1:45 PM - 3:15 PM
[SVC34-P07] Particle Properties and Bubble Microtexture of Drifted and Seafloor Pumice from the 2021 Eruption, Fukutoku-Oka-no-Ba, Japan.
Keywords:Fukutoku-Oka-no-Ba submarine volcano, Phreatomagmatic explosion, Bubble texture analysis, Decompression raterate
Large-scale explosive eruptions, such as Plinian eruptions, are driven by bubble formation and fragmentation of magma. When eruptions occur below the water surface in the shallow sea, the interaction of magma and external water promotes magma fragmentation and causes highly explosive eruptions, which are known as phreatomagmatic eruptions. While small-scale phreatomagmatic eruptions, such as Surtseyan eruptions, are often observed in shallow water environments, large-scale phreatomagmatic explosions have been rarely observed, and their phenomena and mechanisms are poorly understood.
The large-scale explosive eruption occurred at the Fukutoku-Oka-no-Ba submarine volcano in the Izu-Bonin arc in August 2021. The eruption generated large columns and a large amount of pyroclasts was discharged as pumice rafts. The column observed consisted mainly of water vapor and fine ash, which may have different characteristics from those of Plinian eruptions. In this study, we focus on the drifted pumice and seafloor pumice, and estimate the magma ascent process in this eruption by analyzing physical properties and bubble microtextures. Drifted pumice clasts were collected at Nagahama, Yomitan Village, Okinawa Prefecture, and seafloor pumice and volcanic ash were collected by the Shinseimaru cruises (KS-22-05 and KS-22-13).
Seafloor surveys on the western side of the volcano suggest that pumice clasts on the seafloor were deposited by pumice rafts or transported by density currents. The pumice clasts several centimeters in diameter often have heterogeneous microtextures in color and bubble shape probably due to water cooling, making it difficult to identify a representative microtexture caused by the eruption. On the other hand, volcanic ash particles are thought to have been cooled in the air after fragmentation (Tani et al., 2023 IAVCEI), and their shape does not resemble ones caused by water cooling (Self and Sparks, 1978). Probably these volcanic ash particles are not caused by fragmentation due to direct contact with seawater and are expected to have preserved their history of bubble formation and magmatic fragmentation.
Based on measurements of vesicularity and bubble connectivity of each sample, pyroclasts are classified into the following three types. The seafloor pumice was classified into two types: Type A with high vesicularity (85-95%) and high connectivity (>90%), Type B with medium vesicularity (70-85%) and medium connectivity (55-80%), and the drifted pumices are Type C with medium vesicularity (72-87%) and low connectivity (20-50%). Using the method of Mitchell et al. (2021), types A and B sink when about 80% of the connected bubbles are filled with seawater, while type C cannot sink even when 100% of the connected bubbles are filled with seawater. Therefore, one of the factors in the formation of the large pumice rafts may be that the bubbles in the pyroclasts were not sufficiently connected. Pyroclasts with a large number of independent bubbles have been observed in other submarine eruptions, and the process of bubble nucleation and coalescence should be further investigated.
The bubble microtexture of pumice and volcanic ash particles was quantified by image analysis. In all samples, bubbles smaller than 300 μm are nearly spherical in shape and are not considered to be coalesced, which allows estimating the nucleation process from bubble microtexture. The bubble size distribution in the single-log plot is linear and has a large slope in the range of less than 150 μm for drifted pumice and 50-70 μm for seafloor pumice. Such a linear plot occurs when bubble nucleation and growth continue at a constant rate, and a larger slope indicates a higher nucleation rate. Seafloor pumice shows that their nucleation rate increases at later stages of bubble formation, suggesting that the nucleation rate has increased in shallower areas. The calculated bubble number density (BND) was 2.8-4.5 × 1013 m-3, and the decompression rate was estimated to be 0.2-0.3 MPa/s using the method of Fiege and Cichy (2015). These are relatively lower than those of large explosive eruptions, such as Plinian eruptions, suggesting that the observed plume columns are not driven by very high eruption rates.
The large-scale explosive eruption occurred at the Fukutoku-Oka-no-Ba submarine volcano in the Izu-Bonin arc in August 2021. The eruption generated large columns and a large amount of pyroclasts was discharged as pumice rafts. The column observed consisted mainly of water vapor and fine ash, which may have different characteristics from those of Plinian eruptions. In this study, we focus on the drifted pumice and seafloor pumice, and estimate the magma ascent process in this eruption by analyzing physical properties and bubble microtextures. Drifted pumice clasts were collected at Nagahama, Yomitan Village, Okinawa Prefecture, and seafloor pumice and volcanic ash were collected by the Shinseimaru cruises (KS-22-05 and KS-22-13).
Seafloor surveys on the western side of the volcano suggest that pumice clasts on the seafloor were deposited by pumice rafts or transported by density currents. The pumice clasts several centimeters in diameter often have heterogeneous microtextures in color and bubble shape probably due to water cooling, making it difficult to identify a representative microtexture caused by the eruption. On the other hand, volcanic ash particles are thought to have been cooled in the air after fragmentation (Tani et al., 2023 IAVCEI), and their shape does not resemble ones caused by water cooling (Self and Sparks, 1978). Probably these volcanic ash particles are not caused by fragmentation due to direct contact with seawater and are expected to have preserved their history of bubble formation and magmatic fragmentation.
Based on measurements of vesicularity and bubble connectivity of each sample, pyroclasts are classified into the following three types. The seafloor pumice was classified into two types: Type A with high vesicularity (85-95%) and high connectivity (>90%), Type B with medium vesicularity (70-85%) and medium connectivity (55-80%), and the drifted pumices are Type C with medium vesicularity (72-87%) and low connectivity (20-50%). Using the method of Mitchell et al. (2021), types A and B sink when about 80% of the connected bubbles are filled with seawater, while type C cannot sink even when 100% of the connected bubbles are filled with seawater. Therefore, one of the factors in the formation of the large pumice rafts may be that the bubbles in the pyroclasts were not sufficiently connected. Pyroclasts with a large number of independent bubbles have been observed in other submarine eruptions, and the process of bubble nucleation and coalescence should be further investigated.
The bubble microtexture of pumice and volcanic ash particles was quantified by image analysis. In all samples, bubbles smaller than 300 μm are nearly spherical in shape and are not considered to be coalesced, which allows estimating the nucleation process from bubble microtexture. The bubble size distribution in the single-log plot is linear and has a large slope in the range of less than 150 μm for drifted pumice and 50-70 μm for seafloor pumice. Such a linear plot occurs when bubble nucleation and growth continue at a constant rate, and a larger slope indicates a higher nucleation rate. Seafloor pumice shows that their nucleation rate increases at later stages of bubble formation, suggesting that the nucleation rate has increased in shallower areas. The calculated bubble number density (BND) was 2.8-4.5 × 1013 m-3, and the decompression rate was estimated to be 0.2-0.3 MPa/s using the method of Fiege and Cichy (2015). These are relatively lower than those of large explosive eruptions, such as Plinian eruptions, suggesting that the observed plume columns are not driven by very high eruption rates.