11:30 AM - 11:45 AM
[SCG54-09] The eruption sequence and products of the 2023 eruption off the Okinahama beach, Ioto(Iwojima), Japan
Keywords:Izu-Bonin-Mariana arc, alkaline magma series, postcaldera volcanism, phreatomagmatic eruption, island-forming eruption
Introduction
Ogasawara Ioto is a caldera volcano that has been undergoing intensive crustal deformation, geothermal and seismic activity. Since August 2021, eruptions have often occurred off Okinahama in the southern part of the caldera. This report provides an overview of the eruptions during October-December 2023, which temporarily formed a new island.
Sequence of the off Okinahama eruption
Surtseyan eruptions began on Oct. 21, near the southern end of the wave-cut platform where subaqueous eruptions had previously occurred. Oct. 25, the eruption cloud enlarged and the amount of drifting pumice increased. An onshore sand bank formed on the north side of the subaqueous vent on Oct. 30, and then the growth of a subaerial pyroclastic cone began on Oct. 31. The climax of the eruptions occurred on Nov. 3, with frequent explosions accompanied by base surges. The maximum travel distance of base surges was about 160 m and volcanic bombs was about 130 m. The subaerial pyroclastic cone grew to about 145 m in diameter and 25 m high on Nov. 4. From around Nov. 4 to Nov. 9, small-scale lava flows flowed out on the southeast side of the cone, and small-scale Vulcanian-like explosions occurred frequently. Presumably, there was no significant contact between the rising magma and seawater. During this period, wave erosion progressed in the northeast and south sides of the pyroclastic cone where lava was not covered, and sand banks consisting mainly of reworked deposit grew on the north and west sides. After a few days of dormancy, Surtseyan eruption resumed on the west flank of the cone. Subsequent eruptive reconstruction of pyroclastic cone and collapse followed. The eruption ended in mid-December. The pyroclastic cone immediately disappeared by erosion and leaved a small reef mainly composed of lava. The sandbanks continued to move landward and connected to the main island after mid-December. Surtseyan eruptions resumed from Dec. 31 to Jan.4, 2024, but this activity did not produce any new land.
Eruption deposit
The essential materials that drifted to the main island were pumice, scoria, and fragments of volcanic bombs. Large clasts often oxidized to reddish-brown color. Some clasts have welded textures. These may have formed subaerial pyroclastic cone. Fresh gray lava fragments, probably derived from lava flows, were also washed up on sand bank near the eruption site.
Petrographic Features
The whole-rock chemical composition of the essential material are concentrated in the range of SiO2=61.2-61.3 wt% and Total Alkali=10.6-10.8 wt%, and almost the same trachyte as the 2022 subaqueous eruption. The minerals included are plagioclase, clinopyroxene, olivine, Fe-Ti oxide, and apatite. Fe sulfides are occasionally included in the Fe-Ti oxides. No significant differences in assemblages of phenocrysts and groundmass phases. The crystallinity of the magma is lower than the 2022 subaqueous eruption because of small amounts of microlites and microphenocrysts.
Eruptive Volume and Discharge Rate
The volume of the deposit was estimated to be about 5×105 m3 based on truncated cone approximation. For the subaqueous portion, assuming average water depth of 5 m and deposition slope of 2° based on the description by Kaizuka et al. (1985). The daily averaged discharge rate was about 1 m3/s at the peak of the eruption.
Conclusion
The 2023 eruption was a typical of magmatic eruptions in shallow-water. The estimated eruptive volume was small, and no crustal deformation indicative of deflation was observed. Therefore, magma rising in the shallow subsurface of Ioto continues to accumulate without being consumed by eruption. Since the discharge rate appears to have been greater than the 2022 subaqueous eruption, the rate of magma ascent was also larger, which may have resulted in insufficient magma degassing and crystallization.
Acknowledgments
The authors would like to thank JMSDF, JMA Volcanic Observation and Warning Center, and the Asahi Shimbun Co. for their cooperation.
Ogasawara Ioto is a caldera volcano that has been undergoing intensive crustal deformation, geothermal and seismic activity. Since August 2021, eruptions have often occurred off Okinahama in the southern part of the caldera. This report provides an overview of the eruptions during October-December 2023, which temporarily formed a new island.
Sequence of the off Okinahama eruption
Surtseyan eruptions began on Oct. 21, near the southern end of the wave-cut platform where subaqueous eruptions had previously occurred. Oct. 25, the eruption cloud enlarged and the amount of drifting pumice increased. An onshore sand bank formed on the north side of the subaqueous vent on Oct. 30, and then the growth of a subaerial pyroclastic cone began on Oct. 31. The climax of the eruptions occurred on Nov. 3, with frequent explosions accompanied by base surges. The maximum travel distance of base surges was about 160 m and volcanic bombs was about 130 m. The subaerial pyroclastic cone grew to about 145 m in diameter and 25 m high on Nov. 4. From around Nov. 4 to Nov. 9, small-scale lava flows flowed out on the southeast side of the cone, and small-scale Vulcanian-like explosions occurred frequently. Presumably, there was no significant contact between the rising magma and seawater. During this period, wave erosion progressed in the northeast and south sides of the pyroclastic cone where lava was not covered, and sand banks consisting mainly of reworked deposit grew on the north and west sides. After a few days of dormancy, Surtseyan eruption resumed on the west flank of the cone. Subsequent eruptive reconstruction of pyroclastic cone and collapse followed. The eruption ended in mid-December. The pyroclastic cone immediately disappeared by erosion and leaved a small reef mainly composed of lava. The sandbanks continued to move landward and connected to the main island after mid-December. Surtseyan eruptions resumed from Dec. 31 to Jan.4, 2024, but this activity did not produce any new land.
Eruption deposit
The essential materials that drifted to the main island were pumice, scoria, and fragments of volcanic bombs. Large clasts often oxidized to reddish-brown color. Some clasts have welded textures. These may have formed subaerial pyroclastic cone. Fresh gray lava fragments, probably derived from lava flows, were also washed up on sand bank near the eruption site.
Petrographic Features
The whole-rock chemical composition of the essential material are concentrated in the range of SiO2=61.2-61.3 wt% and Total Alkali=10.6-10.8 wt%, and almost the same trachyte as the 2022 subaqueous eruption. The minerals included are plagioclase, clinopyroxene, olivine, Fe-Ti oxide, and apatite. Fe sulfides are occasionally included in the Fe-Ti oxides. No significant differences in assemblages of phenocrysts and groundmass phases. The crystallinity of the magma is lower than the 2022 subaqueous eruption because of small amounts of microlites and microphenocrysts.
Eruptive Volume and Discharge Rate
The volume of the deposit was estimated to be about 5×105 m3 based on truncated cone approximation. For the subaqueous portion, assuming average water depth of 5 m and deposition slope of 2° based on the description by Kaizuka et al. (1985). The daily averaged discharge rate was about 1 m3/s at the peak of the eruption.
Conclusion
The 2023 eruption was a typical of magmatic eruptions in shallow-water. The estimated eruptive volume was small, and no crustal deformation indicative of deflation was observed. Therefore, magma rising in the shallow subsurface of Ioto continues to accumulate without being consumed by eruption. Since the discharge rate appears to have been greater than the 2022 subaqueous eruption, the rate of magma ascent was also larger, which may have resulted in insufficient magma degassing and crystallization.
Acknowledgments
The authors would like to thank JMSDF, JMA Volcanic Observation and Warning Center, and the Asahi Shimbun Co. for their cooperation.