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Abyssal plain
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{{Short description|Flat area on the deep ocean floor}} {{Use dmy dates|date=June 2019}} {{Good article}} [[File:Oceanic basin.svg|thumb|right|Diagrammatic cross-section of an [[oceanic basin]], showing the relationship of the abyssal plain to a [[Continental shelf#Topography|continental rise]] and an [[oceanic trench]]]] [[File:Oceanic divisions.svg|thumb|Depiction of the [[abyssal zone]] in relation to other major [[oceanic zone]]s]] {{aquatic layer topics}} An '''abyssal plain''' is an underwater [[plain]] on the deep [[ocean floor]], usually found at depths between {{convert|3000|and|6000|m}}. Lying generally between the foot of a [[continental rise]] and a [[mid-ocean ridge]], abyssal plains cover more than 50% of the [[Earth]]'s surface.<ref name=CRS2008>{{Cite journal |author1=Craig R. Smith |author2=Fabio C. De Leo |author3=Angelo F. Bernardino |author4=Andrew K. Sweetman |author5=Pedro Martinez Arbizu |title=Abyssal food limitation, ecosystem structure and climate change |journal=Trends in Ecology and Evolution |volume=23 |pages=518–528 |year=2008 |url=http://cmbc.ucsd.edu/Students/Current_Students/SIO277/Smith%20et%20al.%20TREE%202008.pdf |pmid=18584909 |issue=9 |doi=10.1016/j.tree.2008.05.002 |access-date=18 June 2010 |archive-date=20 July 2011 |archive-url=https://web.archive.org/web/20110720075942/http://cmbc.ucsd.edu/Students/Current_Students/SIO277/Smith%20et%20al.%20TREE%202008.pdf |url-status=dead }}</ref><ref name=Vino1997>{{Cite book |author=N.G. Vinogradova |title=The Biogeography of the Oceans |chapter=Zoogeography of the Abyssal and Hadal Zones |volume=32 |pages=325–387 |year=1997 |doi=10.1016/S0065-2881(08)60019-X |series=Advances in Marine Biology |isbn=9780120261321}}</ref> They are among the flattest, smoothest, and least explored regions on Earth.<ref name=Weaver>{{Cite book |title = Geology and Geochemistry of Abyssal Plains |author1 = P.P.E. Weaver |author2 = J. Thomson |author3 = P. M. Hunter |year = 1987 |publisher = Blackwell Scientific Publications |location = Oxford |page = x |isbn = 978-0-632-01744-7 |url = http://sp.lyellcollection.org/cgi/issue_pdf/frontmatter_pdf/31/1.pdf |access-date = 18 June 2010 |url-status = dead |archive-url = https://web.archive.org/web/20101224060317/http://sp.lyellcollection.org/cgi/issue_pdf/frontmatter_pdf/31/1.pdf |archive-date = 24 December 2010 }}</ref> Abyssal plains are key geologic elements of [[oceanic basin]]s. (the other elements being an elevated mid-ocean ridge and flanking [[abyssal hill]]s) The creation of the abyssal plain is the result of the spreading of the seafloor (plate tectonics) and the melting of the lower [[oceanic crust]]. Magma rises from above the [[asthenosphere]] (a layer of the upper [[Mantle (geology)|mantle]]), and as this [[basalt]]ic material reaches the surface at mid-ocean ridges, it forms new oceanic crust, which 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-grained [[sediment]]s, mainly [[clay]] and [[silt]]. Much of this sediment is deposited by [[turbidity current]]s that have been channelled from the [[continental margin]]s along [[submarine canyon]]s into deeper water. The rest is composed chiefly of [[pelagic sediments]]. Metallic [[Manganese nodule|nodules]] are common in some areas of the plains, with varying concentrations of metals, including [[manganese]], [[iron]], [[nickel]], [[cobalt]], and [[copper]]. There are also amounts of carbon, nitrogen, phosphorus and silicon, due to material that comes down and decomposes. Owing in part to their vast size, abyssal plains are believed to be major reservoirs of [[biodiversity]]. They also exert significant influence upon ocean [[Carbon cycle|carbon cycling]], dissolution of [[calcium carbonate]], and [[Carbon dioxide in Earth's atmosphere|atmospheric CO<sub>2</sub> concentrations]] over [[Geologic time scale|time scales]] of a hundred to a thousand years. The structure of abyssal [[ecosystem]]s is strongly influenced by the rate of [[marine snow|flux of food]] to the seafloor and the composition of the material that settles. Factors such as [[climate change]], [[Trawling|fishing practices]], and [[Ocean nourishment|ocean fertilization]] have a substantial effect on patterns of [[primary production]] in the [[photic zone|euphotic zone]].<ref name="CRS2008"/><ref>{{harvnb|Smith|Paterson|Lambshead|Glover|Gooday|Rogers|Sibuet|Kitazato|Galéron|Menot|2008|p=5}}</ref> Animals absorb dissolved oxygen from the oxygen-poor waters. Much dissolved oxygen in abyssal plains came from polar regions that had melted long ago. Due to scarcity of oxygen, abyssal plains are inhospitable for organisms that would flourish in the oxygen-enriched waters above. [[Deep-water coral|Deep sea coral reefs]] are mainly found in depths of 3,000 meters and deeper in the abyssal and [[hadal zone]]s. Abyssal plains were not recognized as distinct [[physiographic]] features of the [[Seabed|sea floor]] until the late 1940s and, until recently, none had been studied on a systematic basis. They are poorly preserved in the [[Geologic record|sedimentary record]], because they tend to be consumed by the subduction process. Due to darkness and a water pressure that can reach about 750 times atmospheric pressure (76 megapascal), abyssal plains are not well explored.
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