Kusatsu-Shirane Volcano is one of the most active volcanoes in Japan. The summit of the volcano comprises three young pyroclastic cones: the Motoshirane Pyroclastic Cone Group (MPCG), which erupted on January 23, 2018, with almost no eruptive precursors; the Ainomine Pyroclastic Cone, of unknown age; and the Shirane Pyroclastic Cone Group (SPCG), which is the site of numerous historical phreatic eruptions. In this study, we present the results of an investigation of the stratigraphy and eruption history of the MPCG. The MPCG consists of overlapping pyroclastic cones including, from south to north, Motoshirane-nishi, the Older and Younger Motoshirane, Kagamiike, and Kagamiike-kita. The stratigraphic relationships and geochemical characteristics of the erupted material suggest that construction of most of these cones involved initial lava flow and subsequent cone-forming stages. Four lava flows, the Daimyozawa, Ishizu, Sessho, and Furikozawa lavas, form the bases of the Motoshirane-nishi, Older Motoshirane, Kagamiike, and Kagamiike-kita cones, respectively. The subsequent cone-forming stages included Vulcanian and phreatomagmatic eruptions. Products of the Kagamiike and Kagamiike-kita eruptions can be correlated with the ca. 4800 cal yr BP 12L Volcanic Sand (Hayakawa and Yui, 1989), which lies at the eastern foot of the volcano, and the juvenile jointed block-rich horizon (after ca. 1500 cal yr BP) on the southern flank of the Kagamiike cone, respectively. In summary, the MPCG eruption sites shifted from south (the Motoshirane-nishi cone) to north (the Kagamiike-kita cone), and magmatic eruptions continued until just after ca. 1500 cal yr BP. Petrological analyses of the erupted material suggest that the MPCG eruptions were fed by a mixture of dacitic magma from the long-lived crustal magma chamber, and repeatedly recharged mafic magmas with different compositions for each pyroclastic cone.

Here, the characteristics of post-LGM (Last Glacial Maximum) incised valley fills and their depositional sequence were examined by analyzing the SKM core collected in the Sukumo coastal lowlands, southwestern Shikoku Island, which has been subject to extensive seismic subsidence due to the large Nankai Trough earthquakes. Here we provide results of sedimentological, radiocarbon dating, and paleoenvironmental analysis. Sediments of the SKM core show clear indications that the succession was influenced by post-glacial sea level change. The Matsuda River incised valley formed during the LGM (26.5-19 ka), and was infilled by fluvial sand and gravel in the late Pleistocene. Following postglacial transgression, sea level rose by 30 m (a.s.l.) at 9.8 ka, and the incised valley became an estuarine environment. As sea level continued to rise, the estuarine environment was replaced by an inner bay mud bottom, and maximum water depth was reached at 7.5 ka. The 7.3 ka Kikai caldera eruption in southern Kyushu Island caused heavy K-Ah ash fall in southwestern Shikoku Island, and frequent large-scale lahars occurred immediately after the ash fall because of the proximity to the volcanic source. After the ash fall, the K-Ah secondary sediments were deposited rapidly in the inner bay environment and caused rapid sea level regression. After 7.0 ka, a delta region began to develop, which might have been due to the large K-Ah ash fall. At 5 ka, sea level reached +2.5 m (a.s.l.), estimated from the Sukumo midden, and is recognized as the Holocene marine limit in this area. This information on relative sea level change during the past 10,000 years suggests that the Sukumo Bay area has not subsided as a result of seismic-induced crustal deformation.

The 50-m-thick uppermost Pleistocene to Holocene succession exposed on Shinjima Island represents part of the sedimentary succession that was deposited in the Aira caldera. The succession was uplifted in 1780 due to magma intrusion, providing an opportunity to study post-caldera volcanism and environmental changes in the Aira caldera. The stratigraphy and origin of this succession, however, remain controversial. Based on careful revision of the classification and stratigraphic relationships of the constituent facies, here we propose the division of the succession into the Lower Shinjima Silt Bed, Shinjima Pumice, Southern Shinjima Pumice, Upper Shinjima Silt Bed, and Moeshima Shell Bed, in ascending stratigraphic order. The Lower and Upper Silt Beds are composed mainly of greenish-brown silt and contain shallow-marine fossil assemblages, representing a 100-200 m-deep inner bay environment. The Moeshima Shell Bed comprises lahar (debris and hyper-concentrated flows) deposits derived from Sakurajima volcano. The Shinjima Pumice and overlying Southern Shinjima Pumice, which are sandwiched between the Lower and Upper Silt Beds, are submarine eruption-fed density current deposits of rhyolitic composition, derived from the Wakamiko caldera over a brief interval of 12-13 cal ka BP. Tephras of known ages from the Sakurajima volcano and Yonemaru maar have been identified in the Upper Shinjima Silt Bed, and the Moeshima Shell Bed also contains pumices derived from the Sakurajima tephras. Numerous shell and wood fragments and organic particles have been collected from the Lower and Upper Sjinjima Silt Beds, Southern Shinjima Pumice, and Moeshima Shell Bed, and ages have been obtained by ¹⁴C dating. These chronological data suggest that the age of the Shinjima succession ranges from 13-2 cal ka BP.

Here we describe in detail turbidite and hemipelagic mud layers in deep-sea sediments from a core collected from the area east of Mikura-Jima Island. The 196 cm long core sample comprises mainly quartz, volcanic glass, scoria, radiolaria, and foraminifera in silty clay. The silty clay is interbedded with two scoriaceous sand layers deposited as turbidites. We identified the boundaries between hemipelagic and turbidite muds in the silty clay using a variation of grain components and physical properties, as follows. At the boundary, radiolarian and foraminiferal abundances increase, and scoria particles decrease rapidly upwards in the uppermost turbidite muds, whereas these grains are present in consistent numbers throughout the hemipelagic mud layers. Lowest wet bulk density values also indicate the boundary. In addition, we obtained ¹⁴C ages of 1296-1171 cal year AD and 2611-2431 cal year BC from the hemipelagic mud layers at 92.5-96.6 and at 33-37 cm depth, respectively.

Numerous paleomagnetic and chronological studies have contributed to the elucidation of the early Miocene (18-16 Ma) clockwise rotation of Southwest Japan associated with the major opening of back-arc basins in the Japan Sea. Paleomagnetic data have been reported previously from an andesitic parallel dike swarm in the Takane area, Takayama City, Gifu Prefecture, Central Japan, showing easterly deflection of the observed magnetization direction, in contrast to the expected paleomagnetic direction for the early Miocene. This must therefore represent clockwise rotation during the opening of the Japan Sea, providing the dike swarm was emplaced prior to its opening. Here we report new K-Ar hornblende ages from an andesite dike from the swarm. Two K-Ar ages of 22.1±1.2 and 21.2±1.2 Ma are statistically indistinguishable, and clearly pre-date the clockwise rotation.