Evidence of the weakly stratified crystalline magma feeding an explosive Plinian eruption: A case study of the Holocene ‘Malalak Tephra’ deposit Tandikat volcano, West Sumatra, Indonesia

Abstract

The Malalak Tephra (MT) in West Sumatra, Indonesia, is a thick (up to 5 m within 5 km distance around the vent) pumice fall deposit that consists of one initial ash layer (F-0) and five main lapilli layers (F-1 and F-2 are referred to as the early stage, and F-3, F-4, and F-5 as the late stage). White pumice (microlite-free) and grey pumice (microlite-bearing) occur as the main juvenile phase in the lapilli fall layers, with a tendency of increasing grey pumice content from the early to the late stage (3–4 and 6–12 vol%, respectively). Both pumice types shared identical petrographic and chemical (glass, plagioclase, and amphibole compositions) features, suggesting a similar magmatic origin. However, a slight increase in phenocrysts content (from 30–45 to 33–58 %), modal pyroxene (from 3 to 11 vol%), and anorthite contents (from An70 to An74) was observed from the early toward the late-stage products. Such evidence, coupled with the fact that the white pumice was relatively colder, more hydrous, and shallower than the grey pumice (based on plagioclase and amphibole geothermobarometers) suggests the occurrence of a weakly stratified crystalline magma reservoir. The ‘less silicic’ magma (as the source of grey pumice) experienced extensive microlite crystallization during magma ascent because undercooling can effectively occur under lower viscosity conditions (lower SiO2), forming microlite-bearing juvenile. Whereas the ‘more silicic’ magma did not experience such an extensive syn-eruptive crystallization due to the relatively higher viscosity condition. Finally, the glass compositions of the Malalak pumice were found to be similar to the V-5 deep-marine tephra layer on the west side of Sumatra, suggesting an approximate volume and eruption age of 7.5–16.1 km3 and 4,860 ± 60 cal. BP, respectively.