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Large-scale volcanic deposit fluidization by dilute pyroclastic density currents


There is increasing evidence that fine-grained deposits of pyroclastic density currents can be remobilized on a large scale, resulting in concentrated flows. These flows can be a major hazard; for example, at Soufrière Hills Volcano (Montserrat) in 1997, some travelled beyond the designated danger zone to inhabited areas. Despite their hazard potential, the scale and generation mechanism of these flows are poorly understood. Here we demonstrate using laboratory experiments and numerical modelling that decompression following the passage of dilute pyroclastic density currents can cause rapid deposit fluidization over areas of several square kilometres and to depths of tens of centimetres. The fluidized volume can be substantially greater than that deposited by the triggering pyroclastic density current because of remobilization of previously emplaced deposits, and the fluidized volume can flow even on slopes of a few degrees. The capacity for remobilization of a deposit is limited by its particle cohesion, which rapidly increases with atmospheric humidity. This mechanism of fluidization should alert us to an under-appreciated volcanic hazard of long-run-out pyroclastic flows that can be generated by remobilization very rapidly and with little warning.

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Fig. 1: Emplacement of a dilute PDC.
Fig. 2: The experimental device made to simulate the unloading of a dilute PDC.
Fig. 3: Results of the decompression experiments.
Fig. 4: Results of the numerical study of the fluidization by the unloading of a dilute PDC.

Data availability

Videos and synchronized pressure data (text files) of characteristic experiments can be downloaded from

Code availability

The numerical code used (open source) is available at The estimation of the pressure and the decompression rate in the field has been done with the version ‘Pyroclastic flows and surges’ of VolcFlow, which is downloadable at


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This is contribution number 590 of the ClerVolc programme of the International Research Center for Disaster Sciences and Sustainable Development of the University Clermont Auvergne. We thank T. Latchimy, E. Régis, E. Brut, J.-L. Fruquière and C. Guillot for their help in the construction of the experimental devices. The comments of G. Valentine contributed substantially to the improvement of the article.

Author information

Authors and Affiliations



Conceptualization: K.K. Laboratory experiments: K.K. & A.P. Numerical code: K.K. Writing—original draft: K.K. and A.P. Writing—review and editing: K.K. Funding acquisition: K.K.

Corresponding author

Correspondence to Karim Kelfoun.

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The authors declare no competing interests.

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Peer review information

Nature Geoscience thanks Greg Valentine, Roberto Sulpizio and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary handling editor: Stefan Lachowycz, in collaboration with the Nature Geoscience team.

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Extended data

Extended Data Fig. 1 Numerical simulation of one laboratory experiment.

(a) Pressure evolution in the deposit for K = 2 × 10−12, 3 × 10−12 and 4 × 10−12 m2. The experimental measurements are given by the white circles. Where the particle pressure (red lines) is equal to or lower than the gas pressure, the granular material is fully fluidized locally. (b) Evolution of the apparent friction angle δ with time for K = 3 × 10−12 m2. During decompression, the flow is fully fluidized down to about 12 cm beneath the surface and the apparent friction angle of less than 10° shows that all the material can flow on steeper slopes for more than 1 s (and the upper surface for 4 s). Because the geometry change induced by the flow was not simulated, results are compared with experiments carried out on the horizontal but the time of fluidization coincides with the flow duration of experiments carried out on a slope and in the same conditions.

Extended Data Fig. 2 Evolution of the ratio Hf/H according to II1 and II2.

The colours give the values of Hf/H of each of the 5000 simulations used to draw the curves (some points overlap exactly). The data used to make the figure are available here: 10.25519/TF5A-8291. They have been made with the numerical code available in the same repository.

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Kelfoun, K., Proaño, A. Large-scale volcanic deposit fluidization by dilute pyroclastic density currents. Nat. Geosci. 16, 499–504 (2023).

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