[S-VC30] 火山・火成活動および長期予測

[SVC30-P01] 火口近傍プリニアン降下堆積物の基底に発達するフルートマークに似た侵食痕

*安田 裕紀¹、前野 深² (1.東京大学先端科学技術研究センター、2.東京大学地震研究所)

Flute marks are erosive features typically observed at the bases of marine sandstones and are formed by erosion due to turbulent eddies of an overlying turbidity current. Although turbulent pyroclastic currents would potentially produce flute marks on the substrate, these marks have rarely been reported in the volcanological literature. We describe flute-like marks developed at the boundary between Plinian and sub-Plinian units of the 34-ka eruption at the Ohachidaira caldera in the Taisetsu volcano group and discuss their formation mechanisms. The flute-like scours are observed at a section 1.8 km of the caldera center (0.9 km from the rim) and show asymmetric cross-sectional shapes that curve downward with the lowest point near its upstream end (toward the caldera); they cut into underlying strata of the immediately preceding sub-Plinian fallout deposits and are filled with pumice lapilli from the later Plinian phase. These scours have lengths of 20–130 cm and are 3–20 cm deep; the ratios of length to depth are 5.0–7.5, values similar to flute marks associated with turbidites. These morphological similarities in cross section suggest that the flute-like marks would likely have been formed in a similar manner to flutes in sedimentary rocks. Another characteristic erosional feature is shear deformation of the top ash layer of the sub-Plinian deposits; this ash layer is locally bent, folded, split, or fragmented. These scours and deformation structures appear to be largely coeval with the basal Plinian pumice lapilli. We interpret these features, based upon their field characteristics, to have been formed by dilute turbulent pyroclastic currents that occurred at the beginning of the Plinian phase. Observations allow the estimation of timescales for formation of the flute-like marks to be at <0.5–5 min, which is rarely obtained from flute marks in sedimentary rocks. The erosional features are observed exclusively within the most proximal sections (~1–2 km from the caldera center), suggesting short runout distances of the erosive pyroclastic currents. Such small dilute pyroclastic currents may be consistent with a local collapse along the margin of the Plinian column.