1:45 PM - 3:15 PM
[SVC36-P01] Generation and magma fragmentation process of a large-scale phreatoplinian eruption inferred from the water content of volcanic glass
Keywords:Phreatoplinian eruption, external water, caldera, Kutcharo volcano
“Phreatoplinian eruption” is one of the phreatomagmatic eruption styles generated in silicic magma systems. Phreatoplinian eruption deposits are mainly voluminous fine ash, dispersed from the vent to several tens ~ hundreds of km, containing abundant accretionary lapilli (Self and Sparks, 1978). However, the generation process and mechanism of phreatoplinian eruptions are still poorly understood due to the lack of direct observations. Understanding the factors that control the eruption style and the explosivity of the phreatomagmatic eruptions at silicic volcanoes is important for hazard assessment. Here, we present a case study of the 40 ka caldera-forming eruption (Kp I: VEI=7) from Kutcharo volcano, based on geological (stratigraphy and lithofacies) and petrological (morphology and chemistry of volcanic glass) data. Water content was estimated by SEM-EDS using the quantitative analysis of oxygen concentration based on Geshi et al. (2017).
The Kp I eruption deposits consist of 7 units (Shibata and Hasegawa, 2022). In ascending order, units 1 to 6 are air-falls and unit 7 is ignimbrite. Units 1 to 6 are thin pumice and thick silty ash alternation layers, mainly consisting of clast-supported accretionary lapilli and considered “phreatoplinin eruption” deposits (bulk volume: 20 km3). These units contain not only highly-vesiculated pumice (by volatile fragmentation), but also blocky-shaped and fine-grained glass shards probably produced by phreatomagmatic (quench) fragmentation. Although glass chemistries show an overall homogeneous composition with SiO2 = 77 wt.%, the water content of matrix glass of pumice (3.0 to 3.5 wt.%) is remarkably lower than that of very fine sand-sized ash (4.0 to 5.0 wt.%).
In Plinian eruptions, the water content of matrix glass represents the residual dissolved water content in the melt at the fragmentation level (e.g., Martel et al, 2000). Our water content data suggest that phreatomagmatic fragmentation generating blocky fine ash occurred earlier at deeper levels in the conduit by interacting with external water before and/or during the volatile fragmentation (Allen and Cas, 1998; Aravena et al., 2018) that generally occurs below a depth of approximately 1,000 m in H2O-saturated felsic magma systems (e.g., Zhang, 1999). For both types of glass shards to have been generated, the margin of the ascending magma would have interacted with external water at deeper levels near the conduit wall resulting in phreatomagmatic fragmentation, whereas volatile fragmentation occurred at the center and/or shallower levels of the ascending magma flow without magma-water interaction. In this model, the conduit system should penetrate through a huge and deep aquifer that preserved/hosted a large amount of external water. The conduit-aquifer system might be related to the preexisting caldera structure formed by the former largest caldera-forming eruption at the Kutcharo volcano (Kp IV: 120 ka; Hasegawa et al., 2016). To eliminate the effect of secondary hydration, we shall use the FT-IR technique in our future work to investigate the concentration of OH- for pumice and blocky-shaped glass shards.
The Kp I eruption deposits consist of 7 units (Shibata and Hasegawa, 2022). In ascending order, units 1 to 6 are air-falls and unit 7 is ignimbrite. Units 1 to 6 are thin pumice and thick silty ash alternation layers, mainly consisting of clast-supported accretionary lapilli and considered “phreatoplinin eruption” deposits (bulk volume: 20 km3). These units contain not only highly-vesiculated pumice (by volatile fragmentation), but also blocky-shaped and fine-grained glass shards probably produced by phreatomagmatic (quench) fragmentation. Although glass chemistries show an overall homogeneous composition with SiO2 = 77 wt.%, the water content of matrix glass of pumice (3.0 to 3.5 wt.%) is remarkably lower than that of very fine sand-sized ash (4.0 to 5.0 wt.%).
In Plinian eruptions, the water content of matrix glass represents the residual dissolved water content in the melt at the fragmentation level (e.g., Martel et al, 2000). Our water content data suggest that phreatomagmatic fragmentation generating blocky fine ash occurred earlier at deeper levels in the conduit by interacting with external water before and/or during the volatile fragmentation (Allen and Cas, 1998; Aravena et al., 2018) that generally occurs below a depth of approximately 1,000 m in H2O-saturated felsic magma systems (e.g., Zhang, 1999). For both types of glass shards to have been generated, the margin of the ascending magma would have interacted with external water at deeper levels near the conduit wall resulting in phreatomagmatic fragmentation, whereas volatile fragmentation occurred at the center and/or shallower levels of the ascending magma flow without magma-water interaction. In this model, the conduit system should penetrate through a huge and deep aquifer that preserved/hosted a large amount of external water. The conduit-aquifer system might be related to the preexisting caldera structure formed by the former largest caldera-forming eruption at the Kutcharo volcano (Kp IV: 120 ka; Hasegawa et al., 2016). To eliminate the effect of secondary hydration, we shall use the FT-IR technique in our future work to investigate the concentration of OH- for pumice and blocky-shaped glass shards.