Chemical weathering of Reference European Volcanic Soils

Abstract

Volcanic rocks are an important component of the weathering regime. They contribute to the geochemical cycles of many elements and are locally significant as a source of nutrient elements in agricultural soils (Chesworth et al. 2004). Geochemistry makes use of major element data in different ways, as for example in rock classification (Rollinson 1993) and in rock and soil material weathering. During weathering and pedogenesis, elements are removed from the parent material or selectively accumulated. So, in homogeneous materials differences in chemistry between samples may simply reflect variation in extent of weathering. Consequently, chemical weathering indices are commonly used for characterizing weathering and soil profiles (Price and Velbel 2003). Chemical weathering indices convert bulk major element contents into a single value for each sample. Weathering indices are typically plotted versus depth in the soil profile, providing a visual representation of changes in bulk chemistry with presumed increase or decrease in weathering. In homogeneous parent rocks, changes of weathering indices with depth are usually gradual and systematic, reflecting continuous leaching of elements as weathering progresses. Due to the frequent burial by new volcanic materials and/or by the gradual or repeated deposition of fresh materials that causes rejuvenation of topsoils, volcanic materials, however, do not always produce systematic trends with depth of elemental composition. The weathering rate of volcanic materials depends on their chemical composition and texture. Volcanic material of rhyodacitic composition weathers at lower rates than material of andesitic or basaltic composition (Kirkman and McHardy 1980). Fine volcanic materials show little resistance to weathering in comparison to other types of parent materials (Shoji 1986). In addition to the parent material, climatic conditions and the age of weathering are important factors. Additionally, Malpas et al. (2001) studying pyroclastic volcanic rocks in Hong Kong, showed that the type and abundance of sesquioxides and clay minerals can significantly modify the geochemical signature of weathering. For this reason, in addition to the chemical weathering indices more frequently reported in literature, we have also measured the abrasion pH, a parameter used as an indicator of rock weathering (Grant 1969) that has been proposed by Ferrari and Magaldi (1983) as an index of potential fertility of soils. The abrasion pH is obtained by grinding soil material in distilled water. Its value is affected by the quantity of cations released from primary minerals and by the amount and type of clay minerals. Consequently, higher abrasion-pH values are expected in soils that are rich in fresh and weatherable minerals. As weathering proceeds and the clay content of the soil increases, the abrasion pH tends to decrease. (Table presented). © Springer-Verlag Berlin Heidelberg 2007.