The depositional processes and gas pore pressure in pyroclastic flows are investigated through scaled experiments on transient, initially fluidized granular flows. The flow structure consists of a sliding head whose basal velocity decreases backwards from the front velocity (U f) until onset of deposition occurs, which marks transition to the flow body where the basal deposit grows continuously. The flows propagate in a fluid-inertial regime despite formation of the deposit. Their head generates underpressure proportional to U 2f whereas their body generates overpressure whose values suggest that pore pressure diffuses during emplacement. Complementary experiments on defluidizing static columns prove that the concept of pore pressure diffusion is relevant for gas-particle mixtures and allow characterization of the diffusion timescale (t d) as a function of the material properties. Initial material expansion increases the diffusion time compared with the nonexpanded state, suggesting that pore pressure is self-generated during compaction. Application to pyroclastic flows gives minimum diffusion timescales of seconds to tens of minutes, depending principally on the flow height and permeability. This study also helps to reconcile the concepts of en masse and progressive deposition of pyroclastic flow units or discrete pulses. Onset of deposition, whose causes deserve further investigation, is the most critical parameter for determining the structure of the deposits. Even if sedimentation is fundamentally continuous, it is proposed that late onset of deposition and rapid aggradation in relatively thin flows can generate deposits that are almost snapshots of the flow structure. In this context, deposition can be considered as occurring en masse, though not strictly instantaneously.
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This work was supported by the Institut de Recherche pour le Développement (IRD, France), and the ANR Volbiflo (France) and ECOS-Conicyt C06U01 and C11U01 (France-Chile) projects. This is Laboratory of Excellence ClerVolc contribution n° 25. The paper benefited from useful reviews of Editor J. White, AE M. Manga, D. Doronzo, and an anonymous reviewer.
Editorial responsibility: M. Manga
Electronic supplementary material
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Distal detail view of a flow generated from the release of a fluidized column of fine (d = 80 μm) glass beads and containing black markers (colored beads, d = 700 μm). The movie is ten times slower than actual, and shows the sliding flow head and the aggrading basal deposit in the flow body. The basal horizontal plate is 1-cm thick. (AVI 8,355 kb)
Flow generated from the release of a fluidized column (20 × 20 cm) of fine (d = 80 μm) glass beads and containing black PVC markers (d = 1–2 mm). The movie is ten times slower than actual. (AVI 29,010 kb)
Flow generated from the release of a dry column (20 × 20 cm) of fine (d = 80 μm) glass beads and containing black PVC markers (d = 1–2 mm). The movie is ten times slower than actual. (AVI 32,477 kb)
Detail view of a flow generated from the release of a fluidized column of fine (d = 80 μm) glass beads and containing black markers (colored beads, d = 700 μm), showing a subtle caterpillar-type motion at the flow front. The movie is fifty times slower than actual. (AVI 12,992 kb)
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Roche, O. Depositional processes and gas pore pressure in pyroclastic flows: an experimental perspective. Bull Volcanol 74, 1807–1820 (2012). https://doi.org/10.1007/s00445-012-0639-4
- Pyroclastic flow
- Density current
- Pore pressure