crustal large scale serpentinized mantle peridotite body in the sulu ultrahigh pressure metamorphic belt eastern china evidence from gravity and magnetic anomalies abstract from analysis of the geological and geophysical data gravity magnetic seismic and petrophysics we propose that geophysical anomalies are produced by a serpentinized mantle peridotite body smpb situated in the middle to lower crust in the sulu belt the smpb was formed by crustal emplacement of mantle peridotites accompanied by ultrahigh pressure uhp metamorphism our finding suggests an emplacement mechanism for the serpentinized mantle wedge smw early in the subduction process this is different from the classic view which holds that the serpentinized forearc mantle is formed by in situ hydration processes blakely et al 2005 the petrophysical properties of the smpb are similar to those of the serpentinized forearc mantle or smw in modern subduction zones worldwide but the formation mechanisms for smpb and smw are different this observation is important for understanding the geodynamic processes that operated in the large uhp metamorphic belt in the dabie sulu area eastern china

constraining spatiotemporal characteristics of magma migration at piton de la fournaise volcano from pre eruptive seismicity volcano tectonic seismicity has been recorded for decades on various volcanoes and is linked with the magma transport and reservoir pressurization yet earthquakes often appear broadly distributed such that magma movement is difficult to infer from its analysis we explore the seismicity that occurred before eruptions at piton de la fournaise in the last 5 years using template matching and relocation techniques we produce a refined image of the summit seismicity demonstrating that most earthquakes are located on a ring structure however only a portion of this structure is activated before each eruption which provides an indication as to the direction of the future eruptive site furthermore we show that the delay between the pre eruptive swarm and the eruption onset is proportional to the distance of the eruptive fissures relative to the summit cone this reveals that the beginning of the intrusion already bears information regarding the future eruption location

ballistic blocks around kīlauea caldera their vent locations and number of eruptions in the late 18th century abstract thousands of ballistic blocks occur around k lauea caldera and record part of the latest major period of explosive activity on the volcano in late 1790 or within a few years thereafter the sizes of the blocks the largest of which is more than 2 m in nominal diameter and differences in rock types allow the definition of at least 6 dispersal lobes of mostly undetermined relative age the orientations of the lobes help approximate the locations of vents or explosion sources on the floor of the caldera now deeply buried by younger lava flows the vents may have been distributed northward for about 2 km from near the site of the modern halema uma u crater and were apparently confined to the western half of the caldera the blocks are entirely lithic except for those in one dispersal lobe which contains cored bombs and blocks as well as juvenile lapilli eruption parameters calculated from eject suggest that the phreatic and phreatomagmatic explosions could have been generated at the water table about 600 m below the high point on the caldera rim

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

explosive eruptions from the interaction of magmatic and hydrothermal systems during flank extension the bellecombe tephra of piton de la fournaise la reunion island piton de la fournaise la reunion island is a very active primarily effusive ocean island volcano the bellecombe tephra represents at least three explosive eruptions that occurred between about 5465 and 2971 calendar years bp near the margin of the present day enclos fouque caldera margin two bellecombe eruptions produced a sequence of two tuff breccias interbedded with tuff the tuff breccias only reach a few hundred meters outside the current caldera margin at petite carriere an old scoria cone 1 km from the enclos fouque margin these two deposits the lower bellecombe tephra are represented by two tuffs with incipient soil formation at the top of each they are overlain by a third unit the upper bellecombe tephra made of bedded lapilli tuff and tuff some reworked in small debris flows off the scoria cone the lapilli increase in size and the beds in thickness southeastward toward chisny volcano and away from the enclos fouque caldera deposits from the upper bellecombe tephra are correlated to sites 5 km northwest of petite carriere and 6 km north of a postulated vent location on the north side of chisny volcano distribution patterns of all bellecombe tephra are consistent with eruption columns that did not rise above 8 km asl the ash fraction of the bellecombe tephra contains three juvenile components a dominant gray vitric basaltic ash an oceanitic olivine rich basalt ash and pyroxene bearing gabbro with a few percent glass it also contains doubly terminated clear quartz grains and olivine and rarer clinopyroxene crystals the lower bellecombe tephra contains an altered brown ash whereas a tan yellow clay rich ash is common in the upper unit lava flows of gray aphyric basalt and oceanite are exposed at the surface and preceded the bellecombe eruptions but the gabbro quartz crystals and hydrothermally altered grains indicate the involvement of the magma hydrothermal system from 0 5 to 2 km depth we propose that the three eruptions of the bellecombe tephra were preceded by voluminous eruptions of lava flows that led to seaward sliding mass movement of the volcano this opened fractures that explosively depressurized the hydrothermal system and incorporated magma and gabbro from the magma system and probably from rock along the fissure in the ejecta this model is consistent with the small erupted volume 150 mm3 for the lower two combined and about the same for the upper tephra and presence of components from a variety of depths if the eruption had excavated downward more surficial components would be expected instead the fissures allowed material from many depths to erupt simultaneously similar eruptions may occur after voluminous eruptions and or when lateral seaward sliding of the volcano occurs

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

the effects of serpentinization on density and magnetic susceptibility a petrophysical model abstract new measurements are presented of initial magnetic susceptibility and density for serpentinized harzburgites from the josephine peridotite oregon an inverse correlation between density and susceptibility which is typical of these rocks and similar rocks from other localities is not explained by simple serpentinization reactions calculations of density and susceptibility as a function of reaction progress for reactions that produce magnetite show susceptibilities that are larger than those observed at low degrees of serpentinization susceptibilities of fe bearing serpentine and brucite are calculated from a molecular field model initially developed for the olivine and orthopyroxene solid solutions at high degrees of serpentinization susceptibilities that are lower than those observed are predicted by reactions where iron is sequestered by serpentine and brucite observed density susceptibility trends of serpentinized harzburgites fall between the values predicted by these limiting reactions the observed properties may be modelled by multiple reactions involved in multi stage serpentinization processes such that the production of magnetite increases with the degree of serpentinization the empirical and calculated properties provide a complementary characterization of density and susceptibility values of serpentinites

observation of the eruption sequence and formation process of a temporary lava lake during the june august 2015 mt raung eruption indonesia using high resolution and high frequency satellite image datasets abstract from june to august 2015 mt raung located at the east end of java indonesia erupted effusively with strombolian activity at the intra caldera cinder cone the effused lava from the base of the cone entirely buried the caldera floor with a lava bed and ultimately formed a temporary lava lake in this study details of the eruption sequence and formation process of the temporary lava lake are analyzed based on a series of high resolution images combined with the high frequency thermal observation results of himawari 8 the volume of effused lava was also estimated using a digital elevation model aster gdem based on the lava bed bathymetry in the caldera and lava bed distribution at several points over the course of the eruption the total effusion volume was estimated to be 7 5 107 m3 and the average effusion rate was 1 4 106 m3 day the activity consisted of four stages the precursory stage was characterized by low level thermal anomalies which are thought to result from high temperature spots at the bottom of several pits formed in the buried vent probably due to magma head partial exposure or gas emission pulses 1 and 2 were effusive stages that formed the lava bed the pattern recognized in the time series variation of thermal anomalies was confirmed to reflect the lava effusion rate using the volumes estimated in this study the lava bed is divided into the lower and upper lava groups which were generated in pulses 1 and 2 respectively except for the fine lobes formed around the margin of the lower lava group the terminal stage also showed low level thermal anomalies most likely caused by gas emissions the intra caldera cone grew to approximately 100 m higher than its initial height by mid august and then collapsed into the conduit at the end of the terminal stage the lava bed increased in thickness through lobe multi layering and later through the addition of lava directly from the vent into the lava bed interior over the course of the lava bed growth the bed outline nearly matched the caldera contours uplift of the lava bed surface caused lateral fluidization of the bed and molten lava was extruded at the bed margins forming fine lobes this process enlarged the zone of fine lobes around the lava bed thus the lava bed growth resulted in the appearance of a lava lake in the middle of pulse 1 a small amount of hot material was thrown onto the slope beyond the caldera rim in the future the caldera floor uplift caused by this activity approximately 49 m will increase the risk of erupted material ejection outside the caldera in a comparison with the 2013 15 nishinoshima eruption which was a similar effusive eruption mt raung was shown to have predominantly large lobes over most of the lava bed while in nishinoshima narrow medium scale lobes occupied the majority of the surface this may be attributed to the high effusion rate of lava for mt raung which was approximately seven times higher on average than the lava effusion rate for nishinoshima

the magnetic properties of metaperidotitic rocks as a function of metamorphic grade implications for crustal magnetic anomalies seventeen samples from the malenco serpentinite in the swiss alps representing systematic prograde metamorphic conditions from prehnite pumpellyite to upper amphibolite are moderately to strongly magnetic average susceptibility of 6 61 10 2 si units due predominantly to the presence of a magnetite rich spinel some greenschist facies samples contain the metal alloy awaruite ni3fe these contain in addition much greater amounts of magnetite small amounts of pyrrhotite in about half the samples complete the contributions to the total magnetization susceptibilities generally decrease with increasing metamorphic grade because of the production of increasing amounts of chromerich spinel which dilutes the magnetite component these results indicate that significant magnetic anomalies should be generated in suture zones containing metamorphosed serpentinite and in areas where serpentinite is being actively subducted

constitutive mass balance relations between chemical composition volume density porosity and strain in metasomatic hydrochemical systems results on weathering and pedogenesis relations characterizing the chemical physical and mechanical changes resulting from metasomatic hydrochemical processes are developed using mass balance models which formally link chemical composition to bulk density mineral density volumetric properties porosity and amount of deformation strain rigorous analysis of aqueous solute transport effects is then made possible in a variety of porous media flow environments including chemical weathering pedogenesis soil formation diagenesis ore deposition and enrichment and metamorphism application of these linear constitutive relations to chemical weathering profiles shows that immobile and locally mobile chemical elements with masses conserved on the scale of soil profiles can be accurately identified from analysis of appropriate data arrays and then used as natural geochemical tracers to infer the nature and extent of hydrochemical weathering processes and volume changes during pedogenesis assumptions commonly made in the past about the supposed immobility of certain elements e g ti and zr become unnecessary quantitative differentiation between the effects of residual and supergene fractionation is then easily made these methods are applied to ni rich laterites developed by weathering of ultramafic rocks showing that during ordinary residual enrichment ni is concentrated by as much as 4 protolith peridotite concentrations this occurs simply by silicate mineral dissolution and removal of chemical elements other than ni e g mg with a corresponding reduction in saprolite density and increase in bulk porosity without significant deformation in contrast laterites with mineable concentrations of ni which are similarly undeformed such as the nickel mountain mine in riddle oregon have experienced in addition to residual enrichment strong supergene enrichment by fractionation of ore elements between a leached zone from which ni is extracted and a complementary enriched zone positioned farther along the direction of ground water flow soil forming processes in podzol chronosequences developed on sandy beach terraces of the mendocino coast of california involved soil column collapse of 60 percent by dissolution of silicate minerals in the albic horizon of al and fe leaching and 70 percent dilation expansion in the overlying organic rich layer by root growth the amount of erosion based upon paleosurface reconstructions using the excess mass of fe al pb ga and cu in the zone of supergene enrichment spodic horizon below the ground water table indicates that subsurface erosion by dissolutional collapse is three times that of surficial erosion finally using published chemical data for ti zr and cr on major bauxite deposits in australia where erosion rates are thought to be low we infer that there may have been major amounts of dissolutional collapse to explain the upwards increase of detrital zircon and rutile in weathering profiles

new evidence of co2 soil degassing anomalies on piton de la fournaise volcano and the link with volcano tectonic structures insu cnrs and la reunion prefecture projet pour la quantification de l alea volcanique a la reunion

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

alpine serpentinites ultramafic magmas and ocean basin evolution the ideas of h h hess hess9 ideas related to the tectonic significance of ultramafic rocks ultramafic magmas evolution of oceanic crust and h 2 o outgassing all emanating from his abiding interest in serpentinite hess recognized the prime importance of ultramafic rocks in understanding the tectonic development of collisional alpine type mountain belts they are not magmatic intrusions as he proposed but tectonic remnants of fossil plate boundaries hess was the formulator of the concept of sea floor spreading his ideas on the nature of oceanic crust changed through the years but he repeatedly argued that oceanic crust was partly or predominantly serpentinite despite the lack of subsequent popularity of this view he was at least partly right serpentinite is a part of oceanic crust notably in tectonically thinned regions along rifted ridges near transform faults and along obliquely rifted continental margins such as the mesozoic western alpine continental margin

internal structure of piton de la fournaise volcano from seismic wave propagation and earthquake distribution abstract arrival times of seismic waves from local earthquakes are inverted for both locating the source and defining the 3 d velocity heterogeneity of piton de la fournaise the lateral heterogeneity of the 2632 m high edifice is resolved as a high velocity plug 1 5 km in diameter surrounded by a low velocity ring which may be interpreted as due to the construction of fournaise on the flank of the older volcano piton des neiges wave mode conversion detected on three component seismograms provides evidence for boundaries of contrasted velocities pre eruptive swarm earthquakes cluster in the high velocity zone under the dolomieu summit crater low strength and cohesion of the surrounding material account for the lack of seismicity for the final 1 3 km radial flow of magma to the vents in enclos fouque beneath the high velocity plug the existence of a body with low velocity for p and even for s waves is well constrained however the walls and base are poorly defined because of the lack of deep earthquakes for sampling the few earthquakes that are located in this depth region usually occur at a depth of around 1 5 km below sea level in the region of the cone this can be considered providing the upper constraint on the lower limit of the aseismic part of the low velocity body the coincidence in time of their occurrence with the swarms above sea level and the eruptions suggests magmatic activation of the low velocity aseismic volume 1 5 km below sea level under the high velocity plug of the cone further down the concentration of seismicity in two swarms between 2 and 4 km under the eastern flank does not allow the structure to be sampled effectively

archeomagnetism of piton de la fournaise bearing on volcanic activity at la reunion island and geomagnetic secular variation in southern indian ocean historical and other recent lavas from piton de la fournaise la reunion island are studied using the large sample archeomagnetic method which provides here paleodirections of the earth magnetic field with confidence cones between 0 9 and 2 5 thus offering a precise record of the geomagnetic field and potential dating constraints ages of the lavas are known thanks to historical chronicles from 1708 or 14c dating and their analysis give information about the directional secular variation sv in a region where observatory measurements are scarce and not available before the end of the 19th century for the past 250 yr we find a high value of inclination 50 to 55 with respect to the geographic latitude 21 s connected with a very restricted directional sv conversely the older volcanic units present a larger sv with magnetic inclinations of between 31 and 46 and declinations from 16 w to 10 e these results which are in reasonable agreement with instrumental measurements made on ships in the vicinity of la reunion allow us to infer that undated lavas of the northern part of the caldera emanated from eruptions during the second half of the 1700s other volcanic products e g mare longue flow piton chisny cone and flows and pointe langevin despite their fresh morphology are necessarily older than about 1500 ad further knowledge of the path of the sv in the more distant past and therefore further archeomagnetic dating is hampered by the lack of a precise chronology through radiometric or other similar methods available models of the geomagnetic field during the last millennium which suffer from the scarcity of data in the southern hemisphere are discussed in the light of our results

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

serpentinized ultramafics and hydrothermal activity at the mid atlantic ridge crest near 15 n the initial occurrence of serpentinized ultramafic rocks at the nontransform intersection of a wall of a rift valley with the wall of a fracture zone is described from a site at the fifteen twenty fracture zone the ultramafics crop out in block faulted terrain on the upper portion of the eastern intersection between the rift valley and fracture zone walls in water depths between 2910 and 3300 m they comprise cumulate harzburgites pyroxenites iherzolites and wehrlites as well as gabbronorites olivine gabbronorites gabbropegmatites and alteration products including serpentinites bastite serpentinites and asbestos the ti zr y relations and relatively constant zr ti ratio in basalts recovered with the ultramafic rocks indicate a cogenetic relation from a common magmatic source that has undergone a late stage differentiation in the lower crust ongoing hydrothermal activity is indicated by chemical anomalies 3he mn in the near bottom water at the ultramafic outcrop the upwelling hydrothermal circulation apparently follows crust penetrating faults that may have controlled the diapiric ascent of the serpentinites and that continue to tap degassing magma and or mantle the observations presented indicate that ultramafic cumulates form beneath the rift valley adjacent to long offset 100 km ridge ridge transform faults where they are serpentinized by hydrothermal processes within the initial 1 106 years of generation of lithosphere at a slow spreading axis the corners formed by the intersections of the walls of a rift valley with both the transform rt corner and the nontransform rn corner portions of these fracture zones are principal loci of diapiric emplacement of serpentinized ultramafics

fracture patterns in the zagros fold and thrust belt kurdistan region of iraq abstract fracture data have been collected in the kurdistan region of iraq which is a poorly accessible and unexplored area of the zagros pre to early folding ne sw striking extensional fractures and nw se striking contractive elements represent the older set affecting the exposed multilayer of the area these latter structures are early syn folding and followed by folding related mesostructural assemblages which include elements striking parallel to the axial trend of major folds longitudinal fractures bedding perpendicular joints and veins and extensional faults belonging to this second fracture set are located in the outer arc of exposed anticlines whilst longitudinal reverse faults locate in the inner arcs consistently these elements are associated with syn folding tangential longitudinal strain the younger two sets are related to e w extension and nne ssw to n s shortening frequently displaying reactivation of the older sets the last shortening event which is described along the entire zagros belt probably relates with the onset of n s compression induced by the northward movement of the arabian plate relative to the eurasian plate in comparison between the inferred palaeostrain directions and the kinematics of recent gps measurements we conclude that the n s compression and the partitioning into nw se trending folds and nw to n trending strike slip faults likely remained unchanged throughout the neogene tectonic history of the investigated area

a field chemical and stable isotope study of subseafloor metamorphism of the josephine ophiolite california oregon the josephine ophiolite is a large complete ophiolite generated in a late jurassic back arc basin along western north america the ophiolite underwent subseafloor metamorphism under a steep thermal gradient as indicated by downward changes in mineralogy and 18o values characteristic mineral zonation includes mica orthoclase and hematite in the lower extrusives greenschist facies assemblages in the upper sheeted dikes and amphibolite facies assemblages in the lower sheeted dikes and high level gabbro cumulate gabbros are only incipiently altered but are depleted in 18o indicating high temperature alteration with fluids that evolved at low water w rock r ratios a second off axis circulation system is recorded by metalliferous sediments that occur 8 21 m above the ophiolite alteration in the upper sheeted dike complex and especially the extrusive sequence is heterogeneous at outcrop scale as indicated by large variations in mineralogy chemistry and 18o values within individual pillows and dikes the heterogenity is due to 1 variations in w r ratio and starting material during alteration by downwelling seawater e g pillow cores versus glassy rims and 2 localized discharge mineralization mostly epidosites superimposed on the background recharge alteration epidosites occur largely as dike parallel stringers irregular replacement of pillow and massive lava and replacement of interpillow and pillow breccia matrices the chemistry of most extrusives and sheeted dikes is characterized by loss of ca and gain in na and mg consistent with alteration by downwelling fluids in addition the sheeted dikes are depleted in k2o and the lower sheeted dikes and high level gabbros are depleted in zn and cu the 18o and d values for most samples from the extrusive sequence indicate alteration by seawater at 50 rotation of the entire crustal sequence at the spreading axis suggests that the ophiolite formed at a slow spreading center where magma chambers were episodic

rapid morphological changes at the summit of an active volcano reappraisal of the poorly documented 1964 eruption of mount etna italy while the eruptive record of mount etna is reasonably complete for the past 400 years the activity of the early and late 1960s which took place at the summit is poorly documented in the scientific literature from 1955 to 1971 the central and northeast craters were the sites of long lived mild strombolian and effusive activity and numerous brief episodes of vigorous eruptive activity which led to repeated overflows of lava onto the external flanks of the volcano a reconstruction of the sequence of the more important of these events based on research in largely obscure and nearly inaccessible sources permits a better understanding of the eruption dynamics and rough estimates of erupted volumes and of the changes to the morphology of the summit area during the first half of 1964 the activity culminated in a series of highly dynamic events at the central crater including the opening of a fissure on the e flank of the central summit cone lava fountains voluminous tephra emission prolonged strong activity with continuous lava overflows and growth of large pyroclastic intracrater cones among the most notable processes during this eruption was the breaching of a section of the crater wall which was caused by lateral pressure of lava ponding within the crater comparison with the apparently similar summit activity of 1999 allows us to state that a lava overflows from large pit craters at the summit are often accompanied by breaching of the crater walls which represents a significant hazard to nearby observers and that b eruptive activity in 1999 was much more complex and voluminous than in 1964 for 1960s standards however the 1964 activity was the most important summit eruption in terms of intensity and output rates for about 100 years causing profound changes to the summit morphology and obliterating definitively the former central crater

origin and emplacement of ophiolites ever since their recognition ophiolites have been a source of controversy although all workers have agreed upon their importance a number of conflicting schools of thought have become unified in the interpretation of ophiolite complexes as fragments of oceanic crust and mantle formed at spreading centers an expanded version of the penrose ophiolite definition is proposed herein called the ophiolite association taking into account rocks which bear upon the origin and significance of ophiolite complexes an ideal ophiolite association includes the following units from bottom to top 1 a crystalline basement and shallow water sedimentary sequence 2 a tectonic unit of thrust slices of continental margin and rise and abyssal sediments and or melange 3 a metamorphic unit as much as a few hundred meters thick generally with higher grade rocks over lower grade ones 4 ultramafic tectonite unit composed dominantly of multiply deformed peridotite dunite and minor chromitite 5 a cumulate complex ultramafic at the base grading to mafic or intermediate at the top 6 a noncumulus unit of varitextured gabbro and minor trondhjemite 7 a sheeted dike complex 8 an extrusive section of massive and pillowed flows and intercalated sediments 9 an abyssal or bathyal sediment sequence which may include radiolarian chert red pelagic limestone metalliferous sediments breccias and or pyroclastic deposits and 10 postemplacement deposits of laterite reef limestone or shallow marine or subaerial sediments this expanded ophiolite association allows greater precision in inferring original origin and emplacement some ophiolite complexes particularly in the tethys herein designated tethyan type exhibit units 1 2 3 and 4 whereas others particularly in the cordilleran belt herein termed cordilleran type lack units 1 2 3 and in places 4 the emplacement of the former probably occurred during collision of a passive margin with a subduction zone or incipient subduction zone emplacement of the cordilleran type generally is not clear cut the nature of the spreading center which produced a given ophiolite is indicated more clearly by the presence or absence of units 5 9 than by chemistry or mineralogy some ophiolite complexes lack complete sections and may have formed along transform faults others contain volcanic pyroclastic deposits instead of pelagic sedimentary sequence and probably formed within or near an arc several ophiolite complexes may have originated by conversion of a transform fault into a subduction zone such conversion may result from normal plate evolution or from major plate reorganization major times of ophiolite emplacements appear to correspond with global plate reorganizations

phreatomagmatic eruptions associated with the caldera collapse during the miyakejima 2000 eruption japan abstract the 2000 ad eruption of miyakejima was characterized by a series of phreatomagmatic eruptions from the subsiding caldera six major eruptive events occurred and they can be divided into the first and second periods separated by a 25 day hiatus the phreatomagmatic eruptions produced a total of 2 1010 kg of tephra which mainly comprised fine grained volcanic ash the tephra layers could be divided into six fall units corresponding to the six major eruptive events the grains of the 2000 tephra consisted mainly of accessory fragments derived from the previous edifice juvenile fragments were identified in four major units the angular and blocky shape of the juvenile fragments showed rapid cooling of magma by external water indicating the mechanical mixing of ground water and magma the removal of the load on the aquifer by caldera subsidence and the intrusion of magma into the decompressed aquifer caused a series of phreatomagmatic eruptions from the subsiding caldera the absence of surface water in the subsiding caldera during the eruption showed that groundwater participated in the phreatomagmatic eruptions rising magma along the ring faults intruded into the decompressed aquifer surrounding the caldera and caused phreatomagmatic explosions the 2000 eruption of miyakejima indicated that the fine grained tephra abundant in hydrothermally altered rock fragments is a possible indicator of the phreatomagmatic eruptions in the hydrothermal system during the caldera formation in basaltic volcanoes

imaging the dynamics of dyke propagation prior to the 2000 2003 flank eruptions at piton de la fournaise reunion island 1 the relationship between shallow magma storage and flank injections in volcanic edifices is rarely documented we analyse the time series of geodetic and seismic data collected at piton de la fournaise volcano la reunion during 9 flank eruptions from 2000 to 2003 the data depicts how magma injections supplied flank eruptions at piton de la fournaise volcano and allows to precisely determine the direction and duration of dike intrusions indicating a two phases pattern consistent over all the eruptions 1 fast vertical migration of the dyke 2 m s from the magma chamber to the surface in 10 to 50 minutes and 2 slower lateral migration 0 2 0 8 m s in tens of minutes to hours controlled by the rift zone geometry

edifice growth deformation and rift zone development in basaltic setting insights from piton de la fournaise shield volcano reunion island abstract the overall morphology of basaltic volcanoes mainly depends on their eruptive activity effusive vs explosive the geometry of the rift zones and the characteristics of both endogenous and exogenous growth processes the origin of the steep geometry of the central cone of piton de la fournaise volcano which is unusual for a basaltic effusive volcano and its deformation are examined with a combination of a detailed morphological analysis field observations gps data from the piton de la fournaise volcano observatory and numerical models the new caldera walls formed during the april 2007 summit collapse reveal that the steep cone is composed of a pyroclastic core inherited from an earlier explosive phase overlapped by a pile of thin lava flows this suggests that exogenous processes played a major role in the building of the steep central cone magma injections into the cone which mainly occur along the n25 30 and n120 rift zones lead to an asymmetric outward inflation concentrated in the cone s eastern half this endogenous growth progressively tilts the southeast and east flanks of the cone and induces the development of a dense network of flank fractures finally it is proposed that intrusions along the n120 rift zone are encouraged by stresses induced by magma injections along the n25 30 rift zone

effective magnetic susceptibility of the oceanic upper mantle derived from magsat data passive ocean continent boundaries have little magnetic signatures at magsat altitude 400 km whereas subducting oceanic lithosphere has well defined magnetic anomalies over some of the subduction zones based on this observation and a thermal evolution model of the oceanic lithosphere subducted along the kuril kamchatka trench we determined an effective magnetic susceptibility of 0 0085 si for the upper 35 km of the oceanic upper mantle

keanakākoʻi tephra produced by 300 years of explosive eruptions following collapse of kīlauea s caldera in about 1500 ce abstract the keanak ko i tephra at k lauea volcano has previously been interpreted by some as the product of a caldera forming eruption in 1790 ce our study however finds stratigraphic and 14c evidence that the tephra instead results from numerous eruptions throughout a 300 year period between about 1500 and 1800 the stratigraphic evidence includes 1 as many as six pure lithic ash beds interleaved in sand dunes made of earlier keanak ko i vitric ash 2 three lava flows from k lauea and mauna loa interbedded with the tephra 3 buried syneruptive cultural structures 4 numerous intraformational water cut gullies and 5 abundant organic layers rich in charcoal within the tephra section interpretation of 97 new accelerator mass spectrometry ams 14c ages and 4 previous conventional ages suggests that explosive eruptions began in 1470 1510 ce and that explosive activity continued episodically until the early 1800s probably with two periods of quiescence lasting several decades k lauea s caldera rather than forming in 1790 predates the first eruption of the keanak ko i and collapsed in 1470 1510 immediately following and perhaps causing the end of the 60 year long 4 6 km3 ail au eruption from the east side of k lauea s summit area the caldera was several hundred meters deep when the keanak ko i began erupting consistent with oral tradition and probably had a volume of 4 6 km3 the caldera formed by collapse but no eruption of lava coincided with its formation a large volume of magma may have quickly drained from the summit reservoir and intruded into the east rift zone perhaps in response to a major south flank slip event leading to summit collapse alternatively magma may have slowly drained from the reservoir during the prolonged ail au eruption causing episodic collapses before the final largest downdrop took place two prolonged periods of episodic explosive eruptions are known at k lauea the keanak ko i and the uw kahuna tephra fiske et al 2009 and both occurred when a deep caldera existed probably with a floor at or below the water table and external water could readily interact with the magmatic system the next period of intense explosive activity will probably have to await the drastic deepening of the present caldera or halema uma u crater or the formation of a new caldera

seismic anisotropy and its precursory change before eruptions at piton de la fournaise volcano la reunion the piton de la fournaise volcano exhibits frequent eruptions preceded by seismic swarms and is a good target to test hypotheses about magmatically induced variations in seismic wave properties we use a permanent station network and a portable broadband network to compare seismic anisotropy measured via shear wave splitting with geodetic displacements ratios of compressional to shear velocity vp vs earthquake focal mechanisms and ambient noise correlation analysis of surface wave velocities and to examine velocity and stress changes from 2000 through 2012 fast directions align radially to the central cone and parallel to surface cracks and fissures suggesting stress controlled cracks high vp vs ratios under the summit compared with low ratios under the flank suggest spatial variations in the proportion of fluid filled versus gas filled cracks secular variations of fast directions and delay times dt between split shear waves are interpreted to sense changing crack densities and pressure delay times tend to increase while surface wave velocity decreases before eruptions rotations of may be caused by changes in either stress direction or fluid pressure these changes usually correlate with gps baseline changes changes in shear wave splitting measurements made on multiplets yield several populations with characteristic delay times measured incoming polarizations and fast directions which change their proportion as a function of time an eruption sequence on 14 october 2010 yielded over 2000 shear wave splitting measurements in a 14 h period allowing high time resolution measurements to characterize the sequence stress directions from a propagating dike model qualitatively fit the temporal change in splitting

magnetic properties of synthetic analogues of the altered olivines of igneous rocks summary a laboratory simulation has been made of the high temperature deuteric alteration of olivine such as occurs in slowly cooled basic intrusions three compositions in the forsterite fayalite solid solution series were synthesized by a three firing method using self buffering and controlled atmosphere to obtain material containing no detectable magnetite fe o as a contaminant 0 005 per cent the prepared olivines were oxidized between 850 and 1280 c at oxygen fugacities of between lo and lop5 atmospheres the composition and concentration of the multiphase oxidation products were studied by magnetic optical electron optical and x ray methods the magnetic constituent of the intergrowth was magnetite below about 1150 c the microstructure is similar to the dendritic intergrowths found for example in the altered olivine of the central layered intrusion of the isle of rhum above about 1150 c the form may be similar to symplectite intergrowths the magnetization process parameters coercive force etc vary systematically with temperature of oxidation to which evidently controls the microstructure of the magnetite the magnetite produced is magnetically hard samples altered at 900 c and 950 c having higher coercive force hc than any magnetite reported in the literature the room temperature properties appear to bridge the published trends found in grown crystal magnetites and crushed grains inferred effective grain sizes of the complex intergrowths range from 0 1 to 3 pm as to increases from 900 to 1200 c the temperature dependence of magnetization process parameters principally h measured between room temperature and the curie point are modelled in terms of i monodomain grains subject to thermal fluctuations ii monodomain grains not subject to thermal fluctuations and iii as multidomain grains model i yields plausible values for effective grain dimensions and length diameter ratios higher to producing larger more equidimensional particles model ii suggests that the relative importance of cubic and uniaxial anisotropies may be influenced by a microstructure dependent uniaxial particle particle interaction term model iii implies that if domain walls are pinned by both surface and volume defects magnetostriction plays a greater role in volume pinning than in surface pinning plausible values of demagnetizing factors appropriate to grains with few domains are produced by model iii

spatio temporal evolution of rockfall activity from 2007 to 2011 at the piton de la fournaise volcano inferred from seismic data abstract seismic data have been used to catalog the location and volume of most of the rockfalls that occurred at the piton de la fournaise volcano from may 2007 just after the major collapse of the dolomieu summit crater floor to may 2011 this catalog made it possible to compare the evolution of the number and volume of rockfalls at a high temporal resolution and to investigate their links with eruptions seismicity deformations and rainfalls affecting the piton de la fournaise volcano results show that the purge of unstable areas created by the dolomieu crater floor collapse occurred in two phases a first phase lasting three months during which the intense rockfall activity immediately following the collapse decreased abruptly and a second phase lasting more than two years during which the daily volume of the rockfalls slowly decreased before reaching a steady state a detailed study of 4 time periods including 3 eruptive cycles indicates that strong seismicity can increase the number of rockfalls furthermore when a dike reaches the surface at the summit of the central cone giving birth to an eruption the associated local forcing can in some cases increase the volume of rockfalls possibly by creating or expanding weak zones in most cases this is not observed for dike intrusions that do not reach the surface the newly created weak zones are most often far from the eruptive fissures of the ongoing eruption but close to the location of the next eruption this suggests that these distant zones are also weakened at depth thus creating preferred paths for future dikes propagating toward the surface the impact of rainfall on rockfall activity was also studied given that la reunion island is subject to very intense rainfall events our results show that rainfall can in some cases trigger rockfalls rapidly with response times of less than a day but not systematically and no triggering rainfall threshold was found

mineralization alteration and hydrothermal metamorphism of the ophiolite hosted turner albright sulfide deposit southwestern oregon the turner albright sulfide deposit part of the josephine ophiolite formed on and below the seafloor during late jurassic volcanism at a back arc spreading center ore fluids were probably localized by faults which were active on the seafloor at the time of sulfide deposition the uppermost massive sulfide formed on the seafloor at hydrothermal vents similar to sulfide depositing hot springs on modern spreading centers the bulk of the sulfide mineralization formed below the seafloor within olivine basalt hyaloclastite erupted near the time of mineralization infiltration of hydrothermal fluid into the hyalo clastite altered the rock to quartz fe chlorite and quartz sericite fe chlorite intense alteration resulted in replacement of the protolith by quartz sulfide the fluid responsible for the hydrothermal alteration was evolved seawater with low ph and mg and high fe sulfide 534s values average 4 7 and indicate contribution of sulfur from both basalt and seawater sulfate the average value of sulfide 34s and the difference between sulfide and contemporaneous seawater sulfate 34s values are similar to ophiolite hosted sulfide deposits in cyprus chlorite rich hydrothermally altered basalt has 18o values of 3 0 6 8 due to high temperature 250 350 c reaction with a seawater derived ore fluid quartz in silicified basalt has higher 18o values of 12 3 15 5 probably reflecting lower temperature recrystallization of amorphous silica formed during initial alteration of basalt mudstone and clinopyroxene basalt above the sulfide horizons were not altered by the ore transporting hydrothermal fluid but these rocks were hydrothermally metamorphosed by altered seawater heated by deep circulation into hot oceanic crust this subseafloor metamorphism produced a mineral assemblage typical of prehnite pumpellyite facies metamorphism exchange with altered seawater increased the whole rock 18o of the basalts to values of 9 4 11 2

investigating the deepest part of a volcano plumbing system evidence for an active magma path below the western flank of piton de la fournaise la reunion island abstract peripheral diffuse degassing of co 2 from the soil occurs across the western flank of piton de la fournaise volcano la reunion island indian ocean along a narrow zone in this area carbon isotopic analysis on soil gas samples highlights significant mixing between magmatic and organic end members the zones with the strongest magmatic signature highest 13 c overlap spatial distribution of hypocenters recorded shortly before and during volcano reactivation and allow discriminating a n135 degassing lineament with a minimum length of 11 km and 140 20 m width such orientation is in accordance with that of an old dyke network along the rift zone and with n120 130 and n140 155 lineaments related to the inheritance of oceanic lithosphere structures our findings show that this n135 lineament represents a preferential magmatic pathway for deep magma transfer below the volcano flank moreover spatial distributions of recent eccentric cones indicate a well founded possibility that future eruptions may by pass the shallow plumbing system of the central area of the volcano taking a lateral pathway along this structure our results also confirm that piton de la fournaise activity is linked to a laterally shifted plumbing system and represent a major improvement in identifying the main high risk area on the densely populated western flank of the volcano

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

rift zones and magma plumbing system of piton de la fournaise volcano how do they differ from hawaii and etna on ocean basaltic volcanoes magma transfer to the surface proceeds by subvertical ascent from the mantle lithosphere through the oceanic crust and the volcanic edifice possibly followed by lateral propagation along rift zones we use a 19 year long database of volcano tectonic seismic events together with detailed mapping of the cinder cones and eruptive fissures to determine the geometry and the dynamics of the magma paths intersecting the edifice of piton de la fournaise volcano we show that the overall plumbing system from about 30 km depth to the surface is composed of two structural levels that feed distinct types of rift zones the deep plumbing system is rooted between piton des neiges and piton de la fournaise volcanoes and has a n30 40 orientation above 20 km below sea level bsl the main axis switches to a n120 orientation which permits magma transfer from the lithospheric mantle to the base of the oceanic crust below the summit of piton de la fournaise the related nw se rift zone is 15 km wide linear spotted by small to large pyroclastic cones and related lava flows and emits slightly alkaline magmas resulting from high pressure fractionation of clinopyroxene olivine this rift zone has low magma production rate of 0 5 3 6 10 3 m 3 s 1 and an eruption periodicity of around 200 years over the last 30 ka seismic data suggest that the long lasting activity of this rift zone result from regional nne ssw extension which reactivates inherited lithospheric faults by the effect of underplating and or thermal erosion of the mantle lithosphere the shallow plumbing system b11 km bsl connects the base of the crust with the central cone it is separated from the deep plumbing system by a relatively large aseismic zone between 8 and 11 km bsl which may represent a deep storage level of magma the shallow plumbing system feeds frequent short lived summit and flank ne and se flanks eruptions along summit and outer rift zones respectively summit rift zones are very active 0 1 0 25 m 3 s 1 short 2 3 km and present an orthogonal pattern confined to the central active cone of piton de la fournaise outer ne and se rift zones are much less active 4 7 3 10 3 m 3 s 1 and extend from inside the enclos fouque caldera to bound the mobile eastern volcano flank we show that the outer rift zones are almost aseismic and are genetically linked to the seaward flank displacements whose most recent events where detected in 2004 and 2007 east flank sliding is itself triggered by shallow b 2 km depth sill intrusions we propose that the subvertical magma intrusions along the perpendicular summit rift zones sill intrusions and subsequent magma injections along the outer rift zones are controlled by cycles of stress permutations we thus tentatively propose that as for piton de la fournaise the regional stress field acting on etna and hawaiian volcanoes is an important parameter in the control of the magma transfer along their deep plumbing system whereas the dynamics of their summit system is governed by shallow stress variations above 5 10 km bsl due to edifice deformation and magma transfers

microstructure of serpentinite mylonites from the josephine ophiolite and serpentinization in retrogressive shear zones california the josephine peridotite has experienced deformational episodes over a wide range of temperatures deformation within the stability field of a serpentine group mineral appears invariably accompanied by retrogressive metamorphism serpentinization two general types of deformed serpentinites have been identified 1 incohesive serpentinites most common which appear to have experienced low temperature predominantly brittle deformation and 2 serpentinite mylonites which have experienced plastic flow near the upper limit of antigorite stability the distinction between these two is based on textural and mineralogical criteria serpentinite mylonites are cohesive homogeneously foliated and typically strongly lineated rocks composed primarily of antigorite and magnetite they occur in planar bounded zones enclosed within the josephine peridotite that were the loci of both noncoaxial deformation and serpentinization we discuss the development of two important shear sense indicators porphyroclast systems and oblique shear band foliations as observed in these rocks porphyroclast systems in serpentinite mylonites are often comprised of cores and mantles of different mineralogy cores are usually composed of primary pyrogenetic minerals whereas mantles including tails are composed of products of the serpentinization reaction these systems are therefore termed porphyroctasts with neocrystallization tails oblique shear bands are common micro structures in serpentinite mylonites shear bands which offset the mylonitic foliation along narrow discrete fractures are termed f type whereas those which offset the mylonitic foliation along narrow ductile micro shear zones are termed d type the latter are considered more reliable for shear sense determinations strains accomplished by movement on shear bands are probably small strain resulting from a volume increase dur ing serpentinization while holding the shear zone width constant is sufficient to produce the shear bands therefore we suggest that oblique shear bands may develop to accom modate volume increases associated with deformation

high temperature hydrothermal alteration of ultrmafic cumulates from the base of the sheeted dikes in the josephine ophiolite nw california ultramafic cumulate pods occur locally just below the base of the sheeted dike complex in the josephine ophiolite in north western california these ultramafic rocks occur with metagabbros and both are extensively intruded by mafic dikes the ultramafic cumulates contain evidence of high temperature hydrothermal alteration early alteration possibly above 900 c which continued to progressively lower temperatures 350 c this hydrothermal alteration has been partially obscured by later low grade regional metamorphism petrographic evidence for the hydrothermal alteration includes 1 hydration of primary clinopyroxene to hornblende 2 supersolvus secondary hornblende 3 subsolvus coexisting hornblende actinolite and actinolite chlorite 4 alteration of hornblende to chlorite 5 alteration of orthopyroxene and olivine to serpentine and 6 serpentine veining the composition of the altering fluid appears to change with temperature

intricate episodic growth of a hawaiian tephra deposit case study of the 1959 kīlauea iki eruption the 1959 k lauea iki eruption on hawai i generated a succession of fountains most reaching hundreds of meters high the 16 episodes of fountaining persisted intermittently with repose periods of between 7 h and 4 days for 36 days they produced tephra deposits that were dispersed several kilometers downwind of the vent and a much larger volume of clastogenic lava which drained into the k lauea iki crater to form a 100 m deep lava lake field data from 211 tephra sample pits downwind of the vent reflect imperfectly the episodic nature of the fountaining behavior only five composite stratigraphic subunits from the total of 16 fountaining episodes can be mapped in the field however isopach maps of these subunits were generated and by the application of empirical deposit thinning relationships volumes of each subunit were estimated in combination with detailed observations made in 1959 our field data allow us to assign stratigraphic subunits to either single or aggregates of several fountaining episodes the most voluminous subunits are linked with the highest fountaining not with the longest in duration in fact significant downwind dispersal was possible only if the top part of the fountains reached above the crater rim which was located about 100 m above the vent the end of high fountaining episodes produced ash rich partings which are found on top of the lapilli sized products of some episodes however the ash rich intervals are strongly attenuated by wind advection and so are only present along the dispersal axes the subunits follow slightly different dispersal axes the maximum spread of 28 degrees between various dispersal axes is predominantly controlled by changing wind directions and spatter cone collapse into the vent which modified its geometry this study outlines the dependency of tephra dispersal on fountain height and emphasizes the capability of an episodic hawaiian fountaining eruption to generate a seemingly monotonous downwind tephra deposit

pigeonholing pyroclasts insights from the 19 march 2008 explosive eruption of kīlauea volcano we think conventionally of volcanic explosive eruptions as being triggered in one of two ways by release and expansion of volatiles dissolved in the ejected magma magmatic explosions or by transfer of heat from magma into an external source of water phreatic or phreatomagmatic explosions we document here an event where neither magma nor an external water source was involved in explosive activity at k lauea instead the eruption was powered by the expansion of decoupled magmatic volatiles released from deeper magma which was not ejected by the eruption and the trigger was a collapse of near surface wall rocks that then momentarily blocked that volatile flux mapping of the advected fall deposit a day after this eruption has highlighted the difficulty of constraining deposit edges from unobserved or prehistoric eruptions of all magnitudes our results suggest that the dispersal area of advected fall deposits could be miscalculated by up to 30 of the total raising issues for accurate hazard zoning and assessment eruptions of this type challenge existing classification schemes for pyroclastic deposits and explosive eruptions and in the past have probably been interpreted as phreatic explosions where the eruptive mechanism has been assumed to involve flashing of groundwater to steam

april 2007 collapse of piton de la fournaise a new example of caldera formation collapse calderas are frequent in the evolution of volcanic systems but very few have formed during historical times piton de la fournaise is one of the world s most active basaltic shield volcanoes the caldera collapse which occurred during the april 2007 lateral eruption is one of the few large documented collapse events on this volcano it helps to understand the mode and origin of caldera collapses in basaltic volcanoes field observations gps and seismic data show that the collapse occurred at an early stage of the eruption the cyclic seismic signal suggests a step by step collapse that directly influenced the lateral eruption rate likely the caldera results from the combined effect of i the progressive collapse of the plumbing system above the magma chamber since 2000 and ii the large amount of magma withdrawal during the early stage of the eruption by both a significant intrusion within the edifice and an important emission rate

retrieving 65 years of volcano summit deformation from multitemporal structure from motion the case of piton de la fournaise la reunion island the structure from motion photogrammetry technique enables use of historical airborne photography to achieve high resolution 3d terrain models we apply this method on piton de la fournaise volcano la reunion which allows a unique opportunity to retrieve high resolution 1 5 0 11 m digital elevation models and precise deformation maps of the volcano since 1950 our results provide evidence that the summit volume increased throughout the study period at a stable rate of 2 2 mm3 yr between 1950 and 2000 increasing by a factor of four to 8 0mm3 yr prior to the major 2007 eruption which was accompanied by summit caldera collapse at the same time summit deformation was asymmetric with 9 2 2 5 m of eastward seaward displacement and 1 3 2 5 m to the west during 1950 2015 our results reveal a temporal evolution in the volcano magma influx rate and deformation tracking these fluxes and the long lived preferential eastern motion is crucial to mitigate risks associated to flank destabilization

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

the 2007 eruptions and caldera collapse of the piton de la fournaise volcano la reunion island from tilt analysis at a single very broadband seismic station seismic records from la reunion island very broadband geoscope station are investigated to constrain the link between the 2007 eruptive sequence and the related caldera collapse of the piton de la fournaise volcano tilt estimated from seismic records reveals that the three 2007 eruptions belong to a single inflation deflation cycle tilt trend indicates that the small volume summit eruption of 18 february occurred during a phase of continuous inflation that started in january 2007 inflation decelerated 24 days before a second short lived small volume eruption on 30 march almost simultaneous with a sudden large scale deflation of the volcano deflation rate which had stabilized at relatively low level increased anew on 1 april while no magma was erupted followed on 2 april by a major distal eruption and on 5 april by a summit caldera collapse long term tilt variation suggests that the 2007 eruptive succession was triggered by a deep magma input

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

the josephine ophiolite northwestern california the 157 m y old josephine ophiolite is a complete ophiolite consisting of the following units harzburgite tectonite cumulate ultramafics cumulate gabbro interlayered with cumulate ultramafics cumulate gabbro high level gabbro and diorite a 1 5 km thick sheeted dike complex and 0 4 km of pillow lava and breccia the ophiolite is conformably overlain by the galice formation consisting of a thin siliceous pelagic sequence overlain by a thick sequence of turbidites having both volcanic arc and continental margin sources the ophiolite and overlying galice formation were intruded by numerous 150 to 151 m y old calc alkaline dikes and sills followed shortly by deformation and regional metamorphism under conditions of prehnite pumpellyite to lower greenschist facies the josephine peridotite makes up the base of the ophiolite and covers 800 km2 it is an alpine type peridotite that represents upper mantle material and the residue of partial melting ultramafic rocks overlying the josephine peridotite are olivine spinel clinopyroxene cumulates that grade upward into clinopyroxene and orthopyroxene olivine plagioclase cumulates and finally to clinopyroxene plagioclase cumulates high level gabbros and diorites are characterized by highly variable textures complex intrusive relationships strongly zoned plagioclase and abundant mafic dikes plagiogranites occur at this level as dikes small intrusions and screens between mafic dikes but they are rare the sheeted dike complex consists of mafic dikes averaging 0 6 m in thickness chilled margins are locally well preserved and many of the dikes have been split and intruded by younger dikes resulting in half and marginless dikes the dike complex is composed of 100 sheeted dikes near the center but contains gabbro screens near the base and pillow lava and breccia screens in its upper parts the dikes are inclined at 40 to bedding after correcting for folding and are inferred to have been rotated shortly after formation the extrusive sequence consists predominantly of pillow lavas but contains abundant pillow breccias and some massive lavas some of the breccias formed as talus deposits whereas others are graded and matrix supported indicating transport and redeposition possibly in an area of rugged submarine topography the dikes and lavas have geochemical affinities to island arc tholeiites but are in some aspects transitional to mid ocean ridge basalts the dikes and lavas are cogenetic with the underlying plutonic sequence as indicated by similarity in crystallization sequence contradictory intrusive relationships between sheeted dikes and high level plutonic rocks and lack of diabase dikes in the lowermost cumulates and harzburgite the upper portions of the ophiolite were hydrothermally metamorphosed at the spreading center resulting in downward changes in mineralogy texture and 18o in addition metalliferous sediments and massive sulfides that occur overlying and within the pillow lavas probably were formed by precipitation from thermal springs a geothermal gradient of 250 350 c km is inferred from mineral assemblages on the basis primarily of its regional setting the ophiolite is inferred to have formed in a back arc basin in particular it is within a thrust sheet sandwiched between a coeval magmatic arc complex and a slightly older arc complex remnant arc this interpretation is consistent with abundant volcanic arc detritus in graywackes overlying the ophiolite the island arc magmatic affinities of the lavas and dikes may be the result of formation of the ophiolite during the earliest stages of back arc spreading

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

variability of ash deposits at piton de la fournaise la reunion island insights into fragmentation processes at basaltic shield volcanoes 25 it is commonly accepted that effusive activity emplaces the main emitted magmatic volume in basaltic 26 shield volcanoes at piton de la fournaise la reunion island france eruptive activity occurs mostly within the 27 non populated enclos fouque caldera and generally does not pose any risk to the population however historical 28 observations recent monitoring data and field work on tephra deposits suggest that some eruptions have produced 29 unusual and unexpected explosive phases a comprehensive sampling of tephra from major historical and recent 30 eruptions on this volcano allowed us to perform systematic componentry grain size chemical and morphological 31 analyses in order to characterize the eruptive dynamics involved in these explosive basaltic eruptions this 32 integrative approach reveals highly variable characteristics of the studied tephra reflecting different fragmentation 33 efficiencies and multiple associated mechanisms primary ductile and partially brittle fragmentation of various 34 intensity of juvenile magma emitted during hawaiian fountaining or mild strombolian explosions were identified 35 as the most common fragmentation mechanisms in particular during the 2014 2018 dominantly effusive eruptive 36 sequence in parallel we distinguished more efficient short lived fragmentation events related to i plug 37 pressurization and brittle fragmentation enhanced by syn eruptive crystallization ii magma water interactions 38 iii rare phreatic explosions and iv secondary fragmentation producing fine ash during caldera collapse we 39 conclude that textural geochemical and morphological analysis make it possible to identify and characterize the 40 variability in eruptive processes with the grain size and the componentry of the ash particles being probably the 41 most important parameters to quantify both the efficiency and the nature of the fragmentation 42 43

remanent magnetization of the oceanic upper mantle skewnesses of the marine and magsat magnetic anomalies of the atlantic ocean are explained in terms of the remanent magnetization gradually acquired by the oceanic lithosphere during its cooling magnetizations of the lower crust and the upper mantle are about 20 50 and 10 30 percent of the magnetization of the oceanic basaltic layer 2a respectively the oceanic upper mantle controls the skewness of the magsat anomalies whereas the skewness of the marine anomalies arise primarily from the oceanic lower crust

structures and evolution of the plumbing system of piton de la fournaise volcano inferred from clustering of 2007 eruptive cycle seismicity analysis of seismic activity associated with the eruptions of 2007 which led to the collapse of the dolomieu crater on april 5th reveals the link between the seismicity and the magma transfers at piton de la fournaise three eruptive phases occurred on february 18th march 30th and april 2nd 2007 at the summit 2 000 m and 600 m high on the south east flank respectively illustrating the three types of eruptions defined for the current piton de la fournaise activity we use cross correlation of seismic waveforms and clustering to improve the earthquake locations and determine the best constrained focal mechanisms with an average of 78 p phase polarities the pre eruptive seismicity of the february and march eruptions is composed of time extended clusters that also preceded other distal eruptions from 2000 to 2007 our analysis shows that the seismic swarm prior to the february eruption initiated the intrusion that led to the april 2007 eruption the seismicity preceding the dolomieu crater collapse consists of numerous but time limited clusters and specific seismic activity that accompanied the dolomieu crater collapse from april 1st to april 5th earthquakes with clvd mechanisms combined with normal faulting sources occurred between 0 8 and 0 km asl until the complete rupture of the shallow magma storage roof this collapse induced the propagation of a de pressurization front and triggered a migration of seismicity from 0 to 8 km along a very narrow path

modeling the lava heat flux during severe effusive volcanic eruption an important impact on surface air quality the april 2007 eruption of piton de la fournaise one of the most active volcanoes in the world was the strongest eruption in recent decades with 230mm3 of lava emitted and more 300kt of so2 degased the surface concentrations of so2 have been measured by the ora air observatory of the reunion island and showed that many stations exceeded the critic threshold for health these high concentrations led to important health issues accompanied by environmental and infrastructure degradations realized with mesonh atmospheric model our simulations show the transport of sulfur and his component between 2 april and 6 april 2007 with a focus on the influence of heat flow from lava for this purpose we have implemented forefire a surface model initially realized to simulate forest fire by adapting it to reproduce the dynamic of a lava flow thus all flows so2 heat vapor co2 co are triggered depending on its dynamic with this first approach our simulations reproduce quite faithfully the surface field observation of so2 provides by ora various sensitivity analyzes exhibit that volcano sulfur distribution was mainly controlled by the lava heat flow without heat flow parameterization the surface concentrations are multiplied by a factor 30 compared to the reference simulation simulations also put in evidence that the 5 april during the height of the eruption changes in meteorological conditions especially weakening of atmospheric boundary layer stability led to various pollutants to be transported in higher altitude 8000m the main consequence is the volcanic pollutants are transported off the east coast of reunion island

dating young 1000 yr lava flow eruptions of piton de la fournaise volcano from size distribution of long lived pioneer trees dating recent lava flows is a critical issue to assess the volcanic hazards for nearby human populations but traditional methods such as radiocarbon dating are often not applicable we propose here a simple statistical method that relates the age of lava flows of the piton de la fournaise volcano pdf to the size of agarista salicifolia ericaceae a long lived pioneer tree that quickly establishes after eruption and able to live 600 years we measured the diameter at base of 711 trees on 20 dated lava flows between 1401 ce and 2007 ce we used a log log linear model to assess the relationship between maximum diameter at base of agarista salicifolia and the age of lava flows and showed a very strong correlation r2 0 987 p 2 2e 16 we then used this calibrated model to estimate the age of 11 lava flows between 1447 ce ci 1349 1531 and 1823 ce ci 1806 1839 these new ages combined with existing radiocarbon ages and historical records indicate three clusters of eruptions 1460 1630 1690 1840 since 1970 ce affecting both the caldera and the flanks of pdf we interpret such discontinuous dynamics made of periods of intense and low activity as evidencing pulses of high magma supply since at least the 11th century overall our work shows that dating lava flow with calibration based on the size distribution of long lived pioneer trees represents an accurate alternative method to redefine the hazard map of lava flow inundation the existence of long lived pioneer trees in several volcanic areas provides the opportunity to use the same framework in order to better understand eruption recurrence patterns

the 2007 caldera collapse of piton de la fournaise volcano source process from very long period seismic signals abstract in april 2007 piton de la fournaise volcano experienced its largest caldera collapse in at least 300 yr this event consisted of a series of 48 subsidence increments characterized by very long period vlp seismic signals equivalent to m w 4 4 to 5 4 source analysis of vlp events suggests a piston like mechanism with a collapsing crack source corresponding to the contraction of the magma reservoir and a single force representing the collapse of the above rock column we show that the collapse dynamics is primarily controlled by magma withdrawal from the reservoir which was likely in a bubbly state at the time of the event similar to the 2018 kilauea collapse we observe a reduction of the time interval between successive subsidence increments which results from the acceleration of magma withdrawal and a progressive weakening of the edifice at the beginning of the sequence

toward continuous quantification of lava extrusion rate results from the multidisciplinary analysis of the 2 january 2010 eruption of piton de la fournaise volcano la reunion the dynamics of the 2 12 january 2010 effusive eruption at piton de la fournaise volcano were examined through seismic and infrasound records time lapse photography so2 flux measurements deformation data and direct observations digital elevation models were constructed for four periods of the eruption thus providing an assessment of the temporal evolution of the morphology the volume and the extrusion rate of the lava flow these data were compared to the continuous recording of the seismic and infrasonic waves and a linear relationship was found between the seismic energy of the tremor and the lava extrusion rate this relationship is supported by data from three other summit eruptions of piton de la fournaise and gives total volume and average lava extrusion rate in good agreement with previous studies we can therefore provide an estimate of the lava extrusion rate for the january 2010 eruption with a very high temporal resolution we found an average lava extrusion rate of 2 4 m3s 1 with a peak of 106 6 m3s 1 during the initial lava fountaining phase we use the inferred average lava extrusion rate during the lava fountaining phase 30 23 m3s 1 to estimate the value of the initial overpressure in the magma reservoir which we found to range from 3 7 106 pa to 5 9 106 pa finally based on the estimated initial overpressure the volume of magma expelled during the lava fountaining phase and geodetic data we inferred the volume of the magma reservoir using a simple mogi model between 0 25 km3 and 0 54 km3 which is in good agreement with previous studies

magnetic structure of the upper kilometer of the marine crust at deep sea drilling project hole 504b eastern pacific ocean deep sea drilling project hole 504b is located in 5 9 m y crust in the eastern pacific ocean about 200 km south of the costa rica rift at 1076 m subbasement it is the deepest penetration of marine crust yet achieved we present here magnetic data from this hole especially from the recently cored leg 83 section which constitutes the lowermost 500 m these data when combined with those of other studies yield not only the deepest but also the most detailed and comprehensive picture of marine magnetic structure at a single site currently available the basement rocks from hole 504b can be divided into three major magnetic units the upper units the top 500 m or so of basement are essentially similar to other shallow marine basement sections consisting of a mixture of various types of extrusive basalt low temperature altered titanomagnetite is the dominant magnetic carrier and the magnetic properties of these rocks are comparable to other shallow marine basalts below the upper units is a 200 m transition zone consisting of a mixture of extrusives and dikes high temperature hydrothermal alteration much of it greenschist facies grade has caused oxidation exsolution of primary titanomagnetite as well as its partial or total replacement by silicates the dominant magnetic carrier is an fe rich titanomagnetite magnetically indistinguishable from pure magnetite the combination of oxidation exsolution and silicate replacement results in a very low natural remanent magnetization nrm the remaining 300 m are in the upper portion of the sheeted dike complex primary titanomagnetite is exsolved but alteration is less intense than in the transition zone nrm values are substantially higher than those of the transition zone due to less silicate replacement of primary titanomagnetite coupled with an uncertain contribution from secondary magnetite which occurs as a silicate alteration product the nrm magnitude is sufficiently high that the sheeted dike complex in this location at least may make a significant if not substantial contribution to the magnetic anomaly overall the magnetic properties of this hole especially the lower 500 m are strongly influenced by postemplacement alteration and may bear little or no resemblance to their values upon initial cooling as hydrothermal temperatures do not appear to have exceeded 400 c the nrm in the lower two sections is apparently a chemical rather than a thermal remanence as such models of marine magnetic structure that combine cooling models of the crust with the assumption that magnetic remanence is purely of thermal origin do not appear sufficient to adequately predict the properties of the deeper crust a better understanding of the nature of postemplacement alteration in the marine crust is required especially its characteristics at high temperatures and its effects on magnetic properties

the spectrum of persistent volcanic flank instability a review and proposed framework based on kīlauea piton de la fournaise and etna abstract persistent motion of the south flank of k lauea volcano hawai i has been known for several decades but has only recently been identified at other large basaltic volcanoes namely piton de la fournaise la reunion and etna sicily thanks to the advent of space geodetic techniques nevertheless understanding of long term flank instability is based largely on the example of k lauea despite the large differences in the manifestations and mechanisms of the process when viewed through a comparative lens for example the rate of flank motion at k lauea is several times that of etna and piton de la fournaise and is accommodated on a slip plane several km deeper than is probably present at the other two volcanoes gravitational spreading also appears to be the dominant driving force at k lauea given the long term steady motion of the volcano s south flank regardless of eruptive intrusive activity whereas magmatic activity plays a larger role in flank deformation at etna and piton de la fournaise k lauea and etna however are both characterized by heavily faulted flanks while piton de la fournaise shows little evidence for flank faulting a helpful means of understanding the spectrum of persistent flank motion at large basaltic edifices may be through a framework defined on one hand by magmatic activity which encompasses both magma supply and edifice size and on the other hand by the structural setting of the volcano especially the characteristics of the subvolcanic basement or subhorizontal intravolcanic weak zones a volcano s size and magmatic activity will dictate the extent to which gravitational and magmatic forces can drive motion of an unstable flank and possibly the level of faulting of that flank while the volcano s structural setting governs whether or not a plane of weakness exists beneath or within the edifice and can facilitate flank slip considering persistent flank instability using this conceptual model is an alternative to using a single volcano as a type example especially given that the example is usually k lauea which defines an extreme end of the spectrum and can provide a basis for understanding why flank motion may or may not exist on other large basaltic volcanoes worldwide

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