Vesicularity, Density, and Crystal Size Distribution of Pumice-Scoria at South-Southeast Flank of Merapi Volcano: A Window Into Explosive Volcanic Eruption

Abstract

Abstract Merapi, one of the most active volcanoes in Java Island of Sunda Arc, is a constant threat to the ten million people who live within its 10-km radius of the summit. In 2010, it experienced an explosive eruptive event, sending pumice-laden pyroclastic density currents (PDC) that extended as far as 15 km down its southeastern flank. We report field observations of PDC deposits on the south-southeast flank of the volcano and calculate the vesicularity, density and crystal size distribution of pumice–scoria fragments. The density of the pumice–scoria is measured by archimedes’ principle by weighting the sample in water. The vesicularity is determined from petrography using the point counting method. Crystal Size distribution (CSD) is determined by images from thin sections using plagioclase crystals to calculate the magma residence time. Based on their physical appearance, the pumice–scoria fragments from this area are grouped into: (1) white pumice, (2) grey pumice, and (3) dark scoria. The density of white pumice is overlapping with the grey pumice group (1.5-2 g/ml); while the dark scoria is the densest among others (>2 g/ml). The vesicularity of pumice–scoria ranges from 7-30%. CSD analysis is done in two most vesicular samples from each group. The CSD result gives both shallow and steep slopes which reflect the crystal size and population density. The slope suggests that magma residence times are (a) 11.49–249.68 years for grey pumice, (b) 5.03–194.54 years for white pumice, and (c) 3.83–273.36 years for dark scoria. This research suggests that the three types of pumice–scoria from the study area generally have similar crystallization path and time. Our CSD data shows that these pumice-scoria samples tend to have steeper slopes than lava samples, indicating a more uneven crystal-growth condition.