4:30 PM - 4:45 PM
[SVC31-21] Magma Ascent Process of the 1977 Sub-Plinian Eruption of Usu Volcano Inferred from Petrological Observations
Keywords:Usu volcano, magnetite, eruption, zoning
1. Introduction:
Usu Volcano is one of the most active volcanoes in Japan and has often caused major disasters. The two most recent eruptions, the 1977 and 2000 eruptions, had similar magma compositions of dacite with SiO2 = 69-70 wt.%, but differed in eruptive style. The former was a sub-Plinian eruption that ejected a large amount of pumice and volcanic ash, while the latter was a phreatomagmatic eruption with a much smaller amount (Soya et al., 2007). Tomiya (2022) considered the reason why the 2000 eruption "failed" to be a sub-Plinian eruption as follows: Cooling and degassing significantly progressed during the ascent of the magma, causing that the magma strongly interacted with the groundwater near the surface. This was based on the detailed FE-EPMA analysis of the zoning profiles of magnetite phenocrysts, which had a significant decrease in Mg/Mn over several tens of μm from the rim. Considering the diffusion rate and partition coefficient, he estimated that the temperature decreased by 50-100°C over a timescale of several tens of hours. In this study, I analyzed zoning profiles of the magnetite phenocrysts in the 1977 sub-Plinian eruptive product, estimated the magma ascent process of the eruption, and compared it with the 2000 eruption.
2. Analysis results of the 1977 eruptive product:
According to Tomiya and Takahashi (2005), magnetite from the 1977 eruption shows a bimodal composition distribution of type-I (low Ti, high Al, high Mg/Mn) and type-II (high Ti, low Al, low Mg/Mn), with the former being at a higher temperature, and the zoning profiles of both types are almost flat (no significant zoning). They considered that the two types of magma mixed just prior to the eruption and erupted in a short time without re-homogenization.
Detailed analysis by this study revealed that slight zoning was observed near the rim. In particular, several magnetite phenocrysts show a slight increase in Mg/Mn, which suggests heating in contrast to the 2000 eruption. The increase in Mg/Mn is observed for both type-I and type-II, implying the presence of magma with a higher temperature than both types. Further detailed analysis is necessary in the future.
3. Comparison of the 1977 and 2000 eruptions:
The 1977 eruption became a sub-Plinian eruption without significant cooling of the ascending magma. In contrast, the 2000 eruption ended in a phreatomagmatic eruption with significant cooling during the ascent. The duration of the precursory earthquake was 30 hours for the former and 4 days for the latter. At Usu Volcano, eruption style and eruption hazards can be greatly affected by whether or not magma ascends rapidly. Therefore, it is important to observe the ascent of magma in real time.
This study was partly supported by the Integrated Program for Next Generation Volcano Research and Human Resource Development (Theme C).
Usu Volcano is one of the most active volcanoes in Japan and has often caused major disasters. The two most recent eruptions, the 1977 and 2000 eruptions, had similar magma compositions of dacite with SiO2 = 69-70 wt.%, but differed in eruptive style. The former was a sub-Plinian eruption that ejected a large amount of pumice and volcanic ash, while the latter was a phreatomagmatic eruption with a much smaller amount (Soya et al., 2007). Tomiya (2022) considered the reason why the 2000 eruption "failed" to be a sub-Plinian eruption as follows: Cooling and degassing significantly progressed during the ascent of the magma, causing that the magma strongly interacted with the groundwater near the surface. This was based on the detailed FE-EPMA analysis of the zoning profiles of magnetite phenocrysts, which had a significant decrease in Mg/Mn over several tens of μm from the rim. Considering the diffusion rate and partition coefficient, he estimated that the temperature decreased by 50-100°C over a timescale of several tens of hours. In this study, I analyzed zoning profiles of the magnetite phenocrysts in the 1977 sub-Plinian eruptive product, estimated the magma ascent process of the eruption, and compared it with the 2000 eruption.
2. Analysis results of the 1977 eruptive product:
According to Tomiya and Takahashi (2005), magnetite from the 1977 eruption shows a bimodal composition distribution of type-I (low Ti, high Al, high Mg/Mn) and type-II (high Ti, low Al, low Mg/Mn), with the former being at a higher temperature, and the zoning profiles of both types are almost flat (no significant zoning). They considered that the two types of magma mixed just prior to the eruption and erupted in a short time without re-homogenization.
Detailed analysis by this study revealed that slight zoning was observed near the rim. In particular, several magnetite phenocrysts show a slight increase in Mg/Mn, which suggests heating in contrast to the 2000 eruption. The increase in Mg/Mn is observed for both type-I and type-II, implying the presence of magma with a higher temperature than both types. Further detailed analysis is necessary in the future.
3. Comparison of the 1977 and 2000 eruptions:
The 1977 eruption became a sub-Plinian eruption without significant cooling of the ascending magma. In contrast, the 2000 eruption ended in a phreatomagmatic eruption with significant cooling during the ascent. The duration of the precursory earthquake was 30 hours for the former and 4 days for the latter. At Usu Volcano, eruption style and eruption hazards can be greatly affected by whether or not magma ascends rapidly. Therefore, it is important to observe the ascent of magma in real time.
This study was partly supported by the Integrated Program for Next Generation Volcano Research and Human Resource Development (Theme C).