Pre-1963 Mount Agung Eruption History and Magma Evolution Based On Petrological, Mineralogical, and Geochemical Analysis

Abstract

Mount Agung is an active stratovolcano on Bali Island. The eruption of Mount Agung, which took place in 1963, was the largest eruption after Mount Krakatoa in 1883 and became one of the most prominent eruptions in the 20th century. Pre-eruption history of Mount Agung has yet to be known in detail. This study aims to determine the history of the pre-1963 volcanic activity of Mount Agung. Based on statistical, petrological, and mineralogical data analyses, the result shows that the pre-1963 eruption of Mount Agung was influenced by three cogenetic magmas that can be divided into four eruption periods, i.e., pre-3200 ± 60 BP, 3200 ± 60 – 1870 ± 40 BP, 1870 ± 40 – 1040 ± 50 BP, and post-1040 ± 50 BP. Mount Agung’s historical activity was marked by the injection of the basaltic magma in the pre-3200 ± 60 BP period and reached its peak in the 3200 ± 60 – 1870 ± 40 BP period. Both periods produced dominant basalt to basaltic andesite lava units (SiO2 51 – 56%). The end of the 3200 ± 60 – 1870 ± 40 BP period was marked by Bukit Pawon parasitic cone formation. In 1870 ± 40 – 1040 ± 50 BP, many pyroclastic flow units were formed in addition to the emergence of basalt to andesite lava units. During this period, magma differentiation continued as indicated by the rising content of SiO2 (51 – 58%), which was controlled by crystallization fractionation. During this period, a small injection of basaltic magma occurred, which caused the magma to mix with the previously differentiated magma. At the beginning of the post-1040 ± 50 BP period, there was a slight injection of basaltic magma. Afterward, magma underwent intensive differentiation coinciding with the increase of SiO2 (53 – 63%), followed by crystal fractionation and slight crustal contamination. The evolved magma can produce pyroclastic fall, pyroclastics flow, and lava units.