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

[SVC30-03] Stratigraphy of lava flows in the Higashi Chokai amphiteater, Chokai volcano, northern Japan arc

*Arata Murayama¹, Tsukasa Ohba¹, Masamichi Ikeda¹, Akira Chiba¹, Yukio Fujimoto (1.Akita University)

Chokai volcano is a stratovolcano located at the back arc of northern Honshu arc and the Higashi Chokai Amphitheater was formed to sector collapse approximately 2,500 years ago. The lava that repeatedly flowed down within the Higashi Chokai Amphitheater after the sector collapse and well preserved topographically. Since ca. 2,500 years ago, at least five relatively large explosive eruptions have occurred, many of which were accompanied by lahars (Ohba et al., 2022). However, the age relationship between the lava flows filling the Higashi Chokai caldera, explosive eruptions, and lahars has not been clarified. Since ca. 2,800 years ago, eruption volume of Chokai volcano has more lava than tephra (Ohba et al., 2024). Clarifying the stratigraphy of lava flows is essential to elucidating history of volcanic activity. We performed topographical analysis, geological survey, and petrological analysis of lava excluding historical lava, and examined the stratigraphy of lava from about 2,500 years ago to before historical eruptions.
Thirty-one lava lobes (lava lobes A-AE) were recognized topographically, and their stratigraphy was determined from stratigraphic relationship. The mineral assemblage of Higashi Chokai Amphitheater lavas generally includes plagioclase, orthopyroxene, clinopyroxene, olivine, and phenocrysts of opaque minerals, and some samples contain hornblende. Some lack olivine phenocrysts. Higashi Chokai Amphitheater lavas are classified into four mineral assemblage types (1: plagioclase + orthopyroxene + clinopyroxene + olivine 2: plagioclase + orthopyroxene + clinopyroxene + olivine + hornblende,3: plagioclase + orthopyroxene + clinopyroxene + hornblende,4: plagioclase + orthopyroxene + clinopyroxene).
Whole-rock chemical analysis shows SiO₂ content varies from 56 to 62 wt%, and many samples are classified as andesite and some as basaltic andesite. Almost all samples are plotted on Medium-K series on the SiO₂-K₂O diagram. In addition, all samples plot in the calc-alkaline series on the SiO₂-FeOt/MgO diagram.
Correlation of lava lobes to define lava units were performed based on the relationship between lava stratigraphy, mineral assemblage and whole-rock composition. As a result, Higashi Chokai Amphitheater lavas ware classified into 11 units.

References

Ohba T., Hayashi S., Ban M., Imura T., Minami Y. and Endo M. (2022): Late Holocene tephrostratigraphy at Chokai volcano, northern Japan, and contribution to hazard assessment. Journal of Disaster Research,17(5), 724-735
Ohba T., Hayashi S., Ban M., Imura T., Ikeda M., Hirata A. and Kikuchi E. (2024): Magma Discharge Step-Diagram of Chokai volcano -Changes in Eruption Volume, Especially since 2,800 Years Ago-