Weathering on volcanic edifices under semiarid climates: insights from a regional assessment of the composition of Fogo Island regoliths (Cape Verde)

Abstract

AbstractIn this study, the geochemistry and mineralogy of regoliths formed on Fogo Island (Cape Verde), a polygenic strato-volcano built during the Quaternary, are used to assess the geomorphological factors that control the early stages of basalt weathering. Fogo Island soils are mainly derived from relatively homogenous silica-undersaturated basaltic rocks. However, a discernible exotic component is recognized in areas most exposed to prevailing winds via ratios of non-mobile elements that are hosted in different amounts by basaltic rocks (e.g. Th, Sc and Ti). The weathering extent is evidenced by a relative depletion in mobile elements (e.g. Na, Ca, Mg) and an enrichment in non-mobile elements (e.g. Ti, Fe, Sc, Al), the decomposition of the most labile minerals (olivines) and the formation of secondary components (phyllosilicates and some Fe-oxides, such as hematite-goethite), along with the enrichment in quartz supplied from non-volcanic areas. Weathering depends on bedrock age and type (pyroclastic deposits v. lava-flows). In particular, soils covering older volcanic units tend to be more affected by chemical alteration than those overlying younger units. In addition, more intense weathering is observed in locations characterized by a combination of moderate elevation, slopes with low gradient and relatively high rainfall. The present investigation shows that even in low humidity environments recently formed basalts are affected by weathering, with the extent of chemical decomposition being mainly determined by the age of surface exposure and local orographic/climatic features.