the mantle convection pattern and force source mechanism of recent tectonic movement in china abstract the runcorn stress equations and 2 30 harmonic coefficients of the geopotential have been applied to determine the mantle convection pattern beneath china the pattern is compared with geophysical and geological observations and it is found that the directional change belts of mantle flows coincide with the major fault belts between tectonic units of china the stress field generated by mantle flows except in the tian shan region also coincide with the stress field of recent tectonic movement in china the tarim and junggar basins are formed by tensional stresses due to divergent mantle convection currents under northwest china the formation of the qinhai xizang tibet plateau is due mainly to the compression of the tarim block and indian plate caused by convergent mantle convection currents the shear fault belts in central china 100 105 e are generated by the running change belt of mantle flows a well known n s seismic zone in eastern china tensional faults grabens lake and sea depressions are related to the eastward displacement of continental lithosphere exerted by eastward dispersal mantle flows under this region this paper provides new material for further study of the force source mechanism of recent tectonic movement from the viewpoint of mantle convection currents

shallow vent architecture during hybrid explosive effusive activity at cordon caulle chile 2011 12 evidence from direct observations and pyroclast textures in june 2011 an eruption of rhyolite magma began at the puyehue cordon caulle volcanic complex southern chile by january 2012 explosive activity had declined from sustained pyroclastic plinian to sub plinian fountaining to mixed gas and ash jetting punctuated by vulcanian blasts this explosive activity was accompanied by synchronous effusion of obsidian lava in a hybrid explosive effusive eruption fortuitous climatic conditions permitted ground based observation and video recording of transient vent dynamics as well as real time collection of proximal juvenile ash as it sedimented from the active plume the main eruptive vent complex and site of lava effusion was represented by two loci of vulcanian blasts within a single tephra cone containing a pancake shaped proto lava dome these blast loci each consisted of clusters of sub vents that expressed correlated shifts in eruption intensity indicating the presence of partially connected and or branching zones of high permeability within the upper conduit pyroclast textures were examined by x ray computed microtomography and their permeability was modeled by lattice boltzmann simulations the porosity 39 to 67 and darcian permeability 3 1x10 15 m2 perpendicular to fabric to 3 8x10 11 m2 parallel to fabric of fine ash emitted during ash jetting indicates that the permeable zones comprised highly sheared tube like bubbly magma and contrast with the low porosity 17 and nul permeability of bombs ejected to hundreds of metres from the vent in vulcanian blasts residual h2o content of ash 0 14 wt and two bombs 0 2 0 25 wt determined by karl fischer titration indicate degassing of this pyroclastic material to near atmospheric pressures ash textures and simple degassing vesiculation models indicate the onset of permeability by ductile processes of shear enhanced bubble coalescence in the upper 1 to 1 5 km of the conduit repeated ash jetting and vulcanian blasts indicate that such ductile processes were not sufficiently effective to accommodate all degassing requirements and additional brittle e g fragmentation events transiently assisted in maintaining an open system degassing regime that permitted long lived hybrid explosive effusive activity the eruption at cordon caulle has provided an excellent opportunity to closely observe key processes of silicic volcanism

fragmentation efficiency of explosive volcanic eruptions a study of experimentally generated pyroclasts abstract products of magma fragmentation can pose a severe threat to health infrastructure environment and aviation systematic evaluation of the mechanisms and the consequences of volcanic fragmentation is very difficult as the adjacent processes cannot be observed directly and their deposits undergo transport related sorting however enhanced knowledge is required for hazard assessment and risk mitigation laboratory experiments on natural samples allow the precise characterization of the generated pyroclasts and open the possibility for substantial advances in the quantification of fragmentation processes they hold the promise of precise characterization and quantification of fragmentation efficiency and its dependence on changing material properties and the physical conditions at fragmentation we performed a series of rapid decompression experiments on three sets of natural samples from unzen volcano japan the analysis comprised grain size analysis and surface area measurements the grain size analysis is performed by dry sieving for particles larger than 250 m and wet laser refraction for smaller particles for all three sets of samples the grain size of the most abundant fraction decreases and the weight fraction of newly generated ash particles up to 40 wt increases with experimental pressure potential energy for fragmentation this energy can be estimated from the volume of the gas fraction and the applied pressure the surface area was determined through argon adsorption the fragmentation efficiency is described by the degree of fine particle generation results show that the fragmentation efficiency and the generated surface correlate positively with the applied energy

magmatic architecture of dome building eruptions at volcan de colima mexico changes in the physical chemical and rheological properties of ascending magma regulate the style of volcanic eruptions volcan de colima s eruptive cycles of lava dome growth and explosions have been thoroughly monitored during the period 1998 2010 and provide a remarkable opportunity for deepening our understanding of the underlying processes responsible for the evolution of magma properties here we integrate direct observation with analytical and experimental data to 1 constrain the configuration of the shallow plumbing system and its influence on eruptive activity 2 describe the rheological behaviour of the magma and 3 assess the conditions that lead to fragmentation and ultimately to explosive eruptions the configuration of the shallow plumbing system was inferred from direct observation of extrusion sites and porosity of the erupted products during the ongoing eruptive phase magma was never extruded from a central vent both explosive and effusive activities were restricted to discrete vents inside the crater extensive field based density measurements on 500 blocks in pyroclastic flow deposits reveal a bimodality of porosity at values of 12 and 26 vol the least porous rocks tend to be altered whereas the more porous rocks are pristine this bimodal distribution combined with the lack of a central vent suggests the presence of a central dense altered plug the fragments of which are entrained during explosive eruptions during effusive periods the plug appears to deflect the ascent of magma at a shallow depth and consequently the site of lava extrusion the rheological properties and deformation induced seismogenic behaviour of the magmas were investigated using a uniaxial deformation apparatus instrumented with acoustic sensors the homogeneity in the physicochemical properties of the erupted magma permits the description of a flow law at eruptive temperature and strain rate conditions the crystal rich magma of volcan de colima exhibits a shear thinning rheology and becomes increasingly brittle at higher strain rates complete failure of magma can be predicted using the material failure forecast method which integrates the acceleration of released acoustic energy throughout the deformation rapid decompression experiments of samples pressurised with argon were performed to assess the fragmentation conditions under which explosive eruptions progress in the absence of gas loss due to permeable flow the pore pressure required to fragment volcanic products is inversely proportional to the porosity at volcan de colima a rapid decompression of 6 mpa is required to fragment magma averaging 26 vol pores and to thereby instigate an explosive eruption analysis of ballistic impacts 4 6 km away from the vent from recent explosive eruptions further requires decompression as great as 20 mpa which is sufficient to disrupt the more porous material as well as a fraction of the denser plug the continuing growth of a lava dome and of a larger more impermeable plug could have consequences for the stability of the edifice and development of stronger and more erratic explosive activity at volcan de colima

experimental determination of drag coefficient for volcanic materials calibration and application of a model to popocatepetl volcano mexico ballistic projectiles 1 this study shows the results of experiments performed in a subsonic wind tunnel to measure drag coefficients cd for volcanic particles emitted by popocatepetl volcano mexico the results indicate that volcanic projectiles move in a range of reynolds number re values where cd is independent of re drag coefficients of volcanic fragments were found between the values of spheres and cubes with low values in this study initial conditions of the ballistic equations were calibrated with the launching kinetic energy from ballistics expelled by popocatepetl volcano during the eruptive event of december 17 1998 the maximum range of the projectiles calculated with the experimentally measured drag coefficient is in better agreement with the field and observational data reported here than using the values of geometrical bodies as in previous studies our study can be useful for definition of safety zones around volcanoes to protect people and infrastructure adequately

three fragmentation mechanisms for highly viscous magma under rapid decompression abstract analysis of the behavior of viscous magma under short term tensile stresses demonstrates that the strain rates anticipated in the eruptive stages of collapse related events may be high enough under certain circumstances to provide fragmentation of the melt by brittle failure mechanisms of fragmentation of a highly viscous vesicular magma by the propagation of an unloading wave by layer by layer vesicle bursting fragmentation wave propagation and by rapid filtration gas flow are outlined fragmentation scenarios involving possible combinations of the three fragmentation mechanisms are evaluated for two models of magma with isolated and with connected pores experimental results for magma fragmentation under high decompression rates appear to support such mechanisms these mechanisms of highly viscous magma fragmentation by rapid decompression probably have their greatest application in cases of explosive disintegration of lava domes and cryptodomes caused by collapse events

primary and secondary fragmentation of crystal bearing intermediate magma abstract crystal rich intermediate magmas are subjected to both primary and secondary fragmentation processes each of which may produce texturally distinct tephra of particular interest for volcanic hazards is the extent to which each process contributes ash to volcanic plumes one way to address this question is by fragmenting pyroclasts under controlled conditions we fragmented pumice samples from soufriere hills volcano shv montserrat by three methods rapid decompression in a shock tube like apparatus impact by a falling piston and milling in a ball mill grain size distributions of the products reveal that all three mechanisms produce fractal breakage patterns and that the fractal dimension increases from a minimum of 2 1 for decompression fragmentation primary fragmentation to a maximum of 2 7 by repeated impact secondary fragmentation to assess the details of the fragmentation process we quantified the shape texture and components of constituent ash particles ash shape analysis shows that the axial ratio increases during milling and that particle convexity increases with repeated impacts we also quantify the extent to which the matrix is separated from the crystals which shows that secondary processes efficiently remove adhering matrix from crystals particularly during milling abrasion furthermore measurements of crystal size distributions before using x ray computed tomography and after by componentry of individual grain size classes decompression driven fragmentation show not only that crystals influence particular size fractions across the total grain size distribution but also that free crystals are smaller in the fragmented material than in the original pumice clast taken together our results confirm previous work showing both the control of initial texture on the primary fragmentation process and the contributions of secondary processes to ash formation critically however our extension of previous analyses to characterisation of shape texture and componentry provides new analytical tools that can be used to assess contributions of secondary processes to ash deposits of uncertain or mixed origin we illustrate this application with examples from shv deposits

energy consumption by magmatic fragmentation and pyroclast ejection during vulcanian eruptions abstract magmatic fragmentation during explosive eruptions consumes a significant amount of energy by the creation of new surface via fracturing this process reduces the energy that can be imparted to the resulting pyroclasts in the form of kinetic energy to date models of vulcanian eruptions have neglected the energy balance consumed by magmatic fragmentation we present a 1 d model of vulcanian eruption that considers the energy balance in rapid decompression of a pressurized magma below a caprock followed by fragmentation and acceleration of particles we then tested the model via decompression experiments at different temperatures room vs 850 c and initial pressure different series of experiments involving either fragmentation of an intact volcanic rock followed by ejection of the particles or simply the ejection of loose particles were performed to quantify the energy consumed by fragmentation and the kinetic energy available to eject the particles the ejection velocities observed in the fragmentation experiments of intact rocks are systematically lower than the ones with loose particles due to the energy consumed by fragmentation this energy depends on the fragmentation threshold i e the minimum pressure required to completely fragment the sample and is inversely proportional to the fraction of pressurized pores we propose that the effective pressure propelling the gas particle mixtures after fragmentation corresponds to the pressure initially stored in the pore fraction minus the fragmentation threshold we applied our caprock model to vulcanian eruptions of popocatepetl volcano february 2003 and colima volcano 10 february 1999 and obtained gas pressures at the onset of fragmentation of 11 13 mpa and 19 22 mpa respectively these pressures might correspond to the gas pressures required to disrupt the caprock fragment the underlying porous magma and eject the ballistics at the observed distances the model presented herein may help describe the dynamics of vulcanian eruptions and improve hazard assessment

spherical shell models of mantle convection with tectonic plates abstract a simple three dimensional spherical model of mantle convection where plates are taken into account in the top boundary condition allows to investigate the plate tectonics mantle convection coupling in a self consistent way avoiding the strong difficulties inherent in the numerical treatment of rheology the plate condition appears efficient in reproducing the earth like features as subduction mid oceanic ridges and hotspots whereas the free slip condition leads to a classical polygonal cell pattern with cylindrical hot plumes surrounded by downwellings the plate condition favors the development of strong linear downwellings associated to passive diverging zones along plate boundaries these cold currents very similar to subductions act the main role in mantle convection they drive the whole circulation in that context hot plumes remain almost independent except if on the long term cold material spreading at the core surface induces a slight migration below a few mm yr of their surface impingement the main result is that plate tectonics appear to be more than a simple mode of organization of the surface movements it is the essence of the earth mantle dynamics

global plate motion and the geoid a physical model summary the effect of lateral variations in the sub lithospheric viscosity i e in the lithosphere asthenosphere coupling is incorporated in dynamic earth models driven by plate velocities if the coupling is stronger under fast moving continents it is shown that a geoid low will develop in their wake and a high in front of them thus the well known low in the indian ocean could be explained at least in part in terms of induced upper mantle dynamics similarly if the thickness of oceanic plates increases with age the models show that trenches should be associated with marked geoid highs whereas ridges could correspond to much weaker geoid lows this also seems to agree with some features of the observed geoid at very long wavelengths the mathematical framework of such dynamic earth models is developed extensively here these models are characterized by lateral viscosity variations inside an outer shell and a purely radial viscosity structure at greater depth their internal flow patterns are only driven by imposed surface velocities not by internal loads

hydrothermal eruptions at unstable crater lakes insights from the boiling lake dominica lesser antilles abstract the boiling lake a major touristic attraction in the morne trois pitons national park dominica s west indies world heritage site is located in an area of high hydrothermal activity in the southern part of the island this spectacular lake is characterized by unstable behavior which has been interpreted to result from the presence of a water body above a fluctuating gas filled or multi phase vent system the system produces a near boiling lake that occasionally drains and refills with water without any simple periodicity despite the fact that such unstable draining lakes are thought to be non eruptive the boiling lake is known to have sourced occasional small hydrothermal eruptions as well as toxic gas emissions which accompany the refilling phase of these drainage events similar explosive events seem also to be likely after landslide slumped rocks from steep altered crater walls cover the vent area in the lake leading to overpressurized conditions in both cases rock expulsion and gas release continue to pose serious threats for tourists and tour guides while conceptual models exist describing possible mechanisms of the boiling lake drainage itself little information exists to date regarding the trigger mechanisms related to the explosive events further no estimation of the eruption processes and dynamics of these events has previously been attempted to address this knowledge gap we employ here a combined field and laboratory based approach in order to i characterize the properties of the dominant lithologies from the crater walls at boiling lake ii determine how the ongoing alteration may affect their petrophysical properties e g porosity and permeability and iii evaluate how water saturated unconsolidated wall rock materials behave when rapidly decompressed from a slightly elevated pressure and temperature conditions here we propose that it is the petrophysical state of the materials involved e g porosity permeability and alteration related strength which controls the style and energy of these eruptions as well as the potential for landslides from the crater walls the experimentally simulated scenarios included in this study are i decompression from only slightly elevated pressure conditions inferred for small hydrothermal eruptions occurring after lake drainage events and ii decompression from rapidly enhanced pressure conditions produced by the capping and overpressurization of the vent area following landslide episodes ejection velocities obtained from rapid decompression experiments were used as input parameters for ballistic trajectory calculations allowing for estimation of the potential area impacted by ejected debris we anticipate that this study will serve as a contribution to the understanding of the hazard potential of future hydrothermal eruptions at unstable crater lakes in general as well as providing direct insights into the ongoing hydrothermal activity at the boiling lake

experimental constraints on phreatic eruption processes at whakaari white island volcano vigorous hydrothermal activity interspersed by sequences of phreatic and phreatomagmatic eruptions occur at whakaari white island volcano new zealand here we investigate the influence of sample type hydrothermally altered cemented ash tuffs and unconsolidated ash lapilli and fragmentation mechanism steam flashing versus gas expansion on fragmentation and ejection velocities as well as on particle size and shape our rapid decompression experiments show that fragmentation and ejection speeds of two ash tuffs cemented by alunite and amorphous opal increase with increasing porosity and that both are significantly enhanced in the presence of steam flashing ejection speeds of unconsolidated samples are higher than ejection speeds of cemented tuffs as less energy is consumed by fragmentation fragmentation dominated by steam flashing results in increased fragmentation energy and a higher proportion of fine particles particle shape analyses before and after fragmentation reveal that both steam flashing and pure gas expansion produce platy or bladed particles from fracturing parallel to the decompression front neither fragmentation mechanisms nor sample type show a significant influence on the shape our results emphasize that under identical pressure and temperature conditions eruptions accompanied by the process of liquid water flashing to steam are significantly more violent than those driven simply by gas expansion therefore phase changes during decompression and cementation are both important considerations for hazard assessment and modeling of eruptions in hydrothermally active environments

the dynamics of volcanic jets temporal evolution of particles exit velocity from shock tube experiments pyroclast ejection during explosive volcanic eruptions occurs under highly dynamic conditions involving great variations in flux particle sizes and velocities this variability must be a direct consequence of complex interactions between physical and chemical parameters inside the volcanic plumbing system the boundary conditions of such phenomena cannot be fully characterized via field observation and indirect measurements alone in order to understand better eruptive processes we conducted scaled and controlled laboratory experiments by performing shock tube experiments at known conditions we defined the influence of physical boundary conditions on the dynamics of pyroclast ejection if applied to nature we are focusing in the near vent processes where independently of fragmentation mechanism impulsively released gas pyroclast mixtures can be observed these conditions can be met during e g strombolian or vulcanian eruptions parts of plinian eruptions or phreatomagmatic explosions the following parameters were varied 1 tube length 2 vent geometry 3 particle load 4 temperature and 5 particle size distribution gas and particles in the experiments are not coupled st 1 the initial overpressure with respect to atmosphere was always at 15mpa we found a positive correlation of pyroclast ejection velocity with 1 particle load 2 diverging vent walls and 3 temperature as well as a negative correlation with 1 tube length and 2 particle size additionally we found that particle load strongly affects the temporal evolution of particle ejection velocity these findings stress the importance of scaled and repeatable laboratory experiments for a better understanding of volcanic phenomena and therefore volcanic hazard assessment

magma fragmentation by rapid decompression the processes leading to magma fragmentation and the generation of pyroclastic debris during explosive volcanic eruptions are of fundamental importance in volcanology observations of explosive eruptions1 3 as well as theoretical analyses of the underlying processes4 have raised the question of whether the rapid decompression of highly viscous vesicular magma that results from the collapse of a lava dome or volcanic edifice can in itself produce explosive magma fragmentation and pyroclast formation here we report the results of a laboratory investigation of pyroclast formation following rapid decompression samples of magma from the 1980 eruption of mount st helens when rapidly depressurized from initial pressures of up to 12 mpa and at temperatures in the range 750 825 c fragmented to form pyroclastic products that are in many respects similar to those formed in real eruptions moreover we observe explosive fragmentation at temperatures well below those normally associated with magmatic processes suggesting that even relatively cool magma bodies can be very hazardous when subjected to rapid unloading events

phreatic activity and hydrothermal alteration in the valley of desolation dominica lesser antilles phreatic eruptions are possibly the most dramatic surface expressions of hydrothermal activity and they remain poorly understood the near absence of precursory signals makes phreatic eruptions unpredictable with respect to both time and magnitude the valley of desolation vod dominica located close to the boiling lake the second largest high temperature volcanic crater lake in the world hosts vigorous hydrothermal activity with hot springs mud pools fumaroles and steaming ground a phreatic or phreatomagmatic eruption from this site is considered to be the most likely scenario for future volcanic activity on dominica yet there is little information regarding the trigger mechanisms and eruption processes of explosive events at this active hydrothermal center and only a very small number of studies have investigated hydrothermal activity in the vod we therefore conducted two field campaigns in the vod to map hydrothermal activity and its surficial phenomena we also investigated alteration processes and their effects on degassing and phreatic eruption processes we collected in situ petrophysical properties of clay rich unconsolidated samples and together with consolidated rock samples we investigated the range of supergene and hydrothermal alteration in the laboratory in addition we performed rapid decompression experiments on unconsolidated soil samples our results show that alteration leads to an increasing abundance of clay minerals and a decrease in both strength and permeability of the rocks in the immediate vicinity of degassing acid sulfate fluids advanced argillic alteration yields a mineral zoning which is influenced by meteoric water the water saturated basal zone is dominated by kaolinite run 0whereas alunite formation is favored at and above the groundwater table where atmospheric oxidation of h2s to h2so4 occurs e g steam heated alteration alteration effects may in turn inhibit degassing at the surface increasing the potential for pressurization in the subsurface and thus lead to phreatic eruptions rapid decompression experiments together with ballistic trajectory calculations constrain estimates of the conditions prior to the 1997 small scale phreatic event in the vod the results presented here may serve as a contribution to the understanding of the hazard potential of ongoing hydrothermal activity within the vod on a broader perspective our results will help evaluate hydrothermal activity in similar areas worldwide which might also have the potential for phreatic eruptions for instance poas costa rica or tongariro and waimangu new zealand

timing magma ascent at popocatepetl volcano mexico 2000 2001 abstract magnetic anomalies may be used to constrain magma ascent and are useful as precursors to eruptions especially when correlated with other geophysical and geochemical data in this paper we present multiparameter data on the magnetics dome morphology geochemistry and seismicity associated with the december 2000 january 2001 eruptions the largest of the recent eruptions at popocatepetl volcano a 6 month data period was studied in order to evaluate the precursors and post eruption processes several cycles of dome construction and destruction occurred from september 2000 through february 2001 in december large amplitude tremor associated with a higher effusion rate resulted in the formation of a large dome which filled the crater to within about 50 m of the lowest part of the crater rim seismic activity in december was marked by many volcanotectonic earthquakes and both high frequency and harmonic tremor on december 12 and 13 an increase in the tremor amplitude was followed by ash eruptions with 1 7 5 km high columns tremor amplitude increased again on december 15 and oscillated for the next four days activity remained high until the end of the month on january 22 an 18 km high plume produced ash and pumice fall to the east as well as pyroclastic flows and mudflows which reached 6 km from the crater the eruption left three concentric explosion pits partially destroying the december dome mixing of a mafic olivine bearing melt with a more evolved magma triggered the larger eruption on january 22 as can be seen from the higher mgo concentrations in some of the ejecta and the presence of a dark andesitic scoria with lower silica content and a white andesitic pumice with higher silica content precursory negative magnetic anomalies up to 5 nt 3 2 nt 5 nt 2 9 nt were associated with the ascent of the larger batches of magma which preceded the increases in seismicity before the december 2000 january 22 vei 3 4 eruptions no significant increases in seismicity were observed prior to the january 22 eruption except for a magnitude 2 4 earthquake on the day of the eruption there was however a 0 8 nt magnetic anomaly which lasted from january 10 to 12 and possibly to january 17 most of the magma ascended in three large batches and several smaller ones from october 22 through december 10 short lived crater domes with small volumes were formed at popocatepetl in september october and november 2000 and in march 2001 even the smaller negative anomalies from september to february were associated with the ascent of several batches of magma they preceded the increases observed in the seismicity and growth of a dome by several days

influence of the fragmentation process on the dynamics of vulcanian eruptions an experimental approach abstract the dynamics of magma fragmentation is a controlling factor in the behavior of explosive volcanic eruptions if porous magma is sufficiently decompressed a fragmentation front develops and travels through the magma at a certain speed fragmentation speed while the resulting particles are being ejected to investigate the influence of the fragmentation process on eruption dynamics we have performed fragmentation experiments in a shock tube apparatus using natural volcanic samples with diverse porosities and different applied pressures 4 20 mpa for each experiment we simultaneously measured the fragmentation speed and ejection velocities the results are consistent with a theoretical model based on a 1 d shock tube theory considering the conservation laws across the fragmentation front our results show that a certain pressure threshold has to be exceeded for fragmentation and ejection of the particles to take place and that the fragmentation speed determines the initial conditions of the expansion of the gas particle mixture the fragmentation process has a controlling influence on the velocity density and mass discharge rate per unit area of the gas particle mixture all factors which can affect eruption dynamics significantly the model presented herein may help describe the dynamics of vulcanian eruptions and improve hazard assessment

experimental investigations on the explosivity of steam driven eruptions a case study of solfatara volcano campi flegrei steam driven eruptions both phreatic and hydrothermal expel exclusively fragments of non juvenile rocks disintegrated by the expansion of water as liquid or gas phase as their violence is related to the magnitude of the decompression work that can be performed by fluid expansion these eruptions may occur with variable degrees of explosivity in this study we investigate the influence of liquid fraction and rock petrophysical properties on the steam driven explosive energy a series of fine grained heterogeneous tuffs from the campi flegrei caldera were investigated for their petrophysical properties the rapid depressurization of various amounts of liquid water within the rock pore space can yield highly variable fragmentation and ejection behaviors for the investigated tuffs our results suggest that the pore liquid fraction controls the stored explosive energy with an increasing liquid fraction within the pore space increasing the explosive energy overall the energy released by steam flashing can be estimated to be 1 order of magnitude higher than for simple argon gas expansion and may produce a higher amount of fine material even under partially saturated conditions the energy surplus in the presence of steam flashing leads to a faster fragmentation with respect to gas expansion and to higher ejection velocities imparted to the fragmented particles moreover weak and low permeability rocks yield a maximum fine fraction using experiments to unravel the energetics of steam driven eruptions has yielded estimates for several parameters controlling their explosivity these findings should be considered for both modeling and evaluation of the hazards associated with steam driven eruptions

solving vening meinesz moritz inverse problem in isostasy vening meinesz inverse problem in isostasy deals with solving for the moho depth from known bouguer gravity anomalies and normal moho depth t 0 known e g from seismic reflection data using

a fragmentation criterion for highly viscous bubbly magmas estimated from shock tube experiments abstract when a highly viscous bubbly magma is sufficiently decompressed layer by layer fracturing propagates through the magma at a certain speed fragmentation speed on the basis of a recent shock tube theory by koyaguchi and mitani koyaguchi t mitani n k 2005 a theoretical model for fragmentation of viscous bubbly magmas in shock tubes journal of geophysical research 110 b10 b10202 doi 10 1029 2004jb003513 gas overpressures at the fragmentation surface are estimated from experimental data on fragmentation speed in shock tube experiments for natural volcanic rocks with various porosities the results show that gas overpressure at the fragmentation surface increases as initial sample pressure increases and sample porosity decreases we propose a new fragmentation criterion to explain the relationship between the gas overpressure at the fragmentation surface the initial pressure and the porosity our criterion is based on the idea that total fragmentation of highly viscous bubbly magmas occurs when the tensile stress at the midpoint between bubbles exceeds a critical value we obtain satisfactory agreement between our simulation and experiment when we assume that the critical value is inversely proportional to the square root of bubble wall thickness this fragmentation criterion suggests that long micro cracks or equivalent flaws e g irregular shaped bubbles that reach the midpoints between bubbles are a dominant factor to determine the bulk strength of the bubbly magma

a numerical study of the effects of boundary conditions on mantle convection models constrained to fit the low degree geoid coefficients a simple mathematical model is set up to study the effects on the mantle convection pattern of boundary conditions at the core mantle boundary and at the surface or the lithosphere asthenosphere boundary uniform viscosity and homogeneous heat source are assumed and the density differences driving convection and the associated boundary undulations are constrained to fit the observed low degree geoid coefficients the most important conclusion is that when the rayleigh number is around 1 4 1 8 times the critical rayleigh number at degree 2 a model having a free lower boundary and a frictional upper boundary gives plate motions strongly disagreeing with the observed motions but a model with a free upper boundary and frictional lower boundary agrees better with the global tectonic features in this case the frictional lower boundary is considered to represent increasing viscosity with depth which is not formally included in the model the correlation between convergent zones of the calculated convection pattern and the subducting slabs indicates that the slabs provide the most plausible driving mechanism of the plate motions

experimental estimates of the energy budget of hydrothermal eruptions application to 2012 upper te maari eruption new zealand sudden hydrothermal eruptions occur in many volcanic settings and may include high energy explosive phases ballistics launched by such events together with ash plumes and pyroclastic density currents generate deadly proximal hazards the violence of hydrothermal eruptions or explosive power depends on the energy available within the driving fluids gas or liquid which also influences the explosive mechanisms volumes durations and products of these eruptions experimental studies in addition to analytical modeling were used here to elucidate the fragmentation mechanism and aspects of energy balance within hydrothermal eruptions we present results from a detailed study of recent event that occurred on the 6th of august 2012 at upper te maari within the tongariro volcanic complex new zealand the eruption was triggered by a landslide from this area which set off a rapid stepwise decompression of the hydrothermal system explosive blasts were directed both westward and eastward of the collapsed area with a vertical ash plume sourced from an adjacent existing crater all explosions ejected blocks on ballistic trajectories hundreds of which impacted new zealand s most popular hiking trail and a mountain lodge 1 4 km from the explosion locus we have employed rocks representative of the eruption source area to perform rapid decompression experiments under controlled laboratory conditions that mimic hydrothermal explosions under controlled laboratory conditions an experimental apparatus for 34 by 70 mm cylindrical samples was built to reduce the influence of large lithic enclaves up to 30 mm in diameter within the rock the experiments were conducted in a temperature range of 250 degrees c 300 degrees c and applied pressure between 4 mpa and 6 5 mpa which span the range of expected conditions below the te maari crater within this range we tested rapid decompression of pre saturated samples from both liquid dominated conditions and the vapor dominated field further we tested dry samples at the same pressure and temperature conditions results showed that host rock lithology and state of the interstitial fluid was a major influence on the fragmentation and ejection processes as well as the energy partitioning clasts were ejected with velocities of up to 160 m s as recorded by high speed camera in addition to rare large clasts analogous to ballistics a large amount of fine and very fine 63 mu m ash was produced in all experiments the efficiency of transformation of the total explosive energy into fragmentation energy was estimated between 10 to 15 depending on the host rock lithology while less than 0 1 of this was converted into kinetic energy our results suggest that liquid to vapor flashing expansion provides an order of magnitude higher energy release than steam expansion which best explains the dynamics of the westward and most energetic directed blast at te maari considering the steam flashing as the primary energy source the experiments suggested that a minimum explosive energy of 7 x 10 10 to 2 x 10 12 j was involved in the te maari blast experimental studies under controlled conditions compared closely to a field example are thus highly useful in providing new insights into the energy release and hazards associated with eruptions in hydrothermal areas c 2016 the author s published by elsevier b v

bombs behaving badly unexpected trajectories and cooling of volcanic projectiles we collected thermal infrared video of two explosive eruptions at stromboli in june 2008 and manually traced the trajectories of 95 particles launched during two eruptions we found that 10 15 of the analyzed trajectories deviated from predicted curves due to collisions causing one particle to travel horizontally more than twice as far as expected furthermore we observed an oscillatory cooling behavior for the airborne pyroclasts with a median period of 0 46 s measured cooling was typically much faster than model predicted cooling with discrepancies of up to 40 between measured cooling and theoretical modeling we interpret the measured cooling curves as resulting from the spinning and twisting and tearing of particles during travel the periodic re exposing of the hotter core of the pyroclasts to the atmosphere may cause the observed oscillations and the spinning may accelerate cooling by enhancing convective heat transfer current volcanic trajectory and cooling models do not account for projectile collisions spinning or tearing and can thus severely underestimate the maximum landing distance and cooling rates of large pyroclasts

depressurization of fine powders in a shock tube and dynamics of fragmented magma in volcanic conduits abstract samples of fine glass beads mean grain size equal to 38 and 95 m have been depressurized within a vertical shock tube these short lived rapid decompressions resemble discrete cannon like vulcanian explosions and produce two phase flows that are inhomogeneous in density in both vertical and horizontal directions because of the presence of bubble like heterogeneities we suggest that also volcanic flows may present similar inhomogeneities in density in the experimental apparatus the flow velocities increase from approximately 1 to 13 m s when the pressure drop increases from approximately 200 to 900 mbar a physical model of the initial velocities of expansions in the shock tube has been applied to a range of volcanic overpressures between 0 1 and 20 mpa suggesting initial velocities of volcanic flows caused by the removal of a rock plug in volcanic conduits between 25 and 400 m s during the experiments at large pressure drops as the mixture expands and moves up the tube the flow front becomes highly irregular and bubble like heterogeneities form the shape of these bubbles becomes distorted and stretched in the turbulent flow during the experiments at relatively small pressure drops the sample oscillates when the particles after the expansion flow back and bounce upward again jets with diameter smaller than that of the tube are ejected from the oscillating samples generating independent pulses large bubble like heterogeneities whose diameter is a significant fraction of the tube diameter can also discretize the flows similar mechanisms in real volcanoes may produce pulse like ejections of gas particle mixtures out of the vent

explosive volcanic eruptions vii the ranges of pyroclasts ejected in transient volcanic explosions an earlier treatment of the motions of pyroclasts produced in discrete volcanic explosions assumed that clasts were ejected into a stationary atmosphere we argue here that the air in the vicinity of an explosion site will itself be moving radially away from the source with a speed comparable to that of the clasts especially at early times after the start of the motion as a result the initial relative velocity of clasts and air will be small in the earlier treatment this relative velocity was inevitably large leading to an overestimate of the atmospheric drag force at early times and a consequent underestimate of the final clast range we set up a simple model of the explosion process which allows us to link the initial clast and air velocities and to find an approximation to the subsequent air flow field integration of the equations of motion for clasts with a wide range of sizes densities initial speeds and initial elevation angles allows the ranges of these clasts to be found clast range data from three documented explosive eruptions are re analysed to yield initial velocities and the implications of these velocities for pre explosion pressure conditions are discussed

the global stress field in the lithosphere obtained from the satellite gravitational harmonics abstract in this paper the lithosphere is considered to be a homogeneous elastic spherical shell for the sake of simplicity and the stress equations for the base of the lithosphere are taken as boundary conditions then the stress equations are obtained for use in the computation of the stress field in the lithosphere with the satellite gravitational harmonic coefficients the 5 5 global stress field in the lithosphere is computed from harmonics of 2 30 the directions of principal stresses of this stress field agree favourably with the directions of principal stresses indicated by mid plate earthquake mechanisms in situ stress measurements and sensitive geological features this result indicates that the drag forces exerted on the base of the lithosphere due to gravitational mantle convection may be among the driving forces of plate motion and a major source for the stress field in the lithosphere

multidisciplinary constraints of hydrothermal explosions based on the 2013 gengissig lake events kverkfjoll volcano iceland hydrothermal explosions frequently occur in geothermal areas showing various mechanisms and energies of explosivity their deposits though generally hardly recognised or badly preserved provide important insights to quantify the dynamics and energy of these poorly understood explosive events furthermore the host rock lithology of the geothermal system adds a control on the efficiency in the energy release during an explosion we present results from a detailed study of recent hydrothermal explosion deposits within an active geothermal area at kverkfjoll a central volcano at the northern edge of vatnajokull on august 15th 2013 a small jokulhlaup occurred when the gengissig ice dammed lake drained at kverkfjoll the lake level dropped by approximately 30 m decreasing pressure on the lake bed and triggering several hydrothermal explosions on the 16th here a multidisciplinary approach combining detailed field work laboratory studies and models of the energetics of explosions with information on duration and amplitudes of seismic signals has been used to analyse the mechanisms and characteristics of these hydrothermal explosions field and laboratory studies were also carried out to help constrain the sedimentary sequence involved in the event the explosions lasted for 40 50 s and involved the surficial part of an unconsolidated and hydrothermally altered glacio lacustrine deposit composed of pyroclasts lavas scoriaceous fragments and fine grained welded or loosely consolidated aggregates interbedded with clay rich levels several small fans of ejecta were formed reaching a distance of 1 km north of the lake and covering an area of approximately 0 3 km2 with a maximum thickness of 40 cm at the crater walls the material volume of approximately 104 m3 has been ejected by the expanding boiling fluid generated by a pressure failure affecting the surficial geothermal reservoir the maximum thermal craterisation and ejection energies calculated for the explosion areas are on the order of 1011 1010 and 109 j respectively comparison of these with those estimated by the volume of the ejecta and the crater sizes yields good agreement we estimate that approximately 30 of the available thermal energy was converted into mechanical energy during this event the residual energy was largely dissipated as heat while only a small portion was converted into seismic energy estimation of the amount of freshly fragmented clasts in the ejected material obtained from sem morphological analyses reveals that a low but significant energy consumption by fragmentation occurred decompression experiments were performed in the laboratory mimicking the conditions due to the drainage of the lake experimental results confirm that only a minor amount of energy is consumed by the creation of new surfaces in fragmentation whereas most of the fresh fragments derive from the disaggregation of aggregates furthermore ejection velocities of the particles 40 50 m s measured via high speed videos are consistent with those estimated from the field the multidisciplinary approach used here to investigate hydrothermal explosions has proven to be a valuable tool which can provide robust constraints on energy release and partitioning for such small size yet hazardous steam explosion events

isostatic geoid anomalies on a sphere long wavelength geoid anomalies due to lateral variations in the density or thickness of a thin layer in local isostatic equilibrium on the surface of a sphere depend sensitively on the assumed state of stress within the layer a number of common intuitive definitions of local isostasy generally believed to be essentially identical are associated with quite different states of isostatic stress and the corresponding theoretical geoid anomalies can vary by more than a factor of 2 this sensitivity of the theoretical anomaly to the exact definition of local isostasy constitutes an obstacle to any proposed program of interpreting or exploiting observed global isostatic geoid anomalies such as those associated with the aging of the oceanic lithosphere

mantle convection plates and hotspots abstract more than 20 years after their discovery plates and hotspots are still awaiting a consistent dynamic model both imply large viscosity variations in the mantle sustained efforts have been targeted on modeling on computer as well as in the laboratory convection in fluids with temperature and pressure dependent viscosity the main results of these studies are reviewed and summarized in a few simple rules several properties of plates such as ridge push or heat flow distribution are well accounted for by convection models with temperature dependent viscosity however the process of subduction is not properly explained by these models one of the simple rules deduced from the study of convection and temperature dependent viscosity states that the viscosity drop across the lower boundary layer adjusts to a low value less than 10 this constrasts strongly with the viscosity variations required by current hotspot formation models which are typically 100 times larger it is suggested that this paradox can be solved if subduction is properly included in the convection models

global stress pattern constrained on deep mantle flow and tectonic features abstract global stress patterns in the lithosphere which are constrained on the deep mantle flow and global tectonic features are investigated the drag forces acting on the bottom of the lithosphere and the boundary generating forces are used in our computing models comparing them with the stress map constructed by in situ stress measurements in boreholes focal mechanisms geological structures and other geophysical and geological observations we find that these forces are the main control of the global lithospheric stress pattern

dynamics of explosive volcanism at unzen volcano an experimental contribution knowledge of the dynamics of magma fragmentation is necessary for a better understanding of the explosive behaviour of silicic volcanoes here we have measured the fragmentation speed and the fragmentation threshold of five dacitic samples 6 7 53 5 vol open porosity from unzen volcano kyushu japan the measurements were carried out using a shock tube based fragmentation apparatus modified after alidibirov and dingwell 1996a b the results of the experimental work confirm the dominant influence of porosity on fragmentation dynamics the velocity of the fragmentation front increases and the value of the fragmentation threshold decreases with increasing porosity further we observe that the fragmentation speed is strongly influenced by the initial pressure difference and the texture of the dacite at an initial pressure difference of 30 mpa the fragmentation speed varies from 34 m s for the least porous sample to 100 m s for the most porous sample these results are evaluated by applying them to the 1990 1995 eruptive activity of unzen volcano emplacements of layered lava dome lobes merapi type pyroclastic flows and minor explosive events dominated this eruption the influence of the fragmentation dynamics on dome collapse and vulcanian events is discussed

permeability control on magma fragmentation fragmentation of porous magma that is subject to gas overpressure is considered to be a crucial process in the generation of explosive volcanic eruptions a decompressive event e g rapid magma ascent landslide dome collapse disrupts the stress equilibrium between the gas phase and the surrounding melt when the gas in the pores is exposed to a pressure gradient it may either fragment the surrounding magma or escape from the magma along an existing pathway of cracks and interconnected bubbles therefore magma permeability can be a decisive parameter in determining if an eruption experiences fragmentation i e whether it is explosive or effusive or exhibits a temporal transition between the two eruptive styles despite the central role that gas permeability may play in the fragmentation of volcanic rocks previous studies have not experimentally verified or quantified this influence based on a comprehensive database of combined permeability and fragmentation experiments we show that high permeability substantially increases the overpressure required to fragment porous volcanic rocks our results allow us to deduce a fragmentation criterion that incorporates gas permeability as well as porosity and internal overpressure this criterion implies that the energy required for fragmentation is less dependent on the actual pore geometry than on the way the void space is interconnected and thus on the contribution of permeable gas flow to decompression

plate kinematics and the gravity field abstract both the system of plate motions and the global gravity field or the geoid are now so precisely known that it seems worthwhile to look for quantitative relationships some aspects such as the general occurrence of positive gravity and geoid anomalies in regions of plate convergence have long been known our aim is to describe the gravitational field in terms of plate kinematic parameters and we present a preliminary step in this direction for four plates pacific nazca indian american we have computed the correlation of the gem 8 geoid heights with reference to an ellipsoid of 1 298 255 ellipticity with distance from the poles of motion and distance from the axes in an absolute frame the geoid tends first to drop from the ridge axes to at least 10 distance and then to rise toward the convergence zones this trend is strongest for the indian plate in collision with eurasia is smaller but very clear for the oceanic pacific and nazca plates and is not developed for the american plate which does not subduct we did not find a consistent relationship for the geoid with distance from the pivots a possible interpretation of the results is the return flow of the large scale mantle circulation

the electrification of volcanic jets and controlling parameters a laboratory study abstract lightning is ubiquitous in large ash rich eruptions however the quantification of the link between volcanic and electrical activities is still missing hindering its potential for monitoring explosive eruptions here we focus on vent lightning i e discharges occurring within the ash laden jet we use a shock tube apparatus generating jets with variable mass of ash grain size distribution and initial overpressure the experimental jet is directed inside a faraday cage where the current flux of electrical charges is measured allowing to estimate the total charge of the plume and the number and magnitude of jet to ground discharges three mechanisms control the electrical structure of the jet i the tribocharging of the ash particles against the shock tube walls ii the particle particle tribocharging in the jet flow iii the particle and charge separation according to the particle size leading to the formation of clusters of electrical charges the number and magnitude of discharges mainly rely on the two latter mechanisms while the particle particle interactions define the total charge in all the clusters which is linked to the observed cumulative magnitude of the discharges the jet structure defines the size of the individual clusters and in turn how the cumulative magnitude is partitioned on a number of discharges finally our experimental relationships between eruptive and electrical parameters are compatible with field observations suggesting that the pattern of discharges recorded by electrical monitoring systems can be used to interpret the structure of volcanic jets and their dynamic evolution

energetics of glass fragmentation experiments on synthetic and natural glasses 1 natural silicate glasses are an essential component of many volcanic rock types including coherent and pyroclastic rocks they span a wide range of compositions occur in diverse environments and form under a variety of pressure temperature conditions in subsurface volcanic environments e g conduits and feeders melts intersect the thermodynamically defined glass transition temperature to form glasses at elevated confining pressures and under differential stresses we present a series of room temperature experiments designed to explore the fundamental mechanical and fragmentation behavior of natural obsidian and synthetic glasses pyrex under confining pressures of 0 1 100 mpa in each experiment glass cores are driven to brittle failure under compressive triaxial stress analysis of the load displacement response curves is used to quantify the storage of energy in samples prior to failure the brittle release of elastic energy at failure and the residual energy stored in the post failure material we then establish a relationship between the energy density within the sample at failure and the grain size distributions d values of the experimental products the relationship between d values and energy density for compressive fragmentation is significantly different from relationships established by previous workers for decompressive fragmentation compressive fragmentation is found to have lower fragmentation efficiency than fragmentation through decompression i e a smaller change in d value with increasing energy density we further show that the stress storage capacity of natural glasses can be enhanced approaching synthetic glasses through heat treatment

the contribution of experimental volcanology to the study of the physics of eruptive processes and related scaling issues a review abstract the experimental approach has become a major tool increasingly used by volcanologists in recent decades to investigate the physics of eruptive processes in complement to field and theoretical works researchers have developed various methodologies to study volcanic phenomena at reduced length scale the works involve natural or analogue materials and their types range from first order tests to identify fundamental processes and make qualitative comparison with field observations to more sophisticated experiments in which precise data obtained in a controlled environment can be used to validate outputs of theoretical models scaling is a central issue to ensure dynamic similarity between the small scale experiments and the large scale volcanic phenomena when natural conditions cannot be simulated due to inherent length scale difference and or technical limitations in this respect dimensionless numbers are used to map physical regimes and to define scaling laws which allow experimental results to be extrapolated to natural scale this review presents the variety of experimental studies conducted to investigate subterraneous and aerial volcanic phenomena involving in particular fluid particle mixtures we focus on the major scaling issues and the physical regimes investigated and we also highlight in a historical perspective some of the major advances achieved through experimental studies finally we conclude on some perspectives for future works

a model for viscous magma fragmentation during volcanic blasts fragmentation of magma containing gas bubbles is of great interest in connection with developing models for the formation of pyroclastics and for volcanic blasts explosions this paper considers the problem of fragmentation of highly viscous 108 pa s or solidified magma containing bubbles with excess gas pressure it is suggested that the fragmentation of magma be considered on the basis of the fragmentation wave theory proposed by nikolsky and khristianovich which is generally applicable to gas dynamic phenomena occurring in mines then it becomes possible to derive the equations of conservation for the fragmentation wave front which moves into a body of magma from its free surface as a result the velocity n of magma fragmentation and the velocity u of the movement of the gas pyroclastic mixture behind the fragmentation wave front are determined calculations show that n can reach 5 m s therefore the duration of the fragmentation of the magma body blast duration proves to be long the suggested model explains the possibility of several explosions during the blast as a result of the fragmentation wave stopping and accounts for the angular shape of pyroclasts by the brittle disruption of interbubble partitions during fragmentation wave propagation through the porous magma body the initiation and cessation of fragmentation are defined by magma porosity magma tensile strength and the pressure differential between gas pressure in pores and the atmospheric pressure the physical model of magma fragmentation developed explains the mechanism of energy release during volcanic blasts of the vulcanian or pelean types

explosive energy during volcanic eruptions from fractal analysis of pyroclasts abstract despite recent advances by means of experiments and high resolution surveys and the growing understanding of the physical processes before and during volcanic eruptions duration and type of eruptive activity still remain highly unpredictable this uncertainty hinders appropriate hazard and associated risk assessment tremendously in an effort to counter this problem experimentally generated pyroclasts have been studied by fractal statistics with the aim of evaluating possible relationships between eruption energy and fragmentation efficiency rapid decompression experiments have been performed on three differently porous sample sets of the 1990 1995 eruption of unzen volcano japan at 850 c and at initial pressure values above the respective fragmentation threshold u kueppers b scheu o spieler d b dingwell fragmentation efficiency of explosive volcanic eruptions a study of experimentally generated pyroclasts j volcanol geotherm res 153 2006 125 135 o spieler b kennedy u kueppers d b dingwell b scheu j taddeucci the fragmentation threshold of pyroclastic rocks epsl 226 2004 139 148 the size distribution of generated pyroclasts has been studied by fractal fragmentation theory and the fractal dimension of fragmentation df a value quantifying the intensity of fragmentation has been measured for each sample results show that size distribution of pyroclastic fragments follows a fractal law i e power law in the investigated range of fragment sizes indicating that fragmentation of experimental samples reflects a scale invariant mechanism in addition df is correlated positively with the potential energy for fragmentation pef while showing a strong influence of the open porosity of the samples results obtained in this work indicate that fractal fragmentation theory may allow for quantifying fragmentation processes during explosive volcanic eruptions by calculating the fractal dimension of the size distribution of pyroclasts it emerges from this study that fractal dimension may be utilised as a proxy for estimating the explosivity of volcanic eruptions by analysing their natural pyroclastic deposits

on the isostatic gravity anomaly and disturbance and their applications to vening meinesz moritz gravimetric inverse problem in this study we showthat the traditionally defined bouguer gravity anomaly needs a correction to become the no topography gravity anomaly and that the isostatic gravity anomaly is better defined by the latter anomaly plus a gravity anomaly compensation effect than by the bouguer gravity anomaly plus a gravitational compensation effect this is because only the newisostatic gravity anomaly completely removes and compensates for the topographic effect f a vening meinesz inverse problem in isostasy deals with solving for the moho depth from the known external gravity field and mean moho depth known e g from seismic reflection data by a regional isostatic compensation using a flat earth approximation h moritz generalized the problem to that of a global compensation with a spherical mean earth approximation the problem can be formulated mathematically as that of solving a non linear fredholm integral equation the solutions to these problems are based on the condition of isostatic balance of the isostatic gravity anomaly and theoretically this assumption cannot be met by the old definition of the isostatic gravity anomaly we show how the moho geometry can be solved for the gravity anomaly gravity disturbance and disturbing potential etc and from a theoretical point of view all these solutions are the same

experimental and analytical modeling of basaltic ash explosions at mount etna italy 2001 1 the summer 2001 flank eruption of mount etna ended with repeated basaltic ash explosions the explosions were similar to small scale vulcanian blasts and probably originated when a plug of magma at the top of the conduit became overpressurized we use three independent methods to estimate the overpressure in the plug 1 to determine the pressure differential required to fragment the plug we use a shock tube apparatus to heat and pressurize ballistic blocks at temperature t up to 900 c and pressure p up to 25 mpa and rapidly decompress them to ambient conditions 2 we apply a model to determine the overpressure generated by microlite growth at depth because pyroclasts from the explosions have 35 vol more microlites than preceding strombolian product and 3 we relate the range of ballistic blocks to their exit velocity and with models for vulcanian eruptions the exit velocity to pressure in the conduit experiments and ejecta range bracket the overpressure driving the explosions between 3 and 5 mpa below 2 5 mpa the blocks would not reach the observed range but above 5 mpa the existing blocks with 20 porosity would not survive the explosions crystallization of 35 vol microlites at a depth of 100 m increases the pressure in the plug to 6 mpa above atmospheric enough to drive the explosions with a possible contribution from deeper volatiles combining the different methods we also constrain the exsolved water at fragmentation at 0 05 wt and provide inferences on how the explosions came to an end

low temperature emplacement of phreatomagmatic pyroclastic flow deposits at the monogenetic mt gambier volcanic complex south australia and their relevance for understanding some deposits in diatremes the occurrence of pyroclastic flow deposits is not restricted to polygenetic and or felsic volcanic systems but can also occur at mafic monogenetic volcanic centres at the c 5 ka mt gambier volcanic complex australia two small volume pyroclastic flow deposits resulted from phreatomagmatic eruptive phases of the blue lake east and the valley lake maar craters the facies descriptions of the two massive poorly sorted lapilli tuff and tuff breccia deposits are given low grade carbonized wood fragments in the deposits mark low emplacement temperatures 180 270 c for these deposits with only c 12 23 of the total thermal energy of the original magma preserved at the time of deposition the blue lake east vent erupted by forming a short lived vertical eruption column which could not be sustained and collapsed the valley lake pyroclastic flow deposit has a highly asymmetric dispersal to the west indicating that this deposit was formed during an eruptive phase with a large westward directed lateral component such massive poorly sorted deposits outside the maar crater indicate that some similar deposits in diatremes may have originated from surface eruptions and not just from subsurface debris jets

towards sub lithospheric stress determination from seismic moho topographic heights and goce data abstract sub lithospheric stresses can be estimated by analysis of gravity field measurements depending on the measured gravimetric quantity different methods can be employed to estimate those sub lithospheric stresses here we further develop the runcorn s theory for estimation of mantle stresses 1967 such that a moho model and full topographic information are used to recover the function from which the stress can be computed by taking derivatives northwards and eastwards we develop new integral equations for such a purpose and recover this function by solving those integral equations locally over the indo pak india pakistan region from 1 a gravimetric moho model computed from the srtm shuttle radar topography mission and the earth gravity model egm2008 2 srtm and the seismic moho model of crust1 0 and 3 data and measurements of the goce gravity field and steady state ocean circulation explorer mission finally we perform a joint inversion of seismic and goce data for the same purpose the numerical results show that the use of a seismic moho model recovers information about the stress field which is not seen in the results derived from a gravimetric moho model a combination of the seismic moho model srtm and goce yields a better stress field than that of either the seismic and or gravimetric data alone the magnitudes of the sub lithospheric stress are computed from the shear stress components over the area and good agreement is seen between the recovered combined stress field the regional tectonic boundaries and the seismicity of the world stress map 2008 database

using microlites to gain insights into ascent conditions of differing styles of volcanism at soufriere hills volcano abstract microlite textural characteristics are analyzed in the products of vulcanian explosions ash venting occurring synchronously with lava effusion syn extrusive ash venting a precursory explosion and lava dome effusion associated with phases 3 and 5 of the current soufriere hills volcano shv eruption 2d microlite population statistics and crystal size distributions are used to infer decompression pathways for each style of volcanism in addition magma ascent rates for each eruptive style were calculated using microlite number density toramaru et al 2008 strong differences between pyroclastic and effusive microlites are observed in 2d population statistics and crystal size distributions indicating higher amounts of smaller microlites 0 0084 to 0 012 mm however syn extrusive ash venting samples have comparatively greater amounts of large microlites the similarity between lava dome and vulcanian explosions at larger microlite sizes suggests comparable crystallization conditions in the deeper conduit system 2 km depth the contrast in eruption style despite similarity at depth suggests a shallow control to eruption processes at shv the greater amounts of large microlites in syn extrusive ash venting compared to the lava dome indicates two distinct decompression pathways within a single conduit inferred to result from differences in ascent rate sluggish conduit margin ascent leading to lower decompression rates is inferred to produce the increased degree of crystal growth observed in syn extrusive ash venting compared with the lava dome calculated magma ascent rates for vulcanian explosions and syn extrusive ash venting have similar ranges while lava effusion is up to a factor of 10 faster differences in magma ascent rate are inferred to result from variations in the depth at which ascent rate is calculated for each eruptive style ascent rate is calculated at microlite nucleation depth using the method of toramaru et al 2008 however the average microlite nucleation depth will vary because of differences in microlite nucleation rate during ascent more small microlites in the lava dome suggest greater nucleation rates in the shallow conduit recording an on average shallower magma ascent rate 2 km depth

pyroclast tracking velocimetry illuminates bomb ejection and explosion dynamics at stromboli italy and yasur vanuatu volcanoes a new image processing technique pyroclast tracking velocimetry was used to analyze a set of 30 high speed videos of strombolian explosions from different vents at stromboli italy and yasur vanuatu volcanoes the studied explosions invariably appear to result from the concatenation of up to a hundred individual pyroclast ejection pulses all these pulses share a common evolution over time including 1 a non linear decrease of the pyroclast ejection velocity 2 an increasing spread of ejection angle and 3 an increasing size of the ejected pyroclasts these features reflect the dynamic burst of short lived gas pockets in which the rupture area enlarges while pressure differential decreases we estimated depth of pyroclast release to be approximately 1 and 8 m below the surface at stromboli and yasur respectively in addition explosions featuring more frequent pulses also have higher average ejection velocities and larger total masses of pyroclasts these explosions release a larger overall amount of energy stored in the pressurized gas by a combination of more frequent and stronger ejection pulses in this context the associated kinetic energy per explosion ranging 103 109 j appears to be a good proxy for the explosion magnitude differences in the pulse defining parameters among the different vents suggest that this general process is modulated by geometrical factors in the shallow conduit as well as magma specific rheology indeed the more viscous melt of yasur compared to stromboli is associated with larger vents producing fewer pulses but larger pyroclasts

integrating puffing and explosions in a general scheme for strombolian style activity strombolian eruptions are among the most common subaerial styles of explosive volcanism worldwide distinctive features of each volcano lead to a correspondingly wide range of variations of magnitude and erupted products but most papers focus on a single type of event at a single volcano here in order to emphasize the common features underlying this diversity of styles we scrutinize a database from 35 different erupting vents including 21 thermal infrared videos from stromboli italy etna italy yasur vanuatu and batu tara indonesia from puffing through rapid explosions to normal explosions with variable ejection parameters and relative abundance of gas ash and bombs using field observations and high speed thermal infrared videos processed by a new algorithm we identify the distinguishing characteristics of each type of activity and how they may relate and interact in particular we record that ash poor normal explosions may be preceded and followed by the onset or the increase of the puffing activity while ash rich explosions are emergent i e with inflation of the free surface followed directly by emission of increasingly large gas pockets overall we see that all strombolian activities form a continuum arising from a common mechanism and are modulated by the combination of two well established controls 1 the length of the bursting gas pocket with respect to the vent diameter and 2 the presence and thickness of a high viscosity layer in the uppermost part of the volcanic conduit

source and behavior of pyroclastic density currents generated by vulcanian style explosions of popocatepetl volcano mexico on 22 january 2001 abstract since 1994 popocatepetl volcano has extruded 85 lava domes each of which has been destroyed by vulcanian style explosions the paroxysmal vulcanian explosion of 22 january 2001 has been unique by generating multiple pyroclastic density currents pdcs scientists and civil protection authorities have used those pdcs to remap the high risk hazard zone of the volcano although little study has been carried out to understand the generation and evolution of those pdcs here we examine 38 stratigraphic sections and analyze the granulometry componentry vesicularity and petrology glass melt inclusions in plagioclase of collected samples to interpret the origins of the vulcanian explosions and the pdc behavior the 22 january event generated two series of dense pdcs that detached from the base of the eruption column the first set of pdcs traveled northwest across a glacier on the volcano partially scouring and melting the ice that hours later generated a lahar two pulses of these pdcs were confined within a large ravine surpassed the slope break and traveled up to 3 km away from the vent the deposits are matrix supported throughout their extent the second set of pdcs traveled to the northeast mainly as unconfined currents that split into two lobes prior to the slope break the farthest traveled current followed a western ravine and reached the forest 3 5 km from the summit the eastern lobe buried a local hiking hut and reached 2 1 km the deposits from these northeastern currents are massive and block supported all deposits volume of 5 71 105 m3 have finger like morphology with levees and frontal snouts all pdcs flowed as a dense basal granular current with turbulent upper clouds at minimum speeds of 7 10 m s in terms of hazards pdcs that interact with glaciers on the volcano are expected to travel farther and potentially generate extensive lahars at present small intermittent patches of glacier occur on the northern upper slopes of the volcano the components of the deposits are in order of abundance black scoria 53 62 wt sio2 banded scoria pumice 63 wt sio2 dome lithics 60 61 wt sio2 accidental lithics and free crystals juvenile pumice and scoria contain phenocrysts of plagioclase an47 10 clinopyroxene en48 3 orthopyroxene en64 1 ilmenite and titanomagnetite set in a rhyolitic glass pumice clast 74 39 wt sio2 or microlite rich matrix scoria clast the dome lithics also contain these phenocrysts as well as rare olivine phenocrysts fe2 fe2 mg 0 1 dissolved h2o co2 contents in melt inclusions hosted by pumice plagioclase phenocrysts suggest the felsic magma was stored at depths of 2 4 km thus the 2001 event was triggered by the injection of mafic magmas into a small volume of felsic magma probably forming dikes left from the previous eruptive episode we propose that the vulcanian explosions tap a column of magma 2 km depth equivalent to the volume that contained the felsic dikes this mechanism has been acting during the destruction of all summit domes of the ongoing eruption of the volcano

a simple global model of plate dynamics and mantle convection cooling and thickening of lithospheric plates with age and subduction result in large scale horizontal density contrasts tending to drive plate motions and mantle flow we quantify the driving forces associated with these density contrasts to determine if they can drive the observed plate motions first two dimensional models are computed to evaluate the effects of assumed rheologies and boundary conditions we are unable to obtain platelike behavior in viscous models with traction free boundary conditions the piecewise uniform velocities distinctive of plate motion can be imposed as boundary conditions and the dynamic consistency of the models evaluated by determining if the net force on each vanishes if the lithosphere has a newtonian viscous rheology the net force on any plate is a strong function of the effective grid spacing used leading to ambiguities in interpretation incorporating a rigid plastic lithosphere which fails at a critical yield stress into the otherwise viscous model removes these ambiguities the model is extended to the actual three dimensional spherical plate geometry the observed velocities of rigid plastic plates are matched to the solution of the viscous stokes equation at the lithosphere asthenosphere boundary body forces from the seismically observed slabs from the thickening of the lithosphere obtained from the actual lithospheric ages and from the differences in structure between continents and oceans are included interior density contrasts such as those resulting from upwellings from a hot bottom boundary layer are assumed to occur on a scale small compared to plate dimensions and are not included the driving forces from the density contrasts within the plates are calculated and compared to resisting forces resulting from viscous drag computed from the three dimensional global return flow and resistance to deformation at converging boundaries the rms residual torque is 30 of the driving torque the density contrasts within the plates themselves can reasonably account for plate motions body forces from convection in the interior may provide only a small net force on the plates at converging boundaries the lithosphere has a yield stress of 100 bars drag at the base of the plates is 5 bars and resists plate motion the net driving forces from subducting slabs and collisional resistance are localized and approximately balance driving forces from lithospheric thickening are distributed over the areas of the plates as is viscous drag the approximate balance of these two forces predicts plate velocities uncorrelated with plate area as observed the model represents a specific case of boundary layer convection the dynamical results are consistent with either upper mantle or mantle wide convection

diagenetic origin of quartz silt in mudstones and implications for silica cycling mudstone the most abundant sedimentary rock type1 composed primarily of clay or silt sized particles contains most of the quartz found in sedimentary rocks2 these quartz grains which are chemically and mechanically resistant and therefore preserve their characteristics well have long been considered to be derived from the continental crust1 here we analyse quartz silt from black shales in the eastern usa dating back to the late devonian period about 370 million years ago using backscattered electron and cathodoluminescence imaging and measure oxygen isotopes with an ion probe our results indicate that up to 100 of the quartz silt in our samples does not originate from the continental crust instead it appears to have precipitated early in diagenesis in algal cysts and other pore spaces3 with silica derived from the dissolution of opaline skeletons of planktonic organisms such as radiolaria and diatoms transformation of early diatoms into in situ quartz silt might explain the time gap between the earliest fossil occurrences of diatoms about 120 myr ago4 and molecular evidence for a much earlier appearance between 266 or even 500 myr ago5 6 moreover if many other mudstone successions show similarly high proportions of in situ precipitated rather than detrital quartz silt the sedimentary record in mudstones may have been misinterpreted in the past with consequences for our estimates of palaeoproductivity as well as our perceptions of the dynamics and magnitude of global biogeochemical cycling of silica

global tectonics and the geoid abstract a digitised tectonic model initially built up for regionalization of rayleigh waves is applied to the geoid in order to define the mean geoid heights of the following regions 3 oceanic regions namely young oceans 0 30 ma middle aged oceans 30 80 ma and old oceans 80 ma trenches and subduction zones mountains and shields the relative importance of the deep sources is damped or enhanced by progressively removing or adding the lower or higher degrees of the geoid a statistical approach allows us to quantify the success of the correlation between tectonics and these filtered geoids significant variations are observed in these correlations for oceanic regions including subduction zones with a cut off between degree 2 and higher degrees for degrees 3 a well known trend is observed high values correspond to young oceans ridges and low values to old oceans high values are also obtained for subduction zones on the contrary and unexpectedly for the degree 2 alone a trend reversal is observed geoid lows are observed over ridges and geoid highs over old oceans trenches give the same geoid amplitude than old oceans clearly this denotes a degree 2 convection pattern connected to plate tectonics in addition it is shown that the minimum and maximum inertia axes of the surface distribution of young oceans and independently of old oceans and trenches coincide with the earth s equatorial inertia axes 74 e and 164 e i e with the equatorial extremes of the degree 2 geoid plate tectonics is uncorrelated with the polar anomaly of the degree 2 geoid namely the flattening which is not accounted for by earth rotation a north south axisymmetric convection with a degree 2 pattern is proposed to explain this extra flattening this model is supported by the latitude dependence of the depth of oceanic ridges

local recovery of lithospheric stress tensor from goce gravitational tensor the sub lithospheric stress due to mantle convection can be computed from gravity data and propagated through the lithosphere by solving the boundary value problem of elasticity for the earth s lit

characterisation of the 1997 vulcanian explosions of soufriere hills volcano montserrat by video analysis the activity of convention at montserrat soufriere hills volcano montserrat during the period 1995 1999 included numerous violent explosions two major cycles of vulcanian explosions occurred in 1997 a first of 13 explosions between 4 and 12 august and a second of 75 between 22 september and 21 october the explosions were short lived events lasting a few tens of seconds during which partial fountain collapse generated pyroclastic surges and pyroclastic flows and buoyant plumes ascended 3 15 km into the atmosphere each explosion discharged on average 3 105 m3 dense rock equivalent dre of magma draining the conduit to depths of 1 2 km the paper focuses on the first few seconds of three explosions of the 75 that occurred in september october 1997 6 october 1997 at 17 50 7 october 1997 at 16 02 and 9 october 1997 at 12 32 physical parameters such as exit velocities magmatic water contents and magma pressures at fragmentation are estimated by following and modelling the ascent of individual momentum dominated finger jets visible on videos during the initial stages of each explosion the model treats each finger jet as an incompressible flow sustained by a steady flux of gas and particles during the few seconds of ascent and produces results that compare favourably with those using a multiphase compressible code run using similar eruptive parameters each explosion reveals a progressive increase in eruptive intensity with time jet exit velocities increasing from 40 m s 1 at the beginning of the explosion up to 140 m s 1 after a few seconds modelling suggests that the first magma to exit was largely degassed whereas that discharged after a few seconds contained up to 2 wt water magma overpressures up to 10 mpa are estimated to have existed in the conduit immediately prior to each explosion progressive increases in jet exit velocity with time over the first few seconds of each explosion provide direct evidence for strong pre eruptive gradients in water content and magma pressure in the upper reaches probably 100 500 m of the conduit fountain collapse occurred during the first 10 20 s of each explosion because the discharging jets had bulk densities up to 100 times that of the atmosphere and were unable to entrain enough air to become buoyant such high eruptive densities were due to the presence of partially degassed magma in the conduit

a theoretical discussion on vening meinesz moritz inverse problem of isostasy the moho surface can be determined according to isostatic theories and the recent vening meinesz moritz vmm theory of isostasy has been successful for this purpose in this paper we will study t

analysis of source characteristics of experimental gas burst and fragmentation explosions generated by rapid decompression of volcanic rocks gas burst and fragmentation explosions induced by rapid decompression of volcanic rocks in a high pressure autoclave have been analyzed we performed experiments from 4 to 20 mpa on 1 loose pyroclasts and 2 pumice from recent eruptions of popocatepetl volcano our aim is to characterize the source mechanism distinguishing the physical processes that operate in the conduit which determine the dynamics of explosive volcanoes for this aim we have analyzed experimentally the parameters that indicate the state and behavior of the conduit i e decompression time fragmentation threshold and speed forces and the partitioning of energy using microseismic monitoring the decompression time curves and the sample rheological properties the initial available potential energy in the system and its partitioning into different types of energies are correlated in space and time with specific stages of the explosive phenomenon such correlations taken together with the energy distribution enable the distinction of the individual physical processes involved and their causal sequence and relationships our observations suggest that in volcanic conduits with regular explosive activity a source mechanism may operate whereby a causal sequence of processes results in a system that undergoes both rapid and stable transitions such behavior may persist over long periods of time

maximum horizontal range of volcanic ballistic projectiles ejected during explosive eruptions at santorini caldera abstract this study investigates the hazard posed by volcanic ballistic projectiles vbps to the santorini islands considering eruption scenarios that include low vei 2 3 and higher energy vei 3 eruptions a model that describes rapid decompression of pressurized magma below a caprock along with its fragmentation and acceleration of particles is utilized for estimating initial velocities during vulcanian style eruptions these initial velocities are inserted into the ballistic equations assuming that vbps have a cube like shape are subjected to gravity drag forces and are launched into a zone of reduced drag four different diameters of vbps are considered 0 35 m 1 0 m 2 0 m 3 0 m and also different values of gas fractions and extent of the reduced drag zone are investigated the results of these calculations show that an area of 1 2 km width along the western coast of thera will be within the maximum range of vbps provided that the eruptive vent will develop either on nea kameni or between nea kameni and thera initial velocities for higher energy eruptions are estimated by considering the conversion efficiency of thermal to kinetic energy for the case of an eruption with vei 4 and a number of vents centered between nea and palea kameni calculations show that the coastal areas of thera and therasia are within the maximum horizontal range of vbps with diameter larger than 0 35 m as the exact position of the eruptive vent seems to be of crucial importance for determining the areas at risk continuous seismic and geodetic monitoring of the caldera is needed in order to decipher its likely location

high speed imaging of strombolian explosions the ejection velocity of pyroclasts explosive volcanic eruptions are defined as the violent ejection of gas and hot fragments from a vent in the earth s crust knowledge of ejection velocity is crucial for understanding and modeling relevant physical processes of an eruption and yet direct measurements are still a difficult task with largely variable results here we apply pioneering high speed imaging to measure the ejection velocity of pyroclasts from strombolian explosive eruptions with an unparalleled temporal resolution measured supersonic velocities up to 405 m s are twice higher than previously reported for such eruptions individual strombolian explosions include multiple sub second lasting ejection pulses characterized by an exponential decay of velocity when fitted with an empirical model from shock tube experiments literature this decay allows constraining the length of the pressurized gas pockets responsible for the ejection pulses these results directly impact eruption modeling and related hazard assessment as well as the interpretation of geophysical signals from monitoring networks

a practical model for organic richness from porosity and resistivity logs a practical method the delta log r technique for identifying and calculating total organic carbon in organic rich rocks has been developed using well logs the method employs the overlaying of a properly scaled porosity log generally the sonic transit time curve on a resistivity curve preferably from a deep reading tool in water saturated organic lean rocks the two curves parallel each other and can be overlain since both curves respond to variations in formation porosity however in either hydrocarbon reservoir rocks or organic rich non reservoir rocks a separation between the curves occurs using the gamma ray curve reservoir intervals can be identified and eliminated from the analysis the separation in organic rich intervals results from two effects the orosity curve responds to the presence of low density low velocity kerogen and the resistivity curve responds to the formation fluid in an immature organic rich rock where no hydrocarbons have been generated the observed curve separation is due solely to the porosity curve response in mature source rocks in addition to the porosity curve response the resistivity increases because of the presence of generated hydrocarbons the magnitude of the curve separation in non reservoirs is calibrated to total organic carbon and maturity and allows for depth profiling of organic richness in the absence of sample data this method allows organic richness to be accurately assessed in a wide variety of lithologies and maturities using common well logs

eruptive and shallow conduit dynamics during vulcanian explosions insights from the episode iv block field of the 1912 eruption of novarupta alaska the study of 1300 juvenile and lithic blocks from a vulcanian phase of the 1912 eruption of novarupta provides new insight into the state of the magma as an eruption passes from sustained plinian to dome growth blocks that were predominantly ballistically ejected were measured and sampled within an 2 3 km radius from vent and supply a picture of a dynamic and complex shallow conduit prior to magma fragmentation in repeated small explosions extreme conduit heterogeneity is expressed in the diverse range of dacitic block types including pumiceous dense banded and variably welded breccia clasts all with varied degrees of surface breadcrusting we present new maps of block lithology and size making episode iv the most thoroughly mapped vulcanian deposit to date sectorial regions rich in specific lithologies together with the block size data suggest multiple small explosions modeling of block trajectories to reproduce the field data indicates that ejection velocities range from 50 to 124 m s with a median of 70 m s we propose that individual explosions originated from a heterogeneous shallow conduit characterized both by the juxtaposition of magma domains of contrasting texture and vesiculation state and by the intimate local mingling of different textures on short vertical and horizontal length scales at the contacts between these domains in our model each explosion disrupted the conduit to only shallow depths and tapped diverse localized pockets within the conduit this contrasts with existing models for repetitive vulcanian explosions and suggests that the dynamics of episode iv were more complex than a simple progressive top down evacuation of a horizontally stratified conduit

seismological analysis of conduit dynamics in fragmentation experiments we simulate gas burst and volcanic explosions under controlled laboratory conditions inducing fragmentation of volcanic rocks by rapid depressurization a series of experiments were performed in a shock tube apparatus at room temperature and a pressure range of 4 to 20 mpa using argon ar gas and particles or pumice samples of different porosities the instrumentation of this system with high precision piezoelectric sensors enabled us to capture elastic waves and to recognize their characteristic signatures by relating these signals to physical processes in the wave field we have been able to characterize the conduit mechanism and the source dynamics we compare and discuss conspicuous features of the waveforms and frequency spectra of these experimental signals with those of volcanic origin despite the fact that these signals are different in amplitude resulting from different scale conditions our observations indicate that the physical processes that occur during simulated explosions and those that occur during volcanic eruptions yield comparable signatures in their respective records the effects of the source receiver configuration and resonance also have significant implications all this suggests that the physical processes e g pressurization and depressurization of a system involve a system response that causes similar distinctive effects independent of the system size reflecting its intrinsic dynamics these similarities imply that powerful constraints on the source mechanisms of volcanic seismicity can emerge from seismic investigations of experimental simulations of volumetric sources such constraints may yield significant advances in the understanding of volcanic conduit dynamics and in the interpretation of seismic unrest at volcanic centers

an analytical model for gas overpressure in slug driven explosions insights into strombolian volcanic eruptions strombolian eruptions common at basaltic volcanoes are mildly explosive events that are driven by a large bubble of magmatic gas a slug rising up the conduit and bursting at the surface gas overpressure within the bursting slug governs explosion dynamics and vigor and is the main factor controlling associated acoustic and seismic signals we present a theoretical investigation of slug overpressure based on magma static and geometric considerations and develop a set of equations that can be used to calculate the overpressure in a slug when it bursts slug length at burst and the depth at which the burst process begins we find that burst overpressure is controlled by two dimensionless parameters v which represents the amount of gas in the slug and a which represents the thickness of the film of magma that falls around the rising slug burst overpressure increases nonlinearly as v and a increase we consider two eruptive scenarios 1 the standard model in which magma remains confined to the vent during slug expansion and 2 the overflow model in which slug expansion is associated with lava effusion as occasionally observed in the field we find that slug overpressure is higher for the overflow model by a factor of 1 2 2 4 applying our model to typical strombolian eruptions at stromboli we find that the transition from passive degassing to explosive bursting occurs for slugs with volume 24 230 m 3 depending on magma viscosity and conduit diameter and that at burst a typical strombolian slug with a volume of 100 1000 m 3 has an internal gas pressure of 1 5 bars and a length of 13 120 m we compare model predictions with field data from stromboli for low energy puffers mildly explosive strombolian eruptions and the violently explosive 5 april 2003 paroxysm we find that model predictions are consistent with field observations across this broad spectrum of eruptive styles suggesting a common slug driven mechanism we propose that paroxysms are driven by unusually large slugs large v

global isostatic geoid anomalies for plate and boundary layer models of the lithosphere abstract isostatic geoid anomalies are usually interpreted using a flat earth one dimensional idealization isostatic anomalies on the spherical self gravitating earth differ from this idealization because 1 degree one terms in the spherical harmonic expansion vanish 2 each term in the spherical harmonic expansion is multiplied by l 2 l 0 5 relative to the flat earth case 3 mass in cones rather than straight sided columns is constant and 4 further deformation of the earth is induced by the gravitational attraction of the deformation caused by the isostatic potential anomaly when the effect of each of these is quantified the second third and fourth nearly cancel leaving the degree one over the horizon effect providing the most important difference calculations of model isostatic geoid anomalies for the spherical analogues developed here of the plate and boundary layer thermal models show that this effect can bias estimates of geoid slopes by over 20 although the effect is usually less than 5 the geoid anomalies for these two models are quite different over old ocean basins but they are unlikely to be distinguishable on the basis of global geoid observations owing to the presence of other larger perturbations in the geoid stripping the effects of plate aging and a hypothetical uniform 35 km thick isostatically compensated continental crust from the observed geoid emphasizes that the largest amplitude geoid anomaly is the geoid low of almost 120 m over west antarctica this anomaly is a factor of two greater in amplitude than the low of 60 m over sri lanka

the fragmentation threshold of pyroclastic rocks in response to rapid decompression porous magma may fragment explosively this occurs when the melt can no longer withstand forces exerted upon it due to the overpressure in included bubbles this occurs at a critical pressure difference between the bubbles and the surrounding magma in this study we have investigated this pressure threshold necessary for the fragmentation of magma here we present the first comprehensive high temperature experimental quantification of the fragmentation threshold of volcanic rocks varying widely in porosity permeability crystallinity and chemical composition we exposed samples to increasing pressure differentials in a high temperature shock tube apparatus until fragmentation was initiated experimentally we define the fragmentation threshold as the minimum pressure differential that leads to complete fragmentation of the pressurized porous rock sample our results show that the fragmentation threshold is strongly dependent on porosity high porosity samples fragment at lower pressure differentials than low porosity samples the fragmentation threshold is inversely proportional to the porosity of the other factors permeability likely affects the fragmentation threshold at high porosity values whereas chemical composition crystallinity and bubble size distribution appear to have minor effects the relationship for fragmentation threshold presented here can be used to predict the minimum pressure differential necessary for the initiation or cessation of the explosive fragmentation of porous magma

a mathematical model of vulcanian eruptions summary the problem of an expansion wave propagating into a saturated magma is solved and used as a model of a vulcanian eruption in our model for an explosive eruption we assume that a drop in pressure leads to the exsolution of magmatic volatiles initially the exsolved vapours create bubbles in the magma we model the subsequent part of the exsolution process in which a foam is created this is believed to be an essential feature of explosive volcanic eruptions the foam also has the advantage that it can be modelled as a mixture or pseudo gas without slippage between the phases eventually the foam breaks up and becomes volcanic ash assuming that the exsolution of vapour is given by henry s law that the temperature is constant that the magma and vapour have equal velocities and neglecting wall friction and gravitational effects an analytic solution for pressure velocity and vapour fraction is obtained for the expanding mixture in a constant area duct the exit velocity u for the mixture is u o0rt0 1 2in p0 p where o0 is the original mass fraction of dissolved vapour r is the gas constant for the vapour t0 the constant temperature and p0 p the pressure ratio across the expansion with o0 1 per cent t0 1000 k and p0 p 100 we find u0 300ms 1 consistent with observations for vulcanian eruptions

physical parameterization of strombolian eruptions via experimentally validated modeling of high speed observations pressurized gas drives explosive volcanic eruptions existing models can predict the amount and pressure of gas in erupting magma but application and testing of such models is currently limited by the accuracy of input parameters from natural systems here we present a new methodology based on a novel integration of 1 high speed imaging and 2 shock tube modeling of volcanic activity in order to derive estimates of sub second variations in the pressure mass and volume of gas that drive the dynamics of unsteady eruptions first we validate the method against laboratoryscale shock tube experiments having validated the method we then apply it to observations of eruptions at stromboli volcano italy finally we use those results for a parametric study of the weight of input parameters on final outputs we conclude that strombolian explosions with durations of seconds result from discrete releases of gas with mass and pressure in the 4 714 kg and 0 10 0 56 mpa range respectively and which occupy the volcano conduit to a depth of 4 190 m these variations are present both among and within individual explosions

martian sub crustal stress from gravity and topographic models the latest martian gravity and topographic models derived from the mars orbiter laser altimeter and the mars global surveyor spacecraft tracking data are used to compute the sub crustal stress field on mars for this purpose we apply the method for a simultaneous determination of the horizontal sub crustal stress component and the crustal thickness based on solving the navier stokes problem and incorporating the vening meinesz moritz inverse problem of isostasy results reveal that most of the martian sub crustal stress is concentrated in the tharsis region with the most prominent signatures attributed to a formation of tharsis major volcanoes followed by crustal loading the stress distribution across the valles marineris rift valleys indicates extensional tectonism this finding agrees with more recent theories of a tectonic origin of valles marineris caused for instance by a crustal loading of the tharsis bulge that resulted in a regional trusting and folding aside from these features the martian stress field is relatively smooth with only a slightly enhanced pattern of major impact basins the signatures of active global tectonics and polar ice load are absent whereas the signature of the hemispheric dichotomy is also missing the long wavelength spectrum of the stress field comprises the signature of additional dichotomy attributed to the isostatically uncompensated crustal load of tharsis volcanic accumulations these results suggest a different origin of the earth s and martian sub crustal stress whereas the former is mainly related to active global tectonics the latter is generated by a crustal loading and regional tectonism associated with a volcanic evolution on mars the additional sub crustal stress around major impact basins is likely explained by a crustal extrusion after impact followed by a moho uplift

a viscous to brittle transition in eruptions through clay suspensions volcanic lakes are often associated with active geothermal circulation mineral alteration and precipitation each of which can complicate the analysis of shallow magma physics geophysical signals and chemical signals the rheology of the lake and associated hydrothermal system affects the eruptive activity as bubbles ascend and burst through the lake producing distinct ejection behavior we investigate such phenomena by conducting scaled experiments in which heated water clay suspensions are decompressed rapidly from relevant pressures after a jet phase of expanding vapor the suspensions break up into ejecta that are either angular or droplet geometry we parameterize these regimes and find a universal clay volume fraction of 0 28 below which the ejecta are form droplets and above which the ejecta are angular we propose a regime diagram for optical observations of active lakes which allows rheological characterization and informs volcanic monitoring

lithospheric stress strain and displacement changes from grace fo time variable gravity case study for sar e pol zahab earthquake 2018 temporal variations in the earth s gravity field can be used for monitoring of lithospheric deformations the network of continuously operating gravity stations is required for this purpose but a global coverage by such network is currently extremely sparse temporal variations in long wavelength part of the earth s gravity field have been however observed by two satellite missions namely the gravity recovery and climate experiment grace and the grace follow on grace fo these satellite gravity observations can be used to study long wavelength deformations of the lithosphere consequently considering the lithosphere as a spherical elastic shell and solving the partial differential equation of elasticity for it the stress strain and displacement inside the lithosphere can be estimated the lower boundary of this shell is assumed to be stressed by mantle convection which has a direct relation to the earth s gravity field according to runcorn s theory changes in gravity field lead to changes in the sub lithospheric stress and the stress propagated throughout the lithosphere in this study we develop mathematical models in spherical coordinates for describing the stress propagation from the sub lithosphere through the lithosphere we then organise a system of observation equations for finding a special solution to the boundary value problem of elasticity in the way that provides a stable solution in contrast models presented in previously published studies are ill posed furthermore we use constants of the solution determined from the boundary stresses to determine the strain and displacements leading to these stresses while in previous studies only the stress has been considered according to rheological properties of the lithosphere we demonstrate a practical applicability of this theoretical model to estimate the stress strain redistribution caused by the sar e pol zahab 2018 earthquake in iran by using the grace fo monthly solutions

resolution limits of fluid overpressures from mineralogy porosity and sonic velocity variations in north sea mudrocks pore pressure identification in mudrocks largely relies on indirect methods including analyses of sonic velocity data the magnitude of pore pressure changes that can be resolved from such data depends on the impact of fluid pressures on well log responses and on the velocity responses to lithological variations that could be mistaken for indications of pore pressure changes analysis of relationships between sonic velocity mineralogy and porosity from 11 north sea wells demonstrated porosity variations of up to 5 from the mean value in mudrocks at any given depth between 1 and 3 5 km 3300 11 500 ft comparisons of these wells with seven wells from the norwegian sea demonstrated that the sonic velocity and density variations are influenced by the content of silica with a biogenic origin but that they can otherwise not in general be related to mineralogy and pore pressure variations the sonic velocity deviations from the mean value are of the same magnitude as those that would be caused by porosity changes of 8 in mudrocks of uniform lithology furthermore the deviations are not significantly reduced when the velocity data are compensated for changes in density porosity detection limits for mechanically compacting mudrocks that are at their maximum effective stress are equivalent to static mud density changes of 0 1 0 15 g cm3 these numbers apply to mudrocks with similar lithological variability to those analyzed in our data set but should be much higher when significant amounts of silica with a biogenic origin are present the impact of effective stress changes on the sonic velocities of chemically compacting as well as of mechanically unloading rocks is much less because small or no porosity changes follow from the effective stress changes in such cases the velocity responses for mechanically unloading and stress insensitive chemical compaction can only be identified from four dimensional time lapse seismic data unless the rocks are close to the fracturing limit these observations imply that reasonably accurate velocity based pore pressure identification in north sea mudrocks requires a knowledge of local mudrock heterogeneity and compaction that cannot easily be obtained as a result sonic velocity data should be treated with considerable caution in pore pressure evaluation

an experimental facility for the investigation of magma fragmentation by rapid decompression a special experimental facility has been developed to investigate the fragmentation of vesicular magma undergoing rapid decompression the facility operates in a regime similar to that of shock tubes and at temperatures up to 950 c and pressures up to 200 bar cylindrical samples diameter ca 17 mm length ca 50 mm undergo rapid decompression in a high temperature high pressure section of the facility following the disruption of a diaphragm separating that section from a low pressure low temperature section actual vesicular magma samples have been experimentally fragmented at elevated temperatures and pressures corresponding to those observed during explosive volcanic eruptions and the resulting pyroclastics have been photographically resolved in flight and collected for physical characterization the results of these experiments show that the rapid decompression of highly viscous vesicular magma can generate pyroclastic ejecta via rapid and complete fragmentation of magma at high temperature this new fragmentation facility is presented as a tool for experimental volcanology under well constrained conditions

on gravity inversion by no topography and rigorous isostatic gravity anomalies we discuss some theoretical aspects and practical consequences of using traditional versus new rigorous formulations of the bouguer and isostatic gravity anomalies disturbances in principle the differences between these two concepts are in the definition of the so called secondary indirect topographic effect site on the gravity data although we follow the tradition to call this effect site we show that it is formally a direct topographic effect dite needed to remove all topographic signal but in practice not regarded as such consequently there is a need for a no topography gravity anomaly which removes all topographic effects leaving the below crust earth transparent for gravity inversion similarly a rigorous isostatic gravity anomaly includes also a compensation effect for the site by using a simple topographic model we confirm a theoretically found ratio of 2 n 1 between the magnitudes of the site and dite by wavelength spherical harmonic degree n both for the bouguer and isostatic gravity anomalies finally global gravity inversions are applied by utilizing the vening meinesz moritz isostatic model to determine the moho geometry using the bouguer gravity disturbances anomalies and the no topography gravity anomalies and the results are compared the numerical results confirm our theoretical findings that the bouguer gravity disturbances and the no topography gravity anomalies provide very similar results a comparison of these gravimetrically computed moho depths with the crust1 0 seismic model shows rms agreements of 4 3 and 4 5 km respectively this is a significant improvement when compared to the moho result obtained by using the bouguer gravity anomalies yielding the rms difference of 7 3 km for the crust1 0 model these results confirm a theoretical deficiency of the classical definition of the bouguer and isostatic gravity anomalies which do not take into consideration the site effects on the topography and its compensation

geoid anomalies in a dynamic earth in order to obtain a dynamically consistent relationship between the geoid and the earth s response to internal buoyancy forces we have calculated potential and surface deformation love numbers for internal loading these quantities depend on the depth and harmonic degree of loading they can be integrated as green functions to obtain the dynamic response due to an arbitrary distribution of internal density contrasts spherically symmetric self gravitating flow models are constructed for a variety of radial newtonian viscosity variations and flow configurations including both whole mantle and layered convection we demonstrate that boundary deformation due to internal loading reaches its equilibrium value on the same time scale as postglacial rebound much less than the time scale for significant change in the convective flow pattern by calculating relaxation times for a series of spherically symmetric viscous earth models for uniform mantle viscosity the geoid signature due to boundary deformations is larger than that due to internal loads resulting in net negative geoid anomalies for positive density contrasts geoid anomalies from intermediate wavelength density contrasts are amplified by up to an order of magnitude geoid anomalies are primarily the result of density contrasts in the interior of convecting layers density contrasts near layer boundaries are almost completely compensated layered mantle convection results in smaller geoid anomalies than mantle wide flow for a given density contrast viscosity stratification leads to more complicated spectral signatures because of the sensitivity of the dynamic response functions to model parameters forward models for the geoid can be used to combine several sources of geophysical data e g subducted slab locations seismic velocity anomalies surface topography to constrain better the structure and viscosity of the mantle

flow in the mantle inferred from the low degree harmonics of the geopotential summary flow in the earth s mantle is inferred from continental drift its distribution may be obtained from the pattern of zones of compression and tension in the earth s crust and from the low harmonics of the geopotential the origin of the flow is held to be convection the depth to which the flow extends will be discussed the relation of the flow to the earthquake mechanism will be considered