Re-evaluation of explosive activities of Iwate Volcano in the last 10,000 years:: Spatial and temporal relationship of phreatic and magmatic explosions

The active Iwate Volcano is located on the volcanic front of the Northeast Japan Arc. Yakushidake (2038 m), which is the youngest stratocone of Iwate Volcano, formed after a large-scale sector collapse associated with the Hirakasa debris avalanche deposit. This study re-examines the explosive eruptive history of the Yakushidake stratocone via tephra-stratigraphic study and radiocarbon dating. New C ages indicate that the Hirakasa debris avalanche occurred at 8.5–9.9 cal ka BP. Twelve phreatic eruption units (labeled here Iwph12 to Iw-ph1, from oldest to youngest) and one argillaceous pyroclastic density flow deposit (the Yunosawa pyroclastic deposit, YPD) were identified by detailed C age dating and X-ray diffraction mineralogical analyses. The average recurrence interval of phreatic eruptions is 500–1000 years. Phreatic tephra deposits crop out around the Yakushidake summit crater, the Ojigokudani fumarolic area, and the Amihariyumoto geothermal area. The thickest phreatic tephra deposit, Iw-ph7, was erupted from Amihariyumoto at 4.0–4.5 cal ka BP. An ashfall volume of ca. 2.3×10 m for the eruption was estimated using the calculation method of Hayakawa (1985). Explosive magmatic eruptions deposited 15 tephra units. Three of the more voluminous explosive units (Iw-SuS, Iw-OdS, and Iw-KS) eruptions have approximate magnitudes of VEI = 3 and were associated with the pyroclastic surge deposits. The repose interval between voluminous explosions is c. 2000 years. The volcanic history of Yakushidake Volcano is dominated by two vigorous eruptive phases: YV1 (associated with tephra deposits Iw-W6d to Iw-OkS) and YV2 (tephra layers Iw-SS to Iw-KS and KSr). In both phases, small-scale magmatic eruptions preceded the more voluminous (VEI = 3) events. There is evidence for recurrent phreatic activity during the vigorous eruptive phases; however, individual magmatic activities and phreatic eruptions do not always coincide.