This session focuses on generation and accumulation processes of magmas, magma-crust interaction and degassing, and modes of eruption, long-term forecast of eruption, dispersal and emplacement of the volcanic products. The discussion spans petrological, geochemical, geophysical, and geological processes related with volcanic activity and products in the past, the present and the future. Hyaloclastite is water-lain volcanic breccia embedded in a matrix of glassy clasts by fragmentation of

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... lava under thermal stress. Fluidal basalt lava tends to form coherent flows like pillow lava and sheet flows. In contrast, viscous lava such as andesite and dacite is more likely to form hyaloclastite. This preference of hyaloclastite on lava composition indicates that mechanical response of solidified lava under stress is strongly dependent on composition. Fracturing of lava occurs when the rate of stress accumulation exceeds the rate of stress relaxation and ultimately reaches the mechanical strength of the lava. The rate of stress relaxation decreases with the increase in lava viscosity. Therefore, hyaloclastite is more common in viscous silicic lava. However, the occurrences of pillow lava of dacite and rhyolite are known from the Ogasawara Islands, Unalaska Island, Oman Ophiolite, etc. Pillow lava is commonly associated with hyaloclastite of the same compositions. These examples demonstrate that factors other than lava composition determines fragmentation of lava. Then, the problem arises what are the governing factors that control the mechanical response of lava under stress. We will address these issues through comparative study on glass, quenched melt, of pillow lava and hyaloclastite of variable compositions spanning from basaltic andesite to rhyolite from the Eocene submarine volcanic strata in Chichijima, Ogasawara Islands.Samples of glass from these sites were analyzed by EPMA for major elements and by SIMS for water contents. Eruption temperatures were estimated by clinopyroxene-liquid geothermometer of Putirka (2008). Crystal number densities of groundmass plagioclase and clinopyroxene were determined on COMPO images and modal abundance of constituent minerals were determined on element distribution maps of EPMA. Bulk viscosity of lava was estimated by the methods of Giordano et al. (