The development of micromorphological features in relation to some mineralogical and chemical properties of volcanic ash soils in highland Papua New Guinea

Abstract

A sequence containing six soil profiles developed within a Late Quaternary, andesitic tephra at different altitudes (1000-2400 m) and under different climatic conditions is described. Allophane dominates the soils above 2000 m, gibbsite those between 1200 and 2000 m and halloysite those below 1200 m. All the soils have isotic plasmic fabrics within the A horizons. A combination of allophane and organic matter, and in the lowest altitude soils halloysite and organic matter, seems to lead to the development of isotic fabric. B horizon plasmic fabrics range from undulic to isotic-argillasepic-indeterminate, with the most sepic plasmic fabrics occurring at lowest altitudes, where halloysite content is highest and wetting-drying cycles are more frequent. Gibbsans formed due to the precipitation of aluminium are prominent features infilling channels within the soils between 1200 and 2000 m, where desilication is most active. Above 2000 m climatic and drainage factors appear to limit the desilication of the alluvial clay features. Below 1200 m, silica and aluminium released by weathering combine to form halloysite under a less intense leaching regime. Gibbsite also was observed to be forming as a pseudomorph of felspar grains in most profiles.