *Earth: Portrait of a Planet*. pp. 41–42.

- McDuff, Russell E.; Heath, G. Ross. ["Ocean 540: Oceanic Lithosphere; Plate Tectonics; Seafloor Topography"](https://web.archive.org/web/20090302101820/http://www2.ocean.washington.edu/oc540/lec01-1/). School of Oceanography, University of Washington. Archived from [the original](http://www2.ocean.washington.edu/oc540/lec01-1/) on 2 March 2009. Retrieved 9 August 2009.

v t e Structure of Earth Shells Crust Mantle Upper mantle Lithospheric mantle Asthenosphere Lower mantle (aka Mesosphere) Core Outer core Inner core Global discontinuities Mohorovičić (crust–mantle) 410 discontinuity (upper mantle) 660 discontinuity (upper mantle) D’’ discontinuity (lower mantle) Core–mantle boundary LLSVPs Inner-core boundary Regional discontinuities Conrad continental crust Gutenberg (upper mantle) Lehmann (upper mantle) Category

v t e Physical oceanography Waves Airy wave theory Ballantine scale Benjamin–Feir instability Boussinesq approximation Breaking wave Clapotis Cnoidal wave Cross sea Dispersion Edge wave Equatorial waves Gravity wave Green's law Infragravity wave Internal wave Iribarren number Kelvin wave Kinematic wave Longshore drift Luke's variational principle Miche criterion Mild-slope equation Radiation stress Rogue wave Draupner wave Rossby wave Rossby-gravity waves Sea state Seiche Significant wave height Soliton Stokes drift Stokes problem Stokes wave Swell Trochoidal wave Tsunami megatsunami Undertow Ursell number Wave action Wave base Wave height Wave nonlinearity Wave power Wave radar Wave setup Wave shoaling Wave turbulence Wave–current interaction Waves and shallow water one-dimensional Saint-Venant equations shallow water equations Wind fetch Wind setup Wind wave model Circulation Atmospheric circulation Baroclinity Boundary current Coriolis force Coriolis–Stokes force Craik–Leibovich vortex force Downwelling Eddy Ekman layer Ekman spiral Ekman transport El Niño–Southern Oscillation General circulation model Geochemical Ocean Sections Study Geostrophic current Global Ocean Data Analysis Project Gulf Stream Humboldt Current Hydrothermal circulation Langmuir circulation Longshore drift Loop Current Modular Ocean Model Ocean current Ocean dynamical thermostat Ocean dynamics Ocean gyre Overflow Princeton Ocean Model Rip current Subsurface ocean current Sverdrup balance Thermohaline circulation shutdown Upwelling Whirlpool Wind generated current World Ocean Circulation Experiment Tides Amphidromic point Earth tide Head of tide Internal tide Lunitidal interval Perigean spring tide Rip tide Rule of twelfths Slack tide Theory of tides Tidal bore Tidal force Tidal power Tidal race Tidal range Tidal resonance Tide gauge Tideline Landforms Abyssal fan Abyssal plain Atoll Bathymetric chart Carbonate platform Coastal geography Cold seep Continental margin Continental rise Continental shelf Contourite Guyot Hydrography Knoll Ocean bank Oceanic basin Oceanic plateau Oceanic trench Passive margin Seabed Seamount Submarine canyon Submarine volcano Plate tectonics Convergent boundary Divergent boundary Fracture zone Hydrothermal vent Marine geology Mid-ocean ridge Mohorovičić discontinuity Oceanic crust Outer trench swell Ridge push Seafloor spreading Slab pull Slab suction Slab window Subduction Transform fault Vine–Matthews–Morley hypothesis Volcanic arc Ocean zones Benthic Deep ocean water Deep sea Littoral Mesopelagic Oceanic Pelagic Photic Surf Swash Sea level Deep-ocean Assessment and Reporting of Tsunamis Global Sea Level Observing System North West Shelf Operational Oceanographic System Sea-level curve Sea level drop Sea level rise World Geodetic System Acoustics Deep scattering layer Ocean acoustic tomography Sofar bomb SOFAR channel Underwater acoustics Satellites Jason-1 OSTM/Jason-2 Jason-3 Related Acidification Argo Benthic lander Color of water DSV Alvin Marginal sea Marine energy Marine pollution Mooring National Oceanographic Data Center Ocean Explorations Observations Reanalysis Ocean surface topography Ocean temperature Ocean thermal energy conversion Oceanography Outline of oceanography Pelagic sediment Sea surface microlayer Sea surface temperature Seawater Science On a Sphere Stratification Thermocline Underwater glider Water column World Ocean Atlas Category Commons Oceans portal

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Adapted from the Wikipedia article [Oceanic crust](https://en.wikipedia.org/wiki/Oceanic_crust) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Oceanic_crust?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.