Subduction is a geological process that takes place at convergent
boundaries of tectonic plates where one plate moves under another and
is forced or sinks due to gravity into the mantle. Regions where this
process occurs are known as subduction zones. Rates of subduction are
typically in centimeters per year, with the average rate of
convergence being approximately two to eight centimeters per year
along most plate boundaries.
Plates include both oceanic crust and continental crust. Stable
subduction zones involve the oceanic lithosphere of one plate sliding
beneath the continental or oceanic lithosphere of another plate due to
the higher density of the oceanic lithosphere. That is, the subducted
lithosphere is always oceanic while the overriding lithosphere may or
may not be oceanic.
Subduction zones are sites that usually have a
high rate of volcanism and earthquakes. Additionally, subduction
zones develop belts of deformation[better source needed]
in the overriding plate's crust in a processes called orogeny part of
which leads to mountain building.
1 General description
2 Theory on origin
3.2 Volcanic activity
3.3 Earthquakes and tsunamis
6 See also
8 External links
Subduction zones are sites of convective downwelling of Earth's
lithosphere (the crust plus the top non-convecting portion of the
Subduction zones exist at convergent plate boundaries
where one plate of oceanic lithosphere converges with another plate.
The descending slab, the subducting plate, is over-ridden by the
leading edge of the other plate. The slab sinks at an angle of
approximately twenty-five to forty-five degrees to Earth's surface.
This sinking is driven by the temperature difference between the
subducting oceanic lithosphere and the surrounding mantle
asthenosphere, as the colder oceanic lithosphere is, on average,
denser. At a depth of approximately 80–120 kilometers, the basalt of
the oceanic crust is converted to a metamorphic rock called eclogite.
At that point, the density of the oceanic crust increases and provides
additional negative buoyancy (downwards force). It is at subduction
zones that Earth's lithosphere, oceanic crust, sedimentary layers and
some trapped water are recycled into the deep mantle.
Earth is so far the only planet where subduction is known to occur.
Subduction is the driving force behind plate tectonics, and without
it, plate tectonics could not occur.
Subduction zones dive down into the mantle beneath 55,000 kilometers
of convergent plate margins (Lallemand, 1999), almost equal to the
cumulative 60,000 kilometers of mid-ocean ridges.
burrow deeply but are imperfectly camouflaged, and geophysics and
geochemistry can be used to study them. Not surprisingly, the
shallowest portions of subduction zones are known best. Subduction
zones are strongly asymmetric for the first several hundred kilometers
of their descent. They start to go down at oceanic trenches. Their
descents are marked by inclined zones of earthquakes that dip away
from the trench beneath the volcanoes and extend down to the
Subduction zones are defined by the
inclined array of earthquakes known as the
Wadati–Benioff zone after
the two scientists who first identified this distinctive aspect.
Subduction zone earthquakes occur at greater depths (up to
600 km) than elsewhere on Earth (typically <20 km depth);
such deep earthquakes may be driven by deep phase transformations,
thermal runaway, or dehydration embrittlement.
The subducting basalt and sediment are normally rich in hydrous
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. The supercritical water, which is hot and more
buoyant than the surrounding rock, rises into the overlying mantle
where it lowers the pressure in (and thus the melting temperature of)
the mantle rock to the point of actual melting, generating magma. The
magmas, in turn, rise (and become labeled diapirs) because they are
less dense than the rocks of the mantle. The mantle-derived magmas
(which are basaltic in composition) can continue to rise, ultimately
to Earth's surface, resulting in a volcanic eruption. The chemical
composition of the erupting lava depends upon the degree to which the
mantle-derived basalt interacts with (melts) Earth's crust and/or
undergoes fractional crystallization.
Above subduction zones, volcanoes exist in long chains called volcanic
arcs. Volcanoes that exist along arcs tend to produce dangerous
eruptions because they are rich in water (from the slab and sediments)
and tend to be extremely explosive. Krakatoa, Nevado del Ruiz, and
Mount Vesuvius are all examples of arc volcanoes. Arcs are also known
to be associated with precious metals such as gold, silver and copper
believed to be carried by water and concentrated in and around their
host volcanoes in rock called "ore".
Theory on origin
This section needs expansion. You can help by adding to it. (September
Although the process of subduction as it occurs today is fairly well
understood, its origin remains a matter of discussion and continuing
Subduction initiation can occur spontaneously if denser oceanic
lithosphere is able to founder and sink beneath adjacent oceanic or
continental lithosphere; alternatively, existing plate motions can
induce new subduction zones by forcing oceanic lithosphere to rupture
and sink into the asthenosphere. 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.
Results from numerical models generally favor induced subduction
initiation for most modern subduction zones, which is supported
by geologic studies, but other analogue modeling shows the
possibility of spontaneous subduction from inherent density
differences between two plates at passive margins, and
observations from the Izu-Bonin-Mariana subduction system are
compatible with spontaneous subduction nucleation.
Furthermore, subduction is likely to have spontaneously initiated at
some point in Earth's history, as induced subduction nucleation
requires existing plate motions, though an unorthodox proposal by A.
Yin suggests that meteorite impacts may have contributed to subduction
initiation on early Earth.
Don L. Anderson
Don L. Anderson has hypothesized that plate tectonics
could not happen without the calcium carbonate laid down by bioforms
at the edges of subduction zones. The massive weight of these
sediments could be softening the underlying rocks, making them pliable
enough to plunge. However, considering that some refractory
minerals used for dating early Earth, such as zircon, are typically
generated in subduction zones and associated with granites and
pegmatites, some of these early dates may have preceded significant
biological activity on Earth.
Subduction zone metamorphism
Main article: Volcanic arc
Oceanic plates are subducted creating oceanic trenches.
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, hence the term volcanic arc. Two
kinds of arcs are generally observed on Earth: island arcs that form
on oceanic lithosphere (for example, the Mariana and the
arcs), and continental arcs such as the Cascade Volcanic Arc, that
form along the coast of continents.
Island arcs are produced by the
subduction of oceanic lithosphere beneath another oceanic lithosphere
(ocean-ocean subduction) while continental arcs formed during
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 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 25% of the total volume of magma produced each year
on Earth (approximately thirty to thirty-five cubic kilometers), much
less than the volume produced at mid-ocean ridges, and they contribute
to the formation of new continental crust. Arc volcanism has the
greatest impact on humans, because many arc volcanoes lie above sea
level and erupt violently. Aerosols injected into the stratosphere
during violent eruptions can cause rapid cooling of Earth's climate
and affect air travel.
Earthquakes and tsunamis
Main article: Megathrust earthquake
The strains caused by plate convergence in subduction zones cause at
least three different types of earthquakes. Earthquakes mainly
propagate in the cold subducting slab and define the Wadati–Benioff
zone. Seismicity shows that the slab can be tracked down to the upper
mantle/lower mantle boundary (approximately six hundred kilometer
Nine of the ten largest earthquakes of the last 100 years were
subduction zone events, 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. 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 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. 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,
Bletery et al. 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
Outer rise earthquakes occur when normal faults oceanward of the
subduction zone are activated by flexture 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.
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 seven hundred
kilometers. These quakes define inclined zones of seismicity known as
Wadati–Benioff zones, after the scientists who discovered them,
which trace the descending lithosphere.
Seismic tomography has helped
detect subducted lithosphere, slabs, deep in the mantle where there
are no earthquakes. About 100 slabs have been described in terms of
depth and their timing and location of subduction. Some subducted
slabs seem to have difficulty to penetrate the major discontinuity in
the mantle, marking the boundary between the upper mantle and lower
mantle, that lies at a depth of about 670 kilometers. Other subducted
oceanic plates can penetrate all the way to the core-mantle boundary.
The great seismic discontinuities in the mantle, at 410 and 670
kilometer depth, are disrupted by the descent of cold slabs in deep
Main article: Orogeny
This section needs expansion. You can help by adding to it. (June
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, or
prism. These accretionary wedges can be identified by ophiolites
(uplifted ocean crust consisting of sediments, pillow basalts, sheeted
dykes, gabbro, and peridotite). This accretion process is thought
by many geologists to be the reason for the crustal growth of western
North America and of the uplift that produced the Rocky Mountains.
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 leading to folding, faulting, crustal thickening and mountain
building. This flat-slab subduction process is thought to be one of
the main causes of mountain building and deformation in South America.
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 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 some that are
Flat-slab subduction (<30°): occurs when subducting lithosphere,
called a slab, subducts horizontally or nearly horizontally. The flat
slab can extend for hundreds of kilometers and can even extend to over
a thousand. That is abnormal, as the dense slab typically sinks at a
much steeper angle directly at the subduction zone. Because subduction
of slabs to depth is necessary to drive subduction zone volcanism
(through the destabilization and dewatering of minerals and the
resultant flux melting of the mantle wedge), 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
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
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. Then, 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.
Steep-angle subduction (>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 making volcanic arcs and fragments of
continental crust wander away from continents over geological times
leaving behind a marginal sea.
Subduction zones are important for several reasons:
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 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
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, as well as 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 earthquakes, making the effects
on using any specific site for disposal unpredictable and possibly
adverse to the safety of longterm disposal.
Divergent double subduction
List of tectonic plate interactions
Paired metamorphic belts
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Look up subduction in Wiktionary, the free dictionary.
Animation of a subduction zone.
From the Seafloor to the Volcano's Top Video about the work of the
Collaborative Research Center (SFB) 574 Volatiles and
Subduction Zones in Chile by GEOMAR I Helmholtz Centre for Ocean
 Realistic animation of plate forming processes beneath divergent
plate margins and destruction of lithosphere beneath convergent plate
margins (~ 9 minutes long).
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