Subduction
Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at the convergent boundaries between tectonic plates. Where one tectonic plate converges with a second plate, the heavier plate dives beneath the other and sinks into the mantle. A region where this process occurs is known as a subduction zone, and its surface expression is known as an arc-trench complex. The process of subduction has created most of the Earth's continental crust. Rates of subduction are typically measured in centimeters per year, with rates of convergence as high as 11 cm/year.
Subduction is possible because the cold and rigid oceanic lithosphere is slightly denser than the underlying asthenosphere, the hot, ductile layer in the upper mantle. Once initiated, stable subduction is driven mostly by the negative buoyancy of the dense subducting lithosphere. The down-going slab sinks into the mantle largely under its own weight.
Earthquakes are common along subduction zones, and fluids released by the subducting plate trigger volcanism in the overriding plate. If the subducting plate sinks at a shallow angle, the overriding plate develops a belt of deformation characterized by crustal thickening, mountain building, and metamorphism. Subduction at a steeper angle is characterized by the formation of back-arc basins.
Subduction and plate tectonics
According to the theory of plate tectonics, the Earth's lithosphere, its rigid outer shell, is broken into sixteen larger tectonic plates and several smaller plates. These plates are in slow motion, due mostly to the pull force of subducting lithosphere. Sinking lithosphere at subduction zones is a part of convection cells in the underlying ductile mantle. This process of convection allows heat generated by radioactive decay to escape from the Earth's interior.The lithosphere consists of the outermost light crust plus the uppermost rigid portion of the mantle. Oceanic lithosphere ranges in thickness from just a few km for young lithosphere created at mid-ocean ridges to around for the oldest oceanic lithosphere. Continental lithosphere is up to thick. The lithosphere is relatively cold and rigid compared with the underlying asthenosphere, and so tectonic plates move as solid bodies atop the asthenosphere. Individual plates often include both regions of the oceanic lithosphere and continental lithosphere.
Subduction zones are where cold oceanic lithosphere sinks back into the mantle and is recycled. They are found at convergent plate boundaries, where the heavier oceanic lithosphere of one plate is overridden by the leading edge of another, less-dense plate. The overridden plate sinks at an angle most commonly between 25 and 75 degrees to Earth's surface. This sinking is driven by the temperature difference between the slab and the surrounding asthenosphere, as the colder oceanic lithosphere is, on average, more dense. Sediments and some trapped water are carried downwards by the slab and recycled into the deep mantle.
So far, Earth is the only planet where subduction is known to occur, and subduction zones are its most important tectonic feature. Subduction is the driving force behind plate tectonics, and without it, plate tectonics could not occur. Oceanic subduction zones are located along of convergent plate margins, almost equal to the cumulative plate formation rate of of mid-ocean ridges.
Sea water seeps into oceanic lithosphere through fractures and pores, and reacts with minerals in the crust and mantle to form hydrous minerals that store water in their crystal structures. Water is transported into the deep mantle via hydrous minerals in subducting slabs. During subduction, a series of minerals in these slabs such as serpentine can be stable at different pressures within the slab geotherms, and may transport a significant amount of water into the Earth's interior. As plates sink and heat up, released fluids can trigger seismicity and induce melting within the subducted plate and in the overlying mantle wedge. This type of melting selectively concentrates volatiles and transports them into the overlying plate. If an eruption occurs, the cycle then returns the volatiles into the oceans and atmosphere.
Structure of subduction zones
Arc-trench complex
The surface expressions of subduction zones are arc-trench complexes. On the ocean side of the complex, where the subducting plate first approaches the subduction zone, there is often an outer trench high or outer trench swell. Here the plate shallows slightly before plunging downwards, as a consequence of the rigidity of the plate. The point where the slab begins to plunge downwards is marked by an oceanic trench. Oceanic trenches are the deepest parts of the ocean floor.Beyond the trench is the forearc portion of the overriding plate. Depending on sedimentation rates, the forearc may include an accretionary wedge of sediments scraped off the subducting slab and accreted to the overriding plate. However, not all arc-trench complexes have an accretionary wedge. Accretionary arcs have a well-developed forearc basin behind the accretionary wedge, while the forearc basin is poorly developed in non-accretionary arcs.
Beyond the forearc basin, volcanoes are found in long chains called volcanic arcs. 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 melting temperature of the mantle rock, generating magma via flux melting. The magmas, in turn, rise as diapirs because they are less dense than the rocks of the mantle. The mantle-derived magmas can ultimately reach the Earth's surface, resulting in volcanic eruptions. The chemical composition of the erupting lava depends upon the degree to which the mantle-derived basalt interacts with Earth's crust or undergoes fractional crystallization. Arc volcanoes tend to produce dangerous eruptions because they are rich in water and tend to be extremely explosive. Krakatoa, Nevado del Ruiz, and Mount Vesuvius are all examples of arc volcanoes. Arcs are also associated with most ore deposits.
Beyond the volcanic arc is a back-arc region whose character depends strongly on the angle of subduction of the subducting slab. Where this angle is shallow, the subducting slab drags the overlying continental crust partially with it, which produces a zone of shortening and crustal thickening in which there may be extensive folding and thrust faulting. If the angle of subduction steepens or rolls back, the upper plate lithosphere will be put in tension instead, often producing a back-arc basin.
Deep structure
The arc-trench complex is the surface expression of a much deeper structure. Though not directly accessible, the deeper portions can be studied using geophysics and geochemistry. Subduction zones are defined by an inclined zone of earthquakes, the Wadati–Benioff zone, that dips away from the trench and extends down below the volcanic arc to the 660-kilometer discontinuity. Subduction zone earthquakes occur at greater depths than elsewhere on Earth ; such deep earthquakes may be driven by deep phase transformations, thermal runaway, or dehydration embrittlement. Seismic tomography shows that some slabs can penetrate the lower mantle and sink clear to the core–mantle boundary. Here the residue of the slabs may eventually heat enough to rise back to the surface as mantle plumes.Subduction angle
Subduction typically occurs at a moderately steep angle by the time it is beneath the volcanic arc. However, anomalous shallower angles of subduction are known to exist as well as some that are extremely steep.- Flat slab subduction occurs when the slab subducts nearly horizontally. The relatively flat slab can extend for hundreds of kilometers under the upper plate. This geometry is commonly caused by the subduction of buoyant lithosphere due to thickened crust or warmer lithosphere. Recent studies have also shown a strong correlation that older and wider subduction zones are related to flatter subduction dips. This provides an explanation as to why flat subduction only presently occur in the eastern pacific as only these regions were old and wide enough to support flat slab subduction and why the Laramide flat slab subduction and South China flat slab subduction were possible. Hu ultimately proposes that a combination of subduction age and slab characteristics provide the strongest controls over subduction dips. Because subduction of slabs to depth is necessary to drive subduction zone volcanism, flat-slab subduction can be invoked to explain volcanic gaps.
The Laramide Orogeny in the Rocky Mountains of the United States is attributed to flat-slab subduction. During this orogeny, 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 occurs in subduction zones where Earth's oceanic crust and lithosphere are cold and thick and have, therefore, lost buoyancy. Recent studies have also correlated steep angled subduction zones with younger and less extensive subduction zones. This would explain why most modern subduction zones are relatively steep. The steepest dipping subduction zone lies in the Mariana Trench, which is also where the oceanic lithosphere 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 the upper plate, creating volcanic arcs and pulling fragments of continental crust away from continents to leave behind a marginal sea.