Abstract
Eight volcanic ashes occur above the Akahoya (K-Ah)Volcanic Ash horizon in piston cores from Beppu Bay. Petrographically, the ashes can be divided into three groups (B, G, and M), by differences in mineral abundances and by the morphology and refractive indices of very fine sand-sized volcanic glass shards. The ashes are named M-2, G-4, B-1, B-2, G-5, M-3, G-6 and B-3 in ascending order.Group B and M ashes characteristically contain abundant glass shards. The shards in Group B ashes are pale brown, whereas those in Group M ashes are colorless. Group G ashes contain distinctive pale greenish-brown glass shards with vesicular to blocky morphology and high refractive index (Nd : 1.554-1.572). Identical petrographic characteristics are seen in glass shards in tephras erupted from Aso and Kuju Volcanoes.On the basis of refractive indices and chemical compositions of the above tephras, it is clear that the Beppu Bay G-6 ash is the Aso N 2 phreatomagmatic eruption tephra. The lower part of the G-5 ash is the Aso N 7 tephra, whereas the upper part is Aso N 6 (Kishimadake scoria) and Aso N 4 (Ojodake scoria). The G-4 ash is correlated with the A 1-8 tephra from Kuju Volcano. The Aso N 2 phreatomagmatic eruption distributed Pele's hair-type glass shards over a distance of 100 km or more. This eruption type is considered responsible for the occurrence of the Aso ashes in Beppu Bay.From sedimentation rates in the Beppu Bay cores and AMS 14C dating, the calendar year age of the G-6 ash is AD 330, that of G-5 2590-1580 BC, and that of G-4 4730-4550 BC.
