Subduction

From Wikipedia for FEVERv2
Jump to navigation Jump to search

Subduction is a geological process in which oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Subduction_sentence_0

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. Subduction_sentence_1

A region where this process occurs is known as a subduction zone, and its surface expression is known as an arc-trench complex. Subduction_sentence_2

The process of subduction has created most of the Earth's continental crust. Subduction_sentence_3

Rates of subduction are typically measured in centimeters per year, with the average rate of convergence being approximately two to eight centimeters per year along most plate boundaries. Subduction_sentence_4

Subduction is possible because cold oceanic lithosphere is slightly more dense than the underlying asthenosphere, the hot, ductile layer in the upper mantle underlying the cold, rigid lithosphere. Subduction_sentence_5

Once initiated, stable subduction is driven mostly by the negative buoyancy of the dense subducting lithosphere. Subduction_sentence_6

The slab sinks into the mantle largely under its own weight. Subduction_sentence_7

Earthquakes are common along the subduction zone, and fluids released by the subducting plate trigger volcanism in the overriding plate. Subduction_sentence_8

If the subducting plate sinks at a shallow angle, the overriding plate develops a belt of deformation characterized by crustal thickening, mountain building, and metamorphism. Subduction_sentence_9

Subduction at a steeper angle is characterized by the formation of back-arc basins. Subduction_sentence_10

Subduction and plate tectonics Subduction_section_0

See also: Plate tectonics Subduction_sentence_11

According to the theory of plate tectonics, the Earth's lithosphere, its rigid outer shell, is broken into sixteen larger tectonic plates and several smaller plates. Subduction_sentence_12

These are in slow motion, due to convection in the underlying ductile mantle. Subduction_sentence_13

This process of convection allows heat generated by radioactive decay to escape from the Earth's interior. Subduction_sentence_14

The lithosphere consists of the outermost light crust plus the uppermost rigid portion of the mantle. Subduction_sentence_15

Oceanic lithosphere ranges in thickness from just a few km for young lithosphere created at mid-ocean ridges to around 100 km (62 mi) for the oldest oceanic lithosphere. Subduction_sentence_16

Continental lithosphere is up to 200 km (120 mi) thick. Subduction_sentence_17

The lithosphere is relatively cold and rigid compared with the underlying asthenosphere, and so tectonic plates move as solid bodies atop the asthenosphere. Subduction_sentence_18

Individual plates often include both regions of oceanic lithosphere and continental lithosphere. Subduction_sentence_19

Subduction zones are where cold oceanic lithosphere sinks back into the mantle and is recycled. Subduction_sentence_20

They are found at convergent plate boundaries, where the oceanic lithosphere of one plate converges with less dense lithosphere of another plate. Subduction_sentence_21

The heavier oceanic lithosphere is overridden by the leading edge of the other plate. Subduction_sentence_22

The overridden plate (the slab) sinks at an angle of approximately twenty-five to seventy-five degrees to Earth's surface. Subduction_sentence_23

This sinking is driven by the temperature difference between the slab and the surrounding asthenosphere, as the colder oceanic lithosphere has, on average, a greater density. Subduction_sentence_24

Sediments and some trapped water are carried downwards by the slab and recycled into the deep mantle. Subduction_sentence_25

Earth is so far the only planet where subduction is known to occur, and subduction zones are its most important tectonic feature. Subduction_sentence_26

Subduction is the driving force behind plate tectonics, and without it, plate tectonics could not occur. Subduction_sentence_27

Oceanic subduction zones are located along 55,000 km (34,000 mi) of convergent plate margins, almost equal to the cumulative 60,000 km (37,000 mi) of mid-ocean ridges. Subduction_sentence_28

Structure of subduction zones Subduction_section_1

Arc-trench complex Subduction_section_2

The surface expression of subduction zones are arc-trench complexes. Subduction_sentence_29

On the ocean side of the complex, where the subducting plate first approaches the subduction zone, there is often an outer trench high or outer trench swell. Subduction_sentence_30

Here the plate shallows slightly before plunging downwards, as a consequence of the rigidity of the plate. Subduction_sentence_31

The point where the slab begins to plunge downwards is marked by an oceanic trench. Subduction_sentence_32

Oceanic trenches are the deepest parts of the ocean floor. Subduction_sentence_33

Beyond the trench is the forearc portion of the overriding plate. Subduction_sentence_34

Depending on sedimentation rates, the forearc may include an accretionary wedge of sediments scraped off the subducting slab and accreted to the overriding plate. Subduction_sentence_35

However, not all arc-trench complexes have an accretionary wedge. Subduction_sentence_36

Accretionary arcs have a well-developed forearc basin behind the accretionary wedge, while the forearc basin is poorly developed in non-accretionary arcs. Subduction_sentence_37

Beyond the forearc basin, volcanoes are found in long chains called volcanic arcs. Subduction_sentence_38

The subducting basalt and sediment are normally rich in hydrous minerals and clays. Subduction_sentence_39

Additionally, large quantities of water are introduced into cracks and fractures created as the subducting slab bends downward. Subduction_sentence_40

During the transition from basalt to eclogite, these hydrous materials break down, producing copious quantities of water, which at such great pressure and temperature exists as a supercritical fluid. Subduction_sentence_41

The supercritical water, which is hot and more buoyant than the surrounding rock, rises into the overlying mantle, where it lowers the melting temperature of the mantle rock, generating magma via flux melting. Subduction_sentence_42

The magmas, in turn, rise as diapirs because they are less dense than the rocks of the mantle. Subduction_sentence_43

The mantle-derived magmas (which are initially basaltic in composition) can ultimately reach the Earth's surface, resulting in volcanic eruptions. Subduction_sentence_44

The chemical composition of the erupting lava depends upon the degree to which the mantle-derived basalt interacts with (melts) Earth's crust or undergoes fractional crystallization. Subduction_sentence_45

Arc volcanoes tend to produce dangerous eruptions because they are rich in water (from the slab and sediments) and tend to be extremely explosive. Subduction_sentence_46

Krakatoa, Nevado del Ruiz, and Mount Vesuvius are all examples of arc volcanoes. Subduction_sentence_47

Arcs are also associated with most ore deposits. Subduction_sentence_48

Beyond the volcanic arc is a back-arc region whose character depends strongly on the angle of subduction of the subducting slab. Subduction_sentence_49

Where this angle is shallow, the subducting slab drags the overlying continental crust, producing a zone of compression in which there may be extensive folding and thrust faulting. Subduction_sentence_50

If the angle of subduction is deep, the crust will be put in tension instead, often producing a back-arc basin. Subduction_sentence_51

Deep structure Subduction_section_3

The arc-trench complex is the surface expression of a much deeper structure. Subduction_sentence_52

Though not direct accessible, the deeper portions can be studied using geophysics and geochemistry. Subduction_sentence_53

Subduction zones are defined by an inclined zone of earthquakes, the Wadati–Benioff zone, that dips away from the trench and extends down to the 660-kilometer discontinuity. Subduction_sentence_54

Subduction zone earthquakes occur at greater depths (up to 600 km (370 mi)) than elsewhere on Earth (typically less than 20 km (12 mi) depth); such deep earthquakes may be driven by deep phase transformations, thermal runaway, or dehydration embrittlement. Subduction_sentence_55

Seismic tomography shows that some slabs are able to penetrate the lower mantle and sink clear to the core-mantle boundary. Subduction_sentence_56

Here the residue of the slabs may eventually heat enough to rise back to the surface as mantle plumes. Subduction_sentence_57

Subduction angle Subduction_section_4

Subduction typically occurs at a moderately steep angle right at the point of the convergent plate boundary. Subduction_sentence_58

However, anomalous shallower angles of subduction are known to exist as well as some that are extremely steep. Subduction_sentence_59

Subduction_unordered_list_0

  • Flat-slab subduction (subducting angle less than 30°) occurs when the slab subducts nearly horizontally. The relatively flat slab can extend for hundreds of kilometers. That is abnormal, as the dense 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 beneath part of the Andes, causing segmentation of the Andean Volcanic Belt 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. The Laramide Orogeny in the Rocky Mountains of United States is attributed to flat-slab subduction. During this orogeny, a broad volcanic gap appeared at the southwestern margin of North America, and deformation occurred much farther inland; it was during this time that the basement-cored mountain ranges of Colorado, Utah, Wyoming, South Dakota, and New Mexico came into being. The most massive subduction zone earthquakes, so-called "megaquakes", have been found to occur in flat-slab subduction zones.Subduction_item_0_0

Subduction_unordered_list_1

  • 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 also where the oceanic crust, of Jurassic age, is the oldest on Earth exempting ophiolites. Steep-angle subduction is, in contrast to flat-slab subduction, associated with back-arc extension of crust, creating volcanic arcs and pulling fragments of continental crust away from continents to leave behind a marginal sea.Subduction_item_1_1

Life cycle of subduction zones Subduction_section_5

Initiation of subduction Subduction_section_6

Although stable subduction is fairly well understood, the process by which subduction is initiated remains a matter of discussion and continuing study. Subduction_sentence_60

Subduction can begin spontaneously if denser oceanic lithosphere is able to founder and sink beneath adjacent oceanic or continental lithosphere through vertical forcing only; alternatively, existing plate motions can induce new subduction zones by horizontally forcing oceanic lithosphere to rupture and sink into the asthenosphere. Subduction_sentence_61

Both models can eventually yield self-sustaining subduction zones, as oceanic crust is metamorphosed at great depth and becomes denser than the surrounding mantle rocks. Subduction_sentence_62

The compilation of subduction zone initiation events back to 100 Ma suggests horizontally-forced subduction zone initiation for most modern subduction zones, which is supported by results from numerical models and geologic studies. Subduction_sentence_63

Some analogue modeling shows, however, the possibility of spontaneous subduction from inherent density differences between two plates at specific locations like passive margins. Subduction_sentence_64

There is evidence this has taken place in the Izu-Bonin-Mariana subduction system. Subduction_sentence_65

Earlier 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 have contributed to subduction initiation on early Earth. Subduction_sentence_66

End of subduction Subduction_section_7

Subduction can continue as long as oceanic lithosphere moves into the subduction zone. Subduction_sentence_67

However, arrival of buoyant crust at a subduction zone can result in its failure, by disrupting downwelling. Subduction_sentence_68

Arrival of continental crust results in a collision or terrane accretion that disrupts subduction. Subduction_sentence_69

Continental crust can subduct to depths of 100 km (62 mi) or more, but then resurfaces. Subduction_sentence_70

Sections of crustal or intraoceanic arc crust greater than 15 km (9.3 mi) in thickness or oceanic plateau greater than 30 km (19 mi) in thickness can distrupt subduction. Subduction_sentence_71

However, island arcs subducted end-on may cause only local disruption, while an arc arriving parallel to the zone can shut it down. Subduction_sentence_72

This has apparently happened with the Ontong Java Plateau and the Vitiaz Trench. Subduction_sentence_73

Effects Subduction_section_8

Metamorphism Subduction_section_9

Main article: Subduction zone metamorphism Subduction_sentence_74

Volcanic activity Subduction_section_10

Main article: Volcanic arc Subduction_sentence_75

Volcanoes that occur above subduction zones, such as Mount St. Helens, Mount Etna and Mount Fuji, lie at approximately one hundred kilometers from the trench in arcuate chains called volcanic arcs. Subduction_sentence_76

Two kinds of arcs are generally observed on Earth: island arcs that form on oceanic lithosphere (for example, the Mariana and the Tonga island arcs), and continental arcs such as the Cascade Volcanic Arc, that form along the coast of continents. Subduction_sentence_77

Island arcs (intraoceanic or primitive arcs) are produced by the subduction of oceanic lithosphere beneath another oceanic lithosphere (ocean-ocean subduction) while continental arcs (Andean arcs) form during subduction of oceanic lithosphere beneath a continental lithosphere (ocean-continent subduction). Subduction_sentence_78

An example of a volcanic arc having both island and continental arc sections is found behind the Aleutian Trench subduction zone in Alaska. Subduction_sentence_79

The arc magmatism occurs one hundred to two hundred kilometers from the trench and approximately one hundred kilometers above the subducting slab. Subduction_sentence_80

This depth of arc magma generation is the consequence of the interaction between hydrous fluids, released from the subducting slab, and the arc mantle wedge that is hot enough to melt with the addition of water. Subduction_sentence_81

It has also been suggested that the mixing of fluids from a subducted tectonic plate and melted sediment is already occurring at the top of the slab before any mixing with the mantle takes place. Subduction_sentence_82

Arcs produce about 10% of the total volume of magma produced each year on Earth (approximately 0.75 cubic kilometers), much less than the volume produced at mid-ocean ridges, but they have formed most continental crust. Subduction_sentence_83

Arc volcanism has the greatest impact on humans because many arc volcanoes lie above sea level and erupt violently. Subduction_sentence_84

Aerosols injected into the stratosphere during violent eruptions can cause rapid cooling of Earth's climate and affect air travel. Subduction_sentence_85

Earthquakes and tsunamis Subduction_section_11

Main article: Megathrust earthquake Subduction_sentence_86

The strains caused by plate convergence in subduction zones cause at least three types of earthquakes. Subduction_sentence_87

These are deep earthquakes, megathrust earthquakes, and outer rise earthquakes. Subduction_sentence_88

Anomalously deep events are a characteristic of subduction zones, which produce the deepest quakes on the planet. Subduction_sentence_89

Earthquakes are generally restricted to the shallow, brittle parts of the crust, generally at depths of less than twenty kilometers. Subduction_sentence_90

However, in subduction zones, quakes occur at depths as great as 700 km (430 mi). Subduction_sentence_91

These quakes define inclined zones of seismicity known as Wadati–Benioff zones which trace the descending slab. Subduction_sentence_92

Nine of the ten largest earthquakes of the last 100 years were subduction zone megathrust earthquakes, which included the 1960 Great Chilean earthquake, which, at M 9.5, was the largest earthquake ever recorded; the 2004 Indian Ocean earthquake and tsunami; and the 2011 Tōhoku earthquake and tsunami. Subduction_sentence_93

The subduction of cold oceanic crust into the mantle depresses the local geothermal gradient and causes a larger portion of Earth to deform in a more brittle fashion than it would in a normal geothermal gradient setting. Subduction_sentence_94

Because earthquakes can occur only when a rock is deforming in a brittle fashion, subduction zones can cause large earthquakes. Subduction_sentence_95

If such a quake causes rapid deformation of the sea floor, there is potential for tsunamis, such as the earthquake caused by subduction of the Indo-Australian Plate under the Euro-Asian Plate on December 26, 2004 that devastated the areas around the Indian Ocean. Subduction_sentence_96

Small tremors which cause small, nondamaging tsunamis, also occur frequently. Subduction_sentence_97

A study published in 2016 suggested a new parameter to determine a subduction zone's ability to generate mega-earthquakes. Subduction_sentence_98

By examining subduction zone geometry and comparing the degree of curvature of the subducting plates in great historical earthquakes such as the 2004 Sumatra-Andaman and the 2011 Tōhoku earthquake, it was determined that the magnitude of earthquakes in subduction zones is inversely proportional to the degree of the fault's curvature, meaning that "the flatter the contact between the two plates, the more likely it is that mega-earthquakes will occur." Subduction_sentence_99

Outer rise earthquakes occur when normal faults oceanward of the subduction zone are activated by flexure of the plate as it bends into the subduction zone. Subduction_sentence_100

The 2009 Samoa earthquake is an example of this type of event. Subduction_sentence_101

Displacement of the sea floor caused by this event generated a six-meter tsunami in nearby Samoa. Subduction_sentence_102

Seismic tomography has helped detect subducted lithosphere, slabs, deep in the mantle where there are no earthquakes. Subduction_sentence_103

About one hundred slabs have been described in terms of depth and their timing and location of subduction. Subduction_sentence_104

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. Subduction_sentence_105

Some subducted slabs seem to have difficulty penetrating the major discontinuity that marks the boundary between upper mantle and lower mantle at a depth of about 670 kilometers. Subduction_sentence_106

Other subducted oceanic plates have sunk all the way to the core-mantle boundary at 2890 km depth. Subduction_sentence_107

Generally slabs decelerate during their descent into the mantle, from typically several cm/yr (up to ~10 cm/yr in some cases) at the subduction zone and in the uppermost mantle, to ~1 cm/yr in the lower mantle. Subduction_sentence_108

This leads to either folding or stacking of slabs at those depths, visible as thickened slabs in Seismic tomography. Subduction_sentence_109

Below ~1700 km, there might be a limited acceleration of slabs due to lower viscosity as a result of inferred mineral phase changes until they approach and finally stall at the core-mantle boundary. Subduction_sentence_110

Here the slabs are heated up by the ambient heat and are not detected anymore ~300 Myr after subduction. Subduction_sentence_111

Orogeny Subduction_section_12

Main article: Orogeny Subduction_sentence_112

Orogeny is the process of mountain building. Subduction_sentence_113

Subducting plates can lead to orogeny by bringing oceanic islands, oceanic plateaus, and sediments to convergent margins. Subduction_sentence_114

The material often does not subduct with the rest of the plate but instead is accreted (scraped off) to the continent, resulting in exotic terranes. Subduction_sentence_115

The collision of this oceanic material causes crustal thickening and mountain-building. Subduction_sentence_116

The accreted material is often referred to as an accretionary wedge, or prism. Subduction_sentence_117

These accretionary wedges can be identified by ophiolites (uplifted ocean crust consisting of sediments, pillow basalts, sheeted dykes, gabbro, and peridotite). Subduction_sentence_118

Subduction may also cause orogeny without bringing in oceanic material that collides with the overriding continent. Subduction_sentence_119

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 cause the upper plate to contract leading to folding, faulting, crustal thickening and mountain building. Subduction_sentence_120

Flat-slab subduction causes mountain building and volcanism moving into the continent, away from the trench, and has been described in North America (i.e. Laramide orogeny), South America and East Asia. Subduction_sentence_121

The processes described above allow subduction to continue while mountain building happens progressively, which is in contrast to continent-continent collision orogeny, which often leads to the termination of subduction. Subduction_sentence_122

Beginnings of subduction on Earth Subduction_section_13

See also: Archean subduction Subduction_sentence_123

Modern-style subduction is characterized by low geothermal gradients and the associated formation of high-pressure low temperature rocks such as eclogite and blueschist. Subduction_sentence_124

Likewise, rock assemblages called ophiolites, associated to modern-style subduction, also indicate such conditions. Subduction_sentence_125

Eclogite xenoliths found in the North China Craton provide evidence that modern-style subduction occurred at least as early as 1.8 Ga ago in the Paleoproterozoic Era. Subduction_sentence_126

Nevertheless, the eclogite itself was produced by oceanic subduction during the assembly of supercontinents at about 1.9–2.0 Ga. Subduction_sentence_127

Blueschist is a rock typical for present-day subduction settings. Subduction_sentence_128

Absence of blueschist older than Neoproterozoic reflect more magnesium-rich compositions of Earth's oceanic crust during that period. Subduction_sentence_129

These more magnesium-rich rocks metamorphose into greenschist at conditions when modern oceanic crust rocks metamorphose into blueschist. Subduction_sentence_130

The ancient magnesium-rich rocks means that Earth's mantle was once hotter, but not that subduction conditions were hotter. Subduction_sentence_131

Previously, lack of pre-Neoproterozoic blueschist was thought to indicate a different type of subduction. Subduction_sentence_132

Both lines of evidence refute previous conceptions of modern-style subduction having been initiated in the Neoproterozoic Era 1.0 Ga ago. Subduction_sentence_133

History of investigation Subduction_section_14

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. Subduction_sentence_134

This theory was to become known as seafloor spreading. Subduction_sentence_135

Since the Earth's circumference has not changed over geologic time, Hess concluded that older seafloor has to be consumed somewhere else, and suggested that this process takes place at oceanic trenches, where the crust would be melted and recycled in the Earth's mantle. Subduction_sentence_136

In 1964, George Plafker conducted research on the Good Friday earthquake in Alaska. Subduction_sentence_137

He concluded that the cause of the earthquake was a megathrust reaction in the Aleutian Trench, a result of the Alaskan continental crust overlapping the Pacific oceanic crust. Subduction_sentence_138

This meant that the Pacific crust was being forced downward, or subducted, beneath the Alaskan crust. Subduction_sentence_139

The concept of subduction would play a role in the development of the plate tectonics theory. Subduction_sentence_140

Importance Subduction_section_15

Subduction zones are important for several reasons: Subduction_sentence_141

Subduction_unordered_list_2

  • Subduction zone physics: Sinking of the oceanic lithosphere (sediments, crust, mantle), by contrast of density between the cold and old lithosphere and the hot asthenospheric mantle wedge, is the strongest force (but not the only one) needed to drive plate motion and is the dominant mode of mantle convection.Subduction_item_2_2

Subduction_unordered_list_3

  • Subduction zone chemistry: The subducted sediments and crust dehydrate and release water-rich (aqueous) fluids into the overlying mantle, causing mantle melting and fractionation of elements between surface and deep mantle 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.Subduction_item_3_3

Subduction_unordered_list_4

  • 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_item_4_4

Subduction_unordered_list_5

  • Subduction zones pose significant threats to lives, property, economic vitality, cultural and natural resources, and quality of life. The tremendous magnitudes of earthquakes or volcanic eruptions can also have knock-on effects with global impact.Subduction_item_5_5

Subduction zones have also been considered as possible disposal sites for nuclear waste in which the action of subduction itself would carry the material into the planetary mantle, safely away from any possible influence on humanity or the surface environment. Subduction_sentence_142

However, that method of disposal is currently banned by international agreement. Subduction_sentence_143

Furthermore, plate subduction zones are associated with very large megathrust earthquakes, making the effects on using any specific site for disposal unpredictable and possibly adverse to the safety of longterm disposal. Subduction_sentence_144

See also Subduction_section_16

Subduction_unordered_list_6

  • Divergent boundary – Linear feature that exists between two tectonic plates that are moving away from each otherSubduction_item_6_6
  • Divergent double subduction – Two parallel subduction zones with different directions are developed on the same oceanic plateSubduction_item_6_7
  • List of tectonic plate interactions – Definitions and examples of the interactions between the relatively mobile sections of the lithosphereSubduction_item_6_8
  • Obduction – The overthrusting of oceanic lithosphere onto continental lithosphere at a convergent plate boundarySubduction_item_6_9
  • Paired metamorphic belts – Sets of juxtaposed linear rock units that display contrasting metamorphic mineral assemblagesSubduction_item_6_10
  • Slab window – A gap that forms in a subducted oceanic plate when a mid-ocean ridge meets with a subduction zone and the ridge is subductedSubduction_item_6_11


Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Subduction.