Generation of air lubrication within pyroclastic density currents

Abstract

Pyroclastic density currents are highly dangerous ground-hugging currents from volcanoes that cause >50% of volcanic fatalities globally. These hot mixtures of volcanic particles and gas exhibit remarkable fluidity, which allows them to transport thousands to millions of tonnes of volcanic material across the Earth’s surface over tens to hundreds of kilometres, bypassing tortuous flow paths and ignoring rough substrates and flat and upsloping terrain. Their fluidity is attributed to an internal process that counters granular friction. However, it is difficult to measure inside pyroclastic density currents to quantify such a friction-defying mechanism. Here we show, through large-scale experiments and numerical multiphase modelling, that pyroclastic density currents generate their own air lubrication. This forms a near-frictionless basal region. Air lubrication develops under high basal shear when air is locally forced downwards by reversed pressure gradients and displaces particles upward. We show that air lubrication is enhanced through a positive feedback mechanism, explaining how pyroclastic density currents are able to propagate over slopes much shallower than the angle of repose of any natural granular material. This discovery necessitates a re-evaluation of hazard models that aim to predict the velocity, runout and spreading of pyroclastic density currents.

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Fig. 1: Synthesizing pyroclastic density currents in large-scale experiments.
Fig. 2: Time-variant flow properties at 5.6 m.
Fig. 3: Height-variant friction and a test of the effective friction coefficient law.
Fig. 4: Origin and potential for air lubrication in experimental pyroclastic density currents.
Fig. 5: Occurrence of basal air lubrication in pyroclastic density currents.

Data availability

The data that support the findings of this study are available from the corresponding author upon request.

Code availability

The code used to produce the DEM-CFD is freely available at https://mfix.netl.doe.gov/.

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Acknowledgements

We thank A. Moebis and K. Kreutz for assistance during the experiments, and K. Arentsen and G. Lube Sr for internal review. This study was supported by the Royal Society of New Zealand Marsden Fund (contract number MAU1506), National Science Foundation (EAR 1650382) and New Zealand Natural Hazards Research Platform (contract number 2015-MAU-02-NHRP).

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Contributions

G.L. designed the experiments and wrote the first draft of the manuscript, which was then revised by all the authors. G.L. and E.C.P.B. conducted and analysed the experiments, and interpreted the data together with J.J. L.F. and T.W. conducted the advection analysis. E.C.P.B. and J.D. conducted the numerical simulations. G.L., S.J.C. and J.J. developed the PELE facility.

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Correspondence to Gert Lube.

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Supplementary information

Supplementary Information

Supplementary Figures, Supplementary Tables and Supplementary References

Supplementary Movie 1

Synthesising pyroclastic flows

Supplementary Movie 2

The air-lubrication of pyroclastic flows

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Lube, G., Breard, E.C.P., Jones, J. et al. Generation of air lubrication within pyroclastic density currents. Nat. Geosci. 12, 381–386 (2019). https://doi.org/10.1038/s41561-019-0338-2

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