Volcano profile · Kyushu, Japan
Aso
Nakadake & the Aso caldera
A vast caldera containing a landscape of central cones — with Nakadake providing the restless modern focus through crater-lake changes, gas, ash and explosive eruptions.
Latest activity and alert levelCheck the live status hub
Aso is continuously monitored by the Japan Meteorological Agency. Use the Volcoholics directory for the latest verified operational status.
View latest Aso status →A volcano on an enormous scale
Aso is not one mountain — it is an entire volcanic landscape
Aso’s caldera measures roughly 25 kilometres north–south by 18 kilometres east–west and contains a cluster of central volcanoes.
The caldera was created by four gigantic pyroclastic-flow eruptions between roughly 270,000 and 90,000 years ago. Volcanism then continued inside it, constructing numerous post-caldera cones.
Today the active focus is Nakadake. Its crater system has produced frequent explosive activity over historical time, including ash eruptions and phreatic or phreatomagmatic explosions.
25 × 18 kmApprox. caldera size
NakadakeActive central cone
~1,506 mNakadake elevation
JMAContinuous monitoring
Aso quick facts
- Setting
- Kyushu, Japan
- System
- Large caldera
- Active focus
- Nakadake No. 1
- Crater feature
- Hyperacidic lake
- Official monitor
- JMA
Activity and behaviour
A restless crater inside a much older giant
Evergreen profile
How modern Aso behaves
Nakadake No. 1 crater is the principal modern vent. During quieter periods it can contain a highly acidic crater lake; active periods can involve strong degassing, ash emission, Strombolian activity and steam- or water-influenced explosions.
Check the latest verified status →Active ventNakadake No. 1Northernmost of the aligned summit craterlets
Quiet-period featureCrater lakeA hyperacidic lake can occupy the active crater
Explosive processPhreatic activityHeated water and rock can be violently fragmented
Eruption productAshExplosive episodes can spread ash well beyond the crater
Near-crater hazardBallistics & PDCsExplosions can affect areas close to Nakadake rapidly
Water changes the story
A crater lake can hide a highly energetic volcanic system
At Nakadake, volcanic heat and gas interact with groundwater and crater water. That means explosive activity does not always require fresh molten magma to be erupted: rapidly heated water can fragment altered rock and old crater deposits.
Volcanic architecture
The modern cones grew inside the scar of colossal ancient eruptions
Aso’s huge depression formed through repeated caldera-forming pyroclastic eruptions. After the last of these, volcanism resumed within the caldera and constructed a cluster of central cones.
GSJ identifies at least 17 independent volcanoes in the central cluster, although earlier centres may have been buried by younger deposits. Nakadake is the presently active member.
Aso calderaAbout 25 km north–south and 18 km east–west.
Central conesA diverse post-caldera volcanic cluster.
NakadakeThe active modern eruptive focus.
Mixed magma typesPost-caldera products range broadly from basaltic to silicic compositions.
How Nakadake works
Magma, gas and water meet beneath an unusually dynamic crater
Nakadake is built mainly from basaltic-andesite and andesite volcanic material. Gas and heat rising from depth interact with a hydrothermal system and, during quieter phases, the crater lake.
The result can range from persistent degassing to ash-rich explosions. Studies of the 2021 ejecta found abundant hydrothermally altered rock and very little fresh volcanic glass, highlighting the important role of the wet crater environment in that eruption.
Monitoring
Watching a crater where water, gas and seismicity all matter
JMA continuously monitors Aso, while Japanese research organisations study its geology and eruptive products. No single measurement tells the whole story, so changes are interpreted across several datasets.
A volcano through time
Aso eruption timeline
Caldera era~270–90 ka
Four gigantic pyroclastic-flow eruptions
The Aso-1 to Aso-4 eruptive sequence built and repeatedly transformed the enormous caldera system.
After ~90 ka
Central cones grow inside the caldera
Post-caldera volcanism constructed numerous new volcanic centres, including the Nakadake system.
AD 553
Oldest Japanese written description of volcanic activity
GSJ records an Aso eruption account in the Tsukushi Fudoki, beginning an exceptionally long historical record.
2014–15
Renewed magmatic eruptive activity
Activity from late 2014 into 2015 included eruption of hot magma and frequent ash emissions.
2015–16
Larger explosive episodes
Explosions in September 2015 and October 2016 produced pyroclastic flows around the crater and widespread ash.
Recent reference event2021
Explosive eruption at Nakadake No. 1 crater
The 20 October eruption generated pyroclastic flows and ashfall; field analysis highlighted abundant hydrothermally altered crater material.
Hazards
The hazards that matter most at Aso
Explosions can throw blocks around the active crater with little time to react.
Hot ash and gas can move rapidly across areas around Nakadake.
Ash can affect communities, transport, agriculture and aviation downwind.
Strong emissions can create hazardous concentrations near the crater.
Steam-driven events can occur through interaction with the hydrothermal system.
Aso’s ancient record demonstrates the system’s capacity for events vastly larger than its typical historical activity.
Myths versus reality
Aso’s giant caldera does not mean every eruption is giant
The caldera is a broad inhabited landscape containing valleys, towns and a cluster of younger central volcanoes.
Nakadake’s lake sits within an active hydrothermal and magmatic system and can change as heat and gas conditions change.
The modern historical record is dominated by much smaller activity centred on Nakadake.
Volcoholics insight
Aso is a lesson in volcanic scale
Stand beside Nakadake and the active crater feels enormous. Zoom out, though, and it becomes only one vent inside a chain of cones, themselves sitting inside the footprint of eruptions so large they reshaped an entire region. Few volcanoes show the difference between everyday activity and deep geological history quite as clearly as Aso.
Questions answered
Aso explained
Is Nakadake the same as Aso?
Nakadake is the currently active central cone within the much larger Aso volcanic system and caldera.
How large is Aso caldera?
It is approximately 25 kilometres north–south and 18 kilometres east–west.
Why does Nakadake sometimes have a crater lake?
During calmer periods water can occupy the active crater, interacting with the volcano’s heat, gases and hydrothermal system.
What happened at Aso in 2021?
An explosive eruption at Nakadake No. 1 crater on 20 October produced pyroclastic flows and ashfall.
Where can I find Aso’s current alert level?
Use the Volcoholics volcano directory for the latest verified summary and JMA for official operational information.
Official science, made readable
Built from the agencies studying Aso
This evergreen profile uses Japanese official monitoring and geological references, while deliberately keeping today’s changing alert level separate from the permanent profile.
Japan Meteorological Agency (JMA)Continuous volcano monitoring, official activity information and the national catalogue of active volcanoes.
Geological Survey of Japan, AISTAso geological mapping, caldera history, eruptive record and field analysis of recent ejecta.