The Role of Crystal Fraction to Relative Magma Viscosity: An Approach to Understand the Explosive Caldera-Forming Volcanism of Raung Volcano

Abstract

Explosive volcanism is one of the deadliest volcanic hazards. The characteristics of the magma chamber that cause explosive eruptions need to be studied more deeply. Raung Volcano in East Java is an example of volcano that has a history of explosive eruptions that marked by a 2 km wide caldera. Petrographic analysis was carried out on selected 4 basaltic lava samples to determine the composition and proportion of minerals. Then, an analysis of the viscosity of the magma was carried out in relative terms using the shape and size configuration of the crystals. The comparison between the percentage of crystals and the relative viscosity value shows a positive correlation which indicates that there is an effect of the number of crystals on the viscosity of the magma. The increase in the viscosity of magma that occurred in the pre-caldera lava eruption of Raung Volcano indicates an intensive crystallization. This is supported by petrographic data which shows a porphyritic texture with a phenocryst of more than 40%. Plagioclase crystals become quite dominant crystals (> 30%). Based on this, it can be interpreted that the crystallization process occurred intensively in the shallow magma pocket of Raung Volcano.