grain size dependence of sediment composition and environmental bias in provenance studies abstract this article investigates both experimentally and theoretically the compositional changes associated with textural effects and hydraulic sorting during sediment transport and deposition which cause systematic distortion in quantitative provenance analysis environmental bias traditional procedures aimed at eliminating textural noise find limited success the gazzi dickinson method cannot remove hydrodynamic related modal variability multiple size window strategies are time consuming narrow or moving size window strategies represent misleading or impractical short cuts being less convenient options than simply analysing each sample in bulk new concepts introduced here unravel the superposed causes of compositional variability in modern sediments intrasample modal variability fundamentally explained by settling equivalence relationships can be accurately modelled mathematically intersample modal variability principally resulting from selective entrainment can be assessed and removed by a simple principle in absence of provenance changes and environmental bias the weighted average density of terrigenous grains srd index should be equal for each sample and each grain size class of each sample to the weighted average density of source rocks by correcting relative abundances of detrital minerals in proportion to their densities we can restore the appropriate srd index for any provenance and subprovenance type in each sample or grain size class modal variability is effectively reduced by this procedure which can be applied confidently to modern sediments deposited by tractive currents in any environment good results are obtained even for placer sands and finest classes where heavy mineral concentration is strongest such srd correction also successfully compensates for biased narrow window modes thus providing a numerical solution of general validity to the problem of environmental bias in sedimentary petrology after compensating for settling equivalence and selective entrainment effects residual size dependent compositional variability may be provenance related minor in ganga brahmaputra sediments provenance related effects are spectacularly displayed in the nile basin where volcaniclastic silt mixes with basement derived quartzofeldspathic sand and wind blown saharan quartz

a miocene coarse volcaniclastic mass flow deposit in the shimane peninsula sw japan product of a deep submarine eruption a subaqueous volcaniclastic mass flow deposit in the miocene josoji formation shimane peninsula is 15 16 m thick and comprises mainly blocks and lapilli of rhyolite and andesite pumices and non to poorly vesiculated rhyolite it can be divided into four layers in ascending order layer 1 is an inversely to normally graded and poorly sorted lithic breccia 0 3 6 m thick layer 2 is an inversely to normally graded tuff breccia to lapilli tuff 6 11 m thick this layer bifurcates laterally into minor depositional units individually composed of a massive lithic rich lower part and a diffusely stratified pumice rich upper part with inverse to normal grading of both lithic and pumice clasts layer 3 is 2 5 3 m thick and consists of interbedded fines depleted pumice rich and pumice poor layers a few centimeters thick layer 4 is a well stratified and well sorted coarse ash bed 1 5 2 m thick the volcaniclastic deposit shows internal features of high density turbidites and contains no evidence for emplacement at a high temperature the mass flow deposit is extremely coarse grained dominated by traction structures and is interpreted as the product of a deep submarine explosive eruption of vesicular magma or explosive collapse of lava

the spatial distribution of grains and crystals in rocks characterisation and analysis of the spatial distribution pattern sdp of grains or crystals in rocks is potentially a powerful technique which can be used to constrain the processes which operate in the formation of rocks a method to quantify the sdp of grains in thin section is presented the distance betwen the centre of a grain and the centre of its nearest neighbour is calculated for all the grains in the sample area to produce a distribution of distances that characterises the spatial pattern of grains in the rock this distribution is then normalised to a random distribution of points with the same population density to give a descriptive value r values of r for rock samples are plotted against porosity modal abundance of other phases in igneous and metamorphic rocks to characterise the sdp the sdp of randomly packed distributions of equal size spheres varies systematically with porosity producing a line on a porosity versus r plot termed the random sphere distribution line rsdl rocks which plot above the rsdl have an ordered sdp and those that plot below a clustered sdp the effects of variation in grain packing order grain sorting compaction and random crystallisation overgrowth on determined r values were investigated using a combination of 3 d sphere models and 2 d texture models the maximum possible value of r is 2 148 corresponding to a perfect section through hexagonal cubic close packing of grains the minimum value of r is dependent on the proportion of grains in the sample volume and may be as low as 1 2 for a sample volume with 30 grains which are clustered variations in size sorting can cause r to vary by approximately 0 25 mechanical compaction of a loose framework of grains results in a higher packing order and an increase in r continued compaction creates a fabric in the texture and r decreases as cluster patterns are developed perpendicular to the principal stress the overgrowth of grains in a touching framework alters the location of the geometric grain centre as the crystal shape changes leading to an increase in r as porosity decreases results reveal that grain sorting compaction and grain overgrowth produce different r versus porosity relationships which can be identified in and therefore used to constrain the relative importance of these processes in rock samples

quenching of steam charged pumice implications for submarine pyroclastic volcanism abstract huge quantities of silicic pumice have been deposited in intra oceanic convergent margin settings throughout earth s history the association of submarine silicic calderas with thick proximal accumulations of pumice lapilli suggests that these pyroclasts were deposited as a direct result of submarine eruptions yet when first erupted these highly vesicular gas filled clasts had densities significantly less than seawater experiments carried out 1 atm on heated pumice samples whose vesicles were charged with steam the dominant component of magmatic volatiles show that buoyancy of freshly erupted submarine pumice is transient upon quenching the phase change of steam to liquid water creates strong negative pore pressures within the pumice vesicles that accelerate the absorption of surrounding water generating high density pumice and promoting rapid clast sinking variations in the physical properties of steam with temperature and pressure have important implications for submarine pyroclastic eruptions firstly highly vesicular pumice can be deposited on the seafloor at temperatures elevated significantly above ambient if they are erupted at sufficient depths to remain wholly submarine 200 m and either the fluid in which they cool contains heated water and or they only absorb sufficient water to sink secondly the rapid increase in density of the eruption column caused by condensation and the transition from buoyant gas filled to denser water saturated pumice lapilli together with turbulent mixing with the surrounding seawater favour collapse and transport of pyroclasts in water supported gravity currents finally this mixing of the ejecta with seawater and the ease of water ingestion into permeable pumice clasts suggest that water supported transport mechanisms can operate as primary dispersal processes in explosive submarine eruptions

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

a comprehensive study of pumice formation and dispersal the cretaio tephra of ischia italy the cretaio tephra is a small volume 0 02 km3 pumice deposit on the island of ischia in italy it was erupted during historical time and is associated with a tuff ring dome complex extruded along a n s trending normal fault system bordering the east side of the mt epomeo resurgent block 14c ages of the deposit average 1860 a b p which is in agreement with the potsherds found in paleosols underlying and overlying the deposit the eruption sequence consists of 7 members most of these members are pumice fall deposits one is an ash and pumice fallout and another is a surge bed the tephra have a trachytic phonolitic composition all chemical characteristics including major and trace elements and sr isotope composition are homogeneous throughout the sequence the vesicularity of pumice clasts does not show any systematic change the erupted magma had the same amount of volatiles the same flux and vesicle growth behavior throughout the eruption sequence grain size analyses of 90 tephra samples representative of all the members at variable distance from the supposed vent have been processed using the sequential fragmentation transport theory in order to define the effects of eruptive fragmentation and subsequent transport on the particle sizes distribution four significant subpopulations have been identified and related to transport mechanisms

products of neptunian eruptions a common pyroclastic facies in subaqueous volcanic successions comprises massive to graded very thick several to tens of meters laterally extensive several kilometers beds of nonwelded pumice lapilli with volumes ranging to tens of cubic kilometers this facies may be overlain by laminated ash or bimodal ash and giant 1 m pumice clasts and underlain by coarse lithic breccia the association is inferred to be the typical product of sustained magmatic volatile driven explosive eruptions from vents at water depths of 1300 200 m we propose the term neptunian for such eruptions and their products the eruption column rapidly mixes with the surrounding water cools increases in density and collapses while remaining under water lithic clasts that are too heavy to be entrained in the column are deposited close to the source forming a neptunian lithic breccia pumice lapilli are rapidly waterlogged and form the dominant component in the collapsing column and in eruption fed water supported density currents neptunian density currents hot buoyant giant pumice clasts continue to rise and may reach the water surface before being waterlogged and settling along with temporarily suspended ash forming neptunian suspension deposits eruption magnitude fragmentation mechanisms and juvenile pyroclast characteristics especially vesicularity are very similar in neptunian and plinian style eruptions but column behavior differs primarily because of the contrasting physical properties of the ambient fluid water versus air

grain size distribution of volcaniclastic rocks 2 characterizing grain size and hydraulic sorting quantification of the grain size distribution of sediments allows interpretation of processes of transport and deposition jutzeler et al 2012 developed a technique to determine grain size distribution of consolidated clastic rocks using functional stereology allowing direct comparison between unconsolidated sediments and rocks here we develop this technique to characterize hydraulic sorting and infer transport and deposition processes we compare computed grain size and sorting of volcaniclastic rocks with field based characteristics of volcaniclastic facies for which transport and depositional mechanisms have been inferred we studied pumice rich subaqueous facies of volcaniclastic rocks from the oligocene ohanapecosh formation ancestral cascades washington usa pliocene dogashima formation izu peninsula honshu japan miocene manukau subgroup northland new zealand and the quaternary sierra la primavera caldera jalisco state mexico these sequences differ in bed thickness grading and abundance of matrix we propose to evaluate grain size and sorting of volcaniclastic deposits by values of their modes matrix proportion 2 mm f 1 and d16 instead of median diameter d50 and standard deviation parameters f 1 and d16 can be uniformly used to characterize and compare sieving and functional stereology data volcaniclastic deposits typically consist of mixtures of particles that vary greatly in density and porosity hydraulic sorting ratios can be used to test whether inferred density of mixed clast populations of pumice and dense clasts are hydraulically sorted with each other considering various types of transport under water evaluation of this ratio for our samples shows that most studied volcaniclastic facies are deposited by settling from density currents and that basal dense clast breccia are emplaced by shear rolling these hydraulic sorting ratios can be applied to any type of clastic rocks and indifferently on consolidated and unconsolidated samples

facies architecture of a continental below wave base volcaniclastic basin the ohanapecosh formation ancestral cascades arc washington usa the 800 m thick oligocene ohanapecosh formation records voluminous sedimentation of volcanic clasts in the ancestral cascades arc washington state usa most volcaniclastic beds are dominated by angular pumice clasts and fiamme of andesitic composition now entirely devitrified and altered all beds are laterally continuous and have uniform thickness fine sandstone and mudstone beds have features typical of deposits from low density turbidity currents and suspension settling erosion surfaces cross beds and evidence of bi directional oscillatory currents i e wave ripples and swaley and hummocky cross stratification are almost entirely absent we infer that the setting was subaqueous and below wave base the abundance of angular pumice clasts crystals and dense volcanic clasts and the extreme thickness of several facies suggest they were derived from magmatic volatile driven explosive eruptions the extremely thick beds are ungraded or weakly graded and lack evidence of hot emplacement suggesting deposition from subaqueous water supported high concentration volcaniclastic density currents some of the thickest beds contain coarse rounded dense clasts at their base and are interbedded with accretionary lapilli bearing mudstone these beds are interpreted to be deposits from subaqueous density currents fed by subaerial pyroclastic flows that crossed the shoreline shallow basaltic intrusions and mafic volcanic breccia composed of scoria lapilli indicate the presence of intra basinal scoria cones that may have been partly subaerial the range in facies in the ohanapecosh formation is typical of below wave base continental lacustrine basins that form in proximity to active volcanic arcs and includes eruption fed and resedimented facies extreme instantaneous aggradation rates are related directly to explosive eruptions and sediment pathways reflect the locations of active volcanoes in contrast to conventional sedimentation processes acting in non volcanic environments

stratigraphy of the kos plateau tuff product of a major quaternary explosive rhyolitic eruption in the eastern aegean greece the kos plateau tuff kpt erupted during a moderate volume explosive rhyolitic event approximately 161 ka from a source south of kos in the eastern aegean sea six major stratigraphic units have been identified from a at the base to f uppermost unit a is a widespread vitric ash fall layer that is thickest 1 5 m and most extensive southeast of the source unit b is a 1 to 2 m thick low angle cross stratified armoured pumice lapilli and ash layer found on kos unit c resembles unit b but includes a greater abundance of lithic lapilli less fine ash is only diffusely stratified and is on kos and west of the source unit d includes a sequence of three non welded 1 to 20 m thick ignimbrites that extend radially 38 km from the source in areas of low topography unit e is a sequence of two non welded 3 to 8 m thick ignimbrites which occur radially from the vent regardless of topography 64 km from source unit f has a 6 m thick basal low angle cross stratified armoured pumice lapilli and ash part probably deposited radially from source the upper part of unit f is a widespread 1 m thick vitric ash fall layer found to at least 50 km from the source these six units represent a change in eruptive conditions from initial and final phreatomagmatic activity depositing fallout and internally stratified pyroclastic density current deposits to dry explosive during the more intense phases of the eruption which generated ignimbrites

grain size distribution of volcaniclastic rocks 1 a new technique based on functional stereology the power of explosive volcanic eruptions is reflected in the grain size distribution and dispersal of their pyroclastic deposits grain size also forms part of lithofacies characteristics that are necessary to determine transport and depositional mechanisms responsible for producing pyroclastic deposits however the common process of welding and rock lithification prevents quantification of grain size by traditional sieving methods for deposits in the rock record here we show that functional stereology can be used to obtain actual 3d volume fractions of clast populations from 2d cross sectional images tests made on artificially consolidated rocks demonstrate successful correlations with traditional sieving method we show that the true grain size distribution is finer grained than its representation on a random 2d section our method allows the original size of vesicular pumice clasts to be estimated from their compacted shapes we anticipate that the original grain size distribution of welded ignimbrites can also be characterized by this method our method using functional stereology can be universally applied to any type of consolidated weakly to non deformed clastic material regardless of grain size or age and therefore has a wide application in geology

3d particle size distributions from 2d observations stereology for natural applications abstract we have developed a general formulation for stereological analysis of particle distributions which is applicable to any particle size or size distribution not limited to log normal unimodal etc we have applied numerical techniques to define intersection probability distributions for any shapes previously only known for spheres and quantified the errors involved in using spherical coefficients for various non spherical particles this stereological technique is based on knowing the probability distribution of random cross sections through various particles so that small circle cross sections can be subtracted from an observed population to provide the 3d size distribution of particles the results indicate that the most important parameter controlling calculated size distribution is particle aspect ratio for a distribution of particles with a specific aspect ratio or range of aspect ratios variations of particle form spherical vs cubic rectangular vs elliptical do not alter the results so the technique can be applied to a range of particle shapes applications can be made in a petrology volcanology and other fields only a few of which can also be treated using expensive x ray techniques

dynamics and deposits generated by the kos plateau tuff eruption controls of basal particle loss on pyroclastic flow transport the explosive eruption of voluminous silicic magmas often produces widespread and massive deposits formed from pyroclastic density currents while these punctuated events dramatically alter the landscape and have potential climate altering impact our understanding of the internal structure and transport dynamics of these eruptions is hampered by a lack of direct observations we utilize the natural boundary conditions encountered by the eruption of the kos plateau tuff to probe its internal structure as well as constrain the neotectonic activity in the region and eruption duration of this moderate to large 60 km3 event at the time of the eruption 161 ka the lower sea level in the mediterranean may have resulted in flows that traversed mostly land to the north of the eruptive vent while flows to the south may have encountered an expanse of water steep topography and overwater transport can impede the transport of the dense basal portions of the flow where particles make multiple or sustained contact with the bed we use an eulerian eulerian lagrangian computational approach coupled with overwater and overland boundary conditions including topography to determine the role of bed load versus suspended load in the transport of these flows we find that a ring vent structure and eruptive fluxes greater than 2 106 m3 s are required for the spatial distribution of the kpt the maximum grain size and deposit locations of the first voluminous ignimbrite unit d can be explained by suspended flow to the south consistent with overwater transport and bed load and suspended load transport to the north consistent with overland transport however the maximum lithic size for the largest and last ignimbrite unit e requires some bed load transport in all directions we propose that the boundary conditions were significantly altered during the course of the eruption through either the in filling of a shallow sea to the south or the development of a thick pumice raft to aid saltation on the basis of the inferred eruptive flux we estimate that the duration of the eruption climax in which most of the material was erupted likely only lasted from a few hours to a day

sequential fragmentation transport theory pyroclast size density relationships and the emplacement dynamics of pyroclastic density currents a case study on the mt st helens usa 1980 eruption abstract pyroclastic density currents pdcs are the most dangerous hazard associated with explosive volcanic eruptions despite recent advancements in the general understanding of pdc dynamics limited direct observation and or outcrop scarcity often hinder the interpretation of specific transport and depositional processes at many volcanoes this study explores the potential of sequential fragmentation transport theory sft cf wohletz et al 1989 a modeling method capable of predicting particle mass distributions based on the physical principles of fragmentation and transport to retrieve the transport and depositional dynamics of well characterized pdcs from the size and density distributions of individual components within the deposits the extensive vertical and lateral exposures through the may 18th 1980 pdc deposits at mt st helens msh provide constraints on pdc regimes and flow boundary conditions at specific locations across the depositional area application to msh deposits suggests that sft parameter distributions can be effectively used to characterize flow boundary conditions and emplacement processes for a variety of pdc lithofacies and deposit locations results demonstrate that 1 the sft approach reflects particle fragmentation and transport mechanisms regardless of variations in initial component distributions consistent with results from previous studies 2 sft analysis reveals changes in particle characteristics that are not directly observable in grain size and fabric data and 3 sft parameters are more sensitive to regional transport conditions than local outcrop scale depositional processes the particle processing trends produced using sft analysis are consistent with the degree of particle processing inferred from lithofacies architectures for all lithofacies examined in this study suspension sedimentation products exhibit much better processing than concentrated current deposits integrated field observations and sft results provide evidence for increasing density segregation within the depositional region of the currents away from source as well as for comparable density segregation processes acting on lithic concentrations and pumice lenses within the current these findings further define and reinforce the capability of sft analysis to complement more conventional pdc study methods significantly expanding the information gained regarding flow dynamics finally this case study demonstrates that the sft methodology has the potential to constrain regional flow conditions at volcanoes where outcrop exposures are limited

influence of convective sedimentation on the formation of widespread tephra fall layers in the deep sea the formation of widespread volcanic ash fall layers in deep sea sediments was investigated experimentally to examine the settling behavior of tephra 20 180 m diameter as it travels from the atmosphere into water using a fallout mass flux rate that was constrained by measurements of distal fallout from the 1980 eruption of mount st helens 0 2 g cm 2 hr the experiments show that particle settling in the water column is dramatically accelerated by the formation of diffuse vertical gravity currents the currents form as a result of convective instabilities that develop in a surface boundary layer when the local particle concentration becomes large at the air water interface the settling velocity of particles drops abruptly and the concentration of particles increases because of the differential in mass flux above and below the fluid surface the implication of the experiments is that deposition of distal tephra fall layers in deep sea sediments may be dominantly controlled by diffuse vertical gravity currents as opposed to passive settling of individual particles through the water column this process greatly reduces the residence time of fine ash in the ocean and diminishes the role of ocean currents in influencing the distribution patterns of individual tephra layers support for this mechanism comes from observations of greatly accelerated tephra settling rates in the south china sea following the 1991 eruption of mount pinatubo in the philippines

total grain size distribution and volume of tephra fall deposits on 17 june 1996 ruapehu volcano new zealand produced a sustained andesitic sub plinian eruption which generated a narrow tephra fall deposit extending more than 200 km from the volcano the extremely detailed data set from this eruption allowed methods for the determination of total grain size distribution and volume of tephra fall deposits to be critically investigated calculated total grain size distributions of tephra fall deposits depend strongly on the method used and on the availability of data across the entire dispersal area the voronoi tessellation method was tested for the ruapehu deposit and gave the best results when applied to a data set extending out to isomass values of 1 g m 2 the total grain size distribution of a deposit is also strongly influenced by the very proximal samples and this can be shown by artificially constructing subsets from the ruapehu database unless the available data set is large all existing techniques for calculations of total grain size distribution give only apparent distributions the tephra fall deposit from ruapehu does not show a simple exponential thinning but can be approximated well by at least three straight line segments or by a power law fit on semi log plots of thickness vs area 1 2 integrations of both fits give similar volumes of about 4 106 m3 integration of at least three exponential segments and of a power law fit with at least ten isopach contours available can be considered as a good estimate of the actual volume of tephra fall integrations of smaller data sets are more problematic

detailed multidisciplinary monitoring reveals pre and co eruptive signals at nyamulagira volcano north kivu democratic republic of congo this paper presents a thorough description of nyamulagira s january 2010 volcanic eruption north kivu democratic republic of congo based on a combination of field observation and ground based and space borne data it is the first eruption in the virunga volcanic province that has been described by a combination of several modern monitoring techniques the 2010 eruption lasted 26 days and emitted 45 5 106 m3 of lava field observations divided the event into four eruptive stages delimited by major changes in effusive activity these stages are consistent with those described by pouclet 1976 for historical eruptions of nyamulagira co eruptive signals from ground deformation seismicity so2 emission and thermal flux correlate with the eruptive stages unambiguous pre eruptive ground deformation was observed 3 weeks before the lava outburst coinciding with a small but clear increase in the short period seismicity and so2 emission the 3 weeks of precursors contrasts with the only precursory signal previously recognized in the virunga volcanic province the short term increase of tremor and long period seismicity which for example were only detected less than 2 h prior to the 2010 eruption the present paper is the most detailed picture of a typical flank eruption of this volcano it provides valuable tools for re examining former mostly qualitative descriptions of historical nyamulagira eruptions that occurred during the colonial period

insights on tephra settling velocity from morphological observations in this study we present a systematic and detailed morphological characterization of tephra particles from different eruptions fontana lapilli masaya nicaragua keanak ko i formation kilauea usa recent dome explosions of soufriere hills volcano montserrat and the calculation of their terminal fall velocity tfv as obtained based on different drag prediction models i e wilson and huang 1979 haider and levenspiel 1989 ganser 1993 and dellino et al 2005 in particular particle sphericity and therefore particle surface area is essential for the calculation of tfv of irregular shape particles but is of complex determination various attempts have been proposed according to our results 2d morphological characterization of volcanic particles is a fast and simple application for a wide range of particle size and provides consistent sphericity and settling velocity values 3d scanning also provides a promising tool for lapilli sized tephra 2 cm in contrast gas adsorption derived surface area is not suitable for the calculation of tfv of volcanic particles mostly because it mainly describes the surface contribution of nanometric pores that are not expected to affect significantly tfv and because bulk sample analysis is representative of neither individual particles nor of the whole particle population settling velocities calculated using values of surface area derived from gas adsorption analyses are up to two orders of magnitude lower than the values obtained through 2d analysis in addition our results also show how the influence of particle shape on tfv increases with particle size in particular calculated tfv converges at small particle sizes 3 regardless of the model applied suggesting that the spherical assumption is appropriate for this size fraction discrepancies with the spherical model are within 10 discrepancies with the spherical model increase with particle size up to about 50 and depend on the choice of both the tfv model and the morphological parameterization used in particular the drag prediction model of ganser 1993 is sensitive to the effect of particle morphology on tfv and is well suited for all sizes and reynolds numbers of typical tephra particles finally our results show how individual size categories whole and half are not associated with individual tfv values but with a range of values which increases with class size nonetheless the half system is associated with a smaller standard deviation than the whole system and is therefore more appropriate for the modeling of tephra dispersal in any case for dispersal modeling purposes it is more appropriate to indicate a range of settling velocities for each size class rather than giving an average value

vertical density currents a review of their potential role in the deposition and interpretation of deep sea ash layers marine tephra layers form important chronostratigraphic markers and may also provide information on eruption dynamics and chronology recent experiments and field data from the 1991 pinatubo eruption challenge tacit assumptions that following atmospheric fallout on the ocean surface marine sedimentation of ash grade material occurs by stokes law settling of individual grains laboratory experiments have demonstrated that vertical density currents can be initiated at representative mass flux rates further experiments described herein involving sustained particle fluxes entering stratified environments demonstrate that ash particles can decouple from the transporting fluid whose vertical motions are limited by the stable density gradient in the ocean vertical density currents generated by ash loading and gravitational destabilization of the water column can overcome the strong stable density gradients in the ocean and transport ash particles vertically 1 3 orders of magnitude faster than possible by stokesian settling greatly reducing their residence time sea bed sedimentation is inferred to involve settling of individual coarse or dense grains through the entire water column plus suspension fallout from the polydisperse turbid layer delivered by convective plumes yielding normal distribution grading the turbid layer may continue laterally as a nepheloid current under the influence of bottom slopes and or geostrophic forces either mechanism possibly modifying the grain size distribution of the suspension and hence the resulting ash layer by transporting selected fractions into different depositional locations consequently and regardless of any subsequent reworking deep sea ash isopachs may not directly reflect an eruptive signal and the ash layers may not directly record the order rate or location of arrival of ash particles at the ocean surface

textural studies of vesicles in volcanic rocks an integrated methodology abstract vesicles in volcanic rocks are frozen records of degassing processes in magmas for this reason their sizes spatial arrangements numbers and shapes can be linked to physical processes that drive magma ascent and eruption although numerous techniques have been derived to describe vesicle textures there is no standard approach for collecting analyzing and interpreting vesicular samples here we describe a methodology for techniques that encompass the entire data acquisition process from sample collection to quantitative analysis of vesicle size and number carefully chosen samples from the lower mean and higher density vesicularity endmembers are characterized using image nesting strategies we show that the texture of even microvesicular samples can be fully described using less than 20 images acquired at several magnifications to cover efficiently the range of existing vesicle sizes a new program foams was designed to perform the quantification stage from vesicle measurement to distribution plots altogether this approach allows substantial reduction of image acquisition and processing time while preserving enough user control to ensure the validity of obtained results we present three cameo investigations on basaltic lava flows scoria deposits and pumice layers to show that this methodology can be used to quantify a wide range of vesicle textures which preserve information on a wide range of eruptive conditions

hydrodynamic behaviour of taupo 1800a pumice implications for the sedimentology of remobilized pyroclasts large scale explosive eruptions from silicic caldera volcanoes can generate huge volumes of pyroclastic material in terrestrial and marine environments on land erosion remobilization and redeposition of this debris is predominantly carried out by running water in the form of precipitation run off conversely in the submarine realm both primary emplacement and subsequent remobilization are influenced by the presence of water as a transporting medium despite this and the number of studies devoted to volcaniclastic sedimentation relatively little attention has been paid to the hydrodynamic behaviour of the particles themselves which ought to underpin any assessment of transport or depositional process this is crucial as many volcanic particles exhibit variable density according to composition and as functions of differing degrees of vesiculation and the extent to which pore space is filled by water and or gaseous phases during transport and deposition investigation of the physical and hydrodynamic properties of taupo 1800a pumice with reference to sedimentary facies developed during the eruption aftermath shows that although buoyant when dry when sufficiently waterlogged cool pumice clasts will sink and behave more like quartzo feldspathic material saturation is apparently achieved by a combination of rapid capillary flooding of large interconnected vesicles and slower diffusional air water exchange in smaller pores low saturated pumice densities result in lower settling velocities and easier entrainment by tractional currents than those for equivalent sized quartzo feldspathic or crystal lithic particles fine grained pumice is conversely harder to entrain because of the frictional interlocking of angular particles these unusual properties of temporary buoyancy variable saturation low density and size dependent cohesion complicate interpretations of the depositional setting and energy of pumiceous sediments and give rise to several unique facies these findings have implications not only for the analysis of remobilized pyroclastic facies in terrestrial and marine environments but also for primary depositional processes during subaqueous explosive volcanism

using hydraulic equivalences to discriminate transport processes of volcanic flows we characterized stratified deposits from the upper toluca pumice at toluca volcano mexico to distinguish the various modes of transport at play in their genesis using the concept of hydraulic equivalence we determined that deposits resulted from a combination of suspended load fallout saltation and rolling in particular some well sorted coarse stratified beds have a single pumice mode most likely indicative of clasts having traveled through both the transport system and the traction bed such beds are likely remnants of the sorting operated within the large scale transport system other coarse beds have pumice and lithic modes suggesting rolling in the traction bed we propose that boundary layer processes control the sorting of those beds and all finer beds by helping to discriminate between transport mechanisms hydraulic equivalences have a general applicability in geophysical flows involving clasts of contrasted densities

pliocene mass failure deposits mistaken as submarine tsunami backwash sediments an example from hornitos northern chile abstract in this study we question the former interpretation of a shallow marine backwash tsunami origin of a conspicuous pliocene coarse clastic unit at hornitos northern chile and instead argue for a debris flow origin for this unit we exclude a relation to a tsunami in general and to the eltanin impact in particular the observed deposit at hornitos was not generated either directly impact triggered tsunami or indirectly submarine mass flow caused by seismic shaking by an impact re calculation of the alleged impact tsunami including consideration of the van dorn effect shows that an impact in the southern ocean did not cause a significant tsunami at hornitos impact related seismic shaking was not able to trigger slides several thousands of kilometers away because the eltanin event was a deep sea impact that did not create a crater additionally the biostratigraphic age of 5 1 2 8 ma of the associated la portada formation is not concurrent with the newly established age of 2 511 0 07 ma for the eltanin impact instead we argue for an origin of the conspicuous unit at hornitos as a debris flow deposit caused by an earthquake in the andean subduction zone in northern chile our re interpretation considers the local synsedimentary tectonic background a comparison to recent submarine tsunami sediments and recent examples of mass wasting deposits along the chilean margin the increased uplift during the pliocene caused oversteepening of the coastal scarp and entailed a contemporaneous higher frequency of seismic events that triggered slope failures and cliff collapses the coarse clastic unit at hornitos represents an extraordinary potentially tsunami generating mass wasting event that is intercalated with mass wasting deposits on a smaller scale

paleoceanographic reconstruction of the late cretaceous oil shale of the negev israel integration of geochemical and stable isotope records of the organic matter the levantine high productivity system was an extensive coastal upwelling that operated in the late cretaceous along the se tethyan margin this study focuses on the top phosphate member of the mishash formation and the oil shale member osm of the ghareb formation latest campanian early maastrichtian which represent the last phase of this high productivity system in the negev israel bulk organic matter toc 13corg 15norg c n and pristane phytane pr ph ratios were studied for reconstruction of seafloor and water column depositional environments our records indicate a gradual decrease with time in surface water productivity in the osm and a marked weakening at the overlying marl member high c n ratio along with relatively low 15norg 4 to 6 and 13corg 29 to 28 probably reflect significant diagenetic preferential loss of nitrogen rich organic compounds enriched with 15n and 13c isotopes e g proteins this along with the low pr ph values 0 11 0 7 indicate oxygen depleted bottom water anoxia dysoxia during the deposition of the top phosphate member and the osm the moderate gradual upward increase in 15norg and in pr ph values and the decrease of toc and c n values from the top phosphate member through the osm indicate transition from anoxic phosphate lower osm to dysoxic middle upper osm conditions this environmental trend is consistent with co occurring foraminiferal assemblages in the studied succession and implies that the benthic species in the negev sequence were adapted to persistent minimum oxygen conditions by performing complete denitrification as recently found in many modern benthic foraminifera

entrance of hot pyroclastic flows into the sea experimental observations the entrance of hot pyroclastic flows into water has been observed in a series of experiments in which shooting granular flows of hot ignimbrite ash 403 c of bulk density near that of water run down a smooth chute and enter a water filled tank at an angle of 26 flows of relatively cool ash 250 c mixing across the water surface over some distance from shore generates steam explosions forming fountains of wet and dry ash and convectively rising fine ash plumes steam explosions increase in strength and lateral extent towards higher ash temperatures and mass fluxes the explosions generate water waves that remain driven by massive fountain fallout across some distance from shore the ash fountains feed pyroclastic surges which advance down tank over water at high speeds underwater plumes of sediment falling from the ash fountains and surges drop coarse load onto the floor whereas the finer load forms a turbidity current high temperature and poor size sorting of pyroclastic flows are key parameters that determine the processes of interaction with water and the associated hazards poor sorting allows for the formation of ash cloud surges moving over water even from cool flows high temperatures cause littoral explosions and facilitate extensive mass transport over water all flows generate tsunami waves by different mechanisms