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[SVC33-04] Magma ascent rates during the 1914 sub-Plinian eruption of Sakurajima, Japan inferred from the glass embayment water contents
Keywords:eruption, magma, Sakurajima, feeder dike
The structure of volcanic conduits and the dynamics of magma flow within them are key factors influencing the magnitude, nature, and temporal progression of volcanic eruptions. In this work, we investigate the variations in the rate of magma ascent during the 1914 Sakurajima eruption by integrating stratigraphic and new petrological data.
The 1914 Taisho eruption of Sakurajima was a large-scale eruption that erupted about 1.5 km3 of andesitic magma during explosive activities in the early stages of the eruption and later as effusive lava outflow. The eruption started with minor explosions, first from the western flank of the volcano, and shortly after in the eastern flank, forming an eruption fissures of 7 km in total length. Within the first one hour from the eruption’s onset, the activity remained relatively weak, generating columns with maximum height within 10 km above sea level. Soon after, the magma discharge rate increased rapidly to a sub-Plinian phase with the plume column reaching about 20 km above sea level at the climax. Thereafter, the eruption waned and gradually shifted to an effusive-dominated phase, with sporadic Vulcanian explosions and episodes of continuous ash emissions.
These eruptive transitions are recorded in the stratigraphy of the eruptions. In Yunodaira area at the western foot of Sakurajima, a fine-grained pumice and pumiceous volcanic ash layer is recognized at the base of the 1914 pyroclastic deposit (Unit T0), which is interpreted as the eruptive product of the initial phase. The coarse-grained pumice layer deposited during the sub-Plinian phase at the climax of the eruption (Units T1 to T2) overlies T0. At Kurokami, at the eastern foot of Sakurajima, the upper part of the coarse-grained pumice layer from the sub-Plinian phase consists of weakly stratified, slightly fine-grained pumice layers (Unit T3), marking the waning of this phase. Unit T3 is then covered by a pumiceous volcanic ash layer (Unit T4), that is the product of the Vulcanian explosions and ash emission of the post-climax phase1.
The water contents and water content profiles measured in the glass embayments of the phenocrysts indicate changes in magma ascent conditions during the eruption. The relatively low water contents (~1.0-1.5 wt%) in the glass embayments of the initial phase products suggest that the magma experienced degassing at lower pressure than the storage condition. The glass embayments in the sub-Plinian phase products show higher water contents (~2.5-3.5 wt%) that are comparable to those of the glass inclusions in the phenocryst. This suggests the magmas were tapped directly from a temporal storage depth (“pre-charged2.3” depth ~1.6 km) that are suggested by the glass inclusions of the phenocrysts. The relatively low water contents in the glass embayments of the post-climax phase products further suggest that the magma experienced degassing at shallow depth. These stratigraphic changes in water contents within the glass embayments show the changing conditions in magma-ascending speed during the explosive phases of the eruption. Particularly, the prolonged degassing at shallow depths experienced by the magma in the initial phase may record the upward propagation process of the feeder dike from the magma reservoir prior to the onset of the eruption, which has significant implications for volcano monitoring and hazard mitigation strategies.
1. Todde et al. (2017) https://doi.org/10.1007/s00445-017-1154-4; 2. Araya et al. (2019) Sci. Rep. https://doi.org/10.1038/s41598-019-38494-x; 3. Araya et al. (2024) J.G.R. https://doi.org/10.1029/2023JB028558
The 1914 Taisho eruption of Sakurajima was a large-scale eruption that erupted about 1.5 km3 of andesitic magma during explosive activities in the early stages of the eruption and later as effusive lava outflow. The eruption started with minor explosions, first from the western flank of the volcano, and shortly after in the eastern flank, forming an eruption fissures of 7 km in total length. Within the first one hour from the eruption’s onset, the activity remained relatively weak, generating columns with maximum height within 10 km above sea level. Soon after, the magma discharge rate increased rapidly to a sub-Plinian phase with the plume column reaching about 20 km above sea level at the climax. Thereafter, the eruption waned and gradually shifted to an effusive-dominated phase, with sporadic Vulcanian explosions and episodes of continuous ash emissions.
These eruptive transitions are recorded in the stratigraphy of the eruptions. In Yunodaira area at the western foot of Sakurajima, a fine-grained pumice and pumiceous volcanic ash layer is recognized at the base of the 1914 pyroclastic deposit (Unit T0), which is interpreted as the eruptive product of the initial phase. The coarse-grained pumice layer deposited during the sub-Plinian phase at the climax of the eruption (Units T1 to T2) overlies T0. At Kurokami, at the eastern foot of Sakurajima, the upper part of the coarse-grained pumice layer from the sub-Plinian phase consists of weakly stratified, slightly fine-grained pumice layers (Unit T3), marking the waning of this phase. Unit T3 is then covered by a pumiceous volcanic ash layer (Unit T4), that is the product of the Vulcanian explosions and ash emission of the post-climax phase1.
The water contents and water content profiles measured in the glass embayments of the phenocrysts indicate changes in magma ascent conditions during the eruption. The relatively low water contents (~1.0-1.5 wt%) in the glass embayments of the initial phase products suggest that the magma experienced degassing at lower pressure than the storage condition. The glass embayments in the sub-Plinian phase products show higher water contents (~2.5-3.5 wt%) that are comparable to those of the glass inclusions in the phenocryst. This suggests the magmas were tapped directly from a temporal storage depth (“pre-charged2.3” depth ~1.6 km) that are suggested by the glass inclusions of the phenocrysts. The relatively low water contents in the glass embayments of the post-climax phase products further suggest that the magma experienced degassing at shallow depth. These stratigraphic changes in water contents within the glass embayments show the changing conditions in magma-ascending speed during the explosive phases of the eruption. Particularly, the prolonged degassing at shallow depths experienced by the magma in the initial phase may record the upward propagation process of the feeder dike from the magma reservoir prior to the onset of the eruption, which has significant implications for volcano monitoring and hazard mitigation strategies.
1. Todde et al. (2017) https://doi.org/10.1007/s00445-017-1154-4; 2. Araya et al. (2019) Sci. Rep. https://doi.org/10.1038/s41598-019-38494-x; 3. Araya et al. (2024) J.G.R. https://doi.org/10.1029/2023JB028558