4:15 PM - 4:30 PM
[SVC34-14] Eruption activities from 2012 to 2022 on Ioto (Iwojima) Volcano, Izu-Bonin arc
Ogasawara Ioto (Iwo Jima) is a caldera volcano with active crustal deformation, geothermal and seismic activity. Since the settlement at the end of the 19th century, small eruptions have been recorded, but many of them were poorly described in terms of their eruption sequence and deposits, and the relationships to other activities were unclear. In this presentation, we will introduce eruptive phenomena observed by NIED in the last 10 years and discuss the mode of activity of submarine caldera volcano during the resurgent doming stage.
Currently, Ioto is in a period of active crustal deformation since 2011, and uplift rate is high around the normal fault zone surrounding Mt. Motoyama, the central resurgent dome of the caldera. From 2012 to 2016, small eruptions occurred in the area west to northwest of Mt. Motoyama. After around 2018, eruptive activity became active again in response to the anew high rate uplifting, and the eruption sites increased on the south and northeast sides, resulting in a distribution that surrounds Motoyama. These eruptions resulted the formation of tuff cones due to the dispersion of air fall ash and ballistic blocks and the outgoing of hot mudflows, often accompanied by the formation of collapse crater. Smaller eruptions were a few cubic meter in volume, and the largest was about 105 m3. Most of the ejecta samples obtained are mixtures of hydrothermally altered minerals and pre-existing weakly altered~unaltered volcanic rock fragments near the surface, suggesting that the eruptions were phreatic. Because of the different assemblage of hydrothermally altered minerals, there may be more than one hydrothermal reservoir.
In addition, the July-August 2022 off Okinahama eruption is considered to have been small scale phreatomagmatic eruption because drifted fresh breadcrust bombs were collected.
The mechanism of the series of eruption activities can be considered as follows. The magma intrusion that created a cone-sheet like body just below the caldera of Ioto has continued. Normal fault zones are formed under the influence of tensile field created by the tip of the intrusion, and the faults are pathway for accumulated hydrothermal fluids. Frequent phreatic eruptions tend to occur at the same period as or slightly later than the timing of high rate uplifting, which is thought to be due to the activation of hydrothermal activity by the addition of volcanic gases emitted from the newer intruding magma, and the displacement of faults, which facilitates the formation of hydrothermal channels.
The off Okinahama Eruption indicates that the tip of the southern intruding magma finally reached the surface, but the trend of crustal deformation and seismic activity has not changed since then, and the eruption style that continues on the northeast coast is still phreatic. This suggests that magmatic activity is stable at shallow depth emplacement and that eruptions to the surface are only small scale, non-essential and localized phenomena.
Careful monitoring of the transition of volcanic activities should continue into the future and to constrain the petrological conditions of the magma from analysis of the ejecta. In addition, since the eruptions occurring in the coastal and subaqueous areas remain unclear, it is considered necessary to have a variety of observational means.
Acknowledgments:
The field survey and instrumental observations were conducted with the cooperation of the Ioto Air Station Support Unit Division of the Japan Maritime Self-Defense Force.
Currently, Ioto is in a period of active crustal deformation since 2011, and uplift rate is high around the normal fault zone surrounding Mt. Motoyama, the central resurgent dome of the caldera. From 2012 to 2016, small eruptions occurred in the area west to northwest of Mt. Motoyama. After around 2018, eruptive activity became active again in response to the anew high rate uplifting, and the eruption sites increased on the south and northeast sides, resulting in a distribution that surrounds Motoyama. These eruptions resulted the formation of tuff cones due to the dispersion of air fall ash and ballistic blocks and the outgoing of hot mudflows, often accompanied by the formation of collapse crater. Smaller eruptions were a few cubic meter in volume, and the largest was about 105 m3. Most of the ejecta samples obtained are mixtures of hydrothermally altered minerals and pre-existing weakly altered~unaltered volcanic rock fragments near the surface, suggesting that the eruptions were phreatic. Because of the different assemblage of hydrothermally altered minerals, there may be more than one hydrothermal reservoir.
In addition, the July-August 2022 off Okinahama eruption is considered to have been small scale phreatomagmatic eruption because drifted fresh breadcrust bombs were collected.
The mechanism of the series of eruption activities can be considered as follows. The magma intrusion that created a cone-sheet like body just below the caldera of Ioto has continued. Normal fault zones are formed under the influence of tensile field created by the tip of the intrusion, and the faults are pathway for accumulated hydrothermal fluids. Frequent phreatic eruptions tend to occur at the same period as or slightly later than the timing of high rate uplifting, which is thought to be due to the activation of hydrothermal activity by the addition of volcanic gases emitted from the newer intruding magma, and the displacement of faults, which facilitates the formation of hydrothermal channels.
The off Okinahama Eruption indicates that the tip of the southern intruding magma finally reached the surface, but the trend of crustal deformation and seismic activity has not changed since then, and the eruption style that continues on the northeast coast is still phreatic. This suggests that magmatic activity is stable at shallow depth emplacement and that eruptions to the surface are only small scale, non-essential and localized phenomena.
Careful monitoring of the transition of volcanic activities should continue into the future and to constrain the petrological conditions of the magma from analysis of the ejecta. In addition, since the eruptions occurring in the coastal and subaqueous areas remain unclear, it is considered necessary to have a variety of observational means.
Acknowledgments:
The field survey and instrumental observations were conducted with the cooperation of the Ioto Air Station Support Unit Division of the Japan Maritime Self-Defense Force.