5:15 PM - 6:30 PM
[SVC28-P12] Abrupt change in eruption style during the 2019-2020 eruption of Nishinoshima, Ogasawara
Keywords:Nishinoshima, Eruption, Drone, Basaltic andesite, Pyroclastic deposits, Lava flows
The style of volcanic eruption often changes significantly during a series of activity. Elucidation of the causes of such changes of eruption is important in prediction of eruption sequence. In the latest eruption of Nishinoshima, Ogasawara, magma discharge rate abruptly increased in June-July 2020, and the eruption style changed from lava flow-dominated to pyroclast-dominated one. In this presentation, we will report geological and geomorphological features and current state of volcanic activity of Nishinoshima, which was clarified by survey and observation on KR20-E06 cruise conducted in December 2020. Also we will discuss the eruption sequence and the causes of the change of volcanic activity based on the results of analysis of eruptive materials obtained by sample return with drone.
The most notable feature on the activity in 2019-2020 is that the entire island was covered with pyroclastic deposits due to the change in eruption style to explosive one. As a result of producing a DEM based on still images acquired by drone and estimating the amount of topographical change before and after the latest eruption, it was unveiled that the new pyroclastic cone accounts for ~60% of the increased volume of ~100 million m3. This value is significantly larger than before the eruption, ~10%. Although lava flows were thickly covered with pyroclastic deposits, some parts are eroded, and lava interior is exposed. In such places, fumarolic activity and high temperature regions are recognized. This situation that even six months after the eruption high temperature regions are widely recognized on the entire surface of the island has not been observed in the previous activity of Nishinoshima. One of plausible reasons is that cooling of lava flows has been delayed due to low thermal conductivity of porous pyroclastic deposits.
Lapilli and ash were collected at multiple locations on the western side of the island by drone. Analysis of the whole rock chemical composition revealed that they are basaltic andesite with 54-55 wt.% in SiO2. This result is almost the same as the value of bulk ash chemistry for samples collected by Meteorological Research Institute, JMA, in July 2020. Thus it is understood that the magma composition has changed significantly from andesite with 59-60 wt.% in SiO2. There are also major changes in phenocrysts, such as increase in An# of plagioclase and the presence of olivine. In addition, changes of incompatible element concentration ratios such as Zr/Y in whole rock composition are identified. Such changes in eruption patterns and magma characteristics suggest that new undifferentiated magma was involved in the latest eruption, apart from the mafic magma that was steadily involved in the previous eruptions. Specifically, there are possibilities that undifferentiated magma rich in volatile components accidentally ascended from the deeper part, or the discharge of a large amount of magma from the shallow magma reservoir due to long-term eruptive activity triggered the movement, redistribution and ascent of deeper magma.
The most notable feature on the activity in 2019-2020 is that the entire island was covered with pyroclastic deposits due to the change in eruption style to explosive one. As a result of producing a DEM based on still images acquired by drone and estimating the amount of topographical change before and after the latest eruption, it was unveiled that the new pyroclastic cone accounts for ~60% of the increased volume of ~100 million m3. This value is significantly larger than before the eruption, ~10%. Although lava flows were thickly covered with pyroclastic deposits, some parts are eroded, and lava interior is exposed. In such places, fumarolic activity and high temperature regions are recognized. This situation that even six months after the eruption high temperature regions are widely recognized on the entire surface of the island has not been observed in the previous activity of Nishinoshima. One of plausible reasons is that cooling of lava flows has been delayed due to low thermal conductivity of porous pyroclastic deposits.
Lapilli and ash were collected at multiple locations on the western side of the island by drone. Analysis of the whole rock chemical composition revealed that they are basaltic andesite with 54-55 wt.% in SiO2. This result is almost the same as the value of bulk ash chemistry for samples collected by Meteorological Research Institute, JMA, in July 2020. Thus it is understood that the magma composition has changed significantly from andesite with 59-60 wt.% in SiO2. There are also major changes in phenocrysts, such as increase in An# of plagioclase and the presence of olivine. In addition, changes of incompatible element concentration ratios such as Zr/Y in whole rock composition are identified. Such changes in eruption patterns and magma characteristics suggest that new undifferentiated magma was involved in the latest eruption, apart from the mafic magma that was steadily involved in the previous eruptions. Specifically, there are possibilities that undifferentiated magma rich in volatile components accidentally ascended from the deeper part, or the discharge of a large amount of magma from the shallow magma reservoir due to long-term eruptive activity triggered the movement, redistribution and ascent of deeper magma.