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[SVC30-P06] Magma-water interaction for a large-scale and explosive phreatomagmatic eruption: Insights into magma fragmentation processes of phreatoplinian eruption
Keywords:magma-water interaction, phreatomagmatic eruption, phreatoplinian eruption, Kutcharo volcano
Phreatomagmatic eruptions induced by the interaction of magma and external water are one of the most common and hazardous eruption styles [1]. Most volcanoes on Earth are on the seafloor, even terrestrial volcanoes often contain large amount of external water such as aquifers, crater/caldera lakes, and glaciers/snow. Therefore, it is important to elucidate the processes of phreatomagmatic activity for future volcanic hazard assessment.
In this study, we evaluated two methods, SEM-EDS [2] and FT-IR [3], for quantifying the water content in volcanic glass shards and attempted to estimate the quenching fragmentation depth of magma during the large-scale phreatomagmatic eruption (Kutcharo-Shoro ash: KSr) that preceded the more recent caldera-forming eruption at the Kutcharo volcano. We also investigated where and how magma-water interaction occurred during magma ascent in the conduit during the phreatoplinian eruption style, for which no direct observations exist.
KSr is mainly thick fine ash consisting of clast-supported accretionary lapilli and is interpreted to be phreatoplinian eruption deposit (ca. 20 km3) [4]. In addition to the presence of micro-fossils of Dinophyceae in KSr [4], and the current Kutcharo caldera was mostly formed by the 120 ka caldera-forming eruption [5] suggesting that the eruption of KSr began in a lacustrine environment (paleo-caldera lake). Fine ash of KSr contains not only micro-pumice/bubble-wall glass produced by volatile-driven fragmentation (V-type), but also poorly or non-vesiculated blocky glass shards with stepped fractures, produced by quenching fragmentation during magma-water interaction (Q-type). The ranges of water contents (H2Ot) of glass shards determined by SEM-EDS and FT-IR are 2.6–6.8 wt.% and 1.1–6.6 wt. % respectively, with both measurements showing unimodal peaks in the range of 3.5–4.0 wt.%. For dissolved H2O species determined by FT-IR analysis, the H2Om and OH contents of V-type glass shards were 0.9–5.2 wt.% and 0.1–1.4 wt.%, respectively, while those of Q-type glass shards were 1.7–4.9 wt.% and 0.4–1.0 wt.%. The H2Om/OH ratios in glass shards are considerably higher than the equilibrium H2Om/OH ratios at 500–800 ℃ in rhyolite, suggesting secondary hydration. Therefore, we determined the fragmentation depth based on the H2Ot contents estimated from the OH contents.
Assuming the lithostatic and hydrostatic pressures, the fragmentation depth is almost ground surface to 1,200 m (0.1–24 MPa) in V-type, and 20–410 m (1.4–11 MPa) in Q-type. V-type has a relatively wide range of fragmentation depths, while those of the Q-type are shallower than the V-type and are concentrated at 100–300 m. These results indicate that some part of the ascending magma was already vesiculated and fragmented at about 1,000 m and the magma-water interaction occurred in the shallow part of the conduit around 100–300 m. Considering the glass shard types, their fragmentation depths, and the paleo-environment, it is likely that the external water came to the conduit through a shallow aquifer from a paleo-caldera lake. The conditions of magma-water interaction during the KSr phretoplinian eruption may have been strongly dependent on the geological environment (e.g., intra-caldera filled deposit and paleo-caldera lake) surrounding the conduit and vent locations in the caldera volcanoes.
References
[1] Houghton et al. (2015) The Encyclopedia of Volcanoes 2nd ed., [2] Geshi et al. (2017) Bull Volcanol Soc Japan, [3] McIntosh et al. (2022) Geology, [4] Shibata and Hasegawa (2024) J Volcanol Geotherm Res, [5] Hasegawa et al. (2016) J Volcanol Geotherm Res
In this study, we evaluated two methods, SEM-EDS [2] and FT-IR [3], for quantifying the water content in volcanic glass shards and attempted to estimate the quenching fragmentation depth of magma during the large-scale phreatomagmatic eruption (Kutcharo-Shoro ash: KSr) that preceded the more recent caldera-forming eruption at the Kutcharo volcano. We also investigated where and how magma-water interaction occurred during magma ascent in the conduit during the phreatoplinian eruption style, for which no direct observations exist.
KSr is mainly thick fine ash consisting of clast-supported accretionary lapilli and is interpreted to be phreatoplinian eruption deposit (ca. 20 km3) [4]. In addition to the presence of micro-fossils of Dinophyceae in KSr [4], and the current Kutcharo caldera was mostly formed by the 120 ka caldera-forming eruption [5] suggesting that the eruption of KSr began in a lacustrine environment (paleo-caldera lake). Fine ash of KSr contains not only micro-pumice/bubble-wall glass produced by volatile-driven fragmentation (V-type), but also poorly or non-vesiculated blocky glass shards with stepped fractures, produced by quenching fragmentation during magma-water interaction (Q-type). The ranges of water contents (H2Ot) of glass shards determined by SEM-EDS and FT-IR are 2.6–6.8 wt.% and 1.1–6.6 wt. % respectively, with both measurements showing unimodal peaks in the range of 3.5–4.0 wt.%. For dissolved H2O species determined by FT-IR analysis, the H2Om and OH contents of V-type glass shards were 0.9–5.2 wt.% and 0.1–1.4 wt.%, respectively, while those of Q-type glass shards were 1.7–4.9 wt.% and 0.4–1.0 wt.%. The H2Om/OH ratios in glass shards are considerably higher than the equilibrium H2Om/OH ratios at 500–800 ℃ in rhyolite, suggesting secondary hydration. Therefore, we determined the fragmentation depth based on the H2Ot contents estimated from the OH contents.
Assuming the lithostatic and hydrostatic pressures, the fragmentation depth is almost ground surface to 1,200 m (0.1–24 MPa) in V-type, and 20–410 m (1.4–11 MPa) in Q-type. V-type has a relatively wide range of fragmentation depths, while those of the Q-type are shallower than the V-type and are concentrated at 100–300 m. These results indicate that some part of the ascending magma was already vesiculated and fragmented at about 1,000 m and the magma-water interaction occurred in the shallow part of the conduit around 100–300 m. Considering the glass shard types, their fragmentation depths, and the paleo-environment, it is likely that the external water came to the conduit through a shallow aquifer from a paleo-caldera lake. The conditions of magma-water interaction during the KSr phretoplinian eruption may have been strongly dependent on the geological environment (e.g., intra-caldera filled deposit and paleo-caldera lake) surrounding the conduit and vent locations in the caldera volcanoes.
References
[1] Houghton et al. (2015) The Encyclopedia of Volcanoes 2nd ed., [2] Geshi et al. (2017) Bull Volcanol Soc Japan, [3] McIntosh et al. (2022) Geology, [4] Shibata and Hasegawa (2024) J Volcanol Geotherm Res, [5] Hasegawa et al. (2016) J Volcanol Geotherm Res