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[SVC31-P14] Nature of drifting pumice from the Fukutoku-Oka-no-Ba 2021 eruption: Pore structure and flotation characteristics of pumice inferred from porosimetry
Keywords:Fukutoku-Oka-no-Ba 2021 eruption, drifting pumice, porosimetry, pore structure, density
The Fukutoku-Oka-no-Ba eruption on August 13-15, 2021 released a large amount of pumice. The pumice rafts have drifted in the Pacific Ocean, reached the islands in Kagoshima and Okinawa Prefecture after October and had various impacts on society. Therefore, research on the nature of drifting pumice is important for volcanological and disaster research. In this study, with the accompanying study of Ishige et al. (2022), we investigate the pore structure and density of pumice, based on the method of volcanic rock porosimetry. We clarify the pore structure and density characteristics of pumice with interest to flotation characteristics.
In this porosimetry, both floating and submerged pumices were measured. Samples were collected on the main island of Okinawa on October 19 and 20 (near Ikeijima) and November 2 and 3 (Azama). We maintained a part of the collected drifting pumice in a container with seawater until just before the porosimetry. Most pumice was floating in the container, but a few pumice were submerged at the bottom. The grain size distribution of drifting pumice has a peak of around 2 to 4 mm (Ishige et al., 2022). Since it is challenging to measure samples with a grain size of 2 to 4 mm, we tried to measure samples with slightly larger grain size. We selected 17 samples with grain volumes ranging from 0.1 to 0.6 cm3. For the grain volume measurement, we used a bulk density measurement device (Micromeritics GeoPyc1365). The GeoPyc1365 can measure the volume of irregularly shaped samples using powder as a pseudo-fluid. A He gas pycnometer (Quantachrome Pentapycnometer) was used to measure the solid density of the powdered pumice and the total volume of solid phase plus isolated pore in the pumice. For solid density measurements, the same mass of floating pumice with grain sizes ranging from 8 mm (-3Φ) to 500 μm (1Φ) were taken and pulverized to prepare powder samples. The finer the grain size, the large number of grains were pulverized.
The total porosity, connected porosity, isolated porosity, dry pumice density and solid density were obtained. For five samples, the total volume of solid phase plus isolated pore could not be measured with sufficient accuracy. The important results obtained are as follows:
1.The solid density decreases from 2.66 to 2.52 g⁄cm3 as the grain size of drifting pumice decreases from 8 mm to 500 μm. The dry pumice density is 0.68 to 0.40 g⁄cm3.
2. Assuming a solid density of 2.62 g⁄cm3, the total porosity is calculated to be 74 to 85 vol%.
3. There are more grains with slightly higher connected porosity (33-55 vol%) in submerged pumice than floating pumice (18-44 vol%).
4. The relationship between total and connected porosity of floating and submerged pumice is similar to those of the Havre 2012 submarine eruption (Mitchell et al., 2021).
5. The relationship between total and connected porosity is similar to those of decompression experiments for silicic magmas (Takeuchi et al., 2009), in which the vesicular texture of pumice is quenched as rapidly as possible under high pressure.
The feature of 1 may reflect the loss of phenocryst with high density in floating pumice as the grain size decreases. The feature of 3 suggests that the dominant factor for the pumice submersion is seawater intrusion into the connected pore with large porosity. The results of 5 suggests that the shallow-water eruption of the Fukutoku-Oka-no-Ba was effective in quenching the vesicular texture of decompressed pumice.
Ishige et al. (2022) Characteristics of drifting pumice from the Fukutoku-Oka-no-Ba 2021 eruption and their relationship to eruption sequence, Japan Geoscience Union Meeting 2022.
Mitchell et al. (2021) BV, 83, 80.
Takeuchi et al. (2009) EPSL, 283, 101-110.
In this porosimetry, both floating and submerged pumices were measured. Samples were collected on the main island of Okinawa on October 19 and 20 (near Ikeijima) and November 2 and 3 (Azama). We maintained a part of the collected drifting pumice in a container with seawater until just before the porosimetry. Most pumice was floating in the container, but a few pumice were submerged at the bottom. The grain size distribution of drifting pumice has a peak of around 2 to 4 mm (Ishige et al., 2022). Since it is challenging to measure samples with a grain size of 2 to 4 mm, we tried to measure samples with slightly larger grain size. We selected 17 samples with grain volumes ranging from 0.1 to 0.6 cm3. For the grain volume measurement, we used a bulk density measurement device (Micromeritics GeoPyc1365). The GeoPyc1365 can measure the volume of irregularly shaped samples using powder as a pseudo-fluid. A He gas pycnometer (Quantachrome Pentapycnometer) was used to measure the solid density of the powdered pumice and the total volume of solid phase plus isolated pore in the pumice. For solid density measurements, the same mass of floating pumice with grain sizes ranging from 8 mm (-3Φ) to 500 μm (1Φ) were taken and pulverized to prepare powder samples. The finer the grain size, the large number of grains were pulverized.
The total porosity, connected porosity, isolated porosity, dry pumice density and solid density were obtained. For five samples, the total volume of solid phase plus isolated pore could not be measured with sufficient accuracy. The important results obtained are as follows:
1.The solid density decreases from 2.66 to 2.52 g⁄cm3 as the grain size of drifting pumice decreases from 8 mm to 500 μm. The dry pumice density is 0.68 to 0.40 g⁄cm3.
2. Assuming a solid density of 2.62 g⁄cm3, the total porosity is calculated to be 74 to 85 vol%.
3. There are more grains with slightly higher connected porosity (33-55 vol%) in submerged pumice than floating pumice (18-44 vol%).
4. The relationship between total and connected porosity of floating and submerged pumice is similar to those of the Havre 2012 submarine eruption (Mitchell et al., 2021).
5. The relationship between total and connected porosity is similar to those of decompression experiments for silicic magmas (Takeuchi et al., 2009), in which the vesicular texture of pumice is quenched as rapidly as possible under high pressure.
The feature of 1 may reflect the loss of phenocryst with high density in floating pumice as the grain size decreases. The feature of 3 suggests that the dominant factor for the pumice submersion is seawater intrusion into the connected pore with large porosity. The results of 5 suggests that the shallow-water eruption of the Fukutoku-Oka-no-Ba was effective in quenching the vesicular texture of decompressed pumice.
Ishige et al. (2022) Characteristics of drifting pumice from the Fukutoku-Oka-no-Ba 2021 eruption and their relationship to eruption sequence, Japan Geoscience Union Meeting 2022.
Mitchell et al. (2021) BV, 83, 80.
Takeuchi et al. (2009) EPSL, 283, 101-110.