5:15 PM - 6:30 PM
[SVC31-P03] Textural study of the 7.3 ka caldera-forming eruption of Kikai Caldera: Correlating bulk density to bubble-size observations
Keywords:Kikai Caldera, Textural Analyses, Bulk Density, Bubble, Fieldwork
The 7.3 eruption of Kikai Caldera is one of the catastrophic events that cause devastating damage on southern Kyushu, Japan (Maeno et al., 2007). A comprehensive study on the detailed stratigraphy and transport-mechanism of its products has been carried out by Maeno et al. (2007). Regardless, the textural study of the 7.3 ka eruptive deposits of Kikai Caldera hasn’t been conducted yet. In addition to the detailed description of the stratigraphic sequence of the eruptive deposits, the bubble size observation in the pumice is useful to characterize the vesicle texture and to gain insights into the eruption dynamics such as decompression rate in the conduit and overpressure condition in the magma chamber prior to the eruption. Integrating the bulk density data to the bubble-size observation for pumices is the aim of this study.
Samples were collected from all of the layers of Unit A, Unit C, and Unit D of the 7.3 ka caldera-forming eruption of Kikai Caldera at three localities, Southern Kyushu (Distal Locality), Takeshima Island (Proximal Locality), and Satsuma-Iojima Island (Proximal Locality). From all the localities, we had confirmed the stratigraphy which is coherent with the results by Maeno et al. (2007). Bulk density analyses were conducted for the pumice grains with the selected size range (2-16 mm) by 3D scanner (Medit Solutonix D700) and measuring software (Hira 3D viewer).
We selected 1131 pumice grains from all localities and measured the wide range (0.17 g/cm3 – 1.31 g/cm3) of pumice bulk density from Unit A, Unit C, and Unit D. From the observation of pumice grains, we identified some physical characteristic such as bubble size, bubble shape, and crystal abundance. We compared each unit and analysed the different physical characteristics from its appearance qualitatively. Thus, from the measured bulk density, Unit A (0.17 g/cm3 – 1.14 g/cm3) has a relatively lower bulk density compared to Unit C (0.27 g/cm3 – 1.31 g/cm3) and Unit D (0.22 g/cm3 – 1.24 g/cm3). We observed the dominant mode bulk of Unit A (0.45 g/cm3) angular and spherical bubble shape, crystal poor, and relatively has smaller bubble size, sometimes has a foamy appearance. On the other hand, the dominant mode of Unit C (0.54 g/cm3) and Unit D (0.50 g/cm3) have similar physical characteristics such as have sub-angular and frequently stretched to elongated bubble shape, have a bigger bubble size, and have prismatic jointing appearance in some grains. The ranges of densities and physical characteristics in pumices may imply that each unit has a different vesiculation and fragmentation process such as different quenching. Thus, the difference fragmentation process affects measurable properties such as shape, vesicularity, permeability, bubble, and crystal size distribution (Rust et al., 2011). In this study, we attempt to reveal the eruption dynamic between each unit by connecting the bulk density with the bubble-size observation that comprises measurement of vesicularity, bubble size distribution (BSD), bubble number density (BND); and crystal contents.
Samples were collected from all of the layers of Unit A, Unit C, and Unit D of the 7.3 ka caldera-forming eruption of Kikai Caldera at three localities, Southern Kyushu (Distal Locality), Takeshima Island (Proximal Locality), and Satsuma-Iojima Island (Proximal Locality). From all the localities, we had confirmed the stratigraphy which is coherent with the results by Maeno et al. (2007). Bulk density analyses were conducted for the pumice grains with the selected size range (2-16 mm) by 3D scanner (Medit Solutonix D700) and measuring software (Hira 3D viewer).
We selected 1131 pumice grains from all localities and measured the wide range (0.17 g/cm3 – 1.31 g/cm3) of pumice bulk density from Unit A, Unit C, and Unit D. From the observation of pumice grains, we identified some physical characteristic such as bubble size, bubble shape, and crystal abundance. We compared each unit and analysed the different physical characteristics from its appearance qualitatively. Thus, from the measured bulk density, Unit A (0.17 g/cm3 – 1.14 g/cm3) has a relatively lower bulk density compared to Unit C (0.27 g/cm3 – 1.31 g/cm3) and Unit D (0.22 g/cm3 – 1.24 g/cm3). We observed the dominant mode bulk of Unit A (0.45 g/cm3) angular and spherical bubble shape, crystal poor, and relatively has smaller bubble size, sometimes has a foamy appearance. On the other hand, the dominant mode of Unit C (0.54 g/cm3) and Unit D (0.50 g/cm3) have similar physical characteristics such as have sub-angular and frequently stretched to elongated bubble shape, have a bigger bubble size, and have prismatic jointing appearance in some grains. The ranges of densities and physical characteristics in pumices may imply that each unit has a different vesiculation and fragmentation process such as different quenching. Thus, the difference fragmentation process affects measurable properties such as shape, vesicularity, permeability, bubble, and crystal size distribution (Rust et al., 2011). In this study, we attempt to reveal the eruption dynamic between each unit by connecting the bulk density with the bubble-size observation that comprises measurement of vesicularity, bubble size distribution (BSD), bubble number density (BND); and crystal contents.