Bull Volcanol (2010) 72:771–789 DOI 10.1007/s00445-010-0363-x
RESEARCH ARTICLE
Structural analysis of the early stages of catastrophic stratovolcano 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 the domains display distinctive structural patterns and kinetic failure of low-angle strata in or under the edifice. Such failures behaviour. Normal faults develop in the toreva domain and produce voluminous, destructive debris avalanches that are a inside the graben, while the hummock domain is characterised major volcanic hazard. At Socompa, Las Isletas-Mombacho by transtensional structures. The hummock domain also over- and Parinacota volcanoes, field studies have shown that thrusts the lower amphitheatre sides, which allows subsequent during catastrophic flank collapse a significant segment of sideways avalanche spreading. Measurements show that their substrata was detached and expelled from beneath the horizontal speeds of the hummock domain are always higher volcanic edifice and formed a mobile basal layer on which the than that of the toreva domain during model collapse. The sliding flanks were transported. Previous studies have main role played by the low-viscosity basal layer during this proposed that gravitational flank spreading was likely type of collapse is to control the size, shape and structural involved in the onset of sudden substrata failure. The early complexity of the sliding flank; it also transmits mass and stages of this particular type of flank collapse can be modelled momentum from the toreva to the hummock domain. under laboratory conditions using analogue models. This allows us to study the development of structures accommo- Keywords Flank collapse . Stratovolcano . Analogue dating early deformation of the sliding flank during cata- models . Early stage . Structures strophic collapse. In the experiments, the detached substratum segment (low-viscosity basal layer) was modelled with a silicone layer, and the overlying stratovolcano with a layered Introduction sand cone. The first structure developed in the models is a graben rooted in the low-viscosity basal layer. This graben Flank destabilization and catastrophic flank collapse have forms the limits of the future avalanche-amphitheatre and been recognized as common and very hazardous phenom- divides the sliding flank into a ‘toreva’ domain (upper sliding ena during the development of stratovolcanoes (Siebert flank) and a ‘hummock’ domain (lower sliding flank). These 1984; Siebert et al. 1987). Flank destabilization may be a long-term process, while catastrophic collapse is a geolog- ically instantaneous event. Slow flank destabilization at Editorial responsibility S. Nakada stratovolcanoes is probably induced by long-term processes : S. D. Andrade B. van Wyk de Vries like tectonic activity (e.g. Lagmay et al. 2000; Vidal and CNRS, IRD, Laboratoire Magmas et Volcans, Merle 2000), magmatic activity/volcano growth (e.g. — OPGC Université Blaise Pascal, Gorshkov 1959, Ando 1979; Donnadieu and Merle 1998; UMR 6524, 5 rue Kessler, 63000 Clermont-Ferrand, France Tibaldi 2001), hydrothermal alteration (e.g. Lopez and Williams 1993; van Wyk de Vries and Francis 1997; Reid S. D. Andrade (*) et al. 2001) and gravitational spreading (Borgia et al. 1992; Instituto Geofísico, Escuela Politécnica Nacional, van Wyk de Vries and Francis 1997). A.P. 17-2759, Quito, Ecuador Catastrophic flank collapse, in contrast, occurs when the e-mail: [email protected] destabilized flank of a stratovolcano suddenly fails, forming 772 Bull Volcanol (2010) 72:771–789 a voluminous landslide and a debris avalanche. The usual sediments or unwelded ignimbrites) contained in a thick trigger proposed to explain these catastrophic failures is the substratum may show ductile behaviour under the weight of sudden increase of pore pressure inside the stratovolcano due a sufficiently large overlying volcano (van Bemellen 1949; to phenomena like earthquakes (Montaldo et al. 1996), Borgia 1994): the main effect is that the ductile layer would magmatic intrusions (Gorshkov 1959; Voight et al. 1981; slowly flow outwards from beneath the volcano. The Elsworth and Voight 1996) or meteoric events (van Wyk de interactions between volcano edifice and substrata are more Vries et al. 2000). Usually, more than one destabilizing complex than this simple postulate, and may display a wide process and more than one trigger may have acted together variety of resulting structures, developed in both the to induce the eventual catastrophic flank collapse. The volcano and the substratum. This is because the interactions evidence indicating the origin of the destabilizing processes are controlled by several independent parameters such as: and the triggers for a specific collapse may be usually found volcano radius, height and cohesion; ductile layer thickness in the stratovolcano edifice and in the debris avalanche and viscosity; and slope of substrata. Various numerical and deposit (DAD). analogue models, as well as natural examples, have been In the present work, we will concentrate on three particular used to test the relevance of these parameters (e.g. Merle examples of flank destabilization and catastrophic collapse: and Borgia 1996; van Wyk de Vries and Matela 1998; Socompa (Chile), Las Isletas-Mombacho (Nicaragua) and Wooller et al. 2004; Delcamp et al. 2008). Parinacota (Chile) volcanoes. Studies carried out on these Gravitational spreading may occur in all directions volcanoes show that the three edifices directly overlie a radially away from the edifice summit, as at Maderas substratum composed of unconsolidated, pumice-rich pyro- volcano (van Wyk de Vries and Borgia 1996), or may occur clastic sequences (Wadge et al. 1995; van Wyk de Vries et al. in one preferential direction involving just one flank, as at 2001; Clavero et al. 2002;Sheaetal.2008). The DADs of Etna (Borgia et al. 1992). Typically, the related structures these volcanoes show that the failure surfaces involved are radial intersecting grabens (rooted in the ductile layer), significant segments of the volcano substratum, which were which tend to spread and flatten the edifice, and circular detached from underneath the volcano and formed a mobile thrust-and-fold belts formed around the base of the volcano low-viscosity basal layer on which the sliding flanks flowed (Fig. 1a) (Merle and Borgia 1996). The radial grabens during catastrophic collapse (van Wyk de Vries et al. 2001; dissect the substratum and the volcano flanks in triangular Shea et al. 2008). Based on their field observations, those segments which, in natural examples, are usually enhanced authors have hypothesised the sequence of structures that by erosion and classically interpreted as evidence for developed in the sliding flank during the early-stages of gravitational spreading (Merle and Borgia 1996; Delcamp catastrophic collapse, in order to explain some of the final et al. 2008). features observed at the DADs. The development of these In general, gravitational spreading is a slow, flank- structures is not likely to be directly observed in nature, stabilizing process, because with time, volcano flanks because catastrophic collapses are short-lived, relatively flatten due to the outward flow of the substratum, uncommon and usually fatal events to observers. becoming less and less prone to catastrophic collapse. The geometric, kinematic and dynamic characteristics of However, there is evidence which suggests that gravita- the catastrophic collapse-early stages of the three volcanoes tional spreading may also act as a flank-destabilizing can be studied under laboratory conditions using analogue process, creating conditions which could favour cata- models. The aim of this study is to observe the sequence of strophic collapses, as proposed for Socompa (van Wyk de structures that accommodate the deformation of a sliding flank Vries et al. 2001), Las Isletas-Mombacho (Shea et al. during the early stages of catastrophic collapse. These 2008) and Parinacota (Clavero et al. 2002). observations may also be seen as corresponding to the early stages of rockslide debris avalanche formation at stratovolca- noes, and so they could be useful in better understanding some The catastrophic collapses of Socompa, Mombacho characteristics of these mass-movements. The experiments and Parinacota here will directly reflect only a few particular natural cases, but we will argue on a more general applicability of the The catastrophic collapse and debris avalanche deposits results. (DAD) of Socompa (Wadge et al. 1995; van Wyk de Vries et al. 2001), Las Isletas-Mombacho (van Wyk de Vries and Francis 1997;Sheaetal.2008) and Parinacota (Clavero et Gravitational spreading and volcano stability al. 2002) volcanoes have been described in detail and the following paragraphs are based on those papers. Some The main postulate of volcano gravitational spreading is general parameters of these volcanoes and corresponding that a layer of unconsolidated materials (i.e. lacustrine DADs are listed in Table 1. When comparing the Bull Volcanol (2010) 72:771–789 773
Fig. 1 Structural sketches of: a the vertical section of a gravita- a tionally spreading volcano (modified from Merle and Borgia, 1996); b Socompa DAD (proximal zone) (van Wyk de Vries et al. 2001), c Las Isletas- Mombacho DAD (Shea et al. 2008)andd Parinacota DAD N 69 15' W 69˚10' W (proximal zone) (Clavero et al. b d ˚ N 2002). e Typical stratigraphic 5 km section of Las Isletas-Mombacho DAD; the average thickness of the deposit is 22 m (modified 18˚10' S H H T from Shea et al. 2008). Fault 24˚20' S symbols and nomenclature are T valid for all figures
H T 3 km 68˚20' W N 84˚54' W c e H
H
T Lake Nicaragua 11˚50' N DNature 4 km
Amphitheatre scar: DAD Flow direction Hummocks H and ridges T Toreva blocks Faults: Normal Reverse Strike-slip Anticline descriptions of the deposits, several features common to facies; and an amphitheatre-shaped depression in the source most volcanic DADs are found. For example, their coarse- zone (Fig. 1b, c, d) (Siebert 1984;Ui1983). breccia nature; the common occurrence of jigsaw-fit cracks Their most remarkable feature, however, is not common in the blocks; the presence of hummocks in medium to to all volcanic DADs: Socompa, Las Isletas-Mombacho and distal deposition zones forming transversal and longitudinal Parinacota display a continuous basal layer composed of ridges; the common preservation of the original edifice elements coming from the substratum directly underlying stratification in the deposit; a block facies and a matrix the respective volcanoes. At Socompa, the basal layer is
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 area mmkm3 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 22 140 774 Bull Volcanol (2010) 72:771–789 composed of unconsolidated ignimbrite, gravels and sands Other remarkable similarities among the Socompa, Las from the Salin formation, and may represent up to 80% of Isletas-Mombacho and Parinacota DADs are: the total DAD volume. At Las Isletas-Mombacho, it is 1. During flow, debris avalanches were able to widely composed of pumice, lithics and crystals coming from the expand laterally, even overflowing the low amphi- Apoyo and Las Sierras units of the substratum (Fig. 1e). At theatre rims (Fig. 1b, c, d). Parinacota, the basal layer is composed of pumiceous 2. Toreva blocks (Reiche 1937) are present inside and in rhyodacitic pyroclastic flows, rhyodacitic breccias and the proximity of the amphitheatres (Fig. 1b, c, d). This minor fluvioglacial deposits from a volcanic succession is less clear for Las Isletas-Mombacho, probably due to predating the edifice construction. These basal layers are the dense vegetation and because the toreva blocks are easy to distinguish because of their contrasting composi- less voluminous than at Socompa and Parinacota. tion with respect to the rest of the DAD and because they show abundant evidence of fluid-like behaviour, such as On the other hand, important differences may be fine grain-size, rounded pumice clasts, discontinuous observed also between the DADs of our examples, and wavy layering and obliterated original stratification, all consist mostly of structures related to the effects of local of which are absent in the in situ substratum outcrops as topography on avalanche-flow during deposition. For well as in the rest of the DAD. Their occurrence has lead example, Socompa avalanche flowed 20–30 km in a to the conclusion that a significant segment of the north-west direction, on a chiefly flat open surface before substratum beneath the respective volcano was detached riding up and being deflected to the north-east by the and involved in the catastrophic failure. This means that mountains of Sierra Almeida and Cordon de Lila. A the main failure surface (faults that form the amphitheatre complex network of normal, thrust and strike-slip faults scar) of the collapse necessarily extended down to the are observed in the DAD, mostly in the segment of the volcano substratum. Moreover, following the field evi- avalanche that was deflected (Kelfoun and Druitt 2005, dence, it has been proposed that once detached, the Kelfoun et al. 2008, Shea and van Wyk de Vries, 2008). substratum probably behaved as a fluidized, low- Conversely, Las Isletas-Mombacho avalanche flowed viscosity basal layer that was expelled from beneath the ∼5 km on an obstacle-free, gentle, smooth slope before it volcano and on which the sliding flank moved during entered the shallow Lake Nicaragua (Fig. 1c). In this near- catastrophic collapse in a process detailed by van Wyk de ideal situation, without topographic barriers to the ava- Vries et al. (2001). lanche flow, the main observed structures are strike-slip and Another remarkable feature of Socompa and Las Isletas- normal faults. Finally, normal, thrust and strike-slip faults Mombacho is the presence of thrust-fold belts at the edifice are reported at Parinacota, where the avalanche flowed on a foot, close to the failure zones and perpendicular to the mainly flat surface, but was strongly channelled by the direction of collapse (Fig. 1b, c). At Parinacota, folds have mountainous morphology close to the volcano (Fig. 1d). not been reported, although no detailed structural study of The quantitative analysis of hummocks at both Parinacota the edifice has been published yet. At Socompa, the parallel and Mombacho DAD’s shows that their volumes and the Loma Alta and La Flexura anticlines display deformed height-to-width ratios tend to decrease with transport substratum sequences, notably the Salin formation. In fact, distance (Clavero et al. 2002; Shea et al. 2008). La Flexura fold, which is cut by the lower-most amphi- In order to explain some of their field observations, theatre scar, is interpreted to represent the front of a Wadge et al. (1995), van Wyk de Vries et al. (2001), and compressive belt that was active before collapse (Fig. 1b) Shea et al. (2008) have suggested that a sequence of (van Wyk de Vries et al. 2001). Similarly, at Mombacho structures developed in the sliding flanks during the early several thrusts involving substratum formations occur in the stages of catastrophic collapse. Wadge et al. (1995) peripheral northern and eastern lower flanks of the volcano proposed a deep-seated main failure surface involving a and seem to be cut by the collapse scar (Fig. 1c); their large segment of the substratum beneath Socompa, as well interpretation is similar to the folds observed at Socompa. as a complex network of faults developing in the sliding For both volcanoes it has been proposed that the compres- flank. In contrast, van Wyk de Vries et al. (2001) and Shea sive belts originated by slow gravitational spreading of the et al. (2008) propose shallow main failure surfaces and a substratum units (Fig. 1a), and that the catastrophic limited segment of substratum detached from beneath collapses were triggered when a segment of the thrust belt Socompa and Mombacho respectively. In the case of failed (van Wyk de Vries and Francis 1997). Similar active Parinacota, Clavero et al. (2002) were more concerned compressive belts, caused by gravity-loading, have been with the DAD emplacement mechanisms and proposed no observed by geophysical methods in other spreading hypothesis on the structures accommodating early collapse volcanoes, for example at Kilauea (Morgan et al. 2003) deformation, except that it was controlled by the original and at Vesuvius (Borgia et al. 2005). edifice faults, fractures and lithologies. Bull Volcanol (2010) 72:771–789 775
Analogue model, geometrical constraints and materials Lateral view used RModel DModel 12 cm 14 cm On the basis of the preceding descriptions, a simplified Silicone layer hModel (7 mm thick) analogue model that would simulate the collapses of 6 cm Socompa, Las Isletas-Mombacho and Parinacota is pro- Stratified dModel sand cone posed. The objective of the model is to study the structures developed in the sliding flank during the early-stages of Rigid surface catastrophic collapse, this is in the short time and distance following the trigger of failure. It is assumed that the whole Plan view volcano-substratum system has been gravitationally spread- ing slowly until the moment of collapse, which marks the beginning of our experiments and observations. In order to reproduce the natural examples, the models R have two principal elements: a stratovolcano and a low- M od viscosity basal layer. In the following paragraphs, the el dModel respective geometry and size of these elements are defined.
The stratovolcanoes D M od Natural stratovolcano shapes tend to be conical and el characterized by a height (hNature) measured from its base Stratified (Tables 1, 2) and slopes of 25-30°. Thus, stratovolcanoes sand cone will be approximated with a conical shape in the models. Silicone layer This shape has been widely used in previous analogue models simulating flank destabilization and catastrophic Fig. 2 Experimental setup and illustration of the main geometrical collapse at stratovolcanoes (Donnadieu and Merle 1998; parameters of the models. Only opening angle (α) and the ductile Lagmay et al. 2000, Vidal and Merle 2000; Acocella 2005). layer offset (dModel) were varied during experiments For practical convenience, the volcanic edifices were modelled with stratified sand cones of constant height natural stratovolcanoes (see Scaling below) and to con- hModel=6 cm and slopes of 25–30°. Thus, a fixed radius strain fault displacements in vertical sections. A mixture of RModel=12 cm was used for all models (Fig. 2, Table 2). 85% white sand and 15% plaster was used for white layers Strata with two different cohesions and colours were with CModel=100 Pa cohesion, and pure black sand was used in order to better simulate the physical behaviour of used for black layers with negligible cohesion CModel=0 Pa
Table 2 Parameters used in the scaling procedure. §: Williams Variable Definition Unit Value et al. (1987); #: Afrouz (1992) and Bell (2000) 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 1 776 Bull Volcanol (2010) 72:771–789
(Table 2). The densities of both kind of layers were similar Vries et al. 2001)and∼200 m for the Apoyo and Las Sierras −3 and averaged at ρModel=1500 kg m (Table 2). Addition- units underneath Mombacho (van Wyk de Vries and Francis ally, during some experiments, the analogue volcano was 1997). There are no estimates for the case of Parinacota. covered with a 0.2–0.3 cm thick pure plaster layer in order The average T of the low-viscosity basal layer was taken to to sharpen the surface visualization of the structures be around 15% the volcano height (h) and, given that model developed. This layer does not significantly affect the stratovolcanoes have a constant hModel=6 cm, then for the mechanical behaviour of the analogue volcano, because experiments TModel=0.7 cm (Table 2). models with and without plaster layer developed the same The material used to simulate the low-viscosity basal layer structures. was a silicone putty (SGM-36) produced by Dow Corning 4 (UK) Ltd., which has a viscosity μModel=2×10 Pa s and a 3 −3 The low-viscosity basal layer density γModel=10 kg m (Weijemars and Schmeling, 1986) (Table 2). The low-viscosity basal layers for experiments First, the natural examples show that the low-viscosity were thus obtained by cutting triangular segments with basal layers should have a roughly arc-shaped external varied dModel and α, from an initial silicone circle 0.7 cm front, defined by the circular thrust-fold belts formed by thick (TModel) and 14 cm radius (DModel)(Fig.2). gravitational spreading (Borgia et al. 1992; van Wyk de Vries and Francis 1997). In the natural examples, the angular lengths (α) of the arcuate fronts are ∼π/3 at Model scaling Socompa and ∼π/6 at Las Isletas-Mombacho (Fig. 1b, c). At Parinacota this cannot be found because it is covered A scaling procedure must be respected for experiments to with post-collapse deposits. The value of α was varied be geometrically, kinematically and dynamically similar to during the experiments (Fig. 2; Table 2). the natural examples. Standard similarity conditions (Hub- Also, in the natural examples the failed fronts of the bert 1937) were established through the 12 variables thrust-fold belts are located at a distance DNature measured involved in the experiment (Table 2). Thus, according to in plan-view from the inferred pre-collapse volcano summit the Buckingham-П theorem, 9 independent dimensionless (Fig. 1c; Table 2). At Socompa and Las Isletas-Mombacho variables must be defined and need to be as similar as the respective DNature are ∼13 and ∼8 km. If their respective possible between models and nature. Geometrical similarity RNature are inferred at ∼10 km and ∼7 km (van Wyk de is guaranteed by the five variables (П1, П2, П3, П4 and Vries et al. 2001; van Wyk de Vries and Francis 1997), then П5) defined in Table 3. The П6 variable relates the density
1.3>DNature/RNature>1.14. For the models, DModel/RModel of the volcano edifice (ρ) to that of the low-viscosity basal should be similar to DNature/RNature. Thus a fixed DModel/ layer (γ). The bulk density of an edifice has been estimated −3 RModel=1.17 was used, so the free arcuate front of the low- at ρNature=2200 kg m (Williams et al. 1987), while the viscosity basal layer was placed at a constant DModel= density of a pumice-rich ignimbrite may be γNature=1400– 14 cm (Fig. 2; Table 2). 2100 kg m−3 (Bell, 2000) (Table 2). As already stated, the main failure surface (faults that form The kinematic and dynamic similarity conditions may be the amphitheatre scar) at Socompa, Las Isletas-Mombacho obtained with the balance of the main forces acting on the and Parinacota necessarily extended to the underlying models and natural examples during the collapse process. substratum during catastrophic collapse. Thus, the lateral These forces are: 1) gravitational (FG), 2) inertial (FI), 3) borders of the substratum segment detached during cata- failure resistance (FR), and 4) viscous (FV), and are defined strophic collapse should be somewhat similar in shape to the as follows: borders of the amphitheatre scars of the natural examples; this F ¼ r g h; ð1Þ is roughly an elongated horse-shoe shape. In order to simplify G the experiments, we have approximated this shape to an isoceles triangle, whose vertex is placed at a horizontal 2 FI ¼ r V ð2Þ distance dModel from stratovolcano summit (Fig. 2). During experiments, dModel was varied from 0 cm (vertex directly where V is a characteristic velocity of the process; underneath volcano summit) to 8 cm (vertex close to volcano ¼ þ = ðÞs s ðÞ foot) and normalized with respect to the fixed RModel=12 cm. FR C h R 1 3 3 So, for the presentation of results we will refer to the ratio if a Navier-Coulomb failure criterion is assumed; d/R, which will vary from 0 to 2/3 (Tables 2, 3). Finally, in the natural examples, the thickness (T) of the FV ¼ m=t; ð4Þ detached substratum segment has been estimated at 300 m for the Salin Formation underneath Socompa (van Wyk de where t is a characteristic time of the process. Bull Volcanol (2010) 72:771–789 777
Table 3 Definition and values of the calculated П-Numbers in Dimensionless variable Definition Value natural examples and the models 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
In (3), h/R represents the value of tan θ, where θ is the natural gravitational, failure resistance and viscous forces. angle of internal friction of the stratovolcano. σ1 and σ3 are However, there are very large differences between models the maximum and minimum principal stresses, expressed and nature for П9=FI/FV and П10=FI/FG, which are mainly by σ1=ρ×g×h, and σ3=(ρ×g×h/3)+μ/t, given that σ3is due to the second power applied to V when calculating FI equal to the stress due to volcano loading plus the stress with (2). This implies that models fail to represent the due to viscous deformation (Merle and Borgia 1996). Thus, inertial forces of nature (Table 3). The differences of П9 the failure resistance force may be expressed as and П10 between models and nature are less pronounced if
tModel is strongly reduced (tModel<1000 s), which suggests FR ¼ C þ h=R ðÞ2FG=3 FV ð5Þ that VModel may in fact be representative of VNature,butonly The velocities of the sliding flank during the Mount St. during the initial moments of analogue model collapse;
Helens catastrophic collapse were measured at 10
FG/FV and П8=FR/FV between models and natural exam- evolution, geometry and kinematics of the structures to be ples suggest that experiments will reproduce fairly well the constrained. 778 Bull Volcanol (2010) 72:771–789
The first series of experiments was designed to explore amphitheatre scar (see Fig. 3a). We consider that SModel the effect of the basal layer geometry on the surface represents a proxy for slide volume, which is hard to structures developed during collapse. This series has two directly measure in our experiments. SModel displays a parts. In part I, d/R=0 (dModel=0 cm) was kept constant and positive linear correlation with α,whend/R is fixed. the value of α was varied from π (half-circle arc) to π/6 This graph also shows that, regardless the value of α, (narrow arc), in steps of π/6. In part II, surface observations when d/R=0, the volcano summit is always involved in were performed in models with variable d/R=0, 1/6, 1/3, the failure (SModel/RModel>1). For example, at Las Isletas- 2/3 (dModel=0,2,4,8cm)andα=π/2, π/3, π/6. Mombacho the summit of the pre-collapse volcano was In the second series of experiments, the model with α=π/3 not involved in failure (Shea et al. 2008;vanWykdeVries was chosen to perform vertical sections and explore the and Francis 1997), and the opposite occurred with internal structures developed during model collapse. This Socompa (van Wyk de Vries et al. 2001; Wadge et al. series has also two parts. In part I, the model had both α=π/3 1995), which suggests contrasting values of d/R between and d/R=0 fixed, and sections were performed at tModel=0, these natural prototypes. 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 First series, Part II (tNature=30s)inmodelswithfixedα=π/3 and variable d/R=1/6, 1/3, 2/3. The effect of variable basal layer position (d/R=0, 1/6, 1/3, Finally, the model with α=π/3 and d/R=0 was used to 2/3) was evaluated in the models with ductile layer measure the surface horizontal velocities, by the analysis of openings of α=π/2, π/3, π/6. At the surface, at the end of the sequential photographs. valid observations (tModel=120min)thesamemajor structures as in Part I develop in the models: an amphitheatre-shaped depression, large toreva blocks and a Results hummocky fan (Fig. 5). It is also observed that with
increasing d/R: 1) amphitheatre length SModel decreases; 2) First Series, Part I the toreva blocks develop less; and, 3) the main failure surfaces are less steep and less deep (Fig. 5). The first point When α was varied and d/R=0 kept constant, the major may be evaluated with surface images, while the two others structures developed at the end of valid observations (tModel= need vertical sections to be confirmed. 120 min) are: 1) an amphitheatre-shaped depression, with In Fig. 4b, the values of SModel/RModel are plotted against sub-vertical walls, on top of the initial basal layer site; 2) d/R. As expected, the diagram shows that SModel/RModel large, tilted slide blocks (toreva blocks) inside the amphi- decreases regularly with increasing d/R. Thus, SModel/RModel theatre, elongated perpendicular to the slip direction; and 3) a varies as a function of both d/R (dModel) and α. This is a fan-shaped zone outside the amphitheatre (proto-avalanche), largely expected, not surprising result, but it is associated with analogue hummocks forming transverse and longitudi- with gradual changes in the structures developed in the nal ridges (Fig. 3a). sliding flank, as well as it gives some important clues to the
When the models have π/2>α>π/6, the amphitheatre is nature of the failure geometry. For example, if the SNature/ relatively narrow and the structures developed are similar to RNature and α values could be measured in a natural those observed in the natural examples (compare Figs. 1b, volcano, then the ratio d/R could be estimated from c, d and 3b). When π>α>π/2, the wide open amphitheatres Fig. 4b. At Socompa SNature/RNature≈1.15 and α≈π/3, then produced do not correspond to the natural examples and, as d/R≈0.1, which implies a deeply rooted basal layer for far as we know, have not been reported at any stratovolcano the collapse, in agreement with the hypothesis of Wadge in continental domains (Fig. 3c). Such wide open amphi- et al. (1995), but in disagreement with that of van Wyk de theatres are often observed, however, in oceanic-island Vries et al. (2001). At Las Isletas-Mombacho, SNature/ shield volcanoes like Lanai and Kilauea in Hawaii (Moore RNature≈0.85 and α≈π/6, then d/R≈0.4, which implies a et al. 1989) or La Palma and El Hierro in the Canary Islands less deeply rooted basal layer, reinforcing the hypoth- (Carracedo et al. 1999). No further analysis of the cases esis of Shea et al. (2008) and van Wyk de Vries and where π>α>π/2 will be done here because our scaling and Francis (1997). For Parinacota, these data cannot be model procedure do not correspond to the conditions at measured because the collapse scar has been buried by Hawaii and the Canary Islands. post-collapse activity. However, given the subjectivity in
In Figure 4a, the values of the model amphitheatre the measurements of RNature, these calculations must be length, SModel, normalized to model radius RModel, are taken only as a first order approximation to the value of plotted against α. SModel wasmeasuredinplanviewfrom d/R and to the actual size of the detached basal layer in model volcano foot (fixed for all experiments) to the top natural cases. Bull Volcanol (2010) 72:771–789 779
Fig. 3 Plan view photographs = /2 5 cm = /2 obtained at tModel=120 min of 5 cm
models with variable α and
I
I
fixed d/R=0. a Model and I I Amphitheatre I H
structural sketch with α=π/2; b I scar I
models with α<π/2; c Models I with α>2π/3. Compare the I SModel I T a I structural sketch with those of I I I natural examples in Fig. 2 I I RModel I H Toreva blocks Avalanche with hummocks 5 cm = /6 5 cm = /3
b
5 cm = 5 /6 5 cm =
c
Second series, Part I faults; we will call this zone the Hummock Domain. Second, a block in the middle and upper flank (including The model with fixed opening angle (α=π/3) and fixed the summit) displays only a few normal faults sub-parallel ductile layer position (d/R=0) was chosen to study the to the top main failure; we will call this zone the Toreva development of early structures using plan-view sequential domain (Fig. 6a). pictures and vertical sections. Pictures were taken every The vertical section at tModel=15 min shows that the 3 min (1.5 s in tNature) during the first 15 min of model initial dominant structure is a listric graben perpendicular to collapse, and then every 5 min (2.5 s in tNature) until tModel= the sliding direction (Fig. 6b). This graben is formed by: 1) 120 min. Experiments were stopped at tModel=15, 30, 60 the main failure surface, composed of large faults verging and120min(tNature=7.5,15,30,60s)toperform towards the volcano summit, which accommodate the longitudinal and transverse vertical sections. formation of both the amphitheatre and the toreva slides;
On the surface, at tModel=15 min (tNature=7.5 s) of model and, 2) antithetic faults verging towards volcano foot that collapse, a part of the main failure delimiting the future accommodate the initial horizontal displacement of the amphitheatre scar is established (Fig. 6a). The faults hummock domain (Fig. 6b). The graben faults are rooted in comprising the main failure are purely extensional in the the silicone layer, and the graben axis separates the summit zone, transtensional in the upper-middle slopes and hummock domain from the toreva domain. A vertical transpressional in the middle-lower slopes. Inside these section performed at tModel=0 min clearly showed that the limits, two sharply different domains are observed in the graben, though poorly developed, was already present and sliding flank. First, a highly fractured zone in the middle that in fact it started to form during model construction. and lower flank is limited by the lateral transpressional This effect is an inevitable side-effect in our models, 780 Bull Volcanol (2010) 72:771–789
a 1.6 In the toreva domain, the parallel, normal faults continue to develop. They limit the toreva blocks which tilt and slide 1.4 coherently towards the base of the volcano (Fig. 7b). 1.2 During this movement, the lower-most formed toreva 1 blocks, initially placed close to the graben axis, are Model
R incorporated in the upper hummock domain where they / 0.8 break-up and start to spread laterally. 0.6 Model d/R =0 The vertical sections confirm the observations in the S d/R =1/6 0.4 surface fault patterns. An antithetic network of normal d/R =1/3 faults is formed and accommodates the strong extension in 0.2 d/R = 2/3 the low-toreva and upper-hummock domains (Fig. 7b). It is 0 5 2
0 worth noting that these antithetic faults are rooted in the / / / 3 2 6 / / 3 6 cohesionless black layers and not in the basal silicone layer. Additionally, the main failure normal faults accommodate b 1.6 the transport of the whole sliding flank and individualize and tilt the toreva blocks. In the frontal hummock domain, = /2 1.4 transtensional faults accommodate lateral spreading; these = /3 faults are rooted in the basal silicone (Fig. 7c). = /6 1.2 Further deformation, between 60 R 1 / domain, notably in the frontal zone which spreads and forms the future avalanche fan (Fig. 8a). The toreva Model 0.8 domain, on the other hand, continues to slide but no new S structures develop; the main failure (amphitheatre scar) is 0.6 thus completely established (Fig. 8b). The vertical sections at tModel=120 min show that most 0.4 of the basal layer has been expelled from beneath the 0 1/6 1/3 2/3 volcano and is now placed at the base of the hummock d/R domain (Fig. 8b, c, d). The arcuate transtensional faults accommodating the deformation in the frontal hummock Fig. 4 Graphs of a SModel/RModel vs. α, and b SModel/RModel vs. d/R obtained by measurements performed in models of the First Series domain continue to develop, but as this domain expands (see text) and thins, it also loses coherence and thus the resulting structures appear more like shear bands in a granular occurring due to the immediate ductile response of the material and not like faults in brittle rock-formations silicone. (Fig. 8d). This indicates that the frontal hummock domain Later, between 30 Fig. 5 Plan view photographs d/R = 0 5 cm d/R = 1/6 5 cm obtained at tModel=120 min of models with fixed α=π/3 and variable d/R. Progressive de- crease of amphitheatre size (Smodel) as well as toreva domain complexity is observed with increasing d/R Toreva domain d/R = 1/3 5 cm d/R = 2/3 5 cm Hummock domain respectively d/R=0, 1/6, 1/3, 2/3 was stopped at tModel= large in our models (d/R>3/5). This hypothesis is rein- 60 min in order to perform vertical sections. In Part II of the forced with the observations performed by Acocella (2005), first series of experiments (see above, Fig. 5), the surface who found similar grabens developing in the sliding flanks analysis already suggested the necessity of vertical sections of similar models, even when the values of d/R are larger in order to clarify two points: with decreasing d/R, 1) toreva than in our experiments (d/R≈7/10). Instead of using blocks seem to develop less; and, 2) the main failure silicone basal layers, Acocella (2005) used thin rigid plates, surfaces appear less steep and less deep. which were displaced horizontally at the base of the model In Fig. 9a, b, c, the vertical sections confirm that the volcano to generate a velocity discontinuity and thus trigger structures developed in both the hummock and the toreva failures. This approach is geologically less realistic than domains are less complex with decreasing d/R. The initial here, as the motor for deformation comes from the graben is, however, present in all models except when d/R= horizontal movement of basal plate and not from gravity. 2/3. We think this absence is just a topographic effect Additionally, although Acocella (2005) recognized the coupled with the behaviour of silicone basal layer, which presence of the grabens in the models, he disregarded them would prevent clear graben development when d/R is too when analysing the experimental results. Fig. 6 a Surface structures, and, tModel= 15 min 5 cm 5 cm b longitudinal section of model Graben with α=π/3 and d/R=0, at t =15 min. In the section, Main Model failure the original model surface cor- a responds to the uppermost black layer; this is valid for all the Section (b) vertical sections presented in the Section (b) following figures. The white Hummock layer on top was added at the domain end of the experiment in order to Toreva Thrust domain fault facilitate the dissection of model = 15 min Listric tModel Section (b) 5 cm faults Graben Antithetic faults b Main failure Basal silicone layer 782 Bull Volcanol (2010) 72:771–789 Fig. 7 a Surface structures, b 5 cm longitudinal section, and c tModel= 60 min Arcuate transtensive transversal section of model faults α π a Main with = /3 and d/RModel=0, at failure tModel=60 min Hummock domain Toreva Thrust domain fault tModel= 60 min Toreva blocks 5 cm Antithetic b network Main failure Basal silicone layer tModel= 60 min 5 cm c Basal silicone layer An attempt to image the internal shape of the main general decrease of the maximum δ values measured in failure surfaces was made using the vertical sections of each failure surface occurs, while the mean δ values stay these models. The main failure surfaces, which are constant except when d/R=0. Regardless the value of d/R, observed in vertical sections as lines, were thus divided in the maximum δ of a given failure surface is always found several segments of RModel/12 (1 cm) length, beginning towards the failure head, in agreement with similar from volcano foot towards the top of the main failure observations by Acocella (2005), which is an expected surface. The dip (δ) of each segment was then measured result for any normal fault in a cohesive material. The plots and plotted against distance from volcano foot. Figure 9d also show that the models with d/R=0–1/6, have a curved shows the results of this procedure. With increasing d/R,a failure surface, with the values of δ continually increasing Fig. 8 a Surfacel structures, b tModel= 120 min 5 cm longitudinal section, c proximal 5 cm transversal section, and d distal transversal section of model with α=π/3 and d/R=0, at Section Section tModel=120 min (b) a (b) Section (c) Section (c) Section Section (d) Section(d) 3 tModel= 120 min Section (b) 5 cm Toreva blocks Antithetic network b Main failure Basal silicone layer tModel= 120 min 5 cm Section (c) Toreva blocks c tModel= 120 min Section (d) Arcuate-Transtensive faults 5 cm d Basal silicone layer Bull Volcanol (2010) 72:771–789 783 Fig. 9 Longitudinal sections dModel= 2 cm 5 cm 5 cm Toreva and sketches of the developed block structures for models with fix Hummock α=π/3 and a d/R=1/6, b d/R=1/ domain a 3, c d/R=2/3, at tModel=60 min for each section. d Variation in Main dip (δ) of the main failure failure surfaces marked in the models Silicone layer shown above and in figure 7b dModel= 4 cm 5 cm 5 cm Toreva block ? Hummock b domain Main failure Silicone layer dModel= 8 cm 5 cm 5 cm Hummock c domain Main failure Silicone layer 80 d/R = 0 d = 60.2˚; St= 16.6˚ d/R = 1/6 60 = 51.1˚; St= 14.8˚ ) s e e r d/R = 1/3 g 40 e = 50.5˚; St= 7.6˚ d ( d/R = 2/3 20 = 49.8˚; St= 4.3˚ Segment # = mean 0 St= standard deviation 1 2 3 4 5 6 7 8 9 101112131415161718 until intersection with the surface (Fig. 9d). This is reflected to depend more on d/R than in α,ifα>π/6. It is worth to by the high standard deviation of the δ values measured in remember here that the velocity range measured in the these cases. The models with d/R=1/3–2/3, on the other models, 0.23 The general effects of α and d/R on the mean horizontal (ViModel) of four points aligned with the collapse axis and velocities (VmModel) of the sliding flanks may be evaluated initially placed in different zones of the sliding flank, are in the models of the first series of experiments (see above). plotted against tModel. The zones of the sliding flank selected The values of VmModel were obtained by measuring the for measurements are, respectively, the upper and lower parts horizontal displacement of the hummock domain leading of both the toreva and hummock domains (Fig. 11b). edge (model volcano foot) after tModel=120 min of model The graph shows that at the beginning of flank collapse, collapse. Figure 10a shows that for a given value of d/R, the the whole toreva domain is clearly slower than the measured VmModel remain constant regardless the value of hummock domain, in agreement with the initial graben α, except in the experiment with α=π/6, where VmModel is development (Fig. 11a). All four zones accelerate during the clearly lower. Conversely, for a given value of α, the first minutes of tModel, but in different amounts, so the VmModel regularly decreases with increasing d/R (Fig. 10b). differences among them soon appear. While the upper-toreva Thus, in general, the mean velocity of the hummock domain accelerates slowly until tModel≈15 min, the lower-toreva leading front during the early stages of flank collapse seems domain continues to strongly accelerate until tModel≈30 min 784 Bull Volcanol (2010) 72:771–789 a 0.8 to spread laterally, which is a motion direction not represented in the plot. After tModel≈45 min, the lower-toreva and the whole ) n 0.6 i hummock domain share similar ViModel, with the lower- m / hummock (avalanche front) having a slightly higher speed. m m ( This reflects that the three zones have entered the regime of l e 0.4 avalanche spreading, where the deformation is accommo- d o M dated by arcuate transtensional faults. The upper-toreva d/R =0 m domain, on the other hand, has a very small ViModel, that V d/R = 1/6 0.2 has decreased exponentially with time since t =15 min. d/R = 1/3 Model The time-evolution of the ViModel described above for the d/R = 2/3 model with α=π/3 could be expected for any model with 0 2 5 0 π α π / / / /2> > /6 and d/R=0, and thus for natural cases that 6 3 2 / / 3 6 match these conditions. If d/R>0, conversely, the ViModel (rad) could be expected to decrease. A contour map of the Vi magnitude at t = b 0.5 Model Model 30 min for the whole sliding flank is shown in Fig. 11c. The = /2 ) = /3 n i a m 0.4 / = /6 UT m m 1,5 LT ( l e UH d ) o 1,25 n i M LH 0.3 m m / SLIDE I V m 1 b m ( SLIDE II l e d 0,75 o M 0.2 i 0 1 1 2 V / / / 0,5 6 3 3 d/R 0,25 α Fig. 10 Graphs of a VmModel vs. , and b VmModel vs. d/R obtained 0 by measurements performed in models of the First Series (see text) 0 153045607590105120 tModel (min) when it reaches the same ViModel as the hummock domain. c tModel= 60 min 5 cm ViModel This ViModel gradient between upper- and lower-toreva (mm/min) domains reflects both the presence of an extensional regime inside the graben and the previous observation that the initial 0.486 lower-toreva is soon incorporated in the hummock domain, 0.405 where it accelerates and deforms (see second series—part I, 0.324 above). On the other hand, both upper- and lower-hummock 0.243 domains accelerate with similar ViModel until tModel≈15 min, 0.162 confirming that the whole hummock domain is initially 0.081 expelled 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, Fig. 11 a Graph of ViModel vs. tModel of four points aligned on the axis of collapse, and placed in different zones of the sliding flank: Upper- but deceleration in the lower-hummock is much greater. This Toreva domain (UT); Lower-Toreva domain (LT); Upper-Hummock difference in deceleration reflects: 1) the presence of a domain (UH); and Lower-Hummock domain (LH). b Initial emplace- temporary compressive regime in the whole hummock ment of each point in the sliding flank. The measured ViNature of the domain, which is accommodated by its thrusting over the Slides I and II of Mount St. Helens catastrophic collapse (Voight 1981) have been scaled and are also reported in the graph (see lateral borders of the amphitheatre scar (see second series-part discussion in the text). c Contour map of the ViModel measured at I above); and, 2) the fact that the lower-hummock has started tModel=60 min for the model with α=π/3 and d/R=0 Bull Volcanol (2010) 72:771–789 785 toreva and hummock domains are clearly distinguishable. gravitational spreading of a ∼2500 m-thick sedimentary The contour patterns broadly reflect extension in the toreva substratum which contained a highly ductile 300 m-thick domain and transtension in the hummock domain, coherent salt layer (Szakács and Krézsek 2006) and slowly flowed with the fault patterns proposed in the Figs. 7a and 11a. from under the volcanoes. No catastrophic collapses involving the salt layer have been reported in this chain. Discussion The velocity measurements The role of gravitational spreading Voight (1981) measured the velocities of the sliding flank during the early-stages of Mount St. Helens catastrophic The main role of gravitational spreading during the kind of collapse. In order to test our models, those velocities have catastrophic collapse described here is establishing the size been scaled using an a priori scaling factor V*=VModel/ −7 and shape of the substratum segment that will be detached VNature=10 , which was chosen explicitly to fit with the and expelled from underneath the volcano during collapse. velocity magnitudes measured in the models (Table 2). Major structures observed in other natural examples like Although this procedure implies only limited dynamic Etna and Kilauea (Borgia et al. 1992; Morgan et al. 2003) similarity, as established by the scaling procedure, it is as well as in analogue models (Merle and Borgia 1996; very interesting to note that the variations of velocities Wooller et al. 2004; Delcamp et al. 2008) suggest that, through time is in fact similar between models and nature. since the beginning of gravitational spreading, the substra- Slide I of Mount St. Helens, which corresponded to the tum is dissected in segments that can have an arcuate lower sliding flank, behaves similarly to the low-hummock thrust-fold front (variable α) and a roughly triangular shape domain of models, with an initial acceleration followed by a (variable d/R). strong deceleration before entering a more stable regime of For Socompa and Parinacota, gravitational spreading slow deceleration. Also Slide II of Mount St. Helens, which most probably acted in one preferential direction due to the corresponded to the higher sliding flank just below the buttress effect exerted by neighbouring topography, as seen summit, behaves similarly to the low-toreva domain of the in analogue models by Merle and Borgia (1996). Con- models, with an initial acceleration tending to approach versely, Las Isletas-Mombacho is not buttressed by neigh- the velocity of Slide I. The measurements of Voight (1981) bouring topography but is placed on a slightly dipping could not be completed because the blast-cloud at Mount St. substratum which, in addition with regional tectonics, Helens obscured further observations. probably controlled the direction of collapse (Shea et al. The Mount St. Helens collapse clearly occurred within 2008; Wooller et al. 2004). However, in these three volcano- the volcano edifice. An analogy to the models can be made substratum systems the evidence of gravitational spreading, if the lower part of the edifice is considered as a potential like radial transtensional grabens in the flanks and summit, ductile layer. In the case of Mount St. Helens this would or thrust-fold belts at volcano foot, are not as marked as in be low-strength breccias, sediments and hydrothermally al- other spreading volcanoes like Etna or Kilauea (Borgia et al. tered material incorporated during the edifice growth and 1992; Morgan et al. 2003). This suggests that, in our natural mobilised by the intrusive and hydrothermal events in 1980. examples: 1) gravitational spreading was active only for a short time-span before catastrophic collapse; 2) coeval Mass and energy transmission volcano activity was rapidly burying the surface evidence of gravitational spreading; and, 3) gravitational spreading Experiments show that, in the case of catastrophic collapses stopped or was reduced after catastrophic collapse. driven by the failure of a substratum segment, the resulting We propose that gravitational spreading may enhance DAD will be essentially formed by elements coming from catastrophic collapse only during its initial stages, when the the hummock domain of the sliding flank and from the segments of substratum have been defined and the volcano detached substratum segment, in agreement with observa- flanks are still steep. If catastrophic collapse does not occur tions in natural examples (van Wyk de Vries et al. 2001; at the beginning, then progressive gravitational spreading Clavero et al. 2002; Shea et al. 2008). These elements would slowly flatten volcano flanks, preventing failure. The (hummock domain and substratum segment) have the least main factor determining whether catastrophic failure or gravitational potential before collapse; however they are the slow spreading occurs is probably the rheology of the most far-travelled, both in experiments and in natural substratum. For example, geophysical surveys performed at examples. the Miocene Calimani-Gurghiu-Harghita volcanic chain These observations suggest that gravitational potential (Romania) show that several volcano flanks were flattened may be transmitted to the hummock domain (lower flank). (and even back-tilted) in one preferential direction by This hypothesis is reinforced by the horizontal velocity 786 Bull Volcanol (2010) 72:771–789 measurements in experiments: the hummock domain is relatively deeply rooted (0.1 In Fig. 12a, a low-viscosity layer is proposed to form related to the sudden formation of a low-viscosity layer in the inside a stratovolcano. The sequence of structures developed substratum, at the base of the sliding flank. The analogue in the early stages of such a catastrophic collapse could be also models show that several features observed in natural DADs described from the point of view of our experiments. In may be the inheritance of structures that accommodated Fig. 12b, an experiment with d/R=0 and α=π/3 was run with early-stage deformation during catastrophic flank collapse. the rigid surface inclined 6° in the direction of collapse, in The following observations can be highlighted from our order to simulate a layer belonging to the internal sequence experiments: of a stratovolcano, like in Fig. 12a. No noticeable differences 1. The main structure accommodating initial collapse is a between the structures developed by the horizontal (Fig. 3b) graben formed in the sliding flank, perpendicular to slip or the inclined (Fig. 12b) layer could be observed. direction. This graben: a) is rooted in the ductile basal The potential involvement of low-viscosity layers be- layer, and, b) divides the sliding flank into a toreva longing to the stratovolcano during a catastrophic collapse domain (higher flank) and a hummock domain (lower would be more difficult to confirm in the corresponding flank). DAD, given that the elements composing the low-viscosity 2. Large listric normal faults form the main failure surface layer are expected to be petrologically very similar to the (amphitheatre scar), and individualise and tilt toreva rest of the deposit, except if hydrothermally altered blocks. The main failure surfaces become less curved formations are involved. Other evidences should be when the detached basal layer is less deeply rooted. searched in the DAD in order to define if whether or not 3. Antithetic normal and oblique transtensional faults a low-viscosity layer drove the catastrophic collapse. accommodate respectively the longitudinal and lateral deformation of the hummock domain. Early hummocks and ridges are formed during this process. Conclusions 4. Oblique thrusting over the lower amphitheatre rims occurs when the hummock domain initiates lateral Analogue models were used to explore the structures spreading, which allows significant lateral spread of the developed during the early stages of catastrophic collapse avalanche at the exit of the amphitheatre. This explains the observations in natural examples (Socompa, Mom- bacho and Parinacota) where the avalanches flowed: a) nearly perpendicular to the slide direction at the exit of the amphitheatre (Fig. 1); and, b) over the lower amphitheatre scars (van Wyk de Vries et al. 2001). 5. Models show that horizontal speed of the hummock a domain is always higher than that of toreva domain in the sliding flank. This reflects an efficient mass and moment transmission from the toreva to the hummock tModel= 120 min 5 cm domain via the low-viscosity basal layer during catastrophic collapse-early stages. 6. At the end of experiments, most of the viscous basal layer has been expelled from underneath the volcano and forms a characteristic layer at the base of the avalanche deposit, as in natural examples (van Wyk de Vries and Francis 1997; van Wyk de Vries et al. 2001). The original stratigraphy of the sliding flank is preserved in the avalanche deposit. 7. Low-viscosity basal layers could be formed not only in b the volcano substratum, but also inside the volcano. In this case, the early-stage sequence of structures accom- modating deformation in the sliding flank should be Fig. 12 a Sketch of the potential main structures formed during the early similar to the ones observed in experiments. stages of a catastrophic collapse driven by the formation of a low- viscosity layer inside a stratovolcano. The structures are inspired in the observations performed in our analogue models. b Plan view photograph Acknowledgements D. Andrade was supported by the Secretaría at tModel=120 min of an experiment run with α=π/3 and d/R=0. The Nacional para la Ciencia y la Tecnología (SENACYT-Ecuador), the rigid surface at the base of the experiment was tilted ∼6° in the direction French Ministry of Foreign Affairs through the French Embassy in of collapse, in order to simulate an inclined internal stratovolcano layer Ecuador, and the Institut de Recherche pour le Développement (IRD, 788 Bull Volcanol (2010) 72:771–789 France). We thank Benjamin Bernard, Alberto de la Fuente and the McGuire WJ, Jones AP, Neuberg J (Eds) Volcano instability on the reviewers William Chadwick and Tim Davies for their critical remarks Earth and other planets. Geol Soc London Spec Pub 110:169–177 on the original manuscript. 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