DEFINITION OF SEAMOUNT

Mountains that rise above the seafloor, typically

to a height of 3,000 feet (1,000 meters) or more, are called

seamounts. Many parts of the abyssal ocean floor, particularly

the western Pacific, are covered by seamounts with different

shapes and relationships to each other. Small flat-topped

seamounts are called guyots and represent intraplate oceanic

volcanoes that grew toward or above the ocean surface and

were eroded to have a flat top as they subsided below sea

level. Many formed near the oceanic ridge system as off-axis

volcanoes, and as the oceanic crust cools and subsides as it

ages and moves away from the mid-ocean ridges, the

seamounts subside below the surface as well. Once below the

surface, waves erode the upper surface making it flat. If the

seamount is in tropical or subtropical climates corals may

form fringing reefs around the volcanic island. As the island

subsides, the reefs keep growing and stay near the surface,

forming coral atoll islands.

Other seamounts are much larger and represent voluminous

outpourings of mafic lava from hot spots. For example,

the Hawaiian-Emperor seamount chain extends for thousands

of kilometers across the Pacific Ocean, tracing the

movement of the Pacific plate with respect to a fairly stationary

hot spot in the underlying mantle. As hot magma

upwelled in this hot spot or mantle plume, it acted as a blowtorch

against the bottom of the Pacific plate, erupting mafic

volcanics as the plate moved overhead. Thus, the youngest

volcanic islands are above the surface on the islands of

Hawaii and Maui and become older progressively to the

northwest. The older islands have cooled and sunk below sea

level in northwestern parts of the Hawaiian-Emperor chain.

More voluminous outpourings of lava than on seamount

chains are found on thick oceanic plateaus.

There are several other types of seamounts that are less

common than the intraplate volcanics. For instance, small

submarine intermediate and felsic volcanoes form near some

island arcs, creating seamounts along convergent boundaries.

Transform faults and aseismic ridges have topographic highs

associated with these structures, and some rifted margins

have extensional horsts that form seamounts along young

passive margins.

See also ATOLL; HOT SPOT; OCEANIC PLATEAU;

OCEANOGRAPHY.

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