Magmatic Evolution of Pre 1963 Eruption of Agung Volcano, Indonesia

Abstract:

Agung volcano is an active stratovolcano in the eastern part of Bali Province, Indonesia. The worst notable eruptions have occurred in 1963, which killed 1,148 people and injured hundreds of people. There was no data on casualties before 1963 volcanic eruption but the eruptions had products largely covering the area of the volcano. Based on the statistical analysis of published data and our petrological and/or mineralogical data, we obtained that the pre-1963 eruption of Agung volcano involving of three individual cogenetic magmas and can be divided into four periods of eruption, i.e. pre-3200 ± 60 BP, 3200 ± 60 BP – 1870 ± 60 BP, 1870 ± 60 BP – 1040 ± 60 BP, and post-1040 ± 60 BP. The eruption periods of pre-3200 ± 60 BP were marked by the start of the injection of more basaltic magma and reached a peak on the eruption periods of 3200 ± 60 BP – 1870 ± 60 BP. These eruption periods were marked by the dominant formation of the basalt to basaltic andesite lava of rock units with SiO₂ content of 51 – 56% and the formation of Bukit Pawon parasitic cone. The period of 1870 ± 60 BP – 1040 ± 60 BP, the Agung volcano eruptions produced basalt to andesite lava and pyroclastic flow of rock units and showed that magma has subsequently undergone of magmatic differentiation with SiO₂ content of 51 – 58%. It is possible that the eruptions were triggered by the injection of new basaltic magma and then the new magma mixed with previous of differentiated magma. The magmatic differentiation process continued during the eruption period of post-1040 ± 60 BP with an increase of SiO₂ content (53 – 63%) to capable produce the pyroclastic fall of rock units.

Keyword: Agung volcano, magma, eruption periods, magmatic differentiation.