2024 Volume 69 Issue 3 Pages 115-141
Sakurajima Volcano, located in the southern part of Aira caldera, southern Kyushu, Japan, has caused at least 17 major eruptive events since about 26 cal ka BP, after the Aira caldera-forming eruption. The Satsuma eruption in 12.8 cal ka BP is the largest activity of this volcano. This study describes the distribution and stratigraphy of the Satsuma eruption deposits based on facies of deposits and component characteristics. The Satsuma eruption deposits are divided into 17 units (P14A to P14Q, in ascending order): P14A to P14E (Group 1: total tephra volume 7.4 km3 (based on Fierstein and Nathenson (1992)) are distributed around southern Kyushu, while P14F to P14 K (Group 2: total 0.48 km3) and P14 L to P14Q (Group 3: total 0.73 km3) are distributed in the west (Satsuma Peninsula) and east (Osumi Peninsula) from Sakurajima Volcano, respectively. The Group 1 shows a transition from a large-scale plinian eruption to a pyroclastic density current (PDC)-generating eruption, followed by widespread poorly-sorted fine ash including accretionary lapilli, and finally another plinian eruption. The PDC deposit (P14B) reaches an approximate radius of 10‒18 km from Sakurajima Volcano. The Group 2 consists mainly of small pumiceous deposits and well-sorted coarse ash deposits, which represent two repetitions of vulcanian and (sub) plinian eruption phases. The Group 3 are composed of relatively small pumiceous deposits, well-sorted coarse- and poorly-sorted fine ash in alternating layers, which indicates the occurrence of vulcanian, (sub) plinian, and phreatomagmatic eruptions. In addition, the Groups 2 and 3 characteristically contain fresh glassy lithics and a few dark-colored vesiculated particles. These suggest that the latter half of Satsuma eruption underwent a variety of eruptive styles due to differences in conduit processes and other factors.
