Changes in Stability of Mountain Slopes of Yakushima Island Analyzed by Variation of Inorganic Content in Peat Deposits

Abstract

Slope stability and its change play important roles in geomorphic development both of mountains and coastal plaines. Changes in slope stability is expected to have been recorded in geological sections at depositional areas close to slopes and/or in mountain areas. Changes in slope stability were investigated on the basis of stratigraphical analysis, especially analysis of variation of inorganic content in peat deposits at moors and a grassland of Yakushima Island (30°20'N, 130°30'E), which is famous as one of the most rainy regions in Japan.
Three localities on an alluvial cone, a flood plain and a gentle slope on a ridge were selected as the study areas (Fig. 1). Using the panning method, the inorganic content is calculated as the ratio of inorganic content against dry bulk weight of each horizon and expressed by percent. The change in inorganic content was concordant with those in facies and water content (Fig. 2, 3), and showed good agreement with that of ignition loss of thick humic soil (Fig. 7). From the sequential changes in degree of plant decomposition and the inorganic content, peat and thick humic soil profiles can be divided into four layers, namely, the Lowest layer, the Lower one, the Middle one, and the Upper one in ascending order. The change in the composition of materials from the Lower to the Middle layer is the most conspicuous. The changes in the inorganic content suggest that the yield of detritus was the most intensive at the Lower layer and was reduced towards the surface.
These inorganic materials on the alluvial cone and the flood plain were transported by mud flows and floods. Therefore, the high content of inorganic materials can be considered to have been caused by heavy rainfall. Consequently these changes in inorganic content roughly suggest the changes in heavy rainfall occurrences. Initiation of the deposition of the Lower layer is presumed to be after 2000BP from the 14C date (2600-2700BP) of the base of peat layer at Hananoego moor and the thickness of the Lowest layer. Therefore, the slope condition on Yakushima Island is considered to become less stable after 2000BP.