Coupling of turbulent and non-turbulent flow regimes within pyroclastic density currents

Abstract

Volcanic eruptions are at their most deadly when pyroclastic density currents sweep across landscapes to devastate everything in their path1,2. The internal dynamics underpinning these hazards cannot be directly observed3. Here we present a quantitative view inside pyroclastic density currents by synthesizing their natural flow behaviour in large-scale experiments. The experiments trace flow dynamics from initiation to deposition, and can explain the sequence and evolution of real-world deposits. We show that, inside pyroclastic density currents, the long-hypothesized non-turbulent underflow and fully turbulent ash-cloud regions4,5 are linked through a hitherto unrecognized middle zone of intermediate turbulence and concentration. Bounded by abrupt jumps in turbulence, the middle zone couples underflow and ash-cloud regions kinematically. Inside this zone, strong feedback between gas and particle phases leads to the formation of mesoscale turbulence clusters. These extremely fast-settling dendritic structures dictate the internal stratification and evolution of pyroclastic density currents and allow the underflows to grow significantly during runout. Our experiments reveal how the underflow and ash-cloud regions are dynamically related—insights that are relevant to the forecasting of pyroclastic density current behaviour in volcanic hazard models.

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Figure 1: Synthesizing pyroclastic density currents in large-scale experiments.
Figure 2: Evolution of the internal flow structure and velocity fields.
Figure 3: Experimental deposit and timescale of deposition.
Figure 4: Internal structure of experimental PDCs.

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Acknowledgements

We are very grateful to G. Kereszturi and R. van Niekerk for assisting with the experiments. We thank S. Sundaresan for helpful discussions regarding mesoscale clusters and A. Freundt for his help on the Laacher See ignimbrites. We also thank K. Arentsen and B. Walsh for internal reviews. This study was partially supported by the Royal Society of New Zealand Marsden Fund (contract no. 15-MAU-085) and the New Zealand Natural Hazards Research Platform (contract no. 2015-MAU-02-NHRP). We are very grateful to A. Neri and A. Burgisser for thoughtful reviews that strengthened the manuscript.

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E.C.P.B. and G.L. designed the experiments and wrote the first draft of the manuscript, which was then revised by all the authors. E.C.P.B. and G.L. conducted the experiments with the help of A.M., who also assisted in the mixture density measurements. E.C.P.B. led the data analyses and their interpretation, assisted by G.L., J.R.J., J.D., S.J.C. and G.A.V. G.L. designed the experimental facility with the help of J.R.J. and S.J.C. All authors concurred with the paper’s content.

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Correspondence to Eric C. P. Breard or Gert Lube.

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Breard, E., Lube, G., Jones, J. et al. Coupling of turbulent and non-turbulent flow regimes within pyroclastic density currents. Nature Geosci 9, 767–771 (2016). https://doi.org/10.1038/ngeo2794

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