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Experimental aqueous fluidization of ignimbrite

Abstract : Experiments were carried out on the aqueous¯uidization behaviour of ignimbrite and the associated formation of¯uidescape pipes. The starting material was an ignimbrite that had been saturated with water under vacuum until 80^15% of the vesicles were ®lled. This aimed to reproduce water-logging conditions of a hot pyroclastic¯ow in contact with water, such as in the case of a lahar or of a pyroclastic¯ow entering the sea. The ignimbrite sample was¯uidized by water at vertical velocities from 0.005 to 7 mm s 21 for durations of 10±180 min. Channelling occurred almost immediately, at even the lowest velocities, and pipe (channel) size increased slightly with time. The pipes had the form of elongated, upwardly¯ared funnels and grew downwards and sideways with time, even under decreasing¯ow conditions. Pipe nucleation and growth generated irregular pressure¯uctuations in the sample, showing that the standard DP±U plots commonly used in¯uidization studies are not useful for coarse-grained, poorly sorted samples. Each pipe was strati®ed internally, with a basal layer rich in dense lithic and crystals, an intermediate layer rich in pumice, and an upper layer rich in ®ne components. As much as 30% of the initial sample mass was elutriated (including platy mica crystals) at the highest¯ow rates. At velocities exceeding 2 mm s 21 (duration of experiment: 10 min) single pipes grew and coalesced rapidly, either forming a single, large pipe or causing the entire sample to become segregation-layered. In natural water-lain sediments, pipes may form near the end of deposition and during compaction, because during transport shear may reduce channelling by water. We also measured the degree of crystal enrichment in pipes. We conclude that the presence of¯uid-escape pipes in ignimbrite-like sediment cannot be used to infer a gas-¯uidized origin of the deposit, since the geometry, granulometry, and degree of crystal enrichment in water-generated pipes are similar to those in pipes formed by gas under dry conditions.
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Contributor : Olivier Roche Connect in order to contact the contributor
Submitted on : Monday, November 30, 2020 - 2:55:09 PM
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  • HAL Id : hal-03031481, version 1



Olivier Roche, T Druitt, R Cas. Experimental aqueous fluidization of ignimbrite. Journal of Volcanology and Geothermal Research, Elsevier, 2001, 112 (1-4), pp.267-280. ⟨hal-03031481⟩



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