Mount Aso

Mount Aso (阿蘇山, Aso-san) or Aso Volcano is the largest active volcano in Japan and among the largest in the world. Located on the island of Kyushu, in Kumamoto Prefecture, it stands within the Aso Kujū National Park.^[1]

Mount Aso is situated in a fairly large caldera (a volcanic crater) with dimensions of approximately 25 kilometers (16 miles) north-south and 18 kilometers (11 miles) east-west. Its tallest peak, Takadake, reaches an impressive elevation of 1,592 meters (5,223 feet) above sea level.^[2]

Approximately 300,000 years ago, a significant eruption occurred, shaping the present-day Aso caldera+. This volcanic event led to the formation of a massive depression (caldera) as the volcanic chamber collapsed. Over time, four major eruptions (Aso 1 – 4) occurred between 300,000 and 90,000 years ago. During these eruptions, copious amounts of pyroclastic flow and volcanic ash were expelled.

The fourth eruption (Aso 4) stands out as the largest. Its volcanic ash covered not only the Kyushu region but also extended as far as Yamaguchi Prefecture. As a result of these cataclysmic events, several volcanic cones emerged within the caldera. Notable among them are Mt. Taka, Mt. Naka (which remains active today), Mt. Eboshi, and Mt. Kishima.^[3]

Aso’s pyroclastic flow deposits (known as welded tuff) found practical use in bridge construction. Kumamoto Prefecture boasts approximately 320 arched stone bridges, including the Tsujun-kyo and Reitai-kyo bridges spanning the Midorikawa River—each recognized as important national cultural properties.^[4]