著者

宮地 直道

出版者

特定非営利活動法人日本火山学会

雑誌

火山. 第2集 (ISSN:04534360)

巻号頁・発行日

vol.29, no.1, pp.17-30, 1984-04-01

被引用文献数

3

---

"Fuji 1707 Tephra" was thrown out from the three craters on the upper southeast flank of Fuji Volcano on December 16 to 31, 1707 (TSUYA, 1955). It is composed of scoria beds named Ho-II to -IV and lowest two pumice beds named Ho-I. Ho-I and -II are probably originated from the second and the third craters and Ho-III and -IV are from the first crater on the basis of distribution of ejecta and characteristics in apparent densities. Ho-I_a, the lower Ho-I, is studied with respect to its mode of eruption, transportation and sedimentation. This bed is composed of white to pale brownish pumice, and is readily recognizable in the field and traceable over 130 km eastward from the source. Ho-I_a is the Plinian pumice-fall deposit and its volume is calculated to be 0.067 km^3 by the method of "Cross section area" (SUZUKI et al., 1973). The axis of maximum pumice diameters (MP), maximum lithic fragment diameters (ML) and median diameters of grains (Md) run along the southern margin of the distribution area. The same feature is recognized in the apparent density distribution for pumice fragments. More than 130 km eastward from the source, such an asymmetric distribution for MP is preserved. These facts suggest that the eruption column of Ho-I_a was affected by the strong westerlies in the upper atmosphere and the local SW winds in the lower one. The present meteorological data support this interpretation. Consequently, it is concluded that the irregular distribution of this Plinian tephra was originated by the differences in wind direction and velocity due to the altitude.