miocene sea level falls related to the geologic history of midway atoll a period of atoll emergence results in the development of a karst surface on the atoll upon resubmergence this surface is preserved as a subsurface solution unconformity that can be identified by petrographic characteristics and a distinctive stable isotope signature imprinted by subaerial or phreatic diagenesis determination of the magnitude of miocene sea level falls by considering subsidence rates and depths to solution unconformities must take into account the lowering of the emergent atoll surface by dissolution previously the depth to a miocene solution unconformity below midway atoll was assumed to record a maximum highstand of sea level that preceded a period of island emergence however because of surface dissolution the solution unconformity must represent some elevation between a past highstand and a subsequent lowstand the subsurface stratigraphy of midway atoll shows that a late miocene sea level fall of 75 125 m below present sea level took place the magnitude of this fall is greater than that previously determined on the basis of stable isotope data from midway atoll but approximates miocene sea level falls determined by seismic stratigraphic methods

3 d reconstruction of ash vesicularity insights into the origin of ash rich explosive eruptions abstract explosive volcanic eruptions are characterized by highly variable degrees of magma fragmentation even during a single eruptive event the increasing amount of fine pyroclasts is often uncritically related to explosive magma water interaction i e hydromagmatic fragmentation here we report examination of two examples of major explosive eruptions from the quaternary vulsini volcanic district central italy in which the fine grained nature of deposits even in near vent settings indicates negligible effect of transport and implies the eruption of highly fragmented magmas sem morphoscopy of the juvenile products rules out extensive ash production due to hydromagmatic fragmentation we apply a recently developed stereo scanning electron microscopy ssem technique proussevitch et al 2011 to determine vesicularity features e g bubble size distribution and bubble number density hereafter bsd and bnd respectively of ash particles ssem analysis provides new insights into magma vesiculation history and fragmentation mechanism leading to major ash rich eruptions we conclude that extensive ash production was related to essentially magmatic processes involving high degrees of decompression in shallow magma reservoirs

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

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

a geophysical interpretation of the 1883 krakatau eruption abstract from the present submarine topography in the vicinity of the krakatau islands it is concluded that the focus of the large explosions was situated to the northwest of the present rakata island the channel between krakatau and sebesi islands was completely blocked by banks of volcanic material immediately after the eruptions and it is suggested that this material was mainly lithic fragments the explosion sequences of krakatau are deduced from the records of sea waves and air waves observed at jakarta the large tsunami was caused by the most violent explosion simultaneously with the largest air waves it is inferred that the origin of the tsunami was a sudden upheaval of the seawater due to a violent explosion and that the height of the tsunami near its source was 30 40 m energy of the explosion is estimated from analyses of the air waves as one order of magnitude greater than that of the 1956 bezymianny eruption thus the 1883 krakatau eruption may have caused explosive removal of more than 10 km3 of material the subsurface structure of krakatau islands after the 1883 eruption is deduced from gravity anomalies it is concluded that at the bottom of the caldera there are deposits of low density in the shape of an inverted cone 8 km in diameter and 1 km in depth from the residual gravity anomaly observed over the caldera one can estimate the mass deficiency there this allows estimates to be made as to the amount of ejecta although large uncertainties remain these data indicate that explosive removal of material was the main process responsible for the disappearance of the northern half of the former rakata krakatau island in the 1883 eruption

stress fields controlling the formation of nested and overlapping calderas implications for the understanding of caldera unrest abstract commonly the formation of a collapse caldera does not necessarily imply the end of the volcanic activity in the area in many cases successive calderas may form close to the previous collapse depression or intersecting it leading to overlapping collapse structures occasionally subsequent caldera collapses may take place at the interior of the first caldera creating nested collapse structures during the last years several authors have investigated numerically how the stress field around magma chambers may favour the formation of collapse calderas assuming that the host rock surrounding the magmatic reservoir behaves as a linear elastic homogeneous medium the numerical models presented in this work study how a caldera collapse may modify the stress field of a volcanic area and hence the conditions for the formation of future collapse calderas our models take into account the effect of the collapse structure considering it as a mechanical discontinuity we also investigate the mechanical influence of the intra and extra caldera deposits on the formation of new calderas all the numerical models are two dimensional assuming plane strain and considering that the surrounding crust behaves as a linear homogeneous elastic material the computational domain corresponds to a cross section of the upper crust 50 25 km and magma chambers are modelled as sill like cavities located at a certain depth below the earth s surface the existing collapse caldera depression is 8 km wide and 2 75 km deep however we consider the caldera infill i e intra caldera material to be 1 25 or 1 75 km thick we assume as loading conditions an underpressure of 10 mpa imposed at the chamber walls that is negative excess pressure in the chamber in some numerical runs we have considered the existence of a previous ring fault by introducing a thin and elongate vertical weak zone at the caldera margins we find that the stress field around shallow level magma chambers favouring the formation of either nested or overlapping caldera structures depend on 1 the dimensions of the previous caldera collapse structure 2 the size of the new magma chamber susceptible to generate the subsequent caldera forming event and 3 the mechanical properties of the surrounding host rock and the syn and post collapse materials moreover results obtained in this paper may be relevant to the interpretation of unrest episodes in active caldera systems our numerical results confirm that the formation of multiple calderas nested overlapping or separate imply the development of new shallow magma chambers different from the one that lead to the first collapse caldera in consequence interpretation of magmatic unrest episodes in active collapse calderas should not be based on the assumption that the responsible for the current unrest is the same system that originated the caldera

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

the tyrrhenian apennines system structural setting and seismotectogenesis abstract the available geological geophysical and seismological data of the tyrrhenian apennines system are briefly reviewed and synthesized in a number of sketch maps integrating and comparing these data a large scale simple shear kinematic model is proposed for the tyrrhenian apennines system that is here interpreted as an oblique asymmetric passive continental margin developed during the progressive counterclockwise rotation of the adriatic foreland from the late miocene to the present in the frame of this working hypothesis the thinning and stretching of the tyrrhenian crust were accommodated by progressive eastward motion of extensional allochthonous slices bounded by low angle west dipping oblique shear zones rotating above a common detachment that penetrates into the mantle lithosphere at any given time the most external eastward moving slice is bounded by a transtensional shear zone at the top and by a transpressional one at the bottom the displacement within the hanging wall of the transtensional shear zone is accommodated by high angle normal faults and en echelon grabens the displacement within the hanging wall of the transpressional shear zone is instead accommodated by co axial en echelon sets of folds thrusts and high angle strike slip faults thus the development of the late miocene pliocene pleistocene contractional strain field of the apennines of italy might not be a consequence of complex subduction processes but subordinate to the deformation processes that lead to the progressive opening of the tyrrhenian sea the seismicity of the crustal apennines of italy is analyzed in the frame of the above history of progressive deformation and a seismotectogenetic model is outlined

tephra fountaining rheomorphism and spatter flow during emplacement of the pitigliano tuffs latera caldera italy abstract toward the end of the main ash flow phase ca 156 ka of the latera caldera central italy explosive eruptions near the nw caldera rim produced coarse grained tephra exposed in 35 m high faces of the toscapomice casa collina quarry the pyroclastic sequence fills a narrow sw ne paleovalley and records the near continuous eruption of a compositionally zoned magma batch white phonolitic pumice clasts abruptly grade upward to and partly intrude black spatter like pumice fragments also of phonolite composition this sequence in turn is overlain by densely welded tuff that grades upward to tephriphonolitic scoriae and ash the upper welded part of this sequence referred to as the vulcanite complessa di pitigliano has long been among the more enigmatic deposits in the latera area this sequence displays evidence for both fall and flow emplacement and is here interpreted as an ash fountain fed deposit that locally welded as spatter yet became sufficiently fluidized in distal parts to behave as a spatter rich pyroclastic flow this mechanism requires that the eruption underwent abrupt transition from a vigorously convecting column to a weakly convecting ash fountain and that this period of explosive activity culminated with unusually high accumulation rates the rapidly emplaced tephra retained sufficient heat to weld by compaction and undergo rheomorphism close to the vent yet contained high enough proportions of fluidized dust to have flowed outward along the northern caldera rim and eventually to have welded by vapor phase sintering

formation of nonintersecting nested calderas insights from analogue models approximately 56 of 60 known nested calderas are pairs of nonintersecting subcircular structures with similar eccentricities the consistent configuration suggests an interdependence in the formation of the two annular structures this possibility has been investigated using 11 analogue models with sand crust analogue overlying silicone magma analogue silicone withdrawal induces collapse within the sand forming two concentric depressions the first formed inner depression bordered by outward dipping reverse ring faults mean dip of 75 induces the collapse of the outer depression bordered by inward dipping normal ring faults mean dip of 84 an original linear relationship is determined among the ratios between the diameters of the two depressions and the depth of the ring faults selected pairs of natural nested calderas show similar proportions highlighting a consistent architecture this suggests that some pairs of nonintersecting nested calderas result from a single collapse event

the evolution of latera caldera central italy in the light of subsurface data subsurface geothermal exploration has considerably added to our knowledge of the latera volcanic complex a syenitic body is located about 2 km below the present day surface k ar data point a 0 9 ma age the primary magma was a silica saturated trachyte undersaturated hauyne bearing products are found near the carbonatic wall rocks and have been interpreted as reaction products subsurface data from deep drilling and geophysical surveys suggest that the latera caldera resulted from three main successive collapse phases i formation of an old caldera now buried related to the eruption of ignimbrites from the syenitic magma chamber ii sinking of the eastern sector as a consequence of the formation of the nearby bolsena caldera 0 3 ma iii multistage formation of the present latera caldera 0 16 ma

prolonged carbonate diagenesis under an evolving late cenozoic climate nullarbor plain southern australia abstract the nullarbor plain in southern australia the largest areal karst on the globe is a 240 000 km2 uplifted succession of cenozoic marine carbonates whose surface has been exposed for 14 to 15 m y the middle miocene nullarbor limestone forms the upper surface of the plain and hosts a complex and prolonged record of meteoric diagenesis such a complete record offers unique insights into the effects of climate tectonics sea level topography and hydrology on the style and placement of numerous diagenetic events in flat low lying carbonate plains alteration took place during three broad phases comprising eight stages that are interpreted to have formed against a background of dramatic climate change middle miocene phase one diagenesis took place under a humid climate and resulted in rapid mineral equilibration calcite cementation extensive karst development and finally widespread lacustrine and palustrine sedimentation resultant palustrine sediments especially terrestrial ooids are now preserved at the surface and in underlying karst cavities latest middle miocene to middle pliocene phase two diagenesis occurred during a prolonged period 8 m y of temperate climate and resulted in initial deep cave dissolution during low sea levels and later shallow cave development in the course of a high sea level onset of a somewhat more arid climate in the latest pliocene led to the development of the modern desolate landscape of the plain this final phase of diagenesis involved creation of solution pits filled with black limestone pebbles open and closed dolines with associated colluvium fill and pervasive pedogenic calcrete the nullarbor plain demonstrates that low lying carbonate plains can have low surficial erosion rates precisely record relative sea level positions be able to have extensive caves with extended periods of arrested calcite precipitation and finally host extensive terrestrial ooid deposits the importance of this comprehensive paragenetic record is its applicability to not only recognize unconformities in the rock record but to better appreciate the climate in which they formed

origin of the calderas and evolution of roccamonfina volcano roman region italy abstract the roccamonfina volcano consists of a volcanic fluvial succession dated 549 ka bp paleoauruncus overlain by a 546 ka bp stratacone main cone formed by leucite bearing lava and tephra the main cone was dissected by four nested calderas a ring fracture concentric with and outside of the central caldera is also present detailed mapping of the volcano inspection of landsat satellite images and radiometric dating establish that the gli stagli and northern calderas subsided between respectively 546 474 and 403 374 ka bp and that the summit part collapsed between 546 and 446 ka bp to form the central caldera a chronologically intermediate state dated in the range 376 323 ka bp was identified between the 549 374 ka bp leucite bearing high k series hks stage i and the 317 96 ka bp leucite free low k series lks stage ii this stage is made up of concurrent hks lks rocks stage ics the central gli stagli and northern calderas are older than blt and wtt as supported by the strong discrepancies between calculated volumes of these pyroclastic rocks and the volume of the caldera also caldera collapses due to sector sliding are improbable as tested by the absence of rock slide avalanche deposits the collapses occurred incrementally through repetitive draining of the magma chamber s resulting from a succession of moderate sized volcanic events gli stagli caldera was infilled by a lake early after collapse so that sediments alternated with leucite tephrite lava flows and tephra within the lake the sequence was overlain by a thick succession of young pyroclastic rocks downfaulting downsagging cut the flat lying gli stagli filling sequence resulting in the old valle caldera successive pyroclastic and debris flows derived by updoming of the central caldera deposits infilled the last structure and downfaulted part of the valle fill producing the young valle caldera

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

on the anatomy of magma chamber and caldera collapse the example of trachy phonolitic explosive eruptions of the roman province central italy abstract textural and compositional features of pyroclastic products erupted during caldera forming events often reveal the tapping of different portions of variably zoned magma chambers due to changing geometries of the conduit vent systems here we report on ultrapotassic trachytic phonolitic explosive eruptions of the roman province central italy which show remarkable changes of textural features and glass compositions in the juvenile material even if the bulk chemical composition is essentially constant in each example the lower eruption sequence contains whitish crystal poor leucite free highly vesicular pumice emplaced by early plinian fallout and or pyroclastic currents upsection the eruption sequence contains black low porphyritic sanidine leucite bearing moderately vesicular scoria or spatter emplaced by major pyroclastic flows red tuff with black scoria and associated co ignimbrite coarse lithic rich breccias this suggests a shift from a central feeder conduit tapping the central part of the magma chamber to a ring fracture vent system tapping the peripheral portions of the magma chamber during caldera collapse key features of these evacuating magma chambers are the thermal and volatile concentration xvol gradients that produce the observed textural and compositional spectrum of trachy phonolitic rock types in particular the degrees of freedom during the crystallization of these ultrapotassic magmas are increased by the variation of the leucite stability field at different ph2o conditions both leucite free and leucite bearing differentiated ultrapotassic rock types can be produced in the course of individual eruptions as a result of pre eruptive conditions in the feeder magma with no need to invoke different differentiation suites related to mantle source heterogeneities of parental magmas

aeromagnetic mapping of the structure of pine canyon caldera and chisos mountains intrusion big bend national park texas analysis of aeromagnetic and gravity data reveals new details of the structure igneous geology and temporal evolution of the prominent enigmatic ca 32 ma pine canyon caldera and the chisos mountains big bend national park texas the main caldera fi lling pine canyon rhyolite the oldest member of the south rim formation is reversely magnetized allowing it to be used as a key marker bed for determining caldera fi ll thickness modeling of gravity and magnetic anomalies indicates that the pine canyon rhyolite is probably thicker in the northeastern part of the caldera lineaments in the magnetic data suggest the presence of buried faults beneath the caldera that may have led to increased downdrop in the northeast versus the southwest allowing a thicker section of caldera fi ll to accumulate there the pine canyon caldera has been interpreted as a downsag caldera because it lacks surfi cial faulting so these inferred faults are the fi rst mapped features there that could be responsible for caldera collapse the caldera boundary correlates well with the margins of a gravity low general features of the caldera match well with basic models of downsag calderas meaning that the pine canyon caldera may be a classic example of downsagging of which few well described examples exist in terms of a geophysical signature the source of a long wavelength magnetic high over the chisos mountains is interpreted as a previously unknown broad intrusion the long axis of which trends parallel to a major crustal boundary related to the ouachita orogeny or an even earlier precambrian margin this feature represents the largest intrusion 28 34 km diameter 1 4 km thick 700 3000 km 3 in volume in an area where relatively small laccoliths are ubiquitous the intrusion most likely represents a long lived 1 m y reservoir replenished by small batches of magma of varying composition as refl ected in the variation of eruptive products from the pine canyon and sierra quemada calderas the intrusion may represent the easternmost occurrence of voluminous tertiary magmatism in the southwestern united states

lateral variations in the taupo ignimbrite abstract the young non welded taupo ignimbrite shows remarkable lateral variations which are documented by granulometric and component analyses and studies of maximum clast size and density the grain size spans practically the entire known ignimbrite field the coarser proximal ignimbrite having a median diameter 100 times greater than the finest distal ignimbrite the content and maximum size of lithic fragments decrease also by a factor of 100 between proximal and distal parts the content of free crystals first rises to reach a peak but thereafter decreases to attain a very low value in far distal exposures the pumice maximum size decreases by a factor of about 10 and the most conspicuously coarse pumice rocks occur in a girdle nearly halfway out from vent to distal limit the pumice in each grain size class decreases in density to half of its near source value in distal ignimbrite the overall outward trend is towards an ignimbrite which consists wholly of fine vitric ash some distal exposures closely approach this condition these variations are accounted for by a combination of processes operating in the moving ash flow one is a continuous fragmentation of pumice leading to a rounding of the clasts a progressive decrease in maximum size the generation of much vitric dust and the liberation of crystals another is a continuous sedimentation of heavy constituents lithics and crystals and an antipathetic rise of lighter coarse pumice towards the top of the flow these processes operated in a moving flow whose upper layers travelled progressively farther from source it is the topmost layers strongly depleted in heavy constituents and enriched in light pumice which have travelled the farthest and constitute the far distal parts of the ignimbrite a number of ignimbrite facies are characterized the ignimbrite proper with its proximal distal and pumice concentration zone facies the deposits which form in the head and are then over ridden by the body of the flow including the fines depleted ignimbrite variant and the heavies enriched ground layer and the ignimbrite veneer deposits which are left behind by the flow which differ little from the ignimbrite except in their landscape mantling form and the occurrence in them of lee side coarse pumice lenses

upper crustal tomographic images of the amiata vulsini geothermal region central italy we computed the three dimensional p wave velocity structure of the upper crust beneath the adjacent amiata and vulsini quaternary volcanoes central italy inverting local earthquake arrival times we used 676 selected earthquakes out of more than 3000 in the m range 1 5 4 0 located between 1977 and 1992 by two contiguous microseismic networks operated by ente nazionale energia elettrica to monitor three geothermal areas namely latera torre alfina and mount amiata the resolving capability of the data set has been verified both with a priori synthetic tests and an a posteriori analysis of the resolution matrix we find a 40 25 7 h km3 crustal volume where we can resolve details as small as 2 to 4 km our analysis is favored by numerous geological and geophysical data collected for geothermal exploration these independent data give us specific constraints for both choosing a starting model and interpreting tomographic results we find high velocity anomalies in the upper 3 km of the crust beneath latera torre alfina and mount amiata geothermal fields related to uplifted limestone units the lateral extension of limestone units is defined by the three dimensional velocity pattern a negative anomaly clearly identifies the radicofani graben a plio quaternary depression filled with low velocity clayey sediments between 1 and 3 km depth a sharp gradient between high velocities 6 9 km s in the northern region and lower velocities in the southern one is recovered at 7 km depth this feature may indicate the transition between the tuscan province uplifted metamorphic crust and thickened carbonate series of the latium province

long term risk in a recently active volcanic system evaluation of doses and indoor radiological risk in the quaternary vulsini volcanic district central italy abstract volcanic rocks in the vulsini volcanic district central italy contain high concentrations of 238u 232th and 40k due to subduction related metasomatic enrichment of incompatible elements in the mantle source coupled with magma differentiation within the upper crust due to their favorable mechanical properties they have been extensively used for construction since the etruscan age in the old buildings of the bolsena village one of the most populated ancient village in the area the major source of indoor radioactivity is 222rn a radioactive noble gas descendant of 238u direct 222rn indoor measurements have detected extremely high values in the old center due to the combined effect of building materials radon fluxes from the volcanic basement and low air exchange rates in these cases the evaluated risk of developing lung cancer within a 75 year lifetime reaches up to 40 for ever smokers simulations of standard rooms built with different tuffs and lavas collected from the vulsini volcanic district have also provided estimations of the effective doses and lifetime risk for radiogenic cancer other than by the method adopted for calculation the total evaluated risk for each volcanic rock depends on different parameters such as radionuclide content radon emanation power occupancy factor and air exchange rate occupancy factor and air exchange rate appear as the only controlling parameters able to mitigate the indoor radiological risk

post collapse volcanic history of calderas on a composite volcano an example from roccamonfina southern italy roccamonfina part of the roman potassic volcanic province is an example of a composite volcano with a complex history of caldera development the main caldera truncates a cone constructed predominantly of this caldera may have been associated with one of the ignimbritic eruptions of the brown leucitic tuff blt around 385 000 yr bp the campagnola tuff the youngest ignimbrite of the blt however drapes the caldera margin and must postdate at least the initial stages of collapse during the subsequent history of the caldera there were several major explosive eruptions the largest of these was that of the galluccio tuff at about 300 000 yr bp it is likely that there was further collapse within the main caldera associated with these eruptions it is of note that despite these subsequent major explosive eruptions later collapse occurred within the confines of the main caldera between eruptions caldera lakes developed producing numerous lacustrine beds within the caldera fill extensive phases of phreatomagmatic activity generated thick sequences of pyroclastic surge and fall deposits activity within the main caldera ended with the growth of a large complex of basaltic trachyandestite lava domes around 150 000 yr bp early in the history of roccamonfina sector collapse on the northern flank of the volcano formed the northern caldera one of the youngest major events on roccamonfina occurred at the head of this northern caldera with explosive activity producing the conca ignimbrite and associated caldera there is no evidence that there was any linkage in the plumbing systems that fed eruptions in the main and northern calderas

gravity fields and the interpretation of volcanic structures geological discrimination and temporal evolution abstract a review of gravity data reflecting shallow concealed structures on volcanoes demonstrates that valuable information may be derived on the geological development of such structures and on the dynamic evolution of active volcanoes to a first approximation all recorded examples of anomalous gravity fields associated with volcanoes are concentric with and have sources that lie within the volcanic edifice positive anomalies with wavelengths less than 20 km and amplitudes up to ca 30 mgal characterise mainly basaltic volcanoes from various tectonic settings and are caused by a relatively dense intrusive complex magma body which contrasts with its surroundings either because the body is more mafic than average or more likely because near surface previously erupted materials are uncompacted negative anomalies with wavelengths up to 100 km and amplitudes up to ca 60 mgal occur over much larger volcanic calderas many of which have erupted highly silicic pyroclastic ash and pumice uncompacted silicic caldera infill with a possible contribution from low density magma bodies is responsible for the observed anomalies there is some evidence for a continuum of gravity anomaly types that corresponds to the geological evolution of volcanic systems generally from primitive rift or subduction related basaltic andesite to mature high level silicic calderas repeated microgravity observations over active volcanoes enable magma movements variations in magma input and changes in magma density to be monitored closely dynamic modelling of volcanic systems is providing new evidence on the behaviour of concealed magma bodies and could have considerable potential for predicting eruptive events

incremental caldera collapse of suswa volcano gregory rift valley kenya suswa volcano located at 1 10 s 36 20 e is quaternary in age following caldera i collapse phonolite lava flows were erupted from the caldera floor centrally erupted phonolite lava flows led to the construction of ol doinyo onyoke lava cone a pit crater on the cone was a precursor to the collapse of caldera ii both of which were generated entirely by magma withdrawal regional decompression caused ring fault bounded block resurgence of the caldera floor

downsag and extension at calderas new perspectives on collapse geometries from ice melt mining and volcanic subsidence structures at calderas may form as a result of precursory tumescence subsidence due withdrawal of magmatic support resurgence and regional tectonism structural reactivation and overprinting are common to explore which types of structures may derive directly from subsidence without other factors evidence is reviewed from pits caused by the melting of buried ice blocks mining subsidence scaled subsidence models and from over 50 calderas this review suggests that complex patterns of peripheral deformation with multiple ring and arcuate fractures both inside and outside caldera rims topographic embayments arcuate graben and concentric zones of extension and compression may form as a direct result of subsidence and do not require a complex subsidence and inflation history downsag is a feature of many calderas and it does not indicate subsidence on an inward dipping ring fault as has been inferred previously where magmatic inflation is absent or slight initial arcuate faults formed during collapse are likely to be multiple and dip outwards to vertical associated downsag causes the peripheries of calderas undergo radial centripetal extension and this accounts for some of the complex peripheral fractures arcuate crevasses graben and some topographic moats the structural boundary of a caldera defined here as the outermost limits of subsidence and related deformation including downsag commonly lies outside ring faults and outside the embayed topographic wall it is likely to be funnel shaped i e inward dipping even though ring and arcuate fractures within it may dip outward inward dipping arcuate normal faults at shallow levels and steep inward dipping contacts between a caldera s fill and walls may both occur at a caldera that has initially subsided on outward dipping ring faults they arise due to peripheral surficial extension gravitational spreading and scarp collapse topographic enlargement at some calderas and the formation of embayments may reflect general progressive downsag and localized downsag respectively these processes may occur in addition to surficial degradation of oversteep ring fault scarps

40 ar 39 ar ages and stratigraphy of the latera caldera italy the latera caldera is a well exposed volcano where more than 8 km3 of mafic silica undersaturated potassic lavas scoria and felsic ignimbrites were emplaced between 380 and 150 ka isotopic ages obtained by 40ar 39ar analysis of single sanidine crystals indicate at least four periods of explosive eruptions from the caldera the initial period of caldera eruptions began at 232 ka with emplacement of trachytic pumice fallout and ignimbrite they were closely followed by eruption of evolved phonolitic magma after roughly 25 ky several phonolitic ignimbrites were deposited and they were followed by phreatomagmatic eruptions that produced trachytic ignimbrites and several smaller ash flow units at 191 ka compositionally zoned magma then erupted from the northern caldera rim to produce widespread phonolitic tuffs tephriphonolitic spatter and scoria bearing ignimbrites after 40 ky of mafic surge deposit and scoria cone development around the caldera rim a compositionally zoned pumice sequence was emplaced around a vent immediately northwest of the latera caldera this activity marks the end of large scale explosive eruptions from the latera volcano at 156 ka

tectonics of nisyros island greece by field and offshore data and analogue modelling abstract we use new on land and offshore structural data and scaled analogue models to analyse the fault tectonic pattern at nisyros island greece which has an active caldera that shows a complicated network of faults fractures and volcano tectonic structures we measured 157 faults that show dominant dip slip normal motions along planes mainly striking ne sw nne ssw and nnw sse inside the caldera dykes necks and morphometric parameters of volcanic domes explosion craters and fumarole pits indicate the control by ne striking discontinuities on magma and gas paths the nnw and ne striking faults bound a major block that underwent repeated downthrow and uplift movements during the late pleistocene holocene experiments with scaled models of caldera resurgence and two magma chambers indicate the formation of an hourglass shaped fault pattern as seen in plan view with an asymmetric increase in the fault offset and a widening of the fault divergence towards the volcano flank all these data suggest that regional fault tectonics and stress state strongly guided magma upwelling and the emplacement of volcanic centres whereas periodical bulging due to the overpressure of a second magma chamber located northwest of the caldera combined with faulting due to tectonic stresses can account for the overall deformation field

the merapi 2010 eruption an interdisciplinary impact assessment methodology for studying pyroclastic density current dynamics abstract the large explosive eruption of merapi volcano indonesia in 2010 presented a key and rare opportunity to study the impacts of a major explosive eruption in a densely populated area pyroclastic density currents pdcs produced throughout the 2010 eruption were unusually destructive causing near complete devastation across a 22 km 2 swath of the densely populated southern flanks and casualties to the end of their runout at 15 5 km from the volcano the majority 120 of the more than 200 fatalities occurred more than 12 km from the volcano where many people were caught in pdcs as they were evacuating the 2010 eruption vei 4 exhibited a range of pdc behaviour in a complex multi stage event that marked a change in eruption behaviour at merapi being the first eruption of this magnitude and style since 1872 this shift in style may mark a change in regime and so understanding the potential impact of such large explosive eruptions is essential for future risk assessment at merapi we describe a new impact assessment methodology that allowed us to collect important empirical geological damage and casualty information and reconstruct impact dynamics associated with the pdcs in contrast to previous pdc impact studies we combined remote field laboratory and gis assessments and were able to enter the affected areas safely and before their disturbance by rains or human activity by integrating the results of our geological damage and medical studies we could reconstruct the spatial and temporal dynamics of the pdcs and their main hazard characteristics our interdisciplinary methods and preliminary findings are discussed here in the areas damaged by pdcs we used empirical damage data and calculations of material and structural resistance to lateral force to estimate approximate dynamic pressures dynamic pressures associated with the 5 november paroxysm exceeded 15 kpa more than 6 km from source and rapidly attenuated over a distance of less than 1 km at the end of the pdc runouts analysis of thermal indicators such as deformed plastic and correlation with information on burns injuries and fires provided estimates of ambient temperatures associated with the pdcs even at the relatively low temperatures estimated for the pdcs 200 300 c they were lethal to people inside as well as outside buildings in part because of the building design that enabled the pdcs to rapidly infiltrate inside such detailed quantitative data can be used to support numerical pdc and impact modelling and risk assessment at dome forming volcanoes providing an improved understanding of the complexity of pdcs and their associated impacts on exposed populations

ongoing long term evolution of an unconformity that originated as a karstic surface in the late miocene a case study from the cayman islands british west indies abstract on grand cayman and cayman brac karst development on the upper surface of the cayman formation during the late miocene lowstand produced the cayman unconformity on grand cayman this led to the development of a deep atoll shaped depression on the western part of the island the ensuing lower pliocene transgression buried that unconformity and led to deposition of sediments that now form the pedro castle formation today on grand cayman the dolostones of the cayman formation remain largely buried on the western half of grand cayman but are widely exposed on the central and eastern parts of the island where they have been subjected to extensive weathering over the last 4 3 ma although the situation is similar on cayman brac uplift of the central core led to removal of the pedro castle formation and much of the cayman formation karst development on both islands reflects the interplay between neogene sea level changes climate and tectonic uplift today weathering is further modifying the cayman unconformity on the eastern parts of both islands the cayman unconformity on the central and eastern parts of grand cayman is characterized by karst landforms including a peripheral rim sinkholes solution widened joints and photolineaments surface traces of joints and or faults the westward tilting of cayman brac 4 3 ma to 400 ka led to more severe weathering of the exposed cayman formation that included 1 enhancement of the peripheral rim and karst features on the upslope margin and 2 higher denudation rates than on grand cayman during the messinian the denudation rate on the west end of grand cayman was 0 03 0 10 mm yr 1 in contrast the denudation rate over the an estimated period of 4 9 to 6 2 ma for the eastern half of grand cayman has been about 0 01 mm yr 1 whereas on the east end of cayman brac it has been 0 03 0 04 mm yr 1 on both islands substantial thicknesses of strata have been lost to erosion and the cayman unconformity has been subject to ongoing modification over a period of 4 9 to 6 2 ma

caldera structure amount of collapse and erupted volumes the case of bolsena caldera italy calderas are common on volcanoes but their structure is seldom visible the 19 km wide bolsena caldera italy formed between 0 6 and 0 2 ma the largely preserved structural rim and subsurface data make bolsena ideal to investigate caldera structure in relation to the subsidence and erupted volume in this paper we use remote sensing field analysis and available subsurface data at the surface the caldera passes from a downsag south rim to a narrow and densely faulted area north rim with outer normal and inner reverse faults the caldera structure on the widely faulted east rim appears to be scale dependent with a staircase like fault zone larger scale horst and graben like structures intermediate scale and domino like structures smaller scale subsurface data indicate asymmetric collapse with a northward increase in subsidence ranging from diffuse to the south to focused to the north deformation at the surface the collapse rate constant between ca 490 and 175 ka was more than magma output between ca 330 and 130 ka highlighting significant 200 m and prolonged 200 ka posteruptive subsidence as the nearby latera caldera west rim of bolsena was mostly active between ca 265 and 160 ka much of the subsidence at bolsena may be related to this activity suggesting a common magmatic reservoir the subsidence related structural variations along the caldera rim and the significant posteruptive subsidence found at bolsena have not been found in other calderas

deciphering the impact of sea level changes and tectonic movement on erosional sequence boundaries in carbonate successions a case study from tertiary strata on grand cayman and cayman brac british west indies abstract the stratigraphic architecture of carbonate successions that develop on geographically isolated islands reflects the balance between sea level highstands lowstands and tectonic activity this is readily apparent in the tertiary carbonate sequences on the cayman islands that are formed of the bluff group which includes the brac formation lower oligocene cayman formation middle miocene and pedro castle formation middle pliocene these strata are overlain by the pleistocene ironshore formation the unconformities that define the boundaries between these formations are variable because some developed during one cycle of past erosion others developed through two or more past erosional cycles and some are still developing today some unconformities like the one at the top of the cayman formation are geographically variable because they underwent different developmental histories in different areas the challenge with architectural complex successions like those on the cayman islands is that of deciphering the impact of sea level changes as opposed to tectonic influences during sea level lowstands the older carbonate successions were exposed on land and modified by surface and subsurface karst development at the same time marine erosional processes affected the coastal areas surface karst modification which commonly produced rugged topographies with erosional relief at least 62 m was controlled largely by rainfall runoff and stratal dip weathering on grand cayman at the end of the miocene for example produced a dish shaped topography with elevated peripheral rims in contrast uplift of the east end of cayman brac between the late pliocene 3 6 ma and 400 ka elevated the basal part of the cayman formation 33 m above sea level subsequent karst development which is still ongoing today removed most of the cayman formation on the eastern part of the island and produced peripheral rims that are higher than those on grand cayman during some lowstands like that between the late pliocene and 400 ka coeval coastal erosion led to the development of coastal benches that cut into the older carbonate strata the combination of karst development in the islands interiors and coastal erosion produced complex rugged topographies that strongly influenced patterns of deposition during the following highstand the complex stratigraphic architecture of the carbonate successions on the cayman islands reflects the variable impact of tectonics karst development and coastal erosion that was associated with each lowstand highstand cycle

morphometric analysis of sinkholes in a karst coastal area of southern apulia italy salento the southern portion of apulia region se italy is a narrow and elongated peninsula in carbonate rocks with prevailing low coastlines locally interrupted by high rock cliffs the long stretches of low coasts are marked by typical karst landforms consisting of collapse sinkholes as observed in many other karst coastal settings worldwide development of sinkholes may be particularly severe along the coasts where both natural and anthropogenic processes contribute to accelerate the dissolution of carbonate rocks and subsidence processes even influencing the coastline evolution following a previous study where the main features of sinkholes at torre castiglione taranto province were investigated and described and a preliminary susceptibility map produced at the light of updated data and elaborations in the present paper we perform a detailed morphometric analysis on the sample of identified sinkholes the main morphometric parameters generally used for sinkhole characterization have been considered in this study shape of the sinkhole azimuth and length of maximum and minimum axes depth elongation ratio and distance from the shorelines each of them is described both as individual parameter and in conjunction with the others in the attempt to identify the main factors controlling development of sinkholes in the area and their evolution as well with regard to this latter aspect beside simple morphometry of the sample of sinkholes at torre castiglione we also focused our attention on the likely relationships existing between distribution and shape of the sinkholes and the tectonic discontinuities to investigate the matter a three stage analysis has been carried out in this study by means of field measurements of the fractures bounding the sinkholes field measurement of the long axes azimuth of the elongated sinkholes comparison of the previously described sets with the strikes of the main regional geological structures the obtained results show in addition to the coincidence of the main regional discontinuity systems with the major axis of elongated sinkholes a clear control exerted by development and evolution of the sinkholes on the formation of coastal inlets and bays eventually the approach here presented may be applied in other karst coastal sinkhole prone areas to gain new knowledge on the genesis and evolution of coastal sinkholes and to properly evaluate the hazard they pose to the anthropogenic environment

improved chronostratigraphic reference curve of late neogene seawater 87sr 86sr we present a reference curve of seawater 87 sr 86 sr variation through the past 7 m y based on 455 samples of planktonic foraminifera from ocean drilling program odp site 758 in the indian ocean this single site curve is superior to extant curves because of several features 1 a continuous sedimentary section free of disturbances 2 a well determined and precise chronostratigraphy that is calibrated to numeric age on the basis of continuous magnetostratigraphy and a refined astronomical time scale 3 a high temporal resolution sample interval averages 15 ka 4 an improved analytical reproducibility and 5 a uniformly high sample quality the gross structure of the site 758 curve is generally similar to that observed in previous work but the increased temporal resolution and decreased analytical scatter of the curve provide improved chronostratigraphic resolution at the 95 confidence level uncertainties on age estimates range from 0 60 m y in the lower pleistocene and upper miocene to 2 03 m y in the middle pliocene moreover this detailed record identifies two periods of steplike increases in seawater 87 sr 86 sr 6 10 to 5 80 ma and 1 46 to 1 13 ma that are characterized by rates of change of 150 10 6 m y these features if verified at other sites indicate abrupt changes in the sr mass balance of the ocean and may be used as precise event markers in late neogene marine sections

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

image analysis and circular statistics for shape fabric analysis applications to lithified ignimbrites computer assisted image analysis can be successfully used to derive quantitative textural data on pyroclastic rock samples this method provides a large number of different measurements such as grain size particle shape and 2d orientation of particle main axes directional or shape fabric automatically and in a relatively short time orientation data reduction requires specific statistical tests mainly devoted to defining the kind of particle distribution pattern the possible occurrence of preferred particle orientation the confidence interval of the mean direction and the degree of randomness with respect to pre assigned theoretical frequency distributions data obtained from image analysis of seven lithified ignimbrite samples from the vulsini volcanic district central italy are used to test different statistics and to provide insight about directional fabrics first the possible occurrence of a significant deviation from a theoretical circular uniform distribution was evaluated by using the rayleigh and tukey 2 tests then the kuiper test was performed to evaluate whether or not the observation fits with a unimodal von mises like theoretical frequency distribution finally the confidence interval of mean direction was calculated with the exception of one sample fpd10 which showed a well developed bimodality all the analysed samples display significant anisotropic and unimodal distributions the minimum number of measurements necessary to obtain reasonable variabilities of the calculated statistics and mean directions was evaluated by repeating random collections of the measured particles at increments of 100 particles for each sample although the observed variabilities depend largely on the pattern of distribution and an absolute minimum number cannot be stated approximately 1500 2000 measurements are required in order to get meaningful mean directions for the analysed samples

application of the gauss theorem to the study of silicic calderas the calderas of la primavera los azufres and los humeros mexico abstract we explored applications including limitations of gauss s theorem to the study of silicic calderas first it enables us to determine the mass deficiency from calderas mass deficiency itself has also other potential applications it enables to make qualitative comparisons between calderas we can use the mass deficiency to test in a quick way and as a preliminary step to a formal gravity inversion for the feasibility of caldera types of simple geometry i e piston subsidence and funnel models this application can be done in a straightforward way once the mass deficiency has been determined for this purpose the mass deficiency is converted to the volume of material missing at the caldera subsequently for example this volume and the respective caldera diameter enable us to estimate the height of the cylinder fitting the piston subsidence model if the obtained parameters are congruent with the known geology and geophysical information then the model may be considered further in the inversion of the gravity data for the detailed structure other simple models i e the funnel model can also be analyzed in this way in particular when working with a piston subsidence caldera type the gauss theorem enables us to estimate the caldera collapse very difficult to obtain based on geologic information alone these possible uses of gauss s theorem are illustrated with the calderas of la primavera los azufres and los humeros caldera mexico the obtained mass deficiency from these calderas follow the linear mass deficiency diameter trend observed for other calderas in particular because of their diameters and mass deficiencies la primavera and krakatau calderas can be considered equiparable this comparison is of the most importance considering that la primavera is located in the neighbourhood of a metropolis guadalajara city since geophysical studies have already established a piston subsidence model for these calderas we assessed gauss s theorem used to estimate their collapses the respective estimated subsidences for la primavera 600 m los humeros 500 m and los azufres calderas 800 and 1200 m respectively are supported by borehole data these relatively small subsidences in relation to many calderas in western us can be explained by the presence of cylindrical to sub spherical magma chambers of smaller size than those underlying the western us calderas of yellowstone valles and long valley for example in an epicontinental setting of young quaternary rocks against precambrian cratonic basement in western us a reassessment of all the available geological and geophysical information was also done in particular the caldera limits were checked and corrected when necessary based on available 2 d gravity modeling gravity gradients and the location of the volcanic vents regional tectonic lineaments might explain many of the particular details of their structures la primavera caldera might have formed through a piecemeal subsidence controlled by the pre caldera tectonics los humeros caldera is an example of a caldera that evolved by incremental growth as the vulsiniani calderas in italy the los azufres caldera comprises two contiguous calderas emplaced in a regional tectonic depression

discontinuities in carbonate successions identification interpretation and classification of some italian examples abstract the growing use of unconformities as bounding surfaces for new types of stratigraphic units seems not to be matched by a corresponding effort on process oriented researches on unconformities themselves this paper aims at giving a contribution to classification and understanding of discontinuities in carbonate successions from the outcrop perspective based on some italian examples a number of integrated criteria geometry sedimentology diagenesis biostratigraphy is proposed to recognise stratigraphic breaks old terms are discussed and sometimes redefined while some new terms are introduced the term discontinuity surface ds is proposed as a general one to indicate any stratigraphic interface where an interruption of sedimentation can be proved a ds can be characterized as a firm ground or a rock ground depending on its coherence at the moment of the renewal of sedimentation polygenic and simple omission surfaces are separated within firm grounds if a recognizable gap is either present or not a third kind of firm ground revealed only by biostratigraphy has been called hidden discontinuity surface two categories of rock grounds are also distinguished hard grounds and inherited rock grounds if respectively generation of the ds and or deposition of the overlying sediments took place in the same environment of the underlying sediments or not a genetic interpretation for each type of discontinuity is proposed the largest variety of ds s occurs in pelagic sediments some discontinuities hard grounds and simple omission surfaces are attributed to increased bottom current activity during sea level falls others polygenic omission surfaces and hidden ds s are interpreted as submarine slide scars either with or without the overprint due to exposure on the seafloor the ds s separating different facies e g platform and pelagic ones are interpreted to be due to tectonics the relevance of the recognized ds s to sequence stratigraphy is briefly discussed

evidence for sub lacustrine volcanic activity in lake bolsena central italy revealed by high resolution seismic data sets abstract the bolsena caldera that formed between 0 6 and 0 2 ma has a well preserved structural rim which makes it an ideal site to study the tectonic and volcanic evolution of calderas however the main area is covered by a 150 m deep lake which makes it rather difficult to investigate the subsurface structure directly to overcome this problem new high resolution hydro acoustic surveys using a multichannel reflection seismic system and a sediment echo sounder system were conducted in september 2012 as space was limited we used a rowing boat towed by a rubber boat to handle a 36 m long and 24 channel streamer to receive seismic reflections produced using a mini gi gun 0 25 l the subsurface structure of lake bolsena was imaged up to a sediment depth of 190 m which is estimated to have filled over a period of 333 kyrs however massive pyroclastic flow deposits found in the deeper parts of the basin indicate an initial infill of volcanic deposits from two adjacent younger calderas the latera w and montefiascone se calderas our data suggest that the caldera has a long history of active volcanism because the lacustrine sediments show post sedimentary influences of geothermal fluids we mapped several mound structures at various stratigraphic depths two volcanic structures outcrop at the modern lake surface implying recent activity one of these structures is hardly covered by sediments and has a crater like feature in its summit the other structure shows a pockmark like depression on top another observable feature is a partially sediment filled crater located in the western part of the lake which further implies the existence of a magma chamber located beneath the bolsena caldera since the late pleistocene and holocene the sedimentation was mainly hemipelagic evidenced by a sediment drape of up to 10 m thick sediment drape on the uppermost sediments beneath the drape we found evidence for a distal tephra layer likely related to an explosive eruption from the campanian volcanic province occurring as the neapolitan yellow tuff our new geophysical data set is a valuable record with a potential to constrain the sedimentary and volcanic evolution of the vulsini volcanic district in areas that have not been assessed previously due to logistical challenges of conducting surveys in water filled settings

analogue models of caldera collapse in strike slip tectonic regimes regional scale faulting particularly in strike slip tectonic regimes is a relatively poorly constrained factor in the formation of caldera volcanoes to examine interactions between structures associated with regional tectonic strike slip deformation and volcano tectonic caldera subsidence we made scaled analogue models tabular sill like inclusions of creamed honey in a sand gypsum mix replicated shallow level granitic magma chambers in the brittle upper crust lateral motion of a base plate sited below half the sand gypsum pack allowed simulation of regional strike slip deformation our experiments modelled 1 strike slip deformation of a homogeneous brittle medium 2 strike slip deformation of a brittle medium containing a passive magma reservoir 3 caldera collapse into sill like magma reservoirs without regional strike slip deformation and 4 caldera collapse into sill like magma reservoirs after regional strike slip deformation our results show that whilst the magma chamber shape principally influences the development and geometry of volcano tectonic collapse structures regional tectonic strike slip faults riedel shears and y shears may affect a caldera s structural evolution in two main ways firstly regional strike slip faults above the magma chamber may form a pre collapse structural grain that is exploited and reactivated during subsidence our experiments show that such faults may preferentially reactivate where tangential to the collapse area and coincident with the chamber margins in this case volcano tectonic extension in the caldera periphery tends to localise on regional tectonic faults that lie just outside the chamber margins in addition volcano tectonic reverse faults may link with and reactivate pre collapse regional tectonic faults that lie just inside the chamber margins secondly where regional tectonic strike slip faults define corners in the magma chamber margin they may halt the propagation of volcano tectonic reverse faults the experiments also highlight the potential difficulties in assessing the relative contributions of volcano tectonic and regional tectonic subsidence processes to the final caldera structure seen in the field disruption of the pre collapse surface by regional tectonic faulting was preserved during coherent volcano tectonic subsidence to produce a caldera floor of differentially subsided fault blocks without definitive evidence for syn eruptive growth faulting thickness changes in caldera fill across such regional tectonic fault blocks in nature could be mistaken as evidence for piecemeal volcano tectonic collapse

paleosols in shallow marine carbonate sequences abstract shallow water carbonate sequences in the geological record commonly exhibit numerous exposure surfaces reflecting emersion of sediment due to progradation processes and sealevel oscillations some ancient platform sequences contain hundreds of such surfaces associated with paleosols fossil soils typically calcretes a wide range of macro and microstructures are associated with these calcretes which are reviewed profile types and stages in profile development can be defined the nature of the exposure surface and associated paleosols can be used to identify the nature of the prevailing climate although hydromorphic effects related to marine flooding of the platforms can modify the paleosols a potentially important use of paleosols is to define the nature of sequence boundaries several studies have identified sets of paleosols with varying maturities reflecting changes in rates of accommodation space creation during sealevel rises and falls

imaging the bracciano caldera system by aeromagnetic data inversion sabatini volcanic district central italy abstract bracciano lake located in central italy 30 km northwest of rome is a volcano tectonic depression developed in pleistocene time representing the sabatini volcanic district svd western eruptive center little is known about bracciano caldera deep structure and potential field methods are appropriate to probe through a volcanic pile in this paper we present a 3d magnetic inversion model of the bracciano caldera system based upon new aeromagnetic data and paleomagnetic measurements carried out by istituto nazionale di geofisica e vulcanologia ingv magnetic modelling provides a reconstruction of bracciano caldera morphology whose extension is about 20 km north south and 15 km east west the maximum collapse depth is up to 1 km and the depocentre area appears localised to the north of the present bracciano lake shoreline considering the geological records of the main bracciano caldera forming events we have interpreted the resulting magnetic model as an overlap of two collapse structures generating a nested system whose location was strictly influenced by tectonic processes our 3d magnetic model confirms and consolidates the validity of the magnetic method to highlight deep structures in volcanic environments

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

evidence of incremental growth in the vulsinian calderas central italy abstract a comparative work on the vulsini calderas has been carried out fundamentally from volcanological field study drilling and gravity anomalies data on the basis of a detailed stratigraphy four volcanic complexes have been recognized in the vulsini volcanic district from the oldest to the youngest paleo bolsena bolsena montefiascone and latera the latter two complexes having a contemporaneous evolution in their activity each complex gave rise to several little calderas caused by piecemeal collapses due to moderate size explosive eruptions or coherent subsidence of a superficial piston like block nevertheless the extensional volcano tectonic regime affecting this area could have accounted for as much of the total subsidence as the caldera forming eruptions bolsena and latera calderas respectively 16 km and 8 km in average diameter represent the main depressions of the vulsini district and their topographic features are still well preserved several collapses forming the main poligenetic calderas of bolsena and latera are masked at present by the bolsena lake the younger volcanic covering the lacustrine deposits and erosion in a minor amount only the little montefiascone and vepe calderas which represent the youngest collapses of the vulsini area have not been masked bolsena caldera could be considered a depression caused mostly by a subsidence hinged to the southwest by down sagging and by collapses which occurred after the most important explosive eruptions of the paleo bolsena bolsena and montefiascone volcanic complexes meanwhile latera caldera is mostly due to a piecemeal collapse linked to several moderate size ignimbrite units which were emplaced throughout the explosive activity of the latera complex therefore the vulsinian calderas did not form by catastrophic eruptions because several and frequent moderate size eruptions downsagging and tectonic hinged subsidence concur indicating an incremental growth

plinian fall deposits from vulsini volcanic district central italy stratigraphic investigations of the vulsini volcanic district indicate the existence of five prominent plinian fall deposits deposited over a period of about 0 34 ma the oldest deposit basal pumices crops out mainly in the peripheral areas and is one of the largest plinian events in the vulsini district with a volume of about 9 km3 and a calculated column height exceeding 30 km subcircular patterns of isopachs and isopleths around the bolsena lake indicate emplacement in wind free conditions and suggest a source vent location in the northernmost sector of the present bolsena lake where it is possible that an older central volcanic structure existed the four younger plinian fall deposits are related to bolsena activity the oldest of these named the ponticello and orvieto bagnoregio pumices are mainly distributed on the north eastern sector of the vulsini district their volumes are an order of magnitude lower than that of the basal unit and the estimated column heights do not exceed 20 km the younger ospedaletto pumice deposit has a ne sw dispersal axis whereas the youngest casetta pumice deposit is found only in the north eastern sector of the district their estimated volumes are respectively 1 2 and 0 1 km3 whereas the inferred vent areas appear to be located slightly to the east of the ponticello and orvieto bagnoregio pumice deposit source areas the chronology of the ospedaletto and casetta pumices indicates that the final plinian activity from the bolsena complex is contemporaneous with the latera activity the decreasing volumes from the oldest to the youngest units together with the progressive shifting northeastwards of the source vents may be related to the volcano tectonic subsidence of the bolsena area the source vents for the post basal pumices events are mainly clustered just north of bolsena village where the greatest displacement during subsidence occurred accordingly source vent shifting appears to be related to the progressive opening of normal faults from the inner to the outer margin of the bolsena depression

cyclic caldera collapse piston or piecemeal subsidence field and experimental evidence many multicycle caldera volcanoes display a complex extracaldera fault system genetically linked to caldera evolution on gran canaria canary islands the miocene tejeda caldera was filled by ignimbrites and epiclastic sediments and intruded by syenites and a major cone sheet swarm obscuring intracaldera structural relations extracaldera radial and circumferential faults and dikes are well exposed we used field data and experiments scaled to the island of gran canaria to clarify the poorly understood structural temporal and genetic relationship of peripheral faults to the central caldera the experimental setup comprises a cone of medium grained sand in which a rubber balloon was repeatedly inflated and deflated balloon inflation resulted in updoming and a dominantly radial fault pattern whereas balloon deflation caused piecemeal caldera collapse and a concentric peripheral fault arrangement on the edifice flanks a complex interplay of repeated inflation and deflation cycles of the experimental magma chamber is found to explain the peripheral fault system of the tejeda caldera on gran canaria and offers insight into the structure of the caldera floor and the mechanisms acting in multicycle caldera volcanoes

open and filled karst features on the cayman islands implications for the recognition of paleokarst the dolostones of the oligocene miocene bluff formation on the cayman islands are characterized by well developed surficial interface and subsurface karst features that are open or filled with speleothems and sediment some of the open caves show evidence of a minimum of two stages of development related to dissolution in the phreatic zone and the vadose zone the development of speleothemic formations e g stalactites stalagmites columns is limited in some caves but extensive in others some of the caves have limited amounts of sediment on their floors in these caves the speleothems and sediments are still formed of caco3 and thus contrast sharply with the dolostone of the bluff formation in which they occur some of the joints sinkholes and caves are filled with breccia caymanite terra rossa terra rossa breccia pisolitic limestone and speleothems although filled such karst features are analogous to the open karst features that occur elsewhere on the islands the filling of joints sinkh

volcanological and petrological evolution of the eastern vulsini district central italy on the basis of recent field mapping data stratigraphic reconstruction and k ar datings a volcanological and petrological study of the bolsena and montefiascone volcanic zones eastern vulsini district has been carried out ten main eruptive phases with both effusive and explosive activity have been recognized these are characterized by a large range of magma compositions belonging to the leucitite basanite and shoshonite magmatic series within each series near primary magmas and slightly to strongly differentiated products are present although in different proportions the great majority of differentiated products mostly trachytic outcrops in the northern sectors of the eastern vulsini district bolsena volcanic zone where lavas or pyroclastics were erupted during moderate to large volume plinian type and ignimbrite forming explosive activity by contrast abundant rather undifferentiated magmas are more frequently found in the southern sectors of the eastern vulsini district montefiascone volcanic zone the structural setting of the sedimentary substratum as well as its lithological characteristics may have played an important role in producing this magma distribution the very shallow position a few hundred meters below the topographic surface of the top of the fracturated carbonate basement would have facilitated the uprising of abundant less evolved magmas in the montefiascone volcanic zone whereas relatively large magma bodies a few kilometer deep can only be inferred in the bolsena volcanic zone bolsena caldera where a front of overthrust sheets of the tuscan and umbrian sedimentology sequences is most probably present in this area source vents of explosive activity show a tendency to shift northeastward with time feeding systems were also differently oriented e w and wnw ese in the southern sectors and nw se and nnw sse in the northern sectors while a ns system was related to a late stage activity of the whole eastern vulsini district