Mount Aso

Aso is really two things at once. The caldera is enormous, roughly twenty-five kilometres across, and it is not empty: towns, rice fields, roads and a railway line sit on its floor, ringed by the old crater wall, which is the clearest way to grasp the scale. Inside it stands a group of newer cones, and one of them, Nakadake, is continuously active, with a milky green pool of acid water and a plume of sulphur dioxide rising straight out of a hole you can walk to the edge of. Around them lie Kusasenri, a grassy bowl with a shallow pond and grazing horses, and the small ash cone of Komezuka.

History

The caldera formed in four great eruptions, Aso 1 to 4, between about 300,000 and 90,000 years ago, when the emptied magma chamber collapsed; Mount Naka, one of the cones that grew afterwards, is still active, and a new eruption began on 20 October 2021. The welded tuff of Aso's pyroclastic flows was used for some 320 arched stone bridges in Kumamoto Prefecture (Wikipedia, Mount Aso).

What's there

• Nakadake crater rim - the active vent and its acid pool, when gas levels allow access.
• The caldera wall - viewpoints on the rim showing towns and fields on the crater floor.
• Kusasenri - a grassland bowl with a shallow pond and horses below the cones.
• Komezuka - a small, neatly formed ash cone with a dip in its top.

When to go

Spring green and autumn give the best conditions; the grasslands are burned off in early spring, which changes the look completely. Summer is humid and often clouded in. Winter can ice the crater road. Always check the day's alert level and gas readings before setting out, because access to the rim is decided at short notice.

Good to know

The crater closes without warning when sulphur dioxide rises, sometimes for weeks, and there is no compensation for a wasted trip; people with asthma or heart conditions are turned away even when it is open. The caldera floor is ordinary countryside rather than a spectacle. Weather shuts the upper area regularly, and public transport up the mountain is limited.

Nearby

The hot spring valleys of Kurokawa and Yufuin to the north, and Kumamoto city to the west.

Key facts