quiescent explosive transitions during dome forming volcanic eruptions using seismicity to probe the volcanic processes leading to the 29 july 2008 vulcanian explosion of soufriere hills volcano montserrat

unique strainmeter observations of vulcanian explosions soufriere hills volcano montserrat july 2003 gravity waves propagated at 30 m s 1 eruption volumes estimated from plume height and strain data are 0 32 0 42 10 6 0 26 0 49 10 6 and 0 81 0 84 10 6 m 3 f or explosions 3 5 respectively consistent with quasi cylindrical conduit drawdown 2 km the duration of vigorous explosion is given by the strain signature indicating mass fluxes of order 10 7 kg s 1 conduit pressures released reflect static weight of porous gas charged magma and exsolution generated overpressures of order 10 mpa citation voight b et al 2010 unique strainmeter observations of vulcanian explosions soufriere hills volcano montserrat july 2003 geophys res lett 37 l00e18 doi 10 1029 2010gl042551

control on the emplacement of the andesite lava dome of the soufriere hills volcano montserrat by degassing induced crystallization lava solidification is controlled by two mechanisms external cooling and gas exsolution the latter inducing crystallization due to increasing liquidus temperature the andesite lava dome of the soufriere hills volcano montserrat is an extrusion dominated by crystallization caused by gas exsolution where cooling is unimportant in controlling emplacement in the magma chamber the magma has an estimated viscosity of 7 106 pa s during ascent gas exsolution caused the magma to extrude in a highly crystalline state with only 5 15 residual melt viscosities in the range 1013 1014 pa s and mechanical strength 1 mpa deformation can be heterogeneous with extrusion along shear zones rheological stiffening in the upper conduit also causes large overpressures shallow seismicity and cyclic patterns of dome extrusion gas rich porphyritic andesites tend to be the least mobile kind of lava because transition from magma into hot crystalline material was reached during ascent

repeated fracture and healing of silicic magma generate flow banding and earthquakes textures in an exceptionally preserved effusive rhyolite conduit at torfajokull iceland indicate that rising magma repeatedly fractured and healed at shallow levels in the conduit rfh process anastomosing tuffisite veins filled by fine grained juvenile clasts were generated by shear fracture of highly viscous magma in the glass transition interval welding of the particulate material during subsequent deformation led to thorough healing of veins allowing repeated fracture of the same body of magma we propose that the rfh process is a rechargeable trigger mechanism for hybrid seismicity and show that the time scale of the process and the fractures formed by it are consistent with the repeat time and magnitude of hybrid earthquakes during silicic eruptions the rfh process may also form the flow banding that is nearly ubiquitous in obsidian

magma degassing buffered by vapor flow through brecciated conduit margins obsidian pyroclasts a common component of rhyolitic tephra preserve a range of volatile contents which has been used to infer syneruptive conditions of magmatic degassing here we show that the textures of obsidian pyroclasts provide information on physical mechanisms of magma flow and degassing along conduit margins obsidian clasts often contain xenoliths sheared bands of lithic powder and textures consistent with magma autobrecciation these features suggest that pyroclastic obsidian primarily forms near conduit walls where magma fragments and reanneals during ascent we use these observations to develop a degassing model for pyroclastic obsidian from the a d 1340 mono craters california eruptions we suggest that degassing was buffered by continual flux of vapor through highly permeable brecciated magma along conduit walls continuous reequilibration of magma with vapor of relatively constant composition not only explains the co 2 h 2 o and d h 2 o data from mono craters pyroclastic obsidian but also requires much lower magmatic co 2 values than the commonly accepted model of closed system degassing taken together the chemical and physical evidence suggests that magma brecciation along conduit walls aids the degassing of ascending rhyolite

transitions between explosive and effusive eruptions of silicic magmas water rich silicic magmas may erupt explosively giving rise to massive columns of fragmented ash1 5 or effusively as viscous bubbly lavas to form lava domes6 8 complex cycles of explosive and effusive eruptions are often observed8 12 although these two eruption mechanisms have been modelled independently1 8 the conditions under which a volcano exhibits a particular style of eruption are not known and the transitions between eruption styles are poorly understood by modelling the ascent of magma along a permeable conduit we show here that for small magma chamber overpressures the style of eruption explosive or effusive is dependent on the magma flow rate we also identify a critical overpressure above which only explosive eruptions occur complex eruption sequences follow naturally for example if a shallow magmatic system is supplied with magma at an intermediate flow rate the eruption will be slow and effusive until the chamber over pressure becomes too large an explosive eruption then relieves the overpressure and an effusive eruption style resumes we suggest that monitoring of temporal variations in chamber overpressure for example by measuring ground deformation can be used to assess whether a passively effusing volcano has the potential to erupt explosively

satellite based constraints on explosive so2 release from soufriere hills volcano montserrat 1 numerous episodes of explosive degassing have punctuated the 1995 2009 eruption of soufriere hills volcano shv montserrat often following major lava dome collapses we use ultraviolet uv and infrared ir satellite measurements to quantify sulfur dioxide so2 released by explosive degassing which is not captured by routine ground based and airborne gas monitoring we find a total explosive so2 release of 0 5 tg which represents 6 of total so2 emissions from shv since july 1995 the majority of this so2 0 4 tg was vented following the most voluminous shv dome collapses in july 2003 and may 2006 based on our analysis we suggest that the so2 burden measured following explosive disruption of lava domes depends on several factors including the instantaneous lava effusion rate dome height above the conduit and the vertical component of directed explosions space based so2 measurements merit inclusion in routine gas monitoring at shv and other dome forming volcanoes

dynamics of continental accretion three dimensional dynamic computer models show how accretionary tectonic margins evolve from the initial plate collision state through a period of plate margin instability and then re establish a stable convergent margin the models illustrate how significant curvature of the orogenic system develops as well as the mechanism for tectonic escape of the back arc region subduction zones where one plate dives under another become congested when they try to accommodate buoyant exotic crust louis moresi et al describe new numerical models of continental accretion that follow the entire process from the initial collision state through a period of plate margin instability to the re establishment of a stable convergent margin the models illustrate how significant curvature of the orogenic system develops as well as the mechanism for tectonic escape of the back arc region subduction zones become congested when they try to consume buoyant exotic crust the accretionary mountain belts orogens that form at these convergent plate margins have been the principal sites of lateral continental growth through earth s history modern examples of accretionary margins are the north american cordilleras and southwest pacific subduction zones the geologic record contains abundant accretionary orogens such as the tasmanides1 along the eastern margin of the supercontinent gondwana and the altaides which formed on the southern margin of laurasia2 in modern and ancient examples of long lived accretionary orogens the overriding plate is subjected to episodes of crustal extension and back arc basin development often related to subduction rollback3 and transient episodes of orogenesis and crustal shortening4 5 6 7 coincident with accretion of exotic crust here we present three dimensional dynamic models that show how accretionary margins evolve from the initial collision through a period of plate margin instability to re establishment of a stable convergent margin the models illustrate how significant curvature of the orogenic system develops as well as the mechanism for tectonic escape of the back arc region the complexity of the morphology and the evolution of the system are caused by lateral rollback of a tightly arcuate trench migrating parallel to the plate boundary and orthogonally to the convergence direction we find geological and geophysical evidence for this process in the tasmanides of eastern australia and infer that this is a recurrent and global phenomenon

petrologic constraints on the decompression history of magma prior to vulcanian explosions at the soufriere hills volcano montserrat abstract a series of 88 vulcanian explosions occurred at the soufriere hills volcano montserrat between august and october 1997 conduit conditions conducive to creating these and other vulcanian explosions were explored via analysis of eruptive products and one dimensional numerical modeling of magma ascent through a cylindrical conduit the number densities and textures of plagioclase microlites were documented for twenty three samples from the events the natural samples all show very high number densities of microlites and 50 by number of microlites have areas 2 pre explosion conduit conditions and decompression history have been inferred from these data by comparison with experimental decompressions of similar groundmass compositions our comparisons suggest quench pressures 13 75 mpa each during ascent from chamber to surface values are consistent with field studies of the same events and statistical analysis of explosion time series data the microlite volume number density trend with depth reveals an apparent transition from growth dominated crystallization to nucleation dominated crystallization at pressures of 7 mpa and lower a concurrent sharp increase in bulk density marks the onset of significant open system degassing apparently due to a large increase in system permeability above 70 vesicularity this open system degassing results in a dense plug which eventually seals the conduit and forms conditions favorable to vulcanian explosions the corresponding inferred depth of overpressure at 250 700 m near the base of the dense plug is consistent with depth to center of pressure estimated from deformation measurements here we also illustrate that one dimensional models representing ascent of a degassing crystal rich magma are broadly consistent with conduit profiles constructed via our petrologic analysis the comparison between models and petrologic data suggests that the dense conduit plug forms as a result of high overpressure and open system degassing through conduit walls

cyclical patterns in volcanic degassing revealed by so2 flux timeseries analysis an application to soufriere hills volcano montserrat abstract cyclical patterns of behaviour in timeseries of seismic and geodetic data at volcanoes are frequently observed during lava dome building eruptions and are particularly well documented from the current eruption of the soufriere hills volcano shv montserrat however the discontinuous nature of many so 2 measurements often preclude the identification and quantitative analysis of cyclical patterns in degassing data here using a long so 2 timeseries from shv with continuous measurements since 2002 we explore for the first time degassing behaviour at a resolution comparable to that possible for seismic and deformation datasets timeseries analysis of flux data spanning 2002 2011 reveals that so 2 emissions at shv exhibit complex cyclicity with dominant cycles evident on both multi year and multi week 50 day timescales these cycles persist through phases of both active extrusion and eruptive pause and show close similarities to periodic components previously identified at shv in timeseries of seismicity ground deformation and lava extrusion the strength of expression or amplitude of degassing cycles particularly on multi week timescales shows distinct temporal variation and appears to correlate with the occurrence and nature of explosive activity occurring in 2002 2009 this suggests that the amplitude of surface gas flux cycles is modulated by physical conditions within the conduit direct quantitative comparison between seismicity dome growth and degassing for eruptive phases 2 2002 2003 and 3 2005 2007 reveals that peaks in so 2 flux appear to correspond broadly to enhanced lava extrusion and elevated seismicity within cycles of 30 50 days however time lags of 2 4 and 7 days are observed between initial low frequency seismic swarms and peaks in dome growth so 2 flux and rockfall event rate respectively multi parameter correlations offer valuable insights into the controls on subsurface gas ascent but further research is required to fully explore the contributions of permeability and overpressure as well as other subsurface processes

volcanic sintering timescales of viscous densification and strength recovery 1 sintering and densification are ubiquitous processes influencing the emplacement of both effusive and explosive products of volcanic eruptions here we sinter ash size fragments of a synthetic national institute of standards and technology viscosity standard glass at temperatures at which the resultant melt has a viscosity of 108 109 pa s at 1bar to assess sintering dynamics under near surface volcanic conditions we track the strength recovery via uniaxial compressive tests we observe that volcanic ash sintering is dominantly time dependent temperature dependent and grain size dependent and may thus be interpreted to be controlled by melt viscosity and surface tension sintering evolves from particle agglutination to viscous pore collapse and is accompanied by a reduction in connected porosity and an increase in isolated pores sintering and densification result in a nonlinear increase in strength micromechanical modeling shows that the pore emanated crack model explains the strength of porous lava as a function of pore fraction and size

conduit processes during the february 11 2010 vulcanian eruption of soufriere hills montserrat abstract we analyzed pumice from the february 11 2010 vulcanian explosion that immediately followed a large dome collapse at soufriere hills volcano we obtained pre explosive values of porosity pressure and depth by combining textural analyses and glass water content determinations our data suggest that the february 2010 explosion evacuated the upper 3 km of the conduit from the dense magma 10 vol porosity it contained the low porosity distribution in the volcanic conduit implies that the magma rising from the reservoir had time to extensively degas during emplacement we use a conduit flow model to characterize the effects of permeability on ascent conditions model input parameters were fitted to match our pre explosive porosity data which yielded first order constraints on conduit radius mass flux outgassing efficiency and permeability this parametric study suggests that efficient lateral gas escape is necessary to explain the low pre explosive porosities steady state solutions fitting the observed range of dome extrusion rate in the month preceding the february 11 event suggest permeabilities

rock mass properties and edifice strength data from pinnacle ridge mt ruapehu new zealand abstract volcanic edifices exhibit spatially variable physical and mechanical properties magmatic intrusions are common at shallow depths within the volcanic edifice and are a poorly understood contributor to this spatial variability intrusion related alteration has been found to weaken rock mass strength through the development of joints and fractures however there is a paucity of research investigating how intrusions affect rock mass strength specific to the geotechnical units that define the rock masses in this study we employ a range of field techniques field permeametry rock hardness assessment rock mass classification and discontinuity mapping to characterise an exposed fossil geothermal system produced by a shallow intrusion at pinnacle ridge mt ruapehu new zealand we find that intrusions detrimentally affect the rock mass characteristics of altered brecciated lava margins the resulting change in rock mass strength may be offset by an increase in intact rock strength as a product of alteration mineral precipitation in microfractures consequently the final strength of the rock mass of the altered brecciated lava margins has the potential to be lowest of any of the geotechnical units in the volcanic edifice we also conclude that these discontinuities increase permeability of the host rock at distances from the intrusion roughly proportional to 1 2 times the thickness of the intrusion itself under near surface conditions the data and conclusions presented in this study help to bridge the gap between the lab and the field scale and have immediate relevance to engineering geology and geothermal applications worldwide and to rock mass classification assessments in volcanic environments

geochemical evidence for relict degassing pathways preserved in andesite andesitic arc volcanoes degas large quantities of volatiles evidence for vapour transport in the erupted lavas is rarely preserved and poorly understood but is crucial for understanding eruption style we present geochemical evidence for the transport of metal bearing vapour in shear zones preserved in lavas erupted from soufriere hills volcano montserrat textural evidence suggests that shear induced brittle failure occurred in a narrow zone at the conduit wall or in the lava dome elevated metal concentrations cu au ag pb zn within the zones indicate that the fractures acted as a transient pathway for metal bearing magmatic gases during slip frictional heating to temperatures of 1000 c caused partial melting at the slip surface resorption of volatiles and metals into the partial melt preserved the geochemical signature of magmatic vapour in the shear zone cordierite which is highly unusual in volcanic rocks crystallised from the peraluminous partial melt with metal bearing sulphides and oxides the shear zones provide the first geochemical evidence for vapour segregation and transport through viscous andesite magmas and provide an insight into controls on eruption style

in situ measurements of plagioclase growth using sims depth profiles of 7li 30si a means to acquire crystallization rates during short duration decompression events numerous petrologic studies have attempted to determine crystal growth rates in volcanic systems through several methods including analyses of crystal size distributions in natural samples and decompression experiments on hydrous magmas experiments have revealed that rim growth on existing crystals will occur under a wide range of decompression conditions and is favored under conditions of low to moderate undercoolings over microlite nucleation which has typically been the focus of decompression induced crystallization studies for this study samples of eruptive clasts were collected from a vulcanian explosion that occurred following the july 12 13 2003 dome collapse of soufriere hills volcano montserrat plagioclase phenocrysts were extracted and examined with secondary ion mass spectrometry sims depth profiling lithium inflection depths within the profiles along with the observed time interval between the peak in the collapse decompression and the explosion that ejected the examined samples were used to calculate the growth rates as a result of magma devolatilization with an average of 8 3 x 10 8 mm s anorthite content of the plagioclase rim growth indicates an average decompression magnitude of 40 mpa inducing an undercooling of 45 c that favors crystal growth over microlite nucleation however variability in the final anorthite contents suggests that not all phenocrysts recorded an equilibrium composition reflecting accurate pressure conditions in such events occurring over short timescales 10 h lithium is a more reliable indicator of decompression induced growth than changing anorthite content due to lithium s rapid diffusivity

strain field analysis on montserrat w i as tool for assessing permeable flow paths in the magmatic system of soufriere hills volcano strain dilatometers have been operated on the volcanic island of montserrat west indies for more than a decade and have proven to be a powerful technique to approach short term dynamics in the deformational field in response to pressure changes in the magmatic system of the andesitic dome building soufriere hills volcano shv we here demonstrate that magmatic activity in each of the different segments of the shv magmatic system shallow dyke conduit upper and lower magma chambers generates a characteristic strain pattern that allows the identification of operating sources in the plumbing system based on a simple scheme of amplitude ratios we use this method to evaluate strain data from selected vulcanian explosions and gas emission events that occurred at shv between 2003 and 2012 our results show that the events were initiated by a short phase of contraction of either one or both magma chambers and a simultaneous inflation of the shallow feeder system the initial phase of the events usually lasted only tens to hundreds of seconds before the explosion gas emission started and the system recovered the short duration of this process points at rapid transport of fluids rather than magma ascent to generate the pressure changes we suggest the propagation of tensile hydraulic fractures as viable mechanism to provide a pathway for fluid migration in the magmatic system at the observed time scale fluid mobilization was initiated by a sudden destabilization of large pockets of already segregated fluid in the magma chambers our study demonstrates that geodetic observables can provide unprecedented insights into complex dynamic processes within a magmatic system commonly assessed by theoretical modeling and petrologic observations

zircon ages and geochemical compositions of the manlay ophiolite and coeval island arc implications for the tectonic evolution of south mongolia abstract numerous small dismembered ophiolite fragments occur in south mongolia but they are very poorly studied the lack of age data and geochemical analysis hampers our understanding of the paleozoic tectonic evolution of the region we conducted detailed studies on the manlay ophiolitic complex and huree volcanic rocks south of the main mongolian lineament mml to provide some constraints on these rocks the manlay ophiolite consists of dunite harzburgite pyroxenite gabbro plagiogranite basalt and chert locally with chromite mineralization in dunite the gabbro and plagiogranite yielded shrimp zircon weighted mean 206 pb 238 u ages of 509 5 ma and 482 4 ma respectively the basalt and dolerite samples of this complex show enrichment in lree and lile and negative nb ta and ti anomalies and the chrome spinel from the chromitite lens in the dunite is characterized by high cr and low tio 2 contents these features suggest a supra subduction zone ssz origin for the ophiolitic complex the huree volcanic rocks ranging from basalt to dacite display enrichment in lree and lile weak eu anomalies and distinctly negative nb ta and ti anomalies consistent with those of typical magmas in a subduction environment an andesite sample from this arc yielded a shrimp 206 pb 238 u zircon age of 487 5 ma which is the oldest reliable age for an island arc in south mongolia recognition of an early paleozoic ophiolitic complex and a coeval island arc indicates that south mongolia underwent a period of active volcanism during late cambrian to ordovician additionally the tuff overlying the ophiolitic complex and a granite intruding the ophiolite have shrimp zircon u pb ages of 391 5 ma and 304 4 ma respectively combining the available data we propose that the early paleozoic subduction accretionary complexes likely constitute the basement of the late paleozoic arc formations and correlate with the lake zone in western mongolia

granites of the southern mongolia carboniferous arc new geochronological and geochemical constraints the crust in southern mongolia is part of the central asian orogenic belt a vast accretionary orogen that records the opening and closure of the palaeo asian ocean in the late proterozoic to palaeozoic the crustal evolution of the region is revealed in basement inliers that also contain intrusion related porphyry ore bodies that are important mineral exploration targets the saykhandulaan inlier in southeast mongolia is a devonian carboniferous segment of island arc crust which is dominantly composed of extrusive and sedimentary lithologies but which also contains the oyut ulaan i type quartz monzonite intrusion a u pb zircon age for the oyut ulaan monzonite indicates emplacement at 330 0 0 5 ma to the east of the saykhandulaan inlier intrusive complexes dominate the neighbouring mandakh inlier new ages are presented for four of these plutons the bronze fox granodiorite 333 6 0 6 ma the narin hudag monzonite 333 2 0 6 ma the shuteen quartz monzonite 325 5 1 0 ma and the north mandakh granite 292 3 0 5 the intrusive bodies of the saykhandulaan and mandakh inliers have two distinct geochronological and geochemical associations 1 mid carboniferous i type monzonites that constitute the most easterly intrusive expression of the southern mongolia carboniferous arc and 2 early permian a type and peralkaline granites that represent a post orogenic phase of voluminous granite emplacement both groups are significantly younger than the nearby oyu tolgoi and tsagaan suvarga cu porphyry ore bodies which have previously been dated as early and late devonian respectively the new data presented here provide constraints on the timing of the transition from island arc magmatism to post collisional extension related magmatism in the region and possible controls on fertile and infertile granitoid intrusions with respect to cu au mineralisation

new geochemical insights into volcanic degassing magma degassing plays a fundamental role in controlling the style of volcanic eruptions whether a volcanic eruption is explosive or effusive is of crucial importance to approximately 500 million people living in the shadow of hazardous volcanoes worldwide studies of how gases exsolve and separate from magma prior to and during eruptions have been given new impetus by the emergence of more accurate and automated methods to measure volatile species both as volcanic gases and dissolved in the glasses of erupted products the composition of volcanic gases is dependent on a number of factors the most important being magma composition and the depth of gas melt segregation prior to eruption this latter parameter has proved difficult to constrain in the past yet is arguably the most critical for controlling eruptive style spectroscopic techniques operating in the infrared have proved to be of great value in measuring the composition of gases at high temporal resolution such methods when used in tandem with microanalytical geochemical investigations of erupted products are leading to better constraints on the depth at which gases are generated and separated from magma a number of recent studies have focused on transitions between explosive and effusive activity and have led to a better understanding of gas melt segregation at basaltic volcanoes other studies have focused on degassing during intermediate and silicic eruptions important new results include the recognition of fluxing by deep derived gases which buffer the amount of dissolved volatiles in the melt at shallow depths and the observation of gas flow up permeable conduit wall shear zones which may be the primary mechanism for gas loss at the cusp of the most explosive and unpredictable volcanic eruptions in this paper i review current and future directions in the field of geochemical studies of volcanic degassing processes and illustrate how the new insights are beginning to change the way in which we understand and classify volcanic eruptions

gas and ash emissions associated with the 2010 present activity of sinabung volcano indonesia abstract sinabung volcano sumatra indonesia awoke from over 1200 years of dormancy with multiple phreatic explosions in 2010 after a period of quiescence sinabung activity resumed in 2013 producing frequent explosions lava dome extrusion and pyroclastic flows from dome and lava flow collapses becoming one of the world s most active volcanoes and displacing over 20 000 citizens this study presents a compilation of the geochemical datasets collected by the indonesian center for volcanology and geological hazard mitigation cvghm from 2010 current 2016 which provides insights into the evolution of the eruption based on observations of so2 emissions ash componentry ash leachate chemistry and bulk ash geochemistry the eruption can be split into six distinct geochemical phases the initial stage of phreatic summit explosions phase a occurred from august september 2010 during which background so2 emissions averaged 550 180 t d 1 s d an eruptive pause phase b starting in october 2010 abruptly ended in september 2013 with a resumption of conduit clearing eruptions phase c this third phase had a relatively modest background so2 emission rate avg 430 310 t d and produced ash consisting of accidental ejecta with high s cl leachate molar ratios 12 0 8 2 max 34 suggestive of deep sourced magma and or the incorporation of hydrothermal sulfur bearing phases lava extrusion at the summit phase d began in mid december 2013 and was accompanied by relatively low so2 emission rates 360 200 t d and lower but variable s cl leachate ratios 6 3 8 5 the most intense phase of the eruption phase e occurred from mid january to late february 2014 following a major lava dome collapse this period included increased lava extrusion rates dozens of large eruptions per day high so2 emission rates average 1680 1070 t d peak 3800 t d and a dramatic drop in s cl ash leachates to ratios average 1 4 0 5 consistent with increased degassing from shallow magma and the clearing out of sulfurous phases from the old hydrothermal system from march 2014 through the time of writing september 2016 sinabung settled into a relatively steady state of lower activity phase f ash emissions now consist of dominantly juvenile andesitic dacitic material with low s cl leachate ratios average 1 1 0 6 in august 2016 so2 emissions started being measured in a continuous manner using a network of permanent scanning doas instruments background so2 emission rates average 450 290 t d for the entirety of phase f but have been progressively decreasing to an average of 250 300 t d since june 2016 this long term gradual decline in so2 emission rates at sinabung since early 2014 is consistent with an apparent decrease in magma supply and lava effusion rates our conceptual degassing model suggests that large explosions and pyroclastic flows could continue in the near term owing to conduit plugging and dome collapses remaining a major threat until the magma supply rate decreases further and the eruption ends

generation of sodium rich magmas from newly underplated basaltic crust sodium rich rocks of trondhjemite tonalite dacite ttd or granodiorite ttg suites form much of precambrian continental crust1 they are thought to have formed by partial melting of subducted oceanic crust2 3 a process that would have been much more widespread early in earth history than at present owing to the higher thermal gradients prevailing at that time4 phanerozoic ttd suites do exist however and seem also to relate to subduction zones5 defant and drummond6 proposed that these suites form where young 25 myr hot oceanic lithosphere is subducted and melts thus locally simulating the conditions that led to widespread crustal growth in the archaean here we describe plutonic and volcanic rocks from the cordillera blanca complex in peru which have characteristics of the high al ttd suite but which were produced above a subduction zone containing a 60 myr old slab we present evidence that the complex formed by partial melting of newly underplated basaltic crust and argue that this mechanism should be considered more generally as an additional way of generating sodium rich arc magmas

pre and syn eruptive geochemistry of volcanic gases from soufriere hills of montserrat west indies soufriere hills fumaroles contained magma derived volatiles before and during the eruption initiated in 1995 but also preserved a typical and quite steady hydrothermal composition chemical changes due to increased boiling and a greater input of oxidizing magmatic gas occurred only at galway s soufriere the most active fumarolic field hydrothermal buffering of the fumaroles has been favoured by their remote location 1 2 km from the eruptive vents and by a preferential degassing of the uprising magma through intrusive conduits under the crater high temperature 720 c gas collected from the extruding lava dome in feb 1996 was chemically and isotopically representative of the magmatic gas stream its composition allows assessment of average eruptive fluxes of h2o co2 and hcl which require the degassing of only 2 5 3 times more magma than erupted

crustal evolution of the saykhandulaan inlier mongolia implications for palaeozoic arc magmatism polyphase deformation and terrane accretion in the southeast gobi mineral belt abstract the saykhandulaan inlier in south east mongolia lies within the central asian orogenic belt caob and records a complex history of palaeozoic tectonism and magmatism associated with terrane accretion on the northern margin of the palaeo asian ocean the inlier spans the boundary between the gobi altai back arc basin terrane in the north and the mandalovoo and gurvansayhan island arc terranes in the south which are notable for their many mineralised intrusions including the oyu tolgoi gold rich copper porphyry deposit results from cross strike transects within the saykhandulaan inlier reveal that it can be subdivided into five parallel east west striking litho tectonic domains 1 the northern slate belt comprising devonian greenschist grade pelites and psammites with deep marine to coastal siliciclastic protoliths 2 the saykhandulaan valley lineament zone svlz a tectonised zone of faulted and lithologically altered volcanic rocks 3 the high strain belt consisting of tightly folded and flattened metamorphosed clastic sedimentary rocks 4 the molasse succession consisting of relatively undeformed coarse conglomerates and sandstones and 5 the oyut ulaan volcanic group a nearly 5 km thick folded carboniferous volcanic succession that hosts the mid carboniferous oyut ulaan mineralised granite the northern slate belt metasedimentary rocks record a northerly cratonic provenance whereas all rocks to the south of the svlz have arc affinities the svlz is thus interpreted to be the boundary between the gobi altai and mandalovoo terranes two major deformation events are documented 1 back arc basin closure and inversion involving regional scale folding and greenschist grade metamorphism in the northern slate and high strain belts 2 contraction associated with mandalovoo terrane accretion and final closure of the palaeo asian ocean to the south following terrane accretion and cessation of subduction crustal extension and strike slip faulting further modified the crustal architecture of the inlier the results presented here provide a useful framework for understanding the crustal evolution of adjacent regions within the southeast gobi mineral belt

late carboniferous bimodal volcanic rocks and coeval a type granite in the suman khad area southwest mongolia implications for the tectonic evolution abstract the volcanic rocks in the suman khad area in southwest mongolia form a bimodal suite consisting mainly of peralkaline rhyolites with subordinate basalts the rhyolite sample collected from the bimodal suite yielded a shrimp zircon u pb age of 314 5 ma mswd 1 41 n 12 which was interpreted to represent formation time of the bimodal volcanic suite the basalts were characterized by enrichment in lile and lree and depletion in hfse indicating their formation was related to subduction processes these features together with their positive end t values 6 3 6 7 suggest that the basalts were likely derived from a depleted mantle source metasomatized by subduction related fluids in various tectonic discrimination diagrams the basalts exhibited a transition from true arc basalts to intraplate basalts and thus were suggested to from in a back arc tectonic setting the rhyolites show a close affinity to a type granites with enrichment in lile and lree depletion in nb ta and ti and positive end t values 6 0 6 4 considering the observed distinct compositional gap between the endmembers of the bimodal suite the rhyolites are proposed to originate from partial melting of juvenile basaltic crustal rocks rather than fractional crystallization of basaltic melt the granite associated with the bimodal volcanic rocks yielded a shrimp zircon u pb ages of 312 5 ma mswd 0 75 n 13 indicating that the granite is contemporaneous with the bimodal volcanic suite the granite samples showed typical a type granitic geochemical affinities and are considered to have been formed by partial melting of crustal rocks in a within plate tectonic setting based on a combination of the available data we suggest that the late carboniferous bimodal volcanic suite together with the coeval a type granites in the suman khad region probably document a back arc basin extensional environment which probably related to the roll back of the paleo asian oceanic plate during the northward subduction under the central mongolia microcontinent

evidence for the development of permeability anisotropy in lava domes and volcanic conduits the ease at which exsolving volatiles can migrate though magma and outgas influences the explosivity of a volcanic eruption volcanic rocks often contain discrete discontinuities providing snapshots of strain localisation processes that occur during magma ascent and extrusion whether these features comprise pathways for or barriers to fluid flow is thus of relevance for volcanic eruption and gas emission modelling we report here on nine discontinuity bearing andesite blocks collected from volcan de colima mexico we present a systematic porosity and permeability study of fifty cores obtained from the blocks collected and interpret the genetic processes of the discontinuities through detailed microstructural examination bands in pumiceous blocks were inferred to be relicts of inhomogeneous bubble expansion which despite significantly increasing porosity do not markedly affect permeability other discontinuities in our blocks are interpreted to be shear strain induced flow banding cavitation porosity and or variably healed fractures in each of these cases an increase in permeability up to around three orders of magnitude was measured relative to the host material a final sample contained a band of lower porosity than the host rock characterised by variably infilled pores in this case the band was an order of magnitude less permeable than the host rock highlighting the complex interplay between dilatant and densifying processes in magma we therefore present evidence for significant permeability anisotropy within the conduit and or dome of a volcanic system we suggest that the abundance and distribution of strain localisation features will influence the escape or entrapment of volatiles and therefore the evolution of pore pressure within active volcanic systems using a simple upscaling model we illustrate the relative importance of permeable structures over different lengthscales strain localisation processes resulting in permeability anisotropy are likely to play an important role in the style magnitude and recurrence interval of volcanic eruptions

late paleozoic to early triassic multiple roll back and oroclinal bending of the mongolia collage in central asia abstract the architecture and mechanics of an orogen can be understood in terms of a system of collages that are characterized by a complex assemblage of multiple components but the fundamental paleogeographic framework and the tectonic relationships between the different components are often insufficiently defined because of unavailable data the central asian orogenic belt caob provides an ideal opportunity to address the fundamental framework of paleogeography and tectonic relationships between the diverse and many components in this huge collage in this paper we review several lines of available evidence which enable us to propose a new tectonic model of huge roll back in the formation of the accretionary tectonics of the mongolian collage in central asia in the early paleozoic the mongolia collage comprised the southern siberian and the tuva mongol oroclines the siberia craton and the mongolia collage jointly formed a giant tadpole shape within the paleo asian and panthanlassic oceans its head siberia was to the south and the tail tuva mongol to the northwest the structures and tectonic zonation of the mongolia collage are characteristic of multiple arcs which have been separately described in detail in different segments southwards from the southern siberia east sayan west sayan gorny altai chara via the lake zone junggar tianshan gobi altai beishan alxa to the manlay hegenshan baolidao solonker segments almost all segments underwent early paleozoic to permian or even triassic frontal subduction and accretion while rifting in the late carboniferous to permian or triassic occurred in the outward oceanward westward advancing mongolian collage therefore we suggest that a huge complex roll back active from the carboniferous to permian or even to late triassic facilitated the formation of the mongolian collage the outward multiple roll back process was compatible and almost coeval with the start of the tuva mongol orocline and rotation of the siberian craton as confirmed by paleomagnetic and structural data during the roll back processes an archipelago paleogeography was formed behind the frontal subduction and accretion in which independent arcs or terranes were amalgamated or collided to form composite arcs or terranes either simultaneously or at slightly different times the roll back process was affected by the collision of the kazakhstan collage along the chara and karamay zones in the early permian the collision of the tarim craton along the south tianshan zone in the early permian the collision of the dunhuang block along the liuyuan zone in the early permian triassic the collision of the alxa block along the qugan qulu zone in the permian and the collision of the north china craton along the solonker zone in the middle late triassic the tectonic styles and architecture of accretionary orogenic belts like the caob are characterized both by the amalgamation of multiple terranes and by oroclinal bending the systematic anatomy of the multiple roll back processes and their interactions with the adjacent collages shed light on the evolving orogenic architecture and the crustal accretionary history of orogens

injection of vesicular magma into an andesitic dome at the effusive explosive transition article i nfo a single vesicular pyroclast collected from a major 11 10 6 m 3 dome collapse eruption of the soufriere hills volcano montserrat 21st september 1997 was found to contain textural and raman evidence for multiple decompression events and magma injection into the dome just prior to the onset of dome collapse quartz plagioclase and hornblende phenocrysts contain closed and burst melt inclusions mis the latter in the form of either foam or cylindrical melt extrusions from cavities on fracture surfaces fractures within ruptured quartz phenocrysts were infilled with glass whereas those within plagioclase are entirely free of glass indicatingrelativelyearlyfracturing decompression ofthequartz thisinterpretationissupportedbyslightly higher measured h2otin burst mis in quartz 0 44 wt 0 07 1 s d compared with plagioclase 0 31 wt 0 05 1 s d indicating equilibration on rupturing of the quartz at higher ph2o h2ot levels in glass from burst mis in plagioclase are consistent with plagioclase fragmentation at h2o saturation pressures b5 mpa at an estimated depth of b210 m within the dome the fragmentation occurred whilst the groundmass glass was still plastic as evident from the presence of stretched vesicles in glass at fracture terminations the fractures in plagioclase are empty of vapour phase precipitates mainly cristobalite that infills matrix vesicles which constrains the fracturing of plagioclase to the last depressurisation event recorded in the pyroclast which was the dome collapse which led to the pyroclast being ejected from the dome we suggest that our study provides the first direct i e non seismic evidence for the injection of gas charged magma into the dome at the onset of dome collapse marking the transition from effusive to explosive eruption and that the magma injection could have provided the triggering mechanism for dome collapse

subduction zone infancy examples from the eocene izu bonin mariana and jurassic california arcs a new model for the earliest stages in the evolution of subduction zones is developed from recent geologic studies of the izu bonin mariana ibm arc system and then applied to late jurassic ophiolites of cailfornia the model accounts for several key observations about the earliest stages in the evolution of the ibm system 1 subduction nucleated along an active transform boundary which separated younger less dense lithosphere in the west from older more dense lithosphere to the east 2 initial arc magmatic activity occupied a much broader zone than existed later 3 initial magmatism extended up to the modern trench over a region now characterized by subnormal heat flow 4 early are magmatism was characterized by depleted tholeiitic and ultra depleted boninitic magmas indicating that melting was more extensive and involved more depleted mantle than is found anywhere else on earth 5 early arc magmatism was strongly extensional with crust forming in a manner similar to slow spreading ridges and 6 crust production rates were 120 to 180 km 3 km ma several times greater than for mature arc systems these observations require that the earliest stages of subduction involve rapid retreat of the trench we infer that this resulted from continuous subsidence of denser lithosphere along the transform fault this resulted in strong extension and thinning of younger more buoyant lithosphere to the west this extension was accompanied by the flow of water from the sinking oceanic lithosphere to the base of the extending lithosphere and the underlying asthenosphere addition of water and asthenospheric upwelling led to catastrophic melting which continued until lithosphere subsidence was replaced by lithospheric subduction application of the subduction zone infancy model to the late jurassic ophiolites of california provides a framework in which to understand the rapid formation of oceanic crust with strong arc affinities between the younger sierran magmatic arc and the franciscan subduction complex provides a mechanism for the formation and subsidence of the great valley forearc basin and explains the limited duration of high t high p metamorphism experienced by franciscan melanges

catastrophic lava dome failure at soufriere hills volcano montserrat 12 13 july 2003 the lava dome collapse of 12 13 july 2003 was the largest of the soufriere hills volcano eruption thus far 1995 2005 and the largest recorded in historical times from any volcano 210 million m3 of dome material collapsed over 18 h and formed large pyroclastic flows which reached the sea the evolution of the collapse can be interpreted with reference to the complex structure of the lava dome which comprised discrete spines and shear lobes and an apron of talus progressive slumping of talus for 10 h at the beginning of the collapse generated low volume pyroclastic flows it undermined the massive part of the lava dome and eventually prompted catastrophic failure from 02 00 to 04 40 13 july 2003 large pyroclastic flows were generated these reached their largest magnitude at 03 35 when the volume flux of material lost from the lava dome probably approached 16 million m3 over two minutes the high flux of pyroclastic flows into the sea caused a tsunami and a hydrovolcanic explosion with an associated pyroclastic surge which flowed inland a vulcanian explosion occurred during or immediately after the largest pyroclastic flows at 03 35 13 july and four further explosions occurred at progressively longer intervals during 13 15 july 2003 the dome collapse lasted approximately 18 h but 170 of the total 210 million m3 was removed in only 2 6 h during the most intense stage of the collapse

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

cyclic shear zone cataclasis and sintering during lava dome extrusion insights from chaos crags lassen volcanic center usa abstract the ascent and extrusion of crystal rich magma is commonly facilitated by deformation partitioned within annular conduit parallel shear zones the physical properties and textures of the shear zone materials where exposed at surface provide a record of ascent and eruption dynamics we describe the shear zone developed in dome c part of chaos crags in the lassen volcanic center california usa the extruded shear zone comprises volcanic fault gouge and variably densified cataclasites the competent cataclasites evidence deep seated gouge production followed by gouge densification within the conduit on the timescale of lava dome ascent textural geochemical and mineralogical data identify solid state sintering as the densification mechanism at the temperatures and pressures in the volcanic conduit solid state sintering causes rapid porosity and permeability loss within the gouge and concomitant material strengthening longer dwell times i e slower ascent allow for more sintering producing stronger denser and less permeable cataclasites at chaos crags we use the extent of sintering quantified by residual porosity to recover minimum in conduit dwell times necessary to produce the observed cataclasites our analysis of the dome c cataclasites suggests a maximum linear ascent rate of 10 m day and a minimum ascent time of 100 days we evaluate the consequences of shear zone lithification by solid state sintering for the eruption of other crystal rich glass poor magmas chaos crags cataclasites preserve evidence of multiple cycles of fracturing cataclasis and re sintering suggesting a mechanism for transitions between effusive and explosive phases of dome building eruptions

real time and in situ assessment of conduit permeability through diverse long period tremors beneath aso volcano japan abstract long period signal lps 0 2 2 s and very long period signal vlp 2 100 s observed in the shallow volcanic plumbing system are typically repetitive and time invariant in their location and source mechanism offering in situ probes of hot fluid transport over the eruption cycles while the amplitude and activity of volcanic tectonic earthquakes and lp events have been commonly used to infer overpressure within their source region one missing link is an observable that may permit inference on the change in the permeability of the conduit plug wall which can regulate the degree of pressurization affect the mechanical strength of the surrounding rock and consequently the likelihood of an upcoming eruption here we show that during the 2011 2016 eruption cycle at aso volcano in japan long period tremor events a vlp of 15 s period with opposite waveform polarity can be systematically detected and categorized as pressurization and depressurization events in the same crack like conduit we suggest that depending on the strength of the surrounding rock and the permeability of the crack like conduit wall plug pressurization due to magmatic heat and vaporization is more likely to occur when a less permeable conduit plug wall can effectively keep the gas inside the crack like conduit on the other hand depressurization is prone to occur if the conduit wall plug permeability is sufficiently high to allow gas to escape from the conduit considering the amplitude of lpt proportional to the conduit overpressure contrasting energetics of these diverse lpt events allows us to define whether the conduit is prone to pressurization or depressurization providing a framework to infer how the permeability of the conduit wall plug may evolve over an eruption cycle

geochemistry and geochronology of mississippian volcanic rocks from sw mongolia implications for terrane subdivision and magmatic arc activity in the trans altai zone abstract baaran and baytag terranes form important part of the trans altai zone of the central asian orogenic belt in the sw mongolia the trans altai zone represents an oceanic domain built by ordovician to devonian ophiolites covered by devonian carboniferous oceanic sediments this oceanic assemblage was intruded by a spectrum of carboniferous volcanic rocks ranging from basalts to rhyolites 325 6 1 4 ma to 351 0 5 9 ma la icp ms u pb dating on zircon that display similar whole rock trace element and sr nd isotopic signatures regardless their silica contents and the terrane they belong to they are normal k calc alkaline rocks whose nmorb normalized multielement patterns show an arc like enrichment in lile large ion lithophile elements and deep negative tnt ta nb ti anomalies based on the relatively high nb yb and th yb ratios the arc was probably formed on immature continental rather than typical oceanic crust low 87sr 86sr i 0 7043 0 7047 high e nd 350 5 2 to 5 8 and the observed in situ hf isotopic variation in dated zircons e hf t 7 4 to 13 are all attributed to a partial melting of primitive and youthful neoproterozoic ordovician arc type lower continental crust metamorphosed intermediate acid igneous rocks and or arc derived greywackes augmented with and most likely facilitated by contemporaneous arc related basaltic intrusions a distinct possibility remains the previously formulated model assuming that the carboniferous magmatism in gobi altai and trans altai zones sampled the juvenile neoproterozoic to cambrian arc crust relaminated under the ordovician devonian oceanic crust in any case the studied arc association together with lithological sequence of ophiolites can be closely correlated with the dulate arc in eastern junggar it is proposed that the two arc sequences form a single belt c 500 km long that represents one of the main known arc structures in the central asian orogenic belt

adjoint inversion of the thermal structure of southeastern australia summary the variation of temperature in the crust is difficult to quantify due to the sparsity of surface heat flow observations and lack of measurements on the thermal properties of rocks at depth we examine the degree to which the thermal structure of the crust can be constrained from the curie depth and surface heat flow data in southeastern australia we cast the inverse problem of heat conduction within a bayesian framework and derive its adjoint so that we can efficiently find the optimal model that best reproduces the data and prior information on the thermal properties of the crust efficiency gains obtained from the adjoint method facilitate a detailed exploration of thermal structure in se australia where we predict high temperatures within precambrian rocks of 650 c due to relatively high rates of heat production 0 9 1 4 w m 3 in contrast temperatures within dominantly phanerozoic crust reach only 520 c at the moho due to the low rates of heat production in cambrian mafic volcanics a combination of the curie depth and heat flow data is required to constrain the uncertainty of lower crustal temperatures to 73 c we also show that parts of the crust are unconstrained if either data set is omitted from the inversion

late paleozoic magmatic record of middle gobi area south mongolia and its implications for tectonic evolution evidences from zircon u pb dating and geochemistry abstract late paleozoic subduction accretion complexes occur widely in middle gobi area and provide a good opportunity for unraveling the paleozoic tectonic evolution of south mongolia the magmatic rocks in the tsavchir hudug district mainly consist of rhyolites and volcaniclastic rocks the rhyolites show enrichment in lree and lile and negative nb ta and ti anomalies indicating genesis in the subduction zone a rhyolite sample from the tsavchir hudug region yielded a shrimp 206pb 238u zircon age of 315 4 ma mswd 0 79 n 15 the andesite overlying the namdain hundy early paleozoic ophiolite shows adakite geochemical features and the two andesite samples yielded shrimp 206pb 238u zircon ages of 325 3 ma mswd 1 6 n 14 and 319 4 ma mswd 0 56 n 13 respectively suggesting that the carboniferous island arc formed on the basis of early paleozoic accretionary complex the granodiorite sample that intrudes the early paleozoic accretion complex with adakite geochemical features yielded a shrimp 206pb 238u zircon age of 333 4 ma mswd 1 6 n 16 representing the late paleozoic island arc intrusive the shrimp u pb analyses for the tuff sandstones that occur associated with early paleozic oceanic inliers in middle gobi area suggest detrital zircons mainly stem from the devonian carboniferous arc the age data obtained from the ophiolite 528 509 ma and tuff sandstone indicate the accretion in middle gobi area lasted from early paleozoic to late paleozoic for at least ca 200 ma suggesting the ocean of the accretionary complex was the major paleo asain ocean basin the subduction related magmatic belt in middle gobi area includes both early paleozoic and late paleozoic island arc activities which is consistent with the accretion duration time obtained from accretionary complex and also attests the argument of major paleo asain ocean basin

unprecedented pressure increase in deep magma reservoir triggered by lava dome collapse 1 the collapse of the soufriere hills volcano lava dome on montserrat in july 2003 is the largest such event worldwide in the historical record here we report on borehole dilatometer data recording a remarkable and unprecedented rapid 600s pressurisation of a magma chamber triggered by this surface collapse the chamber expansion is indicated by an expansive offset at the near dilatometer sites coupled with contraction at the far site by analyzing the strain data and using added constraints from experimental petrology and long term edifice deformation from gps geodesy we prefer a source centered at approximately 6 km depth below the crater for an oblate spheroid with overpressure increase of order 1 mpa and average radius 1 km pressurisation is attributed to growth of 1 3 of gas bubbles in supersaturated magma triggered by the dynamics of surface unloading recent simulations demonstrate that pressure recovery from bubble growth can exceed initial pressure drop by nearly an order of magnitude

geodetic constraints on the shallow magma system at soufriere hills volcano montserrat 1 ground surface tilts recorded proximal to a growing lava dome and sustained through repeated dome collapses provide unique geodetic constraints on the depth size and overpressures of the shallow magmatic system at soufriere hills volcano tilt records identify a single pressure source inflating with periods 3 30 h sustained tilt amplitude ratios for paired stations identify the pressure source at a centroid depth between 740 and 970 m this pressure source and its robust oscillatory response survived a dome collapse that removed the upper 200 m of the magmatic system shallow overpressures of 1 4 mpa are confirmed over a radius of 200 340 m this size suggests pressurization of a fluid saturated fractured rock mass annulus surrounding a narrow magma conduit

insights into eruption dynamics from the 2014 pyroclastic deposits of kelut volcano java indonesia and implications for future hazards abstract on 13 to 14 february 2014 a 4 h long vei 4 eruption occurred at kelut volcano java indonesia pyroclastic density currents pdcs and extensive ash fall led to 7 fatalities and disruption to flights across the asia pacific region new sedimentological descriptions of the pyroclastic deposits from the 2014 eruption were compared with eyewitness and satellite reports to elucidate temporal variations in eruptive dynamics the stratigraphy of the deposits is presented in 3 stages associated with two eruptions that occurred approximately 15 30 min apart stage 1 pdc deposits originate from the smaller onset eruption the pdc deposits from stage 2 and tephra fall deposits from stage 3 originate from the second plinian eruption during the onset eruption stage 1 low energy pdcs were produced that ran out to 70 by vol pdc deposits ran out to 4 7 km from the vent the deposits reflect an increased output of fresh fragmented magma and some conduit widening evidenced by dense lithic fragments vent instabilities and incorporation of dense material into basal margins of the plume led to the marginal collapse and formation of these pdcs stage 3 occurred in the final hour at the peak of the plinian eruption around 01 00 to 02 00 and produced reversely graded lapilli fall deposits with 90 vol pumice from a 26 km high plume this indicates that there was a sustained flux of juvenile magma to the now open vent system and expansion and fragmentation of the gas rich magma was at its most efficient our study of the eruptive sequence of kelut provides some constraints on predicted patterns for future explosive activity critical for further hazard assessment of the volcano since 1901 kelut has erupted on intervals of 1 to 23 years and the 2014 event characterises a typical explosive style of eruption that alternates regularly with effusive dome formation and collapse events this pattern depends on the dynamics of magma renewal to the system and degassing conditions in the shallow magma reservoir and upper conduit if this pattern holds and the next eruption occurs within the next two decades a return of prolonged dome growth could be anticipated

constraints on magma flux from displacements data at merapi volcano java indonesia the displacement field has been monitored in the vicinity of the crater rim at mount merapi indonesia from 1993 to 1997 during this period the volcanic activity has been quasicontinuous with dome growth explosions and pyroclastic flows we measured a nine point network every year with the global positioning system static method interpretation of results is conducted with a three dimensional elastostatic boundary elements code that takes into account topography fractures and complex magma source geometry the inversion technique yields an estimate of the variation with time of the boundary conditions at the magma duct interface together with the probability associated with the best model the young s modulus of the equivalent continuum is found to be very low of the order of 1 gpa a feature which suggests that a viscoelastic behavior may be more appropriate for this rock mass given the observed seismic velocities for the domain of interest a striking compatibility is outlined between observed deformations and the rate of occurrence of multiphase seismic events once the main fractures of the structure have been taken into consideration this suggests that the summit elastic or viscoelastic deformation field is controlled by the magma flux within the duct rather than by magma pressure variations in addition a nonelastic displacement has been identified at the westernmost point of the network this was considered critical for the stability of the summit structure a concern whose validity has been verified a posteriori by the july 1998 explosion

heterogeneous clasts as windows into magma mingling at soufriere hills volcano 1 a clast erupted from soufriere hills volcano during the syn collapse vulcanian explosions of july 2003 reveals several incorporated silicic zones within a more mafic host these incorporated silicics display variable microlite contents that reflect different stages of magma crystallization one zone a shows evidence of heating in several mineralogical features including completely pseudomorphed hornblende crystals that broke down over a minimum time period of 37 78 days the lower limit of this time period roughly correlates with the onset of hybrid seismic swarms prior to the 2003 dome collapse element x ray maps reveal percolation of ca rich melt into the incorporated silicic margins suggesting sustained contact that allowed isolation of portions of the resident magma these characteristics suggest that magma mixing at soufriere hills may not only result from disaggregation of mafic inclusions but also from disaggregation and incorporation of the resident magma into the mafic intrusion

a model of degassing at galeras volcano colombia 1988 1993 galeras volcano reactivated in early 1988 as evidenced by increased seismicity gas emissions and fumarole temperatures and had a series of explosive eruptions in may 1989 during 1989 1990 the volcano was characterized by high so 2 flux reaching 5000 t metric tons day this period of strong degassing was followed by upward movement of magma july september 1991 and the emplacement of a lava dome october november 1991 since november 1991 long period seismicity and so 2 flux have decreased indicating that the conduit has partially sealed itself probably by crystallization and solidification of the magma on july 16 1992 the dome was destroyed by an explosion consistent with pressurization due to sealing and degassing in a nonporous medium continued pressurization beneath the volcano is indicated by eruptions on january 14 march 23 april 4 april 13 and june 7 1993 glass inclusion and matrix glass analyses support a hypothesis of significant magma degassing at depth in the magma chamber and in the conduit this degassing has reduced the h 2 o s and cl contents of the magma but the f content appears to have increased due to crystallization of anhydrous mineral phases and to the low fluid melt partition coefficient for f despite its degassed state galeras is currently a dangerous volcano because the sealing process forces gas saturated magma to release volatile components into confined spaces thereby causing pressurization and explosive eruptions

extrusion cycles during dome building eruptions abstract we identify and quantify controls on the timescales and magnitudes of cyclic periodic volcanic eruptions using the numerical model domeflow de michieli vitturi et al 2010 which was developed by the authors for magma systems of intermediate composition domeflow treats the magma mixture as a liquid continuum with dispersed gas bubbles and crystals in thermodynamic equilibrium with the melt and assumes a modified poiseuille form of the viscous term for fully developed laminar flow in a conduit of cylindrical cross section during ascent magma pressure decreases and water vapor exsolves and partially degasses from the melt as the melt simultaneously crystallizes causing changes in mixture density and viscosity two mechanisms previously proposed to cause periodic eruption behavior have been implemented in the model and their corresponding timescales explored the first applies a stick slip model in which motion of a shallow solid plug is resisted by static dynamic friction as described in iverson et al 2006 for a constant magma supply rate at depth this mechanism yields cyclic extrusion with timescales of seconds to tens of seconds with values generally depending on assumed friction coefficients the second mechanism does not consider friction but treats the plug as a high viscosity newtonian fluid during viscous resistance pressure beneath the degassed plug can increase sufficiently to overcome dome overburden plug weight and viscous forces and ultimately drive the plug from the conduit in this second model cycle periods are on the order of hours and decrease with increasing magma supply rate until a threshold is reached at which point periodicity disappears and extrusion rate becomes steady vanishingly short periods magma volatile content for fixed chamber pressure has little effect on cycle timescales but increasing volatile content increases mass flow rate and cycle magnitude as defined by the difference between maximum and minimum extrusion rates increasing mass flow rate also increases the magnitude of predicted deformation cycles both models have been applied to dome building eruptions using conditions appropriate for the soufriere hills volcano montserrat results suggest that the modeled stick slip mechanism cannot explain the cycles of extrusion and explosion well documented at montserrat timescales of hours however a subset of the simulations for the viscous plug model are consistent with the montserrat data and therefore demonstrate the feasibility of this second formulation

textural and micro petrological variations in the eruptive products of the 2006 dome forming eruption of merapi volcano indonesia implications for sub surface processes the interplay between magma ascent degassing and changing magmatic properties are widely recognized as critical factors controlling the style of silicic volcanic eruptions microlite textures in samples from the prolonged dome forming eruption of merapi in 2006 provide a record of changing magmatic ascent conditions and shallow conduit processes throughout the eruption analysis of microlite textural parameters including measurements of areal number density na mean microlite size crystal aspect ratio and groundmass crystallinity combined with the monitoring record and field observations indicate that magma ascent paths change between continuous ascent at varying rates from a deeper magma storage region to ascent being temporarily stalled at shallow depths in the latter stages of the eruption supporting the idea of an ephemeral shallow magma storage region at merapi plagioclase microlite compositions show evidence of decompression induced degassing often displaying rims of anorthoclase and more k rich alkali feldspar sanidine anorthite contents also support the textural data of later erupted magma being temporarily stalled at shallow depths crystal size distributions csds are interpreted to show that both growth dominated and nucleation dominated crystallisation regimes existed during the 2006 eruption resulting from changing conditions of undercooling t during variable magma ascent paths by contrast microlite textural analysis and feldspar microlite compositions of samples from the fast growing lava dome of the second phase of the 2010 eruption prior to the cataclysmic events on 5 november indicate faster ascent rates a crystallisation regime more strongly dominated by nucleation due to high t and interaction of the 2010 magma with more hotter magma from depth

analysis and forward modeling of seismic anisotropy during the ongoing eruption of the soufriere hills volcano montserrat 1996 2007 1 volcanic stress field analysis has been lauded as a potentially powerful tool for midterm to long term eruption forecasting however because tectonic processes can also produce localized stress field reorientations evidence for a direct causal link between observed stress field reorientations and magmatic activity is of critical importance in this study we show that local stress field reorientations preceding changes in volcanic activity at the soufriere hills volcano montserrat are observable using two independent measures of crustal stress local volcano tectonic earthquake fault plane solutions and measurements of shear wave splitting in regional earthquakes we further demonstrate that the local stress field orientation during a 6 month period preceding the onset of eruptive activity at soufriere hills in 1999 is highly localized and spatiotemporally variable and that the spatial pattern of precursory local stress orientations is consistent with numerically modeled patterns of stress resulting from pressurization of a vertical dike these observations provide compelling evidence for a direct causal link between pressurization of midlevel volcanic conduit systems by ascending magma and precursory local stress field reorientations and demonstrate that seismological analysis can be used to detect subtle local changes in stress that herald eruptive activity

gas storage transport and pressure changes in an evolving permeable volcanic edifice abstract the total volume of gas in a magma dissolved and subsequently exsolved greatly influences the degree of explosiveness of a volcanic system there is a marked contrast between the behaviour of a volcano in an open system compared to one which is closed it is therefore essential to understand the entire degassing process gas transport storage and loss a particular focus of this study is the effect different permeabilities and pressure gradients within a volcanic edifice have on the degree and pattern of the gas velocity gas loss is modelled numerically in two dimensions using a finite element approach which allows the specification of boundary conditions with respect to pressure and different permeability domains within the volcanic edifice by combining the time dependent continuity equation and darcy s law a partial differential equation is derived and solved for the pressure the associated pressure gradient is then used within darcy s law to determine the corresponding gas velocity distribution this method is used not only for stationary systems in equilibrium but also as a time dependent progression it permits the modelling of different situations to study how various volcanic characteristics affect the gas loss the model is used to investigate the change in pressure and gas in response to time dependent scenarios these are a dome collapse or sudden increase in permeability by magma rupture at the conduit margin the formation of cracks within the lava dome and sealing by crystallisation our results show that a combination of high and low permeability regions is required for effective gas storage high permeability allows the gas to enter the system but impermeable areas act to confine the gas thereby increasing its pressure and consequently increasing the amount of gas which may be dissolved in the melt furthermore our results show that permeability is an essential factor influencing the response time to system changes which could be linked in future to deformation and other geophysical observations our model is highly versatile and sheds new light on the understanding of gas storage and transport in a permeable volcanic edifice

shear stress along the conduit wall as a plausible source of tilt at soufriere hills volcano montserrat 1 surface deformations recorded in close proximity to the active lava dome at soufriere hills volcano montserrat can be used to infer stresses within the uppermost 1000 m of the conduit system most deformation source models consider only isotropic pressurisation of the conduit we show that tilt recorded during rapid magma extrusion in 1997 could have also been generated by shear stresses sustained along the conduit wall these stresses are a consequence of pressure gradients that develop along the conduit numerical modelling incorporating realistic topography can reproduce both the morphology and half the amplitude of the measured deformation field using a realistic shear stress amplitude equivalent to a pressure gradient of 3 5 104 pa m 1 along a 1000 m long conduit with a 15 m radius this shear stress model has advantages over the isotropic pressure models because it does not require either physically unattainable overpressures or source radii larger than 200 m to explain the same deformation

automated high time resolution measurements of so2 flux at soufriere hills volcano montserrat we report here the first results from an automated telemetered uv scanning spectrometer system for monitoring so2 emission rates at soufriere hills volcano montserrat two spectrometers receive light by way of a motor driven stepping prism and telescope in order to make vertical scans of the volcanic plume spectral data from these spectrometers situated 2 800 m apart and 4 500 m from the volcano are relayed back to the observatory every 4 5 s via radio modems a full scan of the plume is accomplished every 1 6 min by the time synchronised spectrometers and a so2 emission rate is calculated using the so2 slant concentrations scan angles and plume speeds estimated from the wind speed from a telemetered weather station near to the volcano the plume s position and dimensions are calculated using the angular data from the two spectrometers the plume height varies significantly diurnally and seasonally and is important in order to minimise the error on so2 emission rates the new scanning system scanspec provides so2 emission rates from 08 00 to 16 00 h local time every day preliminary results highlight a number of features of the so2 time series and plume dynamics and give our first indications of the errors and limits of detection of this system so2 emission rates vary widely on all time scales minutes days months this new system has already provided the first real and consistent indication that so2 emission rates vary on a minutes to hours basis which can be correlated with volcanic activity for example rockfall and pyroclastic flow activity it is anticipated that this system at soufriere hills will yield information on shallow processes occurring on short time scales periods of minutes to hours as well as deep processes relating to magma supply rates which will be associated with longer wavelength so2 signals of weeks to months

a volcanic degassing event at the explosive effusive transition 1 a sequence of five vulcanian explosions followed a lava dome collapse in july 2003 at soufriere hills volcano each explosion occurred at t 190 n4 3 where n 1 5 and t is the time s since the decompression rate peak during the collapse instead of a sixth explosion at the predicted time a rapid emission of 97 103 kg so2 was observed by a spectrometer network this event represents the transition from explosive to effusive activity after the last explosion high magma ascent rates were maintained but the critical overpressure explosion criterion was not reached instead degassing and crystallisation in the upper conduit caused horizontal gradients in viscosity and flow rate and brittle failure at the walls when the rate of shear strain exceeded a critical value development of a permeable shear zone allowed gas release relief of overpressure and a return to effusive lava dome building

tracking the permeable porous network during strain dependent magmatic flow abstract rheological variations have been postulated as the cause of transitions from effusive to explosive volcanic eruption style rheology is integrally linked to the composition and textural state porosity crystallinity of magma as well as the stress temperature and strain rate operative during flow this study characterises the rheological behaviour and importantly the evolution of physical properties of two magmas with different crystallinity and porosity from volcan de colima mexico a volcanic system known for its rapid fluctuations in eruption style magma samples deformed in a uniaxial press at a constant stress of 2 8 12 or 24 mpa a constant temperature of 940 945 c comparable to upper conduit or lava dome conditions to strains of 20 or 30 displayed different mechanical behaviour and significant differences in measured strain rates 10 2 10 5 s 1 the evolution of porosity permeability dynamic young s modulus and dynamic poisson s ratio illustrate a complex evolution of the samples manifested as strain hardening visco elastic constant rate and strain weakening deformation both magmas behave as shear thinning non newtonian liquids and viscosity decreases as a function of strain we find that strain localisation during deformation leads to the rearrangement and closure of void space a combination of pores and cracks followed by preferentially aligned fracturing in the direction of the maximum principal stress to form damage zones as well as densification of other areas in a dome setting highly viscous low permeability magmas carry the potential to block volcanic conduits with a magma plug resulting in the build up of pressures in the conduit above a certain threshold of strain dependent upon stress strain rate the initiation propagation and coalescence of fractures leads to mechanical degradation of the magma samples which then supersedes magmatic flow and crystal rearrangement as the dominant form of deformation this results in lower apparent viscosities than those anticipated for magma of such crystallinity especially at high strain rates in a lava dome this could result in dome collapse and the concomitant depressurisation could trigger an explosive eruption

dense welding caused by volatile resorption welding of pyroclastic rocks is generally thought to occur by mechanical expulsion of interstitial gas from a deposit as it compacts under its own weight we propose here that volatile resorption and compression can also be important factors in welding we describe densely welded rocks which cannot be explained by loading and re evaluate the welding process taking into account the effects of volatile resorption into the glassy fragments intra caldera oligocene ignimbrites from the rhodope mountains bulgaria and intrusive tuffs of the loch ba ring dyke mull in scotland are intensely welded but some samples lack foliation eutaxitic texture fiamme and glass shards show no preferred orientation in thin section or on outcrop scale fiamme are sometimes complexy deformed into u or s shapes we propose that these textures are the consequence of volatile dissolution in the glass and gas compression scaling analysis indicates that there are two regimes for gas behaviour following emplacement of hot pyroclastic deposits which depend on the relative characteristic time scales of compaction gas escape and gas resorption a gas escape regime and a gas retention regime during explosive eruption glassy pyroclasts are decompressed to less than one atmosphere pressure and are outgassed during deposition and subsequent burial in a thick hot deposit volatiles will be retained and soluble gases h 2 o can be resorbed back into the glass defining the gas retention regime poorly soluble gases air are compressed to small fractions of their original volume resulting in destruction of pore spaces and vesicles in pumice in some circumstances such as volcanic vents the volume changes involve isotropic strain and implosion of the tuff resorption of water greatly speeds up welding during compactional loading by reducing the viscosity of the glass welding is inhibited not only by lower temperatures but also by entrainment of insoluble atmospheric gases the two regimes also can explain the common presence of post emplacement gas escape pipes in non welded ignimbrites and their rarity in densely welded ignimbrite factors that are likely to promote the gas retention regime include fine grain size low collapsing columns a large thickness of tuff and incorporation of external water

permeability of compacting porous lavas the highly transient nature of outgassing commonly observed at volcanoes is in part controlled by the permeability of lava domes and shallow conduits lava domes generally consist of a porous outer carapace surrounding a denser lava core with internal shear zones of variable porosity here we examine densification using uniaxial compression experiments on variably crystalline and porous rhyolitic dome lavas from the taupo volcanic zone experiments were conducted at 900 c and an applied stress of 3 mpa to 60 strain while monitoring acoustic emissions to track cracking the evolution of the porous network was assessed via x ray computed tomography he pycnometry and relative gas permeability high starting connected porosities led to low apparent viscosities and high strain rates initially accompanied by abundant acoustic emissions as compaction ensued the lavas evolved apparent viscosity increased and strain rate decreased due to strain hardening of the suspensions permeability fluctuations resulted from the interplay between viscous flow and brittle failure where phenocrysts were abundant cracks had limited spatial extent and pore closure decreased axial and radial permeability proportionally maintaining the initial anisotropy in crystal poor lavas axial cracks had a more profound effect and permeability anisotropy switched to favor axial flow irrespective of porosity both crystalline samples compacted to a threshold minimum porosity of 17 19 whereas the crystal poor sample did not achieve its compaction limit this indicates that unconfined loading of porous dome lavas does not necessarily form an impermeable plug and may be hindered in part by the presence of crystals

early paleozoic oceanic inliers and reconstruction of accretionary tectonics in the middle gobi region mongolia evidence from shrimp zircon u pb dating and geochemistry abstract the ophiolites that occur as inliers among the late paleozoic formations in the middle gobi area are crucial for understanding the tectonic evolution of south mongolia in this paper we conducted detailed studies on the namdain hundy ophiolite to provide some constraints on the early paleozoic evolution of the middle gobi region in mongolia the ophiolite mainly consists of ultramafic rocks carbonatation plagiogranite metagabbro basalt and chert the metagabbro and plagiogranite from namdain hundy ophiolite yielded shrimp zircon u pb ages of 528 7 ma and 519 5 ma respectively though most of the volcanic rocks of this ophiolite show supra subduction zone ssz affinity samples with oib and n morb geochemical features were also identified indicating genesis in a forearc setting the granodiorite intruding into the namdain hundy ophiolite yielded a shrimp zircon u pb age of 491 3 ma which constrained the upper age limit of ophiolite emplacement this granodiorite shows adakitic geochemical affinity attesting to the existence of cambrian paleo subduction in south mongolia based on the available data so far we suggest the middle gobi area comprises of the manlay accretion complex the island arc and the biluutiin ovoo back arc basin the spatial configuration of these three tectonic belts suggests that the polarity of the paleo ocean subduction was from south to north in the early paleozoic forming a trench arc basin system south of the central mongolia microcontinent

underplating related adakites in xinjiang tianshan china abstract intermediate to acidic porphyries and dacites in the awulale and sanchakou area of the xinjiang tianshan region china conform to the definition of high sio2 adakite hsa these volcanic or subvolcanic rocks are characterized by high na2o na2o k2o high sr y 51 327 and strong depletions in hree and hfse nb ta and ti despite these characteristics they are distinct from a suite of adakites in the same area that are inferred to be derived from partial melting of subducted slab differences between the two suites are evident mainly in their rock association isotopic ages and compositions of sr and nd isotopes the subducted slab related adakites are associated with common island arc rocks niobium enriched basalt neb and high mg andesite hma formed in the late carboniferous 320 334 ma and are characterized by relative high nd t 3 4 11 6 and low 87sr 86sr i ratios 0 7032 0 7063 in contrast the awulale and sanchakou adakites were generated during the middle to late permian 260 278 ma possess a wide range of low nd t 0 75 5 69 a greater range of 87sr 86sr i 0 7039 0 7054 and have young nd model ages t2dm 472 699 ma in addition their mg 35 56 and compatible element contents mgo 0 93 2 22 wt cr 4 95 16 41 ppm ni 2 9 25 8 ppm are relative low compared to the subducted slab related adakites mg 55 71 mgo 1 22 6 78 wt cr 24 132 ppm ni 2 28 45 61 ppm based on these characteristics the regional association of igneous rocks and evidence from a global geoscience transect ggt the awulale and sanchakou adakites represent a genetically distinct suite in the xinjiang region we proposed that the source of these adakites was basaltic rock underplated to the base of the lower crust that partial melted at the rutile bearing amphibole eclogite facies 650 c and 1 5 gpa minimum pressure 50 km