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.
Subscribe to Journal
Get full journal access for 1 year
only $15.58 per issue
All prices are NET prices.
VAT will be added later in the checkout.
Rent or Buy article
Get time limited or full article access on ReadCube.
All prices are NET prices.
The data that support the findings of this study are available from the corresponding author upon request.
The code used to produce the DEM-CFD is freely available at https://mfix.netl.doe.gov/.
Valentine, G. A. Damage to structures by pyroclastic flows and surges, inferred from nuclear weapons effects. J. Volcanol. Geotherm. Res. 87, 117–140 (1998).
Branney, M. & Kokelaar, P. Pyroclastic Density Currents and the Sedimentation of Ignimbrites Geological Society Memoir No. 27 (Geological Society, London, 2002).
Sulpizio, R., Dellino, P., Doronzo, D. M. & Sarocchi, D. Pyroclastic density currents: state of the art and perspectives. J. Volcanol. Geotherm. Res. 283, 36–65 (2014).
Dufek, J., Esposti Ongaro, T. & Roche, O. in The Encyclopedia of Volcanoes 2nd edn (ed. Sigurdsson, H.) 617–629 (Academic Press, 2015).
Wilson, C. J. N. The role of fluidization in the emplacement of pyroclastic flows: an experimental approach. J. Volcanol. Geotherm. Res. 8, 231–249 (1980).
Wilson, C. J. N. The Taupo eruption, New Zealand. II. The Taupo ignimbrite. Phil. Trans. A Math. Phys. Eng. Sci. 314, 229–310 (1985).
Roche, O., Buesch, D. C. & Valentine, G. A. Slow-moving and far-travelled dense pyroclastic flows during the Peach Spring super-eruption. Nat. Commun. 7, 10890 (2016).
Francis, P. W. & Baker, M. C. W. Mobility of pyroclastic flows. Nature 270, 164–165 (1977).
Girolami, L., Roche, O., Druitt, T. H. & Corpetti, T. Particle velocity fields and depositional processes in laboratory ash flows, with implications for the sedimentation of dense pyroclastic flows. Bull. Volcanol. 72, 747–759 (2010).
Druitt, T. H., Bruni, G., Lettieri, P. & Yates, J. G. The fluidization behaviour of ignimbrite at high temperature and with mechanical agitation. Geophys. Res. Lett. 31, L02604 (2004).
Cas, R. A. F. et al. The flow dynamics of an extremely large volume pyroclastic flow, the 2.08-Ma Cerro Galan Ignimbrite, NW Argentina, and comparison with other flow types. Bull. Volcanol. 73, 1583–1609 (2011).
Sparks, R. S. J. Grain size variations in ignimbrites and implications for the transport of pyroclastic flows. Sedimentology 23, 147–188 (1976).
Roche, O. Depositional processes and gas pore pressure in pyroclastic flows: an experimental perspective. Bull. Volcanol. 74, 1807–1820 (2012).
Melosh, H. J. Acoustic fluidization: a new geologic process? J. Geophys. Res. Solid Earth 84, 7513–7520 (1979).
Dufek, J. & Manga, M. In situ production of ash in pyroclastic flows. J. Geophys. Res. 113, B09207 (2008).
Shreve, R. L. The Blackhawk landslide. Geol. Soc. Am. Spec. Pap. 108, 1–47 (1968).
Lucas, A., Mangeney, A. & Ampuero, J. P. Frictional velocity-weakening in landslides on Earth and on other planetary bodies. Nat. Commun. 5, 3417 (2014).
Lube, G., Huppert, H. E., Sparks, R. S. J. & Hallworth, M. A. Axisymmetric collapses of granular columns. J. Fluid Mech. 508, 175–199 (2004).
National Academies of Sciences, Engineering, and Medicine Volcanic Eruptions and Their Repose, Unrest, Precursors, and Timing (National Academies Press, 2017); https://doi.org/10.17226/24650
Lube, G., Breard, E. C. P., Cronin, S. J. & Jones, J. Synthesizing large-scale pyroclastic flows: experimental design, scaling, and first results from PELE. J. Geophys. Res. Solid Earth 120, 1487–1502 (2015).
Breard, E. C. P. et al. Coupling of turbulent and non-turbulent flow regimes within pyroclastic density currents. Nat. Geosci. 9, 767–771 (2016).
Hogg, A., Lowe, D. J., Palmer, J., Boswijk, G. & Ramsey, C. B. Revised calendar date for the Taupo eruption derived by 14C wiggle-matching using a New Zealand kauri 14C calibration data set. Holocene 22, 439–449 (2012).
Hayashi, J. N. & Self, S. A comparison of pyroclastic flow and debris avalanche mobility. J. Geophys. Res. 97, 9063–9071 (1992).
Taberlet, N., Richard, P., Jenkins, J. T. & Delannay, R. Density inversion in rapid granular flows: the supported regime. Eur. Phys. J. E 22, 17–24 (2007).
Brodu, N., Delannay, R., Valance, A. & Richard, P. New patterns in high-speed granular flows. J. Fluid Mech. 769, 218–228 (2015).
Jop, P., Forterre, Y. & Pouliquen, O. A constitutive law for dense granular flows. Nature 441, 727–730 (2006).
Da Cruz, F., Emam, S., Prochnow, M., Roux, J.-N. & Chevoir, F. Rheophysics of dense granular materials: discrete simulation of plane shear flows. Phys. Rev. E 72, 021309 (2005).
Kelfoun, K. Suitability of simple rheological laws for the numerical simulation of dense pyroclastic flows and long-runout volcanic avalanches. J. Geophys. Res. 116, B08209 (2011).
Forterre, Y. & Pouliquen, O. Flows of dense granular media. Annu. Rev. Fluid Mech. 40, 1–24 (2008).
Leighton, D. & Acrivos, A. The shear-induced migration of particles in concentrated suspensions. J. Fluid Mech. 181, 415–439 (2006).
Garg, R., Galvin, J., Li, T. & Pannala, S. Open-source MFIX-DEM software for gas–solids flows: part I—verification studies. Powder Technol. 220, 122–137 (2012).
Li, T., Garg, R., Galvin, J. & Pannala, S. Open-source MFIX-DEM software for gas–solids flows: part II—validation studies. Powder Technol. 220, 138–150 (2012).
Syamlal, M., Rogers, W. & O’Brien, T. J. MFIX Documentation: Theory Guide (US Department of Energy, 1993).
Gidaspow, D. Multiphase Flow and Fluidization: Continuum and Kinetic Theory Description (Academic Press, Cambridge, 1994).
Druitt, T. H. et al. Small-volume, highly mobile pyroclastic flows formed by rapid sedimentation from pyroclastic surges at Soufriere Hills Volcano, Montserrat: an important volcanic hazard. Mem. Geol. Soc. Lond. 21, 263–279 (2002).
Lube, G., Cronin, S. J., Thouret, J.-C. & Surono. Kinematic characteristics of pyroclastic density currents at Merapi and controls on their avulsion from natural and engineered channels. Geol. Soc. Am. Bull. 123, 1127–1140 (2011).
Iverson, R. M. Regulation of landslide motion by dilatancy and pore pressure feedback. J. Geophys. Res. Earth Surf. 110, F02015 (2005).
Iverson, R. M. & Lahusen, R. G. Dynamic pore-pressure fluctuations in rapidly shearing granular materials. Science 246, 796–799 (1989).
Iverson, R. M. et al. Positive feedback and momentum growth during debris-flow entrainment of wet bed sediment. Nat. Geosci. 4, 116–121 (2011).
Breard, E. C. P., Dufek, J. & Lube, G. Enhanced mobility in concentrated pyroclastic density currents: an examination of a self-fluidization mechanism. Geophys. Res. Lett. 45, 654–664 (2018).
Sheridan, M. F. Emplacement of pyroclastic flows: a review. Geol. Soc. Am. Spec. Pap. 180, 125–136 (1979).
Charbonnier, S. J. & Gertisser, R. Numerical simulations of block-and-ash flows using the Titan2D flow model; examples from the 2006 eruption of Merapi Volcano, Java, Indonesia. Bull. Volcanol. 71, 953–959 (2009).
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).
The authors declare no competing interests.
Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
About this article
Cite this article
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
Impact of Fluidized Granular Flows into Water: Implications for Tsunamis Generated by Pyroclastic Flows
Journal of Geophysical Research: Solid Earth (2020)
Decoding pyroclastic density current flow direction and shear conditions in the flow boundary zone via particle-fabric analysis
Journal of Volcanology and Geothermal Research (2020)
Nature Communications (2020)
Nature Reviews Earth & Environment (2020)
The contribution of experimental volcanology to the study of the physics of eruptive processes, and related scaling issues: A review
Journal of Volcanology and Geothermal Research (2019)