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For the French viticulturist, see Jules Guyot. For the Old French name, see Guiot.
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A guyot /giɘʊ/, also known as a tablemount, is a flat-topped seamount. It was named after the Swiss-American geographer and geologist Arnold Henry Guyot (died 1846). The term was coined around 1922 by Harry Hammond Hess. Guyots are most commonly found in the Pacific Oceanmarker. 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. The surfaces of some of the largest guyots measure 10 km (6 mi). 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 Seamountsmarker are an excellent example of an entire volcanic chain undergoing this process and contain many guyots among their other examples.

We know that underwater guyots were originally above water due to the extensive amount of fossils discovered on the top of guyots. 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 Seamountmarker on the coast of British Columbia, Canadamarker.

One guyot in particular, the Great Meteor Tablemountmarker 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.

Guyots were first seen by Harry Hammond Hess in 1965 who collected data using echo-sounding equipment on a ship he commanded during World War II. The data showed the configuration of the seafloor where he saw that some undersea mountains had flat tops. The geology building at Princeton Universitymarker is, and was, called Guyot Hall (being named after the 18th century geographer Arnold Henry Guyot. Guyot Hall has a flat roof, so Hess called these undersea mountains guyots (because they resembled Guyot Hall). Hess postulated they were once volcanic islands that were beheaded by wave action yet they are now deep under sea level. This idea was used to help bolster the theory of plate tectonics.

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