09:45 〜 10:00
[SCG54-04] Magma–water interaction during the 2022 shallow sea eruption at Ioto, Ogasawara, Japan
キーワード：小笠原硫黄島、海域火山活動、マグマ水蒸気噴火、マグマの含水量、急冷
Submarine eruption ubiquitously occurs on the Earth's surface. The interaction of magma with seawater must occur during the submarine eruption. Thus, the process of interaction is essential for the understanding of the mechanism of submarine eruption.
This study examines a magma-water interaction during the 2022 eruption at Ioto, Ogasawara, Japan, based on the characteristics of texture and water content of eruptive products. The explosive eruptions started to occur off the southern coast of Ioto in July 2022. The style of the eruptions is considered to be a phreatomagmatic eruption in the shallow sea, because of the emission of cock’s tail jet and the depth of the sea of 10-20 m around the vent region. The eruptive activity generated the drifted pumice blocks which were washed up to the southern coast. The pumice blocks develop a chilled margin which must represent rapid cooling due to sea water and petrological characteristics of magma at the eruption. The chilled margin has a vesicularity of 30-40 vol.%, and often shows scratch marks on the surface. The textural characteristics of the chilled margin suggest that magma suffered from outgassing (escape of gas phase from magma) before the magma-water interaction, and the fragments of outgassed magma rubbed each other after the magma-water interaction. The water content in the groundmass glass of the chilled margin was measured to ~0.22 wt.% by using micro FTIR analysis, indicating a quench pressure of ca 4 MPa which is equivalent to the depth of ca 200 m from the volcano edifice.
Here we summarize the mechanism of the 2022 eruption. The magma ascended from the magma chamber located at a depth of 1-2 km through a dyke system (Miwa et al., in prep). The outgassing occurred in the magma during the ascent, which decreases the vesicularity. The outgassed magma interacted with seawater which permeated into the edifice at the depth of 200 m, causing the rapid cooling and fragmentation of magma. The magma fragments were rubbed with each other in a confined dyke, producing the scratch marks. Therefore, we conclude that the interaction of outgassed magma with seawater in the volcano edifice drove the 2022 shallow sea eruption at Ioto volcano.
This study examines a magma-water interaction during the 2022 eruption at Ioto, Ogasawara, Japan, based on the characteristics of texture and water content of eruptive products. The explosive eruptions started to occur off the southern coast of Ioto in July 2022. The style of the eruptions is considered to be a phreatomagmatic eruption in the shallow sea, because of the emission of cock’s tail jet and the depth of the sea of 10-20 m around the vent region. The eruptive activity generated the drifted pumice blocks which were washed up to the southern coast. The pumice blocks develop a chilled margin which must represent rapid cooling due to sea water and petrological characteristics of magma at the eruption. The chilled margin has a vesicularity of 30-40 vol.%, and often shows scratch marks on the surface. The textural characteristics of the chilled margin suggest that magma suffered from outgassing (escape of gas phase from magma) before the magma-water interaction, and the fragments of outgassed magma rubbed each other after the magma-water interaction. The water content in the groundmass glass of the chilled margin was measured to ~0.22 wt.% by using micro FTIR analysis, indicating a quench pressure of ca 4 MPa which is equivalent to the depth of ca 200 m from the volcano edifice.
Here we summarize the mechanism of the 2022 eruption. The magma ascended from the magma chamber located at a depth of 1-2 km through a dyke system (Miwa et al., in prep). The outgassing occurred in the magma during the ascent, which decreases the vesicularity. The outgassed magma interacted with seawater which permeated into the edifice at the depth of 200 m, causing the rapid cooling and fragmentation of magma. The magma fragments were rubbed with each other in a confined dyke, producing the scratch marks. Therefore, we conclude that the interaction of outgassed magma with seawater in the volcano edifice drove the 2022 shallow sea eruption at Ioto volcano.