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Mid-oceanridge

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Oceanicridge,includingablacksmoker

Amid-oceanridgeisageneraltermforanunderwatermountainsystemthatconsistsofvariousmountainranges（chains）,typicallyhavingavalleyknownasariftrunningalongitsspine,formedbyplatetectonics.Thistypeofoceanicridgeischaracteristicofwhatisknownasanoceanicspreadingcenter,whichisresponsibleforseafloorspreading.Theupliftedseafloorresultsfromconvectioncurrentswhichriseinthemantleasmagmaatalinearweaknessintheoceaniccrust,andemergeaslava,creatingnewcrustuponcooling.Amid-oceanridgedemarcatestheboundarybetweentwotectonicplates,andconsequentlyistermedadivergentplateboundary.

Themid-oceanridgesoftheworldareconnectedandformasingleglobalmid-oceanicridgesystemthatispartofeveryocean,makingthemid-oceanicridgesystemthelongestmountainrangeintheworld.Thecontinuousmountainrangeis65,000 km（40,400mi）long（severaltimeslongerthantheAndes,thelongestcontinentalmountainrange）,andthetotallengthoftheoceanicridgesystemis80,000 km（49,700mi）long.[1]

Contents

∙1Description

∙2Formationprocesses

∙3History

o3.1Discovery

o3.2Impact

∙4Listofoceanicridges

o4.1Listofancientoceanicridges

∙5Seealso

∙6References

∙7Externallinks

[edit]Description

Aclose-upshowingamid-oceanridgetopographywithmagmarisingfromachamberbelow,formingnewoceanplatewhichspreadsawayfromridge

Mid-oceanridgesaregeologicallyactive,withnewmagmaconstantlyemergingontotheoceanfloorandintothecrustatandnearriftsalongtheridgeaxes.Thecrystallizedmagmaformsnewcrustofbasalt（knownasMORBforMid-OceanRidgeBasalt）andgabbro.

Therocksmakingupthecrustbelowtheseafloorareyoungestattheaxisoftheridgeandagewithincreasingdistancefromthataxis.NewmagmaofbasaltcompositionemergesatandneartheaxisbecauseofdecompressionmeltingintheunderlyingEarth'smantle.[2]

TheoceaniccrustismadeupofrocksmuchyoungerthantheEarthitself:

mostoceaniccrustintheoceanbasinsislessthan200millionyearsold.Thecrustisinaconstantstateof"renewal"attheoceanridges.Movingawayfromthemid-oceanridge,oceandepthprogressivelyincreases;thegreatestdepthsareinoceantrenches.Astheoceaniccrustmovesawayfromtheridgeaxis,theperidotiteintheunderlyingmantlecoolsandbecomesmorerigid.Thecrustandtherelativelyrigidperidotitebelowitmakeuptheoceaniclithosphere.

SlowspreadingridgesliketheMid-AtlanticRidgegenerallyhavelarge,wideriftvalleys,sometimesasbigas10–20km（6.2–12mi）wideandveryruggedterrainattheridgecrestthatcanhavereliefofuptoa1,000 m（3,300 ft）.Bycontrast,fastspreadingridgesliketheEastPacificRisearenarrow,sharpincisionssurroundedbygenerallyflattopographythatslopesawayfromtheridgeovermanyhundredsofmiles.

[edit]Formationprocesses

Furtherinformation:

Platetectonics

Oceaniccrustisformedatanoceanicridge,whilethelithosphereissubductedbackintotheasthenosphereattrenches.

Therearetwoprocesses,ridge-pushandslabpull,thoughttoberesponsibleforthespreadingseenatmid-oceanridges,andthereissomeuncertaintyastowhichisdominant.Ridge-pushoccurswhenthegrowingbulkoftheridgepushestherestofthetectonicplateawayfromtheridge,oftentowardsasubductionzone.Atthesubductionzone,"slab-pull"comesintoeffect.Thisissimplytheweightofthetectonicplatebeingsubducted（pulled）belowtheoverlyingplatedraggingtherestoftheplatealongbehindit.

Theotherprocessproposedtocontributetotheformationofnewoceaniccrustatmid-oceanridgesisthe"mantleconveyor"（seeimage）.However,therehavebeensomestudieswhichhaveshownthattheuppermantle（asthenosphere）istooplastic（flexible）togenerateenoughfrictiontopullthetectonicplatealong.Moreover,unlikeintheimageabove,mantleupwellingthatcausesmagmatoformbeneaththeoceanridgesappearstoinvolveonlyitsupper400 km（250 mi）,asdeducedfromseismictomographyandfromstudiesoftheseismicdiscontinuityatabout400 km（250 mi）.Therelativelyshallowdepthsfromwhichtheupwellingmantlerisesbelowridgesaremoreconsistentwiththe"slab-pull"process.Ontheotherhand,someoftheworld'slargesttectonicplatessuchastheNorthAmericanPlateareinmotion,yetarenowherebeingsubducted.

Therateatwhichthemid-oceanridgecreatesnewmaterialisknownasthespreadingrate,andisgenerallymeasuredinmm/yr.Thecommonsubdivisionsofspreadingratearefast,medium,andslowwithvaluesgenerallybeing>100 mm/yr,100–55 mm/yr,and55–20 mm/yr,respectively.ThespreadingrateofthenorthAtlanticOceanis~25 mm/yr,whileinthePacificregion,itis80–120 mm/yr.Ridgesthatspreadatrates<20 mm/yrarereferredtoasultraslowspreadingridges（e.g.,theGakkelridgeintheArcticOceanandtheSouthwestIndianRidge）andtheyprovideamuchdifferentperspectiveoncrustalformationthantheirfasterspreadingbrethren.

Themid-oceanridgesystemsformnewoceaniccrust.AscrystallizedbasaltextrudedataridgeaxiscoolsbelowCuriepointsofappropriateiron-titaniumoxides,magneticfielddirectionsparalleltotheEarth'smagneticfieldarerecordedinthoseoxides.TheorientationsofthefieldintheoceaniccrustrecordpreservearecordofdirectionsoftheEarth'smagneticfieldwithtime.Becausethefieldhasreverseddirectionsatirregularintervalsthroughoutitshistory,thepatternofreversalsintheoceancrustcanbeusedasanindicatorofage.Likewise,thepatternofreversalstogetherwithagemeasurementsofthecrustisusedtohelpestablishthehistoryoftheEarth'smagneticfield.

[edit]History

WorldDistributionofMid-OceanicRidges;USGS

[edit]Discovery

Mainarticle:

Platetectonics#Historicalcontext

Mid-oceanridgesaregenerallysubmergeddeepintheocean.Itwasnotuntilthe1950s,whentheoceanfloorwassurveyedindetail,thattheirfullextentbecameknown.

TheVema,ashipoftheLamont-DohertyEarthObservatoryofColumbiaUniversity,traversedtheAtlanticOcean,recordingdataabouttheoceanfloorfromtheoceansurface.AteamledbyMarieTharpandBruceHeezenanalyzedthedataandconcludedthattherewasanenormousmountainchainrunningalongthemiddleofflooroftheAtlantic.Scientistsgavethename"Mid-AtlanticRidge"tothesubmarinemountainrange.

Atfirst,theridgewasthoughttobeaphenomenonspecifictotheAtlanticOcean.However,assurveysoftheoceanfloorcontinuedaroundtheworld,itwasdiscoveredthateveryoceancontainspartsofthemid-oceanridgesystem.AlthoughtheridgesystemrunsdownthemiddleoftheAtlanticOcean,theridgeislocatedawayfromthecenterofotheroceans.

[edit]Impact

AlfredWegenerproposedthetheoryofcontinentaldriftin1912.Hestated:

theMid-AtlanticRidge...zoneinwhichtheflooroftheAtlantic,asitkeepsspreading,iscontinuouslytearingopenandmakingspaceforfresh,relativelyfluidandhotsima[rising]fromdepth.[3]However,Wegenerdidnotpursuethisobservationinhislaterworksandhistheorywasdismissedbygeologistsbecausetherewasnomechanismtoexplainhowcontinentscouldplowthroughoceancrust,andthetheorybecamelargelyforgotten.

Followingthediscoveryoftheworld-wideextentofthemid-oceanridgeinthe1950s,geologistsfacedanewtask:

explaininghowsuchanenormousgeologicalstructurecouldhaveformed.Inthe1960s,geologistsdiscoveredandbegantoproposemechanismsforseafloorspreading.Platetectonicswasasuitableexplanationforseafloorspreading,andtheacceptanceofplatetectonicsbythemajorityofgeologistsresultedinamajorparadigmshiftingeologicalthinking.

Itisestimatedthat20volcaniceruptionsoccureachyearalongearth'smid-oceanridgesandthateveryyear2.5 km2（0.97 sq mi）ofnewseafloorisformedbythisprocess.Withacrustalthicknessof1to2km（0.62to1.2mi）,thisamountstoabout4 km3（0.96 cu mi）ofnewoceancrustformedeveryyear.[citationneeded]

∙

Oceanicridgeanddeepseaventchemistry

∙

Ageofoceaniccrust.Theredismostrecent,andblueistheoldest.

∙

Platesinthecrustoftheearth,accordingtotheplatetectonicstheory

∙

Seafloormagneticstriping

∙

Ademonstrationofmagneticstriping

[edit]Listofoceanicridges

∙AdenRidge

∙ExplorerRidge

∙GordaRidge

∙JuandeFucaRidge

∙CocosRidge

∙American-AntarcticRidge

∙ChileRise

∙EastPacificRise

∙EastScotiaRidge

∙GakkelRidge（Mid-ArcticRidge）

∙Pacific-AntarcticRidge

∙SoutheastIndianRidge

∙CentralIndianRidge

oCarlsbergRidge

∙SouthwestIndianRidge

∙Mid-AtlanticRidge

oKnipovichRidge（betweenGreenlandandSpitsbergen）

oMohnsRidge

oKolbeinseyRidge（NorthofIceland）

oReykjanesRidge（SouthofIceland）

[edit]Listofancientoceanicridges

∙AegirRidge

∙AlphaRidge

∙BellingshausenRidge

∙IzanagiRidge

∙Kula-FarallonRidge

∙Pacific-KulaRidge

∙Pacific-FarallonRidge

∙PhoenixRidge

[edit]Seealso

∙Divergentboundary

∙ListofOceanicLandforms

∙PetrologicalDatabaseoftheOceanFloor

∙Platetectonics

∙Ridge-push

∙Seafloorspreading

∙Slabwindow

∙Famouslocationsonthecontinent

oIceland

oAfarDepression

[e