幸屋火砕流 : 極めて薄く拡がり堆積した火砕流の発見

抄録

A pyroclastic eruption of acidic magma took place at the mouth of Kagoshima Bay, southern Kyushu, 6, 000 years before present. The eruption consists of 4 stages represented by the following deposits, namely air-fall pumice (stage I), pyroclastic flow (stage II), thin bedded ash of pyroclastic flow origin (stage III) and air-fall pyroclastics composing mainly of volcanic ash (stage IV) in ascending order. Their volume, area of distribution, density and composition of essential ejecta etc are shown in Table 1. Data obtained from field study suggest the pyroclastic flow origin of stage II deposit (Koya pyroclastic flow deposit), however the nature of which are quite unique in comparison with pyroclastic flows of common occurrence. 1) Thickness of the pyroclastic flow is less than 2 m, whereas the flow distance attains to some 60 km from the source (Fig. 4). This is approximately 5 times longer than common pyroclastic flows of similar volume (Fig. 15). 2) Density of the essential pumice is much less than 0.5 and is smaller than that of common pyroclastic flows (Fig. 16). 3) Dune-type bedding, grading, local concentration of pumice are common feature in the deposit within an area 20-30 km from the estimated source. These characters suggest the possibility of either extremely delayed vesiculation of essential pumice or unusual deepness of magma chamber so that abundant hot juvenile gas was available for maintaining turbulence in the flow. Location of estimated source is now under the sea at least 10 km far from existing outcrops on land.