subaqueous explosive eruption and welding of pyroclastic deposits silicic tuffs infilling an ancient submarine caldera at mineral king in california show microscopic fabrics indicative of welding of glass shards and pumice at temperatures 500 degrees c the occurrence indicates that subaqueous explosive eruption and emplacement of pyroclastic materials can occur without substantial admixture of the ambient water which would cause chilling intracaldera progressive aggradation of pumice and ash from a thick fast moving pyroclastic flow occurred during a short lived explosive eruption of approximately 26 cubic kilometers of magma in water 150 meters deep the thickness high velocity and abundant fine material of the erupted gas solids mixture prevented substantial incorporation of ambient water into the flow stripping of pyroclasts from upper surfaces of subaqueous pyroclastic flows in general both above the vent and along any flow path may be the main process giving rise to buoyant convective subaqueous eruption columns and attendant fallout deposits

tsunami generation by a rapid entrance of pyroclastic flow into the sea during the 1883 krakatau eruption indonesia 1 the 1883 eruption of krakatau volcano in indonesia was one of the most explosive volcanic events in history it was a marine caldera forming eruption that resulted in voluminous ignimbrite deposits and huge tsunamis we have used numerical simulations to investigate three major mechanisms for tsunami generation caldera collapse phreatomagmatic explosion and pyroclastic flow and have constrained the source parameters computed tsunami characteristics for each hypothesis are compared with observations at locations along the coasts of the sunda strait where tsunami data were obtained immediately after the eruption for the pyroclastic flow hypothesis two types of two layer shallow water models dense and light type models were used under different initial conditions pyroclastic flows are erupted from a circular source following a sine function that assumes waning and waxing phases caldera collapse was performed using a simple piston like plunger model in which collapse duration was assumed to be up to 1 h the phreatomagmatic explosion hypothesis was examined using simple empirical models for underwater explosions in shallow water with explosion energy between 1016 and 1017 j the results show that when a pyroclastic flow with a volume of 5 km3 and an average discharge rate of the order of 107 m3 s enters the sea the computed tsunami heights are broadly consistent with historical records in coastal areas including a tide gauge record at batavia now jakarta we conclude that a pyroclastic flow entering the sea is the most plausible mechanism of the 1883 krakatau tsunami

theoretical analysis of tsunami generation by pyroclastic flows 1 pyroclastic flows are a common product of explosive volcanism and have the potential to initiate tsunamis whenever thick dense flows encounter bodies of water we evaluate the process of tsunami generation by pyroclastic flow by decomposing the pyroclastic flow into two components the dense underflow portion which we term the pyroclastic debris flow and the plume which includes the surge and coignimbrite ash cloud parts of the flow we consider five possible wave generation mechanisms these mechanisms consist of steam explosion pyroclastic debris flow plume pressure plume shear and pressure impulse wave generation our theoretical analysis of tsunami generation by these mechanisms provides an estimate of tsunami features such as a characteristic wave amplitude and wavelength we find that in most situations tsunami generation is dominated by the pyroclastic debris flow component of a pyroclastic flow this work presents information sufficient to construct tsunami sources for an arbitrary pyroclastic flow interacting with most bodies of water

generation mechanism of tsunamis from the 1883 krakatau eruption the 1883 eruption of krakatau generated a destructive tsunami which claimed about 34 000 lives we compute the generation and propagation of tsunamis from three models previously proposed they are 1 large scale caldera collapse of the northern part of krakatau island 2 emplacement of pyroclastic flow deposits 3 submarine explosion we numerically solve the non linear shallow water equations on actual bathymetry all three models displace the same volume 11 5 km 3 of water in different ways we compare the results of our computation with the observed tsunami waveforms at batavia and tsunami wave heights along the coasts of java and sumatra the comparison shows that the submarine explosion model of 1 to 5 min duration best explains the generation of the largest tsunami

paleomagnetic evidence for hot pyroclastic debris flow in the shallow submarine shirahama group upper miocene pliocene japan the cooling history of coarse andesite blocks erupted and deposited proximally in the shallow marine environment has been investigated by paleomagnetic studies and heat conductivity calculations six to 13 samples were cored and measured from six essential andesite blocks 1 5 m in diameter thermal demagnetization techniques were used to detect the directional change of remanence through demagnetization steps each representing different blocking temperatures tb each sample has three components of remanent magnetization c1 c2 and c3 c1 is characterized by a tb of 250 c and an orientation coincident with the present geomagnetic field c2 has a tb of 250 500 c and shows a consistent direction with reversed polarity c3 has a tb of 500 600 c and its orientation is nearly consistent from samples collected from nearby parts of the same clast and are random from samples from different clasts on the basis of the remanence direction of each component the differing tb and the petrography of the samples c1 is interpreted as secondary viscous remanence whereas c2 and c3 are the partial thermoremanent magnetizations obtained during and immediately after emplacement c3 was probably acquired while the blocks were rolling or sliding downslope and c2 formed during continued cooling of the blocks after they came to rest heat conductivity calculations suggest that the blocks were emplaced less than an hour after being erupted if the eruptive vent was only 2 3 km distant as suggested by field relations a downslope transport rate of less than 36 km h is implied

two examples of subaqueously welded ash flow tuffs the visean of southern vosges france and the upper cretaceous of northern anatolia turkey abstract pyroclastic deposits interpreted as subaqueous ash flow tuff have been recognized within archean to recent marine and lacustrine sequences several authors proposed a high temperature emplacement for some of these tuffs however the subaqueous welding of pyroclastic deposits remains controversial the visean marine volcaniclastic formations of southern vosges france contain several layers of rhyolitic and rhyodacitic ash flow tuff these deposits include from proximal to distal settings breccia lapilli and fine ash tuff the breccia and lapilli tuff are partly welded as indicated by the presence of fiamme fluidal and axiolitic structures the lapilli tuff form idealized sections with a lower coarse and welded unit and an upper bedded and unwelded fine ash tuff sedimentary structures suggest that the fine ash tuff units were deposited by turbidity currents welded breccias interbedded in a thick submarine volcanic complex indicate the close proximity of the volcanic source the lapilli and fine ash tuff are interbedded in a thick marine sequence composed of alternating sandstones and shales presence of a marine stenohaline fauna and sedimentary structures attest to a marine depositional environment below storm wave base in northern anatolia thick massive sequences of rhyodacitic crystal tuff are interbedded with the upper cretaceous marine turbidites of the mudurnu basin some of these tuffs are welded as in southern vosges partial welding is attested by the presence of fiamme and fluidal structures the latter are frequent in the fresh vitric matrix these tuff units contain a high proportion of vitroclasis and were emplaced by ash flows welded tuff units are associated with non welded crystal tuff and contain abundant bioclasts which indicate mixing with water during flowage at the base basaltic breccia beds are associated with micritic beds containing a marine fauna the welded and non welded tuff sequences are interbedded in an alternation of limestones and marls these limestones are rich in pelagic microfossils the evidence above strongly suggest that in both examples tuff beds are partly welded and were emplaced at high temperature by subaqueous ash flows in a permanent marine environment the sources of the pyroclastic material are unknown in both cases we propose that the ash flows were produced during submarine fissure eruptions such eruptions could produce non turbulent flows which were insulated by a steam carapace before deposition and welding the welded ash flow tuff deposits of southern vosges and northern anatolia give strong evidence for existence of subaqueous welding

emergence of subaqueous depositional environments in advance of a major ignimbrite eruption capel curig volcanic formation ordovician north wales an example of regional volcanotectonic uplift abstract the garth tuff of the ordovician capel curig volcanic formation comprises a laterally extensive sheet of largely welded rhyolitic ash flow tuff that can be traced from a proximal subaerial to a distal marine environment six facies associations are recognized within subjacent sedimentary successions facies are interpreted as deposits of stream dominated alluvial fans ephemeral streams braidplain lower delta plain delta front and shallow marine environments the development of a large scale coarsening and or shallowing upward sequence reflects the amount of differential tectonic uplift between fault bound highlands and depositional basin there is no evidence for volcanicity associated with sedimentation uplift resulted in aggradation and progradation of fan and braidplain deltas a basinward shift in the loci of dominant sedimentation and a 15 km advance in the position of the pre eruption coastline associated tilting of fault blocks influenced the position of fluvial systems on braidplain with channel belts localizing along one margin in the area of greatest subsidence the spatial correlation between pre emplacement geography and location of probable vent for garth tuff implicates upward and or lateral migration of the magma as the most likely mechanism driving these tectonic changes

growth destruction and facies architecture of effusive and explosive volcanics in the miocene shama basin southwest of saudi arabia subaqueous subaerial volcanism in a lacustrine setting abstract this paper presents for the first time detailed stratigraphical and petrographical analyses of the harrat shama volcanic basin hsvb of miocene age in sw saudi arabia this basin is filled by abundant volcanics intercalated with fluvio lacustrine sedimentary successions these successions are characterized by a syneruptive intereruptive cycle which results from the intercalation periods of volcanics and volcaniclastic sedimentation along with periods of fluvial incision two depositional sequences showing an overall fining upward trend are preserved within the hsvb the first comprises hyaloclastites zeolite bearing bedded tuffs and bedded accretionary lapilli tuffs interpreted as deep lake whereas the second is made up of extensive pumiceous pyroclastic flows peperitic breccias and basaltic lava flows which are in turn capped by siliciclastic deposits of lacustrine deposits the passage between two sequences is defined by lateral variations of the lithofacies and textural features reflecting a change in paleogeography the fining upward volcaniclastic deposits of the hsvb sequence are interpreted to represent the sedimentation pattern induced by tectonic subsidence with the influence of volcaniclastic sediment input indicating well established proximal distal relationship hsvb was evolved through three stages including pre syn and post caldera stages the first two stages correspond to proximal aggradational syn eruptive event whereas the latter stage refers to distal degradational inter eruptive event these stages overlap in time as shown by the alternation of explosive effusive and their resedimented deposits reflecting diverse eruptive sequences formed by different fragmentation processes the difference in the latter coupled with the variability in the magma water interaction and the nature of substrate are responsible for such diversity resulting in progressive environmental changes from subaqueous to subaerial regime from bottom to top the development of such contrast in the depositional sequences reflects variation in the sediment flux and subsidence rate during the evolution of the hsvb

an 18 m thick volcaniclastic interval in pantelleria trough sicily channel deposited from a large gravitative flow during the green tuff eruption abstract a 30 m long piston core from the floor of pantelleria trough penetrated an 18 m thick volcaniclastic interval under and overlain by hemipelagic sediments the distribution and thickness of the interval is mapped in 3 5 khz profiles across the trough core sediments have been analysed for grain size carbonate content detrital petrology and physical properties such as bulk density and magnetic susceptibility chemical analyses of tephra and included minerals have been made by electron microprobe the volcaniclastic interval is correlated with the youngest large explosive eruption on pantelleria the 50 ka green tuff ignimbrite eruption on the basis of the sedimentation rate of overlying sediment calcareous nannofossil and foraminiferal biostratigraphy a calibrated radiocarbon age of 43 47 2 ka possibly infinite in the underlying marls and many similarities in tephra and mineral chemistry and abundance the lower 8 m of the volcaniclastic interval consists of poorly sorted massive units of lapilli tuff interpreted as resulting from the direct flow of a pyroclastic flow into the sea with the energetic hyperconcentrated flows crossing the length of pantelleria trough and eventually freezing on the low gradients the overlying 10 m thick turbidite probably developed from the dilute turbulent pyroclastic flow of the co ignimbrite cloud and the resulting turbulent subaqueous flow eroded bioclastic material from the shelf and deposited a well graded bed from sand through silt to mud the poorer sorting of the volcaniclastic interval compared with the green tuff ignimbrite on land supports the concept that ignimbrite deposits are sorted by aggrading bed processes

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

hydraulics of the august 7 1980 pyroclastic flow at mount st helens washington open channel hydraulic theory for flow of an inviscid fluid down a channel of changing slope and width yields a longitudinal velocity profile that matches measured velocities of the august 7 1980 pyroclastic flow at mount st helens the model is useful for prediction of zones of erosion deposition and hazards it requires only topographic data estimates of volumetric flow rate and a manning coefficient appropriate for the channel

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

an ash flow tuff emplaced in shallow water early miocene koura formation southwest japan abstract the t 4 tuff in non marine shallow water deposits of the early miocene koura formation southwest japan consists mainly of poorly sorted ash pumice and lithic lapilli and is about 20 m thick the tuff grades upward into a parallel to cross stratified surge like layer and further to a fine tuff layer and locally possesses gas escape pipes extending vertically from the base to the top the basal part of the tuff includes ripped up clasts and load marks are common on the sole armored lapilli commonly occur in the upper part especially in the stratified layer the features of the t 4 tuff are mostly those of subaerial ash flow tuff but load marks on the sole and ripped up clasts in the basal part are typical of subaqueous ash flow tuff armored lapilli imply that the ash flow was wet the ash flow from which the t 4 tuff was deposited is interpreted to have advanced across a shallow water body while pushing water aside and becoming wet the t 4 tuff may be a typical facies of ash flows emplaced in quite shallow water bodies

the dali ash island of rhodes greece a problem in interpreting submarine volcanigenic sediments in the southern part of rhodes greece rhyolitic subaqueous pyroclastic deposits are interbedded with tertiary deep water marine sediments the lowermost and best exposed of these deposits the dali ash is described here the deposit has been previously described as a deep water welded subaqueous ignimbrite this paper shows that there is no evidence of welding and texture previously reported were misidentified the dali ash consists of a lower massive unit 5 m thick overlain by a sequence of ash turbidites 2 5 m thick the lower unit was deposited by a high concentration turbidity current and the ash turbidites by dilute turbidity currents foraminifera are dispersed throughout the deposit and indicate that all the sedimentary gravity flows were cold water particulate systems a palaeomagnetic study also suggests they were deposited cold

analysis of earthquake risk in iran based on seismotectonic provinces abstract analysis of seismic risk in iran is made using a seismotectonic provinces model the earthquake data of various seismotectonic provinces are compiled and treated statistically the coefficients of a log linear and a log quadratic magnitude frequency relationship are calculated and return periods for several earthquake magnitudes are found from log linear coefficients seismic risk intensities and ground acceleration for a set of return periods and epicentral distances are estimated and substantial variations in return periods for a given magnitude are noted the provinces which are located southwest of the zagros thrust and northeast of the arabian landmass are the most active in producing earthquakes with magnitude of about 6 in less than a decade whereas the northern and northeastern provinces are capable of producing a magnitude 7 5 earthquake about each century the seismic risk is lowest for esfahan sirgan arabian platform persian gulf kavir in central iran and arvand shatt al arab provinces and the highest seismic risk is for elburz kopet dagh ferdows and fars provinces the expected peak ground acceleration is highest for a time exposure of 30 yrs for fars ferdows and tabas provinces and least for esfahan sirjan for a time exposure of 200 yrs the expected peak ground acceleration is highest for elburz kopet dagh and ferdows provinces

facies architecture of the upper oligocene submarine smrekovec stratovolcano northern slovenia abstract the upper oligocene smrekovec volcanic complex svc represents the remains of a submarine stratovolcano which underwent early miocene tectonic dissection and large scale displacement along the periadriatic line according to the lithofacies architecture abundance and spatial distribution central medial and distal zones of the former volcano edifice have been recognized the zones extend at a distance of 0 2 km 2 5 km and 5 20 km south of the periadriatic line respectively lavas shallow intrusive bodies autobreccia peperite and hyaloclastite deposits occur mainly in the central zone which is extensively altered under hydrothermal conditions characterised by the development of zeolites mainly laumontite prehnite chlorite albite and epidote explosive volcanic activity intensified with the evolution of magma from basaltic andesitic to dacitic gas and water supported eruption fed density flows are assumed their deposits show lithofacies organisation in pyroclastic depositional units pdus similar to either volcaniclastic turbidites type 1 or thin subaqueous ignimbrites type 2 volcaniclastic debris flow deposits are developed as volcaniclastic breccia and sandy debris flow deposits the former occurs in the central and medial zones and the latter occurs in the medial zone only the most widespread lithofacies in the svc are volcaniclastic turbidite deposits three types of sedimentary unit tsu have been distinguished type 1 is 2 5 m thick and dominated by the massive division it mainly occurs in the central zone type 2 is 0 7 3 5 m thick and occurs in the central and medial zones it comprises the massive division bedded fining and thinning upward division and a horizontally wavy and cross laminated division topped by massive fine grained tuff type 3 is 0 1 1 5 m thick and mainly occurs in the distal zone the massive division is absent and thinly bedded and laminated divisions are topped by massive fine grained tuff volcaniclastic turbidites were settled from low density turbidity currents and related suspension clouds mixed volcaniclastic siliciclastic deposits are subordinate in occurrence and restricted to the distal zone they are commonly reworked by bottom currents

source mechanisms of volcanic tsunamis volcanic tsunamis are generated by a variety of mechanisms including volcano tectonic earthquakes slope instabilities pyroclastic flows underwater explosions shock waves and caldera collapse in this review we focus on the lessons that can be learnt from past events and address the influence of parameters such as volume flux of mass flows explosion energy or duration of caldera collapse on tsunami generation the diversity of waves in terms of amplitude period form dispersion etc poses difficulties for integration and harmonization of sources to be used for numerical models and probabilistic tsunami hazard maps in many cases monitoring and warning of volcanic tsunamis remain challenging further technical and scientific developments being necessary and must be coupled with policies of population preparedness

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

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

flow evolution of experimental gravity currents implications for pyroclastic flows at volcanoes abstract gravity current experiments consisting of aqueous glucose flows and turbidity suspensions were performed to investigate mixing and sedimentation processes the evolution from high density to low density flows and the effects of variable grain density the initial solution or suspension was released from a lucite box into a water filled flume 5 5 m long the aqueous glucose solutions had initial densities ranging from 1014 to 1206 kg m 3 the turbidity suspensions consisted of spherical acrylic resins with a narrow grain size of 0 21 0 30 mm 2 25 1 75 and two densities of 1143 and 1196 kg m 3 the initial bulk densities of the turbidity suspensions were 1040 and 1062 kg m 3 both the aqueous glucose flows and the turbidity currents became density stratified with distance with the densest flows exhibiting the largest vertical density gradients for a given initial bulk density the turbidity flows had lower velocities densities viscosities and reynolds numbers due to the effects of sedim

a submarine welded ignimbrite crystal rich sandstone facies association in the cambrian tyndall group western tasmania australia three occurrences of rhyolitic welded ignimbrite are intercalated within a submarine below storm wave base sedimentary succession in the cambrian tyndall group mount read volcanics western tasmania these occurrences are closely associated with very thick crystal rich sandstone facies that is present at this stratigraphic level throughout the tyndall group this facies is interpreted to comprise deposits from syn eruptive crystal rich submarine sediment gravity flows that were generated by interaction of subaerial pyroclastic flows with seawater removal of fine ash and pumice from the submarine flows by hydraulic sorting and flotation resulted in marked crystal enrichment in the deposits rapid essentially syn eruptive aggradation of crystal rich sand led to temporary shoaling so that in some cases subsequent pyroclastic flows deposited welded ignimbrite in shallow marine or possibly subaerial settings e g zig zag hill welded ignimbrite breccia units composed of welded ignimbrite clasts and crystal rich matrix e g comstock and anthony road ignimbrite breccias imply that some welded ignimbrite was submerged providing clasts to syn eruptive submarine crystal rich sediment gravity flows one example of welded ignimbrite cradle mountain link road may have been deposited in an entirely below storm wave base environment the distinctive facies association of welded ignimbrite crystal rich sandstone and ignimbrite clast breccia in the tyndall group exemplifies the submarine record of a major rhyolitic explosive eruption in the source volcanic terrane

the influence of palaeotopography on facies architecture and pyroclastic flow processes of a lithic rich ignimbrite in a high gradient setting the abrigo ignimbrite tenerife canary islands abstract the late pleistocene 188 19 ka abrigo ignimbrite represents the last major explosive caldera forming eruption on tenerife it consists almost entirely of a moderate volume 1 8 km 3 light grey phonolitic lithic rich ignimbrite deposit although a possible abrigo member thin fall deposit is locally preserved beneath the ignimbrite suggesting an initial short lived stable plinian eruption column the ignimbrite deposit is dispersed radially around the lower flanks of the las canadas edifice and correlates to a clast supported lithic breccia on the eastern and northern caldera wall much of the ignimbrite was probably deposited offshore or within the caldera now buried by more recent lavas two major depositional units separated by a co ignimbrite ash cloud deposit and which display dramatic variations in thickness pyroclastic flows were channelised along and deposited within the same valleys that exist today and were commonly continuous and sheet like across the interfluves the basal zones of pyroclastic flows are envisaged as a bed load of coarse lithic lapilli and blocks transported under their own momentum and with the support of the surrounding fine ash gas granular flow

a shallow marine volcaniclastic facies model an example from sedimentary rocks bounding the subaqueously welded ordovician garth tuff north wales u k abstract volcaniclastic sedimentary rocks bounding the ordovician garth tuff in north wales were deposited in a shallow marine setting adjacent to a magmatic arc the volcaniclastic sedimentary rocks are primarily of angular to euhedral quartz angular euhedral feldspar and volcanic rock fragments these grains are of a pyroclastic origin but have been reworked to various degrees sedimentary rock fragments rounded quartz muscovite biotite iron rich chlorite and various clay minerals occur in lesser amounts the sedimentary rocks can be divided into a proximal offshore to foreshore facies association poffa and an offshore to lower shoreface facies association olsfa the sand dominated poffa exposed near capel curig is characterized by large wave ripples hummocky cross stratification co sets of trough cross beds reactivation surfaces plane beds and graded turbidite layers in suspension deposited mudstone water depths calculated from bedding plane exposure of large wave ripples and determined by the presence of hummocky cross stratification and by facies association vary from a few meters to around 30 m 45 50 m theoretical maximum in a 45 m thick stratigraphic section beneath the tuff eight kilometers to the southeast the garth tuff is bounded by a thick sequence of suspension deposited laminated mudstone with thin graded beds of siltstone and fine grained sandstone this facies association olsfa represents the introduction of material by distal turbidites and by pelagic sedimentation the olsfa was deposited in water depths well below storm wave base possibly in excess of 200 m no evidence of storm waves or surface generated currents occur until over 40 m above the tuff this study documents marine sedimentary rocks deposited in water depths ranging from foreshore to offshore as the bounding facies of the garth tuff it is thus reasonable to conclude that the garth tuff was emplaced and welded in water depths greater than the thickness of the tuff in the area of this study

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

submarine silicic volcanism of the healy caldera southern kermadec arc sw pacific i volcanology and eruption mechanisms the submarine healy volcano southern kermadec arc with a 2 2 5 km wide caldera is pervasively mantled with highly vesicular silicic pumice within a water depth of 1 150 1 800 m pumices comprise type 1 white light grey pumice with 30 mm vesicles and weak moderate foliation type 2 grey pumice with millimetre scale laminae flow banded foliation including stretched vesicles 55 mm in length and a minor finely vesicular type 3 pumice all types are sparsely porphyritic with undevitrified glassy groundmass 68 70 sio2 which is microlite and lithic free coexisting pyroxenes yield magma temperatures of 950 c pumice density is 0 5 g cm 3 and vesicularity is 78 83 vesicle size distributions for types 1 and 2 pumice range from 20 m to 20 mm with a strong power law relation with d 2 5 0 4 for vesicles 9 mpa will limit vesicularity to less than the observed 78 83 whereas pressure 6 mpa require a more shallower reconstruction of the edifice and larger volume syn eruptive collapse uniformly high vesicularity is interpreted as evidence of insulation within an eruption column comprising steam and hot pyroclasts most pyroclasts cool condensing and ingesting water into steam inflated vesicles and then sink progression into pyroclastic mode would expand the eruption column displace ambient water reduce the hydrostatic load and further promote vesiculation and fragmentation pyroclasts within the column would quench at these reduced pressures we argue that healy eruptions deeper than 1 000 m cannot be pyroclastic volumes for the lower and upper bounds of edifice size are 2 36 and 3 58 km3 respectively but do not account for intra caldera pumice fill these volumes are considered to be predominantly primary eruption output as shown by a dearth of accessory lithics in all pumice yielding at an average 81 vesicularity eruptive pumice volumes of between 10 and 15 km3 some pyroclasts may have risen to the sea surface and be a correlative of the sea rafted loisels pumice the latter occurs in some new zealand holocene beach sequences and has a estimated age of 590 80 calendar years

distribution and significance of crystalline perlitic and vesicular textures in the ordovician garth tuff wales diverse spherulitic and granular crystalline fabrics perlitic textures and fabrics related to the growth and migration of vesicles occur in the garth tuff a largely welded ordovician ignimbrite defining the distribution of such textures helps to constrain the ignimbrite s cooling and degassing history suites of spherulitic and perlitic textures closely reflect variation in cooling rates seven facies are defined based on the style and intensity of crystallisation 1 a medium to coarsely crystalline equigranular facies 2 an intensely spherulitic facies 3 a sparsely spherulitic facies 4 a pectinate facies 5 a microcrystalline to cryptocrystalline equigranular facies 6 a lithophysal facies and 7 a transitional perlite pectinate facies textural changes from facies 1 to 5 reflect progressively higher cooling rates facies 1 occurs in proximal settings in the ignimbrite s core facies 2 to 5 successively envelop facies 1 with facies 2 becoming the dominant fabric in the ignimbrite s core in medial settings facies 5 is typically developed in the originally glassy perlitic zones at the ignimbrite s welded margins crystallisation under hydrous conditions is reflected by second boiling textures in the sporadically developed lithophysal facies the seventh facies reflects a subtle interplay between cooling hydration and crystallisation which locally prevented perlitic fracturing the distribution of amygdales reflects patterns of volatile migration and entrapment in the lower levels of the ignimbrite amygdales occur in irregular concentrations or rare subvertical pipe like structures pipe like structures attest to fumarolic activity while the ignimbrite was in a rheomorphic state amygdales are widespread and evenly distributed in the upper levels of the ignimbrite however the top of the welding profile is characterised by a thin poorly vesiculated originally vitrophyric horizon that abruptly caps an intense concentration of amygdales ductile and brittle fabrics developed during the upward migration of gas microscopic drag folds occur around some amygdales jigsaw fit to clast rotated breccias originated through both late stage pneumatic fracturing and autobrecciation vaporisation of water at the flow base provided a significant source of volatiles in addition to gas released during cooling and crystallisation secondary alteration has enhanced or modified some fabrics perlitic zones were susceptible to patchy chlorite sericite carbonate diagenetic alteration diagenesis and metamorphism have contributed to the infilling of vesicles

submarine volcaniclastic rocks in this chapter we are concerned with underwater volcaniclastic eruptions and products as well as pyroclastic flows and debris generated on land and transported by mass flowage into the sea submarine fallout deposits derived from land based eruptions are treated in chapter 7 clastic materials redistributed as mass flows or turbidity currents originating from the rapid build up of tephra and epiclastic volcanic debris along coastal parts of volcanoes are also discussed in various places throughout the text e g chap 13

sedimentary signatures of the entrance of coarse grained volcaniclastic flows into the sea the example of the breccia units of the las palmas detritic formation mio pliocene gran canaria eastern atlantic spain abstract the transformations of volcaniclastic gravity flows entering the sea remain poorly understood they are recorded by sedimentary signatures in the volcaniclastic sedimentation of the mio pliocene las palmas detritic formation lpdf of the volcanic island of gran canaria these gravity flows are contemporaneous with an erosive period on the island succeeding the emplacement of the fataga volcanic group and the beginning of the evolution of the roque nublo stratovolcano the sedimentary section that crops out at the mouth of a paleobarranco at las cuevas del guincho comprises subtidal calcareous and clastic sediments affected by wave and storm action units 1 and 3 beach gravels unit 4 volcaniclastic deposits represented by reworked phonolitic pyroclastic deposits unit 1 breccia deposits that contain juvenile and phonolitic gravels and blocks mainly incorporated at the shoreline unit 2 and a debris avalanche deposit at the top unit 5 the breccia deposits display a reversely graded base which disappears distally as a result of seawater incorporation and liquidization of the volcaniclastic mass and mixing with marine sediment this breccia unit is interpreted as having been emplaced by a granular mass flow after transformation of a block and ash flow deposit that entered the sea in the debris avalanche deposits thermal quenching of juvenile basaltic lava clasts and large inclined clastic intrusions of underlying marine sediments suggest that the debris avalanche entered the sea rapid subaqueous emplacement of the granular mass flow and of the debris avalanche deformed the sedimentary substratum water incorporation reduced the influence of the volcaniclastic flows on the substratum ams data confirm the effect of water ingestion and correlative loss of cohesion and allow also reconstruction of paleoflow directions that were normal to the paleoshoreline of the northern coast of gran canaria all these volcaniclastic gravity flow deposits and their textural characteristics are particularly useful in the reconstruction of processes that occur at the subaerial submarine transition and for the location of ancient shorelines

the 1883 eruption of krakatau the 1883 eruption of krakatau was a modest ignimbrite forming event the deposits are primarily coarse grained dacitic non welded ignimbrite large explosions produced pyroclastic flows that entered the sea generating destructive tsunami grain size studies of the ignimbrite suggest that these explosions were not driven by magma seawater interaction the total bulk volume of pyroclastic deposits including co ignimbrite ash is estimated to be 18 21 km3

the behaviour of subaerially produced pyroclastic flows in a subaqueous environment evidence from the roseau eruption dominica west indies abstract the roseau tephra 30 000 yrs b p with its source on the island of dominica represents the largest eruption in the caribbean in the last 200 000 years pyroclastic flows generated by the eruption flowed offshore into the grenada basin giving rise to a widespread layer of sediment gravity flow deposits on land the flow deposits are valley ponded and their granulometric characteristics allow them to be classified as weakly fines depleted ignimbrites offshore the characteristics of the sediment gravity flow deposits are consistent with deposition by numerous small scale events rather than a single large scale event a comparison of the on land and offshore deposits reveals an increase in the crystal lithic ratio in the latter this change can be best explained by the fragmentation of hot pyroclastic material as the subaerial flows mixed with water mixing and fragmentation occurred not at the air water interface but as flows descended the steep submarine slopes of the island of dominica

orbital control of long term moisture in the southwestern usa 1 regional lake sediment records including late pleistocene lake estancia new mexico show that the southwestern usa was wetter than present over the last glacial maximum and drier during the holocene chronology of speleothems that mark groundwater levels in cavenee caverns near lake estancia indicates water level lowering through the cave during the early to middle holocene 8500 yr bp consistent with the timing of desiccation of lake estancia and deflation of the basin floor the speleothem record also shows the same wetter to drier climatic shift during the penultimate glacial and interglacial transition at 134 kyr bp subaqueous crusts formed when cave submerged to 121 kyr bp cave rafts formed during water table drop our data support wet glacials and dry interglacials as being a general feature of pleistocene climate in the southwestern usa the timing suggests that the process is orbitally controlled

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

deformation structures in shale bed indicate flow direction of overlying miocene subaqueous pyroclastic flow thrusts pinch and swell structures and undulations are present within a 2 m thick layered shale bed of miocene age that is overlain by a rhyolite subaqueous pyroclastic flow deposit 2 3 m thick the deformation structures were caused by loading and lateral compression by the subaqueous pyroclastic flow probably analogous to those observed in layered muds deformed by a sand mass advancing across them prominent thrusts strike east west and dip south and the crests of undulations strike east west indicating that the subaqeuous pyroclastic flow moved northward

seismic stratigraphy and geodynamic evolution of christiana basin south aegean arc abstract the analysis and interpretation of vintage and recent seismic profiles across christiana basin in the south aegean arc provided information on the seismic stratigraphy and chrono stratigraphy of the basin the occurrence of volcanic deposits within the sedimentary basin infill and its post miocene geodynamic evolution plio quaternary sediments overlie a messinian erosion surface on the basin s margins while in the basin they have been deposited above the messinian evaporites the basin evolved in two stages during the early stage subsidence of the three main depocenters in the nw w and se part of the basin has been controlled by e w to ese wnw trending faults the end of the early stage is marked by the cessation of activity on the southern marginal fault and the migration of the northern faults towards north where the basin expanded significantly towards south and north the seismic stratigraphy of the younger stage deposits is characterized by the presence of three thick pyroclastic flows they occur only in the eastern part of the basin and have therefore derived from santorini volcanic center since middle pleistocene the youngest one is associated with the minoan eruption while the next one is slightly younger than 0 42 ma and may be linked with activity in peristeria volcano the lower pyroclastic flow has deposited shortly after 1 2 ma and is the thickest 100 msec and widest distributed pyroclastic flow deposit in the basin it is most probably the biggest pyroclastic flow known in the south aegean sea it may be associated with an eruption of the early volcanic centers of akrotiri no evidence of volcanic or volcano clastic deposits derived from west i e from milos volcanic center has been found in christiana basin by taking into consideration the similar two stage structural evolution of the santorini to amorgos area proposed in the literature it is suggested here that since early to middle quaternary the e w trending christiana basin may be considered as an area of extension developed on the right side of the southwestern termination of the ne sw trending dextral santorini amorgos fault zone

tephra deposits associated with a large lava dome collapse soufriere hills volcano montserrat 12 15 july 2003 abstract the 12 13 july 2003 dome collapse at soufriere hills volcano montserrat was the largest event of its kind during the eruption thus far 1995 2005 involving the removal of 210 million m 3 of the lava dome complex over 18 h less than 2 of the total volume of material involved in the dome collapse was deposited on land a pyroclastic density current deposit alongshore and inland from the tar river fan was generated from a single blast originating at the shoreline the blast was caused by the interaction of pyroclastic flows with seawater we propose that at the peak of the lava dome collapse a sharp increase in the volume flux of pyroclastic flows caused substantial displacement of seawater from the shoreline followed by inrush of seawater when the flux decreased a few minutes later the tsunami allowed penetration of seawater into the main body of the pyroclastic flow at the shoreline which led to explosive fragmentation of pyroclastic blocks tephra fall deposits accumulated at a high rate on montserrat causing extensive damage to vegetation and buildings their stratigraphy recorded fallout from high co pyroclastic flow clouds from a vulcanian explosion cloud at the peak in collapse rate caused by the fragmentation of degassed lava dome and from four vulcanian explosion clouds after the dome collapse caused by fragmentation of bubbly magma in the conduit the total tephra fall volume is estimated at 10 20 million m 3

a subaqueous welded tuff from the ordovician of county waterford ireland abstract the metal man tuff mmt from the ordovician of county waterford ireland was emplaced and welded in water depths greater than the thickness of the pyroclastic flow the mmt is the basal member of the middle tramore volcanic formation mtvf of the 5 km thick tramore group the mmt consists of a 10 m thick basal graded zone that represents a pyroclastic flow consisting of angular clasts of black mudstone pumice gray flow banded rhyolite and pink massive rhyolite set in a matrix of non deformed ash shards and pumice maximum grain size grades from large cobbles and small boulders to pebbles the basal 10 30 cm is depleted with respect to the largest boulders resulting in an inversely graded basal layer the basal graded zone passes upward into a transition zone with a strong eutaxitic foliation defined by elongated fiamme of mudstone and flattened pumice overlying this is an upper welded zone with a pronounced eutaxitic foliation columnar jointing flattened ash shards and shards deformed around phenocrysts and spheroids the presence of these features indicate that the deposit is welded was hot and was in motion as the shards deformed the mmt represents a pyroclastic flow that was a hot primary product of an eruption rather than re mobilized cold pyroclastic debris the mmt is bounded by suspension deposited fine grained tuff tuffaceous mudstone and terrigenous mudstone deposited below storm wave base many of the mudstone horizons contain brachipod faunas from shelf depth water nowhere in the 5 km thick tramore group is there terrigenous sandstone evidence of unidirectional flowing water nor any indication of shoreline alluvial environments or subaerial exposure it thus seems reasonable to conclude that the mmt was emplaced and welded subaqueously the geochemistry of the mmt is typical of other high silica 70 78 wt sio2 rhyolite from the ordovician of ireland the chemistry of the mmt is consistent from top to bottom allowing it to be distinguished from associated syndepositional intrusive rock and fine grained tuffs

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

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

the early palaeozoic break up of northern gondwana sedimentology physical volcanology and geochemistry of a submarine volcanic complex in the bavarian facies association saxothuringian basin germany abstract ordovician volcano sedimentary successions of the bavarian facies association in the saxothuringian basin record the continental rift phase of the separation of the saxothuringian terrane from gondwana an 80 m succession from the vogtendorf beds and randschiefer series arenig middle ordovician exposed along the northern margin of the munchberg gneiss massif in northeast bavaria were subjected to a study of their sedimentology physical volcanology and geochemistry the randschiefer series previously has been interpreted as lavas tuffs sandstones and turbidites but the studied ordovician units include four main lithological associations mature sandstones and slates pillowed alkali basalts and derivative mass flow deposits trachyandesitic lavas and submarine pyroclastic flow deposits interbedded with turbidites eight lithofacies have been distinguished based on relict sedimentary structures and textures which indicate deposition on a continental shelf below wave base the explosive phase that generated the pyroclastic succession was associated with the intrusion of dykes and sills and was succeeded by the eruption of pillowed basalts debris flow deposits overlie the basalts ordovician volcanism in this region therefore alternated between effusive and explosive phases of submarine intermediate to mafic volcanism based on geochemical data the volcanic and pyroclastic rocks are classified as basalts and trachyandesites according to their geochemical characteristics especially to their variable concentrations of incompatible elements such as the high field strength elements hfse they can be divided into three groups group i which is formed by massive lavas at the base of the succession has extraordinarily high contents of hfse the magmas of this group were probably derived from a mantle source in the garnet stability field by low ca 1 degrees of partial melting and subsequent fractionation group ii which comprises the pillow lavas at the top of the sequence displays moderate enrichment of hfse this can be explained by a slightly higher degree of melting ca 1 6 for the primary magma group i and ii melts fractionated from their parental magmas in different magma chambers the eruption centres of groups i and ii therefore cannot be the same and the volcanic rocks must have originated from different vents the sills and pyroclastic flow deposits of group iii stem at least partly from the same source as group i rocks of group i most likely mixed together with group ii components during the formation of the group iii flows which became hybridised during eruption transportation and emplacement the sedimentological and geochemical data best support a rift as the tectonic setting of this volcanism analogous to modern continental rift zones hence the rift associated volcanic activity preserved in the vogtendorf beds and randschiefer series represents an early ordovician stage of rift volcanism when the separation of the saxothuringian terrane from gondwana had just commenced

saline and particle driven interfacial intrusions this paper presents a theoretical and experimental investigation into saline and particle driven intrusions along the interface between two layers of dierent densities the conditions at the nose of an intrusion are described in an analysis similar to that applied by benjamin 1968 to boundary gravity currents equations for propagation velocity and front position as functions of relative density are derived these are used in an integral model for intrusions which also includes the eects of sedimentation of particles and detrainment of interstitial fluid the model describes the time evolution of the length of the intrusion and the sediment distribution it produces laboratory experiments were carried out with lock releases of a xed volume of saline or particleladen fluid into a two layer stratication measurements were taken of the intrusion propagation intrusion position and sediment distribution and are found to be in good agreement with the solutions of the integral model horizontal density gradients in a fluid may give rise to gravity currents sometimes called density currents the density gradients can be due to dierences in fluid composition phase temperature or concentration of suspended particulate in a suciently quiescent environment a sharp front may be formed across which a density dierence exists between the current and the surroundings this density dierence drives the mainly horizontal motion of the gravity current when the surrounding fluid is vertically stratied the gravity current can enter it at intermediate levels where its density is comparable with that of the ambient and is then called a gravity intrusion gravity intrusions occur frequently in the atmosphere when rising buoyant air reaches levels where its density matches the ambient density and spreads out horizontally a striking example is the outflow above volcanic plumes which carry ash high into the atmosphere and then spread laterally over many thousands of kilometres gravity intrusions are also an important phenomenon in oceanography a much quoted example is the warm but saline outflow from the mediterranean in the eastern atlantic ocean which produces a saline intrusion at about 1000 m depth particle driven intrusions in the ocean can occur when material is suspended by a river outflow by an underwater landslide from a shelf edge or even articially

dense welding caused by volatile resorption welding of pyroclastic rocks is generally thought to occur by mechanical expulsion of interstitial gas from a deposit as it compacts under its own weight we propose here that volatile resorption and compression can also be important factors in welding we describe densely welded rocks which cannot be explained by loading and re evaluate the welding process taking into account the effects of volatile resorption into the glassy fragments intra caldera oligocene ignimbrites from the rhodope mountains bulgaria and intrusive tuffs of the loch ba ring dyke mull in scotland are intensely welded but some samples lack foliation eutaxitic texture fiamme and glass shards show no preferred orientation in thin section or on outcrop scale fiamme are sometimes complexy deformed into u or s shapes we propose that these textures are the consequence of volatile dissolution in the glass and gas compression scaling analysis indicates that there are two regimes for gas behaviour following emplacement of hot pyroclastic deposits which depend on the relative characteristic time scales of compaction gas escape and gas resorption a gas escape regime and a gas retention regime during explosive eruption glassy pyroclasts are decompressed to less than one atmosphere pressure and are outgassed during deposition and subsequent burial in a thick hot deposit volatiles will be retained and soluble gases h 2 o can be resorbed back into the glass defining the gas retention regime poorly soluble gases air are compressed to small fractions of their original volume resulting in destruction of pore spaces and vesicles in pumice in some circumstances such as volcanic vents the volume changes involve isotropic strain and implosion of the tuff resorption of water greatly speeds up welding during compactional loading by reducing the viscosity of the glass welding is inhibited not only by lower temperatures but also by entrainment of insoluble atmospheric gases the two regimes also can explain the common presence of post emplacement gas escape pipes in non welded ignimbrites and their rarity in densely welded ignimbrite factors that are likely to promote the gas retention regime include fine grain size low collapsing columns a large thickness of tuff and incorporation of external water

syneruptive features and sedimentary processes associated with pyroclastic currents entering the sea the ad 79 eruption of vesuvius bay of naples italy the ad 79 plinian eruption of vesuvius that buried pompeii and herculaneum began with pumice falls deposited towards the se of the volcano followed by pyroclastic currents directed towards the sw these currents reached herculaneum and rapidly entered the sea forming a fan the interpretation of seismic reflection profiles and gravity cores collected off herculaneum documents a submarine fan shaped pyroclastic body at 10 140 m water depth that we interpret as the submarine counterpart of the onshore pyroclastic current deposits this fan c 0 3 km 3 in volume displays a chaotic seismic facies that changes seaward to parallel reflectors and then to wavy reflectors gravity cores reveal a succession consisting of centimetre thick sand or silt sized ash couplets followed by a graded gravelly sand sized bed up to 180 cm thick containing shell fragments and beach derived pebbles overlain by centimetre thick graded and laminated sandy ash layers the depositional textures and sedimentary structures of the submarine pyroclastic fan have been interpreted as the product of the interactions between pyroclasts water waves and tsunamis induced by the ad 79 pyroclastic density currents into the bay of naples

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

submarine crystal tuffs their origin using a lower devonian example from southeastern australia summary the submarine crystal rich volcaniclastics of the lower devonian merrions tuff contain concentrations of angular to euhedral volcanic quartz plagioclase and orthoclase lithic fragments largely altered vitriclasts are minor components and the average matrix content is 37 6 associated dacite andesite and rhyodacitic lavas have average groundmass contents of 64 2 rare shards in the graded pelitic tops of the thick volcaniclastic sedimentation units suggest that the mode of fragmentation originally was by explosive magmatic eruptions the sedimentology of the volcaniclastics suggests subsequent redeposition by cold state mass flow processes the volcaniclastics form an isotopically coherent suite and so redeposition must have occurred essentially contemporaneously with eruption the high crystal fragment concentration in these volcaniclastics is higher than lavas and ignimbrites and suggests some process whereby the groundmass fraction of the erupting magma is selectively removed so concentrating the crystal fraction the crystal rich character of the crystal tuffs is not simply due to explosive eruption several primary and secondary factors processes could have interacted to ultimately produce the crystal rich character these being i eruption of highly crystallized magmas 65 phenocrysts ii concentration of crystals in primary eruption columns iii concentration of crystals in any resulting pyroclastic flows fine vitric ashes being elutriated out and being carried away in accompanying trailing ash clouds iv concentration of crystals in secondary eruption columns generated by the flow of hot pyroclastic flows into the ocean and v concentration of crystals by the elutriation of fines into the trailing fine sediment cloud accompanying submarine mass flows resulting from the slumping of volcaniclastic aggregates from shallow marine subaerial settings

comagmatic granophyric granite in the fish canyon tuff colorado implications for magma chamber processes during a large ash flow eruption the 27 8 ma fish canyon tuff a vast ash flow sheet 5000 km 3 of uniform phenocryst rich dacite is representative of monotonous intermediate eruptions from a magma chamber that lacked compositional gradients sparse small fragments of comagmatic granophyre in late erupted tuff and postcaldera lava having mineral compositions indistinguishable from phenocrysts in the tuff and precaldera lava like rocks record complex events in the fish canyon chamber just prior to eruption sanidine phenocrysts in the granophyre preserve zoning evidence of mingling with andesitic magma then shattering by decompression and volatile loss accompanying early fish canyon eruptions before overgrowth by granophyre the textural and chemical disequilibria indicate that the eruption resulted from batholith scale remobilization of a shallow subvolcanic chamber contrary to previous interpretations of magma storage and phenocryst growth in the lower crust

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