Holocene Eruptive Activity of Yufu-Tsurumi Volcano Group Found from Piston Cores in Beppu Bay, Central Kyushu

Abstract

Yufu-Tsurumi volcano group lies at the northeastern part of Kyushu island, Japan. Based on tephrochronological studies, Yufu-Tsurumi volcano group started their activities more than 35,000 years ago. In the latest eruptions of Yufu-dake, pyroclastic flows and ash falls due to collapse of lava domes emplaced, and one of their ash falls, the Yufu-dake 1 Ash (Yf-1), is well-known as one of the tephra late Quaternary around eastern Central Kyushu. Beppu Bay is located about 5 km on the eastern part of Yufu-Tsurumi volcano group. The Site-P core was obtained with a piston core sampler from central part of Beppu Bay. It has two intercalated thin volcanic ash layers at depth of 3-5 and 5-1 horizon. The layer at depth 3-5 horizon is characterized by the existence of greenish brown or brown micro pumice-type volcanic glass shards with high refractive indices of 1.558-1.563. The 5-1 horizon is chacterized by the occurrence of colorless volcanic glass shards with minerals of low refractive indeices of 1.493-1.503. The one of their volcanic ash layers (the 5-1 horizon) in the core was correlated with the Yufu-dake 1 Ash, based on their morphology and refractive index of glass shards. The eruption age of the Yufu-dake 1 Ash was examined by AMS (accelerator mass spectrometry) -¹⁴C dating method using shells on two levels which are intercalated with volcanic ash layers at depth of 5-1 horizon and was estimated to be about 2,500 year B. P. As a result of this study, the avarage frequency of eruption at Yufu-Tsurumi volcano group is estimated as every 1,000-1,500 years intervals.