An Experimental Model of Unconfined Bubbly Lava Flows

Abstract:

Most lava flows bear bubbles which affect their flow behavior. The 2018 eruption in the lower East Rift Zone (LERZ), on Kīlauea, Hawaii, produced destructive lava flows and provided an opportunity to observe the effect of bubbles on a lava flow. Samples taken from the flow reveal high bubble volume fractions (> 50%). In many places in the flow, bubbles coalesced to produce large (~1 m diameter) bubbles and form shelly Pahoehoe, and promote outgassing along the flow’s path. Bubbles in magma and lava affect magma rheology depending on the Capillary (Ca) number. At low Ca, the bubbles maintain a spherical shape and increase effective viscosity. In contrast, at large Ca, a result of a high shear rate relative to the bubble surface tension, bubbles deform, and act to lower the effective viscosity. Thus, the bubble fraction and strain-rate modulate the flow rate of a lava flow.

To understand the effect of a high volume fraction of bubbles of a range of sizes on a lava flow, we conducted a series of analogue experiments of a bubbly lava flow. We introduced bubbles into corn syrup by a chemical reaction and poured the bubbly fluid on an inclined slope. We observed the bubbly flow using a video camera, and its velocity field was calculated using optical flow. We also monitored the thickness of the bubbly flow by using a laser displacement sensor.

A localized region of faster motion appears at the center of the bubbly flows, in which the thickness of the center is thinner than the surrounding area. These two observations suggest that the bubbles in this region deform to reduce the effective viscosity and that the deformed bubbles connect to promote outgassing. We also found that, at the flow front, large bubbles concentrated in the top part of the flow, overlaying a bubble-poor bottom layer. The vertical separation of bubbles may generate this structure, and the bubble-free bottom layer enhances the flow rate. Our experiments show how bubble deformation and the evolving spatial variations of bubble fraction distribution modulate lava flows