10:15 〜 10:30
[SVC28-24] 諏訪之瀬島火山2020年末からの活発化と火山灰の時代変遷追跡に基づく中長期予測の可能性
キーワード：火山灰、マグマ供給系、諏訪之瀬島、中長期予測
It would be one of the largest interests for geologists engaged in eruption prediction whether or not the deposits of ancient eruption provide any informative sign or precursor for the next serious eruption. Suwanosejima volcano is one of the most active volcanoes in Japan, and continuously releases volcanic ash. Previous studies on the stratigraphy have revealed that this volcano has been active almost continuously for about 4000 years with thick ashy deposit. On the other hand, during this period, sub-Plinian-scale eruptions that settled thick scoria layers have occurred every several hundred to one thousand years (Shimano et al., 2013). Regarding the long-term activity transition and disaster mitigation, we should understand the cause of such bimodality in eruptive activity and the mechanism of transition toward vigorous eruption (eg. VEI >4). It is because similar transition is commonly reported in geological time scale from many active volcanoes that release volcanic ash for a long period of time. It thus would be worth accumulating time-sequence of such long-term record in detail by ancient activity, and comparing with that of current activity. Then, if any sign of larger scale eruption could be obtained in advance during ash eruption period, which is relatively less active, it would surely reduce disasters caused by larger eruptions.
In this study, we report the characteristics of the volcanic ash that erupted during an activation from the end of 2020 up to date, and compare with that of the ejecta over the past 20 years based on systematic petrological analysis of time-series samples. Then, we interpret this time sequence of eruptive products based on the comparison with ash layers leading up to the latest sub-Plinian eruption of the 1813 eruption. The matrix glass composition of the recent 20 years has been relatively similar, including the 2020-21 products, but minor changes are observed with crystallization at shallow depth, probably reflecting magma ascent rate leading to small temporal changes in eruption styles. On the contrary, the pattern of glass composition from the pre-1813 ash up to the climactic sub-plinian phase changes systematically with SiO2 decrease and MgO increase. In addition, MgO increases as a stepwise manner which is followed by K2O increase due to microlite crystallization at shallow depth. We interpret such temporal change in glass composition to reflect evolution of magmatic system as well as magma ascent. Current state of the volcanic activity of Suwanosejima is less active than that for urgent sub-Plinian activity, because there is no clear evidence of large or frequent input of mafic magma as a precursory sign.
In this study, we report the characteristics of the volcanic ash that erupted during an activation from the end of 2020 up to date, and compare with that of the ejecta over the past 20 years based on systematic petrological analysis of time-series samples. Then, we interpret this time sequence of eruptive products based on the comparison with ash layers leading up to the latest sub-Plinian eruption of the 1813 eruption. The matrix glass composition of the recent 20 years has been relatively similar, including the 2020-21 products, but minor changes are observed with crystallization at shallow depth, probably reflecting magma ascent rate leading to small temporal changes in eruption styles. On the contrary, the pattern of glass composition from the pre-1813 ash up to the climactic sub-plinian phase changes systematically with SiO2 decrease and MgO increase. In addition, MgO increases as a stepwise manner which is followed by K2O increase due to microlite crystallization at shallow depth. We interpret such temporal change in glass composition to reflect evolution of magmatic system as well as magma ascent. Current state of the volcanic activity of Suwanosejima is less active than that for urgent sub-Plinian activity, because there is no clear evidence of large or frequent input of mafic magma as a precursory sign.