subduction

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. A subduction zone is an area on Earth where two tectonic plates move towards one another and subduction occurs. 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].

Subduction zones involve an oceanic plate sliding beneath either a continental plate or another oceanic plate. Subduction zones are often noted for their high rates of volcanism, earthquakes, and mountain building. This is because subduction processes result in melt of the mantle that produces a volcanic arc as relatively lighter rock is forcibly submerged.

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 are the opposite of divergent boundaries, where tectonic plates move apart.

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 leading edge of the subducting plate—is overridden by 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.

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 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 be produce dangerous eruptions because they are rich in water (from the slab and sediments) and tend to be extremely explosive. Krakatoa, Nevada 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 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 and cool mantle sinks causing convection cells to form. At points where two downward moving convecting cells meet (cold mantle sinking), convection can occur taking 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.

Theory on origin

There have been some recent theories on the beginnings of subduction and Plate tectonics generally. 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, which was 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" (Mart et al., 2005; Goren et al., 2008). 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, and then it 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]}

Associated volcanic activity

Oceanic plates are subducted creating oceanic trenches.

Volcanoes that occur above subduction zones, such as Mount St. Helens and Mount Fuji, often occur in arcuate chains, hence the term volcanic arc or island arc. Not all "volcanic arcs" are arced: trenches and arcs are often linear.

The magmatism associated with the volcanic arc occurs 100-300 km away from the trench. However, a relationship has been found, which relates volcanic arc location to depth of the subducted crust as defined by the Wadati-Benioff zone. Studies of many volcanic arcs around the world have revealed that volcanic arcs tend to form at a location where the subducted slab has reached a depth of about 100 km. This has implications for the mechanism that causes the magma production at these arcs. 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. Nevertheless, 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.

The absence of volcanism in the Norte Chico region of Chile is believed to be a result of a flat-slab subduction caused by the Juan Fernández Ridge.

Earthquakes and tsunamis

The strains caused by plate convergence in subduction zones cause at least three different types of earthquake.

Nine out of the ten largest earthquakes to occur in the last 100 years were subduction zone events. This includes the 1960 Chilean Earthquake, which at M 9.5 was the largest earthquake ever recorded. 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 ocean-ward 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.

Importance

Cartoon representation of the Subduction Factory, from Y. Tatsumi JAMSTEC.

Subduction zones are important for several reasons:

1. Subduction Zone Physics: Sinking of mantle lithosphere is the strongest force (but not the only one) needed to drive plate motion and is the dominant mode of mantle convection.
2. Subduction Zone Chemistry: The cold subducting plate sinking in subduction zones releases water into the overlying mantle, causing mantle melting and fractionating of elements between surface and deep mantle reservoirs, producing island arcs and continental crust.
3. Subduction zones mix subducted sediments, oceanic crust, and mantle lithosphere with mantle from the overriding plate to produce fluids, 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.^[5]

See also

• Plate tectonics
• List of tectonic plate interactions
• Back-arc basin
• Oceanic rift
• Oceanic trench
• Obduction
• Orogeny
• Slab window

References

External links

Look up subduction in Wiktionary, the free dictionary.

• Animation of a subduction zone.