In geology, subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate and sinks into the mantle as the plates converge. 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 2 to 8 cm per year.
Plates include both oceanic crust and continental crust. Stable subduction zones involve the oceanic crust of one plate sliding beneath the continental crust or oceanic crust of another plate. That is, the subducted crust is always oceanic while the overriding crust may or may not be oceanic. Subduction zones are often noted for their high rates of volcanism, earthquakes, and mountain building.
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.
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. 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. 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), 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.
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.
A model of the initiation of subduction, based on analytic and analog modeling, presumes that the difference of density between two adjacent lithospheric 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". Analog experiments based on this concept were carried out using a centrifuge using a 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. Geophysicist Don L. Anderson has hypothesized 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. 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 preceded the biologic activty on earth.
Volcanoes that occur above subduction zones, such as Mount St. Helens and Mount Fuji, lie 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 form on oceanic lithosphere, such as the Mariana or Tonga island arcs, or continental arc that form 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.
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.
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.
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.
Subduction zones are important for several reasons:
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, this 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 long-term disposal.
Lallemand, S., La Subduction Oceanique, Gordon and Breach, Newark, N. J., 1999.
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