influence of cone morphology on dykes stromboli italy abstract the island of stromboli represents the emerged part of a 2 6 km high composite volcano elongated in a ne direction over the period 100 13 ka bp the morphological evolution of the volcano was characterised by a series of summit caldera collapses between major building phases from 13 ka bp onward a series of large lateral collapses occurred northwestwards in one sector of the cone the results of the field study of the total 94 dyke population is discussed in this paper they show that during the last 100 ka the majority of dykes were injected with a nnegssw to ene wsw strike along a ne sw weakness zone crossing the volcano summit this is interpreted as a volcanic rift the geometry of which is controlled by regional tectonic stresses dykes injected prior to 13 ka bp at lower altitudes are generally steeper than those injected at higher elevations after 13 ka bp dykes were injected along the ne sw weakness zone and parallel and close to the margins of the earliest sector collapse at dips of 20 80 inclined towards the collapse depression i suggest that this is linked to unbuttressing of the collapse depression or to seaward gravity instability of the depression infill deposits extensive emplacement of long dykes along the ne sw weakness zone occurred only when the volcano regained a conical shape some of the dykes emplaced close to the original surface of the cone bent to assume a dip direction parallel to the local slope i propose that this local change of dyke geometry can be explained by the direct influence of the free surface of the cone leading to a rotation of the least principal stress which assumes an orientation perpendicular to the slope

laboratory investigation of nonlinear flow characteristics in rough fractures during shear process abstract to understand the influence of shear behavior on the transporting properties of fluid through a single fracture splitting fractures were made in the laboratory and shear flow tests were carried out under constant normal load conditions the applied normal stress is in the range of 0 5 3 0 mpa before the physical test the fracture s morphology is measured for identification of the roughness at each shear step we performed 5 8 high precise hydraulic tests with different hydraulic gradient the relationship between pressure gradient and volume flow rate demonstrates to be nonlinear and fits very well with forchheimer s and izbash s laws the linear and nonlinear coefficients in forchheimer s law are quite sensitive to shear deformation closure or dilation experienced 1 2 and 1 3 orders of magnitude reduction during shear respectively an empirical equation is proposed to quantify the relationship between linear coefficient and nonlinear coefficient based on the experimental observations the two coefficients in izbash s law are quantified the m value is in the range between 1 06 and 1 41 and the value experiences a reduction of 1 2 orders of magnitude during shear in addition the studied critical reynolds number exhibits a decreasing and increasing variation corresponding to shear contraction and shear dilation of rock fracture for all the cases in this study the critical reynolds number ranges between 1 5 and 13 0

reactivation of basement faults beneath volcanoes a new model of flank collapse in order to explain the presence of voluminous volcanic debris avalanche deposits around a stratovolcano reactivation of vertical faults beneath a volcanic cone has been tested using analogue models reactivation of a single vertical fault beneath a cone generates a normal fault and an upturning of the layers creating a bulge on the flank the upturning induces a flank collapse characterized by a typical horseshoe shaped scar called an avalanche caldera reactivation of two vertical faults beneath a cone also generates a normal fault and a summit bulge this bulge may result from the movement along a reverse fault a large collapse is generated within the angle created by the two vertical faults the angle of the collapse can be up to 140 whereas this angle is typically 120 for a dome intrusion collapse is instantaneous and is favoured by the presence of ductile layers ash and pumice formations in the example considered in a stratovolcano complex the model may be applicable to volcanoes in a state of dormancy or extinction in regions with active regional tectonism we suggest this mechanism of collapse in the case of the cantal stratovolcano massif central france to explain the presence of voluminous volcanic debris avalanche deposits around this volcano

n view reconstruction a new method for morphological modelling and deformation measurement in volcanology abstract we present a method of reconstructing volcanic morphology using multiple digital views n view captured at different angles around an object this approach uses recent advances in the field of computer vision to provide accurate 3 d measurements of volcanic surfaces videogrammetry digital image reconstruction is used as it is best adapted to numerical processing the method is tested and now used in the laboratory on analogue volcanic cones the method begins with calibrating the camera and finding image positions using an accurate n view calibration method this is done by estimating internal and external parameters of the camera using several views of a specially constructed calibration target the n view reconstruction of the real object is then done by iteratively deforming an initial theoretical model of the surface laboratory tests show that reconstruction accuracy is about 10 4 m for a distance between the object and the camera of 0 5 m and is potentially several orders of magnitude higher for surfaces of finer texture and using higher precision sensors this is easily high enough to be useful for the accuracy required for morphological studies it is also sufficient for monitoring most types of volcano deformation the technique has the potential to detect morphology changes of the order of mm use of the method in the field requires a slightly different approach from that in the laboratory textures and lighting are more variable and the sensor and ground control point location and model calibration must be done differently we provide case studies from laboratory tests and qualitative image analysis for two field cases piton de la fournaise indian ocean and santaguito guatemala these illustrate the technique s potential and explore problems of field application using current sensors the method has the potential to provide sufficient precision for fine mm cm scale reconstruction and will represent a valuable simple and flexible tool when compared with classical stereophotogrammetry techniques

catastrophic collapse at stratovolcanoes induced by gradual volcano spreading unlike ordinary mountains which are formed by slow uplift and erosion volcanoes are constructed rapidly as a consequence many are liable to massive flank failures leading to debris avalanches for example at mount st helens in 1980 such failures occur worldwide about once every 25 years ref 1 and even small ones can present a major hazard in particular if far reaching tsunamis are generated as at mayu yama in 1792 ref 2 previous work has tended to emphasize differences in eruption style associated with flank failure2 but here we focus on the fundamental structural causes of failure most volcanic failures are generated by magmatic intrusion and flank spreading3 we present evidence however that mombacho volcano in nicaragua experienced a previously unrecognized type of failure triggered by sub volcanic basement spreading notably collapses related to basement spreading do not require that the volcano be magma tically active and thus flank failure may pose a significant risk even at inactive volcanoes which are rarely monitored

tungurahua volcano ecuador structure eruptive history and hazards abstract tungurahua one of ecuador s most active volcanoes is made up of three volcanic edifices tungurahua i was a 14 km wide andesitic stratocone which experienced at least one sector collapse followed by the extrusion of a dacite lava series tungurahua ii mainly composed of acid andesite lava flows younger than 14 000 years bp was partly destroyed by the last collapse event 2955 90 years ago which left a large amphitheater and produced a 8 km3 debris deposit the avalanche collided with the high ridge immediately to the west of the cone and was diverted to the northwest and southwest for 15 km a large lahar formed during this event which was followed in turn by dacite extrusion southwestward the damming of the chambo valley by the avalanche deposit resulted in a 10 km long lake which was subsequently breached generating another catastrophic debris flow the eruptive activity of the present volcano tungurahua iii has rebuilt the cone to about 50 of its pre collapse size by the emission of 3 km3 of volcanic products two periods of construction are recognized in tungurahua s iii history from 2300 to 1400 years bp high rates of lava extrusion and pyroclastic flows occurred during this period the magma composition did not evolve significantly remaining essentially basic andesite during the last 1300 years eruptive episodes take place roughly once per century and generally begin with lapilli fall and pyroclastic flow activity of varied composition andesite dacite and end with more basic andesite lava flows or crater plugs this pattern is observed in the three historic eruptions of 1773 1886 and 1916 1918 given good age control and volumetric considerations tungurahua iii growth s rate is estimated at 1 5 106 m3 year over the last 2300 years although an infrequent event a sector collapse and associated lahars constitute a strong hazard of this volcano given the 3000 m relief and steep slopes of the present cone a future collapse even of small volume could cover an area similar to that affected by the 3000 year old avalanche the more frequent eruptive episodes of each century characterized by pyroclastic flows lavas lahars as well as tephra falls directly threaten 25 000 people and the agoyan hydroelectric dam located at the foot of the volcano

the volcano tectonic evolution of concepcion nicaragua we describe the evolution of concepcion volcano by integrating regional geology eruptive activity morphology stratigraphy petrology structure and active deformation data this nicaraguan volcano is set close to the back limb of the northwest trending tertiary rivas anticline a regional structure that bounds the southwest side of lake nicaragua concepcion rises 1 600 m above a 1 km thick sequence of quaternary lacustrine mud stones there is no record of volcanism in the lake prior to concepcion in addition the only nearby volcano maderas volcano has not deposited material on concepcion because of the trade winds thus concepcion and made ras too can be considered as pristine volcanic environments unaffected by other centres a topographic rise forms an annulus 20 km in diameter around the cone the rise is created by thrust related folds at the western base where the trade winds have accumulated a thick sequence of tephra and by mud diapirs at the eastern base where only lake mudstones are present four magmatic eruptive episodes exist in the stratigraphic record the first begins with primitive low alumina basalt and subsequently evolves to dacitic compositions the following three episodes begin with high alumina basalts and evolve only to silicic andesites the occurrence of the high alumina basalt after the first episode is indicative of crystal fractionation at lower crustal depths the first episode may be associated with a compressive phase of volcano evolution in this phase the edifice load compresses substrata allowing a longer magma residence time and differentiation in a shallow reservoir possibly located at the density contrast between the lake sediments and the tertiary flysch during the next three episodes the weak sediments below the volcano started to rupture and yield under its increasing load beginning a thrusting diapiring phase of volcano evolution because of outward thrusting vertical and horizontal stresses above the chamber were reduced allowing magma to erupt more easily and to reach a lesser degree of evolution if we consider the future evolution of concepcion the differentiation in the shallow reservoir has probably generated a cumulitic complex which eventually will start to deform and spread beginning another this time plutonic spreading phase this phase which may be beginning now could allow less evolved magmas to be erupted again four components influence the phases of volcano evolution 1 the regional geology that is the boundary condition of the environment 2 the substrata rheology that controls deformation 3 the load of the volcanic edifice and 4 the magma which provides the input of mass and energy our model of volcanic evolution suggests that concepcion is a complex geologic environment the volcanic activity tectonics and hazards can only be constrained through a complete knowledge of the many components of this environment

spatial variations of p residuals and deep structure of the european lithosphere spatial variations of these normalized residuals as large as 2 s were derived from events in different focal regions an average residual at each station was calculated from 23 selected groups compiled from 181 events evenly distributed as to their azimuths and distances the 3 d inversion of the residuals yielded very much the same picture of the relatively high and low velocity provinces for the upper layer of the mantle as compared with the map of average residuals the map of average residuals mainly reflects variations in the thickness of the lithosphere and its thermal state regions of low velocities include the pannonian basin and central carpathians the northern apennines and the po plain as well as the rhine graben the rhenish massif and the north german polish platform high velocities are observed in the south western margin of the east european platform and in three well defined zones situated in the inner part of the western alps in the eastern part of the austrian alps and in the eastern carpathians besides the effects of inhomogeneities also anisotropic effects are anticipated from the spatial diagrams of the residuals which show a similar pattern within large tectonic provinces r

volcanic edifice stability during cryptodome intrusion limit equilibrium analyses were applied to the 1980 mount st helens and 1956 bezymianny failures in order to examine the influence on stability of structural deformation produced by cryptodome emplacement weakening structures associated with the cryptodome include outward dipping normal faults bounding a summit graben and a flat shear zone at the base of the bulged flank generated by lateral push of the magma together with the head of the magmatic body itself these structures serve directly to localize failure along a critical surface with low stability deep within the interior of the edifice this critical surface with the safety coefficient reduced by 25 30 is then very sensitive to stability condition variation in particular to the pore pressure ratio ru and seismicity coefficient n for ru 0 3 or n 0 2 the deep surface suffers catastrophic failure removing a large volume of the edifice flank in the case of mount st helens failure occurred within a material with angle of friction 40 cohesion in the range 105 106 pa and probably significant water pore pressure on 18 may 1980 detachment of slide block i occurred along a newly formed rupture surface passing through the crest of the bulge although sliding of block i may have been helped by the basal shear zone significant pore pressure and a triggering earthquake were required ru 0 3 and n 0 2 detachment of the second block was guided by the summit normal fault the front of the cryptodome and the basal shear zone this occurred along a deep critical surface which was on the verge of failure even before the 18 may 1980 earthquake the stability of equivalent surfaces at bezymianny volcano appears significantly higher thus although magma had already reached the surface weaker materials or higher pore pressure and or seismic conditions were probably required to reach the rupture threshold from our analysis we find that deep seated sector collapses formed by removing the edifice summit cannot generally result from a single slide cryptodome induced deformation does however provide a deep potential slip surface as previously thought it may assist deep seated sector collapse because it favors multiple retrogressive slides this leads to explosive depressurization of the magmatic and hydrothermal systems which undermines the edifice summit and produces secondary collapses and explosive blasts

analogue models of the effect of long term basement fault movement on volcanic edifices long term fault movement under volcanoes can control the edifice structure and can generate collapse events to study faulting effects we explore a wide range of fault geometries and motions from normal through vertical to reverse and dip slip to strike slip using simple analogue models we explore the effect of cumulative sub volcanic fault motions and find that there is a strong influence on the structural evolution and potential instability of volcanoes the variety of fault types and geometries are tested with realistically scaled displacements demonstrating a general tendency to produce regions of instability parallel to fault strike whatever the fault motion where there is oblique slip faulting the instability is always on the downthrown side and usually in the volcano flank sector facing the strike slip sense of motion different positions of the fault beneath the volcano change the location type and magnitude of the instability produced for example the further the fault is from the central axis the larger the destabilised sector also with greater fault offset from the central axis larger unstable volumes are generated such failures are normal to fault strike using simple geometric dimensionless numbers such as the fault dip degree of oblique motion angle of obliquity and the fault position we graphically display the geometry of structures produced the models are applied to volcanoes with known underlying faults and we demonstrate the importance of these faults in determining volcanic structures and slope instability using the knowledge of fault patterns gained from these experiments geological mapping on volcanoes can locate fault influence and unstable zones and hence monitoring of unstable flanks could be carried out to determine the actual response to faulting in specific cases

earthquake induced shallow sildes in volcanic debris soils in march 1987 following two earthquakes and prolonged heavy rain landslides in northern ecuador caused extensive damage on slopes of 35 60 the tectonics of the area and the characteristics of the soil are described attention is focused on the aspects of behaviour which are peculiar to volcanic debris residual soils which may affect the stability of such materials on slopes an analysis of the slides shows the effects of components such as shear strength pore water pressure and seismic loading the presence of a cohesive component of strength related to bonding is confirmed conclusions are drawn as to the mechanisms of the slides which required the presence of both saturated soil and the earthquakes gaps in current knowledge on these soils and future research needs are identified a la suite de deux tremblements de terre et de grosses pluies prolongees des glissements de terrain ont provoque de grands dommages sur des pentes entre 35 et 60 dans les regions septentrionales de la republique de l e

caldera formation at volcan colima mexico by a large large holocene volcanic debris avalanche abstract about 4 300 years ago 10 km 3 of the upper cone of ancestral volcan 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 km 2 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 km 3 yr through repeated eruptions from the post caldera cone as a result the southern slope of volcan 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 volcan 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 rain triggered atenquique volcaniclastic debris flow of october 16 1955 at nevado de colima volcano mexico abstract on october 16 1955 at 10 45 a m local time after three days of intense rain 140 mm that was twice the monthly average precipitation a devastating flood surge formed a volcaniclastic debris flow on the eastern slopes of nevado de colima volcano nearly simultaneous flood surges formed in the arroyo seco los platanos and dos volcanes ravines that coalesced with the larger flow in the atenquique ravine at each confluence with a tributary the flow was diluted the texture and structure of the preserved 1955 deposits near high water marks indicate that the downstream flow was mainly in the lower range of debris flow concentration 60 sediment concentration by weight downstream the tributaries the flood encountered a 0 06 10 6 m 3 water reservoir that failed significantly increasing the surge volume additional entrained sediment also increased the flow volume downstream the flood wave reached the town of atenquique as an 8 9 m catastrophic wave causing the death of more than 23 people the partial destruction of the town and losses of 13 000 000 pesos 1 million us dollars today to a paper mill and company facilities according to eyewitness accounts the flood wave had a peak discharge that lasted ca 10 to 15 minutes at atenquique deposits at the site and the high water marks observed from photographs of the town s church indicate that sediment concentration was ca 60 wt the flood continued for about 1 km to its junction with the tuxpan river where it was diluted by mixing with normal flood flow the deposits covered an area of 1 2 km 2 and had a minimum volume of 3 2 10 6 m 3 the main deposit consists of a single unit averaging 4 m in thickness with weak textural variations that suggest surging within the flood wave the deposit is heterolithologic and consists of boulders set in a matrix of sand size sediment with polymodal or bimodal distributions and normal grading varying with distance from source the town of atenquique has been reconstructed largely within the area inundated by the 1955 flood wave thus creating the conditions for a future disaster a rainfall intensity warning system and an educational program for inhabitants are strategies to mitigate this risk

sangay volcano ecuador structural development present activity and petrology abstract sangay 5230 m the southernmost active volcano of the andean northern volcanic zone nvz sits 130 km above a 32 ma old slab close to a major tear that separates two distinct subducting oceanic crusts southwards quaternary volcanism is absent along a 1600 km long segment of the andes three successive edifices of decreasing volume have formed the sangay volcanic complex during the last 500 ka two former cones sangay i and ii have been largely destroyed by sector collapses that resulted in large debris avalanches that flowed out upon the amazon plain sangay iii being constructed within the last avalanche amphitheater has been active at least since 14 ka bp only the largest eruptions with unusually high plinian columns are likely to represent a major hazard for the inhabited areas located 30 to 100 km west of the volcano however given the volcano s relief and unbuttressed eastern side a future collapse must be considered that would seriously affect an area of present day colonization in the amazon plain 30 km east of the summit andesites greatly predominate at sangay there being few dacites and basalts in order to explain the unusual characteristics of the sangay suite highest content of incompatible elements except y and hree of any nvz suite low y and hree values in the andesites and dacites and high nb la of the only basalt found a preliminary five step model is proposed 1 an enriched mantle in comparison with an morb source or maybe a variably enriched mantle at the site of the sangay prior to quaternary volcanism 2 metasomatism of this mantle by important volumes of slab derived fluids enriched in soluble incompatible elements due to the subduction of major oceanic fracture zones 3 partial melting of this metasomatized mantle and generation of primitive basaltic melts with nb la values typical of the nvz which are parental to the entire sangay suite but apparently never reach the surface and subordinate production of high nb la basaltic melts maybe by lower degrees of melting at the periphery of the main site of magma formation that only infrequently reach the surface 4 afc processes at the base of a 50 km thick crust where parental melts pond and fractionate while assimilating remelts of similar basaltic material previously underplated producing andesites with low y and hree contents due to garnet stability at this depth 5 low pressure fractionation and mixing processes higher in the crust both an enriched mantle under sangay prior to volcanism and an important slab derived input of fluids enriched in soluble incompatible elements two parameters certainly related to the unique setting of the volcano at the southern termination of the nvz apparently account for the exceptionally high contents of incompatible elements of the sangay suite in addition the low cr ni values of the entire suite another unique characteristic of the nvz also requires unusual fractionation processes involving cr spinel and or clinopyroxene either in the upper mantle or at the base of the crust

a structural model guide for geothermal exploration in ancestral mount bao leyte philippines abstract the tongonan geothermal field is the largest producing geothermal field in the philippines having an installed capacity of 700 mw it hosts several major power plants that tap geothermal power from the northern flank of the eroded ancestral mount bao amb volcano in leyte island philippines a structural model guide is presented to delineate exploration targets in other flanks of the 1200 km 2 area of the amb volcano if applied the model constrains the coverage of geothermal exploration to areas where more detailed investigations involving geological geophysical and geochemical methods can be conducted analog sand cone experiments and their comparison with the deformation of the amb volcano were used to interpret wells within the tongonan geothermal field the study shows that existing producing wells in tongonan straddle the crypto philippine fault a n34 w azimuth 326 trending master fault inferred to traverse the base of the amb volcano this master fault induced fracture controlled permeability where fluids in the tongonan geothermal field circulate the structural model suggests that the south southeastern flank of the amb directly above the southern extension of the master philippine fault is an ideal geothermal target amenable to detailed exploration

kizimen volcano kamchatka a future mount st helens we studied the tectonic setting morphology geologic structure history of eruptive activity and evolution of the composition of the erupted material of kizimen volcano kamchatka from the moment of its origination 11 12 thousand years ago to the present time four cycles each 2 3 5 thousand years long were distinguished that characterize the activity of the volcano all of the largest eruptions were dated and their parameters determined we also estimated the volume and the mass of the erupted products the volcanic intensity of eruption of material during periods of high activity and the amount of material the volcano ejected at different stages of its formation it has been shown that the evolution of the composition of the rocks erupted from dacite to basaltic andesite takes place as a result of mixing of dacitic and basaltic magma it is suggested that future eruptions that may take place at kizimen may be similar to those at bandai 1888 and mount st helens 1980 volcanoes

latest pleistocene holocene tectonics of the ecuadorian andes abstract a structural study was carried out in the most tectonically active areas of the ecuadorian andes including part of the interandean valley the cordillera real and the subandean zone since the pleistocene the region has undergone complex wrench tectonics which are still active apparently two different stress states have coexisted within different parts of the andean range during the latest pleistocene and holocene 1 1 the external part of the chain including the cordillera real and the westernmost part of the subandean zone has been subject to a east northeast west southwest direction of greatest principal stress associated motions are partitioned between right lateral oblique thrusting and pure right lateral strike slip faulting along north south to north northeast trending structures as a whole these define a right lateral transpressive shear zone 2 2 in the interandean valley which lies above a major tertiary suture zone deformations are consistent with a north south direction of greatest horizontal principal stress associated with recent development of a left lateral transtensional shear zone in the first zone fault displacements are at least an order of magnitude larger than in the second zone the present tectonics of the area is explained by a kinematic model in which the wedge shaped cordillera real crustal block bounded to the east by a right lateral shear zone and to the west by a left lateral shear zone is being uplifted and extruded northward with respect to the cordillera occidental and to the stable south american foreland this motion is the result of oblique convergence between the nazca and south american plates

multiple sector collapses at stromboli volcano italy how they work this paper demonstrates that four large sector collapses have affected the nw flank of the stromboli volcano in the past 13 ka alternating with growth phases in order to contribute to the evaluation of the critical conditions which trigger lateral collapses a reconstruction of the geometry of each collapse of the volcano edifice in the four stages that preceded the relative collapse events is also presented and a computation of the landslide volume this reconstruction is based on new field data plotted in three dimensions prior to the initial 13 ka collapse the volcano was 1125 100 m high above sea level the collapse had a volume of 2 23 0 87 km3 whereas the pre collapse volcano volume was 218 8 7 7 km3 the next edifice that failed was 900 70 m high a s l the collapse volume was 1 0 54 km3 with a precollapse volcano volume of 201 4 5 4 km3 the edifice then grew to 1000 60 m a s l the third collapse had a volume of 1 08 0 39 km3 and occurred within a volcano with a volume of 209 1 4 6 km3 this was followed by a new growth phase followed by the last collapse with a volume of 0 73 0 22 km3 the volcano volume was about the same as the present one the present active crater zone is at 780 m a s l in the first three collapses sliding surfaces cut the main magma conduit in the last collapse the upper scarp coincided with the conduit location dyking along a main ne trending weakness zone across the volcano summit exerted a lateral force for collapse inception the decrease of the landslide volumes with the age and the concentric scarps of the four collapses suggest that the younger sliding planes tended to become more superficial and to decrease the areal extent this is interpreted as due to a successively weaker eruptive products from dominantly lavas to dominantly pyroclastics and b the feedback effects between collapses and dykes that injected along the lateral segments of the first collapse slide plane

the cohesive naranjo debris flow deposit 10 km3 a dam breakout flow derived from the pleistocene debris avalanche deposit of nevado de colima volcano mexico abstract mass movement processes on volcanic terrains such as landslides and debris avalanches can cause the obstruction of main drainages producing the formation of temporary dams a good example of this occurred 18 5 ka ago when the eastern flank of the nevado de colima volcano collapsed producing a debris avalanche deposit that was previously described as one of the largest in the world the deposit extended from the volcanic summit as far as the pacific coast 120 km away new stratigraphic sedimentological and componentry data suggest that the volcanic collapse of nevado de colima resulted in a debris avalanche that traveled 20 km southeast to the naranjo river there it crashed against a topographic barrier consisting of cretaceous limestones cerro la carbonera and the flow direction was diverted to the south down the naranjo river channel for another 25 km before the avalanche came to a halt the obstruction of the drainage produced a temporary dam that stored ca 1 km3 of water and deposited fluvial and slack water sediments some time after the damming the accumulated water sediment load was able to overtop the obstructing material and to release a breakout flow with a calculated initial flow discharge of 3 5 million m3 s the resulting flood cohesive debris flow followed the channel of the naranjo river and due to the high erodibility of the channel and introduction of substrate material the debris flow progressively increased its volume up to 10 km3 six times its initial volume this study highlights the relevance of evaluating the potential remobilization of debris avalanche deposits to initiate large magnitude cohesive debris flows therefore the hazard and risk analysis of future potential events of this nature must consider the pre eruption conditions and the topography surrounding a volcano

sector collapse forming at casita volcano nicaragua catastrophic sector collapse occurs when a volcano becomes structurally unable to support its own load one process particularly capable of weakening the edifice is hydrothermal activity it can produce high pore pressures and alter strong rock to clays alteration can extend progressively over long periods 100 yr allowing deformation to develop slowly before collapse an important finding is that structures produced by such deformation are recognizable and could permit collapse prediction we present the case of casita nicaragua where hydrothermal activity has been weakening the edifice core causing flank spreading altering original constructional shape and steepening flank slopes one side is slumping outward producing a crescentic scar with a basal bulge we identify this feature as the site of a potential sector collapse with conditions ripe for failure

scaled experiments of volcanic spreading experiments were conducted to study the spreading of volcanic constructs volcanoes are simulated by a sand cone and the volcanic substratum is simulated by a sand layer brittle substratum overlying a silicone layer ductile substratum similarity conditions between natural volcanoes and experimental prototypes led to the definition of dimensionless numbers experiments determine values which predict whether or not spreading takes place of particular importance are the ratio between the thickness of the brittle substratum and the height of the volcano 2 and the brittle ductile ratio of the substratum 3 2 indicates that the volcano must be large enough to break the substratum before spreading occurs whereas 3 controls the style of deformation during spreading these dimensionless numbers change with time reaching values that tend toward those observed for stable configurations experimental values are compared with those from well constrained natural examples it is found that an essential requirement for volcanic spreading is the presence of a low viscosity layer within the substratum flow of the weak layer away from the excess load is responsible for the spreading the overlying edifice displays radial intersecting grabens due to concentric stretching dissected summit areas concentric zones of thrusts and folds form in the substratum around the edifice and diapirs of the ductile substratum rise within the fault zones

large scale failures on domes and stratocones situated on caldera ring faults sand box modeling of natural examples from kamchatka russia edifices of stratocones and domes are often situated eccentrically above shallow silicic magma reservoirs evacuation of such reservoirs forms collapse calderas commonly surrounded by remnants of one or several volcanic cones that appear variously affected and destabilized we studied morphologies of six calderas in kamchatka russia with diameters of 4 to 12 km edifices affected by caldera subsidence have residual heights of 250 800 m and typical amphitheater like depressions opening toward the calderas the amphitheaters closely resemble horseshoe shaped craters formed by large scale flank failures of volcanoes with development of debris avalanches where caldera boundaries intersect such cones the caldera margins have notable outward embayments we therefore hypothesize that in the process of caldera formation these eccentrically situated edifices were partly displaced and destabilized causing large scale landslides the landslide masses are then transformed into debris avalanches and emplaced inside the developing caldera basins to test this hypothesis we carried out sand box analogue experiments in which caldera formation modeled by evacuation of a rubber balloon was simulated the deformation of volcanic cones was studied by placing sand cones in the vicinity of the expected caldera rim at the initial stage of the modeled subsidence the propagating ring fault of the caldera bifurcates within the affected cone into two faults the outermost of which is notably curved outward off the caldera center the two faults dissect the cone into three parts 1 a stable outer part 2 a highly unstable and subsiding intracaldera part and 3 a subsiding graben structure between parts 1 and 2 further progression of the caldera subsidence is likely to cause failure of parts 2 and 3 with failed material sliding into the caldera basin and with formation of an amphitheater like depression oriented toward the developing caldera the mass of material which is liable to slide into the caldera basin and the shape of the resulted amphitheater are a function of the relative position of the caldera ring fault and the base of the cone a cone situated mostly outside the ring fault is affected to a minor degree by caldera subsidence and collapses with formation of a narrow amphitheater deeply incised into the cone having a small opening angle accordingly the caldera exhibits a prominent outward embayment by contrast collapse of a cone initially situated mostly inside the caldera results in a broad amphitheater with a large opening angle i e the embayment of the caldera rim is negligible the relationships between the relative position of an edifice above the caldera fault and the opening angle of the formed amphitheater are similar for the modeled and the natural cases of caldera cone interactions thus our experiments support the hypothesis that volcanic edifices affected by caldera subsidence can experience large scale failures with formation of indicative amphitheaters oriented toward the caldera basins more generally the scalloped appearance of boundaries of calderas in contact with pre caldera topographic highs can be explained by the gravitational influence of topography on the process of caldera formation

morphology of pyroclastic cones and tectonics the relationships between morphology and spatial distribution of 1315 quaternary pyroclastic cones and coeval faulting of the volcanic substrate are analyzed in the following regions with different structural settings tepic rift mexico ethiopian rift mexican volcanic belt canary archipelago and mount etna field data and analog experiments of tephra cone emplacement and collapse enable the definition of a number of parameters which can be used to infer the geometry of the fracture feeding the magma to a pyroclastic cone the strike of the feeding plane is directly related to 1 the elongation of cone base and crater 2 the location of depressions on the crater rim and 3 the alignment of pyroclastic cones in relation to a given vent spacing in addition the strike and dip of faults affect the direction of cone breaching these relationships are valid for volcanic substrate topographic surfaces with an inclination of less than 9 and are especially sensitive to fault escarpment and cone height lava and cone density and fault orientation with respect to the dip of the volcanic substrate topography relations 1 and 2 become more pronounced for regions undergoing extensional tectonics where edifices also have a larger dimension whereas breaching in the direction of the fault dip is more widespread in regions under extension breaching along the fault strike as well as the coincidence between fault strike and vent alignment are more frequent in regions with transcurrent or transtensional tectonics

giant debris avalanches from the colima volcanic complex mexico implications for long runout landslides 100 km and hazard assessment at least two giant volcanic debris avalanche deposits are associated with the colima volcanic complex located in the western part of the trans mexican volcanic belt one avalanche originated from volcan de colima 3820 m probably 4300 yr ago a much larger avalanche originated from nevado de colima 4240 m 18500 yr ago it traveled more than 120 km from its source and covered an area of at least 2200 km 2 almost twice the area of any previously described avalanche deposit this older debris avalanche is the second largest by volume known has the longest travel distance yet reported and has one of the lowest height length ratios 0 04 the avalanche material was probably hot during emplacement and after initial slope failure may have behaved similarly to a pyroclastic flow such large long runout landslides from volcanic or nonvolcanic constructs present a serious geologic hazard that must be considered in risk assessment

pyroclastic density current from the 1888 phreatic eruption of bandai volcano ne japan abstract the great eruption of july 15 1888 at bandai volcano produced large phreatic explosions and a major debris avalanche toward the northern foot this eruption is characterized as follows 1 there was no significant precursor 2 the eruption and collapse of the volcanic edifice were triggered by an earthquake with a magnitude of about 5 3 the initial explosions were accompanied with a pyroclastic density current 4 the main explosive eruption quickly ceased a few minutes after the eruption began 5 a subsequent late stage plume rose buoyantly to a height of 5000 m falling with a heavy warm rain which generated a lahar and 6 ejected materials were made up wholly of pre existing rock fragments with no juvenile glass shards the main branch of the pyroclastic density current cascaded from the summit for a distance of 6 km along the biwazawa valley on the southeast flank the heim coefficient of the reconstructed energy line slope for the density current is 0 223 to 0 296 the estimated velocity using these values is less than 100 m s the density current deposit consists of normally graded parallel bedded or scour fill cross bedded thin layers of coarse ash and lapilli except for coarse massive breccia near the vent the sedimentary structures and grain size data suggest that the pyroclastic materials were transported in traction saltation and true suspension the estimated velocity is competent for transportation of the deposit the time sequence suggests that the seismic shocks and subsequent collapse of the edifice broke the confined hydrothermal system and caused the phreatic explosions the pressurized hydrothermal fluids had been presumably stored due to self sealing of the system because hydrothermal activities had been vigorous at the summit region until the july 15 seismic shocks collapse of the dry and dense initial eruption column generated the density current which spilled toward the opposite side of the crater along the topographic lows the change from the collapse to the late stage ascending plume results from a decrease in the rock fragment contents which made the plume light and wet this is because that a decrease in the fraction of rock fragments causes an increase in the fraction of steam produced within the plume the occurrence of the water flushed ash fall and lahar during the late stage eruption is consistent with this model

the effect of volcanic constructs on rift fault patterns volcanoes in rift situations often show distinctive hour glass fault patterns with increased fault throw as the volcano is approached we present analogue models that show that this is caused by an interaction of the regional stress field with that set up by the volcano mass for faults to be reorientated there must be a ductile layer below the volcano this can be hot crust at mid ocean ridges continental rifts or weak sedimentary strata increased volcano size or mass and lower brittle ductile ratios lead to increased fault curvature whereas an increased regional extension rate decreases the effect volcanoes on one side of a rift may capture it forming the axis of a new rift with curved or en echelon zones on either side by concentrating extension magma is more easily erupted so volcanoes may erupt more magma of less evolved composition a positive feedback between increased extension and magma eruption rate will lead to rift narrowing which can favor the formation of oceanic crust

plio quaternary volcanism in ecuador extensive sampling major element chemistry on over 300 samples and k ar radiometric dating have been carried out on the ecuadorian upper tertiary quaternary volcanoes the results show important space time variations of the volcanic activity between late miocene time and the present in late miocene time a continuous volcanic belt located approximately along the present volcanic front vf affected the whole country from the cuenca basin to the south up to colombia to the north major changes occurred at about 5 ma volcanic activity stopped south of the guayaquil fault belt and never resumed to the north the active volcanic axis shifted eastward to the cordillera real cr area with a simultaneous relative decrease in intensity since early quaternary time the volcanic belt widened westward to the western cordillera where the volcanism resumed at about 1 5 1 0 ma giving rise to the very wide active volcanic zone of ecuador the plio quaternary products show significant longitudinal and latitudinal chemical and mineralogical changes volcanics of the vf and interandean depression contain amphibole and define a calc alkaline trend with a k 2 o content lower than that of the cr products which are characterized by a mostly anhydrous phenocryst assemblage in both areas andesites dominate but extreme compositions basaltic andesites and rhyolites are more diffuse in the cr than the vf no significant transverse zoning has been detected in the northern region north of the chota mira transverse tectonic line the observed temporal and spatial variations are interpreted as a result of the subduction of the carnegie ridge anomalous oceanic crust underthrusting of which began approximately 6 ma ago

faulting assisted lateral collapses and influence on shallow magma feeding system at ollague volcano central volcanic zone chile bolivia andes abstract the role played by the regional tectonics and relationships between volcanic structures and their shallow magmatic feeding system are of paramount importance in understanding multiple lateral collapse events and thus for hazard assessment in active volcanoes ollague 21 18 s 68 11 w andean central volcanic zone chile bolivia border is an active andesite dacite composite volcano developed on a regional nw striking extensional fault system that bisects the volcanic edifice geological and stratigraphic studies on the summit part of the cone together with new 40ar 39ar dating and petrographic and geochemical major trace elements analyses have allowed us to delineate the volcanic history of ollague and recognize multiple deformation and lateral collapse events the evolution of ollague appears to be characterized by four stages of volcano building separated by three main events of deformation of the cone these deformation events developed progressively by downthrow of the sw sector of the volcano 800 ka along the nw striking normal fault propagating from the substrate to a final catastrophic failure 300 ka of the sw sector and debris avalanche emplacement both nw trending regional tectonics and weakness of the edifice caused by the successive deformation and collapse events are responsible for both the geometric variations of the shallow magma feeding system and the coexistence of summit and parasitic dacitic vents central and lateral vents shifted in a sw trending direction i e perpendicular to the tectonic trend and along the axis of the collapse movement and developed along a nw trending alignment i e parallel to the main tectonic trend and collapse scarps

experiments on the indentation process during cryptodome intrusions new insights into mount st helens deformation scaled experiments were carried out to study the deformation of a volcanic edifice by forcible intrusion of a cryptodome as the magma analogue is injected vertically into a sand cone asymmetric deformation is caused by the formation of a curved major shear fault which dips inward from one side of the cone to the opposite edge of the intrusion the path of the ascending silicone deviates to follow the trajectory of the fault and a lateral bulge grows slowly from the footwall of the fault the oblique push makes the bulge migrate outward causing extension upslope to form an asymmetric graben in the hanging wall of the major shear fault we suggest that the pattern of internal deformation within mount st helens prior to the may 18 1980 eruption was similar to that observed in our scaled models

deformed symmetrical volcanoes analog modeling of volcanic cones traversed by strike slip faulting was conducted and the cones were analyzed for deformation characteristics the study shows that symmetrical volcanoes that have undergone basal strike slip offset may be deformed internally without manifesting any change in their conical shape volcanoes deformed by strike slip faulting may already have well developed fractures in their interior yet still appear as a symmetrical cone exhibiting concentric contours when viewed on a topographic map moreover slight changes in the basal shape of the cone induced by strike slip movement can be restored by the relatively faster resurfacing and reshaping processes from the deposition of younger eruptive products these findings pose a subtle but significant point in the assessment of volcanic landslide hazards not all perfect cones are undisturbed the detection of concealed deformation is important because fractures induce further instability in volcanoes and act as slip planes during volcano collapse events there are many examples of symmetrical volcanoes in nature the faultless appearance of such perfect cones can be misleading which requires careful attention in hazards assessment

extension induced by volcanic loading in regional strike slip zones volcanoes and strike slip pull apart structures are commonly found together we use theoretical predictions and analogue modeling to evaluate the effect of volcanic loading in strike slip zones and the converse effect of regional strike slip faults on volcano structure theoretical predictions indicate that regional stress is reoriented by volcano loading to cause extension analogue models indicate that volcanoes in strike slip zones develop extensional pull apart structures a graben typically forms normal to the minimum regional stress axis 3 at a cone cones emplaced onto preexisting faults increase that fault9s tensional component if the substratum is weak enough to permit volcano spreading thrusts form normal to the maximum stress axis 1 a feedback situation can arise i e loading related extension allows more magma influx more eruption and thus increased loading this process can create large volcano tectonic depressions volcano structures may also be used as indicators of regional strike slip movements

experiments on vertical basement fault reactivation below volcanoes scaled modeling has been conducted to study the role played by the reactivation of vertical basement faults in the destabilization process of overlying volcanoes results show that basement fault reactivation induces the formation of faults within the volcanic edifice these faults delimite a central block which is extruded from between the two undeformed lateral parts of the edifice collapse of this central block can occur when heterogeneous cones are used in experiments revealing that mechanical interfaces are of paramount importance in triggering rockslide avalanches collapse structures may display horseshoe shapes in map view and are then surprisingly similar to avalanche scars resulting from failure created by magmatic intrusions in natural volcanoes large scale destabilisation of half the surface of the cone may also occur in a single event depending on the position of the reactivated basement fault below the edifice it is emphasized that the process under consideration may occur on a dormant volcano as well provided that a fault is reactivated below it

digital photogrammetry as a tool in analogue modelling applications to volcano instability abstract three techniques of digital photogrammetry have been applied successfully to laboratory analogue models to study surface displacements caused by various volcano deformation types firstly side perspective videos are used to differentiate profile displacements between cryptodome intrusion models and models deforming by ductile inner core viscous flow both models show similar morphologic features including a bulged flank and an asymmetric upper graben however differences in displacement trajectories of the bulge crest reflect upward intrusion push contrasting with essentially downward displacement vectors of weak core models the other two techniques use vertical views correlated automatically either as time sequence monoscopic views or as coeval stereoscopic pairs this exploits to a maximum the method s potential by imaging surface displacements over the whole model successive monoscopic photograms because they suffer only moderate numerical processing for topographic effect removal can detect very small displacements occurring early in deformation processes as illustrated by analysis of intrusion models the monoscopic method allows prediction of fault locations and main displacement locations it can also anticipate the principal strain directions and separate different deformation stages on the other hand the stereo photogrammetry technique although more complicated provides topography and volume changes as well as pictures of surface displacements in three dimensions results are presented for the spreading of volcano models on a ductile substratum and viscous cored cones we have found digital photogrammetry to be a useful tool for analogue modelling because it provides quantitative data on surface displacements including movement invisible to the eye as well as topographic changes it is a good method for investigating and comparing different deformation mechanisms it is especially useful for interpretation of displacement patterns obtained from monitoring of natural active volcanoes in fact results of the methods used in the laboratory can be directly compared with field data from geodetic or photogrammetric surveys as at mount st helens in 1980

quaternary eruptive history and hazard zone model at nevado del tolima and cerro machin volcanoes colombia the ice clad and fumarolic nevado del tourna volcano 4 39 n 75 20 w south of nevado del ruiz is offset toward the southeast from the axis of the volcanic ruiz tolima massif with respect to the major ne trending strike slip palestina fault it is composed of four units 1 a pre tolima plateau like basement of basaltic andesite lava flows of early quaternary age 2 a dissected ancestral tourna stratovolcano cut by a presumed collapse caldera of middle pleistocene age 3 an older tolima stratovolcano of late pleistocene age partly destroyed by a summit caldera and 4 composite domes of the cone shaped young and present tolima young tolima volcano is an andesitic and dacitic composite cone formed over the past 40 000 years within a 3 km wide caldera that opened around 0 14 ma deposits of welded and nonwelded pumice and scoria flows were emplaced toward the southeast rio combeima and northeast rio totare repeated growth of lava domes over the past 16 000 years is witnessed by thick block lava flows on the southern and eastern flanks and by block and ash or scoria rich pyroclastic flow deposits this activity occurred during at least six eruptives stages as follows el placer ca 16 200 14 000 yr b p romerales ca 13 000 12 300 yr b p canalones ca 11 500 9750 yr b p mesetas ca 7200 4600 yr b p encanto ca 3600 1700 yr b p and nieves historical interactions with the ice cap probably triggered debris flows that partly filled the combeima and totare valleys and formed the holocene terraces on the upper pleistocene volcaniclastic fans of ibagueand venadillo as much as 60 km from the source the latest major activity was a plinian eruption which deposited a pumice fall layer ca 3600 yr b p 0 5 km3 actual volume mainly toward the west and northwest minor tephra falls and debris flows occurred during the historical period before the reported 1918 and 1943 small phreatic events a general hazard zone map shows areas potentially affected by future eruptions both at nevado del tolima and at active cerro machin 12 km southward the extent of areas likely to be affected by tephra falls debris flows pyroclastic flows or surges debris avalanches and lava flows is shown subplinian and plinian eruptions of nevado del tolima were used to represent the moderate and large events to be expected 300 000 people live within a 35 km distance from those volcanoes which have exhibited a behaviour more explosive than nevado del ruiz despite the small sized ice cap debris flows are the most probable hazard for even a minor eruption because of the very steep slope gradient and because of probable interactions of hot eruptive products with ice and snow additionally scoria flows and debris avalanches can be directed toward the southeast and could be transformed into debris flows that would devastate the combeima valley and suburbs of ibaguecity where about 50 000 people live

formation of clastogenic lava flows during fissure eruption and scoria cone collapse the 1986 eruption of izu oshima volcano eastern japan the 1986 eruption of b fissure at izu oshima volcano japan produced among other products one andesite and two basaltic andesite lava flows locally the three flows resemble vent effused holocrystalline blocky or aa lava however remnant clast outlines can be identified at most localities indicating that the flows were spatter fed or clastogenic the basaltic andesite flows are interpreted to have formed by two main processes a reconstitution of fountain generated spatter around vent areas by syn depositional agglutination and coalescence followed by extensional non particulate flow and b syn eruptive collapse of a rapidly built spatter and scoria cone by rotational slip and extensional sliding these processes produced two morphologically distinct lobes in both flows by a earlier non particulate flow of agglutinate and coalesced spatter which formed a thin lobe of rubbly aa lava ca 5 m thick with characteristic open extension cracks revealing a homogeneous holocrystalline interior and b later scoria cone collapse which created a larger lobe of irregular thickness 20 m made of large detached blocks of scoria cone interpreted to have been rafted along on a flow of coalesced spatter the source regions of these lava flows are characterized by horseshoe shaped scarps 30 m high with meso blocks ca 30 m in diameter of bedded scoria at the base one lava flow has a secondary lateral collapse zone with lower ca 7 m scarps backward tilted meso blocks are interpreted to be the product of rotational slip and forward tilted blocks the result of simple toppling squeeze ups of coalesced spatter along the leading edge of the meso blocks indicate that coalescence occurred in the basal part of the scoria cone this low viscosity coalesced spatter acted as a lubricating layer along which basal failure of the scoria cone occurred rotational sliding gave way to extensional translational sliding as the slide mass spread out onto the present caldera floor squeeze ups concentrated at the distal margin indicate that the extensional regime changed to one of compression probably as a result of cooling of the flow front sliding material piled up behind the slowing flow front and coalesced spatter was squeezed up from the interior of the flow through fractures and between rafted blocks the andesite flow although morphologically similar to the other two flows has a slightly different chemical composition which corresponds to the earliest stage of the eruption it is a much smaller lava flow emitted from the base of the scoria cone 2 days after the eruption had ceased this lava is interpreted to have been formed by post depositional coalescence of spatter under the influence of the in situ cooling rate and load pressure of the deposit extrusion occurred through the lower part of the scoria cone and subsequent non particulate flow of coalesced material produced a blocky and aa lava flow the mechanisms of formation of the lava flows described may be more common during explosive eruptions of mafic magma than previously envisaged

analogue modeling of faulting pattern ductile deformation and vertical motion in strike slip fault zones the relationship between faulting ductile deformation and vertical displacement in provinces of strike slip faults were analyzed using analogue modeling the described experiment considered a two layer small scale model for the lithosphere set on a low viscosity fluid that allowed free isostatic compensation the development of the faults and their orientation follow generally riedel s model but fault spacing depends on the thickness of the brittle layer and lateral displacement of the brittle layer was associated with series of deep elongated depressions that developed along the trace of the principal strike slip fault the depressions correspond to local thinning of the brittle layer and the amount of thinning is more than 60 in places the underlying ductile layer displayed two types of superimposed deformation namely a series of tight parallel folds occurring on top of elongated domes the folds are attributed to the compressive component of the strike slip displacement and the updoming to the extensional component the elongated domes are located beneath the superficial depressions and the deeper troughs underlie shallow uplifted structures in the deformed band significant vertical motion was observed along faults that are considered as perfectly strike slip faults according to the classical riedel model the thinning of the brittle layer and the deep deformation of the ductile layer are in good agreement with actual examples of pull apart basins and elongated swells in large strike slip zones a new structural pattern developed during strike slip shear is proposed relating the geometrical relationships between faults folds and elongated domes to the stress axes

volcanic hazards from bezymianny and bandai type eruptions major slope failures are a significant degradational process at volcanoes slope failures and associated explosive eruptions have resulted in more than 20 000 fatalities in the past 400 years the historic record provides evidence for at least six of these events in the past century several historic debris avalanches exceed 1 km3 in volume holocene avalanches an order of magnitude larger have traveled 50 100 km from the source volcano and affected areas of 500 1500 km2 historic eruptions associated with major slope failures include those with a magmatic component bezymianny type and those solely phreatic bandai type the associated gravitational failures remove major segments of the volcanoes creating massive horseshoe shaped depressions commonly of caldera size the paroxysmal phase of a bezymianny type eruption may include powerful lateral explosions and pumiceous pyroclastic flows it is often followed by construction of lava dome or pyroclastic cone in the new crater bandai type eruptions begin and end with the paroxysmal phase during which slope failure removes a portion of the edifice massive volcanic landslides can also occur without related explosive eruptions as at the unzen volcano in 1792 the main potential hazards from these events derive from lateral blasts the debris avalanche itself and avalanche induced tsunamis lateral blasts produced by sudden decompression of hydrothermal and or magmatic systems can devastate areas in excess of 500km2 at velocities exceeding 100 m s 1 the ratio of area covered to distance traveled for the mount st helens and bezymianny lateral blasts exceeds that of many pyroclastic flows or surges of comparable volume the potential for large scale lateral blasts is likely related to the location of magma at the time of slope failure and appears highest when magma has intruded into the upper edifice as at mount st helens and bezymianny debris avalanches can move faster than 100 ms 1 and travel tens of kilometers when not confined by valley walls avalanches can affect wide areas beyond the volcano s flanks tsunamis from debris avalanches at coastal volcanoes have caused more fatalities than have the landslides themselves or associated eruptions the probable travel distance l of avalanches can be estimated by considering the potential vertical drop h data from a catalog of around 200 debris avalanches indicates that the h l rations for avalanches with volumes of 0 1 1 km3 average 0 13 and range 0 09 0 18 for avalanches exceeding 1 km3 h l ratios average 0 09 and range 0 5 0 13 large scale deformation of the volcanic edefice and intense local seismicity precede many slope failures and can indicate the likely failure direction and orientation of potential lateral blasts the nature and duration of precursory activity vary widely and the timing of slope faliure greatly affects the type of associated eruption bandai type eruptions are particularly difficult to anticipate because they typically climax suddenly without precursory eruptions and may be preceded by only short periods of seismicity

volcano spreading controlled by dipping substrata most volcanoes grow on slopes and some tend to fail catastrophically on the downslope side many volcanoes also deform by volcano spreading which may lead to failure we look at the effect of dipping substrata on the potential for spreading and collapse with analogue models the dip is found to strongly control the spreading style rate and direction a distinct change from purely radial spreading occurs even with small 1 substrata tilt structures on the cone and surrounding area are modified according to the underlying dip direction spreading becomes concentrated on downslope sectors where movement is predominantly in the dip direction the degree of structural realignment is a function of the slope angle our models are applied to previously known and new examples of volcano spreading the effect of dipping substrata in confining spreading to sectors increases the potential for deep seated sector collapses this finding provides a mechanism for failure on the downslope side the incorporation of substrata can create very large volume collapses