Diagram of the geological process of subduction
Subduction is a geological process that takes place at convergent boundaries of tectonic plates where one plate moves under another and is forced to sink due to high gravitational potential energy into the mantle. Regions where this process occurs are known as subduction zones.
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.
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. This
means that 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. Furthermore, subduction zones develop belts of deformation and metamorphism in the subducting crust, whose exhumation is part of orogeny and also leads to mountain building in addition to collisional thickening.
General description
Subduction zones are sites of gravitational sinking of Earth's lithosphere (the crust plus the top non-convecting portion of the upper mantle). 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 has, on average, a greater density. At a depth of greater than 60 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 and continental 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.
Oceanic
subduction zones dive down into the mantle beneath 55,000 km
(34,000 mi) of convergent plate margins (Lallemand, 1999), almost equal
to the cumulative 60,000 km (37,000 mi) of mid-ocean ridges. Subduction
zones 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 660-kilometer 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 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.
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
Initiation
Although
the process of subduction as it occurs today is fairly well understood,
its origin remains a matter of discussion and continuing study.
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.
Geophysicist 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.
Modern-style subduction
Modern-style subduction is characterized by low geothermal gradients and the associated formation of high-pressure low temperature rocks such as eclogite and blueschist. Likewise, rock assemblages called ophiolites, associated to modern-style subduction, also indicate such conditions. 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.
Nevertheless, the eclogite itself was produced by oceanic subdcution
during the assembly of supercontinents at about 1.9–2.0 Ga.
Blueschist is a rock typical for present-day subduction settings. Absence of blueschist older than Neoproterozoic reflect more magnesium-rich compositions of Earth's oceanic crust during that period. These more magnesium-rich rocks metamorphose into greenschist at conditions when modern oceanic crust rocks metamorphose into blueschist. The ancient magnesium-rich rocks means that Earth's mantle
was once hotter, but not that subduction conditions were hotter.
Previously, lack of pre-Neoproterozoic blueschist was thought to
indicate a different type of subduction. Both lines of evidence refutes previous conceptions of modern-style subduction having been initiated in the Neoproterozoic Era 1.0 Ga ago.
Effects
Volcanic activity
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 Tonga island 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
The strains caused by plate convergence in subduction zones cause at
least three 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
depth).
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, 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.
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 700 km (430 mi).
These quakes define inclined zones of seismicity known as Wadati–Benioff zones which trace the descending lithosphere.
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 410 km (250 mi)
depth and 670 km (420 mi), are disrupted by the descent of cold slabs in
deep subduction zones. Some subducted slabs seem to have difficulty
penetrating the major discontinuity
that marks the boundary between upper mantle and lower mantle at a
depth of about 670 kilometers. Other subducted oceanic plates have sunk
all the way 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, or prism. These accretionary wedges can be identified by ophiolites (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 leading 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.
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 that are extremely steep.
- Flat-slab subduction (subducting angle less than 30°) occurs when subducting lithosphere, called a 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 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 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. 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 (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 making volcanic arcs and fragments of continental crust wander away from continents over geological times leaving behind a marginal sea.
Importance
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 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 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 earthquakes,
making the effects on using any specific site for disposal
unpredictable and possibly adverse to the safety of longterm disposal.
