Towards a weathering model of mount lamington ash, Papua New Guinea

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The Mount Lamington ash halo has been studied at six sites both close to and distant from the volcano. Negligible alumina occurs in the seepage waters from the ash and so alumina is assumed constant during weathering. In andesitic ashes, weathering to allophane, an approximation to a first order reaction was found in 1968 and is confirmed here. With given infiltration and silica concentration the weathering rate constant decreases with increasing content of weathering minerals and increasing thickness of ash being weathered. The rate constant also increased with increasing infiltration and decreasing median size of the ash. The total weathering rate is the product of the thickness, percentage of ash being weathered, and the weathering rate constant. The total weathering rate is the same for thick, less weathered, proximal ashes as it is for the thin, more weathered, distal ashes. First order kinetics imply a high rate of leaching decreasing with time but continual burial under new ash at a nearly constant rate leads to linear leaching of the upper part of the profile. The leaching rate decreases exponentially with depth and appears to ignore buried topsoil barriers. It is possible to construct weathering models exploring the rate constant and to link it with leaching rates. Two thirds of the loss on weathering is silica and there is a total leaching rate of 3–4 mg SiO 2 cm −2 year −1 as on basalt in Hawaii and on andesite in St. Vincent. Water analyses show silica concentrations in the ash of 17–35 ppm and in draining streamlets of 9–20 ppm. Measured concentrations agree with calculated concentrations from the weathering data. Silica concentrations are similar in other humid tropical rivers.