Geochemical Characterization and Evolution of Soils from Krakatau Islands

The Krakatau islands complex was formed after the huge eruption of the late Mt. Krakatau in 1883. The complex consists of four islands: Rakata, Panjang, Sertung, and Anak Krakatau. While volcanic activity and plant succession have been extensively studied in the Krakatau area, the soils received less attention. This study characterizes the morphology, chemistry, and geochemistry of the soils from the Krakatau islands. Ten sampling sites were selected from Mt. Anak Krakatau, Rakata, Panjang, and Sebesi islands. The elemental concentration of the samples was measured using the X-ray fluorescence (XRF). In addition, chemical weathering indices were calculated from the geochemical elements. Linear discriminant analysis was used to separate materials from the four islands based on their geochemical concentrations (SiO2, Al2O3, Fe2O3, CaO, MgO, K2O, SO3, P2O5, Ti, and Zr). While the four islands were nearby and influenced by the eruption of Mt. Krakatau, the analysis showed that the geochemical characteristics of volcanic ash for each island are distinct. Mahalanobis distance differentiated soils of Anak Krakatau from the others. Base cations loss in Sebesi was larger than in Rakata, Panjang and Anak Krakatau. Meanwhile, desilication was larger in Rakata compared to Sebesi, Panjang, and Anak Krakatau. The following sequence of weathering degree was determined: Sebesi > Rakata = Panjang > Anak Krakatau. Comparison between sites allowed differentiation of origin of the volcanic materials and a calculation of a relative weathering sequence. This study suggests that the geochemical characteristics of the volcanic materials on these islands are unique and can be used to assess its weathering process and pedogenesis.