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

volcaniclastic sediments process interaction and depositional setting of the mio pliocene miura group se japan abstract the mid miocene to early pliocene 14 3 ma miura group exposed on the miura and boso peninsulas around tokyo bay in southern honshu comprises at least a 2 km thick succession of volcaniclastic sediments these accumulated in a forearc basin or basins on the eastern side of a proto izu bonin arc prior to and during accretion of the miura block onto the honshu arc and its subsequent dextral transcurrent eduction the basin fill shows a complex admixture of facies that result from multiple processes which operated on a relatively shallow slope basin the main facies groups include dark coloured mostly coarse grained scoriaceous beds pale coloured muddy silty pumiceous bioturbated sediment yellowish and whitish coloured tuffaceous horizons and chaotic slump debris flow and injection units many of the facies and beds in the older misaki formation can be readily interpreted as the result of a direct pyroclastic fall through the air and water b downslope resedimentation via turbidity currents and related processes in some cases derived from subaqueous base surge pyroclastic flow or vertical density currents c hemipelagic settling commonly under the influence of thermohaline bottom currents in other cases composite beds are believed to result from the interaction of these processes in more complex events the younger hasse formation is dominated by more shallow water marine currents and storm events and in parts by fluvial or proximal fan delta processes the distribution of these different facies illustrates the evolution and shallowing of the miura boso forearc basins during accretion and emplacement

volcanic ash clusters tephra rafts and scavengers abstract much of the volcanic ash deposited in the vicinity of pullman washington from the may 18th eruption of mt st helens in 1980 fell in the form of composite ash clusters 0 25 to 0 5 mm in diameter particles in the clusters ranged from sub micron to more than 40 microns in size the porous clusters rafted large ash particles great distances and scavenged particles of all sizes as the wind blew them eastward the result is poorly sorted ash deposits as far as 644 km east missoula within individual clusters textural relationships suggest that both mechanical interlocking and electrostatic attraction played a role in cluster formation the clusters are very fragile and may never be preserved in the stratigraphic record but it is suggested that this kind of ash transport may well have occurred in the geologic past documentation of this ash cluster fall will promote a better understanding of the physical aspects of the downwind distribution of fine grained volcanic ash the poor size sorting in some ancient ash deposits may thus be explained

on the interaction between convection and crystallization in cooling magma chambers abstract we investigate the interaction of thermal convection and crystallization in large aspect ratio magma chambers because nucleation requires a finite amount of undercooling crystallization is not instantaneous for typical values of the rates of nucleation and crystal growth the characteristic time scale of crystallization is about 10 3 10 4 s roof convection is characterized by the quasi periodic formation and instability of a cold boundary layer its characteristic time scale depends on viscosity and ranges from about 10 2 s for basaltic magmas to about 10 7 s for granitic magmas hence depending on magma viscosity convective instability occurs at different stages of crystallization a single non dimensional number is defined to characterize the different modes of interaction between convection and crystallization using realistic functions for the rates of nucleation and crystal growth we integrate numerically the heat equation until the onset of convective instability we determine both temperature and crystal content in the thermal boundary layer crystallization leads to a dramatic increase of viscosity which acts to stabilize part of the boundary layer against instability we compute the effective temperature contrast driving thermal convection and show that it varies as a function of magma viscosity and hence composition in magmas with viscosities higher than 10 5 poise the temperature contrast driving convection is very small hence thermal convection is weak in low viscosity magmas convective breakdown occurs before the completion of crystallization and involves partially crystallized magma the convective regime is thus characterized by descending crystal bearing plumes and bottom crystallization proceeds both by in situ nucleation and deposition from the plumes we suggest that this is the origin of intermittent layering a form of rhythmic layering described in the skaergaard and other complexes we show that this regime occurs in basic magmas only at temperatures close to the liquidus and never occurs in viscous magmas this may explain why intermittent layering is observed only in a few specific cases

geologic stereo mapping of geologic structures with spot satellite data geologic note 1 the spot imaging system launched in 1986 by the french national space agency cnes provides two significant advances for mapping geological structures from satellite imagery increased spatial resolution of key structural and stratigraphic elements and stereoscopic capabilities for making strike and dip measurements quantitative stereo mapping with spot data may be done using hard copy images and conventional equipment such as stereo plotters or by displaying images on a video monitor in stereo viewing format the latter method is a new remote sensing technique particularly relevant to the exploration community this technique allows development with minor expenditure and training of an effective stereo mapping capability on existing image processing systems or o stand alone personal computers spot stereo data is particularly useful for mapping low amplitude and highly deformed structures surface structure maps generated from stereo imagery data can be applied to exploration at two levels at early stages such maps can be used to establish the style trend size and spatial distribution of structures in a basin and to investigate key tectonic elements at more advanced prospecting stages these maps can be used to constrain subsurface mapping of structures located in areas of poor seismic data quality this latter technique however may require the support of higher resolution stereo aerial photography

early diagenesis impact of organic matter on mass physical properties and processes california continental margin abstract high resolution scales of 2 5 mm to submicrometer interdisciplinary studies of california continental slope sediments 0 80 cm reveal the importance of marine organic material and mineralogy on mass physical properties in fine grained sediments during very early diagenesis the combination of organic material 2 wt with smectite 5 is associated with high depositional porosities low effective stresses low porosity reduction and dewatering and the impedance of interstitial fluid flow living microbiota and nonliving mucopolysaccharides organic matter affecting the physical properties was revealed by transmission electron microscopy and carbon isotope studies calculations using the redfield ratio show that sediments containing high organic carbon 5 wt have hydrated organic material in the pore spaces that can occupy a volume that is up to 20 of the total sediment porosity pore water chemistry and x ray diffraction show evidence of early diagenetic processes that may significantly influence sediment fabric an important geochemical process near the sediment water interface that affects sediment fabric is the rapid dissolution of biogenic opal

fallout of volcanic ash to the deep south china sea induced by the 1991 eruption of mount pinatubo philippines an ongoing sediment trap experiment in the central south china sea provided the opportunity to investigate directly the mode and rates of a medial fallout of volcanic ash following the june 15 1991 eruption of mount pinatubo philippines within less than three days after the release of the major eruption plume ash accumulation 9 kg m 2 was simultaneously recorded by traps positioned in 1190 and 3730 m water depth at a distance of 586 km to the west of the volcano a numerical simulation of the fallout reveals that the vertical trajectories of the pyroclasts in both the atmosphere and ocean were controlled by particle aggregation the aggregation process caused a premature subaerial fallout of fine grained ash after crossing the air sea interface vertical settling of the ash clusters was enhanced by absorption of water leading to settling rates of more than 1670 m d aggregates were observed at all depths and their rapid settling is reflected in identical pyroclast spectra in intermediate and deep water this implies that particle sorting must have been complete in the upper water column and that the fallout was not perturbed by oceanic currents the tephra blankets an area of 37 10 4 km 2 in a fan shaped westward distribution which reflects the prevailing direction of the upper level winds our data show that atmospheric tephra dispersal patterns are recorded almost instantaneously on the deep sea floor and corroborate the key role of deep sea ash layers as highly reliable chronostratigraphic markers and as a geological tool to infer ancient wind fields

the fall dump a new perspective on the role of a shade flora in the annual cycle of diatom production and export flux investigations of diatom fluxes recorded in laminated sediments using scanning electron microscope techniques together with evidence from sediment trap studies have contributed to a reappraisal of the annual cycle of diatom production and export flux we propose that where there is a strong seasonal thermocline and nutricline a number of diatoms hitherto regarded as typical sparse summer flora characteristic of oligotrophic waters are able to generate substantial production at depth these species including rhizosolenia spp stephanopyxis palmeriana thalassiothrix spp and some coscinodiscus spp may represent a shade flora that have adapted to grow in low light conditions and or to regulate their buoyancy to move between a deep nutrient source and the euphotic zone although rates of growth and primary production are substantially lower than species characteristic of spring bloom or upwelling conditions the total primary production integrated over the several month period of summer stratification may be as significant as the spring bloom or greater the term fall or autumn bloom as a counterpart of the spring bloom is therefore a misnomer whereas the spring bloom involves a rapid burst of reproduction and sedimentation the fall dump is the sedimentation of a long lived episode of production lasting the duration of the seasonal thermocline and triggered by the fall winter mixing that breaks down stratification the fall dump may produce as much or in some cases more export production than the spring bloom the results of this study suggest that a reorientation of thinking on diatom ecology and palaeoecology may be required

computer simulation of transport and deposition of the campanian y 5 ash analyses of grain size and modal composition of the campanian tuff ash layer y 5 from 11 deep sea cores have been carried out this layer represents ash fall that has been correlated with the 38 000 y b p campanian ignimbrite thunell et al 1979 a deposit formed by the largest eruption documented in the mediterranean region during the late pleistocene barberi et al 1978 the bulk deposit is bimodal in grain size and dominated by glass shards the calculated mean grain size of the coarse mode of the individual size distributions decreases with distance from the source and progressively approaches a near constant fine mode of approximately 13 microns distal samples are unimodal in grain size these data combined with a set of vertical profiles of wind 10 year average have been used as input to a computer model that simulates fallout of tephra modelling indicates that the downwind variation of grain size of the coarse mode can be accurately reproduced with transport of ash between 5 and 35 km the observed fine mode of the deposit cannot however be generated by transport of ash as individual particles at these elevations such transport would result in deposition of virtually all of the fine ash beyond the studied area deposition of fine ash within the studied distance of 1600 km from source can only occur by fallout as particle aggregates from a high eruption plume or as individual particles from co ignimbrite ash clouds with a maximum elevation of 3 km the large volume of ash in the fine mode 70 wt and the irregularity in azimuth of low level winds argue against major low level transport of co ignimbrite ash rather the ash may have been derived from both a plinian eruption column and high altitude clouds of co ignimbrite ash with settling of fine ash as particle aggregates

crystal capture sorting and retention in convecting magma the mystery of producing strong compositional diversity among suites of comagmatic igneous rocks is investigated by considering the dynamic evolution of basaltic magma in a sheet like chamber a central conclusion is that inward progressing crystallization produces strong viscosity and temperature gradients that promote convection only near the leading edge of the upper thermal boundary layer convection is apparently confined to an essentially isoviscous isothermal region that hugs the downward growing roof zone strong changes in viscosity with crystallization divide the upper and lower thermal boundary layers into regions of decreasing viscosity and crystallinity n called rigid crust n 0 5 mush 0 5 n 0 25 and suspension n 0 25 the strong increase in viscosity near the mush suspension interface acts as a capture front that overtakes and traps slowly settling crystals initial phenocrysts mostly escape capture but crystals nucleated and grown in the suspension zone can escape only if the capture front slows to a critical rate attainable only in bodies thicker than about 100 m escaping crystals are redistributed and sorted by convection driven by the advance of the capture front itself crystal laden plumes traverse the central hot core of the body and deposit partially resorbed and sorted crystals within the lower suspension zone convection is never vigorous but is part of an overall intimate balance between roofward heat loss rigid crust growth crystallization kinetics and transport and sorting of sinking escaped crystals there is a strong similarity between these processes and those producing both varves and saline pan deposits it is clear that lavas lava lakes and sills are indeed examples of true magma chambers strongly exhibiting certain aspects of this over all process these aspects commonly also characterize the large mafic magmatic bodies because strong compositional changes in the residual melt occur largely outward that is at lower temperatures and higher crystallinities of the capture front which is immobile and mostly within rigid crust the possible range in comagmatic compositions available for eruption anywhere within the active magma is very limited this is in broad agreement with the compositional range observed in basaltic lava lakes sills and plutons like skaergaard the tuning of convection crystallization kinetics and phase equilibria in chambers of this type can produce a variety of textures and layering but not a diversity of compositions

deep sea benthic foraminiferal recolonization of the 1991 mt pinatubo ash layer in the south china sea a census count of rose bengal stained benthic foraminifera from the surface area on top of a 2 to 6 cm thick ashfall layer at three deep water stations along the western margin of the philippines exhibits a unique assemblage composition of benthic foraminifera the total number of benthic foraminifera is low and the ratio of living individuals to empty tests is high specific diversity is low with a significant dominance of infaunal morphotypes including species of the genus reophax r scorpiurus r bilocularis and r dentaliniformis which are regarded as successful recolonizers assemblages below the ash layers are diverse and contain many epifaunal suspension feeding agglutinated and calcareous foraminifera the 1991 mt pinatubo eruption caused mass mortality of benthic foraminifera in a vast area of the eastern south china sea followed by step wise recolonization of the ash substrate three years after the eruption the benthic foraminiferal community structure is still far from recovery to background levels

sedimentation of lithogenic particles in the deep ocean abstract investigation of lithogenic particles collected by sediment traps in open ocean stations revealed that the sediment flux increased linearly with depth in the water column this rate of increase decreased with distance of the station from the continent it was largest at the panama basin station and almost negligible at the e hawaii abyssal plain station at the panama basin station smectite flux increased with depth we suggest that smectite resuspended from bottom sediments of the continental slope west of the sediment trap station is advected by easterly deep currents and the suspended particles are then possibly entrapped by large settling particles on the other hand the flux of hemipelagic clay particles kaolinite and chlorite was nearly constant at all depths this can be explained by incorporation of these particles in fecal pellets which then settle from the surface water at the demerara abyssal basin station flux of illite and chlorite particles increased with depth and the flux of smectite was constant a sudden increase of the flux of illite and chlorite was observed near the bottom traps at the sohm abyssal plain station the flux of quartz and feldspar was 10 to 15 of the clay flux

sedimentology of the minoan deep sea tephra layer in the aegean and eastern mediterranean the minoan eruption of santorini resulted in deposition of pyroclastic material over a large area of the aegean sea and eastern mediterranean the eruptive activity commenced with a plinian phase which was followed by a phreatomagmatic phase and ended with numerous pyroclastic flows we present a sedimentological study of the minoan ash deposits found in cores taken throughout the region the cores are divided into three groups 1 those from the sub marine slopes immediately surrounding santorini 2 those from the aegean sea where the topography consists of many steep sided basins and 3 those from the eastern mediterranean where the topography is relatively flat and smooth this last group is designated the distal ash zone the deposits from the aegean sea have been remobilised as turbidity currents grain size distribution analysis reveals that these deposits are bimodal the mdo of the coarse population increasing away from source whilst that of the fine population remains uniform irrespective of distance from source the bimodality is caused by a combination of aggregation of fine particles in ash clouds produced during the last two phases of the eruptive activity and mixing of the coarse plinian and fine phreatomagmatic and co ignimbrite deposits within the turbidity current an unusual feature found in two of the cores is a coarse density sorted layer beneath the bouma a division of several turbidite units this is interpreted as an example of a flow head deposit as described by sparks and wilson 1983 the deposits from the distal ash fall zone are fine grained and unimodal containing phreatomagmatic and co ignimbrite material they show only very minor remobilisation of the original air fall ash due to the flat topography of the area the mdo of the deposit remains uniform irrespective of distance from source and is very similar to that of the fine population found in the bimodal aegean deposits this is interpreted as the result of aggregation of fines in the phreatomagmatic and co ignimbrite ash clouds the deposits in both areas have been extensively bioturbated a study of dispersed ash in the cores to reconstruct the original thickness of the ash layers suggests that ash layers less than 0 62 cm thick will be completely dispersed in the surrounding sediment whilst those thicker than 2 5 cm will be preserved as discrete layers the base being unaffected by bioturbation the deposits on the submarine slopes of santorini were not originally deposited as air fall ash but are derived from the erosion of coastal exposures of non welded ignimbrite they form storm sand layers and are typical of the type of deposit that may be formed by rapid erosion of fresh pyroclastic material after the eruption of an island arc volcano

a reappraisal of mount st helens ash clusters depositional model from experimental observation abstract ash clusters depositing the distal ash of the may 18 1980 eruption of mount st helens are supposed to be generated due to electrostatic forces within the eruption cloud therefore an experimental attempt was made to test electrostatic ash clustering which produced fallout with artificial ash clusters in the laboratory its grain size characteristics were similar to the mount st helens ash proximal fallout was coarse grained but showed a fine tail on the distribution curve medial ash was bimodal with the distribution minimum at 45 63 m distal ash was fine grained 63 m and cluster modifying ash 63 m clustering and subsequent deposition of two different typs of ash aggregates explain the grain size characteristics of the mount st helens and the experimental fallout ash proximal fallout was deposited as single grain clusters composed of a large particle 63 m with adhering fines settling at the grain specific rates of the largest particle distal fallout settled as fine ash clusters made up of ash 90 wt of which were

chemical composition distribution and origin of silicic volcanic ash layers in the greenland iceland norwegian sea explosive volcanism from 10 to 300 ka as recorded in deep sea sediments explosive ocean island volcanism in the greenland iceland norwegian sea gin sea is indicated by marine tephra layers at 10 300 ka peaks of explosive volcanism occurred in oxygen isotope stages 8 7 5 and 1 the depositional age of the tephra was estimated using the oxygen isotope stratigraphy and dating of marine records geochemical analyses of the tephra layers show that all originate from iceland here we report the characteristics of tephra from these major icelandic events in 30 deep sea cores from the gin sea our findings provide constraints on the distribution of tephra from the eruption source for the vedde ash oxygen isotope stage 1 we estimate a minimum fallout area of 2 105 km2 stretching from central greenland in the west and southern sweden in the east to 71 n in the gin sea the magnitude of the eruption and the regional wind conditions controlled the extent and concentrations of these ash fallout events oceanic circulation and differential settling may have affected the distribution and final deposition of ash particles such as bubble wall shards

open ocean convection observations theory and models we review what is known about the convective process in the open ocean in which the properties of large volumes of water are changed by intermittent deep reaching convection triggered by winter storms observational laboratory and modeling studies reveal a fascinating and complex interplay of convective and geostrophic scales the large scale circulation of the ocean and the prevailing meteorology two aspects make ocean convection interesting from a theoretical point of view first the timescales of the convective process in the ocean are sufficiently long that it may be modified by the earth s rotation second the convective process is localized in space so that vertical buoyancy transfer by upright convection can give way to slantwise transfer by baroclinic instability moreover the convective and geostrophic scales are not very disparate from one another detailed observations of the process in the labrador greenland and mediterranean seas are described which were made possible by new observing technology when interpreted in terms of underlying dynamics and theory and the context provided by laboratory and numerical experiments of rotating convection great progress in our description and understanding of the processes at work is being made

geologic stereo mapping of geologic structures with spot satellite data reply the spot imaging system launched in 1986 by the french national space agency cnes provides two significant advances for mapping geological structures from satellite imagery increased spatial resolution of key structural and stratigraphic elements and stereoscopic capabilities for making strike and dip measurements quantitative stereo mapping with spot data may be done using hard copy images and conventional equipment such as stereo plotters or by displaying images on a video monitor in stereo viewing format the latter method is a new remote sensing technique particularly relevant to the exploration community this technique allows development with minor expenditure and training of an effective stereo mapping capability on existing image processing systems or on stand alone personal computers spot stereo data is particularly useful for mapping low amplitude and highly deformed structures surface structure maps generated from stereo imagery data can be applied to exploration at two levels at early stages such maps can be used to establish the style trend size and spatial distribution of structures in a basin and to investigate key tectonic elements at more advanced prospecting stages these maps can be used to constrain subsurface mapping of structures located in areas of poor seismic data quality this latter technique however may require the support of highermore resolution stereo aerial photography less

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

demise of one volcanic zone and birth of another a 12 m y marine record of major rhyolitic eruptions from new zealand ocean drilling program sites 1123 and 1124 provide an unprecedented 12 m y record of major rhyolitic eruptions from the coromandel and taupo volcanic zones of new zealand macroscopic tephras n 197 were dated using magnetostratigraphy supplemented by geochemical correlation with subaerial tephra isothermal plateau fission track ages and orbitally tuned stable isotope data eruptions began in the coromandel volcanic zone ca 12 ma 1 6 1 m y earlier than previously known thereafter volcanism was fairly continuous with a tempo and intensity that increased through the late miocene pliocene and into the quaternary when the taupo volcanic zone formed the transition from the coromandel to the taupo zone previously placed as ca 4 2 ma was seamless without obvious breaks or changes in ash composition this well dated history of long lived and productive volcanism allows for more confident correlation with other circum pacific tephra records thus helping confirm the occurrence of widespread coeval eruptions throughout the region

influence of particle aggregation on deposition of distal tephra from the may 18 1980 eruption of mount st helens volcano the may 18 1980 eruption of mount st helens msh produced an extensive ashfall deposit in washington idaho and montana with a minumum volume of 0 55 km3 tephra an unusual feature of the deposit is the occurrence of a second thickness maximum 325 km ene of msh near ritzville washington grain size and component abundance analysis of samples along the main dispersal axis indicates that ash in this region is very fine grained mean size 22 m poorly sorted polymodal and rich in glass shards and pumice fragments a computer simulation of ash fallout from an atmospherically dispersed eruption plume was developed to evaluate various hypotheses for the origin of the distal ash characteristics particularly the thickness versus distance relationship the model was constrained by observations of the eruption column height elevation of major ash transport lateral spreading of the eruption plume and atmospheric wind structure in the vicinity of msh results of different simulations indicate that the second thickness maximum cannot be attributed to either decreased wind velocities over central washington or injection of fine ash above the horizontal wind velocity maximum near the tropopause for the model to fit the observed characteristics of the deposit significant particle aggregation of ash finer than 63 m must be invoked the best fit occurs when ash less than 63 m is aggregated into particles several hundred microns in diameter with a settling velocity of 0 35 m s support for this process comes from the observation and collection of fragile ash clusters of similar size which fell at pullman washington during the may 18 eruption sorem 1982 the premature fallout of fine ash as particle aggregates is a fundamental process in the origin of the grain size characteristics variations in component abundances and thickness versus distance relationship of the may 18 msh ash fall deposit

generation and dispersal of fine ash and dust by volcanic eruptions abstract the volcanic eruptions which generate the greatest quantities of fine ash and dust are those of ignimbrite forming plinian vulcanian and phreatomagmatic types these are also the eruptions which produce the widest dispersal of this material attributed to the superior height attained by their eruptive columns however much of the fine ash and dust may be rapidly flushed out of the eruptive plume by water particularly in phreatomagmatic eruptions recent studies made on the dispersal and grain size of pyroclastic deposits produced by examples of plinian and phreatomagmatic types have yielded estimates of the quantities of material generated in each grain size class besides the extent of their dispersal not all of the fine volcanic particles are produced by fragmentation at the eruptive vent in ignimbrite eruptions there is good evidence for their large scale generation in and loss from the moving ash flows

bimodal grain size distribution and secondary thickening in air fall ash layers investigation of the ashfall deposit from the 1980 mount st helens eruption reveals that the deposit is bimodal the finer population maintaining a uniform grain size distribution away from source and that the deposit exhibits secondary thickening beyond the 1 cm isopach grain size distribution shows that at least 64 of the deposit is finer than 63 m far more than is generally supposed for plinian eruptions an investigation of other ashfall deposits reveals similar grain size bimodality features which are interpreted as evidence of aggregation of fine particles in the eruption cloud our data suggest that aggregation may be a common process during ash fallout from explosive eruptions and that secondary thickening may often occur providing that other conditions are met

large magnitude middle ordovician volcanic ash falls in north america and europe dimensions emplacement and post emplacement characteristics abstract middle ordovician k bentonites represent some of the largest known fallout ash deposits in the phanerozoic era they cover minimally 2 2 106 km2 in eastern north america and 6 9 105 km2 in northwestern europe and represents the coeval accumulation of plinian and co ignimbrite ash on both laurentia and baltica during the closure of the iapetus ocean the three most widespread beds are the deicke and millbrig k bentonites in north america and the kinnekulle k bentonite in northwestern europe the vents were located near the laurentian margin of iapetus on an arc or microplate undergoing collision with laurentia the volume of ash preserved in the stratigraphie record converted to dense rock equivalent dre of silicic magma is minimally estimated to be 943 km3 for the deicke 1509 km3 for the millbrig and 972 km3 for the kinnekulle the millbrig and kinnekulle beds are coeval and possibly equivalent yielding a combined dre volume of nearly 2500 km3 some unknown but probably large amount of additional ash fell into oceanic regions of the iapetus but these areas became subducted and the ash is not preserved in the geologic record the symmetry of the thickness contours is suggestive that one or more ash clouds interacting with equatorial stratospheric and tropospheric wind patterns dispersed pyroclastic material to both the northwest and southeast in terms of ordovician paleogeography based on grain size measurements and thickness area 1 2 plots we conclude the three beds were each formed from co ignimbrite or possibly phreatoplinian eruption columns analyses of melt inclusions in primary quartz crystals indicate the parental magma contained approximately 4 dissolved water at the time of the eruption this water provided the explosive energy during the initial gas thrust phase the implied fragmentation pressure on the magma would have reduced much of the ejecta to small particles forming a deposit composed largely of single crystals and glassy dust conversion of the ash to k bentonite resulted in a mass loss of approximately 35 mostly in the form of si with lesser amounts of na and k

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

eruption dynamics at pahvant butte volcano utah western usa insights from ash sheet dispersal grain size and geochemical data shallow to emergent subaqueous basaltic explosive eruptions here referred to as surtseyan are a source of pyroclastic material expelled from subaqueous vents and transferred to the water column and atmosphere they can threaten coastal communities in different ways and one of these is through the generation of pyroclastic density currents travelling over water once the eruption becomes subaerial these may be preceded or accompanied by eruption fed currents moving along the sea lake floor one approach for studying these eruptions is through analysis of their deposits but these are commonly either inaccessible under water or poorly preserved by the time they are exposed subaerially the site selected for this work pahvant butte ut usa avoids these limitations the volcano was formed at 18 600 cal year b p in the giant pleistocene lake bonneville by an eruption initiated at a relatively shallow water depth 85 m the water is now drained and deposits of the edifice and of the ash sheet at medial distances on the old lake floor medial lake are accessible and well preserved the ash sheet deposits allow us to investigate dispersal of ash from a surtseyan eruption including the subaqueous phase of edifice building by comparing edifice deposits with ash sheet deposits correlated using geochemistry and textural analysis results suggest that medial deposited ash from pahvant butte travelled at least 25 km from the vent in eruption fed pyroclastic density currents moving along the lake floor this transport began early in the eruption apparently prior to the volcano s subaerial emergence

reconstructing the quaternary evolution of the world s most active silicic volcanic system insights from an 1 65 ma deep ocean tephra record sourced from taupo volcanic zone new zealand abstract the taupo volcanic zone tvz central north island new zealand is the most frequently active quaternary rhyolitic system in the world silicic tephras recovered from ocean drilling programme site 1123 41 47 16 s 171 29 94 w 3290 m water depth in the southwest pacific ocean provide a well dated record of explosive tvz volcanism since 1 65 ma we present major minor and trace element data for 70 quaternary tephra layers from site 1123 determined by electron probe microanalysis 1314 analyses and laser ablation inductively coupled plasma mass spectrometry 654 analyses trace element data allow for the discrimination of different tephras with similar major element chemistries and the establishment of isochronous tie lines between three sediment cores 1123a 1123b and 1123c recovered from site 1123 these tephra tie lines are used to evaluate the stratigraphy and orbitally tuned stable isotope age model of the site 1123 composite record trace element fingerprinting of tephras identifies 4 5 m and 7 9 m thick sections of repeated sediments in 1123a 49 0 53 5 mbsf metres below seafloor and 1123c 48 1 56 0 mbsf respectively these previously unrecognised repeated sections have resulted in significant errors in the site 1123 composite stratigraphy and age model for the interval 1 15 1 38 ma and can explain the poor correspondence between 18o profiles for site 1123 and site 849 equatorial pacific during this interval the revised composite stratigraphy for site 1123 shows that the 70 tephra layers when correlated between cores correspond to 37 38 individual eruptive events tephras 7 of which can be correlated to onshore tvz deposits the frequency of large volume tvz derived silicic eruptions as recorded by the deposition of tephras at site 1123 has not been uniform through time rather it has been typified by short periods 25 50 ka of intense activity bracketed by longer periods 100 130 ka of quiescence the most active period at least 1 event per 7 ka occurred between 1 53 and 1 66 ma corresponding to the first 130 ka of tvz rhyolitic magmatism since 1 2 ma 80 of tephras preserved at site 1123 and the more proximal site 1124 were erupted and deposited during glacial periods this feature may reflect either enhanced atmospheric transport of volcanic ash to these sites up to 1000 km from source during glacial conditions or more speculatively that these events are triggered by changes in crustal stress accumulation associated with large amplitude sea level changes only 8 of the 37 38 site 1123 tephra units 20 can be found in all three cores and 22 tephra units 60 are only present in one of the three cores whether a tephra is preserved in all three cores does not have any direct relationship to eruptive volume instead it is postulated that tephra preservation at site 1123 is patchy and influenced by the vigorous nature of their deposition to the deep ocean floor as vertical density currents at this site at least 5 cores would need to have been drilled within a proximity of 10 s to 100 s of metres of each other to yield a 99 chance of recovering all the silicic tephras deposited on the ocean surface above it in the past 1 65 ma

dynamics of isolated convective regions in the ocean abstract an initially resting ocean of stratification n is considered subject to buoyancy loss at its surface of magnitude b0 over a circular region of radius r at a latitude where the coriolis parameter is f initially the buoyancy loss gives rise to upright convection as an ensemble of plumes penetrates the stratified ocean creating a vertically mixed layer however as deepening proceeds horizontal density gradients at the edge of the forcing region support a geostrophic rim current which develops growing meanders through baroclinic instability eventually finite amplitude baroclinic eddies sweep stratified water into the convective region at the surface and transport convected water outward and away below setting up a steady state in which lateral buoyancy flux offsets buoyancy loss at the surface in this final state quasi horizontal baroclinic eddy transfer dominates upright plume convection by using parcel theory to consider the energy transformations taking place it is shown that the de

sediment fluxes and varve formation in santa barbara basin offshore california results from a year long sediment trap study provide the first direct observations of seasonal variability in sediment fluxes to the sea floor in santa barbara basin offshore california and are used to evaluate varve formation in this basin sediment fluxes throughout the year are dominated by lithogenic material biogenic sediments primarily silica are of secondary importance we conclude that the combination of high lithogenic fluxes and low biogenic fluxes during the fall winter period results in the deposition of dark laminae the onset of upwelling and high surface productivity in the spring results in high biogenic silica fluxes that last into the summer the relative contribution of silica to the total sediment flux is highest during this spring summer period and we suggest that light laminae are formed at that time this is consistent with previous suggestions that sediment input is the primary factor controlling varve formation in this basin

the 26 5 ka oruanui eruption new zealand an introduction and overview abstract the 26 5 ka oruanui eruption from taupo volcano in the central north island of new zealand is the largest known wet eruption generating 430 km3 of fall deposits 320 km3 of pyroclastic density current pdc deposits mostly ignimbrite and 420 km3 of primary intracaldera material equivalent to 530 km3 of magma erupted magma is 99 rhyolite and 90 pdc deposits range from mm to cm thick ultra thin veneers enclosed within fall material to 200 m thick ignimbrite in proximal areas the farthest travelled 90 km most energetic pdcs velocities 100 m s 1 occurred during phase 8 but the most voluminous pdc deposits were emplaced during phase 10 grain size variations in the pdc deposits are complex with changes seen vertically in thick proximal accumulations being greater than those seen laterally from near source to most distal deposits modern lake taupo partly infills the caldera generated during this eruption a 140 km2 structural collapse area is concealed beneath the lake while the lake outline reflects coeval peripheral and volcano tectonic collapse early eruption phases saw shifting vent positions development of the caldera to its maximum extent indicated by lithic lag breccias occurred during phase 10 the oruanui eruption shows many unusual features its episodic nature wide range of depositional conditions in fall deposits of very wide dispersal and complex interplay of fall and pdc activity

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

sediment laden fresh water above salt water nonlinear simulations when a layer of particle laden fresh water is placed above clear saline water both double diffusive and rayleigh taylor instabilities may arise the present investigation extends the linear stability analysis of burns meiburg j fluid mech vol 691 2012 pp 279 314 into the nonlinear regime by means of two and three dimensional direct numerical simulations dns the initial instability growth in the dns is seen to be consistent with the dominant modes predicted by the linear stability analysis the subsequent vigorous growth of individual fingers gives rise to a secondary instability and eventually to the formation of intense plumes that become detached from the interfacial region the simulations show that the presence of particles with a stokes settling velocity modifies the traditional double diffusive fingering by creating an unstable nose region in the horizontally averaged profiles located between the upward moving salinity and the downward moving sediment interface the effective thickness l s l c of the salinity sediment interface grows diffusively as does the height h of the nose region the ratio h l s initially grows and then plateaus at a value that is determined by the balance between the flux of sediment into the rose region from above the double diffusive rayleigh taylor flux out of the nose region below and the rate of sediment accumulation within the nose region for small values of h l s leqslant o 0 1 double diffusive fingering dominates while for larger values h l s geqslant o 0 1 the sediment and salinity interfaces become increasingly separated in space and the dominant instability mode becomes rayleigh taylor like a scaling analysis based on the results of a parametric study indicates that h l s is a linear function of a single dimensionless grouping that can be interpreted as the ratio of inflow and outflow of sediment into the nose region the simulation results furthermore indicate that double diffusive and rayleigh taylor instability mechanisms cause the effective settling velocity of the sediment to scale with the overall buoyancy velocity of the system which can be orders of magnitude larger than the stokes settling velocity while the power spectra of double diffusive and rayleigh taylor dominated flows are qualitatively similar the difference between flows dominated by fingering and leaking is clearly seen when analysing the spectral phase shift for leaking dominated flows a phase locking mechanism is observed which intensifies with time hence the leaking mode can be interpreted as a fingering mode which has become phase locked due to large scale overturning events in the nose region as a result of a rayleigh taylor instability

flow head deposits in ash turbidites during the bronze age minoan eruption of santorini volcano greece a volcanic ash layer was spread over the southern aegean and the eastern mediterranean contemporaneous slumping of ash fall deposits which accumulated on the sea floor generated density currents that entered basins in the southern aegean sea beneath the t a divisions of the resulting turbidites we have observed a separate depositional layer this layer is coarser grained than the base of the overlying t a division shows a lower half enriched in crystals and lithic rock and an upper half enriched in pumice shards and is depleted in silt size components compared to the t a division we propose that this layer formed by the incorporation of water into the head of a sediment laden density current resulting in fluidization and expansion of the head and the segregation and sedimentation of the coarser and denser particles this layer was subsequently overridden by the body of the flow that deposits the overlying t a division