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

discovery of a magma chamber and faults beneath a mid atlantic ridge hydrothermal field crust at slow spreading ridges is formed by a combination of magmatic and tectonic processes with magmatic accretion possibly involving short lived crustal magma chambers the reflections of seismic waves from crustal magma chambers have been observed beneath intermediate and fast spreading centres but it has been difficult to image such magma chambers beneath slow spreading centres owing to rough seafloor topography and associated seafloor scattering in the absence of any images of magma chambers or of subsurface near axis faults it has been difficult to characterize the interplay of magmatic and tectonic processes in crustal accretion and hydrothermal circulation at slow spreading ridges here we report the presence of a crustal magma chamber beneath the slow spreading lucky strike segment of the mid atlantic ridge the reflection from the top of the magma chamber centred beneath the lucky strike volcano and hydrothermal field is approximately 3 km beneath the sea floor 3 4 km wide and extends up to 7 km along axis we suggest that this magma chamber provides the heat for the active hydrothermal vent field above it we also observe axial valley bounding faults that seem to penetrate down to the magma chamber depth as well as a set of inward dipping faults cutting through the volcanic edifice suggesting continuous interactions between tectonic and magmatic processes

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

an integrated model of magma chamber conduit and column for the analysis of sustained explosive eruptions abstract explosive volcanic eruptions comprise a complex series of processes involving withdrawal from the magma chamber magma ascent along the conduit and eruption column dynamics numerous studies have modeled the different sub domains of a volcanic system but their interplay has seldom been analyzed to this end we developed c 3 c cubed that stands for chamber conduit and column a new integrated model that describes the dynamics of an explosive eruption as a series of steady state regimes and as a function of geometry and initial conditions of the magma reservoir we used global sensitivity analysis to quantify the role of the relevant model parameters and describe the interplay between the different volcanic sub domains in particular we analyzed the evolution of a sustained explosive eruption in order to identify the conditions for buoyant super buoyant and collapsing columns input data were based on field reconstructions of quaternary explosive eruptions in the vulsini volcanic district roman province central italy model results show that 1 the column regime although affected by complex interactions among several factors mostly depends on the conduit radius the volatile content i e supersaturation concentration at the top of the chamber and length of the conduit in decreasing level of importance 2 the amount of mass erupted is independent of the conduit radius and depends mostly on volatile supersaturation the radius of the magma chamber the length of the conduit and the overpressure at the conduit inlet 3 the mass flow rate column height and duration of the eruption are largely controlled by the conduit radius 4 the flow pressure and density at the conduit exit are mostly controlled by the conduit inlet overpressure at the onset of the eruption and by the length of the conduit at the end of the eruption 5 the exit velocity from the conduit is mostly controlled by the volatile content the length of the conduit and the inlet overpressure in this model framework and with specific reference to selected plinian events of the vulsini volcanic district simulation results show that column collapse is not achieved for reasonable eruption durations order of hours and conduit widths tens of meters this is consistent with field reconstructions suggesting that column collapse did not likely occur and that pyroclastic flows were therefore generated by independent mechanisms from ring fissures and or multiple vents concomitant to caldera collapse

the role of vegetation in the formation of anabranching channels in an ephemeral river northern plains arid central australia as the distribution and abundance of vegetation in drylands is often controlled by the greater availability of water along river channels riparian vegetation has the potential to influence significantly dryland river form process and behaviour this paper demonstrates how a small indigenous shrub the inland teatree melaleuca glomerata influences the formation and maintenance of anabranching channels in a reach of the ephemeral marshall river northern plains arid central australia here the marshall is characterized by ridge form anabranching where water and sediment are routed through subparallel multiple channels of variable size which occur within a typically straight channel train channels are separated by channel train ridges narrow flow aligned vegetated features or by wider islands by providing a substantial element of boundary roughness dense stands of teatrees growing on channel beds or atop the ridges and islands influence flow velocities flow depths and sediment transport resulting in flow diversion bank and floodplain erosion and especially sediment deposition ridges and islands represent a continuum of forms and their formation and development can be divided into a three stage sequence involving teatree growth and alluvial sedimentation 1 teatrees colonize a flat sandy channel bed initiating the formation of ridges by lee side accretion individual ridges grow laterally vertically and longitudinally and maintain a geometrically similar streamlined lemniscate form that presents minimum drag 2 individual ridges grow in size and interact with neighbouring ridges causing the lemniscate forms to become distorted ridges in the lee of other ridges tend to be protected from the erosive effects of floods and survive whereas individual teatrees or small ridges exposed to flow concentrated between larger ridges tend to be removed 3 ridges lengthen and coalesce with downstream ridges eventually subdividing the channel train into well defined anabranches this sequence turns a channel initially obstructed with dense and chaotic stands of teatrees into a well organized system of ridge form anabranches in the moderate to low gradient marshall river which is colonized by an abundance of within channel vegetation and subject to declining downstream discharges this helps to minimize flow resistance thereby maintaining an efficient water and sediment flux copyright 2000 john wiley sons ltd

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

seismic images of active magma systems beneath the east pacific rise between 17 05 and 17 35 s seismic reflection data from the east pacific rise between 17 059 and 17 359s image a magma lens that varies regularly in depth and width as ridge morphology changes confirming the notion that axial morphology can be used to infer ridge magmatic state however at 17 269s where the ridge is locally shallow and broad the magma lens is markedly shallower and wider than predicted from regional trends in this area submersible dives reveal recent volcanic eruptions these observations indicate that it is where the width and depth of the magma chamber differ from regional trends indicating an enhanced magmatic budget that is diagnostic of current magmatism

formation and development of normal fault calderas and the initiation of large explosive eruptions the ring fractures that form most collapse calderas are steeply inward dipping shear fractures i e normal faults at the surface of the volcano within which the caldera fault forms the tensile and shear stresses that generate the normal fault caldera must peak at a certain radial distance from the surface point above the center of the source magma chamber of the volcano numerical results indicate that normal fault calderas may initiate as a result of doming of an area containing a shallow sill like magma chamber provided that the area of doming is much larger than the cross sectional area of the chamber and that the internal excess pressure in the chamber is smaller than that responsible for doming this model is supported by the observation that many caldera collapses are preceded by a long period of doming over an area much larger than that of the subsequently formed caldera when the caldera fault does not slip eruptions from calderas are normally small nearly all large explosive eruptions however are associated with slip on caldera faults during dip slip on and doming of a normal fault caldera the vertical stress on part of the underlying chamber suddenly decreases this may lead to explosive bubble growth in this part of the magma chamber provided its magma is gas rich this bubble growth can generate an excess fluid pressure that is sufficiently high to drive a large fraction of the magma out of the chamber during an explosive eruption

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

conceptual and numerical models of ring fault formation abstract most ring faults of collapse calderas are primarily shear fractures the initiation and development of which depends on the state of stress in the host rock the state of stress in a volcano is controlled by the loading conditions such as magma chamber geometry and pressure but also by the mechanical properties of its rock units and structures such as existing contacts faults and joints ring fault formation is thus essentially a problem in rock physics field observations show that some ring faults are dip slip whereas others are partly faults shear fractures and partly ring dykes extension fractures although slip on existing ring faults is much more common in basaltic edifices shield volcanoes than in true composite volcanoes in both types of volcanoes most caldera unrest periods do not result in ring fault slip here i present new conceptual and numerical models of caldera formation in volcanoes with shallow spherical circular or oblate ellipsoidal sill like magma chambers in the layered models the host rock above the chamber is composed of 30 comparatively thin layers with stiffnesses young s moduli alternating between 1 gpa and 100 gpa the chamber itself is located in a single thick layer the crustal segment hosting the chamber is either 20 km or 40 km wide but has a constant thickness of 20 km the loading conditions considered are 1 a crustal segment subject to 5 mpa tension 2 crustal segment subject to excess magmatic pressure of 10 mpa at the bottom doming of the volcanic field containing the chamber 3 a combination of tension and doming and 4 chamber subject to underpressure negative excess pressure of 5 mpa the main results are as follows 1 excess pressure and underpressure in a chamber normally favour dyke injection rather than ring fault formation 2 for doming or tension a spherical magma chamber favours dyke injection except when the layer hosting the chamber is very soft 10 gpa or one with recent dyke injections in which case the surface stress field favours ring fault formation 3 for a sill like chamber in a 20 km wide crustal segment a ring fault can be generated by either tension or tension and doming for a 40 km wide segment doming alone is sufficient to generate a ring fault 4 since individual layers in a volcano may develop different local stresses stress field homogenisation through all the layers between the chamber and the surface is a necessary condition for ring fault formation 5 because the mechanical properties of the layers that constitute basaltic edifices are more uniform than those that constitute true composite volcanoes it follows that stress field homogenisation and thus ring fault formation or slip is more commonly reached in basaltic edifices than in composite volcanoes 6 both for basaltic edifices and composite volcanoes the stress fields most likely to initiate ring faults are those generated around sill like chambers subject to tension doming or both

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

the new worldwide collapse caldera database ccdb a tool for studying and understanding caldera processes collapse calderas are one of the most important volcanic structures not only because of their hazard implications but also because of their high geothermal energy potential and their association with mineral deposits of economic interest the objective of this work is to describe a new general worldwide collapse caldera database ccdb in order to provide a useful and accessible tool for studying and understanding caldera collapse processes the principal aim of the ccdb is to update the current field based knowledge on calderas merging together the existing databases and complementing them with new examples found in the bibliography and leaving it open for the incorporation of new data from future studies this database does not include all the calderas of the world but it tries to be representative enough to promote further studies and analyses we have performed a comprehensive compilation of published field studies of collapse calderas including more than 200 references and their information has been summarized in a database linked to a geographical information system gis application thus it is possible to visualize the selected calderas on a world map and to filter them according to different features recorded in the database e g age structure the information recorded in the ccdb can be grouped in seven main information classes caldera features properties of the caldera forming deposits magmatic system geodynamic setting pre caldera volcanism caldera forming eruption sequence and post caldera activity additionally we have added two extra classes the first records the references consulted for each caldera the second allows users to introduce comments on the caldera sample such as possible controversies concerning the caldera origin a further purpose of this work is to construct the ccdb web page in this web page where registered users can acquire the current database version as well as to propose corrections or updates and to exchange information with other registered members also involved in the study of caldera collapse processes additionally the ccdb includes a formulary that will facilitate the incorporation of new calderas into the database

experimental studies of collapse calderas we present experiments to investigate the formation of calderas using inflatable balloons in a medium of fused alumina powder this system provides a model of a magma chamber in an homogeneous elastic media which scales to volcanic systems in terms of geometry and strength of the media three kinds of experiment were performed i the balloon was inflated and then deflated simulating pre emption tumescence and caldera collapse ii the balloon was deflated without prior inflation simulating caldera formation without pre eruption tumescence iii three balloons were inflated and deflated in a line simulating multiple magma chambers the influence of magma chamber shape was investigated using different shaped balloons balloon inflation formed a dome with a surface pattern of shallow polygonal fractures centrally and radial fractures peripherally uplift occurs along deep inward dipping concentric reverse faults deflation generated depressions with an outer set of concentric ring fractures surrounding a central funnel formed by sagging along numerous minor faults some of which formed during doming during collapse the concentric reverse faults were converted to normal faults and the radial normally faulted fractures were converted to reverse faults without prior tumescence an outer set of near vertical ring faults surround a flat central depression due to collapse of a coherent block an inner set of outward dipping reverse ring faults also develop the experimental caldera area increased with balloon size and increased as the balloon depth decreased for a given balloon volume and depth collapse area is significantly smaller if doming had occurred prior to collapse arcuate fractures can provide the pathways for magma ascent during pre emption tumescence smith bailey 1968 but the shallow radial and polygonal fracture systems may also influence vent location changes in fault geometry and sense of movement extension to compression and vice versa occur when subsidence initiates and can account for changes in vent location that occur in many eruptions multiple overlapping collapse calderas are associated with magma chamber migration whereas nested calderas can result from the activity of a single magma chamber

caldera morphology in the western galapagos and implications for volcano eruptive behavior and mechanisms of caldera formation abstract caldera morphology on the six historically active shield volcanoes that comprise isabela and fernandina islands the two westernmost islands in the galapagos archipelago is linked to the dynamics of magma supply to and withdrawal from the magma chamber beneath each volcano caldera size e g volumes 2 9 times that of the caldera of kilauea hawai i the absence of well developed rift zones and the inability to sustain prolonged low volumetric flow rate flank eruptions suggest that magma storage occurs predominantly within centrally located chambers at the expense of storage within the flanks the calderas play an important role in the formation of distinctive arcuate fissures in the central part of the volcano repeated inward collapse of the caldera walls along with floor subsidence provide mechanisms for sustaining radially oriented least compressive stresses that favor the formation of arcuate fissures within 1 2 km outboard of the caldera rim variations in caldera shape depth to diameter ratio intra caldera bench location and the extent of talus slope development provide insight into the most recent events of caldera modification which may be modulated by the episodic supply of magma to each volcano a lack of correlation between the volume of the single historical collapse event and its associated volume of erupted lava precludes a model of caldera formation linked directly to magma withdrawal rather caldera collapse is probably the result of accumulated loss from the central storage system without sufficient recharge and as has been suggested for kilauea may be aided by the downward drag of dense cumulates and intrusives

volcanological study of the great tambora eruption of 1815 the 1815 eruption of tambora is one of the largest explosive volcanic events of the past 10 000 yr by a conservative estimate 175 km3 of nepheline normative trachyandesitic pyroclastic material equivalent to about 50 km3 of dense rock was erupted in 24 h major activity began with a brief plinian phase followed by pyroclastic flows much of the ignimbrite that was produced shows evidence of deposition by turbulent flows a 6 7 km caldera about 1 km deep formed at the end of the pyroclastic flow phase caldera volume closely matches that of magma ejected plinian and co ignimbrite ash fall 1 cm thick covered 500 000 km2 of the java sea and surrounding islands accounting for more than two thirds of the magma volume erupted this distal tephra is known only from 1815 reports indicating that some great eruptions especially those from island volcanoes may be virtually undetectable in the geologic record without supporting information such as deep sea core data land based volume estimates may be insufficient to characterize eruption magnitude in many cases

temporal evolution of flow conditions in sustained magmatic explosive eruptions abstract the temporal evolution of fundamental flow conditions in the magma chamber plus conduit system such as pressure velocity mass flow rate erupted mass etc during sustained magmatic explosive eruptions was investigated to this aim simplified one dimensional and isothermal models of magma chamber emptying and conduit flow were developed and coupled together the chamber model assumed an homogeneous composition of magma and a vertical profile of water content the chamber could have a cylindrical elliptical or spherical rigid geometry inside the chamber magma was assumed to be in hydrostatic equilibrium both before and during the eruption since the time scale of pressure variations at the conduit inlet of the order of hours is much longer than the travel time of magma in the conduit of the order of a few minutes the flow in the conduit was assumed as at steady state the one dimensional mass and momentum balance equations were solved along a circular conduit with constant diameter assuming choked flow conditions at the exit bubble nucleation was considered when the homogeneous flow pressure dropped below the nucleation pressure given the total water content and the solubility law above the nucleation level bubbles and liquid magma were considered in mechanical equilibrium the same equilibrium assumption was made above the fragmentation level between gas and pyroclasts due to the hydrostatic hypothesis the integration of the density distribution in the chamber allowed to obtain the total mass in the chamber as a function of pressure at the chamber top and through the conduit model as a function of time simulation results pertaining to rhyolitic and basaltic magmas defined at the volcanic eruption mechanism modeling workshops durham nh 2002 nice france 2003 are presented important flow variables such as pressure density velocity shear stress in the chamber and conduit are discussed as a function of time and magma chamber and conduit properties results indicate that vent variables react in different ways to the pressure variation of the chamber pressure density and mass flow rate show relative variations of the same order of magnitude as the conduit inlet pressure whereas velocity is more constant in time sill like chambers produce also significantly longer and more voluminous eruptions than dike like chambers water content stratification in the chamber and the increase of chamber depth significantly reduce the eruption duration and volume maximum erupted mass fractions of about 0 2 are computed for small water saturated and shallow chambers

on magma chamber evolution during slow effusive eruptions 1 we explore some of the controls on the evolution of a slow effusive eruption imposed by processes in a shallow magma reservoir we envisage that the emplacement of basaltic magma into a shallow evolved crustal reservoir generates sufficient overpressure to trigger the slow effusive eruption of the overlying layer of evolved magma the subsequent evolution of the system then depends on a number of factors including 1 the volatile contents of the two magmas 2 the rate of any continuing influx of basalt 3 the cooling heating rates of the two layers of magma and 4 the eruption rate of the silicic magma where we assume that the crystals and bubbles remain in the layer in which they formed any exsolved volatiles in the magma increase the compressibility of the magma in a waning system this tends to increase the volume of material which needs to be erupted to relieve a given overpressure by an amount of order 10 100 and hence increases the duration of the eruption typical calculations suggest that eruptions from magma chambers of size 1 10 km3 may persist for times of order 1 10 years if the main body of silicic magma is saturated in volatiles in addition cooling of the basalt leads to formation of dense crystals which lowers the pressure however if the magma becomes saturated exsolution of volatile gases occurs tending to increase the pressure again in a waning system this tends to extend the eruption duration and increase the mass erupted from the chamber for chamber volumes of 1 10 km3 cooling rates of 10 6 k s can increase the eruption volume by a factor of order 10 rapid cooling may also lead to an initial increase in the eruption rate while the chamber pressure is controlled by the rate of production of new bubbles if there is an influx of basalt to the chamber which is maintained during the eruption then the evolution of the eruption is primarily dependent on this input rate although the cooling of the basalt may increase the eruption rate to values in excess of this input rate especially at an early stage in the eruption the complex and nonlinear evolution of the eruption rate predicted by our simple model illustrates the sensitivity of the eruption rate to the magma volatile content and magma cooling rates such understanding is key in developing quantitative models for hazard assessments of slow effusive eruptions

hydrofracturing stress measurements in the iceland research drilling project drill hole at reydarfjordur iceland two independent suites of hydrofracturing stress measurements were conducted in the top 600 m of the iceland research drilling project deep hole at reydarfjordur east iceland as indicated by the continuously extracted drill core the tested section consists of tertiary subaerial tholeiitic lava flows cut by many basaltic dikes the density of the basalt was used to estimate the vertical stress v in megapascals at depth d in meters v 0 027d in the 33 to 270 m depth range the horizontal principal stresses hmax and hmin as calculated from hydrofracturing results increase gradually with depth but at a lower rate than the vertical stress hmin 2 1 0 016d hmax 3 3 0 016d in the 300 to 550 m depth range a major perturbation occurs with both horizontal stress magnitudes rapidly rising between 300 and 400 m and then uniformly decreasing between 400 and 550 m to a level apparently predictable by the relationships obtained in the top 270 m conditions favoring normal faulting v hmax hmin were established in the 540 to 580 m range and are conjectured to persist at greater depths since the vertical stress gradient is higher than that of hmax and hmin this would be in accord with the predominant fault type in east iceland the relative low horizontal stress values and the small difference between them are supportive of ocean floor spreading theories and contradict near surface high compressive stresses previously determined by hast 1973 in east iceland the large increase in horizontal stress in the range of 300 to 500 m depth is not understood a correlation appears to exist between the zone of almost unintruded thick lava flows 200 500 m and that of high stresses an intuitive model suggesting locked in stresses in this zone is proposed the hydrofracturing determined overall mean direction of hmax is n50 e while a more selective sampling of results yields n40 e the measured stress directions appear to be reasonably compatible with the local structure such as dike direction nne and lava dip wsw significantly hmax direction is subparallel to the n20 e trend of the nearest fissure swarms in the axial rift zone this almost coincidence of directions like the measured stress magnitudes is also in agreement with the model of spreading lithospheric plates being pulled apart

thermal structure of the accreting earth the energy associated with the accretion of the earth and the segregation of the core is more than sufficient to melt the entire earth in order to understand the thermal evolution of the early earth it is necessary to study the relevant heat transfer mechanisms in this paper we postulate the existence of a global magma ocean and carry out calculations of the heat flux through it in order to determine its depth in the solid mantle heat is transferred by the upward migration of magma this magma supplies the magma ocean the increase in the mantle liquidus with depth pressure is the dominant effect influencing heat transfer through the magma ocean we find that a magma ocean with a depth of the order of 20 km would have existed as the earth accreted we conclude that the core segregated and an atmosphere was formed during accretion

volcanism and the dynamics of open magma chambers the repeated eruption of many volcanoes is now accounted for by events taking place in a subterranean magma chamber which is open in the sense that the inflow of material to the chamber from below leads to a magma pressure p which exceeds the lithostatic pressure pl by an amount greater than the tensile strength of the chamber walls before the contents of the magma chamber have reached thermodynamic and geochemical equilibrium this is the condition which must be satisfied when an eruption occurs we have now derived a relationship giving the critical volume of added material v required to cause an eruption as a function of magma compressibility and the volume by which the chamber expands

the permeability controlled accumulation of primary magma the initial accumulation of primary magma occurs just after the mantle has become permeable the accumulation is caused by the compaction of the residuum which either may be controlled by the rate of creep or by the rate of flow of the interstitial melt experimental results suggest that the rate of compaction is controlled by the permeability and a model for the accumulation process is worked out on this basis the compaction causes the formation of a lower compaction boundary and an upper layer of melt the ascending mantle of plumes and convection currents will form layers of melt situated 20 100 m apart the type of partial melting for this accumulation is critical melting

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

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

pressure evolution during explosive caldera forming eruptions abstract caldera forming eruptions of silicic magmas result from a complex coupling of the mechanics and fluid dynamics of the associated magma chamber field studies of caldera forming eruption products suggest that great pressure variations occur inside the magma chamber and associated conduits during these eruptions pressure evolution during explosive caldera forming eruptions is investigated through a simple model that describes the first order quantitative behaviour of the chamber we consider a piston like model that assumes a coherent block subsiding along circular sub vertical ring faults into the magma chamber this subsidence occurs after significant decompression of the chamber by an initial central vent eruption we assume that the initial pressure distribution in the chamber is magmastatic once collapse has begun the chamber roof is supported by the magma so that magma pressure at the chamber roof increases to lithostatic we suggest that pressure variations during caldera forming eruptions are mainly controlled by variations in magma volatile content regardless of what induces the formation of ring faults the model suggests that the occurrence of explosive caldera forming events depends on the strength of the chamber walls and the depth water content and aspect ratio of the magma chamber no significant differences exist between model results for a cylindrical or a more realistic elliptical magma chamber geometry of comparable aspect ratio assuming a constant strength of the host rock the mass fraction of magma that must be erupted during the central vent phase in order to trigger caldera collapse ranges for deep gas poor chambers from a few percent up to 40 for shallow gas rich chambers the model suggests that zoned chambers tend to collapse earlier than homogeneous chambers dike shaped chambers will erupt less magma than sill like chambers before caldera collapse initiates although dike like geometries are not associated with stress fields appropriate to create ring faults the model suggests that once initiated caldera collapse will tend to force out most or all of the volatile rich magma from the chamber for volatile rich magma chambers the total volume of erupted magma during caldera forming event is of the same order as the chamber volume the model also explains the variation in the erupted mass during the different phases of explosive caldera forming eruptions and is in good agreement with natural examples

widespread uplift and trapdoor faulting on galapagos volcanoes observed with radar interferometry volcanic uplift caused by the accumulation of magma in subsurface reservoirs is a common precursor to eruptions1 2 but for some volcanoes uplift of metres or more has not yet led to an eruption3 here we present displacement maps of volcanoes in the galapagos islands constructed using satellite radar interferometry that might help explain this dichotomy we show that all but one of the seven volcanoes on the islands of isabela and fernandina deformed during 1992 99 cerro azul and fernandina erupted4 5 6 during the observation period and show evidence of inflation co eruptive deflation and shallow dyke intrusion in contrast the largest volcano sierra negra has not erupted yet exhibits spatially and temporally variable deformation with a maximum uplift of 2 7 m between 1992 and 1999 which can be modelled by a shallow inflating sill inflation during 1997 98 however was accompanied by trapdoor faulting on a steeply dipping fracture system within the caldera repeated trapdoor faulting over geological time has formed an arcuate intra caldera ridge within sierra negra and may have acted to relax stresses above the magma chamber inhibiting summit eruptions similar processes may help explain large uplift unaccompanied by eruptive activity at other volcanoes

formation of collapse calderas collapse calderas are common in many volcanic regions but the mechanics of their formation is not well understood i propose that collapse calderas may form as a result of the growth of associated shallow magma chambers excess magmatic pressure in a shallow chamber gives rise to tensile stresses at the surface but the location and magnitude of the maximum tensile stress depend on the shape of and depth d to the top of the chamber this model indicates that penny shaped chambers i e sill like chambers of circular horizontal cross sections are most likely to initiate collapse calderas for such a chamber with horizontal radius a the higher the a d radius depth ratio the greater the probability of forming a collapse caldera