combining local and remote infrasound recordings from the 2009 redoubt volcano eruption abstract the explosive phase of the 2009 redoubt volcano eruption produced predominantly short duration high amplitude infrasound signals recorded up to 4500 km away all 19 numbered explosive events were recorded at a local microphone dfr 12 km as well as at an infrasound array in fairbanks alaska i53us 547 km most with high signal to noise ratios the local microphone provides an estimate of the source parameters and comparison between the two datasets allows the unique opportunity to evaluate acoustic source term estimation at a remote array high waveform similarity between dfr and i53us occurs during much of the explosive phase due to strong stratospheric ducting permitting accurate source constraints inferred from i53us data cross correlation analysis after applying a hilbert transform to the i53us data shows how the acoustic energy has passed through a single caustic as predicted by ray theory similar to previous studies significant low frequency infrasound from redoubt recorded at i53us is coincident with high altitude ash emissions the largest events also produced considerable energy at greater than 50 s periods likely related to the initial oscillations of the volcanic plume or jet many of the explosive events have emergent onsets somewhat unusual for explosive short duration eruptions comparison of the satellite derived so2 emissions with the relative amount of acoustic energy at i53us shows a very high statistically significant correlation this study reiterates the utility of using remote infrasound arrays for detection of hazardous emissions and characterization of large volcanic eruptions and demonstrates how under typical meteorological conditions remote infrasound arrays can provide an accurate representation of the acoustic source

magnetic fabrics of the miocene ignimbrites from west cameroon implications for pyroclastic flow source and sedimentation abstract the miocene ignimbrites of mounts bambouto and bamenda located in the central part of cameroon volcanic line are generally made of welded and non welded massive lapilli tuff and lithic breccias these discontinuous deposits cover a total area of 180 km 2 with thickness ranging from 25 to 200 m the different facies contain several lithic fragments of mainly trachytic nature the devitrified matrix of the welded ignimbrites is constituted by sanidine anorthoclase quartz plagioclase clinopyroxene biotite fe ti oxides and devitrified fiammes anisotropy of magnetic susceptibility ams is used to characterize magnetic fabrics and to provide an estimate of flow direction of each ignimbrite sheet magnetic mineralogy results from different flow units show that titanomagnetite titanohematite maghemite and goethite with grain size ranging from coarse md to very fine sp are the main magnetic carriers of these ignimbrites inferred transport directions based on the ams data and field indicators show that bambouto caldera is the source of main pyroclastic deposits of mount bambouto in southwestern mount bamenda santa mbu caldera or bambouto caldera constituted the probable emission center of mbengwi bamenda and mbu ignimbrite sheets whereas magnetic fabrics of bambili sabga and big babanki ignimbrites demonstrate that these deposits were emitted from a northeastern source namely oku vent in mount oku a small number of subvertical ams fabrics correspond to rocks possibly modified by an elutriation process

modeling width adjustment in meandering channels widening in sinuous channels occurs when the retreat of the outer bank exceeds the advance of the opposite bank an analytical model is presented to simulate width adjustment in meandering channels of non cohesive bank material resulting from bank erosion of two interactive processes basal erosion and bank collapse bank collapse refers to the avalanche of non cohesive material in the upper part of bank above water surface resulting from over steepening of the bank surface due to basal erosion the rate of basal erosion including lateral erosion and bed degradation is calculated as a function of the longitudinal gradient of sediment transport rate and strength of secondary flow the transverse bed slope is treated as a variable that increases as channel sinuosity increases until the transverse bed slope reaches its maximum value by simplifying the bank collapse process for non cohesive materials the present study shows that the rate of bank line retreat is determined by lateral erosion rate near bank bed degradation rate sediment grain size and difference between flow depth and bank height the time dependent widening processes of two meandering channels in the laboratory are selected to test applicability of the model the result shows that the simulated bank lines at individual time intervals closely match the experimental measurements whether the sinuosity of a meandering channel will increase or decrease is primarily determined by didtribution of the lngitudinal gradient of sediment transport rate along the channel

characteristics of short term slow slip events estimated from deep low frequency tremors in shikoku japan 1 we have investigated the characteristics of short term slow slip events sses recurrence interval and size distribution and the slip rate at the transition zone on the plate interface beneath the shikoku region japan using nonvolcanic deep low frequency dlf tremors on the basis of a proportional relationship between the seismic moment of sse observed geodetically and the total size of dlf tremors of the corresponding episode we estimated the seismic moment due to the slip on the plate interface from the dlf tremors and a temporal variation in the cumulative seismic moment the recurrence interval of major short term sses is 6 months in the western area and 3 months in the central and the eastern areas the size distribution of short term sses as well as dlf tremors is approximated by an exponential law rather than by a power law showing a different scaling for regular earthquakes the average slip rate at the transition zone estimated from the cumulative seismic moment with time of sses is 4 2 cm yr 3 3 cm yr and 4 9 cm yr in the western central and eastern areas respectively these values compensate for the difference between the convergence rate at the trench and the slip deficit rate at the transition zone of the subducting philippine sea plate in other words the slip rate estimated from the dlf tremors provides a constraint of the slip deficit rate at the transition zone on the plate interface

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

criteria for interpreting kimberlite as coherent insights from the muskox and jericho kimberlites nunavut canada the jurassic muskox and jericho kimberlites northern slave province nunavut canada contain a variety of facies exhibiting different geometries contact relationships internal organisation country rock abundance and olivine shapes although many have similar matrix groundmass mineralogies and textures five facies are examined that either have characteristics consistent with coherent rocks in general i e intrusive and extrusive non fragmental rocks or are mineralogically and texturally similar to kimberlite described as coherent or apparent coherent three facies are interpreted as coherent on the basis of 1 geological setting 2 apparent porphyritic texture 3 sharp contacts with fragmental kimberlite 4 relative abundance of elongate and unbroken olivine crystals and 5 paucity of country rock xenoliths while the remaining two facies are interpreted as fragmental on the basis of 1 the gradational contacts with demonstrably fragmental kimberlite 2 relative abundance and range of sizes of country rock lithic clasts and 3 numerous broken olivine crystals comparisons are made with coherent and apparent coherent kimberlite from the literature our three coherent facies are similar to literature reported coherent kimberlite dykes hosted in country rock ckd in terms of internal organisation low abundance of country rock xenoliths and apparent porphyritic texture conversely our two fragmental facies share attributes with previously described pipe filling coherent and apparent coherent kimberlite ckpf in terms of geometry internal organisation and abundance of country rock xenoliths we conclude that ckd and most ckpf although similar in matrix groundmass mineralogy and texture can be distinguished on the basis of internal organisation country rock lithic clast abundance texture e g apparent porphyritic texture and possibly olivine crystal shapes and suggest that fragmental kimberlite is more common than reported

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

scaling behavior and source mechanism of tremor recorded at erebus volcano ross island antarctica abstract erebus is an intraplate stratovolcano of phonolitic composition located at the southern end of terror rift within the west antarctica rift system the volcanic edifice of erebus exhibits a summit plateau that hosts the main crater and a smaller inner crater that contains a persistent lava lake volcano seismic signals are recorded at erebus by a permanent network of short period and broadband seismometers installed around the volcano edifice we investigate the duration amplitude scaling of tremor recorded by the closest 0 6 2 km broadband stations to the inner crater during may 2002 april 2004 in order to elucidate the physical processes taking place at its source after screening the data for signals related to non volcanic sources such as iceberg calving we obtained 72 tremor events spectral analysis reveals that these events were either harmonic exhibiting several overtones or broadband but there were also mixed events that contained both of these two states all tremor episodes considered in this study follow an exponential duration amplitude distribution that implies a scale bound source process this precludes the possibility that any of these signals is related to iceberg calving near ross island or that rock fracturing processes play any role in tremor generation the large viscosity of phonolitic magma 105 3 pa s suggests that source mechanisms which require high flow speeds in order to excite tremor are unlikely to operate at erebus on the other hand oscillations of co2 bubble clouds formed at localized asperities within the magma chamber represent a scale bound source that is also consistent with the observed oxidation state of ascending gas and with the deep 5 7 km location of the tremor source these results are important for understanding tremor processes at volcanoes which similar to erebus exhibit vigorous convection in their shallow plumbing system

miocene phreatomagmatic volcanism at tihany pannonian basin hungary abstract a late miocene 7 56 ma maar volcanic complex tihany maar volcanic complex tmvc is preserved in the pannonian basin and is part of the bakony balaton highland volcanic field base surge and fallout deposits were formed around maars by phreatomagmatic explosions caused by interactions between water saturated sediments and alkali basalt magma carrying peridotite lherzolite xenoliths as well as pyroxene and olivine megacrysts subsequently nested maars functioned as a sediment trap where deposition built up gilbert type delta sequences at the onset of eruption magma began to interact with a moderate amount of groundwater in the water saturated sand as eruption continued phreatomagmatic blasts excavated downward into limestones providing access to abundant karst water and deeper to sandstones and schist both providing large amount of fracture filling water at the surface this wet eruption led to the emplacement of massive tuff breccias by fall surge mudflow and gravity flow deposition the nature of the tmvc maar eruptions and their deposits appears to depend on the hydrological condition of the karst and or fracture filling aquifer which varies seasonally with rainfall and spring runoff the west and east maar volcanoes of tmvc are interpreted to represent low water input from the karst and or fracture filling aquifer summer vent whereas the east maar is interpreted to have formed when abundant karst and or fracture filling water was available spring vent

real time seismic amplitude measurement rsam a volcano monitoring and prediction tool seismicity is one of the most commonly monitored phenomena used to determine the state of a volcano and for the prediction of volcanic eruptions although several real time earthquake detection and data acquisition systems exist few continuously measure seismic amplitude in circumstances where individual events are difficult to recognize or where volcanic tremor is prevalent analog seismic records provide a quick visual overview of activity however continuous rapid quantitative analysis to define the intensity of seismic activity for the purpose of predicing volcanic eruptions is not always possible because of clipping that results from the limited dynamic range of analog recorders at the cascades volcano observatory an inexpensive 8 bit analog to digital system controlled by a laptop computer is used to provide 1 min average amplitude information from eight telemetered seismic stations the absolute voltage level for each station is digitized averaged and appended in near real time to a data file on a multiuser computer system raw realtime seismic amplitude measurement rsam data or transformed rsam data are then plotted on a common time base with other available volcano monitoring information such as tilt changes in earthquake activity associated with dome building episodes weather and instrumental difficulties are recognized as distinct patterns in the rsam data set rsam data for domebuilding episodes gradually develop into exponential increases that terminate just before the time of magma extrusion mount st helens crater earthquakes show up as isolated spikes on amplitude plots for crater seismic stations but seldom for more distant stations weather related noise shows up as low level long term disturbances on all seismic stations regardless of distance from the volcano implemented in mid 1985 the rsam system has proved valuable in providing up to date information on seismic activity for three mount st helens eruptive episodes from 1985 to 1986 may 1985 may 1986 and october 1986 tiltmeter data the only other telemetered geophysical information that was available for the three dome building episodes is compared to rsam data to show that the increase in rsam data was related to the transport of magma to the surface thus if tiltmeter data is not available rsam data can be used to predict future magmatic eruptions at mount st helens we also recognize the limitations of rsam data two examples of rsam data associated with phreatic or shallow phreatomagmatic explosions were not preceded by the same increases in rsam data or changes in tilt associated with the three dome building eruptions

mechanisms of hydrovolcanic pyroclast formation grain size scanning electron microscopy and experimental studies pyroclasts produced by explosive magma water interactions are of various sizes and shapes data from analysis of over 200 samples of hydrovolcanic ash are interpreted by comparison with experimentally produced ash grain size and scanning electron microscopy sem reveal information on the formation of hydrovolcanic pyroclasts strombolian explosions result from limited water interaction with magma and the pyroclasts produced are dominantly centimeter sized with increasing water interaction hydrovolcanism increases in explosivity to surtseyan and vulcanian activity these eruptions produce millimeter to micron sized pyroclasts the abundance of fine ash 63 m diameter increases from 5 to over 30 percent as water interaction reaches an explosive maximum this maximum occurs with interactions of virtually equal volumes of melt and water five dominant pyroclast shape types determined by sem result from hydrovolcanic fragmentation 1 blocky and equant 2 vesicular and irregular with smooth surfaces 3 moss like and convoluted 4 spherical or drop like and 5 plate like types 1 and 2 dominate pyroclasts greater than 100 m in diameter types 3 and 4 are typical of fine ash type 5 pyroclasts characterize ash less than 100 m in diameter resulting from hydrovolcanic fragmentation after strong vesiculation fragmentation mechanisms observed in experimental melt water interactions result from vapor film generation expansion and collapse fragments of congealed melt are products of several alternative mechanisms including stress wave cavitation detonation waves and fluid instability mixing all result in rapid heat transfer these mechanisms can explain the five observed pyroclast shapes stress wave fracturing cavitation of the melt results from high pressure and temperature gradients at the melt water interface simultaneous brittle fracture and quenching produces type 1 pyroclasts type 2 develops smooth fused surfaces due to turbulent mixing with water after fracture and before quenching fluid instabilities promote turbulent mixing of melt and water and produce fine ash this kind of fragmentation occurs during high energy explosions the increased melt surface area due to fine fragmentation promotes high efficiency heat exchange between the melt and water shapes of resulting pyroclasts are determined by maximum surface area type 3 and surface tension effects type 4 type 5 pyroclasts result from nearly simultaneous vesicle burst and melt water fragmentation

flow around a cylinder shallow water modeling with diffusion dispersion the equations for two dimensional 2d horizontal flow are obtained by vertical depth integration of the continuity and momentum equations diffusion dispersion terms appear containing turbulent and dispersion stresses turbulent stresses are expressed with the eddy viscosity concept dispersion stresses are evaluated using velocity distributions along and across the curved streamlines the differential equations are solved using the maccormack scheme the numerical simulation was done for three runs the expected alteration of the flow field around the cylinder is evident notably the wake behind the cylinder for comparison two similar runs were performed in a laboratory channel the velocity vectors upstream from the cylinder and along its sides are in reasonably good agreement however very close to the cylinder the simulation sometimes underpredicts the velocities downstream from the cylinder agreement is satisfactory both inside and outside the wake the flow depths at the centerline both upstream and downstream from the cylinder are also in good agreement however along the cylinder circumference the simulated flow depths are lower than the observed ones

character mass distribution and origin of tephra fall deposits from the 2009 eruption of redoubt volcano alaska highlighting the significance of particle aggregation abstract the 2009 eruption of redoubt volcano included 20 tephra producing explosions between march 15 2009 and april 4 2009 utc next generation radar nexrad data show that plumes reached heights between 4 6 km and 19 km asl and were distributed downwind along nearly all azimuths of the volcano explosions lasted between 0 8 mm thick including communities along the kenai peninsula 80 100 km and the city of anchorage 170 km trace ash on march 15 a small 4 6 km asl phreatic explosion containing minor non juvenile ash erupted through the summit ice cap the first five magmatic explosions events 1 5 occurred within a 6 hour period on march 23 plumes rose to heights between 5 5 km and 14 9 km asl during 2 to 20 minute duration explosions and were dispersed mainly along a nne trajectory trace ash fall was reported as far as fairbanks owing to a shift in wind direction and heavy snowfall during these events field discrimination among many of these layers was possible all deposits comprise a volumetrically significant amount of particle aggregates yet only event 5 deposits contain coarse clasts including glacier ice the most voluminous tephra fall was deposited on march 24 event 6 from a 15 minute explosion that sent a plume to 18 3 km asl and dispersed tephra to the wnw within 10 km of the vent this deposit contains 1 11 cm pumice clasts in a matrix of 1 2 mm aggregate lapilli a small dome was presumably emplaced between march 23 and march 26 and was subsequently destroyed during 1 14 minute magmatic explosions of events 7 8 march 26 that sent plumes between 8 2 km and 19 km asl ash fell along a broad swath to the ese covering communities along the kenai peninsula with up to 1 mm of ash proximal deposits are largely composed of aggregate lapilli of 1 2 mm with very little coarse juvenile material events 9 18 march 27 sent plumes between 5 2 km and 15 5 km asl during particle size data showing a preponderance of fine ash even in the most proximal locations along with the abundance of aggregate lapilli documented in most samples confirms that particle aggregation played a significant role in the 2009 eruption and induced premature fallout of fine ash

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

gondwanan late jurassic and cretaceous palaeosurfaces of the australian craton abstract cretaceous and especially early cretaceous marine transgressions played an important part in the evolution of the australian landscape they covered at least 40 of the present australian craton and about 45 of the entire present continent in so doing they preserved in unconformity extensive areas of the pre existing or gondwana landscape though substantial gondwana elements have been ehumed at the margins of the depositional basins as a result of the cainozoic stripping of the sedimentary cover clear examples are demonstrated in the isa highlands the peake and davenport ranges and the northermost flinders ranges the basinal areas subsided under the weight of water and sediment while adjacent land areas were uplifted flexure continentale or randschwelle those that were already of low relief were dissected and regoliths were eroded but substantial areas of the gondwana plains in etch form were preserved on divides as in the gawler and hamersley ranges others were reduced to low relief by rivers graded to cretaceous shorelines as in the southern and central flinders ranges elsewhere the gondwana landscapes were preserved more or less intact as in the western yilgarn craton the mt lofty ranges and possibly the higher ridges of the macdonnell ranges the nature of the gondwana landscape varied according to structure but inselberg landscapes were widely developed and have been exhumed for instance in the western pilbara and in the eastern musgrave block kulgera region the gondwana landscape was already complex with stratigraphic conditions suitable for the exhumation of archaean preterozoic and cambrian forms for instance already in place re exposure later took place so that very old elements form integral parts of the present landscape exhumed gondwana landscapes have been preserved by burial and the stripping of some of the etch forms may have taken place in the later cainozoic so that their survival poses few difficulties other etch surfaces like those of the gawler and the hamersley ranges demonstrably were stripped in the early cretaceous and eocene respectively and they like epigene earliest tertiary and mesozoic features call for explanation various factors like resistant bedrock unequal erosion surfaces which stand high and dry interior situation and reinforcement effects go some way to relieving though they do not resolve the problem

dune migration in a steep coarse bedded stream during 1986 and 1987 migrating bed forms composed of coarse sand and fine gravel d50 1 8 to 9 1 mm were documented in the north fork toutle river at kid valley washington at flow velocities ranging from 1 6 to 3 4 m s 1 and depths of 0 8 to 2 2 m the bed forms predominantly lower regime dunes were studied with a sonic depth sounder transducer suspended in the river at a stationary point twelve temporal depth sounding records were collected during storm runoff and nearly steady average streamflow with record durations ranging from 37 to 261 min waveform height was defined by dune front heights which ranged from 12 to 70 cm a weak correlation between flow depth and the standard deviation of bed elevation was noted dune front counts and spectral analyses of the temporal records showed that dune crests passed the observation point every 2 to 5 min dunes were often superposed on larger bed forms with wave periods between 10 and 30 min gradual changes in waveform height and periodicity occurred over several hours during storm runoff the processes of dune growth and decay were both time dependent and affected by changes in streamflow rates of migration for typical dunes were estimated to be 3 cm s 1 and dune wavelengths were estimated to be 6 to 7 m

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

application of a roughness length representation to parameterize energy loss in 3 d numerical simulations of large rivers 1 recent technological advances in remote sensing have enabled investigation of the morphodynamics and hydrodynamics of large rivers however measuring topography and flow in these very large rivers is time consuming and thus often constrains the spatial resolution and reach length scales that can be monitored similar constraints exist for computational fluid dynamics cfd studies of large rivers requiring maximization of mesh or grid cell dimensions and implying a reduction in the representation of bedform roughness elements that are of the order of a model grid cell or less even if they are represented in available topographic data these subgrid elements must be parameterized and this paper applies and considers the impact of roughness length treatments that include the effect of bed roughness due to unmeasured topography cfd predictions were found to be sensitive to the roughness length specification model optimization was based on acoustic doppler current profiler measurements and estimates of the water surface slope for a variety of roughness lengths this proved difficult as the metrics used to assess optimal model performance diverged due to the effects of large bedforms that are not well parameterized in roughness length treatments however the general spatial flow patterns are effectively predicted by the model changes in roughness length were shown to have a major impact upon flow routing at the channel scale the results also indicate an absence of secondary flow circulation cells in the reached studied and suggest simpler two dimensional models may have great utility in the investigation of flow within large rivers

modelling free forming meander evolution in a laboratory channel using three dimensional computational fluid dynamics abstract the present study investigated the use of computational fluid dynamics cfd in predicting the formation development and migration of free forming meander bends the three dimensional cfd model computed water flow and sediment transport in alluvial channels and predicted vertical and horizontal bed changes different algorithms and parameters were tested to provide an insight into the application range of cfd when modelling free forming meander formation the computational domain was discretized by an unstructured grid a control volume method was used for the discretization of the navier stokes equations for the flow calculation and of the convection diffusion equation for the sediment transport calculation turbulence was modelled by the k e turbulence model the simulation was started from an initially straight grid with neither sediment feed nor any perturbation at the inflow boundary the model computed the river bed evolution over a real time period of 3 d results were compared with laboratory experiments and showed that the cfd model can predict many of the characteristics of the alluvial meander formation and migration however some limitations and uncertainties exist that have to be clarified in future investigations

long term landscape evolution in australia the main features of the australian physical landscape are of the order of 107 108 years old this contradicts the widely held view that little of the earth s topography predates the quaternary and that erosion cycles are carried to planation within tens of millions of years much of the australian landscape must have developed over similar timescales to that of the tectonic evolution of the continent itself the study of the geomorphology of such ancient terrains may therefore be seriously deficient unless it is considered within the context of continental scale tectonic development application of this approach shows that there are strong links between the geomorphology of australia and plate movements ocean spreading plate convergence tectonostratigraphic terranes orogenesis and epeirogenesis the most important factor contributing to the survival of ancient landscapes in australia is the low rate of denudation which the continent has experienced during the mesozoic and cenozoic this is largely a consequence of orogenic stability although the absence of significant quaternary glaciation may also be of importance however in order for landforms to have survived over such timespans denudation must not only have been low but must also have been highly localized over space and time this has been the case both on a regional scale with long term denudation rates of 0 2 m ma 1 in central australia contrasting with higher rates along the continental margins and on a local scale with denudation confined to valleys leaving divides and interfluves almost unscathed

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

structures and grain size distribution in surge deposits as a tool for modelling the dynamics of dilute pyroclastic density currents at la fossa di vulcano aeolian islands italy abstract turbulent dilute pyroclastic density currents surges represent the most common and hazardous type of eruptive event at la fossa di vulcano during the volcanic history these currents have frequently passed over the topographic barrier of la fossa caldera maintaining a high velocity we interpret lateral facies variation in terms of fluid dynamic behaviour by analyses of structures thickness and grain size of correlated surge layers from the palizzi succession using these data we validate a sedimentation model of traction fed by a low concentration turbulent and stratified current by the use of a sedimentological model we have then reconstructed the fluid dynamic parameters of the current and calculated the maximum horizontal distance that solid particles travelled before deposition results have been checked against actual deposit features consistency of data indicates that the method used in this study even if it is approximate and needs further refinement by specific experimental approaches and numerical simulations gives a reasonable indication of surge characters such as velocity and density which are relevant for hazard calculation and risk assessment

experimental study of the response of a gravel streambed to increased sediment supply if increased sediment supply to a river channel exceeds its transport capacity deposition necessarily occurs as the bed adjusts to accommodate the increased supply both the mean and spatial patterns in bed elevation and grain size may change and an ability to understand their relative importance is needed to predict bed response we report on an experiment in a field scale flume in which sediment supply is increased to a gravel bed with alternate bars sediment was recirculated in the experiments but augmented in two steps after which the bed was allowed to reach a new steady state the transport rate at the end of the experiment was three times larger than at the start high resolution sediment flux and topographic measurements grain size derived from photographs and hydrodynamic modeling allow us to document the topographic and textural response of the bed to increased sediment supply the spatial patterns of bed topography and texture were forced by the flume setup and the initial and final steady states included long stationary alternate bars with associated grain size sorting the transient bed contained several scales of shorter wavelength migrating bedforms superimposed on and temporarily replacing the stationary alternate bars bed topography and textural patterns adjusted to increased sediment supply over different timescales bed slope and mean stress increased directly with sediment supply rate to produce a new transport steady state in a time about 2 5 times the minimum needed to deposit the required sediment wedge indicating a trap efficiency of about 40 for the aggrading wedge adjustments in local topography and sorting primarily in the form of smaller migrating bars continued for a period approximately equal to that required to initially reach transport steady state copyright 2013 john wiley sons ltd

rothenberg scoria cone east eifel a complex strombolian and phreatomagmatic volcano rothenberg scoria cone eifel formed by an alternation of three strombolian and three phreatomagmatic eruptive phases eruptions took place from up to six vents on a 600 m long fissure building an early tuff ring and then two coalescing scoria cones strombolian volcanism dominated volumetrically as the supply of external water was severely limited magma water interaction only occurred during the opening stages of eruption at any vent when discharge rates were low and the fragmentation surface was below the water table the phreatomagmatic deposits consist of relatively well sorted fall beds and only minor surge deposits they contain juvenile clasts with a wide range of vesicularity and grain size implying considerable heterogeneity in the assemblage of material ejected by the phreatomagmatic explosions the transition from phreatomagmatic to strombolian eruption at any vent was rapid and irreversible and strombolian volcanism persisted even when eruption rates are inferred to have waned at the close of each eruptive phase as by then the fragmentation surfaces were high in the growing cones and water was denied access to the magma the strombolian deposits are relatively homogenous consisting of alternating coarser and finer grained well sorted fall beds erupted during periods of open vent eruption and partial blockage of the vent respectively the intervals of strombolian eruption were always a delicate balance between discharge of freely degassing magma and processes such as ponding of degassed magma in the vent collapse of the growing cones and repeated recycling of clasts through the vent clear evidence of the instability of the rothenberg cones is preserved in numerous unconformities within deposits of the inner walls of the cones the close of strombolian phases was probably marked by a decreasing supply of magma to the vents accompanied by ponding and stagnation of lava in the craters

remote observations of eruptive clouds and surface thermal activity during the 2009 eruption of redoubt volcano abstract volcanoes often erupt explosively and generate a variety of hazards including volcanic ash clouds and gaseous plumes these clouds and plumes are a significant hazard to the aviation industry and the ground features can be a major hazard to local communities here we provide a chronology of the 2009 redoubt volcano eruption using frequent low spatial resolution thermal infrared tir mid infrared mir and ultraviolet uv satellite remote sensing data the first explosion of the 2009 eruption of redoubt volcano occurred on march 15 2009 utc and was followed by a series of magmatic explosive events starting on march 23 utc from march 23 april 4 2009 satellites imaged at least 19 separate explosive events that sent ash clouds up to 18 km above sea level asl that dispersed ash across the cook inlet region in this manuscript we provide an overview of the ash clouds and plumes from the 19 explosive events detailing their cloud top heights and discussing the variations in infrared absorption signals we show that the timing of the tir data relative to the event end time was critical for inferring the tir derived height and true cloud top height the ash clouds were high in water content likely in the form of ice which masked the negative tir brightness temperature difference btd signal typically used for volcanic ash detection the analysis shown here illustrates the utility of remote sensing data during volcanic crises to measure critical real time parameters such as cloud top heights changes in ground based thermal activity and plume cloud location

shallow seated controls on styles of explosive basaltic volcanism a case study from new zealand abstract the pyroclastic deposits of many basaltic volcanic centres show abrupt transitions between contrasting eruptive styles e g hawaiian versus strombolian or dry magmatic versus wet phreatomagmatic these transitions are controlled dominantly by variations in degassing patterns magma ascent rates and degrees of interaction with external water we use crater hill a 29 ka explosive effusive monogenetic centre in the auckland volcanic field new zealand as a case study of the transitions between these end member eruptive styles the crater hill eruption took place from at least 4 vents spaced along a nne trending 600 m long fissure that is contained entirely within a tuff ring generated during the earliest eruption phases early explosive phases at crater hill were characterised by eruption from multiple unstable and short lived vents later dominantly extrusive volcanism took place from a more stable point source most of the crater hill pyroclastic deposits were formed in 3 phreatomagmatic p and 4 dry magmatic m episodes forming in turn the outer tuff ring and maar crater p1 m1 p2 and scoria cone 1 m2 m4 this activity was followed by formation of a lava shield and scoria cone 2 purely wet activity is represented by the bulk of p1 and p2 and purely dry activity by much of m2 m4 however m1 and parts of m2 and m4 show evidence for simultaneous eruptions of differing style from adjacent vents and rapid variations in the extent and timing of magma water interaction at each vent the nature of the wall rock lithics and these rapid variations in inferred water magma ratios imply interaction was occurring mostly at depths of 80 m and the vesicularity patterns in juvenile clasts from these and the p beds imply that rapid degassing occurred at these shallow levels we suggest that abrupt transitions between eruptive styles in time and space at crater hill were linked to changes in the local magma supply rate and patterns and vigour of degassing during the final metres of ascent

the crater facies kimberlite system of tokapal bastar district chhattisgarh india abstract discovery of diamondiferous kimberlites in the mainpur kimberlite field raipur district chhattisgarh in central india encouraged investigation of similar bodies in other parts of the bastar craton the earlier known tokapal ultramafic intrusive body located beyond the 19 km milestone in tokapal village along the jagdalpur geedam road was reinterpreted as crater facies kimberlite its stratigraphic position in the meso neoproterozoic intracratonic sedimentary indravati basin makes it one of the oldest preserved crater facies kimberlite systems ground and limited subsurface data dug tube wells and exploratory boreholes have outlined an extensive surface area 550 ha of the kimberlite the morphological and surface color features of this body on enhanced satellite images suggest that there is a central feeder surrounded by a collar and wide pyroclastic apron exploration drilling indicates that the central zone probably corresponds to a vent overlain by resedimented volcaniclastic epiclastic rocks that are surrounded by a 2 km wide spread of pyroclastic rocks lapilli tuff tuff ash beds and volcaniclastic breccia drill holes also reveal that kimberlitic lapilli tuffs and tuffs are sandwiched between the kanger and jagdalpur formations and also form sills within the sedimentary sequence of the indravati basin the lapilli tuffs are commonly well stratified and display slumping base surges and lava flows occur in the southern part of the tokapal system the geochemistry and petrology of the rock correspond to average group i kimberlite with a moderate degree of contamination however the exposed rock is intensely weathered and altered with strong leaching of mobile elements ba rb sr layers of vesicular fine grained glassy material represent kimberlitic lava flows tuffs containing juvenile lapilli with pseudomorphed olivine macrocrysts are set in a talc serpentine carbonate matrix with locally abundant spinel and sphene garnet has not been observed and phlogopite is very rare very limited microdiamond testing two 18 kg samples proved negative however the composition of chromite grains indicate crystallization in the diamond stability field

observations of volcanic tremor at mount st helens volcano digital recordings of ground motion during tremor episodes accompanying eruptions at mount st helens volcano in the state of washington on august 7 and october 16 18 1980 are studied the spectra of the vertical component waveforms contain at least two dominant peaks at 1 0 and 1 3 hz for all events recorded during both eruptions that were studied spectra of horizontal ground motion show peaks at 0 9 and 1 1 hz the relative amplitude of the two peaks changes between tremor episodes and during single tremor episodes and shows no consistent relation to amplitude of ground motion spectra of long period earthquakes are very similar to those of tremor events suggesting that tremor is composed of many long period earthquakes that occur over a period of time the unique waveform of tremor events observed at mount st helens must be due to a source effect since the relative amplitude of the two dominant peaks changes during tremor episodes the path effect on tremor waveforms is small since there are no peaks in the spectra of waveforms recorded during tectonic earthquakes occurring in the vicinity of mount st helens the consistency of the location of the spectral peaks for the wide range of tremor amplitudes means that there must be a physical length at the source that is constant independent of the amplitude of motion at the source variations in amplitude of motion generated during tremor events must be due to variations in the force driving the tremor amplitudes of ground motion varies between 0 11 and 4 7 m seismic moment rates during the two eruptions are calculated using the model of aki et al 1977 and found to vary between 6 1018 and 1 1020 dynes cm s 1 which are larger than values found by aki et al 1977 who studied amplitudes of shallow tremor events recorded during the october 1963 eruption of kilauea volcano in hawaii study of tremor amplitudes recorded at corvallis oregon leads to the conclusion that tremor accompanying the cataclysmic may 18 1980 eruption was at least one order of magnitude larger in amplitude than tremor during august and october

redistribution of velocity and bed shear stress in straight and curved open channels by means of a bubble screen laboratory experiments open channel beds show variations in the transverse direction due to the interaction between downstream flow cross stream flow and bed topography which may reduce the navigable width or endanger the foundations of structures the reported preliminary laboratory study shows that a bubble screen can generate cross stream circulation that redistributes velocities and hence would modify the topography in straight flow the bubble generated cross stream circulation cell covers a spanwise extent of about four times the water depth and has maximum transverse velocities of about 0 2 ms 1 in sharply curved flow it is slightly weaker and narrower with a spanwise extent of about three times the flow depth it shifts the counter rotating curvature induced cross stream circulation cell in the inwards direction maximum bubble generated cross stream circulation velocities are of a similar order of magnitude to typical curvature induced cross stream circulation velocities in natural open channel bends the bubble screen technique is adjustable reversible and ecologically favorable detailed data on the 3d flow field in open channel bends is provided which can be useful for validation of numerical models

subterranean fragmentation of magma during conduit initiation and evolution in the shallow plumbing system of the small volume jagged rocks volcanoes hopi buttes volcanic field arizona usa monogenetic volcanoes have limited magma supply and lack long lived sustained magma plumbing systems they erupt once often from multiple vents and sometimes over several years and are rarely or never re activated eruptive behavior is very sensitive to physical processes e g volatile exsolution magma water interaction occurring in the later stages of magma ascent at shallow crustal depths 1 km which yield a spectrum of eruptive styles including weak to moderate explosive activity violent phreatomagmatism and lava effusion jagged rocks complex in the late miocene hopi buttes volcanic field arizona usa exposes the frozen remnants of the feeding systems for one or a few monogenetic volcanoes it provides information on how a shallow magmatic plumbing system evolved within a stable non marine sedimentary basin and the processes by which magma flowing through dikes fragmented and conduits were formed we have identified three main types of fragmental deposits 1 buds which emerge from dikes 2 pyroclastic massifs and 3 diatremes these represent three different styles and intensities of shallow depth magma fragmentation they may develop successively and at different sites during the evolution of a monogenetic volcano the deposits consist of a mixture of pyroclasts with varying degrees of welding and country rock debris in various proportions pyroclasts are commonly welded together but also reveal in places features consistent with phreatomagmatism such as blocky shapes dense groundmasses and composite clasts loaded and cored the extent of fragmentation and the formation of subterranean open space controlled the nature of the particles and the architecture and geometry of these conduit structures and their deposits

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

minimum magnitude of completeness in earthquake catalogs examples from alaska the western united states and japan we mapped the minimum magnitude of complete reporting m c for alaska the western united states and for the junec earthquake catalog of japan mc was estimated based on its departure from the linear frequency magnitude relation of the 250 closest earthquakes to grid nodes spaced 10 km apart in all catalogs studied mc was strongly heterogeneous in offshore areas the mc was typically one unit of magnitude higher than onshore on land also mc can vary by one order of magnitude over distance less than 50 km we recommend that seismicity studies that depend on complete sets of small earthquakes should be limited to areas with similar mc or the minimum magnitude for the analysis has to be raised to the highest com mon value of mc we believe that the data quality as reflected by the mc level should be used to define the spatial extent of seismicity studies where mc plays a role the method we use calculates the goodness of fit between a power law fit to the data and the observed frequency magnitude distribution as a function of a lower cutoff of the magnitude data mc is defined as the magnitude at which 90 of the data can be modeled by a power law fit mc in the 1990s is approximately 1 2 0 4 in most parts of california 1 8 0 4 in most of alaska aleutians and panhandle excluded and at a higher level in the junec catalog for japan various sources such as ex plosions and earthquake families beneath volcanoes can lead to distributions that cannot be fit well by power laws for the hokkaido region we demonstrate how neglecting the spatial variability of m c can lead to erroneous assumptions about deviations from self similarity of earthquake scaling

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

sedimentologic and stratigraphic constraints on emplacement of the star kimberlite east central saskatchewan abstract diamond bearing kimberlites in the fort a la corne region east central saskatchewan consist primarily of extra crater pyroclastic deposits which are interstratified with lower cretaceous albian and cenomanian marine marginal marine and continental sediments approximately 70 individual kimberlite occurrences have been documented the star kimberlite occurring at the southeastern end of the main fort a la corne trend has been identified as being of economic interest and is characterized by an excellent drill core database integration of multi disciplinary data sets has helped to refine and resolve models for emplacement of the star kimberlite detailed core logging has provided the foundation for sedimentological and volcanological studies and for construction of a regionally consistent stratigraphic and architectural framework for the kimberlite complex micropaleontologic and biostratigraphic analysis of selected sedimentary rocks and u pb perovskite geochronology on kimberlite samples have been integrated to define periods of kimberlite emplacement radiometric age determination and micropaleontologic evidence support the hypothesis that multiple kimberlite eruptive phases occurred at star the oldest kimberlite in the star body erupted during deposition of the predominantly continental strata of the lower mannville group cantuar formation kimberlites within the cantuar formation include terrestrial airfall deposits as well as fluvially transported kimberlitic sandstone and conglomerate successive eruptive events occurred contemporaneous with deposition of the marginal marine upper mannville group pense formation kimberlites within the pense formation consist primarily of terrestrial airfall deposits fine to medium grained cross stratified kimberlitic olivine dominated sandstone in this interval reflects reworking of airfall deposits during a regional marine transgression the location of the source feeder vents of the cantuar and pense kimberlite deposits has not been identified the youngest and volumetrically most significant eruptive events associated with the star kimberlite occur within the predominantly marine lower colorado group joli fou and viking formations kimberlite beds which occur at several horizons within these units consist of subaerial and marine fall deposits the latter commonly exhibiting evidence of wave reworking black shale encased resedimented kimberlite beds likely deposited as subaqueous debris flows and turbidites are particularly common in the lower colorado group during its multi eruptive history the star kimberlite body is interpreted to have evolved from a feeder vent and overlying positive relief tephra ring into a tephra cone initial early joli fou volcanism resulted in formation of a feeder vent 200 m diameter and tephra ring subsequent eruptions dominated by subaerial deposits partly infilled the crater and constructed a tephra cone a late joli fou eruption formed a small 70 m diameter feeder pipe slightly offset to the nw of the early joli fou feeder vent deposits from this event further infilled the crater and were deposited on top of early joli fou kimberlite proximal to the vent and sediments of the joli fou formation distal to the vent the shape of the tephra cone was modified during multiple marine transgression and regression cycles coeval with deposition of the lower colorado group resulting in wave reworked kimberlite sand along the fringes of the cone and kimberlitic event deposits tempestites turbidites debris flows in more distal settings

bed load transport equation on arbitrarily sloping beds this work presents a simple tool to evaluate bed load transport intensity and direction on arbitrarily sloping beds with local longitudinal and transversal inclinations up to 25 and in the case of uniform sediments and low values of the applied shields stress the tool is composed of a set of equations which fit the results obtained via an iterative procedure by the semiempirical model recently proposed in 2003 by parker seminara and solari the tool provides a fully nonlinear description of bed load transport which overcomes the limitations of linear formulations developed in the case of negligible local bed inclinations the proposed tool can be easily implemented in any morphodynamic model to describe the evolution of the bottom topography and to capture the dynamics of relevant sloping beds

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 fractal and multifractal dimension of volcanic ash particles contour a test study on the utility and volcanological relevance abstract image processing analysis is used to check the ability of the fractal dimension for quantitatively describing the shape of volcanic ash particles digitized scanning electron microscopy images of fine pyroclasts from the eruptions of monte pilato rocche rosse lipari italy are investigated to test the efficiency of the fractal dimension to discriminate between particles of different eruptive processes multivariate analysis of multiple fractal components allows distinction between magmatic particles and phreatomagmatic particles which however is less significant than the discrimination obtained in previous studies by the use of simple adimensional shape parameters approximation of the actual particle boundary and the not rotation invariant nature of the fractal data frequently result in a significant scatter of data points in the mandelbrot richardson plot such behavior obscures in some cases the actual information of particle shape and renders the discriminating power of fractal analysis less effective than classical shape descriptors data less affected by scatter reveal that phreatomagmatic particles of the monte pilato rocche rosse eruptions are true mono fractals whereas magmatic particles are multifractals the textural small scale fractal of magmatic particles is similar to the fractal dimension value of phreatomagmatic particles and is attributed to the rheological behavior of melt upon brittle fragmentation the structural large scale fractal of magmatic particles refers to the walls of ruptured vesicles that lay on the particle outline the high difference between the values of the textural and structural fractals of magmatic particles of the monte pilato rocche rosse eruptions suggests two distinct and independent processes in the formation of such pyroclasts at the scales corresponding to the textural fractal the fragmentation process is independent of vesicles magmatic fragmentation is not simply related to growth expansion interference and explosion of vesicles but to a brittle cracking of the highly viscous melt likely related to rapid decompression

fragmentation in kimberlite products and intensity of explosive eruption the explosive eruption of kimberlite magma is capable of producing a variety of pyroclast sizes shapes and textures however all pyroclastic deposits of kimberlite comprise two main types of pyroclasts 1 pyroclasts of kimberlite with or without enclosed olivine crystals and 2 olivine crystals which lack coatings of kimberlite here we propose two hypotheses for how kimberlite magmas are modified due to explosive eruption 1 olivine crystals break during kimberlite eruption and 2 kimberlite melt can be efficiently separated from crystals during eruption these ideas are tested against data collected from field study and image analysis of coherent kimberlite and fragmental kimberlite from kimberlite pipes at diavik nt olivines are expected to break because of rapid pressure changes during the explosive eruption disruption of kimberlite magma and pyroclast production is driven by ductile deformation processes rather than by brittle fragmentation the extent to which melt separates from olivine crystals to produce kimberlite free crystals is a direct consequence of the relative proportions of gas melt and crystals lastly the properties of juvenile pyroclasts in deposits of pyroclastic kimberlite are used to index the relative intensity of kimberlite eruptions a fragmentation index is proposed for kimberlite eruption based on a crystal size distributions of olivine and on b ratios of selvage free olivine pyroclasts to pyroclasts of kimberlite with or without olivine crystals

changes in morphometric meander parameters identified on the karoon river iran using remote sensing data abstract river meander dynamics and mobility are important indicators of environmental change related to climate changes and anthropogenic activities at local and river basin scales the aim of the present study is to identify morphological changes of the karoon river in iran using high accuracy maps and landsat satellite images by analyses during the time period 1989 2008 in this study 20 meandering reaches were analyzed over a 128 km long river reach located in the middle part of the karoon river iran morphometric indicators such as river width w meander neck length l axis length a radius of curvature r water flow length s and sinuosity of meander c were extracted for the identified meanders the results of a paired t test showed that river width w and meander neck length l have significantly changed during the study period 1989 2008 with an increase of 3 5 m for w and a decrease of 274 m for l spearman correlation analysis has shown that meander parameter changes are highly correlated to each other the parameters that do not have significant correlation together are c with w and l w and l and l with s and a during the period of the study the flow length and river sinuosity decreased for the whole river reach by 4 77 km and 0 11 respectively analysis of land use land cover categories 1989 and 2008 using the support vector machine svm and kernel function method served as one of the tools for interpretation of the meander parameter changes these changes can be attributed not only to lu lc riparian vegetation to agriculture area ratio but also to dam construction in the upstream part of the river that leads to major hydrological regime and sediment transfer alteration sediment extraction may also be an important factor

depositional age and provenance of the marboo formation from shrimp u pb zircon geochronology implications for the early palaeoproterozoic tectonic evolution of the kimberley region western australia abstract the marboo formation is the oldest rock unit in the hooper complex in the west kimberley region of western australia the complex forms an elongate belt 350 km long and at least 100 km wide of deformed and metamorphosed palaeoproterozoic low to high grade metasedimentary rocks and mafic and felsic igneous rocks the marboo formation consists of a sequence of turbiditic metasedimentary rocks derived from the northwest an area of crust now buried beneath the unconformably overlying speewah and kimberley basins shrimp u pb dating of detrital zircons from a low grade quartz wacke indicate that the provenance terrain contributed zircons dated at around 2500 to 2400 ma 2300 ma 2200 to 2050 ma 1970 ma and 1910 ma the mount joseph migmatite formed by in situ melting of the marboo formation and zircons interpreted as detrital in origin give ages of 2500 to 2000 ma a maximum depositional age is provided by a population of zircon cores dated at 1872 9 ma growth of rims on zircons is dated at 1861 5 ma and is interpreted as the age of high grade metamorphism a minimum age for deposition is provided by the richenda microgranodiorite which forms a high level intrusive stock into the marboo formation and is dated at 1858 5 ma comparison with recent shrimp ages from the halls creek group in the east kimberley indicates that the turbidites of the marboo formation were deposited deformed metamorphosed and intruded by granitoid before the majority of the turbiditic metasedimentary rocks in the olympio formation were deposited this is not consistent with deposition of the turbiditic rocks in the same basin the assumption of contemporaneous turbidite deposition throughout the kimberley region has been taken as evidence of intracratonic palaeoproterozoic orogeny however these results provide support for plate tectonic models and the recognition of tectonostratigraphic terranes within the kimberley region detrital zircons from the marboo formation suggest that unexposed crust beneath the speewah and kimberley basins is largely early palaeoproterozoic in age rather than archaean as previously assumed the spread in ages indicates that the time period between formation of late archaean basement in northern australia and the formation of palaeoproterozoic orogens was marked by ongoing crust formation and was not a period of quiessence

channel responses to varying sediment input a flume experiment modeled after redwood creek california article i nfo storage flume aggradation degradation at the reach scale a channel adjusts to sediment supply and flow through mutual interactions among channel form bed particle size and flow dynamics that govern river bed mobility sediment can impair the beneficial uses of a river but the timescales for studying recovery following high sediment loading in the field setting make flume experiments appealing we use a flume experiment coupled with field measurements in a gravel bed river to explore sediment transport storage and mobility relations under various sediment supply conditions our flume experiment modeled adjustments of channel morphology slope and armoring in a gravel bed channel under moderate sediment increases channel bed elevation increased and sediment output increased but channel planform remained similar to pre feed conditions during the following degradational cycle most of the excess sediment was evacuated from the flume and the bed became armored under high sediment feed channel bed elevation increased the bed became smoother mid channel bars and bedload sheets formed and water surface slope increased concurrently output increased and became more poorly sorted during the last degradational cycle the channel became armored and channel incision ceased before all excess sediment was removed selective transport of finer material was evident throughout the aggradational cycles and became more pronounced during degradational cycles as the bed became armored our flume results of changes in bed elevation sediment storage channel morphology and bed texture parallel those from field surveys of redwood creek northern california which has exhibited channel bed degradation for 30 years following a large aggradation event in the 1970s the flume experiment suggested that channel recovery in terms of reestablishing a specific morphology may not occur but the channel may return to a state of balancing sediment supply and transport capacity published by elsevier b v

experiments on bar formation in a straight flume 2 graded sediment laboratory experiments have been carried out in a large laboratory flume using a strongly bimodal sediment mixture in a range of flow conditions such that the initially flat bed of the flume became unstable enhancing the development of alternate bars in order to elucidate the effect of grain sorting the present experiments are compared with a previous set of experiments performed in the same flume under similar hydraulic conditions but using a nearly uniform sand with a mean geometric grain diameter equal to that characterizing present experiments the comparison suggests that sediment heterogeneity may appreciably affect bed form characteristics the development of smallscale ripples and mesoscale dunes sediment waves tended to be inhibited thus allowing a decrease in flow resistance owing to the bimodal character of the adopted mixture at low values of the bed shear stress a condition of partial transport was attained for which only the finer grained portion of the mixture was observed to move while the coarsergrained fractions remained essentially immobile throughout the experimental run however complete mobilization of all size fractions was observed to occur in runs carried out at higher slopes i e at higher bed shear stress in order to generate an alternate bar pattern selective transport of individual grain size fractions coupled with the characteristic bar topography pattern led to an intense longitudinal sorting which accreted the coarser particles on bar crests furthermore bar migration caused through scour and fill a significant vertical sorting as a consequence of these sorting processes and in accordance with previous experimental observations bar height turns out to be invariably damped with respect to uniform sediment experiments the trend exhibited by the wavelengths is less clear and suggests that in the present experiments sorting effects do not enhance the clear shortening of bar wavelengths typically observed in other series of flume experiments carried out with weakly bimodal mixtures

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

duration amplitude relationships of volcanic tremor and earthquake swarms preceding and during the 2009 eruption of redoubt volcano alaska abstract duration amplitude relationships were studied for tremor episodes and earthquake swarms occurring during the 2009 eruption of redoubt volcano alaska duration amplitude distribution plots were generated daily from january 1 to may 31 and fit with both an exponential law and power law comparing r2 values of the fit for both laws showed that the exponential law fit better for days in which volcanic tremor and earthquake swarms occurred while the power law fit better for other days fitting segments of seismic data with both an exponential and a power law leads to a metric that has potential for volcano monitoring r2exp r2pow the ratio of the r2 fits using the exponential law and the power law the ratio r2exp r2pow tended to be greater than 1 when volcanic activity or precursory seismic activity was occurring and less than 1 when no volcano seismic activity was occurring duration amplitude plots were generated for episodes of volcanic tremor that were identified by the r2exp r2pow 1 method and compared in an attempt to identify changes that may have occurred during the eruption stronger episodes of volcanic tremor showed higher characteristic amplitudes maximum heights of the plumes generated by the explosions showed a positive correlation with the characteristic amplitude of the concurrent tremor

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

characteristics and alteration origins of matrix minerals in volcaniclastic kimberlite of the muskox pipe nunavut canada abstract the matrix of volcaniclastic kimberlite vk from the muskox pipe northern slave province nunavut canada is interpreted to represent an overprint of an original clastic matrix muskox vk is subdivided into three different matrix mineral assemblages that reflect differences in the proportions of original primary matrix constituents temperature of formation and nature of the altering fluids using whole rock x ray fluorescence xrf whole rock x ray diffraction xrd microprobe analyses back scatter electron bse imaging petrography and core logging we find that most matrix minerals serpentine phlogopite chlorite saponite monticellite fe ti oxides and calcite lack either primary igneous or primary clastic textures the mineralogy and textures are most consistent with formation through alteration overprinting of an original clastic matrix that form by retrograde reactions as the deposit cools or in the case of calcite by precipitation from ca bearing fluids into a secondary porosity the first mineral assemblage consists largely of serpentine phlogopite calcite fe ti oxides and monticellite and occurs in vk with relatively fresh framework clasts alteration reactions driven by deuteric fluids derived from the juvenile constituents promote the crystallisation of minerals that indicate relatively high temperatures of formation 400 c lower temperature minerals are not present because permeability was occluded before the deposit cooled to low temperatures thus shielding the facies from further interaction with fluids the other two matrix mineral assemblages consist largely of serpentine phlogopite calcite diopside and chlorite they form in vk that contains more country rock which may have caused the deposit to be cooler upon emplacement most framework components are completely altered suggesting that larger volumes of fluids drove the alteration reactions these fluids were likely of meteoric provenance and became heated by the volcaniclastic debris when they percolated into the vk infill most alteration reactions ceased at temperatures 200 c as indicated by the absence or paucity of lower temperature phases in most samples such as saponite recognition that muskox vk contains an original clastic matrix is a necessary first step for evaluating the textural configuration which is important for reconstructing the physical processes responsible for the formation of the deposit

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

do alternate bars affect sediment transport and flow resistance in gravel bed rivers in natural gravel bed streams the complex topography of the bed can cause variation of flow resistance and sediment transport previous studies have shown that in addition to grain resistance skin friction flow resistance is also caused by bank roughness channel bars bed undulations and channel curvature sediment transport is similarly influenced by the complex topography and the transport rate can vary spatially a three dimensional 3d numerical model was used to generate a detailed description of the flow and bedload transport fields in gravel bed rivers here we quantify the reach averaged hydraulic resistance and sediment transport regime that prevails when self formed alternate river bars are present and compare with the regime that would prevail were no bars are present we do this by comparing the results of i a 3d morphodynamic model in which bars form as a consequence of flow bed interaction and ii an equivalent one dimensional 1d case which refers to flat bed conditions but otherwise corresponding to identical average velocity and bed slope the 3d numerical model is applied to generate different bed topographies of alternate bars at regime morphological equilibrium then extended to non equilibrium conditions for decreasing shear stress within a sensitivity analysis context the contribution of grain resistance is estimated with the local values of the bed shear stress while bar resistance results from the overall deviation of the flow field from that occurring in the flat bed configuration the local sediment transport in both the longitudinal and transverse directions is computed with the local shields stress and local bed inclination the calculations result in a method for correcting 1d models to account for the total sediment transport and resistance in a cross section due to 3d effects of alternate bars we term the resulting relations morphologically averaged sediment transport and resistance equations copyright 2012 john wiley sons ltd

local high slope effects on sediment transport and fluvial bed form dynamics 1 we investigate the morphodynamic evolution of an arbitrarily tilted coarse mobile bed under the action of a free surface water flow the analysis is based on the experimental observations of the evolution of a mobile bed from its initial laterally tilted configuration to the final laterally flat state due to the presence of a lateral component of bed load transport measurements of the bed topography at different times allow one to reconstruct the flattening process the experimental observations are interpreted by means of a three dimensional numerical hydromorphodynamic model employing different relationships linear and nonlinear to evaluate bed load intensity and direction the analysis shows that the inclusion of nonlinear gravitational effects in the description of the bed load leads to a more satisfactory description of the bed evolution this finding opens important issues on how the inclusion of the nonlinear gravitational effects on the bed load can affect the morphodynamic evolution of complex bed topography occurring in natural rivers to investigate this point an application to the evolution of river bars is presented

a roughness corrected index of relative bed stability for regional stream surveys abstract quantitative regional assessments of streambed sedimentation and its likely causes are hampered because field investigations typically lack the requisite sample size measurements or precision for sound geomorphic and statistical interpretation we adapted an index of relative bed stability rbs for data calculated from a national stream survey field protocol to enable general evaluation of bed stability and anthropogenic sedimentation in synoptic ecological surveys rbs is the ratio of bed surface geometric mean particle diameter dgm divided by estimated critical diameter dcbf at bankfull flow based on a modified shield s criterion for incipient motion application of rbs to adequately depict bed stability in complex natural streams however has been limited because typical calculations of rbs do not explicitly account for reductions in bed shear stress that result from channel form roughness we modified the index rbs to incorporate the reduction in bed shear stress available for sediment transport that results from the hydraulic resistance of large wood and longitudinal irregularities in channel dimensions form roughness based on dimensional analysis we derived an adjustment to bankfull shear stress by multiplying the bankfull hydraulic radius rbf by the one third power of the ratio of particle derived resistance to total hydraulic resistance cp ct 1 3 where both resistances are empirically based calculations we computed cp using a keulegan equation relating resistance to relative submergence of bed particles we then derived an empirical equation to predict reach scale hydraulic resistance ct from thalweg mean depth thalweg mean residual depth and large wood volume based on field dye transit studies in which total hydraulic resistance ct was measured over a wide range of natural stream channel complexity including manipulation of large wood volumes we tested our estimates of ct and rbs by applying them to data from a summer low flow probability sample of 104 wadeable stream reaches in the coastal ecoregion of oregon and washington usa stream discharges calculated using these ct estimates compared favorably with velocity area measurements of discharge during summer low flow and with the range of 1 to 2 year recurrence floods scaled by drainage area at u s geological survey gauged sites in the same region log rbs ranged from 4 2 to 0 98 in the survey region dgm ranged from silt to boulders while estimated bankfull critical diameter d cbf ranged from very fine gravel to large boulders the median value of d cbf adjusted for form roughness influences averaged 40 inter quartile range 28 to 59 of the unadjusted estimate dcbf log rbs was consistently negatively related to human disturbances likely to produce excess sediment inputs or hydrologic alteration log rbs ranged from 1 9 to 0 5 in the streams within the lower quartile of human disturbance in their basin and riparian areas and was substantially lower 4 2 to 1 1 in streams within the upper quartile of human disturbance the synoptic survey methods and designs we used appear adequate to evaluate regional patterns in bed stability and sedimentation and their general relationship to human disturbances although the rbs concept also shows promise for evaluating sediment and bed stability in individual streams our approach is relatively coarse so site specific assessments using these rapid field methods might prudently be confined to identifying severe cases of sedimentation or channel alteration greater confidence to discern subtle differences in site specific assessments could be gained by calculating rbs using more precise field measurements of channel slope bed particle size and bankfull dimensions and by refining our adjustments for energy loss from channel form roughness

bar resistance in gravel bed streams at bankfull stage total flow resistance in gravel bed streams with high width depth ratios and low sinuosities is primarily a result of grain and bar roughnesses the relative importance of these two roughness elements has not been established the relative importance of grain and bar resistance is examined for 12 straight and divided gravel bed reaches at bankfull stage by using the keulegan equation to determine a resistance division of energy slope the remaining slope is assumed to represent the relative importance of bar resistance bar resistance accounts for 50 to 75 of the total resistance in these reaches calculated values of reach averaged bar slope correlate closely with field measurements of bar magnitude

continuous measurements of discharge from a horizontal acoustic doppler current profiler in a tidal river 1 acoustic doppler current profilers adcps can be mounted horizontally at a river bank yielding single depth horizontal array observations of velocity across the river this paper presents a semideterministic semistochastic method to obtain continuous measurements of discharge from horizontal adcp hadcp data in a tidal river in the deterministic part single depth velocity data are converted to specific discharge by applying the law of the wall which requires knowledge of local values of the bed roughness length z0 a new filtration technique was developed to infer cross river profiles of z0 from moving boat adcp measurements width averaged values of z0 were shown to be predominantly constant in time but differed between ebb and flood in the stochastic part of the method specific discharge was converted to total discharge on the basis of a model that accounts for the time lag between flow variation in the central part of the river and flow variation near the banks model coefficients were derived using moving boat adcp data the consistency of mutually independent discharge estimates from hadcp measurements was investigated to validate the method analyzing river discharge and tidal discharge separately inaccuracy of the method is attributed primarily to mechanisms controlling transverse exchange of momentum which produce temporal variation in the discharge distribution over the cross section specifically development of river dunes may influence the portion of the discharge concentrated within the range of the hadcp

characterization of seismic events during the 2009 eruption of redoubt volcano alaska abstract seismic events were automatically detected and characterized throughout the 2009 eruption of redoubt volcano alaska on a single short period station located 3 km from the volcanic crater a total of 126 789 individual events were identified from continuous recording of seismic data from january 1 to june 30 average 29 events per hour using a short term average long term average detection algorithm nine metrics were computed for this suite of events including event duration inter event time event rate peak amplitude peak to peak amplitude root mean square rms amplitude peak frequency center frequency and frequency index eight swarms were identified with event rates exceeding 100 events per hour the first two occurred in late january and are attributed to high amplitude spasmodic tremor five additional swarms were manually repicked including swarms on february 26 27 march 20 23 march 27 march 29 and april 2 4 three of these swarms immediately preceded major explosions including march 20 23 march 27 and april 2 4 a final swarm on may 2 9 was re picked using a correlation detection scheme we identified 146 event families that occurred within this suite of selected events using a cross correlation technique seven explosions were each immediately preceded by one or more event families events from the dominant family during each of these periods was additionally re picked using correlation detection the procession of event metrics and occurrence of event families formed a complex distribution throughout the eruption a single station approach was used to gain a fine scale view of variations in seismic behavior at redoubt with a focus on potential indicators of impending explosions these techniques may serve an important role in future real time eruption monitoring efforts

on the formation of deep seated subterranean peperite like magma sediment mixtures abstract in addition to large amounts of fragmented country rocks maar diatreme volcanoes commonly eject juvenile lapilli and cauliflower bombs containing many dispersed xenoliths and or xenocrysts derived from the country rocks surrounding the diatremes formation of these lapilli and bombs is attributed to thermohydraulic explosions in the root zones of the diatremes and their consequences the thermohydraulic explosions release shock waves which are capable of intensely fragmenting the surrounding country rocks vaporisation of water superheated during thermohydraulic explosions and its expansion to lower pressure causes ejection of major parts of the fragmented country rocks surviving sections of fragmented country rocks may collapse or slide into the partially evacuated root zone and thus form a mass flow deposit subsequent to each explosion when prior to a new explosion magma rises from the underlying feeder dyke into the root zone it intrudes this clastic debris inflates it and may mingle with the debris to form a peperite subsequent explosions can fragment and eject fragments of this peperite in the form of juvenile ash grains juvenile lapilli and bombs containing xenoliths or xenocrysts from the country rock debris lack of further explosions but continued rise and intrusion of magma will cause emplacement of such inflated peperite masses as plugs

flow and bed profile in meandering sand silt rivers mathematical models for defining three dimensional flow and bed topography in sinuous channels with suspendable bed material are presented it is shown theoretically that the secondary flow in sinuous channels shows a reduction in its magnitude below that of a uniformly curved channel and also displays a phase lag relative to the channel plan form the model for bed topography is derived by considering the sediment balance for bed load and suspended load the transport rate and direction all of which are governed by the three dimensional flow a laboratory test supports the model the present study has made it possible to predict the large scale bed topography of meandering sand silt rivers including such features as the magnitude and the location of local scour and point bar deposits

re os isotopic evidence for archean lithospheric mantle beneath the kimberley block western australia we report sm nd and the first re os isotopic data as well as platinum group element concentrations for two of the highest grade diamond deposits in the kimberley block of western australia whole rock sm nd isotopic data for the 1200 ma argyle olivine lamproite and an argyle peridotite xenolith yield unradiogenic initial isotopic compositions nd 3 2 to 6 0 and depleted mantle model ages of 1750 to 2000 ma these data indicate that shallow mantle light rare earth element enrichment of the argyle lamproite source probably occurred during the proterozoic hooper orogeny sm nd isotopic data for the 800 ma seppelt kimberlite yield a radiogenic initial isotopic composition nd 1 8 and a t dm model age of 1200 ma isotopic features consistent with worldwide group i kimberlite occurrences re os isotopic data yield unradiogenic initial isotopic compositions for the argyle lamproite and peridotite xenoliths os 2 to 6 and seppelt kimberlite and kimberlitic chromites os 7 to 8 whereas data for a picroilmenite megacryst from the 800 ma maude creek kimberlite yield a radiogenic initial os isotopic composition os 27 our modeling suggests that the os isotopic composition of the argyle lamproite and peridotites may be explained as mixtures of a ca 3000 ma refractory mantle component as represented by the kimberlitic chromites and a ca 1500 ma enriched mantle component as represented by the picroilmenite megacryst the xenoliths and chromites yield an imprecise re os isochron age of 3400 ma giving a strong indication that the kimberley block is underlain by archean continental lithospheric mantle and that this mantle was largely unaffected during early proterozoic orogenesis

scale amplification of natural debris flows caused by cascading landslide dam failures debris flows are typically caused by natural terrain landslides triggered by intense rainfalls if an incoming mountain torrent collapses a series of landslide dams large debris flows can form in a very short period moreover the torrent can amplify the scale of the debris flow in the flow direction the catastrophic debris flows that occurred in zhouqu china on 8 august 2010 were caused by intense rainfall and the upstream cascading failure of landslide dams along the gullies in the wake of the incident a field study was conducted to better understand the process of cascading landslide dam failures and the formation of debris flows this paper looks at the geomorphic properties of the debris flow gullies estimates the peak flow discharges at different locations using three different methods and analyzes the key modes i e different landslide dam types and their combinations of cascading landslide dam failures and their effect on the scale amplification of debris flows the results show that five key modes in luojiayu gully and two modes in sanyanyu gully accounted for the scale amplification of downstream debris flows in the zhouqu event this study illustrates how the hazardous process of natural debris flows can begin several kilometers upstream as a complex cascade of geomorphic events failure of landslide dams and erosion of the sloping bed can scale to become catastrophic discharges neglecting recognition of these hazardous geomorphic and hydrodynamic processes may result in a high cost

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

nonlinear modeling of mean flow redistribution in curved open channels 1 phenomena concerning flow morphology and water quality in rivers are often investigated by means of a depth integrated flow model coupled to a sediment transport model and a water quality model in such depth integrated models the vertical structure of the flow is represented by a closure submodel which mainly has to account for the secondary circulation which 1 redistributes by advection the flow the boundary shear stresses the sediment transport and dissolved and suspended matter 2 causes the direction of the bed shear stress to deviate from the direction of the depth averaged velocity and thereby influences the bed topography and 3 gives rise to additional friction losses as compared with straight uniform flow the commonly used linear closure submodels are shown to fail in reproducing essential features in moderately to strongly curved flow because they neglect the feedback between the downstream velocity and the secondary circulation a nonlinear closure submodel taking this feedback into account is proposed and shown to yield results that compare well with experimental data even for very strongly curved flow the feedback effects turn out to be controlled almost exclusively by a single parameter which enables their parameterization in a relatively simple way this control parameter also helps to objectively distinguish weak moderate and strongly curved flows the proposed closure submodel has the potential of improving the performance of depth integrated flow sediment water quality models without much extra computational effort

seismic evolution of the 1989 1990 eruption sequence of redoubt volcano alaska abstract redoubt volcano in south central alaska erupted between december 1989 and june 1990 in a sequence of events characterized by large tephra eruptions pyroclastic flows lahars and debris flows and episodes of dome growth the eruption was monitored by a network of five to nine seismic stations located 1 to 22 km from the summit crater notable features of the eruption seismicity include 1 small long period events beginning in september 1989 which increased slowly in number during november and early december 2 an intense swarm of long period events which preceded the initial eruptions on december 14 by 23 hours 3 shallow swarms 0 to 3 km of volcano tectonic events following each eruption on december 15 4 a persistent cluster of deep 6 to 10 km volcano tectonic earthquakes initiated by the eruptions on december 15 which continued throughout and beyond the eruption 5 an intense swarm of long period events which preceded the eruptions on january 2 and 6 nine additional intervals of increased long period seismicity each of which preceded a tephra eruption hypocenters of volcano tectonic earthquakes suggest the presence of a magma source region at 6 10 km depth earthquakes at these depths were initiated by the tephra eruptions on december 15 and likely represent the readjustment of stresses in the country rock associated with the removal of magma from these depths the locations and time history of these earthquakes coupled with the eruptive behavior of the volcano suggest this region was the source of most of the erupted material during the 1989 1990 eruption this source region appears to be connected to the surface by a narrow pipe like conduit as inferred from the hypocenters of volcano tectonic earthquakes concentrations of shallow volcano tectonic earthquakes followed each of the tephra eruptions on december 15 these shocks may represent stress readjustment in the wall rock related to the removal of magma and volatiles at these depths this shallow zone was the source area of the majority of long period seismicity through the remainder of the eruption the long period seismicity likely reflects the pressurization of the shallow portions of the magmatic system

seismic observations of redoubt volcano alaska 1989 2010 and a conceptual model of the redoubt magmatic system abstract seismic activity at redoubt volcano alaska has been closely monitored since 1989 by a network of five to ten seismometers within 22 km of the volcano s summit major eruptions occurred in 1989 1990 and 2009 and were characterized by large volcanic explosions episodes of lava dome growth and failure pyroclastic flows and lahars seismic features of the 1989 1990 eruption were 1 weak precursory tremor and a short 23 hour long intense swarm of repetitive shallow long period lp events centered 1 4 km below the crater floor 2 shallow volcano tectonic vt and hybrid earthquakes that separated early episodes of dome growth 3 13 additional swarms of lp events at shallow depths precursory to many of the 25 explosions that occurred over the more than 128 day duration of eruptive activity and 4 a persistent cluster of vt earthquakes at 6 to 9 km depth in contrast the 2009 eruption was preceded by a pronounced increase in deep lp dlp events at lower crustal depths 25 to 38 km that began in mid december 2008 two months of discontinuous shallow volcanic tremor that started on january 23 2009 a strong phreatic explosion on march 15 and a 58 hour long swarm of repetitive shallow lp events the 2009 eruption consisted of at least 23 major explosions between march 23 and april 5 again accompanied by shallow vt earthquakes several episodes of shallow repetitive lp events and dome growth continuing until mid july increased vt earthquakes at 4 to 9 km depth began slowly in early april possibly defining a mid crustal magma source zone magmatic processes associated with the 2009 eruption seismically activated the same portions of the redoubt magmatic system as the 1989 1990 eruption although the time scales and intensity vary considerably among the two eruptions the occurrence of precursory dlp events suggests that the 2009 eruption may have involved the rise of magma from lower crustal depths based on the evolution of seismicity during the 1989 1990 and 2009 eruptions the redoubt magmatic system is envisioned to consist of a shallow system of cracks extending 1 to 2 km below the crater floor a magma storage or source region at roughly 3 to 9 km depth and a diffuse magma source region at 25 to 38 km depth close tracking of seismic activity allowed the alaska volcano observatory to successfully issue warnings prior to many of the hazardous explosive events that occurred in 2009

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

the seismicity of the 2009 redoubt eruption article redoubt volcano erupted in march 2009 following 6 months of precursory seismic activity the 4 5 month long eruptive sequence was accompanied by phreatic and magmatic explosions periods of steady dome growth lahars seismic swarms extended episodes of volcanic tremor and changes in the background seismicity rate this study presents a seismic chronology of the eruption and places it in context with the variety of other geological and geophysical data that were recorded during the eruptive period we highlight 6 notable seismic swarms 3 of which preceded large explosions the swarms varied from an hour to several days in duration and contained tens to over 7000 earthquakes many of the swarms were dominated by low frequency type earthquakes that contained families of repeating events seismic tremor varied considerably in frequency amplitude and duration during the eruption with distinct characteristics accompanying different types of volcanic activity the explosion signals during march 23 24 were the most energetic and the explosions on march 26 29 contained proportionally more low frequency energy 0 033 0 3 hz two seismic stations were particularly well suited to recording lahars that flowed down the drift river valley data from these stations showed that lahars were generated by the majority of the explosion events as well as during the continuous eruptive activity on march 29 when no large explosions occurred we also examine the seismicity which occurred outside of the explosion and swarm episodes and find several families of repeating vt earthquakes which begin shortly before the april 4 explosion and that continue

reconstructing eruption processes of a miocene monogenetic volcanic field from vent remnants waipiata volcanic field south island new zealand the miocene waipiata volcanic field new zealand is an eroded phreatomagmatic intracontinental volcanic field formed during a period of weak lithospheric extension the field includes remnants of at least 55 volcanoes in an area of 5000 km2 vent filling deposits comprising predominantly lava e g plugs necks lava flows or dykes often associated with thin basal phreatomagmatic pyroclastic deposits were classified as type 1 vents and are inferred to be the remnants of scoria cones vents represented by predominantly pyroclastic infill are classified as type 2 vents and are inferred to have been the substructures of phreatomagmatic tuff ring and or maar volcanoes type 3 vent complexes are groups of closely spaced or overlapping vents with voluminous preserved lava flows they are inferred to be the remnants of volcanoes comprising adjoining to coalescing maars and tuff rings with magmatic explosive and effusive products pyroclastic rocks of most of the waipiata vents record initial phreatomagmatic explosive activity fuelled by groundwater followed by strombolian style eruptions aligned and clustered vents are accommodated to structural features of the regional basement rock otago schist

on the volcanology of the gibeon kimberlite field namibia kimberlite volcanism in the upper cretaceous gibeon kimberlite field southern namibia consisting of at least 42 diatremes and a number of associated dykes is closely related to carbonatitic and ultrabasic volcanic and intrusive activity which occurred at the margin of the field the volcanology of the diatremes and dykes as well as their structural setting is reported here because of the paleohydrogeological setting and since juvenile kimberlite occurring in dykes intrusive plugs and spherical lapilli is devoid of vesicles a phreatomagmatic eruption mechanism is proposed for the genesis of the kimberlite diatremes karoo dolerite basalt and sediment xenoliths in the diatremes provide evidence for the former extent of karoo strata at the time of eruption

secondary current and river meander formation a small perturbation stability analysis is developed for investigation of the role of the secondary current accompanying channel curvature in the initiation and early development of meanders of alluvial and ice or rock incised streams a small sinusoidal perturbation in the channel alignment of an initially straight prismatic channel is introduced the velocity of the secondary flow is calculated for uniform quasi steady flow conditions by application of the equation of the conservation of moment of momentum the formulation is then closed by introducing the assumption that the differential between the outer and inner banks rate of boundary erosion dissolution or melting for incised channels or sediment discharge for alluvial channels is proportional to the strength of the secondary flow this formulation leads to a linear differential equation which is solved for its orthogonal components which give the rates of meander growth and downstream migration it is shown that the amplitude of the meanders tends to increase and that the meanders migrate downstream the dominant wavelength and the corresponding phase shift between channel meandering and the velocity of the spiral motion are calculated as those corresponding to the fastest growth rate the results are found to be in good agreement with data reported by others

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

experiments on bar formation in a straight flume 1 uniform sediment laboratory experiments have been carried out in a large laboratory flume using a nearly uniform sand and under controlled steady flow conditions such as to ensure the development of alternate bars on a ripple and or dune covered bed the interaction between small scale and large scale bed forms is found to enhance a modulation in time and in space of the bar pattern moreover the flow depth decrease experienced by the flow field when approaching bar fronts leading to a progressive reduction of the dimensions of small scale bed forms may influence appreciably the overall flow resistance experimental values of bar wavelength and of bar celerity are compared with the theoretical estimates obtained from a model developed within the classical framework of linear stability analysis in particular the model accounts for the local variability of friction coefficient and water level for the secondary helical flow effects and for the influence of longitudinal slope on sediment transport quantitatively satisfactory predictions of bar wavelength appear to be possible in spite of the fact that in various runs a correct estimation of flow resistance and flow discharge appears to be relatively difficult

velocity redistribution in curved rectangular channels the main velocity redistribution in steady flow through curved conduits of shallow rectangular cross section is considered its mechanism is analysed using a mathematical model of steady incompressible laminar flow in coiled rectangular pipes the transverse transport of main flow momentum by the secondary circulation is shown to be the principal cause of this velocity redistribution the importance of the side wall regions even in shallow channels is assessed and the neglect of the influence of the side walls in the commonly applied simplified models of flow through shallow curved channels is shown to be strongly limiting in case of long bends with a rectangular cross section

selective sorting and abrasion of river gravel ii applications most gravel rivers show a tendency for characteristic grain size to decrease in the downstream direction over scales of tens or hundreds of kilometers it has been surmised that this downstream fin

origin of the joya honda maar san luis potosi mexico abstract joya honda is a quaternary maar of unusual type from the mexican basin and range province its 300 m deep crater is excavated in cretaceous limestones the surrounding tephra deposit which in places is 100 m thick begins with a series of weakly indurated pyroclastic surge and fall layers that we interpret as dry surge deposits these are overlain by the main sequence of strongly indurated massive tuff breccias that we interpret as wet surge deposits joya honda formed subaerially from the interaction of groundwater with rapidly ascending intraplate type basanitic magma carrying peridotitic mantle xenoliths local aquifer characteristics controlled the style of eruption and the nature of the deposits groundwater in the limestone hosted aquifer beneath joya honda was apparently contained within solution enhanced fractures at the onset of the eruption magma began to interact with a moderate amount of groundwater producing the dry surge deposits which are typical of deposits found at many maars and tuff rings as the eruption continued the crater grew and the hydromagmatic blasts fractured the limestones around the explosion foci a marked increase in the water magma ratio of the system followed when a large fracture or a portion of the limestone with enhanced secondary permeability was intersected by the expanding crater subsequent phreatomagmatic explosions occurred in a system with groundwater flow rates several orders of magnitude larger than in the initial dry surge stage at the maar rim these wet eruptions led to the emplacement of massive tuff breccias through a combination of fallout surges and mudflows these steeply dipping tuff breccias are similar to deposits found at many tuff cones juvenile clasts in the near vent deposits show marked upward increases in both hydration palagonitization and vesicularity the increased palagonitization with height in the section appears to be a consequence of the overall increased wetness of the eruption with time correlating with greater carbonate cementation and lithification in the upper part of the deposit the transition toward higher vesicularity is interpreted as evidence of a gradual reduction in the confining pressure for the ascending magma prior to explosive fragmentation perhaps related to unroofing during progressive excavation of the overlying maar crater thus joya honda does not support maar formation models that invoke downward displacement of explosion foci caused by formation of a cone of depression in the aquifer in order to maintain the confining pressure for the hydromagmatic blasts