安達太良火山西麓,酸川流域に分布するラハール堆積物 : 過去14000年間の層序・年代と堆積過程
Stratigraphy, Chronology and Depositional Processes of Lahar Deposits in the Sukawa River System, Adatara Volcano, Northeast Japan

Kyoko S. KATAOKA, Narumi KAMINO, Yoshitaka NAGAHASHI, Katsuhiko KIMURA
2015 Bulletin of the Volcanological Society of Japan  
Previously known lahar deposits along the Sukawa River, western slope of Adatara volcano, consist of at least 7 muddy (cohesive) units and were formed during the last 2500 years. This paper reports stratigraphy, chronology and depositional processes of the newly found lahar units that were deposited older than 2500 years ago. 14 C dating for wood fragments within lahar deposits and intercalated paleosol layers indicates that at least 18 lahar events occurred in the area during the last 14000
more » ... rs. The newly found lahar units comprise 10 muddy matrix-rich units and one bouldery gravel unit. The muddy matrix-rich units are very poorly sorted with pebble to cobble sized andesite clasts and hydrothermally altered clasts. Bouldery gravel unit is filled with sandy matrix and composed of clasts with oxidized rind locally. Bulk chemistry of these lahar units indicates that high sulfur content in the muddy units whereas Fe2O3 content is higher in the bouldery gravel units. The muddy units are derived from syn-or post-eruptive lahars in relations with phreatic eruptions or degradation of hydrothermally altered source rocks nearby the Numanotaira crater at the summit. The bouldery unit distributed farther downstream may have resulted from a non-cohesive debris flow or an outburst flood from the crater or a volcanically dammed upstream area. The presence of oxidized clasts indicates that during traveling, the watery flow entrained clasts which were exposed on a gravelly riverbed in an inter-eruptive period. The two types of lahar deposits show flows with different characteristics (cohesive and non-cohesive) that originated from Adatara volcano. Proximal eruptive units in the summit area sometimes can be degraded easily and overprinted by other eruptive units that result in a paucity and difficulty in understanding the precise eruption history of a volcano. Therefore, approaching from analysis of distally deposited lahar units will give a hint to assess not only a lahar trigger but also the types, frequency and volume of volcanic eruptions.
doi:10.18940/kazan.60.4_461 fatcat:nkcvv7mlo5cabciieiccysr2cm