10:45 AM - 11:00 AM
[SVC31-13] Stratigraphic facies and eruptive sequence of the 12.8 cal ka BP Satsuma Eruption of Sakurajima Volcano
Keywords:Sakurajima Volcano, Satsuma eruption, eruptive sequence, plinian eruption, PDC deposit
The Sakurajima Satsuma (P14) tephra (hereafter Satsuma tephra), which erupted about 12.8 cal ka BP at Sakurajima volcano, is the first deposits of Stage 2a (Younger Kitadake stage), characterized by extremely explosive pumiceous eruptions and massive lava flow. This tephra is the largest tephra in the eruption history of this volcano and shows an unusual concentric distribution as tephra. This eruption (hereafter Satsuma eruption) is the only eruption that produced pyroclastic density currents outside of the Sakurajima island. Therefore, it is important to clarify the sequence of this eruption in order to understand the overview of the largest eruption in Sakurajima volcano, to understand the sequence of large-scale eruptions in general, and to consider disaster prevention in the surrounding area. However, the stratigraphy of the Satsuma tephra has not been precisely defined in previous studies, and there are problems with the stratigraphic comparison between Satsuma and Osumi Peninsulas. In addition, the detailed eruption sequence of the Satsuma eruption has not been fully understood. In this study, we conducted a geological survey of the Satsuma tephra around the Aira caldera and analyzed the deposits and their components. The sequence of the Satsuma eruption is discussed based on these results.
The Satsuma tephra is stratigraphically divided into 17 units (P14A to P14Q, in ascending order). In this study, we divided them into Group 1 (P14A-P14E), Group 2 (P14F-P14K), and Group 3 (P14L-P14Q) based on the characteristics of the main axes of distribution. Group 1 tephra is distributed in a circle around the Aira Caldera area. P14A and P14E are large pumice fall layers. P14B is pyroclastic density current (PDC) deposit. P14C is pumice fall layer with coarse ash. P14D is a poor-sorted fine ash fall layer including accretionary lapilli. The PDC deposit (P14B) is distributed both in Satsuma and Osumi Peninsulas and reaches an approximate radius of 10 to 18 km from Sakurajima volcano. This unit transitions to P14C as it tracks into the distance. The thickness of P14D does not change significantly from proximal to distal. This unit is especially thin near the source where P14B is deposited thickly. Group 2 deposits are distributed mainly in the southwest to west direction (on Satsuma Peninsula side) from the volcano. It consists of well-sorted coarse ash fall layers (P14F-H, P14J) and relatively small pumice fall layers (P14I, P14K), which include abundant lithic fragments. Group 2 and 3 deposits are characterized by the presence of black to brown glassy lithic fragments and dark colored vesiculated particles. Group 3 tephra is mainly distributed in the east-northeast to southeast direction (on Osumi Peninsula side) from Sakurajima volcano. It consists of relatively small pumice fall layers (P14L, P14P (U)), well-sorted coarse ash fall layers (P14M-N, P14P (L)), and poor-sorted fine ash fall layers (P14O, P14Q) containing accretionary lapilli. The tephra volume of each group is estimated to be 7.4 km3 (2.8 km3; minimum estimate) for Group 1, 0.48 km3 (0.38 km3) for Group 2, and 0.73 km3 (0.38 km3) for Group 3.
These results suggest the following sequence of the Satsuma eruption. The Group 1 deposits show a transition from a large-scale plinian eruption to a PDC-generating eruption, followed by widespread poorly-sorted fine ash including accretionary lapilli, and finally subsequent plinian eruption. The distribution of fine ash (P14D) is very complicated, so it is more likely to be co-PDC ash deposit than general ash fall deposit. In the Satsuma eruption, Group 1 deposits have the largest eruption volume, accounting for more than 70% of the total Satsuma tephra and reaching its climax early in the Satsuma eruption. Group 2 tephra is considered to have alternated between two phases of large-scale vulcanian and subplinian eruptions. Thereafter, the wind direction changed significantly toward Osumi Peninsula, and Group 3 is thought to have experienced phreatomagmatic eruptions in addition to subplinian and large-scale vulcanian eruptions. Based on recent eruption records and current meteorological conditions, it is likely that the Satsuma eruption occurred between summer and autumn. The stratigraphic and compositional characteristics of the Satsuma tephra revealed in this study will provide important basic information for understanding the mechanism that caused this eruption and its complicated eruptive transition.
The Satsuma tephra is stratigraphically divided into 17 units (P14A to P14Q, in ascending order). In this study, we divided them into Group 1 (P14A-P14E), Group 2 (P14F-P14K), and Group 3 (P14L-P14Q) based on the characteristics of the main axes of distribution. Group 1 tephra is distributed in a circle around the Aira Caldera area. P14A and P14E are large pumice fall layers. P14B is pyroclastic density current (PDC) deposit. P14C is pumice fall layer with coarse ash. P14D is a poor-sorted fine ash fall layer including accretionary lapilli. The PDC deposit (P14B) is distributed both in Satsuma and Osumi Peninsulas and reaches an approximate radius of 10 to 18 km from Sakurajima volcano. This unit transitions to P14C as it tracks into the distance. The thickness of P14D does not change significantly from proximal to distal. This unit is especially thin near the source where P14B is deposited thickly. Group 2 deposits are distributed mainly in the southwest to west direction (on Satsuma Peninsula side) from the volcano. It consists of well-sorted coarse ash fall layers (P14F-H, P14J) and relatively small pumice fall layers (P14I, P14K), which include abundant lithic fragments. Group 2 and 3 deposits are characterized by the presence of black to brown glassy lithic fragments and dark colored vesiculated particles. Group 3 tephra is mainly distributed in the east-northeast to southeast direction (on Osumi Peninsula side) from Sakurajima volcano. It consists of relatively small pumice fall layers (P14L, P14P (U)), well-sorted coarse ash fall layers (P14M-N, P14P (L)), and poor-sorted fine ash fall layers (P14O, P14Q) containing accretionary lapilli. The tephra volume of each group is estimated to be 7.4 km3 (2.8 km3; minimum estimate) for Group 1, 0.48 km3 (0.38 km3) for Group 2, and 0.73 km3 (0.38 km3) for Group 3.
These results suggest the following sequence of the Satsuma eruption. The Group 1 deposits show a transition from a large-scale plinian eruption to a PDC-generating eruption, followed by widespread poorly-sorted fine ash including accretionary lapilli, and finally subsequent plinian eruption. The distribution of fine ash (P14D) is very complicated, so it is more likely to be co-PDC ash deposit than general ash fall deposit. In the Satsuma eruption, Group 1 deposits have the largest eruption volume, accounting for more than 70% of the total Satsuma tephra and reaching its climax early in the Satsuma eruption. Group 2 tephra is considered to have alternated between two phases of large-scale vulcanian and subplinian eruptions. Thereafter, the wind direction changed significantly toward Osumi Peninsula, and Group 3 is thought to have experienced phreatomagmatic eruptions in addition to subplinian and large-scale vulcanian eruptions. Based on recent eruption records and current meteorological conditions, it is likely that the Satsuma eruption occurred between summer and autumn. The stratigraphic and compositional characteristics of the Satsuma tephra revealed in this study will provide important basic information for understanding the mechanism that caused this eruption and its complicated eruptive transition.