A climosequence of soils on late quaternary volcanic ash in highland Papua New Guinea

Abstract

Some chemical, physical and mineralogical properties of three soils developed in volcanic ash at altitudes of 1040 m, 1720 m, and 2350 m in Papua New Guinea's Enga Province are presented. Silt-fraction mineralogy and total chemical analyses show that fresh ash occurs in the upper approximately 30 cm of profile at each site. This fresh ash is probably only a few thousand years old and overlies an older weathered ash (Tomba Tephra;more than 50000 years old). At the lowest site the majority of the primary minerals (predominantly amphiboles, volcanic glass and felspar, and some pyroxenes) have been decomposed by weathering to produce a high clay content. With increasing altitude, greater proportions of silt-sized, unweathered and partially altered primary minerals are recognised and molar ratios of calcium, magnesium, sodium and potassium to aluminum increase accordingly. In the clay fractions, allophane with Al/Si ratios of approximately 2.0 is dominant at the highest site, whereas with decreasing altitude lower proportions of allophane occur, Al/Si ratios decline and halloysite becomes dominant. Gibbsite is found in all three profiles