9:30 AM - 9:45 AM
[SCG54-03] Basaltic old knoll at the northeastern flank of Nishinoshima
Keywords:Nishinoshima, submarine volcano, research cruise
We performed a research cruise KM23-14 using R/V Kaimei from November 9 to 28, 2023, aiming at the geologic survey of submarine volcanoes and volcanic islands in the Izu-Bonin arc, including Miyakejima, Sofu seamount, and Nishinoshima. During the cruise, we performed KM-ROV, dredge, and piston-coring surveys to investigate the volcanic activities recorded in the undersea area of Nishinoshima. We also performed unmanned aerial vehicle (UAV) operation for on-land observation of Nishinoshima.
Since 2013, Nishinoshima shows intermittent volcanic activities where the episodes from 1 to 4 have been recognized [1]. The volcanic ejecta of episode 1-3 activities was andesite lava while the episode 4 eruption (2019-2020) produced dacite pumice and basalt to basaltic-andesite clasts [1,2]. The Nishinoshima volcano during our cruise showed continuous emission of white steam from inside and outside of the crater and several white-to-yellow precipitates were seen on the mountain slopes.
The KM-ROV operations were performed on an undersea knoll locating northeast of the island, where a low magnetic anomaly was observed by the previous ship surveys. We first performed geomagnetic survey for an area of approximately 100 × 100 meters using a towing-type magnetometer. Subsequently, seafloor observation and sampling were performed according to the obtained geomagnetic distribution. The large portion of the observed area was covered with a thick layer of the black sandy ash which was scarcely consolidated.
The rock samples collected from outcrops at the peak of the knoll (Fig. a) were covered with a thin Mn-film of approximately 0.1 mm. Assuming a growth rate of Mn-film to be 1 mm / Myr, this thin film gives the knoll’s age of ~ 0.1 Myr. The collected rocks were weakly vesiculated without obvious structure. The phenocrysts of olivine, clinopyroxene, and plagioclase were recognized in a crystal-rich groundmass for most samples. The olivine compositions were Fo77 to Fo85 depending on samples, where rims of olivine grains were transformed to iddingsite (Fig. b).
In one sample (KMROV251-R02), significant layering structure with well elongated vacancy was recognized. This sample is free from olivine and contains plagioclase, pigeonite, augite, and Fe-oxide phenocrysts. Fe-oxide grains contain melt inclusions with SiO2 content of approximately 70 mass% which is comparable with the melt inclusions reported from the episode 1-3 activity of Nishinoshima [1]. The elongated vacancies commonly contain idiomorphic cristobalite, indicating chemical-vapor deposition (CVD) origin (Fig. c). The CVD cristobalite is known to be a common phase in submarine volcanic activities and is recently reported from many submarine volcanoes [3-5]. This sample also contains deposition of tuff breccia containing olivine and less silicic volcanic glass with SiO2 ~57 mass%, yielding identical value to those of the episode 4 activity [1].
The whole rock composition of the studied rocks (except for cristobalite-bearing KMROV251-R02) show basaltic composition. The development of Mn-coating and strong iddingsitization indicate this basaltic knoll is old compared to historical activities of Nishinoshima. Recently, Tamura et al. [2] indicated that the activity of the basaltic magma has been generated throughout the entire history of Nishinoshima. Our new findings of the old basaltic knoll support this idea and will provide a fruitful information about the growth history of this large volcano.
1 Maeno et al. (2021) https://doi.org/10.3389/feart.2021.773819
2 Tamura et al. (2023) https://doi.org/10.3389/feart.2023.1137416
3 Ikegami et al. (2018) https://doi.org/10.3389/feart.2018.00147
4 Hamada et al. (2023) https://doi.org/10.1016/j.jvolgeores.2022.107738
5 Yoshida et al. (2023) https://doi.org/10.1111/iar.12498
Since 2013, Nishinoshima shows intermittent volcanic activities where the episodes from 1 to 4 have been recognized [1]. The volcanic ejecta of episode 1-3 activities was andesite lava while the episode 4 eruption (2019-2020) produced dacite pumice and basalt to basaltic-andesite clasts [1,2]. The Nishinoshima volcano during our cruise showed continuous emission of white steam from inside and outside of the crater and several white-to-yellow precipitates were seen on the mountain slopes.
The KM-ROV operations were performed on an undersea knoll locating northeast of the island, where a low magnetic anomaly was observed by the previous ship surveys. We first performed geomagnetic survey for an area of approximately 100 × 100 meters using a towing-type magnetometer. Subsequently, seafloor observation and sampling were performed according to the obtained geomagnetic distribution. The large portion of the observed area was covered with a thick layer of the black sandy ash which was scarcely consolidated.
The rock samples collected from outcrops at the peak of the knoll (Fig. a) were covered with a thin Mn-film of approximately 0.1 mm. Assuming a growth rate of Mn-film to be 1 mm / Myr, this thin film gives the knoll’s age of ~ 0.1 Myr. The collected rocks were weakly vesiculated without obvious structure. The phenocrysts of olivine, clinopyroxene, and plagioclase were recognized in a crystal-rich groundmass for most samples. The olivine compositions were Fo77 to Fo85 depending on samples, where rims of olivine grains were transformed to iddingsite (Fig. b).
In one sample (KMROV251-R02), significant layering structure with well elongated vacancy was recognized. This sample is free from olivine and contains plagioclase, pigeonite, augite, and Fe-oxide phenocrysts. Fe-oxide grains contain melt inclusions with SiO2 content of approximately 70 mass% which is comparable with the melt inclusions reported from the episode 1-3 activity of Nishinoshima [1]. The elongated vacancies commonly contain idiomorphic cristobalite, indicating chemical-vapor deposition (CVD) origin (Fig. c). The CVD cristobalite is known to be a common phase in submarine volcanic activities and is recently reported from many submarine volcanoes [3-5]. This sample also contains deposition of tuff breccia containing olivine and less silicic volcanic glass with SiO2 ~57 mass%, yielding identical value to those of the episode 4 activity [1].
The whole rock composition of the studied rocks (except for cristobalite-bearing KMROV251-R02) show basaltic composition. The development of Mn-coating and strong iddingsitization indicate this basaltic knoll is old compared to historical activities of Nishinoshima. Recently, Tamura et al. [2] indicated that the activity of the basaltic magma has been generated throughout the entire history of Nishinoshima. Our new findings of the old basaltic knoll support this idea and will provide a fruitful information about the growth history of this large volcano.
1 Maeno et al. (2021) https://doi.org/10.3389/feart.2021.773819
2 Tamura et al. (2023) https://doi.org/10.3389/feart.2023.1137416
3 Ikegami et al. (2018) https://doi.org/10.3389/feart.2018.00147
4 Hamada et al. (2023) https://doi.org/10.1016/j.jvolgeores.2022.107738
5 Yoshida et al. (2023) https://doi.org/10.1111/iar.12498