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'{{Refimprove|date=March 2008}} [[File:Age oceanic lithosphere, Muller et al., 2008.jpg|thumb|400px|Age of oceanic lithosphere; youngest (red) is along spreading centers.]] '''Seafloor spreading''' is a process that occurs at [[mid-ocean ridge]]s, where new [[oceanic crust]] is formed through [[volcano|volcanic activity]] and then gradually moves away from the ridge. Seafloor spreading helps explain [[continental drift]] in the theory of [[plate tectonics]]. When oceanic plates [[Divergent boundary|diverge]], tensional stress causes fractures to occur in the [[lithosphere]].The motivating force for seafloor spreading ridges is tectonic plate pull rather than magma pressure, although there is typically significant magma activity at spreading ridges.<ref>{{Cite journal|last=Tan|first=Yen Joe|last2=Tolstoy|first2=Maya|last3=Waldhauser|first3=Felix|last4=Wilcock|first4=William S. D.|title=Dynamics of a seafloor-spreading episode at the East Pacific Rise|url=http://www.nature.com/doifinder/10.1038/nature20116|journal=Nature|volume=540|issue=7632|pages=261–265|doi=10.1038/nature20116}}</ref> At a spreading center [[Basalt|basaltic magma]] rises up the fractures and cools on the ocean floor to form new [[seabed]]. [[Hydrothermal vent]]s are common at spreading centers. Older rocks will be found farther away from the spreading zone while younger rocks will be found nearer to the spreading zone. Additionally spreading rates determine if the ridge is a fast, intermediate, or slow. As a general rule, fast ridges see spreading rate of more than 9&nbsp;cm/year. Intermediate ridges have a spreading rate of 4–9&nbsp;cm/year while slow spreading ridges have a rate less than 4&nbsp;cm/year.<ref>{{Cite journal|last=Tan|first=Yen Joe|last2=Tolstoy|first2=Maya|last3=Waldhauser|first3=Felix|last4=Wilcock|first4=William S. D.|title=Dynamics of a seafloor-spreading episode at the East Pacific Rise|url=http://www.nature.com/doifinder/10.1038/nature20116|journal=Nature|volume=540|issue=7632|pages=261–265|doi=10.1038/nature20116}}</ref> Earlier theories (e.g. by [[Alfred Wegener]] and [[Alexander du Toit]]) of continental drift postulated that [[continent]]s "ploughed" through the sea. The idea that the seafloor itself moves (and also carries the continents with it) as it expands from a central axis was proposed by [[Harry Hammond Hess|Harry Hess]] from [[Princeton University]] in the 1960s.<ref>{{Cite book |first=H. H. |last=Hess |chapterurl=http://www.mantleplumes.org/WebDocuments/Hess1962.pdf |chapter=History of Ocean Basins |date=November 1962 |title=Petrologic studies: a volume to honor A. F. Buddington |editor=A. E. J. Engel |editor2=Harold L. James |editor3=B. F. Leonard |place=Boulder, CO |publisher=Geological Society of America |pages=599–620 |accessdate=8 September 2010 }}</ref> The theory is well accepted now, and the phenomenon is known to be caused by [[convection]] currents in the [[asthenosphere]], which is ductile, or [[Plasticity (physics)|plastic]], and the brittle lithosphere.<ref>{{cite journal|last1=Elsasser|first1=Walter M.|title=Sea-Floor Spreading as Thermal Convection|journal=Journal of Geophysical Research|volume=76|pages=1101|year=1971|doi=10.1029/JB076i005p01101|bibcode=1971JGR....76.1101E}}</ref>{{clarify|date=October 2016}} ==Incipient spreading== {{unreferenced section|date=April 2015}} [[Image:Plates tect2 en.svg|thumb|Plates in the crust of the earth, according to the [[plate tectonics]] theory]] In the general case, sea floor spreading starts as a [[rift (geology)|rift]] in a [[Continental plate|continental land mass]], similar to the Red Sea-[[East Africa Rift]] System today.<ref>{{Cite journal|last=Makris|first=J.|last2=Ginzburg|first2=A.|date=1987-09-15|title=Sedimentary basins within the Dead Sea and other rift zones The Afar Depression: transition between continental rifting and sea-floor spreading|url=http://www.sciencedirect.com/science/article/pii/0040195187901867|journal=Tectonophysics|volume=141|issue=1|pages=199–214|doi=10.1016/0040-1951(87)90186-7}}</ref> The process starts by heating at the base of the continental crust which causes it to become more plastic and less dense. Because less dense objects rise in relation to denser objects, the area being heated becomes a broad dome (see [[isostasy]]). As the crust bows upward, fractures occur that gradually grow into rifts. The typical rift system consists of three rift arms at approximately 120 degree angles. These areas are named [[triple junction]]s and can be found in several places across the world today. The separated margins of the [[continent]]s evolve to form [[passive margin]]s. Hess' theory was that new seafloor is formed when magma is forced upward toward the surface at a mid-ocean ridge. If spreading continues past the incipient stage described above, two of the rift arms will open while the third arm stops opening and becomes a 'failed rift'. As the two active rifts continue to open, eventually the continental crust is attenuated as far as it will stretch. At this point basaltic oceanic crust begins to form between the separating continental fragments. When one of the rifts opens into the existing ocean, the rift system is flooded with seawater and becomes a new sea. The [[Red Sea]] is an example of a new arm of the sea. The East African rift was thought to be a "failed" arm that was opening somewhat more slowly than the other two arms, but in 2005 the [[Ethiopia]]n Afar Geophysical Lithospheric Experiment<ref>{{Cite journal|last=Bastow|first=Ian D.|last2=Keir|first2=Derek|last3=Daly|first3=Eve|date=2011-06-01|title=The Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE): Probing the transition from continental rifting to incipient seafloor spreading|url=http://specialpapers.gsapubs.org/content/478/51|journal=Geological Society of America Special Papers|language=en|volume=478|pages=51–76|doi=10.1130/2011.2478(04)|issn=0072-1077}}</ref> reported that in the [[Afar (region)|Afar region]] last September,{{When|date=March 2017}} a 60&nbsp;km fissure opened as wide as eight meters. During this period of initial flooding the new sea is sensitive to changes in climate and [[eustasy]]. As a result, the new sea will evaporate (partially or completely) several times before the elevation of the rift valley has been lowered to the point that the sea becomes stable. During this period of evaporation large evaporite deposits will be made in the rift valley. Later these deposits have the potential to become hydrocarbon seals and are of particular interest to [[petroleum]] geologists. Sea floor spreading can stop during the process, but if it continues to the point that the continent is completely severed, then a new [[ocean basin]] is created. The Red Sea has not yet completely split Arabia from Africa, but a similar feature can be found on the other side of Africa that has broken completely free. South America once fit into the area of the [[Niger Delta]]. The Niger River has formed in the failed rift arm of the triple junction. ==Continued spreading and subduction== [[Image:Ridge render.jpg|thumb|300px|right|Spreading at a mid-ocean ridge]] As new seafloor forms and spreads apart from the [[mid-ocean ridge]] it slowly cools over time. Older seafloor is therefore colder than new seafloor, and older oceanic basins deeper than new oceanic basins due to [[isostasy]]. If the diameter of the earth remains relatively constant despite the production of new crust, a mechanism must exist by which crust is also destroyed. The destruction of oceanic crust occurs at [[subduction]] zones where oceanic crust is forced under either continental crust or oceanic crust. Today, the Atlantic basin is actively spreading at the Mid-Atlantic Ridge. Only a small portion of the oceanic crust produced in the Atlantic is subducted. However, the plates making up the Pacific Ocean are experiencing subduction along many of their boundaries which causes the volcanic activity in what has been termed the [[Pacific Ring of Fire|Ring of Fire]] of the Pacific Ocean. The Pacific is also home to one of the world's most active spreading centres (the [[East Pacific Rise]]) with spreading rates of up to 13&nbsp;cm/yr. The Mid-Atlantic Ridge is a "[[wiktionary:textbook|textbook]]" slow-spreading centre, while the East Pacific Rise is used as an example of fast spreading. The differences in spreading rates affect not only the geometries of the ridges but also the geochemistry of the basalts that are produced.<ref>{{cite book|last=Bhagwat|first=S.B.|title=Foundation of Geology Vol 1|year=2009|publisher=Global Vision Publishing House|isbn=9788182202764|page=83|url=https://books.google.com/books?id=Fkw99wNiyQ0C&pg=PA83}}</ref> Since the new oceanic basins are shallower than the old oceanic basins, the total capacity of the world's ocean basins decreases during times of active sea floor spreading. During the opening of the [[Atlantic Ocean]], sea level was so high that a [[Western Interior Seaway]] formed across [[North America]] from the [[Gulf of Mexico]] to the [[Arctic Ocean]]. ==Debate and search for mechanism== At the [[Mid-Atlantic Ridge]] (and in other areas), material from the upper [[Mantle (geology)|mantle]] rises through the faults between oceanic plates to form new [[Crust (geology)|crust]] as the plates move away from each other, a phenomenon first observed as [[continental drift]]. When [[Alfred Wegener]] first presented a hypothesis of continental drift in 1912, he suggested that continents ploughed through the ocean crust. This was impossible: oceanic crust is both more dense and more rigid than continental crust. Accordingly, Wegener's theory wasn't taken very seriously, especially in the United States. Since then, it has been shown that the motion of the continents is linked to seafloor spreading. In the 1960s, the past record of [[geomagnetic reversal]]s was noticed by observing the magnetic stripe "anomalies" on the ocean floor.<ref>{{cite journal|doi=10.1038/199947a0|title=Magnetic Anomalies Over Oceanic Ridges|year=1963|author=Vine, F. J.|journal=Nature|volume=199|pages=947–949|last2=Matthews|first2=D. H.}}</ref> This results in broadly evident "stripes" from which the past magnetic field polarity can be inferred by looking at the data gathered from simply towing a magnetometer on the sea surface or from an aircraft. The stripes on one side of the mid-ocean ridge were the mirror image of those on the other side. The seafloor must have originated on the Earth's great fiery welts, like the [[Mid-Atlantic Ridge]] and the East Pacific Rise. The driver for seafloor spreading in plates with [[active margin]]s is the weight of the cool, dense, subducting slabs that pull them along. The magmatism at the ridge is considered to be "passive upswelling", which is caused by the plates being pulled apart under the weight of their own slabs.<ref>{{cite journal|doi=10.1038/311615a0|title=India–Eurasia collision chronology has implications for crustal shortening and driving mechanism of plates|year=1984|author=Patriat, Philippe|journal=Nature|volume=311|pages=615|last2=Achache|first2=José|issue=5987|bibcode = 1984Natur.311..615P }}</ref> This can be thought of as analogous to a rug on a table with little friction: when part of the rug is off of the table, its weight pulls the rest of the rug down with it. ==Sea floor global topography: half-space model== To first approximation, [[sea floor]] global topography in areas without significant [[subduction]] can be estimated by the half-space model.<ref>{{cite journal |last1=Davis |first1=E.E |last2=Lister |first2=C. R. B. |title=Fundamentals of Ridge Crest Topography |journal=Earth and Planetary Science Letters |publisher=North-Holland Publishing Company |volume=21 |pages=405–413 |year=1974 |doi=10.1016/0012-821X(74)90180-0 |bibcode=1974E&PSL..21..405D}}</ref> In this model, the seabed height is determined by the [[oceanic lithosphere]] temperature, due to thermal expansion. [[Oceanic lithosphere]] is continuously formed at a constant rate at the [[mid-ocean ridge]]s. The source of the lithosphere has a half-plane shape (x = 0, z < 0) and a constant temperature T₁. Due to its continuous creation, the lithosphere at x > 0 is moving away from the ridge at a constant velocity v, which is assumed large compared to other typical scales in the problem. The temperature at the upper boundary of the lithosphere (z=0) is a constant T₀ = 0. Thus at x = 0 the temperature is the [[Heaviside step function]] <math>T_1\cdot\Theta(-z)</math>. Finally, we assume the system is at a quasi-[[steady state]], so that the temperature distribution is constant in time, i.e. T=T(x,z). By calculating in the frame of reference of the moving lithosphere (velocity v), which have spatial coordinate x' = x-vt, we may write T = T(x',z,t) and use the [[heat equation]]: <math>\frac{\partial T}{\partial t} = \kappa \nabla^2 T = \kappa\frac{\partial^2 T}{\partial^2 z} + \kappa\frac{\partial^2 T}{\partial^2 x'}</math> where <math>\kappa</math> is the [[thermal diffusivity]] of the mantle lithosphere. Since T depends on x' and t only through the combination <math>x = x'+vt</math>, we have: <math>\frac{\partial T}{\partial x'} = \frac{1}{v}\cdot\frac{\partial T}{\partial t}</math> Thus: <math>\frac{\partial T}{\partial t} = \kappa \nabla^2 T = \kappa\frac{\partial^2 T}{\partial^2 z} + \frac{\kappa}{v^2}\frac{\partial^2 T}{\partial^2 t}</math> We now use the assumption that <math>v</math> is large compared to other scales in the problem; we therefore neglect the last term in the equation, and get a 1-dimensional diffusion equation: <math>\frac{\partial T}{\partial t} = \kappa\frac{\partial^2 T}{\partial^2 z}</math> with the initial conditions <math>T(t=0) = T_1\cdot\Theta(-z)</math>. The solution for <math>z\le 0</math> is given by the [[error function]] <math>\operatorname{erf}</math>: :<math>T(x',z,t) = T_1 \cdot \operatorname{erf} \left(\frac{z}{2\sqrt{\kappa t}}\right)</math>. Due to the large velocity, the temperature dependence on the horizontal direction is negligible, and the height at time t (i.e. of sea floor of age t) can be calculated by integrating the thermal expansion over z: :<math> h(t) = h_0 + \alpha_\mathrm{eff} \int_0^{\infty} [T(z)-T_1]dz = h_0 - \frac{2}{\sqrt{\pi}}\alpha_\mathrm{eff}T_1\sqrt{\kappa t} </math> where <math>\alpha_\mathrm{eff}</math> is the effective volumetric [[thermal expansion]] coefficient, and h₀ is the mid-ocean ridge height (compared to some reference). Note that the assumption the v is relatively large is equivalently to the assumption that the thermal diffusivity <math>\kappa</math> is small compared to <math>L^2/T</math>, where L is the ocean width (from [[mid-ocean ridges]] to [[continental shelf]]) and T is its age. The effective thermal expansion coefficient <math>\alpha_\mathrm{eff}</math> is different from the usual thermal expansion coefficient <math>\alpha</math> due to [[isostasy|isostasic]] effect of the change in water column height above the lithosphere as it expands or retracts. Both coefficients are related by: :<math> \alpha_\mathrm{eff} = \alpha \cdot \frac{\rho}{\rho-\rho_w}</math> where <math>\rho \sim 3.3 g/cm^3</math> is the rock density and <math>\rho_0 = 1 g/cm^3</math> is the density of water. By substituting the parameters by their rough estimates: <math>\kappa \sim 8\cdot 10^{-7}</math> m²/s, <math>\alpha \sim 4\cdot 10^{-5}</math>&nbsp;°C⁻¹ and T₁ ~1220&nbsp;°C (for the Atlantic and Indian oceans) or ~1120&nbsp;°C (for the eastern Pacific), we have: :<math>h(t) \sim h_0 - 350 \sqrt{t}</math> for the eastern Pacific Ocean, and: :<math>h(t) \sim h_0 - 390 \sqrt{t}</math> for the Atlantic and the Indian Ocean, where the height is in meters and time is in millions of years. To get the dependence on x, one must substitute t = x/v ~ Tx/L, where L is the distance between the ridge to the [[continental shelf]] (roughly half the ocean width), and T is the ocean age. == See also == {{Commons category|Seafloor spreading}} * [[Divergent boundary]] * [[Mid-ocean ridge]] * [[Morley-Vine-Matthews hypothesis]] ==References== {{reflist}} ==External links== {{Wikibooks |Historical Geology|Seafloor spreading}} * [http://www.oceanexplorer.noaa.gov/explorations/03fire/logs/ridge.html Animation of a mid-ocean ridge] {{physical oceanography|expanded=other}} {{DEFAULTSORT:Seafloor Spreading}} [[Category:Geological processes]] [[Category:Plate tectonics]] [[Category:Oceanographical terminology]]'