火山灰中の本質ガラスの石基組織からみる桜島火山2012年7月24日南岳山頂火口噴火

—特にマイクロライトの特徴について—

抄録

On the 24^th July, 2012, a large scale explosion occurred at Minamidake Summit crater of Sakurajima volcano, for the first time in last 1.5 years. This eruption is characterized by a clear, large preceding inflation ca.22 hours before the eruption. The juvenile glass particles in the volcanic ash of this eruption have less amount of microlite than those of Showa crater. The crystal size of microlite is also smaller. In contrast, their microlite number densities (MND) of plagioclase and pyroxenes are similar to those of Showa crater. According to the results of the decompression experiments by previous studies, the variations of crystallinity of microlite with the constant MND can be explained by the difference in the length of the duration for decompression, and/or the duration until the quench after the decompression. Considering these results, the juveniles of the summit eruption are derived from the eruption induced by relatively rapid decompression with rapid quench, and the juveniles from Showa crater are the products of the eruptions accompanied with the relatively slower decompression and/or the longer annealing after decompression. Therefore, it is interpreted that the 24^th July, 2012 eruption was caused by rapid magma ascent with much shorter stagnation in the conduit. In contrast, the eruptions at Showa crater might have been induced by slower magma ascent and/or longer stagnation at a shallower depth of the conduit. This interpretation agrees with the data of geophysical observations. On matrix glass chemistry, juveniles of this summit eruption show the distinct trend from those of Showa crater. This feature can be produced by the difference in mode of microlite, which are crystallized during magma ascent. Accordingly, it is possible to evaluate the difference in magma ascent process, using the matrix glass chemistry.