DEFINITION OF OCEAN BASIN

The surface of the Earth is divided into two

fundamentally different types of crust, including relatively

light quartz and plagioclase-rich sial, forming the continental

regions, and relatively dense olivine and pyroxene-rich sima

underlying the ocean basins. The ocean basins may be

defined as submarine topographic depressions underlain by

oceanic (simatic) crust. Ocean basins are quite diverse in size,

shape, depth, characteristics of the underlying seafloor topography,

and types of sediments deposited on the oceanic crust.

The largest ocean basins include the Pacific, Atlantic, Indian,

and Arctic Oceans, and the Mediterranean Sea, whereas

dozens of smaller ocean basins are located around the globe.

The ocean basins depths were first extensively explored

by the H.M.S. Challenger in the 1800s, using depth reading

from a weight attached to a several-kilometer-long cable that

was dropped to the ocean floor. Results from these studies

suggested that the oceans were generally about 3–4 miles

(5–6 km) in depth. Later, with the development of echosounding

technologies and war-induced mapping efforts, the

variety of seafloor topography became appreciated. Giant

submarine mountain chains were recognized where the depth

rises to 1.7 miles (2.7 km), and these were later understood

to be oceanic ridges where new oceanic crust is created.

Deep-sea trenches with depths exceeding 5 miles (8 km) were

delineated and later recognized to be subduction zones where

oceanic crust is sinking back into the mantle. Other anomalous

regions of thick oceanic crust (and reduced depths) were

recognized, including large oceanic plateaux where excessive

volcanism produced thick crust over large regions, and smaller

seamounts (or guyots) where smaller, off-ridge volcanism

produced isolated submarine mountains. Some of these rose

above sea level, were eroded by waves, and grew thick reef

complexes as they subsided with the cooling of the oceanic

crust. Such guyots and coral atolls were made famous by

Charles Darwin, in his study of coral reefs of the Pacific

Ocean basin.

Pelagic sediments are deposited in the ocean basins, and

generally form a blanket of sediments draping over preexisting

topography. Carbonate rocks produced mainly by the

tests of foraminifera and nannofossils may be deposited on

the ocean ridges and guyots that are above the CCD (carbonate

compensation depth), above which the seawater is saturated

with CaCO3, and below which it dissolves in the water.

Below this, sediments comprise red clays and radiolarian and

diatomaceous ooze. Manganese nodules are scattered about

on some parts of the ocean floor.

The abyssal plains are relatively flat parts of the ocean

basins where the deep parts of the seafloor topography have

been filled in with sediments, forming flat plains. Some of

these abyssal plains are quite large, such as the 386,100-

square-mile (1 million-km2) Angolan abyssal plain in the

South Atlantic, and the 1,428,578-square-mile (3.7 millionkm2)

abyssal plain in the Antarctic Ocean basin. Other

abyssal plains are much smaller, such as the 1,003-squaremile

(2,600-km2) Alboran Sea in the Mediterranean. Abyssal

plains may also be characterized and distinguished on the

basis of their sediment composition, their geometry, depth,

and volume and thickness of the sediments they contain.

See also ABYSSAL PLAINS; CORALS; OCEANIC CRUST; PLATE

TECTONICS; REEF.

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