Depositional features and emplacement mechanism of the 1991-96 Unzen pyroclastic flows, Japan.

More than 9400 Merapi-type dacite pyroclastic flows (totaling 0.2 km³) were produced from lava dome collapse at Unzen Volcano, western Japan from May 24, 1991 to May 1, 1996. The thickness of a flow unit ranges 20 cm- 5 m. Most flow units show reverse grading of larger clasts. The clasts are angular to subrounded. The layer 2a (<20 cm thick, ML<3-8 cm) occur at the bottom of the deposit. Clasts in the layer 2a are relatively rounded than the upper part. Massive to laminated ash-cloud surge beds (<20 cm thick) sometimes occur between flow units. A complete succession of pyroclastic surge deposits that occurred from Sep. 15, 1991 to June 24, 1993 was found in the trench section excavated by FFPRI and NAFES (S2). The Sep. 15, 1991 pyroclastic surge deposits and the June 23-24, 1993 surge and flow deposits are observed in the trench section. Lobes and levees are observed at the surface of the deposit (S5, S6). Estimated yield strengths of the flow at the depositional stage were 10³-10⁴ Pa.

The reverse grading, layer 2a, fine materials in the matrix, and video images suggest that large clasts in the Unzen pyroclastic flow were transported by saltation, traction and collisional dispersive force, and deposited forming basal traction carpets. Lobes and levees on the surface indicate that the matrix of the Unzen pyroclastic flow had a yield strength at the time of deposition. Therefore, the emplacement mechanism of the Unzen pyroclastic flows can be explained by the high-density turbidity current model and the slow-moving basal density-modified grained flows, which producing many lobes at the bottom due to accumulation of clasts on a gentle slope.