Mid-ocean ridge 1821971 226107810 2008-07-16T21:18:48Z Betterusername 6677551 {{Refimprove|date=October 2007}} [[Image:World Distribution of Mid-Oceanic Ridges.gif|right|350px|thumb|World Distribution of Mid-Oceanic Ridges; USGS]] [[Image:Ridge render.jpg|thumb|right|300px|Oceanic ridge]] [[Image:oceanic spreading.svg|thumb|right|220px|Oceanic crust is formed at an oceanic ridge, while the lithosphere is subducted back into the asthenosphere at trenches.]] A '''mid-ocean ridge''' or '''mid-oceanic ridge''' is an underwater [[mountain]] range, typically having a valley known as a [[rift]] running along its axis, formed by [[plate tectonics]]. This type of oceanic ridge is characteristic of what is known as an '''oceanic spreading center'''. The uplifted sea floor results from [[convection]] currents which rise in the mantle as [[magma]] at a linear weakness in the [[oceanic crust]], and emerge as [[lava]], creating new crust upon cooling. A mid-ocean ridge demarcates the boundary between two [[tectonic plate]]s, and consequently is termed a [[divergent plate boundary]]. The mid-ocean ridges of the world are connected and form a single global mid-oceanic ridge system that is part of every ocean, making the mid-oceanic ridge system the longest [[mountain range]] in the world. The continuous mountain range is {{convert|65000|km|mi|-2|abbr=on}} long and the total length of the system is {{convert|80000|km|mi|-2|abbr=on}}<ref>Cambridge Encyclopedia 2005 - Oceanic ridges</ref>. ==Description== Mid-ocean ridges are geologically active, with new [[magma]] constantly emerging onto the ocean floor and into the crust at and near rifts along the ridge axes. The crystallized magma forms new crust of [[basalt]] and [[gabbro]]. The rocks making up the crust below the sea floor are youngest at 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]]. The [[oceanic crust]] is made up of rocks much younger than the Earth itself: most oceanic crust in the ocean basins is less than 200 million years old. The crust is in a constant state of "renewal" at the ocean ridges. Moving away from the mid-ocean ridge, ocean depth progressively increases; the greatest depths are in [[ocean trench]]es. As the [[oceanic crust]] moves away from the ridge axis, the [[peridotite]] in the underlying mantle cools and becomes more rigid. The crust and the relatively rigid peridotite below it make up the oceanic [[lithosphere]]. ==Formation processes== There are two processes, ridge-push and slab-pull, thought to be responsible for the spreading seen at mid-ocean ridges, and there is some uncertainty as to which is dominant. Ridge-push occurs when the weight of the ridge pushes the rest of the tectonic plate away from the ridge, often towards a [[subduction zone]]. At the subduction zone, "slab-pull" comes into effect. This is simply the weight of the tectonic plate being subducted (pulled) below the overlying plate dragging the rest of the plate along behind it. The other process proposed to contribute to the formation of new oceanic crust at mid-ocean ridges is the "mantle conveyor" (see image). However, there have been some studies which have shown that the upper [[Mantle (geology)|mantle]] ([[asthenosphere]]) is too plastic (flexible) to generate enough [[friction]] to pull the tectonic plate along. Moreover, unlike in the image above, mantle upwelling that causes magma to form beneath the ocean ridges appears to involve only the mantle above about 400 kilometers' (250&nbsp;mi) depth, as deduced from [[seismic tomography]] and from studies of the seismic discontinuity at about 400 kilometers. The relatively shallow depths from which the upwelling mantle rises below ridges are more consistent with the "slab-pull" process. On the other hand, some of the world's largest tectonic plates such as the [[North American Plate]] are in motion, yet are nowhere being subducted. The rate at which the mid-ocean ridge creates new material is known as the spreading rate, and is generally measured in mm/yr. The common subdivisions of spreading rate are fast, medium and slow, whose values are generally >100 mm/yr, between 100 and 55 mm/yr and 55 to 20 mm/yr, respectively for full rates. The spreading rate of the north Atlantic Ocean is ~ 25 mm/yr, while in the Pacific region, it is 80–120 mm/yr. Ridges that spread at rates <20 mm/yr are referred to as ultraslow spreading ridges (e.g., the Gakkel ridge in the Arctic Ocean and the Southwest Indian Ridge) and they provide a much different perspective on crustal formation than their faster spreading brethren. The mid-ocean ridge systems form new oceanic crust. 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 in the oceanic crust record preserve a record of directions of the [[Earth's magnetic field]] with time. Because the field has reversed directions at irregular intervals throughout its history, the pattern of reversals in the ocean crust can be used as an indicator of age. Likewise, the pattern of reversals together with age measurements of the crust is used to help establish the history of the Earth's magnetic field. ==Discovery== Because a mid-ocean ridge is submerged at very deep depths in the ocean, its existence was not even known until the 1950s, when it was discovered through surveys of the ocean floor conducted by research ships. More specifically, the ''[[Research Vessel Vema|Vema]]'', a ship of the Lamont-Doherty Geological Observatory of [[Columbia University]], traversed the [[Atlantic Ocean]], recorded data about the ocean floor from the ocean surface. A team lead by [[Marie Tharp]] and [[Bruce C. Heezen|Bruce Heezen]] analyzed the data and concluded that there was an enormous mountain chain running along the middle of the Atlantic. The mountain range was named the [[Mid-Atlantic Ridge]]; it remains the most famous part of the mid-ocean ridge. It is the reason for the "mid-ocean" part of the title of this article, since it is only in the Atlantic that the ridge system is in the center of the ocean. At first, it was thought to be a phenomenon specific to the [[Atlantic Ocean]], because nothing like such a massively long undersea mountain chain had ever been discovered before. However, as surveys of the ocean floor continued to be conducted around the world, it was discovered that every ocean contained parts of the mid-ocean ridge. ==Impact== [[Image:Plates tect2 en.svg|thumb|300px|Plates in the crust of the earth, according to the [[plate tectonics]] theory]] [[Alfred Wegener]] proposed the theory of [[continental drift]] in 1912. However, the theory was dismissed by [[geologist]]s 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 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 [[sea floor spreading]]. [[Plate tectonics]] was a suitable explanation for sea floor 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 20 volcanic eruptions occur each year along earth's mid-ocean ridges and that every year 2.5 square kilometers of new sea floor is formed by this process. With a crustal thickness of 1 to 2 kilometers. This amounts to about 4 cubic kilometers of new ocean crust formed each year.{{Fact|date=January 2008}} ==List of oceanic ridges== *[[Chile Rise]] *[[Cocos Ridge]] *[[East Pacific Rise]] *[[Explorer Ridge]] *[[Gakkel Ridge]] (Mid-Arctic Ridge) *[[Gorda Ridge]] *[[Juan de Fuca Ridge]] *[[Mid-Atlantic Ridge]] *[[Pacific-Antarctic Ridge]] *[[Reykjanes Ridge]] *[[Central Indian Ridge]] *[[Southeast Indian Ridge]] *[[Southwest Indian Ridge]] ==List of ancient oceanic ridges== *[[Phoenix Ridge]] *[[Izanagi Ridge]] *[[Kula Ridge]] *[[Farallon Ridge]] *[[Bellingshausen Ridge]] *[[Aegir Ridge]] == See also == *[[Iceland]] *[[List of landforms#Coastal and oceanic landforms|List of Oceanic Landforms]] ==References== {{reflist}} ==External links== * [http://www.tectonic-forces.org An explanation of relevant tectonic forces] * [http://pubs.usgs.gov/gip/dynamic/baseball.html Mid-Ocenic ridge, like baseball seam (The Dynamic Earth, USGS)] [[Category:Geological processes]] [[Category:Physical oceanography]] [[Category:Plate tectonics]] [[Category:Oceanic ridges| ]] [[cs:Středooceánský hřbet]] [[da:Oceanryg]] [[de:Mittelozeanischer Rücken]] [[et:Ookeani keskahelik]] [[es:Dorsal oceánica]] [[eo:Mezoceana dorso]] [[fr:Dorsale (géologie)]] [[ko:해령]] [[hr:Srednjeoceanski hrbat]] [[id:Mid Oceanic Ridge]] [[it:Dorsale oceanica]] [[hu:Óceánközépi hátság]] [[nl:Mid-oceanische rug]] [[ja:海嶺]] [[pl:Grzbiet śródoceaniczny]] [[pt:Dorsal oceânica]] [[ru:Срединно-океанический хребет]] [[sk:Stredooceánsky chrbát]] [[sr:Средњеокеански гребен]] [[fi:Keskiselänne]] [[sv:Mittoceanisk rygg]] [[ta:நடுக்கடல் முகடு]] [[uk:Океанічні хребти]] [[zh:中洋脊]]