5:15 PM - 6:45 PM
[SVC26-P04] Magmatic activity of the 1977-1978 eruption of Usu Volcano inferred from magnetite analysis
Keywords:Usu volcano, 1977-1978 eruption, dacite magma, eruptive sequence, magnetite, zoning profile
During a volcanic eruption, the eruption style often changes, and the hazards also change accordingly. Therefore, it is necessary to know the mechanism of change in eruption styles and make accurate predictions. Since magma underground drives eruptions, it is important to know at what depth and what state the magma is. In this study, I will consider this problem through petrographic analysis, mainly targeting magnetite, using the 1977-1978 eruption of Usu Volcano as an example.
The 1977-1978 eruption of Usu Volcano has been observed in detail, and geophysical observations and geological and petrographic data are abundant (e.g., Katsui et al., 1978; Niida et al., 1980; Yokoyama et al., 1981; Oba et al., 1983; Yoshida, 1995). At the beginning, sub-Plinian eruptions that ejected well-vesiculated pumice occurred (Us-1977-I, II, III & IV), followed by explosions that ejected poorly vesiculated rock fragments (e.g., Us-1977-SB), and then the eruption stopped (1st stage eruption). However, the cryptodome (Usu Shinzan) began to grow, and the next eruption activity occurred (2nd stage eruption), repeating from phreatic eruption to phreatomagmatic/magmatic eruption (Us-1978). In this study, I focused on magnetite, which records magmatic processes over a relatively short timescale (less than days to weeks) due to its rapid elemental diffusion, and petrographically analyzed its compositional zoning profiles to estimate the pre-eruptive magma processes.
Regarding the sub-Plinian eruptions in 1977, I reported in JpGU 2023 (Tomiya, 2023) that just prior to the eruption, the magma chamber was heated by the injection of high-temperature magma, causing it to rise rapidly and erupt explosively. This study revealed that the cooling process is noticeable in Us-1977-SB and Us-1978. In particular, for Us-1978, silica minerals were abundant in the groundmass, indicating that the magma had remained for some time at low temperature and low pressure. Therefore, it is likely that the magma that caused the 1978 eruption did not come directly from the magma chamber, but from a shallow intrusion that had formed since 1977. This is consistent with the fact that an aseismic area and low-velocity anomaly were observed at a depth of about 1-2 km during the 1978 eruption.
Accordingly, heating, cooling, and storage depth of magma can affect the eruption style. Thus, if we can grasp the state of magma during an eruption, for example by rapid analysis of volcanic ash, it may be possible to monitor and predict the progress of the eruption.
The 1977-1978 eruption of Usu Volcano has been observed in detail, and geophysical observations and geological and petrographic data are abundant (e.g., Katsui et al., 1978; Niida et al., 1980; Yokoyama et al., 1981; Oba et al., 1983; Yoshida, 1995). At the beginning, sub-Plinian eruptions that ejected well-vesiculated pumice occurred (Us-1977-I, II, III & IV), followed by explosions that ejected poorly vesiculated rock fragments (e.g., Us-1977-SB), and then the eruption stopped (1st stage eruption). However, the cryptodome (Usu Shinzan) began to grow, and the next eruption activity occurred (2nd stage eruption), repeating from phreatic eruption to phreatomagmatic/magmatic eruption (Us-1978). In this study, I focused on magnetite, which records magmatic processes over a relatively short timescale (less than days to weeks) due to its rapid elemental diffusion, and petrographically analyzed its compositional zoning profiles to estimate the pre-eruptive magma processes.
Regarding the sub-Plinian eruptions in 1977, I reported in JpGU 2023 (Tomiya, 2023) that just prior to the eruption, the magma chamber was heated by the injection of high-temperature magma, causing it to rise rapidly and erupt explosively. This study revealed that the cooling process is noticeable in Us-1977-SB and Us-1978. In particular, for Us-1978, silica minerals were abundant in the groundmass, indicating that the magma had remained for some time at low temperature and low pressure. Therefore, it is likely that the magma that caused the 1978 eruption did not come directly from the magma chamber, but from a shallow intrusion that had formed since 1977. This is consistent with the fact that an aseismic area and low-velocity anomaly were observed at a depth of about 1-2 km during the 1978 eruption.
Accordingly, heating, cooling, and storage depth of magma can affect the eruption style. Thus, if we can grasp the state of magma during an eruption, for example by rapid analysis of volcanic ash, it may be possible to monitor and predict the progress of the eruption.