Genesis of tephra-derived soils from the Roccamonfina volcano, South Central Italy

The properties of seven pedons, formed in three different tephra deposits inside the main caldera of the Roccamonfina volcano, were examined to better understand the genesis of these soils. Three of these soils derived from trachytic ash deposits are deep and porous and have andic properties, imogolite, and high allophane content. In these ash-derived soils, neogenesis of crystalline clays is limited but include halloysite and hydroxy-interlayered vermiculite. Hydrated halloysite is present only in the deepest horizons of two of these pedons. By contrast, the deep and clayey soil formed from alkali basaltic scoria and the shallow and slowly permeable soils derived from consolidated trachyandesitic tuffs have weak andic properties. In these soils, allophane content is negligible and pedogenesis induces the neogenesis of crystalline clays dominated by halloysite. Unlike the ash-derived soils, hydrated halloysite is present through most of the pedons. Climate, topography, and vegetation do not appear to be limiting factors for allophane formation in the area. The hydraulic properties of the parent material, possibly together with the time of pedogenesis, apparently affect the different pedogenetic trends of these soils. Allophanic soils developed in younger, porous, permeable ash deposits where rapid glass weathering is favored and Al and Si are readily available for allophane formation. By contrast, non-allophanic soils developed in older, less porous, and less permeable scoria and consolidated tuffs where pedogenesis induces the neogenesis of crystalline clays and hydrated halloysite is probably formed by in situ weathering of volcanic glass. Allophanic and non-allophanic soils are then coexisting in similar landscapes under similar climatic conditions.