Origin and Classification of the Surface Soils by Some Color Analyses

- a Case Study Around the Aso Volcano, Kyushu, Southwestern Japan -

Abstract

The black-colored soil, the surface eolian deposits of the dry land of Japan, has been called various names and was misunderstood when the origin was volcanic ashes. In the vicinity of Aso volcano, it is clear that there are the volcanic ashes and non-volcanic ashes as the parent material of sediment, so this area is suitable for survey of surface soils. Therefore, we investigated three sites, Sensuikyo site（parent material of soils are volcanic ash） , Izumikawa site（volcanic ash dominant）, and Daikanbo site containing almost non-volcanic ash in the parent material. Field observation revealed that the black color of soils at the Sensuikyo site closest to the eruption center is due to colored minerals. The surface soils of the Daikanbo site far from the eruption center are “Kuroboku”（andosols）and loamy soil. It was revealed that the micro charcoal is involved in the color of these soils. The aims of this study are the investigation of the origin of surface soils and the classification of the surface soils at the Izumikawa site nearer the eruption center based on results at Daikanbo site. Decolorization treatment, the micro charcoal analysis and color analysis of the samples obtained from Daikanbo site were performed to obtain the criteria for the topsoil classification. Based on this criterion got from the examination, we explained the main cause of the coloration of the topsoil at Izumikawa site. From the results, we can conclude that coloring was caused by soluble humus, colored minerals, pedogenesis and others. As described above, the investigated results focusing on the surface soils parent materials as aeolian deposits were collected, and the surface soils classification by main coloring materials was performed.