RESEARCH ARTICLE

Structural analysis of the early stages of catastrophicstratovolcano flank-collapse using analogue models

S. Daniel Andrade & Benjamin van Wyk de Vries

Received: 15 December 2008 /Accepted: 23 February 2010 /Published online: 30 March 2010# Springer-Verlag 2010

Abstract Many major volcanic flank collapses involve thefailure of low-angle strata in or under the edifice. Such failuresproduce voluminous, destructive debris avalanches that are amajor volcanic hazard. At Socompa, Las Isletas-Mombachoand Parinacota volcanoes, field studies have shown thatduring catastrophic flank collapse a significant segment oftheir substrata was detached and expelled from beneath thevolcanic edifice and formed a mobile basal layer on which thesliding flanks were transported. Previous studies haveproposed that gravitational flank spreading was likelyinvolved in the onset of sudden substrata failure. The earlystages of this particular type of flank collapse can be modelledunder laboratory conditions using analogue models. Thisallows us to study the development of structures accommo-dating early deformation of the sliding flank during cata-strophic collapse. In the experiments, the detached substratumsegment (low-viscosity basal layer) was modelled with asilicone layer, and the overlying stratovolcano with a layeredsand cone. The first structure developed in the models is agraben rooted in the low-viscosity basal layer. This grabenforms the limits of the future avalanche-amphitheatre anddivides the sliding flank into a ‘toreva’ domain (upper slidingflank) and a ‘hummock’ domain (lower sliding flank). These

domains display distinctive structural patterns and kineticbehaviour. Normal faults develop in the toreva domain andinside the graben, while the hummock domain is characterisedby transtensional structures. The hummock domain also over-thrusts the lower amphitheatre sides, which allows subsequentsideways avalanche spreading. Measurements show thathorizontal speeds of the hummock domain are always higherthan that of the toreva domain during model collapse. Themain role played by the low-viscosity basal layer during thistype of collapse is to control the size, shape and structuralcomplexity of the sliding flank; it also transmits mass andmomentum from the toreva to the hummock domain.

Keywords Flank collapse . Stratovolcano . Analoguemodels . Early stage . Structures

Introduction

Flank destabilization and catastrophic flank collapse havebeen recognized as common and very hazardous phenom-ena during the development of stratovolcanoes (Siebert1984; Siebert et al. 1987). Flank destabilization may be along-term process, while catastrophic collapse is a geolog-ically instantaneous event. Slow flank destabilization atstratovolcanoes is probably induced by long-term processeslike tectonic activity (e.g. Lagmay et al. 2000; Vidal andMerle 2000), magmatic activity/volcano growth (e.g.Gorshkov 1959, Ando 1979; Donnadieu and Merle 1998;Tibaldi 2001), hydrothermal alteration (e.g. Lopez andWilliams 1993; van Wyk de Vries and Francis 1997; Reidet al. 2001) and gravitational spreading (Borgia et al. 1992;van Wyk de Vries and Francis 1997).

Catastrophic flank collapse, in contrast, occurs when thedestabilized flank of a stratovolcano suddenly fails, forming

Editorial responsibility S. Nakada

S. D. Andrade :B. van Wyk de VriesCNRS, IRD, Laboratoire Magmas et Volcans,OPGC—Université Blaise Pascal,UMR 6524, 5 rue Kessler,63000 Clermont-Ferrand, France

S. D. Andrade (*)Instituto Geofísico, Escuela Politécnica Nacional,A.P. 17-2759,Quito, Ecuadore-mail: dandrade@igepn.edu.ec

Bull Volcanol (2010) 72:771–789DOI 10.1007/s00445-010-0363-x

a voluminous landslide and a debris avalanche. The usualtrigger proposed to explain these catastrophic failures is thesudden increase of pore pressure inside the stratovolcano dueto phenomena like earthquakes (Montaldo et al. 1996),magmatic intrusions (Gorshkov 1959; Voight et al. 1981;Elsworth and Voight 1996) or meteoric events (van Wyk deVries et al. 2000). Usually, more than one destabilizingprocess and more than one trigger may have acted togetherto induce the eventual catastrophic flank collapse. Theevidence indicating the origin of the destabilizing processesand the triggers for a specific collapse may be usually foundin the stratovolcano edifice and in the debris avalanchedeposit (DAD).

In the present work, we will concentrate on three particularexamples of flank destabilization and catastrophic collapse:Socompa (Chile), Las Isletas-Mombacho (Nicaragua) andParinacota (Chile) volcanoes. Studies carried out on thesevolcanoes show that the three edifices directly overlie asubstratum composed of unconsolidated, pumice-rich pyro-clastic sequences (Wadge et al. 1995; van Wyk de Vries et al.2001; Clavero et al. 2002; Shea et al. 2008). The DADs ofthese volcanoes show that the failure surfaces involvedsignificant segments of the volcano substratum, which weredetached from underneath the volcano and formed a mobilelow-viscosity basal layer on which the sliding flanks flowedduring catastrophic collapse (van Wyk de Vries et al. 2001;Shea et al. 2008). Based on their field observations, thoseauthors have hypothesised the sequence of structures thatdeveloped in the sliding flank during the early-stages ofcatastrophic collapse, in order to explain some of the finalfeatures observed at the DADs. The development of thesestructures is not likely to be directly observed in nature,because catastrophic collapses are short-lived, relativelyuncommon and usually fatal events to observers.

The geometric, kinematic and dynamic characteristics ofthe catastrophic collapse-early stages of the three volcanoescan be studied under laboratory conditions using analoguemodels. The aim of this study is to observe the sequence ofstructures that accommodate the deformation of a sliding flankduring the early stages of catastrophic collapse. Theseobservations may also be seen as corresponding to the earlystages of rockslide debris avalanche formation at stratovolca-noes, and so they could be useful in better understanding somecharacteristics of these mass-movements. The experimentshere will directly reflect only a few particular natural cases,but we will argue on a more general applicability of theresults.

Gravitational spreading and volcano stability

The main postulate of volcano gravitational spreading isthat a layer of unconsolidated materials (i.e. lacustrine

sediments or unwelded ignimbrites) contained in a thicksubstratum may show ductile behaviour under the weight ofa sufficiently large overlying volcano (van Bemellen 1949;Borgia 1994): the main effect is that the ductile layer wouldslowly flow outwards from beneath the volcano. Theinteractions between volcano edifice and substrata are morecomplex than this simple postulate, and may display a widevariety of resulting structures, developed in both thevolcano and the substratum. This is because the interactionsare controlled by several independent parameters such as:volcano radius, height and cohesion; ductile layer thicknessand viscosity; and slope of substrata. Various numerical andanalogue models, as well as natural examples, have beenused to test the relevance of these parameters (e.g. Merleand Borgia 1996; van Wyk de Vries and Matela 1998;Wooller et al. 2004; Delcamp et al. 2008).

Gravitational spreading may occur in all directionsradially away from the edifice summit, as at Maderasvolcano (van Wyk de Vries and Borgia 1996), or may occurin one preferential direction involving just one flank, as atEtna (Borgia et al. 1992). Typically, the related structuresare radial intersecting grabens (rooted in the ductile layer),which tend to spread and flatten the edifice, and circularthrust-and-fold belts formed around the base of the volcano(Fig. 1a) (Merle and Borgia 1996). The radial grabensdissect the substratum and the volcano flanks in triangularsegments which, in natural examples, are usually enhancedby erosion and classically interpreted as evidence forgravitational spreading (Merle and Borgia 1996; Delcampet al. 2008).

In general, gravitational spreading is a slow, flank-stabilizing process, because with time, volcano flanksflatten due to the outward flow of the substratum,becoming less and less prone to catastrophic collapse.However, there is evidence which suggests that gravita-tional spreading may also act as a flank-destabilizingprocess, creating conditions which could favour cata-strophic collapses, as proposed for Socompa (van Wyk deVries et al. 2001), Las Isletas-Mombacho (Shea et al.2008) and Parinacota (Clavero et al. 2002).

The catastrophic collapses of Socompa, Mombachoand Parinacota

The catastrophic collapse and debris avalanche deposits(DAD) of Socompa (Wadge et al. 1995; van Wyk de Vrieset al. 2001), Las Isletas-Mombacho (van Wyk de Vries andFrancis 1997; Shea et al. 2008) and Parinacota (Clavero etal. 2002) volcanoes have been described in detail and thefollowing paragraphs are based on those papers. Somegeneral parameters of these volcanoes and correspondingDADs are listed in Table 1. When comparing the

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descriptions of the deposits, several features common tomost volcanic DADs are found. For example, their coarse-breccia nature; the common occurrence of jigsaw-fit cracksin the blocks; the presence of hummocks in medium todistal deposition zones forming transversal and longitudinalridges; the common preservation of the original edificestratification in the deposit; a block facies and a matrix

facies; and an amphitheatre-shaped depression in the sourcezone (Fig. 1b, c, d) (Siebert 1984; Ui 1983).

Their most remarkable feature, however, is not commonto all volcanic DADs: Socompa, Las Isletas-Mombacho andParinacota display a continuous basal layer composed ofelements coming from the substratum directly underlyingthe respective volcanoes. At Socompa, the basal layer is

Amphitheatre scar:

Toreva blocksH Hummocksand ridges

Flow direction

Normal Reverse Strike-slipFaults:

T

11 5̊0' N

84 5̊4' WN

LakeNicaragua

H

H

T

68 2̊0' W

24 2̊0' S

T

H

Nb5 km

4 km

Anticline

T

H

H

T

3 km

N69 1̊5' W 69 1̊0' W

18 1̊0' S

d

c

DNature

e

a

DAD

Fig. 1 Structural sketches of: athe vertical section of a gravita-tionally spreading volcano(modified from Merle andBorgia, 1996); b Socompa DAD(proximal zone) (van Wyk deVries et al. 2001), c Las Isletas-Mombacho DAD (Shea et al.2008) and d Parinacota DAD(proximal zone) (Clavero et al.2002). e Typical stratigraphicsection of Las Isletas-MombachoDAD; the average thickness ofthe deposit is 22 m (modifiedfrom Shea et al. 2008). Faultsymbols and nomenclature arevalid for all figures

Table 1 General geometric data of the three main natural DAD examples presented in the text

Volcano Summit height (a.s.l.) Relief (max.) hNature DAD volume DAD runout DAD aream m km3 km km2

Socompa 6,100 2,900 25 40 490

Las Isletas - Mombacho 1,345 ∼1,300 ∼1.2 12 57

Parinacota 6,350 1,800 >6 22 140

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composed of unconsolidated ignimbrite, gravels and sandsfrom the Salin formation, and may represent up to 80% ofthe total DAD volume. At Las Isletas-Mombacho, it iscomposed of pumice, lithics and crystals coming from theApoyo and Las Sierras units of the substratum (Fig. 1e). AtParinacota, the basal layer is composed of pumiceousrhyodacitic pyroclastic flows, rhyodacitic breccias andminor fluvioglacial deposits from a volcanic successionpredating the edifice construction. These basal layers areeasy to distinguish because of their contrasting composi-tion with respect to the rest of the DAD and because theyshow abundant evidence of fluid-like behaviour, such asfine grain-size, rounded pumice clasts, discontinuouswavy layering and obliterated original stratification, allof which are absent in the in situ substratum outcrops aswell as in the rest of the DAD. Their occurrence has leadto the conclusion that a significant segment of thesubstratum beneath the respective volcano was detachedand involved in the catastrophic failure. This means thatthe main failure surface (faults that form the amphitheatrescar) of the collapse necessarily extended down to thevolcano substratum. Moreover, following the field evi-dence, it has been proposed that once detached, thesubstratum probably behaved as a fluidized, low-viscosity basal layer that was expelled from beneath thevolcano and on which the sliding flank moved duringcatastrophic collapse in a process detailed by van Wyk deVries et al. (2001).

Another remarkable feature of Socompa and Las Isletas-Mombacho is the presence of thrust-fold belts at the edificefoot, close to the failure zones and perpendicular to thedirection of collapse (Fig. 1b, c). At Parinacota, folds havenot been reported, although no detailed structural study ofthe edifice has been published yet. At Socompa, the parallelLoma Alta and La Flexura anticlines display deformedsubstratum sequences, notably the Salin formation. In fact,La Flexura fold, which is cut by the lower-most amphi-theatre scar, is interpreted to represent the front of acompressive belt that was active before collapse (Fig. 1b)(van Wyk de Vries et al. 2001). Similarly, at Mombachoseveral thrusts involving substratum formations occur in theperipheral northern and eastern lower flanks of the volcanoand seem to be cut by the collapse scar (Fig. 1c); theirinterpretation is similar to the folds observed at Socompa.For both volcanoes it has been proposed that the compres-sive belts originated by slow gravitational spreading of thesubstratum units (Fig. 1a), and that the catastrophiccollapses were triggered when a segment of the thrust beltfailed (van Wyk de Vries and Francis 1997). Similar activecompressive belts, caused by gravity-loading, have beenobserved by geophysical methods in other spreadingvolcanoes, for example at Kilauea (Morgan et al. 2003)and at Vesuvius (Borgia et al. 2005).

Other remarkable similarities among the Socompa, LasIsletas-Mombacho and Parinacota DADs are:

1. During flow, debris avalanches were able to widelyexpand laterally, even overflowing the low amphi-theatre rims (Fig. 1b, c, d).

2. Toreva blocks (Reiche 1937) are present inside and inthe proximity of the amphitheatres (Fig. 1b, c, d). Thisis less clear for Las Isletas-Mombacho, probably due tothe dense vegetation and because the toreva blocks areless voluminous than at Socompa and Parinacota.

On the other hand, important differences may beobserved also between the DADs of our examples, andconsist mostly of structures related to the effects of localtopography on avalanche-flow during deposition. Forexample, Socompa avalanche flowed 20–30 km in anorth-west direction, on a chiefly flat open surface beforeriding up and being deflected to the north-east by themountains of Sierra Almeida and Cordon de Lila. Acomplex network of normal, thrust and strike-slip faultsare observed in the DAD, mostly in the segment of theavalanche that was deflected (Kelfoun and Druitt 2005,Kelfoun et al. 2008, Shea and van Wyk de Vries, 2008).Conversely, Las Isletas-Mombacho avalanche flowed∼5 km on an obstacle-free, gentle, smooth slope before itentered the shallow Lake Nicaragua (Fig. 1c). In this near-ideal situation, without topographic barriers to the ava-lanche flow, the main observed structures are strike-slip andnormal faults. Finally, normal, thrust and strike-slip faultsare reported at Parinacota, where the avalanche flowed on amainly flat surface, but was strongly channelled by themountainous morphology close to the volcano (Fig. 1d).The quantitative analysis of hummocks at both Parinacotaand Mombacho DAD’s shows that their volumes and theheight-to-width ratios tend to decrease with transportdistance (Clavero et al. 2002; Shea et al. 2008).

In order to explain some of their field observations,Wadge et al. (1995), van Wyk de Vries et al. (2001), andShea et al. (2008) have suggested that a sequence ofstructures developed in the sliding flanks during the earlystages of catastrophic collapse. Wadge et al. (1995)proposed a deep-seated main failure surface involving alarge segment of the substratum beneath Socompa, as wellas a complex network of faults developing in the slidingflank. In contrast, van Wyk de Vries et al. (2001) and Sheaet al. (2008) propose shallow main failure surfaces and alimited segment of substratum detached from beneathSocompa and Mombacho respectively. In the case ofParinacota, Clavero et al. (2002) were more concernedwith the DAD emplacement mechanisms and proposed nohypothesis on the structures accommodating early collapsedeformation, except that it was controlled by the originaledifice faults, fractures and lithologies.

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Analogue model, geometrical constraints and materialsused

On the basis of the preceding descriptions, a simplifiedanalogue model that would simulate the collapses ofSocompa, Las Isletas-Mombacho and Parinacota is pro-posed. The objective of the model is to study the structuresdeveloped in the sliding flank during the early-stages ofcatastrophic collapse, this is in the short time and distancefollowing the trigger of failure. It is assumed that the wholevolcano-substratum system has been gravitationally spread-ing slowly until the moment of collapse, which marks thebeginning of our experiments and observations.

In order to reproduce the natural examples, the modelshave two principal elements: a stratovolcano and a low-viscosity basal layer. In the following paragraphs, therespective geometry and size of these elements are defined.

The stratovolcanoes

Natural stratovolcano shapes tend to be conical andcharacterized by a height (hNature) measured from its base(Tables 1, 2) and slopes of 25-30°. Thus, stratovolcanoeswill be approximated with a conical shape in the models.This shape has been widely used in previous analoguemodels simulating flank destabilization and catastrophiccollapse at stratovolcanoes (Donnadieu and Merle 1998;Lagmay et al. 2000, Vidal and Merle 2000; Acocella 2005).For practical convenience, the volcanic edifices weremodelled with stratified sand cones of constant heighthModel=6 cm and slopes of 25–30°. Thus, a fixed radiusRModel=12 cm was used for all models (Fig. 2, Table 2).

Strata with two different cohesions and colours wereused in order to better simulate the physical behaviour of

natural stratovolcanoes (see Scaling below) and to con-strain fault displacements in vertical sections. A mixture of85% white sand and 15% plaster was used for white layerswith CModel=100 Pa cohesion, and pure black sand wasused for black layers with negligible cohesion CModel=0 Pa

Variable Definition Unit Value

Model (M) Nature (N) Ratio M/N

h Edifice height m 0.06 2900–1300 2×10−5–5×10−5

R Edifice radius m 0.12 10000–7000 1.2×10−5–1.7×10−5

C Edifice cohesion Pa 0–100 10−4–10−7 # 0–10−5

ρ Edifice density kg m3 1500 2200 § 0,7

α Basal layer angle rad π–π/6 π/6–π/3 1–0.33

D Basal layer length m 0.14 13000–8000 10−5–1.8×10−5

d Basal layer vertex distance m 0–0.08 ?? –

T Basal layer thickness m 0.007 300–200 2.3×10−5–3.5×10−5

μ Basal layer viscosity Pa s 20000 107 0.002

γ Basal layer density kg m3 1000 1500–2100 0.5

t Observation time s 7200 60 120

V Velocity m s−1 10−5–10−6 100–10 10−6–10−7

g Gravity acceleration m s−2 9.8 9.8 1

Table 2 Parameters used in thescaling procedure. §: Williamset al. (1987); #: Afrouz (1992)and Bell (2000)

Rigid surface

Stratifiedsand cone

Silicone layer(7 mm thick)

Lateral view

Stratifiedsand cone

Plan view

RledoM

RModel

12 cmDModel14 cm

Silicone layer

DledoM

dModel

hModel

6 cm dModel

Fig. 2 Experimental setup and illustration of the main geometricalparameters of the models. Only opening angle (α) and the ductilelayer offset (dModel) were varied during experiments

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(Table 2). The densities of both kind of layers were similarand averaged at ρModel=1500 kg m−3 (Table 2). Addition-ally, during some experiments, the analogue volcano wascovered with a 0.2–0.3 cm thick pure plaster layer in orderto sharpen the surface visualization of the structuresdeveloped. This layer does not significantly affect themechanical behaviour of the analogue volcano, becausemodels with and without plaster layer developed the samestructures.

The low-viscosity basal layer

First, the natural examples show that the low-viscositybasal layers should have a roughly arc-shaped externalfront, defined by the circular thrust-fold belts formed bygravitational spreading (Borgia et al. 1992; van Wyk deVries and Francis 1997). In the natural examples, theangular lengths (α) of the arcuate fronts are ∼π/3 atSocompa and ∼π/6 at Las Isletas-Mombacho (Fig. 1b, c).At Parinacota this cannot be found because it is coveredwith post-collapse deposits. The value of α was variedduring the experiments (Fig. 2; Table 2).

Also, in the natural examples the failed fronts of thethrust-fold belts are located at a distance DNature measuredin plan-view from the inferred pre-collapse volcano summit(Fig. 1c; Table 2). At Socompa and Las Isletas-Mombachothe respective DNature are ∼13 and ∼8 km. If their respectiveRNature are inferred at ∼10 km and ∼7 km (van Wyk deVries et al. 2001; van Wyk de Vries and Francis 1997), then1.3>DNature/RNature>1.14. For the models, DModel/RModel

should be similar to DNature/RNature. Thus a fixed DModel/RModel=1.17 was used, so the free arcuate front of the low-viscosity basal layer was placed at a constant DModel=14 cm (Fig. 2; Table 2).

As already stated, the main failure surface (faults that formthe amphitheatre scar) at Socompa, Las Isletas-Mombachoand Parinacota necessarily extended to the underlyingsubstratum during catastrophic collapse. Thus, the lateralborders of the substratum segment detached during cata-strophic collapse should be somewhat similar in shape to theborders of the amphitheatre scars of the natural examples; thisis roughly an elongated horse-shoe shape. In order to simplifythe experiments, we have approximated this shape to anisoceles triangle, whose vertex is placed at a horizontaldistance dModel from stratovolcano summit (Fig. 2). Duringexperiments, dModel was varied from 0 cm (vertex directlyunderneath volcano summit) to 8 cm (vertex close to volcanofoot) and normalized with respect to the fixed RModel=12 cm.So, for the presentation of results we will refer to the ratiod/R, which will vary from 0 to 2/3 (Tables 2, 3).

Finally, in the natural examples, the thickness (T) of thedetached substratum segment has been estimated at 300 mfor the Salin Formation underneath Socompa (van Wyk de

Vries et al. 2001) and ∼200 m for the Apoyo and Las Sierrasunits underneath Mombacho (van Wyk de Vries and Francis1997). There are no estimates for the case of Parinacota.The average T of the low-viscosity basal layer was taken tobe around 15% the volcano height (h) and, given that modelstratovolcanoes have a constant hModel=6 cm, then for theexperiments TModel=0.7 cm (Table 2).

The material used to simulate the low-viscosity basal layerwas a silicone putty (SGM-36) produced by Dow Corning(UK) Ltd., which has a viscosity μModel=2×10

4Pa s and adensity γModel=10

3kg m−3 (Weijemars and Schmeling, 1986)(Table 2). The low-viscosity basal layers for experimentswere thus obtained by cutting triangular segments withvaried dModel and α, from an initial silicone circle 0.7 cmthick (TModel) and 14 cm radius (DModel) (Fig. 2).

Model scaling

A scaling procedure must be respected for experiments tobe geometrically, kinematically and dynamically similar tothe natural examples. Standard similarity conditions (Hub-bert 1937) were established through the 12 variablesinvolved in the experiment (Table 2). Thus, according tothe Buckingham-П theorem, 9 independent dimensionlessvariables must be defined and need to be as similar aspossible between models and nature. Geometrical similarityis guaranteed by the five variables (П1, П2, П3, П4 andП5) defined in Table 3. The П6 variable relates the densityof the volcano edifice (ρ) to that of the low-viscosity basallayer (γ). The bulk density of an edifice has been estimatedat ρNature=2200 kg m−3 (Williams et al. 1987), while thedensity of a pumice-rich ignimbrite may be γNature=1400–2100 kg m−3 (Bell, 2000) (Table 2).

The kinematic and dynamic similarity conditions may beobtained with the balance of the main forces acting on themodels and natural examples during the collapse process.These forces are: 1) gravitational (FG), 2) inertial (FI), 3)failure resistance (FR), and 4) viscous (FV), and are definedas follows:

FG ¼ r� g � h; ð1Þ

FI ¼ r� V 2 ð2Þwhere V is a characteristic velocity of the process;

FR ¼ C þ h=R� s1� s3ð Þ ð3Þif a Navier-Coulomb failure criterion is assumed;

FV ¼ m=t; ð4Þwhere t is a characteristic time of the process.

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In (3), h/R represents the value of tan θ, where θ is theangle of internal friction of the stratovolcano. σ1 and σ3 arethe maximum and minimum principal stresses, expressedby σ1=ρ×g×h, and σ3=(ρ×g×h/3)+μ/t, given that σ3 isequal to the stress due to volcano loading plus the stressdue to viscous deformation (Merle and Borgia 1996). Thus,the failure resistance force may be expressed as

FR ¼ C þ h=R� 2FG=3� FVð Þ ð5ÞThe velocities of the sliding flank during the Mount St.

Helens catastrophic collapse were measured at 10<VNature<100 m s−1 by Voight (1981) and were used in (2). Given thatour analysis corresponds to the initial stages of catastrophiccollapse, then an observation time span of 0<tNature<60 s hasbeen also approximated from the measurements performed atMount St. Helens by Voight (1981) and was used in (4)(Table 2). Although no direct measurements exist for thebasal layer viscosity during natural catastrophic collapses,it may be estimated in the order of μNature=10

7Pa s fromdiverse numerical analysis and simulations of debrisavalanche flows (Sousa and Voight 1995; Dade andHuppert 1998; Kelfoun and Druitt 2005); that value wasalso used in (4) (Table 2). Finally, the cohesions (C) ofintact basalts and other lavas are in the order of 106–108Pa, while volcanic ash and tephra have cohesions inthe order of 102–105Pa (Afrouz, 1992; Bell, 2000).Consequently, a 104<CNature<10

7Pa range was used in(5) and is assumed to represent the range of cohesionspresent in natural stratovolcanoes, with coarse tephra andash represented in the lower end of the range, and lavaflows in the higher (Table 2).

The ratios obtained by combining the forces defined in(1), (2), (4) and (5) give the four additional dimensionlessvariables listed in Table 3, that are needed to complete thescaling procedure. The remarkable concordance of П7=FG/FV and П8=FR/FV between models and natural exam-ples suggest that experiments will reproduce fairly well the

natural gravitational, failure resistance and viscous forces.However, there are very large differences between modelsand nature for П9=FI/FV and П10=FI/FG, which are mainlydue to the second power applied to V when calculating FIwith (2). This implies that models fail to represent theinertial forces of nature (Table 3). The differences of П9and П10 between models and nature are less pronounced iftModel is strongly reduced (tModel<1000 s), which suggeststhat VModel may in fact be representative of VNature, but onlyduring the initial moments of analogue model collapse;thus, the longer the model is observed, the less represen-tative VModel will be. Nevertheless, the experiments werealways observed for tModel=7200 s (2 h) given that severalinteresting features are developed in this time span. Inorder to improve the presentation of data, a fixed scalingfactor t*= tModel/tNature=120 will be used hereafter, so eachsecond of tNature scales to 120 seconds (2 min) of tModel.The velocities measured in the experiments and presentedbelow will be then only referential and not to scale.

Model procedure

The low-viscosity basal layer (silicone slice) was placedover a smooth horizontal rigid surface, and the modelstratovolcano was built directly on top, respecting thegeometrical constraints imposed by the scaling procedure(Fig. 2). Surface structures developed in the models wererecorded with plan-view sequential photographs. Internalstratovolcano structures, on the other hand, were recordedwith vertical sections cut in the models at several instantsduring collapse, and parallel and perpendicular to the slipdirection at each instant. This notably implied the repetitionof the same experiment for several different time periods.The combined surface and internal observations allowed theevolution, geometry and kinematics of the structures to beconstrained.

Dimensionless variable Definition Value

Model Nature

П1 Height/Radius of edif. 0.5 0.3–0.2

П2 Basal layer thickness/Edif. height 0.12 0.1–0.15

П3 Basal layer length/Edif. radius 1.17 1.3–1.14

П4 Basal layer vertex dist./Edif. radius (d/R) 0–0.66 ??

П5 α angular distance (αR/H) 6.3–1.05 5.6–1.8

П6 Edif./Basal layer density 1.5 1.1

П7 Gravitational/Viscous forces 317 370–170

П8 Frictional/Viscous forces 104–140 65–130

П9 Inertial/Viscous forces 2×10−4–2×106 1.32–132

П10 Inertial/Gravity forces 7×10−7–7×10−9 0.35–0.0035

Table 3 Definition and valuesof the calculated П-Numbers innatural examples and the models

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The first series of experiments was designed to explorethe effect of the basal layer geometry on the surfacestructures developed during collapse. This series has twoparts. In part I, d/R=0 (dModel=0 cm) was kept constant andthe value of α was varied from π (half-circle arc) to π/6(narrow arc), in steps of π/6. In part II, surface observationswere performed in models with variable d/R=0, 1/6, 1/3,2/3 (dModel=0, 2, 4, 8 cm) and α=π/2, π/3, π/6.

In the second series of experiments, the model with α=π/3was chosen to perform vertical sections and explore theinternal structures developed during model collapse. Thisseries has also two parts. In part I, the model had both α=π/3and d/R=0 fixed, and sections were performed at tModel=0,15, 30, 60, 120 min (tNature=0, 7.5, 15, 30, 60 s). In part II,vertical sections were always performed at tModel=60 min(tNature=30 s) in models with fixed α=π/3 and variabled/R=1/6, 1/3, 2/3.

Finally, the model with α=π/3 and d/R=0 was used tomeasure the surface horizontal velocities, by the analysis ofthe sequential photographs.

Results

First Series, Part I

When α was varied and d/R=0 kept constant, the majorstructures developed at the end of valid observations (tModel=120 min) are: 1) an amphitheatre-shaped depression, withsub-vertical walls, on top of the initial basal layer site; 2)large, tilted slide blocks (toreva blocks) inside the amphi-theatre, elongated perpendicular to the slip direction; and 3) afan-shaped zone outside the amphitheatre (proto-avalanche),with analogue hummocks forming transverse and longitudi-nal ridges (Fig. 3a).

When the models have π/2>α>π/6, the amphitheatre isrelatively narrow and the structures developed are similar tothose observed in the natural examples (compare Figs. 1b,c, d and 3b). When π>α>π/2, the wide open amphitheatresproduced do not correspond to the natural examples and, asfar as we know, have not been reported at any stratovolcanoin continental domains (Fig. 3c). Such wide open amphi-theatres are often observed, however, in oceanic-islandshield volcanoes like Lanai and Kilauea in Hawaii (Mooreet al. 1989) or La Palma and El Hierro in the Canary Islands(Carracedo et al. 1999). No further analysis of the caseswhere π>α>π/2 will be done here because our scaling andmodel procedure do not correspond to the conditions atHawaii and the Canary Islands.

In Figure 4a, the values of the model amphitheatrelength, SModel, normalized to model radius RModel, areplotted against α. SModel was measured in plan view frommodel volcano foot (fixed for all experiments) to the top

amphitheatre scar (see Fig. 3a). We consider that SModel

represents a proxy for slide volume, which is hard todirectly measure in our experiments. SModel displays apositive linear correlation with α, when d/R is fixed.This graph also shows that, regardless the value of α,when d/R=0, the volcano summit is always involved inthe failure (SModel/RModel>1). For example, at Las Isletas-Mombacho the summit of the pre-collapse volcano wasnot involved in failure (Shea et al. 2008; van Wyk de Vriesand Francis 1997), and the opposite occurred withSocompa (van Wyk de Vries et al. 2001; Wadge et al.1995), which suggests contrasting values of d/R betweenthese natural prototypes.

First series, Part II

The effect of variable basal layer position (d/R=0, 1/6, 1/3,2/3) was evaluated in the models with ductile layeropenings of α=π/2, π/3, π/6. At the surface, at the end ofvalid observations (tModel=120 min) the same majorstructures as in Part I develop in the models: anamphitheatre-shaped depression, large toreva blocks and ahummocky fan (Fig. 5). It is also observed that withincreasing d/R: 1) amphitheatre length SModel decreases; 2)the toreva blocks develop less; and, 3) the main failuresurfaces are less steep and less deep (Fig. 5). The first pointmay be evaluated with surface images, while the two othersneed vertical sections to be confirmed.

In Fig. 4b, the values of SModel/RModel are plotted againstd/R. As expected, the diagram shows that SModel/RModel

decreases regularly with increasing d/R. Thus, SModel/RModel

varies as a function of both d/R (dModel) and α. This is alargely expected, not surprising result, but it is associatedwith gradual changes in the structures developed in thesliding flank, as well as it gives some important clues to thenature of the failure geometry. For example, if the SNature/RNature and α values could be measured in a naturalvolcano, then the ratio d/R could be estimated fromFig. 4b. At Socompa SNature/RNature≈1.15 and α≈π/3, thend/R≈0.1, which implies a deeply rooted basal layer forthe collapse, in agreement with the hypothesis of Wadgeet al. (1995), but in disagreement with that of van Wyk deVries et al. (2001). At Las Isletas-Mombacho, SNature/RNature≈0.85 and α≈π/6, then d/R≈0.4, which implies aless deeply rooted basal layer, reinforcing the hypoth-esis of Shea et al. (2008) and van Wyk de Vries andFrancis (1997). For Parinacota, these data cannot bemeasured because the collapse scar has been buried bypost-collapse activity. However, given the subjectivity inthe measurements of RNature, these calculations must betaken only as a first order approximation to the value ofd/R and to the actual size of the detached basal layer innatural cases.

778 Bull Volcanol (2010) 72:771–789

Second series, Part I

The model with fixed opening angle (α=π/3) and fixedductile layer position (d/R=0) was chosen to study thedevelopment of early structures using plan-view sequentialpictures and vertical sections. Pictures were taken every3 min (1.5 s in tNature) during the first 15 min of modelcollapse, and then every 5 min (2.5 s in tNature) until tModel=120 min. Experiments were stopped at tModel=15, 30, 60and 120 min (tNature=7.5, 15, 30, 60 s) to performlongitudinal and transverse vertical sections.

On the surface, at tModel=15 min (tNature=7.5 s) of modelcollapse, a part of the main failure delimiting the futureamphitheatre scar is established (Fig. 6a). The faultscomprising the main failure are purely extensional in thesummit zone, transtensional in the upper-middle slopes andtranspressional in the middle-lower slopes. Inside theselimits, two sharply different domains are observed in thesliding flank. First, a highly fractured zone in the middleand lower flank is limited by the lateral transpressional

faults; we will call this zone the Hummock Domain.Second, a block in the middle and upper flank (includingthe summit) displays only a few normal faults sub-parallelto the top main failure; we will call this zone the Torevadomain (Fig. 6a).

The vertical section at tModel=15 min shows that theinitial dominant structure is a listric graben perpendicular tothe sliding direction (Fig. 6b). This graben is formed by: 1)the main failure surface, composed of large faults vergingtowards the volcano summit, which accommodate theformation of both the amphitheatre and the toreva slides;and, 2) antithetic faults verging towards volcano foot thataccommodate the initial horizontal displacement of thehummock domain (Fig. 6b). The graben faults are rooted inthe silicone layer, and the graben axis separates thehummock domain from the toreva domain. A verticalsection performed at tModel=0 min clearly showed that thegraben, though poorly developed, was already present andthat in fact it started to form during model construction.This effect is an inevitable side-effect in our models,

Avalanchewith hummocks

Toreva blocks

IIIII

II

II

II

II I

I I I

5 cm

Amphitheatrescar

T

H

H

5 cm = /3

= /2

= 5 /6 =

= /65 cm

5 cm 5 cm

b

= /2

SModel

RModel

5 cm

a

c

Fig. 3 Plan view photographsobtained at tModel=120 min ofmodels with variable α andfixed d/R=0. a Model andstructural sketch with α=π/2; bmodels with α<π/2; c Modelswith α>2π/3. Compare thestructural sketch with those ofnatural examples in Fig. 2

Bull Volcanol (2010) 72:771–789 779

occurring due to the immediate ductile response of thesilicone.

Later, between 30<tModel<60 min (15<tNature<30 s),the structures rapidly develop (Fig. 7a). On the surface, themain failure faults show no change in their kinematics andthe fracture patterns are enhanced. A new normal faultdevelops parallel to the previous top of the main failure,extending it well behind the volcano summit and creating anew toreva block.

In the hummock domain, the fracture network suggeststhat extension in the sliding direction is dominant near thelimit with the toreva domain, and that transtensional arcuatefaults accommodate the deformation in the frontal zone(Fig. 7a). This implies that the frontal hummock domain isfirst partially transported out from the amphitheatre as acoherent block and then it spreads laterally once it is lessconstrained by the lower amphitheatre lateral walls. As aconsequence, the frontal hummock domain partially over-rides the lower lateral amphitheatre walls and forms thrustfaults that could develop into avalanche lobes (Fig. 7a).

In the toreva domain, the parallel, normal faults continueto develop. They limit the toreva blocks which tilt and slidecoherently towards the base of the volcano (Fig. 7b).During this movement, the lower-most formed torevablocks, initially placed close to the graben axis, areincorporated in the upper hummock domain where theybreak-up and start to spread laterally.

The vertical sections confirm the observations in thesurface fault patterns. An antithetic network of normalfaults is formed and accommodates the strong extension inthe low-toreva and upper-hummock domains (Fig. 7b). It isworth noting that these antithetic faults are rooted in thecohesionless black layers and not in the basal silicone layer.Additionally, the main failure normal faults accommodatethe transport of the whole sliding flank and individualizeand tilt the toreva blocks. In the frontal hummock domain,transtensional faults accommodate lateral spreading; thesefaults are rooted in the basal silicone (Fig. 7c).

Further deformation, between 60<tModel<120 min(30<tNature<60 s), is mainly concentrated in the hummockdomain, notably in the frontal zone which spreads andforms the future avalanche fan (Fig. 8a). The torevadomain, on the other hand, continues to slide but no newstructures develop; the main failure (amphitheatre scar) isthus completely established (Fig. 8b).

The vertical sections at tModel=120 min show that mostof the basal layer has been expelled from beneath thevolcano and is now placed at the base of the hummockdomain (Fig. 8b, c, d). The arcuate transtensional faultsaccommodating the deformation in the frontal hummockdomain continue to develop, but as this domain expandsand thins, it also loses coherence and thus the resultingstructures appear more like shear bands in a granularmaterial and not like faults in brittle rock-formations(Fig. 8d). This indicates that the frontal hummock domainhas entered a stage of pervasive deformation at the scale ofthe grains, where shear zones are approximately as thick asindividual layers. It must be noticed that the original strataof the volcano are still preserved in the frontal hummockdomain, although it has been strongly deformed. This isalso characteristic of volcanic DADs in nature.

The evolution of the internal structures described herefor the model with α=π/3 could in general be expected forany model with π/2>α>π/6, when d/R=0. Part I of the firstseries of experiments already showed that the generalfracture networks and the main structures developed duringcollapse are very similar for all models with π/2>α>π/6when d/R=0 (Fig. 3a, 3b).

Second series, Part II

The internal effects of varying d/R were evaluated inmodels with fixed α=π/3. For this, each model run with

0.4

0.6

0.8

1

1.2

1.4

1.6

0 1/6 1/3 2/3

= /2

= /3

= /6

S Mod

el/R

Mod

elS M

odel

/RM

odel

d/R

b

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

56/

23/2/3/6/0

a

d/R = 0d/R = 1/6d/R = 1/3d/R = 2/3

Fig. 4 Graphs of a SModel/RModel vs. α, and b SModel/RModel vs. d/Robtained by measurements performed in models of the First Series(see text)

780 Bull Volcanol (2010) 72:771–789

respectively d/R=0, 1/6, 1/3, 2/3 was stopped at tModel=60 min in order to perform vertical sections. In Part II of thefirst series of experiments (see above, Fig. 5), the surfaceanalysis already suggested the necessity of vertical sectionsin order to clarify two points: with decreasing d/R, 1) torevablocks seem to develop less; and, 2) the main failuresurfaces appear less steep and less deep.

In Fig. 9a, b, c, the vertical sections confirm that thestructures developed in both the hummock and the torevadomains are less complex with decreasing d/R. The initialgraben is, however, present in all models except when d/R=2/3. We think this absence is just a topographic effectcoupled with the behaviour of silicone basal layer, whichwould prevent clear graben development when d/R is too

large in our models (d/R>3/5). This hypothesis is rein-forced with the observations performed by Acocella (2005),who found similar grabens developing in the sliding flanksof similar models, even when the values of d/R are largerthan in our experiments (d/R≈7/10). Instead of usingsilicone basal layers, Acocella (2005) used thin rigid plates,which were displaced horizontally at the base of the modelvolcano to generate a velocity discontinuity and thus triggerfailures. This approach is geologically less realistic thanhere, as the motor for deformation comes from thehorizontal movement of basal plate and not from gravity.Additionally, although Acocella (2005) recognized thepresence of the grabens in the models, he disregarded themwhen analysing the experimental results.

Section (b)tModel= 15 min 5 cm

Basal silicone layer

GrabenListricfaults

Antitheticfaults

Mainfailure

5 cm

Mainfailure

Hummockdomain

Torevadomain

Section(b)

Graben

Thrustfault

a

b

5 cm

Section (b)

tModel= 15 minFig. 6 a Surface structures, and,b longitudinal section of modelwith α=π/3 and d/R=0, attModel=15 min. In the section,the original model surface cor-responds to the uppermost blacklayer; this is valid for all thevertical sections presented in thefollowing figures. The whitelayer on top was added at theend of the experiment in order tofacilitate the dissection of model

5 cmd/R = 2/3

d/R = 1/6 5 cm

Toreva domain

5 cmd/R = 0

5 cmd/R = 1/3

Hummock domain

Fig. 5 Plan view photographsobtained at tModel=120 min ofmodels with fixed α=π/3 andvariable d/R. Progressive de-crease of amphitheatre size(Smodel) as well as toreva domaincomplexity is observed withincreasing d/R

Bull Volcanol (2010) 72:771–789 781

An attempt to image the internal shape of the mainfailure surfaces was made using the vertical sections ofthese models. The main failure surfaces, which areobserved in vertical sections as lines, were thus divided inseveral segments of RModel/12 (1 cm) length, beginningfrom volcano foot towards the top of the main failuresurface. The dip (δ) of each segment was then measuredand plotted against distance from volcano foot. Figure 9dshows the results of this procedure. With increasing d/R, a

general decrease of the maximum δ values measured ineach failure surface occurs, while the mean δ values stayconstant except when d/R=0. Regardless the value of d/R,the maximum δ of a given failure surface is always foundtowards the failure head, in agreement with similarobservations by Acocella (2005), which is an expectedresult for any normal fault in a cohesive material. The plotsalso show that the models with d/R=0–1/6, have a curvedfailure surface, with the values of δ continually increasing

5 cm

Basal silicone layer

5 cm

Basal silicone layer

Toreva blocksAntitheticnetwork

Mainfailure

HummockdomainTorevadomain

Mainfailure

Arcuatetranstensive

faults

Thrustfault

5 cm

a

b

tModel= 60 min

tModel= 60 min

tModel= 60 min

c

Fig. 7 a Surface structures, blongitudinal section, and ctransversal section of modelwith α=π/3 and d/RModel=0, attModel=60 min

5 cm

Basal silicone layer

Arcuate-Transtensive faults

Section(d)

5 cmtModel= 120 min

Section (c)

Section 3

Section (c)Section(d)

5 cm

tModel= 120 minSection (c)

Section (d)tModel= 120 min

5 cmToreva blocks

d

a

c

5 cm Toreva blocks

Basal silicone layer

Antitheticnetwork

Mainfailure

Section(b)

Section(b)

tModel= 120 min Section (b)

b

Fig. 8 a Surfacel structures, blongitudinal section, c proximaltransversal section, and d distaltransversal section of modelwith α=π/3 and d/R=0, attModel=120 min

782 Bull Volcanol (2010) 72:771–789

until intersection with the surface (Fig. 9d). This is reflectedby the high standard deviation of the δ values measured inthese cases. The models with d/R=1/3–2/3, on the otherhand, have more planar failure surfaces, with the values ofδ rapidly reaching a level where they stay relatively stableuntil intersection with the surface. They thus display lowstandard deviation of the δ values.

Velocity measurements

The general effects of α and d/R on the mean horizontalvelocities (VmModel) of the sliding flanks may be evaluatedin the models of the first series of experiments (see above).The values of VmModel were obtained by measuring thehorizontal displacement of the hummock domain leadingedge (model volcano foot) after tModel=120 min of modelcollapse. Figure 10a shows that for a given value of d/R, themeasured VmModel remain constant regardless the value ofα, except in the experiment with α=π/6, where VmModel isclearly lower. Conversely, for a given value of α, theVmModel regularly decreases with increasing d/R (Fig. 10b).Thus, in general, the mean velocity of the hummock domainleading front during the early stages of flank collapse seems

to depend more on d/R than in α, if α>π/6. It is worth toremember here that the velocity range measured in themodels, 0.23<VmModel<0.47 mm/min, can not be scaled tothe natural examples as the scaling procedure showed thatexperiments are not dynamically similar. This does notmean, however, that the correlations between the velocitiesand the geometrical parameters are incorrect.

More detailed speed measurements were performedusing sequential image analysis of the model with α=π/3and d/R=0. In Fig. 11a, the instantaneous horizontal speeds(ViModel) of four points aligned with the collapse axis andinitially placed in different zones of the sliding flank, areplotted against tModel. The zones of the sliding flank selectedfor measurements are, respectively, the upper and lower partsof both the toreva and hummock domains (Fig. 11b).

The graph shows that at the beginning of flank collapse,the whole toreva domain is clearly slower than thehummock domain, in agreement with the initial grabendevelopment (Fig. 11a). All four zones accelerate during thefirst minutes of tModel, but in different amounts, so thedifferences among them soon appear. While the upper-torevaaccelerates slowly until tModel≈15 min, the lower-torevadomain continues to strongly accelerate until tModel≈30 min

dModel= 8 cm

dModel= 2 cm 5 cm

5 cm

Silicone layer

5 cm

c

Torevablock

Hummockdomain

Silicone layer

5 cm

aMain

failure

Mainfailure

0

20

40

60

80

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

d/R = 0= 60.2˚; St= 16.6˚

d/R = 1/6= 51.1˚; St= 14.8˚

d/R = 1/3= 50.5˚; St= 7.6˚

d/R = 2/3= 49.8˚; St= 4.3˚

Segment #

(seerged)

= meanSt= standard deviation

d

5 cmdModel= 4 cmTorevablock ?

Silicone layer

5 cm

bMain

failure

Hummockdomain

Hummockdomain

Fig. 9 Longitudinal sectionsand sketches of the developedstructures for models with fixα=π/3 and a d/R=1/6, b d/R=1/3, c d/R=2/3, at tModel=60 minfor each section. d Variation indip (δ) of the main failuresurfaces marked in the modelsshown above and in figure 7b

Bull Volcanol (2010) 72:771–789 783

when it reaches the same ViModel as the hummock domain.This ViModel gradient between upper- and lower-torevadomains reflects both the presence of an extensional regimeinside the graben and the previous observation that the initiallower-toreva is soon incorporated in the hummock domain,where it accelerates and deforms (see second series—part I,above).

On the other hand, both upper- and lower-hummockdomains accelerate with similar ViModel until tModel≈15 min,confirming that the whole hummock domain is initiallyexpelled from inside the amphitheatre as a coherent block(Fig. 11a). Then, between tModel≈15 min and tModel≈30 min, both upper- and lower-hummock domains decelerate,but deceleration in the lower-hummock is much greater. Thisdifference in deceleration reflects: 1) the presence of atemporary compressive regime in the whole hummockdomain, which is accommodated by its thrusting over thelateral borders of the amphitheatre scar (see second series-partI above); and, 2) the fact that the lower-hummock has started

to spread laterally, which is a motion direction not representedin the plot.

After tModel≈45 min, the lower-toreva and the wholehummock domain share similar ViModel, with the lower-hummock (avalanche front) having a slightly higher speed.This reflects that the three zones have entered the regime ofavalanche spreading, where the deformation is accommo-dated by arcuate transtensional faults. The upper-torevadomain, on the other hand, has a very small ViModel, thathas decreased exponentially with time since tModel=15 min.

The time-evolution of the ViModel described above for themodel with α=π/3 could be expected for any model withπ/2>α>π/6 and d/R=0, and thus for natural cases thatmatch these conditions. If d/R>0, conversely, the ViModel

could be expected to decrease.A contour map of the ViModel magnitude at tModel=

30 min for the whole sliding flank is shown in Fig. 11c. The

0

0,25

0,5

0,75

1

1,25

1,5

0 15 30 45 60 75 90 105 120

UT

LT

UH

LH

SLIDE I

SLIDE II

tModel (min)

iV

le doM

(ni

m/m

m)

b

a

0.081

0.162

0.243

0.324

0.405

0.486

ViModel

(mm/min)tModel= 60 min 5 cmc

Fig. 11 a Graph of ViModel vs. tModel of four points aligned on the axisof collapse, and placed in different zones of the sliding flank: Upper-Toreva domain (UT); Lower-Toreva domain (LT); Upper-Hummockdomain (UH); and Lower-Hummock domain (LH). b Initial emplace-ment of each point in the sliding flank. The measured ViNature of theSlides I and II of Mount St. Helens catastrophic collapse (Voight1981) have been scaled and are also reported in the graph (seediscussion in the text). c Contour map of the ViModel measured attModel=60 min for the model with α=π/3 and d/R=0

0

0.2

0.4

0.6

0.8

d/R = 1/6d/R = 1/3

d/R = 2/3

mV

ledoM

( ni

m/m

m)

(rad)

56/

23/2/3/6/0

a

0.2

0.3

0.4

0.5

0

6/1

3/1

3/2

= /2

= /3

= /6

b

mV

ledoM

( ni

m/m

m)

d/R

d/R = 0

Fig. 10 Graphs of a VmModel vs. α, and b VmModel vs. d/R obtainedby measurements performed in models of the First Series (see text)

784 Bull Volcanol (2010) 72:771–789

toreva and hummock domains are clearly distinguishable.The contour patterns broadly reflect extension in the torevadomain and transtension in the hummock domain, coherentwith the fault patterns proposed in the Figs. 7a and 11a.

Discussion

The role of gravitational spreading

The main role of gravitational spreading during the kind ofcatastrophic collapse described here is establishing the sizeand shape of the substratum segment that will be detachedand expelled from underneath the volcano during collapse.Major structures observed in other natural examples likeEtna and Kilauea (Borgia et al. 1992; Morgan et al. 2003)as well as in analogue models (Merle and Borgia 1996;Wooller et al. 2004; Delcamp et al. 2008) suggest that,since the beginning of gravitational spreading, the substra-tum is dissected in segments that can have an arcuatethrust-fold front (variable α) and a roughly triangular shape(variable d/R).

For Socompa and Parinacota, gravitational spreadingmost probably acted in one preferential direction due to thebuttress effect exerted by neighbouring topography, as seenin analogue models by Merle and Borgia (1996). Con-versely, Las Isletas-Mombacho is not buttressed by neigh-bouring topography but is placed on a slightly dippingsubstratum which, in addition with regional tectonics,probably controlled the direction of collapse (Shea et al.2008; Wooller et al. 2004). However, in these three volcano-substratum systems the evidence of gravitational spreading,like radial transtensional grabens in the flanks and summit,or thrust-fold belts at volcano foot, are not as marked as inother spreading volcanoes like Etna or Kilauea (Borgia et al.1992; Morgan et al. 2003). This suggests that, in our naturalexamples: 1) gravitational spreading was active only for ashort time-span before catastrophic collapse; 2) coevalvolcano activity was rapidly burying the surface evidenceof gravitational spreading; and, 3) gravitational spreadingstopped or was reduced after catastrophic collapse.

We propose that gravitational spreading may enhancecatastrophic collapse only during its initial stages, when thesegments of substratum have been defined and the volcanoflanks are still steep. If catastrophic collapse does not occurat the beginning, then progressive gravitational spreadingwould slowly flatten volcano flanks, preventing failure. Themain factor determining whether catastrophic failure orslow spreading occurs is probably the rheology of thesubstratum. For example, geophysical surveys performed atthe Miocene Calimani-Gurghiu-Harghita volcanic chain(Romania) show that several volcano flanks were flattened(and even back-tilted) in one preferential direction by

gravitational spreading of a ∼2500 m-thick sedimentarysubstratum which contained a highly ductile 300 m-thicksalt layer (Szakács and Krézsek 2006) and slowly flowedfrom under the volcanoes. No catastrophic collapsesinvolving the salt layer have been reported in this chain.

The velocity measurements

Voight (1981) measured the velocities of the sliding flankduring the early-stages of Mount St. Helens catastrophiccollapse. In order to test our models, those velocities havebeen scaled using an a priori scaling factor V*=VModel/VNature=10

−7, which was chosen explicitly to fit with thevelocity magnitudes measured in the models (Table 2).Although this procedure implies only limited dynamicsimilarity, as established by the scaling procedure, it isvery interesting to note that the variations of velocitiesthrough time is in fact similar between models and nature.Slide I of Mount St. Helens, which corresponded to thelower sliding flank, behaves similarly to the low-hummockdomain of models, with an initial acceleration followed by astrong deceleration before entering a more stable regime ofslow deceleration. Also Slide II of Mount St. Helens, whichcorresponded to the higher sliding flank just below thesummit, behaves similarly to the low-toreva domain of themodels, with an initial acceleration tending to approachthe velocity of Slide I. The measurements of Voight (1981)could not be completed because the blast-cloud at Mount St.Helens obscured further observations.

The Mount St. Helens collapse clearly occurred withinthe volcano edifice. An analogy to the models can be madeif the lower part of the edifice is considered as a potentialductile layer. In the case of Mount St. Helens this wouldbe low-strength breccias, sediments and hydrothermally al-tered material incorporated during the edifice growth andmobilised by the intrusive and hydrothermal events in 1980.

Mass and energy transmission

Experiments show that, in the case of catastrophic collapsesdriven by the failure of a substratum segment, the resultingDAD will be essentially formed by elements coming fromthe hummock domain of the sliding flank and from thedetached substratum segment, in agreement with observa-tions in natural examples (van Wyk de Vries et al. 2001;Clavero et al. 2002; Shea et al. 2008). These elements(hummock domain and substratum segment) have the leastgravitational potential before collapse; however they are themost far-travelled, both in experiments and in naturalexamples.

These observations suggest that gravitational potentialmay be transmitted to the hummock domain (lower flank).This hypothesis is reinforced by the horizontal velocity

Bull Volcanol (2010) 72:771–789 785

measurements in experiments: the hummock domain isalways much faster than the toreva domain (Fig. 11).Additionally, the vertical sections show that only exten-sional structures develop between the toreva and hummockdomains (Figs. 6, 7, 8), and thus no direct horizontal pushof the toreva on the hummock domain occurs. Thus, thegravitational potential transmission from the toreva domainto the hummock domain must take place via the low-viscosity basal layer. The transmission is effected by theejection of the heavily-loaded ductile substrata from underthe toreva domain and its ejection into the hummockdomain. This adds mass and momentum to the hummockdomain and provides a thickened ductile layer on which theavalanche carapace can spread more rapidly.

The models suggest, however, that this mass-energytransmission should be a short-lived event, limited to thetime-span while the toreva domain is in contact withsignificant volumes of the low-viscosity basal layer. Thehigher velocities observed in the hummock domain duringthe first 15 min of model collapse reflect this effect(Figs. 7b, 11a). Afterwards, the basal layer is mainly placedbeneath the hummock domain, whose movement anddeformation are controlled by the ductile behaviour of thesilicone under the weight of sand. The torevas, starved ofbasal substrata, come to a standstill.

The natural examples

Several similarities between the natural examples and themodels have been observed. For instance, our resultspermit us to propose that Socompa had a deeply rooted(d/R≈0.1) basal layer detachment (see above, first series,part II). This is in concordance with some important fea-tures like: 1) the presence and distribution of prominenttoreva blocks, 2) the extraordinary DAD volume (Table 1)with respect to the size of the volcano, 3) the large amountof basal layer in the DAD, and 4) the size of the avalancheamphitheatre (Figs. 1b, 4b).

For Las Isletas-Mombacho, the results suggest that thecollapse was driven by a basal layer detachment less deeplyrooted than at Socompa (d/R≈0.4). This would explain thatat Las Isletas: 1) the DAD shows poorly developed or notoreva blocks (Fig. 1); 2) the avalanche amphitheatre isshallow and does not dissect the volcano summit (van Wykde Vries and Francis, 1997); 3) the basal layer is moreabundant in the distal zone of the DAD than in the proximalzone (Shea et al., 2008).

The case of Parinacota is different because it isimpossible to directly measure the values of α and SNature/RNature in the volcano and thus to estimate the ratio d/Rfrom Fig. 4 (see first series—part II above). Conversely,based on the results of the experiments, the following fieldevidence would suggest that the failure of Parinacota was

relatively deeply rooted (0.1<d/R<0.4): 1) the considerablevolume of the DAD with respect to edifice size (>6 km3 fora 1800 m height edifice); 2) the fact that prominent torevablocks are present and distributed all around the westernand south-western foot of the volcano (the latter notablysuggests a strong lateral avalanche spread close to thevolcano); 3) the basal layer of the DAD is as present indistal as in proximal outcrops (Clavero et al., 2002).

More general cases

The behavior, size and shape of the low-viscosity basallayer is very important in developing the early-stagestructures observed in our experiments. As describedabove, the scaling and the model procedures of ourexperiments were mainly based on the particular character-istics of our three natural examples. This particular case offlank collapse was chosen because it included a specificelement, the low-viscosity basal layer, which was relativelyeasy to model with laboratory materials. The validity of ourobservations is initially limited to the cases of catastrophiccollapses driven by the failure of a substratum segment thatacquires sudden low-viscosity behaviour.

If experiments were performed with a thinner basal layerthe deformation rate would be expected to be lower. In suchcases there would be less material to be extruded from underthe volcano. The consequence of this may be that only limiteddeformation occurs in the model, so collapse would bearrested, or there would be a smaller deposit, with less run out.

However, we have already indicated some similaritiesbetween Mount St. Helens and our models, and we proposethat our observations may have a more general validity. Wespeculate that low-viscosity layers, similar to the onesdescribed and used in experiments, may develop not only inthe substratum but also inside stratovolcanoes, and thusdrive more shallow catastrophic collapses. This hypothet-ical low-viscosity layer belongs initially to the stratovolca-no sequence and may be originally composed of weakmaterial like poorly-consolidated proximal pyroclastics,sequences of coarse-grained tephra, pyroclastic flows, oreven weathered blocky lava flows and proximal sedimen-tary fans. Hydrothermal activity may also contribute toweaken such layers by alteration. As the stratovolcanogrows, this layer is subject to increased stresses, whichultimately may reach failure limits. Such a weak layer issimilar that suggested by Oehler et al. (2004). Duringstratovolcano development, regional tectonic activity ormagmatic intrusions are capable to defining the triangularsegment of weak layer to be detached (Donnadieu andMerle 1998; Lagmay et al. 2000; Vidal and Merle 2000).Eventually, only the final trigger is needed for the weaklayer to fail, suddenly acquire a low-viscosity and drivecatastrophic collapse.

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In Fig. 12a, a low-viscosity layer is proposed to forminside a stratovolcano. The sequence of structures developedin the early stages of such a catastrophic collapse could be alsodescribed from the point of view of our experiments. InFig. 12b, an experiment with d/R=0 and α=π/3 was run withthe rigid surface inclined 6° in the direction of collapse, inorder to simulate a layer belonging to the internal sequenceof a stratovolcano, like in Fig. 12a. No noticeable differencesbetween the structures developed by the horizontal (Fig. 3b)or the inclined (Fig. 12b) layer could be observed.

The potential involvement of low-viscosity layers be-longing to the stratovolcano during a catastrophic collapsewould be more difficult to confirm in the correspondingDAD, given that the elements composing the low-viscositylayer are expected to be petrologically very similar to therest of the deposit, except if hydrothermally alteredformations are involved. Other evidences should besearched in the DAD in order to define if whether or nota low-viscosity layer drove the catastrophic collapse.

Conclusions

Analogue models were used to explore the structuresdeveloped during the early stages of catastrophic collapse

related to the sudden formation of a low-viscosity layer in thesubstratum, at the base of the sliding flank. The analoguemodels show that several features observed in natural DADsmay be the inheritance of structures that accommodatedearly-stage deformation during catastrophic flank collapse.

The following observations can be highlighted from ourexperiments:

1. The main structure accommodating initial collapse is agraben formed in the sliding flank, perpendicular to slipdirection. This graben: a) is rooted in the ductile basallayer, and, b) divides the sliding flank into a torevadomain (higher flank) and a hummock domain (lowerflank).

2. Large listric normal faults form the main failure surface(amphitheatre scar), and individualise and tilt torevablocks. The main failure surfaces become less curvedwhen the detached basal layer is less deeply rooted.

3. Antithetic normal and oblique transtensional faultsaccommodate respectively the longitudinal and lateraldeformation of the hummock domain. Early hummocksand ridges are formed during this process.

4. Oblique thrusting over the lower amphitheatre rimsoccurs when the hummock domain initiates lateralspreading, which allows significant lateral spread of theavalanche at the exit of the amphitheatre. This explainsthe observations in natural examples (Socompa, Mom-bacho and Parinacota) where the avalanches flowed: a)nearly perpendicular to the slide direction at the exit ofthe amphitheatre (Fig. 1); and, b) over the loweramphitheatre scars (van Wyk de Vries et al. 2001).

5. Models show that horizontal speed of the hummockdomain is always higher than that of toreva domain inthe sliding flank. This reflects an efficient mass andmoment transmission from the toreva to the hummockdomain via the low-viscosity basal layer duringcatastrophic collapse-early stages.

6. At the end of experiments, most of the viscous basallayer has been expelled from underneath the volcanoand forms a characteristic layer at the base of theavalanche deposit, as in natural examples (van Wyk deVries and Francis 1997; van Wyk de Vries et al. 2001).The original stratigraphy of the sliding flank ispreserved in the avalanche deposit.

7. Low-viscosity basal layers could be formed not only inthe volcano substratum, but also inside the volcano. Inthis case, the early-stage sequence of structures accom-modating deformation in the sliding flank should besimilar to the ones observed in experiments.

Acknowledgements D. Andrade was supported by the SecretaríaNacional para la Ciencia y la Tecnología (SENACYT-Ecuador), theFrench Ministry of Foreign Affairs through the French Embassy inEcuador, and the Institut de Recherche pour le Développement (IRD,

a

tModel= 120 min 5 cm

b

Fig. 12 a Sketch of the potential main structures formed during the earlystages of a catastrophic collapse driven by the formation of a low-viscosity layer inside a stratovolcano. The structures are inspired in theobservations performed in our analogue models. b Plan view photographat tModel=120 min of an experiment run with α=π/3 and d/R=0. Therigid surface at the base of the experiment was tilted ∼6° in the directionof collapse, in order to simulate an inclined internal stratovolcano layer

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France). We thank Benjamin Bernard, Alberto de la Fuente and thereviewers William Chadwick and Tim Davies for their critical remarkson the original manuscript.

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