著者

早田 勉

出版者

Japan Association for Quaternary Research

雑誌

第四紀研究 (ISSN:04182642)

巻号頁・発行日

vol.27, no.4, pp.297-312, 1989-03-31 (Released:2009-08-21)

参考文献数

22

被引用文献数

10 12

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Haruna volcano, situated in the central part of Japan, erupted twice in the 6th century, from Futatsu-dake crater. The first eruption, which occurred in the early part of the 6th century, was set set off by a low-temperature phreatomagmatic eruption. The initially ejected very fine ash accumulated as accretionary lapilli and muddy rainfalls. Later, the eruption changed to hot pyroclastic flow effusions, which contained many essential lithics. These pyroclastic flow effusions included small-scale phreatic eruptions. The ash had formed ash clouds that then accumulated on each pyroclastic flow deposit. This tephra sequence was named the Haruna-Shibukawa tephra formation(Hr-S).These pyroclastic flow encroached on an older village, Nakasuji, situated on the eastern flank of Haruna volcano. The pyroclastic flow (S-5) burned and destroyed many houses. Because its deposit was very thinly laminated, it took the form of a hot pyroclastic surge, which spread over the eastern side of Haruna volcano, causing widespread damage there before changing to mud flows and floods and damaging rice fields in the area.The second eruption, which occurred in the middle or later part of the 6th century, is characterized by plinian eruptions and pyroclastic flow effusions. This tephra sequence was named the Haruna-lkaho tephra formation (Hr-I). The pumice ejected in the plinian eruptions was deposited, in a layer about 3cm thick, on Soma city, 200km from the vent.An older village, Kuroimine, situated about 10km from the vent, was buried by a layer of pumice about 200cm thick. Because pumice oxidized by the flames of burning houses is observed from the bottom to near the top of the pumice fall deposit, we can confirm that the greater part of the pumice accumulated during a period of hours. A house was crushed by the coarser part of the pumice fall deposit (1-6). The pyroclastic flows, which caused columns to collapse, moved and accumulated along the valleys before changing into mud flows and floods. They also caused heavy damage to rice fields and farms.In Gunma Prefecture, it may well be that villages, rice fields, and farms damaged by volcanic eruptions in the same way as Nakasuji village and Kuroimine village were damaged will be discovered. The data in relation to past volcanic hazards, obtained by joint research between archaeology and volcanology, will contribute to predicting volcanic disasters.

著者

早田 勉

出版者

Japan Association for Quaternary Research

雑誌

第四紀研究 (ISSN:04182642)

巻号頁・発行日

vol.28, no.4, pp.269-282, 1989 (Released:2009-08-21)

参考文献数

42

被引用文献数

10 7

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A very important problem for Quaternary research is determining when human beings first settled the Japanese islands. Recently many artifacts of the Early Paleolithic age, dating from before 30, 000 years ago, have been discovered in the northern part of Sendai plain, North Japan. The age of these artifacts has been determined mainly by radiometric dating methods.On the other hand, tephrochronology is an effective technique for establishing Quaternary stratigraphy in the Japanese islands and their surrounding area. The author investigated the age of horizons bearing artifacts on representative sites of that age from the viewpoint of tephrochronology.The stratigraphy of proximal tephra layers is indicated in Fig. 1. Useful widespread tephra layers for chronological study of this area are Toya (ca. 90, 000-100, 000y.B.P.), On-Pm I (ca. 80, 000y.B.P.), Aso-4 (ca. 70, 000y.B.P.) and AT (ca. 21, 000-22, 000y.B.P.). Toya, On-Pm I, Aso-4 and AT were discovered from the horizons between IcP and KtA, IcP and KtA, N-N and N-Y, and N-Y and NK-U, respectively. At Babadan A site, artifacts of the Early Paleolithic age were excavated from horizons in Ando soil found below IwP, just below and above IcP, between IcP and KtA, just above N-Y, and between N-Y and NK-U. Consequently, the artifacts excavated from the horizons below KtA belong to the Early Paleolithic age. At Zazaragi site, another representative site belonging to the Early Paleolithic age, the artifacts were excavated from the horizons of orange-colored volcanic ash (the lower part of Yasuzawa volcanic ash). During the excavation, this deposit was regarded as an airfall tephra layer. But the author interpreted this deposit as a weathered part of a pyroclastic flow deposit (N-Y3) on the basis of the following observations:(1) The boundary of the lower part of Yasuzawa volcanic ash and lower pyroclastic flow deposit is irregular and not clear in some place.(2) It is massive and not layered. There is no intercalation of soil.(3) It is not well sorted.(4) It includes charcoals.(5) It has segregation pipes. Some of them develop at the boundary of the lower part between the Yasuzawa volcanic ash and the lower pyroclastic flow deposit.The author believes that the artifacts were incorporated into pyroclastic flow and carried to their present site.The horizons from which artifacts of the Early Paleolithic age in Japan have been excavated are plotted in Fig. 8. It has been confirmed that some widespread tephra layers originated from gigantic eruptions and covered the area surrounding the Sea of Japan (cf. MACHIDA and ARAI, 1933). The author believes that common chronological studies of Paleolithics in this area are made feasible by tephrochronology.

著者

早田 勉 八木 浩司

出版者

Japan Association for Quaternary Research

雑誌

第四紀研究 (ISSN:04182642)

巻号頁・発行日

vol.30, no.5, pp.369-378, 1991-12-25 (Released:2009-08-21)

参考文献数

40

被引用文献数

8 8

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Stratigraphic studies of local tephras distributed around the foot of Quaternary volcanoes in Northeastern Honshu, Japan have been carried out since the beginning of the 1969's. Since the discovery of the AT tephra in Fukushima Basin in 1976, a total of seven late Quaternary widespread tephras have been found, interbedded at many localities with local tephras. These are the B-Tm, K-Ah, AT, DKP, Aso-4, Pm-I and Toya tephras, in descending order. They link the stratigraphy of local tephras throughout the late Pleistocene with those in the central and southern parts of Japan. Consequently they are useful in studies of late Quaternary tectonic movements, geomorphic history relating to climatic change, volcanic activities and archaeology in Northeastern Honshu.Ignimbrites of the Lower and Middle Pleistocene have been found in the vicinity of Mt. Hakkoda, along the upper course of the Tama River, Onikobe and Shirakawa areas. Widespread tephras, predating the last interglacial, have not been discovered there yet. It is necessary to find them to support the development of geochronology and other related studies of the Middle Pleistocene in Northeastern Japan.