A guyot is a flat-topped underwater mountain that rises from the ocean floor, formed by volcanic activity that once pushed it above the sea surface. What sets it apart from an ordinary seamount is that distinctive flat summit, sitting more than 200 meters (660 feet) below sea level. These are essentially drowned islands, volcanoes that were worn down to sea level and then slowly sank beneath the waves.
How Guyots Differ From Seamounts
Seamounts and guyots are both undersea volcanic mountains, and the terms sometimes get used interchangeably. But there’s an important geological distinction. A seamount formed underwater and never broke the ocean’s surface. A guyot, by contrast, was once an island. It rose above the waterline, got eroded flat by waves and landslides, and then submerged again as the seafloor beneath it slowly sank. That flat top is the signature feature. If you could drain the ocean, a guyot would look like a mesa or a table mountain, while a typical seamount would still have a peaked or rounded summit.
How a Guyot Forms
The life cycle of a guyot starts the same way any volcanic island begins: magma pushes up through the ocean floor, building a mountain that eventually breaks the surface. For a while, it exists as a volcanic island, potentially with steep slopes and active eruptions. But once volcanic activity stops, destruction takes over.
The main force that flattens these islands is mass wasting, essentially large-scale landsliding triggered partly by earthquake activity along moving tectonic plates. Research comparing the Hawaiian and Canary Island chains found that this process, not wave erosion, is primarily responsible for leveling the volcanic slopes. Waves do play a role, but mainly as a finishing touch, polishing the surface smooth after landslides have already done the heavy work. The full process of reducing a volcanic peak to a flat guyot surface takes between one and four million years.
Once the island has been worn flat to sea level, the tectonic plate it sits on keeps moving. As it drifts away from the mid-ocean ridge where the plate originally formed (which sits higher due to heat), the crust cools, contracts, and sinks. The flattened island gradually drops below the waterline and keeps sinking. In the Pacific, where guyots are most common, their summits typically sit 1,000 to 2,000 meters (3,300 to 6,600 feet) below the surface today.
Where Guyots Are Found
There are a few hundred known guyots worldwide, and roughly half of them are in the western and central Pacific Ocean. This concentration makes sense: the Pacific plate is the largest and oldest oceanic plate, and it has been moving steadily northwest for well over 100 million years, carrying a long procession of volcanic islands away from their birthplaces and into deeper water.
Notable guyot chains in the North Pacific include those associated with the Emperor Seamounts (the older, submerged extension of the Hawaiian chain), the Marcus-Wake Seamounts, the Dutton Ridge, and chains near the Gulf of Alaska. The oldest guyots sit on the oldest seafloor in the western Pacific, where the ocean crust dates back roughly 150 million years. The youngest are found in the Gulf of Alaska, where the crust is newer.
The Hawaiian Islands themselves offer a living demonstration of the process. The Big Island of Hawaii is still volcanically active. Moving northwest along the chain, each island is progressively older, more eroded, and lower. Eventually the chain disappears beneath the surface entirely, becoming the Emperor Seamount chain, a string of guyots stretching toward the Aleutian Trench.
Why Guyots Matter to Science
Guyots played a surprisingly important role in one of the biggest scientific breakthroughs of the 20th century: the theory of plate tectonics. During World War II, a Princeton geologist named Harry Hess was serving as a Navy captain aboard a transport ship. He kept the ship’s depth-sounding equipment running continuously between battles, mapping the ocean floor as he went. What he found were hundreds of these oddly flat-topped underwater mountains, which he named “guyots” after Arnold Guyot, the Swiss-born founder of Princeton’s geology department.
The existence of guyots posed a puzzle. If these mountains had clearly been eroded flat at sea level, how did they end up thousands of feet underwater? In 1962, Hess proposed his theory of seafloor spreading, which offered an elegant answer. New ocean crust forms at mid-ocean ridges and slowly moves outward, cooling and sinking as it goes. Guyots were volcanoes that had risen above the surface, been worn flat, and then been carried into deeper water by the spreading seafloor. This theory also explained why ocean sediments contain no fossils older than 180 million years (older crust gets recycled back into the Earth at subduction zones) and why so little sediment has accumulated on the ocean floor despite billions of years of opportunity. Hess’s work became one of the foundational pillars of plate tectonic theory.
Life on Guyot Summits
Even submerged hundreds or thousands of meters below the surface, guyots are biological hotspots compared to the surrounding deep ocean floor. Their flat summits and steep flanks create changes in ocean currents, pushing nutrient-rich deep water upward. This upwelling supports denser concentrations of plankton, which in turn attract fish, deep-sea corals, sponges, and other marine life. For species living in the deep ocean, a guyot summit is essentially an oasis, a shallow platform surrounded by vast stretches of much deeper water. This makes guyots significant not just as geological curiosities but as important features of deep-sea ecosystems, particularly in the vast, otherwise sparsely populated mid-Pacific.