Kusatsu-Shiranesan

Geography & Climate

Kusatsu-Shiranesan is located in Japan, within the Northeast Japan Volcanic Arc of the broader Northwestern Pacific Volcanic Regions. Situated at 36.62° N, 138.53° E in the Northern Hemisphere, the volcano lies within a temperate climate zone. At 2,165 meters above sea level, Kusatsu-Shiranesan rises above the surrounding terrain into montane or subalpine conditions. The elevation creates distinct ecological zones along its flanks, from forested lower slopes to exposed rocky terrain near the summit. 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

Kusatsu-Shiranesan 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 Kusatsu-Shiranesan, 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

Kusatsu-Shiranesan has 20 recorded eruptions in the geological database, spanning from 1902 CE to 2018 CE. The most powerful recorded event was a severe eruption capable of regional ash fall and pyroclastic flows in 1932 CE, reaching VEI 3 on the Volcanic Explosivity Index. With an average interval of roughly 6 years between eruptions over a span of 116 years, this is one of the more frequently active volcanoes in the database. This persistent activity suggests a robust and well-supplied magma system beneath the volcano. With eruptive activity as recently as 2018 CE, Kusatsu-Shiranesan remains an actively monitored volcano.

Significance

With a maximum recorded VEI of 3, Kusatsu-Shiranesan has produced eruptions capable of generating pyroclastic flows and significant ash fall in the surrounding area. While not among the largest eruptions globally, VEI 3 events pose serious hazards to local populations and can disrupt regional air travel. Its 20 recorded eruptions make it one of the most prolific volcanoes in our database, reflecting a persistent and well-documented eruptive history that has provided volcanologists with extensive data for understanding eruption patterns and forecasting future activity. Given its recent activity, Kusatsu-Shiranesan is closely monitored by geological survey organizations to provide early warning of future unrest.

GVP Reference Summary

The Kusatsu-Shiranesan complex, located immediately north of Asama volcano, consists of a series of overlapping pyroclastic cones and three crater lakes. The andesitic-to-dacitic volcano was formed in three eruptive stages beginning in the early to mid-Pleistocene. The Pleistocene Oshi pyroclastic flow produced extensive welded tuffs and non-welded pumice that covers much of the E, S, and SW flanks. The latest eruptive stage began about 14,000 years ago. Historical eruptions have consisted of phreatic explosions from the acidic crater lakes or their margins. Fumaroles and hot springs that dot the flanks have strongly acidified many rivers draining from the volcano. The crater was the site of active sulfur mining for many years during the 19th and 20th centuries.

— Smithsonian Institution, Global Volcanism Program