On the Kanto Ash Beds in the Middle and Southern Parts of the Boso Peninsula

Abstract

Studies of the grain-size analyses, mineral analyses, chemical analyses and X-ray analyses were carried out on the weathered volcanic ash beds at 13 locations of the Boso peninsula (Fig. 1).
Their volcanic ash beds are divided into three types, according to the composition of mafic minerals, clay minerals and the chemical composition. The characteristics of the types are summarized as follows:
(1) The volcanic ash beds of the first type are fine (Md: 4.7-5.0) and 30-60% of the mafic minerals in these beds is olivine (Fig. 3). Allophane is dominant clay mineral.
(2) The second type of ash beds have much iron minerals but less of olivine among mafic minerals (Fig. 3). The clay mineral of these beds is mostly halloysite (Figs. 4 and 5).
(3) The volcanic ash beds of the third type have much of volcanic glass and the contents of SiO₂, Na₂O and K₂O are more than those of the other ash beds (Table 2).