Amid-ocean ridge (MOR) is aseafloor mountain system formed byplate tectonics. It typically has a depth of about 2,600 meters (8,500 ft) and rises about 2,000 meters (6,600 ft) above the deepest portion of anocean basin. This feature is whereseafloor spreading takes place along adivergent 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 newseafloor and oceaniclithosphere results frommantle upwelling in response to plate separation. The melt rises asmagma at the linear weakness between the separating plates, and emerges aslava, creating newoceanic crust and lithosphere upon cooling.
The first discovered mid-ocean ridge was theMid-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 abaseball. The mid-ocean ridge system thus is the longest mountain range on Earth, reaching about 65,000 km (40,000 mi).
Most mid-ocean ridges of the world are connected and formthe Ocean Ridge, a global mid-oceanic ridge system that is part of everyocean, making it thelongestmountain range in the world. The continuous mountain range is 65,000 km (40,400 mi) long (several times longer than theAndes, the longest continental mountain range), and the total length of the oceanic ridge system is 80,000 km (49,700 mi) long.[1]
At thespreading center on a mid-ocean ridge, the depth of the seafloor is approximately 2,600 meters (8,500 ft).[2][3] 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 thelithosphere where depth is measured). Thedepth-age relation can be modeled by the cooling of a lithosphere plate[4][5] ormantle half-space.[6] 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.[6] The overall shape of ridges results fromPrattisostasy: 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 mantlelithosphere (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.[4][5]
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 belowCurie 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 theEarth's magnetic field with time. Because the field has reversed directions at known intervals throughout its history, the pattern ofgeomagnetic 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.[2][3][7][8]
Spreading rates range from approximately 10–200 mm/yr.[2][3] Slow-spreading ridges such as the Mid-Atlantic Ridge have spread much less far (showing a steeper profile) than faster ridges such as theEast Pacific Rise (gentle profile) for the same amount of time and cooling and consequent bathymetric deepening.[2] Slow-spreading ridges (less than 40 mm/yr) generally have largerift valleys, sometimes as wide as 10–20 km (6.2–12.4 mi), and very rugged terrain at the ridge crest that can haverelief of up to 1,000 m (3,300 ft).[2][3][9][10] By contrast, fast-spreading ridges (greater than 90 mm/yr) such as the East Pacific Rise lack rift valleys. The spreading rate of theNorth Atlantic Ocean is ~ 25 mm/yr, while in thePacific region, it is 80–145 mm/yr.[11] The highest known rate is over 200 mm/yr in theMiocene on the East Pacific Rise.[12] Ridges that spread at rates <20 mm/yr are referred to as ultraslow spreading ridges[3][13] (e.g., theGakkel Ridge in theArctic Ocean and theSouthwest Indian Ridge).
The spreading center or axis commonly connects to atransform 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 calledfracture zones. At faster spreading rates the axes often displayoverlapping spreading centers that lack connecting transform faults.[2][14] 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.[2] Ultra-slow spreading ridges form both magmatic and amagmatic (currently lack volcanic activity) ridge segments without transform faults.[13]
Mid-ocean ridges exhibit activevolcanism andseismicity.[3] 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 existingocean 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 ofdecompression melting in the underlyingEarth's mantle.[15] Theisentropic upwelling solid mantle material exceeds thesolidus temperature and melts.
The crystallized magma forms a new crust ofbasalt known asMORB for mid-ocean ridge basalt, andgabbro below it in thelower oceanic crust.[16] Mid-ocean ridge basalt is atholeiitic basalt and is low inincompatible elements.[17][18]Hydrothermal vents fueled by magmatic and volcanic heat are a common feature at oceanic spreading centers.[19][20] A feature of the elevated ridges is their relatively high heat flow values, of about 1–10 μcal/cm2s,[21] or roughly 0.04–0.4 W/m2.
Most crust in the ocean basins is less than 200 million years old,[22][23] which is much younger than the 4.54 billion yearage of Earth. This fact reflects the process of lithosphere recycling into the Earth's mantle duringsubduction. As the oceanic crust and lithosphere moves away from the ridge axis, theperidotite in the underlying mantle lithosphere cools and becomes more rigid. The crust and the relatively rigid peridotite below it make up theoceanic lithosphere, which sits above the less rigid and viscousasthenosphere.[3]
Age of oceanic crust. Red is most recent, and blue is the oldest.
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 oceantrenches. Two processes,ridge-push andslab pull, are thought to be responsible for spreading at mid-ocean ridges.[24] 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.[25] 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.[24][26]
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 deepconvection (see image).[27][28] However, some studies have shown that theupper mantle (asthenosphere) is too plastic (flexible) to generate enoughfriction to pull the tectonic plate along.[29][30] Moreover, mantle upwelling that causes magma to form beneath the ocean ridges appears to involve only its upper 400 km (250 mi), as deduced fromseismic tomography and observations of the seismic discontinuity in the upper mantle at about 400 km (250 mi). On the other hand, some of the world's largest tectonic plates such as theNorth American plate andSouth American plate are in motion, yet only are being subducted in restricted locations such as theLesser Antilles Arc andScotia 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.[31]
Increased rates ofseafloor spreading (i.e. the rate of expansion of the mid-ocean ridge) have caused the global (eustatic) sea level to rise over very long timescales (millions of years).[32][33] 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.[34]
Sealevel change can be attributed to other factors (thermal expansion, ice melting, andmantle convection creatingdynamic topography[35]). 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.[36]
The 100 to 170 meters higher sea level of theCretaceous 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.[34]
Impact on seawater chemistry and carbonate deposition
Magnesium/calcium ratio changes at mid-ocean ridges
Seafloor spreading on mid-ocean ridges is a global scaleion-exchange system.[37] Hydrothermal vents at spreading centers introduce various amounts ofiron,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.[38]
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.[39] A lower Mg/Ca ratio favors the precipitation of low-Mgcalcitepolymorphs ofcalcium carbonate (calcite seas).[40][41]
Slow spreading at mid-ocean ridges has the opposite effect and will result in a higher Mg/Ca ratio favoring the precipitation ofaragonite and high-Mg calcite polymorphs ofcalcium carbonate (aragonite seas).[41]
Experiments show that most modern high-Mg calcite organisms would have been low-Mg calcite in past calcite seas,[42] 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.[39][42]
The first indications that a ridge bisects theAtlantic Ocean basin came from the results of the BritishChallenger expedition in the nineteenth century.[43] Soundings from lines dropped to the seafloor were analyzed by oceanographersMatthew Fontaine Maury andCharles Wyville Thomson and revealed a prominent rise in the seafloor that ran down the Atlantic basin from north to south.Sonarecho sounders confirmed this in the early twentieth century.[44]
It was not until afterWorld War II, when the ocean floor was surveyed in more detail, that the full extent of mid-ocean ridges became known. TheVema, a ship of theLamont–Doherty Earth Observatory ofColumbia University, traversed the Atlantic Ocean, recording echo sounder data on the depth of the ocean floor. A team led byMarie Tharp andBruce 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[45] and fresh lavas were found in the rift valley.[46] Also, crustal heat flow was higher here than elsewhere in the Atlantic Ocean basin.[47]
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. TheGerman Meteor expedition traced the mid-ocean ridge from theSouth Atlantic into theIndian 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.[2][3]
Alfred Wegener proposed the theory ofcontinental 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 hotsima [rising] from depth".[48] 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 howcontinents could plow through oceancrust, 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 forseafloor spreading. The discovery of mid-ocean ridges and the process of seafloor spreading allowed forWegener's theory to be expanded so that it included the movement of oceanic crust as well as the continents.[49] Plate tectonics was a suitable explanation for seafloor spreading, and the acceptance of plate tectonics by the majority of geologists resulted in a majorparadigm shift in geological thinking.
It is estimated that along Earth's mid-ocean ridges every year 2.7 km2 (1.0 sq mi) of new seafloor is formed by this process.[50] With a crustal thickness of 7 km (4.3 mi), this amounts to about 19 km3 (4.6 cu mi) of new ocean crust formed every year.[50]
Oceanic ridge and deep sea vent chemistry
Plates in the crust of the earth, according to theplate tectonics theory
^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.Bibcode:1982AREPS..10..155M.doi:10.1146/annurev.ea.10.050182.001103.
^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.
^abMiller, Kenneth G. (2009). "Sea Level Change, Last 250 Million Years".Encyclopedia of Paleoclimatology and Ancient Environments. Encyclopedia of Earth Sciences Series. Springer, Dordrecht. pp. 879–887.doi:10.1007/978-1-4020-4411-3_206.ISBN978-1-4020-4551-6.
^abRies, Justin B. (2004-11-01). "Effect of ambient Mg/Ca ratio on Mg fractionation in calcareous marine invertebrates: A record of the oceanic Mg/Ca ratio over the Phanerozoic".Geology.32 (11): 981.Bibcode:2004Geo....32..981R.doi:10.1130/g20851.1.ISSN0091-7613.
^Hsü, Kenneth J. (2014-07-14).Challenger at sea : a ship that revolutionized earth science. Princeton, New Jersey.ISBN9781400863020.OCLC889252330.{{cite book}}: CS1 maint: location missing publisher (link)
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