Geochemical and mineralogical composition of the 2018 volcanic deposits of Mt. Anak Krakatau

Abstract

Mount Anak Krakatau erupted on 22nd December 2018, resulting in a large deposit of fresh volcanoclastic materials on the island's surface. As the age of these new soil materials is known precisely, and the island is isolated from human activities, studying the ash and pyroclastic materials of Anak Krakatau will provide insights on the first stages of pedogenesis in tropical volcanic regions. This paper reports for the first time the chemical and geochemical characteristics of the fresh volcanic materials of Mt. Anak Krakatau. Samples were collected at 63 days and 82 days after the eruption. Remobilization of newly deposit volcanic materials occurred immediately after the eruption. Rill and gully erosion were observed on the surface of the island. Two types of samples were collected and analysed: volcanic ash (< 2 mm) and lapilli (2–64 mm). The pHH2O of the samples had a mean value of 5.9 (std. dev. 0.6). The CEC of the materials was very low but variable (3.11 ± 2.3 cmolc kg−1) and the cations were easily leached (Mg > Ca > K > Na). Microscopic analysis and XRD patterns showed that the materials were dominated by plagioclases, with labradorite being the dominant species. XRF analysis indicated that there were two types of volcanic materials: the first group had lower SiO2 concentration (mean 39%) and lower Al2O3 (mean 10%) but a higher concentration of Fe2O3 (mean 18.9%), CaO (15.6%), K2O (1.7%), SO4 (2.9%), P2O5 (2.8%), and heavier elements (Mn, Zn, Cu, Ti, Sr, Zr, Cr) and Ag2O (0.76%). The second group was more common with basaltic-andesitic composition with a mean concentration of 54.9% SiO2, 17.2% Al2O3, and 8.5% Fe2O3. Anak Krakatau eruptions and volcanic deposits are rare events; basic information provided in this report offers valuable information on the initial stage of pedogenesis and can be used as a baseline for future pedogenesis study.