synthetic signatures of volcanic ash cloud particles from x band dual polarization radar weather radar retrieval in terms of detection estimation and sensitivity of volcanic ash plumes is dependent not only on the radar system specifications but also on the range and ash cloud distribution the minimum detectable signal can be increased for a given radar and ash plume scenario by decreasing the observation range and increasing the operational frequency and also by exploiting possible polarimetric capabilities for short range observations in proximity of the volcano vent a compact portable system with relatively low power transmitter may be evaluated as a suitable compromise between observational and technological requirements this paper starting from the results of a previous study and from the aforementioned issues is aimed at quantitatively assessing the optimal choices for a portable x band system with a dual polarization capability for real time ash cloud remote sensing the physical electromagnetic model of ash particle distributions is systematically reviewed and extended to include nonspherical particle shapes vesicular composition silicate content and orientation phenomena the radar backscattering response at x band is simulated and analyzed in terms of self consistent polarimetric signatures for ash classification purposes and correlation with ash concentration for quantitative retrieval aims an x band radar system sensitivity analysis to ash concentration as a function of radar specifications range and ash category is carried out in trying to assess the expected system performances and limitations

observations of volcanic clouds in their first few days of atmospheric residence the 1992 eruptions of crater peak mount spurr volcano alaska abstract satellite so2 and ash measurements of mount spurr s three 1992 volcanic clouds are compared with ground based observations to develop an understanding of the physical and chemical evolution of volcanic clouds each of the three eruptions with ratings of volcanic explosivity index three reached the lower stratosphere 14 km asl but the clouds were mainly dispersed at the tropopause by moderate to strong 20 40 m s tropospheric winds three stages of cloud evolution were identified first heavy fallout of large 500 m pyroclasts occurred close to the volcano 25 km from the vent during and immediately after the eruptions and the cloud resembled an advected gravity current second a much larger highly elongated region marked by a secondary mass maximum occurred 150 350 km downwind in at least two of the three events this was the result of aggregate fallout of a bimodal size distribution including fine 25 m ash that quickly depleted the solid fraction of the volcanic cloud for the f

comparative proximal features of the main plinian deposits campanian ignimbrite and pomici di base of campi flegrei and vesuvius abstract the proximal plinian fall deposits of the campanian ignimbrite ci 38 ky fedele et al 2008 and pomici di base pdb 18 ky bertagnini et al 1998 have been investigated in order to understand the contribution of each part of the plume to the proximal sedimentation following houghton et al 2004b we consider three main transport regimes jet phase producing facies fb buoyant region of the plume producing facies fa and direct lateral ejection producing facies fc as well documented in medial locations sparks et al 1992 1997 ernst et al 1996 transport regimes can develop different facies even in proximal locations according to the dynamics of the eruptive column our proximal deposits show stratification and diffuse bedding allowing us to introduce two new facies stratified fa sfa and diffuse bedded fb dbfb these facies retain the transport regime previously proposed for fa buoyant plume and fb jet phase but their lithological features are influenced by near vent depositional conditions lithology and sedimentological data grain size componentry maximum clasts suggest that most of the sedimentation occurred mainly from the buoyant plume with simultaneous contribution from the other two different dynamic regimes coarse clasts falling from the lower margins of the plume strongly affected the sedimentation of the ci proximal fall deposit with a minor contribution from lithic clasts ballistically emplaced and partial collapses of the plume forming pyroclastic density currents in contrast the pdb proximal fall deposit was strongly affected by coarse clasts emplaced directly from the vent through parabolic trajectories with very little contribution of material emplaced from the lower part of the plume these differences can be attributed to different vent conduit processes acting during the eruptions

formation of scoria cone during explosive eruption at izu oshima volcano japan 1 scoria cones have been regarded to be formed by accumulation of ballistic bombs ejected by mild eruptions however more recent geological investigations show that some scoria cones could be formed during explosive eruptions here we demonstrate how the scoria cone of the 1986 izu oshima eruption was formed during the explosive eruption we measured particle fractionation of the cone and propose a theoretical model to explain the observation the model considers lateral transport of particles by turbulent eddies particles that reached characteristic column radius l are laterally transported to l where they starts to free fall we obtained 1 2 and 2 5 for larger and smaller particles respectively which is consistent to the observation in the model extensive fallout takes place at the base of the column where it expands rapidly we suggest that lateral particle projection and the rapid column expansion are the key processes to form the cones

formation of high grade ignimbrites part ii a pyroclastic suspension current model with implications also for low grade ignimbrites analogue experiments in part i led to the conclusion that pyroclastic flows depositing very high grade ignimbrite move as dilute suspension currents in the thermo fluid dynamical model developed the degree of cooling of expanded turbulent pyroclastic flows dynamically evolves in response to entrainment of air and mass loss to sedimentation initial conditions of the currents are derived from column collapse modeling for magmas with an initial h2o content of 1 3 wt erupting through circular vents and caldera ring fissures the flows spread either longitudinally or radially from source up to a runout distance that increases with higher mass flux but decreases with higher gas content temperature bottom slope and coarser initial grain size progressive dilution by entrainment and sedimentation causes pyroclastic currents to transform into buoyant ash plumes at the runout distance the ash plumes reach stratospheric heights and distribute 30 80 of the erupted material as widespread co ignimbrite ash pyroclastic suspension currents with initial mass fluxes of 107 1012 kg s can spread for tens of kilometers with only limited cooling although they move as supercritical strongly entraining currents for the eruption conditions considered here with increasing eruption mass flux cooling during passage through the fountain diminishes while cooling during flow transport increases the net effect is that eruption temperature exerts the prime control on emplacement temperature pyroclastic suspension currents can form welded ignimbrite across their entire extent if eruption temperature is to 1 3 tmw the minimum welding temperature high eruption rates a large fraction of fine ash and a ring fissure vent favor the formation of extensive high grade ignimbrite for very hot eruptions producing sticky partially molten pyroclasts analysis of particle aggregation systematics shows that factors favoring longer runout also favor more efficient aggregation which reduces runout as a result very high grade ignimbrites cannot spread more than a few tens of kilometers from their source in cooler pyroclastic currents particles do not aggregate and the sedimentation process may involve re entrainment of particles which potentially leads to more extensive cooling and longer runout such effects however are only significant when net erosion of substrate occurs model results can be employed to estimate mass flux and duration of ignimbrite eruptions from measured ignimbrite masses and aspect ratios the model also provides an alternative explanation of the observed decrease in h lratios with ignimbrite mass

crystal settling in a vigorously converting magma chamber there has been much debate concerning the mechanism of fractional crystallization in magma chambers the traditional hypothesis of crystal settling has been widely replaced by the concept of in situ crystallization coupled with compositional convection observations from layered intrusions however are equivocal1 4 doubts have been raised about crystal settling on theoretical grounds because convective velocities in magma chambers are often much greater than the crystal settling velocities predicted by stokes law5 but there has been no experimental study of crystal settling in such vigorous convection here we present physical considerations and laboratory experiments which show that the phenomenon of particle settling in these conditions can be accounted for by a simple theory application of this theory to crystal settling in magma chambers suggests that crystal settling may be an efficient differentiation mechanism at least in basaltic magma chambers despite large convective velocities

the 0 57 ma plinian eruption of the granadilla member tenerife canary islands an example of complexity in eruption dynamics and evolution the granadilla member is one of the most widely dispersed and largest volume pyroclastic units at tenerife canary islands and represents the culminating eruption to a second cycle of explosive volcanism of the las canadas edifice the member dared at 0 57 ma comprises a plinian fall deposit the granadilla pumice which is overlain by ignimbrite up to 30 m thick the granadilla pumice is up to 9 m thick approximately 10 km from sourer pyle b t value is 5 35 km and is subdivided into four fail units unit 1 is up to 1 2 m thick and is further divisible into another four pumice fall subunits based on bedding and grainsize differences unit 2 is a thin but distinctive ash layer similar to2 cm thick and its wide dispersal 550 km 2 constant thickness planar laminations and ash aggregate textures collectively indicate a phreatoplinian fall origin the lithic rich nature and abundance of unaltered lithic fragments reflect magma interaction with aquifer derived water at depth unit 3 less than or equal to1 8 m thick records a reversal to dry plinian eruptive activity unit 4 the thickest of the fall units up to 6 3 m thick records the maximum dispersal and intensity of the eruption pyle b t and b c values are 5 7 and 6 3 km respectively best illustrated by the presence of large pumice bombs up to 30 cm diameter at distances up to 20 km from vent and reverse grading of lithic and pumice clasts the widespread 500 km 2 nonwelded and pumice rich granadilla ignimbrite unit 5 records the collapse of the plinian eruption column the ignimbrite has a simple sheet like geometry but exhibits a complex internal stratigraphy the base of the ignimbrite locally cuts down through the underlying granadilla pumice removing it entirely indicating up to 9 m of erosion by the pyroclastic flows a coarse vent derived lithic breccia horizon towards the top of the ignimbrite is interpreted to record the onset of caldera collapse late in the eruption minimum volume estimates for the granadilla pumice and ignimbrite are 5 2 and 5 km 3 respectively the dispersal area deposit characteristics and exponential thickness and clast size decay relationships with isopach area 1 2 are consistent with dispersal and fallout from the umbrella region of a moderately high similar to 17 to greater than or equal to 25 km plinian column we propose that the eruption involved two vents probably aligned along a ne sw fissure within the las canadas caldera c 2000 elsevier science b v all rights reserved

spreading hypothesis of a particle plume abstractintegral models of single phase plumes are often closed using the entrainment hypothesis which assumes the entrainment velocity is proportional to a characteristic plume velocity but the corresponding theoretical development for modeling particle plumes has received less attention in this paper an integral model is developed by proposing a new spreading hypothesis for particle plumes in which the fluid phase spreading rate is taken as that of a single phase plume whereas the particle phase spreading rate is a function of the particle phase and fluid phase average vertical velocity the change in momentum flux of the particle and fluid phases are calculated by considering the cross sectional integrated buoyant and drag forces acting on the particles and their reaction force acting on the fluid mixing characteristics of the particle plume can then be determined the model was validated by laboratory particle plume experiments conducted for various particle sizes and initial plume to particle

probabilistic modeling of tephra dispersal hazard assessment of a multiphase rhyolitic eruption at tarawera new zealand 1 the tarawera volcanic complex comprises 11 rhyolite domes formed during five major eruptions between 17 000 b c and a d 1886 the first four of which were predominantly rhyolitic the only historical event erupted about 2 km3 of basaltic tephra fall a d 1886 the youngest rhyolitic event erupted a tephra fall volume more than 2 times larger and covered a wider area northwest and southeast of the volcano a d 1315 kaharoa eruption we have used the kaharoa scenario to assess the tephra fall hazard from a future rhyolitic eruption at tarawera of a similar scale the plinian phase of this eruption consisted of 11 discrete episodes of vei 4 we have developed an advection diffusion model tephra that allows for grain size dependent diffusion and particle density a stratified atmosphere particle diffusion time within the rising plume and settling velocities that include reynolds number variations along the particle fall simulations are run in parallel on multiple processors to allow a significant implementation of the physical model and a fully probabilistic analysis of inputs and outputs tephra is an example of a class of numerical models that take advantage of new computational tools to forecast hazards as conditional probabilities far in advance of future eruptions three different scenarios were investigated for a comprehensive tephra fall hazard assessment upper limit scenario eruption range scenario and multiple eruption scenario hazard curves and probability maps show that the area east and northeast of tarawera would be the most affected by a kaharoa type eruption

scales and causes of heterogeneity in bars in a large multi channel river rio parana argentina to date published studies of alluvial bar architecture in large rivers have been restricted mostly to case studies of individual bars and single locations relatively little is known on how the depositional processes and sedimentary architecture of km scale bars vary within a multi km reach or over several 100s km downstream this study presents ground penetrating radar gpr and core data from 11 km scale bars from the rio parana argentina the investigated bars are located between 30 km upstream and 540 km downstream of the rio parana rio paraguay confluence where a significant volume of fine grained suspended sediment is introduced into the network bar scale cross stratified sets with lengths and widths up to 600 m and thicknesses up to 12 m enable the distinction of large river deposits from stacked deposits of smaller rivers but are only present in half the surface area of the bars up to 90 of bar scale sets are found on top of finer grained ripple laminated bar trough deposits bar scale sets make up as much as 58 of the volume of the deposits in small incipient mid channel bars but this proportion decreases significantly with increasing age and size of the bars contrary to what might be expected a significant proportion of the sedimentary structures found in the rio parana is similar in scale to those found in much smaller rivers in other words large river deposits are not always characterised by big structures that allow a simple interpretation of river scale however the large scale of the depositional units in big rivers causes small scale structures such as ripple sets to be grouped in thicker co sets which indicate river scale even when no obvious large scale sets are present the results also show that the composition of bars differs between the studied reaches upstream and downstream of the confluence with the rio paraguay relative to other controls on downstream fining the tributary input of fine grained suspended material from the rio paraguay causes a marked change in the composition of the bar deposits compared to the upstream reaches the sedimentary architecture of the downstream reaches in the top 5 m of mid channel bars shows i an increase in the abundance and thickness up to m scale of laterally extensive 100s of metres fine grained layers ii an increase in the percentage of deposits comprised of ripple sets to 40 in the upper bar deposits and iii an increase in bar trough deposits and a corresponding decrease in bar scale cross strata

settling driven convection a mechanism of sedimentation from stratified fluids convection driven by sediment particles may play an important role in sedimentation from the base of buoyant hypopycnal plumes for example fluvial plumes in stratified estuaries and lakes black smokers on the ocean floor volcanic clouds and coastal currents in addition to the well known double diffusive convection mechanism another mode of convective instability development is by settling across the density interface we performed laboratory experiments to investigate this fingering convective instability mechanism and its effect on particle distribution in the water column and deposition at the bed a simple theoretical model of finger formation at a fluid density interface is developed based on an analogy with thermal plume formation at a flat heated plate this model which involves a thickening interface layer that becomes gravitationally unstable relative to the ambient fluid is in good agreement with measurements of finger size and instability wavelength from visualization experiments since fingering at the density interface drives larger scale convection in the fluid below a mass balance model of the lower layer assuming strong mixing i e uniform sediment concentration is successfully applied to predict sediment concentration in the water column and deposition at the bed strong mixing can be assumed since convective velocities are usually much greater than the particle fall velocities as convection proceeds the sediment concentrations in the two layers approach each other and convection will die out using the model equations we develop analytical expressions for the time when convection ceases and the portion of sediment remaining in the water column

on the hydraulic properties of coarse textured sediments at intermediate water contents 1 a modified steady state head control method was used to obtain on identical samples direct measurements of soil moisture retention volumetric water content versus matric potential and unsaturated hydraulic conductivity k as a function of both and the minimum values for the undisturbed coarse textured samples were as low as 400 cm whereas the values were as low as 0 04 of the 79 samples 41 contained a high gravel fraction 2 mm size that ranged from 20 to 71 by weight the remaining samples were sandy with very little gravel fraction we examined similarities and differences between the two soil types in their retention and k slopes for the intermediate moisture regime the retention data for the gravelly soils fell within a narrower range than for the sandy type the water capacity d d estimates for the two soil types showed similar variability although both soils are coarse textured important differences were noted in two integral measures for and k measurements the mean and variance for the campbell pore size distribution parameter b and k slope estimates for the gravelly type were larger than those for the sandy type a unique relation was noted between b and the relationship was similar for the two soil types with a very dissimilar particle size distribution

tertiary quaternary drainage of the pre glacial mackenzie basin abstract pre glacial drainage in the northwestern mackenzie region was established in early tertiary time with the uplift of the mountains during the laramide orogeny and persisted until near the end of the pleistocene rivers arose in the mackenzie and richardson mountains and followed a generally eastward direction across piedmont surfaces from the mountains towards the plains the evidence to support this hypothesis includes geomorphology provenance of sediments reconstruction of paleo surfaces and stratigraphy two major drainage systems developed one to the atlantic ocean and another to the arctic ocean the atlantic drainage is composed of the northwestern tributaries of an integrated drainage system that flowed towards hudson bay traces of this drainage are found in the canyon ranges of the mackenzie mountains iroquois upland and in the norman range of the franklin mountains the arctic drainage included two main basins the porcupine basin that drained from west to east across the richardson mountains and the anderson basin that included the peel and snake rivers as its tributaries late wisconsinan laurentide glaciation deranged these drainage systems establishing the porcupine river as a tributary of the yukon river and integrating the tertiary drainage systems into the mackenzie river system flowing to the arctic ocean

the role of discharge variability in the formation and preservation of alluvial sediment bodies abstract extant planform based facies models for alluvial deposits are not fully fit for purpose because they over emphasise plan form whereas there is little in the alluvial rock record that is distinctive of any particular planform and because the planform of individual rivers vary in both time and space accordingly existing facies models have limited predictive capability in this paper we explore the role of inter annual peak discharge variability as a possible control on the character of the preserved alluvial record data from a suite of modern rivers for which long term gauging records are available and for which there are published descriptions of subsurface sedimentary architecture are analysed the selected rivers are categorized according to their variance in peak discharge or the coefficient of variation cvqp standard deviation of the annual peak flood discharge over the mean annual peak flood discharge this parameter ranges over the rivers studied between 0 18 and 1 22 allowing classification of rivers as having very low 0 90 annual peak discharge variance deposits of rivers with very low and low peak discharge variability are dominated by cross bedding on various scales and preserve macroform bedding structure allowing the interpretation of bar construction processes rivers with moderate values preserve mostly cross bedding but records of macroform processes are in places muted and considerably modified by reworking rivers with high and very high values of annual peak discharge variability show a wide range of bedding structures commonly including critical and supercritical flow structures abundant in situ trees and transported large woody debris and their deposits contain pedogenically modified mud partings and generally lack macroform structure such a facies assemblage is distinctively different from the conventional fluvial style recorded in published facies models but is widely developed both in modern and ancient alluvial deposits this high peak variance style is also distinctive of rivers that are undergoing contraction in discharge over time because of the gradual annexation of the channel belt by the establishment of woody vegetation we propose that discharge variability both inter annual peak variation and flashiness may be a more reliable basis for classifying the alluvial rock record than planform and we provide some examples of three classes of alluvial sediment bodies representing low intermediate and high very high discharge variability from the rock record that illustrate this point

deposition of cohesive sediment from turbulent plumes gravity currents and turbidity currents models for the deposition of cohesive sediment from turbulent plumes or buoyant jets gravity currents and turbidity currents are provided in this paper the cohesive sediment is made up of small particles that aggregate together to form larger flocs which are in turn broken up by turbulent shear the equilibrium mean floc size and thus the equilibrium mean fall speed is a function of the turbulent dissipation rate and the sediment concentration the flows are modeled by using integral and box models with dissipation related to bulk flow properties for plumes it is shown that there is a well defined equilibrium fall speed at the virtual origin and that the fall speed changes relatively slowly in the momentum dominated part of the flow within one jet length or so of the source if the flocs are assumed to adjust instantaneously to their equilibrium size an integral model for a turbulent plume carrying cohesive sediment can be described in terms of two parameters the angle between the plume an

origin of modal and rhythmic igneous layering by sedimentation in a convecting magma chamber experimental investigations of convecting particle laden fluids show two regimes for convection driven by cooling from above1 in very dilute suspensions convection will maintain a homogeneous distribution of particles throughout the convecting layer provided that particle fall velocities are small compared with turbulent fluid velocities above a critical concentration convection is unable to keep the particles suspended so the particles settle leaving behind a layer of convecting fluid virtually free of particles here we apply these results to cooling magma chambers in which crystallization leads to an increase in suspended crystal content with time discrete sedimentation events are predicted each time the concentration exceeds the critical value for common igneous minerals critical concentrations are very small typically 0 002 0 03 wt and layers of the order of centimetres to a few metres thick will result because minerals of different density and size have different critical concentrations and settling velocities complex fluctuations in sedimentation rate and mineral proportions can occur in a multi component melt this may lead to either regular repetitive cycles or more complex fluctuations the process is confined to low viscosity magmas such as basalts in which the crystals are able to separate from the active thermal boundary layer during convection

a 6 year time series of hydrothermal plumes over the cleft segment of the juan de fuca ridge the causal links between magmatic tectonic processes and hydrothermal venting along the mid ocean ridge axis will be difficult to discern without broadband observations of the temporal variability of hydrothermal activity i report here on temporal variability in the intensity and distribution of chronic and event plumes generated by continuous and episodic hydrothermal discharge at two vent fields on the cleft segment of the juan de fuca ridge hydrothermal temperature and light attenuation c anomalies were comprehensively mapped between 1 and 4 times per year during nine cruises from 1986 to 1991 intensity of and c in the chronic plume overlying the north cleft vent field showed more variability between years than within a given year both and c were exceptionally high in 1986 fell to moderate levels during 1988 to 1990 and declined to minimum values in 1991 the chronic plume over the south cleft vent field varied similarly superimposed on the chronic plumes were brief and intense releases of hydrothermal fluid that created event plumes distinguishable by their symmetry and exceptional rise height event plumes at the north cleft field ranged from the 1986 megaplume with a rise height exceeding 1000 m to smaller events in 1987 and 1989 with rise heights of 400 m chronic hydrothermal heat 660 277 1 mw and mass 204 86 1 g s 1 fluxes from the north cleft field were determined by assuming these fluxes were equivalent to the long term net advection of a mean cross section of the vertically integrated and c in the plume this 6 year history of hydrothermal plumes indicates that episodic discharge has been an important component of the recent hydrothermal flux and that the aggregate heat and mass fluxes declined significantly though not consistently between 1986 and 1991

volcanic ash cloud retrieval by ground based microwave weather radar the potential of ground based microwave weather radar systems for volcanic ash cloud detection and quantitative retrieval is evaluated the relationship between radar reflectivity factor ash concentration and fall rate is statistically derived for various eruption regimes and ash sizes by applying a radar reflectivity microphysical model to quantitatively evaluate the ash detectability by weather radars a sensitivity analysis is carried out by simulating synthetic ash clouds and varying ash concentration and size as a function of the range radar specifications are taken from typical radar systems at s c and x band a prototype algorithm for volcanic ash radar retrieval varr is discussed starting from measured single polarization reflectivity the statistical inversion technique to retrieve ash concentration and fall rate is based on two cascade steps namely 1 classification of eruption regime and volcanic ash category and 2 estimation of ash concentration and fall rate expected accuracy of the varr algorithm estimates is evaluated using a synthetic data set an application of the varr technique is finally shown taking into consideration the eruption of the grinodotacutemsvoumltn volcano in iceland on november 2004 volume scan data from a doppler c band radar which is located at 260 km from the volcano vent are processed by means of the varr algorithm examples of the achievable varr products are presented and discussed

particle aggregation in volcanic eruption columns we develop a model to describe the formation of aggregates in a volcanic eruption column the model combines a description of the rate of collision and sticking of particles with a model of the vertical transport in the eruption column we thereby determine the evolution of the grain size distribution as a function of height in the eruption column we consider aggregates in which liquid water provides the binding agent for sufficiently large eruption columns we find that this limits the vertical extent of the zone where aggregates may form since near the source all water is in the vapor form while in the upper part of the column the mixture becomes very cold and freezes however in many cases we find that the particles spend sufficient time in the central region where the water is in the liquid form that a substantial amount of aggregation occurs furthermore we predict that owing to the reduction in the binding efficiency of water as particle size increases gilbert and lane 1994 there is a relatively narrow size distribution of aggregates at the plume top we also model the airfall deposits associated with this aggregate rich distribution of particles which is injected to the top of the eruption column we show that the relatively narrow size distribution of particles at the top of the column coupled with the gravitational settling and transport by ambient winds may lead to enhanced deposition close to the source and in some cases secondary thickening of the deposit the relatively near source deposition of fine ash in these deposits is associated with the fallout of aggregates as a simple application we show that the present dynamical aggregation model is consistent with the secondary thickening of the deposit from the may 18 1980 eruption of mount st helens

provenance of quaternary and modern alluvial deposits of the amazonian floodplain brazil inferred from major and trace elements and pb nd sr isotopes abstract chemical and pb nd and sr isotope data from floodplain sedimentary deposits in the solimoes river northwest brazil constrain the variability of the deposits through the quaternary and help reconstruct the paleogeography of amazonia since the uplift of the andes compared to the modern alluvial deposits in the active channel the quaternary terrace deposits in the solimoes river have a higher chemical index of alteration cia higher sio2 al2o3 ratios higher concentrations of immobile elements higher radiogenic sr isotopic compositions more negative end t 0 values and older nd tdm ages these geochemical characteristics indicate that the quaternary sediment sources were composed by more felsic and or recycled old continental crust derived rock materials than the younger source areas which contributed major arc volcanic materials to the flood plain deposits this shift in the composition of the sediment source areas indicates that a significant reorganization of the architecture of the solimoes river has occurred since the uplift of the andes

composition and dissolution of black smoker particulates from active vents on the juan de fuca ridge during two atlantis ii alvin cruises to the juan de fuca ridge in 1984 active high temperature 140 284 c vents were sampled for black smoker particulates using the grassle pump individual mineral phases were identified using standard x ray diffraction and petrographic procedures in addition elemental compositions and particle morphologies were determined by x ray energy spectrometry and scanning electron microscope x ray energy spectrometry techniques the vent particulates from the southern juan de fuca ridge vent sites were highly enriched in s si fe zn and cu and were primarily composed of sphalerite wurtzite pyrite pyrrhotite barite chalcopyrite cubanite hydrous iron oxides and elemental sulfur two additional unidentified phases which were prevalent in the samples included an fe si phase and a ca si phase the grain sizes of the individual particle phases ranged from 100 m for the fe si particles grain size and current meter data were used in a deposition model of individual phase dispersal for many of the larger sulfide and sulfate particles the model predicts dispersal to occur over length scales of only several hundreds of meters the high temperature black smokers from the more northerly endeavour segment vents were highly enriched in fe s ca cu and zn and were primarily composed of anhydrite chalcopyrite sphalerite barite sulfur pyrite and other less abundant metal sulfide minerals the grain sizes of the individual particles ranged from 10 m to slightly larger than 500 m the composition and size distributions of the mineral phases are highly suggestive of high temperature mixing between vent fluids and seawater a series of field and laboratory studies were conducted to determine the rates of dissolution of several sulfate and sulfide minerals the dissolution rates ranged over more than 3 orders of magnitude from 3 2 10 8 cm s 1 for anhydrite to 1 2 10 12 cm s 1 for chalcopyrite the results indicate that for some minerals particularly anhydrite and marcasite total dissolution occurs within a few hours to a few weeks of their formation for other more stable minerals including pyrite sphalerite and chalcopyrite the time required for total dissolution is much longer and consequently individual crystals may be expected to persist in the sediments for considerable periods of time after deposition

application of channel belt scaling relationships to early middle jurassic source to sink system evolution in the southern junggar basin abstract a wide range of key scaling relationships obtained from modern fluvial systems can be used to estimate the drainage areas in ancient source to sink systems in this study these relationships are applied to reconstruct source to sink systems of the early middle jurassic xishanyao formation in the southern junggar basin where meandering fluvial and deltaic successions are developed both the lower channel belt thickness and the bankfull channel belt thickness are measured and are used to estimate the scale of drainage systems that were associated with the seven fluvial systems in three sequences sequence 1 2 and 3 in ascending order the results indicate that the channel belt thickness had significant variation through time and space the average values of the bankfull thickness of the seven fluvial systems are translated into drainage area which range from 63 000 157 000 km2 and the maximum drainage length which range from 559 to 934 km the delta volume of the seven fluvial systems range from 2 01 km3 to 10 35 km3 from sequence 1 to sequence 3 detrital mineral compositions of sandstones show a gradual change from lithic arkose to feldspathic litharenite provenance fields on ternary plots and the characteristics of sandstone components show that the sandstone sources gradually shifted from the southwest dominated yilinhabierga mountains to the southeast dominated bogda mountains sediment provenance analysis together with paleogeographic maps and analysis of sedimentary records suggests that orogenic building of the bogda mountains was responsible for the progressive eastward shifting of the main fluvial channels and for the modification of drainage areas the mismatched pattern of channel sizes between the rivers developed in the early middle jurassic and in the modern northern tian shan front is due to late cenozoic north south crustal shortening which had caused the drainage network reorganization and the evolution of fluvial styles our work provides a complete profile of sediment dispersal from source to sink for better understanding of the paleogeographic evolution in the southern junggar basin this source to sink approach could also be applicable to other lacustrine basins in china

grain size variation of tephra derived from volcanic umbrella clouds the relationships among the thickness and grain size of tephra fall deposits and the volumetric flow rate of their source umbrella clouds are analytically obtained the logarithm of the ratio of the probability distribution function based on grain size inr f in fall deposits at two localities from the vent r 1 and r 2 respectively has a linear relationship with the particle settling velocity v as in 1n r f frac pi r 2 2 r 1 2 nu q 1n a where q is the volumetric flow rate of the umbrella cloud and a is a constant for a given pair of localities the volumetric flow rate of the umbrella cloud can be estimated from granulometric data using this formula generally the thickness distance relationship of tephra fall deposits depends on the initial grain size distribution and the volumetric flow rate of the umbrella cloud the empirical relationship of the exponential thinning behaviour can be extrapolated towards infinite distance only for a specific initial grain size which is similar to a log normal distribution with 2 5 otherwise it holds only in a limited range of distances in applying these results to the 1991 eruption of mt pinatubo it is shown that the volumetric flow rate of the umbrella cloud during the climactic phase of 15 june was approximately 5x1010 m3 s which is fairly consistent with the expansion rate of the umbrella cloud observed in the satellite images

controls on the geometry of incised valleys in the basal quartz unit lower cretaceous western canada sedimentary basin in southern alberta multiple episodes of fluvial erosion occurred during lower mannville basal quartz time early cretaceous creating a series of incised valley systems this study examines two of these valley networks in order to illustrate the geomorphic and genetic complexity of incised valley systems both valley networks are filled by a transgressive succession of fluvial to estuarine deposits isopach mapping of these deposits over an area of approximately 10 000 km 2 in southern alberta shows that both systems are dendritic to rectilinear in character with the trunk valley flowing from south to north along the length of the western canada sedimentary basin these systems were fed from the west south and east by a network of tributaries eustatic sea level changes may have played a role in initiating river incision but regional tectonic movements had an important influence on the history of accommodation change the periods of erosion appear to have been relatively brief because the valleys are narrow although confinement within cohesive mudstone rich paleosols of the underlying incised valley deposits may have slowed valley widening a relatively wet paleoclimate may also have contributed to the formation of narrow valleys by favoring the existence of single channel rivers the general morphology of the alberta foreland basin together with the distribution of unfilled relief associated with a preceding valley system and or of less resistant slightly older deposits determined the location of the trunk valleys uplift of the bow island arch sweetgrass arch to the east may have been responsible for a shift in the valley headwaters between the formation of the two valley systems synerosional block faulting in response to flexural loading had a significant influence on the geometry of the valley network through its control on the location and geometry of major tributary valleys the trunk valley is also significantly narrower and more deeply incised where it crosses the most pronounced uplifted block enhanced erosion at tributary junctions and valley bends produced localized areas ca 2 3 km in diameter where the valley fill is up to five times thicker and considerably coarser grained than the adjacent valley deposits such localized scour fills represent prime petroleum exploration targets

a gravity current model for the may 18 1980 mount st helens plume observations of the stratospheric plume from the may 18 1980 mount st helens eruption suggest that it spread in the crosswind direction as an intrusive gravity current as it was transported downwind grain size analyses of the plinian tephra are consistent with this model suggesting that to distances of many hundreds of kilometers turbulent atmospheric diffusion played a secondary role in plume spreading and tephra dispersal

the plinian eruptions of 1912 at novarupta katmai national park alaska the three day eruption at novarupta in 1912 consisted of three discrete episodes episode i began with plinian dispersal of rhyolitic fallout layer a and contemporaneous emplacement of rhyolitic ignimbrites and associated proximal veneers the plinian column was sustained throughout most of the interval of ash flow generation in spite of progressive increases in the proportions of dacitic and andesitic ejecta at the expense of rhyolite accordingly plinian layer b which fell in unbroken continuity with purely rhyolitic layer a is zoned from 99 to 15 rhyolite and accumulated synchronously with emplacement of the correspondingly zoned ash flow sequence in mageik creek and the valley of ten thousand smokes vtts only the andesiterichest flow units that cap the flow sequence lack a widespread fallout equivalent indicating that ignimbrite emplacement barely outlasted the plinian phase on near vent ridges the passing ash flows left proximal ignimbrite veneers that share the compositional zonation of their valley filling equivalents but exhibit evidence for turbulent deposition and recurrent scour episode ii began after a break of a few hours and was dominated by plinian dispersal of dacitic layers c and d punctuated by minor proximal intraplinian flows and surges after another break dacitic layers f and g resulted from a third plinian episode iii intercalated with these proximally are thin intraplinian ignimbrites and several andesite rich fall flow layers both cd and fg were ejected from an inner vent 400 m wide nested within that of episode i into which the rhyolitic lava dome novarupta was still later extruded two finer grained ash layers settled from composite regional dust clouds layer e which accumulated during the d f hiatus includes a contribution from small contemporaneous ash flows and layer h settled after the main eruption was over both are distinct layers in and near the vtts but distally they merge with cd and fg respectively they are largely dacitic but include rhyolitic shards that erupted during episode i and were kept aloft by atmospheric turbulence published models yield column heights of 23 26 km for a 22 25 km for cd and 17 23 km for fg and peak mass eruption rates of 0 7 1x108 0 6 2x108 and 0 2 0 4x108 kg s 1 respectively fallout volumes adjusted to reflect calculated redistribution of rhyolitic glass shards are 8 8 km3 4 8 km3 and 3 4 km3 for episodes i ii and iii microprobe analyses of glass show that as much as 0 4 km3 of rhyolitic glass shards from eruptive episode i fell with cde and 1 1 km3 with fgh most of the rhyolitic ash in the dacitic fallout layers fell far downwind se of the vent near the rhyolite dominated ignimbrite however nearly all of layers e and h are dacitic showing that the downwind rhyolitic ash is of co plinian rather than co ignimbrite origin

bifurcation of volcanic plumes in a crosswind bent over buoyant jets distorted by a crosscurrent develop a vortex pair structure and can bifurcate to produce two distinct lobes which diverge from one another downwind the region downwind of the source between the lobes has relatively low proportions of discharged fluid factors invoked by previous workers to cause or enhance bifurcation include buoyancy release of latent heat at the plume edge by evaporating water droplets geometry and orientation of the source and the encounter with a density interface on the rising path of the plume we suggest that the pressure distribution around the vortex pair of a rising plume may initially trigger bifurcation we also report new experimental observations confirming that bifurcation becomes stronger for stronger bent over plumes identifying that bifurcation can also occur for straight edged plumes but gradually disappears for stronger plumes which form a gravity current at their final level and spread for a significant distance against the current observations from satellites and the ground are reviewed and confirm that volcanic plumes can show bifurcation and a large range of bifurcation angles many of the bifurcating plumes spread out at the tropopause level and suggest the tropopause may act on the plumes as a density interface enhancing bifurcation even for quite moderate bifurcation angles the two plume lobes become rapidly separated downwind by distances of tens of kilometers such bifurcating plumes drifting apart can only result in bilobate tephra fall deposits the tephra fall deposit from the 16 km elevation se spreading bifurcating volcanic plume erupted on 15 may 1981 from mt pagan was sampled by previous workers and clearly displayed bilobate characteristics examples of bilobate tephra fall deposits are reviewed and their origin briefly discussed bilobate deposits are common and may result from many causes plume bifurcation should be considered one of the possible mechanisms which can account for come examples of bilobate tephra fall deposits

sedimentological analysis of the tephra from the 12 15 august 1991 eruption of hudson volcano the tephra fallout from the 12 15 august 1991 explosive eruption of hudson volcano cordillera de los andes 45 54 s 72 58 w chile was dispersed on a narrow elongated ese sector of patagonia covering an area on land of more than 100 000 km2 the elongated shape of the deposit together with the relatively coarse mean and median values of the particles at a considerable distance from the vent were the result of strong winds blowing to the southeast during the eruption the thickness of the fall deposit decreases up to 250 km ese from hudson volcano where it begins to thicken again secondary maxima are well developed at approximately 500 km from the vent secondary maxima together with grainsize bimodality in individual layers and in the bulk deposit suggest that particle aggregation played an important role in tephra sedimentation the fallout deposit is well stratified with alternating fine grained and coarsegrained layers which is probably a result of strong eruptive pulses followed by relatively calm periods and or changes in the eruptive style from plinian to phreatoplinian the tephra is mostly composed of juvenile material the coarse mode mostly pumice shifts to finer sizes with distance from the volcano the fine mode mostly glass shards is always about 5 6 phi glass shards and pumice are mostly light gray to colorless however considerable amounts of dark poorly vesiculated blocky shards suggest a hydromagmatic component in the eruption a land based tephra volume of 4 35 km3 was estimated and a total volume of 7 6 km3 arose from an extrapolation which took into account the probable volume sedimented in the sea bulk density ranges from 0 9 to 1 10 gr cm3 beyond 110 km from the vent rather uniform density values measured in crushed samples 2 45 2 50 gr cm3 at all distances from the vent reveal a relatively homogeneous composition mean and median sizes decrease rapidly up to 270 km from the vent beyond that point they are more or less constant whereas the maximum size 1 phi shows a steady decrease up to 550 km a concomitant improvement in sorting is observed this is attributed to sorting due to wind transport combined with particle aggregation at different times and distances from the vent the hudson tephra fallout shares some strikingly similar features with the mount st helens 18 may 1980 and quizapu 1932 eruptions

three dimensional sedimentary architecture of a large mid channel sand braid bar jamuna river bangladesh the three dimensional subsurface alluvial architecture of a large approximately 3 km long 1 km wide 12 m high mid channel sand braid bar in the jamuna river bangladesh is described evolution of the bar and its depositional characteristics are assessed from a unique combination of ground penetrating radar surveys vibracoring and trenching that are allied to a series of bathymetric surveys taken during growth of the bar over a 29 month period this methodology permits identification of the formative processes of different packages of braid bar sedimentation and provides a facies model for deposition within the entire bar mid channel bar growth occurred in a region of flow expansion and was probably initiated by the stalling and amalgamation of large dunes these dunes created a bar core that grew by i propagation of a downstream accreting slipface ii vertical accretion through stacking of dunes on both bar stoss and top and iii lateral accretion on the bar margins during recession of the flood hydrograph braid bar sedimentation is dominated by four radar facies 1 large scale predominantly planar dipping reflections interpreted as cross stratification up to 8 m in height and greater than 100 m in width that is produced by the cross channel migration of bar margins 2 medium scale trough shaped and planar discontinuous reflections interpreted as cross stratification up to 4 m in height and 300 m wide that is deposited from large sinuous crested sand dunes 3 discontinuous reflections up to 2 m high and 30 m wide interpreted as small scale trough cross stratification that is the product of smaller sinuous crested dunes and 4 high amplitude undulating reflections interpreted as mud drapes deposited in regions of slow flow often in the bar tail region at low stage dune size decreases vertically within the bar in response to the progressively shallower flows on the emerging bar top later evolution of the bar as one anabranch channel became dominant created a 1 5 km extension to the bar tail with an 8 m high angle of repose bar margin slipface formed by flow transverse to the long axis of the bar seven styles of deposition can be defined that constitute the alluvial architecture bar margin slipface vertical accretion in channel bar top vertical accretion upstream accretion lateral accretion downstream accretion and low stage mud drapes a model of braid bar sedimentation is presented that shares many similarities with previous studies of smaller sand bed braid bars with the dominance of dune scale cross stratification the presence of large scale bar margin cross stratification and the occurrence of lateral vertical upstream and downstream accretion however the contribution of the bar margin facies to the preserved stratigraphy highlighted herein may have been underestimated in previous models of braided rivers in which the braid bars were migrating slowly this study suggests a scale invariance in several aspects of mid channel bar sedimentation in sand bed rivers and proposes a model of braid bar sedimentation that may be applied widely within studies of braided alluvial architecture

quantitative shape measurements of distal volcanic ash 1 large scale volcanic eruptions produce fine ash 200 m which has a long atmospheric residence time 1 hour or more and can be transported great distances from the volcanic source thus becoming a hazard to aircraft and public health ash particles have irregular shapes so data on particle shape size and terminal velocities are needed to understand how the irregular shaped particles affect transport processes and radiative transfer measurements in this study a methodology was developed to characterize particle shapes sizes and terminal velocities for three ash samples of different compositions the shape and size of 2500 particles from 1 distal fallout 100 km of the 14 october 1974 fuego eruption basaltic 2 the secondary maxima 250 km of the 18 august 1992 spurr eruption andesitic and 3 the miocene ash hollow member nebraska rhyolitic were measured using image analysis techniques samples were sorted into 10 to 19 terminal velocity groups 0 6 59 0 cm s using an air elutriation device grain size distributions for the samples were measured using laser diffraction aspect ratio feret diameter and perimeter measurements were found to be the most useful descriptors of how particle shape affects terminal velocity these measurement values show particle shape differs greatly from a sphere commonly used in models and algorithms the diameters of ash particles were 10 120 larger than ideal spheres at the same terminal velocity indicating that irregular particle shape greatly increases drag gas adsorption derived surface areas are 1 to 2 orders of magnitude higher than calculated surface areas based on measured dimensions and simple geometry indicating that particle shapes are highly irregular correction factors for surface area were derived from the ash sample measurements so that surface areas calculated by assuming spherical particle shapes can be corrected to reflect more realistic values

assessment of depositional ash loadings from the 2009 eruption of mt redoubt abstract volcanic ash deposited onto ice and snow in the arctic has the potential to perturb the regional radiation balance by altering the surface reflectivity in order to determine changes in surface albedo loading fields of deposits must be quantified loading fields are defined as the total mass of ash deposited over an area we assessed the areal extent and loading of ash deposits from the 2009 eruption of mt redoubt using an eulerian volcanic ash transport and dispersion model fall3d combined with satellite and deposit observations because direct observations are often limited in remote arctic regions we devised a novel method for modeling ash deposit loading fields for the entire eruption based on best fit parameters of a well studied eruptive event the major land depositing eruptive events events 2 6 on march 23 24 were chosen for analyses realistic ranges of model parameters were selected based on the well described event 5 march 23 and were estimated for other events the model results were validated against nasa a train satellite data and field measurements reported by the alaska volcano observatory overall good to moderate agreement was found a total cumulative deposit area of 3 7 106 km2 was produced and loadings ranged from 7000 3000 g m 2 near the vent to

interaction between volcanic plumes and wind during the 2010 eyjafjallajokull eruption iceland 1 estimates of volcanic source mass flux currently deduced from observations of plume height are crucial for ash dispersion models for aviation and population hazard this study addresses the role of the atmospheric wind in determining the height at which volcanic plumes spread in the atmosphere and the relationship between source mass flux and plume height in a wind field we present a predictive model of volcanic plumes that describes the bending over of the plume trajectory in a crosswind and show that model predictions are in accord with a dataset of historic eruptions if the profile of atmospheric wind shear is described the wind restricts the rise height of volcanic plumes such that obtaining equivalent rise heights for a plume in a windy environment would require an order of magnitude increase in the source mass flux over a plume in a quiescent environment our model calculations are used to calibrate a semi empirical relationship between the plume height and the source mass flux that explicitly includes the atmospheric wind speed we demonstrate that the model can account for the variations in plume height observed during the first explosive phase of the 2010 eyjafjallajokull eruption using independently measured wind speeds and show that changes in the observed plume height are better explained by changing meteorology than abrupt changes in the source mass flux this study shows that unless the wind is properly accounted for estimates of the source mass flux during an explosive eruption are likely to be very significant underpredictions of the volcanic source conditions

structure of a horizontal sediment laden momentum jet abstractpartially treated wastewater discharge from marine outfall diffusers often contains inorganic and organic solids that may settle close to the source and give rise to sludge banks that affect benthic ecology an experimental and theoretical investigation of the structure and settling mechanisms of a horizontal sediment momentum jet in stagnant ambient has been carried out the cross sectional particle concentration distribution of a horizontal sediment jet is measured for the first time bottom deposition measurements are made for a total of 41 experiments of momentum jets laden with sand particles or synthetic spherical glass particles the sediment jet can be characterised by a sedimentation length scale lm defined by the jet momentum flux and particle settling velocity the measured longitudinal sediment deposition rate can be described by a semiempirical log normal distribution based on lm in the instantaneous particle distribution the sediment fallout is concentrated in a central region and

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

sedimentation from turbulent jets and plumes theoretical models are developed for the sedimentation from the margins of a particle laden axisymmetric turbulent buoyant plume in a still environment and for an axisymmetric turbulent momentum jet the models assume that the mass of each individual size fraction of sediment carried in a parcel of fluid decreases exponentially with time for relatively coarse particles the fallout models predict that the sediment deposition beyond a distance r on the ground expressed in log units should decay linearly with distance away from the vent for the momentum jet and should decrease with r1 3 for the buoyant plume the exponential decay constant j is proportional to the terminal fall velocity vt of the particles in both cases and inversely proportional to the square root of the initial momentum flux m0 for the jet fallout jj vtmo 1 2 and to the third power of the initial buoyancy flux fo for the plume fallout jp vtfo 1 3 smaller particles are affected by reentrainment caused by the turbulent eddies sweeping ambient fluid back into the plume or jet and thus reincorporating some particles that were released from the flow at greater heights this is taken into account by introducing a reentrainment coefficient into the theoretical models with the assumption that the coefficient has a constant value for a plume of given strength in new experiments fallout occurs from the margins of particle laden fresh water buoyant jets and plumes in a tank of salty water and sedimentation is measured on the tank floor two experiments were weakly affected by reentrainment and show excellent agreement with the simple theory for smaller particles and increasingly buoyant plumes and strong jets particle reentrainment is important the experimental data are fitted by the new reentrainment theory confirming that values of the reentrainment coefficient are approximately constant for a given flow a settling number is defined as the ratio of the characteristic velocity of the jet or plume to the particle settling velocity for 1 reentrainment seems to reach an equilibrium state for which the reentrainment coefficient is a constant of value 0 1 for jets and 0 4 for plumes irrespective of flow strength or particle size the plume experiments indicate that the value of the reentrainment coefficient is strongly dependent on plume strength and particle size for slightly less than 1 the general principles of sedimentation from turbulent plumes and jets are applied to the fallout of pumice from volcanic eruption columns and of metalliferous particles from black smokers on the ocean floor for volcanic eruptions the results provide an explanation for the near vent overthickening of tephra fall deposits and imply that lithic and pumice fragments from small lapilli up to at least 1 m diameter blocks are efficiently reentrained into eruption columns the size of particles reentrained in hydrothermal plumes is predicted to vary from less than 100 m in weakly buoyant plumes up to over 1000 m in megaplumes

variability of gardner s α for coarse textured sediments vadose zone soils in arid regions often contain a high gravel fraction 2 mm size this paper examines for coarse textured soils variability of the slope of gardner s 1958 unsaturated hydraulic conductivity k versus matric potential relation k kse the steady state head control method was used to obtain unsaturated k for tensions as high as 386 cm a total of 79 samples were analyzed in the laboratory the gravel fraction for the 41 gravelly samples ranged from 20 to 71 by weight the remaining 38 samples were sandy with no gravel fraction the measured k values for the gravelly samples fall within a narrow range and are well within the range of measured k values for the sandy samples the bulk of both gravelly and sandy samples can be described using a single slope exponential model gardner s and the intercept k0 are based on a least squares fit to the late time measurements at lower the mean and variance of for the gravelly samples are lower compared with the sandy type the values for the gravelly soils follow a normal distribution whereas the k0 estimates and the laboratory measured ks follow lognormal distributions for the sandy type k0 and ks follow lognormal distributions the values appear to be related to the median particle diameter d50 and the slope of the particle size distribution a clear reduction in variability of is apparent with an increase in d50 suggesting that the unsaturated k and variability for the gravelly soils can be estimated within narrow ranges the flow weighted characteristic pore size m is clearly dependent on the soil type m values for the gravelly soils are characterized by a smaller mean and variance than for the sandy type this helps explain the finding that the range in unsaturated k for the gravelly soils is less than for the sandy type at higher tensions the mean and variance of and lnk0 as well as the cross correlation between and lnk0 are important inputs to stochastic models of unsaturated flow the implications of their variability on flow in heterogeneous media are discussed

the hawkesbury sandstone south of sydney australia triassic analogue for the deposit of a large braided river abstract the hawkesbury sandstone is a triassic sheet sandstone extensively exposed in the sydney basin new south wales particularly along the coast near sydney unidirectional paleoflow in the sandstone its freshwater biota and abundant mudrock intraclasts indicate fluvial deposition sheet morphology low paleocurrent variance abundant erosion surfaces and the paucity of in situ mudrocks point to a braided fluvial system three facies assemblages have been recognized stratified sandstone massive sandstone and a minor mudrock assemblage the stratified sandstone assemblage is dominated by stacked sets of planar cross strata and minor trough cosets in sequences 6 23 m thick bounded by erosion surfaces significant paleocurrent changes between channel sequences indicate that they were initiated by the avulsion of major channel systems in some cases the channel sequences fine upward with planar cross stratal sets overlain by trough sets that decrease in magnitude upwards fining up into mudrocks the massive sandstone assemblage occurs principally as massive sandstone in elongate erosional features oriented transverse to paleoflow the massive sandstone commonly contains large mudrock intraclasts and is attributed to failure of high channel banks and or large bedforms during falling water stages the mudrock assemblage comprises rippled and horizontally laminated fine sandstone siltstone and shale with minor mudstone it is attributed primarily to floodplain deposition but abandoned channel fills are also present the hawkesbury sandstone does not conform to existing models for braided fluvial deposition in that planar cross strata accumulated in deeper parts of channels whereas trough cross strata formed in shallower water the large scale of planar cross strata sets up to 7 5 m thick mudrock intraclasts up to 38 m long bank collapse scars up to 11 m deep and abandoned channel fills up to 18 m deep show that the hawkesbury river was very large with deep main channels and high but variable discharge

correcting laboratory measured moisture retention data for gravels vadose zone soils in desert environments often contain a high gravel fraction 2 mm size laboratory measurements of moisture retention are typically made on the fine fraction 2 mm size the measurements are then corrected for field conditions by accounting for the gravel fraction in the sample a frequently used correction procedure for gravelly soils is tested a total of 23 samples containing gravel fractions up to 80 by weight were analyzed in the laboratory retention data on 2 mm size fraction were obtained and compared with measurements on bulk soil and gravel field samples results indicate that for the relatively wet moisture regime considered in this study retention data for gravelly soils can be estimated reasonably well by correcting the pressure cell measurements for the field bulk density and the fraction of fines present in the field samples further work is needed on testing the correction procedure for the dry moisture regime

deposition from particle laden round turbulent horizontal buoyant jets in stationary and coflowing receiving fluids results are presented from a series of laboratory experiments investigating the characteristic features of particle laden round turbulent buoyant jets discharged horizontally into stationary and coflowing receiving fluids for the volumetric source concentrations of particles tested 0 1 the presence of the particle load was found to have no significant influence on mean buoyant jet trajectories deposition patterns on the bed of the receiving water container indicated the existence of two separate sedimentation processes for discharges into stationary or coflowing ambients namely 1 a relatively concentrated narrow band of particle accumulation associated with near source fallout from the buoyant jet margins and 2 a broader and more disperse downstream depositional fan associated with particle fallout from the radially expanding surface gravity current formed by the impingement of the buoyant jet with the free surface of the receiving fluid scaling arguments have been developed and applie

a coupled model for dispersal of tephra during sustained explosive eruptions abstract we present a model for fallout of tephra based on approximate analytical solutions of the advection diffusion transport equation as usual the model requires the distribution of mass throughout the eruptive column as a boundary condition an input which is normally imposed ad hoc by means of analytical expressions we propose an alternative way to assess this input based on coupling the model of fallout with already existing simple models for volcanic plumes flow along the volcanic conduit and magma chamber withdrawal the final outcome is a one way coupled model that involves all the spatial domains simultaneously and has a good compromise between physical accuracy and computational efficiency the coupled model outputs not only maps of isopachs but also several relevant eruptive parameters such as column height mass eruption rate or duration of the eruption time dependency is introduced via a quasi steady approach that is considering a succession of steady states we apply the model to simulate the 2 ka subplinian eruption of montana blanca tenerife island

composition and sedimentation of hydrothermal plume particles from north cleft segment juan de fuca ridge in 1990 and 1991 particles from buoyant and neutrally buoyant hydrothermal plumes above hydrothermal vents at the north cleft segment of the juan de fuca ridge were sampled to study their changing composition and fluxes away from the vent field in the rising buoyant plume 75 of the p v cr and as scavenging from seawater by hydrothermal precipitates occurs in the first 50 m above the vent cu and zn are most enriched in buoyant plume particles collected from the first few meters above the vent however the degree of enrichment decreases very rapidly with increased height above the vents due to sedimentation of the more dense cu and zn rich sulfide phases using the plume data coupled with the results of our analysis of sediment trap samples we estimated that more than 99 and 99 9 respectively of the total hydrothermal fe and mn produced at the vent field are transported beyond the vent field and dispersed in the open ocean

the sedimentology and alluvial architecture of a large braid bar rio parana argentina abstract this paper presents 30 km of ground penetrating radar gpr data from a mid channel bar in the sixth largest river in the world the rio parana argentina gpr profiles with depth of penetration up to 12 m below the bar surface were collected from a sandy braid bar 3 km long by 1 km wide on a grid with a 200 to 400 m spacing two facies were found to dominate the sedimentary architecture the principal facies 83 of total facies comprises trough and planar cross strata related to the migration of dunes with the thickness of the cross strata decreasing towards the bar surface the second significant facies 15 is high angle generally 10 20 strata that typically form by accretion at the bar margins or bartail clay drapes the rio parana gpr surveys are compared with other gpr studies of sandy braid bars of different sizes from the south saskatchewan wisconsin and jamuna rivers the dominance of dune deposits is ubiquitous to all rivers with each also possessing a significant proportion of large scale high angle strata however two differences were found to exist between the deposits of these rivers 1 the compound bar deposits of smaller rivers contained greater proportions of the fills of cross bar channels which suggests a potential role for discharge as a factor in shaping the alluvial architecture through its impact on the frequency of sediment reworking over the bar tops and 2 the thickness of large scale high angle sets decreases with the age of the bar which suggests that the deposits of older bars may provide more useful geometrical analogues for interpreting ancient successions than smaller transient or recent bar forms that have undergone only limited modification

an algorithm for the detection and characterisation of volcanic plumes using thermal camera imagery volcanic plumes are turbulent mixtures of particles and gas which are injected into the atmosphere during a volcanic eruption depending on the intensity of the eruption plumes can rise from a few tens of metres up to many tens of kilometres above the vent and thus present a major hazard for the surrounding population currently however few if any algorithms are available for automated plume tracking and assessment here we present a new image processing algorithm for segmentation tracking and parameters extraction of convective plume recorded with thermal cameras we used thermal video of two volcanic eruptions and two plumes simulated in laboratory to develop and test an efficient technique for analysis of volcanic plumes we validated our method by two different approaches first we compare our segmentation method to previously published algorithms next we computed plume parameters such as height width and spreading angle at regular intervals of time these parameters allowed us to calculate an entrainment coefficient and obtain information about the entrainment efficiency in strombolian eruptions our proposed algorithm is rapid automated while producing better visual outlines compared to the other segmentation algorithms and provides output that is at least as accurate as manual measurements of plumes

stokes settling and particle laden plumes implications for deep sea mining and volcanic eruption plumes turbulent buoyant plumes moving through density stratified environments transport large volumes of fluid vertically eventually the fluid reaches its neutral buoyancy level at which it intrudes in

sediment laden gravity currents with reversing buoyancy there are many natural occurrences of sediment laden gravity currents in which the density of the interstitial fluid is less than that of the ambient fluid although the bulk density of the current is greater such currents are driven by the excess density of suspended particles however after sufficient particles have sedimented the current will become buoyant cease its lateral motion and ascend to form a plume examples of such currents include brackish underflows in deltas turbidity currents and pyroclastic flows experimental studies are described which show that due to sedimentation sediment laden gravity currents decelerate more rapidly than saline currents of the same density there is little difference in the experiments between a sediment laden current with neutrally buoyant interstitial fluid and one with buoyant interstial fluid until sufficient sediment has been lost to cause the latter kind of current to lift off a marked deceleration is then observed and a plume is generated with lift off occurring along the length of the current the resulting buoyant plume then generates a gravity current below the upper surface of the fluid in the tank the deposit from a current with buoyant fluid shows a fairly abrupt decrease in thickness beyond the lift off distance and has a flatter profile than that from a simple sediment current a theoretical model is presented which is based on the two layer shallow water equations and incorporates a model of the sedimentation in which particles are assumed to be uniformly suspended by the turbulence of the current the model shows good agreement with the observed lengths of the experimental currents as a function of time and predicts the lift off distance reasonably well these processes have implications for the behaviour of turbidity currents the interpretation of turbidites mixing processes in the oceans and the lift off of pyroclastic flows

particle recycling in the tag hydrothermal plume abstract analysis of th isotope distributions in the tag hydrothermal plume yields evidence for pronounced 234 th 230 th isotope fractionation this fractionation can be attributed to a combination of four processes 1 particle ageing 2 differential scavenging of 234 th and 230 th 3 differential focusing of 234 th and 230 th and 4 recycling of old particles we present arguments for the importance of the last of these processes particle recycling particles in the tag hydrothermal plume exhibit grain sizes which fall in the range 1 10 m applying the range of densities reported for amorphous iron oxyhydroxide and iron oxide mineral phases stokes law predicts that these particles should achieve terminal settling velocities ranging between 4 and 8 m d 1 for coarse grained 5 m radius particles and 0 2 0 3 m d 1 for fine grained particles 1 m radius these velocities have been used in a physical model of re entrainment and recycling in neutrally buoyant plumes our results indicate that particle recycling and re entrainment in hydrothermal plumes may be important over length scales of 1 10 km the model predicts that the proportion of particles re entrained is independent of particle size but increases with plume strength for plumes with volumetric fluxes greater than 10 3 m 3 s 1 the proportion of particles re entrained exceeds 50 in agreement with previous calculations based upon 230 th distributions sampling strategies in the past have typically concentrated upon processes operating within 1 km of active vent fields and will need to be revised accordingly in the future

upper bar deposits in large amazon rivers occurrence morphology and internal structure abstract although large anabranching rivers are a very common element in the greatest modern drainage basins the recognition of such systems in the rock record is hampered by insufficient sedimentological information from active rivers that is particularly the case of bar top depositional environments in large anabranching rivers motivating the survey of six selected areas of seasonally emergent bars in the solimoes and japura rivers western brazilian amazonia using ground penetrating radar sub bottom profiler and multibeam echosounder as well as direct observation sediment sampling gpr surveys and field observations indicate these bars are dominated by 0 5 to 4 m tall subaqueous dunes with the local presence of metric angle of repose bar side surfaces downstream accretion components are characterized by radar facies indicating horizontal to low angle downstream dipping cosets of dm scale to cm scale cross strata lateral accretion components are characterized by radar facies indicating m scale high angle cross strata sets passing laterally to low angle and inclined cosets composed of dm scale cross strata sets upstream accretion components are characterized by radar facies indicating upstream inclined cosets of m and dm scale cross strata sets with local mud drapes preservation of accretionary bar forms in these periodically exposed areas is supported by calculation of the coefficient of variation of peak discharge which indicates low interannual peak discharge variability over the bar tops

origins and energetics of maar volcanoes examples from the ultrapotassic sabatini volcanic district roman province central italy maar volcanoes represent a common volcano type which is produced by the explosive interaction of magma with external water here we provide information on a number of maars in the ultrapotassic sabatini volcanic district svd roman province as young as 90 ka the svd maars are characterised in terms of crater and ejecta ring morphologies eruptive successions and magma compositions in light of the local substrate settings with the aim of assessing magma water interaction conditions eruption energetics and genetic mechanisms feeder magmas spanned the whole svd differentiation trend from trachybasalts shoshonites to phonolites from the ejected lithic fragments from aquifer rocks the range of depth of magma water explosive interaction is estimated to have been mostly at 400 600 m below ground level with a single occurrence of surficial interaction in palustrine lacustrine environment in particular the interaction with external water may have triggered the explosive behaviour of poorly differentiated magmas whereas it may have acted only as a late controlling factor of the degree of fragmentation and eruption style for the most differentiated magma batches during low flux ascent in an incipiently fragmented state crater sizes ejecta volumes and ballistic data allow a reconstruction of the energy budget of svd maar forming eruptions erupted tephra volumes from either monogenetic or polygenetic maars ranged 0 004 0 07 km3 during individual maar forming eruptions with corresponding total magma thermal energies of 8 1015 4 1017 j based on energy partitioning and volume balance of erupted magmas and lithic fractions vs crater holes we consider the different contributions of explosive excavation of the substrate vs subsidence in forming the svd maar craters following available models based on crater sizes highly variable fractions 5 50 of the magma thermal energies would have been required for crater excavation it appears that subsidence may have played a major role in some svd maars characterised by low lithic contents whilst substrate excavation became increasingly significant with increasing degrees of aquifer fragmentation

replenishment of magma chambers by light inputs magma chambers particularly those of basaltic composition are often replenished by an influx of magma whose density is less than that of the resident magma this paper describes the fundamental fluid mechanics involved in the replenishment by light inputs if denotes the uniform density of the resident magma and that of the input the situation is described by the reduced gravity g g the volume flux q and the viscosities of the resident and input magmas e and i respectively the nondimensional reynolds numbers ree g q3 1 5 e and rei g q3 1 5 i and chamber geometry then completely specify the system for sufficiently low values of the two reynolds numbers each less than approximately 10 the input rises as a laminar conduit for larger values of the reynolds numbers the conduit may break down and exhibit either a varicose or a meander instability and entrain some resident magma at still larger reynolds numbers the flow will become quite unsteady and finally turbulent the values of the reynolds numbers at which these transitions occur have been documented by a series of experiments with water glycerine and corn syrup if the input rises as a turbulent plume significant entrainment of the resident magma can take place the final spatial distribution of the mixed magma depends on the geometry of the chamber if the chamber is much wider than it is high the mixed magma forms a compositionally stratified region between the roof and a sharp front above uncontaminated magma in the other geometrical extreme the input magma is mixed with almost all of the resident magma if the density of the resident magma is already stratified the input plume may penetrate only part way into the chamber even though its initial density is less than that of the lowest density resident magma the plume will then intrude horizontally and form a hybrid layer at an intermediate depth this provides a mechanism for preventing even primitive basaltic magmas of minimum density from erupting at the surface by conducting an experiment using aqueous solutions we show that entrainment can lead to crystallization of the magma in the input plume by making it locally supersaturated all these effects are discussed and illustrated by photographs of laboratory experiments

how do we identify big rivers and how big is big abstract big rivers are the trunk rivers that carry the water and sediment load from major orogens or that drain large areas of a continent identifying such rivers in the ancient record is a challenge some guidance may be provided by tectonic setting and sedimentological evidence including the scale of architectural elements and clues from provenance studies but such data are not infallible guides to river magnitude the scale of depositional elements is the most obvious clue to channel size but evidence is typically sparse and inadequate and may be misleading for example thick fining upward successions may be tectonic cyclothems two examples of the analysis of large ancient river systems are discussed here in order to highlight problems of methodology and interpretation the hawkesbury sandstone triassic of the sydney basin australia is commonly cited as the deposit of a large river on the basis of abundant very large scale crossbedding an examination of very large outcrops of this unit including a coastal cliff section 6 km long near sydney showed that even with 100 exposure there are ambiguities in the determination of channel scale it was concluded in this case that the channel dimensions of the hawkesbury rivers were about half the size of the modern brahmaputra river the tectonic setting of a major ancient fluvial system is commonly not a useful clue to river scale the hawkesbury sandstone is a system draining transversely from a cratonic source into a foreland basin whereas most large rivers in foreland basins flow axially and are derived mainly from the orogenic uplifts e g the large tidally influenced rivers of the athabasca oil sands alberta epeirogenic tilting of a continent by the dynamic topography process may generate drainages in unexpected directions for example analyses of detrital zircons in upper paleozoic mesozoic nonmarine successions in the sw united states suggests significant derivation from the appalachian orogen many large rivers in se asia flow along structural trends generated during the himalayan orogeny e g brahmaputra jamuna irrawaddy mekong red but evidence for large channel systems is not always present where it might be expected for example in the gulf of thailand up to 8 km of cenozoic nonmarine sediments have been recorded in fault bounded basins but seismic time slice images of the pliocene holocene section indicate rivers of modest dimensions the existing major trunk river the chao phraya which empties into the gulf at bangkok may be but a remnant of the mekong which was tectonically diverted to its present course at some point during the late cenozoic

experimental studies of particle laden plumes experiments are described in which buoyant plumes of fresh water and solid particles are generated in a tank of salt water dependence of the behavior of the plumes on particle concentration and grain size was studied at low concentrations behavior was similar to single component plumes and the principal effect of the particles was to reduce the buoyancy flux measurements of particle concentration across the plumes demonstrated that there were substantial amounts of particle reentrainment from an outer sedimenting veil reentrainment caused the plume margins to become denser than the ambient fluid total particle flux in the plumes increased with height and can be several times greater than the flux at the plume source plumes with high particle concentration exhibit several kinds of convective instability which generated gravity currents and rapid sedimentation gravity currents were generated at the margins of concentrated plumes these flows are attributed to reentrainment causing the flow margins to have excess density when the plume density was close to but less than the density of the ambient fluid a fountain forms high in the plumes which collapsed asymmetrically to generate a gravity current on one side of the plume when the density of the particle laden flow was greater than the ambient fluid a symmetrical fountain formed and gravity currents moved across the tank floor depositing sediment at a well defined distance from the source buoyant fluid ascended from the flow fronts to form a ring of plumes around the central fountain these plumes coalesced to form a broad buoyant cloud the plumes are generated by the sedimentation from the gravity currents which resulted in the fresh water and some entrained particles being released all the experimental plumes ascended to the top of the tank and spread out as gravity currents beneath the upper surface particles often fall from this layer in discrete parcels of particle rich fluid with shapes similar to thermals the fall velocities were much greater than the terminal velocities of individual particles and the sedimenting region is inhomogeneous these observations indicate that sedimentation is enhanced by convective instabilities when particle rich fluid is emplaced above particle free fluid the experiments indicate that there may be several different mechanisms of generating pyroclastic flows and surges from the collapse of volcanic eruptions columns the experiments also demonstrate that the sedimentation of particles from a plume has an important influence on dynamical behavior

the october 1902 plinian eruption of santa maria volcano guatemala abstract the october 1902 eruption of santa maria volcano guatemala was one of the largest this century it was preceded by a great earthquake on april 19 centered at the volcano as well as numerous other major earthquakes the 18 20 hour long plinian eruption on october 25 produced a column at least 28 km high reaching well into the stratosphere the airfall pumice deposit covered more than 1 2 million km2 with a trace of ash and was only two meters thick at the vent white dacitic pumice dark gray scoriaceous basalt with physically and chemically mixed intermediate pumice and loose crystals of plagioclase hornblende hypersthene biotite and magnetite make up the juvenile components of the deposit lithic fragments are of volcanic plutonic and metamorphic origin the plinian deposit is a fine grained crystal rich single pumice fall unit and shows inverse grading mapping of the deposit gives a volume of 8 3 km3 within the one mm isopach crystal concentration studies show that the true volume erupted was at least 20 km3 equivalent to 8 5 km3 of dense dacite and that 90 of the ejecta was less than 2 mm in diameter the plinian volume eruption rate averaged 1 2 105 m3s 1 and the average gas muzzle velocity of the column exceeded 270 ms 1 a total of 8 3 1018 j of energy were released by the eruption a knowledge of both theoretically derived eruption parameters and contemporary information allows a detailed analysis of eruption mechanisms this eruption was the major stratospheric aerosol injection in the 1902 1903 period however mid to low latitude northern hemisphere temperature deviation data for the years following the eruption show no significant temperature decrease this may be explained by the sulfur poor nature of dacite magmas suggesting that volatile composition rather than mass of volatiles is the controlling parameter in climatic response to explosive eruptions

sedimentation from buoyant jets an integral model is developed to describe sedimentation from a turbulent buoyant jet injected at an angle into a stationary uniform ambient fluid entrainment is modeled using the standard entrainment assumption and sediment is assumed to fall from the jet where the outward component of the fall velocity normal to the jet boundary exceeds the inward entrainment velocity when appropriately scaled by source momentum and buoyancy fluxes turbulent buoyant jets may be described in terms of a single parameter the angle 0 between the flow and the horizontal at the virtual origin which is close to the actual source for large initial densimetric froude numbers including sedimentation in the model introduces a further parameter ws which is the ratio of the fall speed of the sedimenting particles to a typical entrainment velocity and so ws is a nondimensional fall speed an important result is that this ratio is independent of the source speed so that the proportion of the sediment load deposited nea

the role of discharge variability in determining alluvial stratigraphy we illustrate the potential for using physics based modeling to link alluvial stratigraphy to large river morphology and dynamics model simulations validated using ground penetrating radar data from the rio parana argentina demonstrate a strong relationship between bar scale set thickness and channel depth which applies across a wide range of river patterns and bar types we show that hydrologic regime indexed by discharge variability and flood duration exerts a first order influence on morphodynamics and hence bar set thickness and that planform morphology alone may be a misleading variable for interpreting deposits indeed our results illustrate that rivers evolving under contrasting hydrologic regimes may have very similar morphology yet be characterized by marked differences in stratigraphy this realization represents an important limitation on the application of established theory that links river topography to alluvial deposits and highlights the need to obtain field evidence of discharge variability when developing paleoenvironmental reconstructions model simulations demonstrate the potential for deriving such evidence using metrics of paleocurrent variance

patterns of anabranching channels the ultimate end member adjustment of mega rivers abstract large fluvial systems adjust to a combination of controls to form distinctive channels which represent a dominant factor in the evolution of floodplain geomorphology and sedimentology fluvial geomorphology has commonly classified river channels into meandering straight and braiding patterns which are seen to represent a continuum of channel geometry anabranching patterns rivers with multiple channels however are characteristic of many rivers the identification of a combination of variables that discriminates specific channel patterns has been a significant focus of research in fluvial geomorphology the development of this body of knowledge however has been established from medium and small rivers and laboratory flume studies very few of these research ideas developed from analysis of large fluvial systems this paper assesses the pattern of channel adjustment of large fluvial systems by employing hydraulic geometry discharge w d slope grain size stream power specific stream power and froude number q mean 1000 m 3 s the study demonstrates that methods currently used to discriminate channel patterns are not useful when applied to very large rivers further with the exception of the lower mississippi alluvial rivers with mean annual discharges greater than 17 000 m 3 s here classified as mega rivers do not generate single thread meandering or typical braided patterns these mega rivers develop anabranching patterns

the danube river inception evidence for a 4 ma continental scale river born from segmented paratethys basins before reaching the black sea the danube river passes through a string of para tethyan vienna pannonian and western and eastern dacian basins the key question is when and how did the danube river became a continental scale river with a drainage similar to present new data presented here show that the late miocene deep water strata in the black sea have a significant sediment source and depositional style change at about 4 ma however the presence of active miocene basins within the danube catchment raises questions about the timing of danube river inception and whether the upstream palaeogeography would have allowed or disallowed delivery of large sediment volumes to the deep water black sea stratigraphic analyses in the pannonian and dacian basins reveal a phase of coeval sedimentary fill of the basins along the danube at about 4 ma this multi basin observation points to a concurrent basin fill model rather than the basin fill and spill or messinian type lowstand models previously proposed for danube inception this article is protected by copyright all rights reserved

near field variability of the tag non buoyant plume 26 n mid atlantic ridge we report the initial results of a nine hour monitoring of the non buoyant plume above the tag hydrothermal vent field at 26 n on the mid atlantic ridge a ctd conductivity temperature and pressure and nephelometer particle sensor were continuously raised and lowered through the non buoyant plume for nine hours at a site situated 100 m south of the hydrothermal mound this method permits a detailed record of the variability of the non buoyant plume close to the vent field to be made our results show that the particulate maximum of the non buoyant plume appears 150 m lower in the water column at the start and end of the record compared with profiles recorded towards the middle of the record a corresponding structure also appears in the temperature and salinity records with the result that the plume particle maximum resides on the 27 8035 isopycnal throughout modelling of the rise height of an axisymmetric plume can only account for 95 m of the variability in the final level of neutral buoyancy plume bending due to currents is a likely explanation for the recorded temporal variability

the effect of total grain size distribution on the dynamics of turbulent volcanic plumes abstract the impact of explosive volcanic plumes on climate and on air traffic strongly depends on the concentration and grain size distribution gsd of pyroclastic fragments injected into the atmosphere accurate and robust modelling of the evolution of gsd during pyroclast transport from the vent to the ash cloud is therefore crucial for the assessment of major volcanic hazards analysis of field deposits from various recent plinian eruptions shows that their total gsd is well described by a power law as expected from the physics of magma fragmentation with an exponent d ranging from 3 0 to 3 9 by incorporating these measured gsd into the initial conditions of a steady state 1d model of explosive eruption columns we show that they have a first order impact on the dynamical behaviour of explosive eruption columns starting from an initial value of d the model tracks the evolution of gsd in the column and calculates the dynamical consequences of particle sedimentation the maximum height reached by the column one of the first order results relevant to aircraft safety changes by 30 for mass fluxes of 10 7 kg s 1 or larger and by 45 85 for mass fluxes between 105 and 10 7 kg s 1 depending on exponent d we compare our predictions to a specially assembled set of geologic field data and remote sensing observations from 10 plinian eruptions for which maximum column height and mass flux are known independently the incorporation of realistic power law gsd in the model greatly improves the predictions which opens new perspectives for estimation of ash load and gsd in volcanic clouds from near real time measurements available from satellite payloads our results also contribute to the improvement of volcanic source term characterization that is required input for meteorological dispersion models

particle segregation of an eruption plume as revealed by a comprehensive analysis of tephra dispersal theory and application abstract a new way to deduce particle segregation in an uprising eruption column is developed this method subdivides an eruption column into horizontal slices and particle dispersal from each column slice is calculated using an advection diffusion model named tephra2 a grid search is employed to obtain the particle segregation from each column slice and other unknown parameters vent position diffusion coefficient and particle density the initial conditions for the calculation were mass loading data collected on the ground wind profile and minimum empirical parameters this method is applied to the 1986b eruption of izu oshima volcano japan which was a sub plinian eruption with a vertically uprising column of 10 km high a previous study of this eruption employed a gravity current model which assumes that most of the tephra segregation took place at the column umbrella transition and at the base of the umbrella cloud the gravity current model was used to calculate the base height of the umbrella cloud from the mass loading distribution on the ground and the calculated base height 7 9 km is consistent with the observed column height however the present study shows that most of the particles segregated from the lower parts of the column 1 3 km high and no significant fallout from the upper part of the column was observed even though the segregation pattern is different between the present and past studies the total mass and total grain size distribution are similar this coincidence is attributed to the use of the same decay function of mass loading in both models both assume the exponential decay of mass loading as a function of the area enclosed by the isopleth although the physical meaning of the decay rate is different in the two models this conclusion also implies that the shape of the decay function itself cannot be used to determine which model is appropriate in describing the source column however the applicability of the gravity current model should be justified when the decay rate is low and when an unreasonably high diffusion coefficient is obtained by inversion techniques that use advection diffusion models the new approach of this study also enables the calculation of the total mass and total grain size distribution without hand drawn isopleth maps and uses all of the point data with error evaluations for each point data the calculation results e g the erupted mass are shown with associated errors

constraining particle size dependent plume sedimentation from the 17 june 1996 eruption of ruapehu volcano new zealand using geophysical inversions weak subplinian plinian plumes pose frequent hazards to populations and aviation yet many key parameters of these particle laden plumes are to date poorly constrained this study recovers the particle size dependent mass distribution along the trajectory of a well constrained weak plume by inverting the dispersion process of tephra fallout we use the example of the 17 june 1996 ruapehu eruption in new zealand and base our computations on mass per unit area tephra measurements and grain size distributions at 118 sample locations comparisons of particle fall times and time of sampling collection as well as observations during the eruption reveal that particles smaller than 250 m likely settled as aggregates for simplicity we assume that all of these fine particles fell as aggregates of constant size and density whereas we assume that large particles fell as individual particles at their terminal velocity mass fallout along the plume trajectory follows distinct trends between larger particles d 250 m and the fine population d 250 m that are likely due to the two different settling behaviors aggregate settling versus single particle settling in addition we computed the resulting particle size distribution within the weak plume along its axis and find that the particle mode shifts from an initial 1 mode to a 2 5 mode 10 km from the vent and is dominated by a 2 5 to 3 mode 10 180 km from vent where the plume reaches the coastline and we do not have further field constraints the computed particle distributions inside the plume provide new constraints on the mass transport processes within weak plumes and improve previous models the distinct decay trends between single particle settling and aggregate settling may serve as a new tool to identify particle sizes that fell as aggregates for other eruptions

sedimentation of tephra by volcanic plumes i theory and its comparison with a study of the fogo a plinian deposit sao miguel azores sedimentation of ejecta from volcanic plumes has been studied as a function of distance from the source in the fogo a plinian deposit sao miguel azores the fogo a trachytic pumice deposit is reversely graded and can be divided into two parts on the basis of pumice colour abundance of syenite accessory lithic clasts and distribution the lower syenite poor part was dispersed to the south and was clearly influenced by wind the upper syenite rich part is coarsegrained and has a nearly symmetrical distribution around the vent elongation of isopachs to the east indicate a weak wind influence the grain size variations of lithic and crystal components in the upper coarse part were studied total accumulation and accumulation per unit area expressed in kg m2 show good fits to a gaussian function at distances greater than 7 km for grain diameters less than 2 cm these results agree with a theoretical model for a radially spreading turbulent current moving over a quiescent fluid the gaussian coefficient is shown to be a function of grain size and the flow rate of material into the umbrella region of the eruption column the coefficient is therefore also a function of column height the column height deduced from these data is 21 km which is in broad agrrement with the column height of 27 km deduced from maximum clast dispersal using the method of carey and sparks 1986 the accumulation of clasts larger than 2 cm agrees with a theory for the fallout of clasts from the margins of the ascending eruption column which treats the plume as a succession of large eddies that decrease their mass of particles as an exponential function of time calculations are also presented for the influence of the radial inflow of surrounding air into the column on the deposition of clasts these calculations constrain the wind speed during the later part of the fogo a eruption to be at most a few metres per second the study has allowed four different dynamic categories of clast behaviour to be recognised in eruption columns

evidence for the control of the geochemistry of amazonian floodplain sediments by stratification of suspended sediments in the amazon abstract holocene and historical amazonian floodplain deposits collected from two cores of the varzea do curuai flooded area brazil were analysed for major and trace element geochemistry as well as nd sr isotopic compositions 21 samples the ta11 and ta14 cores 110 cm and 270 cm in depth respectively were collected at different locations in the varzea near a channel inlet connecting the amazon river to the varzea and at the centre of the varzea respectively the two cores represent records of sedimentation on different time scales with ta11 covering the last 100 years and ta14 extending back to 5600 years cal bp although the sediments are generally coarser in ta11 than in ta14 the major and trace element concentrations cr th and th sc and eu anomalies and nd sr isotopic compositions in both cores fail to show any clear variations with depth however there are chemical differences between the two analysed cores the ta14 sediments have higher al si and chemical index of alteration cia values than those of ta11 the ta14 sediments are enriched in th u y nb ree cs rb v and ni but show slightly depleted mgo cao and sr and more strongly depleted na2o zr and hf compared to ta11 in addition the nd sr isotopic compositions of the ta11 sediment core are on the whole similar to the solimoes suspended particulate matter spm whereas ta14 has a similar nd sr isotopic composition compared with the spm of the amazon river at obidos these differences are best explained by chemical stratification of the spm of the amazon river during flooding of the amazon river coarser grained particulates supplied by the solimoes river are deposited in the deepest environments near the channel inlet as recorded in the ta11 sediment core by contrast finer grained suspended sediments derived from the madeira river are transported into the shallower environments of the varzea system and deposited as a result of flow expansion and loss of carrying power as recorded in the ta14 sediment core using the nd isotopic compositions and concentrations of each sediment core we calculate between 20 and 85 of the sediment input to the ta14 core site is derived from the madeira river while between 52 and 100 of the sediment input to the ta11 core sample is supplied by the solimoes river similar calculation using the sr isotopic compositions and concentrations of ta11 and ta14 sediments gave similar results with between 32 and 49 of ta14 sediments deriving from the solimoes river while between 70 and 100 of ta11 sediments originate from the solimoes river finally our findings lead us to question whether particular amazon deposits that do not integrate the entire range of spm present in the river are representative of the overall sediment load provided today and in the past by global rivers

sub surface dynamics and eruptive styles of maars in the colli albani volcanic district central italy protezione civile italy 2004 2006 agreement with istituto nazionale di geofisica e vulcanologia ingv project v3 1

sedimentation from gravity currents generated by turbulent plumes sedimentation from radially spreading gravity currents generated at the top of ascending sediment laden plumes is described by a model which assumes that sediment is dispersed homogeneously by turbulence in the gravity current resulting in an exponential decrease in the concentration of sediment with time as particles settle out of the lower boundary of the current for radial spreading this model predicts a gaussian distribution of sediment accumulation away from the source with an exponential constant b which depends on flow rate q and particle settling velocity v b nv q in the experiments described sedimentation occurs from gravity currents generated by ascent of buoyant particle laden plumes of fresh water in a tank of salty water the sediment accumulation shows close agreement with the theoretical model and the gaussian decay constant b can be determined from a maximum in the accumulated mass of sediment per unit distance and from the slope of the line in s s0 br2 where r is the radial distance s is the sediment mass flux per unit area and s0 is the value of s at r 0 data from the dispersal of volcanic ejecta from a large c 24 km high plinian eruption column in the azores also show good agreement with the theory confirming that it is general and independent of scale and the nature of the fluid the experimental data also show a change in sedimentation behaviour at distances from the source corresponding to the corner of the plume where it diverts into a lateral gravity current and there is an abrupt decrease in vertical velocity sedimentation of coarse grain sizes between the source and the corner occurs from the inclined plume margins and does not behave as predicted by the theoretical model

transport and sedimentation dynamics of transitional explosive eruption columns the example of the 800 bp quilotoa plinian eruption ecuador abstract impact of large scale explosive eruptions largely depends on the dynamics of transport dispersal and deposition of ash by the convective system in fully convective eruptive columns ejected gases and particles emitted at the vent are vertically injected into the atmosphere by a narrow buoyant column and then dispersed by atmosphere dynamics on a regional scale in fully collapsing explosive eruptions ash partly generated by secondary fragmentation is carried and dispersed by broad co ignimbrite columns ascending above pyroclastic currents in this paper we investigate the transport and dispersion dynamics of ash and lapillis during a transitional plinian eruption in which both plinian and co ignimbrite columns coexisted and interacted the 800 bp eruptive cycle of quilotoa volcano ecuador produced a well exposed tephra sequence our study shows that the sequence was accumulated by a variety of eruptive dynamics ranging from early small phreatic explosions to sustained magmatic plinian eruptions to late phreatomagmatic explosive pulses the eruptive style of the main 800 bp plinian eruption u1 progressively evolved from an early fully convective column plinian fall bed to a late fully collapsing fountain dense density currents passing through an intermediate transitional eruptive phase fall syn plinian dilute density currents in the transitional u1 regime height of the convective plinian column and volume and runout of the contemporaneous pyroclastic density currents generated by partial collapses were inversely correlated the convective system originated from merging of co plinian and co surge contributions this hybrid column dispersed a bimodal lapilli and ash fall bed whose grain size markedly differs from that of classic fall deposits accumulated by fully convective plinian columns sedimentological analysis suggests that ash dispersion during transitional eruptions is affected by early aggregation of dry particle clusters

large barchanoid dunes in the amazon river and the rock record implications for interpreting large river systems abstract the interpretation of large river deposits from the rock record is hampered by the scarcity of direct observations of active large river systems that is particularly true for deep channel environments where tens of meters deep flows dominate these conditions are extremely different from what is found in smaller systems from which current facies models were derived mbes and shallow seismic surveys in a selected area of the upper amazonas river in northern brazil revealed the presence of large compound barchanoid dunes along the channel thalweg the dunes are characterized by v shaped concave downstream crest lines and convex up longitudinal profiles hundreds of meters wide up to 300 m in wavelength and several meters high based on the morphology of compound dunes expected preserved sedimentary structures are broad large scale low angle concave up and downstream cross strata passing laterally and downstream to inclined cosets examples of such structures from large river deposits in the rock record are described in the silurian serra grande group and the cretaceous sao sebastiao and marizal formations in northeastern brazil as well as in triassic hawkesburry sandstone in southeastern australia and the plio pleistocene ica formation in the western amazon all these sedimentary structures are found near channel base surfaces and are somewhat coarser than the overlying fluvial deposits favoring the interpretation of thalweg depositional settings the recognition of large barchanoid dunes as bedforms restricted to river thalwegs and probably to large river systems brings the possibility of establishing new criteria for the interpretation of fluvial system scale in the rock record sedimentary structures compatible with the morphological characteristics of these bedforms seem to be relatively common in large river deposits given their initial recognition in five different fluvial successions in brazil and australia potentially enabling substantial improvements in facies models for large rivers

assessing quasi equilibrium fining and concavity of present rivers a modelling approach abstract the aim of the paper is presenting a very simple but reliable tool to reproduce the evolution of alluvial rivers worldwide typically characterized by longitudinal profile concavity and downstream bottom fining a zero dimensional two reach two grainsize hydro morphological model has been developed to simulate the long term evolution of alluvial watercourses up to the present state starting from the basin orogeny firstly the model is described and tested against six hypothetic cases to show the quasi stationary behaviour of profile concavity and bottom fining then it is applied to four real watercourses having diverse size hydrological and morphoclimatic characteristics the adige ob parana and zambezi rivers these are characterized by a different age indicated by the estimated time elapsed from the last significant tectonic episode in their basin ranging from 10 to 300 million years for the four cases for all the simulated rivers it has been confirmed a satisfactory replication by the model of the measured slopes and concavity of the longitudinal profile in the present conditions unfortunately only a limited comparison of the computed sediment fining in the downstream direction was possible as very few information on spatially distributed measured grainsize composition along the studied river systems is available

tarawera 1886 new zealand a basaltic plinian fissure eruption abstract new zealand s biggest and most destructive volcanic eruption of historical times was that of tarawera in 1886 the resulting scoria fall has a dispersal very similar in extent to that of the vesuvius a d 79 pumice fall and is one of the few known examples of a basaltic deposit of plinian type a new estimate of the volume 2 km 3 is significantly greater than previous estimates the basalt came mainly from a 7 km length of fissure and emission and exit velocity were fairly uniform along at least 4 km of it this is one of the few documented examples of a plinian eruption from a fissure vent primary welding of the scoria fall resulted where the accumulation rate exceeded about 250 mm min 1 a model of the eruption dynamics is proposed which leads to an estimate of 28 km for the height of the eruption cloud and implies a magma volatile fraction of 1 5 3 violent phreatic explosions occurred in the southwestern extension of the fissure across the rotomahana geothermal field and it is thought that some of the water responsible for the power of the plinian eruption came from this source though its amount was not sufficient to turn the eruption into a phreatoplinian one

development of tectonic cyclothems in rift pull apart and foreland basins sedimentary response to episodic tectonism the thickest part of asymmetric rift pull apart and foreland basin fills commonly consists of large scale hundreds to thousands of metres thick tectonically generated cyclothems of fine grained marine lacustrine or longitudinal fluvial deposits and coarse grained transverse braid plain or alluvial fan deposits the appearance of coarse clastics in these basin fills is typically noted as the time of tectonically rejuvenated source area uplift based on the conceptual tie between relief and coarse grain size and on the application of the davis theory of landform development we propose the opposite interpretation that the commencement of fine grained sedimentation above coarse grained deposits in a tectonic cyclothem is the best indicator of renewed tectonic activity this reinterpretation is more consistent with 1 modern examples 2 the consideration of source area and sedimentary basin geomorphology 3 the disparity between the reaction rates of the various environments to subsidence 4 the disparity between the rates of tectonic uplift and erosion and 5 the controls on elastic wedge progradation in our model extensive coarse grained clastic wedge progradation is an indicator of tectonically quiescent phases

combined effects of total grain size distribution and crosswind on the rise of eruptive volcanic columns abstract the maximum height of an explosive volcanic column h depends on the 1 4th power of the eruptive mass flux q and on the 3 4th power of the stratification of the atmosphere n expressed as scaling laws this relationship has made h a widely used proxy to estimate q two additional effects are usually included to produce more accurate and robust estimates of q based on h particle sedimentation from the volcanic column which depends on the total grain size distribution tgsd and the atmospheric crosswind both coarse tgsd and strong crosswind have been shown to decrease strongly the maximum column height and tgsd which also controls the effective gas content in the column influences the stability of the column however the impact of tgsd and of crosswind on the dynamics of the volcanic column are commonly considered independently we propose here a steady state 1d model of an explosive volcanic column rising in a windy atmosphere that explicitly accounts for particle sedimentation and wind together we consider three typical wind profiles uniform linear and complex with the same maximum wind velocity of 15 m s 1 subject to a uniform wind profile the calculations show that the maximum height of the plume strongly decreases for any tgsd the effect of tgsd on maximum height is smaller for uniform and complex wind profiles than for a linear profile or without wind the largest differences of maximum heights arising from different wind profiles are observed for the largest source mass fluxes 107 kg s 1 for a given tgsd compared to no wind conditions the field of column collapse is reduced for any wind profile and tgsd at the vent an effect that is the strongest for small mass fluxes and coarse tgsd provided that the maximum plume height and the wind profile are known from real time observations the model predicts the mass discharge rate feeding the eruption for a given tgsd we apply our model to a set of eight historical volcanic eruptions for which all the required information is known taking into account the measured wind profile and the actual tgsd at the vent substantially improves by 30 the agreement between the mass discharge rate calculated from the model based on plume height and the field observation of deposit mass divided by eruption duration relative to a model taking into account tgsd only this study contributes to the improvement of the characterization of volcanic source term required as input to larger scale models of ash and aerosol dispersion

results of the eruptive column model inter comparison study this study compares and evaluates one dimensional 1d and three dimensional 3d numerical models of volcanic eruption columns in a set of different inter comparison exercises the exercises were designed as a blind test in which a set of common input parameters was given for two reference eruptions representing a strong and a weak eruption column under different meteorological conditions comparing the results of the different models allows us to evaluate their capabilities and target areas for future improvement despite their different formulations the 1d and 3d models provide reasonably consistent predictions of some of the key global descriptors of the volcanic plumes variability in plume height estimated from the standard deviation of model predictions is within 20 for the weak plume and 10 for the strong plume predictions of neutral buoyancy level are also in reasonably good agreement among the different models with a standard deviation ranging from 9 to 19 the latter for the weak plume in a windy atmosphere overall these discrepancies are in the range of observational uncertainty of column height however there are important differences amongst models in terms of local properties along the plume axis particularly for the strong plume our analysis suggests that the simplified treatment of entrainment in 1d models is adequate to resolve the general behaviour of the weak plume however it is inadequate to capture complex features of the strong plume such as large vortices partial column collapse or gravitational fountaining that strongly enhance entrainment in the lower atmosphere we conclude that there is a need to more accurately quantify entrainment rates improve the representation of plume radius and incorporate the effects of column instability in future versions of 1d volcanic plume models

a global sensitivity analysis of the plumerise model of volcanic plumes abstract integral models of volcanic plumes allow predictions of plume dynamics to be made and the rapid estimation of volcanic source conditions from observations of the plume height by model inversion here we introduce plumerise an integral model of volcanic plumes that incorporates a description of the state of the atmosphere includes the effects of wind and the phase change of water and has been developed as a freely available web based tool the model can be used to estimate the height of a volcanic plume when the source conditions are specified or to infer the strength of the source from an observed plume height through a model inversion the predictions of the volcanic plume dynamics produced by the model are analysed in four case studies in which the atmospheric conditions and the strength of the source are varied a global sensitivity analysis of the model to a selection of model inputs is performed and the results are analysed using parallel coordinate plots for visualisation and variance based sensitivity indices to quantify the sensitivity of model outputs we find that if the atmospheric conditions do not vary widely then there is a small set of model inputs that strongly influence the model predictions when estimating the height of the plume the source mass flux has a controlling influence on the model prediction while variations in the plume height strongly effect the inferred value of the source mass flux when performing inversion studies the values taken for the entrainment coefficients have a particularly important effect on the quantitative predictions the dependencies of the model outputs to variations in the inputs are discussed and compared to simple algebraic expressions that relate source conditions to the height of the plume