Subduction

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Geometry of a subduction zone - insets to show accretionary prism
and partial melting of hydrated asthenosphere

In geology,
subduction is the process that takes place at convergent
boundaries by which one tectonic plate moves
under another tectonic plate, sinking into the Earth's mantle, as the
plates converge. These 3D regions of mantle downwellings are known as
"Subduction Zones". A subduction zone is an area on Earth where two tectonic plates
move towards one another and one slides under the other. Rates of subduction are
typically measured in centimeters per year, with the average rate of convergence
being approximately 2 to 8 centimeters per year (about the rate a fingernail
grows).[1]

Plates can be formed from either oceanic lithosphere or continental
lithosphere. Stable subduction zones involve an oceanic
plate sliding beneath either a continental plate or
another oceanic plate (that is, the subducted plate is always oceanic while the
subducting plate may or may not be oceanic). Subduction zones are often noted
for their high rates of volcanism, earthquakes, and mountain building.
Continental
collision results where a continental plate is subducted.

Orogenesis, or
mountain-building, occurs when large pieces of material on the subducting plate
(such as island arcs) are pressed into
the overriding plate. These areas are subject to many earthquakes, which are caused
by the interactions between the subducting slab and the mantle, the
volcanoes, and (when applicable) the mountain-building related to island arc
collisions.[citation
needed]

Contents

1 General
description

2 Theory on
origin

3 Effects

3.1 Volcanic
activity

3.2 Earthquakes and
tsunamis

3.3 Orogeny

4 Subduction
angle

5 Importance

6 See also

7 References

8 External
links

[edit]
General description

Subduction zones mark sites of convective downwelling of the Earth's
lithosphere (the crust plus the top
brittle portion of the upper mantle). Subduction zones exist at convergent plate
boundaries where one plate of oceanic lithosphere
converges with another plate. The down-going slab -- the subducting
plate—is overridden by the leading edge of the other plate. The slab sinks at an
angle of approximately 25 to 45 degrees to the surface of the Earth. At a depth
of approximately 80–120 km, the basalt of the oceanic slab is
converted to a metamorphic rock called eclogite. At this point, the
density of the oceanic lithosphere increases and it is carried into the mantle by the
downwelling convective currents. It is at subduction zones that the Earth's
lithosphere, oceanic crust, sedimentary layers,
and some trapped water are recycled into the deep mantle. Earth is the only
planet where subduction is known to occur. Without subduction, plate tectonics could
not exist.

Subduction zones dive down into the mantle beneath 55,000 km of
convergent plate margins (Lallemand, 1999), almost equal to the cumulative
60,000 km of mid-ocean ridges. Subduction zones burrow deeply but are
imperfectly camouflaged, and we can use geophysics and geochemistry 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 660 km
discontinuity. 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 enormously greater
depths than elsewhere on Earth, where seismicity is limited to the outermost
20 km of the solid Earth.

The subducting basalt and sediment are normally rich in hydrous minerals
and clays. 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. These magmas, in turn, rise, because they are less dense than
the rocks of the mantle. These mantle-derived magmas (which are basaltic in
composition) can continue to rise, ultimately to the 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 (a) interacts with (melts)
the Earth's crust and/or (b) 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 - again believed to be carried by water
and concentrated in and around their host volcanoes in rock termed "ore".

Subduction results from convection in the asthenosphere. The heat from the
core of the earth that is imparted to the mantle causes the mantle to convect
much the way boiling water convects in a pan on the stove. Hot mantle at the
core-mantle boundary rises while cool mantle sinks, causing convection cells to
form. At points where two downward moving convecting cells meet (cold mantle
sinking), convection can occur, forcing the oceanic crust below either
continents or other oceanic crust. Continental crust tends to override oceanic
crust because it consists of less dense granite compared to the basalt of the
oceanic crust.

[edit]
Theory on origin

Although the process of subduction as it occurs today is fairly well
understood, its origin remains a matter of discussion and ongoing study. A
recent paper by V.L. Hansen in Geology presented a
hypothesis that mantle upwelling and similar thermal processes, combined with an
impact from an extraterrestrial source, would give the early earth the
discontinuities in the crust for the subduction of the denser material
underneath lighter material.[2]

A model of the initiation of subduction, based on analytic and analog
modeling, presumes that the difference of density between two adjacent
lithopsheric slabs is sufficient to lead to the initiation of subduction. The
analytic part of the model shows that where two lithospheric slabs of different
densities are positioned one next to the other, maximum differential lithostatic
pressure would occur at the base of the denser slab directed towards the lighter
one. The resulting strain would lead to the rotation of the contact zone between
the slabs to dip towards the lighter slab, and the dip would be reduced until
offset along the contact zone would be enabled. The parameters that constrain
the rotation of the contact zone are known as "Argand Numbers".[3][4]
Analog experiments based on this concept were carried out using a centrifuge,
comprising lighter and denser brittle and ductile "lithosphere" floating on
still denser "asthenosphere". The analog experiments suggested that the
initiation of subduction started with the penetration of the denser ductile
"lithosphere" below its lighter counterpart. Consequently, the lighter
"lithosphere" was uplifted, then collapsed on the denser slab, increasing the
load on its edge and driving the denser sequence further under the lighter slab.
It was presumed further that once the denser "lithosphere" was set below the
lighter one, it underwent conversion to eclogite which increased its density and
drove it to subduct into the "asthenosphere". The rate of this part of the
subduction process was determined by friction. Reduction of slab friction in
nature could result from serpentinization and other water-related processes.[3][4]

According to the geophysicist Don
L. Anderson, it is conceivable that plate tectonics could not happen without
the calcium carbonate laid
down by living beings at the edges of subduction zones. The massive weight of
these sediments could be softening the underlying rocks, making them pliable
enough to plunge.[5]

[edit]
Effects

[edit]
Volcanic activity

Oceanic plates are subducted
creating oceanic
trenches.

Volcanoes
that occur above subduction zones, such as Mount St. Helens and Mount
Fuji, at ~ 100 km from the trench in arcuate chains, hence the term volcanic
arc. Two kinds of arcs are generally observed on Earth: island
arcs that formed on oceanic lithosphere, such as the Mariana or Tonga island arcs, or
continental arc that
formed on the continent, such as the Cascade Volcanic
Arc. Island arcs are produced by the subduction of oceanic lithosphere
beneath another oceanic lithosphere (oceanic subduction), while continental arcs
formed during subduction of oceanic lithosphere beneath a continental
lithosphere.

The arc magmatism occurs 100–200 km away from the trench and ~
100 km from the subducting slab. This depth of arc magma generation is the
consequence of the interaction between fluids, released from the subducting
slab, and the arc mantle wedge that is hot enough to generate hydrous melting.
Arcs produce about 25% of the total volume of magma produced each year on Earth
(~30–35 km³), 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 the Earth's climate and affect
air travel.

[edit]
Earthquakes and
tsunamis

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 (~ 600 km depth).

Nine out of the ten largest earthquakes to occur in the last 100 years were
subduction zone events. This includes 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 the earth to deform in a more brittle fashion
than it would in a normal geothermal gradient setting. Because earthquakes can
only occur when a rock is deforming in a brittle fashion, subduction zones can
create large earthquakes. If such an earthquake 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 Eurasian Plate on
December 26, 2004 that devastated the
areas around the Indian Ocean. Small tremors that create small, non-damaging
tsunamis occur frequently.

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 Samoa earthquake of
2009 is an example of this type of event. Displacement of the sea floor caused
by this event generated a 6m tsunami in nearby Samoa.

Anomalously deep events are a characteristic of subduction zones which
produce the deepest earthquakes on the planet.
Earthquakes are generally restricted to the shallow, brittle parts of the crust,
generally at depths of less than 20 km. However, in subduction zones,
earthquakes occur at depths as great as 700 km. These earthquakes 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 in regions where there are no
earthquakes. Some subducted slabs seem not to be able to penetrate the major discontinuity in the
mantle that lies at a depth of about 670 km, whereas 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 km depth - are disrupted by the descent of cold slabs in deep
subduction zones.

[edit]
Orogeny

Main article: Orogeny

This section requires expansion.

Subducting plates can bring island arcs and sediments to convergent margins.
This material often does not subduct with the rest of the plate, but instead is
accreted to the continent in the form of exotic
terranes. These cause crustal thickening and mountain-building.

[edit]
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 some extremely steep.

Flat-slab subduction (<30°): occurs when subducting lithosphere,
called a slab, subducts horizontally or nearly-horizontally. The flat slab can
extend for hundreds of to over a thousand kilometers. This 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 subdution 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 respectively. Around Taitao Peninsula
flat-slab dubduction 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.[6]
During this time, 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.

Steep-angle subduction (>70°): occurs in subduction zones where
earth's oceanic crust and
lithosphere are old and thick and have therefore lost buoyancy. At present the
steepest dipping subduction zone lies the Marianas Trench where
also the earths oldest oceanic crust lies (Jurassic age), if exempting
ophiolites. Steep-angle
subduction is in contrast to flat-slab subduction associated with back-arc
extension[7]
of crust making volcanic arcs and fragments of continental crust wander away
from continents over geological times leaving behind a marginal sea.

[edit]
Importance

Cartoon representation of the Subduction
Factory.

Subduction zones are important for several reasons[citation
needed]:

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
crust.

Subduction zones drag down subducted oceanic sediments, oceanic crust, and
mantle lithosphere that interact with the hot asthenospheric mantle from the overriding
plate to produce calc-alkaline series
melts, ore deposits, and continental crust.

Subduction zones have also been considered as possible disposal
sites for nuclear waste, where the action would carry the material into the
planetary mantle, safely away
from any possible influence on humanity or the surface environment, but this
method of disposal is currently banned by international agreement.[8]

[edit]
See also

List
of tectonic plate interactions

Back-arc basin

Oceanic trench

Obduction

Slab window

[edit]
References

This article needs additional citations
for verification.
Please help improve
this article by adding citations to reliable
sources. Unsourced material may be challenged
and removed.
(May 2011)

^ Defant, M. J. (1998). Voyage of Discovery: From the
Big Bang to the Ice Age. Mancorp. pp. 325. ISBN 0931541611.

^ Hansen, Vicki L. (December 2007). "Subduction origin
on early Earth: A hypothesis". Geology 35 (12): 1059–1062. doi:10.1130/G24202A.1. Univ. of Minnesota-Duluth.

^ a
b
Mart, Y., Aharonov, E., Mulugeta, G., Ryan,
W.B.F., Tentler, T., Goren, L. (2005). "Analog modeling of the initiation of
subduction". Geophys. J. Int. 160 (3): 1081–1091. doi:10.1111/j.1365-246X.2005.02544.x.

^ a
b
Goren, L., E. Aharonov, G. Mulugeta, H. A.
Koyi, and Y. Mart (2008). "Ductile Deformation of Passive Margins: A New
Mechanism for Subduction Initiation". J. Geophys. Res. 113:
B08411. Bibcode 2008JGRB..11308411G. doi:10.1029/2005JB004179.

^
Harding, Stephan. Animate Eart. Science, Intuition and Gaia. Chelsea
Green Publishing, 2006, p. 114. ISBN
1-933392-29-0

^ W. P. Schellart, D. R. Stegman, R. J. Farrington, J.
Freeman, and L. Moresi (16 July 2010). "Cenozoic Tectonics of Western North
America Controlled by Evolving Width of Farallon Slab". Science
329 (5989): 316–319. doi:10.1126/science.1190366. PMID 20647465.

^ Lallemand, Serge; Heuret, Arnauld; Boutelier, David
(8 September 2005). "On the
relationships between slab dip, back-arc stress, upper plate absolute motion,
and crustal nature in subduction zones". Geochemistry Geophysics
Geosystems 6 (9): Q09006. Bibcode 2005GGG.....609006L. doi:10.1029/2005GC000917. http://perso-sdt.univ-brest.fr/~jacdev/pdf/1.pdf.

^ World Nuclear Association

Stern, R.J. (2002). "Subduction zones".
Reviews of Geophysics 40 (4): 1012. Bibcode 2002RvGeo..40.1012S. doi:10.1029/2001RG000108.

Stern, R.J. (1998). "A Subduction Primer
for Instructors of Introductory Geology Courses and Authors of Introductory
Geology Textbooks". J. Geoscience Education 46:
221–228.

Tatsumi, Y. (2005). "The Subduction
Factory: How it operates on Earth". GSA Today 15 (7): 4–10. doi:10.1130/1052-5173(2005)015[4:TSFHIO]2.0.CO;2.

Lallemand, S., La Subduction Oceanique, Gordon and Breach, Newark, N. J.,
1999.

[edit]
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