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

本文建立了火山爆发的理论模型，并提出了一种利用粒度数据估算火山爆发强度的方法。第一个是描述岩浆通过火山管道的稳定流动的模型。该模型预测了一个新的制度的岩浆上升流由于液相和气相之间的相对运动。这一结果可以解释安山质岩浆喷发方式的多样性。第二个是一个数值代码来模拟火山羽流和扩散伞云的湍流。该模型使我们能够估计强度的爆发性火山喷发的遥感观测的喷发cloud.The粒度的方法适用于火山碎屑沉积物从皮纳图博1991年的喷发。结果表明，超过70wt%的总喷出物和60wt. %的斑晶在喷发的岩浆中被高度破碎，成为非常细的颗粒（直径为几十微米或更小），因此它们在爆炸喷发期间广泛分散在大气中。细玻璃的形状和玻璃碎片与晶体碎片初始粒度分布的差异表明，岩浆破碎的主要机制是气泡的聚并，这一方法也适用于浅间1783和水濑1813喷发的喷出物。结果，成功地重建了浅间1783年爆发的爆发序列。

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.