14:45 〜 15:15
[SCG62-10] 玄武岩質マグマの火山噴火における気泡合体のダイナミクス
★招待講演
キーワード：気泡合体、玄武岩質噴火、粘性流体力学
Bubbles in basaltic eruptions vary widely in size, ranging from microns to tens of meters. The variation in bubble size appears to be linked to the style and explosivity of basaltic eruptions. Bubble coalescence is a key process governing the evolution of bubble size. In basaltic magmas, buoyancy is the primary driving force behind bubble motion, leading to higher mobility and frequent coalescence. In this talk, I review the physics of buoyancy-driven bubble coalescence in basaltic magmas and discuss its impact on eruption style and explosivity.
In highly viscous liquids, ascending bubbles exhibit complex interactions: they experience both repulsive and attractive forces due to the flow field generated around them. First, I introduce a theory for the buoyancy-driven coalescence of two bubbles in basaltic magma [1]. The governing equations for the motion and deformation of two bubbles are derived using the theory of viscous fluid mechanics. These equations predict the separation distance between bubbles and identify critical conditions for collision and coalescence. The frequency of buoyancy-driven coalescence is determined in terms of bubble size and size ratio. For smaller bubbles (<1 cm), they remain spherical, and the surrounding flow pushes them apart. In contrast, larger bubbles (>1 cm) can deform, aligning with each other and changing their ascent direction to move closer together.
Next, I introduce a theory for the buoyancy-driven coalescence of multiple bubbles, incorporating the effects of bubble expansion [2]. The governing equations for the evolution of bubble size distribution through coalescence are derived based on a kinetic theory for dilute particle suspensions. These equations predict that an initially single-peaked size distribution evolves into a power-law form within short timescales ranging from 45 minutes to 3 days. This rapid evolution is driven by a positive feedback loop in bubble coalescence: as bubbles merge, their increased size enhances mobility, leading to even more frequent coalescence. This process may result in the formation of giant bubbles several meters in size, which have been observed in weak basaltic eruptions. The critical magma ascent velocity required for giant bubble formation is identified and found to be consistent with observations from Izu-Oshima and Kīlauea.
[1] Maruishi, T., & Toramaru, A. (2022). Effect of bubble deformation on the coalescence of two ascending bubbles in a viscous liquid. Physics of Fluids, 34(4).
[2] Maruishi, T., & Toramaru, A. (2024). Rapid coalescence of bubbles driven by buoyancy force: Implication for slug formation in basaltic eruptions. Journal of Geophysical Research: Solid Earth, 129(11), e2024JB029130
In highly viscous liquids, ascending bubbles exhibit complex interactions: they experience both repulsive and attractive forces due to the flow field generated around them. First, I introduce a theory for the buoyancy-driven coalescence of two bubbles in basaltic magma [1]. The governing equations for the motion and deformation of two bubbles are derived using the theory of viscous fluid mechanics. These equations predict the separation distance between bubbles and identify critical conditions for collision and coalescence. The frequency of buoyancy-driven coalescence is determined in terms of bubble size and size ratio. For smaller bubbles (<1 cm), they remain spherical, and the surrounding flow pushes them apart. In contrast, larger bubbles (>1 cm) can deform, aligning with each other and changing their ascent direction to move closer together.
Next, I introduce a theory for the buoyancy-driven coalescence of multiple bubbles, incorporating the effects of bubble expansion [2]. The governing equations for the evolution of bubble size distribution through coalescence are derived based on a kinetic theory for dilute particle suspensions. These equations predict that an initially single-peaked size distribution evolves into a power-law form within short timescales ranging from 45 minutes to 3 days. This rapid evolution is driven by a positive feedback loop in bubble coalescence: as bubbles merge, their increased size enhances mobility, leading to even more frequent coalescence. This process may result in the formation of giant bubbles several meters in size, which have been observed in weak basaltic eruptions. The critical magma ascent velocity required for giant bubble formation is identified and found to be consistent with observations from Izu-Oshima and Kīlauea.
[1] Maruishi, T., & Toramaru, A. (2022). Effect of bubble deformation on the coalescence of two ascending bubbles in a viscous liquid. Physics of Fluids, 34(4).
[2] Maruishi, T., & Toramaru, A. (2024). Rapid coalescence of bubbles driven by buoyancy force: Implication for slug formation in basaltic eruptions. Journal of Geophysical Research: Solid Earth, 129(11), e2024JB029130