5:15 PM - 7:15 PM
[SVC33-P09] Distribution of the Kikai-Akahoya ash in central and southern Kyushu
Keywords:Caldera-forming eruption, Co-ignimbrite ash, Kikai Akahoya ash
Most large-scale caldera-forming eruptions generate voluminous pyroclastic density currents (PDCs) with a runout distance of 10–100 km at their climax phases. These PDCs further generate buoyant plumes (co-PDC plumes) and ash fallout deposits (co-ignimbrite ash fall deposits). Co-ignimbrite ash fall deposits can exceed 100 km³ in eruptive volume and are much voluminous and more widespread than normal Plinian fallouts. Therefore, understanding the transport and depositional processes of the co-ignimbrite ash deposits is crucial for elucidating the mechanism of caldera-forming eruptions. However, few previous studies have conducted detailed geological surveys of co-ignimbrite ash fall deposits. This study shows the distribution and sedimentary structure of a co-ignimbrite ash fall deposit of the 7.3 ka Kikai caldera eruption (Akahoya ash) in southern and central Kyushu and presents a preliminary discussion of its transport and depositional processes.
The Akahoya ash exhibits a normal grading, consisting of a basal part with coarse pumice, free crystals, and accretionary lapilli, and a main part with fine vitric ash rich in bubble-wall glass shards, except in small areas near the source PDCs (Koya ignimbrite). Accretionary lapilli in the basal part are rarely observed within the range of the source PDCs but become increasingly abundant in regions 5–10 km beyond their maximum extent. Thickness and grain size distributions of the Akahoya ash indicate a dispersal axis trending overall east to east-southeast. Thickness does not show a simple monotonic decrease with distance from the Kikai caldera. A localized increase, reaching almost 100 cm, is observed around the Kirishima volcano and the Kyushu Mountains. In contrast, the maximum grain sizes of the coarse pumice and accretionary lapilli in the basal part decrease systematically with distance from the Kikai caldera. Their decreasing trends are similar to those of large-scale Plinian and phreatoplinian eruptions, in which tephra transport is primarily driven by gravitational flow within an umbrella cloud.
Based on these observations, we propose a conceptual model to describe the transport and depositional processes of the Akahoya ash. Within the co-PDC plume that generated the Akahoya ash, coarse components (coarse pumice and accretionary lapilli) with higher terminal velocities were transported by gravitational flow within the umbrella cloud and deposited earlier than the fine component. The accretionary lapilli likely formed within the co-PDC plume as it ascended and expanded. In contrast, the fine vitric ash, which dominates the main part of the Akahoya ash, remained suspended in the atmosphere for a long time and settled more slowly than the coarser components. The localized increase in thickness observed around the Kirishima volcano and the Kyushu Mountains may have resulted from aggregation processes facilitated by specific meteorological conditions.
The Akahoya ash exhibits a normal grading, consisting of a basal part with coarse pumice, free crystals, and accretionary lapilli, and a main part with fine vitric ash rich in bubble-wall glass shards, except in small areas near the source PDCs (Koya ignimbrite). Accretionary lapilli in the basal part are rarely observed within the range of the source PDCs but become increasingly abundant in regions 5–10 km beyond their maximum extent. Thickness and grain size distributions of the Akahoya ash indicate a dispersal axis trending overall east to east-southeast. Thickness does not show a simple monotonic decrease with distance from the Kikai caldera. A localized increase, reaching almost 100 cm, is observed around the Kirishima volcano and the Kyushu Mountains. In contrast, the maximum grain sizes of the coarse pumice and accretionary lapilli in the basal part decrease systematically with distance from the Kikai caldera. Their decreasing trends are similar to those of large-scale Plinian and phreatoplinian eruptions, in which tephra transport is primarily driven by gravitational flow within an umbrella cloud.
Based on these observations, we propose a conceptual model to describe the transport and depositional processes of the Akahoya ash. Within the co-PDC plume that generated the Akahoya ash, coarse components (coarse pumice and accretionary lapilli) with higher terminal velocities were transported by gravitational flow within the umbrella cloud and deposited earlier than the fine component. The accretionary lapilli likely formed within the co-PDC plume as it ascended and expanded. In contrast, the fine vitric ash, which dominates the main part of the Akahoya ash, remained suspended in the atmosphere for a long time and settled more slowly than the coarser components. The localized increase in thickness observed around the Kirishima volcano and the Kyushu Mountains may have resulted from aggregation processes facilitated by specific meteorological conditions.