{"id":1670,"date":"2015-04-29T11:29:25","date_gmt":"2015-04-29T11:29:25","guid":{"rendered":"http:\/\/www.bluehabitats.org\/?page_id=1670"},"modified":"2015-05-04T09:20:51","modified_gmt":"2015-05-04T09:20:51","slug":"mid-ocean-ridge","status":"publish","type":"page","link":"https:\/\/bluehabitats.org\/?page_id=1670","title":{"rendered":"Mid-ocean ridge"},"content":{"rendered":"
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Mid-ocean spreading ridges<\/em> – \u201cthe linked major mid-oceanic mountain systems of global extent\u201d (IHO, 2008).\u00a0 Spreading ridges are distinguished from other ridges in this study (see definition of ridges).\u00a0 They were mapped by hand based on their appearance as ridge-like features that coincide with the youngest ocean crust as mapped by M\u00fcller et al. (1997) in their \u201cEarthByte\u201d digital age grid of the ocean floor.\u00a0 Spreading ridges that were not visible in the SRTM30_PLUS bathymetry (100 m contours; Becker et al., 2009) were not included in our interpretation, but there is otherwise no vertical size limitation on spreading ridges (they overlay the abyssal plains, hills or mountains classification layers in different locations).\u00a0 The mid-ocean spreading ridge covers the largest fraction of abyssal zone in the Arctic Ocean, where it characterises 4.76% of the area of abyssal zone, and it is absent from the Mediterranean and Black Sea.\u00a0 The greatest area of mid-ocean ridges occurs in the South Pacific Ocean where this feature type covers an area of 1,868,490 km2<\/sup>.<\/p>\n

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Mid-ocean ridges statistics (after Harris et al., 2014).\u00a0 The percentage areas refer to ocean basin areas and the percentage of the abyssal zone that is mid-ocean ridge.<\/p>\n\n\t\t<\/div>\n\t<\/div>\n

[vc_table vc_table_theme=”classic_blue” allow_html=””]Ocean,Area%20km2,Mid-ocean%20ridge%20area%25,%25Area%20of%20abyssal%20zone%20that%20is%20Mid-Ocean%20Ridge,Number%20of%20mid-ocean%20ridge%20segments,Average%20area%20of%20mid-ocean%20ridge%20segments%20km2|Arctic%20Ocean,254%2C630,1.96,4.76,5,50%2C930|Indian%20Ocean,1%2C547%2C910,2.17,2.46,34,45%2C530|Mediterranean%20%26%20Black%20Sea%20,0,0,0,0,0|North%20Atlantic%20,677%2C630,1.51,2.01,5,135%2C530|North%20Pacific%20,840%2C300,1.02,1.22,24,35%2C010|South%20Atlantic,1%2C166%2C750,2.89,3.19,9,129%2C640|South%20Pacific%20,1%2C868%2C490,2.14,2.31,30,62%2C280|Southern%20Ocean,343%2C740,1.69,2.02,9,38%2C190|All%20Oceans,6%2C699%2C460,1.85,2.19,106,63%2C200[\/vc_table]\n\t
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If all the water were removed from the ocean basins, there would be revealed a chain of mountains located in the middle of the oceans and encircling the world, like the seems of a baseball, along a total length of over 75,000 km.\u00a0 Mid-ocean ridges are created by the upwelling of basaltic lava and lateral rifting of ocean crust, thus forming a rift valley system. The mid-ocean ridges are the earth\u2019s largest volcanic system, accounting for more than 75% of all volcanic activity on the planet.\u00a0 The heat from this volcanism is dispersed by hydrothermal circulation of seawater.\u00a0 Hot seawater venting from the seafloor supports strange benthic communities that have evolved to survive by using the hydrogen sulphide dissolved in the hot fluid.<\/p>\n

The deepest parts of ocean basins are located in troughs or trenches, where ocean crust is being subducted beneath continental or other ocean crust into the earth\u2019s mantle.\u00a0 Therefore, the deepest parts of the oceans are not located in the middle, but rather closer to land where the subduction trenches are found.\u00a0 The upwelling of lava, rifting apart of the mid-ocean ridges and the lateral spreading of ocean crust is the driving force of plate tectonics<\/em>.\u00a0 The basaltic mid-ocean ridges are boundaries along which two ocean plates are pulled apart by the force of gravity.\u00a0 The pulling force is provided by the sinking of cooled, condensed ocean crust in deep ocean trenches at the other end of the ocean plate \u201cconveyor belt\u201d.\u00a0 Hence the mid-ocean ridge appears elevated because the surrounding ocean floor has cooled and subsided around it, which is different to mountains formed on land that are formed by land being pushed up by lateral compression.\u00a0 The age of the ocean crust is young nearest to the mid-ocean ridges and is generally older closer to the continents.<\/p>\n

Deep sea vent communities<\/em><\/strong><\/p>\n

One of the most remarkable discoveries in oceanographic research is the existence of thriving colonies of animals located on the mid-ocean spreading ridges, feeding on sulfide-rich fluids escaping from hydrothermal vents. \u00a0Deep sea vent communities comprise entirely separate ecosystems, decoupled from solar-powered life on the earth\u2019s surface, having evolved to utilize organic matter synthesized by hydrogen sulfide reducing bacteria. Over 300 endemic species have been found near the vents, including corals, clams, shrimps, crabs and the now famous giant, red-tipped tubeworms, 4 m tall creatures that flourish in waters close to the hot springs.<\/p>\n

Discovery<\/em><\/strong><\/p>\n

Hydrothermal vent communities were first discovered by marine geologists while investigating processes of geothermal heat flux from mid-ocean spreading ridges (van Dover, 2000).\u00a0 Heat derived from volcanic eruptions along the spreading ridges is released into the ocean, but in the 1960\u2019s and early 1970\u2019s the heat transfer mechanisms were not well understood.\u00a0 It was thought that seawater might move through the hot volcanic rocks to dissipate heat by convection processes, but hot springs had never been observed on the seafloor.\u00a0 Frequent earthquakes (80 per minute) and unusually warm water temperatures had been recorded near the ridges (Corliss et al., 1979), but there was no evidence for any deep sea \u201cvents\u201d as such.<\/p>\n

On Feb. 17, 1977, the submersible Alvin,<\/em> descended over 2,000 m to the Galapagos Spreading Center seeking evidence for hydrothermal circulation.\u00a0 A trail of white clamshells strewn across the dark volcanic rock background led the submersible onwards until, suddenly, the submersible crew observed \u201ccoming out of small cracks cutting across the lava terrain was warm shimmering water that quickly turned cloudy blue, as manganese and other chemicals in solution began to precipitate out of the warm water and were deposited on the lava surface, where they formed a brown stain.\u201d (Robert Ballard, Oceanus Magazine,<\/em> 1977).\u00a0 The first hydrothermal vent had been discovered.<\/p>\n

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Hydrothermal vents: (A) Map showing vent sites and different ridge communities along the mid-ocean ridges of the world (from GOODS bioregionalisation). \u00a0(B) Photograph taken by Charles Fisher, Woods Hole Oceanographic Institute of a vent-plume at the Mariner vent field on the Valu Fa Ridge near Fiji, South Pacific Ocean (C) giant tubeworms Riftia pachyptila<\/em> at 13\u00b0N on the East Pacific Rise, Copyright Ifremer \u2013Phare Expedition communication@ifremer.fr<\/a> \u00a0(D) swarming vent fauna, mid-Atlantic ridge (University of Bremen akluegel@uni-bremen.de<\/a>).<\/p>\n\n\t\t<\/div>\n\t<\/div>\n<\/div><\/div><\/div><\/div><\/div><\/div><\/div><\/div>

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(A) Map showing vent sites and different ridge communities along the mid-ocean ridges of the world (from GOODS bioregionalisation).<\/div><\/a><\/div><\/div><\/div><\/div>
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(B) Photograph taken by Charles Fisher, Woods Hole Oceanographic Institute of a vent-plume at the Mariner vent field on the Valu Fa Ridge near Fiji, South Pacific Ocean<\/div><\/a><\/div><\/div><\/div><\/div>
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giant tubeworms Riftia pachyptila at 13\u00b0N on the East Pacific Rise, Copyright Ifremer \u2013Phare Expedition communication@ifremer.fr<\/div><\/a><\/div><\/div><\/div><\/div>
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swarming vent fauna, mid-Atlantic ridge (University of Bremen akluegel@uni-bremen.de). <\/div><\/a><\/div><\/div><\/div><\/div><\/div>
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Geothermal heat flux<\/em><\/strong><\/p>\n

The sulphides, sulphates and oxides precipitated from the hot (over 350\u00b0C) fluids escaping from deep sea vents form constructional, chimney-like features over 10 m in height.\u00a0 Vent chimneys are most commonly less than 20 m in height, but one over 45 m in height, named \u201cGodzilla\u201d, occurs on the Juan de Fuca Ridge.\u00a0 On exiting the chimney, the minerals in the hot fluid precipitate rapidly, forming what are described as black or white \u201csmokers\u201d.\u00a0 Mounds of precipitated pyrite-chalcopyrite several meters high cover the seafloor around the vents (Corliss et al., 1979).<\/p>\n

The vents and their associated chemosynthetic communities are located on the axial ridge of the rift valley, where the geothermal heat flux is highest.\u00a0 Typically only a few km in width, axial ridges generally comprise a rocky substrate devoid of sediment cover, although ridge morphology varies depending upon the seafloor spreading rate:\u00a0 at slow spreading rates of less than 40 mm\/yr, the paired flanks of the rift valley are far apart (5 to 15 km) and the valley depth is large (1 to 3 km deep).\u00a0 In contrast, fast spreading ridges do not exhibit a well-defined rift valley; rather the ridge supports a linear caldera or eruptive fissure 50 to 1000 m wide and only a few 10\u2019s of meters deep (MacDonald et al., 1991).\u00a0 The east Pacific rise has the fastest spreading ocean crust, with rates of from 90 to 170 mm\/yr.<\/p>\n

Factors for conservation<\/em><\/strong><\/p>\n

Of significance for conservation is the correlation between the number of vent sites on slow versus fast spreading ridges; fast spreading ridges on the East Pacific Rise may have one active site for every 5 km of ridge crest, whereas vents along the slow spreading mid-Atlantic ridge occur only once in every 100 to 350 km (see van Dover, 2000, for details).\u00a0 Based on these figures, we can estimate that, whereas the fast-spreading ridges in the Pacific, Indian and Southern Oceans may support as many as 12,000 vent sites, the slow spreading mid-Atlantic ridge supports only on the order of 40 or so vents (i.e. there are perhaps only 40 hydrothermal vents along the entire length of the mid-Atlantic ridge in the North and South Atlantic Ocean).\u00a0 Clearly, the identification and conservation of the isolated mid-Atlantic ridge vent sites should be a priority for the global community, in terms of the protection of these ecosystems.<\/p>\n

The communities that inhabit hydrothermal vents exhibit high degrees of endemism and diversity.\u00a0 The average biomass associated with vents is an order of magnitude at least larger than that associated with the surrounding deep sea environment.\u00a0 Clams and tubeworms characterise the assemblage of animals living near the vents.\u00a0 They host symbiotic, chemosynthetic bacteria that convert sulphides into organic matter.\u00a0 Other chemosynthetic organisms make filamentous mats and biofilms that provide food for grazers and deposit feeders.\u00a0 At the apex of the food web are predators and scavengers such as spider crabs, who move in from the surrounding abyssal areas.\u00a0 Some species of predators and scavengers have evolved to become vent specialists and are found only on vents (Van Dover, 2000).<\/p>\n

Connectivity between hydrothermal vents<\/em><\/strong><\/p>\n

Factors that play a part in the connectivity between vents are the rate of vent formation, vent longevity, vent distribution, spacing between active vents, the temperature of venting seawater and bottom current regime at the vent site.\u00a0 The first factors are all related to the rate at which new ocean crust is formed.\u00a0 Thus a fundamental piece of information is the spreading rate of the ocean ridge system and the amount volcanic activity.\u00a0 As noted above, volcanic activity along the mid-Atlantic ridge is generally much less than in the Pacific, and the spacing between individual vent communities is much larger in the Atlantic than in the Pacific.<\/p>\n

All of the water in the world ocean is estimated to flow through the mid-ocean ridges and escape through hydrothermal vents once in every 10 to 100 million years.\u00a0 For comparison, the rivers refill the oceans via the hydrological cycle about once every 30,000 years.\u00a0 Therefore the absolute flux of water via river inflow is equal to around 300 to 3,000 times the flux of seawater through hydrothermal vents.\u00a0 Although the total amount of water escaping from deep sea vents is not large, the fact that vents may occur along the entire length of the mid-ocean ridge, and also on the flanks of erupting undersea volcanos, suggests this process is of global significance.<\/p>\n

Larvae spawning from a vent point source must rely upon bottom currents for dispersal, rather than the flow of water escaping from the vent.\u00a0 Axial ridges typically rise over 1,000 m in height above the level of the abyssal plains and thus interact with the flow of bottom water.\u00a0 Their linear geometry tends to deflect bottom currents to flow parallel to the ridge, enhancing larvae dispersal and colonization of vent sites along the axial ridge.\u00a0 In contrast, rift valleys and other basins perched between the parallel axial ridges may contain isolated volumes of water that have sluggish flow rates limiting larvae dispersal and colonization of vent sites.\u00a0 For these reasons the distribution patterns of today’s vent fauna display the strong imprint of the timing and geometry of ancient plate boundaries (Tunnicliffe and Fowler, 1996).<\/p>\n

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References<\/h5>\n

Becker, J.J., Sandwell, D.T., Smith, W.H.F., Braud, J., Binder, B., Depner, J., Fabre, D., Factor, J., Ingalls, S., Kim, S.H., Ladner, R., Marks, K., Nelson, S., Pharaoh, A., Trimmer, R., Von Rosenberg, J., Wallace, G., Weatherall, P., 2009. Global Bathymetry and Elevation Data at 30 Arc Seconds Resolution: SRTM30_PLUS. Marine Geodesy 32, 355-371.<\/p>\n

Corliss, J.B., Dymond, J., Gordon, L.I., Edmond, J.M., von Herzen, R.P., Ballard, R.D., Green, K., Williams, D., Bainbridge, A., Crane, K., van Andel, T.H., 1979. Submarine thermal springs on the Galapagos Rift. Science 203, 1073-1083.<\/p>\n

Harris, P.T., MacMillan-Lawler, M., Rupp, J., Baker, E.K., 2014. Geomorphology of the oceans. Marine Geology 352, 4-24.<\/p>\n

IHO, 2008. Standardization of Undersea Feature Names: Guidelines Proposal form Terminology, 4th ed. International Hydrographic Organisation and Intergovernmental Oceanographic Commission, Monaco, p. 32.<\/p>\n

Macdonald, K.S., Scheirer, D.S., Carbotte, S.M., 1991. Mid-ocean ridges: discontinuities, segments and giant cracks. Science 253, 986-994.<\/p>\n

Muller, R.D., Roest, W.R., Royer, J.Y., Gahagan, L.M., Sclater, J.G., 1997. Digital Isochrons of the World’s Ocean Floor. Journal of Geophysical Research 102, 3211-3214.<\/p>\n

Tunnicliffe, V., Fowler, M.R., 1996. Influence of sea-floor spreading on the global hydrothermal vent fauna. Nature 379, 531 – 533.<\/p>\n

Van Dover, C., 2000. The ecology of deep-sea hydrothermal vents. Princeton University Press, Princeton, New Jersey.<\/p>\n\n\t\t<\/div>\n\t<\/div>\n<\/div><\/div><\/div><\/div>\n<\/div>","protected":false},"excerpt":{"rendered":"

Mid-ocean spreading ridges – \u201cthe linked major mid-oceanic mountain systems of global extent\u201d (IHO, 2008).\u00a0 Spreading ridges are distinguished from other ridges in this study (see definition of ridges).\u00a0 They were mapped by hand based on their appearance as ridge-like features that coincide with the youngest ocean crust as mapped by M\u00fcller et al. (1997)…<\/p>\n","protected":false},"author":3,"featured_media":0,"parent":1646,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"_acf_changed":false,"footnotes":""},"class_list":["post-1670","page","type-page","status-publish","hentry","description-off"],"acf":[],"_links":{"self":[{"href":"https:\/\/bluehabitats.org\/index.php?rest_route=\/wp\/v2\/pages\/1670","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/bluehabitats.org\/index.php?rest_route=\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/bluehabitats.org\/index.php?rest_route=\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/bluehabitats.org\/index.php?rest_route=\/wp\/v2\/users\/3"}],"replies":[{"embeddable":true,"href":"https:\/\/bluehabitats.org\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=1670"}],"version-history":[{"count":4,"href":"https:\/\/bluehabitats.org\/index.php?rest_route=\/wp\/v2\/pages\/1670\/revisions"}],"predecessor-version":[{"id":1793,"href":"https:\/\/bluehabitats.org\/index.php?rest_route=\/wp\/v2\/pages\/1670\/revisions\/1793"}],"up":[{"embeddable":true,"href":"https:\/\/bluehabitats.org\/index.php?rest_route=\/wp\/v2\/pages\/1646"}],"wp:attachment":[{"href":"https:\/\/bluehabitats.org\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=1670"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}