Shallow crystallization of eruptive magma inferred from volcanic ash microtextures: a case study of the 2018 eruption of Shinmoedake volcano, Japan

Abstract

The occurrence of groundmass crystals reveals the shallow conduit process of magmas, which affects the behavior of eruptions. Here, we analyzed groundmass microtextures of ash samples from the 2018 eruption of Shinmoedake volcano, Japan, to evaluate the change of magma ascent conditions during the eruption sequence. The eruptive activity changed from ash venting (Phase 1: March 1–6) to lava effusion with continuous ash-laden plumes (Phase 2: March 6–9) and then shifted to Vulcanian explosions (Phase 3: March 10–April 5). Non-juvenile particles were abundant in Phase 1, whereas juvenile particles were dominant in Phases 2 and 3. Vesicular juvenile particles were more abundant in Phase 2 than Phase 3. The lower microlite crystallinity and groundmass SiO2 concentrations of the vesicular particles indicate that they were sourced from magma that ascended rapidly. Abundant nanolites were observed in the black interstitial glass of juvenile particles under an optical microscope, whereas few nanolites were observed in the transparent ones. The presence of nanolites can be explained by the dehydration of silicate melt, as well as cooling and oxidation between fragmentation and quenching. Temporal changes in the ash componentry show that the eruption activity started from the erosion of the pre-existing vent plug (Phase 1), shifted to the simultaneous eruption of bubble-bearing and outgassed magmas (Phase 2), and concluded with explosions of the stagnant lava (Phase 3), thereby demonstrating the sequence of vent opening and extrusion and stagnation of magma. Therefore, ash microtextures are valuable for monitoring the shallow conduit process of eruptive magma.

A chemical threshold controls nanocrystallization and degassing behaviour in basalt magmas

“…7a) in the order of seconds 47 . These observations suggest that nanocrystallization may occur before or after fragmentation and, in fact, recent detailed studies of volcanic deposits go in this direction [44][45][46]104 .…”

“…However, our results have implications that are potentially further-reaching than the laboratory, since nanocrystals have been identified in various rocks formed during explosive eruptions 21,32,33,[37][38][39][40][41][42][43][44][45][46][47][48][49][50][51] .…”

Permutation entropy variations in seismic noise before and after eruptive activity at Shinmoedake volcano, Kirishima complex, Japan

“…The decreasing PE pattern before the 2018 eruption was less clear at all stations. The seismicity, tilt, and seismic velocity records also exhibited no significant precursory signals prior to the eruption and this was probably caused by the rapid ascent of magma (Yamada et al 2019;Nishida et al 2020;Matsumoto and Geshi 2021). Another possibility is that the decreasing pattern occurred several months prior to the 2018 eruption and the observation period of PE needs to be extended in order to identify such a decrease.…”

“…On the other hand, by 2018 the aquifer probably had shrunk considerably and the PE spike feature was likely generated by a different mechanism. Matsumoto and Geshi (2021) identified that the ash emission materials mainly consisted of particles from the hydrothermal and nonhydrothermal altered part of the 2011 solidified lava and from the material derived from the 2018 ascending magma head. Hence, the spike in PE values just before the 2018 eruption was probably associated with fracturing of the 2011 solidified magma as it was being pushed out of the conduit by fresh magma (Fig.…”

Seismic background level growth can reveal slowly developing long-term eruption precursors

“…Shinmoe-dake became active following a phreatic eruption in August 2008. Subsequently, two magmatic eruptions occurred in 2011 and 2017-2018, erupting 2.1-2.7 × 10 7 m 3 in DRE 30 and 1.5 × 10 7 m 3 of lava 31,32 , respectively. The 2011 event was the rst magmatic eruption of Shinmoe-dake in 300 years 30 .…”