Understanding of magmatic eruptions and prediction of future eruptions in an active volcano, Zao

早火山最年轻阶段（30ka至今）有3个活跃期（约31 ~ 29ka、7.5 ~ 4.1 ka、2.0 ka至今）。大量中小规模喷发形成的火山碎屑岩堆是火山活动的主要产物。根据产物的岩石学特征，研究了三个时期的岩浆供给系统。从岩石学和矿物学特征看，这三个时期喷发的岩石为橄榄石+辉石玄武岩安山岩-安山岩，由两个端元岩浆混合形成。估算的长英质端元在不同时期是相似的，安山岩（硅含量约为60%）和正辉石(Mg#=ca。64)、斜辉石(mg# =ca。68)，斜长石（An=ca.65）斑晶。各期喷发岩的基性端元均为玄武岩、橄榄石（Fo=ca.80）和斜长石（An=ca.90）斑晶，但第二期喷发岩的总体MgO、Cr和Ni含量高于前两期。在第二和第三时期，岩石的二氧化硅含量从58%暂时下降到55- 56%，然后恢复到58%，这些变化可以解释为参与混合的玄武岩岩浆的不同百分比。这些特征表明，基性端元岩浆在不同时期是不同的，可能已经在地壳的较深处储存了大约3.5到2.0天。近距离观察过去约0.8 ky喷发的岩石化学成分，可以看到Zr含量向上部逐渐减少（Cr含量增加）至少两次，这可能对应于玄武岩岩浆向较浅的安山岩岩浆房的渐进注入，估计每次注入的持续时间小于0.2 ky。

In the youngest stage (30 ka to present) of the Zao volcano, three active periods (ca. 31 to 29 ka, 7.5 to 4.1 ka, and 2.0 to present) can be seen. Piles of pyroclastic rocks by numerous small to medium sized eruptions are main products of the activities. In this study we examined the magma feeding system in the three periods, based on the petrologic features of the products.Rocks erupted in the three periods are olivine+-pyroxene basaltic andesite to andesite, and these were formed by mixing of two end-member magmas, judged from the petrographic and mineralogic features. The estimated felsic end-members are similar among the periods, andesite (ca.60% in silica content) with orthopyroxene (Mg#=ca.64), clinopyroxene (Mg#=ca.68), plagioclase (An=ca.65) phenocrysts. The estimated mafic end-members are basalt with olivine (Fo=ca.80) and plagioclase (An=ca.90) phenocrysts in all periods, however, the bulk MgO, Cr and Ni contents of the erupted rocks are higher in the second period than in the other two periods. During the second and third periods, silica contents of the rocks decreased temporally from 58 to 55-56 % and recovered up to 58 %, and these variations can be explained by the different percentages of the basaltic magma involved in the mixing. Those features are suggesting that the mafic end-member magmas are distinct among periods, and may have been stored in the deeper part of the crust for ca.3.5 to 2.0 ky.Looking at the chemical compositions of rocks in the past ca.0.8-ky eruptions closely, gradual decrease in Zr (and increase in Cr) contents toward upper part can be seen at least twice, which may correspond to the progressive injection of the basaltic magma to the shallower andesitic magma chamber, and it is estimated that the duration of each injection is less than 0.2 ky.