九州地方の第四紀テフラ研究 巨大火砕流堆積物の第四紀学的諸問題

Abstract

This paper outlines the previous studies of many Quaternary gigantic pyroclastic flow deposits widely distributed in Kyushu in terms of Quaternary studies: (1) age, distribution, and source, and (2) influence on the Jomon Culture of Kyushu in the Holocene and on late Pleistocene slope erosion of Yaku and Tane islands.Seven gigantic pyroclastic flows are recognized in the late Pleistocene: Koya (source: Kikai caldera, age: 6, 300yBP), Ito (Aira caldera, 21, 000-22, 000yBP), Aso-4 (Aso caldera, 70, 000yBP), Nagase (Kikai caldera, 75, 000yBP), Ata (Ata caldera, 85, 000yBP), Aso-3 (Aso caldera, 105, 000yBP) and Torihama (Ata caldera, 100, 000-150, 000yBP) pyroclastic flows. Co-ignimbrite ash falls associated with all of them are found in distal areas more than 1, 000km distant from their sources. The ages, estimated by stratigraphic positions of those ash falls as well as radiometric datings, indicate that the eruptions of gigantic pyroclastic flows concentrate in the early stage of the late Pleistocene. Those pyroclastic flows showing circular distribution extend to a distance of 100-150km from the source.In contrast, the age, distribution, and source of middle-early Pleistocene gigantic pyroclastic flows are not sufficiently clarified, except for the Aso-2, Aso-1, Kakuto and Shimokado pyroclastic flows in the late stage of the middle Pleistocene.A clear difference in Jomon pottery between the layer above K-Ah ash associated with Koya pyroclastic flows and that beneath it, is widely recognized in Kyushu, suggesting that Koya pyroclastic flows eruption played an important role in the change in Jomon culture.We can often recognize slope deposits, including blocks of Nagase pyroclastic flows deposits in Yaku and Tane islands. This may suggest that unstable conditions occurred on the slopes over a wide area around the Kikai caldera owing to this eruption.