the diversity of flow structures in felsic dykes abstract numerous structures and textures which can be related to magma flow were observed in felsic dykes intruding late mesozoic granitoid plutons in the jiaodong peninsula in eastern shandong province eastern china these flow structures may be classified into two categories interior and peripheral the former group includes magmatic bands various types of folds e g injection sheath similar and disharmonic folds rotation of phenocrysts magmatic foliation or lineation and crenulation whereas the latter includes hot tool marks and quarrying structures magmatic banding resulted from shearing of mingled magma during magma flow in the dykes the magma seemed to flow rapidly probably triggering turbulence in some thick dykes interaction at the contact between the hot moving magma and the cold stationary wallrock sometimes produced the peripheral structures a few measurements of hot tool marks and of magmatic lineation reveal a roughly horizontal flow of magma within these dykes for the dominant ne sw striking dyke set in the laoshan granitoid pluton the felsic magma probably ascended on or to the sw of the pluton to feed the dyke swarm and then flowed laterally to drive the horizontal propagation of dykes

geology of the san cristobal volcanic complex nicaragua abstract the san cristobal volcanic complex is located in northwestern nicaragua near the city of chinandego five volcanic centers occur within this group the youngest and tallest volcano is san cristobal which erupted repeatedly throughout the 16th and 17th centuries renewed low level volcanic activity beginning in 1971 continues to the present examination of prehistoric deposits indicates that strombolian to moderately strong vulcanian eruptions are typical of san cristobal with effusion of lava possible from both the summit and lower flanks of the cone of potential volcanic hazards associated with this mountain the most serious to neighboring urban areas is heavy tephra fall the four other volcanoes of the san cristobal complex are in various stages of development and erosion only la casita shows evidence of possible future eruptive activity extrusive rocks of the volcanic complex are dominantly tholeiitic to calc alkaline basalts and andesites showing chemical affinity with island arc lavas dacitic domes and pumice layers are associated with at least two other volcanoes in the complex

controls on caldera structure results from analogue sandbox modeling we conducted scaled analogue sandbox models of caldera formation in order to understand the effects of chamber depth and orientation on the spatial and temporal development of calderas dry sand contained in a 1 m diameter cylinder served as a crustal rock analogue and a water filled 0 6 m diameter rubber bladder served as an analogue magma chamber scaling parameters included a length ratio l of 2 5 10 5 and a stress ratio of 1 8 2 4 10 5 in contrast to some previous analogue models the viscosity of the fluid in the chamber and its withdrawal rate were properly scaled generally deformation began with broad sagging followed by an arcuate or linear outward dipping fault that formed on one side of the caldera this fault propagated laterally around the caldera in both directions sometimes joining other faults and typically forming an overall polygonal structure as subsidence continued the caldera grew incrementally outward and progressively formed a series of concentric outward dipping faults lastly a peripheral zone of extension and pronounced sagging and commonly an inward dipping outer fault related to extension developed at the surface as the depth of the chamber increased 1 the area of faulting decreased 2 the symmetry of the caldera was affected and 3 the coherence of the subsiding block decreased tilting the chamber caused highly asymmetric subsidence to occur in this case faults formed first where the bladder was shallowest subsidence then shifted rapidly to where the bladder was deepest producing an elongate trapdoor caldera that was deepest where the bladder was deepest our experiments highlight the roles of sagging and faulting during caldera subsidence surface fault patterns both in our experiments and at natural calderas are frequently not circular the aspect ratio of the block above the magma chamber controls the shape of the caldera which is frequently polygonal the faults at natural calderas determine locations and migration of eruptive vents the degree of subsidence the style of postcal dera resurgent magmatism and the extent of hydrothermal circulation our experiments reveal details of how calderas grow outward incrementally and demonstrate that asymmetric subsidence along linear and arcuate faults is common to many calderas

how summit calderas collapse on basaltic volcanoes new insights from the april 2007 caldera collapse of piton de la fournaise volcano in april 2007 piton de la fournaise volcano experienced a caldera collapse during its largest historical eruption we present here a structural analysis both of the caldera and the surrounding area and precise gps data recorded with a dense gps network specifically dedicated to the analysis of deformation related to the summit collapse structures despite a collapse of more than 300 m in the central zone the geometry of the new caldera is similar in map view to that of the pre existing collapsed structure which was formed from the coalescence of several pit craters the caldera shows an asymmetric inner geometry with sub vertical walls in the nw quadrant and steep scarps composed of inward tilted blocks in the southern half the presence of preserved polished surfaces on the lower part of the sub vertical scarp indicates that it corresponds to the caldera north western ring fault the april 2007 caldera collapse led to the development and the reactivation of concentric fractures on the caldera rim mostly along the southern limit of the caldera gps data show that fractures result from radial extensional stresses that are restricted within the first tens of meters of the caldera edge gps data also reveal that the caldera collapse was coeval with a centripetal deflation whose magnitude is largest along the southern half of the caldera the displacements recorded by gps result from both a general deflation due to magma withdrawal from piton de la fournaise s summit magma chamber and additional local effects related to the caldera collapse comparison of the caldera collapses at piton de la fournaise miyakejima and fernandina reveals striking similarities with cyclic seismic signals accompanying small scale deflation inflation cycles this strongly suggests a common mode of collapse hence we propose a unifying model of caldera collapse in basaltic setting in which the inward deflation due to magma withdrawal from the magma chamber prevents the collapse of the caldera roof until the gravitational stress acting on the rock column above the magma chamber exceeds the shear strength along pre existing ring faults the downward displacement stops when the pressure increase into the magma chamber is able to again sustain the rock column the succession of 1 inward deflation that prevents the collapse 2 collapse due to gravitational stress and 3 stopping of the downward motion is repeated many times the frequency of the cycles is influenced by the rate of magma withdrawal and by the amount of intrusion of magma along the ring faults

chronology and products of the 2000 eruption of miyakejima volcano japan lateral migration of magma away from miyakejima volcanic island japan generated summit subsidence associated with summit explosions in the summer of 2000 an earthquake swarm beneath miyakejima began on the evening of 26 june 2000 followed by a submarine eruption the next morning strong seismic activity continued under the sea from beneath the coast of miyakejima to a few tens of kilometers northwest of the island summit eruptive event began with subsidence of the summit on 8 july and both explosions and subsidence continued intermittently through july and august the most intense eruptive event occurred on 18 august and was vulcanian to subplinian in type ash lofted into the stratosphere fell over the entire island and abundant volcanic bombs were erupted at this time another large explosion took place on 29 august this generated a low temperature pyroclastic surge which covered a residential area on the northern coast of the island the total volume of tephra erupted was 9 3 106 m3 dre much smaller than the volume of the resulting caldera 6 108 m3 migration of magma away from miyakejima was associated with crustal extension northwest of miyakejima and coincident shrinkage of miyakejima island itself during july august 2000 this magma migration probably caused stoping of roof rock into the magma reservoir generating subsurface cavities filled with hydrothermal fluid and or magmatic foam and formation of a caldera oyama caldera at the summit interaction of hydrothermal fluid with ascending magma drove a series of phreatic to phreatomagmatic eruptions it is likely that new magma was supplied to the reservoir from the bottom during waning stage of magma s migration resulting in explosive discharge on 18 august the 18 august event and phreatic explosions on 29 august produced a conduit system that allowed abundant so2 emission as high as 460 kg s 1 after the major eruptive events were over at the time of writing inhabitants of the island about 3 000 have been evacuated from miyakejima for more than 3 years

a new shipboard three component magnetometer the three components of the geomagnetic field were measured at sea by stcm shipboard three component magnetometer with a relative accuracy of 50 25 nt the stcm was controlled by a microcomputer and the three geomagnetic components were calculated by reducing the magnetic field due to the ship s remanent and induced magnetic moments the only assumption being that the magnetic field is linearly related to the magnetic moments although stcm has no particular magnetic compensation device it can be set at any place on board of any kind of ship in the kh82 5 cruise the three component measurements were conducted along latitude 42 n north of the mendocino fracture zone where there are well defined magnetic anomaly lineations trending north and south stcm measured the north component anomalies with very small amplitudes compared to the east and downward component anomalies which shows that lineations trend north and south the three measured component anomalies are almost identical to the anomalies

how the development of magma chambers affects collapse calderas insights from an overview here we review the studies of pluton emplacement and caldera collapse proposing a model linking the two processes the shallow rise of magma occurs by dyking and its emplacement occurs along major anisotropies many plutons are emplaced at subhorizontal discontinuities forming sills these will eventually grow forming laccoliths the most common mechanism to store shallow magma calderas are a surface expression of these reservoirs and can be approximated by a piston cylinder which sinks due to 1 underpressure in the reservoir producing outward dipping reverse faults 2 overpressure within a silllike chamber undergoing doming and 3 overpressure within a laccolith generating apical tensile stresses in the latter two cases inward dipping normal faults form we suggest that collapse calderas are the surface expression of pressure variations within laccoliths or tabular intrusions their geometric relationships depend on the shape and aspect ratio of the intrusion and its pressure conditions tabular intrusions generating ring faults mainly at their tips form calderas with a similar width to the intrusion laccoliths may generate ring faults along the intrusion roof forming calderas narrower than the intrusion the outward or inward dip of the caldera faults results from the underpressure or overpressure conditions within the

asymmetric growth of collapsed caldera by oblique subsidence during the 2000 eruption of miyakejima japan abstract oblique development of the ring faults reflecting the structural heterogeneities inside the volcano formed many asymmetric structures of miyakejima 2000 ad caldera the asymmetry includes a offset location of the ring faults with respect to the associated shallow magma chamber b unequal outward migration of the caldera wall 600 m at the southeastern rim but only 200 m at the northwestern rim c development of tilted terrace only at the southeastern caldera margin d eruption sites and fumaroles being confined to the southern part of the caldera geophysical data including ground deformation and seismic activity indicates the offset of the location of the magma chamber about 2 km south of the caldera center on the surface the ring faults propagated from the deflating magma chamber obliquely about 30 degrees toward the summit the oblique subsidence of the cylindrical block formed a wider instable zone particularly in the southeastern side of the ring fault that enhanced the larger outward migration of the caldera rim and also caused the formation of the outer half ring fault bordering the tilting slope at the southern part ascending pass of the buoyant magma along the tilted ring faults was concentrated in the southern half of the caldera and consequently the distributions of the eruption sites and fumaroles are localized in the southern half part of the caldera the structure of the miyakejima 2000 caldera with complete development of the ring faults its high roof aspect ratio and oblique subsidence is clearly distinguishable from trapdoor type caldera the oblique development of the ring faults can be controlled by the mechanical contrast between the solidified conduits and surrounding fragile volcanic edifice asymmetric development of the miyakejima caldera shows that the collapsed calderas are potential indicators of the heterogeneous structures inside of the volcano particularly in the case of small size caldera

understanding caldera structure and development an overview of analogue models compared to natural calderas abstract understanding the structure and development of calderas is crucial for predicting their behaviour during periods of unrest and to plan geothermal and ore exploitation geological data including that from analysis of deeply eroded examples allow the overall surface setting of calderas to be defined whereas deep drillings and geophysical investigations provide insights on their subsurface structure collation of this information from calderas worldwide has resulted in the recent literature in five main caldera types downsag piston funnel piecemeal trapdoor being viewed as end members despite its importance such a classification does not adequately examine a the structure of calderas particularly the nature of the caldera s bounding faults and b how this is achieved including the genetic relationships among the five caldera types various sets of analogue models specifically devoted to study caldera architecture and development have been recently performed under different conditions apparatus materials scaling parameters stress conditions the first part of this study reviews these experiments which induce collapse as a result of underpressure or overpressure within the chamber analogue the experiments simulating overpressure display consistent results but the experimental depressions require an exceptional amount of doming seldom observed in nature to form therefore these experiments are not appropriate to understand the structure and formation of most natural calderas the experiments simulating underpressure reveal a consistent scenario for caldera structure and development regardless of their different boundary conditions these show that complete collapse proceeds through four main stages proportional to the amount of subsidence progressively characterized by 1 downsag 2 reverse ring fault 3 peripheral downsag 4 peripheral normal ring fault the second part of this study verifies the possibility that these latter calderas constitute a suitable analogue to nature and consists of a comprehensive comparison of the underpressure experiments to natural calderas this shows that all the experimental structures as well as their progressive development are commonly observed at natural calderas highlighting a consistency between models and nature as the shallow structure of experimental calderas corresponds to a precise architecture at depth it provides a unique key to infer the deeper structure of natural calderas recognizing diagnostic surface features within a caldera will thus allow it to be categorized within a precise structural and evolutionary context the general relationship between the evolutionary stage of a caldera and its d s diameter subsidence ratio allows such a quantification with stage 1 calderas characterized by d s 40 stage 2 by 18 the four stages adequately explain the architecture and development of the established caldera end members along a continuum where one or more end members downsag piston funnel piecemeal trapdoor may correspond to a specific stage while such a continuum is controlled by progressive subsidence specific collapse geometries will result from secondary contributory factors roof aspect ratio collapse symmetry pre existing faults these considerations allow proposing an original classification of calderas incorporating their structural and genetic features

the pit craters and pit crater filling lavas of masaya volcano lava flowing into a pit crater will become entrapped to form an inactive lava lake at masaya volcano nicaragua pit filling lavas are exposed in the walls of nindiri santiago and san pedro pits mapping of these lavas shows that fill can involve emplacement of both a a and pahoehoe with single fill units ranging in thickness from 2 to 22 m thick units with columnar joints were emplaced as simple inactive lava lakes during high effusion rate episodes sequences of thinner units which can form pit floor shields or compound lakes were emplaced at lower effusion rates lava withdrawal caused unsupported sections of three 20 m thick units to subside resulting in unit flexure and faulting and viscous peeling features reveal that subsidence occurred while at least one unit was still partially molten where withdrawal has not occurred fill sequences are flat lying and symmetrically distributed around the feeder structures cinder cones and dykes the filled nindiri pit holds 5 107 m3 of lava in a 215 m thick sequence partial fill of santiago pit with 1 107 m3 of lava has filled the pit with a 110 m thick lava sequence of which 50 has been consumed by formation of a secondary pit altogether 6 4 107 m3 of lava was erupted into nindiri and santiago during 1525 1965 with 94 of this volume remaining pit contained the remainder forms a north flank lava flow field pit development and filling is a dynamic and ephemeral process having short lived effects on volcano morphology where pits develop and fill over hours to centuries however pits play an important role in shaping an edifice representing lava sinks and controlling whether lavas are trapped or able to spread onto the flanks

volcanic and hydrothermal processes in submarine calderas the kulo lasi example sw pacific abstract the study area is located at the transition between the northern end of the tonga trench and the north fiji fracture zone where tectonic movements are reputed to be the fastest in the world to the southeast of futuna island a broad area of volcanism occurs within a region characterized by a change in the tectonic fabric between a ne sw oriented volcanic graben and the n s oriented alofi ridge in 2010 the active volcano kulo lasi which represents the most recent volcanic episode in the futuna area was discovered in the center of this extensive volcanic zone kulo lasi is a 20 km diameter shield volcano that rises 400 m above the seafloor it is composed of basaltic to trachy andesitic lava with no obvious geochemical affinity with the tonga subduction that occurs 500 km to the east the central caldera is 5 km in diameter and 300 m deep and is located at a water depth of 1500 m diving operations with the submersible nautile and high resolution auv mapping have revealed the presence of numerous active and inactive hydrothermal fields on the floor and the walls of the caldera four tectono volcanic stages can be distinguished at kulo lasi caldera in stage 1 the shield volcano is built annular reverse faults develop at the summit and control circulation of water rock dominated hydrothermal fluids and high temperature alteration of rocks along the nascent normal faults mixing of hydrothermal fluids with seawater is favored along normal superficial faults leading to the formation of low temperature fe mn mineralization at the summit of the volcano during stage 2 the caldera collapse gradually revealing outcrops of the altered and mineralized zones formed during stage 1 as the magma chamber cools and collapses less heat is available as a result medium to low temperature

complexities in the tectonic evolution of the west philippine basin abstract the oceanic west philippine basin can be subdivided into a main basin a northwest sub basin and a southern sub basin these basins have undergone different tectonic histories magnetic anomalies 21 17 are present in the main basin and indicate that it formed by two limbed seafloor spreading the now extinct spreading center is located within a zone of rough topography associated with the northwest trending central basin fault cbf the morphology of the cbf is primarily the result of post spreading tectonism the magnetic anomaly pattern of the main basin does not extend into the southern sub basin or the northwest sub basin geometrical constraints indicate that the southern sub basin did not form as part of the southern limb of the main basin the southern sub basin may have formed immediately before a ridge jump to the main basin spreading axis alternatively possible shallower crustal depth in the southern sub basin may indicate that the sub basin is younger than the main basin and formed separately from it in the northwest sub basin which originated as part of the main basin the dominant topographic trends are perpendicular to those in the main basin this difference as well as higher heat flow and shallower depth in the northwest sub basin indicate that it has undergone a deformational event which did not affect the main basin this deformation may be related to subduction along the east luzon margin in mid tertiary time the gagua ridge an uplifted sliver of oceanic crust along the western edge of the northwest sub basin may have originated during this event

stability and instability of quiescently active volcanoes the case of masaya nicaragua quiescently active volcanoes are enigmatic due to their restlessness but lack of eruptive activity i present a model of coupled conduit convection and foam accumulation to explain degassing behavior of masaya an active volcano in nicaragua that is currently emitting large amounts of gas but not erupting gas rich magma is transported through a conduit 2 6 m in radius and then released into a shallow reservoir the magma is degassed in the reservoir and forms a foam 1 3 m thick at the top with bubbles 20 60 m in diameter the foam layer is stable because the input of gas into the foam is balanced by gas release through a vent to the surface if the foam layer is destabilized the volcano can erupt explosively the most likely cause of foam destabilization is a large injection of volatile rich magma from deeper levels into the shallow conduit reservoir system thereby increasing magma fluxes and gas fluxes exsolving large gas bubbles and reducing surface tension of the magma

vector magnetic analysis within the southern ayu trough equatorial western pacific summary we present the analysis of magnetic field data of the ayu trough a divergent plate margin located in the southern philippine sea plate the survey which mapped both axial and offaxis regions of the southern ayu trough recorded the total field using a proton precession magnetometer towed behind the ship and the vector magnetic field using a shipboard threecomponent fluxgate magnetometer together with multibeam bathymetry the two magnetic field anomalies show good agreement indicating that the reduction of vector field data was successful our inspection of e w profiles shows that there is little correlation in magnetic anomalies between adjacent profiles and lack of symmetry with respect to the trough axis however a careful examination of gridded magnetic anomaly map reveals that symmetry may be found to the north of 1 n along n120e which is roughly the direction of spreading at this region as revealed by the fine scale seafloor fabric by our survey vector magnetic anomalies exhibit important differences with strike of magnetic boundaries within the inner region between tobi and mapia ridges trending predominantly n s whereas in the outer region they are almost e w this finding suggests that the outer region appears to retain the magnetization and crustal structure of the old west philippine basin while the inner region is inherited from the e w seafloor spreading in the ayu trough the short along axis segmentation and complex seafloor morphology caused by slow oblique spreading may explain the lack of symmetry in the magnetic anomalies with respect to the trough axis and a wider occurrence of 3 d magnetic source crust roughly to the south of 1 n

comprehensive analysis of marine magnetic vector anomalies we present a comprehensive description of the analysis of marine magnetic vector anomalies the marine magnetic vector field can be measured with a shipboard three component magnetometer stcm the stcm has the advantage of measuring three components but it suffers from several types of noise sources because the principles of its measurement are more complex than that of a proton precession magnetometer we have estimated the quality of stcm data constructed an optimal filter and evaluated the reliability of the filtered data then we have organized an analytical procedure composed of four steps 1 the discrimination between two and three dimensional magnetic sources 2 the determination of magnetic boundaries 3 the calculation of the boundary strikes and 4 the estimation of the boundary magnetization contrasts we show examples of these procedures using the data near the east pacific rise 28 s 31 s collected during leg 5 of the gloria expedition all the results of the above processing are merged in order to illustrate the distinct pattern of the seafloor magnetization in the surveyed area

mechanical and geometric controls on the structural evolution of pit crater and caldera subsidence 1 pit craters and calderas are volcanic depressions produced by subsidence of a magma reservoir roof to identify how geometric and mechanical factors may influence the structural evolution of this subsidence we used two dimensional distinct element method numerical models the reservoir host rock was represented as an assemblage of bonded circular particles that interact according to elastic frictional laws varying particle and bond properties produced a range of bulk material properties characteristic of natural rock masses fracturing results when bonds break once their shear or tensile strength is exceeded the magma reservoir was represented as a region of nonbonded low friction particles withdrawal of magma was simulated by incrementally reducing the area of the reservoir particles resultant gravity driven failure and subsidence of the reservoir roof were explicitly replicated interaction of the roof s strength young s modulus thickness diameter ratio t d and the reservoir s shape yields a variety of model structures and subsidence styles in conceptual terms four end member subsidence styles developed 1 central sagging favored by low strength and low t d 2 central snapping favored by high strength low t d and a sill like reservoir shape 3 single central block favored by low to intermediate strength high young s modulus and intermediate t d and 4 multiple central blocks favored by high strength low young s modulus and high t d most model realizations incorporated some combination of each style however the models provide a geomechanical framework for understanding natural pit crater or caldera structures as at nindiri nicaragua fernandina galapagos dolomieu la reunion and miyakejima japan

the structure and formation of mud volcano summit calderas circular depressions bounded by inward dipping faults found at the upper terminations of large mud volcano systems 500 m diameter are termed mud volcano summit calderas from new mapping and comparison with previously identified examples we describe a series of common structural and morphological features found at a number of calderas and propose a mechanism for caldera formation a typical example consists of concentric deformational and volcanic zones including an outermost topographic rim inward dipping circular fault system moat and raised central pedestal of freshly extruded mud volcanic sediment this distinctive moat and pedestal morphology can be explained in terms of the quantity and rheology of extruded mud and appears characteristic of calderas from the south caspian basin and elsewhere the association of fresh mud volcanic deposits with the calderas in combination with extensive literature on caldera modelling leads us to conclude that the most likely causal mechanism is subsidence as a result of evacuation of fluids and sediment from shallow structural levels during eruptions summit calderas mapped in this study characterize dormant periods of mud volcanism in subaerial and submarine settings and appear to be a common structural element in the extrusive domain of mud volcano systems

comparison of gravity anomaly between mature and immature intra oceanic subduction zones in the western pacific abstract the western pacific hosts major subduction systems such as izu bonin mariana and tonga kermadec but also less conspicuous systems such as yap mussau and hjort trenches which constitute the young incomplete or ultraslow member in the evolutionary spectrum of subduction zones we used satellite derived gravity data to compare well developed and immature subduction systems it is shown that at spatial resolution 10 20 km or so the satellite data have accuracy comparable to ship board gravity measurements over intra oceanic subduction zones in the isostatic residual gravity anomaly map the width of non isostatically compensated region of the mature subduction zones is much wider than that of immature ones more importantly when the gravitational attraction due to seafloor is removed a large difference exists between the mature and immature subduction zones in the overriding plate side mature subduction zones exhibit broad low gravity anomalies of 200 250 mgal centered at distances of 150 200 km from the trench which are not found over immature subduction zones the cause of the broad low gravity anomalies over mature subduction zones is debatable due to lack of information on the deep crust and upper mantle structure and property we discuss the following four causes 1 serpentinization of the upper mantle beneath the forearc 2 presence of partial melt in the mantle wedge caused by release of volatiles from the slab frictional heating and distributed by mantle circulation 3 difference in density structure between the overriding and subducting plates caused by difference in age and thermal structures with and without compositional stratification between crust and mantle and 4 anomalous thickness of the arc not explained by isostasy our analysis suggests that serpentinization cannot explain the observed gravity anomaly which appears 150 200 km from the trench although the extent and distribution of partial melt within the mantle wedge remain in question to our best estimate partial melting contributes little

gravity structure of piton de la fournaise volcano and inferred mass transfer during the 2007 crisis abstract the structure and mass transfers at piton de la fournaise volcano are investigated using new on land and offshore measurements the structures are defined using 2 and 3d modelling short wavelength positive anomalies to the w and nw of the central area are interpreted as shallow piles of thick dense lava flows which have filled volcano tectonic depressions several hundreds of meters deep this finding provides a new insight into the evolution of the volcano conversely the currently active central zone is associated with a negative short wavelength anomaly explained by the presence of thin highly vesiculated and fractured lava flows building the central cone low density formations underlying the rivieres des remparts and langevin areas are tentatively interpreted as breccias related to erosion and or to southward directed mass wasting topographic offshore highs in the continuation of the se and ne rift zones of piton de la fournaise are characterized by negative gravity anomalies and reversed magnetic anomalies their interpretation as hyaloclastites and pillow lavas provides a coherent explanation for both types of signals if we admit that they were primarily constructed during the alizes stage two main deeper dense structures are also found the grand brule complex and a complex beneath the plaine des sables and the enclos fouque both are identified as hypovolcanic intrusive complexes the grand brule complex 10 18 km in plan view shows a marked ns elongated geometry unlike the other comparable complexes on the island the plaine des sables enclos fouque complex a more or less circular structure 5 km in diameter is attributed to the ancient shield stage it is centered on the enclos fault suggesting that the paleo center of the ancient shield was located about 1 5 km eastward of the previously proposed location an apparent contradiction was noted between the seismic and gravity images of the internal structure in particular a high velocity cylinder beneath the summit was not detected in the gravity models this contradiction was resolved by considering the relative sensitivity of each method finally the gravity dataset has allowed us to explore the gravity changes that accompanied the major eruptive and volcano tectonic crisis of april 2007 350 m collapse of the summit and emission of 120 106 m3 of lava the change in the mass distribution of the edifice was large enough to be detected in the survey data collected in the central area before and after the crisis however the accuracy was too low for the precise assessment of the mass transfers

the april 2007 eruption and the dolomieu crater collapse two major events at piton de la fournaise la reunion island indian ocean abstract after 10 years of high activity and 24 eruptions piton de la fournaise pdf produced on april 2 to may 1 2007 one of its most voluminous and intense eruption since at least one century the eruption focused at 590 m elevation in the grand brule on the south east flank of piton de la fournaise 7 km away from summit it was located close to the southern cliff of the grand brule volcano tectonic collapse the rempart du tremblet and to the nearby village called le tremblet and it highly rattled and incommoded the inhabitants there eleven hours after the beginning of the eruption the lava flows reached the sea and created 0 45 km 2 of new land until the end of eruption in the night of april 5 the rock column beneath dolomieu crater started to collapse with a piston like mechanism into the magma chamber forming within less than 24 h the most recent and well documented caldera we report in this paper the chronology of these two related major events in the recent activity of pdf seen by the scientific networks of the volcanological observatory ovpf ipgp and field observations which allowed to back up the scenario of the caldera formation and to quantify these two exceptional episodes

volcanic craters pit craters and high level magma feeding systems of a mafic island arc volcano ambrym vanuatu south pacific

morphological studies of the ayu trough philippine sea caroline plate boundary the ayu trough lies on the southern boundary between the philippine sea plate and caroline plate although this trough may be the best place to study the evolution and kinematics of the philippine sea plate the origin and evolution of this trough are poorly understood our geophysical and morphological surveys in the northern part of the ayu trough revealed that the trough shares morphological similarities with slow spreading lid ocean ridges the seafloor ages and an average spreading rate of the trough were inferred from the avcrage length of the ridge segments distribution of sediment thickness and basement subsidence based on the seismic section at 3 30 n the opening of the trough started about 25 ma with an average half spreading rate 4 1 mm yr this spreading rate is much slower than some previous estimates the present magma production rate and activity of the trough is estimated to be even lower than the 25 m y average the seafloor depth of the trough axis and relief of the axial valley are approximately twice as deep as that of active lid ocean ridges the trough axis valley is covered with sediment in contrast with the axial valleys of active lid ocean ridges sedimentation at the trough axis shows that the spreading rate has been quite slow for the last 2 5 ma

geomorphology and structural development of the nested summit crater of lascar volcano studied with terrestrial laser scanner data and analogue modelling abstract many volcano summits host craters that are partially overlapping the formation of such nested craters has been commonly interpreted as vent migration here we present an additional mechanism that may explain the geometry of nested craters at volcanoes lascar volcano the most active volcano of the central volcanic zone in the chilean andes hosts ene wsw trending summit craters that are partially overlapping nested details on the evolution and interaction between the different craters remain unclear to create a robust dataset terrestrial laser scanner tls data were collected at the summit of lascar in 2013 the resulting topographic data set consisting of more than 15 million data points with centimetre sampling allows visualising almost the complete eastern edifice of the volcano s summit from the tls data a digital elevation model dem and a slope map were generated allowing us to create a lineament map and quantify the observed morphological and structural features to further improve our understanding of the processes responsible for the formation of the craters and geomorphology we designed sandbox analogue models results suggest that one of the craters is a parasite crater formed as a consequence of ongoing activity in the adjacent crater our data suggest that the nested craters have all been modified since the last major eruption in 1993 by near surface effects associated with cooling compaction and gravitational sliding of the crater floor infill as the active crater deepens the adjacent inactive crater extends and partially slumps towards the active one understanding the structural development of these nested craters is relevant for assessing potential future eruption sites thus making lascar a dynamic target for a detailed morphology study these findings may similarly be applied to other volcanoes where nested craters have developed

explosive volcanism vei 6 without caldera formation insight from huaynaputina volcano southern peru through examination of the vent region of volcan huaynaputina peru we address why some major explosive eruptions do not produce an equivalent caldera at the eruption site here in 1600 more than 11 km3 dre vei 6 were erupted in three stages without developing a volumetrically equivalent caldera fieldwork and analysis of aerial photographs reveal evidence for cryptic collapse in the form of two small subsidence structures the first is a small non coherent collapse that is superimposed on a cored out vent this structure is delimited by a partial ring of steep faults estimated at 0 85 by 0 95 km collapse was non coherent with an inwardly tilted terrace in the north and a southern sector broken up along a pre existing local fault displacement was variable along this fault but subsidence of approximately 70 m was found and caused the formation of restricted extensional gashes in the periphery the second subsidence structure developed at the margin of a dome the structure has a diameter of 0 56 km and crosscuts the non coherent collapse structure subsidence of the dome occurred along a series of up to seven concentric listric faults that together accommodate approximately 14 m of subsidence both subsidence structures total 0 043 km3 in volume and are much smaller than the 11 km3 of erupted magma crosscutting relationships show that subsidence occurred during stages ii and iii when 2 km3 was erupted and not during the main plinian eruption of stage i 8 8 km3

hydrothermal fluid venting in the offshore sector of campi flegrei caldera a geochemical geophysical and volcanological study the ongoing unrest at the campi flegrei caldera cfc in southern italy is prompting exploration of its poorly studied offshore sector we report on a multidisciplinary investigation of the secca delle fumose sdf a submarine relief known since antiquity as the largest degassing structure of the offshore sector of cfc we combined high resolution morpho bathymetric and seismo stratigraphic data with onshore geological information to propose that the present day sdf morphology and structure developed during the initial stages of the last cfc eruption at monte nuovo in ad 1538 we suggest that the sdf relief stands on the eastern uplifted border of a n s trending graben like structure formed during the shallow emplacement of the monte nuovo feeding dike we also infer that the high angle bordering faults that generated the sdf relief now preferentially allow the ascent of hot brines with an equilibrium temperature of 179 c thereby sustaining hydrothermal degassing on the seafloor systematic vertical seawater profiling shows that hydrothermal seafloor venting generates a sizeable co2 ph and temperature anomaly in the overlying seawater column data for the seawater vertical profile can be used to estimate the co2 and energy heat outputs from the sdf area at 50 tons day 0 53 kg s and 80mw respectively in view of the cause effect relationship with the monte nuovo eruption and the substantial gas and energy outputs we consider that the sdf hydrothermal system needs to be included in monitoring programs of the ongoing cfc unrest this article is protected by copyright all rights reserved

the november 2002 eruption at piton de la fournaise volcano la reunion island ground deformation seismicity and pit crater collapse an eruption on the eastern flank of piton de la fournaise volcano started on 16 november 2002 after 10 months of quiescence after a relatively constant level of activity during the first 13 days of the eruption lava discharge volcanic tremor and seismicity increased from 29 november to 3 december lava effusion suddenly ceased on 3 december while shallow earthquakes beneath the dolomieu summit crater were still recorded at a rate of about one per minute this unusual activity continued and increased in intensity over the next three weeks ending with the formation of a pit crater within dolomieu based on ground deformation measured by rapid static and continuous gps and an extensometer seismic data and lava effusion patterns the eruptive period is divided into five stages 1 slow summit inflation and sporadic seismicity 2 rapid summit inflation and a short seismic crisis 3 rapid flank inflation onset of summit deflation sporadic seismicity accompanied by stable effusion 4 flank inflation coupled with summit deflation intense seismicity and increased lava effusion and finally 5 little deflation intense shallow seismicity and the end of lava effusion we propose a model in which the pre intrusive inflation of stage 1 in the months preceding the eruption was caused by a magma body located near sea level the magma reservoir was the source of an intrusion rising under the summit during stage 2 in stage 3 the magma ponded at a shallow level in the edifice while the lateral injection of a radial dike reached the surface on the eastern flank of the basaltic volcano causing lava effusion pressure decrease in the magmatic plumbing system followed resulting in upward migration of a collapse front forming a subterranean column of debris by faulting and stoping this caused intense shallow seismicity increase in discharge of lava and volcanic tremor at the lateral vent in stage 4 and eventually the formation of a pit crater in stage 5

depth determinations of shallow hydrothermal systems by self potential and multi scale wavelet tomography in hydrothermal studies the depth of the hydrothermal system is always required but rarely known via traditional geophysical exploration techniques while previous studies have shown that continuous wavelet transform algorithms applied to self potential data can theoretically determine the depth of the hydrothermal fluids this study uses multi scale wavelet tomography with multiple wavelets field measurements and geophysical models to accurately determine this depth on stromboli waita and masaya volcanoes multi scale wavelet tomography of field measurements gives reproducible depth results supported by independent geophysical measurements and models and accurately locates the main water flow paths at shallow depths unlike other traditional geophysical methods multi scale wavelet tomography using self potential data is a low cost tool to rapidly determine depths of the shallowest hydrothermal structures this approach has the potential to significantly enhance our ability to locate geothermal systems and monitor active volcanoes

relative scale and the strength and deformability of rock masses abstract the strength and deformation of rocks depend strongly on the degree of fracturing which can be assessed in the field and related systematically to these properties appropriate mohr envelopes obtained from the rock mass rating rmr classification system and the hoek brown criterion for outcrops and other large scale exposures of fractured rocks show that rock mass cohesive strength tensile strength and unconfined compressive strength can be reduced by as much as a factor often relative to values for the unfractured material the rock mass deformation modulus is also reduced relative to young s modulus a cook book example illustrates the use of rmr in field applications the smaller values of rock mass strength and deformability imply that there is a particular scale of observation whose identification is critical to applying laboratory measurements and associated failure criteria to geologic structures

on the emplacement of the voisey s bay intrusion labrador canada the mesoproterozoic voisey s bay intrusion is a small 10 km3 mafic intrusive complex forming part of the 20 000 km2 nain plutonic suite of labrador canada and hosting a world class orthomagmatic ni cu sulfide ore deposit an extensive database of drill core and down hole televiewer data provides a wealth of subsurface three dimensional information on the intrusion the intrusion consists of the upper eastern deeps chamber and lower western deeps chamber connected via a complex network of dikes the eastern deeps chamber can be divided into six units that are distinguished based on geometrical styles dominant lithologies and the occurrence of bodies of wall rock partly separating units unit i occurring at the highest levels of the chamber is tabular and consists dominantly of olivine gabbro with minor normal textured orthocumulate troctolite units at midlevels of the eastern deeps chamber iia iib iic are sill shaped bodies consisting of normal troctolite i e orthocumulate exhibiting tiling of plagioclase laths with local variably textured troctolite i e mesocumulate locally pegmatitic locally dynamically recrystallized units from the lowermost level iiia iiib are irregularly shaped and consist of variable troctolite breccias containing centimeter sized xenoliths of host rock paragneiss and strongly sulfide mineralized rocks at least three different feeder dikes deliver magmas associated with unit iiib the overall dike system has an upward curving geometry such that the dike dips shallow upward and as they approach the eastern deeps chamber this is analogous to fault geometries produced during caldera collapse or cauldron subsidence evolving emplacement styles reflect internal igneous processes and intermittent normal sense activity along pre to synemplacement brittle wall rock structures high temperature normal sense shear zones are prominent within the chamber within 100 m of its basal contact and are interpreted to have formed in response to late stage normal faulting of the chamber floor the geometry and internal structure of the eastern deeps chamber are indicative of top down incremental emplacement associated with floor subsidence our findings provide a kinematic and dynamic framework for petrogenetic models of voisey s bay and other intrusion hosted ni cu sulfide deposits

philippine sea and east asian plate tectonics since 52 ma constrained by new subducted slab reconstruction methods we reconstructed philippine sea and east asian plate tectonics since 52 ma from 28 slabs mapped in 3 d from global tomography with a subducted area of 25 of present day global oceanic lithosphere slab constraints include subducted parts of existing pacific indian and philippine sea oceans plus wholly subducted proto south china sea and newly discovered east asian sea mapped slabs were unfolded and restored to the earth surface using three methodologies and input to globally consistent plate reconstructions important constraints include the following 1 the ryukyu slab is 1000 km n s too short to account for 20 philippine sea northward motion from paleolatitudes 2 the marianas pacific subduction zone was at its present location 200 km since 48 10 ma based on a 1000 km deep slab wall 3 the 8000 2500 km east asian sea existed between the pacific and indian oceans at 52 ma based on lower mantle flat slabs 4 the caroline back arc basin moved with the pacific based on the overlapping coeval caroline hot spot track these new constraints allow two classes of philippine sea plate models which we compared to paleomagnetic and geologic data our preferred model involves philippine sea nucleation above the manus plume 0 150 e near the pacific east asian sea plate boundary large philippine sea westward motion and post 40 ma maximum 80 clockwise rotation accompanied late eocene oligocene collision with the caroline pacific plate the philippine sea moved northward post 25 ma over the northern east asian sea forming a northern philippine sea arc that collided with the sw japan ryukyu margin in the miocene 20 14 ma

stable and unstable phases of elevated seismic activity at the persistently restless telica volcano nicaragua abstract telica volcano nicaragua is a persistently restless volcano with daily seismicity rates that can vary by orders of magnitude without apparent connection to eruptive activity low frequency lf events are dominant and peaks in seismicity rate show little correlation with eruptive episodes presenting a challenge for seismic monitoring and eruption forecasting a short period seismic station teln has been operated on telica s summit since 1993 and in 2010 the installation of a six station broadband seismic and eleven station continuous gps network the tesand network was completed to document in detail the seismic characteristics of a persistently restless volcano between our study period of november 2009 and may 2013 over 400 000 events were detected at the tesand summit station tbtn with daily event rates ranging from 5 to 1400 we present spectral analyses and classifications of 200 000 events recorded by the tesand network between april 2010 and march 2013 and earthquake locations for a sub set of events between july 2010 and february 2012 in 2011 telica erupted in a series of phreatic vulcanian explosions six months before the 2011 eruption we observe a sudden decrease in lf events concurrent with a swarm of high frequency hf events followed by a decline in overall event rates which reached a minimum at the eruption onset we observe repeated periods of high and low seismicity rates and suggest these changes in seismicity represent repeated transitions between open system and closed system degassing we suggest that these short and long term transitions between open to closed system degassing form part of a long term pattern of stable vs unstable phases at telica stable phases are characterised by steady high rate seismicity and represent stable open system degassing whereas unstable phases are characterised by highly variable seismicity rates and represent repeated transitions from open to closed system degassing where the system is unable to sustain steady open system degassing these observations have implications for seismic monitoring at persistently restless volcanoes as the recognition of unstable seismic phases may indicate the open system degassing process cannot be sustained and explosions are likely

field observations on the initial lava flow and the fracture system developed during the early days of the stromboli 2007 eruption abstract the february april 2007 stromboli eruption was followed by numerous scientists and well documented from an integrated geophysical analysis by contrast direct visual observations on the summit craters and bastimento area were difficult during the first three days of the eruption because of bad weather conditions in this paper we report field observations on the fracture system after the crisis of february 27th and before the summit explosion on march 15th the observations were focused on the early lava flow emplacement and deformation at elevations between 400 and 700 m a s l i e between the summit crater area and the lateral fissure eruption along the sciara del fuoco sdf

sinkholes pit craters and small calderas analog models of depletion induced collapse analyzed by computed x ray microtomography volumetric depletion of a subsurface body commonly results in the collapse of overburden and the formation of enclosed topographic depressions such depressions are termed sinkholes in karst terrains and pit craters or collapse calderas in volcanic terrains this paper reports the first use of computed x ray microtomography mu ct to image analog models of small scale similar to 2 km diameter high cohesion overburden collapse induced by depletion of a near cylindrical stock like body time lapse radiography enabled quantitative monitoring of the evolution of collapse structure velocity and volume moreover mu ct scanning enabled nondestructive visualization of the final collapse volumes and fault geometries in three dimensions the results illustrate two end member scenarios 1 near continuous collapse into the depleting body and 2 near instantaneous collapse into a subsurface cavity formed above the depleting body even within near continuously collapsing columns subsidence rates vary spatially and temporally with incremental accelerations the highest subsidence rates occur before and immediately after a surface depression is formed in both scenarios the collapsing overburden column undergoes a marked volumetric expansion such that the volume of subsurface depletion substantially exceeds that of the resulting topographic depression in the karst context this effect is termed bulking and our results indicate that it may occur not only at the onset of collapse but also during progressive subsidence in the volcanic context bulking of magma reservoir overburden rock may at least partially explain why the volume of magma erupted commonly exceeds that of the surface depression

relationships between volcano gravitational spreading and magma intrusion volcano spreading with its characteristic sector grabens is caused by outward flow of weak substrata due to gravitational loading this process is now known to affect many present day edifices a volcano intrusive complex can form an important component of an edifice and may induce deformation while it develops such intrusions are clearly observed in ancient eroded volcanoes like the scottish palaeocene centres or in geophysical studies such as in la reunion or inferred from large calderas such as in hawaii the canaries or galapagos volcanoes volcano gravitational spreading and intrusive complex emplacement may act simultaneously within an edifice we explore the coupling and interactions between these two processes we use scaled analogue models where an intrusive complex made of golden syrup is emplaced within a granular model volcano based on a substratum of a ductile silicone layer overlain by a brittle granular layer we model specifically the large intrusive complex growth and do not model small scale and short lived events such as dyke intrusion that develop above the intrusive complex the models show that the intrusive complex develops in continual competition between upward bulging and lateral gravity spreading the brittle substratum strongly controls the deformation style the intrusion shape and also controls the balance between intrusive complex spreading and ductile layer related gravitational spreading in the models intrusive complex emplacement and spreading produce similar structures to those formed during volcano gravitational spreading alone i e grabens folds en echelon fractures therefore simple analysis of fault geometry and fault kinetic indicators is not sufficient to distinguish gravitational from intrusive complex spreading except when the intrusive complex is eccentric from the volcano centre however the displacement fields obtained for 1 a solely gravitational spreading volcano and for 2 a gravitational spreading volcano with a growing and spreading intrusive complex are very different consequently deformation fields like those obtained from geodetic monitoring can give a strong indication of the presence of a spreading intrusive complex we compare the models with field observations and geophysical evidence on active volcanoes such as la reunion island indian ocean ometepe island nicaragua and eroded volcanic remnants such as ardnamurchan scotland and suggest that a combination between gravitational and intrusive complex spreading has been active

the west philippine basin an eocene to early oligocene back arc basin opened between two opposed subduction zones 1 based on geological and geophysical data collected from the west philippine basin and its boundaries we propose a comprehensive cenozoic history of the basin our model shows that it is a back arc basin that developed between two opposed subduction zones rifting started around 55 ma and spreading ended at 33 30 ma the initial spreading axis was parallel to the paleo philippine arc but became inactive when a new spreading ridge propagated from the eastern part of the basin reaching the former one at an r r r triple junction spreading occurred mainly from this second axis with a quasi continuous counter clockwise rotation of the spreading direction the gagua and palau kyushu ridges acted as transform margins accommodating the opening arc volcanism occurred along the palau kyushu ridge eastern margin during the whole opening of the basin whereas the paleo philippine arc decreased its activity between 43 and 36 ma the western margin underwent a compressive event in late eocene early oligocene time leading to the rising of the gagua ridge and to a short subduction episode along eastern luzon in the western part of the basin the spreading system was highly disorganized due to the presence of a mantle plume overlapping spreading centers and ridge jumps occurred toward the hot region and a microplate developed shortly after the end of the spreading a late stage of amagmatic extension occurred between 30 and 26 ma in the central part of the basin being responsible for the deep rift valley that cut across the older spreading fabric

characterization of the in situ magnetic architecture of oceanic crust hess deep using near source vector magnetic data marine magnetic anomalies are a powerful tool for detecting geomagnetic polarity reversals lithological boundaries topographic contrasts and alteration fronts in the oceanic lithosphere our aim here is to detect lithological contacts in fast spreading lower crust and shallow mantle by characterizing magnetic anomalies and investigating their origins we conducted a high resolution near bottom vector magnetic survey of crust exposed in the hess deep tectonic window using the remotely operated vehicle rov isis during rrs james cook cruise jc21 in 2008 hess deep is located at the western tip of the propagating rift of the cocos nazca plate boundary near the east pacific rise epr 2 15 n 101 30 w rov isis collected high resolution bathymetry and near bottom magnetic data as well as seafloor samples to determine the in situ lithostratigraphy and internal structure of a section of epr lower crust and mantle exposed on the steep 20 dipping south facing slope just north of the hess deep nadir ten magnetic profiles were collected up the slope using a three axis fluxgate magnetometer mounted on rov isis we develop and extend the vertical magnetic profile vmp approach of tivey 1996 by incorporating for the first time a three dimensional vector analysis leading to what we here termed as vector vertical magnetic profiling approach we calculate the source magnetization distribution the deviation from two dimensionality and the strike of magnetic boundaries using both the total field fourier transform inversion approach and a modified differential vector magnetic analysis overall coherent long wavelength total field anomalies are present with a strong magnetization contrast between the upper and lower parts of the slope the total field anomalies indicate a coherently magnetized source at depth the upper part of the slope is weakly magnetized and magnetic structure follows the underlying slope morphology including a bench and lobe shaped steps imaged by microbathymetry the lower part of the slope is strongly magnetized with a gradual reduction in amplitude from east to west across the slope surface morphology and recent drilling results indicate that the slope has been affected by mass wasting but the observation of internally coherent magnetization distributions within the upper and lower slopes suggest that the disturbance is surficial we attribute the spatial differences in magnetization distribution to the combination of changes in in situ lithology and depth to the source these survey lines document the first magnetic profiles that capture the gabbro ultramafic and possibly dike gabbro boundaries in fast spreading lower crust

influence of ring faulting in localizing surface deformation at subsiding calderas abstract caldera unrest can lead to major volcanic eruptions analysis of subtle subsidence or inflation at calderas helps understanding of their subsurface volcanic processes and related hazards several subsiding calderas have shown similar patterns of ground deformation composed of broad subsidence affecting the entire volcanic edifice and stronger localized subsidence focused inside the caldera physical models of internal deformation sources used to explain these observations typically consist of two magma reservoirs at different depths in an elastic half space however such models ignore important subsurface structures such as ring faults that may influence the deformation pattern here we use both analog subsidence experiments and boundary element modeling to study the three dimensional geometry and kinematics of caldera subsidence processes evolving from an initial downsag to a later collapse stage we propose that broad subsidence is mainly caused by volume decrease within a single magma reservoir whereas buried ring fault activity localizes the deformation within the caldera omitting ring faulting in physical models of subsiding calderas and using multiple point sill like sources instead can result in erroneous estimates of magma reservoir depths and volume changes

experimental study of caldera formation scaled experiments have been carried out on caldera collapse mechanisms using silicone as analogue magma and dry sand as analogue rock experiments were carried out in two and three dimensions using a range of roof aspect ratios thickness width 0 2 to 4 5 appropriate for caldera collapse they reveal a general mechanism of collapse only weakly dependent on the shape of the reservoir for low roof aspect ratios 1 subsidence starts by flexure of the roof and the formation of outward dipping reverse ring faults which in turn trigger formation of peripheral inward dipping normal ring faults the subsidence always occurs asymmetrically in cross section the reverse faults delimit a coherent piston bounded on each side by an annular zone of inwardly tilted strata located between the reverse and normal ring fault sets the surface depression consists of a nondeformed area piston surrounded by an annular extensional zone tilted strata for high aspect ratios 1 multiple reverse faults break up the roof into large pieces and subsidence occurred as a series of nested wedges 2 d or cones 3 d the extensional zone dominates the surface depression in the case where preexisting regional faults do not play a major role the collapse mechanics of calderas probably depends strongly on the roof aspect ratio calderas with low roof aspect ratios are predicted to collapse as coherent pistons along reverse faults the annular extensional zone might be the source of the large landslides that generate intracaldera megabreccias collapse into magma reservoirs with high roof aspect ratios may be the origin of some funnel calderas where explosive reaming is not dominant

new geological insights and structural control on fluid circulation in la fossa cone vulcano aeolian islands italy accepted 1 june 2009 available online 21 june 2009 keywords electrical resistivity self potential soil co2 degassing temperature fluid circulation hydrothermal system structural boundary vulcano la fossa cone electric resistivity tomography ert self potential sp soil co2 ux and temperature are used to study the inner structure of la fossa cone vulcano aeolian islands nine profiles were performed across the cone with a measurement spacing of 20 m the crater rims of la fossa cone are underlined by sharp horizontal resistivity contrasts sp co2 flux and temperature anomalies underline these boundaries which we interpret as structural limits associated to preferential circulation of fluids the pietre cotte crater and gran cratere crater enclose the main hydrothermal system identified at the centre of the edifice on the base of low electrical resistivity values b20 m and strong co2 degassing sp and temperature anomalies in the periphery the hydrothermal activity is also visible along structural boundaries such as the punte nere forgia vecchia and palizzi crater rims and at the base of the cone on the southern side of the edifice along a fault attributed to the nwmain tectonic trend of the island inside the punte nere crater the ert sections show an electrical resistive body that we interpret as an intrusion or a dome this magmatic body is reconstructed in 3d using the available ert profiles its shape and position with respect to the pietre cotte crater fault allows replacing this structure in the chronology of the development of the volcano it corresponds to a late phase of activity of the punte nere edifice considering the position of the sp soil co2 flux and temperature maxima and the repartition of conductive zones related to hydrothermal circulation with respect to the main structural features la fossa cone could be considered as a relevant example of the strong influence of preexisting structures on hydrothermal fluid circulation at the scale of a volcanic edifice

a geochemical and geophysical investigation of the hydrothermal complex of masaya volcano nicaragua abstract masaya volcano nicaragua is a persistently active volcano characterized by continuous passive degassing for more than 150 years through the open vent of santiago crater this study applies self potential soil co 2 and ground temperature measurements to highlight the existence of uprising fluids associated to diffuse degassing structures throughout the volcano the diffuse degassing areas are organized in a semi circular pattern and coincide with several visible and inferred surface volcanic structures cones fissure vents and likely consist of a network of buried faults and dykes that respectively channel uprising flow and act as barrier to gravitational groundwater flow water depths have been estimated by multi scale wavelet tomography of the self potential data using wavelets from the poisson kernel family compared to previous water flow models our water depth estimates are shallower and mimic the topography typically less than 150 m below the surface between 2006 and 2010 the depths of rising fluids along the survey profiles remained stable suggesting that hydrothermal activity is in a steady state this stable activity correlates well with the consistency of the volcanic activity expressed at the surface by the continuously passive degassing when compared to previous structural models of the caldera floor it appears that the diffuse degassing structures have an important effect on the path that shallow groundwater follows to reach the laguna de masaya in the eastern part of the caldera the hydrogeological system is therefore more complex than previously published models and our new structural model implies that the flow of shallow groundwater must bypass the intrusions to reach the laguna de masaya furthermore these diffuse degassing structures show clear evidence of activity and must be connected to a shallow magmatic or hydrothermal reservoir beneath the caldera as such the heat budget for masaya must be significantly larger than previously estimated

three hawaiian calderas an origin through loading by shallow intrusions the calderas of kilauea and mauna loa are highly dynamic structures and in the 200 year historic period have varied in volume by a factor of 2 and gained or lost 1 km3 per century the deeply eroded caldera of the extinct koolau volcano in oahu is wider than active hawaiian calderas and its lavas have a strong centripetal dip and funnel structure not evident at kilauea or mauna loa the differences can be attributed to the different erosion depths and the time integrated subsidence profile of kilauea is also a stepped funnel having its apex at halemaumau koolau caldera is the focus of an extraordinarily intense dike complex and an intriguing feature is the great diminution in dike concentration into the caldera it is thought that dike injection in any part of the complex generally continued until it reached 50 to 65 in outer parts of the caldera the complex was maintained at or rebuilt to this value despite subsidence in the center of the caldera where the positive bouguer anomaly is centered subsidence evidently greatly outpaced the capacity of dike injections to rebuild the complex assuming the same dike injection rate as kilauea and mauna loa yields a volumetric subsidence rate in koolau caldera exceeding 1 km3 per century hawaiian calderas are much more dynamic than calderas of silicic volcanoes shaped by frequent small events instead of a few great ones the temporal and volumetric correspondence of historical subsidence events with eruptions is poor and this and the high subsidence rates argue for a caldera forming mechanism that consumes the subsided rocks it is suggested that subsidence is caused by the great localized excess load of intrusive rocks carrying the center of the volcano into the thermally weakened lithosphere above the hawaiian hot spot it is envisaged that under steady state conditions the magma chamber rises as the injection of intrusions causes the level of neutral buoyancy at which the chamber is located to ascend and keeps pace with subsidence

instabilities in the summit region of mount etna during the 1999 eruption during the 1999 eruption of mount etna sicily the summit of the volcano changed dramatically lavas erupted from a fissure on the southern flank of se cone formed a large compound flow field that buried a substantial part of the northern wall of valle del bove and the ground between there and se cone hot mass flow deposits formed on the eastern and western flanks of the chasm following a period of intense strombolian activity at the same time a new vent opened on the southern flank of se cone and there was a major rockfall from its summit seven weeks later part of the outer western rim of bocca nuova crater collapsed during another period of intense strombolian activity and lava emerged at a high effusion rate through the breached crater wall following a subsequent collapse of the bocca nuova crater rim a hot avalanche flowed a few hundred metres on top of the previous lava flow field similar deposits have been described on other volcanoes but their importance in the evolution of the summit of etna has not previously been recognised

constraints on magma transfers and structures involved in the 2003 activity at piton de la fournaise from displacement data piton de la fournaise reunion island had shown 8 months of high activity during 2003 including four eruptions and one intrusion the monitoring networks provided by the volcanological observatory of piton de la fournaise show two different kinds of behavior strong and rapid deformations linked with the magma injections and a slight continuous intereruptive inflation of the summit area inversed modeling based on the deformation data using a three dimensional elastostatic boundary element code combined with seismicity provides new insights into the volcanic feeding system involved in the 2003 activity 1 the 2003 eruptions are fed from a pressurized magma chamber located beneath the summit at 300 m above sea level 2 a vertical path was activated between the magma chamber and the surface during the december 2002 collapse allowing the magma to rise into it and leading to the initial summit eruption in may 2003 3 the august and september 2003 lateral eruptions had been fed by dikes intruding along the same vertical pathway at an estimated velocity of 1 1 to 1 3 m s before intruding the flank into the rift zones at a velocity of 0 2 to 0 8 m s this last point suggests a strong structural control on the dike pathway

a new method for precise determination of the position and strike of magnetic boundaries using vector data of the geomagnetic anomaly field summary we present a new method which provides precise determination of the position and strike of magnetic boundaries using vector data of the geomagnetic anomaly field assuming a 2 d magnetic structure the intensity of spatial differential vectors isdv exhibits a peak at the boundary of magnetic structures the strike of the magnetic boundary is obtained using the fact that the magnetic field is zero in the direction parallel to the boundary we have also applied this method to 3 d magnetic structures our method requires only the relative changes of the three components which are more accurately measured than their actual magnitudes we have applied this method to data from the southern juan de fuca ridge obtained on the r v hakuho maru expedition in 1988 kh 88 3 the results are summarized in magnetic boundary strike diagrams mbsd these are maps that show the position strike and character of the magnetic boundaries mbsd provides valuable information for understanding the magnetic structure of oceanic crust such as magnetic lineations pseudo faults fracture zones and the central anomaly magnetic high camh therefore relatively few vector magnetic profiles provide information similar to extensive total intensity geomagnetic surveys

explosive activity of the summit cone of piton de la fournaise volcano la reunion island a historical and geological review summit explosive activity and collapses that form pit craters and calderas represent major volcanic hazards on a dominantly effusive frequently active volcano like piton de la fournaise only three summit collapse events 1986 2002 2007 have been recorded since the foundation of the piton de la fournaise volcano observatory ovpf in 1979 and two of them 1986 and 2007 were associated with weak phreatic activity at piton de la fournaise the normal explosive activity consists of short lived and mild 20 m high lava fountains which quickly evolve into strombolian activity during the eruptions based on comprehensive literature review and high resolution image analysis of surface outcrops and summit caldera walls we reconstructed the time distribution of recent explosive events phreatomagmatic phreatic and their link with summit collapses and lateral flank effusive eruptions in historical time post 1640 ce we recognise two main clusters of explosive events frequent and violent phreatomagmatic to phreatic explosions occurred during the oldest cluster 1708 1878 and alternated with long lasting periods years to decades of summit effusive activity in contrast scarce and on average weak explosions occurred during the youngest cluster 1897 2012 when discrete and short lived 6 months effusive eruptions represent the main eruptive dynamics historical summit collapses pit craters and caldera all localised at the top of the summit cone were related to voluminous lateral eruptions and were followed by a significant decrease in eruptive rate however magma draining during lateral eruptions was not systematically associated with summit collapses or explosions the long lasting occurrence of magma at very shallow depth below the volcano summit followed by a rapid lateral drainage apparently represents a critical condition favouring magma groundwater interaction to produce explosive activity the prehistoric growth of the piton de la fournaise summit cone results chiefly from long lasting to continuous activity centred below its western side bory crater containing lava lakes high lava fountains long lasting effusive activity lava lakes ash plumes and block ejections were common types of eruptive dynamics in the historical past between 1640 and 1878 in this perspective short lived small volume eruptions and long pauses up to six years during the last century of activity of piton de la fournaise can be considered as a lull despite the high frequency of eruption 1 eruption 9 months on average temporal and spatial variations in recurrence rate and eruptive dynamics of basaltic volcanism such as those recognised at piton de la fournaise should be considered in the formulation of hazard assessments and in the interpretation of precursory patterns

a volcanotectonic survey of ascraeus mons mars 1 ascraeus mons is one of the largest volcanoes on mars it is replete with well preserved features that can be used to understand its volcanotectonic evolution previous studies of this volcano focused on specific features and were limited by the quality and coverage of contemporary data our objective is to review and enhance the existing developmental model for ascraeus by considering all endogenic surface features on the volcano we surveyed the volcano s caldera complex flank terraces pit structures sinuous rilles arcuate grabens and small vents we report the spatial and temporal distributions of these features appraise their proposed formation mechanisms in light of our mapping results and propose a detailed geological history for ascraeus mons an initial shield building phase was followed by the formation of a summit caldera complex and small parasitic cones while compression due to flexure of the supporting basement led to extensive terracing of the shield flanks an eruptive hiatus followed ending with the construction of expansive rift aprons to the northeast and southwest against later extensive flank resurfacing in the late amazonian continued flexure formed arcuate grabens concentric to the edifice localized eruption and surface flow of a fluid agent lava and or water from within the volcano then produced a population of rilles on the lower flanks finally in a change of flank tectonic regime from compression to extension pit crater chains and troughs developed on the main shield and rift aprons eventually coalescing to form large embayments at the northeast and southwest base of the volcano

onset of caldera collapse during ignimbrite eruptions the rapid depressurisation and discharge of volatile rich silicic magma chamber and the onset of caldera collapse associated with ignimbrite eruption is investigated using combined theoretical and experimental approaches we first present a scaling analysis of the force balance on the roof of a depressurising magma chamber this provides a failure criterion for the chamber roof and shows that the chamber underpressure required to trigger coherent piston collapse along a vertical or steeply outward dipping reverse ring fault increases with the roof aspect ratio r thickness width the failure criterion is validated by a series of experiments that reveal a transition with increasing roof aspect ratio from piston collapse to either 1 no collapse low reservoir underpressure or 2 non coherent collapse high reservoir underpressure then taking into account the failure criterion obtained with the scaling analysis a theoretical model allows calculation of the erupted chamber volume fraction required to trigger caldera collapse we consider a two stage model with an initially overpressured chamber that becomes progressively underpressured as the eruption proceeds the main input parameters are the chamber depth vertical extent and ellipticity in plan view the magma water content the cohesion and coefficient of internal friction of the roof and the ring fault dip in the case of a mechanically homogeneous roof and a single large and rapid eruption the erupted volume fraction required to trigger piston collapse increases with the roof aspect ratio until the chamber is totally emptied before initiation of any collapse at r 1 1 4 the exact value depending mainly on the ring fault dip and to a lesser extent on the coefficient of internal friction of the roof as shown by experiments collapse into reservoirs with r 1 1 4 probably cannot collapse as a piston and if a surface caldera forms at all collapse is more likely to occur non coherently data for the onset of caldera collapse during seven well documented major ignimbrite eruptions are in broad agreement with the model

episodes of pit crater collapse documented by seismology at piton de la fournaise calderas are oval fault bounded depressions in volcanic regions the formation of a small analog a summit pit crater has followed a ten month cycle of eruptive activity at piton de la fournaise volcano reunion island indian ocean in 1985 six short episodes of summit seismic activity with shocks indicating shear failures have been followed by eruptions in five cases the eruptions occurred near the base of the summit cone which is at 2 5 km elevation inside the enclos fouque an ancient caldera 7 km in diameter in another instance the summit seismic crisis did not evolve to an eruption but was followed by a seismic crisis 4 km deeper in march 1986 low level activity both at shallow depth beneath the summit and around 4 km depth to the southeast a simultaneous occurrence not previously observed culminated by a summital swarm on march 18 activity resumed its low level and five hours later a small and short lived eruption occurred at mid distance between the summit and the rim of enclos fouque one day later a flank eruption began at 1000 m elevation outside of the enclos 9 km southeast of the summit four days later effusion began near sea level 14 km southeast of the summit the shift of activity from the vent inside to that outside the enclos occurred whilst both shallow summital and deeper flank seismicity were sustained but without notable shallow activity between one vent and the next then the seismic activity which in all five previous instances stopped when the vents opened persisted for ten days increasing in energy release rate activity only consisted of shallow shocks in the summit area whilst eruption was low on the flank 9 km then 14 km away the focal mechanisms are different from all those previously determined they correspond neither to shear failure nor tensile fracturing by fluid overpressure they suggest cracking due to volume increase in a low fluid flow environment which may be consistent with the simultaneous tapping of magma deeper in the edifice a final paroxysmal seismic crisis lasting one and a half days led to the collapse of a 200 m diameter 80 m deep pit crater inside the dolomieu summital caldera on march 29 this crisis released an order of magnitude more energy than previous activity the long period signature of these seismic events indicates low stress drop with respect to source dimensions sources are shallow however rather than directly marking the progressive development of a depression at the surface they appear to correspond to fractures opening above the retreating magma and progressive stoping of a cavity similarities with seismic observations during the last phase of the fernandina galapagos 8 km2 caldera collapse of june 1968 suggest that the same kind of mechanical behaviour occurred with a scaling of three orders of magnitude in seismic moments and in volumes of collapsed blocks between the two cases

volcanic rifting at martian grabens 1 a large fraction of surface extension on mars occurred at segmented grabens having width length ratios akin to oceanic rifts on earth association with volcanic landforms such as pit craters clearly suggests interconnection between tectonic and magmatic processes a martian rift evolution model is proposed on the basis of new geomorphological and structural interpretations of imagery high resolution digital elevation models dems scaled experimental modeling and three dimensional boundary element modeling of magmatic and tectonic processes and a comparison with terrestrial rifts the dems were obtained from mars observer laser altimeter viking orbiter stereo images or a combination of both comparison of terrestrial rifts included afar iceland and the east pacific rise the ambient extensional stress field induced by regional body forces is combined at depth with decompression melting and mantle plume thermal anomaly resulting in emplacement of elongated magma reservoirs along the grabens injection of dikes above the reservoirs and flood basalt eruption result in a reservoir underpressurization of up to hundreds of mpa and induces surface collapse each collapse event is associated with an eruption of volcanic volumes akin to those of individual flow eruptions in large terrestrial igneous provinces the geometry and mechanisms of graben formation and surface collapse are described and used to infer reservoir depth and width we conclude from this study that giant dike swarms akin to typical giant terrestrial dike swarms are unlikely to underlie the volcanic martian grabens on the basis one graben one dike rather every volcanic graben segment appears to own a local dike swarm perhaps analogous to the dike swarms in the icelandic fissure zones index terms 5475 planetology solid surface planets tectonics 8149 5480 planetology solid surface planets volcanism 8450 6225 planetology solar system objects mars 8121 tectonophysics dynamics convection currents and mantle plumes 8010 structural geology fractures and faults keywords rifting graben pit crater magma chamber dyke

new insights into the genesis of the miocene collapse structures of the island of gozo malta central mediterranean sea the large palaeosinkholes located in the nw of gozo central mediterranean sea malta offer excellent exposures that provide information on the geometry and kinematics of large karst related collapse structures detailed geological analysis of these peculiar palaeosinkholes indicates that deep seated evaporite dissolution is the most feasible hypothesis to explain their formation according to the following evidence 1 several structures have been formed by progressive foundering of cylindrical blocks with limited internal deformation as revealed by the synsedimentary subsidence recorded by their miocene sedimentary fill this subsidence mechanism is more compatible with interstratal dissolution of evaporites than karstification and cave development in limestone formations 2 the dimensions and deformation style of the palaeosinkholes are similar to those of other collapse structures related to deep seated dissolution of salt bearing evaporites 3 the arcuate monocline associated with some of these collapse structures is also a characteristic feature of subsidence related to dissolution of evaporites however no major evaporite formations have been documented so far in the subsurface of the malta platform supplementary material detailed descriptions of the collapse structures of the island of gozo malta central mediterranean sea are available at www geolsoc org uk sup18808

from structure to erosion controlled subsiding calderas evidence thresholds and mechanics collapse calderas evolve by increasing their depth diameter ratio to properly characterize caldera evolution a structural s d ratio between structural subsidence and ring fault diameter ss ds and a topographic s d ratio between topographic caldera depth and topographic caldera width st dt are considered we review the evolution of the a d 2000 miyakejima caldera with two concentric ring faults at earlier collapsing stages and erosion of its wall accumulating debris on the floor at later collapsing stages while st dt and ss ds show a similar increase at initial stages when ss ds 0 33 the ss ds becomes significantly different from st dt while continuous caldera subsidence monotonically increases ss ds the erosion of the wall and the filling of the floor decrease st dt this evolution finds close similarities with recent caldera collapses of krakatau 1883 katmai 1912 fernandina 1968 tolbachik 1975 1976 pinatubo 1991 and dolomieu 2007 analog experiments mimic the observed variation evolving from a depression controlled by the activity of the double ring faults to that controlled by the erosion of the wall and sedimentation at the floor these natural and modeling results show that the control on the shape of mature calderas ss ds 0 07 and approaching ss ds 0 3 0 4 passes from a mainly structural to a mainly erosional control both st dt and ss ds are needed to describe the evolution of a collapse and the processes accompanying it evaluating st dt and ss ds allows proper description of the precise evolutionary stage of a caldera and of the relative importance of the structural and erosional processes and allows making semiquantitative comparisons between evolutionary stages

complex circular subsidence structures in tephra deposited on large blocks of ice varða tuff cone oraefajokull iceland several broadly circular structures up to 16 m in diameter into which higher strata have sagged and locally collapsed are present in a tephra outcrop on southwest oraefajokull southern iceland the tephra was sourced in a nearby basaltic tuff cone at var a the structures have not previously been described in tuff cones and they probably formed by the melting out of large buried blocks of ice emplaced during a preceding jokulhlaup that may have been triggered by a subglacial eruption within the oraefajokull ice cap they are named ice melt subsidence structures and they are analogous to kettle holes that are commonly found in proglacial sandurs and some lahars sourced in ice clad volcanoes the internal structure is better exposed in the var a examples because of an absence of fluvial infilling and reworking and erosion of the outcrop to reveal the deeper geometry the ice melt subsidence structures at var a are a proxy for buried ice they are the only known evidence for a subglacial eruption and associated jokulhlaup that created the ice blocks the recognition of such structures elsewhere will be useful in reconstructing more complete regional volcanic histories as well as for identifying ice proximal settings during palaeoenvironmental investigations

pits rifts and slumps the summit structure of piton de la fournaise a clear model of structures and associated stress fields of a volcano can provide a framework in which to study and monitor activity we propose a volcano tectonic model for the dynamics of the summit of piton de la fournaise la reunion island indian ocean the summit contains two main pit crater structures dolomieu and bory two active rift zones and a slumping eastern sector all of which contribute to the actual fracture system dolomieu has developed over 100 years by sudden large collapse events and subsequent smaller drops that include terrace formation small intra pit collapse scars and eruptive fissures are located along the southern floor of dolomieu the western pit wall of dolomieu has a superficial inward dipping normal fault boundary connected to a deeper ring fault system outside dolomieu an oval extension zone containing sub parallel pit related fractures extends to a maximum distance of 225 m from the pit at the summit the main trend for eruptive fissures is n80 normal to the north south rift zone the terraced structure of dolomieu has been reproduced by analogue models with a roof to width ratio of approximately 1 suggesting an original magma chamber depth of about 1 km such a chamber may continue to act as a storage location today the east flank has a convex concave profile and is bounded by strike slip fractures that define a gravity slump this zone is bound to the north by strike slip fractures that may delineate a shear zone the southern reciprocal shear zone is probably marked by an alignment of large scoria cones and is hidden by recent aa lavas the slump head intersects dolomieu pit and may slide on a hydrothermally altered layer known to be located at a depth of around 300 m our model has the summit activity controlled by the pit crater collapse structure not the rifts the rifts become important on the mid flanks of the cone away from pit related fractures on the east flank the superficial structures are controlled by the slump we suggest that during pit subsidence intra pit eruptions may occur during tumescence however the pit system may become blocked and a flank eruption is more likely intrusions along the rift may cause deformation that subsequently increases the slump s potential to deform conversely slumping may influence the east flank stress distribution and locally control intrusion direction these predictions can be tested with monitoring data to validate the model and eventually improve monitoring

review of ten years of volcano deformations recorded by the ground based insar monitoring system at stromboli volcano a tool to mitigate volcano flank dynamics and intense volcanic activity abstract stromboli volcano southern italy is one of the most monitored volcano in the world with a surveillance network that includes a permanently sited ground based sar interferometer gbinsar this work is the review of the gbinsar data gained from the last decade of monitoring activity the analysis of the entire dataset of gbinsar measurements allowed the assessment of the deformation field of the northern part of the summit crater area and the sciara del fuoco depression in detail the main displacements recognized can be related to different factors 1 the inflation deflation respectively immediately before and after each new effusive event 2 the bulging of localized sectors of the volcano involved in the vent opening 3 the gravitational sliding of the sciara del fuoco infill 4 the movement of lava flows accelerations in this sector are related to sheet intrusions while the possibility of vent opening associated with small sliding or catastrophic flank failure are related to highly overpressurized sheets able to produce high displacement rate in the sciara del fuoco in the summit crater area the increases in the displacement rate are related to the pressurization of the shallow conduit system as the consequence of the variation in the magma level magmastatic pressure or to the lateral magma migration lateral conduit expansion or dike intrusion in response to the increase of the overpressure component fluctuations in the displacement rate in the summit crater area can be related to the magma overturning within the conduit with the increases in displacement rate during the upwelling of less dense magma while displacement rate decreases as the degassed magma column is pushed out from the conduit lava flows or overflows instead the decrease in the displacement rate without coeval lava outpouring could be related to the sink of the degassed magma due to density contrast between the gas poor and the gas charged magmas using the displacement rate in the summit crater area as a proxy for the variation in the pressure condition in conduit both magmastatic and overpressure components thresholds for the crises characterized by the occurrence of overflows eventually associated with major explosions and flank effusions eventually associated with paroxysmal explosions are identified small conduit overpressure will produce overflows sometimes associated with crater rim collapses while large magma overpressure will laterally expand the conduit forming ne sw striking sheets feeding eruptive vents at the base of the summit crater area and within the sciara del fuoco generating conditions of instability that can evolve into catastrophic collapse of the instable flank

bathymetry and magnetic anomalies in the havre trough and southern lau basin from rifting to spreading in back arc basins abstract the havre trough and lau basin are active back arc basins related to pacific australian plate convergence these back arc basins provide insight into the evolution of rifting through various stages including a final stage of oceanic spreading we carried out a swath bathymetry and magnetic anomaly survey focused on the less studied havre trough and southern lau basin the western portion of the whole havre trough is deeper and more thickly sedimented whereas shallow bathymetry and volcanic edifices are dominant in the east we suggest that crustal rifting progresses asymmetrically with respect to the trough axis the active tectono magmatic zone migrates trenchward during ongoing widening rifting along normal faults trending in the longitudinal trough strike occurs in the southern havre trough volcanism is restricted to a line along the normal faults circular or elongated knolls in the northern havre trough indicate that en echelon type segmented rifting of short strike length and successive volcanic intrusion takes place volcanism in this area is dominated by diapir like magma intrusion into an extended and thinned arc crust the southernmost tip of the spreading center of lau basin is defined by the deep graben striking at n20 e located at 24 00 s 177 10 w

relationship between caldera collapse and magma chamber withdrawal an experimental approach collapse calderas have received considerable attention due to their link to earth s ore deposits and geothermal energy resources but also because of their tremendous destructive potential although calderas have been investigated through fieldwork numerical models and experimental studies some important aspects on their formation still remain poorly understood one key issue concerns the volume of magmas involved in caldera forming eruptions we perform analogue experiments to correlate the structural evolution of a collapse with the erupted magma chamber volume fraction the experimental device consists of a transparent box 60 60 40 cm filled with dry quartz sand and a water filled latex balloon as a magma chamber analogue evacuation of water through a pipe causes a progressive deflation of the balloon that leads to a collapse of the overlying structure the experimental design allows to record the temporal evolution of the collapse and to track the evolution of fractures and faults we study the appearance and development of specific brittle structures such as surface fractures or internal reverse faults and correlate each different structure with the corresponding removed magma chamber volume fraction we also determine the critical conditions for caldera onset experimental results show that at any stage of caldera developments the experimental relationship between volume fraction and chamber roof aspect ratio fits a logarithmic curve it implies that volume fractions required to trigger caldera collapse are lower for chambers with low aspect ratios shallow and wide than for chambers with high aspect ratios deep and small these results are in agreement with natural examples and previous theoretical studies

growth of phreatomagmatic explosion craters a model inferred from suoana crater in miyakejima volcano japan abstract a subvertical cross section of a maar diatreme volcano is exposed in the wall of the a d 2000 caldera on miyakejima volcano the maar suoana is one of the lateral vents of this volcano and it was inferred to be formed in the 7th century the subvertical wall of the a d 2000 caldera truncated the suoana maar crater at its center revealing the near comlete cross section of this small maar diatreme volcano exposed in the cross section are a 400 m wide maar crater an associated tuff ring with a maximum thickness of 20 m a diatreme extending vertically to a depth of about 220 m from the floor of the maar crater and a feeder dike connected to the base of the diatreme the depth of the diatreme structure is about 260 m from the original ground surface the outline of the diatreme resembles an upward opening funnel with an almost vertical wall below 560 m asl and an upward flaring wall above 560 m asl coarse grained volcanic breccia fills the diatreme the deposits of which can be divided into 6 units based on lithological and structural characteristics the upper half of the diatreme is filled with landslide deposits mainly derived from the surrounding crater wall the bottom of the diatreme is occupied by massive explosion breccia some coherent blocks were detached from the wall of the diatreme and preserved in the diatreme fill the y shaped cross sectional geometry of the suoana diatreme is the result of a combination of an underground subsidence in the lower part of the diatreme and the surface landslide in its upper part the inwardly inclined stratification of pyroclastic rock units and development of many small faults in the diatreme filling breccia indicate successive collapse and deformation of these materials within the diatreme during the eruption the upper part of the diatreme was formed by the landslides of the crater wall induced by the subsidence of the crater floor discharge of tephra from the bottom of the diatreme caused infill subsidence which induced sliding of the inner wall of the crater as a result the topographic diameter of the crater became much larger than that of the diatreme itself the tephra ring surrounding the crater consists mainly of pyroclastic fall deposits

the geometry of volcano flank terraces on mars abstract flank terraces are subtle expansive structures on the slopes of many large martian shield volcanoes several terrace formation hypotheses including self loading lithospheric flexure magma chamber tumescence volcano spreading and shallow gravitational slumping have been suggested terraces are not readily visible on photogeological data consequently terrace geometry has not yet been comprehensively described terrace provenance therefore is poorly understood we used three dimensional mars orbiter laser altimeter mola data to characterise the geometry of these elusive structures with a view to understanding better the role that flank terraces play in the tectonic evolution of volcanoes on mars terraces have a broad convex upward profile in section and a systematic fish scale imbricate stacking pattern in plan they are visible at all elevations on at least nine disparate martian volcanoes terrace like features also occur on three shield volcanoes on earth an observation not recorded before analysis of a suite of morphometric parameters for flank terraces showed that they are scale invariant with similar proportions to thrust faults on earth we compared predicted formation geometries to our terrace observations and found that only lithospheric flexure can fully account for the morphology distribution and timing of terraces as a volcano flexes into the lithosphere beneath it its upper surface will experience a net reduction in area resulting in the formation of outward verging thrusts we conclude therefore that flank terraces are fundamental volcanotectonic structures that they are the surface expressions of thrust faults probably formed by lithospheric flexure and that they are not restricted to mars

evidence of multiple thermokarst events in northeastern france and southern belgium during the two last glaciations a discussion on features caused by ground ice growth and decay in late pleistocene fluvial deposits paris basin france bertran et al 2018 abstract the past thermokarst activities in valleys of northern france and belgium covered the upper weichselian and the upper saalian periods to develop in western europe thermokarst first requires an accumulation of ground ice close to the surface progressively stored along the glacial time permafrost aggradation it is regionally uncommon during the early glacials especially on the plateau but frequent during the upper pleniglacials in valleys these features mostly relate to various frost mounds created by injection and segregated ices the role of ice wedges is really very limited in this zone of southern extent of the european palaeo permafrost on plateau and terraces thermokarst events are mostly susceptible to occur during the coldest part of the glacial with a more progressive warming or a retrogressive thermokarst triggered by erosion as in arctic today deformations are more gradual in direct relation with the rheological properties of the sediments and usually local drainage they are in concurrence with the vegetation dynamic that will limit its expression thermokarst events are in first order orbitally forced under control of a maximum insolation and a minimum in precession as well as during the weichselian and the saalian they are moreover associated with abrupt warming transmitted by dansgaerd oeschger events snowiness and mild winter temperatures are probably the main triggers for thermokarst activity as of today other events can be triggered by solar activity as at 20 ka or perhaps enhanced by major ash splay as during the mis 6b zeiffen interstadial thermokarst events are usually followed on the continent by a reorganization of the rivers from braided to meandering systems

insight into ground deformations at lascar volcano chile from sar interferometry photogrammetry and gps data implications on volcano dynamics and future space monitoring abstract we present a detailed study of lascar volcano chile based on the combination of satellite aerial and ground based data in order i to better characterize the deformation style of andean explosive volcanoes and ii to provide new insights on the potential of space techniques to monitor active volcanic deformations on such edifices lascar is one of the most active volcanoes in central andes characterized by a recent cyclic activity additionally it is located in favourable conditions for radar imaging lascar thus offers very good conditions for studying large to small scale ground deformations associated with volcano dynamics the analysis of insar synthetic aperture radar interferometry time series data from the european and japanese satellites ers jers acquired between 1993 and 2000 encompassing three eruptive events confirmed the absence of broad far field deformation signal thus during the recent activity of lascar we discard significant magmatic input at depth the following approaches were used to improve the insar signal noise ratio in order to detect possible local deformation we carried out a quantitative evaluation of the potential tropospheric contribution in insar interferograms for the salar de atacama lascar area using radar asar envisat and spectrometer modis data we also used an accurate aerial photogrammetric and gps constrained dem in our insar data reprocessing we find a co eruptive ground deformation confined into the summit crater for the 1995 eruption this deformation has spatial dimension of 500 by 400 m and relates to a subsidence of crater floor up to 17 mm we interpret it as pressure or volume decrease at subsurface levels below the active crater our study made it possible to image a new near field volcanic deformation confined within the summit crater of the lascar volcano it also demonstrates that the combination of precise photogrammetry dem and insar data can significantly improve our ability to remotely sense subtle surface deformation on these explosive volcanoes this methodology might contribute to better understand volcano dynamics and to complement their monitoring in remote areas

caldera types how end members relate to evolutionary stages of collapse 1 currently proposed caldera types downsag piston funnel piecemeal trapdoor are mainly based on field evidence and are each viewed as end members an overview of recent experiments on caldera formation under different conditions shows remarkably consistent results and suggests four experimental stages characterized by progressive subsidence distinctive structural features of each stage are found in many calderas highlighting an overall consistency the evolutionary stages adequately explain the architecture and development of the established caldera end members along a continuum where one or more end member may correspond to a specific stage while such a continuum is controlled by progressive subsidence specific collapse geometries will result from secondary factors such as roof aspect ratio collapse symmetry and pre existing faults

kinematic analysis of vertical collapse on volcanoes using experimental models time series 1 volcanoes are often associated with vertical collapse due to deeper magma withdrawal calderas are the most notable type of vertical collapse on the summit of volcanoes caldera collapse has been observed and monitored only at miyakejima japan 2000 dolomieu reunion 2007 and fernandina galapagos 1968 highlighting our limited knowledge on its kinematic behavior here we use experimental models to investigate the kinematic evolution of calderas and vertical collapses in general we extract velocity and strain fields using the particle image velocimetry piv technique generating time series experimental vertical collapses undergoing constant subsidence velocity show three main kinematic behaviors 1 continuous collapse whose velocity is similar to the source subsidence velocity 2 incremental collapse with episodic stepwise accelerations along pre existing ring structures 3 sudden collapse resulting from the upward migration of a cavity only for t d 2 t and d are the depth and width of the magma chamber respectively and without ring structures the velocity in the collapsing column may increase up to four orders of magnitude with regard to the constant subsidence velocity of the source comparison to nature suggests that 1 there are close kinematic similarities with monitored collapse calderas explaining their incremental subsidence after the development of ring structures 2 sudden pit crater formation is induced by the upward propagation of cavities due to magma removal at depth and in absence of ring structures 3 all these types of vertical collapses have a consistent mechanism of formation and kinematic behavior function of t d and the presence absence of ring structures

early syn rift evolution in the west cameros basin upper jurassic nw iberian range spain and pedogenetic calcretes in early syn rift alluvial systems upper jurassic west cameros basin northern spain reply first we would like to highlight our appreciation of dr platt s and dr wright s comments and the opportunity they give us to discuss some aspects of our papers sacristan horcajada et al 2015 2016 improving the final state of our work knowledge about sedimentology of alluvial systems has had an important development in recent years with new proposals for models in rift basins where tectonics play a predominant role nichols and fisher 2007 hartley et al 2010 weismann et al 2010 fielding et al 2012 the differential behavior of the extensional structures during the earliest syn rift stage tithonian in the west cameros basin played a key role in the architecture of the various alluvial systems sacristan horcajada et al 2015 as well as the distribution of various calcrete sequences sacristan hocajada et al 2016 we discuss the points raised drs platt and wright in the same order they established with respect to this section of the discussion we consider it important to emphasize several points 1 the types of early syn rift alluvial systems studied from our point of view in the case of the nuestra senora de brezales formation it would be an oversimplification to refer only to alluvial fans the brezales fm abbreviation of nuestra senora de brezales fm see below has been interpreted as the result of clastic sedimentation in alluvial systems alluvial and fluvial fans developed in generally small halfgrabens specifically in our recent studies detailed facies analysis stratigraphic framework and paleogeographic reconstructions have proved that the arrangement of the depositional system was complex and heterogeneous due to the differential behavior of the extensional structures that controlled each halfgraben sacristan horcajada et al 2012b sacristan horcajada et al 2013 sacristan horcajada et al 2015 this different tectonic activity produced that each halfgraben basin displayed different sedimentary architectures and alluvial records although they were developed under similar climatic conditions

formation of the april 2007 caldera collapse at piton de la fournaise volcano insights from gps data abstract on april 5th 2007 a major collapse of the dolomieu summit crater occurred at piton de la fournaise volcano la reunion island to constrain the origin and the dynamics of the collapse we modelled continuous gps displacements preceding and accompanying this event our results reveal that the dolomieu collapse formed by a piston like mechanism in which the fast draining of the shallow magma reservoir at the beginning of the march 30th may 1st 2007 eruption caused a gravity driven downward displacement of its roof during the five days preceding the collapse summit deflation was caused by volume change through the fractured rock column above the shallow magma chamber due to the closure of voids or small scale cavities or the upward stoping and migration of an underground cavity since 2000 weakening of the rock column by repeated refilling and draining of the shallow magma chamber favoured its destabilization on april 5th 7 h before the onset of the collapse the deflation source was located at 300 m depth revealing the initiation of the collapse at very shallow depth the sudden collapse on april 5th was followed by continuous cyclic subsidence until april 6th each cycle was characterized by a progressive inward deflation of the summit zone and ended by sudden summit outward displacements caused by stress relaxation following a collapse event subsidence of the stoped column in the magma reservoir during the collapse of the crater on april 5th and 6th acted as a piston thus increasing the eruption rate the collapse occurred in only 24 h by successive events and increased the depth of the dolomieu crater by 340 m after april 6th only minor readjustments of the summit cone occurred with slight summit deflation and small landslides of the caldera walls

sub surface structures and collapse mechanisms of summit pit craters abstract summit pit craters are found in many types of volcanoes and are generally thought to be the product of collapse into an underpressured reservoir caused by magma withdrawal we investigate the mechanisms and structures associated with summit pit crater formation by scaled analogue experiments and make comparisons with natural examples models use a sand plaster mixture as analogue rock over a cylinder of silicone simulating an underpressured magma reservoir experiments are carried out using different roof aspect ratios roof thickness roof width of 0 2 2 they reveal two basic collapse mechanisms dependant on the roof aspect ratio one occurs at low aspect ratios 1 as illustrated by aspect ratios of 0 2 and 1 outward dipping reverse faults initiated at the silicone margins propagates through the entire roof thickness and cause subsidence of a coherent block collapse along the reverse faults is accommodated by marginal flexure of the block and tension fractures at the surface aspect ratio of 0 2 or by the creation of inward dipping normal faults delimiting a terrace aspect ratio of 1 at an aspect ratio of 1 overhanging pit walls are the surface expressions of the reverse faults experiments at high aspect ratio 1 2 reveal a second mechanism in this case collapse occurs by stopping which propagates upwards by a complex pattern of both reverse faults and tension fractures the initial underground collapse is restricted to a zone above the reservoir and creates a cavity with a stable roof above it an intermediate mechanism occurs at aspect ratios of 1 1 1 2 in this case stopping leads to the formation of a cavity with a thin and unstable roof which collapses suddenly the newly formed depression then exhibits overhanging walls surface morphology and structure of natural examples such as the summit pit craters at masaya volcano nicaragua have many of the features created in the models indicating that the internal structural geometry of experiments can be applied to real examples in particular the surface area and depth of the underpressured reservoir can be roughly estimated we present a morphological analysis of summit pit craters at volcanoes such as kilimanjaro tanzania san cristobal telica and masaya nicaragua and ubinas peru and indicate a likely type of subsidence and possible position of the former magma reservoir responsible for collapse in each case

origin and evolution of the west philippine basin a new interpretation abstract combined analyses of the magnetic lineations and seafloor structures of the west philippine basin show that it was formed by spreading from the central basin spreading center in two distinctly different spreading phases before and after subduction was initiated along the palau kyushu trend at 45 ma b p these episodes are recognized by differences in the strike of the magnetic lineations spreading rates density and strike of fracture zones and basement relief from 60 45 ma b p spreading was ne sw relative to present orientations at a half rate of 44 mm a at about 45 ma b p the spreading direction changed to a more n s direction with a reconfiguration of the central basin spreading center into numerous short e w segments offset by closely spaced n s transform faults spreading slowed to a half rate of 18 mm a and ceased at 35 ma b p thus the west philippine basin originated at 45 ma b p by the trapping of normal ocean crust west of the initial subduction along the palau kyushu trend the 45 35 ma b p period represents the dying phase of spreading on the central basin spreading center following isolation of the west philippine basin from the plate driving forces of the pacific

insights into the kinematics of a volcanic caldera drop probabilistic finite source inversion of the 1996 bardarbunga iceland earthquake article i nfo the accurate modelling and prediction of volcanic eruptions depends critically on information concerning the interaction between the caldera and the underlying magma chamber knowledge concerning the kinematics of a caldera in the course of an eruption is therefore essential here we provide detailed seismological constraints on the kinematics of a volcanic caldera drop and the geometry of a caldera ring fault for this we performed a finite source inversion of the september 29 1996 bardarbunga iceland earthquake that was caused by caldera subsidence our methodology is based on spectral element simulations of seismic wave propagation through a realistic model of the icelandic crust and upper mantle a particularly robust feature is the initiation of the rupture in the north western part of the ring fault that is about 10 km in diameter from there it spread to the other fault segments within about 3 s without invoking super shear propagation sensu stricto we can explain this unusually fast rupture propagation by the triggering of fault segments through p waves that propagated across the caldera our results favour outward dipping fault segments in the western half of the ring fault while the eastern half is preferentially inward dipping this variability may reflect structural heterogeneities or an irregular magma chamber geometry the individual segments of the caldera ring fault radiated approximately equal amounts of energy this indicates that the caldera dropped coherently as one single block the work presented here is intended to aid in the design of realistic models of magma chamber and caldera dynamics

constraints on the geomorphological evolution of the nested summit craters of lascar volcano from high spatio temporal resolution terrasar x interferometry small scale geomorphological changes that are associated with the formation development and activity of volcanic craters and eruptive vents are often challenging to characterize as they may occur slowly over time can be spatially localized and difficult or dangerous to access using high spatial and high temporal resolution synthetic aperture radar sar imagery collected by the german terrasar x tsx satellite in spotlight mode in combination with precise topographic data as derived from pleiades 1a satellite data we investigate the surface deformation within the nested summit crater system of lascar volcano chile the most active volcano of the central andes our aim is to better understand the structural evolution of the three craters that comprise this system to assess their physical state and dynamic behavior and to link this to eruptive activity and associated hazards using multi temporal sar interferometry mt insar from ascending and descending orbital geometries we retrieve the vertical and east west components of the displacement field this time series indicates constant rates of subsidence and asymmetric horizontal displacements of all summit craters between june 2012 and july 2014 as well as between january 2015 and march 2017 the vertical and horizontal movements that we observe in the central crater are particularly complex and cannot be explained by any single crater formation mechanism rather we suggest that short term activities superimposed on a combination of ongoing crater evolution processes including gravitational slumping cooling and compaction of eruption products as well as possible piston like subsidence are responsible for the small scale geomorphological changes apparent in our data our results demonstrate how high temporal resolution synthetic aperture radar interferometry insar time series can add constraints on the geomorphological evolution and structural dynamics of active crater and vent systems at volcanoes worldwide