[S-VC34] Volcanic and igneous activities, and these long-term forecasting

[SVC34-03] Explosive eruption record since 10 ka from tephrostratigraphy of Chokai volcano.

*Sota Kosuga¹, Tsukasa Ohba¹, Masao Ban², Takumi Imura² (1.Akita University, 2.Yamagata University)

Keywords:tephra, ash componentry, eruption history

Chokai Volcano, located on the back-arc side of the northeastern Honshu arc, consists of East Chokai, centered on the Shinzan summit, and West Chokai, centered on Chokai Lake. The volcanic edifice formation history is classified into three stages: Stage I, Stage II, and Stage III, with Stage II further subdivided into IIa, IIb, and IIc, and Stage III into IIIa and IIIb (Hayashi, 1984). Major explosive eruptions between the 2.8 ka Saruana lava eruption and the 10th century (To-a) eruption occurred at 2.8 ka, 2.5 ka, 2.1–1.9 ka, 1.8 ka, 1.6 ka, and 871 CE (Ohba et al., 2022, 2024). However, the eruption chronology and history before these events remain poorly understood. Additionally, multiple tephra deposits from explosive eruptions after the To-a eruption have been identified, but their detailed stratigraphy has yet to be fully established. Therefore, this study aims to elucidate the history of explosive eruptions older than 2.8 ka and younger than 1.1 ka. A tephra survey was conducted on the slopes of Chokai Volcano, with volcanic ash and soil samples collected from five locations. Soil samples were taken from directly beneath tephra layers and subjected to AMS radiocarbon dating at the Yamagata University Center for Accelerator Mass Spectrometry. Volcanic ash componentry analysis was performed under a stereomicroscope following the classification scheme of Ohba et al. (2022), distinguishing seven types of particles: irregular black ash particles (IBJ), blocky black ash particles (BBJ), irregular colorless glassy ash (IGJ), altered lithic particles (AL), pale gray lithic particles (PGL), Crystal particles (XL), and volcanic pisolite ash (PA). IBJ, BBJ, and IGJ were interpreted as juvenile components. The observed tephra layers predominantly consist of well-sorted coarse to fine volcanic ash, with some poorly sorted volcanic lapilli layers. Their colors range from reddish-brown and pale yellow to light and dark gray. Key layers included the widespread To-a tephra, tephra deposits associated with the 2.5 ka sector collapse, and the Torinoumi scoria, due to their distinctive lithological characteristics. AMS radiocarbon dating of the soil beneath four tephra layers at an outcrop in Senjojogahara, within the West Chokai horseshoe-shaped caldera, yielded ages of 5936 ± 25 yr BP, 6026 ± 25 yr BP, 6466 ± 26 yr BP, and 7479 ± 28 yr BP. Tephra layers dated between 10 ka and 6 ka are distributed exclusively around Chokai Lake. The proportion of juvenile components in the analyzed tephra layers ranged from 6% to 93%, with most layers containing 10–30% juvenile materials. Many tephra layers were rich in altered lithic fragments. The explosive eruption history of Chokai Volcano over the past 10 ka can be summarized as follows. The Torinoumi scoria eruption occurred before 10 ka. Between approximately 10 ka and 6 ka, volcanic activity in West Chokai was intense, characterized by frequent phreatomagmatic eruptions. Around 4.6 ka, the center of activity shifted from West Chokai to East Chokai, where it has continued to the present. While most eruptions were phreatomagmatic, volcanic ash component analysis suggests that a Vulcanian eruption occurred around 3.4 ka. In addition to the previously identified eruptions at 2.8 ka, 2.5 ka, 2.1–1.9 ka, 1.8 ka, 1.6 ka, and 871 CE, this study revealed that an eruption also occurred around 1200 CE.