Total grain size distribution of a mafic subplinian tephra, TB-2, from the 1986 Izu-Oshima eruption, Japan: An estimation based on a theoretical model of tephra dispersal

Total grain size distribution was estimated using field data and theoretical modelling. Isopleth maps of each size were constructed at intervals of 1 phi based on the detailed sampling and analysis of a subplinian deposit, TB-2—a product of the climatic phase of the 1986 eruption of the Izu-Oshima volcano, Japan. The umbrella spreading of the eruptive plume is analysed from isopleth maps based on a theoretical model of tephra dispersal (Bursik, M.I., Sparks, R.S.J., Gilbert, J.S., Carey, S.N., 1992. Sedimentation of tephra by volcanic plumes: I. Theory and its comparison with a study of the Fogo A Plinian deposit, Sao Miguel (Azores). Bull. Volcanol. 54, 329–344). The dispersal of particles in the diameter range of 32–4 mm agrees with the model, and the deduced column height is consistent with the visual observation. Using field data and the theoretical model, the erupted mass of each size fraction is estimated, and the total grain size distribution and total mass were obtained. The observed mass loading decay as a function of the area enclosed by the isopleth is revised using the calculated terminal fall velocity and flux of the plume in the umbrella region. The total grain size distribution of TB-2 indicates an intermediate nature between Plinian and Hawaiian eruption styles.