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{{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 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 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 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. The mid-ocean ridge system thus is the longest mountain range on Earth, reaching about {{convert|65000|km|abbr=on}}.

Global system

(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 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.WEB,oceanservice.noaa.gov/facts/midoceanridge.html, What is the longest mountain range on earth?, Ocean Facts, NOAA, 17 October 2014,

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 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 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 RidgeMid-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=}}.{{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 }} 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 depth-age relation can be modeled by the cooling of a lithosphere plateJOURNAL, Sclater, John G., Anderson, Roger N., Bell, M. Lee, 1971-11-10, Elevation of ridges and evolution of the central eastern Pacific, Journal of Geophysical Research, 76, 32, 7888–7915, 1971JGR....76.7888S, 10.1029/jb076i032p07888, 2156-2202, JOURNAL, Parsons, Barry, Sclater, John G., 1977-02-10, An analysis of the variation of ocean floor bathymetry and heat flow with age, Journal of Geophysical Research, 82, 5, 803–827, 1977JGR....82..803P, 10.1029/jb082i005p00803, 2156-2202, or (Seafloor spreading#Seafloor global topography: cooling models|mantle half-space.)JOURNAL, Davis, E.E, Lister, C. R. B., 1974, Fundamentals of Ridge Crest Topography, Earth and Planetary Science Letters, 21, 4, 405–413, 1974E&PSL..21..405D, 10.1016/0012-821X(74)90180-0, 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. The overall shape of ridges results from Pratt isostacy: 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.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 points 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.JOURNAL, Vine, F. J., Matthews, D. H., 1963, Magnetic Anomalies Over Oceanic Ridges, Nature, en, 199, 4897, 947–949, 10.1038/199947a0, 0028-0836, 1963Natur.199..947V, 4296143, JOURNAL, Vine, F. J., 1966-12-16, Spreading of the Ocean Floor: New Evidence, Science, en, 154, 3755, 1405–1415, 10.1126/science.154.3755.1405, 17821553, 0036-8075, 1966Sci...154.1405V, 44362406, Spreading rates range from approximately 10–200 mm/yr.BOOK, Mid-ocean ridges, Searle, Roger, 9781107017528, New York, 842323181, 2013-09-19, 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. Slow-spreading ridges (less than 40 mm/yr) generally have large rift valleys, sometimes as wide as 10–20 km (6.2–12.4 mi), and very rugged terrain at the ridge crest that can have relief of up to {{convert|1000|m|abbr=on}}.JOURNAL, Macdonald, Ken C., 1977, Near-bottom magnetic anomalies, asymmetric spreading, oblique spreading, and tectonics of the Mid-Atlantic Ridge near lat 37°N, Geological Society of America Bulletin, en, 88, 4, 541, 10.1130/0016-7606(1977)882.0.CO;2, 0016-7606, 1977GSAB...88..541M, JOURNAL, Macdonald, K. C., 1982, Mid-Ocean Ridges: Fine Scale Tectonic, Volcanic and Hydrothermal Processes Within the Plate Boundary Zone, Annual Review of Earth and Planetary Sciences, 10, 1, 155–190, 1982AREPS..10..155M, 10.1146/annurev.ea.10.050182.001103, By contrast, fast-spreading ridges (greater than 90 mm/yr) such as the East Pacific Rise lack rift valleys. The spreading rate of the North Atlantic Ocean is ~ 25 mm/yr, while in the Pacific region, it is 80–145 mm/yr.JOURNAL, Argus, Donald F., Gordon, Richard G., DeMets, Charles, 2010-04-01, Geologically current plate motions, Geophysical Journal International, en, 181, 1, 1–80, 10.1111/j.1365-246X.2009.04491.x, 0956-540X, 2010GeoJI.181....1D, free, The highest known rate is over 200 mm/yr in the Miocene on the East Pacific Rise.JOURNAL, Wilson, Douglas S., 1996, Fastest known spreading on the Miocene Cocos-Pacific Plate Boundary, Geophysical Research Letters, en, 23, 21, 3003–3006, 10.1029/96GL02893, 1944-8007, 1996GeoRL..23.3003W, Ridges that spread at rates