1:45 PM - 3:15 PM
[O11-P76] Thin section observation of volcanic pisolite by the ROCK-GUARD method and estimation of the formation process
Keywords: Volcanic pisolite, Thin sections, Tuff, Granite, Michel-Levy interference color chart, Hail
(Motivation and purpose) In April 2023, we visited Higashimatsuyama City and Hatoyama Town in Saitama Prefecture during a field trip by the Department of Earth Sciences, and collected volcanic pisolite from the Okuda Tuff of the Shogunzawa Formation that had fallen on the outcrop. In addition, we also collected volcanic pisolite from a visit by our advisory teacher about 30 years ago, and decided to create thin section of volcanic pisolite to investigate the structure and constituent minerals of the volcanic pisolite, as well as to estimate the formation process of the volcanic pisolite.
(Method) Volcanic pisolite is very soft, so a piece of granite is drilled out to the shape of the volcanic pisolite , and then the slide glass is placed around it. The volcanic pisolite and the granite are simultaneously cut to prevent the volcanic pisolite from crumbling (this method of producing thin section is called the ROCK-GUARD method). Polishing is stopped when the slice is thicker than 0.03 mm, but the interference colors of the minerals in the granite are matched to the Michel-Levy interference color chart, and the thickness of the slice is fixed, allowing the identification of the minerals in the volcanic pisolite.
(Results) Using a polarizing microscope, we observed thin slices of volcanic peanuts obtained using the ROCK-GUARD method and identified relatively large minerals. Based on the ratio of minerals, we estimated that the peanuts were formed by the eruption of a rhyolitic magma volcano. Based on the structure, which resembles a concentric circle, we estimated that the volcanic peanuts were formed when volcanic ash floating in the sky during a volcanic eruption was absorbed into the hail that was forming in the cumulonimbus cloud, just as hail is formed in a cumulonimbus cloud. So they carried out a re-creation experiment in a freezer. They hung a seed crystal inside the freezer, sprayed it with water using a spray bottle to freeze it, then moved it to a case containing volcanic ash, hung it there, and sprayed volcanic ash on it using an air compressor. By repeating this process, they succeeded in creating a concentric mass.
(Conclusion) From the thin section observation and the reproduction experiment, it can be inferred that the volcanic pisolite in the Shogunzawa Formation Okuda Tuff was formed when volcanic ash floating in the sky was absorbed when hail formed in cumulonimbus clouds. In the future, we would like to take a picture of the cross section of the lump created in the reproduction experiment in a freezer and compare it with the internal structure of the volcanic pisolite.
(Method) Volcanic pisolite is very soft, so a piece of granite is drilled out to the shape of the volcanic pisolite , and then the slide glass is placed around it. The volcanic pisolite and the granite are simultaneously cut to prevent the volcanic pisolite from crumbling (this method of producing thin section is called the ROCK-GUARD method). Polishing is stopped when the slice is thicker than 0.03 mm, but the interference colors of the minerals in the granite are matched to the Michel-Levy interference color chart, and the thickness of the slice is fixed, allowing the identification of the minerals in the volcanic pisolite.
(Results) Using a polarizing microscope, we observed thin slices of volcanic peanuts obtained using the ROCK-GUARD method and identified relatively large minerals. Based on the ratio of minerals, we estimated that the peanuts were formed by the eruption of a rhyolitic magma volcano. Based on the structure, which resembles a concentric circle, we estimated that the volcanic peanuts were formed when volcanic ash floating in the sky during a volcanic eruption was absorbed into the hail that was forming in the cumulonimbus cloud, just as hail is formed in a cumulonimbus cloud. So they carried out a re-creation experiment in a freezer. They hung a seed crystal inside the freezer, sprayed it with water using a spray bottle to freeze it, then moved it to a case containing volcanic ash, hung it there, and sprayed volcanic ash on it using an air compressor. By repeating this process, they succeeded in creating a concentric mass.
(Conclusion) From the thin section observation and the reproduction experiment, it can be inferred that the volcanic pisolite in the Shogunzawa Formation Okuda Tuff was formed when volcanic ash floating in the sky was absorbed when hail formed in cumulonimbus clouds. In the future, we would like to take a picture of the cross section of the lump created in the reproduction experiment in a freezer and compare it with the internal structure of the volcanic pisolite.