Why Does Japan Have So Many Volcanoes?

Japan lies along the northwestern margin of the Pacific Ocean, where multiple tectonic plates converge, descend and deform the crust.

Two major oceanic plates drive much of the country’s volcanism:

• The Pacific Plate
• The Philippine Sea Plate

These plates descend beneath crustal blocks carrying the Japanese islands. As the sinking oceanic crust reaches greater depths, minerals within it release water and other volatile compounds.

Those fluids rise into the mantle wedge above the descending plate. They lower the melting temperature of the mantle and promote partial melting.

The resulting magma is less dense than the surrounding rock. It rises through fractures, faults and zones of weakness in the crust.

Some magma erupts at the surface. Other magma accumulates in reservoirs where it cools, crystallizes, mixes with new magma and releases gas.

This process produces chains of volcanoes roughly parallel to Japan’s deep ocean trenches.

Why Japanese Volcanoes Are So Diverse

Japan’s volcanoes do not all erupt the same type of magma. Their chemistry and behavior depend on:

• The depth and geometry of the descending plate
• The composition of the mantle
• The thickness and structure of the crust
• The time magma spends in underground reservoirs
• Magma mixing
• Gas content
• Interaction with groundwater
• Interaction with crater lakes or seawater

Some Japanese volcanoes produce fluid basaltic lava. Others erupt viscous andesite or dacite capable of generating lava domes, powerful explosions and pyroclastic flows.

Many alternate between several eruption styles over time.

Japan’s Complex Tectonic Setting

The Japanese archipelago lies at the meeting point of several major and smaller tectonic plates.

Depending on the tectonic model used, these include:

• The Pacific Plate
• The Philippine Sea Plate
• The Eurasian or Amur Plate
• The North American or Okhotsk Plate

The exact boundaries between some continental blocks remain the subject of scientific refinement, but the broad pattern is clear: oceanic plates descend beneath Japan from the east and south.

The Japan Trench

East of northern Honshu, the Pacific Plate descends beneath northeastern Japan along the Japan Trench.

This subduction zone feeds the volcanoes of Tōhoku and contributes to major megathrust earthquakes.

The Kuril Trench

Northeast of Hokkaido, the Pacific Plate descends along the Kuril Trench.

This system feeds the Kuril volcanic arc, which continues through eastern Hokkaido and northward toward the Russian Kuril Islands and Kamchatka.

The Izu–Ogasawara Trench

South of Tokyo, the Pacific Plate descends beneath the Philippine Sea Plate along the Izu–Ogasawara Trench.

This produces a long oceanic volcanic arc containing active islands, submarine calderas and rapidly evolving volcanic vents.

The Nankai Trough

South of central and western Japan, the Philippine Sea Plate descends beneath southwest Japan along the Nankai Trough.

This tectonic system contributes to volcanism in central Honshu and Kyushu while also generating major earthquake and tsunami hazards.

Why Volcanoes Are Set Back From the Trench

Volcanoes do not normally sit directly above the trench.

The descending plate must reach sufficient depth to release water and generate substantial mantle melting. As a result, the volcanic front forms inland or on the arc side of the trench.

The distance between trench and volcanic front varies according to the angle of subduction and crustal structure.

Japan’s Major Volcanic Arcs

Japan contains several connected but geologically distinct volcanic arcs.

Kuril Arc

The Kuril Arc extends through eastern Hokkaido toward the Kuril Islands.

Important Hokkaido volcanoes associated with this system include:

• Shiretoko-Iozan
• Rausudake
• Mashu
• Akan
• Meakan

Northeast Japan Arc

The Northeast Japan Arc runs through western Hokkaido and northern Honshu.

Major volcanoes include:

• Hokkaido-Komagatake
• Usu
• Tarumae
• Iwate
• Akita-Komagatake
• Iwaki
• Hakkoda
• Iwatesan
• Zao
• Azuma
• Adatara
• Bandai

Izu Arc

The Izu Arc extends southward from central Honshu through the Izu Peninsula and Izu Islands.

It includes:

• Hakone
• Izu-Tobu volcanic field
• Izu-Oshima
• Miyakejima
• Hachijojima
• Aogashima
• Bayonnaise Rocks
• Sumisujima

Ogasawara or Bonin Arc

Farther south, the volcanic chain continues through the Ogasawara Islands and numerous submarine volcanoes.

Important systems include:

• Nishinoshima
• Ioto, formerly called Iwo Jima
• Fukutoku-Okanoba
• Kaitoku Seamount
• Submarine calderas and volcanic banks

Southwest Japan Arc

The Southwest Japan Arc includes volcanoes influenced by subduction of the Philippine Sea Plate.

It extends through parts of central Honshu, western Honshu and Kyushu.

Ryukyu Arc

The Ryukyu Arc extends southwest from Kyushu through the Ryukyu Islands toward Taiwan.

Volcanoes include:

• Satsuma-Iojima
• Kuchinoerabujima
• Suwanosejima
• Nakanoshima
• Iotorishima

Major Volcanic Regions of Japan

Hokkaido

Hokkaido contains stratovolcanoes, lava domes, calderas and crater lakes associated with the Kuril and Northeast Japan arcs.

Major systems include:

• Meakan
• Tokachidake
• Usu
• Tarumae
• Hokkaido-Komagatake
• Kuttara
• Akan
• Mashu
• Shikotsu
• Daisetsu

Tōhoku

Northern Honshu contains a long chain of volcanic mountains extending through Aomori, Akita, Iwate, Yamagata, Miyagi and Fukushima prefectures.

Notable volcanoes include:

• Iwaki
• Hakkoda
• Iwate
• Akita-Komagatake
• Chokai
• Zao
• Azuma
• Adatara
• Bandai
• Hiuchigatake

Central Honshu

Central Japan contains Mount Fuji, Ontake, Asama, Kusatsu-Shirane, Hakone, Yakedake and numerous volcanic complexes.

This region is especially important because volcanoes intersect with:

• Major cities
• High-speed rail corridors
• Expressways
• Power infrastructure
• National parks
• Popular climbing routes

Kyushu

Kyushu contains some of Japan’s largest and most active volcanic systems.

They include:

• Aso
• Unzen
• Kirishima
• Sakurajima
• Aira Caldera
• Ikeda and Kaimondake
• Kikai Caldera

Izu and Ogasawara Islands

South of Tokyo, an oceanic volcanic chain extends for more than 1,000 kilometers.

Many volcanoes rise only partly above sea level, while others remain completely submarine.

Ryukyu and Tokara Islands

Southwest of Kyushu, small volcanic islands form part of the northern Ryukyu Arc.

Suwanosejima is particularly active, while Kuchinoerabujima has repeatedly required evacuations.

Major Volcanoes in Japan

Mount Fuji: Japan’s Highest and Most Famous Volcano

Mount Fuji, or Fujisan, rises to 3,776 meters southwest of Tokyo. Its near-symmetrical cone has become one of the most recognizable volcanic landscapes on Earth.

Fuji is Japan’s highest mountain and its largest polygenetic volcano.

Although the mountain has not erupted for more than three centuries, it remains active.

How Mount Fuji Formed

Modern Fuji developed through several overlapping stages of volcanic construction.

The present cone grew above older volcanic systems, including Komitake and Older Fuji.

Repeated eruptions produced:

• Basaltic lava flows
• Scoria cones
• Ash deposits
• Flank vents
• Lava tubes
• Debris-flow deposits

More than 100 flank vents and cones surround the mountain.

This is important because the next eruption does not have to begin at the summit. Magma could open a new vent on the flank.

The Jōgan Eruption of 864–866

The Jōgan eruption generated extensive lava flows from vents on Fuji’s northwestern flank.

Lava entered and divided a large lake, contributing to the formation of the modern Fuji Five Lakes landscape.

The Aokigahara lava flow created rugged forested terrain containing caves and lava tubes.

The Hōei Eruption of 1707–1708

Fuji’s most recent confirmed eruption began in December 1707.

The eruption opened vents on the southeastern flank and formed the Hōei craters.

Unlike many earlier Fuji eruptions, the event was strongly explosive.

It produced:

• Large quantities of ash and scoria
• Widespread tephra fall
• Darkened skies
• Damage to fields and villages
• Ashfall across Edo, modern Tokyo
• Long-term flooding and sediment problems in downstream rivers

The eruption produced little or no major lava flow. Its principal hazard was explosive tephra production.

Could Mount Fuji Erupt Again?

Yes. Fuji is an active volcano with a long history of repeated eruptions.

Three centuries of quiet do not mean the magma system is extinct.

Potential future activity could include:

• Flank fissures
• Scoria-cone formation
• Lava flows
• Explosive ash eruptions
• Ballistic projectiles
• Snowmelt lahars
• Debris flows during heavy rain

Mount Fuji Ashfall and Tokyo

A Hōei-scale eruption could send ash toward densely populated central Japan if winds carry the plume eastward.

Tokyo would not necessarily be destroyed by lava or pyroclastic flows, but ash could cause cascading disruption.

Potential consequences include:

• Airport closures
• Railway disruption
• Road closures
• Electrical failures
• Water-treatment problems
• Mechanical damage
• Reduced solar-power generation
• Roof loading during rain
• Respiratory irritation
• Supply-chain disruption

Sakurajima: Japan’s Persistently Active Urban Volcano

Sakurajima rises from the northern part of Kagoshima Bay in southern Kyushu.

It is a post-caldera volcano built within the enormous Aira Caldera.

Kagoshima City lies only several kilometers across the bay, making Sakurajima one of the clearest examples of a persistently active volcano beside a major urban area.

Minamidake and Showa Craters

Modern activity has concentrated mainly at:

• Minamidake summit crater
• Showa crater on the eastern flank

Eruptions commonly produce:

• Ash plumes
• Vulcanian explosions
• Ballistic blocks
• Volcanic lightning
• Pyroclastic material
• Shock waves

The 1914 Taishō Eruption

Sakurajima’s 1914 eruption was one of Japan’s largest twentieth-century volcanic events.

Strong earthquake activity preceded the eruption, allowing many residents to evacuate.

Major fissures opened on both sides of the volcano.

The eruption produced:

• Powerful explosions
• Heavy ashfall
• Lava fountains
• Extensive lava flows
• Earthquakes
• Damage across the Kagoshima region

Lava filled the narrow channel separating Sakurajima from the Ōsumi Peninsula, transforming the former island into a peninsula.

Living With Frequent Ashfall

Residents around Kagoshima have adapted to repeated ashfall.

Measures include:

• Special ash collection bags
• Reinforced shelters
• Designated evacuation routes
• School safety procedures
• Volcanic ash forecasts
• Regular evacuation exercises

Adaptation reduces risk, but it cannot eliminate the possibility of a much larger eruption.

Could Sakurajima Produce a Larger Eruption?

Yes. Persistent small and moderate explosions do not release all magma accumulating beneath the broader Aira system.

Scientists monitor deformation across the caldera because long-term magma accumulation could influence future activity.

Aira Caldera: The Giant System Beneath Kagoshima Bay

Aira Caldera occupies the northern half of Kagoshima Bay.

It formed through major explosive eruptions long before modern Sakurajima developed.

Sakurajima is therefore not an isolated volcano. It is the active post-caldera cone of a much larger magmatic system.

The Aira Caldera-Forming Eruption

A major eruption approximately 30,000 years ago generated widespread pyroclastic flows and ash.

The eruption dispersed the Aira-Tanzawa tephra across large parts of Japan.

Caldera collapse followed the withdrawal of a large volume of magma.

Why Aira Matters Today

The caldera lies beside Kagoshima and contains an active magma system.

Modern monitoring examines:

• Crustal inflation
• Earthquake activity
• Gas emissions
• Sakurajima eruption rates
• Long-term magma accumulation

A future caldera-forming eruption is far less likely in any given year than ordinary Sakurajima activity, but Aira’s geologic history demonstrates the maximum scale of volcanism possible in southern Kyushu.

Aso: One of Earth’s Great Volcanic Calderas

Aso is a vast caldera in central Kyushu.

It measures roughly 24 kilometers across and contains towns, farms, roads and railways.

A group of central cones rises from the caldera floor. Nakadake is the most frequently active modern vent.

How Aso Caldera Formed

Aso developed through four major explosive eruption cycles between roughly 300,000 and 90,000 years ago.

The largest, known as Aso-4, generated enormous pyroclastic flows and widespread ash deposits.

Pyroclastic-flow deposits from Aso reached far beyond the modern caldera and covered large areas of Kyushu.

Nakadake Crater

Nakadake contains an active crater that may hold a crater lake during quieter periods.

Activity can include:

• Gas emissions
• Steam plumes
• Ash eruptions
• Ballistic blocks
• Strombolian activity
• Phreatic explosions

Volcanic Gas Hazard

Sulfur dioxide and other gases can accumulate around the crater, especially during weak wind or unfavorable weather.

Access restrictions may be imposed even when no major eruption is occurring.

People with respiratory or cardiovascular conditions are particularly vulnerable.

Could Aso Produce Another Caldera-Forming Eruption?

Geologically, yes. Aso remains an active magmatic system.

However, caldera-forming eruptions are exceptionally rare.

Modern unrest is far more likely to involve Nakadake ash emissions, gas releases or moderate explosions than an Aso-4-scale event.

Learn how giant volcanic calderas form →

Unzen: Lava Domes, Pyroclastic Flows and Sector Collapse

Unzen is a volcanic complex on the Shimabara Peninsula of western Kyushu.

It is famous for two very different disasters:

• The 1792 sector collapse and tsunami
• The 1990–1995 lava-dome eruption and pyroclastic flows

The 1792 Unzen–Mayuyama Disaster

Earthquakes and volcanic unrest destabilized Mayuyama, a large dome east of the main volcanic complex.

A massive landslide swept toward Shimabara and entered the Ariake Sea.

The landslide displaced water and generated a destructive tsunami that struck communities on both sides of the bay.

The combined landslide and tsunami killed approximately 15,000 people, making it Japan’s deadliest historic volcanic disaster.

The 1990–1995 Eruption

Unzen reawakened in 1990 after nearly two centuries of quiet.

Viscous magma emerged and constructed a lava dome at Fugen-dake.

Sections of the unstable dome repeatedly collapsed.

These collapses generated block-and-ash pyroclastic flows that raced down the slopes.

The June 1991 Pyroclastic Flow

On June 3, 1991, a pyroclastic flow killed 43 people, including scientists, journalists, firefighters and residents.

The victims included volcanologists Maurice and Katia Krafft and Harry Glicken.

The tragedy demonstrated that pyroclastic flows can travel beyond apparently active collapse zones and change direction according to topography.

Lessons From Unzen

• Lava domes can remain unstable for years.
• Small dome collapses can generate lethal pyroclastic flows.
• Exclusion zones must account for uncertainty.
• Media and observers may underestimate expanding hazards.
• Volcanic landslides can generate tsunamis.

Mount Ontake: The Deadly 2014 Steam-Driven Eruption

Mount Ontake is a large stratovolcano in central Honshu and an important sacred mountain.

Thousands of hikers visit its summit region during favorable weather.

On September 27, 2014, Ontake erupted suddenly while many climbers were near the summit.

What Happened in 2014?

The eruption was primarily phreatic, meaning it was driven by pressurized steam and hydrothermal fluids rather than a large volume of newly erupted magma.

The explosion produced:

• Ash clouds
• Ballistic blocks
• Hot surges
• Poor visibility
• Rapidly changing conditions

Sixty-three people died or remained missing, making it Japan’s deadliest volcanic eruption since World War II.

Why Was Warning So Difficult?

Phreatic eruptions can develop with subtle or short-lived precursors.

Unlike large magmatic eruptions, they may not produce prolonged ground inflation, major gas changes or sustained earthquake swarms.

Hydrothermal pressure can rise quickly beneath sealed or altered rock.

Lessons From Ontake

The disaster led to stronger attention to:

• Volcano information for hikers
• Emergency shelters
• Helmet availability
• Summit communication systems
• Rapid alert dissemination
• Phreatic-eruption research

Mount Asama: A Frequently Active Volcano Near Tokyo

Mount Asama lies northwest of Tokyo on the border of Gunma and Nagano prefectures.

It is one of Honshu’s most active volcanoes.

The 1783 Tenmei Eruption

Asama’s 1783 eruption was one of the most destructive Japanese eruptions of the Edo period.

Activity included:

• Explosive ashfall
• Pumice eruptions
• Pyroclastic flows
• Lava flows
• Debris avalanches
• Lahars and river flooding

Volcanic debris entered river systems and affected communities far downstream.

The eruption occurred during a period of severe climatic and agricultural hardship and contributed to regional food shortages.

Modern Activity

Asama has produced repeated ash eruptions and explosive events during the twentieth and twenty-first centuries.

Its location near transport corridors, resorts and populated areas makes even moderate activity disruptive.

Kirishima: A Complex of Active Craters in Southern Kyushu

Kirishima is not one single cone. It is a volcanic complex containing numerous stratovolcanoes, craters, cones and crater lakes.

Important vents include:

• Shinmoedake
• Ioyama
• Ohachi
• Karakunidake
• Takachihonomine

Shinmoedake

Shinmoedake produced a major eruptive episode in 2011.

The eruption included:

• Explosive ash plumes
• Pumice fall
• Lava accumulation inside the crater
• Volcanic lightning
• Air-traffic disruption

Further explosive activity occurred during 2017 and 2018.

Ioyama

Ioyama is a hydrothermally active area with fumaroles and altered ground.

Its activity demonstrates that unrest can migrate between different parts of a volcanic complex.

Why Multi-Vent Systems Are Difficult to Monitor

At volcanic complexes such as Kirishima, the next eruption may not occur at the vent that erupted most recently.

Scientists must monitor earthquakes, deformation and gas across a broad area.

Kikai Caldera: Japan’s Hidden Submarine Giant

Kikai Caldera lies mostly beneath the sea south of Kyushu.

Parts of the caldera rim rise above sea level as islands, including Satsuma-Iojima and Takeshima.

The Akahoya Eruption

Approximately 7,300 years ago, Kikai produced the enormous Akahoya eruption.

The event generated:

• Widespread ashfall
• Large pyroclastic flows
• Caldera collapse
• Severe disruption across southern Japan

Pyroclastic flows crossed the sea and reached parts of southern Kyushu.

The Kikai-Akahoya ash layer is an important archaeological and geological marker across Japan.

Modern Activity

Post-caldera volcanism continues at Satsuma-Iojima and nearby submarine vents.

Kikai is not considered extinct simply because its largest eruption occurred thousands of years ago.

Suwanosejima: One of Japan’s Most Persistently Active Volcanoes

Suwanosejima is a volcanic island in the Tokara chain south of Kyushu.

Its Otake crater frequently produces explosive activity.

Typical eruptions include:

• Ash plumes
• Strombolian explosions
• Ballistic ejecta
• Incandescent crater activity
• Volcanic lightning

A small community lives on the island, making reliable alerts and exclusion zones essential.

Frequent activity does not guarantee that every eruption will remain small.

Kuchinoerabujima: Explosive Eruptions and Island Evacuations

Kuchinoerabujima is a volcanic island southwest of Kyushu.

Modern activity centers mainly at Shindake.

The 2015 Eruption

A powerful eruption in May 2015 generated an ash plume and pyroclastic flows.

Residents were evacuated from the island.

The event demonstrated the logistical difficulty of evacuating a small volcanic island by sea.

Main Hazards

• Explosive ash eruptions
• Pyroclastic flows
• Ballistic projectiles
• Volcanic gas
• Lahars
• Coastal evacuation difficulties

Hokkaido Volcanoes

Hokkaido contains some of Japan’s most scenic and hazardous volcanic landscapes.

Meakan

Meakan is a complex volcano in eastern Hokkaido with several active craters.

Activity includes:

• Steam emissions
• Ash eruptions
• Volcanic gas
• Hydrothermal activity

Tokachidake

Tokachidake is an active stratovolcano in central Hokkaido.

Its 1926 eruption generated snowmelt lahars that killed many people.

Snow-covered volcanoes can produce destructive mudflows even when eruption volume is moderate.

Usu

Usu rises on the southern rim of Toya Caldera.

It has repeatedly produced:

• Strong earthquake swarms
• Rapid ground deformation
• New vents
• Phreatic explosions
• Lava domes

Its 2000 unrest was successfully recognized before eruption, allowing evacuation.

Tarumae

Tarumae is a stratovolcano near Lake Shikotsu and the city of Tomakomai.

A lava dome occupies its summit crater.

Hokkaido-Komagatake

Hokkaido-Komagatake is an andesitic stratovolcano with a history of explosive eruptions and debris avalanches.

Major Hokkaido Calderas

Hokkaido contains several large calderas:

• Shikotsu
• Toya
• Kuttara
• Akan
• Mashu
• Kutcharo

Tōhoku Volcanoes

The mountainous backbone of northern Honshu contains a long series of active volcanoes.

Bandai

Bandai erupted catastrophically in 1888.

A steam-driven explosion destabilized part of the volcano, generating a debris avalanche that buried villages and dammed rivers.

The new lakes created by the avalanche became part of the modern Bandai-Asahi landscape.

Zao

Zao is a volcanic complex famous for the acidic Okama crater lake.

Potential hazards include:

• Phreatic explosions
• Ballistic blocks
• Ashfall
• Gas emissions
• Crater-lake disturbance

Azuma

The Azuma volcanic group contains active fumaroles, hydrothermal areas and crater systems west of Fukushima.

Adatara

Adatara includes an active hydrothermal crater known as Numano-taira.

A deadly gas incident in 1997 highlighted the danger of volcanic depressions where toxic gases can accumulate.

Akita-Komagatake

Akita-Komagatake is a basaltic-andesitic volcanic complex containing calderas and active cones.

Iwate

Mount Iwate is a large stratovolcano near Morioka.

Earthquake swarms and deformation during the late 1990s prompted intensive monitoring, although no eruption followed.

Central Japan Volcanoes

Kusatsu-Shirane

Kusatsu-Shirane is a volcanic complex containing crater lakes and active hydrothermal systems.

In January 2018, a sudden eruption occurred near the Motoshirane area, killing one person and injuring others at a ski resort.

The event showed that activity can begin at a vent not considered the principal focus of unrest.

Hakone

Hakone is a large caldera southwest of Tokyo.

Modern activity centers on the Owakudani geothermal area.

Earthquake swarms, steam emissions and small phreatic eruptions can disrupt tourism and transportation.

Yakedake

Yakedake is an active volcano in the Northern Japanese Alps.

Its steep terrain and popular hiking routes create a significant exposure risk.

Niigata-Yakeyama

Niigata-Yakeyama is a lava-dome-bearing stratovolcano capable of explosive eruptions and pyroclastic flows.

Norikura

Norikura is a broad volcanic complex in central Honshu containing numerous peaks and crater features.

Kyushu Volcanoes

Kyushu contains Japan’s highest concentration of large calderas and frequently active volcanic centers.

Kaimondake

Kaimondake is a symmetrical stratovolcano near the southern tip of the Satsuma Peninsula.

Its last major historic eruption occurred during the ninth century.

Ikeda Caldera

Lake Ikeda occupies a caldera associated with explosive eruptions in southern Kyushu.

Satsuma-Iojima

Satsuma-Iojima forms part of the rim of Kikai Caldera.

Its Iodake cone releases persistent volcanic gas and contains active fumaroles.

Yufu and Tsurumi

These volcanic systems rise near Beppu, one of Japan’s best-known geothermal and hot-spring regions.

The Izu–Ogasawara Volcanic Arc

The Izu–Ogasawara Arc stretches south from central Honshu into the western Pacific.

It includes inhabited volcanic islands, remote oceanic cones and submarine volcanoes.

Izu-Oshima

Izu-Oshima contains Mihara crater and lies relatively close to Tokyo.

Its 1986 eruption produced lava fountains and fissure eruptions, leading to the evacuation of the island.

Miyakejima

Miyakejima erupted in 2000.

Summit collapse formed a large caldera, and massive sulfur-dioxide emissions forced a prolonged evacuation.

Residents eventually returned, but gas masks and monitoring remained part of daily life.

Aogashima

Aogashima is a small inhabited island located inside a larger volcanic structure.

Historic eruptions during the eighteenth century caused deaths and evacuation.

Ioto

Ioto is an unusually active uplifted volcanic island.

Rapid ground deformation, hydrothermal activity and submarine eruptions occur around the island.

Its persistent uplift indicates continuing magma and fluid movement beneath the surface.

Japan’s Submarine Volcanoes and New Islands

Many Japanese volcanoes lie below sea level.

Submarine eruptions may initially be detected through:

• Discolored seawater
• Steam plumes
• Floating pumice
• Gas bubbles
• Satellite thermal anomalies
• New islands
• Reports from aircraft or ships

Nishinoshima

Nishinoshima is an active volcanic island in the Ogasawara chain.

Repeated eruptions since 2013 produced extensive lava flows and dramatically enlarged the island.

Its development allows scientists to observe:

• Island growth
• Coastal erosion
• New ecosystem formation
• Transition from submarine vent to stable island

Fukutoku-Okanoba

Fukutoku-Okanoba is a shallow submarine volcano south of Ioto.

Its 2021 eruption generated a towering plume, temporary land and enormous pumice rafts.

Pumice drifted across the western Pacific and affected ports, boats and coastal communities.

Bayonnaise Rocks

Bayonnaise Rocks is a submarine volcanic area south of Tokyo.

Activity has repeatedly produced discolored water and submarine eruptions.

Sumisujima

Sumisujima, also called Smith Rocks, is a mostly submarine caldera complex with steep exposed rocks.

Why Submarine Eruptions Can Be Violent

At shallow depth, seawater interacting with hot magma may flash into steam and fragment magma explosively.

At greater depth, water pressure suppresses rapid steam expansion.

Explore submarine volcanoes and seamounts →

Japan’s Great Calderas

Japan contains numerous calderas created by collapse after large explosive eruptions.

Are Japanese Calderas Supervolcanoes?

The term “supervolcano” is informal and often misleading.

Several Japanese calderas have produced extremely large eruptions, but each system has a different magma supply, eruption history and current state.

A caldera’s existence does not mean that another giant eruption is imminent.

Explore what scientists mean by supervolcano →

Major Historic and Prehistoric Eruptions in Japan

Explore the world’s most important historic eruptions →

Volcanic Hazards in Japan

Japan’s volcanoes produce nearly every major type of volcanic hazard.

Explosive Eruptions

Gas-rich magma can fragment violently and generate eruption columns, ashfall and ballistic ejecta.

Pyroclastic Flows

Pyroclastic flows are fast-moving currents of hot gas, ash and rock.

They may form through:

• Eruption-column collapse
• Lava-dome collapse
• Explosive destruction of a dome
• Dense overflow from a crater

Ballistic Blocks

Explosions can throw large blocks and volcanic bombs around the vent.

Ballistics were a primary cause of death during the 2014 Ontake eruption.

Lava Flows

Lava flows usually move more slowly than pyroclastic flows, but they can bury roads, homes, fields and infrastructure.

Lava Domes

Viscous magma may accumulate above a vent and form an unstable dome.

Dome collapse can generate pyroclastic flows, as demonstrated at Unzen.

Lahars

Rain, snowmelt or crater-lake water can mix with ash and debris to form volcanic mudflows.

Japan’s heavy rainfall, typhoons and seasonal snow increase lahar risk.

Learn how lahars form and travel →

Debris Avalanches

A volcanic flank may collapse because of earthquakes, magma intrusion, hydrothermal alteration or gravity.

Bandai and Unzen provide major Japanese examples.

Volcanic Gas

Volcanoes release sulfur dioxide, carbon dioxide, hydrogen sulfide and other gases.

Gas can threaten visitors and communities even when no major ash eruption occurs.

Crater-Lake Hazards

Crater lakes may conceal active hydrothermal systems.

Heating or magma-water interaction can trigger steam explosions, gas releases and lahars.

Volcanic Lightning

Colliding ash particles may separate electrical charge and generate lightning inside an eruption cloud.

Discover how volcanic lightning forms →

Aviation Hazards

Ash can damage jet engines, abrade cockpit windows and contaminate aircraft systems.

Japan’s busy airspace makes rapid ash advisories essential.

Read the complete guide to volcanic hazards →

Volcanic Ashfall and Urban Disruption in Japan

Volcanic ash consists of fragments of rock, minerals and volcanic glass smaller than two millimeters.

It is not soft fireplace ash.

Why Ash Is So Disruptive

Ash can:

• Reduce visibility
• Make roads slippery
• Stop rail services
• Close airports
• Damage engines
• Block ventilation systems
• Contaminate water
• Short-circuit electrical equipment
• Cover crops
• Cause respiratory irritation
• Accumulate heavily on roofs when wet

Sakurajima Ashfall

Kagoshima regularly experiences ashfall from Sakurajima.

Communities have developed routines for cleaning streets, protecting schools and disposing of ash.

Mount Fuji Ash Scenario

A major Fuji eruption could affect the Tokyo region without lava approaching the city.

Fine ash could disrupt transport, electricity, telecommunications, water treatment and supply chains across the most densely developed part of Japan.

Ash Cleanup

Ash should not simply be washed into drainage systems because it can harden, clog pipes and overwhelm treatment facilities.

Dry sweeping may also place particles back into the air.

Effective cleanup requires controlled collection, protective equipment and organized disposal.

Can Japanese Volcanoes Cause Tsunamis?

Yes. Japanese volcanic systems can generate tsunamis through several mechanisms:

• Volcanic flank collapse
• Submarine explosions
• Caldera collapse
• Underwater landslides
• Pyroclastic flows entering the sea
• Debris avalanches entering bays

The 1792 Unzen Tsunami

The collapse of Mayuyama entered the Ariake Sea and displaced a large volume of water.

The resulting tsunami struck both the Shimabara Peninsula and the opposite coast.

Submarine Volcanoes

Shallow underwater eruptions in the Izu–Ogasawara and Ryukyu arcs can disturb the sea locally.

Not every submarine eruption produces a tsunami. Large and rapid water displacement is normally required.

Explore volcanic tsunamis and island collapse →

How Japan Monitors Its Volcanoes

Japan operates one of the world’s most extensive volcano-observation networks.

The Japan Meteorological Agency coordinates national monitoring and volcanic warnings.

Other institutions include:

• Geological Survey of Japan
• National Research Institute for Earth Science and Disaster Resilience
• Geospatial Information Authority of Japan
• Universities and volcano observatories
• Local governments

Seismic Monitoring

Seismometers detect earthquakes caused by:

• Rock fracture
• Magma movement
• Gas movement
• Hydrothermal pressure
• Crater explosions

Scientists examine earthquake number, depth, location, waveform and migration.

Volcanic Tremor

Volcanic tremor is a sustained seismic vibration that may reflect moving fluids, gas release or continuous eruptive activity.

Ground Deformation

Inflation may indicate magma or pressurized fluids accumulating underground.

Monitoring methods include:

• GNSS and GPS
• Tiltmeters
• Strainmeters
• Leveling surveys
• Satellite radar interferometry

Gas Monitoring

Changes in sulfur dioxide, carbon dioxide and other gases can indicate changing magma supply or conduit conditions.

Thermal Monitoring

Infrared cameras and satellites detect changes in crater temperature, lava and fumarolic areas.

Webcams and Visual Observations

Cameras track plume height, crater glow, ash emissions and surface changes.

Satellite Monitoring

Satellites can detect:

• Ash clouds
• Sulfur-dioxide plumes
• Thermal anomalies
• Ground deformation
• New lava flows
• Submarine discolored water

Volcanic Ash Advisory Centers

The Tokyo Volcanic Ash Advisory Center issues aviation information for a large part of East Asia and the northwestern Pacific.

Can Eruptions Be Predicted Exactly?

No. Scientists can identify unrest and estimate likely scenarios, but they cannot reliably provide an exact eruption time, vent location and eruption size far in advance.

Explore volcano monitoring and eruption forecasting →

Japan’s Volcanic Alert Levels

Japan uses volcanic warnings and alert levels to communicate activity and recommended action.

The five-level system generally progresses from normal awareness to evacuation, although the exact areas and restrictions differ between volcanoes.

Not every active volcano uses exactly the same alert framework. Marine volcanoes and remote islands may use different warning formats.

Alert levels describe current hazard management. They do not guarantee that activity will rise sequentially from Level 1 to Level 5.

Living With Volcanoes in Japan

Volcanoes are hazards, but they also provide resources and landscapes central to Japanese life.

Hot Springs

Japan’s onsen culture depends on geothermal heat circulating groundwater through volcanic and tectonic systems.

Geothermal Energy

Volcanic regions contain significant geothermal resources.

Development must balance energy production with national parks, hot-spring businesses and environmental protection.

Fertile Soils

Weathered ash and volcanic deposits can create productive agricultural soils.

Tourism

Mount Fuji, Aso, Hakone, Kirishima, Hokkaido’s caldera lakes and many other volcanic landscapes attract millions of visitors.

Disaster Preparedness

Communities near active volcanoes use:

• Hazard maps
• Evacuation plans
• Public drills
• Emergency shelters
• Ashfall forecasts
• Road and trail closures
• School preparedness

Volcano Shelters

Some popular mountains have reinforced shelters designed to provide limited protection from ballistic projectiles and ash.

Shelters are not safe against every hazard, particularly large pyroclastic flows or lava.

Japanese Volcanoes in Culture, Religion and Folklore

Volcanoes are deeply embedded in Japanese spiritual and cultural traditions.

Mount Fuji

Fuji has long been regarded as sacred.

It is associated with:

• Shinto traditions
• Buddhist practice
• Mountain asceticism
• Pilgrimage
• Poetry
• Woodblock prints
• National identity

Volcano Shrines

Shrines near active volcanoes often reflect attempts to honor, understand or spiritually contain dangerous natural forces.

Onsen Culture

Hot springs have influenced settlement, healing traditions, inns and tourism throughout Japan.

Volcanoes as Both Sacred and Dangerous

Japanese cultural relationships with volcanoes do not treat them only as destructive objects.

They are mountains, watersheds, sacred sites, tourist destinations and sources of fertile land and hot water.

Where Could Japan’s Next Major Eruption Occur?

No scientist can identify the exact next volcano, date and eruption size.

Future activity is most likely at systems with persistent or repeated historical unrest.

These include:

• Sakurajima
• Suwanosejima
• Aso
• Kirishima
• Kuchinoerabujima
• Asama
• Kusatsu-Shirane
• Meakan
• Tokachidake
• Izu–Ogasawara submarine volcanoes

Mount Fuji Scenario

Fuji will erupt again eventually, but its long repose makes timing difficult.

A future eruption might involve:

• A new flank vent
• Lava flows
• Scoria eruption
• Ashfall
• Snowmelt lahars

Sakurajima Scenario

Frequent explosions are expected to continue, but larger lava-producing or sustained explosive activity remains possible.

Aso and Kirishima Scenario

Moderate crater eruptions, ashfall and gas releases are much more likely in the near term than giant caldera-forming events.

Submarine Eruption Scenario

New submarine activity could generate pumice rafts, temporary islands, ash plumes and maritime exclusion zones.

Japan’s Greatest Practical Risk

The most disruptive future event may not be the largest eruption geologically.

A moderate eruption in the wrong location and wind conditions could shut airports, stop railways, interrupt power and affect millions of people.