debris jets in continental phreatomagmatic volcanoes a field study of their subterranean deposits in the coombs hills vent complex antarctica abstract the ferrar large igneous province of antarctica contains significant mafic volcaniclastic deposits some of which are interpreted to fill large vent complexes such a complex was re examined at coombs hills to map individual steep sided cross cutting bodies in detail and we found several contrasting types two of which are interpreted to have filled subterranean passageways forcefully opened from below into existing non consolidated debris these transient conduits were opened because of the propagation of debris jets upward moving streams of volcaniclastic debris steam magmatic gases liquid water droplets following explosive magma aquifer interaction some debris jets probably remained wholly subterranean whereas others made it to the surface but the studied outcrops do not allow us to differentiate between these cases the pipes filled with country rock rich lapilli tuff or tuff breccia are interpreted to have formed following phreatomagmatic explosions occurring near the walls or floor of the vent complex causing fragmentation of both magma and abundant country rock material in contrast some of the cross cutting zones filled with basalt rich tuff breccia or lapilli tuff could have been generated following explosions taking place within pre existing basalt bearing debris well away from the complex walls or floor we infer that once focused jets were formed they did not incorporate significant amounts of existing debris while travelling through them instead incorporation of fragments from the granular host took place near explosion sites other basalt rich tuff breccia zones accompanied by domains of in situ peperite and coherent basalt pods are inferred to have originated by less violent processes

the mechanism of surtseyan volcanism during eruption the vent of a surtseyan volcano is occupied by a highly mobile slurry of tephra hyaloclastite and water as magma ascends rapidly through the slurry mixing occurs due to velocity shear acceleration of the fluid fluid contacts and mass incorporation consequent expansion of the admixed water causes the jetting and continuous up rush activity that characterizes surtseyan volcanism there is no evidence of the widely accepted fuel coolant interaction mechanism

morphology and dynamics of explosive vents through cohesive rock formations shallow explosive volcanic processes such as kimberlite volcanism and phreatomagmatic and phreatic activity produce volcanic vents exhibiting a wide variety of morphologies including vertical pipes and v shaped vents in this study we report on experimental and numerical models designed to capture a range of vent morphologies in an eruptive system using dimensional analysis we identified key governing dimensionless parameters in particular the gravitational stress to fluid pressure ratio 2 p gh and the fluid pressure to host rock strength ratio 3 p c we used combined experimental and numerical models to test the effects of these parameters the experiments were used to test the effect of 2 on vent morphology and dynamics a phase diagram demonstrates a separation between two distinct morphologies with vertical structures occurring at high values of 2 and diagonal ones at low values of 2 the numerical simulations were used to test the effect of 3 on vent morphology and dynamics in the numerical models we see three distinct morphologies vertical pipes are produced at high values of 3 diagonal pipes at low values of 3 and horizontal sills at intermediate values of 3 our results show that vertical pipes form by plasticity dominated yielding in high energy systems high 2 and 3 whereas diagonal and horizontal vents dominantly form by fracturing in lower energy systems low 2 and 3 although our models are two dimensional they suggest that circular pipes result from plastic yielding of the host rock in a high energy regime whereas v shaped volcanic vents result from fracturing of the host rock in lower energy systems

an analytical study for 1 dimensional steady flow in volcanic conduits the effects of magma properties viscosity and volatile content and geological conditions radius and length of conduit on the dynamics of explosive eruptions are analytically investigated on the basis of a simplified model for 1 dimensional steady flow in a volcanic conduit the conduit flow is characterized by bubbly flow before magma fragmentation and by gas pyroclast flow after magma fragmentation the features of the flow are largely determined by the balance of viscous effects of the bubbly flow and expansion of the gas pyroclast flow under a given boundary condition those effects are described by two non dimensional numbers and the parameter represents the ratio of effects of wall friction and gravity in the bubbly flow the length of the bubbly flow region decreases as increases the parameter represents the ratio of pressures at the vent and at fragmentation level the length of the gas pyroclast flow region decreases as increases three regimes of conduit flow are defined according to when is smaller than a certain value determined by initial volatile content and length of conduit the conduit flow is composed mainly of the gas pyroclast flow region regime 1 on the other hand when 10 the conduit flow is composed mainly of the bubbly flow region regime 3 regime 2 is the intermediate regime between regimes 1 and 3 the flow variables such as level of fragmentation and magma discharge rate are sensitive to magmatic properties and geological conditions in regime 2 a graphical method to estimate the effects of these parameters is also proposed

eruption processes and deposit characteristics at the monogenetic mt gambier volcanic complex se australia implications for alternating magmatic and phreatomagmatic activity the 5 ka mt gambier volcanic complex in the newer volcanics province australia is an extremely complex monogenetic volcanic system that preserves at least 14 eruption points aligned along a fissure system the complex stratigraphy can be subdivided into six main facies that record alternations between magmatic and phreatomagmatic eruption styles in a random manner the facies are 1 coherent to vesicular fragmental alkali basalt effusive hawaiian spatter and lava flows 2 massive scoriaceous fine lapilli with coarse ash strombolian fallout 3 bedded scoriaceous fine lapilli tuff violent strombolian fallout 4 thin medium bedded undulating very fine lapilli in coarse ash dry phreatomagmatic surge modified fallout 5 palagonite altered cross bedded medium lapilli to fine ash wet phreatomagmatic base surges and 6 massive palagonite altered very poorly sorted tuff breccia and lapilli tuff phreato vulcanian pyroclastic flows since most deposits are lithified to quantify the grain size distributions gsds image analysis was performed the facies are distinct based on their gsds and the fine ash to coarse fine ash ratios these provide insights into the fragmentation intensities and water magma interaction efficiencies for each facies the eruption chronology indicates a random spatial and temporal sequence of occurrence of eruption styles except for a magmatic horizon of effusive activity occurring at both ends of the volcanic complex simultaneously the eruption foci are located along nw se trending lineaments indicating that the complex was fed by multiple dykes following the subsurface structures related to the tartwaup fault system possible factors causing vent migration along these dykes and changes in eruption styles include differences in magma ascent rates viscosity crystallinity degassing and magma discharge rate as well as hydrological parameters

possible creep related tilt precursors obtained in the central apennines italy and in the southern caucasus georgia daily averaged tilt component data from two sites of the central apennines italy and of the southern caucasus georgia respectively revealed intermediate term tilts as possible precursors to earthquakes m 3 0 4 7 which occurred in the above mentioned seismic areas within a distance of 50 km from the sites a good temporal correlation as well as a fair spatial correspondence between these residual tilts with amplitude and duration of some microradians and months respectively and main shocks were pointed out by removing both secular trends and seasonal thermoelastic effects from the raw tilts an attempt was made to justify the above mentioned results based on the assumption that the observed intermediate term preseismic tilts are the manifestation of aseismic creep episodes of comparable duration in the fault materials of thrust faults close to the tilt sites the mechanism refers to a strain field slowly propagating from the preparation focal area to the tilt site through crustal blocks separated by weak transition zones this propagation is thought to be the cause of the local aseismic fault slip recorded by the tiltmeters previously both discrete structures and strain propagation effects were revealed in the central apennines and are thought also to exist in the southern caucasus as in the past the rheological properties of fault materials are revealed as viscoelastic ones in fact creep equations obtained by applying several viscoelastic models on our data proved to fit quite well some of the observed tilt precursors producing viscosity and rigidity values very similar to those reported in literature

numerical modeling of preseismic slow movements of crustal blocks caused by quasi horizontal tectonic forces abstract a 2d model is proposed for slow crustal movements caused by subhorizontal forces applied to a system of rigid blocks separated by transition zones faults filled with viscoelastic material the aim of the modeling is to explain the main features of tilt anomalies observed prior to earthquakes it is assumed that an earthquake is prepared within the fault zone where macroproperties of the fault gouge material are changed due to the increase of microfractures the fault zones are inclined and the blocks are allowed to move in both vertical and horizontal directions as well as to rotate numerical experiments were performed for systems of two and seven blocks with different viscosity and rigidity of the fault material the shape of the anomalies as well as the time delay and decay of the anomaly amplitude with distance from the earthquake preparation zone are explained by the model

topographic controls on a phreatomagmatic maar diatreme eruption field and numerical results from the holocene dotsero volcano colorado usa we analyze the eruptive products of holocene dotsero volcano colorado usa 4 15 ka which record evidence of a progression from effusive magmatic activity through explosive phreatomagmatic maar forming activity to a final explosive magmatic phase the nature of the deposits suggests that the irregular and mountainous pre eruptive topography strongly influenced the formation of directed pyroclastic density currents pdcs during the phreatomagmatic phase where the topographically high northern crater rim acted as a barrier and promoted transport to the south furthermore the crater shape strongly controlled the grain size of the final deposits providing further evidence that deposit grain size can be a misleading proxy for fragmentation or explosion intensity we test these hypotheses as gleaned from fieldwork by implementing a multiphase numerical model to simulate discrete explosions and their resulting pdcs at maar diatreme volcanoes we confirm that a crater shape where one rim is higher than the other promotes multiple pulses of particle transport in the direction of the lower rim the initial pdcs contain both coarse and fine particles whereas pdcs generated by later pulses contain predominately finer particles this mechanism provides an alternative explanation to the formation of lapilli tuff tuff couplets often found at maar diatreme tephra rings including dotsero volcano additional modeling shows how the crater depth can control grain size of the resulting pdc deposits in the case of a deep crater and a relatively weak explosion it is possible that only the fine particles escape following collapse of the eruptive column resulting in a fine grained deposit this work allows us to improve models developed to describe emplacement of pdcs for different crater geometries and informs on the dynamics of maar diatreme eruptions but also highlights the limitations of our knowledge of key parameters especially those related to the processes of magma water interaction and syn eruptive hydrogeology during such events

deformation of the mitaka rhombus strain buildup following the 1923 kanto earthquake central honshu japan frequently repeated surveys of a local network near tokyo provide considerable detail on the horizontal strain changes immediately adjacent to the rupture zone of the m 8 1 kanto earthquake changes in the three independent tensor strain components are determined for 35 epochs during 1916 1980 to a precision of about 1 5 strain no significant shear or elongational strain changes preceded the earthquake a change in dilational strain is marginally significant postseismic movements are large accumulating to 40 or more of the coseismic strain drop they have at least two time scales a distinct short term transient lasting less than a year is explained well by slip or localized deformation that occurs downdip of the coseismic rupture plane and has a time constant of about 3 months more complex longer period postseismic movements continue for at least another 10 years their precise duration as well as the transition to presumably steadier interseismic deformation is obscured by notable irregularities in the strain buildup at mitaka as a result neither the interseismic strain rate nor the occurrence time of the next kanto earthquake can be reliably estimated from these observations

viscoelastic stress relaxation on deep fault sections as a possible source of very long period elastic waves observations of very long period elastic waves are attributed to silent earthquakes events which are presumed to occur at depth in a nonbrittle regime the fault at depth is modeled as a thin viscoelastic gouge layer embedded in an elastic medium the source of silent earthquakes is envisaged as a prolonged slow slippage of the fault faces which yields a radiation spectrum strongly depleted in the high frequency components and dominated by the near field term the results of the model are compared with the dominant frequencies mean duration and amplitude of a long period perturbation recorded at trieste in the years preceding the 1976 friuli italy earthquake the role that silent earthquakes may play in the overall earthquake mechanism in particular as precursory phenomena is discussed

deep bedded ultramafic diatremes in the missouri river breaks volcanic field montana usa 1 km of syn eruptive subsidence the ultramafic eocene missouri river breaks volcanic field mrbvf montana usa includes over 50 diatremes emplaced in a mostly soft substrate the current erosion level is 1 3 1 5 km below the pre eruptive surface exposing the deep part of the diatreme structures and some dikes five representative diatremes are described here they are 200 375 m across and have sub vertical walls their infill consists mostly of 55 90 bedded pyroclastic rocks fine tuffs to coarse lapilli tuffs with concave upward bedding and 45 10 non bedded pyroclastic rocks medium lapilli tuffs to tuff breccias the latter zones form steep columns 15 135 m in horizontal dimension which cross cut the bedded pyroclastic rocks megablocks of the host sedimentary formations are also present in the diatremes some being found 1 km or more below their sources the diatreme infill contains abundant lithic clasts and ash sized particles indicating efficient fragmentation of magma and country rocks the spherical to sub spherical juvenile clasts are non vesicular they are accompanied by minor accretionary lapilli and armored lapilli the deposits of dilute pyroclastic density currents are locally observed our main interpretations are as follows 1 the observations strongly support phreatomagmatic explosions as the energy source for fragmentation and diatreme excavation 2 the bedded pyroclastic rocks were deposited on the crater floor and subsided by 1 0 1 3 km to their current location with subsidence taking place mostly during the eruption 3 the observed non bedded pyroclastic columns were created by debris jets that punched through the bedded pyroclastic material the debris jets did not empty the mature diatreme occupying only a fraction of its width and some debris jets probably did not reach the crater floor 4 the mature diatreme was nearly always filled and buttressed by pyroclastic debris at depth there was never a 1 3 1 5 km deep empty hole with sub vertical walls otherwise the soft substrate would have collapsed inward which it only did near the surface to create the megablocks we infer that syn eruptive subsidence shifted down bedded pyroclastic material and shallow sedimentary megablocks by 0 8 1 1 km or more after which limited post eruptive subsidence occurred this makes the mrbvf diatremes an extreme end member case of syn eruptive subsidence in the spectrum of possibilities for maar diatreme volcanoes worldwide

multiphase flow above explosion sites in debris filled volcanic vents insights from analogue experiments abstract discrete explosive bursts are known from many volcanic eruptions in maar diatreme eruptions they have occurred in debris filled volcanic vents when magma interacted with groundwater implying that material mobilized by such explosions passed through the overlying and enclosing debris to reach the surface although other studies have addressed the form and characteristics of craters formed by discrete explosions in unconsolidated material no details are available regarding the structure of the disturbed debris between the explosion site and the surface field studies of diatreme deposits reveal cross cutting steep sided zones of non bedded volcaniclastic material that have been inferred to result from sedimentation of material transported by debris jets driven by explosions in order to determine the general processes and deposit geometry resulting from discrete explosive injections of entrained particles through a particulate host we ran a series of analogue experiments specific volumes of compressed 0 5 2 5 mpa air were released in bursts that drove gas particle dispersions through a granular host the air expanded into and entrained coloured particles in a small crucible before moving upward into the host white particles each burst drove into the host an expanding cavity containing air and coloured particles total duration of each run recorded with high speed video was approximately 0 5 1 s the coloured beads sedimented into the transient cavity this same behaviour was observed even in runs where there was no breaching of the surface and no coloured beads ejected a steep sided body of coloured beads was left that is similar to the cross cutting pipes observed in deposits filling real volcanic vents in which cavity collapse can result not only from gas escape through a granular host as in the experiments but also through condensation of water vapour a key conclusion from these experiments is that the geometry of cross cutting volcaniclastic deposits in volcanic vents is not directly informative of the geometry of the intrusions that formed them an additional conclusion is that complex structures can form quickly from discrete events

simulating maar diatreme volcanic systems in bench scale experiments maar diatreme eruptions are incompletely understood and explanations for the processes involved in them have been debated for decades this study extends bench scale analogue experiments previously conducted on maar diatreme systems and attempts to scale the results up to both field scale experimentation and natural volcanic systems to produce a reconstructive toolkit for maar volcanoes these experimental runs produced via multiple mechanisms complex deposits that match many features seen in natural maar diatreme deposits the runs include deeper single blasts series of descending discrete blasts and series of ascending blasts debris jet inception and diatreme formation are indicated by this study to involve multiple types of granular fountains within diatreme deposits produced under varying initial conditions it is not possible to infer the energies of single blasts in multiple blast series from the final deposits the depositional record of blast sequences can be ascertained from the proportion of fallback sedimentation versus maar ejecta rim material the final crater size and the degree of overturning or slumping of accessory strata quantitatively deeper blasts involve a roughly equal partitioning of energy into crater excavation energy versus mass movement of juvenile material whereas shallower blasts expend a much greater proportion of energy in crater excavation supplementary materials five video files 12 figures and three tables are available at http www geolsoc org uk sup18890

a 2 d model for tilt and strain fields associated to earthquakes in crustal block structures abstract a 2 d model for slow crustal movements including tilt and strain anomalies before earthquakes as well as shape and time lag of each preseismic anomaly with respect to the time of origin of the corresponding earthquake is proposed the model represents a numerical study to evaluate the behavior of a series of crustal rigid blocks separated by narrow fault zones filled with viscoelastic material rheology of the fault material is described by constitutive equations of standard linear solid vertical displacement and rotation of the blocks result from the vertical uplift of one central block due to a vertical force displacements and tilts of blocks are computed from a system of linear differential equations which are solved numerically the main characteristic feature of the model is that the viscoelastic parameters of the fault zone where fracture occurs at the boundary of the rising block may change in time simulating the earthquake preparation numerical modelling shows that tilt behavior at successive blocks reveals anomalies which are similar in shape to the observed ones the anomalies are of opposite signs in adjacent blocks and show a time lag with distance from the fracturing zone

rheological models and interpretation of postglacial uplift accepted 1989 february 1 received 1989 january 28 in original form 1988 september 26 summary paper investigates four generalized maxwebl bodies gmbs and the classical maxwell body cmb with respect to their potential to explain postglacial uplift the gmbs are the lomnitz body defined by a logarithmic creep law the q power law body defined by a power law dependence of the quality factor on frequency the caputo body defined by a special stress strain relation with memory and the burgers body the burgers body is a 4 parameter model the other gmbs have 3 and the cmb has only 2 parameters theoretical uplift curves are calculated for simple space time models of the canadian and fennoscandian deglaciations and hedgehog inversion yields the successful model parameters the gmbs explain the data better than the cmb the parameters of the burgers body are only rather weakly constrained because of trade offs among them from the success of the gmbs we conclude that long time scale transient creep is a possible rheological behaviour of mantle rocks viscosity extrapolation from the time constant of postglacial uplift about 10 ka to the time constant of mantle convection roughly 100 ma is made with the effective viscosity and yields approximately ld2 pas q power law body loz3 pas caputo body and loz5 pas lomnitz body hence postglacial uplift does not strongly constrain the long time scale mantle viscosity it provides only a lower bound about loz1 pas this value follows from data interpretation with the cmb model in which steady state creep is present alone rayleigh number estimates for whole mantle convection are clearly supercritical for the cmb the q power law body and the caputo body but only about critical or even subcritical for the lomnitz body both postglacial uplift and mantle convection is satisfactorily explained by the cmb the q power law body and the caputo body

volcanology of tuzo pipe gahcho kue cluster root diatreme processes re interpreted abstract the middle cambrian 540 ma gahcho kue kimberlite field is situated about 275 km ene of yellowknife nwt canada the kimberlites were emplaced into 2 6 ga archean granitic rocks of the yellowknife supergroup four larger kimberlite bodies 5034 tesla tuzo and hearne as well as a number of smaller pipes and associated sheets occur in the field in plan view the tuzo pipe has a circular outline at the surface and it widens towards deeper levels the pipe infill consists of several types of coherent and fragmental kimberlite facies coherent or apparent coherent possibly welded kimberlite facies dominate at depth but also occur at shallow levels as dikes intruded late in the eruptive sequence or individual coherent kimberlite clasts the central and shallower portions of the pipe consist of several fragmental kimberlite varieties that are texturally classified as tuffisitic kimberlites the definition geometry and extent of the geological units are complex and zones controlled by vertical elements are most significant the fluidal outlines of some of the coherent kimberlite clasts suggest that at least some are the product of disruption of magma that was in a semi plastic state or even of welded material ragged clasts at low levels are inferred to form part of a complex peperite like system that intrudes the base of the root zone a variable often high abundance of local wall rock xenoliths between and within the kimberlite phases is observed varying in size from sub millimeter to several tens of meters wall rock fragments are common at all locations within the pipe but are especially frequent in a domain with a belt like geometry between 120 and 200 m depth in the pipe steeply outward dipping bedded deposits made up of wall rock fragments occur in deep levels of the pipe and are especially common under the downward widening roof segments the gradational contact relationships of these deposits with the surrounding kimberlite bearing rocks as well as their location suggest that they formed more or less in situ different breccia facies inside the pipe suggest an origin by slumping grain flows rock fall or pyroclastic deposition the shape and facies architecture of the tuzo pipe suggests that the studied section of the pipe lies at a root zone diatreme transitional structural level composite coherent kimberlite clasts imply that recycling processes were active over time while reworked wall rock rich deposits and ductily deformed clasts of welded kimberlite point to the presence of temporary cavities in the root zone the emplacement of the tuzo pipe did not occur in a single violent explosion but involved repetitive volcanic explosions alternating with periods of relative quiescence the observed features are typical of phreatomagmatic processes which may include phases of less explosive magmatic activity

tilt measurements and seismicity in central italy over a period of approximately three years tilt data obtained from july 1983 january 1986 at three sites in the central apennines italy have been examined a filtering of raw recordings indicated the existence of residual anomalies attempting a correlation between such anomalies and regional seismicity 3 0 magnitudem 5 4 has allowed us to interpret the anomalies as possible preseismic variations the results seem to confirm the existence of a relation between precursor time t earthquake magnitude m and epicentral distance r finally the presence of a time delay between the onsets of the anomalies detected at different sites for the same earthquake gives further credit to the possibility of the existence of their propagation

geology of the victor kimberlite attawapiskat northern ontario canada cross cutting and nested craters abstract the pipe shapes infill and emplacement processes of the attawapiskat kimberlites including victor contrast with most of the southern african kimberlite pipes the attawapiskat kimberlite pipes are formed by an overall two stage process of 1 pipe excavation without the development of a diatreme sensu stricto and 2 subsequent pipe infilling the victor kimberlite comprises two adjacent but separate pipes victor south and victor north the pipes are infilled with two contrasting textural types of kimberlite pyroclastic and hypabyssal like kimberlite victor south and much of victor north are composed of pyroclastic spinel carbonate kimberlites the main features of which are similar clast supported discrete macrocrystal and phenocrystal olivine grains pyroclastic juvenile lapilli mantle derived xenocrysts and minor country rock xenoliths are set in serpentine and carbonate matrices these partly bedded juvenile lapilli bearing olivine tuffs appear to have been formed by subaerial fire fountaining airfall processes the victor south pipe has a simple bowl like shape that flares from just below the basal sandstone of the sediments that overlie the basement the sandstone is a known aquifer suggesting that the crater excavation process was possibly phreatomagmatic in contrast the pipe shape and internal geology of victor north are more complex the northwestern part of the pipe is dominated by dark competent rocks which resemble fresh hypabyssal kimberlite but have unusual textures and are closely associated with pyroclastic juvenile lapilli tuffs and country rock breccias volcaniclastic kimberlite current evidence suggests that the hypabyssal like kimberlite is in fact not intrusive and that the northwestern part of victor north represents an early formed crater infilled with contrasting extrusive kimberlites and associated breccias the remaining main part of victor north consists of two macroscopically similar but petrographically distinct pyroclastic kimberlites that have contrasting macrodiamond sample grades the juvenile lapilli of each pyroclastic kimberlite can be distinguished only microscopically the nature and relative modal proportion of primary olivine phenocrysts in the juvenile lapilli are different indicating that they derive from different magma pulses or phases of kimberlite and thus represent separate eruptions the initial excavation of a crater cross cutting the earlier northwestern crater was followed by emplacement of phase i a low grade olivine phenocryst rich pyroclastic kimberlite and the subsequent eruption of phase ii a high grade olivine phenocryst poor pyroclastic kimberlite as two separate vents nested within the original phase i crater the second eruption was accompanied by the formation of an intermediate mixed zone with moderate grade thus the final pyroclastic pipe infill of the main part of the victor north pipe appears to consist of at least three geological macrodiamond grade zones in conclusion the victor kimberlite was formed by several eruptive events resulting in adjacent and cross cutting craters that were infilled with either pyroclastic kimberlite or hypabyssal like kimberlite which is now interpreted to be of probable extrusive origin within the pyroclastic kimberlites of victor north there are two nested vents a feature seldom documented in kimberlites elsewhere this study highlights the meaningful role of kimberlite petrography in the evaluation of diamond deposits and provides further insight into kimberlite emplacement and volcanism

models of crustal deformation this review is principally concerned with theoretical models of crustal deformation developed during this quadrennium observational data as well as topics in lithospheric flexure and isostasy are covered in companion reviews in this volume progress has been made toward constructing both new inelastic and inhomogeneous elastic models of earthquake induced crustal deformation most of the inelastic models involve dislocations or green s functions in an elastic or anelastic lithosphere which overlies a maxwell viscoelastic asthenosphere

geology of the mwadui kimberlite shinyanga district tanzania abstract the mwadui pipe represents the largest diamondiferous kimberlite ever mined and is an almost perfectly preserved example of a kimberlitic crater in fill albeit without the tuff ring the geology of mwadui can be subdivided into five geological units viz the primary pyroclastic kimberlite pk re sedimented volcaniclastic kimberlite deposits rvk granite breccias subdivided into two units the turbidite deposits and the yellow shales listed in approximate order of formation the pk can be further subdivided into two units lithic rich ash and lapilli tuffs which dominate the succession and lithic poor juvenile rich ash and lapilli tuffs the lower crater is well bedded down to at least 684 m from present surface extent of current drill data the bedding is defined by the presence of juvenile rich lapilli tuffs vs lithic rich lapilli tuffs and the systematic variation in granite content and clast size within much of the lithic rich lapilli tuffs four distinct types of bedding have been identified in the pyroclastic deposits diffuse zones characterised by increased granite abundance and size and upward fining units represent the dominant types throughout the deposit lateral heterogeneity was observed in addition to the vertical changes suggesting that the eruption was quite heterogeneous or that more than one vent may have been present the continuous nature of the bedding in the pyroclastic material and the lack of ash partings suggest deposition from a high concentration ejecta sustained eruption column at times e g the massive very diffusely stratified deposits the paucity of tractional bed forms suggest near vertical particle trajectories i e a clear air fall component but the poorly sorted matrix supported nature of the deposits suggest that pyroclastic flow and or surge processes may also have been active during the eruption available diamond sampling data were examined and correlated with the geology data derive from the old 120 37 m 200 61 m 300 92 m and 1200 ft 366 m levels pits sunk during historical mining operations drill logs as well as more recent bench mapping correlating macro diamond sample data and geology shows a clear relationship between diamond grade and lithology localised enrichment and dilution of the primary diamond grade has taken place in the upper reworked volcaniclastic deposits due to post eruptive sedimentary in fill processes clear distinction can be drawn between upper re sedimented and lower pyroclastic crater deposits at mwadui both from a geological and diamond grade perspective finally an emplacement model for the mwadui kimberlite is proposed geological evidence suggests that little or no sedimentary cover existed at the time of emplacement the nature of the bedding within the pyroclastic deposits and the continuity of the bedding in the vertical dimension suggest that the eruption was continuous but that the eruption column may have been heterogeneous both petrologically as well as geometrically volcanic activity appears to have ceased thereafter and the crater was gradually filled with granite debris from the unstable crater walls and re sedimented volcaniclastic material derived from the tuff ring the mwadui kimberlite exhibits marked similarities compared to the orapa kimberlite in botswana

no preseismic evidence from hydrogeochemical parameters on the occasion of the april 29 1991 georgian earthquake caucasus abstract the behaviour of helium content in thermal waters and of water level in deep wells was investigated on the occasion of the strong m 6 9 georgian earthquake which occurred in the caucasus on april 29 1991 data were collected from a measurement network located in georgia just one significant preseismic variation appeared in the water level which seemed to be connected with the georgian earthquake in other wells only some co seismic and post seismic effects are visible no statistically viable variation was noted in the helium content in contrast clear anomalies in these parameters were observed on the occasion of the 1988 spitak earthquake with the same magnitude which occurred in the caucasus we prepose that the location of the epicentre with respect to the measurement network was the reason for this disagreement we therefore suggest the introduction of a coefficient c into relationships that give the size of the premonitory area as a function of the magnitude of the forthcoming earthquake this coefficient ranging from 0 to 1 can take into account the extent to which the focal zone belongs to different tectonic structures and the extent to which the measurement network coincides with these structures

maar diatreme phreatomagmatism at hopi buttes navajo nation arizona usa remnants of an extensive maar diatreme volcanic field are magnificently exposed at various depths of erosion in the hopi buttes volcanic field of northeastern arizona field and petrographic studies of both the maar and diatreme elements of a selection of volcanoes within the field show that 1 lower sections of the maar rim sequences are typically rich in sandy mudrock derived from the pre eruptive mio pliocene bidahochi formation and the muddy bidahochi sediment was soft and wet at the time of maar eruptions 2 beds higher within the rims contain generally increased proportions of sandstone clasts from the triassic wingate formation in the diatremes late emplaced breccia has deeper seated lithics than more marginal breccia emplaced earlier and 3 many vents are topped by megacryst enriched scoria and spatter and deep seated xenoliths are known only from upper diatreme and craterfilling tephra these observations show that 1 eruptions at hopi buttes involved interaction of magma with unconsolidated mudrock at shallow levels and the phreatomagmatic processes that provided the bulk of the energy involved in the violent eruptions were driven by the interaction of magma and wet sediment 2 the locus of explosive activity migrated downward as eruptions progressed and 3 the closing stage of many eruptions was characterized by rapid magma rise and relative depletion of water

experimental birth of a maar diatreme volcano abstract maar diatreme eruptions are hazardous to people and infrastructure and are also linked to the formation of the kimberlitic variety of diatremes which is important economically processes occurring in the subsurface diatreme and their relation to surface eruptions are not yet well understood we conducted field scale experiments using analog materials to shed more light on these processes especially the formation of the proto diatreme during the first explosions of a maar eruption specifically a series of buried explosions in a prepared layered substrate pad produced craters extra crater deposits and sub crater deposits analogous to volcanic maar craters tephra rings and incipient diatremes post explosion substrate excavation revealed that single large explosions produce sub crater deposits extending nearly to the crater rim crest the same energy divided into three blasts either co located or at different depths with the same epicenter produced narrower and sometimes deeper sub crater deposits even though the final sizes of the craters were similar to that produced by the single large blast the sub crater deposits have an upper zone with domains from different substrate depths and an underlying zone distinguished primarily by being more loosely packed than the original substrate videos show surface motion extending beyond the post shot crater rim and largely vertical ejection and fallback of material into the footprint of these deposits especially for the explosions that occurred below optimal depth of burial we infer that much of the loosely packed material was disassembled vertically transported to different heights during the explosions then fell back without significant relative lateral movement of grains however subvertical fallback did produce apparent cross cutting structures in shallow sub crater deposits one explosion ejected material from the deepest substrate horizon but it was redeposited only within the crater and is unrepresented in the ejecta rim implications of the experiments for maar diatreme volcanoes including some kimberlite pipes are as follows 1 vertical focusing of deep explosions in the diatreme explains the deficit of deep wallrock lithics observed at maar volcanoes 2 direct vertical fallback is possibly an important process forming diatreme deposits especially during the earliest stages and 3 even in our limited simulation the number and scaled depth of explosions clearly affect proto diatreme size and structure

shallow earthquakes in a viscoelastic shear zone with depth dependent friction and rheology summary most crustal earthquakes of the world are observed to occur within a seismogenic layer which extends from the earth s surface to a depth of a few tens of kilometres at most a model is proposed in which the shear zone along a transcurrent plate margin is represented as a viscoelastic medium with depth dependent power law rheology a frictional resistance linearly increasing with depth is assumed on a vertical transcurrent fault within the shear zone such a model is able to reproduce a continuous transition from the brittle behaviour of the upper crust to the ductile behaviour at depth assuming that the shear zone is subjected to a constant strain rate from the opposite motions of the two adjacent plates it is found that there exists a maximum depth h below which tectonic stress can never reach the frictional threshold this may be identified as the maximum depth of earthquake nucleation the value of h is consistent with observations for plausible values of the model parameters the stress evolution in the shear zone is calculated in the linear approximation of the constitutive equation a change in rigidity with depth which is also introduced in the model may reproduce the high vertical gradient of shear stress which has been measured across the san andreas fault and the fact that most earthquakes are nucleated at some depth in the seismogenic layer a crack which drops the ambient stress to the dynamic frictional level is then introduced in the model to this aim a crack solution is employed without a stress singularity at its edges which is compatible with a frictional stress threshold criterion for fracture a constraint on the vertical friction gradient is obtained if such cracks are assumed to be entirely confined within the seismogenic layer

large phreatomagmatic vent complex at coombs hills antarctica wet explosive initiation of flood basalt volcanism in the ferrar karoo lip the mawson formation and correlatives in the transantarctic mountains and south africa record an early eruption episode related to the onset of ferrar karoo flood basalt volcanism mawson formation rocks at coombs hills comprise mainly 80 vol structureless tuff breccia and coarse lapilli tuff cut by irregular dikes and sills within a large vent complex 30 km2 quenched juvenile fragments of generally low but variable vesicularity accretionary lapilli and country rock clasts within vent fill and pyroclastic density current deposits point to explosive interaction of basalt with groundwater in porous country rock and wet vent filling debris metre scale dikes and pods of coherent basalt in places merge imperceptibly into peperite and then into surrounding breccia steeply dipping to sub vertical depositional contacts juxtapose volcaniclastic rocks of contrasting componentry and grainsize these sub vertical tuff breccia zones are inferred to have formed when jets of debris steam water passed through unconsolidated vent filling deposits these jets of debris may have sometimes breached the surface to form subaerial tephra jets which fed subaerial pyroclastic density currents and fall deposits others however probably died out within vent fill before reaching the surface allowing mixing and recycling of clasts which never reached the atmosphere most of the ejecta that did escape the debris filled vents was rapidly recycled as vents broadened via lateral quarrying of country rock and bedded pyroclastic vent rim deposits which collapsed along the margins into individual vents the unstratified poorly sorted deposits comprising most of the complex are capped by tuff lapilli tuff and tuff breccia beds inferred to have been deposited on the floor of the vent complex by pyroclastic density currents development of the extensive coombs hills vent complex involved interaction of large volumes of magma and water we infer that recycling of water as well as recycling of pyroclasts was important in maintaining water supply for phreatomagmatic interactions even when aquifer rock in the vent walls lay far from eruption sites as a consequence of vent complex widening the proportion of recycled water increased with vent complex size in the same way that the proportion of recycled tephra did though water recycling leaves no direct rock record the volcaniclastic deposits within the vent complex show through their lithofacies structural architecture lithofacies characteristics and particle properties clear evidence for extensive and varied recycling of material as the complex evolved

the geology of kimberlite pipes of the ekati property northwest territories canada abstract this paper reviews key characteristics of kimberlites on the ekati property nwt canada to date 150 kimberlites have been discovered on the property five of which are mined for diamonds the kimberlites intrude archean basement of the central slave craton numerous proterozoic diabase dykes intrude the area the precambrian rocks are overlain by quaternary glacial sediments no phanerozoic rocks are present however mudstone xenoliths and disaggregated sediment within the kimberlites indicate that late cretaceous and tertiary cover likely

magma water interactions in subaqueous and emergent basaltic in the subaqueous growth and emergence of a basaltic volcano clasts are formed by one or a combination of 1 explosive release of magmatic volatiles 2 explosive expansion and collapse of steam formed at magma water contact surfaces 3 explosive expansion of steam following enclosure of water in magma or entrapment of water close to magma and 4 cooling contraction these processes named respectivelymagmatic explosivity contact surface steam explosivity bulk interaction steam explosivity andcooling contraction granulation can be enhanced by mutual interaction and feedback the first three explosive processes are limited at certain water depths hydrostatic pressures and become increasingly vigorous at shallower levels the depth of onset of magmatic explosivity depends largely on juvenile volatile content it is up to 200 m for tholeiitic magmas and up to 1 km for alkalic magmas at the depth where formation of clastic deposits becomes predominant over effusion of lavas magmatic explosivity is subordinate to steam explosivity as a clast forming process the upward transition to accumulation of dominantly clastic deposits is not simply related to the onset of substantial exsolution of magmatic volatiles and can occur without it contact surface explosivity commonly requires initiation by a vigorous impact between magma and water and although no certain depth limit is known likelihood of such explosivity decreases rapidly with depth clast generation by bulk interaction explosivity appears to be restricted to depths much shallower than that of the critical pressure of water which in sea water is at about 3 km cooling contraction granulation can occur in any depth of water but at shallow levels may be replaced by contact surface explosivity during continuous eruption under water tephra can be ejected and deposited within a cupola of steam such that rapid quenching does not occur emergent volcanoes are characterized by distinctive steam explosive activity that results primarily from a bulk interaction between rapidly ascending magma and a highly mobile slurry of clastic material water and steam the water gets into the vent by flooding across or through the top of the tephra pile and violent explosions cease when this access is sealed the eruptions during emergence of surtsey and capelinhos typify the distinctive explosive activity the style and controls of which are different from those of maar volcanoes

seismo induced effects in the near earth space combined ground and space investigations as a contribution to earthquake prediction abstract the paper aims at giving a few methodological suggestions in deterministic earthquake prediction studies based on combined ground based and space observations of earthquake precursors up to now what is lacking is the demonstration of a causal relationship with explained physical processes and looking for a correlation between data gathered simultaneously and continuously by space observations and ground based measurements coordinated space and ground based observations imply available test sites on the earth surface to correlate ground data collected by appropriate networks of instruments with space ones detected on board of leo satellites at this purpose a new result reported in the paper is an original and specific space mission project esperia and two instruments of its payload the esperia space project has been performed for the italian space agency and three esperia instruments arina and lazio particle detectors and egle search coil magnetometer have been built and tested in space the egle experiment started last april 15 2005 on board the iss within the eneide mission the launch of arina occurred on june 15 2006 on board the resurs dk 1 russian leo satellite as an introduction and justification to these experiments the paper clarifies some basic concepts and critical methodological aspects concerning deterministic and statistic approaches and their use in earthquake prediction we also take the liberty of giving the scientific community a few critical hints based on our personal experience in the field and propose a joint study devoted to earthquake prediction and warning

time and space variations in magmatic and phreatomagmatic eruptive processes at easy chair lunar crater volcanic field nevada usa the products of monogenetic volcanoes often record complex sequences of eruptive processes easy chair volcano lunar crater volcanic field nevada usa was formed by a monogenetic eruption along a 2 5 km long series of en echelon fissure vents hawaiian to strombolian fountains along the fissures dominated initial activity producing a series of agglomerate ramparts focusing of eruptive activity to two central vents and the formation of two overlapping scoria cones followed the early phase fountain fed lavas from those cones merged to form a channel that fed lava onto a flow field at the foot of the cones focusing of subsurface magma flow toward the central conduits may have reduced magma flux in the remaining fissures and the southern segment s entered a phase of phreatomagmatic explosions that destroyed the early agglomerate rampart and formed a maar and tephra ring composed of lapilli tuff rich in clasts derived from pre easy chair lavas and early agglomerates the eruption closed with a minor phase of magmatic activity that deposited scoria lapilli and bombs on top of the phreatomagmatic deposits the eruptive sequence indicates that relatively low hazard strombolian to hawaiian activity can be replaced by more hazardous phreatomagmatic explosions well into a monogenetic eruption

the importance of the transport system in shaping the growth and form of kimberlite volcanoes transport of kimberlite magma from the mantle to the surface is a complex process that involves many often sudden and dramatic changes in transport style the pace and magnitude of changes in the transport system acclerate as the magma nears the surface culminating in eruptions during which multiple transport events take place simultaneously as well as single transport events that change significantly from their point of origin to final deposition understanding the range of transport styles recorded by kimberlite deposits is key to describing the type and style of eruptions the growth of cones and craters and the distribution geometry and structure of the deposits that make up kimberlite volcanoes in an economic context building a clear picture of the processes that shape deposits is essential for selecting exploration targets and evaluating the grade and value of diamond bearing kimberlites variations in diamond grade reflect differences in the diamond content of different magma parcels erupted during the lifetime of the kimberlite volcano sorting during transport of eruption products or re working of diamonds during crater growth cone collapse and erosion deep transport systems control delivery of magma and diamonds to eruption sites the initial stages of kimberlite transport involve rise of magma in narrow interconnected networks of fluidfilled fractures there is compelling evidence that kimberlite magmas are extracted from their source region and erupted without significant residence times in the crust many single kimberlite volcanoes have significant variations in diamond grade between different kimberlite phases suggesting eruption of distinct perhaps unrelated magma parcels at the same site

stratigraphy and evolution of the holocene aljojuca maar volcano serdan oriental basin eastern trans mexican volcanic belt and implications for hazard assessment abstract aljojuca maar is located in the eastern trans mexican volcanic belt within the serdan oriental basin which is characterized by quaternary bimodal monogenetic volcanism aljojuca maar has an irregular shape with an eastern embayment structure that forms an e w alignment with at least three older scoria cones to the east the crater walls expose a sequence of pre maar volcaniclastic deposits with a lava flow linked to the older scoria cones and a paleosol at its top the latter indicates that a significant hiatus occurred between the growth of the scoria cones and the eruption of the aljojuca maar whose deposits overlie the soil strong interactions of the ascending magma with groundwater led to the emplacement of a 30 m thick maar sequence our detailed stratigraphic study shows that the eruption can be grouped into two main stages comprising eight stratigraphic units deposited during five eruptive phases the first stage reflects the development of the main crater a maar sensu stricto and is recorded by six stratigraphic units from a to f the eruption began with a strombolian pulse followed by phreatomagmatic activity unit a forming dilute pyroclastic density currents in the second phase phreatomagmatic explosions prevailed with different degrees of fragmentation from juvenile and lithic rich lapilli fallout unit b to less diluted pyroclastic density currents unit c and a spatially restricted strombolian pulse the third phase includes a main vent enlargement event represented by a chaotic and heterolithic breccia unit d in the fourth phase dilute pyroclastic density current deposits unit e dominate with minor chaotic breccias unit f the second stage fifth phase includes the occurrence of a possibly fissural e w structure which formed a scallop in the eastern flank of the maar eastern embayment structure breaking the previously formed main crater s eastern flank and producing a 4 m chaotic breccia unit g followed by the occurrence of a dilute pyroclastic density current unit h whose energy enabled the current to climb to the closest scoria cone the migration of the explosive loci toward the eastern flank of the maar reflects a tectonic control by the dominant regional east trending structural pattern several radiocarbon ages support that the eruption of aljojuca took place between 2 7 ka age of charcoal fragments under the ejecta ring and 2 2 ka age of the oldest lake sediments in the maar crater this has important implications for the hazard assessment of volcanism in this area which had not been considered previously as a potentially active system

recent advances in the geology of koffiefontein mine free state province south africa abstract the complex internal geology of the koffiefontein pipe has contributed to the marginal nature of the mine the key to this is the presence of a large zone dominated by down rafted country rock karoo sediment and dolerite xenoliths recent work indicates that the kimberlite pipe at koffiefontein consists of precursor dykes the west and east fissures and the main pipe in which two main eruptive phases have been recognized groundmass spinel compositions have been used to provide a chemical fingerprint of each lithology there is evidence for at least three magma batches each with its own chemical signature cross cutting contact relationships were used to determine the emplacement sequence the characterization of the different internal geological units permitted the development of a three dimensional 3d model of the pipe both main eruptive phases viz the speckled west kimberlite and the speckled east kimberlite comprise volcaniclastic kimberlite they are separated by a large irregular mass of kimberlite that contains abundant country rock xenoliths comprising varying proportions of karoo mudstone and dolerite as well as probable bedded crater facies fragments this zone of contamination dilutes the grade of the kimberlites affects the geotechnical stability and adversely affects the economics of the mine

linear and nonlinear inverse rheologies of rocks abstract the generalized constitutive equations of anelastic media are considered and discussed in three dimensions 3d and in terms of two material functions as a generalized standard linear solid sls the inverse problem is considered and it is seen that one may not determine the material functions using the data of one standard creep or relaxation experiment data from both experiments would be needed if the medium is a generalized viscoelastic one then the material function can be determined with one experiment only the creep is determined in the generalized viscoelastic 3d case simulating a 3d experiment using the creep compliance obtained from the relaxation modulus determined from constant strain rate uniaxial experiments caputo 1983 it is seen that the one dimensional 1d model mostly used to model rock mechanics laboratory experiments which in practice are 3d experiments and not rigorously 1d gives the same value of the creep rate obtained using as load the difference between 1 and the confining pressure 3 the case of nonlinear viscoelastic stress strain relation is also discussed and a relation is found to determine the creep rate from 3d experiments at constant strain rate a check made with polycrystalline halite at high pressure and large strain rate gives satisfactory results

radon and rilt anomalies detected before the irpinia south italy earthquake of november 23 1980 at great distances from the epicenter some investigators of istitute of physics and istitute of geology of the university of rome are carrying out measurements of tilt and groundwater radon content in sites located in central italy beginning from june to november 1980 anomalous variations in the radon content have been detected in rome and rieti sites water samples for discrete measurements of radon content were drawn periodically from underground strata the amplitude of the anomalies were 25 and 170 compared with the background level from about march 15 a bubble type tiltmeter installed at l aquila site showed a first anomalous tilting with a time scale of about two mounths and an amplitude of 15 rad a second anomalous tilting about 20 rad in amplitude was observed from the middle of may to november the seismicity m 4 0 4 7 within about 100 km of the sites seems to justify only the first anomalous tilting on november 23 1980 a strong m 6 5 earthquake took place in irpinia southern italy at about 250 km from rome rieti and l aquila sites we suggest to assume both the radon content anomalies and the second anomalous tilting as precursors of the irpinia earthquake

maar diatreme volcanoes a review maar diatreme volcanoes are produced by explosive eruptions that cut deeply into the country rock a maar is the crater cut into the ground and surrounded by an ejecta ring while the diatreme structure continues downward and encloses diatreme and root zone deposits here we attempt an evenhanded review of maar diatreme volcanology that extends from mafic to kimberlitic varieties and from historical maar eruptions to deeply eroded or mined diatreme structures we conclude that maar diatreme eruptions are episodic ejecta rings provide invaluable insight into eruption processes and sequence but are incomplete records of diatreme formation deposits within the diatreme structure include in varying proportions lower unbedded deposits sometimes typified by subvertical contacts among domains of debris emplaced sequentially and upper bedded deposits formed by sedimentation on surfaces open to the atmosphere a basal root zone comprises the transition from coherent magmatic feeder dike to clastic deposits formed by fragmentation of magma and enclosing country rock root zones are irregular in form and the clastic deposits are typically intruded by contorted dikes irregular root zone like chaotic breccias cut by contorted dikes are also present within diatreme deposits where they represent intra diatreme fragmentation zones and record changes in the location of the explosion locus during eruption

very slow moving crustal strain disturbances abstract a model of discrete crustal blocks separated by a thin weak transition zone whose stress strain relations contain derivatives of real order implying a memory mechanism in the medium seems to give a good explanation for the low velocity values 0 03 15 cm s of slow moving crustal strain disturbances tilt and strain anomalies observed before moderate and large earthquakes are thought to be generated in the focal zone of the forthcoming earthquake due to the spatial and temporal redistribution of the physical processes that occur during the preparation time of the earthquake on the basis of experimental results it has been assumed that these anomalies propagate from the hypocentral area in addition post seismic deformations show the effects of propagation the anomalies observed have a duration and a velocity ranging from minutes to months and from o o5 cm s to 10 cm s respectively the velocity of migration of post seismic deformation also lies in this range the velocity of propagating strain disturbances is obtained as a function of the frequency and of the linear dimension of the oscillating blocks the velocity values calculated using this model range from 0 1 cm s to 10 cm s for viscosities of 10 9 10 11 g cm 1 s 2 z 0 1 z 0 5 and blocksizes 1 4 10 6 cm at frequencies of 10 5 10 8 s 1

rapid injection of particles and gas into non fluidized granular material and some volcanological implications in diatremes and other volcanic vents steep bodies of volcaniclastic material having differing properties particle size distribution proportion of lithic fragments etc from those of the surrounding vent filling volcaniclastic material are often found it has been proposed that cylindrical or cone shaped bodies result from the passage of debris jets generated after phreatomagmatic explosions or other discrete subterranean bursts to learn more about such phenomena we model experimentally the injection of gas particulate dispersions through other particles analogue materials glass beads or sand and a finite amount of compressed air are used in the laboratory the gas is made available by rapidly opening a valve therefore the injection of gas and coloured particles into a granular host is a brief 1 s discrete event comparable to what occurs in nature following subterranean explosions the injection assumes a bubble shape while expanding and propagating upwards in reaction the upper part of the clastic host moves upward and outward above the bubble forming a dome the doming effect is much more pronounced for shallow injection depths thin hosts with dome angles reaching more than 45 significant surface doming is also observed for some full scale subterranean blasts e g buried nuclear explosions so it is not an artefact of our setup what happens next in the experiments depends on the depth of injection and the nature of the host material with shallow injection into a permeable host glass beads the compressed air in the bubble is able to diffuse rapidly through the roof meanwhile the coloured beads sediment into the transient cavity which is also closing laterally because of inward directed granular flow of the host depending on the initial gas pressure in the reservoir the two phase flow can erupt or not non erupting injections produce cylindrical bodies of coloured beads whereas erupting runs produce flaring upward or conical deposits changing the particle size of the host glass beads does not have a large effect under the size range investigated 100 200 to 300 400 m doubling the host thickness injection depth requires a doubling of the initial gas pressure to produce similar phenomena such injections whether erupting or wholly subterranean provide a compelling explanation for the origin and characteristics of multiple cross cutting bodies that have been documented for diatreme and other vent deposits

the role of fluidisation in the formation of volcaniclastic kimberlite grain size observations and experimental investigation abstract deep steep sided kimberlite pipes in southern africa are normally partially filled with multiple units of a characteristically massive structureless volcaniclastic rock here termed massive volcaniclastic kimberlite mvk these units can be arranged in a pseudo concentric nested pipes within pipes structure several key observations on typical mvk from the venetia kimberlite pipes south africa indicate that fluidisation may have been an important process during its emplacement these include 1 the thorough mixing together of juvenile components and of lithic components derived from all stratigraphic levels of the country rock and 2 a matrix composed of void filling minerals related to hydrothermal metamorphism mainly diopside and serpentine which indicates that the primary deposit had a high porosity 50 and 3 particle size distributions poor in fine ash 10 cm analogue experiments utilising a sand bed laced with marker horizons were conducted in order to investigate the role of fluidisation in the formation of mvk high pressure gas fed through the bed from a point source resulted in the formation of an upwardly diverging pipe structure of thoroughly mixed fluidised material bounded by undisturbed sand strong upward gas flow along the centre of the pipe is characteristically bubbly and results in internal circulation with downward moving regions at the pipe margins analysis of the experimental results suggests that the gas velocity decreases linearly with height and has a gaussian distribution of vertical velocity in the cross flow direction a decrease in the gas flow rate resulted in the development of nested pipes reminiscent of the pipes within pipes structures in kimberlite pipes the results suggest that fluidisation could be a major control on the structure and sorting of deposits in kimberlite pipes we propose that the fluidisation of particles in a kimberlite pipe occurs during the final stages of the eruption when erupting pyroclasts and incorporated fragmented lithic material can no longer be ejected out of the deep wide conduit this bed is subject to quasi sustained fluidisation by the erupting gas particle flow which imparts on it the distinctive characteristics of a massive well mixed poorly sorted deposit

gps constraints on continental deformation in the africa arabia eurasia continental collision zone and implications for the dynamics of plate interactions 1 the gps derived velocity field 1988 2005 for the zone of interaction of the arabian african nubian somalian and eurasian plates indicates counterclockwise rotation of a broad area of the earth s surface including the arabian plate adjacent parts of the zagros and central iran turkey and the aegean peloponnesus relative to eurasia at rates in the range of 20 30 mm yr this relatively rapid motion occurs within the framework of the slow moving 5 mm yr relative motions eurasian nubian and somalian plates the circulatory pattern of motion increases in rate toward the hellenic trench system we develop an elastic block model to constrain present day plate motions relative euler vectors regional deformation within the interplate zone and slip rates for major faults substantial areas of continental lithosphere within the region of plate interaction show coherent motion with internal deformations below 1 2 mm yr including central and eastern anatolia turkey the southwestern aegean peloponnesus the lesser caucasus and central iran geodetic slip rates for major block bounding structures are mostly comparable to geologic rates estimated for the most recent geological period 3 5 myr we find that the convergence of arabia with eurasia is accommodated in large part by lateral transport within the interior part of the collision zone and lithospheric shortening along the caucasus and zagros mountain belts around the periphery of the collision zone in addition we find that the principal boundary between the westerly moving anatolian plate and arabia east anatolian fault is presently characterized by pure left lateral strike slip with no fault normal convergence this implies that extrusion is not presently inducing westward motion of anatolia on the basis of the observed kinematics we hypothesize that deformation in the africa arabia eurasia collision zone is driven in large part by rollback of the subducting african lithosphere beneath the hellenic and cyprus trenches aided by slab pull on the southeastern side of the subducting arabian plate along the makran subduction zone we further suggest that the separation of arabia from africa is a response to plate motions induced by active subduction

transient two dimensional dynamics in the upper conduit of a rhyolitic eruption a comparison of closure models for the granular stress to elucidate the role of particle collisions in redistributing momentum after fragmentation a numerical study was performed comparing the behavior of an inviscid or collision less granular material with a granular material whose viscosity and pressure were modeled using kinetic theory the granular viscosity calculation is sensitive to particle size and three particle sizes were considered 0 0002 m 0 002 m and 0 02 m a critical volume fraction of gas 0 75 identifies the onset of fragmentation dividing the region of rhyolitic magma with dispersed bubbles from the region of turbulent gas with dispersed particles the transient simulations can be divided into two dynamic regimes an initial shock followed by a transition to steady state during the initial shock phase treatment of the granular pressure and viscosity led to greater particle velocities relative to the inviscid calculation due to the development of higher gas pressure at fragmentation however as steady state is approached the viscosity slows the particle phase relative to the inviscid counterpart for particles greater than a millimeter the modeled sub millimeter particle velocity was insensitive to the treatment of the granular pressure and viscosity centimeter scale particles have a much higher granular viscosity up to 10 1 pa s and the kinetic theory calculation is thus relevant in these conditions after reaching steady state the differential velocity between gas and particles at the conduit exit correlates with particle size for both the inviscid and granular viscosity calculations negligible differential velocity develops for sub millimeter particles but centimeter scale particles exit the conduit with 5 differential velocity between phases

immense vent complex marks flood basalt eruption in a wet failed rift coombs hills antarctica large lava effusions can have impressive explosive antecedents although our picture of flood basalt is overwhelmingly effusive phreatomagmatic eruptions have preceded quiet effusion of some flood basalts and reflect the same influence of vent architecture and hydrology on eruptive style as seen for small volume eruptions the scale of phreatomagmatic deposits associated with flood basalts can be huge at coombs hills a vast but otherwise typical phreatomagmatic vent complex is exposed over more than 25 km 2 and its features are interpreted to reflect processes of tephra jet eruptions with diatreme development similar vent complexes are probably the source of laharic deposits reported elsewhere in the transantarctic mountains and in the karoo province of south africa

compound maar crater and co eruptive scoria cone in the lunar crater volcanic field nevada usa abstract bea s crater lunar crater volcanic field nevada usa consists of two coalesced maar craters with diameters of 440 m and 1050 m combined with a co eruptive scoria cone that straddles the northeast rim of the larger crater the two craters and the cone form an alignment that parallels many local and regional structures such as normal faults and is interpreted to represent the orientation of the feeder dyke near the surface the maar formed among a dense cluster of scoria cones the cone cluster topography resulted in crater rim that has a variable elevation these older cones are composed of variably welded agglomerate and scoria with differing competence that subsequently affected the shape of bea s crater tephra ring deposits associated with phreatomagmatic maar forming eruptions are rich in basaltic lithics derived from