DEFINITION OF abyssal plains Flat, generally featureless plains that formlarge areas on the seafloor. In the Atlantic Ocean, abyssalplains form large regions on either side of the Mid-AtlanticRidge, covering the regions from about 435–620 miles(700–1,000 km), and they are broken occasionally by hillsand volcanic islands such as the Bermuda platform, CapeVerde Islands, and the Azores. The deep abyssal areas in thePacific Ocean are characterized by the presence of moreabundant hills or seamounts, which rise up to 0.6 miles (1km) above the seafloor. Therefore, the deep abyssal region ofthe Pacific is generally referred to as the abyssal hills insteadof the abyssal plains. Approximately 80–85 percent of thePacific Ocean floor lies close to areas with hills andseamounts, making the abyssal hills the most common landformon the surface of the Earth.Many of the sediments on the deep seafloor (the abyssalplain) are derived from erosion of the continents and are carriedto the deep sea by turbidity currents, wind (e.g., volcanicash), or released from floating ice. Other sediments, knownas deep-sea oozes, include pelagic sediments derived frommarine organic activity. When small organisms die, such asdiatoms in the ocean, their shells sink to the bottom and overtime can create significant accumulations. Calcareous oozeoccurs at low to middle latitudes where warm water favorsthe growth of carbonate-secreting organisms. Calcareousoozes are not found in water that is more than 2.5–3 miles(4–5 km) deep, because this water is under such high pressurethat it contains dissolved CO2 that dissolves carbonate shells.Siliceous ooze is produced by organisms that use silicon tomake their shell structure.
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