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how does subduction change the ocean floor

Written By Sunday, 6 February 2022 Add Comment Edit

A geological process at convergent tectonic plate boundaries where i plate moves under the other

Diagram of the geological procedure of subduction

Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth'due south drapery at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a 2nd 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 chaff.[1] Rates of subduction are typically measured in centimeters per yr, with the average charge per unit of convergence being approximately ii to eight centimeters per year along most plate boundaries.[2]

Subduction is possible because the cold oceanic lithosphere is slightly denser than the underlying asthenosphere, the hot, ductile layer in the upper mantle underlying the cold, rigid lithosphere. In one case initiated, stable subduction is driven more often than not by the negative buoyancy of the dense subducting lithosphere. The slab sinks into the mantle largely under its weight.[three]

Earthquakes are common along the subduction zone, and fluids released by the subducting plate trigger volcanism in the overriding plate. If the subducting plate sinks at a shallow bending, the overriding plate develops a belt of deformation characterized by crustal thickening, mountain building, and metamorphism. Subduction at a steeper angle is characterized by the formation of back-arc basins.[iv]

Subduction and plate tectonics [edit]

According to the theory of plate tectonics, the Earth'south lithosphere, its rigid outer shell, is broken into sixteen larger tectonic plates and several smaller plates. These are in slow motion, due to convection in the underlying ductile mantle. This process of convection allows oestrus generated by radioactive decay to escape from the Earth'due south interior.[5]

The lithosphere consists of the outermost calorie-free crust plus the uppermost rigid portion of the pall. Oceanic lithosphere ranges in thickness from just a few km for immature lithosphere created at mid-ocean ridges to around 100 km (62 mi) for the oldest oceanic lithosphere.[6] Continental lithosphere is up to 200 km (120 mi) thick.[seven] The lithosphere is relatively common cold and rigid compared with the underlying asthenosphere, and and so tectonic plates movement equally solid bodies atop the asthenosphere. Individual plates often include both regions of the oceanic lithosphere and continental lithosphere.

Subduction zones are where the cold oceanic lithosphere sinks dorsum into the curtain and is recycled.[4] [8] They are found at convergent plate boundaries, where the oceanic lithosphere of one plate converges with the less dense lithosphere of another plate. The heavier oceanic lithosphere is overridden by the leading edge of the other plate.[vi] The overridden plate (the slab) sinks at an bending of approximately twenty-five to seventy-5 degrees to Globe's surface.[9] This sinking is driven past the temperature departure between the slab and the surrounding asthenosphere, as the colder oceanic lithosphere has, on average, a greater density.[6] Sediments and some trapped water are carried down by the slab and recycled into the deep pall.[ten]

Earth is and so far the but planet where subduction is known to occur, and subduction zones are its most important tectonic feature. Subduction is the driving strength backside plate tectonics, and without it, plate tectonics could not occur.[11] Oceanic subduction zones are located along 55,000 km (34,000 mi) of convergent plate margins,[12] almost equal to the cumulative threescore,000 km (37,000 mi) of mid-body of water ridges.[13]

Structure of subduction zones [edit]

Arc-trench circuitous [edit]

The surface expression of subduction zones are arc-trench complexes. On the ocean side of the complex, where the subducting plate get-go approaches the subduction zone, there is often an outer trench high or outer trench neat. Here the plate shallows slightly before plunging downwards, every bit a consequence of the rigidity of the plate.[14] The betoken where the slab begins to plunge down is marked by an oceanic trench. Oceanic trenches are the deepest parts of the bounding main floor.

Beyond the trench is the forearc portion of the overriding plate. Depending on sedimentation rates, the forearc may include an accretionary wedge of sediments scraped off the subducting slab and accreted to the overriding plate. However, not all arc-trench complexes have an accretionary wedge. Accretionary arcs have a well-developed forearc basin behind the accretionary wedge, while the forearc bowl is poorly adult in non-accretionary arcs.[15]

Beyond the forearc bowl, volcanoes are found in long chains called volcanic arcs. The subducting basalt and sediment are usually rich in hydrous minerals and clays. Additionally, large quantities of water are introduced into cracks and fractures created equally the subducting slab bends downwardly.[16] During the transition from basalt to eclogite, these hydrous materials suspension downwards, producing copious quantities of water, which at such great pressure and temperature exists as a supercritical fluid.[17] The supercritical water, which is hot and more than buoyant than the surrounding rock, rises into the overlying curtain, where information technology lowers the melting temperature of the mantle rock, generating magma via flux melting.[eighteen] The magmas, in turn, rising as diapirs because they are less dense than the rocks of the drapery.[19] The mantle-derived magmas (which are initially basaltic in limerick) tin can ultimately attain the Earth's surface, resulting in volcanic eruptions. The chemical limerick of the erupting lava depends upon the degree to which the mantle-derived basalt interacts with (melts) Globe'south chaff or undergoes partial crystallization. Arc volcanoes tend to produce dangerous eruptions because they are rich in water (from the slab and sediments) and tend to exist extremely explosive.[20] Krakatoa, Nevado del Ruiz, and Mount Vesuvius are all examples of arc volcanoes. Arcs are too associated with almost ore deposits.[19]

Beyond the volcanic arc is a dorsum-arc region whose grapheme depends strongly on the bending of subduction of the subducting slab. Where this angle is shallow, the subducting slab drags the overlying continental crust, producing a zone of compression in which in that location may be extensive folding and thrust faulting. If the angle of subduction is deep, the crust will be put in tension instead, often producing a back-arc basin.[21]

Deep construction [edit]

The arc-trench complex is the surface expression of a much deeper structure. Though not directly accessible, the deeper portions can be studied using geophysics and geochemistry. Subduction zones are divers by an inclined zone of earthquakes, the Wadati–Benioff zone, that dips away from the trench and extends downward to the 660-kilometer aperture. Subduction zone earthquakes occur at greater depths (upwardly to 600 km (370 mi)) than elsewhere on Earth (typically less than twenty km (12 mi) depth); such deep earthquakes may be driven by deep stage transformations, thermal delinquent, or aridity embrittlement.[22] [23] Seismic tomography shows that some slabs can penetrate the lower mantle[24] [25] and sink clear to the cadre–drapery boundary.[26] Hither the residue of the slabs may eventually heat plenty to rise back to the surface equally drape plumes.[27] [28]

Subduction angle [edit]

Subduction typically occurs at a moderately steep bending right at the point of the convergent plate boundary. Nevertheless, anomalous shallower angles of subduction are known to exist too every bit some that are extremely steep.[29]

  • Flat-slab subduction (subducting angle less than 30°) occurs when the slab subducts most horizontally. The relatively apartment slab can extend for hundreds of kilometers. That is abnormal, as the dumbo slab typically sinks at a much steeper angle. Because subduction of slabs to depth is necessary to drive subduction zone volcanism, flat-slab subduction can be invoked to explain volcanic gaps.

Flat-slab subduction is ongoing below office of the Andes, causing segmentation of the Andean Volcanic Chugalug into four zones. The flat-slab subduction in northern Peru and the Norte Chico region of Chile is believed to be the result of the subduction of two buoyant aseismic ridges, the Nazca Ridge and the Juan Fernández Ridge, respectively. Around Taitao Peninsula flat-slab subduction is attributed to the subduction of the Chile Rise, a spreading ridge.[30] [31]

The Laramide Orogeny in the Rocky Mountains of the United States is attributed to apartment-slab subduction.[32] During this orogeny, a wide volcanic gap appeared at the southwestern margin of N America, and deformation occurred much farther inland; it was during this fourth dimension that the basement-cored mountain ranges of Colorado, Utah, Wyoming, South Dakota, and New Mexico came into being. The most massive subduction zone earthquakes, and so-called "megaquakes", have been institute to occur in flat-slab subduction zones.[33]

  • Steep-angle subduction (subducting angle greater than 70°) occurs in subduction zones where Earth's oceanic crust and lithosphere are old and thick and have, therefore, lost buoyancy. The steepest dipping subduction zone lies in the Mariana Trench, which is likewise where the oceanic crust, of Jurassic age, is the oldest on Globe exempting ophiolites. Steep-angle subduction is, in contrast to apartment-slab subduction, associated with back-arc extension[34] of crust, creating volcanic arcs and pulling fragments of continental chaff abroad from continents to get out backside a marginal bounding main.

Life cycle of subduction zones [edit]

Initiation of subduction [edit]

Although stable subduction is adequately well understood, the process by which subduction is initiated remains a matter of discussion and continuing report. Subduction tin can brainstorm spontaneously if the denser oceanic lithosphere can founder and sink beneath the adjacent oceanic or continental lithosphere through vertical forcing simply; alternatively, existing plate motions can induce new subduction zones past horizontally forcing the oceanic lithosphere to rupture and sink into the asthenosphere.[35] [36] Both models can eventually yield self-sustaining subduction zones, as the oceanic crust is metamorphosed at great depth and becomes denser than the surrounding mantle rocks. The compilation of subduction zone initiation events dorsum to 100 Ma suggests horizontally-forced subduction zone initiation for most modernistic subduction zones,[36] which is supported by results from numerical models[37] [38] and geologic studies.[39] [forty] Some analogue modeling shows, nevertheless, the possibility of spontaneous subduction from inherent density differences between two plates at specific locations like passive margins.[41] [42] There is bear witness this has taken place in the Izu-Bonin-Mariana subduction organization.[43] [44] Before in Earth's history, subduction is likely to have initiated without horizontal forcing due to the lack of relative plate motion, though an unorthodox proposal by A. Yin suggests that meteorite impacts may take contributed to subduction initiation on early Earth.[45]

Stop of subduction [edit]

Subduction can continue every bit long as the oceanic lithosphere moves into the subduction zone. Even so, the arrival of buoyant chaff at a subduction zone tin event in its failure, by disrupting downwelling. The inflow of continental crust results in a collision or terrane accretion that disrupts subduction.[46] Continental chaff can subduct to depths of 100 km (62 mi) or more than but then resurfaces.[47] [28] Sections of crustal or intraoceanic arc crust greater than 15 km (9.iii mi) in thickness or oceanic plateau greater than 30 km (19 mi) in thickness can disrupt subduction. However, island arcs subducted terminate-on may cause only local disruption, while an arc arriving parallel to the zone can shut it downwardly.[46] This has happened with the Ontong Java Plateau and the Vitiaz Trench.[48]

Effects [edit]

Metamorphism [edit]

Subduction zones host a unique variety of rock types created by the high-pressure, low-temperature conditions a subducting slab encounters during its descent.[49] The metamorphic atmospheric condition the slab passes through in this process creates and destroys water begetting (hydrous) mineral phases, releasing water into the mantle. This water lowers the melting point of drape stone, initiating melting.[50] Agreement the timing and conditions in which these aridity reactions occur, is key to interpreting drapery melting, volcanic arc magmatism, and the formation of continental crust.[51]

A metamorphic facies is characterized by a stable mineral assemblage specific to a pressure-temperature range and specific starting material. Subduction zone metamorphism is characterized by a low temperature, high-ultrahigh pressure metamorphic path through the zeolite, prehnite-pumpellyite, blueschist, and eclogite facies stability zones of subducted oceanic crust.[52] Zeolite and prehnite-pumpellyite facies assemblages may or may not be nowadays, thus the onset of metamorphism may only be marked by blueschist facies conditions.[53] Subducting slabs are composed of basaltic chaff topped with pelagic sediments;[54] however, the pelagic sediments may be accreted onto the forearc-hanging wall and not subducted.[55] Well-nigh metamorphic stage transitions that occur inside the subducting slab are prompted by the dehydration of hydrous mineral phases. The breakdown of hydrous mineral phases typically occurs at depths greater than 10 km.[56] Each of these metamorphic facies is marked by the presence of a specific stable mineral assemblage, recording the metamorphic conditions undergone but the subducting slab. Transitions between facies causes hydrous minerals to dehydrate at certain pressure-temperature conditions and can therefore be tracked to melting events in the drape beneath a volcanic arc.

Volcanic activity [edit]

Volcanoes that occur higher up subduction zones, such as Mount St. Helens, Mount Etna, and Mountain Fuji, lie approximately one hundred kilometers from the trench in arcuate chains chosen volcanic arcs. Two kinds of arcs are generally observed on Earth: island arcs that form on the oceanic lithosphere (for example, the Mariana and the Tonga isle arcs), and continental arcs such as the Cascade Volcanic Arc, that grade along the declension of continents. Island arcs (intraoceanic or primitive arcs) are produced by the subduction of oceanic lithosphere below another oceanic lithosphere (body of water-body of water subduction) while continental arcs (Andean arcs) form during the subduction of oceanic lithosphere beneath a continental lithosphere (ocean-continent subduction).[57] An example of a volcanic arc having both island and continental arc sections is constitute behind the Aleutian Trench subduction zone in Alaska.[58]

The arc magmatism occurs 1 hundred to two hundred kilometers from the trench and approximately ane hundred kilometers above the subducting slab. This depth of arc magma generation is the event of the interaction between hydrous fluids, released from the subducting slab, and the arc mantle wedge that is hot enough to cook with the add-on of water.[59] It has also been suggested that the mixing of fluids from a subducted tectonic plate and melted sediment is already occurring at the pinnacle of the slab before whatsoever mixing with the drapery takes place.[sixty]

Arcs produce about 10% of the total volume of magma produced each year on Earth (approximately 0.75 cubic kilometers), much less than the book produced at mid-ocean ridges,[61] but they have formed near continental crust.[4] Arc volcanism has the greatest impact on humans because many arc volcanoes lie above bounding main level and erupt violently. Aerosols injected into the stratosphere during violent eruptions can cause rapid cooling of Earth'south climate and bear upon air travel.[59]

Earthquakes and tsunamis [edit]

Global map of subduction zones, with subducted slabs contoured past depth

The strains caused by plate convergence in subduction zones cause at least three types of earthquakes. These are deep earthquakes, megathrust earthquakes, and outer rise earthquakes.

Anomalously deep events are a characteristic of subduction zones, which produce the deepest quakes on the planet. Earthquakes are mostly restricted to the shallow, breakable parts of the crust, generally at depths of less than twenty kilometers. However, in subduction zones, quakes occur at depths equally not bad as 700 km (430 mi). These quakes define inclined zones of seismicity known every bit Wadati–Benioff zones which trace the descending slab.[62]

Nine of the 10 largest earthquakes of the last 100 years were subduction zone megathrust earthquakes, which included the 1960 Keen Chilean convulsion, which, at Thou 9.five, was the largest earthquake e'er recorded; the 2004 Indian Ocean earthquake and tsunami; and the 2022 Tōhoku earthquake and tsunami. The subduction of cold oceanic crust into the pall depresses the local geothermal slope and causes a larger portion of World to deform in a more brittle mode than it would in a normal geothermal gradient setting. Because earthquakes tin occur only when a stone is deforming in a breakable way, subduction zones can cause big earthquakes. If such a quake causes rapid deformation of the sea floor, there is potential for tsunamis, such as the earthquake acquired by subduction of the Indo-Australian Plate under the Euro-Asian Plate on December 26, 2004, that devastated the areas effectually the Indian Ocean. Small tremors which cause small, nondamaging tsunamis, as well occur frequently.[62]

A written report published in 2022 suggested a new parameter to determine a subduction zone's power to generate mega-earthquakes.[63] By examining subduction zone geometry and comparing the caste of curvature of the subducting plates in not bad historical earthquakes such equally the 2004 Sumatra-Andaman and the 2022 Tōhoku earthquake, it was determined that the magnitude of earthquakes in subduction zones is inversely proportional to the caste of the fault's curvature, meaning that "the flatter the contact betwixt the two plates, the more likely it is that mega-earthquakes will occur."[64]

Outer rise earthquakes occur when normal faults oceanward of the subduction zone are activated by flexure of the plate equally it bends into the subduction zone.[65] The 2009 Samoa earthquake is an example of this type of event. Displacement of the sea floor caused by this consequence generated a half-dozen-meter tsunami in nearby Samoa.

Seismic tomography has helped notice subducted lithosphere, slabs, deep in the pall where in that location are no earthquakes. Virtually one hundred slabs accept been described in terms of depth and their timing and location of subduction.[66] The great seismic discontinuities in the mantle, at 410 km (250 mi) depth and 670 km (420 mi), are disrupted by the descent of cold slabs in deep subduction zones. Some subducted slabs seem to have difficulty penetrating the major discontinuity that marks the boundary betwixt the upper mantle and lower drapery at a depth of nearly 670 kilometers. Other subducted oceanic plates have sunk to the core–drapery boundary at 2890 km depth. Generally, slabs decelerate during their descent into the mantle, from typically several cm/yr (up to ~10 cm/year in some cases) at the subduction zone and in the uppermost drape, to ~1 cm/yr in the lower pall.[66] This leads to either folding or stacking of slabs at those depths, visible as thickened slabs in Seismic tomography. Below ~1700 km, there might be a express dispatch of slabs due to lower viscosity every bit a consequence of inferred mineral stage changes until they approach and finally stall at the core–mantle boundary.[66] Here the slabs are heated upwardly by the ambient oestrus and are not detected anymore ~300 Myr later subduction.[66]

Orogeny [edit]

Orogeny is the process of mountain edifice. Subducting plates can lead to orogeny by bringing oceanic islands, oceanic plateaus, and sediments to convergent margins. The fabric oft does non subduct with the rest of the plate but instead is accreted (scraped off) to the continent, resulting in exotic terranes. The collision of this oceanic textile causes crustal thickening and mountain-building. The accreted fabric is frequently referred to as an accretionary wedge or prism. These accretionary wedges tin be identified by ophiolites (uplifted bounding main crust consisting of sediments, pillow basalts, sheeted dykes, gabbro, and peridotite).[67]

Subduction may also cause orogeny without bringing in oceanic fabric that collides with the overriding continent. When the subducting plate subducts at a shallow angle underneath a continent (something called "flat-slab subduction"), the subducting plate may have enough traction on the bottom of the continental plate to crusade the upper plate to contract to lead to folding, faulting, crustal thickening, and mountain edifice. Flat-slab subduction causes mountain building and volcanism moving into the continent, away from the trench, and has been described in N America (i.e. Laramide orogeny), South America, and East Asia.[66]

The processes described above allow subduction to continue while mountain building happens progressively, which is in contrast to continent-continent collision orogeny, which ofttimes leads to the termination of subduction.

Beginnings of subduction on Earth [edit]

Modern-style subduction is characterized by depression geothermal gradients and the associated formation of high-force per unit area depression-temperature rocks such equally eclogite and blueschist.[68] [69] Likewise, rock assemblages called ophiolites, associated with modern-manner subduction, besides indicate such weather.[68] Eclogite xenoliths found in the North China Craton provide prove that modern-way subduction occurred at to the lowest degree every bit early on as 1.8 Ga ago in the Paleoproterozoic Era.[68] All the same, the eclogite itself was produced by oceanic subduction during the assembly of supercontinents at about one.9–ii.0 Ga.

Blueschist is a stone typical for nowadays-twenty-four hour period subduction settings. The absenteeism of blueschist older than Neoproterozoic reflects more than magnesium-rich compositions of Globe'southward oceanic chaff during that period.[seventy] These more magnesium-rich rocks metamorphose into greenschist at conditions when modern oceanic crust rocks metamorphose into blueschist.[70] The ancient magnesium-rich rocks mean that Earth'southward drape was one time hotter, but not that subduction weather were hotter. Previously, the lack of pre-Neoproterozoic blueschist was thought to indicate a unlike type of subduction.[70] Both lines of evidence refute previous conceptions of modernistic-mode subduction having been initiated in the Neoproterozoic Era 1.0 Ga ago.[68] [lxx]

History of investigation [edit]

Harry Hammond Hess, who during World War II served in the United States Navy Reserve and became fascinated in the ocean floor, studied the Mid-Atlantic Ridge and proposed that hot molten rock was added to the crust at the ridge and expanded the seafloor outward. This theory was to go known as seafloor spreading. Since the Earth'due south circumference has not changed over geologic fourth dimension, Hess concluded that older seafloor has to exist consumed somewhere else, and suggested that this process takes place at oceanic trenches, where the crust would exist melted and recycled in the Earth'southward mantle.[71]

In 1964, George Plafker researched the Good Friday earthquake in Alaska. He ended that the cause of the convulsion was a megathrust reaction in the Aleutian Trench, a result of the Alaskan continental crust overlapping the Pacific oceanic crust. This meant that the Pacific crust was beingness forced downwardly, or subducted, beneath the Alaskan crust. The concept of subduction would play a role in the development of the plate tectonics theory.[72]

First geologic attestations of the "subduct" words date to 1970,[73] In ordinary English language to subduct, or to subduce (from Latin subducere, "to lead abroad")[74] are transitive verbs requiring a subject to perform an activeness on an object not itself, hither the lower plate, which has then been subducted ("removed"). The geological term is "consumed," which happens the geological moment the lower plate slips under, even though information technology may persist for some time until its remelting and dissipation. In this conceptual model, plate is continually beingness used up.[75] The identity of the subject, the consumer, or agent of consumption, is left unstated. Some sources accept this subject field-object construct.

Geology makes to subduct into an intransitive verb and a reflexive verb. The lower plate itself is the subject. Information technology subducts, in the sense of retreat, or removes itself, and while doing so, is the "subducting plate." Moreover, the word slab is specifically attached to the "subducting plate," even though in English the upper plate is just as much of a slab.[76] The upper plate is left hanging, so to speak. To express it geology must switch to a different verb, typically to override. The upper plate, the subject, performs the activeness of overriding the object, the lower plate, which is overridden.[77]

Importance [edit]

Subduction zones are important for several reasons:

  • Subduction zone physics: Sinking of the oceanic lithosphere (sediments, crust, mantle), past the dissimilarity of density between the cold and old lithosphere and the hot asthenospheric drape wedge, is the strongest force (but not the simply one) needed to drive plate motion and is the dominant mode of curtain convection.
  • Subduction zone chemistry: The subducted sediments and crust dehydrate and release h2o-rich (aqueous) fluids into the overlying mantle, causing mantle melting and fractionation of elements between the surface and deep pall reservoirs, producing island arcs and continental crust. Hot fluids in subduction zones also alter the mineral compositions of the subducting sediments and potentially the habitability of the sediments for microorganisms.[78]
  • Subduction zones drag down subducted oceanic sediments, oceanic crust, and mantle lithosphere that interact with the hot asthenospheric mantle from the over-riding plate to produce calc-alkaline series melts, ore deposits, and continental crust.
  • Subduction zones pose significant threats to lives, property, economical vitality, cultural and natural resources, and quality of life. The tremendous magnitudes of earthquakes or volcanic eruptions tin as well have knock-on furnishings with global bear on.[79]

Subduction zones have too been considered as possible disposal sites for nuclear waste in which the action of subduction itself would comport the fabric into the planetary mantle, safely away from any possible influence on humanity or the surface environment. However, that method of disposal is currently banned past international understanding.[fourscore] [81] [82] [83] Furthermore, plate subduction zones are associated with very large megathrust earthquakes, making the effects of using any specific site for disposal unpredictable and possibly agin to the safety of long-term disposal.[81]

See too [edit]

  • Compaction simulation
  • Divergent boundary – Linear feature that exists betwixt two tectonic plates that are moving away from each other
  • Divergent double subduction – Tectonic procedure in which two parallel subduction zones with different directions are adult on the aforementioned oceanic plate
  • Listing of tectonic plate interactions – Definitions and examples of the interactions between the relatively mobile sections of the lithosphere
  • Obduction – Overthrusting of oceanic lithosphere onto continental lithosphere at a convergent plate boundary
  • Paired metamorphic belts – Sets of juxtaposed linear rock units that display contrasting metamorphic mineral assemblages
  • Ring of Fire – Region around the rim of the Pacific Ocean where many volcanic eruptions and earthquakes occur
  • Slab window – Type of gap in a subducted oceanic plate
  • Wilson Wheel – Geophysical model of the opening and endmost of rifts

References [edit]

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