contrasting styles of welding observed in the proximal askja 1875 eruption deposits i regional welding abstract as an alternative to classical welding models of fall deposits due to the progressive accumulation of hot tephra which then weld we describe here welded deposits on the northern 1875 caldera rim of askja volcano that have welded due to the influence of hot discrete spatter bombs impacting into and supplying heat to a halo of surrounding tephra this style of welding we term local welding in contrast to regional welding which is described elsewhere carey r j houghton b f thordarson t 2008 contrasting styles of welding observed in the proximal askja 1875 eruption deposits i regional welding j volcanol geotherm res 171 1 19 doi 10 1016 j jvolgeores 2007 11 020 locally welded deposits are associated with the rhyolitic plinian phase of the 1875 eruption of askja volcano two distinct welding units w1 and w2 are interbedded with plinian fall on the northern caldera rim and grade outwards to weakly dispersed non welded fall spatter bombs are found in both welding units but vary in their characteristic sizes and internal features in the w1 unit simple bombs with homogeneous internal characteristics up to 60 cm in diameter are found in the w2 unit large discrete spatter bombs with complex internal features range up to 9 m in diameter we describe here two case studies showing the effects of a single small spatter bombs b multiple small spatter bombs and c large discrete spatter bombs varying in size vertical and lateral profiles through welding zones reveal that the primary controls on local welding are the availability of supplied or added heat and the loading capacity of the spatter bomb local welding grades are much higher than that of regional welding as the combined effects of heat compaction and insulation can provide suitable conditions which lead to dense welding and proximal to the spatter bomb rheomorphic flowage if heating and loading exceed the critical requirement for welding porosity loss via matrix welding and vesicle collapse occurs to a point where further strain must be accommodated as shearing and ductile flowage the spatter bombs are found only within the weakly dispersed welding units and are the final erupted products of each fountaining phase their low viscosities are evident by their deformation on impact and fluidal forms and hold some important clues to eruption dynamics in the shallow conduit and vent regions

crystallization and welding variations in a widespread ignimbrite sheet the rattlesnake tuff eastern oregon usa the 7 05 ma rattlesnake tuff covers ca 9000 km2 but the reconstructed original coverage was between 30000 and 40000 km2 thicknesses are remarkably uniform ranging between 15 and 30 m for the most complete sections only 13 of the area is covered with tuff thicker than 30 m to a maximum of 70 m the present day estimated tuff volume is 130 km3 and the reconstructed magma volume of the outflow is 280 km3 dre dense rock equivalent the source area of the tuff is inferred to be in the western harney basin near the center of the tuff distribution based mainly on a radial exponential decrease in average pumice size and is consistent with a general radial decrease in welding and degree of post emplacement crystallization rheomorphic tuff is found to a radius of 40 60 km from the inferred source four facies of welding and four of post emplacement crystallization are distinguishable they are non welded incipiently welded partially welded and densely welded zones and vapor phase pervasively devitrified spherulite and lithophysae zones the vapor phase pervasively devitrified and lithophysae zones are divided into macroscopically distinguishable subzones at constant thickness 20 3 m and over a distance of 1 3 km nonrheomorphic sections can cary between two extremes a entirely vitric sections grading from nonwelded to incipiently welded and b highly zoned sections highly zoned sections have a basal non to densely welded vitric tuff overlain by a spherulite zone that grades upward through a lithophysae dominated zone to a zone of pervasive devitrification which in turn is overlain by a zone of vapor phase crystallization and is capped by partially welded vitric tuff a three dimensional welding and crystallization model has been developed based on integrating local and regional variations of 85 measured sections strong local variations are interpreted to be the result of threshold governed welding and crystallization controlled by residence time above a critical temperature which is achieved through differences in thickness and accumulation rate

fiamme degassing structures and their implications for the post emplacement temperatures and h2o contents of high grade ignimbrites abstract a new process in post emplacement ignimbrite degassing is proposed from observations of unusual structures in a high grade ignimbrite in sardinia the structures consist of fiamme decorated above and below by vesicles termed here fiamme degassing structures fds the relative volumes of vesicles and fiamme have a positive linear correlation and the ratio between the volume of vesicles above and below fiamme asymmetry ratio is 1 wt at

hillslope storage boussinesq model for subsurface flow and variable source areas along complex hillslopes 2 intercomparison with a three dimensional richards equation model 1 the boussinesq equation for subsurface flow in an idealized sloping aquifer of unit width has recently been extended to hillslopes of arbitrary geometry by incorporating the hillslope width function w x into the governing equation where x is the flow distance along the length of the hillslope troch et al 2003 introduction of a source sink term n allows simulation of storm interstorm sequences in addition to drainage processes while a function sc x representing the maximum subsurface water storage can be used to account for surface saturation response in variable source areas activated by the saturation excess mechanism of runoff generation the model can thus simulate subsurface flow and storage dynamics for nonidealized more realistic hillslope configurations in this paper we assess the behavior of this relatively simple one dimensional model in a series of intercomparison tests with a fully three dimensional richards equation model special attention is given to the discretization and setup of the boundary and initial conditions for seven representative hillslopes of uniform convergent and divergent plan shape drainage and recharge experiments are conducted on these hillslopes for both gentle 5 and steep 30 bedrock slope angles the treatment and influence of the drainable porosity parameter are also considered and for the uniform idealized hillslope case the impact of the unsaturated zone is examined by running simulations for different capillary fringe heights in general terms the intercomparison results show that the hillslope storage boussinesq model is able to capture the broad shapes of the storage and outflow profiles for all of the hillslope configurations in specific terms agreement with the richards equation results varies according to the scenario being simulated the best matches in outflow hydrographs were obtained for the drainage experiments suggesting a greater influence of the unsaturated zone under recharge conditions due to transmission of water throughout the hillslope in the spatiotemporal water table response a better match was observed for convergent than divergent hillslopes and the bedrock slope angle was not found to greatly influence the quality of the agreement between the two models on the basis of the intercomparison experiments we make some suggestions for further development and testing of the hillslope storage model

recharge induced groundwater flow over a plane sloping bed solutions for steady and transient flow using physical and numerical models 1 the free surface profile and outflow hydrograph for groundwater under conditions of steady uniform recharge followed by recession have been studied in viscous fluid model tests using a sloping bed with a gradient of 0 2 the data have been compared with the nonlinear boussinesq model and a modification of that model simulating the outflow seepage surface obtained from a finite difference solution of the free boundary problem it is shown that for a given bed slope there is close to a linear relationship between outflow and storage raised to a power n where n ranges from almost 2 for zero slopes to just above 1 at a gradient of 0 3 there is also a simple relationship between outflow and storage in the final stage of recession when a drainage front occurs on the bed these results lead to simple algorithms for prediction of the outflow under unsteady recharge conditions

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

exhumed upper mantle within the ocean continent transition on the northern west iberia margin evidence from prestack depth migration and total tectonic subsidence analyses 1 the west iberia continental margin contains an enigmatic north south ridge of serpentinized peridotite located within the ocean continent transition we used prestack depth migration and total tectonic subsidence analysis to constrain the basement type adjacent to the ridge in order to learn more about its emplacement prestack depth migration was performed on segments of the nine east west lines from the iberia seismic experiment ise 97 and total tectonic subsidence analyses were performed on the depth sections the seismic data and subsidence analyses are consistent with exhumed upper mantle occurring immediately landward of the ridge suggesting that the ridge is located within a broad zone of exhumed mantle that has been serpentinized the subsidence analyses also provide minimum bounds on the thickness of the serpentinized layer which extends from the zone of exhumed mantle landward under thinned continental crust exhumed upper mantle has previously been identified in the wider ocean continent transition to the south in the southern iberia abyssal plain and we conclude that similar but narrower exposures occur to the north in the galicia bank area while upper mantle appears to have been tectonically exhumed along all the ise 97 lines it does not always form a distinct basement ridge there is no well defined ridge on three seismic profiles in the middle of the ise 97 survey where it is well developed the peridotite ridge parallels a deeply penetrating west dipping normal fault and the three middle lines appear to represent a transfer zone where this normal fault is not well developed

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

rheology of porous volcanic materials high temperature experimentation under controlled water pressure abstract we present a suite of 21 high temperature uniaxial deformation experiments performed on 25 by 50 mm unjacketed cores of porous 0 8 sintered rhyolitic ash the experiments were performed at both atmospheric dry and elevated water pressure conditions wet experiments used a constant displacement rate of 2 5 10 6 m s 1 corresponding to a strain rate o of 5 10 5 s 1 a single experiment was run at 2 5 10 5 m s 1 o 5 10 4 s 1 dry experiments were conducted mainly at 900 c but also included a suite of lower temperature experiments at 850 800 and 750 c wet experiments were performed at 650 c under water pressures of 1 2 5 and 5 mpa and at a fixed p h 2 o of 2 5 mpa for temperatures of 385 450 and 550 c during deformation strain is manifest by shortening of the cores reduction of porosity flattening of ash particles and radial increase of the cores the continuous reduction of porosity leads to a dynamic transient strain dependent rheology and requires strain to be partitioned between a volume porosity loss and a shear radial increase component these data demonstrate the effect of porosity on the rheology of dry and hydrous melts the effect of increasing porosity is to expand the window for ductile deformation for dry melts by delaying the onset of brittle deformation by 50 c 875 c to 825 c the effect is more pronounced in hydrous melts 0 67 0 78 wt h 2 o where the ductile to brittle transition is depressed by 140 to 150 c increasing water pressure also delays the onset of strain hardening due to compaction driven porosity reduction these rheological data are pertinent to any volcanic processes involving high temperature porous magmas e g magma flow in conduits welding of pyroclastic materials

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

deposition of rheomorphic ignimbrite d mogan formation gran canaria canary islands spain rheomorphic ignimbrite d 13 4 ma upper mogan formation on gran canaria a multiple flow single cooling unit is divided into four major structural zones that differ in fabric and finite strain of deformed pyroclasts their structural characteristics indicate contrasting deformation mechanisms during rheomorphic flow the zones are a a basal zone vitrophyre with pure uniaxial flattening perpendicular to the foliation b an overlying shear zone characterized by asymmetric fabrics and a significantly higher finite strain with an ellipsoid geometry similar to stretched oblate bodies c a central zone with a finite strain geometry similar to that of the underlying shear zone but without evidence of a rotational strain component and d a slightly deformed to non deformed top zone where the almost random orientation of subspherical pyroclasts suggests preservation of original syn depositional clast shapes rheomorphic flow in d is the result of syn to post depositional remobilization of a hot pyroclastic flow as shown by kinematic modeling based on a the overall vertical structural zonation suggested by finite strain and fabric analysis b the relation of shear sense to topography c the interrelationship of the calculated vertical cooling progression at the base of the flow formation of vitrophyre and the related vertical changes in strain geometry d the complex lithification history and e the consequent mechanisms of deformational flow rheomorphic flow was caused by load pressure due to an increase in the vertical accumulation of pyroclastic material on a slope of generally 6 8 we suggest that every level of newly deposited pyroclastic flow material of d first passed through a welding process that was dominated by compaction pure flattening before rheomorphic deformation started

boiling over dense pyroclastic density currents during the formation of the 100 km 3 huichapan ignimbrite in central mexico stratigraphic and lithofacies analysis abstract a lithofacies analysis of the huichapan ignimbrite has been undertaken to evaluate its depositional history from large pyroclastic density currents the huichapan ignimbrite is a massive ignimbrite sheet with a maximum runout of at least 55 km and thickness variations between 6 and 80 m the lower portion of the huichapan ignimbrite consists of a large plateau 100 km3 69 km3 as dense rock equivalent dre of massive ignimbrites with welding variations from densely welded to partly welded devitrification and high temperature vapor phase alteration the lower part grades laterally to moderately welded and non devitrified ignimbrites these variations are interpreted as the sedimentation of density stratified pyroclastic density currents erupted as boiling over pulses from the huichapan donguinyo caldera complex at a continuous rate supporting deposition by quasi steady progressive aggradation of sustained and hot currents to the north of the caldera the lower portion of the ignimbrite consists of a small plateau

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

variations in welding characteristics within the plinian air fall deposit of the middle pumice eruption santorini greece abstract the welded plinian air fall deposit of the middle pumice a mp a eruption santorini greece 144 6 ka was analysed in order to document welding characteristics and determine the factors that control welding due to the sintering of hot ash to block sized pyroclasts there are vertical and lateral variations in welding intensity with welding increasing upwards within deposit sections and decreasing laterally away from the source four welding grades are distinguished a densely welded b slightly welded c tack welded and d non welded lateral welding zones are defined by the highest welding grade observed zone a densely welded 3 7 km pumice density increases and porosity decreases with welding with the most densely welded part having a density of 2290 kg m 3 and a porosity of 5 distal non welded pumices have a density of 370 kg m 3 and porosities of 75 clast oblateness varies from 0 79 in the densely welded proximal deposit sections to 0 54 in the distal non welded deposits in southern thera strain determined using the rf method indicates that the deposit is moderately flattened but relatively undeformed the mp a deposit is dacitic to andesitic in composition becoming more mafic with stratigraphic height where the degree of welding is highest welding is controlled by geochemical variations compactional load and local variations in accumulation rate and clast sizes

experimental and textural investigation of welding effects of compaction sintering and vapor phase crystallization in the rhyolitic rattlesnake tuff abstract the abrupt changes in character of variably welded pyroclastic deposits have invited decades of investigation and classification we conducted two series of experiments using ash from the nonwelded base of the rhyolitic rattlesnake tuff of oregon usa to examine conditions of welding one series of experiments was conducted at atmospheric pressure 1 at in a muffle furnace with variable run times and temperature and another series was conducted at 5 mpa and 600 c in a cold seal apparatus with variable run times and water contents we compared the results to a suite of incipiently to densely welded natural samples of the rattlesnake tuff experiments at 1 at required a temperature above 900 c to produce welding which is in excess of the estimated pre eruptive magmatic temperature of the tuff the experiments also yielded globular clast textures unlike the natural tuff during the cold seal experiments the gold sample capsules collapsed in response to sample densification textures and densities that closely mimic the natural suite were produced at 5 mpa 600 c and 0 4 wt h2o over run durations of hours to 2 days clast deformation and development of foliation in 2 week runs were greater than in natural samples both more and less water reduced the degree of welding at otherwise constant run conditions for 5 mpa experiments changes in the degree of foliation of shards and of axial ratios of bubble shards and non bubble mainly platy shards are consistent with early densification related to compaction and partial rotation of shards into a foliation subsequent densification was associated with viscous deformation as indicated by more sintered contacts and deformation of shards sintering local fusion of shard shard contacts was increasingly important with longer run times higher temperatures and greater pressures during runs with high water concentrations sintering was rare and adhesion between clasts was dominated by precipitation of sublimates in pore spaces a few tenths wt h2o in the rhyolite glass promote the development of welding by sharp reduction of glass viscosity large amounts of water inhibit welding by creating surface sublimates that interfere with sintering and may exert fluid pressure counter to lithostatic load if sintering and vapor phase sublimates seal permeability in the tuff

cooling degassing and compaction of rhyolitic ash flow tuffs a computational model previous models of degassing cooling and compaction of rhyolitic ash flow deposits are combined in a single computational model that runs on a personal computer the model applies to a broader range of initial and boundary conditions than riehle s earlier model which did not integrate heat and mass flux with compaction and which for compound units was limited to two deposits model temperatures and gas pressures compare well with simple measured examples the results indicate that degassing of volatiles present at deposition occurs within days to a few weeks compaction occurs for weeks to two to three years unless halted by devitrification near emplacement temperatures can persist for tens of years in the interiors of thick deposits even modest rainfall significantly chills the upper parts of ash deposits but compaction in simple cooling units ends before chilling by rainwater influences cooling of the interior of the sheet rainfall does however affect compaction at the boundaries of deposits in compound cooling units because the influx of heat from the overlying unit is inadequate to overcome heat previously lost to vaporization of water three density profiles from the matahina ignimbrite a compound cooling unit are fairly well reproduced by the model despite complexities arising from numerous cooling breaks uncertainties in attempts to correlate in detail among the profiles may be the result of the non uniform distribution of individual deposits regardless it is inferred that model compaction is approximately valid thus the model should be of use in reconstructing the emplacement history of compound ash deposits for inferring the depositional environments of ancient deposits and for assessing how long deposits of modern ash flows are capable of generating phreatic eruptions or secondary ash flows

viscosity of a teide phonolite in the welding interval abstract the viscosity of a natural phonolitic composition with variable amounts of h2o has been experimentally determined the starting materials were crystal free phonolitic glasses from montana blanca situated within the las canadas caldera of teide dry phonolitic melt viscosities were determined using concentric cylinder viscometry in the low viscosity range the glassy quench products of these runs were then hydrated by high pressure synthesis in a piston cylinder apparatus to generate a suite of samples with water contents ranging from 0 02 to 3 75 wt samples thus hydrated were quenched rapidly and prepared cut and polished for the determination of water contents by infrared spectroscopy before and after experimental viscometry the viscosities of the melts dry and hydrated were determined at 1 bar using a micropenetration technique samples were stable under the measurement conditions up to 3 75 wt h2o homogeneity of water content was confirmed by infrared spectroscopy and total water contents were calculated using absorptivity coefficients for compositions extremely close to that investigated here the variation of viscosity as a function of water content and temperature can be described in the high viscosity interval of relevance to many welding processes by the non arrhenian expression 1 log 10 5 900 0 286 ln h 2 o 10775 4 394 8 h 2 o t 148 7 21 65 ln h 2 o whereas the high viscosity range alone is adequately described by the arrhenian expression 2 log 10 10 622 0 738 ln h 2 o 17114 3 590 4 h 2 o 1 t where is the viscosity in pa s h2o is the water content in wt and t is the temperature in k these results are particularly useful for the scaling of conditions extant during the welding of phonolitic products of montana blanca the welding of glassy phonolitic rocks is enhanced by the lower viscosity of these melts with respect to calcalkaline rhyolites the ratio of viscosities of phonolitic to calcalkaline rhyolitic melts is a complex function of temperature and water content and reaches up to 104 5 at 0 1 wt h2o and 500 c abundant evidence of welding and remobilisation of pyroclastic and spatter products of teide system volcanism are consistent with these experimental observations

cooling dynamics of spatter fed phonolite obsidian flows on tenerife canary islands abstract relaxation geospeedometry has been applied to two series of clastogenic obsidian flows on tenerife to determine their thermal history across the glass transition the phonolite flows investigated were both generated by lava fountaining activity followed by rheomorphism of the deposits the detailed sampling resolution within the two series enabled an accurate quantification of their thermal history cooling rates within the investigated spatter fed flows vary over more than two orders of magnitude the highest cooling rates of 0 39 k min were modeled for the central vesiculated part of one flow the dense basal obsidian layers of both flows were cooled at substantially lower rates of 0 0042 and 0 0028 k min respectively there appears to be an influence of in situ vesiculation processes on the thermal budget of the investigated flows in addition the slow cooling rates for the basal portions of both flows seem to be associated with a stage of thermal buffering continual advective heat transport of hot material along a basal shear plane may sustain elevated temperatures associated with quasi isothermal annealing within this decollement numerical simulations based on conductive heat loss concepts fail to resolve the cooling history quantified through relaxation geospeedometry for the investigated flows the effects of vesiculation and thermal annealing on the cooling behavior of the clastogenic flows across the glass transition are discussed in the light of these new data in addition viscometric data on these phonolites are used to correlate the known cooling rates to viscosities at the glass transition

storage dependent drainable porosity for complex hillslopes 1 in hydraulic groundwater theory the parameter drainable porosity f a storage coefficient that accounts for the effect of the unsaturated zone on water table dynamics is usually treated as a constant for shallow unconfined aquifers the value of this parameter however depends on the depth to the water table and the water retention characteristics of the soil in this study an analytical expression for f as a function of water table depth is derived under the assumption of quasi steady state hydraulic equilibrium in this way accounting in part for the effects of the unsaturated zone on groundwater dynamics the derived expression is implemented in the nonlinear hillslope storage boussinesq hsb model troch et al 2003 to simulate the drainage response of complex hillslopes the model s behavior is analyzed by comparison to 1 the hsb model with a constant value for f and 2 measurements of water tables and outflow hydrographs on a 6 0 2 5 0 5 m laboratory hillslope experiment the comparison is conducted for a pure drainage case on two different hillslope shapes linearly convergent and divergent and for three different slope inclinations 5 10 and 15 comparison 1 is run in an uncalibrated and a fully calibrated mode and it enables us to evaluate the effect of a dynamic state dependent value for f on model output comparison 2 allows us to test the hsb model on several hillslope configurations and to analyze whether the concept of a storage dependent f enhances the model performance the comparison of the hsb models to the measurements from the laboratory hillslopes shows that it is possible to capture the general features of the outflow hydrograph during a drainage experiment using either one of the hsb models overall the original constant f hsb model with one fitting parameter more than the revised hsb model shows a slightly better fit on the hydrographs when compared to the revised variable f hsb model however the peak outflow values the first few minutes after initiation of the experiments are better captured by the revised hsb model the revised hsb model s performance in simulating water table movements is much more accurate than that of the original hsb model the improved match of the revised hsb model to piezometric measurements is worth stressing because the ability to model water tables is a key attribute of the model making it possible to investigate phenomena such as saturation excess runoff also noteworthy is the good match between the revised hsb model and the outflow measurements without any calibration for the divergent slopes the changing values of the calibrated drainable porosity parameter for the original hsb model as different configurations are simulated slope angle plan shape initial conditions together with the ability of the revised hsb model to more accurately simulate water table dynamics clearly demonstrates the importance of regarding drainable porosity as a dynamic storage dependent parameter

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

textural evidence for high grade ignimbrites formed by low explosivity eruptions parana magmatic province southern brazil abstract the parana etendeka province is a lower cretaceous huge bimodal tholeiitic volcanic province 1 million km 3 that predated the gondwana breakup its silicic portion makes up a total volume of at least 20 000 km 3 and in southern brazil it comprises the chapeco porphyritic high ti trachydacites dacites and the palmas microporphyritic aphyric low ti dacites rhyolites the widespread silicic sheets are debated in the literature because they bear similarities between lavas and high grade ignimbrites here we provide new observations and interpretations for flow units with large dark and vesicle poor lens shaped blobs surrounded by a light colored matrix the textural features macro to micro scale of these blobs are different from typical pumice and or fiamme and support a low explosivity pyroclastic origin possibly low column fountain eruptions with discharge rates high enough to produce laterally extensive high grade ignimbrites such an interpretation combined with a conspicuous absence of lithic fragments in the deposits is aligned with a lack of identified calderas in the parana etendeka province maximum timescales of crystallization associated with the juvenile blobs and estimated from csd slopes are on the order of millennia for phenocryst populations and on the order of decades for microphenocryst populations

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

cooling and welding history of the bishop tuff in adobe valley and chidago canyon california abstract a numerical model for cooling and welding of ash flow tuffs was developed to obtain a quantitative understanding of the welding compaction that occurs during the cooling of ash flow sheets and to evaluate the relative effects of emplacement temperature thickness and emplacement histories on compaction zonation density profiles of large ash flow sheets this model includes a set of two phase compressible navier stokes equations for the transport of mass momentum and energy for a variable density volatile outfluxing through a porous media ash flow tuff with this model we calculated theoretical compaction profiles density vs elevation to match actual data measured in sections of bishop tuff compaction profiles from the adobe basin indicate that there were 3 periods of non deposition 5 10 and 40 days separating 4 distinct rapidly emplaced packets rep of pyroclastic flow deposition in contrast the 3 periods of quiescence 400 20 and 800 days between the 4 reps in chidago canyon were considerably longer these data constrain models of eruption for the bishop tuff by implying an eruption cycle that lasted longer than 3 years the average calculated deposition temperatures for all 4 reps in the adobe basin are fairly uniform at 667 33 c in chidago canyon two of the reps had a higher deposition temperature 660 c than the other two 534 c

nuees ardentes of 22 november 1994 at merapi volcano java indonesia nuees ardentes associated with dome collapse on 22 november 1994 at merapi volcano traveled to the south southwest as far as 6 5 km and collectively accumulated roughly 2 5 3 million cubic meters of deposits the damaged area comprises 9 5 km2 and is covered by two nuee ardente facies a conventional merapi type valley fill block and ash flow facies and a pyroclastic surge facies the proximal deposits reflect the accumulation of dozens of nuees ardentes with many subsidiary flow units the distal deposits are more simply organized as only a few individual events reached to distances 3 5 km the stratigraphic relationships north of turgo hill indicate that the surge deposits are a facies of particularly mobile nuees ardentes that also deposited channeled block and ash flow facies they further suggest that the surge facies beyond the channel margins correlate laterally with a finer grained sublayer locally developed at the base of the block and ash flow facies eyewitness reports suggest that the emplacement of the block and ash flow facies in the distal part of the boyong river may have followed by a short time interval the destruction and deposition of the surge facies at turgo village the stratigraphy is in accord with the eyewitness reports the surge facies was emplaced by a dilute surge current detached from the same dome collapse nuee ardente that as a separate flow unit subsequently emplaced the distal block and ash deposit in the boyong valley the detachment occurred at higher elevations likely at or above the slope break at about 2000 m elevation this flow separation enabled the surge current to shortcut over the landscape and to emplace its deposit even as the block and ash flow continued its tortuous southward movement in the boyong channel dome collapse nuee ardente activity formed the bulk of the eruption which was accompanied by virtually no significant vertical summit explosive activity

absorption and scattering in ground penetrating radar analysis of the bishop tuff 1 ground penetrating radar gpr signals are attenuated by both absorption and scattering we performed low frequency 100 mhz gpr surveys at the volcanic tableland of the bishop california tuff to evaluate the factors that control gpr depth of investigation and to develop insight into the capabilities of such radars for mars the subsurface reflection character was very different for two different commercial systems used together they revealed both internal welding contacts in the tuff and an abundance of discrete scatterers attenuation coefficients were computed from profiles that showed distributed scattering the semilogarithmic signal decay is directly analogous to seismic coda the absorption intrinsic loss was determined to be 1 db m from low frequency vertical electric soundings the residual attenuation that is the attenuation in the absence of absorption is attributed to scattering scattering attenuation of 1 db m at 25 50 mhz corresponds to mean free paths as short as 4 m a fraction of the two way propagation distances of 20 40 m therefore the bishop tuff is formally a strong scatterer to gpr the mean free path is also comparable to the subsurface radar wavelength in this case maximizing scattering loss the scatterers themselves likely originate as welding heterogeneities contrasts in dielectric constant due to density differences may be supplemented by moisture variations on mars scattering is likely to contribute significant losses to gpr signals in all but the most uniform materials and unfrozen thin films of water in the lower cryosphere could influence both absorption and scattering

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

the history and dynamics of a welded pyroclastic dam and its failure the 2 360 bp eruption of mount meager british columbia began as an explosive dacitic sub plinian eruption that waned rapidly to a sustained period of vulcanian eruption triggered dome collapse events producing voluminous block and ash flow baf deposits the earliest baf deposits accumulated rapidly enough immediately downslope of the vent to retain heat and weld using the deposit as a paleoviscometer determines an effective viscosity of 109 1010 pa s during welding this prolific production of hot lava and block and ash flows in a steep mountainous terrain created a 110 m high largely impermeable dam capped by permeable non welded baf deposits and unconsolidated avalanche deposits that blocked the flow of the lillooet river and created a temporary lake the welded pyroclastic dam was compromised and overtopped at least once before the peak dam height was reached renewed eruption caused buildup of the dam to a maximum of 780 m above sea level asl and grew the temporary lake to an elevation of 740 m asl and a minimum volume of 0 55 km3 the rise of lake level led to catastrophic failure of the top of the dam generating an outburst flood that carved a canyon through most of the dam and resulted in a voluminous lahar that is traced at least 65 km downstream based on current flow rates of the lillooet river the lake would have overtopped the final dam at a minimum of 39 65 days after its formation the peak deluge lasted approximately 8 h and eroded a 2 5 km long canyon into the still hot dam core before returning to background flow rates

ranking welding intensity in pyroclastic deposits welding of pyroclastic deposits involves flattening of glassy pyroclasts under a compactional load at temperatures above the glass transition temperature progressive welding is recorded by changes in the petrographic e g fabric and physical e g density properties of the deposits mapping the intensity of welding can be integral to studies of pyroclastic deposits but making systematic comparisons between deposits can be problematical here we develop a scheme for ranking welding intensity in pyroclastic deposits on the basis of petrographic textural observations e g oblateness of pumice lapilli and micro fabric orientation and measurements of physical properties including density porosity point load strength and uniaxial compressive strength our dataset comprises measurements on 100 samples collected from a single cooling unit of the bandelier tuff and parallel measurements on 8 samples of more densely welded deposits the proposed classification comprises six ranks of welding intensity ranging from unconsolidated rank i to obsidian like vitrophyre rank vi and should allow for reproducible mapping of subtle variations in welding intensity between different deposits the application of the ranking scheme is demonstrated by using published physical property data on welded pyroclastic deposits to map the total accumulated strain and to reconstruct their pre welding thicknesses

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

compaction and gas loss in welded pyroclastic deposits as revealed by porosity permeability and electrical conductivity measurements of the shevlin park tuff pyroclastic flows produced by large volcanic eruptions commonly densify after emplacement processes of gas escape compaction and welding in pyroclastic flow deposits are controlled by the physical and thermal properties of constituent material through measurements of matrix porosity permeability and electrical conductivity we provide a framework for understanding the evolution of pore structure during these processes using data from the shevlin park tuff in central oregon united states and from the literature we find that over a porosity range of 0 70 matrix permeability varies by almost 10 orders of magnitude from 10 20 to 10 11 m 2 with over three orders of magnitude variation at any given porosity part of the variation at a given porosity is due to permeability anisotropy where oriented core samples indicate higher permeabilities parallel to foliation horizontally than perpendicular to foliation vertically this suggests that pore space is flattened during compaction creating anisotropic crack like networks a geometry that is supported by electrical conductivity measurements we find that the power law equation k 1 1 3 10 21 5 2 provides the best approximation of dominant horizontal gas loss where k 1 permeability and porosity application of kozeny carman fluid flow approximations suggests that permeability in the shevlin park tuff is controlled by crack or disk like pore apertures with minimum widths of 0 3 and 7 5 m we find that matrix permeability limits compaction over short times but deformation is then controlled by competition among cooling compaction water resorption and permeable gas escape these competing processes control the potential for development of overpressure and secondary explosions and the degree of welding in the deposit processes that are applicable to viscous densification of volcanic deposits in general further the general relationships among porosity permeability and pore geometry are relevant for flow of any fluid through an ignimbritic host

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

snake river sr type volcanism at the yellowstone hotspot track distinctive products from unusual high temperature silicic super eruptions a new category of large scale volcanism here termed snake river sr type volcanism is defined with reference to a distinctive volcanic facies association displayed by miocene rocks in the central snake river plain area of southern idaho and northern nevada usa the facies association contrasts with those typical of silicic volcanism elsewhere and records unusual voluminous and particularly environmentally devastating styles of eruption that remain poorly understood it includes 1 large volume lithic poor rhyolitic ignimbrites with scarce pumice lapilli 2 extensive parallel laminated medium to coarse grained ashfall deposits with large cuspate shards crystals and a paucity of pumice lapilli many are fused to black vitrophyre 3 unusually extensive large volume rhyolite lavas 4 unusually intense welding rheomorphism and widespread development of lava like facies in the ignimbrites 5 extensive fines rich ash deposits with abundant ash aggregates pellets and accretionary lapilli 6 the ashfall layers and ignimbrites contain abundant clasts of dense obsidian and vitrophyre 7 a bimodal association between the rhyolitic rocks and numerous coalescing low profile basalt lava shields and 8 widespread evidence of emplacement in lacustrine alluvial environments as revealed by intercalated lake sediments ignimbrite peperites rhyolitic and basaltic hyaloclastites basalt pillow lava deltas rhyolitic and basaltic phreatomagmatic tuffs alluvial sands and palaeosols many rhyolitic eruptions were high mass flux large volume and explosive vei 6 8 and involved h2o poor low 18o metaluminous rhyolite magmas with unusually low viscosities partly due to high magmatic temperatures 900 1 050 c sr type volcanism contrasts with silicic volcanism at many other volcanic fields where the fall deposits are typically plinian with pumice lapilli the ignimbrites are low to medium grade non welded to eutaxitic with abundant pumice lapilli or fiamme and the rhyolite extrusions are small volume silicic domes and coulees sr type volcanism seems to have occurred at numerous times in earth history because elements of the facies association occur within some other volcanic fields including trans pecos texas etendeka parana lebombo the english lake district the proterozoic keewanawan volcanics of minnesota and the yardea dacite of australia

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

steady annual and monthly recharge implied by deep unconfined aquifer flow abstract we consider the response of a deep unconfined horizontal aquifer to steady annual and monthly recharge a groundwater divide and a zero head reservoir constrain the aquifer so that sinusoidal monthly and aperiodic annual recharge fluctuations create transient specific discharge near the reservoir and an unsteady water table elevation inland one existing and two new long term data sets from the plymouth carver aquifer in southeastern massachusetts calibrate and confirm hydraulic properties in a set of analytical models geohydrology and simulated groundwater flow 1992 data and a new power law for tritiugenic helium to tritium ratios calibrate the steady recharge that drives the classical parabolic model of steady hydraulics applied hydrogeology 2001 observed water table and gradient fluctuations calibrate the transient recharge models in the latter regard monitoring wells within 1 km of buttermilk bay exhibit appreciable specific discharge and reduced water table fluctuations we apply trans am geophys union 32 1951 238 periodic model to the monthly hydraulics and a recharge convolution integral j hydrol 126 1991 315 to annual flow an infiltration fraction of 0 79 and a consumptive use coefficient of 1 08 10 8 m s c relate recharge to precipitation and daylight weighted temperature across all three time scales errors associated with this recharge relation decrease with increasing time scale

a formula for computation of time varying recharge of groundwater comment this paper presents a formula for computation of time varying recharge of groundwater the formula is derived from the basic equation of groundwater flow on an inclined impervious base this analytical expression can also be used to compute evaporation rate if there is no recharge of groundwater readings from electronic data loggers show that recharge of groundwater is highly variable depending on the patterns of rainfall and antecedent conditions of the soil

a rare occurrence of a crater filling clastogenic extrusive coherent kimberlite victor northwest ontario canada kimberlite pipes can contain significant proportions of dark and dense kimberlite that have mostly been interpreted as intrusive coherent hypabyssal in origin this study reports a well documented occurrence of a fresh intra crater clastogenic extrusive coherent kimberlite that is concluded to have formed as a result of lava fountaining this paper focuses on a dark dense competent generally crystal rich massive kimberlite unit within the victor northwest kimberlite pipe ontario canada using a comprehensive volcanological and petrographic analysis of all available drill cores it is shown that this unit has a fresh well crystallised coherent groundmass and is extrusive and pyroclastic in origin the proposed clastogenic coherent extrusive origin is based on deposit morphology gradational contacts to enveloping pyroclastic units as well as the presence of remnant pyroclast outlines and angular broken olivines this paper and an increasing number of other studies suggest that fragmental extrusive coherent kimberlite in intra crater settings may be more common than previously thought the emplacement history and volcanology of these pipes need to be reconsidered based on the emerging importance of this particular kimberlite facies

the 2 0 1 88 ga paleoproterozoic evolution of the southern amazonian craton brazil an interpretation inferred by lithofaciological geochemical and geochronological data abstract the study of paleoproterozoic rocks is crucial for understanding earth s tectonic evolution during the time when most of the modern crust and ore deposits were formed the rocks of the brazilian amazonian craton record some of the most complete and best preserved paleoproterozoic magmatic and volcanic episodes on earth following previous investigations we present new lithofaciological and stratigraphic records of the felsic rocks of the tapajos mineral province tmp 2 1 88 ga and the sao felix do xingu region sfx 1 88 ga which combined with new petrological and geochronological data help providing a more complete understanding of the tectonic magmatic and volcanological evolution of the amazonian craton this magmatism volcanism is thought to be formed in a late post orogenic to extentional regime confirmed by the new geochemical data presented here the transition from late convergent to extensional tectonic setting could register the beginning of the taphrogenesis that marked the amazonian craton throughout the mesoproterozoic the volcanological approach of this contribution can serve as a strategy for the modelling of the evolution of precambrian volcano sedimentary basins around the world the large amount of rocks analyzed are divided into primary and secondary volcaniclastic products depending on if they resulted from a direct volcanic activity pyroclastic or processes that reworked pyroclastic fragments furthermore the deposits are subdivided into massive and stratified depending on their primary mechanisms of transport and emplacement by confirming the results from previous studies our study permits to depict a more precise paleo environmental picture of the processes that occurred in the amazonian craton during the late paleoproterozoic in particular the presence of large regional scale fissural systems and caldera collapses produced large silicic explosive volcanic eruptions also accompanied by the emission of large volume effusive products although studies on the amazonian craton are still scarce and controversial the present study provides new evidence that this volcanism may have formed one of the largest silicic large igneous provinces slip on earth our data also confirm that at least two major paleoproterozoic periods of formation of volcanic rocks exist in the amazonian craton this point is of great relevance for any future interpretation of the geological evolution of this craton

a numerical model of volatile behavior in nonwelded cooling pyroclastic deposits the solution of the governing transport equations for transient volatile behavior in a cooling pyroclastic deposit reveals dominant mechanisms that control cooling and degassing the numerical model and algorithm presented here are time accurate and suitable for operation on a personal computer this model takes into account the combined effects of cooling and low volatile outfluxing on the pressure temperature and volatile mass content in a deposit sample deposit simulations compare favorably with observations made by kozu 1934 and kienle and swanson 1980 other exemplary calculations of 20 and 40 m thick pyroclastic deposits indicate that the time required for total volatile outflux approximately 21 and 83 days respectively is small compared with that required for the cooling effects of the upper and lower margins to come in contact in excess of 70 and 265 days respectively the thermal behavior of the modeled deposits appears to be dominated by diffusive heat transfer while the pressure field and volatile content within each sheet are dominated by hydrodynamic effects

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

on the welding of pyroclasts from very low viscosity magmas examples from kimberlite volcanoes abstract lithofacies in kimberlite pipes in southern africa exhibit features consistent with welded pyroclastic rocks these include flared conduit filling geometries abundant lithic clasts lithic clast layers subhorizontal clast fabrics gradational contacts with volcaniclastic rocks and sintered and coalesced globular ash and lapilli and melt coated particles the welding dynamics of kimberlite pyroclasts differ from those of glassy vesiculated pyroclasts in silicic volcanic systems low melt viscosity 0 1 pa s results in the efficient separation of volatiles and melt and the breakup of magma into nonvesicular spherical droplets the glassy state is difficult to form in silica deficient magmas because the crystallization kinetics are fast in low viscosity melts three pyroclastic lithofacies are recognized that primarily relate to the state of the initial melt phase in the pyroclasts on deposition 1 densely welded kimberlite forms where the initial melt phase in pyroclasts remains as pure mel

rheology of arc dacite lavas experimental determination at low strain rates andesitic dacitic volcanoes exhibit a large variety of eruption styles including explosive eruptions endogenous and exogenous dome growth and kilometer long lava flows the rheology of these lavas can be investigated through field observations of flow and dome morphology but this approach integrates the properties of lava over a wide range of temperatures another approach is through laboratory experiments however previous studies have used higher shear stresses and strain rates than are appropriate to lava flows we measured the apparent viscosity of several lavas from santiaguito and bezymianny volcanoes by uniaxial compression between 1 109 and 1 315 k at low shear stress 0 085 to 0 42 mpa low strain rate between 1 1 10 8 and 1 9 10 5 s 1 and up to 43 7 total deformation the results show a strong variability of the apparent viscosity between different samples which can be ascribed to differences in initial porosity and crystallinity deformation occurs primarily by compaction with some cracking and or vesicle coalescence our experiments yield apparent viscosities more than 1 order of magnitude lower than predicted by models based on experiments at higher strain rates at lava flow conditions no evidence of a yield strength is observed and the apparent viscosity is best approached by a strain rate and temperature dependent power law equation the best fit for santiaguito lava for temperatures between 1 164 and 1 226 k and strain rates lower than 1 8 10 4 s 1 is log eta text app 0 738 9 24 times 10 3 t k 0 654 cdot log dot varepsilon where app is apparent viscosity and dot varepsilon is strain rate this equation also reproduced 45 data for a sample from bezymianny with a root mean square deviation of 0 19 log unit pa s applying the rheological model to lava flow conditions at santiaguito yields calculated apparent viscosities that are in reasonable agreement with field observations and suggests that internal shear heating may be significant ongoing heat source within these flows enabling highly viscous lava to travel long distances

some analytical solutions of the linearized boussinesq equation with recharge for a sloping aquifer subsurface flow from a hillslope can be described by the hydraulic groundwater theory as formulated by the boussinesq equation several attempts have been made to solve this partial differential equation and exact solutions have been found for specific situations in the case of a sloping aquifer brutsaert 1994 suggested linearizing the equation to calculate the unit response of the hillslope in this paper we first apply the work of brutsaert by assuming a constant recharge to the groundwater table the solution describes the groundwater table levels and the outflow in function of time then an analytical expression is derived for the steady state solution by allowing time to approach infinity this steady state water table is used as an initial condition to derive another analytical solution of the boussinesq equation this can then be used in a quasi steady state approach to compute outflow under changing recharge conditions subsurface flow from a hillslope can be described by the hydraulic groundwater theory as formulated by the boussinesq equation several attempts have been made to solve this partial differential equation and exact solutions have been found for specific situations in the case of a sloping aquifer brutsaert 1994 suggested linearizing the equation to calculate the unit response of the hillslope in this paper we first apply the work of brutsaert by assuming a constant recharge to the groundwater table the solution describes the groundwater table levels and the outflow in function of time then an analytical expression is derived for the steady state solution by allowing time to approach infinity this steady state water table is used as an initial condition to derive another analytical solution of the boussinesq equation this can then be used in a quasi steady state approach to compute outflow under changing recharge conditions

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

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

emplacement temperatures of the november 22 1994 nuee ardente deposits merapi volcano java abstract a study of emplacement temperatures was carried out for the largest of the 22 november 1994 nuee ardente deposits at merapi volcano based mainly on the response of plastic and woody materials subjected to the hot pyroclastic current and the deposits and to some extent on eyewitness observations the study emphasizes the turgo kaliurang area in the distal part of the area affected by the nuee ardente where nearly 100 casualties occurred the term nuee ardente as used here includes channeled block and ash flows and associated ash clouds of surge and fallout origins the emplacement temperature of the 8 m thick channeled block and ash deposit was relatively high 550 c based mainly on eyewitness reports of visual thermal radiance emplacement temperatures for ash cloud deposits a few cm thick were deduced from polymer objects collected at turgo and kaliurang most polymers do not display a sharp melting range but polyethylene terephthalate used in water bottles melts between 245 and 265 c and parts of the bottles that had been deformed during fabrication molding turn a milky color at 200 c the experimental evidence suggests that deposits in the turgo area briefly achieved a maximum temperature near 300 c whereas those near kaliurang were

magma expansion and fragmentation in a propagating dyke abstract the influence of magma expansion due to volatile exsolution and gas dilation on dyke propagation is studied using a new numerical code many natural magmas contain sufficient amounts of volatiles for fragmentation to occur well below earth s surface magma fragmentation has been studied for volcanic flows through open conduits but it should also occur within dykes that rise towards earth s surface the characteristics of volatile rich magma flow within a hydraulic fracture are studied numerically the mixture of melt and gas is treated as a compressible viscous fluid below the fragmentation level and as a gas phase carrying melt droplets above it the numerical code solves for elastic deformation of host rocks the flow of the magmatic mixture and fracturing at the dyke tip with volatile free magma a dyke fed at a constant rate in a uniform medium adopts a constant shape and width and rises at a constant velocity with volatiles involved magma expands and hence the volume flux of magma increases with no fragmentation this enhanced flux leads to acceleration and thinning of the dyke simple scaling laws allow accurate predictions of dyke width and ascent rate for a wide range of conditions with fragmentation dyke behaviour is markedly different due to the sharp drop of head loss that occurs in gas rich fragmented material large internal overpressures develop below the dyke tip and induce swelling of the nose region leading to deceleration of the dyke these results are applied to the two month long period of volcanic unrest that preceded the may 1980 eruption of mount st helens an initial phase of rapid earthquake migration from the 7 8 km deep reservoir to shallow levels was followed by very slow progression of magma within the edifice such behaviour can be accounted for by magma fragmentation at the top of a dyke

fabrics facies and flow through a large volume ignimbrite pampa de oxaya chile the miocene oxaya formation exposed along the western andean slope in northern chile represents one of the largest ignimbrite provinces on earth in this study magnetic fabric data were acquired from a 1 km long core drilled vertically through a single cooling unit of the oxaya formation the ca 22 ma cardones ignimbrite samples for magnetic analysis were obtained every 20 m from the fine grained matrix of the core detailed measurements of the variation in bulk magnetic properties including natural remanent magnetization nrm susceptibility and anisotropy of magnetic susceptibility ams were used to monitor changes in magnetic mineralogy as well as changes in the strength and orientation of the magnetic fabric throughout the flow ams ellipsoid orientation and shape reflect rapid deposition from a concentrated granular fluidized flow and constrain both the location of the source caldera for this catastrophic eruption and processes of transport and deposition in this large volume ignimbrite after utilizing the magnetic remanence to correct for rotation about the core axis well grouped and imbricated petrofabric orientations reveal a well defined sw 247 transport direction down a proto western andean slope indicating syn or post welding flow and confirming the deformed lauca caldera as the likely source of the eruption systematic variations in fabric shape t and intensity p with depth reveal a predominately oblate fabric toward the top and base of the flow and predominately prolate fabrics in the center these vertical changes in fabric reveal that this massive apparently homogeneous deposit has a systematic layering this layering reflects depth dependent temperature variations temporal changes in the flow boundary zone during deposition and post depositional processes

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

experiments and models on h2o retrograde solubility in volcanic systems we present a suite of 36 high temperature 900 1100 c experiments performed on 10 10 mm unjacketed cores of rhyolitic obsidian from hrafntinnuhryggur krafla iceland under atmospheric pressure the obsidian is bubble and crystal free with an h2o content of 0 11 4 wt the obsidian cores were heated above the glass transition temperature tg held for 0 25 24 h then quenched during each experiment the volume of the samples increased as h2o vapor filled bubbles nucleated and expanded uniquely the bubbles did not nucleate on the surface of the core nor escape conserving mass during all experiments within each isothermal experimental suite the cores increased in volume with time until they reached a maximum after which continued heating caused no change in volume measured by he pycnometry we interpret these t t conditions as representing thermochemical equilibrium between the melt and exsolved vapor these experiments are modeled to recover the 1 atm temperature dependent solubility of water in the rhyolite melt our results define the magnitude of retrograde solubility 7 1 10 3 wt h2o per 100 c and provide estimates of the enthalpy and entropy of the h2o exsolution reaction h 17 8 kj mol s 107 j k mol we conclude by modeling the implications of retrograde solubility for the glass transition temperatures tg of cooling volcanic systems at pressures relevant to volcanic conduits and the earth s surface all volcanic systems cool the effects of retrograde solubility are to allow melts to rehydrate by h2o dissolution as they cool isobarically thereby depressing tg and expanding the melt window ultimately the melt is quenched at higher h2o contents and lower temperatures where the isobaric retrograde solubility curve catches the evolving tg

glass transition temperatures of natural hydrous melts a relationship with shear viscosity and implications for the welding process abstract glass transition temperatures tg have been determined for natural multicomponent melts using differential scanning calorimetry trachytic dacitic phonolitic and basaltic base compositions have been analysed over a range of water contents up to 3 75 wt for each sample tg has been obtained over a range of cooling heating rates using the extrapolated onset and the peak temperatures in heat capacity temperature curves tg of all compositions are strongly reduced by increasing water content particularly for the first 1 wt added base composition also has an effect with the lowest tg occurring in the peralkaline phonolite suite for all samples a clear dependence on the cooling heating rate has been recorded these results have been compared with rheological investigations on the same samples on the basis of the equivalence of the shear and enthalpic relaxation process timescales we provide a method to predict the shear viscosity at the glass transition for all the melts investigated in this study both dry and hydrous our determinations of tg provide a lower limit for the time temperature envelope that gives rise to densely welded deposits and constraints on their emplacement temperature furthermore by using the viscosity values predicted at the glass transition we suggest that welding processes may occur over timescales on the order of tens of seconds to tens of minutes at tg

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

magnetic fabric welding texture and strain fabric in the nuraxi tuff sardinia italy anisotropy of magnetic susceptibility ams has been used to interpret flow directions in ignimbrites but no study has demonstrated that the ams fabric corresponds to the flow fabric in this paper we show that the ams and strain fabric coincide in a high grade ignimbrite the nuraxi tuff a miocene rhyolitic ignimbrite displaying a wide variability of rheomorphic features and a well defined magnetic fabric natural remanent magnetization nrm data indicate that the magnetization of the tuff is homogeneous and was acquired at high temperatures by ti magnetite crystals comparison between the magnetic fabric and the deformation features along a representative section shows that ams and anisotropy of isothermal remanent magnetization airm fabric are coaxial with and reproduce the shape of the strain ellipsoid magnetic tests and scanning electron microscopy observations indicate that the fabric is due to trails of micrometer size pseudo single domain magnetically interacting magnetite crystals microlites formed along discontinuities such as shard rims and vesicle walls mimicking the petrofabric of the tuff the fabric was thus acquired after deposition before late rheomorphic processes and accurately mimics homogeneous deformation features of the shards during welding processes and mass flow

welded block and ash flow deposits from mount meager british columbia canada abstract the 2360 years b p eruption of mount meager british columbia produced a succession of rarely observed welded block and ash flow deposits and non welded equivalents we report on these rhyodacitic block and ash flow deposits by describing the deposits documenting the nature of the welding process and establishing the origins of these pyroclastic deposits variations in welding intensity are tracked by image analysis of field texture maps petrographic study of shard morphology and measurements of sample density and porosity pyroclast oblateness from image analysis of texture maps provides a qualitative record of welding intensity but underestimates the amount of compactional e g volume strain when compared to estimates from physical property measurements e g density porosity unconsolidated deposits have an average matrix porosity of 41 38 45 and clasts have an average porosity of 32 24 40 in unconsolidated deposits isolated porosity is pervasive 50 m shortening of the thickest deposit from 162 m to the current 112 m the pyroclastic flow deposits result from accumulation in a narrow confined river valley the accumulation was sufficiently rapid to keep the deposits above their glass transition temperature thereby allowing the succession to weld as a single cooling unit the nature and distribution of isolated porosity in the juvenile clasts of rhyodacite suggest an explosive rather than effusive origin for these block and ash flow deposits that may be analogous to some vulcanian eruption triggered dome collapse processes observed at soufriere hills volcano montserrat rather than gravitational collapse

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

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

paleoproterozoic felsic volcanism of the tapajos mineral province southern amazon craton brazil abstract amazonian rocks record one amongst the most complete and best preserved paleoproterozoic magmatic episodes on earth the present contribution documents the extremely well preserved paleoproterozoic architecture of a series of felsic rocks found in the tapajos mineral province tmp located in the western part of para state southern amazon craton north of brazil these rocks are the first to be investigated to comprehend based on their textural evidences their emplacement mechanisms textural characterization allowed to identify three main facies with as following reported 1 chaotic breccia group 2 eutaxitic eutax group and 3 parataxitic paratax group vitrophyric textures given the superb preservation of our samples the investigated rocks are grouped according to their grade of welding into a wide variety of lithofacies from very low grade to high grade and rheomorphic ignimbrites in the paratax group strong similarities with banding in lava flows are observed based on the presented data we discuss the effusive or explosive origin of the observed flow mechanisms

spatter stability constraining accumulation rates and temperature conditions with experimental bomb morphology we have developed the first experimental methodology to create a volcanic spatter pile using molten basalt this method permits reproduction of thermal conditions that yield the wide variety of spatter morphologies observed in nature the morphology of the clasts is most strongly controlled by the time the clast spends above the glass transition temperature which is in turn affected by the rate of accumulation and cooling of the deposit also spatter piles that remain hotter over longer durations experience increased fusion between clasts less void space between clasts and generally larger aspect ratios our experimental method successfully replicated natural microcrystal textures rheology and clast size work is still therefore required to achieve realistic vesicle distribution and deposit void space based on presented experimental work we estimate emplacement conditions of southern idaho spatter vents to have been 850 900 c with eruption temperatures closer to 1000 1100 c the rapid decrease from eruption temperature to effective emplacement temperature is the result of clast flight as well as equilibrating with the cooler surrounding material the morphology of the natural clasts matches experiments that have accumulation rates of 2 5 4 5 m h which also is consistent with the few measurements made at active eruptions finally we provide a constraint on the temperatures and accumulation rates that can lead to the construction of fused spatter features as well as provide the steps for future experiments to investigate other aspects such as compression impact and larger sizes of spatter formation by adapting our methodology

deformation of the wineglass welded tuff and the timing of caldera collapse at crater lake oregon abstract four types of deformation occur in the wineglass welded tuff on the northeast caldera rim of crater lake a vertical tension fractures b ooze outs of fiamme c squeeze outs of fiamme and d horizontal pull apart structures the three types of plastic deformation b d developed in the lower part of the wineglass welded tuff where degree of welding and density are maximum deformation originated from concentric normal faulting and landsliding as the caldera collapsed the degree of deformation of the wineglass welded tuff increases toward the northeast part of the caldera where plastic deformation occurred more easily because of a higher emplacement temperature probably due to proximity to the vent the probable glass transition temperature of the wineglass welded tuff suggests that its emplacement temperature was 750 c where the tuff is densely welded calculation of the conductive cooling history of the wineglass welded tuff and the preclimactic cleetwood lava flow under assumptions of a initially isothermal sheet and uniform properties suggests that a caldera collapse occurred a maximum of 9 days after emplacement of the wineglass welded tuff and that b the period between effusion of the cleetwood lava flow and onset of the climactic eruption was

paleointensity estimates from ignimbrites an evaluation of the bishop tuff 1 ash flow tuffs or ignimbrites typically contain fine grained magnetite spanning the superparamagnetic to single domain size range that should be suitable for estimating geomagnetic field intensity however ignimbrites may have a remanence of thermal and chemical origin as a result of the complex magnetic mineralogy and variations in the thermal and alteration history we examined three stratigraphic sections through the 0 76 ma bishop tuff where independent information on postemplacement cooling and alteration is available as a test of the suitability of ignimbrites for paleointensity studies thermomagnetic curves suggest that low ti titanomagnetite tc 560 c 580 c is the dominant phase with a minor contribution from a higher tc phase s significant remanence unblocking above 580 c suggests that maghemite and or titano maghemite is an important contributor to the remanence in most samples we obtained successful paleofield estimates from remanence unblocked between 440 c and 580 c for 46 of 89 specimens 15 sites at two of three total localities these specimens represent a range of degrees of welding and have variable alteration histories and yet provide a consistent paleofield estimate of 43 0 t 3 2 equivalent to a vadm of 7 8 1022 am2 the most densely welded sections of the tuff have emplacement temperatures inferred to be as high as 660 c suggesting that the remanence may be primarily thermal in origin though a contribution from thermochemical remanence cannot be excluded these results suggest that ignimbrites may constitute a viable material for reliable paleointensity determinations

low temperature thermochronology in the peruvian central andes implications for long term continental denudation timing of plateau uplift canyon incision and lithosphere dynamics abstract in peru the western edge of the 4 5 km high western cordillera is cut by a 3 km deep canyon to understand incision by the cotahuasi ocona river and the regional uplift history of this orogenic plateau capped by volcanic rocks 26 crystalline rock samples were collected for low temperature thermochronology from vertical profiles parallel and perpendicular to the canyon rock cooling histories confirm that most plateau denudation had occurred prior to 24 ma but plateau incision peaked after c 14 9 ma in response to rapid surface uplift the abrupt occurrence of a rock heating event is also detected during middle miocene time this was either a response to the emplacement of low conductivity regionally extensive ignimbritic caprock or a response to crustal scale fluid circulation caused by wet melting of the overriding plate when magmatism resumed c 24 ma the potential for thermochronology to provide information on past geothermal gradients is discussed showing how it can be used as a proxy for understanding change in subducting slab dynamics with oscillations in subduction angle having perhaps been the main on off switch for magmatism in this cordilleran setting supplementary material fission track and apatite u th he results and apatite chemistry indicators are available at http www geolsoc org uk sup18405

low dimensional modeling of hillslope subsurface flow relationship between rainfall recharge and unsaturated storage dynamics 1 we present a coupling between the one dimensional richards equation for vertical unsaturated flow and the one dimensional hillslope storage boussinesq equation hsb for lateral saturated flow along complex hillslopes here the capillary fringe is included in the flow domain as an integral part of the boussinesq aquifer the coupling allows quantitative investigation of the role of unsaturated storage in the relationship between rainfall and recharge the coupled model hsb coupled is compared to the original hsb model hsb original and a three dimensional richards equation re based model taken to be the benchmark on a set of seven synthetic hillslopes ranging from convergent to divergent using hsb original the water tables are overestimated and the outflow rates are generally underestimated and there is no delay between rainfall and recharge the coupled model however shows a remarkably good match with the re model in terms of outflow rates and the delay between rainfall and recharge is captured well we also see a clear improvement in the match to the water tables even though the values are still overestimated for some hillslope shapes in particular the convergent slopes we show that for the hillslope configurations and scenarios examined in this paper it is possible to reproduce hydrographs and water table dynamics with a good degree of accuracy using a low dimensional hydrological model

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