10:45 〜 11:00
[SVC29-05] 気泡を含む低粘性マグマの破砕と流れ
キーワード：破砕、マグマ、気泡
Low-viscosity basaltic magma is the most abundant, and understanding how it fragments and flows is required for hazard assessment. I would like to introduce recent studies on these two topics based on the observation in the 2018 lower East Rift Zone eruption of Kīlauea Volcano, Hawai'i.
Lava fountains are a phenomenon seen in low-viscosity magma, where the magma appears to behave fluidally. The dominant fragmentation mechanism in such a fountain is inertia driven, which produces a population of large fluidal pyroclasts. However, in the 2018 eruption, a subpopulation of smaller and more vesicular pyroclasts is generated. The more hazardous small pyroclasts are generated when sufficient volcanic gas is released in the fountain, suggesting that adiabatic gas expansion may be a clue. That is, adiabatic gas expansion lowers its temperature and cools the outer surface of liquid pyroclasts below the glass transition temperature. The rigid crust fragments as the hot interior attempts to expand due to further volatile diffusion from the melt into bubbles. Adiabatic expansion of volcanic gas occurs in all eruptions. Brittle fragmentation induced by rapid adiabatic cooling may be a widespread process, although of varying importance, in explosive eruptions.
A cavernous shelly paho'eho'e has been reported for lava flows in Hawaii and observed in the 2018 eruption of Kīlauea Volcano. The standard explanation for forming such cavities is an emptying out of the lava beneath a solidified crust, similar to a small lava tube. However, some observed flow patches do not have an outlet through which to discharge the lava. We simulated this process using analogue experiments. The buoyant bubbles concentrate at the top of the flow front, while the liquid-rich layer generated at the bottom by vertical bubble separation lubricates the bottom boundary. Our experiments suggest that large hollow voids may be generated by bubble accumulation at the top of the flow front.
References:
Namiki, A., Patrick, M.R., Manga, M., & Houghton, B.F. (2021) Brittle fragmentation by rapid gas separation in a Hawaiian fountain. Nature Geoscience 14, 242–247. https://doi.org/10.1038/s41561-021-00709-0
Namiki, A., Lev, E., Birnbaum, J., & Baur, J. (2022). An experimental model of unconfined bubbly lava flows: Importance of localized bubble distribution. Journal of Geophysical Research: Solid Earth, 127, e2022JB024139. https://doi. org/10.1029/2022JB024139
Lava fountains are a phenomenon seen in low-viscosity magma, where the magma appears to behave fluidally. The dominant fragmentation mechanism in such a fountain is inertia driven, which produces a population of large fluidal pyroclasts. However, in the 2018 eruption, a subpopulation of smaller and more vesicular pyroclasts is generated. The more hazardous small pyroclasts are generated when sufficient volcanic gas is released in the fountain, suggesting that adiabatic gas expansion may be a clue. That is, adiabatic gas expansion lowers its temperature and cools the outer surface of liquid pyroclasts below the glass transition temperature. The rigid crust fragments as the hot interior attempts to expand due to further volatile diffusion from the melt into bubbles. Adiabatic expansion of volcanic gas occurs in all eruptions. Brittle fragmentation induced by rapid adiabatic cooling may be a widespread process, although of varying importance, in explosive eruptions.
A cavernous shelly paho'eho'e has been reported for lava flows in Hawaii and observed in the 2018 eruption of Kīlauea Volcano. The standard explanation for forming such cavities is an emptying out of the lava beneath a solidified crust, similar to a small lava tube. However, some observed flow patches do not have an outlet through which to discharge the lava. We simulated this process using analogue experiments. The buoyant bubbles concentrate at the top of the flow front, while the liquid-rich layer generated at the bottom by vertical bubble separation lubricates the bottom boundary. Our experiments suggest that large hollow voids may be generated by bubble accumulation at the top of the flow front.
References:
Namiki, A., Patrick, M.R., Manga, M., & Houghton, B.F. (2021) Brittle fragmentation by rapid gas separation in a Hawaiian fountain. Nature Geoscience 14, 242–247. https://doi.org/10.1038/s41561-021-00709-0
Namiki, A., Lev, E., Birnbaum, J., & Baur, J. (2022). An experimental model of unconfined bubbly lava flows: Importance of localized bubble distribution. Journal of Geophysical Research: Solid Earth, 127, e2022JB024139. https://doi. org/10.1029/2022JB024139