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

apparent welding textures in altered pumice rich rocks fiamme and eutaxitic texture are common in ancient non welded pumice breccias the fiamme are phyllosilicate rich lenses that define a bedding parallel foliation resembling eutaxitic texture in welded ignimbrites pumice breccias in the cambrian mount read volcanics australia are composed of tube pumice clasts bubble wall shards plagioclase crystal fragments and volcanic lithic clasts and contain dark green fiamme and stylolites these sericite or chlorite sericite fiamme are aligned roughly parallel to regional bedding and are enclosed in pale pink feldspar quartz altered domains of uncompacted randomly oriented tube pumice clasts and bubble wall shards similarly pumice breccias in the miocene green tuff belt in the hokuroku basin japan comprise tube pumice clasts fiamme bubble wall shards crystal fragments and non vesicular volcanic clasts the dark green wispy smectite or chlorite fiamme are aligned parallel to bedding and are enclosed in domains of variably clay zeolite and sericite altered uncompacted pumice clasts and shards the fiamme in these examples vary in length 0 5 mm to 1 m length to height ratio 3 1 to 40 1 shape e g flame like feathery branching bow tie wedge and internal texture porphyritic massive vesicular or stylolitic the fiamme define an anastomosing bedding parallel pre tectonic eutaxitic texture delicate feathery terminations and locally preserved vesicles indicate that the fiamme are altered compacted pumice clasts the presence of bedding parallel stylolites and solution seams undeformed shards and pumice clasts adjacent to the fiamme and the overprinting of fiamme by the regional cleavage suggest that the fiamme formed during diagenesis prior to tectonic deformation and that compaction did not involve welding fiamme and eutaxitic texture in these examples resulted from the alteration dissolution and mechanical compaction of cold tube pumice clasts during diagenesis and lithification

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

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

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

galileo at io results from high resolution imaging during late 1999 early 2000 the solid state imaging experiment on the galileo spacecraft returned more than 100 high resolution 5 to 500 meters per pixel images of volcanically active io we observed an active lava lake an active curtain of lava active lava flows calderas mountains plateaus and plains several of the sulfur dioxide rich plumes are erupting from distal flows rather than from the source of silicate lava caldera or fissure often with red pyroclastic deposits most of the active flows in equatorial regions are being emplaced slowly beneath insulated crust but rapidly emplaced channelized flows are also found at all latitudes there is no evidence for high viscosity lava but some bright flows may consist of sulfur rather than mafic silicates the mountains plateaus and calderas are strongly influenced by tectonics and gravitational collapse sapping channels and scarps suggest that many portions of the upper 1 kilometer are rich in volatiles

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

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

the effect of water on the viscosity of a haplogranitic melt under p t x conditions relevant to silicic volcanism the viscosities of hydrous haplogranitic melts synthesized by hydrothermal fusion at 2 kbar pressure and 800 to 1040 c have been measured at temperatures just above the glass transition and at a pressure of 1 bar using micropenetration techniques the micropenetration viscometry has been performed in the viscosity range of 109 pa s to 1012 pa s the samples ranged in water content from 0 4 to 3 5 wt for samples with up to 2 5 wt h2o the water contents have been determined using infrared spectroscopy obtained before and after each viscometry experiment to be constant over the duration of the measurements above this water content a measurable loss of water occurs during the viscometry the viscosity data illustrate an extremely nonlinear decrease in viscosity with added water the viscosity drops drastically with the addition of 0 5 wt of water and then shallows out at water contents of 2 wt an additional viscosity datum point obtained from the analysis of fluid inclusions in a water saturated hpg8 confirms a near invariance of the viscosity with the addition of water between 2 and 6 wt these measurements may be compared directly with the data of hess et al 1995 in press for the effects of excess alkali and alkaline earth oxides on the viscosity of hpg8 also obtained at 1 bar the viscosity of the melts compared on an equivalent molar basis increases in the order h2o li2o na2o k2o rb2o cs2o bao sro cao mgo beo the extraordinary decrease in melt viscosity with added water is poorly reproduced by the calculation scheme of shaw 1972 for the range of water contents investigated here the speciation of water in the quenched glasses can be used to quantify the dependence of the viscosity on hydroxyl content considering only the hydroxyl groups as active fluidizers in the hydrous melts the nonlinearity of the viscosity decrease and the difference with the effects of the alkali oxides becomes larger consequences for degassing calcalkaline rhyolite are discussed

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

growth destruction and facies architecture of effusive and explosive volcanics in the miocene shama basin southwest of saudi arabia subaqueous subaerial volcanism in a lacustrine setting abstract this paper presents for the first time detailed stratigraphical and petrographical analyses of the harrat shama volcanic basin hsvb of miocene age in sw saudi arabia this basin is filled by abundant volcanics intercalated with fluvio lacustrine sedimentary successions these successions are characterized by a syneruptive intereruptive cycle which results from the intercalation periods of volcanics and volcaniclastic sedimentation along with periods of fluvial incision two depositional sequences showing an overall fining upward trend are preserved within the hsvb the first comprises hyaloclastites zeolite bearing bedded tuffs and bedded accretionary lapilli tuffs interpreted as deep lake whereas the second is made up of extensive pumiceous pyroclastic flows peperitic breccias and basaltic lava flows which are in turn capped by siliciclastic deposits of lacustrine deposits the passage between two sequences is defined by lateral variations of the lithofacies and textural features reflecting a change in paleogeography the fining upward volcaniclastic deposits of the hsvb sequence are interpreted to represent the sedimentation pattern induced by tectonic subsidence with the influence of volcaniclastic sediment input indicating well established proximal distal relationship hsvb was evolved through three stages including pre syn and post caldera stages the first two stages correspond to proximal aggradational syn eruptive event whereas the latter stage refers to distal degradational inter eruptive event these stages overlap in time as shown by the alternation of explosive effusive and their resedimented deposits reflecting diverse eruptive sequences formed by different fragmentation processes the difference in the latter coupled with the variability in the magma water interaction and the nature of substrate are responsible for such diversity resulting in progressive environmental changes from subaqueous to subaerial regime from bottom to top the development of such contrast in the depositional sequences reflects variation in the sediment flux and subsidence rate during the evolution of the hsvb

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

rheomorphic structures in a high grade ignimbrite the nuraxi tuff sulcis volcanic district sw sardinia italy deposits of the 15 8 ma nuraxi explosive eruption crop out in the sulcis volcanic district southwestern sardinia they consist of a decimeter thick pumice fallout layer overlain with no apparent temporal break by several tens of meters thick high grade ignimbrite the eruption was from a vent probably located 20 30 km north of s pietro island and it tapped a compositionally uniform rhyolite magma reservoir sio2 70 72 wt bearing about 20 vol crystals the ignimbrite consists of a a lower ignimbrite li 2 m thick fine grained and glassy grading upward into a partly devitrified lapilli tuff and b an upper ignimbrite ui that is an intensely welded to lava like lapilli tuff the deposit has an inferred maximum runout of about 80 km and overall dispersal thickness characteristic of a low aspect ratio ignimbrite over most of its outcrop the tuff rests on a flat horizontal paleosurface and forms an ignimbrite plateau with uniform thickness and structural characteristics however in some areas it shows extensive folding and over thickening as a result of rheomorphic flow into topographic depressions structural analysis of the ignimbrite reveals the presence of a pervasive foliation and lineation as well as macroscopic and microscopic flow structures occurrence of abundant widely distributed stretched cavities in the tuff indicates that a substantial amount of gas was trapped in the tuff during aggradation the observed pattern of structures of the ignimbrite plateau accords well with a conceptual model in which syn depositional structures are formed during non particulate basal laminar flow due to agglutination of juvenile particles within the lower part of the pyroclastic density current the flow was controlled by the substantial transfer of momentum from the high speed particulate upper part of the current to the top of the non particulate part of the current and the onset of a complex relationship between welding flow and sedimentation processes it is proposed in particular that elongated gas pockets played a major role in guiding the laminar shear within the non particulate part of the current in flat horizontal areas eventually promoting brittle failure of the deforming tuff mass along shear planes

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

rheomorphism of welded tuffs abstract peralkaline welded tuffs from the islands of gran canaria canary islands and pantelleria italy show abundant evidence for post depositional flow it is demonstrated that rheomorphism or secondary mass flowage can occur in welded tuffs of ignimbrite and air fall origin the presence of a linear fabric is taken as the diagnostic criterion for the recognition of the process deposition on a slope is an essential condition for the development of rheomorphism after compaction and welding internal structures produced during rheomorphic flow can be studied by the methods of structural geology and show similar dispositions to comparable features in sedimentary slump sheets it is shown that secondary flowage can occur in welded tuffs emplaced on gentle slopes provided that the apparent viscosity of the magma is sufficiently low compositional factors favor the development of rheomorphism in densely welded tuffs of peralkaline type

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

miocene rhyolitic welded tuff infilling a funnel shaped eruption conduit shiotani southeast of matsue sw japan at shiotani sw japan rhyolitic welded tuff forms a steep sided funnel shaped body confined by paleogene granitic rocks to an elliptical area 1 1 5 km across the shiotani welded tuff is pervasively welded and foliated concordantly with the contact that dips inward at angles of 70 90 in contrast nearby contemporary volcaniclastic deposits are non welded and gently inclined near the contact with the granite the tuff is plastically deformed and shows lineations that plunge inward at angles of 40 65 lithic and crystal clasts in the rheomorphic outer part are rotated in a plane normal to the foliations and parallel to the lineations indicating downward flow of the welded tuff the geometry and internal structures suggest that the shiotani welded tuff was emplaced and welded in a funnel shaped eruption conduit upon collapse of a plinian or phreatoplinian eruption column the majority of the conduit filling pyroclasts probably fell back en masse into the conduit heat and steam from underlying magma and diffusion of interstitial volatiles into the glass perhaps reduced the viscosity of juvenile pyroclasts and facilitated welding in the conduit especially at deep levels the hot welded pyroclasts then flowed down the conduit wall during welding compaction and retreat of the magma these processes resulted in increased welding toward the contacts and welding foliations concordant with the steep wall emplacement of nearby correlative volcaniclastic mass flow deposits in a shelf to upper bathyal environment suggests a possibility that when active the shiotani conduit was under the sea welding compaction would occur even under the sea provided that the steam generated in the upper part of the conduit fill prevented water access

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

sedimentation and welding processes of dilute pyroclastic density currents and fallout during a large scale silicic eruption kikai caldera japan abstract sedimentation and welding processes of the high temperature dilute pyroclastic density currents and fallout erupted at 7 3 ka from the kikai caldera are discussed based on the stratigraphy texture lithofacies characteristics and components of the resulting deposits the welded eruptive deposits unit b were produced during the column collapse phase following a large plinian eruption and preceding an ignimbrite eruption and can be divided into two subunits units b l and b u unit b l is primarily deposited in topographic depressions on proximal islands and consists of multiple thin u shows densely welded stratified facies composed of alternating lithic rich and pumice rich layers the layers mantle lower units and are sometimes viscously deformed by ballistics the sedimentary characteristics of unit b l such as welded stratified or cross stratified facies indicate that high temperature dilute pyroclastic density currents were repeatedly generated from limited magma water interactions it is thought that dense brittle particles were segregated in a turbulent current and were immediately buried by deposition of hot lighter pumice rich particles and that this process repeated many times it is also suggested that the depositional temperature of eruptive materials was high and the eruptive style changed from a normal plinian eruption through surge generating explosions unit b l into an agglutinate dominated fallout eruption unit b u on the basis of field data welded pyroclastic surge deposits could be produced only under specific conditions such as 1 rapid accumulation of pyroclastic particles sufficiently hot to weld instantaneously upon deposition and 2 elastic particles interactions with substrate deformation these physical conditions may be achieved within high temperature and highly energetic pyroclastic density currents produced by large scale explosive eruptions

volcanic geology of hadriaca patera and the eastern hellas region of mars hadriaca patera is a low relief volcano in the southern highlands of mars northeast of the hellas basin layered friable deposits composing the extensive channeled flanks of the volcano surround a well defined summit caldera containing late stage eruptive products the morphologic characteristics of the channels suggest erosion by groundwater sapping and surface runoff the erosional morphology of the volcano the lack of lava flow features and the friable nature of the flank materials indicate that hadriaca patera consists predominantly of pyroclastic deposits gravity driven flow models demonstrate that the distribution of flank materials can be attributed to the emplacement of pyroclastic flows both magmatic and hydromagmatic eruption models are viable for the magmatic case the necessary mass eruption rates 107 108 kg s ejection velocities 400 m s and volatile contents 1 5 3 0 wt h2o are consistent with parameters derived for terrestrial plinian eruptions for the hydromagmatic case the required energy conversion efficiencies are comparable to those of laboratory experiments and the inferred permeability of the martian crust allows large amounts of groundwater to be transported rapidly at flow rates of 103 104 m3 s into the region models of cooling during emplacement indicate that welding of pyroclastic flows can occur at large distances hundreds of kilometers from a source vent on mars the layering within hadriaca patera could be attributed to welding of pyroclastic flows that would control its susceptibility to erosion morphologic similarities between hadriaca and tyrrhena paterae suggest a similar volcanic history with an early pyroclastic dominated shield building phase followed by effusive eruptions at their summit calderas and on the flank of tyrrhena patera the formation of the extensive ridged plains of hesperia planum following the formation of the highland paterae supports the interpretation of a transition from explosive to effusive volcanism in the eastern hellas region a progressive depletion of volatiles on mars is consistent with the morphologic properties of highland paterae and other central vent volcanoes a predominance of hydrovolcanic eruptions in the development of hadriaca and tyrrhena paterae would imply that the transition in volcanic eruption style can be attributed to a volatile depletion of the crust whereas magmatic eruptions at the paterae would be indicative of temporal changes in martian magmas

morphological structural and textural variations in the 1988 1990 andesite lava of lonquimay volcano chile the 1988 1990 eruption of lonquimay volcano chile 38 s formed a 10 2 km long andesite lava with a volume of 0 23 km 3 over a period of 13 months the lava extrusion rate decreased with time as chamber pressure and vent dimensions decreased the velocity of the flow front decreased exponentially with distance from vent as a consequence of cooling and the increase of apparent viscosity at the flow front the lava developed a central channel which decreased in width and depth with time three prominent lava levees were formed on each margin and resulted from abandonment as the channel decreased in width as a result of a rapid decrease of flow rate over the first 100 days of activity a fourth major levee developed in february during a brief period of flow rate increase down the main channel but its walls were gradually exposed as the lava depth again decreased due to declining flow rate the structure of lava levees depended on their age and longevity of the flow in the adjacent channel initial levees were formed in the first few days as the lava spread laterally and then retreated leaving levees of massive lava more mature rubble levees were formed during the next month by the lava pushing and then shearing aa and blocky breccia which formed on the cooling flow margin fragmentation and abrasion formed a characteristic zonation in the levees a basal zone consists of very poorly sorted matrix rich breccia with very rounded vesicular clasts and bimodal grain size distribution the basal breccia zone strongly resembles block and ash flow deposits this zone passes up into a zone of clast supported clinker breccia which becomes increasingly matrix poor and coarser with clasts becoming more angular upwards the crest of the levee is composed of large 10 100 cm angular to subangular blocks with no matrix the zoned levees form after the active lava channel suddenly narrows lava depth initially increases and breccias are deposited on the channel margins and acquire the zoned structure by progressive shearing and accretion of clinkery aa breccia the lava level then drops exposing the steep inner scarp of a levee the most mature levee type formed in a long lived channel over several months the outer wall of the levee consists of zoned breccia but the inner wall consists of a massive curving wall of strongly foliated lava with well developed horizontal striations and ductile reidel shears the massive foliated facies is a consequence of prolonged flow which coats strongly sheared lava onto the inner levee wall scanning electron microscopy shows that the aa clinker clasts and foliated lava from the levee walls form at low melt fractions 15 in the last three months of the eruption the flow front ceased to advance but thickened as lava drained from proximal regions and intruded into the interior of the distal lava the last stages of lava movement were characterized by updoming in the central channel a lava surface feature named here armadillo structure was formed by deformation of the cooler but still ductile lava crust the deformation caused by underflow produced reidel shears dipping upstream and doming of the lava due to rotation of the shear planes the study demonstrates that lava morphology structure and texture are strongly influenced by variations of effusion rate local flow rate channel topography and thermal maturity of the lava which is reflected in downstream changes in viscosity

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

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

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

early holocene deglaciation of expedition and strand fiords canadian high arctic terrestrial and marine records provide the first reconstruction of past glacial events on west central axel heiberg island lateral moraines along expedition fiord define an early holocene ice margin when a trunk glacier occupied the inner half of the fiord limited sediment accumulation in the outer fiord 8 4 ka bp principally by calving of an unstable ice front to a position well upvalley of the present ice limit rapid deglaciation is evidenced by i an absence of ice marginal landforms and related marine biofacies ii a scarcity of raised marine sediments and iii exceptionally thin sediments 5 20 m in the inner fiord a transition from a hiatella arctica mya truncata macrofossil assemblage to an astarte borealis mya truncata assemblage reflects a change from moderate to low energy depositional environments at the fiord head by 7 9 ka bp the extensive ice advance into expedition fi

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