What Is An Abyssal Plain?

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Tubeworms and coral thousands of meters deep in the abyssal plain below the Gulf of Mexico and Atlantic Ocean off of the coast of Florida in the United States.

What is an Abyssal Plain?

An abyssal plain refers to a flat area in the seabed at an abyssal depth (10,000 to 20,000 feet). The larger abyssal plains may be thousands of kilometers in length and breadth, and are generally adjacent to a continent. Very little surface variation is observed in these abyssal plains, and they may vary in depth by only 10 to 100 centimeters per kilometer of horizontal distance.

Where are Abyssal Plains Found?

Approximately 40% of the Earth’s ocean floor is made up of abyssal plains, while the rest of the ocean bed topography is comprises by underwater hills, deep trenches, and mountain chains. The abyssal plains are found in most major ocean basins of our planet, but are more common in the Atlantic Ocean, less common in the Indian Ocean, and quite rare in the Pacific. The Sohm Plain in the North Atlantic Ocean, covering an area of 900,000 square kilometers, is one of the largest abyssal plains in the world.

Formation and Composition of Abyssal Plains

Accumulation of land-derived sediments over long periods of time on the ocean floor gives rise to the flat surfaces characteristic of an abyssal plain. The accumulation of these deposits, which average 1 kilometer in thickness, smooth out the irregular topography on the ocean bed. However, in some areas where sediments incompletely bury the underlying topography, outcrops of hill groups or volcanic hills might mar the regularity of these abyssal plains. Sediments of silt, sand, and gravel, as well as volcanic ash, wind-blown dust, and chemical precipitates from continental margins, enter the oceanic waters to form dense sediment-laden slurries which are deposited on the ocean floor by turbidity-driven currents. Layers of such sediments accumulate over elongated periods of time, creating these abyssal plains. Besides inorganic matter, organic matter, including the microscopic remains of innumerable organisms, also become deposited as layers in these abyssal plains.

Biology of Abyssal Plains

Though initially estimated to be deserted habitats, scientists have more recently discovered a high microbial biodiversity in the abyssal plains. The oceanographic expeditions conducted by the Census of Diversity of Abyssal Marine Life have discovered the existence of about 2,000 bacterial, 250 protozoan, and 500 invertebrate species in the abyssal plains of the earth's oceans. The Abyssobrotula galatheae and the Pseudoliparis amblystomopsis are the two deepest marine-living fish species recorded up to the present date. While some of the abyssal zone species, like the protozoan foraminiferans, are virtually universal in their distribution, others are endemic to some specific abyssal plains or trenches, such as certain isopod crustaceans. However, due to the mind-boggling depths of the abyssal plains, exploration of such habitats demands great deals of time, effort, and budgeting, which leads to the generation of only a low volume of data regarding the abyssal plain ecosystems, at least for now.

Significance of the Abyssal Plains

The abyssal plains of the world are areas of high ecological, commercial, and strategic importance. As the fish in the upper zones of the ocean rapidly become depleted due to overfishing, marine fisheries are expected to exploit ever deeper living sea fishes, which, due to their slow turnover and longer life cycles, stand chances of becoming threatened in even quicker time periods than those currently commercially fished closer to the sea surface. The abyssal plains might also be exploited in the future for legal or illegal purposes, including the dumping of hazardous wastes like radioactive waste, being used as a burial ground for disposed ships and oil rigs, and for the emplacement of sewage and dredge spoils from coastal areas. The abyssal plains might also be the future sites of mineral, oil, and gas extraction, making the deep-lying ocean bed susceptible to environmental degradation already being seen having catastrophic effects on land and in shallower waters.

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