new global marine gravity model from cryosat 2 and jason 1 reveals buried tectonic structure gravity models are powerful tools for mapping tectonic structures especially in the deep ocean basins where the topography remains unmapped by ships or is buried by thick sediment we combined new radar altimeter measurements from satellites cryosat 2 and jason 1 with existing data to construct a global marine gravity model that is two times more accurate than previous models we found an extinct spreading ridge in the gulf of mexico a major propagating rift in the south atlantic ocean abyssal hill fabric on slow spreading ridges and thousands of previously uncharted seamounts these discoveries allow us to understand regional tectonic processes and highlight the importance of satellite derived gravity models as one of the primary tools for the investigation of remote ocean basins

conditions and timescales for welding block and ash flow deposits welding of pyroclastic deposits to reform a coherent rock mass is a common phenomenon especially for pumiceous pyroclastic density current deposits i e ignimbrites however and despite the pervasive abundance of block and ash flow baf deposits in the geological and modern record instances of strongly welded baf deposits are few here we present a series of high temperature 800 900 c compaction experiments designed to map the conditions deposit thickness stress and temperature viscosity and timescales that permit or inhibit the welding of baf deposits our experiments were performed on unconsolidated aggregates containing an ash and lapilli component derived from crushed and sieved lava blocks containing 25 crystals taken from the well documented welded baf deposit at mount meager volcano british columbia canada the experiments demonstrate that welding efficiency increases with increasing time and temperature progressive welding is expressed by increasing axial strain porosity loss and bulk density the rate of change of each of these physical properties reduces as welding progresses microstructural analysis of the experimental products shows that the loss of interclast porosity during welding results from the progressive sintering and amalgamation of vitric fragments and that the pore shape changes from sub equant pores to stretched lenses sandwiched between vitric and crystal fragments the coincidence between the microstructure and rock physical properties of the natural and experimental samples highlight that we have successfully reproduced welded baf in the laboratory furthermore our permeability measurements highlight a hysteresis in the return journey of the there and back again volcanic permeability cycle expressed by an increase in permeability due to vesiculation and fragmentation followed by a decrease due to welding this hysteresis cannot be described by a single porosity permeability power law relationship and reflects the change in pore shape and connectivity during welding finally we show that a simple model for welding can accurately forecast the welding timescales of the baf deposit at mount meager as reconstructed from the collapse of the lillooet river valley dam using our experimental data we use this validation as a platform to provide a universal window for the welding of baf deposits also applicable for comparable welded deposits e g welded autobreccias in block lavas and lava domes for a broad range of deposit thickness or stress and effective viscosity

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

emplacement of the rocche rosse rhyolite lava flow lipari aeolian islands the rocche rosse lava flow marks the most recent rhyolitic extrusion on lipari island italy and preserves evidence for a multi stage emplacement history due to the viscous nature of the advancing lava 108 to 1010 pa s indicators of complex emplacement processes are preserved in the final flow this study focuses on structural mapping of the flow to highlight the interplay of cooling crust formation and underlying slope in the development of rhyolitic lavas the flow is made up of two prominent lobes small 0 2 m scale folding and a channelled geometry foliations dip at 2 4 over the flatter topography close to the vent and up to 30 50 over steeper mid flow topography brittle faults tension gashes and conjugate fractures are also evident across flow heterogeneous deformation is evident through increasing fold asymmetry from the vent due to downflow cooling and stagnation a steeper underlying topography mid flow led to development of a channelled morphology and compression at topographic breaks resulted in fold superimposition in the channel we propose an emplacement history that involved the evolution through five stages each associated with the following flow regimes 1 initial extrusion crustal development and small scale folding 2 extensional strain stretching lineations and channel development over steeper topography 3 compression at topographic break autobrecciation lobe development and medium scale folding 4 progressive deformation with stagnation large scale folding and re folding and 5 brittle deformation following flow termination the complex array of structural elements observed within the rocche rosse lava flow facilitates comparisons to be made with actively deforming rhyolitic lava flows at the chilean volcanoes of chaiten and cordon caulle offering a fluid dynamic and structural framework within which to evaluate our data

viscous heating in silicate melts an experimental and numerical comparison 1 the transition from newtonian to non newtonian flow of silicate melts is commonly manifested as shear thinning at conditions of high stress and strain rate shear thinning may strongly influence the dynamics of magmatic flows but the details of its microscopic origins are not fully understood here we consider viscous heating and thermomechanical coupling as a potential cause of shear thinning we compare the results of laboratory uniaxial compression experiments of a silicate melt with the results of thermomechanical numerical simulations corresponding to the experimental setup both the experimental and numerical results concord and indicate that the reduction of the temperature dependent viscosity in flowing silicate melts is a result of viscous heating viscous heating was quantified for glasses with viscosities ranging from 108 to 1011 pa s and strain rates from 10 5 to 100 s 1 the results of 48 compression experiments indicate that the transition from newtonian to non newtonian flow in the silicate melt occurs at a brinkmann number i e ratio of heat gained to heat lost around 1 whereas brittle behavior dominates the melt deformation when the deborah number i e ratio of viscoelastic relaxation time to characteristic deformation time is larger than around 0 01 the observed viscous heating significantly contributes to the viscosity decrease observed in high stress strain rate experiments and questions our current understanding of the non newtonian deformation behavior of silicate melts

explosive origin of silicic lava textural and δd h2o evidence for pyroclastic degassing during rhyolite effusion a long standing challenge in volcanology is to explain why explosive eruptions of silicic magma give way to lava a widely cited idea is that the explosive to effusive transition manifests a two stage degassing history whereby lava is the product of non explosive open system gas release following initial explosive closed system degassing direct observations of rhyolite eruptions indicate that effusive rhyolites are in fact highly explosive as they erupt simultaneously with violent volcanic blasts and pyroclastic fountains for months from a common vent this explosive and effusive overlap suggests that pyroclastic processes play a key role in rendering silicic magma sufficiently degassed to generate lava here we use precise h isotope and magmatic h2o measurements and textural evidence to demonstrate that effusion results from explosion s lavas are the direct product of brittle deformation that fosters batched degassing into transient pyroclastic channels tuffisites that repetitively and explosively vent from effusing lava our measurements show specifically that d h ratios and h2o contents of a broad suite of explosive and effusive samples from chaiten volcano hydrous bombs plinian pyroclasts tuffisite veins and lava define a single and continuous degassing trend that links wet explosive pyroclasts 1 6 wt h2o d 76 4 to dry obsidian lavas 0 13 wt h2o d 145 7 this geochemical pattern is best fit with batched degassing model that comprises small repeated closed system degassing steps followed by pulses of vapour extraction this degassing mechanism is made possible by the action of tuffisite veins which by tapping already vesicular or brecciated magma allow batches of exsolved gas to rapidly and explosively escape from relatively isolated closed system domains and large tracts of conduit magma by giving them long range connectivity even though tuffisite veins render magma degassed and capable of effusing they are nonetheless the avenues of violent gas and particle transport and thus have the potential to drive explosions when they become blocked or welded shut thus the effusion of silicic lava traditionally thought to be relatively benign process presents a particularly hazardous form of explosive volcanism

welding insights from high temperature analogue experiments abstract the rheological behavior of pyroclastic deposits during welding is incompletely understood and is based on a surprisingly small number of experimental studies here we present results from a new experimental apparatus comprising an automated uniaxial compression load frame that can run constant load up to 1150 kg or constant displacement rate 10 6 to 0 25 cm s tests at elevated temperatures 1100 c deformation experiments were performed on pre fabricated cylinders 4 5 cm diameter 6 cm length of soda lime silica glass beads n 32 sintered rhyolite ash n 7 and cores of pumiceous rhyodacite n 6 experimental runs used strain rates from 10 5 to 10 3 s 1 and stresses of 0 to 5 24 mpa temperatures varied from 535 to 650 c for experiments on soda lime silica glass beads and 825 to 950 c for natural materials in all cases experimental cores showed a strain dependent rheology that is more strongly affected by temperature than by load or strain rate results from these experiments are used to develop a relationship in which the effective viscosity e of the experimental cores is predicted by e o exp 5 3 f 1 f where 0 is melt viscosity and f is sample porosity this rheological model provides a means for exploring the relative roles of emplacement temperature load and glass transition temperature in welding of pyroclastic deposits

timescales of porosity and permeability loss by solid state sintering abstract the physical properties of granular geologic materials are transient a variety of densification processes operate throughout earth s crust one such process is solid state sintering which causes crystalline clasts to coalesce in the absence of fluids or melt solid state sintering operates wherever unconsolidated granular materials are subjected to elevated pressures and temperatures for protracted periods of time there are however few studies that constrain the conditions and timescales for densification and lithification of crystalline geologic materials by solid state sintering here we present the results of hot pressing experiments designed to cause a natural glass free volcanic fault gouge to undergo solid state sintering unconsolidated starting materials subjected to volcanic temperatures 700 900 c and pressures 20 70 mpa are transformed by solid state sintering into more coherent porous composites over a period of 4 60 hours the relative density and competence of experimental products increase and porosity and permeability decrease as sintering pressure temperature and time increase we use the experimental results to develop a robust densification model that predicts time dependent porosity and permeability loss at a pressure temperature range that includes volcanic and some upper crustal environments in these environments solid state sintering causes reductions in porosity and permeability of 0 35 and 104 m2 respectively over days to months depending on pressure temperature conditions applied to volcanic environments the short timescales of solid state sintering driven permeability loss and lithification can dictate the efficiency of outgassing and therefore modulate eruption style i e explosive vs effusive

tectonic erosion of the peruvian forearc lima basin by subduction and nazca ridge collision 1 subsidence of lima basin part of the peruvian forearc is controlled by tectonic erosion by the subducting nazca plate multichannel seismic reflection data coupled with age and paleowater depth constraints derived from ocean drilling program odp coring now allow the rates of erosion to be reconstructed through time in trenchward locations the forearc has experienced limited recent relative uplift 700 850 m likely due to preferential basal erosion under the center of lima basin long term subsidence driven by basal tectonic erosion dominates and is fastest closest to the trench since 47 ma eocene up to 148 km of the plate margin have been lost at an average rate of up to 3 1 km myr 1 appoximately 110 km of that total appears to be lost since 11 ma implying much faster average rates of trench retreat 10 km myr 1 since collision of the nazca ridge with the lima basin at 11 ma although there is no clear subsidence event at odp site 679 during the time at which nazca ridge was subducting beneath this part of the forearc 4 11 ma the more trenchward odp sites 682 and 688 show significant deepening after 11 ma indicating that subduction of the ridge accelerates tectonic erosion long term rates of crustal erosion in the region of lima basin are greater than estimates of regional arc magmatic productivity implying that such margins are net sinks of continental crust

fusion characteristics of volcanic ash relevant to aviation hazards the fusion dynamics of volcanic ash strongly impacts deposition in hot parts of jet engines in this study we investigate the sintering behavior of volcanic ash using natural ash of intermediate composition erupted in 2012 at santiaguito volcano guatemala a material science procedure was followed in which we monitored the geometrical evolution of cylindrical shaped volcanic ash compact upon heating from 50 to 1400 c in a heating microscope combined morphological mineralogical and rheological analyses helped define the evolution of volcanic ash during fusion and sintering and constrain their sticking potential as well as their ability to flow at characteristic temperatures for the ash investigated 1240 c marks the onset of adhesion and flowability the much higher fusibility of ash compared to that of typical test sands demonstrates for the need of a more extensive fusion characterization of volcanic ash in order to mitigate the risk posed on jet engine operation

subduction of seamounts at the java trench a view with long range sidescan sonar abstract we describe here a 1300 km by 45 km gloria long range sidescan sonar swath along the eastern java trench the swath images the trench axis a narrow strip of oceanic crust to the south and the toe of the accretionary wedge to the north between 108 and 120 e sonar images of the ocean crust show a pattern of normal faults typical of the outer wall of trenches these result from tension related to the bending of the oceanic lithosphere into the subduction zone a number of sub circular seamounts are also seen some of which are currently being subducted isolated ponds of flat lying sediments occur in the trench axis although along much of its length the trench is devoid of such sediment accumulations on the inner trench wall the accretionary wedge is recognised by its distinctive grainy texture with the grain aligned sub parallel to the deformation front where subducting seamounts are colliding with the wedge large crescentic areas of very high backscattering correlate with re entrants in the deformation front and large indentations in the wedge the high backscattering within these collision scars is interpreted to arise from outcropping strata and talus covered slopes resulting from erosion of an inner trench wall shortened and oversteepened by collision with a seamount all of the sediment ponds in the trench axis occur in close association with collision scars strongly suggesting localised erosion of the accretionary wedge in the scar areas however the volume of the indentations appears to be an order of magnitude greater than the volume of eroded sediments deposited in the trench indicating that erosion cannot be the primary mechanism by which indentations are created most of the missing material must be displaced landward by thrusting and folding ahead of the seamount our observations at the java trench are broadly comparable with those made at the japan trench by french and japanese workers however some differences in the detail of the collision process are evident in particular oversteepening and erosion occur from the earliest stage of collision at the java trench while uplift and understeepening initially dominate at the japan trench differences in the effects of subducting seamounts in the two areas are attributed to contrasts between the relatively unlithified strata of the java trench inner wall and the more lithified strata of the non accreting japan trench inner wall

focused subaerial erosion during ridge subduction impact on the geomorphology in south central peru data obtained from low temperature thermochronometers such as apatite fission track and u th he are combined with morphometric information extracted from digital elevation models this combination shows several geomorphological effects that are caused by the migration of the nazca ridge along the peruvian coastal margin offshore the depth of the deep sea trench decreases by 1500 m where the nazca ridge collides with the continental south american plate onshore the ridge causes an uplift of at least 800 m in the coastal cordillera this uplift results in a westward shift of the coastline thereby focusing and increasing erosion in the uplifted areas at the trailing edge the shelf subsides and the coastline retreats eastwards producing at least part of the indentation observed between paita and pisco the ridge acts therefore like a wave uplifting the andean margin as it traverses inland and southwards leaving a clear fingerprint on the topographic evolution of the peruvian coastal margin

how voluminous rhyolite lavas mimic rheomorphic ignimbrites eruptive style emplacement conditions and formation of tuff like textures silicic lavas of large volume and relatively great areal extent are not uncommon in association with the tracks of mantle plume hotspots across the continents these units are commonly eroded and or poorly exposed and their interpretation as lavas has been controversial the morphology of a 15 km 3 calc alkalic rhyolitic unit 75 to 77 wt sio 2 on the owyhee plateau of southwestern idaho shows that it is a true lava flow emplaced by effusion the eruption tapped a zoned magma chamber with a small volume of nearly aphyric magma underlain by a much larger volume of less evolved crystal rich magma magmatic temperatures were about 830 c and h 2 o contents were about 2 to 2 75 wt eruptive activity first involved explosive venting of the nearly aphyric magma followed by effusion of the crystal rich magma movement of the lava over and around the tephra deposits formed large discrete lava lobes one flow lobe reached 9 km from the vent though the unit is a true lava flow a diverse suite of fragmental textures including rock that closely resembles welded tuff formed by lava flow processes during the time the flow was active

crystal rich lava dome extrusion during vesiculation an experimental study abstract lava dome forming eruptions represent a common eruptive style and a major hazard at numerous active volcanoes worldwide the extrusion mechanics of crystal rich lava domes and the influence of volatiles on the transition from viscous to brittle behaviour during lava dome extrusion remain unclear understanding how gas exsolution and crystallinity control effusive versus explosive eruption behaviour is essential here we present new experimental results on the rheology of synthesised crystal rich 50 to 80 vol quartz crystals hydrous 4 2 wt h2o in the glass dacite samples which vesiculate from 5 to 27 vol gas bubbles at high temperatures from glass transition temperature to 797 c during deformation conducted in a parallel plate viscometer constant stress at 0 63 0 64 mpa and variable strain rates ranging from 8 32 10 8 to 3 58 10 5 s 1 the experiments reproduce certain aspects of lava dome deformation in volcanic conduits during vesiculation of the residual melt instigated in the experiments by increasing temperature during gas exsolution i e nucleation and growth of gas pressurised bubbles and volume inflation we find that the rheological lubrication of the system during deformation is strongly dictated by the initial crystallinity at crystal contents

fracture healing and strength recovery in magmatic liquids cycles of fracture and healing in magma are important controls on outgassing time scales and repetitive seismicity at silicic volcanoes here we experimentally drove silicate melts at 109 1011 pa s to tensile failure measuring the strength during fracture of the otherwise liquid material we then took the same melts with parallel contact surfaces and closed the fracture under compressive stress and recorded the evolution of tensile strength of the interface healed for different times we provide a semi empirical model for fracture healing time scales useful for volcanic applications as the time available for healing is increased strength nonlinearly recovers toward that of unfractured glass we parameterized the healing kinetics as a three stage process 1 relaxation of the compressive stress 2 fracture surface surface wetting and 3 diffusive removal of the interface during welding of these surfaces in air we observed that micropores are trapped along the fracture plane which may inhibit complete healing and provide a textural record of relict fracture planes in volcanic glasses we conclude that at magmatic conditions fracture healing is efficient in crystal poor melts and it could rapidly seal outgassing pathways over eruptive time scales contributing to cyclic behavior associated with recurring gas and ash explosions and outgassing events

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

episodic development of a convergent margin inferred from structures and processes along the southern chile margin seismic reflection data acquired in the vicinity of isla mocha across the southern coast of chile image structures formed along the continental margin and reveal an episodic history of accretion nonaccretion and possibly erosion structures formed at the toe of the continental slope suggest frontal accretion of to 1 km of trench fill seismic images also reveal that a small accretionary wedge 20 30 km wide abuts the truncated continental metamorphic basement that extends seaward from beneath the shelf the small size of the accretionary wedge on three profiles examined here is not consistent with a long history of accretion with the current deformational style as current rates of frontal accretion could have accumulated all of the existing accretionary wedge in less than 1 2 m y this is a small fraction of convergence history along this margin and the current accretionary mode has not been consistently maintained in the past the isla mocha region is located between the temperate climate of central chile and the glacial climate of southern chile and climatic conditions in this region have likely fluctuated sufficiently to cause significant variation in trench sediment supply accretionary and nonaccretionary or erosional episodes are probably linked to temporal variations in trench sediment thickness as suggested by observations along the chile margin currently thick trench sediment correlates with accretion along the southern chile margin and thin trench sediment correlates with nonaccretion tectonic erosion as near the chile ridge and from the juan fernandez ridge to northern chile the isla mocha region also lies 900 1000 km north of the chile triple junction and the chile ridge lies approximately 2000 km to the west and has not yet collided and affected the margin near isla mocha this part of the precollision zone provides an excellent reference to examine the effects of chile ridge collision in the development of the chile margin the most apparent effect of subduction of the buoyant young crust of the chile ridge is a shallow trench that is nearly devoid of sediment consequently the triple junction is undergoing nonaccretion or erosion and the accretionary complex near the triple junction remains smaller than to the north or south because the current phase of rapid accretion elsewhere in the trench has bypassed the triple junction region the interplay of subduction zone processes such as trench sedimentation and ridge collision has resulted in an episodic development of the margin and produced a discontinuous record of convergence history within the accretionary wedge

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

structure and tectonics of the erosional convergent margin off antofagasta north chile 23 30 s subduction erosion has dominated the evolution of the north chile convergent continental margin since at least the mesozoic we investigate the structure of the antofagasta 23 s sector of this margin along a transect using coincident wide angle and near vertical seismic profiling and gravity data a 2 d velocity field of the overriding and subducting plates was obtained using joint refraction and reflection travel time tomography a velocity derived density distribution was used to model marine gravity data and substantiate the velocity model the gravity and velocity models imply that the overriding plate is mainly made of arc type igneous basement the upper plate is constructed of two main rock bodies separated by a subhorizontal layer defined by a velocity inversion the top coincident with a reflection in near vertical seismic images the seismic boundary is interpreted as a detachment separating an upper extended domain with large scale normal faulting from a lower domain apparently undergoing a different type of deformation velocity derived porosity indicates that the front of the margin is probably fluid saturated and disaggregated by fracturation as a consequence of frontal subduction erosion fluids carried into the subduction channel within slope debris filling underthrusting grabens reduce basal friction and probably induce hydrofracturing and basal erosion along the underside of the overriding plate at depths greater than 20 km porosity and density values imply that most fluids have been exhausted and the lower part of the upper plate is structurally coherent and little fractured the change in physical properties leads to increased mechanical coupling along the plate boundary and occurs at the updip limit of the distribution of aftershocks of the 1995 antofagasta earthquake m w 8 0 defining the seismogenic zone

processes of seamount subduction in the mariana and izu bonin trenches abstract morphology of oceanic plate seamounts near the mariana and izu bonin trenches in the western pacific indicates that the response of these seamounts to subduction related tectonic processes is a function of two factors seamount size and the degree of fracturing of the surrounding sea floor as the oceanic plate bends prior to subduction seamounts smaller than about 40 km in diameter are fractured on the same scale as or to a greater degree than the surrounding sea floor larger seamounts 40 100 km in diameter show less evidence of fracturing prior to subduction since there is no evidence of accretion of large amounts of plate material onto the mariana fore arc it is probable that plate seamounts are subducted at least partially intact the subduction of plate seamounts beneath the mariana arc may be responsible for some areas of uplift on the outer mariana fore arc broad areas of uplift on the mariana and bonin fore arcs are associated in some cases with apparent subduction of ridges of large greater than 100 km diameter seamounts the large seamounts at the western end of the michelson ridge appear to be breaking up into a series of blocks separated by en echelon fractures that are oriented oblique to the trench axis and lie in line with the convergence direction of the pacific plate such large seamounts may interrupt normal subduction processes this suggestion is consistent with the occurrence of gaps in benioff zone seismicity where chains of such large seamounts abut the trenches it appears likely that these large seamounts are also eventually subducted although they may first undergo significant fracturing and reorientation

high temperature deformation of volcanic materials in the presence of water we describe an experimental apparatus used to perform deformation experiments relevant to the volcanological sciences the apparatus supports low load high temperature deformation experiments under dry and wet conditions on natural and synthetic samples the experiments recover the transient rheology of complex melt porosity solids volcanic materials during uniaxial deformation the key component to this apparatus is a steel cell designed for high temperature deformation experiments under controlled water pressure experiments are run under constant displacement rates or constant loads the range of accessible experimental conditions include 25 1100 c load stresses 0 to 150 mpa strain rates 10 6 to 10 2 1 s and fluid pressures 0 150 mpa the apparatus is calibrated against standard values of viscosity using constant load experiments on cores of nist nbs 717a borosilicate glass we also report results of constant displacement rate 10 6 m s experiments on porous 70 sintered cores of ash from the rattlesnake tuff the cores of volcanic ash were deformed in experiments under ambient dry and elevated water pressures wet dry experiments at 870 c show an increase in effective viscosity 10 9 5 to 10 10 4 pa s with increasing strain 30 due to porosity reduction during compaction experiments under 1 3 mpa p h 2 o recover lower values of apparent viscosity 10 9 2 to 10 9 4 pa s despite being run at lower temperatures 640 665 c the wet experiments also do not show a rise in viscosity with increased strain decreasing porosity as observed in dry experiments rather the presence of an h 2 o fluid phase expands the window of viscous deformation and delays the onset of strain hardening that normally accompanies porosity reduction

assembling an ignimbrite mechanical and thermal building blocks in the bishop tuff california abstract the building blocks that control ignimbrite physical characteristics are here defined as mechanical flow units and flow packages and thermal cooling units ignimbrite construction from these mechanical and thermal building blocks is illustrated by a longitudinal profile of the bishop tuff in the owens river gorge california in the bishop ignimbrite flow unit boundaries can be defined only locally but known or inferred multiple flow units combine to form eruptive packages showing imbricate offlapping relationships such that each package is thickest successively farther from the source in these packages the presence or absence of flow unit boundaries does not necessarily reflect directly the presence or absence respectively of time breaks thus massive ignimbrite need not be the product of continuous accumulation progressive aggradation the intrinsic thermal properties of each package or parts thereof have combined with local thicknesses of accumulated material to control the weld

the permeability evolution of tuffisites and implications for outgassing through dense rhyolitic magma there is growing evidence that outgassing through transient fracture networks exerts an important control on conduit processes and explosive effusive activity during silicic eruptions indeed the first modern observations of rhyolitic eruptions have revealed that degassed lava effusion may depend upon outgassing during simultaneous pyroclastic venting the outgassing is thought to occur as gas and pyroclastic debris are discharged through shallow fracture networks within otherwise low permeability conduit plugging lava domes however this discharge is only transient as these fractures become clogged and eventually blocked by the accumulation and sintering of hot melt rich pyroclastic debris drastically reducing their permeability and creating particle filled tuffisites in this study we present the first published permeability measurements for rhyolitic tuffisites using samples from the recent rhyolitic eruptions at chaiten 2008 2009 and cordon caulle 2011 2012 in chile to place constraints on tuffisite permeability evolution we combine 1 laboratory measurements of the porosity and permeability of tuffisites that preserve different degrees of sintering 2 theoretical estimates on grainsize and temperature dependent sintering timescales and 3 h2o diffusion constraints on pressure time paths the inferred timescales of sintering driven tuffisite compaction and permeability loss spanning seconds in the case of compaction driven sintering to hours surface tension driven sintering coincide with timescales of diffusive degassing into tuffisites observed vent pulsations during hybrid rhyolitic activity extrusive behaviour coincident with intermittent explosions and more broadly timescales of pressurisation accompanying silicic lava dome extrusion we discuss herein the complex feedbacks between fracture opening closing and sintering and their role in outgassing rhyolite lavas and mediating hybrid explosive effusive activity

friendly fire engineering a fort wall in the iron age there is widespread evidence that the walls of iron age forts across europe were set on fire causing partial melting of the stonework followed by either recrystallization or glass formation on cooling a process termed vitrification the motivation for fort wall firing has remained speculative since its first description in 1777 since the suggestion of mackie 1969 that fort vitrification might destabilize fort walls the debate as to motives has focused on combative or destructive intentions here a multidisciplinary analysis of experimental fort wall samples shows that in fact vitrification results in strengthening not weakening the strengthening involves diffusive and viscous sintering of material aggregates and size dependent heat transfer these new results support a long since dismissed idea that iron age fort walls were intentionally set ablaze in order to fortify the walls

constraints on rheology of obsidian lavas based on mesoscopic folds the geometry of mesoscopic single and multiple layer folds in rhyolitic obsidian flows is investigated folds are composed of obsidian embedded in a matrix of pumice folds form by buckling processes as indicated by discontinuous deformation between obsidian and pumice layers and by the geometries of wavetrains buckling occurs through a continuum of styles controlled largely by the thickness ratio of pumice to obsidian n styles of folds include chevron harmonic polyharmonic disharmonic and single layer assemblages harmonic and chevron folds are observed for small values of n for large values of n folds buckle independently of one another and form disharmonic and single layer assemblages wavelength to thickness ratios of single layer folds are compared to theoretically predicted ratios for newtonian and power law fluids as a means of estimating shear viscosity ratios of obsidian and pumice while all folds indicate that bubble free rhyolite is more viscous during flow than bubbly rhyolite estimates of shear viscosity ratio based on newtonian theory 010 500 may exceed estimates based on power law rheologies by more than an order of magnitude newtonian buckling theory involves a number of simplifications and does not account for the possibly complex rheology of bubble bearing rhyolite 1999 elsevier science ltd all rights reserved

high rates of arc consumption by subduction processes some consequences new estimates of long term consumption of japanese and peruvian continental fore arc margins together with recent drilling data ocean drilling program on some other erosional island arcs such as izu bonin mariana or tonga allow the calculation of a mean rate of 7 3 km m y landward migration of the arc trench system on both east and west sides of the pacific basin during cenozoic time such relative arcward retreat counterbalances seaward migration of the slab hinge line to accommodate opening of the atlantic ocean retreat can thus be interpreted as a delay in seaward or retrograde migration due to upper mantle resistance to contraction of the pacific basin mass transfers in subduction zones must be reconsidered in the light of these new estimates especially the balance between net growth and loss of continental crust in recent geologic time a 50 m y period is long enough to erode the initial width of the volcanic arc and fore arc massif of any subduction zone assuming a mean trench volcanic arc landward migration of 5 km m y and a mean trench volcanic arc distance of 250 km this circumstance thus raises the question of whether the izu bonin mariana and tonga arcs are older than their reported eocene age

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

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

mineralogy and geochemistry of volcanic and plutonic rocks from the boundaries of the caroline plate tectonic implications abstract chemical and mineralogical data from rocks dredged at six sites along plate boundaries around the caroline sea define at least three separate igneous suites three sites were located in the sorol trough along the caroline pacific plate boundary and three sites were located in the ayu trough along the caroline philippine plate boundary geophysical data suggest that both features have originated through processes of crustal extension weathered basalts with mid ocean ridge basalt morb affinities were recovered from the base of the northern margin of the sorol trough fresh transitional to alkalic pillow basalts from a seamount in the central part of the trough have geochemical and sr isotopic characteristics similar to enriched morb or volcanics associated with off ridge volcanism a suite of tholeiitic rocks consisting of high mg basalt ferrobasalt and a variety of mafic to ultramafic rocks dredged from the western section of the sorol trough has chemical and mineralogical characteristics of other intraplate tholeiitic suites like those from hawaii the ferrobasalts could have been derived from a high mg basaltic liquid through fractional crystallization of olivine clinopyroxene and plagioclase the geochemical data indicate that both morb and hot spot type sources have been involved in the generation of magmas beneath the sorol trough furthermore the highly metamorphosed and tectonized mafic to ultramafic rocks recovered with the tholeiites suggest that a significant component of shear accompanied crustal accretion along leaky transform faults within the sorol trough the chemical and isotopic characteristics of fresh to weathered pillow lava fragments from the axial rift and eastern margin of the ayu trough suggest that a morb type source was tapped during crustal accretion along a central spreading centre an assemblage of metabasalts metagabbros and silicic plutonic rocks plagiogranites recovered from a scarp along the western margin of the trough have petrologic and chemical features analogous to rocks from plutonic complexes in ophiolites and some oceanic ridges the plagiogranites could be differentiates from a relatively hydrous morb magma but cannot be related to the slightly lil enriched basalts in the ayu trough by crystal fractionation two separate mantle sources are implicated metamorphic features in the plutonic rocks are inferred to be the result of hydrothermal and dynamic events associated with the inception of spreading and to subsequent tectonism which exposed them the petrochemistry of rocks from the ayu and sorol troughs is consistent with the hypothesis that crustal accretion occurred along the northern and western boundaries of the caroline plate and further suggests that a variety of sub oceanic mantle sources have been involved in the genesis of the crust in each region

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

basalt unveiling fluid filled fractures inducing sediment intra void transport ephemerally examples from katla 1918 abstract this article documents textures within basaltic katla 1918 pyroclasts where particle filled fractures and bubbles have been observed these features are analogous to tuffisite veins particle filled fractures which represent the preserved remains of transient degassing pathways in shallow conduits such fractures have long been considered restricted to high viscosity silicic melts however through bse images and compositional maps we have identified similar tuffisite like features in crystal poor basalt pyroclasts from the 1918 ce subglacial eruption of katla iceland k1918 clast textures record transient mobility of juvenile lithic particles melt droplets and gas through magmatic fractures and connected vesicles key evidence includes 1 the presence of variably sintered fine ash particles within variably healed fractures and vesicles present in 80 of clasts analysed 2 compositional maps that reveal the presence of foreign particles within preserved and healed permeable pathways and 3 lower vesicularities immediately surrounding fracture walls suggestive of diffusive volatile loss into a permeable network the 1918 ce eruption of katla occurred under a thick glacier however the ice was quickly breached owing partly to the explosive nature of the eruption we propose that the formation and preservation of these transient permeable networks have been facilitated by rapid decompression of a relatively volatile rich magma in a confined subglacial environment with combined magmatic and phreatomagmatic fragmentation followed by rapid quenching by meltwater tuffisite veins in rhyolite demonstrate repeated fracture healing cycles which drive incremental release of overpressured gas and help to defuse explosive eruptions interestingly the permeable network at katla failed to defuse the 1918 ce eruption which involved a particularly violent subglacial eruptive phase it is unclear whether this demonstrates an inability of mafic tuffisite like features to efficiently degas magma perhaps owing to the especially transient nature of permeable pathways in low viscosity magmas or an ability to enhance fragmentation by providing infiltration pathways for external water the latter scenario may explain the rapid melting of the overlying glacier as the large surface area to volume ratio of fractured magma would allow rapid heat transfer nevertheless we document a previously unrecognised texture in basaltic magmas it is intriguing why it has not to the best of our knowledge been documented elsewhere have these permeable pathways been overlooked in the past e g mistaken for bad sample preparation or not noticed without high magnification bse images and are in fact a widespread phenomenon in subglacial and other basalts or do our samples in fact represent a rarely preserved texture either way they offer a new insight into the degassing and fragmentation of subglacial basalt

melt fracturing and healing a mechanism for degassing and origin of silicic obsidian we present water content transects across a healed fault in pyroclastic obsidian from lami pumice cone lipari italy using synchrotron fourier transform infrared spectroscopy results indicate that rhyolite melt degassed through the fault surface transects define a trough of low water content coincident with the fault trace surrounded on either side by high water content plateaus plateaus indicate that obsidian on either side of the fault equilibrated at different pressure temperature p t conditions before being juxtaposed the curves into the troughs indicate disequilibrium and water loss through diffusion if we assume constant t melt equilibrated at pressures differing by 0 74 mpa before juxtaposition and the fault acted as a low p permeable path for h2o that diffused from the glass within time scales of 10 and 30 min assuming constant p instead melt on either side could have equilibrated at temperatures differing by as much as 100 c before being brought together water content on the fault trace is particularly sensitive to post healing diffusion its preserved value indicates either higher temperature or lower pressure than the surroundings indicative of shear heating and dynamic decompression our results reveal that water contents of obsidian on either side of the faults equilibrated under different p t conditions and were out of equilibrium with each other when they were juxtaposed due to faulting immediately before the system was quenched degassing due to faulting could be linked to cyclical seismic activity and general degassing during silicic volcanic activity and could be an efficient mechanism of producing low water content obsidian

statistical evidence of transitioning open vent activity towards a paroxysmal period at volcan santiaguito guatemala during 2014 2018 abstract long term eruptive activity at the santiaguito lava dome complex guatemala is characterised by the regular occurrence of small to moderate size explosions from the active caliente dome between november 2014 and december 2018 we deployed a seismo acoustic network at the volcano which recorded several changes in the style of eruption including a period of elevated explosive activity in 2016 here we use a new catalogue of explosions to characterise changes in the eruptive regime during the study period we identify four different phases of activity based on changes in the frequency and magnitude of explosions at the two ends of the spectrum of repose times we find pairs of explosions with near identical seismic and acoustic waveforms recorded within 1 10 min of one another and larger explosions with recurrence times on the order of days to weeks the magnitude frequency relationship for explosions at santiaguito is well described by a power law we show that changes in b value between eruptive regimes reflect temporal and spatial changes in rupture mechanisms likely controlled by variable magma properties we also demonstrate that the distribution of inter explosion repose times between and within phases is well represented by a poissonian process the poissonian distribution describing repose times changes between and within phases as the source dynamics evolve we find that changes in source properties restrict the extrapolation of explosive behaviour to within a given eruptive phase limiting the potential for long term assessments of anticipated eruptive behaviour at santiaguito

rheology of welding inversion of field constraints at present the mechanisms and rheological behaviour of pyroclastic deposits during welding and compaction are poorly understood here we explore the extent to which the rheological properties of pyroclastic deposits are constrained by physical property distributions in welded ignimbrite physical properties of samples from a 20 m section of the bandelier tuff new mexico are used as proxies for strain the observed strain et is ascribed to a combination of a time dependent viscous compaction ev and a time independent mechanical compaction em described by

new insight into a subduction related orogen a reappraisal of the geotectonic framework and evolution of the japanese islands abstract the geotectonic framework and the evolutionary history of the japanese islands need revision in accordance with the various geophysical geological evidence gathered by new methodologies in the recent years including seismic tomography vibroseis ground breaking seismic experiments and detrital zircon chronology these investigations have addressed various themes such as 1 seismic profile of the crust and mantle beneath the japanese islands 2 high precision ages of the protoliths of high p t metamorphic rocks and 3 provenance of terrigenous clastics the results have led to a number of important findings including 1 detection of a large mass of slab around the mantle boundary layer suggesting the long term oceanic subduction beneath japan 2 confirmation of the subhorizontal piled nappe structure for the entire crust of japan 3 finding a new high p t metamorphosed accretionary complex unit that represents the youngest blueschist in japan 4 finding of the oldest early cambrian arc batholith and cover sediments and 5 the identification of plural arc batholiths which have already been erased from the surface based on a synthesis of these new data this article presents a re evaluation of the conventional geotectonic subdivision of the subduction related orogen in japan re definition of the elements and their mutual boundaries and re consideration of the geotectonic evolution of the japanese islands in particular the historical change in provenance suggests that proto japan has experienced large scale tectonic erosion in multiple stages and the corresponding large amounts of continental crust materials were subducted for understanding the orogenic growth of japan during the last ca 500 million years the significance of tectonic erosion coupled with continental contraction as well as the oceanward accretionary growth requires further attention

permeability evolution during non isothermal compaction in volcanic conduits and tuffisite veins implications for pressure monitoring of volcanic edifices abstract explosive volcanic eruptions are destructive geological phenomena that pose hazards of significant socioeconomic impact and potential loss of life effective risk mitigation and decision making prior to and during volcanic crises require real time monitoring of gas overpressure the single most important driving force for explosive eruptions development and release of gas overpressure are regulated by gas loss through permeable pathways that are inherently transient here we use geometry dependent conductive cooling models in concert with the most up to date welding and permeability models to assess the potential for freezing in permeability within 1 conduit filling pyroclastic deposits and 2 tuffisite veins within the edifice we find that both geometry and dimension of each deposit dictate its thermal evolution and with that its transient outgassing capacity rapid cooling of thin sheet like tuffisite veins preserves high porosities and permeabilities in contrast wide cylindrical conduit filling deposits cool slowly and permeability is annihilated over a period of minutes to hours this highlights that conduit filling deposits lose their outgassing capacity through welding while tuffisite veins previously thought to rapidly seal can form long lived outgassing features we use the model results to calculate the time dependent gas flow partitioning between both degassing lithologies based on the reconstructed outgassing pattern we outline the potential to use the gas flow balance between the central conduit and distal fumaroles fed by tuffisite veins as a simple tool to monitor gas overpressure within a volcanic edifice

critical porosity a key to relating physical properties to porosity in rocks discovering accurate relations between porosity and seismic wave velocities in porous rocks has been an important topic of rock physics for decades such relations if they actually exist are critical when seismic and or sonic data are to be used to infer porosity and porosity related properties e g permeability strength saturation in situ these inferences became especially important in recent years with the rapid development of seismic and sonic methods which are applied to the characterization of heterogeneous reservoirs and recovery monitoring

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

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

welding of pyroclastic conduit infill a mechanism for cyclical explosive eruptions vulcanian style eruptions are small to moderate sized singular to cyclical events commonly having volcanic explosivity indices of 1 3 they produce pyroclastic flows disperse tephra over considerable areas and can occur as precursors to larger e g plinian eruptions the fallout deposits of the 2360 b p eruption of mount meager bc canada contain bread crusted blocks of welded breccia as accessory lithics they display a range of compaction welding intensity and provide a remarkable opportunity to constrain the nature and timescales of mechanical processes operating within explosive volcanic conduits during repose periods between eruptive cycles we address the deformation and porosity permeability reduction within natural pyroclastic deposits infilling volcanic conduits we measure the porosity permeability and ultrasonic wave velocities for a suite of samples and quantify the strain recorded by pumice clasts we explore the correlations between the physical properties and deformation fabric based on these correlations we reconstruct the deformation history within the conduit model the permeability reduction timescales and outline the implications for the repressurization of the volcanic conduit our results highlight a profound directionality in the measured physical properties of these samples related to the deformation induced fabric gas permeability varies drastically with increasing strain and decreasing porosity along the compaction direction of the fabric but varies little along the elongation direction of the fabric the deformation fabric records a combination of compaction within the conduit and postcompaction stretching associated with subsequent eruption model timescales of these processes are in good agreement with repose periods of cyclic vulcanian eruptions

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

rheology of basaltic ash from stromboli volcano inferred from intermittent compression experiments abstract after ash dominated strombolian eruption considerable amount of ash falls back to the volcanic conduit by the fallback ash the formation of a dense near surface region is expected because it is compacted by weights of its own and other fallback clasts the dense cap causes the gas pressure inside the conduit to increase which promotes a forthcoming eruption under such conditions the rheology of the fallback ash plays an important role in this cyclical eruption mechanism however little attention has been given to the point we examined the rheology of ash collected at stromboli volcano via intermittent compression experiments changing temperature and compression time rate the elastic modulus the viscosity and the relaxation time were estimated by a viscoelastic maxwell model the results show that the elasticity and the viscosity dominantly depend on the compression rate and the temperature respectively although both of them vary with time thus the ash behaves either elastically or viscously depending on the external conditions and time the viscoelastic characteristics can be explained by magnitude relationships between the characteristic relaxation time and times for compression and rest processes this indicates that the balance of the time scales is key to determining the rheological characteristics

kimberlite emplacement temperatures from conodont geothermometry abstract kimberlites are mantle derived ultramafic rocks preserved in volcanic and sub volcanic edifices and are the main primary source of diamonds the temperatures of formation transport eruption and deposition remain poorly constrained despite their importance for understanding the petrological and thermodynamic properties of kimberlite magmas and styles of volcanic eruption here we present measured values of colour alteration indices cai for conodonts recovered from 76 paleozoic carbonate xenoliths found within 11 pipes from the chidliak kimberlite field on baffin island nunavut canada the dataset comprises the largest range of cai values 1 5 to 8 and the highest cai values reported to date for kimberlite hosted xenoliths thermal models for cooling of the chidliak kimberlite pipes and synchronous heating of conodont bearing xenoliths indicate time windows of 10 20 000 h and for these short time windows the measured cai values indicate heating of the xenoliths to temperatures of 225 to 925 c we equate these temperatures with the minimum temperatures of the conduit filling kimberlite deposit i e emplacement temperature t e the majority of the xenoliths record cai values of between 5 and 6 5 suggesting heating of xenoliths to temperatures of 460 c 735 c the highest cai values are consistent with being heated to 700 c 925 c and establish the minimum conditions for welding or formation of clastogenic kimberlite deposits lastly we use t e variations within and between individual pipes in conjunction with the geology of the conduit filling deposits to constrain the styles of explosive volcanic eruption

subduction erosion crustal structure and an evolutionary model of the northern yap subduction zone new observations from the latest geophysical survey the yap subduction zone is a distinctive erosive margin with an extremely slow convergence rate the high relief of the subducting plate generated by horst and graben structures and seamounts leads to attenuation of the crust in this study we present the latest geophysical data collected by the chinese research vessel kexue to investigate subduction erosion at the yap subduction zone and develop subduction models for yap subduction zone structures we show an anomalous distance between the yap trench and the adjacent volcanic arc the steep slope of the trench arcward a frontal prism and rare sediment in the trench all typical features of erosive margins we propose that the high relief subducting plate has led to erosion of the overlying plate and that different subduction processes controlled locally by the topography of the subducting plate have modified the yap subduction zone numerous normal faults increase the relief of the seafloor entering the trench and an anomalously large slope angle along the trench reflects uplift of the fore arc high via seamount subduction in addition the thin crust of the subducted horst and graben structures along with the normal faults might have eroded the fore arc crust and subsequently eroded the yap arc crust during subduction these subduction erosional processes at the yap trench provide one of the best cases of an erosive margin in the world particularly for a subduction zone with an extremely slow convergence rate

the west philippine basin an eocene to early oligocene back arc basin opened between two opposed subduction zones 1 based on geological and geophysical data collected from the west philippine basin and its boundaries we propose a comprehensive cenozoic history of the basin our model shows that it is a back arc basin that developed between two opposed subduction zones rifting started around 55 ma and spreading ended at 33 30 ma the initial spreading axis was parallel to the paleo philippine arc but became inactive when a new spreading ridge propagated from the eastern part of the basin reaching the former one at an r r r triple junction spreading occurred mainly from this second axis with a quasi continuous counter clockwise rotation of the spreading direction the gagua and palau kyushu ridges acted as transform margins accommodating the opening arc volcanism occurred along the palau kyushu ridge eastern margin during the whole opening of the basin whereas the paleo philippine arc decreased its activity between 43 and 36 ma the western margin underwent a compressive event in late eocene early oligocene time leading to the rising of the gagua ridge and to a short subduction episode along eastern luzon in the western part of the basin the spreading system was highly disorganized due to the presence of a mantle plume overlapping spreading centers and ridge jumps occurred toward the hot region and a microplate developed shortly after the end of the spreading a late stage of amagmatic extension occurred between 30 and 26 ma in the central part of the basin being responsible for the deep rift valley that cut across the older spreading fabric

seismic observations at the yap islands and the northern yap trench abstract we conducted two seismic surveys in the yap region to investigate tectonic activity in this area one was conducted in the northern yap trench for ten days using ocean bottom seismometers the other was conducted on the yap islands for eight months using a small array from these observations we detected many earthquakes beneath the yap region and found a characteristic pattern of seismicity many earthquakes occurred in the inner trench slope no seismicity was observed in the axial region of the trench and a few earthquakes occurred in the outer trench slope this pattern is similar to the typical pattern in active subduction zones this result indicates that the yap trench is still actively subducting we did not detect earthquakes deeper than 40 km during the observations

progressive growth of the cerro bayo cryptodome chachahuen volcano argentina implications for viscous magma emplacement cryptodome and dome collapse is associated with volcanic hazards such as explosive eruptions pyroclastic flows and volcanic edifice collapse study of the growth and evolution of volcanic domes

textural and geochemical constraints on andesitic plug emplacement prior to the 2004 2010 vulcanian explosions at galeras volcano colombia hazardous sequences of vulcanian explosions are thought to result from the repeated emplacement and destruction of degassed highly crystalline magma plugs in the shallow conduit of arc volcanoes the processes governing the timing and magnitude of these explosions are thought to be related to magma ascent rate and efficiency of degassing and crystallisation we study a rare suite of time constrained ballistic bombs from the 2004 2010 period of activity of galeras volcano to reconstruct magma plug architecture prior to six individual vulcanian explosions we find that each plug was vertically stratified with respect to crystallinity vesicularity and melt volatile content melt composition and viscosity we interpret this structure as resulting from multiple bubble nucleation events and degassing driven crystallisation during multi step ascent of the magma forming the plug followed by spatially variable crystallisation within the plug under contrasting conditions of effective undercooling created by degassing we propose that the shallow conduit evolved from more open degassing conditions during 2004 2008 to more closed conditions during 2009 2010 this resulted in explosions becoming smaller and less frequent over time during 2004 2008 then larger and more frequent over time during 2009 2010 this evolution was controlled by changing average ascent rates and is recorded by systematic changes in plagioclase microlite textures our results suggest that small volume vulcanian explosions 105 m3 should generally be associated with longer repose times hundreds of days and produce ballistics characterised by small numbers of large prismatic plagioclase microlites larger volume vulcanian explosions 106 m3 should be associated with shorter repose times tens of days and produce ballistics characterised by high numbers of small more tabular plagioclase microlites

fragility and an extremely low shear modulus of high porosity silicic magma abstract the rheology and strength of bubbly magma govern eruption dynamics by determining the possibility of fragmentation of ascending magmas they are also required parameters for understanding seismic monitoring we measured the rheology and strength of high porosity rhyolitic magma at 500 950 c the measured shear modulus and strength are several orders of magnitude lower than bubble free rhyolite melt implying that high porosity magma cannot avoid fracturing during magma ascent the occurrence of fractures is observed in the low temperature magma 800 c in this temperature range the measured attenuation is low that is the elastic energy originated by deformations avoids attenuation and is stored in the bubbly magma until released by fracturing q 1 the newly found porosity dependent strength based on our measurements comprehensively explains three different fragmentation criteria that have been previously proposed independently our measurements also show that the shear modulus becomes lower by increasing porosity which can slow the shear wave velocity these results suggest that knowing the attenuation of the seismic wave is useful to evaluate magma temperature and the possibility of a fragmentation event that may determine subsequent volcanic activities

the integration of jers 1 and ers sar in differential interferometry for measurement of complex glacier motion complex glacier motion in the grove mountains region antarctica is measured using four pass differential synthetic aperture radar sar interferometry insar the components of the motion vector field are resolved using a 44 day separation japanese earth resources satellite 1 jers 1 insar pair and a european remote sensing satellite 1 2 ers 1 2 tandem insar pair the 44 day temporal baseline provides the sensitivity required to observe the range of ice motion around 8 10 m a 1 and the 1 day short baseline provides the best choice for glacier digital elevation model reconstruction it is remarkable that the scattering field of the jers 1 pair remained coherent over the long time interval and the interferometric fringes are clear the overall ice flow is from east to west downslope and towards lambert glacier the regional flow is obstructed by nunataks extending north south with two wide gaps two narrow glaciers flow past the nunataks and rejoin each other at the downstream end regional morphology and the resolved flow in the grove mountains area suggests that ice flow is channeled throughout this eastern flank of lambert glacier

rapid pulses of uplift subsidence and subduction erosion offshore central america implications for building the rock record of convergent margins integrated ocean drilling program iodp expedition 334 to southern costa rica central america documented unprecedented subduction erosion in an area of active seismic slip widespread subduction erosion of the upper plate initiated when the cocos ridge an overthickened aseismic ridge arrived at the middle america trench subduction erosion was coeval with the rapid formation of deposition centers on the forearc of the upper plate the completely recovered shelf sequence constrains a short 2 m y interval of extreme subsidence 1200 m with a rapid pulse occurring during the first 0 3 m y this event removed an estimated 1 2 106 km3 of forearc material at a rate of 1690 km3 m y km of trench during a time of rapid 1035 m m y shelf sediment accumulation at this erosive margin a sediment starved trench persisted in spite of abundant sediment supply because subduction erosion led to the creation of forearc basins similar rapid pulses of subduction erosion may punctuate the evolution of many margins contributing disproportionately to crustal recycling at subduction zones with implications for the evolution of continental crust and mountain belts and recycling of continental material into the mantle

petrology geochemistry and tectonic implications of volcanics dredged from the intersection of the yap and mariana trenches abstract rocks dredged from the forearc very close to the intersection of the yap and mariana trenches include a suite of highly depleted arc tholeiites and several samples of transitional to slightly alkaline basalt the tholeiites range from magnesian quartz tholeiites with 0 46 0 6 tio2 to andesites with up to 62 sio2 and 8 2 feo all show pronounced lree depletion and have very low contents of ba and sr they are postulated to have been produced by partial melting of upper mantle peridotite residual after morb extraction following influx of hydrous fluids from the subducted slab while these fluids were responsible for small enrichments in ba k rb and sr in melts generated lree were not involved in the metasomatism and the strong lree depletion probably reflects the unmodified depleted source peridotite the second lava suite includes slightly ne normative ti augite bearing basalts with convex upward ree patterns showing slight lree depletion la sm n 0 76 the chemical features of these basalts support affinities with basalts erupted during the earliest stages of backarc basin opening a k ar age on one sample 7 8 1 3m y is in good agreement with the initial opening of the mariana trough the tectonic significance of the dredged arc tholeiite suite is less obvious a k ar age of10 8 0 4 my on one andesite and the occurrence of similar lavas in dredges from at least 300 km along the length of the yap arc suggest that subduction was occurring beneath the yap arc in the late miocene after overthrusting of the yap greenschist allochthon and while calc alkaline arc magmatism was occurring further north on the west mariana ridge we suggest that the depleted arc tholeiites in dredge 1438 were generated by abnormally shallow melting of upper mantle beneath the yap forearc following subduction beneath this area of young hot sorol trough crust these arc tholeiites represent a magma type transitional between more typical arc tholeiites e g tongan and high mg andesites and boninites

models for viscosity and shear localization in bubble rich magmas abstract bubble content influences magma rheology and thus styles of volcanic eruption increasing magma vesicularity affects the bulk viscosity of the bubble melt suspension and has the potential to promote non newtonian behavior in the form of shear localization or brittle failure here we present a series of high temperature uniaxial deformation experiments designed to investigate the effect of bubbles on the magma bulk viscosity the starting materials are cores of natural rhyolitic obsidian synthesized to have variable vesicularity 0 66 the foamed cores were deformed isothermally t 750 c at atmospheric conditions using a high temperature uniaxial press under constant displacement rates strain rates between 0 5 1 10 4 s 1 and to total strains of 10 40 the viscosity of the bubble free melt 0 was measured by micropenetration and parallel plate methods to establish a baseline for experiments on the vesicle rich cores at the experimental conditions rising vesicle content produces a marked decrease in bulk viscosity that is best described by a two parameter empirical equation log 10 bulk log 10 0 1 47 1 0 48 our parameterization of the bubble melt rheology is combined with maxwell relaxation theory to map the potential onset of non newtonian behavior shear localization in magmas as a function of melt viscosity vesicularity and strain rate for low degrees of strain i e as in our study the rheological properties of vesicular magmas under different flow types pure vs simple shear are indistinguishable for high strain or strain rates where simple and pure shear viscosity values may diverge our model represents a maximum boundary condition vesicular magmas can behave as non newtonian fluids at lower strain rates than unvesiculated melts thereby promoting shear localization and explosive or non explosive magma fragmentation the extent of shear localization in magma influences outgassing efficiency thereby affecting magma ascent and the potential for explosivity

time scales of compaction in volcanic systems we used 12 high temperature deformation experiments on fabricated cores of rhyolite ash to explore the effect of porosity on the rheology of pyroclastic deposits during deformation the cores of ash accommodate strain mainly by shortening and reduction of porosity porosity loss creates a strain dependent rheology expressed by a marked and continuous increase in effective viscosity the effective viscosity graphic 1 of pyroclastic materials is predicted as a function of porosity graphic 2 and melt viscosity graphic 3 o by graphic 4 the optimal value for the parameter based on the experimental data is 0 78 0 15 these experiments provide constraints on the time scales of compaction and flow processes in volcanic systems under variable conditions i e temperature load stress our results indicate that welding and compaction processes in pyroclastic deposits e g ignimbrites conduits can occur on time scales of tens of minutes to hours at these time scales welding is fully decoupled from cooling of the deposits and may be coupled to depositional processes within volcanic conduits welding processes operate on fragmented magma rapidly enough to eliminate porosity in a matter of hours thereby sealing off permeability and supporting cyclical vulcanian style explosive eruptions 1 embed inline graphic 1 gif 2 embed inline graphic 2 gif 3 embed inline graphic 3 gif 4 embed inline graphic 4 gif

the java margin revisited evidence for subduction erosion off java the eastern sunda margin off indonesia from central java to sumba island remains a little investigated subduction zone contrary to its well studied northwestern segment whereas large portions of the sunda margin are considered a classical accretionary zone subduction characteristics along the central java sector indicate erosive processes as the dominant mode of mass transfer the tectonic framework of the central java margin with a convergence rate of 6 7 cm yr insignificant sediment input and a pronounced seafloor roughness where the oceanic roo rise is subducting underneath java facilitates subduction erosion evidence for erosion comes from newly acquired geophysical data off central java local erosive processes in the wake of seamount subduction are documented by a high resolution bathymetric survey and result in an irregular trend of the deformation front sculpted by seamount collision scars subduction of oceanic basement relief leads to large scale uplift of the forearc as recorded on a reflection seismic profile and to a dismemberment of the previous outer forearc high giving way to isolated topographic elevations the broad retreat of the java trench and deformation front above the leading edge of the roo rise has exposed an area of approximately 25 000 km2 of deeper seafloor formerly covered by the previous frontal prism frontal erosion coincides with a steepening of the lower slope angle in the central java sector compared to the neighbouring segments in global compilations the key geological parameters of the central java margin lie in the erosive regime reflecting the interplay of basement relief subduction negligible sediment supply and a high convergence rate on the evolution of the margin

a general model for welding of ash particles in volcanic systems validated using in situ x ray tomography welding occurs during transport and deposition of volcanic particles in diverse settings including pyroclastic density currents volcanic conduits and jet engines welding rate influences hazard relevant processes and is sensitive to water concentration in the melt we characterize welding of fragments of crystal free water supersaturated rhyolitic glass at high temperature using in situ synchrotron source x ray tomography continuous measurement of evolving porosity and pore space geometry reveals that porosity decays to a percolation threshold of 1 3 vol at which bubbles become isolated and welding ceases we develop a new mathematical model for this process that combines sintering and water diffusion which fits experimental data without requiring empirically adjusted parameters a key advance is that the model is valid for systems in which welding is driven by confining pressure surface tension or a combination of the two we use the model to constrain welding timescales in a wide range of volcanic settings we find that volcanic systems span the regime divide between capillary welding in which surface tension is important and pressure welding in which confining pressure is important our model predicts that welding timescales in nature span seconds to years and that this is dominantly dependent on the particle viscosity or the evolution of this viscosity during particle degassing we provide user friendly tools written in python and in excel to solve for the evolution of porosity and dissolved water concentration during welding for user defined initial conditions

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

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

bulk and particle strain analysis in high temperature deformation experiments experimental data alone are not sufficient to describe the rheology of deformed geomaterials to fully characterize a material s rheological properties independent verification of deformation mechanisms is required here we use standard image analysis techniques to semi quantify the physical changes in experimentally deformed cores of soda lime silica glass beads and rhyolite ash previously described by quane and russell quane s l russell j k 2005a welding insights from high temperature analogue experiments j volcanol geotherm res 142 67 87 the properties we measure by image analysis include porosity radial bulging and particle elongation the image analysis measurements combined with digital output from the experiments allow us to determine the amount of total axial and radial strain accumulated by the bulk sample eb and by individual particles ep we demonstrate that these metrics of strain are nearly equal to the one dimensional strain recorded by the deformation apparatus em and sample shortening es confirming that all strain introduced by the deformation apparatus is being transferred into both the bulk sample and individual particles we also show that eb is manifest as two discrete components axial ea and radial er strain we use these independent components of strain accumulation to show that despite having nearly identical strain time and stress strain deformation paths glass bead cores and rhyolite ash cores have strikingly different mechanisms of strain accumulation in the higher porosity rhyolite ash cores axial strain dominates implying that under the conditions present natural glassy particulate geomaterials deform almost entirely by porosity loss

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

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

nonisothermal viscous sintering of volcanic ash volcanic ash is often deposited in a hot state volcanic ash containing glass deposited above the glass transition interval has the potential to sinter viscously both to itself particle particle and to exposed surfaces here we constrain the kinetics of this process experimentally under nonisothermal conditions using standard glasses in the absence of external load this process is dominantly driven by surface relaxation in such cases the sintering process is rate limited by the melt viscosity the size of the particles and the melt vapor interfacial tension we propose a polydisperse continuum model that describes the transition from a packing of particles to a dense pore free melt and evaluate its efficacy in describing the kinetics of volcanic viscous sintering we apply our model to viscous sintering scenarios for cooling crystal poor rhyolitic ash using the 2008 eruption of chaiten volcano as a case example we predict that moderate linear cooling rates of 0 1 c min 1 can result in the common observation of incomplete sintering and the preservation of pore networks

welding processes in volcanology insights from field experimental and modeling studies this volume is a collection of ten papers largely derived from a special session welding processes in volcanology organized for the 2003 meeting of eug egs agu in nice france and brings together field experimental and modeling studies we briefly review advances in understanding welding as represented by this volume and point to future lines of inquiry with an emphasis on understanding the timescales of welding the metrics of strain and feedback relationships between compaction porosity permeability loss fluid transport and rheology of the deposit our analysis suggests that within the realm of field and textural studies more systematic and quantitative data are necessary to evaluate strain histories related to welding and the timing of welding relative to other processes during eruption and emplacement a new generation of experiments is needed in which load strain rate and volatile pressure can be controlled numerical models are needed to further explore the relationships and feedbacks between processes that contribute to welding d 2004 elsevier b v all rights reserved

the strength of heterogeneous volcanic rocks a 2d approximation abstract volcanic rocks typically contain heterogeneities in the form of crystals and pores we investigate here the influence of such heterogeneity on the strength of volcanic rocks using an elastic damage mechanics model in which we numerically deform two dimensional samples comprising low strength elements representing crystals and zero strength elements representing pores these circular elements are stochastically generated so that there is no overlap in a medium representing the groundmass our modelling indicates that increasing the fraction of pores and or crystals reduces the strength of volcanic rocks and that increasing the pore fraction results in larger strength reductions than increasing the crystal fraction the model also highlights an important weakening role for pore diameter but finds that crystal diameter has a less significant influence for strength to account for heterogeneity pores and crystals we propose an effective medium approach where we define an effective pore fraction p v p v p v g where v p and v g are the pore and groundmass fractions respectively highly heterogeneous samples containing high pore and or crystal fractions will therefore have high values of p and vice versa when we express our numerical samples more than 200 simulations spanning a wide range of crystal and pore fractions in terms of p we find that their strengths can be described by a single curve for a given pore diameter to provide a predictive tool for the strength of heterogeneous volcanic rocks we propose a modified version of 2d solution for the sammis and ashby 1986 pore emanating crack model a micromechanical model designed to estimate strength using microstructural attributes such as porosity pore radius and fracture toughness the model reformulated to include p and therefore crystal fraction captures the strength curves for our numerical simulations over a sample heterogeneity range relevant to volcanic systems we find that published experimental data for which the porosity and crystal fraction is known correlate well with our modelled curves adding validity to our approach we present herein a 2d analytical tool to estimate the strength of volcanic rock that can be used when the porosity crystal fraction and maximum pore diameter are known

seamount subduction erosion in the nankai trough and its potential impact on the seismogenic zone seamount subduction along subduction zone plate boundary thrusts has long been implicated as a mechanism for abrasion and tectonic erosion of the base of the overriding plate however tectonic erosion processes have not been examined in detail with high quality three dimensional 3 d seismic reflection imaging in 1999 we acquired 3 d seismic reflection data from the nankai trough subduction zone to image the plate boundary fault and the overlying accretionary wedge structure fortuitously these data reveal a small to 1 km high basement ridge that has subducted to 7 km subseafloor updip from the basement ridge a 1 km thick sequence of sediment from the base of the accretionary wedge appears to be missing we interpret these data as evidence for tectonic erosion of the base of the accretionary wedge following the basement ridge subduction tectonic erosion has removed more than 25 km 3 from the updip edge of the seismogenic zone and carried it down into the seismogenic zone the tectonically eroded sediments are presumed to enhance fault zone fluid content potentially reducing fault zone effective stress and may temporarily inhibit earthquake rupture potential after the passage of the ridge the boundary fault returns to its former position in a period of enhanced underplating

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

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

origin and tectonic evolution of the caroline ridge and the sorol trough western tropical pacific from admittance and a tectonic modeling analysis abstract an independent caroline sea plate surrounded by prominent seismotectonic features is said to exist in the western tropical pacific the caroline sea plate is a transient feature existing only when the caroline philippine sea plate boundary is jumping to the caroline pacific boundary the origin and the tectonic evolution of the caroline ridge sorol trough region which forms the northern part of the caroline pacific plate boundary are examined using a standard admittance analysis a tectonic modeling study and additional direct gravity and magnetic analyses the observed admittance function is fitted well by shallow airy model in the 250 125 km 0 025 k 1 wavelength region whose average crustal thickness t c is 20 km and also by thin plate flexure model of isostasy within the diagnostic waveband of 250 80 km 0 025 k 1 with an effective elastic thickness t e of 5 7 km a simple interpretation of the observed admittance function is that the local airy model of isostasy is the main support mechanism in the study region for the longer wavelength more fundamental topography corresponding to that of the caroline ridge the caroline ridge is inferred to have formed in a near ridge tectonic setting on top of oceanic lithosphere which was young thin and weak at the time of emplacement the tectonic model study after an explanation for rifting along the mendana fracture zone in the nazca plate suggests that the sorol trough formed in the 17 7 ma interval splitting the caroline ridge into two longitudinal halves the rifting along the sorol trough which took place in the manner described in weissel and karner 1989 flexural uplift of rift flanks due to mechanical unloading of the lithosphere during extension j geophys res 94 13 919 13 950 necking of unbroken lithosphere post dated the formation of the caroline ridge by 10 15 m y during which time t e in the region increased to 5 10 km the rifting necking along the sorol trough represents a second loading event in the region the combined gravity effects of the rifting as well as the increase in the t e in the region from essentially nil to 5 10 km is reflected in the agreement between the observed admittance and the regional model with a 5 7 km t e in the wavelength region 80 km these results are consistent with the findings and the suggestions of the previous investigators in the region in that 1 the caroline ridge was constructed in the late oligocene by hot spot volcanism on top of oligocene seafloor of the caroline sea back arc basin 2 the eauripik rise genetically related to the caroline ridge formed on young and weak caroline basin lithosphere and is supported by airy manner crustal thickening and that 3 the lithosphere of the caroline ridge was young at the time of rifting necking along the sorol trough on the basis of the current results and the findings of the previous workers a tentative hypothetical rheological model for the crustal density structure of the caroline ridge sorol trough is also given the caroline sea plate appears to be bounded in the north and south by e w trending boundaries that deform diffusely while more normal narrow n s trending boundaries exhibiting more localized albeit slow deformation bound the plate in the east and west

the role of melt fracture degassing in defusing explosive rhyolite eruptions at volcan chaiten explosive volcanic eruptions of silicic magma often evolve towards non explosive emissions of lava the mechanisms underlying this transition remain unclear however a widely cited idea holds that shear induced magma fragmentation plays a critical role by fostering volatile loss from fragmentary magma and through ash filled cracks termed tuffisite we test this hypothesis by measuring h2o concentrations within glassy tuffisite from the 2008 2011 rhyolitic eruption at volcan chaiten chile we show that while h2o concentrations decrease next to tuffisite veins and at the margins of obsidian fragments the depletions cannot account for the disparity in h2o between explosively and effusively erupted rhyolite tuffisite vein lifetimes derived from diffusion modeling min to h imply degassing rates that are too slow to effectively degas magma unless the magma was entirely fragmented to mm or smaller particles this level of brecciation may locally develop near conduit margins but is unrealistic for entire conduits the primary role of melt fracturing may therefore be to provide gas escape pathways for more efficient degassing of permeable vesicular magma in the conduit interior

from seamount accretion to tectonic erosion formation of osa melange and the effects of cocos ridge subduction in southern costa rica 1 the costa rica portion of the middle america trench mat is characterized by active tectonic erosion a process that causes the removal of material from the base of the upper plate as the plate boundary migrates upward offshore studies demonstrate accelerated subduction erosion starting at the miocene pliocene boundary as the result of subduction of thickened galapagos related crust as presently represented by the cocos ridge the subduction of the cocos ridge also caused uplift and exposure of the outer forearc on the osa peninsula which offers a window to explore the tectonic evolution of the area the rocks outcropping on osa peninsula are a middle eocene middle miocene melange dominated by basalt chert and limestone resulting from accretion of seamounts the accretion dominated period of the mat evolution ended at the miocene pliocene boundary when thickened crust a paleo cocos ridge produced at the galapagos hot spot arrived at the trench the thick crust caused uplift and severe tectonic erosion of the accretionary edifice allowing exhumation of the osa melange the change from accretion to erosion caused the outer forearc to be offset along subvertical faults that define small kilometer to tens of kilometers size blocks that are going through differential vertical movements in response to the morphology of the subducting ridge subduction accretion and erosion are two processes that can alternate in time and space or coexist along the same margin so that mass removal can develop on a previously growing margin and completely remove an accretionary prism

subduction of a late cretaceous seamount of the louisville ridge at the tonga trench a model of normal and accelerated tectonic erosion the hotspot generated louisville ridge is a 4000 km chain of seamounts typically 2 2 5 km high and 10 40 km in diameter and an underlying crustal swell 1 5 km high and 100 km wide trending nnw across the southwestern pacific the northwest end of the ridge collides with the north trending tonga trench 26 s which just north of that point is exceptionally deep 10 8 km and lacks both a turbidite wedge and a bordering accretionary complex the collision zone is moving rapidly southward multichannel seismic reflection data in the collision zone show a west dipping subsurface platform 2 3 km beneath the lower western trench slope which is interpreted as the flat summit of a subducted guyot motuku of the louisville chain projected eastward the summit plain passes 1 2 km above the trench floor dredging of the nearby inner trench wall recovered uppermost cretaceous maestrichtian oceanic pelagic sediments interpreted to be fragments of the sedimentary mantle of a subducted louisville seamount the principal effects of hotspot ridge collision with a sediment starved trench are 1 the impacting seamounts are subducted rather than accreted and 2 although some seamount rocks are temporarily accreted the inner trench wall is tectonically eroded arcward at rates possibly as high as 50 km m y accelerated tectonic erosion is related to 1 fracturing shearing and general weakening of arc substrate rocks as they are lifted by the swell penetrated by impacting seamounts and left to collapse as the ridge moves away 2 a more effective removal of weakened rock in underthrusting grabens which are larger in the crustal swell 3 a possible elevation of the subduction decollement to account for the removal of as much as 30 000 km of material from a 400 km sector of the trench in 1 million years and 4 a reduction in supply of arc derived debris resulting from the gap in arc volcanism accompanying subduction of the ridge normal tectonic erosion in the tonga trench is apparently minor and we conclude that the bulk of the 37 000 km of material which fills subducting grabens each million years is arc derived volcanic and pelagic sediment

evidence for welding of a block and ash pyroclastic flow deposit the case of cerro bravo volcano colombia cerro bravo is an active composite volcano located in the san diego cerro machin volcano tectonic province scvtp the northernmost volcanic chain in the colombian andes among the products associated with the volcano there is a peculiar indurated deposit emplaced between 7 and 2 9 ka which resembles a block and ash pyroclastic flow deposit through field observations physical property analyses petrography and scanning electron microscopy sem we demonstrate that this induration is the result of welding processes high density deformed glassy fragments with a common orientation low porosity fiamme and sintering are the most visible characteristics the evidence indicates that welding reached up to rank iv in a i to vi classification scheme we suggest that welding is associated with 1 high emplacement temperature related to the syneruptive destruction of a growing endogenous dome and 2 a low glass transition temperature related to the influence of upward water diffusion after emplacement of the flow over wet ground thus with emplacement temperatures between 670 and 540 c and water contents between 0 5 and 1 2 wt the welding process was unusually efficient calculations made indicate that for the deposit to present the observed characteristics of welding the emplacement temperature needed to exceed the threshold of the glass transition temperature for a time of 5 6 days and have an effective viscosity of 1012 pa s as a result of the welding the deposit decreased its thickness by between 15 and 7 m this work contributes to the knowledge of welding processes and stresses the unusual occurrence of welding in this type of pyroclastic flow deposit which thus requires special emplacement conditions in terms of temperature and water content