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

structural evolution of calderas insights from two dimensional discrete element simulations the manner in which calderas develop is a key question in volcanology and has important implications for associated volcanogenic risk and geothermal and ore exploitation to better understand the structural evolution of calderas i use a discrete element model of a frictional cover undergoing piston like subsidence at its base simulating magma chamber deflation and cover collapse these novel simulations capture not only the initiation of cal deras but also much of the complexity of faulting during their later development in all models both normal and reverse faults accommodate deeper subsidence at higher structural levels curved outward dipping reverse faults are consistently the first structures to develop subsequent caldera growth is mainly the result of movement on vertical or steeply inward dipping normal faults this may be followed by a later phase of lateral collapse on shallower normal faults the roof aspect ratio r cover thickness piston width is seen to be an important factor calderas with low aspect ratios r 1 exhibit a coherent central subsiding block while those with higher aspect ratios have a complexly faulted internal structure

geochronology and stratigraphy of the ignimbrites from the 21 30 s to 23 30 s portion of the central andes of northern chile the central volcanic zone cvz of the andes which extends from 14 to 28 s is one of the largest provinces of late tertiary to recent ignimbrite volcanism in the world in the 21 30 s to 23 30 s portion of the cvz in northern chile ignimbrite volcanism was initiated at the beginning of the late miocene 10 4 ma ago and continued until the recent the stratigraphy is dominated by the products of five major dacitic ignimbrite eruptions 100 km3 the artola ignimbrite member 9 4 ma the sifon ignimbrite member 8 3 ma the pelon ignimbrite member 5 5 ma the puripicar ignimbrite member 4 2 ma and the atana ignimbrite member 4 1 ma at least three of these represent only portions of larger ignimbrite sheets erupted from centres of sw bolivia two of the ignimbrites the sifon and atana ignimbrites must represent erupted volumes in excess of 1000 km3 each the major units which represent 90 of the total volume of ignimbrite in this area are intercalated with smaller 50 km3 less extensive volcanic units some of which were erupted from source structures within the study area the upper tertiary to recent volcanic stratigraphy has been revised using the large ignimbrites as the major stratigraphic horizons the ignimbrite formations can be divided into two groups on a geographic basis those in the northern and central portions of the study area are known as the san bartolo group while those in the southern portion belong to the silapeti group these two groups appear to be contemporaneous although the silapeti group may represent a period of ignimbrite volcanism which was initiated later about 6 ma ago the ignimbrite formation of sw bolivia and the punense arancaniano formation of nw argentina are equivalent to the san bartolo and silapeti groups based on correlation of the stratigraphy into adjacent areas in sw bolivia and nw argentina the beginning of the late miocene 10 4 ma marks the onset of the most intense phase ignimbrite volcanism in the cvz this phase of volcanism is suggested to be the result of crustal melting in response to crustal thickening during the quechua phase of deformation and subduction approximately 10 000 km3 of ignimbrite erupted over a period of 9 ma in the 21 30 23 30 s portion and led to the development of a major ignimbrite province

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

catastrophic caldera forming eruptions thermomechanics and implications for eruption triggering and maximum caldera dimensions on earth abstract approximately every 100 000 years the earth experiences catastrophic caldera forming supereruptions that are considered to be one of the most hazardous natural events on earth utilizing new temperature dependent viscoelastic numerical models that incorporate a mohr coulomb failure criterion we find that eruptive failure of the largest magma chambers is a function of the geometry of the overlying roof and the location of the brittle ductile transition in particular the ductile halo created around the hot magma chamber buffers increasing overpressures and prevents pressure relief via magmatic injection from the magma chamber the numerical results indicate that as chamber volume increases the higher temperatures in the host rock and the decrease in the roof aspect ratio cause a shift from reservoir triggered eruption to an external roof triggered mechanism specifically as overpressure increases within the largest magma chambers extensive uplift in the overlying roof promotes the development of through going faults that may trigger eruption and caldera collapse from above we find that for magma chamber volumes 103 km3 and roof aspect ratios depth width

genetic links between ash flow calderas and associated ore deposits as revealed by large scale thermo mechanical modeling abstract in addition to a number of epithermal ore deposits some porphyry type ore deposits are also hosted by ash flow calderas spatial correlation between ash flow calderas and these deposits is observed in several parts of the world but temporal correlation between the mineralization and the episode of caldera formation remains unclear especially because geochronological data indicate that the mineralization may date to several million years before or after caldera formation because caldera collapse represents only a very short event in a long time scale evolution between a silicic magma chamber and its host rocks we have developed a thermo mechanical model of upper crustal and magmatic system behavior during the pre and post caldera formation stages major petrophysical properties of ash flow caldera related rocks are incorporated into the model which accounts for brittle elasto ductile rheology and heat transfer and reproduces fault initiation and various caldera collapse scenarios large scale mechanisms of initiation of thermal and structural traps for epithermal ore deposits are described in particular we found that the outer sides of caldera border faults are associated with both anomalous heat transfer and a high fracture density the finite size of the insulating intracaldera units forces the accumulated heat excess to be laterally transferred across the border faults in most cases the model predicts y shaped fault geometry which is surprisingly similar to that of mineralized faults in the field variations in lithostatic pressure on the magma chamber roof and in the vicinity of the border faults can reach 10 to 20 mpa with deep overpressured zones close to the inner edge of the chamber corners despite the fact that hydrothermal convection and fluid geochemistry are not included in the model these thermal and mechanical results are believed to constrain the conditions for mineralization processes our results show that in the presence of regional extension the development of caldera border faults may be suppressed to the advantage of deep seated fractures clustered above the central part of the magma chamber thus favoring conditions for the formation of porphyry type ore deposits finally we demonstrate that well before the eruption the gradual emplacement of a hot and large silicic magma chamber may create significant extensional stress in the brittle upper crust resulting in breakup and pre eruptive normal faulting and thus allowing for conditions of pre caldera mineralization

fracturing of volcanic systems experimental insights into pre eruptive conditions conditions for fracturing are a primary control on the behaviour of volcanic systems especially during the approach to eruption we here present the results of deformation experiments under simulated volcanic conditions on a porhyritic andesite from ancestral mount shasta andesite was chosen as a representative material because it is common at subduction zone volcanoes among both erupted products and country rock we deformed the lava in tension and triaxial compression tests at strain rates of 10 5 s 1 confining pressures from 0 to 50 mpa and temperatures up to 900 degrees c we also concurrently recorded acoustic emissions ae in order to monitor cracking activity the results show that deformation behaviour changes significantly in the temperature range 600 750 degrees c thus as temperatures increased across this interval the tensile fracture toughness increased from 2 5 0 5 mpa m 1 2 to 3 5 1 mpa m 1 2 the compressive strength decreased from 110 30 mpa to 55 35 mpa at 900 degrees c and the corresponding young s modulus decreased from 20 4 gpa to 6 4 gpa the changes occur when the deformation of the sample changes from elastic brittle to brittle ductile behaviour which we attribute to the blunting of crack tips due to melting of the glass phase and enhanced crystal plasticity at high temperature ae activity was observed in all experiments indicating that earthquakes can be generated not only in country rock but also in hot magma such as may be found in lava domes and at the margins of magma conduits in addition the trends in accelerating ae event rates before sample failure were comparable to those seen in earthquakes before some volcanic eruptions and a minimum in the seismic b value coincided with sample failure applied to volcanic systems the results suggest that 1 andesite strength and elasticity will not be affected by temperature or pressure beyond similar to 10 100 m from active magma 2 before eruptions fractures propagate preferentially through weaker horizons in a mechanically heterogeneous volcano and 3 volcanic rocks have characteristic seismic b values that are perturbed during the approach to bulk failure each of these conditions provides quantitative constraints on models for seismic precursors to eruption or intrusion c 2009 elsevier b v all rights reserved

the basic ingredients of the north atlantic storm track part i land sea contrast and orography understanding and predicting changes in storm tracks over longer time scales is a challenging problem particularly in the north atlantic this is due in part to the complex range of forcings land sea contrast orography sea surface temperatures etc that combine to produce the structure of the storm track the impact of land sea contrast and midlatitude orography on the north atlantic storm track is investigated through a hierarchy of gcm simulations using idealized and semirealistic boundary conditions in a highresolution version of the hadley centre atmosphere model hadam3 this framework captures the largescale essence of features such as the north and south american continents eurasia and the rocky mountains enabling the results to be applied more directly to realistic modeling situations than was possible with previous idealized studies the physical processes by which the forcing mechanisms impact the large scale flow and the midlatitude storm tracks are discussed the characteristics of the north american continent are found to be very important in generating the structure of the north atlantic storm track in particular the southwest northeast tilt in the upper tropospheric jet produced by southward deflection of the westerly flow incident on the rocky mountains leads to enhanced storm development along an axis close to that of the continent s eastern coastline the approximately triangular shape of north america also enables a cold pool of air to develop in the northeast intensifying the surface temperature contrast across the eastern coastline consistent with further enhancements of baroclinicity and storm growth along the same axis

trace element partitioning between alkali feldspar and peralkalic quartz trachyte to rhyolite magma part i systematics of trace element partitioning new alkali feldspar felsic magma trace element partition coefficients d values for rb sr ba eu y zr nb ga zn trivalent ree be cs hf pb th and u for 30 samples of peralkalic quartz trachyte and rhyolite are presented d values of incompatible elements vary systematically with melt polymerization parameters increasing with whole rock silica but decreasing in rocks with higher agpaitic indices a i mol na k al d values for sr and ba evaluated to be accurate vary systematically with crystal chemistry probably substituting for na in the ca poor alkali feldspar phases apparent d values for sr and ba from pre quaternary systems are fraught with contamination and analytical errors respectively and should be used with caution d eu decreases exponentially with a i ranging from compatible in weakly peralkalic a i 1 2 rocks these regular variations strongly suggest that partition coefficients for these elements may be predicted accurately if whole rock and crystal chemical parameters are known

magma driven subcritical crack growth and implications for dike initiation from a magma chamber 1 the purpose of this paper is to explore a viscoelastic energy dissipation theory for subcritical dike growth from a magma chamber the theoretical relationship between the dike growth velocity and dike length is established using the viscoelastic subcritical crack growth theory proposed by the first author and the solutions of stress intensity factor at the crack tip derived by a perturbation method effects of magma chamber over pressure buoyancy and viscoelastic properties of the host rock on the subcritical growth rate are included in the model the numerical results indicate that the viscous energy dissipation of the host rock could allow a short dike to slowly grow on the order of 10 7 10 5 m s under modest over pressure and to accelerate when the stress intensity factor increases close to the fracture toughness followed by the unstable dike propagation the proposed theory provides a reasonable understanding of dike initiation process from a magma chamber

temperature dependent viscoelastic modeling of ground deformation application to etna volcano during the 1993 1997 inflation period abstract we used the finite element method fem for modeling time dependent ground deformation due to volcanic pressure sources embedded in a viscoelastic medium especially in volcanic areas the presence of heterogeneous materials and high temperatures produce a lower effective viscosity of the earth s crust that calls for considering the thermal regime of crustal volume surrounding the magmatic sources we propose a thermo mechanical numerical model for evaluating the temperature dependency of the viscoelastic solution both temperature distributions and ground deformation are evaluated by solving an axi symmetric problem to estimate the effects of thermo viscoelastic response of the medium the thermo mechanical model permits to evidence that viscoelastic relaxation is responsible for significant time dependent variations in long term deformation these effects may be relevant for the interpretation and quantitative assessments of the pressure changes within magmatic sources with this in mind we reviewed the ground deformation observed on etna volcano during the 1993 1997 inflation period by setting up a fully 3d temperature dependent viscoelastic model since 1993 different geodetic measurements edm gps sar and leveling data identified an inflationary phase characterized by a uniform and continuous expansion of the overall volcano edifice that was not perturbed by eruptive activity the numerical model including significant viscoelastic material and reduced crustal rigidity around the magmatic source enables to produce deformation comparable with those obtained from elastic model requiring a significantly lower pressure for a purely elastic model with the same geometry and rigidity the pressure change necessary to describe the 1993 through 1997 inflation is around 320 mpa whereas for the viscoelastic model a pressure increase of about 200 mpa is required

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

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

the flow dynamics of an extremely large volume pyroclastic flow the 2 08 ma cerro galan ignimbrite nw argentina and comparison with other flow types the 2 08 ma cerro galan ignimbrite cgi represents a 630 km3 dense rock equivalent vei 8 eruption from the long lived cerro galan magma system 6 ma it is a crystal rich 35 60 pumice 10 generally and lithic poor 5 generally rhyodacitic ignimbrite lacking a preceding plinian fallout deposit the cgi is preserved up to 80 km from the structural margins of the caldera but almost certainly was deposited up to 100 km from the caldera in some places only one emplacement unit is preserved in proximal to medial settings and in most distal settings suggesting constant flow conditions but where the pyroclastic flow moved into a palaeotopography of substantial valleys and ridges it interacted with valley walls resulting in flow instabilities that generated multiple depositional units often separated by pyroclastic surge deposits the cgi preserves a widespread sub horizontal fabric defined by aligned elongate pumice and lithic clasts and minerals e g biotite a sub horizontal anisotropy of magnetic susceptibility fabric is defined by minute magnetic minerals in all localities where it has been analysed the cgi is poor in both vent derived accessory lithics and locally derived lithics from the ground surface accidental lithics locally derived lithics are small 20 cm and were not transported far from source points all data suggest that the pyroclastic flow system producing the cgi was characterised throughout by high sedimentation rates resulting from high particle concentration and suppressed turbulence at the depositional boundary layer despite being a low aspect ratio ignimbrite based on these features we question whether high velocity and momentum are necessary to account for extensive flow mobility it is proposed that the cgi was deposited by a pyroclastic flow system that developed a substantial high particle concentration granular under flow which flowed with suppressed turbulence high particle concentration and fine ash content hindered gas loss and maintained flow mobility in order to explain the contemporaneous maintenance of high particle concentration high sedimentation rate at the depositional boundary layer and a high level of mobility it is also proposed that the flow s was continuously supplied at a high mass feeding rate it is also proposed that internal gas pressure within the flow directed downwards onto the substrate over which the flow was passing reduced the friction between the flow and the substrate and also enhanced its mobility the pervasive sub horizontal fabric of aligned pumice lithic and even biotite crystals indicates a consistent horizontal shear force existed during transport and deposition in the basal granular flow consistent with the existence of a laminar shearing granular flow regime during the final stages of transport and deposition

the role of magma chamber fault interaction in caldera forming eruptions this paper examines the role of the position and orientation of a regional fault in the roof of a magma chamber on stress distribution mechanical failure and dyking using 2d finite element numerical simulations the study pertains to the magma chamber behavior in the relatively short time intervals of several hundreds to thousand of years the magma chamber is represented as an elliptical inclusion eccentricity a b 0 12 at a relative depth h a of 0 9 the fault has a 45 dip and is represented by a frictionless fracture the temperature field in the host rock is calculated assuming a quasisteady state thermal regime that develops through periodic episodes of magma supply the rheology of the surrounding rocks is treated using viscoelasticity with temperature activated strain rate dependent viscosity strain weakening of the rocks in the ductile zone is described within the frame of the dynamic power law model the magma pressure is coupled with the deformation of the rock mass hosting the chamber including the fault the variation of magma pressure in response to magma supply and chamber deformation is calculated in the elastic and viscoelastic regimes the latter corresponds to slow filling while the former represents a filling time much less than the viscous relaxation time scale the resulting equation of state for the magma chamber couples the magma pressure with the chamber volume in the elastic regime and with the filling rate for the viscoelastic regime analysis of stresses is used to predict dyke propagation conditions and the mechanical failure of the chamber roof for different fault positions and magma overpressures results show that an outward dipping fault located on the periphery of the chamber roof hinders the propagation of dykes to the surface causing magma to accumulate under the footwall of the fault at high to moderate overpressures 30 40 mpa the fault causes localized shear failure and chamber roof collapse that might lead to the first stage of a caldera forming eruption

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

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

conditions for the growth of a long lived shallow crustal magma chamber below mount pelee volcano martinique lesser antilles arc 1 the compositional homogeneity of silicic andesites emitted during the 13 500 year long last eruptive cycle of mount pelee suggests that the physical state of the magma chamber was largely unmodified during this period experimental phase equilibria on mount pelee recent products indicate that pre eruptive magma temperatures and pressures were in the range of 875 900 c and 2 0 5 kbar respectively the estimated average eruption rate was 7 5 10 4 km3 a with an average volume of about 0 3 km3 per eruption an analytical model for a spherical magma chamber indicates that a magma flux of 4 5 10 4 km3 a can maintain a magma chamber of 0 3 km3 above 875 c below mount pelee however observations of plutons suggest that magma chambers may grow by addition of sheet like intrusions with numerical simulation we show that a minimum sheet accretion rate of a few centimeters per year is required to grow a persistently active magma chamber independently of the intruded volumetric flux this minimum injection rate is higher if the heat transfer is enhanced by convection processes the limited ability of an arc crust to extend and accommodate dikes suggests that magma injections are sills or if they are dikes that most of the volume injected in the magma chamber is eventually erupted in a conductively cooling igneous body formed by sills injected at a rate of a few centimeters per year most of the injected magma completely solidifies and only a small part of the body about 10 20 is above 875 c and able to feed eruptions

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

implications of incremental emplacement of magma bodies for magma differentiation thermal aureole dimensions and plutonism volcanism relationships abstract field observations and geophysical data indicate that many igneous bodies grow by amalgamation of successive magma pulses that commonly take the shape of horizontal sheets sills emplacement styles and emplacement rates of magma bodies have fundamental implications on magma differentiation country rock metamorphism and assimilation and for the formation of large magma chambers in the upper crust when a magma body begins to grow by slow accretion of sills each successive intrusion solidifies before the injection of the next one when the system is thermally mature sill temperatures equilibrate above the solidus melts accumulate and older sills can re melt the time needed for each magma injection to cool down and equilibrate with its surrounding is short relatively to the total emplacement time of the body the transition from a mafic crystal poor magma to a partially molten rock that retains a highly differentiated melt is fast whereas the resulting evolved residual melt can reside in the crust for protracted periods as long as temperatures in the system are relatively low highly differentiated melts are generated which may explain the bi modal character and the absence of intermediate compositions in some magmatic provinces the level of emplacement of successive magma pulses controls the shape of the thermal anomaly associated with the magma body growth metamorphism partial melting and assimilation of the country rock are favoured if successive magma sheets are emplaced at or close to the country rock magma body boundary if the magma emplacement rate is low the size of the thermal aureole is controlled by the size of one pulse and not by the size of the entire igneous body understanding emplacement of magma bodies is fundamental for our understanding of the plutonism volcanism relationship magma emplacement rates of several centimetres per year are needed for a magma body to evolve into a large magma chamber able to feed large silicic explosive eruptions the time averaged emplacement rates of plutons are lower than this critical emplacement rate eruptions of 100s to 1000s cubic kilometres of silicic products show that such high volumes of magmas can accumulate in the upper crust this suggests that the emplacement of magma bodies is a multi timescale process with the development of large magma chambers corresponding to the highest magma fluxes because they control magmatic processes and the impact of magma intrusion on the country rock future studies should focus on magma emplacement rates and on magma emplacement geometries these studies should integrate field observation on plutons and geophysical data on active magmatic systems coupled with laboratory experiments and numerical simulations

stratigraphy of the toba tuffs and the evolution of the toba caldera complex sumatra indonesia during the past 1 2 m y a magma chamber of batholithic proportions has developed under the 100 by 30 km toba caldera complex four separate eruptions have occurred from vents within the present collapse structure which formed from eruption of the 2800 km3 youngest toba tuff ytt at 74 ka eruption of the three older toba tuffs alternated from calderas situated in northern and southern portions of the present caldera the northern caldera apparently developed upon a large andesitic stratovolcano the calderas associated with the three older tuffs are obscured by caldera collapse and resurgence resulting from eruption of the ytt samosir island and the uluan block are two sides of a single resurgent dome that has resurged since eruption of the ytt samosir island is composed of thick ytt caldera fill whereas the uluan block consists mainly of the oldest toba tuff ott in the past 74000 years lava domes have been extruded on samosir island and along the caldera s western ring fracture this part of the ring fracture is the site of the only current activity at toba updoming and fumarolic activity the toba eruptions document the growth of the laterally continuous magma body which eventually erupted the ytt repose periods between the four toba tuffs range between 0 34 and 0 43 m y and give insights into pluton emplacement and magmatic evolution at toba