[S-VC30] Volcanic and igneous activities, and these long-term forecasting

[SVC30-P16] Emplacement temperature estimation of Koya pyroclastic flow deposits of Kikai caldera volcano by paleomagnetic measurement

*Reina NAKAOKA¹, Himika ONO², Takahiro TOYODA², Keiko SUZUKI³ (1.Graduate School of Science , Kobe University, 2.Faculty of Science, Kobe University, 3.Kobe Ocean-Bottom Exploration Center, Kobe University)

Kikai Caldera is an active caldera volcano located about 50 km south of the Satsuma Peninsula, mostly on the seafloor. Several caldera eruptions have occurred in the Kikai caldera, and large-scale pyroclastic flows have erupted (Ono, et al., 1982). The most recent eruption, the 7.3 ka Kikai-Akahoya eruption, began with a Plinian eruption, which deposited Funakura pumice fall and Funakura pyroclastic flow. Subsequently, a large-scale pyroclastic flow, the Koya pyroclastic flow, was generated as a result of caldera collapse with Akahoya ash fall deposits (Ono, et al., 1982; Maeno and Taniguchi, 2007). The Koya ignimbrite are distributed in the caldera walls of Takeshima and Satsuma Iwo Jima as thick as 30 m, and are thinly distributed within 1 m thick on the surrounding islands 40-60 km across the sea. On land, the Koya ignimbrite has been known as a low aspect ratio ignimbrite because it is extremely thinly deposited over a wide area in the southern Satsuma and Osumi Peninsulas (Ui, 1973). Reflection seismic surveys by the Kobe University T/S Fukae Maru have also revealed extensive thick deposits corresponding to the Koya ignimbrite on the surrounding seafloor (Shimizu et al., 2024). However, how large-scale pyroclastic flows generated in the sea area were separated and flowed across the sea and the seafloor has not been investigated using quantitative information such as temperature. In this study, we estimate the emplacement temperature of the Koya ignimbrite across the sea area from paleomagnetic measurements and investigate its flow and depositional processes. On Satsuma Iwo Jima, 10 oriented block samples (lithic and pumice) from each of the two sites and 5 oriented matrix samples from one site were collected from the lag breccia part of the lowest Koya ignimbrite at two sites. On Takeshima, 7 to 9 oriented block samples (pumice, scoria, and lithic) were collected from each of the four stratigraphic levels. Across the sea, on the Osumi Peninsula, 50-60 km from the caldera rim, oriented matrix samples were collected at three sites, 4-9 samples each. Stepwise thermal demagnetization and principal component analyses were conducted on each sample. The remanent magnetization of the lag-breccia and lowest volcanic ash in the Koya ignimbrite at Satsuma Iwo Jima is estimated to have been mainly emplaced above 590 or 640°C because the maximum demagnetization temperatures are stable up to 590 or 640°C and their directions are aligned. On the other hand, pumice, scoria, and lithic fragments in the Koya ignimbrite on Takeshima showed no stable magnetic components and no evidence of high-temperature emplacement. Remanent magnetization of matrix samples collected in the Osumi Peninsula is mainly stable with a maximum demagnetization temperature of 350-550°C, and their directions are aligned in the same direction within the same location, suggesting that they were emplaced at 350-550°C.
The chemical composition of the volcanic glass in Koya ignimbrite near the source is characterized by the presence of only high-Si glass in the lower part and an increase in low-Si glass content toward the upper part, reaching more than 20%. Similarly, in the distal Koya ignimbrite, only high-Si glass is contained in the lower part, and low-Si glass increases toward the upper part, but its content is about 5%. Thus, it is presumed that the early stages of the Koya pyroclastic flow reached distal areas, but when the eruption progressed and the low-Si glass content increased to more than 20%, the pyroclastic flow did not reach the distal areas and was deposited only proximal areas. Therefore, it is possible that the Koya pyroclastic flow was deposited at high temperature near the source in the early stage of the eruption as a lag-breccia part and remained at high temperature in the distal area, but as the eruption progressed, it was cooled by external water at the source and deposited at low temperature only in the proximal area.