When Was the Last Time Japan's Mount Aso Erupted and How Active Is the Volcano?

Mount Aso, located on the island of Kyushu in southwest Japan, is one of the world's largest volcanoes.

According to the Japan Meteorological Agency (JMA), Mount Aso comprises the Aso caldera and post-caldera central cones.

A caldera is described as a circular depression in the Earth that's larger than an ordinary crater, the Aso Volcano Disaster Prevention Council website explains.

Spanning around 15.5 miles north-south and 11.1 miles east-west in diameter, the Aso caldera is one of the world's largest collapsed volcanoes, according to the Aso UNESCO Global Geopark, which lies within the Aso Kuju National Park, home to Mount Aso.

According to the Aso UNESCO Global Geopark, there is no freestanding mountain known as Mount Aso, but experts refer to the entire area up to the caldera's outer rim as Aso Volcano.

The JMA explains the Aso caldera was formed by four "gigantic pyroclastic-flow eruptions" from around 90,000 to 270,000 years ago, while post-caldera central cones were formed soon after then.

The Nakadake Volcano is one of Japan's most active volcanoes. It is the only active central cone of Mount Aso and only its northernmost crater (No. 1 crater) has been active in the past 80 years.

The Nakadake No. 1 crater is occupied by a hyperacidic crater lake during its calm periods. It is characterized by ash as well as "strombolian eruptions and phreatic or phreatomagmatic explosions" during active periods, the JMA says.

People near Mount Aso in Japan.
People walk near Mount Aso, an active volcano on Japan's on Kyushu island, pictured on December 14, 2019. CHARLY TRIBALLEAU/AFP via Getty Images

When Did Japan's Mount Aso Last Erupt?

The JMA said the volcano erupted on Wednesday, October 20, at 11:43 a.m. local time at the No. 1 Nakadake crater.

The eruption prompted four people to walk off the volcano, which is a popular hiking destination.

Japan's Chief Cabinet Secretary Hirokazu Matsuno told reporters that no injuries have been reported so far. Local authorities are working to establish whether there are any climbers left on the mountain.

A level 3 volcanic activity alert, which is the third-highest level on the five-tiered scale, was issued by the JMA. The alert applies to the city of Aso, the town of Takamori as well as the village of Minami-Aso. Those near the volcano were warned not to approach the area.

The JMA reported volcanic ash had blown more than around half a mile from the crater and reached a height of about 11,500 feet.

The latest eruption follows a small-scale one that occurred Thursday.

How Active Is the Aso Volcano?

The Aso Volcano has been erupting sporadically for decades, the NASA Earth Observatory website explains.

According to the Smithsonian Institution's Global Volcanism Program, there have been 38 separate eruptions since 1950, with the most recent beginning on December 8, 2014. All of the eruptions recorded by the program took place at Nakadake Volcano.

According to NASA, the Nakadake crater has periodically produced "small explosive eruptions" since 2003. In late November 2014, the volcano was reported to be "producing a steady ash plume," NASA said at the time.

When Mount Aso erupted in November 2014, it had last blown up 22 years earlier.

Mount Aso's Nakadake crater.
A view of Mount Aso, with the green area being the Nakadake crater area, pictured on January 15, 2018 in Japan. Zhizhao Wu/Getty Images