The Japanese Committee on Tephrochronology made a distribution map of the Quaternary volcanic products in Japan (Fig. 1, original scale :1/2, 000, 000). As the compiler of this map, the author gives some descriptive notes about the distribution and the related problems.
1. The main sources of this map are geological and soil maps made by various scholars and organizations, and unpublished tephra distribution maps made by the committee members. The degree of accuracy of tephra distribution on this map varies for different regions; more reliable for Hokkaido, Central Honshu and Southern Kyushu, and less for Japan Sea side and for Shikoku. In general, under 10cm thick tephra layers are not illustrated.
2. In this map, not only tephra layers, but also pyroclastic flow deposits and lavas are shown. Moreover, both of them are classified chronologically into those of the Holocene and the Pleistocene. It is noticeable that almost all Pleistocene tephra layers in this map belong to the upper Pleistocene age, because the greater part of tephra layers of the middle and lower Pleistocene have been eroded away.
3. The pattern of the tephra layer distribution suggests that the prevailing wind direction during both the Holocene and the Pleistocene was from the west. Furthermore, the total thickness of the tephra layers called Tachikawa and Musashino Loam (fig. 2), which were derived from Mt. Fuji during the Würm glacial age suggests that the average wind direction of that age was similar to that of the present mean direction, i. e. W10°S in 2 to 20km high in the upper atmosphere over the Japanese Islands. From the stand-point of tephrochronology, the types of tephra distribution in Japan-zonally extended tephra layers on the meridionally elongataed islands-have both week and strong points in determining the Quaternaty geochronology. The former lies in difficulty in the correlation of tephra layers in the north-south direction, because of few cases which show the overlapping relations between tephra layers of north and south regions; the latter is the case of correlation between the west volcanic and the east coastal regions by the use of east and west extended tephra layers.
4. The volumes of some remarkable tephra layers and the above-mentioned Loam from Mt. Fuji are cited in table. From these several examples, the total volume of the Quaternary tephra layers in Japan are roughly estimated over 1, 000km3.
