Studies on the Red Colored Soil Dervied from Basaltic Andesite of Shibayama

On Shibayama, a hill situated at the northern extremity of Nijozan Volcanic Group, a red colored soil (abbreviated as red soil) is distributed contacting with a brown soil derived from the same basaltic andesitic origin. Under the brown soil, floaters were found just as in commonly weathered basaltic soils, whereas intermediates which keep the original texture were seen under the red soil. To discuss the forming process and specificities of this red soil, some pedological studies were made on these soils, the red intermediates, two other brown basaltic soils for comparison, and their parent rocks. Shibayama basaltic andesite was a compact grayish black rock with phenocrysts of olivine, augite, and plagioclase. Its groundmass was mainly composed of plagioclase. With increasing red color in intermediates, olivine phenocrysts were first altered to chlorite and/or opaque clayey materials. Each sand fraction contained magnetite, augite (except Shibayama red soil), glass, and plagioclase. The predominant clay mineral in Shibayama red soil was montmorillonite, and in other soil samples, dehydrated halloysite. Shibayama red soil had more clay fraction than other brown soils, and cation exchange capacity of this specimen was very high, corresponding to its high montmorillonite content. The base content decreased and total iron content increased from the fresh basaltic andesite, through the red intermediates, to the red soil. Both total and free iron content were higher in the brown soil than in the red soil, while the content of hematite form iron oxide was higher in the red soil than in the brown soils. Based on the results thus obtained from field observation, as well as mineralogical and chemical researches, together with the data of some reports by other research workers on the red colored soils, it is assumed that the original material of Shibayama red basaltic soil had suffered certain primary alteration due to some geological effects before soil formation proceeded