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

relationship between dike and volcanic conduit distribution in a highly eroded monogenetic volcanic field san rafael utah usa we mapped 63 conduits 2000 dike segments and 12 sills in the san rafael subvolcanic field utah united states where this pliocene magmatic system is eroded to a depth of 0 8 km and is exceptionally well exposed although the number of mapped conduits dikes and sills might represent minimums depending on the level of erosion and exposure mapped dikes are more numerous around the areally extensive sills and interact with sills and conduits in complex ways we analyze conduit distribution using kernel density methods and compare results with dike and sill distribution we find that the distribution of conduits matches the major features of dike distribution including development of clusters and distribution of outliers these statistical models are then applied to the distributions of volcanoes in several recently active volcanic fields where intrusion distributions must be inferred from very sparse data and compared with san rafael conduit distribution this comparison supports the use of statistical models in probabilistic hazard assessment for distributed volcanism specifically renewed dike intrusion and potential eruptions in active basaltic systems can be assessed probabilistically from the distribution of older volcanoes in distributed volcanic systems

reconstructing paleoenvironment eruption mechanism and paleomorphology of the pliocene pula maar hungary abstract pula maar is a partially eroded pliocene maar diatreme volcano part of the mio pliocene bakony balaton highland volcanic field the surficial remnant of the maar diatreme volcano consists of 1 a distinct depression with a thick post eruptive lacustrine alginite sediment infill interbedded with coarse grained volcaniclastic sediments 2 a narrow marginal zone inside the depression consisting of primary pyroclastic rock units that are interpreted to be partly collapsed and subsided blocks of entire sections from the tephra ring formerly surrounding the maar crater depression and 3 coarse grained volcaniclastic debris flow deposits closely associated with the collapsed primary pyroclastic rock units in the marginal zone the presence of coherent lava rocks below the crater fill units their distribution pattern and their association with scoriaceous beds indicate that after the maar diatreme forming phreatomagmatic explosive activity small 100 m scale scoria and or spatter cones erupted in the maar crater these cones are the likely source of the lava flows that partially filled the maar crater basin the widespread dm to m thick basaltic sand and or silt units at the base of the post eruptive crater filling sedimentary succession are interpreted to be reworked volcaniclastic material from the intra maar scoria spatter cones as well as from the tephra ring based on comparative analyses of 53 core descriptions this study reveals that the original maar crater basin was larger than previously suggested the deep level of the maar crater is reconstructed to be a northeast southwest elongated depression currently forming a c 50 m deep basin geomorphological considerations suggest that most of the phreatomagmatic pyroclastic rocks are composed of base surge and tephra fall deposits around the deep maar depression these allochthonous rock units form a 50 400 m wide zone of proximal tuff ring sequences the formation of this zone is inferred to be a result of a combination of syn eruptive subsidence due to mass deficit in the rigid triassic dolomite basement caused by the phreatomagmatic explosions as well as post eruptive subsidence of the crater and diatreme filling successions due to diagenetic compaction the facies in the centre of the maar lake is a soft laminated alginite mainly botryococcus colonies diatom frustles calcium carbonate crystals clay minerals in the section exposed in the pula open cast mine a single turbiditic layer is present this layer originated in a landslide which possibly could have been caused by either syn eruptive earthquake and or a sudden post eruptive subsidence event of the diatreme fill

complex interaction between strombolian and phreatomagmatic eruptions in the quaternary monogenetic volcanism of the catalan volcanic zone ne of spain the catalan volcanic zone cvz at the ne of the iberian peninsula is one of the quaternary alkaline volcanic provinces of the european rifts system the cvz has been active during the last 12 ma despite the fact that this volcanism is significant in extension and volume and that eruptions have also occurred in holocene times it is mostly unknown compared to the contemporaneous alkaline volcanism in other parts of western and central europe volcanism younger than 0 5 ma is mostly concentrated in an area of about 100 km2 located between the main cities of olot and girona this basaltic volcanic field comprises more than 50 monogenetic cones including scoria cones lava flows tuff rings and maars magmatic eruptions range from hawaiian to violent strombolian phreatomagmatism is also common and has contributed to the construction of more than a half of these volcanic edifices frequently associated with the strombolian activity but also independently giving rise to a large variety of eruptive sequences we describe the main characteristics of this volcanism and analyse in particular the successions of deposits that form some of these volcanoes and discuss the potential causes of such a wide diversity of eruptive sequences we find that the main cause of such complex eruptive behaviour resides in the stratigraphic structural and hydrogeological characteristics of the substrate above which the volcanoes were emplaced rather than on the compositional characteristics of the erupting magma as they do not show significant variations among the different volcanoes studied

sounding of the topside ionosphere plasmasphere based on gps measurements from champ initial results 1 beside the gps radio occultation measurements of the troposphere and ionosphere the champ satellite tracks presently up to 8 gps satellites simultaneously for navigation the received dual frequency gps signals provide valuable information on the ionization state of the topside ionosphere and plasmasphere this paper briefly describes a new developed model assisted technique to improve the three dimensional electron density estimation above the champ orbit from these gps measurements application of this assimilation algorithm provides a 2 d electron density slice of the ionosphere plasmasphere system within the champ orbit plane from the champ altitude up to gps orbit heights we estimate the accuracy of the gps tec calibration and perform a first validation by comparing the retrieved electron density of the ionosphere plasmasphere slice at champ altitude to simultaneous electron density measurements of the langmuir probe onboard champ

violent strombolian or not using ground penetrating radar to distinguish deposits of low and high energy scoria cone eruptions scoria cones often grow by the accumulation of individual particles transported ballistically in strombolian type eruptions alternative models of cone formation suggest that cones are built primarily of fallout from the eruption column in more explosive eruptions often termed violent strombolian currently the distinction between normal strombolian and violent strombolian is based on direct observations of eruptions or by inference of eruption characteristics from mapping of tephra fall deposits unfortunately medial to distal tephra fall deposits erode rapidly leaving behind only the near vent facies of scoria cones which are thicker and less easily eroded therefore a tool that is capable of delineating differences between low energy strombolian deposits and higher energy violent strombolian deposits from investigation of the preserved scoria edifice is necessary ground penetrating radar imaging of cerro negro an active basaltic scoria cone in nicaragua has revealed details of cone deposits at depths of up to 12 m the record of the 1992 eruption which was observed to be violent strombolian shows quantifiable differences from normal strombolian near vent facies including reflections in the downwind profile that are continuous for hundreds of meters and through the slope break the ability to differentiate between tephra fallout and ballistically emplaced deposits at cerro negro suggests ground penetrating radar imaging may be useful in distinguishing eruptive style in older scoria cones where the medial to distal tephra deposits are eroded or buried

the role of external environmental factors in changing eruption styles of monogenetic volcanoes in a mio pleistocene continental volcanic field in western hungary abstract the occurrence shape structure and eruption style of monogenetic volcanoes such as maars tuff rings tuff cones and scoria cones are generally governed by several internal composition of the magma magmatic flux ascent rate viscosity volatile contents and external conditions regional and local tectonics topography and the presence of surfacial ground and meteoric water these controlling factors are together responsible for the eruption style distribution pattern volcanic facies architecture and morphology of the monogenetic volcanic landforms the late miocene to pleistocene bakony balaton highland volcanic field bbhvf in western hungary is a typical small sized at least 6 volcanic cycles have been identified by cluster analysis time gap between the cycles were vary from 1 66 up to 0 06 ma while the average eruption recurrence rate was 0 1078 ma event the time volume diagram of the volcanism of bbhvf have shown time predictive behavior combined with low magma flux total preserved volume 2 867 km3 and output rates 0 53 km3 ma for the entire volcanic field and 0 90 km3 ma for the last 5 cycles suggesting that volcanism was largely tectonically controlled and not magmatically controlled furthermore the topographic differences between the northern elevated and southern basin like parts of the volcanic field are also important in local differences in dominant fragmentation style because the elevated part of the field was prone to host large more magmatically evolved volcanoes than on the lower water saturated unconsolidated sediments which favored to the magma water interaction driven phreatomagmatic fragmentation the third controlling parameter which seems to play an important role in controlling the eruptive style of monogenetic volcanism at bbhvf was the paleoclimate fluctuation especially during the time interval of 3 0 to 2 5 ma thus mainly the long term environmental changes e g aridification have been response the shifting fragmentation style from phreatomagmatic to more magmatic ones

modelled atmospheric temperatures and global sea levels over the past million years the climate has passed through a series of glacials and interglacials over the past million years but the nature of this cyclicity in terms of temperature ice volume and sea level and the underlying causes are not well known bintanja et al use a new method to deduce a one million year time series of these variables the reconstructed records are much longer than other methods have provided for any of these variables individually the most intense glacial stages were 17 c colder than today and most of the continental ice was present in north america strong cooling in the beginning of glacials was found to precede ice sheet build up these findings may shed light on the causes of ice age cycles marine records of sediment oxygen isotope compositions show that the earth s climate has gone through a succession of glacial and interglacial periods during the past million years but the interpretation of the oxygen isotope records is complicated because both isotope storage in ice sheets and deep water temperature affect the recorded isotopic composition1 2 3 4 5 separating these two effects would require long records of either sea level or deep ocean temperature which are currently not available here we use a coupled model of the northern hemisphere ice sheets6 and ocean temperatures forced to match an oxygen isotope record for the past million years compiled from 57 globally distributed sediment cores to quantify both contributions simultaneously we find that the ice sheet contribution to the variability in oxygen isotope composition varied from ten per cent in the beginning of glacial periods to sixty per cent at glacial maxima suggesting that strong ocean cooling preceded slow ice sheet build up the model yields mutually consistent time series of continental mean surface temperatures between 40 and 80 n ice volume and global sea level we find that during extreme glacial stages air temperatures were 17 1 8 c lower than present with a 120 10 m sea level equivalent of continental ice present

an investigation of motions of the equatorial anomaly crest using more than 350 ionospheric images reconstructed tomographically studies on the motion of the anomaly crest and its geophysical implications are carried out on an average day the crest forms at 09 00 lt and in the next two hours moves poleward with a speed of about 1 per hour as it intensifies this poleward motion is slowed as the crest reaches its highest latitude where it stays for several hours until early afternoon thereafter the crest starts to weaken as it recedes with a speed of about 0 5 per hour equatorward during 12 00 14 00 lt the crest latitude is found to correlate with the fountain strength and the total number of electrons in a cross sectional plane at the observational longitude of the whole equatorial ionosphere

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

comparisons of ionospheric electron density distributions reconstructed by gps computerized tomography backscatter ionograms and vertical ionograms global positioning system gps computerized ionosphere tomography cit and ionospheric sky wave ground backscatter radar are both capable of measuring the large scale two dimensional 2 d distributions of ionospheric electron density ied here we report the spatial and temporal electron density results obtained by gps cit and backscatter ionogram bsi inversion for three individual experiments both the gps cit and bsi inversion techniques demonstrate the capability and the consistency of reconstructing large scale ied distributions to validate the results electron density profiles obtained from gps cit and bsi inversion are quantitatively compared to the vertical ionosonde data which clearly manifests that both methods output accurate information of ionopsheric electron density and thereby provide reliable approaches to ionospheric soundings our study can improve current understanding of the capability and insufficiency of these two methods on the large scale ied reconstruction

depositional mechanics and sequences of base surges songaksan tuff ring cheju island korea the songaksan mount in the southwestern part of cheju island korea is a taalian tuff ring produced by phreatomagmatic explosions at an aquifer a detailed analysis of proximal to distal facies changes reveals that the tuff ring sequence can be represented by 21 sedimentary facies one lateral facies sequence lfs and three vertical facies sequences vfs the vfs 1 and 2 are representative of facies relationships in horizontal near vent deposits the vfs 1 comprises scour fill bedded tuff inversely graded tuff massive tuff and laminated tuff from base to top the vfs 2 is a variant of the vfs 1 replaced by an inversely graded lapilli tuff unit at the base the sequences suggest traction carpet suspension and minor traction sedimentation from a high concentration near vent base surge the lfs 1 and the vfs 3 are distilled from outward dipping flank deposits both sequences begin with disorganized lapilli tuff followed successively by stratified lapilli tuff dune bedded lapilli tuff very thinbedded tuff and accretionary lapilli they are suggestive of waning base surge which decreases in particle concentration suspended load fall out rate and flow regimes with an increase in traction and sorting these facies sequences suggest that a base surge experiences flow transformation with its flow characters changing with time and space a near vent base surge is turbulent uniformly mixed and highly concentrated and produces scour fill bedded tuff as capacity decreases the surge transforms into a dense and laminar underflow and a dilute and turbulent upper part gravity transformation depositing inversely graded massive and normally graded lapilli tuff ensuing loss of sediment load and mixing of ambient air result in flow dilution surface transformation stratified and dune bedded units are produced by tractional processes of turbulent and low concentration surge further dilution causes deceleration and cooling and results in precipitation of moistened ash and accretionary lapilli from suspension

gis techniques and statistical models in evaluating landslide hazard geographical information systems gis and numerical cartography may greatly facilitate the development and use of statistical models for the assessment of regional landslide hazard from a small drainage basin located in central italy relevant geological and geomorphological factors were collected and processed by applying gis technology in particular modules were used which both generate high fidelity digital terrain models and automatically partition the terrain into main slope units the resulting information was then analysed by discriminant analysis which enabled landslide hazard and risk to be evaluated in each slope unit although not lacking drawbacks the method proved to be a feasible and cost effective approach to landslide susceptibility assessment and mapping

laschamp and mono lake geomagnetic excursions recorded in new zealand eight basaltic lavas from the auckland volcanic field new zealand record three distinct sets of excursional geomagnetic field directions and low paleointensities however the timing and therefore paleomagnetic significance of these records have been poorly understood radiocarbon k ar and thermoluminescence dating constrain these lavas to have erupted during the last 75 ka a period during which as many as three excursions have been recorded differentially at several northern and fewer southern hemisphere sites forty 40ar 39ar incremental heating experiments conducted on groundmass from seven of these excursional lavas indicate that they erupted during at least two periods at 39 1 4 1 and 31 6 1 8 ka coincident with 40ar 39ar astrochronologic and 14c ages determined for the laschamp and mono lake excursions respectively samples from a lava flow associated with a third cluster of virtual geomagnetic poles vgps are complicated by low concentrations of radiogenic argon in the presence of excess argon and thus yield discordant age spectra and an imprecise age of 26 6 8 1 ka our findings indicate that the laschamp and mono lake excursions until recently identified unequivocally and isotopically dated only in the northern hemisphere were globally synchronized at 40 1 and 32 1 ka however the vgps of lavas that record the laschamp excursion in new zealand and france are inconsistent with a simple clockwise looping geometry inferred from vgp paths obtained in eight marine sediment cores spanning the laschamp excursion we suggest that the differences in vgps recorded at the various sites are significant and may point to non axial dipole components and lower mantle control on transitional fields during short lived excursions

localization of volcanic activity topographic effects on dike propagation eruption and conduit formation magma flow in a dike rising in a crack whose strike runs from a highland or a ridge to an adjacent lowland has been modeled to determine the effect of topography on the flow it is found that there is a distinct tendency for the flow to be diverted away from the highland end of the strike toward the lowland separation of the geometric effect of the topography from its effect on lateral confining stresses on the crack indicates that both contribute to the effect but that the effect of stress is less important although this analysis explains a tendency for volcanic eruptions to occur in low lands it does not preclude eruptions on highlands the particular configuration modeled mimics topography around the proposed nuclear waste repository at yucca mountain nevada so that the results may indicate some reduction in the volcanic hazard to the site

a generic classification for the morphological and spatial complexity of volcanic and other landforms landform studies require that morphological identification description and classification use an accurate and precise taxonomy this communication offers taxonomic criteria useful for landform classification as well as spatial statistical investigations of landform patterns the spatial statistical modeling of landform patterns poses new challenges concerning the systematic characterization of landforms for study within a gis geographical information system for example the sundry descriptions and non diagnostic descriptors for volcanoes are many and appear to have promoted the adoption of somewhat imprecise and confusing classes this communication presents a generic three tiered classification as a basic description of the complexity of the landform regardless of scale tectonic and petrologic setting climatic regime state of degradation or local definition importantly it offers a taxonomy that is useful to the geographical analysis of point pattern fields using spatial statistical techniques the scheme offered here was used to characterize mostly monogenetic volcanoes however the classification scheme may also be usable for other landscape features for instance such features may include landslide scars thermokarst dolines kettle and kame topography drumlins dome crescentic and star dunes pingos rootless cones lava domes and coulees and impact craters

revised conceptual model for maar diatremes subsurface processes energetics and eruptive products diatremes are debris filled structures beneath maars that result from many magma water phreatomagmatic explosions during a monogenetic volcano s lifetime a long standing model requires deepening explosions due to water table drawdown that eject progressively deeper seated country rock from the explosion sites while the overlying diatreme and its surface crater widen due to subsidence a revised model is proposed wherein explosions can take place at any level within a diatreme at a given time most effectively venting material from near surface explosions deep seated country rock lithics in tephra deposits record stepwise vertical mixing of material by upward directed debris jets and downward subsidence rather than direct ejection from deep explosions juvenile and lithic clasts erupted during a given explosion may have had a complex history within the diatreme and need not directly reflect fragmentation or brecciation during the explosion that ejects them

proximal tephra hazards recent eruption studies applied to volcanic risk in the auckland volcanic field new zealand abstract auckland new zealand is unique in being a metropolitan area built on an active volcanic field despite the small size and intensity of auckland eruptions the risk from tephra fall is high because of the high density of buildings and lifelines the nature of this threat can be evaluated by comparisons with historical strombolian and hawaiian eruptions which have occurred in non populated areas cone building phases of such eruptions are typically protracted i e weeks to months in duration prolonging the period during which emergency managers will have to fine tune mitigation for numerous parameters such as fluctuations in intensity and wind shifts rapid cone growth during future eruptions will define a region of some 30 to 100 ha where complete destruction will occur on a time scale of hours the cost of this destruction is likely to range between nz 200m and nz 1 4b ca us 130m to us 900m beyond this we have modeled the cumulative long term effect of the build up of a downwind blanket of lapilli and ash by estimating accumulation rates for three phases of the 1959 k lauea iki eruption in hawai i the effect of changing wind direction was evaluated using low level wind data from auckland these results show that intervals between 4 and 100 h will lapse before onset of significant damage to buildings

influences on the variability of eruption sequences and style transitions in the auckland volcanic field new zealand abstract monogenetic basaltic volcanism is characterised by a complex array of eruptive behaviours reflecting spatial and temporal variability of the magmatic properties e g composition eruptive volume magma flux as well as environmental factors at the vent site e g availability of water country rock geology faulting these combine to produce changes in eruption style over brief periods minutes to days in many eruption episodes monogenetic eruptions in some volcanic fields often start with a phreatomagmatic vent opening phase that later transforms into dry magmatic explosive or effusive activity with a strong variation in the duration and importance of this first phase such an eruption sequence pattern occurred in 83 of the known eruption in the 0 25 my old auckland volcanic field avf new zealand in this investigation the eruptive volumes were compared with the sequences of eruption styles preserved in the pyroclastic record at each volcano of the avf as well as environmental influencing factors such as distribution and thickness of water saturated semi to unconsolidated sediments topographic position distances from known fault lines the avf showed that there is no correlation between ejecta ring volumes and environmental influencing factors that is valid for the entire avf in contrary using a set of comparisons of single volcanoes with well known and documented sequences resultant eruption sequences could be explained by predominant patterns of the environment in which these volcanoes were erupted based on the spatial variability of these environmental factors a first order susceptibility hazard map was constructed for the avf that forecasts areas of largest likelihood for phreatomagmatic eruptions by overlaying topographical and shallow geological information combining detailed phase by phase breakdowns of eruptive volumes and the event sequences of the avf along with the new susceptibility map more realistic eruption scenarios can be developed for different parts of the volcanic field this approach can be applied to tailoring field and sub field specific hazard forecasting at similar volcanic fields worldwide

environmental changes in shamei lagoon hainan island china interactions between natural processes and human activities abstract the sources and burial patterns of organic matter in shamei lagoon hainan china were investigated by analyzing the collected sediment cores in terms of the variations in total organic carbon toc total nitrogen tn stable carbon isotopic composition 13c and 15n and the ratio of total organic carbon to total nitrogen toc tn on such a basis the environmental evolution and primary productivity changes of the lagoon were revealed and the impacts of both natural environmental changes and human activities were identified downcore variation patterns of the parameters representing sources and burial flux of organic matter were predominantly related to natural evolution of the lagoon from 1800 to 1950 and they were increasingly affected by human activities afterwards in the period 1800 1900 the water exchange capacity of the lagoon declined substantially but it was still suitable for the growth of marine phytoplankton as is indicated by relatively high proportion and burial flux of organic matter originated from marine sources during 1900 1950 the lagoon was transformed into a freshwater environment after 1950 due to strong influence by human activities the local freshwater phytoplankton became the major source of buried organic matter at the same time marine lost its niche and its burial flux was much lower than that of its freshwater counterpart the large increment of buried freshwater organic matter also indirectly reflects the eutrophication trend since 1950

crust mantle structure of the central north island new zealand based on seismological observations abstract two geophysical features characterise the anomalous crust mantle structure beneath central north island of new zealand the apparent thinning of the felsic crust by a factor of at least 50 and upper mantle p wave velocities pn phase about 10 less than normal 7 3 7 4 km s pn velocities increase slowly to a maximum of 7 8 km s at a depth of about 80 100 km above a depth of 15 km seismic p wave velocities vp are 6 km s or less representing what is likely to be a mix of the original greywacke crust that has been stretched along with an unknown volume of intruded igneous rocks that are in various stages of cooling defining the moho is problematic in this dynamic region below a depth of 15 km there is a continuous increase in vp such that velocities 7 km s are reached at a depth as shallow as 20 km on a east west line and 7 4 km s on a north south line we interpret these wave speeds as a transition zone into low pn velocities rather than high lower crustal values of vp pn wave speeds as low as 7 4 km s are detected by tomographic methods to depths of at least 65 km and laterally over an area of at least 10 000 km2 a 1 4 partial melt in the mantle wedge beneath the central north island is estimated based on qp seismic anisotropy low values of pn and amplitudes of deep reflections these upper mantle features are more indicative of asthenosphere than lithosphere and coupled with the geological evidence for rapid uplift here at 5 ma are consistent with convective removal of the mantle lithosphere from western and central north island in late miocene times

felsic maar diatreme volcanoes a review felsic maar diatreme volcanoes host major ore deposits but have been largely ignored in the volcanology literature especially for the diatreme portion of the system here we use two mexican tuff rings as analogs for the maar ejecta ring new observations from one diatreme and the economic geology literature on four other mineralized felsic maar diatremes to produce an integrated picture of this type of volcano the ejecta rings are up to 50 m thick and extend laterally up to 1 5 km from the crater edge in two mexican examples the lower part of the ejecta ring is dominated by pyroclastic surge deposits with abundant lithic clasts up to 80 at hoya de estrada these deposits display low angle cross bedding dune bedforms undulating beds channels bomb sags and accretionary lapilli and are interpreted as phreatomagmatic rhyolitic juvenile clasts at tepexitl have only 0 25 vesicles in this portion of the ring the upper parts of the ejecta ring sequences in the mexican examples have a different character lithic clasts can be less abundant the grain size is typically coarser and the juvenile clasts can be different in character with some more vesicular fragments fragmentation was probably shallower at this stage the post eruptive maar crater infill is known at wau and consists of reworked pyroclastic deposits as well as lacustrine and other sediments underneath are bedded upper diatreme deposits interpreted as pyroclastic surge and fall deposits the upper diatreme and post eruptive crater deposits have dips larger than 30 at wau with approximately centroclinal attitudes at still lower structural levels the diatreme pyroclastic infill is largely unbedded montana tunnels and kelian are good examples of this at cerro de pasco the pyroclastic infill seems bedded despite about 500 m of post eruptive erosion relative to the pre eruptive surface the contact between the country rocks and the diatreme is sometimes characterized by country rock breccias kelian mt rawdon pyroclastic rocks in the diatreme are typically poorly sorted and ash rich they contain a heterolithic mix of juvenile clasts and lithic clasts from various stratigraphic levels megablocks derived from the ejecta ring or the country rocks are often found in the diatremes evidence for multiple explosions is in the form of steep crosscutting pyroclastic bodies within some diatremes and fragments of pyroclastic rocks within other pyroclastic facies pyroclastic rocks are cut by coherent felsic dikes and plugs which may have been feeders to lava domes at the surface allowing for the difference in magma composition felsic maar diatreme volcanoes have many similarities with their ultramafic to mafic equivalents differences include a common association with felsic domes inside the crater or just outside wau although the domes within the crater may be destroyed during the eruption hoya de estrada tepexitl the dikes and plugs feeding and invading felsic diatremes seem larger the processes of phreatomagmatic explosions involving felsic magmas may be different

the temporal evolution of three magmatic systems in the west eifel volcanic field germany abstract the quaternary west eifel volcanic field is the surface expression of a mantle plume that is thought to contain 1 melt the volcanoes range in age from 0 7 ma to 10 000 yr bp and the average recurrence rate of volcanism is around 3 5 10 4 eruptions per year giving a mean time between eruptions of 2875 years assuming that the field is monogenetic the presence of mafic and ultramafic xenoliths is taken as evidence for rapid single stage emplacement of magmas at velocities around 0 5 km h however the presence of clinopyroxene that preserves evidence of polybaric crystallisation belies this simple model this study uses variations in the fo content of olivine in mantle derived xenoliths and in olivine xenocrysts enclosed in cumulate xenoliths to calculate the time scale of evolution of three volcanic centres in the eifel field core rim variations in olivine are interpreted to result from fe mg interdiffusion between it and a surrounding melt using experimental determinations of the interdiffusion coefficient and approximations of magma temperature contact times have been calculated using planar and radial diffusion models the three volcanic centres studied were meerfelder maar gemundener maar and baarley the cumulate xenoliths at meerfelder maar formed at a depth of 13 km and record a magma emplacement event at between 1 and 3 years prior to eruption two suites of mantle derived olivine clinopyroxenites record a further two events at 1 8 days and 3 h prior to eruption the final event is interpreted to record the eruption of this volcano and the 3 h contact time records the time required for transport of peridotite xenoliths from the mantle to surface giving an average emplacement velocity of around 15 km h the cumulate xenoliths at gemundener maar crystallised at depths of between 4 and 22 km there are four distinct events recorded by the zoned olivine grains in the clinopyroxene rich cumulates from this locality the maximum residence time is similar to that at meerfelder maar however in this case the final magma resided in the chamber for between 23 days and 2 7 months prior to eruption allowing enough time for any entrained peridotite xenoliths to be filtered out the cumulate xenoliths from the baarley locality record a much longer history than those at the other localities in this case initially peridotitic olivine has been completely re equilibrated over 175 years prior to eruption in common with the other two localities there is evidence of multiple intrusive events at depths up to 7 km prior to final eruption the duration of the final eruption event and the interpreted depth to source of the xenoliths indicate an emplacement velocity of around 3 km h the data derived from modelling of diffusion profiles in olivine from cumulate xenoliths indicate that the magmatic systems feeding these three volcanoes were dynamic and very short lived in addition these data can be used to place constraints on the rate of cumulate formation in subvolcanic magma chambers on the basis of the data presented here it would seem that an appreciable thickness of cumulates can form over relatively short time spans days to a few years

ionospheric electron concentration imaging using gps over the usa during the storm of july 2000 a new technique for ionospheric imaging is demonstrated during a severe geomagnetic storm of 15th july 2000 the three dimensional time dependent imaging algorithm is applied to multi directional ground based gps data and yields spatial maps of electron concentration this technique is demonstrated by showing a series of images of the mid latitude ionosphere over the usa during the storm of july 2000 a strong uplift in the height of the f layer is observed after 20 ut on 15th july followed by a severe latitudinal electron concentration gradient independent verification of the images is provided by the ionosondes and incoherent scatter radar data the gps images reveal the large scale dynamics of the ionosphere during the disturbed conditions and show the potential of geophysical imaging for storm time ionospheric studies

a two layer model of the ionosphere using global positioning system data we present a new approach to model the ionosphere based on gps data previous authors have used models with an unique shell in this case we have included a second shell to account for the distribution of the electrons in the outer part of the ionosphere we have analyzed the ionospheric electron content of a region above 30 degrees in declination in different conditions of ionospheric activity using the kalman filter the data used has been obtained from the international gps service for geodynamics igs network simultaneously we have studied the receiver and transmitter differential biases showing the effects of neglecting the outer part of the ionosphere in the model it appears a systematic variations for the receivers depending on its latitude not for the satellites

mixed deposits of complex magmatic and phreatomagmatic volcanism an example from crater hill auckland new zealand a series of alternating phreatomagmatic wet and magmatic dry basaltic pyroclastic deposits forming the crater hill tuff ring in new zealand contains one unit m1 which can only be interpreted as the products of mixing of ejecta from simultaneous wet and dry explosions at different portions of a multiple vent system the principal characteristics of m1 are a rapid lateral changes in the thicknesses of and proportions in juvenile components in individual beds and b wide ranges of juvenile clast densities in every sample m1 appears to have been associated with an elongate source of highly variable and fluctuating magma water ratios and magma discharge rates this contrasts with the only other documented mixed wet and dry basaltic pyroclastic deposits where mixing from two point sources of quite different but stable character has been inferred

numerical simulation of basaltic lava flows in the auckland volcanic field new zealand implication for volcanic hazard assessment monogenetic volcanic fields such as the auckland volcanic field avf new zealand are common on the earth s surface and are typically dominated by basaltic lava flows up to 10 s of km long in monogenetic volcanic fields located in close proximity to human population and infrastructure lava flows are a significant threat in this study lava flow emplacement conditions for some basaltic eruptions of the avf were reconstructed using the thermo rheological magflow model eight existing lava flows in the avf were simulated using magflow and eruptive volumes measured from light detection and ranging lidar derived digital terrain models dtms fitting the simulations to the dimensions of actual lava flows provides insight into their emplacement mechanisms and conditions such as effusion rate and probable eruption durations by looking at emplacement in different settings the likely magma ascent rate for studied avf eruptions is calculated to have been on the order of 0 1 m s in the avf the typical estimated duration of past lava flows was from a minimum of 2 days for small volume flows such as little rangitoto 0 0015 km3 up to 83 days for large volume flows such as three kings 0 078 km3 the three best fitting simulations were used to establish eruption scenarios for future volcanic hazard mapping for the avf inferences of eruption duration that will be useful for developing realistic emergency management plans and recovery scenarios for this densely populated volcanic field are also provided

the fekete hegy balaton highland hungary soft substrate and hard substrate maar volcanoes in an aligned volcanic complex implications for vent geometry subsurface stratigraphy and the palaeoenvironmental setting the fekete hegy volcanic complex is located in the centre of the bakony balaton highland volcanic field in the pannonian basin which formed from the late miocene to pliocene period the eruption of at least four very closely clustered maar volcanoes into two clearly distinct types of prevolcanic rocks allows the observation and comparison of hard substrate and soft substrate maars in one volcanic complex the analyses of bedding features determination of the proportion of accidental lithic clasts granulometry and age determination helped to identify and distinguish the two types of maar volcanoes ascending magma interacted with meteoric water in karst aquifers in mesozoic carbonates as well as in porous media aquifers in neogene unconsolidated wet siliciclastic sediments the divided basement setting is reflected by distinct bedding characteristics and morphological features of the individual volcanic edifices as well as a distinct composition of pyroclastic rocks country rocks in hard substrate maars have a steep angle of repose leading to the formation of steep sided cone shaped diatremes enlargement and filling of these diatreme is mainly a result of shattering material by fci related shock waves and wall rock collapse during downward penetration of the explosion locus country rocks in soft substrate maars have much shallower angles of repose leading to the formation of broad bowl shaped structures or irregular depressions enlargement and filling of these diatremes is mainly the result of substrate collapse for example due to liquefaction of unconsolidated material by fci related shock waves and its emplacement by gravity flows the fekete hegy is an important example illustrating that the substrate of a volcanic edifice has to be taken into account as an important interface which can have major control on phreatomagmatic eruptions and the resulting characteristics of the volcanic complex

investigating the consequences of urban volcanism using a scenario approach i development and application of a hypothetical eruption in the auckland volcanic field new zealand abstract what happens when a city has a volcanic eruption within its boundaries to explore the consequences of this rare but potentially catastrophic combination we develop a detailed multi hazard scenario of an auckland volcanic field avf eruption the avf underlies new zealand s largest city auckland we start with an existing avf unrest scenario sequence and develop it through a month long hypothetical eruption based on geologic investigations of the avf and historic similar eruptions from around the world we devise a credible eruption sequence and include all volcanic hazards that could occur in an avf eruption in consultation with civil defence and emergency management staff we create a series of evacuation maps for before during and after the hypothetical eruption sequence our result is a versatile scenario with many possible applications developed further in companion papers that explore eruption consequences on transportation and water networks however here we illustrate one application evaluating the consequences of an eruption on electricity service provision in a collaborative approach between scientists and electricity service providers we evaluate the impact of the hypothetical eruption to electricity generation transmission and distribution infrastructure we then evaluate how the impacted network functions accounting for network adaptations e g diverting power away from evacuated areas site access and restoration factors we present a series of regional maps showing areas with full service rolling outages and no power as a result of the eruption this illustrative example demonstrates how a detailed scenario can be used to further understand the ramifications of urban volcanism on local and regional populations and highlights the importance of looking beyond damage to explore the consequences of volcanism

forecasting transitions in monogenetic eruptions using the geologic record spatial forecasting of volcanism and associated hazards in intraplate monogenetic volcanic fields is subject to large uncertainties in both data and models we demonstrate a novel logistic regression method for mapping phreatomagmatic magmatic eruption transition susceptibility using near surface hydrologic topographic and geologic data the method is illustrated on the auckland volcanic field the location of new zealand s largest city auckland environmental factors examined for possible influence included the thickness of water saturated and porous sediments substrate type and geology vent elevation and distance from the nearest fault by comparing these factors with the volumes and styles of past eruption sequences a location specific eruption sequence forecasting model was constructed recognizing that larger and or longer eruptions are more likely to exhaust vent area sources of water estimating volcanic hazard susceptibility in this way allows more effective planning and improved preeruption preparedness between eruptions and during future volcanic crises

a pleistocene pyroclastic poor maar from central anatolia turkey influence of a local fault on a phreatomagmatic eruption in central anatolia the late pleistocene narkoy maar formed on a fault zone a 5 m thick pyroclastic succession erupted from a crater 900 m in diameter the presence of gas fumeroles hot springs and hydrothermal alteration zones indicates that the area is geothermally active the products of the maar are large blocks and lapilli to coarse ash planar bedding and cross stratification are well developed within coarse ash beds the maar deposits were generated by a single period of phreatomagmatic explosion with two episodes the first episode produced wet eruption clouds while the second one was relatively dry in nature the estimate of the volume of pyroclastic ejecta compared with the dimensions of the maar demonstrates a material deficiency which is attributed to a large volume of gas emanated during the phreatomagmatic explosion along a fault presently cross cutting the maar in addition a relatively small volume of magma intruded along the fault and mixed with meteoric water in the fault zone to produce the phreatomagmatic eruption the existence of a hydrothermal system in the fault zone is important in affecting the eruption style and evolution of narkoy maar

the structure of basaltic scoria and reticulite and inferences for vesiculation foam formation and fragmentation in lava fountains in this investigation pyroclast structures are used to constrain degassing in basaltic lava fountains vesicle size shape number density interconnectedness and packing character are quantified and related to 1 the kinetics of bubble nucleation and growth 2 the structural evolution of magmatic foams and 3 the influence of vesiculation rate on magma fragmentation measurements made on a diverse suite of pyroclasts from kilauea volcano indicate that basaltic foams evolve through an initially disordered closed celled spherical state to a well ordered open celled polyhedral state as the vesicularity rises from 75 to 98 the structural changes occur rapidly 10 s in the conduit and fountain in response to an intense vesiculation burst vesicle size distribution systematics indicate bubble nucleation rates 2 104 events cm 3s 1 that are approximately three orders of magnitude greater than those found for effusive eruptive activity bubble growth rates 9 10 4 cm s exceed effusive estimates by a factor of 3 the observed runaway rate of bubble production indicates strong supersaturations at the onset of nucleation we speculate that the rise speed of the magma as it reaches the level where significant volatile exsolution begins determines the intensity of the vesiculation burst and hence the vigor of the eruption rapid expansion and acceleration of the magma under these conditions may provide the impetus for fragmentation

ionospheric electron concentration imaging using combination of leo satellite data with ground based gps observations over china space based global positioning system gps data onboard low earth orbit satellites are used together with ground based gps observations to simultaneously image ionospheric electron density ied and it aims to improve the spatial resolution of ionospheric tomography high precision dual frequency gps data from the crustal movement observation network of china and several international gps service stations and the constellation observation system for meteorology ionosphere and climate cosmic are used to invert a time series of the ied profiles over china and surroundings some retrieved electron density profiles are compared with the results from cosmic occultation data processed by cosmic data analysis and archival center and the peak density in f2 nmf2 is verified by ionosonde data experimental results demonstrate the imaging effectiveness due to cosmic data used in the inversion and the potential benefits using cosmic data in such system are also discussed

scoria cone formation through a violent strombolian eruption irao volcano sw japan scoria cones are common volcanic features and are thought to most commonly develop through the deposition of ballistics produced by gentle strombolian eruptions and the outward sliding of talus however some historic scoria cones have been observed to form with phases of more energetic violent strombolian eruptions e g the 1943 1952 eruption of paricutin central mexico the 1975 eruption of tolbachik kamchatka maintaining volcanic plumes several kilometers in height sometimes simultaneous with active effusive lava flows geologic evidence shows that violent strombolian eruptions during cone formation may be more common than is generally perceived and therefore it is important to obtain additional insights about such eruptions to better assess volcanic hazards we studied irao volcano the largest basaltic monogenetic volcano in the abu monogenetic volcano group sw japan the geologic features of this volcano are consistent with a violent strombolian eruption including voluminous ash and fine lapilli beds on order of 10 1 km3 dre with simultaneous scoria cone formation and lava effusion from the base of the cone the characteristics of the volcanic products suggest that the rate of magma ascent decreased gradually throughout the eruption and that less explosive strombolian eruptions increased in frequency during the later stages of activity during the eruption sequence the chemical composition of the magma became more differentiated a new k ar age determination for phlogopite crystallized within basalt dates the formation of irao volcano at 0 4 0 05 ma

shallow marine records of pyroclastic surges and fallouts over water in jeju island korea and their stratigraphic implications explosive volcanism results in a wide range of volcaniclastic deposits in many of earth9s subaerial and subaqueous environments in this paper we introduce a unique shallow marine volcaniclastic deposit from jeju island korea for which the materials were transported to the water surface by pyroclastic clouds and then settled from the surface as they were entrained in the water the deposition occurred under alternating currents and still waters which is most plausibly attributed to tidal processes mud flasers or drapes intercalated in the deposit which indicate periods of slack water during tidal cycles suggest that the deposit accumulated in a very short period of a fortnight or a month about a million times faster than the adjacent sedimentary strata because of the unusually high sedimentation rate the volcaniclastic deposit could record the usual fair weather processes in the depositional site at a resolution that is almost never provided by ordinary sedimentary deposits this finding highlights the biases in earth9s stratigraphic records and teaches us that volcanic deposits commonly regarded as the products of catastrophic events can in some cases record more faithfully the ordinary and usual processes that nonvolcanic deposits cannot

localization of volcanic activity 2 effects of pre existing structure abstract the capture of vertically rising dikes by faults is examined with a combination of an analytic solution numerical simulations and field observations one of the numerical solutions and the field observations also provide insight into the intrusion of sills into the hanging wall of faults that are reactivated when lubricated by magma the approximate analytic solution indicates that dike capture by a fault will be limited primarily to steeply dipping faults capture will become easier meaning that it will be possible for faults with shallower dips as the intersection of the dike with the fault occurs at smaller depths capture will also be easier as it is harder for crack to grow in the hanging wall this may be interpreted in terms of a higher tensile strength or in terms of shorter pre existing cracks or higher fracture toughness numerical analysis using a discrete element code confirms this conclusion but also shows that the analytic solutions overestimate the difficulty of intruding a pre existing fault the maximum deviation occurring for faults dipping at 45 this finding that dike capture by faults should be restricted to high angle faults and shallow depths is supported by field observations of exhumed miocene basaltic volcanic terrain in central nevada usa differences in style seen in other regions are attributed to both the rock properties and the regional stress field the numerical solutions also indicate that synintrusive fault slip is likely to lead to formation of sills in the hanging wall at depths of a few hundred meters or less a finding that is also consistent with the field observations in nevada

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

a model for calculating eruptive volumes for monogenetic volcanoes implication for the quaternary auckland volcanic field new zealand abstract monogenetic basaltic volcanism is characterised by a complex array of behaviours in the spatial distribution of magma output and also temporal variability in magma flux and eruptive frequency investigating this in detail is hindered by the difficulty in evaluating ages of volcanic events as well as volumes erupted in each volcano eruptive volumes are an important input parameter for volcanic hazard assessment and may control eruptive scenarios especially transitions between explosive and effusive behaviour and the length of eruptions erosion superposition and lack of exposure limit the accuracy of volume determination even for very young volcanoes in this study a systematic volume estimation model is developed and applied to the auckland volcanic field in new zealand in this model a basaltic monogenetic volcano is categorised in six parts subsurface portions of volcanoes such as diatremes beneath phreatomagmatic volcanoes or crater infills are approximated by geometrical considerations based on exposed analogue volcanoes positive volcanic landforms such as scoria spatter cones tephras rings and lava flow were defined by using a light detection and ranging lidar survey based digital surface model dsm finally the distal tephra associated with explosive eruptions was approximated using published relationships that relate original crater size to ejecta volumes considering only those parts with high reliability the overall magma output converted to dense rock equivalent for the post 250 ka active auckland volcanic field in new zealand is a minimum of 1 704 km3 this is made up of 1 329 km3 in lava flows 0 067 km3 in phreatomagmatic crater lava infills 0 090 km3 within tephra tuff rings 0 112 km3 inside crater lava infills and 0 104 km3 within scoria cones using the minimum eruptive volumes the spatial and temporal magma fluxes are estimated at 0 005 km3 km2 and 0 007 km3 ka the temporal volumetric evolution of auckland is characterised by an increasing magma flux in the last 40 ky which is inferred to be triggered by plate tectonics processes e g increased asthenospheric shearing and backarc spreading of underneath the auckland region

spatial distribution of cones and satellite detected lineaments in the pali aike volcanic field southernmost patagonia insights into the tectonic setting of a neogene rift system abstract the relationships between the distribution and morphometric features of eruptive structures scoria and spatter cones maar tuff rings and the fracture network were investigated in the pliocene quaternary pali aike volcanic field southernmost patagonia argentina chile the alkali basaltic basanitic magmas which erupted in this area have nearly primary magma compositions and often bear mantle xenoliths hence magma ascent from deep seated reservoirs was probably very fast with no significant stagnation at crustal levels field surveys and satellite image analysis led to the identification of up to 467 eruptive structures and four main nw se ne sw e w and n s fracture systems the spatial distribution of eruptive cones and fractures was investigated through the computation of power law exponents df for self similar clustering the self similarity of cones and fractures was defined between lower and upper cut offs which were in turn related to the thickness of the fractured mechanical layer the fractal character of cones and fracture distribution clustering in the pali aike volcanic field area was thus correlated with crustal thickness the self similarity of fractures was used to establish the relative chronology of the detected fracture systems the self similar clustering exponent is highest in the e w and nw se fracture systems df 1 78 and 1 77 respectively and lowest in the n s system df 1 65 the self similar clustering of eruptive structures is well defined df 1 45 the intense volcano tectonic activity in the pali aike area marks a major pliocene quaternary phase in the development of the magellan neogene rift system

maar diatreme geometry and deposits subsurface blast experiments with variable explosion depth basaltic maar diatreme volcanoes which have craters cut into preeruption landscapes maars underlain by downward tapering bodies of fragmental material commonly cut by hypabyssal intrusions diatremes are produced by multiple subsurface phreatomagmatic explosions although many maar diatremes have been studied the link between explosion dynamics and the resulting deposit architecture is still poorly understood scaled experiments employed multiple buried explosions of known energies and depths within layered aggregates in order to assess the effects of explosion depth and the morphology and compaction of the host on the distribution of host materials in resulting ejecta the development of subcrater structures and deposits and the relationships between them experimental craters were 1 2 m wide analysis of high speed video shows that explosion jets had heights and shapes that were strongly influenced by scaled depth physical depth scaled against explosion energy and by the presence or absence of a crater jet properties in turn controlled the distribution of ejecta deposits outside the craters and we infer that this is also reflected in the diverse range of deposit types at natural maars ejecta were dominated by material that originated above the explosion site and the shallowest material was dispersed the farthest subcrater deposits illustrate progressive vertical mixing of host materials through successive explosions we conclude that the progressive appearance of deeper seated material stratigraphically upward in deposits of natural maars probably records the length and time scale for upward mixing through multiple explosions with ejection by shallow blasts rather than progressive deepening of explosion sites in response to draw down of aquifers

amplified hazard of small volume monogenetic eruptions due to environmental controls orakei basin auckland volcanic field new zealand orakei maar and tuff ring in the auckland volcanic field is an example of a basaltic volcano in which the style and impacts of the eruption of a small volume of magma were modulated by a fine balance between magma flux and groundwater availability these conditions were optimised by the pre 85 ka eruption being hosted in a zone of fractured and variably permeable plio pleistocene mudstones and sandstones orakei maar represents an end member in the spectrum of short lived basaltic volcanoes where substrate conditions rather than the magmatic volatile content was the dominant factor controlling explosivity and eruption styles the eruption excavated a crater 80 m deep that was subsequently filled by slumped crater wall material followed by lacustrine and marine sediments the explosion crater may have been less than 800 m in diameter but wall collapse and wave erosion has left a 1 000 m diameter roughly circular basin a tuff ring around part of the maar comprises dominantly base surge deposits along with subordinate fall units grain size texture and shape characteristics indicate a strong influence of magma water and magma mud interactions that controlled explosivity throughout the eruption but also an ongoing secondary role of magmatic gas driven expansion and fragmentation the tuff contains 70 of material recycled from the underlying plio pliestocene sediments which is strongly predominant in the 2 fraction the magmatic clasts are evolved alkali basalt consistent with the eruption of a very small batch of magma the environmental impact of this eruption was disproportionally large when considering the low volume of magma involved dre 0 003 km3 hence this eruption exemplifies one of the worst case scenarios for an eruption within the densely populated auckland city destroying an area of 3 km2 by crater formation and base surge impact an equivalent scenario for the same magma conditions without groundwater interaction would yield a scoria spatter cone with a diameter of 400 550 m destroying less than a tenth of the area affected by the orakei event

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

monogenetic volcanism personal views and discussion monogenetic volcanism produces small volume volcanoes with a wide range of eruptive styles lithological features and geomorphic architectures they are classified as spatter cones scoria or cinder cones tuff rings maars maar diatremes and tuff cones based on the magma water ratio dominant eruption styles and their typical surface morphotypes the common interplay between internal such as the physical chemical characteristics of magma and external parameters such as groundwater flow substrate characteristics or topography plays an important role in creating small volume volcanoes with diverse architectures which can give the impression of complexity and of similarities to large volume polygenetic volcanoes in spite of this volcanic facies complexity we defend the term monogenetic volcano and highlight the term s value especially to express volcano morphotypes this study defines a monogenetic volcano a volcanic edifice with a small cumulative volume typically 1 km3 that has been built up by one continuous or many discontinuous small eruptions fed from one or multiple magma batches this definition provides a reasonable explanation of the recently recognized chemical diversities of this type of volcanism

the origin of alkaline magmas in an intraplate setting near a subduction zone the ngatutura basalts north island new zealand abstract the ngatutura basalts are one of a series of pliocene quaternary alkalic basalt volcanic fields in north island new zealand they are situated in an intraplate tectonic setting behind the currently active taupo volcanic zone and 300 km above the subducting slab the volcanic field consists of 16 small volume monogenetic volcanic centres composed mainly of eroded scoria cones and lava flows that occupy an extensional tectonic environment characterized by ne striking block faulting in some cases the faults have controlled the localization of volcanic vents the lavas have restricted compositions ranging from hawaiites to nepheline hawaiites and are characterized by enriched lile lree and hfs elements with particularly high nb and ta low ba nb and high zr y and ce n yb n ratios nepheline hawaiites are slightly more differentiated than hawaiites and have higher ce n yb n ratios petrogenetic modelling suggests that the range in composition was mainly controlled by fractional crystallization of olivine clinopyroxene and minor plagioclase and titanomagnetite which is consistent with the modal phenocryst abundances fractionation is explained by side wall crystallization and flowage differentiation during rapid ascent rather than gravitative settling in a magma chamber ngatutura magmas were probably derived from an enriched garnet lherzolite source within the low velocity mantle the process of source enrichment is speculative but our preferred model calls on metasomatizing fluids in the low velocity zone there is no geochemical evidence for any influence of the subducted slab on their composition even though they overlie the pacific plate subduction zone this implies that the extent of subduction related contamination in the mantle wedge is not pervasive but is confined to a limited region overlying the subducted slab also the deep mantle plume responsible for alkalic magmatism must have originated above the slab because it seems unlikely that such a plume could have occurred at a deeper level and penetrate the slab without some evidence this therefore limits the depth of origin of these deep mantle plumes to less than 300 km

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

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

historic hydrovolcanism at deception island antarctica implications for eruption hazards deception island antarctica is the southernmost island of the south shetland archipelago in the south atlantic volcanic activity since the eighteenth century along with the latest volcanic unrest episodes in the twentieth and twenty first centuries demonstrates that the volcanic system is still active and that future eruptions are likely despite its remote location the south shetland islands are an important touristic destination during the austral summer in addition they host several research stations and three summer field camps deception island is characterised by a quaternary caldera system with a post caldera succession and is considered to be part of an active dispersed monogenetic volcanic field historical post caldera volcanism on deception island involves monogenetic small volume vei 2 3 eruptions such forming cones and various types of hydrovolcanic edifices the scientific stations on the island were destroyed or severely damaged during the eruptions in 1967 1969 and 1970 mainly due to explosive activity triggered by the interaction of rising or erupting magma with surface water shallow groundwater and ice we conducted a detailed revision field petrology and geochemistry of the historical hydrovolcanic post caldera eruptions of deception island with the aim to understand the dynamics of magma water interaction as well as characterise the most likely eruptive scenarios from future eruptions we specifically focused on the crimson hill estimated age between 1825 and 1829 and kroner lake estimated age between 1829 and 1912 eruptions and 1967 1969 and 1970 events by describing the eruption mechanisms related to the island s hydrovolcanic activity data suggest that the main hazards posed by volcanism on the island are due to fallout ballistic blocks and bombs and subordinate dilute pdcs in addition deception island can be divided into five areas of expected activity due to magma water interaction providing additional data for correct hazard assessment on the island

the role of phreatomagmatism in a plio pleistocene high density scoria cone field llancanelo volcanic field mendoza argentina abstract the plio pleistocene llancanelo volcanic field together with the nearby payun matru field comprises at least 800 scoria cones and voluminous lava fields that cover an extensive area behind the andean volcanic arc beside the scoria cones in the llancanelo field at least six volcanoes show evidence for explosive eruptions involving magma water interaction these are unusual in the context of the semi arid climate of the eastern andean ranges the volcanic structures consist of phreatomagmatic derived tuff rings and tuff cones of olivine basalt composition malacara and jarilloso tuff cones were produced by fallout of a range of dry to wet tephra the malacara cone shows more evidence for a predominance of wet emplaced units with a steep slump slope characterized by many soft sediment deformation structures such as undulating bedding planes truncated beds and water escape features the piedras blancas and carapacho tuff rings resulted from explosive eruptions with deeper explosion loci these cones are hence dominated by lapilli tuff and tuff units emplaced mainly by wet and or dry pyroclastic surges carapacho is the only centre that appears to have started with phreatomagmatic eruptions with lowermost tephra being rich in non volcanic country rocks the presence of deformed beds with impact sags slumping textures asymmetrical ripples dunes cross and planar lamination syn volcanic faulting and accretionary lapilli beds indicate an eruption scenario dominated by excessive water in the transportational and depositional regime this subordinate phreatomagmatism in the llancanelo volcanic field suggests presence of ground and or shallow surface water during some of the eruptions each of the tuff rings and cones are underlain by thick fractured multiple older lava units these broken basalts are inferred to be the horizons where rising magma interacted with groundwater the strong palagonitization at each of the phreatomagmatic cones formed hard beds resistant to erosion and therefore the volcanic landforms are well preserved

degradation of quaternary cinder cones in the cima volcanic field mojave desert california basaltic cinder cones in the cima volcanic field record a detailed history of progressive erosion in the arid environment of the eastern mojave desert these cones range in age from 0 015 m y to 1 09 0 08 m y as dated by radiocarbon or k ar analyses of the youngest lava flows from each cone cone heights range from 50 to 155 m and basal widths range from 400 to 915 m on younger cones height width ratios average 0 17 and crater width cone width ratios average 0 42 the degradational morphology of these cones displays several trends that are closely related to cone age 1 crater width cone width ratios decrease from 0 48 on the youngest cone to 0 21 on the oldest cone with a preserved crater 2 mean maximum side slopes tan sc decrease from 0 575 on the youngest cone to an average of 0 41 on the oldest cones studied 3 debris apron height cone height ratios increase from 0 10 on the youngest cone to an average of 0 34 on the oldest cones 4 cone drainage evolves from irregularly spaced rills and gullies on the youngest cone to regularly spaced gullies on 0 20 to 0 35 m y old cones to valleys as much as 110 m wide and 10 m deep on cones 0 59 m y old and older these trends form the basis of an empirical model of cinder cone degradation in arid environments this model documents 1 an erosional loss of 15 of cone volume during the first million years 2 progressive decline of cone slope at an average rate of 0 006 103 yr and cone height at an average rate of 2 25 cm 103 yr 3 initial rapid stripping of the loose cinder mantle from upper cone slopes accompanied by rapid debris apron formation and 4 a gradual transition between 0 25 and 0 6 m y from relatively uniform stripping of upper slopes to localized fluvial dissection of both cone slopes and debris apron this transition is apparently controlled by a concomitant change from diffuse subsurface drainage within the pervious cinder mantle to concentrated surface flow across the heterogeneous assemblage of agglutinate layers dikes and ponded flows of the cone interior

phreatomagmatic eruptions through unconsolidated coastal plain sequences maungataketake auckland volcanic field new zealand abstract maungataketake is a monogenetic basaltic volcano formed at 85 89 ka in the southern part of the auckland volcanic field avf new zealand it comprises a basal 1100 m diameter tuff ring with a central scoria spatter cone and lava flows the tuff ring was formed under hydrogeological and geographic conditions very similar to the present the tuff records numerous density stratified wet base surges that radiated outward up to 1 km decelerating rapidly and becoming less turbulent with distance the pyroclastic units dominantly comprise fine grained expelled grains from various sedimentary deposits beneath the volcano mixed with a minor component of juvenile pyroclasts 35 vol subtle lateral changes relate to deceleration with distance and vertical transformations are minor pointing to stable explosion depths and conditions with gradual transitions between units and no evidence for eruptive pauses this volcano formed within and on 60 m thick plio pleistocene poorly consolidated highly permeable shelly sands and silts kaawa formation capped by near impermeable water saturated muds tauranga group these sediments rest on moderately consolidated miocene aged permeable turbiditic sandstones and siltstones waitemata group magma water fuelled thermohydraulic explosions remained in the shallow sedimentary layers excavating fine grained sediments without brittle fragmentation required on the whole the resulting cool wet pyroclastic density currents were of low energy the unconsolidated shallow sediments deformed to accommodate rapidly rising magma leading to development of complex sill like bodies and a range of magma water contact conditions at any time the weak saturated sediments were also readily liquefied to provide an enduring supply of water and fine sediment to the explosion loci changes in magma flux and or subsequent stabilisation of the conduit area by a lava ring barrier led to ensuing strombolian and fire fountaining eruption phases future eruptions in littoral environments around auckland are likely to be of this type producing base surges that rapidly decrease in energy over short runout distances 1 km

improving the vertical resolution of ionospheric tomography with gps occultations we combine gps met data from 29 occultations and igs ground data collected from 160 stations around the world to perform stochastic tomography of the ionosphere with a 4 20 20 global grid of voxels extending from 200 to 650 km above the mean surface of the earth a correlation functional approach that limits the spatial high frequency content of the images is used and a kalman filter is applied in the time direction the combination of ground and occultation data and the use of smoothing techniques is robust enough for vertical resolution in this four layer model analysis we discuss the role of noise on the choice of the correct range of eigenvalues in the inversion problem and the impact of occultation data showing that ground data alone is insufficient for vertical resolution even in a three layer noise less simulation

reconstructing the eruption magnitude and energy budgets for the pre historic eruption of the monogenetic 5 ka mt gambier volcanic complex south eastern australia understanding explosive volcanic eruptions especially phreatomagmatic eruptions their intensities and energy budgets is of major importance when it comes to risk and hazard studies with only a few historic occurrences of phreatomagmatic activity a large amount of our understanding comes from the study of pre historic volcanic centres which causes issues when it comes to preservation and vegetation in this research we show that using 3d geometrical modelling it is possible to obtain volume estimates for different deposits of a pre historic complex monogenetic centre the mt gambier volcanic complex south eastern australia using these volumes we further explore the energy budgets and the magnitude of this eruption vei 4 including dispersal patterns eruption columns varying between 5 and 10 km dispersed towards north east to south to further our understanding of intraplate monogenetic eruptions involving phreatomagmatic activity we also compare which thermodynamic model fits best in the creation of the maar crater of mt gambier the major explosion dominated model or the incremental growth model in this case the formation of most of the craters can best be explained by the latter model

controls on volcanism at intraplate basaltic volcanic fields a broad range of controlling mechanisms is described for intraplate basaltic volcanic fields ibvfs in the literature these correspond with those relating to shallow tectonic processes and to deep mantle plumes accurate measurement of the physical parameters of intraplate volcanism is fundamental to gain an understanding of the controlling factors that influence the scale and location of a specific ibvf detailed volume and geochronology data are required for this however these are not available for many ibvfs in this study the primary controls on magma genesis and transportation are established for the pliocene recent newer volcanics province nvp of south eastern australia as a case study for one of such ibvf the nvp is a large and spatio temporally complex ibvf that has been described as either being related to a deep mantle plume or upper mantle and crustal processes we use innovative high resolution aeromagnetic and 3d modelling analysis constrained by well log data to calculate its dimensions volume and long term eruptive flux our estimates suggest volcanic deposits cover an area of 23 100 530 km2 and have a preserved dense rock equivalent of erupted volcanics of least 680 km3 and may have been as large as 900 km3 the long term mean eruptive flux of the nvp is estimated between 0 15 and 0 20 km3 ka which is relatively high compared with other ibvfs our comparison with other ibvfs shows eruptive fluxes vary up to two orders of magnitude within individual fields most examples where a range of eruptive flux is available for an ibvf show a correlation between eruptive flux and the rate of local tectonic processes suggesting tectonic control limited age dating of the nvp has been used to suggest there were pulses in its eruptive flux which are not resolvable using current data these changes in eruptive flux are not directly relatable to the rate of any interpreted tectonic driver such as edge driven convection however the nvp and other ibvfs used for comparison have long term eruptive fluxes that are considerably less than definitive plume related volcanic systems along with their spatio temporal patterns and other analysis it is suggested that the nvp and the vast majority of low and high flux ibvfs appear to be the result of tectonic processes without requiring additional thermal input from a deep mantle source considering a control on volcanism by tectonic processes the range of eruptive flux of ibvfs is related to variations in the rate of the effecting tectonic process mantle composition and the size of the mantle source zone where melt generation and accumulation is taking place

strombolian and effusive activity as precursors to phreatomagmatism eruptive sequence at maars of the pinacate volcanic field sonora mexico the pinacate volcanic field sonora mexico contains 400 cinder cones and eight maars it is noteworthy that most of the maar forming phreatomagmatic eruptions were immediately preceded by effusive and strombolian activity rather than occurring when magma first approached the surface the strombolian activity may have facilitated access of groundwater to the conduits in this arid region the field evidence suggests that phreatomagmatism is inhibited unless the magma flux is low relative to the rate of water supply and unless the top of the magma column has subsided probably below the water table the latter condition is difficult to prove in the absence of direct observation and alternative hypotheses involving disturbance of the conduit system are considered the spatial distribution of maars in the pinacate and the lithology of their associated tuffaceous ejecta both may reflect the course of an ancient river channel whose permeable gravels were pierced by the magmatic conduits

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