petrology geochemistry and environmental significance of silcrete calcrete intergrade duricrusts at kang pan and tswaane central kalahari botswana calcrete silcrete intergrade duricrusts are an important component of the kalahari group sediments of central southern africa and yet have neither been analysed systematically nor in any detail in this study the petrological and geochemical characteristics of suites of calcrete silcrete and intergrade duricrusts from two fresh relatively deep exposures at kang pan and tswaane adjacent to the okwa valley in the central kalahari botswana are described the duricrust profile at kang pan consists of a highly indurated crystalline non pedogenic calcrete which has been extensively silicified by chalcedony and rarely cryptocrystalline silica or microquartz silicification is most extensive in lower parts of the profile where replacement is related to groundwater fluctuations and in upper sections due to periodic flooding by ephemeral surface water the exposure at tswaane consists of a sequence of pale green glauconitic non pedogenic silcrete and cal silcrete overlain by non pedogenic calcrete all of which have formed within sediments situated upon granitoid gneiss bedrock the siliceous duricrusts are dominated by cryptocrystalline silica cements and appear to have developed through the replacement of a preexisting non pedogenic calcrete these siliceous duricrusts have also been calcified at a later date during the formation of the overlying calcrete to produce a complex range of silica carbonate cements at both sites the style and type of silicification present appears to be determined by the duration of wetting and the permeability of the precursor calcrete geochemical evidence indicates a lack of chemical weathering profile development within the granitoid gneiss bedrock and considerable differences between the chemical signature of the bedrock and combined duricrusts from kang and tswaane this suggests that bedrock made a minimal contribution in terms of silica and carbonate species to duricrust formation and that the majority of cementing agents were non local it would therefore appear likely that the geomorphological context of each site had a major influence upon the development of calcrete silcrete and intergrade duricrust cements

revised calendar date for the taupo eruption derived by 14c wiggle matching using a new zealand kauri 14c calibration data set taupo volcano in central north island new zealand is the most frequently active and productive rhyolite volcano on earth its latest explosive activity about 1800 years ago generated the spectacu

submarine silicic caldera at the front of the izu bonin arc japan voluminous seafloor eruptions of rhyolite pumice myojin knoll caldera a submarine rhyolitic center 400 km south of tokyo is one of nine silicic calderas along the northern 600 km of the izu bonin ogasawara arc and the first anywhere to receive detailed submersible based study the caldera slightly smaller than the crater lake structure in oregon is 6 7 km in diameter its inner walls are 500 900 m high and it has a remarkably flat floor at 1400 m below sea level mbsl the caldera collapse volume is 18 km 3 suggesting that more than 40 km 3 of pumiceous tephra may have been erupted at the time the caldera formed precaldera seafloor eruptions built a broad volcanic edifice consisting of overlapping composite volcanoes made of rhyolitic lavas shallow intrusions and a variety of volcaniclastic deposits including thick accumulations of rhyolitic pumice erupted at 900 500 mbsl the caldera forming eruption produced a 150 200 m deposit of nonwelded fines depleted pumice that resembles a colossal layer of popcorn at the top of the caldera wall freshly erupted pumice behaved as sinkers or floaters depending on the environment in which it cooled the pumice clasts deposited proximally and exposed in the caldera wall were likely quenched in eruption columns that remained below sea level this pumice ingested seawater and sank as gases filling its vesicles cooled particularly as steam in its vesicles condensed to liquid water some eruption columns may have broken through the sea surface and entered the air especially during vigorous phases of the caldera forming eruption these pumices had the opportunity to ingest air as they cooled becoming floaters as they fell back to the sea these could have been carried distally on the sea surface by the combined effects of ocean currents and wind the age of the caldera is unknown but it may be as young as several thousand years its magmatic system at depth retains sufficient heat to sustain an actively growing intracaldera kuroko type polymetallic sulfide deposit rich in gold and silver and topped by chimneys emitting fluids as hot as 278 c sufficient time has elapsed however for a 250 m high postcaldera dome to grow on the caldera floor and for the caldera rim to be deeply scalloped by slumping

the 1995 1998 eruption of the soufriere hills volcano montserrat wi eruption of the soufriere hills volcano began on 18 july 1995 after three years of elevated seismic activity four months of increasingly vigorous phreatic activity culminated in mid november 1995 with the initiation of dome growth growth rates increased unevenly through early march 1996 with fluctuations on time scales from hours to months since march 1996 gravitational collapse of the unstable dome flank has affected an ever increasing area with pyroclastic flows surges and ashfalls major collapse of the eastern flank on 17 september 1996 resulted in a sub plinian explosive eruption later that day by february 1997 the dome had outgrown the confines of the crater and begun to spill into the surrounding valleys a large collapse on 25 june 1997 caused pyroclastic flows and surges on the northern flanks and resulted in the only deaths of the eruption in august september and october 1997 vulcanian explosions followed further collapses on the western and northern flanks the largest event of the eruption occurred on 26 december 1997 with failure of the southwestern flank of the volcano producing a debris avalanche and large dome collapse pyroclastic flows dome growth ceased in early march 1998 but residual volcanic activity has continued and consists of ash venting mild explosions and dome collapse pyroclastic flows

the analysis of the influence of pumice shape on its terminal velocity 1 pumice particles represent the basic ingredient of many large explosive eruptions and form as a result of magma fragmentation inside the conduit at the onset of eruption fragmental pumices are expelled at high velocity from the crater by the overpressure of gas liberated on explosion the resulting multiphase flow is forced through atmosphere by a variety of transportation mechanisms and pumices eventually decouple from the gas flow settling down to form pyroclastic deposits here we propose new experimental data of terminal velocity together with a quantitative shape analysis of a wide range of pumice particles the resulting model allows predicting the terminal velocity of pumice by means of easily measured particle characteristics with an average error of 12 which compares favourably with previous models

eruption of soufriere hills 1995 2009 from an offshore perspective insights from repeated swath bathymetry surveys this contribution provides an analysis of the 1995 2009 eruptive period of soufriere hills volcano montserrat from a unique offshore perspective the methodology is based on five repeated swath bathymetric surveys the difference between the 2009 and 1999 bathymetry suggests that at least 395 mm3 of material has entered the sea this proximal deposit reaches 95 m thick and extends 7km from shore however the difference map does not include either the finer distal part of the submarine deposit or the submarine part of the delta close to the shoreline we took both contributions into account by using additional information such as that from marine sediment cores by march 2009 at least 65 of the material erupted throughout the eruption has been deposited into the sea this work provides an excellent basis for assessing the future activity of the soufriere hills volcano including potential collapse and other volcanoes on small islands

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

pillow lavas and the leidenfrost effect field observations of the formation of pillows by hot de gassed lava entering the sea have proved that their generation involves the underwater exposure and movement of incandescent material this is made possible by the leidenfrost effect the phenomenon whereby film boiling replaces the usual nucleated boiling above a certain temperature thereby forming an insulating sheath of vapour around any sufficiently hot body immersed in a liquid only on cooling below a certain temperature defined here as the nukiyama temperature will rapid heat exchange occur between water and hot gas free lava near the surface this could produce phreatic explosions and extensive clouds of steam

submarine pyroclastic deposits formed at the soufriere hills volcano montserrat 1995 2003 what happens when pyroclastic flows enter the ocean the soufriere hills volcano montserrat west indies has undergone a series of dome growth and collapse events since the eruption began in 1995 over 90 of the pyroclastic material produced has been deposited into the ocean sampling of these submarine deposits reveals that the pyroclastic flows mix rapidly and violently with the water as they enter the sea the coarse components pebbles to boulders are deposited proximally from dense basal slurries to form steep sided near linear ridges that intercalate to form a submarine fan the finer ash grade components are mixed into the overlying water column to form turbidity currents that flow over distances 30 km from the source the total volume of pyroclastic material off the east coast of montserrat exceeds 280 106 m3 with 65 deposited in proximal lobes and 35 deposited as distal turbidites

the saturation behaviour of pumice and some sedimentological implications abstract caldera forming explosive eruptions from rhyolitic volcanoes typically produce huge quantities of pyroclastic material blanketing the landscape and triggering long term voluminous and widespread sedimentary responses pumice is a major component of such eruptions but its behaviour in the sedimentary environment is little understood its most notable feature is its very low density which frequently enables pumice to float on water for long periods of time before eventually waterlogging enough to sink once fully saturated pumice behaves in a broadly similar way to normal clastic material in terms of its hydrodynamic behaviour modified by its lower density however relatively little is known about its earlier behaviour in the sedimentary environment and in particular the manner and rate by which initially dry pumice absorbs water in this paper we develop a mathematical model which predicts that the time required for a pumice clast to saturate is proportional to the square of its radius tests on samples of real pumice accord with predicted trends and match lacustrine pumiceous facies observed in the field the ability to predict the time required for large pumice clasts to sink may provide a tool for estimating the timescales over which certain rapid post eruptive sedimentary processes operate

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

effects of water depth on pumice formation in submarine domes at sumisu izu bonin arc western pacific domes of the sumisu volcanic complex western pacific having summits at ocean depths of 1100 600 245 and 95 m are mantled with compositionally identical rhyolitic pumice that has similar vesicularity but that varies systematically in distribution size and surface texture suggesting that facies and morphology can serve as useful indicators of eruption depth at depths 500 m the pumice formed a thick carapace on dense rhyolite and disintegrated by quench fracture and mechanical failure into a jumble of giant meters to tens of meters polyhedral blocks with smooth curviplanar surfaces that display a single quenched margin vesiculation was arrested on eruption in seawater in all but the interior of the thickest carapace at 500 m depths the pumice occurs as an apron of blocky giant and smaller rough textured clasts enclosed by quenched margins and pockmarked by coarse cm vesicles no carapace pumice occurs and the summit is composed of craggy dense dome rock these shallower water pumice clasts resemble those spalled from historic submarine dome forming eruptions that buoyed to the sea surface we interpret spalling to result from vent derived weak volatile driven explosions that take place at water depths 500 m our study shows that an increase in hydrostatic pressures over a range of 12 mpa reduces volatile driven explosivity for subaqueous rhyolitic dome forming eruptions but does not affect vesicularity we conclude that meter size highly vesicular pumice is diagnostic of subaqueous dome eruptions in water depths of at least 1300 m and its morphology can be used to distinguish between explosive and effusive origins

isovolumetric weathering and silcrete formation southern cape province south africa calculations of absolute elemental gains and losses based on isovolumetric bedrock alteration are applied to deep weathering profiles and silcrete developed on dwyka tillite and bokkeveld shale bedrock weathering to kaolinite and illite yields sufficient sio2 to account for observed thicknesses of associated silcrete absolute accumulation of sio2 in silcrete is complemented by substantial absolute depletion of al2o3 absolute accumulation of tio2 in both silcrete and associated weathered material together with authigenic anatase concentrations in silctete recorded during micromorphological investigations indicate tio2 mobilization in a low ph environment ph 3 75 during silcrete formation under these low ph conditions increased al2o3 solubility led to replacement of kaolinite and illite by a proportion of sio2 retained in the weathering profile through restricted drainage

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

interbedded late quaternary turbidites and contourites in flemish pass off southeast canada their recognition origin and temporal variation abstract the recognition of contourite and turbidite deposits in deep marine sequences can be challenging as deposits may show similar sedimentary characteristics in flemish pass a small 1100 m deep perched basin along the southeast canadian continental margin both types of deposits occur late quaternary sediments include thin sand layers interbedded with hemipelagic muds the distribution age sedimentary structures and petrology of these sands were used to distinguish between turbidite and contourite deposits which were then characterized by detailed grain size distribution for understanding the conditions leading to deposition of each type seven sediment facies each with a distinctive grain size distribution were recognized they are 1 transported contourites 2 winnowed beds 3 possible jokulhlaup deposits 4 sandy turbidites 5 muddy turbidites 6 hemipelagic sediment and 7 turbidites from well sorted materials sand packets comprised of several stacked turbidite sand beds some with erosive contacts were correlated from the margin to the basin floor showing clear distal fining and creating a small lobe the temporal distribution of the sand beds indicates that most turbidites and contourites were deposited at stadial to interstadial transitions this linkage may be related to variations in the oceanic circulation in flemish pass which winnowed turbidites and may have triggered turbidity currents by moving sands into canyon heads

paleomagnetic evidence for high temperature emplacement of the 1883 subaqueous pyroclastic flows from krakatau volcano indonesia the paroxysmal 1883 eruption of krakatau volcano in indonesia discharged at least 6 5 km3 dense rock equivalent of pyroclastic material into the shallow waters of the sunda straits within a 15 km radius of the volcano progressive thermal demagnetization studies of individually oriented pumice clasts from a core sample of the submarine pyroclastic deposits show that 41 out of 47 clasts exhibit single component remanence with mean inclination of 2 4 the partial thermoremanent magnetization components of both pumice and lithic clasts are well grouped in orientation indicating that substantial cooling of clasts must have occurred following deposition estimated subaqueous emplacement temperature for such clasts is greater than 500 c rare two component lithic fragments exhibit inflection points on vector endpoint diagrams that mark the temperature below which the fragments acquired magnetization of similar orientation these inflection points range from 350 to 550 c indicating a minimum subaqueous emplacement temperature of 350 c paleomagnetic evidence for high emplacement temperature supports the hypothesis that proximal 1883 submarine pyroclastic deposits resulted from entrance of hot subaerially generated pyroclastic flows into the sea similar deposits have been interpreted from the geologic record but this is the first documented example of submarine pyroclastic flows from a historic eruption the krakatau deposits thus serve as an important modern analog for the study of pyroclastic flow seawater interactions

late pleistocene stratigraphy of iodp site u1396 and compiled chronology offshore of south and south west montserrat lesser antilles marine sediments around volcanic islands contain an archive of volcaniclastic deposits which can be used to reconstruct the volcanic history of an area such records hold many advantages over often incomplete terrestrial data sets this includes the potential for precise and continuous dating of intervening sediment packages which allow a correlatable and temporally constrained stratigraphic framework to be constructed across multiple marine sediment cores here we discuss a marine record of eruptive and mass wasting events spanning 250 ka offshore of montserrat using new data from iodp expedition 340 as well as previously collected cores by using a combination of high resolution oxygen isotope stratigraphy ams radiocarbon dating biostratigraphy of foraminifera and calcareous nannofossils and clast componentry we identify five major events at soufriere hills volcano since 250 ka lateral correlations of these events across sediment cores collected offshore of the south and south west of montserrat have improved our understanding of the timing extent and associations between events in this area correlations reveal that powerful and potentially erosive density currents traveled at least 33 km offshore and demonstrate that marine deposits produced by eruption fed and mass wasting events on volcanic islands are heterogeneous in their spatial distribution thus multiple drilling coring sites are needed to reconstruct the full chronostratigraphy of volcanic islands this multidisciplinary study will be vital to interpreting the chaotic records of submarine landslides at other sites drilled during expedition 340 and provides a framework that can be applied to the stratigraphic analysis of sediments surrounding other volcanic islands

bulldozing the core mantle boundary localized seismic scatterers beneath the caribbean sea images of the scattering field near the core mantle boundary cmb beneath the caribbean were obtained from migration processing of pkikp precursors recorded by a broadband seismic array in the southern caribbean from earthquakes in the western pacific strong seismic scatterers were found to be concentrated near the cmb north of colombia and are surrounded by high velocity anomalies associated with the subducted farallon plate the relative location between the imaged scatterers and the surrounding high velocity anomalies leads us to speculate that the observed seismic scatterers are remnants of the subducted slab as the farallon plate penetrated into the lower mantle segregation allowed its basaltic crust to descend more quickly and to pool in front of the major portion of the subducting slab the ponding oceanic crustal material could be further collected or bulldozed by the residual motion of the subducting plate forming a localized region with strong seismic scatterers

combinations of volcanic flank and seafloor sediment failure offshore montserrat and their implications for tsunami generation recent seafloor mapping around volcanic islands shows that submarine landslide deposits are common and widespread such landslides may cause devastating tsunamis but accurate assessment of tsunami hazard relies on understanding failure processes and sources here we use high resolution geophysical data offshore from montserrat in the lesser antilles to show that landslides around volcanic islands may involve two fundamentally different sources of sediment island flank and larger seafloor sediment failures and can occur in multiple stages a combination of these processes produces elongate deposits with a blocky centre associated with island flank collapse surrounded by a smoother surfaced deposit that is dominated by failed seafloor sediment the failure of seafloor sediment is associated with little marginal accumulation and involves only limited downslope motion submarine landslide deposits with similar blocky and smooth surfaced associations are observed in several locations worldwide but the complex emplacement processes implied by this morphological relationship can only be revealed by high resolution geophysical data such complexity shows that the volume of landslide deposits offshore of volcanic islands cannot simply be used in tsunami models to reflect a single stage collapse of primary volcanic material by applying predictive equations for tsunami amplitude to investigate general scenarios of volcanic island landslide generation we show that the tsunami hazard associated with volcanic island collapse remains highly significant volcanic flank failures even if relatively small may generate large local tsunamis but associated seafloor sediment failures even if they have a much greater volume have a substantially lower potential for tsunami generation

submarine deposits from pumiceous pyroclastic density currents traveling over water an outstanding example from offshore montserrat iodp 340 pyroclastic density currents have been observed to both enter the sea and to travel over water for tens of kilometers here we identified a 1 2 m thick stratified pumice lapilli ash cored at site u1396 offshore montserrat integrated ocean drilling program iodp expedition 340 as being the first deposit to provide evidence that it was formed by submarine deposition from pumice rich pyroclastic density currents that traveled above the water surface the age of the submarine deposit is ca 4 ma and its magma source is similar to those for much younger soufriere hills deposits indicating that the island experienced large magnitude subaerial caldera forming explosive eruptions much earlier than recorded in land deposits the deposit s combined sedimentological characteristics are incompatible with deposition from a submarine eruption pyroclastic fall over water or a submarine seafloor hugging turbidity current derived from a subaerial pyroclastic density current that entered water at the shoreline the stratified pumice lapilli ash unit can be subdivided into at least three depositional units with the lowermost one being clast supported the unit contains grains in five separate size modes and has a 12 phi range particles are chiefly subrounded pumice clasts lithic clasts crystal fragments and glass shards pumice clasts are very poorly segregated from other particle types and lithic clasts occur throughout the deposit fine particles are weakly density graded we interpret the unit to record multiple closely spaced 2 d hot pyroclastic density currents that flowed over the ocean releasing pyroclasts onto the water surface and settling of the various pyroclasts into the water column our settling and hot and cold flotation experiments show that waterlogging of pumice clasts at the water surface would have been immediate the overall poor hydraulic sorting of the deposit resulted from mixing of particles from multiple pulses of vertical settling in the water column attesting to complex sedimentation slow settling particles were deposited on the seafloor together with faster descending particles that were delivered at the water surface by subsequent pyroclastic flows the final sediment pulses were eventually deflected upon their arrival on the seafloor and were deposited in laterally continuous facies this study emphasizes the interaction between products of explosive volcanism and the ocean and discusses sedimentological complexities and hydrodynamics associated with particle delivery to water

submarine deposition of volcaniclastic material from the 1995 2005 eruptions of soufriere hills volcano montserrat abstract soufriere hills volcano montserrat has been erupting since 1995 during the current eruption a large part of the material produced by the volcano has been transported into the sea modifying the morphology of the submarine flanks of the volcano we present a unique set of swath bathymetric data collected offshore from montserrat in 1999 2002 and 2005 from 1999 to 2002 pyroclastic flows associated with numerous dome collapses entered the sea to produce 100 mm 3 deposit from 2002 to 2005 the 290 mm 3 submarine deposit is mainly from the 12 13 july 2003 collapse these data allow us to estimate that by may 2005 at least 482 mm 3 of material had been deposited on the sea floor since 1995 we compare on land characteristics and volumes of dome collapse events with the submarine deposits and propose a new analysis of their emplacement on the submarine flanks of the volcano the deposition mechanism shows a slope dependence with the maximum thickness of deposit before the break in the slope probably because of the type of the dense granular flow involved we conclude that from 1995 to 2005 more than 75 of the erupted volume entered the sea

tephra dispersal from myojinsho japan during its shallow submarine eruption of 1952 1953 a new and detailed bathymetric map of the myojinsho shallow submarine volcano provides a framework to interpret the physical volcanology of its 1952 1953 eruption especially how the silicic pyroclasts both primary and reworked enlarged the volcano and were dispersed into the surrounding marine environment myojinsho 420 km south of tokyo along the izu ogasawara arc was the site of approximately 1000 phreatomagmatic explosions during the 12 5 month eruption these explosions shattered growing dacite domes producing dense clasts that immediately sank into the sea minor amounts of pumice floated on the sea surface after some of these events the myojinsho cone has slopes of almost precisely 21 in the depth range 300 700 m we interpret this to be the result of angle of repose deposition of submarine pyroclastic gravity flows that traveled downslope in all directions many of these gravity flows resulted from explosions and associated dome collapse but others were likely triggered by the remobilization of debris temporarily deposited on the summit and steep upper slopes of the cone tephra was repeatedly carried into air in subaerial eruption columns and fell into the sea within 1 2 km of the volcano s summit entering water as deep as 400 m because the fall velocity of single particles decreased by a factor of 30 in passing from air into the sea we expect that the upper part of the water column was repeatedly choked with hyperconcentrations of fallout tephra gravitational instabilities within these tephra choked regions could have formed vertical density currents that descended at velocities greater than those of the individual particles they contained upon reaching the sea floor many of these currents probably continued to move downslope along myojinsho s submarine slopes fine tephra was elutriated from the rubbly summit of the volcano by upwelling plumes of heated seawater that persisted for the entire duration of the eruption ocean currents carried this tephra to distal areas where it presumably forms a pyroclastic component of deep sea sediment

emplacement processes of submarine volcaniclastic deposits iodp site c0011 nankai trough during iodp nantroseize expedition 322 four packages of tuffaceous sandstones tst 1 2 3a 3b were recovered within a moderately lithified and bioturbated silty claystone succession in the late miocene 7 6 to 9 1 ma upper part of the middle shikoku basin deposits to assess the emplacement processes of the tuffaceous sandstones we investigate modal and geochemical compositions of 24 thin sections that reveal systematic vertical changes within each bed tst 1 2 and 3b are single beds whereas tst 3a is composed of at least two beds suggesting several rapidly succeeding sedimentation events the beds are density graded such that low density pyroclasts including pumice lapilli are enriched at the top whereas dense lithic components and minerals are enriched at the bottom the volcanic glass particles pumice and shards that are the dominant modal constituents of each sandstone bed have homogeneous geochemical compositions in each bed moreover tst 1 2 and 3a glass compositions overlap completely but tst 3b glass has a different composition as is analogously observed for the mineral compositions this unique multistage approach of sedimentological and geochemical methods facilitated the detailed investigation of distal volcano derived probably tsunamogenic turbidites in order to contribute to the distinction between primary and secondary induced mass flows we interpret that all tuffaceous sandstones were emplaced by turbidity currents that were formed during major explosive volcanic eruptions however while tst 1 2 and 3a turbidity currents formed by the entry into the ocean of voluminous pyroclastic flows erupted at a volcano on mainland japan tst 3b was emplaced by a turbidity current formed by a shallow submarine or subaerial volcanic eruption at the izu bonin arc where it collided with japan these results regarding distal turbidites encourage the revisiting of older marine deposits in the scope of hazard evaluation through past events especially in regions near to volcanic sources

geochemical mass balance and oxygen isotope constraints on silcrete formation and its paleoclimatic implications in southern australia abstract two geographically distinct silcrete associations are present in southern australia inland and eastern these were sampled in central south australia and central victoria respectively at each site both silicified and immediately adjacent unsilicified parent material were collected analytical data from these pairs were used to construct isocons assuming zr immobility and to calculate the volume change and amount of silica introduced during silicification these results together with whole rock oxygen isotope compositions were used to determine the 18o of the introduced silica the results show that the eastern silcretes in central victoria are probably linked genetically to the associated basalts weathering of which supplied the introduced silica this conclusion is based on the close spatial connection between the two as well as the substantial amount of introduced silica in the silcretes greater than in the inland silcretes resulting in volume increases in some eastern silcretes oxygen isotopic calculations for the silcretes indicate that the silica precipitated from groundwaters at temperatures slightly higher than present conditions silcrete formation apparently occurred during the miocene and pliocene basalts in victoria younger than pliocene lack associated silcrete and may reflect the much wetter climate in southeastern australia at that time the inland silcretes of central south australia can be divided into pedogenic the most common and groundwater varieties the pedogenic silcretes which show typical soil features like columnar and nodular textures contain moderate amounts of introduced silica that precipitated by evaporation from saline groundwaters for the groundwater silcretes which have massive textures and formed at or close to the water table insufficient data are available to determine the mode of formation the inland pedogenic silcretes have probably been forming from the eocene miocene to the present implying that conditions of seasonally high evaporation have occurred in central australia during this time period thus silcrete formation depends on a complex interplay between climate and silica supply and it is impossible to generalize that the presence of silcrete is indicative of a particular climate likewise the elemental composition of silcretes particularly ti content is not necessarily of climatic significance nevertheless detailed geochemical and oxygen isotopic studies of a silcrete and its parent material can elucidate the mechanisms of silcrete formation and if evaporation is indicated as a major factor in silcrete formation then the climate at the time was likely to have been at least seasonally arid

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

presence of large pumice blocks on tierra del fuego and south shetland islands shorelines from 1962 south sandwich islands eruption abstract we report on the source of pumice found on beaches of tierra del fuego argentina and the south shetland islands antarctica north and south of the drake passage petrological studies chemical analyses and experiments on this pumice indicate it is most likely from a 1962 submarine eruption in the south sandwich islands previous work stated that clockwise marine currents carried such pumice as far as tasmania our findings show that the currents continued to transport the pumice to drake passage nearly completing a circuit around antarctica and up to southern argentina showing that this material can follow complex pathways for more than 20 000 km

evidence for carbonate platform failure during rapid sea level rise ca 14 000 year old bioclastic flow deposits in the lesser antilles bioclastic flow deposits offshore from the soufriere hills volcano on montserrat in the lesser antilles were deposited by the largest volume sediment flows near this active volcano in the last 26 kyr the volume of these deposits exceeds that of the largest historic volcanic dome collapse in the world which occurred on montserrat in 2003 these flows were most probably generated by a large submarine slope failure of the carbonate shelf comprising the south west flank of antigua or the east flank of redonda adjacent islands that are not volcanically active the bioclastic flow deposits are relatively coarse grained and either ungraded or poorly graded and were deposited by non cohesive debris flow and high density turbidity currents the bioclastic deposit often comprises multiple sub units that cannot be correlated between core sites some located just 2 km apart multiple sub units in the bioclastic deposit result from either flow reflection stacking of multiple debris flow lobes and or multi stage collapse of the initial landslide this study provides unusually precise constraints on the age of this mass flow event that occurred at ca 14 ka few large submarine landslides have been well dated but the slope failures that have been dated are commonly associated with periods of rapid sea level change

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

the preservation potential of ash layers in the deep sea the example of the 1991 pinatubo ash in the south china sea following the eruption of mount pinatubo on 15 june 1991 volcanic ash was transported westward to the south china sea in an atmospheric plume falling out and settling to the sea floor within days and forming an up to 10 cm thick layer on an area 400 000 km 2 immediately after deposition surviving deepburrowing animals re opened their connection to the sea floor to obtain water for respiration and or food take up later small sized meiofauna and then macrofauna re colonized the sea floor mixing newly deposited organic fluff with the underlying ash consequently ash deposits thinner than 1 mm have not often been observed as a continuous layer when cored six years after the eruption while ash about 2 mm thick is now patchily bioturbated in areas covered by ash thicker than 5 mm mixing by benthic animals is controlled mainly by the adaptation of the burrowing fauna to variations in grain size the rate of background sedimentation the availability of benthic food on and within the sediment and pore water oxygen levels with respect to these factors four provinces can be distinguished i along the philippines margin run off from land fuels primary production that in turn leads to a high benthic food content the benthic fauna is adapted to a variable grain size and rapid sedimentation therefore mixing is intense and the preservation potential of the ash layer is low ii in areas affected by deposition of hyperpycnites and turbidites i e in canyons in front of river mouths and in the manila trench the ash layer is preserved due to rapid burial iii the area to the west to about 116 e receives low amounts of benthic food benthic mixing is less intense and the preservation potential of the ash is high iv the central south china sea where the ash is thinner than 3 cm is affected by intense wind mixing and upwelling and the benthic food content is high thus the chance that the ash will be preserved as a sharp based layer is low consequently the style of ash preservation has palaeo environmental significance older buried and burrowed event layers provide further information to elucidate the fate of the 1991 pinatubo ash layer in general their appearance fits with observations in the recent

sediment sorting in the deposits of turbidity currents created by experimental modeling of explosive subaqueous eruptions abstract deposits of turbidity currents induced by subaqueous volcanic eruptions are increasingly recognized in settings ranging from lakes to the deep sea these eruption fed density currents have high current temperatures and low density particles which affect current dynamics and deposit features field data indicate that deposit thickness and grain size decrease downflow at a changing rate this trend is similar for deposits of noneruptive turbidity currents but with volcanic activity the deposits often exhibit a break in slope of the profile of deposit thickness and grain size distribution here we present laboratory results on density currents that address the effects of variations in grain density and water temperature on flow properties and depositional processes parallel runs were performed under identical flow conditions using quartz sand and pyroclastic sediment each grain type was used in one series of runs in which a hot density current enters a cold environment and one series in which

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

volcanological study of the great tambora eruption of 1815 the 1815 eruption of tambora is one of the largest explosive volcanic events of the past 10 000 yr by a conservative estimate 175 km3 of nepheline normative trachyandesitic pyroclastic material equivalent to about 50 km3 of dense rock was erupted in 24 h major activity began with a brief plinian phase followed by pyroclastic flows much of the ignimbrite that was produced shows evidence of deposition by turbulent flows a 6 7 km caldera about 1 km deep formed at the end of the pyroclastic flow phase caldera volume closely matches that of magma ejected plinian and co ignimbrite ash fall 1 cm thick covered 500 000 km2 of the java sea and surrounding islands accounting for more than two thirds of the magma volume erupted this distal tephra is known only from 1815 reports indicating that some great eruptions especially those from island volcanoes may be virtually undetectable in the geologic record without supporting information such as deep sea core data land based volume estimates may be insufficient to characterize eruption magnitude in many cases

a hot miocene subaqueous scoria flow deposit in the shimane peninsula sw japan abstract a subaqueous scoria flow deposit occurs in the miocene takashibiyama formation in sw japan faunal assemblages in associated sediments suggest an outer shelf to upper bathyal environment with water depth likely between 200 and 1500 m the deposit 5 7 m thick is similar to subaerial scoria flow deposits containing mainly non to moderately vesiculated hyalopilitic basaltic andesite lapilli and coarse ash the essential basaltic andesite clasts are variably vesiculated even within single clasts and are commonly cracked with curviplanar surfaces whereas the inner parts are cracked in a pattern similar to perlitic cracks the clasts are blocky with curviplanar surfaces if non to poorly vesiculated and are fibrous with irregular shapes and flattened by compaction if more vesiculated at one proximal location fines depleted rootless pipes 5 90 cm wide and 1 3 m long occur and spherical to amoebic vesicles 0 01 0 3 mm in diameter and filled with stilbite quartz or greenish brown clay are common in the fine ash matrix and in siltstone clasts contained in the deposit the vesicles represent open spaces once supported by gas and the rootless pipes are interpreted as gas escape pipes magnetization vectors of essential clasts are dispersed not at random but around a single direction suggesting that the clasts gained thermoremanent magnetism mainly during cooling in the flow or during post emplacement cooling and then rotated to some extent by differential movement of the flow or by gas escape in the deposit the vesicularity shape and texture of essential clasts suggest a phreatomagmatic eruption involving magmatic vesiculation and dynamic interaction of magma and ambient water the perlitic cracks in the inside of the essential clasts and the paleomagnetic vectors of the essential clasts indicate that the flow was initially hot and that the deposit cooled slowly internal depositional features change rapidly over only 500 m rootless pipes and vesicles in the fine ash matrix and siltstone clasts are therefore absent at distal locations indicating that an initially cohesionless debris flow quickly became sufficiently turbulent to mix with ambient seawater

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

physical volcanology of the 2 050 bp caldera forming eruption of okmok volcano alaska in the aleutian volcanic chain usa the 2 050 50 bp collapse of okmok caldera generated pyroclasts that spread over 1 000 km2 on umnak island after expelling up to 0 25 km3 dre of rhyodacitic plinian air fall and 0 35 km3 dre of andesitic phreatomagmatic tephra the caldera collapsed and produced the 29 km3 dre okmok ii scoria deposit which is composed of valley ponding poorly sorted massive facies and over bank stratified facies with planar and cross bedding geological and sedimentological data suggest that a single density current produced the okmok ii deposits by segregating into a highly concentrated base and an overriding dilute cloud the dense base deposited massive facies whereas the dilute cloud sedimented preferentially on hills as stratified deposits the pyroclastic current spread around okmok in an axisymmetric fashion encountering topographic barriers on the southwest reaching unalaska island across an 8 km strait on the east and reaching the shoreline of umnak in the other directions a kinematic model burgisser and bergantz earth planet sci lett 202 405 418 2002 was used to show how decoupling of the pyroclastic current was triggered by both sea entrance and interaction with the topography in the former case the dense part of the current and the lithics transported by the dilute cloud went underwater in the latter case topographical barriers noticeably decelerated both parts of the decoupled current and favored sedimentation by partial or complete blocking the resulting unloading of the dilute current drastically reduced the runout distance by triggering an early buoyant lift off

discharge of pyroclastic flows into the sea during the 1996 1998 eruptions of the soufriere hills volcano montserrat explosive eruptions of the soufriere hills volcano on the island of montserrat in the west indies generated pyroclastic flows that reached the sea on the east and southwest coasts between november 1995 and july 1998 discharge of the flows produced two pyroclastic deltas off of the tar and white river valleys a marine geological survey was conducted in july 1998 to study the submarine extensions of both deltas detailed profiles of depth and sub bottom structure were obtained using a chirp ii bubble pulser system these profiles were compared with pre eruption bathymetric data in order to identify areas of recent deposition and erosion deposition off the tar and white river valleys was thickest nearest the coastline and deltas and extended into deeper water up to 5 km from shore the total volume of submarine pyroclastic deposits as of july 1998 was 73 106 m3 dre submarine pyroclastic deposits off the tar river valley made up more than two thirds of the total volume 55 106 m3 dre and covered an area of approximately 5 0 km2 which included the delta the volume of submarine pyroclastic deposits in the white river area 18 106 m3 dre is probably underestimated due to the lack of precise pre eruption bathymetric data in areas greater than 2 km from shore growth of pyroclastic deltas at the mouths of the tar and white river valleys continued to the edge of the submarine shelf where there was a steep break in slope in the tar river area pyroclastic material was distributed down the steep shelf break and into deeper water at least a few kilometers from shore the material spread out radially forming a submarine fan where distribution was primarily controlled by bathymetry and slope

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

volcaniclastic resedimentation in distal fluvial basins induced by large volume explosive volcanism the ebisutoge fukuda tephra plio pleistocene boundary central japan the ebisutoge fukuda tephra plio pleistocene boundary central japan has a well recorded eruptive style history magnitude and resedimentation styles despite the absence of a correlative volcanic edifice this tephra was ejected by an extremely large magnitude and complex volcanic eruption producing more than 400 km 3 total volume of volcanic materials volcanic explosivity index 7 which extended more than 300 km away from the probable eruption centre remobilization of these ejecta occurred progressively after the completion of a series of eruptions resulting in thick resedimented volcaniclastic deposits in spatially separated fluvial basins more than 100 km from the source facies analysis of resedimented volcaniclastic deposits was carried out in distal fluvial basins the distal tephra 100 300 km from the source comprises two different lithofacies primary pyroclastic fall deposits and reworked volcaniclastic deposits the resedimented volcaniclastic succession shows five distinct sedimentary facies interpreted as debris flow deposits facies a hyperconcentrated flow deposits facies b channel fill deposits facies c floodplain deposits with abundant flood flow deposits facies d and floodplain deposits with rare flood deposits facies e resedimented volcaniclastic materials at distal locations originated from unconsolidated deposits of a climactic large ignimbrite forming eruption factors controlling inter and intrabasinal facies changes are 1 temporal change of introduced volcaniclastic materials into the basin 2 proximaldistal relationship and 3 distribution pattern of pyroclastic flow deposits relative to drainage basins thus studies of the ebisutoge fukuda tephra have led to a depositional model of volcaniclastic resedimentation in distal areas after extremely large magnitude eruptions an aspect of volcaniclastic deposits that has often been ignored or poorly understood

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

dispersal of volcaniclasts during deep sea eruptions settling velocities and entrainment in buoyant seawater plumes abstract we use tank experiments to measure settling rates of deep sea volcaniclastic material recovered from the arctic 85 e gakkel ridge and pacific juan de fuca ridge loihi seamount oceans we find that clast size and shape exert a strong influence on settling velocity with velocities of 30 cm s for large 8 mm blocky clasts compared to velocities of 2 5 cm s for small

pyroclastic flows and surges over water an example from the 1883 krakatau eruption pyroclastic deposits from the 1883 eruption of krakatau are described from areas northeast of the volcano on the islands of sebesi sebuku and lagoendi and the southeast coast of sumatra massive and poorly stratified units formed predominantly from pyroclastic flows and surges that traveled over the sea for distances up to 80 km granulometric and lithologic characteristics of the deposits indicate that they represent the complement of proximal subaerial and submarine pyroclastic flow deposits laid down on and close to the krakatau islands the distal deposits exhibit a decrease in sorting coefficient median grain size and thickness with increasing distance from krakatau crystal fractionation is consistent with the distal facies being derived from the upper part of gravitationally segregated pyroclastic flows in which the relative amount of crystal enrichment and abundance of dense lithic clasts diminished upwards the deposits are correlated to a major pyroclastic flow phase that occurred on the morning of 27 august at approximately 10 a m energetic flows spread out away from the volcano at speeds in excess of 100 km h and traveled up to 80 km from source the flows retained temperatures high enough to burn victims on the sw coast of sumatra historical accounts from ships in the sunda straits constrain the area affected by the flows to a minimum of 4x103 km2 at the distal edge of this area the flows were relatively dilute and turbulent yet carried enough material to deposit several tens of centimeters of tephra the great mobility of the krakatau flows from the 10 a m activity may be the result of enhanced runout over the sea it is proposed that the generation of steam at the flow seawater interface may have led to a reduction in the sedimentation of particles and consequently a delay in the time before the flows ceased lateral motion and became buoyantly convective the buoyant distal edge of these ash and steam laden clouds lifted off into the atmosphere leading to cooling condensation and mud rain

subaerial to submarine transitions in early miocene pyroclastic flow deposits southern san joaquin basin california dacitic pyroclastic flow deposits within the tecuya formation of the san emigdio mountains california exhibit a clear transition from subaerial to submarine deposition across the southeastern margin of the san joaquin basin the dacitic pyroclastic deposits are part of an informal volcanic member within the tecuya formation consisting of a lower dacitic unit and an upper basaltic unit volcanogenic facies transitions within each of these units can be traced westward obliquely across paleoshoreline for 15 km depositional environments of the dacitic unit are well constrained by underlying nonvolcanic siliciclastic and overlying basaltic deposits the lowermost dacitic unit consists of pyroclastic surge and fallout deposits which overlie nonmarine alluvial fan deposits and were produced by subaerial phreatomagmatic eruptions a second magmatic eruptive stage resulted in deposition of dacitic pyroclastic flows on top of the subaerial surge and fallout deposits and within marine nearshore to outer shelf settings these pyroclastic flows generated from subaerial vents were high concentration poorly fluidized flows which traveled down paleoslope into the marine part of the basin upon reaching water the hot pyroclastic flows continued to flow subaqueously partially mixed with water and were quenched transitions to cool submarine pyroclastic debris flows may have occurred surface transformations of the submarine pyroclastic flows pyroclastic debris flows generated dilute pyroclastic turbidity flows which at times became detached and continued to flow independently downslope progradation of submarine pyroclastic flow deposits to the west into deeper marine settings likely accompanied the growth of subaerial volcanic vents to the east a second probably submarine vent also produced submarine pyroclastic flow deposits in the western part of the basin deposition of reworked tuff and epiclastic debris flows within marine settings accompanied primary pyroclastic surge fallout and flow deposition the tecuya volcanic member is an important ancient example in which the depositional environments of subaerial to submarine pyroclastic deposits are well constrained by bounding units the characteristics of the subaerial and submarine pyroclastic flow deposits within the tecuya volcanic member thus may be useful in other ancient volcaniclastic successions which lack bounding units of obvious paleoenvironmental origin

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

fallout of pyroclastic debris from submarine volcanic eruptions volcanic fallout deposits on land being widespread and accessible for study have received much attention and have revealed a great deal about subaerial eruption mechanisms in contrast virtually nothing is known about equivalent deposits produced by submarine volcanoes despite the probable abundance of such material in today s oceans and in accreted volcanic arc terrains many submarine deposits may form by the fallout of debris to the sea floor downcurrent from the umbrella region of submarine eruption columns experiments on water saturated pumice and pieces of rock lithics show that particles settling to the sea floor at terminal velocities of 10 to 50 centimeters per second will display conspicuous bimodality of particle diameters pieces of pumice may be five to ten times as large as codeposited lithic fragments similar material erupted into the air and deposited on land displays less well developed bimodality pumice diameters are generally two to three times as large as associated lithics submarine fallout deposits are therefore distinctive and may be used to indicate a subaqueous origin for some of the great thicknesses of nonfossiliferous volcanic debris contained in ancient volcanic terrains worldwide whose environment of deposition has been uncertain

explosive destruction of a pliocene hot lava dome underwater dogashima japan transition from effusive to explosive volcanism is common during subaerial eruptions and here we demonstrate that this behavior is also possible underwater the pyroclastic facies produced underwater are distinctive and can be used to distinguish subaqueous from subaerial eruptions and depositional settings the pliocene dogashima formation izu peninsula japan is a pumice rich succession originally deposited in an open marine below wave base setting jutzeler et al 2014a a thick clast supported gray andesite breccia composed of very coarse dense andesite clasts with quenched margins was sourced from disintegration of an active lava dome overall the gray andesite breccia is gradationally to sharply overlain by thick graded clast supported white pumice breccia chiefly composed of angular pumice clasts and free broken crystals regional setting and distinctive facies show that this succession was produced by a fully underwater magmatic volatile driven pumice forming explosive eruption the gradational contact between the two breccias compositional similarities rare mingled clasts and fluidal textures in the gray andesite clasts suggest that the explosive eruption destroyed a hot lava dome and generated an eruption fed high concentration density current in most places the coarsest hot lava dome fragments were deposited first followed by the lower density white pumice clasts the low amount of fine 2 mm components well developed hydraulically controlled grading and sorting clast angularity and very coarse dome derived clasts some including well defined quenched margins and common fluidal textures distinguish the products of subaqueous effusive to explosive eruptions from their subaerial counterparts

characteristics of submarine pumice rich density current deposits sourced from turbulent mixing of subaerial pyroclastic flows at the shoreline field and experimental assessment this study investigates the types of subaqueous deposits that occur when hot pyroclastic flows turbulently mix with water at the shoreline through field studies of the znp marine tephra in japan and flume experiments where hot tephra sample interacted with water the znp is a very thick pumice rich density current deposit that was sourced from subaerial pyroclastic flows entering the japan sea in the pliocene notable characteristics are well developed grain size and density grading lithic rich base pumice rich middle and ash rich top preponderance of sedimentary lithic clasts picked up from the seafloor during transport fine ash depletion in coarse facies and presence of curviplanar pumice clasts flume experiments provide a framework for interpreting the origin and proximity to source of the znp tephra on contact of hot tephra sample with water steam explosions produced a gas supported pyroclastic density current that advanced over the water while a water supported density current was produced on the tank floor from the base of a turbulent mixing zone experimental deposits comprise proximal lithic breccia medial pumice breccia and distal fine ash experiments undertaken with cold water saturated slurries of tephra sample and water did not produce proximal lithic breccias but a medial basal lithic breccia beneath an upper pumice breccia results suggest the characteristics and variations in znp facies were strongly controlled by turbulent mixing and quenching proximity to the shoreline and depositional setting within the basin presence of abundant curviplanar pumice clasts in submarine breccias reflects brittle fracture and dismembering that can occur during fragmentation at the vent or during quenching subsequent transport in water supported pumiceous density currents preserves the fragmental textures careful study is needed to distinguish the products of subaerial versus subaqueous eruptions

deep submarine pyroclastic eruptions theory and predicted landforms and deposits submarine pyroclastic eruptions at depths greater than a few hundred meters are generally considered to be rare or absent because the pressure of the overlying water column is sufficient to suppress juvenile gas exsolution so that magmatic disruption and pyroclastic activity do not occur consideration of detailed models of the ascent and eruption of magma in a range of sea floor environments shows however that significant pyroclastic activity can occur even at depths in excess of 3000 m in order to document and illustrate the full range of submarine eruption styles we model several possible scenarios for the ascent and eruption of magma feeding submarine eruptions 1 no gas exsolution 2 gas exsolution but no magma disruption 3 gas exsolution magma disruption and hawaiian style fountaining 4 volatile content builds up in the magma reservoir leading to hawaiian eruptions resulting from foam collapse 5 magma volatile content insufficient to cause fragmentation normally but low rise speed results in strombolian activity and 6 volatile content builds up in the top of a dike leading to vulcanian eruptions we also examine the role of bulk interaction steam explosivity and contact surface steam explosivity as processes contributing to volcaniclastic formation in these environments we concur with most earlier workers that for magma compositions typical of spreading centers and their vicinities the most likely circumstance is the quiet effusion of magma with minor gas exsolution and the production of somewhat vesicular pillow lavas or sheet flows depending on effusion rate the amounts by which magma would overshoot the vent in these types of eruptions would be insufficient to cause any magma disruption the most likely mechanism of production of pyroclastic deposits in this environment is strombolian activity due to the localized concentration of volatiles in magma that has a low rise rate magmatic gas collects by bubble coalescence and ascends in large isolated bubbles which disrupt the magma surface in the vent producing localized blocks bombs and pyroclastic deposits another possible mode of occurrence of pyroclastic deposits results from vulcanian eruptions these deposits being characterized by the dominance of angular blocks of country rocks deposited in the vicinity of a crater should be easily distinguishable from strombolian and hawaiian eruptions however we stress that a special case of the hawaiian eruption style is likely to occur in the submarine environment if magmatic gas buildup occurs in a magma reservoir by the upward drift of gas bubbles in this case a layer of foam will build up at the top of the reservoir in a sufficient concentration to exceed the volatile content necessary for disruption and hawaiian style activity the deposits and landforms are predicted to be somewhat different from those of a typical primary magmatic volatile induced hawaiian eruption specifically typical pyroclast sizes might be smaller fountain heights may exceed those expected for the purely magmatic hawaiian case cooling of descending pyroclasts would be more efficient leading to different types of proximal deposits and runout distances for density flows would be greater potentially leading to submarine pyroclastic deposits surrounding vents out to distances of tens of meters to a kilometer in addition flows emerging after the evacuation of the foam layer would tend to be very depleted in volatiles and thus extremely poor in vesicles relative to typical flows associated with hawaiian style eruptions in the primary magmatic gas case we examine several cases of reported submarine volcaniclastic deposits found at depths as great as 3000 m and conclude that submarine hawaiian and strombolian eruptions are much more common than previously suspected at mid ocean ridges furthermore the latter stages of development of volcanic edifices seamounts formed in submarine environments are excellent candidates for a wide range of submarine pyroclastic activity due not just to the effects of decreasing water depth but also to 1 the presence of a summit magma reservoir which favors the buildup of magmatic foams enhancing hawaiian style activity and episodic dike emplacement which favors strombolian style eruptions and 2 the common occurrence of alkalic basalts the co2 contents of which favor submarine explosive eruptions at depths greater than tholeiitic basalts these models and predictions can be tested with future sampling and analysis programs and we provide a checklist of key observations to help distinguish among the eruption styles

timing and emplacement dynamics of newly recognised mass flow deposits at 8 12ka offshore soufriere hills volcano montserrat how submarine stratigraphy can complement subaerial eruption histories this contribution describes two mass movement deposits total volume 0 5 km3 identified in seven marine cores located 8 to 15 km offshore southern montserrat west indies the deposits were emplaced in the last 35 ka and have not previously been recognised in either the subaerial or distal submarine records age constraints provided by radiocarbon dating show that an explosive volcanic eruption occurred at ca 8 12 ka emplacing a primary eruption related deposit that overlies a large 0 3 km3 reworked bioclastic and volcaniclastic flow deposit formed from a shelf collapse between 8 and 35 ka the origin of these deposits has been deduced through the correlation of marine sediment cores component analysis and geochemical analysis the 8 12 ka primary volcanic deposit was likely derived from a highly erosive pyroclastic flow from the soufriere hills volcano that entered the ocean and mixed with the water column forming a water supported density current previous investigations of the eruption record suggested that there was a hiatus in activity at the soufriere hills volcano between 16 and 6 ka the ca 8 12 ka eruptive episode identified here shows that this hiatus was shorter than previously hypothesised and thus highlights the importance of obtaining an accurate and complete marine record of events offshore from volcanic islands and incorporating such data into eruption history reconstructions comparisons with the submarine deposit characteristics of the 2003 dome collapse also suggests that the 8 12 ka eruptive episode was more explosive than eruptions from the current eruptive episode

overview of the eruption of soufriere hills volcano montserrat 18 july 1995 to december 1997 the onset of phreatic volcanic activity at the soufriere hills volcano montserrat on 18 july 1995 followed a three year period of heightened volcano seismic activity beneath the island phreatic explosions gave way to continuous eruption of juvenile andesitic magma in the form of a lava dome on or around 15 november 1995 magma production rate has varied leading to changes in eruptive style an explosive eruption on 17 september 1996 followed a period of enhanced dome growth and large scale gravitational collapses from its eastern flank increasing dome volume led to stressing and overtopping of the confining crater walls to the southwest north and west during early 1997 sustained high magma production rate since june 1997 has led to three periods of major gravitational dome collapses followed by vulcanian explosive eruptions dome growth re started immediately after the cessation of the latest of these explosive phases in october 1997 and continues as of december 1997

subaqueous pyroclastic flows and ignimbrites an assessment an assessment of the literature on subaqueous pyroclastic flows and their deposits shows that the term pyroclastic flow is frequently used loosely to describe primary hot gas rich pyroclastic flows mass flows which resulted from the transformation of gassupported flows into water supported ones and secondary mass flows carrying redeposited pyroclastic debris based on subaerial pyroclastic flows the term pyroclastic flow should be restricted to demonstrably hot gas rich mass flows of pyroclastic debris using this definition very few examples of subaqueous pyroclastic deposits with evidence for hot emplacement and of having been wholly submerged have been described in the majority of these cases the evidence for a hot state of emplacement and for the subaqueous nature of the host depositional environment is inadequate the only unequivocal cases of hot pyroclastic flow deposits with adequate supporting evidence are the ordovician nearshore shallow marine ignimbrites of ireland and wales and miocene ignimbrites of southwest japan resulting from the passage of subaerially erupted pyroclastic flows into shallow water other possible examples are near vent dense clast deposits in the donzurobo formation of japan possible submarine intra caldera ponded ignimbrite successions in california and wales and near vent pumiceous deposits of ramsay island wales all other purported cases are either clearly the result of water supported mass flow transportation and deposition debris avalanches debris flows turbidity currents or lack adequate supporting evidence regarding the heat state or the palaeoenvironment only the shallow marine ignimbrites of ireland and wales show adequate evidence of welding but even these could have been nearly wholly exposed above sea level when welding occurred we conclude that when pyroclastic flows enter water they are generally disrupted explosively and or ingest water and transform into water supported mass flows and we suggest the various scenarios in which this occurs there is no evidence to suggest that welding in wholly subaqueous environments is common

sumisu volcano izu bonin arc japan site of a silicic caldera forming eruption from a small open ocean island sumisu volcano was the site of an eruption during 30 60 ka that introduced 48 50 km3 of rhyolite tephra into the open ocean environment at the front of the izu bonin arc the resulting caldera is 8 10 km in diameter has steep inner walls 550 780 m high and a floor averaging 900 m below sea level in the course of five research cruises to the sumisu area a manned submersible two rovs a deep tow camera sled and dredge samples were used to study the caldera and surrounding areas these studies were augmented by newly acquired single channel seismic profiles and multi beam seafloor swath mapping caldera wall traverses show that pre caldera eruptions built a complex of overlapping dacitic and basaltic edifices that eventually grew above sea level to form an island about 200 m high the caldera forming eruption began on the island and probably produced a large eruption column we interpret that prodigious rates of tephra fallback overwhelmed the sumisu area forming huge rafts of floating pumice choking the nearby water column with hyperconcentrations of slowly settling tephra and generating pyroclastic gravity currents of water saturated pumice that traveled downslope along the sea floor thick compositionally similar pumice deposits encountered in odp leg 126 cores 70 km to the south could have been deposited by these gravity currents the caldera rim presently at ocean depths of 100 400 m is mantled by an extensive layer of coarse dense lithic clasts but syn caldera pumice deposits are only thin and locally preserved the paucity of syn caldera pumice could be due to the combined effects of proximal non deposition and later erosion by strong ocean currents post caldera edifice instability resulted in the collapse of a 15 sector of the eastern caldera rim and the formation of bathymetrically conspicuous wavy slump structures that disturb much of the volcano s surface

submarine eruption fed and resedimented pumice rich facies the dogashima formation izu peninsula japan in the izu peninsula japan the pliocene pumice rich dogashima formation 4 55 0 87 ma displays exceptional preservation of volcaniclastic facies that were erupted and deposited in a below wave base marine setting it includes high concentration density current deposits that contain clasts that were emplaced hot indicating an eruption fed origin the lower part of the dogashima 2 unit consists of a very thick sequence 1 m pumice and ash rich bed overlying the breccias may be derived from delayed settling of pyroclasts from suspension in dogashima 1 and 2 thick cross and planar bedded facies composed of sub rounded pumice clasts are intercalated with eruption fed facies implying inter eruptive mass wasting on the flank of a submarine volcano and reworking and resedimentation by high energy tractional currents in a below wave base environment

anatomy of a submarine pyroclastic flow and associated turbidity current july 2003 dome collapse soufriere hills volcano montserrat west indies the 12 to 13 july 2003 andesite lava dome collapse at the soufriere hills volcano montserrat provides the first opportunity to document comprehensively both the sub aerial and submarine sequence of events for an eruption numerous pyroclastic flows entered the ocean during the collapse depositing approximately 90 of the total material into the submarine environment during peak collapse conditions as the main flow penetrated the air ocean interface phreatic explosions were observed and a surge cloud decoupled from the main flow body to travel 2 to 3 km over the ocean surface before settling the bulk of the flow was submerged and rapidly mixed with sea water forming a water saturated mass flow efficient sorting and physical differentiation occurred within the flow before initial deposition at 500 m water depth the coarsest components 60 of the total volume were deposited proximally from a dense granular flow while the finer components 40 were efficiently elutriated into the overlying part of the flow which evolved into a far reaching turbidity current

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

new insights into landslide processes around volcanic islands from remotely operated vehicle rov observations offshore montserrat submarine landslide deposits have been mapped around many volcanic islands but interpretations of their structure composition and emplacement are hindered by the challenges of investigating deposits directly here we report on detailed observations of four landslide deposits around montserrat collected by remotely operated vehicles integrating direct imagery and sampling with sediment core and geophysical data these complementary approaches enable a more comprehensive view of large scale mass wasting processes around island arc volcanoes than has been achievable previously the most recent landslide occurred at 11 5 14 ka deposit 1 1 7 km3 and formed a radially spreading hummocky deposit that is morphologically similar to many subaerial debris avalanche deposits hummocks comprise angular lava and hydrothermally altered fragments implying a deep seated central subaerial collapse inferred to have removed a major proportion of lavas from an eruptive period that now has little representation in the subaerial volcanic record a larger landslide deposit 2 10 km3 occurred at 130 ka and transported intact fragments of the volcanic edifice up to 900 m across and over 100 m high these fragments were rafted within the landslide and are best exposed near the margins of the deposit the largest block preserves a primary stratigraphy of subaerial volcanic breccias of which the lower parts are encased in hemipelagic mud eroded from the seafloor landslide deposits south of montserrat deposits 3 and 5 indicate the wide variety of debris avalanche source lithologies around volcanic islands deposit 5 originated on the shallow submerged shelf rather than the terrestrial volcanic edifice and is dominated by carbonate debris

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

an improved quantitative measure of the tendency for volcanic ash plumes to form in water implications for the deposition of marine ash beds laboratory experiments and numerical simulations have shown that volcanic ash particles immersed in water can either settle slowly and individually or rapidly and collectively as particle laden plumes the ratio of timescales for individual and collective settling in the form of analytical expressions provides a dimensionless quantitative measure of the tendency for such plumes to grow and persist which has important implications for determining particle residence times and deposition rates however existing measures in the literature assume that collective settling obeys stokes law and is therefore controlled by the balance between gravitational forces and viscous drag despite plume development actually being controlled by the balance between gravitational forces and inertial drag even in the absence of turbulence during early times this paper presents a new measure for plume onset which takes into account the inertial drag controlled rather than viscous drag controlled nature of plume growth and descent a parameter study comprising a set of numerical simulations of small scale volcanic ash particle settling experiments highlights the effectiveness of the new measure and by comparison with an existing measure in the literature also demonstrates that the timescale of collective settling is grossly under estimated when assuming that plume development is slowed by viscous drag furthermore the formulation of the new measure means that the tendency for plumes to form can be estimated from the thickness and concentration of the final deposit the magnitude and duration of particle flux across the water s surface do not need to be known the measure therefore permits the residence times of particles in a large body of water to be more accurately and practically determined and allows the improved interpretation of layers of volcaniclastic material deposited at the seabed

pyroclastic flows generated by gravitational instability of the 1996 97 lava dome of soufriere hills volcano montserrat numerous pyroclastic flows were produced during 1996 97 by collapse of the growing andesitic lava dome at soufriere hills volcano montserrat measured deposit volumes from these flows range from 0 2 to 9 106 m flows range from discrete single pulse events to sustained large scale dome collapse events flows entered the sea on the eastern and southern coasts depositing large fans of material at the coast small runout distance 1 km flows had average flow front velocities in the order of 3 10 m s while flow fronts of the larger runout distance flows up to 6 5 km advanced in the order of 15 30 m s many flows were locally highly erosive field relations show that development of the fine grained ash cloud surge component was enhanced during the larger sustained events periods of elevated pyroclastic flow productivity and sustained dome collapse events are linked to pulses of high magma extrusion rates

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

the waterlogging of floating objects we consider the dynamical processes by which a dense porous object floating in a body of liquid becomes waterlogged and sinks we first generalize the classic model of capillary rise in a porous medium to present an analytically tractable model of the process which is valid for objects that are very shallow compared to their horizontal extent we find an analytical expression for the time taken for the object to sink under this approximation we use a series of boundary integral simulations to show that decreasing the horizontal extent of the object decreases the time taken to sink we find that the results of these numerical simulations are in good quantitative agreement with a series of laboratory experiments finally we discuss the implications of our work for pumice fragments often found floating in open water after a volcanic eruption occasionally even supporting plant animal and human remains

diverse patterns of ascent degassing and eruption of rhyolite magma during the 1 8 ka taupo eruption new zealand evidence from clast vesicularity k lauea hawaii like vesuvius and etna is a frequently active volcano associated with a national park which is a focus for volcano tourism two contrasting summit eruptions at k lauea in 2008 2009 and 1959 emphasize how even small eruptions on populous volcanoes have significant impact on both hawaii volcanoes national park hvnp and the surrounding communities the effects are typically more indirect and social than direct physical but have significant fiscal repercussions the former include both positive benefits and negative impacts as an illustration direct consequences of the 1959 eruption were the rerouting of the major park access road and destruction of a summer camp but benefits included an unprecedented surge in visitor numbers and construction of nature trails that are now the most visited asset in the park both positive and negative impacts have increased in intensity with time due to growth in the resident population and visitor numbers for example the entire park has been evacuated twice during the ongoing 2008 2009 eruption

sedimentology of the krakatau 1883 submarine pyroclastic deposits the majority of tephra generated during the paroxysmal 1883 eruption of krakatau volcano indonesia was deposited in the sea within a 15 km radius of the caldera two syneruptive pyroclastic facies have been recovered in scuba cores which sampled the 1883 subaqueous pyroclastic deposit the most commonly recovered facies is a massive textured poorly sorted mixture of pumice and lithic lapilli to block sized fragments set in a silty to sandy ash matrix this facies is indistinguishable from the 1883 subaerial pyroclastic flow deposits preserved on the krakatau islands on the basis of grain size and component abundances a less common facies consists of well sorted planarlaminated to low angle cross bedded vitric enriched silty ash entrance of subaerial pyroclastic flows into the sea resulted in subaqueous deposition of the massive facies primarily by deceleration and sinking of highly concentrated deflated components of pyroclastic flows as they traveled over water the basal component of the deposit suggests no mixing with seawater as inferred from retention of the fine ash fraction high temperature of emplacement and lack of traction structures and no significant hydraulic sorting of components the laminated facies was most likely deposited from low concentration pyroclastic density currents generated by shear along the boundary between the submarine pyroclastic flows and seawater the krakatau deposits are the first well documented example of true submarine pyroclastic flow deposition from a modern eruption and thus constitute an important analog for the interpretation of ancient sequences where subaqueous deposition has been inferred based on the facies characteristics of encapsulating sedimentary sequences

construction of volcanic records from marine sediment cores a review and case study montserrat west indies detailed knowledge of the past history of an active volcano is crucial for the prediction of the timing frequency and style of future eruptions and for the identification of potentially at risk areas subaerial volcanic stratigraphies are often incomplete due to a lack of exposure or burial and erosion from subsequent eruptions however many volcanic eruptions produce widely dispersed explosive products that are frequently deposited as tephra layers in the sea cores of marine sediment therefore have the potential to provide more complete volcanic stratigraphies at least for explosive eruptions nevertheless problems such as bioturbation and dispersal by currents affect the preservation and subsequent detection of marine tephra deposits consequently cryptotephras in which tephra grains are not sufficiently concentrated to form layers that are visible to the naked eye may be the only record of many explosive eruptions additionally thin reworked deposits of volcanic clasts transported by floods and landslides or during pyroclastic density currents may be incorrectly interpreted as tephra fallout layers leading to the construction of inaccurate records of volcanism this work uses samples from the volcanic island of montserrat as a case study to test different techniques for generating volcanic eruption records from marine sediment cores with a particular relevance to cores sampled in relatively proximal settings i e tens of kilometres from the volcanic source where volcaniclastic material may form a pervasive component of the sedimentary sequence visible volcaniclastic deposits identified by sedimentological logging were used to test the effectiveness of potential alternative volcaniclastic deposit detection techniques including point counting of grain types component analysis glass or mineral chemistry colour spectrophotometry grain size measurements xrf core scanning magnetic susceptibility and x radiography this study demonstrates that a set of time efficient non destructive and high spatial resolution analyses e g xrf core scanning and magnetic susceptibility can be used effectively to detect potential cryptotephra horizons in marine sediment cores once these horizons have been sampled microscope image analysis of volcaniclastic grains can be used successfully to discriminate between tephra fallout deposits and other volcaniclastic deposits by using specific criteria related to clast morphology and sorting standard practice should be employed when analysing marine sediment cores to accurately identify both visible tephra and cryptotephra deposits and to distinguish fallout deposits from other volcaniclastic deposits

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

late pleistocene tephrochronology of marine sediments adjacent to montserrat lesser antilles volcanic arc the recent history of the soufriere hills volcano montserrat lesser antilles volcanic arc is deduced using data obtained from a submarine core collected in 2002 the core contains concentrations of ash and several tephra layers which are identified by the abundance of glass shards dense and poorly vesiculated particles and scoria the tephra layers have been dated using micropalaeontology and stable isotope stratigraphy tephra layers in a marine sediment core off the coast of montserrat record the volcanic history of south soufriere hills soufriere hills volcano back to 250 ka eight layers are composed of dense juvenile ash related to dome eruptions five of which can be directly correlated to dated domes or related pyroclastic flow sequences on land six layers are composed of pumiceous glassy ash and relate to significant explosive eruptions a marker sequence of basalt tephra layers is dated at 124 147 ka and is correlated with construction of the south soufriere hills basaltic stratocone pelagic sediments between the main tephra layers have low abundances of volcanogenic components c 10 4 years of dormancy or low activity

the hazards of eruptions through lakes and seawater abstract eruptions through crater lakes or shallow seawater referred to here as subaqueous eruptions present hazards from hydromagmatic explosions such as base surges lahars and tsunamis which may not exist at volcanoes on dry land we have systematically compiled information from eruptions through surface water in order to understand the circumstances under which these hazards occur and what disastrous effects they have caused in the past subaqueous eruptions represent only 8 of all recorded eruptions but have produced about 20 of all fatalities associated with volcanic activity in historical time excluding eruptions that have resulted in about a hundred deaths or less lahars have killed people in the largest number of historical subaqueous eruptions 8 followed by pyroclastic flows excluding base surges 5 tsunamis 4 and base surges 2 subaqueous eruptions have produced lahars primarily on high 1000 m steep sided volcanoes containing small

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

on the emplacement of ignimbrite in shallow marine environments abstract the emplacement of primary pyroclastic flow deposits on the sea floor can occur 1 by nonturbulent dense flows that conserve their heat under water or 2 because the flow either turbulent or nonturbulent temporarily pushes back the shoreline the feasibility of the second mechanism has been investigated theoretically and experimentally using a simple analogue system the experiments involved sustained turbulent saline currents of different densities flowing into a sectorial tank of ambient fluid that was either lighter or denser than the current the currents in all cases displaced the entire layer of ambient fluid up to a distance r irrespective of whether they were denser or lighter r is analogous to the shoreline displacement distance up to which hot ignimbrite can be emplaced by a pyroclastic flow entering the sea by varying flow rate density contrast and depth of the ambient fluid layer a simple equation for r is found that is in good agreement with the experimental data the effect of seawater boiling on shoreline displacement was also investigated by experiments using currents of carbonate solution that produced gas when flowing into a layer of ambient acid the distance r was not reduced in these experiments the equations show that pyroclastic flows of about 10 km 3 or more are capable of pushing back the sea at least a couple of kilometres and possibly more in areas of extensive shallow water enabling hot ignimbrite to be laid down on the seabed at depths corresponding to a few tens of metres shoreline displacement is thus a feasible mechanism for the emplacement of welded ignimbrite in shallow marine settings emplacement of subaerially erupted ignimbrite below storm wave base is not ruled out by the calculations but requires a combination of high discharge rate sustained eruption low density contrast between flow and water steep bathymetric gradient and small island volcano application of the model to the 1883 krakatau eruption suggests that seawater displacement may explain the occurrence of primary ignimbrite on the seabed 10 km northwest of the volcano

the 1986 eruption of mount st augustine field test of a hazard evaluation the 1986 eruption of mount st augustine an island volcano in cook inlet of southcentral alaska provided a good test of previously prepared volcanic hazard models for that volcano the 1986 eruption was generally similar to other historic eruptions an initial vent clearing explosive eruptive phase in late march of 1986 removed part of the dome emplaced at the end of the last eruption 1976 pyroclastic flows reached the sea during this phase and airborne ash spread over much of cook inlet including anchorage the initial explosive phase was followed by dome growth which continued over much of the spring and late summer of 1986 pyroclastic flows much less intense than those of march 1986 accompanied dome growth volcanic hazards from the 1986 eruption were generally within the limits set by previous hazard studies pyroclastic flows were confined to a very high hazard zone on and immediately surrounding augustine island airborne ash and volcanic gases presented a moderate hazard over much of cook inlet and presented a serious hazard to aircraft encountering the ash plume the potential for a large debris avalanche that could enter cook inlet and result in a tsunami existed during the 1986 eruption previous work on mount st augustine underestimated the landslide related hazard and more work is needed to assess fully the potential of this hazard these new data when incorporated into existing hazard assessments will improve an already successful hazard evaluation for mount st augustine assuming that augustine island will never be developed far field hazards associated with ashfalls and tsunamis are the most serious real time satellite and radar imagery can potentially provide adequate warnings about ash plumes but the warning time for tsunami hazards is short improvements in the tsunamis warning system are needed to reduce the hazard

tsunami deposits from major explosive eruptions an example from the 1883 eruption of krakatau inundation of coastal areas by tsunamis during the 1883 eruption of krakatau volcano led to the deposition of unusual pumice enriched deposits fractal analysis of pumice shapes and lithologic characterization of the deposits suggest that the source of the abundant pumiceous material was widespread pumice rafts on the surface of the sunda straits that formed by fallout and pyroclastic flow activity the rafts contained pumices rounded by particle to particle abrasion and were strongly depleted in dense components such as lithics and crystals by differential settling stranding of the floating pumice is inferred to have occurred during the receding phase of tsunamis after they had inundated low lying coastal areas other pumice bearing tsunami deposits contain significant amounts of coral fragments and nonvolcanic beach sediment these units represent redeposition of beach and shallow water sediments that were mixed with varying proportions of primary pyroclastic material the krakatau example illustrates the great diversity of lithofacies that may occur in deposits formed from volcanogenic tsunamis recognition of such deposits in coastal areas near centers of active explosive volcanism may provide an additional criterion with which to assess volcanic hazards

spatiotemporal evolution of a marine caldera forming eruption generating a low aspect ratio pyroclastic flow 7 3 ka kikai caldera japan implication from near vent eruptive deposits abstract the vei 7 7 3 ka caldera forming eruption of the kikai caldera occurred in a shallow sea and caused devastating damage to the prehistoric human settlements of southern kyushu japan this holocene activity at this volcano records eruptions with compositional range of 56 to 74 wt in sio2 spanning with the climactic eruption in this study stratigraphy component and lithology of the pyroclastic deposits were analyzed at near vent islands satsuma iwo jima and take shima in order to reconstruct this eruption stratigraphic sections are characterized by plinian pumice fall deposits unit a intraplinian flow deposits unit b climactic pyroclastic flow deposits units c1 c3 and co ignimbrite ash fall deposits unit d in total the estimated magma volume in the system was 70 80 km3 dre and the eruption therefore represents the evacuation of a major silicic magmatic system the plinian stage phase 1 is subdivided into an initial small phase and a second large one the column height in the second phase was estimated to be 40 43 km the total tephra volume of this stage was estimated to be 40 km3 the magma discharge rate has been calculated from the column height data to be 2 108 kg s the eruption duration is also estimated to be a minimum of approximately 28 h collapse of the column phase 2 produced unit b which consists of multiple thin lithic rich or pumice rich layers or pods including welded pumice fall layers the deposits are characterized by stratified or cross stratified facies and display various degree of welding these sedimentary characteristics indicate that during the plinian column collapse high temperature turbulent density currents were generated where dense pyroclasts were well segregated resulting in the lithic rich layers or pods phase 3 is characterized by units c1 c3 unit c1 shows non welded stratified facies which consist of lithic and crystals including quenched juvenile materials as a minor constituent unit c2 displays welded stratified facies which consist of lithic rich layers and pumice rich layers these two subunits occur only in topographic lows in satsuma iwo jima unit c3 is thickest and poorly sorted non welded massive deposit which includes fragments of welded tuff from underlying units in proximal regions these facts indicate that multiple pyroclastic density currents produced units c1 and c2 in the near vent area and were followed by the main sustained current producing unit c3 a low aspect ratio ignimbrite distributed over a wide area of southern kyushu across the sea varying extents of magma water interactions started during phase 2 continuing during the early stages of phase 3 and diminished during the climactic c3 ignimbrite stage in addition collapse of the caldera may have started before unit c deposition based on the evidence of a fault overlain by unit c on the caldera rim the collapse may have initiated water access to the magma the source appears to have been biased toward the western side of the caldera the holocene evolution of the kikai volcano records the existence of a large silicic magma system at depths of about 7 km that coexisted with or was regularly recharged with mafic magma

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

coring disturbances in iodp piston cores with implications for offshore record of volcanic events and the missoula megafloods piston cores collected from iodp drilling platforms and its predecessors provide the best long term geological and climatic record of marine sediments worldwide coring disturbances affecting the original sediment texture have been recognized since the early days of coring and include deformation resulting from shear of sediment against the core barrel basal flow in due to partial stroke loss of stratigraphy fall in sediment loss through core catchers and structures formed during core recovery and on deck transport the most severe disturbances occur in non cohesive sandy facies which are particularly common in volcanogenic environments and submarine fans although all of these types of coring disturbances have been recognized previously our contribution is novel because it provides an easily accessible summary of methods for their identification this contribution gives two specific examples on the importance of these coring disturbances we show how suck in of sediments during coring artificially created very thick volcaniclastic sand layers in cores offshore montserrat and martinique lesser antilles we then analyze very thick structureless sand layers from the escanaba trough inferred to be a record of the missoula mega floods these sand layers tend to coincide with the base of core sections and their facies suggest coring disturbance by basal flow in destroying the original structure and texture of the beds we conclude by outlining and supporting iodp led initiatives to further reduce and identify coring disturbances and acknowledge their recent successes in drilling challenging sand rich settings such as during iodp expedition 340