Geology of Ontake Volcano

 Ontake Volcano, an active stratovolcano situated at the southern end of the Hida Mountain Range (the Japanese Northern Alps) in central Japan. The volcano is altitude of 3,067 m (the highest peak named Kengamine), spans a width of 20 km and has a total edifice volume of approximately 50 km³ (without fall tephras in distal area).

 The Ontake Volcano is bifurcated into two primary sections: the older volcano (active during the period 0.78–0.39 Ma) and the younger volcano (active post 0.1 Ma). The older volcano’s products range from basalt to dacite, with a SiO₂ composition of 51–65 wt.%, while the younger volcano’s products range from basaltic andesite to rhyolite, with a SiO₂ composition of 52–74 wt.%.

 The older volcano’s activity is further divided into two stages: the formation of the Yukawa Volcano (Older Stage 1: 0.78–0.64 Ma), characterized by a rich presence of pyroclastic materials, and the formation of the Otaki Volcano (Older Stage 2: 0.64– 0.39 Ma), composed of a lava mainly. Yukawa Tephra 5, a tephra erupted during the early Yukawa Volcano activity, correlates with the Byk-E tephra, which serves as the reference layer for the Chibanian GSSP in the Kokumoto Formation of the Boso Peninsula.

 The younger volcano’s activity is divided into five stages: the Mamahaha Volcano (Younger Stage 1: 100–80 ka), the Marishiten Volcano (Younger Stage 2: 80–50 ka), the Yonnoike Volcano (Younger Stage 3: 40–30 ka), the Ichinoike Volcano (Younger Stage 4: 30–11.7 ka), and the Holocene volcanic activity (Younger Stage 5: 11.7 ka to present). During Stage 1 of the younger volcano, large-scale Plinian eruptions were frequent, resulting in the widespread dispersion of tephra, such as the On-Pm1 tephra. The average magma eruption rate in the younger Ontake Volcano is 0.3 km³/ky.

 Documented eruptions of the Ontake Volcano have been phreatic since 1979, occurring in 1979 (to 1980), 1991, 2007, and 2014. The eruptions in 1991 and 2007 were relatively minor, with the volume of erupted tephra being in the tens of cubic meters. The 1979 and 2014 eruptions were of VEI = 2 scale, generating pyroclastic density current and ejected ballistic materials, along with the ash fall. In 1984, an Mj6.8 earthquake at the foot of the volcano led to the collapse of a 3.4×10⁷ m³ volcanic edifice (known as Ontake Kuzure or Denjogawa Kuzure), causing a debris avalanche that extended up to 12 km south.

　 前を読む　 次を読む