A guyot is an isolated, flat-topped seamount, which is an underwater volcanic mountain rising abruptly from the deep ocean floor. Unlike typical conical seamounts, its summit has been truncated, or leveled, by erosional forces. Guyots, also known as tablemounts, are completely submerged, often by thousands of meters. American geologist Harry Hess coined the term in 1945 after recognizing these features while mapping the ocean floor during World War II. He named them in honor of the Swiss-American geographer Arnold Henry Guyot.
Defining Physical Characteristics
The most recognizable physical attribute of a guyot is its broad, flat summit, contrasting sharply with the pointed peaks of most underwater volcanoes. These flat tops often measure over 10 kilometers in diameter, sometimes reaching over 100 kilometers. To be formally classified as a guyot, the flat summit must be submerged more than 200 meters below the sea surface.
The flanks are generally steep, rising abruptly from the abyssal plain with slopes approaching 20 degrees near the summit. Total height, measured from the surrounding seafloor, typically ranges from 2 to 5 kilometers. In the Pacific Ocean, where they are most numerous, the summit usually rests between 1,000 and 2,000 meters below sea level.
The Geological Process of Formation
The creation of a guyot is a multi-stage process that begins with intense volcanic activity, often triggered by a mantle hotspot. Basaltic magma erupts onto the seafloor, gradually building a massive shield volcano that rises from the deep ocean floor. If this volcanic activity is prolonged, the seamount eventually breaches the ocean surface, becoming a volcanic island.
The second phase is marine abrasion, which levels the island’s peak. Once exposed near sea level, powerful wave action and weathering erode the summit. This erosion creates the characteristic flat platform, or wave-cut bench. In tropical climates, this bench may be capped by shallow-water limestones and coral reefs.
The final stage is tectonic subsidence, where the flat-topped structure sinks deep beneath the waves. As the oceanic lithosphere moves away from the spreading center or hotspot, it cools and contracts. This cooling increases the density of the underlying crust, causing it to sink via isostatic adjustment.
This carries the eroded summit down to its modern depths, typically between 1,000 and 2,000 meters. The rate of subsidence can range from 20 to 60 meters per million years. Fossilized shallow-water corals and rounded volcanic cobbles found on the deep summit provide physical evidence of this history of emergence, erosion, and submergence.
Global Distribution and Scientific Importance
Guyots are present in nearly all the world’s oceans, though most are concentrated in the Pacific Ocean. The North Pacific contains significant numbers, including the Hawaiian-Emperor seamount chain, which traces the movement of the Pacific plate over a fixed mantle plume. Approximately 283 guyots have been identified globally.
These submerged mountains hold scientific value as paleoceanographic markers. Representing ancient wave-cut platforms, their flat summits act as a “fossil shoreline,” preserving a record of past sea levels and crustal subsidence rates. Dating the volcanic rock and fossilized reef material allows scientists to track the speed and direction of tectonic plate movement.
Guyots also function as deep-sea biodiversity hotspots. Their elevated structure disrupts deep-ocean currents, causing nutrient-rich water to well up. This supports dense communities of marine life, including endemic species of fish, corals, and invertebrates.