Resumen
Forty-three soils (130 horizons), sampled by the geo-pedological mission organized by the State University of Gent (Belgium) in 1962 on Isla Santa Cruz (Galápagos Islands), were analysed in order to determine their degree of chemical evolution. Several weathering indices (Weathering Index of Parker – WIP –, Chemical Index of Alteration – CIA –, Chemical Index of Weathering – CIW –, Plagioclase Index of Alteration – PIA – and Silica–Titania Index – STI –) and multivariate statistical analysis (principal components analysis), based on chemical composition, were used. With the only exception of the STI, the indices were highly correlated (r > 0.85). The highest WIP and STI values (20.9 ± 8.2 and 70.2 ± 2.2 respectively) were found for soils developed on basalt flows near the coast. Slightly lower values (WIP 16.8 + 5.1 and STI 61 ± 3.4) were shown by brown soils developed from basaltic flows at elevations between 140 and 225 m a.s.l. While the lowest values (WIP 9 ± 5 and STI 47 ± 6.8), representing the more weathered materials, were found for soils located at the highest elevations (> 400 m a.s.l.) and mostly developed on pyroclastic materials (tuff and tephra). As the chemical composition of the geological material (basalt and tephra) is highly homogeneous, the degree of weathering is likely to depend on climatic conditions controlled by altitude and orientation. On the windward slopes of the island a gradient of increasing weathering is observed from the arid conditions predominant at the coast to elevations of 400–500 m a.s.l., where much more humid conditions prevail. Principal component analysis on elemental composition also supported the interpretation that the degree of weathering (first component) and soil horizonation (second component) are both related to climatic conditions. Both, the variation of the chemical indices and the principal components of the geochemical composition are related to the bioclimatic zones: soils with the lowest degree of weathering are located in the arid coastal zone; slightly higher intensity was found for soils located in the transition and Scalesia zones; while the most weathered soils appear in the brown zone. Compared to other volcanic soils studied in the literature, soils from Isla Santa Cruz are in the upper range of chemical weathering intensity, only comparable to soils from Azores Islands and Canary Islands (Tenerife and La Gomera) developed on basalts, under oceanic conditions.
