spine growth and seismogenic faulting at mt unzen japan the concluding episode of activity during the recent eruption of mt unzen october 1994 to february 1995 was characterized by incremental spine extrusion accompanied by seismicity analysis of the seismic record reveals the occurrence of two dominant long period event families associated with a repeating nondestructive source mechanism which we attribute to magma failure and fault controlled ascent we obtain constraints on the slip rate and distance of faulting events within these families that analysis is complemented by an experimental thermomechanical investigation of fault friction in mt unzen dacitic dome rock using a rotary shear apparatus at variable slip rates and normal stresses a power density threshold is found at 0 3 mw m 2 above which frictional melt forms and controls the shear resistance to slip inducing a deviation from byerlee s frictional law homogenized experimentally generated pseudotachylytes have a similar final chemistry thickness and crystal content facilitating the construction of a rheological model for particle suspensions this is compared to the viscosity constrained from the experimental data to assess the viscous control on fault dynamics the onset of frictional melt formation during spine growth is constrained to depths below 300 m for an average slip event this combination of experimental data viscosity modeling and seismic analysis offers a new description of material response during conduit plug flow and spine growth showing that volcanic pseudotachylyte may commonly form and modify fault friction during faulting of dome rock this model furthers our understanding of faulting and seismicity during lava dome formation and is applicable to other eruption modes

geology of the don manuel igneous complex central chile implications for igneous processes in porphyry copper systems the don manuel igneous complex and associated porphyry copper mineralization in the andean cordillera of central chile demonstrate similarities between intrusive complexes associated with porphyry copper deposits and arc volcanoes that generate porphyritic volcanics the don manuel igneous complex intrusions progressed from quartz monzonite through rhyolite and biotite tonalite to intermediate porphyritic and basaltic andesite dikes which intrude the older units mineralization is associated with the biotite tonalite and intermediate porphyries which also contain the greatest abundance of mafic enclaves zoning patterns within plagioclase phenocrysts suggest that the later intermediate porphyries comprise a hybridized suite formed by magma mixing new zircon u pb ages and whole rock ar ar ages indicate that the don manuel igneous complex was emplaced between ca 4 and 3 6 ma the time scale for the episodic intrusion of the don manuel igneous complex units is similar to observed episodicity of eruption and degassing events in active arc volcanoes observations from the don manuel igneous complex are consistent with the close spatial and temporal association of mineralization with episodic intrusion and interaction between silicic and mafic magmas during emplacement the observations are also consistent with the hypothesis that mafic magma provides a source of sulfur for porphyry copper deposit formation

the challenging retrieval of the displacement field from insar data for andesitic stratovolcanoes case study of popocatepetl and colima volcano mexico abstract despite the ability of synthetic aperture radar sar interferometry to measure ground motion with high resolution application of this remote sensing technique to monitor andesitic stratovolcanoes remains limited specific acquisition conditions characterizing andesitic stratovolcanoes mainly vegetated areas with large elevation ranges induce low signal coherence as well as strong tropospheric artefacts that result in small signal to noise ratio we propose here a way to mitigate these difficulties and improve the sar measurements we derive ground motions for two of the most active mexican stratovolcanoes popocatepetl and colima volcano from the time series of sar data acquired from december 2002 to august 2006 the sar data are processed using a method that combines both persistent scatterers and small baseline approaches stratified tropospheric delays are estimated for each interferogram using inputs from the global atmospheric model narr up to a maximum of 10 rad km these delays are validated using spectrometer data as well as the correlation between the wrapped phase and the elevation the tropospheric effect is removed from the wrapped phase in order to improve the unwrapping process on popocatepetl we observe no significant deformation the colima summit area exhibits a constant subsidence rate of more than 1 cm year centered on the summit but enhanced reaching more than 2 cm year around the 1998 lava flow we model this subsidence considering both a deflating magma source at depth and the effect of the eruptive deposits load

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

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

petrology and geochemistry of lava and ash erupted from volcan colima mexico during 1998 2005 abstract lava n 8 and bulk ash samples n 6 erupted between july 1999 and june 2005 were investigated to extend time series compositional and textural studies of the products erupted from volcan colima since 1869 in particular we seek to evaluate the possibility that the current activity will culminate in major explosive plinian style event similar to that in 1913 lava samples continue to show relatively heterogeneous whole rock compositions with some significant mafic spikes 1999 2001 as have prevailed since 1976 groundmass sio2 contents continue trends to lower levels that have prevailed since 1961 in the direction of the still lower groundmass sio2 contents found in 1913 scoriae importantly ash samples from investigated vulcanian style explosive eruptions in 2005 are devoid of particles with micro vesiculated groundmass textures such textures characterized the 1913 scoriae signifying expansion of in situ magmatic gas as the propellant of the 1913 eruption all magmas erupted since 1913 appear to have arrived in the upper volcanic conduit system in a degassed state the small to moderate vulcanian style explosive eruptions which have been common since 1999 16 000 events have blasted ash clouds as high as 11 km a s l and sent pyroclastic flows out to distances of 5 km these eruptions do not appear to be powered by expansion of in situ magmatic gas new small lava domes have been observed in the crater prior to many explosive eruptions these plugs of degassed lava may temporarily seal the conduit and allow the build up of magmatic gases streaming upward from below ahead of rising and degassing magma in this interpretation when gas pressure exceeds the strength of the plug seal in the upper conduit an explosive vulcanian style eruption occurs alternatively these explosive eruptions may represent interactions of hot rock and groundwater phreato magmatic

field based density measurements as tool to identify preeruption dome structure set up and first results from unzen volcano japan for an improvement in the quality of conduit flow and dome related explosive eruption models knowledge of the preeruption or precollapse density of the rocks involved is necessary as close investigation is impossible during eruption the best substitute comes from quantitative investigation of the eruption deposits the porosity of volcanic rocks is of primary importance for the eruptive behaviour and accordingly a key parameter for realistic models of dome stability and conduit flow fortunately this physical property may be accurately determined via density measurements we developed a robust battery powered device for rapid and reliable density measurements of dry rock samples in the field the density of the samples sealed in plastic bags at 250 mbar is determined using the archimedean principle we have tested the device on the deposits of the 1990 1995 eruption of unzen volcano japan short setup and operation times allow up to 60 measurements per day under fieldwork conditions the rapid accumulation of correspondingly large data sets has allowed us to acquire the first statistically significant data set of clast density distribution in block and ash flow deposits more than 1100 samples with a total weight of 2 2 tons were measured the data set demonstrates that the deposits of the last eruptive episode at unzen display a bimodal density distribution with peaks at 2 0 0 1 and 2 3 0 1 g cm3 corresponding to open porosity values of 20 and 8 vol respectively we use this data set to link the results of laboratory based fragmentation experiments to field studies at recently active lava domes

interpretation of porosity in dacite lava domes as ductile brittle failure textures abstract pores in dacite lavas of three volcanoes in japan unzen yakedake and daisen have characteristics unlike vesicles most of the pores have ragged walls marked by the protrusion of groundmass crystals and phenocrysts the shapes of these pores range from irregular to planar many pores occur in pressure shadows adjacent to phenocrysts some pores are crack like and pass around phenocrysts and have blunt terminations most of the pores are interpreted as the result of failure of the magma by flow during ductile brittle transition this phenomenon known as cavitation is well established in hydraulics and the material sciences cavitation of the lavas in this study was promoted by steep slope of emplacement subtle mechanical contrasts between bands and strain intensification around rigid crystals and lithic fragments cavitation in lava can extract volatiles from magma causing premature embrittlement and weakening cavitation is inferred to be critical to the initiation of lava front pyroclastic flows

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

gravitational slope deformation of a resurgent caldera new insights from the mechanical behaviour of mt nuovo tuffs ischia island italy abstract ischia island italy is an impressive example of the rare phenomenon of caldera resurgence the emplacement and replenishment of magmas at shallow depth resulted in a vertical uplift of about 900 m concentrated in the western portion of mt epomeo 789 m a s l as a consequence of this uplift the island has experienced several slope instabilities at different scales since the holocene from shallow mass movements to large rock and debris avalanches these mass wasting events which mobilised large volumes of greenish alkali trachytic tuff the mt epomeo green tuff megt were strictly related to volcano tectonic activity and the interaction between the volcanic slopes and the hydrothermal system beneath the island deep seated gravitational slope deformation dsgsd at mt nuovo located adjacent to densely populated coastal villages is an ongoing process that covers an area of 1 6 km 2 the mt nuovo dsgsd involves a rock mass volume of 190 mm 3 and is accommodated by a main shear zone and a series of sub vertical fault zones associated with high angle joint sets to improve our understanding of this gravity induced process we performed a physical porosity and permeability and mechanical uniaxial and triaxial deformation experiments characterisation of two ignimbrite deposits both from the megt that form a significant component of the nw sector of mt epomeo the main conclusions drawn from our experiments are twofold first the presence of water dramatically reduces the strength of the tuffs suggesting that the movement of fluids within the hydrothermal system could greatly impact slope stability second the transition from brittle dilatant to ductile compactant behaviour in the tuffs of the megt occurs at a very low effective pressure analogous to a depth of a couple of hundred metres and that this transition is likely moved closer to the surface in the presence of water we hypothesise that compactant porosity decreasing behaviour at the base of the layer could therefore facilitate slope instability although our results show that transient exposure to 300 c does not influence the short term strength of the tuff we speculate that the high in situ temperature could increase the efficiency of brittle and compactant creep and therefore increase the rate of slope deformation taken together our experimental data highlight a potentially important role for the hydrothermal system that reaches a minimum depth of 1 km in dictating the dsgsd at mt nuovo an understanding of this deformation process is not only important for the proximal coastal villages at risk of engulfment by a large debris avalanche but also for the towns and cities along the coast of the gulf of naples that are at risk to a secondary consequence of such an avalanche a tsunami wave

exogenous and endogenous growth of the unzen lava dome examined by satellite infrared image analysis abstract phase 1 of the most recent eruption of unzen volcano japan occurred between 1991 and 1993 and involved the endogenous and exogenous addition of magma to a growing summit lava dome the temporal variation between endogenous and exogenous styles of dome growth has an important bearing on the hazard posed by dome collapse and explosive events however such data are difficult to acquire and have previously only been obtained in a fully quantitative manner for the mount st helens dome in that case using detailed photogrammetry here we investigate these competing dome growth processes at unzen via analysis of their thermal emission signals present in shortwave infrared swir satellite imagery we find that both the endogenous and exogenous growth styles exhibit identifiable swir signatures whose magnitude can be related to their respective magma supply rates we find that the swir radiant area corresponding to endogenous growth is around four times larger than that corresponding to exogenous growth at the same magma discharge rate analysis of time series imagery indicates that like mount st helens unzen experienced a temporally decreasing magma discharge rate during phase 1 as time progressed and the discharge rate fell the growth style of the unzen dome changed from exogenous only through exogenous decreasing and endogenous increasing to endogenous only thus relative growth rates at the endogenous and exogenous areas of the unzen dome appear to be at least partly controlled by the overall magma discharge rate as with mount st helens the cause of this relationship was most likely the formation and strengthening of a brittle crust at the surface vent which impeded exogenous growth more effectively as time elapsed and the discharge rate fell these results suggest the similarity of the processes operating at the unzen and mount st helens domes though important differences remain in that dome growth was basically continuous at unzen whilst at mount st helens growth episodes were interspersed by significant periods of repose

dispersal of key subplinian plinian tephras from hekla volcano iceland implications for eruption source parameters hekla is the most active silicic volcano in iceland with 18 subplinian plinian eruptions since ad 1104 in the period 1970 to 2000 the frequency of such eruptions increased to once every decade hekla is currently inflated to above the levels observed prior to the most recent eruptions in 1991 and 2000 the next eruption could pose a hazard to air traffic between north america and europe because explosive eruptions of hekla independent of size typically start with a subplinian or plinian phase that produces a sustained ash plume we present an overview of five of the largest historical hekla eruptions taking place in 1104 1158 1300 1693 and 1766 these eruptions cover a compositional range of rhyolite to andesite previously estimated volcanic explosivity index vei values of 4 5 and are characterised by contrasting wind dispersal dispersal axes nw ne new isopach maps show both greater deposit thicknesses in the proximal region and wider dispersal than previously inferred resulting in different volume estimates minimal values ranging between 0 18 and 0 91 km3 new isopleth maps were also compiled and resulted in inferred plume heights of about 13 25 km these changes in the estimated values of volume and mass eruption rates have large implications on the forecasting and impacts of future hekla eruptions

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

identification of structural controls in an active lava dome with high resolution dems volcan de colima mexico monitoring the topography of active lava domes is critical for detecting changes that may trigger or influence collapse or explosive activity internal dome structure and conditions are more difficult to elucidate but also play vital roles here we describe the exposure following an explosion of significant scarps in the active dome at volcan de colima mexico that are interpreted as evidence of brittle failure planes and a complex internal dome morphology in the first use of automated 3d computer vision reconstruction techniques structure from motion and multi view stereo sfm mvs on an active volcanic dome we derive high resolution surface models from oblique and archive photographs taken with a consumer camera the resulting 3d models were geo referenced using features identified in a web sourced orthoimage no ground based measurements were required in december 2010 the dome 2 14 106 m3 had a flat upper surface reflecting an overall ductile emplacement regime between then and may 2011 a period of low explosivity was accompanied by a small volume loss 0 4 105 m3 and arcuate steps appeared in the dome surface suggesting the presence of localized planes of weakness the complex array of summit scarps was exposed following a significant explosion in june 2011 and is interpreted to be the surface expression of fault planes in the dome the 1 m resolution dems indicated that the region of greatest volume loss was not coincident with the assumed location of the conduit and that heterogeneity within the dome may have been important during the june explosion

long period earthquakes and co eruptive dome inflation seen with particle image velocimetry dome growth and explosive degassing are fundamental processes in the cycle of continental arc volcanism johnson et al present novel optical geodetic observations to constrain the sources responsible for long period volcanic earthquakes which are coincident with frequent explosive eruptions at santiaguito volcano guatemala they find that acceleration in deformation of the volcanic dome extracted from high resolution optical image processing is coincident with recorded long period seismic sources based on these observations abrupt mass shift of solidified domes conduit magma or magma pads seem to be part of the mechanism responsible for generating long period earthquakes at silicic volcanic systems this study presents optical geodetic observations to constrain the sources responsible for long period volcanic earthquakes which are coincident with frequent explosive eruptions at santiaguito volcano guatemala it is found that acceleration in deformation of the volcanic dome extracted from the high resolution optical image processing is coincident with recorded long period seismic sources on the basis of these observations abrupt mass shift of solidified domes conduit magma or magma pads seem to be part of the mechanism responsible for generating long period earthquakes at silicic volcanic systems dome growth and explosive degassing are fundamental processes in the cycle of continental arc volcanism1 2 because both processes generate seismic energy geophysical field studies of volcanic processes are often grounded in the interpretation of volcanic earthquakes3 although previous seismic studies have provided important constraints on volcano dynamics4 5 6 such inversion results do not uniquely constrain magma source dimension and material properties here we report combined optical geodetic and seismic observations that robustly constrain the sources of long period volcanic earthquakes coincident with frequent explosive eruptions at the volcano santiaguito in guatemala the acceleration of dome deformation extracted from high resolution optical image processing is shown to be associated with recorded long period seismic sources and the frequency content of seismic signals measured across a broadband network these earthquake sources are observed as abrupt subvertical surface displacements of the dome in which 20 50 cm uplift originates at the central vent and propagates at 50 m s 1 towards the 200 m diameter periphery episodic shifts of the 20 80 m thick dome induce peak forces greater than 109 n and reflect surface manifestations of the volcanic long period earthquakes a broad class of volcano seismic activity that is poorly understood and observed at many volcanic centres worldwide7 on the basis of these observations the abrupt mass shift of solidified domes conduit magma or magma pads may play a part in generating long period earthquakes at silicic volcanic systems

catastrophic collapse at stratovolcanoes induced by gradual volcano spreading unlike ordinary mountains which are formed by slow uplift and erosion volcanoes are constructed rapidly as a consequence many are liable to massive flank failures leading to debris avalanches for example at mount st helens in 1980 such failures occur worldwide about once every 25 years ref 1 and even small ones can present a major hazard in particular if far reaching tsunamis are generated as at mayu yama in 1792 ref 2 previous work has tended to emphasize differences in eruption style associated with flank failure2 but here we focus on the fundamental structural causes of failure most volcanic failures are generated by magmatic intrusion and flank spreading3 we present evidence however that mombacho volcano in nicaragua experienced a previously unrecognized type of failure triggered by sub volcanic basement spreading notably collapses related to basement spreading do not require that the volcano be magma tically active and thus flank failure may pose a significant risk even at inactive volcanoes which are rarely monitored

micromechanics of brittle faulting and cataclastic flow in mount etna basalt understanding how the strength of volcanic rocks varies with stress state pressure and microstructural attributes is fundamental to understanding the dynamics and tectonics of a volcanic system and also very important in applications such as geothermics or reservoir management in volcanic environments in this study we investigated the micromechanics of deformation and failure in basalt focusing on samples from mount etna we performed 65 uniaxial and triaxial compression experiments on nominally dry and water saturated samples covering a porosity range between 5 and 16 at effective pressures up to 200 mpa dilatancy and brittle faulting were observed in all samples with porosity of 5 water saturated samples were found to be significantly weaker than comparable dry samples shear enhanced compaction was observed at effective pressures as low as 80 mpa in samples of 8 porosity microstructural data revealed the complex interplay of microcracks pores and phenocrysts on dilatant failure and inelastic compaction in basalt the micromechanics of brittle failure is controlled by wing crack propagation under triaxial compression and by pore emanated cracking under uniaxial compression especially in the more porous samples the mechanism of inelastic compaction in basalt is cataclastic pore collapse in agreement with a recent dual porosity model

tungurahua volcano ecuador structure eruptive history and hazards abstract tungurahua one of ecuador s most active volcanoes is made up of three volcanic edifices tungurahua i was a 14 km wide andesitic stratocone which experienced at least one sector collapse followed by the extrusion of a dacite lava series tungurahua ii mainly composed of acid andesite lava flows younger than 14 000 years bp was partly destroyed by the last collapse event 2955 90 years ago which left a large amphitheater and produced a 8 km3 debris deposit the avalanche collided with the high ridge immediately to the west of the cone and was diverted to the northwest and southwest for 15 km a large lahar formed during this event which was followed in turn by dacite extrusion southwestward the damming of the chambo valley by the avalanche deposit resulted in a 10 km long lake which was subsequently breached generating another catastrophic debris flow the eruptive activity of the present volcano tungurahua iii has rebuilt the cone to about 50 of its pre collapse size by the emission of 3 km3 of volcanic products two periods of construction are recognized in tungurahua s iii history from 2300 to 1400 years bp high rates of lava extrusion and pyroclastic flows occurred during this period the magma composition did not evolve significantly remaining essentially basic andesite during the last 1300 years eruptive episodes take place roughly once per century and generally begin with lapilli fall and pyroclastic flow activity of varied composition andesite dacite and end with more basic andesite lava flows or crater plugs this pattern is observed in the three historic eruptions of 1773 1886 and 1916 1918 given good age control and volumetric considerations tungurahua iii growth s rate is estimated at 1 5 106 m3 year over the last 2300 years although an infrequent event a sector collapse and associated lahars constitute a strong hazard of this volcano given the 3000 m relief and steep slopes of the present cone a future collapse even of small volume could cover an area similar to that affected by the 3000 year old avalanche the more frequent eruptive episodes of each century characterized by pyroclastic flows lavas lahars as well as tephra falls directly threaten 25 000 people and the agoyan hydroelectric dam located at the foot of the volcano

faulting within the mount st helens conduit and implications for volcanic earthquakes the 2004 2008 eruption of mount st helens produced seven dacite spines mantled by cataclastic fault rocks comprising an outer fault core and an inner damage zone these fault rocks provide remarkable insights into the mechanical processes that accompany extrusion of degassed magma insights that are useful in forecasting dome forming eruptions the outermost part of the fault core consists of finely comminuted fault gouge that is host to 1 to 3 mm thick layers of extremely fine grained slickenside bearing ultracataclasite interior to the fault core there is an 2 m thick damage zone composed of cataclastic breccia and sheared dacite and interior to the damage zone there is massive to flow banded dacite lava of the spine interior structures and microtextures indicate entirely brittle deformation including rock breakage tensional dilation shearing grain flow and microfaulting as well as gas and fluid migration through intergranular pores and fractures in the damage zone slickenside lineations and consistent orientations of riedel shears indicate upward shear of the extruding spines against adjacent conduit wall rocks paleomagnetic directions demagnetization paths oxide mineralogy and petrology indicate that cataclasis took place within dacite in a solidified steeply dipping volcanic conduit at temperatures above 500 c low water content of matrix glass is consistent with brittle behavior at these relatively high temperatures and the presence of tridymite indicates solidification depths of more than a million small and low frequency drumbeat earthquakes with coda magnitudes m d 2 slip patches possibly along observed extensive principal shear zones within the fault core but probably not along the smaller riedel shears although our field and structural data are compatible with stick slip models they do not rule out seismic and infrasound models that call on resonance of steam filled fractures to generate the drumbeat earthquakes we suggest that stick slip and gas release processes may be coupled and that regardless of the source mechanism the distinctive drumbeat earthquakes are proving to be an effective precursor for dome forming eruptions our data document a continuous cycle of deformation along the conduit margins beginning with episodes of fracture in the damage zone and followed by transfer of motion to the fault core we illustrate the cycle of deformation using a hypothetical cross section of the mount st helens conduit extending from the surface to the depth of magmatic solidification

thermal resilience of microcracked andesitic dome rocks abstract the strength of the rocks forming a lava dome informs on its structural stability important for volcanic hazard assessments dome forming rocks are persistently challenged by thermal stresses from recurring eruptive events that may reduce their strength and jeopardise the structural stability of the dome here we present a series of experiments to better understand the impact of thermal stresses on the strength of an andesitic dome rock from volcan de colima mexico a volcano that has witnessed some substantial dome collapses in recent years uniaxial compressive strength ucs was first tested at room temperature on as collected samples and samples that had undergone either slow heated and cooled at 1 c min or shock heated at 1 c min and shock cooled in cold water thermal stressing to target temperatures of 400 700 c slow and shock cooling thermal stressing did not measurably alter sample strength connected porosity or permeability ucs tests performed at high in situ temperatures 400 700 c however showed an increase in sample strength and stiffness we interpret that the resistance of this rock to thermal stresses results from both the presence of abundant pre existing microcracks and the thermal stability of its mineral assemblage unchanged physical properties for the thermally stressed samples deformed at room temperature suggests that the pre existing microcracks close and reopen respectively as the rock expands and contracts during heating and cooling to accommodate the volumetric changes without further microcracking the increase in strength and stiffness at high in situ temperatures can be explained by the closure of microcracks due to thermal expansion these observations suggest that the strength of microcracked dome rocks 1 may be slightly higher when hot below the glass transition of the groundmass glass although upscaled strength estimates highlight that dome strength will be largely unchanged by an increase in temperature 2 may only be reduced following the first thermal stressing event and 3 may not be further reduced by repeated thermal stressing events therefore thermal perturbations often observed at active domes may not as perhaps expected repeatedly degrade the strength of individual blocks forming the lava dome and therefore may not jeopardise dome stability

dynamics of seismogenic volcanic extrusion at mount st helens in 2004 05 the 2004 05 eruption of mount st helens exhibited sustained near equilibrium behaviour characterized by relatively steady extrusion of a solid dacite plug and nearly periodic shallow earthquakes here we present a diverse data set to support our hypothesis that these earthquakes resulted from stick slip motion along the margins of the plug as it was forced incrementally upwards by ascending solidifying gas poor magma we formalize this hypothesis with a dynamical model that reveals a strong analogy between behaviour of the magma plug system and that of a variably damped oscillator modelled stick slip oscillations have properties that help constrain the balance of forces governing the earthquakes and eruption and they imply that magma pressure never deviated much from the steady equilibrium pressure we infer that the volcano was probably poised in a near eruptive equilibrium state long before the onset of the 2004 05 eruption

influence of alteration on the mechanical behaviour and failure mode of andesite implications for shallow seismicity and volcano monitoring volcanoes often host hydrothermal systems that alter the host rock to understand the influence of alteration on mechanical behaviour of edifice forming rock we performed a series of triaxial deformation experiments on variably altered andesite from mt ruapehu new zealand under constant effective pressure under the imposed conditions andesite with intermediate argillic alteration deforms in a brittle manner forming fractures by contrast andesite with advanced argillic alteration deforms in a ductile manner with sample failure driven by distributed cataclastic pore collapse we consider this the result of an increase in porosity and clay content with increasing alteration ancillary experiments highlight that the brittle ductile transition occurs at lower effective pressure i e at shallower depths in andesites with advanced argillic alteration relative to unaltered andesites of comparable porosity we conclude that advanced argillic alteration can create an anomalous shallow ductile zone which has important implications for fluid flow and pre eruptive seismicity

heat flow at the proposed appalachian ultradeep core hole adcoh site tectonic implications the heat flow in northwestern south carolina at the appalachian ultradeep core hole adcoh site area is approximately 55 mw m this data supplements other data to the east in the piedmont and atlantic coastal plain provinces where heat flows 55 mw m are characteristic of post and late synmetamorphic granitoids piedmont heat flow and heat generation data for granites metagranites and one slate belt site in a zone approximately parallel to major structural appalachian trends define a linear relation tectonic truncation of heat producing crust at a depth of about 8 km a depth equal to the slope of the heat flow heat production line is proposed to explain the linear relation using the value of reduced heat flow estimated from this empirical relation and assuming thicknesses of heat producing crust defined by new adcoh seismic data the heat flow and heat production at the adcoh site are consistent with a depth to the base of the inner piedmont crystalline allochthon of about 5 5 km seismic data at the adcoh site confirm that the inner piedmont is tectonically truncated at about 5 5 km by the blue ridge master decollement temperatures at 10 km at the adcoh site are predicted to be less than 200 c

the valle del bove mount etna its origin and relation to the stratigraphy and structure of the volcano abstract the valle del bove is a major horseshoe shaped depression some 5 km in diameter and over 1000 m deep carved into the eastern flank of mount etna volcano the origin of the valle del bove has been a subject of debate for over a century but recent models relate it to the collapse of the ancient trifoglietto ii centre of etna either by caldera collapse or phreatomagmatic explosive activity careful morphological analysis shows the valle del bove to have formed after the post trifoglietto ii ellittico and leone centres and therefore to be considerably younger than the trifoglietto ii centre it has also been argued that the pyroclastic deposits that occur on the flanks of the volcano in close spatial relationship with the valle del bove were produced by the same event these pyroclastics are divided on the basis of age and composition into two groups the upper and lower tephra deposits the stratigraphy petrology and radiocarbon date of the lower tephra deposits indicate a trifoglietto ii age at least 20 000 years before the formation of the valle del bove the upper tephra deposits have a strong phreatomagmatic character and are closest in age to the valle del bove there is no evidence however to link them with its formation morphological study shows the valle del bove to consist of a series of coalescing scallops indicative of formation by landsliding the material removed by slope failure and subsequent fluvial transport followed a valley to the coast where it forms a fan shaped accumulation the chiancone consisting of volcaniclastics of fluvial and mudflow origin it is argued that the valle del bove did not result from simple caldera collapse but formed from a series of collapses caused by gravitational slope failure the chiancone is considered to be the product of this collapse

fracture and compaction of andesite in a volcanic edifice the failure mode of lava dilatant or compactant depends on the physical attributes of the lava primarily the porosity and pore size and the conditions under which it deforms the failure mode for edifice host rock has attendant implications for the structural stability of the edifice and the efficiency of the sidewall outgassing of the volcanic conduit in this contribution we present a systematic experimental study on the failure mode of edifice forming andesitic rocks porosity from 7 to 25 from volcan de colima mexico the experiments show that at shallow depths 1 km while low porosity 10 lava remains dilatant the failure of high porosity lava is compactant and driven by cataclastic pore collapse although inelastic compaction is typically characterised by the absence of strain localisation we observe compactive localisation features in our porous andesite lavas manifest as subplanar surfaces of collapsed pores in terms of volcano stability faulting in the upper edifice could destabilise the volcano leading to an increased risk of flank or large scale dome collapse while compactant deformation deeper in the edifice may emerge as a viable mechanism driving volcano subsidence spreading and destabilisation the failure mode influences the evolution of rock physical properties permeability measurements demonstrate that a throughgoing tensile fracture increases sample permeability i e equivalent permeability by about a factor of two and that inelastic compaction to an axial strain of 4 5 reduces sample permeability by an order of magnitude the implication of these data is that sidewall outgassing may therefore be efficient in the shallow edifice where rock can fracture but may be impeded deeper in the edifice due to compaction the explosive potential of a volcano may therefore be subject to increase over time if the progressive compaction and permeability reduction in the lower edifice cannot be offset by the formation of permeable fracture pathways in the upper edifice the mode of failure of the edifice host rock is therefore likely to be an important factor controlling lateral outgassing and thus eruption style effusive versus explosive at stratovolcanoes

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

the influence of porosity and vesicle size on the brittle strength of volcanic rocks and magma volcanic rocks and magma display a wide range of porosity and vesicle size a result of their complex genesis while the role of porosity is known to exert a fundamental control on strength in the brittle field less is known as to the influence of vesicle size to help resolve this issue here we lean on a combination of micromechanical sammis and ashby s pore emanating crack model and stochastic rock failure and process analysis code modelling the models show for a homogenous vesicle size that an increase in porosity in the form of circular vesicles from 0 to 40 and or vesicle diameter from 0 1 to 2 0 mm results in a dramatic reduction in strength for example uniaxial compressive strength can be reduced by about a factor of 5 as porosity is increased from 0 to 40 the presence of vesicles locally amplifies the stress within the groundmass and promotes the nucleation of vesicle emanating microcracks that grow in the direction of the applied macroscopic stress as strain increases these microcracks continue to grow and eventually coalesce leading to macroscopic failure vesicle clustering which promotes the overlap and interaction of the tensile stress lobes at the north and south poles of neighbouring vesicles and the increased ease of microcrack interaction is encouraged at higher porosity and reduces sample strength once a microcrack nucleates at the vesicle wall larger vesicles impart higher stress intensities at the crack tips allowing microcracks to propagate at a lower applied macroscopic stress larger vesicles also permit a shorter route through the groundmass for the macroscopic shear fracture this explains the reduction in strength at higher vesicle diameters at a constant porosity the modelling highlights that the reduction in strength as porosity or vesicle size increases is nonlinear the largest reductions are observed at low porosity and small vesicle diameters in detail we find that vesicle diameter can play an important role in dictating strength at low porosity but is largely inconsequential above 15 porosity vesicle clustering and stress lobe interaction are implicit at high porosity regardless of the vesicle diameter in the case of an inhomogeneous vesicle size the microcracks grow from the largest vesicles and brittle strength is closer to that of the largest vesicle end member the results of this study highlight the important role of vesicle size and the complex interplay between porosity and vesicle size in controlling the brittle strength of volcanic rocks and magma

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

pore pressure embrittlement in a volcanic edifice the failure mode of porous rock in compression dilatant or compactant is largely governed by the overlying lithostatic pressure and the pressure of pore fluids within the rock wong solid earth 102 3009 3025 1997 both of which are subject to change in space and time within a volcanic edifice while lithostatic pressure will tend to increase monotonously with depth due to the progressive accumulation of erupted products pore pressures are prone to fluctuations during periods of volcanic unrest for example an increase in pore fluid pressure can result in rock fracture even at depths where the lithostatic pressure would otherwise preclude such dilatant behaviour a process termed pore fluid induced embrittlement we explore this phenomenon through a series of targeted triaxial experiments on typical edifice forming andesites from volcan de colima mexico we first show that increasing pore pressure over a range of timescales on the order of 1 min to 1 day can culminate in brittle failure of otherwise intact rock irrespective of the pore pressure increase rate we record comparable accelerations in acoustic emission and strain prior to macroscopic failure we further show that oscillating pore fluid pressures can cause iterative and cumulative damage ultimately resulting in brittle failure under relatively low effective mean stress conditions we find that macroscopic failure occurs once a critical threshold of damage is surpassed suggesting that only small increases in pore pressure may be necessary to trigger failure in previously damaged rocks finally we observe that inelastic compaction of volcanic rock as we may expect in much of the deep edifice can be overprinted by shear fractures due to this mechanism of embrittlement pore fluid induced embrittlement of edifice rock during volcanic unrest is anticipated to be highest closer to the conduit and as a result may assist in the development of a fractured halo zone surrounding the conduit potentially explaining commonly observed near conduit outgassing at many active volcanoes further rock embrittlement at depth may create transient outgassing pathways by linking fracture networks near the edifice to larger scale regional fault systems our experimental results affirm that pore pressure fluctuations associated with volcanic unrest may play a crucial role in dictating the evolution of a volcanic system

fumarole monitoring with a handheld infrared camera volcan de colima mexico 2006 2007 abstract this paper describes a methodology for the monitoring of fumarole temperatures at medium ranges 6 km using a handheld infrared camera wavelength range 8 13 5 m as a relationship between fumarole temperatures gas flux and volcanic activity has been demonstrated by a number of studies fumarole temperature data has a potential use as a monitoring tool volcan de colima is an andesitic stratovolcano with a 300 m diameter summit crater formed by the destruction of the 2004 lava dome by a series of explosions in 2005 between january 2006 and august 2007 sequences of thermal images were recorded from a viewpoint 6 km to the north during regular 24 48 hour monitoring excursions the temperatures of fumaroles on the crater rim and the ground surface on the volcano s flanks were measured a methodology was developed to remove data affected by clouds or volcanic water vapour based on rates of temperature change and scatter within the data for the remaining data it is demonstrated mathematically that at this range typical variations in atmospheric transmissivity will affect the apparent temperatures by 2 c while a 25 change in fumarole heat flux would change it by 5 10 c the mean night time apparent temperature of the fumaroles was calculated for each excursion and showed an irregular decline over the 19 month period subtracting the radiant heat flux of flank rocks from those of the fumaroles removes seasonal variations and gives the clearest view of trends in the fumarole heat flux a sharp drop in fumarole temperature during february 2007 coincided with the emergence of a lava dome in the crater the declining fumarole temperature is interpreted to reflect decreasing gas flux from the crater in line with a change in eruptive regime from frequent small ash rich explosions to slow effusion of lava

chemical and isotopic composition of fumarolic gases and the so2 flux from volcan de colima mexico between the 1994 and 1998 eruptions volcan de colima is the most active volcano in mexico including a significant effusive eruption in 1991 and a weak explosive event in 1994 geochemical monitoring of the volcano started in january 1996 with collection of fumarolic gases approximately every 3 months from two sites in the summit crater z3 800 c and z2 400 c chemical compositions of volcanic gases from the z2 fumarole showed a peak in s cl in the middle of 1997 14 15 months before the start of the 1998 lava eruption high temperature volcanic vapors from z3 changed progressively in water isotopic composition and showed a weak trend of increasing hcl hf and a peak in s cl at the same time starting from the summer of 1997 the volcanic water became gradually enriched in deuterium which probably indicated an increasing contribution from deep less degassed magma a negative correlation between d and cl content for the z3 fumarolic gases is explained by shallow degassing of the colima hot lava plug this hypothesis is consistent with published data on cl contents and solubilities in magmas and supported by the observed positive trends of s cl and hcl hf with d for the z3 fumarole until september 1998 the so2 flux from the volcano was at a low level 100 tons day and often below the detection limit of the instrument 30 tons day a sharp increase in the so2 flux was recorded 1 month before the eruption started it jumped from 400 50 tons day to 1600 tons day 2 days before new lava emerged on 20 november 1998 the so2 flux correlated well with seismicity

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

microcracking during triaxial deformation of porous rocks monitored by changes in rock physical properties i elastic wave propagation measurements on dry rocks we present results from two series of triaxial deformation experiments performed on dry samples of two sandstones darley dale and gosford carried out at confining pressures from 25 mpa to 200 mpa over this pressure range the mode of failure in both these sandstones passes from localized brittle failure with a clear through going fault to distributed cataclastic flow during these experiments stress strain compressional wave velocity vp and shear wave velocity vs measurements were made simultaneously in the direction of the maximum principal compressive loading axis initial application of the hydrostatic confining pressure causes both vp and vs to increase and upon raising the axial stress above the confining pressure both velocities increase further at first generally by only a few percent but then decrease as dilatant crack growth commences during dilatancy vs decreases proportionately more than vp and this decrease is generally of the order of 10 15 these velocity measurements allow changes in rock physical properties to be calculated along with the axial and transverse crack volume density parameters x and z the results from two selected tests are analysed in detail these tests were chosen because they exhibit a typical brittle shear failure and b typical ductile cataclastic flow respectively the full interrogating elastic waveforms were also recorded during testing and these have been used to calculate the seismic quality factors qp and qs to our knowledge this is the first time this has been reported for rock samples undergoing triaxial deformation the changes in q values generally exhibit similar trends to those observed in the velocity measurements but the percentage changes in q are an order of magnitude greater suggesting that this parameter is a more sensitive measure of dilatant crack damage the measurements on dry rock samples reported here provide the basis for comparison with measurments of changes in complementary physical properties made on water saturated rock samples under the same experimental conditions and reported in a companion paper in this issue read et al 1995

heat flow in the piedmont and atlantic coastal plain of the southeastern united states seventy four new heat flow values have been determined at sites from new jersey to georgia mostly in the piedmont and atlantic coastal plain provinces relatively high heat flows are characteristic of synmetamorphic and postmetamorphic granites exposed in the piedmont and occurring in the basement beneath the sediments of the atlantic coastal plain the piedmont heat flow and heat generation values determined in granites metagranites and one slate belt site lie in a belt approximately parallel to major structural trends in the appalachians these values fall on or near the regression line q mw m 2 29 8 n 1 5 mw m 2 7 8 km n 0 475a w m 3 r 0 97477 it is proposed that the occurrence of a thrust fault that truncates granites at a depth d of 7 8 km is directly responsible for this linear relation in the eastern piedmont basement core from beneath the sediments of the atlantic coastal plain was recovered at ten sites results from five of these sites plus one site in the baltimore gabbro define a second linear relation with a slope d 8 0 q 0 380 km r 0 99550 approximately parallel to the piedmont relation but with an intercept of 48 2 0 8 mw m 2 these two relations may represent different heat flow provinces in the southeastern united states or the linear relations may simply be a consequence of differences in the thicknesses of the upper heat producing layer at least as far east as postmetamorphic granites sampled under the coastal plain

continental fault structure and the shallow earthquake source plate boundaries in continental crust are generally less sharply defined than in the oceans with seismicity spread over broad areas interplate displacements appear to be largely accommodated by networks of major fault zones a simple 2 level model for these important structures accounts for the depth distribution of most continental earthquakes and for the observed range of faulting styles and associated rock deformation textures the model consists of a seismogenic frictional slip regime overlying quasi plastic mylonite belts wherein shearing is largely accommodated aseismically due mainly to the changing response of quartz to deformation with increasing temperature shear resistance increases with depth to a peak value in the vicinity of the frictiona1 quasi plastic transition and then decreases rapidly the depth to which microseismic activity extends appears inversely related to regional heat flow and can be satisfactorily modelled as the frictional quasi plastic transition for different geotherms using laboratory determined flow laws for quartz bearing rocks larger earthquake ruptures m 5 5 tend to nucleate near the base of the seismogenic regime in the region inferred to have the highest shear resistance and concentration of distortional strain energy consideration is also given to the depression of isotherms and seismic activity in regions of thrusting and to the question of the downward continuation of major fault zones through the lithosphere decoupling of the upper crust on flat lying shear zones may accompany higher level dip slip and perhaps in some circumstances strike slip faulting being favoured by above average continental heat flow and a high quartz content in the middle or deep crust the average level of deviatoric stress within the seismogenic regime remains an outstanding problem

fault textures in volcanic conduits evidence for seismic trigger mechanisms during silicic eruptions it is proposed that fault textures in two dissected rhyolitic conduits in iceland preserve evidence for shallow seismogenic faulting within rising magma during the emplacement of highly viscous lava flows detailed field and petrographic analysis of such textures may shed light on the origin of long period and hybrid volcanic earthquakes at active volcanoes there is evidence at each conduit investigated for multiple seismogenic cycles each of which involved four distinct evolutionary phases in phase 1 shear fracture of unrelaxed magma was triggered by shear stress accumulation during viscous flow forming the angular fracture networks that initiated faulting cycles transient pressure gradients were generated as the fractures opened which led to fluidisation and clastic deposition of fine grained particles that were derived from the fracture walls by abrasion fracture networks then progressively coalesced and rotated during subsequent slip phase 2 developing into cataclasite zones with evidence for multiple localised slip events fluidisation and grain size reduction phase 2 textures closely resemble those formed on seismogenic tectonic faults characterised by friction controlled stick slip behaviour increasing cohesion of cataclasites then led to aseismic distributed ductile deformation phase 3 and generated deformed cataclasite zones which are enriched in metallic oxide microlites and resemble glassy pseudotachylite continued annealing and deformation eventually erased all structures in the cataclasite and formed microlite rich flow bands in obsidian phase 4 overall the mixed brittle ductile textures formed in the magma appear similar to those formed in lower crustal rocks close to the brittle ductile transition with the rheological response mediated by strain rate variations and frictional heating fault processes in highly viscous magma are compared with those elsewhere in the crust and this comparison is used to appraise existing models of volcano seismic activity based on the textures observed it is suggested that patterns of long period and hybrid earthquakes at silicic lava domes reflect friction controlled stick slip movement and eventual healing of fault zones in magma which are an accelerated and smaller scale analogue of tectonic faults

precursory seismicity of the 1994 eruption of popocatepetl volcano central mexico popocatepetl volcano is located in the central mexican volcanic belt within a densely populated region inhabited by over 20 million people the eruptive history of this volcano indicates that it is capable of producing a wide range of eruptions including plinian events after nearly 70 years of quiescence popocatepetl reawakened in december 21 1994 the eruptive activity has continued up until the date of this submission and has been characterized by a succession of lava dome growth and destruction episodes similar to events that have apparently been typical for popocatepetl since the fourteenth century in this regime the episodes of effusive and moderately explosive activity alternate with long periods of almost total quiescence in this paper we analyze five years of volcano tectonic seismicity preceding the initial eruption of the current episode the evolution of the v t seismicity shows four distinct stages which we interpret in terms of the internal processes which precede an eruption after a long period of quiescence the thermal effects of a magma intrusion at depth the fracturing related to the slow development of magma related fluid pathways the concentration of stress causing a protracted acceleration of this process and a final relaxation or redistribution of the stress shortly before the initial eruption are reflected in the rates of v t seismic energy release a hindsight analysis of this activity shows that the acceleration of the seismicity in the third stage asymptotically forecast the time of the eruption the total seismic energy release needed to produce an eruption after a long period of quiescence is related to the volume of rock that must be fractured so imposing a characteristic threshold limit for polygenetic volcanoes limit that was reached by popocatepetl before the eruption

shallow crustal mechanics from volumetric strain data insights from soufriere hills volcano montserrat volumetric strain data from the 29 july 2008 vulcanian explosion of the soufriere hills volcano provide an excellent opportunity to probe the mechanical properties of the volcanic edifice and the shallow crust beneath montserrat we use finite element analysis to constrain the geometry of the shallow plumbing system as well as edifice and crustal mechanical properties for mechanically plausible pressure drops associated with dilatational volumetric strains recorded during the explosive activity our results from both forward and inverse models indicate a conduit radius of 40 m and a length of 1500 m and hence much larger conduit dimensions than previously suggested in order to fit the syneruptive volumetric strain data for a conduit pressure drop of 10 mpa the conduit needs to be surrounded by a halo of mechanically compliant rocks for which the best fit models indicate a width of 600 m and a young s modulus of 1 gpa young s moduli for the main edifice and the shallow crust up to 1000 m below sea level are found to be 10 gpa our best fit inverse model predicts a syneruptive radial conduit contraction by 0 24 m with a corresponding volume loss of 0 1 mm3 implying only partial emptying of the conduit upon the explosion this study demonstrates the critical role of edifice and shallow crustal mechanics for strain partitioning on montserrat our findings may have implications for the assessment of conduit processes at other dome building volcanoes

persistent summit subsidence at volcan de colima mexico 1982 1999 strong evidence against mogi deflation this paper re examines recent ground deformation measurements at volcan de colima mexico to test the hypothesis that the observed movements are in response to pressure changes within a sub volcanic magma chamber as suggested for other volcanoes by mogi 1958 earthq res inst 36 99 134 measurements of vertical ground deformation across the summit dome complex of volcan de colima from a precise levelling network between 1982 and 1999 together with vertical and horizontal displacements derived from dual frequency gps measurements in 1994 and 1997 show continuous subsidence the deformation pattern derived from the levelling shows that subsidence increases towards the summit the closest stations to the summit 1 1 km distant show a mean subsidence rate of 5 mm per year compared to the reference station at 2 3 km distance which may be subsiding itself vertical displacements of individual stations since 1982 show that the subsidence has been fairly continuous the gps stations which are distributed more widely and include some close to the edge of the active dome confirm summit subsidence the largest measured value a decrease of 280 mm or 93 mm subsidence per year was obtained at the edge of the dome horizontal displacements measured during 1994 1997 also show the largest values at the summit but these are much smaller than the vertical displacements with the maximum rate of 23 mm per year recorded close to the dome it is conclusively shown that these measured movements cannot be due to deflation of a buried mogi source as vertical and horizontal displacements for some stations are in contrary directions to those predicted by the model and there is no consistent pattern to the horizontal movements we attribute the measured deformation to downslope creep settling and compaction of the edifice gravitational spreading or a combination of these processes

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

multiple timescales of cyclical behaviour observed at two dome forming eruptions article i nfo cyclic behaviour over a range of timescales is a well documented feature of many dome forming volcanoes but has not previously been identified in high resolution seismic data from volcan de colima mexico using daily seismic count datasets from volcan de colima and soufriere hills volcano montserrat this study explores par allels in the long term behaviour of seismicity at two long lived systems datasets are examined using multiple techniques including fast fourier transform detrended fluctuation analysis and probabilistic distribution analysis and the comparison of results from two systems reveals interesting parallels in sub surface processes operating at both systems patterns of seismicity at both systems reveal complex but broadly similar long term temporal patterns with cycles on the order of 50 to 200 days these patterns are consistent with previously published spectral analyses of so2 flux time series at soufriere hills volcano and are attributed to variations in the movement of magma in each system detrended fluctuation analysis determined that both volcanic systems showeda systematic relationship betweenthe numberofseismic eventsand the relative roughness ofthetime series and explosionsatvolcande colimashoweda1 5 2 yearcycle neither observation hasaclear explanatory mechanism atvolcandecolima analysisofreposeintervalsbetweenseismiceventsshowslong termbehaviour that responds to changes in activity at the system similar patterns for both volcanic systems suggest a common process or processes driving the observed signal but it is not clear from these results alone what those processes may be furtherattempts to modelconduitprocessesateach volcanomustaccountfor thesimilarities anddiffer ences in activity within each system the identification of some commonalities in the patterns of behaviour dur ing long lived dome forming eruptions atandesiticvolcanoes provides a motivation for investigating further use of time series analysis as a monitoring tool

dynamic basis of volcanic spreading the established models of the structure and dynamics of a typical hawaiian volcano lead inevitably to a paradox finite element viscoelastic calculations using tecton melosh and raefsky 1980 1981 predict that a compressional stress field should characterize their upper flanks instead extension is observed the paradox is solved by postulating that volcanic evolution is determined by feedback processes related to growth and spreading of the volcanic edifice many examples of the process of spreading are documented in the geologic literature on volcanoes of a very broad size range and very different geologic settings the process is similar across many orders of magnitude in size the deformation of the substratum related to magma intrusion during lava dome building events is at the lower end of the scale the spreading of seafloor at oceanic ridges is at the higher end of the scale volcanoes may evolve from small constructs into huge shield volcanoes and perhaps into ocean plates key elements for volcanic spreading are 1 the existence of a weak basal layer and 2 a sufficiently high mass and magma influx to drive the process as the mass of a volcano grows it passes through five phases which may overlap repeat or be omitted building compressing thrusting intruding and spreading during the building phase the mass of a volcano does not significantly affect the stress field of the substratum primitive magmas are likely to be erupted as the mass increases the volcano subsides and becomes characterized by a compressional stress field which inhibits any further intrusion of magma into the volcanic edifice magma may differentiate in the crust allowing for large caldera forming eruptions to occur in the next phase thrusting begins on a decollement usually located at the foot of the edifice while the stress field on the upper slopes of the volcano remains compressional the stress at the base close to the center becomes extensional allowing the magma to intrude and differentiate at the base of the edifice a basal intrusive complex begins to grow explosive eruptions may be expected finally spreading of this complex creates an extensional stress field on the upper flanks of the volcano confining compression to the lower slopes of the edifice the extension allows primitive magma to erupt again at the summit the process of spreading seems to have a fundamental influence on the structural and magmatic evolution of volcanoes at every scale this process results from coupling and feedback between the gravitational and thermal fields thus studying this process may improve our understanding of the relation between structural dynamics and magma evolution and of the origin and evolution of volcanic seismicity and eruptions

effect of groundwater abstraction on fen ecosystems summary quantifying the effects of groundwater abstraction on fen ecosystems located in discharge areas can be complicated the water level in fens is close to the terrain surface most of the year and it is controlled by a relatively constant groundwater exfiltration it is difficult to measure the exfiltration fluxes and thus water level data is typically used to evaluate if the ecosystem is affected the paper presents collected data and analysis from a case study where the hydrological effect of groundwater abstraction on rich fens and springs in a danish river valley has been studied the natural hydrological conditions have been monitored intensively since march 2007 in the early spring 2009 two full scale pumping test were conducted in the regional high yield limestone aquifer the evaluation focuses on three isolated fens covering an area of approximately 5000 m 2 each and two natural springs all within a distance of 1 5 km to a planned well field in the river valley the interaction between groundwater and surface water is strongly affected by low permeable sediments these sediments reduce the direct discharge to the river and have a large impact on the functioning and presence of the rich fen and spring habitats continuous water level data from deep and shallow wells in the fens reflect the hydrological conditions large differences in water level drawdown during dry summer periods have been observed in the monitored fens and it is hypothesised that these differences are directly related to the groundwater seepage rate two pumping tests with duration of 3 weeks were performed at two different locations and the effects were monitored with 20 pressure transducers in monitoring wells and by continuous flow logging in springs in the area a significant reduction of the flow in the springs has been measured while the on the water level in the fens is difficult to distinguish from the natural variations it is concluded that hydrological modelling is a necessary tool in order to predict the long term effects of pumping in the fen areas the collected data gives valuable knowledge on the hydrology of the monitored ecosystems and the response towards groundwater abstraction and forms a solid foundation for hydrological modelling

pathways for degassing during the lava dome eruption of mount st helens 2004 2008 the ability of volatiles to escape rising magma regulates the explosivity of a volcanic system during silicic lava dome eruptions strain localization at the conduit margin occurs during magma ascent creating a damage halo with implications for gas escape here we report the first systematic study of permeability network anisotropy across the marginal shear zone of the a d 2004 2008 lava dome at mount st helens washington state usa the results show increasingly large permeability anisotropy of as much as four orders of magnitude over 4 m moving from the interior of the spine through the damage halo we find the permeability to be essentially isotropic in the spine interior but highly anisotropic in the damage zone and fault core our examination of the dome rocks reveals that the permeability anisotropy depends strongly on the presence of vertically oriented shear layers here we show that the rate of escape of volatiles will be several orders of magnitude higher vertically through a conduit margin shear zone than horizontally into the conduit wall

influence of macro fractures and micro fractures on permeability and elastic wave velocities in basalt at elevated pressure fractures are ubiquitous on all scales in crustal rocks the investigation of fractures and their influence on physical and transport properties of rocks is therefore essential for understanding of many key problems in seismology volcanology and rock engineering in crystalline rocks pore water is primarily stored in and migrates through networks of cracks and fractures at all scales it is therefore essential to know how fluid flow in such fracture networks responds to the elevated pressures found at depth here we report results from an investigation of changes in fluid permeability and associated changes in p wave and s wave velocities at elevated effective pressure for intact macro fractured and micro fractured samples of seljadur basalt in all cases permeability decreases and both wave velocities increase with increasing effective pressure permeability decreases were smallest in the intact material from approximately 10 19 m 2 to 3 x 10 20 m 2 intermediate in the micro fractured material from approximately 5 x 10 17 m 2 to 1 x 10 17 m 2 and largest in the macro fractured material from approximately 3 x 10 15 m 2 to 9 x 10 19 m 2 for a material containing both micro fractures and macro fractures the closure of macro fractures dominated the permeability reduction at low pressure with the closure of micro fractures exerting an increasing influence at higher pressure c 2010 elsevier b v all rights reserved

natural fractures in tight gas volcanic reservoirs and their influences on production in the xujiaweizi depression songliao basin china lower cretaceous volcanic rocks that contain major natural fractures are the main natural gas reservoirs in the deep xujiaweizi depression of the northern songliao basin in china natural fractures are mainly tectonic fractures in addition to some shrinkage dissolution and cryptoexplosive fractures analyzing various sources of data revealed that fractures are generally better developed in the rhyolites and trachytes of the crater and near crater group also tectonic fractures are developed to a higher degree in the hanging wall of faults with the degree of development decreasing with increasing distance from the fault planes more than 85 of these fractures are found to be unfilled and when connected by pores provide the main effective storage volume for these volcanic reservoirs up to 75 natural gas production is closely related to the linear density p10 of unfilled tectonic fractures and the angle between the orientation of unfilled tectonic fracture and the corresponding direction of the maximum horizontal stress component the results show that natural gas production is elevated when the average p10 exceeds 2 m 1 and the average included angle is less than 30 finally it was concluded that within the study area unfilled tectonic fractures of near east west strikes which have smaller included angles 30 with respect to the maximum horizontal stress direction are under minimal compressive stresses and make the best fluid flow pathways in the reservoir

time dependent permeability evolution in compacting volcanic fracture systems and implications for gas overpressure abstract volcanic eruptions are driven by the ascent of volatile laden magma the capacity of a volcano system to outgas these volatiles its permeability controls the explosive potential and fractures at volcanic conduit margins play a crucial role in tempering eruption explosivity by acting as outgassing pathways however these fractures are often filled with hot volcanic debris that welds and compacts over time meaning that these permeable pathways have a finite lifetime while numerous studies emphasize that permeability evolution is important for regulating pressure in shallow volcanic systems how and when this occurs remains an outstanding question in volcanology in this contribution we show that different pressure evolution regimes can be expected across a range of silicic systems as a function of the width and distribution of fractures in the system the timescales over which they can outgas a function of depth and temperature and the permeability of the host material we define outgassing diffusive relaxation and pressure increase regimes which are distinguished by comparing the characteristic timescales over which they operate moreover we define a critical permeability threshold which determines in concert with characteristic timescales of diffusive mass exchange between the pore and melt phases whether systems fracture and outgas efficiently or if a volcano will be prone to pressure increases incomplete healing and explosive failure

flank instability assessment at kick em jenny submarine volcano grenada lesser antilles a multidisciplinary approach using experiments and modeling kick em jenny kej located ca 8 km north of the island of grenada is the only active submarine volcano of the lesser antilles volcanic arc previous investigations of kej revealed that it lies within a collapse scar inherited from a past flank instability episode to assess the likelihood of future collapse we employ here a combined laboratory and modeling approach lavas collected using a remotely operated vehicle rov provided samples to perform the first rock physical property measurements for the materials comprising the kej edifice uniaxial and triaxial deformation experiments showed that the dominant failure mode within the edifice host rock is brittle edifice fractures such as those at champagne vent will therefore assist the outgassing of the nearby magma filled conduit favoring effusive behavior these laboratory data were then used as input parameters in models of slope stability first relative slope stability analysis revealed that the sw to n sector of the volcano displays a deficit of mass volume with respect to a volcanoid ideal 3d surface slope stability analysis using a limit equilibrium method lem showed that kej is currently stable since all values of stability factor or factor of safety fs are greater than unity the lowest values of fs were found for the sw nw sector of the volcano the sector displaying a mass volume deficit although currently stable kej may become unstable in the future instability severe reductions in fs could result for example from overpressurization due to the growth of a cryptodome our modeling has shown that instability induced flank collapse will most likely initiate from the sw nw sector of kej therefore mobilizing a volume of at least ca 0 7 km3 the mobilization of ca 0 7 km3 of material is certainly capable of generating a tsunami that poses a significant hazard to the southern islands of the west indies

groundwater surface water interaction between an esker aquifer and a drained fen summary groundwater discharge from an esker aquifer to a fen was studied to understand relevant hydrological processes for groundwater surface water interaction in an esker peatland hillslope piezometric levels of the peat layer and esker sand layer were continuously monitored and compared to climate data groundwater exfiltration points were spatially mapped and related to peat depth the study showed a clear interaction between fen surface water and sand esker groundwater although the hydraulic conductivity of peat was low and the peat depth thick in the artesian aquifer beneath the fen the piezometric head showed a clear response to precipitation and evapotranspiration wetting of the peat is rapidly seen as pressure increase in the confined aquifer this provides new information on hydraulics of wetlands found in aquifer discharge areas groundwater exfiltrated through thick peat layers in vertical preferential point discharges demonstrating how double porosity provides flow through the peat layer with low hydraulic conductivity in the peat matrix the pipe flow channels were found in the deep peat area in the transition zones from deep peat to shallow peat diffuse seepage through the ditch bottom was found where ditches cut into the sand layer increased discharge through the ditch bed reduced the flow resistance at the aquifer boundary which can lower the groundwater level in the esker the results show new processes in the groundwater discharge zone that are useful for groundwater modelling and policy development for future groundwater protection

seismotectonic deformation of the volcanic edifice prior to the 1998 lava eruption of volcan de colima mexico the andesitic stratovolcano volcan de colima is one of the most active volcanoes in mexico the recent eruption of volcan de colima began in november 1998 and was preceded by a 12 month period of seismic activity that included five earthquake swarms about 600 events with magnitudes from 0 5 to 2 7 were located within a 50 km2 area extending northward from the crater of volcan de colima to the pleistocene volcano nevado de colima the majority of hypocenters within this area did not exceed 5 km depth below sea level we investigated earthquake focal mechanisms and seismotectonic deformations of the volcanic edifice focal mechanisms during four earthquake swarms indicated normal faulting associated with extensional processes which is in agreement with the general stress regime near the volcano revealed by field measurements of fault slips earthquakes in the fifth swarm had focal mechanisms associated with inverse faulting showing a significant change in the stress situation just before the beginning of the eruption the calculated deformations varied from 1 3 10 11 to 2 7 10 9 the first swarm of november december 1997 resulted in a n s horizontal elongation that was two times greater than the e w horizontal shortening the volume was also subject to vertical shortening the second and third swarms observed in march and may 1998 showed uniform horizontal n s and e w elongations accompanied by a vertical elongation of the volume in june july 1998 the situation of november december 1997 was repeated with n s horizontal elongation greater than the e w horizontal shortening of the volume accompanied by intensive vertical shortening during the last swarm of october november 1998 slight e w elongation of the seismic volume was accompanied by strong n s shortening and very slight vertical shortening we assume that the seismic activity prior to the 1998 eruption of volcan de colima developed along two intersecting tectonic structures the n s trending colima rift and the e w trending system of faults associated with tamazula fault during the first stage november 1997 july 1998 the passageway for magma was developed along the tamazula fault system under horizontal extension without any surface manifestation in october november 1998 the seismic events began to cluster along the colima rift structures under predominantly compressional stresses this condition culminated with the extrusion of andesitic block lava from the summit crater

thermal imaging and analysis of short lived vulcanian explosions at volcan de colima mexico abstract vulcanian explosions present a major hazard at many active volcanoes but they also provide useful insights into the underlying behaviour of the volcanic system and therefore require close monitoring thermal infrared cameras are an effective tool for imaging vulcanian explosion plumes since they capture detailed temperature information and can reveal the internal dynamics of the plume forming explosions high spatial resolution thermal images of 200 small to moderate sized vulcanian explosions from the summit crater of volcan de colima mexico recorded between 2006 and 2011 were analysed to distinguish different event types and develop an explosion classification scheme explosions display a broad spectrum of sizes and characteristics ranging between two typical end members large impulsive events producing rapidly ascending explosion plumes up to heights of 600 1600 m above the crater rim and small diffusive events with plumes restricted to heights 1 to 29 m s 1 a time series of thermal imagery throughout the period 2006 2011 reveals a weak relationship between apparent plume temperatures and lava dome extrusion with the highest explosion temperatures coinciding with the onset of dome growth in early 2007 temporal variations in the source locations of explosions across the summit crater are also identified and appear to show a close relationship to the patterns of lava dome growth and thermal evolution with explosion source locations associated with the highest temperature thermal features

the porosity of pyroclasts as an indicator of volcanic explosivity abstract the porosity of pyroclasts is a valuable indicator of the character of a volcanic eruption here we analyse the porosity distribution of deposits of 17 different volcanic eruptions covering a broad range of inferred magma properties and observed eruption styles histograms of porosity versus pyroclast frequency show distinctive features in terms of the mean variance modality and skewness we present characteristic porosity distributions for five different eruption style categories dome forming explosive basaltic cryptodome forming subplinian plinian ultraplinian and phreatomagmatic we find that the mean porosity values of the deposits correlate positively with the volcanic explosivity index vei of newhall and self 1982 regardless of the degree of complexity of the individual distributions we then propose causes of the deviations from a general trend in terms of magma viscosity degassing efficiency and cap rock confinement finally a generalised correlation between clast porosity and explosivity of an eruption is developed that should help together with conventional field techniques facilitate the interpretation of pyroclastic sequences even in cases where no direct observation of the eruption exists

degassing processes during lava dome growth insights from santiaguito lava dome guatemala abstract eruptions of intermediate magma may be explosive or effusive the development of open system degassing has been proposed as a pre requisite for effusion of intermediate magma however processes leading to open system degassing are poorly understood to better understand degassing processes during lava dome extrusion we report high temporal resolution so2 emission rate measurements collected with an ultra violet imaging camera at santiaguito guatemala santiaguito is an ideal case study as the dome lava is compositionally very similar to products of the 1902 plinian eruption of the parental santa maria volcano we find that degassing is weak 0 4 1 kg s 1 but continuous and explosions are associated with small increases in emission rates up to 2 3 kg s 1 continuous repose degassing occurs through a shallow cap rock which likely represents a proto crust on the block lava flow which is extruded from the same vent the continual permeability of the upper conduit argues against a mechanism of explosion triggering in which gas pressure builds beneath a viscous cap rock or plug rather we consider degassing data better consistent with a model of shear fracturing at the conduit margins using field constraints we model the viscosity of santiaguito magma as a function of depth and show that conditions for shear fracturing are met from 150 600 m to the surface this is in line with independent estimates of explosion initiation depth we show that repose timescales are orders of magnitude longer than the timescale for shear fracture and suggest that explosions are triggered when a continuous network of smaller scale fractures develops at which point decompression occurs and an explosion is triggered fracture healing occurs by viscous relaxation however near to the surface where viscosity is highest an unconsolidated gouge layer may develop our model implies that the observed explosions are a by product of extrusion shear fracturing can drive open system degassing of crystal rich intermediate magma at shallow levels in the conduit as high magma viscosity is able to overcome the low strain rates associated with slow ascent of magma

evolution of the mechanics of the 2004 2008 mt st helens lava dome with time and temperature the 2004 2008 eruption of mount st helens washington usa formed a typical example of a pelean spiny lava dome with solid spines of crystalline silicic lava extruded along shear zones bounded by fault gouge cashman et al 2008 iverson et al 2006 creation of and movement along shear zones in this erupting lava recorded as shallow micro earthquakes were thus key controls on eruption style and rate in addition to dome stability as eruption style and lava dome stability often change within individual eruptions it is important to identify how the strength of dome rocks changes with temperature time texture and extrusion rate however critical values of the shear strength of dome lava at eruptive temperatures have never been measured we have found in controlled laboratory experiments simulating eruption conditions that dome lava strength increased during the course of the 2004 2008 eruption of mount st helens as the textures became more crystalline and less porous increasing lava strength during the eruption would lead to deformation becoming increasingly localised to shear zones explaining why the extrusion style and centre were so consistent acoustic emissions and microstructure analysis indicate that fracturing was more localised at higher temperatures resulting in higher strengths at eruptive temperature compared to ambient temperatures for later erupted samples the strength of these later erupted samples at eruptive temperatures increased markedly with strain rate this would damp any increases in effusion rate that may result from changes in magma pressure explaining the steady effusion rate at mount st helens from 2004 to 2008

coupled effect of magma degassing and rheology on silicic volcanism abstract explosive volcanism such as the 1991 mt pinatubo philippines and the 2008 mt chaiten chile eruptions is caused by violent vesiculation of hydrous magma however gas may efficiently separate from magma owing to the enhancement of gas permeability by shear deformation of magma flowing in a volcanic conduit this makes it difficult to maintain the driving force of explosive volcanism although explosive volcanism is actually common here we propose that shear localization in a volcanic conduit controls the eruption style and explosivity based on deformation experiments of vesicular magma linked with synchrotron radiation x ray radiography and computed tomography we observed for the first time in situ that the shear localization caused magma fracturing and formed a slip plane and thus inhibited deformation and outgassing elsewhere we also observed the compaction of vesicular magma into a dense lava as a result of outgassing when shear localization did not occur in a natural setting shear localizes along the edges of a volcanic conduit where the strain rate is high causing a highly permeable fracturing layer to form at the conduit s edge and leaving less sheared and less outgassed magma at its center the less outgassed magma in the center may ascend rapidly and cause explosive volcanism non explosive lava effusion may occur only when shear localization does not occur effectively this new view explains the rapid ascent of viscous magma and the formation of pyroclasts with contrasting vesicularity pyroclastic obsidian and highly vesiculated pumice

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

reconstructing magma failure and the degassing network of dome building eruptions volcanic eruptions are regulated by the rheology of magmas and their ability to degas both detail the evolution of stresses within ascending subvolcanic magma but as magma is forced through the ductile brittle transition new pathways emerge as cracks nucleate propagate and coalesce constructing a permeable network current analyses of magma dynamics center on models of the glass transition neglecting important aspects such as incremental strain accommodation and the key monitoring tool of seismicity here in a combined methods study we report the first high resolution 20 m neutron computed tomography and microseismic monitoring of magma failure under controlled experimental conditions the data reconstruction reveals that a competition between extensional and shear fracturing modes controls the total magnitude of strain to failure and importantly the geometry and efficiency of the permeable fracture network that regulates degassing events extrapolation of our findings yields magma ascent via strain localization along conduit margins thereby providing an explanation for gas and ash explosions along arcuate fractures at active lava domes we conclude that a coupled deformation seismicity analysis holds a derivation of fracture mechanisms and network and thus holds potential application in forecasting technologies

from rock to magma and back again the evolution of temperature and deformation mechanism in conduit margin zones abstract explosive silicic volcanism is driven by gas overpressure in systems that are inefficient at outgassing the zone at the margin of a volcanic conduit thought to play an important role in the outgassing of magma and therefore pore pressure changes and explosivity is the boundary through which heat is exchanged from the hot magma to the colder country rock using a simple heat transfer model we first show that the isotherm for the glass transition temperature whereat the glass within the groundmass transitions from a glass to an undercooled liquid moves into the country rock when the magma within the conduit can stay hot or into the conduit when the magma is quasi stagnant and cools on the centimetric scale over days to months we then explore the influence of a migrating viscous boundary on compactive deformation micromechanisms in the conduit margin zone using high pressure effective pressure of 40 mpa high temperature up to 800 c triaxial deformation experiments on porous andesite our experiments show that the micromechanism facilitating compaction in andesite is localised cataclastic pore collapse at all temperatures below the glass transition of the amorphous groundmass glass t g i e rock in this regime porosity is only reduced within the bands of crushed pores the porosity outside the bands remains unchanged further the strength of andesite is a positive function of temperature below the threshold t g due to thermal expansion driven microcrack closure the micromechanism driving compaction above t g i e magma is the distributed viscous flow of the melt phase in this regime porosity loss is distributed and is accommodated by the widespread flattening and closure of pores we find that viscous flow is much more efficient at reducing porosity than cataclastic pore collapse and that it requires stresses much lower than those required to form bands of crushed pores our study therefore highlights that temperature excursions can result in a change in deformation micromechanism that drastically alters the mechanical and hydraulic properties of the material within the conduit margin zone with possible implications for pore pressure augmentation and explosive behaviour

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

permeability reduction of fractured rhyolite in volcanic conduits and its control on eruption cyclicity repetitive explosions of silicic volcanoes are thought to be triggered by volcanic gases in the shallow parts of the conduit to trap the volcanic gases in the conduit the lava dome or plug must be impermeable however silicic magma undergoes brittle fracturing during ascent because its viscosity increases via dehydration and groundmass crystallization since fractured magma has high permeability the reduction of permeability is a key process that allows gases to accumulate leading to an explosion we performed uniaxial compression experiments on rhyolite fragments at temperatures of 700 900 c to investigate the permeability reduction of fractured rhyolite experimental results show that permeability is controlled by porosity and permeability reduction can be caused by compaction of fractured magma based on the observed relationship between permeability and porosity we calculate the time scale of permeability reduction i e the transition from permeable to impermeable magma the calculated time scale 100 1000 s is much shorter than the eruption repose 1 10 h observed at sakurajima volcano japan this result implies that an impermeable lava dome and plug formed during the eruption repose in contrast the repose time 20 40 min of santiaguito volcano guatemala is shorter than the time scale of permeability reduction 10 h this indicates that gases could not have accumulated before volcanic explosions and that other mechanisms could control eruption cyclicity

causes and consequences of pressurisation in lava dome eruptions abstract high total and fluid pressures develop in the interior of high viscosity lava domes and in the uppermost parts of the feeding conduit system as a consequence of degassing two effects are recognised and are modelled quantitatively first large increases in magma viscosity result from degassing during magma ascent strong vertical gradients in viscosity result and large excess pressures and pressure gradients develop at the top of the conduit and in the dome calculations of conduit flow show that almost all the excess pressure drop from the chamber in an andesitic dome eruption occurs during the last several hundred metres of ascent second microlites grow in the melt phase as a consequence of undercooling caused by gas loss rapid microlite growth can cause large excess fluid pressures to develop at shallow levels theoretically closed system microlite crystallization can increase local pressure by a few tens of mpa although build up of pressure will be countered by gas loss through permeable flow and expansion by viscous flow microlite crystallization is most effective in causing excess gas pressures at depths of a few hundred metres in the uppermost parts of the conduit and dome interior some of the major phenomena of lava dome eruptions can be attributed to these pressurisation effects including spurts of growth cycles of dome growth and subsidence sudden onset of violent explosive activity and disintegration of lava during formation of pyroclastic flows the characteristic shallow level long period and hybrid seismicity characteristic of dome eruptions is attributed to the excess fluid pressures which are maintained close to the fracture strength of the dome and wallrock resulting in fluid movement during formation of tensile and shear fractures within the dome and upper conduit

aquifer peatland connectivity in southern quebec canada in areas where peatlands are abundant they are likely to play a significant role in the hydrological and hydrogeological dynamics of a watershed although individual case studies are reported in the literature there is a large range of aquifer peatland interactions and there is a need to understand the controls of these interactions the objectives of this study were 1 to better understand aquifer peatland connections and how these may be predicted by geology and geomorphic location and 2 to provide a variety of reference sites for glacial geological settings slope and depression peatlands were studied in the abitibi temiscamingue region and in the st lawrence lowlands two contrasting regions of southern quebec a total of 12 transects that span a shallow aquifer peatland interface were instrumented with piezometers field investigations included peatland characterization monthly water level monitoring and continuous hydraulic head measurements with pressure transducers the results indicate that 7 of the 12 transects receive groundwater from the surrounding shallow aquifer at the peatland margin four lateral flow patterns were identified and associated with slope peatlands parallel inflow and divergent flow and with depression peatlands convergent flow and parallel outflow vertical hydraulic gradients suggest that water flows mainly downwards i e from the peatland to the underlying mineral deposits vertical connectivity appears to decrease as the distance from the peatland margin increases all of these exchanges are important components in the sustainability of peatland hydrogeological functions the regional comparison of aquifer peatland flow dynamics performed in this study provides a new set of referenced data for the assessment of aquifer peatland connectivity

source location of long period seismicity at volcan de colima mexico this paper presents an analysis of seismicity associated with the volcanic activity of volcan de colima mexico and recorded in the period november 2005 april 2006 during a field survey by the istituto nazionale di geofisica e vulcanologia ingv osservatorio vesuviano the observatorio vulcanologico de colima of colima university and the instituto andaluz de geofisica university of granada three different types of volcanic earthquakes have been identified on the basis of their spectral properties type a 0 3 1 hz type b 1 5 hz and type c 3 4 hz results of polarization analysis applied to type a events show a predominance of radial motion indicating that the wavefield comprises compressional waves p and shear waves polarized in the vertical plane sv while the signal always begins with a negative polarity type a b and c earthquakes have been located using both a flat layered model and a 3d model including topography hypocentre distributions indicate that the source of type a signals is very shallow and confined to a small volume lying about 1 km below the crater in contrast the source of type b and c events is significantly deeper with most hypocentres located in a volume of about 1 km3 centred at 2 5 3 km depth a cluster analysis based on the cross correlation among the waveforms of different events recorded at the same station was applied to type a earthquakes only two clusters which include only a small percentage of events were found indicating that earthquake families were uncommon during the period of our survey

stratovolcano growth by co eruptive intrusion the 2008 eruption of tungurahua ecuador 1 volcanic edifices are constructed by a combination of erupted material and internal growth we use the l band satellite alos to produce insar measurements at tungurahua volcano ecuador we find a maximum of 17 5 cm of uplift on the upper western flank between december 2007 and march 2008 coincident with an eruption in february 2008 the deformation can be modeled using an ellipsoidal or sill like source within the edifice the models require an elongated aspect ratio with a length of 4 6 km the intruded volume of 1 2 106 m3 is roughly equivalent to the bulk erupted volume of 1 5 106 m3 and together the values are roughly equal to the long term magma flux the question remains whether this uplift is permanent thus contributing to the internal growth of the edifice or temporary in which case the magma will be released in a future eruption

determining flow type shear rate and shear stress in magmas from bubble shapes and orientations we interpret the shear environments that produced bubble textures in obsidian samples using the results of theoretical numerical and experimental studies on the deformation of bubbles in shear flows in particular we use the shapes and orientations of bubbles vesicles in obsidian to estimate shear rates and shear stresses and assess flow type simple vs pure shear this technique can be used to determine shear rates in volcanic conduits the origin of pyroclastic obsidian and the emplacement history and dynamics of obsidian flows a spherical bubble in a viscous fluid subjected to a low reynolds number steady flow field deforms until it reaches a steady shape and orientation bubble deformation is governed by the competing stresses from shearing that deforms and surface tension that rerounds the ratio of these stresses is the capillary number ca because the relationships among ca bubble orientation and shape differ for pure and simple shear we can distinguish between these flow types using preserved bubble geometries furthermore because ca is a function of shear rate we can use relationships between ca and the magnitude of deformation to calculate shear rates when melt viscosity and surface tension are known to demonstrate the potential of the technique we examine three obsidian samples chosen for diversity of origin and texture two of the samples have low crystallinities and banding defined by layers of different vesicularity bubble geometries in a spatter fed obsidian flow sample record deformation by pure shear whereas a juvenile obsidian clast from a pyroclastic fall deposit records predominantly simple shear a third sample from an obsidian flow has banding marked by variable concentrations of microlites and shows bubble deformation most consistent with dominantly simple shear and some bubble relaxation the highest shear rate and shear stress are recorded by the pyroclastic obsidian shear rate 0 01 s 1 shear stress 60 kpa the spatter fed obsidian flow sample deformed by pure shear records the lowest shear stress 4 9 kpa the obsidian flow sample deformed by simple shear records the lowest shear rates 10 6 6 10 6 9 s 1 despite high shear stresses 22 36 kpa because of the high viscosity of the degassed rhyolite 0 13 wt water

origins of mount st helens cataclasites experimental insights the 2004 2006 eruption of mount st helens produced a sequence of lava domes characterized by a 1 3 m thick outer carapace of highly brecciated and comminuted dacite fault rocks this outer layer of fault rocks is proposed to be a physical manifestation of the drumbeat microseismicity such that magma extrusion occurred via integration of rapid co seismic slip along small displacement 5 mm faults a suite of deformation experiments performed on samples of mount st helens msh dacite under confining pressures of 0 25 50 and 75 mpa at room temperature and an average displacement rate of 10 4 s were run to reproduce fault textures found in nature the msh dacite starting material has low porosity 7 8 and a microcrystalline groundmass with little glass a subsidiary set of deformation experiments on dacite samples from the 2006 augustine eruption was run under identical experimental conditions to evaluate the effect of increased porosity 20 24 on failure mechanisms the msh dacite samples show a progressive increase in peak strength with increasing confining pressure are strong peak stress at 75 mpa is 700 mpa and failed by localized brittle behavior characterized by macroscopic fractures and rapid stress drops in contrast augustine dacite deformed by distributed cataclastic flow and is much weaker peak stress at 75 mpa is 220 mpa we propose that the generation of low porosity dacite was an important variable in promoting wholesale localized faulting and the attendant drumbeat microseismicity at mount st helens microstructures of gouge developed experimentally at room temperature are remarkably similar to those developed at 730 c at msh mode i microcracking shear fracture of grains and grain size comminution occurred in both natural and experimental fault rocks laser grain size particle analyses show peaks at 1 5 m for experimental run products vs peaks at 1 9 and 4 m for natural msh gouge we conclude that because the msh lava had solidified prior to faulting temperature is secondary in importance in the formation of the gouge material based on the amount of fault displacement per microseismic event the number of drumbeats and the aggregate radial thickness 1 3 m of the gouge we calculate that the 3 6 m day eruption rate allows for gouge filled slip surfaces having thicknesses of 0 8 5 mm and strike lengths of 98 190 m per seismic event