5:15 PM - 7:15 PM
[SVC34-P14] Eruption processes of Yt-Pm4 at Yokodake volcano, northern Yatsugatake
Keywords:Yokodake, northern Yatsugatake, pumice, Yt-Pm4, eruption process
Yokodake Volcano, located at the northern end of Yatsugatake volcanic group in central Japan, is the only active volcano in the group. Yokodake volcano composed of nine andesitic lava flows (Y1-Y9) and one large-scale pumice deposit (Yt-Pm4, Kawachi, 1974). Based on geomorphological observations, Yt-Pm4 eruption was estimated to occur between the Y2 lava flow eruption and one of the Y3-6 lava flow eruptions (Kawachi et al., 1978). Radiocarbon dating of paleosol immediately below Yt-Pm4 indicates an age of 34-35 cal kyr BP (Shitaoka et al., 2023). The eruption volume is estimated at 2.0*10-1 km3 DRE (Oishi and Suzuki, 2004), which is an order of magnitude larger than the average volume of 1.8*10-2 km3 for lava flows emplaced after Y3 (Kawachi et al., 1978). Yt-Pm4 is a distinctive eruption for Yokodake, which has mainly produced lava flows. However, eruptive and magmatic conditions about Yt-Pm4 were less well understood. In this study, we conducted a geological investigation of Yt-Pm4, focusing on pumice grain size, vesicularity, componentry, and magnetic susceptibility in order to examine the eruptive processes.
Yt-Pm4 deposits were identified at more than 10 outcrops on the eastern foot of Yokodake volcano. The deposits consist of well-vesiculated, yellowish-brown, angular pumice, with andesitic lithic fragments and obsidian. Except for one proximal outcrop discussed later, pumice density and magnetic susceptibility increase continuously towards the upperpart of the deposits. There is no typical fine ash layers within the deposits, suggesting the deposit derives from single erutpion. At an outcrop 1.5 km east of the summit, the Yt-Pm4 reaches its maximum thickness of about 3 m. This outcrop has several depositional units: yellowish-brown pumice unit with a poor fine-grained matrix (hereafter referred to as g layer) and dark brown pumice unit with matrix rich (e layer). The pumice in the e layer exhibits a dark brown surface but the inner part shows yellowish-brown, like that of the g layer. The g-layer pumice has a mean maximum grain size of 4.3 cm, which is larger than that of the e layer (3.6 cm) and shows a slightly higher vesicularity. Magnetic susceptibility values for the g-layer pumice are concentrated around 4.0-5.0*10-6 m3/kg, whereas those for the e-layer pumice exhibit a wider range, extending up to approximately 1.0*10-5 m3/kg. The proportions of lithic fragments and obsidian vary inversely; in layers with a higher proportion of lithic fragments, the obsidian content is lower. Overall, the e layer contains approximately 13% more lithic fragments than the g layer.
These observations suggest that the g layer, characterized by highly vesiculated and coarser pumice, was emplaced during the climax phase of the eruption. In contrast, the e layer, which contains less vesiculated pumice with higher magnetic susceptibility, likely experienced a lower cooling rate sufficient to allow the crystallization of magnetic minerals. These variations may reflect fluctuations in the intensity of the eruption column. However, such characteristics were not observed at other outcrops located more than 5 km from Yokodake, suggesting that these characteristics are preserved only in proximal deposits near the vent.
Yt-Pm4 deposits were identified at more than 10 outcrops on the eastern foot of Yokodake volcano. The deposits consist of well-vesiculated, yellowish-brown, angular pumice, with andesitic lithic fragments and obsidian. Except for one proximal outcrop discussed later, pumice density and magnetic susceptibility increase continuously towards the upperpart of the deposits. There is no typical fine ash layers within the deposits, suggesting the deposit derives from single erutpion. At an outcrop 1.5 km east of the summit, the Yt-Pm4 reaches its maximum thickness of about 3 m. This outcrop has several depositional units: yellowish-brown pumice unit with a poor fine-grained matrix (hereafter referred to as g layer) and dark brown pumice unit with matrix rich (e layer). The pumice in the e layer exhibits a dark brown surface but the inner part shows yellowish-brown, like that of the g layer. The g-layer pumice has a mean maximum grain size of 4.3 cm, which is larger than that of the e layer (3.6 cm) and shows a slightly higher vesicularity. Magnetic susceptibility values for the g-layer pumice are concentrated around 4.0-5.0*10-6 m3/kg, whereas those for the e-layer pumice exhibit a wider range, extending up to approximately 1.0*10-5 m3/kg. The proportions of lithic fragments and obsidian vary inversely; in layers with a higher proportion of lithic fragments, the obsidian content is lower. Overall, the e layer contains approximately 13% more lithic fragments than the g layer.
These observations suggest that the g layer, characterized by highly vesiculated and coarser pumice, was emplaced during the climax phase of the eruption. In contrast, the e layer, which contains less vesiculated pumice with higher magnetic susceptibility, likely experienced a lower cooling rate sufficient to allow the crystallization of magnetic minerals. These variations may reflect fluctuations in the intensity of the eruption column. However, such characteristics were not observed at other outcrops located more than 5 km from Yokodake, suggesting that these characteristics are preserved only in proximal deposits near the vent.