5:15 PM - 7:15 PM
[SVC34-P06] Emplacement temperature of the Koya pyroclastic flow across the sea from the Kikai caldera
The Kikai caldera is a caldera volcano located about 50 km south of Kyushu, and several caldera eruptions with large-scale pyroclastic flows have occurred (Ono, et al., 1982). The most recent eruption, the 7.3 ka Kikai-Akahoya eruption, initiated by a Plinian eruption and 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 is thick with lag breccias at the caldera walls of Takeshima and Satsuma Iwo Jima, with a maximum thickness of 30 m at Takeshima. On the distal islands 30-60 km across the sea from the caldera rim, the ignimbrite is thin, about 1 m thick, and have been known as a low aspect ratio ignimbrite (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 flow generated in sea area is affected by seawater as they flow over the sea has not been examined 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. For the distal areas, 45-65 km away from the caldera rim (35-60 km at sea), 6-8 oriented matrix samples were collected from 3, 5, and 3 sites on the Satsuma and Osumi Peninsulas and Tanegashima, respectively. 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 estimated to have emplaced at < 150 °C. For the distal matrix samples, it is estimated from the maximum demagnetization temperature of stable magnetization that they were deposited at 150-400°C in the Satsuma Peninsula, 350-450°C in the Osumi Peninsula, and 150-350°C in the Tanegashima, and that the Osumi Peninsula, with shorter sea distance, tended to be emplaced at higher temperature than the Satsuma Peninsula or Tanegashima.
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. Thus, it is concluded that the Koya ignimbrite was initially emplaced at high temperature near the source as a lag-breccia part, and that it was cooled by seawater as it crossed the sea, and retained its high temperature and was deposited. As the eruption progressed, the flow was cooled by ambient water at the source and deposited at low temperatures only in the proximal area.
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. Thus, it is concluded that the Koya ignimbrite was initially emplaced at high temperature near the source as a lag-breccia part, and that it was cooled by seawater as it crossed the sea, and retained its high temperature and was deposited. As the eruption progressed, the flow was cooled by ambient water at the source and deposited at low temperatures only in the proximal area.