Gas retention in fine-grained pyroclastic flow materials at high temperatures

Abstract

International audienceThe ability of a dense pyroclastic flow to maintain high gas pore pressure, and hence low friction, during runout is determined by (1) the strengths and longevities of gas sources, and (2) the ability of the material to retain residual gas once those sources become ineffective. The latter is termed the gas retention capacity. Gas retention capacity in a defluidizing granular material is governed by three timescales: one for the evacuation of bubbles (t be ; brief and not considered in this paper), one for hindered settling from the expanded state (t sett), and one for diffusive release of residual pore pressure from the non-expanded state (t diff). The relative magnitides of t sett and t diff depend on bed thickness, t sett dominating in thin systems and t diff in thick ones. Three pyroclastic flow materials, two ignimbrites and a block-and-ash flow sample, were studied experimentally to investigate expansion behaviour under gas flow and to determine gas retention times. Effects of particle size were evaluated by using two size cuts (<4 mm and <250 μm) from each sample. Careful drying of the materials was necessary to avoid effects of humidity-related cohesion. Two sets of experiments were carried out: (1) expansion in the non-bubbling regime at 50–200°C, (2) bed collapse tests from the initially bubbling state at 50–550°C. Provided that gas channelling was avoided by gentle stirring, all the samples exhibited a regime of uniform expansion prior to the onset of bubbling. Fine particle size (in particular high fines content), low particle density and high temperature all favoured smoother fluidization by increasing the maximum expansion possible in the non-bubbling state. An empirical equation describing the uniform expansion of the materials was determined. High temperature also favoured greater gas partitioning into the dense phase of the bubbling bed, as well (in finer-grained samples) as higher voidage in the settled bed. Large values of t sett and t diff were favoured by fine particle size. Temperature had less influence, suggesting that experimental results at low temperatures (50–200°C) can be extrapolated to higher temperatures. Gas retention times provide insight into the ability of pyroclastic flows in expanded (t sett) or non-expanded (t diff) flow states to retain gas once air ingestion or gas production have become ineffective. Finer-grained pyroclastic flows are expected to retain gas longer, and hence to have higher apparent ‘mobilities', than coarser-grained ones of comparable volume, as has been observed on Montserrat

Dynamics of pyroclastic density currents: Conditions that promote substrate erosion and self-channelization — Mount St Helens, Washington (USA)

“…Unit III is massive, poorly-sorted, and shows little to no evidence of elutriation or segregation of lithics or pumice, suggesting a highly concentrated current with high internal pore pressure where size/density segregation was suppressed (Druitt, 1995;Druitt et al, 2007). The lack of pumice breakage indicates particle-particle interactions were buffered by a high internal pore pressure and thus a highly mobile, inertial flow behavior over much of the runout distance (cf.…”

Particle velocity fields and depositional processes in laboratory ash flows, with implications for the sedimentation of dense pyroclastic flows

“…The operating temperature of all experiments was fixed at 170°C, which was high enough to avoid humidity-related cohesive effects (Druitt et al 2007). All experiments were limited to the non-bubbling state (U mf ≤U g <U mb ), and the bed expansion was defined as…”

“…They travel at high speeds and constitute one of the most important hazards around active volcanoes (e.g., Wilson 1986;Druitt 1998;Freundt and Bursik 2001). Their ability to travel large distances on slopes as gentle as a few degrees has been attributed to non-equilibrium gas pore pressure and associated fluidization effects generated either by gases released internally or by entrainment of air (Sparks 1976;Wilson 1980;Druitt et al 2007). Development of quantitative models of pyroclastic flows is a priority for modern volcanology, and this requires better understanding of their physical properties.…”

“…Hot ash differs from glass beads in that it can be expanded considerably (up to ∼40%) above loose packing when fluidized above U mf , allowing the effect of initial expansion on flow kinematics to be investigated. Heating the ash to ∼200°C was necessary to reduce interparticle cohesion due to atmospheric humidity, allowing the ash to behave as a group-A material (Druitt et al 2007). The ash-flow experiments showed that the runout of initially expanded flows scaled with the initial expansion and were in the range of 5.5-10 h 0 , the first emplacement phase being governed by gravitational acceleration, and the second and third phases being dominated by both gravity and hindered settling.…”

“…Some fine-grained powders (group A of Geldart 1973) expand uniformly between U mf and the minimum bubbling threshold, U mb (>U mf ). If the gas supply to a uniformly expanded bed is cut, the bed re-sediments from the base upwards by hindered settling in what is called a bed-collapse test (Lettieri et al 2000;Druitt et al 2007).…”