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Experimental aqueous fluidization of ignimbrite
Journal of Volcanology and Geothermal Research 112 (2001) 267±280 www.elsevier.com/locate/jvolgeores
Experimental aqueous ¯uidization of ignimbrite O. Roche a,*, T.H. Druitt a, R.A.F. Cas a,b a
Laboratoire Magmas and Volcans (UMR 6524 and CNRS), Universite Blaise Pascal, 5 Rue Kessler, 63038 Clermont-Ferrand, France b Department of Earth Sciences, Monash University, Melbourne, Australia Received 26 June 2000; revised 14 June 2001; accepted 14 June 2001
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. q 2001 Elsevier Science B.V. All rights reserved. Keywords: ignimbrite; water ¯uidization; experiments; ¯uid-escape pipes; segregation
1. Introduction Fluidization is the process whereby the upward, focussed ¯ow of a ¯uid (gas or water) through a particulate aggregate can support and entrain grains. This may generate distinctive structures, grainsize sorting, * Corresponding author. Present address: Department of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK. Tel.: 144117-9545243; fax: 144-117-9253385. E-mail address: [email protected] (O. Roche).
and density segregation effects. Fluidization is used in many industrial systems, and may occur in some geological settings. In sedimentary and volcanic systems, ¯uidization process can occur during high rates of deposition of particulate dispersions, as grains fall to the depositional surface and displace ¯uid upwards. It can also occur in stationary deposited aggregates where these are ¯uidized by a ¯uid sourced from below, e.g. during dewatering and compaction leading to interstitial pore overpressures. Fluidization processes and effects have most commonly been
0377-0273/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0377-027 3(01)00246-3
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studied in aqueous terrigenous sediments (Lowe and LoPiccolo, 1974; Lowe, 1975; Postma, 1983; Johnson, 1986), and in gas-supported pyroclastic ¯ows and their deposits, ignimbrites (Walker, 1971; Wilson, 1980, 1984). Aqueous ¯uidization is potentially a signi®cant process in settings where the deposits of pyroclastic ¯ows rich in pumice and ash are remobilized by water, either subaerially (e.g. lahars) or under water, or where the ¯ows themselves enter the sea, as occurs in many modern and ancient maritime volcanic provinces (Cas and Wright, 1991). In this latter case, many of these ¯ows are known to have gone under water (e.g. Krakatau, Mandeville et al., 1994, 1996. Some may have retained initially a gassupported integrity. Others may have transformed by syn-eruptive and post-eruptive resedimentation into water-supported mass ¯ows (Cas and Wright, 1991), which, like their terrigenous sediment counterparts such as turbidity currents and debris ¯ows, are likely to have experienced varying degrees of syn-depositional and syn-compaction ¯uidization. Resulting structures in ignimbritic deposits should be preserved in the geological record, but few have yet been recorded (Crandell, 1971; Best, 1989, 1992), probably because few attempts have been made to look for them. The formation of aqueous ¯uidization features in such volcanic deposits has not been studied experimentally. In experiments, the relative motion of particles and ¯uid is accounted for by introducing a ¯uid ¯ux at the base of a ®xed bed. Experiments have been previously carried out in engineering aqueous and gas ¯uidization systems (Kunii and Levenspiel, 1991), and on aqueous ¯uidization processes in terrigenous and synthetic aggregates (Nichols et al., 1994; Nichols 1995). Gas ¯uidization processes have been studied in ignimbrite samples by Wilson (1980, 1984). In this study, we summarize new experiments on aqueous ¯uidization of ignimbrite samples, and we describe the structures generated as a guide to the characteristics likely to be found in the geological record. We focus on ¯uid-escape pipes generated by the upward migration of pore water through the ignimbrite. We show that pipes form very easily and rapidly in water-saturated ignimbrite, even at low rates of water ¯ux. Such pipes have been generated experimentally by ¯uidi-
zation of water-saturated, layered sediment (Nichols et al., 1994), by liquefaction and remobilization of sand (Nichols, 1995), and by sedimentation of poorly sorted concentrated dispersions (Druitt, 1995). The experiments reported here differ from those cited above in that the starting material is coarser and more poorly sorted, and it contains particles of two different densities. We also document both size- and density-related segregation effects, as ignimbrite contains particles of two discrete densities (pumice clasts and lithic clasts/crystals). It is important to note that we ¯uidized an already ®nes-depleted aerial ignimbrite, due to transport of the ¯ow in air. Distinguishing gas-¯uidized (subaerial) ignimbrites from water-¯uidized ignimbrite-derived sediments (such as lahar deposits or those deposited in aqueous settings) is a problem commonly encountered by volcanologists, and evidence for the origin of deposits and environments is often ambiguous (Cas and Wright, 1991). The aim of the paper is to compare the effects of water ¯uidization of ignimbrite with those of gas ¯uidization of similar material (Walker, 1971; Wilson, 1980, 1984). This is important as we seek criteria to distinguish between gas- and watersaturated ignimbrite. For this reason, we report segregation-layering processes, grain size distribution, component abundance and crystal-enrichment factors (Walker, 1972) in water-¯uidized samples, and we make comparisons with data reported for natural lahar deposits (Best, 1989, 1992) as well as those for experimentally gas-¯uidized ignimbrites (Wilson, 1980, 1984). 2. Aqueous ¯uidization Fluidization is the process whereby a bed of granular material is transformed into a ¯uid-like state by an upward ¯ow of ¯uid (Richardson, 1971; Kunii and Levenspiel, 1991). If the upward ¯uid drag force on the particles is suf®cient to support their weight, the bed is said to be completely ¯uidized. Complete ¯uidization is restricted to certain classes of granular materials. If only some of the particles are supported, the bed is said to be partially ¯uidized. We focus here on ¯uidization by water (Stanley-Wood et al., 1990). Two fundamentally different types of ¯uidization, particulate and aggregative, have been recognized in
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and Levenspiel, 1991): #1=2 )21 ( " 18m rf 2:335 2 1:744c 2 Ut 1 rs 2 rf gd rs 2 rf gd 2 2
Fig. 1. Theoretical minimum ¯uidization velocity (Umf) and terminal fall velocity (Ut) for particles of two densities as function of particle size (spherical particles, emf 0:4).
¯uidized systems (Richardson, 1971; Wilson, 1980, 1984; Kunii and Levenspiel, 1991). Particulate (homogeneous) ¯uidization occurs in granular materials that are monodisperse (single particle size) or weakly polydisperse (multiple particle sizes) and with a small density contrast between the different particles. At low water ¯ow rates, the bed is said to be ®xed and the system is governed by Darcy's law. With increasing ¯ow rate, the bed expands smoothly and instability-generated heterogeneities are negligible. The entire bed is ¯uidized when the minimum ¯uidization velocity Umf is reached. Umf is given by the Ergun equation (Kunii and Levenspiel, 1991): 1:75 3 ce mf
dUmf rf m
2
150 1 2 emf dUmf rf 1 m c 2 e3mf
d3 rf rs 2 rf g m2
1
where r s is particle density, r f the ¯uid density, m the ¯uid viscosity, d is particle diameter, c the particle sphericity, and e mf is the voidage at minimum ¯uidization. As velocity is increased above Umf, there is uniform expansion of the dispersion until, at U U t (terminal fall velocity of the particles), the particles are fully entrained and elutriated from the system. Terminal velocity is calculated readily from (Kunii
Fig. 1 shows curves for Umf and Ut as a function of grain size in the range of interest. The curves are for spherical particles c 1; an e mf value of 0.4, and two particle densities (2650 and 1090 kg m 23) that correspond to lithic fragments and water-saturated pumice clasts respectively in our experiments (see Section 3). The ratio U t =Umf varies widely across the range of grain sizes shown, from ,80 at d , 64 mm (.4 f ) to ,10 at d . 8 mm (, 2 3 f ). Complete ¯uidization is possible for a polydisperse material only if Umf of the coarsest component is less than Ut of the ®nest. For the parameters speci®ed above, this corresponds to materials with a total grainsize range between 3 and 5 f for particles of same density, and materials with a total grain-size range between 1 and 6 f for particles of the different densities considered here. For particles of the same density, this is equivalent to sorting of 0.6±1.0 if the grain-size distribution is approximately log-normal. Many ignimbrites, including that studied here, have sorting values higher than 1 (Cas and Wright, 1987), showing that complete ¯uidization by water is not possible. A similar conclusion was reached by Wilson (1980, 1984) in the case of gas ¯uidization of hot pyroclasts during transport and deposition. In aggregative ¯uidization, the formation and growth of instabilities generates heterogeneities. In most gas±solid systems, either monodisperse or polydisperse, once U is increased above Umb ($Umf), the excess gas passes through the bed as bubbles, and the surrounding particulate material remains poorly expanded (Richardson, 1971; Wilson, 1980, 1984; Kunii and Levenspiel, 1991). Bubbling is rare in liquid±solid systems, and occurs only at very high ¯uid velocity and if particles are very dense like tungsten or lead (Richardson, 1971). A particular type of instability in aggregative ¯uidization results in channelling of the ¯uid phase and in the formation of vertical pipes. Pipe formation occurs preferentially in polydisperse systems and in dispersions of small grain size and high grain density. In some well sorted dispersions, pipes appear throughout the bed at low
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Table 1 Grain-size and component composition (wt%) of the ignimbrite sample. Grain-size analyses were carried out by sieving ($5 f ) or with a Coulter Multisizer (,5 f ). Pumice, lithic, and crystal components in each 1 f size fraction of 5 f or larger were counted with eye or under binocular and converted to weight percent. Proportions of components for fraction smaller than 5 f are assumed to be the same as for f 5
f 23 22 21 0 1 2 3 4 5 6 7 8 9 10 Total
Pumice 0.9 1.8 2.4 2.4 5.7 4.7 4.5 4.5 9.6 5.5 5.7 2.6 0.6 0.1 51
Crystals
Lithic
± ± 0.1 0.7 2.2 2.1 4.6 5.2 3.1 1.8 1.8 0.8 0.2 ±
± 1 2.2 4.1 2.5 4.1 5.6 5.2 0.7 0.4 0.4 0.2 ± ±
22.6
26.4
Total 0.9 2.8 4.7 7.2 10.4 10.9 14.7 14.9 13.4 7.7 7.9 3.6 0.8 0.1 100
velocities, but disappear at velocities above Umf (Richardson, 1971). In highly polydisperse systems, the pipes may be very stable because elutriation of ®ne components within the pipe increases local permeability and enhances channelling. It is common that particles of different sizes and densities segregate from each other during the ¯uidization of polydisperse materials. Segregation is most effective if the contrasts in size Umf coarse=Umf fine . 2 and density are high, and occurred readily in the experiments on poorly sorted ignimbrite reported here. 3. Experimental methods The experiments used sub-16 mm samples of 0.58 Ma non-welded trachytic ignimbrite collected at Neschers, 30 km south of Clermont-Ferrand in the French Massif Central. The grain-size distribution and componentry of the ignimbrite are shown in Table 1. The sample has the coarse grain size and poor sorting typical of many ignimbrites (Walker, 1971). The graphic mean Mz and inclusive graphic standard deviation s I (a measure of sorting) (Folk and Ward,
1957) are 2.8 and 2.7 f , respectively. The principal crystals in the ignimbrite are plagioclase, quartz, amphibole, biotite, and oxides. Biotite crystals are typically platy in shape and consequently have low terminal fall velocities in water, as do shards and pumice fragments. The samples were saturated with water under vacuum prior to each experiment. Whitham and Sparks (1986) have shown that hot pumice clasts immersed in water rapidly become water-logged. This is because the hot gas in the pores contracts upon cooling, sucking in the water. In order to simulate the properties of hot pumiceous ignimbrite which had come into contact with water, the pre-dried cold sample was saturated with water under vacuum. Density measurements on twelve representative pumice lapilli and blocks (5±10 cm in diameter), with vesicularities in the range 77±86% showed that 80 ^ 15% of the vesicles were ®lled by water, giving a bulk wet pumice density of 1090 ^ 90 kg m 23 : The starting thickness of the sample bed was always between 11 and 18 cm (Fig. 2). The pressure drop across the bed was measured using two sidemounted PDCR 42 pressure transducers with a range of 0±7500 Pa and precision of ^1 Pa. Tests showed that ¯ow was laminar and uniform under all experimental conditions. Particles elutriated from the sample were collected for grain-size analysis. Tests using monodisperse and weakly polydisperse beds of silicon carbide and glass beads gave reproducible results. Ten experiments, each with a new sample from the same ignimbrite, were carried out at ¯ow velocities in the range 0.005±7 mm s 21 for durations between 10 and 180 min (Table 2). After seven of the experiment (4±10; Table 2), the sample was extruded from the rig, frozen, and serially sectioned to examine internal structures.
4. Experimental results 4.1. General observations Complete ¯uidization was not expected in the ignimbrite sample, because the sorting s I 2:7 was greater than the critical value of ,1 (Fig. 1). In fact, incipient, partial ¯uidization was found dif®cult
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Fig. 2. (a) Experimental apparatus. (b) Enlargement of the ¯uidization rig. The cylindrical rig, 14 cm in diameter, was fed by a water supply from a header tank. The hydrostatic head was kept constant by means of a stopcock, thus permitting a given ¯ow rate to be maintained over long periods. Inside the rig, the water ¯ow was ®rst de¯ected radially from the outlet value by a horizontal plate, then rendered laminar by a 10 cm thick layer of packed drinking straws. Uniform ¯ow across the rig was generated by a 2 cm thick layer of 2 mm lead shot, supported by a metal grid, overlain by a sheet of 32 mm nylon mesh on which rested the sample.
to achieve under the experimental conditions. Even at velocities as low as 0.3 mm s 21, and in some cases as soon as the experiment commenced, one or more pipes appeared in the bed and ¯ow became strongly channelled. In many cases, the pipes were more sheetlike in form than cylindrical, but we use the term pipe for convenience. The pipes were not edge effects, but formed throughout the sample and tended to grow and merge laterally with increasing ¯uid velocity, and with time if the velocity was held constant. The tendency for pipe formation was reduced if the sample was gently mixed immediately prior to an experiment
(Richardson, 1971). In experiments at high ¯ow rates (Experiments 6, 7 and 8; Table 2), the pipes merged completely. Fine components discharged from the pipes either accumulated on the surface of the bed as a ¯uidized scum, or were elutriated from the rig. The weight percentage of the initial bed elutriated from the rig was 25% in Experiment 6 (2 mm s 21) and 30% in Experiment 7 (3 mm s 21). Pipes also formed in the bed due to sedimentation effects, without introduction of water from below. Prior to each experiment, the loosely packed dispersion was mixed very slowly by hand to induce
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Table 2 Experiments carried out during this study Experiment
Duration (min)
U (mm s 21)
Aim of experiment
1 2 3 4 5 6 7 8 9 10
50 80 50 10 10 10 10 10 80 180
0±3 0±3 0±3 0.5 1 2 3 7 1 0.5
DP=U graph at increasing±decreasing U DP=U graph at increasing±decreasing U DP=U graph at decreasing U Internal structure Internal structure Internal structure and granulometry Internal structure and granulometry Internal structure and granulometry Internal structure Internal structure
homogenization. Subsequent settling caused expulsion of small amounts of pore water which formed subvertical channels. The pipes formed in this way were much smaller (mm-sized) than those generated once ¯uidization began because the vertical water ¯ux lasted only a few seconds during sedimentation. It is possible that these incipient pipes acted as nucleii for larger ones once ¯uidization began. 4.2. Pressure curves and pipe growth The ¯uidization behaviour of a material is conventionally recorded in terms of the pressure drop DP across the bed as a function of super®cial ¯uid velocity U, calculated from the volumetric ¯ow rate assuming that the container is empty (Richardson, 1971; Wilson, 1980, 1984; Kunii and Levenspiel, 1991). The pressure across the ®xed bed at zero ¯ow is hydrostatic because the particles are selfsupporting. As the ¯ow is increased U , Umf ; DP rises, until at Umf there is a break in slope of the pressure curve e emf : Above Umf, the bed expands (e increases), DP remains approximately constant in particulate ¯uidization although it can decrease rapidly in aggregative ¯uidization, due to ¯uid channelling caused by pipes. In this paper, the theoretical curve (DP versus U) does not often match the curves that depict the ¯uidization behaviour of the chosen material. The DP versus U curves for experiments 1±3 are shown in Fig. 3. In experiments 1 and 2, ¯ow velocity was increased from 0 to 3 mm s 21, then decreased again, over periods of 50 (Experiment 1) and 80 (Experiment 2) min. Umf cannot be de®ned in these
experiments because the ¯uidization behaviour is very heterogeneous and pressure transducers give only a local value of DP. In experiment 1 (Fig. 3a), DP increased up to a velocity of 0.3 mm s 21 (region A), when several small pipes nucleated. The initial DP increase suggests that some ¯uidization of the ®nest components occurred. Even at such small ¯ow rates, ®ne particles were being elutriated from the rig. Initiation of a small pipe at 0.3 mm s 21 resulted in strongly channelled ¯ow and effectively stalled ¯uidization. Shortly after, one small pipe on the side of the container diametrically opposite to the pressure transducers increased dramatically in size (DP maximum). As a result, the ¯ow of water was abruptly channelled into that pipe and away from the transducers, and a drop in DP was registered (region B). The DP decrease continued as ¯ow rate was further increased. Since no further pipes formed, the system approached a steady state with strong partitioning of ¯ow through the one main pipe. On decreasing the ¯ow, the water ¯ux through the pipe diminished, but DP stayed constant. In experiment 2 (Fig. 3b), pipes appeared immediately at the lowest ¯ow velocities possible with the apparatus (0.005 mm s 21). One of these formed next to the lowermost pressure transducer, causing an abrupt increase in DP above hydrostatic as soon as the experiment began. When the water ¯ux is concentrated in a pipe adjacent to the basal transducer, the measured DP increases because the ¯ow velocity is locally high. If, on the other hand, ¯ow is diverted away from the transducer, DP drops, as in experiment 1. In experiment 2, DP varied in a saw-tooth manner as the pipe network evolved, sometimes diverting
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¯ow towards the transducer, sometimes away. As the ¯ow rate was decreased, DP dropped in a smooth fashion, without abrupt changes. In experiment 3 (Fig. 3c) the ¯ow was increased rapidly to 3 mm s 21 then decreased steadily over 50 min to assess the effect of decreasing ¯ow conditions on pipe development. The initial DP at high ¯ow rate was small because water was channelled strongly away from the transducers through a pipe on the opposite side of the rig. With decreasing ¯ow, two sudden increases in DP occurred, the ®rst when a pipe
Fig. 3. Variation of pressure drop (DP) and bed thickness (H) with ¯ow velocity U in three experiments. 1 hPa is 100 Pa. See text for discussion of details and Table 2 for experimental conditions. In experiment 1 (a) and experiment 2 (b) the ¯ow velocity was ®rst increased slowly, then decreased slowly. In experiment 3 (c) the ¯ow velocity was increased rapidly to 3 mm s 21, then decreased slowly.
Fig. 4. Experimental samples frozen, then sawn, following ¯uidization. (a) Experiment 5. Upward ¯aring pipes (p) separated by regions of unmodi®ed ignimbrite (i, also analysed). In each pipe, the abundance of dense lithic fragments increases towards the base, and that of pumice and ®ne components towards the top. (b) Experiment 7. The pipes have amalgamated so that the entire sample is segregation-layered. Three layers are observed: lithic-rich (1), pumice-rich (2) and ®nes-rich (3).
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nucleated next to the basal transducer, and the second when that pipe suddenly grew in width. These data show the rather surprising feature that pipes nucleate and grow even under decreasing ¯ow conditions. This is not likely to be an artefact of the water distributor system, as tests showed that ¯ow was uniform. In all three experiments, the thickness of the bed dropped as the ¯ow velocity either increased or decreased. This is explained by the progressive elutriation of ®nes combined with the settling of larger particles as they re-orient and compact. The increase in thickness at moderate (increasing) ¯ow rates in experiment 2 (Fig. 3b) was due to the formation of a capping layer of ®nes discharged from newly formed pipes. 4.3. Internal structure of the samples Experiments 4±10 were carried out to document the internal structures of samples that had been ¯uidized at different velocities and for different durations (Table 2). At velocities of 0.5±1 mm s 21 for 10 min (Experiments 4 and 5), the resulting sample consisted of two discrete phases (Fig. 4a): unmodi®ed starting material (analyses made), and pipes in which the sample had been sorted by focused ¯uid ¯ow. Each pipe had the shape of an upward ¯aring funnel, and pipes were more sheet-like in form than cylindrical (Fig. 5). The sample within each pipe showed segregation layering. The base was enriched in dense lithic and crystal components and depleted in ®nes, the central part was enriched in pumice lapilli and the top was enriched in ®nes (Figs. 4a and 5). Note that these were not ®nes that were elutriated from the bed, then settled back once the ¯uid ¯ow ceased, but represented material that migrated to the top of the pipe during ¯uidization. In experiment 5, the ¯ared tops of several pipes merged, so that the pumice-rich and ®nes-rich layers occupied the entire upper half of the sample (Fig. 4a). If the same ¯ow rates were maintained (80±180 min; Experiment 9 and 10), the pipes were signi®cantly wider, but unmodi®ed ignimbrite still remained at the base of the sample (Fig. 5). Fig. 5. Serial sections showing the form of a pipe from experiment 10. The pipe has an elongated sheet-like form in three dimensions. Dense lithic and crystals (black) are concentrated at the base and pumice (white) and ®ne components (dots) at the top.
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Fig. 6. Grain size and componentry of ¯uidized samples from experiments 6, 7, and 8. See Table 1 for comparisons and Table 2 for the experimental conditions. Component proportions were not determined for f . 5: In experiment 6, the composition is that of a single large pipe. In 7 and 8, the entire samples were segregation-layered. Lithic-rich: base enriched in dense lithic and crystals; pumice-rich: middle enriched in pumice; ®nes-rich: top enriched in ®ne components; elutriated: ®ne ash entrained by the ¯ow of water and carried out of the bed.
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At higher ¯ow velocities (2, 3, and 7 mm s 21; Experiments 6±8), pipe growth was so rapid that the pipes merged completely during the 10-min duration runs, such that either a single large pipe formed (Experiment 6) or the entire sample became segregation layered (Experiments 7 and 8) (Fig. 4b). To summarize, each pipe expanded with time (under either increasing or decreasing ¯ow conditions) downwards and sideways into the unmodi®ed ignimbrite. Within each pipe, the high local ¯uid ¯ux probably caused partial ¯uidization and the migration of dense components to the base, and light and/or ®ne components to the top. At suf®ciently high ¯ow rates or durations, the pipes coalesced and the layering extended through the whole sample. 4.4. Granulometry and component analysis Fig. 6 shows grain-size histograms for the lithicrich, pumice-rich, ®nes-rich, and elutriated fractions of samples after ¯uidization for 10 min at velocities of 2, 3, and 7 mm s 21 (Experiments 6±8). In experiments 7 and 8, the entire sample became segregationlayered, and so the stated velocities are those `seen' by the whole sample. In experiment 8, the ®nes-rich layer was very thin and impossible to sample. In experiment 6, the analyses are of different layers within a single large pipe and the actual ¯uid velocity experienced by the segregated material slightly exceeded 2 mm s 21. Each bed is normally graded both in particle size and density, with the ®nest and lowest density components concentrated upwards. Basal layers are uniformly enriched in dense lithic fragments and depleted of ®nes compared with the starting ignimbrite (Table 1 and Fig. 6). The percentage of ,63 mm (3 f ) ash decreases progressively with increasing ¯uid velocity in the basal layers. The crystal content also increases in the same sense. Pumicerich layers show strong enrichment in $2 mm pumice, resulting in a weak bimodality. The ®nesrich top layers in experiments 6 and 7 are strongly enriched in vitric ash, but have a coarse tail of pumice lapilli up to 8 mm (23 f ). Elutriated fractions are in each case well sorted and enriched in vitric ash. At 2 mm s 21, there is a sharp cut-off in grain-size at 125 mm (4 f ); at higher velocities, a small percentage of 125±250 mm fraction
appears as well as a small amount of pumice. Crystals present in the elutriated fractions are mostly platy biotite. In Fig. 7 the results are shown as cumulative curves and on a plot of s I versus Mz (Folk and Ward, 1957). In all cases, the three segregation layers and elutriated fraction are better sorted than the initial ignimbrite. With increasing ¯ow rate, the lithic-rich and pumicerich layers became better sorted as more and more ®ne particles were removed progressively from them. The ®nes-rich layer and elutriated fraction became correspondingly poorer sorted. In each experiment, the lithic-rich and pumice-rich layers are coarser than the starting ignimbrite, whereas the ®nes-rich layer and elutriated ash are ®ner. In general, Mz of the lithic-rich and pumice-rich layers are insensitive to ¯ow velocity and decrease by only 1 f from 2 to 7 mm s 21. The lowest value of s I attained by the lithic-rich layers was 1.6 f , which is comparable to that of lithic-rich segregations generated by gas ¯uidization of ignimbrite (Wilson, 1984). Fig. 8 shows the abundance of particles smaller than 63 mm relative to the initial ignimbrite. The lithic-rich layers are depleted in ®nes by as much as ten-fold. The ®nes-rich layers have about the same proportions than the ignimbrite. The elutriated fraction is enriched in ®nes. We have used the crystal concentration method of Walker (1972) to quantify the upward migration of ®nes during ¯uidization, and to aid comparison with segregation structures in gas-¯uidized ignimbrite. Walker (1972) de®ned a crystal enrichment factor, EF: EF
C 2 P1 P2 C1
3
where C2 =P2 is the mass ratio of free crystals to pumice in the ,2 mm fraction of the sample, and C1 =P1 is the same for arti®cially crushed pumice clasts. Glass shards and crystals are hard to distinguish in size fractions less than 32 mm f 5; especially when feldspar is present. In carrying out the component analyses to calculate EF, we considered two alternative assumptions: (1) that the C=P ratio for f . 5 is equal to that for f 5: This yields a maximum value of C=P for the whole sample. (2) C=P is zero for f . 5; which gives a minimum C=P estimate for the whole sample.
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Fig. 7. (a), (b), (c). Cumulative grain-size curves of ¯uidized samples from experiment 6, 7, and 8. See Table 2 for the experimental conditions. L: lithic-rich and crystal-rich layer; P: pumice-rich; F: ®nes-rich; E: elutriated fraction. The grain size composition of the starting ignimbrite is shown by a heavy line in each ®gure. (d): Mz ± s I diagram for the different layers generated in experiments 6, 7 and 8. The numbers are the ¯ow velocity in mm s 21. The graphic mean Mz and inclusive graphic standard deviation s I (a measure of sorting; Folk and Ward, 1957) are 2.8 f and 2.7 f , respectively, where M z f16 1 f50 1 f84 =3 and s I f84 2 f16 =4 1 f95 2 f5 =6:6:
EF values for the starting ignimbrite and experimental samples are listed in Table 3. The average C1 =P1 for three large pumice clasts (5±10 cm in diameter) from the Neschers Ignimbrite is 0:18 ^ 0:02; corresponding to a phenocryst content of 15 ^ 1wt%: The EF (EFign) is 4:0 ^ 1:8; and is typical of many ignimbrites (Walker, 1972; Sparks and Walker, 1977). EF for ¯uidized samples that are greater than
EFign indicate depletion of vitric ash (and enrichment in crystals), whereas values that are less than EFign indicate the opposite. In each experimental sample, EF decreases progressively from base to top, re¯ecting concentration of ®nes upwards in the bed. The basal, lithic-rich samples, and in two cases the pumice-rich layers, have all lost vitric ash compared to the original ignimbrite. There is a progressive
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Fig. 8. Relative abundance of ,63 mm particles relative to the initial ignimbrite in various fractions from experiments 6, 7, and 8. The numbers are the ¯ow velocity in mm s 21.
increase in average EF from 5 to 22 with increasing ¯uid velocity in the lithic-rich layers. The ®nes-rich layers and elutriated fraction in all three experiments have been enriched in vitric ash. 5. Discussions In order to assess if ¯uidization characteristics of ignimbrite-like deposits can be used to distinguish those formed under aqueous conditions from those formed under gaseous conditions, we now compare the experimental behaviour of ignimbrite ¯uidized by water with that of ignimbrite ¯uidized by gas
(Wilson, 1980, 1984. Our experiments have revealed several similarities of the water- and gas-¯uidized systems. The poor sorting, abundance of ®ne components, and large range of particle densities in watersaturated ignimbrite all favour aggregative behaviour, ¯uid channelling, and ef®cient particle segregation, as in gas-¯uidized counterparts. The present experiments show that ¯ow instabilities and pipe production commence at very low ¯uid velocities (0.005 mm s 21 or less) in this poorly permeable material. This occurs because focusing of ¯uid ¯ow locally increases sediment permeability, leading to a positive feedback effect. The pipes thus generated are upward-¯aring funnels that grow larger (both sideways and downwards) and coalesce with time, even with decreasing ¯ow velocity. Focusing of ¯uid through each pipe results in local ¯ow velocities that exceed the calculated average over the whole container. This results in partial ¯uidization of a signi®cant fraction of the particles in the pipes and vertical sorting by size and density. Dense components (lithic lapilli plus crystals) concentrate towards the base of each pipe, ®ne fractions (ash) at the top, and pumice lapilli at intermediate levels. The ®nest particles are entrained by the ¯uid ¯ow and elutriated from the bed. At high ¯ow velocities, single pipes coalesce, the entire bed becomes segregationlayered, and each layer is better sorted than the starting ignimbrite. In the present experiments this occurred at ¯uid velocities of $3 mm s 21 sustained for 10 min (Experiment 7 and 8), but not at 0.5 mm s 21 for 180 min (Experiment 10). The amount
Table 3 Crystal enrichment data for the initial ignimbrite and for experimentally ¯uidized samples Experiment
U (mm s 21)
Sample
C2 =P2 max
C2 =P2 min
EF
6 6 6 6 7 7 7 7 8 8 8
2 2 2 2 3 3 3 3 7 7 7
Initial ignimbrite Elutriated Fines-rich layer Pumice-rich layer Lithic-rich layer Elutriated Fines-rich layer Pumice-rich layer Lithic-rich layer Elutriated Pumice-rich layer Lithic-rich layer
0.87 0.26 0.19 0.45 0.92 0.49 0.47 1.13 1.62 0.36 1.02 3.87
0.45 0.03 0.12 0.39 0.78 0.08 0.29 0.99 1.52 0.09 0.98 3.79
4:0 ^ 1:8 0:9 ^ 0:8 0:9 ^ 0:3 2:5 ^ 1:0 5:0 ^ 1:1 1:8 ^ 1:4 2:3 ^ 0:9 6:3 ^ 1:3 9:2 ^ 1:6 1:4 ^ 1:0 5:9 ^ 0:9 22:4 ^ 3:4
O. Roche et al. / Journal of Volcanology and Geothermal Research 112 (2001) 267±280
of ash elutriated increased with increasing ¯uid velocity. It also increased with time in a given experiment, even under decreasing ¯ow conditions. Pressure measurements are not useful in ¯uidization studies of coarse-grained, poorly sorted samples because the transducers record only the local ¯uidization state and curves of DP±U are irregular due to continuous pipe nucleation and growth (e.g. Wilson, 1980). As in the present experiments, gas-¯uidized ignimbrite exhibits aggregative behaviour with the general characteristics listed above (Wilson, 1980, 1984). Pipes form readily; they are depleted in pumice lapilli and ®ne ash, and enriched in lithic lapilli and crystals. Fines-rich and pumice-rich layers commonly form at the tops of the bed. At high gas velocities, the entire bed becomes segregated into distinct layers. In both gaseous and aqueous systems, the sorting of the segregated material may approach values of 1.5 or less. However, gas-¯uidized systems show some differences from the water-¯uidized systems reported here. In the gaseous system, pipes appear only at high ¯uid velocity, if U . Ump (,1±2 cm s 21) as de®ned by Wilson (1980). They commonly broaden downwards, and layering tends to form by sedimentation of the particles within the pipes under gravity rather than by their coalescence. Bubbling is also a common feature. Once formed, pipes focus the gas ¯ow and inhibit initiation of new pipes, and bed expansion can reach up to 20%. In our experiments, pipes enlarge very easily even at decreasing ¯ow rate, and bed expansion is not observed. Fluid-escape pipes may form readily in natural water-saturated, ignimbrite-like deposits, provided that a source of ¯uidising water exists. Fluid-escape pipes and sheets have been recognized in a range of rapidly deposited water-lain sediments, including turbidites, debris-¯ow deposits, ¯uvial sands (Lowe and LoPiccolo, 1974; Lowe, 1975; Postma, 1983; Johnson, 1986), and also in lahar deposits (Crandell, 1971; Best, 1989, 1992. Possible mechanisms of pipe formation in natural sediments include channelling of escaping pore ¯uid during rapid sedimentation from a high-concentration aqueous mass ¯ow (Best, 1989; Druitt, 1995), or during re-sedimentation following liquefaction (Nichols, 1995). Pipes are not likely to form during transport because shear during ¯owage tends to inhibit channelling and pipe formation
279
(Richardson, 1971). In the present experiments, slow mixing of the sample during ¯uidization severely reduced pipe development. Pipes are most likely to initiate during the ®nal stages of sedimentation and compaction of a dense dispersion as the rate of horizontal shear approaches zero. The largest pipes might be expected to occur in the thickest deposits, and near the top where the ¯ux of escaping water is maintained longest (Best, 1989). In this context, ¯uid-escape pipes are expected to be found in lahar deposits as well as in ignimbrite-like deposits from pyroclastic ¯ows that have entered the sea and mixed with water (Cas and Wright, 1991). In the case of subaerial, water-supported mass-¯ow deposits such as those from lahars, the source of ¯uidising water may be limited and it may be for this reason that the waterescape pipes in these deposits are generally small and not very common (Crandell, 1971; Best, 1992). Large pipes may form best in aqueous environments, as suggested by some submarine and ¯uvial mass-¯ow deposits, where rapid loading and overpressuring of underlying water-logged sediment layers could cause vertical ¯uxes of water sustained over long periods (Lowe and LoPiccolo, 1974; Postma, 1983; Johnson, 1986). The rapid emplacement of thick mass-¯ow deposits formed when voluminous pyroclastic ¯ows enter, and mix with, the sea may also generate favourable conditions for sustained vertical ¯ux of water. In many cases, the pipes in our experiments were sheetlike (vertically) rather than cylindrical in form, even though there was no shear. Care should thus be taken in using elongate ¯uid-escape pipes as shear-sense indicators in natural sediments. Crystal enrichment factors (EF) generated in the present experiments using water (,1±22) are similar to those determined for gas-¯uidized ignimbrites in the ®eld. The average EF calculated by Walker (1972) in nine sub-aerial ignimbrites ranged from 3.5 to 14 for the main ignimbrite, and from 15 to 27 for associated lithic-rich layers and gas-escape pipes. The average EF calculated by Sparks and Walker (1977) for seven co-ignimbrite ash-fall deposits was 0.72. Water-¯uidized ignimbrites (e.g. lahars or subaqueous) and gas-¯uidized ignimbrites are hard to distinguish in the ®eld and this question is a matter of debate (Cas and Wright, 1991). Walker (1971, 1972) proposed that the presence of ¯uid-escape pipes could suggest ¯uidization by gas and could be
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used as evidence for a subaerial origin with no subsequent remobilization by water. However, observation of pipes in lahar deposits (Crandell, 1971; Best, 1989), in experiments using concentrated dispersions (Druitt, 1995), and their formation in the present experiments indicate that elutriation pipes may also form if subaerial ignimbrites become water-saturated (lahars) or if pyroclastic ¯ows enter an aqueous environment. Consequently, neither the existence of segregation pipes and layers, nor of high crystal-enrichment factors, are suf®cient evidence for a gas-¯uidized origin of an ignimbrite-like deposit, unless the pipes are rooted on carbonized vegetation. Acknowledgements Rob Nichols kindly provided advice on the construction of the ¯uidization rig. Reviews of the manuscript by Richard Fisher and Jocelyn McPhie, and review of an earlier version by Colin Wilson were helpful. References Best, J.L., 1989. Fluidization pipes in volcaniclastic mass ¯ows, Volcan Hudson, Southern Chile. Terra Nova 1, 203±208. Best, J.L., 1992. Sedimentology and event timing of a catastrophic volcaniclastic mass ¯ow, Volcan Hudson, Southern Chile. Bull. Volcanol. 54, 299±318. Cas, R.A.F., Wright, J.V., 1987. Volcanic successions, modern and ancient. Chapman & Hall, London, 528 pp. Cas, R.A.F., Wright, J.V., 1991. Subaqueous pyroclastic ¯ows and ignimbrites: an assessment. Bull. Volcanol. 53, 357±380. Crandell, D.R., 1971. Post-glacial lahars from Mount Rainier volcano, Washington. U.S. Geol. Surv. Prof. Pap. 677. Druitt, T.H., 1995. Settling behaviour of concentrated dispersions and some volcanological applications. J. Volcanol. Geotherm. Res. 65, 27±39. Folk, R.L., Ward, W.C., 1957. Brazos River bar: a study of the signi®cance of grain size parameters. J. Sedim. Petrol. 27, 3±26.
Johnson, S.Y., 1986. Water-escape structures in coarse-grained, volcaniclastic, ¯uvial deposits of the Ellensburg formation, South-Central Washington. J. Sedim. Petrol. 56, 905±910. Kunii, D., Levenspiel, O., 1991. Fluidization Engineering, 2nd ed. Butterworth, London. Lowe, D., 1975. Water escape structures in coarse-grained sediments. Sedimentology 22, 157±204. Lowe, D.R., LoPiccolo, R., 1974. The characteristics and origins of dish and pillars structures. J. Sedim. Petrol. 44, 484±501. Mandeville, C.W., Carey, S., Sigurdsson, H., King, J., 1994. Paleomagnetic evidence for high-temperature emplacement of the 1883 subaqueous pyroclastic ¯ows from Krakatau volcano, Indonesia. J. Geophys. Res. 99, 9487±9504. Mandeville, C.W., Carey, S., Sigurdsson, H., 1996. Sedimentology of the Krakatau 1883 submarine pyroclastic deposits. Bull. Volcanol. 57, 512±529. Nichols, R.J., 1995. The liqui®cation and remobilization of sandy sediments. In: Hartley, A.J., Prosser, D.J. (Eds.), Characterization of Deep Marine Clastic Systems. Geol. Soc. London Spec. Publ. 94, 63±76. Nichols, R.J., Sparks, R.S.J., Wilson, C.J.N., 1994. Experimental studies of the ¯uidization of layered sediments and formation of ¯uid-escape structures. Sedimentology 41, 233±253. Postma, G., 1983. Water escape structure in the context of a depositional model of a mass ¯ow dominated conglomeratic fan-delta (Abrioja Formation, Spain). Sedimentology 30, 91±103. Richardson, J.F., 1971. Incipient ¯uidization and particulate systems. In: Davidson, J.F., Harrison, D. (Eds.), Fluidization. Academic Press, London, pp. 25±64. Sparks, R.S.J., Walker, G.P.L., 1977. The signi®cance of vitricenriched air-fall ashes associated with crystal-enriched ignimbrites. J. Volcanol. Geotherm. Res. 2, 329±341. Stanley-Wood, N.G., Obata, E., Takahashi, H., Ando, K., 1990. Liquid ¯uidization curves. Powder Technol. 60, 61±70. Walker, G.P.L., 1971. Grainsize characteristics of pyroclastics deposits. J. Geol. 79, 696±714. Walker, G.P.L., 1972. Crystal concentration in ignimbrites. Contrib. Mineral. Petrol. 36, 135±146. Whitham, A.G., Sparks, R.S.J., 1986. Pumice. Bull. Volcanol. 48, 209±223. Wilson, C.J.N., 1980. The role of ¯uidization in the emplacement of pyroclastic ¯ows: an experimental approach. J. Volcanol. Geotherm. Res. 8, 231±249. Wilson, C.J.N., 1984. The role of ¯uidization in the emplacement of pyroclastic ¯ows, 2: experimental results and their interpretation. J. Volcanol. Geotherm. Res. 20, 55±84.
Experimental aqueous ¯uidization of ignimbrite O. Roche a,*, T.H. Druitt a, R.A.F. Cas a,b a
Laboratoire Magmas and Volcans (UMR 6524 and CNRS), Universite Blaise Pascal, 5 Rue Kessler, 63038 Clermont-Ferrand, France b Department of Earth Sciences, Monash University, Melbourne, Australia Received 26 June 2000; revised 14 June 2001; accepted 14 June 2001
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. q 2001 Elsevier Science B.V. All rights reserved. Keywords: ignimbrite; water ¯uidization; experiments; ¯uid-escape pipes; segregation
1. Introduction Fluidization is the process whereby the upward, focussed ¯ow of a ¯uid (gas or water) through a particulate aggregate can support and entrain grains. This may generate distinctive structures, grainsize sorting, * Corresponding author. Present address: Department of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK. Tel.: 144117-9545243; fax: 144-117-9253385. E-mail address: [email protected] (O. Roche).
and density segregation effects. Fluidization is used in many industrial systems, and may occur in some geological settings. In sedimentary and volcanic systems, ¯uidization process can occur during high rates of deposition of particulate dispersions, as grains fall to the depositional surface and displace ¯uid upwards. It can also occur in stationary deposited aggregates where these are ¯uidized by a ¯uid sourced from below, e.g. during dewatering and compaction leading to interstitial pore overpressures. Fluidization processes and effects have most commonly been
0377-0273/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0377-027 3(01)00246-3
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studied in aqueous terrigenous sediments (Lowe and LoPiccolo, 1974; Lowe, 1975; Postma, 1983; Johnson, 1986), and in gas-supported pyroclastic ¯ows and their deposits, ignimbrites (Walker, 1971; Wilson, 1980, 1984). Aqueous ¯uidization is potentially a signi®cant process in settings where the deposits of pyroclastic ¯ows rich in pumice and ash are remobilized by water, either subaerially (e.g. lahars) or under water, or where the ¯ows themselves enter the sea, as occurs in many modern and ancient maritime volcanic provinces (Cas and Wright, 1991). In this latter case, many of these ¯ows are known to have gone under water (e.g. Krakatau, Mandeville et al., 1994, 1996. Some may have retained initially a gassupported integrity. Others may have transformed by syn-eruptive and post-eruptive resedimentation into water-supported mass ¯ows (Cas and Wright, 1991), which, like their terrigenous sediment counterparts such as turbidity currents and debris ¯ows, are likely to have experienced varying degrees of syn-depositional and syn-compaction ¯uidization. Resulting structures in ignimbritic deposits should be preserved in the geological record, but few have yet been recorded (Crandell, 1971; Best, 1989, 1992), probably because few attempts have been made to look for them. The formation of aqueous ¯uidization features in such volcanic deposits has not been studied experimentally. In experiments, the relative motion of particles and ¯uid is accounted for by introducing a ¯uid ¯ux at the base of a ®xed bed. Experiments have been previously carried out in engineering aqueous and gas ¯uidization systems (Kunii and Levenspiel, 1991), and on aqueous ¯uidization processes in terrigenous and synthetic aggregates (Nichols et al., 1994; Nichols 1995). Gas ¯uidization processes have been studied in ignimbrite samples by Wilson (1980, 1984). In this study, we summarize new experiments on aqueous ¯uidization of ignimbrite samples, and we describe the structures generated as a guide to the characteristics likely to be found in the geological record. We focus on ¯uid-escape pipes generated by the upward migration of pore water through the ignimbrite. We show that pipes form very easily and rapidly in water-saturated ignimbrite, even at low rates of water ¯ux. Such pipes have been generated experimentally by ¯uidi-
zation of water-saturated, layered sediment (Nichols et al., 1994), by liquefaction and remobilization of sand (Nichols, 1995), and by sedimentation of poorly sorted concentrated dispersions (Druitt, 1995). The experiments reported here differ from those cited above in that the starting material is coarser and more poorly sorted, and it contains particles of two different densities. We also document both size- and density-related segregation effects, as ignimbrite contains particles of two discrete densities (pumice clasts and lithic clasts/crystals). It is important to note that we ¯uidized an already ®nes-depleted aerial ignimbrite, due to transport of the ¯ow in air. Distinguishing gas-¯uidized (subaerial) ignimbrites from water-¯uidized ignimbrite-derived sediments (such as lahar deposits or those deposited in aqueous settings) is a problem commonly encountered by volcanologists, and evidence for the origin of deposits and environments is often ambiguous (Cas and Wright, 1991). The aim of the paper is to compare the effects of water ¯uidization of ignimbrite with those of gas ¯uidization of similar material (Walker, 1971; Wilson, 1980, 1984). This is important as we seek criteria to distinguish between gas- and watersaturated ignimbrite. For this reason, we report segregation-layering processes, grain size distribution, component abundance and crystal-enrichment factors (Walker, 1972) in water-¯uidized samples, and we make comparisons with data reported for natural lahar deposits (Best, 1989, 1992) as well as those for experimentally gas-¯uidized ignimbrites (Wilson, 1980, 1984). 2. Aqueous ¯uidization Fluidization is the process whereby a bed of granular material is transformed into a ¯uid-like state by an upward ¯ow of ¯uid (Richardson, 1971; Kunii and Levenspiel, 1991). If the upward ¯uid drag force on the particles is suf®cient to support their weight, the bed is said to be completely ¯uidized. Complete ¯uidization is restricted to certain classes of granular materials. If only some of the particles are supported, the bed is said to be partially ¯uidized. We focus here on ¯uidization by water (Stanley-Wood et al., 1990). Two fundamentally different types of ¯uidization, particulate and aggregative, have been recognized in
O. Roche et al. / Journal of Volcanology and Geothermal Research 112 (2001) 267±280
269
and Levenspiel, 1991): #1=2 )21 ( " 18m rf 2:335 2 1:744c 2 Ut 1 rs 2 rf gd rs 2 rf gd 2 2
Fig. 1. Theoretical minimum ¯uidization velocity (Umf) and terminal fall velocity (Ut) for particles of two densities as function of particle size (spherical particles, emf 0:4).
¯uidized systems (Richardson, 1971; Wilson, 1980, 1984; Kunii and Levenspiel, 1991). Particulate (homogeneous) ¯uidization occurs in granular materials that are monodisperse (single particle size) or weakly polydisperse (multiple particle sizes) and with a small density contrast between the different particles. At low water ¯ow rates, the bed is said to be ®xed and the system is governed by Darcy's law. With increasing ¯ow rate, the bed expands smoothly and instability-generated heterogeneities are negligible. The entire bed is ¯uidized when the minimum ¯uidization velocity Umf is reached. Umf is given by the Ergun equation (Kunii and Levenspiel, 1991): 1:75 3 ce mf
dUmf rf m
2
150 1 2 emf dUmf rf 1 m c 2 e3mf
d3 rf rs 2 rf g m2
1
where r s is particle density, r f the ¯uid density, m the ¯uid viscosity, d is particle diameter, c the particle sphericity, and e mf is the voidage at minimum ¯uidization. As velocity is increased above Umf, there is uniform expansion of the dispersion until, at U U t (terminal fall velocity of the particles), the particles are fully entrained and elutriated from the system. Terminal velocity is calculated readily from (Kunii
Fig. 1 shows curves for Umf and Ut as a function of grain size in the range of interest. The curves are for spherical particles c 1; an e mf value of 0.4, and two particle densities (2650 and 1090 kg m 23) that correspond to lithic fragments and water-saturated pumice clasts respectively in our experiments (see Section 3). The ratio U t =Umf varies widely across the range of grain sizes shown, from ,80 at d , 64 mm (.4 f ) to ,10 at d . 8 mm (, 2 3 f ). Complete ¯uidization is possible for a polydisperse material only if Umf of the coarsest component is less than Ut of the ®nest. For the parameters speci®ed above, this corresponds to materials with a total grainsize range between 3 and 5 f for particles of same density, and materials with a total grain-size range between 1 and 6 f for particles of the different densities considered here. For particles of the same density, this is equivalent to sorting of 0.6±1.0 if the grain-size distribution is approximately log-normal. Many ignimbrites, including that studied here, have sorting values higher than 1 (Cas and Wright, 1987), showing that complete ¯uidization by water is not possible. A similar conclusion was reached by Wilson (1980, 1984) in the case of gas ¯uidization of hot pyroclasts during transport and deposition. In aggregative ¯uidization, the formation and growth of instabilities generates heterogeneities. In most gas±solid systems, either monodisperse or polydisperse, once U is increased above Umb ($Umf), the excess gas passes through the bed as bubbles, and the surrounding particulate material remains poorly expanded (Richardson, 1971; Wilson, 1980, 1984; Kunii and Levenspiel, 1991). Bubbling is rare in liquid±solid systems, and occurs only at very high ¯uid velocity and if particles are very dense like tungsten or lead (Richardson, 1971). A particular type of instability in aggregative ¯uidization results in channelling of the ¯uid phase and in the formation of vertical pipes. Pipe formation occurs preferentially in polydisperse systems and in dispersions of small grain size and high grain density. In some well sorted dispersions, pipes appear throughout the bed at low
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Table 1 Grain-size and component composition (wt%) of the ignimbrite sample. Grain-size analyses were carried out by sieving ($5 f ) or with a Coulter Multisizer (,5 f ). Pumice, lithic, and crystal components in each 1 f size fraction of 5 f or larger were counted with eye or under binocular and converted to weight percent. Proportions of components for fraction smaller than 5 f are assumed to be the same as for f 5
f 23 22 21 0 1 2 3 4 5 6 7 8 9 10 Total
Pumice 0.9 1.8 2.4 2.4 5.7 4.7 4.5 4.5 9.6 5.5 5.7 2.6 0.6 0.1 51
Crystals
Lithic
± ± 0.1 0.7 2.2 2.1 4.6 5.2 3.1 1.8 1.8 0.8 0.2 ±
± 1 2.2 4.1 2.5 4.1 5.6 5.2 0.7 0.4 0.4 0.2 ± ±
22.6
26.4
Total 0.9 2.8 4.7 7.2 10.4 10.9 14.7 14.9 13.4 7.7 7.9 3.6 0.8 0.1 100
velocities, but disappear at velocities above Umf (Richardson, 1971). In highly polydisperse systems, the pipes may be very stable because elutriation of ®ne components within the pipe increases local permeability and enhances channelling. It is common that particles of different sizes and densities segregate from each other during the ¯uidization of polydisperse materials. Segregation is most effective if the contrasts in size Umf coarse=Umf fine . 2 and density are high, and occurred readily in the experiments on poorly sorted ignimbrite reported here. 3. Experimental methods The experiments used sub-16 mm samples of 0.58 Ma non-welded trachytic ignimbrite collected at Neschers, 30 km south of Clermont-Ferrand in the French Massif Central. The grain-size distribution and componentry of the ignimbrite are shown in Table 1. The sample has the coarse grain size and poor sorting typical of many ignimbrites (Walker, 1971). The graphic mean Mz and inclusive graphic standard deviation s I (a measure of sorting) (Folk and Ward,
1957) are 2.8 and 2.7 f , respectively. The principal crystals in the ignimbrite are plagioclase, quartz, amphibole, biotite, and oxides. Biotite crystals are typically platy in shape and consequently have low terminal fall velocities in water, as do shards and pumice fragments. The samples were saturated with water under vacuum prior to each experiment. Whitham and Sparks (1986) have shown that hot pumice clasts immersed in water rapidly become water-logged. This is because the hot gas in the pores contracts upon cooling, sucking in the water. In order to simulate the properties of hot pumiceous ignimbrite which had come into contact with water, the pre-dried cold sample was saturated with water under vacuum. Density measurements on twelve representative pumice lapilli and blocks (5±10 cm in diameter), with vesicularities in the range 77±86% showed that 80 ^ 15% of the vesicles were ®lled by water, giving a bulk wet pumice density of 1090 ^ 90 kg m 23 : The starting thickness of the sample bed was always between 11 and 18 cm (Fig. 2). The pressure drop across the bed was measured using two sidemounted PDCR 42 pressure transducers with a range of 0±7500 Pa and precision of ^1 Pa. Tests showed that ¯ow was laminar and uniform under all experimental conditions. Particles elutriated from the sample were collected for grain-size analysis. Tests using monodisperse and weakly polydisperse beds of silicon carbide and glass beads gave reproducible results. Ten experiments, each with a new sample from the same ignimbrite, were carried out at ¯ow velocities in the range 0.005±7 mm s 21 for durations between 10 and 180 min (Table 2). After seven of the experiment (4±10; Table 2), the sample was extruded from the rig, frozen, and serially sectioned to examine internal structures.
4. Experimental results 4.1. General observations Complete ¯uidization was not expected in the ignimbrite sample, because the sorting s I 2:7 was greater than the critical value of ,1 (Fig. 1). In fact, incipient, partial ¯uidization was found dif®cult
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271
Fig. 2. (a) Experimental apparatus. (b) Enlargement of the ¯uidization rig. The cylindrical rig, 14 cm in diameter, was fed by a water supply from a header tank. The hydrostatic head was kept constant by means of a stopcock, thus permitting a given ¯ow rate to be maintained over long periods. Inside the rig, the water ¯ow was ®rst de¯ected radially from the outlet value by a horizontal plate, then rendered laminar by a 10 cm thick layer of packed drinking straws. Uniform ¯ow across the rig was generated by a 2 cm thick layer of 2 mm lead shot, supported by a metal grid, overlain by a sheet of 32 mm nylon mesh on which rested the sample.
to achieve under the experimental conditions. Even at velocities as low as 0.3 mm s 21, and in some cases as soon as the experiment commenced, one or more pipes appeared in the bed and ¯ow became strongly channelled. In many cases, the pipes were more sheetlike in form than cylindrical, but we use the term pipe for convenience. The pipes were not edge effects, but formed throughout the sample and tended to grow and merge laterally with increasing ¯uid velocity, and with time if the velocity was held constant. The tendency for pipe formation was reduced if the sample was gently mixed immediately prior to an experiment
(Richardson, 1971). In experiments at high ¯ow rates (Experiments 6, 7 and 8; Table 2), the pipes merged completely. Fine components discharged from the pipes either accumulated on the surface of the bed as a ¯uidized scum, or were elutriated from the rig. The weight percentage of the initial bed elutriated from the rig was 25% in Experiment 6 (2 mm s 21) and 30% in Experiment 7 (3 mm s 21). Pipes also formed in the bed due to sedimentation effects, without introduction of water from below. Prior to each experiment, the loosely packed dispersion was mixed very slowly by hand to induce
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Table 2 Experiments carried out during this study Experiment
Duration (min)
U (mm s 21)
Aim of experiment
1 2 3 4 5 6 7 8 9 10
50 80 50 10 10 10 10 10 80 180
0±3 0±3 0±3 0.5 1 2 3 7 1 0.5
DP=U graph at increasing±decreasing U DP=U graph at increasing±decreasing U DP=U graph at decreasing U Internal structure Internal structure Internal structure and granulometry Internal structure and granulometry Internal structure and granulometry Internal structure Internal structure
homogenization. Subsequent settling caused expulsion of small amounts of pore water which formed subvertical channels. The pipes formed in this way were much smaller (mm-sized) than those generated once ¯uidization began because the vertical water ¯ux lasted only a few seconds during sedimentation. It is possible that these incipient pipes acted as nucleii for larger ones once ¯uidization began. 4.2. Pressure curves and pipe growth The ¯uidization behaviour of a material is conventionally recorded in terms of the pressure drop DP across the bed as a function of super®cial ¯uid velocity U, calculated from the volumetric ¯ow rate assuming that the container is empty (Richardson, 1971; Wilson, 1980, 1984; Kunii and Levenspiel, 1991). The pressure across the ®xed bed at zero ¯ow is hydrostatic because the particles are selfsupporting. As the ¯ow is increased U , Umf ; DP rises, until at Umf there is a break in slope of the pressure curve e emf : Above Umf, the bed expands (e increases), DP remains approximately constant in particulate ¯uidization although it can decrease rapidly in aggregative ¯uidization, due to ¯uid channelling caused by pipes. In this paper, the theoretical curve (DP versus U) does not often match the curves that depict the ¯uidization behaviour of the chosen material. The DP versus U curves for experiments 1±3 are shown in Fig. 3. In experiments 1 and 2, ¯ow velocity was increased from 0 to 3 mm s 21, then decreased again, over periods of 50 (Experiment 1) and 80 (Experiment 2) min. Umf cannot be de®ned in these
experiments because the ¯uidization behaviour is very heterogeneous and pressure transducers give only a local value of DP. In experiment 1 (Fig. 3a), DP increased up to a velocity of 0.3 mm s 21 (region A), when several small pipes nucleated. The initial DP increase suggests that some ¯uidization of the ®nest components occurred. Even at such small ¯ow rates, ®ne particles were being elutriated from the rig. Initiation of a small pipe at 0.3 mm s 21 resulted in strongly channelled ¯ow and effectively stalled ¯uidization. Shortly after, one small pipe on the side of the container diametrically opposite to the pressure transducers increased dramatically in size (DP maximum). As a result, the ¯ow of water was abruptly channelled into that pipe and away from the transducers, and a drop in DP was registered (region B). The DP decrease continued as ¯ow rate was further increased. Since no further pipes formed, the system approached a steady state with strong partitioning of ¯ow through the one main pipe. On decreasing the ¯ow, the water ¯ux through the pipe diminished, but DP stayed constant. In experiment 2 (Fig. 3b), pipes appeared immediately at the lowest ¯ow velocities possible with the apparatus (0.005 mm s 21). One of these formed next to the lowermost pressure transducer, causing an abrupt increase in DP above hydrostatic as soon as the experiment began. When the water ¯ux is concentrated in a pipe adjacent to the basal transducer, the measured DP increases because the ¯ow velocity is locally high. If, on the other hand, ¯ow is diverted away from the transducer, DP drops, as in experiment 1. In experiment 2, DP varied in a saw-tooth manner as the pipe network evolved, sometimes diverting
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¯ow towards the transducer, sometimes away. As the ¯ow rate was decreased, DP dropped in a smooth fashion, without abrupt changes. In experiment 3 (Fig. 3c) the ¯ow was increased rapidly to 3 mm s 21 then decreased steadily over 50 min to assess the effect of decreasing ¯ow conditions on pipe development. The initial DP at high ¯ow rate was small because water was channelled strongly away from the transducers through a pipe on the opposite side of the rig. With decreasing ¯ow, two sudden increases in DP occurred, the ®rst when a pipe
Fig. 3. Variation of pressure drop (DP) and bed thickness (H) with ¯ow velocity U in three experiments. 1 hPa is 100 Pa. See text for discussion of details and Table 2 for experimental conditions. In experiment 1 (a) and experiment 2 (b) the ¯ow velocity was ®rst increased slowly, then decreased slowly. In experiment 3 (c) the ¯ow velocity was increased rapidly to 3 mm s 21, then decreased slowly.
Fig. 4. Experimental samples frozen, then sawn, following ¯uidization. (a) Experiment 5. Upward ¯aring pipes (p) separated by regions of unmodi®ed ignimbrite (i, also analysed). In each pipe, the abundance of dense lithic fragments increases towards the base, and that of pumice and ®ne components towards the top. (b) Experiment 7. The pipes have amalgamated so that the entire sample is segregation-layered. Three layers are observed: lithic-rich (1), pumice-rich (2) and ®nes-rich (3).
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nucleated next to the basal transducer, and the second when that pipe suddenly grew in width. These data show the rather surprising feature that pipes nucleate and grow even under decreasing ¯ow conditions. This is not likely to be an artefact of the water distributor system, as tests showed that ¯ow was uniform. In all three experiments, the thickness of the bed dropped as the ¯ow velocity either increased or decreased. This is explained by the progressive elutriation of ®nes combined with the settling of larger particles as they re-orient and compact. The increase in thickness at moderate (increasing) ¯ow rates in experiment 2 (Fig. 3b) was due to the formation of a capping layer of ®nes discharged from newly formed pipes. 4.3. Internal structure of the samples Experiments 4±10 were carried out to document the internal structures of samples that had been ¯uidized at different velocities and for different durations (Table 2). At velocities of 0.5±1 mm s 21 for 10 min (Experiments 4 and 5), the resulting sample consisted of two discrete phases (Fig. 4a): unmodi®ed starting material (analyses made), and pipes in which the sample had been sorted by focused ¯uid ¯ow. Each pipe had the shape of an upward ¯aring funnel, and pipes were more sheet-like in form than cylindrical (Fig. 5). The sample within each pipe showed segregation layering. The base was enriched in dense lithic and crystal components and depleted in ®nes, the central part was enriched in pumice lapilli and the top was enriched in ®nes (Figs. 4a and 5). Note that these were not ®nes that were elutriated from the bed, then settled back once the ¯uid ¯ow ceased, but represented material that migrated to the top of the pipe during ¯uidization. In experiment 5, the ¯ared tops of several pipes merged, so that the pumice-rich and ®nes-rich layers occupied the entire upper half of the sample (Fig. 4a). If the same ¯ow rates were maintained (80±180 min; Experiment 9 and 10), the pipes were signi®cantly wider, but unmodi®ed ignimbrite still remained at the base of the sample (Fig. 5). Fig. 5. Serial sections showing the form of a pipe from experiment 10. The pipe has an elongated sheet-like form in three dimensions. Dense lithic and crystals (black) are concentrated at the base and pumice (white) and ®ne components (dots) at the top.
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Fig. 6. Grain size and componentry of ¯uidized samples from experiments 6, 7, and 8. See Table 1 for comparisons and Table 2 for the experimental conditions. Component proportions were not determined for f . 5: In experiment 6, the composition is that of a single large pipe. In 7 and 8, the entire samples were segregation-layered. Lithic-rich: base enriched in dense lithic and crystals; pumice-rich: middle enriched in pumice; ®nes-rich: top enriched in ®ne components; elutriated: ®ne ash entrained by the ¯ow of water and carried out of the bed.
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At higher ¯ow velocities (2, 3, and 7 mm s 21; Experiments 6±8), pipe growth was so rapid that the pipes merged completely during the 10-min duration runs, such that either a single large pipe formed (Experiment 6) or the entire sample became segregation layered (Experiments 7 and 8) (Fig. 4b). To summarize, each pipe expanded with time (under either increasing or decreasing ¯ow conditions) downwards and sideways into the unmodi®ed ignimbrite. Within each pipe, the high local ¯uid ¯ux probably caused partial ¯uidization and the migration of dense components to the base, and light and/or ®ne components to the top. At suf®ciently high ¯ow rates or durations, the pipes coalesced and the layering extended through the whole sample. 4.4. Granulometry and component analysis Fig. 6 shows grain-size histograms for the lithicrich, pumice-rich, ®nes-rich, and elutriated fractions of samples after ¯uidization for 10 min at velocities of 2, 3, and 7 mm s 21 (Experiments 6±8). In experiments 7 and 8, the entire sample became segregationlayered, and so the stated velocities are those `seen' by the whole sample. In experiment 8, the ®nes-rich layer was very thin and impossible to sample. In experiment 6, the analyses are of different layers within a single large pipe and the actual ¯uid velocity experienced by the segregated material slightly exceeded 2 mm s 21. Each bed is normally graded both in particle size and density, with the ®nest and lowest density components concentrated upwards. Basal layers are uniformly enriched in dense lithic fragments and depleted of ®nes compared with the starting ignimbrite (Table 1 and Fig. 6). The percentage of ,63 mm (3 f ) ash decreases progressively with increasing ¯uid velocity in the basal layers. The crystal content also increases in the same sense. Pumicerich layers show strong enrichment in $2 mm pumice, resulting in a weak bimodality. The ®nesrich top layers in experiments 6 and 7 are strongly enriched in vitric ash, but have a coarse tail of pumice lapilli up to 8 mm (23 f ). Elutriated fractions are in each case well sorted and enriched in vitric ash. At 2 mm s 21, there is a sharp cut-off in grain-size at 125 mm (4 f ); at higher velocities, a small percentage of 125±250 mm fraction
appears as well as a small amount of pumice. Crystals present in the elutriated fractions are mostly platy biotite. In Fig. 7 the results are shown as cumulative curves and on a plot of s I versus Mz (Folk and Ward, 1957). In all cases, the three segregation layers and elutriated fraction are better sorted than the initial ignimbrite. With increasing ¯ow rate, the lithic-rich and pumicerich layers became better sorted as more and more ®ne particles were removed progressively from them. The ®nes-rich layer and elutriated fraction became correspondingly poorer sorted. In each experiment, the lithic-rich and pumice-rich layers are coarser than the starting ignimbrite, whereas the ®nes-rich layer and elutriated ash are ®ner. In general, Mz of the lithic-rich and pumice-rich layers are insensitive to ¯ow velocity and decrease by only 1 f from 2 to 7 mm s 21. The lowest value of s I attained by the lithic-rich layers was 1.6 f , which is comparable to that of lithic-rich segregations generated by gas ¯uidization of ignimbrite (Wilson, 1984). Fig. 8 shows the abundance of particles smaller than 63 mm relative to the initial ignimbrite. The lithic-rich layers are depleted in ®nes by as much as ten-fold. The ®nes-rich layers have about the same proportions than the ignimbrite. The elutriated fraction is enriched in ®nes. We have used the crystal concentration method of Walker (1972) to quantify the upward migration of ®nes during ¯uidization, and to aid comparison with segregation structures in gas-¯uidized ignimbrite. Walker (1972) de®ned a crystal enrichment factor, EF: EF
C 2 P1 P2 C1
3
where C2 =P2 is the mass ratio of free crystals to pumice in the ,2 mm fraction of the sample, and C1 =P1 is the same for arti®cially crushed pumice clasts. Glass shards and crystals are hard to distinguish in size fractions less than 32 mm f 5; especially when feldspar is present. In carrying out the component analyses to calculate EF, we considered two alternative assumptions: (1) that the C=P ratio for f . 5 is equal to that for f 5: This yields a maximum value of C=P for the whole sample. (2) C=P is zero for f . 5; which gives a minimum C=P estimate for the whole sample.
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Fig. 7. (a), (b), (c). Cumulative grain-size curves of ¯uidized samples from experiment 6, 7, and 8. See Table 2 for the experimental conditions. L: lithic-rich and crystal-rich layer; P: pumice-rich; F: ®nes-rich; E: elutriated fraction. The grain size composition of the starting ignimbrite is shown by a heavy line in each ®gure. (d): Mz ± s I diagram for the different layers generated in experiments 6, 7 and 8. The numbers are the ¯ow velocity in mm s 21. The graphic mean Mz and inclusive graphic standard deviation s I (a measure of sorting; Folk and Ward, 1957) are 2.8 f and 2.7 f , respectively, where M z f16 1 f50 1 f84 =3 and s I f84 2 f16 =4 1 f95 2 f5 =6:6:
EF values for the starting ignimbrite and experimental samples are listed in Table 3. The average C1 =P1 for three large pumice clasts (5±10 cm in diameter) from the Neschers Ignimbrite is 0:18 ^ 0:02; corresponding to a phenocryst content of 15 ^ 1wt%: The EF (EFign) is 4:0 ^ 1:8; and is typical of many ignimbrites (Walker, 1972; Sparks and Walker, 1977). EF for ¯uidized samples that are greater than
EFign indicate depletion of vitric ash (and enrichment in crystals), whereas values that are less than EFign indicate the opposite. In each experimental sample, EF decreases progressively from base to top, re¯ecting concentration of ®nes upwards in the bed. The basal, lithic-rich samples, and in two cases the pumice-rich layers, have all lost vitric ash compared to the original ignimbrite. There is a progressive
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Fig. 8. Relative abundance of ,63 mm particles relative to the initial ignimbrite in various fractions from experiments 6, 7, and 8. The numbers are the ¯ow velocity in mm s 21.
increase in average EF from 5 to 22 with increasing ¯uid velocity in the lithic-rich layers. The ®nes-rich layers and elutriated fraction in all three experiments have been enriched in vitric ash. 5. Discussions In order to assess if ¯uidization characteristics of ignimbrite-like deposits can be used to distinguish those formed under aqueous conditions from those formed under gaseous conditions, we now compare the experimental behaviour of ignimbrite ¯uidized by water with that of ignimbrite ¯uidized by gas
(Wilson, 1980, 1984. Our experiments have revealed several similarities of the water- and gas-¯uidized systems. The poor sorting, abundance of ®ne components, and large range of particle densities in watersaturated ignimbrite all favour aggregative behaviour, ¯uid channelling, and ef®cient particle segregation, as in gas-¯uidized counterparts. The present experiments show that ¯ow instabilities and pipe production commence at very low ¯uid velocities (0.005 mm s 21 or less) in this poorly permeable material. This occurs because focusing of ¯uid ¯ow locally increases sediment permeability, leading to a positive feedback effect. The pipes thus generated are upward-¯aring funnels that grow larger (both sideways and downwards) and coalesce with time, even with decreasing ¯ow velocity. Focusing of ¯uid through each pipe results in local ¯ow velocities that exceed the calculated average over the whole container. This results in partial ¯uidization of a signi®cant fraction of the particles in the pipes and vertical sorting by size and density. Dense components (lithic lapilli plus crystals) concentrate towards the base of each pipe, ®ne fractions (ash) at the top, and pumice lapilli at intermediate levels. The ®nest particles are entrained by the ¯uid ¯ow and elutriated from the bed. At high ¯ow velocities, single pipes coalesce, the entire bed becomes segregationlayered, and each layer is better sorted than the starting ignimbrite. In the present experiments this occurred at ¯uid velocities of $3 mm s 21 sustained for 10 min (Experiment 7 and 8), but not at 0.5 mm s 21 for 180 min (Experiment 10). The amount
Table 3 Crystal enrichment data for the initial ignimbrite and for experimentally ¯uidized samples Experiment
U (mm s 21)
Sample
C2 =P2 max
C2 =P2 min
EF
6 6 6 6 7 7 7 7 8 8 8
2 2 2 2 3 3 3 3 7 7 7
Initial ignimbrite Elutriated Fines-rich layer Pumice-rich layer Lithic-rich layer Elutriated Fines-rich layer Pumice-rich layer Lithic-rich layer Elutriated Pumice-rich layer Lithic-rich layer
0.87 0.26 0.19 0.45 0.92 0.49 0.47 1.13 1.62 0.36 1.02 3.87
0.45 0.03 0.12 0.39 0.78 0.08 0.29 0.99 1.52 0.09 0.98 3.79
4:0 ^ 1:8 0:9 ^ 0:8 0:9 ^ 0:3 2:5 ^ 1:0 5:0 ^ 1:1 1:8 ^ 1:4 2:3 ^ 0:9 6:3 ^ 1:3 9:2 ^ 1:6 1:4 ^ 1:0 5:9 ^ 0:9 22:4 ^ 3:4
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of ash elutriated increased with increasing ¯uid velocity. It also increased with time in a given experiment, even under decreasing ¯ow conditions. Pressure measurements are not useful in ¯uidization studies of coarse-grained, poorly sorted samples because the transducers record only the local ¯uidization state and curves of DP±U are irregular due to continuous pipe nucleation and growth (e.g. Wilson, 1980). As in the present experiments, gas-¯uidized ignimbrite exhibits aggregative behaviour with the general characteristics listed above (Wilson, 1980, 1984). Pipes form readily; they are depleted in pumice lapilli and ®ne ash, and enriched in lithic lapilli and crystals. Fines-rich and pumice-rich layers commonly form at the tops of the bed. At high gas velocities, the entire bed becomes segregated into distinct layers. In both gaseous and aqueous systems, the sorting of the segregated material may approach values of 1.5 or less. However, gas-¯uidized systems show some differences from the water-¯uidized systems reported here. In the gaseous system, pipes appear only at high ¯uid velocity, if U . Ump (,1±2 cm s 21) as de®ned by Wilson (1980). They commonly broaden downwards, and layering tends to form by sedimentation of the particles within the pipes under gravity rather than by their coalescence. Bubbling is also a common feature. Once formed, pipes focus the gas ¯ow and inhibit initiation of new pipes, and bed expansion can reach up to 20%. In our experiments, pipes enlarge very easily even at decreasing ¯ow rate, and bed expansion is not observed. Fluid-escape pipes may form readily in natural water-saturated, ignimbrite-like deposits, provided that a source of ¯uidising water exists. Fluid-escape pipes and sheets have been recognized in a range of rapidly deposited water-lain sediments, including turbidites, debris-¯ow deposits, ¯uvial sands (Lowe and LoPiccolo, 1974; Lowe, 1975; Postma, 1983; Johnson, 1986), and also in lahar deposits (Crandell, 1971; Best, 1989, 1992. Possible mechanisms of pipe formation in natural sediments include channelling of escaping pore ¯uid during rapid sedimentation from a high-concentration aqueous mass ¯ow (Best, 1989; Druitt, 1995), or during re-sedimentation following liquefaction (Nichols, 1995). Pipes are not likely to form during transport because shear during ¯owage tends to inhibit channelling and pipe formation
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(Richardson, 1971). In the present experiments, slow mixing of the sample during ¯uidization severely reduced pipe development. Pipes are most likely to initiate during the ®nal stages of sedimentation and compaction of a dense dispersion as the rate of horizontal shear approaches zero. The largest pipes might be expected to occur in the thickest deposits, and near the top where the ¯ux of escaping water is maintained longest (Best, 1989). In this context, ¯uid-escape pipes are expected to be found in lahar deposits as well as in ignimbrite-like deposits from pyroclastic ¯ows that have entered the sea and mixed with water (Cas and Wright, 1991). In the case of subaerial, water-supported mass-¯ow deposits such as those from lahars, the source of ¯uidising water may be limited and it may be for this reason that the waterescape pipes in these deposits are generally small and not very common (Crandell, 1971; Best, 1992). Large pipes may form best in aqueous environments, as suggested by some submarine and ¯uvial mass-¯ow deposits, where rapid loading and overpressuring of underlying water-logged sediment layers could cause vertical ¯uxes of water sustained over long periods (Lowe and LoPiccolo, 1974; Postma, 1983; Johnson, 1986). The rapid emplacement of thick mass-¯ow deposits formed when voluminous pyroclastic ¯ows enter, and mix with, the sea may also generate favourable conditions for sustained vertical ¯ux of water. In many cases, the pipes in our experiments were sheetlike (vertically) rather than cylindrical in form, even though there was no shear. Care should thus be taken in using elongate ¯uid-escape pipes as shear-sense indicators in natural sediments. Crystal enrichment factors (EF) generated in the present experiments using water (,1±22) are similar to those determined for gas-¯uidized ignimbrites in the ®eld. The average EF calculated by Walker (1972) in nine sub-aerial ignimbrites ranged from 3.5 to 14 for the main ignimbrite, and from 15 to 27 for associated lithic-rich layers and gas-escape pipes. The average EF calculated by Sparks and Walker (1977) for seven co-ignimbrite ash-fall deposits was 0.72. Water-¯uidized ignimbrites (e.g. lahars or subaqueous) and gas-¯uidized ignimbrites are hard to distinguish in the ®eld and this question is a matter of debate (Cas and Wright, 1991). Walker (1971, 1972) proposed that the presence of ¯uid-escape pipes could suggest ¯uidization by gas and could be
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used as evidence for a subaerial origin with no subsequent remobilization by water. However, observation of pipes in lahar deposits (Crandell, 1971; Best, 1989), in experiments using concentrated dispersions (Druitt, 1995), and their formation in the present experiments indicate that elutriation pipes may also form if subaerial ignimbrites become water-saturated (lahars) or if pyroclastic ¯ows enter an aqueous environment. Consequently, neither the existence of segregation pipes and layers, nor of high crystal-enrichment factors, are suf®cient evidence for a gas-¯uidized origin of an ignimbrite-like deposit, unless the pipes are rooted on carbonized vegetation. Acknowledgements Rob Nichols kindly provided advice on the construction of the ¯uidization rig. Reviews of the manuscript by Richard Fisher and Jocelyn McPhie, and review of an earlier version by Colin Wilson were helpful. References Best, J.L., 1989. Fluidization pipes in volcaniclastic mass ¯ows, Volcan Hudson, Southern Chile. Terra Nova 1, 203±208. Best, J.L., 1992. Sedimentology and event timing of a catastrophic volcaniclastic mass ¯ow, Volcan Hudson, Southern Chile. Bull. Volcanol. 54, 299±318. Cas, R.A.F., Wright, J.V., 1987. Volcanic successions, modern and ancient. Chapman & Hall, London, 528 pp. Cas, R.A.F., Wright, J.V., 1991. Subaqueous pyroclastic ¯ows and ignimbrites: an assessment. Bull. Volcanol. 53, 357±380. Crandell, D.R., 1971. Post-glacial lahars from Mount Rainier volcano, Washington. U.S. Geol. Surv. Prof. Pap. 677. Druitt, T.H., 1995. Settling behaviour of concentrated dispersions and some volcanological applications. J. Volcanol. Geotherm. Res. 65, 27±39. Folk, R.L., Ward, W.C., 1957. Brazos River bar: a study of the signi®cance of grain size parameters. J. Sedim. Petrol. 27, 3±26.
Johnson, S.Y., 1986. Water-escape structures in coarse-grained, volcaniclastic, ¯uvial deposits of the Ellensburg formation, South-Central Washington. J. Sedim. Petrol. 56, 905±910. Kunii, D., Levenspiel, O., 1991. Fluidization Engineering, 2nd ed. Butterworth, London. Lowe, D., 1975. Water escape structures in coarse-grained sediments. Sedimentology 22, 157±204. Lowe, D.R., LoPiccolo, R., 1974. The characteristics and origins of dish and pillars structures. J. Sedim. Petrol. 44, 484±501. Mandeville, C.W., Carey, S., Sigurdsson, H., King, J., 1994. Paleomagnetic evidence for high-temperature emplacement of the 1883 subaqueous pyroclastic ¯ows from Krakatau volcano, Indonesia. J. Geophys. Res. 99, 9487±9504. Mandeville, C.W., Carey, S., Sigurdsson, H., 1996. Sedimentology of the Krakatau 1883 submarine pyroclastic deposits. Bull. Volcanol. 57, 512±529. Nichols, R.J., 1995. The liqui®cation and remobilization of sandy sediments. In: Hartley, A.J., Prosser, D.J. (Eds.), Characterization of Deep Marine Clastic Systems. Geol. Soc. London Spec. Publ. 94, 63±76. Nichols, R.J., Sparks, R.S.J., Wilson, C.J.N., 1994. Experimental studies of the ¯uidization of layered sediments and formation of ¯uid-escape structures. Sedimentology 41, 233±253. Postma, G., 1983. Water escape structure in the context of a depositional model of a mass ¯ow dominated conglomeratic fan-delta (Abrioja Formation, Spain). Sedimentology 30, 91±103. Richardson, J.F., 1971. Incipient ¯uidization and particulate systems. In: Davidson, J.F., Harrison, D. (Eds.), Fluidization. Academic Press, London, pp. 25±64. Sparks, R.S.J., Walker, G.P.L., 1977. The signi®cance of vitricenriched air-fall ashes associated with crystal-enriched ignimbrites. J. Volcanol. Geotherm. Res. 2, 329±341. Stanley-Wood, N.G., Obata, E., Takahashi, H., Ando, K., 1990. Liquid ¯uidization curves. Powder Technol. 60, 61±70. Walker, G.P.L., 1971. Grainsize characteristics of pyroclastics deposits. J. Geol. 79, 696±714. Walker, G.P.L., 1972. Crystal concentration in ignimbrites. Contrib. Mineral. Petrol. 36, 135±146. Whitham, A.G., Sparks, R.S.J., 1986. Pumice. Bull. Volcanol. 48, 209±223. Wilson, C.J.N., 1980. The role of ¯uidization in the emplacement of pyroclastic ¯ows: an experimental approach. J. Volcanol. Geotherm. Res. 8, 231±249. Wilson, C.J.N., 1984. The role of ¯uidization in the emplacement of pyroclastic ¯ows, 2: experimental results and their interpretation. J. Volcanol. Geotherm. Res. 20, 55±84.