Seafloor spreading

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.

Age of oceanic lithosphere
Age of oceanic lithosphere; youngest (red) is along spreading centers.

History of study

Earlier theories by Alfred Wegener and Alexander du Toit of continental drift postulated that continents in motion "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 central rift axis 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.[1][2] The phenomenon is known today as plate tectonics. In locations where two plates move apart, at mid-ocean ridges, new seafloor is continually formed during seafloor spreading.

Significance

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 not subducting are driven by gravity sliding off the elevated mid-ocean ridges a process called ridge push.[4] At a spreading center, basaltic magma rises up the fractures and cools on the ocean floor to form new seabed. Hydrothermal vents 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. 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 rate). 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 mmmm/year. Intermediate ridges have a spreading rate of 40–90 mm/year while slow spreading ridges have a rate less than 40 mm/year.[5][6][7]:2

Spreading center

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.[8] Within the crustal accretion zone is a 1-2 km-wide neovolcanic zone where active volcanism occurs.[9][10]

Incipient spreading

Plates tect2 en
Plates in the crust of the earth, according to the plate tectonics theory

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.[11] 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.

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' or aulacogen. 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 and upper mantle lithosphere 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 more slowly than the other two arms, but in 2005 the Ethiopian Afar Geophysical Lithospheric Experiment[12] reported that in the Afar region, September 2005, a 60 km fissure opened as wide as eight meters.[13] 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.

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.[14]

Continued spreading and subduction

Ridge render
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 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 145 +/- 4 mm/yr between the Pacific and Nazca plates.[15] The Mid-Atlantic Ridge is a slow-spreading center, while the East Pacific Rise is 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.[6] The differences in spreading rates affect not only the geometries of the ridges but also the geochemistry of the basalts that are produced.[16]

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 mid-ocean ridges), 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 plowed 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.

At first the driving force for spreading was argued to be convection currents in the mantle.[17] Since then, it has been shown that the motion of the continents is linked to seafloor spreading by the theory of plate tectonics.[4] In the 1960s, the past record of geomagnetic reversals was noticed by observing the magnetic stripe "anomalies" on the ocean floor.[18] This results in broadly evident "stripes" from which the past magnetic field polarity can be inferred by looking at the data gathered from 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, or slab pull. The magmatism at the ridge is considered to be passive upwelling, which is caused by the plates being pulled apart under the weight of their own slabs.[4][19] 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. However, the Mid-Atlantic ridge itself is not bordered by plates that are being pulled into subduction zones. In this case the plate are sliding apart over the mantle upwelling in the process of ridge push.[4]

Polarityshift
magnetic stripes formed during seafloor spreading

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 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[20][21] or mantle half-space in areas without significant subduction.[22]

In the half-space model,[22] the seabed height is determined by the oceanic lithosphere 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.[22] 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 . Finally, we assume the system is at a quasi-steady state, so that the temperature distribution is constant in time, i.e.

By calculating in the frame of reference of the moving lithosphere (velocity v), which have spatial coordinate we may write and use the heat equation:

where is the thermal diffusivity of the mantle lithosphere.

Since T depends on x' and t only through the combination we have:

Thus:

We now use the assumption that 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:

with the initial conditions

The solution for is given by the error function:

.

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:

where 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 is small compared to , where L is the ocean width (from mid-ocean ridges to continental shelf) and T is its age.

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

where is the rock density and is the density of water.

By substituting the parameters by their rough estimates:

we have:

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

References

  1. ^ Hess, H. H. (November 1962). "History of Ocean Basins" (PDF). In A. E. J. Engel; Harold L. James; B. F. Leonard (eds.). Petrologic studies: a volume to honor A. F. Buddington. Boulder, CO: Geological Society of America. pp. 599–620.
  2. ^ Dietz, Robert S. (1961). "Continent and Ocean Basin Evolution by Spreading of the Sea Floor". Nature. 190 (4779): 854–857. doi:10.1038/190854a0. ISSN 0028-0836.
  3. ^ Tan, Yen Joe; Tolstoy, Maya; Waldhauser, Felix; Wilcock, William S. D. (2016). "Dynamics of a seafloor-spreading episode at the East Pacific Rise". Nature. 540 (7632): 261–265. Bibcode:2016Natur.540..261T. doi:10.1038/nature20116. PMID 27842380.
  4. ^ a b c d Forsyth, Donald; Uyeda, Seiya (1975-10-01). "On the Relative Importance of the Driving Forces of Plate Motion". Geophysical Journal International. 43 (1): 163–200. Bibcode:1975GeoJ...43..163F. doi:10.1111/j.1365-246x.1975.tb00631.x. ISSN 0956-540X.
  5. ^ Macdonald, Ken C. (2019), "Mid-Ocean Ridge Tectonics, Volcanism, and Geomorphology", Encyclopedia of Ocean Sciences, Elsevier, pp. 405–419, doi:10.1016/b978-0-12-409548-9.11065-6, ISBN 9780128130827, retrieved 2019-09-15
  6. ^ a b Macdonald, K. C. (1982). "Mid-Ocean Ridges: Fine Scale Tectonic, Volcanic and Hydrothermal Processes Within the Plate Boundary Zone". Annual Review of Earth and Planetary Sciences. 10 (1): 155–190. Bibcode:1982AREPS..10..155M. doi:10.1146/annurev.ea.10.050182.001103.
  7. ^ Searle, Roger (2013). Mid-ocean ridges. New York: Cambridge. ISBN 9781107017528. OCLC 842323181.
  8. ^ Luyendyk, Bruce P.; Macdonald, Ken C. (1976-06-01). "Spreading center terms and concepts". Geology. 4 (6): 369. Bibcode:1976Geo.....4..369L. doi:10.1130/0091-7613(1976)4<369:sctac>2.0.co;2. ISSN 0091-7613.
  9. ^ Daignieres, Marc; Courtillot, Vincent; Bayer, Roger; Tapponnier, Paul (1975). "A model for the evolution of the axial zone of mid-ocean ridges as suggested by icelandic tectonics". Earth and Planetary Science Letters. 26 (2): 222–232. Bibcode:1975E&PSL..26..222D. doi:10.1016/0012-821x(75)90089-8.
  10. ^ McClinton, J. Timothy; White, Scott M. (2015-03-01). "Emplacement of submarine lava flow fields: A geomorphological model from the Niños eruption at the Galápagos Spreading Center". Geochemistry, Geophysics, Geosystems. 16 (3): 899–911. Bibcode:2015GGG....16..899M. doi:10.1002/2014gc005632. ISSN 1525-2027.
  11. ^ Makris, J.; Ginzburg, A. (1987-09-15). "Sedimentary basins within the Dead Sea and other rift zones The Afar Depression: transition between continental rifting and sea-floor spreading". Tectonophysics. 141 (1): 199–214. Bibcode:1987Tectp.141..199M. doi:10.1016/0040-1951(87)90186-7.
  12. ^ Bastow, Ian D.; Keir, Derek; Daly, Eve (2011-06-01). The Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE): Probing the transition from continental rifting to incipient seafloor spreading. Geological Society of America Special Papers. 478. pp. 51–76. doi:10.1130/2011.2478(04). hdl:2158/1110145. ISBN 978-0-8137-2478-2. ISSN 0072-1077.
  13. ^ Grandin, R.; Socquet, A.; Binet, R.; Klinger, Y.; Jacques, E.; Chabalier, J.-B. de; King, G. C. P.; Lasserre, C.; Tait, S. (2009-08-01). "September 2005 Manda Hararo-Dabbahu rifting event, Afar (Ethiopia): Constraints provided by geodetic data" (PDF). Journal of Geophysical Research. 114 (B8): B08404. Bibcode:2009JGRB..114.8404G. doi:10.1029/2008jb005843. ISSN 2156-2202.
  14. ^ Burke, K (1977-05-01). "Aulacogens and Continental Breakup". Annual Review of Earth and Planetary Sciences. 5 (1): 371–396. doi:10.1146/annurev.ea.05.050177.002103. ISSN 0084-6597.
  15. ^ DeMets, Charles; Gordon, Richard G.; Argus, Donald F. (2010). "Geologically current plate motions". Geophysical Journal International. 181 (1): 52. doi:10.1111/j.1365-246X.2009.04491.x.
  16. ^ Bhagwat, S.B. (2009). Foundation of Geology Vol 1. Global Vision Publishing House. p. 83. ISBN 9788182202764.
  17. ^ Elsasser, Walter M. (1971-02-10). "Sea-floor spreading as thermal convection". Journal of Geophysical Research. 76 (5): 1101–1112. doi:10.1029/JB076i005p01101.
  18. ^ Vine, F. J.; Matthews, D. H. (1963). "Magnetic Anomalies Over Oceanic Ridges". Nature. 199 (4897): 947–949. Bibcode:1963Natur.199..947V. doi:10.1038/199947a0.
  19. ^ 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.
  20. ^ Sclater, John G.; Anderson, Roger N.; Bell, M. Lee (1971-11-10). "Elevation of ridges and evolution of the central eastern Pacific". Journal of Geophysical Research. 76 (32): 7888–7915. Bibcode:1971JGR....76.7888S. doi:10.1029/jb076i032p07888. ISSN 2156-2202.
  21. ^ Parsons, Barry; Sclater, John G. (1977-02-10). "An analysis of the variation of ocean floor bathymetry and heat flow with age". Journal of Geophysical Research. 82 (5): 803–827. Bibcode:1977JGR....82..803P. doi:10.1029/jb082i005p00803. ISSN 2156-2202.
  22. ^ a b c Davis, E.E; Lister, C. R. B. (1974). "Fundamentals of Ridge Crest Topography". Earth and Planetary Science Letters. 21 (4): 405–413. Bibcode:1974E&PSL..21..405D. doi:10.1016/0012-821X(74)90180-0.

External links

Back-arc basin

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 (the oceanic trench is wandering in the seafloor direction) 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.

Burma Plate

The Burma Plate is a minor tectonic plate or microplate located in Southeast Asia, sometimes considered a part of the larger Eurasian Plate. The Andaman Islands, Nicobar Islands, and northwestern Sumatra are located on the plate. This island arc separates the Andaman Sea from the main Indian Ocean to the west.

To its east lies the Sunda Plate, from which it is separated along a transform boundary, running in a rough north-south line through the Andaman Sea. This boundary between the Burma and Sunda plates is a marginal seafloor spreading centre, which has led to the opening up of the Andaman Sea (from a southerly direction) by "pushing out" the Andaman-Nicobar-Sumatra island arc from mainland Asia, a process which began in earnest approximately 4 million years ago.

To the west is the much larger India Plate, which is subducting beneath the western facet of the Burma Plate. This extensive subduction zone has formed the Sunda Trench.

Calcite sea

A calcite sea is one in which low-magnesium calcite is the primary inorganic marine calcium carbonate precipitate. An aragonite sea is the alternate seawater chemistry in which aragonite and high-magnesium calcite are the primary inorganic carbonate precipitates. The Early Paleozoic and the Middle to Late Mesozoic oceans were predominantly calcite seas, whereas the Middle Paleozoic through the Early Mesozoic and the Cenozoic (including today) are characterized by aragonite seas ).

The most significant geological and biological effects of calcite sea conditions include rapid and widespread formation of carbonate hardgrounds , calcitic ooids , calcite cements, and the contemporaneous dissolution of aragonite shells in shallow warm seas. Hardgrounds were very common, for example, in the calcite seas of the Ordovician and Jurassic, but virtually absent from the aragonite seas of the Permian.Fossils of invertebrate organisms found in calcite sea deposits are usually dominated by either thick calcite shells and skeletons, were infaunal and/or had thick periostraca, or had an inner shell of aragonite and an outer shell of calcite. This was apparently because aragonite dissolved quickly on the seafloor and had to be either avoided or protected as a biomineral.Calcite seas were coincident with times of rapid seafloor spreading and global greenhouse climate conditions. Seafloor spreading centers cycle seawater through hydrothermal vents, reducing the ratio of magnesium to calcium in the seawater through metamorphism of calcium-rich minerals in basalt to magnesium-rich clays. This reduction in the Mg/Ca ratio favors the precipitation of calcite over aragonite. Increased seafloor spreading also means increased volcanism and elevated levels of carbon dioxide in the atmosphere and oceans. This may also have an effect on which polymorph of calcium carbonate is precipitated. Further, high calcium concentrations of seawater favor the burial of CaCO3, thereby removing alkalinity from the ocean, lowering seawater pH and reducing its acid/base buffering.

Central Basin Spreading Center

Central Basin Spreading Center (formerly Central Basin Fault) is a seafloor spreading center of the West Philippine Sea Basin.

Drummond Matthews

Drummond Hoyle Matthews FRS (5 February 1931 – 20 July 1997), known as "Drum", was a British marine geologist and geophysicist and a key contributor to the theory of plate tectonics. His work, along with that of fellow Briton Fred Vine and Canadian Lawrence Morley, showed how variations in the magnetic properties of rocks forming the ocean floor could be consistent with, and ultimately help confirm, Harry Hammond Hess's 1962 theory of seafloor spreading. In 1989 he was awarded the Geological Society of London's highest honour, the Wollaston Medal.

Easter Microplate

Easter Plate is located to the west of Easter Island off the west coast of South America in the middle of the Pacific Ocean, bordering the Nazca plate to the east and the Pacific plate to the west. It was discovered from looking at earthquake distributions that were offset from the previously perceived Nazca-Pacific Divergent boundary. This young plate is 5.25 million years old and is considered a microplate because it is small with an area of approximately 160,000 km2. Seafloor spreading along the Easter microplate's borders have some of the highest global rates, ranging from 50 to 140 mm/yr.

Geodynamics

Geodynamics is a subfield of geophysics dealing with dynamics of the Earth. It applies physics, chemistry and mathematics to the understanding of how mantle convection leads to plate tectonics and geologic phenomena such as seafloor spreading, mountain building, volcanoes, earthquakes, faulting and so on. It also attempts to probe the internal activity by measuring magnetic fields, gravity, and seismic waves, as well as the mineralogy of rocks and their isotopic composition. Methods of geodynamics are also applied to exploration of other planets.

Gulf of California Rift Zone

The Gulf of California Rift Zone (GCRZ) is the northernmost extension of the East Pacific Rise which extends some 1,300 km (800 mi) from the mouth of the Gulf of California to the southern terminus of the San Andreas Fault at the Salton Sink.

The GCRZ is an incipient rift zone akin to the Red Sea Rift. In the GCRZ continental crust originally associated with the North American Plate has been pulled apart by tectonic forces and is being replaced by newly formed oceanic crust and seafloor spreading. The rifting has resulted in the transfer of the Baja California Peninsula to the Pacific Plate.

Hydrosphere

The hydrosphere (from Greek ὕδωρ hydōr, "water" and σφαῖρα sphaira, "sphere") is the combined mass of water found on, under, and above the surface of a planet, minor planet or natural satellite. Although Earth's hydrosphere has been around for longer than 4 billion years, it continues to change in size. This is caused by seafloor spreading and continental drift, which rearranges the land and ocean.It has been estimated that there are 1,386 million cubic kilometres (333,000,000 cubic miles) of water on Earth. This includes water in liquid and frozen forms in groundwater, oceans, lakes and streams. Saltwater accounts for 97.5% of this amount, whereas fresh water accounts for only 2.5%. Of this fresh water, 68.9% is in the form of ice and permanent snow cover in the Arctic, the Antarctic and mountain glaciers; 30.8% is in the form of fresh groundwater; and only 0.3% of the fresh water on Earth is in easily accessible lakes, reservoirs and river systems.The total mass of Earth's hydrosphere is about 1.4 × 1018 tonnes, which is about 0.023% of Earth's total mass. At any given time, about 20 × 1012 tonnes of this is in the form of water vapor in the Earth's atmosphere (for practical purposes, 1 cubic meter of water weighs one tonne). Approximately 71% of Earth's surface, an area of some 361 million square kilometers (139.5 million square miles), is covered by ocean. The average salinity of Earth's oceans is about 35 grams of salt per kilogram of sea water (3.5%).

Magnetic anomaly

In geophysics, a magnetic anomaly is a local variation in the Earth's magnetic field resulting from variations in the chemistry or magnetism of the rocks. Mapping of variation over an area is valuable in detecting structures obscured by overlying material. The magnetic variation in successive bands of ocean floor parallel with mid-ocean ridges is important evidence supporting the theory of seafloor spreading, central to plate tectonics.

Marine transgression

A marine transgression is a geologic event during which sea level rises relative to the land and the shoreline moves toward higher ground, resulting in flooding. Transgressions can be caused either by the land sinking or the ocean basins filling with water (or decreasing in capacity). Transgressions and regressions may be caused by tectonic events such as orogenies, severe climate change such as ice ages or isostatic adjustments following removal of ice or sediment load.

During the Cretaceous, seafloor spreading created a relatively shallow Atlantic basin at the expense of deeper Pacific basin. This reduced the world's ocean basin capacity and caused a rise in sea level worldwide. As a result of this sea level rise, the oceans transgressed completely across the central portion of North America and created the Western Interior Seaway from the Gulf of Mexico to the Arctic Ocean.

The opposite of transgression is regression, in which the sea level falls relative to the land and exposes former sea bottom. During the Pleistocene Ice Age, so much water was removed from the oceans and stored on land as year-round glaciers that the ocean regressed 120 m, exposing the Bering land bridge between Alaska and Asia.

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 along 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 and the outline of the ridges across the ocean floor appears similar to the seam of a baseball. The mid-ocean ridge system thus is the longest mountain range on Earth, reaching about 65,000 km (40,000 mi).

Ocean chemistry

Ocean chemistry, also known as marine chemistry, is influenced by plate tectonics and seafloor spreading, turbidity currents, sediments, pH levels, atmospheric constituents, metamorphic activity, and ecology. The field of chemical oceanography studies the chemistry of marine environments including the influences of different variables.

Paleomagnetism

Paleomagnetism (or palaeomagnetism in the United Kingdom) is the study of the record of the Earth's magnetic field in rocks, sediment, or archeological materials. Certain minerals in rocks lock-in a record of the direction and intensity of the magnetic field when they form. This record provides information on the past behavior of Earth's magnetic field and the past location of tectonic plates. The record of geomagnetic reversals preserved in volcanic and sedimentary rock sequences (magnetostratigraphy) provides a time-scale that is used as a geochronologic tool. Geophysicists who specialize in paleomagnetism are called paleomagnetists.

Paleomagnetists led the revival of the continental drift hypothesis and its transformation into plate tectonics. Apparent polar wander paths provided the first clear geophysical evidence for continental drift, while marine magnetic anomalies did the same for seafloor spreading. Paleomagnetism continues to extend the history of plate tectonics back in time and are applied to the movement of continental fragments, or terranes.

Paleomagnetism relied heavily on new developments in rock magnetism, which in turn has provided the foundation for new applications of magnetism. These include biomagnetism, magnetic fabrics (used as strain indicators in rocks and soils), and environmental magnetism.

Progradation

In sedimentary geology and geomorphology, the term progradation refers to the growth of a river delta farther out into the sea over time. This occurs when the mass balance of sediment into the delta is such that the volume of incoming sediment is greater than the volume of the delta that is lost through subsidence, sea-level rise, and/or erosion.

Progradation can be caused by:

Periods of sea-level fall which result in marine regression. This can occur during major continental glaciations within ice ages, be caused by changes in the rates of seafloor spreading that affects the volume of the ocean basins, or tectonic effects on the regional mantle density structure that can change the geoid elevation.

Extremely high sediment input, such as by the Huang He (Yellow River) in China, which drains the Loess plateau, or from high sediment loads in proglacial rivers.

Sheeted dyke complex

A sheeted dyke complex or sheeted dike complex is a normal component of an ophiolite, a piece of oceanic crust that has been emplaced within a sequence of continental rocks. In the original formation environment below the sea floor the dykes acted as feeders for the overlying sequence of extrusive rocks, typically pillow lavas forming a layer of the oceanic crust . As each injection of a dyke represents one increment of seafloor spreading, each dyke was normally intruded into earlier dykes. The dykes are typically dolerites but plagiogranites (trondhjemites) often form a significant part of the complex.

South Greenland Triple Junction

The South Greenland Triple Junction was a geologic triple junction in the North Atlantic Ocean that divided the North American, Greenland and Eurasian plates. It existed during the Paleogene and consisted of the Mid-Labrador and Mid-Atlantic ridges. The triple junction became extinct when seafloor spreading along the Mid-Labrador Ridge ceased during the Eocene.

Tectonics of the South China Sea

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.

The geological history can be classified into five tectonic evolutionary stages. (1) rift system development (2) sea floor spreading, (3) subsidence of the South China Sea, (4) closure of the South China Sea Basin and (5) uplift of Taiwan.

Vine–Matthews–Morley hypothesis

The Vine–Matthews–Morley hypothesis, also known as the Morley–Vine–Matthews hypothesis, was the first key scientific test of the seafloor spreading theory of continental drift and plate tectonics.

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