Investigation of variable aeration of monodisperse mixtures: implications for pyroclastic density currents

The high mobility of dense pyroclastic density currents (PDCs) is commonly attributed to high gas pore pressures. However, the influence of spatial and temporal variations in pore pressure within PDCs has yet to be investigated. Theory suggests that variability in the fluidisation and aeration of a current will have a significant control on PDC flow and deposition. In this study, the effect of spatially heterogeneous gas pore pressures in experimental PDCs was investigated. Sustained, unsteady granular currents were released into a flume channel where the injection of gas through the channel base was controlled to create spatial variations in aeration. Maximum current front velocity results from high degrees of aeration proximal to the source, rather than lower sustained aeration along the whole flume channel. However, moderate aeration (i.e. ~ 0.5 minimum static fluidisation velocity (Umf_st)) sustained throughout the propagation length of a current results in greater runout distances than currents which are closer to fluidisation (i.e. 0.9 Umf_st) near to source, then de-aerating distally. Additionally, although all aerated currents are sensitive to channel base slope angle, the runout distance of those currents where aeration is sustained throughout their lengths increases by up to 54% with an increase of slope from 2° to 4°. Deposit morphologies a primarily controlled by the spatial differences in aeration, where there is a large decrease in aeration the current forms a thick depositional wedge. Sustained gas-aerated granular currents are observed to be spontaneously unsteady, with internal sediment waves travelling at different velocities.

Related papers:

title: Pyroclastic flows from the 1991 eruption of Unzen volcano, Japan
abstract: Pyroclastic flows from Unzen were generated by gravitational collapse of the growing lava dome. As soon as the parental lobe failed at the edge of the dome, spontaneous shattering of lava occurred and induced a gravity flow of blocks and finer debris. The flows had a overhanging, tongue-like head and cone- or rollershaped vortices expanding outward and upward. Most of the flows traveled from 1 to 3 km, but some flows reached more than 4 km, burning houses and killing people in the evacuated zone of Kita-kamikoba on the eastern foot of the volcano. The velocities of the flows ranged from 15 to 25 m/s on the gentle middle flank. Observations of the flows and their deposits suggest that they consisted of a dense basal avalanche and an overlying turbulent ash cloud. The basal avalanche swept down a topographic low and formed to tongue-like lobe having well-defined levees; it is presumed to have moved as a non-Newtonian fluid. The measured velocities and runout distances of the flows can be matched to a Bingham model for the basal avalanche by the addition of turbulent resistance. The rheologic model parameters for the 29 May flow are as follows: the density is 1300 kg/m3, the yield strength is 850 Pa, the viscosity is 90 Pa s, and the thickness of the avalanche is 2 m. The ash cloud is interpreted as a turbulent mixing layer above the basal avalanche. The buoyant portions of the cloud produced ash-fall deposits, whereas the dense portions moved as a surge separated from the parental avalanche. The ash-cloud surges formed a wide devastated zone covered by very thin debris. The initial velocities of the 3 June surges, when they detached from avalanches, are determined by the runout distance and the angle of the energy-line slope. A comparison between the estimated velocities of the 3 June avalanches and the surges indicates that the surges that extended steep slopes along the avalanche path, detached directly from the turbulent heads of the avalanches. The over-running surge that reached Kita-Kamikoba had an estimated velocity higher than that of the avalanche; this farther-travelled surge is presumed to have been generated by collapse of a rising ash-cloud plume.

title: Pyroclastic density currents and the sedimentation of ignimbrites
abstract: Pyoclastic density currents are awesome volcanic phenomena that can wreak destruction on a regional scale and can impact global climate. They deposit ignimbrites, which include vast impact lansdscape-modifying sheets with volumes exceeding 1000 km3.This book takes stock of our understanding of pyroclastic density currents and presents a new conceptual framework for investigating how ignimbrites are deposited. It integrates the results of field-based studies, laboratory experiments and numerical modelling, including work on clastic sedimentologym and industrial particle transport. Topics covered include the behaviour or particulate currents, mechanisms of clast support and segregation, interpreting ignimbrite lithofacies and architectures, and future research directions. The new approach focuses on processes and conditions within the lower flow-boundary zone of currents. Superb diagrams explain many new concepts, while the 95 photographs make an explanatiry atlas of deposit types. This is essential reading for workers investigating volcanic hazards, and for anyone wishing to interpret modern or ancient ignimbrites, as well as other catastrophically emplaced sediments.

“Given the depth of scholarship that they have brought to the subject, the power of their arguments, and the degree of synthesis with other fields, this would seemto qualify as a seminal work… I think that this will be the paper on the topic that others will have to contend with for many years to come.” Marcus Bursik, State University of New York

title: The flow dynamics of an extremely large volume pyroclastic flow, the 2.08-Ma Cerro Galán Ignimbrite, NW Argentina, and comparison with other flow types
abstract: The 2.08-Ma Cerro Galán Ignimbrite (CGI) represents a >630-km3 dense rock equivalent (VEI 8) eruption from the long-lived Cerro Galán magma system (∼6 Ma). It is a crystal-rich (35–60%), pumice (<10% generally) and lithic-poor (<5% generally) rhyodacitic ignimbrite, lacking a preceding plinian fallout deposit. The CGI is preserved up to 80 km from the structural margins of the caldera, but almost certainly was deposited up to 100 km from the caldera in some places. Only one emplacement unit is preserved in proximal to medial settings and in most distal settings, suggesting constant flow conditions, but where the pyroclastic flow moved into a palaeotopography of substantial valleys and ridges, it interacted with valley walls, resulting in flow instabilities that generated multiple depositional units, often separated by pyroclastic surge deposits. The CGI preserves a widespread sub-horizontal fabric, defined by aligned elongate pumice and lithic clasts, and minerals (e.g. biotite). A sub-horizontal anisotropy of magnetic susceptibility fabric is defined by minute magnetic minerals in all localities where it has been analysed. The CGI is poor in both vent-derived (‘accessory’) lithics and locally derived lithics from the ground surface (‘accidental’) lithics. Locally derived lithics are small (<20 cm) and were not transported far from source points. All data suggest that the pyroclastic flow system producing the CGI was characterised throughout by high sedimentation rates, resulting from high particle concentration and suppressed turbulence at the depositional boundary layer, despite being a low aspect ratio ignimbrite. Based on these features, we question whether high velocity and momentum are necessary to account for extensive flow mobility. It is proposed that the CGI was deposited by a pyroclastic flow system that developed a substantial, high particle concentration granular under-flow, which flowed with suppressed turbulence. High particle concentration and fine-ash content hindered gas loss and maintained flow mobility. In order to explain the contemporaneous maintenance of high particle concentration, high sedimentation rate at the depositional boundary layer and a high level of mobility, it is also proposed that the flow(s) was continuously supplied at a high mass feeding rate. It is also proposed that internal gas pressure within the flow, directed downwards onto the substrate over which the flow was passing, reduced the friction between the flow and the substrate and also enhanced its mobility. The pervasive sub-horizontal fabric of aligned pumice, lithic and even biotite crystals indicates a consistent horizontal shear force existed during transport and deposition in the basal granular flow, consistent with the existence of a laminar, shearing, granular flow regime during the final stages of transport and deposition.

title: The Taupo eruption, New Zealand. II. The Taupo Ignimbrite
abstract: The ca. 30 km3 Taupo ignimbrite was erupted as a climax to the ca. AD 186 Taupo eruption in the central North Island of New Zealand. It was erupted as a single vent-generated flow unit over a time period of ca. 400 s and was emplaced very rapidly (locally at more than 250-300 m s-1) and violently. The parent flow reached 8 0 + 10 km from source in all directions, crossed all but one of the mountains within its range and only stopped when it ran out of material. The ignimbrite is divisible into layers 1 and 2, and a distant facies which combines features of both layers. Layer 1 was generated as a result of strong fluidization in the flow head, caused by air ingestion, and consists of two main facies. ) is a pumiceous, mildly to strongly fines-depleted unit, generated by the expulsion of material from the flow front, and termed the jetted deposits. The overlying layer 1 (H) is a thinner, crystal- and lithic-rich, fines-depleted unit, generated by the sedimentation of coarse/dense constituents segregated out by strong fluidization within the flow head and termed the ground layer. Layer 2 consists of two facies with similar compositions but contrasting morphologies; during emplacement, material left behind by the flow body partially drained into depressions to form the valley-ponded ignimbrite, leaving the veneer deposit as a thin, landscape mantling layer on interfluves. The distant facies occurs in some outermost hilly areas of the ignimbrite where the flow velocity remained high but its volume had shrunk through deposition so that air ingestion fluidization affected the whole flow. The ignimbrite shows great lateral variations. Each facies, or variants therein, exhibits systematic degrees of development with varying distances from vent. Near vent, the flow consisted of a series of batches of material which by ca. 25 km had coalesced into a single wavy flow and by ca. 40 km into a single wave. Out to ca. 13 km, the flow was rather dilute and highly turbulent as it deflated from the collapsing eruption column. Beyond this distance it was fairly concentrated, being less than 100% expanded over its non-fluidized compacted state, and had acquired a fluidization-induced stable density stratification, which strongly suppressed turbulence in the flow body. Deflation from the eruption column was largely complete by ca. 13 km but influenced the flow as far as 20-25 km from vent. Grainsize and compositional parameters measured in the ignimbrite show lateral variations which equal or exceed the entire spectrum of published ignimbrite data. The flow had deflated and coalesced from the eruption column by ca. 20 km from vent. Beyond this distance most lateral variations are modelled by considering the flow to be a giant fluidized bed. As the flow moved, material was deposited from its base, and hence predictable vertical variations in the model fluidized bed are comparable with lateral variations in the ignimbrite. The agreement is excellent, and, in particular, discontinuities in the nature of the ignimbrite at 55-60 km from vent suggest that the more distal ignimbrite represents a vast segregation layer generated above the moving flow. Differences between the model and variations of some parameters reflect the influence of kinetic processes, such as shearing and local fluidization, that operated regardless of the bulk flow composition. The strong fluidization in the flow is a result of the high flow velocities (promoting air ingestion), not vice versa as is often accepted. Contrasts in the natures of layers 1 and 2 imply that the first material erupted contained significantly coarser, and a higher content of, lithics than the bulk of the flow. During emplacement, this earlier material was depleted by deposition and diluted by material introduced from the flow body. Systematic regional variations also occur in the ignimbrite: for example, it contains lower crystal: lithic ratios and higher density pumice in a northeasterly sector, and vice versa to the southwest. Ignimbrite found in mountainous areas shows changes consistent with its derivation from the upper, more mobile and pumiceous top of the flow. Fluidization processes generated structures and facies in the ignimbrite on various scales. Individual segregation bodies found at any exposure show features mimicking those of the ground layer, i.e. fines depletion and crystal- and lithic-enrichment. Fluidization-induced grading visible at individual exposures accounts for the great range of grading styles seen in the valley-ponded ignimbrite, and strong fluidization has locally generated an upper fines- and pumice- rich segregation layer (here termed layer 2c). On the largest scale, fluidization was primarily responsible for the generation of the layer 1 deposits, and for the grainsize and compositional zonation within the flow that produced the lateral variations in the ignimbrite. Ingested and heated air is inferred to have been the most important gas source for fluidization within the flow, although several other gas sources were locally dominant. It is clear that the thickness, grainsize and composition of the ignimbrite at any point are not simply related to values of these parameters in either the originally erupted material or the parent flow, and that, except for its density, the dimensions and composition of the parent flow cannot be directly inferred from the ignimbrite.

title: The plinian eruptions of 1912 at Novarupta, Katmai National Park, Alaska
abstract: The three-day eruption at Novarupta in 1912 consisted of three discrete episodes. Episode I began with plinian dispersal of rhyolitic fallout (Layer A) and contemporaneous emplacement of rhyolitic ignimbrites and associated proximal veneers. The plinian column was sustained throughout most of the interval of ash flow generation, in spite of progressive increases in the proportions of dacitic and andesitic ejecta at the expense of rhyolite. Accordingly, plinian Layer B, which fell in unbroken continuity with purely rhyolitic Layer A, is zoned from >99% to ∼15% rhyolite and accumulated synchronously with emplacement of the correspondingly zoned ash flow sequence in Mageik Creek and the Valley of Ten Thousand Smokes (VTTS). Only the andesiterichest flow units that cap the flow sequence lack a widespread fallout equivalent, indicating that ignimbrite emplacement barely outlasted the plinian phase. On near-vent ridges, the passing ash flows left proximal ignimbrite veneers that share the compositional zonation of their valley-filling equivalents but exhibit evidence for turbulent deposition and recurrent scour. Episode II began after a break of a few hours and was dominated by plinian dispersal of dacitic Layers C and D, punctuated by minor proximal intraplinian flows and surges. After another break, dacitic Layers F and G resulted from a third plinian episode (III); intercalated with these proximally are thin intraplinian ignimbrites and several andesite-rich fall/flow layers. Both CD and FG were ejected from an inner vent <400 m wide (nested within that of Episode I), into which the rhyolitic lava dome (Novarupta) was still later extruded. Two finer-grained ash layers settled from composite regional dust clouds: Layer E, which accumulated during the D-F hiatus, includes a contribution from small contemporaneous ash flows; and Layer H settled after the main eruption was over. Both are distinct layers in and near the VTTS, but distally they merge with CD and FG, respectively; they are largely dacitic but include rhyolitic shards that erupted during Episode I and were kept aloft by atmospheric turbulence. Published models yield column heights of 23–26 km for A, 22–25 km for CD, and 17–23 km for FG; and peak mass eruption rates of 0.7–1x108, 0.6–2x108, and 0.2–0.4x108 kg s-1, respectively. Fallout volumes, adjusted to reflect calculated redistribution of rhyolitic glass shards, are 8.8 km3, 4.8 km3, and 3.4 km3 for Episodes I, II, and III. Microprobe analyses of glass show that as much as 0.4 km3 of rhyolitic glass shards from eruptive Episode I fell with CDE and 1.1 km3 with FGH. Most of the rhyolitic ash in the dacitic fallout layers fell far downwind (SE of the vent); near the rhyolite-dominated ignimbrite, however, nearly all of Layers E and H are dacitic, showing that the downwind rhyolitic ash is of ‘co-plinian’ rather than co-ignimbrite origin.

title: The 17 July 1999 block-and-ash flow (BAF) at Colima Volcano: New insights on volcanic granular flows from textural analysis
abstract: On July 17 1999, a strong explosion occurred at Colima Volcano (Mexico) that produced a 10 km high eruptive column. The partial column collapse originated a block-and-ash flow (BAF) that flowed to the south, along the San Antonio and Montegrande ravines, travelling 3.3 km from the volcano summit. The flow filled the ravines with a volume estimated at 7.9 × 10 5  m 3 . The erosion of these deposits occurred between 1999 and 2002 (time of sampling), providing excellent longitudinal outcrops that allowed their detailed textural study. The study was carried out by means of quantitative textural analysis: (1) Rosiwal intersections, for carrying out vertical granulometric profiles; (2) total grain-size analysis, from − 11 to + 9 ϕ; and (3) Fourier and fractal analysis of the particle morphology. Grain size and morphometric parameters obtained with these methods were used to identify vertical and longitudinal variation patterns in the BAF deposit. The grain size variations allowed to infer the main particle segregation mechanisms that acted during transport and deposition of the studied BAFs. The two methods used for studying the particle shape morphologies yielded results with different accuracy and reliability. In particular, fractal analyses have been found to be the most effective in describing the particle support mechanisms that acted during transport and deposition of the studied BAFs. The results highlight the importance of the information obtained by means of these techniques, and provide new insights in transportation and deposition mechanisms of BAFs.

title: Topographic Controls on Pyroclastic Density Current Dynamics: Insight from 18 May 1980 Deposits at Mount St. Helens, Washington (USA)
abstract: Our ability to interpret the deposits of pyroclastic density currents (PDCs) is critical for understanding the transport and depositional processes that control PDC dynamics. This paper focuses on the influence of slope on flow dynamics and criticality as recorded in PDC deposits from the 18 May 1980 eruption of Mt. St. Helens (USA). PDC deposits are found along the steep flanks (10°–30°) and across the pumice plain (~ 5°) up to 8 km north of the volcano. Granulometry, componentry and descriptions of depositional characteristics (e.g., bedform morphology) are recorded with distance from source. The pumice plain deposits are primarily thick (3–12 m), massive and poorly-sorted, and represent deposition from a series of concentrated PDCs. By contrast, the steep flank deposits are stratified to cross-stratified, suggesting deposition from PDCs where turbulence strongly influenced transport and depositional processes. We propose that acceleration of the concentrated PDCs along the steep flanks resulted in thinning of the concentrated, basal region of the current(s). Enhanced entrainment of ambient air, and autofluidization from upward fluxes of air from substrate interstices and plunging breakers across rugged, irregular topography further inflated the currents to the point that the overriding turbulent region strongly influenced transport and depositional mechanisms. Acceleration in combination with partial confinement in slot canyons and high surface roughness would also increase basal shear stress, further promoting shear and traction transport in the basal region of the current. Conditions along the steep flank resulted in supercritical flow, as recorded by regressive bedforms, which gradually transitioned to subcritical flow downstream as the concentrated basal region thickness increased as a function of decreasing slope and flow energy. We also find that (1) PDCs were erosive into the underlying granular substrate along high slopes (> 25°) where currents were partially confined in steep slot canyons, suggesting that basal shear stress is an important control on erosive capacity, and (2) bedform amplitude, wavelength and the presence of regressive bedforms increase with increasing slope and proximity to source along the steep flank, suggesting a link between bedform morphology, flow velocity, and flow criticality. While our results indicate that slope and irregular topography strongly influence PDC dynamics, criticality and erosive capacity, the influence of these conditions on ultimate flow runout distance is unclear. The work here also highlights the issue that relationships between the controls on bedform size and morphology in density stratified flows remain poorly constrained, limiting our ability to extract important information about the currents that produced them. These final two points warrant further exploration through the combination of field, experimental and numerical approaches.

title: Small-volume, highly mobile pyroclastic flows formed by rapid sedimentation from pyroclastic surges at Soufrière Hills Volcano, Montserrat: an important volcanic hazard
abstract: Gravitational collapses of the lava dome at Soufrière Hills Volcano on 25 June and 26 December 1997 generated pyroclastic surges that spread out over broad sectors of the landscape and laid down thin, bipartite deposits. In each case, part of the settling material continued to move upon reaching the ground and drained into valleys as high-concentration granular flows of hot (120-410°C) ash and lapilli. These surge-derived pyroclastic flows travelled at no more than 10 m s-1 but extended significantly beyond the limits of the parent surge clouds (by 3 km on 25 June and by 1 km on 26 December). The front of the 25 June flow terminated in a valley about 50 m below a small town that was occupied at the time. Despite their small deposit volumes (5-9 x 104m3), the surge-derived pyroclastic flows travelled as far as many of the Soufrière Hills block-and-ash flows on slopes as low as a few degrees, reflecting a high degree of mobility. An analysis of the deposits from 26 December suggests that sediment accumulation rates of at least several millimetres per second were sufficient to generate pyroclastic flows by suspended-load fallout from pyroclastic surges on Montserrat. Surge-derived pyroclastic flows are an important, and hitherto underestimated, hazard around active lava domes. At Montserrat they formed by sedimentation over large catchment areas and drained into valleys different from those affected by the primary block-and-ash flows and pyroclastic surges, thereby impacting areas not anticipated to be vulnerable in prior hazards analyses. The deposits are finer-grained than those of other types of pyroclastic flow at Soufrière Hills Volcano; this may aid their recognition in ancient volcanic successions but, along with valley-bottom confinement, reduces the preservation potential.

title: Emplacement of the Taupo ignimbrite by a dilute turbulent flow
abstract: AN ignimbrite is a pumice-rich deposit that records the passage of a ground-hugging ash flow (a 'pyroclastic flow') generated by the collapse of a volcanic eruption column1. Geologists study such deposits to reconstruct the parent eruptions and to predict the consequences of future eruptions. The 1,800-yr-old Taupo ignimbrite of New Zealand has been interpreted to represent en masse emplacement by an avalanche-like flow with a volumetric solids concentration in excess of 30% (ref. 2). The evidence for this is equivocal, however, and here we propose the alternative view that the deposit was emplaced by a relatively dilute and turbulent density current3. We present an isothermal, hydraulic model, the results of which, taken together with existing observations, suggest that the total flow rate of solids and gas was about 40km3s–1 for around 15 minutes. This intense flux resulted in a flow which had a near-vent solids concentration of 0.3% by volume, was about 1 km thick, and travelled outward from the vent with a typical speed of 200 ms–1. In view of the good agreement between predicted and observed radial trends in the Taupo deposit, we suggest that the origin of other ignimbrites with similar characteristics should be reconsidered.

title: Ignimbrite types and ignimbrite problems
abstract: A spectrum of ignimbrite emplacement types exists, ranging from the “conventional” high-aspect ratio (H.A.R.I.) type, emplaced relatively quietly and passively in valleys, to the low-aspect ratio (L.A.R.I.) type, emplaced cataclysmically. Features of the L.A.R.I., such as a remarkable ability to scale mountains and cross open water and a strong fines-depletion of part of their deposits, stem from a high flow velocity which may result from an extremely high magma discharge rate. Being less rare than large-volume H.A.R.I. eruptions covering the same area, L.A.R.I. eruptions are a much more immediate volcanic hazard. Being thin and inconspicuous, a L.A.R.I. may easily be overlooked when determining the past record of a volcano. Another ignimbrite spectrum depends on variations in particle viscosity during emplacement and extends from the low-grade (water-cooled?) ignimbrite which is totally non-welded even if >50 m thick, to the high-grade (superheated?) one which is densely welded even if One problem of ignimbrite eruptions is the origin of the extensive associated ash fall, comparable in volume to the ignimbrite. This ash may be: (a) pre-ignimbrite Plinian pumice; (b) co-ignimbrite ash, containing material lost from both the eruptive column and ash flow; (c) phreatoplinian, due to the entry of significant amounts of water into the vent; (d) phreatoplinian co-ignimbrite, due to explosions at rootless vents where ash flows enter water from land. Another problem is the origin of associated well-sorted and sometimes wavy-bedded deposits. These deposits may be from: (a) base surges, related to either the collapsing column or entry of water to the vent; (b) base surges, due to explosions at rootless vents where ash flows enter water from land; (c) fines depletion in, and deposition from, the strongly fluidized head of the ash flow; (d) standing waves in a high-velocity ash flow; (e) pyroclastic surges springing from the ash flow; (f) superficial turbulence in the topmost fractions of the ash flow as it comes to rest. Major problems concern the relationship between pyroclastic surges and flows, the ability of one to change into the other, and the distinction between their deposits. Thus, the May 18th 1980 “directed blast” of Mount St. Helens is widely regarded as a surge, yet produced deposits having many characteristics of a L.A.R.I.. Understanding the behaviour of the fine ash and dust fraction is thought to be critical to the solution of these problems.

title: Reconciling Pyroclastic Flow and Surge: the Multiphase Physics of Pyroclastic Density Currents.
abstract: Two end-member types of pyroclastic density current are commonly recognized: pyroclastic surges are dilute currents in which particles are carried in turbulent suspension and pyroclastic flows are highly concentrated flows. We provide scaling relations that unify these end-members and derive a segregation mechanism into basal concentrated flow and overriding dilute cloud based on the Stokes number ( S T ), the stability factor ( Σ T ) and the dense–dilute condition ( D D ). We recognize five types of particle behaviors within a fluid eddy as a function of S T and Σ T : (1) particles sediment from the eddy, (2) particles are preferentially settled out during the downward motion of the eddy, but can be carried during its upward motion, (3) particles concentrate on the periphery of the eddy, (4) particles settling can be delayed or ‘fast-tracked’ as a function of the eddy spatial distribution, and (5) particles remain homogeneously distributed within the eddy. We extend these concepts to a fully turbulent flow by using a prototype of kinetic energy distribution within a full eddy spectrum and demonstrate that the presence of different particle sizes leads to the density stratification of the current. This stratification may favor particle interactions in the basal part of the flow and D D determines whether the flow is dense or dilute. Using only intrinsic characteristics of the current, our model explains the discontinuous features between pyroclastic flows and surges while conserving the concept of a continuous spectrum of density currents.

title: The motion of a finite mass of granular material down a rough incline
abstract: Rock, snow and ice masses are often dislodged on steep slopes of mountainous regions. The masses, which typically are in the form of innumerable discrete blocks or granules, initially accelerate down the slope until the angle of inclination of the bed approaches the horizontal and bed friction eventually brings them to rest. The present paper describes an initial investigation which considers the idealized problem of a finite mass of material released from rest on a rough inclined plane. The granular mass is treated as a frictional Coulomb-like continuum with a Coulomb-like basal friction law. Depth-averaged equations of motion are derived; they bear a superficial resemblance to the nonlinear shallow-water wave equations. Two similarity solutions are found for the motion. They both are of surprisingly simple analytical form and show a rather unanticipated behaviour. One has the form of a pile of granular material in the shape of a parabolic cap and the other has the form of an M -wave with vertical faces at the leading and trailing edges. The linear stability of the similarity solutions is studied. A restricted stability analysis, in which the spread is left unperturbed shows them to be stable, suggesting that mathematically both are possible asymptotic wave forms. Two numerical finite-difference schemes, one of Lagrangian, the other of Eulerian type, are presented. While the Eulerian technique is able to reproduce the M -wave similarity solution, it appears to give spurious results for more general initial conditions and the Lagrangian technique is best suited for the present problem. The numerical predictions are compared with laboratory experiments of Huber (1980) involving the motion of gravel released from rest on a rough inclined plane. Although in these experiments the continuum approximation breaks down at large times when the gravel layer is only a few particle diameters thick, the general features of the development of the gravel mass are well predicted by the numerical solutions.

title: Gas retention in fine-grained pyroclastic flow materials at high temperatures
abstract: The 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 (tbe ; brief and not considered in this paper), one for hindered settling from the expanded state (tsett), and one for diffusive release of residual pore pressure from the non-expanded state (tdiff). The relative magnitides of tsett and tdiff depend on bed thickness, tsett dominating in thin systems and tdiff 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 tsett and tdiff 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 (tsett) or non-expanded (tdiff) 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.

title: Pore fluid pressure diffusion in defluidizing granular columns
abstract: Pore fluid pressure variations play an important role in the motion of natural granular flows like debris and pyroclastic flows. Pore pressure in a defluidizing air-particle bed was investigated by means of experiments and numerical modeling. Experiments consisted of recording the defluidization process, measured as the decay of the basal pore fluid pressure in initially aerated granular mixtures. Mixtures were aerated to different degrees of fluidization by introducing a vertical air flux at the base of a granular column. The degree of fluidization was characterized by the parameter bo (pore fluid pressure/lithostatic pressure). Bed expansion occurred for bo > 0.8–0.9, with maximum expansions near 8% at bo $1. Pore pressure diffusion in our mixtures was modeled by a simple diffusion equation, taking into account a variable diffusion coefficient. When mixtures were expanded (bo > 0.8–0.9), continuous consolidation introduced nonlinearities in the diffusion coefficients, which retarded the decay of pore pressure. In contrast, for non-expanded mixtures, the diffusion coefficient remained constant (linear diffusion). Our results highlight that mixture compressibility can effectively reduce the pressure diffusion coefficient in initially expanded granular mixtures, thus increasing the duration of pressure diffusion. In our experiments, as well as for most self-consolidating natural granular mixtures, changes in permeability due to mixture consolidation appear to be negligible for the defluidizing process, as they are counteracted by changes in porosity and because the fluid behaves as incompressible, even when the fluid is air.

title: Depositional processes and gas pore pressure in pyroclastic flows: an experimental perspective
abstract: The depositional processes and gas pore pressure in pyroclastic flows are investigated through scaled experiments on transient, initially fluidized granular flows. The flow structure consists of a sliding head whose basal velocity decreases backwards from the front velocity (Uf) until onset of deposition occurs, which marks transition to the flow body where the basal deposit grows continuously. The flows propagate in a fluid-inertial regime despite formation of the deposit. Their head generates underpressure proportional to Uf2 whereas their body generates overpressure whose values suggest that pore pressure diffuses during emplacement. Complementary experiments on defluidizing static columns prove that the concept of pore pressure diffusion is relevant for gas-particle mixtures and allow characterization of the diffusion timescale (td) as a function of the material properties. Initial material expansion increases the diffusion time compared with the nonexpanded state, suggesting that pore pressure is self-generated during compaction. Application to pyroclastic flows gives minimum diffusion timescales of seconds to tens of minutes, depending principally on the flow height and permeability. This study also helps to reconcile the concepts of en masse and progressive deposition of pyroclastic flow units or discrete pulses. Onset of deposition, whose causes deserve further investigation, is the most critical parameter for determining the structure of the deposits. Even if sedimentation is fundamentally continuous, it is proposed that late onset of deposition and rapid aggradation in relatively thin flows can generate deposits that are almost snapshots of the flow structure. In this context, deposition can be considered as occurring en masse, though not strictly instantaneously.

title: Grain size variations in ignimbrites and implications for the transport of pyroclastic flows
abstract: Ignimbrite flow units commonly show reverse grading of large pumice clasts and normal grading of large lithic clasts. Ignimbrites show coarse-tail grading, in which particles beneath a critical diameter, ranging from 64 to 2 mm, are ungraded. Above this size the larger the clast diameter the more pronounced the segregation. The grading is consistent with the theoretical settling rates of particles in a dispersion with a high particle concentration. Ignimbrite flow units show a reversely graded, fine grained basal layer which is attributed to the action of boundary forces during flow. Ignimbrites are commonly associated with cross-stratified pyroclastic surge deposits and fine ash fall deposits formed in the same eruption. The fine ash fall deposit is depleted in crystals and is thought to be the deposit of the fine turbulent cloud observed making up the upper parts of nuees ardentes. 
 
 
 
Pyroclastic flows are postulated to be dense, poorly expanded partly fluidized debris flows. Only its fine grained components can be fluidized by gas. Pyroclastic flows are believed to behave as a dispersion of larger clasts in a medium of fluidized fines, which acts as a lubricant similar to water in mud-flows. Poor sorting in ignimbrites is attributed to high particle concentrations not turbulence. Many pyroclastic flows may be laminar in their movement with apparent viscosities, deduced from the lateral grading of large lithic clasts, in the range 101−103 poise.

title: Effects of pore pressure in pyroclastic flows : numerical simulation and experimental validation
abstract: Pyroclastic flows are mixtures of volcanic gases and particles that can be very hazardous owing to their fluid-like behavior. One possible mechanism to explain this behavior is the reduction of particles friction due to the internal gas pore pressure. To verify this hypothesis, we present a numerical model of a granular flow with high initial pore pressure that decreases with time as the gas-particle mixture propagates. First, we validate the model by reproducing laboratory experiments. Then, the numerical code is applied to pyroclastic flows of Lascar volcano (1993 eruption, Chile). The simulation reproduces the runout and the morphological features of the deposits, with lateral levees, a central channel, and a lobate front. Our results support the hypothesis of the role of gas pore pressure in pyroclastic flows and explain both the fluid-like behavior of the flows and the formation of lateral levees.

title: Ignimbrite depositional facies: the anatomy of a pyroclastic flow
abstract: A model is presented for the depositional regimes in a pyroclastic flow, to explain the origin of facies in the Taupo ignimbrite and to compare these with other published examples. In this model, a pyroclastic flow consists of a head, a body and a tail. The head, which is where fluidization caused by air ingestion occurs, generates layer 1 deposits. On lithological grounds these are divided into layers 1(P) which is fines-depleted and rich in pumice, and 1(H) which is fines-depleted and rich in lithics and crystals. Layer 1(P) represents material thrown forwards from the flow head and is termed the jetted deposits.  represents material sedimented from within the flow head and is termed the ground layer. The body represents the bulk of the flow and the tail is its trailing part which is slowed by ground friction; these parts generate the layer 2 deposits which include the valley-pond ignimbrite and its associated ignimbrite veneer deposit. Localized depositional modes within the body and tail generate distinctive coarse pumice concentration zones and pumiceous lee-side lenses behind obstacles. At its outer limits all of the flow on interfluves is affected by air-ingestion fluidization, producing the distant facies which combines features of both layer 1 and 2 deposits. One important conclusion of this work is that the thicknesses and compositions of the various facies are not related in any simple way to the thickness and composition of the parent flow.

title: Gas release rates from pyroclastic flows: a assessment of the role of fluidisation in their emplacement
abstract: Estimates are presented for the rates of release of dissolved water from the particles in a pyroclastic flow by diffusion. Velocities of gas escaping from pyroclastic flows of different thicknesses are calculated. For quite small residual gas contents (0.2 to 0.8% H2O), gas velocities of 10 to over 100 cm/sec during the first 103 sec of release are estimated for flows of thickness 1 to 20 m, which experimental studies demonstrate are the velocities required to fluidise fine to medium ash. Flows with high residual gas contents or large volume (thick flows) are likely to be substantially fluidised by exsolving gas. 30% to over 60% of the particles in such flows are predicted to be fluidised. Fluidisation is thus believed to be an important mechanism in the flow and in determining the mobility of the large magnitude, prehistoric pyroclastic flows which formed extensive ignimbrite sheets. Small pyroclastic flows, however, of the magnitude observed in several historic eruptions are not believed to be fluidised, because of their low residual gas contents, small volume, and the substantial amount of cooling that occurs during their emplacement.

title: Settling behaviour of concentrated dispersions and some volcanological applications
abstract: Experiments have been carried out on the settling of non-aggregating, poorly sorted aqueous dispersions laced with lowdensity particles in order to gain insight into the sedimentation behaviour of volcaniclastic flows which contain pumice. Two poorly sorted dispersions of silicon carbide particles with continuous size distributions were studied; plastic beads were added as a light component. Experiments were carried out at solids concentrations between 10 and 60 vol%. Dispersions of low concentration generated normally graded deposits in which particles of different densities were in approximate hydraulic equivalence. Above a critical concentration (45 to 60% depending on the grain-size distribution) particle segregation was suppressed, and the dispersion settled to form a virtually ungraded bed. Hindered settling at intermediate concentrations generated normally graded deposits with elutriation pipes, “floating” rafts of plastic, and capping layers of fines elutriated by escaping pore fluid. Buoyancy forces resulted in strong hydraulic inequivalence between light and dense components in the deposits from intermediate concentrations. Two mechanisms of elutriation-pipe growth were observed. At intermediate concentrations, pipes grew progressively upwards in the deposit as the latter aggraded, whereas at high concentrations they nucleated and grew uniformly throughout the whole dispersion. Elutriation pipes form readily during sedimentation of some poorly sorted aqueous dispersions; the presence of elutriation pipes cannot be used alone to distinguish between deposits of epiclastic and pyroclastic flows. Reverse grading of pumice, pumice rafts, elutriation pipes, and layers of elutriated fines should characterise deposits from pumice-rich submarine mass flows. Despite differences in the fluid phase, the experiments also have implications for fluidization mechanisms in pyroclastic flows. Hindered settling of a concentrated, poorly sorted dispersion is a type of self-fluidization in which the sedimentation of large particles causes a sufficiently strong vertical fluid flux to fluidize or elutriate smaller ones. Hindered settling is intrinsic to all pyroclastic flows as they spread out and deflate across the landscape, and may reinforce other gas sources as an effective fluidization mechanism during emplacement of type 2 and type 3 ignimbrites and of coignimbrite lag breccias.

title: Autofluidization of pyroclastic flows propagating on rough substrates as shown by laboratory experiments
abstract: This study investigates the influence of the substrate surface roughness on the emplacement mechanisms of pyroclastic flows. We carried out laboratory experiments on gravitational flows generated from the release of initially fluidized or nonfluidized columns of fine particles (diameter d = 0.08 mm) in a horizontal channel. The roughness of the channel base was uniform in each experiment, created by gluing particles of diameter d0 = 0.08 to 6 mm to the base. Other things being equal, the flow runout distance increased with the channel base roughness (d0) to a maximum of about twice that of flows on a smooth substrate when d0 = 1.5–3 mm, before decreasing moderately at higher roughness values of d0 = 6 mm. Long runout originated mainly during the late stages of emplacement as flow deceleration was strongly reduced at high substrate roughness. This was caused by (partial) autofluidization due to an upward air flux escaping from the substrate interstices in which flow particles settled. Autofluidization was evidenced by high pore fluid pressure measurements at the base of initially nonfluidized flows and also by reduced flow runout when the interstices were initially partially filled so that less air was available. Furthermore, the runout distance of flows of large particles (d = 0.35 mm), which could not be fluidized by the ascending air flux, was independent of the substrate roughness. This study suggests that autofluidization caused by air escape from the interstices of a rough substrate is one important mechanism to explain the common long runout distance of pyroclastic flows even on subhorizontal topographies.

title: A proximal ignimbrite breccia facies on santorini , Greece
abstract: Prominent among the pyroclastic deposits of Santorini are several thick, widespread lithic breccia deposits, which are found in intimate association with ignimbrite. At least three of these breccias are interpreted, on the basis of field and grain-size criteria, as having originated by the segregation of lithic clasts from active pyroclastic flows. They therefore record the occurrence of three large, previously unrecognized ignimbrite-forming eruptions of the volcano. The breccias of the 18,500 yr. B.P. Cape Riva eruption include two types. The first type is a thin, basal ground breccia, which overlies a strong erosion surface. This breccia shows pinch and swell structures and is strongly enriched in lithic and crystal components. It is considered to have formed by strong fluidization due to incorporation of air into the head of an active pyroclastic flow. The second, and predominant, type consists of thick co-ignimbrite lag breccias (up to 25 m), which overlie the ground breccia. These deposits are generally clast-supported, poorly sorted breccias which in places grade both vertically and laterally into non-welded pumiceous ignimbrite. They consist of well-defined, normally graded units which show coarse tail grading of lithic and pumice clasts. Each breccia unit is underlain by a thin, inversely graded ignimbrite basal layer, and correlates laterally with a flow unit of the associated ignimbrite. The lag breccias are therefore thick equivalents of the 2b lithic concentration zones of Sparks et al. The lag breccias and ignimbrite contain abundant lithic segregation structures that are characteristic of strong gas fluidization. These structures, the presence of basal layers, and the gradation into normal ignimbrite, suggest that the lag breccias originated by the segregation of lithic clasts within the bodies of dense, but strongly fluidized pyroclastic flows. The Cape Riva breccias occur within a few kilometers of their source vent and are interpreted as proximal facies of their associated ignimbrite. The presence of the ground breccia indicates that within this distance, the pyroclastic flows had developed the head and body regions characteristic of gravity currents. The deflation of the pyroclastic flow bodies, within a few kilometers from source, to particle concentrations sufficient to permit the generation of basal layers and coarse tail grading, is incompatible with present theories of column collapse. It is postulated that high pressures at the base of the collapsing Cape Riva eruption column were sufficient to significantly compress the dilute particle-gas mixture of the column close to the source vent. Subsequent sedimentation, as the pyroclastic flows moved laterally, increased the density further to the point where the observed sedimentary features could form. Simultaneous decompression of the gas phase resulted in strong fluidization, and the segregation of the lag breccias.

title: On the apparent viscosity of a fluidized bed
abstract: The fluid-dynamic analogy of a typical spherical particle in a fluidized bed as the same particle suspended under terminal conditions in a pseudo-fluid (composed of the fluid and all the other suspended particles) is shown to yield predictive estimates for the apparent viscosity of fluidized beds, in good quantitative agreement with reported experimental measurements for particle concentrations of up to ∼ 40 % . For higher concentrations the predictions fall progressively below measured values, leading to order-of-magnitude underestimates for dense beds at close to minimum fluidization conditions. This latter phenomenon might be ascribable to the dominance, under these conditions, of particle–particle interactions. At high particle concentrations, these would give rise to the observed phenomenon of liquid- and gas-fluidized bed apparent viscosities approaching very similar values, and gas pressure having no influence at all.

title: Pore fluid pressure and internal kinematics of gravitational laboratory air‐particle flows: Insights into the emplacement dynamics of pyroclastic flows
abstract: The emplacement dynamics of pyroclastic flows were investigated through noninvasive measurements of the pore fluid pressure in laboratory air-particle flows generated from the release of fluidized and nonfluidized granular columns. Analyses of high-speed videos allowed for correlation of the pressure signal with the flow structure. The flows consisted of a sliding head that caused underpressure relative to the ambient, followed by a body that generated overpressure and at the base of which a deposit aggraded. For initially fluidized flows, overpressure in the body derived from advection of the pore pressure generated in the initial column and decreased by diffusion during propagation. Relatively slow diffusion caused the pore pressure in the thinner flow to be larger than lithostatic at early stages. Furthermore, partial auto-fluidization, revealed in initially nonfluidized flows, also occurred and contributed to maintain high pore pressure, whereas dilation or contraction of the air-particle mixture with associated drag and/or pore volume variation transiently led the pressure to decrease or increase, respectively. The combination of all these processes resulted in long-lived high pore fluid pressure in the body of the flows during most of their emplacement. In the case of the initially fluidized and slightly expanded (∼3–4%) flows, (at least) ∼70%–100% of the weight of the particles was supported by pore pressure, which is consistent with their inertial fluid-like behavior. Dense pyroclastic flows on subhorizontal slopes are expected to propagate as inertial fluidized gas-particle mixtures consisting of a sliding head, possibly entraining basement-derived clasts, and of a gradually depositing body.

title: The effects of gas flow on granular currents
abstract: Currents of particles have been quite successfully modelled using techniques developed for fluid gravity currents. These models require the rheology of the currents to be specified, which is determined by the interaction between particles. For relatively small slow currents, this is determined primarily through friction, which can be controlled and reduced by fluidizing the particles, so that they may become much more mobile. Recent results cannot be predicted using many of the proposed models, and may be defined by the interaction between the particles and the fluid through which they are passing. However, in addition, particles that are only initially fluidized also form currents that are also mobile, but otherwise are different from continuously fluidized currents. The mobility of these currents appears not to be connected to the time taken for them to degas. This suggests that defining the continuous stresses on the particle current may not be sufficient to understand its motion and that a challenge for the future is to understand the structure of these flows and how this affects their motion.

title: Experiments on deaerating granular flows and implications for pyroclastic flow mobility
abstract: Granular flows were generated by the release of beds of particles in various fluidized states, which then deaerated in a horizontal channel. We describe characteristics of the flows and their deposits. Morphological similarities between deposits in experiments and in the field suggest that pyroclastic flow deposits form from a fluidized mixture. The experiments show that slightly expanded, fluidized flows are more mobile than non-fluidized flows of equivalent volume and material composition. They travel to a fixed distance from their source, which depends only weakly on their initial degree of fluidization. Flows of fine particles (<100 μm) deaerate slowly and are highly mobile. Pyroclastic flows commonly have large amounts of fine ash, which may have a controlling influence on their high mobility.

title: Experimental study of gas-fluidized granular flows with implications for pyroclastic flow emplacement
abstract: Experiments have been carried out on initially gas-aerated and gas-fluidized granular flows propagating into a horizontal channel. After lateral acceleration following release of the originally fluidized bed, two contrasting flow behaviors were observed, which reflected the degree of initial fluidization and the grain size. Initial fluidization disrupts the interparticle contact network, which controls internal friction of the static bed. The flow regime then depends on the timescale needed to reestablish a strong contact network, and this time increases as the grain size decreases. Initially aerated and fluidized flows of coarse particles (>≈100 μm) and initially aerated flows of fine particles (<≈100 μm) behave as their nonfluidized counterparts and they propagate as a wedge, with decelerating velocities so that the front position increases as the ∼0.8 power of time. In contrast, initially fluidized flows of fine particles propagate for most of their duration at constant thickness and frontal velocity in a similar fashion to the slumping regime of buoyancy-driven gravity currents of Newtonian fluids. We have determined a Froude number Fr for such flows of ≈2.6 consistent with published data from experimental and theoretical investigations on inviscid fluids. This implies that internal particle friction can be neglected in describing the dynamics of initially fluidized, concentrated fine granular flows. However, all flows are characterized by a short, final stopping phase whose timescale gives an estimate of the kinetics required to reestablish a strong contact network and form a static deposit. These results suggest that fines-rich pyroclastic flows may propagate as inviscid fluids for most of their emplacement.

title: Enhanced run-out of dam-break granular flows caused by initial fluidization and initial material expansion
abstract: We report results of the run-out of experimental dam-break flows in a horizontal channel generated from the collapse of columns of fine (75 $$\upmu $$μm) particles fluidized at various degrees. We find that the flow run-out (x) made dimensionless by the initial column length ($$x_{o})$$xo) is a power function of the initial column height-to-length ratio (r), as shown in previous works with non-fluidized flows. The run-out of flows initially fluidized at different degrees is accounted by $$x/x_{o }=\alpha r^{n}$$x/xo=αrn. For initially non-fluidized flows, our values of $$\alpha $$α are significantly higher than those reported earlier for flows of coarser granular material ($$>$$>0.15 mm), showing that finely grained flows have longer run-outs compared to their coarser counterparts. The coefficient $$\alpha $$α is a function of the initial degree of fluidization, with a higher growth above 93 % of fluidization, which coincides with the onset of bed expansion, and it accounts for a flow run-out increase being up to more than twice that of non-fluidized flows. The parameter $$\alpha $$α is well correlated with the amount of initial bed expansion, which undergoes a sharp transition at high degrees of fluidization that has shown to be an important mechanism for reducing flow friction. Our results are consistent with earlier findings that showed that bed expansion significantly increases pore pressure diffusion timescales in static columns, suggesting that the long run-out of initially expanded finely grained flows is due to their ability to diffuse pore pressure slowly.

title: Dynamic pore-pressure variations induce substrate erosion by pyroclastic flows
abstract: Field evidence shows that pyroclastic flows can entrain blocks from underlying substrates formed by earlier geological events, yet, counterintuitively, they are less likely to erode unconsolidated layers of fine particles. Here we report laboratory experiments that reproduce these seemingly contradictory observations and also offer a means to infer pyroclastic flow velocity. Experiments demonstrate that the sliding head of a granular flow generates a dynamic upward pore-pressure gradient at the flow-substrate interface. Associated upward air flux is enough to fluidize a substrate of fines, so that particles are not entrained individually but the substrate instead is subject to small shear instabilities. In contrast, coarse particles forming a non-fluidized substrate are lifted at a critical upward force due to the pore-pressure gradient, according to their individual masses, which provides a basis for a model to calculate the flow velocity. Application to the 18 May 1980 pyroclastic flow deposits at Mount St. Helens (Washington State, USA) gives velocities of ∼9-13 m s-1 at ∼6-7 km from the vent on gentle slopes (<4°-6°), in agreement with field observations at this volcano and at others.

title: Aerated granular flow over a horizontal rigid surface
abstract: The effect of a vertical gas flow on the dynamics of a coulombic granular material moving over a horizontal rigid porous surface has been studied experimentally and theoretically. The presence of a fluidizing gas significantly alters the granular flow dynamics. When the gas velocity, ug, is below the minimum fluidization velocity, umf, the effect of the gas is to reduce the angle of repose θ from the value measured in the absence of a gas flow. When material is poured from a point source onto a horizontal surface it forms a pile, which adjusts through episodic avalanching to a self-similar conical shape. Under these conditions, the development of the pile is determined by the local force balance on individual particles and its extent may be expressed in terms of the volume of particles added and the angle of repose. A volume of material is poured continuously from a point source onto a surface according to Qtα. Below the minimum uidization velocity, a quasi-static description gives the encroachment distance of the granular pile as l = (2Q/(2π/3)n−1 tan θ)1/n+1tα/n+1 where n = 1 for a planar release and n = 2 for an axisymmetric release. A continuum description of fluidized granular flow has been developed by vertically averaging the mass and momentum conservation equations and including the momentum exchange between the gas and granular flow. The bulk movement is driven along the ground by horizontal gradients of particle pressure and is resisted by a viscous drag force due to the particles moving horizontally through a vertical gas flow. Above the minimum fluidization velocity the character of the granular flow is significantly altered and takes on fluid-like properties. The model predicts the shape of the fluidized granular pile and that the encroachment distance grows as l = λnα (Q(ug + umf) / ε)1/n+2t α+1/n+2, where ε is the void fraction in the bed and λnα is a constant. Below the conditions for fluidization (ug < umf), the pile of granular material grows quasi-statically when t > t∗, where t∗ ∼ (εn+1Qug + umf) / μ2+n (umf − ug)2+n) 1/1+n−α corresponds to the critical time when frictional forces are comparable to gradients of particle pressure and the drag force. Numerical solutions describing the granular flow are presented. Experimental observations of the shape and extent of planar and axisymmetric granular flows when α = 1 compare well with theoretical predictions for various values of particle volume flux Q, time t, and gas flow rate ug. The mathematical description of fluidized granular flows along rigid surfaces indicates a strong analogy with buoyancy-driven flows in a porous medium. This analogy permits extension of our description to include flows down slopes and the effect of internal stratification.

title: Experimental study of dense pyroclastic density currents using sustained, gas-fluidized granular flows
abstract: We present the results of laboratory experiments on the behaviour of sustained, dense granular flows in a horizontal flume, in which high-gas pore pressure was maintained throughout the flow duration by continuous injection of gas through the flume base. The flows were fed by a sustained (0.5–30 s) supply of fine (75 ± 15 μm) particles from a hopper; the falling particles impacted an impingement surface at concentrations of ~3 to 45 %, where they densified rapidly to generate horizontally moving, dense granular flows. When the gas supplied through the flume base was below the minimum fluidization velocity of the particles (i.e. aerated flow conditions), three flow phases were identified: (i) an initial dilute spray of particles travelling at 1–2 m s−1, followed by (ii) a dense granular flow travelling at 0.5–1 m s−1, then by (iii) sustained aggradation of the deposit by a prolonged succession of thin flow pulses. The maximum runout of the phase 2 flow was linearly dependent on the initial mass flux, and the frontal velocity had a square-root dependence on mass flux. The frontal propagation speed during phase 3 had a linear relationship with mass flux. The total mass of particles released had no significant control on either flow velocity or runout in any of the phases. High-frequency flow unsteadiness during phase 3 generated deposit architectures with progradational and retrogradational packages and multiple internal erosive contacts. When the gas supplied through the flume base was equal to the minimum fluidization velocity of the particles (i.e. fluidized flow conditions), the flows remained within phase 2 for their entire runout, no deposit formed and the particles ran off the end of the flume. Sustained granular flows differ significantly from instantaneous flows generated by lock-exchange mechanisms, in that the sustained flows generate (by prolonged progressive aggradation) deposits that are much thicker than the flowing layer of particles at any given moment. The experiments offer a first attempt to investigate the physics of the sustained pyroclastic flows that generate thick, voluminous ignimbrites.

title: Experimental Study on the Effective Particle Diameter of a Packed Bed with Non-Spherical Particles
abstract: An experimental study is conducted to determine the characteristics of frictional pressure drops of fluid flow in porous beds packed with non-spherical particles. The objective is to examine the applicability of the Ergun equation to flow resistance assessment for packed beds with non-spherical particles. The experiments are carried out on the POMECOFL facility at KTH. Hollow spheres and cylinders are used to pack the beds. Either water or air is chosen as the working fluid. The experimental data show that the Ergun equation is applicable to all the test beds if the effective particle diameter used in the equation is chosen as the equivalent diameter of the particles, which is the product of Sauter mean diameter and shape factor of the particles in each bed.

title: Types of gas fluidization
abstract: The behaviour of solids fluidized by gases falls into four clearly recognizable groups, characterized by density difference (ϱs–ϱf) and mean particle size. The most easily recognizable features of the groups are: powders in group A exhibit dense phase expansion after minimum fluidization and prior to the commenment of bubbling; those in group B bubble at the minimum fluidization velocity; those in group C are difficult to fluidize at all and those in group D can form stable spouted beds. A numerical criterion which distinguishes between groups A and B has been devised and agrees well with published data. Generalizations concerning powders within a group can be made with reasonable confidence but conclusions drawn from observations made on a powder in one group should not in general be used to predict the behaviour of a powder in another group.

title: Influence of slope angle on pore pressure generation and kinematics of pyroclastic flows: insights from laboratory experiments
abstract: The influence of slope angle on pore pressure generation and kinematics of fines-rich pyroclastic flows was investigated through laboratory experiments. Granular flows were generated by the release of a column of fine glass beads (d = 0.08 mm) in an inclined channel (0–30°). The granular column could be fluidized while the channel base was either smooth or made rough by glued beads of 3 mm diameter. Pore pressure measurements reveal that the degree of autofluidization, caused by air escaping from the substrate interstices into which flow particles settled, was high at all slope angles. Flow runout increase due to autofluidization, however, was reduced at slope angle higher than ∼12° because of the occurrence of a strong deceleration phase that limited the flow duration. This is probably caused by the combination of flow head thinning at increased slope angle and settling of particles into the substrate interstices until the flow ran out of mass. Analysis of high-speed videos suggests that ingestion of ambient air at the flow front did not occur, even on steep slopes of 30°. Experiments at inclinations close to (25°) or slightly higher (30°) than the repose angle of the granular material (28.5°) revealed the formation of a thin basal deposit that was then eroded as the flow thickness and velocity gradually decreased. Our study suggests that air escape from substrate interstices in nature can be a significant external cause of pore pressure generation that favors low energy dissipation and long runout distances of pyroclastic flows on moderate topographies.

title: Fundamental changes of granular flow dynamics, deposition, and erosion processes at high slope angles: Insights from laboratory experiments
abstract: Entrainment of underlying debris by geophysical flows can significantly increase the flow deposit extent. To study this phenomenon, analog laboratory experiments have been conducted on granular column collapse over an inclined channel with and without an erodible bed made of similar granular material. Results show that for slope angles below a critical value θc, between 10° and 16°, the run out distance rf depends only on the initial column height h0 and is unaffected by the presence of an erodible bed. On steeper slopes, the flow dynamics change fundamentally, with a slow propagation phase developing after flow front deceleration, significantly extending the flow duration. This phase has characteristics similar to those of steady uniform flows. Its duration increases with increasing slope angle, column volume, column inclination with respect to the slope and channel width, decreasing column aspect ratio (height over length), and in the presence of an erodible bed. It is independent, however, of the maximum front velocity. The increase in the duration of the slow propagation phase has a crucial effect on flow dynamics and deposition. Over a rigid bed, the development of this phase leads to run out distances rf that depend on both the initial column height h0 and length r0. Over an erodible bed, as the duration of the slow propagation phase increases, the duration of bed excavation increases, leading to a greater increase in the run out distance compared with that over a rigid bed (up to 50%). This effect is even more pronounced as bed compaction decreases.

title: An exceptionally widespread ignimbrite with implications for pyroclastic flow emplacement
abstract: PUMiCE-rich pyroclastic flows leave deposits called ignimbrites1, which are common in the eruption records of arc and intracontinental volcanoes2,3. Pyroclastic flows represent the most dangerous manifestation of volcanism4 because of their volumes, rapid emplacement at high temperatures, and capacity to extend >100 km from the source. However, maximum distances travelled and extents of areas destroyed by pyroclastic flows remain poorly known. Historic examples have been relatively small, the largest reaching to only ∼35 km from the vent5, but prehistoric ignimbrites over 150 km across are known3,6–10. The largest ignimbrites are invariably partly buried or eroded, and only minimum estimates of their size (and hence of their destructive capacity) can be made. Here we describe the ∼l-Myr-old Kidnappers ignimbrite, erupted from the Taupo Volcanic Zone in New Zealand, which we have correlated for ≥385 km and which thus represents the most wide-spread ignimbrite yet known. We suggest that this wide distribution primarily reflects a coupling of high initial flow velocities with large mass fluxes, and that favourable flow paths and emplacement of flows over a wet substrate may have aided their mobility.

title: Shock waves, dead zones and particle-free regions in rapid granular free-surface flows
abstract: Shock waves, dead zones and particle-free regions form when a thin surface avalanche of granular material flows around an obstacle or over a change in the bed topography. Understanding and modelling these flows is of considerable practical interest for industrial processes, as well as for the design of defences to protect buildings, structures and people from snow avalanches, debris flows and rockfalls. These flow phenomena also yield useful constitutive information that can be used to improve existing avalanche models. In this paper a simple hydraulic theory, first suggested in the Russian literature, is generalized to model quasi-two-dimensional flows around obstacles. Exact and numerical solutions are then compared with laboratory experiments. These indicate that the theory is adequate to quantitatively describe the formation of normal shocks, oblique shocks, dead zones and granular vacua. Such features are generated by the flow around a pyramidal obstacle, which is typical of some of the defensive structures in use today.

title: A reappraisal of ignimbrite emplacement: progressive aggradation and changes from particulate to non-particulate flow during emplacement of high-grade ignimbrite
abstract: We propose a mechanism by which massive ignimbrite and layered ignimbrite sequences — the latter liable to have been previously interpreted as multiple flow units-form by progressive aggradation during sustained passage of a single particulate flow. In the case of high-temperature eruptive products the mechanism simplifies interpretation of problematic deposits that exhibit pronounced vertical and lateral variations in texture, including between non-welded, eutaxitic, rheomorphic (lineated) and lava-like. Agglutination can occur within the basal part of a hot density-stratified flow. During initial incursion of the flow, agglutinate chills and freezes against the ground. During sustained passage of the flow, agglutination continues so that the non-particulate (agglutinate) layer thickens (aggrades) and becomes mobile, susceptible to both gravity-induced motion and traction-shear imparted by the overriding particulate part of the flow. The particulate to non-particulate (P-NP) transition occurs in and just beneath a depositional boundary layer, where disruptive collisions of hot viscous droplets give way, via sticky grain interactions, to fluidal behavior following adhesion. Because they have different rheologies, the particulate and non-particulate flow components travel at different velocities and respond to topography in different ways. This may cause detachment and formation of two independent flows. The P-NP transition is controlled by factors that influence the rheological properties of individual erupted particles (strain rate, temperature, and composition including volatiles), by cooling and volatile exsolution during transport, and by the particle-size population and concentration characteristics of the depositional boundary layer. At any one location along the flow path one or more of these can change through time (unsteady flow). Thus the P-NP transition can develop momentarily or repeatedly during the passage of an unsteady flow, or it can occur continuously during the passage of a quasi-steady flow supplied by a sustained explosive eruption. Vertical facies successions developed in the deposit (high-grade ignimbrite) reflect temporal changes in flow steadiness and in material supplied at source. The P-NP transition is also influenced by factors that affect flow behaviour, such as topography. It may occur at any location laterally between a proximal site of deflation (e.g. a fountain-fed lava) and a flow's distal limit, but it most commonly occurs throughout a considerable length of the flow path. Up-sequence variations in welding-deformation fabric (between oblate uniaxial to triaxial and prolate) reflect evolving characteristics of the depositional boundary layer (e.g. fluctuations from direct suspension-sedimentation to deposition via traction carpets or traction plugs), as well as possible modifications resulting from subsequent, post-depositional hot loading and slumping. Similar processes can also account for lateral lithofacies gradations in conduits and vents filled with welded tuff. Our consideration of high-grade ignimbrites has implications for ignimbrite emplacement in general, and draws attention to the limitations of the widely accepted models of emplacement involving mainly high-concentration non-turbulent transport and en masse ‘freezing’ of high-yield-strength plug flows.

title: Temporal and spatial evolution of a waxing then waning catastrophic density current revealed by chemical mapping
abstract: We reconstruct the behavior of a catastrophic sustained radial pyroclastic density current as it waxed then waned during its brief lifespan. By subdividing the deposit into 8 time slices using a chemical tracer, we show that the sustained current initially was topographically restricted, but that its leading edge advanced in all directions, encroaching upon and gradually ascending hills. During peak fl ow the current reached its maximum extent and overtopped all topographic highs. After this, and while the current direction from source was maintained, the leading edge gradually retreated sourceward. High-resolution analysis of the depositional architecture reveals how the fl ow dynamics evolved and runout distance of the sustained density current rapidly increased then decreased, refl ecting the dominant infl uence of changing mass fl ux, as demonstrated in numerical models but not previously distinguished in a natural deposit.

title: Fluidization and de-aeration of pyroclastic mixtures : The influence of fines content, polydispersity and shear flow
abstract: Fluidization of pyroclastic solids has long been indicated as one key to explain the enhanced mobility of dense pyroclastic gravity currents and their associated hazard. However there is a lack of characterization of the actual pattern and extent of fluidization establishing in real pyroclastic flows and some authors still raise arguments about the relevance of fluidization to the mobility of dense pyroclastic gravity currents. The present paper addresses the fluidization of pyroclastic granular solids with a specific focus on the analysis of factors that may promote homogeneous fluidization and retard solids de-aeration and consolidation. These factors include fines content, particle polydispersity and the establishment of shear flow. The influence of fines content and particle polydispersity has been assessed by analyzing the fluidization behaviour of mixtures of pyroclastic solids characterized by different and properly tailored particle size distributions and fines content. Pyroclastic mixtures, extracted from a Neapolitan Yellow Tuff deposit, were nearly homogeneous as to the lithological composition. The experimental campaign has been carried out with a lab-scale segmented fluidization column 120 mm ID. Experiments included quasi-steady fluidization of granular mixtures at variable gas superficial velocity as well as collapse tests from pre-set fluidized states. Results of the collapse tests were independently analyzed by looking at the time-series of the pressure drop across the bed as well as at the bed surface level as a function of time, recorded with a high-speed video camera. The study has been pursued further to analyze the effect of shear on the fluidization behaviour of pyroclastites. To this end the experimental apparatus has been modified and equipped with a rotating cylinder which could be used to impose a given shear rate to the fluidized bed. Shear rates were comparable to those that are likely to establish in real pyroclastic flows. Quasi-steady fluidization experiments and collapse tests were carried out under either stationary or sheared fluidization conditions. We compared the fluidization behaviour of mixtures characterized by different fines contents. Moreover we compared the fluidization behaviour recorded under either stationary or sheared flow conditions. Results of experimental tests highlight that both the presence of a fine fraction and the establishment of shear flow significantly influence the fluidization behaviour of pyroclastic mixtures, promoting homogeneous fluidization of the granular mixtures. The range of gas superficial velocities within which bubble-free fluidization occurs is significantly broadened by the presence of fines, by the polydisperse nature of the mixtures and by the establishment of shear. The de-aeration/consolidation times determined by the collapse tests are correspondingly much longer. Altogether, results of the present study confirm that key features of dense pyroclastic density currents, like the large fines fractional content, the polydisperse nature of the particle size distribution and strongly sheared flow conditions, exert a significant influence on fluidization, promote suppression of bubbling, retard solids de-aeration, enhance the mobility of the solids.

title: A conceptual model for block-and-ash flow basal avalanche transport and deposition, based on deposit architecture of 1998 and 1994 Merapi flows
abstract: The 1998 block-and-ash flow (BAF) deposits on the western flanks of Merapi were described both immediately following emplacement, and after one and two wet seasons of erosion. This afforded a unique combination of data on deposit morphology and internal structure from proximal to distal reaches. The deposits of the basal avalanches from individual BAF events were classified into three major and two minor facies in channelised reaches, primarily resulting from effects of channel confinement and slope. Termination reaches were classified into three facies, related to preevent topography. The deposit facies architecture was interpreted to understand particle transport and deposition within the framework of granular flow systematics. 
 
The transport model we propose for these basal avalanches involves unsteady flow conditions, generated by pulses of collapse as the source, and/or by development of kinetic waves during the granular flow. The coarsest clasts along with low-density material migrated to the wave/pulse fronts within these flows. Whereas, in bulk flow, grain dispersive pressures and kinetic sieving result in coarsest clasts being buoyed to the flow top, with this tendency being greatest on higher slopes (=greater kinetic energy) and with greater flow confinement (maximising particle collisions). 
 
Deposition begins as the flow reaches near its full extent. An initial pulse or flow wave freezes rapidly from its base upward and its rear forward as it looses kinetic energy and dispersive pressure, such that an upper higher energy part of the flow is still able to move downstream over the frozen base. Later waves or pulses then accrete progressively horizontally upslope banking up onto or overtopping earlier deposited sediment. Hence, the overall flow freezes from its front to its tail. At termination reaches in unconfined areas, the overflowing upper part of flow gradually deposits from its base upward, resulting in a slightly tapering sheet formed deposit, marked by a front made up of wood fragments and large boulders. On sudden slope breaks in termination areas, piling up of sediment occurs in flat reaches, producing whaleback type forms as the flows rapidly lost competence. 
 
One implication of this model is that each kinetic wave or flow-pulse crest developed within the basal avalanche represents a portion of greater flow depth (or stage height). Hence, development of these crests control the ability of parts of the basal avalanche to escape from confined channels and generate unexpected hazards.

title: Stratigraphy, sedimentology and inferred flow dynamics from the July 2015 block-and-ash flow deposits at Volcán de Colima, Mexico
abstract: The July 2015 block-and-ash flow (BAF) events represent the first documented series of large-volume and long-runout BAFs generated from sustained dome collapses at Volcan de Colima. This eruption is particularly exceptional at this volcano due to (1) the large volume of BAF material emplaced (0.0077 ± 0.001 km3), (2) the long runout reached by the associated BAFs (max. ~ 10 km), and (3) the short period (~ 18 h) over which two main long-sustained dome collapse events occurred (on 10 and 11 July, respectively). Stratigraphy and sedimentology of the 2015 BAF deposits exposed in the southern flank of the volcano based on lithofacies description, grain size measurements and clast componentry allowed the recognition of three main deposit facies (i.e., valley-confined, overbank and ash-cloud surge deposits). Correlations and lithofacies variations inside three main flow units from both the valley-confined and overbank deposits left from the emplacement of the second series of BAFs on 11 July provide detailed information about: (1) the distribution, volumes and sedimentological characteristics of the different units; (2) flow parameters (i.e., velocity and dynamic pressure) and mobility metrics as inferred from associated deposits; and (3) changes in the dynamics of the different flows and their material during emplacement. These data were coupled with geomorphic analyses to assess the role of the topography in controlling the behaviour and impacts of the successive BAF pulses on the volcano flanks. Finally, these findings are used to propose a conceptual model for transport and deposition mechanisms of the July 2015 BAFs at Volcan de Colima. In this model, deposition occurs by rapid stepwise aggradation of successive BAF pulses. Flow confinement in a narrow and sinuous channel enhance the mobility and runout of individual channelized BAF pulses. When these conditions occur, the progressive valley infilling from successive sustained dome-collapse events promote the overspill and lateral spreading of the upper and marginal regions of the main flow body, generating highly mobile overbank flows that travel outside of the main valley. Volume- and distance-dependent critical channel capacities for the generation of overbank flows are used to better estimate the inundation area of these hazardous unconfined pyroclastic flows. These results highlight the importance of including and correctly assessing the hazards posed by large volume and long runout BAFs associated with frequent, small VEI, sustained dome-collapse eruptions.