種子島の火山灰土壌に関する2・3の問題について

抄録

Tanegashima island is located about 43km to the south of Kyushu, Japan and belongs to perhumid subtropical zone. The bed-rocks of the island are tertiary sandstones and shales, and the ground surface is covered with volcanic ejecta, of which the origin is considered to be the volcanic islands in the west (Kikaicaldera). The repeated falls of volcanic ejecta since pleistocene, strong weathering, severe erosin and re-sedimentation are supposed to be main causes of the complex stratigraphy of the soil parent materials of the island. To obtain some information on the stratigraphy and the origin of each horizon, mineralogical and physico-chemical studies were carried out on two different soil profiles (Anno and Nishinoomote). 1. Horizons of the soils were divided into the following 5 groups according to the difference of the facies. Group 1; weathered volcanic fine gravel and pumiceous layer ; Akahoya-like layer (glassy, coarse textured), almost pure volcanic ejecta. Group 2; humic surface horizon (glassy, coarse textured), derived from volcanic ejecta with some non-volcanic materials (weathered sandstones and shales) Group 3; weathered volcanic ash horizon (fine textured), derived from volcanic ejecta with various degree of admixture of non-volcanic materials (weathered sandstones and shales) Group 4; horizons with pebbles of shale (fine textured), derived from the transported materials of tertiary shale with some volcanic ejecta. Group 5; horizons with breccia of shale (fine textured), derived from residual or colluvial materials of tertiary shale with some volcanic ejecta. 2. Holocene and pleistocene tephra of the profiles of Anno (northeastern coast of Tanegashima island) were divided into six members, which shows 6 times of repeated deposit since pleistocene.