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Abstract Melt composition, temperature, and crystallinity are often seen as the three most important characteristics driving lava rheology, which controls eruptive behavior. Traditional methods of measuring the viscosity of crystallizing basalts often yield different mineral characteristics to natural samples and are typically bubble-free. To quantify the viscosity of basalts inclusive of bubble and crystal cargo, we developed a new technique to measure high-temperature three-phase isothermal lava viscosity and applied it to samples from the 2018 eruption of Kīlauea. This new experimental technique begins at subliquidus temperatures, preserving original phenocrysts. A short experimental duration allows for the retention of most of the original bubble population (19%–31% vs. 36% in the original lava) and accurate replication of crystal textures from field samples, as documented in quenched postexperiment samples. The observed rheological behavior in these experiments, conducted at syneruptive temperatures (1150–1105 °C) and strain rates (0.4–18 s–1), should therefore be representative of the lava flows. We measured average viscosities of 116 Pa·s at 1150 °C to 167 Pa·s at 1115 °C (i.e., only 10%–25% higher than calculated liquid viscosities at those temperatures) and a maximum of 1800 Pa·s at 1105 °C. These results are much lower than viscosity measured in traditional bubble-free experiments, which plateaued at ~14,000 Pa·s at 1115 °C. Our results suggest the effect of bubbles in three-phase magmas may be greater than predicted by models based on two-phase bubbly liquids, and this effect must be included in realistic lava flow rheology models. The method proposed here supplies a framework for providing the necessary experimental constraints.
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An Experimental Model of Unconfined Bubbly Lava Flows: Importance of Localized Bubble Distribution
Abstract 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.
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- Award ID(s):
- 1929008
- PAR ID:
- 10445422
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Solid Earth
- Volume:
- 127
- Issue:
- 6
- ISSN:
- 2169-9313
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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