Abyssal plain

An abyssal plain is an underwater plain on the deep ocean floor, usually found at depths between 3,000 metres (9,800 ft) and 6,000 metres (20,000 ft). 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. They are among the flattest, smoothest, and least explored regions on Earth. Abyssal plains are key geologic elements of oceanic basins (the other elements being an elevated mid-ocean ridge and flanking abyssal hills). An abyssal plain is an underwater plain on the deep ocean floor, usually found at depths between 3,000 metres (9,800 ft) and 6,000 metres (20,000 ft). 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. They are among the flattest, smoothest, and least explored regions on Earth. Abyssal plains are key geologic elements of oceanic basins (the other elements being an elevated mid-ocean ridge and flanking abyssal hills). 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), and as this basaltic 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 sediments, mainly clay and silt. Much of this sediment is deposited by turbidity currents that have been channelled from the continental margins along submarine canyons into deeper water. The rest is composed chiefly of pelagic sediments. Metallic nodules are common in some areas of the plains, with varying concentrations of metals, including manganese, iron, nickel, cobalt, and copper. 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 cycling, dissolution of calcium carbonate, and atmospheric CO2 concentrations over time scales of a hundred to a thousand years. The structure of abyssal ecosystems are strongly influenced by the rate of flux of food to the seafloor and the composition of the material that settles. Factors such as climate change, fishing practices, and ocean fertilization have a substantial effect on patterns of primary production in the euphotic zone. Abyssal plains were not recognized as distinct physiographic features of the sea floor until the late 1940s and, until very recently, none had been studied on a systematic basis. They are poorly preserved in the sedimentary record, because they tend to be consumed by the subduction process. The ocean can be conceptualized as being divided into various zones, depending on depth, and presence or absence of sunlight. Nearly all life forms in the ocean depend on the photosynthetic activities of phytoplankton and other marine plants to convert carbon dioxide into organic carbon, which is the basic building block of organic matter. Photosynthesis in turn requires energy from sunlight to drive the chemical reactions that produce organic carbon. The stratum of the water column nearest the surface of the ocean (sea level) is referred to as the photic 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 the euphotic zone (also referred to as the epipelagic zone, or surface zone). The lower portion of the photic zone, where the light intensity is insufficient for photosynthesis, is called the dysphotic zone (dysphotic means 'poorly lit' in Greek). The dysphotic zone is also referred to as the mesopelagic zone, or the twilight zone. Its lowermost boundary is at a thermocline of 12 °C (54 °F), which, in the tropics generally lies between 200 and 1000 metres. The euphotic zone is somewhat arbitrarily defined as extending from the surface to the depth where the light intensity is approximately 0.1–1% of surface sunlight irradiance, depending on season, latitude and degree of water turbidity. In the clearest ocean water, the euphotic zone may extend to a depth of about 150 metres, or rarely, up to 200 metres. Dissolved substances and solid particles absorb 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. The dysphotic zone, where light intensity is considerably less than 1% of surface irradiance, extends from the base of the euphotic zone to about 1000 metres. Extending from the bottom of the photic zone down to the seabed is the aphotic zone, a region of perpetual darkness. Since the average depth of the ocean is about 4300 metres, the photic zone represents only a tiny fraction of the ocean's total volume. However, due to its capacity for photosynthesis, the photic zone has the greatest biodiversity and biomass of all oceanic zones. Nearly all primary production in the ocean occurs here. Life forms which inhabit the aphotic zone are often capable of movement upwards through the water column into the photic zone for feeding. Otherwise, they must rely on material sinking from above, or find another source of energy and nutrition, such as occurs in chemosynthetic archaea found near hydrothermal vents and cold seeps. The aphotic zone can be subdivided into three different vertical regions, based on depth and temperature. First is the bathyal zone, extending from a depth of 1000 metres down to 3000 metres, with water temperature decreasing from 12 °C (54 °F) to 4 °C (39 °F) as depth increases. Next is the abyssal zone, extending from a depth of 3000 metres down to 6000 metres. The final zone includes the deep oceanic trenches, and is known as the hadal zone. This, the deepest oceanic zone, extends from a depth of 6000 metres down to approximately 11000 metres. Abyssal plains are typically in the abyssal zone, at depths from 3000 to 6000 metres.