Volcanostratigraphy and Evolution Characterization Using Morphological and Geochemical Analysis, and Field Observation for Quaternary Balerang and Rajabasa Volcanoes, Lampung-Indonesia

Abstract:

Balerang and Rajabasa are Quaternary volcanic complex with active fumaroles though no historic eruptions were recorded. These volcanoes are stratovolcanoes and located in the volcanic front of Sunda arc. Although some studies reported the geological setting of these volcanoes in these past few years, their detailed volcanostratigraphy and evolution are still unclear. Morphological analysis and field observation were used to understand the detail of volcanostratigraphy and the evolution of volcanic edifices. Morphological characteristics were analyzed by areal and linear analysis using perspective view of DEM. Sample data from Balerang and Rajabasa volcanoes were collected as well and compared each other. By analyzing the both data, it was elucidated that this volcanic complex has erupted from 4 different vents, and produced both lavas and pyroclastics. This result was strengthened by constructing a fence diagram and reconciling each of the layers. The shifting of vents was strongly influenced by active dextral Great Sumatran Fault. Geochemical study was conducted as well, and this study shows that the composition of Balerang’s magma has undergone a reversal evolution from dacite to basaltic andesite. The lavas of the early product shows dacitic (%SiO₂~65.76-66.24), middle stage shows andesitic (%SiO₂ ~62.19), and the late stage shows basaltic andesite (%SiO₂ ~55.19). On the other hand, Rajabasa magma is nearly constant between andesitic to dacitic (%SiO₂ ~60.15-65.52).