Dense Pyroclastic Flows Research Articles

Particle-size segregation in self-channelized granular flows

Geophysical mass flows such as debris flows, dense pyroclastic flows and snow avalanches can self-channelize on shallow slopes. The confinement afforded by formed levees helps to maintain the flow depth, and hence mobility, allowing self-channelized flows to run out significantly farther than unconfined, spreading flows. Levee formation and self-channelization are strongly associated with particle-size segregation, but can also occur in monodisperse flows. This paper uses the monodisperse depth-averaged theory of Rochaet al.(J. Fluid Mech., vol. 876, 2019, pp. 591–641), which incorporates a hysteretic friction law and second-order depth-averaged viscous terms. Both of these are vital for the formation of a travelling wave that progressively deposits a pair of levees just behind the front. The three-dimensional velocity field is reconstructed in a frame moving with the front assuming Bagnold flow. This enables a bidisperse particle-size segregation theory to be used to solve for the large and small particle concentrations and particle paths in three-dimensions, for the first time. The model shows that the large particles tend to segregate to the surface of the flow, forming a carapace that extends over the centre of the channel, as well as along the external sides and base of the levee walls. The small particles segregate downwards, and are concentrated in the main channel and in the inner levee walls. This supports the contention that a low-friction channel lining provides a secondary mechanism for run-out enhancement. It is also shown that the entire theory scales with particle diameter, so experiments with millimetre-sized particles provide important insights into geophysical-scale flows with boulders and smaller rock fragments. The model shows that self-channelization does not need particle-size segregation to occur, but supports the hypothesis that particle-size segregation and the associated frictional feedback can significantly enhance both the flow mobility and the levee strength.

Stress level effect on mobility of dry granular flows of angular rock fragments

Granular flows of angular rock fragments such as rock avalanches and dense pyroclastic flows are simulated numerically by means of the discrete element method. Since large-scale flows generate stresses that are larger than those generated by small-scale flows, the purpose of these simulations is to understand the effect that the stress level has on flow mobility. The results show that granular flows that slide en mass have a flow mobility that is not influenced by the stress level. On the contrary, the stress level governs flow mobility when granular flow dynamics is affected by clast agitation and collisions. This second case occurs on a relatively rougher subsurface where an increase of the stress level causes an increase of flow mobility. The results show also that as the stress level increases, the effect that an increase of flow volume has on flow mobility switches sign from causing a decrease of mobility at low stress level to causing an increase of mobility at high stress level. This latter volume effect corresponds to the famous Heim’s mobility increase with the increase of the volume of large rock avalanches detected so far only in the field and for this reason considered inexplicable without resorting to extraordinary mechanisms. Granular flow dynamics is described in terms of dimensionless scaling parameters in three different granular flow regimes. This paper illustrates for each regime the functional relationship of flow mobility with stress level, flow volume, grain size, channel width, and basal friction.

Granular mixture deflation and generation of pore fluid pressure at the impact zone of a pyroclastic fountain: Experimental insights

We present the results of analogue laboratory experiments on pyroclastic fountain collapse. Mixtures of air and glass beads ranging in size around 75 ± 15 μm, with Stokes number of ~10−3−101 and representative of 0.1–1 mm sized particles in nature, were released from a hopper at heights of 0.45–2.95 m above the base of a horizontal channel. Free fall caused continuous dilation of the granular material and led to mean particle concentrations of ~9–36 vol%, with concentration inversely proportional to drop height, before the particles impacted the channel base. Decoupling between the particles and the ambient air upon impact caused deflation of the mixture, which then propagated laterally as a dense granular flow overridden by a dilute suspension. Measurements at the impingement surface revealed that pore fluid pressure, generated through high air-particle relative velocity during deflation, counterbalanced up to ~50% of the weight of the emerging granular flow. The runout distance of the dense flow increased linearly with the fall height, similar to published results on unidirectional flows generated from collapse of packed granular columns. This suggests that the runout of flows resulting from release of granular material is controlled essentially by conversion of potential to kinetic energy and that the initial particle concentration is a second order parameter. We conclude that fountaining of pyroclastic material containing large amounts of particles with Stokes numbers of the order 10−3−101 can generate dense pyroclastic flows with some degree of pore fluid pressure.

Stresses at the base of dry and dense flows of angular rock fragments in 3-D discrete element modeling: Scaling of basal stress fluctuations versus grain size, flow volume and channel width

We simulate granular flows of angular rock fragments by means of a three-dimensional discrete element modeling to study the basal stresses that these flows exert on the subsurface. These granular flows have different grain sizes and different flow volumes and they model dry rock avalanches and dense pyroclastic flows. These flows travel on four different concave–upward chutes that represent channels on a mountainside or on the flank of a volcano. Each chute has a different width. The stress data demonstrate the validity of a linear relation between two scaling parameters: D and ψ. Parameter D is a scaled basal stress deviation that is equivalent to a scaled particle agitation. Particle agitation is ultimately responsible for the energy dissipation that governs the mobility of dense geophysical flows in nature. Parameter ψ contains grain size, flow volume and channel width. This second parameter is equal to the product of the reciprocal of characteristic numbers of fragments in granular flows. Since these numbers of particles are dimensionless, the linear relation is valid at any scale, either in the laboratory or in nature.

Slow-moving and far-travelled dense pyroclastic flows during the Peach Spring super-eruption.

Explosive volcanic super-eruptions of several hundred cubic kilometres or more generate long run-out pyroclastic density currents the dynamics of which are poorly understood and controversial. Deposits of one such event in the southwestern USA, the 18.8 Ma Peach Spring Tuff, were formed by pyroclastic flows that travelled >170 km from the eruptive centre and entrained blocks up to ∼70–90 cm diameter from the substrates along the flow paths. Here we combine these data with new experimental results to show that the flow's base had high-particle concentration and relatively modest speeds of ∼5–20 m s−1, fed by an eruption discharging magma at rates up to ∼107–108 m3 s−1 for a minimum of 2.5–10 h. We conclude that sustained high-eruption discharge and long-lived high-pore pressure in dense granular dispersion can be more important than large initial velocity and turbulent transport with dilute suspension in promoting long pyroclastic flow distance.

Sedimentology and geomorphology of the deposits from the August 2006 pyroclastic density currents at Tungurahua volcano, Ecuador.

The deposits of the pyroclastic density currents from the August 2006 eruption of Tungurahua show three facies associations depending on the topographic setting: the massive, proximal cross-stratified, and distal cross-stratified facies. (1) The massive facies is confined to valleys on the slopes of the volcano. It contains clasts of >1 m diameter to fine ash material, is massive, and interpreted as deposited from dense pyroclastic flows. Its surface can exhibit lobes and levees covered with disk-shaped and vesicular large clasts. These fragile large clasts must have rafted at the surface of the flows all along the path in order to be preserved, and thus imply a sharp density boundary near the surface of these flows. (2) The proximal cross-stratified facies is exposed on valley overbanks on the upper part of the volcano and contains both massive coarse-grained layers and cross-stratified ash and lapilli bedsets. It is interpreted as deposited from (a) dense pyroclastic flows that overflowed the gentle ridges of valleys of the upper part of the volcano and (b) dilute pyroclastic density currents created from the dense flows by the entrainment of air on the steep upper flanks. (3) The distal cross-stratified facies outcrops as spatially limited, isolated, and wedge-shaped bodies of cross-stratified ash deposits located downstream of cliffs on valleys overbanks. It contains numerous aggrading dune bedforms, whose crest orientations reveal parental flow directions. A downstream decrease in the size of the dune bedforms, together with a downstream fining trend in the grain size distribution are observed on a 100-m scale. This facies is interpreted to have been deposited from dilute pyroclastic density currents with basal tractional boundary layers. We suggest that the parental flows were produced from the dense flows by entrainment of air at cliffs, and that these diluted currents might rapidly deposit through “pneumatic jumps”. Three modes are present in the grain size distribution of all samples independently of the facies, which further supports the interpretation that all three facies derive from the same initial flows. This study emphasizes the influence of topography on small volume pyroclastic density currents, and the importance of flow transformation and flow-stripping processes.

Dune bedforms produced by dilute pyroclastic density currents from the August 2006 eruption of Tungurahua volcano, Ecuador

A series of pyroclastic density currents were generated at Tungurahua volcano (Ecuador) during a period of heightened activity in August 2006. Dense pyroclastic flows were confined to valleys of the drainage network, while dilute pyroclastic density currents overflowed on interfluves where they deposited isolated bodies comprising dune bedforms of cross-stratified ash exposed on the surface. Here, the description, measurement, and classification of more than 300 dune bedforms are presented. Four types of dune bedforms are identified with respect to their shape, internal structure, and geometry (length, width, thickness, stoss and lee face angles, and stoss face length). (1) “Elongate dune bedforms” have smooth shapes and are longer (in the flow direction) than wide or thick. Internal stratification consists of stoss-constructional, thick lensoidal layers of massive and coarse-grained material, alternating with bedsets of fine laminae that deposit continuously on both stoss and lee sides forming aggrading structures with upstream migration of the crests. (2) “Transverse dune bedforms” show linear crests perpendicular to the flow direction, with equivalent lengths and widths. Internally, these bedforms exhibit finely stratified bedsets of aggrading ash laminae with upstream crest migration. Steep truncations of the bedsets are visible on the stoss side only. (3) “Lunate dune bedforms” display a barchanoidal shape and have stratification patterns similar to those of the transverse ones. Finally, (4) “two-dimensional dune bedforms” are much wider than long, exhibit linear crests and are organized into trains. Elongate dune bedforms are found exclusively in proximal deposition zones. Transverse, lunate, and two-dimensional dune bedforms are found in distal ash bodies. The type of dune bedform developed varies spatially within an ash body, transverse dune bedforms occurring primarily at the onset of deposition zones, transitioning to lunate dune bedforms in intermediate zones, and two-dimensional dune bedforms exclusively on the lateral and distal edges of the deposits. The latter are also found where flows moved upslope. Elongate dune bedforms were deposited from flows with both granular-based and tractional flow boundaries that possessed high capacity and competence. They may have formed in a subcritical context by the blocking of material on the stoss side. We do not interpret them as antidune or “chute-and-pool” structures. The dimensions and cross-stratification patterns of transverse dune bedforms are interpreted as resulting from low competence currents with a significant deposition rate, but we rule out their interpretation as “antidunes”. A similar conclusion holds for lunate dune bedforms, whose curved shape results from a sedimentation rate dependent on the thickness of the bedform. Finally, two-dimensional dune bedforms were formed where lateral transport exceeds longitudinal transport; i.e., in areas where currents were able to spread laterally in low velocity zones. We suggest that the aggrading ash bedsets with upstream crest migration were formed under subcritical flow conditions where the tractional bedload transport was less important than the simultaneous fallout from suspension. This produced differential draping with no further reworking. We propose the name “regressive climbing dunes” for structures produced by this process. A rapid decrease in current velocity, possibly triggered by hydraulic jumps affecting the entire parent flows, is inferred to explain their deposition. This process can in principle hold for any kind of particulate density current.Electronic supplementary materialThe online version of this article (doi:10.1007/s00445-013-0762-x) contains supplementary material, which is available to authorized users.

Granular pressure at the base of dry flows of angular rock fragments as a function of grain size and flow volume: A relationship from laboratory experiments

Experiments are carried out by releasing angular rock fragments down a curved chute and by measuring the basal pressures that are exerted by the granular flows on the basal containing surface (the substrate). The purpose of these experiments is to understand the mechanisms of energy dissipation and interaction with the ground of rock avalanches and dense pyroclastic flows. Our data show that collisions due to particle agitation affect significantly the basal interaction of granular flows. In particular, our experiments reveal that particle agitation per unit of flow mass increases as grain size increases or as flow volume decreases (with all the other features the same). This is so because as grain size increases or as flow volume decreases (with all the other features the same), there are fewer particles in the flows and the agitation due to the interaction with the rough containing boundary surfaces penetrates relatively more inside the flows. The analysis of the experimental data generates a linear relationship between particle agitation (expressed as a dimensionless basal pressure deviation) and a parameter which is directly proportional to the square of grain size and inversely proportional to the cube root of flow volume. This relationship shows the intrinsic ability of the granular flows to dissipate more energy (larger particle agitation per unit of flow mass) or less energy (smaller particle agitation per unit of flow mass) as a function of flow volume and grain size.

A semi-empirical method to calculate the permeability of homogeneously fluidized pyroclastic material

This work postulates that highly polydisperse materials have an effective size distribution that controls permeability. Existence of such effective distribution implies that not all clasts participate to the permeable network resisting to gas flow and that clasts smaller than the minimal effective size are elutriated. When this concept is coupled to a generalized Blake–Kozeny equation, the resulting semi-empirical law links permeability to material properties only (bed void fraction, clast sizes and densities). After calibration of an experimental constant, it is able to replicate within ±0.6log unit experimentally measured permeabilities of both loosely packed and expanded beds made of highly polydisperse (from 1μm to 4mm) pyroclastic deposits that were resampled so as to ensure homogeneous fluidization. The presence of an experimentally calibrated constant and the necessary absence of segregation during fluidization limit the extrapolation of the proposed law to any pyroclastic bed. Satisfactory fitting of the experimental values, however, confirms that the permeability of homogeneously fluidized beds is controlled by a balance between settling and elutriation. This balance suggests a first-order link between permeability and bed expansion, which has implications on the kinetics of dense pyroclastic flows.

Effects of flow volume and grain size on mobility of dry granular flows of angular rock fragments: A functional relationship of scaling parameters

Flows of angular rock fragments are released down a concave upward chute in the laboratory to study their mobility. This mobility is measured as the reciprocal of the apparent coefficient of friction that is equal to the vertical drop of the center of mass of the granular material divided by its horizontal distance of travel. Our experiments show that the finer the grain size (with all the other features the same), the larger the mobility of the center of mass. We believe this to be due to the fact that in finer grain size flows there are less agitated particles per unit of flow mass, so that these flows dissipate less energy per unit of travel distance. Our experiments show also that the larger the volume (with all the other features the same), the larger the apparent coefficient of friction. We believe this to be so because the frontal portion of a flow reaches the less steep part of a curved slope and stops before the rear portion, preventing the rear portion and the center of mass from traveling farther downhill. This phenomenon (which is more prominent in larger‐volume flows, whose rear and frontal ends are more distant) counteracts the decrease of energy dissipation per unit of flow mass because of the decrease of particle agitation per unit of flow mass that is expected when the volume of a flow increases (with all the other features the same). Our analysis generates a functional relationship between the dimensionless apparent coefficient of friction and a scaling parameter whose numerator is equal to the mean grain size multiplied by the cube root of the deposit volume and whose denominator is the square of the channel width. The purpose of our experiments is to understand the dynamics of rock avalanches and dense pyroclastic flows.

Suitability of simple rheological laws for the numerical simulation of dense pyroclastic flows and long-runout volcanic avalanches

[1] The rheology of volcanic rock avalanches and dense pyroclastic flows is complex, and it is difficult at present to constrain the physics of their processes. The problem lies in defining the most suitable parameters for simulating the behavior of these natural flows. Existing models are often based on the Coulomb rheology, sometimes with a velocity-dependent stress (e.g., Voellmy), but other laws have also been used. Here I explore the characteristics of flows, and their deposits, obtained on simplified topographies by varying source conditions and rheology. The Coulomb rheology, irrespective of whether there is a velocity-dependent stress, forms cone-shaped deposits that do not resemble those of natural long-runout events. A purely viscous or a purely turbulent flow can achieve realistic velocities and thicknesses but cannot form a deposit on slopes. The plastic rheology, with (e.g., Bingham) or without a velocity-dependent stress, is more suitable for the simulation of dense pyroclastic flows and long-runout volcanic avalanches. With this rheology, numerical flows form by pulses, which are often observed during natural flow emplacement. The flows exhibit realistic velocities and deposits of realistic thicknesses. The plastic rheology is also able to generate the frontal lobes and lateral levees which are commonly observed in the field. With the plastic rheology, levee formation occurs at the flow front due to a divergence of the driving stresses at the edges. Once formed, the levees then channel the remaining flow mass. The results should help future modelers of volcanic flows with their choice of which mechanical law corresponds best to the event they are studying.

A two-layer approach to modelling the transformation of dilute pyroclastic currents into dense pyroclastic flows

Most models of volcanic ash flows assume that the flow is either dilute or dense, with dynamics dominated by fluid turbulence or particle collisions, respectively. However, most naturally occurring flows feature both of these end members. To this end, a two-layer model for the formation of dense pyroclastic basal flows from dilute, collapsing volcanic eruption columns is presented. Depth-averaged, constant temperature, continuum conservation equations to describe the collapsing dilute current are derived. A dense basal flow is then considered to form at the base of this current owing to sedimentation of particles and is modelled as a granular avalanche of constant density. We present results which show that the two-layer model can predict much larger maximum runouts than would be expected from single-layer models, based on either dilute or dense conditions, as the dilute surge can outrun the dense granular flow, or vice versa, depending on conditions.

Pore fluid pressure and internal kinematics of gravitational laboratory air‐particle flows: Insights into the emplacement dynamics of pyroclastic flows

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.

Large particle segregation, transport and accumulation in granular free-surface flows

Particle size segregation can have a significant feedback on the motion of many hazardous geophysical mass flows such as debris flows, dense pyroclastic flows and snow avalanches. This paper develops a new depth-averaged theory for segregation that can easily be incorporated into the existing depth-averaged structure of typical models of geophysical mass flows. The theory is derived by depth-averaging the segregation-remixing equation for a bi-disperse mixture of large and small particles and assuming that (i) the avalanche is always inversely graded and (ii) there is a linear downslope velocity profile through the avalanche depth. Remarkably, the resulting ‘large particle transport equation’ is very closely related to the segregation equation from which it is derived. Large particles are preferentially transported towards the avalanche front and then accumulate there. This is important, because when this is combined with mobility feedback effects, the larger less mobile particles at the front can be continuously shouldered aside to spontaneously form lateral levees that channelize the flow and enhance run-out. The theory provides a general framework that will enable segregation-mobility feedback effects to be studied in detail for the first time. While the large particle transport equation has a very simple representation of the particle size distribution, it does a surprisingly good job of capturing solutions to the full theory once the grains have segregated into inversely graded layers. In particular, we show that provided the inversely graded interface does not break it has precisely the same solution as the full theory. When the interface does break, a concentration shock forms instead of a breaking size segregation wave, but the net transport of large particles towards the flow front is exactly the same. The theory can also model more complex effects in small-scale stratification experiments, where particles may either be brought to rest by basal deposition or by the upslope propagation of a granular bore. In the former case the resulting deposit is normally graded, while in the latter case it is inversely graded. These completely opposite gradings in the deposit arise from a parent flow that is inversely graded, which raises many questions about how to interpret geological deposits.

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

We conducted laboratory experiments on dam-break flows of sub-250-µm volcanic ash, generated by the release of gas-fluidized and variably non-expanded to expanded (up to 35%) beds, in order to gain insights into the internal kinematics of pyroclastic flows. The flows were typically several cm thick and had frontal speeds of up to ∼2 m s−1. High-speed videos taken through the transparent sidewall of the 3-m-long channel were analyzed with a particle-tracking algorithm, providing a spatial and temporal description of transport and sedimentation. The flows deposited progressively as they traveled down the flume, being consumed by sedimentation until they ran out of volume. Deposition commenced 5–20 cm rearward of the flow front and (for a given expansion) proceeded at a rate independent of distance from the lock gate. Deposit aggradation velocities were equal to those inferred beneath quasi-static bed collapse tests of the same ash at the same initial expansions, implying that shear rates of up to ∼300 s−1 have no measurable effect on aggradation rate. The initially non-expanded (and just fluidized) flow deposited progressively at a rate indicative of an expansion of a few percent, perhaps due to shear-induced Reynolds dilation during initial slumping. The fronts of the flows slid across the flume floor on very thin basal shear layers, but once deposition commenced a no-slip condition was established at the depositional interface. Within the flows, the trajectory of the constituent particles was linear and sub-horizontal. The velocities of the particles increased with height above the depositional interface, reached a maximum, then declined slightly towards the flow surface, perhaps due to air drag. At a given location, the velocity profiles were translated upwards as the deposit aggraded. The results show that even cm-thin, poorly expanded flows of ash deposit progressively, as inferred for many pyroclastic flows. The change from (frontal) slip to (rearward) no-slip conditions at the bases of the laboratory flows are qualitatively consistent with some textural features of pyroclastic flow deposits.

Modeling dense pyroclastic basal flows from collapsing columns

A two layer model for the formation of dense pyroclastic basal flows from dilute, collapsing volcanic eruption columns is presented. The collapsing dilute current is described by depth averaged, isothermal, continuum conservation equations. The dense basal flow is modelled as a granular avalanche of constant density. Simulations demonstrate that pyroclastic flow formation and behavior is dependent upon slope conditions when the dilute part of the current has lost most of its mass. The dilute current runout increases with decreasing particle size and increasing initial column height. If the dilute current has transferred its mass to the dense basal flow on volcanic slopes with inclination angle greater than the friction angle of the basal flow, then the basal flow will continue to propagate until frictional forces bring it to rest. If the dilute current terminates on lower angled slopes, frictional forces dominate the basal flow preventing further front propagation.

On the dynamics of volcanic columns: A comparison of field data with a new model of negatively buoyant jets

Explosive volcanic eruptions propel high-velocity turbulent jets into the atmosphere and are one of the most powerful and dangerous turbulent flows on Earth. Such eruptions are particularly difficult to predict due to their unusual dynamics that allow the jet to form a high buoyant Plinian plume or dense pyroclastic flows when the column collapses. A major goal of physical volcanology has been to predict quantitatively the limit between the flow regimes, as a function of exit conditions specified at the vent, which requires a physical model constrained by geologic field data. A first generation of quasi-analytic models, developed over 20–30 years although qualitatively successful, has failed to achieve this goal, because they overestimate by more than one order of magnitude the mass flux at the transition. There has been no consensus on whether this mostly reflects poor knowledge of geologic parameters or inadequacies in the physical description of the dynamics. Direct numerical simulations bring considerable advances in the study of unsteady phenomena that occur within the volcanic column, but their complexity makes the deciphering of the underlying dynamics challenging. Here, we present a basis for a new generation of 1D-models that includes a more sophisticated description of the rate of entrainment of air into the jet as a function of its buoyancy. In this framework, we demonstrate that the inconsistency between previous 1D models and field data is due to the omission of a key ingredient in the modeling of turbulence: reduction of entrainment due to the high density of the volcanic jet. We show that negative buoyancy near the base of the jet reduces the coefficient governing turbulent mixing to much less than the value commonly assumed based on experimental studies of neutrally buoyant jets and positively buoyant plumes. The results of our new model, allowing for variable entrainment as a function of local Richardson number, greatly improve the quantitative prediction of the transition as constrained by field data from a large number of eruptions.

Phreatomagmatic and phreatic eruptions in locally extensive settings of Southern Central Andes: The Tocomar Volcanic Centre (24°10′S–66°34′W), Argentina

Two Pleistocene eruptions are studied, linked to an increment in the tectonic activity of an extensive system related to Calama–Olacapato–El Toro Fault (COT Fault). The first one, consisting of dense pyroclastic flows and pyroclastic surges interbedded with fall deposits, was caused by a phreatoplinian eruption. The second one, consisting of a succession of pyroclastic surges, derived from a phreatic eruption. Both eruption vents have been located in en echelon normal faults with an NNE trend, and conjugated to strike faults following an NNW trend. In both episodes pyroclastic deposits and ballistic blocks follow an E–SSE trend, the direction to which the fault planes and topographical slope lean. The obsidianic deposit filling the first eruption vent has the same trend and dip as the main fault plane and provides (preliminary) evidences of syntectonic emplacement. On the other hand, pyroclastic surge deposits linked to the second episode offer evidences of syndepositional faulting with listric growth faults. It is concluded that this local horizontal area linked to the COT fault was intensely active during the Pleistocene and triggered both eruptions.

Relation between dry granular flow regimes and morphology of deposits: formation of levées in pyroclastic deposits

Experiments on dry granular matter flowing down an inclined plane are performed in order to study the dynamics of dense pyroclastic flows. The plane is rough, and always wider than the flow, focusing this study on the case of laterally unconfined (free boundary) flows. We found that several flow regimes exist depending on the input flux and on the inclination of the plane. Each flow regime corresponds to a particular morphology of the associated deposit. In one of these regimes, the flow reaches a steady state, and the deposit exhibits a levée/channel morphology similar to those observed on small pyroclastic flow deposits. The levées result from the combination between lateral static zones on each border of the flow and the drainage of the central part of the flow after the supply stops. Particle segregation features are created during the flow, corresponding to those observed on the deposits of pyroclastic flows. Moreover, the measurements of the deposit morphology (thickness of the channel, height of the levées, width of the deposit) are quantitatively related to parameters of the dynamics of the flow (flux, velocity, height of the flow), leading to a way of studying the flow dynamics from only measurements of the deposit. Some attempts to make extensions to natural cases are discussed through experiments introducing the polydispersity of the particle sizes and the particle segregation process.

Fall vs flow activity during the 1991 climactic eruption of Pinatubo Volcano (Philippines)

Six years after the 1991 Mt. Pinatubo eruption, deep erosion incisions into the pyroclastic deposits accumulated around the volcano enabled us to investigate the stratigraphy of the climactic deposits both in valley bottoms and on contiguous ridges. Stratigraphic relationships between fall, flow, and surge deposits in the Marella drainage system indicate that during the climactic eruption a progressive shift occurred from an early convective regime, to a transitional regime feeding both the plinian convective column and mostly dilute density currents, to a fully collapsing regime producing mostly dense pyroclastic flows. Syn-plinian dilute density currents (surges) propagated up to ~10 km from the crater, both along valley bottoms and on contiguous ridges of the Marella Valley, whereas post-plinian pyroclastic flows had greater runout (~13 km), were confined to valleys and were not associated with significant surges. Stratigraphic study and grain-size analyses allow the identification of three types of intra-plinian deposits: (a) lower and often coarse-grained surge deposits, emplaced during the accumulation of the coarsest portion of the fallout bed at time intervals of ~16–24 min; (b) upper fine-grained surge deposits, interstratified with the fine-grained portion of the fall bed and emplaced at shorter time intervals of ~3–13 min; and (c) small-volume, channel-confined, massive pumiceous flow deposits interbedded with the upper surges in the upper fine-grained fall bed. Maximum clast size isopleths of 1.6 and 0.8 cm for lithics (ML) and 2.0 and 4.0 cm for pumices (MP) show almost symmetrical distribution around the vent, indicating that the passing of the typhoon Yunya during the climactic eruption had little effect on trajectories of high-Reynold-number clasts. Significant distortion was, however, observed for the 3.2-cm ML and 6.0-cm MP proximal isopleths, whose patterns were probably influenced by the interaction of the clasts falling from column margins with the uprising co-ignimbrite ash plumes. Application of the Carey and Sparks (1986) model to the undisturbed isopleths generated by the umbrella cloud yields a maximum column height of ~42 km, in good agreement with satellite measurements. Systematic stratigraphic and vertical grain-size studies of the plinian fall deposit in the Marella Valley, combined with satellite data and eyewitness accounts, reveal that the carrying capacity of the convective column and related fallout activity peaked in the early phase of the eruption, beginning slightly before 13:41 and gradually declined until its cessation 3 h later. Most of the pumiceous pyroclastic flow deposits were emplaced after the end of the fallout activity at ~16:30 but before the summit caldera collapse at approximately 19:11. Only a small volume of pumiceous flow deposits accumulated after the final caldera collapse. In contrast to the previous reconstruction of Holasek et al. (1996), which interpreted the progressive lowering of the column, documented by satellite data, as due to a decreasing mass eruption rate, we suggest that a progressive shift from a plinian column to a large co-ignimbrite column could also account for such a variation.