Kujusan

Geography & Climate

Kujusan is located in Japan, within the Nankai Volcanic Arc of the broader Western Pacific Volcanic Regions. Situated at 33.09° N, 131.25° E in the Northern Hemisphere, the volcano lies within a subtropical climate zone. With a summit elevation of 1,791 meters above sea level, Kujusan is a moderately sized peak that remains accessible to hikers and researchers for much of the year. The volcanic landform is characterized as a composite, which describes the physical shape and structure of the volcanic edifice as observed from the surface.

Geological Context

Kujusan sits in a subduction zone, where one tectonic plate dives beneath another, creating intense heat and pressure that generates magma. Subduction zones are responsible for many of the world's most explosive volcanoes and deadliest eruptions. For communities in Japan near Kujusan, this tectonic setting means the volcano is capable of producing powerful explosive eruptions, pyroclastic flows, and lahars that can threaten populated areas within tens of kilometers of the summit. The dominant rock type is andesite / basaltic andesite, a dark, fine-grained volcanic rock that forms from rapidly cooling, low-viscosity lava. Basaltic eruptions tend to be less explosive and produce fluid lava flows that can travel long distances. While less immediately dangerous than explosive eruptions, basaltic lava flows can destroy structures and infrastructure in their path, and volcanic gases released during these eruptions can affect air quality over a wide area.

Eruption History Summary

Kujusan has 14 recorded eruptions in the geological database, spanning from 9160 BCE to 1995 CE. The most powerful recorded event was a cataclysmic eruption with ash columns reaching the stratosphere in 100 BCE, reaching VEI 4 on the Volcanic Explosivity Index. Notable eruptions include 370 CE (VEI 3), 100 BCE (VEI 4), 990 BCE (VEI 4). This level of eruptive frequency indicates a persistently active volcanic system that warrants ongoing monitoring. The most recent eruption in 1995 CE places this volcano within the modern era of volcanological observation.

Significance

With a maximum recorded VEI of 4, Kujusan has demonstrated the capacity for significant explosive eruptions that can impact regional aviation, agriculture, and communities tens of kilometers from the summit. VEI 4 eruptions produce ash columns reaching 15-25 kilometers into the atmosphere. With 14 recorded eruptions, Kujusan has a substantial eruptive history that provides valuable data for understanding volcanic behavior and assessing future hazards in the Western Pacific Volcanic Regions region. Given its recent activity, Kujusan is closely monitored by geological survey organizations to provide early warning of future unrest.

GVP Reference Summary

Kujusan is a complex of stratovolcanoes and lava domes lying NE of Aso caldera in north-central Kyushu. The group consists of 16 andesitic lava domes, five andesitic stratovolcanoes, and one basaltic cone. Activity dates back about 150,000 years. Six major andesitic-to-dacitic tephra deposits, many associated with the growth of lava domes, have been recorded during the Holocene. Eruptive activity has migrated systematically eastward during the past 5000 years. The latest magmatic activity occurred about 1600 years ago, when Kurodake lava dome at the E end of the complex was formed. The first reports of historical eruptions were in the 17th and 18th centuries, when phreatic or hydrothermal activity occurred. There are also many hot springs and hydrothermal fields. A fumarole on Hosho lava dome was the site of a sulfur mine for at least 500 years. Two geothermal power plants are in operation at Kuju.

— Smithsonian Institution, Global Volcanism Program