Fluid dynamics study on dynamics of eruption clouds (particularly on the significance of fragmentation processes)

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Theoretical modeling of explosive eruptions and a method to estimate intensity of eruption from granulometric data are developed in this study.Two types of theoretical models are established. The first one is a model to described a steady magma flow through a volcanic conduit. The model predicts a new regime of magma ascent flow due to relative motion between liquid and gas phases. The results may explain diversity of eruption style of silicic to andesitic magmas. The second one is a numerical code to simulate the turbulent flow of volcanic plumes and spreading umbrella clouds. The model enable us to estimate intensity of explosive volcanic eruptions from the remote-sensing observations of the eruption clouds.The granulometric method is applied to the tephra-fall deposits from the Pinatubo 1991 eruption. The results show that more than 70 wt % of the total ejecta and 60 wt. % of the phenocrysts in erupted magma were highly fragmented to become a very fine particles (a few tens of micron or less in diameter), so that they are extensively dispersed in the atmosphere during the explosive eruption. The shape of fine glass and difference in initial size distribution between glass fragments and crystal fragments indicate that the main mechanism of fragmentation is coalescence of bubbles in the magma.The similar method are also applied to the ejecta of the Asama 1783 eruption and those of the Suwanose 1813 eruption. As a results, the sequence of the explosive eruption in the Asama 1783 eruption was successfully reconstructed.