Continental shelfFrom Wikipedia, the free encyclopediaMarine habitats

Anatomy of a continental shelf of the south eastern coast of the United States

Littoral zone Intertidal zone Estuaries Kelp forests Coral reefs Ocean banks Continental shelf Neritic zone Straits Pelagic zone Oceanic zone Seamounts Hydrothermal vents Cold seeps Demersal zone Benthic zone

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Thecontinental shelfis an underwater landmass which extends from acontinent, resulting in an area of relatively shallow water known as ashelf sea. Much of the shelves were exposed duringglacial periodsandinterglacial periods.The shelf surrounding anislandis known as aninsular shelf.Thecontinental margin, between the continental shelf and theabyssal plain, comprises a steep continental slope followed by the flatter continental rise.Sedimentfrom the continent above cascades down the slope and accumulates as a pile of sediment at the base of the slope, called the continental rise. Extending as far as 500km from the slope, it consists of thick sediments deposited byturbidity currentsfrom the shelf and slope.[1]The continental rise'sgradientis intermediate between the slope and the shelf, on the order of 0.51.[2]Under theUnited Nations Convention on the Law of the Sea, the name continental shelf was given a legal definition as the stretch of theseabedadjacent to the shores of a particular country to which it belongs.Contents[hide] 1Geographical distribution 2Topography 3Sediments 4Biota 5Economic significance 6See also 7Notes 8References 9External linksGeographical distribution[edit]

The global continental shelf, highlighted in cyanThe width of the continental shelf varies considerably it is not uncommon for an area to have virtually no shelf at all, particularly where the forward edge of an advancingoceanic platedives beneathcontinental crustin an offshoresubduction zonesuch as off the coast ofChileor the west coast ofSumatra. The largest shelf theSiberian Shelfin theArctic Ocean stretches to 1,500 kilometers (930mi) in width. TheSouth China Sealies over another extensive area of continental shelf, theSunda Shelf, which joinsBorneo, Sumatra, andJavato the Asian mainland. Other familiar bodies of water that overlie continental shelves are theNorth Seaand thePersian Gulf. The average width of continental shelves is about 80km (50mi). The depth of the shelf also varies, but is generally limited to water shallower than 150m (490ft).[3]The slope of the shelf is usually quite low, on the order of 0.5; vertical relief is also minimal, at less than 20m (66ft).[4]Though the continental shelf is treated as aphysiographicprovince of theocean, it is not part of the deep ocean basin proper, but the flooded margins of the continent.[5]Passive continental marginssuch as most of theAtlanticcoasts have wide and shallow shelves, made of thick sedimentary wedges derived from long erosion of a neighboring continent.Active continental marginshave narrow, relatively steep shelves, due to frequentearthquakesthat move sediment to the deep sea.[6]Topography[edit]

The shelf usually ends at a point of increasing slope[7](called theshelf break). The sea floor below the break is thecontinental slope. Below the slope is thecontinental rise, which finally merges into the deep ocean floor, theabyssal plain. The continental shelf and the slope are part of thecontinental margin.The shelf area is commonly subdivided into theinner continental shelf,mid continental shelf, andouter continental shelf, each with their specificgeomorphologyandmarine biology.The character of the shelf changes dramatically at the shelf break, where the continental slope begins. With a few exceptions, the shelf break is located at a remarkably uniform depth of roughly 140m (460ft); this is likely a hallmark of past ice ages, when sea level was lower than it is now.[8]The continental slope is much steeper than the shelf; the average angle is 3, but it can be as low as 1 or as high as 10.[9]The slope is often cut withsubmarine canyons. The physical mechanisms involved in forming these canyons were not well understood until the 1960s.[10]Sediments[edit]The continental shelves are covered byterrigenoussediments; that is, those derived from erosion of the continents. However, little of the sediment is from currentrivers; some 60-70% of the sediment on the world's shelves isrelict sediment, deposited during the last ice age, when sea level was 100120 m lower than it is now.[11]Sediments usually become increasingly fine with distance from the coast; sand is limited to shallow, wave-agitated waters, while silt and clays are deposited in quieter, deep water far offshore.[12]These shelf sediments accumulate at an average rate of 30cm/1000 years, with a range from 1540cm.[13]Though slow by human standards, this rate is much faster than that for deep-seapelagic sediments.Biota[edit]Continental shelves teem with life, because of the sunlight available in shallow waters, in contrast to the biotic desert of the oceans'abyssal plain. Thepelagic(water column) environment of the continental shelf constitutes theneritic zone, and thebenthic(sea floor) province of the shelf is thesublittoral zone.[14]Though the shelves are usually fertile, ifanoxicconditions prevail during sedimentation, the deposits may overgeologic timebecomesourcesforfossil fuels.Economic significance[edit]The relatively accessible continental shelf is the best understood part of the ocean floor. Most commercial exploitation from the sea, such as metallic-ore, non-metallic ore, andhydrocarbonextraction, takes place on the continental shelf. Sovereign rights over their continental shelves up to a depth of 200 metres or to a distance where the depth of waters admitted of resource exploitation were claimed by the marine nations that signed theConvention on the Continental Shelfdrawn up by the UN'sInternational Law Commissionin 1958. This was partly superseded by the 1982United Nations Convention on the Law of the Sea.[15]which created the 200 nautical mile exclusive economic zone and extended continental shelf rights for states with physical continental shelves that extend beyond that distance.The legal definition of a continental shelf differs significantly from the geological definition. UNCLOS states that the shelf extends to the limit of thecontinental margin, but no less than 200 nautical miles from thebaseline. Thus inhabited volcanic islands such as theCanaries, which have no actual continental shelf, nonetheless have a legal continental shelf, whereas uninhabitable islands have no shelf.See also[edit]Environment portal

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Baseline Continental Island Continental shelf pump Continental shelf of Russia Exclusive economic zone International waters Land bridge Outer Continental Shelf Region of Freshwater Influence Territorial waters Passive marginPassive marginFrom Wikipedia, the free encyclopediaApassive marginis the transition betweenoceanicandcontinental lithospherewhich is not an active platemargin.

Rifting to Spreading TransitionIt is constructed bysedimentationabove an ancientrift, now marked by transitional lithosphere. Continental rifting creates new ocean basins. Eventually the continental rift forms amid-ocean ridgeand the locus ofextensionmoves away from thecontinent-ocean boundary. The transition between the continental and oceanic lithosphere that was originally created by rifting is known as a passive margin.

Passive Continental MarginContents[hide] 1Global distribution 2Key components 2.1Active vs. passive margins 2.2Morphology 2.3Cross-section 3Subsidence mechanisms 4Classification of passive margins 4.1Geometry of passive margins 4.1.1Rifted margin 4.1.2Sheared margin 4.1.3Transtensional margin 4.2Nature of transitional crust 4.2.1Non-volcanic rifted margin 4.2.2Volcanic rifted margin 4.3Heterogeneity of transitional crust 4.3.1Simple transitional crust 4.3.2Complex transitional crust 4.4Sedimentation 4.4.1Constructional 4.4.2Starved 5Formation 6Economic significance 7Law of the Sea 8See also 9ReferencesGlobal distribution[edit]

Map showing the distribution of Earth's passive margins (yellow swaths).Passive margins are found at every ocean and continent boundary that is not marked by a strike-slip fault or a subduction zone. Passive margins define the region around theAtlantic Ocean,Arctic Ocean, and westernIndian Ocean, and define the entire coasts ofAfrica,Greenland,IndiaandAustralia. They are also found on the east coast ofNorth AmericaandSouth America, in westernEuropeand most ofAntarctica. EastAsiaalso contains some passive margins.Key components[edit]Active vs. passive margins[edit]This refers to whether a crustal boundary between oceanic lithosphere and continental lithosphere is aplateboundary or not.Active marginsare found on the edge of a continent where subduction occurs. These are often marked byupliftandvolcanic mountain beltson the continental plate. Less often there is a strike-slip fault, as defines the southern coastline of W.Africa. Most of the easternIndian Oceanand nearly all of thePacific Oceanmargin are examples of active margins. While a weld between oceanic and continental lithosphere is called a passive margin, it is not an inactive margin. Active subsidence, sedimentation, growth faulting, pore fluid formation and migration are all active processes on passive margins. Passive margins are only passive in that they are not active plate boundaries.Morphology[edit]

Bathymetric profile across a typical passive margin. Note that vertical scale is greatly exaggerated relative to the horizontal scale.Passive margins consist of both onshorecoastal plainand offshorecontinental shelf-slope-rise triads. Coastal plains are often dominated by fluvial processes, while the continental shelf is dominated by deltaic and longshore current processes. The great rivers (Amazon.Orinoco,Congo,Nile,Ganges,Yellow,Yangtze, andMackenzierivers) drain across passive margins. Extensiveestuariesare common on mature passive margins. Although there are many kinds of passive margins, themorphologiesof most passive margins are remarkably similar. Typically they consist of a continental shelf, continental slope, continental rise, and abyssal plain. The morphological expression of these features are largely defined by the underlying transitional crust and the sedimentation above it. Passive margins defined by a large fluvial sediment budget and those dominated by coral and other biogenous processes generally have a similar morphology. In addition, the shelf break seems to mark the maximumNeogenelowstand, defined by the glacial maxima. The outer continental shelf and slope may be cut by greatsubmarine canyons, which mark the offshore continuation of rivers.At high latitudes and during glaciations, the nearshore morphology of passive margins may reflect glacial processes, such as thefjordsofNorwayandGreenland.Cross-section[edit]

Transitional crust composed of stretched and faulted continental crust. Note: vertical scale is greatly exaggerated relative to horizontal scale.

Cross-section through transitional crust of a passive margin. Transitional crust as a largely volcanic construct. Note: vertical scale is greatly exaggerated relative to horizontal scale.The main features of passive margins lie underneath the external characters. Beneath passive margins the transition between the continental and oceanic crust is a broad transition known as transitional crust. The subsided continental crust is marked bynormal faultsthat dip seaward. The faulted crust transitions into oceanic crust and may be deeply buried due tothermal subsidenceand the mass of sediment that collects above it. Thelithospherebeneath passive margins is known as transitional lithosphere. The lithosphere thins seaward as it transitions seaward to oceanic crust. Different kinds of transitional crust form, depending on how fast rifting occurs and how hot the underlying mantle was at the time of rifting. Volcanic passive margins represent one endmember transitional crust type, the other endmember (amagmatic) type is the rifted passive margin. Volcanic passive margins they also are marked by numerousdykesandigneous intrusionswithin the subsided continental crust. There are typically a lot of dykes formed perpendicular to the seaward-dipping lava flows and sills. Igneous intrusions within the crust cause lava flows along the top the subsided continental crust and form seaward-dipping reflectors.Subsidence mechanisms[edit]Passive margins are characterized by thick accumulations of sediments. Space for these sediments is called accommodation and is due to subsidence of especially the transitional crust. Subsidence is ultimately caused by gravitational equilibrium that is established between the crustal tracts, known asisostasy. Isostasy controls the uplift of the rift flank and the subsequent subsidence of the evolving passive margin and is mostly reflected by changes inheat flow. Heat flow at passive margins changes significantly over its lifespan, high at the beginning and decreasing with age. In the initial stage, the continental crust and lithosphere is stretched and thinned due to plate movement (plate tectonics) and associated igneous activity. The very thin lithosphere beneath the rift allows the upwelling mantle to melt by decompression. Lithospheric thinning also allows theasthenosphereto rise closer to the surface, heating the overlying lithosphere byconductionandadvectionof heat by intrusive dykes. Heating reduces the density of the lithosphere and elevates the lower crust and lithosphere. In addition,mantle plumesmay heat the lithosphere and cause prodigious igneous activity. Once a mid-oceanic ridge forms and seafoor spreading begins, the original site of rifting is separated into conjugate passive margins (for example, the eastern US and NW African margins were parts of the same rift in earlyMesozoictime and are now conjugate margins) and migrates away from the zone of mantle upwelling and heating and cooling begins. The mantle lithosphere below the thinned and faulted continental oceanic transition cools, thickens, increases in density and thus begins to subside. The accumulation of sediments above the subsiding transitional crust and lithosphere further depresses the transitional crust.Classification of passive margins[edit]There are four different perspectives needed to classify passive margins:1. map-view formation geometry (rifted, sheared, and transtensional),2. nature of transitional crust (volcanic and non-volcanic),3. whether the transitional crust represents a continuous change from normal continental to normal oceanic crust or this includes isolated rifts and stranded continental blocks (simple and complex), and4. sedimentation (carbonate-dominated, clastic-dominated, or sediment starved).The first describes the relationship between rift orientation and plate motion, the second describes the nature of transitional crust, and the third describes post-rift sedimentation. All three perspectives need to be considered in describing a passive margin. In fact, passive margins are extremely long, and vary along their length in rift geometry, nature of transitional crust, and sediment supply; it is more appropriate to subdivide individual passive margins into segments on this basis and apply the threefold classification to each segment.Geometry of passive margins[edit]Rifted margin[edit]This is the typical way that passive margins form, as separated continental tracts move perpendicular to the coastline. This is how the CentralAtlanticopened, beginning inJurassictime. Faulting tends to belistric:normal faultsthat flatten with depth.Sheared margin[edit]Sheared margins form where continental breakup was associated withstrike-slip faulting. A good example of this type of margin is found on the south-facing coast of west Africa. Sheared margins are highly complex and tend to be rather narrow. They also differ from rifted passive margins in structural style and thermal evolution during continental breakup. As theseafloor spreadingaxis moves along the margin, thermal uplift produces a ridge. This ridge traps sediments, thus allowing for thick sequences to accumulate. These types of passive margins are less volcanic.Transtensional margin[edit]This type of passive margin develops where rifting is oblique to the coastline, as is now occurring in theGulf of California.Nature of transitional crust[edit]Transitional crust, separating true oceanic and continental crusts, is the foundation of any passive margin. This forms during the rifting stage and consists of two endmembers: Volcanic and Non-Volcanic. This classification scheme only applies to rifted and transtensional margin; transitional crust of sheared margins is very poorly known.Non-volcanic rifted margin[edit]Non-volcanic marginsare formed when extension is accompanied by littlemantlemelting and volcanism. Non-volcanic transitional crust consists of stretched and thinned continental crust. Non-volcanic margins are typically characterized by continentward-dipping seismic reflectors (rotated crustal blocks and associated sediments) and low P-wave velocities ( 7km) and large thicknesses of the LCBs are evidence that supports the case for plume-fed accretion (mafic thickening) underplating the crust during continental breakup. LCBs are located along the continent-ocean transition but can sometimes extend beneath the continental part of the rifted margin (as observed in the mid-Norwegian margin for example). In the continental domain, there are still open discussion on their real nature, chronology, geodynamic and petroleum implications.[1]Example of volcanic margins: The Yemen margin The East Australian margin The West Indian margin The Hatton-Rockal margin The U.S East Coast The mid-Norwegian margin The Brazilian margins The Namibian marginHeterogeneity of transitional crust[edit]Simple transitional crust[edit]Passive margins of this type show a simple progression through the transitional crust, from normal continental to normal oceanic crusts. The passive margin offshoreTexasis a good example.Complex transitional crust[edit]This type of transitional crust is characterized by abandonedriftsand continental blocks, such as theBlake Plateau,Grand Banks, orBahama Islandsoffshore eastern Florida.Sedimentation[edit]A fourth way to classify passive margins is according to the nature ofsedimentationof the mature passive margin. Sedimentation continues throughout the life of a passive margin. Sedimentation changes rapidly and progressively during the initial stages of passive margin formation because rifting begins on land, becoming marine as the rift opens and a true passive margin is established. Consequently, the sedimentation history of a passive margin begins with fluvial, lacustrine, or other subaerial deposits, evolving with time to Depending on how the rifting occurred, when, how and what type of sediment varies.Constructional[edit]Constructional margins are the "classic" mode of passive margin sedimentation. Normal sedimentation results from thetransportanddepositionof sand, silt, and clay byriversviadeltasand redistribution of these sediments bylongshore currents. The nature of sediments can change remarkably along a passive margin, due to interactions between carbonate sediment production, clastic input from rivers, and alongshore transport. Whereclastic sedimentinputs are small,biogenic sedimentationcan dominate especially nearshore sedimentation. TheGulf of Mexicopassive margin along the southern United States is an excellent example of this, with muddy and sandy coastal environments down current (west) from theMississippi River Deltaand beaches ofcarbonatesand to the east. The thick layers of sediment gradually thin with increasing distance offshore, depending on subsidence of the passive margin and the efficacy of offshore transport mechanisms such asturbidity currentsandsubmarine channels.Development of the shelf edge and its migration through time is critical to the development of a passive margin. The location of the shelf edge break reflects complex interaction between sedimentation, sealevel, and the presence of sediment dams. Coral reefs serve as bulwarks that allow sediment to accumulate between them and the shore, cutting off sediment supply to deeper water. Another type of sediment dam results from the presence ofsalt domes, as are common along theTexasandLouisianapassive margin.Starved[edit]Sediment-starved margins produce narrow continental shelves and passive margins. This is especially common in arid regions, where there is little transport of sediment by rivers or redistribution by longshore currents. The Red Sea is a good example of a sediment-starved passive margin.Formation[edit]

There are three main stages in the formation of passive margins:1. In the first stage a continental rift is established due to stretching and thinning of the crust and lithosphere by plate movement. This is the beginning of the continental crust subsidence. Drainage is generally away from the rift at this stage.2. The second stage leads to the formation of an oceanic basin, similar to the modernRed Sea. The subsiding continental crust undergoes normal faulting as transitional marine conditions are established. Areas with restricted sea water circulation coupled with arid climate create evaporite deposits. Crust and lithosphere stretching and thinning are still taking place in this stage. Volcanic passive margins also have igneous intrusions and dykes during this stage.3. The last stage in formation happens only when crustal stretching ceases and the transitional crust and lithosphere subsides as a result of cooling and thickening (thermal subsidence). Drainage starts flowing towards the passive margin causing sediment to accumulate over it.Economic significance[edit]Passive margins are important reservoirs ofpetroleum. Mann et al. (2001) classified 592 giant oil fields into six basin and tectonic-setting categories, and noted that continental passive margins account for 31% of giants. Continental rifts (which are likely to evolve into passive margins with time) contain another 30% of the world's giant oil fields. Basins associated with collision zones and subduction zones are where most of the remaining giant oil fields are found.Passive margins are petroleum storehouses because these are associated with favorable conditions for accumulation and maturation of organic matter. Early continental rifting conditions led to the development ofanoxicbasins, large sediment and organic flux, and the preservation of organic matter that led to oil and gas deposits. Crude oil will form from these deposits. These are the localities in which petroleum resources are most profitable and productive. Productive fields are found in passive margins around the globe, including theGulf of Mexico, westernScandinavia, and WesternAustralia.Law of the Sea[edit]International discussions about who controls the resources of passive margins are the focus ofLaw of the Seanegotiations. Continental shelves are important parts of nationalexclusive economic zones, important for seafloor mineral deposits (including oil and gas) and fisheries.See also[edit] Convergent boundary Divergent boundary Plate tectonics Continental shelfReferencesAbyssal fanFrom Wikipedia, the free encyclopediaAbyssal fans, also known asdeep-sea fans,underwater deltas, andsubmarine fans, are underwater geological structures associated with large-scalesediment depositionand formed byturbidity currents. They can be thought of as an underwater version ofalluvial fansand can vary dramatically in size, with widths from several kilometres to several thousands of kilometres (seeBengal Fan).[1]Contents[hide] 1Formation 2See also 3References 4SourcesFormation[edit]Abyssal (or submarine) fans are formed from turbidity currents.Turbidity currents start when something, for example an earthquake (or just the inherent instability of newly deposited sediments), triggers sediments to be pushed over the edge of thecontinental shelfand down thecontinental slope, creating a submarine landslide. A dense slurry ofmudsandsandsaccelerates towards the foot of the slope until the gradient levels off and the turbidity current slows. The slowing current has a reduced ability to transport sediments and deposition of the coarser grains begins, creating a submarine fan. The current continues to slow down as it moves towards the continental rise until it reaches the level bottom of the ocean. This final result is a series of graded sediments of sand, silt and mud and these are known asturbidites, as described by theBouma sequence.Abyssal plainFrom Wikipedia, the free encyclopedia

Diagrammatic cross-section of anoceanic basin, showing the relationship of the abyssal plain to acontinental riseand anoceanic trench

Depiction of theabyssal zonein relation to other majoroceanic zonesAquatic layers

Pelagic

Photic

Epipelagic

Aphotic

Mesopelagic

Bathyalpelagic

Abyssopelagic

Hadopelagic

Demersal

Benthic

Stratification

Pycnocline

Isopycnal

Chemocline

Halocline

Thermocline

Thermohaline

Marine habitats

Lake stratification

Aquatic ecosystems

Wild fisheries

Anabyssal plainis an underwaterplainon the deepocean floor, usually found at depths between 3000 and 6000m. Lying generally between the foot of acontinental riseand amid-ocean ridge, abyssal plains cover more than 50% of theEarths surface.[1][2]They are among the flattest, smoothest and least explored regions on Earth.[3]Abyssal plains are key geologic elements ofoceanic basins(the other elements being an elevated mid-ocean ridge and flankingabyssal hills). In addition to these elements,activeoceanic basins (those that are associated with a movingplate tectonicboundary) also typically include anoceanic trenchand asubduction zone.Abyssal plains were not recognized as distinctphysiographicfeatures of thesea flooruntil the late 1940s and, until very recently, none had been studied on a systematic basis. They are poorly preserved in thesedimentary record, because they tend to be consumed by the subduction process.[3]The creation of the abyssal plain is the end result of spreading of the seafloor (plate tectonics) and melting of the loweroceanic crust. Magma rises from above theasthenosphere(a layer of the uppermantle) and as thisbasalticmaterial reaches the surface at mid-ocean ridges it forms new oceanic crust. This is constantly pulled sideways by spreading of the seafloor. Abyssal plains result from the blanketing of an originally uneven surface of oceanic crust by fine-grainedsediments, mainlyclayandsilt. Much of this sediment is deposited byturbidity currentsthat have been channelled from thecontinental marginsalongsubmarine canyonsdown into deeper water. The remainder of the sediment is composed chiefly ofpelagic sediments. Metallicnodulesare common in some areas of the plains, with varying concentrations of metals, includingmanganese,iron,nickel,cobalt, andcopper. These nodules may provide a significant resource for futureminingventures.Owing in part to their vast size, abyssal plains are currently believed to be a major reservoir ofbiodiversity. The abyss also exerts significant influence upon oceancarbon cycling, dissolution ofcalcium carbonate, andatmospheric CO2concentrationsover timescales of 1001000years. The structure and function of abyssalecosystemsare strongly influenced by the rate offlux of foodto the seafloor and the composition of the material that settles. Factors such asclimate change,fishing practices, andocean fertilizationare expected to have a substantial effect on patterns ofprimary productionin theeuphotic zone. This will undoubtedly impact the flux of organic material to the abyss in a similar manner and thus have a profound effect on the structure, function and diversity of abyssal ecosystems.[1][4]Contents[hide] 1Oceanic zones 2Formation 3Discovery 4Terrain features 4.1Hydrothermal vents 4.2Cold seeps 5Biodiversity 6Exploitation of resources 7List of abyssal plains 8See also 9References 10Bibliography 11External linksOceanic zones[edit]Main article:Oceanic zone

Pelagic zonesThe ocean can be conceptualized as being divided into variouszones, depending on depth, and presence or absence ofsunlight. Nearly alllife formsin the ocean depend on thephotosyntheticactivities ofphytoplanktonand other marineplantsto convertcarbon dioxideintoorganic carbon, which is the basic building block oforganic matter. Photosynthesis in turn requires energy from sunlight to drive the chemical reactions that produce organic carbon.[5]The stratum of thewater columnnearest the surface of the ocean (sea level) is referred to as thephotic zone. The photic zone can be subdivided into two different vertical regions. The uppermost portion of the photic zone, where there is adequate light to support photosynthesis by phytoplankton and plants, is referred to as theeuphotic zone(also referred to as theepipelagic zone, orsurface zone).[6]The lower portion of the photic zone, where the light intensity is insufficient for photosynthesis, is called thedysphotic zone(dysphotic means "poorly lit" in Greek).[7]The dysphotic zone is also referred to as themesopelagic zone, or thetwilight zone.[8]Its lowermost boundary is at athermoclineof 12C (54F), which, in thetropicsgenerally lies between 200 and 1000metres.[9]The euphotic zone is somewhat arbitrarily defined as extending from the surface to the depth where the light intensity is approximately 0.11% of surface sunlightirradiance, depending onseason,latitudeand degree of waterturbidity.[6][7]In the clearest ocean water, the euphotic zone may extend to a depth of about 150metres,[6]or rarely, up to 200metres.[8]Dissolved substancesandsolid particlesabsorb and scatter light, and in coastal regions the high concentration of these substances causes light to be attenuated rapidly with depth. In such areas the euphotic zone may be only a few tens of metres deep or less.[6][8]The dysphotic zone, where light intensity is considerably less than 1% of surface irradiance, extends from the base of the euphotic zone to about 1000metres.[9]Extending from the bottom of the photic zone down to theseabedis theaphotic zone, a region of perpetual darkness.[8][9]Since the average depth of the ocean is about 4300metres,[10]the photic zone represents only a tiny fraction of the oceans total volume. However, due to its capacity for photosynthesis, the photic zone has the greatest biodiversity andbiomassof all oceanic zones. Nearly all primary production in the ocean occurs here. Life forms which inhabit the aphotic zone are often capable ofmovement upwards through the water columninto the photic zone for feeding. Otherwise, they must rely onmaterial sinking from above,[1]or find another source of energy and nutrition, such as occurs inchemosyntheticarchaeafound nearhydrothermal ventsandcold seeps.The aphotic zone can be subdivided into three different vertical regions, based on depth and temperature. First is thebathyal zone, extending from a depth of 1000metres down to 3000metres, with water temperature decreasing from 12C (54F) to 4C (39F) as depth increases.[11]Next is theabyssal zone, extending from a depth of 3000metres down to 6000metres.[11]The final zone includes the deep oceanic trenches, and is known as thehadal zone. This, the deepest oceanic zone, extends from a depth of 6000metres down to approximately 11000metres.[2][11]Abyssal plains are typically located in the abyssal zone, at depths ranging from 3000 to 6000metres.[1]The table below illustrates the classification of oceanic zones:ZoneSubzone (common name)Depth of zoneWater temperatureComments

photiceuphotic(epipelagic zone)0200metreshighly variable

disphotic(mesopelagic zone, or twilight zone)2001000metres4C or 39F highly variable

aphoticbathyal10003000metres412C or 3954F

abyssal30006000metres04C or 3239F[12]water temperature may reach as high as 464C (867F) nearhydrothermal vents[13][14][15][16][17]

hadalbelow 6000metres[18]12.5C or 3436F[19]ambient water temperature increases below 4000metres due toadiabatic heating[19]

Formation[edit]See also:Plate tectonicsandMantle convection

Oceanic crustis formed at amid-ocean ridge, while thelithosphereissubductedback into theasthenosphereatoceanic trenches

Age of oceanic crust (red is youngest, and blue is oldest)Oceanic crust, which forms thebedrockof abyssal plains, is continuously being created at mid-ocean ridges (a type ofdivergent boundary) by a process known asdecompression melting.[20]Plume-related decompression melting of solid mantle is responsible for creating ocean islands like theHawaiian islands, as well as the ocean crust at mid-ocean ridges. This phenomenon is also the most common explanation forflood basaltsandoceanic plateaus(two types oflarge igneous provinces). Decompression melting occurs when the uppermantleispartially meltedintomagmaas it moves upwards under mid-ocean ridges.[21][22]This upwelling magma then cools and solidifies byconductionandconvectionof heat to form newoceanic crust.Accretionoccurs as mantle is added to the growing edges of atectonic plate, usually associated withseafloor spreading. The age of oceanic crust is therefore a function of distance from the mid-ocean ridge.[23]The youngest oceanic crust is at the mid-ocean ridges, and it becomes progressively older, cooler and denser as it migrates outwards from the mid-ocean ridges as part of the process calledmantle convection.[24]Thelithosphere, which rides atop theasthenosphere, is divided into a number of tectonic plates that are continuously being created and consumed at their oppositeplate boundaries. Oceanic crust and tectonic plates are formed and move apart at mid-ocean ridges. Abyssal hills are formed by stretching of the oceanic lithosphere.[25]Consumption or destruction of the oceanic lithosphere occurs atoceanic trenches(a type ofconvergent boundary, also known as a destructive plate boundary) by a process known assubduction. Oceanic trenches are found at places where the oceanic lithospheric slabs of two different plates meet, and the denser (older) slab begins to descend back into the mantle.[26]At the consumption edge of the plate (the oceanic trench), the oceanic lithosphere has thermally contracted to become quite dense, and it sinks under its own weight in the process of subduction.[27]The subduction process consumes older oceanic lithosphere, so oceanic crust is seldom more than 200 million years old.[28]The overall process of repeated cycles of creation and destruction of oceanic crust is known as theSupercontinent cycle, first proposed byCanadiangeophysicistandgeologistJohn Tuzo Wilson.New oceanic crust, closest to the mid-oceanic ridges, is mostly basalt at shallow levels and has a ruggedtopography. The roughness of this topography is a function of the rate at which the mid-ocean ridge is spreading (the spreading rate).[29]Magnitudes of spreading rates vary quite significantly. Typical values for fast-spreading ridges are greater than 100mm/yr, while slow-spreading ridges are typically less than 20mm/yr.[21]Studies have shown that the slower the spreading rate, the rougher the new oceanic crust will be, and vice versa.[29]It is thought this phenomenon is due tofaultingat the mid-ocean ridge when the new oceanic crust was formed.[30]These faults pervading the oceanic crust, along with their bounding abyssal hills, are the most common tectonic and topographic features on the surface of the Earth.[25][30]The process of seafloor spreading helps to explain the concept ofcontinental driftin the theory of plate tectonics.The flat appearance of mature abyssal plains results from the blanketing of this originally uneven surface of oceanic crust by fine-grained sediments, mainly clay and silt. Much of this sediment is deposited from turbidity currents that have been channeled from the continental margins along submarine canyons down into deeper water. The remainder of the sediment comprises chiefly dust (clay particles) blown out to sea from land, and the remains of smallmarine plantsandanimalswhich sink from the upper layer of the ocean, known aspelagic sediments. The total sediment deposition rate in remote areas is estimated at two to three centimeters per thousand years.[31][32]Sediment-covered abyssal plains are less common in the Pacific Ocean than in other major ocean basins because sediments from turbidity currents are trapped in oceanic trenches that border the Pacific Ocean.[33]During parts of theMessinian salinity crisismuch of theMediterranean Sea's abyssal plain was an empty hot dry salt-floored sink.[34][35][36][37]Discovery[edit]See also:Bathymetry

Location of theChallenger Deepin theMariana TrenchThe landmark scientificexpedition(December 1872 May 1876) of the BritishRoyal Navysurvey shipHMSChallengeryielded a tremendous amount ofbathymetricdata, much of which has been confirmed by subsequent researchers. Bathymetric data obtained during the course of the Challenger expedition enabled scientists to draw maps,[38]which provided a rough outline of certain major submarine terrain features, such as the edge of thecontinental shelvesand theMid-Atlantic Ridge. This discontinuous set of data points was obtained by the simple technique of takingsoundingsby lowering long lines from the ship to the seabed.[38]The Challenger expedition was followed by the 18791881 expedition of theJeannette, led byUnited States NavyLieutenantGeorge Washington DeLong. The team sailed across theChukchi Seaand recordedmeteorologicalandastronomicaldata in addition to taking soundings of the seabed. The ship became trapped in theice packnearWrangel Islandin September 1879, and was ultimately crushed and sunk in June 1881.[39]TheJeannetteexpedition was followed by the 18931896 ArcticexpeditionofNorwegianexplorerFridtjof Nansenaboard theFram, which proved that theArctic Oceanwas a deep oceanic basin, uninterrupted by any significant land masses north of theEurasiancontinent.[40][41]Beginning in 1916, Canadian physicistRobert William Boyleand other scientists of the Anti-Submarine Detection Investigation Committee (ASDIC) undertook research which ultimately led to the development ofsonartechnology.Acoustic soundingequipment was developed which could be operated much more rapidly than the sounding lines, thus enabling theGerman Meteor expeditionaboard the German research vesselMeteor(192527) to take frequent soundings on east-west Atlantic transects. Maps produced from these techniques show the major Atlantic basins, but the depth precision of these early instruments was not sufficient to reveal the flat featureless abyssal plains.[42][43]As technology improved, measurement of depth,latitudeandlongitudebecame more precise and it became possible to collect more or less continuous sets of data points. This allowed researchers to draw accurate and detailed maps of large areas of the ocean floor. Use of a continuously recordingfathometerenabled Tolstoy & Ewing in the summer of 1947 to identify and describe the first abyssal plain. This plain, located to the south ofNewfoundland, is now known as theSohm Abyssal Plain.[44]Following this discovery many other examples were found in all the oceans.[45][46][47][48][49]TheChallenger Deepis the deepest surveyed point of all of Earth's oceans; it is located at the southern end of theMariana Trenchnear theMariana Islandsgroup. The depression is named after HMSChallenger, whose researchers made the first recordings of its depth on 23 March 1875 atstation 225. The reported depth was 4,475fathoms(8184meters) based on two separate soundings. On 1 June 2009, sonar mapping of the Challenger Deep by theSimradEM120multibeam sonar bathymetrysystem aboard theR/VKilo Moanaindicated a maximum depth of 10971meters (6.82 miles). The sonar system usesphaseandamplitudebottom detection, with an accuracy of better than 0.2% of water depth (this is an error of about 22meters at this depth).[50][51]Terrain features[edit]Hydrothermal vents[edit]

In thisphase diagram, the green dotted line illustrates theanomalous behavior of water. The solid green line marks themelting pointand the blue line theboiling point, showing how they vary with pressure.Main article:Hydrothermal ventA rare but important terrain feature found in the abyssal and hadal zones is the hydrothermal vent. In contrast to the approximately 2 C ambient water temperature at these depths, water emerges from these vents at temperatures ranging from 60 C up to as high as 464 C.[13][14][15][16][17]Due to the highbarometric pressureat these depths, water may exist in either its liquid form or as asupercritical fluidat such temperatures.At a barometric pressure of 218atmospheres, thecritical pointof water is 375 C. At a depth of 3,000meters, the barometric pressure of sea water is more than 300atmospheres (as salt water isdenserthan fresh water). At this depth and pressure, seawater becomes supercritical at a temperature of 407 C (see image). However the increase in salinity at this depth pushes the water closer to its critical point. Thus, water emerging from the hottest parts of some hydrothermal vents,black smokersandsubmarine volcanoescan be asupercritical fluid, possessing physical properties between those of agasand those of aliquid.[13][14][15][16][17]Sister Peak(Comfortless Cove Hydrothermal Field,448S1222W, elevation 2996m),Shrimp FarmandMephisto(Red Lion Hydrothermal Field,448S1223W, elevation 3047m), are three hydrothermal vents of the black smoker category, located on the Mid-Atlantic Ridge nearAscension Island. They are presumed to have been active since an earthquake shook the region in 2002.[13][14][15][16][17]These vents have been observed to ventphase-separated, vapor-type fluids. In 2008, sustained exit temperatures of up to 407 C were recorded at one of these vents, with a peak recorded temperature of up to 464 C. Thesethermodynamicconditions exceed the critical point of seawater, and are the highest temperatures recorded to date from the seafloor. This is the first reported evidence for directmagmatic-hydrothermalinteraction on a slow-spreading mid-ocean ridge.[13][14][15][16][17]Cold seeps[edit]

Tubewormsandsoft coralsat acold seeplocated at 3000meters depth on theFlorida Escarpment.Eelpouts, agalatheid crab, and analvinocarid shrimpare feeding on chemosyntheticmytilidmussels.Main article:Cold seepAnother unusual feature found in the abyssal and hadal zones is thecold seep, sometimes called acold vent. This is an area of the seabed where seepage ofhydrogen sulfide,methaneand otherhydrocarbon-rich fluid occurs, often in the form of a deep-seabrine pool. The first cold seeps were discovered in 1983, at a depth of 3200meters in theGulf of Mexico.[52]Since then, cold seeps have been discovered in many other areas of theWorld Ocean, including theMonterey Submarine Canyonjust offMonterey Bay, California, theSea of Japan, off the Pacific coast ofCosta Rica, off the Atlantic coast of Africa, off the coast of Alaska, and under anice shelfinAntarctica.[53]Biodiversity[edit]Marine habitats

Littoral zone Intertidal zone Estuaries Kelp forests Coral reefs Ocean banks Continental shelf Neritic zone Straits Pelagic zone Oceanic zone Seamounts Hydrothermal vents Cold seeps Demersal zone Benthic zone

v t e

See also:Deep sea communities,Deep sea creature,Deep sea fish,Demersal fishandBenthosThough the plains were once assumed to be vast,desert-like habitats, research over the past decade or so shows that they teem with a wide variety ofmicrobiallife.[54][55]However, ecosystem structure and function at the deep seafloor have historically been very poorly studied because of the size and remoteness of the abyss. Recentoceanographicexpeditions conducted by an international group of scientists from theCensus of Diversity of Abyssal Marine Life(CeDAMar) have found an extremely high level of biodiversity on abyssal plains, with up to 2000 species of bacteria, 250 species ofprotozoans, and 500 species ofinvertebrates(worms,crustaceansandmolluscs), typically found at single abyssal sites.[56]New species make up more than 80% of the thousands of seafloor invertebrate species collected at any abyssal station, highlighting our heretofore poor understanding of abyssal diversity and evolution.[56][57][58][59]Richer biodiversity is associated with areas of knownphytodetritusinput and higher organic carbon flux.[60]Abyssobrotula galatheae, aspeciesof cusk eel in thefamilyOphidiidae, is among the deepest-living species of fish. In 1970, one specimen wastrawledfrom a depth of 8370meters in thePuerto Rico Trench.[61][62][63]The animal was dead, however, upon arrival at the surface. In 2008, thehadal snailfish(Pseudoliparis amblystomopsis)[64]was observed and recorded at a depth of 7700meters in theJapan Trench. These are, to date, the deepest living fish ever recorded.[11][65]Other fish of the abyssal zone include the fishes of theIpnopidaefamily, which includes the abyssal spiderfish (Bathypterois longipes), tripodfish (Bathypterois grallator), feeler fish (Bathypterois longifilis), and the black lizardfish (Bathysauropsis gracilis). Some members of this family have been recorded from depths of more than 6000meters.[66]CeDAMar scientists have demonstrated that some abyssal and hadal species have a cosmopolitan distribution. One example of this would be protozoanforaminiferans,[67]certain species of which are distributed from the Arctic to the Antarctic. Other faunal groups, such as thepolychaeteworms andisopodcrustaceans, appear to be endemic to certain specific plains and basins.[56]Many apparently uniquetaxaofnematodeworms have also been recently discovered on abyssal plains. This suggests that the very deep ocean has fosteredadaptive radiations.[56]The taxonomic composition of the nematode fauna in the abyssal Pacific is similar, but not identical to, that of the North Atlantic.[60]A list of some of the species that have been discovered or redescribed by CeDAMar can be foundhere.Eleven of the 31 described species ofMonoplacophora(aclassofmollusks) live below 2000meters. Of these 11 species, two live exclusively in the hadal zone.[68]The greatest number of monoplacophorans are from the eastern Pacific Ocean along the oceanic trenches. However, no abyssal monoplacophorans have yet been found in the Western Pacific and only one abyssal species has been identified in the Indian Ocean.[68]Of the 922 known species ofchitons(from thePolyplacophoraclass of mollusks), 22 species (2.4%) are reported to live below 2000meters and two of them are restricted to the abyssal plain.[68]Although genetic studies are lacking, at least six of these species are thought to be eurybathic (capable of living in a wide range of depths), having been reported as occurring from thesublittoralto abyssal depths. A large number of the polyplacophorans from great depths areherbivorousorxylophagous, which could explain the difference between the distribution of monoplacophorans and polyplacophorans in the world's oceans.[68]Peracaridcrustaceans, including isopods, are known to form a significant part of the macrobenthic community that is responsible for scavenging on large food falls onto the sea floor.[1][69]In 2000, scientists of theDiversity of the deep Atlantic benthos(DIVA 1) expedition (cruise M48/1 of the German research vessel RVMeteor III) discovered and collected three new species of theAsellotasuborderofbenthicisopods from the abyssal plains of theAngola Basinin the SouthAtlantic Ocean.[70][71][72]In 2003, De Broyer et al. collected some 68,000 peracarid crustaceans from 62 species from baited traps deployed in theWeddell Sea,Scotia Sea, and off theSouth Shetland Islands. They found that about 98% of the specimens belonged to theamphipodsuperfamilyLysianassoidea, and 2% to the isopod familyCirolanidae. Half of these species were collected from depths of greater than 1000meters.[69]In 2005, theJapan Agency for Marine-Earth Science and Technology(JAMSTEC) remotely operated vehicle,KAIKO, collected sediment core from the Challenger Deep. 432 living specimens of soft-walled foraminifera were identified in the sediment samples.[73][74]Foraminifera are single-celledprotiststhat construct shells. There are an estimated 4,000 species of living foraminifera. Out of the 432 organisms collected, the overwhelming majority of the sample consisted of simple, soft-shelled foraminifera, with others representing species of the complex, multi-chambered genera Leptohalysis and Reophax. Overall, 85% of the specimens consisted of soft-shelledallogromids. This is unusual compared to samples of sediment-dwelling organisms from other deep-sea environments, where the percentage of organic-walled foraminifera ranges from 5% to 20% of the total. Small organisms with hard calculated shells have trouble growing at extreme depths because the water at that depth is severely lacking in calcium carbonate.[75]While similar lifeforms have been known to exist in shallower oceanic trenches (>7,000m) and on the abyssal plain, the lifeforms discovered in the Challenger Deep may represent independent taxa from those shallower ecosystems. This preponderance of soft-shelled organisms at the Challenger Deep may be a result of selection pressure. Millions of years ago, the Challenger Deep was shallower than it is now. Over the past six to nine million years, as the Challenger Deep grew to its present depth, many of the species present in the sediment of that ancient biosphere were unable to adapt to the increasing water pressure and changing environment. Those species that were able to adapt may have been the ancestors of the organisms currently endemic to the Challenger Deep.[73]Polychaetes occur throughout the Earth's oceans at all depths, from forms that live asplanktonnear the surface, to the deepest oceanic trenches. The robot ocean probeNereusobserved a 23cm specimen (still unclassified) of polychaete at the bottom of the Challenger Deep on 31 May 2009.[74][76][77][78]There are more than 10,000 described species of polychaetes; they can be found in nearly every marine environment. Some species live in the coldest ocean temperatures of the hadal zone, while others can be found in the extremely hot waters adjacent to hydrothermal vents.Within the abyssal and hadal zones, the areas around submarine hydrothermal vents and cold seeps have by far the greatest biomass and biodiversity per unit area. Fueled by the chemicals dissolved in the vent fluids, these areas are often home to large and diverse communities ofthermophilic,halophilicand otherextremophilicprokaryoticmicroorganisms(such as those of the sulfide-oxidizingBeggiatoagenus), often arranged in largebacterial matsnear cold seeps. In these locations, chemosynthetic archaea and bacteria typically form the base of the food chain. Although the process of chemosynthesis is entirely microbial, these chemosynthetic microorganisms often support vast ecosystems consisting of complex multicellular organisms throughsymbiosis.[79]These communities are characterized by species such asvesicomyid clams,mytilidmussels,limpets, isopods,giant tube worms,soft corals,eelpouts,galatheid crabs, andalvinocarid shrimp. The deepest seep community discovered thus far is located in theJapan Trench, at a depth of 7700meters.[11]Probably the most important ecological characteristic of abyssal ecosystems is energy limitation. Abyssal seafloor communities are considered to befood limitedbecausebenthicproduction depends on the input ofdetritalorganic materialproduced in the euphotic zone, thousands of meters above.[80]Most of the organic flux arrives as anattenuated rainof small particles (typically, only 0.52% of net primary production in the euphotic zone), which decreases inversely with water depth.[9]The small particle flux can be augmented by thefall of larger carcassesand downslope transport of organic material near continental margins.[80]Exploitation of resources[edit]See also:Deep sea miningandOffshore drillingIn addition to their high biodiversity, abyssal plains are of great current and future commercial and strategic interest. For example, they may be used for the legal and illegal disposal of large structures such as ships andoil rigs,radioactive wasteand otherhazardous waste, such asmunitions. They may also be attractive sites fordeep-sea fishing, andextraction of oil and gasand otherminerals. Future deep-seawaste disposalactivities that could be significant by 2025 includeemplacement of sewage and sludge,carbon-dioxide sequestration, and disposal ofdredge spoils.[81]Asfish stocksdwindle in the upper ocean, deep-seafisheriesare increasingly being targeted for exploitation. Becausedeep sea fishare long-lived and slow growing, these deep-sea fisheries are not thought to be sustainable in the long term given current management practices.[81]Changes in primary production in the photic zone are expected to alter the standing stocks in the food-limited aphotic zone.Hydrocarbon exploration in deep water occasionally results in significantenvironmental degradationresulting mainly from accumulation of contaminateddrill cuttings, but also fromoil spills. While theoil gusherinvolved in theDeepwater Horizon oil spillin theGulf of Mexicooriginates from awellheadonly 1500meters below the ocean surface,[82]it nevertheless illustrates the kind ofenvironmental disasterthat can result from mishaps related tooffshore drillingfor oil and gas.Sediments of certain abyssal plains contain abundant mineral resources, notablypolymetallic nodules. These potato-sizedconcretionsof manganese, iron, nickel, cobalt, and copper, distributed on the seafloor at depths of greater than 4000meters,[81]are of significant commercial interest. The area of maximum commercial interest for polymetallic nodule mining (called thePacific nodule province) lies ininternational watersof the Pacific Ocean, stretching from 118157, and from 916N, an area of more than 3 million km.[83]The abyssalClarion-Clipperton Fracture Zone(CCFZ) is an area within the Pacific nodule province that is currently under exploration for its mineral potential.[60]Eight commercial contractors are currently licensed by theInternational Seabed Authority(anintergovernmental organizationestablished to organize and control all mineral-related activities in the international seabed area beyond the limits ofnational jurisdiction) to explore nodule resources and to test mining techniques in eightclaim areas, each covering 150,000km.[83]When mining ultimately begins, each mining operation is projected to directly disrupt 300800km of seafloor per year and disturb thebenthic faunaover an area 510 times that size due to redeposition of suspended sediments. Thus, over the 15-year projected duration of a single mining operation, nodule mining might severely damage abyssal seafloor communities over areas of 20,000 to 45,000km (a zone at least the size ofMassachusetts).[83]Limited knowledge of thetaxonomy,biogeographyandnatural historyofdeep sea communitiesprevents accurate assessment of the risk of speciesextinctionsfrom large-scale mining. Data acquired from the abyssal North Pacific and North Atlantic suggest that deep-sea ecosystems may be adversely affected by mining operations on decadal time scales.[81]In 1978, a dredge aboard theHughes Glomar Explorer, operated by the American miningconsortiumOcean Minerals Company(OMCO), made a mining track at a depth of 5000meters in the nodule fields of the CCFZ. In 2004, theFrenchResearch Institute for Exploitation of the Sea (IFREMER) conducted theNodinautexpedition to this mining track (which is still visible on the seabed) to study the long-term effects of this physical disturbance on the sediment and its benthic fauna. Samples taken of the superficial sediment revealed that its physical and chemical properties had not shown any recovery since the disturbance made 26 years earlier. On the other hand, the biological activity measured in the track by instruments aboard the mannedsubmersiblebathyscapheNautiledid not differ from a nearby unperturbed site. This data suggests that the benthic fauna and nutrient fluxes at the watersediment interface has fully recovered.[84]Oceanic trenchFrom Wikipedia, the free encyclopedia

Oceanic crust is formed at anoceanic ridge, while the lithosphere is subducted back into the asthenosphere at trenches.Theoceanic trenchesare hemispheric-scale long but narrow topographic depressions of the sea floor. They are also the deepest parts of the ocean floor. Oceanic trenches are a distinctive morphological feature ofconvergentplate boundaries. Along convergent plate boundaries, plates move together at rates that vary from a few mm to over ten cm per year. A trench marks the position at which the flexed,subductingslabbegins to descend beneath another lithospheric slab. Trenches are generally parallel to avolcanicisland arc, and about 200km (120mi) from avolcanic arc. Oceanic trenches typically extend 3 to 4km (1.9 to 2.5mi) below the level of the surrounding oceanic floor. The greatest ocean depth to be sounded is in theChallenger Deepof theMariana Trench, at a depth of 11,034m (36,201ft) below sea level. Oceanic lithosphere moves into trenches at a global rate of about 3km2/yr.[1]Contents[hide] 1Geographic distribution 2History of the term "trench" 3Trench rollback 3.1Processes involved 3.2Mantle interactions 4Morphologic expression 5Filled trenches 6Accretionary prisms and sediment transport 7Water and biosphere 8Empty trenches and subduction erosion 9Factors affecting trench depth 10Deepest oceanic trenches 11Notable oceanic trenches 12Ancient oceanic trenches 13Notes 14See also 15ReferencesGeographic distribution[edit]

Major Pacific trenches (1-10) and fracture zones (11-20): 1. Kermadec 2. Tonga 3. Bougainville 4. Mariana 5. Izu-Ogasawara 6. Japan 7. KurilKamchatka 8. Aleutian 9. Middle America 10. Peru-Chile 11. Mendocino 12. Murray 13. Molokai 14. Clarion 15. Clipperton 16. Challenger 17. Eltanin 18. Udintsev 19. East Pacific Rise (S-shaped) 20. Nazca RidgeThere are about 50,000km (31,000mi) ofconvergent plate margins, mostly around thePacific Oceanthe reason for the reference Pacific-type marginbut they are also in the easternIndian Ocean, with relatively short convergent margin segments in theAtlantic Oceanand in theMediterranean Sea. Trenches are sometimes buried and lack bathymetric expression, but the fundamental structures that these represent mean that the great name should also be applied here. This applies toCascadia,Makran, southernLesser Antilles, andCalabrian trenches. Trenches along withvolcanic arcsand zones ofearthquakesthat dip under the volcanic arc as deeply as 700km (430mi) are diagnostic of convergent plate boundaries and their deeper manifestations,subduction zones. Trenches are related to but distinguished from continental collision zones (like that between India and Asia to form theHimalaya), wherecontinental crustenters the subduction zone. When buoyant continental crust enters a trench, subduction eventually stops and the convergent plate margin becomes a collision zone. Features analogous to trenches are associated with collisions zones; these are sediment-filled foredeeps referred to as peripheral foreland basins, such as that which theGanges Riverand Tigris-Euphrates rivers flow along.History of the term "trench"[edit]Trenches were not clearly defined until the late 1940s and 1950s. Thebathymetryof the ocean was of no real interest until the late 19th and early 20th centuries, with the initial laying ofTransatlantic telegraph cableson the seafloor between the continents. Even then the elongated bathymetric expression of trenches was not recognized until well into the 20th century. The term trench does not appear inMurrayand Hjorts (1912) classicoceanographybook. Instead they applied the term deep for the deepest parts of the ocean, such asChallenger Deep. Experiences fromWorld War Ibattlefields emblazoned the concept of thetrench warfareas an elongate depression defining an important boundary, so it was no surprise that the term trench was used to describe natural features in the early 1920s. The term was first used in a geologic context by Scofield two years after the war ended to describe a structurally controlled depression in theRocky Mountains. Johnstone, in his 1923 textbookAn Introduction to Oceanography, first used the term in its modern sense for any marked, elongate depression of the sea bottom.During the 1920s and 1930s,Felix Andries Vening Meineszdeveloped a uniquegravimeterthat could measuregravityin the stable environment of a submarine and used it to measure gravity over trenches. His measurements revealed that trenches are sites ofdownwellingin the solid Earth. The concept of downwelling at trenches was characterized by Griggs in 1939 as the tectogene hypothesis, for which he developed an analogue model using a pair of rotating drums.World War IIin the Pacific led to great improvements of bathymetry in especially the western and northern Pacific, and the linear nature of these deeps became clear. The rapid growth of deep sea research efforts, especially the widespread use of echosounders in the 1950s and 1960s confirmed the morphological utility of the term. The important trenches were identified, sampled, and their greatest depths sonically plumbed. The heroic phase of trench exploration culminated in the 1960 descent of theBathyscapheTrieste, which set an unbeatable world record by diving to the bottom of the Challenger Deep. FollowingRobert S. Dietz andHarry Hess articulation of the seafloor spreading hypothesis in the early 1960s and the plate tectonic revolution in the late 1960s the term trench has been redefined withplate tectonicas well as bathymetric connotations.Trench rollback[edit]Although trenches would seem to be positionally stable over time, it is hypothesized that some trenches, particularly those associated with subduction zones where two oceanic plates converge, retrograde, that is, they move backward into the plate which is subducting, akin to a backward-moving wave. This has been termedtrench rollbackorhinge retreat(alsohinge rollback). This is one explanation for the existence ofback-arc basins.Slab rollback can also be referred to as hinge/trench retreat, or trench rollback.Slab rollback is a process which occurs during thesubductionof two tectonic plates resulting in the seaward motion of the trench. Forces acting perpendicular to the slab (portion of the subducting plate within the mantle) at depth are responsible for the backward migration of the slab in the mantle and ultimately the movement of the hinge and trench at the surface.[2]The driving force for rollback is the negative buoyancy of the slab with respect to the underlying mantle[3]as well as the geometry of the slab.[4]Back-arc basinsare often associated with slab rollback due to extension in the overriding plate as a response to the subsequent subhorizontal mantle flow from the displacement of the slab at depth.[5]Processes involved[edit]Several forces are involved in the processes 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) which is the pressure supplied 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 resisisting force from the surrounding mantle opposes the slab pull forces. Interactions with the 660-km discontinuity will cause a deflection due to the buoyancy at the phase transition (F660).[4]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 will occur. 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.[5]Seismic tomographyprovides evidence for slab rollback. Results demonstrate high temperature anomalies within the mantle suggesting subducted material is present in the mantle.[6]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 ofophiolites.Slab rollback is not always a continuous process suggesting an episodic nature.[3]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.[4]Nearby continental collisions have an effect on slab rollback. Continental collisions induce mantle flow and extrusion of mantle material which results in stretching and arc-trench rollback.[5]In the area of the Southeast Pacific, there have been several rollback events resulting in the formation of numerous back-arc basins.[3]Mantle interactions[edit]Interactions with themantlediscontinuities 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.[4]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.[2]Extension rates are altered when the slab interacts with the discontinuities within the mantle at 410km and 660km depth. Slabs can either penetrate directly into the lower mantle, or can be retarded due to the phase transition at 660km depth creating a difference in buoyancy. An increase in retrograde trench migration (slab rollback) (24cm/yr) is a result of flattened slabs at the 660-km discontinuity where the slab does not penetrate into the lower mantle.[7]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 (~13cm/yr) such as the Mariana arc, Tonga arcs.[7]Morphologic expression[edit]

ThePeru-Chile TrenchTrenches are centerpieces of the distinctive physiography of a convergent plate margin. Transects across trenches yield asymmetric profiles, with relatively gentle (~5) outer (seaward) slope and a steeper (~1016) inner (landward) slope. This asymmetry is due to the fact that the outer slope is defined by the top of the downgoing plate, which must bend as it starts its descent. The great thickness of the lithosphere requires that this bending be gentle. As the subducting plate approaches the trench, it is first bent upwards to form theouter trench swell, then descends to form the outer trench slope. The outer trench slope is disrupted by a set of subparallel normalfaultswhich staircase the seafloor down to the trench. The plate boundary is defined by the trench axis itself. Beneath the inner trench wall, the two plates slide past each other along the subductiondecollement, the seafloor intersection of which defines the trench location. The overriding plate containsvolcanic arc(generally) and aforearc. The volcanic arc is caused by physical and chemical interactions between the subducted plate at depth andasthenospheric mantleassociated with the overriding plate. The forearc lies between the trench and the volcanic arc. Forearcs have the lowest heatflow from the interior Earth because there is noasthenosphere(convecting mantle) between the forearc lithosphere and the cold subducting plate.The inner trench wall marks the edge of the overriding plate and the outermost forearc. The forearc consists ofigneousandmetamorphiccrust, and this crust acts as buttress to a growing accretionary prism (sediments scraped off the downgoing plate onto the inner trench wall, depending on how much sediment is supplied to the trench). If the flux of sediments is high, material will be transferred from the subducting plate to the overriding plate. In this case an accretionary prism grows and the location of the trench migrates progressively away from the volcanic arc over the life of the convergent margin. Convergent margins with growing accretionary prisms are called accretionary convergent margins and make up nearly half of all convergent margins. If the sediment flux is low, material will be transferred from the overriding plate to the subducting plate by a process of tectonic ablation known as subduction erosion and carried down the subduction zone. Forearcs undergoing subduction erosion typically expose igneous rocks. In this case, the location of the trench will migrate towards the magmatic arc over the life of the convergent margin. Convergent margins experiencing subduction erosion are called nonaccretionary convergent margins and comprise more than half of convergent plate boundaries. This is an oversimplification, because different parts of a convergent margin can experience sediment accretion and subduction erosion over its life.The asymmetric profile across a trench reflects fundamental differences in materials and tectonic evolution. The outer trench wall and outer swell comprise seafloor that takes a few million years to move from where subduction-related deformation begins near the outer trench swell until sinking beneath the trench. In contrast, the inner trench wall is deformed by plate interactions for the entire life of the convergent margin. The forearc is continuously subjected to subduction-relatedearthquakes. This protracted deformation and shaking ensures that the inner trench slope is controlled by the angle of repose of whatever material it is composed of. Because they are composed of igneous rocks instead of deformed sediments, non-accretionary trenches have steeper inner walls than accretionary trenches.Filled trenches[edit]The composition of the inner trench slope and a first-order control on trench morphology is determined bysedimentsupply. Active accretionary prisms are common for trenches nearcontinentswhere largeriversorglaciersreach the sea and supply great volumes of sediment which naturally flow to the trench. These filled trenches are confusing because in aplate tectonicsense they are indistinguishable from other convergent margins but lack thebathymetric expressionof a trench. TheCascadiamargin of the northwest USA is a filled trench, the result of sediments delivered by the rivers of the NW USA and SW Canada. TheLesser Antillesconvergent margin shows the importance of proximity to sediment sources for trench morphology. In the south, near the mouth of theOrinocoRiver, there is no morphological trench and the forearc plus accretionary prism is almost 500km (310mi) wide. The accretionary prism is so large that it forms the islands ofBarbadosandTrinidad. Northward the forearc narrows, the accretionary prism disappears, and only north of 17N the morphology of a trench is seen. In the extreme north, far away from sediment sources, thePuerto Rico Trenchis over 8,600m (28,200ft) deep and there is no active accretionary prism. A similar relationship between proximity to rivers, forearc width, and trench morphology can be observed from east to west along theAlaskan-Aleutianconvergent margin. The convergent plate boundary offshore Alaska changes along its strike from a filled trench with broad forearc in the east (near the coastal rivers of Alaska) to a deep trench with narrow forearc in the west (offshore the Aleutian islands). Another example is theMakranconvergent margin offshore Pakistan and Iran, which is a trench filled by sediments from theTigris-EuphratesandIndusrivers. Thick accumulations ofturbiditesalong a trench can be supplied by down-axis transport of sediments that enter the trench 1,0002,000km (6201,240mi) away, as is found for thePeru-Chile Trenchsouth ofValparasoand for the Aleutian Trench. Convergence rate can also be important for controlling trench depth, especially for trenches near continents, because slow convergence causes the capacity of the convergent margin to dispose of sediment to be exceeded.There an evolution in trench morphology can be expected as oceans close and continents converge. While the ocean is wide, the trench may be far away from continental sources of sediment and so may be deep. As the continents approach each other, the trench may become filled with continental sediments and become shallower. A simple way to approximate when the transition from subduction to collision has occurred is when the plate boundary previously marked by a trench is filled enough to rise above sealevel.Accretionary prisms and sediment transport[edit]Accretionary prisms grow by frontal accretion, whereby sediments are scraped off,bulldozer-fashion, near the trench, or byunderplatingof subducted sediments and perhapsoceanic crustalong the shallow parts of the subduction decollement. Frontal accretion over the life of a convergent margin results in younger sediments defining the outermost part of the accretionary prism and the oldest sediments defining the innermost portion. Older (inner) parts of the accretionary prism are much more lithified and have steeper structures than the younger (outer) parts. Underplating is difficult to detect in modern subduction zones but may be recorded in ancient accretionary prisms such as the Franciscan Group of California in the form of tectonic mlanges and duplex structures. Different modes of accretion are reflected in morphology of the inner slope of the trench, which generally shows three morphological provinces. The lower slope comprises imbricate thrust slices that form ridges. The mid slope may comprise a bench or terraces. The upper slope is smoother but may be cut bysubmarine canyons. Because accretionary convergent margins have high relief, are continuously deformed, and accommodate a large flux of sediments, they are vigorous systems of sediment dispersal and accumulation.Sediment transportis controlled by submarinelandslides, debris flows,turbidity currents, andcontourites. Submarine canyons transport sediment frombeachesand rivers down the upper slope. These canyons form by channelized turbidites and generally lose definition with depth because continuous faulting disrupts the submarine channels. Sediments move down the inner trench wall via channels and a series of fault-controlled basins. The trench itself serves as an axis of sediment transport. If enough sediment moves to the trench, it may be completely filled so that turbidity currents are able to carry sediments well beyond the trench and may even surmount the outer swell. Sediments from the rivers of SW Canada and NW USA spill over where the Cascadia trench would be and cross theJuan de Fuca plateto reach the spreading ridge several hundred kilometres to the west.The slope of the inner trench slope of an accretionary convergent margin reflects continuous adjustments to the thickness and width of the accretionary prism. The prism maintains a critical taper, established in conformance withMohrCoulomb theoryfor the pertinent materials. A package of sediments scraped off the downgoing lithospheric plate will deform until it and the accretionary prism that it has been added to attain a critical taper (constant slope) geometry. Once critical taper is attained, the wedge slides stably along its basal decollement. Strain rate and hydrologic properties strongly influence the strength of the accretionary prism and thus the angle of critical taper. Fluid pore pressures modify rock strength and are important controls of critical taper angle. Low permeability and rapid convergence may result in pore pressures that exceed lithostatic pressure and a relatively weak accretionary prism with a shallowly tapered geometry, whereas high permeability and slow convergence result in lower pore pressure, stronger prisms, and steeper geometry.TheHellenic Trenchof theHellenic arcsystem is unusual because this convergent margin subductsevaporites. The slope of the surface of the southern flank of theMediterranean Ridge(its accretionary prism) is low, about 1, which indicates very low shear stress on the decollement at the base of the wedge. Evaporites influence the critical taper of the accretionary complex, as their mechanical properties differ from those of siliciclastic sediments, and because of their effect upon fluid flow and fluid pressure, which controleffective stress. In the 1970s, the linear deeps of the Hellenic trench south ofCretewere interpreted to be similar to trenches at other subduction zones, but with the realization that the Mediterranean Ridge is an accretionary complex, it became apparent that the Hellenic trench is actually a starved forearc basin, and that the plate boundary lies south of the Mediterranean Ridge.[8]Water and biosphere[edit]The volume of water escaping from within and beneath theforearcresults in some of Earths most dynamic and complex interactions between aqueous fluids and rocks. Most of this water is trapped in pores and fractures in the upper lithosphere and sediments of the subducting plate. The average forearc is underrun by a solid volume of oceanic sediment that is 400m (1,300ft) thick. This sediment enters the trench with 50-60%porosity. These sediments are progressively squeezed as they are subducted, reducing void space and forcing fluids out along the decollement and up into the overlying forearc, which may or may not have an accretionary prism. Sediments accreted to the forearc are another source of fluids. Water is also bound in hydrous minerals, especiallyclaysandopal. Increasing pressure and temperature experienced by subducted materials converts the hydrous minerals to denser phases that contain progressively less structurally bound water. Water released by dehydration accompanying phase transitions is another source of fluids introduced to the base of the overriding plate. These fluids may travel through the accretionary prism diffusely, via interconnected pore spaces in sediments, or may follow discrete channels along faults. Sites of venting may take the form of mud volcanoes or seeps and are often associated with chemosynthetic communities. Fluids escaping from the shallowest parts of a subduction zone may also escape along the plate boundary but have rarely been observed draining along the trench axis. All of these fluids are dominated by water but also contain dissolved ions and organic molecules, especiallymethane. Methane is often sequestered in an ice-like form (methane clathrate, also called gas hydrate) in the forearc. These are a potential energy source and can rapidly break down. Destabilization of gas hydrates has contributed to global warming in the past and will likely do so in the future.Chemosyntheticcommunities thrive where cold fluids seep out of the forearc. Cold seep communities have been discovered in inner trench slopes down to depths of 7000 m in the western Pacific, especially around Japan, in the Eastern Pacific along North, Central and South America coasts from the Aleutian to the Peru-Chile trenches, on the Barbados prism, in the Mediterranean, and in the Indian Ocean along the Makran and Sunda convergent margins. These communities receive much less attention than the chemosynthetic communities associated withhydrothermal vents. Chemosynthetic communities are located in a variety of geological settings: above over-pressured sediments in accretionary prisms where fluids are expelled through mud volcanoes or ridges (Barbados, Nankai and Cascadia); along active erosive margins with faults; and along escarpments caused by debris slides (Japan trench, Peruvian margin). Surface seeps may be linked to massive hydrate deposits and destabilization (e.g. Cascadia margin). High concentrations of methane andsulfidein the fluids escaping from the seafloor are the principal energy sources for chemosynthesis.Empty trenches and subduction erosion[edit]Trenches distant from an influx of continental sediments lack an accretionary prism, and the inner slope of such trenches is commonly composed of igneous or metamorphic rocks. Non-accretionary convergent margins are characteristic of (but not limited to) primitive arc systems. Primitive arc systems are those built on oceanic lithosphere, such as the Izu-Bonin-Mariana, Tonga-Kermadec, and Scotia (South Sandwich) arc systems. The inner trench slope of these convergent margins exposes the crust of the forearc, including basalt, gabbro, and serpentinized mantle peridotite. These exposures allow easy access to study the lower oceanic crust and upper mantle in place and provide a unique opportunity to study the magmatic products associated with the initiation of subduction zones. Most ophiolites probably originate in a forearc environment during the initiation of subduction, and this setting favors ophiolite emplacement during collision with blocks of thickened crust. Not all non-accretionary convergent margins are associated with primitive arcs. Trenches adjacent to continents where there is little influx of sediments carried by rivers, such as the central part of the Peru-Chile Trench, may also lack an accretionary prism.Igneous basement of a nonaccretionary forearc may be continuously exposed by subduction erosion. This transfers material from the forearc to the subducting plate and can be accomplished by frontal erosion or basal erosion. Frontal erosion is most active in the wake of seamounts being subducted beneath the forearc. Subduction of large edifices (seamount tunneling) oversteepens the forearc, causing mass failures that carry debris towards and ultimately into the trench. This debris may be deposited in graben of the downgoing plate and subducted with it. In contrast, structures resulting from subduction erosion of the base of the forearc are difficult to recognize from seismic reflection profiles, so the possibility of basal erosion is difficult to confirm. Subduction erosion may also diminish a once-robust accretionary prism if the flux of sediments to the trench diminishes.Nonaccretionary forearcs may also be the site ofserpentinemud volcanoes. These form where fluids released from the downgoing plate percolate upwards and interact with cold mantle lithosphere of the forearc. Mantleperidotiteis hydrated intoserpentinite, which is much less dense than peridotite and so will rise diapirically when there is an opportunity to do so. Some nonaccretionary forearcs are subjected to strong extensional stresses, for example the Marianas, and this allows buoyant serpentinite to rise to the seafloor where they form serpentinite mud volcanoes. Chemosynthetic communities are also found on non-accretionary margins such as the Marianas, where they thrive on vents associated with serpentinite mud volcanoes.Factors affecting trench depth[edit]

ThePuerto Rico TrenchThere are several factors that control the depth of trenches. The most important control is the supply of sediment, which fills the trench so that there is nobathymetricexpression. It is therefore not surprising that the deepest trenches (deeper than 8,000m (26,000ft)) are all nonaccretionary. In contrast, all trenches with growing accretionary prisms are shallower than 8,000m (26,000ft). A second order control on trench depth is the age of the lithosphere at the time of subduction. Becauseoceanic lithospherecools and thickens as it ages, it subsides. The older the seafloor, the deeper it lies and this determines a minimum depth from which seafloor begins its descent. This obvious correlation can be removed by looking at the relative depth, the difference between regional seafloor depth and maximum trench depth. Relative depth may be controlled by the age of the lithosphere at the trench, the convergence rate, and the dip of the subducted slab at intermediate depths. Finally, narrow slabs can sink and roll back more rapidly than broad plates, because it is easier for underlyingasthenosphereto flow around the edges of the sinking plate. Such slabs may have steep dips at relatively shallow depths and so may be associated with unusually deep trenches, such as theChallenger Deep.Deepest oceanic trenches[edit]TrenchOceanMaximum Depth

Mariana TrenchPacific Ocean11,034m (36,201ft)

Tonga TrenchPacific Ocean10,882m (35,702ft)

Philippine TrenchPacific Ocean10,545m (34,596ft)

KurilKamchatka TrenchPacific Ocean10,542m (34,587ft)

Kermadec TrenchPacific Ocean10,047m (32,963ft)

Izu-Bonin Trench (Izu-Ogasawara Trench)Pacific Ocean9,810m (32,190ft)

Japan TrenchPacific Ocean9,504m (31,181ft)

Puerto Rico TrenchAtlantic Ocean8,800m (28,900ft)

South Sandwich TrenchAtlantic Ocean8,428m (27,651ft)

Peru-Chile TrenchorAtacama TrenchPacific Ocean8,065m (26,460ft)

Notable oceanic trenches[edit]TrenchLocation

Aleutian TrenchSouth of theAleutian Islands, west ofAlaska

Bougainville TrenchSouth ofNew Guinea

Cayman TrenchWesternCaribbean Sea

Cedros Trench(inactive)Pacific coast ofBaja California

Hikurangi TrenchEast ofNew Zealand

Izu-Ogasawara TrenchNearIzuandBoninislands

Japan TrenchNortheastJapan

Kermadec Trench*Northeast ofNew Zealand

Kuril-Kamchatka Trench*NearKuril islands

Manila TrenchWest ofLuzon, Philippines

Mariana Trench*WesternPacific ocean; east ofMariana Islands

Middle America TrenchEastern Pacific Ocean; off coast ofGuatemala,El Salvador,Nicaragua,Costa Rica

New Hebrides TrenchWest ofVanuatu(New Hebrides Islands).

Peru-Chile TrenchEastern Pacific ocean; off coast ofPeru&Chile

Philippine Trench*East of thePhilippines

Puerto Rico TrenchBoundary ofCaribbean SeaandAtlantic ocean

Puysegur trenchSouthwest ofNew Zealand

Ryukyu TrenchEastern edge of Japan'sRyukyu Islands

South Sandwich Trench

Sunda TrenchCurves from south ofJavato west ofSumatraand theAndaman and Nicobar Islands

Tonga Trench*NearTonga

Yap TrenchWestern Pacific ocean; betweenPalau Islandsand Mariana Trench

(*) The 5 deepest trenches in the worldAncient oceanic trenches[edit]TrenchLocation

Intermontane TrenchWesternNorth America; betweenIntermontane Islandsand North America

Insular TrenchWestern North America; betweenInsular Islandsand Intermontane Islands

Farallon TrenchWestern North America

Tethyan TrenchSouth ofTurkey,Iran,TibetandSoutheast Asia

AtollFrom Wikipedia, the free encyclopediaFor other senses of this word, seeatoll (disambiguation).

Satellite picture of theAtafuatoll inTokelauin thePacific Ocean.Anatoll(/tl/or/tl/),[1]sometimes called acoral atoll, is a ring-shapedcoral reefincluding a coral rim that encircles alagoonpartially or completely. There may becoral islands/cayson the rim.[2](p60)[3]The coral of the atoll often sits atop the rim of an extinctseamountorvolcanowhich has eroded or subsided partially beneath the water. The lagoon forms over the volcanic crater orcalderawhile the higher rim remains above water or at shallow depths that permit the coral to grow and form the reefs. For the atoll to persist, continued erosion or subsidence must be at a rate slow enough to permit reef growth upwards and outwards to replace the lost height.[4]Contents[hide] 1Usage 2Distribution and size 3Formation 4Investigation by the Royal Society of London into the formation of coral reefs 5United States national monuments 6See also 7References 8External linksUsage[edit]The wordatollcomes from theDhivehi(anIndo-Aryan languagespoken on theMaldive Islands) wordatholhu(Dhivehi:,[tu]), meaning an administrative subdivision.OEDIts first recorded use inEnglishwas in 1625 asatollonCharles Darwinrecognized its indigenous origin and coined, in hisThe Structure and Distribution of Coral Reefs, the definition of atolls as "circular groups of coral islets" that is synonymous with "lagoon-island".[5](p2)More modern definitions ofatolldescribe them as "annular reefs enclosing a lagoon in which there are nopromontoriesother than reefs andisletscomposed of reefdetritus"[6]or "in an exclusively morphological sense, [as] a ring-shaped ribbon reef enclosing a lagoon".[7]Distribution and size[edit]

NASAsatellite image of some of theatolls of the Maldives, which consists of 1,322 islands arranged into 26 atolls.

Nukuorofrom space. CourtesyNASA.

Los Roques Archipelagoin Venezuela, the largest marine National Park in Latin America,[8]from space. CourtesyNASA.The distribution of atolls around the globe is instructive: most of the world's atolls are in the Pacific Ocean (with concentrations in theTuamotu Islands,Caroline Islands,Marshall Islands,Coral Sea Islands, and the island groups ofKiribati,TuvaluandTokelau) andIndian Ocean(theAtolls of the Maldives, theLakshadweep Islands, theChagos Archipelagoand theOuter Islandsof theSeychelles). TheAtlantic Oceanhas no large group