The Moving Seafloor Is Swallowed Up Again by the Mantle Along

Geological process at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and so gradually moves away from the ridge

Age of oceanic lithosphere; youngest (cherry-red) is forth spreading centers

Seafloor spreading or Seafloor spread is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic action and then gradually moves away from the ridge.

History of written report [edit]

Earlier theories by Alfred Wegener and Alexander du Toit of continental migrate postulated that continents in motility "plowed" through the fixed and immovable seafloor. The idea that the seafloor itself moves and also carries the continents with information technology equally it spreads from a primal rift axis was proposed past Harold Hammond Hess from Princeton University and Robert Dietz of the U.Southward. Naval Electronics Laboratory in San Diego in the 1960s.^{[ane] [two]} The phenomenon is known today as plate tectonics. In locations where two plates movement autonomously, at mid-ocean ridges, new seafloor is continually formed during seafloor spreading.

Significance [edit]

Seafloor spreading helps explain continental drift in the theory of plate tectonics. When oceanic plates diverge, tensional stress causes fractures to occur in the lithosphere. The motivating force for seafloor spreading ridges is tectonic plate slab pull at subduction zones, rather than magma pressure, although there is typically significant magma activity at spreading ridges.^[3] Plates that are non subducting are driven by gravity sliding off the elevated mid-sea ridges a process called ridge push.^[4] At a spreading center, basaltic magma rises up the fractures and cools on the ocean floor to grade new seabed. Hydrothermal vents are common at spreading centers. Older rocks will be institute farther abroad from the spreading zone while younger rocks will be found nearer to the spreading zone.

Spreading rate is the charge per unit at which an ocean basin widens due to seafloor spreading. (The rate at which new oceanic lithosphere is added to each tectonic plate on either side of a mid-bounding main ridge is the spreading half-rate and is equal to half of the spreading charge per unit). Spreading rates determine if the ridge is fast, intermediate, or slow. As a general rule, fast ridges have spreading (opening) rates of more than 90 mm/year. Intermediate ridges have a spreading rate of twoscore–xc mm/year while slow spreading ridges take a charge per unit less than 40 mm/year.^{[five] [half dozen] [7] : 2} The highest known rate was over 200 mm/yr during the Miocene on the Eastward Pacific Rise.^[8]

In the 1960s, the past record of geomagnetic reversals of Globe'south magnetic field was noticed past observing magnetic stripe "anomalies" on the ocean floor.^{[9] [10]} This results in broadly evident "stripes" from which the past magnetic field polarity can be inferred from data gathered with a magnetometer towed on the sea surface or from an aircraft. The stripes on one side of the mid-ocean ridge were the mirror prototype of those on the other side. Past identifying a reversal with a known historic period and measuring the altitude of that reversal from the spreading heart, the spreading half-rate could exist computed.

magnetic stripes formed during seafloor spreading

In some locations spreading rates have been institute to be asymmetric; the half rates differ on each side of the ridge crest by almost v percent.^{[11] [12]} This is thought due to temperature gradients in the asthenosphere from mantle plumes near the spreading eye.^[12]

Spreading center [edit]

Seafloor spreading occurs at spreading centers, distributed along the crests of mid-sea ridges. Spreading centers end in transform faults or in overlapping spreading centre offsets. A spreading centre includes a seismically active plate purlieus zone a few kilometers to tens of kilometers broad, a crustal accretion zone within the purlieus zone where the ocean crust is youngest, and an instantaneous plate boundary - a line within the crustal accretion zone demarcating the two separating plates.^[13] Inside the crustal accretion zone is a i-2 km-wide neovolcanic zone where active volcanism occurs.^{[14] [xv]}

Incipient spreading [edit]

In the full general case, seafloor spreading starts as a rift in a continental state mass, similar to the Red Sea-Eastward Africa Rift Organization today.^[16] The process starts by heating at the base of the continental crust which causes information technology to become more plastic and less dumbo. Because less dense objects rise in relation to denser objects, the area existence heated becomes a wide dome (run into isostasy). As the crust bows upwards, fractures occur that gradually grow into rifts. The typical rift organisation consists of three rift arms at approximately 120-degree angles. These areas are named triple junctions and tin can be constitute in several places across the world today. The separated margins of the continents evolve to form passive margins. Hess' theory was that new seafloor is formed when magma is forced upward toward the surface at a mid-sea ridge.

If spreading continues by the incipient phase described above, ii of the rift arms volition open while the third arm stops opening and becomes a 'failed rift' or aulacogen. Equally the two active rifts keep to open, somewhen the continental crust is attenuated equally far as information technology will stretch. At this point basaltic oceanic crust and upper mantle lithosphere begins to form between the separating continental fragments. When i of the rifts opens into the existing bounding main, the rift system is flooded with seawater and becomes a new bounding main. The Ruby Ocean is an example of a new arm of the sea. The East African rift was thought to be a failed arm that was opening more slowly than the other two arms, only in 2005 the Ethiopian Afar Geophysical Lithospheric Experiment^[17] reported that in the Afar region, September 2005, a 60 km fissure opened every bit wide as viii meters.^[xviii] During this flow of initial flooding the new sea is sensitive to changes in climate and eustasy. Every bit a result, the new sea volition evaporate (partially or completely) several times earlier 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 exist made in the rift valley. Subsequently these deposits have the potential to become hydrocarbon seals and are of item interest to petroleum geologists.

Seafloor spreading tin can cease during the process, but if information technology continues to the point that the continent is completely severed, then a new bounding main basin is created. The Scarlet Sea has not however completely split Arabia from Africa, just a like feature can exist found on the other side of Africa that has broken completely free. Due 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.^[19]

Continued spreading and subduction [edit]

Spreading at a mid-ocean ridge

As new seafloor forms and spreads apart from the mid-ocean ridge it slowly cools over fourth dimension. Older seafloor is, therefore, colder than new seafloor, and older oceanic basins deeper than new oceanic basins due to isostasy. If the bore of the globe remains relatively constant despite the product of new crust, a mechanism must exist by which chaff is also destroyed. The destruction of oceanic crust occurs at subduction zones where oceanic crust is forced under either continental crust or oceanic chaff. Today, the Atlantic basin is actively spreading at the Mid-Atlantic Ridge. Simply a small-scale portion of the oceanic chaff produced in the Atlantic is subducted. Yet, 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 Ring of Fire of the Pacific Body of water. The Pacific is also home to one of the world'south most active spreading centers (the E Pacific Rising) with spreading rates of upwards to 145 +/- iv mm/yr betwixt the Pacific and Nazca plates.^[twenty] The Mid-Atlantic Ridge is a wearisome-spreading centre, while the East Pacific Ascent is an example of fast spreading. Spreading centers at slow and intermediate rates exhibit a rift valley while at fast rates an axial loftier is institute within the crustal accretion zone.^[6] The differences in spreading rates affect not only the geometries of the ridges merely too the geochemistry of the basalts that are produced.^[21]

Since the new oceanic basins are shallower than the one-time 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, ocean level was and so high that a Western Interior Seaway formed beyond N America from the Gulf of United mexican states to the Arctic Ocean.

Debate and search for mechanism [edit]

At the Mid-Atlantic Ridge (and in other mid-ocean ridges), material from the upper drape rises through the faults between oceanic plates to form new crust every bit the plates move away from each other, a phenomenon offset observed as continental drift. When Alfred Wegener first presented a hypothesis of continental migrate in 1912, he suggested that continents plowed through the sea crust. This was impossible: oceanic crust is both more than dense and more rigid than continental crust. Accordingly, Wegener'south theory wasn't taken very seriously, especially in the United States.

At first the driving force for spreading was argued to be convection currents in the mantle.^[22] Since then, it has been shown that the motion of the continents is linked to seafloor spreading by the theory of plate tectonics, which is driven by convection that includes the crust itself as well.^[4]

The driver for seafloor spreading in plates with active margins is the weight of the cool, dumbo, subducting slabs that pull them along, or slab pull. The magmatism at the ridge is considered to exist passive upwelling, which is acquired by the plates existence pulled apart under the weight of their own slabs.^{[4] [23]} This tin can be thought of as coordinating to a rug on a tabular array with footling friction: when function of the rug is off of the table, its weight pulls the rest of the carpeting down with it. Withal, the Mid-Atlantic ridge itself is not bordered by plates that are being pulled into subduction zones, except the modest subduction in the Bottom Antilles and Scotia Arc. In this case the plates are sliding apart over the mantle upwelling in the process of ridge push button.^[iv]

Seafloor global topography: cooling models [edit]

The depth of the seafloor (or the height of a location on a mid-ocean ridge in a higher place a base-level) is closely correlated with its historic period (age of the lithosphere where depth is measured). The age-depth relation can be modeled by the cooling of a lithosphere plate^{[24] [25] [26] [27]} or pall one-half-infinite in areas without meaning subduction.^[28]

Cooling drapery model [edit]

In the mantle one-half-space model,^[28] the seabed height is determined by the oceanic lithosphere and pall temperature, due to thermal expansion. The simple result is that the ridge pinnacle or body of water depth is proportional to the square root of its age.^[28] Oceanic lithosphere is continuously formed at a constant rate at the mid-ocean ridges. The source of the lithosphere has a half-plane shape (x = 0, z < 0) and a abiding temperature T ₁. 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 T ₀ = 0. Thus at 10 = 0 the temperature is the Heaviside step part ${\displaystyle T_{ane}\cdot \Theta (-z)}$ . The arrangement is assumed to be at a quasi-steady state, so that the temperature distribution is constant in fourth dimension, i.eastward. ${\displaystyle T=T(ten,z).}$

Past calculating in the frame of reference of the moving lithosphere (velocity v), which has spatial coordinate ${\displaystyle 10'=x-vt,}$ ${\displaystyle T=T(10',z,t).}$ and the heat equation is:

${\displaystyle {\frac {\partial T}{\fractional t}}=\kappa \nabla ^{2}T=\kappa {\frac {\partial ^{two}T}{\partial ^{2}z}}+\kappa {\frac {\partial ^{ii}T}{\partial ^{2}x'}}}$

where ${\displaystyle \kappa }$ is the thermal diffusivity of the mantle lithosphere.

Since T depends on x' and t only through the combination ${\displaystyle ten=x'+vt,}$ :

${\displaystyle {\frac {\partial T}{\fractional x'}}={\frac {1}{v}}\cdot {\frac {\partial T}{\partial t}}}$

Thus:

${\displaystyle {\frac {\partial T}{\fractional t}}=\kappa \nabla ^{2}T=\kappa {\frac {\partial ^{2}T}{\partial ^{2}z}}+{\frac {\kappa }{v^{2}}}{\frac {\fractional ^{2}T}{\partial ^{2}t}}}$

It is assumed that ${\displaystyle v}$ is large compared to other scales in the problem; therefore the terminal term in the equation is neglected, giving a 1-dimensional diffusion equation:

${\displaystyle {\frac {\fractional T}{\partial t}}=\kappa {\frac {\partial ^{2}T}{\partial ^{2}z}}}$

with the initial conditions

${\displaystyle T(t=0)=T_{ane}\cdot \Theta (-z).}$

The solution for ${\displaystyle z\leq 0}$ is given past the error function:

${\displaystyle T(ten',z,t)=T_{1}\cdot \operatorname {erf} \left({\frac {z}{2{\sqrt {\kappa t}}}}\right)}$ .

Due to the big velocity, the temperature dependence on the horizontal management is negligible, and the height at fourth dimension t (i.east. of sea floor of historic period t) tin can be calculated by integrating the thermal expansion over z:

${\displaystyle h(t)=h_{0}+\alpha _{\mathrm {eff} }\int _{0}^{\infty }[T(z)-T_{1}]dz=h_{0}-{\frac {two}{\sqrt {\pi }}}\alpha _{\mathrm {eff} }T_{1}{\sqrt {\kappa t}}}$

where ${\displaystyle \blastoff _{\mathrm {eff} }}$ is the effective volumetric thermal expansion coefficient, and h₀ is the mid-ocean ridge height (compared to some reference).

The assumption that v is relatively large is equivalent to the assumption that the thermal diffusivity ${\displaystyle \kappa }$ is small compared to ${\displaystyle Fifty^{2}/A}$ , where L is the ocean width (from mid-sea ridges to continental shelf) and A is the age of the ocean basin.

The effective thermal expansion coefficient ${\displaystyle \alpha _{\mathrm {eff} }}$ is different from the usual thermal expansion coefficient ${\displaystyle \alpha }$ due to isostasic effect of the change in water column height higher up the lithosphere equally it expands or retracts. Both coefficients are related by:

${\displaystyle \alpha _{\mathrm {eff} }=\alpha \cdot {\frac {\rho }{\rho -\rho _{westward}}}}$

where ${\displaystyle \rho \sim three.iii\ \mathrm {g} \cdot \mathrm {cm} ^{-3}}$ is the stone density and ${\displaystyle \rho _{0}=one\ \mathrm {thou} \cdot \mathrm {cm} ^{-3}}$ is the density of water.

By substituting the parameters by their rough estimates:

${\displaystyle {\brainstorm{aligned}\kappa &\sim 8\cdot 10^{-7}\ \mathrm {thou} ^{2}\cdot \mathrm {southward} ^{-1}\\\alpha &\sim iv\cdot ten^{-5}\ {}^{\circ }\mathrm {C} ^{-1}\\T_{1}&\sim 1220\ {}^{\circ }\mathrm {C} &&{\text{for the Atlantic and Indian oceans}}\\T_{i}&\sim 1120\ {}^{\circ }\mathrm {C} &&{\text{for the eastern Pacific}}\finish{aligned}}}$

we have:^[28]

${\displaystyle h(t)\sim {\begin{cases}h_{0}-390{\sqrt {t}}&{\text{for the Atlantic and Indian oceans}}\\h_{0}-350{\sqrt {t}}&{\text{for the eastern Pacific}}\stop{cases}}}$

where the meridian is in meters and time is in millions of years. To become the dependence on ten, one must substitute t = x/5 ~ Ax/L, where L is the distance between the ridge to the continental shelf (roughly half the body of water width), and A is the ocean bowl historic period.

Rather than pinnacle of the ocean floor ${\displaystyle h(t)}$ in a higher place a base or reference level ${\displaystyle h_{b}}$ , the depth of the bounding main ${\displaystyle d(t)}$ is of interest. Because ${\displaystyle d(t)+h(t)=h_{b}}$ (with ${\displaystyle h_{b}}$ measured from the sea surface) we tin can detect that:

${\displaystyle d(t)=h_{b}-h_{0}+350{\sqrt {t}}}$ ; for the eastern Pacific for example, where ${\displaystyle h_{b}-h_{0}}$ is the depth at the ridge crest, typically 2600 m.

Cooling plate model [edit]

The depth predicted past the square root of seafloor age derived above is too deep for seafloor older than 80 million years.^[27] Depth is improve explained by a cooling lithosphere plate model rather than the cooling drapery one-half-space.^[27] The plate has a constant temperature at its base and spreading edge. Analysis of depth versus age and depth versus square root of age data immune Parsons and Sclater^[27] to estimate model parameters (for the North Pacific):

~125 km for lithosphere thickness

${\displaystyle T_{ane}\thicksim 1350\ {}^{\circ }\mathrm {C} }$ at base and young edge of plate

${\displaystyle \blastoff \thicksim 3.ii\cdot 10^{-v}\ {}^{\circ }\mathrm {C} ^{-1}}$

Assuming isostatic equilibrium everywhere beneath the cooling plate yields a revised historic period depth relationship for older sea floor that is approximately correct for ages as young as 20 meg years:

${\displaystyle d(t)=6400-3200\exp {\bigl (}-t/62.8{\bigr )}}$ meters

Thus older seafloor deepens more slowly than younger and in fact can exist assumed almost constant at ~6400 yard depth. Parsons and Sclater concluded that some manner of mantle convection must apply heat to the base of operations of the plate everywhere to forestall cooling downwards beneath 125 km and lithosphere contraction (seafloor deepening) at older ages.^[27] Their plate model also allowed an expression for conductive heat menstruum, q(t) from the body of water floor, which is approximately constant at ${\displaystyle ane\cdot 10^{-6}\mathrm {cal} \,\mathrm {cm} ^{-ii}\mathrm {sec} ^{-ane}}$ beyond 120 meg years:

${\displaystyle q(t)=11.iii/{\sqrt {t}}}$

Meet also [edit]

• Divergent boundary – Linear feature that exists between two tectonic plates that are moving away from each other
• Vine–Matthews–Morley hypothesis – Offset fundamental scientific test of the seafloor spreading theory of continental drift and plate tectonics
• DSV ALVIN the research submersible that explored spreading centers in the Atlantic (Project FAMOUS) and Pacific Oceans (RISE project).

References [edit]

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External links [edit]

• Blitheness of a mid-bounding main ridge

mooretheented.blogspot.com

Source: https://en.wikipedia.org/wiki/Seafloor_spreading

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