Mineralogy and weathering processes in Recent and Holocene tephra deposits of the Pacific Northwest, USA

Abstract Weathering and evolution of minerals in tephra deposits in the Cascade Range of Washington State, USA, are influenced by climate, vegetation, time, microtopography, and in the vicinity of Mt. St. Helens, naturally acidic precipitation. We have observed the interaction of these factors in Spodosols developed on Holocene tephra in the central Cascades, and in Entisols formed on the 1980 pyroclastic flow of Mt. St. Helens. Weathering regimens in these tephritic soils can be qualitatively characterized by the soil solution composition. Weathering reactions in the Spodosol are driven by organic acids in the upper soil horizons and by carbonic acid in the lower B and C horizons. The mineral assemblage in the Spodosol reflects, in part, the chemical regimen dictated by each proton donor. The presence of smectite in the highly acidic E horizon (pH