Petrological research on magma behavior during volcanic eruptions

火山喷发过程中的减压和脱气过程使岩浆泡化，熔体结晶。因此，出现了熔体成分和岩石结构的多样性。本研究旨在通过对上述岩石学特征的研究，了解喷发过程中的岩浆行为。在2000年宫城岛火山喷发期间，玄武岩安山岩岩浆在8月初被榴辉玄武岩岩浆取代。这些岩浆的熔融包裹体具有不同的主体成分，其化学成分包括挥发分含量彼此之间差别不大。提出了计算火山玻璃上氧逸度的方法，估算出两种岩浆的氧逸度均接近N0。火山喷发的粒度分析表明，火山喷发碎屑大、分散小，属于云雾岩浆喷发的特征，而不是岩浆喷发。根据USU 2000年喷发产物的结晶度和气泡度，粗略估计了岩浆的上升速度；其中提出了两阶段泡化事件。由于熔岩在近地表淬火前后发生破碎，岩石学上的区分变得非常重要。岩层中结晶程度和脱气作用的程度，后者是由熔岩的孔隙度估计的，与1991-95年温泉喷发期间低频地震的深度密切相关。我们预测了低频地震的成因与泡沫岩浆的碎裂和相关的挥发分流动的关系。我们介绍了一种模型，在该模型中，更富含挥发分的岩浆可以通过较大的应变速率和较高的减压速率来破碎。

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.