TEXTURE AND TOPOGRAPHY OF VOLCANIC ASH BEDS

Abstract

This paper deals with the effects of agency of soil erosion working on the texture of volcanic ash beds. The surveyed area is the northeastern foot of Asama Volcano. In this area, heavy rainfalls during the rainy season wash down large quantities of soils from the ground surface and deposit them in lower parts of the land. Minor external agencies such as corrasion and deposition due to rain wash accelerate erosion of soils, and degradation, though on a samall scale, is fairly active. As a result, perpetual cultivation becomes impossible where the topography is such that illustrated in Fig. 2 and 5. The writers selected some localities where the effects of such minor agencies are distinctly observed, and carried out topographic measurementand investigation of sedimentational conditions and texture of volcanic ash beds, and contem-pl.ated on the land degradation. The localities are indicated in Fig. 1 and 4. Samples of the black ash right below point A in Fig. 2 and Fig. 5 were collected in a vertical cylindrical form. After solidified, the samples were made into thin sections, which were prepared in the horizontal and the vertical (to the bedding plane) directions at 3 to 5cm intervals. Therefore, the number of thin sections merely for the samples below point D in Fig. 2 amounted to about 90. The study of the texture of these numerous thin sections has revealed the following facts. The black ash comprises several beds of a common texture. Apparently these beds constitute one stratum, but actually they are made of several thin layers differing in texture. The difference in texture has probably resulted from differing degrees of agency, i, e., extent of rain wash which takes place in every rainfall. The micrographic photo shows a thin section cut vertical to the bedding plane. The section represents the texture of the thin layer in the thick black ash. The layesrs are thicker at the base of the slopes and their texture is granular in most cases. Under the microscope, the layers are found to contain minute grain of brown volcanic ash. Thus, from the thickness of volcanic ash beds in outcrops, the status of cultivation on the ground surface (the number of years of cultivation), and from the environment of the black ash sedimentation as revealed by the texture, it has become known that the action of degradation is unexpectedly strong and quick in the areas composed of soft sedimentary beds.