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

shrimp u pb zircon dating of grenville age events in the western part of the musgrave block central australia shrimp u pb zircon dating of structurally constrained felsic orthogneiss samples in the western musgrave block has been used to delineate discrete magmatic and metamorphic events at c 1300 and c 1200 ma the dating of pre d1 and post d1 felsic orthogneiss constrains d1 to have occurred at 1312 16 to 1324 4 ma this is the first geochronological study to identify such a metamorphic and deformation event in the musgrave block d1 was accompanied by a major magmatic event involving the emplacement of voluminous felsic orthogneiss between 1296 and 1324 ma zircon overgrowths on numerous igneous zircon cores give a consistent age of c 1200 ma reflecting zircon growth during a second high grade metamorphic event d2 this c 1200 ma metamorphic event was followed by the intrusion of a c 1190 ma megacrystic granite the c 1300 and c 1200 ma events in the musgrave block can be tentatively correlated with metamorphic events in the albany fraser orogen and the windmill islands and bunger hills in east antarctica a major continuous grenville age orogenic belt joining these areas may have represented a plate boundary between the pre rodinian proto australian continent and proto antarctica during the formation of rodinia in the mesoproterozoic

timing the breakup of a proterozoic supercontinent evidence from australian intracratonic basins in the late proterozoic several broad shallow intracratonic depressions appeared across a vast area of central australia the basins which all contain shallow marine to fluvial successions appear to have been tenuously interconnected through much of their history analysis of their fill by means of tectonic subsidence curves suggests that they are the product of two separate and distinct periods of crustal extension one at about 900 ma and a second at about 600 ma these extensional episodes were probably the result of failed rifting events that almost fragmented the australian continent during the late proterozoic the second period of extension almost certainly relates to the breakup of a proterozoic supercontinent the results suggest that sediments preserved in the relatively protected environment of interior basins may provide a more subtle record of major tectonic events than continental margin sequences that were exposed to major continental interactions

field investigation of frazil jam evolution a case study using field measurements of frazil jams from 1982 to 1989 in the hequ reach of the yellow river this paper discusses both the mechanism of evolution of a frazil jam and the associated variation in water level the variation in water level depends not only on the thickness of the frazil jam but also on the ice discharge an empirical power law relationship between the dimensionless ice thickness and froude number is established it has been found that the critical froude number under an ice cover condition describing the transition from an ice jam to a hanging dam overloaded ice jam is about 0 075 with a dimensionless ice thickness of about 0 4 interestingly the results from the st lawrence river show a similar tendency with similar limits

sediment slump structures a review of diagnostic criteria and application to an example from newfoundland abstract in the study of the history of deformed sedimentary rocks it is important that the cause of the deformation and the timing of deformation relative to metamorphic and tectonic events be assessed there is a continuous gradation between the deformation of freshly deposited sediments by gravitational forces and the deformation of well lithified sediments by tectonic forces so determining the degree to which gravity tectonism and lithification influenced deformation can be very difficult the characteristics that may be considered in determining the origin of deformational structures in sedimentary rocks can be divided into five categories ductile deformation structures 1 can provide definitive criteria for recognizing post lithification deformation but not pre lithification deformation this also holds for 2 brittle deformation structures and decollements overprinting of sediment reworking or remobilization structures 3 are the best criteria for recognizing pre lithification structures fabrics 4 can be very useful but are as yet poorly understood and there are misconceptions in the literature about what sort of fabrics are or are not found in unlithified sediments spatial relationships 5 may also tell us a great deal but are most useful when taken in conjunction with other types of evidence complex fold patterns in sandstones on farmer head north central newfoundland provide a case study for criteria that may be used to determine the degree of lithification during deformation although these folds have been interpreted as slump folds by earlier workers fabrics and spatial relationships point to a tectonic post lithification origin for the structures a conclusion which has significant implications for the interpretation of the regional geology

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

precambrian tectonics and the physicochemical evolution of the continental crust i the gabbro eclogite transition revisited abstract we report the first occurrence of retrograde eclogite shear zones cutting proterozoic granulites from australia the musgrave range granulites were metamorphosed at about 12 kbar pressure and 850 900 c the granulites then cooled isobarically and developed a variety of lower temperature mineral reaction coronas typical of garnet granulites these rocks are cut by a major shear zone the davenport shear in which lower temperature fine grained eclogite developed this demonstrates that mafic rocks in the deep crust can cool down into the thermodynamic stability field of eclogite without undergoing transformation to eclogite we agree with harte et al 1981 that rocks lacking a fluid phase in stable tectonic regions are unlikely to maintain local equilibrium at ambient geotherm conditions until depths well within the upper mantle are reached it is proposed that many isobaric high pressure granulites could have cooled down into the eclogite field before their later excavation to the surface and did not transform to an eclogite mineralogy the inability of the lower crust to transform into eclogite by retrograde reaction has important implications for the seismic petrologic and geochemical character of the lower crust the continental moho cannot be caused by the gabbro eclogite transition preservation of relatively low density high temperature granulite and gabbroic mineral assemblages in the lower continental crust prohibits major recycling of lower crust back into the mantle once it is removed from active plate boundaries however in island arcs with magmatically thickened crust mafic rocks at a depth of 60 to 70 km will be in the eclogite stability field at high enough temperatures 800 900 c for eclogite to rapidly form and sink back into the mantle this could result in a net accreted crust of broadly andesitic composition in contrast later growth of continental crust by basalt underplating in stabilised shield regions will not result in subsequent eclogite formation and lower crustal recycling back into the mantle will not occur thus estimates of the average composition of the continental crust which are based on a mass balance of plate margin basaltic input from the mantle will not be adequate

ice regime of the lower peace river and ice jam flooding of the peace athabasca delta the peace athabasca delta pad in northern alberta is one of the world s largest inland freshwater deltas home to large populations of waterfowl muskrat beaver and free ranging wood bison beginning in the mid 1970s a paucity of ice jam flooding in the lower peace river has resulted in prolonged dry periods and considerable reduction in the area covered by lakes and ponds that provide a habitat for aquatic life in the pad region using archived hydrometric data and in situ observations the ice regime of the lower peace is described and quantified setting the stage for identification of the conditions that lead to ice jam flooding and replenishment of delta habitat the first such condition is the occurrence of a mechanical as opposed to a thermal breakup event second the river flow should be at least 4000 m3 s and third an ice jam should form within the last 50 km of the peace river the type of breakup event depends on the freeze up stage and spring flow the former has increased as a result of flow regulation and the latter has decreased owing to changing climatic patterns both trends tend to inhibit the occurrence of mechanical breakups and contribute to less frequent ice jam flooding potential mitigation strategies are discussed copyright 2006 crown in the right of canada published by john wiley sons ltd

tephra fountaining rheomorphism and spatter flow during emplacement of the pitigliano tuffs latera caldera italy abstract toward the end of the main ash flow phase ca 156 ka of the latera caldera central italy explosive eruptions near the nw caldera rim produced coarse grained tephra exposed in 35 m high faces of the toscapomice casa collina quarry the pyroclastic sequence fills a narrow sw ne paleovalley and records the near continuous eruption of a compositionally zoned magma batch white phonolitic pumice clasts abruptly grade upward to and partly intrude black spatter like pumice fragments also of phonolite composition this sequence in turn is overlain by densely welded tuff that grades upward to tephriphonolitic scoriae and ash the upper welded part of this sequence referred to as the vulcanite complessa di pitigliano has long been among the more enigmatic deposits in the latera area this sequence displays evidence for both fall and flow emplacement and is here interpreted as an ash fountain fed deposit that locally welded as spatter yet became sufficiently fluidized in distal parts to behave as a spatter rich pyroclastic flow this mechanism requires that the eruption underwent abrupt transition from a vigorously convecting column to a weakly convecting ash fountain and that this period of explosive activity culminated with unusually high accumulation rates the rapidly emplaced tephra retained sufficient heat to weld by compaction and undergo rheomorphism close to the vent yet contained high enough proportions of fluidized dust to have flowed outward along the northern caldera rim and eventually to have welded by vapor phase sintering

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

monazite behaviour and age significance in poly metamorphic high grade terrains a case study from the western musgrave block central australia abstract integrated in situ textural chemical and electron microprobe age analysis of monazite grains in a migmatitic metapelitic gneiss from the western musgrave block central australia has identified evidence for multiple events of growth and recrystallisation during poly metamorphism in the mesoproterozoic garnet sillimanite bearing metapelite underwent partial melting and segregation to palaeosome and leucosome during metamorphism between 1330 and 1296 ma with monazite grains in leucosome recording crystallisation at 1300 ma monazite breakdown during melting is inferred to have occurred in the palaeosome during a subsequent granulite facies event at 1200 ma deformation and metamorphism of leucosome and palaeosome resulted in partial disturbance of ages and potential minor growth on 1300 ma monazite in leucosome growth of new high y hree monazite in palaeosome domains occurred during garnet breakdown in the presence of sillimanite to cordierite and spinel as a result of post peak isothermal decompression diffusive enrichment of resorbed garnet rims in y hree suggests garnet breakdown occurred slower than volume diffusion of ree monazite in both palaeosome and leucosome were subsequently partially to penetratively recrystallised during a retrogression event that is suggested to have occurred at 1150 1130 ma the intensity of recrystallisation and disturbance of ages appears linked to proximity to retrogressed garnet porphyroblasts and their occurrence in the relatively reactive or fertile local environments provided by the palaeosome mesosome volumes which caused localised changes in retrogressive fluids towards compositions more aggressive to monazite like reaction textures it is apparent that domainal equilibrium and reaction may control or at least strongly influence monazite ree and u th pb chemistry and hence ages

ice jam caused fluvial gullies and scour holes on northern river flood plains abstract two anomalous fluvial landforms gullies and scour holes eroded into flood plains bordering meandering and braiding river channels have not been previously reported we observed these features along the milk river in southern alberta canada and northern montana usa which has a history of frequent 50 probability of recurrence and high magnitude 12 probability of recurrence greater than bankfull ice jam floods gullies have palmate and narrow linear shapes with open ends downvalley and measure up to 208 m long 139 m wide 3 5 m deep below bankfull channel ice jams reroute river water across meander lobes and cause headward gully erosion where flow returns to the main channel erosion of the most recent gully was observed during the record 1996 ice breakup flood and ice jams scour holes bowl shaped closed depressions eroded by water vortices beneath and between grounded ice jam blocks measure up to 91 m long 22 m wide 4 5 m deep ice jam caused gullies may be precursors to the formation of u shaped oxbow lakes and multiple channels common in many northern rivers

river ice jams theory case studies and applications the theory of river ice jams as developed by several investigators over the past two decades is reviewed and two methods for analyzing case studies are developed the first method is based on a d

river ice break up freeze up a review of climatic drivers historical trends and future predictions river ice plays a fundamental role in biological chemical and physical processes that control freshwater regimes of the cold regions moreover it can have enormous economic implications for river based developments all such activities and processes can be modified significantly by any changes to river ice thickness composition or event timing and severity this paper briefly reviews some of the major hydraulic mechanical and thermodynamic processes controlling river ice events and how these are influenced by variations in climate a regional and temporal synthesis is also made of the observed historical trends in river ice break up freeze up occurrence from the eurasian and north american cold regions this involves assessment of several hydroclimatic variables that have influenced past trends and variability in river ice break up freeze up dates including air temperature indicators e g seasonal temperature 0 c isotherm dates and various degree days and large scale atmospheric circulation patterns or teleconnections implications of future climate change on the timing and severity of river ice events are presented and discussed in relation to the historical trends attention is drawn to the increasing trends towards the occurrence of mid winter break up events that can produce especially severe flood conditions but prove to be the most difficult type of event to model and predict

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

welded tuffs deformed into megarheomorphic folds during collapse of the mcdermitt caldera nevada oregon the montana mountains contain part of a resurgent dome complex within the southwestern sector of the mcdermitt caldera exposed intracaldera units consist of welded tuffs lavas domes and epiclastic deposits the majority of volcanic units have a comenditic composition but a few units are nonperalkaline and biotite bearing a deformed series of comenditic welded tuffs occur in a strongly folded rheomorphic zone more than 500 m thick the geometry of vitrophyres flow bands foliations and lineations causes the rheomorphic zone to superficially resemble a series of intrusive domes however the contacts between units are conformable the vitrophyres are continuous from one fold to the next and the units display pumice shards and other features common in welded tuffs the deformed tuff sequence lies on top of biotite tuffs and epiclastic deposits that are tilted and faulted but not folded and beneath relatively undeformed tuffs and lake deposits this relationship suggests that their large scale folds in the deformed sequence developed shortly after caldera collapse

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

eruption and emplacement of a basaltic welded ignimbrite during caldera formation on gran canaria the 14 1 ma old composite ignimbrite cooling unit p1 45 km3 on gran canaria comprises a lower mixed rhyolite trachyte tuff a central rhyolite basalt mixed tuff and a slightly rhyolite contaminated basaltic tuff at the top the basaltic tuff is compositionally zoned with a an upward change in basalt composition to higher mgo content 4 3 5 2 wt b variably admixed rhyolite or trachyte commonly 5 wt and c an upward increasing abundance of basaltic and plutonic lithic fragments and cognate cumulate fragments the basaltic tuff is divided into three structural units i the welded basaltic ignimbrite which forms the thickest part c 95 vol and is the main subject of the present paper ii poorly consolidated massive bomb and block rich beds interpreted as phreatomagmatic pyroclastic flow deposits and iii various facies of reworked basaltic tuff tuff unit i is a basaltic ignimbrite rather than a lava flow because of the absence of top and bottom breccias radial sheet like distribution around the central tejeda caldera thickening in valleys but also covering higher ground and local erosion of the underlying p1 ash a gradual transition from dense rock in the interior to ash at the top of the basaltic ignimbrite reflects a decrease in welding the shape of the welding profile is typical for emplacement temperatures well above the minimum welding temperature a similar transition occurs at the base where the ignimbrite was emplaced on cold ground in distal sections in proximal sections the base is dense where it was emplaced on hot felsic p1 tuff the intensity of welding especially at the base and the presence of spherical particles and of mantled and composite particles formed by accretion and coalescence in a viscous state imply that the flow was a suspension of hot magma droplets the flow most likely had to be density stratified and highly turbulent to prevent massive coalescence and collapse model calculations suggest eruption through low pyroclastic fountains 1000 m high with limited cooling during eruption and turbulent flow from an initial temperature of 1160 c the large volume of 26 km3 of erupted basalt compared with only 16 km3 of the evolved p1 magmas and the extremely high discharge rates inferred from model calculations are unusual for a basaltic eruption it is suggested that the basaltic magma was erupted and emplaced in a fashion commonly only attributed to felsic magmas because it utilized the felsic p1 magma chamber and its ring fissure conduits evolution of the entire p1 eruption was controlled by withdrawal dynamics involving magmas differing in viscosity by more than four orders of magnitude the basaltic eruption phase was initially driven by buoyancy of the basaltic magma at chamber depth and continued degassing of felsic magma but most of the large volume of basalt magma was driven out of the reservoir by subsidence of a c 10 km diameter roof block which followed a decrease in magma chamber pressure during low viscosity basaltic outflow

geological strain analysis a manual for the rf φ technique by richard j lisle pergamon press oxford 1985 viii 99 pp 15 hard cover 8 95 soft cover

downstream transition of river ice jams the granular mass theory of river ice jams is extended to their downstream transition as a means of improving understanding of conditions at jam toe the theory is coupled with the equations of motion under and seepage through the jam and the resulting differential equations are solved numerically the present model applies to wide rectangular channels and involves several coefficients whose precise values in nature are unknown example calculations are performed to illustrate the sensitivity of the model to the various coefficients preliminary application of the model to two case studies resulted in plausible findings with regard to grounding of ice jams

dynamic ice formation in channels as a driver for stream aquifer interactions 1 this research introduces and provides evidence of a novel mechanism of stream aquifer exchange caused by dynamic ice formation on streams at a stream site in southwestern wisconsin we document a significant fluctuation in stream depth and the potentiometric surface in the adjacent aquifer during periods of ice formation suggesting that stream aquifer interactions may be influenced by these transient events four years of winter stream data document that dynamic ice formation 1 causes an average increase in stream depth of 106 2 affects stream depth on 20 of days from december through february and 3 substantially alters stream aquifer interactions by reducing the hydraulic gradient toward the channel during ice formation events dynamic ice formation represents a potentially widespread mechanism for altering stream aquifer interactions and associated biogeochemical transformations the sensitivity of this process to air temperature indicates a need for further research on the impacts of climate warming

exposed cross sections through the continental crust implications for crustal structure petrology and evolution abstract the continental crust is exposed in cross section at numerous sites on the earth s surface these exposures which appear to have formed by obduction along great faults during continental collision may be recognized by exposures of deep crustal rocks exhibiting asymmetric patterns of metamorphic grade and age across the faults and by distinctive bouguer anomaly patterns reflecting dipping basement structure and an anomalously deep mantle from an examination of five complexes which meet these criteria it is concluded that the most prominent layering in the crust is not compositional but metamorphic the lower crust consists of granulite facies rocks of mafic to intermediate composition while the intermediate and shallow levels consist predominantly of amphibolite facies gneisses and greenschist facies supracrustal rocks respectively post metamorphic granitic intrusions are common at intermediate to shallow levels position of discontinuities in refraction velocity where present commonly correspond to changes in composition or metamorphic grade with depth the continental crust is characterized by lateral and vertical heterogeneities of varying scale which are the apparent cause of the complex seismic reflections recorded by cocorp field observations coupled with geochemical data indicate a complex evolution of the lower crust which can include anatexis multiple deformation polymetamorphism and reworking of older crustal material the complexity of the crust is thus the result of continuous evolution by recycling and metamorphism through time in a variety of tectonic environments

marine emplacement of welded ignimbrite the ordovician pitts head tuff north wales the pitts head welded ignimbrite records a subaerial large volume eruption and associated pyroclastic current that flowed into the sea the current 15 km from source was steadily sustained with high mass flux and high particle concentration towards its base and it entered the sea without substantial mixing with water and thus without large scale hydroexplosivity and without general cooling it continued to aggrade ignimbrite at 580 c for at least 3 4 km from the original shoreline in water initially 50 m deep entirely subaqueous hot state progressive aggradation and welding of the ignimbrite occurred where the water could not be wholly displaced by the current although eventually the deposit displaced the shore 4 km seawards wet sea floor sediments buried by the ignimbrite were heated and locally fluidized by steam and several square kilometres of hot ignimbrite with variable thicknesses of sedimentary substrate detached and slid downslope directions of sliding and the order of piling up of slide sheets are shown by hot state rheomorphic deformation fabrics and the geometric relations of detachment surfaces extensional disruption of the ignimbrite is marked by breaks in the sheet via which fluidized sediments were mobilized locally to form rootless vents both the initial incursion of the pyroclastic current into the sea and the subsequent submarine sliding of the ignimbrite are likely to have caused tsunamis similar occurrences at modern coastlines presently susceptible to incursion of high mass flux pyroclastic currents e g taupo volcanic zone new zealand neapolitan region italy vicinity of manila taal philippines would according to tsunami propagation behaviour cause significant near and far field coastal hazard

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

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