Bulletin of Volcanology (2023) 85:67
https://doi.org/10.1007/s00445-023-01682-9
RESEARCH ARTICLE
Cohesional behaviours in pyroclastic material and the implications
for deposit architecture
Nemi Walding1,2
· Rebecca Williams2
· Pete Rowley3
· Natasha Dowey4
Received: 20 June 2023 / Accepted: 11 October 2023 / Published online: 27 October 2023
© The Author(s) 2023
Abstract
Pyroclastic density currents (PDCs) are hazardous, multiphase currents of heterogeneous volcanic material and gas. Moisture
(as liquid or gas) can enter a PDC through external (e.g., interaction with bodies of water) or internal (e.g., initial eruptive activity style) processes, and the presence of moisture can be recorded within distinct deposit layers. We use analogue
experiments to explore the behaviour of pyroclastic material with increasing addition of moisture from 0.00–10.00% wt.
Our results show that (1) the cohesivity of pyroclastic material changes with the addition of small amounts of moisture, (2)
small increases in moisture content change the material properties from a free-flowing material to a non-flowable material,
(3) changes in moisture can affect the formation of gas escape structures and fluidisation profiles in pyroclastic material,
(4) gas flow through a deposit can lead to a moisture profile and resulting mechanical heterogeneity within the deposit and
(5) where gas escape structure growth is hindered by cohesivity driven by moisture, pressure can increase and release in an
explosive fashion. This work highlights how a suite of varied gas escape morphologies can form within pyroclastic deposits
resulting from moisture content heterogeneity, explaining variation in gas escape structures as well as providing a potential
mechanism for secondary explosions.
Keywords Cohesion · Gas escape · Fluidisation · Secondary explosions · Volcaniclastics · Volcanology
Introduction
Pyroclastic density currents (PDC) are hazardous, rapidly
moving and often high-temperature volcanic phenomena.
These currents are multiphase mixtures of heterogeneous
juvenile material, atmospheric gas, and accessory lithic fragments. The high mobility of PDCs has in part been attributed
to the onset of fluidisation (Sparks 1976; 1978; Wilson 1984;
Branney and Kokelaar 1992, 2002; Roche 2012; Aravena
et al. 2021; Breard et al. 2017; 2023): the upward movement
of gas counterbalances the force of gravity and supports the
Editorial responsibility: L. Pioli
* Nemi Walding
n.walding-2021@hull.ac.uk
1
Energy and Environment Institute, University of Hull, HU6
7TQ, Hull, UK
2
School of Environmental Sciences, University of Hull, Cohen
Building, HU6 7RX, Hull, UK
3
University of Bristol, Bristol, UK
4
Sheffield Hallam University, Sheffield, UK
flow (Sparks 1976; Branney and Kokelaar 2002; Cocco et al.
2014). The ability of the material to flow, or its ‘flowability’,
depends upon interparticle forces (Van der Waals, electrostatic or capillary forces). These forces can be influenced by
bulk composition and material physical properties such as
particle size, density, shape and moisture content (Roche et al.
2004; Rios 2006; Druitt et al. 2007; Leturia et al. 2014).
Fluidisation in PDCs can be initiated from formation and
maintained throughout the course of the flow by transport on
steep slopes, flow channelisation (Kelfoun and Gueugneau
2022), substrate evaporation (i.e., steam generated from interaction with surfaces with moisture content or bodies of water),
bulk self-fluidisation or ambient air entrainment (Sparks 1978;
Branney and Kokelaar 2002; Chedeville and Roche 2015; Valentine and Sweeny 2018; Kelfoun and Gueugneau 2022, Breard
et al. 2023). Sedimentation fluidisation (or, hindered settling)
and particle-self fluidisation is the interstitial gas movement
driven by particle settling and compaction (Aravena et al. 2021;
Roche 2012; Breard et al. 2017; Chedeville and Roche 2018).
On or after deposition, the material will defluidise and particles
segregate forming gas escape structures (i.e., fines depleted
elutriation pipes) (Wilson 1980; Cas and Wright 1991).
13
Vol.:(0123456789)
67
Page 2 of 18
Previous analogue investigations into fluidisation behaviours of pyroclastic material and segregation structures have
been completed on dry (0% water content; Wilson 1980;
1984) and saturated (80 ± 15% water content; Roche et al.
2001) natural pyroclastic material. Experiments completed
by Wilson (1980; 1984) used non-cohesive, poorly sorted
pyroclastic mixtures and added an influx of gas into the
deposit. This resulted in poor fluidisation, along with the
creation of gas escape structures dictated by particle size
and density. In a study by Roche et al. (2001), aqueous fluidisation within a water-saturated deposit of volcanic material was investigated. These findings revealed that fluidescape pipes formed easily under conditions of low water
flux, leading to localised separation of particle sizes and
densities. Combining the results from both experiments, we
can conclude that natural pyroclastic material will exhibit
aggregative fluidisation, where fluidisation is inhomogeneous throughout the deposit through creation of bubbling
and channelling (Branney and Kokelaar 2002; PachecoHoyos et al. 2020). This behaviour arises due to the particle
size and density range, regardless of the medium used for
fluidisation.
Understanding how moisture, through adsorption of
atmospheric humidity, impacts powder material has important industrial applications. Experiments have explored fluidisation behaviours of industrial material with the addition
of small volumes of moisture by controlling environment
humidity levels. With the introduction of moisture into a
material, Van der Waals forces are no longer dominant and
liquid bridges connect particles through capillary cohesion;
resulting in poor fluidisation behaviours (Wormsbecker and
Pugsley 2008; Ludwig et al. 2020; Yehuda and Kalman
Bulletin of Volcanology (2023) 85:67
2020). A study by Wormsbecker and Pugsley (2008) looked
at gas fluidisation behaviours on a semi-saturated (30, 20, 15
and 5 wt.% moisture) powder. Results showed a significant
change in fluidisation behaviour associated with the addition
of moisture, which were observed in conjunction with the
drying states of the material from 30 to 5 wt.%.
Moisture in PDCs and their resulting
deposits
Moisture (i.e., water vapour or liquid water) can enter a PDC
system during formation at source or as PDCs propagate
(Fig. 1). Eruption columns can be water rich due to phreatomagmatic interaction (Self and Sparks 1978; Hurwitz
et al. 2003; Houghton et al. 2015; Shimizu et al. 2023) or
atmospheric conditions (Vecino et al. 2022). During transport,
internal clasts of juvenile magma will exsolve and release
water vapour and other volatiles. Experiments have highlighted how magmatic clasts may hold 22–86% vol. residual
gas and water content during initiation of PDC transport
(Sparks 1978). Gas diffusion times of water content depend
on particle size, where larger more porous clasts are thought
to release gas more rapidly, and temperature changes, where
cooler temperature reduce diffusivity of gas (Sparks 1978).
Moisture may be introduced through a combination of
atmospheric (e.g., humidity; Pepin et al. 2017; Camuffo
2019), topographic (e.g., height; Barclay et al. 2006; Duane
et al. 2008; Hartmann 2016), climatic (e.g., global location;
Barclay et al. 2006) and meteorological (e.g., precipitation)
conditions. Furthermore, periods of intense rainfall have
been suspected and observed to affect the onset of volcanic
Fig. 1 Schematic illustration of a PDC interacting with sources of moisture across a landscape which have the potential to enter the PDC system
and resulting deposits
13
Page 3 of 18 67
Bulletin of Volcanology (2023) 85:67
activity (Barclay et al. 2006; Sahoo et al. 2022 and references therein). Matthews et al. (2009) documented that
within 24 h of heavy rainfall, the probability of lava dome
collapse at Soufriere Hills Volcano, Montserrat (during the
period 1998–2003), increased, resulting in higher moisture
availability to the resulting PDCs.
Interaction with external bodies of water (i.e., streams,
lakes, sea, snow; Dartevelle et al. 2002; Cole et al. 1998;
2002), water saturated substrate (Moyer and Swanson 1987;
Brown and Branney 2013; Gilbertson et al. 2020) or by the
incorporation of vegetation (as observed at Mount Pelé,
1902; Mount St Helens, 1980; Montserrat, 2002; and Fuego
Volcano, 2018) can also contribute to moisture within the
PDC system. Therefore, we expect moisture content in PDCs
and their resultant deposits to be variable in time and space,
for example, we can expect high water contents in deposits
near bodies of water (i.e., following PDC interaction with
a lake) than perhaps in areas where very small amounts of
juvenile water are contributing to the overall moisture content (i.e., exsolving juvenile clasts).
The presence of moisture within PDCs can be demonstrated by the presence of peculiar features in their deposit.
Moisture has been linked to the formation of wet ash aggregates (e.g., pellets) in pyroclastic deposits (Brown et al.
2010; Van Eaton and Wilson 2013), to elutriation pipes that
are rooted in areas of evaporating moisture (i.e., vegetation
or water-laden sediments; Pacheco-Hoyos et al. 2020) or
by secondary hydroeruptions forming in deposits overlying moisture-rich areas (e.g., Mount St. Helens; Moyer and
Swanson 1987). The influence of these relatively small additions of moisture into a PDC system has been largely ignored
in analogue and experimental studies, due to the difficulty of
using and controlling the characteristics of moisture-affected
material. Therefore, prior to experiments, the material is
generally dried to remove any residual moisture (Druitt et al.
2004, 2007; Girolami et al. 2008; 2015).
Capillary cohesion
The presence of moisture in a PDC, or in a subsequent
deposit, will result in cohesional forces within the interparticulate space. A PDC can reach temperatures > 1000 °C
and the resulting deposit can maintain high temperatures
for extended periods of time (Dufek 2016; Riehle et al.
1995), and it has been assumed that at these temperatures,
the dominant cohesive forces will be electrostatic and Van
der Waals forces (Branney and Kokelaar 2002). However,
with increasing distance and entrainment, temperatures will
decrease (Benage et al. 2016; Dellino et al. 2021; Pensa et al.
2023), and the introduction of moisture will likely lead to
the formation of capillary bridges (‘capillary condensation’; Ma et al. 2019), resulting in a change of the dominant
interparticulate forces. This is described in Telling et al.
(2013), where electrostatic attraction has been observed to
be dominant only where water vapour (i.e., humidity) was
lower than 71% and in Chigira and Yokoyama (2005), where
capillary cohesion became the dominant cohesive force with
the addition of moisture into the granular material.
Previous studies have shown that an increase in water
content and moisture leads to a drastic change in the physical properties of a given material. For example, in sands,
capillary forces were seen to affect the tensile strength of
the material until reaching a water-saturated state (Kim and
Sture 2008; Chen et al. 2021). Therefore, at lower temperatures it is highly likely that the introduction of moisture into
the dynamic (current) and static (deposit) regions will induce
variations in material properties. Changes in tensile strength
and yield stress may determine how resistant a material is to
shear and erode and are important in understanding the flow
properties of a material (Pierrat and Caram 1997; LaMarche
et al. 2016). Within a PDC deposit, such changes may also
influence defluidisation through gas escape.
The experiments detailed herein assess the impact of
the addition of small volumes of moisture within natural
pyroclastic material. We explore the resulting variations in
terms of fluidisation and particle segregation behaviours.
Our results provide new and novel insights into the variation
of gas escape behaviours and resulting secondary explosions
in a defluidising PDC deposit.
Methodology
Source material and sample preparation
Unconsolidated material collected in 2009 from the 2006
Tungurahua, Ecuador, eruptions (provided by U. Küppers,
LMU Munich) has been subjected to a range of characterisation tests to elucidate flowability properties and variations
with moisture content. The studied material is dark grey/
brown and andesitic in chemistry and was sampled from
the PDC material deposited during the 2006 VEI 3 eruption
(Eychenne et al. 2012). The PDCs, formed from the destabilisation of erupted deposits at the vent (Douillet et al. 2013),
reached a maximum of 8.5 km from source and descended
2600–3000 m altitude (Hall et al. 2007; Kelfoun et al. 2009).
Samples were dried in an 80 °C oven for 24 h to ensure
the removal of residual and adsorbed moisture, and agglomerations were broken up by sieving prior to addition of water.
The experiments were completed using 6 samples of the
Tungurahua pyroclastic material (Fig. 2, V1–V6). Samples V1, V4, V5 and V6 were sieved into desired particle
size distributions, whereas samples V2 and V3 were kept
as sampled at source (ranging from > 74 to 300 µm). For
the series of characterisation tests, water was added to the
13
13
72.93
107.5
115.0
153.5
347.3
557.1
2.5–297.3
15–425
5–1000
20–650
10–1000
10–1000
3.734
3.320
3.140
2.710
1.568
0.812
35.76
21.41
19.57
0.19
0.11
0.05
0.428
0.710
0.868
0.252
0.758
0.445
Well
Moderate
Moderate
Very well
Moderate
Well
0.77
0.79
0.74
0.84
0.80
0.79
1.891
− 0.524
− 0.562
0.141
− 0.103
0.589
17.00
2.576
4.140
8.536
2.480
8.277
56.3 × 10−6
21.89 × 10−6
22.9 × 10−6
26.18 × 10−6
55.6 × 10−6
112.84 × 10−6
Geldart group
Geometric mean
(µm)
Sorting
(𝜎G )
Sphericity
Skewness (Sk Kurtosis
(K ) ∅
)∅
Sauter mean
(m)
3.776
3.215
3.118
2.703
1.508
0.833
Particle analysis of the pyroclastic material was undertaken
using a CAMSIZER X2. This uses particle imaging to build
particle shape and size characteristics for dry samples. The
maximum resolution for particle size and shape of the CAMSIZER is 0.8 µm per pixel. Particles were sieved prior to
using the CAMSIZER and samples < 1000 µm were used.
Any results from the CAMSIZER erroneously returned
as > 1460 µm were removed. The CAMSIZER results
allowed us to calculate the sphericity and cumulative size
of the samples. The latter were run through GRADISTAT
(Blott and Pye 2001) to obtain the particle size mean ((x̅) ∅),
median (∅), range (µm), sorting index ((σ) ∅), sorting (σG),
skewness ((Sk) ∅), kurtosis ((K) ∅) and geometric mean
(µm). Using methods from Breard et al. (2019), we were
then able to calculate the Sauter mean (m) and fines content
(%) of the material. All characteristics of each sample are
presented in Table 1.
V1
V2
V3
V4
V5
V6
Particle size and shape analysis
Fines content Sorting index
(%)
(𝜎 ) ∅
Material characterisation and cohesive behaviour
tests
Particle size Particle size
range (µm)
median ∅
samples based on weight percentage (0.00, 0.25, 0.50, 1.00,
2.50, 5.00, 7.50, 10.00%). Finally, samples were stirred thoroughly to ensure a homogeneous moisture distribution. All
experiments were carried out at room temperature. Whilst
the role of temperature may be important in natural material, we were unable to control for this variable in these
experiments.
Material Particle size
mean ( x) ∅
Fig. 2 Particle mass fraction of volcanic material
A
A, B
A, B
A, B
A, B
A, B
Bulletin of Volcanology (2023) 85:67
Page 4 of 18
Table 1 Particle size mean (logarithmic), particle size median (log), particle range, fines content (< 63 µm), geometric mean, logarithmic (ϕ) method of moments used for mean, sorting, sphericity, skewness and kurtosis. Sauter mean diameter calculated from Breard et al. (2019). Geldart group classification (Geldart 1973) based on mean size of particle
67
Page 5 of 18 67
Bulletin of Volcanology (2023) 85:67
Geldart’s classification of powders
Geldart (1973) classified powders into four distinctive
groups (A–D), each defined by their fluidisation behaviours,
which are influenced by particle size and density. These
behaviours span a spectrum from ‘very poor’ to ‘excellent’.
Group A (30–100 µm) and B (100 µm–1 mm) powders
exhibit the most favourable behaviours and expand during
fluidisation. On the other hand, Group C, comprising the
finest particles (< 20 µm), is governed by interparticulate
forces. Group D (> 1 mm) demands higher gas velocities for
effective fluidisation. Both group C and D present moderate to very poor fluidisation behaviours, often expressing as
slugging, channelling and spouting (Leturia et al. 2014). The
pyroclastic materials used in these experiments (Fig. 2) have
particle size distributions from 2.5 to 1000 µm, and using
the particle mean should exhibit fluidisation behaviours typically of Groups A and B in Geldart’s classification.
Bulk and tapped density
Given that volcanic ash displays uneven and angular characteristics (see sphericity; Table 1), not all spaces between
particles are eliminated. Bulk and tapped density were calculated for dry samples herein to characterise cohesive behaviour prior to the addition of water (method adapted from
United States Pharmacopeia 2015).
Bulk density (𝜌b ) was obtained by pouring 100 g of the
volcanic material into a 250-mL cylinder and levelling when
needed. The unsettled volume was measured, and bulk density calculated using Eq. 1. This procedure was completed
three times per sample.
The cylinder was tapped at 150 taps/min, with volume
measured every minute until levelled. Using the unsettled
apparent volume and final tapped volume, the tapped density
( 𝜌t ; Eq. 2), Carr’s index (CI; Eq. 3) and the Hausner ratio
(HR; Eq. 4) were calculated, where m is mass (g), Vo is the
unsettled apparent volume (mL) and Vf is the final tapped
volume (mL) (Moondra et al. 2018).
𝜌b= m
(1)
𝜌t= m
(2)
V0
The Carr’s index and Hausner ratio are indicative of flowability and interparticulate behaviours (Hausner 1981) and
are useful tools in determining a materials ability to fluidise and flow (Table 2). The Carr’s index measures the
strength and compressibility of a material (Eq. 3; Moondra
et al. 2018). The Hausner ratio determines the packing of
the material and how prone the material is to compaction
from external forces (Eq. 4, Yu and Hall 1994; Abdullah and
Geldart 1999). A material with a low Hausner ratio indicates
better flowability. These parameters are calculated from bulk
and tapped density measurements.
Bulk and tapped density measurements describe the mass
and volume ratio of a powder or granular material, without
and with packing, respectively (Amidon et al. 2017). Tapped
density experiments reflect the maximum density achievable through packing. The differences observed in bulk and
tapped density measurements are influenced by cohesive
attributes of particles (Deb et al. 2018) and can be impacted
by the shape and size of a material (Amidon et al. 2017).
Table 2 Relationship between Carr’s compressibility index, Hausner
ratio, and flowability behaviours. From (Gorle and Chopade (2020)
CI
HR
Flowability
≤ 10
11–15
16–20
21–25
26–31
32–37
> 38
1.00–1.11
1.12–1.18
1.19–1.25
1.26–1.34
1.35–1.45
1.46–1.59
> 1.60
Excellent
Good
Fair
Passable
Poor
Very Poor
Very Very Poor
Vf
(
𝜌 − 𝜌b
CI = 100 t
𝜌t
HR =
)
(3)
𝜌t
𝜌b
(4)
Angle of repose
The angle of repose (AoR) refers to the static friction coefficient and the angle of internal friction and can be investigated
through static (funnel) and dynamic (rotating cylinder drum)
methods (Beakawi Al-Hashemi & Baghabra Al-Amoudi
2018) to explore cohesive behaviours of a material (Montanari et al. 2017). AoR results can be interpreted in terms of
understanding the flowability of a material (Table 3).
Table 3 Flowability based on angle of repose results (Beakawi AlHashemi and Baghabra Al-Amoudi 2018)
Flowability
Angle of repose (°)
Very free flowing
Free flowing
Fair to passable flow
Cohesive
Very cohesive (non-flowing)
< 30
30–38
38–45
45–55
> 55
13
67
Bulletin of Volcanology (2023) 85:67
Page 6 of 18
To determine the static angle of repose (SAoR) for each
experiment, samples of 100 g of material were released
from a funnel held 3.5 cm over a circular platform (Av
diameter = 12 cm). The height of the cone was measured,
and the angle of repose calculated using Eq. 5 (Beakawi
Al-Hashemi and Baghabra Al-Amoudi 2018), where h is
height and D is base diameter (mm). When the material
did not release freely from the funnel, the material was
lightly agitated. If the height of the cone reached the base
of the funnel, then the funnel was incrementally moved
vertically to accommodate the growing cone. This was
repeated three times for each experiment.
( )
2h
SAoR (◦ ) = tan−1
(5)
D
Dynamic angle of repose (DAoR) was determined by
rotating 100 g of material in a clear cylindrical drum at
a constant rate (Smith 2020). This was recorded on video
and critical angle (the maximum angle prior to collapse)
measurements analysed using ImageJ (Schneider et al.
2012). This was repeated three times.
Fluidisation behaviour tests
Experiments to determine the fluidisation behaviours
of the pyroclastic material with increasing moisture
contents were completed using a rectangular, near2D fluidisation chamber with a porous base (following Gilbertson et al. 2020). Homogeneous samples of
200 g of pyroclastic material and water were placed
into the chamber and carefully levelled. A manometer
probe recorded basal pore pressure changes during each
experiment. Gas velocity of dry compressed air (cm/s)
was increased incrementally until either a stable, channelised bubbling fluidisation state was achieved, or large
amounts of winnowing or pressure build-up occurred. To
limit the effects of drying from basal air flow, experiments were carried out with gradual increases in gas
flow rate (0.050–0.208 cm/s) for dry sediments and
0.451–0.764 cm/s for moisture added sediments) over a
period of 01:11–23:51 min.
Table 4 Loose and tapped bulk
density, the Hausner ratio, Carr
index and flowability
13
Limitations
To better isolate the effect of moisture on material behaviours, a number of other parameters linked to natural PDCs,
and their deposits were either kept the same or constrained.
In nature, pyroclastic material is more polydisperse and
showcases a wider distribution of size, density, shape,
composition and temperature than the material used in this
work. The limitations on the particle size in our experiments
originate from (1) the size of the fluidisation tank and the
maximum gas velocity, which dictate the range of material
particle sizes that can be used, and (2) the need for effective
control of the influence of moisture addition. For instance,
the finer fractions of material, characterised by an increased
surface area, are more likely to be affected by moisturerelated effects (Huang et al. 2009). The fines content of a
material governs fluidisation, and fine material will readily fluidise at a lower minimum fluidisation velocity than
a coarser material (e.g., blocks from a block and ash flow)
(Gilbertson et al. 2020). The samples used in this study are
analogous to natural fine fractions of pyroclastic material,
such as the fines content of a lapilli-tuff ignimbrite facies,
an ash-dominated ignimbrite facies or a block and ash flow
deposit.
Our work provides insight to the role of moisture content on pyroclastic material. In the discussion, we begin to
explore how other parameters (e.g., size, shape) may impact
the cohesivity and material behaviour where appropriate.
A multivariate analysis to quantify the relative control of
moisture versus a wide range of other parameters would be
an important follow-on study to this work.
Results
Material properties
The bulk and tapped densities were calculated for pyroclastic
samples ranging in sizes from 3.8 ϕ (V1) to coarsest 0.8 ϕ
(V6). With increasing particle size, bulk and tapped densities generally decrease (Table 4). Material flowability, as
determined by the Hausner ratio and Carr index, is good
Material
Loose bulk density Tapped bulk
(kg m-3)
density (kg m-3)
Hausner ratio
Carr index
Flowability
V1
V2
V3
V4
V5
V6
1310
1320
1380
1320
1280
1180
1.08
1.18
1.17
1.07
1.12
1.15
7.73
15.13
14.28
6.59
10.68
13.37
Excellent
Good
Good
Excellent
Excellent
Good
1420
1550
1610
1420
1440
1370
Bulletin of Volcanology (2023) 85:67
(V2, V3, V6) and excellent (V1, V4, V5) under the 0% moisture conditions. The change in flowability between V5 and
V6 likely reflects the large increase in geometric mean from
347 (V5) to 557 mm (V6) (Table 1). The excellent flowability in V1, V4 and V5 is likely related to the smaller particle
range in V1 and V4 and the low fines content in V5.
The static angle of repose (SAoR) increases with increasing moisture across all volcanic samples (V1–6; Fig. 3). For
the 0% moisture condition, the SAoR ranges from 21 (V2,
V4, V5) to 23° (V1, V3). Interestingly, these results show
that under 0% moisture conditions, the SAoR is broadly
similar (within 2°) regardless of particle size or sorting
(Fig. 4a).
When increasing moisture contents to 5%, the SAoR
values increase to approximately double those achieved
with 0% moisture, reaching from 42 (V5, V6) to 47° (V4).
However, this relationship is not linear with increasing
moisture content (Fig. 4a). All materials show a rapid
increase in SAoR with moisture to around 25°. But beyond
a moisture content of 0.5%, a division is evident between
Page 7 of 18 67
the fine and coarse mixtures; those with higher Sauter
mean diameters (V2, V3 and V4) quickly increase to SAoR
values of ~ 45° at moisture contents of 2.5%, before becoming invariant with additional moisture content. Mixtures
with large Sauter mean diameters (V5, V6) mostly show
a more gradual increase in SAoR with moisture content.
V5, with a Sauter mean diameter of 55.6 × 10−6 m, shows
an intermediate behaviour, whilst V6 with a Sauter mean
diameter of 112.9 × 10−6 m shows a more linear relationship for SAoR with moisture between 0.5 and 5%. However, V1, with a Sauter mean diameter of 56.3 × 10−6 m,
shows a rapid increase, similar to V2, V3 and V4. This
may be due to the high fines content and sorting index
(Table 1).
The sphericity of the samples is shown in Table 1. Samples used are from the same parent material; therefore, we
see no large variation in particle shape (0.74–0.84 in sphericity). In general, the finer material is slightly more angular than the coarser material. We conclude that any differences in sphericity have not influenced the results, and small
Fig. 3 Representative static angle of repose (SAoR) cone formation of V1–V6. Numbers next to each cone show the average cone height (°)
13
67
Bulletin of Volcanology (2023) 85:67
Page 8 of 18
Fig. 4 a SAoR for volcanic material with varying moisture percentages with standard deviation error bars; b DAoR critical angle of volcanic
material with varying moisture percentages with standard deviation error bars
changes in sphericity are not directly related to the cohesive
behaviours seen in our experiments.
The results indicate that SAoR is sensitive to increasing
values of water in the materials and that relatively small
weight percentages can produce very different cohesivities
within the mixtures. It is notable that fines-rich mixtures are
particularly sensitive to moisture related cohesion, notably
at < 2% moisture, and this is thought to be due to increased
surface area and Sauter mean diameter.
Figure 4 also shows the relationship of dynamic angle of
repose experiments (Fig. 4b). Generally, and similar to the
SAoR results, there is an increase in the DAoR with increasing moisture. However, in experiments with increasing moisture levels (> 2.50%), the material was observed to clump,
slide and stick to the outer walls of the drum, complicating
the results. Nonetheless, it is important to observe that the
Sauter mean relationships detected within the SAoR experiments are not replicated in the DAoR tests.
Bubbling gas escape (Fig. 5a) is seen initially in most
experiments, where gas bubbles rise from the influx of
gas within the deposit. With increasing gas flux, this can
lead to channelling, where the material is sorted through
vertical channels or via pipe structures forming within the
deposit (Fig. 5b). Drying profiles that migrate through the
deposit are shown in Fig. 5c. As drying migrates with nonuniformity in the vertical deposit, formation of areas of wet
lobes and bubbling dry pockets can be observed, referred
to as pocketing (Fig. 5c). Explosive channelling can also
be observed in some experiments (Fig. 5d); as the material
dries, the upper wet deposit inhibits gas escape and causes a
pressure increase and subsequent release (Online Resource
1). Finally, under the highest moisture contents, the material does not form any of the gas escape structures outlined
above. Instead, pressure builds until the deposit fractures
into cracks where gas can easily permeate through (Fig. 5e;
Online Resource 2).
0.00% moisture
Fluidisation experiments
Fluidisation behaviours were described via sidewall video
analysis of the fluidisation chamber. The observation of gas
escape structures (i.e., bubbling, channelling, pocketing,
explosive channelling, cracking; Fig. 5a–e) and gas velocity measurements were recorded at varying moisture levels
(Fig. 6).
13
At 0.00% moisture for mixtures with moderate sorting (i.e.,
V2, V3, V5), fine material migrates through gas escape
channels (Fig. 5b) in the lower portion of the deposit. The
observation of minimum bubbling (Umb) is first seen in the
upper fine fraction of the deposit at 0.11 (V2), 0.08 (V3) and
0.42 (V5) cm/s. There is often a separation of fines bubbling
in the upper layer, with a mid-area of coarse channelling
Page 9 of 18 67
Bulletin of Volcanology (2023) 85:67
deposit and channelling in the lower. This reflects the slight
particle size variation of the material used, and therefore, the
Umb of the coarser material (Fig. 5b).
0.25% moisture
At 0.25% moisture contents, similar behaviours are observed
for V3 (Umb 0.069 cm/s), V5 (Umb 0.22 cm/s) and V6 (Umb
1.25 cm/s) as are observed for 0.00% moisture. For V2 (Umb
0.15 cm/s), V1 (Umb 0.13 cm/s) and V4 (Umb 0.15 cm/s),
bubbling begins at the base of the deposit. However, as the
surrounding wet deposit begins to dry, this dry material
becomes incorporated into the bubbling deposit. In V2, we
again see a separation of channelling and bubbling in the
lower and upper deposit.
0.50% moisture
At 0.50% moisture, a drying profile can be observed throughout most of the deposit (V4, V5, V6). In the V4 sample, as
drying at the base moves throughout the deposit, dry material begins to bubble (Umb 0.28 cm/s), and pressure slowly
increases. This is released suddenly (explosive channelling)
at 0.54 cm/s through a large channel which cuts through
the wet, upper part of the deposit. As the surrounding wet
material then begins to dry, it becomes incorporated into
the bubbling deposit. In the V5 sample, the drying profile
forms lobes of wet material and pockets of dry material. The
dry pockets slowly grow until reaching the upper deposit
and begin to bubble (Umb 1.04 cm/s). With continued drying as the experiment progresses, similar behaviours to the
0.25% and 0.00% moisture level experiments are observed.
After the drying profile has moved through the deposit of
V6, similar behaviours to the 0.25% and 0.00% moisture
experiments are observed (Umb 1.60 cm/s).
For the V3 material, channels of coarser material begin
to slowly move towards the surface. Material begins to dry
and is then incorporated into the bubbling deposit (Umb
0.14 cm/s).
Fig. 5 Examples of the structures recognised across the experiments
1.00% moisture
(Umc) at 0.13 (V2) and 0.10 (V3) cm/s as fines are being elutriated, and a coarse material layer at the base of the deposit.
Bubbling only affects the finer material.
In the mixtures that are well to very well sorted (i.e., V1,
V4), bubbles rise uniformly throughout the whole deposit
with a Umb of 0.07 (V1) and 0.19 (V4) cm/s. Within the
more coarse, well sorted material (V6), bubbles migrate in
a sluggish motion from the base of the deposit, with mostly
bubbling (Umb 1.60 cm/s) occurring in the upper half of the
At 1.00% moisture, V1, V2, and V4 show portions of material at the base of the deposit drying in pockets. The dry
material begins to bubble (Umb 0.35, V1; 0.49, V2; 0.42, V4
cm/s) and as the surrounding wet material begins to dry, it is
incorporated into the bubbling deposit. In V5 and V6, a distinctive drying profile moves through the deposit. Again, this
creates dry pockets of bubbling material (Umb 1.32, V5; 1.81
V6 cm/s) and wet lobes. In V3, pressure slowly builds as gas
velocity is increased. Pressure is suddenly released through
the formation of an explosive channel (Umb 0.35 cm/s). The
13
67
Bulletin of Volcanology (2023) 85:67
Page 10 of 18
Fig. 6 A–F Fluidisation profiles of V1–V6 with increasing moisture (0.00–10.00%). Symbols show gas escape structure formation
dry deposit then begins to bubble (Umb 0.35 cm/s) and is
slowly incorporated into the surrounding drying material.
2.50% moisture
At 2.50%, behaviours of V4 show similar results to 1.00%
moisture content: as the base dries, bubbling pockets are
formed (Umb 0.70 cm/s) in-between lobes of wet material.
In V4, pressure builds until it is suddenly released through
an explosive channel (Umb 2.15 cm/s).
13
5.00% moisture
At 5.00%, V2 shows the deposit drying at the base which
forms drying and bubbling (Umb 0.35 cm/s) in pockets, and
wet lobes.
7.50% moisture
At 7.50%, a clear drying profile forms through the V4
deposit; cracks begin to form and move through the deposit
Bulletin of Volcanology (2023) 85:67
until reaching the top and collapsing into pieces (U mb
3.82 cm/s). As gas moved through cracks, there was no dramatic rise and release in pressure.
10.00% moisture
Finally, at 10.00%, V3 forms a clear drying profile within
the deposit. Pressure builds before being released suddenly
at Umb 3.82 cm/s. This forms a large crack in-between wet
material. V6 shows a clear drying profile, as pressure slowly
rises as small pockets eventually form and dry material
begins to bubble (Umb 4.17 cm/s).
Key observations
The fluidisation experiments clearly demonstrate how small
additions of water into pyroclastic material can greatly
impact fluidisation behaviours and resulting gas escape
structures of a defluidising pyroclastic deposit. Two key
observations are apparent in the experiments: (1) the drying
profile and (2) pressure build up and release.
The dynamics of the drying profile, as the moisture content is impacted by the fluidising gas, exert a strong control
on the distribution of gas escape features, with variations
controlled by the grain size of the materials.
As gas flux is increased, a drying profile can move from
the base to the top of the deposit. The drying profile forms
more easily within the coarser materials (V3–V6). The profile initially rises uniformly across the bed, before becoming
irregular as it reaches the top of the deposit. These profiles
are noted as they highlight vertical and lateral moisture heterogeneity within the deposit, and their irregular structure
determines the formation of drying pockets and wet lobes
(Fig. 5c). At low moisture percentages (< 2.50%), the drying
pocket bubble and the wet lobes begin to dry before being
incorporated into the pockets. However, at high moisture
contents (> 2.50%), moisture-rich lobes remain throughout
the experiment, even at high gas velocities. This shows that
within a defluidising deposit, a drying profile will lead to
lateral and vertical variations in moisture.
In experiments with moisture contents of 0.50–10.00%,
explosive channelling (V3, V4) and cracking (V3, V4) can
occur. Across the experiments with 0.50–5.00% moisture
contents, a wet impermeable cap was observed to form
above the drier underlying deposits, with progressive drying of the vertical profile. Pressure builds under the cohesive
cap and continues to rise with increasing gas velocity. This
eventually results in explosive channelling and a sudden
basal pressure drop as the overburden pressure is exceeded.
In higher moisture level (5.00–10.00%) experiments, the
deposit does not dry as a relatively uniform rising profile.
Instead, pressure builds as the gas velocity is increased until
Page 11 of 18
67
cracks form in the deposit. These cracks act as effective gas
escape structures and release the pore pressure.
Discussion
The impact of moisture on pyroclastic material and PDC
behaviour is poorly understood, with previous detailed
investigations of fluidisation in pyroclastic material having
focused on dry (Wilson 1980) and saturated (Roche et al.
2001) end members. However, direct observations have
shown that variable amounts of moisture can enter a PDC
system (Cole et al. 1998, 2002; Lipman 2019; Vecino et al.
2022), and accretionary lapilli and ash pellets are believed to
provide evidence for the presence of moisture within PDCs
(Branney and Kokelaar 2002; Brown et al. 2010; Druitt
2014).
Our results show that for the pyroclastic material used
within these experiments, (1) the cohesivity of pyroclastic
material alters drastically, even with very small concentrations of moisture, (2) moisture addition into pyroclastic
material can change flow property behaviours from free
flowing to non-flowing, (3) changes in moisture affects
fluidisation profiles and gas escape structures, (4) a defluidising deposit can lead to a drying profile, and therefore
lateral and vertical heterogeneity within the deposit, and
(5) pressure can increase where gas escape is hindered by
moisture, which can cause dramatic releases of pressure in
an explosive fashion. Here, we discuss the implications of
these findings.
Gas escape structures
A variety of gas escape structures were observed in the fluidisation experiments, with many of them related to moisture content. Here, we define three main types of behaviour
(Table 5). In type 1 (< 0.50% moisture), we see partial fluidisation and segregation of heterogenous material through
bubbling and channelling. In a material with a smaller size
range, small vertical bubbling occurs across the entirety of
Table 5 Types of behaviour of gas escape observed with increasing
moisture in volcanic material
Moisture range
Bubbling
Channeling
Drying profile
Pocketing
Explosive channeling
Cracking
Type 1
Type 2
Type 3
0.00–0.25%
Yes
Yes
No
No
No
No
0.50–5.00%
Yes
Yes
Yes
Yes
Yes
No
7.50, 10.00%
Yes
No
Yes
Yes
No
Yes
13
67
Page 12 of 18
the deposit. During type 2 (0.50–5.0% moisture), an irregular drying profile develops and moves through the deposit
from the base. As the drying profile grows, dry pockets of
bubbling material begin to form in between irregular lobes
of wet material. Explosive channelling also occurs, which
releases pressure and facilitates quicker drying of the whole
deposit. Finally, during type 3 (7.5–10.0% moisture), similar lobe and pocket structures are formed to type 2 but are
accompanied by cracking processes, where fractures in the
wet material form to accommodate rapid gas escape. Our
experiments represent a defluidising deposit with dry air;
in nature, we may expect to see defluidisation of moist air,
through contact with bodies of water, for example. Our
results demonstrate that moisture addition may hinder or
prevent fluidisation and gas escape; therefore, a wet air flux
may display strikingly different results. This could not be
tested experimentally in this work and would benefit from
future investigation.
Roche et al. (2001) investigated the fluidisation behaviour of pyroclastic material where the material was saturated
with water (aqueous state) and subjected to an increase in
fluid velocity. The findings of Wilson (1980; 1984) and the
experiments herein demonstrate gas escape structures forming from an aerated fluidisation state with an increase in
gas velocity. In all experiments, aggregative behaviour was
observed, and the gas escape structures that formed were
consistently depleted in fines and enriched in dense and
coarse material. Importantly, Roche et al.’s (2001) research
revealed that aqueous gas escape structures (pipes) formed at
lower fluid velocities than the aerated structures in Wilson’s
work (1980) (Fig. 7). This is due to water having a lower
terminal velocity than air.
Figure 7 shows that by increasing moisture within a sample, higher gas velocities are required for aggregative fluidisation. Values from Wilson (1980) are based on the first formation of pipes at 0% moisture. Our values are from the first
formation of gas escape structures at varying moisture percentages (i.e., bubbling at lower moisture percentages and
explosive cracking at the highest). Results from Roche et al,
(2001) are based on initial pipe formation at 80% moisture.
Between 10 and 80% moisture, a change in the dominant
fluidising medium is inferred, from gas to water. Instead
of impeding early fluidisation structures, a large increase
in moisture leads to more regular structures forming. This
can be explained by changing particle-water states with
increasing moisture. Future investigation covering increments between 10 and 80% moisture may be able to define
this behaviour change. Aggregative fluidisation mechanisms
will result in the segregation of particles through gas escape
structures, where fines are winnowed. The nature of segregation will depend on the particle concentration and the
size, shape, density and relative proportions of clasts (Sparks
1976; Wilson 1980; 1984; Branney and Kokelaar 2002). We
13
Bulletin of Volcanology (2023) 85:67
Fig. 7 The gas velocity (cm/s) required to initiate gas escape structures depending on moisture percentage (%). Symbols represent different experimental suites from this study (circles), Wilson (1980)
(hourglass) and Roche et al. (2001) (crosses)
find that the moisture content of the deposit also controls this
process; segregation structures can change dynamically with
drying or become hindered with increasing moisture influence. This is due to our material being in a predominantly
capillary state (Kim and Hwang 2003; Kim and Sture 2008).
At higher levels of moisture, particles reach a more saturated
state, are completely supported by capillary bonds and fluidisation is no longer inhibited (as seen in Roche et al. 2001;
Kim and Hwang 2003). We observe that even small moisture
influences (as low as 0.50% of weight percentage) into the
pyroclastic material used in these experiments may control
the formation and nature of gas escape structures.
Application to natural gas escape structures
Our results show that introducing moisture into pyroclastic
materials may cause changes in gas escape morphology. Gas
escape structures have been recorded and described extensively within field volcanological literature (e.g., Fisher and
Schmincke 1984; Cioni et al. 2015; Pacheco-Hoyos et al.
2020). They have been described as pods and pipes displaying single or branching patterns, or as lenticular, curvilinear
and crescentic shaped (Wilson 1980; Branney and Kokelaar
2002; Pacheco-Hoyos et al. 2020). They can be spatially
arranged within individual layers or can move through multiple layers and are often fines depleted. Our results demonstrate varied morphologies, including vertical channels,
Bulletin of Volcanology (2023) 85:67
sub-vertical cracks and pods (created by moisture-rich lobes
and dry pockets).
Changes in gas escape structures in pyroclastic deposits are thought to be dominated by heterogeneity within the
material (e.g., size, density, shape; Wilson 1984; PachecoHoyos et al. 2020). We propose that varying moisture levels
will also influence changes in gas escape morphology and
may explain circumstances where morphological changes
are observed when other conditions appear unchanged. More
detailed documentation of morphology of field examples
may allow for improved interpretations of depositional
environment.
Mechanism for secondary explosions
Secondary explosions in pyroclastic deposits form due to the
interaction between water and hot material (Van Westen and
Daag 2005). Water in contact with hot pyroclastic material
will convert into steam and expand, causing sudden explosive decompression. Secondary explosions form large craters
(20–80 m depth) can remobilise large volumes of pyroclastic
material and can occur for years after the initial eruption (the
1991 Mount Pinatubo generated secondary explosions for up
to a year; Riehle et al. 1995; Van Westen and Daag 2005).
Riehle et al. (1995) modelled cooling, degassing and compaction behaviours within thick pyroclastic deposits. High
temperatures were most likely to remain elevated within
deposits > 50 m thick, with temperatures cooling mostly by
groundwater and rainfall. Keating (2005) modelled that the
addition of water on a hot deposit can result in increasing
pore pressure, in turn exceeding the overburden pressure.
This can result in secondary explosions.
Moyer and Swanson (1987) described three styles of secondary explosions—passive degassing (least explosive), ash
fountaining and explosive cratering (most explosive)—controlled by thermal energy and the permeability of the overlying material. Analogue experiments investigating the mechanisms of secondary explosions have been performed by
Gilbertson et al. (2020). They identified that vertical changes
in size fractions, and therefore, a vertical profile of minimum fluidisation velocities, resulted in secondary phreatic
explosions. In the experiments of Gilbertson et al. (2020),
a deposit capped with coarser material formed an upward
doming bed leading to an explosive release of material. This
was due to a drag-induced system. The fine-particle layer
below acted as a lower minimum fluidisation layer that was
unable to fluidise the overlying, coarser layer, resulting in
pressure increase and release of gas and particles.
Secondary explosions occur in deposits marked by the
occurrence of an active and mobile pore pressure gradient
associated with a vertical variation in permeability. Results
from our experiments show that increasing moisture levels within the fluidised deposit can lead to impermeable
Page 13 of 18
67
layers forming through drying at the base of the deposit. By
increasing moisture throughout our experiments, we exhibit
passive degassing (0%), ash fountaining (> 0.50% wt.) and
explosive cratering (> 0.50% wt.) behaviours as described
in Moyer and Swanson (1987). After lithostatic pressure of
the impermeable wet cap is overcome, explosive channelling (> 0.50% wt.) and cracking (> 7.50% wt.) occurs (the
‘explosive cratering’ of Moyer and Swanson 1987). Similarly, to these works, our results demonstrate the impact of
intermediate permeability on secondary explosion styles. We
argue that the change from passive degassing to explosive
cratering is not only a consequence of thermal energy in the
system, but also of internal degassing of a partially fluidised
deposit.
Critically, our results suggest a potential new mechanism
for secondary explosions that form in a moisture-influenced
material (Fig. 8a). In our experiments, the addition of water
during deposition results in increased cohesion and tensile
strength. As the deposit dries from the base, we see a shift
in gas escape as the material begins to dry and bubble. In
our model (Fig. 8a), the upper moisture-rich layer inhibits
passive degassing and leads to increased pore pressure. With
increasing pressure in the deposit, the overburden strength
of the wet material is compromised. The result is a sudden pressure release by explosive channelling and cracking,
which mimics similar behaviours seen in secondary explosions in pyroclastic deposits.
In a dry deposit later moistened by water (i.e., precipitation), the upper moisture-rich layers of material will create
an overall denser material (Fig. 8b). Secondary explosions
were observed following the Mt. St. Helens 1980 and Mt.
Pinatubo 1991 events (Keating 2005) and were attributed to
variations in the permeability of pyroclastic deposits caused
by the presence of water (e.g., rainfall and lacustrine environments) (Moyer and Swanson 1987; Manville et al. 2002).
It is thought that high pressure towards the base of these
pyroclastic deposits (caused by vaporisation of water) led
to low-permeable layers preventing the balancing of pore
pressures throughout the deposit, which resulted in explosive
depressurization (Keating 2005). Keating (2005) suggests
that after emplacement, hydrological re-establishment may
begin to occur, and interaction with hot overlying pyroclastic
material may result in the formation of secondary phreatic
explosions.
Our moisture influenced model may provide an explanation for the observations of secondary explosions in deposits
that have aggraded with the presence of water (e.g., secondary explosions followed the previous location of the Rogue
River; Druitt and Bacon 1986) (Fig. 8a) and that have interacted with rain (e.g., Mt. Pinatubo, Daag and Westen 1996)
(Fig. 8b). Rainfall may create a moisture-rich cap to the
deposit that is impermeable to degassing from the lower
deposit. The increased moisture from the rain would result
13
67
Page 14 of 18
Bulletin of Volcanology (2023) 85:67
Fig. 8 Moisture-influenced model of secondary explosion formation by a a defluidising wet deposit and b a defluidising dry deposit with external influences of water
in an increased cohesivity, and therefore, tensile strength,
of the material. With gas escape inhibited, pressure may
continue to build until the overburden pressure is reached,
and degassing is then allowed to escape through a secondary
explosion in the deposit.
Implications for deposit remobilisation
and preservation
The fine ash fraction of a fines-rich deposit will contribute to
increased packing of the material (Lam and Nakagawa 1993;
Averardi et al. 2020). In the influence of moisture, where
moisture has a greater effect on fines, we would expect the
deposit to hold an increased moisture content in the fines
portion in comparison to a coarser portion. Within a material with > 30% volume of fines, the stress forces begin to
13
be dominated by the fine fraction (Li et al. 2020; Breard
et al. 2023). A large volume of fines, both with and without moisture, may dramatically alter the deposition and the
preservation potential of these layers. Our results show that
small amounts of moisture appear to increase the cohesivity of pyroclastic material. A more cohesive deposit may be
more resistant to erosion, and remobilisation, meaning that
moist layers may be more likely to be preserved in volcanic
successions. Additionally, the formation of a drying profile
demonstrates both vertical and lateral variation due to an
undulating contact between wet and dry material, resulting in vertical and lateral changes in the tensile strength
of a deposit. Therefore, erodibility and preservation of layers may be variable. Future work should aim to quantify
cohesivity of material directly and determine yield stress
and tensile strength. This will enhance our understanding
Bulletin of Volcanology (2023) 85:67
of material properties and behaviours, allowing us to comprehensively assess their implications for erodibility, remobilisation and preservation.
Implications for PDC flow dynamics
The material behaviours revealed by the BTD, SAoR and
DAoR tests raise important questions regarding the impact
of moisture within a dynamic, moving PDC. The experiments show that in a dry (0% moisture) state, the material
analysed has a low cohesivity as evaluated by the friction
angles. As well as fines concentration, sorting is seen to play
a key role (Table 1). This can be seen in V1 (5% SAoR: 45°),
which is well sorted and displays the largest volumes of fines
(35.76%), whereas V4 (5% SAoR: 47°) is very well sorted
and has one of the lowest volumes of fine material (0.11%).
The excellent flowability seen in V4 may result from its
sorting and resulting packing behaviour, which is known to
affect flow behaviour (Breard et al. 2023). The DAoR results
show contrasting behaviours versus the SAoR results when
comparing the Sauter mean diameter. This could suggest that
particle size has a greater control on material behaviour in
static regimes compared to dynamic regimes. Such an observation has wide ranging implications. For example, Breard
et al. (2023) suggests that long run-out distances in block
and ash flows (BAFs) were a result of large degrees of fragmentation, with the current becoming more fines rich, and
subsequently, the deposit displaying higher packing. These
particle size changes result in a dynamically evolving flow,
where fines formation and increasing packing behaviour
reflect elevated pore pressure within the flow (Breard et al.
2023). Our experiments (V1, V4) show that both fines content and packing can contribute to good flowability behaviours, with implications for the resulting run-out distance of
PDCs. Our experiments are limited by particle size distribution and do not contain any blocks so are only modelling
the behaviours of the finest fraction. However, it is the fines
fraction that controls the fluidisation of PDCs (Gilbertson
et al. 2020). The role of large blocks in affecting fluidization
and flowability of PDCs is an important avenue for future
research (Sparks 1976; Branney and Kokelaar 2002).
The material with the largest volume of fines (V1, V2) is
shown to exhibit more cohesive behaviours with increasing
moisture (i.e., higher SAoR angle). Sample V4, which is
more well sorted, has the highest increase in SAoR values
when moisture is added. Our work demonstrates the important role of moisture, even in small amounts, in changing
flowability behaviours. We can build a hypothesis that the
addition of moisture into a PDC during propagation, with
increasing fragmentation of particles and packing during
flow, can be a factor in controlling run-out distances—
higher moisture contents reduce flowability so may reduce
Page 15 of 18
67
maximum runout distances, particularly in flows with
enhanced fragmentation.
Our results also indicate that the introduction of moisture reduces material fluidization. Expanding on this, future
research exploring moisture on fluidised currents would be
a valuable extension of the current static fluidisation experiments. This would allow for a better assessment of how
moisture influences fluidisation and the resulting behaviours
in PDCs.
Conclusion
This work offers insights into the influence of moisture on
the behaviour and characteristics of materials deposited from
pyroclastic density currents. Our results demonstrate that for
certain pyroclastic material, (1) the cohesivity of pyroclastic
material changes drastically, even for relatively small additions of moisture (> 0.50%), (2) an increase in moisture can
entirely alter flow property behaviour from a free flowing
to a non-flowing material, (3) changes in moisture impact
fluidisation profiles and gas escape structures, (4) a defluidising deposit can lead to lateral and vertical heterogeneity
within the deposit, and (5) pressure can increase where gas
escape is hindered by cohesive substrates driven by moisture
content, resulting in secondary explosions. Our results build
on previous models of secondary explosions in deposits and
support the idea that they are formed because of the development of an impermeable capping layer, here created by
the addition of moisture. This work further proposes that
moisture within a defluidising deposit profile may hinder
or change the formation of gas escape structures, which can
then lead to pressure increase and release, with implications
for the interpretations of the structures within the deposits.
Overall, these findings suggest that moisture plays a critical
role in PDC flow dynamics and their deposits, with implications for erodibility, preservation potential and our broader
understanding of deposit architecture.
Supplementary Information The online version contains supplementary material available at https://doi.org/10.1007/s00445-023-01682-9.
Acknowledgements N. W. was supported by the EU Horizon 2020
Programme (Project GEOSTICK 712525). We thank Gilbertson et al.
(2020) at the University of Bristol for the use of the fluidisation chamber. Ulrich Küppers is thanked for supplying the pyroclastic material
used in these experiments.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long
as you give appropriate credit to the original author(s) and the source,
provide a link to the Creative Commons licence, and indicate if changes
were made. The images or other third party material in this article are
included in the article’s Creative Commons licence, unless indicated
13
67
Page 16 of 18
otherwise in a credit line to the material. If material is not included in
the article’s Creative Commons licence and your intended use is not
permitted by statutory regulation or exceeds the permitted use, you will
need to obtain permission directly from the copyright holder. To view a
copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
References
Abdullah EC, Geldart D (1999) The use of bulk density measurements
as flowability indicators. Powder Technol 102(2):151–165. https://
doi.org/10.1016/S0032-5910(98)00208-3
Amidon GE, Meyer PJ, Mudie DM (2017) Particle, powder, and compact characterization. In: Developing Solid Oral Dosage Forms,
2nd edn. Academic Press, pp 271–293. https://doi.org/10.1016/
B978-0-12-802447-8.00010-8
Aravena A, Chupin L, Dubois T, Roche O (2021) The influence of
gas pore pressure in dense granular flows: numerical simulations versus experiments and implications for pyroclastic density currents. Bull Volcanol 83:1–20. https://doi. org/ 10. 1007/
s00445-021-01507-7
Averardi A, Cola C, Zeltmann SE, Gupta N (2020) Effect of particle
size distribution on the packing of powder beds: a critical discussion relevant to additive manufacturing. Mater Today Commun 24:100964. https://doi.org/10.1016/j.mtcomm.2020.100964.
(ISSN 2352-4928)
Barclay J, Johnstone JE, Matthews AJ (2006) Meteorological monitoring of an active volcano: implications for eruption prediction. J
Volcanol Geoth Res 150(4):339–358. https://doi.org/10.1016/j.
jvolgeores.2005.07.020
Beakawi Al-Hashemi HM, Baghabra Al-Amoudi OS (2018) A review
on the angle of repose of granular materials. Powder Technol
330:397–417. https:// doi. org/ 10. 1016/j. powtec. 2018. 02. 003.
(ISSN 0032-5910)
Benage MC, Dufek J, Mothes PA (2016) Quantifying entrainment in pyroclastic density currents from the Tungurahua eruption, Ecuador: integrating field proxies with numerical simulations. Geophys Res Lett
43(13):6932–6941. https://doi.org/10.1002/2016GL069527
Blott SJ, Pye K (2001) GRADISTAT: a grain size distribution and
statistics package for the analysis of unconsolidated sediments.
Earth Surf Proc Land 26:1237–1248
Branney M, Kokelaar P (1992) A reappraisal of ignimbrite emplacement: progressive aggradation and changes from particulate to
non-particulate flow during emplacement of high-grade ignimbrite. Springer-Verlag, pp 504–520
Branney M, Kokelaar P (2002) Pyroclastic density currents and the
sedimentation of ignimbrites. The Geological Society, London
Breard E, Dufek J, Lube G (2017) Enhanced mobility in concentrated
pyroclastic density currents: an examination of a self-fluidization
mechanism. Geophys Res Lett 45:654–664. https://doi.org/10.
1002/2017GL075759
Breard E, Jones J, Fullard L, Lube G, Davies C, Dufek J (2019) The
permeability of volcanic mixtures—implications for pyroclastic
currents. J Geophys Res Solid Earth 124(2):1343–1360. https://
doi.org/10.1029/2018JB016544
Breard E, Dufek J, Charbonnier S, Gueugneau V, Giachetti T, Walsh
B (2023) The fragmentation-induced fluidisation of pyroclastic
density currents. Nat Commun 14(1):2079. https:// doi. org/ 10.
1038/s41467-023-37867-1
Brown RJ, Branney MJ (2013) Internal flow variations and diachronous
sedimentation within extensive, sustained, density-stratified pyroclastic density currents flowing down gentle slopes, as revealed
by the internal architectures of ignimbrites on Tenerife. Bull Volcanol 75(7):1–24. https://doi.org/10.1007/s00445-013-0727-0
13
Bulletin of Volcanology (2023) 85:67
Brown RJ, Branney MJ, Maher C, Davila-Harris P (2010) Origin of
accretionary lapilli within ground-hugging density currents: evidence from pyroclastic couplets on Tenerife. Bull Geol Soc Am
122(1–2):305–320. https://doi.org/10.1130/B26449.1
Camuffo D (2019) Theoretical grounds for humidity. In: Microclimate
for Cultural Heritage. Elsevier, pp 43–59. https://doi.org/10.1016/
B978-0-444-64106-9.00003-1.
Cas RA, Wright JV (1991) Subaqueous pyroclastic flows and ignimbrites: an assessment. Bull Volcanol 53(5):357–380. https://doi.
org/10.1007/BF00280227
Chédeville C, Roche O (2015) Influence of slope angle on pore pressure generation and kinematics of pyroclastic flows: insights from
laboratory experiments. Bull Volcanol 77(11):96. https://doi.org/
10.1007/s00445-015-0981-4
Chédeville C, Roche O (2018) Auto fluidization of collapsing bed of
fine particles: implications for the emplacement of pyroclastic
flows. J Volcanol Geotherm Res 368:91–99. https://doi.org/10.
1016/j.jvolgeores.2018.11.007
Chen D, Zheng J, Zhang C, Guan D, Li Y, Yigang Wang (2021)
Critical shear stress for erosion of sand-mud mixtures and pure
mud. Front Marine Sci 8:713. https:// doi. org/ 10. 3389/ fmars.
2021.713039
Chigira M, Yokoyama O (2005) Weathering profile of non-welded ignimbrite and the water infiltration behaviour within it in relation to
the generation of shallow landslides. Eng Geol 78(3–4):187–207.
https://doi.org/10.1016/j.enggeo.2004.12.008
Cioni R, Pistolesi M, Rosi M (2015) Plinian and subplinian eruptions.
In: The encyclopedia of volcanoes, 2nd edn. Academic Press, pp
519–535
Cocco R, Reddy SB, Knowlton KT (2014) Back to basics introduction
to fluidization. Available at: www.aiche.org/cep
Cole PD, Calder ES, Druitt TH, Hoblitt R, Robertson R, Sparks RSJ,
Young SR (1998) Pyroclastic flows generated by gravitational
instability of the 1996–97 Lava Dome of Soufriere Hills Volcano, Montserrat. Geophys Res Lett 25(18):3425–3428. https://
doi.org/10.1029/98GL01510
Cole PD, Calder ES, Sparks RSJ, Clarke AB, Druitt TH, Young SR,
Herd RA, Harford CL, Norton GE (2002) Deposits from domecollapse and fountain-collapse pyroclastic flows at Soufriere Hills
Volcano, Montserrat. Geol Soc 21:231–262
Daag A, Westen CJ (1996) Cartographic modelling of erosion in
pyroclastic flow deposits of Mount Pinatubo, Philippines. ITC
J 2:110–124
Dartevelle S, Ernst GGJ, Stix J, Bernard A (2002) Origin of the Mount
Pinatubo climactic eruption cloud: implications for volcanic hazards and atmospheric impacts. Geology 7:663
Deb PK, Abed SN, Jaber AMY, Tekade RK (2018) Particulate level
properties and its implications on product performance and processing. Dosage Form Design Parameters 2:155–220. https://doi.
org/10.1016/b978-0-12-814421-3.00005-1
Dellino P, Dioguardi F, Isaia R, Sulpizio R, Male D (2021) The impact
of pyroclastic density currents duration on humans: the case of the
AD 79 eruption of Vesuvius. Sci Rep 11:4959. https://doi.org/10.
1038/s41598-021-84456-7
Douillet GA, Pacheco DA, Kueppers U, Letort J, Tsang-Hin-Sun E,
Bustillos J, Hall M, Ramon P, Dingwell DB (2013) Dune bedforms produced by dilute pyroclastic density currents from the
August 2006 eruption of Tungurahua volcano. Ecuador Bull Volcanol 75:762. https://doi.org/10.1007/s00445-013-0762-x
Druitt TH (2014) New insights into the initiation and venting of the
Bronze-Age eruption of Santorini (Greece), from component
analysis. Bull Volcanol 76(2):794. https:// doi. org/ 10. 1007/
s00445-014-0794-x
Druitt TH, Bacon CR (1986) Lithic breccia and ignimbrite erupted during the collapse of Crater Lake Caldera, Oregon. J Volcanol Geoth
Res 29(1–4):1–32. https://doi.org/10.1016/0377-0273(86)90038-7
Bulletin of Volcanology (2023) 85:67
Druitt TH, Bruni G, Lettieri P, Yates JG (2004) The fluidization behaviour of ignimbrite at high temperature and with mechanical agitation. Geophys Res Lett 31:L02604. https://doi.org/10.1029/2003G
L018593
Druitt TH, Avard G, Bruni G, Lettieri P, Maez F (2007) Gas retention in
fine-grained pyroclastic flow materials at high temperatures. Bull
Volcanol 69:881–901. https://doi.org/10.1007/s00445-007-0116-7
Duane WJ, Pepin NC, Losleben ML, Hardy DR (2008) General characteristics of temperature and humidity variability on Kilimanjaro,
Tanzania. Arct Antarct Alp Res 40(2):323–334. https://doi.org/
10.1657/1523-0430
Dufek J (2016) The fluid mechanics of pyroclastic density currents.
Annu Rev Fluid Mech 48:459–485. https://doi.org/10.1146/annur
ev-fluid-122414-034252
Eychenne J, Pennec JLL, Troncoso L, Gouhier M, Nedelec JM (2012)
Causes and consequences of bimodal grain-size distribution of
tephra fall deposited during the August 2006 Tungurahua eruption
(Ecuador). Bull Volcanol 74(1):187–205. https://doi.org/10.1007/
s00445-011-0517-5
Fisher RV, Schmincke HU (1984) Pyroclastic flow deposits. In: Pyroclastic Rocks. Springer Berlin, Heidelberg, pp 186–230. https://
doi.org/10.1007/978-3-642-74864-6_8.
Geldart D (1973) Types of gas fluidization. Powder Technol 7:285–
292. https://doi.org/10.1016/0032-5910(73)80037-3
Gilbertson MA, Taylor A, Mitchell S, Rust AC (2020) A fluidisation
mechanism for secondary hydroeruptions in pyroclastic flow
deposits. Front Earth Sci 8:324. https:// doi. org/ 10. 3389/ feart.
2020.00324
Girolami L, Druitt T, Roche O, Khrabrykh ZV (2008) Propagation
and hindered settling of laboratory ash flows. J Geophys Res
B02202:113. https://doi.org/10.1029/2007JB005074
Girolami L, Druitt TH, Roche O (2015) Towards a quantitative understanding of pyroclastic flows: effects of expansion on the dynamics of laboratory fluidized granular flows. J Volcanol Geotherm
Res 296:31–39. https://doi.org/10.1016/j.jvolgeores.2015.03.008.
(ISSN 0377-0273)
Gorle AP, Chopade SS (2020) Liquisolid technology: preparation, characterization and applications. J Drug Deliv Ther 10(3-s):295–307.
https://doi.org/10.22270/jddt.v10i3-s.4067
Hall M, Mothes P, Ramon P, Arellano S, Barba D, Palacios P (2007)
Dense pyroclastic flows of the 16 -17 August 2006 eruption of
Tungurahua Volcano, Ecuador. American Geophysical Union,
Spring Meeting 2007, Abstract V33A-03
Hartmann DL (2016) Global physical climatology, 2nd edn. In: Vol
103 of International Geophysics. Newnes
Hausner HH (1981) Powder characteristics and their effect on powder
processing. Powder Technol 30(1):3–8. https://doi.org/10.1016/
0032-5910(81)85021-8
Houghton B, White JDL, Van Eaton AR (2015) Phreatomagmatic and
related eruption styles. In: Sigurdsson H, Houghton B, Rymer
H, Stix J, McNutt S (eds), The Encyclopedia of Volcanoes. Elsevier, pp 537–552. https://doi.org/10.1016/B978-0-12-385938-9.
00030-4
Huang Q, Zhang H, Zhu J (2009) Experimental study on fluidization
of fine powders in rotating drums with various wall friction and
baffled rotating drums. Chem Eng Sci 64(9):2234–2244. https://
doi.org/10.1016/j.ces.2009.01.047
Hurwitz S, Kipp KL, Ingebritsen SE, Reid ME (2003) Groundwater
flow, heat transport, and water table position within volcanic
edifices: implications for volcanic processes in the Cascade
Range. J Geophys Res 108(B12). https://doi.org/10.1029/2003j
b002565
Keating GN (2005) The role of water in cooling ignimbrites. J Volcanol
Geotherm Res 142(1-2 SPEC. ISS.):145–171. https://doi.org/10.
1016/j.jvolgeores.2004.10.019
Page 17 of 18
67
Kelfoun K, Gueugneau V (2022) A unifying model for pyroclastic
surge genesis and pyroclastic flow fluidization. Geophys Res Lett
49:e2021GL096517. https://doi.org/10.1029/2021GL096517
Kelfoun K, Samaniego P, Palacios P, Barba D (2009) Testing the suitability of frictional behaviour for pyroclastic flow simulation by
comparison with a well-constrained eruption at Tungurahua volcano (Ecuador). Bull Volcanol 71:1057–1075. https://doi.org/10.
1007/s00445-009-0286-6
Kim TH, Hwang C (2003) Modelling of tensile strength on moist granular earth material at low water content. Eng Geol 69(3–4):233–
244. https://doi.org/10.1016/s0013-7952(02)00284-3
Kim TH, Sture S (2008) Capillary-induced tensile strength in unsaturated sands. Can Geotech J 45(5):726–737. https:// doi. org/ 10.
1139/T08-017
Lam D, Nakagawa M (1993) Packing of Particles (Part 2): Effect of
extra pore volume on packing density of mixtures of monosized
spheres. J Ceram Soc Jpn 101(11):1234–1238
LaMarche CQ, Miller AW, Liu P, Hrenya C (2016) Linking microscale predictions of capillary forces to macro-scale fluidization
experiments in humid environments. AIChE J 62(10):3585–3597.
https://doi.org/10.1002/aic.15281
Leturia M, Benali M, Lagarde S, Ronga I, Saleh K (2014) Characterization of flow properties of cohesive powders: a comparative
study of traditional and new testing methods. Powder Technol
253:406–423. https://doi.org/10.1016/j.powtec.2013.11.045
Li WC, Deng G, Liang XQ, Sun XX, Wang SW, Lee LM (2020) Effects
of stress state and fine fraction on stress transmission in internally unstable granular mixtures investigated via discrete element
method. Powder Technol 367:659–670. https://doi.org/10.1016/j.
powtec.2020.04.024
Lipman PW (2019) When ignimbrite meets water: mega scale gasescape structures formed during welding. Geology 47(1):63–66.
https://doi.org/10.1130/G45772.1
Ludwig B, Millington-Smith D, Dattani R, Adair JH, Posatko EP,
Mawby LM, Ward SK, Sills CA (2020) Evaluation of the hydrodynamic behaviour of powders of varying cohesivity in a fluidized bed using the FT4 Powder Rheometer®. Powder Technol
371:106–114. https://doi.org/10.1016/j.powtec.2020.05.042
Ma Y, Evans TM, Phillips N, Cunningham N (2019) Modelling
the effect of moisture on the flowability of a granular material. Meccanica 54(4–5):667–681. https:// doi. org/ 10. 1007/
s11012-018-0901-8
Manville V, Segschneider B, White JDL (2002) Hydrodynamic behaviour of Taupo 1800a pumice: implications for the sedimentology
of remobilized pyroclasts. Sedimentology 49(5):955–976. https://
doi.org/10.1046/j.1365-3091.2002.00485.x
Matthews AJ, Barclay J, Johnstone JE (2009) The fast response of
volcano-seismic activity to intense precipitation: triggering of
primary volcanic activity by rainfall at Soufrière Hills Volcano,
Montserrat. J Volcanol Geoth Res 184(3–4):405–415. https://doi.
org/10.1016/j.jvolgeores.2009.05.010
Montanari D, Agostini A, Bonini M, Corti G, Ventisette CD (2017)
The use of empirical methods for testing granular materials in
analogue modelling. Materials 10(6):635. https://doi.org/10.3390/
ma10060635
Moondra S, Maheshwari R, Tanja N, Tekade M, Tekadle RK (2018)
Bulk level properties and its role in formulation development and
processing. In: Advances in Pharmaceutical Product Development
and Research, Dosage Form Design Parameters, vol 2. Academic
Press, pp 221–256. https://doi.org/10.1016/B978-0-12-814421-3.
00006-3.
Moyer TC, Swanson DA (1987) Secondary hydroeruptions in pyroclastic-flow deposits: examples from Mount St. Helens. J Volcanol
Geotherm Res 32(4):299–319. https:// doi. org/ 10. 1016/ 03770273(87)90081-3
13
67
Page 18 of 18
Pacheco-Hoyos JG, Aguirre-Díaz GJ, Dávila-Harris P (2020) Elutriation pipes in ignimbrites: an analysis of concepts based on
the Huichapan Ignimbrite, Mexico. J Volcanol Geotherm Res
403:107026. https://doi.org/10.1016/j.jvolgeores.2020.107026
Pensa A, Giordano G, Corrado S, Petrone PP (2023) A new hazard
scenario at Vesuvius: deadly thermal impact of detached ash cloud
surges in 79CE at Herculaneum. Sci Rep 13:5622. https://doi.org/
10.1038/s41598-023-32623-3
Pepin NC, Pike G, Schaefer M, Boston CM, Lovell H (2017) A comparison of simultaneous temperature and humidity observations
from the SW and NE slopes of Kilimanjaro: the role of slope
aspect and differential land-cover in controlling mountain climate.
Global Planet Change 157:244–258. https://doi.org/10.1016/j.
gloplacha.2017.08.006
Pierrat P, Caram HS (1997) Tensile strength of wet granula materials.
Powder Technol 97(2):83–93. https:// doi. org/ 10. 1016/ S00325910(96)03179-8
Riehle JR, Miller TF, Bailey RA (1995) Cooling, degassing and compaction of rhyolitic ash flow tuffs: a computational model. Bull
Volcanol 57:319–336. https://doi.org/10.1007/BF00301291
Rios M (2006) Developments in powder flow testing. Pharm Technol
30(2):38–49
Roche O (2012) Depositional processes and gas pore pressure in
pyroclastic flows: an experimental perspective. Bull Volcanol.
74:1807–1820. https://doi.org/10.1007/s00445-012-0639-4
Roche O, Druitt TH, Cas RAF (2001) Experimental aqueous fluidization of ignimbrite. J Volcanol Geotherm Res 112(1–4):267–280.
https://doi.org/10.1016/S0377-0273(01)00246-3
Roche O, Gilbertson MA, Phillips JC, Sparks RS (2004) Experimental study of gas-fluidized granular flows with implications
for pyroclastic flow emplacement. J Geophys Res Solid Earth
109:B10201. https://doi.org/10.1029/2003JB002916
Sahoo S, Tiwari DK, Panda D, Kundu B (2022) Eruption cycles of
Mount Etna triggered by seasonal climatic rainfall. J Geodyn 149.
https://doi.org/10.1016/j.jog.2021.101896
Schneider CA, Rasband WS, Eliceiri KW (2012) NIH Image to ImageJ:
25 years of image analysis. Nat Methods 9:671–675
Self S, Sparks RSJ (1978) Characteristics of widespread formed by the
interaction of silicic magma and water pyroclastic deposits. Bull
Volcanol 41:196–212
Shimizu HA, Koyaguchi T, Suzuki YJ (2023) Dynamics and deposits
of pyroclastic density currents in magmatic and phreatomagmatic
eruptions revealed by a two-layer depth-averaged model. Geophysical Research Letters 50(16):616. https://doi.org/10.1029/
2023GL104616
Smith GM (2020) Propagation of aerated pyroclastic density current
analogues: flow behaviour and the formation of bedforms and
deposits. Dissertation, University of Hull
Sparks RSJ (1976) Grain size variations in ignimbrites and implications for the transport of pyroclastic flows. Sedimentology
13
Bulletin of Volcanology (2023) 85:67
23(2):147–188. https://doi.org/10.1111/j.1365-3091.1976.tb000
45.x
Sparks RSJ (1978) Gas release rates from pyroclastic flows: a assessment of the role of fluidisation in their emplacement. Bull Volcanol 41(1):1–9. https://doi.org/10.1007/BF02597679
Telling J, Dufek J, Shaikh A (2013) Ash aggregation in explosive volcanic eruptions. Geophys Res Lett 40(10):2355–2360. https://doi.
org/10.1002/grl.50376
The United States Pharmacopeial Convention (2015) Bulk density and
tapped density of powders - US pharmacopeia (USP), 〈616〉 Bulk
Density and Tapped Density of Powders. Available at: https://
www.usp.org/sites/default/files/usp/document/harmonization/genchapter/bulk_density.pdf (Accessed: 02 June 2023).
Valentine G, Sweeney M (2018) Compressible flow phenomena at
inception of lateral density currents fed by collapsing gas-particle
mixtures. J Geophys Res: Solid Earth 123(2):1286–1302. https://
doi.org/10.1002/2017JB015129
Van Eaton AR, Wilson CJN (2013) The nature, origins and distribution
of ash aggregates in a large-scale wet eruption deposit: Oruanui,
New Zealand. J Volcanol Geoth Res 250:129–154. https://doi.org/
10.1016/j.jvolgeores.2012.10.016
van Westen CJ, Daag AS (2005) Analysing the relation between rainfall
characteristics and lahar activity at Mount Pinatubo, Philippines.
Earth Surf Process Landf 30(13):1663–1674. https://doi.org/10.
1002/esp.1225
Vecino MCD, Rossi E, Freret-Logeril V, Fries A, Gabellini P, Lemus
J, Pollastri S, Poulidis AP, Iguchi M and Bonadonna C (2022)
Aerodynamic characteristics and genesis of aggregates at
Sakurajima Volcano, Japan. Sci Rep 14. https://doi.org/10.1038/
s41598-022-05854-z
Wilson CJN (1980) The role of fluidization in the emplacement of
pyroclastic claws: an experimental approach. J Volcanol Geotherm
Res 8(2–4):231–249. https:// doi. org/ 10. 1016/ 0377- 0273(80)
90106-7
Wilson CJN (1984) The role of fluidization in the emplacement of
pyroclastic flows, 2: experimental results and their interpretation.
J Volcanol Geoth Res 20(1–2):55–84. https://doi.org/10.1016/
0377-0273(84)90066-0
Wormsbecker M, Pugsley T (2008) The influence of moisture on the
fluidization behaviour of porous pharmaceutical granule. Chem
Eng Sci 63(16):4063–4069. https://doi.org/10.1016/j.ces.2008.
05.023
Yehuda T, Kalman H (2020) Geldart classification for wet particles.
Powder Technol 362:288–300. https://doi.org/10.1016/j.powtec.
2019.11.073
Yu AB, Hall JS (1994) Packing of fine powders subjected to tapping.
Powder Technol 78(3):247–256. https://doi.org/10.1016/00325910(93)02790-H