Seamounts and Volcanoes in the Ocean

Home | Category: Physical Oceanography


Undersea volcanic eruption

There are a number of active volcanoes under the sea, with deepest yet discovered about five kilometers below the surface in the Caymen Trench in Caribbean. The Caymen Trench is the world’s deepest undersea volcanic rift. It is still largely unexplored and was the setting for the James Cameron film “The Abyss”. About 100 million years ago, oxygen disappeared from much of the world’s oceans, wiping out a third of all ocean life. Many scientists think that sulphur produced by volcanoes was the culprit. They think that the amount of sulfur in the water changed the amount of phosphorous, which is vital for phytoplankton growth and this triggered an anoxic event depriving the oceans of oxygen.

Volcanic eruptions occur only in certain places and do not occur randomly. This is because the Earth’s crust is broken into a series of slabs known as tectonic plates. These plates are rigid, but they “float” on a hotter, softer layer in the Earth's interior. As the plates move, they spread apart, collide, or slide past each other. Sixty percent of all active volcanoes occur at the boundaries between tectonic plates. Most volcanoes are found along a belt, called the “Ring of Fire” that encircles the Pacific Ocean. Some volcanoes, like those that form the Hawaiian Islands, occur in the interior of plates at areas called “hot spots.” [Source: NOAA]

Although most of the active volcanoes we see on land occur where plates collide, the greatest number of the Earth's volcanoes are hidden from view, occurring on the ocean floor along spreading ridges.

Websites and Resources: National Oceanic and Atmospheric Administration (NOAA); “Introduction to Physical Oceanography” by Robert Stewart , Texas A&M University, 2008 ; Woods Hole Oceanographic Institute ; Cousteau Society ; Monterey Bay Aquarium

World's Biggest Recorded Underwater Eruption Gave Birth to 820-Meter-High Volcano

Hunga Tonga eruption in January 2022

In 2018, the largest active underwater eruption ever recorded gave birth to a skyscraper-size underwater volcano. According to Live Science: Scientists discovered the 2,690-foot-tall (820 meters) volcano in the western Indian Ocean, off Madagascar, following a puzzling spate of earthquakes that struck near what is normally a seismically quiet area. After collecting geological data, including information from a 2019 underwater survey of the region, the team realized that there was a new submarine volcano about 1.5 times the height of New York's One World Trade Center. What's more, this new "baby" draws from the deepest volcanic magma reservoir known to scientists. "The source of the magma, the reservoir, is very deep" — about 34 miles (55 kilometers) underground, study lead researcher Nathalie Feuillet, a marine geoscientist at the Paris Institute of Earth Physics (IPGP) - University of Paris, told Live Science. "This is the first time in volcanology that we can see such a deep reservoir at the base of the lithosphere," the outer shell of Earth that includes the upper mantle and crust. [Source: Laura Geggel, Live Science, October 21, 2021

Between May 2018 and May 2021, more than 11,000 detectable earthquakes shook Mayotte, a small island and French territory between Madagascar and Mozambique. The most powerful earthquake was a magnitude 5.9, but there were also strange seismic hums, or very-low-frequency earthquakes, that originated deep underground; they couldn't be felt at the surface but were picked up by seismometers around the world. These very-low-frequency earthquakes are associated with volcanic activity. This sudden seismic activity was surprising, given that only two earthquakes have been detected near Mayotte since 1972 and, until now, the most recent volcanic activity — a layer of pumice in a lagoon near the island — was left at least 4,000 years ago, the researchers wrote in the study.

In July 2018, scientists realized that Mayotte was moving eastward at about 7.8 inches (20 centimeters) a year, according to GPS data. At that point in time, the island had only three or four GPS stations, so scientists installed global navigation satellite systems and ocean-bottom seismometers around the island to learn more about the geological changes occurring there. The findings were extraordinary: The combined land and ocean-bottom seismometers picked up 17,000 events between February and May 2019, the researchers found.

In May 2019, Feuillet and her colleagues had the opportunity to go on a voyage aboard the research vessel Marion Dufresne. The team knew there had been a magmatic event east of Mayotte, but they weren't sure if the magma had stayed deep under the crust or if it had erupted onto the seafloor. "We expected to see something, but it was not certain," Feuillet said. In a 2019 post, she wrote, "On board, we put in place a protocol to analyse the seismic signals recorded by the OBS [ocean-bottom seismometers]. The teams operated around the clock, broken down into shifts, and we were able to precisely locate, in less than 2 weeks, the nearly 800 largest earthquakes (of magnitudes between 3.5 and 4.9)."

Hunga Tonga eruption in January 2022

Their efforts paid off: "We discovered that these earthquakes were, for the most part, located in an area quite close to the island (10 km [6 miles] from the east coast of the island) but were deep (between 20 and 50 km [12 to 31 miles] deep)," Feuillet wrote. Then, the vessel's multibeam echo sounder, which sends out sound waves to map the seafloor and water column, found something "very big" about 31 miles east of Mayotte, Feuillet said. It was an underwater volcano with a pyramid-shaped edifice measuring about 1.2 cubic miles (5 cubic km). This volcano was completely new; it wasn't there in 2014, according to the previous survey by France's Naval Hydrographic and Oceanographic Service.

According to the 2014 survey, that area was "almost flat at around 3,300 m [10,827 feet] below sea level," the researchers wrote in the study. As of May 2019, the newly minted volcano's summit rose to 8,465 feet (2,580 m) below sea level. The volume of material that this volcano spawned is 30 to 1,000 times larger than other documented deep-sea eruptions. It's more than three times larger than the 2012 Havre eruption in New Zealand and 2.5 times larger than the 2014 Bardarbunga eruption in Iceland, which was Iceland's largest eruption in the past 200 years.

It appears that plate tectonic movement led lava in the asthenosphere, the upper layer of the mantle directly below the rigid lithosphere, to move upward. This magma flowed upward in geologic dikes, which could explain the earthquakes and the subsequent massive eruption. What's more, this eruption doesn't appear to be the first one near Mayotte. "Large lava flows and cones on the upper slope and onshore Mayotte indicate that this has occurred in the past," the researchers wrote in the study. The team is monitoring the region for more earthquakes and volcanic activity. "It's still erupting," Feuillet said. "The last evidence for lava at the seafloor was in January 2021." The study was published online Aug. 26 in the journal Nature Geoscience.

Undersea Methane-Spewing Mud Volcano Created During Ice Age Explosion

Ocean explorers in the Arctic discovered an underwater volcano spewing mud and methane from inside another, larger crater that probably formed after a catastrophic blowout at the end of the last ice age. Live Science reported: Researchers spotted the unusual feature about 80 miles (130 kilometers) south of Norway's Bear Island, or Bjørnøya, in the Barents Sea. The volcano, which the team named the Borealis Mud Volcano, is only the second of its kind discovered in Norwegian waters. "Exploring the seabed and discovering new methane [seeps] is like finding hidden treasures," said Stefan Buenz, a professor at The Arctic University of Norway (University of Tromsø) and co-leader of the Advancing Knowledge of Methane in the Arctic (AKMA) expedition that made the discovery. "Every time we go down to the seabed, we get the feeling that we have just begun to understand the great and incredible diversity of such [seep] systems," Buenz said [Source: Sascha Pare, Live Science, May 16, 2023]

mud volcano in Norway

A submarine mud volcano is a geological structure formed by an expulsion of muddy fluid and gas, predominantly methane. The Borealis Mud Volcano measures roughly 23 feet (7 meters) in diameter and is about 8 feet (2.5 m) tall. On May 7, the scientists used a remote-controlled rover to capture footage of the small mount continuously emitting a muddy fluid, which the researchers say is rich in methane. Methane is a powerful greenhouse gas once it reaches the atmosphere and contributes to climate change.

The volcano sits in the middle of another, much larger crater, which is 984 feet (300 m) wide and 82 feet (25 m) deep. The exceptional formation sits 1,312 feet (400 m) below the sea surface and likely resulted from a sudden and massive methane eruption after the last glacial period, 18,000 years ago, according to the statement. "Seeing an underwater eruption in real time reminds me how 'alive' our planet is," Giuliana Panieri, a professor in geology at The Arctic University of Norway and the leader of the expedition, said in the statement.

The researchers found the volcano's flanks teeming with animal life feeding off carbonate crusts — mineral crusts formed when microorganisms consume methane and produce bicarbonate as a byproduct, according to a 2019 study in the International Journal of Environmental Research and Public Health — that formed thousands of years ago. They spied sea anemones, sponges, corals, starfish, sea spiders and diverse crustaceans.

The only other known mud volcano in Norwegian waters is the Håkon Mosby volcano. This 0.6-mile-wide (1 km) feature was discovered 4,100 feet (1,250 m) below the water’s surface on the seabed south of Svalbard in 1995, according to the University of Bergen's Center for Geobiology. Underwater mud volcanoes are difficult to spot and map, but researchers estimate there could be hundreds or thousands of them on the seafloor globally, according to a 2021 chapter in Lecture Notes in Earth System Sciences.

These volcanoes provide a rare window into geological processes occurring deep below Earth's crust, since they spout mainly water, minerals and fine sediment from these depths. They also offer clues about previous environments and conditions on Earth, and could give an insight into systems on other planets, Panieri said. The AKMA expedition is a three-part mission investigating methane activity in Arctic waters. Scientists are now on the lookout for similar formations in the Arctic. "We do not rule out the possibility of discovering other mud volcanoes in the Barents Sea," Panieri said.


Bear Seamount

Seamounts are isolated or comparatively isolated elevations rising 1000 meters or more from the sea floor and with small summit areas. Mainly volcanic in origin, they compare in size to land-based mountains and are probably more numerous. They take many forms such as flat-topped guyots, pinnacles and rolling hills that can stretch for 80 kilometers or more. About half of them are in the Pacific Ocean and the remainder are in the Atlantic and Indian Oceans. New estimates suggest that, taken together, seamounts encompass about 28.8 million square kilometers of the Earth's surface. That's larger than deserts, tundra, or any other single land-based global habitat on the planet. [Source: National Geographic, NOAA]

Seamounts can be taller than 3,050 meters (10,000 feet). They can be isolated or part of large mountain chains. The New England Seamount contains more than 30 peaks that stretch 1,600 kilometers (994 miles) from the coast of New England. Many seamounts form at volcanic “hotspots” — near the tectonic plane boundaries. Typically, they are cone shaped, but often have other prominent features such as craters, linear ridges and crevices. There is a broad size distribution for seamounts but to be classified as a seamount, the feature must have a vertical relief of at least 1,000 meters (3,300 feet) above the surrounding seafloor. [Source: NOAA]

According to National Geographic: Seamounts generally form when volcanic mountains rise up from the seafloor but fail to reach the surface (those that break the surface become islands). Scientists estimate that there are some 100,000 seamounts at least one kilometer (3,281 feet) high. But if you include others that range from small hills to rolling mountains, there may be as many as a million of them. We’ve seen little of these oases of life in the deep. Of all Earth’s seamounts, marine biologists have studied only a few hundred. More finely detailed maps of the surface of Mars may exist than of the remotest parts of the ocean floor. [Source: Gregory S. Stone, National Geographic, September 2012]

Seamounts are home to many commercial fish and are therefore very beneficial to our economy. Seamounts are also important to the field of medicine, as any number of undiscovered species may lead to new drugs or medical treatments. But, worldwide, seamounts are threatened. More and more, deep-sea fishing trawlers drag nets weighted with heavy chains across seamounts to catch schools of fish that congregate around them. In the process the nets destroy long-lived and slow-growing corals, sponges, and other invertebrates. Once these underwater communities are disrupted, it can take hundreds, even thousands, of years for them to reestablish themselves.

Satellite-Based High-Resolution Radar Reveals 27,000 Seamounts

Data from high-definition radar satellites published in April 2023 in the journal Earth and Space Science revealed more than 19,000 seamounts. Live Science reported: Prior to this, only one-quarter of Earth's seafloor had been mapped using sonar, which uses sound waves to detect objects hidden underwater. A 2011 sonar census found more than 24,000 seamounts, or undersea mountains formed by volcanic activity. However, there are more than 27,000 seamounts that remain uncharted by sonar, according to the Science article. “It’s just mind boggling,” David Sandwell, a marine geophysicist at the Scripps Institution of Oceanography who worked on the survey, told Science magazine. [Source: Jennifer Nalewicki, Live Science, April 28, 2023]

Okeanos seamount

“However, the new study shows that scientists don’t need to rely on sonar surveys to investigate what’s going on under the ocean. Radar satellites not only measure an ocean's height but can also see what's lurking in the water's inky depths, offering a better representation of the topography of the seafloor. Scientists pulled data from several satellites, including the European Space Agency's CryoSat-2, and found that they could detect underwater mounds as small as 3,609 feet (1,100 meters) tall, which is the lower limit of what constitutes a seamount, according to the Science article.

With this technology, scientists predict they can estimate the heights of small undersea volcanoes to an accuracy of approximately 370 meters (1,214 feet), according to the study. So far, researchers have mapped a collection of seamounts in the northeast Atlantic Ocean that could help explain the evolution of a mantle plume that feeds more than 100 volcanoes in Iceland. These updated maps will also provide a better understanding of ocean currents and "upwellings," which occur when water from the bottom of the ocean churns upward to the surface — a phenomenon that scientists think could be "concentrated at seamounts and ridges," according to the Science article. "There's a zoo of interesting things that happen when you have topography," Brian Arbic, a physical oceanographer at the University of Michigan in Ann Arbor who wasn't involved with the study, told Science.

Seamount Biodiversity

Once thought to be little more than hazards to submarine navigation, they are regarded by scientists today as biological hotspots that support a dazzling array of marine life. The biological richness of seamount habitats results from the shape of these undersea mountains. Thanks to the steep slopes of seamounts, nutrients are carried upwards from the depths of the oceans toward the sunlit surface, providing food for creatures ranging from corals to fish to crustaceans. [Source: National Geographic, NOAA]

Seamounts often have a high level of biological productivity because they provide habitats for many species of plants and animals. Over 200 species of sea creatures have been observed at a single guyot in the New England Seamount. Seamounts are great locations to discover new species because each seamount houses different types of animals, including many that can only be found in guyot habitats.

When currents collide with a seamount at their base they push cold nutrient-rich waters upwards towards the surface, providing food for life that live at the tops of the mounts. Currents above a seamount can form an eddy, trapping organisms that normally migrate up and down. These trapped species provide food for predators near the top. Currents that travel up one side of a seamount can form rolls and eddies as they move down the opposite side.

Many types of corals live on the sides of the seamounts at different depths. The plentiful sea life also includes sponges, crustaceans, gastropods and fish. New species are being discovered all the time. Among the creatures most commonly seen are brittle stars, sea urchins, lobster, batfish, shrimps, crabs, octopuses, sea fans, hydroids and star fish.

Exploring a Seamount in a Submersible

Gregory S. Stone wrote in National Geographic Sealed in our submersible, DeepSee, we are untied, drifting, a tiny dot on the immense Pacific Ocean. Outside of the sub are cameras, hydraulics, thrusters, and hundreds of other essential parts that will keep us safe. Pilot Avi Klapfer floods the ballast tanks, and we sink, surrounded by bubbles. It’s like falling into a glass of champagne, and we feel appropriately giddy. Three of us are crammed inside DeepSee’s five-foot sphere, surrounded by communication equipment, pressure valves, controls, snacks, cameras, special bags to urinate in: everything we need for our quest to reach a seamount named Las Gemelas. Its cluster of peaks, rarely seen up close before, rises from the bottom of the Pacific near Cocos Island, 300 miles southwest of Cabo Blanco in Costa Rica. The highest peak here is more than 7,500 feet tall. [Source: Gregory S. Stone, National Geographic, September 2012]

Marianas Seamounts

We turn a ghostly greenish blue in the light, kept dim so we can see outside. Clear, pulsing jellies glide gently in the dark, bouncing off the sub in every direction. A black-and-white manta ray flexes its wings and soars past for a look. We are still in the photic zone, where sunlight penetrates and provides energy for countless microscopic, photosynthetic ocean plants that create much of the Earth’s oxygen. Then we descend farther. The ocean is pitch-black.

At about 700 feet the sub’s dazzling lights bring the bottom into view. Klapfer maneuvers deftly, but the current is strong, and we may not be able to stay down for too long. Suddenly something just beyond the lights rises from the otherwise featureless seafloor. We joke that maybe we’ve found a new wreck, but instead it is a volcanic remnant, perhaps millions of years old. Within minutes a muffled whir tells us that Klapfer has reversed the thrusters and is bringing the sub into position to hover inches from the bottom, inside an ancient, circular vent of the now extinct volcano that forms Las Gemelas. Its sculptured walls look like the facade of a deep-sea cathedral.

Scientists don’t often explore their slopes firsthand—or even their shallower summits: living mazes of hard coral, sponges, and sea fans circled by schools of fish, some of them orange roughy that have lived to be more than a hundred years old. A prickly shark cruises among the volcanic cliffs and crevices of Las Gemelas. This slow-moving deepwater predator lives on and around the tops of seamounts, capitalizing on the large number of resident fish, crustaceans, and other prey down the food chain.

This is the last of our five dives in DeepSee, after a week of calling Las Gemelas home. During our time here, we have observed the animals that live on the summit of this seamount and the pelagic, or marine, invertebrates that occupy the water column around it....Our sub surfaces after five hours—all too soon. We stow our gear aboard Argo and begin the long haul back to our landlocked lives, where we will analyze our data and add one more piece to the puzzle of our global ocean.

Bundt-Cake-Shape Seamount Discovered off California

A seamount with an unusual shape was discovered in 2023 about 184 nautical miles off Cape Mendocino, which is about 300 miles northwest of Sacramento. The underwater mountains rises about 1,000 meters (3,300 feet), has “relatively steep, smooth sides” and is rounded, with a diameter of nearly 915 meters (3,000 feet).“The immediate reaction from the team was that it looked like a Bundt cake. It’s very round and with steep sides and a curved top that slopes into a crater in the center,” according to Neah Baechler, the lead surveyor for Saildrone, the research company that found it, “One side of the top rim is higher than the other, forming a gradual summit. ... The top is slightly bulbous, due to variability in slope.” [Source: Mark Price, Sacramento Bee, March 22, 2023]

The Sacramento Bee reported: The summit of the seamount is about 3,460 meters (11,352 feet) below sea level. Meanwhile, the crater at the top of it is 366 meters (1,200 feet) deep, data shows. Saildrone discovered the seamount while mapping the seafloor in a partnership with the National Oceanic Atmospheric Administration and the Bureau of Ocean Energy Management.

Previously collected radar and satellite data hinted something might be sticking up off the seafloor in the area, “but the seamount turned out to be one much larger feature,” Saildrone reports. “Discovering a new seafloor feature is always noteworthy, but this feature is wedged between the Mendocino and Pioneer fracture zones, placing it in a very dynamic area,” Baechler said.

The U.S. Geological Survey reports “there are at least 63 seamounts” off California, and most of them are 100 miles offshore and a mile deep. It’s estimated they are 10 million to 25 million years old, and none are active volcanoes, the USGS says. Saildrone found the new mount in February, during a mission to map “previously unexplored areas around Alaska’s Aleutian Islands and off the California coast,” officials say.

Brian Connon, vice president of ocean mapping for Saildrone, said “No shipwrecks were noted during during data collection, but we did see a whole array of fascinating features, including long finger ridges, likely from faulting with intricate drainage textures, submarine canyons and basins, and the Aleutian trench” which is over 6.9 kilometers (4.3 miles) deep.

Image Sources: Wikimedia Commons; YouTube, NOAA

Text Sources: National Oceanic and Atmospheric Administration (NOAA); “Introduction to Physical Oceanography” by Robert Stewart , Texas A&M University, 2008 ; Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.

Last Updated May 2023

This site contains copyrighted material the use of which has not always been authorized by the copyright owner. Such material is made available in an effort to advance understanding of country or topic discussed in the article. This constitutes 'fair use' of any such copyrighted material as provided for in section 107 of the US Copyright Law. In accordance with Title 17 U.S.C. Section 107, the material on this site is distributed without profit. If you wish to use copyrighted material from this site for purposes of your own that go beyond 'fair use', you must obtain permission from the copyright owner. If you are the copyright owner and would like this content removed from, please contact me.