Mt. Aso (阿蘇山: Aso-san) is the largest active volcano in Japan, and is among the largest in the world. It stands in Kumamoto Prefecture, on the island of Kyushu. Its peak is 1592 m above sea level. Its caldera is 25km across north to south and 18km across east to west. The caldera has a circumference of around 120 km (75 miles), although sources vary on the exact distance.

The Range of Mt. Aso

The central cone group of Aso The central cone group consists of five mountains: Mt. Neko, Mt. Taka, Mt. Naka, Mt. Eboshi, and Mt. Kishima. The highest point is the 1592m of Mt. Taka. The crater of Mt. Naka, the west side of which is accessible by road, contains an active volcano which continuously emits smoke and occasional eruptions.

Aso as a Caldera Volcano The present Aso caldera formed as a result of four huge caldera eruptions occuring over a range of 30 - 90,000 years ago. The caldera, one of the largest in the world, contains the city of Aso as well as Aso Takamori-cho and South Aso-mura. The somma enclosing the caldera extends about 18km east to west and about 25km north to south. Viewpoints from the somma overlooking the caldera are perched upon lava formed prior to the volcanic activity which created the present caldera.

The Range of Pyroclastic Flow Plateau Ejecta from the huge caldera eruption 90,000 years ago covers more than 600km³ and roughly equals the volume of Mt. Fuji; it is presumed that the pyroclastic flow covered half of Kyushu.

History

Aso volcano group The eruption which formed the present somma occured approximately 300,000 years ago. Huge Caldera Eruption Four large-scale eruptions (Aso 1~4) occured during a period extending from 300,000 to 90,000 years ago. As large amounts of pyroclastic flow and volcanic ash were emitted from the volcanic chamber, a huge depression (caldera) was formed as the chamber collapsed. The 4th eruption (Aso4) was the largest, with volcanic ash covering the entire Kyushu region and even extinding to Yamaguchi Prefecture.

Central Cone Group Mt. Taka, Mt. Naka, Mt. Eboshi, and Mt. Kishima are cones formed following the 4th above-mentioned huge caldera eruption. Mt. Naka remains active today. It is presumed that Mt. Neko is older than the 4th huge caldera eruption.