13:45 〜 14:00
[SVC30-01] 長期間(>1000年)にわたって複数回の噴火で形成された間宮岳テフラリング：古地磁気学的検討
キーワード：古地磁気方位、テフラリング、マール-カルデラ複合体
Maar volcanoes are surrounded by tephra rings that consist of well-bedded tephra layers. Due to the undisturbed nature of the successive tephra layers, tephra-ring deposits are generally considered to be emplaced over a short time interval (days to years) without significant time breaks; hence, maars are generally interpreted as being monogenetic. In this presentation I describe paleomagnetic evidence for polygenetic origin of the Mamiyadake tephra ring which was formed during maar-forming eruptions at Ohachidaira volcano, central Hokkaido, Japan. In this study, oriented block samples (mostly scoria clasts) were collected from >30 sites throughout the sequence of the Mamiyadake tephra ring for paleomagnetic analysis. Like other maar-tephra-ring deposits elsewhere, pyroclastic layers making up the Mamiyadake tephra ring show no sedimentological evidence for significant time breaks, but with one unconformity between the layers occurring in the southwest rim. The paleomagnetic directions obtained from the ring deposits significantly vary vertically through the sequence. This significant variations in paleomagnetic directions between the sites are interpreted as recording snapshots of paleomagnetic secular variation (PSV) of the geomagnetic field over time. By using the average rate of PSV in Japan calculated from the Holocene archaeomagnetic and paleomagnetic records, I conclude that the deposits of the Mamiyadake tephra ring were emplaced intermittently over a time span of at least 1000 yr. The results of this study suggest a possibility of maar’s longer lifespans (hundreds to thousands of years) that are at least three orders of magnitude longer than previously thought.