10:00 AM - 10:15 AM
[SVC49-05] Linking petrological and geophysical observations: A case study of the 2011 eruption of Shinmoedake volcano
Keywords:Eruption dynamics, Magma fragmentation, Petrological and geophysical observations
The petrological data above indicate the following scenario. During the first sub-plinian eruption, magma experienced outgassing and microlite crystallization, resulting in the formation of relatively low porosity magma with high microlite crystallinity. The degree of outgassing decreased during the second sub-plinian eruption and the microlite crystallinity decreased. The magma erupted by the final sub-plinian eruption experienced outgassing and crystallization similar to that of the first sub-plinian eruption. The variation in microlite crystallinity can be explained by considering the change in magma decompression rate and/or the change in the final pressure at which the magma is quenched (e.g., Riker et al., 2015).
Linking the petrological and geophysical observations allows us to understand more details of temporal evolution of explosive eruptions. Geodetic data indicated that the magma fluxes were almost constant during the three sub-plinian eruptions, whereas the pressure in the magma chamber monotonically decreased corresponding to the eruptions (Kozono et al., 2013). These observations are counterintuitive because it is commonly expected that the flux decreases in response to the decrease in the pressure of the magma chamber under the assumption of magma chamber of constant volume. However, these paradoxical observations (at least those from the first and second sub-plinian eruption) may be qualitatively explained by considering that magma fragmentation pressure increased, as recorded in the groundmass of pumices, i.e., the decrease in microlite crystallinity observed from the first to the second sub-plinian eruption. According to the steady conduit flow model (Kozono and Koyaguchi, 2009; Koyaguchi, 2016), even when the magma chamber pressure decreases, the magma flux can be kept constant if the fragmentation pressure slightly increases so that the length of gas-pyroclastic flow regime in the conduit increases, i.e., the level of the fragmentation surface descends.