Oceanic trenches are prominent, long, narrow topographic depressions of the ocean floor. They are typically Template:Convert wide and Template:Cvt below the level of the surrounding oceanic floor, but can be thousands of kilometers in length. There are about Template:Cvt of oceanic trenches worldwide, mostly around the Pacific Ocean, but also in the eastern Indian Ocean and a few other locations. The greatest ocean depth measured is in the Challenger Deep of the Mariana Trench, at a depth of Template:Cvt below sea level.
Oceanic trenches are a feature of the Earth's distinctive plate tectonics. They mark the locations 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. Oceanic lithosphere moves into trenches at a global rate of about Template:Cvt per year.Template:Sfn A trench marks the position at which the flexed, subducting slab begins to descend beneath another lithospheric slab. Trenches are generally parallel to and about Template:Cvt from a volcanic arc.
Much of the fluid trapped in sediments of the subducting slab returns to the surface at the oceanic trench, producing mud volcanoes and cold seeps. These support unique biomes based on chemotrophic microorganisms. There is concern that plastic debris is accumulating in trenches and threatening these communities.
Geographic distributionEdit
There are approximately Template:Convert of convergent plate margins worldwide. These are mostly located around the Pacific Ocean, but are also found in the eastern Indian Ocean, with a few shorter convergent margin segments in other parts of the Indian Ocean, in the Atlantic Ocean, and in the Mediterranean.Template:Sfn They are found on the oceanward side of island arcs and Andean-type orogens.Template:Sfn Globally, there are over 50 major ocean trenches covering an area of 1.9 million km2 or about 0.5% of the oceans.Template:Sfn
Trenches are geomorphologically distinct from troughs. Troughs are elongated depressions of the sea floor with steep sides and flat bottoms, while trenches are characterized by a V-shaped profile.Template:Sfn Trenches that are partially infilled are sometimes described as troughs, for example the Makran Trough.Template:Sfn Some trenches are completely buried and lack bathymetric expression as in the Cascadia subduction zone,Template:Sfn which is completely filled with sediments.Template:Sfn Despite their appearance, in these instances the fundamental plate-tectonic structure is still an oceanic trench. Some troughs look similar to oceanic trenches but possess other tectonic structures. One example is the Lesser Antilles Trough, which is the forearc basin of the Lesser Antilles subduction zone.Template:Sfn Also not a trench is the New Caledonia trough, which is an extensional sedimentary basin related to the Tonga-Kermadec subduction zone.Template:Sfn Additionally, the Cayman Trough, which is a pull-apart basin within a transform fault zone,Template:Sfn is not an oceanic trench.
Trenches, along with volcanic arcs and Wadati–Benioff zones (zones of earthquakes under a volcanic arc) are diagnostic of convergent plate boundaries and their deeper manifestations, subduction zones.Template:SfnTemplate:SfnTemplate:Sfn Here, two tectonic plates are drifting into each other at a rate of a few millimeters to over Template:Convert per year. At least one of the plates is oceanic lithosphere, which plunges under the other plate to be recycled in the Earth's mantle.
Trenches are related to, but distinct from, continental collision zones, such as the Himalayas. Unlike in trenches, in continental collision zones continental crust enters a subduction zone. When buoyant continental crust enters a trench, subduction comes to a halt and the area becomes a zone of continental collision. Features analogous to trenches are associated with collision zones. One such feature is the peripheral foreland basin, a sediment-filled foredeep. Examples of peripheral foreland basins include the floodplains of the Ganges River and the Tigris-Euphrates river system.Template:Sfn
History of the term "trench"Edit
Trenches were not clearly defined until the late 1940s and 1950s. The bathymetry of the ocean was poorly known prior to the Challenger expedition of 1872–1876,Template:Sfn which took 492 soundings of the deep ocean.Template:Sfn At station #225, the expedition discovered Challenger Deep,Template:Sfn now known to be the southern end of the Mariana Trench. The laying of transatlantic telegraph cables on the seafloor between the continents during the late 19th and early 20th centuries provided further motivation for improved bathymetry.Template:Sfn The term trench, in its modern sense of a prominent elongated depression of the sea bottom, was first used by Johnstone in his 1923 textbook An Introduction to Oceanography.Template:SfnTemplate:Sfn
During the 1920s and 1930s, Felix Andries Vening Meinesz measured gravity over trenches using a newly developed gravimeter that could measure gravity from aboard a submarine.Template:Sfn He proposed the tectogene hypothesis to explain the belts of negative gravity anomalies that were found near island arcs. According to this hypothesis, the belts were zones of downwelling of light crustal rock arising from subcrustal convection currents. The tectogene hypothesis was further developed by Griggs in 1939, using an analogue model based on a pair of rotating drums. Harry Hammond Hess substantially revised the theory based on his geological analysis.Template:Sfn
World War II in the Pacific led to great improvements of bathymetry, particularly in the western Pacific. In light of these new measurements, the linear nature of the deeps became clear. There was a rapid growth of deep sea research efforts, especially the widespread use of echosounders in the 1950s and 1960s. These efforts confirmed the morphological utility of the term "trench." Important trenches were identified, sampled, and mapped via sonar.
The early phase of trench exploration reached its peak with the 1960 descent of the Bathyscaphe Trieste to the bottom of the Challenger Deep. Following Robert S. Dietz' and Harry Hess' promulgation of the seafloor spreading hypothesis in the early 1960s and the plate tectonic revolution in the late 1960s, the oceanic trench became an important concept in plate tectonic theory.Template:Sfn
MorphologyEdit
Oceanic trenches are Template:Convert wide and have an asymmetric V-shape, with the steeper slope (8 to 20 degrees) on the inner (overriding) side of the trench and the gentler slope (around 5 degrees) on the outer (subducting) side of the trench.Template:SfnTemplate:Sfn The bottom of the trench marks the boundary between the subducting and overriding plates, known as the basal plate boundary shearTemplate:Sfn or the subduction décollement.Template:Sfn The depth of the trench depends on the starting depth of the oceanic lithosphere as it begins its plunge into the trench, the angle at which the slab plunges, and the amount of sedimentation in the trench. Both starting depth and subduction angle are greater for older oceanic lithosphere, which is reflected in the deep trenches of the western Pacific. Here the bottoms of the Marianas and the Tonga–Kermadec trenches are up to Template:Convert below sea level. In the eastern Pacific, where the subducting oceanic lithosphere is much younger, the depth of the Peru-Chile trench is around Template:Convert.Template:Sfn
Though narrow, oceanic trenches are remarkably long and continuous, forming the largest linear depressions on earth. An individual trench can be thousands of kilometers long.Template:Sfn Most trenches are convex towards the subducting slab, which is attributed to the spherical geometry of the Earth.Template:Sfn
The trench asymmetry reflects the different physical mechanisms that determine the inner and outer slope angle. The outer slope angle of the trench is determined by the bending radius of the subducting slab, as determined by its elastic thickness. Since oceanic lithosphere thickens with age, the outer slope angle is ultimately determined by the age of the subducting slab.Template:SfnTemplate:Sfn The inner slope angle is determined by the angle of repose of the overriding plate edge.Template:Sfn This reflects frequent earthquakes along the trench that prevent oversteepening of the inner slope.Template:Sfn
As the subducting plate approaches the trench, it bends slightly upwards before beginning its plunge into the depths. As a result, the outer trench slope is bounded by an outer trench high. This is subtle, often only tens of meters high, and is typically located a few tens of kilometers from the trench axis. On the outer slope itself, where the plate begins to bend downwards into the trench, the upper part of the subducting slab is broken by bending faults that give the outer trench slope a horst and graben topography. The formation of these bending faults is suppressed where oceanic ridges or large seamounts are subducting into the trench, but the bending faults cut right across smaller seamounts. Where the subducting slab is only thinly veneered with sediments, the outer slope will often show seafloor spreading ridges oblique to the horst and graben ridges.Template:Sfn
SedimentationEdit
Trench morphology is strongly modified by the amount of sedimentation in the trench. This varies from practically no sedimentation, as in the Tonga-Kermadec trench, to completely filled with sediments, as with the Cascadia subduction zone. Sedimentation is largely controlled by whether the trench is near a continental sediment source.Template:Sfn The range of sedimentation is well illustrated by the Chilean trench. The north Chile portion of the trench, which lies along the Atacama Desert with its very slow rate of weathering, is sediment-starved, with from 20 to a few hundred meters of sediments on the trench floor. The tectonic morphology of this trench segment is fully exposed on the ocean bottom. The central Chile segment of the trench is moderately sedimented, with sediments onlapping onto pelagic sediments or ocean basement of the subducting slab, but the trench morphology is still clearly discernible. The southern Chile segment of the trench is fully sedimented, to the point where the outer rise and slope are no longer discernible. Other fully sedimented trenches include the Makran Trough, where sediments are up to Template:Convert thick; the Cascadia subduction zone, which is completed buried by Template:Convert of sediments; and the northernmost Sumatra subduction zone, which is buried under Template:Convert of sediments.Template:Sfn
Sediments are sometimes transported along the axis of an oceanic trench. The central Chile trench experiences transport of sediments from source fans along an axial channel.Template:Sfn Similar transport of sediments has been documented in the Aleutian trench.Template:Sfn
In addition to sedimentation from rivers draining into a trench, sedimentation also takes place from landslides on the tectonically steepened inner slope, often driven by megathrust earthquakes. The Reloca Slide of the central Chile trench is an example of this process.Template:Sfn
Erosive versus accretionary marginsEdit
Convergent margins are classified as erosive or accretionary, and this has a strong influence on the morphology of the inner slope of the trench. Erosive margins, such as the northern Peru-Chile, Tonga-Kermadec, and Mariana trenches, correspond to sediment-starved trenches.Template:Sfn The subducting slab erodes material from the lower part of the overriding slab, reducing its volume. The edge of the slab experiences subsidence and steepening, with normal faulting. The slope is underlain by relative strong igneous and metamorphic rock, which maintains a high angle of repose.Template:Sfn Over half of all convergent margins are erosive margins.Template:Sfn
Accretionary margins, such as the southern Peru-Chile, Cascadia, and Aleutians, are associated with moderately to heavily sedimented trenches. As the slab subducts, sediments are "bulldozed" onto the edge of the overriding plate, producing an accretionary wedge or accretionary prism. This builds the overriding plate outwards. Because the sediments lack strength, their angle of repose is gentler than the rock making up the inner slope of erosive margin trenches. The inner slope is underlain by imbricated thrust sheets of sediments. The inner slope topography is roughened by localized mass wasting.Template:Sfn Cascadia has practically no bathymetric expression of the outer rise and trench, due to complete sediment filling, but the inner trench slope is complex, with many thrust ridges. These compete with canyon formation by rivers draining into the trench. Inner trench slopes of erosive margins rarely show thrust ridges.Template:Sfn
Accretionary prisms grow in two ways. The first is by frontal accretion, in which sediments are scraped off the downgoing plate and emplaced at the front of the accretionary prism.Template:Sfn As the accretionary wedge grows, older sediments further from the trench become increasingly lithified, and faults and other structural features are steepened by rotation towards the trench.Template:Sfn The other mechanism for accretionary prism growth is underplatingTemplate:Sfn (also known as basal accretionTemplate:Sfn) of subducted sediments, together with some oceanic crust, along the shallow parts of the subduction decollement. The Franciscan Group of California is interpreted as an ancient accretionary prism in which underplating is recorded as tectonic mélanges and duplex structures.Template:Sfn
EarthquakesEdit
Frequent megathrust earthquakes modify the inner slope of the trench by triggering massive landslides. These leave semicircular landslide scarps with slopes of up to 20 degrees on the headwalls and sidewalls.Template:Sfn
Subduction of seamounts and aseismic ridges into the trench may increase aseismic creep and reduce the severity of earthquakes. Contrariwise, subduction of large amounts of sediments may allow ruptures along the subduction décollement to propagate for great distances to produce megathrust earthquakes.Template:Sfn
Trench rollbackEdit
Trenches seem positionally stable over time, but scientists believe that some trenches—particularly those associated with subduction zones where two oceanic plates converge—move backward into the subducting plate.Template:SfnTemplate:Sfn This is called trench rollback or retreat, hinge rollback or retreat, slab rollback or retreat and is one explanation for the existence of back-arc basins.
Forces perpendicular to the slab (the portion of the subducting plate within the mantle) are responsible for steepening of the slab and, ultimately, the movement of the hinge and trench at the surface.Template:Sfn These forces arise from the negative buoyancy of the slab with respect to the mantleTemplate:Sfn modified by the geometry of the slab itself.Template:Sfn The extension in the overriding plate, in response to the subsequent subhorizontal mantle flow from the displacement of the slab, can result in formation of a back-arc basin.Template:Sfn
Processes involvedEdit
Several forces are involved in the process of slab rollback. Two forces acting against each other at the interface of the two subducting plates exert forces against one another. The subducting plate exerts a bending force (FPB) that supplies pressure during subduction, while the overriding plate exerts a force against the subducting plate (FTS). The slab pull force (FSP) is caused by the negative buoyancy of the plate driving the plate to greater depths. The resisting force from the surrounding mantle opposes the slab pull forces. Interactions with the 660-km discontinuity cause a deflection due to the buoyancy at the phase transition (F660).Template:Sfn The unique interplay of these forces is what generates slab rollback. When the deep slab section obstructs the down-going motion of the shallow slab section, slab rollback occurs. The subducting slab undergoes backward sinking due to the negative buoyancy forces causing a retrogradation of the trench hinge along the surface. Upwelling of the mantle around the slab can create favorable conditions for the formation of a back-arc basin.Template:Sfn
Seismic tomography provides evidence for slab rollback. Results demonstrate high temperature anomalies within the mantle suggesting subducted material is present in the mantle.Template:Sfn Ophiolites are viewed as evidence for such mechanisms as high pressure and temperature rocks are rapidly brought to the surface through the processes of slab rollback, which provides space for the exhumation of ophiolites.
Slab rollback is not always a continuous process suggesting an episodic nature.Template:Sfn The episodic nature of the rollback is explained by a change in the density of the subducting plate, such as the arrival of buoyant lithosphere (a continent, arc, ridge, or plateau), a change in the subduction dynamics, or a change in the plate kinematics. The age of the subducting plates does not have any effect on slab rollback.Template:Sfn Nearby continental collisions have an effect on slab rollback. Continental collisions induce mantle flow and extrusion of mantle material, which causes stretching and arc-trench rollback.Template:Sfn In the area of the Southeast Pacific, there have been several rollback events resulting in the formation of numerous back-arc basins.Template:Sfn
Mantle interactionsEdit
Interactions with the mantle discontinuities play a significant role in slab rollback. Stagnation at the 660-km discontinuity causes retrograde slab motion due to the suction forces acting at the surface.Template:Sfn Slab rollback induces mantle return flow, which causes extension from the shear stresses at the base of the overriding plate. As slab rollback velocities increase, circular mantle flow velocities also increase, accelerating extension rates.Template:Sfn Extension rates are altered when the slab interacts with the discontinuities within the mantle at 410 km and 660 km depth. Slabs can either penetrate directly into the lower mantle, or can be retarded due to the phase transition at 660 km depth creating a difference in buoyancy. An increase in retrograde trench migration (slab rollback) (2–4 cm/yr) is a result of flattened slabs at the 660-km discontinuity where the slab does not penetrate into the lower mantle.Template:Sfn This is the case for the Japan, Java and Izu–Bonin trenches. These flattened slabs are only temporarily arrested in the transition zone. The subsequent displacement into the lower mantle is caused by slab pull forces, or the destabilization of the slab from warming and broadening due to thermal diffusion. Slabs that penetrate directly into the lower mantle result in slower slab rollback rates (~1–3 cm/yr) such as the Mariana arc, Tonga arcs.Template:Sfn
Hydrothermal activity and associated biomesEdit
As sediments are subducted at the bottom of trenches, much of their fluid content is expelled and moves back along the subduction décollement to emerge on the inner slope as mud volcanoes and cold seeps. Methane clathrates and gas hydrates also accumulate in the inner slope, and there is concern that their breakdown could contribute to global warming.Template:Sfn
The fluids released at mud volcanoes and cold seeps are rich in methane and hydrogen sulfide, providing chemical energy for chemotrophic microorganisms that form the base of a unique trench biome. Cold seep communities have been identified in the inner trench slopes of the western Pacific (especially JapanTemplate:Sfn), South America, Barbados, the Mediterranean, Makran, and the Sunda trench. These are found at depths as great as Template:Convert.Template:Sfn The genome of the extremophile Deinococcus from Challenger Deep has sequenced for its ecological insights and potential industrial uses.Template:Sfn
Because trenches are the lowest points in the ocean floor, there is concern that plastic debris may accumulate in trenches and endanger the fragile trench biomes.Template:Sfn
Deepest oceanic trenchesEdit
Recent measurements, where the salinity and temperature of the water was measured throughout the dive, have uncertainties of about Template:Convert.Template:Sfn Older measurements may be off by hundreds of meters.
| Trench | Ocean | Lowest Point | Maximum Depth | Source | |
|---|---|---|---|---|---|
| Mariana Trench | Pacific Ocean | Challenger Deep | Template:Convert | Template:Sfn<ref name=":0">{{#invoke:citation/CS1|citation | CitationClass=web
}}</ref> |
| Tonga Trench | Pacific Ocean | Horizon Deep | Template:Convert | Template:Sfn | |
| Philippine Trench | Pacific Ocean | Emden Deep | Template:Convert | Template:Sfn | |
| Kuril–Kamchatka Trench | Pacific Ocean | Template:Convert | Template:Sfn | ||
| Kermadec Trench | Pacific Ocean | Template:Convert | Template:Sfn | ||
| Izu–Bonin Trench (Izu–Ogasawara Trench) | Pacific Ocean | Template:Convert | Template:Sfn | ||
| New Britain Trench | Pacific Ocean (Solomon Sea) | Planet Deep | Template:Convert | Template:Sfn | |
| Puerto Rico Trench | Atlantic Ocean | Milwaukee Deep | Template:Convert | Template:Sfn<ref>{{#invoke:citation/CS1|citation | CitationClass=web
}}</ref> |
| South Sandwich Trench | Atlantic Ocean | Meteor Deep | Template:Convert | Template:Sfn<ref name="Sonar Images">{{#invoke:citation/CS1|citation | CitationClass=web
}}</ref> |
| Peru–Chile Trench or Atacama Trench | Pacific Ocean | Richards Deep | Template:Convert | Template:Sfn | |
| Japan Trench | Pacific Ocean | 8,412 m (27,498 ft) | Template:Sfn | ||
| Cayman Trench | Atlantic Ocean | Caribbean Deep | Template:Convert | Template:Sfn | |
| South Sandwich Trench | Southern Ocean | Factorian Deep | Template:Convert | Template:Sfn<ref name="Sonar Images"/> | |
| Sunda Trench | Indian Ocean | Java Deep | Template:Convert | Template:Sfn<ref name=":0" /> | |
| Mauritius Trench | Indian Ocean | Mauritius Point | Template:Convert | Template:Sfn | |
| India Trench | Indian Ocean | Between India & Maldives | Template:Convert | Template:Sfn | |
| Ceylon Trench | Indian Ocean | Sri Lanka Deep | Template:Convert | Template:Sfn | |
| Somalia Trench | Indian Ocean | Somali Deep | Template:Convert | Template:Sfn | |
| Madagascar Trench | Indian Ocean | Madagascar Deep | Template:Convert | Template:Sfn | |
| Puerto Rico Trench | Atlantic Ocean | Rio Bermuda Deep | Template:Convert | Template:Sfn | |
| Mid-Atlantic Ridge | Arctic Ocean | Molloy Deep | Template:Convert | Template:Sfn<ref name=":0"/> |
Notable oceanic trenchesEdit
| Trench | Location |
|---|---|
| Aleutian Trench | South of the Aleutian Islands, west of Alaska |
| Bougainville Trench | South of New Guinea |
| Cayman Trench | Western Caribbean |
| Cedros Trench (inactive) | Pacific coast of Baja California |
| Hikurangi Trough | East of New Zealand |
| Hjort Trench | Southwest of New Zealand |
| Izu–Ogasawara Trench | Near Izu and Bonin islands |
| Japan Trench | East of Japan |
| Kermadec Trench * | Northeast of New Zealand |
| Kuril–Kamchatka Trench * | Near Kuril Islands |
| Manila Trench | West of Luzon, Philippines |
| Mariana Trench * | Western Pacific Ocean; east of Mariana Islands |
| Middle America Trench | Eastern Pacific Ocean; off coast of Mexico, Guatemala, El Salvador, Nicaragua, Costa Rica |
| New Hebrides Trench | West of Vanuatu (New Hebrides Islands). |
| Peru–Chile Trench | Eastern Pacific Ocean; off coast of Peru & Chile |
| Philippine Trench * | East of the Philippines |
| Puerto Rico Trench | Boundary of Caribbean and Atlantic Ocean |
| Puysegur trench | Southwest of New Zealand |
| Ryukyu Trench | Eastern edge of Japan's Ryukyu Islands |
| South Sandwich Trench | East of the South Sandwich Islands |
| Sunda Trench | Curves from south of Java to west of Sumatra and the Andaman and Nicobar Islands |
| Tonga Trench * | Near Tonga |
| Yap Trench | Western Pacific Ocean; between Palau Islands and Mariana Trench |
(*) The five deepest trenches in the world
Ancient oceanic trenchesEdit
| Trench | Location |
|---|---|
| Intermontane Trench | Western North America; between the Intermontane Islands and North America |
| Insular Trench | Western North America; between the Insular Islands and the Intermontane Islands |
| Farallon Trench | Western North America |
| Tethys Trench | South of Turkey, Iran, Tibet and Southeast Asia |
See alsoEdit
- Glossary of landforms
- List of submarine topographical features
- Mid-ocean ridge
- Physical oceanography
- Ring of Fire
ReferencesEdit
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