Numazawa

Geography & Climate

Numazawa is located in Japan, within the Northeast Japan Volcanic Arc of the broader Northwestern Pacific Volcanic Regions. Situated at 37.44° N, 139.57° E in the Northern Hemisphere, the volcano lies within a temperate climate zone. With a summit elevation of 835 meters above sea level, Numazawa is a moderately sized peak that remains accessible to hikers and researchers for much of the year. The volcanic landform is characterized as a shield, which describes the physical shape and structure of the volcanic edifice as observed from the surface.

Geological Context

Numazawa 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 Numazawa, 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 dacite, an intermediate to silica-rich volcanic rock. This composition typically produces more viscous magma, leading to more explosive eruptions with ash columns and pyroclastic flows. Andesitic and dacitic magmas build pressure before erupting, which is why volcanoes with this rock chemistry are often associated with Plinian-style eruptions that can send ash tens of kilometers into the atmosphere and disrupt aviation across entire regions.

Eruption History Summary

Numazawa has 1 recorded eruption in the geological database, spanning from 3400 BCE to 3400 BCE. The most powerful recorded event was a paroxysmal eruption with devastating regional consequences in 3400 BCE, reaching VEI 5 on the Volcanic Explosivity Index.

Significance

With a maximum recorded VEI of 5, Numazawa has demonstrated the capacity for paroxysmal eruptions comparable to the 1980 eruption of Mount St. Helens. Eruptions of this scale can devastate areas within 20-30 kilometers and produce ash fall that disrupts aviation and agriculture across hundreds of kilometers.

GVP Reference Summary

Numazawa is a small, 2-km-wide dominantly dacitic-to-rhyolitic caldera constructed within an older Pliocene caldera. Eruptions of the 110,00 years before present (BP) rhyolitic Shibahara pyroclastic-fall deposit and the 71,000 BP dacitic Mukuresawa lava dome were followed by the dacitic plinian Mizunuma eruption about 45,000 BP, emplacement of the Sozan lava dome at 43,000 BP and the Sozan lava dome at about 20,000 BP. The Numazawako pumice flow and plinian eruption about 4600 years BP resulted in formation of the 1.5 x 2 km Numazawako caldera, now largely filled by a caldera lake.

— Smithsonian Institution, Global Volcanism Program