Study of the groundmass textures of volcanic products from Asama volcano and its implications for prediction of eruption.

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Asama volcano, central Japan was active during September to December, 2004. Among its several eruptions, vulcanian eruptions of Sept 1st. and Sept 23rd. were relatively intensive. We described the ascent and crystallization process of the magma during these three weeks as follows. Before Sept. 1st, a column of new magma had risen into the pre existing andesite body beneath the crater floor. On Sept.1st, the built up gas pressure surpassed the tensile strength of the andesite body to result in a vulcanian eruption. This eruption provided fragments of the pre-Sept.1st andesite, along with smaller amounts of the essential breadcrust pumices, the glass of which contain 0.4-0.8wt.% of water based on the infrared absorption spectra measurement. Because the cap rock had disappeared, the magma could uplift, upper portion of which effused and made a dome inside the crater. It in turn became a cap rock for the succeeding magma. On Sept. 23rd, next vulcanian eruption took place, the ejecta of this eruption consisting of the lithic fragments and the essential scoria with 0.1wt% water.The textural and chemical analyses of the essential materials ; pumice of Sept.1st and scoria of Sept 23rd, reveal the crystallization process in a chamber-vent system. Following the crystallization of phenocrysts in a deeper magma chamber, two crystallization stages are preserved in crystal size distribution of microlites both in the pumice and scoria. First stage : Magma left the chamber and rose through complex path-steps with ever increasing nucleation. Second stage : In a shallower part in the vent, numerous microlites precipitated rapidly under a high undercooling condition. The similarity between the both CSD patterns implies that their magma had a common ascent history, although groundmass in the scoria has lower crystallinity than that in the pumice, suggesting that the scoria magma stood lower in the magma column.