Convergent boundary

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A convergent boundary is an area on Earth where two or more lithospheric plates collide. One plate eventually slides beneath the other causing a process known as subduction. The subduction zone can be defined by a plane where many earthquakes occur, called the Benioff Zone. [1] These collisions happen on scales of millions to tens of millions of years and can lead to volcanism, earthquakes, orogenesis, destruction of lithosphere, and deformation. Convergent boundaries occur between oceanic-oceanic lithosphere, oceanic-continental lithosphere, and continental-continental lithosphere. The geologic features related to convergent boundaries vary depending on crust types.

Plate tectonics The scientific theory that describes the large-scale motions of Earths lithosphere

Plate tectonics is a scientific theory describing the large-scale motion of seven large plates and the movements of a larger number of smaller plates of the Earth's lithosphere, since tectonic processes began on Earth between 3 and 3.5 billion years ago. The model builds on the concept of continental drift, an idea developed during the first decades of the 20th century. The geoscientific community accepted plate-tectonic theory after seafloor spreading was validated in the late 1950s and early 1960s.

Subduction A geological process at convergent tectonic plate boundaries where one plate moves under the other

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 gravity into the mantle. Regions where this process occurs are known as subduction zones. Rates of subduction are typically in centimeters per year, with the average rate of convergence being approximately two to eight centimeters per year along most plate boundaries.

Wadati–Benioff zone Planar zone of seismicity corresponding with the down-going slab

A Wadati–Benioff zone is a planar zone of seismicity corresponding with the down-going slab in a subduction zone. Differential motion along the zone produces numerous earthquakes, the foci of which may be as deep as about 670 kilometres. The term was named for the two seismologists, Hugo Benioff of the California Institute of Technology and Kiyoo Wadati of the Japan Meteorological Agency, who independently discovered the zones.

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Plate tectonics is driven by convection cells in the mantle. Convection cells are the result of heat generated by radioactive decay of elements in the mantle escaping to the surface and the return of cool materials from the surface to the mantle. [2] These convection cells bring hot mantle material to the surface along spreading centers creating new crust. As this new crust is pushed away from the spreading center by the formation of newer crust, it cools, thins, and becomes denser. Subduction initiates when this dense crust converges with the less dense crust. The force of gravity helps drive the subducting slab into the mantle. Evidence supports that the force of gravity will increase plate velocity. [3] As the relatively cool subducting slab sinks deeper into the mantle, it is heated causing dehydration of hydrous minerals. This releases water into the hotter asthenosphere, which leads to partial melting of asthenosphere and volcanism. Both dehydration and partial melting occurs along the 1000 °C isotherm, generally at depths of 65 – 130  km. [4] [5]

Simplified schematic of different convergent boundary types. Continental-continental destructive plate boundary.svg
Simplified schematic of different convergent boundary types.

Some lithospheric plates consist of both continental and oceanic lithosphere. In some instances, initial convergence with another plate will destroy oceanic lithosphere, leading to convergence of two continental plates. Neither continental plate will subduct. It is likely that the plate may break along the boundary of continental and oceanic crust. Seismic tomography reveals pieces of lithosphere that have broken off during convergence.

Continental crust Layer of rock that forms the continents and continental shelves

Continental crust is the layer of igneous, sedimentary, and metamorphic rocks that forms the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial because its bulk composition is richer in silicates and aluminium minerals and has a lower density compared to the oceanic crust, called sima which is richer in magnesium silicate minerals and is denser. Changes in seismic wave velocities have shown that at a certain depth, there is a reasonably sharp contrast between the more felsic upper continental crust and the lower continental crust, which is more mafic in character.

Oceanic crust The uppermost layer of the oceanic portion of a tectonic plate

Oceanic crust is the uppermost layer of the oceanic portion of a tectonic plate. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramafic cumulates. The crust overlies the solidified and uppermost layer of the mantle. The crust and the solid mantle layer together constitute oceanic lithosphere.

Seismic tomography is a technique for imaging the subsurface of the Earth with seismic waves produced by earthquakes or explosions. P-, S-, and surface waves can be used for tomographic models of different resolutions based on seismic wavelength, wave source distance, and the seismograph array coverage. The data received at seismometers are used to solve an inverse problem, wherein the locations of reflection and refraction of the wave paths are determined. This solution can be used to create 3D images of velocity anomalies which may be interpreted as structural, thermal, or compositional variations. Geoscientists use these images to better understand core, mantle, and plate tectonic processes.

Subduction zones

Subduction zones are areas where one lithospheric plate slides beneath another at a convergent boundary due to lithospheric density differences. These plates dip at an average of 45° but can vary. Subduction zones are often marked by an abundance of earthquakes, the result of internal deformation of the plate, convergence with the opposing plate, and bending at the oceanic trench. Earthquakes have been detected to a depth of 670  km. The relatively cold and dense subducting plates are pulled into the mantle and help drive mantle convection. [6]

Oceanic - Oceanic Convergence

In collisions between two oceanic plates, the cooler, denser oceanic lithosphere sinks beneath the warmer, less dense oceanic lithosphere. As the slab sinks deeper into the mantle, it releases water from dehydration of hydrous minerals in the oceanic crust. This water reduces the melting temperature of rocks in the asthenosphere and causes partial melting. Partial melt will travel up through the asthenosphere, eventually, reach the surface, and form volcanic island arcs.

Continental - Oceanic Convergence

When oceanic lithosphere and continental lithosphere collide, the dense oceanic lithosphere slides beneath the less dense continental lithosphere. An accretionary wedge forms on the continental crust as deep-sea sediments and oceanic crust are scraped from the oceanic plate. Volcanic arcs form on continental lithosphere as the result of partial melting due to dehydration of the hydrous minerals of the subducting slab.

Continental - Continental Convergence

Some lithospheric plates consist of both continental and oceanic crust. Subduction initiates as oceanic lithosphere slides beneath continental crust. As the oceanic lithosphere subducts to greater depths, the attached continental crust is pulled closer to the subduction zone. Once the continental lithosphere reaches the subduction zone, subduction processes are altered as continental lithosphere is more buoyant and resists subduction beneath other continental lithosphere. A small portion of the continental crust may be subducted until the slab breaks, allowing the oceanic lithosphere to continue subducting, hot asthenosphere to rise and fill the void, and rebound of the continental lithosphere. [7] Evidence of this continental rebound include ultrahigh pressure metamorphic rocks which form at depths of 90 – 125 km that are exposed at the surface. [8]

Volcanism and Volcanic Arcs

The oceanic crust contains hydrated minerals such as the amphibole group. During subduction, oceanic lithosphere is heated and metamorphosed causing dehydration of these hydrous minerals contained within basalts, releasing water into the asthenosphere. The release of water into the asthenosphere leads to partial melting. Partial melting allows the rise of more buoyant, hot material and can lead to volcanism at the surface and emplacement of plutons in the subsurface. This processes which generate magma are not entirely understood.

Where these magmas reach the surface they create volcanic arcs. Volcanic arcs can form as island arc chains or as arcs on continental crust. Three series of volcanic rocks generally form arcs, Tholeiitic (low iron basalts), calc-alkaline (moderately enriched in potassium and incompatible elements), and alkaline (highly enriched in potassium) which are very rare. [5]

Back arc basins

Back arc basins form behind a volcanic arc and are associated with extensional tectonics and high heat flow, often being home to seafloor spreading centers. These spreading centers are like mid ocean ridges, though the magma composition of back arc basins is generally more varied and contains a higher water content than mid ocean ridge magmas. [9] Back arc basins are often characterized by thin, hot lithosphere. Opening of back arc basins are still being studied but it’s possible that movement of hot asthenosphere into lithosphere causes extension. [10]

Oceanic trenches

Oceanic trenches are narrow topographic lows that mark convergent boundaries or subduction zones. Oceanic trenches can be average 50 – 100  km wide and can be several thousand kilometers long. Oceanic trenches form as a result of bending of the subducting slab. Depth of oceanic trenches seems to be controlled by age of the oceanic lithosphere being subducted. [5] Sediment fill in oceanic trenches varies and generally depends on abundance of sediment input from surrounding areas. An oceanic trench, the Mariana Trench, is the deepest point of the ocean at a depth of approximately 11,000 m.

Earthquakes

Earthquakes are common along convergent boundaries. A region of high earthquake activity, the Benioff zone, generally dips 45° and marks the subducting plate. Earthquakes will occur to a depth of 670  km along the Benioff. Both compressional and extensional forces act along convergent boundaries. On the inner walls of trenches, compressional faulting or reverse faulting occurs due to the relative motion of the two plates. Reverse faulting scrapes off ocean sediment and leads to the formation of an accretionary wedge. Reverse faulting can lead to massive earthquakes, such as the magnitude 9.1 Sumatra earthquake of 2004.  Tensional or normal faulting occurs on the outer wall of the trench, likely due to bending of the down going slab. [11]

Accretionary Wedge

Accretionary wedges (also called accretionary prisms) form as sediment is scraped from the subducting lithosphere and emplaced against the overriding lithosphere. These sediments include igneous crust, turbidite sediments, and pelagic sediments. Imbricate thrust faulting along a basal decollement surface occurs in accretionary wedges as forces continue to compress and fault these newly added sediments. [5] The continued faulting of the accretionary wedge leads to overall thickening of the wedge. [12] Seafloor topography plays some role in accretion, especially emplacement of igneous crust. [13]

Convergent Boundaries and Natural Disasters

Some of the deadliest natural disasters have occurred due to convergent boundary processes. The 2004 Indian Ocean earthquake and tsunami was triggered by an earthquake along the convergent boundary of the Indian plate and Burma microplate and killed over 200,000 people.  The 2011 tsunami off the coast of Japan, which caused 16,000 deaths and did $360 billion USD in damage, was caused by a magnitude 9 earthquake along the convergent boundary of the Eurasian plate and Pacific Plate.

Examples

See also

Related Research Articles

Oceanic trench Long and narrow depressions of the sea floor

Oceanic trenches are topographic depressions of the sea floor, relatively narrow in width, but very long. These oceanographic features are the deepest parts of the ocean floor. Oceanic trenches are a distinctive morphological feature of convergent plate boundaries, along which lithospheric plates move towards each other at rates that vary from a few millimeters to over ten centimeters per year. A trench marks the position at which the flexed, subducting slab begins to descend beneath another lithospheric slab. Trenches are generally parallel to a volcanic island arc, and about 200 km (120 mi) from a volcanic arc. Oceanic trenches typically extend 3 to 4 km below the level of the surrounding oceanic floor. The greatest ocean depth measured is in the Challenger Deep of the Mariana Trench, at a depth of 11,034 m (36,201 ft) below sea level. Oceanic lithosphere moves into trenches at a global rate of about 3 km2/yr.

Obduction was originally defined by Coleman to mean the overthrusting of oceanic lithosphere onto continental lithosphere at a convergent plate boundary where continental lithosphere is being subducted beneath oceanic lithosphere.

Island arc arc-shaped archipelago

Island arcs are long chains of active volcanoes with intense seismic activity found along convergent tectonic plate boundaries. Most island arcs originate on oceanic crust and have resulted from the descent of the lithosphere into the mantle along the subduction zone. They are the principal way by which continental growth is achieved.

Ridge push or sliding plate force is a proposed driving force for plate motion in plate tectonics that occurs at mid-ocean ridges as the result of the rigid lithosphere sliding down the hot, raised asthenosphere below mid-ocean ridges. Although it is called ridge push, the term is somewhat misleading; it is actually a body force that acts throughout an ocean plate, not just at the ridge, as a result of gravitational pull. The name comes from earlier models of plate tectonics in which ridge push was primarily ascribed to upwelling magma at mid-ocean ridges pushing or wedging the plates apart.

Forearc The region between an oceanic trench and the associated volcanic arc

A forearc is the region between an oceanic trench and the associated volcanic arc. Forearc regions are found at convergent margins, and include any accretionary wedge and forearc basin that may be present. Due to tectonic stresses as one tectonic plate rides over another, forearc regions are sources for great thrust earthquakes

Volcanic arc A chain of volcanoes formed above a subducting plate

A volcanic arc is a chain of volcanoes formed above a subducting plate, positioned in an arc shape as seen from above. Offshore volcanoes form islands, resulting in a volcanic island arc. Generally, volcanic arcs result from the subduction of an oceanic tectonic plate under another tectonic plate, and often parallel an oceanic trench. The oceanic plate is saturated with water, and volatiles such as water drastically lower the melting point of the mantle. As the oceanic plate is subducted, it is subjected to greater and greater pressures with increasing depth. This pressure squeezes water out of the plate and introduces it to the mantle. Here the mantle melts and forms magma at depth under the overriding plate. The magma ascends to form an arc of volcanoes parallel to the subduction zone.

Magmatism geological process

Magmatism is the emplacement of magma within and at the surface of the outer layers of a terrestrial planet, which solidifies as igneous rocks. It does so through magmatic activity or igneous activity, the production, intrusion and extrusion of magma or lava. Volcanism is the surface expression of magmatism.

A submarine, undersea, or underwater earthquake is an earthquake that occurs underwater at the bottom of a body of water, especially an ocean. They are the leading cause of tsunamis. The magnitude can be measured scientifically by the use of the moment magnitude scale and the intensity can be assigned using the Mercalli intensity scale.

Accretionary wedge The sediments accreted onto the non-subducting tectonic plate at a convergent plate boundary

An accretionary wedge or accretionary prism forms from sediments accreted onto the non-subducting tectonic plate at a convergent plate boundary. Most of the material in the accretionary wedge consists of marine sediments scraped off from the downgoing slab of oceanic crust, but in some cases the wedge includes the erosional products of volcanic island arcs formed on the overriding plate.

Mariana Plate A small tectonic plate west of the Mariana Trench

The Mariana Plate is a micro tectonic plate located west of the Mariana Trench which forms the basement of the Mariana Islands which form part of the Izu-Bonin-Mariana Arc. It is separated from the Philippine Sea Plate to the west by a divergent boundary with numerous transform fault offsets. The boundary between the Mariana and the Pacific Plate to the east is a subduction zone with the Pacific Plate subducting beneath the Mariana. This eastern subduction is divided into the Mariana Trench, which forms the southeastern boundary, and the Izu-Ogasawara Trench the northeastern boundary. The subduction plate motion is responsible for the shape of the Mariana plate and back arc.

This is a list of articles related to plate tectonics and tectonic plates.

Tectonic subsidence is the sinking of the Earth's crust on a large scale, relative to crustal-scale features or the geoid. The movement of crustal plates and accommodation spaces created by faulting create subsidence on a large scale in a variety of environments, including passive margins, aulacogens, fore-arc basins, foreland basins, intercontinental basins and pull-apart basins. Three mechanisms are common in the tectonic environments in which subsidence occurs: extension, cooling and loading.

Subduction zone metamorphism Changes of rock due to pressure and heat near a subduction zone

A subduction zone is a region of the earth's crust where one tectonic plate moves under another tectonic plate; oceanic crust gets recycled back into the mantle and continental crust gets created by the formation of arc magmas. Arc magmas account for more than 20% of terrestrially produced magmas and are produced by the dehydration of minerals within the subducting slab as it descends into the mantle and are accreted onto the base of the overriding continental plate. Subduction zones host a unique variety of rock types created by the high-pressure, low-temperature conditions a subducting slab encounters during its descent. The metamorphic conditions the slab passes through in this process creates and destroys water bearing (hydrous) mineral phases, releasing water into the mantle. This water lowers the melting point of mantle rock, initiating melting. Understanding the timing and conditions in which these dehydration reactions occur, is key to interpreting mantle melting, volcanic arc magmatism, and the formation of continental crust.

A continental arc is a type of volcanic arc occurring as an "arc-shape" topographic high region along a continental margin. The continental arc is formed at an active continental margin where two tectonic plates meet, and where one plate has continental crust and the other oceanic crust along the line of plate convergence, and a subduction zone develops. The magmatism and petrogenesis of continental crust are complicated: in essence, continental arcs reflect a mixture of oceanic crust materials, mantle wedge and continental crust materials.

Flat slab subduction is characterized by a low subduction angle beyond the seismogenic layer and a resumption of normal subduction far from the trench. A slab refers to the subducting lower plate. Although, some would characterize flat slab subduction as any shallowly dipping lower plate as in western Mexico. Flat slab subduction is associated with the pinching out of the asthenosphere, an inland migration of arc magmatism, and an eventual cessation of arc magmatism. The coupling of the flat slab to the upper plate is thought to change the style of deformation occurring on the upper plate's surface and form basement-cored uplifts like the Rocky Mountains. The flat slab also may hydrate the lower continental lithosphere and be involved in the formation of economically important ore deposits. During the subduction, a flat slab itself may be deformed, or buckling, causing sedimentary hiatus in marine sediments on the slab. The failure of a flat slab is associated with ignimbritic volcanism and the reverse migration of arc volcanism. Multiple working hypotheses about the cause of flat slabs are subduction of thick, buoyant oceanic crust (15–20 km) and trench rollback accompanying a rapidly overriding upper plate and enhanced trench suction. The west coast of South America has two of the largest flat slab subduction zones. Flat slab subduction is occurring at 10% of subduction zones.

Subcontinental lithospheric mantle

The subcontinental lithospheric mantle (SCLM) is the uppermost solid part of Earth's mantle associated with the continental lithosphere.

Divergent double subduction Two parallel subduction zones with different directions are developed on the same oceanic plate

Divergent double subduction is a special type of subduction system where two parallel subduction zones with different directions are developed on the same oceanic plate. In conventional plate tectonics theory, an oceanic plate subducts under another plate and new oceanic crust is generated somewhere else, commonly along the other side of the same plates However, in divergent double subduction, the oceanic plate subducts on two sides. This results in the closure of ocean and arc-arc collision. This concept was first proposed and applied to the Lachlan fold belt in southern Australia. Since then, geologists have applied this model to other regions such as the Solonker Suture Zone of the Central Asian Orogenic belt, the Jiangnan Orogen, the Lhasa–Qiangtang collision zone and the Baker terrane boundary. Active examples of this system are 1) the Molucca Sea Collision Zone in Indonesia, in which the Molucca Sea plate subducts below the Eurasian plate and the Philippine Sea plate on two sides, and 2) the Adria microplate in the Central Mediterranean, subducting both on its western side and on its eastern side . Note that the term "divergent" is used to describe one oceanic plate subducting in different directions on two opposite sides. It should not be confused with use of the same term in 'divergent plate boundary' which refers to a spreading center that separates two plates moving away from each other.

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