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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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This content will become publicly available on November 20, 2025
Real-time, in situ viscosity mapping of active lava
Abstract Viscosity is a fundamental physical property that controls lava flow dynamics, runout distance, and velocity, which are critical factors in assessing and mitigating risks associated with effusive eruptions. Natural lava viscosity is driven by a dynamic interplay among melt, crystals, and bubbles in response to the emplacement conditions. These conditions are challenging to replicate in laboratory experiments, yet this remains the most common method for quantifying lava rheology. Few in situ viscosity measurements exist, but none of those constrains the spatial evolution of viscosity along an entire active lava flow field. Here, we present the first real-time, in situ viscosity map of active lava as measured in the field at Litli-Hrútur, Iceland. We precisely measured a lava viscosity increase of over two orders of magnitude, associated with a temperature decrease, crystallinity increase, and vesicularity decrease from near-vent to distal locations, crossing the pāhoehoe–‘a‘ā transition. Our data expand the limited database of three-phase lava viscosity, which is crucial for improvements and validation of the current numerical, experimental, and petrological approaches used to estimate lava viscosity. Further, this study showcases that field viscometry provides a rapid, accurate, and precise assessment of lava viscosity that can be implemented in eruptive response modeling of lava transport.
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- PAR ID:
- 10599031
- Publisher / Repository:
- GSA
- Date Published:
- Journal Name:
- Geology
- Volume:
- 53
- Issue:
- 2
- ISSN:
- 0091-7613
- Page Range / eLocation ID:
- 181 to 185
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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