the crystallization of lava lakes thermal convection driven by the kinetic undercooling of solidification can produce compositional and textural variations in an initially uniform lava flow that is cooled predominantly from above we show that such kinetic undercooling is sufficient to drive convection at high rayleigh numbers even in shallow lava lakes we then employ a theoretical model that includes the effects of convection at high rayleigh number to make predictions of the complete evolution of the lava we show that predictions of the rate of growth of solid crust at the surface of the lake do not differ much from predictions made by ignoring effects due to convection however convective motions do shorten the time for complete solidification because when they are driven by kinetic undercooling there is internal crystallization in addition to that occurring near the cooled upper boundary of the lava this important effect which is absent from purely conductive models predicts stratification and zonation of the lava in agreement with field observations we also examine the effect of taking into account the heat transfer to the country rock below the lava lake and determine how the evolution of the lava lake varies with the viscosity of the lava

fluid and melt pathways in the central chilean subduction zone near the 2010 maule earthquake 35 36 s as inferred from magnetotelluric data

field temperature measurements at erta ale lava lake ethiopia the shield volcano erta ale situated in the danakil depression ethiopia is known for its active lava lake in february 2001 our team visited this lake located inside an 80 m deep pit to perform field temperature measurements the distribution and variation of temperature inside the lake were obtained on the basis of infrared radiation measurements performed from the rim of the pit and from the lake shores the crust temperature was also determined from the lake shores with a thermocouple to calibrate the pyrometer we estimated an emissivity of the basalt of 0 74 from this experiment through the application of the stefan boltzmann law we then obtained an estimate of the total radiative heat flux constrained by pyrometer measurements of the pit and visual observations of the lake activity taking into account the atmospheric convective heat flux the convected magma mass flux needed to balance the energy budget was subsequently derived and found to represent between 510 and 580 kg s 1 the surface circulation of this mass flux was also analyzed through motion processing techniques applied to video images of the lake electronic supplementary material to this paper can be obtained by using the springer link server located at http dx doi org 10 1007 s00445 002 0224 3

strombolian explosions 1 a large bubble breaking at the surface of a lava column as a source of sound strombolian activity consists of a series of explosions caused by the breaking of large overpressurized bubbles at the surface of the magma column acoustic pressure has been measured for 36 explosions at stromboli we propose that sound is generated by the vibration of the bubble before it bursts oscillations are driven by an initial overpressure inside the bubble assumed to be initially at rest just below the magma air interface inertia effects cause the bubble to overshoot its equilibrium radius then the bubble becomes underpressurized and contracts because of gas compressibility these oscillations are only slightly damped by viscous effects in the magma layer above the bubble the bubble cannot complete more than one cycle of vibration because of instabilities developing on the magma layer that lead to its breaking near the minimum radius assuming a simple geometry we model this vibration and constrain the radius and length of the bubble and the initial overpressure by fitting a synthetic waveform to the measured acoustic pressure the fit between synthetic and observed waveforms is very good both for frequency 60 rad s 1 and amplitude the initial bubble radius is 1 m and the length varies between several and a few tens of meters from the initial overpressure approximately 105 pa we calculate the maximum radial velocity of ejecta 30 m s 1 the generally good agreement between data and predictions of our model leads us to suggest that acoustic measurements are a powerful tool for the understanding of eruption dynamics

decomposition location and persistence of seismic signals recovered from continuous tremor at erta ale ethiopia abstract subband decomposition and reconstruction sdr provides a quantitative semi automated rapid means of analyzing volcanic tremor which can recover signals that dominate different parts of the frequency spectrum while preserving polarization and phase here applied to data from a two week experiment at erta ale in nov dec 2003 we demonstrate that the sdr method can identify signals corresponding to source processes active in several different physical regions of the erta ale caldera different recovered signals suggest that the continuous tremor at erta ale is a composite of several seismic sources including conduit resonance bubble bursting phenomena and degassing at fumaroles these signals are seen throughout the 2003 experiment suggesting that several competing source models of volcanic tremor can produce signals at the same volcanic conduit system simultaneously

4d velocity of strombolian eruptions and man made explosions derived from multiple doppler radar instruments abstract we used a novel system of three continuous wave doppler radars to successfully record the directivity of i strombolian explosions from the active lava lake of erebus volcano antarctica ii eruptions at stromboli volcano italy and iii a man made explosion in a quarry erebus volcano contains a convecting phonolite lava lake presumably connected to a magma chamber at depth it is one of the few open vent volcanoes that allow a direct observation of source processes during explosions its lava lake is the source of frequent violent strombolian explosions caused by large gas bubbles bursting at the lake surface the exact mechanism of these bubble bursts is unclear as is the mechanism of the creation of the infrasound signal accompanying the explosions we use the doppler radar data to calculate the directivity of strombolian eruptions at erebus this allows us to derive information about the expected type of infrasound source pattern i e the role of a dipole in addition to the monopole signature and the physical structure of the volcano we recorded 10 large explosions simultaneously with three radars enabling us to calculate time series of 3d directivity vectors i e effectively 4d which describe the direction of preferred expansion of the gas bubble during an explosion such directivity information allows a comparison to dipole infrasound radiation patterns recorded during similar explosions only a few weeks later video observations of explosions support our interpretation of the measurements we conclude that at erebus the directivity of explosions is mainly controlled by random processes since the geometry of the uppermost conduit is assumed to have a large effect on the directivity of explosions the results suggest a largely symmetrical uppermost conduit with a vertical axis of symmetry for infrasound recordings a significant dipole signature can be expected in addition to the predominant monopole signature

conduit dynamics and post explosion degassing on stromboli a combined uv camera and numerical modeling treatment recent gas flux measurements have shown that strombolian explosions are often followed by periods of elevated flux or gas codas with durations of order a minute here we present uv camera data from 200 events recorded at stromboli volcano to constrain the nature of these codas for the first time providing estimates for combined explosion plus coda so2 masses of 18 225 kg numerical simulations of gas slug ascent show that substantial proportions of the initial gas mass can be distributed into a train of daughter bubbles released from the base of the slug which we suggest generate the codas on bursting at the surface this process could also cause transitioning of slugs into cap bubbles significantly reducing explosivity this study is the first attempt to combine high temporal resolution gas flux data with numerical simulations of conduit gas flow to investigate volcanic degassing dynamics

grain growth kinetics in olivine aggregates abstract grain growth kinetics were studied in hot pressed fine grained olivine aggregates experimental conditions include pressures of 0 1 mpa 300 mpa and 1 gpa and temperatures of 1473 1573 and 1673 k with or without the presence of water at the initial stage of grain growth growth of larger grains occurs rapidly consuming smaller grains which results in significant pore entrapment almost homogeneous grain size distribution is established in this process after this stage grain size distribution remains almost homogeneous in most cases it was found that the presence of water enhances the grain growth kinetics when the amount of water is limited however when the amount of water is large the water containing pores significantly inhibit the grain boundary migration therefore the enhancement of grain growth due to the presence of water is due to the role of dissolved water in olivine rather than the mass transport through water abnormal grain growth was found in some samples in these cases normal grain growth was inhibited by the presence of a significant amount of water filled pores or grains with significantly larger size than the rest occurred in the starting materials in 0 1 mpa runs significant porosity developed and the grain growth rates were significantly lower than in those at high confining pressures these results show that both water and pores have important effects on grain growth kinetics in olivine when the effects of pores or secondary phases in inhibiting grain growth are unimportant the grain growth rate in olivine is very fast growth to 100 m size will occur in 102 to 103 hours at 1500 k thus fine grain size in olivine will be maintained only for a short geological time unless the grain boundaries are effectively pinned by secondary particles

decadal persistence of cycles in lava lake motion at erebus volcano antarctica article history studies of erebus volcano s active lava lake have shown that many of its observable properties gas composition surface motion and radiant heat output exhibit cyclic behaviour with a period of 10 min we investigate the multi year progression of the cycles in surface motion of the lake using an extended but intermittent dataset of thermal infrared images collected by the mount erebus volcano observatory between 2004 and 2011 cycles with a period of 5 18 min are found to be a persistent feature of the lake s behaviour and no obvious long term change is observed despite variations in lake level and surface area the times at which gas bubbles arrive at the lake s surface are found to be random with respect to the phase of the motion cycles suggesting that the remarkable behaviour of the lake is governed by magma exchange rather than an intermittent flux of gases from the underlying magma reservoir

ground based thermal imaging of lava lakes at erebus volcano antarctica abstract mount erebus a large intraplate stratovolcano dominating ross island antarctica hosts the world s only active phonolite lava lakes the main manifestation of activity at erebus volcano in december 2004 was as the presence of two convecting lava lakes within an inner crater the long lived ray lake 1400 m2 in area was the site of up to 10 small strombolian eruptions per day a new but short lived 1000 1200 m2 lake formed at werner vent in december 2004 sourced by lava flowing from a crater formed in 1993 by a phreatic eruption we measured the radiative heat flux from the two lakes in december 2004 using a compact infrared ir imaging camera daily thermal ir surveys from the main crater rim provide images of the lava lake surface temperatures and identify sites of upwelling and downwelling the radiative heat outputs calculated for the ray and werner lakes are 30 35 mw and 20 mw respectively we estimate that the magma flux needed to sustain the combined heat loss is 250 710 kg s 1 that the minimum volume of the magma reservoir is 2 km3 and that the radius of the conduit feeding the ray lake is 2 m

sustaining persistent lava lakes observations from high resolution gas measurements at villarrica volcano chile active lava lakes as the exposed upper part of magmatic columns are prime locations to investigate the conduit flow processes operating at active degassing volcanoes persistent lava lakes require a constant influx of heat to sustain a molten state at the earth s surface several mechanisms have been proposed to explain how such heat transfer can operate efficiently these models make contrasting predictions with respect to the flow dynamics in volcanic conduits and should result in dissimilar volatile emissions at the surface here we look at high frequency so2 fluxes plume composition thermal emissions and aerial video footage from the villarrica lava lake in order to determine the mechanism sustaining its activity we found that while fluctuations are apparent in all datasets none shows a stable periodic behaviour these observations suggest a continuous influx of volatiles and magma to the villarrica lava lake we suggest that ascending volatile rich and descending degassed magmas are efficiently mixed within the volcanic conduit resulting in no clear periodic oscillations in the plume composition and flux we compare our findings to those of other lava lakes where equivalent gas emission time series have been acquired and suggest that gas flux magma viscosity and conduit geometry are key parameters determining which flow mechanism operates in a given volcanic conduit the range of conduit flow regimes inferred from the few studied lava lakes gives a glimpse of the potentially wide spectrum of conduit flow dynamics operating at active volcanoes

cyclic degassing of erebus volcano antarctica field observations have previously identified rapid cyclic changes in the behaviour of the lava lake of erebus volcano in order to understand more fully the nature and origins of these cycles we present here a wavelet based frequency analysis of time series measurements of gas emissions from the lava lake obtained by open path fourier transform infrared spectroscopy this reveals i a cyclic change in total gas column amount a likely proxy for gas flux with a period of about 10 min and ii a similarly phased cyclic change in proportions of volcanic gases which can be explained in terms of chemical equilibria and pressure dependent solubilities notably the wavelet analysis shows a persistent periodicity in the co2 co ratio and strong periodicity in h2o and so2 degassing the peaks of the cycles defined by maxima in h2o and so2 column amounts coincide with high co2 co ratios and proportionally smaller increases in column amounts of co2 co and ocs we interpret the cycles to arise from recharge of the lake by intermittent pulses of magma from shallow depths which degas h2o at low pressure combined with a background gas flux that is decoupled from this very shallow magma degassing

stability and instability of quiescently active volcanoes the case of masaya nicaragua quiescently active volcanoes are enigmatic due to their restlessness but lack of eruptive activity i present a model of coupled conduit convection and foam accumulation to explain degassing behavior of masaya an active volcano in nicaragua that is currently emitting large amounts of gas but not erupting gas rich magma is transported through a conduit 2 6 m in radius and then released into a shallow reservoir the magma is degassed in the reservoir and forms a foam 1 3 m thick at the top with bubbles 20 60 m in diameter the foam layer is stable because the input of gas into the foam is balanced by gas release through a vent to the surface if the foam layer is destabilized the volcano can erupt explosively the most likely cause of foam destabilization is a large injection of volatile rich magma from deeper levels into the shallow conduit reservoir system thereby increasing magma fluxes and gas fluxes exsolving large gas bubbles and reducing surface tension of the magma

correlations between so2 flux seismicity and outgassing activity at the open vent of villarrica volcano chile 1 the characteristics of the open vent activity of villarrica volcano chile were studied in detail by integrating visual observations of the lava lake analysis of the seismic tremor and measurements of so2 flux the outgassing activity comprises a persistent gas plume emission from the bottom of the crater as well as frequent explosive events three main styles of bubble bursting were identified at the surface of the active lava lake seething magma small short lived lava fountains and strombolian explosions seething magma consists of continual burst of relatively small bubbles a few meters in diameter with varying strength over the entire surface of the lava lake small lava fountains seen as a vigorous extension of seething magma commonly have durations of 20 120 s and reach 10 40 m high above the lava lake correlations between seismicity and visual observations indicate that the seismic tremor is mostly caused by the explosive outgassing activity furthermore for different periods between 2000 and 2006 during which the activity remained comparable the real time seismic amplitude measurement system rsam and so2 emission rates show a very good correlation higher so2 emissions appeared to be related to higher levels of the lava lake stronger bubble bursting activity and changes in the morphology and texture of the crater floor background low levels of activity correspond to a lava lake located 80 m below the crater rim small and or blocky morphology of the roof seismic amplitude rsam lower than 25 units few volcano tectonic earthquakes and daily averages of so2 emissions lower than 600 mg d

sound produced by the rapidly inflating santiaguito lava dome guatemala 1 vertical inflation of the caliente lava dome at santiaguito guatemala occurs coincidentally with the onset of explosive eruptions and produces infrasound that is generally peaked between 0 5 and 2 hz with amplitude of up to 5 pa at 1 km from vent inflation of up to 1 m progresses rapidly within 2 s and can encompass the entire surface of the 200 m diameter dome we use particle image velocimetry to quantify the time history of dome uplift and demonstrate that deflection of a volcano s solid surface can generate infrasound waves with amplitudes of a few pa the volumetric displacement history is used to model linear volumetric acoustic sources both compact and of finite extent synthetic waveforms match recorded infrasound in terms of timing and frequency content amplitude fit is very good for a station located 3 3 km from the vent and less good for stations shielded by near vent topography

video and seismic observations of strombolian eruptions at erebus volcano antarctica abstract between 1986 and 1990 the eruptive activity of erebus volcano was monitored by a video camera with on screen time code and recorded on video tape corresponding seismic and acoustic signals were recorded from a network of 6 geophones and 2 infrasonic microphones two hundred strombolian explosions and three lava flows which were erupted from 7 vents were captured on video in december 1986 the strombolian eruptions ejected bombs and ash in november 1987 large bubble bursting strombolian eruptions were observed the bubbles burst when the bubble walls thinned to 20 cm explosions with bomb flight times up to 14 5 s were accompanied by seismic signals with our local size estimate unified magnitudes m u up to 2 3 explosions in pools of lava formed by flows in the inner crater were comparatively weak changes in eruptive activity occurred in 1987 when the lava lake was buried by a landslide from the crater wall two new vents formed and seismic activity peaked as the landslide was ingested lava flows from a vent on the eastern side of the crater formed small lakes and a vent on the north began to flow in 1990 by december 1990 the entire floor of the inner crater was buried by up to 20 000 m 3 of new lava different families of nearly identical eruption earthquakes occurred each year whose foci were contained within small shallow volumes immediately after several bubble bursting eruptions clear views of the empty vent were recorded the vent was seen to taper downwards to about half its diameter at the bottom our observations confirm models of strombolian eruptions suggesting they arise from gas slugs rising through a conduit into a flared vent

increased flow speed on a large east antarctic outlet glacier caused by subglacial floods large ice streams and outlet glaciers drain greenland and antarctica an observed acceleration of ice velocity in one of these outlet glaciers byrd glacier east antarctica coincides with a large water discharge from two subglacial lakes allowing direct attribution of the change in glacier dynamics to the water drainage network beneath the ice

the basal stress distribution of ice stream e antarctica inferred by control methods the irregular spatial distribution and velocity independence of basal friction derived from landsat measured surface velocity suggests that ice stream flow is not controlled by the properties of a deformable basal till alone rigid bedrock substrata may contact the base of the ice stream in small 100 km2 areas where the velocity field displays strong vorticity and where the ice stream surface appears rumpled in landsat images

stability of lava lakes a physical model of a generic lava lake system is developed we derive the requisite conditions for the existence of an equilibrium lava lake in which magmastatic pressure at the base of the conduit balances the pressure in the underlying magmatic reservoir the stability of this lava lake system is tested by investigating the response of the system to perturbation we develop a graphical method based on the system s pressure depth profile to predict the subsequent behaviour of the system despite the simplicity of the modelled system we find a broad behavioural spectrum initially the rise of bubbles through the magma is ignored in this case both stable long lived lava lakes and unstable lakes that are prone to sudden draining are predicted the stability of the system is shown to be controlled by lake conduit geometry the solubility and gas expansion laws and the magma s volatile content we show that an unstable lake must collapse to a new stable equilibrium subsequent recharge of the system by for example conduit overturn would promote a return to the original equilibrium giving rise to cyclic behaviour such a mechanism is consistent with lava lake behaviour during the 1983 1984 pu u o o eruption of kilauea when the rise of bubbles through the magma is considered our model predicts that stable lakes must drain over time we therefore deduce that persistently degassing stable lava lakes such as those observed at mt erebus antarctica and mauna ulu kilauea hawaii must have an effective conduit convection mechanism or an exogenous supply of bubbles from depth

bi directional flows in constrained systems we consider the exchange flow of relatively dense viscous fluid in a container connected by a vertical pipe to a container beneath it initially full of relatively light fluid a non dimensional value for the flux of dense fluid down the tube is determined experimentally as a function of the ratio of the two viscosities and the reynolds number the experimental data are satisfactorily collapsed using dimensional analysis and balancing buoyancy inertial and viscous forces as appropriate a theoretical analysis assuming steady axisymmetric motion captures a considerable part but not all of the processes involved the paper discusses quantitative applications of the results to the movement of magma in volcanic conduits the concepts indicate how bi directional convection in the conduit between a lava lake and a magma reservoir deep in the crust is the essential ingredient in the explanation of the long standing problem that the amount of degassing of sulphur dioxide from a lava lake in a volcanic crater can exceed by many orders of magnitude that consistent with the amount of lava solidified in the crater movies are available with the online version of the paper

modeling lava lake heat loss rheology and convection 1 measurements at erta ale s lava lake and theoretical equations for lake rheology density driven convection and thermally driven plume ascent allow the constraint of lake dynamics cooling and crystallization expected from surface heat losses imply a viscosity increase from 150 pa s to 300 1800 pa s for cooled surface layers convection is expected to proceed vigorously under low viscosity conditions driving rapid 0 1 0 4 m s 1 surface motions and sluggishly under moderate to high viscosity conditions to drive slower motions 0 08 m s 1 convection is likely driven by small 6 kg m 3 density differences where surface cooling can influence lake rheology and explain variable rates of surface convective motion

magma dynamics crystallization and chemical differentiation of the 1959 kilauea iki lava lake hawaii revisited abstract using constraints from an extensive database of geological and geochemical observations along with results from fluid mechanical studies of convection in magma chambers we identify the main physical processes at work during the solidification of the 1959 kilauea iki lava lakes in turn we investigate their quantitative influence on the crystallization and chemical differentiation of the magma and on the development of the internal structure of the lava lake in contrast to previous studies vigorous stirring in the magma driven predominately by the descent of dense crystal laden thermal plumes from the roof solidification front and the ascent of buoyant compositional plumes due to the in situ growth of olivine crystals at the floor is predicted to have been an inevitable consequence of very strong cooling at the roof and floor the flow is expected to have caused extensive but imperfect mixing over most of the cooling history of the magma producing minor compositional stratification at the roof and thermal stratification at the floor the efficient stirring of the large roof cooling is expected to have resulted in significant internal nucleation of olivine crystals which ultimately settled to the floor additional forcing due to either crystal sedimentation or the ascent of gas bubbles is not expected to have increased significantly the amount of mixing in addition to convection in the magma circulation driven by the convection of buoyant interstitial melt in highly permeable crystal melt mushes forming the roof and the floor of the lava lake is envisaged to have produced a net upward flow of evolved magma from the floor during solidification in the floor zone mush convection may have caused the formation of axisymmetric chimneys through which evolved magma drained from deep within the floor into the overlying magma and potentially the roof we hypothesize that the highly evolved pipe like vertical olivine rich bodies vorbs bull volcanol 43 1980 675 observed in the floor zone of the lake are fossil chimneys in the roof zone buoyant residual liquid both produced at the roof solidification front and gained from the floor as a result of incomplete convective mixing is envisaged to have percolated or leaked into the overlying highly permeable cumulate displacing less buoyant interstitial melt downward the results from rayleigh fractionation type models formulated using boundary conditions based on a quantitative understanding of the convection in the magma indicate that most of the incompatible element variation over the height of the lake can be explained as a consequence of a combination of crystal settling and the extensive but imperfect convective mixing of buoyant residual liquid released from the floor solidification front the remaining chemical variation is understood in terms of the additional influences of mush convection in the roof and floor on the vertical distribution of incompatible elements although cooling was concentrated at the roof of the lake the floor zone is found to be thicker than the roof zone implying that it grew more quickly the large growth rate of the floor is explained as a consequence of a combination of the substantial sedimentation of olivine crystals and more rapid in situ crystallization due to both a higher liquidus temperature and enhanced cooling resulting from imperfect thermal and chemical mixing

methods of determining the rheological properties of magmas at sub liquidus temperatures non vesicular basaltic melts behave as newtonian fluids at temperatures above their liquidus and their viscosities can be calculated using a method developed by shaw 1972 and bottinga and weill 1972 because many igneous processes involve the flow of silicates at sub liquidus temperatures numerous attempts have been made to calculate the interactive effects of the crystal phase however current methods are appropriate only for relatively low crystal concentrations and they assume newtonian behaviour we argue that this assumption is invalid when bubble or crystal concentrations exceed 30 at higher concentrations factors other than concentration become increasingly important these include particle shape and size distribution a method which takes these factors into account is tested on a range of suspensions including magmas at sub liquidus temperatures and rheological properties calculated using this method agree closely with the measured values we also demonstrate that an equation which was introduced to explain large differences in measured apparent viscosities during the cooling and crystallisation of mount st helens dacite murase et al 1985 and which is currently used to calculate the rheological properties of crystallising lavas generates viscosities which may be in error by several orders of magnitude this difference is argued to be caused by a combination of factors including the ten orders of magnitude range in the strain rates utilised during the mount st helens measurements causing orders of magnitude difference in the resulting apparent viscosities rheological data on crystallising silicic melts are reinterpreted taking into account the non newtonian rheology of the magmas and changes in activation energy of flow during crystallisation

geometrical force balance in glaciology 1 the analytical force balance traditionally used in glaciology relates gravitational forcing to ice surface slope for sheet flow and to ice basal buoyancy for shelf flow it is unable to represent stream flow as a transition from sheet flow to shelf flow by having gravitational forcing gradually passing from being driven by surface slope to being driven by basal buoyancy downslope along the length of an ice steam this is a serious defect because ice streams discharge up to 90 of ice from ice sheets into the sea the defect is overcome by using a geometrical force balance that includes basal buoyancy represented by the ratio of basal water pressure to ice overburden pressure as a source of gravitational forcing when combined with the mass balance the geometrical force balance provides a holistic approach to ice flow in which resistance to gravitational flow must be summed upstream from the calving front of an ice shelf this is not done in the analytical force balance and it provides the ice thinning rate required by gravitational collapse of ice sheets as interior ice is downdrawn by ice streams

slug stability in flaring geometries and ramifications for lava lake degassing long lived persistently active lava lakes are rare but have been studied extensively because they may provide a natural laboratory for studying magmatic convection in basaltic and other low viscosity volcanoes however lava lakes differ from other volcanic systems in various ways particularly geometry the presence of the lava lake requires a sudden and significant flaring of the conduit the goal of this paper is to advance our understanding of the effect that a sudden increase in the conduit width has on the stability of large gas slugs and bubbles ascending from the plumbing system we investigate this question by linking analog laboratory experiments to direct numerical simulations in two dimensions we find that the rapid change in the geometry associated with the transition from the conduit to the lava lake causes large gas slugs to break up the nondimensional regime over which we observe the breakup depends sensitively on the flare angle of the lava lake and the importance of inertial effects in the flow implying that degassing related eruptive activity at open system volcanoes is sensitive to the upper conduit and lake geometries applying this idea to the likely geometries and viscosities of actual lava lakes our study suggests that all lava lakes except erebus fall well within the regime where large gas slugs are prone to breakup

infrasonic tracking of large bubble bursts and ash venting at erebus volcano antarctica abstract explosive degassing at erebus volcano produces infrasound that can be used to locate characterize and quantify eruptive activity from multiple vents we use a three element distributed microphone network to pinpoint eruption sources and track the activity at the prominent vents through time eruptive mechanisms for both source types are analyzed in conjunction with the telemetered time synced video imagery we identify two commonly active vents corresponding to the large often 10 m diameter bubble bursts at the free surface of a persistent phonolitic lava lake ray lake and the less frequent ash rich eruptions from a constricted vent active vent located 80 m from the lava lake during a 3 month study interval from 6 january to 13 april 2006 we identified and mapped more than 350 eruptive sources from the lava lake and 20 sources from the ash vent lava lake events are characterized by high amplitude infrasonic transients that reflect rapid less than a few s acceleration and rupture of magma bubble films followed by an explosion of pressurized gases precise infrasonic localization of the lava lake events to accuracies of a few m indicates variable bubble source locations across a 40 by 50 m region spanning the lava lake spatial variability is corroborated by the video data in contrast degassing from the ash vent produces longer duration tens of s lower amplitude transients that reflect diminished impulsivity and an extended degassing duration features that are corroborated by video because infrasound networks can operate continuously in all weather conditions and during both diurnal and seasonal polar darkness and are easily incorporated into automatic processing they significantly contribute to the completeness and quantification of eruption catalogues for erebus

influence of grain size evolution on convective instability 1 grain size one of the most important microstructural properties of materials evolves during creep deformation to minimize the free energy of polycrystalline aggregates we apply a model of grain size evolution to the study of convective instability of cooling boundary layers the grain size evolution model is coupled to a composite rheology where the deformation rate is the sum of that due to dislocation creep and grain size sensitive diffusion creep the onset of convection is sensitive to grain growth rates and the initial grain size the formation of convective instabilities is enhanced by stresses induced by plate motions therefore small scale convection is more likely to occur beneath fast moving plates in finite amplitude convection grain size evolution leads to high viscosity in regions where convective stresses are low and can induce viscosity contrasts exceeding one order of magnitude such viscosity contrasts are sufficient to influence the dynamics of convection often leading to domains which remain isolated from the well mixed convecting fluid a composite viscosity including diffusion creep which has a lower activation energy than dislocation creep reduces the effective temperature dependence of viscosity

fluid migration in the mantle wedge influence of mineral grain size and mantle compaction mineral grain size in the mantle affects fluid migration by controlling mantle permeability the smaller the grain size the less permeable the mantle is mantle shear viscosity also affects fluid migration by controlling compaction pressure high mantle shear viscosity can act as a barrier to fluid flow here we investigate for the first time their combined effects on fluid migration in the mantle wedge of subduction zones over ranges of subduction parameters and patterns of fluid influx using a 2 d numerical fluid migration model our results show that fluids introduced into the mantle wedge beneath the forearc are first dragged down dip by the mantle flow due to small grain size 1 mm and high mantle shear viscosity that develop along the base of the mantle wedge increasing grain size with depth allows upward fluid migration out of the high shear viscosity layer at sub arc depths fluid introduced into the mantle wedge at post arc depths migrate upwards due to relatively large grain size in the deep mantle wedge forming secondary fluid pathways behind the arc fluids that reach the shallow part of the mantle wedge spread trench wards due to the combined effect of high mantle shear viscosity and advection by the inflowing mantle and eventually pond at 55 65 km depths these results show that grain size and mantle shear viscosity together play an important role in focusing fluids beneath the arc

magmatic degassing at erta ale volcano ethiopia abstract here we report measurements of the chemical composition and flux of gas emitted from the central lava lake at erta ale volcano ethiopia made on 15 october 2005 we determined an average so 2 flux of 0 69 0 17 kg s 1 using zenith sky ultraviolet spectroscopy of the plume and molar proportions of magmatic h 2 o co 2 so 2 co hcl and hf gases to be 93 58 3 66 2 47 0 06 0 19 and 0 04 respectively by open path fourier transform infrared ftir spectrometry together these data imply fluxes of 7 3 0 7 0 008 0 03 and 0 004 kg s 1 for h 2 o co 2 co hcl and hf respectively these are the first ftir spectroscopic observations at erta ale and are also some of the very few gas measurements made at the volcano since the early 1970s gerlach t m 1980b investigation of volcanic gas analyses and magma outgassing from erta ale lava lake afar ethiopia journal of volcanology and geothermal research 7 3 4 415 441 we identify significant increases in the proportion of h 2 o in the plume with respect to both co 2 and so 2 across this 30 year interval which we attribute to the depletion of volatiles in magma that sourced effusive eruptions during the early 1970s and or to fractional magma degassing between the two active pit craters located in the summit caldera

gas driven lava lake fluctuations at erta ale volcano ethiopia revealed by modis measurements the long lived lava lake of erta ale volcano ethiopia is remotely monitored by moderate resolution imaging spectroradiometers modis installed on satellites the normalised thermal index nti wright et al remote sens environ 82 135 155 2002 is shown to be proportional to the volume of the lava lake based on visual observations the lava lake s variable level can be plausibly related to a stable foam i e a mixture composed of densely packed non coalescing bubbles in suspension within a liquid this foam is trapped at the top of the magma reservoir and its thickness changes in response to the gas flux feeding the foam being successively turned on and off the temporal evolution of the foam thickness and the resulting variation of the volume of the lava lake is calculated numerically by assuming that the gas flux feeding the foam initially constant and homogeneous since december 9 2002 is suddenly stopped on december 13 2002 and not restarted before may 2003 the best fit between the theoretical foam thickness and the level of the lava lake deduced from the nti provides an estimate of both the reservoir radius 155 170 m and the gas flux feeding the foam 5 5 10 3 7 2 10 3 m 3 s 1 when existing this is in agreement with previous estimates from acoustic measurements bouche et al earth planet sci lett 295 37 48 2010 the very good agreement between the theoretical foam thickness and that deduced from modis data shows for the first time the existence of a regime based on the behaviour of a stable foam whose spreading towards the conduit wide conduit condition can explain the long lived activity our predictive model which links the gas flux at the vent to the foam spreading could potentially be used on any volcano with a long lived activity the underlying gas flux and the horizontal surface area of the magma reservoir can then be deduced by combining modelling to continuous measurements of gas flux the lava lake when high often shows regular rise and fall of its level we have recognised a minimum of 26 very well marked cycles between january 2001 and december 13 2002 corresponding to a typical return time of 10 8 2 3 days and a gas volume of 8 3 105 2 0 105 m 3 this corresponds to a gas volume fraction in the reservoir equal to 0 023 0 063 the yearly gas flux estimated between december 13 2002 and september 27 2004 varies between 2 3 10 6 and 5 9 10 6 m 3 s 1 at the depth of the reservoir the long time series provided by infra red sensors mounted on satellites could be used on any persistent volcano to detect potential periodic variations in the level of lava lakes or lava columns providing that the vent has a funnel shape as often and is sufficiently large

a co2 gas precursor to the march 2015 villarrica volcano eruption we present here the first volcanic gas compositional time series taken prior to a paroxysmal eruption of villarrica volcano chile our gas plume observations were obtained using a fully autonomous multi component gas analyser system multi gas in the 3 month long phase of escalating volcanic activity that culminated into the march 3 2015 paroxysm the largest since 1985 our results demonstrate a temporal evolution of volcanic plume composition from low co2 so2 ratios 0 65 2 7 during november 2014 january 2015 to co2 so2 ratios up to 9 then after the h2o co2 ratio simultaneously declined to 38 in the same temporal interval we use results of volatile saturation models to demonstrate that this evolution toward co2 enriched gas was likely caused by unusual supply of deeply sourced gas bubbles we propose that separate ascent of over pressured gas bubbles originating from at least 20 35 mpa pressures was the driver for activity escalation toward the march 3 climax

acoustic multipole source model for volcanic explosions and inversion for source parameters summary volcanic explosions are accompanied by strong acoustic pressure disturbances in the atmosphere with a proper source model these acoustic signals provide invaluable information about volcanic explosion dynamics far field solutions to volcanic infrasound radiation have been derived above a rigid half space boundary and a simple inversion method was developed based on the half space model acoustic monopole and dipole sources were estimated simultaneously from infrasound waveforms stability of the inversion procedure was assessed in terms of variances of source parameters and the procedure was reliable with at least three stations around the infrasound source application of this method to infrasound observations recorded at tungurahua volcano in ecuador successfully produced a reasonable range of source parameters with acceptable variances observed strong directivity of infrasound radiation from explosions at tungurahua are successfully explained by the directivity of a dipole source model the resultant dipole axis in turn shows good agreement with the opening direction of the vent at tungurahua which is considered to be the origin of the dipole source the method is general and can be utilized to study any monopole dipole or combined sources generated by explosions

focusing of upward fluid migration beneath volcanic arcs effect of mineral grain size variation in the mantle wedge we use numerical models to investigate the effects of mineral grain size variation on fluid migration in the mantle wedge at subduction zones and on the location of the volcanic arc previous coupled thermal grain size evolution t gse models predict small grain size 1 mm in the corner flow of the mantle wedge a down dip grain size increase by two orders of magnitude along the base of the mantle wedge and finer grain size in the mantle wedge for colder slab subduction zones we integrate these t gse modeling results with a fluid migration model in which permeability depends on grain size and fluid flow through a moving mantle matrix is driven by fluid buoyancy and dynamic pressure gradients induced by mantle flow our modeling results indicate that fluids introduced along the base of the mantle wedge beneath the forearc are initially dragged down dip by corner flow due to the small grain size and low permeability immediately above the slab as grain size increases with depth permeability increases resulting in upward fluid migration fluids released beneath the arc and the backarc are also initially dragged down dip but typically are not transported as far laterally before they begin to travel upward as the fluids rise through the backarc mantle wedge they become deflected towards the trench due to the effect of mantle inflow the combination of down dip migration in the forearc and trench ward migration in the backarc results in pathways that focus fluids to the arc this article is protected by copyright all rights reserved

paleowattmeters a scaling relation for dynamically recrystallized grain size during dislocation creep mineral grains often evolve to a stable size dictated by the deformation conditions we suggest that grain size evolution during deformation is determined by the rate of mechanical work provided that other elements of microstructure have achieved steady state and that the dissipation rate is roughly constant then changes in internal energy will be proportional to changes in grain boundary area if normal grain growth and dynamic grain size reduction occur simultaneously then the steady state grain size is determined by the balance of those rates a scaling model using these assumptions and published grain growth and mechanical relations matches stress grain size relations for quartz and olivine rocks with no fitting for marbles the model also explains scatter not rationalized by assuming that recrystallized grain size is a function of stress alone when extrapolated to conditions typical for natural mylonites the model is consistent with field constraints on stresses and strain rates

evidence for a frozen bed byrd glacier antarctica ice thickness computed within the fjord region of byrd glacier on the assumptions that byrd glacier is in mass balance equilibrium and that ice velocity is entirely due to basal sliding are on average 400 m less than measured ice thicknesses along a radio echo profile we consider four explanations for these differences 1 active glacier ice is separated from a zone of stagnant ice near the base of the glacier by a shear zone at depth 2 basal melting rates are some 8 m yr 3 internal shear occurs with no basal sliding in much of the region above the grounding zone or 4 internal creep and basal sliding contribute to the flow velocity in varying proportions above the grounding zone large gradients of surface strain rate seem to invalidate the first explanation computed values of basal shear stress 140 to 200 kpa provide insufficient factional heat to melt the ice demanded by the second explanation both the third and fourth explanations were examined by making simplifying assumptions that prevented a truly quantitative evaluation of their merit nevertheless there is no escaping the qualitative conclusion that internal shear contributes strongly to surface velocities measured on byrd glacier as is postulated in both these explanations

seismic characteristics of variable convection at erta ʻale lava lake ethiopia abstract the active summit lava lake of erta ale volcano ethiopia offers a unique opportunity to study magma convection in february 2002 we collected a multiparametric set of seismic thermal and video data these data indicate that the lake cycled between periods characterized by low 0 01 0 08 m s 1 and high 0 1 0 4 m s 1 convection rates typically lasting tens to hundreds of minutes three seismometers placed around the active crater recorded continuous tremor with a dominant frequency of 2 hz and energy at frequencies from 0 8 to 12 hz here we characterize the seismic signature of each regime by its spectral content wavefield polarization and tremor source location for both regimes the wavefield is mostly rectilinear azimuths and incidence angles are consistent with p waves originating in one of two locations the north edge of the active lava lake or a region 100 150 m ene of the lava lake because both regimes are dominated by a low frequency rectilinearly polarized wavefield we investigate the source location using a method that solves for location and isotropic source power by a weighted least squares amplitude based inversion of seismic data we find that tremor source regions are unique to each convective regime although some location overlap exists when tremor is located in short time windows wavefield composition suggests that the convective phases may share a common source process but their differing locations indicate that either the source region is non stationary or a second source skews the location during the high convective phase tremor polarization and location suggests that the low frequency tremor is caused by bubble coalescence and bursting in a conduit whose surface is the lava lake the higher frequency signal associated with the high convective regime is associated with a scattered more complex wavefield superimposed on the low frequency background tremor caused by bubble bursting and cracking of cooled crust at the lava lake surface

experimental analysis of bubble driven magma motion in the conduit for persistently active open vent volcanoes this study uses analog experiments to understand the role of bubbles in inducing conduit convection for persistently degassing lava lake systems to do so air was fluxed through an initially stagnant column of liquid and the resulting return flow was measured the dynamics suggested by the experimental results is compared with that of quiescent persistently active volcanoes with a focus on eight volcanoes that exhibit summit lava lakes we find that magma flux is a function of combined gas flux conduit size and magma rheology experiments with high gas fluxes through low viscosity liquid took on turbulent characteristics which correspond to a high degree of return flow whereas lower gas fluxes through high viscosity liquids yielded slug flow which corresponded to less return flow we model the magma flux due to bubble ascent and find that gas driven liquid flow can yield faster flow rates than other mechanisms at work in volcanic conduits this can explain the discrepancy between previous estimates of magma flow in conduits and relatively fast lava lake surface velocity observations we show how bubble driven convective flow can work alongside density driven convection and discuss the depths in the conduit where each are likely to dominate the system

effect of solid flow above a subducting slab on water distribution and melting at convergent plate boundaries 1 hydrous fluids derived by dehydration of the downgoing slab at convergent plate boundaries are thought to provoke wet melting in the wedge above the downgoing plate we have investigated the distribution of hydrous fluid and subsequent melt in the wedge using two dimensional models that include solid mantle flow and associated temperature distributions along with buoyant fluid migration and melting solid mantle flow deflects hydrous fluid from their buoyant vertical migration through the wedge melting therefore does not occur directly above the region where hydrous fluids are released from the slab a melting front develops where hydrous fluids first encounter mantle material hot enough to melt wet melting is influenced by solid flow through the advection of fertile mantle material into the wet melting region and the removal of depleted material the region of maximum melting occurs where the maximum flux of water from slab mineral dehydration reaches the wet melting region the extent of melting f and melt production rates increase with increasing convergence rate and grain size due to increased temperatures along the melting front and to increased fractions of water reaching the melting front respectively the position of isotherms above the wet solidus varies with increasing slab dip and thereby also influences f and melt production rates applying the understanding of wet melting from this study to geochemical studies of the aleutians may help elucidate the processes influencing fluid migration and melt production in that region estimates of the timescale of fluid migration seismic velocity variation and attenuation are also investigated

the role of large bubbles detected from acoustic measurements on the dynamics of erta ale lava lake ethiopia the activity at the surface of the lava lake on erta ale volcano ethiopia shows that large bubbles are regularly breaking at a fixed position on the lava lake this is also where the small lava fountains are sometimes produced since this location is likely to be directly above the volcanic conduit feeding the lava lake we have done continuous measurements between march 22 and 26 2003 to understand the degassing of a volcano in permanent activity the bubble size has been first estimated from videos which once combined with the acoustic pressure can constrain the source of the sound the gas volume and overpressure stayed roughly constant between 36 700 m3 and 4 103 1 8 104 pa respectively simultaneous thermal measurements showed regular peaks which occurred when the crust was broken by a large bubble hence gave a direct indication on the typical return time between the bubbles 1 h these spherical cap bubbles had a high reynolds number 4600 20000 therefore a wake periodically unstable formed and detached from the bubble bottom the bubbly wake if the detachment occurs close to the surface can explain the duration of lava fountains measured on the videos the periodic arrival of bubbly wakes which mostly detach from the driving spherical cap within the lava lake could explain the absence of cooling at erta ale erebus antartica villarica chile and nyiragongo democratic republic of congo without invoking a convective downflow of magma in the conduit as previously done

origin of the sound generated by strombolian explosions during strombolian eruptions large bubbles break at the surface of the lava column and produce sound acoustic pressure recorded during several explosions on the eastern vent of stromboli volcano shows a pattern consistent between explosions the well marked oscillation contains only very low frequencies around 7 hz and is followed by a signal containing now both higher frequencies and lower frequency less than 4 hz the generation of sound is explained by the vibration 7 hz and bursting higher frequencies of large bubbles followed by the drainage of magma down in the conduit less than 4 hz the interpretation suggests that 1 the bubble vibrates before bursting and 2 the vibration prior to bursting yields more energy than the bursting itself the radius of bubbles responsible for explosions is determined to be close to 1 meter in agreement with observations of a few conduit sizes

response of subglacial sediments to basal freeze on 2 application in numerical modeling of the recent stoppage of ice stream c west antarctica 1 ross ice streams supply over 90 of the ice volume flowing out of the ross sector of the west antarctic ice sheet wais stoppage of ice stream c isc ca 150 years ago appears to have pushed this sector of wais from negative into positive mass balance joughin and tulaczyk 2002 we propose an explanation for the unsteady behavior of isc using a new numerical ice stream model which includes an explicit treatment of a subglacial till layer when constrained by initial conditions emulating prestoppage geometry dynamics and mass balance of isc the model yields a rapid 100 years stoppage of the main ice stream trunk the stoppage is triggered by basal freeze on which consolidates and strengthens the subglacial till our numerical simulations produce results consistent with a number of existing observations for example continuing activity of the two tributaries of isc the model always yields rapid stoppage unless we specify ice stream width that is smaller than its prestoppage values maximum of 80 km we conjecture that if isc was active for at least a few thousand years before slowdown its width was significantly smaller than today to sustain the long active phase ice stream width is a key control that helps determine whether ice stream flow is sustainable over a long term our work indicates that the recent stoppage of ice stream c could have been part of inherent ice stream cyclicity and it leaves open the possibility that other active ice streams may evolve in the future toward rapid shutdowns

a few remarks on the kinetics of static grain growth in rocks static grain growth is a relatively simple transformation in which grain size increases under driving forces caused by grain and interphase boundary curvature given the relative simplicity of the protocol for grain growth experiments measurements of grain boundary mobility show surprising variations boundary mobilities during grain growth are affected by solute and impurity chemistry chemical fugacity of trace and major elements pore size and number pore fluid chemistry the presence of melts and the presence of solid second phases as well as temperature and pressure all of these factors may exert influence on grain growth of rocks in natural situations and many are also present during the laboratory experiments provided that the necessary kinetics parameters are known bounds may be placed on the interface mobility when pores partial melts or solutes are present to predict the rate of grain growth in natural situations will require improved laboratory data and careful consideration of the thermodynamic conditions likely to be encountered in nature

a global synthesis of lava lake dynamics abstract active lava lakes represent a variety of open vent volcanism in which a sizable body of lava accumulates at the top of the magma column constrained by the vent and or crater geometry the longevity of lava lakes reflects a balancing of cooling and outgassing occurring at the surface and input of hot and gas rich magma from below due to their longevity and relative accessibility lava lakes provide a natural laboratory for studying fundamental volcanic processes such as degassing convection and cooling this article examines all seven lakes that existed at the time of writing in 2018 located in the pacific antarctica africa and south and central america these lakes span all tectonic environments and a range of magma compositions we focus on analysis of the lake surface motion using image velocimetry which reveals both similarities and contrasts in outgassing and lake dynamics when comparing the different lakes we identify two categories of lake behavior organized erta ale nyiragongo k lauea after 2011 and erebus and chaotic villarrica masaya marum this division does not map directly to lake size viscosity gas emission rate or temperature instead when examined together we find that the lakes follow a linear relationship between average surface speed and the ratio of total gas flux to lake surface area this relationship points to the combined importance of both flux and lake size in addition to the total volume of gas emission and suggests that a shared deep mechanism controls the supply of heat and gas to all lakes on the other hand the differences between chaotic and organized lakes highlight the important role of the geometry of the conduit lake transition which superimposes a shallow signal on that of the deep circulation the spatial patterns of surface motion we document suggest that the release of gas bubbles at chaotic lakes is more efficient i e bubbles are less likely to be retained and recycled compared with organized lakes in addition the data presented here indicate that the solidified crust of organized lakes plays a role in regulating convection and outgassing in lava lakes

hydration adjacent to a deeply subducting slab the roles of nominally anhydrous minerals and migrating fluids subduction related transport of water into the mantle has significant dynamical and geochemical implications dehydration of hydrous phases within the slab can introduce water into the transition zone and lower mantle potentially hydrating nominally anhydrous minerals nam and impacting the viscosity and density structure of the mantle over a wide area we present models of fluid transport and mantle hydration in the vicinity of a deeply subducting slab focusing on the fate of water released by deep dehydration reaction in the subducted serpentinized mantle a sharp decrease in water storage capacity across the lower boundary of the transition zone may produce secondary dehydration of hydrated nam leading to precipitation of a hydrous fluid and heterogeneous hydration of the transition zone rapid fluid migration relative to the solid flow field can lead to a broad region of diffuse hydration within the upper mantle wedge and the potential for localized melt regions at the top of the transition zone coincident with fluid pathways slower fluid migration instead implies that the fluid phase can be transported deep into the lower mantle water stabilized in nam and as a free fluid can initiate upwelling within and above the transition zone a less abrupt change in water storage capacity across the base of the transition zone leads to high nam water contents in a channel adjacent to the slab where viscosity is reduced however seismic and electromagnetic observations of hydration in the transition zone are most compatible with a sudden drop of water storage capacity

identification of variable convective regimes at erta ale lava lake abstract during february 17 19 2002 we collected a combined thermal and seismic data set for persistent lava lake activity at erta ale volcano ethiopia these data indicate that the lake cycled between periods characterized by low 0 01 0 08 m s 1 and high 0 1 0 4 m s 1 surface velocities typically lasting tens to hundreds of minutes these periods of high and low velocity motion define periods of vigorous and sluggish convection respectively spectral analysis revealed that vigorous convection periods were characterized by high frequencies and energies in the thermal data and an increased presence of high frequency energy in the seismic data the data show that vigorous periods were characterized by formation of hot short lived plastic crusts and sluggish periods by cooler longer lived brittle crusts here the higher transit velocity across the lake surface from upwelling zones of crust formation to downwelling zones of crust destruction during the vigorous periods decreases the crust lifetime this in turn decreases the total cooling and thickening experienced by a plate of crust moving across the lake surface two scenarios can be envisaged to explain such convection cycles the first relates variable convection rates to changes in the volume flux and rheology of magma entering the lake in the second cyclic convection is set up by the generation of convective instabilities within the lake in this case cooling of a surface layer generates a slow moving viscous increasingly dense convection layer at the lake surface which is consumed and replaced during overturn

thermo rheological magma control on the impact of highly fluid lava flows at mt nyiragongo 1 in january 2002 mount nyiragongo erupted foiditic lavas that covered the southern volcano flank devastating vast urban areas lava flows originated from vents at different heights on the eruptive fissure displayed different velocities from tens of km h at the highest vents to slow advance 0 1 1 km h in goma town several km away from the volcano to understand the different behavior of lava flows and their threat to the local population we undertook a multidisciplinary study involving textural and rheological measurements and numerical simulations of heat transfer during magma ascent we demonstrate that pre eruptive cooling and syn eruptive undercooling of magma determined the different rheological behavior of lava flows erupted from vents at diverse heights venting at lower altitudes is expected to produce viscous slowly advancing lavas although development of fluid faster flows should be included among possible future eruptive scenarios

implications of grain size evolution on the seismic structure of the oceanic upper mantle abstract we construct a 1 d steady state channel flow model for grain size evolution in the oceanic upper mantle using a composite diffusion dislocation creep rheology grain size evolution is calculated assuming that grain size is controlled by a competition between dynamic recrystallization and grain growth applying this grain size evolution model to the oceanic upper mantle we calculate grain size as a function of depth seafloor age and mantle water content the resulting grain size structure is used to predict shear wave velocity v s and seismic quality factor q for a plate age of 60 myr and an olivine water content of 1000 h 10 6 si we find that grain size reaches a minimum of 15 mm at 150 km depth and then increases to 20 30 mm at a depth of 400 km this grain size structure produces a good fit to the low seismic shear wave velocity zone lvz in oceanic upper mantle observed by surface wave studies assuming that the influence of hydrogen on anelastic behavior is similar to that observed for steady state creep further it predicts a viscosity of 10 19 pa s at 150 km depth and dislocation creep to be the dominant deformation mechanism throughout the oceanic upper mantle consistent with geophysical observations we predict larger grain sizes than proposed in recent studies in which the lvz was explained by a dry mantle and a minimum grain size of 1 mm however we show that for a 1 mm grain size diffusion creep is the dominant deformation mechanism above 100 200 km depth inconsistent with abundant observations of seismic anisotropy from surface wave studies we therefore conclude that a combination of grain size evolution and a hydrated upper mantle is the most likely explanation for both the isotropic and anisotropic seismic structure of the oceanic upper mantle our results also suggest that melt extraction from the mantle will be significantly more efficient than predicted in previous modeling studies that assumed grain sizes of 1 mm

depth of the decoupling plate interface and thermal structure under arcs subduction proceeds as thrust movements at the boundary between the subducting plate and the rigid overlying plate at depth below this mechanically decoupling plate interface the subducting plate couples with the overlying mantle and flow is induced in the mantle wedge this induced flow is the main factor that controls thermal structure under arcs depth of this decoupling to coupling transition at the plate interface is estimated from observed surface heat flow in the northeastern japan arc using a two dimensional numerical simulation of thermal structure the estimated depth is about 70 km the termination depth of thrust earthquakes agrees well with the depth of the coupling transition which indicates that these earthquakes occur at the decoupling plate interface the mechanism of this decoupling plate interface is discussed in this paper it is believed that water associated with the subducting oceanic crust reduces the frictional stress at the plate interface and thrust movement thus becomes possible petrological studies show that dehydration reaction in the subducting oceanic crust terminate at about 2 0 gpa and have little temperature dependence termination depths of thrust earthquakes estimated from seismic studies are nearly the same 60 80 km for various arcs this indicates that depth of the decoupling to coupling transition is controlled by the termination depth of dehydration reaction in the subducting oceanic crust across arc heat flow profiles observed in various subduction zones show similar features heat flow is very low in the forearc region and increases steeply at the volcanic front considering that the depth of the slab is about 100 km at the volcanic front for various arcs this heat flow feature is closely related to the depth of the slab thus it is concluded that thermal structure under arcs is mainly controlled by the depth of the pressure dependent dehydration reactions in the subducting oceanic crust

transportation of h2o beneath the japan arcs and its implications for global water circulation abstract in order to understand the transportation and circulation of h2o in subduction zones geological and seismological observations and the corresponding quantitative modeling are reviewed and consolidated with a special emphasis on the japan arcs the japan arcs are ideal for such a study in that 1 they are very active magmatically and seismically with abundant high quality observations and 2 subduction parameters change significantly along the japan arcs allowing one to investigate how the variations of input parameters affect the resultant structures and processes first numerical models for the transportation of h2o and melting are introduced the generation and migration of aqueous fluid its interaction with the convecting solid and melting are considered based on the realistic phase relationships of the peridotitic and basaltic systems application of the models to the japan arcs and comparison between the model results and the observations distribution of volcanoes and seismic structures along the japan arcs are then discussed the three cases from the ne central and sw japan arcs clarify the following points first an aqueous fluid released from the subducting oceanic crust forms a serpentinite layer in the mantle wedge just above the subducting slab maintaining a condition close to equilibrium in terms of hydration dehydration reactions during aqueous fluid migration second most of the h2o is subducted to a depth where serpentine and chlorite in the serpentinite layer break down this depth depends on the thermal structure of the slab and is greater for the older plate beneath the japan arcs as a consequence aqueous fluid and melt are in general not supplied straight upward to the volcanic front this result is rather different from the conventional view finally based on the understanding obtained from the japan arcs the global budget and circulation of h2o are discussed even after the completion of dehydration of major hydrous mineral phases in the subducting materials especially at the base of the mantle wedge just above the subducting slab nominally anhydrous phases of the upper mantle assemblage i e olivine pyroxenes garnet can carry a significant amount of h2o 1 1 1011 to 7 8 1011 kg yr 1 into the deep mantle the minimum estimate is comparable to the outflow of h2o from the mantle at mid ocean ridges and hotspots while the maximum estimate greatly exceeds it the large estimated range is mainly due to the uncertainty of the maximum h2o contents in the nominally anhydrous minerals of the upper mantle which controls the net influx of h2o into the deep mantle through subduction zones water subduction magmatism global circulation

systematic variation in the depths of slabs beneath arc volcanoes summary the depths to the tops of the zones of intermediate depth seismicity beneath arc volcanoes are determined using the hypocentral locations of engdahl et al these depths are constant to within a few kilometres within individual arc segments but differ by tens of kilometres from one arc segment to another the range in depths is from 65 km to 130 km inconsistent with the common belief that the volcanoes directly overlie the places where the slabs reach a critical depth that is roughly constant for all arcs the depth to the top of the intermediate depth seismicity beneath volcanoes correlates neither with age of the descending ocean floor nor with the thermal parameter of the slab this depth does however exhibit an inverse correlation with the descent speed of the subducting plate which is the controlling factor both for the thermal structure of the wedge of mantle above the slab and for the temperature at the top of the slab we interpret this result as indicating that the location of arc volcanoes is controlled by a process that depends critically upon the temperature at the top of the slab or in the wedge of mantle immediately below the volcanic arc

a community benchmark for subduction zone modeling we have developed a suite of benchmarks to facilitate the comparison of numerical models for the dynamics and thermal structure of subduction zones the benchmark cases are based on a thermomechanical approach in which the slab is prescribed kinematically and the wedge flow is computed dynamically we propose various cases to investigate the influence of boundary conditions and rheology on wedge flow and resulting thermal structure a comparison between the codes suggest that accurate modeling of the thermal field requires a good implementation of the velocity discontinuity along the seismogenic zone and high resolution in the thermal boundary layers a minor modification to the boundary conditions of the wedge flow is also necessary to avoid a pressure singularity that exists in analytical solutions of the cornerflow model

experimentally based water budgets for dehydrating slabs and consequences for arc magma generation phase diagrams of hydrous mid ocean ridge mor basalts to 330 km depth and of hydrous peridotites to 250 km depth are compiled for conditions characteristic for subduction zones a synthesis of our experimentally determined phase relations of chlorite lawsonite epidote zoisite amphibole paragonite chloritoid talc and phengite in basalts and of phase relations from the literature of serpentine talc chlorite amphibole and phase a in ultramafics permits calculation of h2o contents in hydrous phase assemblages that occur in natural compositions this yields the information necessary to calculate water budgets for descending slabs starting from low grade blueschist conditions 10 20 km depth with h2o contents between 5 and 6 wt for hydrated oceanic crust complete dehydration is achieved between 70 and 300 km depth as a function of individual slab geotherms hydrous phases which decompose at depth below volcanic arcs are lawsonite zoisite chloritoid and talc phengite in mafic compositions and chlorite and serpentine in peridotite approximately 15 35 of the initially subducted h2o are released below volcanic arcs the contribution of amphibole dehydration to the water budget is small 5 20 and occurs at relatively shallow depth 65 90 km in any predicted thermal structure dehydration is a combination of a stepwise and a continuous process through many different reactions which occur simultaneously in the different portions of the descending slab such a dehydration characteristic is incompatible with single phase dehydration models which focus fluid flow through a unique major dehydration event in order to explain volcanic fronts as a consequence of continuously progressing dehydration water ascending from the slab will be generally available to depth of ca 150 200 km the fluid rising from the subducting lithosphere will cause partial melting in the hot portion of the mantle wedge we propose that the volcanic front simply forms above the mantle wedge isotherm where the extent of melting is sufficient to allow for the mechanical extraction of parental arc magmas thermal models show that such an isotherm ca 1300oc locates below volcanic fronts slab surface depths below such an isotherm are compatible with the observed depths of the slab surface below volcanic fronts 1998 elsevier science b v all rights reserved

insights into explosive activity at closely spaced eruptive vents using infrasound signals example of mt etna 2008 eruption abstract on 13 may 2008 an eruptive fissure opened on mount etna s eastern flank feeding both explosive activity and lava effusion from multiple vents for about 14 months during the investigated may september 2008 eruptive period infrasound recordings from a 4 station sparse network allowed tracking of the explosive activity in terms of location and dynamics in order to focus on activity from the eruptive fissure the infrasonic events generated by the summit craters were selected by using both spectral features and time delays between pairs of stations and excluded from our analysis then to accurately locate events from the fissure we used a composite method based on the semblance and brightness functions this enabled the study of the co existence of more than one infrasound source and or its migration along the eruptive fissure hence results permitted us to discriminate the number of active vents and their location along the fissure even when due to poor weather conditions it was not possible to access the vents or carry out direct observations the eruptive activity was characterised by variations in the number of active vents according to the overall intensity of the eruptive event variability of the infrasound waveforms highlighted either that distinct vents produced signals with different waveforms or that single vents generated different events during distinct periods of time or finally both the previous phenomena we applied the strombolian bubble vibration model to model waveform differences and attributed the signal variations to bubble radius changes

volcano infrasound a review abstract exploding volcanoes which produce intense infrasound are reminiscent of the veritable explosion of volcano infrasound papers published during the last decade volcano infrasound is effective for tracking and quantifying eruptive phenomena because it corresponds to activity occurring near and around the volcanic vent as opposed to seismic signals which are generated by both surface and internal volcanic processes as with seismology infrasound can be recorded remotely during inclement weather or in the dark to provide a continuous record of a volcano s unrest moreover it can also be exploited at regional or global distances where seismic monitoring has limited efficacy this paper provides a literature overview of the current state of the field and summarizes applications of infrasound as a tool for better understanding volcanic activity many infrasound studies have focused on integration with other geophysical data including seismic thermal electromagnetic radiation and gas spectroscopy and they have generally improved our understanding of eruption dynamics other work has incorporated infrasound into volcano surveillance to enhance capabilities for monitoring hazardous volcanoes and reducing risk this paper aims to provide an overview of volcano airwave studies from analog microbarometer to modern pressure transducer and summarizes how infrasound is currently used to infer eruption dynamics it also outlines the relative merits of local and regional infrasound surveillance highlights differences between array and network sensor topologies and concludes with mention of sensor technologies appropriate for volcano infrasound study

an analogue experimental model of depth fluctuations in lava lakes lava lakes consisting of molten degassing lava in summit craters of active basaltic volcanoes sometimes exhibit complex cycles of filling and emptying on time scales of hours to weeks such as recorded at pu u o o in hawaii and oldoinyo lengai in tanzania here we report on a new series of analogue laboratory experiments of two phase flow in a reservoir conduit lava lake system which spontaneously generates oscillations in the depth of liquid within the lake during the recharge phase gas supplied from a subsurface reservoir of degassing magma drives liquid magma up the conduit causing the lake to fill as the magmastatic pressure in the lake increases the upward supply of magma driven by the gas bubbles falls eventually the upflow becomes unstable and liquid drains downwards from the lake driven by the magmastatic pressure of the overlying lake suppressing the ascent of any more bubbles from the chamber at a later stage once the lake has drained sufficiently the descent speed of liquid through the conduit decreases below the ascent speed of the bubbles and the recharge cycle resumes application of a quantitative model of the experiments to the natural system is broadly consistent with field data

registering imagery to icesat data for measuring elevation changes on byrd glacier antarctica 1 we present a new approach to derive control information from icesat data that enables rigorous registration of aerial and satellite imagery the technique based on matching terrain features identified from icesat measurements and aerial imagery opens the door to transform results of previous studies to a global reference frame we demonstrate the proposed methodology with historical aerial photographs to determine surface changes between 1979 and 2004 over byrd glacier this is important because there is no satellite radar altimetry coverage south of 81 5 s which limits mass balance knowledge of outlet glaciers draining the east antarctic ice sheet through the southern transantarctic mountains our study indicates that the grounded part of byrd glacier is close to being in balance however we observe large thinning on the floating part of the glacier probably induced by increased basal melting citation schenk t b csatho c j van der veen h brecher y ahn and t yoon 2005 registering imagery to icesat data for measuring elevation changes on byrd glacier antarctica geophys res lett 32 l23s05 doi 10 1029 2005gl024328

geophysical source conditions for basaltic lava from santorini volcano based on geochemical modeling abstract santorini volcano sits 145 km above the aegean wadati benioff zone where the african plate subducts northward beneath eurasia there are only a few localities in the whole aegean where basaltic lavas primitive enough to constrain mantle processes beneath the aegean arc can be found in this work we analyzed one such locality a basalt lava from the southern part of santorini we apply a suite of petrological tools primacalc2 and abs5 in sequence to estimate magma chamber conditions primary magma composition mantle melting conditions and slab dehydration conditions back calculation modeling based on major element chemistry yields shallow magma chamber conditions of p 0 02 gpa fo 2 qfm 2 and 1 wt h 2 o in the primary magma the estimated major element composition of this primary magma then leads to estimated mantle melting conditions of 2 1 gpa 1353 c and f 8 whereas a more precise estimate derived from trace element modeling implies 1 7 gpa 1323 c and f 18 furthermore the trace element model implies a slab flux derived from 4 6 gpa 150 km slab depth the estimated slab depth magma segregation conditions and magma chamber depth are all consistent with seismic observations supporting slab dehydration in the seismically imaged steep slab interval and flux melting in a relatively hot mantle wedge

dynamics of mount nyiragongo lava lake inferred from thermal imaging and infrasound array abstract lava lakes provide a direct observation window into processes which usually remain hidden such as magma convection and outgassing dynamics we here report a coupled analysis of thermal infrared footage and infrasound array recordings at mount nyiragongo d r congo and derive a conceptual model of the lava lake s convective system and outgassing mechanism we suggest that surface flow results from a horizontal pressure gradient at the surface of the lake driving the crust from high pressure regions where hot upwelling magma impinges the surface to low pressure regions where cold downwelling magma pulls away from the surface the ascending current of this convection cell carries gas pockets which once at the surface are dragged across the lake into downwelling sinks such sinks are characterized by persistent chaotic bubble bursting spattering whose intensity and position are tracked from infrasound array analysis fluctuations of these are observed but have not been correlated with oscillations of the lava lake level nor with the variations of surface velocities both recorded from infrared footage we also report the activity of a new eruptive vent which opened early 2016 near an inner circular fracture of the crater s third terrace we show that the vent s activity was intermittent alternating between explosive strombolian activity and effusive activity the latter produced lava flows which spread on the crater s last terrace before cascading into the active lava lake although no significant change in the lake behavior was witnessed while the new eruptive vent was active increased attention should be addressed as this new activity could reflect over pressurization of the shallow magmatic system the variety of phenomena captured by this study complements and expands observations reported at other low viscosity lava lakes chiefly kilauea hawai i and erta ale ethiopia despite nyiragongo s more vigorous convective regime where multiple convective cells can operate simultaneously we suggest that the mechanisms controlling the surface motion and outgassing are similar at all three systems pointing to generic processes governing the dynamics of low viscosity lava lakes

acoustic measurements of the 1999 basaltic eruption of shishaldin volcano alaska 1 origin of strombolian activity the 1999 basaltic eruption of shishaldin volcano alaska usa displayed both classical strombolian activity and an explosive subplinian plume strombolian activity at shishaldin occurred in two major phases following the subplinian activity in this paper we use acoustic measurements to interpret the strombolian activity acoustic measurements of the two strombolian phases show a series of explosions that are modeled by the vibration of a large overpressurised cylindrical bubble at the top of the magma column results show that the bubble does not burst at its maximum radius as expected if the liquid film is stretched beyond its elasticity but bursting occurs after one cycle of vibration as a consequence of an instability of the air magma interface close to the bubble minimum radius during each strombolian period estimates of bubble length and overpressure are calculated using an alternate method based on acoustic power we estimate gas velocity to be 30 60 m s in very good agreement with synthetic waveforms although there is some variation within these parameters bubble length and overpressure for the first strombolian phase are found to be c82f11 m and 0 083 mpa for the second strombolian phase bubble length and overpressure are estimated at 24f12 m and 0 15 mpa for the first 17 h after which bubble overpressure shows a constant increase reaching a peak of 1 4 mpa just prior to the end of the second strombolian phase this peak suggests that at the time the magma in the conduit may contain a relatively large concentration of small bubbles maximum total gas volume and gas fluxes at the surface are estimated to be 3 3 10 7 and 2 9 10 3 m 3 s for the first phase and 1 0 10 8 and 2 2 10 3 m 3 s for the second phase this gives a mass flux of 1 2 10 3 and 8 7 10 2 kg s respectively for the first and the second strombolian phases

textural and geophysical characterization of explosive basaltic activity at villarrica volcano 1 villarrica volcano chile is one of the most active volcanoes in south america its activity is currently characterized by continuous degassing from a summit lava lake vent punctuated by explosive events during november 2004 a multidisciplinary experiment was deployed for a 10 d period to define the style of emission and infer shallow conduit dynamics at this basaltic center this involved collection of thermal seismic and infrasonic data to describe the background activity confined inside the crater and use of samples to texturally and chemically characterize the ejecta from more energetic explosions able to attain the crater rim the background activity was characterized by gas bursting with a frequency of 9 events per minute this involved emission of gas puffs fed by bubble bursting with larger bursts emplacing sheets of magma onto the lower crater walls the ejecta population from the more energetic events was characterized by the coexistence of both scoriae and golden pumice these two types of clasts have different textures but identical glass compositions suggesting that they underwent different conduit histories the golden pumice is interpreted as the expanding inner part of a short lived jet fed by a rapidly ascending magma batch the scoria forms the outer portion of the jet and comprises degassed material entrained during passage of the fresh batch through material residing in the upper most portion of the conduit we thus have a largely degassed upper column that feeds persistent bubble bursting through which fresh batches occasionally rise to feed events of relatively higher energy

the dynamics of slug trains in volcanic conduits evidence for expansion driven slug coalescence strombolian volcanism is a ubiquitous form of activity driven by the ascent and bursting of bubbles of slug morphology whilst considerable attention has been devoted to understanding the behaviour of individual slugs in this regime relatively little is known about how inter slug interactions modify flow conditions recently we reported on high temporal frequency strombolian activity on etna in which the larger erupted slug masses were followed by longer intervals before the following explosion than the smaller bursts pering et al 2015 we hypothesised that this behaviour arose from the coalescence of ascending slugs causing a prolonged lag before arrival of the next distinct bubble here we consider the potential importance of inter slug interactions for the dynamics of strombolian volcanism by reporting on the first study into the behaviour of trains of ascending gas slugs scaled to the expansion rates in volcanic conduits this laboratory analogue study illustrates that slugs in trains rise faster than individual slugs and can be associated with aspects of co current flow the work also highlights that coalescence and inter slug interactions play an important role in modulating slug train behaviour we also report for the first time on slug coalescence driven by vertical expansion of the trailing slug a process which can occur even where the leading slug base ascent velocity is greater than that of the trailing slug

surface velocity determination on large polar glaciers by aerial photogrammetry aerial photogrammetric block triangulation a standard and well developed technique for extending accurate control for mapping into the interior of a region from a few points of known position on its perimeter can be readily adapted to determine surface velocities on bodies of ice which are too large and often too crevassed to be studied effectively by conventional ground surveying velocities are calculated from the changes in positions of the same natural surface features determined from photography of two or more epochs and the elapsed time this method is capable of providing many uniformly spaced measurements over the whole moving ice surface thus allowing the production of maps of velocity and strain rate which are valuable in analyzing the ice flow regime results from measurements completed some years ago on byrd glacier one of the largest outlet glaciers from the east antarctic plateau are presented as an example of what the method can yield by means of doppler satellite surveying relative positions of control points for each photography epoch can be determined with sub meter accuracy making the technique suitable also in regions where no fixed land features exist a brief description of a project under way in such an area on ice stream b in west antarctica is given

evolution of a small hydrothermal eruption episode through a mud pool of varying depth and rheology white island nz white island volcano new zealand was a host to multiple hydrothermal eruptive episodes within a mud sulphur pool in 2013 although hydrothermal activity is common at white island past events have largely gone undescribed in favour of the larger phreatomagmatic and magmatic eruptions here we detail the first and longest hydrothermal episode of 2013 lasting from 15 january to 7 february using video and photo analysis from tour operators and staff responsible for monitoring the volcano differences in the dominant bubble burst style across this episode led to the classification of four distinct eruption regimes 1 multiple irregular bursts on the pool surface 2 larger distinct symmetric hemispheres with starbursts and or followed by mud heaves 3 no initial pool surface deformation but a vertical steam jet followed by a sometimes large directed mud heave and 4 no lake and continuous pulsating dry ash and block venting the progression through these regimes is associated with a lowering lake level and a concomitantly increasing viscosity of the pool which initially comprises a low viscosity muddy water and partially evaporates to yield a shallow layer of high viscosity mud that ends with the complete drying up of the mud pool formation of primary mud hemispheres or gas jets is followed by heaves or secondary upheaval events the heights of these heaves are used as a measure of explosivity heights increase from 8 m during regime 1 on 15 january to 102 m during regime 3 on 28 january venting of dry mud during regime 4 developed on 29 january before a regression back to regime 1 took place on 7 february as the pool re established through observations of the shapes of ejected mud clots we propose that the increasing explosivity of higher number regimes is primarily due to increasing slug bubble lengths teamed with increasing mud pool viscosity we attribute a lesser control to the decreasing depth of the pool during its progressive desiccation which may in turn influence the bubble burst depth occasionally visible yellowing of the steam gas plume led us to suggest that elemental sulphur may also be present in the conduit and may also play a role in regulating bubble release dynamics although evidence for magmatic phreatomagmatic eruptions was present during eruptions later in 2013 we found no evidence for juvenile magma in the january february eruption episode described here however we concur with other investigators that magma was probably intruded to shallow levels and may have driven heat and gas flux our explanation for the correlation of pool depth mud viscosity and eruption regime is based on a conceptual model in which a pool is perched above a two phase hydrothermal system and is sensitive to changes in the heat and gas flux from shallow magma the variable release of gas and thermal perturbations in the course of the january february eruptive episode impacted the pool level the water to sediment ratio in the pool and thus its viscosity and in turn modulated the eruption regime the varying degree of explosivity throughout this episode calls for a new consideration of pool properties in assessing eruption hazards at this frequently visited volcano we additionally emphasise that ballistic hazards from small eruptions exist coupled with a range of seismic signals and that the hazard was greatest during infrasound tremor

sulfur dioxide emissions and degassing behavior of erebus volcano antarctica abstract emission rates of sulfur dioxide so 2 were measured at erebus volcano antarctica in december between 1992 and 2005 since 1992 so 2 emissions rates are normally distributed with a mean of 61 27 mg d 1 0 7 0 3 kg s 1 n 8064 the emission rates vary over minutes hours days and years hourly and daily variations often show systematic and cyclic trends long wavelength large amplitude trends appear related to lava lake area and both are likely controlled by processes occurring at depth time series analysis of continuous sequences of measurements obtained over periods of several hours reveals periodicity in so 2 output ranging from 10 to 360 min with a 10 min cycle being the most dominant closed and open system degassing models are considered to explain observed variable degassing rates closed system degassing is possible as rheological stiffening and stick slip may occur within the system however the timescales represented in these models do not fit observations made on erebus open system degassing and convection fits the observations collected as the presented models were developed for a system similar to erebus in terms of degassing eruptive activity and process repose time we show that with the observed emission rate 0 71 kg s 1 and a crystal content of 30 magma will cool 65 c to match observed heat fluxes this cooling is sufficient enough to drive convection

grain size distribution in the mantle wedge of subduction zones 1 mineral grain size plays an important role in controlling many processes in the mantle wedge of subduction zones including mantle flow and fluid migration to investigate the grain size distribution in the mantle wedge we coupled a two dimensional 2 d steady state finite element thermal and mantle flow model with a laboratory derived grain size evolution model in our coupled model the mantle wedge has a composite olivine rheology that incorporates grain size dependent diffusion creep and grain size independent dislocation creep our results show that all subduction settings lead to a characteristic grain size distribution in which grain size increases from 10 to 100 m at the most trenchward part of the creeping region to a few centimeters in the subarc mantle despite the large variation in grain size its effect on the mantle rheology and flow is very small as 90 of the deformation in the flowing part of the creeping region is accommodated by grain size independent dislocation creep the predicted grain size distribution leads to a downdip increase in permeability by 5 orders of magnitude this increase is likely to promote greater upward migration of aqueous fluids and melts where the slab reaches 100 km depth compared with shallower depths potentially providing an explanation for the relatively uniform subarc slab depth seismic attenuation derived from the predicted grain size distribution and thermal field is consistent with the observed seismic structure in the mantle wedge at many subduction zones without requiring a significant contribution by the presence of melt

acoustic source characterization of impulsive strombolian eruptions from the mount erebus lava lake abstract we invert for acoustic source volume outflux and momentum imparted to the atmosphere using an infrasonic network distributed about the erupting lava lake at mount erebus ross island antarctica by modeling these relatively simple eruptions as monopole point sources we estimate explosively ejected gas volumes that range from 1 000 m 3 to 24 000 m 3 for 312 lava lake eruptions recorded between january 6 and april 13 2006 though these volumes are compatible with bubble volumes at rupture as estimated from explosion video records departures from isotropic radiation are evident in the recorded acoustic wavefield for many eruptions a point source acoustic dipole component with arbitrary axis orientation and strength provides precise fit to the recorded infrasound this dipole source axis corresponding to the axis of inferred short duration material jetting varies significantly between events physical interpretation of dipole orientation as being indicative of eruptive directivity is corroborated by directional emissions of ejecta observed in erebus eruption video footage although three azimuthally distributed stations are insufficient to fully characterize the eruptive acoustic source we speculate that a monopole with a minor amount of oriented dipole radiation may reasonably model the primary features of the recorded infrasound for these eruptions

magmatic focusing to mid ocean ridges the role of grain size variability and non newtonian viscosity melting beneath mid ocean ridges occurs over a region that is much broader than the zone of magmatic emplacement that forms the oceanic crust magma is focused into this zone by lateral transport this focusing has typically been explained by dynamic pressure gradients associated with corner flow or by a sub lithospheric channel sloping upward toward the ridge axis here we discuss a novel mechanism for magmatic focusing lateral transport driven by gradients in compaction pressure within the asthenosphere these gradients arise from the co variation of melting rate and compaction viscosity the compaction viscosity in previous models was given as a function of melt fraction and temperature in contrast we show that the viscosity variations relevant to melt focusing arise from grain size variability and non newtonian creep the asthenospheric distribution of melt fraction predicted by our models provides an improved explanation of the electrical resistivity structure beneath one location on the east pacific rise more generally we find that although grain size and non newtonian viscosity are properties of the solid phase their effect on melt transport beneath mid ocean ridges is more profound than their effect on the mantle corner flow

control of the location of the volcanic front in island arcs by aqueous fluid connectivity in the mantle wedge the water released from descending oceanic lithosphere is thought to have an important role in subduction zone magmatism as this water might trigger partial melting of the mantle wedge above the subducting plate1 2 3 4 5 6 7 8 if however there is incomplete wetting of mineral grain boundaries in the mantle that is the dihedral angle9 at the triple junctions between grains is more than 60 then the water would not form an interconnected network and might instead be trapped as interstitial fluid in the mantle peridotite the water would then be transported to deeper parts of the mantle rather than triggering partial melting here we use dihedral angle data to estimate the connectivity of an aqueous fluid phase in a model upper mantle mineral assemblage forsterite at pressures from 3 to 5 gpa corresponding to depths of 80 150 km by combining these data with previous results10 11 we find that the dihedral angle is greater than 60 at low pressure and temperature 1 000 c at 2 gpa and 800 c at 4 gpa and lower than 60 at higher pressures and temperatures suggesting that wetting is incomplete below these conditions this indicates that the connectivity of water in hydrous upper mantle peridotite at convergent plate boundaries might control the position of the volcanic front in island arcs