An Experimental Model of Unconfined Bubbly Lava Flows: Importance of Localized Bubble Distribution

• Published in: Journal of geophysical research. Solid earth 2022-06, Vol.127 (6), p.n/a
• Main Authors: Namiki, Atsuko, Lev, Einat, Birnbaum, Janine, Baur, Jasper
• Format: Article
• Language: English
• Subjects: Analogs; bubble; Bubbles; Crystals; Deceleration; Deformation; Deformation effects; Distribution; Flow structures; Flow velocity; Geophysics; Lava; lava flow; Lava flows; Liquid phases; Localization; Lubrication; Mathematical analysis; Rapid flow; Rheological properties; Rheology; Segregation; Separation; Shape; Slopes; Solid phases; Spatial distribution; Syrup; Thickness; Velocity; Velocity distribution; Vertical separation; Viscosity; Voids
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1029/2022JB024139

Most lava flows carry bubbles and crystals in suspension. From earlier works, it is known that spherical bubbles increase the effective viscosity while bubbles deformed by rapid flow decrease it. Changes in the spatial distribution of bubbles can lead to variable rheology and flow localization and thus modify the resulting lava flow structure and morphology. To understand the roles of bubble and solid phase crystal distributions, we conducted a series of analog experiments of high bubble fraction suspensions. We poured the analog lava on an inclined slope, observed its shape, calculated the velocity field, and monitored its local thickness. A region of localized rapid flow and low vesicularity, whose thickness is thinner than the surrounding area, develops at the center of the bubbly flows. These features suggest that the locally higher liquid fraction decreases the effective viscosity, increases the fluid density, and accelerates the flow. We also found that a halted particle‐bearing bubbly flow can resume flowing. We interpret this to result from the upward vertical separation of bubbles, which generates a liquid‐rich layer at the bottom of the flow. In our experiment, bubbles are basically spherical and decrease the flow velocity, while our estimate suggests that bubbles in natural lava flows could increase or decrease flow velocity. Downstream decreases in flow velocity stops the bubble deformation and can cause a sudden increase of effective viscosity. The vertical segregation of the liquid phase at the slowed flow front may be a way to generate a cavernous shelly paho’eho’e. Plain Language Summary Lava flows can cover large areas and pose a hazard to buildings and infrastructure. To assess this hazard, we need to know what determines the shape and velocity of the lava flow. Most lava flows include bubbles, which can decelerate and accelerate lava flows depending on the bubble size and the flow velocity. Thus, heterogeneous bubble distribution may affect the lava flow shape and velocity. We simulated a lava flow by pouring a bubbly syrup as a lava analog on an inclined slope. We found that a more bubbly region flows slowly. 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. These results show that bubble localization within a lava flow can be a source of variations of shape and velocity in natural lava flows. In the field, we so