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Most volcanic activity on Earth happens beneath the ocean. Hidden below kilometers of seawater, submarine volcanoes create new oceanic crust, build enormous seamount chains, power hydrothermal vents and occasionally produce explosive eruptions, pumice rafts, tsunamis or temporary volcanic islands.
This guide explains what submarine volcanoes and seamounts are, where they form, how water depth changes eruption behavior, how scientists detect underwater eruptions and which hazards are realistic. It also explores pillow lava, guyots, hydrothermal vents, seamount ecosystems, oceanic plateaus and major volcanic systems such as Axial Seamount, Hunga Tonga and Kamaʻehuakanaloa.
Submarine Volcanoes in Brief
- Most of Earth’s volcanic activity occurs beneath the oceans, especially along mid-ocean ridges.
- A submarine volcano is an underwater volcanic system; a seamount is the mountain or structure it builds.
- Deep eruptions are commonly effusive because high water pressure suppresses gas expansion.
- Shallow eruptions can become explosive when magma interacts with seawater.
- Underwater eruptions are detected through earthquakes, hydroacoustic signals, sonar, water chemistry and occasional satellite observations.
- The most important hazards include local tsunamis, flank collapse, pumice rafts, ash, gas release and navigation problems.
- Most submarine eruptions are too deep to affect the atmosphere or global climate.
What Is a Submarine Volcano?
A submarine volcano is any volcanic system that erupts beneath the surface of an ocean, sea or large lake. It may form a small vent on the seafloor, an enormous mountain, a volcanic ridge, a caldera or a chain of overlapping volcanic centers.
Some submarine volcanoes are active and erupt repeatedly. Others are dormant or extinct but remain preserved as large seafloor landforms.
Submarine volcanoes can occur:
- Along mid-ocean ridges
- Above subduction zones
- Along island arcs
- Within continental rifts flooded by the sea
- Above mantle hotspots
- Along faults and fractures in oceanic crust
Most never rise above sea level. Those that do may become volcanic islands, only to be eroded, collapse or sink beneath the ocean again.
What Is a Seamount?
A seamount is a large underwater mountain rising from the seafloor but remaining below the ocean surface. Most seamounts are volcanic in origin.
They range from small isolated cones to enormous volcanic structures several kilometers high and hundreds of kilometers across.
A seamount may be:
- Actively erupting
- Dormant
- Extinct
- Part of a volcanic chain
- Part of a mid-ocean ridge
- A remnant of an eroded volcanic island
The word describes the physical landform rather than its present activity. An extinct underwater volcano is still a seamount, while an active seamount is also a submarine volcano.
Seamount vs. Submarine Volcano
| Feature | Seamount | Submarine volcano |
|---|---|---|
| Meaning | An underwater mountain or volcanic structure | An underwater volcanic system capable of erupting |
| Activity | May be active, dormant or extinct | Usually refers to active or potentially active volcanism |
| Visibility | Remains below sea level | May remain submerged or temporarily breach the surface |
| Main significance | Seafloor topography, ecology and geological history | Magma movement, eruptions and volcanic hazards |
| Tsunami relevance | Possible through collapse or landslides | Possible through explosion, collapse or landslides |
In simple terms, a submarine volcano is the active geological system, while a seamount is the mountain it builds.
Where Do Submarine Volcanoes Form?
Submarine volcanoes form wherever magma rises through oceanic or submerged continental crust. Three tectonic settings account for most underwater volcanism:
- Mid-ocean ridges
- Subduction zones and volcanic arcs
- Oceanic hotspots
Each setting creates different magma compositions, landforms and eruption styles.
Mid-Ocean Ridge Volcanoes
Mid-ocean ridges form where tectonic plates move apart and hot mantle rises beneath the gap. As pressure decreases, part of the mantle melts and produces basaltic magma.
The magma rises through fractures, erupts onto the seafloor and forms new oceanic crust.
The global mid-ocean ridge system is Earth’s longest continuous volcanic zone. It circles the planet beneath the oceans and produces an enormous share of global volcanism.
Typical ridge activity
- Basaltic lava eruptions
- Dike intrusions
- Earthquake swarms
- Pillow-lava formation
- Hydrothermal venting
- Crustal spreading
Most eruptions are deep, effusive and invisible from the surface.
Iceland provides a rare place where part of a mid-ocean ridge rises above sea level. Explore this setting in Iceland Volcanoes & Rift Eruptions.
Submarine Volcanoes at Subduction Zones
Subduction occurs where one tectonic plate sinks beneath another. Water and other volatile substances released from the descending plate promote melting in the overlying mantle.
The resulting magma feeds volcanic arcs. Many arc volcanoes form islands, while others remain completely submerged.
Submarine arc volcanoes occur throughout:
- The Tonga–Kermadec Arc
- The Mariana Arc
- The Aleutian Arc
- The Izu–Bonin Arc
- The Lesser Antilles Arc
- Indonesia and the western Pacific
Arc magmas may be more viscous and gas-rich than typical mid-ocean ridge basalt. Shallow arc volcanoes can therefore produce explosive eruptions, caldera collapse and volcanic tsunamis.
Explore the wider tectonic setting in Pacific Ring of Fire Volcanoes & Earthquakes.
Oceanic Hotspots and Seamount Chains
Hotspots can produce long chains of submarine volcanoes as an oceanic plate moves across a relatively persistent source of magma.
A typical hotspot volcano may pass through several stages:
- A submarine volcano begins growing on the ocean floor.
- Repeated eruptions build a large seamount.
- The summit eventually reaches sea level.
- A volcanic island forms.
- The island moves away from the active hotspot.
- Erosion and subsidence gradually lower it.
- The volcano eventually becomes a seamount or guyot again.
The Hawaiian–Emperor chain is the classic example. Most of the chain is underwater, even though its best-known volcanoes form the Hawaiian Islands.
Explore the process in Hawaiian Volcanoes & Hotspot Volcanism and Canary Islands, La Palma & Madeira Hotspot.
How Do Underwater Volcanic Eruptions Work?
Underwater eruptions are controlled by the same basic processes as eruptions on land: magma rises, pressure decreases, gases expand and molten rock reaches the surface.
However, seawater creates several major differences.
- Water pressure suppresses gas expansion at depth.
- Cold seawater rapidly chills lava.
- Magma–water interaction can cause fragmentation.
- Buoyancy transports ash, bubbles and pumice through the water column.
- Hydrothermal circulation transfers heat and chemicals into the ocean.
Depth is one of the most important controls on eruption behavior.
How Water Depth Controls Underwater Eruptions
Deep submarine eruptions
At great depth, the pressure of the overlying ocean suppresses gas expansion. Basaltic lava commonly erupts quietly and forms pillow lava, sheet flows and fragmented volcanic glass.
Deep eruptions may produce little or no visible sign at the ocean surface.
Intermediate-depth eruptions
At moderate depths, eruptions may produce lava flows, gas plumes, water discoloration and hydrothermal activity. Some fragmentation occurs, but the water pressure still limits explosive expansion.
Shallow submarine eruptions
Near the ocean surface, pressure is lower and magma can interact violently with seawater. Rapid steam generation and magma fragmentation may produce:
- Explosive jets
- Fine volcanic ash
- Base surges
- Pumice
- Volcanic lightning
- Temporary islands
- Tsunamis
The most disruptive submarine eruptions are therefore often shallow rather than deep.
Pillow Lava Explained
Pillow lava forms when fluid lava erupts underwater and its surface cools almost instantly.
A flexible crust develops around the molten interior. Continued pressure causes the crust to split, allowing another rounded lobe to emerge. Repetition of this process creates piles of bulbous or elongated structures resembling stacked pillows.
Pillow lava is especially common:
- Along mid-ocean ridges
- At deep submarine vents
- Around underwater shield volcanoes
- Where lava enters the ocean from land
The outer surface consists of rapidly chilled volcanic glass, while the interior may be more crystalline.
Ancient pillow lavas preserved on land provide important evidence that a rock sequence originally formed underwater.
Pumice Rafts
Pumice rafts form when gas-rich magma fragments into lightweight pumice that remains buoyant at the ocean surface.
A large submarine eruption can release billions of floating pumice fragments that gather into rafts covering extensive areas.
Pumice rafts may:
- Drift for months or years
- Cross major ocean basins
- Damage or clog vessel intakes
- Wash ashore far from the eruption
- Carry marine organisms between islands
- Provide the first visible evidence of an underwater eruption
As pumice absorbs water and becomes colonized by organisms, it gradually sinks or breaks apart.
How Do Underwater Volcanoes Create Islands?
A volcanic island forms when repeated submarine eruptions build a volcanic edifice high enough to reach above sea level.
The earliest exposed island may be unstable because it consists of loose ash, pumice and fragmented material. Waves can erode it rapidly.
An island is more likely to survive when later lava flows protect and strengthen the original deposits.
Temporary volcanic islands
Some islands appear after shallow eruptions but disappear within months or years because:
- Waves erode loose volcanic ash
- The new cone collapses
- The seafloor subsides
- Storms remove unconsolidated material
Long-lived volcanic islands
Islands such as Hawaii, Iceland and many Pacific archipelagos survived because repeated lava eruptions built large, durable volcanic structures.
Notable modern island-forming eruptions include Surtsey near Iceland, Nishinoshima in Japan and several phases of activity at Hunga Tonga–Hunga Haʻapai.
What Is a Guyot?
A guyot is a flat-topped seamount.
Most guyots began as volcanic islands or shallow seamounts. Waves eroded their summits near sea level, creating a relatively flat surface.
Later, the volcano moved away from its magma source, cooled and subsided beneath the ocean.
The basic sequence is:
- A submarine volcano grows.
- It reaches or approaches sea level.
- Waves erode the summit.
- Volcanic activity declines.
- The oceanic crust cools and sinks.
- The flattened volcano becomes a submerged guyot.
Guyots preserve evidence of former islands and long-term movement of tectonic plates.
Hydrothermal Vents and Black Smokers
Hydrothermal vents form where seawater enters fractures in the oceanic crust, is heated by magma or hot rock and returns to the seafloor carrying dissolved minerals.
At black smokers, extremely hot mineral-rich fluid meets cold seawater. Metals and sulfide minerals precipitate, creating dark plumes and chimney-like structures.
Hydrothermal fluids may contain:
- Iron
- Copper
- Zinc
- Sulfur compounds
- Silica
- Hydrogen
- Methane
Why hydrothermal vents matter
They:
- Transfer heat from Earth’s crust into the ocean
- Alter oceanic rock
- Concentrate metal deposits
- Support unique ecosystems
- Provide clues about early life on Earth
- Offer possible analogues for extraterrestrial oceans
Vent ecosystems rely on chemosynthesis rather than sunlight. Microorganisms use chemical energy from hydrogen sulfide, methane or hydrogen to produce organic matter.
Seamount Ecology and Biodiversity
Seamounts are not only volcanic structures. They can also create important marine habitats.
By rising above the surrounding seafloor, seamounts alter ocean currents and may concentrate nutrients, plankton and marine life.
Seamount ecosystems can support:
- Deep-sea corals
- Sponges
- Crustaceans
- Fish
- Sharks
- Whales and other migrating animals
Rocky slopes provide hard surfaces where corals and sponges can attach, while currents deliver food.
Ecological vulnerability
Many seamount organisms grow slowly and are easily damaged by:
- Bottom trawling
- Mining
- Pollution
- Ocean warming
- Changes in oxygen levels
Not every seamount is an ecological hotspot, but many provide isolated habitats with distinctive biological communities.
Oceanic Plateaus and Underwater Mega-Volcanoes
Oceanic plateaus are enormous elevated regions of oceanic crust commonly formed by exceptionally large volumes of basaltic magma.
They are much larger than ordinary seamounts and may cover hundreds of thousands or millions of square kilometers.
Examples include:
- The Ontong Java Plateau
- Shatsky Rise
- Kerguelen Plateau
- Manihiki Plateau
Tamu Massif
Tamu Massif is a vast volcanic structure within Shatsky Rise in the northwest Pacific. It has sometimes been described as one of the largest volcanoes on Earth by area.
Its enormous size does not mean it is currently dangerous. It formed during ancient seafloor volcanism and is inactive today.
Terms such as “megavolcano” or “largest volcano discovered” can be misleading when they imply present unrest. These structures are important records of ancient magma production, not evidence of an impending eruption.
How Do Scientists Detect Submarine Eruptions?
Most underwater eruptions cannot be watched directly. Scientists detect them by combining several indirect signals.
Earthquake swarms
Rising magma fractures rock and can produce clusters of earthquakes. Offshore swarms may indicate dike intrusion, fault movement or eruption.
Hydroacoustic signals
Explosions, earthquakes and collapsing volcanic structures generate sound waves that travel efficiently through seawater.
Hydrophones can detect these signals across large distances.
T-waves
Some earthquake energy becomes trapped within the ocean sound channel and travels as hydroacoustic T-waves. These signals can help locate underwater seismic and volcanic activity.
Water-column anomalies
Research vessels and autonomous instruments may detect:
- Increased temperature
- Dissolved gases
- Changes in water chemistry
- Particles and volcanic ash
- Hydrothermal plumes
Satellite observations
Satellites occasionally reveal:
- Pumice rafts
- Discolored water
- Steam clouds
- New islands
- Ash plumes
- Surface temperature anomalies
Seafloor instruments
Pressure sensors, seismometers, hydrophones and deformation instruments can record eruptions directly on the seabed.
Axial Seamount is one of the best-monitored submarine volcanoes because a network of instruments records its earthquakes, inflation and eruptions.
How Are New Underwater Volcanoes and Seamounts Discovered?
Many “newly discovered” underwater volcanoes are not newly formed. They are ancient structures revealed by improved mapping.
Multibeam sonar
Research vessels send sound waves toward the seafloor and measure the returning echoes. Multibeam sonar can reveal:
- Volcanic cones
- Calderas
- Lava flows
- Collapse scars
- Faults
- Hydrothermal fields
Satellite gravity measurements
Large underwater mountains subtly affect Earth’s gravity field and the shape of the ocean surface. Satellite measurements can therefore reveal previously unmapped seamounts.
Earthquake patterns
Unexpected offshore earthquake swarms may lead scientists to investigate and map a previously unknown volcanic center.
Water chemistry
Heat, gases and mineral-rich plumes can reveal active hydrothermal systems and possible volcanic vents.
Remotely operated vehicles
ROVs and autonomous underwater vehicles can photograph lava, collect samples and map volcanic structures in detail.
Only part of the global seafloor has been mapped at modern high resolution, so many volcanic structures remain poorly known.
Submarine Volcano Hazards
Most underwater eruptions are deep and pose little direct danger to people. The highest risks come from shallow eruptions and structural collapse.
Potential hazards include:
- Local and regional tsunamis
- Submarine landslides
- Caldera collapse
- Ash clouds
- Pumice rafts
- Volcanic gases
- Floating debris
- Damage to submarine cables and pipelines
- Navigation hazards
- Coastal ashfall
Pumice and shipping
Floating pumice can clog cooling-water systems, damage propellers and interfere with navigation.
Ash and aviation
If an eruption becomes subaerial or produces a plume above sea level, ash can become an aviation hazard.
Submarine infrastructure
Lava flows, earthquakes and landslides may damage communication cables, pipelines and scientific equipment on the seafloor.
Can Submarine Volcanoes Cause Tsunamis?
Yes, but lava entering the ocean is rarely the main cause.
Volcanic tsunamis are more commonly generated when a volcanic event rapidly displaces a large volume of water.
Possible mechanisms include:
- Submarine landslides
- Flank collapse
- Caldera collapse
- Large explosions
- Pyroclastic flows entering the ocean
- Atmospheric pressure waves
Most waves generated by submarine volcanic events are local or regional. Basin-wide tsunamis are much less common.
Large island-collapse scenarios are frequently exaggerated online. The more realistic hazard is usually a damaging local wave close to an unstable volcanic island or shallow submarine volcano.
Continue with Submarine Landslides & Seafloor Collapse and Volcanic Hazards Explained.
Famous Submarine Volcanoes and Seamounts
Axial Seamount
Axial Seamount lies on the Juan de Fuca Ridge off the western coast of North America. It is one of the best-monitored submarine volcanoes in the world.
Its eruptions in 1998, 2011 and 2015 produced basaltic lava flows with little visible surface disturbance.
Hunga Tonga–Hunga Haʻapai
This shallow volcanic system in the Tonga Arc produced the extraordinary 2022 eruption, generating a global atmospheric pressure wave, volcanic lightning and destructive tsunamis.
Kick-’em-Jenny
Kick-’em-Jenny is an active submarine volcano north of Grenada in the Caribbean. Its shallow summit and eruptive history make it important for navigation and local tsunami monitoring.
Kamaʻehuakanaloa
Formerly known as Lōʻihi, Kamaʻehuakanaloa is a young submarine volcano southeast of Hawaii. It represents the early submarine stage of Hawaiian hotspot volcanism.
Monowai
Monowai is an active submarine volcano in the Kermadec Arc. Its summit has repeatedly grown and collapsed, and much of its activity is detected through acoustic and bathymetric measurements.
West Mata
West Mata in the western Pacific was observed erupting on the deep seafloor, providing rare direct images of explosive submarine activity at great depth.
Nishinoshima
Nishinoshima is a volcanic island south of Japan that has expanded significantly during repeated eruptions, demonstrating how submarine activity can build durable land above sea level.
Tamu Massif
Tamu Massif is a gigantic inactive volcanic structure within Shatsky Rise. It illustrates the scale of ancient submarine volcanism but poses no known modern eruption threat.
Major Underwater Eruptions and Volcanic Events
Hunga Tonga–Hunga Haʻapai, 2022
The January 2022 eruption was one of the most powerful volcanic explosions observed in the modern satellite era.
It produced:
- A towering eruption plume
- Global atmospheric waves
- Volcanic lightning
- Regional and distant tsunamis
- Major destruction in Tonga
- An unusually large injection of water vapor into the stratosphere
The eruption demonstrated that shallow submarine volcanoes can create hazards extending far beyond the immediate vent.
Axial Seamount eruptions
Eruptions at Axial Seamount in 1998, 2011 and 2015 reshaped parts of the seafloor with basaltic lava while producing little drama at the ocean surface.
These events are representative of the majority of submarine volcanism: active, geologically important and largely invisible.
Havre pumice-raft eruption, 2012
The Havre submarine volcano in the southwest Pacific produced an enormous pumice raft visible from satellites.
Later seafloor mapping revealed a large and complex eruption involving lava domes, ash and widespread pumice.
Home Reef eruptions
Home Reef in Tonga has repeatedly formed temporary islands and pumice rafts. Some newly created land has later been eroded by waves.
Surtsey, 1963–1967
Surtsey formed south of Iceland when a submarine eruption reached the surface. Early explosive interaction with seawater was followed by lava flows that helped preserve the island.
Common Myths About Submarine Volcanoes
Myth: Every underwater eruption causes a tsunami
False. Most deep submarine eruptions produce lava flows and only minor water displacement.
Myth: A newly discovered seamount is a newly formed volcano
False. Many seamounts are millions of years old and have only recently been mapped.
Myth: The largest underwater volcano is the most dangerous
False. Size does not indicate current activity. Many enormous seamounts and oceanic plateaus are extinct.
Myth: Underwater volcanoes always erupt explosively
False. High water pressure suppresses gas expansion, so many deep eruptions are effusive.
Myth: Every seamount will eventually become an island
False. Most stop growing before reaching sea level.
Myth: Submarine eruptions cause major global cooling
Usually false. Deep eruptions release material into the ocean rather than the stratosphere.
Myth: Water always suppresses volcanic explosions
False. Deep water suppresses gas expansion, but shallow magma–water interaction can dramatically increase fragmentation and explosivity.
Comparison of Submarine Volcanic Features
| Feature | Formation | Typical activity | Main significance |
|---|---|---|---|
| Submarine volcano | Magma erupts beneath water | Lava flows, explosions and hydrothermal activity | Active underwater volcanism |
| Seamount | Volcanic mountain grows above the seafloor | May be active, dormant or extinct | Topography, ecosystems and plate history |
| Guyot | Island or shallow seamount is eroded and later subsides | Usually extinct | Evidence of former sea-level exposure |
| Submarine caldera | Collapse after magma withdrawal | Explosive eruptions, domes and later unrest | Large volcanic collapse system |
| Mid-ocean ridge | Plates separate and basaltic magma rises | Frequent effusive eruptions | Formation of new oceanic crust |
| Oceanic plateau | Exceptionally large basaltic magma production | Ancient large-volume volcanism | Large igneous province beneath the ocean |
| Hydrothermal vent | Seawater circulates through hot crust | Release of hot mineral-rich fluids | Chemical exchange and chemosynthetic ecosystems |
| Volcanic island | Submarine volcano grows above sea level | Subaerial and submarine eruptions | Island formation and coastal hazards |
Frequently Asked Questions About Submarine Volcanoes
Are most volcanoes on Earth underwater?
Yes. Most volcanic activity occurs beneath the oceans, particularly along the global mid-ocean ridge system.
What is the difference between a seamount and a submarine volcano?
A seamount is an underwater mountain, usually volcanic in origin. A submarine volcano is an underwater volcanic system that is active or potentially active. Many submarine volcanoes build seamounts.
How do submarine volcanoes erupt?
Magma rises through fractures in the oceanic crust and erupts onto the seafloor. Deep eruptions commonly produce pillow lava, while shallow eruptions may become explosive through interaction with seawater.
Why are deep underwater eruptions less explosive?
High water pressure suppresses the expansion of volcanic gases. This favors effusive lava eruptions rather than large explosive plumes.
Can submarine volcanoes cause tsunamis?
Yes, but the largest tsunami risk often comes from flank collapse, submarine landslides, caldera collapse or major explosions rather than lava flows alone.
What is pillow lava?
Pillow lava consists of rounded or elongated lava lobes formed when fluid lava erupts underwater and cools rapidly against seawater.
What is a pumice raft?
A pumice raft is a floating mass of lightweight volcanic pumice produced by an underwater or coastal eruption. It can drift across the ocean for months or years.
Can an underwater volcano form a new island?
Yes. Repeated eruptions can build a volcano above sea level. Some new islands survive, while others are quickly destroyed by erosion and collapse.
What is a guyot?
A guyot is a flat-topped seamount that was once near or above sea level, where waves eroded its summit before it subsided beneath the ocean.
How do scientists detect underwater eruptions?
Scientists use earthquake swarms, hydrophones, sonar, water chemistry, pressure sensors, seafloor instruments and occasional satellite observations of pumice rafts, ash or water discoloration.
Are newly discovered seamounts newly formed?
Usually not. Many are ancient volcanoes that have only recently been detected or mapped in detail.
Do submarine volcanoes affect climate?
Most do not because their gases and ash remain in the ocean. Climate effects generally require an eruption plume that reaches the atmosphere and injects sulfur into the stratosphere.
Are seamounts important for marine life?
Yes. Some seamounts alter currents, concentrate nutrients and provide rocky habitats for corals, sponges, fish and other marine organisms.
What is the largest submarine volcano?
The answer depends on how individual volcanic structures are defined. Tamu Massif is one of the largest known submarine volcanic structures by area, but it is ancient and inactive.
What is the most active submarine volcano?
Several submarine volcanoes are highly active, including Monowai and Axial Seamount. Exact rankings are difficult because most underwater eruptions are poorly observed.
How deep is the deepest active volcano?
Active volcanic vents occur several kilometers beneath the ocean surface along deep mid-ocean ridges and subduction systems. Exact records change as new vents are mapped and observed.
