debris jets in continental phreatomagmatic volcanoes a field study of their subterranean deposits in the coombs hills vent complex antarctica abstract the ferrar large igneous province of antarctica contains significant mafic volcaniclastic deposits some of which are interpreted to fill large vent complexes such a complex was re examined at coombs hills to map individual steep sided cross cutting bodies in detail and we found several contrasting types two of which are interpreted to have filled subterranean passageways forcefully opened from below into existing non consolidated debris these transient conduits were opened because of the propagation of debris jets upward moving streams of volcaniclastic debris steam magmatic gases liquid water droplets following explosive magma aquifer interaction some debris jets probably remained wholly subterranean whereas others made it to the surface but the studied outcrops do not allow us to differentiate between these cases the pipes filled with country rock rich lapilli tuff or tuff breccia are interpreted to have formed following phreatomagmatic explosions occurring near the walls or floor of the vent complex causing fragmentation of both magma and abundant country rock material in contrast some of the cross cutting zones filled with basalt rich tuff breccia or lapilli tuff could have been generated following explosions taking place within pre existing basalt bearing debris well away from the complex walls or floor we infer that once focused jets were formed they did not incorporate significant amounts of existing debris while travelling through them instead incorporation of fragments from the granular host took place near explosion sites other basalt rich tuff breccia zones accompanied by domains of in situ peperite and coherent basalt pods are inferred to have originated by less violent processes

a physical theory of snow gliding snow glide slip of the entire snowpack over the ground is not observed unless water reaches the ground interface in this paper snow gliding is approached from the point of view that a perfect s

unbedded diatreme deposits reveal maar diatreme forming eruptive processes standing rocks west hopi buttes navajo nation usa diatremes provide partial records of how dyke delivered magma periodically interacting with water produces the largest known cylindrical conduit structures of any volcano type we address how pre eruptive country rock is disrupted and redistributed to form the diatreme structure during an eruption by establishing the internal architecture of the unbedded diatreme at standing rocks west and the volumes and sources of wall rock within the diatreme and in a complementary tephra ring at teshim both in the hopi buttes volcanic field the unbedded diatreme is dominantly exposed as a massif comprising multiple subvertical columns interpreted as ephemeral conduit deposits of well mixed poorly sorted composite and juvenile pyroclast rich deposits that truncate marginal layered deposits plus a peripheral heterolithic country rock breccia wall rock clasts are scarce in the massif and were mainly sourced from 75 to 245 m depth below the paleosurface by contrast the tephra ring has abundant wall rock fragments dominantly from the upper 75 m of the pre eruptive sedimentary sequence the diatreme structure is interpreted to have been formed by many small volume explosions fed by a low flux of basaltic magma late stage explosive activity was rooted mostly within pyroclastic debris at shallow to intermediate depths within the diatreme structure this resulted in damped and shifting intra diatreme explosions and jets that facilitated gradual mixing recycling and remobilization of debris in the diatreme with incremental addition of juvenile material and possibly a local rise in the crater floor

facies distribution of ejecta in analog tephra rings from experiments with single and multiple subsurface explosions the volume grain size and depositional facies of material deposited outside an explosion crater ejecta are sensitive to the depth of the explosion the explosion energy and the presence or absence of a crater before the explosion we detonate buried chemical explosives as an analog for discrete volcanic explosions in experiments to identify unique characteristics of proximal medial and distal ejecta facies and their distribution from a range of scaled depths in undisturbed and cratered ground ejecta are here discussed in terms of three facies 1 proximal ejecta which form a constructional landform around a crater 2 medial ejecta which form a continuous sheet deposit that thins much more gradually with distance and 3 distal ejecta that are deposited as isolated clasts the extent of proximal ejecta away from the crater relative to crater size is not sensitive to scaled depth but the volume proportion of proximal ejecta to the total ejecta deposit is sensitive to the presence of a crater and scaled depth medial ejecta distribution and volume contributions are both sensitive to the presence of a crater and to scaled depth distal ejecta distance is dependent on scaled depth and the presence of a crater while the volume proportion of distal ejecta is less sensitive to scaled depth or presence of a crater experimental facies and deposit structures inferred from observations of jet dynamics are used to suggest facies associations anticipated from eruptions dominated by explosions of different scaled depth configurations we emphasize that significant differences in tephra ring deposits can result from the effects of scaled depth and preexisting craters on ejecta dynamics and are not necessarily related to fundamental differences in explosion mechanisms or degree of magma fragmentation

suspended sediment transport in the deepwater navigation channel yangtze river estuary china in the dry season 2009 1 observations over spring and neap tidal cycles 1 the in situ data in the deepwater navigation channel dnc yangtze river estuary yre china in the dry season 2009 shows spring tides associated with greater maximum velocities more mixing less stratification and diffused fluid mud whereas neap tides are associated with smaller maximum velocities greater stratification inhibited mixing and stratified fluid muds the balance of salt flux indicates the seaward salt transport is dominated by fluvial flows and the landward salt transport is generated by compensation flows during spring tides but shear effects during neap tidal cycles the balance of suspended sediment flux illustrates the offshore sediment transport is dominated by fluvial flows as well but the onshore transport is induced by tidal pumping effects on spring tides and shear effects on neaps the suspended sediment transport is strongly affected by the salinity distribution and salinity gradient induced stratification in the dnc the spring neap asymmetry is generated by the estuarine gravitational circulation during low flow conditions while the flood ebb asymmetric stratification within a tidal cycle is due to the semidiurnal tidally movement of the salt front

reconstruction of a kimberlite eruption using an integrated volcanological geochemical and numerical approach a case study of the fox kimberlite nwt canada abstract an integrated approach involving volcanology geochemistry and numerical modelling has enabled the reconstruction of the volcanic history of the fox kimberlite pipe the observed deposits within the vent include a basal massive poorly sorted matrix supported lithic fragment rich eruption column collapse lapilli tuff extensive vent widening during the climactic magmatic phase of the eruption led to overloading of the eruption column with cold dense country rock lithic fragments dense juvenile pyroclasts and olivine crystals triggering column collapse 40 dilution of the kimberlite by granodiorite country rock lithic fragments is observed both in the physical componentry of the rocks and in the geochemical signature where enrichment in al2o3 and na2o compared to average values for coherent kimberlite is seen the wide deep open vent provided a trap for a significant proportion of the collapsing column material preventing large scale run away in the form of pyroclastic flow onto the ground surface although minor flows probably also occurred a massive to diffusely bedded poorly sorted matrix supported accretionary lapilli bearing lithic fragment rich lapilli tuff overlies the column collapse deposit providing evidence for a late phreatomagmatic eruption stage caused by the explosive interaction of external water with residual magma correlation of pipe morphology and internal stratigraphy indicate that widening of the pipe occurred during this latter stage and a thick granodiorite cobble boulder breccia was deposited ash and accretionary lapilli rich tephra deposited on the crater rim during the late phreatomagmatic stage was subsequently resedimented into the vent incompatible elements such as nb are used as indicators of the proportion of the melt fraction or kimberlite ash retained or removed by eruptive processes when compared to average coherent kimberlite the ash rich deposits exhibit 30 loss of fines whereas the column collapse deposit exhibits 50 loss this shows that despite the poorly sorted nature of the column collapse deposit significant elutriation has occurred during the eruption indicating the existence of a high sustained eruption column the deposits within fox record a complex eruption sequence showing a transition from a probable violent sub plinian style eruption driven by instantaneous exsolution of magmatic volatiles to a late phreatomagmatic eruption phase mass eruption rate and duration of the sub plinian phase of the eruption have been determined based on the dimensions of milled country rock boulders found within the intra vent deposits calculations show a short lived eruption of one to eleven days for the sub plinian magmatic phase which is similar in duration to small volume basaltic eruptions this is in general agreement with durations of kimberlite eruptions calculated using entirely different approaches and parameters such as predictions of magma ascent rates in kimberlite dykes

measurement of size and settling velocity of suspended aggregates on the northern california continental shelf abstract a high resolution video camera attached to a miniature sediment trap was used to make time lapse images of suspended aggregates settling through the water column the video trap system was placed 2 m above the seabed in the strataform study area off eureka california and was programmed to take a 7 second time sequence of settling particles every 6 h over a 4 month period the camera field of view within the trap is 15 mm and the minimum particle resolution is approximately 130 m data analysis was carried out with a video frame grabber and digital imaging software to determine the particle size shape and settling velocity results of the first deployment of this system september 1995 show that particle sizes elliptical nominal diameter determined from the video images ranged from 130 to 740 m with a modal size of 300 m the average form factor ratio of short to long semi axis of the aggregates was 0 71 measured settling velocities varied from 0 09 to 8 13 mm s in terms of weight percent or mass the median size of the aggregate distribution is 600 m analysis of settled particles collected from the sediment trap shows disaggregated or component grains ranging from 1 0 to 63 m with a modal size of 26 m and a median size of 11 m additionally an analysis was carried out to separate the relative importance of particle shape and excess density on settling velocity and these results were compared to observations using the equations for settling velocity of spheres and prolate spheroids and substituting published relationships for size versus excess density into these equations the settling velocity for combinations of shape and excess density has been calculated results show that differences between sphere and prolate spheroid with major axis both parallel and perpendicular to flow approximations caused less than a 13 difference in computed settling velocity in contrast the various size excess density relationships used to compute settling velocity caused order of magnitude deviations from measured values in all cases however the approximations of settling velocity for aggregates provide a significantly better fit to data than approximations based on disaggregated sediment size distribution

experiments with vertically and laterally migrating subsurface explosions with applications to the geology of phreatomagmatic and hydrothermal explosion craters and diatremes we present results of experiments that use small chemical explosive charges buried in layered aggregates to simulate the effects of subsurface hydrothermal and phreatomagmatic explosions at varying depths and lateral locations extending earlier experimental results that changed explosion locations only along a vertical axis the focus is on the resulting crater size and shape and subcrater structures final crater shapes tend to be roughly circular if subsurface explosion epicenters occur within each other s footprints defined as the plan view area of reference crater produced by a single explosion of a given energy as predicted by an empirical relationship craters are elongate if an epicenter lies somewhat beyond the footprint of the previous explosion such that their footprints overlap but if epicenters are too far apart the footprints do not overlap and separate craters result explosions beneath crater walls formed by previous blasts tend to produce inclined laterally directed ejecta jets while those beneath crater centers are vertically focused lateral shifting of explosion sites results in mixing of subcrater materials by development of multiple subvertical domains of otherwise pure materials which progressively break down with repeated blasts and by ejection and fallback of deeper seated material that had experienced net upward displacement to very shallow levels by previous explosions a variably developed collar of material that experienced net downward displacement surrounds the subvertical domains the results demonstrate key processes related to mixing and ejection of materials from different depths during an eruptive episode at a maar diatreme volcano as well as at other phreatomagmatic and hydrothermal explosion sites

effects of explosion energy and depth to the formation of blast wave and crater field explosion experiment for the understanding of volcanic explosion we made field explosion experiments as an analogue of volcanic explosion to understand the relationship between the explosion condition and the resultant surface phenomena the main parameters we employed were explosion depth and explosion energy through the experiments we confirmed that scaled depth which is the depth divided by cube root of energy is the main parameter determining the properties of explosive volcanism the energy assigned to blast wave decreased exponentially against the scaled depth the scaled crater diameter became maximum when the scaled depth was about 4 10 3 m j scaled crater diameters by nuclear chemical subsurface and some volcanic explosions were almost the same from the scaling law the overpressure at crater rim was estimated to be several mpa which corresponded to typical rock strength probably the ground forming materials were broken inside the area where overpressure exceeded their strength

settling velocities of fine suspended particles in the changjiang estuary china abstract point sampled suspended sediment concentration profiles grain size largely c of sediment in suspension the best fit equation for the field settling velocity of fine particles in the changjiang estuary can be expressed by the power law ws 2 37 c 0 84 r2

deep bedded ultramafic diatremes in the missouri river breaks volcanic field montana usa 1 km of syn eruptive subsidence the ultramafic eocene missouri river breaks volcanic field mrbvf montana usa includes over 50 diatremes emplaced in a mostly soft substrate the current erosion level is 1 3 1 5 km below the pre eruptive surface exposing the deep part of the diatreme structures and some dikes five representative diatremes are described here they are 200 375 m across and have sub vertical walls their infill consists mostly of 55 90 bedded pyroclastic rocks fine tuffs to coarse lapilli tuffs with concave upward bedding and 45 10 non bedded pyroclastic rocks medium lapilli tuffs to tuff breccias the latter zones form steep columns 15 135 m in horizontal dimension which cross cut the bedded pyroclastic rocks megablocks of the host sedimentary formations are also present in the diatremes some being found 1 km or more below their sources the diatreme infill contains abundant lithic clasts and ash sized particles indicating efficient fragmentation of magma and country rocks the spherical to sub spherical juvenile clasts are non vesicular they are accompanied by minor accretionary lapilli and armored lapilli the deposits of dilute pyroclastic density currents are locally observed our main interpretations are as follows 1 the observations strongly support phreatomagmatic explosions as the energy source for fragmentation and diatreme excavation 2 the bedded pyroclastic rocks were deposited on the crater floor and subsided by 1 0 1 3 km to their current location with subsidence taking place mostly during the eruption 3 the observed non bedded pyroclastic columns were created by debris jets that punched through the bedded pyroclastic material the debris jets did not empty the mature diatreme occupying only a fraction of its width and some debris jets probably did not reach the crater floor 4 the mature diatreme was nearly always filled and buttressed by pyroclastic debris at depth there was never a 1 3 1 5 km deep empty hole with sub vertical walls otherwise the soft substrate would have collapsed inward which it only did near the surface to create the megablocks we infer that syn eruptive subsidence shifted down bedded pyroclastic material and shallow sedimentary megablocks by 0 8 1 1 km or more after which limited post eruptive subsidence occurred this makes the mrbvf diatremes an extreme end member case of syn eruptive subsidence in the spectrum of possibilities for maar diatreme volcanoes worldwide

glide avalanche response to an extreme rain on snow event snoqualmie pass washington usa rain on snow events trigger immediate and delayed avalanches as liquid water penetrates the snowpack we present results from an extreme rain on snow event that triggered a glide avalanche near snoqualmie pass washington usa snoqualmie pass recorded 463 cm of snowfall from 13 december 2008 to 6 january 2009 this period of snowfall was followed by a strong southwesterly tropical flow that resulted in an extreme rain on snow event sensors at snoqualmie pass recorded 285 mm of precipitation over a 52 hour period flooding slush flows landslides and avalanches resulted from the influx of precipitation snow heights decreased rapidly over the period with settlement rates approaching 80 mm h 1 liquid water infiltrated and flowed through the snowpack within a few hours of the arrival of rain yet many of the major avalanches occurred 12 30 or more hours after the onset of rain and water outflow a glide avalanche occurred 30 hours after the onset of rain and the establishment of drainage through the snowpack increasing glide rates correlate with periods of rapid snow settlement here glide rates approached 670 mm h 1 although glide and settlement rates increased during periods of intense precipitation glide failure occurred 8 hours after peak precipitation and outflow

the 1976 1982 strombolian and phreatomagmatic eruptions of white island new zealand eruptive and depositional mechanisms at a wet volcano white island is an active andesitic dacitic composite volcano surrounded by sea yet isolated from sea water by chemically sealed zones that confine a long lived acidic hydrothermal system within a thick sequence of fine grained volcaniclastic sediment and ash the rise of at least 106 m3 of basic andesite magma to shallow levels and its interaction with the hydrothermal system resulted in the longest historical eruption sequence at white island in 1976 1982 about 107 m3 of mixed lithic and juvenile ejecta was erupted accompanied by collapse to form two coalescing maar like craters vent position within the craters changed 5 times during the eruption but the vents were repeatedly re established along a line linking pre 1976 vents the eruption sequence consisted of seven alternating phases of phreatomagmatic and strombolian volcanism strombolian eruptions were preceded and followed by mildly explosive degassing and production of incandescent blocky juvenile ash from the margins of the magma body phreatomagmatic phases contained two styles of activity a near continuous emission of gas and ash and b discrete explosions followed by prolonged quiescence the near continuous activity reculted from streaming of magmatic volatiles and phreatic steam through open conduits frittering juvennile shards from the margins of the magma and eroding loose lithic particles from the unconsolidated wall rock the larger discrete explosions produced ballistic block aprons downwind lobes of fall tephra and cohesive wet surge deposits confined to the main crater the key features of the larger explosions were their shallow focus random occurrence and lack of precursors and the thermal heterogeneity of the ejecta this white island eruption was unusual because of the low discharge rate of magma over an extended time period and because of the influence of a unique physical and hydrological setting the low rate of magma rise led to very effective separation of magmatic volatiles and high fluxes of magmatic gas even during phreatic phases of the eruption while true strombolian phases did occur more frequently the decoupled magmatic gas rose to interact with the conduit walls and hydrothermal system producing phreatomagmatic eruptions the form of these wet explosions was governed by a delicate balance between erosion and collapse of the weak conduit walls if the walls were relatively stable fine ash was slowly eroded and erupted in weak near continous phreatomagmatic events when the walls were unstable wall collapse triggered larger discrete phreatomagmatic explosions

in situ observations of floc settling velocities in glacier bay alaska abstract in situ floc settling velocities and diameters of particles ranging in size from 0 63 to 5 05 mm equivalent circular diameter were measured under a buoyant discharge plume by deploying a bottom tripod mounted floc camera assembly fca in tarr inlet glacier bay alaska these observations were used to estimate floc effective densities three results emerge from this work first fits of settling velocity and effective density to diameter are consistent with expressions published for other environments suggesting that common controls on floc size and settling velocity operate across diverse marine environments second the raw data show considerable scatter with upper and lower 95 prediction intervals on settling velocity and excess density differing by about a factor of 7 analysis of sources of error suggests that the variability is caused by differences in component grain composition among flocs and turbulent stirring within the stilling box third bin averaged effective densities and settling velocities are highly correlated with diameter thus while it is not possible based on diameter to predict accurately the settling velocity of a single floc it is possible to estimate the mean settling velocity of a population of like sized flocs

tidal resuspension and transport processes of fine sediment within the river plume in the partially mixed changjiang river estuary china a personal perspective abstract this paper summarizes process oriented field and numerical studies undertaken on the river plume in the partially mixed changjiang river estuary both estuarine circulation and tidal asymmetry are of great importance to the fine sediment processes mean suspended sediment concentration c and bottom shear stress are the dominant physical parameters controlling the flocculation and settling velocities ws of mud flocs in suspension two important physical processes are revealed by acoustic imaging i e i the near bed impulsive resuspension and ii the transport processes driven by fine sediment induced plumes during a spring tide a turbidity maximum associated with a suspended sediment front is observed its formation is caused mainly by tidal asymmetry near bed periodic tidal resuspension and turbulence suppression by suspension salinity stratifications a conceptual sketch of the turbidity maximum is cautiously proposed for the changjiang river estuary four different settling velocity equations taking flocculation into account have different effects on the modeled concentration profiles of fine sediment apparently cao and wang 1994 pp 252 253 would be the best for spring tide and thorn 1982 fig 3 page 66 for neap tide both tidal acceleration and tidal deceleration have strong effects on the concentration profiles of fine sediment within the changjiang river estuary

unconventional maar diatreme and associated intrusions in the soft sediment hosted mardoux structure gergovie france a miocene age volcanic hypabyssal structure comprising volcaniclastic deposits and mafic intrusions is exposed with vertical relief of 110 m on the side of gergovie plateau auvergne france three main volcaniclastic facies are 1 fluidal tuff breccia composed of juvenile basalt and sediment clasts with dominantly fluidal shapes with several combinations of basalt and sediment within individual clasts 2 thickly bedded lapilli tuff composed of varying proportions of fine grained sediment derived from oligocene miocene lacustrine marls and mudstones and basaltic lapilli blocks and bombs 3 planar bedded tuff forming thin beds of fine to coarse ash size sedimentary material and basalt clasts intrusive bodies in the thickly bedded lapilli tuff range from irregularly shaped and anastomosing dikes and sills of meters to tens of meters in length to a main feeder dike that is up to 20 m wide and that flares into a spoon shaped sill at 100 m in diameter and 10 20 m thick in the eastern part of the structure volcaniclastic deposits and structural features suggest that ascending magma entrained soft saturated sediment host material into the feeder dike and erupted fluidal magma and wet sediment via weak strombolian like explosions host sediment and erupted material subsided to replace the extracted sediments producing the growth subsidence structure that is similar to upper diatreme facies in typical maar diatremes but lacks evidence for explosive disruption of diatreme fill irregularly shaped small intrusions extended from the main feeder dike into the diatreme and many were disaggregated due to shifting and subsidence of diatreme fill and recycled via eruption the mardoux structure is an unconventional maar diatreme in that it was produced mainly by weak explosive activity rather than by violent phreatomagmatic explosions and is an example of complex coupling between soft sediment and ascending magma

terminal settling velocity of commonly occurring sand grains published measurements of terminal settling velocity for commonly occurring sands are used to develop three equations which join into a single segmented curve of dimensionless form results are noticeably different from those for spheres of similar diameter and permit calculation of the settling velocity for usual sand grains without specification of exact grain shape for quartz in water the three equations of different settling regimes correspond approximately to very fine sand fine to coarse sand and very coarse sand

tephra ring interpretation in light of evolving maar diatreme concepts stracciacappa maar central italy abstract maar diatreme volcanoes in their simplest form are thought to result primarily from discrete subsurface phreatomagmatic explosions tephra ring deposits around maar craters are commonly interpreted in terms of a framework wherein explosions begin at shallow levels and migrate downward ejecting progressively deeper seated lithic clasts that contribute to the tephra rings additionally variations in grain size of the deposits are sometimes inferred to record different degrees of fragmentation which are in turn related to variations in magma water ratios at the explosion sites recent detailed studies of diatremes and of maar tephra rings suggest a different conceptual model wherein explosions can happen at various vertical and lateral locations within a diatreme during its eruptive lifetime rather than being limited to simple downward migration experiments numerical modeling and field data indicate that most explosions deeper than about 200 250 m will not vent to the surface but instead contribute to mixing within the diatremes arrival of deep seated lithics at the surface is related to this mixing process experiments also indicate that explosion phenomena can result in a range of emplacement mechanisms and grain sizes in tephra rings even in the absence of variations in explosion mechanisms fragmentation efficiency here we document the tephra ring of stracciacappa maar central italy and interpret its eruption and emplacement history in terms of evolving conceptual models along with a comparison to more traditional interpretations because most elements of both conceptual models are viable the work points to an important need to develop criteria to distinguish between the various inferred mechanisms

complex bombs of phreatomagmatic eruptions role of agglomeration and welding in vents of the 1886 rotomahana eruption tarawera new zealand 1 basaltic bombs ejected from a rotomahana crater during the 1886 tarawera eruption in new zealand are of two fundamental types cored bombs which have single large fragments generally of rhyolite wall rock material as their cores and loaded bombs which contain abundant small fragments dispersed through the bombs similar lithic bearing bombs are common products of other phreatomagmatic eruptions worldwide cored bombs originate when a large wall rock fragment is entrained in basaltic melt and then ejected with an adhering skin of basalt in contrast loaded bombs are constructed by agglomeration of multiple fragments of basalt among which smaller rhyolitic wall rock fragments are dispersed different styles and rates of bomb loading are reflected in the bombs textures and compositions these textures provide insight into the nature of fragmentation during the eruption the processes of recycling that produce multiply ejected clasts versus recapitulated ones those where early formed clasts were reingested in magma and then formed the cores of new bombs the degree and timescale of interaction of magma with vent walls and the timescales over which lithic material from the vent walls is incorporated into complex bombs prior to their ejection because fragments of rhyolitic pumice are readily refused their condition constrains estimates of the timing of fragmentation and agglomeration the presence of nonfused rhyolite pumice within many loaded bombs implies that the processes forming and quenching the bombs cannot have exceeded several seconds in contrast other bombs contain patches of compositionally modified basalt with isolated rhyolite derived feldspar xenocrysts that indicate complete fusing melting and assimilation of entrained rhyolitic debris the bomb population overall indicates that conditions within this rotomahana vent were thermally and physically heterogeneous and that recycling of debris was ubiquitous both as recycling of large individual clasts and by fallback and aggregation of material which was reingested in magma before being reejected as new bombs

full depth avalanche occurrences caused by snow gliding coquihalla british columbia canada snow glide is the translational slip of the entire snowpack over a sloping ground surface and it is thought that rapid rates of snow glide precede full depth avalanches the nature of avalanches that release at the ground makes them difficult to predict and difficult to control using explosives on slope instrumentation comprised of stainless steel glide shoes was used to measure rates of snow glide for two winters on a bedrock slope adjacent to the coquihalla highway cascade mountains british columbia canada climate data and avalanche occurrences were recorded by the british columbia ministry oftransportation and highways our results show that the supply of free water to the snow ground interface by rain or snow melt is the most important influence on full depth avalanche release full depth avalanche release responds to rainfall and snowmelt events within 12 24 hours occasionally full depth avalanches occur unexpectedly during clear cold periods snowmelt by radiation is thought to contribute enough meltwater during these cold periods to induce higher rates of snow glide and full depth avalanche release the results also indicate that snow glide alone is not a reliable indicator for full depth avalanche release

overcoming the stauchwall viscoelastic stress redistribution and the start of full depth gliding snow avalanches 1 when a full depth tensile crack opens in the mountain snowcover internal forces are transferred from the fracture crown to the stauchwall the stauchwall is located at the lower limit of a gliding zone and must carry the weight of the snowcover the stauchwall can fail leading to full depth snow avalanches or it can withstand the stress redistribution the snowcover often finds a new static equilibrium despite the initial crack we present a model describing how the snowcover reacts to the sudden transfer of the forces from the crown to the stauchwall our goal is to find the conditions for failure and the start of full depth avalanches the model balances the inertial forces of the gliding snowcover with the viscoelastic response of the stauchwall we compute stresses strain rates and deformations during the stress redistribution and show that a new equilibrium state is not found directly but depends on the viscoelastic properties of the snow which are density and temperature dependent during the stress redistribution the stauchwall encounters stresses and strain rates that can be much higher than at the final equilibrium state because of the excess strain rates the stauchwall can fail in brittle compression before reaching the new equilibrium snow viscosity and the length of the gliding snow region are the two critical parameters governing the transition from stable snowpack gliding to avalanche flow the model reveals why the formation of gliding snow avalanches is height invariant and how technical measures to prevent snowpack glide can be optimized to improve avalanche mitigation

experimental birth of a maar diatreme volcano abstract maar diatreme eruptions are hazardous to people and infrastructure and are also linked to the formation of the kimberlitic variety of diatremes which is important economically processes occurring in the subsurface diatreme and their relation to surface eruptions are not yet well understood we conducted field scale experiments using analog materials to shed more light on these processes especially the formation of the proto diatreme during the first explosions of a maar eruption specifically a series of buried explosions in a prepared layered substrate pad produced craters extra crater deposits and sub crater deposits analogous to volcanic maar craters tephra rings and incipient diatremes post explosion substrate excavation revealed that single large explosions produce sub crater deposits extending nearly to the crater rim crest the same energy divided into three blasts either co located or at different depths with the same epicenter produced narrower and sometimes deeper sub crater deposits even though the final sizes of the craters were similar to that produced by the single large blast the sub crater deposits have an upper zone with domains from different substrate depths and an underlying zone distinguished primarily by being more loosely packed than the original substrate videos show surface motion extending beyond the post shot crater rim and largely vertical ejection and fallback of material into the footprint of these deposits especially for the explosions that occurred below optimal depth of burial we infer that much of the loosely packed material was disassembled vertically transported to different heights during the explosions then fell back without significant relative lateral movement of grains however subvertical fallback did produce apparent cross cutting structures in shallow sub crater deposits one explosion ejected material from the deepest substrate horizon but it was redeposited only within the crater and is unrepresented in the ejecta rim implications of the experiments for maar diatreme volcanoes including some kimberlite pipes are as follows 1 vertical focusing of deep explosions in the diatreme explains the deficit of deep wallrock lithics observed at maar volcanoes 2 direct vertical fallback is possibly an important process forming diatreme deposits especially during the earliest stages and 3 even in our limited simulation the number and scaled depth of explosions clearly affect proto diatreme size and structure

inssev an instrument to measure the size and settling velocity of flocs in situ abstract an instrument has been developed to observe the settling of individual flocs in turbid water in order to measure size and settling velocity spectra of estuarine cohesive suspended sediments inssev in situ settling velocity instrument is bed mounted and comprises a computer controlled decelerator chamber that collects a sample of water from which some of the suspended matter is allowed to enter the top of a settling column the settling flocs are viewed using a miniature video system subsequent analysis of video tapes can provide direct measurements of size and settling velocity of individual flocs down to 20 m from this information floc density can be estimated the main feature of the instrument is its ability to video flocs in situ irrespective of the concentration in the estuary with as little disturbance to their hydrodynamic environment as possible preliminary field testing in the tamar estuary has given useful results in flow velocities up to 0 4 m s 1 and in concentrations up to 200 mg 1 1 the video recordings show large numbers of low density flocs with multiple structures linked by fine organic fibres

in situ particle size spectra and density of particle aggregates in a dredging plume abstract an in situ laser particle sizer the lisst 100 was used to describe the spatial variation of beam attenuation coefficient in situ particle size spectra and aggregate densities in a dredging plume in the sound oresund between denmark and sweden the results proved that the above mentioned parameters varied significantly within the investigated length of the plume approximately 2 km it is shown how the small single primary particles aggregate and change the in situ particle size spectra into a slightly better sorted and coarser size distribution and at the same time how the mean density of particles aggregates decreases significantly this shift in the state in which the particles exist in the water effectively changes the optical response of the mass of particles suspended in the water it is shown that adequate correlations between mass concentrations and beam attenuation coefficients can only be obtained if parameters describing the in situ quality and state of the mass of particles e g standard deviation of the size spectra and volume concentration is included in the regression from the spatial variation in mean density and in situ particle size it was possible to calculate the spatial difference in settling velocity it was found that the difference in settling velocity was only about a factor of 1 7 because increasing in situ particle size was counter balanced by decreasing mean density furthermore the time scale of flocculation within the plume was found to be in the order of 50 min

maar diatreme geometry and deposits subsurface blast experiments with variable explosion depth basaltic maar diatreme volcanoes which have craters cut into preeruption landscapes maars underlain by downward tapering bodies of fragmental material commonly cut by hypabyssal intrusions diatremes are produced by multiple subsurface phreatomagmatic explosions although many maar diatremes have been studied the link between explosion dynamics and the resulting deposit architecture is still poorly understood scaled experiments employed multiple buried explosions of known energies and depths within layered aggregates in order to assess the effects of explosion depth and the morphology and compaction of the host on the distribution of host materials in resulting ejecta the development of subcrater structures and deposits and the relationships between them experimental craters were 1 2 m wide analysis of high speed video shows that explosion jets had heights and shapes that were strongly influenced by scaled depth physical depth scaled against explosion energy and by the presence or absence of a crater jet properties in turn controlled the distribution of ejecta deposits outside the craters and we infer that this is also reflected in the diverse range of deposit types at natural maars ejecta were dominated by material that originated above the explosion site and the shallowest material was dispersed the farthest subcrater deposits illustrate progressive vertical mixing of host materials through successive explosions we conclude that the progressive appearance of deeper seated material stratigraphically upward in deposits of natural maars probably records the length and time scale for upward mixing through multiple explosions with ejection by shallow blasts rather than progressive deepening of explosion sites in response to draw down of aquifers

lateral vent migration during phreatomagmatic and magmatic eruptions at tecuitlapa maar east central mexico abstract tecuitlapa maar located in the eastern central volcanic belt of mexico is a late quaternary 100 m deep 1 km diameter crater with an alignment of scoria cones building in height to the east within it analysis of stratigraphic sections around the maar indicates that the phreatomagmatic eruption began in the eastern part of the crater with interaction between the basaltic magma and liquefied tuffaceous sediments the explosion locus gradually moved westward as evidenced by bomb sag trajectories duneform axes and facies changes producing an elliptical crater the eruption then dried out and began to produce scoria spatter cones with nested craters that young in age and climb in height to the east along the same alignment exhibited by the phreatomagmatic eruption the lithic lapilli and ash in the deposits throughout the eruption are from the 10 40 m thick water saturated tuffaceous sediments partially exposed in the maar crater walls rather than the underlying lavas and limestones this implies that the vent migrated laterally rather than vertically during the eruption and this lack of deepening of the explosion loci may be related to the high lithologic contrast between the non consolidated sediments and the underlying fractured bedrock water movement in the bedrock would have been by fracture flow whereas movement within the sediments was through liquefaction failure and fluidization of the deposit low magma supply rates would allow collapse of the non lithified walls of the dike producing mingling and phreatomagmatic explosions whereas the same magma supply rate in fractured bedrock may seal off water access to the magma the alignment of the vents is parallel to regional structural trends so was probably set in the underlying bedrock and the distance that the vent migrated is likely related to the overall dike length the lack of fluidized sediment coating the scoria cone clasts both in the cones and in the deposits in the walls of the maar implies that the phreatomagmatic eruptions used up most of the liquefied sediment which ended the phreatomagmatic portion of the eruption the migration of the scoria cone vent loci can be explained by an actual migration of the vents or because the evidence on their locations is based on surficial crater overlap relations by a fissure eruption that closed up from the west to the east producing later activity at the taller eastern vents

large phreatomagmatic vent complex at coombs hills antarctica wet explosive initiation of flood basalt volcanism in the ferrar karoo lip the mawson formation and correlatives in the transantarctic mountains and south africa record an early eruption episode related to the onset of ferrar karoo flood basalt volcanism mawson formation rocks at coombs hills comprise mainly 80 vol structureless tuff breccia and coarse lapilli tuff cut by irregular dikes and sills within a large vent complex 30 km2 quenched juvenile fragments of generally low but variable vesicularity accretionary lapilli and country rock clasts within vent fill and pyroclastic density current deposits point to explosive interaction of basalt with groundwater in porous country rock and wet vent filling debris metre scale dikes and pods of coherent basalt in places merge imperceptibly into peperite and then into surrounding breccia steeply dipping to sub vertical depositional contacts juxtapose volcaniclastic rocks of contrasting componentry and grainsize these sub vertical tuff breccia zones are inferred to have formed when jets of debris steam water passed through unconsolidated vent filling deposits these jets of debris may have sometimes breached the surface to form subaerial tephra jets which fed subaerial pyroclastic density currents and fall deposits others however probably died out within vent fill before reaching the surface allowing mixing and recycling of clasts which never reached the atmosphere most of the ejecta that did escape the debris filled vents was rapidly recycled as vents broadened via lateral quarrying of country rock and bedded pyroclastic vent rim deposits which collapsed along the margins into individual vents the unstratified poorly sorted deposits comprising most of the complex are capped by tuff lapilli tuff and tuff breccia beds inferred to have been deposited on the floor of the vent complex by pyroclastic density currents development of the extensive coombs hills vent complex involved interaction of large volumes of magma and water we infer that recycling of water as well as recycling of pyroclasts was important in maintaining water supply for phreatomagmatic interactions even when aquifer rock in the vent walls lay far from eruption sites as a consequence of vent complex widening the proportion of recycled water increased with vent complex size in the same way that the proportion of recycled tephra did though water recycling leaves no direct rock record the volcaniclastic deposits within the vent complex show through their lithofacies structural architecture lithofacies characteristics and particle properties clear evidence for extensive and varied recycling of material as the complex evolved

recycling in debris filled volcanic vents eruptions through debris filled vents produce deposits containing magmatic juvenile lithic and recycled clasts recycled clasts are exposed to multiple transportation and fragmentation events we used experiments with multiple subsurface explosions to track clasts and highlight dominant recycling processes in eruptions through analog debris filled vents recycled clasts include those that fall back into and reside in the vent for extended time periods and those that return to the vent through crater growth or collapse clasts are recycled by any combination of lofting and fallback of material in the crater by explosion jets mixing and churning of material at depth in the debris fill and redistribution of extra crater deposits by explosion induced excavation or slumping we compare experimental processes with natural deposits that preserve recycling signatures from discrete explosions through debris filled vents such as maar diatremes strombolian vents and hydrothermal craters clasts may not preserve textures diagnostic of their complete recycling histories but can be used to infer if that history occurred in part in the vent debris or in the eruptive jet experiment results and natural deposits suggest that for volcanic craters that undergo multiple explosions clasts likely undergo some form of recycling before final deposition outside the craters the underestimation of recycled clast contributions to deposits can lead to inaccurate estimates of thermal budgets and eruption processes

estuarine flocs their size settling velocity and density the size and settling velocity of flocs of suspended sediments from the chesapeake bay were measured in laboratory experiments and in the field the curves between floc diameter and settling velocity showed a non stoke s relationship that was best described by the following settling velocity cm s 1 73 floc diameter cm 0 78 furthermore the relationship between floc diameter and settling velocity indicated that the floe density decreased as the floes increased in size in addition to changing density a cylinder shape was found to be the best shape factor

scaling multiblast craters general approach and application to volcanic craters most volcanic explosions leave a crater in the surface around the center of the explosions such craters differ from products of single events like meteorite impacts or those produced by military testing because they typically result from multiple rather than single explosions here we analyze the evolution of experimental craters that were created by several detonations of chemical explosives in layered aggregates an empirical relationship for the scaled crater radius as a function of scaled explosion depth for single blasts in flat test beds is derived from experimental data which differs from existing relations and has better applicability for deep blasts a method to calculate an effective explosion depth for nonflat topography e g for explosions below existing craters is derived showing how multiblast crater sizes differ from the single blast case sizes of natural caters radii and volumes are not characteristic of the number of explosions nor therefore of the total acting energy that formed a crater also the crater size is not simply related to the largest explosion in a sequence but depends upon that explosion and the energy of that single blast and on the cumulative energy of all blasts that formed a crater the two energies can be combined to form an effective number of explosions that is characteristic for the crater evolution the multiblast crater size evolution has implications on the estimates of volcanic eruption energies indicating that it is not correct to estimate explosion energy from crater size using previously published relationships that were derived for single blast cases

vertical suspension profile in the changjiang estuary abstract by using an acoustic suspended sediment monitor assm four patterns of vertical suspension profile have been obtained at spring tide 28 29 may 1994 in the changjiang estuary type i is l shaped at flood tide the vertical gradients in suspended sediment concentration ssc were small

folding of passive layers and forms of minor structures near terminations of blind thrust faults application to the central appalachian blind thrust abstract the existence of detachment surfaces or decollement zones beneath folded rocks of the valley and ridge and plateau provinces of the appalachians has been recognized as an important condition of folding large folds at the border between the two provinces resulted primarily from repetition of strata by thrusting of blocks over ramp faults that connect detachement surfaces at different horizons some investigators have suggested that folds in the plateau province of pennsylvania were produced by splay faults arising from detachment surfaces but field observations and theoretical analyses by sherwin and by wiltschko chapple suggest that the folds are a result of buckling of multilayered rocks above a decollement an exception may be the burning springs anticline in west virginia which appears to have formed at the termination of a detachment surface investigation of the translation of an homogenous viscous material above a flat detachment surface that terminates laterally indicates that the termination produces a broad low amplitude anticline in passive layering as a result of thickening induced by a gradient of shear stresses in the vertical direction this thickening above the termination of a detachment is a mechanism of folding if the viscous fluid contained mechanical layering the fold would become amplified by buckling computations of stresses in the material indicate that minor faults should be generated first near the termination of the flat detachment surface the burning springs anticline probably was initiated by termination of a detachment surface and subsequently amplified by buckling

recycling of magmatic clasts during explosive eruptions estimating the true juvenile content of phreatomagmatic volcanic deposits the juvenile content of phreatomagmatic deposits contains both first cycle juvenile clasts derived from magma at the instant of eruption and recycled juvenile clasts which were fragmented and first ejected by earlier explosions during the eruption but fell back or collapsed into the vent recycled juvenile clasts are similar to accessory and accidental lithics in that they contribute no heat to further magma water interaction but previously no effective criteria have been defined to separate them from first cycle juvenile clasts we have investigated componentry parameters vesicularity clast morphology and extent of mud coating which in specific circumstances can distinguish between first cycle juvenile clasts involved in only one explosion and such recycled juvenile clasts phreatomagmatic fall deposits commonly show gross grainsize and sorting characteristics identical to deposits of purely dry or magmatic eruptions however the abundance of non juvenile clasts in pyroclastic deposits is a sensitive indicator of the involvement of external water if this component is calculated including recycled juvenile clasts with accidental and accessory clasts the contrast is even more striking data from a holocene maar deposit in taupo volcanic zone new zealand suggest that the first cycle juvenile component of the deposits is less than one third of that determined by simple juvenile lithic crystal componentry

impacts of human activity on the late holocene development of the subaqueous yangtze delta china as shown by magnetic properties and sediment accumulation rates development of the yangtze delta during the late holocene and its relationship to human activities in the drainage basin was analyzed using data from 16 cores collected from distributaries to the

characteristics of snow gliding on rock summary discussion our data from gliding on steep rock show the following characteristics i highly fluctuating values of glide speed are found particularly in early winter and late spring those characteristics are however consistent with gliding theory 2 data for two winters showed that glide rates tend to be similar at the same location for winters of similar character 3 our studies of spatial variation show that glide speed is higher below the step in the rock where snow has separated from the rock in the snow cover than above it there are probably two contributing effects i the presence of the free surface at the uphill surface of the snow allows easier downslope motion of the gliding slab and 2 the open crack can help to concentrate melt water at the glide interface by serving as an open reservoir for surface melt water to concentrate 4 we cannot conclusively say whether glide rates are on average faster during the day than at night better scrutiny of meteorological parameters is needed to understand gliding on a diurnal time scale two winters gliding data do not yield a clear answer 5 we have presented simple analytical expressions for the longitudinal force on restraining barriers in deep snow covers mcclung and larsen 1989 and on roofs when gliding is present the character of the problem on a warm smooth roof is obviously of a different character than that of a deep snow cover bu t it may be approached from a similar mechanics formalism in the zero friction limit equation 3b constants from the glide constitutive equation do not appear mcclung and others snow gliding on rocks one aspect of gliding which is not included in our simple analytical formulae for barrier forces is time dependence even though gliding shows highly fluctuat ing time dependence our equations are based on quasi static formulations a complete time dependent solution is not available yet and its implications for engineering problems have not been fully considered our simple analysis of the fluctuations eq uation 8 and physics of snow gliding shows consistency with the proposed gliding constitutive equation

geology of a complex kimberlite pipe k2 pipe venetia mine south africa insights into conduit processes during explosive ultrabasic eruptions k2 is a steep sided kimberlite pipe with a complex internal geology geological mapping logging of drillcore and petrographic studies indicate that it comprises layered breccias and pyroclastic rocks of various grain sizes lithic contents and internal structures the pipe comprises two geologically distinct parts k2 west is a layered sequence of juvenile and lithic rich breccias which dip 20 45 inwards and k2 east consists of a steep sided pipe like body filled with massive volcaniclastic kimberlite nested within the k2 pipe the layered sequence in k2 west is present to 900 m below present surface and is interpreted as a sequence of pyroclastic rocks generated by explosive eruptions and mass wasting breccias generated by rock fall and sector collapse of the pipe walls both processes occurred in tandem during the infill of the pipe several breccia lobes extend across the pipe and are truncated by the steep contact with k2 east dense pyroclastic rocks within the layered sequence are interpreted as welded deposits k2 east represents a conduit that was blasted through the layered breccia sequence at a late stage in the eruption this phase may have involved fluidisation of trapped pyroclasts with loss of fine particles and comminution of coarse clasts we conclude that the k2 kimberlite pipe was emplaced in several distinct stages that consisted of an initial explosive enlargement followed by alternating phases of accumulation and ejection

differentiating the effects of advection and resuspension on suspended sediment concentrations in a turbid estuary abstract suspended sediment concentration ssc has a significant impact on the estuarine environment and its morphological evolution at any given location the temporal variability of depth averaged ssc is due to a combination of two processes horizontal advection and local resuspension in this study we investigated the sediment dynamics at three anchored monitoring stations close to the maximum turbidity zone of the changjiang estuary and developed a box model to differentiate the effects of advection and resuspension further settling velocities were estimated using the adv reynolds flux method excluding the advection ssc component we found that predicted changes in advection and resuspension induced sscs were consistent with the bottom shear stress and accretion erosion observations the combination of observed bed accretion erosion changes and the predicted advection induced sscs indicates that the advective transport of suspended sediment is an important process in accelerating persistent erosion at the monitoring stations although ssc variations due to advection and resuspension are of similar magnitudes our model results indicate that if resuspension dominates the resuspension induced component can reach up to twice the magnitude of the advection induced component we conclude that the box model is a valuable tool for evaluating subaqueous delta accretion erosion in response to sediment reduction caused by upstream dam construction and climate change

a study of in situ sediment flocculation in the turbidity maxima of the yangtze estuary abstract in order to improve our understandings of temporal and vertical variations of sediment flocculation dynamics within the turbidity maxima tm of the highly turbid yangtze estuary ye we deployed lisst 100c a laser instrument for in situ monitor of the sizes and concentrations of flocculated particles in a wet season field data in terms of vertical profiles of flow velocity suspended sediment concentration ssc salinity flocculated particle size distribution and volume concentration were obtained based on field works conducted at consecutive spring moderate and neap tides data analyses show that the mean floc diameters d m were in the range of 14 95 m and flocculation exhibited strong temporal and vertical variations within a tidal cycle and between spring neap cycles larger d m were observed during high and low slack waters and the averaged floc size at neap tide was found 57 larger than at spring tide effective density of flocs decreased with the increase of floc size and fractal dimension of flocs in the ye was mainly between 1 5 and 2 1 we also estimated the settling velocity of flocs by 0 04 0 6 mm s 1 and the largest settling velocity occurred also at slack waters moreover it is found that turbulence plays a dominant role in the flocculation process floc size decreases significantly when the shear rate parameter g is 2 3 s 1 suggesting the turbulence breaking force combined effects of fine sediment flocculation enhanced settling process and high sediment concentration resulted in a large settling flux around high water which can in part explain the severe siltation in the tm of the ye thus shedding lights on the navigation channel management

structure and eruptive mechanisms at surtsey volcano iceland glassy basalt tuff was the primary material cored in 1979 from a 181 m deep drill hole on the east tuff ring of surtsey volcano despite the fact that the hole extends 122 m below sea level all the core is similar to the exposed tephra composing the two tuff rings of the island the tuff includes abundant accretionary lapilli and tuff vesicles indicating that it was all deposited subaerially during the growth of the tuff rings repeated hydromagmatic explosion cycles began with a series of intermittent tephra finger explosions leading up to continuous uprush explosions which lasted for several minutes to several hours this nozzle like continuous activity produced eruption columns 100 250 m in diameter and 500 2000 m in height which probably quarried several hundred metres below the ground surface the continuous uprush explosion type provides a reasonable mechanism to excavate a diatreme from the top down during construction of the tuff rings concentric faults repeatedly downdropped a funnel like structure 400 800 m in diameter several hundred metres thus accounting for the presence of subaerially deposited tephra in the drill core far beneath sea level ring dykes later intruded upward along these faults and fed small lava flows heat in the surface tephra probably originated primarily from these shallow intrusions

a new instrument for measuring settling velocities in situ abstract an in situ settling cylinder for determining particle settling velocities has been deployed during a field experiment in san pablo bay california the instrument is a spring loaded cylinder which traps approximately 31 of water and suspended sediment between two end plates mounted between the end plates is a vertical array of five miniature nephelometers positioned 4 20 cm below the top end plate which continuously monitor 5 hz sampling rate suspended sediment concentration at each level as particles settle the resulting settling curve can be divided into components with characteristic settling velocities and a settling velocity distribution obtained for the suspension the results are conservative estimates of settling velocities for the suspension in situ estimates of particle settling velocity are higher than stokes settling velocities calculated for the disaggregated grain size distribution of a suspended sediment sample indicating that the sediment in suspension is in the form of aggregates the aggregates have settling velocities as high as those of sand size quartz spheres