Volcanic Debris Avalanche Deposits Research Articles

Volcanic debris avalanche deposits and their significance in the architecture and evolution of the Miocene-Quaternary Călimani-Gurghiu-Harghita volcanic range (Eastern Transylvania, Romania)

The Călimani-Gurghiu-Harghita volcanic range (CGH) represents the surface manifestation of the post-collisional magmatism that followed the docking of its basement, the Dacia Mega Unit, onto the East European margin. Between ca. 11 and 0.3 Ma, detachment of subducted slab, magma generation, and volcanic activity in CGH propagated south-southeastward, sub-parallel with the collisional Eastern Carpathian Orogen, resulting in the construction of a series of mostly composite volcanoes dominated by calc-alkaline lavas and pyroclastic products. The generally well-preserved axial row of the CGH volcanic edifices is accompanied by voluminous volcaniclastic deposits of which sources and emplacement history persist as a matter of debate. In the standard view reflected in regional scale geologic maps, these deposits represent products of complete erosion of hypothetical earlier volcanoes. Other interpretation, however, suggest that they are at least partially constituted by voluminous volcanic debris avalanche deposits (VDADs).By combining evidence from field observation and GIS analysis with published and new data from petrography, bulk-rock geochemistry, and K/Ar geochronology, here we demonstrate that VDADs vastly dominate these volcanoclastic deposits. In addition to two previously identified VDADs (Western Călimani and Vârghiş), we show that further six VDADs (Eastern Călimani, Fâncel-Lăpuşna, Ostoroş, Ivo-Cocoizaş, Luci, and Pilişca) were generated due to the sector failure of a series of CGH volcanoes, most of which still preserve well-distinguishable collapse scars. Based on our estimates, debris avalanches with volumes between <1 km3 and > 100 km3 displaced 0.4–39% of the source edifices, transferring nearly a third of their cumulated volcanic output to the observed volcaniclastic deposits.K/Ar age constraints suggest VDAD formation closely followed in space and time the migrating volcanic activity between ca. 7.8 and 1.5 Ma. Relying on this age pattern, collapse scar orientations, and VDAD runout directions, we propose that regional scale deformation of the Dacia lithosphere imposed by progressive detachment of the underlying subducted slab, manifested on the surface as a transient subsidence–uplift couple traveling along CGH as well as opening of intramountain basins at the eastern flank of the range, was responsible for basement tilting and faulting that destabilised the CGH volcanic edifices. Seismic activity associated with slab detachment and shallow crustal fault system as well as explosive volcanic eruptions may have triggered the collapse of these edifices, leading to the generation of the VDADs.

Distributed stress fluidisation: Insights into the propagation mechanisms of the Abona volcanic debris avalanche (Tenerife) through a novel method for indurated deposit sedimentological analysis

Introduction: Volcanic debris avalanches mobilise large volumes and achieve long runouts with high destructive potential. However, the propagation processes that generate them are not currently explained by theoretical or numerical models, which are unable to represent deposit observations. Evaluation of the dynamics represented in deposits is therefore vital for constraining su ch models. The Abona volcanic debris avalanche deposit is located on the southern flank of the island of Tenerife, Spain. The deposit exhibits universal microfracturing and cataclasis. Fluidal features such as fluidal mixing of lithological units and diffuse boundaries, and mixed matrix are observed throughout the deposit.Methods: Field description including sedimentology and facies identification and the evaluation of their distribution have allowed the generation of a new conceptual model for the propagation dynamics of this volcanic debris avalanche, and potentially others with similar properties. The deposit is indurated making the detailed study of its sedimentology difficult, especially clast-size analysis. A novel method utilising structure from motion photogrammetry and photographic sampling was employed.Results: The universal cataclasis of the material and fluidal features suggest that the lack of a major competent material component allowed the mass to fragment and enabled fluidised granular flow behaviour. It is proposed that shear was periodically distributed throughout the body of the avalanche in chaotic temporary shear networks rearranging according to the instantaneous distribution of the mass. Stress and agitation were not temporally or spatially homogenous during propagation. This is also reflected in the unsystematic erosion of the substrate according to the variable basal shear accommodation.Discussion: It is proposed that lithological properties are potentially a determining factor for the propagation mechanisms, stress distribution, and consequently the evolution of a volcanic debris avalanche from the initial collapse to its emplacement. This study highlights the importance of dedicated field examinations of sedimentological, morphological, and structural features for providing constraints for models of volcanic debris avalanche dynamics and the factors dictating them. The novel methodology proposed has the potential of broadening the number of events that can be studied and enhancing the understanding of these complex and hazardous phenomena.

The propagation and emplacement mechanisms of the Tenteniguada volcanic debris avalanche (Gran Canaria): Field evidence for brittle fault-accommodated spreading

The Tenteniguada volcanic debris avalanche deposit is located on the east of the island of Gran Canaria, Spain. Its internal structure is composed of a complex assemblage of extensional features and shearing structures including normal faults, horst and graben, brittle/ductile boudinage and clastic dike injections. Examination of these features in the field and evaluation of their distribution have allowed the generation of a new conceptual model for the transport and emplacement of this debris avalanche, and potentially others. In the majority of the deposit, the degree of disaggregation is low, with large portions of the original edifice preserved, although displaced by brittle deformation. Greater disaggregation is observed deeper and in the more distal section of the deposit. The findings suggest that the propagation of the volcanic debris avalanche was most likely facilitated by the normal fault-accommodated spreading and extension of the mass, with the majority of stress focused in fault zones. The greater disaggregation exhibited in the deeper and the more distal part of the deposit is likely to be due to greater stress accommodation from fault convergence and momentum transfer respectively. The abundance of competent lava lithologies and scarcity of weaker material that could be easily disaggregated is the most likely reason Tenteniguada did not fully evolve from a slide to a granular flow, and therefore generated a deposit which bears resemblance to non-volcanic blockslide deposits. Therefore, lithological properties are potentially a vital factor for the propagation mechanisms, distribution of stress and consequently the evolution of a debris avalanche from the initial collapse to its emplacement. The present study highlights the importance of dedicated field examinations of sedimentological, morphological, and structural features for providing constraints for models of debris avalanche propagation mechanisms and the factors dictating them.

Volcanic debris avalanche transport and emplacement at Chimpa volcano (Central Puna, Argentina): Insights from morphology, grain-size and clast surficial textures

Understanding the flow dynamics in debris avalanches is an important tool to advance the knowledge about lateral failures of volcanoes, a fundamental step towards an accurate risk assessment and mitigation in volcanic areas. We describe the morphological, grain-size, and clast surficial textures of the Casana volcanic debris avalanche deposit emplaced by the sector collapse of Chimpa volcano (Central Puna, Argentina). We focused our analysis on the volcanic debris avalanche deposit, characterized by ridges, reduction in downflow matrix grainsize, jigsaw-cracked blocks in the whole extent of the deposit, collision superficial textures in grains (fractures, percussion marks, and voids). Sedimentological and textural analysis show a progressive disintegration and fracturing of the larger particles with greater distance in a dry, granular flow with minimal internal deformation during propagation. Casana avalanche behaved like a rigid body in the proximal area and as a granular flow in the medial and distal reach. Block sliding mechanisms generated the toreva block in the proximal region whereas granular flow produced the debris avalanche deposits. This is an example of how different mechanisms can interact during debris avalanche emplacement, which are strictly related to the cause of the volcano instability, the lithology and the degree of alteration of the source mass, and its interaction with paleotopography. Although the geological risk of the proposed study area is low, the study of Casana VDAD is important to understand similar processes in other volcanoes providing constraints to hazard assessment.

Mid-Holocene lateral collapse of Antuco volcano (Chile): debris avalanche deposit features, emplacement dynamics, and impacts

Antuco (37.4°S, 71.4°W; Chile) is a dominantly basaltic stratovolcano whose original ~ 3300 m altitude main cone experienced a catastrophic sector collapse at ~ 7.1 cal ka BP, producing a volcanic debris avalanche deposit (VDAD) with hummocky surface and ~ 6.4 km3 of volume. We carried out geological studies of its debris avalanche deposit, which was distributed to the W and displays a longitudinal facies transformation from edifice’s megablocks and block to mixed facies in distal areas (up to 25 km from the scar). Our observations support the behavior of the avalanche beginning as a translational slide, and then as plug flow when confined within the Laja River valley. Clay abundance and high content of hydrothermally altered material may suggest active participation of water; flow velocities are estimated to ~100 m s−1. We primarily identify the steep-sided flanks of the cone, and hydrothermal alteration promoted the edifice instability, while basement seismogenic structures may have ultimately triggered the landslide. Subsequent landslide-led events include the transformation of the volcanic activity with explosive eruptions producing a sequence of dilute pyroclastic density currents (PDCs) ending ~3.4 ky BP, and extensive lava effusion rapidly reconstructing the collapsed edifice. Moreover, the Antuco VDAD also blocked the natural output of the Laja Lake, increasing its level by ~200 m and then triggering cataclysmic outburst floods by dam rupture, preserved as high-energy alluvial beds with ages between 2.8 and 1.7 ky BP. The Antuco constitutes an excellent example of a critical chain of events initiated by a stratovolcano lateral collapse and warns for detailed hazard investigations to better comprehend its related impacts.

Construction and Destruction of Bontău Composite Volcano in the Extensional Setting of Zărand Basin during Miocene (Apuseni Mts., Romania)

The Eastern part of the Miocene Zărand extensional basin witnessed the generation and evolution of the largest composite volcano in Apuseni Mts., named recently Bontău. The volcano is filling the basin at the junction between the South and North Apuseni Mountains. The Bontău Volcano is known to be active roughly between ~14–10. In spite of heavily forested and poorly exposed volcanic deposits, it was possible to identify its complex evolution. The volcano suggests an original oval-shaped edifice base currently showing a north-oriented horseshoe-shaped debris avalanche eroded crater. The early effusive volcanic activity was contemporaneous with the emplacement of individual and/or clustered volcanic lava Domes. Late-stage summit dome generation was followed by several volcanic collapses all around the volcanic edifice producing large volcanic debris avalanche deposits (DADs), accompanied by numerous debris flows all around the volcano periphery. Thick pumice pyroclastic flow deposits found below DADs at the periphery may suggest that the slope failures were proceeded by a Plinian eruption. The debris avalanche crater is the last event in the volcano evolution exposing several intrusive andesitic-dioritic bodies and associated hydrothermal and mineralization processes, most probably including the former central vent area of the volcano. The volcano proximal effusive and explosive deposits display a change in the composition of the erupting magma (increased SiO2 from 53.4% to 60.6%) that resulted in an increase of viscosity and the construction of the summit lava domes. Such domes are however only found as various size blocks in DADs. The volcanism connects with the two steps of geotectonic evolution of the Zărand Basin: The initial construction period during regional extension started ~16 Ma up to 12.3–12.1 when the Bontău volcano and surrounding domes were generated. The second period, younger than 12 Ma, corresponds to NW-SE compressional tectonics developed only in the Bontău volcano with summit dome generation and, finally, assists volcano destruction and DADs generation. Newly performed geochemical and Sr and Nd isotopic data studies attest to a calc-alkaline character and suggest an evolution via assimilation-fractional crystallization processes of a dominant MORB-like mantle source magma. Also, they confirm the amphibole (±pyroxene) andesites to be the most evolved lithology. The stepwise changes in fracture propagation in the Zărand extensional setting along with a change to more hydrated and fractionated magma favored in ~4 Myrs of the evolution of the Bontău volcano lead to multiple pulses of the longest-lived magma chamber in the whole Miocene volcanism of the Apuseni Mts.

Grain size distribution and sedimentology in volcanic mass-wasting flows: implications for propagation and mobility

The sedimentological characteristics of mass-wasting flow deposits are important for assessing the differences between phenomena and their propagation and emplacement mechanisms. In the present study, nine volcanic debris avalanche deposits and eight lahar deposits are considered, from the literature. Their sedimentology is expressed in the descriptive statistics: median grain size, sand, gravel and finer particle fractions, skewness and sorting. Analysis of the data confirms that lahars and debris avalanches diverge in their grain size distribution and in their evolution during propagation. Water saturation in lahars is the main factor enabling debulking, a mechanism that is not recorded in the data derived from debris avalanches deposits. On the contrary, evidence of comminution of particles due to particle-particle interactions is observed in debris avalanches, and not in lahars. These findings support previous studies suggesting that although water content in debris avalanches plays a role in propagation, the effects of inertial collision of solid fragments are more important than fluid effects, confirming that particle-particle interactions are the main factor influencing the mobility of non-saturated mass wasting flows.

Subaerial and Subaqueous Investigations of Volcanic Debris Avalanche and Lahar Deposits on the Northern Coast of Ulleung Island, Korea

Chun, J.-H. and Lee, H.-J., 2019. Subaerial and subaqueous investigations of volcanic debris avalanche and lahar deposits on the northern coast of Ulleung Island, Korea. In: Jung, H.-S.; Lee, S.; Ryu, J.-H., and Cui, T. (eds.), Advances in Remote Sensing and Geoscience Information Systems of Coastal Environments. Journal of Coastal Research, Special Issue No. 90, pp. 369-376. Coconut Creek (Florida), ISSN 0749-0208.Volcanic debris avalanche and lahar deposits associated with small-scale lava dome collapse have been poorly documented in volcanic islands. The Chusan Formation was emplaced on the collapsed northern flank of Ulleung Island, Korea. Subaerial and subaqueous investigations of the collapsed northern part of Ulleung Island have been performed based on shaded relief images generated from a digital elevation model (DEM), a GeoEye satellite image, outcrop observations, and multi-beam echosounder and high-resolution Chirp sub-bottom profiling systems. The Chusan Formation is an elongated (length, 800 m; width, 250 m) system of valley-confined deposits connected to the Albong lava dome within the Nari caldera depression. A 40-m-thick outcrop of the Chusan Formation consists of three aggradational units that were emplaced by lahars, volcanic debris avalanches, and mixed deposition due to these processes. The peat layer between the overlying Chusan Formation and underlying reworked sediments was dated at 3,070–3,275 cal B.P., matching the emplacement of the Chusan Formation. Equivalent subaqueous deposits of the Chusan Formation were not detected in the northern shelf; thus, large-scale caldera collapse deposits covered the marine terrace before emplacement of the Chusan Formation. The valley-confined Chusan Formation is the result of an aggradational succession of lahar and volcanic debris avalanche deposits associated with the Albong lava dome collapse, corresponding to the most recent volcanic activity of Ulleung Island.

Fault textures in volcanic debris-avalanche deposits and transformations into lahars: The Pichu Pichu thrust lobes in south Peru compared to worldwide avalanche deposits

Structures, lithofacies and semi-quantitative sedimentological analysis of volcanic debris-avalanche deposits are essential to constrain syn-emplacement processes along avalanche fault zones in the setting of mass-flow runout distances. At a distance of 28km from the extinct Pichu Pichu compound volcano, we show how internal structures of thrust lobes, faulted as to form horst- and graben-like structures, have recorded the brecciation with granular segregations. Cataclastic gradients have been recognized in more than eleven breccia structures with statistical grain-size parameters, among them fractal D-values between 1.96 and 2.53 related to an extensional fracturing in DADs with fractal D-values between 2 and 2.5 leading to granular disaggregation with fractal D-values between 2.13 and 2.22. Primary fractures related to transport processes and the secondary cataclasis produced during emplacement are quantified as having <50% matrix, while the avalanche transformations into lahar deposits occur with >50% matrix. The hybrid lithofacies in graben-like structures may be the precursory stages of the matrix transformation into lahar deposits.Volcanic debris-avalanche structures of eleven worldwide volcanoes that belong to the Central Andes and the Cascades volcanic arc have been compared using sedimentary and statistical methods (156 sieved samples and Shape Preferred Orientation measured on 30,342 clasts). Median diameter, sorting index, skewness and kurtosis values help differentiate four deposits and processes: (1) the proximal gravitational collapse deposits; (2) the avalanche thrust lobes, located in inverted extensional fault zones, and (3) the hybrid and mixed deposits with polymodal distributions; and (4) the bimodal, transformed matrix with clast sorting. The fractal D-values of avalanche matrix range between 2.3 and 2.7, suggesting that extensional fractures have led to granular disaggregation.From the available statistical dataset, a few equations have been developed that help pointing to a syn-emplacement cataclastic gradient along avalanche fault zone. Three textural classes of avalanche fault zones have been identified based on the ellipse/a/b values of ~2.14 for the planar collapse in fault zone, 1.75 to 2 for the crushing of volcanic avalanche breccias, and <1.7 due to the thermal effect of clast fragmentation. Inherited clast shapes with a/b=2.5 and ellipse=4 suggests a co-evolution of avalanche lithofacies. A sedimentological classification of avalanche fault zones proposed here has been related to the syn-emplacement processes of DAD structural units. As a result, the semi-quantitative texture dataset helps constrain the different stages of avalanche runout and the transformation steps into lahar deposits.

Volcanic avalanche fault zone with pseudotachylite and gouge in French Massif Central

Structures and textures with sedimentological variations at different scales of the lithofacies assemblage help us to constrain the basal kinematic transition from non-depositional to depositional conditions during volcanic avalanche emplacement. In the well-exposed impact-sheared contact along volcanic avalanche fault zone in the French Massif Central, we observe how the granular textures of the pseudotachylite and fault gouge have recorded the propagation of shock wave with granular oscillatory stress. Sequential events of basal aggradation along avalanche fault zone have been established related to fractal D-values, temperature pressure regime and oscillatory stress during slow wave velocity.A typical lithofacies assemblage with a reverse grading shows the pseudotachylite and fault gouge. A cataclastic gradient is characterised by the fractal D-values from 2.7 in jigsaw breccias with pseudotachylite partial melt, to 2.6 in the polymodal gouge. Shock, brecciation and comminution produce cataclastic shear bands in the pseudotachylite and quartz microstructures along the basal contact of the volcanic debris-avalanche deposit. Gouge microstructures show granular segregation, cataclasis with antithetic rotational Riedel shear, and an arching effect between the Riedel shear bands. X-ray microtomography provided 3D microfabrics along the clastic vein in the sandy-gouge. From the available statistical dataset, a few equations have been developed implicating the same cataclastic origin with a co-genetic evolution of lithofacies.An impact wave during primary shear propagation may contribute to produce hydroclastic matrix, pseudotachylite partial melt and proximal gouge thixotropy with v~50m/s and a T<654°C. The interseismic period with oscillatory stress is related to crushed clasts and basaltic melt around ~800°C, Riedel shear bands with granular segregation along the fault gouge. The secondary shock by matrix-rich avalanche (ΔP=10GPa, T≥1000–1500°C) contributes to quartz microstructures along the avalanche basal contact and quartz spheroids in microscopic cataclastic shear bands. Decompression around 654–800°C is related to tertiary sub-vertical oscillations with a backward moving shock and antithetic rotational fault megablock. Semi-quantitative analyses of seismogenic fault basement contribute to establish the localised conditions related to sequential aggradation along volcanic avalanche fault zone.

Climate influence on volcano edifice stability and fluvial landscape evolution surrounding Mount Ruapehu, New Zealand

Large volcanic debris avalanches are triggered by failure of the steep flanks of long-lived composite cones. Their huge deposits change the landscape and drainage pattern surrounding stratovolcanoes for thousands of years. At Mt. Ruapehu, New Zealand, we identified seven major flank-collapse events that produced debris avalanches travelling down pre-existing river catchments for up to 90km from source. In two cases the extreme mass flux into the river valleys led to their complete truncation from the volcano, while four drainage systems were subsequently re-established along similar pathways influenced by regional strike-slip faulting, which caused localized graben formation. In all cases the volcanic debris-avalanche deposits currently form distinctive plateaus at or near the highest topographic elevations of each river valley margin. The timing of the flank failures indicate that inter-eruptive cone destabilization of Mt. Ruapehu is affected by climate change and occurs most commonly during interstadials when glaciers on the cone are in retreat, whereas syn-eruptive collapses are most prominent during cold stages. Dated debris-avalanche deposit levels, along with those of up to four stadial-related aggradational gravel terraces between c. 125 and 18ka, were used to calculate regional uplift rates in this area. Rates of between 0.2±0.1mmyr−1 to 3.8±0.8mmyr−1 are found for four river systems dissecting the central North Island of New Zealand. In three cases incision below the diamicton sequences and into the basement, allowed quantification of sediment-flux rates into the Tasman Sea of 107,000±1,200m3yr−1 to 177,000±3,500m3yr−1 since debris-avalanche emplacement.

The extreme mobility of debris avalanches: A new model of transport mechanism

AbstractLarge rockslide‐debris avalanches, resulting from flank collapses that shape volcanoes and mountains on Earth and other object of the solar system, are rapid and dangerous gravity‐driven granular flows that travel abnormal distances. During the last 50 years, numerous physical models have been put forward to explain their extreme mobility. The principal models are based on fluidization, lubrication, or dynamic disintegration. However, these processes remain poorly constrained. To identify precisely the transport mechanisms during debris avalanches, we examined morphometric (fractal dimension and circularity), grain size, and exoscopic characteristics of the various types of particles (clasts and matrix) from volcanic debris avalanche deposits of La Réunion Island (Indian Ocean). From these data we demonstrate for the first time that syn‐transport dynamic disintegration continuously operates with the increasing runout distance from the source down to a grinding limit of 500 µm. Below this limit, the particle size reduction exclusively results from their attrition by frictional interactions. Consequently, the exceptional mobility of debris avalanches may be explained by the combined effect of elastic energy release during the dynamic disintegration of the larger clasts and frictional reduction within the matrix due to interactions between the finer particles.

Early Holocene collapse of Volcán Parinacota, central Andes, Chile: Volcanological and paleohydrological consequences

The catastrophic gravitational collapse of the Old Cone of Volcan Parinacota produced a 6 km 3 debris avalanche that traveled ∼22 km and covered more than 150 km 2 . The Upper Lauca drainage, a broad high-altitude basin in the Chilean Altiplano, was permanently altered by the collapse. Although the eruptive history of Parinacota before and after the debris avalanche is well known from petrologic and geochronologic studies, previous age limits on the debris avalanche (based on multiple chronometers) span ca. 8–20 ka. New cosmogenic surface-exposure ages from boulders atop the deposit are based on a regionally calibrated production rate of in situ 10 Be and indicate that the avalanche occurred at 8.8 ± 0.5 ka. These data demonstrate that cosmogenic 10 Be surface-exposure dating can be successfully applied to quartz-bearing, volcanic debris avalanche deposits, and that this method offers a distinct advantage over 14 C chronologies that provide only minimum or maximum age limits. The 8.8 ka exposure age for the debris avalanche (1) agrees with 14 C age limits of paleosol material incorporated in the debris avalanche, (2) requires a voluminous initial phase of postcollapse volcanism with an eruptive rate exceeding that of recent cone-building episodes at most continental arc volcanoes, and (3) suggests that volcano collapse did not result in the formation of Lago Chungara, but instead led to a major expansion of a preexisting closed basin.

Tephrochronology and distribution characteristics using LiDAR DEMs of the Nasu volcanic debris avalanche deposits

Tephrochronology and distribution characteristics using LiDAR DEMs of the Nasu volcanic debris avalanche deposits

Volcanic facies of the debris avalanche deposit of Tata Sabaya Volcano, Central Andes.

Debris avalanches associated with partial sector collapse of volcanic edifices are common phenomena in the evolution of a volcano. These flows are formed by edifice instability, which could be due to several factors such as the presence of hydrothermally altered areas, climatic changes, earthquakes, shallow magma intrusions (cryptodomes and/or dykes) and/or fault activity beneath or close to the volcano. The final product of this avalanche flow, known as debris avalanche deposit (DAD) shows typical hummocky and closed drainage morphologies. More than 14 volcanic centers of the Central Andes have volcanic DAD&rsquo;s, being Tata Sabaya (Bolivia) one of them. The collapse that has originated the deposit could have been generated by a combination of magmatic and seismic activity in the volcano. The deposit associated to the partial collapse of Tata Sabaya volcano is distributed on its lower southern flank and partially fills the north-western part of the Salar de Coipasa basin. The deposit covers a minimum area of 230 km2 and has an estimated minimum volume of 6&plusmn;1 km3. The deposit is formed by 6 different types of hummocks, according to their compositions: lavic, pyroclastic, sedimentary, mixed, pyroclastic breccia and basaltic-andesitic hummocks. Based on the predominant hummock type and the spatial distribution, the deposit has been divided into 6 different facies (Toreva Block Facies, Volcanic Hummocks Facies, Central Facies, Sedimentary Hummocks Facies, Mixed Facies and Pyroclastic Breccia Hummocks Facies). Taking into account the facies distribution and their characteristics, we infer the pre-collapse structure of the volcano.

Runout of the Socompa volcanic debris avalanche, Chile: a mechanical explanation for low basal shear resistance

We propose a mechanical explanation for the low basal shear resistance (about 50 kPa) previously used to simulate successfully the complex, well-documented deposit morphology and lithological distribution produced by emplacement of the 25 km3 Socompa volcanic debris avalanche deposit, Chile. Stratigraphic evidence for intense basal comminution indicates the occurrence of dynamic rock fragmentation in the basal region of this large granular mass flow, and we show that such fragmentation generates a basal shear stress, retarding motion of the avalanche, that is a function of the flow thickness and intact rock strength. The topography of the Socompa deposit is realistically simulated using this fragmentation-derived resistance function. Basal fragmentation is also compatible with the evidence from the deposit that reflection of the avalanche from topography caused a secondary wave that interacted with the primary flow.

Development of characteristic volcanic debris avalanche deposit structures: New insight from distinct element simulations

Large-scale catastrophic collapse of volcanic edifices is a relatively common phenomenon in the geologic record. However, the processes that occur during debris avalanche emplacement remain poorly understood and must generally be inferred from analysis of avalanche deposits in the field, which are recognized to contain a suite of recurrent features, such as conical surface hummocks and toreva block ridges. This study uses the distinct element numerical modelling method to investigate debris avalanche emplacement processes, because it allows brittle deformation of the failure mass to be examined. After describing the material calibration and model setup necessary for modelling the geomechanical behaviour of a failed edifice flank, a sequence of emplacement snapshots is described. The simulated debris avalanche is seen to evolve from initial block sliding, through extension with horst and graben structures defined by propagation and offset of large-scale discontinuities, to a final stage in which the avalanche flows down-valley. The numerical model is consistent with emplacement theory in the literature and allows for a more precise view of the processes at work during failure and the deposits created thereby. For example, toreva blocks are formed in the medial and proximal sections of the failure through top-down propagation of normally offset discontinuities developed from upper surface tension. A mechanism for surface hummock formation is also recognized involving the retention of surface blocks. As these and other characteristic deposit features are formed from a flank with homogeneous material properties, brittle deformation of the initially listric-shaped failure mass and associated macroscopic structural evolution are recognized as important factors in failure evolution and subsequent deposit morphology.

Distinguishing volcanic debris avalanche deposits from their reworked products: the Perrier sequence (French Massif Central)

Debris avalanches associated with volcanic sector collapse are usually high-volume high-mobility phenomena. Debris avalanche deposit remobilisation by cohesive debris flows and landslides is common, so they can share textural characteristics such as hummocks and jigsaw cracks. Distinguishing original deposits from reworked products is critical for geological understanding and hazard assessment because of their different origin, frequency and environmental impact. We present a methodology based on field evidence to differentiate such epiclastic breccias. Basal contact mapping constrained by accurate altitude and location data allows the reconstruction of deposit stratigraphy and geometry. Lithological analysis helps to distinguish the different units. Incorporation structures, kinematic indicators and component mingling textures are used to characterise erosion and transport mechanisms. We apply this method to the enigmatic sequence at Perrier (French Massif Central), where four units (U1–U4) have been interpreted either as debris flow or debris avalanche deposits. The sequence results from activity on the Monts Dore Volcano about 2 Ma ago. The epiclastic units are matrix supported with an almost flat top. U2 and U3 have clear debris flow deposit affinities such as rounded clasts and intact blocks (no jigsaw cracks). U1 and U4 have jigsaw cracked blocks with matrix injection and stretched sediment blocks. U1 lacks large blocks (>10 m wide) and has a homogenous matrix with an upward increase of trapped air vesicle content and size. This unit is interpreted as a cohesive debris flow deposit spawned from a debris avalanche upstream. In contrast, U4 has large mega-blocks (up to 40 m wide), sharp contacts between mixed facies zones with different colours and numerous jigsaw fit blocks (open jigsaw cracks filled by monogenic intra-clast matrix). Mega-blocks are concentrated near the deposit base and are spatially associated with major substratum erosion. This deposit has a debris avalanche distal facies with local debris flow affinities due to partial water saturation. We also identify two landslide deposits (L1 and L2) resulting from recent reworking that has produced a similar facies to U1 and U4. These are distinguishable from the original deposits, as they contain blocks of mixed U1/U4 facies, a distinctly less consolidated and more porous matrix and a fresh hummocky topography. This work shows how to differentiate epiclastic deposits with similar characteristics, but different origins. In doing so, we improve understanding of present and past instability of the Monts Dore and identify present landslide hazards at Perrier.

Landscape and sedimentary response to catastrophic debris avalanches, western Taranaki, New Zealand

Catastrophic volcanic debris avalanches reshape volcanic edifices with up to half of pre-collapse cone volumes being removed. Deposition from this debris avalanche deposit often fills and inundates the surrounding landscape and may permanently change the distribution of drainage networks. On the weakly-incised Mt. Taranaki ring-plain, volcanic debris avalanche deposits typically form a large, wedge shape (in plan view), over all flat-lying fans. Following volcanic debris avalanches a period of intense re-sedimentation commonly begins on ring-plain areas, particularly in wet or temperate climates. This is exacerbated by large areas of denuded landscape, ongoing instability in the scarp/source region, damming of river/stream systems, and in some cases inherent instability of the volcanic debris avalanche deposits. In addition, on Mt. Taranaki, the collapse of a segment of the cone by volcanic debris avalanche often generates long periods of renewed volcanism, generating large volumes of juvenile tephra onto unstable and unvegetated slopes, or construction of new domes with associated rock falls and block-and-ash flows. The distal ring-plain impact from these post-debris avalanche conditions and processes is primarily accumulation of long run-out debris flow and hyperconcentrated flow deposits with a variety of lithologies and sedimentary character. Common to these post-debris avalanche units is evidence for high-water-content flows that are typically non-cohesive. Hence sedimentary variations in these units are high in lateral and longitudinal exposure in relation to local topography. The post-collapse deposits flank large-scale fans and hence similar lithological and chronological sequences can form on widely disparate sectors of the ring plain. These deposits on Mt. Taranaki provide a record of landscape response and ring-plain evolution in three stages that divide the currently identified Warea Formation: 1) the deposition of broad fans of material adjacent to the debris avalanche unit; 2) channel formation and erosion of Stage 1 deposits, primarily at the contact between debris avalanche deposits and the Stage 1 deposits and the refilling of these channels; and 3) the development of broad tabular sheet flows on top of the debris avalanche, leaving sediments between debris avalanche mounds. After a volcanic debris avalanche, these processes represent an ever changing and evolving hazard-scape with hazard maps needing to be regularly updated to take account of which stage the sedimentary system is in.

Characterisation of a Pleistocene debris-avalanche deposit in the Tenteniguada Basin, Gran Canaria Island, Spain

We studied a large debris-avalanche deposit of Pleistocene age in the Tenteniguada Basin, Gran Canaria Island, Spain. This deposit, which is well preserved because it is mostly covered by basanite lava flows, has distinctive matrix and block facies, hummocky topography and internal structures typical of debris avalanches. However, neither syneruptive lavas nor some characteristic features of volcanic debris-avalanche deposits, such as a stratovolcano edifice or a horseshoe-shaped crater, are present. The occurrence of internal features characteristic of volcanic avalanche deposits could be attributed to the volcanic materials involved in the movement rather than to the triggering of the avalanche during a volcanic eruption. The conditioning factors are shown to be associated with specific structural and hydrological conditions, such as the presence of old volcanic domes, strength reduction of the rocks, effective stress decrease, active gully erosion and water table rise during Pleistocene humid episodes. We finally suggest that the possible triggering factor of the avalanche was a neighbouring volcanic or tectonic earthquake.