Powerful phreatic explosion at Mount Asosan, Alert Level raised, Japan


A powerful phreatic explosion took place at Asosan’s Nakadake crater at 02:43 UTC (14:43 JST) on October 20, 2021, prompting the Japan Meteorological Agency (JMA) to raise the Alert Level from 2 to 3 at 02:48 UTC. The eruption came one week after volcanic tremors detected under the volcano suggested hot water and magma are moving underground.

While there were no reports of injuries, the eruption produced pyroclastic flows that streamed about 1.3 km (0.8 miles) west of the crater and ejected large volcanic rocks nearly 1 km (0.62 miles) to the south.1

Ash cloud reached a height of 3.5 km (11 500 feet) above the crater.


Local police have confirmed that 11 people who were climbing the volcano have come down and are safe.

JMA is urging people to refrain from entering the danger zone in Aso City, Takamori Town, and Minamioso Village.

Geological summary

The 24 km (14.9 miles) wide Asosan caldera was formed during four major explosive eruptions from 300 000 to 90 000 years ago. These produced voluminous pyroclastic flows that covered much of Kyushu.

The last of these, the Aso-4 eruption, produced more than 600 km3 (144 mi3) of airfall tephra and pyroclastic-flow deposits.

A group of 17 central cones was constructed in the middle of the caldera, one of which, Nakadake, is one of Japan’s most active volcanoes. It was the location of Japan’s first documented historical eruption in 553 AD.

The Nakadake complex has remained active throughout the Holocene. Several other cones have been active during the Holocene, including the Kometsuka scoria cone as recently as about 210 CE.

Historical eruptions have largely consisted of basaltic to basaltic-andesite ash emission with periodic strombolian and phreatomagmatic activity.

The summit crater of Nakadake is accessible by toll road and cable car, and is one of Kyushu’s most popular tourist destinations.2


1 Japan’s Moun Aso erupts, warning issued – NHK

2 Asosan – Geological summary – GVP

Featured image credit: Jiji Press (stillshot)


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  1. This volcano was already giving signs, but watching on the first camera of the news, there were many people nearby, they are flirting with danger, they need a class with the Spaniards to avoid predictable tragedies. In these cases there may not be a second chance.

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