A Climosequence of Soils on Late Quaternary Volcanic Ash in Highland Papua New Guinea

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Chartres, C. J.
Pain, C. F.
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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, weathered and partially altered primary minerals are recognized and molar ratios of calcium, magnesium, sodium and potassium to aluminum increase accordingly. In the clay fractions, cellophane 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.
Library Journal, 32, p. 131-155