Mountains · Kumamoto
Mount Aso
阿蘇山あそさんAso-sanA living volcano inside a caldera large enough to hold a city.
Mount Aso (阿蘇山) in Kumamoto Prefecture is a volcanic complex inside one of the largest calderas on Earth, about 25 km north to south and 18 km east to west. Its central cones – the Aso Five Peaks – rise to 1,592 m at Takadake, and the Nakadake crater is among Japan's most active vents.
Overview
Mount Aso is not one mountain but a landscape: a huge caldera whose outer rim forms a ring of grassy cliffs, and inside it a cluster of younger cones, the Aso Gogaku (Five Peaks) – Takadake, Nakadake, Nekodake, Eboshidake and Kishimadake. The towns of Aso and Minamiaso lie on the caldera floor, together with rice fields, hot springs and railway lines.
Nakadake's crater, emitting steam from an acidic lake, has been the focus of visits for more than a century and is one of the country's most closely watched volcanoes. From the outer rim – above all the Daikanbō viewpoint – the whole caldera and the smoking central cones can be seen at once, with the jagged Nekodake to the east.
Landscape and formation
The Aso caldera formed through four enormous eruptions between about 270,000 and 90,000 years ago. The last, Aso-4, spread pyroclastic flows across much of Kyūshū and even to Yamaguchi on Honshū, and its ash fell as far as Hokkaidō. Subsequent collapse created the basin, which was once filled by a lake before it drained through the Tateno gap in the west.
The central cones grew within the caldera afterwards. Nakadake remains active, with repeated ash eruptions and occasional explosive events; Nekodake is an older, deeply eroded cone. The grasslands of the rim and the springs of the caldera, fed by rain soaking through the volcanic deposits, support agriculture across central Kyūshū.
Highlights
Nakadake crater
中岳火口Nakadake kakōThe active crater with a steaming acid lake, reached by road when access is permitted.
Takadake
高岳TakadakeThe highest of the Five Peaks, 1,592 m, with views over the caldera.
Kusasenri
草千里ヶ浜Kusasenri-ga-hamaA grassy crater plain with a pond and grazing horses, below Eboshidake.
Daikanbō
大観峰DaikanbōThe classic viewpoint on the northern caldera rim, overlooking the Five Peaks.
Komezuka
米塚KomezukaA perfectly shaped small scoria cone, said to be a heap of rice given by the god of Aso.
Ecology
Aso's grasslands, among the largest in Japan, host plants and butterflies that are relics of the cooler, more open landscapes of the ice age. Their survival depends on the annual spring burning and on grazing by red Aso cattle.
Flora
Miyama-kirishima
ミヤマキリシマmiyama-kirishimaPink azaleas cover the upper slopes of Nakadake and Takadake in late May and early June.
Susuki grass
ススキsusukiSilver pampas grass dominates the rim grasslands in autumn.
Hana-shinobu
ハナシノブhanashinobuA blue Jacob's ladder confined to the Aso grasslands, critically endangered.
Fauna
Aso akaushi (red cattle)
あか牛akaushiBrown cattle grazed on the grasslands, part of the system that keeps them open.
Ōruri-shijimi
オオルリシジミōruri-shijimiA rare blue butterfly of the Aso grasslands.
Culture
Aso Jinja, the Higo Ichinomiya, venerates Takeiwatatsu-no-mikoto, the legendary grandson of the first emperor, who is said to have drained the caldera lake by kicking open the western rim. The hereditary priestly Aso family ruled the region in the Middle Ages. The crater itself was worshipped, and Buddhist temples once stood near Nakadake. The Aso grasslands and their agricultural system are recognised as a Globally Important Agricultural Heritage System.
Aso Jinja
阿蘇神社Aso JinjaOne of Kyūshū's oldest shrines, whose great gate, damaged in the 2016 Kumamoto earthquake, has been restored.
Aso Volcano Museum
阿蘇火山博物館Aso Kazan HakubutsukanA museum at Kusasenri with live camera views of the crater.
History
c. 90,000 years ago
The Aso-4 eruption completes the formation of the caldera.
553
Earliest recorded eruption in Japanese chronicles.
1934
Aso National Park is designated, one of the first in Japan.
2014
Aso is recognised as a UNESCO Global Geopark.
2016
The Kumamoto earthquakes cause landslides and destroy the Aso Ōhashi bridge; Nakadake erupts later that year.
Through the seasons
Spring
Controlled burning blackens the grasslands in March; fresh green follows in April and May, and azaleas bloom from late May.
Summer
Green grasslands and grazing cattle; afternoon thunderstorms and fog are common.
Autumn
Silver susuki grass waves across the rim; clear days in October and November.
Winter
Frost and occasional snow on the Five Peaks; the caldera often fills with morning fog, best seen from Daikanbō.
Routes and trails
Sensuikyō – Takadake
Up the steep Bakaone ridge from Sensuikyō, about 2.5 hours; moderate, rocky and exposed to wind.
Sunasenri – Takadake
From the Nakadake area across the volcanic desert of Sunasenri and along the crater rim, 1.5–2 hours; open only when the crater area is accessible.
Eboshidake and Kusasenri
A short climb from Kusasenri to Eboshidake, about 1 hour; easy.
Practical notes
- Aso station is on the JR Hōhi Line, about one and a half hours from Kumamoto; buses run to the crater area.
- Access to the Nakadake crater changes with volcanic gas and activity; people with asthma or heart conditions are advised not to approach. Check the Japan Meteorological Agency and local notices.
- Stay on trails in the grasslands; parts are private grazing land.
Frequently asked questions
Can visitors look into the Nakadake crater?
When volcanic activity and gas levels allow, visitors can approach the crater rim and see its steaming green acid lake. Access is often restricted, sometimes for long periods; the Japan Meteorological Agency and the local authorities publish current information.
How big is the Aso caldera?
About 25 km from north to south and 18 km from east to west, with a circumference of roughly 100 km. Towns, railways and rice fields lie on its floor, home to tens of thousands of people.
How long does it take to climb Takadake?
From Sensuikyō on the north-east side via the Bakaone ridge, about 2.5 hours up; from the Nakadake area via the Sunasenri plain, about 1.5–2 hours when that approach is open.
What are the Aso grasslands?
Vast semi-natural grasslands on the caldera rim and slopes, kept open for more than a thousand years by grazing, mowing and controlled burning (noyaki) each spring.
Related
Sources
- 1.日本の主な山岳標高 (opens in a new tab) — 国土地理院 (Geospatial Information Authority of Japan)
- 2.阿蘇くじゅう国立公園 (opens in a new tab) — 環境省 (Ministry of the Environment)
- 3.火山登山者向けの情報提供ページ（阿蘇山） (opens in a new tab) — 気象庁 (Japan Meteorological Agency)
- 4.日本ジオパークネットワーク (opens in a new tab) — 日本ジオパークネットワーク (Japanese Geoparks Network)