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.     

Large debris avalanches and associated eruptions in the Holocene eruptive history of Shiveluch Volcano, Kamchatka, Russia

 Shiveluch Volcano, located in the Central Kamchatka Depression, has experienced multiple flank failures during its lifetime, most recently in 1964. The overlapping deposits of at least 13 large Holocene debris avalanches cover an area of approximately 200 km² of the southern sector of the volcano. Deposits of two debris avalanches associated with flank extrusive domes are, in addition, located on its western slope. The maximum travel distance of individual Holocene avalanches exceeds 20 km, and their volumes reach ∼3 km³. The deposits of most avalanches typically have a hummocky surface, are poorly sorted and graded, and contain angular heterogeneous rock fragments of various sizes surrounded by coarse to fine matrix. The deposits differ in color, indicating different sources on the edifice. Tephrochronological and radiocarbon dating of the avalanches shows that the first large Holocene avalanches were emplaced approximately 4530–4350 BC. From ∼2490 BC at least 13 avalanches occurred after intervals of 30–900 years. Six large avalanches were emplaced between 120 and 970 AD, with recurrence intervals of 30–340 years. All the debris avalanches were followed by eruptions that produced various types of pyroclastic deposits. Features of some surge deposits suggest that they might have originated as a result of directed blasts triggered by rockslides. Most avalanche deposits are composed of fresh andesitic rocks of extrusive domes, so the avalanches might have resulted from the high magma supply rate and the repetitive formation of the domes. No trace of the 1854 summit failure mentioned in historical records has been found beyond 8 km from the crater; perhaps witnesses exaggerated or misinterpreted the events. Received: 18 August 1997 / Accepted: 19 December 1997     

Debris-avalanche deposits of the Milo Lahar sequence and the opening of the Valle del Bove on Etna volcano (Italy)

Previously undescribed debris-avalanche deposits occur in two locations downslope from the open end of the Valle del Bove. These outcrops comprise unstratified, ungraded deposits of metre-scale lava blocks in a matrix of weathered and fractured lava clasts. The avalanche deposits are unconformably overlain by matrix- to clast-supported conglomerates, representing debris-flow and interbedded fluvial deposits, that constitute most of the Milo Lahar sequence. We present evidence that the Milo Lahar sequence, which crops out just at the exit of the Valle del Bove, formed during the opening and enlargement of this depression. The presence of the avalanche deposits at the base of the Milo Lahar sequence indicates that catastrophic landslides were involved in the formation of the Valle del Bove. The composition of lavas in the debris avalanche deposits is similar to that of most of the Ellittico volcanic sequence exposed along the northern wall of the Valle del Bove. Radiocarbon dates of 8400 and 5300 years BP from the base and top, respectively, of the debris-flow sequence indicate that the Milo Lahars are correlative with the exposed part of the Chiancone deposit. The basal lahars of the Chiancone, which contain lava blocks whose compositions partially overlap that of blocks in the avalanche deposits, may have formed by water concentration in the distal end of the avalanche causing transformation to debris, or alternatively by reworking of the avalanche deposit.     

A Miocene coarse volcaniclastic mass-flow deposit in the Shimane Peninsula,SW Japan: product of a deep submarine eruption?

 A subaqueous volcaniclastic mass-flow deposit in the Miocene Josoji Formation, Shimane Peninsula, is 15–16 m thick, and comprises mainly blocks and lapilli of rhyolite and andesite pumices and non- to poorly vesiculated rhyolite. It can be divided into four layers in ascending order. Layer 1 is an inversely to normally graded and poorly sorted lithic breccia 0.3–6 m thick. Layer 2 is an inversely to normally graded tuff breccia to lapilli tuff 6–11 m thick. This layer bifurcates laterally into minor depositional units individually composed of a massive, lithic-rich lower part and a diffusely stratified, pumice-rich upper part with inverse to normal grading of both lithic and pumice clasts. Layer 3 is 2.5–3 m thick, and consists of interbedded fines-depleted pumice-rich and pumice-poor layers a few centimeters thick. Layer 4 is a well-stratified and well-sorted coarse ash bed 1.5–2 m thick. The volcaniclastic deposit shows internal features of high-density turbidites and contains no evidence for emplacement at a high temperature. The mass-flow deposit is extremely coarse-grained, dominated by traction structures, and is interpreted as the product of a deep submarine, explosive eruption of vesicular magma or explosive collapse of lava. Received: 10 January 1996 / Accepted: 23 February 1996     

Transport and emplacement mechanisms of channelised long-runout debris avalanches,Ruapehu volcano,New Zealand

The steep flanks of composite volcanoes are prone to collapse, producing debris avalanches that completely reshape the landscape. This study describes new insights into the runout of large debris avalanches enhanced by topography, using the example of six debris avalanche deposits from Mount Ruapehu, New Zealand. Individual large flank collapses (>1 km³) produced all of these units, with four not previously recognised. Five major valleys within the highly dissected landscape surrounding Mount Ruapehu channelled the debris avalanches into deep gorges (≥15 m) and resulted in extremely long debris avalanche runouts of up to 80 km from source. Classical sedimentary features of debris avalanche deposits preserved in these units include the following: very poor sorting with a clay-sand matrix hosting large subrounded boulders up to 5 m in diameter, jigsaw-fractured clasts, deformed clasts and numerous rip-up clasts of late-Pliocene marine sediments. The unusually long runouts led to unique features in distal deposits, including a pervasive and consolidated interclast matrix, and common rip-up clasts of Tertiary mudstone, as well as fluvial gravels and boulders. The great travel distances can be explained by the debris avalanches entering deep confined channels (≥15 m), where friction was minimised by a reduced basal contact area along with loading of water-saturated substrates which formed a basal lubrication zone for the overlying flowing mass. Extremely long-runout debris avalanches are most likely to occur in settings where initially partly saturated collapsing masses move down deep valleys and become thoroughly liquified at their base. This happens when pore water is available within the base of the flowing mass or in the sediments immediately below it. Based on their H/L ratio, confined volcanic debris avalanches are two to three times longer than unconfined, spreading flows of similar volume. The hybrid qualities of the deposits, which have some similarities to those of debris flows, are important to recognise when evaluating mass flow hazards at stratovolcanoes.     

Late Pleistocene flank collapse of Zempoala volcano (Central Mexico) and the role of fault reactivation

Zempoala is an extinct Pleistocene (∼ 0.7–0.8 Ma) stratovolcano that together with La Corona volcano (∼ 0.9 Ma) forms the southern end of the Sierra de las Cruces volcanic range, Central Mexico. The volcano consists of andesitic and dacitic lava flows and domes, as well as pyroclastic and epiclastic sequences, and has had a complex history with several flank collapses. One of these collapses occurred during the late Pleistocene on the S–SE flank of the volcano and produced the Zempoala debris avalanche deposit. This collapse could have been triggered by the reactivation of two normal fault systems (E–W and NE–SW), although magmatic activity cannot be absolutely excluded. The debris avalanche traveled 60 km to the south, covers an area of 600 km² and has a total volume of 6 km³, with a calculated Heim coefficient (H/L) of 0.03. Based on the textural characteristics of the deposit we recognized three zones: proximal, axial, and lateral distal zone. The proximal zone consists of debris avalanche blocks that develop a hummocky topography; the axial zone corresponds with the main debris avalanche deposit made of large clasts set in a sandy matrix, which transformed to a debris flow in the lateral distal portion. The deposit is heterolithologic in composition, with dacitic and andesitic fragments from the old edifice that decrease in volume as bulking of exotic clasts from the substratum increase. Several cities (Cuernavaca, Jojutla de Juárez, Alpuyeca) with associated industrial, agricultural, and tourism activities have been built on the deposit, which pose in evidence the possible impact in case of a new event with such characteristics, since the area is still tectonically active.     

Erosion by pyroclastic flows on Lascar Volcano, Chile

 Pyroclastic flows generated in the 19–20 April 1993 eruption of Lascar Volcano, Chile, produced spectacular erosion features. Scree and talus were stripped from the channels and steep slopes on the flanks of the volcano. Exposed bedrock and boulders suffered severe abrasion, producing smoothed surfaces on coarse breccias and striations and percussion marks on bedrock and large boulders. Erosional furrows developed with wavelengths of 0.5–2 m and depths of 0.1–0.3 m. Furrows commonly nucleated downstream of large boulders or blocks, which are striated on the upstream side, and thereby produced crag-and-tail structures. Erosive features were produced where flows accelerated through topographic restrictions or where they moved over steep slopes. The pyroclastic flows are inferred to have segregated during movement into lithic-rich and pumice-rich parts. Lithic-rich deposits occur on slopes up to 14°, whereas pumice-rich deposits occur only on slopes less than 4°, and mainly at the margins and distal parts of the 1993 fan. The lithic-rich deposits contain large (up to 1 m) lithic clasts eroded from the substrate and transported from the vent, whereas pumice-rich deposits contain only small (typically <2 cm) lithic clasts. These observations suggest that lithic clasts segregated to the base of the flows and were responsible for much of the erosive phenomena. The erosive features, distribution of lithic clasts and deposit morphology indicate that the 1993 flows were highly concentrated avalanches dominated by particle interactions. In some places the flows slid over the bedrock causing abrasion and long striations which imply that large blocks were locked in fixed positions for periods of about 1 s. However, shorter striae at different angles, impact marks, segregation of the deposits into pumice- and lithic-rich parts, and mixing of bedrock-derived lithic clasts throughout the deposits indicate that clasts often had some freedom of movement and that jostling of particles allowed internal mixing and density segregation to occur within the flows. Received: 15 July 1996 / Accepted: 15 January 1997     

Generation of pyroclastic flows and surges by hot-rock avalanches from the dome of Mount St. Helens volcano,USA

Several hot-rock avalanches have occurred during the growth of the composite dome of Mount St. Helens, Washington between 1980 and 1987. One of these occurred on 9 May 1986 and produced a fan-shaped avalanche deposit of juvenile dacite debris together with a more extensive pyroclastic-flow deposit. Laterally thinning deposits and abrasion and baking of wooden and plastic objects show that a hot ash-cloud surge swept beyond the limits of the pyroclastic flow. Plumes that rose 2–3 km above the dome and vitric ash that fell downwind of the volcano were also effects of this event, but no explosion occurred. All the facies observed originated from a single avalanche. Erosion and melting of craterfloor snow by the hot debris caused debris flows in the crater, and a small flood that carried juvenile and other clasts north of the crater. A second, broadly similar event occured in October 1986. Larger events of this nature could present a significant volcanic hazard.     

Internal structural variations in a debris-avalanche deposit from ancestral Mount Shasta,California, USA

Various parameters of the internal structure of a debris-avalanche deposit from ancestral Mount Shasta (size and percentage of block facies in each exposure, number and width of jigsaw cracks, and number of rounded clasts in matrix facies) were measured in order to study flow and emplacement mechanisms. Three types of coherent blocks were identified: blocks of massive or brecciated lava flows or domes, blocks of layered volcaniclastic deposits, and blocks of accidental material, typically from sedimentary units underlying Shasta Valley. The mean maximum dimension of the three largest blocks of layered volcaniclastic material is 220 m, and that of the lava blocks, 110 m. This difference may reflect plastic deformation of blocks of layered volcaniclastic material; blocks of massive or brecciated volcanic rock deformated brittly and may have split into several smaller blocks. The blocks in the deposit are one order of magnitude larger, and the height of collapse 1100 m higher, than the Pungarehu debris-avalanche deposit at Mount Egmont, New Zealand, although the degree of fracturing is about the same.This suggests either that the Shasta source material was less broken, or that the intensity of any accompanying explosion was smaller at ancestral Mount Shasta. The Shasta debris-avalanche deposit covered the floor of a closed basin; the flanks of the basin may have retarded the opening of jigsaw cracks and the formation of stretched and deformed blocks such as those of the Pungarehu debris-avalanche deposit.     

Morphology and emplacement of an unusual debris-avalanche deposit at Jocotitlán volcano,Central Mexico

A pre-historic collapse of the northeastern flank of Jocotitlán Volcano (3950 m), located in the central part of the Trans Mexican Volcanic Belt, produced a debris-avalanche deposit characterized by surficial hummocks of exceptional size and conical shape. The avalanche covered an area of 80 km², had an apparent coefficient of friction (H/L)_of 0.11, a maximum runout distance of 12 km, and an estimated volume of 2.8 km³. The most remarkable features of the Jocotitlán debris avalanche deposit are: the several steep (29–32°) conical proximal hummocks (up to 165 m high), large tansverse ridges (up to 205 m high and 2.7 km long) situated at the base of the volcano, and the steep 15–50 m thick terminal scarp. Proximal conical hummocks and parallel ridges that can be visually fitted back to their pre-collapse position on the mountain resulted from a sliding mode of emplacement. Steep primary slopes developed as a result of the accumulation of coarse angular clasts at the angle of repose around core clasts that are decameters in size. Distal hummocks are commonly smaller, less conical, and clustered with more diffuse outlines. Field evidence indicates that the leading distal edge of the avalanche spilled around certain topographic barriers and that the distal moving mass had a yield strength prior to stopping. In the NE sector, the avalanche was suddenly confined by topographically higher lacustrine and volcaniclastic deposits which as a result were intensely thrust-faulted, folded, and impacted by large clasts that separated from the avalanche front. Post-emplacement loading also induced normal faulting of these soft, locally water-rich sediments. The regional tectonic pattern, N-NE direction of flank failure, and the presence of a major normal fault which intersects the volcano and is parallel to the orientation of the Acambay graben located 10 km to the N suggest a genetic relationship between the extensional tectonic stress regime and triggering of catastrophic slope failure. The presence of a 3-m-thick sequence of pumice and obsidian-rich pyroclastic surge and fall tephra directly overlying the debris-avalanche deposit indicates that magma must have been present within the edifice just prior to the catastrophic flank failure. The breached crater left by the avalanche has mostly been filled by dacitic domes and lava flows. The youngest pryroclastic surge deposits on the upper flanks of the volcano have an historical C¹⁴ age of 680±80 yearsBp (Ad 1270±80). Thus Jocotitlán volcano, formerly believed to be extinct, should be considered potentially active. Because of its close proximity to Mexico-City (60 km), the most populous city in the world, reactivation could engender severe hazards.     

The enormous Chillos Valley Lahar: an ash-flow-generated debris flow from Cotopaxi Volcano, Ecuador

 The Chillos Valley Lahar (CVL), the largest Holocene debris flow in area and volume as yet recognized in the northern Andes, formed on Cotopaxi volcano's north and northeast slopes and descended river systems that took it 326 km north–northwest to the Pacific Ocean and 130+ km east into the Amazon basin. In the Chillos Valley, 40 km downstream from the volcano, depths of 80–160 m and valley cross sections up to 337 000 m² are observed, implying peak flow discharges of 2.6–6.0 million m³/s. The overall volume of the CVL is estimated to be ≈3.8 km³. The CVL was generated approximately 4500 years BP by a rhyolitic ash flow that followed a small sector collapse on the north and northeast sides of Cotopaxi, which melted part of the volcano's icecap and transformed rapidly into the debris flow. The ash flow and resulting CVL have identical components, except for foreign fragments picked up along the flow path. Juvenile materials, including vitric ash, crystals, and pumice, comprise 80–90% of the lahar's deposit, whereas rhyolitic, dacitic, and andesitic lithics make up the remainder. The sand-size fraction and the 2- to 10-mm fraction together dominate the deposit, constituting ≈63 and ≈15 wt.% of the matrix, respectively, whereas the silt-size fraction averages less than ≈10 wt.% and the clay-size fraction less than 0.5 wt.%. Along the 326-km runout, these particle-size fractions vary little, as does the sorting coefficient (average=2.6). There is no tendency toward grading or improved sorting. Limited bulking is recognized. The CVL was an enormous non-cohesive debris flow, notable for its ash-flow origin and immense volume and peak discharge which gave it characteristics and a behavior akin to large cohesive mudflows. Significantly, then, ash-flow-generated debris flows can also achieve large volumes and cover great areas; thus, they can conceivably affect large populated regions far from their source. Especially dangerous, therefore, are snow-clad volcanoes with recent silicic ash-flow histories such as those found in the Andes and Alaska. Received: 28 April 1997 / Accepted: 19 August 1997     

Late-Pleistocene flank collapse triggered by dome growth at Tacaná volcano, México-Guatemala, and its relationship to the regional stress regime

During late Pleistocene time, the extrusion of an andesitic dome at the summit of Tacaná volcano caused the collapse of its northwestern flank. The stratocone collapse was nearly parallel to the σ _min stress direction suggesting that failure was controlled by the regional stress field. The event produced a debris avalanche that was channelized in the San Rafael River and moved 8 km downstream. The deposit covered a minimum area of 4 km², had a volume of 0.8 ± 0.5 km³, with an H/L (vertical drop to horizontal transport distance ratio) of ~0.35, defining a degree of mobility that is atypical for volcanic debris avalanches. The flank failure undermined the summit dome leading to its collapse and the generation of a series of block-and-ash flows that were emplaced in quick succession and covered the avalanche surface. The collapse event left a 600-m-wide summit amphitheatre with a 30-degree opening to the northwest, and >200 m thick debris that blocked the San Rafael River. Remobilization of this material produced debris flows that eroded the primary deposits and cascaded into the Coatán River. After the collapse, the activity of Tacaná continued with the emission of the Agua Zarca lava flow dated at 10 ± 6 ka (⁴⁰Ar/³⁹Ar), and pyroclastic surges dated at 10,610 + 330/−315 yr BP (¹⁴C), which provide a minimum age for the collapse event. During the Holocene, Tacaná has been very active producing explosive and effusive eruptions that ended with the extrusion of two summit domes that today occupy the amphitheatre. The 1950 and 1986 phreatic outbursts occurred along the Pleistocene collapse scar. Currently ~300,000 inhabitants live within a 35 km radius of Tacaná, and could conceivably be impacted by future events of similar magnitude.     

Paleomagnetic estimate of the emplacement temperature of the long-runout Nevado de Colima volcanic debris avalanche deposit, Mexico

Stoopes and Sheridan have mapped a volcanic debris avalanche of Nevado de Colima which has an exceptionally long runout (120 km) and low fall-height to length ratio (H/L = 0.04). We present paleomagnetic results from this volcanic debris avalanche deposit which provide evidence that this avalanche was emplaced at elevated temperatures. The majority of samples, collected from lithic clasts in the volcanic debris avalanche deposit, exhibit two-component remanent magnetizations with a low-temperature component (25–350°C) which is well grouped about the geomagnetic field direction at Colima and a high-temperature component (350–580°C) which is randomly oriented. Although the temperature of the deposit most likely varied with distance from the volcanic source and the thickness of the deposit, our results suggest an emplacement temperature of approximately 350°C at intermediate distances (18–26 km) from the source. In order for the rock clasts (20–40 cm diameter) to be heated to these temperatures, the avalanche was most likely the results of a magmatic, Bezymianny-type eruption. The mixing of hot, juvenile gases with the clasts provides an explanation for the high degree of fluidization of this material, as evidenced by the long runout of this avalanche deposit.     

Caldera formation at Volcán Colima, Mexico, by a large large holocene volcanic debris avalanche

About 4,300 years ago, 10 km³ of the upper cone of ancestral Volcán Colima collapsed to the southwest leaving a horseshoe-shaped caldera 4 km in diameter. The collapse produced a massive volcanic debris avalanche deposit covering over 1550 km² on the southern flanks of the volcano and extending at least 70 km from the former summit. The avalanche followed a steep topographic gradient unobstructed by barriers, resulting in an unusually high area/volume ratio for the Colima deposit. The apparent coefficient of friction (fall height/distance traveled) for the Colima avalanche is 0.06, a low value similar to those of other large-volume deposits. The debris avalanche deposit contains 40–75% angular volcanic clasts from the ancestral cone, a small proportion of vesicular blocks that may be juvenile, and in distal exposures, rare carbonate clasts plucked from the underlying surface by the moving avalanche. Clasts range in size to over 20 m in diameter and are brecciated to different degrees, pulverized, and surrounded by a rock-flour matrix. The upper surface of the deposit shows prominent hummocky topography with closed depressions and surface boulders. A thick, coarse-grained, compositionally zoned scoria-fall layer on the upper northeastern slope of the volcano may have erupted at the time of collapse. A fine-grained surge layer is present beneath the avalanche deposit at one locality, apparently representing an initial blast event. Most of the missing volume of the ancestral volcano has since been restored at an average rate of 0.002 km³/yr through repeated eruptions from the post-caldera cone. As a result, the southern slope of Volcán Colima may again be susceptible to collapse. Over 200,000 people are now living on primary or secondary deposits of the debris avalanche, and a repetition of this event would constitute a volcanic disaster of great magnitude.Ancestral Volcán Colima grew on the southern, trenchward flank of the earlier and larger volcano Nevado de Colima. Trenchward collapse was favored by the buttressing effect of Nevado, the rapid elevation drop to the south, and the intrusion of magma into the southern flank of the ancestral volcano. Other such trenchward-younging, paired volcanoes are known from Mexico, Guatemala, El Salvador, Chile, and Japan. The trenchward slopes of the younger cones are common sites for cone collapse to form avalanche deposits, as occurred at Colima and Popocatepetl in Mexico and at San Pedro Volcano in Chile.     

The 1984 nuée-ardente deposits of Merapi volcano,Central Java,Indonesia: stratigraphy,textural characteristics,and transport mechanisms

For many centuries Merapi volcano has generated hot avalanches of blocks, lapilli and ashes, derived from the destruction of partially solidified, viscous lava domes (Merapi-type nuées ardentes). On 15 June 1984, at least four nuées ardentes came down the southwest slope of the Merapi, the first and the last being responsible for more than 99% of the deposits which are now exposed. The first nuée ardente, a Merapi-type nuée ardente, was produced by the destruction of the dome, travelled 7 km from the crater, leaving a measured deposit, 2.7 m thick, 4 km from the crater, near its upper depositional limit, regularly increasing to a maximum measured thickness of 12 m at the front of the deposit. The lower contact is sharp, non-erosive, with pines still rooted in the underlying paleosol. The deposit consists of 50% ash, 33% lapilli, and 17% blocks, with two subpopulations (one Rosin and one normal), and is finespoor, with less than 4% of fine ash (d finer than 4 ). The deposit displays reverse population grading of both vesiculated and massive clasts, and of the maximum grain size. The maximum size significantly increases regularly down-current over most of the exposed length of unit 1, and bed thickness increases for the entire length of the deposit. The deposit of the second nuée ardente is only 6–21 cm thick, and of very limited lateral extent. It is a normally graded, coarse to fine ash, with a finespoor base. The third unit consists of fines-poor, normally graded coarse ash, exposed in low-amplitude (20–40 cm), 12-m-wavelength dunes. The deposit of the fourth nuée ardente rests in sharp erosive contact on the underlying unit, increasing in thickness down-flow. It consists of transitional coarse and fine-grained strata, 6–130 c cm thick, dipping 5–10° down-flow. The deposit, made up of two subpopulations (one Rosin and one normal), is normally graded over the entire bed, but coarsegrained strata are reversely graded. The relative content of vesiculated clasts increases up-bed in both strata types, from 12% at the base to 40% at the top. The characteristics of unit 1 suggest that it accumulated from a concentrated suspension of cohesionless solids exhibiting non-Newtonian behavior, where dispersive pressure played an important role in the suspension of the clasts. Units 2 and 3 were probably deposited from dilute turbulent suspensions, whereas the upper unit (4) is a classic example of deposition from a high-density turbulent suspension leading to the formation of multiple traction carpets driven by the overlying, lower-density, surge. The horizontal distance travelled by a hot rock avalanche may be influenced by its transport mechanism. Debris flows are mobile on very low slopes-as low as 1°-whereas grain flows, even density-modified grain flows, require relatively high slopes-more than 6° at Merapi-to maintain their mobility. If the present Merapi dome were to collapse and produce a debris flow, its present volume coupled with the minimal 1.5 km vertical drop could travel a distance ranging between 15 and 30 km. However, if transport were by grain flow mechanisms, the mass could come to rest as it reaches a 5–10° slope.     

Lateral variations within coarse co-ignimbrite lithic breccias of the Kos Plateau Tuff, Greece

 Coarse, co-ignimbrite lithic breccia, Ebx, occurs at the base of ignimbrite E, the most voluminous and widespread unit of the Kos Plateau Tuff (KPT) in Greece. Similar but generally less coarse-grained basal lithic breccias (Dbx) are also associated with the ignimbrites in the underlying D unit. Ebx shows considerable lateral variations in texture, geometry and contact relationships but is generally less than a few metres thick and comprises lithic clasts that are centimetres to a few metres in diameter in a matrix ranging from fines bearing (F2: 10 wt.%) to fines poor (F2: 0.1 wt.%). Lithic clasts are predominantly vent-derived andesite, although clasts derived locally from the underlying sedimentary formations are also present. There are no proximal exposures of KPT. There is a highly irregular lower erosional contact at the base of ignimbrite E at the closest exposures to the inferred vent, 10–14 km from the centre of the inferred source, but no Ebx was deposited. From 14 to <20 km from source, Ebx is present over a planar erosional contact. At 16 km Ebx is a 3-m-thick, coarse, fines-poor lithic breccia separated from the overlying fines-bearing, pumiceous ignimbrite by a sharp contact. This grades downcurrent into a lithic breccia that comprises a mixture of coarse lithic clasts, pumice and ash, or into a thinner one-clast-thick lithic breccia that grades upward into relatively lithic-poor, pumiceous ignimbrite. Distally, 27 to <36 km from source Ebx is a finer one-clast-thick lithic breccia that overlies a non-erosional base. A downcurrent change from strongly erosional to depositional basal contacts of Ebx dominantly reflects a depletive pyroclastic density current. Initially, the front of the flow was highly energetic and scoured tens of metres into the underlying deposits. Once deposition of the lithic clasts began, local topography influenced the geometry and distribution of Ebx, and in some cases Ebx was deposited only on topographic crests and slopes on the lee-side of ridges. The KPT ignimbrites also contain discontinuous lithic-rich layers within texturally uniform pumiceous ignimbrite. These intra-ignimbrite lithic breccias are finer grained and thinner than the basal lithic breccias and overlie non-erosional basal contacts. The proportion of fine ash within the KPT lithic breccias is heterogeneous and is attributed to a combination of fluidisation within the leading part of the flow, turbulence induced locally by interaction with topography, flushing by steam generated by passage of pyroclastic density currents over and deposition onto wet mud, and to self-fluidisation accompanying the settling of coarse, dense lithic clasts. There are problems in interpreting the KPT lithic breccias as conventional co-ignimbrite lithic breccias. These problems arise in part from the inherent assumption in conventional models that pyroclastic flows are highly concentrated, non-turbulent systems that deposit en masse. The KPT coarse basal lithic breccias are more readily interpreted in terms of aggradation from stratified, waning pyroclastic density currents and from variations in lithic clast supply from source. Received: 21 April 1997 / Accepted: 4 October 1997     

Multiple edifice failures, debris avalanches and associated eruptions in the Holocene history of Shiveluch volcano, Kamchatka, Russia

 Investigation of well-exposed volcaniclastic deposits of Shiveluch volcano indicates that large-scale failures have occurred at least eight times in its history: approximately 10,000, 5700, 3700, 2600, 1600, 1000, 600 ¹⁴C BP and 1964 AD. The volcano was stable during the Late Pleistocene, when a large cone was formed (Old Shiveluch), and became unstable in the Holocene when repetitive collapses of a portion of the edifice (Young Shiveluch) generated debris avalanches. The transition in stability was connected with a change in composition of the erupting magma (increased SiO₂ from ca. 55–56% to 60–62%) that resulted in an abrupt increase of viscosity and the production of lava domes. Each failure was triggered by a disturbance of the volcanic edifice related to the ascent of a new batch of viscous magma. The failures occurred before magma intruded into the upper part of the edifice, suggesting that the trigger mechanism was indirectly associated with magma and involved shaking by a moderate to large volcanic earthquake and/or enhancement of edifice pore pressure due to pressurised juvenile gas. The failures typically included: (a) a retrogressive landslide involving backward rotation of slide blocks; (b) fragmentation of the leading blocks and their transformation into a debris avalanche, while the trailing slide blocks decelerate and soon come to rest; and (c) long-distance runout of the avalanche as a transient wave of debris with yield strength that glides on a thin weak layer of mixed facies developed at the avalanche base. All the failures of Young Shiveluch were immediately followed by explosive eruptions that developed along a similar pattern. The slope failure was the first event, followed by a plinian eruption accompanied by partial fountain collapse and the emplacement of pumice flows. In several cases the slope failure depressurised the hydrothermal system to cause phreatic explosions that preceded the magmatic eruption. The collapse-induced plinian eruptions were moderate-sized and ordinary events in the history of the volcano. No evidence for directed blasts was found associated with any of the slope failures. Received: 28 June 1998 / Accepted: 28 March 1999     

Effects of the Little Ice Age on avalanche boulder tongues in the French Alps (Massif des Ecrins)

Lichens of the subspecies Rhizocarpon geographicum s.l were measured on 25 avalanche boulder tongues in the Massif des Ecrins to elucidate the Little Ice Age history of avalanche activity. Results show: (1) an increase of lichen size from the median to the distal zone of deposits, and a decrease from the edges to the centre; (2) three types of lichen settlement. From the uppermost to the median zone, lichens are absent, because avalanche activity is very active. Down‐slope, lichens occur in two different zones: the median zone is colonized by 5–20 mm size lichens on sides of blocks protected from the abrasional action of avalanches, while in the distal zone lichen diameters are largest (>30 mm) and occur on all sides of the blocks. The spatial distribution of the lichens and their size according to elevation make it possible to distinguish different phases during which avalanche activity has increased. At high elevation, the avalanche activity was at a maximum before ad 1650 and between ad 1730 and 1830. During these two periods avalanches had suf?cient magnitude to reach the basal zone of the deposits. At low elevation since ad 1650 the magnitude and frequency of avalanches have declined. Copyright © 2004 John Wiley & Sons, Ltd.     

Pyroclastic flow deposits of the 1991 eruption of Volcán de Colima,Mexico

The April 16, 1991, eruption of Volcán de Colima represents a classical example of partial dome collapse with the generation of progressively longer-runout, Merapi-type pyroclastic flows that traveled up to 4 km along the El Cordoban gullies (East, Central and West). The flows filled the gullies with block-and-ash flow deposits up to 10 m thick, of which, after 7 years of erosion, only remnants remained in the El Cordoban West and East gullies. The El Cordoban Central gully, however, provided a well-preserved and incised longitudinal section of the 1991 deposits. The deposits were emplaced as proximal and distal facies, separated by a change in slope angle from >30° to <20°. The proximal facies consists of massive, clast-supported flow units (up to 1 m thick) with andesite blocks locally supported by a matrix of coarse ash and devoid of segregation structures or grading. The distal facies consists of a massive, matrix-supported deposit up to 8 m thick, which contains dispersed andesite blocks in a fine ash matrix. In the distal facies, a train of blocks marks flow-unit upper boundaries and, although sorting is poor, some grading is present. Thin, finely stratified, or dune-bedded layers of fine ash material are locally present above or below units of both facies. Sedimentologic parameters show that the size or fraction of large pyroclasts (larger than –1 ) decreases from proximal to distal facies, as the percentage of matrix (0 to 4 ) increases, especially immediately beyond the break in slope. We propose that the propagation of the Colima pyroclastic flows is critically dependent on local slope angle, the presence of erodible slope debris, and the decrease in grain size with distance from the vent. The progressive fining is probably caused by some combination of erosion, clast breakup and deposition of larger pyroclasts, and is itself influenced by the slope angle. In the proximal region, the flows moved as granular avalanches, in which interacting grains ground each other and erosion occurred to produce an overriding dilute ash cloud. The maximum runout distance of the avalanches was controlled by the angle of repose of the material, and the volume and grain size of source and eroded material. Because the slope angle is close to the repose angle for this debris, granular avalanches were not able to propagate far beyond the change in slope. If, however, an avalanche had enough mass in finer grain size fractions, at least part of the flow continued beyond the break in slope and across the volcano apron, propagating in a turbulent state and depositing surge layers, or in an otherwise settling-modified state and depositing block-and-ash flow layers.Editorial responsibility: T Druitt