THE DEVELOPMENT OF MT. FUJI AND MT. HAKONE VOLCANOES ANALYSED FROM THE TEPHROCHRONOLOGICAL STUDY IN THE OOISO HILLS

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Mt. Fuji and Mt. Hakone volcanoes lie in the south-west of the Kanto plain and supplied a vast amount of tephras (pyroclastic-fall deposits) through late Quaternary times. The tephras deposited are thicker near the volcanoes and are considered as useful key beds for analysing the history of the volcanoes and the development of terraces. The present paper is devoted primarily to a study of the history of the volcanic activities of Mt. Fuji and Mt. Hakone from the tephrochronological study in the Ooiso hills, where abundant tephra layers and Quaternary sediments are found. In the first place, the most important key bed, "Tokyo pumice", about which we have as yet very little information in the studied area, has been discovered as a conspicuous pumice bed accompanied with the younger pumice flow deposit of Mt. Hakone volcano. As the Tokyo pumice is a useful horizon marker of the lower Musashino loam formation, the younger pumice flow deposit is placed stratlgraphlcally in this horizon, which is different from those which were reported before. In the next place, the Pleistocene tephra layers in the Ooiso hills are divided into three formations ; the Younger loam, the Kissawa loam and the Tsuchizawa loam. These results are obtained from the relation between tephras and geomorphic surfaces. An idealized geological section of the north-east of the Ooiso hills is shown in Fig. 7. The Kissawa loam, the Kissawa formation and the Kissawa surface are the new geological names. The latter two may have been formed under a remarkable transgression prior to the Holocene epoch, which can be correlated to the Sangamon interglacial. Fig. 8 shows the relationship between the thickness of a tephra and the geological age. The former is expressed by relative figures measured at a certain distance from the crater along the axis of distribution, because this is considered to represent the total volume of a fall unit which is in turn a function of explosivity of an eruption. The tephras of the two volcanoes are interbedded each other., but can be distinguished from the petrological point of view; those of Mt. Hakone are pumiceous deposits of two pyroxene andesitic compositions, and those of Mt. Fuji are scoriaceous of olivine basalt. As shown in this figure, the following results are to be found.1) The activity of Mt. Fuji started later than that of Mt. Hakone.2) Mt. Fuji successively erupted through the time of late Pleistocene until about the beginning of the Holocene. This period of activity was called the "Older Fuji I" as compared with the period of the "Older Fuji II", the effluent time producing lava flows, and with the "Younger Fuji" of middle to late Holocene (Machida, 1967).3) The activity of Mt. Hakone volcano continued for a long time gradually decreasing in explosivity. From the precise interpretation, the several periods of activity are recognized; I, II (the Older Somma stage I and II), III (the Younger Somma stage) and IV (the Younger Pumice flow and the Central Cone stages accompanying the caldera depression in the younger stage). Those results are obtained from the correlation between the stratigraphy of tephra and the development of the volcanic edifice. And then it is clear that the three greatest explosions occurred after each long quiescent period of activity shown by the Tsuchizawa pumice, the Kissawa A-pumice and the Tokyo pumice. The volcanic activity became less explosive as well as less frequent in the IV stage, while several central cones were constructed.
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THE DEVELOPMENT OF MT. FUJI AND MT. HAKONE VOLCANOES ANALYSED FROM THE TEPHROCHRONOLOGICAL STUDY IN THE OOISO HILLS

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