the leka opholite complex central norwegian caledonides field characteristics and geotectonic significance the leka ophiolite complex loc of lower ordovician age u pb zircon age of 497 2 ma contains the following components 1 a lower strongly deformed harzburgite unit representing the depleted upper mantle this unit becomes progressively richer in dunite towards 2 a sequence of ultramafic cumulates unconformably overlying the harzburgite tectonite the crystallization sequence in the ultramafic cumulates is olivine chromite clinopyroxene orthopyroxene in the south western part of the ultramafic cumulate sequence layers of metagabbro appear interlayered with banded dunite wehrlite the above mentioned rock sequences may be in tectonic contact with 3 layered metagabbros which give way upwards to laminated and vari textured metagabbro within the laminated vari textured metagabbro is the first appearance of 4 metabasalt dykes which increase upwards in abundance from scattered single dykes to a 100 sheeted dyke swarm associated with the vari textured metagabbro and metabasalt dykes are minor acid intrusions within the upper part of the dyke swarm is the first appearance of 5 pillow lavas which are associated with thick volcaniclastic metasediments at apparently higher stratigraphic levels geochemically the metabasalt dykes and associated pillow lavas range from predominant island arc tholeiite iat to morb character and typical metaboninites are also present this range of geochemical composition as well as the crystallization sequence of the cumulates strongly suggests that the first stages in the formation of the loc was by spreading above a subduction zone the pillow lavas associated with volcaniclastic rocks are of morb or wpb type and these probably represent the later spreading stages of the loc in a back arc basin but at sufficient distance to be unaffected by the geochemical fingerprints of the subduction process

26al in chondrules from unequilibrated l chondrites onset and duration of chondrule formation in the early solar system abstract al mg isotope records in sixteen ferromagnesian chondrules from five unequilibrated ordinary chondrites uocs four l chondrites with petrologic grades varying from 3 0 to 3 2 and one l ll chondrite with petrologic grade 3 1 were studied using an ion microprobe to look for presence of the now extinct nuclide 26 al at the time of their formation uocs of low petrologic grades were selected to avoid possible thermal perturbation of al mg isotope records in the analyzed chondrules the presence of resolved 26 mg excess due to decay of 26 al in al rich phases such as glassy mesostasis and plagioclase was found in all the chondrules barring a sole exception the inferred initial 26 al 27 al ratios in the chondrules range from 0 6 1 9 10 5 and most of them have values lying between 0 6 1 1 10 5 our data supports a late formation of chondrules relative to cais suggested in previous studies and indicates a time lag of at least 1 5 ma between cai formation and onset of an intense episode of chondrule formation in the early solar system that lasted less than a million years the paucity of chondrules 26 al 27 al 10 5 rules out active chondrule forming events at earlier epochs the presence of rare chondrules with initial 26 al 27 al 5 in uocs of low petrologic grades 3 0 to 3 2 analyzed by us and also reported previously in uocs of petrologic grade ranging up to 3 4 most probably represent parent body processes leading to disturbance of al mg isotope systematics in these cases an extended duration of chondrule formation lasting more than a couple of million years suggested in some earlier studies needs reassessment a long duration of chondrule formation is also difficult to reconcile with the problem of storage isolation mixing and assimilation of chondrules into their meteorite parent bodies

a 1 d evolutionary model for icy satellites applied to enceladus abstract we develop a long term 1 d evolution model for icy satellites that couples multiple processes water migration and differentiation geochemical reactions and silicate phase transitions compaction by self gravity and ablation the model further considers the following energy sources and sinks tidal heating radiogenic heating geochemical energy released by serpentinization or absorbed by mineral dehydration gravitational energy and insolation and heat transport by conduction convection and advection we apply the model to enceladus by guessing the initial conditions that would render a structure compatible with present day observations assuming the initial structure to have been homogeneous assuming the satellite has been losing water continually along its evolution we postulate that it was formed as a more massive more icy and more porous satellite and gradually transformed into its present day state due to sustained long term tidal heating we consider several initial compositions and evolution scenarios and follow the evolution for the age of the solar system testing the present day model results against the available observational constraints our model shows the present configuration to be differentiated into a pure icy mantle several tens of km thick overlying a rocky core composed of dehydrated rock at the center and hydrated rock in the outer part for enceladus it predicts a higher rock ice mass ratio than previously assumed and a thinner ice mantle compatible with recent estimates based on gravity field measurements although obviously the model cannot be used to explain local phenomena it sheds light on the internal structure invoked in explanations of localized features and activities

a frictional law for volcanic ash gouge abstract volcanic provinces are structurally active regions undergoing continual deformation along faults within such fault structures volcanic ash gouge containing both crystalline and glassy material may act as a potential fault plane lubricant here we investigate the frictional properties of volcanic ash gouges with varying glass fractions using a rotary shear apparatus at a range of slip rates 1 3 1300 mm s and axial stresses 0 5 2 5 mpa we show that the frictional behaviour of volcanic ash is in agreement with byerlee s friction law at low slip velocities irrespective of glass content the results reveal a common non linear reduction of the friction coefficient with slip velocity and yield a frictional law for fault zones containing volcanic ash gouge textural analysis reveals that strain localisation and the development of shear bands are more prominent at higher slip velocities 10 mm s the textures observed here are similar to those recorded in ash gouge at the surface of extrusive spines at mount st helens usa we use the rate weakening component of the frictional law to estimate shear stress resistance reductions associated with episodic seismogenic slip events that accompany magma ascent pulses we conclude that the internal structure of volcanic ash gouge may act as a kinematic marker of exogenic dome growth

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

volcano dome dynamics at mount st helens deformation and intermittent subsidence monitored by seismicity and camera imagery pixel offsets the surface deformation field measured at volcanic domes provides insights into the effects of magmatic processes gravity and gas driven processes and the development and distribution of internal dome structures here we study short term dome deformation associated with earthquakes at mount st helens recorded by a permanent optical camera and seismic monitoring network we use digital image correlation dic to compute the displacement field between successive images and compare the results to the occurrence and characteristics of seismic events during a 6 week period of dome growth in 2006 the results reveal that dome growth at mount st helens was repeatedly interrupted by short term meter scale downward displacements at the dome surface which were associated in time with low frequency large magnitude seismic events followed by a tremor like signal the tremor was only recorded by the seismic stations closest to the dome we find a correlation between the magnitudes of the camera derived displacements and the spectral amplitudes of the associated tremor we use the dic results from two cameras and a high resolution topographic model to derive full 3 d displacement maps which reveals internal dome structures and the effect of the seismic activity on daily surface velocities we postulate that the tremor is recording the gravity driven response of the upper dome due to mechanical collapse or depressurization and fault controlled slumping our results highlight the different scales and structural expressions during growth and disintegration of lava domes and the relationships between seismic and deformation signals

the june july 1993 seismo acoustic event at coaxial segment juan de fuca ridge evidence for a lateral dike injection the coaxial segment juan de fuca ridge was the site of an intense swarm of earthquakes that began at 21 43 gmt on june 26 1993 julian day 177 the swarm started near 46 15 n and migrated northward over the next 40 hours to 46 36 n where the majority of 676 events occurred during the following 3 weeks of activity the earthquakes propagated nne at a velocity of 0 3 0 1 m s 1 from the southern to northern swarm sites the activity went undetected by land based seismic networks along the oregon and washington coasts suggesting that the earthquakes were all m 4 0 the character of this earthquake swarm is very similar to dike injections observed at krafla and kilauea volcanoes the earthquake activity and subsequent migration probably represent a lateral dike injection into faults and fissures comprising the coaxial segment the reservoir acting as the dike s source likely resides beneath or to the south of the initial southern swarm of earthquakes the large magma supply at axial volcano cannot be ruled out as the source for the coaxial dike even though axial volcano exhibited no earthquake activity related to the coaxial swarm the t wave earthquake swarm reported here is the first deep ocean observation of volcanic seismicity associated with what most likely was a mid ocean ridge dike injection event

frictional melting processes in planetary materials from hypervelocity impact to earthquakes frictional melting is the result of the conversion of mechanical deformation to heat under adiabatic conditions of slip within planetary materials which are mainly natural ceramics frictional melting occurs at high strain rates typically 10 2 s 1 and at slip velocities greater than 0 1 m s 1 t he pathway to friction melting is controlled by the mechanical properties of a rock s constituent minerals especially fracture toughness minerals with the lowest fracture toughnesses and breakdown temperatures are preferentially comminuted and fused to form the melt the product is a polyphase suspension comprising mineral and rock fragments enclosed in a liquid matrix this cools to form the rock type known as pseudotachylyte and at even higher strain rates it forms shock veins in meteorites and in impact craters which may contain high pressure mineral polymorphs the generation of melt on sliding surfaces can lubricate earthquake faults facilitate the post shock modification of impact craters and make landslides more hazardous

volcanic drumbeat seismicity caused by stick slip motion and magmatic frictional melting during volcanic eruptions solidifying magma ascends through the volcanic conduit often accompanied by repetitive drum beat seismicity laboratory experiments on magma samples from soufriere hills volcano montserrat and mount st helens volcano usa show that viscous melt formed at the surface between the rising magma and conduit walls can temporarily halt magma ascent accentuating the cyclical seismicity

spreading structure of the troodos ophiolite cyprus orientations of dikes within the sheeted dike complex of the troodos ophiolite reveal primary spreading structure produced at a complex ridge transform intersection three structural grabens are defined by listric and planar normal faults and rotated dikes that dip symmetrically toward graben axes faults flatten at depth into a detachment within the upper parts of the plutonic complex large exhalative massive sulfide deposits occur within the pillow sections of two of the grabens and appear to be associated with underlying altered and mineralized normal fault zones that channeled hydrothermal fluids we suggest that the grabens are fossil axial valleys produced by successive eastward jumps of an approximately north trending present coordinates slow spreading ridge crest a simple model for ridge migration indicates eastward jumps of 8 13 km changes in ridge orientation are suggested by changes in trends of dikes and graben axes comparison of the pattern of dikes near the arakapas fault zone with the structure of active ridge transform intersections suggests that the fault is a right offset sinistral transform in contrast to earlier models in which a ridge to the west of the exposed troodos complex was proposed

the marinduque intra arc basin philippines basin genesis and in situ ophiolite development in a strike slip setting the marinduque basin is a marine intra arc basin in the north central philippine volcanic arc system a suite of marine geophysical observations show the basin as a whole to be rhombic in shape with its long axis trending north northwest a conspicuous volcanic ridge trends east northeast across the basin dividing it into two smaller active depocenters symmetric linear magnetic anomalies striking parallel to and centered over the central volcanic ridge indicate that it formed by extension in a north south direction by a process analogous to sea floor spreading geometric and temporal relations between the central volcanic ridge and several en echelon sets of north trending left stepping faults indicate that sea floor spreading occurred in the extensional stepover zone of a left lateral strike slip fault system and that the marinduque basin as a whole is a composite pull apart basin whose floor is in part composed of oceanic type crust the evolution of the central volcanic ridge presents an actualistic model for in situ development and emplacement of ophiolites in an island arc setting the presently active left lateral philippine fault zone truncated the eastern flank of the basin as well as the central volcanic ridge following their initial formation shortening associated with a probable restraining bend in the philippine fault has resulted in uplift of basin flank sediments which have been thrust back toward the basin interior the philippine fault has also displaced fragments of the ridge and basin floor toward the north the overall history of the marinduque basin further suggests that strike slip processes may play an important role in the origin and growth of intra arc basins such basins may ultimately become isolated in a forearc or backarc setting

the structure of sheeted dikes and associated rocks in north arm massif bay of islands ophiolite complex and the intrusive process at oceanic spreading centers the structures of north arm mountain ophiolitic basalt and diabase indicate that these formed as a frozen lid to an underlying magma chamber in a system analogous to present day mid ocean spreading systems volcanics consist of pillow lavas flows and breccias volcanics grade downward into sheeted dikes by increase in dike frequency across a transition commonly less than 50 m thick sheeted dikes subparallel throughout dip away from the paleospreading axis located to the west of present ophiolite position as indicated by chilled margin bias implying rotation of the lid down and away from the axis one way chilling statistics dike thickness and dike intrusion sequence analyses all point toward a relatively narrow but complex zone of dike intrusion the mean measured dike thickness is 0 90 m diabase contains zones of fracturing subparallel to dike trends interpreted to reflect oceanic fissuring and faulting these commonly terminate downward in foliated gabbro and or lineated amphibolite diabas

thermal vesiculation during volcanic eruptions a textural examination of volcanic ash erupted from santiaguito volcano in guatemala coupled with an analysis of the geophysical signals indicates that rapid heating during fault friction can cause melting and vesiculation development of bubbles of hydrated silicic magma thus strongly affecting magma strength and eruptive behaviour

rheological transitions in high temperature volcanic fault zones silicic magma experiences shear induced brittle fracturing during its ascent resulting in the formation of a magmatic fault at the conduit margin once the fault is formed frictional behavior of the fault controls the magma ascent process we observed torsional deformation of a magmatic fault gouge in situ at temperatures of 800 and 900 c using synchrotron radiation x ray radiography the torsional deformation rate was set at 0 1 10 rpm corresponding to equivalent slip velocities of 2 27 10 5 1 74 10 3 m s 1 and shear strain rates of 0 014 1 16 s 1 the normal stresses used were 1 5 and 10 mpa the magmatic fault showed frictional sliding as well as viscous flow even above the glass transition temperature the transition between frictional sliding and viscous flow depends on temperature deformation rate and normal stress on the fault at 900 c the fault showed viscous deformation at a normal stress of 10 mpa while frictional sliding was predominant at 800 c we propose the ratio of timescales of fault healing and deformation as a criterion for transition between frictional sliding and viscous flow the experimentally calibrated criterion infers that frictional sliding is predominant from 500 m in depth during explosive eruption this may explain rapid magma ascent without efficient outgassing frictional heating would in turn enhance fault healing resulting in the reverse transition from frictional sliding to viscous flow followed by deceleration of magma ascent therefore cyclic transitions between frictional sliding and viscous flow are a possible explanation for the cyclic behavior of lava effusion

frictional melting of gabbro under extreme experimental conditions of normal stress acceleration and sliding velocity 1 with the advent of high velocity shear apparatus several experimental studies have been performed in recent years improving our understanding of the evolution of fault strength during seismic slip however these experiments were conducted under relatively low normal stress 20 mpa and using small cylindrical samples where a large gradient in slip velocity exists across the sliding surface given the above limitations the extrapolation of these experimental results to natural conditions is not trivial here we present results from an experimental study on gabbroic rocks using a newly developed rotary shear apparatus capable of reaching higher normal stress up to 50 mpa on ring shaped samples 30 50 mm internal external diameter and allowing precise control of the imposed slip velocity function the results confirm that steady state shear stress during the melt lubricated phase of the experiment depends on normal stress in the form of a power law equation as predicted by theoretical models however the exponent appears closer to 0 5 contrary to the theoretical prediction of 0 25 we observe no systematic dependence of shear stress on acceleration but increasing deceleration drastically decreases the recovery of friction during final slip we find that the slip weakening distance decreases inversely with increasing normal stress in agreement with theoretical considerations and decreases with increasing slip rate extrapolation of the slip weakening distance to natural conditions predicts a slip velocity for ancient seismic events of 0 3 1 m s when compared with field estimates these values compare well with seismological estimates

characteristics and consequences of lava dome collapse at ruawahia taupo volcanic zone new zealand the eruption of large rhyolitic lava domes may be accompanied by the formation of large block and ash flows this may be linked to the style of dome extrusion whether it forms a series of individual lobes flows or spines exogenous or grows by internal inflation endogenous lava domes can transition from one extrusive style to another as a result of a change in extrusion rate or the formation of facilitating structures such as shear zones how this change can affect large rhyolitic lava domes is unclear as there are few historically recorded rhyolitic dome eruptions here we present structures at ruawahia lava dome a well exposed 700 year old lava dome how these facilitating structures enable exogenous extrusion at ruawahia dome and link this to collapse episodes along the fringes of the dome during growth ruawahia dome is part of the tarawera dome complex a chain of domes running parallel to regional structures across the okataina caldera complex in the taupo volcanic zone new zealand ruawahia dome consists of 1 a high porosity 44 52 crystalline 65 dre locally brecciated carapace facies with rare bread crusting and ropey flow textures 2 a core facies of dominantly low to moderate porosity 20 25 with elongate vesicles that mark weak flow bands and 3 thin 5 m thick interior breccia zones flow bands at ruawahia are complex and do not fit with hypothesised flow band orientations attributed to a single phase of exogenous or endogenous dome growth inward dipping flow bands on ramp structures on the flow surface suggest a flow like coulee morphology however steeply dipping and multidirectional flow bands on the edges of the dome challenge this hypothesis widespread block and ash flow deposits have been sourced from the leading dome fronts to the nw and se these collapse events left behind inflated and bread crusted outcrops on these dome fronts suggesting syn eruption collapse events that led to expansion of a hot pressurised dome interior we consider ruawahia erupted from multiple aligned vents either as lobes confined within the crater of a pyroclastic cone formed during the initial plinian phase of the eruption or those able to flow down the cone flank the confined lobes formed steep internal breccia zones as individual dome lobes extruded past one another lobes that were able to overcome the pyroclastic cone rim or where the vent was outside the crater were able to flow down the flanks as bulldozing thickening flows with dominantly ductile interiors and brittle exteriors these flows collapsed as the front thickened possibly due to a decrease in gradient producing widespread block and ash flows the removal of lava associated with collapse generated a decompression event which resulted in fragmentation cracking and vesiculation in the hot interior of the lava flows these events left behind a re vesiculated and bread crusted lava flow front and produced block and ash flows with abundant breadcrust bombs that reached the base of tarawera

field characteristics of laterally emplaced dikes anatomy of an exhumed miocene dike swarm in reydarfjordur eastern iceland abstract many dikes of the tertiary breiddalur dike swarm in reydarfjordur have a great lateral extent but are vertically upward and or downward discontinuous field relationships indicate that they are not the feeders of the lava pile they intrude their characteristics can be best interpreted as them constituting the fossil expression of lateral magma emplacement analogous to that occurring in the actively spreading krafla volcanic center of the neovolcanic zone average dike thickness increases clearly with depth and the thickness versus depth relationship is different from that described for other tertiary dike swarms in eastern iceland it is suggested that dikes represent the extent of finite spreading which at higher levels in the crust is expressed by fissures and other extensional structures

geophysical evolution of saturn s satellite phoebe a large planetesimal in the outer solar system abstract saturn s satellite phoebe is the best characterized representative of large outer solar system planetesimals thanks to the close flyby by the cassini spacecraft in june 2004 we explore the information contained in phoebe s physical properties density and shape which are significantly different from those of other icy objects in its size range phoebe s higher density has been interpreted as evidence that it was captured probably from the proto kuiper belt first we demonstrate that phoebe s shape is globally relaxed and consistent with a spheroid in hydrostatic equilibrium with its rotation period this distinguishes the satellite from rubble piles that are thought to result from the disruption of larger proto satellites we numerically model the geophysical evolution of phoebe accounting for the feedback between porosity and thermal state we compare thermal evolution models for different assumptions on the formation of phoebe in particular the state of its water amorphous or crystalline we track the evolution of porosity and thermal conductivity as well as the destabilization of amorphous ice or clathrate hydrates while rubble piles may never reach temperatures suitable for porous ice to creep and relax we argue that phoebe s shape could have relaxed due to heat from the decay of 26al provided that this object formed less than 3 myr after the production of the calcium aluminum inclusions this is consistent with the idea that phoebe could be an exemplar of planetesimals that formed in the transneptunian region and later accreted onto outer planet satellites either during the satellite s formation stage or still later during the late heavy bombardment

fault rheology beyond frictional melting during earthquakes comminution and frictional heating both contribute to the dissipation of stored energy with sufficient dissipative heating melting processes can ensue yielding the production of frictional melts or pseudotachylytes it is commonly assumed that the newtonian viscosities of such melts control subsequent fault slip resistance rock melts however are viscoelastic bodies and at high strain rates they exhibit evidence of a glass transition here we present the results of high velocity friction experiments on a well characterized melt that demonstrate how slip in melt bearing faults can be governed by brittle fragmentation phenomena encountered at the glass transition slip analysis using models that incorporate viscoelastic responses indicates that even in the presence of melt slip persists in the solid state until sufficient heat is generated to reduce the viscosity and allow remobilization in the liquid state where a rock is present next to the melt we note that wear of the crystalline wall rock by liquid fragmentation and agglutination also contributes to the brittle component of these experimentally generated pseudotachylytes we conclude that in the case of pseudotachylyte generation during an earthquake slip even beyond the onset of frictional melting is not controlled merely by viscosity but rather by an interplay of viscoelastic forces around the glass transition which involves a response in the brittle solid regime of these rock melts we warn of the inadequacy of simple newtonian viscous analyses and call for the application of more realistic rheological interpretation of pseudotachylyte bearing fault systems in the evaluation and prediction of their slip dynamics

pseudostratigraphy and origin of the early proterozoic payson ophiolite central arizona the payson ophiolite ca 1 73 ga exposed within the early proterozoic orogenic belt of central arizona is a shallow dipping pseudostratigraphic sequence of gabbro sheeted dikes and submarine volcanic rocks partly disjointed by later intrusion and deformation an older basement complex occurs as roof pendants in the gabbro and screens in the sheeted dike complex and was tilted and eroded prior to development of the ophiolite gabbro dike mingling and mutual intrusion in the transition from gabbro to sheeted dikes indicates that the dikes are rooted in the gabbro analysis of detailed measured sections of well exposed sheeted dikes 600 m reveals 90 100 dikes with lateral variations in the proportions of tonalitic or dacitic dikes and basement screens the transition from sheeted dikes to submarine basalts is abrupt and marked by a stratigraphically continuous zone of intensely altered rocks the sheeted dike complex pseudostratigraphic structure and synmagmatic hydrothermal alteration of the payson ophiolite are diagnostic of a crustal section formed by sea floor spreading the payson ophiolite intrudes and is intruded by arc granitoids and is overlain by arc derived volcanic and sedimentary rocks a model of an intra arc basin formed along an arc parallel strike slip fault is proposed to account for 1 screens of older arc crust 2 inferred arc parallel extension and 3 juxtaposition of the ophiolite with a distinct arc terrane prior to regional convergent deformation and accretion of the arc to north america during the yavapai orogeny ca 1 70 ga

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

growth and evolution of small porous icy bodies with an adaptive grid thermal evolution code i application to kuiper belt objects and enceladus abstract we present a new 1 dimensional thermal evolution code suited for small icy bodies of the solar system based on modern adaptive grid numerical techniques and suited for multiphase flow through a porous medium the code is used for evolutionary calculations spanning 4 6 10 9 yr of a growing body made of ice and rock starting with a 10 km radius seed and ending with an object 250 km in radius initial conditions are chosen to match two different classes of objects a kuiper belt object and saturn s moon enceladus heating by the decay of 26al as well as long lived radionuclides is taken into account several values of the thermal conductivity and accretion laws are tested we find that in all cases the melting point of ice is reached in a central core evaporation and flow of water and vapor gradually remove the water from the core and the final present structure is differentiated with a rocky highly porous core of 80 km radius and up to 160 km for very low conductivities outside the core due to refreezing of water and vapor a compact ice rich layer forms a few tens of km thick except in the case of very high conductivity if the ice is initially amorphous as expected in the kuiper belt the amorphous ice is preserved in an outer layer about 20 km thick we conclude by suggesting various ways in which the code may be extended

slip weakening distance of faults during frictional melting as inferred from experimental and natural pseudotachylytes the slip weakening distance d c over which a fault weakens during seismogenic fault motion is one of most important parameters controlling fault instability a paradox over the last decade has been the source of the large difference between laboratory measured values of d c using conventional friction experiments 10 5 10 3 m and seismically determined d c values for natural earthquakes 10 1 10 0 m d c is a difficult quantity to estimate for natural faults but this study attempts to determine the order of d c values for natural faults containing pseudotachylytes based on a comparison between experimental and natural pseudotachylytes experiments at a constant slip velocity of 0 85 m sec and a normal stress of 1 4 mpa on gabbro have revealed that the progressive growth of a molten layer causes marked slip weakening slip weakening a reduction in frictional strength with displacement and the growth of the molten layer an increase in the thickness of the molten layer with displacement are both characterized by exponential changes with nearly the same characteristic distances the comparison suggests that the order of d c can be estimated from the relationship between pseudotachylyte thickness along its generation fault and fault displacement sibson s 1975 data from the outer hebrides exhibit a similar pseudotachylyte thickness versus fault displacement curve with a characteristic distance of 0 38 m this is the same order of magnitude as seismically inferred values of d c the effect of frictional melting is a possible solution for the d c paradox although it is not the only possible solution since frictional melting along large faults is rather uncommon

the nature and significance of magma chamber margins in ophiolites examples from the norwegian caledonides abstract large scale intrusive contacts with associated marginal series have been encountered within norwegian ophiolite complexes at karmoy solund and leka the contacts limit individual magma chambers and are found at different structural levels of the plutonic suites examples of magma chamber margins adjacent to interlayered ultramafic and gabbroic rocks modally layered gabbros high level gabbros and sheeted dykes are described the nature of the intrusive boundaries and the presence of partially resorbed xenoliths in the vicinity of the intrusive margins suggest that stoping and assimilation have been important mechanisms during the development of the magma chambers characteristic marginal series are developed along the intrusive boundaries the thicknesses and appearance of these series vary with depth in the complexes whereas the marginal series are well developed within the uppermost levels of the plutonic complexes exhibiting rock types such as microgabbro massive gabbro and magnetite gabbro the marginal series observed at lower levels are thinner and also devoid of chilled facies rocks and magnetite gabbros the marginal series may be subdivided into border and roof series the latter are characterized by an intimate relationship with sheeted dykes which comprise dyke swarms formed both prior to during and subsequent to crystallization of the roof series based on these relationships the dykes can be subdivided into rooted and rootless dykes a multiple magma chamber model with magma chambers migrating from a low to a high level within the oceanic crust is proposed on the basis of the observed features

fusion by earthquake fault friction stick or slip field observations of pseudotachylites and experimental studies of high speed friction indicate that melting on a slipping interface may sig nificantly affect the magnitude of shear stresses resisting slip we investigate the effects of rock melting on the dynamic friction using theoretical models of shear heating that couple heat transfer thermodynamics of phase transitions and fluid mechanics results of laboratory experiments conducted at high order of m s slip velocities but low order of mpa normal stresses suggest that the onset of frictional melting may give rise to substantial increases in the effective fault strength presumably due to viscous effects however extrapolation of the modeling results to in situ conditions suggests that the efficiency of viscous braking is significantly reduced under high normal and shear stresses when transient increases in the dynamic fault strength due to fusion are not sufficient to inhibit slip decreases in the effective melt viscosity due to shear heating and melting of clasts drastically decrease the dynamic friction resulting in a nearly complete stress drop thermal runaway the amount of energy dissipation associated with the formation of pseudo tachylites is governed by the temperature dependence of melt viscosity and the average clast size in the fault gouge prior to melting clasts from a coarse grained gouge have lower chances of survival in a pseudotachylite due to a higher likelihood of non equilibrium overheating the maximum temperature and energy dissipation attainable on the fault surface are ultimately limited by either the rock solidus via viscous braking and slip arrest or liquidus via thermal runaway and vanishing resistance to sliding our modeling results indicate that the thermally activated fault strengthening and rupture arrest are unlikely to occur in most mafic protoliths but might be relevant for quartz rich rocks especially at shallow 5 7 km depths where the driving shear stress is relatively low

necking and fracking may explain stationary seismicity and full degassing in volcanic silicic spine extrusion abstract volcanic seismicity during silicic spine eruptions often involves recurrent excitation of similar sources at stationary depth just beneath the crater the mechanics of volcanic spine extrusion may be compared to those of high temperature industrial metal working we thus use slip line field theory to assess stress strain and faulting in ascending magma which although hot behaves as a solid earthquake fault plane solutions during the 09 2004 08 2005 eruptions of mount st helens are generally consistent with shrinking of magma rising across a conduit bottle neck among 215 fault plane solutions thrust and vertical fault planes prevail with fewer normal or strike slip faults constriction across the neck and vertical shear along the conduit walls thus predominate dynamic discrete element modeling reproduces repetitive nucleation and growth of thrust faults within such a neck the pressure drop across the neck s core secondary tension boosts crack opening and hence gas extraction such natural fracking could promote full magma degassing contributing to the typically low explosivity of silicic spine extrusion

modeling serpentinization applied to the early evolution of enceladus and mimas abstract serpentinization is an exothermic reaction which involves the metamorphic transition of ultramafic rocks and liquid water into hydrated serpentine minerals it has been suggested that serpentinization may be a fundamental process in the development of icy planetary objects prompting a reaction runaway in which heat from serpentinization in a localized region of the body could result in principle in the melting of ice throughout the body we use a 1 dimensional adaptive grid thermal evolution code suited for small and medium sized icy bodies of the solar system which we apply to enceladus and mimas the satellites of saturn the code is used to investigate the multiphase flow of water through a porous rocky medium thus giving us a detailed look on the internal distribution of mass and energy we consider heating by serpentinization reactions accounting for the phase transition of the chemically altered rock and the water lost in the process short lived and long lived radionuclides gravitational potential energy and the introduction of ammonia to modify the thermodynamical properties of water ammonia solution we test several cases with different initial compositions our results show that in a short term radioactive heating scenario enceladus will fully differentiate into a rocky inner core topped by a very thin icy crust considering only long term radioactive heating in the absence of any short term radionuclides serpentinization in enceladus can be triggered providing ammonia is present in the water however long term triggered serpentinization is confined only to the inner core we conclude that serpentinization may have been triggered in enceladus during its early evolution and not in mimas leading to the dichotomy between them based on the assumptions that went into our model we may conclude that the formation time of saturn s satellites was around 3 myr after the formation of calcium aluminum inclusions

frictional melt and seismic slip 1 frictional melt is implied in a variety of processes such as seismic slip ice skating and meteorite combustion a steady state can be reached when melt is continuously produced and extruded from the sliding interface as shown recently in a number of laboratory rock friction experiments a thin low viscosity high temperature melt layer is formed resulting in low shear resistance a theoretical solution describing the coupling of shear heating thermal diffusion and extrusion is obtained without imposing a priori the melt thickness the steady state shear traction can be approximated at high slip rates by the theoretical form ss n1 4 a under a normal stress n slip rate v radius of contact area r a is a dimensional normalizing factor and w is a characteristic rate although the model offers a rather simplified view of a complex process the predictions are compatible with experimental observations in particular we consider laboratory simulations of seismic slip on earthquake faults a series of high velocity rotary shear experiments on rocks performed for n in the range 1 20 mpa and slip rates in the range 0 5 2 m s 1 is confronted to the theoretical model the behavior is reasonably well reproduced though the effect of radiation loss taking place in the experiment somewhat alters the data the scaling of friction with n r and v in the presence of melt suggests that extrapolation of laboratory measures to real earth is a highly nonlinear nontrivial exercise

the rise and fall of periodic drumbeat seismicity at tungurahua volcano ecuador abstract highly periodic drumbeat long period lp earthquakes have been described from several andesitic and dacitic volcanoes commonly accompanying incremental ascent and effusion of viscous magma however the processes controlling the occurrence and characteristics of drumbeat and lp earthquakes more generally remain contested here we use new quantitative tools to describe the emergence evolution and degradation of drumbeat lp seismicity at the andesitic tungurahua volcano ecuador in april 2015 the signals were recorded during an episode of minor explosive activity and ash emission without lava effusion and are the first to be reported at tungurahua during the ongoing 17 yrs of eruption following four days of high levels of continuous and pulsed tremor highly periodic lp earthquakes first appear on 10 april over the next four days inter event times and event amplitudes evolve through a series of step wise transitions between stable behaviors each involving a decrease in the degree of periodicity families of similar waveforms persist before during and after drumbeat activity but the activity levels of different families change coincidentally with transitions in event rate amplitude and periodicity a complex micro seismicity initiation sequence shows pulse like and stepwise changes in inter event times and amplitudes in the hours preceding the onset of drumbeat activity that indicate a partial de coupling between event size and rate the observations increase the phenomenology of drumbeat lp earthquakes and suggest that at tungurahua they result from gas flux and rapid depressurization controlled by shear failure of the margins of the ascending magma column

1 ma east pacific rise oceanic crust and uppermost mantle exposed by rifting in hess deep equatorial pacific ocean abstract a series of dives with the french submersible nautile has been conducted in the region of the galapagos triple junction at the tip of the cocos nazca propagator most of the dives were devoted to the study of hess deep a rift valley produced by rifting of oceanic crust created at the axis of the very fast spreading east pacific rise the dives have enabled the structures and lithologies to be assessed and confirmed earlier dredging evidence for abundant basal crust and mantle rocks on an intra rift ridge located to the north of the axis of hess deep this shows that this situation is not specific not slow spreading ridges a nearly complete and unique log of young 1 ma crust and uppermost mantle representative of very fast accretion can be reconstructed even though the section is discontinuous and highly dismembered we propose two models that can explain the exposure on the ocean floor of the deep foundation of the oceanic crust one model stresses symmetry and vertical tectonics through isostasy diapirism of serpentinized mantle another model requires a larger amount of extension and emphasizes asymmetry with denudation of the upper crust through low angle detachment faulting in both models which differ mainly in the structure at depth amagmatic extension of east pacific rise crust by the cocos nazca propagator tip results in uplift and denudation of lower crust and upper mantle

formation of uppermost oceanic crust the uppermost portion of oceanic crust extrusive basalts underlain and fed by diabasic dikes represents the chilled roof zone or lid to mid oceanic spreading center magma chambers this rigid lid forms and evolves through a steady state process of flexure subsidence and internal rotation all occurring within a couple of kilometers of the spreading axis the lid accommodates lateral spreading with dikes emplaced along a unique axis dikes tap underlying magma to feed overlying flows basalts accumulating along the axis of the lid load the lid which responds by flexing down and away from the intrusive axis this subsidence affects lid structure in the following ways emplaced dikes are tilted away from and basalt flows are tilted toward the intrusive axis lid subsidence and section rotation extend the base of the lid more rapidly at the axis than elsewhere consequently dikes preferentially intrude at the existing axis to maintain existing axis shape and relative position through continued spreading loading and flexure of the lid plus faster accretion at the base of the lid as a result of lid rotation cause the lid to be uplifted distal to the axis this creates flanking marginal highs and generates extension structures fissuring and faulting flanking and paralleling the loading extrusive axis the crustal structure and geometry produced by this process is compatible with structure and topography observed at present day mid oceanic spreading centers and with structure observed in ophiolites particularly the bay of islands ophiolite

two modes of back arc spreading back arc spreading is initiated with rifting along the volcanic zone of an island arc under extensional tectonics the width of the volcanic zone of the arc which is closely related to the dip angle of the subducting slab should cause a variation in the style of initial rifting and subsequent back arc spreading two modes of back arc spreading single rift type and multirift type are introduced from the comparative study of the bonin arc and the japan sea divergent tectonics in an island arc above a steeply dipping slab will cause a single rift system in a narrow volcanic zone but will cause a multirift system in a broad volcanic zone which is related to a shallow dipping angle of subduction

does an inter flaw length control the accuracy of rupture forecasting in geological materials multi scale failure of porous materials is an important phenomenon in nature and in material physics from controlled laboratory tests to rockbursts landslides volcanic eruptions and earthquakes a key unsolved research question is how to accurately forecast the time of system sized catastrophic failure based on observations of precursory events such as acoustic emissions ae in laboratory samples or on a larger scale small earthquakes until now the length scale associated with precursory events has not been well quantified resulting in forecasting tools that are often unreliable here we test the hypothesis that the accuracy of the forecast failure time depends on the inter flaw distance in the starting material we use new experimental datasets for the deformation of porous materials to infer the critical crack length at failure from a static damage mechanics model the style of acceleration of ae rate prior to failure and the accuracy of forecast failure time both depend on whether the cracks can span the inter flaw length or not a smooth inverse power law acceleration of ae rate to failure and an accurate forecast occurs when the cracks are sufficiently long to bridge pore spaces when this is not the case the predicted failure time is much less accurate and failure is preceded by an exponential ae rate trend finally we provide a quantitative and pragmatic correction for the systematic error in the forecast failure time valid for structurally isotropic porous materials which could be tested against larger scale natural failure events with suitable scaling for the relevant inter flaw distances

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

extreme frictional processes in the volcanic conduit of mount st helens usa during the 2004 2008 eruption abstract the 2004 2008 eruption of mount st helens saw the extrusion of seven high viscosity spines and formation of discrete shear zones along the conduit margin at spine 7 this shear zone consists of four structurally distinct layers the outer surface gouge l1 crosscuts a dark banded layer l2 which grades into a moderately sheared layer l3 and undeformed rock l4 inside the spine field observations porosity measurements geochemistry mineralogy microstructure crystal size and shape distribution kinetic properties and magnetic analyses chart the evolution of deformation processes and products throughout the eruption gouge formation was concomitant with characteristic microseismic drumbeats at depths 0 5 1 km in addition the seismic record shows two larger earthquakes with similar seismic signatures in august 2006 which we conclude represent larger slip amounts along the conduit margin of spine 7 extensive slip resulted in frictional heating on the order of several hundreds of degrees melting the highly viscous crystalline ascending magma plug and forming a pseudotachylyte high ambient temperatures in the conduit resulted in near equilibrium melting and slow recrystallisation thus impeding the development of signature pseudotachylyte characteristics and hindering identification thus frictional melting and recrystallisation in ascending magma plugs may be a common but unidentified phenomena at composite volcanoes worldwide

seismic and experimental insights into eruption precursors at volcan de colima we combine geophysical and experimental observations to interpret preeruptive unrest at volcan de colima in 1998 17 893 volcanic earthquakes were detected between 1 october and 31 december 1998 including 504 clusters using seismic ambient noise interferometry we observe a drop in velocity prior to the eruption linked to damage accumulation during magma ascent this is supported by experimental observations where static stress causes a velocity decrease prior to failure furthermore we observe acoustic emission clusters during the experiments with lower porosity samples producing higher numbers of repeaters this behavior introduces tensile failure as an additional viable mechanism for clusters during magma ascent the findings suggest that preeruptive magma ascent may be monitored to variable degrees of accuracy via descriptions of damage accumulation and associated seismic velocity changes

experimental generation of volcanic pseudotachylytes constraining rheology volcanic systems are highly dynamic environments that comprise rocks and magmas which in the process of strain localisation e g in catastrophic flank collapse or dome building events are candidate materials for the occurrence of frictional melting and the formation of pseudotachylytes we evaluate the frictional behaviour of a plagioclase two pyroxene and glass bearing andesite and introduce an approach to constrain the rheology of frictional melts frictional slip at a rate of 1 3 m s under an axial stress of 1 5 mpa induces heating at a rate approximating 130 c per meter of slip and frictional melting of the andesite occurs at 1000 c which corresponds to a peak in shear stress with continuing displacement the shear stress decreases by 25 from the peak strength and stabilises while the temperature equilibrates to 1230 1290 c the shear stress for the system is congruent with a non arrhenian temperature dependent frictional melt rheology exhibiting a non newtonian viscosity evolving from approximately 104 2 to 103 4 pa s post experiment analysis show a gradation from 1 the host rock to 2 an extraordinary zone of unmelted yet viscously deformed material to 3 a thin outer region of chemically heterogeneous protomelts and to 4 an inner region of chemically homogeneous frictionally generated modified melt in the core of the slip zone we discuss the role and identification of frictional melting in volcanic systems and the implication of viscous remobilisation of magmatic plugs during frictional slip of glass bearing volcanic materials

frictional melt homogenisation during fault slip geochemical textural and rheological fingerprints abstract volcanic environments often represent structurally active settings where strain localisation can promote faulting frictional deformation and subsequent melting along fault planes such frictional melting is thermodynamically a disequilibrium process initiated by selective melting of individual mineral phases and softening of volcanic glass at its glass transition as a response to rapid frictional heating the formation of a thin melt layer on a fault plane surface can drastically accelerate or terminate slip during fault motion a comprehensive understanding of the physical and chemical properties of the frictional melt is required for a full assessment of slip mechanisms as frictional rheology depends on the contributions from selectively melted mineral and glass phases as well as the physical effects of restite fragments suspended in the frictional melt here we experimentally investigate the impact of host rock mineralogy on the compositional and textural evolution of a frictional melt during slip high velocity rotary shear hvr experiments were performed under controlled volcanically relevant coseismic conditions 1 m s 1 slip rate and 1 mpa normal stress using three intermediate dome lavas with contrasting mineral assemblages sampled from volcanic systems where fault friction is evident 1 an amphibole bearing andesite soufriere hills volcano montserrat 2 an amphibole poor dacite santiaguito dome complex guatemala and 3 an amphibole free andesite volcan de colima mexico for each sample five hvr experiments were terminated at different stages of frictional melt evolution namely 1 at the onset of melting 2 upon formation of a steady state melt layer and 3 after 5 m 4 10 m and 5 15 m of slip at steady state conditions progressive mixing and homogenisation of selective single phase melts within the frictional melt layer through double diffusion convection demonstrates the control of melt composition on slip behaviour amphiboles melted preferentially leading to consistently lower shear stress 1 mpa less than amphibole poor samples and pronounced shear weakening during the frictional melting of amphibole bearing lavas the results highlight the implications of mineral assemblages on fault slip including conduit flow processes which may influence the style of eruptions and run out distances of rapid granular flows

comparison of short term seismic precursors and explosion parameters during the 2016 2017 bogoslof eruption in december 2016 the unmonitored bogoslof volcano unexpectedly began erupting producing at least 70 explosions over the next 8 5 months roughly half of the explosions were preceded by observable short term precursory activity defined here as seismic activity ending within 30 min of the explosion onset or an earthquake swarm ending within a few hours before the explosion onset five different types of precursors were noted during the bogoslof eruption non swarm earthquakes earthquake swarms rapid rate events tremor and chromatic tremor most of the precursors that were detectable by the nearby seismic networks 45 km away occurred during the first 3 months of the eruption with nearly all precursory earthquake swarms occurring during this period a hydrophone deployed 7 km northeast of the summit for the last 3 months of the eruption detected weaker activity that was not recorded by the more distant seismic networks we describe the observed precursors and discuss their inter relations overall the precursors seem indicative of processes occurring throughout the volcanic system and suggest possible changes in the system during the eruption we then compare the short term precursors with explosion parameters such as plume height duration and occurrence of lightning to look for relations that may provide more insight into the eruption dynamics and subsurface processes and help to further improve forecasting we find no obvious relations between any of the precursor or explosion parameters suggesting that their use in forecasting eruption characteristics at bogoslof is limited

early proterozoic assembly of tectonostratigraphic terranes in southwestern north america a 500 km wide early proterozoic orogenic belt in arizona and adjacent areas is divided into lithotectonic blocks by northeast and north trending shear zones structural and u pb zircon studies suggest that these blocks may have experienced different tectonic histories prior to their juxtaposition by thrust and strike slip movements on shear zones a northwestern province here called the yavapai province is composed of at least five tectonic blocks and was assembled at about 1700 ma a southeastern province here called the mazatzal province is composed of at least three blocks and was assembled and juxtaposed with the northwestern province during the mazatzal orogeny between 1695 and 1630 ma the diversity of seemingly incompatible 1700 ma tectonic regimes now in close proximity to each other and the absence of systematic cross strike changes in age and character of deformation plutonism and metamorphism suggest a model whereby the orogenic belt developed and north america grew by assembly of d

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

cyclic extrusion of a lava dome based on a stick slip mechanism abstract lava dome eruptions are sometimes characterised by large periodic fluctuations in extrusion rate over periods of hours that may be accompanied by vulcanian explosions and pyroclastic flows we consider a simple system of nonlinear equations describing a 1d flow of lava extrusion through a deep elastic dyke feeding a shallower cylindrical conduit in order to simulate this short period cyclicity stick slip conditions depending on a critical shear stress are assumed at the wall boundary of the cylindrical conduit by analogy with the behaviour of industrial polymers in a plastic extruder the elastic dyke acts like a barrel and the shallower cylindrical portion of the conduit as a die for the flow of magma acting as a polymer when we applied the model to the soufriere hills volcano montserrat for which the key parameters have been evaluated from previous studies cyclic extrusions with periods from 3 to 30 h were readily simulated matching observations the model also reproduces the reduced period of cycles observed when a major unloading event occurs due to lava dome collapse

shear localisation strain partitioning and frictional melting in a debris avalanche generated by volcanic flank collapse abstract the arequipa volcanic landslide deposit to the east of arequipa peru originated from the pichu pichu volcanic complex covering an area 100 km2 the debris avalanche deposit exhibits internal flow structures and basal pseudotachylytes we present field microstructural and chemical observations from slip surfaces below and within the deposit which show varying degrees of strain localisation at one locality the basal shear zone is localised to a 1 2 cm thick extremely sheared layer of mixed ultracataclasite and pseudotachylyte containing fragments of earlier frictional melts rheological modelling indicates brittle fragmentation of the melt may have occurred due to high strain rates at velocities of 31 m s 1 and that frictional melting is unlikely to provide a mechanism for basal lubrication elsewhere we observe a

cassini observes the active south pole of enceladus cassini has identified a geologically active province at the south pole of saturn s moon enceladus in images acquired by the imaging science subsystem iss this region is circumscribed by a chain of folded ridges and troughs at 55 s latitude the terrain southward of this boundary is distinguished by its albedo and color contrasts elevated temperatures extreme geologic youth and narrow tectonic rifts that exhibit coarse grained ice and coincide with the hottest temperatures measured in the region jets of fine icy particles that supply saturn s e ring emanate from this province carried aloft by water vapor probably venting from subsurface reservoirs of liquid water the shape of enceladus suggests a possible intense heating epoch in the past by capture into a 1 4 secondary spin orbit resonance

brittle ductile deformation and tensile rupture of dome lava during inflation at santiaguito guatemala gas and ash explosions at the santiaguito dome complex guatemala commonly occur through arcuate fractures following a 5 to 6 min period of inflation observed in long period seismic signals observation of active faults across the dome suggests a strong shear component but as fault propagation generally proceeds through the coalescence of tensile fractures we surmise that explosive eruptions require tensile rupture here we assess the effects of temperature and strain rate on fracture propagation and the tensile strength of santiaguito dome lavas indirect tensile tests were conducted on samples with a porosity range of 3 30 and over diametral displacement rates of 0 04 0 004 and 0 0004 mm s at room temperature the tensile strength of dome rock is rate independent within the range tested and inversely proportional to the porosity of the material at eruptive temperatures we observe an increasingly ductile response at either higher temperature or lower displacement rate where ductile deformation is manifest by a reduction in loading rate during constant deformation rate tests resulting in slow tearing viscous flow and pervasive damage we propose a method to conduct indirect tensile tests under volcanic conditions using a modification of the brazilian disc testing protocol and use brittleness indices to classify deformation modes across the brittle ductile transition we show that a degree of ductile damage is inevitable in the lava core during explosions at the santiaguito dome complex and discuss how strain leading to rupture controls fracture geometry which would impact gas pressure release or buildup and regulate explosive activity

differences of precursory seismic energy release for the 2007 effusive dome forming and 2014 plinian eruptions at kelud volcano indonesia abstract early warnings of the timing style and size of volcanic eruptions made on the basis of seismic monitoring are important for mitigation of the hazards associated with volcanic eruptions we calculated the rates of seismic energy release at kelud volcano indonesia preceding both the 2007 effusive lava dome eruption and the 2014 plinian eruption for the 2007 eruption there was first a step wise increase in the rate of release of seismic energy followed by a gradual decrease until the eruption for the 2014 eruption the rate of energy release was almost constant until 10 days before the eruption then gradually increased until there was a dramatic acceleration during the last day before the eruption the total seismic energy released by the 2007 and 2014 eruptions were 3 10 8 j and 2 2 10 9 j respectively the relative difference between the ejected volumes of the 2007 and 2014 eruptions is roughly equivalent to that of the total seismic energy released preceding the eruptions we conducted several trials of the material failure forecast method ffm applying it to the square root of the rate of seismic energy released to assess its usefulness for predicting the timing of eruptions the predicted onset of the 2007 eruption was one day earlier than the actual eruption and the seismic energy release preceding the actual eruption did not increase dramatically as suggested by the ffm model several trials of ffm for the 2014 eruption predicted the onset of the eruption to be three days to two weeks later than the actual eruption even as the end time of the ffm time window approaches several hours to 10 min before the actual eruption onset the predicted eruption onset is still 3 h later than the actual onset thus the results of our ffm modeling did not show the method to be useful for predicting eruption times for kelud volcano the differences of the ffm modeling results for the 2007 and 2014 eruptions likely reflect the differences in the rate of energy release immediately before the two eruptions a deceleration before the 2007 eruption and an acceleration before the 2014 eruption these differences may reflect differences in precursory processes for effusive lava dome and plinian eruptions

the transition from endogenous to exogenous growth of lava domes with the development of shear bands the transition from an endogenous to an exogenous regime of lava dome growth must be achieved by the formation of discontinuities within the dome such transitions and vice versa are an important characteristic of most long lived lava domes and often coincide with significant changes in the dynamics of magma supply and lava dome collapse events for the purpose of this paper following recent experimental and observational evidence we assume that such a transition occurs when shear bands are generated a model for the formation of shear bands and therefore the growth transition within a dome and coupled conduit domain is presented shear bands are most likely to initiate at the junction of the conduit and base of the dome where the shear stress experienced between new lava entering the dome and existing lava is greatest stress accumulation within the shear bands is likely to lead to brittle shear resulting in the formation of fractures finite element modelling of lava flow shows that such shear bands only develop for certain extrusion rates and lava viscosities similarly the growth regime of the lava dome will depend upon the extrusion rate and viscosity within the conduit which is largely controlled by volatile loss and the growth of crystals in the upper part of the conduit we consider a simplified rheology during lava dome growth considering isothermal conditions with crystal growth the development of shear bands in the conduit is explored with a numerical model parameterized with values appropriate for soufriere hills volcano montserrat during october to december 1996 this lava dome forming eruption experienced a transition from endogenous to exogenous growth as it grew in height by about 90 m modelling indicates that the observed fall in magma extrusion rate from about 2 0 m3s 1 to 0 5 m3s 1 as a result of the increased pressure head from the dome and the evolution in viscosity could have subsequently changed the dome growth regime due to the development of shear bands our models provide insight into the shear stress fields possible within the conduit and the shear stresses required for shear band development

volcanic eruption of the mid ocean ridge along the east pacific rise crest at 9 45 52 n direct submersible observations of seafloor phenomena associated with an eruption event in april 1991 in april 1991 we witnessed from the submersible alvin a suite of previously undocumented seafloor phenomena accompanying an in progress eruption of the mid ocean ridge on the east pacific rise crest at 9 45 n 52 n the volume of the eruption could not be precisely determined although comparison of pre and post eruption seabeam bathymetry indicate that any changes in ridge crest morphology resulting from the eruption were 10 m high effects of the eruption included 1 increased abundance and redistribution of hydrothermal vents disappearance of numerous vent communities and changes in characteristics of vent fauna and mineral deposits within the eruption area since december 1989 2 murkiness of bottom waters up to tens of meters above the seafloor due to high densities of suspended mineral and biogenic particulates 3 destruction of a vent community by lava flows mass wasting and possible hydrovolcanic explosion at a site known as tubeworm barbecue in the axial summit caldera asc at 9 50 6 n 4 near critical temperatures of hydrothermal vent fluids ranging up to 403 c 5 temporal variations over a 2 week interval in both temperatures and chemical isotopic compositions of hydrothermal fluids 6 unusual compositions of end member vent fluids with ph values ranging to a record low of 2 5 salinities ranging as low as 0 3 wt nacl one twelfth that of seawater and dissolved gases reaching high concentrations 65 mmol l for both co2 and h2s 7 venting at temperatures above 380 c of visually detectable white vapor that transformed to plumes of gray smoke a few centimeters above vent orifices 8 disorganized venting of both high temperature fluids black and gray smoke and large volumes of cooler diffuse hydrothermal fluids directly from the basaltic seafloor rather than from hydrothermal mineral constructions 9 rapid and extensive growth of flocculent white bacterial mats species unknown on and under the seafloor in areas experiencing widespread venting of diffuse hydrothermal fluid and 10 subseafloor downslope migration of magma normal to the ridge axis in a network of small scale 1 5 m diameter lava tubes and channels to distances at least 100 200 m outside the asc we suggest that in april 1991 intrusion of dikes in the eruption area to 200 m beneath the asc floor resulted in phase separation of fluids near the tops of the dikes and a large flux of vapor rich hydrothermal fluids through the overlying rubbly cavernous lavas low salinities and gas rich compositions of hydrothermal fluids sampled in the eruption area are appropriate for a vapor phase in a seawater system undergoing subcritical liquid vapor phase separation boiling and phase segregation hydrothermal fluids streamed directly from fissures and pits that may have been loci of lava drainback and or hydrovolcanic explosions these fissures and pits were lined with white mats of a unique fast growing bacteria that was the only life associated with the brand new vents the prolific bacteria which covered thousands of square meters on the ridge crest and were also abundant in subseafloor voids may thrive on high levels of gases in the vapor rich hydrothermal fluids initially escaping the hydrothermal system white bacterial particulates swept from the seafloor by hydrothermal vents swirled in an unprecedented biogenic blizzard up to 50 m above the bottom the bacterial proliferation of april 1991 is likely to be a transient bloom that will be checked quickly either by decline of dissolved gas concentrations in the fluids as rapid heat loss brings about cessation of boiling and or by grazing as other organisms are re established in the biologically devastated area

non newtonian rheological law for highly crystalline dome lavas volcanic eruption models are hampered by the lack of multiphase magmatic flow laws most rheological models estimate the viscosity of multiphase lavas via the einstein roscoe equation but this simplification cannot be used for high crystallinity and it does not consider the non newtonian strain rate dependence of viscosity we carried out parallel plate experiments on natural samples to simulate multiphase lava deformation under various stresses and strain rates multiphase lavas exhibit an important component of shear thinning and appear to invalidate the adequacy of einstein roscoe based formulations for highly crystalline lava rheology the remarkable singular dependence of viscosity on strain rate yields a novel universal rheology law at eruptive temperatures t i e log 0 993 8974 t 0 543 log our work reveals the importance of considering microcracking and viscous dissipation at very high strain rate 10 3 s 1 explaining the occurrence of seismic swarms along the conduit margins and consequently supporting plug like magma ascent models

petrological architecture of a magmatic shear zone a multidisciplinary investigation of strain localisation during magma ascent at unzen volcano japan shearing of magma during ascent can promote strain localisation near the conduit margins anymechanical and thermal discontinuities associated with such events may alter the chemical physicaland rheological stability of the magma and thus its propensity to erupt lava spines can record suchprocesses preserving a range of macroscopic and microscopic deformation textures attributed toshearing and friction as magma ascends through the viscous brittle transition here we use a multi disciplinary approach combining petrology microstructures crystallography magnetics and experi mentation to assess the evidence role and extent of shearing across a marginal shear zone of the1994 1995 lava spine at unzen volcano japan our results show that crystals can effectively moni tor stress conditions during magma ascent with viscous remobilisation crystal plasticity and com minution all systematically increasing towards the spine margin accompanying this we find anincrease in mineral destabilisation in the form of pargasitic amphibole breakdown displaying tex tural variations across the shear zone from symplectitic to granular rims towards the spine margin in addition the compaction of pores chemical and textural alteration of interstitial glass and mag netic variations all change systematically with shear intensity the strong correlation between thedegree of shearing crystal deformation and disequilibrium features together with distinct magneticproperties implies a localised thermal input due to shear and frictional processes near the conduitmargin during magma ascent this was accompanied by late stage or post emplacement fluid andgas induced alteration of the gouge as well as oxidation and glass devitrification understandingand recognising evidence for strain localisation during magma ascent may therefore be vital whenassessing factors that regulate the style of volcanic eruptions which may provide insights into thecryptic shifts from effusive to explosive activity as observed at many active lava domes

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

progressive reactivation of the volcanic plumbing system beneath tolbachik volcano kamchatka russia revealed by long period seismicity abstract after lying dormant for 36 yr the tolbachik volcano of the klyuchevskoy group started to erupt on 27 november 2012 we investigate the preparatory phase of this eruption via a statistical analysis of the temporal behavior of long period lp earthquakes that occurred beneath this volcanic system the lp seismicity occurs close to the surface beneath the main volcanic edifices and at 30 km depth in the vicinity of a deep magmatic reservoir the deep lp earthquakes and those beneath the klyuchevskoy volcano occur quasi periodically while the lp earthquakes beneath tolbachik are clustered in time as the seismicity rate increased beneath tolbachik days before the eruption the level of the time clustering decreased we interpret this as a manifestation of the evolution of the volcano plumbing system we suggest that when a plumbing system awakes after quiescence multiple cracks and channels are reactivated simultaneously and their interaction results in the strong time clustering of lp earthquakes with time this network of channels and cracks evolves into a more stable state with an overall increased permeability where fluids flow uninhibited throughout the plumbing system except for a few remaining impediments that continue to generate seismic radiation the inter seismic source interaction and the level of earthquake time clustering in this latter state is weak this scenario suggests that the observed evolution of the statistical behavior of the shallow lp seismicity beneath tolbachik is an indicator of the reactivation and consolidation of the near surface plumbing system prior to the tolbachik eruption the parts of the plumbing system above the deep magmatic reservoir and beneath the klyuchevskoy volcano remain in nearly permanent activity as demonstrated by the continuous occurrence of the deep lp earthquakes and very frequent klyuchevskoy eruptions this implies that these parts of the plumbing system remain in a stable permeable state and contain a few weakly interacting seismogenic sources our results provide new constraints on future mechanical models of the magmatic plumbing systems and demonstrate that the level of time clustering of lp earthquakes can be a useful parameter to infer information about the state of the plumbing system

branching sheeted dikes and seafloor spreading within an early proterozoic intra arc basin the payson ophiolite 1 73 ga is exposed as a shallow dipping pseudostratigraphic sequence of gabbro sheeted dikes and submarine volcanic rocks with screens and roof pendants of older arc crust the sheeted dike complex is particularly well exposed and analysis of measured sections reveals a threefold increase in the average dike width with depth in the complex magmatic temperatures and slow cooling at the base of the sheeted dike complex indicated by gabbro dike mingling and mutual intrusion and coarse textures may have facilitated the growth of wide dikes in contrast extreme alteration of the top of the sheeted dike complex suggests that hydrothermal circulation dominated and caused rapid cooling as thin dikes the connection between wide dikes at depth with thinner dikes toward the top requires a new model where dikes branch upward in response to the strong vertical gradient in the rate of cooling and solidification the branching dike model has important implications for the timing and spatial distribution of seafloor phenomena in the neovolcanic zone of spreading centers

modeling kuiper belt objects charon orcus and salacia by means of a new equation of state for porous icy bodies abstract we use a one dimensional adaptive grid thermal evolution code to model kuiper belt objects charon orcus and salacia and compare their measured bulk densities with those resulting from evolutionary calculations at the end of 4 6 gyr our model assumes an initial homogeneous composition of mixed ice and rock and follows the multiphase flow of water through the porous rocky medium consequent differentiation and aqueous chemical alterations in the rock heating sources include long lived radionuclides serpentinization reactions release of gravitational potential energy due to compaction and crystallization of amorphous ice the density profile is calculated by assuming hydrostatic equilibrium to be maintained through changes in composition pressure and temperature to this purpose we construct an equation of state suitable for porous icy bodies with radii of a few hundred km based on the best available empirical studies of ice and rock compaction and on comparisons with rock porosities in earth analog and solar system silicates we show that the observed bulk densities can be reproduced by assuming the same set of initial and physical parameters including the same rock ice mass ratio for all three bodies we conclude that the mass of the object uniquely determines the evolution of porosity and thus explains the observed differences in bulk density the final structure of all three objects is differentiated with an inner rocky core and outer ice enriched mantle the degree of differentiation too is determined by the object s mass

hot pressing in conduit faults during lava dome extrusion insights from mount st helens 2004 2008 abstract rhyodacitic volcanoes such as mount st helens msh soufriere hills mount unzen and mount pelee erupt spines mantled by layers of magma derived cataclasite and fault gouge msh produced seven lava spines from 2004 2008 composed of low porosity compositionally uniform crystalline dacite dome extrusion was attended by continuous drumbeat seismicity derived from faulting along the conduit margin at 0 5 1 km depth and evidenced by the enveloping gouge layers we describe the properties of the gouge derived fault rocks including laboratory measurements of porosity and permeability the gouge varies from unconsolidated powder to lithified low porosity low permeability fault rocks we reconstruct the subsurface ascent of the msh magma using published field observations and create a model that reconciles the diverse properties of the gouge with conditions in the conduit during ascent i e velocity temperature we show lithification of the gouge to be driven by hot pressing processes wherein the combination of elevated temperature confining pressure and dwell time cause densification and solid state sintering of the comminuted crystal rich glass poor gouge the degree of gouge lithification corresponds with residence time in the conduit such that well lithified materials reflect extended times in the subsurface due to slower ascent rates with this insight we suggest that gouge competence can be used as a first order estimate of lava ascent rates furthermore we posit gouge lithification which reduces porosity and permeability inhibits volcanic outgassing thereby increasing the potential for explosive events at spine producing volcanoes

frictional faulting model for harmonic tremor before redoubt volcano eruptions volcanic tremor can be caused by small earthquakes occurring within the volcano mechanical modelling of volcanic tremor generated at redoubt volcano alaska suggests that high frequency tremor is the result of stick slip motion in faults within the volcano conduit

early proterozoic ophiolite central arizona the 1 73 ga payson ophiolite is a pseudostratigraphic sequence of mafic plutons dike swarms sheeted dikes and submarine basalts that intruded and erupted upon a 1 75 1 76 ga magmatic arc complex the composition of the sheeted dikes is tholeiitic basalt minor andesite with island arc affinities the submarine basalts are overlain by dacitic breccias and a thick section of turbidites with ca 1 72 ga ash beds the entire sequence was deformed intruded by ca 1 70 ga granites and unconformably overlain by fluvial to shallow shelf sediments although most of the 1 8 1 6 ga juvenile crust of arizona consists of magmatic arc rocks the payson ophiolite is unique and is interpreted to have formed the floor of an intra arc basin the ophiolite developed in situ on the older arc basement as opposed to being thrust over it the basin was accreted to the continent by ca 1 70 ga

the josephine ophiolite northwestern california the 157 m y old josephine ophiolite is a complete ophiolite consisting of the following units harzburgite tectonite cumulate ultramafics cumulate gabbro interlayered with cumulate ultramafics cumulate gabbro high level gabbro and diorite a 1 5 km thick sheeted dike complex and 0 4 km of pillow lava and breccia the ophiolite is conformably overlain by the galice formation consisting of a thin siliceous pelagic sequence overlain by a thick sequence of turbidites having both volcanic arc and continental margin sources the ophiolite and overlying galice formation were intruded by numerous 150 to 151 m y old calc alkaline dikes and sills followed shortly by deformation and regional metamorphism under conditions of prehnite pumpellyite to lower greenschist facies the josephine peridotite makes up the base of the ophiolite and covers 800 km2 it is an alpine type peridotite that represents upper mantle material and the residue of partial melting ultramafic rocks overlying the josephine peridotite are olivine spinel clinopyroxene cumulates that grade upward into clinopyroxene and orthopyroxene olivine plagioclase cumulates and finally to clinopyroxene plagioclase cumulates high level gabbros and diorites are characterized by highly variable textures complex intrusive relationships strongly zoned plagioclase and abundant mafic dikes plagiogranites occur at this level as dikes small intrusions and screens between mafic dikes but they are rare the sheeted dike complex consists of mafic dikes averaging 0 6 m in thickness chilled margins are locally well preserved and many of the dikes have been split and intruded by younger dikes resulting in half and marginless dikes the dike complex is composed of 100 sheeted dikes near the center but contains gabbro screens near the base and pillow lava and breccia screens in its upper parts the dikes are inclined at 40 to bedding after correcting for folding and are inferred to have been rotated shortly after formation the extrusive sequence consists predominantly of pillow lavas but contains abundant pillow breccias and some massive lavas some of the breccias formed as talus deposits whereas others are graded and matrix supported indicating transport and redeposition possibly in an area of rugged submarine topography the dikes and lavas have geochemical affinities to island arc tholeiites but are in some aspects transitional to mid ocean ridge basalts the dikes and lavas are cogenetic with the underlying plutonic sequence as indicated by similarity in crystallization sequence contradictory intrusive relationships between sheeted dikes and high level plutonic rocks and lack of diabase dikes in the lowermost cumulates and harzburgite the upper portions of the ophiolite were hydrothermally metamorphosed at the spreading center resulting in downward changes in mineralogy texture and 18o in addition metalliferous sediments and massive sulfides that occur overlying and within the pillow lavas probably were formed by precipitation from thermal springs a geothermal gradient of 250 350 c km is inferred from mineral assemblages on the basis primarily of its regional setting the ophiolite is inferred to have formed in a back arc basin in particular it is within a thrust sheet sandwiched between a coeval magmatic arc complex and a slightly older arc complex remnant arc this interpretation is consistent with abundant volcanic arc detritus in graywackes overlying the ophiolite the island arc magmatic affinities of the lavas and dikes may be the result of formation of the ophiolite during the earliest stages of back arc spreading

on the generation mechanisms of fluid driven seismic signals related to volcano tectonics the generation mechanics of fluid driven volcano seismic signals and their evolution with time remains poorly understood we present a laboratory study aiming to better constrain the time evolution of such signals across temperature conditions 25 to 175 c in order to simulate a bubbly liquid simulations used pressures equivalent to volcanic edifices up to 1 6 km in depth using a triaxial deformation apparatus equipped with an array of acoustic emission ae sensors we investigate the origin of fluid driven seismic signals by rapidly venting the pore pressure through a characterized damage zone during the release of water at 25 c broadband signals were generated with frequencies ranging from 50 to 160 khz however the decompression of a water steam phase at 175 c generated a bi modal spectrum of different signals in the range 100 khz and 160 khz these new results are consistent with natural signals from active volcanoes such as mt etna and highlight the role of fluid and gas phases such as bubbly liquids in generating different types of volcano tectonic seismicity

natural and experimental evidence of melt lubrication of faults during earthquakes melt produced by friction during earthquakes may act either as a coseismic fault lubricant or as a viscous brake here we estimate the dynamic shear resistance f in the presence of friction induced melts from both exhumed faults and high velocity 1 28 meters per second frictional experiments exhumed faults within granitoids tonalites indicate low f at 10 kilometers in depth friction experiments on tonalite samples show that f depends weakly on normal stress extrapolation of experimental data yields f values consistent with the field estimates and well below the byerlee strength we conclude that friction induced melts can lubricate faults at intermediate crustal depths

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