抄録
Taiheizan pyroclastic deposits (3,710+130y.B.P.) are the youngest pyroclastics of Sambe Volcano, southwest Japan. The deposits are distributed mainly on the southwestern and eastern flanks of the volcano. We preliminary report the depositional process of the deposits on the southwestern flank, and quantitative perspective on the flow properties of block-and-ash flow. Depositional processes suggest that more than 6 times block-and-ash flows associated with small-scale pyroclastic falls and surges occurred during the youngest volcanic active stage (Taiheizan stage). These block-and-ash flows are considered to have derived from partial collapse of growing lava dome, which exits on the present main part of Sambe Volcano. Dacite block-and-ash flow type pyroclastic flow deposits are distributed along the Hayamizu River up to 9 km from the source area. A typical flow unit is composed of four depositional subunits which are (1) the poorly sorted boulder-sized blocks with normal to reverse grading, (2) poorly sorted boulder-to cobble-sized blocks with gas segregation pipes, (3) well sorted plane stratified lapilli and ash, and (4) well sorted ash with normal grading, in ascending order. The fomer two subunits can be interpreted as the main body deposits of block-and-ash flow, in which grains must be supported mainly by grain collide dispersion and fluidization, respectively. The third subunit of stratified lapilli and ash indicates the deposits of ash cloud surge with turbulence grain support mechanism. Depositional facies of the fourth subunit suggests the suspension fallout deposits from ash cloud. The flow viscosity is quantitatively estimated by assuming possible flow velocity. The visccsity of the upper main body of the block-and-ash flow was about 0.72 to 4.7 poise, and that of ash cloud surge was about 0.18 to 0.75 poise.
