Mount Aso (Asosan)

Japan's Largest Active Caldera — A City Inside a Volcano

Elevation

1,592 m

Last Eruption

2021

Type

Caldera

Country

Japan

Location

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Volcanic Hazards & Risk Assessment

Primary Hazards

Pyroclastic flows and surges
Large explosive eruptions (VEI 4+)
Ash fall and tephra deposits
Lahars and debris flows

Risk Level

Population at RiskHigh

Infrastructure RiskHigh

Aviation RiskSignificant

Geological Composition & Structure

Rock Types

Primary

Unknown

Silica Content

Varied composition

Tectonic Setting

Unknown

Intraplate setting with hotspot or regional volcanic activity.

Age & Formation

Epoch

Unknown

Evidence

Unknown

Eruption Statistics & Analysis

Metric	Value	Global Ranking	Significance
Total Recorded Eruptions	Unknown	Low	Moderately active volcano
Maximum VEI	VEI Unknown	Minor	Local impact potential
Recent Activity	5 years ago	Very Recent	Currently active

Monitoring & Alert Status

Monitoring Networks

Japan Meteorological Agency (JMA)

Real-time seismic monitoring

Geological Survey of Japan

Geochemical monitoring

Global Volcanism Program

International eruption database

Current Status

Active

Recent volcanic activity detected. Continuous monitoring in place.

Authority Sources

Smithsonian GVP

Official volcano database

JMA Volcano Info

Japan Meteorological Agency

Related Volcanoes

Other Volcanoes in Japan

Sakurajima
Caldera with post-caldera stratovolcano
Asamayama
Complex volcano
Mount Fuji
Stratovolcano
Izu-Oshima
Stratovolcano

Interesting Facts

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The Aso-4 eruption approximately 90,000 years ago produced over 600 km³ of pyroclastic deposits — enough material to bury the entire island of Manhattan under more than 20 km of volcanic debris.

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Approximately 50,000 people live inside the Aso caldera, making it one of the only places on Earth where a sizable city exists within the structure of an active volcano.

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Aso's 187 recorded eruptions make it one of the most frequently erupting volcanoes in the world — averaging approximately one eruption every 54 years across 10,000 years, but far more frequent in historical times.

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The eruption of 553 CE at Nakadake was the first volcanic eruption ever documented in Japanese written records.

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Nakadake's crater lake (Yudamari) has a pH of approximately 1 — roughly as acidic as concentrated hydrochloric acid — making it one of the most acidic natural water bodies on Earth.

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The 2016 eruption at Nakadake sent an eruption column 11 km high — the tallest observed at Aso in modern times — and was followed just days later by the devastating M7.0 Kumamoto earthquake sequence.

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The Aso caldera has a circumference of approximately 128 km and an area of roughly 350 km² — large enough to contain a small city, farms, rail lines, and an airport.

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A 2014 Kobe University study estimated that a repeat of the Aso-4 eruption could deposit lethal pyroclastic flows across an area home to approximately 7 million people within two hours.

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Concrete gas shelters line the paths near Nakadake crater, providing emergency refuge for tourists in the event of sudden phreatic eruptions or toxic gas surges.

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The Aso Shrine, one of Japan's oldest Shinto shrines, has stood within the caldera since at least the 6th century CE — it was severely damaged by the 2016 Kumamoto earthquakes and has since been restored.

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Six tourists were killed and approximately 90 injured in the 1953 phreatomagmatic eruption at Nakadake, prompting Japan's first modern volcanic hazard management protocols for tourist access.

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The Kusasenri grasslands inside the caldera are maintained by annual controlled burning (noyaki), a centuries-old practice that creates the pastoral landscape iconic in Japanese images of Kyushu.

Frequently Asked Questions

Is Mount Aso still active?

Yes, Mount Aso is one of the most active volcanoes in Japan. Nakadake, the most active of the 17 central cones within the Aso caldera, has produced 187 recorded eruptions spanning approximately 10,000 years, with the most recent occurring in October 2021. The volcano has erupted at least once in nearly every century since the 6th century CE. The Japan Meteorological Agency monitors Aso continuously, and the volcanic alert level is regularly adjusted based on seismic, gas, and crater lake observations. Future eruptions at Nakadake are considered virtually certain, though predicting the timing and magnitude remains challenging.

When did Mount Aso last erupt?

Mount Aso's most recent eruption occurred on October 14, 2021, when the Nakadake crater produced an explosive eruption generating an ash column approximately 3,500 m high. The eruption deposited ash on nearby communities and produced pyroclastic flows that traveled about 1.6 km from the crater, though they remained within the restricted access zone. No casualties were reported. Prior to this, a more significant VEI 3 eruption occurred on October 7–8, 2016, producing an 11 km eruption column and causing substantial damage to infrastructure near the crater.

How big is the Aso caldera?

The Aso caldera is one of the largest volcanic calderas in the world, measuring approximately 25 km north-south by 18 km east-west, with a circumference of roughly 128 km and an area of about 350 km². The caldera was formed by four major eruptions between 300,000 and 90,000 years ago. For scale, the caldera is large enough to contain a small city — approximately 50,000 people live inside it, along with farms, roads, rail lines, hot spring resorts, and Aso Kumamoto Airport just outside the western wall. The caldera is significantly larger than famous examples like Crater Lake in Oregon (8 × 10 km) or Santorini in Greece (11 × 7 km).

Can you visit the Aso crater?

Yes, when volcanic conditions permit. The Nakadake crater is accessible by a toll road from the Aso area, with a shuttle bus taking visitors to near the crater rim. At JMA Alert Level 1, visitors can walk to the viewing platforms and see the turquoise-green Yudamari crater lake. Access is restricted at Alert Level 2 (1 km exclusion zone) and closed entirely at Level 3 (2 km exclusion zone). Even at Level 1, the crater may be closed when SO₂ gas concentrations exceed safe limits. Concrete gas shelters are positioned along the paths for emergency protection. Entry fees are approximately JPY 1,200 for the shuttle bus roundtrip.

What type of volcano is Aso?

Aso is classified as a caldera volcano — a large volcanic depression formed by the collapse of a magma chamber after massive eruptions. The 24-km-wide caldera was formed by four caldera-forming eruptions between 300,000 and 90,000 years ago. Within the caldera, 17 post-caldera cones have been built, the most active being Nakadake. This makes Aso a complex volcanic system: a caldera with active stratovolcanic cones inside it. The dominant rock types are andesite and basaltic andesite, produced by subduction of the Philippine Sea Plate beneath the Eurasian Plate. Aso's eruption style at Nakadake ranges from Strombolian to phreatomagmatic explosions.

Could the Aso-4 supereruption happen again?

While a caldera-forming eruption of the Aso-4 scale (600+ km³ of pyroclastic material) is theoretically possible, volcanologists consider such an event extremely unlikely on human timescales — the last caldera-forming eruption at Aso occurred approximately 90,000 years ago, and the current state of the magmatic system shows no signs of accumulating the enormous magma volume necessary for such an eruption. However, the possibility cannot be entirely ruled out, and a 2014 Kobe University study highlighted that a repeat event could affect approximately 7 million people with lethal pyroclastic flows. Japan's national disaster preparedness plans include contingencies for large-scale volcanic events, though the primary concern at Aso remains the more frequent small-to-moderate eruptions at Nakadake.

How many people died in Aso eruptions?

The best-documented fatal eruption at Aso occurred on April 27, 1953, when a phreatomagmatic eruption at Nakadake killed 6 tourists and injured approximately 90 others who were visiting the crater rim. Several earlier eruptions, including the 1872–1873 event, also caused fatalities, though historical records are less precise about death tolls. No deaths have been reported from the 2016 or 2021 eruptions, partly due to improved monitoring and alert systems. The primary risk of future fatalities comes from sudden phreatic eruptions at the heavily visited Nakadake crater, where thousands of tourists may be present on busy days.

Why do people live inside the Aso caldera?

Approximately 50,000 people live inside the Aso caldera primarily because of the exceptionally fertile volcanic soils that support productive agriculture, the geothermal hot springs that attract tourism, and centuries of cultural attachment to the landscape. The caldera floor is relatively flat and sheltered, making it suitable for settlement. Communities have occupied the caldera since ancient times, and the Aso Shrine — one of Japan's oldest — has been located there since at least the 6th century CE. While the hazard from Nakadake's eruptions is real, the frequency of eruptions is well-known and managed through Japan's advanced monitoring and warning systems, and the caldera-forming eruptions that created the depression are extremely rare events.

What is the difference between Aso and Nakadake?

Aso (Asosan) refers to the entire volcanic complex, including the 24-km-wide caldera and all 17 central cones within it. Nakadake is one specific cone — the most active — located within the Aso caldera. All historical eruptions since 553 CE have occurred at Nakadake, which has a summit crater hosting the acidic Yudamari crater lake. When media reports say 'Mount Aso erupts,' they are almost always referring to activity at the Nakadake cone specifically. The Smithsonian Global Volcanism Program catalogs all eruptions under the single entry 'Asosan' (volcano number 282110), as the various cones share the same underlying magmatic system.

How does Aso compare to Yellowstone?

Both Aso and Yellowstone are caldera systems capable of supereruptions, but they differ significantly in scale and current activity. Yellowstone's caldera (~72 km wide) is about three times larger than Aso's (~24 km), and Yellowstone's last supereruption (~640,000 years ago) was more recent than Aso's Aso-4 event (~90,000 years ago). However, Aso is far more frequently active — Nakadake has erupted 187 times in approximately 10,000 years, while Yellowstone has not erupted in approximately 70,000 years. Aso has 50,000 residents inside its caldera; Yellowstone has none (it is a national park). Yellowstone is considered a dormant supervolcano, while Aso is an active caldera with a persistently erupting central cone.