1:45 PM - 3:15 PM
[SVC36-P07] Eruption age estimation and distribution of the Y5 and Y6 lava flows at Yokodake volcano, Northern Yatsugatake, Japan
Keywords:lava, age estimation, distributioon, yokodake,northern yatsugatake
Yokodake volcano, located among the northern Yatsugatake volcano group in central Japan, is the only active volcano in the group. Although the volcano has been currently quiet, nine lava flow eruptions (Y1-Y9) and one large explosive eruption (Yt-Pm4) had occurred (Kawachi, 1974). The eruption ages of the latest two lava flows are reported to be 0.6 ka and 3.4 ka by Nitta et al. (2020), but the ages of the other lava flows were not determined. The eruption age of Yt-Pm4 is estimated to be about ca. 35 cal ka BP by radiocarbon dating (Oishi, 2015), but the stratigraphy of the lavas and the tephra was not established; Ohba and Kawachi (1997) suggested Yt-Pm4 was erupted before the Y6 lava eruption based on the chemical composition of the volcanic materials, while Kawachi et al. (1978) suggested Yt-Pm4 eruption might have occurred between Y2 and Y5 eruption. In this study, we determined the eruption age of the Y5 and Y6 lava flows using paleomagnetic methods and reexamined the distribution of the flows.
Fifty-six and forty-nine paleomagnetic samples were collected from the Y5 lava flow and Y6 lava flow, respectively. We determined the components of remanent magnetization from PThD experiments. Paleointensities were estimated by using the IZZI method (Yu and Tauxe, 2005).
Paleointensity of Y5 lava is estimated to be 54.0-57.4 μT. Compared with the paleointensity curve (0.1-7 ka; Cai et al., 2016, 5ka-150 ka; Channell et al., 2009), ages of about 0.6-4.0 ka, 48.2-52.7 ka, 80 ka, and 92 ka were estimated. Paleodirection of magnetic components obtained from Y5 and Y6 lava is Dec=9.7°, Inc=49.5° (α95=7.2°) and Dec=-16.1°, Inc=47.1° (α95=20.0°), respectively. Palaeomagnetic age was estimated by comparing our results with paleodirection obtained from Lake Biwa sediments (Hayashida et al., 2007). Several paleodirection age was estimated. Combining the paleodirection ages with the paleointensity ages, the paleomagnetic age of Y5 lava was inferred as ca 50 ka or older.
In addition, we found that some lava blocks, sampled within the distribution of Y5 lava by Kawachi (1974), showed different paleomagnetic and petrological characteristics from Y5 lava samples. The thermoluminescence (TL) glow curves of the Y5 lava are different from those of the Y5 lava by TL measurements, suggesting that the TL characteristics of these lava are different from those of the Y5 lava. It is suggested that the lava distribution differs from Kawachi's previously reported area (1974).
Fifty-six and forty-nine paleomagnetic samples were collected from the Y5 lava flow and Y6 lava flow, respectively. We determined the components of remanent magnetization from PThD experiments. Paleointensities were estimated by using the IZZI method (Yu and Tauxe, 2005).
Paleointensity of Y5 lava is estimated to be 54.0-57.4 μT. Compared with the paleointensity curve (0.1-7 ka; Cai et al., 2016, 5ka-150 ka; Channell et al., 2009), ages of about 0.6-4.0 ka, 48.2-52.7 ka, 80 ka, and 92 ka were estimated. Paleodirection of magnetic components obtained from Y5 and Y6 lava is Dec=9.7°, Inc=49.5° (α95=7.2°) and Dec=-16.1°, Inc=47.1° (α95=20.0°), respectively. Palaeomagnetic age was estimated by comparing our results with paleodirection obtained from Lake Biwa sediments (Hayashida et al., 2007). Several paleodirection age was estimated. Combining the paleodirection ages with the paleointensity ages, the paleomagnetic age of Y5 lava was inferred as ca 50 ka or older.
In addition, we found that some lava blocks, sampled within the distribution of Y5 lava by Kawachi (1974), showed different paleomagnetic and petrological characteristics from Y5 lava samples. The thermoluminescence (TL) glow curves of the Y5 lava are different from those of the Y5 lava by TL measurements, suggesting that the TL characteristics of these lava are different from those of the Y5 lava. It is suggested that the lava distribution differs from Kawachi's previously reported area (1974).