11:15 AM - 11:30 AM
[SVC31-03] Limited eruption condition to produce unsinkable pumice inferred from the floating pumice of Fukutoku-Oka-no-Ba 2021 eruption
Keywords:floating pumice, pore connectivity, crystal content, submarine eruption, subaerial eruption, decompression experiment
This study discusses the relationship between pumice porosity and floatability, magmatic properties and eruption processes. Comparing the pumice data from the Fukutoku-Oka-no-Ba (FOB) 2021 eruption with pumice data of submarine, subaerial eruptions and magma decompression experiments in the total porosity-pore connectivity space, we suggest that the condition to produce unsinkable pumice may be narrowly limited in the case of submarine eruption by crystal-poor to intermediate magmas that are quenched at a water depth where high total porosity and low pore connectivity is achieved.
We established a porosity measurement technique for millimeter-to-centimeter-sized pumice particles and applied it to the floating pumice that erupted during the FOB 2021 eruption. The total, connected, and isolated porosities were acquired for floating pumice particles. As studied for the floating pumice from past submarine eruptions, most floating pumice particles are rich in isolated porosity (>30 vol%) and are thus unsinkable even when the connected porosity in the pumice are saturated with the seawater (Takeuchi et al., 2022, Volcanol. Soc. Japan 2022 Fall Meeting). Comparing the data of natural pumices from subaerial eruptions (Colombier et al., 2017, EPSL), the total porosity-pore connectivity relationship shows that most pumice and scoria have high connectivities of more than 70 vol% and are plotted in the condition of sinkable pumice. The total and connected porosities from the decompression experiments were measured for quenched samples of vesiculated, hydrous silicic magmas without crystals (Takeuchi et al. 2009, EPSL) or with crystals (Graham et al. 2023, EPSL). The crystal-free samples with 70–80 vol% total porosity have pore connectivity in the condition of unsinkable pumice, which is close to data on the pumice of FOB 2021. For crystal-rich pumices (40 vol% crystal content), the pore connectivity gradually increases with an increase in the total porosity from 20 to 70 vol%. The total porosity-pore connectivity relationship for crystal-rich pumices varies in the condition of sinkable pumice and can not enter the condition of unsinkable pumice. For crystal-intermediate pumices (20 vol% crystal content), the total porosity-pore connectivity approaches the boundary separating the conditions and appears to enter the condition of unsinkable pumice with higher total porosity. Considering the crystal content of FOB 2021 pumices (17 vol%), we suggest that the magmas could produce the quenched pumice which satisfies the condition of unsinkable pumice.
Generally, in decompression experiments, the sample is kept at a high temperature and pressure, and then decompressed to the targeted pressure. The decompressed magmas underwent vesiculation, including bubble nucleation, growth, coalescence, and shape relaxation. Finally, the samples were quenched for a much shorter time than that during natural eruptions. Based on the study of natural subaerial pumices, Gardner et al. (1996, BV) concluded that the delayed expansion of pumice occurs in the range of melt viscosities in subaerial eruptions. Delayed expansion can cause isolated pores to expand and transform into connected pores. In contrast, vesicular magmas are effectively quenched in submarine eruptions, as in decompression experiments, resulting in producing more isolated pores in the pumice.
We established a porosity measurement technique for millimeter-to-centimeter-sized pumice particles and applied it to the floating pumice that erupted during the FOB 2021 eruption. The total, connected, and isolated porosities were acquired for floating pumice particles. As studied for the floating pumice from past submarine eruptions, most floating pumice particles are rich in isolated porosity (>30 vol%) and are thus unsinkable even when the connected porosity in the pumice are saturated with the seawater (Takeuchi et al., 2022, Volcanol. Soc. Japan 2022 Fall Meeting). Comparing the data of natural pumices from subaerial eruptions (Colombier et al., 2017, EPSL), the total porosity-pore connectivity relationship shows that most pumice and scoria have high connectivities of more than 70 vol% and are plotted in the condition of sinkable pumice. The total and connected porosities from the decompression experiments were measured for quenched samples of vesiculated, hydrous silicic magmas without crystals (Takeuchi et al. 2009, EPSL) or with crystals (Graham et al. 2023, EPSL). The crystal-free samples with 70–80 vol% total porosity have pore connectivity in the condition of unsinkable pumice, which is close to data on the pumice of FOB 2021. For crystal-rich pumices (40 vol% crystal content), the pore connectivity gradually increases with an increase in the total porosity from 20 to 70 vol%. The total porosity-pore connectivity relationship for crystal-rich pumices varies in the condition of sinkable pumice and can not enter the condition of unsinkable pumice. For crystal-intermediate pumices (20 vol% crystal content), the total porosity-pore connectivity approaches the boundary separating the conditions and appears to enter the condition of unsinkable pumice with higher total porosity. Considering the crystal content of FOB 2021 pumices (17 vol%), we suggest that the magmas could produce the quenched pumice which satisfies the condition of unsinkable pumice.
Generally, in decompression experiments, the sample is kept at a high temperature and pressure, and then decompressed to the targeted pressure. The decompressed magmas underwent vesiculation, including bubble nucleation, growth, coalescence, and shape relaxation. Finally, the samples were quenched for a much shorter time than that during natural eruptions. Based on the study of natural subaerial pumices, Gardner et al. (1996, BV) concluded that the delayed expansion of pumice occurs in the range of melt viscosities in subaerial eruptions. Delayed expansion can cause isolated pores to expand and transform into connected pores. In contrast, vesicular magmas are effectively quenched in submarine eruptions, as in decompression experiments, resulting in producing more isolated pores in the pumice.