日光白根火山1649年噴火の復元

抄録

Nikko-Shirane Volcano located on the border of Gunma and Tochigi prefectures had the largest eruption on the historic records in AD 1649. We reconstructed the eruption event based on the geological mapping of the pyroclastic fall deposit and craters at the summit, ¹⁴C dating of soil underlying the pyroclastic fall deposit and interpretation of historic records. The pyroclastic fall deposit is observed in a 10×6 km area around Nikko-Shirane Volcano and thickens to the summit of Mt. Shirane. The pyroclastic fall deposit is preserved at＞4 km east from the summit and observed 5-8 cm thick around Lake Yunoko and 20 cm thick in maximum around the southern part of Senjogahara. Based on the historic records of the 1649 eruption, the craters with about 220 m in long axis diameter and 30 m deep located next to a small shrine at the summit were opened. Thus, the 1649 eruption is considered to occur at the summit of Mt. Shirane and pyroclastic materials fell east to southeast ward. The total mass of pyroclastic fall deposit is estimated at 2×10⁷-3×10⁷ m³ which is a digit larger than the previous report, and it is comparable to Volcanic Explosive Index＝3 and Magnitude＝3.4-3.6. The pyroclastic material contains essential vesicular vitreous particles consisting 1-48 % (mean 19 %) of component in 250-2000 μm fraction. Combination of the essential particles in the 1649 pyroclastic materials suggests that a magmatic eruption was occurred during the 1649 eruption. The essential particles are concentrated in three principal distribution axes of the pyroclastic fall deposit extending to the east, southeast and west. However, the pyroclastic fall deposit is composed of a lot of fine particles, indicating that the 1649 eruption would be possible of a phreatomagmatic eruption triggered by magma intrusion to an aquifer below the volcanic body. Around the time of the eruption, lahar occurred at the western valley of Mt. Shirane and flowed through Ohirogawara to the Nigamatazawa River.