Petrological research on magma behavior during volcanic eruptions

火山喷发时的减压脱气过程使岩浆泡化，熔体结晶。其结果是，熔体成分和岩石结构的差异。本研究旨在通过对上述岩石学特征的阅读，了解火山喷发过程中岩浆的行为。在2000年的三宅岛火山喷发中，在8月上旬，作为喷发源的玄武安山岩岩浆被斑状玄武岩岩浆所取代。这些岩浆中的熔体包裹体具有不同的体相组成，其化学组成包括挥发分含量基本上没有差别。发展了计算火山玻璃氧逸度的方法，估算了两种岩浆的氧逸度压力均接近NNO。火山碎屑的粒度分析显示大的破碎和小的分散，其特征是蒸汽岩浆喷发而不是岩浆喷发。根据乌苏2000年喷发产物的结晶度和泡状度，粗略估计了岩浆上升速度，并提出了两个阶段的泡状事件。由于熔岩在近地表淬火前后发生碎裂，因此从岩石学上区分两者变得非常重要。基块结晶度和脱气程度（后者是根据熔岩的孔隙度估算的）与1991-95年云仙火山喷发期间低频地震的深度密切相关。我们预期低频地震与泡沫岩浆的碎裂和挥发物的相关流动有成因联系。我们提出了一个模型，在这个模型中，更富含挥发分的岩浆可以被更大的应变速率破碎，伴随着高的减压速率。

Decompression and degassing process during volcanic eruptions magma becomes vesiculated and crystallization of melt occur. As a result, diversities of melt compositions and rock texture appear. In the research, we aimed at understanding of magmatic behaviors during eruptions, by reading the above petrological characteristics. Samples from Miyakejima, Usu, and Unzen volcanoes were investigated.During the 2000 eruption at Miyakejima, basaltic andesite magma that generated eruption was replaced with phyric basalt magma in the early August. Melt inclusions from these magmas with different bulk compositions have the chemistry not distinct largely to each other, including volatile contents. The method to calculate oxygen fugacity on volcanic glass was developed, and the pressure was estimated to be near NNO for both magmas. Grain size analysis of the tephra showed large fragmentation and small dispersion, characterizing phreatomagmatic eruption rather than magmatic eruption.The magma ascent speed was estimated roughly, based on crystallinity and vesicularity of the 2000 eruption products at Usu ; where.two-stage vesiculation events were proposed. As fragmentation of lavas occurred before and after quenching near surface, petrological distinguishing of one from the another becomes important for discussion.The extents of crystallinity in groundmass and degassing, the latter of which was estimated from porosity of lavas, are linked intimately to depths of low-frequency earthquakes during the 1991-95 eruption at Unzen. We expected the genetic relationship of low-frequency earthquakes with fragmentation of foamy magma and the associated flowage of volatiles. We introduced a model in which magma more enriched in volatile can be broken by larger strain rate, accompanied by high decompression rate.