気泡組織・サイズ分布から見た, 有珠山2000年噴火でのマグマ上昇と発泡プロセス(<特集>2000年有珠山噴火 (3))

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Study on products of phreatomagmatic eruption on 31 March 2000 at Usu volcano gives insight into timing of bubble nucleation and gas escape from system during magma ascent from the reservoir. The essential fragrnents (pumice and micropumice) vary in vesicularity which is negatively correlated to water content in the rhyolitic groundmass glass. This implies that variable vesicularity resulted from different degree of water exsolution from melt to bubbles, rather than different degree of gas escape. The difference in water exsolution probably reflects the different timing of quenching, where micropumices were quenched in various depths by the aquifer whereas pumice was not quenched. Number density of bubbles in essential fragments changes with increasing vesicularity; it once increases with vesicularity and then decreases. The vesicular texture indicates that the decrease of the number density resulted mainly from bubble coalescence. Bubble size distributions (BSD) for less vesiculated micropumices are composed of two linear parts in log-linear plot. The slope and intercept values for smaller bubbles are higher than those for larger ones, indicating that nucleation rate has increased during a continuous nucleation. This is in accordance with the change in BSD with increasing vesicularity; number of small bubbles increases rapidly, while that of larger bubbles is nearly constant. Because micropumices were quenched around the aquifer, nucleation rate is inferred to have increased before magma's reaching there.
2002-11-29

気泡組織・サイズ分布から見た, 有珠山2000年噴火でのマグマ上昇と発泡プロセス(<特集>2000年有珠山噴火 (3))

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