1977年有珠山噴出物の経時変化 : とくに塩基溶脱について

抄録

For the object of investigating the temporal changes of chemical properties in the new tephra erupted from Usu Volcano in 1977, Some experiments and field investigations were performed. Field investigations were continued for three years. The results obtained are summarized as follows : (1) Erupted new tephra deposit was supersaturated with exchangeable cations, such as Ca^<++>, Na^+, K^+ and Mg^<++>, and it was alkaline (pH=7〜8). Exchangeable sites were occupied with Ca^<++>, Na^+, K^+ ang Mg^<++>. Ca^<++> was predominant constituent of the bases. Na^+ content in the tephra was more than that in general soil. High content of Na^+ was considered to be one of the characteristic of the tephra. (2) From the result of the leaching model experiment and field investigation, the leaching of water-soluble bases from ash and pumice was rapid in the early stage, but in the late stage it was moderate. Especially the leaching of water-soluble bases from pumice in the late stage was more moderate. It seemed that the difference of interpore content between ash and pumice influenced the leaching pattern and rate. The content of exchangeable bases leached from ash and pumice was less than that of water-soluble bases. (3) Exchangeable Mg^<2+> content was increased slightly in the leaching model experiment. It seemed to suggest that alteration from non-exchangeable form as the unit of the mineral crystal to exchangeable form exceeded the leaching content. (4) pH of erupted tephra deposit increased at the early stage in experiment, but afterward decreased. It was assumed that the alteration of the ion component ratio was related to pH change. (5) Dealumination and deironation were recognized. It seemed that these changes were accompanied with the weathering of erupted tephra deposit.