TEXTURAL STUDY AND ERUPTION DYNAMICS OF THE MALALAK TEPHRA (TANDIKAT
VOLCANO), WEST SUMATRA, INDONESIA
Abstract
The Malalak Tephra (sourced from Tandikat volcano) is considered one of
the youngest and most voluminous tephra deposits (VEI-5) in West
Sumatra, Indonesia. However, several constraints such as deposit
characteristics and eruption dynamics, remain poorly understood.
Therefore, this study focuses on the linkage between temporal variation
(stratigraphy) with componentry, bulk density, and juvenile textures
such as vesicles and phenocrysts of the Malalak Tephra deposit. This
deposit mainly consists of five pumice fall layers (F-1, F-2, F-3, F-4,
and F-5) with gradational contact between each layer (repetitions of
normal and reverse grading). White and grey pumices occur as the main
juvenile phase. White pumice has a higher crystallinity (avg. of 44 %)
and more evolved chemical composition of glass (73–78 wt. % SiO2)
compared to grey pumice (average crystallinity 37 % with 71–77 wt. %
SiO2), suggesting that both pumices originated from different magma
chambers and mingled just before and during the eruption. Grey pumice is
typically denser than white pumice, with varying bulk density of
0.65–1.78 g/cm3 and 0.48–1.09 g/cm3 for grey pumice and white pumice,
respectively. Bulk density was observed to increase (for white and grey
pumices) from the main stage (F-1 and F-2) to the closing stage (F-3,
F-4, and F-5). Because bulk density is strongly dependent on bulk
vesicularity, and bulk vesicularity is controlled by vesicle number
density (which is a function of decompression rate), it can be inferred
that the main stage was relatively more explosive compared to the
closing stage.