Iwatesan (Japan): Recorded Eruptions and Historical Activity

Iwatesan is a complex volcano located in Japan at a subduction zone and continental crust tectonic setting (elevation: 2038.0 meters; latitude: 39.853, longitude: 141.001). Maintained in the Smithsonian Global Volcanism Program records under volcano number 283240, the volcano has a documented history featuring 26 confirmed eruptions.

The official chronological record spans from an earliest recorded eruption starting around 6450 BCE (with an uncertainty of 1600 years, determined via tephrochronology correlation) up to the latest confirmed eruption beginning and ending on July 15, 1919. The profile's last known eruption year is recorded as 1919.

Overview of Recorded Eruptions

The 26 confirmed eruptions cataloged for Iwatesan utilize varying methods of evidence, ranging from modern human observations and reports to scientific dating techniques. For prehistoric and older events, researchers apply tephrochronology correlations and radiocarbon dating methods (such as calibrated 14C isotopic analysis) with varying degrees of year uncertainty. For instance, an early Holocene event dates to 6450 BCE with a 1600-year uncertainty, while later prehistoric markers include events around 4350 BCE (uncertainty of 500 years) in the Higashi-Iwate activity area and 2050 BCE (uncertainty of 200 years).

Historical accounts provide precise dates and parameters for later activity. For example, an eruption occurred from March 26 to March 27, 1686 CE, reaching a Volcanic Explosivity Index (VEI) of 3.0 based on reported observations. A subsequent confirmed eruption took place on July 15, 1919 CE, on the west side of the Onigajo caldera (O-jigokudani) with a VEI of 1.0.

Volcanic Explosivity Index and Scale of Activity

Across the recorded history of Iwatesan, the Volcanic Explosivity Index (VEI)—a relative scale rating the explosiveness of volcanic eruptions—has a largest recorded value of 3.0. Out of the 26 confirmed eruptions, 7 have recorded VEI observations. The remaining events lack documented VEI values, reflecting the challenges in quantifying ancient or unobserved prehistoric activity.