EXPERIMENTAL AQUEOUS FLUIDIZATION OF IGNIMBRITE

Abstract

Experiments were carried out on the aqueous fluidization behaviour of ignimbrite and the associated formation of fluid-escape pipes. The starting material was an ignimbrite that had been saturated with water under vacuum until 80+/-15% of the vesicles were filled. This aimed to reproduce water-logging conditions of a hot pyroclastic flow in contact with water, such as in the case of a lahar or of a pyroclastic flow entering the sea. The ignimbrite sample was fluidized by water at vertical velocities from 0.005 to 7mms-1 for durations of 10-180min. 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 flared funnels and grew downwards and sideways with time, even under decreasing flow conditions. Pipe nucleation and growth generated irregular pressure fluctuations in the sample, showing that the standard ΔP-U plots commonly used in fluidization studies are not useful for coarse-grained, poorly sorted samples. Each pipe was stratified internally, with a basal layer rich in dense lithic and crystals, an intermediate layer rich in pumice, and an upper layer rich in fine components. As much as 30% of the initial sample mass was elutriated (including platy mica crystals) at the highest flow rates. At velocities exceeding 2mms-1 (duration of experiment: 10min) 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 fluid-escape pipes in ignimbrite-like sediment cannot be used to infer a gas-fluidized 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.