abyssal plains

Flat, generally featureless plains that form
large areas on the seafloor. In the Atlantic Ocean, abyssal
plains form large regions on either side of the Mid-Atlantic
Ridge, covering the regions from about 435–620 miles
(700–1,000 km), and they are broken occasionally by hills
and volcanic islands such as the Bermuda platform, Cape
Verde Islands, and the Azores. The deep abyssal areas in the
Pacific Ocean are characterized by the presence of more
abundant hills or seamounts, which rise up to 0.6 miles (1
km) above the seafloor. Therefore, the deep abyssal region of
the Pacific is generally referred to as the abyssal hills instead
of the abyssal plains. Approximately 80–85 percent of the
Pacific Ocean floor lies close to areas with hills and
seamounts, making the abyssal hills the most common landform
on the surface of the Earth.
Many of the sediments on the deep seafloor (the abyssal
plain) are derived from erosion of the continents and are carried
to the deep sea by turbidity currents, wind (e.g., volcanic
ash), or released from floating ice. Other sediments, known
as deep-sea oozes, include pelagic sediments derived from
marine organic activity. When small organisms die, such as
diatoms in the ocean, their shells sink to the bottom and over
time can create significant accumulations. Calcareous ooze
occurs at low to middle latitudes where warm water favors
the growth of carbonate-secreting organisms. Calcareous
oozes are not found in water that is more than 2.5–3 miles
(4–5 km) deep, because this water is under such high pressure
that it contains dissolved CO2 that dissolves carbonate shells.
Siliceous ooze is produced by organisms that use silicon to
make their shell structure.
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