5:15 PM - 6:30 PM
[SVC31-P01] PHENO-BUBBLE NUMBER DENSITY (PBND) AS INDICATOR OF OVERPRESSURE IN PRE-ERUPTIVE MAGMA CHAMBER
Keywords:Pheno-bubble, Pheno-bubble number density, Magma chamber, Overpressure, Explosive eruptions, Bulk density
Magma chamber overpressure which caused by the oversaturation of preexisting bubbles (hereafter referred as “pheno-bubble” as a mimic of “phenocryst” in the common terminology of crystals in magma chamber) has been considered as one of the most important factors for triggering explosive volcanic eruptions. However, such qualitative and quantitative information of pheno-bubbles which are preserved in juvenile materials (e.g. pumice and scoria) is not yet provided. Therefore, in this study, we present the qualitative and quantitative study of pheno-bubble by examining natural samples which were obtained from 16 volcanoes, ranging from the VEI (volcanic explosivity index) 3 to 8 eruptions with various eruption style (subplinian to caldera forming eruptions) and various magma compositions (basaltic andesite to rhyolite). Based on our 2D-bubble size distributions (BSDs) data, we confirm that pheno-bubble can be defined as bubbles with diameter larger than approximately 0.1 mm, regardless to its compositions. We found that the number densities of pheno-bubble (PBND) in pre-eruptive magma chamber exhibit positive correlation with VEI. Particularly, VEI 3-5 eruptions have typically smaller PBNDs value (0.07 to 0.3x1011 m-3) compared to VEI 6-8 eruptions (0.8 to 2x1011 m-3). This suggest that the large and long-lived magma chambers experience with more intense heterogeneous nucleation of bubbles (or second boiling) thus results in more intense supersaturation of volatiles and provides larger amount of overpressure. In addition, we also found that bulk density has strong dependence on PBND value, as supported by the fact that juvenile materials from VEI 3-5 eruptions are typically denser than VEI 6-8 eruptions (avg. 0.6-1.4 gr/cm3 and 0.3-0.8 gr/cm3, respectively).