5:15 PM - 6:45 PM
[SCG54-P08] Measurement of connected and isolated porosities by water-displacement method in studies of drifting pumice
Keywords:drifting pumice, porosimetry, connected porosity, water-displacement method, pumice floatability
Pumice porosity is an important physical property governing pumice behavior in drifting pumice phenomena occurring in volcanoes in the Sea. This study reports on technical examination for the application of porosity measurement using a simple water-displacement method to drifting pumice.
The floatability of pumice in water depends on the isolated porosity and the degree of water saturation in the connected pores. To obtain connected and isolated porosities, measurement of skeletal density is required. Skeletal density is calculated by dividing the sample mass by the sum of the volume of the solid phase and isolated pores and is often measured by a precise instrument using gas-displacement methods. Takeuchi et al. (2023, Fall Meeting of the Volcanological Society of Japan) developed a water-displacement method that can perform all the density measurements necessary for porosity calculations, including bulk density and solid density, as well as skeletal density, using inexpensive equipment. In this water-displacement method, a water-impregnation using a vacuum (Yokoyama and Takeuchi, 2009, JGR) is an important procedure, which is used to impregnate as much water as possible into the connected pores in the pumice. In this method, bulk density measurement by the water-displacement method performed by Gardner et al. (1996, BV) is also performed at the same time. The water displacement method was applied to pumice from subaerial eruptions and yielded results similar to instrumented psudofluid- and gas-displacement methods. The measurement can be applied to not only single particle samples, but also multiparticle samples, and the average density and porosity of the multiparticle samples can be measured.
Drifting pumice can be divided into two categories: sinkable and unsinkable pumices. Sinkable pumices float due to air trapped in the connected pores and sink when they lose air (Fauria et al., 2017, EPSL). Unsinkable pumices do not sink due to high isolated porosity, as long as the porosity remains the same state. During the measurement of the water-displacement method, the pumice floatation is affected during the procedure of the water-impregnation and the measurement using specific gravity bottle. It is unknown in advance whether the pumice is an unsinkable or sinkable. In the measurement for an unsinkable pumice, metal weights are used to suppress the sample floatation. The volume of the weights should be measured in advance by the water-displacement method. After the water-impregnation, a sinkable pumice will sink. However, it may take time for a sinkable pumice to sink. A pumice floating after water-impregnation (unsinkable pumice) has the skeletal density lower than the density of water. As the solid density of volcanic rocks is between 2.2 and 2.9 g/cm3 (Takeuchi et al., 2021, JVGR), the skeletal density lower than the density of water means that the pumice contains abundant isolated pores. The water-impregnation procedure allows the pumice sample to determine either sinkable or unsinkable. If seawater is used for impregnation, the pumice sample can be evaluated for its floatability with respect to seawater.
The apparatuses used in the water-displacement method in this study are an electronic balance, a vacuum desiccator with a modified lid, a vacuum pump, and a specific gravity bottle. The above apparatuses are necessary when measuring small samples of a few cm3 in sample volume. For the larger sample, the precise measurement can be performed without using precise apparatuses. For example, we examine the effectiveness of aspirators instead of vacuum pumps and sample bottles or pails instead of specific gravity bottles, as the least expensive apparatuses, regarding the accuracy of the measurements. If measurement using a pail is accurate, it will be possible to measure particles of several tens of cm in diameter. Measurement with inexpensive apparatuses will reduce the cost hurdle for density and porosity measurement of pumice and has the potential for widespread use in research and science education in schools.
The floatability of pumice in water depends on the isolated porosity and the degree of water saturation in the connected pores. To obtain connected and isolated porosities, measurement of skeletal density is required. Skeletal density is calculated by dividing the sample mass by the sum of the volume of the solid phase and isolated pores and is often measured by a precise instrument using gas-displacement methods. Takeuchi et al. (2023, Fall Meeting of the Volcanological Society of Japan) developed a water-displacement method that can perform all the density measurements necessary for porosity calculations, including bulk density and solid density, as well as skeletal density, using inexpensive equipment. In this water-displacement method, a water-impregnation using a vacuum (Yokoyama and Takeuchi, 2009, JGR) is an important procedure, which is used to impregnate as much water as possible into the connected pores in the pumice. In this method, bulk density measurement by the water-displacement method performed by Gardner et al. (1996, BV) is also performed at the same time. The water displacement method was applied to pumice from subaerial eruptions and yielded results similar to instrumented psudofluid- and gas-displacement methods. The measurement can be applied to not only single particle samples, but also multiparticle samples, and the average density and porosity of the multiparticle samples can be measured.
Drifting pumice can be divided into two categories: sinkable and unsinkable pumices. Sinkable pumices float due to air trapped in the connected pores and sink when they lose air (Fauria et al., 2017, EPSL). Unsinkable pumices do not sink due to high isolated porosity, as long as the porosity remains the same state. During the measurement of the water-displacement method, the pumice floatation is affected during the procedure of the water-impregnation and the measurement using specific gravity bottle. It is unknown in advance whether the pumice is an unsinkable or sinkable. In the measurement for an unsinkable pumice, metal weights are used to suppress the sample floatation. The volume of the weights should be measured in advance by the water-displacement method. After the water-impregnation, a sinkable pumice will sink. However, it may take time for a sinkable pumice to sink. A pumice floating after water-impregnation (unsinkable pumice) has the skeletal density lower than the density of water. As the solid density of volcanic rocks is between 2.2 and 2.9 g/cm3 (Takeuchi et al., 2021, JVGR), the skeletal density lower than the density of water means that the pumice contains abundant isolated pores. The water-impregnation procedure allows the pumice sample to determine either sinkable or unsinkable. If seawater is used for impregnation, the pumice sample can be evaluated for its floatability with respect to seawater.
The apparatuses used in the water-displacement method in this study are an electronic balance, a vacuum desiccator with a modified lid, a vacuum pump, and a specific gravity bottle. The above apparatuses are necessary when measuring small samples of a few cm3 in sample volume. For the larger sample, the precise measurement can be performed without using precise apparatuses. For example, we examine the effectiveness of aspirators instead of vacuum pumps and sample bottles or pails instead of specific gravity bottles, as the least expensive apparatuses, regarding the accuracy of the measurements. If measurement using a pail is accurate, it will be possible to measure particles of several tens of cm in diameter. Measurement with inexpensive apparatuses will reduce the cost hurdle for density and porosity measurement of pumice and has the potential for widespread use in research and science education in schools.