東伊豆単成火山地域の噴火史2 : 主として32ka以前の火山について(＜特集＞堆積物による火山噴火史研究)

抄録

We reveal the eruptive history of the Higashi Izu monogenetic volcano field by tephrochronology and loess-chronometry and describe mainly volcanoes older than 32,000 years ago. The eruption of the volcano field started at about 150,000 years ago. Twelve tephra from volcanoes outside the Izu Peninsula were used as key beds. We found four eruptive fissures ; Takatsukayama-Sukumoyama, Kadono-Umenokidaira, Koike-Oike, and Numanokawa fissures, along each of which two to five monogenetic volcanoes erupted simultaneously. Interbedding of a distal wide spread tephra between eruptive deposits proper to the volcano field (the KIP-4 pumice between the Takatsukayama-Sukumoyama tephra, and the Hakone Da-4 pumice between the Kadono-Umenokidaira tephra) means that an eruption of a volcano outside the Izu Peninsula occurred simultaneously with the eruption of the volcano field. Vent locations of the volcano field were limited to the northern half of the present distribution during 80,000-150,000 years ago and were expanded in the later stage. Average frequency of eruption in the volcano field is calculated to be one/7,900 years for the period of 40,000-150,000 years ago, and one/2,500 years for the past 40,000 years. Average discharge rate of magma is calculated to be 0.64 kg/s before 40,000 years ago, and 2.5 kg/s for the past 40,000 years. Thus, both the average frequency of eruption and the magma discharge rate are higher for the past 40,000 years than those in the earlier stage. Many of the eruptions of andesite magma occurred later than 14,500 years ago, and the ones of dacite-rhyolite magma occurred only for the past 3,200 years. The cumulative pattern of the discharge mass of magma from the whole volcano field against time shows no clear predictability.