Ridge push

Last updated

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.

Contents

Mechanics

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 (Mid-ocean Ridge).png
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]

Related Research Articles

<span class="mw-page-title-main">Plate tectonics</span> Movement of Earths lithosphere

Plate tectonics is the scientific theory that Earth's lithosphere comprises a number of large tectonic plates, which have been slowly moving since about 3.4 billion years ago. The model builds on the concept of continental drift, an idea developed during the first decades of the 20th century. Plate tectonics came to be accepted by geoscientists after seafloor spreading was validated in the mid-to-late 1960s.

<span class="mw-page-title-main">Oceanic trench</span> Long and narrow depressions of the sea floor

Oceanic trenches are prominent, long, narrow topographic depressions of the ocean floor. They are typically 50 to 100 kilometers wide and 3 to 4 km below the level of the surrounding oceanic floor, but can be thousands of kilometers in length. There are about 50,000 km (31,000 mi) of oceanic trenches worldwide, mostly around the Pacific Ocean, but also in the eastern Indian Ocean and a few other locations. The greatest ocean depth measured is in the Challenger Deep of the Mariana Trench, at a depth of 10,920 m (35,830 ft) below sea level.

<span class="mw-page-title-main">Seafloor spreading</span> Geological process at mid-ocean ridges

Seafloor spreading, or seafloor spread, 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.

<span class="mw-page-title-main">Lithosphere</span> Outermost shell of a terrestrial-type planet or natural satellite

A lithosphere is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust and the lithospheric mantle, the topmost portion of the upper mantle that behaves elastically on time scales of up to thousands of years or more. The crust and upper mantle are distinguished on the basis of chemistry and mineralogy.

<span class="mw-page-title-main">Asthenosphere</span> Highly viscous, mechanically weak, and ductile region of Earths mantle

The asthenosphere is the mechanically weak and ductile region of the upper mantle of Earth. It lies below the lithosphere, at a depth between ~80 and 200 km below the surface, and extends as deep as 700 km (430 mi). However, the lower boundary of the asthenosphere is not well defined.

<span class="mw-page-title-main">Subduction</span> A geological process at convergent tectonic plate boundaries where one plate moves under the other

Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the heavier plate dives beneath the second plate and sinks into the mantle. A region where this process occurs is known as a subduction zone, and its surface expression is known as an arc-trench complex. The process of subduction has created most of the Earth's continental crust. Rates of subduction are typically measured in centimeters per year, with rates of convergence as high as 11 cm/year.

Obduction is a geological process whereby denser oceanic crust is scraped off a descending ocean plate at a convergent plate boundary and thrust on top of an adjacent plate. When oceanic and continental plates converge, normally the denser oceanic crust sinks under the continental crust in the process of subduction. Obduction, which is less common, normally occurs in plate collisions at orogenic belts or back-arc basins.

<span class="mw-page-title-main">Transform fault</span> Plate boundary where the motion is predominantly horizontal

A transform fault or transform boundary, is a fault along a plate boundary where the motion is predominantly horizontal. It ends abruptly where it connects to another plate boundary, either another transform, a spreading ridge, or a subduction zone. A transform fault is a special case of a strike-slip fault that also forms a plate boundary.

<span class="mw-page-title-main">Convergent boundary</span> Region of active deformation between colliding tectonic plates

A convergent boundary is an area on Earth where two or more lithospheric plates collide. One plate eventually slides beneath the other, a process known as subduction. The subduction zone can be defined by a plane where many earthquakes occur, called the Wadati–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.

<span class="mw-page-title-main">Island arc</span> Arc-shaped archipelago formed by intense seismic activity of long chains of active volcanoes

Island arcs are long chains of active volcanoes with intense seismic activity found along convergent tectonic plate boundaries. Most island arcs originate on oceanic crust and have resulted from the descent of the lithosphere into the mantle along the subduction zone. They are the principal way by which continental growth is achieved.

<span class="mw-page-title-main">Mid-ocean ridge</span> Basaltic underwater mountain system formed by plate tectonic spreading

A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about 2,600 meters (8,500 ft) and rises about 2,000 meters (6,600 ft) 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.

<span class="mw-page-title-main">Magmatism</span> Emplacement of magma on the outer layers of a terrestrial planet, which solidifies as igneous rocks

Magmatism is the emplacement of magma within and at the surface of the outer layers of a terrestrial planet, which solidifies as igneous rocks. It does so through magmatic activity or igneous activity, the production, intrusion and extrusion of magma or lava. Volcanism is the surface expression of magmatism.

<span class="mw-page-title-main">Back-arc basin</span> Submarine features associated with island arcs and subduction zones

A back-arc basin is a type of geologic basin, found at some convergent plate boundaries. Presently all back-arc basins are submarine features associated with island arcs and subduction zones, with many found in the western Pacific Ocean. Most of them result from tensional forces, caused by a process known as oceanic trench rollback, where a subduction zone moves towards the subducting plate. Back-arc basins were initially an unexpected phenomenon in plate tectonics, as convergent boundaries were expected to universally be zones of compression. However, in 1970, Dan Karig published a model of back-arc basins consistent with plate tectonics.

Slab pull is a geophysical mechanism whereby the cooling and subsequent densifying of a subducting tectonic plate produces a downward force along the rest of the plate. In 1975 Forsyth and Uyeda used the inverse theory method to show that, of the many forces likely to be driving plate motion, 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%.

Slab suction is one of the four main forces that drive plate tectonics. It creates a force that pulls down plates as they are subducting and speeds up their movement, creating larger amounts of displacement.

<span class="mw-page-title-main">Opening of the North Atlantic Ocean</span> Breakup of Pangea

The opening of the North Atlantic Ocean is a geological event that has occurred over millions of years, during which the supercontinent Pangea broke up. As modern-day Europe and North America separated during the final breakup of Pangea in the early Cenozoic Era, they formed the North Atlantic Ocean. Geologists believe the breakup occurred either due to primary processes of the Iceland plume or secondary processes of lithospheric extension from plate tectonics.

A continental arc is a type of volcanic arc occurring as an "arc-shape" topographic high region along a continental margin. The continental arc is formed at an active continental margin where two tectonic plates meet, and where one plate has continental crust and the other oceanic crust along the line of plate convergence, and a subduction zone develops. The magmatism and petrogenesis of continental crust are complicated: in essence, continental arcs reflect a mixture of oceanic crust materials, mantle wedge and continental crust materials.

<span class="mw-page-title-main">Plate theory (volcanism)</span>

The plate theory is a model of volcanism that attributes all volcanic activity on Earth, even that which appears superficially to be anomalous, to the operation of plate tectonics. According to the plate theory, the principal cause of volcanism is extension of the lithosphere. Extension of the lithosphere is a function of the lithospheric stress field. The global distribution of volcanic activity at a given time reflects the contemporaneous lithospheric stress field, and changes in the spatial and temporal distribution of volcanoes reflect changes in the stress field. The main factors governing the evolution of the stress field are:

  1. Changes in the configuration of plate boundaries.
  2. Vertical motions.
  3. Thermal contraction.

Intraplate volcanism is volcanism that takes place away from the margins of tectonic plates. Most volcanic activity takes place on plate margins, and there is broad consensus among geologists that this activity is explained well by the theory of plate tectonics. However, the origins of volcanic activity within plates remains controversial.

Magmatism along strike-slip faults is the process of rock melting, magma ascent and emplacement, associated with the tectonics and geometry of various strike-slip settings, most commonly occurring along transform boundaries at mid-ocean ridge spreading centres and at strike-slip systems parallel to oblique subduction zones. Strike-slip faults have a direct effect on magmatism. They can either induce magmatism, act as a conduit to magmatism and magmatic flow, or block magmatic flow. In contrast, magmatism can also directly impact on strike-slip faults by determining fault formation, propagation and slip. Both magma and strike-slip faults coexist and affect one another.

References

  1. 1 2 3 4 5 6 7 Turcotte, D.L.; Schubert, G. (2002). "Plate Tectonics". Geodynamics (2 ed.). Cambridge University Press. pp. 1–21. ISBN   0-521-66186-2.
  2. 1 2 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. Bibcode:1992JGR....9711915M. doi:10.1029/91JB01123.
  3. 1 2 3 4 5 6 DiVenere, Vic (May 21, 2017). "Driving Forces of Plate Motions". Columbia University, Earth and Space Sciences. Retrieved April 7, 2018.
  4. 1 2 3 4 5 6 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. 1 2 3 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. 1 2 Hess, H. H. (January 1962). 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. 1 2 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. 1 2 3 4 5 Bott, M.H.P. (1991). "Ridge push and associated plate interior stress in normal and hot spot regions". Tectonophysics. 200 (1–3): 17–32. Bibcode:1991Tectp.200...17B. doi:10.1016/0040-1951(91)90003-b.
  12. 1 2 3 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.
  13. 1 2 3 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. Bibcode:1984JGR....8910043R. doi:10.1029/JB089iB12p10043.
  14. 1 2 3 4 5 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. Bibcode:1992JGR....9711905S. doi:10.1029/91JB01798.
  15. 1 2 Mahatsente, R (2017). "Global Models of Ridge-Push Force, Geoid, and Lithospheric Strength of Oceanic plates". Pure and Applied Geophysics. 174 (12): 4395–4406. Bibcode:2017PApGe.174.4395M. doi:10.1007/s00024-017-1647-2. S2CID   135176611.
  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. Bibcode:2000Tecto..19..814G. doi: 10.1029/1999TC001152 .
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.