Presentation Information
[O12-P53]Differences in the characteristics of three types of Iwo Jima pumice using tephra layer identification methods.
*rikuto iida1, *kouki rin1 (1. Shizuoka Prefectural Iwata Minami High School)
Keywords:
Pumice,Apparent density,Microscope slide,Colour of pumice
Purpose
It is known that Iwo Jima pumice can be classified into three types based on color differences: brown Kokuto, black Black Kokuto, and Koge Kokuto, which is a mixture of the two. In this study, we analyzed the differences in characteristics among these three types of pumice using the tephra identification method, with the aim of elucidating the formation process of the pumice.
Data and Analysis Methods.
In this experiment, based on the tephra identification method, we sequentially analyzed the following six data sets:
A triangular diagram illustrating the color analysis of the pumice and its results.Photographs of the pumice and its cross-sections.Mineral composition of the pumice.Thin sections of the three types of pumice.Apparent density of the three types of pumice.Chemical composition of the volcanic glass in each of the three types of pumice.
The methods used to obtain the above data are as follows:
Photographs of the pumice cross-sections were taken, and color analysis was performed using the image processing software Macari to create a triangular diagram.Photographs were taken of the three types of pumice and their cross-sections.The collected pumice was crushed using an agate mortar, ground, and then sieved; particles with a grain size of 63-125 um were randomly extracted. Subsequently, for each sample, 200 particles were examined twice using a binocular stereomicroscope to identify volcanic glass, minerals, and rock fragments based on macroscopic characteristics.Photographs of the prepared thin sections were taken through a microscope.Pumice was placed in a beaker, and its volume was measured based on the weight of the water that overflowed.For powder samples, those with larger quantities were placed in PVC rings designed for powdered samples, while those with smaller quantities were placed in aluminum crimp cells. The samples were then compressed and solidified using a compression molding machine. Subsequently, they were placed in an X-ray fluorescence analyzer for measurement.
Results
The results obtained from the above analysis are as follows.
Black Kokuto was black, Kokuto was brown, and the black portion of Koge Kokuto showed a tendency very similar to Black, while the brown portion was similar to Kokuto.Black had large bubbles and a high number of them. Kokuto, on the other hand, had small bubbles and a low number of them. The black portion of Koge Kokuto exhibited the same characteristics as Black, and the brown portion of Koge Kokuto showed the same characteristics as Kokuto.All three types contained volcanic glass, feldspar, and rock fragments, with proportions of 93%, 5%, and 2%, respectively.The "Black" variety had large bubbles and a thick matrix, while the "Kokuto" variety had small bubbles and a thin matrix. Furthermore, the matrix of the Black variety was black, while that of the Kokuto variety was light brown. The black portion of the Koge-Kokuto variety was similar to the Black variety,
and the brown portion exhibited the same characteristics as the Kokuto variety.The density was 0.325 g/cm3 for the Black variety, 0.392 g/cm3 for the Kokuto variety, and 0.382 g/cm3 for the Koge-Kokuto variety.The Na2O + K2O values for all volcanic glasses fell within the range of 10.5-13.5%. Although the values differed for other components, they generally fell within a similar range.
Discussion
The color differences among the three types of pumice are not caused by differences in mineral composition or chemical composition compared to other rocks.Since the black portion of Kogekokuto shares many similarities with Black Kokuto, and the brown portion of Kogekokuto shares many similarities with Kokuto, the black part of Kogekokuto corresponds to Black Kokuto, and the brown part corresponds to Kokuto. Kogekokuto was formed when the boundary between Kokuto and Kogekokuto on a single large piece of Iwo Jima pumice cracked for some reason, and the edges were worn down during drifting.Since the color of the matrix appeared to differ, the difference in the chemical composition of the matrix is the cause of the color variation.
These three points were considered.
Future Prospects
Finally, since the color of the matrix appeared to differ based on thin-section observations, we plan to analyze the color and chemical composition of the matrix to elucidate the factors causing this color difference.
We would like to once again express our gratitude to Dr. Ayumu Nishihara of Tokoha University, Dr. Akira Aoshima of the Fujinokuni Museum, and our advisor, Dr. Hiroyuki Kurematsu, for their cooperation in conducting this research.
References
Hiroshi Machida, Fusao Arai. New Edition: Volcanic Ash Atlas. The University of Tokyo Press, 2003.
It is known that Iwo Jima pumice can be classified into three types based on color differences: brown Kokuto, black Black Kokuto, and Koge Kokuto, which is a mixture of the two. In this study, we analyzed the differences in characteristics among these three types of pumice using the tephra identification method, with the aim of elucidating the formation process of the pumice.
Data and Analysis Methods.
In this experiment, based on the tephra identification method, we sequentially analyzed the following six data sets:
A triangular diagram illustrating the color analysis of the pumice and its results.Photographs of the pumice and its cross-sections.Mineral composition of the pumice.Thin sections of the three types of pumice.Apparent density of the three types of pumice.Chemical composition of the volcanic glass in each of the three types of pumice.
The methods used to obtain the above data are as follows:
Photographs of the pumice cross-sections were taken, and color analysis was performed using the image processing software Macari to create a triangular diagram.Photographs were taken of the three types of pumice and their cross-sections.The collected pumice was crushed using an agate mortar, ground, and then sieved; particles with a grain size of 63-125 um were randomly extracted. Subsequently, for each sample, 200 particles were examined twice using a binocular stereomicroscope to identify volcanic glass, minerals, and rock fragments based on macroscopic characteristics.Photographs of the prepared thin sections were taken through a microscope.Pumice was placed in a beaker, and its volume was measured based on the weight of the water that overflowed.For powder samples, those with larger quantities were placed in PVC rings designed for powdered samples, while those with smaller quantities were placed in aluminum crimp cells. The samples were then compressed and solidified using a compression molding machine. Subsequently, they were placed in an X-ray fluorescence analyzer for measurement.
Results
The results obtained from the above analysis are as follows.
Black Kokuto was black, Kokuto was brown, and the black portion of Koge Kokuto showed a tendency very similar to Black, while the brown portion was similar to Kokuto.Black had large bubbles and a high number of them. Kokuto, on the other hand, had small bubbles and a low number of them. The black portion of Koge Kokuto exhibited the same characteristics as Black, and the brown portion of Koge Kokuto showed the same characteristics as Kokuto.All three types contained volcanic glass, feldspar, and rock fragments, with proportions of 93%, 5%, and 2%, respectively.The "Black" variety had large bubbles and a thick matrix, while the "Kokuto" variety had small bubbles and a thin matrix. Furthermore, the matrix of the Black variety was black, while that of the Kokuto variety was light brown. The black portion of the Koge-Kokuto variety was similar to the Black variety,
and the brown portion exhibited the same characteristics as the Kokuto variety.The density was 0.325 g/cm3 for the Black variety, 0.392 g/cm3 for the Kokuto variety, and 0.382 g/cm3 for the Koge-Kokuto variety.The Na2O + K2O values for all volcanic glasses fell within the range of 10.5-13.5%. Although the values differed for other components, they generally fell within a similar range.
Discussion
The color differences among the three types of pumice are not caused by differences in mineral composition or chemical composition compared to other rocks.Since the black portion of Kogekokuto shares many similarities with Black Kokuto, and the brown portion of Kogekokuto shares many similarities with Kokuto, the black part of Kogekokuto corresponds to Black Kokuto, and the brown part corresponds to Kokuto. Kogekokuto was formed when the boundary between Kokuto and Kogekokuto on a single large piece of Iwo Jima pumice cracked for some reason, and the edges were worn down during drifting.Since the color of the matrix appeared to differ, the difference in the chemical composition of the matrix is the cause of the color variation.
These three points were considered.
Future Prospects
Finally, since the color of the matrix appeared to differ based on thin-section observations, we plan to analyze the color and chemical composition of the matrix to elucidate the factors causing this color difference.
We would like to once again express our gratitude to Dr. Ayumu Nishihara of Tokoha University, Dr. Akira Aoshima of the Fujinokuni Museum, and our advisor, Dr. Hiroyuki Kurematsu, for their cooperation in conducting this research.
References
Hiroshi Machida, Fusao Arai. New Edition: Volcanic Ash Atlas. The University of Tokyo Press, 2003.