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

A mid-ocean ridge (MOR) is an underwater mountain system formed by plate tectonics, the longest mountain range on Earth. It consists of various mountains linked in chains, typically having a valley known as a rift running along its spine. This type of oceanic mountain ridge is characteristic of what is known as an 'oceanic spreading center', which is responsible for seafloor spreading. The production of new seafloor results from mantle upwelling in response to plate spreading; this isentropic upwelling solid mantle material eventually exceeds the solidus and melts. The buoyant melt rises as magma at a linear weakness in the oceanic crust, and emerges as lava, creating new crust upon cooling. A mid-ocean ridge demarcates the boundary between two tectonic plates, and consequently is termed a divergent plate boundary.

Oceanic crust is the uppermost layer of the oceanic portion of a tectonic plate. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramafic cumulates. The crust overlies the solidified and uppermost layer of the mantle. The crust and the solid mantle layer together constitute oceanic lithosphere.

A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.

## History of study

Earlier theories (e.g. by Alfred Wegener and Alexander du Toit) of continental drift postulated that continents "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 Hess from Princeton University in the 1960s. [1] The theory is well accepted now, and the phenomenon is known to be caused by convection currents in the asthenosphere, which is ductile, or plastic, and the brittle lithosphere (crust and upper mantle). [2] [ clarification needed ]

Alfred Lothar Wegener was a German polar researcher, geophysicist and meteorologist.

Alexander Logie du Toit FRS was a geologist from South Africa and an early supporter of Alfred Wegener's theory of continental drift.

A continent is one of several very large landmasses of the world. Generally identified by convention rather than any strict criteria, up to seven regions are commonly regarded as continents. Ordered from largest in area to smallest, they are: Asia, Africa, North America, South America, Antarctica, Europe, and Australia.

## Significance

Continental drift is the theory that the Earth's continents have moved over geologic time relative to each other, thus appearing to have "drifted" across the ocean bed. The speculation that continents might have 'drifted' was first put forward by Abraham Ortelius in 1596. The concept was independently and more fully developed by Alfred Wegener in 1912, but his theory was rejected by many for lack of any motive mechanism. Arthur Holmes later proposed mantle convection for that mechanism. The idea of continental drift has since been subsumed by the theory of plate tectonics, which explains that the continents move by riding on plates of the Earth's lithosphere.

Plate tectonics is a scientific theory describing the large-scale motion of seven large plates and the movements of a larger number of smaller plates of the Earth's lithosphere, since tectonic processes began on Earth between 3 and 3.5 billion years ago. The model builds on the concept of continental drift, an idea developed during the first decades of the 20th century. The geoscientific community accepted plate-tectonic theory after seafloor spreading was validated in the late 1950s and early 1960s.

In plate tectonics, a divergent boundary or divergent plate boundary is a linear feature that exists between two tectonic plates that are moving away from each other. Divergent boundaries within continents initially produce rifts which eventually become rift valleys. Most active divergent plate boundaries occur between oceanic plates and exist as mid-oceanic ridges. Divergent boundaries also form volcanic islands which occur when the plates move apart to produce gaps which molten lava rises to fill.

Seafloor spreading occurs at spreading centers, distributed along the crests of mid-ocean ridges. Spreading centers end in transform faults or in overlapping spreading center offsets. A spreading center includes a seismically active plate boundary zone a few kilometers to tens of kilometers wide, a crustal accretion zone within the boundary zone where the ocean crust is youngest, and an instantaneous plate boundary - a line within the crustal accretion zone demarcating the two separating plates. [6] Within the crustal accretion zone is a 1-2 km-wide neovolcanic zone where active volcanism occurs. [7] [8]

A transform fault or transform boundary is a plate boundary where the motion is predominantly horizontal. It ends abruptly and is connected to another transform, a spreading ridge, or a subduction zone.

Overlapping spreading centers (OSC) are a feature of spreading centers at mid-ocean ridges.

In the general case, seafloor spreading starts as a rift in a continental land mass, similar to the Red Sea-East Africa Rift System today. [9] 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 junctions and can be found 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.

Isostasy is the state of gravitational equilibrium between Earth's crust and mantle such that the crust "floats" at an elevation that depends on its thickness and density.

A triple junction is the point where the boundaries of three tectonic plates meet. At the triple junction each of the three boundaries will be one of 3 types - a ridge (R), trench (T) or transform fault (F) - and triple junctions can be described according to the types of plate margin that meet at them. Of the many possible types of triple junction only a few are stable through time. The meeting of 4 or more plates is also theoretically possible but junctions will only exist instantaneously.

A passive margin is the transition between oceanic and continental lithosphere that is not an active plate margin. A passive margin forms by sedimentation above an ancient rift, now marked by transitional lithosphere. Continental rifting creates new ocean basins. Eventually the continental rift forms a mid-ocean ridge and the locus of extension moves away from the continent-ocean boundary. The transition between the continental and oceanic lithosphere that was originally created by rifting is known as a passive margin.

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 Ethiopian Afar Geophysical Lithospheric Experiment [10] reported that in the Afar region, September 2005, a 60 km fissure opened as wide as eight meters. [11] 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.

The Red Sea is a seawater inlet of the Indian Ocean, lying between Africa and Asia. The connection to the ocean is in the south through the Bab el Mandeb strait and the Gulf of Aden. To the north lie the Sinai Peninsula, the Gulf of Aqaba, and the Gulf of Suez. The Red Sea is a Global 200 ecoregion. The sea is underlain by the Red Sea Rift which is part of the Great Rift Valley.

Ethiopia, officially the Federal Democratic Republic of Ethiopia, is a country in the northeastern part of Africa, popularly known as the Horn of Africa. It shares borders with Eritrea to the north, Djibouti to the northeast, and Somalia to the east, Sudan to the northwest, South Sudan to the west, and Kenya to the south. With over 102 million inhabitants, Ethiopia is the most populous landlocked country in the world and the second-most populous nation on the African continent that covers a total area of 1,100,000 square kilometres (420,000 sq mi). Its capital and largest city is Addis Ababa, which lies a few miles west of the East African Rift that splits the country into the Nubian Plate and the Somali Plate.

Petroleum is a naturally occurring, yellowish-black liquid found in geological formations beneath the Earth's surface. It is commonly refined into various types of fuels. Components of petroleum are separated using a technique called fractional distillation, i.e. separation of a liquid mixture into fractions differing in boiling point by means of distillation, typically using a fractionating column.

Seafloor 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.

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 Ring of Fire of the Pacific Ocean. The Pacific is also home to one of the world's most active spreading centers (the East Pacific Rise) with spreading rates of up to 13  cm/yr. The Mid-Atlantic Ridge is a "textbook" slow-spreading center, while the East Pacific Rise is used as an example of fast spreading. Spreading centers at slow and intermediate rates exhibit a rift valley while at fast rates an axial high is found within the crustal accretion zone. [4] The differences in spreading rates affect not only the geometries of the ridges but also the geochemistry of the basalts that are produced. [12]

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 rises through the faults between oceanic plates to form new 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 by the theory of plate tectonics. In the 1960s, the past record of geomagnetic reversals was noticed by observing the magnetic stripe "anomalies" on the ocean floor. [13] 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 margins 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. [14] [15] 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

The depth of the seafloor (or the height of a location on a mid-ocean ridge above a baselevel) 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 [16] [17] or mantle half-space in areas without significant subduction. [18]

In the half-space model, [18] 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 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 ${\displaystyle T_{1}\cdot \Theta (-z)}$. Finally, we assume the system is at a quasi-steady state, so that the temperature distribution is constant in time, i.e. ${\displaystyle T=T(x,z).}$

By calculating in the frame of reference of the moving lithosphere (velocity v), which have spatial coordinate ${\displaystyle x'=x-vt,}$ we may write ${\displaystyle T=T(x',z,t).}$ and use the heat equation:

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

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

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

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

Thus:

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

We now use the assumption that ${\displaystyle v}$ 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:

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

with the initial conditions

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

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

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

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

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

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

Note that the assumption the v is relatively large is equivalently to the assumption that the thermal diffusivity ${\displaystyle \kappa }$ is small compared to ${\displaystyle L^{2}/T}$, where L is the ocean width (from mid-ocean ridges to continental shelf) and T is its age.

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 above the lithosphere as it expands or retracts. Both coefficients are related by:

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

where ${\displaystyle \rho \sim 3.3\ \mathrm {g} \cdot \mathrm {cm} ^{-3}}$ is the rock 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 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:

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

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 ~ 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.

## Related Research Articles

A lithosphere is the rigid, outermost shell of a terrestrial-type planet, or natural satellite, that is defined by its rigid mechanical properties. On Earth, it is composed of the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater. The outermost shell of a rocky planet, the crust, is defined on the basis of its chemistry and mineralogy.

A convergent boundary is an area on Earth where two or more lithospheric plates collide. One plate eventually slides beneath the other causing a process known as subduction. The subduction zone can be defined by a plane where many earthquakes occur, called the Benioff Zone. These collisions happen on scales of millions to tens of millions of years and can lead to volcanism, earthquakes, orogenesis, destruction of lithosphere, and deformation. Convergent boundaries occur between oceanic-oceanic lithosphere, oceanic-continental lithosphere, and continental-continental lithosphere. The geologic features related to convergent boundaries vary depending on crust types.

The East Pacific Rise is a mid-oceanic ridge, a divergent tectonic plate boundary located along the floor of the Pacific Ocean. It separates the Pacific Plate to the west from the North American Plate, the Rivera Plate, the Cocos Plate, the Nazca Plate, and the Antarctic Plate. It runs south from the Gulf of California in the Salton Sea basin in Southern California to a point near 55° S, 130° W, where it joins the Pacific-Antarctic Ridge trending west-southwest towards Antarctica, near New Zealand. Much of the rise lies about 3200 km (2000 mi) off the South American coast and rises about 1,800–2,700 m (6,000–9,000 ft) above the surrounding seafloor.

The supercontinent cycle is the quasi-periodic aggregation and dispersal of Earth's continental crust. There are varying opinions as to whether the amount of continental crust is increasing, decreasing, or staying about the same, but it is agreed that the Earth's crust is constantly being reconfigured. One complete supercontinent cycle is said to take 300 to 500 million years. Continental collision makes fewer and larger continents while rifting makes more and smaller continents.

Extensional tectonics is concerned with the structures formed by, and the tectonic processes associated with, the stretching of a planetary body's crust or lithosphere.

Back-arc basins are geologic basins, submarine features associated with island arcs and subduction zones. They are found at some convergent plate boundaries, presently concentrated in the western Pacific Ocean. Most of them result from tensional forces caused by oceanic trench rollback and the collapse of the edge of the continent. The arc crust is under extension or rifting as a result of the sinking of the subducting slab. Back-arc basins were initially a surprising result for plate tectonics theorists, who expected convergent boundaries to be zones of compression, rather than major extension. However, they are now recognized as consistent with this model in explaining how the interior of Earth loses heat.

The Izu–Bonin–Mariana (IBM) arc system is a tectonic-plate convergent boundary. The IBM arc system extends over 2800 km south from Tokyo, Japan, to beyond Guam, and includes the Izu Islands, Bonin Islands, and Mariana Islands; much more of the IBM arc system is submerged below sealevel. The IBM arc system lies along the eastern margin of the Philippine Sea Plate in the Western Pacific Ocean. It is the site of the deepest gash in Earth's solid surface, the Challenger Deep in the Mariana Trench.

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The Red Sea Rift is a spreading center between two tectonic plates, the African Plate and the Arabian Plate. It extends from the Dead Sea Transform fault system, and ends at an intersection with the Aden Ridge and the East African Rift, forming the Afar Triple Junction in the Afar Depression of the Horn of Africa.

In fluid dynamics, the mild-slope equation describes the combined effects of diffraction and refraction for water waves propagating over bathymetry and due to lateral boundaries—like breakwaters and coastlines. It is an approximate model, deriving its name from being originally developed for wave propagation over mild slopes of the sea floor. The mild-slope equation is often used in coastal engineering to compute the wave-field changes near harbours and coasts.

This is a list of articles related to plate tectonics and tectonic plates.

The Aegir Ridge is an extinct segment of the Mid-Atlantic Ridge in the far-northern Atlantic Ocean. It marks the initial break-up boundary between Greenland and Norway, along which seafloor spreading was initiated at the beginning of the Eocene epoch to form the northern Atlantic Ocean. Towards the end of the Eocene, the newly forming Kolbeinsey Ridge propagated northwards from Iceland, splitting the Jan Mayen Microcontinent away from the Greenland Plate. As the Kolbeinsey Ridge formed, so activity on the Aegir Ridge reduced, ceasing completely at the end of the Oligocene epoch when the Kolbeinsey Ridge reached the Jan Mayen Fracture Zone.

The South China Sea Basin is one of the largest marginal basins in Asia. South China Sea is located to the east of Vietnam, west of Philippines and the Luzon Strait, and north of Borneo. Tectonically, it is surrounded by the Indochina Block on the west, Philippines Sea plate on the east, Yangtze Block to the north. A subduction boundary exists between the Philippines Sea Plate and the Asian Plate. The formation of the South China Sea Basin was closely related with the collision between the Indian Plate and Eurasian Plates. The collision thickened the continental crust and changed the elevation of the topography from the Himalayan orogenic zone to the South China Sea, especially around the Tibetan Plateau. The location of the South China Sea makes it a product of several tectonic events. All the plates around the South China Sea Basin underwent clockwise rotation, subduction and experienced an extrusion process from the early Cenozoic to the Late Miocene.

Propagating rifts are seafloor features associated with spreading centers at mid-ocean ridges and back-arc basins. They are more commonly observed on faster rate spreading centers. These features are formed by the lengthening of one spreading segment at the expense of an offset neighboring spreading segment. Hence, these are remnant features produced by migration of the tip of a spreading center. In other words, as the tip of a spreading center migrates or grows, the plate itself grows at the expense of the shrinking plate, transferring lithosphere from the shrinking plate to the growing plate.

## References

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math>\kappa \sim 8\cdot 10^{-7}