The purpose of this symposium is two-fold: first, to review recent developments in tephrochronological studies in various districts of Japan, and second, to verify the impact of tephrogenic explosive eruptions on human beings and their environment. Several problems concerning the Japanese tephrochronology are discussed, for example: (1) standardization of petrographic characterizations for the identification of tephra layers, (2) identification of pre-Late Quaternary widespread tephra layers, (3) revision and refinement of several radiometric ages for specific tephra layers, (4) application of the tephrochronology established on land to the deep sea chronology, (5) long distance correlation of terraces and their sediments using widespread tephra layers, (6) establishing the relationship between tephrogenic eruptions and climatic change, and (7) examining the process and mechanism of the impact of explosive eruptions on human beings.
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.
A tephrochronological study has been carried out in the Chugoku and Shikoku Districts, paying special attentions to the widespread marker tephras from Mts. Sanbe and Daisen, as well as tephras from volcanoes in Kyushu. Chemical analysis of the major element composition of glass shards and/or magnetite grains collected from each tephra layer, using the electron microprobe technique, has been quite useful in identifying the individual tephra. The stratigraphy, chronology, and distribution of tephras, and their implications for Late Quaternary events are summarized as follows: (1) Aira Tn (AT; erupted about 25, 000BP), and Kikai Akahoya (K-Ah; erupted about 6, 300BP) ashes, of Kyushu origin, occur as the most distinctive markers. Both are easily identified by the major element composition of their glass shards. The Aso-4 ash is another maker tephra of Kyushu origin, distinguishable by the major element composition of its magnetite. (2) Of the many tephras erupted from Sanbe volcano, the Sanbe Kisuki (SKP), Unnan (SUn), and Ikeda (SIP) pumice layers are the most extensive and representative. The SUn and SIP pumice layers are identified in a marine core taken in the Japan Sea to the north of Tottori, 200km away from Mt. Sanbe. (3) Of the Daisen tephras, the Matsue (DMP), Kurayoshi (DKP) and Kusadanihara (KsP) pumice layers occur most extensively. KsP, of ca 18ka, was recognized in a marine core from the Japan Sea to the west of Sakata City, 600km away from Mt. Daisen. This correlates with the San'in 2 ash. (4) The followings are evident from the relationship of tephrochronology and geomorphic development in the Chugoku and Shikoku Districts: maximum sea level regression in the Late Pleistocene had occurred before the deposition of AT ash in the Izumo Plain; there were three stages of dune sand formation in the San'in District: the first was prior to the deposition of DMP, the second was between the eruptions of DMP and Aso-4, and the third was after the deposition of K-Ah; (5) Some of the unidentified tephras were examined and showed that an ash layer sampled from Pleistocene gravels near Ozu City, Ehime Pref., and one from the lowermost part of the Oetakayama pyroclastic formation in the Tsunozu Group are similar to the Guminoki ash and Yellow III of the Osaka Group respectively, from the viewpoint of the major element composition of glass shards or glassy matrix.
Pliocene-Pleistocene sequences deposited in lacustrine and bay environments are widely distributed in the Kinki District, and many water-based volcanic ash layers are intercalated in those sediments, though active volcanoes of Quaternary age are not located. The main interest in the study of Kinki district tephras is related to tephra identification for use as chronostratigraphic horizons. The history of research on tephras is divided into three stages. During the first stage, fundamental tephra discrimination was carried out on the basis of thickness, color, grain size, and succession in the field. In the second stage, petrographic characters (heavy mineral composition, glass shape, refractive index, magnetic characteristics, etc.) were analyzed for tephra identification. Recently, we have carried out tephra identification on the basis of the single grain method using EDX chemical analysis and refractive index data obtained by the thermal immersion method (RIMS 86), adding petrographic data. One of the recent topics of tephra study in the Kinki District is obtaining continuous and sequential sections in Lake Biwa and Osaka Bay. In Osaka Bay, the volcanic glass stratigraphy labeled V1-V17 (in descending order) was constructed using 65 drilling core samples. Two of them are correlated with the widespread tephra, and V17 is correlated with the Azuki volcanic ash layer that is the most valuable key bed in the Osaka Group. In Lake Biwa, two drilling operations (at 200m depth and 1, 400m depth) were carried out. The uppermost part (T bed) is composed mainly of massive clay and reaches a thickness of 250m. It is thought that this continuous clay includes most of the tephra that has fallen in the Kinki District during about 400, 000 years. Thirty-eight volcanic ash layers are intercalated in the T bed. Some of them (especially in the upper part) are correlated with some of the widespread tephras. The Lake Biwa and Osaka Bay sequences are very valuable in the identification of a tephrostratigraphy for the Kinki District.
The purpose of this paper is twofold. One is to review tephra studies in the Kanto and Chubu areas of central Japan, and the other is to identify important problems that remain to be studied. In these areas, there are marny tephra layers, well preserved fluvial and marine terraces, and glacial deposits, that are stratigraphically related to the tephras. Such favorable conditions have stimulated Quaternary researchers and volcanologists, and consequently there have been many tephra studies done in the Kanto-Chubu areas. Tephra study is more advanced in this region than in any other areas of Japan. The tephrostratigraphy of the late Pleistocene is relatively well established (Figure 1). This results from many studies, ranging from those carried out in the 1960's during the time of the KANTO LOAM RESEARCH GROUP (1965), to those being carried out at present. In particular, studies of widespread tephras throughout Japan, initiated by MACHIDA and ARAI's (1976) work on the AT ash in the 1970's, contributed to the determination of stratigraphic relations between distant tephra layers. Now it is necessary to construct a middle Pleistocene tephrochronology of the Kanto area. Widespread tephras should play a significant role. Here, the authors present new data on several recently discovered widespread tephras. The Omachi APm tephra beds (A1Pm-A5Pm), originating in the Hida Mountains at 300-350ka, are widely distributed, from northern Chubu to the north Kanto. A4Pm and A5Pm sandwich a vitric ash layer composed mainly of bubble-walled glass shards, identified as Nuka 1 Ash from the eastern foot of Yatsugatake Volcano. This vitric ash layer probably correlates with the Kakuto pyroclastic flow deposit (Kt-U) in southern Kyushu. Moreover, another vitric ash layer, the Sanada Ash (SnA), is found above A5Pm. A tephra similar to SnA is found on the Oiso Hills in the southern Kanto, and is called the Beige Ash, with an estimated age of 240ka. SnA and the Beige Ash are similar to the Torihama pyroclastic flow deposit (Th) in south Kyushu, suggesting their correlation.
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.
The critical problems remaining for the study of the Quarternary tephrochronology of Hokkaido in the 1980s were the lack of extensive marker tephras and of well-defined chronological data beyond the range of 14C method. In the last ten years, these problems were partly solved for the Holocene and Late Pleistocene with the recognition of a number of remarkable wide-spread tephra-layers including B-Tm (<1ka), Aso-4 (70ka), Toya (90ka) and KHb (100ka). The finding of Aso-4 and Toya layers enabled to correlate the tephrochronology of Hokkaido with that of Honshu and Kyushu. Meanwhile, information on Early to Middle Pleistocene tephras is still insufficient for establishing a comprehensive chrono-stratigraphy in most of Hokkaido, with the exception of the region around the Tokachi Plain. The tephrostratigraphy in eastern Hokkaido is described in this paper in order to summarize recent progress and to show the outline of Quaternary tephras in Hokkaido. The stratigraphy of pyroclastic flow deposits from Akan and Kutcharo volcanoes, which have the longest history of explosive activities in Hokkaido, has been revised and interrelated through stratigraphic revision and extensive application of microprobe techniques to glass shards. Intervening airfall tephras with different petrographic characteristics from tephras occurring in eastern Hokkaido are introduced as possible marker tephras of Early to Middle Pleistocene age. Chronological data for Middle Pleistocene tephras are still very few, but their stratigraphic positions in relation to major transgressions are revealed. Temporal and regional variation of potassium content in glass shards and maffic mineral assembrage is examined to give general ideas of source volcanoes and ages of unidentified tephra layers of Early to Middle Pleistocene.
Tephra provide extremely useful sedimentary beds for Quaternary research because they are highly reliable time markers. Tephra studies in Japan have advanced primarily on this basis. Because tephra themselves are products of volcanic eruptions it is quite possible to study explosive eruptions using tephra beds. The thickness of the beds tells us the magnitude of the eruption, while the dispersal pattern of the tephra tells us the intensity of the eruption. The mode of eruption is known by the grain-size characteristics of the tephra deposits. The life history of a volcano can be revealed by alternations of tephra and loess beds.
Volcanic eruptions influence land surfaces in various ways and to varying degrees, both direclly and indirectly. Pyroclastic fall and flow deposits fill or covers land surfaces, resulting in damage to vegetation via rapid accumulation of tephra and through thermal effects. Large-scale pyroclastic flows and debris avalanches may cause a variety of topographical changes of extraordinary scale. The former often cover undulating topography with thick flow accumulations, creating flat surfaces that are preserved as pyroclastic flow uplands. The latter may create buried valleys, eroding valley bottoms and walls, and damaging vegetation and soils. The basic properties and characteristics of blast phenomena were first clarified and illustrated in detail by the 1980Mt. St. Helens eruption. When a large scale debris avalanche enters the sea or a lake, it generates a tsunami which affects the coastal areas over wide areas. After rapid deposition of tephra, the surface tephra materials are easily moved by running water and wind action. Sand dunes composed of secondary tephras are common, especially in Hokkaido. Debris flows and mud flows commonly occur in volcanoes such as Usu and Sakurajima which exhibit continuous small scale tephra-producing activity. These flows contribute to the growth of volcanic fans. Strong rainfalls sometimes trigger mud and debris flows, which contribute to developing volcanic fans, on both active volcanoes and older highly dissected volcanoes. Such volcanoes have extensive volcanic fans surrounding their cones.
Major volcanic eruptions have often been followed by global cooling. The year 1816, often called the “year without a summer”, is well known for its extremely cool summer temperatures, extending over western Europe and eastern North America. This unusually cool weather was attributed to the eruption of Mt. Tambora in Indonesia, in 1815. This paper reviews the relationship between major volcanic eruptions and global climatic changes, based on recent articles in the field of climatology. The global mean temperature tends to decrease for 2 to 3 years after major volcanic eruptions (ANGELL and KORSHOVER, 1985). The degree of cooling and its regional extent depend upon various factors, such as, the latitude of the volcanoes, the eruption season, and the type of gases ejected. In general, the quantity of sulfur is probably the most important factor when we consider the impact of volcanic eruptions on global climate variation. It should be noted that temperature variations after major eruptions often show regional differences. This is connected with the modified global circulation patterns caused by changes in terrestrial radiation budgets after such eruptions. The Little Ice Age of the 16th to 19th centuries is known for its world wide cool climates. Time series of acidities in Greenland ice cores indicate frequent volcanic activities during the Little Ice Age (HAMMER et al., 1980). Over time intervals of 10 to 100 years, volcanic eruptions produce the greatest impact on global climatic change.
In this paper we discuss the effects of Aira-Tn Ash (AT) eruption on ecosystem, based on fossil pollen and diatom assemblages obtained from central and northern Honshu in Japan. This eruption occurred during the Last Glacial Maximum (21, 000-22, 000 or 24, 000 years ago) in the Aira Caldera in southern Kyushu. The fossil pollen accumulation rate and pollen assemblages from AT deposits show that the ash fall occurred in the spring. We conclude that the AT fall occurred during, and accelerated, the last stage of climate change to a colder phase, and of attendant vegetation change to coniferous forests, that began about 30, 000 years ago. The AT fall also caused a rise in level and a change in pH from acid to alkaline of the sedimentary environment water. Some of these phenomena changed in proportion to volume or thickness of ash. It seems that the AT eruption caused widespread dry climatic conditions that were felt, at least to some degree, over long distances.
A volcanic eruption can instantly cause total destruction to the surrounding environment and its inhabitants, and may also produce secondary disasters over a much wider area. The major eruptions in Japanese prehistory were those which produced the Kikai-Akahoya tephra (K-Ah) in 6, 300 BP, and the Aira-Tn tephra (AT) at 21, 000∼22, 000 BP, both of which scattered volcanic ash over an extremely wide area, and influenced climate and vegetation on a global scale. Such immense eruptions devastated their immediate vicinities with ash storms, accompanied by hot gas and pyroclastic flows, possibly killing entire species of animals and plants. In the case of the K-Ah eruption, there is evidence that such destruction extended over a radius of more than 100km, whereas the AT eruption is thought to have been even larger, and affected an even larger area. The K-Ah eruption occurred at the Kikai caldera in Kyushu, and the resulting thick layer of volcanic ash can be observed within the Jomon Period horizon in southern Kyushu. This deposit is used as a key layer to separate the Earliest and Early Jomon Periods, represented by the Senokan pottery at the bottom of the K-Ah layer and the Todoroki type at the top. The sudden shift in these types of pottery is thought to have derived from the total extinction of the Earliest Jomon population, undoubtedly as a result of the eruption, followed by the arrival of an unrelated group of people after the tragedy. The effects of the eruption reached as far as Shikoku, the Kinki, and the Tokai regions. The earth's surface was probably covered by thick volcanic precipitation, which also deposited on the floors of bays, killing much of the animal and plant food resources. It was during this cataclysmic event that shellmounds containing Shioya type pottery were abandoned, or totally vanished in the Tokai region. A chronological study of the Paleolithic stone tool assemblages associated with the AT tephra is in progress. It is generally acknowledged that the tool component preceding the AT eruption is homogeneous throughout Japan, but that tool assemblages following the eruption show regional differences, with a greater variety of tool types and an increase in the number of sites. However, in the Musashino region, where one of the first chronologies of Paleolithic culture in Japan was established, this transition in tool assemblage occurs during a much later period (i. e., upper Layer V to lower Layer IV of the Tachikawa Loam sequence), while the AT tephra is observed within Layer VI, associated with knife shaped tools made from obsidian flakes. The gap is several thousand years. To resolve this apparent contradiction, archaeologists must intensively study how the precipitation of large amounts of volcanic ash actually affected the sites and artifacts over a long period of time.
This paper presents an example of the correlation of Early to Middle Pleistocene tephras in Japan. The Ng-1 Ash, intercalated in the Middle Pleistocene Negoya Formation, and tephras which have similar characteristics from various localities in central Japan have been examined. The Ng-1 Ash consists of glass shards, feldspar, quartz and heavy minerals (biotite, amphibole and orthopyroxene). Some tephras at other localities are correlated with the Ng-1 Ash, based on their chemical compositions and refractive indices, their horizons and other characteristics. The age of the Ng-1 Ash is estimated to be 0.3 Ma, and it appears that the source volcano is located in the northern Japan Alps. Investigations covering a wide area are needed to verify both the age and the location determinations.
Drill core samples from Lake Biwa provide important data for reconstructing Quaternary environmental changes. Of the many components of the cores, the tephra layers should give the best data for chronological studies. The present paper deals with the re-examination of these tephra layers, based on petrographic analyses and recent tephrochronological results for Japan. Determinations of the refractive indices of volcanic glass and orthopyroxene and hornblende phenocrysts, together with other criteria show that approximately half of the tephra layers are widespread tephras originating from large caldera volcanoes in Kyushu, and the remainder are possibly from Mt. Daisen and other volcanoes in Honshu. Almost all tephras identified here have been dated by a number of radiometric and stratigraphic methods and hence give more reliable, and younger, ages for the Biwa Lake sediments than previously obtained.