ブルカノ式噴火の発生要因としての火道浅部マグマの増圧過程

抄録

Vulcanian eruptions are relatively frequent eruptive phenomena in island arc volcanism. The high frequency provides us many opportunities to observe the eruptive behavior and to sample the erupted materials. Vulcanian activity of the Sakurajima volcano located at southern Kyushu in Japan has continued since 1955 and been precisely monitored by Sakurajima Volcano Observatory. Therefore, the vulcanian eruptions of Sakurajima volcano can be a common, interesting target to study the eruptive behavior from the various viewpoints of material analysis, geophysical observation and numerical modeling. Recently, many researchers have focused on pressurization processes of magmas in shallow conduits to cause vulcanian eruptions. In this paper, we review several studies on observations and models of generation of excess pressure in shallow conduit magmas just before vulcanian eruptions. We evaluate the possibility of pressurization processes by dehydration-induced crystallization and buoyancy of vesicular conduit magmas. The results of this study indicate that the dehydration-induced crystallization is not effective for excess pressure generation, if crystallizing magma in shallow conduit contains a significant amount of exsolved gas phase. Instead, excess pressure can be generated in a shallow conduit by buoyancy of vesicular conduit magma, when partially solidified magma at the vent of conduit prevents magma ascent and the conduit is filled with vesicular magmas from magma chamber located at ca. 3 km depth to the shallow conduit.