Historical eruptions at Aso Volcano, Japan, spanning the last 800 years: insights from a high-resolution tephra record

Abstract Nakadake Volcano in Aso Caldera, central Kyushu, southwest Japan, is among the most active volcanoes in Japan. Eruption records date back to 553 CE, with over 150 eruptions documented since 1239 CE. However, details of individual eruptions and their geological correlations remain unclear. We conducted a trench survey ~ 1 km west of the Nakadake crater to reconstruct the volcano’s historical eruptive sequence. Here, we describe the tephra layers, radiocarbon dating, grain size analysis, microscopic observation, and chemical analysis. The 6-m-high trench revealed 336 thin ash layers (average thickness, 1.8 cm), indicating frequent small-scale eruptions. Based on tephra characteristics, analytical results, and 14 C dating, the sequence was divided into four zones: Zone A (0–158 cm, mid-1700s to present), Zone B (163–333 cm, late 1400s to early 1700s), Zone C (343–416 cm, mid-1300s to mid-1400s), and Zone D (416–595 cm, 1239 CE to mid-1300s). Zone D tephras, primarily dark coarse ash layers rich in juvenile glass shards, suggest dominant ash emissions and strombolian eruptions. Zone C tephras indicate that dominant eruptive styles transitioned from magmatic to phreatic or phreatomagmatic activity. Zone B tephras feature alternating light-colored fine ash layers with lithic fragments and altered minerals, indicating frequent phreatic and/or phreatomagmatic eruptions over 200–250 years. Zone A tephras, mostly dark coarse ash layers, imply dominance by ash emissions and strombolian eruptions in the last 250 years. The total bulk tephra volume for the past 800 years was ~ 0.35 km 3 , yielding an average discharge rate of 0.2 km 3 /ky (dense rock equivalent), consistent with the last 67,000 years. This suggests that eruptive activity at Nakadake over the past 800 years was typical for the post-caldera stage of Aso Volcano, supported by a stable magma supply system with minimal chemical variation. However, eruption types varied between Zones A and D. Frequent phreatic/phreatomagmatic eruptions between the late 1400s and early 1700s likely disrupted human activities, potentially contributing to the decline of the Furubochu mountain temple site (1–2 km west of the crater). This study highlights the historical eruption transitions and provides insights for predicting future volcanic activity and mitigating related hazards at Nakadake Volcano.