Oceanic crust - wikipedia Oceanic crust Colors indicate the age of oceanic crust, wherein red indicates the youngest age, and blue indicates the oldest age. The lines represent tectonic plate boundaries. Oceanic crust is the uppermost layer of the oceanic portion of a tectonic plate. The crust overlies the solidified and uppermost layer of the mantle. The crust and the solid mantle layer together constitute oceanic lithosphere. Oceanic crust is primarily composed of mafic rocks, or sima, which is rich in iron and magnesium. It is thinner than continental crust, or ( er it is denser, having a mean density of about 2.9 grams per cubic centimeter as opposed to continental crust which has a density of about 2.7 grams per cubic centimeter. The crust is the result of erupted mantle material originating from below the plate, cooled and in most instances, modified chemically by seawater. 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. Contents 1 Composition 1.1 Geochemistry 1.2 Life cycle 2 Magnetic anomalies 3 See also 4 Notes 5 References Composition 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 ( sections of oceanic crust that are thrust onto and preserved on the continents ), comparisons of the seismic structure of the oceanic crust with laboratory determinations of seismic velocities in known rock types, and samples recovered from the ocean floor by submersibles, dredging ( especially from ridge crests and fracture zones ) and drilling. Oceanic crust is significantly simpler than continental crust and generally can be divided in three layers. According to mineral physics experiments, at lower mantle pressures, oceanic crust becomes denser than the surrounding mantle. Layer 1 is on an average 0.4 km thick. It consists of unconsolidated or semiconsolidated sediments, usually thin or even not present near the mid-ocean ridges but thickens farther away from the ridge. Near the continental margins sediment is terrigenous, meaning derived from the land, unlike deep sea sediments which are made of tiny shells of marine organisms, usually calcareous and siliceous, or it can be made of volcanic ash and terrigenous sediments transported by turbidity currents. Layer 2 could be divided into two parts : layer 2A -- 0.5 km thick uppermost volcanic layer of glassy to finely crystalline basalt usually in the form of pillow basalt, and layer 2B -- 1.5 km thick layer composed of diabase dikes. Layer 3 is formed by slow cooling of magma beneath the surface and consists of coarse grained gabbros and cumulate ultramafic rocks. It constitutes over two - thirds of oceanic crust volume with almost 5 km thickness. Geochemistry The most voluminous volcanic rocks of the ocean floor are the mid-oceanic ridge basalts, which are derived from low - potassium tholeiitic magmas. These rocks have low concentrations of large ion lithophile elements ( LILE ), light rare earth elements ( LREE ), volatile elements and other highly incompatible elements. There can be found basalts enriched with incompatible elements, but they are rare and associated with mid-ocean ridge hot spots such as surroundings of Galapagos Islands, the Azores and Iceland. Life cycle Oceanic crust is continuously being created at mid-ocean ridges. As plates diverge at these ridges, magma rises into the upper mantle and crust. As it moves away from the ridge, the lithosphere becomes cooler and denser, and sediment gradually builds on top of it. The youngest oceanic lithosphere is at the oceanic ridges, and it gets progressively older away from the ridges. As the mantle rises it cools and melts, as the pressure decreases and it crosses the 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 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 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. 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. The oceanic lithosphere subducts at what are known as 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. 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 and north - west Atlantic - both are about up to 180 - 200 million years old. However, parts of the eastern Mediterranean Sea are remnants of the much older Tethys ocean, at about 270 and up to 340 million years old. Magnetic anomalies Main article : 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. 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 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 Mohorovičić discontinuity Plate tectonics Notes Jump up ^ Rogers, N. ; Blake, S. ; Burton, K. An introduction to our dynamic planet. Cambridge University Press. p. 19. ISBN 978 - 0 - 521 - 49424 - 3. Retrieved January 2008. Check date values in : access - date = ( help ) Jump up ^ Cogley 1984 Jump up ^ H. Elderfield ( 2006 ). The Oceans and Marine Geochemistry. Elsevier. pp. 182 --. ISBN 978 - 0 - 08 - 045101 - 5. Jump up ^ 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. Jump up ^ 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. Jump up ^ 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. Jump up ^ D.R. Bowes ( 1989 ) The Encyclopedia of Igneous and Metamorphic Petrology, Van Nostrand Reinhold ISBN 0 - 442 - 20623 - 2 Jump up ^ 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. Jump up ^ Jon Erickson ( 14 May 2014 ). Plate Tectonics : Unraveling the Mysteries of the Earth. Infobase Publishing. pp. 83 --. ISBN 978 - 1 - 4381 - 0968 - 8. Jump up ^ Clare P. Marshall, Rhodes W. Fairbridge ( 1999 ) Encyclopedia of Geochemistry, Kluwer Academic Publishers ISBN 0 - 412 - 75500 - 9 Jump up ^ `` Understanding plate motions ( This Dynamic Earth, USGS ) ''. pubs.usgs.gov. Retrieved 2017 - 04 - 16. Jump up ^ C.M.R. Fowler ( 2005 ) The Solid Earth ( 2nd Ed. ), Cambridge University Press ISBN 0 - 521 - 89307 - 0 Jump up ^ McKenzie, Dan ; Jackson, James ; Priestley, Keith ( May 2005 ). `` Thermal structure of oceanic and continental lithosphere ''. Earth and Planetary Science Letters. Elsevier. 233 ( 3 - 4 ). Jump up ^ Condie, K.C. 1997. Plate Tectonics and Crustal Evolution ( 4th Edition ). 288 page, Butterworth - Heinemann Ltd. Jump up ^ Müller, R. Dietmar ( April 2008 ). `` Age, spreading rates, and spreading asymmetry of the world 's ocean crust ''. Geochemistry, Geophysics, Geosystems. American Geophysical Union. 9 ( 4 ). Jump up ^ Benson, Emily ( 15 August 2016 ). `` World 's oldest ocean crust dates back to ancient supercontinent ''. www.newscientist.com. New Scientist. Retrieved 11 September 2016. Jump up ^ `` Researcher uncovers 340 million year - old oceanic crust in the Mediterranean Sea using magnetic data ''. www.sciencedaily.com. Science Daily. 15 August 2016. Retrieved 11 September 2016. Jump up ^ 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. ISSN 2156 - 2202. doi : 10.1029 / JB073i006p02069. References Marshak, Stephen ( 2005 ) Earth : Portrait of a Planet ( 41 - 87 ) McDuff, Russell E. ; Heath, G. Ross. `` Ocean 540 : Oceanic Lithosphere ; Plate Tectonics ; Seafloor Topography ''. School of Oceanography, University of Washington. Retrieved August 2009. Check date values in : access - date = ( help ) 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 Fetch Gravity wave Green 's law Infragravity wave Internal wave Iribarren number Kelvin wave Kinematic wave Longshore drift Luke 's variational principle Mild - slope equation Radiation stress Rogue wave Rossby wave Rossby - gravity waves Sea state Seiche Significant wave height Soliton Stokes boundary layer Stokes drift Stokes wave Swell Trochoidal wave Tsunami megatsunami Undertow Ursell number Wave action Wave base Wave height 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 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 Geostrophic current Global Ocean Data Analysis Project Gulf Stream Halothermal circulation Humboldt Current Hydrothermal circulation Langmuir circulation Longshore drift Loop Current Modular Ocean Model Ocean dynamics Ocean gyre Princeton Ocean Model Rip current Subsurface currents Sverdrup balance Thermohaline circulation shutdown Upwelling Whirlpool 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 water Tidal bore Tidal force Tidal power Tidal race Tidal range Tidal resonance Tide gauge Tideline Landforms Abyssal fan Abyssal plain Atoll Bathymetric chart Coastal geography Cold seep Continental margin Continental rise Continental shelf Contourite Guyot Hydrography 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 Vine -- Matthews -- Morley hypothesis Oceanic crust Outer trench swell Ridge push Seafloor spreading Slab pull Slab suction Slab window Subduction Transform fault 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 Future sea level Global Sea Level Observing System North West Shelf Operational Oceanographic System Sea - level curve Sea level rise World Geodetic System Acoustics Deep scattering layer Hydroacoustics Ocean acoustic tomography Sofar bomb SOFAR channel Underwater acoustics Satellites Jason - 1 Jason - 2 ( Ocean Surface Topography Mission ) Jason - 3 Related Argo Benthic lander Color of water DSV Alvin Marginal sea Marine energy Marine pollution Mooring National Oceanographic Data Center Ocean Ocean exploration Ocean observations Ocean reanalysis Ocean surface topography Ocean thermal energy conversion Oceanography Pelagic sediment Sea surface microlayer Sea surface temperature Seawater Science On a Sphere Thermocline Underwater glider Water column World Ocean Atlas Commons Retrieved from `` https://en.wikipedia.org/w/index.php?title=Oceanic_crust&oldid=804668380 '' Categories : Plate tectonics Structure of the Earth Oceanographical terminology Hidden categories : CS1 errors : dates Talk Contents About Wikipedia भोजपुरी Català Čeština Dansk Deutsch Eesti Español Euskara فارسی Français Gaeilge 한국어 हिन्दी Hrvatski Bahasa Indonesia Íslenska Italiano Latviešu Bahasa Melayu Nederlands Norsk Norsk nynorsk Polski Português Русский Simple English Slovenčina Српски / srpski Srpskohrvatski / српскохрватски Türkçe Українська Tiếng Việt 中文 Edit links This page was last edited on 10 October 2017, at 12 : 22. 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Question: earth's crust under oceanic is composed of this rock type
Answer: mafic rocks, or sima