🌋VolcanoAtlas

Zaozan

Japan's Five-Colored Crater Lake Volcano

Elevation

1,841 m

Last Eruption

1940

Type

Complex volcano

Country

Japan

Location

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Volcanic Hazards & Risk Assessment

Primary Hazards

  • Lava flows and fountaining
  • Volcanic gas emissions
  • Local explosive activity

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

MetricValueGlobal RankingSignificance
Total Recorded EruptionsUnknownLowModerately active volcano
Maximum VEIVEI UnknownMinorLocal impact potential
Recent Activity86 years agoHistoricalRecently 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

Normal
No recent activity. Routine monitoring continues.

Authority Sources

Smithsonian GVP
Official volcano database
JMA Volcano Info
Japan Meteorological Agency

Related Volcanoes

All Japan Volcanoes
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Complex volcano Volcanoes
By volcano type
Northern Honshu (Tohoku)
Regional volcanoes

Other Volcanoes in Japan

Interesting Facts

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Zaozan's Okama crater lake has a pH of approximately 1.3 — among the most acidic natural water bodies in Japan and comparable to concentrated hydrochloric acid.

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The lake earned the nickname 'Goshikinuma' (Five-Colored Lake) because its color shifts between emerald green, turquoise, and milky blue depending on light conditions and dissolved mineral chemistry.

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Zaozan has produced 51 recorded eruptions over 9,600 years — one of the longest eruption records of any volcano in Japan, documented since the 8th century CE.

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The famous 'juhyo' (snow monsters) on Zao's slopes are Maries' fir trees completely encased in rime ice and snow, forming bizarre sculptural shapes found at very few locations worldwide.

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The Zao Onsen hot springs have been used for bathing for reportedly over 1,900 years, heated by the same volcanic hydrothermal system that powers the eruptions.

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Despite not erupting since 1940, Zaozan has experienced multiple episodes of heightened volcanic unrest including seismic swarms, new fumarolic areas, and changes in Okama lake chemistry.

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The 85+ year repose since 1940 is the longest in Zaozan's 1,100+ year documented historical record — a statistical anomaly that has prompted scientific discussion about future eruption potential.

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Zaozan's name derives from the Buddhist deity Zao Gongen, reflecting centuries of mountain ascetic (shugendo) practice on its slopes.

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The 2014 phreatic eruption of Mount Ontake, which killed 63 people, prompted nationwide upgrades to volcanic monitoring at similar tourist-destination volcanoes including Zaozan.

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Zaozan's volcanic complex is built on granitic basement rock, giving it a relatively small total volume (~7 km3 for the central complex) compared to volcanoes built on softer substrates.

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In the 19th century alone, Zaozan erupted 14 times — nearly once every 7 years — compared to zero eruptions in the 85+ years since 1940.

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The Zao Echo Line scenic road, which provides vehicular access to Okama crater lake viewpoints, is one of the most popular mountain driving routes in Tohoku.

Frequently Asked Questions

Is Zaozan (Mount Zao) an active volcano?
Yes, Zaozan is classified as an active volcano by the Japan Meteorological Agency (JMA). Although it has not erupted since 1940, it has produced 51 eruptions over approximately 9,600 years and shows ongoing signs of volcanic activity including fumarolic emissions, an intensely acidic crater lake (pH ~1.3), periodic seismic swarms, and ground deformation. JMA monitors the volcano continuously with seismometers, GPS, gas sensors, and webcams. The agency has raised the volcanic alert level on several occasions in recent decades in response to increased seismic activity and thermal changes, underscoring that Zaozan remains a volcano capable of eruption.
When did Zaozan last erupt?
Zaozan's most recent eruption occurred in 1940 — a minor VEI 1 phreatic (steam-driven) event from the Okama crater. This makes the current repose period over 85 years, the longest gap between eruptions in the volcano's 1,100+ year documented historical record. Before 1940, eruptions occurred in 1927 (uncertain) and 1905. The 19th century was far more active, with 14 eruptions recorded. The extended quiet period does not mean Zaozan is becoming dormant — fumarolic activity and periodic unrest indicate the volcanic system remains capable of erupting.
What is the Okama crater lake?
Okama is a strikingly colorful crater lake within the Goshikidake tuff cone at the summit of Zaozan, at approximately 1,670 m elevation. The lake is roughly 350 m in diameter and 60 m deep, with intensely acidic water (pH ~1.3) sustained by volcanic gas input from fumaroles beneath and around the lake. Its waters shift color between emerald green, turquoise, and milky blue depending on light conditions, dissolved mineral concentrations, and sulfur content — earning the nickname 'Goshikinuma' (Five-Colored Lake). It is one of the most photographed natural features in northern Japan and the primary summer tourist attraction at Zao.
Is it safe to visit Zaozan?
Zaozan is generally safe to visit and attracts hundreds of thousands of visitors annually to its ski resorts, onsen, and crater lake viewpoints. However, it is an active volcano, and JMA may restrict access to the Okama crater area during periods of elevated volcanic unrest. The 2014 phreatic eruption of Mount Ontake, which killed 63 hikers at a similar tourist-destination volcano, demonstrated the potential for sudden eruptions without significant precursory warning. Visitors should check JMA's current volcanic alert level before visiting the crater area, stay on designated trails, and heed any access restrictions. The ski resorts and onsen towns at lower elevations are at minimal risk during typical eruptions.
What are the snow monsters at Zao?
The 'snow monsters' (juhyo in Japanese) are a natural phenomenon unique to a handful of locations in Japan, with Zao being the most famous. They form when supercooled fog and wind-driven snow coat Maries' fir (Abies mariesii) trees on the upper mountain slopes, building thick layers of rime ice and compacted snow that completely encase the trees in bizarre, sculptural shapes — some resembling monsters, animals, or abstract figures. The juhyo typically form between January and March above approximately 1,200 m elevation. They are a major winter attraction, viewable from the ski slopes and ropeway, with night-time illumination tours a signature experience.
How do I get to Zaozan from Tokyo?
The most convenient route is via the Tohoku Shinkansen from Tokyo Station to either Yamagata Station (approximately 2.5 hours) or Shiroishi-Zao Station (approximately 2 hours). From Yamagata, buses run to Zao Onsen ski resort (approximately 40 minutes). In summer, the Zao Echo Line scenic road provides access to Okama crater lake viewpoints. From Sendai (approximately 1 hour by car), the approach is via the Zao Echo Line from the east. The ski resort area has hotels, ryokan, and onsen accommodations. Year-round ropeway service provides upper mountain access.
Could Zaozan have a dangerous eruption?
Yes. While Zaozan's typical eruptions are small phreatic events (VEI 1-2), the geological record includes a VEI 4 eruption around 300 CE that was significantly more powerful. Even smaller phreatic eruptions pose genuine danger to visitors near the crater — the 2014 Mount Ontake disaster showed that sudden phreatic explosions can generate lethal ballistic projectiles and surges without significant warning. Lahars are another concern: an eruption displacing the acidic Okama lake water could generate mudflows. The combination of active volcanic system, large visitor numbers, and potential for rapid-onset phreatic events makes Zaozan a volcano that requires ongoing vigilance despite its peaceful recent history.
What type of eruptions does Zaozan produce?
Zaozan's historical eruptions have been predominantly phreatic (steam-driven) — caused by volcanic heat boiling groundwater, which then explodes to the surface carrying rock fragments, ash, and steam. Most have been VEI 1-2, producing relatively small eruption columns, ballistic projectiles, and minor ashfall. The eruptions originate from the Okama crater within the Goshikidake tuff cone. However, the VEI 4 eruption of ~300 CE likely involved a magmatic component (fresh magma reaching the surface), demonstrating that Zaozan is capable of more explosive activity. Scientists monitor for signs that future eruptions might involve magma rather than just heated groundwater, as magmatic eruptions are typically more powerful and hazardous.