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Ash Finger Formation at the Base of Spreading Volcanic Clouds: Insights from Analogue Experiments

Carolina Diaz-Vecino1, Allan Fries1, Paul A. Jarvis2, Costanza Bonadonna1

  • Affiliations:  1 Département des Sciences de la Terre, Université de Genève, Genève, Suisse 2 GNS Science | Te Pū Ao, Lower Hutt, New Zealand   

  • Presentation type: Poster

  • Presentation time: Thursday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 88

  • Programme No: 3.12.14

  • Theme 3 > Session 12

Abstract

Tephra sedimentation following explosive volcanic eruptions is commonly modelled as governed by the settling of individual particles. However, collective settling processes, such as settling-driven gravitational instabilities (SDGIs), can shorten the atmospheric residence time of volcanic ash particles. These instabilities generate downward-propagating plumes known as fingers that descend faster than the terminal fall velocities of fine ash (< 63 μm). This enhanced sedimentation can affect deposit interpretations and dispersion forecasts. However, despite being commonly observed below eruption plumes and clouds, ash fingers have not yet been included in ash dispersal models. While previous studies mostly focused on finger formation in static, density-stratified fluids, we investigated fingers beneath particle-bearing gravity currents in lock-release experiments to better simulate spreading volcanic plumes. Using a dense ambient fluid and a buoyant gravity current containing 40 µm glass beads (1 g l⁻¹ concentration), we employed particle image velocimetry to study interactions between shear and SDGIs. By varying the ambient fluid density, we simulated ash cloud spreading at different speeds. Qualitative results revealed two finger formation mechanisms: classical SDGIs and a vorticity-driven process. Quantitatively, we found that boundary layer shear reduced finger formation but did not alter their sedimentation behaviour. Once formed, the fingers exhibited entrainment and velocity characteristics similar to those in static configurations. These findings suggest that while shear inhibits finger formation, it does not significantly affect their overall role in sedimentation dynamics, highlighting the importance of incorporating ash fingers into volcanic ash dispersion models.