Introduction
A guyot (pronounced /ˈɡiː.oʊ, ɡiːˈoʊ/; also called a tablemount) is a volcanic submarine landform defined by an isolated seamount whose summit is distinctly flattened and lies submerged more than 200 m (≈660 ft) below mean sea level. These flat tops can form extensive, relatively level platforms with diameters that may exceed 10 km (≈6 mi), reflecting substantial, planar summit areas.
Guyots occur as discrete volcanic edifices rather than as elements of continuous ridge systems and are distributed throughout the world’s ocean basins—reported in every ocean except the Arctic, with the greatest concentration in the Pacific. Examples such as the Bear Seamount in the northern Atlantic illustrate their presence outside the Pacific basin. Morphologically, guyots are analogous to terrestrial flat-topped features (e.g., mesas or tablelands), the principal distinction being their permanently submerged setting.
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History
The term “guyot” was introduced in 1945 by geologist Harry Hammond Hess, who, while commanding a World War II vessel, used echo‑sounding equipment to collect bathymetric profiles and identified a class of undersea mountains with unusually flat summits; he named them after the Department of Geosciences building at Princeton. These features are seamounts whose planar tops contrast with the pointed cones of typical volcanic edifices and whose present deep submergence belies morphological evidence of former emergence above sea level. Hess proposed that guyots began as volcanic islands whose summits were planed off by wave action at sea level and were later carried downward, accounting for flat tops now found well below the ocean surface. The discovery exemplified how mid‑20th‑century naval sonar enabled systematic mapping of submarine topography and the recognition of large‑scale seafloor forms, and it supplied concrete evidence of vertical and horizontal crustal change—supporting concepts of seafloor evolution and the developing theory of plate tectonics.
Formation
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Guyots develop through a predictable sequence of volcanic construction, surface modification, and long-term subsidence. Seamounts—often produced by intermittent eruptions fed from mantle-derived magma at relatively stationary intraplate heat sources (hotspots)—build volcanic edifices on oceanic lithosphere. When such a seamount grows high enough to approach or breach sea level, wave abrasion and carbonate production by reef organisms act together to truncate and plane the summit, producing a broad, flattened top. If the volcanic relief stabilizes near sea level, coral growth can encircle the volcano and form fringing reefs that may evolve into a closed ring atoll as the volcanic cone becomes more subdued.
Once volcanism wanes, the subsequent history of the edifice is dominated by thermal, isostatic and biological processes. As the lithospheric plate bearing the volcano moves away from its magmatic source, the cooling lithosphere increases in density and undergoes isostatic subsidence; this gradual sinking lowers the entire volcanic complex. Mechanical erosion by waves and currents is most effective in the shallow, near-surface zone, so the characteristic flat summit produced prior to subsidence is commonly preserved beneath the reach of vigorous surface erosion once the feature has sunk. Where corals can match moderate rates of subsidence, an atoll may persist; with continued sinking that outpaces reef growth, the carbonate rim falls below the photic zone and the structure becomes a submerged, flat-topped mountain—a guyot. The depth of a guyot thus records the duration and magnitude of post‑volcanic subsidence.
Chains of seamounts and guyots record the interaction between mantle heat sources and plate motion. Linear trends of seamount chains mark the direction in which the lithospheric plate moved over a stationary hotspot, and variations in spacing, size and morphology furnish empirical constraints on lithospheric rheology and the episodic nature of volcanism. The Pacific Ocean, with tens of thousands of seamounts, provides abundant examples of these processes; the Hawaiian–Emperor chain in particular documents the full progression from active volcanic island through reef development and atoll formation to eventual subsidence into guyots. On a broader scale, the same thermal evolution of the lithosphere that drives island subsidence also explains large-scale seafloor topography: relatively shallow, high ridges where lithosphere is young and hot, and deeper abyssal plains and trenches where lithosphere is older, cooler and denser.
Characteristics of Guyots
Guyots, or tablemounts, are flat-topped submarine volcanic edifices defined by a pronounced slope (commonly around 20°) and, for formal classification, a minimum vertical relief of approximately 900 m (3,000 ft). While many volcanic rises on the seafloor fail to reach this threshold—ranging from a few hundred metres down to under 100 m—guyots can achieve very large sizes; for example, the Great Meteor Tablemount in the northeast Atlantic exceeds 4,000 m (13,000 ft) in relief and spans roughly 110 km in diameter, illustrating the upper limits of their morphological expression. Even larger, more extensive volcanic provinces that extend for hundreds of kilometres are categorized separately as oceanic plateaus.
Size statistics reinforce a clear distinction between flat-topped guyots and steeper seamounts: the average planform area of known guyots is about 3,313 km2 (1,279 sq mi), substantially greater than the mean seamount area of roughly 790 km2 (310 sq mi), indicating different growth histories and geomorphic evolution. Globally, 283 guyots have been identified, with the greatest concentration in the North Pacific (119) and substantial numbers in the South Pacific (77) and South Atlantic (43); fewer occur in the Indian (28) and North Atlantic (8) Oceans, the Southern Ocean (6), and the Mediterranean (2). Guyots are generally absent from the Arctic proper, although isolated reports note at least one feature near the Fram Strait.
Ecologically and biogeochemically, guyots function as localized productivity and carbon-cycling hotspots. They are associated with elevated surface chlorophyll a, increased rates of carbon incorporation, shifts in phytoplankton community composition, and distinctive benthic and pelagic assemblages, along with spatially variable organic-matter accumulation. These combined effects make guyots important nodes for biological aggregation and enhanced carbon fluxes within the open-ocean environment.