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

patterns of deposition from experimental turbidity currents with reversing buoyancy turbidity currents are turbulent sediment laden gravity currents which can be generated in relatively shallow shelf settings and travel downslope before spreading out across deep water abyssal plains because of the natural stratification of the oceans and or fresh water river inputs to the source area the interstitial fluid within which the particles are suspended will often be less dense than the deep water ambient fluid consequently a turbidity current may initially be denser than the ambient sea water and propagate as a ground hugging flow but later reverse in buoyancy as its bulk density decreases through sedimentation to become lower than that of the ambient sea water when this occurs all or part of the turbidity current lofts to form a buoyant sediment laden cloud from which further deposition occurs deposition from such lofting turbidity currents containing a mixture of fine and coarse sediment suspended in light interstitial fluid is explored through analogue laboratory experiments complemented by theoretical analysis using a box and cloud model particular attention is paid to the overall deposit geometry and to the distributions of fine and coarse material within the deposit a range of beds can be deposited by bimodal lofting turbidity currents lofting may encourage the formation of tabular beds with a rapid pinch out rather than the gradually tapering beds more typical of waning turbidity currents lofting may also decouple the fates of the finer and coarser sediment depending on the initial flow composition the coarse fraction can be deposited prior to or during buoyancy reversal while the fine fraction can be swept upwards and away by the lofting cloud an important feature of the results is the non uniqueness of the deposit architecture different initial current compositions can generate deposits with very similar bed profiles and grading characteristics highlighting the difficulty of reconstructing the nature of the parent flow from field data it is proposed that deposit emplacement by lofting turbidity currents is common in the geological record and may explain a range of features observed in deep water massive sands thinly bedded turbidite sequences and linked debrites depending on the parent flow and its subsequent development for example a lofting flow may lead to a well sorted largely ungraded or weakly graded bed if the fines are transported away by the cloud however a poorly sorted largely ungraded region may form if during buoyancy reversal high local concentrations and associated hindered settling effects develop at the base of the cloud

coupling between suspended sediment distribution and turbulence structure in a laboratory turbidity current 1 a detailed laboratory experiment reveals the coupling between flow and suspended sediment properties velocity turbulence concentration and granulometry in a low density depositional turbidity current a model is proposed that explains spatial variation in the coupling through size segregation and mixing controlled by the interplay between upward directed turbulence and gravitational settling of suspended particles the head of the flow is strongly turbulent relatively well mixed dilute and coarse grained because upward directed turbulence related to frontal shear and mixing with ambient water generally outbalances particle settling the body is grain size and density stratified with strong turbulence confined to upper and lower flow boundaries here settling is generally more important than turbulence contrasting dynamics of different particle sizes modify the general patterns recognized in the head and body thus preferential settling of the coarsest particles produces normal grading in the lower head in the lower body highly turbulent relatively low downstream velocity fluid packages interpreted as upward moving basal shear induced ejections alternate with less turbulent higher downstream velocity fluid packages relatively coarse grains react to these flow changes by alternate vertical mixing and preferential accumulation near the flow s base whereas the distribution of finer grains remains unaffected kelvin helmholtz waves generated by upper boundary shear penetrate the upper body the zone of maximum flow velocity is typically weakly turbulent and appears to form a barrier to turbulence induced particle exchange between the lower and upper flow regions the coupling model entails periodic changes in grain size measures near the flow s base which may explain the formation of lamination and banding in natural turbidites

mobility of pyroclastic flows and surges at the soufriere hills volcano montserrat the soufriere hills volcano on montserrat has produced avalanche like pyroclastic flows formed by collapse of the unstable lava dome or explosive activity pyroclastic flows associated with dome collapse generate overlying dilute surges which detach from and travel beyond their parent flows the largest surges partially transform by rapid sedimentation into dense secondary pyroclastic flows that pose significant hazards to distal areas different kinds of pyroclastic density currents display contrasting mobilities indicated by ratios of total height of fall h run out distance l area inundated a and volume transported v dome collapse flow mobilities characterised by either l h or a v2 3 resemble those of terrestrial and extraterrestrial cold rockfalls dade and huppert 1998 in contrast fountain fed pumice flows and fine grained secondary pyroclastic flows travel slower but for comparable initial volumes and heights can inundate greater areas

migration of mid ocean ridge volcanic segments the propagation of small offset volcanic spreading centre segments along mid ocean ridge crests may reflect absolute motion of the plate boundary relative to the underlying mesospheric frame such a relationship could be caused by a purely vertical flow of the mantle under spreading centres and would have value in constraining past plate motions from non transform trends generated during along ridge propagation and in linking the major element variability of oceanic crust and upper mantle to the bulk composition and temperatures of mantle ascending under mid ocean ridges

transport capacity of pyroclastic density currents experiments and models of substrate flow interaction 1 one of the more distinctive features of many ignimbrites is the presence of large lithics some greater than meter scale and pumices that have been transported great distances 10 km from the eruptive vent sometimes over steep terrain and expanses of water in many cases these particles have been transported much further than can be explained by aerodynamic forces and ballistic trajectories we examine the forces responsible for transport of large clasts and examine in detail the momentum transfer occurring when particles interact with their boundaries we performed a suite of experiments and numerical simulations to quantify the mass and momentum transfer that occurs when particles interact with a pumice bed substrate and with water substrate two geologically motivated flow end members we find that clasts transported in dilute currents are particularly sensitive to the nature of the boundary and while large particles can skip several times on a water substrate they travel less far than particles that impact pumice bed substrates all else being equal large particles in dense pyroclastic density currents are themselves relatively insensitive to the details of their boundaries however one of the most important ways boundary conditions influence large particles is not through direct interaction but by changing the local concentration of fine particles momentum transfer from fine particles to large particles appears to be required to transport large clasts great distances if initially dense flows become dilute during transport then the transport capacity of large particles in the flow is substantially decreased

turbulent transport and deposition of the ito pyroclastic flow determinations using anisotropy of magnetic susceptibility the ito pyroclastic flow erupted about 22 000 years ago from aira caldera in southern kyushu japan flow directions were determined by anisotropy of magnetic susceptibility ams which measured the preferential alignment of magnetite microphenocrysts usually 0 25 mm in diameter the microphenocrysts are aligned with the long axis of the clasts during strain and fracture in the vent the grains are then deposited parallel to the flow direction with an imbrication there is no evidence of rolling of clasts or nonflow parallel lineations and thus ams can be used to determine flow directions that occurred immediately before deposition beyond 30 km from the center of the caldera flow directions were predominantly down paleogradient indicating expanded flow and that the depositional system was gravity driven and largely decoupled from the transport system within 30 km measured flow directions are random indicating that sedimentation occurred from a depositional system that was closely coupled to the turbulent transport system individual flow directions at all sites except one varied more than the analytical error demonstrating that a variety of flow directions existed even within a small area of the flow this implies that the depositional system was turbulent

east pacific rise 8 10 30 n evolution of ridge segments and discontinuities from seamarc ii and three dimensional magnetic studies magnetic bathymetric and seamarc ii side scan sonar data from the ridge flanks adjoining the overlapping spreading center osc at 9 03 n on the east pacific rise epr are used to establish the evolution of this ridge axis discontinuity and adjacent ridge segments within the past 2 4 m y a three dimensional inversion for magnetization shows the discontinuity evolved from one of small offset 2 km to an 8 km offset osc within the past approximately 1 0 m y the osc has left a v shaped discordant zone indicating an average southward migration of 42 km m y since the beginning of anomaly 2 time 1 8 m y the west flank discordant zone consists of discrete abandoned overlap basins bounded by highly magnetized ridge tips with anomalous lineations while the east flank discordant zone is a broad swath of high magnetization anomalous lineations and greater depths kinematic modelling indicates that southward migration has not been steady but has been accomplished by a series of episodic propagation events with rates ranging from 500 mm yr ridge tips have been abandoned at intervals of approximately 50 000 100 000 years both by linkage of neighboring ridge segments and by self decapitation magnetic modelling suggests rotations of overlap basins of 25 consistent with predictions of kinematic modelling orientation of magnetic isochrons indicate a 3 6 counterclockwise change in pacifie cocos plate motion within the past 1 m y assuming orthogonal spreading throughout this time although the discontinuity currently located at 9 03 n existed prior to this change in plate motion its growth into a right stepping 8 km offset osc may be causally related to this change the current configuration of intratransform spreading within the siqueiros has developed since onset of the brunhes anomaly 0 73 m y possibly in response to this recent counterclockwise change in plate motion in addition to the osc there is evidence for several small discontinuities offsetting the ridge during anomaly 2 time which have since migrated along the ridge and vanished an axial magnetization high is observed along the crest of the epr change in the magnitude of the axial magnetization high or shifts in its location relative to the bathymetric axis coincide with several devals within the area magnetization maxima are found at ridge transform intersections and the osc these highs may reflect presence of highly fractionated feti basalts erupted from magma chambers with restricted supply located adjacent to both large and short offset ridge axis discontinuities discrete magnetization highs are found off axis adjacent to clipperton and siqueiros fracture zones indicating that production of feti basalts at ridge transform intersections is episodic magnetizations are higher north of clipperton than south of it possibly reflecting a long lived starvation in magma supply north of clipperton and more abundant supply to the south most seamounts within the area have the same magnetization polarity as surrounding seafloor indicating that seamount formation occurs near the ridge axis the exceptions lie near magnetic reversal boundaries and are consistent with seamount formation as close as 10 km from the ridge axis few pronounced magnetization highs are associated with seamounts indicating that the magnetized source layer is no thicker beneath them than surrounding seafloor or that seamount basalts are substantially less magnetic than normal mid ocean ridge basalts

the effects of grain density variation on turbidity currents and some implications for the deposition of carbonate turbidites abstract sediment grains in carbonate turbidity currents vary greatly in size shape surface texture and density depending on the evolution of the source carbonate platform in time and space facies character and distribution may vary from traditional siliciclastic turbidite models not only because of the difference in composition but also because variation in grain properties influences flow and depositional behavior experimental gravity flows were generated to investigate one control on this variation the experiments used silicon carbide 3214 kg m 3 and glass ballotini 2650 kg m 3 with a similar grain size distribution in varying proportions to assess the influence of particle density on flow and sedimentation processes mixed particle density flows were more efficient than single particle density flows flow propagation was faster in flows with greater initial bulk density determined by grain density and suspended sediment concentration with mixed particle suspensions flow propagation velocity was maintained for longer due to the presence of low settling velocity particles as a result mixed particle density flows transported more sediment downstream compared to single particle density flows of similar initial suspended sediment volume concentration because of the wider range of grain types in carbonate turbidity currents they may more effectively transport coarse sand grade sediment to more distal sites in a marine basin compared to siliciclastic counterparts of equivalent initial volume this results in differences in bed and facies geometry and distribution

an experimental investigation of density stratified inertial gravity currents turbidity currents and pyroclastic density currents may originate as stratified flows or develop stratification during propagation analogue density stratified laboratory currents are described using layers of salt solutions with different concentrations and depths to create the initial vertical stratification the evolving structure of the flow depends on the distribution of the driving buoyancy between the layers b proportional to the layer volumes and densities and their density ratio q when the lower layer contains more salt than the upper layer and so has a greater proportion of the driving buoyancy b 0 ae5 this layer travels to the nose of the current by mixing into the back of the head along the body wake density interface to produce a homogeneous flow q fi 1 or overtaking leading to streamwise stratification q fi 0 timescales describing the mixing between the layers and the streamwise separation of the layers are used to understand these flow behaviours and are in accordance with the experimental observations distance time measurements of the flow front show that strongly stratified flows initially travel faster than weakly stratified flows but during their later stages they travel more slowly in natural flows that are stratified in concentration and grain size internal features such as stepwise grading gradual upward fining and reverse grading could be produced depending on b and q stratification may also be expected to affect interactions with topography and overall fan architecture

fluid inclusion studies of the felbertal scheelite deposit fluid inclusion measurements on quartz scheelite beryl fluorite and calcite in the metamorphosed felbertal scheelite deposit display two main types of fluid inclusions 1 h2o co2 fluid inclusions are characterized by variable amounts of co2 up to 18 wt they show two or three phases at room temperature the bulk homogenization temperatures for the inclusions range between 269 c and 357 c the calculated salinities are between 2 2 and 7 8 wt nacl equivalent for the late co2 bearing fluid inclusions a methane component is evident from microthermometrical data tmclath 10 0 c combined with tmco2 56 6 c and from raman microprobe analyses 2 aqueous two phase fluid inclusions with salinities in the range between 0 and 11 wt nacl equivalent their homogenization temperatures are scattered between 100 c and 360 c

origin and mode of emplacement of lithic rich breccias at aso volcano japan geological paleomagnetic and petrological reconstruction abstract takajosan breccia rocks are distributed around the southwestern caldera rim of the aso volcano in japan they are characterized by coarse lithic breccias with a pumiceous matrix the proximal coarse lithic breccias are divided into the lower massive unit and the upper stratified unit the lower massive lithic breccias tend to transform laterally into tuff breccias and pumiceous lapilli tuffs paleomagnetic results showed that all of the deposits were deposited at high temperatures of 175 560 c this was also supported by geological characteristics such as spatter clasts clasts with a bread crust texture and weakly welded parts these features clearly show that the deposits originated from pyroclastic density currents pdcs the dense lithic rich lithofacies low vesicularity of pumice lack of plinian fall deposits and radial distribution indicate that the deposits were derived from boil over pdcs rather than plinian column collapse pdcs the sio2 contents of the matrix glasses of the proximal lower massive breccia showed a progressive decrease from the bottom toward the upper part we interpret that this chemical variation corresponds to chemical zonation of the magma chamber this indicates that the massive deposits aggraded progressively from the base upwards progressive aggradation rather than through en masse freezing the vertical lithofacies changes of the proximal breccias from the lower massive to the upper stratified units indicate that a sustained current in a quasi steady state switched to an unsteady current with the progression of the volcanic activity

measurements and analysis of setting velocities of natural quartz sand grains abstract settling velocities in water were measured for natural quartz sand grains in the sieve size range 0 75 to 1 50 0 35 1 68 mm obtained from beach sand samples the grains were selected so as to provide a wide range of roundness from very angular to very well rounded r 0 6 5 1 in a first set of experiments only two axial diameters were measured under a microscope the longest axial diameter d1 and the intermediate diameter di in a second data set the smallest axial diameter ds was measured as well settling velocities were measured in a 6 meter long settling tube it is found that the intermediate grain diameter is on average equal to the nominal diameter computed as dn dsdid1 1 3 this permits a simpler analysis of grain settling through microscopic measurements of di alone but does not allow for corrections of grain shape effects the measured settling velocity wm is then compared with the settling velocity of a sphere ws calculated using di as the sphere diameter and the relationship wm 0 977w0 913s is found to be consistent with the data an alternative approach is provided where ws is calculated directly from the sieve diameter and then corrected to the actual settling velocity wm for that mean grain size the results thus provide graphs and empirical relationships which permit the evaluation of actual grain settling rates rather than settling velocities of spheres from the sieve diameter or from microscopic measurements of di for individual grains these procedures provide a mean settling velocity that takes into consideration the average degree of grain irregularities found in natural sands a more detailed analysis is then conducted to examine the effects of roundness and sphericity on the settling of individual grains it is found that grain roundness has no measurable effect on the settling rate the settling velocity is shown to be a function of grain sphericity either expressed as the corey shape factor or e shape factor grain asymmetries ust also be important accounting for much of the scatter in the results

structural and volcanic expression of a fast slipping ridge transform ridge plate boundary sea marc i and photographic surveys at the clipperton transform fault a high resolution near bottom survey has been conducted of the clipperton transform fault and adjoining segments of the east pacific rise epr using the sea marc i side looking sonar system and the lamont doherty geological observatory olympus based camera system the transform fault zone tfz is a narrow well defined belt of transform parallel lineaments which varies along strike from a single sharp edged notch to a complex band of subparallel lineaments up to 1 km wide the tfz is set within a 5 km wide band of unusually fine grained side scan texture which could indicate nonbasaltic seafloor and or pervasively sheared and mass wasted basaltic crust the fine grained swath is surrounded by constructional volcanic terrain with no hint of strike slip motion this observation puts an upper limit of 5 km on the extent of lateral migration of the tfz in the last 1 5 m y both ridge transform intersections rtis are dominated by bathymetric highs located on the old plate opposite the spreading center a mantling of fresh looking constructional volcanic terrain on side scan images suggests that the highs are built in part by recent extrusive and intrusive volcanism thermal expansion may also play a part the epr south of clipperton has recently experienced extrusion of high effusion rate basalts burial of faults and fissures by lava flows and development of vigorous hydrothermal circulation on the epr north of clipperton the axial zone of faults and fissures tapers toward the transform fault this may reflect a change in the shape or size of the underlying shallow level magma feeders as a function of distance from the site of magma upwelling or distance toward the transform fault

deep tow and sea beam studies of dueling propagating ridges on the east pacific rise near 20 40 s nontransform ridge axis discontinuities such as propagating rifts overlapping spreading centers oscs and saddle points occur frequently along the mid ocean ridge system we used sea beam and deep tow to study an example of a relatively large nontransform offset near 20 40 s on the east pacific rise epr the two spreading center tips are 15 km apart and separated by a diffuse shear zone a broad v shaped wake of disturbed topography and magnetics occurs on the flanks of the epr near the offset on a fine scale the offset is tectonically much more complex than either the typical osc or propagating rift structures for two reasons first it is a dueling propagator in which structures produced by successive short term episodes of northward and southward propagation have overprinted each other with migration toward the south prevailing over the long term second its offset distance is such that its tectonic behavior may be transitional from that of a propagating rift to that of an osc system the offset s rate of migration to the south has been 20 mm yr over the last 2 m y far less than total opening rate of 162 mm yr distinct propagation events may have been much more rapid however surging north and south at rates of 160 230 mm yr in the process several fossil spreading ridges 30 50 km long have been abandoned and rafted to the side we propose a fine scale tectonic evolution in which a propagating spreading center either curves inside or runs outside of a soon to be abandoned propagating rift tip we call this process self decapitation dueling propagation at 20 40 s may be caused by competing magmatic sources at shallow points along the epr one to the north near 20 s and one to the south near 21 30 s the 20 40 s dueling propagator occurs at a long wavelength low in the axial depth profile between these competing sources in general the division of spreading centers into segments separated by ridge axis discontinuities occurs at low points along the axis and may be caused by the pattern of upper mantle convection and the factors governing flow of magma along the strike of the ridge

lithic breccia and ignimbrite erupted during the collapse of crater lake caldera oregon abstract the climactic eruption of mount mazama 6845 y b p vented a total of 50 km3 of compositionally zoned rhyodacitic to basaltic magma from a a single vent as a plinian pumice fall deposit and the overlying wineglass welded tuff and b ring vents as ignimbrite and coignimbrite lithic breccia accompanying the collapse of crater lake caldera new field and grain size data for the ring vent products are presented in this report the coarse grained poorly bedded clast supported lithic breccia extends as far as 18 km from the caldera center like the associated ignimbrite the breccia is compositionally zoned both radially and vertically and silicic mixed and mafic types can be recognized based on the proportion of rhyodacitic pumice matrix fractions in silicic breccias are depleted of fines and are lithic and crystal enriched relative to silicic ignimbrite due to vigorous gas sorting during emplacement ignimbrite occurs as a proximal veneer deposit overlying the breccia a medial 8 to 25 km from the caldera center compositionally zoned valley fill as much as 110 m thick and an unzoned distal 20 km facies which extends as far as 55 km from the caldera breccia within 9 km of the caldera center is interpreted as a coignimbrite lag breccia formed within the deflation zone of the collapsing ring vent eruption columns expanded pyroclastic flows of the deflation zone were probably vertically graded in both size and concentration of blocks as recently postulated for some turbidity currents an inflection in the rate of falloff of lithic clast size within the lithic breccia at 9 km may mark the outer edge of the deflation zone or may be an artifact of incomplete exposure the onset of ring vent activity at mt mazama was accompanied by a marked increase in eruptive discharge pyroclastic flows were emplaced as a semicontinuous stream as few ignimbrite flow unit boundaries are evident as eruption from the ring vents progressed flow runout distance and the extent of breccia deposition decreased due to a greater internal flow friction and b decreasing eruption column heights effect b probably resulted from a progressive decrease in magmatic gas content and discharge rate waning discharge may have been promoted by the tapping of more viscous crystal rich magma collapse of conduit walls and declining caldera collapse rate

east pacific rise from siqueiros to orozco fracture zones along strike continuity of axial neovolcanic zone and structure and evolution of overlapping spreading centers we have used the sea beam multibeam echo sounding system to survey the east pacific rise epr from 8 20 n to 18 30 n obtaining complete coverage of the epr axial neovolcanic zone and all intervening transform faults here we focus on the epr neovolcanic zone and the definition of overlapping spreading centers ocs s between the orozco and siqueiros transform faults the neovolcanic zone is narrow 0 5 2 0 km and continuous along strike and occurs within and near a strike continuous axial summit graben the neovolcanic zone and summit graben reside along the crest of a volcanic axial high which is 2 10 km wide and which continues uninterrupted along strike for 40 140 km within 20 50 km of an intersection with a transform fault the axial neovolcanic zone narrows and deepens and within only 3 6 km of the intersection the neovolcanic zone turns sharply into the transform valley at seven locations between the siqueiros and orozco transform faults the axial neovolcanic zone is discontinuous and is laterally offset a short distance 1 15 km in contrast with a classic ridge transform ridge plate boundary however offset rise terminations overlap each other by a distance approximately 3 times greater than their offset they curve toward each other and often one merges into the other along strike separating the osc s is an elliptical overlap basin up to 600 m deep whose long axis is subparallel to the adjacent spreading centers the overlap basin is characterized by volcanic constructional edifices a key difference between these offsets and small transform faults in the atlantic is that there are no transform fault structures in the overlap basin which link the offset rise axes a continuous axial depth profile reveals a long wavelength 20 60 km undulation of the rise axis which may be associated with variations in the magmatic budget along strike the lowest points in the axial depth profile occur near transform faults and at osc s suggesting that these features are associated with along strike minima in the magmatic budget we suggest that major phases of volcanic activity propagate episodically along the axis away from centers of magmatic inflation and that osc s develop where magmatic pulses fail to meet due to misalignment of fracture systems sinuous bends in the rise axis or other heterogeneities transform faults fail to develop on fast spreading centers where the lateral offsets are small 15 km because the lithosphere is too thin and weak to maintain a classic rigid plate ridge transform fault pattern the osc geometry is unstable and evolves rapidly and the overlapping rises may propagate at rates faster than the local spreading rate one of the two osc s prevails while the other is abandoned while the locations of osc s may not be fixed relative to the rise axis they may tend to recur near the same place creating scars in older lithosphere which resemble fracture zones but whose fine scale structure is quite different there is excellent agreement between the observed structure of osc s and laboratory studies of osc s using wax models photoelastic studies of overlapping cracks and boundary element modeling of overlapping cracks seismic refraction and reflection near two osc s suggests that large offset osc s have separate magma chambers beneath the offset rise axes while small offset osc s may share a single axial magma chamber three dimensional inversion of magnetic anomalies suggests that significant petrologic anomalies may occur near some osc tips waxing and waning of overlapping magma chambers may give rise to the eruption of highly fractionated feti basalts and associated magnetic anomalies

petrologic consequences of rift propagation on oceanic spreading ridges abstract the production of anomalously differentiated lava compositions at several mid ocean spreading centers can be attributed to magmatic processes associated with propagating rifts the degree of differentiation attained by magmas beneath oceanic spreading ridges depends mainly on the balance between cooling rate and the supply rate of new magma to shallow chambers low supply rates and moderate cooling rates allow advanced degrees of closed system fractionation to occur high supply rates result in open systems in which magma compositions are buffered by frequent replenishment with new hot magma propagating rift tips are a special class of ridge transform intersection in which the balance between cooling and supply rates is conducive to the development of advanced degrees of differentiation over an expanded length of ridge this balance is affected by the spreading rate the propagation rate of the rift the length of the bounding transform and proximity to hotspots maximum compositional variability and maximum degree of differentiation occur within 50 km of propagating rift tips and subsequently diminish with increasing distance rifts that propagate through plates in directions approximating their absolute motion relative to the lower mantle are characterized by the presence of anomalously differentiated lavas over longer ridge segments than are rifts that propagate against their absolute motion geochemical anomalies may persist though changing in degree and extent for several million years on ridge segments that stop propagating the concept of magnetic telechemistry is generally supported by our study but in the vicinity of hotspots magnetic anomaly amplitude may be controlled more by bathymetric and or thickened magnetic layer effects than by geochemistry

a box model for non entraining suspension driven gravity surges on horizontal surfaces the propagation of and the deposition from a turbulent gravity current generated by the release of a finite volume of a dense particle suspension is described by a box model the approximate model consists of a set of simple equations a predetermined depth dependent leading boundary condition and one experimentally determined parameter describing the trailing boundary condition it yields predictions that agree well with existing laboratory observations and more complex theoretical models of non eroding non entraining suspension driven flows on horizontal surfaces the essential features of gravity surge behaviour have been observed and are captured accurately by the box model these include the increased rate of downstream loss of flow momentum with increased particle setting velocity the existence of maxima in the thickness of proximal deposits and the downstream thinning of distal deposits our approximation for the final run out distance xr of a surge in deep water is given by xr3 g oq3o w2s 1 5 where g o is the initial reduced gravity of the surge qo the initial two dimensional volume and ws the average settling velocity of the particles in the suspension a characteristic thickness of the resulting deposit is given by oqo xr where oo is the initial volumetric fraction of sediment suspended in the surge our analysis provides additional insight into other features of gravity surge dynamics and deposits including the potential for the thickening of currents with time the maintenance of inertial conditions and the potential for strong feedback in the sorting of particle sizes in the downstream direction at travel distances approaching xr box model approximations for the evolution of gravity surges thus provide a useful starting point for analyses of some naturally occurring turbidity surges and their deposits

resedimentation of cold pumiceous ignimbrite into water facies transformations simulated in flume experiments deposits and transport processes resulting from the resedimentation of cold unconsolidated ignimbrite into water were simulated by flume experiments the ignimbrite sample used was poorly sorted 2 4 3 fine ash rich 2000 kg m 3 and pumice 500 to ca 1300 kg m 3 as a result of the binding forces of the ash matrix the experiments involved resedimentation from a steep front onto the floor with or without an initial ramp of the water filled tank under both still and wave generated conditions larger discrete collapse events were induced by oversteepening the sample front and by undercutting from wave action the mass of the collapse and proportion of pore space water strongly influenced the style of resedimentation and the deposits initial collapse events were from the top of the steep front and fell onto the floor the largest densest clasts were deposited as a lithic lag in a proximal sediment wedge or rolled down to a break in slope fine ash was transported in dilute turbidity currents and coarse unsaturated pumice clasts floated off moderate collapse events generated high density turbidity currents trapping pumice in the flow causing them to saturate these low density pumice clasts were easily remobilized by wave activity and passing currents and accumulated on the gentle slope at the bottom of the resedimented deposit large collapse events slumped producing poorly sorted mounds similar in texture to the original starting material as the matrix of the ignimbrite sample became saturated with water moderate and large collapse events generated debrisflows and slurries that deposited massive poorly sorted deposits furthermore once more gentle slopes were established between the sample and deposit small cascading grainflows deposited lithic clasts on the upper slopes and levees of pumice at the terminus of low relief ash channels the experiments show that excluding large collapse events and debrisflows resedimenting ignimbrite in water is effective at segregating low density pumice clasts from dense lithic clasts and fine ash experiments using fine ash poor ignimbrite and well sorted quartz sand for comparison formed an inherently unstable initial steep front that immediately collapsed by continuous grain avalanches the grainflow deposits had textures similar to the fines poor starting material

lidar derived morphology of the 1993 lascar pyroclastic flow deposits and implication for flow dynamics and rheology abstract pumice flows are potentially destructive volcanic events that derive from eruption column collapse and whose dynamics are poorly understood the challenges in studying these flows include the lack of constraints on the dynamics kinematics and initial conditions that control their emplacement we present a morphological study of the distal deposits lobes of the pumice flows resulting from the 1993 eruption at lascar chile the surface geometry of the lobes was measured in detail using a lidar device which allowed for detailed characterisation of their morphology consisting of central channel and lateral levees and terminal frontal snout in particular we find that the ratio of channel levee height as a function of the ratio of the distance between levees total width of the lobe has a characteristic curve for these pumice flow lobes our analysis of several of the lascar pumice lobe deposits south east sector identified several dimensionless groups of the available parameters which when compared against published results from both experimental and numerical investigations allowed us to constrain crucial kinematic and dynamic information on the terminal phase of the pumice flows notably we estimate the velocity of the terminal phase of pumice flows to be 5 10 m s froude numbers of 1 5 2 are comparable with values found for experimental granular flows height width aspect ratios for the levee channel section of the pumice lobes are similar to those for experimental flows although these same aspect ratios for the snout are much larger for the natural deposits than their small scale analogues finally we discuss the possible emplacement dynamics of the terminal lascar 1993 pumice flows a pseudo reynolds number based on the velocity estimation is found to be up to 100 times larger for the pumice flows than experimental scale flows this suggests that the flow retarding frictional forces for large scale flows are relatively unimportant compared to flows at smaller scales mechanical effects such as fluidisation mobilisation of material lying on the slopes over which they propagate and lubrication due to polydispersivity could provide an explanation for their ability to propagate on shallow slopes 6 11

the role of viscous particle segregation in forming chromite layers from slumped crystal slurries insights from analogue experiments the question of how the mineral layering in layered intrusions forms has been extensively debated for many decades there are many types of layering and it is of course possible that a number of mechanisms are involved of particular interest is how chromite layers form because these may contain valuable metals such as platinum group elements in addition to cr one model for the formation of these layers is that they formed through slumping of semi consolidated cumulates from the margins of the intrusion into the magma chamber during this slumping the grains are sorted by density and or size differences this study examines the viability of this process using analogue modelling starting materials beads and glycerine were scaled to match the density and size of the minerals chromite orthopyroxene and plagioclase present in layered mafic ultramafic layered intrusions and to match the density and viscosity of the silicate magma a perspex flume tank divided by a removable partition at one end was fully filled with glycerine a homogenized mixture of the beads was placed in the smaller partition of the tank representing the margins of the magma chamber the tank was then inclined between 16 and 45 the partition was removed and the beads flowed into the main part of the box the experiments were recorded by video camera allowing us to follow the dynamics of the flow during each run segregation of the beads was observed in the final deposits the larger less dense beads representing plagioclase concentrated at the top of the flow with the intermediate sized and medium density beads representing orthopyroxene in the middle and the smaller denser beads representing chromite at the bottom thus mimicking natural examples in experiments where the angle of inclination was low long thin layers formed such as those found in the bushveld complex in experiments where the angle of inclination was high thick but short layers formed a dimensionless analysis allows better understanding of the dynamics of the flow at the macroscopic scale the flow regime is strongly influenced by the viscosity of the fluid and is considered macro viscous where the role of the interstitial liquid is non negligible

internal flow variations and diachronous sedimentation within extensive sustained density stratified pyroclastic density currents flowing down gentle slopes as revealed by the internal architectures of ignimbrites on tenerife during a protracted explosive eruption at least four laterally extensive and sustained pyroclastic density currents radiated across the flanks of las canadas volcano tenerife each pyroclastic current developed marked local and regional spatial variations in response to the incised gently concave substrate topography the locations of these variations shifted in space as rapid sedimentation from the current progressively buried and modified the topography this complex shifting response of the density currents to minor topographic variations has been reconstructed in high resolution over a wide area 500 km2 using the internal architecture of cryptic time surfaces entrachrons marked by compositional zoning in the deposit including variations in clast types valley side field relations reveal that the currents were density stratified yet at a single instant in time the lower levels of each current comprised a granular fluid at some locations but were fully dilute and turbulent at others moreover the locations of these variations shifted geographically as the topography changed during the eruption the variations within the current are recorded by numerous superbly exposed gradational transitions from various stratified to massive lithofacies both laterally and in the downcurrent direction individual currents were regionally widespread and travelled 15 km but deposited only in longitudinally restricted localised zones that spanned several small valleys and interfluves the currents bypassed slopes up and downcurrent of the restricted depositional zones without depositing the locations of deposition then gradually shifted with time such that the extensive deposit sheet was gradually assembled beneath the sustained current in a diachronous fashion onlap relationships of internal entrachrons reveal that the base of the ignimbrite sheet and even the bases of individual flow units are markedly diachronous deposition of a flow unit commenced and ceased at different times in different places this study suggests that in hazard assessments a models of density currents that incorporate only pre existing topography e g from dems may give misleading results in the case of sustained currents because sedimentation from these significantly modifies the topography during emplacement altering flow paths b frequencies and scales of previous pyroclastic currents determined from pyroclastic successions are likely to be significantly under estimated because currents commonly bypass without leaving a deposit record and c even where preservation appears to be complete an ignimbrite at a single exposure commonly will not record the current s entire flow history at that site

comparison of sphericity indices as related to the hydraulic equivalence of settling grains six of the most commonly used sphericity indices are examined to determine which is most suitable for predicting the settling velocity of nonspherical grains direct comparison is facilitated by normalizing the different measures for the same set of pebbles which range in shape from spheroids ellipsoids and disks to blades regression analysis of the normalized sphericity indices plotted against the settling ratio a modified dynamic shape factor shows that the best index is the hofmann 1994 shape entropy followed by the sphericity measures of aschenbrenner 1956 and wadell 1932 the corey 1949 sneed and folk 1958 and janke 1966 shape factors are less suitable for a wide range of particle shapes this is in contrast to some previous comparisons of the same indices

lateral variations in the taupo ignimbrite abstract the young non welded taupo ignimbrite shows remarkable lateral variations which are documented by granulometric and component analyses and studies of maximum clast size and density the grain size spans practically the entire known ignimbrite field the coarser proximal ignimbrite having a median diameter 100 times greater than the finest distal ignimbrite the content and maximum size of lithic fragments decrease also by a factor of 100 between proximal and distal parts the content of free crystals first rises to reach a peak but thereafter decreases to attain a very low value in far distal exposures the pumice maximum size decreases by a factor of about 10 and the most conspicuously coarse pumice rocks occur in a girdle nearly halfway out from vent to distal limit the pumice in each grain size class decreases in density to half of its near source value in distal ignimbrite the overall outward trend is towards an ignimbrite which consists wholly of fine vitric ash some distal exposures closely approach this condition these variations are accounted for by a combination of processes operating in the moving ash flow one is a continuous fragmentation of pumice leading to a rounding of the clasts a progressive decrease in maximum size the generation of much vitric dust and the liberation of crystals another is a continuous sedimentation of heavy constituents lithics and crystals and an antipathetic rise of lighter coarse pumice towards the top of the flow these processes operated in a moving flow whose upper layers travelled progressively farther from source it is the topmost layers strongly depleted in heavy constituents and enriched in light pumice which have travelled the farthest and constitute the far distal parts of the ignimbrite a number of ignimbrite facies are characterized the ignimbrite proper with its proximal distal and pumice concentration zone facies the deposits which form in the head and are then over ridden by the body of the flow including the fines depleted ignimbrite variant and the heavies enriched ground layer and the ignimbrite veneer deposits which are left behind by the flow which differ little from the ignimbrite except in their landscape mantling form and the occurrence in them of lee side coarse pumice lenses

directed blasts and blast generated pyroclastic density currents a comparison of the bezymianny 1956 mount st helens 1980 and soufriere hills montserrat 1997 eruptions and deposits we compare eruptive dynamics effects and deposits of the bezymianny 1956 bz mount st helens 1980 msh and soufriere hills volcano montserrat 1997 shv eruptions the key events of which included powerful directed blasts each blast subsequently generated a high energy stratified pyroclastic density current pdc with a high speed at onset the blasts were triggered by rapid unloading of an extruding or intruding shallow magma body lava dome and or cryptodome of andesitic or dacitic composition the unloading was caused by sector failures of the volcanic edifices with respective volumes for bz msh and shv c 0 5 2 5 and 0 05 km3 the blasts devastated approximately elliptical areas axial directions of which coincided with the directions of sector failures we separate the transient directed blast phenomenon into three main parts the burst phase the collapse phase and the pdc phase in the burst phase the pressurized mixture is driven by initial kinetic energy and expands rapidly into the atmosphere with much of the expansion having an initially lateral component the erupted material fails to mix with sufficient air to form a buoyant column but in the collapse phase falls beyond the source as an inclined fountain and thereafter generates a pdc moving parallel to the ground surface it is possible for the burst phase to comprise an overpressured jet which requires injection of momentum from an orifice however some exploding sources may have different geometry and a jet is not necessarily formed a major unresolved question is whether the preponderance of strong damage observed in the volcanic blasts should be attributed to shock waves within an overpressured jet or alternatively to dynamic pressures and shocks within the energetic collapse and pdc phases internal shock structures related to unsteady flow and compressibility effects can occur in each phase we withhold judgment about published shock models as a primary explanation for the damage sustained at msh until modern 3d numerical modeling is accomplished but argue that much of the damage observed in directed blasts can be reasonably interpreted to have been caused by high dynamic pressures and clast impact loading by an inclined collapsing fountain and stratified pdc this view is reinforced by recent modeling cited for shv in distal and peripheral regions solids concentration maximum particle size current speed and dynamic pressure are diminished resulting in lesser damage and enhanced influence by local topography on the pdc despite the different scales of the blasts devastated areas were respectively 500 600 and 10 km2 for bz msh and shv and some complexity involving retrogressive slide blocks and clusters of explosions their pyroclastic deposits demonstrate strong similarity juvenile material composes 50 of the deposits implying for the blasts a dominantly magmatic mechanism although hydrothermal explosions also occurred the character of the magma fragmented by explosions highly viscous phenocryst rich variable microlite content determined the bimodal distributions of juvenile clast density and vesicularity thickness of the deposits fluctuates in proximal areas but in general decreases with distance from the crater and laterally from the axial region the proximal stratigraphy of the blast deposits comprises four layers named a b c d from bottom to top layer a is represented by very poorly sorted debris with admixtures of vegetation and soil with a strongly erosive ground contact its appearance varies at different sites due to different ground conditions at the time of the blasts the layer reflects intense turbulent boundary shear between the basal part of the energetic head of the pdc and the substrate layer b exhibits relatively well sorted fines depleted debris with some charred plant fragments its deposition occurred by rapid suspension sedimentation in rapidly waning high concentration conditions layer c is mainly a poorly sorted massive layer enriched by fines with its uppermost part laminated created by rapid sedimentation under moderate concentration weakly tractive conditions with the uppermost laminated part reflecting a dilute depositional regime with grain by grain traction deposition by analogy to laboratory experiments mixing at the flow head of the pdc created a turbulent dilute wake above the body of a gravity current with layer b deposited by the flow body and layer c by the wake the uppermost layer d of fines and accretionary lapilli is an ash fallout deposit of the finest particles from the high rising buoyant thermal plume derived from the sediment depleted pyroclastic density current the strong similarity among these eruptions and their deposits suggests that these cases represent similar source transport and depositional phenomena

segmentation of mid ocean ridges studies of mid ocean ridges in the pacific and atlantic oceans show that the volcanism that forms the oceanic crust along the spreading plate boundaries is concentrated at regular intervals related to spreading rate this observation and a new calculation for a rayleigh taylor type of gravitational instability of a partially molten mantle region growing under spreading centres yield reasonable estimates of upper mantle viscosities

the three dimensional seismic velocity structure of the east pacific rise near latitude 9 30 n three dimensional images of crustal seismic structure beneath the east pacific rise show pronounced axial heterogeneity over distances of a few kilometres a linear high velocity anomaly approximately 1 2 km in width and restricted to the uppermost 1 km of the crust is centred on the rise axis an axial low velocity anomaly at depths of 1 3 km varies in amplitude along the axis consistent with a zone of higher crustal temperatures midway between two discontinuities in the morphology of the rise axis this apparent thermal segmentation along axis is consistent with injection of mantle derived melt midway along a locally linear 12 km long segment of the rise

the proximal facies of the tosu pyroclastic flow deposit erupted from aso caldera japan the tosu pyroclastic flow deposit a low aspect ratio ignimbrite lari has widely distributed breccia facies around aso caldera japan the proximal facies 9 34 km away from the source consists of 3 different lithofacies from bottom to top a lithic enriched and fines depleted fd facies a lithic enriched li facies with an ash matrix and a fines and pumice enriched ni facies modes of emplacement of fd li and ni are interpreted as ground layer 2b lithic concentration zone and normal ignimbrite respectively these stratigraphic components in the tosu originated from the flow head fd and the flow body li and ni and were generated by a single column collapse event remarkably thick fd and li in contrast to thin ni suggest that due to high mobility most ash and punice fragments in the tosu were carried and deposited as ni in the distal area heavier components were selectively deposited as fd and li in the proximal area the rate of falloff of lithic clast size in the tosu shows an inflection at 20 km from the source in a survey of well documented pyroclastic flows the inflection distance of a lari is generally greater than that of a high aspect ratio ignimbrite so that the eruption of the former is probably more intense than the latter

stratified flow in pyroclastic surges stratified flow theory is applied to pyroclatic surges in an effort to gain insight into transport dynamics during explosive eruptions particle transport is assumed to be by turbulent suspension and calculations contained herein show that this is likely for many cases including the 18 may 1980 blast at mount st helens the discussion centers on the rouse number pn which represents a ratio of particle settling velocity to scale of turbulence the brunt vaisala frequency n which is the maximum possible frequency of internal waves the froude number fr representing the ratio of inertial forces to gravitational forces and the richardson number ri a ratio of buoyant restoring forces to turbulent mixing forces the velocity or flow power dependence of bed form wavelength in surge deposits is related to a velocity dependence of wavelength of internal waves in the turbulent surge this produces a decrease in dune wavelength with increasing distance from vent migration direction of bed forms is related tofr as it is defined for a continuously stratified flow proximal to distal facies variations in surge deposits reflect increasingpn andri as the flows move away from their sources this produces the progression from sandwave to massive to planar facies with increasing distance from vent where the long axis of topography is at low angles to the flow direction massive facies in topographic lows may from concurrently with sandwave facies on highs due to the higher particle concentration in the lows where long axis of topography is at high angles to flow direction denser lower parts of the surge may be dammed or blocked blocked material tends to form massive flows that may move down slope independent of the overriding surge a model incorporating turbulent transport stratified flow and time evolution of pyroclastic surges is proposed for deposits which have been attributed to both pyroclastic flow and pyroclastic surge transport by various workers during the initial high energy waxing phase of the eruptive event pn is sufficiently low that only coarse but poorly sorted material is deposited to form relatively coarse bottom layers as the event wanes remaining finer material is deposited through a thin bed load to produce overlying bedded and cross bedded veneer deposits throughout most of the event blocking occurs to produce relatively thick and massive deposits in valley bottoms

northern east pacific rise magnetic anomaly and bathymetric framework the oceanic crust in the eastern pacific between 7on and 30on and east of 127ow contains a fairly complete history of the spreading centers associated with the east pacific rise since 25 m y b p late oligocene in this paper we have summarized the seafloor spreading magnetic anomaly data and the bathymetric data that reflect the record of this tectonic history the well defined magnetic lineations north of the clarion fracture zone in the mouth of the gulf of california and on the east flank of the east pacific rise epr are carefully examined and used to provide a guide for interpreting the spreading pattern between the clarion and clipperton fracture zones southward of the rivera fracture zone over the mathematician ridge and over the entire epr east of the mathematician ridge between the rivera and siqueiros fracture zones the bathymetric data provide a trace of the fracture zone pattern in each of the above mentioned areas the fracture zone bathymetry and the seafloor spreading magnetic lineations on the epr south of the rivera fracture zone have a distinctive fanning pattern caused by close poles of rotation and plate boundary reorganizations all these data provide a good record of the plate reorganizations in the middle miocene at magnetic anomaly 5a time 12 5 to 11 m y b p in the late miocene at magnetic anomaly 3 4 time 6 5 m y b p and in the pliocene at magnetic anomaly 2 3 time 3 5 m y b p several abandoned spreading centers including the mathematician ridge were left behind as a result of these reorganizations the mathematician ridge is shown to be a set of ridges and trough whose origin is related to the tectonic activity associated with each of the above mentioned reorganizations since anomaly 5a

rare earth element behaviour associated with strata bound scheelite mineralisation broken hill australia abstract strata bound scheelite mineralisation occurs within calc silicates of a metamorphosed proterozoic volcano sedimentary sequence and is spatially related to lenticular bodies of basaltic volcanics tourmalinites and quartz gahnite rocks a pre metamorphic alteration halo has been detected within the enclosing basalts in the form of elevated fe as and eu contents which were formed by reducing and or high temperature hydrothermal fluids changing eh conditions during alteration gave rise to ce enriched basalts and sediments proximal and distal to the mineralisation the calc silicate horizon represents a hydrothermally altered and metamorphosed carbonate rich sediment as indicated by calc silicate pods containing primary sedimentary ree patterns the calc silicate horizon displays a lateral zonation from an unmineralised hydrothermally altered lree depleted core to mineralised lree depleted hree and eu enriched margins the wide range of ree patterns and the distinctive variation in ree spectra from the centre to the margins is due to the progressive dissolution of carbonates and the deposition of scheelite with decreasing temperature from the w fe as eu enriched ore fluids

geologic sulfur isotope and fluid inclusion studies of the sannae w mo mine republic of korea comparison of sulfur isotope systematics in korean w deposits the sannae tungsten molybdenum mine is located approximately 60 km north of pusan within the gyeongsang basin of the southern korean peninsula the hydrothermal vein type deposits which comprise the mine occur within an upper cretaceous magnetite series quartz monzonite stock which intrudes middle cretaceous sedimentary and volcanic rocks mineralization can be separated into three stages with declining temperature early molybdenum main tungsten and late carbonate stages fluid inclusion data suggest that most of the molybdenite was deposited at temperatures between 550 degrees and 350 degrees c wolframite and scheelite between 450 degrees and 300 degrees c and carbonates between 300 degrees and 180 degrees c salinities of mineralizing hydrothermal fluids ranged from 5 to 25 equiv wt percent nacl with a marked decrease in salinity between main tungsten and late carbonate stages fluid inclusion evidence of boiling suggests a range of pressures from about 100 to 300 bars this range is consistent with a depth of mineralization near 1 250 m under pressure conditions that alternated between hydrostatic and lithostatic sulfur isotope compositions of sulfides from the mine have the following ranges of delta 34 s values molybdenite 5 2 to 6 0 per mil and pyrite 6 0 to 7 1 per mil the delta 34 s sub h 2 s values calculated for these minerals are consistent with an igneous source of sulfur with a delta 34 s sub sigma s value near 4 0 to 5 0 per mil the geology mineral paragenesis and sulfur isotope and fluid inclusion data of the sannae w mo mine are very similar to data from other korean tungsten deposits the total range of delta 34 s sub h 2 s values of sulfides for all the korean tungsten deposits is 2 to 7 per mil however there is a narrow range of delta 34 s values for individual deposits the sulfur isotope compositions and systematics of korean tungsten deposits are nearly identical to those of japanese deposits associated with magnetite series granitoids and may be explained by the separation of a sulfur bearing magmatic fluid from a granitic melt under different oxidizing conditions

transport of pyroclastic flows across the sea during the explosive rhyolitic eruption of the kos plateau tuff greece the kos plateau tuff kpt is dominated by a sequence of nonwelded rhyolitic ignimbrites some of which were deposited more than 60 km from source on islands in the eastern aegean sea greece palaeo shoreline reconstruction at the time of the kpt eruption 161 ka suggests that northward directed pyroclastic flows travelled over an extensive landmass although the proximal zone may have been sea in comparison flows directed to the southeast and east most likely crossed wide expanses of sea depositing ignimbrite on islands and peninsulas the ignimbrites include basal coarse lithic rich breccias overlain by thick massive pumiceous zones and generally have low abundances of fine ash they are interpreted to have been deposited from pyroclastic flows that were relatively dilute and stratified with lower denser lithic rich zones and upper expanded pumice rich zones high velocity inherited from the intensity of the eruption combined with turbulence and low bulk density favoured transport across water the pyroclastic flows remained hot even after crossing 35 km of the sea heat capacity equations indicate that a depth of 1 cm of water ingested over the outflow path would have cooled the flows by only a few degrees the ignimbrites deposited prior to the eruption climax have distributions restricted by 300 m high topographic barriers and they thicken in topographic depressions these ignimbrites show significant decreases in grain size proportion of lithic clasts and thickness with distance from source in comparison during the eruption climax the coarsest most voluminous and widespread kpt ignimbrite was deposited this ignimbrite has relatively even thickness and uniform componentry and texture regardless of topographic relief or distance from source textural analysis indicates that crossing 35 km water to the southeast had little effect on the pyroclastic flow generated during the eruption climax except for the loss of the coarsest 20 cm vent and conduit derived lithic clasts preferential loss of lithic clasts is related to the mechanism by which they were transported on land large dense clasts could be saltated over the ground surface at the base of the pyroclastic flow in the case of transport over water these large dense clasts sank and were permanently removed from the transport system outflow across water had a greater effect on the pre climax pyroclastic flows the ignimbrites formed at this stage are significantly thinner and slightly finer grained and lack basal lithic breccias and lithic clasts 5 cm where the pyroclastic flows crossed the sea reflecting the lower velocities of the flows all kpt ignimbrites to the east of the source were deposited from pyroclastic flows that crossed 25 km of water ignimbrites in this sector are markedly finer grained possibly as a result of blocking of coarser pyroclasts by a topographic barrier close to source

a tectonic model for ridge transform ridge plate boundaries implications for the structure of oceanic lithosphere abstract the first order geologic and morphologic relationships at along and proximal to ridge transform ridge plate boundaries are used to construct an empirical and speculative tectonic model the distinctive but variable morphotectonic fabric and crustal structure of the transform domain are the product of a tectonic continuum that ranges from very slow rates of strain associated with very thick edges of lithosphere slowly slipping ridge transfrom ridge plate boundaries to extremely high rates of strain associated with very thin edges of lithosphere fast slipping ridge transform ridge plate boundaries the geometry of a ridge transform intersection necessitates the juxtaposition of a relatively cold thick edge of lithosphere against the truncated end of an accreting plate boundary the cold face of lithosphere cools the adjacent wedge of asthenosphere rising beneath the axis of accretion and restricts the amount of partial melting thus attenuating the volume of basaltic melt segregated from the asthenosphere per unit time the shallow level manifestation of this cold edge effect is a thinner oceanic crust at depth the evolving lithosphere thickens rapidly as the cold edge is approached creating changes in the properties of the young lithosphere and forming a weld of upper mantle material against the cold edge of truncating lithosphere the ridge transform weld of upper mantle material creates a shear couple in the lithosphere underlying the intersection resulting in the progressive reorientation of the maximum tensile stress from normal to the ridge axis at some distance from the ridge transform intersection to an oblique angle near the boundary the brittle carapace of oceanic crust that overlies the mantle weld will deform accordingly with the development of oblique dip slip faults the model predicts that the geologic expressions of this cold boundary effect will become more dramatic with increasing thickness of the truncating edge at very slow rates of accretion 100 km juxtapose thick 30 50 km edges of lithosphere against the accreting plate boundary all but nullifying the processes that lead to the emplacement of normal oceanic lithosphere in this environment a relatively strong mantle weld will form at the ridge transform intersection and the thick edges of opposing lithosphere along the length of the transform will tend to confine the strike slip tectonism to a relatively narrow and temporally stable principal transform displacement zone in contrast at very fast rates of accretion large offset transforms 100 km place relatively thin edges of lithosphere

addressing complexity in laboratory experiments the scaling of dilute multiphase flows in magmatic systems the kinematic and dynamic scaling of dilute multiphase mixtures in magmatic systems is the only guarantee for the geological verisimilitude of laboratory experiments we present scaling relations that can provide a more complete framework to scale dilute magmatic systems because they explicitly take into account the complexity caused by the feedback between particles crystal bubble or pyroclast and the continuous phase liquid or gas we consider three canonical igneous systems magma chambers volcanic plumes and pyroclastic surges and we provide estimates of the proposed scaling relations for published experiments on those systems dilute magmatic mixtures can display a range of distinct dynamical regimes that we characterize with a combination of average eulerian properties and instantaneous lagrangian variables the eulerian properties of the mixtures yield the reynolds number re which indicates the level of unsteadiness in the continuous phase the lagrangian acceleration of particles is a function of the viscous drag and gravity forces and from these two forces are derived the stokes number st and the stability number rt two dimensionless numbers that describe the dynamic behavior of the particles within the mixture the compilation of 17 experimental studies relevant for pyroclastic surges and volcanic plumes indicates that there is a need for experiments above the mixing transition ren10 4 and for scaling st and rt among the particle dynamic regimes present in surges and plumes some deserve special attention such as the role of mesoscale structures on transport and sedimentary processes or the consequences of the transition to turbulence on particle gathering and dispersal the compilation of seven experimental studies relevant to magma bodies indicates that in the laminar regime crystals mostly follow the motion of the melt and thus the physical state of the system can be approximated as single phase in the transition to turbulence magmas can feature spatially heterogeneous distributions of laminar regions and important velocity gradients this heterogeneity has a strong potential for crystals sorting in conclusion the re st rt framework demonstrates

mobility of a large volume pyroclastic flow emplacement of the campanian ignimbrite italy abstract the trachytic campanian ignimbrite originally exposed over a 30 000 km 2 area around naples italy is the product of a highly energetic gas rich eruption the deposit lies in valleys and isolated watersheds and in its medial and distal extent to the south north and east of naples the campanian pyroclastic current encountered mountains exceeding 1000 meters anisotropy of magnetic susceptibility ams measurements indicate that the campanian pyroclastic current the transport system traveled radially outward from the phlegrean fields area but the ignimbrite forming flow the deposition system that developed from the base of the transport system moved downslope from mountainsides to valleys including slopes facing the eruption source and flowed down drainage systems from intermontane basins the campanian pyroclastic current flowed 35 km over the water of the bay of naples to deposit 43 m of ignimbrite on the south shore and also overtopped a 685 1000 m high ridge of the sorrento peninsula to deposit more on the other side the distribution of the ignimbrite and the measured flow directions suggest that the campanian pyroclastic current moved across the landscape as an expanded and therefore turbulent decompressing flow rather than as a high density nonturbulent sheet like current moving over mountains by momentum acquired by eruption column collapse strong expansion is corroborated by shard morphology that indicates derivation from highly inflated pumice and suggests vesicles must have been at least 80 by volume of the original magma

sulfur isotope studies of the felbertal scheelite deposit eastern alps the felbertal scheelite deposit is the largest known strata bound tungsten concentration it lies in an up to 400 m thick rock pile in the lowermost part of the volcanic rock sequence probably of the early paleozoic habach formation both ore fields eastern and western have been affected by variscan and alpine metamorphism and tectonism resulting in a remobilization of the ore mineralization this ore deposit and the neighboring rocks show a strikingly low sulfur content the eastern field with one major orebody has very little sulfide mineralization the western field with 8 orebodies k1 k8 and two remobilized vein zones s1 and s2 reveals somewhat more minor sulfide enrichments that are mainly within and around the k1 and k2 orebodies and in some parts of the interlayered schist sequence sulfur isotope compositions of 90 sulfide minerals 37 pyrrhotite 20 chalcopyrite 19 pyrite and 11 molybdenite and or ws2 mos2 solid solutions and 3 pb bi sulfosalts including 7 sulfides within scheelite grains from 60 ore and host rock samples have been determined with a standard error of less than 0 2 per mil all data range from 3 6 to 4 3 34s there are small differences in the sulfur isotope values from place to place and in time from the first and second to the third generation in the western field the k1 orebody differs from other orebodies k2 k4 k7 due to isotopically heavier 34s values the three scheelite generations show differences in the 34s values of the sulfide microphases within scheelite grains from 1 0 to 4 3 per mil for the first and the second and from 1 8 to 3 3 per mil for the third generation sulfide phases within molybdoscheelites may have crystallized under the same conditions as the other coeval sulfide minerals in the same orebody they commonly formed later than scheelite these changes may be explained using data from ohmoto and rye 1979 small changes in temperature ph and or f o 2 may result in large changes in the 34s values with the precipitation of isotopically heavier sulfides under more reducing conditions only four samples with sulfide mineral pairs show isotopic equilibrium all others display some disequilibrium we suggest that the sulfides in the ores and surrounding volcanogenic host rocks formed contemporaneously from the same hydrothermal ore fluids and that the sulfur species in these fluids may have been dominantly h2s

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

density surges two dimensional experiments some new aspects of the development of density surges in channels are developed from recent experimental results a density surge is characterized by the flow of a finite amount of dense fluid this dense fluid may be a saline water solution a saline water solution with tracers sand in suspension or a pure suspension sand in suspension in fresh water the experimental results show in particular that 1 the velocity of the surge is effectively proportional to the square root of the initial volume 2 the velocity of the surge increases with the increase of the initial density of the heavy fluid proportionally to the square root of the ratio of density difference to the ambient fluid density 3 the speed of the surge increases again with an increase of the slope for pure suspensions the experiments show that velocities slow down much more quickly than for saline solutions with the same initial density due to loss of negative buoyancy the effects of the initial density and of the size of the grains on the distribution of sediments are described

the influence of palaeotopography on facies architecture and pyroclastic flow processes of a lithic rich ignimbrite in a high gradient setting the abrigo ignimbrite tenerife canary islands abstract the late pleistocene 188 19 ka abrigo ignimbrite represents the last major explosive caldera forming eruption on tenerife it consists almost entirely of a moderate volume 1 8 km 3 light grey phonolitic lithic rich ignimbrite deposit although a possible abrigo member thin fall deposit is locally preserved beneath the ignimbrite suggesting an initial short lived stable plinian eruption column the ignimbrite deposit is dispersed radially around the lower flanks of the las canadas edifice and correlates to a clast supported lithic breccia on the eastern and northern caldera wall much of the ignimbrite was probably deposited offshore or within the caldera now buried by more recent lavas two major depositional units separated by a co ignimbrite ash cloud deposit and which display dramatic variations in thickness pyroclastic flows were channelised along and deposited within the same valleys that exist today and were commonly continuous and sheet like across the interfluves the basal zones of pyroclastic flows are envisaged as a bed load of coarse lithic lapilli and blocks transported under their own momentum and with the support of the surrounding fine ash gas granular flow

settling behaviour of concentrated dispersions and some volcanological applications abstract experiments have been carried out on the settling of non aggregating poorly sorted aqueous dispersions laced with lowdensity particles in order to gain insight into the sedimentation behaviour of volcaniclastic flows which contain pumice two poorly sorted dispersions of silicon carbide particles with continuous size distributions were studied plastic beads were added as a light component experiments were carried out at solids concentrations between 10 and 60 vol dispersions of low concentration generated normally graded deposits in which particles of different densities were in approximate hydraulic equivalence above a critical concentration 45 to 60 depending on the grain size distribution particle segregation was suppressed and the dispersion settled to form a virtually ungraded bed hindered settling at intermediate concentrations generated normally graded deposits with elutriation pipes floating rafts of plastic and capping layers of fines elutriated by escaping pore fluid buoyancy forces resulted in strong hydraulic inequivalence between light and dense components in the deposits from intermediate concentrations two mechanisms of elutriation pipe growth were observed at intermediate concentrations pipes grew progressively upwards in the deposit as the latter aggraded whereas at high concentrations they nucleated and grew uniformly throughout the whole dispersion elutriation pipes form readily during sedimentation of some poorly sorted aqueous dispersions the presence of elutriation pipes cannot be used alone to distinguish between deposits of epiclastic and pyroclastic flows reverse grading of pumice pumice rafts elutriation pipes and layers of elutriated fines should characterise deposits from pumice rich submarine mass flows despite differences in the fluid phase the experiments also have implications for fluidization mechanisms in pyroclastic flows hindered settling of a concentrated poorly sorted dispersion is a type of self fluidization in which the sedimentation of large particles causes a sufficiently strong vertical fluid flux to fluidize or elutriate smaller ones hindered settling is intrinsic to all pyroclastic flows as they spread out and deflate across the landscape and may reinforce other gas sources as an effective fluidization mechanism during emplacement of type 2 and type 3 ignimbrites and of coignimbrite lag breccias

a model for generation of ash clouds by pyroclastic flows with application to the 1980 eruptions at mount st helens washington pyroclastic flows are defined as consisting of a dense basal avalanche underlying a turbulent ash cloud the generation of ash clouds by the basal avalanche is commonly assumed to be closely related either to fluidization mechanisms within the avalanche or to airflow flow front interactions these mechanisms carry ash upward from the basal avalanche into the ash cloud in currents of gas or air but so far no mechanisms have been proposed to keep the ash in turbulent suspension in the ash cloud here i consider the ash cloud turbulence that is generated in two ways by temperature differences between the basal avalanche and overriding air and by the motions of the blocks lapilli and ash on the outer surface of the basal avalanche this turbulence would carry ash from saltation into turbulent suspension at the base of the ash cloud and this transition would simultaneously occur from the upper surface and front of the basal avalanche this approach quantifies one type of airflow flow front interaction the analysis incorporates studies of heat and mass transfer from rough surfaces during high reynolds number flow equations for conservation of mass momentum heat energy and ash particle concentration are formulated and presented in integral form model results show that for typical avalanche velocities and temperatues temperature differences between the basal avalanche and the surrounding air are insignificant in generating the initial turbulence within the ash cloud instead the initial distribution of particle sizes in the ash cloud is controlled by the turbulence generated by the saltating cascading particles in the basal avalanche study of the morphology of observed ash clouds suggests that thermally driven convection often becomes significant hundreds of meters or more behind the flow front and this may result from deposition of significant quantities of ash from the ash cloud onto the basal avalanche the variations in ash cloud heights morphologies and particle sizes that are predicted by this model are consistent with published observations of small pyroclastic flows at mount st helens in 1980

geochemical and structural studies of the lamont seamounts seamounts as indicators of mantle processes abstract a chain of seamounts located just west of the east pacific rise epr axis at 09 55 n herein called the lamont seamounts have been investigated using sea marc i and sea beam sonar surveys alvin submersible dives deep sea camera tows and dredging the most recently active volcanic vents on the summits of these volcanoes are located at the periphery of each caldera or crater rim or at the margins of caldera floors the distribution of recent volcanic vents and the morphology of the summit plateaus suggest the possible existence of ring fractures on these seamounts ring fractures could be important in focusing post caldera collapse eruptions around summit craters and building summit plateaus structural and morphological data and the petrology of recovered lavas suggest that calderas in the lamont seamounts probably formed by collapse associated with magma withdrawal into large primary vertical magma conduit systems rather than by repeated inflation deflation cycles associated with steady state shallow level magma chambers calderas in the lamont seamounts have evolved through coalescence of adjoining craters and the plan view shape and morphology of each caldera suggest that primary magma conduits have migrated beneath the summit of each seamount through time compared to lavas from the adjacent epr the majority of lamont seamount and cone lavas are more primitive depleted in incompatible elements and compositionally heterogeneous variations in light rare earth elements relative to major elements e g tio 2 na 2 o na 2 o cao and radiogenic isotopes cannot be ascribed to differences in the amount of partial melting of a single n type morb source rather it appears that lamont seamount lavas were derived from heterogeneous mantle sources variably depleted in highly incompatible elements comparison with lavas from the adjacent ridge crest suggests that magma plumbing and ascent mechanisms beneath the lamont seamounts are separate from those along the epr in particular seamount magmas appear to reflect near primary melting events in the mantle whereas epr lavas are the products of shallow level crystallization and mixing in crustal magma chambers absence of long lived magma chambers within the lamont seamounts may account for the low level of hydrothermal activity observed throughout the chain furthermore because of the tight control on sample location and the recovery of lavas having distinct parents from individual volcanoes as well as the lamont chain as a whole we propose that the scale of mantle chemical heterogeneities is less than 5 km

evolution of fluids and metamorphic ore remobilization in the felbertal scheelite deposit eastern alps abstract the felbertal scheelite deposit displays a variety of host rocks and ore types macroscopic and microscopic evidence indicates the formation of the ore mineralization in a geothermal system including explosion and subaqueous surface phenomena at an active continental margin in upper proterozoic upper riphaean lower vendian time the alpine metamorphism and tectonism caused a pervasive metamorphic hydrothermal ore redeposition and a spatial redistribution from smaller volumes to larger volumes of low grade ores especially in the western field microthermometrical studies on quartz scheelite beryl calcite and fluorite in the metamorphosed felbertal scheelite deposit show two types of fluid inclusions aqueous fluid inclusions with variable amounts of co 2 up to 20 wt as well as aqueous fluid inclusions without a measurable co 2 content both types are of alpine origin and were trapped during the alpine retrograde metamorphism therefore the microthermometrical data do not record the conditions of the original pre alpine tungsten formation however it was possible to construct a path for the fluid evolution and the cogenetic ore redeposition during the alpine retrograde metamorphism from the microthermometrical and petrographical data

analogue study of particle segregation in pyroclastic density currents with implications for the emplacement mechanisms of large ignimbrites analogue flume experiments were conducted to investigate the transport and sedimentation behaviour of turbulent pyroclastic density currents the experimental currents were scaled approximately to the natural environment in three ways 1 they were fully turbulent 2 they had a very wide range of particle sizes and associated rouse numbers the ratio of particle settling velocity to effective turbulent eddy velocity in the current and 3 they contained particles of two different densities two sets of surge type experiments were conducted in a 5 m long water filled lock exchange flume at five different volumetric particle concentrations from 0 6 to 23 in one set one component experiments the currents contained just dense particles in the other set two component experiments they contained both light and dense particles in equal volume proportions in both sets of experiments the population of each component had a log normal size distribution in the two component experiments the size range of the light particle population was selected in order to be in hydrodynamic equivalence with that of the dense particles dense particles were normally graded both vertically and downstream in the deposits from both sets of experiments the mass loading normalized to the initial mass of the suspension and grain size of the dense component in the deposits decreased with distance from the reservoir and were insensitive to initial total particle concentration in the currents on the other hand in the two component experiments the light particles were extremely sensitive to concentration they were deposited in hydrodynamic equivalence with the dense particles from dilute currents but were segregated efficiently at concentrations higher than a few per cent with increasing particle concentration the large light particles were carried progressively further down the flume because of buoyancy effects deposits from the high concentration currents exhibited reverse vertical grading of the large light particles efficient segregation of the light component was observed even if the bulk density of the current was less than that of the light particles in both sets of experiments marked inflexions in the rate of downstream decline in mass loading and maximum grain size of the dense component can be attributed to the presence of two different particle settling regimes in the flow 1 particles with rouse numbers 2 5 which did not respond to the turbulence and settled rapidly and 2 particles with rouse numbers 2 5 which followed the turbulent eddies and settled slowly the results are applied to the transport and sedimentation dynamics of pyroclastic density currents that generate large widespread ignimbrites field data fail to reveal significant departures from aerodynamic equivalence between pumice and lithic clasts in three such ignimbrites the particulate loads of some large ignimbrites are transported principally in turbulent suspensions of low concentration in some ignimbrites the well developed inflexions in curves of maximum lithic ml size vs distance can be attributed to the existence of distinct high and low rouse number particle settling regimes that mark the transition from an overcharged state to one in which the residual particulate load is transported more effectively by turbulence

progressive assembly of a massive layer of ignimbrite with a normal to reverse compositional zoning the zaragoza ignimbrite of central mexico the zaragoza ignimbrite and two enclosing rhyodacite pumice fall layers were emplaced during the 15 km3 dre 0 1 ma zaragoza eruption from los humeros volcanic centre 180 km east of mexico city the ignimbrite comprises several massive flow units the largest of which locally exceeds 20 m in thickness and is regionally traceable it comprises massive lapilli ash with vertical elutriation pipes and has a fine grained inverse graded base and a pumice concentration zone at the top it also exhibits an unusual gradational double vertical compositional zonation that is widely traceable a basal rhyodacitic 67 6 69 wt sio2 zone grades up via a mixed zone into a central andesitic 58 62 wt sio2 zone which in turn grades up into an upper rhyodacitic 67 6 69 wt sio2 zone zoning is also defined by vertical variations in lithic clast populations we infer that pyroclastic fountaining fed initially rhyodacite pumice clasts to a sustained granular fluid based pyroclastic density current the composition of the pumice clasts supplied to the current then gradually changed first to andesite and then back to rhyodacite inverse grading at the base of the massive layer may reflect initial waxing flow competence the pumice concentration at the top of the massive layer is entirely rhyodacitic and was probably deposited during waning stages of the current when the supply of andesitic pumice clasts had ceased the return to rhyodacitic composition may have been the result of eruption conduit modification during collapse of los potreros caldera marked in the ignimbrite by a widespread influx of hydrothermally altered lithic blocks and or a decrease in draw up depth from a compositionally stratified magma chamber as the eruptive mass flux waned the massive layer of ignimbrite thins locally to less than 2 m yet it still shows the double zonation correlation of the zoning suggests that the thin massive layer is stratigraphically condensed and aggraded relatively slowly during the same time interval as did the much thicker 50 m massive layer

spatial variations in the composition of turbidites due to hydrodynamic fractionation 1 this study uses measurements from physical experiments to document turbidity currents which are density currents composed of suspended sediment and water to be effective at hydrodynamically fractionating minerals on the basis of grain density and grain shape alone resulting in large scale spatial variations in the composition of their deposits while grain composition varies spatially the population sampled at any one location is hydrodynamically equivalent spatial variations in composition of the deposits are modeled using exponential decay functions which are based on initial concentration of grain types and their respective differences in settling velocity we further discuss implications of this process for addressing practical geophysical problems in which mineralogical distributions are important such as provenance and geochronology studies subsurface imaging and predicting bulk properties of subsurface reservoirs

quantitative analysis of submarine flow deposit shape in the marnoso arenacea formation what is the signature of hindered settling from dense near bed layers abstract submarine sediment density flows are one of the volumetrically most important processes for sediment transport across earth the sediment concentration of flows that reach the deep ocean has never been measured directly and understanding these long runout flows remains a major challenge the miocene marnoso arenacea formation in the italian apennines is the only ancient sequence where individual submarine sediment density flow deposits single beds have been mapped out for more than 100 km down flow here we document the external shape and internal architecture of thirty two individual beds that record flow evolution and can be compared to deposit shapes in mathematical or experimental models the large number of beds allows modes of flow behavior to be identified larger volume turbidites are typically dominated by massive ta or planar laminated tb sandstone intervals that have a broad thickness maximum this shape is important because it suggests that massive and planar laminated sandstones record hindered settling from dense near bed layers which have high 10 by volume sediment concentrations previously some authors have inferred that planar laminated sandstones tb are deposited mainly by dilute flows the position of the broad thickness maximum moves basinward as the volume of sand in the flow increases this is consistent with mathematical modeling that suggests the position of the thickness maximum depends on flow thickness flow speed and sediment settling velocity as well as sediment concentration variations in seawater entrainment rate and local changes in seafloor gradient smaller volume turbidite sandstone intervals are finer grained and dominated by ripple cross lamination tc and have a near exponential decay in thickness that is consistent with deposition from a dilute sediment suspension the rate of near exponential thinning is controlled by sandstone volume in contrast turbidite mudstone intervals show an approximately linear increase in thickness with distance flows that entered the basin in opposite directions produced turbidite mudstone intervals that thicken towards the same location indicating that muddy turbidity currents can drain back over long distances to basinal lows

metamorphosed hydrothermal eruption breccia conglomerate in the felbertal scheelite deposit eastern alps austria abstract a metamorphosed hydrothermal eruption breccia conglomerate has been found in recently opened galleries in the western field of the felbertal scheelite deposit it belongs to the k2 ore body and covers a stratiform quartz mass and surrounding host rocks the quartz mass shows features very similar to siliceous sinters in recent explosion craters e g waiotapu new zealand the breccia displays a variable thickness commonly up to 60 cm it is continuous for at least 100 m along strike and is composed of rock fragments from the surrouding and underlying host rocks including basaltic felsic and quartz rocks these fragments reveal a variable degree of rounding due to subaqueous reworking and an elongation due to tectonism and metamorphism with increasing distance from the quartz mass the quantity of rock fragments in the breccia diminishes rapidly laterally the metamorphosed breccia conglomerate horizon interfingers with biotite amphibolites this breccia indicates 1 at least one hydrothermal eruption 2 the existence of a subaqueous erosional surface 3 the ejection of underlying and surrounding host rocks of the scheelite bearing series 4 the pre existence of the quartz mass and a bimodal rock assemblage including basaltic and felsic volcanites with an earlier tungsten mineralization mainly in quartz fragments and 5 a tungsten mineralization coeval with the formation of the matrix this matrix shows enrichments in w sn mo li be au bi as f p ba th and u

the occurrence and origin of prominent massive pumice rich ignimbrite lobes within the late pleistocene abrigo ignimbrite tenerife canary islands abstract the 0 196 ma lithic rich abrigo ignimbrite on tenerife canary islands contains localised massive coarse pumice rich ignimbrite lobes mprils they typically form low ridges up to 2 m high with axes parallel to the flow direction and in cross section they range from symmetrical to asymmetrical and highly skewed lobate bedforms generally with flat bases the major components are rounded pebble to cobble sized phonolitic pumice clasts within an ignimbritic matrix of ash fine lithics and minor crystals which varies from lithic rich to lithic poor commonly there is a vertical increase in pumice concentration from matrix supported texture at the base to clast supported at the top accompanied by an increase in pumice clast size mprils often thin and grade laterally perpendicular to current flow into planar pumice concentration zones they occur at one or more stratigraphic levels as either solitary lobes associated with flat topography or as multiple onlapping lobes or within a laterally complex stratified pumice rich ignimbrite facies lcspis near palaeotopographic highs mprils are original depositional features not erosional in origin and are derived from a larger pyroclastic flow it is likely that pumice was segregated to the upper and outer regions of the parent flow causing a significant rheological contrast with the lower lithic rich zone the more pumice rich parts are interpreted to have detached from the parent flow as it decelerated onto gentler slopes or interacted with topographic highs and raced ahead as mobile derivative pyroclastic flows the flow parallel ridge shape of mprils may be a result of fingering within these flows or concentration of pumice within the intermediary clefts deposition occurred en masse at the termination of the flow front the resultant lobate deposits were then overridden and mantled by normal ignimbrite facies from either a later flow pulse or the following main part of the parent flow

age and evolution of scheelite hosting rocks in the felbertal deposit eastern alps u pb geochronology of zircon and titanite u pb isotope analyses of zircon and titanite extracted from different rocks of the felbertal scheelite deposit yield the following information 1 an age of 593 22 ma 2 is obtained for zircon crystallization in the scheelite bearing matrix of an eruption breccia in the western ore field 2 discordant zircons from an elongated up to 8 m thick scheelite rich quartzite body in the eastern ore field give an upper intercept age of 544 5 ma this quartzite contains a laminated fine grained scheelite mineralization 3 zircons from a small granitoid intrusion of the western ore field reveal an age of 336 16 ma and concordant titanites document an age of 282 2 ma for variscan amphibolite facies metamorphism both events granitoid intrusion and later metamorphism caused ore re mobilization including the formation of yellowish fluorescent molybdo scheelite porphyroblasts 4 for a narrow lamprop 1hyric dike in the western ore field a concordant titanite age of 283 7 ma is obtained this age is identical with the titanites from the amphibolite facies metamorphic intrusion tiny scheelite grains were tapped by the dike from pre existing scheelite mineralizations in the truncated host rocks 5 alpine metamorphism at 31 4 ma did not exceed lowermost amphibolite facies conditions and it caused scheelite re mobilization on a minor scale only producing bluish fluorescent porphyroblasts in quartz veinlets and veins as well as bluish fluorescent scheelite rims around older scheelite grains moreover crosscutting alpine fissure fillings show bluish fluorescent inclusion free scheelite 6 the preservation of variscan titanites the absence of alpine titanite growth and the large degree of variscan scheelite re mobilization demonstrate that amphibolite facies metamorphism in the felbertal area has a variscan age this result clearly documents variscan tectono metamorphism to be the dominant event instead of the hitherto surmised alpine metamorphism this multi stage evolution of the felbertal ore bodies corroborates the view that tungsten deposits are conditioned by several succeeding thermal events leading to a series of stages that ultimately produce high grade scheelite concentrations these high grade ores predominately occur along shear zones of different age accompanied by the formation of large volumes of low grade scheelite mineralizations along host rock foliations and quartz veinlets and veins

decoupling of pyroclastic currents hazards assessments gravitational settling of fragments gas elutriation and boundary layer processes cause concentration values to change within different parts of sediment gravity flows during movement these processes result in flow transformations density stratification and decoupling of single currents into separate sediment gravity flows density can change by an increase in solids concentration in basal zones of currents by downward fall of particles or by incorporation of particles from the substrate thereby leaving their upper parts more dilute in some flows upward streaming gases and elutriation of fines cause dilute turbulent currents to develop at the top of a dense current increasing concentration dampens turbulent action transforming the turbulent to the nonturbulent state commonly at the current s base conversely dilute parts of currents become or remain turbulent currents consisting of turbulent and nonturbulent parts can decouple by shearing past one another because of differing flow properties with each part taking separate paths the manner by which a pyroclastic flow moves affects its predicted runout distance its distribution as continuous sheets or in isolated basins and whether it can cross barriers such as high mountain ranges or water bodies during the eruption of unzen volcano on june 3 1991 decoupling of a pyroclastic surge from the top of a pyroclastic flow caused the deaths of harry glicken maurice krafft katia krafft and 40 japanese citizens mapping volcanic hazard zones requires that consideration be given to decoupling processes to help prevent similar tragedies in the future

synthesizing large scale pyroclastic flows experimental design scaling and first results from pele pyroclastic flow eruption large scale experiment pele is a large scale facility for experimental studies of pyroclastic density currents pdcs it is used to generate high energy currents involving 500 6500 m3 natural volcanic material and air that achieve velocities of 7 30 m s 1 flow thicknesses of 2 4 5 m and runouts of 35 m the experimental pdcs are synthesized by a controlled eruption column collapse of ash lapilli suspensions onto an instrumented channel the first set of experiments are documented here and used to elucidate the main flow regimes that influence pdc dynamic structure four phases are identified 1 mixture acceleration during eruption column collapse 2 column slope impact 3 pdc generation and 4 ash cloud diffusion the currents produced are fully turbulent flows and scale well to natural pdcs including small to large scales of turbulent transport pele is capable of generating short pulsed and sustained currents over periods of several tens of seconds and dilute surge like pdcs through to highly concentrated pyroclastic flow like currents the surge like variants develop a basal 0 05 m thick regime of saltating rolling particles and shifting sand waves capped by a 2 5 4 5 m thick turbulent suspension that grades upward to lower particle concentrations resulting deposits include stratified dunes wavy and planar laminated beds and thin ash cloud fall layers concentrated currents segregate into a dense basal underflow of 0 6 m thickness that remains aerated this is capped by an upper ash cloud surge 1 5 3 m thick with 100 to 10 4 vol particles their deposits include stratified massive normally and reversely graded beds lobate fronts and laterally extensive veneer facies beyond channel margins

lithofacies and architecture of a basinwide tuff unit in the miocene eoil basin se korea modes of pyroclastic sedimentation changes in eruption style and implications for basin configuration the miocene eoil basin se korea is a small half graben basin that was rifted by both extensional and dextral strike slip deformations during backarc opening of the east sea sea of japan the basin was filled by fluviolacustrine sediments and abundant basaltic and dacitic volcanic deposits the paljori tuff is a 2 20 m thick basinwide dacitic volcaniclastic unit that is intercalated in the topmost part of a fluviolacustrine formation the tuff consists of two pyroclastic units units ii and iii which are underlain and overlain by two resedimented volcaniclastic units units i and iv unit i at the base is composed mainly of fine grained resedimented tephra unit ii is a massive and stratified tuff produced by a dense pyroclastic density current that was ponded in the southwestern part of the basin unit iii is a massive lapilli tuff emplaced by a voluminous and turbulent pyroclastic density current that swept across the basin toward the northeast without ponding or blocking by intrabasinal highs and basin margin relief and unit iv at the top is composed of coarse grained resedimented tephra the general coarsening up of these units the lack of nonvolcaniclastic sedimentary interbeds and the offset stacking of the units toward the northeast suggest an episodic short duration eruption from the southwest that waxed in mass eruption rate and changed eruption style from phreatomagmatic to magmatic these units show basin scale variations in geometry including 1 thick accumulations of unit i in the northeastern and southwestern extremities of the basin 2 ponding of unit ii in the southwestern part of the basin 3 truncation of unit iii in the same area and 4 thickening of unit iv toward the northeastern basin margin these provide important clues to the distribution of and temporal changes in accommodation space and hence the configuration and structural evolution of the eoil basin it is inferred that the greatest subsidence occurred at the southwestern and northeastern corners of the basin the southwest northeast trending axes of the basin plunge gently toward the northeast and the major intrabasinal faults of the basin were produced before the eruption of the paljori tuff the basinwide preservation of the tuff and its fairly uniform thickness suggest that the basin was undergoing rapid extension and subsidence because of the onset of the rift climax the paljori tuff shows that important clues to and high resolution records of the large scale configuration and structural evolution of a sedimentary basin can be obtained from the study of lithofacies variations and the architecture of a single basinwide volcaniclastic unit

experiments on bidisperse constant volume gravity currents propagation and sediment deposition laboratory experiments show that the propagation and sedimentation patterns of particle laden gravity currents are strongly influenced by the size of suspended particles the main series of experiments consisted of fixed volume releases of dilute mixtures containing two sizes of silicon carbide particles 25 m and 69 m mean diameter within a 6 m flume polydisperse experiments involved mixtures of five different particle sizes and variation of the amounts of the finest and coarsest particles all variables apart from the initial relative proportions of particles were identical in the experiments the effects of mixing different proportions of fine and coarse particles is markedly non linear adding small amounts of fine sediment to a coarse grained gravity current has a much larger influence on flow velocity run out distance and sedimentation patterns than adding a small amount of coarse sediment to a fine grained gravity current the experiments show that adding small amounts of fine particles to a coarse grained current results in enhanced flow velocities because the fine sediment remains suspended and maintains an excess current density for a much longer time thus the distance to which coarse particles are transported increases substantially as the proportion of fines in the flow is increased our experiments suggest that sandy turbidity currents containing suspended fines will be much more extensive than turbidity currents composed of clean sand

dynamics and deposition of sediment bearing multi pulsed flows and geological implication copyright 2019 sepm society for sedimentary geology previous studies on dilute multi pulsed subaqueous saline flows have demonstrated that pulses will inevitably advect forwards to merge with the flow front on the assumption that pulse merging occurs in natural scale turbidity currents it was suggested that multi pulsed turbidites that display vertical cycles of coarsening and fining would transition laterally to single pulsed normally graded turbidites beyond the point of pulse merging in this study experiments of dilute single and multi pulsed sediment bearing flows turbidity currents are conducted to test the linkages between downstream flow evolution and associated deposit structure experimental data confirm that pulse merging occurs in laboratory scale turbidity currents however only a weak correspondence was seen between longitudinal variations in the internal flow dynamics and the vertical structure of deposits multi pulsed deposits were documented but transitioned to single pulsed deposits before the pulse merging point this early transition is attributed to rapid sedimentation related depletion of the coarser grained suspended fraction in the laboratory setting whose absence may have prevented the distal development of multi pulsed deposits this factor complicates estimation of the transition point in natural scale turbidite systems

stratification and particle segregation in flowing polydisperse suspensions with applications to the transport and sedimentation of pyroclastic density currents abstract pyroclastic density currents pdcs are particle laden gravity driven flows of volcanic origin they typically contain solid particles with a wide range of sizes and densities resulting in complicated segregation effects during transport and sedimentation this paper describes experimental high density aqueous suspension flows scaled approximately to turbulent pyroclastic density currents it provides information on the development of concentration and particle size gradients in such flows and their relationships to vertical and stream wise grading in the resulting deposit the currents were generated by lock exchange release of initially homogeneous suspensions of highly polydispersed particles with concentrations of 13 8 23 0 vol size distributions were chosen to encompass a wide range of rouse pn numbers from particles that were fully supported by turbulence in the currents pn 2 5 to those whose motion was essentially decoupled from turbulent eddies pn 2 5 as in many pyroclastic density currents experimental currents of two different particle populations were studied one containing particles of a single density and another with particles of two different densities in each case the current head was sampled at different locations along the flume in order to study the evolution of vertical gradients in particle size concentration and density distribution in the propagating flow these gradients were then related to grading observed in the resulting deposits differential settling resulted in rapid segregation of particles of different sizes and densities in the currents and the development of both vertical and longitudinal gradients in particle concentrations sizes and proportions longitudinal gradients developed as vertical gradients were pulled out longitudinally by the vertical velocity profile in the fluid gradient development involved principally large particles capable of moving independently of turbulent eddies in the fluid the segregation behaviour of large light particles equivalent to pumice in the natural system in the bidensity currents was strongly influenced by buoyancy forces in the dense initial suspension sedimentation of large light particles was delayed and reverse vertical and longitudinal grading of these particles was generated in the resulting deposit reverse grading in the deposit was not caused by flotation of the large light particles which were normally stratified in the current during transport deposition of large light particles was probably delayed by a thin boundary layer of sufficiently high bulk density at the base of the rapidly evolving density stratified current vertical or longitudinal reverse grading of large pumices in pyroclastic density current deposits does not necessarily imply flotation during transport

clast sizes in terrestrial and martian ignimbrite lag deposits large scale pyroclastic flows ignimbrites are produced when a dense mixture of volcanic gas and pyroclasts is erupted into an atmosphere and forms a collapsed fountain like structure sparks et al 1978 over a vent a physical model of the resulting fluid flow field can be devised heslop 1987 for any planetary environment heslop and wilson 1988 which allows the pressure and velocity of the dusty gas mixture emerging from the vent to be deduced calculation of the drag forces exerted by the dusty gas on relatively large clasts allows us to deduce the sizes of the largest clasts of a given density which can just be transported through the vent to contribute to a proximal lag breccia deposit we find that ignimbrites on earth are potentially capable of transporting out of the vent significantly larger clasts than those commonly found in lag deposits martian ignimbrite lag breccia deposits should contain near vent clasts at least twice as large as those in terrestrial deposits produced in eruptions with the same volatile contents such clasts should be detectable from future mars orbiting spacecraft furthermore the near vent clast sizes in martian ash fall deposits will be at least a factor of 100 times smaller than those in proximal ignimbrite deposits when coupled with other observations of the morphology of near vent deposits the detection of the presence of coarse clasts should provide a strong indication of the eruption mechanism which emplaced them

emplacement of the taupo ignimbrite by a dilute turbulent flow an ignimbrite is a pumice rich deposit that records the passage of a ground hugging ash flow a pyroclastic flow generated by the collapse of a volcanic eruption column1 geologists study such deposits to reconstruct the parent eruptions and to predict the consequences of future eruptions the 1 800 yr old taupo ignimbrite of new zealand has been interpreted to represent en masse emplacement by an avalanche like flow with a volumetric solids concentration in excess of 30 ref 2 the evidence for this is equivocal however and here we propose the alternative view that the deposit was emplaced by a relatively dilute and turbulent density current3 we present an isothermal hydraulic model the results of which taken together with existing observations suggest that the total flow rate of solids and gas was about 40km3s 1 for around 15 minutes this intense flux resulted in a flow which had a near vent solids concentration of 0 3 by volume was about 1 km thick and travelled outward from the vent with a typical speed of 200 ms 1 in view of the good agreement between predicted and observed radial trends in the taupo deposit we suggest that the origin of other ignimbrites with similar characteristics should be reconsidered

stratigraphy of the toba tuffs and the evolution of the toba caldera complex sumatra indonesia during the past 1 2 m y a magma chamber of batholithic proportions has developed under the 100 by 30 km toba caldera complex four separate eruptions have occurred from vents within the present collapse structure which formed from eruption of the 2800 km3 youngest toba tuff ytt at 74 ka eruption of the three older toba tuffs alternated from calderas situated in northern and southern portions of the present caldera the northern caldera apparently developed upon a large andesitic stratovolcano the calderas associated with the three older tuffs are obscured by caldera collapse and resurgence resulting from eruption of the ytt samosir island and the uluan block are two sides of a single resurgent dome that has resurged since eruption of the ytt samosir island is composed of thick ytt caldera fill whereas the uluan block consists mainly of the oldest toba tuff ott in the past 74000 years lava domes have been extruded on samosir island and along the caldera s western ring fracture this part of the ring fracture is the site of the only current activity at toba updoming and fumarolic activity the toba eruptions document the growth of the laterally continuous magma body which eventually erupted the ytt repose periods between the four toba tuffs range between 0 34 and 0 43 m y and give insights into pluton emplacement and magmatic evolution at toba

axial processes along a segment of the east pacific rise 10 12 n axial segments of the east pacific rise are made up of individual volcanoes each volcano has a distinct magma composition and shows a systematic variation in the fractional crystallization with distance along the axis from the central chamber hydrothermal venting lava morphology and tectonics also vary along the axis lavas erupted near transform faults from the tips of propagating rifts in overlapping spreading centres and from near axis seamounts have different and variable compositions

high magnetization zones near spreading center discontinuities abstract three dimensional inversions of the magnetic field reveal the presence of zones of high magnetization over the propagating limb of large spreading center discontinuities the presence of high magnetization zones is confirmed by rock magnetic measurements where available magnetization highs are associated with basalts which tend to have high fe contents and low mg numbers these data suggest that high magnetization zones observed over large ridge axis discontinuities are associated with highly differentiated basalts enriched in iron following christie and sinton 1 such highly evolved basalts may be the result of shallow level crystal fractionation in small magma bodies with a low supply rate these small magma bodies are postulated to correspond to the first stages in the development of a sub axial magmatic system as a result of the propagation of one of the limbs of the offset into older lithosphere because high magnetization zones at large ridge axis discontinuities often correspond to gaps in the along axis extent of a seismically detectable magma chamber these magma bodies may be smaller than a few hundred meters rock magnetic measurements suggest that the enrichment in iron associated with increased differentiation may be accompanied in a few cases by an increase in the concentration of titanomagnetite within the basalts and in the magnetization of the rocks however the exact relationship between high magnetization intensities and iron enrichment is complex and unclear and may be significantly affected by factors such as magnetic mineralogy and crystallization history

the influence of variable topography on the depositional behaviour of pyroclastic density currents the examples of the upper pollara eruption salina island southern italy abstract stratigraphic reconstruction of the upper pollara eruption has allowed for the inference of eruptive mechanisms and the distillation of a sedimentological model for pyroclastic density currents pdcs moving across variable topography the pre eruptive topography in the study area was characterised by a tuff ring like morphology with both inward and outward dipping slopes highly viscous moderately porphyritic dacitic to rhyolitic magmas fed the eruption which was characterised by a vulcanian eruptive style the stratigraphic succession was divided into five eruption units eus which result from different phases of the eruption separated by stases sustained columns occurred only during eu1 while pdc generation dominates eu2 5 lithofacies analysis of the pdc deposits indicates the prevalence of massive coarse grained deposits on the inner slopes of the pollara crater which are interpreted as the deposits of a flow boundary zone dominated by granular flow or fluid escape regimes dune bedded massive to stratified lithofacies dominate the outer slopes of the pollara crater and are interpreted as the deposits of pdcs with flow boundary zones in which traction played a major role thin massive pdc deposits are exposed on the sub horizontal malfa terrace and are interpreted as representative of flow boundary zones dominated by a granular flow regime the occurrence of stacked deposits indicates that most of the pdcs were characterised by unsteady pulsatory behaviour with development of trains of pulses during their transport the downcurrent lithofacies transitions observed for the upper pollara deposits have finally been compared with other similar lithofacies associations which have been described for short lived pdcs at tuff rings in order to discuss the influence of pre eruptive topography on lithofacies association