User:BrucePL/sandbox/MOR edits

REVISED 6/18  Done

Cross section of Mid-ocean ridge

A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of ~ 2,600 meters (8,500 ft) and rises about two kilometers above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place at a divergent 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 new seafloor and oceanic lithosphere results from mantle upwelling in response to plate separation. The melt rises as magma at the linear weakness in the oceanic crust, and emerges as lava, creating new crust and lithosphere upon cooling. The Mid-Atlantic Ridge 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 called mid-ocean ridges. Mid-ocean ridges around the globe are linked by plate tectonic boundaries to appear like the seam of a baseball. The mid-ocean ridge system thus comprises the longest mountain range on Earth, reaching about 65,000 km.

EXISTING 6/18 EDITED 6/21

Morphology[edit]

 Done See also: Seafloor spreading § Sea floor global topography: half-space model

At the mid-ocean ridge spreading center the depth of the seafloor is ~2,600 meters (8,500 ft)(KCM EOS) 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 closely correlated with its age (age of the lithosphere where depth is measured). The age-depth relation can be modeled by the cooling of a lithosphere plate^[13][14] or mantle half-space.^[9] A good approximation is that the depth of the seafloor at a location on a spreading mid-ocean ridge proportional to the square root of the age of the seafloor.(D&L 74) The overall shape of ridges results from Pratt isostacy: close to the ridge axis there is hot, low-density mantle supporting the oceanic crust. As the oceanic plate cools, away from the ridge axis, the oceanic mantle lithosphere (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.(13,14)

Spreading rates range from ~1 to 20 cm/yr.^[1][2] Slow-spreading ridges like the Mid-Atlantic Ridge have spread much less far (showing a narrower profile) than faster ridges like the East Pacific Rise (wider profile) for the same amount of time and cooling and consequent bathymetric deepening.^[12] Slow-spreading ridges (less than 5 cm/yr) generally have large rift valleys, sometimes as wide as 10–20 km (6.2–12.4 mi), and very rugged terrain at the ridge crest that can have relief of up to a 1,000 m (3,300 ft).(Searle, KCM FAMOUS) By contrast, fast-spreading ridges (greater than 8 cm/yr) such as the East Pacific Rise lack rift valleys. These have narrow, sharp ridge crests surrounded by generally flat topography that slopes away from the crest over many hundreds of miles.^[2]

The spreading center or axis, commonly connects to a transform 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 called fracture zones. At faster spreading rates the axes often display overlapping spreading centers that lack connecting transform faults.^[11][12] The depth of the axis varies in a systematic way with shallower depths mid-way between offsets such as transform faults and overlapping spreading centers dividing the axis into segments; this is believed due to variations in magma supply to the spreading center.^[12] Ultra-slow spreading ridges (less than 2 cm/yr), such as the Southwest India and the Arctic Ridges form both magmatic and amagmatic (currently lack volcanic activity) ridge segments without transform faults.^[10]

EXISTING 6/18 EDITED 6/20

Volcanism[edit]

 Done Mid-ocean ridges exhibit active volcanism and seismicity.^[1](Searle) 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 existing ocean 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 of decompression melting in the underlying Earth's mantle.^[3] (Wilson 93) The isentropic upwelling solid mantle material exceeds the solidus temperature and melts. The crystallized magma forms new crust of basalt known as MORB for mid-ocean ridge basalt, and gabbro below it in the lower oceanic crust.^[4] (Michael 09) Mid-ocean ridge basalt is a tholeiitic basalt and is low in incompatible elements.^[5][6] (Hyndman 85; Blatt 96) Hydrothermal vents are a common feature at oceanic spreading centers.^[7][8] (Spiess 80; Martin 08)

The oceanic crust and lithosphere 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.(RLL et al 85; DM etal 97) As the oceanic crust and lithosphere moves away from the ridge axis, the peridotite in the underlying mantle lithosphere cools and becomes more rigid. The crust and the relatively rigid peridotite below it make up the oceanic lithosphere, which sits above the less rigid and viscous asthenosphere.^[1] (Searle)

EXISTING JUNE 28

Formation processes[edit]

Further information: Plate tectonics  Done Oceanic lithosphere is formed at an oceanic ridge, while the lithosphere is subducted back into the asthenosphere at ocean trenches. Two processes, ridge-push and slab pull, are thought to be responsible for spreading at mid-ocean ridges. (F&U 75) Ridge push refers to the gravitation sliding of the ocean plate that is raised above the hotter asthenospere, thus creating a body force causing sliding of the plate downslope. Turcotte & Schubert

 Done GRAVITY BODY FORCE Ridge-push occurs when the growing bulk 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.

In slab pull the weight of a tectonic plate being subducted (pulled) below an overlying plate drags the rest of the plate along behind it. The slab pull mechanism is considered to be contributing more than the ridge push.^[17] (Harff et al 2014)

EXISTING 9/11/19  Done 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 (asthenosphere) is too plastic (flexible) to generate enough friction to pull the tectonic plate along. Moreover, mantle upwelling that causes magma to form beneath the ocean ridges appears to involve only its upper 400 km (250 mi), as deduced from seismic tomography and from studies of the seismic discontinuity at about 400 km (250 mi). 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.

 Done 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 deep convection (see image).[Holmes; Hess] However, some studies have shown that the upper mantle (asthenosphere) is too plastic (flexible) to generate enough friction to pull the tectonic plate along. Moreover, mantle upwelling that causes magma to form beneath the ocean ridges appears to involve only its upper 400 km (250 mi), as deduced from seismic tomography and from observations of the seismic discontinuity at about 400 km (250 mi). 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 EDIT FOLLOWS 11/22/19 North American Plate and South American plate are in motion, yet only are being subducted in restricted locations such as the Lesser Antilles Arc and Scotia Arc, pointing to action by the ridge push body force on these plates.

 Done The rate at which the mid-ocean ridge creates new material is known as the spreading rate, and is typically measured in mm/yr. As a general rule, fast ridges have spreading (opening) rates of more than 90 mm/year.^[1] Intermediate ridges have a spreading rate of 40–90 mm/year while slow spreading ridges have a rate less than 40 mm/year.^[1][2][18] 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^[2] (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.

THIS IS VINE MATTHEWS

 Done The mid-ocean ridge systems form new oceanic crust. As crystallized basalt extruded at a ridge axis cools below Curie 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 in the oceanic crust preserve a record of directions of the Earth's magnetic fieldwith time. Because the field has reversed directions at irregular intervals throughout its history, the pattern of geomagnetic reversals in the ocean crust can be used as an indicator of age.^[1][2] 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.

EXISTING 9/11/19

EDITED 9/17/19;

Global system

 Done

World distribution of mid-oceanic ridges

MOVED TO TOP

The mid-ocean ridges of the world are connected and form theOcean Ridge, a single global mid-oceanic ridge system that is part of every ocean, making it the longest mountain range in the world. The continuous mountain range is 65,000 km (40,400 mi)long (several times longer than the Andes, the longest continental mountain range), and the total length of the oceanic ridge system is 80,000 km (49,700 mi)long.

REVISED BELOW 9/17/19

 Done

Increased rates of sea-floor spreading(i.e. the expansion of the mid-ocean ridge) has caused global (eustatic) sea-level to rise over very long timescales (millions of years).

The high sea levels that occurred during the Cretaceous Period(144–65 Ma) can only be attributed to plate tectonicssince thermal expansion and the absence of ice sheets by themselves cannot account for the fact that sea levels were 100–170 meters higher than today.

Increased sea-floor spreading means that the hot young crust at the mid-ocean ridge will form at a faster rate than it can be destroyed at subduction zones. The mid-ocean ridge will then expand and form a broader ridge, taking up more space in the ocean basinand causing sea levels to rise.

Sea-level changecan be attributed to other factors (thermal expansion, ice melting). 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 sea-floor spreading along the mid-ocean ridges.

Discovery[edit]

 Done 6/21/19

Main article: Plate tectonics § Development of the theory

The first indications that a ridge bisects the Atlantic Ocean basin came from the results of the British Challenger Expedition in the nineteenth century.(Hsu) Soundings from lines dropped to the seafloor were analyzed by oceanographers Matthew Fontaine Maury and Charles Wyville Thomson and revealed a prominent rise in the seafloor that ran down the Atlantic basin from north to south. Sonar echo sounders confirmed this in the early twentieth century.(Bunch & Hellmans)

It was not until after World War II, when the ocean floor was surveyed in more detail, that the full extent of mid-ocean ridges became known. The Vema, a ship of the Lamont-Doherty Earth Observatory of Columbia University, traversed the Atlantic Ocean, recording echo sounder data on the depth of the ocean floor. A team led by Marie Tharp and Bruce Heezen concluded that there was an enormous mountain chain running up the middle of the Atlantic Ocean. Scientists named it the 'Mid-Atlantic Ridge'.

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. The German Meteor Expedition traced the mid-ocean ridge from the South Atlantic into the Indian 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 mid-ocean ridges are located away from the center of other ocean basins. (KCM EOS)

FROM SFS ARTICLE 9/18/19

Seafloor depth on a MOR: Cooling half-space 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 (age of the lithosphere where depth is measured). The age-depth relation can be modeled by the cooling of a lithosphere plate (MCKENZIE 67; SCLATER & FRANCHETEAU 70; ; PARSONS AND SCLATER 77) or mantle half-space in areas without significant subduction.

Cooling mantle model

In the MANTLE half-space model,(D&L SFS) the seabed height is determined by the oceanic lithosphere AND MANTLE temperature, due to thermal expansion. The simple result is that the ridge height or ocean depth is proportional to the square root of its age.(D&L SFS) 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 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 $T_{1}\cdot \Theta (-z)$. 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:

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

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

ITALICS Since T depends on x' and t only through the combination $x=x'+vt,$ we have:

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

Thus:

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

We now use the assumption that $v$ 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:

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

with the initial conditions

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

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

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

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) ITALICS can be calculated by integrating the thermal expansion over z:

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

where $\alpha _{\mathrm {eff} }$ is the effective volumetric thermal expansion coefficient, and ITALICS 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 $\kappa$ is small compared to $L^{2}/A$, where L is the ocean width (from mid-ocean ridges to continental shelf) and A is THE OCEAN BASIN age.

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

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

where $\rho \sim 3.3\ \mathrm {g} \cdot \mathrm {cm} ^{-3}$ is the rock density and $\rho _{0}=1\ \mathrm {g} \cdot \mathrm {cm} ^{-3}$ is the density of water.

By substituting the parameters by their rough estimates:

${\begin{aligned}\kappa &\sim 8\cdot 10^{-7}\ \mathrm {m} ^{2}\cdot \mathrm {s} ^{-1}\\\alpha &\sim 4\cdot 10^{-5}\ {}^{\circ }\mathrm {C} ^{-1}\\T_{1}&\sim 1220\ {}^{\circ }\mathrm {C} &&{\text{for the Atlantic and Indian oceans}}\\T_{1}&\sim 1120\ {}^{\circ }\mathrm {C} &&{\text{for the eastern Pacific}}\end{aligned}}$

we have:

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

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 ~ Ax/L, where L is the distance between the ridge to the continental shelf (roughly half the ocean width), and A is the OLDEST AGE OF THE OCEAN CRUST ocean age.

********************* below by BPL

Rather than height of the ocean floor $h(t)$above a base or reference level $h_{b}$, the depth of the ocean $d(t)$is of interest. Because $d(t)+h(t)=h_{b}$(with $h_{b}$ measured from the ocean surface) we can find that:

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

Cooling plate model

The depth predicted by the square root of seafloor age derived above is too deep for seafloor older than 80 million years. (P&S) Depth is better explained by a cooling lithosphere plate model rather than the cooling mantle half-space. (P&S) 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 to estimate model parameters (for the North Pacific):

~125 km for lithosphere thickness

$T_{1}\thicksim 1350\ {}^{\circ }\mathrm {C}$ at base and young edge of plate

$\alpha \thicksim 3.2\cdot 10^{-5}\ {}^{\circ }\mathrm {C} ^{-1}$

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

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

Thus older seafloor deepens more slowly than younger and in fact can be assumed almost constant at ~6400 m depth. Parsons and Sclater concluded that some style of mantle convection must apply heat to the base of the plate everywhere to prevent cooling down below 125 km and lithosphere contraction (seafloor deepening) at older ages. (P&S)

Their plate model also allowed an expression for conductive heat flow, q(t) from the ocean floor, which is approximately constant at $1\cdot 10^{-6}\mathrm {cal} \,\mathrm {cm} ^{-2}\mathrm {sec} ^{-1}$ beyond 120 million years: (P&S)

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

Unlinked references

1. Searle, Roger (2013). Mid-ocean ridges. New York: Cambridge. ISBN 9781107017528. OCLC 842323181.
2. "What is the longest mountain range on earth?". Ocean Facts. NOAA. Retrieved 17 October 2014.
3. Roger Buck, W.; Delaney, T.; Karson, A.; Lagabrielle, Yves (1998). Faulting and Magmatism at Mid‐Ocean Ridges | Geophysical Monograph Series. Washington DC American Geophysical Union Geophysical Monograph Series. Geophysical Monograph Series. 106. Bibcode:1998GMS...106.....R. doi:10.1029/gm106. ISBN 9781118664506.