由布-鶴見火山群の池代火砕流堆積物と地代北部火砕物の岩相と残留磁化

抄録

The Ikeshiro pyroclastic-flow deposit was emplaced on the northern foot of Yufu volcano ca. 2,000 years ago, probably originated from repeated collapse of the Ikeshiro lava dome. The Ikeshiro-Hokubu volcaniclastic deposit overlies the Ikeshiro pyroclastic-flow deposit at its eastern end of distribution. Progressive thermal demagnetization results indicate existence of one or more stable magnetic components in all samples from the Ikeshiro pyroclastic-flow deposit and the Ikeshiro-Hokubu volcaniclastic deposit. Components of magnetization from the lkeshiro-Hokubu samples are divergent, while the low temperature ones from the lkeshiro samples, isolate below about 450℃, show in better grouping of the direction (Dec=-0.8°, Inc=32.1°, α₉₅=10.1°). The Ikeshiro pyroclastic-fiow deposit is interpreted to have been emplaced at less than the Curie Point, while the Ikeshiro-Hokubu volcaniclastic deposit equal to the ambient temperature. The number of components from the Ikeshiro-Hokubu samples increase in proportion to the degree of oxidation. This implies that the lkeshiro lava dome grew both exogenously and endogenously and generated pyroclastic fiows which emplaced the Ikeshiro pyroclastic-fiow deposit. After the halt of dome eruption, debris flow occurred on the northern slope to emplace the Ikeshiro-Hokubu volcaniclastic deposit.