The Alteration Sequence of the Ontake Volcanic Ashes

Abstract

The Ontake pyroclastic fall deposits, which have been accumulated in Pleistocene time, show widespread occurrence in the eastern areas of the Ontake volcano in Honshû. In this study, the weathering sequence of these deposits was given, and it was concluded that the alteration followed the sequence of volcanic ash→allophane→halloysite in the order of progressive alteration.
In view of the sedimentary environments, the pyroclastic fall deposits may be divided into water-laid, sea-laid and air-laid sediments.
In the case of “Pm I” pumice fall bed which served as a key bed in these pyroclastic fall deposits, the degree of alteration is reflected by the delicate shade of different environmental conditions. That is to say, in air-laid sediment argillization is remakable, where as, in water-laid and sea-laid sediments, the volcanic glasses are usually prevented from conspicuous alteration, and only small amount of allophane is recognized. These facts suggest that the sufficient water supply and the retaining of dead water in the sediments might be essential factors in progressive alteration.
In general, during the course of weathering, alkalies and alkaline-earth metals might be removed by slightly acid water. The fact that iron and manganese are concentrated as hydroxide at the base of pumice bed may be explained by more alkaline conditions at this site. Under these physicochemical environments, silica and alumina might be leached out as colloidal forms and precipitate as gels. The usual occurrence of allophane, α-cristobalite and gibbsite in the weathering profile will support this view.
The features of alteration mentioned above, may indicate that the hydrogen ion concentration and the redox potential as a function of it, are the dominant factor to controll the mineral species which were formed in the course of weathering of these pyroclastic fall deposits.