5:15 PM - 6:45 PM
[SCG54-P03] The shallow conduit process of the 2020 violent Strombolian eruption of Nishinoshima inferred from the volcanic ash
Keywords:Violent Strombolian eruption, Volcanic ash, Shallow conduit process, Nishinoshima, Microlite, Mafic magma
Nishinoshima volcano has erupted intermittently since 2013, and Episode 4 eruption occurred from December 2019 to August 2020. During Episode 4, the magma composition changed from andesitic (~60 wt% SiO2) to basaltic andesitic (~55 wt% SiO2), and the eruption style changed to violent Strombolian. Previous studies suggest that the violent Strombolian eruption was caused by the ascent and mixing of volatile-rich low-SiO2 magma into the existing high-SiO2 magma chamber right before or during Episode 4 (Maeno et al., 2021; Kaneko et al., 2022). However, the eruption style of mafic magma is also controlled by the shallow conduit process. In this study, textual and chemical analyses were performed for the groundmass of volcanic ash from the violent Strombolian eruption in Nishinoshima Episode 4 to examine the shallow conduit process and mechanism of the violent Strombolian eruption. We took backscattered electron (BSE) images of 59 ash particles, using FE-SEM (JEOL JSM-IT700HR, Shizuoka University), and crystallinities and microlite number densities were quantified for each ash particle by image analyses if the BSE images. In addition, chemical analyses of groundmass minerals and glass were performed using FE-EPMA (JEOL JXA-8530FPlus, Earthquake Research Institute, The University of Tokyo).
About 90% of the ash particles were glassy particles with various amounts of microlites in the groundmass, and others consisted of crystalline particles and free crystals; we focused on the glassy particles. The groundmass consisted of microlites of plagioclase, clinopyroxene, olivine, Fe-Ti oxides, and glass. Nanorites were observed in the groundmass glass. Small pieces of high-crystalline magma were often found incorporated into low-crystalline magma.
The groundmass crystal contents of plagioclase and mafic minerals were 20.8-49.9 vol% and 6.5-18.9 vol%, respectively, and the total crystallinity was 30.7-69.4 vol%. The total crystallinity of most ash particles is larger than the threshold value at which the liquid-solid transition occurs, indicating brittle fragmentation occurred. The microlite number density-crystallinity relations suggest that plagioclase and clinopyroxene crystallized under lower and higher degrees of undercooling, respectively.
The bulk composition of each ash particle was determined from the glass and mineral abundances and chemical compositions; the results were consistent with the XRF result of Maeno et al. (2021). We then performed equilibrium crystallization simulations using MELTS (Gualda et al., 2012) for the melt of this bulk composition and the results were compared to the measured groundmass crystallinity. The comparison indicates that the maximum temperature was ~1100°C, which is consistent with pyroxene geothermometry. About half of the ash particles were plotted along the 1 atm isobars. In addition, a small number of ash particles were plotted along the isobar below 30 MPa, suggesting that the solidified magma stuck to the conduit wall at < 1 km was incorporated into the ascending magma.
Present results suggest that the magma of the Episode 4 eruption ascended the conduit at ~1100°C and increased its crystallinity through decompression-driven crystallization, causing a liquid-solid transition near the surface, resulting in brittle fragmentation and explosion. The magma that stuck to the conduit wall or fell near the vent after the eruption cooled and solidified under 1 atm to form a plug, which suppressed outgassing and further enhanced the explosivity of the eruption. The magma ascent velocity of a violent Strombolian eruption may be relatively small because lava flow simultaneously effuses. Therefore, we thought that the high groundmass crystallinity of the magma due to relatively low temperature plays an important role in causing an intense explosion of a violent Strombolian eruption.
About 90% of the ash particles were glassy particles with various amounts of microlites in the groundmass, and others consisted of crystalline particles and free crystals; we focused on the glassy particles. The groundmass consisted of microlites of plagioclase, clinopyroxene, olivine, Fe-Ti oxides, and glass. Nanorites were observed in the groundmass glass. Small pieces of high-crystalline magma were often found incorporated into low-crystalline magma.
The groundmass crystal contents of plagioclase and mafic minerals were 20.8-49.9 vol% and 6.5-18.9 vol%, respectively, and the total crystallinity was 30.7-69.4 vol%. The total crystallinity of most ash particles is larger than the threshold value at which the liquid-solid transition occurs, indicating brittle fragmentation occurred. The microlite number density-crystallinity relations suggest that plagioclase and clinopyroxene crystallized under lower and higher degrees of undercooling, respectively.
The bulk composition of each ash particle was determined from the glass and mineral abundances and chemical compositions; the results were consistent with the XRF result of Maeno et al. (2021). We then performed equilibrium crystallization simulations using MELTS (Gualda et al., 2012) for the melt of this bulk composition and the results were compared to the measured groundmass crystallinity. The comparison indicates that the maximum temperature was ~1100°C, which is consistent with pyroxene geothermometry. About half of the ash particles were plotted along the 1 atm isobars. In addition, a small number of ash particles were plotted along the isobar below 30 MPa, suggesting that the solidified magma stuck to the conduit wall at < 1 km was incorporated into the ascending magma.
Present results suggest that the magma of the Episode 4 eruption ascended the conduit at ~1100°C and increased its crystallinity through decompression-driven crystallization, causing a liquid-solid transition near the surface, resulting in brittle fragmentation and explosion. The magma that stuck to the conduit wall or fell near the vent after the eruption cooled and solidified under 1 atm to form a plug, which suppressed outgassing and further enhanced the explosivity of the eruption. The magma ascent velocity of a violent Strombolian eruption may be relatively small because lava flow simultaneously effuses. Therefore, we thought that the high groundmass crystallinity of the magma due to relatively low temperature plays an important role in causing an intense explosion of a violent Strombolian eruption.