15:30 〜 15:45
[MIS18-13] Drilling of Kikai caldera volcano for elucidation of magma plumbing system and flow-deposition processes of large ignimbrite
★Invited Papers
キーワード：鬼界カルデラ火山、マグマ供給系、海底火砕流
Caldera volcanoes universally exist in volcanic regions with continental crust and their activities are characterized by catastrophic caldera-forming eruptions (CCFEs) that discharge more than 40 km3 of silicic magma as pyroclastics. The CCFEs are rare but extremely hazardous events. Mechanisms of magma generation and eruption in the caldera volcanoes are important to understand igneous processes in continental crust and mitigate volcanic hazard.
Kikai caldera volcano in the southwest Japan arc caused three CCFEs in the past, has a caldera of 19×22 km formed by the latest 7.3-ka CCFE, and is active at the present. So far, we have reported a giant lava dome inside the caldera that was formed at the post caldera stage and accurate volume estimation of the latest CCFE. We have been comprehensively researching it to know all about it with surveys of geology, bathymetry, seismic reflection, geomagnetic field, and resistivity structure and analyses of volcanic products in the Kikai caldera region. Our purposes are to clarify the current situation of magma beneath the volcano, mechanisms and evolution of the magma plumbing system, and transportation and deposition processes of submarine pyroclastics.
In our project on Kikai caldera volcano, drilling, coring, and dredge are conducted for the last two purposes. Some volcanics samples by coring and dredge in Kikai sea area were obtained in 2019 and 2020. These samples with the previously obtained samples clearly show temporal change of petrological features of the Kikai magmas in whole-rock composition. The cored samples were obtained down to the second CCFE deposit (95 ka) and show different occurrence of the submarine pyroclastic deposits from the subaerial ones. In order to advance understanding of evolution of the Kikai magmas and sedimentological processes of the submarine pyroclastics, further drilling at a proximal subaerial area of Kikai caldera volcano is needed, because Kikai caldera volcano which is a submarine volcano gives us only limited exposure of its ejecta and is not fully understood in terms of its detailed history and occurrence of pyroclastic ejecta. Collecting volcanics samples throughout the activity of Kikai caldera volcano allow us to elucidate magmatic evolution in petrological and geochemical characteristics. Comparison between the submarine and subaerial pyroclastic deposits in the same eruption clearly shows key processes of pyroclastic transportation and deposition in submarine condition; volcanic fields that have both the submarine and subaerial deposits from the same eruption source are rare and precious.
We propose new drilling of 300 m depth at two sites in Takeshima island which is a subaerial area of Kikai caldera volcano to proceed with the investigations mentioned above, and submitted the preliminary proposal to the ICDP. The two sites are near the outcrops of the first and second CCFEs (140 ka and 95 ka, respectively). We plan to clarify eruption sequences of the older two CCFEs which are much less understood and activity of the volcano at the early stage, and analyze petrographical and geochemical characterization of the samples with many analyses such as major/trace elements composition, isotope composition, volatile content for whole-rock, volcanic glass, and phenocryst. On the basis of these results, we attack to reveal mechanisms of magma processes for caldera volcanoes. We plan to analyze sedimentary structure, grain size, and component for the submarine and subaerial pyroclastic deposits, compare the sedimentological characteristics between them, and reconstruct traveling and sedimentation of pyroclastic clasts produced by submarine eruptions.
Kikai caldera volcano in the southwest Japan arc caused three CCFEs in the past, has a caldera of 19×22 km formed by the latest 7.3-ka CCFE, and is active at the present. So far, we have reported a giant lava dome inside the caldera that was formed at the post caldera stage and accurate volume estimation of the latest CCFE. We have been comprehensively researching it to know all about it with surveys of geology, bathymetry, seismic reflection, geomagnetic field, and resistivity structure and analyses of volcanic products in the Kikai caldera region. Our purposes are to clarify the current situation of magma beneath the volcano, mechanisms and evolution of the magma plumbing system, and transportation and deposition processes of submarine pyroclastics.
In our project on Kikai caldera volcano, drilling, coring, and dredge are conducted for the last two purposes. Some volcanics samples by coring and dredge in Kikai sea area were obtained in 2019 and 2020. These samples with the previously obtained samples clearly show temporal change of petrological features of the Kikai magmas in whole-rock composition. The cored samples were obtained down to the second CCFE deposit (95 ka) and show different occurrence of the submarine pyroclastic deposits from the subaerial ones. In order to advance understanding of evolution of the Kikai magmas and sedimentological processes of the submarine pyroclastics, further drilling at a proximal subaerial area of Kikai caldera volcano is needed, because Kikai caldera volcano which is a submarine volcano gives us only limited exposure of its ejecta and is not fully understood in terms of its detailed history and occurrence of pyroclastic ejecta. Collecting volcanics samples throughout the activity of Kikai caldera volcano allow us to elucidate magmatic evolution in petrological and geochemical characteristics. Comparison between the submarine and subaerial pyroclastic deposits in the same eruption clearly shows key processes of pyroclastic transportation and deposition in submarine condition; volcanic fields that have both the submarine and subaerial deposits from the same eruption source are rare and precious.
We propose new drilling of 300 m depth at two sites in Takeshima island which is a subaerial area of Kikai caldera volcano to proceed with the investigations mentioned above, and submitted the preliminary proposal to the ICDP. The two sites are near the outcrops of the first and second CCFEs (140 ka and 95 ka, respectively). We plan to clarify eruption sequences of the older two CCFEs which are much less understood and activity of the volcano at the early stage, and analyze petrographical and geochemical characterization of the samples with many analyses such as major/trace elements composition, isotope composition, volatile content for whole-rock, volcanic glass, and phenocryst. On the basis of these results, we attack to reveal mechanisms of magma processes for caldera volcanoes. We plan to analyze sedimentary structure, grain size, and component for the submarine and subaerial pyroclastic deposits, compare the sedimentological characteristics between them, and reconstruct traveling and sedimentation of pyroclastic clasts produced by submarine eruptions.