Mount Aso

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For the city in Kumamoto Prefecture, please see Aso, Kumamoto.

The steaming crater of Mt. Naka

Mount 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 Kyūshū. Its peak is 1592 m above sea level. Its caldera is 25 km across north to south and 18 km across east to west. The caldera has a circumference of around 120 km (75 miles), although sources vary on the exact distance.

[edit] The Range of Mt. Aso

The central cone group of Aso consists of five peaks: Mt. Neko, Mt. Taka, Mt. Naka, Mt. Eboshi, and Mt. Kishima. The highest point is the 1592 m 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 has occasional eruptions.

The present Aso caldera formed as a result of four huge caldera eruptions occurring 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 18 km east to west and about 25 km 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.

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

[edit] History

The far edge of the Aso caldera.

The eruption which formed the present somma occurred approximately 300,000 years ago.

Four large-scale eruptions (Aso 1 – 4) occurred 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 fourth eruption (Aso 4) was the largest, with volcanic ash covering the entire Kyūshū region and even extending to Yamaguchi Prefecture.

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