A
guyot also known as a
tablemount, is an isolated underwater volcanic mountain (
seamount), with a flat top over 200 meters (660 feet) below the surface of the sea. The diameters of these flat summits can exceed 10 km (6 miles).
The guyot was named after the Swiss-American geographer and geologist
Arnold Henry Guyot (died 1884). The term was coined by
Harry Hammond Hess. Guyots are most commonly found in the
Pacific Ocean. Guyots show evidence of having been above the surface with gradual subsidence through stages from fringed
reefed mountain,
coral atoll, and finally a flat topped submerged mountain. Their flatness is due to erosion by waves, winds, and atmospheric processes.
|
Satellite Image of some of the atolls of the Maldives
by NASA. A total of 1322 islands arranged into
26 atolls make up the country |
The steepness gradient of most guyots is about 20 degrees. To technically be considered a guyot or tablemount, they must stand at least 3000 ft (900 m) tall. However, there are many undersea mounts that can range from just less than 300 ft to around 3000 ft. Very large oceanic volcanic constructions, hundreds of kilometers across, are called
oceanic plateaus. Seamounts are made by extrusion of lavas piped upward in stages from sources within the Earth's mantle to vents on the seafloor. Seamounts provide data on movements of tectonic plates on which they ride, and on the
rheology of the underlying
lithosphere. The trend of a seamount chain traces the direction of motion of the lithospheric plate over a more or less fixed heat source in the underlying
asthenosphere part of the Earth's mantle. There are thought to be an estimated 2,000 seamounts in the Pacific basin. The
Emperor Seamounts are an excellent example of an entire volcanic chain undergoing this process and contain many guyots among their other examples.
Another factor contributing to the guyots being underwater has to do with the oceanic ridges, such as the Mid-Atlantic Ridge in the Atlantic Ocean. Mid-ocean ridges gradually spread apart over time, due to molten lava being pushed up under the surface of the earth and creating new rock. As the mid-ocean ridges spread apart, the guyots move with them, thus continually sinking deeper into the depths of the ocean. Thus, the greater amount of time that passes, the deeper the guyots become. Although guyots can be hundreds of millions of years old, there have been some recently discovered guyots that were only formed within the last 1 million years, including
Bowie Seamount on the
coast of British Columbia,
Canada.
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Map of Bowie Seamount |
One guyot in particular, the
Great Meteor Tablemount in the Northeast Atlantic Ocean, stands at more than 4000 m (13,120 ft). The guyot's diameter is 110 km (70 mi).Guyots are also associated with specific lifeforms and varying amounts of organic matter. Local increases in chlorophyll a, enhanced carbon incorporation rates and changes in phytoplankton species composition were associated with the seamount.