Undersea Volcanoes and Seamounts: Features, Eruptions and Life Found on Them | Sea Life, Islands and Oceania

VOLCANOES IN THE OCEAN

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) noaa.gov; “Introduction to Physical Oceanography” by Robert Stewart , Texas A&M University, 2008 uv.es/hegigui/Kasper ; Woods Hole Oceanographic Institute whoi.edu ; Cousteau Society cousteau.org ; Monterey Bay Aquarium montereybayaquarium.org

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

Hunga Tonga eruption in January 2022

In 2018, an unprecedented underwater eruption created a skyscraper-size submarine volcano off Mayotte, a small French island located between Madagascar and Mozambique, in the western Indian Ocean. Scientists first noticed a sudden swarm of earthquakes in an area that had been largely quiet for decades. GPS data also showed Mayotte drifting eastward at nearly 8 inches (20 centimeters) per year. A study on the volcano was published online August 26, 2021 in the journal Nature Geoscience.[Source: Laura Geggel, Live Science, October 21, 2021]

A 2019 research mission confirmed the cause of earthquakes and movement: a brand-new volcano rising 2,690 feet (820 meters) from the seafloor about 31 miles east of the island. The edifice, which did not exist in 2014 surveys, came from a massive magma reservoir located unusually deep—about 55 kilometers (34 miles) beneath the surface, the deepest volcanic source ever documented. The eruption released 30 to 1,000 times more material than other known deep-sea eruptions.

From May 2018 to May 2021, more than 11,000 detectable earthquakes shook Mayotte, The most powerful earthquake was magnitude 5.9. There were also strange seismic hums, or very-low-frequency earthquakes, originating deep underground. These earthquakes could not be felt on the surface, but seismometers around the world detected them. These earthquakes are associated with volcanic activity. The sudden increase in seismic activity was surprising given that only two earthquakes had been detected near Mayotte since 1972.

Seismometers revealed magma traveling upward through dikes from the asthenosphere as tectonic forces pushed it toward the surface. The region shows evidence of earlier eruptions as well, and monitoring continues. According to researchers, the volcano was still active as recently as early 2021.

Undersea Antarctic Volcano Triggers 85,000 Earthquakes

A long-dormant underwater volcano near Antarctica, the Orca Seamount, triggered a swarm of over 85,000 earthquakes between August and November 2020 due to a large magma intrusion. The intense activity is the largest seismic unrest ever recorded in the area and is attributed to hot magma pushing through the Earth's crust. Researchers used seismic and satellite data to confirm the magma movement, which caused the ground on nearby King George Island to shift by more than 10 centimeters. [Source: Tia Ghose, Live Science, April 27, 2022

Hunga Tonga eruption in January 2022

The quakes were likely caused by a "finger" of hot magma poking into the crust, research found. "There have been similar intrusions in other places on Earth, but this is the first time we have observed it there," study co-author Simone Cesca, a seismologist at the GFZ German Research Centre for Geosciences in Potsdam, told Live Science. "Normally, these processes occur over geologic time scales," as opposed to over the course of a human life span, Cesca said. "So in a way, we are lucky to see this." Researchers suspect the November quake opened fractures that depressurized a magma dike and may have triggered a submarine eruption at the Orca Seamount. However, confirmation would require a expedition to remap the seafloor and compare it with earlier bathymetry.

The swarm of earthquakes in the Bransfield Strait — where the Phoenix Plate subducts beneath Antarctica — revealed unusual activity beneath the Orca Seamount, a large underwater volcano rising 2,950 feet (900 m) from the seafloor. Beginning in 2020, seismic stations on nearby King George Island recorded thousands of small quakes, along with two major events: magnitude 5.9 in October and 6.0 in Novembe Using regional seismic data, GPS ground-displacement measurements, and satellite radar, scientists reconstructed the episode and found that the island shifted about 4.3 inches (11 cm). Only a small portion of that motion came from the earthquakes themselves; most appears to have been driven by magma forcing its way into the crust.

Theory Links Ocean Volcanoes Eruptions to the Breakup of the Continents

In a paper published November 11, 2025 in the journal Nature Geoscience, researchers theorized that some mid-ocean volcanoes form from long-lived waves in Earth’s mantle, triggered by the breakup of supercontinents tens of millions ago that stripped pieces of continental crust from below and carried them oceanward to feed volcanic eruptions. [Source: Stephanie Pappas, Live Science, November 14, 2025]

The study suggests that after continents rift apart, lingering mantle instabilities continue to “peel” crust from the base of continents. This continental debris then enters the mantle and later resurfaces in mid-ocean volcanoes with unusually continent-like magma — such as the Christmas Island Seamount in the Indian Ocean.

These volcanoes have long puzzled scientists because their magmas contain minerals that resemble continental rather than oceanic crust. Earlier explanations invoked recycled subducted crust or deep mantle plumes, but neither fully accounted for the diversity of magma compositions.

mud volcano in Norway

Analyzing volcanic rocks from the Walvis Ridge off Africa and using computer models, the team found evidence that mantle waves generated by continental breakup can sweep crustal fragments into the mantle within a few million years, with the enriched material reappearing in eruptions 5–15 million years later. This supply peaks roughly 50 million years after rifting.

Rock samples from Christmas Island match this pattern: volcanism began about 10 million years after India split from Antarctica and Australia, producing continent-rich magmas that gradually became more oceanic over the next 40–60 million years. The findings show that the mantle continues to respond to continental breakup long after oceans open, reshaping its composition and fueling volcanic chains far from their continental origins.

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]

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.

Life on Undersea Volcanoes

Italy’s Aeolian Islands sit atop one of the Mediterranean’s most active volcanic systems, where magma, gases, and earthquakes continuously reshape both land and seafloor. Despite these harsh conditions, marine life persists and even thrives. Panarea hosts acidic “nature’s Jacuzzis,” where CO -rich bubbles rise from hydrothermal vents. The water is so corrosive that almost no corals or shell-forming species survive; only hardy organisms like anaerobic bacteria and a few worms endure. Scientists monitor the vent noise here as a potential early warning of volcanic activity. [Source: Laurent Ballesta, National Geographic, June 2023, nationalgeographic.com ]

A nearby undersea valley—nicknamed the Valley of 200 Volcanoes—features hundreds of tall iron-oxide chimneys, each venting superheated gases. Despite extreme acidity and heat, pioneering species such as flatworms, sea spiders, and specialized algae cling to the chimneys’ surfaces. At Stromboli, the “Lighthouse of the Mediterranean,” near-constant eruptions send debris crashing into the sea. Underwater, life recolonizes rapidly: gorgonian corals reemerge, dogfish juveniles appear, and vibrant algae forests return even to recently buried slopes.

Scientists also found millions of Pacific white skate eggs clustered around an active volcano nearly a mile underwater off Canada’s west coast. Lead researcher Cherisse Du Preez says the find defies expectations about where life can survive. The shark-related skates lay their eggs—golden, purse-shaped capsules—near geothermal vents because the deep sea is frigid and the added heat dramatically improves incubation. The site, discovered during the first dives on the volcano, appears to host a nursery of roughly 2.6 million eggs, which may take up to 10 years to hatch. Researchers also recorded the first-ever footage of a female skate laying an egg. The volcano’s mineral-rich warm fluids create a rare hotspot that allows these animals to survive in waters once thought too cold to support them. [Source: Doyle Rice, USA TODAY, May 21, 2025; Sascha Pare, Live Science, July 19, 2023]

Seamounts

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

It is believed to be that the number of seamounts that are at least 1,000 meters (3,280 feet) high exceeds 100,000. Despite this, less than one-tenth of a percent of the seamounts in the world have been explored.

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.

Seamounts provide crucial rocky habitats for deep-sea corals, sponges, and other invertebrates because hard substrates are difficult to find in the ocean. Most of the seafloor is covered by loose mud. "Seamounts can be too steep for mud to stick to, and some animals really thrive on the sides," Jon Copley, a professor of deep-sea ecology and ocean exploration at the University of Southampton in the U.K, previously told Live Science. "When one sticks up, it creates strong currents for filter feeders to grow up into the water and catch food." [Source: Sascha Pare, Live Science, November 23, 2023]

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.

Seamounts off the California Coast

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) noaa.gov; “Introduction to Physical Oceanography” by Robert Stewart , Texas A&M University, 2008 uv.es/hegigui/Kasper ; 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 December 2025