collapse driven large eruptions abstract for a typical poroelastic shallow crustal magma chamber about 0 1 of the mafic magma and about 4 of the felsic magma are erupted and or injected during magma chamber rupture magma chambers with volumes of the order of several tens to several hundred cubic kilometres as are common are thus sufficiently large to supply magma to small or moderate eruptions for large eruptions however a much higher percentage of the magma volume must be squeezed out of the chamber for an ordinary poroelastic chamber the excess pressure in the chamber falls exponentially during the eruption for a large eruption to be possible however the excess pressure must be essentially maintained until the very end of the eruption here i show that caldera collapse can maintain the excess pressure through forced magma chamber volume reduction in which case a resulting large eruption would be the consequence not the cause of the collapse i also show that ring fault dip partly controls the size and intensity volumetric flow or effusion rate of the eruption if the ring fault dips inward a normal fault the displacement is stable the volumetric flow rate intensity remains essentially constant during the collapse and the magma chamber remains active following the collapse by contrast if the ring fault dips outward a reverse fault the displacement is unstable the volumetric flow rate normally increases dramatically during the collapse and the magma chamber may be entirely destroyed during the collapse

segmented lateral dyke growth in a rifting event at barðarbunga volcanic system iceland crust at many divergent plate boundaries forms primarily by the injection of vertical sheet like dykes some tens of kilometres long1 previous models of rifting events indicate either lateral dyke growth away from a feeding source with propagation rates decreasing as the dyke lengthens2 3 4 or magma flowing vertically into dykes from an underlying source5 6 with the role of topography on the evolution of lateral dykes not clear here we show how a recent segmented dyke intrusion in the bar arbunga volcanic system grew laterally for more than 45 kilometres at a variable rate with topography influencing the direction of propagation barriers at the ends of each segment were overcome by the build up of pressure in the dyke end then a new segment formed and dyke lengthening temporarily peaked the dyke evolution which occurred primarily over 14 days was revealed by propagating seismicity ground deformation mapped by global positioning system gps interferometric analysis of satellite radar images insar and graben formation the strike of the dyke segments varies from an initially radial direction away from the bar arbunga caldera towards alignment with that expected from regional stress at the distal end a model minimizing the combined strain and gravitational potential energy explains the propagation path dyke opening and seismicity focused at the most distal segment at any given time and were simultaneous with magma source deflation and slow collapse at the bar arbunga caldera accompanied by a series of magnitude m 5 earthquakes dyke growth was slowed down by an effusive fissure eruption near the end of the dyke lateral dyke growth with segment barrier breaking by pressure build up in the dyke distal end explains how focused upwelling of magma under central volcanoes is effectively redistributed over long distances to create new upper crust at divergent plate boundaries

surface stresses associated with arrested dykes in rift zones many theoretical models predict that arrested dykes may generate major grabens at rift zone surfaces arrested dyke tips in eroded rift zones however are normally not associated with major grabens or normal faults that could be generated by dyke induced stresses ahead of the tips and normal faults and grabens tend to be less common in those parts of eroded rift zones where dykes are comparatively abundant similarly there are feeder dykes as well as dykes arrested a few metres below the surface that do not generate faults or grabens at the surface here i propose that this discrepancy between theoretical models and field observations may be explained by the mechanical layering of the crust numerical models presented here show that abrupt changes in young s moduli layers with high dyke normal compressive stresses stress barriers and weak horizontal contacts have large effects on the dyke induced stress fields for the models considered the surface tensile stresses induced by arrested dykes are normally too small to lead to significant fault or graben formation at the rift zone surface the only significant dyke induced surface tensile stresses 2 mpa in these models are for a dyke tip arrested at 1 km depth below the surface of a rift zone with a weak contact at 400 m depth and subject to extension that tensile stress however peaks above the ends of the weak horizontal contact which in the model considered occur at distances of 4 km to either side of the dyke and shows no simple relation to the depth to the dyke tip thus for a layered crust with weak contacts straightforward inversion of surface geodetic data to infer dyke geometries may result in unreliable results

on the effect of crustal layering on ring fault initiation and the formation of collapse calderas collapse calderas can be attributed to subsidence of the magma chamber roof along bounding sub vertical normal faults ring faults after a decompression of the magma chamber it has previously been shown that for ring faults to initiate and thus facilitate collapse the stress field both at the surface and around the magma chamber must satisfy specific critical conditions here we present new numerical models that use a finite element method to investigate the effects of crustal layering on local stress field distribution results are compared with existing criteria for ring fault initiation different subsurface scenarios were simulated by varying the stiffness young s modulus of layers placed above the magma chamber and the host rock in which the chamber is seated we consider depressurisation of a magma chamber so as to simulate magma withdrawal results indicate that mechanical layering is a further first order variable in the rare achievement of stress conditions required for ring fault formation and may be influential in facilitating or inhibiting caldera collapse we show that for a given geometrical set up the magnitude and position of maximum tensional stress at the earth s surface are influenced by the occurrence and relative distribution of mechanically stiff or soft lithologies above the magma chamber for example tensional stress at surface may be reduced by the presence of stiff layers e g lavas or increased by soft layers e g pyroclastic units compared to generic simulations using a homogeneous background medium overall we find that the presence of mechanically soft material promotes surface fracture initiation in addition we suggest that the position of peak tensional stress at surface derived by the numerical models does not represent that related to the position of the bounding ring fault but is instead related to the position of initial tensional fractures appearing prior to the collapse faults

stratigraphy and remanence acquisition of a palaeomagnetic reversal in alluvial siwalik rocks of pakistan a high resolution record of a palaeomagnetic reversal is documented in miocene alluvial rocks of pakistan we examined lateral variability of lithostratigraphy and palaeomagnetic stratigraphy through the same palaeomagnetic reversal in six correlated sections each section contains one or more palaeomagnetic sites with directions between fully reversed and fully normal the position of the reversal illustrates local relief in the study area and the time transgressive nature of certain stratigraphic units variability in the thickness of the transitional interval indicates contemporaneous variability in sediment accumulation rates different characteristics of palaeomagnetic remanence are associated with the depositional and postdepositional history of these sediments variability in patterns of remanence behaviour is the basis for inferences about post depositional processes we discuss two magnetic parameters that express coherency of palaeomagnetic samples the maximum angle of deviation and the circular standard deviation of particular interest are samples with incoherent palaeomagnetic signals the incoherency of samples is inversely correlated with the thickness of the transitional interval a low rate of sediment accumulation suggested by a thin transitional interval may facilitate a prolonged period of remagnetization through pedogenic or hydrological processes alternatively transition intervals defined by coherent magnetization may be thin as a result of pedogenically induced incoherency

effect of stress fields on magma chamber stability and the formation of collapse calderas 1 the summits of many of the earth s and other planets larger volcanoes are occupied by calderas that formed by collapse into an evacuating underlying magma chamber these collapse calderas are typically several tens of square kilometers in area and are commonly elliptical in shape we show that the long axes of late quaternary collapse calderas in the kenya rift valley the western basin and range province the snake river yellowstone plateau and the iceland rift zone are parallel to the upper crustal minimum horizontal stress direction sh as determined by independent criteria we suggest that circular magma chambers beneath these volcanoes became elliptical by stress induced spalling of their chamber walls by a mechanism that is analogous to the formation of breakouts in boreholes and tunnels in breakouts the hole becomes elongate parallel to the far field minimum stress in the kenya rift late pleistocene caldera collapse was accompanied by a 45 rotation of sh and an increase in the magnitude of the maximum horizontal stress sh the breakout model predicts increasingly unstable caldera walls under these conditions a possible explanation for the sudden appearance of so many collapse events in a volcanic setting that had never experienced them before this mechanism of stress change induced collapse may have played a role in other caldera settings

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

formation of oskjuvatn caldera at askja north iceland mechanism of caldera collapse and implications for the lateral flow hypothesis the oskjuvatn caldera at askja volcano north iceland was formed as a result of an explosive eruption at askja on 28th 29th march 1875 oskjuvatn is one of the youngest collapse calderas on earth and its initiation and growth were observed and documented by explorers and geologists working in the askja region between 1875 and 1932 it is an example of caldera formation where the volume of the caldera collapse far exceeds the volume of associated erupted products the discrepancy between the collapse volume and associated erupted products has been used by previous workers as a justification for the hypothesis that the injection of lateral dykes from askja fed the february to october 1875 n jahraun fissure eruptions at the sveinagja graben some 60 km north of askja however historical accounts documenting the caldera formation in sufficient detail show clearly that oskjuvatn formed by piecemeal collapse taking over 40 years to reach its current form we use these accounts to undertake a detailed examination of the stages of caldera collapse and to compare them with the known magmatic output of askja in the years following the 1875 eruption arcgis software was used to calculate the volume of oskjuvatn during the various stages of collapse and the volume of material erupted after 1875 while a dyke extending between askja and sveinagja may be implied to account for the missing volume our results demonstrate that the volume of oskjuvatn in july 1875 closely corresponds to the volume of rhyolitic material erupted on 28th 29th march 1875 in addition geochemical evidence indicates that the n jahraun magmas were sourced from a deeper reservoir rather than by lateral injection from askja s shallow crustal magma chamber we therefore suggest that the injection of sills and dykes in a coherent intrusion complex beneath askja central volcano combined with background deflation of the magma storage zone beneath oskjuvatn caldera over 30 to 40 years following 1875 satisfactorily accommodates the volume required to form oskjuvatn caldera

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

magma chamber volume changes associated with ring fault initiation using a finite sphere model application to the aira caldera japan abstract we have derived an analytical solution expressing the magma chamber volume change required for ring fault caldera initiation in terms of a finite sphere model we consider the conditions by which magma chamber contraction volume reduction could contribute to the initiation of a ring fault at the surface above the chamber using the elastic plastic theory of host rock behaviour the model does not consider underpressure pressure less than lithostatic in the chamber but only magma chamber volume reduction or shrinkage the model is heuristic in the sense that we do not explore the detailed mechanical condition for the volume reduction apart from its being related to magma flow out of the chamber but rather focus on the consequences of the reduction for ring fault initiation and dip the solution obtained for the finite sphere model consists of a leading term that is proportional to the cube of the magma chamber depth and a second term that is proportional to the cube of the magma chamber radius the second term reflects explicitly the effects of a finite source radius whereas the first term corresponds to a solution for a point source model this follows because in a point source model the magma chamber radius is zero so that the second term drops out the equation that plays the key role in obtaining this solution is a linear function relating the radius of the caldera to the magma chamber depth the proportionality coefficient of this equation is dimensionless and depends not only on the elastic constants of the medium but also on the depth d and radius a of the magma chamber given by e a d in addition because this coefficient controls a geometric parameter related to the overall form of the ring fault our results indicate that the ring fault shape and size is decided by the magma chamber depth and size as well as by the host rock elastic properties and angle of internal friction the results indicate that the associated ring fault is inward dipping and that the magma chamber radius can be estimated from the fault radius and dip we applied the analytical solution to the ring fault of the aira caldera in southern kyushu japan the calculations indicate that the aira ring fault could have been initiated at the surface as a result of 2 8 reduction in the volume of a magma chamber with a radius of 4 7 km and at a depth of 9 1 km these results agree with independent information on the aira magma chamber we conclude that the present solution yields useful first order information on magma chambers associated with ring faults

calderas and caldera structures a review abstract calderas are important features in all volcanic environments and are commonly the sites of geothermal activity and mineralisation yet it is only in the last 25 years that a thorough three dimensional study of calderas has been carried out utilising studies of eroded calderas geophysical analysis of their structures and analogue modelling of caldera formation as more data has become available on calderas their individuality has become apparent a distinction between caldera caldera complex cauldron and ring structure is necessary and new definitions are given in this paper descriptions of calderas based on dominant composition of eruptives basaltic peralkaline andesitic dacitic rhyolitic can be used and characteristics of each broad group are given styles of eruption may be effusive or explosive with the former dominant in basaltic calderas and the latter dominant in andesitic dacitic rhyolitic and peralkaline calderas four end member collapse styles occur plate or piston piecemeal trapdoor and downsag but many calderas have multiple styles features of so called funnel and chaotic calderas proposed in the literature can be explained by other collapse styles and the terms are considered unnecessary ground deformation comprises subsidence or collapse essential characteristics of a caldera and uplifting doming and fracturing due to tumescence and or resurgence frequent but not essential collapse may occur on pre existing structures such as regional faults or on faults created during the caldera formation and the shape of the collapse area will be influenced by depth size and shape of the magma chamber the final morphology of a caldera will depend on how the caldera floor breaks up whether collapse takes place in one event or multiple events whether vertical movement is spasmodic or continuous throughout the eruptive sequence and whether blocks subside uniformly or chaotically at one or more collapse centres a meaningful description of any caldera should therefore include number of collapse events presence or absence of resurgence caldera floor coherency caldera floor collapse geometry and dominant composition of eruptives

diffusive transfer of oxygen from seamount basaltic crust into overlying sediments an example from the clarion clipperton fracture zone highlights geochemical evidence confirms exchange between sediments and the basaltic crust fluids o2 exchange between the sediment and the basaltic crust produces a fully oxic sediment column transport reaction modeling was performed to quantify the exchange of o2 and nutrients abstract the clarion clipperton fracture zone ccfz in the pacific ocean is characterized by organic carbon starved sediments and meter scale oxygen penetration into the sediment furthermore numerous seamounts occur throughout its deep sea plain which may serve as conduits for low temperature hydrothermal seawater circulation through the oceanic crust recent studies in deep sea environments of the pacific and atlantic oceans have suggested and presented evidence of dissolved constituent exchange between the seawater flowing in the basaltic crust and the pore water of the overlying sediments through high resolution pore water oxygen and nutrient measurements we examined fluxes and geochemical interactions between the seamount basaltic basement and pore waters of the overlying sediments at three sites located on a radial transect from the foot of teddy bare a small seamount in the ccfz at three sites located 1000 700 and 400 m away from the foot of the seamount we found that oxygen concentrations initially decrease with sediment depth but start to increase at depths of 3 and 7 m toward the basaltic basement nitrate view the mathml sourceno3 concentrations mirror the oxygen concentration profiles as they increase with sediment depth but decrease towards the basement these profiles suggest an upward diffusion of oxygen from seawater circulating within the seamount crust into the overlying basal sediments and a downward diffusion of view the mathml sourceno32 from sediment pore water into the basaltic crust at one site we determined that the 87sr 86sr ratios of the bottom water and of the deep sediment near the basaltic crust are similar further supporting diffusive exchange between basaltic crust fluids and sediment pore water transport reaction modeling performed at two of the study sites revealed that 1 the diffusive flux of oxygen from the basaltic basement outpaces the oxygen consumption through organic matter oxidation and nitrification in the basal sediments and 2 the nutrient exchange between the sediment and the underlying basaltic crust occurs at orders of magnitude lower rates than between the upper sediment and the overlying bottom water these results further show that the diffusion of oxygen from the seamount basaltic basement into the overlying pore waters affects the preservation of organic compounds and helps to maintain a completely oxygenated sedimentary column at all 3 sites near the seamount

surface displacements resulting from magma chamber roof subsidence with application to the 2014 2015 bardarbunga holuhraun volcanotectonic episode in iceland abstract the conditions which lead to caldera collapse are still poorly constrained as there have only been four possibly five well documented caldera forming events in the past century the geodetic signals produced during chamber roof subsidence or chamber volume reduction shrinkage in general are not well documented or understood in particular when two or more geodetic sources are operating and providing signals at the same time it is important to be able to estimate the likely contribution of each simultaneous activities of different geodetic sources are common and include pressure changes in magma chambers reservoirs occurring at the same time as dyke emplacement here we present results from numerical models designed to simulate the subsidence of a magma chamber roof either directly chamber shrinkage or through ring fault displacement and the induced surface deformation and crustal stresses we consider chamber depths at 3 km 5 km and 7 km below the crustal surface using both non layered isotropic and layered anisotropic crustal models we also model the effects of a caldera lake and of a thick ice cover ice sheet on top of the caldera the results suggest that magma chamber roof subsidences between 20 m and 100 m generate large tens of centimetres vertical and in particular horizontal displacements at the surfaces of the ice and the crust out to distances of up to tens of kilometres from the caldera chamber centre crustal layering tends to reduce but increasing chamber depth to enlarge the horizontal and vertical surface displacements applying the results to the ice subsidence in the bardarbunga caldera during the 2014 2015 bardarbunga holuhraun volcanotectonic episode indicates that the modelled ice displacements are less than those geodetically measured also the geodetically measured crustal displacements are less than expected for a 60 m chamber roof subsidence the modelling results thus suggest that only part of the ice subsidence is due to chamber roof subsidence the other part being related to flow in and down bending of the ice we show that such a flow is likely within the caldera as a result of the stress induced by the 45 km long regional dyke emplaced primarily in vertical magma flow during the episode this conclusion is further supported by the model results suggesting that the ring fault piston like displacements must have been much less than the total 60 m ice subsidence or else faults with tens of metres displacements would have cut through the ice these are not observed we suggest that the ring fault subsidence was triggered by small doming of the volcanic field and system hosting the bardarbunga caldera and that this doming occurred as a result of magma inflow and pressure increase in a deep seated reservoir the doming is confirmed by gps measurements and supported by the seismicity results the magmatic pressure increase in the reservoir was in terms of the present model responsible for the regional dyke emplacement the holuhraun eruption and part of the stress concentration around and displacement of the bardarbunga caldera

geometrical and mechanical constraints on the formation of ring fault calderas ash flow plate subsidence piston like calderas are bounded by a set of arcuated sub vertical collapse faults named ring faults experimental studies on caldera formation performed mostly using spherical or cylindrical magma chamber geometries find that the resulting ring faults correspond to steeply outward dipping reverse faults and show that pre existing fractures developed during pre eruptive phases of pressure increase may play a major role in controlling the final collapse mechanism a situation that should be expected in small to medium sized ring fault calderas developed on top of composite volcanoes or volcanic clusters on the other hand some numerical experiments indicate that large sill like elongated magma chambers may induce collapse due to roof bending without fault reactivation as seems to occur in large plate subsidence calderas formed independently of pre existing volcanoes also numerical experiments allow the formation of nearly vertical or steeply inward dipping normal ring faults in contrast with most of the analogue models using a thermoelastic model we investigate the geometrical and mechanical conditions to form ring fault calderas in particular the largest ones without needing a previous crust fracturing results are given in terms of two dimensionless geometrical parameters namely and e the former is the chamber extension to chamber depth ratio whereas the latter stands for the chamber eccentricity we propose that the e pair determinates two different types of ring fault calderas with different associated collapse regimes ring fault region a is related to large plate subsidence calderas i e andean calderas or western us calderas for which few depressurisation is needed to set up a collapse initially governed by flexural bending of the chamber roof in contrast ring fault region b is related to small to moderate sized calderas i e composite volcano calderas for which much depressurisation is needed our opinion is that collapse requires in the latter case reactivation of pre existing fractures and it is therefore more complex and history dependent

ground deformations in collapsed caldera structures abstract a method is presented to analyze the effect of stress strain discontinuities on the ground deformations generated by a pressure source this is meant to simulate the effects due to caldera structures likely to present fractured zones at the borders of the collapsed area a method originally developed by crouch 1976 to solve plane strain problems has been used to simulate deformation curves for several source and discontinuity geometries the main result is that the location of the discontinuities controls the extension of the deformed zone and always reduces it with respect to a continuous medium with respect to a homogeneous medium the presence of lateral discontinuities also acts towards lowering the overpressure required to produce a given amount of deformation these results indicate that when analyzing ground deformations in calderas the use of classical methods involving continuous media should be avoided or at least taken with caution these methods in fact assume that the extension of the deformed zone is only linked to the source depth some examples of ground deformations in active calderas have been analyzed in the framework of the results obtained from theoretical modeling four calderas recently affected by ground deformations have been considered rabaul new guinea campi flegrei italy long valley and yellowstone u s a the effects of collapsed structures on the deformation field are possibly evidenced for all the four calderas at rabaul and campi flegrei the fracture systems mainly affecting the ground deformations probably represent younger innermost collapses and are well evidenced by seismicity studies ground deformations are here concentrated in an area much smaller than the one enclosed by geologically visible caldera rims in particular at rabaul the effect of the innermost collapse can explain the high concentration of the uplift in the period 1971 1985 previously modeled by a very shallow source 1 3 km in terms of overpressure in the main magma chamber probably located at 4 5 km of depth

the 2007 8 volcanic eruption on jebel at tair island red sea observed by satellite radar and optical images we use high resolution optical images and interferometric synthetic aperture radar insar data to study the september 2007 january 2008 jebel at tair eruption comparison of pre and post eruption optical images reveals several fresh ground fissures a new scoria cone near the summit and that 5 9 0 1 km2 of new lava covered about half of the island decorrelation in the insar images indicates that lava flowed both to the western and to the northeastern part of the island after the start of the eruption while later lavas were mainly deposited near the summit and onto the north flank of the volcano from the insar data we also estimate that the average thickness of the lava flows is 3 8 m resulting in a bulk volume of around 2 2 107 m3 we observe no volcano wide pre or post eruption uplift which suggests that the magma source may be deep the co eruption interferograms on the other hand reveal local and rather complex deformation we use these observations to constrain a tensile dislocation model that represents the dike intrusion that fed the eruption the model results show that the orientation of the dike is perpendicular to the red sea rift implying that the local stresses within the volcanic edifice are decoupled from the regional stress field

elliptical calderas in active tectonic settings an experimental approach abstract caldera volcanoes form due to collapse of a magma chamber roof into the underlying magma chamber many field theoretical and experimental studies have postulated that calderas are delimited by reverse ring faults and are surrounded by peripheral concentric normal faults in the simplest theoretical scenario circular magma chambers produce circular calderas many calderas however are elliptical in shape particularly those in extensional and compressive tectonic settings several factors may explain elliptical calderas the first is the presence of an elliptical magma chamber established by for instance preferential intrusion along pre existing basement structures or differential spalling of the magma chamber walls the second is the overlap nesting of several discrete calderas to form a single larger elliptical structure the third is asymmetric subsidence the fourth is variable pre collapse topography a fifth possible factor is distortion of the caldera faults by the regional stress field during caldera formation a sixth factor is the post collapse distortion of the caldera structure due to continued regional deformation to better understand relationships between caldera surface expression reservoir geometry and regional tectonic stresses we conducted scaled analogue experiments these experiments examined the impact of regional stress and associated structures on calderas formed during evacuation of reservoirs circular rubber balloons of known dimensions and depths the results show that in principle calderas produced in compression extension experiments are elongated parallel to the direction of minimum horizontal compressive stress despite the chamber beneath being circular in plan view as a consequence model ring fault orientation varied from steeply dipping where striking perpendicular to the minimum horizontal regional compressive stress to shallower dips where striking parallel to the minimum horizontal regional compressive stress this leads us to suggest that the influence of a regional stress field on caldera fault orientation during and or after caldera formation may be significant in the development of elliptical calderas in addition such variation of caldera ring fault dip from steep to relatively shallow could influence location and behaviour of ring fissure eruptions

caldera subsidence in areas of variable topographic relief results from analogue modeling calderas form in volcanic areas commonly associated with topographic relief pre existing topography plays an important role in the style of the caldera subsidence topography increases the load and affects the principle stress trajectories located between the roof of the magma chamber and the surface the morphology internal structure and temporal evolution of calderas are therefore sensitive to the local topography we carried out scaled analogue experiments to investigate the effect of pre existing topographic relief on caldera subsidence by modeling the presence of stratocones and plateaus of variable mass diameter and positions prior to collapse we induced collapse in sandbox experiments by withdrawing water from a rubber bladder to simulate caldera collapse into a large shallow reservoir deformation was first manifested by sagging of the sand at the surface followed by the upward propagation of a set of subsidence controlling faults from the bladder in experiments with no topography these faults usually reached the surface near the centre of the cylinder as evacuation and incremental growth progressed a second outer set of subsidence controlling faults developed this outer set dominated the subsidence by increasing the topographic load by 6 the subsidence efficiency increased producing calderas up to approximately 20 deeper the inner set of subsidence controlling faults steepened and controlled most of the collapse by contrast to the experiments without topography adding load also reduced outward growth of peripheral sagging and development of extensional faults leading to a diameter of sagging 20 smaller than with no topography a comparative analysis of our results with mount mazama at crater lake oregon and glass mountain at long valley california supports the interpretation that the addition of topographic load modifies the evolution of the major caldera faults

volcaniclastic sedimentation in and around an ordovician subaqueous caldera lower rhyolitic tuff formation north wales the lower rhyolitic tuff formation of ordovician age in north wales records the collapse infilling and subsequent resurgence of a volcanic caldera with an original diameter of about 15 km this volcanic center controlled patterns of volcaniclastic sedimentation providing enough topographic relief for both a shallow lagoon depositional basin within the caldera and a source of sediment derived from the rim within the caldera sediment consists of tuffaceous laminated siltstone and immature coarse grained volcaniclastic sandstone containing plane beds ripple cross laminations symmetrical wave ripples and hummocky cross stratification coarse grained matrix supported conglomerate layers and layers of ash flow tuffs are also present these sediments accumulated in shallow water above fair weather wave base conglomerate units represent debris flows from the caldera rim a nearby shoreline and elevated areas associated with resurgent domes sedimentation outside of the caldera consisted of deposition of background suspension and volcanic ash suspension and turbidite deposition on a pyroclastic apron the outer margin of the apron was dominated by fine grained suspension and turbidite deposition whereas the inner margin of the apron contains hummocky cross stratification and other evidence of reworking by episodic storm waves local highs with associated shallow water sedimentation existed outside the caldera even though deposited in high energy marine environments all sedimentary rocks are both texturally and mineralogically very immature this textural immaturity differs from the typical very mature marine sediments and was caused by rapid depositional rates and a local volcanic sediment source

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

dike emplacement at bardarbunga iceland induces unusual stress changes caldera deformation and earthquakes a 45 km long regional dike was emplaced over a period of 2 weeks in august 2014 at the boundary between the east and north volcanic zones in iceland this is the first regional dike emplacement in iceland monitored with modern geophysical networks the importance of which relates to regional dikes feeding most of the large fissure e g eldgja 934 and laki 1783 and lava shield e g early holocene skjaldbreidur and trolladyngja eruptions during this time the dike generated some 17 000 earthquakes more than produced in iceland as a whole over a normal year the dike initiated close to the bardarbunga volcano but gradually extended to the northeast until it crossed the boundary between the east volcanic zone evz and the north volcanic zone nvz we infer that the strike of the dike changes abruptly at a point from about n45 e coinciding with the trend of the evz to n15 e coinciding with the trend of the nvz this change in strike occurs at latitude 64 7 exactly the same latitude at which about 10 ma dikes in east iceland change strike in a similar way this suggests that the change in the regional stress field from the southern to the northern part of iceland has been maintained at this latitude for 10 million years analytical and numerical models indicate that the dike induced stress field results in stress concentration around faults and particularly shallow magma chambers and calderas in its vicinity such as tungnafellsjokull kverkfjoll and askja in particular the dike has induced high compressive shear and tensile stresses at the location of the bardarbunga shallow chamber and caldera ring fault where numerous earthquakes occurred during the dike emplacement many of which have exceeded m5 the largest m5 7 the first segment of the dike induced high tensile stresses in the nearby part of the bardarbunga magma chamber ring fault resulting in radially outward injection of a dike from the chamber at a high angle to the strike of the regional dike the location of maximum stress at bardarbunga fluctuates along the chamber ring fault boundary in harmony with dike size and or pressure changes and encourages ring dike formation and associated magma flow within the chamber caldera collapse and or eruption in some of these volcanoes is possible most likely in bardarbunga but depends largely on the future development of the regional dike

analogue models of collapse calderas and resurgent domes collapse calderas and resurgent domes are a common association related to inflation deflation processes in volcanic systems the structure of calderas and domes depends upon the permitted relative movements of crustal volumes at depth the so called space problem in order to study the structures of collapse calderas and resurgent domes and to take the space problem into account several analogue models were made dry quartz sand was used to simulate the rheology of the brittle crust while newtonian silicone putty located at the base of the sand pack simulated the ductile behaviour of the magma a piston moved the silicone putty downward or upward inducing collapse and doming within the sand three separate sets of experiments simulated 1 caldera collapse 2 resurgence and 3 superimposition of resurgence on collapse and vice versa collapse experiments are characterized by the development of two concentric depressions the first formed depression is bordered by outward dipping reverse ring faults the subsequent outer concentric depression is bordered by inward dipping normal ring faults the deformation pattern during resurgence is a function of the overburden thickness t and the dome diameter d for higher t d ratios a dome forms bordered by inward dipping high angle reverse ring faults outward dipping normal ring faults develop at late stages for lower t d ratios the dome shows at late stages a crestal depression accompanied by radial fractures subsequently an apical extrusion of silicone occurs the superimposition of resurgence over collapse and vice versa is characterized by the complete reactivation with opposite kinematics of all the pre existing ring faults during inversion both in caldera and resurgence reverse ring faults form in the early stages due to differential uplift extensional structures subsequently form to accommodate gravitational collapse during the activity of the reverse faults the experiments and the overall similarities with nature suggest that the activity of both reverse and normal faults constitutes a possible solution to the space problem during major collapses or resurgences