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Recent Progress in Tephra Studies in Japan1) 第 四 紀 研 究 (The Quaternary Research) 30 (2) p. 141-149 July 1991 Recent Progress in Tephra Studies in Japan1) Hiroshi MACHIDA1) This paper presents an overview of the development of tephrochronology in Japan, and discusses several problems regarding the Japanese Quaternary. The impact of developments in such subjects as soil science, archaeology, radiometric dating and many other disciplines of Quaternary research has stimulated the tephrochronology in studies of earlier periods. However, recent advances seem more closely related to indigenous factors. Since the early 1970's, systematization of tephrochronology and fundamental characterizations for the identification of widespread tephras, occuring in and around the Japanese Islands, have initiated revision and refinement of regional stratigraphies, revolutionary studies of tephra for determining the nature and effects of explosive eruptions, and new applications to several aspects of Quaternary studies. A compilation of tephra catalogue for the Japan region provides fundamental data for correlations on land and in sea and for determining the recurrence period of cataclysmic explosive volcanism. I. Volcanoes in Japan as sources Honshu, the Izu-Ogasawara and the Ryukyu of tephra island arcs are characterized by stratovolcanoes with or without calderas, producing a large Plate tectonic models show that the volcanic quantity of lava and tephra of various com- zones of Japan can be divided into two: ast and position. Accordingly, a more or less continu- west (SUGIMURA, 1960).The east volcanic zone ous mantle of tephra exists over some 40% of the lies on the edge of the North American Plate four main islands, resulting in strong control of which is being underthrust by the subducting landforms and subsurface materials. oceanic Pacific Plate. The west zone is being It is for only about 60 years that the tephra has formed by the interaction between the Eurasian received special attention whereas the lava has Plate and the Philippine Sea Plate. Large quan- been the subject of volcanological study for tities of lava and tephra, mainly of rhyolitic and around 100 years in Japan. In this paper I andesitic composition, have erupted throughout review the recent developments in tephrochro- the Quaternary from about 200 volcanic centers, nology emphasizing that tephra is very useful of which about 80 have continued their activity tool for Quaternary research in Japan. into historic times. In the west Japan volcanic zone, Kyushu is II. Recent developments in characterized by clusters of large caldera vol- tephra studies in Japan canoes, which are sites of large-scale explosive The history of tephra studies in Japan can be volcanism producing a number of ignimbrite and divided into four periods: 1) early 1930s to widespread airfall tephra sheets, as mentioned 1950s, 2) 1950s to early 1960s, 3) early 1960s to later. In the east volcanic zone, numbers of late 1970s, 4) late 1970s to the present. large-scale explosive volcanism occurred, Period 1: Pioneer studies on classification, particularly in Hokkaido and northern Honshu mapping, age estimation and other aspects of caldera volcanoes, producing extensive tephra Holocene tephras were carried out with soil sheets. Other volcanic centers or districts in survey in Hokkaido (URAGAMI et al., 1933). 1) Received 5 February 1991. Accepted 13 May 1991. 2) Department of Geography, Faculity of Science, Tokyo Metropolitan University. 142 The Quaternary Research Vol. 30 No. 3 July 1991 Studies of Pleistocene tephras in the Kanto plain, remote districts with one another but also to certral-east Honshu, were started by HARADA approach various stratigraphical and volcanolog- (1943). ical problems as described below. Period 2: Archaeological discovery of Ages of tephrogenic eruptions in the last 1.5ka palaeolithic culture in upper Pleistocene tephra have been determined by historical documenta- sequences of the Kanto and the first use of the 14C tion together with archaeology and dendrochro- dating method stimulated interdisciplinary nology. The majority of tephras ejected tephra studies. In addition, tephra sequences between c. 30ka and 1.5ka BP have been dated were studied in detail together with geomorphic by the 14C method. Moreover, the use of other surfaces and various Quaternary sediments dating techniques, including fission-track, (KANTO LOAM RESEARCH GROUP, 1965) and with uranium series, thermo-luminescence, electron palaeontologic and geomagnetic studies (ITIHARA spin resonance and K-Ar methods, means that et al., 1975). the tephrochronological approach can be Period 3: Progress in tephrostratigraphy, employed beyond the limit of 14C dating. and radiometric dating and other Quaternary Indeed, a number of ages obtained by various research led to the establishment of a standard methods from the same tephra at different chronology for late Quaternary sequences in the localities can be checked with each other, South Kanto and for the early to middle resulting in high-quality age control. Also, the Pleistocene in the Kansai. In addition, sys- strictly identified tephra sequences should play tematization of tephrochronology through an important role in revision and refinement of volcanological interpretations of tephra and various radiometric ages. Fig. 1 is the time- fundamental characterization studies for tephra space diagram of the late-middle Quaternary identification were introduced (NAKAMURA et al., representative tephras and associated marine 1963: KOBAYASHI, 1969). terrace sequences from Kyushu to Hokkaido. Period 4: Discovery of a very widespread The increase in the number of dated marker- tephra layer (MACHIDA and ARAI, 1976) encouraged tephras has greatly contributed to the the volcanological studies on large-scale understanding of Quaternary problems. explosive volcanism and greatly contributed to IV. Widespread marker-tephra layers Quaternary studies over extensive areas, including the Japanese islands and adjacent seas. As a result of progress in characterization and Revision and refinement of proximal strati- identification of tephra, two very widespread graphy were associated with the identification of tephra layers from gigantic explosive volcanism such widespread marker-tephras. Land and of south Kyushu caldera volcanoes, Aira and sea correlation and chronology have been one of Kikai, were discovered and identified in un- the subjects of current studies. usually extensive areas, i.e., throughout Japan and the Sea of Japan as well as over the floor of III. Characterization and age determina- the Pacific Ocean to the south of Honshu tion for identification of tephra (MACHIDA and ARAI, 1976, 1978, 1983). Both Characterization of marker-tephra layers for tephra layers, Aira-Tn ash (AT in abbreviation) identification constitutes the basis for tephra of 22ka and Kikai-Akahoya ash (K-Ah) of 6.3ka, studies and is carried out from various view- provide distinct datum planes available for many points. Recently detailed descriptions have applications of Quaternary sciences in the Japan beengiven, not only of observable specific fea- region. These results encouraged the identifi- tures in the field and mineral assemblages cation of other widespread tephra layers in- available in proximal tephras but also of the cluding those of continental volcano origin (B- refractive indices and chemical compositions Tm and U-Oki). The petrographic characteris- ofvolcanic glass and specified minerals (ARAI, tics of fine-grained tephras have been thoroughly 1972; ARAMAKI and UI, 1976). As a result, it has investigated not only on land but also in cores become possible not only to correlate tephras in from abyssal sediments around Japan. Many 1991年7月 第 四 紀 研 究 第30巻 第3号 143 ■ Pleistocene marine formation □ filltop fluvial formation △ glacial till Fig. 1 Time-space diagram of marker-tephras with the representative marine, fluvial and glacial sequence for the last 300ka tephra layers have been successfully identified, this type is closely associated with gigantic the most extensive from the late Quaternary are caldera collapse and should give a cataclysmic shown in Table 1. Fig. 2 shows approximate impact on the environment over extensive areas. outer limits of such widespread tephra layers of 2) Plinian tephra; Air-fall tephra produced by late Quaternary age as hitherto described. plinian eruption. A number of pumice fall The above-mentioned widespread tephras can layers of this type are found throughout Japan. be classified into three types according to The eruptions were often followed by extrusions differences in their mode of ejection and of pyroclastic flow deposits. distribution: 3) Unusually large-scale pyroclastic flow 1) Co-ignimbrite tephra; Air-fall deposits deposits; These are represented by the Aira-Ito predominantly of fine-grained vitric ashes pyroclastic flow of the Aira caldera and the Aso- producing at the same time as huge pyroclastic 4 pyroclastic flow of the Aso caldera, and occur flows (MACHTDA and ARAI, 1976; PARKS and extensively in a concentric pattern centered at WALKER, 1977). The volume of tephra of this the source different from that of plinian tephras. type often exceeds that of the associated Recent tephra studies revealed that such pyroclastic flow deposits. Hence eruption of catalysmic pyroclastic flows can usually be 144 The Quaternary Research Vol. 30 No. 3 July 1991 Fig. 2 Map showing general distribution of the representative marker-tephras of the late Pleistocene in Japan and adjacent areas 1991年7月 第 四 紀 研 究 第30巻 第3号 145
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