View source for Mid-ocean ridge

{{Short description|Basaltic underwater mountain system formed by plate tectonic spreading}}
{{Broader|Undersea mountain range}}

[[File:Mid-ocean ridge cut away view.png|thumb|upright=2|Mid-ocean ridge cross-section (cut-away view)]]

A '''mid-ocean ridge''' ('''MOR''') is a [[undersea mountain range|seafloor mountain system]] formed by [[plate tectonics]]. It typically has a depth of about {{Convert|2600|m|ft|sp=us}} and rises about {{Convert|2000|m|ft|sp=us}} above the deepest portion of an [[ocean basin]]. This feature is where [[seafloor spreading]] takes place along a [[Divergent boundary|divergent plate boundary]]. The rate of seafloor spreading determines the morphology of the crest of the mid-ocean ridge and its width in an ocean basin.

The production of new [[seafloor]] and oceanic [[lithosphere]] results from [[Mantle (geology)|mantle]] upwelling in response to plate separation. The melt rises as [[magma]] at the linear weakness between the separating plates, and emerges as [[lava]], creating new [[oceanic crust]] and lithosphere upon cooling.

The first discovered mid-ocean ridge was the [[Mid-Atlantic Ridge]], which is a spreading center that bisects the North and South Atlantic basins; hence the origin of the name 'mid-ocean ridge'. Most oceanic spreading centers are not in the middle of their hosting ocean basis but regardless, are traditionally called mid-ocean ridges. Mid-ocean ridges around the globe are linked by plate tectonic boundaries and the trace of the ridges across the ocean floor appears similar to the seam of a [[Baseball (ball)|baseball]]. The mid-ocean ridge system thus is the longest mountain range on Earth, reaching about {{convert|65000|km|abbr=on}}.

== Global system ==<!-- Oceanic Ridge redirects here -->
[[File:World Distribution of Mid-Oceanic Ridges.gif|right|thumb|upright=1.75|World distribution of mid-oceanic ridges]]

The mid-ocean ridges of the world are connected and form ''the'' Ocean Ridge, a single global mid-oceanic ridge system that is part of every [[ocean]], making it the [[List of mountain ranges#By length|longest]] [[mountain range]] in the world. The continuous mountain range is {{convert|65000|km|mi|-2|abbr=on}} long (several times longer than the [[Andes]], the longest continental mountain range), and the total length of the oceanic ridge system is {{convert|80000|km|mi|-2|abbr=on}} long.<ref name="longest mountain range">{{cite web|url=http://oceanservice.noaa.gov/facts/midoceanridge.html|title=What is the longest mountain range on earth?|website=Ocean Facts|publisher=NOAA|access-date=17 October 2014}}</ref>

== Description ==
[[File:(Manuscript painting of Heezen-Tharp World ocean floor map by Berann).jpg|thumb|Map of [[Marie Tharp]] and [[Bruce Heezen]], painted by [[Heinrich C. Berann]] (1977), showing the [[Terrain|relief]] of the ocean floors with the system of mid-ocean ridges]]
[[File:Mid-ocean ridge topography.gif|thumb|upright=1.35|A mid-ocean ridge, with magma rising from a chamber below, forming new [[Lithosphere#Oceanic lithosphere|oceanic lithosphere]] that spreads away from the ridge]]
[[File:Þingvellir National Park, Bláskógabyggð (6969755432).jpg|thumb|upright|[[Rift zone]] in [[Þingvellir National Park]], Iceland. The island is a sub-aerial part of the [[Mid-Atlantic Ridge]]]]

=== Morphology ===
{{See also|Seafloor spreading#Seafloor global topography: cooling models|Seafloor depth versus age}}At the [[spreading center]] on a mid-ocean ridge, the depth of the seafloor is approximately {{Convert|2600|meters|ft|abbr=}}.<ref name="Macdonald-2019">{{Citation|last=Macdonald|first=Ken C.|chapter=Mid-Ocean Ridge Tectonics, Volcanism, and Geomorphology|date=2019|pages=405–419|publisher=Elsevier|language=en|doi=10.1016/b978-0-12-409548-9.11065-6|isbn=9780128130827|title=Encyclopedia of Ocean Sciences|s2cid=264225475 }}</ref><ref name="Searle-2013" /> On the ridge flanks, the depth of the seafloor (or the height of a location on a mid-ocean ridge above a base-level) is correlated with its age (age of the [[lithosphere]] where depth is measured). The [[Seafloor depth versus age|depth-age relation]] can be modeled by the cooling of a lithosphere plate<ref name="Sclater-1971">{{Cite journal|last1=Sclater|first1=John G.|last2=Anderson|first2=Roger N.|last3=Bell|first3=M. Lee|date=1971-11-10|title=Elevation of ridges and evolution of the central eastern Pacific|journal=Journal of Geophysical Research|volume=76|issue=32|pages=7888–7915|bibcode=1971JGR....76.7888S|doi=10.1029/jb076i032p07888|issn=2156-2202}}</ref><ref name="Parsons-1977">{{Cite journal|last1=Parsons|first1=Barry|last2=Sclater|first2=John G.|date=1977-02-10|title=An analysis of the variation of ocean floor bathymetry and heat flow with age|journal=Journal of Geophysical Research|volume=82|issue=5|pages=803–827|bibcode=1977JGR....82..803P|doi=10.1029/jb082i005p00803|issn=2156-2202}}</ref> or [[Seafloor spreading#Seafloor global topography: cooling models|mantle half-space.]]<ref name="Davis-1974">{{cite journal|last1=Davis|first1=E.E|last2=Lister|first2=C. R. B.|year=1974|title=Fundamentals of Ridge Crest Topography|journal=Earth and Planetary Science Letters|volume=21|issue=4|pages=405–413|bibcode=1974E&PSL..21..405D|doi=10.1016/0012-821X(74)90180-0}}</ref> A good approximation is that the depth of the seafloor at a location on a spreading mid-ocean ridge is proportional to the square root of the age of the seafloor.<ref name="Davis-1974" /> The overall shape of ridges results from [[Wallace Pratt|Pratt]] [[isostasy]]: close to the ridge axis, there is a hot, low-density mantle supporting the oceanic crust. As the oceanic plate cools, away from the ridge axis, the oceanic mantle [[lithosphere]] (the colder, denser part of the mantle that, together with the crust, comprises the oceanic plates) thickens, and the density increases. Thus older seafloor is underlain by denser material and is deeper.<ref name="Sclater-1971" /><ref name="Parsons-1977" />

''Spreading rate'' is the rate at which an ocean basin widens due to seafloor spreading. Rates can be computed by mapping marine magnetic anomalies that span mid-ocean ridges. As crystallized basalt extruded at a ridge axis cools below [[Curie point]]s of appropriate iron-titanium oxides, magnetic field directions parallel to the Earth's magnetic field are recorded in those oxides. The orientations of the field preserved in the oceanic crust comprise a record of directions of the [[Earth's magnetic field]] with time. Because the field has reversed directions at known intervals throughout its history, the pattern of [[geomagnetic reversals]] in the ocean crust can be used as an indicator of age; given the crustal age and distance from the ridge axis, spreading rates can be calculated.<ref name="Macdonald-2019" /><ref name="Searle-2013" /><ref>{{Cite journal|last1=Vine|first1=F. J.|last2=Matthews|first2=D. H.|date=1963|title=Magnetic Anomalies Over Oceanic Ridges|journal=Nature|language=en|volume=199|issue=4897|pages=947–949|doi=10.1038/199947a0|issn=0028-0836|bibcode=1963Natur.199..947V|s2cid=4296143}}</ref><ref>{{Cite journal|last=Vine|first=F. J.|date=1966-12-16|title=Spreading of the Ocean Floor: New Evidence|journal=Science|language=en|volume=154|issue=3755|pages=1405–1415|doi=10.1126/science.154.3755.1405|pmid=17821553|issn=0036-8075|bibcode=1966Sci...154.1405V|s2cid=44362406}}</ref>

Spreading rates range from approximately 10–200&nbsp;mm/yr.<ref name="Macdonald-2019" /><ref name="Searle-2013">{{Cite book|title=Mid-ocean ridges|last1=Searle |first1=Roger |isbn=9781107017528|location=New York|oclc=842323181|date = 2013-09-19}}</ref> Slow-spreading ridges such as the Mid-Atlantic Ridge have spread much less far (showing a steeper profile) than faster ridges such as the [[East Pacific Rise]] (gentle profile) for the same amount of time and cooling and consequent bathymetric deepening.<ref name="Macdonald-2019" /> Slow-spreading ridges (less than 40&nbsp;mm/yr) generally have large [[rift valley]]s, sometimes as wide as 10–20&nbsp;km (6.2–12.4&nbsp;mi), and very rugged terrain at the ridge crest that can have [[Terrain|relief]] of up to {{convert|1000|m|abbr=on}}.<ref name="Macdonald-2019" /><ref name="Searle-2013" /><ref>{{Cite journal|last=Macdonald|first=Ken C.|date=1977|title=Near-bottom magnetic anomalies, asymmetric spreading, oblique spreading, and tectonics of the Mid-Atlantic Ridge near lat 37°N|journal=Geological Society of America Bulletin|language=en|volume=88|issue=4|pages=541|doi=10.1130/0016-7606(1977)88<541:NMAASO>2.0.CO;2|issn=0016-7606|bibcode=1977GSAB...88..541M}}</ref><ref name="Macdonald-1982">{{Cite journal|last=Macdonald|first=K. C.|date=1982|title=Mid-Ocean Ridges: Fine Scale Tectonic, Volcanic and Hydrothermal Processes Within the Plate Boundary Zone|journal=Annual Review of Earth and Planetary Sciences|volume=10|issue=1|pages=155–190|bibcode=1982AREPS..10..155M|doi=10.1146/annurev.ea.10.050182.001103}}</ref> By contrast, fast-spreading ridges (greater than 90&nbsp;mm/yr) such as the East Pacific Rise lack rift valleys. The spreading rate of the [[North Atlantic Ocean]] is ~ 25&nbsp;mm/yr, while in the [[Pacific]] region, it is 80–145&nbsp;mm/yr.<ref>{{Cite journal|last1=Argus|first1=Donald F.|last2=Gordon|first2=Richard G.|last3=DeMets|first3=Charles|date=2010-04-01|title=Geologically current plate motions|journal=Geophysical Journal International|language=en|volume=181|issue=1|pages=1–80|doi=10.1111/j.1365-246X.2009.04491.x|issn=0956-540X|bibcode=2010GeoJI.181....1D|doi-access=free}}</ref> The highest known rate is over 200&nbsp;mm/yr in the [[Miocene Era|Miocene]] on the East Pacific Rise.<ref>{{Cite journal|last=Wilson|first=Douglas S.|date=1996|title=Fastest known spreading on the Miocene Cocos-Pacific Plate Boundary|journal=Geophysical Research Letters|language=en|volume=23|issue=21|pages=3003–3006|doi=10.1029/96GL02893|issn=1944-8007|bibcode=1996GeoRL..23.3003W}}</ref> Ridges that spread at rates <20&nbsp;mm/yr are referred to as ultraslow spreading ridges<ref name="Searle-2013" /><ref name="Dick-2003" /> (e.g., the [[Gakkel Ridge]] in the [[Arctic Ocean]] and the [[Southwest Indian Ridge]]).

The spreading center or axis commonly connects to a [[transform fault]] oriented at right angles to the axis. The flanks of mid-ocean ridges are in many places marked by the inactive scars of transform faults called [[fracture zone]]s. At faster spreading rates the axes often display [[overlapping spreading centers]] that lack connecting transform faults.<ref name="Macdonald-2019" /><ref>{{Cite journal|last1=Macdonald|first1=Ken C.|last2=Fox|first2=P. J.|date=1983|title=Overlapping spreading centres: new accretion geometry on the East Pacific Rise|journal=Nature|volume=302|issue=5903|pages=55–58|bibcode=1983Natur.302...55M|doi=10.1038/302055a0|s2cid=4358534|issn=1476-4687}}</ref> The depth of the axis changes in a systematic way with shallower depths between offsets such as transform faults and overlapping spreading centers dividing the axis into segments. One hypothesis for different along-axis depths is variations in magma supply to the spreading center.<ref name="Macdonald-2019" /> Ultra-slow spreading ridges form both magmatic and amagmatic (currently lack volcanic activity) ridge segments without transform faults.<ref name="Dick-2003">{{Cite journal|last1=Dick|first1=Henry J. B.|last2=Lin|first2=Jian|last3=Schouten|first3=Hans|date=November 2003|title=An ultraslow-spreading class of ocean ridge|journal=Nature|volume=426|issue=6965|pages=405–412|bibcode=2003Natur.426..405D|doi=10.1038/nature02128|issn=1476-4687|pmid=14647373|s2cid=4376557}}</ref>

=== Volcanism ===
Mid-ocean ridges exhibit active [[volcanism]] and [[seismicity]].<ref name="Searle-2013" /> The oceanic crust is in a constant state of 'renewal' at the mid-ocean ridges by the processes of seafloor spreading and plate tectonics. New magma steadily emerges onto the ocean floor and intrudes into the existing [[Oceanic crust|ocean crust]] at and near rifts along the ridge axes. The rocks making up the crust below the seafloor are youngest along the axis of the ridge and age with increasing distance from that axis. New magma of basalt composition emerges at and near the axis because of [[Igneous rock#Decompression|decompression melting]] in the underlying [[Earth's mantle]].<ref>{{cite book |title=Igneous petrogenesis |author=Marjorie Wilson |publisher=Chapman & Hall |year=1993 |isbn=978-0-412-53310-5 |location=London |author-link=Marjorie Wilson}}</ref> The [[isentropic]] upwelling solid mantle material exceeds the [[Solidus (chemistry)|solidus]] temperature and melts.

{{Anchor|MORB}}The crystallized magma forms a new crust of [[basalt]] known as '''MORB''' for mid-ocean ridge basalt, and [[gabbro]] below it in the [[lower oceanic crust]].<ref>{{Cite journal |last1=Michael |first1=Peter |last2=Cheadle |first2=Michael |date=February 20, 2009 |title=Making a Crust |journal=Science |volume=323 |issue=5917 |pages=1017–18 |doi=10.1126/science.1169556 |pmid=19229024 |s2cid=43281390}}</ref> Mid-ocean ridge basalt is a [[tholeiitic basalt]] and is low in [[incompatible element]]s.<ref>{{cite book |title=Petrology of igneous and metamorphic rocks |author=Hyndman, Donald W. |publisher=McGraw-Hill |year=1985 |isbn=978-0-07-031658-4 |edition=2nd}}</ref><ref>{{cite book |title=Petrology |author=Blatt, Harvey |author2=Robert Tracy |publisher=Freeman |year=1996 |isbn=978-0-7167-2438-4 |edition=2nd |name-list-style=amp}}</ref> [[Hydrothermal vent]]s fueled by magmatic and volcanic heat are a common feature at oceanic spreading centers.<ref>{{Cite journal |last1=Spiess |first1=F. N. |last2=Macdonald |first2=K. C. |last3=Atwater |first3=T. |last4=Ballard |first4=R. |last5=Carranza |first5=A. |last6=Cordoba |first6=D. |last7=Cox |first7=C. |last8=Garcia |first8=V. M. D. |last9=Francheteau |first9=J. |date=1980-03-28 |title=East Pacific Rise: Hot Springs and Geophysical Experiments |journal=Science |language=en |volume=207 |issue=4438 |pages=1421–1433 |doi=10.1126/science.207.4438.1421 |pmid=17779602 |issn=0036-8075 |bibcode=1980Sci...207.1421S |s2cid=28363398}}</ref><ref>{{Cite journal |last1=Martin |first1=William |last2=Baross |first2=John |last3=Kelley |first3=Deborah |last4=Russell |first4=Michael J. |date=2008-11-01 |title=Hydrothermal vents and the origin of life |journal=Nature Reviews Microbiology |volume=6 |issue=11 |pages=805–814 |doi=10.1038/nrmicro1991 |issn=1740-1526 |pmid=18820700 |s2cid=1709272}}</ref> A feature of the elevated ridges is their relatively high heat flow values, of about 1–10 μcal/cm<sup>2</sup>s,<ref>{{Citation |title=Chapter 2 The World's Oceanic Ridge System |date=1982-01-01 |url=https://www.sciencedirect.com/science/article/pii/S0422989408709449 |work=Elsevier Oceanography Series |volume=33 |pages=51–139|editor-last=Hekinian |editor-first=R. |series=Petrology of the Ocean Floor |publisher=Elsevier |doi=10.1016/S0422-9894(08)70944-9 |isbn=9780444419675 |language=en |access-date=2020-10-27}}</ref> or roughly 0.04–0.4 W/m<sup>2</sup>.

Most crust in the ocean basins is less than 200 million years old,<ref>Larson, R.L., W.C. Pitman, X. Golovchenko, S.D. Cande, JF. Dewey, W.F. Haxby, and J.L. La Brecque, Bedrock Geology of the World, W.H. Freeman, New York, 1985.</ref><ref>{{Cite journal |last1=Müller |first1=R. Dietmar |last2=Roest |first2=Walter R. |last3=Royer |first3=Jean-Yves|last4=Gahagan |first4=Lisa M. |last5=Sclater |first5=John G. |date=1997-02-10 |title=Digital isochrons of the world's ocean floor |journal=Journal of Geophysical Research: Solid Earth|language=en |volume=102|issue=B2 |pages=3211–3214 |bibcode=1997JGR...102.3211M |doi=10.1029/96JB01781 |doi-access=free}}</ref> which is much younger than the 4.54 billion year [[age of Earth]]. This fact reflects the process of lithosphere recycling into the Earth's mantle during [[subduction]]. As the oceanic crust and lithosphere moves away from the ridge axis, the [[peridotite]] in the underlying mantle lithosphere cools and becomes more rigid. The crust and the relatively rigid peridotite below it make up the [[oceanic lithosphere]], which sits above the less rigid and viscous [[asthenosphere]].<ref name="Searle-2013" />
[[File:Earth seafloor crust age 1996 - 2.png|thumb|upright=1.35|Age of oceanic crust. Red is most recent, and blue is the oldest.]]

==Driving mechanisms==
{{Further|Plate tectonics}}
[[File:Oceanic spreading.svg|thumb|right|upright=1.35|Oceanic crust is formed at an oceanic ridge, while the lithosphere is subducted back into the asthenosphere at trenches.]]

The oceanic lithosphere is formed at an oceanic ridge, while the lithosphere is subducted back into the asthenosphere at ocean [[Oceanic trench|trenches]]. Two processes, [[ridge-push]] and [[slab pull]], are thought to be responsible for spreading at mid-ocean ridges.<ref name="Forsyth-1975">{{Cite journal|last1=Forsyth|first1=D.|last2=Uyeda|first2=S.|date=1975-10-01|title=On the Relative Importance of the Driving Forces of Plate Motion|journal=Geophysical Journal International|language=en|volume=43|issue=1|pages=163–200|doi=10.1111/j.1365-246X.1975.tb00631.x|issn=0956-540X|bibcode=1975GeoJ...43..163F|doi-access=free}}</ref> Ridge push refers to the gravitational sliding of the ocean plate that is raised above the hotter asthenosphere, thus creating a body force causing sliding of the plate downslope.<ref>{{Cite book|title=Geodynamics|url=https://archive.org/details/geodynamics00dltu|url-access=limited|last1=Turcotte|first1=Donald Lawson|last2=Schubert|first2=Gerald|year=2002|isbn=0521661862|edition=2nd|location=Cambridge|pages=[https://archive.org/details/geodynamics00dltu/page/n15 1]–21|oclc=48194722}}</ref> In slab pull the weight of a tectonic plate being subducted (pulled) below an overlying plate at a subduction zone drags the rest of the plate along behind it. The slab pull mechanism is considered to be contributing more than the ridge push.<ref name="Forsyth-1975" /><ref>{{cite book|title=Encyclopedia of Marine Geosciences|last1=Harff|first1=Jan|last2=Meschede|first2=Martin|last3=Petersen|first3=Sven|last4=Thiede|first4=Jörn|chapter=Driving Forces: Slab Pull, Ridge Push |publisher=Springer Netherlands|year=2014|isbn=978-94-007-6644-0|edition=2014|pages=1–6|doi=10.1007/978-94-007-6644-0_105-1}}</ref>

A process previously proposed to contribute to plate motion and the formation of new oceanic crust at mid-ocean ridges is the "mantle conveyor" due to deep [[Mantle convection|convection]] (see image).<ref>{{Cite Q| Q61783012|author-last=Holmes|author-first=A.|}}</ref><ref>{{Citation|last=Hess|first=H. H.|chapter=History of Ocean Basins|date=1962|chapter-url=https://pubs.geoscienceworld.org/books/book/847/chapter/3918672/|pages=599–620|editor-last=Engel|editor-first=A. E. J.|publisher=Geological Society of America|language=en|doi=10.1130/petrologic.1962.599|isbn=9780813770161|access-date=2019-09-11|editor2-last=James|editor2-first=Harold L.|editor3-last=Leonard|editor3-first=B. F.|title=Petrologic Studies}}</ref> However, some studies have shown that the [[Upper mantle (Earth)|upper mantle]] ([[asthenosphere]]) is too plastic (flexible) to generate enough [[friction]] to pull the tectonic plate along.<ref>{{Cite journal|last=Richter|first=Frank M.|date=1973|title=Dynamical models for sea floor spreading|journal=Reviews of Geophysics|language=en|volume=11|issue=2|pages=223–287|doi=10.1029/RG011i002p00223|issn=1944-9208|bibcode=1973RvGSP..11..223R}}</ref><ref>{{Cite journal|last=Richter|first=Frank M.|date=1973|title=Convection and the large-scale circulation of the mantle|journal=Journal of Geophysical Research |language=en|volume=78|issue=35|pages=8735–8745|doi=10.1029/JB078i035p08735|issn=2156-2202|bibcode=1973JGR....78.8735R}}</ref> Moreover, mantle upwelling that causes magma to form beneath the ocean ridges appears to involve only its upper 400&nbsp;km (250&nbsp;mi), as deduced from [[seismic tomography]] and observations of the seismic discontinuity in the upper mantle at about 400&nbsp;km (250&nbsp;mi). On the other hand, some of the world's largest tectonic plates such as the [[North American plate]] and [[South American plate]] are in motion, yet only are being subducted in restricted locations such as the [[Lesser Antilles|Lesser Antilles Arc]] and [[Scotia Arc]], pointing to action by the ridge push body force on these plates. Computer modeling of the plates and mantle motions suggest that plate motion and mantle convection are not connected, and the main plate driving force is slab pull.<ref>{{Cite journal|last1=Coltice|first1=Nicolas|last2=Husson|first2=Laurent|last3=Faccenna|first3=Claudio|last4=Arnould|first4=Maëlis|date=2019|title=What drives tectonic plates?|journal=Science Advances|language=en|volume=5|issue=10|pages=eaax4295|doi=10.1126/sciadv.aax4295|issn=2375-2548|pmc=6821462|pmid=31693727|bibcode=2019SciA....5.4295C}}</ref>

==Impact on global sea level ==

Increased rates of [[seafloor spreading]] (i.e. the rate of expansion of the mid-ocean ridge) have caused the global ([[Eustatic sea level|eustatic]]) sea level to rise over very long timescales (millions of years).<ref>{{Cite journal|last=Pitman|first=Walter C.|date=1978-09-01|title=Relationship between eustacy and stratigraphic sequences of passive margins|journal=GSA Bulletin|language=en|volume=89|issue=9|pages=1389–1403|doi=10.1130/0016-7606(1978)89<1389:RBEASS>2.0.CO;2|issn=0016-7606|bibcode=1978GSAB...89.1389P}}</ref><ref>{{Cite book|last1=Church|first1=J.A.|last2=Gregory|first2=J.M.|pages=[https://archive.org/details/encyclopediaofoc0000unse/page/2599 2599–2604]|doi=10.1006/rwos.2001.0268|title=Encyclopedia of Ocean Sciences|year=2001|isbn=9780122274305|s2cid=129689280 |url=https://archive.org/details/encyclopediaofoc0000unse/page/2599}}</ref> Increased seafloor spreading means that the mid-ocean ridge will then expand and form a broader ridge with decreased average depth, taking up more space in the ocean basin. This displaces the overlying ocean and causes sea levels to rise.<ref name="Miller-2009" />

[[Sea level rise|Sealevel change]] can be attributed to other factors ([[thermal expansion]], ice melting, and [[mantle convection]] creating [[dynamic topography]]<ref>{{Cite journal|last1=Muller|first1=R. D.|last2=Sdrolias|first2=M.|last3=Gaina|first3=C.|last4=Steinberger|first4=B.|last5=Heine|first5=C.|date=2008-03-07|title=Long-Term Sea-Level Fluctuations Driven by Ocean Basin Dynamics|journal=Science|language=en|volume=319|issue=5868|pages=1357–1362|doi=10.1126/science.1151540|pmid=18323446|bibcode=2008Sci...319.1357M|s2cid=23334128|issn=0036-8075}}</ref>). Over very long timescales, however, it is the result of changes in the volume of the ocean basins which are, in turn, affected by rates of seafloor spreading along the mid-ocean ridges.<ref>{{Cite book|chapter-url=http://linkinghub.elsevier.com/retrieve/pii/B012227430X002555|last=Kominz|first=M.A.|pages=[https://archive.org/details/encyclopediaofoc0000unse/page/2605 2605–2613]|doi=10.1006/rwos.2001.0255|title=Encyclopedia of Ocean Sciences|year=2001|isbn=9780122274305|chapter=Sea Level Variations over Geologic Time|url=https://archive.org/details/encyclopediaofoc0000unse/page/2605|publisher=San Diego : Academic Press}}</ref>

The 100 to 170 meters higher sea level of the [[Cretaceous|Cretaceous Period]] (144–65 Ma) is partly attributed to plate tectonics because thermal expansion and the absence of ice sheets only account for some of the extra sea level.<ref name="Miller-2009">{{Cite book|title=Encyclopedia of Paleoclimatology and Ancient Environments|last=Miller|first=Kenneth G.|date=2009|publisher=Springer, Dordrecht|pages=879–887|doi=10.1007/978-1-4020-4411-3_206|chapter = Sea Level Change, Last 250 Million Years|series = Encyclopedia of Earth Sciences Series|isbn = 978-1-4020-4551-6}}</ref>

== Impact on seawater chemistry and carbonate deposition ==
{{Main|Ocean chemistry}}
[[File:MgCaRatioChanges.jpg|thumb|upright=1.25|Magnesium/calcium ratio changes at mid-ocean ridges]]
Seafloor spreading on mid-ocean ridges is a global scale [[Ion exchange|ion-exchange]] system.<ref name="Stanley-1999">Stanley, S.M. and Hardie, L.A., 1999. Hypercalcification: paleontology links plate tectonics and geochemistry to sedimentology. ''GSA today'', ''9''(2), pp.1–7.</ref> Hydrothermal vents at spreading centers introduce various amounts of [[iron]], [[sulfur]], [[manganese]], [[silicon]], and other elements into the ocean, some of which are recycled into the ocean crust. [[Helium-3]], an isotope that accompanies volcanism from the mantle, is emitted by hydrothermal vents and can be detected in plumes within the ocean.<ref>Lupton, J., 1998. Hydrothermal helium plumes in the Pacific Ocean. ''Journal of Geophysical Research: Oceans'', ''103''(C8), pp.15853-15868.</ref>

Fast spreading rates will expand the mid-ocean ridge causing basalt reactions with seawater to happen more rapidly. The magnesium/calcium ratio will be lower because more magnesium ions are being removed from seawater and consumed by the rock, and more calcium ions are being removed from the rock and released into seawater. Hydrothermal activity at the ridge crest is efficient in removing magnesium.<ref name="Coggon-2010">{{Cite journal|last1=Coggon|first1=R. M.|last2=Teagle|first2=D. A. H.|last3=Smith-Duque|first3=C. E.|last4=Alt|first4=J. C.|last5=Cooper|first5=M. J.|date=2010-02-26|title=Reconstructing Past Seawater Mg/Ca and Sr/Ca from Mid-Ocean Ridge Flank Calcium Carbonate Veins|journal=Science|language=en|volume=327|issue=5969|pages=1114–1117|doi=10.1126/science.1182252|pmid=20133522|issn=0036-8075|bibcode=2010Sci...327.1114C|s2cid=22739139}}</ref> A lower Mg/Ca ratio favors the precipitation of low-Mg [[calcite]] [[Polymorphism (materials science)|polymorphs]] of [[calcium carbonate]] ([[calcite sea]]s).<ref>{{Cite journal|last1=Morse|first1=John W.|last2=Wang|first2=Qiwei|last3=Tsio|first3=Mai Yin|date=1997|title=Influences of temperature and Mg:Ca ratio on CaCO3 precipitates from seawater|journal=Geology|language=en|volume=25|issue=1|pages=85|doi=10.1130/0091-7613(1997)025<0085:IOTAMC>2.3.CO;2|issn=0091-7613|bibcode=1997Geo....25...85M}}</ref><ref name="Hardie-1999">{{Cite journal|last1=Hardie|first1=Lawrence|last2=Stanley|first2=Steven|date=February 1999|title=Hypercalcification: Paleontology Links Plate Tectonics and Geochemistry to Sedimentology|url=https://www.geosociety.org/gsatoday/archive/9/2/pdf/i1052-5173-9-2-1.pdf|journal=GSA Today|volume=9|issue=2|pages=1–7}}</ref>

Slow spreading at mid-ocean ridges has the opposite effect and will result in a higher Mg/Ca ratio favoring the precipitation of [[aragonite]] and high-Mg calcite polymorphs of [[calcium carbonate]] ([[aragonite sea]]s).<ref name="Hardie-1999" />

Experiments show that most modern high-Mg calcite organisms would have been low-Mg calcite in past calcite seas,<ref name="Ries-2004">{{Cite journal|last=Ries|first=Justin B.|date=2004-11-01|title=Effect of ambient Mg/Ca ratio on Mg fractionation in calcareous marine invertebrates: A record of the oceanic Mg/Ca ratio over the Phanerozoic|journal=Geology|volume=32|issue=11|pages=981|doi=10.1130/g20851.1|issn=0091-7613|bibcode=2004Geo....32..981R}}</ref> meaning that the Mg/Ca ratio in an organism's skeleton varies with the Mg/Ca ratio of the seawater in which it was grown.

The mineralogy of reef-building and sediment-producing organisms is thus regulated by chemical reactions occurring along the mid-ocean ridge, the rate of which is controlled by the rate of sea-floor spreading.<ref name="Coggon-2010" /><ref name="Ries-2004" />

==History==

===Discovery===
{{Main|Plate tectonics#Development of the theory}}

The first indications that a ridge bisects the [[Atlantic Ocean]] basin came from the results of the British [[Challenger expedition]] in the nineteenth century.<ref>{{Cite book|title=Challenger at sea : a ship that revolutionized earth science|last1=Hsü |first1=Kenneth J. |isbn=9781400863020|location=Princeton, New Jersey|oclc=889252330|date = 2014-07-14}}</ref> Soundings from lines dropped to the seafloor were analyzed by oceanographers [[Matthew Fontaine Maury]] and [[Charles Wyville Thomson]] and revealed a prominent rise in the seafloor that ran down the Atlantic basin from north to south. [[Sonar]] [[Echo sounding|echo sounders]] confirmed this in the early twentieth century.<ref>{{Cite book|title=The history of science and technology : a browser's guide to the great discoveries, inventions, and the people who made them, from the dawn of time to today|last=Bunch, Bryan H.|date=2004|publisher=Houghton Mifflin|others=Hellemans, Alexander, 1946–|isbn=0618221239|location=Boston|oclc=54024134|url=https://archive.org/details/isbn_9780618221233}}</ref>

It was not until after [[World War II]], when the ocean floor was surveyed in more detail, that the full extent of mid-ocean ridges became known. The ''[[Research Vessel Vema|Vema]]'', a ship of the [[Lamont–Doherty Earth Observatory]] of [[Columbia University]], traversed the Atlantic Ocean, recording echo sounder data on the depth of the ocean floor. A team led by [[Marie Tharp]] and [[Bruce C. Heezen|Bruce Heezen]] concluded that there was an enormous mountain chain with a rift valley at its crest, running up the middle of the Atlantic Ocean. Scientists named it the 'Mid-Atlantic Ridge'. Other research showed that the ridge crest was seismically active<ref>{{Cite book|title=Seismicity of the Earth and Associated Phenomena|last1=Gutenberg|first1=B.|last2=Richter|first2=C. F.|publisher=Princeton Univ. Press|year=1954|pages=309}}</ref> and fresh lavas were found in the rift valley.<ref>{{Cite journal|last=Shand|first=S. J.|date=1949-01-01|title=Rocks of the Mid-Atlantic Ridge|journal=The Journal of Geology|volume=57|issue=1|pages=89–92|doi=10.1086/625580|issn=0022-1376|bibcode=1949JG.....57...89S|s2cid=131014204}}</ref> Also, crustal heat flow was higher here than elsewhere in the Atlantic Ocean basin.<ref>{{Cite journal|last1=Day|first1=A.|last2=Bullard|first2=E. C.|date=1961-12-01|title=The Flow of Heat through the Floor of the Atlantic Ocean|journal=Geophysical Journal International|language=en|volume=4|issue=Supplement_1|pages=282–292|doi=10.1111/j.1365-246X.1961.tb06820.x|issn=0956-540X|bibcode=1961GeoJ....4..282B|doi-access=free}}</ref>

At first, the ridge was thought to be a feature specific to the Atlantic Ocean. However, as surveys of the ocean floor continued around the world, it was discovered that every ocean contains parts of the mid-ocean ridge system. The [[German Meteor expedition]] traced the mid-ocean ridge from the [[South Atlantic]] into the [[Indian Ocean]] early in the twentieth century. Although the first-discovered section of the ridge system runs down the middle of the Atlantic Ocean, it was found that most mid-ocean ridges are located away from the center of other ocean basins.<ref name="Macdonald-2019" /><ref name="Searle-2013" />

===Impact of discovery: seafloor spreading===
[[Alfred Wegener]] proposed the theory of [[continental drift]] in 1912. He stated: "the Mid-Atlantic Ridge ... zone in which the floor of the Atlantic, as it keeps spreading, is continuously tearing open and making space for fresh, relatively fluid and hot [[Sima (geology)|sima]] [rising] from depth".<ref>{{Cite journal|doi=10.1130/0091-7613(1981)9<25:MCOEDA>2.0.CO;2|last=Jacoby |first=W. R. |title=Modern concepts of earth dynamics anticipated by Alfred Wegener in 1912 |journal=Geology |volume=9 |issue=1 |pages=25–27 |date=January 1981 |bibcode=1981Geo.....9...25J}}</ref> However, Wegener did not pursue this observation in his later works and his theory was dismissed by geologists because there was no mechanism to explain how [[continent]]s could plow through ocean [[crust (geology)|crust]], and the theory became largely forgotten.

Following the discovery of the worldwide extent of the mid-ocean ridge in the 1950s, geologists faced a new task: explaining how such an enormous geological structure could have formed. In the 1960s, geologists discovered and began to propose mechanisms for [[seafloor spreading]]. The discovery of mid-ocean ridges and the process of seafloor spreading allowed for [[Alfred Wegener|Wegener's]] theory to be expanded so that it included the movement of oceanic crust as well as the continents.<ref>{{Cite news|url=http://www.nationalgeographic.org/encyclopedia/seafloor-spreading/|title=seafloor spreading|date=2015-06-08|publisher=National Geographic Society|access-date=2017-04-14}}</ref> Plate tectonics was a suitable explanation for seafloor spreading, and the acceptance of plate tectonics by the majority of geologists resulted in a major [[paradigm shift]] in geological thinking.

It is estimated that along Earth's mid-ocean ridges every year {{convert|2.7|km2|mi2|abbr=on}} of new seafloor is formed by this process.<ref name="Cogné-2006">{{Cite journal|last1=Cogné|first1=Jean-Pascal|last2=Humler|first2=Eric|date=2006|title=Trends and rhythms in global seafloor generation rate: SEAFLOOR GENERATION RATE|journal=Geochemistry, Geophysics, Geosystems|language=en|volume=7|issue=3|pages=n/a|doi=10.1029/2005GC001148|s2cid=128900649|url=https://hal.archives-ouvertes.fr/hal-00149322/file/Cogn-_et_al-2006-Geochemistry%2C_Geophysics%2C_Geosystems.pdf}}</ref> With a crustal thickness of {{Convert|7|km|mi|abbr=on|sp=us|lk=}}, this amounts to about {{convert|19|km3|mi3|abbr=on}} of new ocean crust formed every year.<ref name="Cogné-2006" />

<gallery mode="packed" align="center">
File:Deep_Sea_Vent_Chemistry_Diagram.svg|Oceanic ridge and deep sea vent chemistry
File:Plates tect2 en.svg|Plates in the crust of the earth, according to the [[plate tectonics]] theory
File:Oceanic.Stripe.Magnetic.Anomalies.Scheme.svg|Seafloor [[magnetic striping]]
File:Polarityshift.gif|A demonstration of magnetic striping
</gallery>

==List of mid-ocean ridges==
{{main category|Oceanic ridges}}
* {{annotated link|Aden Ridge}}
* {{annotated link|Cocos Ridge}}
* {{annotated link|Explorer Ridge}}
*[[Galápagos Islands#Geology|Galapagos Spreading Center]] - an east–west trending mid-ocean ridge east of the eponymous islands between the [[Nazca plate|Nazca]] and Cocos plates
* {{annotated link|Gorda Ridge}}
* {{annotated link|Juan de Fuca Ridge}}
* {{annotated link|South American–Antarctic Ridge}}
* {{annotated link|Chile Rise}}
* {{annotated link|East Pacific Rise}}
* {{annotated link|Gakkel Ridge}}(Mid-Arctic Ridge)
* {{annotated link|Pacific-Antarctic Ridge}}
* {{annotated link|Central Indian Ridge}}
** {{annotated link|Carlsberg Ridge}}
* {{annotated link|Southeast Indian Ridge}}
* {{annotated link|Southwest Indian Ridge}}
* {{annotated link|Mid-Atlantic Ridge}}
** {{annotated link|Kolbeinsey Ridge}} 
** {{annotated link|Mohns Ridge}}
** {{annotated link|Nikolai Mikhailovich Knipovich|Knipovich}} Ridge (between Greenland and Spitsbergen)
** {{annotated link|Reykjanes Ridge}} (south of Iceland)

===List of ancient oceanic ridges===
* {{annotated link|Aegir Ridge}}
* {{annotated link|Alpha Ridge}}
* {{annotated link|Kula-Farallon Ridge}}
* {{annotated link|Mid-Labrador Ridge}}
* {{annotated link|Pacific-Farallon Ridge}}
* {{annotated link|Pacific-Kula Ridge}}
* {{annotated link|Phoenix Ridge}}

== See also ==
{{Portal|Oceans|Volcanoes|Geography|Geology}}
* {{annotated link|Afar Triangle}}
* {{annotated link|Geography of Iceland}}
* {{annotated link|List of landforms#Coastal and oceanic landforms|List of oceanic landforms}}
*[[Ocean chemistry]]
*[[Oceanic crust]]
* {{annotated link|Petrological Database of the Ocean Floor}}
*[[Project FAMOUS]] – first crewed submersible study of the rift valley of the Mid-Atlantic Ridge
*[[RISE project (oceanography)|RISE project]] – discovery of black smokers hydrothermal systems on the East Pacific Rise
* {{annotated link|Slab window}}
* {{annotated link|Submarine volcano}}
*[[Vine–Matthews–Morley hypothesis|Vine-Matthews-Morely]] hypothesis; explains relation of marine magnetic anomalies to seafloor spreading.

==References==
{{reflist}}

==External links==
{{Commons category}}
* [http://www.tectonic-forces.org An explanation of relevant tectonic forces]
* [http://pubs.usgs.gov/gip/dynamic/baseball.html Mid-Oceanic ridge, like baseball seam (The Dynamic Earth, USGS)]
* [https://web.archive.org/web/20160601003941/http://www.ridge2000.org/index.php Ridge2000, Studying Mid-Ocean Ridges from Mantle to Microbe]

{{mid-ocean ridges}}
{{Earth's landforms}}
{{physical oceanography}}
{{Authority control}}

[[Category:Oceanic ridges| ]]
[[Category:Coastal and oceanic landforms]]
[[Category:Geological processes]]
[[Category:Oceanographical terminology]]
[[Category:Physical oceanography]]
[[Category:Plate tectonics]]
[[Category:Volcanic landforms]]