unbedded diatreme deposits reveal maar diatreme forming eruptive processes standing rocks west hopi buttes navajo nation usa diatremes provide partial records of how dyke delivered magma periodically interacting with water produces the largest known cylindrical conduit structures of any volcano type we address how pre eruptive country rock is disrupted and redistributed to form the diatreme structure during an eruption by establishing the internal architecture of the unbedded diatreme at standing rocks west and the volumes and sources of wall rock within the diatreme and in a complementary tephra ring at teshim both in the hopi buttes volcanic field the unbedded diatreme is dominantly exposed as a massif comprising multiple subvertical columns interpreted as ephemeral conduit deposits of well mixed poorly sorted composite and juvenile pyroclast rich deposits that truncate marginal layered deposits plus a peripheral heterolithic country rock breccia wall rock clasts are scarce in the massif and were mainly sourced from 75 to 245 m depth below the paleosurface by contrast the tephra ring has abundant wall rock fragments dominantly from the upper 75 m of the pre eruptive sedimentary sequence the diatreme structure is interpreted to have been formed by many small volume explosions fed by a low flux of basaltic magma late stage explosive activity was rooted mostly within pyroclastic debris at shallow to intermediate depths within the diatreme structure this resulted in damped and shifting intra diatreme explosions and jets that facilitated gradual mixing recycling and remobilization of debris in the diatreme with incremental addition of juvenile material and possibly a local rise in the crater floor

the san venanzo maar and tuff ring umbria italy eruptive behaviour of a carbonatite melilitite volcano the late pleistocene san venanzo maar and nearby pian di celle tuff ring in the san venanzo area of umbria central italy appear to represent different aspects of an eruptive cycle accompanied by diatreme formation approximately 6x106 m3 of mostly lapillisized juvenile ejecta with lesser amounts of lithics and 1x106 m3 of lava were erupted the stratigraphy indicates intense explosive activity followed by lava flows and subvolcanic intrusions the pyroclastic material includes lithic breccia derived from vent and diatreme wall erosion roughly stratified lapilli tuff deposited by concentrated pyroclastic surge chaotic scoriaceous pyroclastic flow and inverse graded grain flow deposits the key feature of the pyroclastics is the presence of concentric shelled lapilli generated by accretion around the lithics during magma ascent in the diatreme conduits the rock types range from kalsilite leucite olivine melilitite lavas and subvolcanic intrusions to carbonatite phonolite and calcitic melilitite pyroclasts juvenile ejecta contain essential calcite whose composition and texture indicate a magmatic origin pyroclastic carbonatite activity is also indicated by the presence of carbonatite ash beds the san venanzo maar forming event is believed to have been trigered by fluid rich carbonatite phonolite magma the eruptive centre the moved to the pian di celle tuff ring where the eruption of degassed olivine melilititic magma and late intrusions ended magmatic activity in the area in both volcanoes the absence of phreatomagmatic features together with the presence of large amounts of primary calcite suggests carbonatite segregation and violent exsolution of co2 which flowing through the diatremes produced the peculiar intrusive pyroclastic facies and triggered explosions

sub volcanic development of kimberlite pipes evidence from the lace and voorspoed group ii kimberlites south africa abstract the lace and voorspoed kimberlites occur on the kaapvaal craton south africa and form part of the kroonstad group ii kimberlite orangeite cluster the lace kimberlite is composed of a main pipe and a satellite blind pipe the latter of which does not reach the current land surface 30 m below the current land surface and is not observed connecting with the main pipe at depth the main pipe increases in size from 100 m to 250 m in diameter at depth the voorspoed kimberlite pipe is the largest of the cluster and is dominantly infilled with massive layers up to 200 m thick of resedimented volcaniclastic kimberlite rvk coherent kimberlite ck identified at all three pipes is described here in order to constrain their formation distinct volcaniclastic kimberlite vk at the lace main pipe is also described here and is characterised by a highly clastic fragmental quartz rich and mud rich interclast material the vk is interpreted to form by resedimentation of extra crater deposits into a deep 1600 m open vent the ck from both lace occurrences is characterised by coherent to globular segregationary textures generally high proportions of crustal xenoliths and the complete absence of karoo basalt xenoliths which are ubiquitous in vk infilling south african cretaceous kimberlites the increase in pipe size with depth associated with the occurrence of ck at the main pipe and the occurrence of ck within the satellite blind pipe strongly favours an interpretation of formation in a hypabyssal setting rather than by pyroclastic processes ck is also described from the voorspoed kimberlite where it occurs at depth along the south western margin of the pipe the ck is xenolith rich the xenolith population is entirely from the immediate sidewall lithologies and has an incipient globular segregationary texture the contact with the rvk in the centre of the pipe is sharp indicating that the ck was cross cut by a period of eruption prior to the formation of spatially associated rvk the pipe margins at similar depths along the northern and eastern sections of the pipe are characterised by the occurrence of contact breccias which have a gradational change to lower degrees of brecciation away from the pipe the ck at voorspoed is interpreted to form by the intrusion of magmatic kimberlite at depth which invades a previously formed contact breccia and incorporates large proportions of crustal xenoliths from the immediate sidewall the ck occurring at the lace and voorspoed kimberlite are interpreted to form in a sub volcanic hypabyssal setting during a stage of pre conditioning of the country rock prior to eruptive emplacement of the pipe a model for the development of a sub volcanic embryonic pipe is presented for the formation of ck at the kroonstad kimberlite cluster

volcaniclastic dykes tell on fracturing explosive eruption and lateral collapse at stromboli volcano italy abstract in the upper part of the stromboli volcano in the le croci and bastimento areas two dyke like bodies of volcanic breccia up to two metre thick crosscut and intrude the products of vancori and neostromboli volcanoes we describe the lithofacies association of these unusual volcaniclastic dykes interpret the setting of dyke forming fractures and the emplacement mechanism of internal deposits and discuss their probable relationships with the explosive eruption and major lateral collapse events that occurred at the end of the neostromboli period the dyke volcaniclastic deposits contain juvenile magmatic fragments pyroclasts suggesting a primary volcanic origin their petrographic characteristics are coincident with the neostromboli products the architecture of the infilling deposits comprises symmetrically nested volcaniclastic units separated by sub vertical boundaries which are parallel to the dyke margins the volcanic units are composed of distinctive lithofacies the more external facies is composed of fine and coarse ash showing sub vertical laminations parallel to the contact wall the central facies comprises stratified lithic rich breccia and lapilli tuff whose stratification is sub horizontal and convolute discordant to the dyke margins only at le croci dyke the final unit shows a massive tuff breccia facies the volcaniclastic dykes experienced a polyphasic geological evolution comprising three stages the first phase consisted in fracturing explosive intrusion related to magma rising and upward injection of magmatic fluids and pyroclasts the second phase recorded the dilation of fractures and their role as pyroclastic conduits in an explosive eruption possibly coeval with the lateral collapse of the neostromboli lava cone finally in the third phase the immediately post eruption mass flow remobilization of pyroclastic deposits took place on the volcano slopes

volcanic debris avalanche deposits of the upper maronne valley cantal volcano france evidence for contrasted formation and transport mechanisms abstract the deposits of two volcanic debris avalanches vda i and ii that occur in the upper maronne valley northwest sector of cantal volcano france were studied to establish their mechanisms of formation transport and deposition these two volcanic debris avalanches that clearly differ with regard to their structures textures and extensions exemplify the wide spectrum of events associated with large scale sector collapse vda i is voluminous 1 km3 in the upper maronne valley and widespread the deposits comprise two distinct facies the block facies that forms the intermediate and upper part of the unit and the mixed facies that crops out essentially at the base of the unit the block facies consists of more or less brecciated lava block and ash flow breccia and pumice flow tuff megablocks set in breccias resulting from block disaggregation mixing and differential movements are almost absent in this part of the vda the mixed facies consists of breccias rich in fine particles that originate from block disagregation as well as being picked up from the substratum during movement mixing and differential movements are predominant in this zone analysis of fractures on lava megablocks suggests that shear stress during the initial sliding is the principal cause of fracture these data strongly indicate that vda i is purely gravitational and argue for a model in which the initial sliding mass transforms into a flow due to differential in situ fragmentation caused by the shear stress vda ii is restricted to low topography areas its volume in the studied area is about 0 3 km3 the deposits consist of brecciated rounded blocks and megablocks set in a fine grained matrix composed essentially of volcanic glass this unit is stratified with a massive layer that contains all the megablocks at the base and in the intermediate part and in the upper part a normally graded layer that contains only blocks

on the first experimental phreatomagmatic explosion of a kimberlite melt abstract detailed field investigations of kimberlite pipes of the upper cretaceous gibeon kimberlite field in southern namibia revealed geologic features which do neither agree with extensive nor with explosive vesiculation in the pipe hanaus 2 vesicle free magmatic kimberlite intruded the diatreme as a plug here we report on the first experiments in which phreatomagmatic explosions were produced with remolten natural non fragmental magmatic kimberlite from the hanaus 2 central plug as a best fit model kimberlite magma system after water was injected into the melt under controlled conditions the resulting water melt mix was triggered by a low energy shock wave 400 m s

gas fluidisation in an experimental tapered bed insights into processes in diverging volcanic conduits abstract gas fluidisation is commonly invoked as an important process during the formation of vent fill deposits in diverging kimberlite pipes in this study we performed a series of laboratory experiments to determine the gas fluidisation behaviour of particles in both confined straight sided and tapered containers as an analogue to volcaniclastic materials infilling a kimberlite pipe in the presence of a gas flow we investigated the effects of taper angle bed height and gas flow rate on fluidisation behaviour focusing on medium coarse grained particles 106 212 m we show that beds in straight sided containers become homogeneously fluidised whereas beds in tapered containers become heterogeneous with fluidisation limited to a central roughly hyperboloid shaped region either side of the well mixed fluidised core marginal wedge shaped regions remain unfluidised and the width of unfluidised regions decreases with increasing gas flux the unfluidised wedges are internally laminated and slip downwards when a critical proportion of the bed is fluidised 90 this generates a conveyor belt type mechanism of particle transport these experimental observations demonstrate how fluctuations in gas velocity can produce steep internal boundaries between laminated and well mixed regions the observations also show how marginal inward dipping layered sequences could slip into deep parts of kimberlite pipes our results provide a framework for interpreting the volcaniclastic lithofacies of kimberlite pipes

3d crystal size distributions a case study on quantifying olivine populations in kimberlites abstract new advances in textural analysis and imaging allow the possibility for the true distribution of mineral phases in a rock to be quantified to high resolution also we are able to explore the relations between 2d to 3d datasets more rigorously in this study the application of 2d and 3d textural analysis to the quantification of olivine populations in kimberlites is investigated crystal size distribution studies csd are carried out on two kimberlite samples 1 one from the venetia k2 kimberlite pipe south africa and 2 one from the dutoitspan kimberlite pipe kimberley south africa a 3d x ray ct study is undertaken to investigate the application of this developing technique in kimberlite research and to test the 2d and 3d data sets csd profiles show markedly curved upwards profiles synonymous with fragmented csds supporting an initial crystal population which is modified by fragmentation of crystals on ascent raw plots of 2d area size frequency provide a powerful first estimate of the population 3d statistics in terms of volume size frequency allowing a quick analysis of olivine phases during exploration which can be quality controlled using more detailed 3d studies where necessary the 3d data set provides accurate analysis over a range of 6 0 5 mm below which the separation of grains becomes very difficult additionally the 3d data provides valuable information on crystal shape which can further be used to distinguish different olivine populations both 2d and 3d textural studies using state of the art textural analysis techniques have been successfully tested on complex kimberite textures and will prove useful techniques in the routine quantification of key indicator phases in kimberlite analysis

factors controlling the internal facies architecture of maar diatreme volcanoes most if not all kimberlite pipes show a multitude of facies types which imply that the pipes were emplaced under an episodic re occurrence of eruptive phases often with inter mittent phases of volcanic quiescence the majority of these facies can be related to either the fragmentation behaviour of the magma during emplacement or changing conditions dur ing sedimentation of volcaniclastic deposits as well as their alteration and compaction after deposition an additional factor controlling pipe facies architecture is the degree of mobility of the loci of explosions in the explosion chambers of the root zone or root zones at the base of the maar diatreme volcano in a growing pipe the root zone moves downward and with that movement the overlying diatreme enlarges both in size and diameter however during the life span of the volcano the explosion chamber can also move upward backintothelower diatreme where renewedexplosionsresult inthedestructionofolderdepositsandtheirstructures nextto vertical shifts of explosion chambers the loci of explosions can also move laterally along the feeder dyke or dyke swarm this mobility of explosion chambers results in a highly com plex facies architecture in which a pipe can be composed of several separate root zones that are overlain by an amalgam ated crosscutting diatreme and maar crater with several lobes pipe complexity is amplified by periodic changes of the fragmentation behaviour and explosivity of kimberlite mag ma recent mapping and logging results of canadian and african kimberlite pipes suggest that kimberlite magma frag mentation ranges from highly explosive with abundant entrained country rock fragments to weakly explosive spatter like production with scarce xenoliths on occasions spatter may even reconstitute and form a texturally coherent deposit on the crater floor in addition ascending kimberlite magma can pass the loci of earlier fragmentation events in the root zone and intrudes as coherent hypabyssal kimberlite dykes in high pipe levels or forms extrusive lava lakes or flows on the crater floor or the syneruptive land surface respectively this highly variable emplacement behaviour is typical for basaltic maar diatreme volcanoes and since similar deposits can also be found in kimberlites it can be concluded thatalsothevolcanologicalprocessesleadingtothesedeposits are similar to the ones observed in basaltic pipes

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

effect of burial and compaction on soil magnetic properties results from soil paleosol sequences in the himalayan foreland india abstract the study based on 395 stratigraphically ordered pedogenic and associated non pedogenic horizons in the himalayan foreland basin indicated a significant depth control for the routine rock magnetic parameters lf fd arm sirm b cr and s ratio the study representing eight fluvial sections with overburdens varying from 0 to 5 km for holocene to 11 ma interval spread over an area of 9000 km 2 infer at least three major phases of mineral magnetic response to soil physico chemical changes these are phase i early burial compaction dissolution and diagenetic changes up to an overburden of 1 5 km leading to substantial loss in ultra fine pedogenic ferrimagnetic oxides phase ii late burial diagenetic transformations up to 2 5 km of overburden leading to formation of antiferromagnetic oxides probably accelerated by the high crustal heat flows of the region and phase iii development of stable antiferromagnetic phase after 2 5 km of burial consequent to phase ii these results indicate a greater control of burial related changes over the routine mineral magnetic parameters than that due to paleoclimatic variations for paleosols of 1 5 km overburden or 2 ma of age in the himalayan foreland basin

subsidence breccias in kimberlite pipes an application of fractal analysis abstract a particular variety of volcanogenic country rock breccia is described a contact breccia that has been identified at venetia river ranch and wimbledon kimberlite pipes the contact breccia is clast supported with no juvenile kimberlite component has tightly packed angular fragments with occasional rounding of smaller particles and has a shear fabric dipping towards the center of each kimberlite pipe or volcanic event clasts have preferred orientations parallel to the fabric photographs of the breccia in the open pit face and measured data from drill core are analyzed specifically to quantify the clast size distributions and clast shapes by means of fractal analysis the fractal dimension is one means of characterizing the breccia because the dimension can be specific to a fragmentation mechanism clast size distribution fractal dimensions in the coarser particles greater than circa 3 cm range from greater than 3 for nonsheared breccia down to circa 2 3 for the sheared breccia breccia characterization based on this fractal analysis suggests that fragmentation occurred initially from confined high energy explosions followed by collapse and abrasion by subsequently gravity induced rockmass subsidence all studied contact breccias produced a distinctive fractal signature in the finer particles

geology of the gahcho kue kimberlite pipes nwt canada root to diatreme magmatic transition zones abstract the cambrian gahcho kue kimberlite cluster includes four main pipes that have been emplaced into the archaean basement granitoids of the slave craton each of the steep sided pipes were formed by the intrusion of several distinct phases of kimberlite in which the textures vary from hypabyssal kimberlite hk to diatreme facies tuffisitic kimberlite breccia tkb the tkb displays many diagnostic features including abundant unaltered country rock xenoliths pelletal lapilli serpentinised olivines and a matrix composed of microlitic phlogopite and serpentine without carbonate the hk contains common fresh olivine set in a groundmass composed of monticellite phlogopite perovskite serpentine and carbonate a number of separate phases of kimberlite display a magmatic textural gradation from tkb to hk which is characterised by a decrease in the proportion of pelletal lapilli and country rock xenoliths and an increase in groundmass crystallinity proportion of fresh olivine and the degree of xenolith digestion the pipe shapes and infills of the gahcho kue kimberlites are similar to those of the classic south african pipes particularly those of the kimberley area similar intrusive magmatic emplacement processes are proposed in which the diatreme zone results from the degassing after breakthrough of the intruding magma column the transition zones represent frozen degassing fronts the style of emplacement of the gahcho kue kimberlites is very different from that of many other pipes in canada such as at lac de gras fort a la corne or attawapiskat

mechanically disrupted and chemically weakened zones in segmented dike systems cause vent localization evidence from kimberlite volcanic systems deformation and alteration zones along kimberlite dikes hold clues as to how point source volcanic vents can localize along sheet like intrusions brittle deformation zones occur in host rock adjacent to kimberlite intrusions of the swartruggens kimberlite dike swarm south africa deformation includes local fracturing and brecciation and is associated with relay zones between offset dike segments breccia zones indicate dilation and hydraulic fracturing and some zones along the dikes were affected by chemical corrosion forming fresh cores surrounded by onion skin concentric shells of altered rock the alteration was caused by either exsolved magmatic volatiles moving in advance of the magma through the fracture or by hydrothermal fluids consideration of the time scales needed for chemical corrosion of host rock requires intrusions to stall at depth prior to transport to higher crustal levels highly disrupted offsets could be preferred locations for explosive activity and initial vent formation as dikes approach the surface a kimberlite pipe forms after magma breaks through to the surface the altered zones are reamed out and fresh cores in spheroidally altered rock are incorporated into the pipe fill along with more angular country rock material as observed in layered volcanic breccias in kimberlite pipes at the venetia mine south africa this model may have wider implications for the localization of conduits along dikes in other volcanic systems dike segmentation provides weak zones where hydrothermal fluids and magmatic volatiles can be preferentially channeled chemical corrosion can further weaken these zones which may then become the locus for initial phreatic and phreatomagmatic explosions creating shallow vents that can then channel magma to the surface during eruption

the rock mechanics of kimberlite volcanic pipe excavation abstract the rock mechanics theory for deformation of open pit and underground mining excavations can be used to better understand aspects of the growth and geometry of kimberlite volcanic pipes large scale rock mass behaviour around an excavation such as a volcanic pipe is dependent on the rock mass strength the in situ rock stress conditions and the excavation geometry rock mass strength is empirically derived from the intact rock strength and quantification of the shear strength frequency and orientation of rock discontinuities tensile slope or sidewall failure typically occurs in shallow level 0 1000 m conditions in which pre existing structures shear due to gravity driven forces the orientation of the pre existing structures provides an important control on the size shape and position of the rock mass failure slope failures are shown to influence the development of many kimberlite volcanic craters and the distribution of layered volcaniclastic facies in the crater explosions or removal of key blocks in a pipe sidewall can cause undercutting and collapse of the sidewall with the rock unravelling along pre existing structures towards surface many lithic rich breccias commonly found on the margins of kimberlite pipes are interpreted to form in this way at intermediate 1000 m to deep 2500 m depths the increasing compressive stress can cause fractures to develop around an excavation stress induced fractures should cause scaling of a volcanic pipe s sidewalls and expansion perpendicular to the maximum component of compressive stress a larger horizontal tectonic stress ratio and a lower internal pipe pressure will promote pipe sidewall fracturing volcanic pipe expansion will also be increased in rate by i mechanical sidewall erosion by flowing magma and or pyroclasts ii by failure on large intersecting fault or dyke structures and iii by reduction in the country rock mass strength by volcanic explosions the final pipe shape and distribution of some internal facies is therefore a consequence of the dominant rock mechanical failure processes in the pipe sidewalls the type of failure is dependant on the rock mass strength geological structures stress and depth below surface

a model for stress controlled pipe growth abstract the rock mechanics theory for deformation of underground mining excavations under high stress conditions can be used to explain the growth and geometry of volcanic pipes in an underground excavation stress concentrates greatest on the sides of an excavation perpendicular to the principal vector of compression if the stress is high enough fractures will develop causing scaling of the tunnel sidewalls and tunnel growth perpendicular to the principal vector of compression pre existing structures aid the physical mechanisms of pipe growth such as gravitational collapse explosive fragmentation and turbulent erosion and by reducing the strength of the rock mass should also aid stress induced scaling universal distinct element code numerical modelling demonstrated in this study reproduces the stress conditions around a circular pipe under uniaxial compression and simulates pipe growth as wedges bounded by failed pre existing joints form around the pipe and are assimilated into the pipe the results show how a volcanic pipe will tend to grow perpendicular to the principal vector of compression if the internal magma pressure is low or absent the orientations of the pre existing joints affect the exact direction of pipe growth in a predictable manner examples from other publications demonstrate that the model is consistent even in extensional tectonic environments case studies from kimberlite occurrences in the limpopo belt at finsch mine and the gross brukkaros volcanic complex demonstrate that dykes and magmatic bodies of kimberlite with high overpressures during emplacement normally have geometries trending near parallel to the principal vector of compression yet pipes or parts of pipes that underwent strong underpressures during emplacement often ending up comprising fragmental volcaniclastic infill have an elongation near perpendicular to the same vector thus the stress induced pipe growth model is demonstrated to be important for some pipes the study of volcanic pipe and dyke shapes can therefore be used to determine the stress regime at the time of emplacement and to distinguish between kimberlite occurrences formed at different times within different stress tensors

the spectrum of basaltic feeder systems from effusive lava eruption to explosive eruption at miyakejima volcano japan basaltic feeder systems exposed in the caldera wall of miyakejima volcano are classified into three groups 1 effusive feeders 2 moderately explosive feeders and 3 highly explosive feeders the surface deposits and feeder systems reveal a wide variation in the explosivity of the eruptions that produced them ranging from non explosive lava effusions to violent explosive eruptions despite the apparent lack of influence of external water effusive feeders are filled with coherent non fragmented intrusive rock indicating no significant fragmentation in the feeder system the other two types of feeder systems consist of a coherent dike in their deeper part and a pyroclastic fill in their uppermost part their uppermost parts show an upward flaring shape the transition from coherent intrusion to pyroclastic fill in the feeder systems suggests underground fragmentation of the rising magma the depth of the coherent pyroclastic transition is deeper 20 150 m in highly explosive feeders than in the moderately explosive feeders 20 m and coincides with the depth at which the system flares upwards presence of lithic fragments derived from the host rock in the products of the highly explosive feeder systems indicates the removal of the wall rock by explosive activity this observation suggests that the fragmentation of rising magma promoted the enlargement of the feeder systems to form their upward flaring shapes by mechanical erosion and wall collapse

the measurement of apparent velocity and azimuth using adaptive processing techniques on data from the warramunga seismic array summary adaptive processing of multichannel data can be achieved by cross correlating the signal on each channel with a velocity and azimuth filtered composite signal in an iterative manner in this way it is possible to accurately determine the arrival times of the wavefront at each sensor this technique which is currently used for the autematic analysis of seismic events recorded at the lasa and norsar arrays has been used as the basis of a software system developed for processing data from the medium aperture warramunga wra array in northern australia this system has been developed for a small and relatively inexpensive computer installation and permits the rapid direct measurement of apparent velocity and azimuth of signals coherent across most or all of the array the position of wra relative to the earth sactive seismic zones and in particular the belt north of australia results in multiple p wave signals from single events being frequently recorded emphasis was therefore placed on assessing the capabilities of adaptive processing for making measurements not only on the first arrival but also on its immediate coda the effects of signal multiplicity on parameter measurement were studied for both real and synthetic data using a number of different processing variables such as convergence criteria parts of waveform used and window positioning the results show that several measurements must be made across a waveform in order to identify systematic drifts shown to result from interference effects experiments using inverse filtering were performed in an attempt to improve the resolution

a new approach to kimberlite facies terminology using a revised general approach to the nomenclature of all volcanic rocks and deposits descriptive to genetic although kimberlite pipes bodies are usually the remains of volcanic vents in vent deposits and subvolcanic intrusions the terminology used for kimberlite rocks has largely developed independently of that used in mainstream volcanology existing kimberlite terminology is not descriptive and includes terms that are rarely used used differently and even not used at all in mainstream volcanology in addition kimberlite bodies are altered to varying degrees making application of genetic terminology difficult because original components and depositional textures are commonly masked by alteration this paper recommends an approach to the terminology for kimberlite rocks that is consistent with usage for other volcanic successions in modern terrains the eruption and emplacement origins of deposits can often be readily deduced but this is often not the case for old variably altered and deformed rock successions a staged approach is required whereby descriptive terminology is developed first followed by application of genetic terminology once all features including the effects of alteration on original texture and depositional features together with contact relationships and setting have been evaluated because many volcanic successions consist of both primary volcanic deposits as well as volcanic sediments terminology must account for both possibilities

arrested diatreme development standing rocks east hopi buttes navajo nation usa abstract maar diatreme volcanoes defined by their relatively large pyroclastic debris filled subsurface structures and craters that cut into the pre eruptive land surface are typically found in small volume mafic to ultramafic monogenetic volcanic fields diatremes are associated with strong explosions throughout most of their development focused along feeder dikes and generally attributed to magma water interaction or high magmatic volatiles detailed mapping of the magnificently exposed standing rocks east sre diatreme shows evidence of additional eruptive complexity and offers new insights into how the plumbing and vent structures of small volume volcanoes evolve during an eruption sre is part of a larger basanitic volcanic complex that includes several diatremes formed along a series of irregular offset nw se trending dikes exposed 300 m below the pre eruptive land surface its similarly oriented elliptical shaped diatreme structure comprises predominantly country rock lithic rich breccia of coarse inhomogeneously mixed wall rock blocks sourced from above and below the current surface plus sparse juvenile material domains of pyroclastic deposits crosscut the country rock breccia deposits and the best exposed is the nw massif rising 35 m above the current erosional surface it represents a cross section of an evolving crater floor and comprises matrix rich lapilli tuff and spatter deposits cut by irregularly distributed dikes some with very complex textures the most significant deposit in terms of volume is an unbedded lapilli tuff that is poorly sorted and has a well mixed population of wall rock and juvenile clast varieties thus resembling deposits typical of diatremes it is overlain by and locally intercalated with spatter deposits and this irregular contact demarcates the base of what was during eruption an uneven evolving crater floor the generally massive variably welded spatter deposits constitute mostly lapilli sized juvenile clasts with fluidal folded over shapes and ropy surfaces subordinate thermally altered wall rock and variegated domains of lapilli tuff sre shows a progressive transition from fissure to diatreme and overall evolution from more explosive to weakly explosive eruption styles recorded at the conduit crater transition diatreme development was initiated by deep quarrying explosive eruptions along a fissure to form the country rock rich breccia only parts of the fissure remained active as magma feeding the highly explosive eruptions along the fissure localized into discrete point sources forming the matrix rich lapilli tuff deposits these superimposed deposits record the passage of multiple debris jets and subvertical fallback from shallow cratering arising from explosions triggered by magma water interaction at numerous discrete sites however instead of continuing to build a well formed diatreme the system switched to weak spattering with intermittent explosive activity and near surface dike emplacement into the unconsolidated anisotropic pyroclastic debris of the crater floor dominant spatter from strombolian style bursts accumulated on the topographically varied evolving unstable syn eruptive crater floor and led to local failure and remobilization this study demonstrates how the combination of fissure behavior and sensitivity of the shallow plumbing system to local conditions during an eruption can lead to a decrease in eruptive footprint within the diatreme structure and an overall decrease in explosivity resulting in the arrested development of an immature diatreme

an integrated model of kimberlite ascent and eruption diatremes are carrot shaped bodies forming the upper parts of very deep magmatic intrusions of kimberlite rock these unusual enigmatic and complex features are famous as the source of diamonds here we present a new model of kimberlite ascent and eruption emphasizing the extremely unsteady nature of this process to resolve many of the seemingly contradictory characteristics of kimberlites and diatremes dyke initiation in a deep co2 rich source region in the mantle leads to rapid propagation of the dyke tip below which co2 fluid collects with a zone of magmatic foam beneath when the tip breaks the surface of the ground gas release causes a depressurization wave to travel into the magma this wave implodes the dyke walls fragments the magma and creates a ringing fluidization wave together these processes form the diatreme catastrophic magma chilling seals the dyke no precursor to the eruption is felt at the surface and the processes are complete in about an hour

kimberlite wall rock fragmentation processes venetia k08 pipe development current kimberlite pipe development models strongly advocate a downward growth process with the pipe cutting down onto its feeder dyke by means of volcanic explosions evidence is presented from the k08 kimberlite pipe in venetia mine south africa which suggests that some pipes or sub components of pipes develop upwards the k08 pipe in pit exposure comprises 90 vol chaotic mega breccia of country rock clasts gneiss and schist and 10 vol coherent kimberlite sub horizontal breccia layers tens of metres thick are defined by lithic clast size variations and contain zones of shearing and secondary fragmentation textural studies of the breccias and fractal statistics on clast size distributions are used to characterize sheared and non sheared breccia zones and to deduce a fragmentation mechanism breccia statistics are compared directly with the statistics of fragmented rock produced from mining processes in order to support interpretations results are consistent with an initial stage of brecciation formed by upward moving collapse of an explosively pre conditioned hanging wall into a sub terranean volcanic excavation our analysis suggests that the pre conditioning is most likely to have been caused by explosions either phreatic or phreatomagmatic in nature with a total energy output of 2 7 109 kj 656 t of tnt a second stage of fragmentation is interpreted as shearing of the breccia caused by multiple late kimberlite intrusions and possible bulk movement of material in the pipe conduit related to adjacent volcanism in the k02 pipe

distinct kimberlite pipe classes with contrasting eruption processes abstract field and scott smith field m scott smith b h 1999 contrasting geology and near surface emplacement of kimberlite pipes in southern africa and canada proc 7th int kimb conf eds gurney et al 1 214 237 propose that kimberlite pipes can be grouped into three types or classes classical or class 1 pipes are the only class with characteristic low temperature diatreme facies kimberlite in addition to hypabyssal and crater facies kimberlite class 2 and 3 pipes are characterized only by hypabyssal and crater facies kimberlite in an increasing number of class 1 pipes a new kimberlite facies transitional facies kimberlite is being found in most cases this facies forms a zone several metres wide at the interface between the hypabyssal and diatreme facies the transitional facies exhibits textural and mineralogical features which are continuously gradational between the hypabyssal and the diatreme types the textural gradations are from a coherent magmatic texture to one where the rock becomes increasingly magmaclastic and this is accompanied by concomitant mineralogical gradations involving the decline and eventual elimination of primary calcite at the expense of microlitic diopside both transitional and diatreme facies kimberlites are considered to have formed in situ from intruding hypabyssal kimberlite magma as a consequence of exsolution of initially co 2 rich volatiles from the volatile rich kimberlite magma the transitional facies is initiated by volatile exsolution at depths of about 3 km below the original surface with subsequent cracking through to the surface and resultant rapid decompression the further catastrophic exsolution of volatiles and their expansion leads to the formation of the diatreme facies thus diatreme facies kimberlite and class 1 pipes are emplaced by essentially magmatic processes rather than by phreatomagmatism distinctly different petrographic features characterize crater facies kimberlite in each of the three pipe classes in crater facies kimberlites of class 1 pipes small pelletal magmaclasts and abundant microlitic diopside are characteristic these features appear to reflect the derivation of the crater facies material from the underlying diatreme zone most class 2 pipes have shallow craters and the crater facies rocks are predominantly pyroclastic kimberlites with diagnostic amoeboid lapilli which are sometimes welded and have vesicles as well as glass possible kimberlite lava also occurs at two class 2 pipes in n angola the possible presence of lava as well as the features of the pyroclastic kimberlite is indicative of hot kimberlite magma being able to rise to levels close to the surface to form class 2 pipes most class 3 kimberlites have very steep craters and crater facies rocks are predominantly resedimented volcaniclastic kimberlites in some cases characterized by the presence of abundant angular magmaclasts which are petrographically very similar to typical hypabyssal facies kimberlite found in class 1 pipes the differences in crater facies kimberlite of the three classes of pipe reflect different formation and depositional processes as well as differences in kimberlite composition specifically volatile composition kimberlite forming pipe classes 1 and 3 is thought to be relatively water rich and is emplaced by processes involving magmatic exsolution of volatiles the kimberlite magma forming class 2 pipes is co 2 rich can rise to shallow levels and can initiate phreatomagmatic emplacement processes

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

the internal geology and emplacement history of the renard 2 kimberlite superior province quebec canada abstract the renard 2 kimberlite is located in the otish mountains region of quebec canada and is one of the largest pipes in the renard cluster the cluster consists of nine kimberlite bodies and was discovered in 2001 by ashton mining of canada inc and its joint venture partner soquem inc renard 2 was emplaced into archean meta greywacke derived migmatite gneiss and granite of the opinaca subprovince of the eastern superior province at approximately 640 5 2 8 ma an undetermined amount of erosion has occurred since emplacement with the present surface expression of the pipe estimated to be 0 75 ha this kimberlite is interpreted as a steep sided diatreme with minor irregularities in the external shape the dominant infill is a massive volcaniclastic kimberlite mvk that is classified as tuffisitic kimberlite breccia tkb and is characterized by a high proportion of granitoid country rock xenoliths a second dominant infill is a texturally complex less diluted coherent kimberlite ck characterized locally by a transitional textures between ck and tkb surrounding the diatreme is a significant zone of variable width comprised of extensively brecciated country rock kimberlite and referred to as marginal breccia in addition to the two main rock types infilling the pipe a number of hypabyssal kimberlite hk dykes and irregular shaped intrusions occur throughout the body along the pipe contacts within the marginal breccia and in the surrounding country rock geological features displayed by renard 2 are similar to those described from class 1 kimberlites of the kimberley area of south africa the gahcho kue cluster of canada and the pimenta bueno kimberlite field of brazil the economic evaluation of renard 2 is in progress and to date has included extensive diamond and reverse circulation drilling as well as the collection of an underground bulk sample results from material sampled from renard 2 including a 2449 tonne bulk sample suggest renard 2 has an estimated diamond content of 83 cpht carats per hundred tonnes a three dimension geology model of the pipe has been developed following the investigation of drill cores subsurface mapping and petrography combined with diamond studies and geophysics the model produced is being used to guide and direct the evaluation of the kimberlite and unravel the emplacement history of the pipe

welded tuff infilling a volcanic vent at weolseong republic of korea a plug of parataxitically welded tuff with an elliptical 700 400 m cross section crops out in weolseong district republic of korea it intrudes late cretaceous granodiorite and extrusive tuffs of the chisulryoung volcanic formation and displays a steeply inclined welding foliation concordant with the intrusive contacts and a subvertical stretching lineation these fabrics are interpreted as resulting from laminar shearing in a boundary layer during deposition of the tuff by agglutination along the walls of a vent which served as a feeder for extrusive ignimbrites

a rare occurrence of a crater filling clastogenic extrusive coherent kimberlite victor northwest ontario canada kimberlite pipes can contain significant proportions of dark and dense kimberlite that have mostly been interpreted as intrusive coherent hypabyssal in origin this study reports a well documented occurrence of a fresh intra crater clastogenic extrusive coherent kimberlite that is concluded to have formed as a result of lava fountaining this paper focuses on a dark dense competent generally crystal rich massive kimberlite unit within the victor northwest kimberlite pipe ontario canada using a comprehensive volcanological and petrographic analysis of all available drill cores it is shown that this unit has a fresh well crystallised coherent groundmass and is extrusive and pyroclastic in origin the proposed clastogenic coherent extrusive origin is based on deposit morphology gradational contacts to enveloping pyroclastic units as well as the presence of remnant pyroclast outlines and angular broken olivines this paper and an increasing number of other studies suggest that fragmental extrusive coherent kimberlite in intra crater settings may be more common than previously thought the emplacement history and volcanology of these pipes need to be reconsidered based on the emerging importance of this particular kimberlite facies

volcanological features of a low viscosity melt the carbonatitic gross brukkaros volcanic field namibia in the upper cretaceous gross brukkaros volcanic field southern namibia a radial dyke system surrounds a dome structure and its 74 closely related carbonatite diatremes this paper focuses on volcanological features which seem to be typical for a low viscosity melt in various settings such as dykes sills and diatremes the total or near absence of vesicles in carbonatite ash grains and lapilli inside the diatremes is evidence against explosive exsolution of volatile phases and in favour of a phreatomagmatic fragmentation mechanism and thus for a phreatomagmatic eruption mechanism of the carbonatite diatremes

on the welding of pyroclasts from very low viscosity magmas examples from kimberlite volcanoes abstract lithofacies in kimberlite pipes in southern africa exhibit features consistent with welded pyroclastic rocks these include flared conduit filling geometries abundant lithic clasts lithic clast layers subhorizontal clast fabrics gradational contacts with volcaniclastic rocks and sintered and coalesced globular ash and lapilli and melt coated particles the welding dynamics of kimberlite pyroclasts differ from those of glassy vesiculated pyroclasts in silicic volcanic systems low melt viscosity 0 1 pa s results in the efficient separation of volatiles and melt and the breakup of magma into nonvesicular spherical droplets the glassy state is difficult to form in silica deficient magmas because the crystallization kinetics are fast in low viscosity melts three pyroclastic lithofacies are recognized that primarily relate to the state of the initial melt phase in the pyroclasts on deposition 1 densely welded kimberlite forms where the initial melt phase in pyroclasts remains as pure mel

miocene rhyolitic welded tuff infilling a funnel shaped eruption conduit shiotani southeast of matsue sw japan at shiotani sw japan rhyolitic welded tuff forms a steep sided funnel shaped body confined by paleogene granitic rocks to an elliptical area 1 1 5 km across the shiotani welded tuff is pervasively welded and foliated concordantly with the contact that dips inward at angles of 70 90 in contrast nearby contemporary volcaniclastic deposits are non welded and gently inclined near the contact with the granite the tuff is plastically deformed and shows lineations that plunge inward at angles of 40 65 lithic and crystal clasts in the rheomorphic outer part are rotated in a plane normal to the foliations and parallel to the lineations indicating downward flow of the welded tuff the geometry and internal structures suggest that the shiotani welded tuff was emplaced and welded in a funnel shaped eruption conduit upon collapse of a plinian or phreatoplinian eruption column the majority of the conduit filling pyroclasts probably fell back en masse into the conduit heat and steam from underlying magma and diffusion of interstitial volatiles into the glass perhaps reduced the viscosity of juvenile pyroclasts and facilitated welding in the conduit especially at deep levels the hot welded pyroclasts then flowed down the conduit wall during welding compaction and retreat of the magma these processes resulted in increased welding toward the contacts and welding foliations concordant with the steep wall emplacement of nearby correlative volcaniclastic mass flow deposits in a shelf to upper bathyal environment suggests a possibility that when active the shiotani conduit was under the sea welding compaction would occur even under the sea provided that the steam generated in the upper part of the conduit fill prevented water access

complex proximal sedimentation from plinian plumes the example of tarawera 1886 the 1886 eruption of tarawera new zealand was unusual for a plinian eruption because it involved entirely basaltic magma originated in a 17 km long fissure and produced extremely overthickened proximal deposits with a complex geometry this study focuses on an 8 km long segment cutting across mount tarawera where over 50 point source vents were active during the 5 5 h eruption a detailed characterization of the proximal deposits is developed and used to interpret the range of styles and intensities of the vents including changes with time we identify the four vents that contributed most heavily to the plinian fall and evaluate the extent to which current volcanic plume models are compatible with the depositional patterns at tarawera three proximal units are mapped that have phreatomagmatic magmatic and phreatomagmatic characteristics respectively within the magmatic proximal unit beds of like character are grouped into packages and delineated on scaled cross sections package dispersal is quantified by measuring the linear thickness half distance t1 2 in the planes of the fissure walls most packages have localized dispersals low t1 2 indicating that strombolian style activity dominated most vents the more widely dispersed packages high t1 2 reflect contributions from additional transport regimes that were more vigorous but still contributed considerable material to the proximal region we conclude that the geometry of the observed proximal deposits requires three modes of fall transport 1 fallout from the upper portions of the plinian plumes produced principally by vents in four craters 2 sedimentation from the margins of the lower portions of the plinian plumes including the jets and possibly the lower convective regions and 3 ejection by weak strombolian style explosions from vents that did not contribute significant volumes of particles to the high plume we suggest that the curvature of the velocity profile across the jet region of each plume 1 4 km height was important and that the lower velocity at the margins allowed proximal deposition of a large volume of material with a wide grain size range

explosive rhyolite tuya formation classic examples from kerlingarfjoll iceland rhyolite eruptions in iceland mostly take place at long lived central volcanoes examples of which are found associated with each of the present day rift zone ice caps subglacial eruptions at kerlingarfjoll central volcano produced rhyolite tuyas that are notable for their exposures of early erupted pyroclastic material observations from a number of these edifices are synthesised into a general model for explosive rhyolite tuya formation eruptions begin with violent phreatomagmatic explosions that generate massive tuff mt but the influence of water quickly declines leading to the formation of massive lapilli tuffs mlt containing magmatically fragmented vesicular pumice and ash these are deposited rapidly near the vent probably by moist pyroclastic density currents confined by ice but not within a meltwater lake the explosive effusive transition is controlled by the ascent rate and gas content of the magma an unusual obsidian rich massive lapilli tuff lithofacies omlt is identified and interpreted as pyroclastic material that was intruded into gas fluidised deposits at the explosive effusive transition the effusive phase of eruption involves the emplacement of intrusions and lava caps intrusions of lava into the early erupted phreatomagmatic deposits are characterised by peperitic margins and the formation of hyaloclastite intrusions into stratigraphically higher levels of the pyroclastic material show more limited interaction with the host tephra and have microcrystalline cores large lava bodies with columnar jointed margins cap the tuyas and have intrusive basal contacts with the tephras the main influence of the ice is to confine the rhyolite eruptive products to immediately above the vent region this is in contrast to subglacial basaltic tuya forming eruptions which are characterised by the formation of meltwater lakes phreatomagmatic fragmentation and subaqueous deposition the lack of meltwater storage may reduce the potential for large jokulhlaups

problems with an in vent column collapse model for the emplacement of massive volcaniclastic kimberlite a discussion of in vent column collapse as an alternative model for massive volcaniclastic kimberlite emplacement an example from the fox kimberlite ekati diamond mine nwt canada by porritt et al j volcanol geotherm res 174 90 102

root zone processes in the phreatomagmatic pipe emplacement model and consequences for the evolution of maar diatreme volcanoes abstract the understanding of processes within the root zone of maar diatreme volcanoes is important for the interpretation of the geology volcanology and even hazard assessment of these volcanoes in the phreatomagmatic model of pipe formation the irregularly shaped root zone is the site of the phreatomagmatic explosions and thus functions as the engine for pipe formation in this model the root zone grows over a period of time in a series of many single thermohydraulic i e phreatomagmatic explosions the explosions initially occur close to the surface and with ongoing explosive activity penetrate towards deeper levels the ejection of country rock clasts from the root zone results in a mass deficiency in the root zone that causes the overlying tephra and the adjacent country rocks to subside passively in a sinkhole like fashion into the root zone many phreatomagmatic eruptions consequently result in the formation of a cone shaped diatreme thus with ongoing eruptions the cone shaped diatreme has to grow systematically both in depth and diameter during its growth processes in the lower diatreme levels successively destroy the upper levels of the evolving root zone at the surface the maar crater in turn reacts to the underlying subsidence processes and also grows both in depth and diameter thermohydraulic explosions which fragment both magma and the surrounding country rocks mostly occur within the bottom part of the root zone violent explosions in small pipes may clear the overlying diatreme for a short period of time before tephra fall and collapse of the walls of the new crater refill the small initial diatreme in larger pipes via expansion of the mixture of highly pressurized water vapor juvenile gas phases and explosively produced tephra the confined and expanding eruption cloud has to pierce through the diatreme fill in a feeder conduit in order to erupt diatreme clearing events in large pipes are difficult or impossible to maintain since the explosive force in the root zone is only in exceptional instances strong enough to lift or entrain the entire diatreme tephra knowledge of the genetic relationships between root zones and diatremes is critical to understand pipe growth processes the combination of such processes can lead to substantial variation in volcanic behavior and thus produce fundamentally different volcano and rock types it is the purpose of this paper to outline important features of root zones and suggest their significance for the genesis and evolution of maar diatreme and related volcanoes

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

development of spheroidal composite bombs by welding of juvenile spinning and isotropic droplets inside a mafic eruption column abstract it is generally assumed that spheroidal volcanic bombs are formed during cooling of molten clots pulled up into spheres by the surface tension of the magma also volcanic bombs are regarded as non fragmental structures i e made of coherent material we describe non coherent spheroidal bombs of mafic composition from the tuff cinder cone of the cabezo segura volcano calatrava volcanic province spain that exhibit features consistent with welding of essentially non vesicular juvenile spherical droplets it is assumed that the eruption of a low viscosity magma containing suspended phenocrysts mantle clasts and cognate lithic clasts resulted in the efficient separation of volatiles and melt and the explosive fragmentation of the magma into different types of pyroclasts the finest magma fragments would yield melt blobs achneliths or lava spray ca 1 5 m small droplets largely made up also of melt and isolated solid clasts phenocryst microphenocryst and lithic the largest lava fragments incorporated phenocrysts microphenocrysts and lithic clasts all these types of clasts would be erupted in a rapid gas thrust that prevented rapid collision among particles massive aggregation and premature collapse by defluidization the rotation of the largest magma droplets during ascent in a fluidized system produced spinning droplets similar to pelletal lapilli the collision and aggregation of the fluidal clasts might start as soon as the pyroclasts gas mixture was thrust out and progressively decelerated a model is proposed for the formation of spheroidal composite bombs assuming fluidal clasts sintering either by coalescence or agglutination and welding of constituent pyroclasts inside the eruption column prior to bomb accumulation on the field

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

syn and posteruptive hazards of maar diatreme volcanoes abstract maar diatreme volcanoes represent the second most common volcano type on continents and islands this study presents a first review of syn and posteruptive volcanic and related hazards and intends to stimulate future research in this field maar diatreme volcanoes are phreatomagmatic monogenetic volcanoes they may erupt explosively for days to 15 years above the preeruptive surface a relatively flat tephra ring forms below the preeruptive surface the maar crater is incised because of formation and downward penetration of a cone shaped diatreme and its root zone during activity both the maar crater and the diatreme grow in depth and diameter inside the diatreme which may penetrate downwards for up to 2 5 km fragmented country rocks and juvenile pyroclasts accumulate in primary pyroclastic deposits but to a large extent also as reworked deposits ejection of large volumes of country rocks results in a mass deficiency in the root zone of the diatreme and causes the diatreme fill to subside thus the diatreme represents a kind of growing sinkhole due to the subsidence of the diatreme underneath the maar crater is a subsidence crater and also grows in depth and diameter with ongoing activity as long as phreatomagmatic eruptions continue the tephra ring grows in thickness and outer slope angle syneruptive hazards of maar diatreme volcanoes are earthquakes eruption clouds tephra fall base surges ballistic blocks and bombs lahars volcanic gases cutting of the growing maar crater into the preeruptive ground formation of a tephra ring fragmentation of country rocks thus destruction of area and ground changes in groundwater table and potential renewal of eruptions the main hazards mostly affect an area 3 to possibly 5 km in radius distal effects are comparable to those of small eruption clouds from polygenetic volcanoes syneruptive effects on infrastructure people animals vegetation agricultural land and drainage are pointed out posteruptive hazards concern erosion and formation of lahars inside the crater a lake usually forms and diverse types of sediments accumulate in the crater volcanic gases may be released in the crater compaction and other diagenetic processes within the diatreme fill result in its subsidence this posteruptive subsidence of the diatreme fill and thus crater floor is relatively large initially but will decrease with time it may last millions of years various studies and monitoring are suggested for syn and posteruptive activities of maar diatreme volcanoes erupting in the future the recently formed maar diatreme volcanoes should be investigated repeatedly to understand more about their syneruptive behaviour and hazards and also their posteruptive topographic limnic and biologic evolution and potential posteruptive hazards for future maar diatreme eruptions a hazard map with four principal hazard zones is suggested with the two innermost ones having a joint radius of up to 5 km areas that are potentially endangered by maar diatreme eruptions in the future are pointed out

volcanology of the aries micaceous kimberlite central kimberley basin western australia abstract the neoproterozoic 815 4 4 3 ma aries kimberlite intrudes the king leopold sandstone and the carson volcanics in the central kimberley basin northern western australia aries is comprised of a n nne trending series of three diatremes and associated hypabyssal kimberlite dykes and plugs the diatremes are volumetrically dominated by massive clast supported accidental lithic rich kimberlite breccias that were intruded by hypabyssal macrocrystic phlogopite kimberlite dykes and plugs with variably uniform to globular segregationary textured groundmasses lower diatreme facies accidental lithic rich breccias probably formed through fall back of debris into the vent with a major contribution from the collapse of the vent walls these massive breccias are overlain by a sequence of bedded volcaniclastic breccias in the upper part of the north lobe diatreme abundant poorly vesicular to nonvesicular juvenile kimberlite ash and lapilli with morphologies that are indicative of phreatomagmatic fragmentation processes occur in a reversely graded volcaniclastic kimberlite breccia unit at the base of this sequence this unit and overlying bedded accidental lithic rich breccias are interpreted to be sediment gravity flow deposits including possible debris flows derived from the collapse of the crater walls and or tephra ring deposits that surrounded the crater diatreme forming eruptions may have been initiated by magma water interactions along fracture and joint controlled aquifers within the king leopold sandstone the current level of exposure of the diatremes probably extends from the lower diatreme facies up into the base of a bedded upper diatreme sequence

formation of clastogenic lava flows during fissure eruption and scoria cone collapse the 1986 eruption of izu oshima volcano eastern japan the 1986 eruption of b fissure at izu oshima volcano japan produced among other products one andesite and two basaltic andesite lava flows locally the three flows resemble vent effused holocrystalline blocky or aa lava however remnant clast outlines can be identified at most localities indicating that the flows were spatter fed or clastogenic the basaltic andesite flows are interpreted to have formed by two main processes a reconstitution of fountain generated spatter around vent areas by syn depositional agglutination and coalescence followed by extensional non particulate flow and b syn eruptive collapse of a rapidly built spatter and scoria cone by rotational slip and extensional sliding these processes produced two morphologically distinct lobes in both flows by a earlier non particulate flow of agglutinate and coalesced spatter which formed a thin lobe of rubbly aa lava ca 5 m thick with characteristic open extension cracks revealing a homogeneous holocrystalline interior and b later scoria cone collapse which created a larger lobe of irregular thickness 20 m made of large detached blocks of scoria cone interpreted to have been rafted along on a flow of coalesced spatter the source regions of these lava flows are characterized by horseshoe shaped scarps 30 m high with meso blocks ca 30 m in diameter of bedded scoria at the base one lava flow has a secondary lateral collapse zone with lower ca 7 m scarps backward tilted meso blocks are interpreted to be the product of rotational slip and forward tilted blocks the result of simple toppling squeeze ups of coalesced spatter along the leading edge of the meso blocks indicate that coalescence occurred in the basal part of the scoria cone this low viscosity coalesced spatter acted as a lubricating layer along which basal failure of the scoria cone occurred rotational sliding gave way to extensional translational sliding as the slide mass spread out onto the present caldera floor squeeze ups concentrated at the distal margin indicate that the extensional regime changed to one of compression probably as a result of cooling of the flow front sliding material piled up behind the slowing flow front and coalesced spatter was squeezed up from the interior of the flow through fractures and between rafted blocks the andesite flow although morphologically similar to the other two flows has a slightly different chemical composition which corresponds to the earliest stage of the eruption it is a much smaller lava flow emitted from the base of the scoria cone 2 days after the eruption had ceased this lava is interpreted to have been formed by post depositional coalescence of spatter under the influence of the in situ cooling rate and load pressure of the deposit extrusion occurred through the lower part of the scoria cone and subsequent non particulate flow of coalesced material produced a blocky and aa lava flow the mechanisms of formation of the lava flows described may be more common during explosive eruptions of mafic magma than previously envisaged

structural analysis and implicit 3d modelling of high grade host rocks to the venetia kimberlite diatremes central zone limpopo belt south africa abstract the beit bridge complex of the central zone cz of the limpopo belt hosts the 519 6 ma venetia kimberlite diatremes deformed shelf or platform type supracrustal sequences include the mount dowe malala drift and gumbu groups comprising quartzofeldspathic units biotite bearing gneiss quartzite metapelite metacalcsilicate and ortho and para amphibolite previous studies define tectonometamorphic events at 3 3 3 1 ga 2 7 2 5 ga and 2 04 ga detailed structural mapping over 10 years highlights four deformation events at venetia rules based implicit 3d modelling in leapfrog geo provides an unprecedented insight into cz ductile deformation and sheath folding d 1 juxtaposed gneisses against metasediments d 2 produced a pervasive axial planar foliation s 2 to isoclinal f 2 folds sheared lithological contacts and s 2 were refolded into regional open predominantly southward verging e w trending f 3 folds intrusion of a hornblendite protolith occurred at high angles to incipient s 2 constrictional prolate d 4 shows moderately ne plunging azimuths defined by elongated hornblendite lenses andalusite crystals in metapelite crenulations in fuchsitic quartzite and sheath folding d 4 overlaps with a 1 2 03 2 01 ga regional m 3 metamorphic overprint b transpressional deformation at 2 2 1 9 ga and c 2 03 ga transpressional dextral shearing and thrusting around the cz and d formation of the avoca bellavue and baklykraal sheath folds and parallel lineations

geology of a complex kimberlite pipe k2 pipe venetia mine south africa insights into conduit processes during explosive ultrabasic eruptions k2 is a steep sided kimberlite pipe with a complex internal geology geological mapping logging of drillcore and petrographic studies indicate that it comprises layered breccias and pyroclastic rocks of various grain sizes lithic contents and internal structures the pipe comprises two geologically distinct parts k2 west is a layered sequence of juvenile and lithic rich breccias which dip 20 45 inwards and k2 east consists of a steep sided pipe like body filled with massive volcaniclastic kimberlite nested within the k2 pipe the layered sequence in k2 west is present to 900 m below present surface and is interpreted as a sequence of pyroclastic rocks generated by explosive eruptions and mass wasting breccias generated by rock fall and sector collapse of the pipe walls both processes occurred in tandem during the infill of the pipe several breccia lobes extend across the pipe and are truncated by the steep contact with k2 east dense pyroclastic rocks within the layered sequence are interpreted as welded deposits k2 east represents a conduit that was blasted through the layered breccia sequence at a late stage in the eruption this phase may have involved fluidisation of trapped pyroclasts with loss of fine particles and comminution of coarse clasts we conclude that the k2 kimberlite pipe was emplaced in several distinct stages that consisted of an initial explosive enlargement followed by alternating phases of accumulation and ejection

internal vs external forcing in shallow marine diatreme formation a case study from the iblean mountains se sicily central mediterranean a model of diatreme evolution in a shallow marine setting is based on a multi disciplinary analysis of diatremes in the iblean mountains sicily the approach includes stratigraphic volcanological structural petrologic and compositional data we invoke a complex interplay of internal rapid ascent and pyroclastic fragmentation of a volatile co2 rich nephelinitic magma at depth and external factors these comprise hydroclastic explosions due to near surface interaction of the rising particle volatile mixture with seawater and water saturated lime mud other external factors contributing to diatreme formation include regional and local tectonics graben formation in pull apart motion combined with lateral pipe enlargement by bedrock spalling and radial block subsidence into the diatreme pipe we suggest that fragmentation of volatile rich magma due to internal eruption forcing was fundamental in the formation of the iblean shallow marine diatremes internal and external factors may act to a variable degree however during diatreme evolution in general

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

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

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

lithofacies characteristics of diatreme deposits examples from a basaltic volcanic field of sw sardinia italy abstract a deeply eroded diatreme field consisting in several decametric sized vertical mainly clastic volcanic bodies of basaltic composition is described for the first time in the variscan basement of sw sardinia the recognition and description of four different lithofacies in these diatremes allowed discussion of the role of the different processes which control magma eruption and conduit infilling and making general inferences about diatremes the studied diatremes have a cross sectional shape from elliptical to sub triangular and are slightly elongated nearly parallel to the main foliation of the intruded meta sedimentary rocks foliation of host rocks is locally reoriented or folded close to the contact with the diatremes suggesting that magma possibly rose to the surface through fissures oriented nearly parallel to host rock foliation textural features of the volcanic bodies show many analogies with kimberlitic diatremes despite the difference in petrography and composition juvenile lapilli are mainly made by ghosts of mafic phenocrysts olivine and clinopyroxene set in a groundmass formed by plagioclase microlites immersed in a cryptocrystalline chlorite rich matrix the four lithofacies were described mainly based on the shape and physical features of the clasts and textural anisotropy a globular juvenile rich lapilli tuff facies gjlt an angular juvenile rich lapilli tuff facies ajlt a lithic rich lapilli tuff facies lirlt and a coherent lava like facies coh all the clastic lithofacies are generally well sorted and typically lack a fine grained matrix juvenile fragments are lapilli sized and from equant to oblate in axial ratio and from rounded globular to very angular in shape conversely lithic clasts are largely variable in shape and size and are mainly represented by basement derived clasts the absence of bedding the scarcity of the coherent facies and the dominance of clast supported structureless volcaniclastic facies suggest that the outcropping portion of these volcanic bodies represents the lower diatreme zone the presence of diffuse welding and the globular shapes of some juvenile fragments together with their vesicularity suggest that magma fragmentation was mainly driven by magmatic gas exsolution occurring at a deeper level respect to classical basaltic explosive activity textural features facies association and facies architecture of the studied deposits are suggestive of an important affinity with kimberlitic and other ultramafic diatremes

physical characteristics of kimberlite and basaltic intraplate volcanism and implications of a biased kimberlite record we assess bias in the record of kimberlite volcanism by using newly acquired size data on more than 900 kimberlite bodies from 12 kimberlite fields eroded to depths of between 0 m and 1200 m and by a comparison with intraplate monogenetic basaltic volcanic fields eroded kimberlite fields are composed of pipes or diatremes and dikes and within any one kimberlite field regardless of erosion level kimberlite bodies vary in area at earth s surface over 2 3 orders of magnitude typically 60 70 of the bodies are

eruption of kimberlite magmas physical volcanology geomorphology and age of the youngest kimberlitic volcanoes known on earth the upper pleistocene holocene igwisi hills volcanoes tanzania the igwisi hills volcanoes ihv tanzania are unique and important in preserving extra crater lavas and pyroclastic edifices they provide critical insights into the eruptive behaviour of kimberlite magmas that are not available at other known kimberlite volcanoes cosmogenic 3he dating of olivine crystals from ihv lavas and palaeomagnetic analyses indicates that they are upper pleistocene to holocene in age this makes them the youngest known kimberlite bodies on earth by 30 ma and may indicate a new phase of kimberlite volcanism on the tanzania craton geological mapping global positioning system surveying and field investigations reveal that each volcano comprises partially eroded pyroclastic edifices craters and lavas the volcanoes stand 102 to 106 pa s for kimberlite attributed to degassing and in vent cooling each volcano is inferred to be the result of a small volume short lived days to weeks monogenetic eruption the eruptive processes of each igwisi volcano were broadly similar and developed through three phases 1 fallout of lithic bearing pyroclastic rocks during explosive excavation of craters and conduits 2 fallout of juvenile lapilli from unsteady eruption columns and the construction of pyroclastic edifices around the vent and 3 effusion of degassed viscous magma as lava flows these processes are similar to those observed for other small volume monogenetic eruptions e g of basaltic magma

evolution and volcanic hazards of taapaca volcanic complex central andes of northern chile taapaca volcanic complex is a large dacitic volcano 35 km 3 located in the western border of the active zone of the central andes of northern chile apart from early poorly preserved silicic andesites taapaca volcanic complex has generated remarkably similar porphyritic hornblende biotite dacites with distinctive sanidine megacrysts for at least 1 5 ma the main products of the volcano are dacite lavas and domes with associated block and ash flow deposits there have also been several sector collapses to generate debris avalanches which are closely associated with volcanic blasts and episodes of dome growth four stages of evolution are recognized with volcanism occurring in short bursts between much longer periods of dormancy volcanism has built a substantial stratovolcano and has migrated 4 5 km towards the sw with time late pleistocene to holocene activity has involved at least three sector collapses of the hydrothermally altered flanks and domes volcanic blasts block and ash flows debris avalanches and lahars have been distributed down the southwestern flanks these areas are the main populated part of the chilean altiplano and the location of the main road between bolivia and the pacific ocean coast a future eruption will threaten these areas and the regional economy