[S-VC47] 火山・火成活動と長期予測

[SVC47-P19] ロンボク地域サマラス-リンジャニ火山のカリウム-アルゴン年代：カルデラ形成噴火に先行する火山活動位置の固定化

*土志田 潔¹、高田 亮²、橘川 貴史³ (1.電力中央研究所、2.産業技術総合研究所、3.セレス)

キーワード：第四紀, カルデラ, インドネシア, スンダ弧, 放射年代測定, 質量分別補正法

K-Ar dating of Pliocene to Quaternary lavas consisting large volcanoes in Lombok are preformed to determine the active periods of Samalas-Rinjani volcano complex (VC) prior to the caldera eruption in 1257 CE. In order to accurately date the hand specimen of young lavas, non-spiked argon ratios are analysed to estimate the amount of mass fractionation of argon isotopes at the time of solidification of the lavas. Consistent ages were obtained for replicate analyses of the four young lavas from Samalas in the range of 0.08-0.04 Ma. Two samples are estimated to have initial argon ratios that are fractionated from atmospheric values. The ages form distinct groups that correspond to the active periods of volcano clusters: 2.7 Ma and 2.0-1.8 Ma for West Lombok VC, 0.5-0.4 Ma for East Lombok VC and 1.0 Ma to present for Samalas-Rinjani. Samalas-Rinjani system should be defined as single volcano complex based on the relative duration of each active period. Rinjani and the current activity of Segara Anak caldera are correlated to the younger stages of Samalas-Rinjani. The location of volcanism has been relatively stable for the past 0.4 million years in both Lombok and Sumbawa, which hosts Tambora volcano. Caldera-forming eruptions of the two regions (the 1257 eruption and the 1815 eruption at Tambora) occurred at the volcanoes with 1000 km³ class edifice that had formed through 0.1 to 0.2 million years of volcanic activity. This contrasts clearly with the migration of volcanic activity from 5 to 0.7 million years BP in the two regions.