DEFINITION OF OCEANIC PLATEAU

Regions of anomalously thick oceanic

crust and topographically high seafloor are known as oceanic

plateaus. Many have oceanic crust that is 12.5–25 miles

(20–40 km) thick and rise thousands of meters above surrounding

oceanic crust of normal thickness. The Caribbean

ocean floor represents one of the best examples of an oceanic

plateau, with other major examples including the Ontong-

Java Plateau, Manihiki Plateau, Hess Rise, Shatsky Rise, and

Mid-Pacific Mountains. All of these oceanic plateaus contain

thick piles of volcanic and subvolcanic rocks representing

huge outpourings of lava, most erupted in a few million

years. They typically do not show the magnetic stripes that

characterize normal oceanic crust produced at oceanic ridges

and are thought to have formed when mantle plume heads

reached the base of the lithosphere, releasing huge amounts

of magma. Some oceanic plateaus have such large volumes of

magma that the total magmatic flux in the plateaus would

have been similar to or larger than all of the magma erupted

at the mid-ocean ridges during the same interval.

The Caribbean seafloor preserves 5–7.5-mile (8–21-km)

thick oceanic crust formed before about 85 million years ago

in the eastern Pacific Ocean. This unusually thick ocean

floor was transported eastward by plate tectonics, where

pieces of the seafloor collided with South America as it

passed into the Atlantic Ocean. Pieces of the Caribbean

oceanic crust are now preserved in Colombia, Ecuador,

Panama, Hispaniola, and Cuba, and some scientists estimate

that the Caribbean oceanic plateau may have once been

twice its present size. In either case, it represents a vast outpouring

of lava that would have been associated with significant

outgassing with possible consequences for global

climate and evolution.

The western Pacific ocean basin contains several large

oceanic plateaus, including the 20-mile (32-km) thick crust of

the Alaskan-sized Ontong-Java Plateau, which is the largest

outpouring of volcanic rocks on the planet. It apparently

formed in two intervals, at 122 million and 90 million years

ago, entirely within the ocean, and represents magma that

rose in a plume from deep in the mantle and erupted on the

seafloor. It is estimated that the volume of magma erupted in

the first event was equivalent to that of all the magma being

erupted at mid-ocean ridges at the present time. Sea levels rose

by more than 10 meters in response to this volcanic outpouroceanic

ing. The gases released during these eruptions are estimated to

have raised average global temperatures by 23°F (13°C).

See also FLOOD BASALT.

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