Temporal change of the mode of eruptive activity and the magma plumbing system of Sakurajima Volcano since 20th century : Implications for forecast future eruptive activity
Abstract
Sakurajima volcano is a post-caldera volcano of Aira caldera and has repeated large plinian eruptions with dormant periods in AD 1471, AD 1779 and AD 1914. After AD 1914 eruption, medium scale of lava effusion occurred in AD 1946. Since AD 1955, frequent vulcanian eruptions have repeated until now. Thus, mode of eruptive activity of the volcano has changed since 20th century. Based on temporal change of petrological features of these eruptive materials, We discuss the relationship between the mode of eruptive activity and magma system to forecast the future eruptive activity. The rocks of AD 1471 and AD 1779 eruptions are CPX-OPX dacite, in which normally and reversely zoned pyroxene and plagioclase phenocrysts coexist. In addition, compositional distribution of plagioclase phenocrysts is bi-modal. These suggest that these rocks are mixing products between dacitic and andesitic magmas. This is consistent with compositional variations of whole-rock chemistry for these rocks. On the other hand, the rocks of AD 1914 and AD 1946 eruption often contain olivine phenocrysts. Plagioclase and pyroxenes phenocrysts in these rocks show similar features to those of AD 1471 and AD 1779 eruptions, suggesting that these rocks are also mixing products of two end-member magmas, dacitic and andesitic ones. However, olivine phenocrysts are much magnesian compared with pyroxenes phenocrysts, indicating that these olivine phencorysts are derived from another basaltic magma. Thus, the basaltic magma injected into the mixed magma between dacitic and andesitic ones. Mixing among three magmas has been recognized since 20th century. The rocks from frequent eruptions since AD 1955 also contain minor amount of olivine phenocrysts, suggesting the injection of basaltic magma has continued. In 1970's and AD 1987 periods, relatively larger scale of vulcanian eruptions had occurred. The rocks from these periods contain considerable amount of olivine phenocrysts, indicating mixing ratio of the basaltic magma was relatively larger than that of other periods. Thus, the effect of the injection would correlate with the scale of eruption. According to geophysical monitoring, voluminous magma has accumulated beneath the Aira caldera, and part of the magma has moved beneath Sakurajima volcano to erupt. Our petrological analysis suggests that monitored voluminous magma beneath Aira caldera would be dacitic magma, in which andesitic magma has repeatedly injected. Before 20th century, the mixed magma had moved toward the volcano to accumulate for large plinian eruptions. In contrast, injection of basaltic magma has repeated to mix with the pre-existed mixed magma since 20th century. This injection would occur beneath the volcano, because timing of the injection must not be so long before eruption. In other words, frequent eruptions had been triggered by the injection. In addition, large scale of the injection would cause larger eruptions. Temporal change of mode of eruptive activity since 20th must be related with frequent injection of basaltic magma. In order to forecast future eruptive activity, detecting of movement of the basaltic magma should be important.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.V21E2541N
- Keywords:
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- 8400 VOLCANOLOGY