Ridge push

Ridge push (also known as gravitational sliding) or sliding plate force is a proposed driving force for plate motion in plate tectonics that occurs at mid-ocean ridges as the result of the rigid lithosphere sliding down the hot, raised asthenosphere below mid-ocean ridges. Although it is called ridge push, the term is somewhat misleading; it is actually a body force that acts throughout an ocean plate, not just at the ridge, as a result of gravitational pull. The name comes from earlier models of plate tectonics in which ridge push was primarily ascribed to upwelling magma at mid-ocean ridges pushing or wedging the plates apart.

Mechanics

Ridge Push (Mid-ocean Ridge)
Diagram of a mid-ocean ridge showing ridge push near the mid-ocean ridge and the lack of ridge push after 90 Ma

Ridge push is the result of gravitational forces acting on the young, raised oceanic lithosphere around mid-ocean ridges, causing it to slide down the similarly raised but weaker asthenosphere and push on lithospheric material farther from the ridges.[1]

Mid-ocean ridges are long underwater mountain chains that occur at divergent plate boundaries in the ocean, where new oceanic crust is formed by upwelling mantle material as a result of tectonic plate spreading and relatively shallow (above ~60 km) decompression melting.[1] The upwelling mantle and fresh crust are hotter and less dense than the surrounding crust and mantle, but cool and contract with age until reaching equilibrium with older crust at around 90 Ma.[1][2][3] This produces an isostatic response that causes the young regions nearest the plate boundary to rise above older regions and gradually sink with age, producing the mid-ocean ridge morphology.[1] The greater heat at the ridge also weakens rock closer to the surface, raising the boundary between the brittle lithosphere and the weaker, ductile asthenosphere to create a similar elevated and sloped feature underneath the ridge.[3]

These raised features produce ridge push; gravity pulling down on the lithosphere at the mid-ocean ridge is mostly opposed by the normal force from the underlying rock, but the remainder acts to push the lithosphere down the sloping asthenosphere and away from the ridge.[1][3] Because the asthenosphere is weak, ridge push and other driving forces are enough to deform it and allow the lithosphere to slide over it, opposed by drag at the Lithosphere-Asthenosphere boundary and resistance to subduction at convergent plate boundaries.[3] Ridge push is mostly active in lithosphere younger than 90 Ma, after which it has cooled enough to reach thermal equilibrium with older material and the slope of the Lithosphere-Asthenosphere boundary becomes effectively zero.[2]

History

Early ideas (1912-1962)

Despite its current status as one of the driving forces of plate tectonics, ridge push was not included in any of Alfred Wegener's 1912-1930 proposals of continental drift, which were produced before the discovery of mid-ocean ridges and lacked any concrete mechanisms by which the process might have occurred.[4][5][6] Even after the development of acoustic depth sounding and the discovery of global mid-ocean ridges in the 1930s, the idea of a spreading force acting at the ridges was not mentioned in scientific literature until Harry Hess's proposal of seafloor spreading in 1960, which included a pushing force at mid-ocean ridges as a result of upwelling magma wedging the lithosphere apart.[4][7][8][9]

Gravitational models

In 1964 and 1965, Egon Orowan proposed the first gravitational mechanism for spreading at mid-ocean ridges, postulating that spreading can be derived from the principles of isostasy. In Orowan's proposal, pressure within and immediately under the elevated ridge is greater than the pressure in the oceanic crust to either side due to the greater weight of overlying rock, forcing material away from the ridge, while the lower density of the ridge material relative to the surrounding crust would gradually compensate for the greater volume of rock down to the depth of isostatic compensation.[10][11] Similar models were proposed by Lliboutry in 1969, Parsons and Richer in 1980, and others.[11] In 1969, Hales proposed a model in which the raised lithosphere of the mid-ocean ridges slid down the elevated ridge, and in 1970 Jacoby proposed that the less dense material and isostasy of Orowan and others' proposals produced uplift which resulted in sliding similar to Hales' proposal.[11] The term "ridge push force" was coined by Forsyth and Uyeda in 1975.[11][12]

Significance

Early models of plate tectonics, such as Harry Hess's seafloor spreading model, assumed that the motions of plates and the activity of mid-ocean ridges and subduction zones were primarily the result of convection currents in the mantle dragging on the crust and supplying fresh, hot magma at mid-ocean ridges.[4][7] Further developments of the theory suggested that some form of ridge push helped supplement convection in order to keep the plates moving, but in the 1990s, calculations indicated that slab pull, the force that a subducted section of plate exerts on the attached crust on the surface, was an order of magnitude stronger than ridge push.[1][4][6][10][11][12] As of 1996, slab pull was generally considered the dominant mechanism driving plate tectonics.[4][6][12] Modern research, however, indicates that the effects of slab pull are mostly negated by resisting forces in the mantle, limiting it to only 2-3 times the effective strength of ridge push forces in most plates, and that mantle convection is probably much too slow for drag between the lithosphere and the asthenosphere to account for the observed motion of the plates.[1][4][13] This restores ridge push as one of the dominant factors in plate motion.

Opposing forces

Ridge push is primarily opposed by plate drag, which is the drag force of the rigid lithosphere moving over the weaker, ductile asthenosphere.[3][14] Models estimate that ridge push is probably just sufficient to overcome plate drag and maintain the motion of the plate in most areas.[14][15] Slab pull is similarly opposed by resistance to the subduction of the lithosphere into the mantle at convergent plate boundaries.[3][14]

Notable qualifications

Research by Rezene Mahatsente indicates that the driving stresses caused by ridge push would be dissipated by faulting and earthquakes in plate material containing large quantities of unbound water, but they conclude that ridge push is still a significant driving force in existing plates because of the rarity of intraplate earthquakes in the ocean.[15]

In plates with particularly small or young subducting slabs, ridge push may be the predominant driving force in the plate's motion.[13][14] According to Stefanick and Jurdy, the ridge push force acting on the South American plate is approximately 5 times the slab pull forces acting at its subducting margins because of the small size of the subducting slabs at the Scotia and Caribbean margins.[14] The Nazca plate also experiences relatively small slab pull, approximately equal to its ridge push, because the plate material is young (no more than 50 million years old) and therefore less dense, with less tendency to sink into the mantle.[13] This also causes the subducting Nazca slab to experience flat slab subduction, one of the few places in the world where this currently occurs.[16]

References

  1. ^ a b c d e f g Turcotte, D.L.; Schubert, G. (2002). "Plate Tectonics". Geodynamics (2 ed.). Cambridge University Press. pp. 1–21. ISBN 0-521-66186-2.
  2. ^ a b Meijer, P.T.; Wortel, M.J.R.; Zoback, Mary Lou (1992). "The dynamics of motion of the South American Plate". Journal of Geophysical Research: Solid Earth. 97 (B8): 11915–11931. doi:10.1029/91JB01123.
  3. ^ a b c d e f DiVenere, Vic (May 21, 2017). "Driving Forces of Plate Motions". Columbia University, Earth and Space Sciences. Retrieved April 7, 2018.
  4. ^ a b c d e f Earle, Steven (2016). "Plate Tectonics". Physical Geology. CreateSpace Independent Publishing Platform. ISBN 9781537068824.
  5. ^ Hughes, Patrick (2007-08-15). "Wegener, Alfred Lothar (1880-1930)". Van Nostrand's Scientific Encyclopedia. Hoboken, NJ, USA: John Wiley & Sons, Inc. doi:10.1002/0471743984.vse9783. ISBN 978-0471743989.
  6. ^ a b c Kious, W. Jacquelyne; Tilling, Robert (1996). This Dynamic Earth: The Story of Plate Tectonics. Washington, D.C.: United States Govt Printing Office. ISBN 0-16-048220-8.
  7. ^ a b Hess, H. H. Petrologic Studies. USA: Geological Society of America. pp. 599–620. doi:10.1130/petrologic.1962.599. ISBN 0813770165.
  8. ^ "Harry Hess 1906-1969". PBS. 1998. Retrieved April 28, 2018.
  9. ^ "Hess proposes sea-floor spreading 1960". PBS. 1998. Retrieved April 28, 2018.
  10. ^ a b Orowan, E. (1964-11-20). "Continental Drift and the Origin of Mountains: Hot creep and creep fracture are crucial factors in the formation of continents and mountains". Science. 146 (3647): 1003–1010. doi:10.1126/science.146.3647.1003. ISSN 0036-8075. PMID 17832393.
  11. ^ a b c d e Bott, M.H.P. (1991). "Ridge push and associated plate interior stress in normal and hot spot regions". Tectonophysics. 200 (1–3): 17–32. doi:10.1016/0040-1951(91)90003-b.
  12. ^ a b c 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. doi:10.1111/j.1365-246x.1975.tb00631.x. ISSN 0956-540X.
  13. ^ a b c Richardson, R.M.; Cox, B.L. (1984). "Evolution of oceanic lithosphere: A driving force study of the Nazca Plate". Journal of Geophysical Research: Solid Earth. 89 (B12): 10043–10052. doi:10.1029/JB089iB12p10043.
  14. ^ a b c d e Stefanick, M; Jurdy, D.M. (1992). "Stress observations and driving force models for the South American Plate". Journal of Geophysical Research: Solid Earth. 97 (B8): 11905–11913. doi:10.1029/91JB01798.
  15. ^ a b Mahatsente, R (2017). "Global Models of Ridge-Push Force, Geoid, and Lithospheric Strength of Oceanic plates". Pure and Applied Geophysics. 174 (12): 4395–4406. doi:10.1007/s00024-017-1647-2.
  16. ^ Gutscher, M.A.; Spakman, W.; Bijwaard, H.; Engdalh, E.R. (2000). "Geodynamics of flat subduction: Seismicity and tomographic constraints from the Andean margin". Tectonics. 19 (5): 814–833. doi:10.1029/1999TC001152.
Bahama Banks

The Bahama Banks are the submerged carbonate platforms that make up much of the Bahama Archipelago. The term is usually applied in referring to either the Great Bahama Bank around Andros Island, or the Little Bahama Bank of Grand Bahama Island and Great Abaco, which are the largest of the platforms, and the Cay Sal Bank north of Cuba. The islands of these banks are politically part of the Bahamas. Other banks are the three banks of the Turks and Caicos Islands, namely the Caicos Bank of the Caicos Islands, the bank of the Turks Islands, and wholly submerged Mouchoir Bank. Further southeast are the equally wholly submerged Silver Bank and Navidad Bank north of the Dominican Republic.

Continental drift

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.

Dynamic topography

The term dynamic topography is used in geodynamics to refer to elevation differences caused by the flow within the Earth's mantle.

Earth's crust

The Earth's crust is a thin shell on the outside of the Earth, accounting for less than 1% of Earth's volume. It is the top component of lithosphere: a division of Earth's layers that includes the crust and the upper part of the mantle. The lithosphere is broken into tectonic plates that move, allowing heat to escape from the interior of the Earth into space.

The crust lies on top of the mantle, a configuration that is stable because the upper mantle is made of peridotite and so is significantly denser than the crust. The boundary between the crust and mantle is conventionally placed at the Mohorovičić discontinuity, a boundary defined by a contrast in seismic velocity.

The crust of the Earth is of two distinctive types:

Oceanic: 5 km (3 mi) to 10 km (6 mi) thick and composed primarily of denser, more mafic rocks, such as basalt, diabase, and gabbro.

Continental: 30 km (20 mi) to 50 km (30 mi) thick and mostly composed of less dense, more felsic rocks, such as granite.Because both continental and oceanic crust are less dense than the mantle below, both types of crust "float" on the mantle. This is isostasy, and it's also one of the reasons continental crust is higher than oceanic: continental is less dense and so "floats" higher. As a result, water pools in above the oceanic crust, forming the oceans.

The temperature of the crust increases with depth, reaching values typically in the range from about 200 °C (392 °F) to 400 °C (752 °F) at the boundary with the underlying mantle. The temperature increases by as much as 30 °C (54 °F) for every kilometer locally in the upper part of the crust, but the geothermal gradient is smaller in deeper crust.

Gorda Ridge

The Gorda Ridge (41°36'19.6"N 127°22'03.1"W), a tectonic spreading center, is located roughly 200 kilometres (120 mi) off the northern coast of California and southern Oregon. Running NE – SW it is roughly 300 kilometres (190 mi) in length. The ridge is broken into three segments; the northern ridge, central ridge, and the southern ridge, which contains the Escanaba Trough.

Grenville orogeny

The Grenville orogeny was a long-lived Mesoproterozoic mountain-building event associated with the assembly of the supercontinent Rodinia. Its record is a prominent orogenic belt which spans a significant portion of the North American continent, from Labrador to Mexico, as well as to Scotland.

Grenville orogenic crust of mid-late Mesoproterozoic age (c. 1250–980 Ma) is found worldwide, but generally only events which occurred on the southern and eastern margins of Laurentia are recognized under the "Grenville" name.These orogenic events are also known as the Kibaran orogeny in Africa and the Dalslandian orogeny in Western Europe.

Inversion (geology)

In structural geology inversion or basin inversion relates to the relative uplift of a sedimentary basin or similar structure as a result of crustal shortening. This normally excludes uplift developed in the footwalls of later extensional faults, or uplift caused by mantle plumes. "Inversion" can also refer to individual faults, where an extensional fault is reactivated in the opposite direction to its original movement.

The term negative inversion is also occasionally used to describe the reactivation of reverse faults and thrusts during extension.

The term "inversion" simply refers to the fact that a relatively low-lying area is uplifted – the rock sequence itself is not normally inverted.

List of submarine volcanoes

A list of active and extinct submarine volcanoes and seamounts located under the world's oceans. There are estimated to be 40,000 to 55,000 seamounts in the global oceans. Almost all are not well-mapped and many may not have been identified at all. Most are unnamed and unexplored. This list is therefore confined to seamounts that are notable enough to have been named and/or explored.

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).

Oceanic plateau

An oceanic or submarine plateau is a large, relatively flat elevation that is higher than the surrounding relief with one or more relatively steep sides.There are 184 oceanic plateaus covering an area of 18,486,600 km2 (7,137,700 sq mi), or about 5.11% of the oceans. The South Pacific region around Australia and New Zealand contains the greatest number of oceanic plateaus (see map).

Oceanic plateaus produced by large igneous provinces are often associated with hotspots, mantle plumes, and volcanic islands — such as Iceland, Hawaii, Cape Verde, and Kerguelen. The three largest plateaus, the Caribbean, Ontong Java, and Mid-Pacific Mountains, are located on thermal swells. Other oceanic plateaus, however, are made of rifted continental crust, for example Falkland Plateau, Lord Howe Rise, and parts of Kerguelen, Seychelles, and Arctic ridges.

Plateaus formed by large igneous provinces were formed by the equivalent of continental flood basalts such as the Deccan Traps in India and the Snake River Plain in the United States.

In contrast to continental flood basalts, most igneous oceanic plateaus erupt through young and thin (6–7 km (3.7–4.3 mi)) mafic or ultra-mafic crust and are therefore uncontaminated by felsic crust and representative for their mantle sources.

These plateaus often rise 2–3 km (1.2–1.9 mi) above the surrounding ocean floor and are more buoyant than oceanic crust. They therefore tend to withstand subduction, more-so when thick and when reaching subduction zones shortly after their formations. As a consequence, they tend to "dock" to continental margins and be preserved as accreted terranes. Such terranes are often better preserved than the exposed parts of continental flood basalts and are therefore a better record of large-scale volcanic eruptions throughout Earth's history. This "docking" also means that oceanic plateaus are important contributors to the growth of continental crust. Their formations often had a dramatic impact on global climate, such as the most recent plateaus formed, the three, large, Cretaceous oceanic plateaus in the Pacific and Indian Ocean: Ontong Java, Kerguelen, and Caribbean.

Physical oceanography

Physical oceanography is the study of physical conditions and physical processes within the ocean, especially the motions and physical properties of ocean waters.

Physical oceanography is one of several sub-domains into which oceanography is divided. Others include biological, chemical and geological oceanography.

Physical oceanography may be subdivided into descriptive and dynamical physical oceanography.Descriptive physical oceanography seeks to research the ocean through observations and complex numerical models, which describe the fluid motions as precisely as possible.

Dynamical physical oceanography focuses primarily upon the processes that govern the motion of fluids with emphasis upon theoretical research and numerical models. These are part of the large field of Geophysical Fluid Dynamics (GFD) that is shared together with meteorology. GFD is a sub field of Fluid dynamics describing flows occurring on spatial and temporal scales that are greatly influenced by the Coriolis force.

Plate tectonics

Plate tectonics (from the Late Latin tectonicus, from the Greek: τεκτονικός "pertaining to building") 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 3.3 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.

The lithosphere, which is the rigid outermost shell of a planet (the crust and upper mantle), is broken into tectonic plates. The Earth's lithosphere is composed of seven or eight major plates (depending on how they are defined) and many minor plates. Where the plates meet, their relative motion determines the type of boundary: convergent, divergent, or transform. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along these plate boundaries (or faults). The relative movement of the plates typically ranges from zero to 100 mm annually.Tectonic plates are composed of oceanic lithosphere and thicker continental lithosphere, each topped by its own kind of crust. Along convergent boundaries, subduction, or one plate moving under another, carries the lower one down into the mantle; the material lost is roughly balanced by the formation of new (oceanic) crust along divergent margins by seafloor spreading. In this way, the total surface of the lithosphere remains the same. This prediction of plate tectonics is also referred to as the conveyor belt principle. Earlier theories, since disproven, proposed gradual shrinking (contraction) or gradual expansion of the globe.Tectonic plates are able to move because the Earth's lithosphere has greater mechanical strength than the underlying asthenosphere. Lateral density variations in the mantle result in convection; that is, the slow creeping motion of Earth's solid mantle. Plate movement is thought to be driven by a combination of the motion of the seafloor away from spreading ridges due to variations in topography (the ridge is a topographic high) and density changes in the crust (density increases as newly formed crust cools and moves away from the ridge). At subduction zones the relatively cold, dense crust is "pulled" or sinks down into the mantle over the downward convecting limb of a mantle cell. Another explanation lies in the different forces generated by tidal forces of the Sun and Moon. The relative importance of each of these factors and their relationship to each other is unclear, and still the subject of much debate.

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.

Sedimentary basin

Sedimentary basins are regions of Earth of long-term subsidence creating accommodation space for infilling by sediments. The subsidence can result from a variety of causes that include: the thinning of underlying crust, sedimentary, volcanic, and tectonic loading, and changes in the thickness or density of adjacent lithosphere. Sedimentary basins occur in diverse geological settings usually associated with plate tectonic activity. Basins are classified structurally in various ways, with a primary classifications distinguishing among basins formed in various plate tectonic regime (divergent, convergent, transform, intraplate), the proximity of the basin to the active plate margins, and whether oceanic, continental or transitional crust underlies the basin. Basins formed in different plate tectonic regimes vary in their preservation potential. On oceanic crust, basins are likely to be subducted, while marginal continental basins may be partially preserved, and intracratonic basins have a high probability of preservation. As the sediments are buried, they are subjected to increasing pressure and begin the process of lithification. A number of basins formed in extensional settings can undergo inversion which has accounted for a number of the economically viable oil reserves on earth which were formerly basins.

Slab pull

Slab pull is that part of the motion of a tectonic plate caused by its subduction. In 1975 Forsyth and Uyeda showed using inverse theory methods that of the many likely driving forces of plates slab pull was the strongest. Plate motion is partly driven by the weight of cold, dense plates sinking into the mantle at oceanic trenches. This force and slab suction account for almost all of the force driving plate tectonics. The ridge push at rifts contributes only 5 to 10%.

Carlson et al. (1983) in Lallemandet al. (2005) defined the slab pull force as:

Where:

K is 4.2g (gravitational acceleration = 9.81 m/s2) according to McNutt (1984);
Δρ = 80 kg/m3 is the mean density difference between the slab and the surrounding asthenosphere;
L is the slab length calculated only for the part above 670 km (the upper/lower mantle boundary);
A is the slab age in Ma at the trench.

The slab pull force manifests itself between two extreme forms:

Between these two examples there is the evolution of the Farallon Plate: from the huge slab width with the Nevada, the Sevier and Laramide orogenies; the Mid-Tertiary ignimbrite flare-up and later left as Juan de Fuca and Cocos plates, the Basin and Range Province under extension, with slab break off, smaller slab width, more edges and mantle return flow.

Some early models of plate tectonics envisioned the plates riding on top of convection cells like conveyor belts. However, most scientists working today believe that the asthenosphere does not directly cause motion by the friction of such basal forces. The North American Plate is nowhere being subducted, yet it is in motion. Likewise the African, Eurasian and Antarctic Plates. Ridge push is thought responsible for the motion of these plates.

The subducting slabs around the Pacific Ring of Fire cool down the Earth and its Core-mantle boundary. Around the African Plate upwelling mantle plumes from the Core-mantle boundary produce rifting including the African and Ethiopian rift valleys.

Slab suction

Slab suction forces are one of the major plate tectonic driving forces. Slab suction occurs when a subducting slab drives flow in the nearby mantle. This flow then exerts shear tractions on nearby plates. This driving force is important when the slabs (or portions thereof) are not strongly attached to the rest of their respective tectonic plate. They cause both the subducting and overriding plate to move in the direction of the subduction zone. Slab suction is the weakest of the three major forces involved in plate motion, the others being slab pull, the strongest, and ridge push.

Tension (geology)

In geology, the term "tension" refers to a stress which stretches rocks in two opposite directions. The rocks become longer in a lateral direction and thinner in a vertical direction. One important result of tensile stress is jointing in rocks. However, tensile stress is rare because most subsurface stress is compressive, due to the weight of the overburden.

Undersea mountain range

Undersea mountain ranges are mountain ranges that are mostly or entirely underwater, and specifically under the surface of an ocean. If originated from current tectonic forces, they are often referred to as a mid-ocean ridge. In contrast, if formed by past above-water volcanism, they are known as a seamount chain. The largest and best known undersea mountain range is a mid-ocean ridge, the Mid-Atlantic Ridge. It has been observed that, "similar to those on land, the undersea mountain ranges are the loci of frequent volcanic and earthquake activity".

Wave base

The wave base, in physical oceanography, is the maximum depth at which a water wave's passage causes significant water motion. For water depths deeper than the wave base, bottom sediments and the seafloor are no longer stirred by the wave motion above.

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