| Popis: |
The Kusatsu-Shirane Volcano is one of the most active composite volcanoes in Japan. All the historical eruptions, except for the 2018 eruption of the Motoshirane Pyroclastic Cone Group, have occurred in and around the Shirane Pyroclastic Cone Group (SPCG). Thus far, the eruption ages and styles of the Yumi-ike Maar and the Shirane-Minami Crater Chain (SMCC), located at the southern foot of SPCG, are not understood. In this study, we re-examine the eruption history of the volcano based on the stratigraphy revealed by three trench surveys, petrography, whole-rock composition, hydrothermal mineral assemblages, and new radiocarbon ages. Three 3 m-deep trenches (RH, SJ, and KSE trenches) were dug on the southern to eastern foot of the SPCG and four key deposits were identified. In ascending order, there are (A) the thick heterogeneous white clayey ash deposit rich in hydrothermal alteration minerals derived from the SPCG; (B) a mixture of pyroclastic surge and fallout deposits with juvenile jointed pyroclasts derived from the Yumi-ike Maar; (C) fallout ash from the AD 1882 eruption, a stratigraphically uppermost thin white clayey ash-fall deposit derived from the SPCG. Additionally, the funnel-shaped pyroclastic dyke is found only at the RH trench at the southern end of the SMCC; its strike coincides with the alignment of the SMCC craters, and it is accompanied by white clayey ash-fall deposits. Our study revealed that these key deposits (except for the ash-fall from the AD 1882 eruption) correspond to three eruptions, with the largest phreatic eruption occurring at the SPCG after ca. 7,000 cal BP; the phreatomagmatic eruption that formed the Yumi-ike Maar at ca. 1,400 cal BP (ca. 600 cal AD); and the phreatic eruptions that formed the SMCC after ca. 1,000 cal BP (ca. 1,000 cal AD). The eruption of the Yumi-ike Maar is likely to be the latest magmatic eruption at the Kusatsu-Shirane Volcano. |
