what evidence do we have to prove that tectonic plates are moving apart at the mid-oceanic ridges?

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

Historic period of oceanic lithosphere; youngest (red) is along spreading centers

Seafloor spreading or Seafloor spread is a process that occurs at mid-bounding main ridges, where new oceanic chaff is formed through volcanic activeness then gradually moves away from the ridge.

History of study [edit]

Before theories by Alfred Wegener and Alexander du Toit of continental drift postulated that continents in move "plowed" through the fixed and immovable seafloor. The idea that the seafloor itself moves and also carries the continents with it as it spreads from a key rift centrality was proposed by Harold Hammond Hess from Princeton University and Robert Dietz of the U.S. Naval Electronics Laboratory in San Diego in the 1960s.^{[one] [2]} The miracle is known today as plate tectonics. In locations where 2 plates move apart, 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 forcefulness for seafloor spreading ridges is tectonic plate slab pull at subduction zones, rather than magma pressure, although at that place is typically significant magma activity at spreading ridges.^[iii] Plates that are non subducting are driven by gravity sliding off the elevated mid-ocean ridges a process chosen ridge push.^[4] At a spreading centre, basaltic magma rises upwards the fractures and cools on the sea floor to form new seabed. Hydrothermal vents are mutual at spreading centers. Older rocks will be found farther away from the spreading zone while younger rocks will exist plant nearer to the spreading zone.

Spreading rate is the rate 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-ocean ridge is the spreading half-rate and is equal to half of the spreading charge per unit). Spreading rates decide if the ridge is fast, intermediate, or slow. Equally a general dominion, fast ridges take spreading (opening) rates of more than ninety mm/twelvemonth. Intermediate ridges take a spreading charge per unit of xl–90 mm/year while slow spreading ridges accept a rate less than 40 mm/yr.^{[five] [6] [seven] : 2} The highest known charge per unit was over 200 mm/yr during the Miocene on the East Pacific Ascent.^[8]

In the 1960s, the past record of geomagnetic reversals of Earth'due south magnetic field was noticed by observing magnetic stripe "anomalies" on the ocean flooring.^{[9] [ten]} 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 ane side of the mid-ocean ridge were the mirror image of those on the other side. Past identifying a reversal with a known historic period and measuring the distance of that reversal from the spreading center, the spreading half-rate could be computed.

magnetic stripes formed during seafloor spreading

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

Spreading middle [edit]

Seafloor spreading occurs at spreading centers, distributed along the crests of mid-bounding main ridges. Spreading centers stop in transform faults or in overlapping spreading center offsets. A spreading centre includes a seismically active plate boundary zone a few kilometers to tens of kilometers broad, a crustal accession zone within the boundary zone where the ocean crust is youngest, and an instantaneous plate boundary - a line inside the crustal accretion zone demarcating the 2 separating plates.^[13] Within the crustal accretion zone is a 1-two km-wide neovolcanic zone where active volcanism occurs.^{[14] [fifteen]}

Incipient spreading [edit]

In the general case, seafloor spreading starts as a rift in a continental land mass, similar to the Red Sea-East Africa Rift Arrangement today.^[sixteen] The procedure starts by heating at the base of the continental crust which causes information technology to get more than plastic and less dense. Because less dense objects rise in relation to denser objects, the area existence heated becomes a broad dome (meet isostasy). As the crust bows upwardly, fractures occur that gradually grow into rifts. The typical rift system consists of three rift arms at approximately 120-caste 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-ocean ridge.

If spreading continues by the incipient stage described above, two of the rift arms will open up while the third arm stops opening and becomes a 'failed rift' or aulacogen. As the two active rifts continue to open, eventually the continental crust is attenuated equally far as it volition stretch. At this point basaltic oceanic crust and upper mantle lithosphere begins to class between the separating continental fragments. When one of the rifts opens into the existing sea, the rift organization is flooded with seawater and becomes a new bounding main. The Red Sea is an example of a new arm of the sea. The Eastward African rift was thought to be a failed arm that was opening more slowly than the other two arms, but in 2005 the Ethiopian Distant Geophysical Lithospheric Experiment^[17] reported that in the Afar region, September 2005, a lx km fissure opened as wide as 8 meters.^[18] During this period of initial flooding the new bounding main 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 indicate that the ocean becomes stable. During this period of evaporation big evaporite deposits will be fabricated in the rift valley. Later these deposits have the potential to become hydrocarbon seals and are of particular interest to petroleum geologists.

Seafloor spreading can stop during the process, merely if it continues to the indicate that the continent is completely severed, then a new body of water basin is created. The Red Sea has not withal completely split up Arabia from Africa, but a like characteristic can exist plant on the other side of Africa that has broken completely gratuitous. S America once fit into the expanse of the Niger Delta. The Niger River has formed in the failed rift arm of the triple junction.^[19]

Connected spreading and subduction [edit]

Spreading at a mid-ocean ridge

As new seafloor forms and spreads apart from the mid-body of water 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 bore of the earth remains relatively constant despite the product 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 Sea are experiencing subduction along many of their boundaries which causes the volcanic activity in what has been termed the Band of Burn down of the Pacific Bounding main. The Pacific is also home to i of the earth'south about agile spreading centers (the Eastward Pacific Rising) with spreading rates of upwardly to 145 +/- 4 mm/yr between the Pacific and Nazca plates.^[20] The Mid-Atlantic Ridge is a irksome-spreading eye, while the East Pacific Ascent is an example of fast spreading. Spreading centers at dull and intermediate rates exhibit a rift valley while at fast rates an axial high is plant within the crustal accretion zone.^[half-dozen] The differences in spreading rates affect non only the geometries of the ridges merely also the geochemistry of the basalts that are produced.^[21]

Since the new oceanic basins are shallower than the sometime oceanic basins, the full capacity of the world'southward ocean basins decreases during times of active sea floor spreading. During the opening of the Atlantic Sea, sea level was so high that a Western Interior Seaway formed across North America from the Gulf of United mexican states to the Arctic Ocean.

Debate and search for machinery [edit]

At the Mid-Atlantic Ridge (and in other mid-bounding main ridges), textile from the upper mantle rises through the faults betwixt oceanic plates to form new crust as the plates move abroad 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 plowed through the body of water crust. This was impossible: oceanic crust is both more dumbo and more rigid than continental crust. Appropriately, Wegener's theory wasn't taken very seriously, peculiarly in the Us.

At first the driving force for spreading was argued to exist convection currents in the mantle.^[22] Since so, information technology 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.^[iv]

The commuter for seafloor spreading in plates with active margins is the weight of the cool, dense, subducting slabs that pull them along, or slab pull. The magmatism at the ridge is considered to be passive upwelling, which is acquired by the plates being pulled apart under the weight of their own slabs.^{[4] [23]} This tin be idea of every bit analogous to a carpet on a tabular array with fiddling friction: when part of the rug is off of the table, its weight pulls the rest of the rug downwards with it. Still, the Mid-Atlantic ridge itself is not bordered by plates that are being pulled into subduction zones, except the minor subduction in the Lesser Antilles and Scotia Arc. In this case the plates are sliding apart over the mantle upwelling in the procedure of ridge push button.^[four]

Seafloor global topography: cooling models [edit]

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

Cooling mantle model [edit]

In the mantle one-half-infinite model,^[28] the seabed height is determined by the oceanic lithosphere and drape temperature, due to thermal expansion. The uncomplicated issue is that the ridge height 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-body of water ridges. The source of the lithosphere has a half-plane shape (x = 0, z < 0) and a constant temperature T ₁. Due to its continuous creation, the lithosphere at 10 > 0 is moving away from the ridge at a constant velocity v, which is assumed big compared to other typical scales in the trouble. The temperature at the upper boundary of the lithosphere (z = 0) is a constant T ₀ = 0. Thus at x = 0 the temperature is the Heaviside step role ${\displaystyle T_{1}\cdot \Theta (-z)}$ . The system is assumed to be at a quasi-steady state, so that the temperature distribution is constant in fourth dimension, i.east. ${\displaystyle T=T(x,z).}$

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

${\displaystyle {\frac {\fractional T}{\partial t}}=\kappa \nabla ^{2}T=\kappa {\frac {\partial ^{2}T}{\fractional ^{ii}z}}+\kappa {\frac {\partial ^{2}T}{\partial ^{2}10'}}}$

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

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

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

Thus:

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

It is assumed that ${\displaystyle 5}$ is large compared to other scales in the problem; therefore the last term in the equation is neglected, giving a ane-dimensional diffusion equation:

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

with the initial weather

${\displaystyle T(t=0)=T_{1}\cdot \Theta (-z).}$

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

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

Due to the large velocity, the temperature dependence on the horizontal management is negligible, and the height at time t (i.due east. of bounding main flooring of age t) tin can be calculated by integrating the thermal expansion over z:

${\displaystyle h(t)=h_{0}+\blastoff _{\mathrm {eff} }\int _{0}^{\infty }[T(z)-T_{1}]dz=h_{0}-{\frac {2}{\sqrt {\pi }}}\alpha _{\mathrm {eff} }T_{1}{\sqrt {\kappa t}}}$

where ${\displaystyle \alpha _{\mathrm {eff} }}$ 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 pocket-size compared to ${\displaystyle L^{ii}/A}$ , where Fifty is the ocean width (from mid-sea ridges to continental shelf) and A is the age of the ocean basin.

The constructive 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 h2o cavalcade height above the lithosphere as it expands or retracts. Both coefficients are related by:

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

where ${\displaystyle \rho \sim 3.three\ \mathrm {m} \cdot \mathrm {cm} ^{-iii}}$ is the stone density and ${\displaystyle \rho _{0}=1\ \mathrm {g} \cdot \mathrm {cm} ^{-3}}$ is the density of water.

By substituting the parameters by their rough estimates:

${\displaystyle {\begin{aligned}\kappa &\sim viii\cdot ten^{-7}\ \mathrm {m} ^{2}\cdot \mathrm {s} ^{-1}\\\alpha &\sim four\cdot x^{-5}\ {}^{\circ }\mathrm {C} ^{-i}\\T_{1}&\sim 1220\ {}^{\circ }\mathrm {C} &&{\text{for the Atlantic and Indian oceans}}\\T_{ane}&\sim 1120\ {}^{\circ }\mathrm {C} &&{\text{for the eastern Pacific}}\end{aligned}}}$

we take:^[28]

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

where the height is in meters and time is in millions of years. To go the dependence on ten, one must substitute t = x/five ~ Ax/L, where L is the distance between the ridge to the continental shelf (roughly half the ocean width), and A is the sea basin age.

Rather than pinnacle of the ocean floor ${\displaystyle h(t)}$ above a base or reference level ${\displaystyle h_{b}}$ , the depth of the ocean ${\displaystyle d(t)}$ is of interest. Because ${\displaystyle d(t)+h(t)=h_{b}}$ (with ${\displaystyle h_{b}}$ measured from the ocean surface) nosotros tin notice that:

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

Cooling plate model [edit]

The depth predicted by the square root of seafloor age derived above is besides deep for seafloor older than fourscore 1000000 years.^[27] Depth is improve explained by a cooling lithosphere plate model rather than the cooling mantle 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 allowed Parsons and Sclater^[27] to judge 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

${\displaystyle \alpha \thicksim three.2\cdot x^{-5}\ {}^{\circ }\mathrm {C} ^{-1}}$

Assuming isostatic equilibrium everywhere beneath the cooling plate yields a revised age depth relationship for older sea floor that is approximately correct for ages every bit young as twenty million 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 be assumed about abiding at ~6400 m depth. Parsons and Sclater concluded that some way of mantle convection must utilise heat to the base of the plate everywhere to foreclose cooling down below 125 km and lithosphere contraction (seafloor deepening) at older ages.^[27] Their plate model also allowed an expression for conductive rut flow, q(t) from the ocean flooring, which is approximately abiding at ${\displaystyle one\cdot 10^{-6}\mathrm {cal} \,\mathrm {cm} ^{-2}\mathrm {sec} ^{-1}}$ beyond 120 million years:

${\displaystyle q(t)=11.3/{\sqrt {t}}}$

See too [edit]

• Divergent boundary – Linear feature that exists between two tectonic plates that are moving away from each other
• Vine–Matthews–Morley hypothesis – First 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 (Ascension projection).

References [edit]

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22. ^ Elsasser, Walter One thousand. (1971-02-10). "Body of water-floor spreading as thermal convection". Journal of Geophysical Research. 76 (5): 1101–1112. Bibcode:1971JGR....76.1101E. doi:10.1029/JB076i005p01101.
23. ^ Patriat, Philippe; Achache, José (1984). "India–Eurasia collision chronology has implications for crustal shortening and driving mechanism of plates". Nature. 311 (5987): 615. Bibcode:1984Natur.311..615P. doi:10.1038/311615a0. S2CID 4315858.
24. ^ McKenzie, Dan P. (1967-12-xv). "Some remarks on heat flow and gravity anomalies". Journal of Geophysical Research. 72 (24): 6261–6273. Bibcode:1967JGR....72.6261M. doi:ten.1029/JZ072i024p06261.
25. ^ Sclater, J. G.; Francheteau, J. (1970-09-01). "The Implications of Terrestrial Oestrus Flow Observations on Current Tectonic and Geochemical Models of the Crust and Upper Drape of the Globe". Geophysical Journal International. 20 (v): 509–542. Bibcode:1970GeoJ...20..509S. doi:10.1111/j.1365-246X.1970.tb06089.10. ISSN 0956-540X.
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External links [edit]

• Animation of a mid-ocean ridge

synnotcamestich.blogspot.com

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

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