[S-VC36] Volcanic and igneous activities, and these long-term forecasting

[SVC36-P12] Emplacement temperature estimation of Kikai-Akahoya deposits of Kikai Caldera volcano by paleomagnetic measurements

*Reina NAKAOKA¹, Takahiro Toyoda², Keiko Suzuki-Kamata¹ (1.Kobe Ocean-Bottom Exploration Center, Kobe University, 2.Faculty of Science, Kobe University)

Keywords:Kikai caldera, pyroclastic flow, emplacement temperature

Kikai caldera volcano is located about 50 km south of the Satsuma Peninsula and is an active volcano that exists mostly on the seafloor. Caldera eruptions with large-scale pyroclastic flows have repeated at Kikai caldera volcano (Ono, et al., 1982). The latest eruption of the 7.3 ka Kikai Akahoya eruption began as a Plinian eruption, depositing Funakura pumice fall and Funakura pyroclastic flows. Subsequently, a large-scale pyroclastic flow, the Koya ignimbrite, occurred with Akahoya ash over a wide area (Ono, et al., 1982; Maeno and Taniguchi, 2007). Koya ignimbrite was deposited on the caldera walls of Takeshima and Satsuma-Iwo-jima with about 30 m thick, and thin deposits with a 1 m thick or less were deposited on Satsuma Peninsula, Osumi Peninsula, and surrounding islands, 40-60 km across the sea. On land, the Koya ignimbrite have been known as a low aspect ignimbrite because they are deposited very thinly over a wide area in the southern part of the Satsuma and Osumi Peninsula (Ui, 1973). Seismic reflection surveys by T.S. Fukae Maru of Kobe University have also revealed that the deposits corresponding to the Koya ignimbrite was deposited thickly over a wide area on the surrounding seafloor (Shimizu, in Prep.). However, how the Koya ignimbrite generated in the sea area flowed and deposited on the sea surface and sea floor has not been investigated using quantitative information such as temperature. The purpose of this study is to estimate the cooling process and flow and depositional process of the Koya ignimbrite as they crossed the sea based on their emplacement temperatures. In this presentation, we attempted to estimate the emplacement temperature at Satsuma Iwo Jima and Takeshima Island located on the caldera wall, which is the initial condition of ignimbrite before they cross the sea. On Satsuma Iwo Jima, 10 oriented samples of lag breccia and 5 oriented samples of volcanic ash in the lowest part of the Koya ignimbrite were collected at two sites and at one site, respectively. On Takeshima Island, 7 oriented samples (3 pumice, 3 scoria, and a lithic fragment) in the middle of the thickness of the ignimbrite were collected at one site. We also collected 10 and 6 samples of pumice from the Funakura pumice fall deposit and welded part from the Funakura pyroclastic flow deposit, respectively, which were deposited prior to the Koya ignimbrite during the Kikai Akahoya eruption on Satsuma Iwo Jima. Each sample was conducted to stepwise thermal demagnetization experiments and principal component analysis. Most of the remanent magnetization of the lowest part of the Koya ignimbrite of Satsuma-Iwojima, the lag-breccia and volcanic ash, starts ~200°C and is stable up to 590 or 640°C. The direction of the remanent magnetization is aligned with the direction of the earth's magnetic field at that time, suggesting that it was emplaced at more than 590 or 640°C. 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 temperatures emplacement. The magnetization of Funakura pumice fall and pyroclastic flow deposits collected from Satsuma Iwo Jima have stable magnetization from ~300°C to 590°C and from ~500°C to 640°C, respectively, and the magnetic direction is aligned with the direction of the earth's magnetic field at that time, suggesting that they were emplaced at more than 590°C and 640°C, respectively. The difference between the mean magnetic direction of the Funakura pyroclastic flow and that of the lower part of the Koya ignimbrite suggests that the lag breccia was not heated by the lower Funakura pyroclastic flow after deposition, but was heated to a high temperature above 590°C by juvenile materials in the conduit and pyroclastic flow and then deposited. We will measure the emplacement temperature after passing through the sea area and study the flow and emplacement process of ignimbrite generated in the sea area.