PETROLOGY, MINERALOGY, AND GEOCHEMISTRY OF VOLCANIC ROCKS FROM PRE 1963 ERUPTION OF MOUNT AGUNG, KARANGASEM DISTRICT, BALI PROVINCE

• Main Author: Syafitri, Aulia
• Format: Final Project
• Language: Indonesia
• Subjects: Geologi, hidrologi & meteorologi
• Online Access: https://digilib.itb.ac.id/gdl/view/53264
• Institution: Institut Teknologi Bandung

Mount Agung eruption in 1963 was the largest eruption after Mount Krakatau in 1883 and became one of the important eruptions of the 20th century. But, the prior eruption history of Mount Agung has yet to be known in detail. This study aims to determine the petrological, mineralogical, and geochemical characteristics, rock-forming magma, and the history of the pre-1963 volcanic activity of Mount Agung. The research area is located in Karangasem District, Bali Province. This study used 18 samples of Mount Agung lava which all were analyzed petrographically dan 6 samples used for mineral chemistry analysis using SEM-EDS. The geochemical analysis was carried out using published journal data of 64 volcanic rock samples. Based on SiO2 vs. K2O and rock unit correlation, the Mount Agung eruption was influenced by three cogenetic magmas and can be divided into four eruption periods, pre 3200 ± 60 BP, 3200 ± 60 BP – 1870 ± 40 BP, 1870 ± 40 BP – 1040 ± 50 BP, and post 1040 ± 50 BP. The pre 3200 ± 60 BP period produced basaltic andesite (SiO2 53–56 wt%) containing plagioclase phenocryst (An38–89), augite, enstatite, magnetite, and olivine (Fo62–78), embedded in glass groundmass, plagioclase microlites, and fine opaque minerals. The 3200 ± 60 – 1870 ± 40 BP period produced basalt – basaltic andesite (SiO2 51–56 wt%) consist of plagioclase (An67–83), enstatite, augite, magnetite, and olivine (Fo60–71), embedded in glass groundmass, plagioclase microlites, and fine opaque crystals. The 1870 ± 40 – 1040 ± 50 BP period produced basalt – andesite (SiO2 51–58 wt%) containing plagioclase phenocryst (An43–88), augite, enstatite, magnetite, and olivine (Fo59–71), in glass groundmass, plagioclase microlites, fine opaque and pyroxene crystals. The post 1040 ± 50 BP period produced basaltic andesite – dacite (SiO2 53–63 wt%) consist of plagioclase (An38–86), augite, enstatite, magnetite, and olivine (Fo66–76), embedded in glass groundmass, plagioclase microlite, fine opaque and pyroxene crystals. Mount Agung's volcanic activity was caused by the subduction of the Indo-Australian Plate within the Eurasian Plate (active continental margin) during the Quaternary Age. The history of the pre-1963 volcanic activity of Mount Agung was marked by the initiation of basaltic magma injection with calc-alkaline affinity in pre 3200 ± 60 BP period and reached the peak in 3200 ± 60 – 1870 ± 40 BP period. Both periods produced dominant lava units and the end of 3200 ± 60 – 1870 ± 40 BP was marked by the eruption of Bukit Pawon, the parasitic cone of Mount Agung. In the 1870 ± 40 – 1040 ± 50 BP period, apart from the emergence of lava units, many pyroclastic flow units were also formed. In this period the magma continued to differentiate which was controlled by the crystal fractionation and it is possible that there is a small amount of basaltic magma injection which generated magma mixing with previously differentiated magma. Early in the post 1040 ± 50 BP period, there was a small amount of basaltic magma injection. Then, magma undergoes intensive differentiation of crystal fractionation and a little crustal contamination, forming pyroclastic fall, pyroclastic flow, and lava units.