Hachijojima

Geography & Climate

Hachijojima is located in Japan, within the Izu Volcanic Arc of the broader Northwestern Pacific Volcanic Regions. Situated at 33.14° N, 139.77° E in the Northern Hemisphere, the volcano lies within a subtropical climate zone. With a summit elevation of 854 meters above sea level, Hachijojima 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

Hachijojima 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 Hachijojima, 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 basalt / picro-basalt, 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

Hachijojima has 19 recorded eruptions in the geological database, spanning from 8020 BCE to 1707 CE. The most powerful recorded event was a paroxysmal eruption with devastating regional consequences in 2050 BCE, reaching VEI 5 on the Volcanic Explosivity Index. Notable eruptions include 350 BCE (VEI 3), 2050 BCE (VEI 5), 2450 BCE (VEI 4). This level of eruptive frequency indicates a persistently active volcanic system that warrants ongoing monitoring.

Significance

With a maximum recorded VEI of 5, Hachijojima 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. With 19 recorded eruptions, Hachijojima has a substantial eruptive history that provides valuable data for understanding volcanic behavior and assessing future hazards in the Northwestern Pacific Volcanic Regions region.

GVP Reference Summary

Hachijojima, in the central Izu Islands about 300 km S of Tokyo, consists of two small Quaternary dominantly basaltic stratovolcanoes forming an elongated NW-SE-trending island. The eroded Pleistocene-to-Holocene Higashiyama volcano occupies the SE end of the 14-km-long island. The symmetrical Holocene Nishiyama volcano the NW end has cones on the SE flank. The small volcanic island of Kojima lies several kilometers W of Hachijojima. Growth of Higashiyama began several tens of thousands of years ago, and included the formation of two small calderas. The initial submarine and early subaerial eruptions of Nishiyama took place from 10,000 to 8,000 years before present (BP). Its latest major activity, from the early Holocene until about 4,000 BP, was restricted to flank eruptions. Eruptions recorded since the 15th century have been restricted to the summit of Nishiyama and a submarine vent of unknown location.

— Smithsonian Institution, Global Volcanism Program