中部日本,八ヶ岳地域の火山活動期の再検討  [in Japanese] Reexamination of volcanic activity of Yatsugatake area, central Japan  [in Japanese]

八ヶ岳地域の火山活動時期を層序学的・年代学的・古地磁気学的手法を用いて再検討した.その結果,八ヶ岳地域の火山活動は,北八ヶ岳地域では約1.2〜0.8Maと約0.5Ma以降,南八ヶ岳地域では約0.5〜0.1Maであることが明らかになった.0.8Ma以前の火山活動は,北八ヶ岳地域のみでみられることから,この時期の火山活動で生じた火山を八柱火山群と新たに定義する.そして,0.5Ma以降の南・北八ヶ岳の両地域の火山活動で生じた火山を八ヶ岳火山群と再定義する.八柱火山群には,噴出中心が4ケ所以上存在する.それらはNW-SE方向に配列し,八ヶ岳火山群のN-S方向の配列とは異なる.八柱火山群の西方には,霧ケ峰・和田峠火山岩類など同じ時期の噴出物が分布しており,前期更新世の八ヶ岳中信高原地域では東西に広がった火山活動があったと考えられる.

We revised the volcanic history of the Yatsugatake area, located in the eastern area of Nagano Prefecture, central Japan, by means of volcanostratigraphy, K-Ar age determination and magnetostratigraphy. As a result, it was clarified that the volcanic activities occurred at around 1.2-0.8 Ma and 0.5-0 Ma in the Northern Yatsugatake area, and at around 0.5-0.1 Ma in the Southern Yatsugatake area. As the volcanic activities older than 0.8 Ma are restricted only in the northern area, the volcanoes in this area are newly defined as "Yabashira Volcanoes". We also redefined "Yatsugatake Volcanoes" as those which were formed in both northern and southern areas after 0.5 Ma. The Yabashira Volcanoes have more than four eruptive centers, which are lined up in the NW-SE direction. The arrangement of these volcanic centers is different from the N-S arrangement of the volcanic centers of the Yatsugatake Volcanoes. To the west of the Yabashira Volcanoes, the volcanic ejecta of the same age including those in Yashigamine, Kirigamine and Wadatoge areas are distributed, forming a volcanic field spreading in the E-W direction within the area of Yatsugatake-Chushin Highland area.