5:15 PM - 6:30 PM
[SVC30-P04] Reinvestigation of Holocene eruption history of Nikko-Shirane Volcano, central Japan
Keywords:Nikko-Shirane Volcano, Eruption history, Geology, Active volcano, Volcanic stratigraphy, Radio carbon age
Holocene eruption history of Nikko-Shirane Volcano was reinvestigated, based on the geological mapping, 5–10 m digital elevation model reading, and 14C ages, and correlated with the tephra stratigraphy. Nikko-Shirane Volcano (peak Mt. Shirane: 2578 m) is located on the border of Gunma and Tochigi prefecture, central Japan. It has three crater area, located at Mt. Shirane, Chinoikejigoku and Mt. Zazen (2317 m). The oldest Marunuma Lava is overlying Miocene basement, Kinugawa Rhyolites, and covered with thin soil and Asama-Itahana yellow (As-YP) tephra of 17 ka suggesting that Nikko-Shirane Volcano has been started eruption around 17 ka. Mt. Zazen is composed of Zazen Lava and Zazen'yama Pyroclastic Cone which has a 200 m-long crater on top. Eruption age of Zazen Lava is estimated to 7.6–5.7 ka (cal: calendar age).
There are seven tephra layers on Zazen Lava, from the lowest to the upper layers, Nks-G, Nks-F, Nks-E, Nks-D (defined in this study), Haruna-Futatsudake-Ikaho (Hr-FP: 6th century: Machida and Arai, 2003), Nks-C and Nks-A (Kusano and Ishizuka, 2020). Eruption age of Nks-G and Nks-D are estimated to 5.7–5.5 ka (cal) and 3.1–2.9 ka (cal). These 14C ages and soil sedimentary rate between these tephra suggest that Nks-F was erupted in 5.2 ka, and Nks-E was erupted in 4.7 ka. Based on the volcanic stratigraphy, modal composition of tephra, petrography and geochemistry, Nks-F, Nks-E and Nks-D are correlated with Goshikinuma Pyroclastic Cone, Shirane-gongen Pyroclastic Cone and Shiranesan Lava, respectively.
We described Jigokunagi Pyroclastic Flow Deposit along a western valley of Mt. Shirane. It has more than 7 m in thickness, and consists of unwelded, block and ash flow deposit. It composed of volcanic blocks with fresh, dense and weakly vesiculated andesite with cooling joints, ash matrix of the same composition, and some charcoals. The pyroclastic flow deposit is covered with soil and Hr-FP. Petrographic feature and bulk-rock composition of these volcanic blocks is similar to that of Shiranesan Lava. Charcoal in the pyroclastic flow deposit show 3.3–3.2 ka. These lines of evidence indicate that the latest Shiranesan Lava was erupted around 3 ka and the crumbling of the lava formed Jigokunagi Pyroclastic Flow Deposit.
We conclude that magmatic eruption in Nikko-shirane Volcano occurs once in 300–1,500 years in recent 7,500 years.
There are seven tephra layers on Zazen Lava, from the lowest to the upper layers, Nks-G, Nks-F, Nks-E, Nks-D (defined in this study), Haruna-Futatsudake-Ikaho (Hr-FP: 6th century: Machida and Arai, 2003), Nks-C and Nks-A (Kusano and Ishizuka, 2020). Eruption age of Nks-G and Nks-D are estimated to 5.7–5.5 ka (cal) and 3.1–2.9 ka (cal). These 14C ages and soil sedimentary rate between these tephra suggest that Nks-F was erupted in 5.2 ka, and Nks-E was erupted in 4.7 ka. Based on the volcanic stratigraphy, modal composition of tephra, petrography and geochemistry, Nks-F, Nks-E and Nks-D are correlated with Goshikinuma Pyroclastic Cone, Shirane-gongen Pyroclastic Cone and Shiranesan Lava, respectively.
We described Jigokunagi Pyroclastic Flow Deposit along a western valley of Mt. Shirane. It has more than 7 m in thickness, and consists of unwelded, block and ash flow deposit. It composed of volcanic blocks with fresh, dense and weakly vesiculated andesite with cooling joints, ash matrix of the same composition, and some charcoals. The pyroclastic flow deposit is covered with soil and Hr-FP. Petrographic feature and bulk-rock composition of these volcanic blocks is similar to that of Shiranesan Lava. Charcoal in the pyroclastic flow deposit show 3.3–3.2 ka. These lines of evidence indicate that the latest Shiranesan Lava was erupted around 3 ka and the crumbling of the lava formed Jigokunagi Pyroclastic Flow Deposit.
We conclude that magmatic eruption in Nikko-shirane Volcano occurs once in 300–1,500 years in recent 7,500 years.