18o 16o mapping and hydrogeology of a short lived 10 years fumarolic 500 c meteoric hydrothermal event in the upper part of the 0 76 ma bishop tuff outflow sheet california 18 o 16 o data from the 200 m thick 0 76 ma bishop tuff outflow sheet provide evidence for a vigorous short lived 10 years high temperature fumarolic meteoric hydrothermal event this is proved by 1 the juxtaposition in the upper partially welded bishop tuff of low 18 o groundmass glass 18 o 5 to 3 with coexisting quartz and feldspar phenocrysts having magmatic 18 o values 8 7 0 3 7 5 0 3 and 2 the fact that these kinds of 18 o 16 o signatures correlate very well with morphological features and mapped zones of fumarolic activity profiles of 18 o with depth in the bishop tuff within the fumarole area define a 40 to 50 m thick low 18 o stratigraphic zone that is sandwiched between the essentially unwelded near surface portion of the tuff and an underlying densely welded black tuff that displays magmatic 18 o 16 o values shallow dipping columnar joints and other fumarolic features i e subhorizontal tubular conduits and steep fissures correlate very well with these pervasively devitrified low 18 o zones the base of the low 18 o zone is extremely sharp 3 per meter and is located directly above the transition from partially welded tuff to densely welded black tuff the observed average whole rock 18 o depletions within this low 18 o zone are about 6 7 requiring meteoric water rock ratios in excess of 0 24 in mass units rainfall on the surface of the tuff would not have been high enough to supply this much h 2o in the short lifetime of fumarolic activity suggesting that some recharge must have been from groundwater flow through the upper part of the tuff above the sloping 1 5 top of the impermeable lower zone this is compatible with the observation that the fumarolic areas roughly correlate with the preeruptive regional drainage pattern some of this recharge may in part have been from the lake that filled long valley caldera which was dammed by the bishop tuff up to the level of this boundary between the partially and densely welded zones 7000 ft the elevation of the highest long valley lake shorelines gazis et al had previously shown that the 2 8 ma intracaldera chegem tuff from the caucasus mountains exhibits exactly the same kind of 18 o signature that we have correlated with fossil fumaroles in the bishop tuff outflow sheet although not recognized as such by mcconnell et al 18 o 16 o data from drill hole samples from the intracaldera bishop tuff in long valley also display this characteristic 18o signature i e analogous 18 o depth profiles as well as low 18 o groundmass coexisting with high 18 o feldspar phenocrysts this fumarolic 18 o 16 o signature is observed to much greater depths 650 750 m in the intracaldera tuffs 1500 m thick than it is in the 200 m thick bishop tuff outflow sheet 80 m depth

porosity and permeability in volcanic rocks a case study on the serie tobifera south patagonia argentina the middle to late jurassic serie tobifera belongs to the chon aike province and extends all over patagonia and the antarctic peninsula it consists largely of ignimbrites epiclastics and rhyolitic lavas and was considered only a minor reservoir rock for oil with fracture permeability petrographic and petrophysical data in selected core samples from the austral basin were collected to determine the processes controlling the porosity and permeability of these volcanic rocks the sequence of processes occurring during cooling and in the post cooling stages can modify sometimes substantially their original petrophysic characteristics the results show that the highest porosity and permeability occur in rocks with quench fractured glasses and in non welded ignimbrites with gas pipe structures followed by autobrecciated rhyolites welded ignimbrites massive glasses and fresh rhyolites have the lowest permeabilities the new data indicate that tectonic fracturing is not as significant as was considered before and application of these concepts are relevant in the assesment of volcanic reservoir quality

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

permeability and fracture patterns in extrusive volcanic rocks implications from the welded santana tuff trans pecos texas the santana tuff of trans pecos texas is a nonwelded to thoroughly welded fractured vitric crystal tuff the unsaturated welded portions of the santana tuff were examined to determine its matrix permeability and fracture patterns more than 1 100 permeability measurements on tuff surfaces were taken with a gas minipermeameter independent variables included the degree of surficial weathering the presence of rock varnish and the orientation of the surface with respect to direction of emplacement and flattening the mean permeabilities for fresh weathered and varnished surfaces were 55 33 md 5 03 md and 3 31 md respectively the fresh surface measurements are significantly different from the other two surface types both of which were from statistically equivalent populations analyses on cut surfaces showed about a 30 decrease in permeability perpendicular to flow direction and flattening this trend was also evident on the varnished and weathered surfaces weathering rinds including the varnish decrease the permeability in thin section the rinds exhibit a thin layer of dark minerals mostly fe oxides sem photographs of the weathered and varnished surfaces show a lamellate texture indicative of coating by smectitic clay particles fracture patterns were examined at six outcrops spanning 9 km a pavement was mapped at one outcrop rose diagrams and stereonets were prepared from more than 300 observations although the composite data show two main orientations of nearly vertical cooling fractures the fracture trends vary considerably even between out crops in proximity to each other field observations indicate that fractures are continuous across individual flow boundaries at the mapped outcrop the fracture density and trace length are fractal between the 0 5 m and 3 m range the fractal dimension of fracture density was 1 44 and the length of the fracture was scale dependent but a fractal dimension of fracture lengths was not able to be determined a lower percentage of abutting fractures was observed here than has been noted in similar studies in nevada this study demonstrates that weathering along fractures reduces fracture matrix interactions in such rocks and that fracture patterns on a regional scale may be difficult to characterize

spatter and welded air fall deposits generated by fire fountaining eruptions cooling of pyroclasts during transport and deposition abstract according to the physico mathematical model presented here juvenile ballistic fragments can travel through the atmosphere during fire fountaining eruptions preserving enough heat to agglutinate or to weld on impact with the ground s surface even a few kilometres from the source area the range is mostly related to exit velocity and ejection angle as well as to such typical properties of the transported particles as size density and shape the selective nature of the transport system can give rise to inverse lateral size density grading the most important interdependent factors that control the degree of final agglutination or welding are accumulation rate grain size and duration of deposition in the investigated grain size range an accumulation rate of 0 2 m h appears as a minimum value to promote welding this value cannot be considered as an absolute threshold because it can be significantly lowered by increasing the grain size duration of deposition appears as another critical factor because it controls the maximum temperature of the whole deposit and its final thickness and therefore the cooling rate of its main portion after the end of deposition in the case of high accumulation rates of coarse particles but during a short period of time primary agglutination may be the dominant factor in controlling the final sintering of particles conversely high accumulation rates of coarse particles during a long period of time give rise to both primary agglutination and welding

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

columnar jointing in vapor phase altered non welded cerro galan ignimbrite paycuqui argentina columnar jointing is thought to occur primarily in lavas and welded pyroclastic flow deposits however the non welded cerro galan ignimbrite at paycuqui argentina contains well developed columnar joints that are instead due to high temperature vapor phase alteration of the deposit where devitrification and vapor phase crystallization have increased the density and cohesion of the upper half of the section thermal remanent magnetization analyses of entrained lithic clasts indicate high emplacement temperatures above 630 c but the lack of welding textures indicates temperatures below the glass transition temperature in order to remain below the glass transition at 630 c the minimum cooling rate prior to deposition was 3 0 10 3 8 5 10 2 c min depending on the experimental data used for comparison alternatively if the deposit was emplaced above the glass transition temperature conductive cooling alone was insufficient to prevent welding crack patterns average 4 5 sides to each polygon and column diameters average 75 cm are consistent with relatively rapid cooling where advective heat loss due to vapor fluxing increases cooling over simple conductive heat transfer the presence of regularly spaced complex radiating joint patterns is consistent with fumarolic gas rise where volatiles originated in the valley confined drainage system below joint spacing is a proxy for cooling rates and is controlled by depositional thickness valley width we suggest that the formation of joints in high temperature non welded deposits is aided by the presence of underlying external water where vapor transfer causes crystallization in pore spaces densifies the deposit and helps prevent welding

electrical conductivity and pore space topology of merapi lavas implications for the degassing of porphyritic andesite magmas pore space topology is studied in recent compact angular blocks and scoria fragments om block and ash flow deposits of merapi volcano indonesia connected porosity and electrical conductivity of cored clay4 ee samples are measured in the laboratory at varying fluid salini ty the electrical brmation fiactor tortuosity and cementation index are derived and the pore space topology isbund to be porosity dependent the electrical flow pattern is controlled by crack like paths in compact angular block and by vesicles geometry in more porous scoria agments because merapi lavas are highly viscous and crystal rich we infer that the development of pore connectedness and vesicle coalescence is promoted by shear strain along with ascent rate considerations we conclude that degassing of merapi magma occurs mostly in the conduit during ascent

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

ignimbrites of amazonis planitia region of mars a series of postulated ignimbrite units is mapped in the amazonis memnonia and aeolis quadrangles of mars the units cover about 2 2 106 km2 within a broad but discontinuous and irregular belt trending east west along the highland lowland boundary the ignimbrites overlie parts of the western and southern aureole materials of olympus mons but are embayed in places by the lava plains of the lowlands stratigraphic relations between the basalt flows from the tharsis montes region and the ignimbrites are not clearly defined crater counts suggest that the younger ignimbrites postdate the lava flows crater counts per square kilometer for the ignimbrites range from 7 29 1 95 10 4 to 6 36 2 01 10 5 for craters larger than 1 km in diameter the ignimbrite materials form thick 100 m extensive relatively flat sheets that are smooth to grooved or gently undulating grooved surfaces appear to be yardangs and in most places are not alined with prevailing wind directions the seven mapped ignimbrite units are characterized by morphologic expression stratigraphic position and crater counts similarities to ignimbrites in the pancake range of central nevada include 1 rounded patches of smooth high albedo nonwelded material superposed on jointed low albedo welded material 2 local complementary joint sets in welded materials and 3 thick flow sheets of great areal extent that follow but subdue underlying topography four major eruptive centers occur in areas where units are thickest and where a dominant nnw sse structural trend is expressed locally by unit margins elongate collapse features and normal faulting a minimum volume of 3 85 106 km3 for the deposits has been calculated from thickness estimates based on shadow measurements and crater rim height relations

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

secondary hydroeruptions in pyroclastic flow deposits examples from mount st helens abstract secondary hydroeruptions occur in pyroclastic flow deposits when water or ice is trapped beneath hot pyroclastic debris and rapidly heated to steam these eruptions display various styles of activity including fumarolic degassing tephra fountaining and explosive cratering the deposits which occupy the layer 3 stratigraphic position on the top of pyroclastic flow units can be distinguished from ash cloud material by lateral thickness variation clast composition and other sedimentary features the ejecta of secondary hydroeruptions comprise a subset of hydrovolcanic pyroclastic deposits a small secondary hydroeruption observed on the mount st helens pumice plain in 1981 produced tephra that was emplaced ballistically by deposition from base surges and by fallout from an eruption column stratigraphic descriptions and grain size analysis of the ejecta from several secondary craters on the pumice plain demonstrate that the bedforms produced by a hydroeruption change with crater diameter in particular craters of small diameter are surrounded by interbedded ripple laminated ash horizons and nonstratified fines depleted units large craters have ejecta ramparts comprised of coarse dunes and antidunes these bedform changes are related to a progressive increase in eruptive energy which produces base surges of greater power and eruptive columns of greater height we suggest that the style of activity displayed during a secondary hydroeruption is controlled by both the total thermal energy of the system and the permeability of the pyroclastic overburden

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

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

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

volcanic settings and their reservoir potential an outcrop analog study on the miocene tepoztlan formation central mexico abstract the reservoir potential of volcanic and associated sedimentary rocks is less documented in regard to groundwater resources and oil and gas storage compared to siliciclastic and carbonate systems outcrop analog studies within a volcanic setting enable to identify spatio temporal architectural elements and geometric features of different rock units and their petrophysical properties such as porosity and permeability which are important information for reservoir characterization despite the wide distribution of volcanic rocks in mexico their reservoir potential has been little studied in the past in the valley of mexico situated 4000 m above the neogene volcanic rocks groundwater is a matter of major importance as more than 20 million people and 42 of the industrial capacity of the mexican nation depend on it for most of their water supply here we present porosity and permeability data of 108 rock samples representing five different lithofacies types of the miocene tepoztlan formation this 800 m thick formation mainly consists of pyroclastic rocks mass flow and fluvial deposits and is part of the southern transmexican volcanic belt cropping out south of the valley of mexico and within the two states of morelos and mexico state porosities range from 1 4 to 56 7 average porosity is 24 8 generally permeabilities are low to median 0 2 933 3 md with an average permeability of 88 5 md the lavas are characterized by the highest porosity values followed by tuffs conglomerates sandstones and tuffaceous breccias on the contrary the highest permeabilities can be found in the conglomerates followed by tuffs tuffaceous breccias sandstones and lavas the knowledge of these petrophysical rock properties provides important information on the reservoir potential of volcanic settings to be integrated to 3d subsurface models

post depositional fracturing and subsidence of pumice flow deposits lascar volcano chile unconsolidated pyroclastic flow deposits of the 1993 eruption of lascar volcano chile have with time become increasingly dissected by a network of deeply penetrating fractures the fracture network comprises orthogonal sets of decimeter wide linear voids that form a pseudo polygonal grid visible on the deposit surface in this work we combine shallow surface geophysical imaging tools with remote sensing observations and direct field measurements of the deposit to investigate these fractures and their underlying causal mechanisms based on ground penetrating radar images the fractures are observed to have propagated to depths of up to 10 m in addition orbiting radar interferometry shows that deposit subsidence of up to 1 cm year 1 occurred between 1993 and 1996 with continued subsidence occurring at a slower rate thereafter in situ measurements show that 1 m below the surface the 1993 deposits remain 5 c to 15 c hotter 18 years after emplacement than adjacent deposits based on the observed subsidence as well as estimated cooling rates the fractures are inferred to be the combined result of deaeration thermal contraction and sedimentary compaction in the months to years following deposition significant environmental factors including regional earthquakes in 1995 and 2007 accelerated settling at punctuated moments in time the spatially variable fracture pattern relates to surface slope and lithofacies variations as well as substrate lithology similar fractures have been reported in other ignimbrites but are generally exposed only in cross section and are often attributed to formation by external forces here we suggest that such interpretations should be invoked with caution and deformation including post emplacement subsidence and fracturing of loosely packed ash rich deposits in the months to years post emplacement is a process inherent in the settling of pyroclastic material

fossil and active fumaroles in the 1912 eruptive deposits valley of ten thousand smokes alaska fumaroles in the ash flow sheet emplaced during the 1912 eruption of novarupta were intensely active throughout the valley of ten thousand smokes vtts when first studied in 1917 fumarole temperatures recorded in 1919 were as hot as 645 c influx of surface waters into the hot ash flow sheet provided the fluid flow to sustain the fumaroles but also enhanced cooling so that by the mid 1930 s vigorous activity survived only in the vent region configuration and distribution of high temperature fissure fumaroles tens of meters long that are prevalent in the middle and upper vtts were controlled largely by sintering and degree of welding which in turn controlled fracturing and permeability of the ash flow tuff one fracture type developed parallel to the enclosing valley walls during compaction of the ash flow sheet another type extends across the vtts nearly perpendicular to the flow direction a third type of randomly oriented fractures developed as cooling contraction cracks during vapor phase devitrification in distal parts of the ash flow sheet where the tuff is nonwelded prominent fumaroles have irregular funnel shaped morphologies fumarole distribution in the nonwelded part of the ash flow sheet is concentrated above pre emplacement river channels the hottest longest lived fumaroles occurred in the upper vtts near the 1912 vent where the ash flow sheet is thicker more indurated and on average more mafic richer in dacite and andesite in contrast to the thinner nonwelded rhyolitic tuff in the distal part of the sheet fumarolic activity was less intense in the distal part of the tuff because of lower emplacement temperatures more diffuse fumarole conduits in the nonwelded tuff and the thinness of the ash flow sheet chemical leaching of ash flow tuff by hot rising fluids took place adjacent to fumarolic conduits in deep parts of the fumaroles deposition of incrustation minerals the components of which were carried upward by fumarolic gases took place in the upper part of the ejecta mostly in the fallout layers the permeability difference between the ash flow tuff and the overlying coarse dacite fallout was a critical factor in promoting the abrupt gradients in temperature pressure and fo2 that resulted in deposition of minerals from the fumarolic gases the permeability difference between nonwelded ash flow tuff and overlying fine grained fall layers in the lower vtts is less pronounced the total mass of fumarolically deposited minerals appears large at first glance owing to the conspicuous coloration by fe minerals the mass is appreciably less than is apparent however because most incrustations are composed largely of ejecta coated or cemented by fine grained fumarolic minerals a large mass of unstable incrustation minerals mainly chlorides and sulfates reported during the 1917 1919 studies have since been removed by dissolution and weathering in the vent region argillic alteration that followed high temperature degassing is localized along arcuate subsidence fractures in fallback ejecta at widely scattered residual orifices fumarolic gases presently are near neutral steam and temperatures are as hot as 90 c

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

fumarole distribution morphology and encrustation mineralogy associated with the 1986 eruptive deposits of mount st augustine alaska numerous rootless fumaroles were developed on pyroclastic flows and a lava flow generated during the march 1986 eruptive cycle of mount st augustine gases issued from fumarole vents with four different shapes fissure phreatic explosion crater single multiple ovoid opening and diffuse multiple opening fumarole distribution and morphology were controlled by preeruption drainage and topography as well as by the thickness compaction and settling of the flow deposits fumarole temperatures measured in june and july 1986 ranged from 75 394 c varying amounts of colorful and often roughly zoned encrustations are associated with all fumarole vent shapes only six types of crystalline phases were detected by x ray diffraction with gypsum the most abundant mineral followed by anhydrite sulfur tridymite halite and soda alum scanning electron microscopy and energy dispersive x ray analysis revealed a number of amorphous phases mainly halogen rich as well as other minor crystalline phases the mineral assemblages in the encrustations suggest formation conditions for these deposits within a general range of 25 250 c in an oxidizing environment many of the amorphous phases are metastable and upon cooling of the fumarole lose nonstructural water and crystallize to more stable forms the high halogen contents of the fumarole condensates and the mineralogy chemistry and morphology of the encrustations support leaching of the andesitic ash and lava flow by condensed acid vapors as the primary source for the chemical components contained in the encrustations comparison of traceelement sr ba v co ni and cr contents in unaltered and altered ash suggests that trace element distribution follows a pattern of isomorphic substitution in the encrustation phases

welded breccias in andesite lavas flow breccias formed at the margins of blocky andesite lavas can be reheated welded and sometimes deformed to form rocks reminiscent of welded pyroclastic rocks reheating occurs due to advection of heat from the flow to the basal breccias surface breccias also infill extensional crevasses which are later closed and compressed strongly deformed welded breccia bands form within the flow levees the angular breccia clasts can be deformed into fiamme

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

determining the extinction cross section of aggregating fish when fish are aggregated over a flat bottom and fish and bottom echoes can be distinguished it is possible to determine the fish extinction cross section by a simple application of the echo integration method the theory for this is developed measurements at 38 khz are presented for aggregations of the same 1983 year class of herring over flat bottomed fjord areas in 1988 1990 and 1991 the ratio of extinction and backscattering cross sections is found to lie in the approximate range from 1 2 2 3 depending on fish size and time of day

volcanic dilemma flow or blow volcanic eruptions can be either slow outpourings of lava or explosive ejections of fragmented rock in his perspective dingwell discusses the physical and chemical properties of magma that lead to these two styles of eruption new experiments and computer simulations are pointing to the involvement of solidlike behavior of liquid magma when it makes a transition to a glass

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

a new middle cambrian radiodont from north china implications for morphological disparity and spatial distribution of hurdiids abstract hurdiidae a clade of radiodonta that has attracted widespread attention in recent years is notable for their massive tripartite cephalic carapace and a wide variety of feeding habits although the hurdiidae has a cosmopolitan distribution in the cambrian their miaolingian occurrences are restricted to laurentia here we describe a new hurdiid radiodont cordaticaris striatus gen et sp nov based on well preserved specimens from the middle cambrian miaolingian drumian zhangxia formation in shandong province north china this new taxon is characterized by a heart shaped central element with linear ornament frontal appendages equipped with nine blade like endites including seven subequal elongate endites a node bearing peytoia type oral cone and a trunk covered with rows of setal structures cordaticaris not only enriches the taxonomic diversity of hurdiidae but also further documents the great morphological disparity of hurdiid sclerotized body elements the discovery of soft bodied fossils in the zhangxia formation also stresses the great potential of the miaolingian strata of north china for the preservation of non biomineralized organisms

fumarolic pipes in the tshirege member of the bandelier tuff on the pajarito plateau jemez mountains new mexico the objective of this study is to demonstrate the relationships among devitrification vapor phase alteration localization of gas emanations into fumarolic pipes and initial deformation of the ash flow sheet during cooling and lithification utilizing a unique and temporary exposure of the tshirege member of the bandelier tuff near los alamos new mexico we identify several zones of distinctly preserved fossil fumarolic activity the fumarolic zones vary in width from a few centimeters to more than a meter almost ubiquitously these zones demonstrate fines depletion induration of the margins upward flaring geometries and intense fracturing of overlying geologic units the fumaroles were preferentially located on post welding early formed cooling joints that vented to the surface after the vapor phase alteration stage the pipes were regularly spaced at distances of approximately 4 5 m n s to 7 m e w in turn the pipes were covered by a surge deposit and overlying tuff which rapidly lithified the overlying tuff was then brecciated during continued fumarolic pipe emissions geochemical evaluations confirm the presence of high temperature mineral scapolite indicative of transport of hot volcanic gases through these zones the pipe centers and walls are depleted in sio2 and enriched in al2o3 and feo the overlying tuff breccia zones are enriched in al2o3 feo and mgo and depleted in sio2 nao and k2o from comparison to other ignimbrite cooling histories the fissures fumaroles and structures observed all likely formed in the first few decades after the deposition of the upper tshirege subunits this may have significant implications as to timing of initial cooling fractures and subsequent consolidation of gas emission pathways