Quaternary Volcanoes > Active Volcanoes > Sakurajima

Volcanic Rocks of Sakurajima Volcano

   The lava and pyroclastic material that erupted from Sakurajima Volcano consists of medium-K series andesite–dacite. The compositions clearly differ between eruption periods. ( Fig. 7). The Younger Kitadake Volcano is dacitic. The Older Minamidake Volcano is andesitic. The Younger Minamidake Volcano that erupted during the Tenpyo-Hoji eruption is andesitie–dacite. During the Bunmei eruption, mostly felsic dacite was expelled. After the An-ei eruption, the SiO2 content of the ejecta had decreased, and the Taisho and Showa eruptions erupted andesite.

   When a comparison of ejecta between that of the younger Minamidake volcano and the Younger Kitadake/Older Minamidake Volcanoes was performed for the same SiO2 content, there were no differences in K2O, Rb, Ba and other incompatible elements. However, the ejecta from the Younger Minamidake Volcano were systematically enriched in TiO2, FeO, Na2O, P2O3, Zr and Y, but poor in MgO and CaO (Uto et al., 2005). Thus, the magmatic supply system had changed at the outset of volcanic activities at the Younger Minamidake. The magma supply system change is striking for trace element ratio like Zr/Th ( Fig. 8 -1). This ratio decreased modestly from the Older Kitadake ejecta to the Older Minamidake ejecta, but there was a rapidly increasing trend for Younger Minamidake ejecta. However, the mafic inclusions in the Younger Kitadake Lavas have a Zr/Th ratio that is even higher than these. The 87Sr/86Sr isotope ratio exhibits a positive correlation with the SiO2 content, so time changes cannot be treated simply. However, the 87Sr/86Sr isotope ratio tends to decrease with age as an overall trend ( Fig. 8 -2). The lowest 87Sr/86Sr isotope ratio in the Sakurajima volcano is from the mafic inclusions of the Younger Kitadake Lavas.

   Essential to the research into the evolution of the magmatic supply system of Sakurajima Volcano are the magma chambers of Aira Caldera. When the caldera formed, the 87Sr/86Sr isotopic ratio represented by the Ito Pyroclastic Flow Deposits were high, and felsic magma from the earth' s crust with a low Zr/Th ratio had been expelled. The existence of a magma chamber under the caldera even after the felsic magma was expelled suggests that the Shinjima Pyroclastic Flow erupted from Wakamiko Caldera about 16,000 years ago.

   On the other hand, for mafic end-members, the 87Sr/86Sr isotope ratio represented by the mafic inclusions in the Younger Kitadake Lavas are low and there is mafic magma from mantle with a high Zr/Th ratio.

   It appears that Sakurajima Volcano is formed by andesite-dacite magma of an intermediate composition or a mixture of both, and broadly as the age becomes younger, it appears that the proportion of mafic magma originating from the mantle increases (Uto et al., 2005). However, the chemical composition of the felsic end-members is not clear, as it seems that they are replaced by ones of different origin for each eruption period (Takahashi et al., 2011).

   Thus, it appears that the build-up of mafic magma in the magma chamber that lead to the Taisho eruption is still taking place because the chemical composition of ejecta from the Taisho/Showa eruptions to the current eruptions make identical trend changes, and because of the uplift that has occurred at the center of Aira Caldera since the Taisho eruption.

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