subduction
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Subduction is a geological process in which the oceanic lithosphere is
recycled Recycling is the process of converting waste materials into new materials and objects. The Energy recycling, recovery of energy from waste materials is often included in this concept. The recyclability of a material depends on its ability t ...
into the
Earth's mantle Earth's mantle is a layer of silicate mineral, silicate rock between the Earth's crust, crust and the Earth's outer core, outer core. It has a mass of 4.01 × 1024 kg and thus makes up 67% of the mass of Earth. It has a thickness of making up a ...
at convergent boundaries. Where the oceanic lithosphere of a
tectonic plate Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large te ...
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 the average rate of convergence being approximately two to eight centimeters per year along most plate boundaries. Subduction is possible because the cold oceanic lithosphere is slightly denser than the underlying
asthenosphere The asthenosphere () is the mechanically weak and ductility (Earth science), ductile region of the upper mantle (Earth), upper mantle of Earth. It lies below the lithosphere, at a depth between ~ below the surface, and extends as deep as . Howeve ...
, the hot, ductile layer in the upper mantle underlying the cold, rigid lithosphere. Once initiated, stable subduction is driven mostly by the negative buoyancy of the dense subducting lithosphere. The slab sinks into the mantle largely under its weight.
Earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth Earth is the third planet from the Sun and the only astronomical object ...
s are common along the subduction zone, and fluids released by the subducting plate trigger
volcanism Volcanism, vulcanism or volcanicity is the phenomenon of eruption of molten rock (magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evi ...
in the overriding plate. 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 at a steeper angle is characterized by the formation of
back-arc basin A back-arc basin is a type of geologic Structural basin, basin, found at some convergent boundary, convergent plate boundaries. Presently all back-arc basins are submarine features associated with island arcs and subduction zones, with many found ...
s.


Subduction and plate tectonics

According to the theory of
plate tectonics Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large te ...
, the Earth's
lithosphere A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust (geology), crust and the portion of the upper mantle (geology), mantle that behaves elastically on time sca ...
, its rigid outer shell, is broken into sixteen larger
tectonic plates Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large te ...
and several smaller plates. These are in slow motion, due to
convection Convection is single or Multiphase flow, multiphase fluid flow that occurs Spontaneous process, spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoya ...
in the underlying ductile mantle. This process of convection allows heat generated by
radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive Decay chain, disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nucl ...
to escape from the Earth's interior. The lithosphere consists of the outermost light crust plus the uppermost rigid portion of the mantle. Oceanic lithosphere ranges in thickness from just a few km for young lithosphere created at
mid-ocean ridge A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a Diverge ...
s to around for the oldest oceanic lithosphere. Continental lithosphere is up to thick. The lithosphere is relatively cold and rigid compared with the underlying
asthenosphere The asthenosphere () is the mechanically weak and ductility (Earth science), ductile region of the upper mantle (Earth), upper mantle of Earth. It lies below the lithosphere, at a depth between ~ below the surface, and extends as deep as . Howeve ...
, and so tectonic plates move as 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 back into the mantle and is recycled. 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. The overridden plate (the '' slab'') sinks at an angle of approximately 25 to 75 degrees to Earth's surface. 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.
Sediments Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently sediment transport, transported by the action of wind, water, or ice or by the force of gravity acting on the particles. ...
and some trapped water are carried downwards by the slab and recycled into the deep mantle. Earth is so far the only planet where subduction is known to occur, and subduction zones are its most important tectonic feature. Subduction is the driving force behind
plate tectonics Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large te ...
, and without it, plate tectonics could not occur. Oceanic subduction zones are located along of convergent plate margins, almost equal to the cumulative of mid-ocean ridges. Sea water seeps into the oceanic lithosphere through fractures and pores, and reacts with minerals in the crust and mantle to form hydrous minerals (such as serpentine) that store water in their crystal structures. Water is transported into the deep mantle ''via'' hydrous minerals in subducting slabs. During subduction, a series of minerals in these slabs such as serpentine can be stable at different pressures within the slab geotherms, and may transport significant amount of water into the Earth's interior. As plates sink and heat up, released fluids can trigger seismicity and induce melting within the subducted plate and in the overlying mantle wedge. This type of melting selectively concentrates volatiles and transports them into the overlying plate. If an eruption occurs, the cycle then returns the volatiles into the oceans and atmosphere


Structure of subduction zones


Arc-trench complex

The surface expression of subduction zones are arc-trench complexes. 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''. Here the plate shallows slightly before plunging downwards, as a consequence of the rigidity of the plate. The point where the slab begins to plunge downwards is marked by an ''
oceanic trench Oceanic trenches are prominent long, narrow topography, topographic depressions of the ocean floor. They are typically wide and below the level of the surrounding oceanic floor, but can be thousands of kilometers in length. There are about of ...
''. Oceanic trenches are the deepest parts of the ocean floor. Beyond the trench is the '' forearc'' portion of the overriding plate. Depending on sedimentation rates, the forearc may include an
accretionary wedge An accretionary wedge or accretionary prism forms from sediments Accretion (geology), accreted onto the non-Subduction, subducting tectonic plate at a Convergent boundary, convergent plate boundary. Most of the material in the 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 basin is poorly developed in non-accretionary arcs. Beyond the forearc basin, volcanoes are found in long chains called ''
volcanic arc A volcanic arc (also known as a magmatic arc) is a belt of volcanoes formed above a Subduction, subducting oceanic tectonic plate, with the belt arranged in an arc shape as seen from above. Volcanic arcs typically parallel an oceanic trench, wit ...
s''. The subducting basalt and sediment are normally rich in
hydrous In chemistry, a hydrate is a substance that contains water or its constituent elements. The chemical state of the water varies widely between different classes of hydrates, some of which were so labeled before their chemical structure was understo ...
minerals and clays. Additionally, large quantities of water are introduced into cracks and fractures created as the subducting slab bends downward. 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 A supercritical fluid (SCF) is any substance at a temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer. Thermometers a ...
. 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 Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...
via flux melting. The magmas, in turn, rise as
diapir A diapir (; , ) is a type of igneous intrusion In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of Intrusive rock, intrusive igneous rock that forms by crystallization of magma slowly cooling below the su ...
s because they are less dense than the rocks of the mantle. The mantle-derived magmas (which are initially basaltic in composition) can ultimately reach the Earth's surface, resulting in volcanic eruptions. 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. Arc volcanoes tend to produce dangerous eruptions because they are rich in water (from the slab and sediments) and tend to be extremely explosive.
Krakatoa Krakatoa (), also transcribed (), is a caldera in the Sunda Strait between the islands of Java and Sumatra in the Indonesian province of Lampung. The caldera is part of a volcanic island group (Krakatoa archipelago) comprising four islands. Tw ...
,
Nevado del Ruiz The Nevado del Ruiz (), also known as La Mesa de Herveo ( en, Mesa of Herveo, the name of the nearby town) is a volcano on the border of the Departments of Colombia, departments of Caldas Department, Caldas and Tolima Department, Tolima in Colom ...
, and
Mount Vesuvius Mount Vesuvius ( ; it, Vesuvio ; nap, 'O Vesuvio , also or ; la, Vesuvius , also , or ) is a Somma volcano, somma-stratovolcano located on the Gulf of Naples in Campania, Italy, about east of Naples and a short distance from the shore. ...
are all examples of arc volcanoes. Arcs are also associated with most
ore Ore is natural Rock (geology), rock or sediment that contains one or more valuable minerals, typically containing metals, that can be mined, treated and sold at a profit.Encyclopædia Britannica. "Ore". Encyclopædia Britannica Online. Ret ...
deposits. Beyond the volcanic arc is a back-arc region whose character depends strongly on the angle 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 there may be extensive folding and
thrust fault A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks. Thrust geometry and nomenclature Reverse faults A thrust fault is a type of reverse fault that has a dip of 45 degrees or less. If ...
ing. If the angle of subduction is deep, the crust will be put in tension instead, often producing a
back-arc basin A back-arc basin is a type of geologic Structural basin, basin, found at some convergent boundary, convergent plate boundaries. Presently all back-arc basins are submarine features associated with island arcs and subduction zones, with many found ...
.


Deep structure

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 Geophysics () is a subject of natural science concerned with the physical processes and Physical property, physical properties of the Earth and its surrounding space environment, and the use of quantitative methods for their analysis. The term ...
and
geochemistry Geochemistry is the science that uses the tools and principles of chemistry to explain the mechanisms behind major geological systems such as the Earth's crust and its oceans. The realm of geochemistry extends beyond the Earth, encompassing the e ...
. Subduction zones are defined by an inclined zone of
earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth Earth is the third planet from the Sun and the only astronomical object ...
s, the Wadati–Benioff zone, that dips away from the trench and extends down to the 660-kilometer discontinuity. Subduction zone earthquakes occur at greater depths (up to ) than elsewhere on Earth (typically less than depth); such deep earthquakes may be driven by deep phase transformations,
thermal runaway Thermal runaway describes a process that is accelerated by increased temperature, in turn releasing Thermal energy, energy that further increases temperature. Thermal runaway occurs in situations where an increase in temperature changes the condi ...
, or dehydration embrittlement. Seismic tomography shows that some slabs can penetrate the lower mantle and sink clear to the core–mantle boundary. Here the residue of the slabs may eventually heat enough to rise back to the surface as
mantle plume A mantle plume is a proposed mechanism of convection within the Earth's mantle, hypothesized to explain anomalous volcanism. Because the plume head partially melts on reaching shallow depths, a plume is often invoked as the cause of volcanic hots ...
s.


Subduction angle

Subduction typically occurs at a moderately steep angle right at the point of the convergent plate boundary. However, anomalous shallower angles of subduction are known to exist as well as some that are extremely steep. * 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 The Andes, Andes Mountains or Andean Mountains (; ) are the longest continental mountain range in the world, forming a continuous highland along the western edge of South America South America is a continent entirely in the Western ...
, 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 The Chile Ridge, also known as the Chile Rise, is a submarine Mid-ocean ridge, oceanic ridge formed by the Divergent Plate Boundary, divergent plate boundary between the Nazca Plate and the Antarctic Plate. It extends from the triple junction of ...
, a
spreading ridge A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a Diverge ...
. The Laramide Orogeny in the
Rocky Mountains The Rocky Mountains, also known as the Rockies, are a major mountain range and the largest mountain system in North America. The Rocky Mountains stretch in great-circle distance, straight-line distance from the northernmost part of western Can ...
of the
United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country Continental United States, primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., ...
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 A basement or cellar is one or more Storey, floors of a building that are completely or partly below the storey, ground floor. It generally is used as a utility space for a building, where such items as the Furnace (house heating), furnace, ...
-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. * Steep-angle subduction (subducting angle greater than 70°) occurs in subduction zones where Earth's
oceanic crust Oceanic crust is the uppermost layer of the oceanic portion of the Plate tectonics, tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike (geology), dike complex, and the lower oceanic crust, composed of troct ...
and lithosphere are old and thick and have, therefore, lost buoyancy. The steepest dipping subduction zone lies in the
Mariana Trench The Mariana Trench is an oceanic trench located in the western Pacific Ocean, about east of the Mariana Islands; it is the deep sea, deepest oceanic trench on Earth. It is crescent-shaped and measures about in length and in width. The maxi ...
, which is also where the oceanic crust, of
Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The J ...
age, is the oldest on Earth exempting
ophiolite An ophiolite is a section of Earth's oceanic crust and the underlying upper mantle (Earth), upper mantle that has been uplifted and exposed above sea level and often emplaced onto continental crustal rocks. The Greek word ὄφις, ''ophis'' ...
s. 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 This is a list of seas of the Ocean#World ocean, World Ocean, including marginal seas, areas of water, various gulfs, bights, bays, and straits. Terminology * Ocean – the four to seven largest named bodies of water in the World Ocean, all o ...
.


Life cycle of subduction zones


Initiation of subduction

Although stable subduction is fairly well understood, the process by which subduction is initiated remains a matter of discussion and continuing study. Subduction can begin spontaneously if the denser oceanic lithosphere can founder and sink beneath the adjacent oceanic or continental lithosphere through vertical forcing only; alternatively, existing plate motions can induce new subduction zones by horizontally forcing the oceanic lithosphere to rupture and sink into the asthenosphere. 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 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. Some analogue modeling shows, however, the possibility of spontaneous subduction from inherent density differences between two plates at specific locations like passive margins. There is evidence this has taken place in the Izu-Bonin-Mariana subduction system. 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.


End of subduction

Subduction can continue as long as the oceanic lithosphere moves into the subduction zone. However, the arrival of buoyant crust at a subduction zone can result in its failure, by disrupting downwelling. The arrival of continental crust results in a ''collision'' or ''terrane accretion'' that disrupts subduction. Continental crust can subduct to depths of or more but then resurfaces. Sections of crustal or intraoceanic arc crust greater than in thickness or oceanic plateau greater than in thickness can disrupt subduction. However, island arcs subducted end-on may cause only local disruption, while an arc arriving parallel to the zone can shut it down. This has happened with the Ontong Java Plateau and the Vitiaz Trench.


Effects


Metamorphism

Subduction zones host a unique variety of rock types created by the high-pressure, low-temperature conditions a subducting slab encounters during its descent. The metamorphic conditions the slab passes through in this process creates and destroys water bearing (hydrous) mineral phases, releasing water into the mantle. This water lowers the melting point of mantle rock, initiating melting. Understanding the timing and conditions in which these dehydration reactions occur, is key to interpreting mantle melting, volcanic arc magmatism, and the formation of continental crust. A
metamorphic facies A metamorphic facies is a set of mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs nat ...
is characterized by a stable mineral assemblage specific to a pressure-temperature range and specific starting material. Subduction zone
metamorphism Metamorphism is the transformation of existing rock (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of , and often also at elevated pressure or in the presence of ...
is characterized by a low temperature, high-ultrahigh pressure metamorphic path through the
zeolite Zeolites are Microporous material, microporous, crystalline aluminosilicate materials commonly used as commercial adsorption, adsorbents and Catalysis, catalysts. They mainly consist of silicon, aluminium, oxygen, and have the general formula ・y ...
, prehnite-pumpellyite, blueschist, and
eclogite Eclogite () is a metamorphic rock Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock ( protolith) is subjected to temperatures greater than and, ofte ...
facies stability zones of subducted oceanic crust.Zheng, Y.-F., Chen, R.-X., 2017. Regional metamorphism at extreme conditions: Implications for orogeny at convergent plate margins. Journal of Asian Earth Sciences 145, 46-73.
Zeolite Zeolites are Microporous material, microporous, crystalline aluminosilicate materials commonly used as commercial adsorption, adsorbents and Catalysis, catalysts. They mainly consist of silicon, aluminium, oxygen, and have the general formula ・y ...
and prehnite-pumpellyite facies assemblages may or may not be present, thus the onset of metamorphism may only be marked by blueschist facies conditions. Subducting slabs are composed of basaltic crust topped with pelagic sediments; however, the pelagic sediments may be accreted onto the forearc-hanging wall and not subducted. Most metamorphic phase transitions that occur within 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. 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 mantle beneath a volcanic arc.


Volcanic activity

Volcano A volcano is a rupture in the Crust (geology), crust of a Planet#Planetary-mass objects, planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and volcanic gas, gases to escape from a magma chamber below the surface. On Ear ...
es that occur above subduction zones, such as Mount St. Helens,
Mount Etna Mount Etna, or simply Etna ( it, Etna or ; scn, Muncibbeḍḍu or ; la, Aetna; grc, Αἴτνα and ), is an active stratovolcano on the east coast of Sicily, Italy, in the Metropolitan City of Catania, between the cities of Messina, ...
, and
Mount Fuji , or Fugaku, located on the island of Honshu, Honshū, is the List of mountains and hills of Japan by height, highest mountain in Japan, with a summit elevation of . It is the second-highest volcano located on an island in Asia (after Mount Ke ...
, lie approximately one hundred kilometers from the trench in arcuate chains called
volcanic arc A volcanic arc (also known as a magmatic arc) is a belt of volcanoes formed above a Subduction, subducting oceanic tectonic plate, with the belt arranged in an arc shape as seen from above. Volcanic arcs typically parallel an oceanic trench, wit ...
s. Two kinds of arcs are generally observed on Earth: island arcs that form on the oceanic lithosphere (for example, the Mariana and the
Tonga Tonga (, ; ), officially the Kingdom of Tonga ( to, Puleʻanga Fakatuʻi ʻo Tonga), is a Polynesia, Polynesian country and archipelago. The country has List of islands and towns in Tonga, 171 islands – of which 45 are inhabited. Its tota ...
island arcs), and
continental arc A continental arc is a type of volcanic arc occurring as an "arc-shape" Topography, 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 plat ...
s such as the Cascade Volcanic Arc, that form along the coast of continents. 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 the subduction of oceanic lithosphere beneath a continental lithosphere (ocean-continent subduction). An example of a volcanic arc having both island and continental arc sections is found behind the Aleutian Trench subduction zone in Alaska. The arc magmatism occurs one hundred to two hundred kilometers from the trench and approximately one hundred kilometers above the subducting slab. This depth of arc
magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...
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. 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. 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 Continental crust is the layer of igneous rock, igneous, sedimentary rock, sedimentary, and metamorphic rocks that forms the geological continents and the areas of shallow seabed close to their shores, known as Continental shelf, continental s ...
. Arc volcanism has the greatest impact on humans because many arc volcanoes lie above sea level and erupt violently.
Aerosols An aerosol is a suspension (chemistry), suspension of fine solid particles or liquid Drop (liquid), droplets in air or another gas. Aerosols can be natural or Human impact on the environment, anthropogenic. Examples of natural aerosols are fog o ...
injected into the stratosphere during violent eruptions can cause rapid cooling of Earth's
climate Climate is the long-term weather pattern in an area, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorologi ...
and affect air travel.


Earthquakes and tsunamis

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 generally restricted to the shallow, brittle parts of the crust, generally at depths of less than twenty kilometers. However, in subduction zones, quakes occur at depths as great as . These quakes define inclined zones of seismicity known as Wadati–Benioff zones which trace the descending slab. 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 An earthquake and a tsunami, known as the Boxing Day Tsunami and, by the scientific community, the Sumatra–Andaman earthquake, occurred at 07:58:53 local time (UTC+07:00, UTC+7) on 26 December 2004, with an epicentre off the west coast of no ...
; and the 2011 Tōhoku earthquake and tsunami. 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. Because earthquakes can occur only when a rock is deforming in a brittle fashion, subduction zones can cause large earthquakes. If such a quake causes rapid deformation of the sea floor, there is potential for
tsunami A tsunami ( ; from ja, 津波, lit=harbour wave, ) is a series of waves in a water body caused by the displacement of a large volume of water, generally in an ocean or a tsunamis in lakes, large lake. Earthquakes, volcanic eruptions and othe ...
s, 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. Small tremors which cause small, nondamaging tsunamis, also occur frequently. A study published in 2016 suggested a new parameter to determine a subduction zone's ability to generate mega-earthquakes. 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." 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. The 2009 Samoa earthquake is an example of this type of event. Displacement of the sea floor caused by this event generated a six-meter tsunami in nearby Samoa. Seismic tomography has helped detect subducted lithosphere, slabs, deep in the mantle where there are no earthquakes. About one hundred slabs have been described in terms of depth and their timing and location of subduction. The great seismic discontinuities in the mantle, at depth and , 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 between the upper mantle and lower mantle at a depth of about 670 kilometers. Other subducted oceanic plates have sunk to the core–mantle boundary at 2890 km depth. 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. 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 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. Here the slabs are heated up by the ambient heat and are not detected anymore ~300 Myr after subduction.


Orogeny

Orogeny is the process of mountain building. Subducting plates can lead to orogeny by bringing oceanic islands, oceanic plateaus, and sediments to convergent margins. 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. The collision of this oceanic material causes crustal thickening and mountain-building. The accreted material is often referred to as an
accretionary wedge An accretionary wedge or accretionary prism forms from sediments Accretion (geology), accreted onto the non-Subduction, subducting tectonic plate at a Convergent boundary, convergent plate boundary. Most of the material in the accretionary wedge ...
or prism. These accretionary wedges can be identified by
ophiolites An ophiolite is a section of Earth's oceanic crust and the underlying upper mantle (Earth), upper mantle that has been uplifted and exposed above sea level and often emplaced onto continental crustal rocks. The Greek word ὄφις, ''ophis'' ...
(uplifted ocean crust consisting of sediments, pillow basalts, sheeted dykes, gabbro, and peridotite). Subduction may also cause orogeny without bringing in oceanic material 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 cause the upper plate to contract to lead to folding, faulting, crustal thickening, and mountain building. 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. 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.


Beginnings of subduction on Earth

Modern-style subduction is characterized by low geothermal gradients and the associated formation of high-pressure low-temperature rocks such as
eclogite Eclogite () is a metamorphic rock Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock ( protolith) is subjected to temperatures greater than and, ofte ...
and blueschist. Likewise, rock assemblages called
ophiolites An ophiolite is a section of Earth's oceanic crust and the underlying upper mantle (Earth), upper mantle that has been uplifted and exposed above sea level and often emplaced onto continental crustal rocks. The Greek word ὄφις, ''ophis'' ...
, associated with modern-style subduction, also indicate such conditions.
Eclogite Eclogite () is a metamorphic rock Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock ( protolith) is subjected to temperatures greater than and, ofte ...
xenolith A xenolith ("foreign rock") is a rock (geology), rock fragment (Country rock (geology), country rock) that becomes enveloped in a larger rock during the latter's development and solidification. In geology, the term ''xenolith'' is almost exclusiv ...
s found in the
North China Craton The North China Craton is a continental crustal block with one of Earth's most complete and complex records of Igneous rock, igneous, Sedimentary rock, sedimentary and Metamorphic rock, metamorphic processes. It is located in northeast China, Inn ...
provide evidence that modern-style subduction occurred at least as early as 1.8  Ga ago in the Paleoproterozoic Era. Nevertheless, the eclogite itself was produced by oceanic subduction during the assembly of supercontinents at about 1.9–2.0 Ga. Blueschist is a rock typical for present-day subduction settings. The absence of blueschist older than
Neoproterozoic The Neoproterozoic Era is the unit of geologic time from 1 billion to 538.8 million years ago. It is the last era of the Precambrian The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of Earth' ...
reflects more magnesium-rich compositions of Earth's
oceanic crust Oceanic crust is the uppermost layer of the oceanic portion of the Plate tectonics, tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike (geology), dike complex, and the lower oceanic crust, composed of troct ...
during that period. These more magnesium-rich rocks metamorphose into
greenschist Greenschists are metamorphic rocks that formed under the lowest temperatures and pressures usually produced by regional metamorphism, typically and 2–10 kilobars (). Greenschists commonly have an abundance of green minerals such as Chlorite ...
at conditions when modern oceanic crust rocks metamorphose into blueschist. The ancient magnesium-rich rocks mean that
Earth's mantle Earth's mantle is a layer of silicate mineral, silicate rock between the Earth's crust, crust and the Earth's outer core, outer core. It has a mass of 4.01 × 1024 kg and thus makes up 67% of the mass of Earth. It has a thickness of making up a ...
was once hotter, but not that subduction conditions were hotter. Previously, the lack of pre-Neoproterozoic blueschist was thought to indicate a different type of subduction. Both lines of evidence refute previous conceptions of modern-style subduction having been initiated in the Neoproterozoic Era 1.0 Ga ago.


History of investigation

Harry Hammond Hess, who during
World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the World War II by country, vast majority of the world's countries—including all of the great power ...
served in the
United States Navy Reserve The United States Navy Reserve (USNR), known as the United States Naval Reserve from 1915 to 2005, is the Reserve components of the United States Armed Forces, Reserve Component (RC) of the United States Navy. Members of the Navy Reserve, called R ...
and became fascinated in the ocean floor, studied the
Mid-Atlantic Ridge The Mid-Atlantic Ridge is a mid-ocean ridge (a divergent or constructive plate boundary) located along the floor of the Atlantic Ocean The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers ...
and proposed that hot molten rock was added to the crust at the ridge and expanded the seafloor outward. This theory was to become known as seafloor spreading. Since the
Earth's circumference Earth's circumference is the distance around Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large list of largest lakes and seas in the Solar System, volumes of water can be ...
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 Earth's mantle is a layer of silicate mineral, silicate rock between the Earth's crust, crust and the Earth's outer core, outer core. It has a mass of 4.01 × 1024 kg and thus makes up 67% of the mass of Earth. It has a thickness of making up a ...
. In 1964, George Plafker researched the Good Friday earthquake in
Alaska Alaska ( ; russian: Аляска, Alyaska; ale, Alax̂sxax̂; ; ems, Alas'kaaq; Central Alaskan Yup'ik language, Yup'ik: ''Alaskaq''; tli, Anáaski) is a U.S. state, state located in the Western United States on the northwest extremity o ...
. 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. This meant that the Pacific crust was being forced downward, or ''subducted'', beneath the Alaskan crust. The concept of subduction would play a role in the development of the
plate tectonics Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large te ...
theory. First geologic attestations of the "subduct" words date to 1970, In ordinary English ''to subduct'', or ''to subduce'' (from Latin ''subducere'', “to lead away”) are
transitive verb A transitive verb is a verb that accepts one or more object (grammar), objects, for example, 'cleaned' in ''Donald cleaned the window''. This contrasts with intransitive verbs, which do not have objects, for example, 'panicked' in ''Donald panick ...
s requiring a subject to perform an action on an object not itself, here 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 it may persist for some time until its remelting and dissipation. In this conceptual model, plate is continually being used up. The identity of the subject, the consumer, or agent of consumption, is left unstated. Some sources accept this subject-object construct. Geology makes ''to subduct'' into an
intransitive verb In grammar, an intransitive verb is a verb whose context does not entail a direct object (linguistics), object. That lack of Transitivity (grammar), transitivity distinguishes intransitive verbs from transitive verbs, which entail one or more o ...
and a reflexive verb. The lower plate itself is the subject. It 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. 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 action of overriding the object, the lower plate, which is overridden.


Importance

Subduction zones are important for several reasons: * Subduction zone physics: Sinking of the oceanic
lithosphere A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust (geology), crust and the portion of the upper mantle (geology), mantle that behaves elastically on time sca ...
(sediments, crust, mantle), by the contrast of
density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek language, Greek letter Rho (letter), rho), although the Latin letter ''D'' ca ...
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 Mantle convection is the very slow creep (deformation), creeping motion of Earth's solid silicate Earth's mantle, mantle as convection currents carrying Earth's internal heat budget, heat from the interior to the planet's surface. The Earth's su ...
. * Subduction zone chemistry: The subducted sediments and crust dehydrate and release water-rich (
aqueous An aqueous solution is a solution in which the solvent A solvent (s) (from the Latin language, Latin ''wikt:solvo#Latin, solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a Solution (chemistry), solutio ...
)
fluids In physics, a fluid is a liquid, gas, or other material that continuously Deformation (physics), deforms (''flows'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are Matter, substances wh ...
into the overlying mantle, causing mantle melting and
fractionation Fractionation is a separation process in which a certain quantity of a mixture (of gases, solids, liquids, enzymes, or isotopes, or a Suspension (chemistry), suspension) is divided during a phase transition, into a number of smaller quantities ...
of elements between the surface and deep mantle reservoirs, producing island arcs and
continental crust Continental crust is the layer of igneous rock, igneous, sedimentary rock, sedimentary, and metamorphic rocks that forms the geological continents and the areas of shallow seabed close to their shores, known as Continental shelf, continental s ...
. Hot fluids in subduction zones also alter the mineral compositions of the subducting sediments and potentially the habitability of the sediments for microorganisms. * 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, 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 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. However, that method of disposal is currently banned by international agreement. Furthermore, plate subduction zones are associated with very large
megathrust earthquake Megathrust earthquakes occur at convergent boundary, convergent plate boundaries, where one tectonic plate is forced underneath another. The earthquakes are caused by slip along the thrust fault that forms the contact between the two plates. Thes ...
s, making the effects of using any specific site for disposal unpredictable and possibly adverse to the safety of long-term disposal.


See also

* Compaction simulation * * * * * * * *


References


Additional reading

* *


External links


The Subduction Zone Initiation Database
The latest knowledge about the formation of subduction zones


From the Seafloor to the Volcano's Top
Video about the work of the Collaborative Research Center (SFB) 574 Volatiles and Fluids in Subduction Zones in Chile by GEOMAR I Helmholtz Centre for Ocean Research Kiel.
Plate Tectonics Basics 1 - Creation and Destruction of Oceanic Lithosphere
University of Texas at Dallas (~ 9 minutes long).
Atlas of the Underworld
– mapping of subducted plates in the Earth’s mantle and their geological interpretation {{Authority control Plate tectonics Geological processes Lithosphere