formation of an a ā lava delta insights from time lapse multibeam bathymetry and direct observations during the stromboli 2007 eruption five consecutive multibeam bathymetries collected before during and after the 2007 stromboli eruption combined with visual inspections allowed us to document the morphological evolution of an a lava fed delta and to reconstruct the main processes acting during its submarine emplacement the 2007 stromboli delta extended down to 600 m water depth and covered an area of 420 103 m2 with a maximum thickness of 65 m and a total estimated volume of 7 106 m3 i e three times larger than its subaerial counterpart the lava delta grew mainly through the emplacement of discrete lobes about 50 150 m in size lobes were fed from point sources along the paleoshoreline and their subaqueous pathways seem to be mainly controlled by the submarine morphology with flows mostly filling in depressions left by previous lobes the main controlling factors on the lobe morphology and thickness are the effusion rates and the pre eruption morphology i e the geometry and gradients of the basal surface data also shows that sudden slope failure of portions of the submarine delta may occur simultaneously with accretion implying that a significant part of the delta material can be transported to greater depths by submarine gravity flows the present study is relevant for future monitoring and hazard assessment during the growth of active lava fed deltas as well as for a better interpretation of ancient volcaniclastic successions inland

a geomagnetic excursion in the brunhes epoch recorded in new zealand basalts abstract anomalous geomagnetic directions are recorded in the thermoremanent magnetisation trm of alkali basalt lavas from the auckland volcanic field new zealand the concordance of paleomagnetic directions both in different sites from individual eruptive centres and in sites from several different centres as well as their relatively short age span compared with that of the field as a whole indicates that these anomalous directions have a geomagnetic origin the directions which are dated between approximately 25 ka and 50 ka do not represent full reversals of the geomagnetic field and are interpreted as excursions very few excursions have been reported from the southern hemisphere this is the first such record known from igneous rocks and further supports the notion that excursions are global phenomena the excursions here are tentatively correlated with those reported from lake mungo mono lake and laschamp

implications for intraplate volcanism and back arc deformation in northwestern new zealand from joint inversion of receiver functions and surface waves summary we employ a joint inversion of teleseismic receiver functions and surface wave phase velocities to determine the shear wave velocity structure in the crust and upper mantle beneath northwestern new zealand receiver functions primarily contain information on velocity contrasts while surface waves are sensitive to the average shear velocity with depth by performing a joint inversion we reduce the limitations of each method resulting in a more robust shear wave model inversion results reveal regions of low shear wave velocity of 2 8 km s 1 in the mid crust 10 19 km depth and 4 0 km s 1 in the upper mantle 70 90 km depth beneath quaternary intraplate basalt fields we infer that the mid crustal low velocity zones lvzs are bodies of partial melt most likely rhyolite intrusions we suggest that the upper mantle lvz is caused by the melt producing regions of the upper mantle and is a source for the basalts of the auckland volcanic field this is in agreement with models for a shallow upper mantle source rather than a deep seated mantle plume for the auckland volcanism average shear velocities for the upper crust are 3 4 3 6 km s 1 increasing to 3 6 4 0 km s 1 in the lower crust the moho is interpreted to be 29 1 km deep in the southern end of the array shallowing to 26 1 km towards the edge of the continental shelf of northern new zealand low upper mantle shear velocities of 4 2 0 1 km s 1 are observed and are thought to represent a small percentage of partial melt in the upper mantle beneath this back arc region these low velocities extend to greater depth 90 km beneath the southern stations indicating that mantle deformation associated with the mantle wedge to the south may extend as far north as the southern end of our array near surface low velocity layers extend to 6 km depth beneath a station at the northern tip of new zealand likely representing the expression of a thick sequence of sediments from the northland allochthon and the more recent northland basin

the distinction between geomagnetic excursions and reversals summary two recent studies of the geomagnetic eld in the last 1 myr have found 14 excursions large changes in direction lasting 5 10 kyr each six of which are established as global phenomena by correlation between diierent sites the older picture of the geomagnetic eld enjoying long periods of stable polarity may not therefore be correct instead the eld appears to suier many dramatic changes in direction and concomitant reduction in intensity for 10 20 per cent of the time during excursions the eld may reverse in the liquid outer core which has timescales of 500 yr or less but not in the solid inner core where the eld must change by diiusion with a timescale of 3 kyr this timescale is consistent with the remarkably uniform duration of well dated excursions the disparity of dynamical timescales between the inner and outer cores a factor of 10 is consistent with the 10 excursions between full reversals

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

delineation of a large ultramafic massif embedded within a major sw pacific suture using gravity methods abstract gravity studies have delineated the largest ultramafic massif in new zealand embedded within a buried major sw pacific crustal suture zone this suture records terrane collision onto the gondwana margin during the mesozoic and separates a forearc terrane from an outboard accretionary prism terrane it can be traced throughout the length of new zealand as the junction magnetic anomaly and contains the permian dun mountain ophiolite belt which in the south island of new zealand is characterized by a string of isolated ultramafic massifs in a sheared matrix of serpentinite and sediment our analysis reveals a steep gravity gradient at the suture boundary which is attributed to a newly recognised density contrast 0 1 mg m 3 between terranes of the forearc and the accretionary prism the massif itself is marked by the occurrence of a strong elongate residual gravity anomaly 120 g u extending 50 km along the suture and coincident with the junction magnetic anomaly it is modelled at its southern end as a dense 15 km wide source body extending to at least 6 km in depth in conjunction with detailed aeromagnetic data this modeling indicates the presence of a spindle shaped ultramafic massif analogous to but larger than similar bodies found within the dun mountain ophiolite belt elsewhere this fabric of sheared serpentinites enclosing ultramafic massifs therefore extends at least the length of new zealand and probably beyond in part it may result from accretion of asperities in the subducting plate but it is also due to disruption of larger ultramafic bodies during subsequent strike slip motion which caused the remarkable linearity of the dun mountain belt given the common occurrence of the plate tectonic processes involved it is likely that such structures can be found in other regions around the world using similar geophysical potential field methods

lidar based quantification of lava flow susceptibility in the city of auckland new zealand abstract lava flows represent one of the most significant volcanic hazards from basaltic monogenetic volcanoes such as spatter cones scoria cones maars and tuff rings they are common features emanating from parasitic vents on the flanks of polygenetic volcanoes and in dominantly flat lying intraplate volcanic fields the auckland volcanic field avf is a volcanic field that has been active for the last ca 250 ka hosting at least 50 monogenetic volcanoes morphometric parameters of lava flows such as volume length thickness and area were used to quantify the potential lava flow inundation susceptibility to new zealand s most densely populated area the city of auckland based on an airborne light detection and ranging lidar digital surface model dsm the morphometric parameters of fifteen studied flows included average length of 2 5 km range 0 7 6 5 km overall average thickness of 14 8 m range 3 4 43 8 m average of maximum thicknesses of 48 2 m range 18 3 180 5 m average area occupied of 5 1 km 2 range 0 4 25 1 km 2 and average volume of 0 12 km 3 range 0 005 1 km 3 based on these parameters and a lidar derived dsm the present topography was classified into sea topographic depressions low lying areas prone to inundation by an average lava flow buffer zones prone to inundation only by extremely thick lava flows and peaks or ridges which are unlikely to be overtopped in monogenetic fields each new vent occurs in a new location creating uncertainty around the spatial location of the volcanic hazard thus this research provides a general vent location independent approach to describe the lava flow susceptibility for a potentially active monogenetic volcanic field what this analysis reveals is that the city of auckland can be divided into two distinct areas with strongly different susceptibility to lava flow inundation the southern part of the city is predominantly flat without hindrance to lava flow whereas the hilly northern and central part has many ridges that can limit or channelise lavas these contrasting properties must be accounted for in scenario based or probabilistic hazard and risk models developed for the avf

the spatial and temporal cost of volcanic eruptions assessing economic impact business inoperability and spatial distribution of risk in the auckland region new zealand volcanic risk assessment has historically concentrated on quantifying the frequency magnitude and potential diversity of physical processes of eruptions and their consequent impacts on life and property a realistic socio economic assessment of volcanic impact must however take into account dynamic properties of businesses and extend beyond only measuring direct infrastructure property loss the inoperability input output model heralded as one of the 10 most important accomplishments in risk analysis over the last 30 years kujawaski syst eng 9 281 295 2006 has become prominent over the last decade in the economic impact assessment of business disruptions we develop a dynamic inoperability input output model to assess the economic impacts of a hypothetical volcanic event occurring at each of 7270 unique spatial locations throughout the auckland volcanic field new zealand this field of at least 53 volcanoes underlies the country s largest urban area the auckland region which is home to 1 4 million people and responsible for 35 3 nz 201481 2 billion of the nation s gdp statistics new zealand 2015 we apply volcanic event characteristics for a small medium scale volcanic eruption scenario and assess the economic impacts of an average eruption in the auckland region economic losses are quantified both with and without business mitigation and intervention responses in place we combine this information with a recent spatial hazard probability map bebbington and cronin bull volcanol 73 1 55 72 2011 to produce novel spatial economic activity at risk maps our approach demonstrates how business inoperability losses sit alongside potential life and property damage assessment in enhancing our understanding of volcanic risk mitigation

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

new criteria for systematic mapping and reliability assessment of monogenetic volcanic vent alignments and elongate volcanic vents for crustal stress analyses abstract linear arrays of monogenetic volcanic vent alignments represent an important source of stress strain data from volcanic regions of the earth and other planets currently however there is no standard methodology for mapping vent alignments or for assessing alignment reliability which are essential to defining a robust stress datum from vent alignments here we define a systematic method for defining monogenetic vent alignments by mapping the locations and shapes of vents elongation of vents into elliptical shapes is directly related to the trend of subsurface fissures that control alignments elongated vents therefore are critical for defining reliable regional vent alignments and also provide an independent means for assessing stress directions our reliability assessment system a b c d for vent alignments is based on the number of vents the standard deviation of vent center points from a best fit line the numbers and types of elongate vents the standard deviation of long axes of elliptical vents from a best fit line and in cases where alignments lack elongate vents by vent spacing distances we quantified morphometric parameters of well defined fissure fed vent alignments from polygenetic volcanoes and platform volcanic fields to establish appropriate threshold values for reliability assessments in this new method the combined analysis of vent locations and shapes optimizes the process of alignment mapping and the assessment of the reliability of alignments to be incorporated in stress analyses we provide quality ranking schemes for stress data derived from reliability assessed vent alignments and from elongate vents we demonstrate the utility of this new mapping and assessment methodology by analyzing monogenetic cinder cone fields within a platform volcanic field the san francisco volcanic field in arizona and within a volcanic field on the flank of a polygenetic volcano mount morning in antarctica

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

shallow sill and dyke complex in western hungary as a possible feeding system of phreatomagmatic volcanoes in soft rock environment abstract neogene alkaline basaltic rocks in the western pannonian basin are eroded remnants of maars tuff rings tuff cones scoria cones and lava fields the erosion level of these volcanoes is deep enough to expose diatreme zones associated with the phreatomagmatic volcanoes the erosion level is deeper yet in the west exposing shallow dyke and sill swarms related to former intra plate volcanoes the basanitic sills are irregular in shape and their lateral extent is highly variable individual sills reach a thickness of a few tens of metres and they commonly form dome like structures with rosette like radial columnar joint patterns the largest sill system identified in this region is traceable over kilometres and forms a characteristic ridge running north east to south west elevation differences in the position of the basanitic sills within an otherwise undisturbed layer cake like siliciclastic succession indicate emplacement of the basanite magma at multiple levels over kilometre scale distances the margins of sills in the system are irregular at a dm to mm scale undulating contacts of the sills together with gentle thermal alteration in the host sediment over cm to dm distances indicate the soft but not necessarily wet state of the host deposits at the time sills were intruded parts of the sill complex show a complicated relationship with the host sediment in form of peperitic zones and irregularly shaped disrupted peperite textures this is interpreted to reflect inhomogenities in water content and rheology of the siliciclastic deposits during intrusion the current summit of the longest continuous ridge preserves a small diatreme that seems to cut through an otherwise disk like sill indicating of relationship between sill emplacement and phreatomagmatic explosive eruptions

blocky versus fluidal peperite textures developed in volcanic conduits vents and crater lakes of phreatomagmatic volcanoes in mio pliocene volcanic fields of western hungary abstract volcanic fields in the pannonian basin western hungary comprise several mio pliocene volcaniclastic successions that are penetrated by numerous mafic intrusions peperite formed where intrusive and extrusive basaltic magma mingled with tuff lapilli tuff and non volcanic siliciclastic sediments within vent zones peperite is more common in the pannonian basin than generally realised and may be also important in other settings where sediment sequences accumulate during active volcanism hajagos hegy an erosional remnant of a maar volcano was subsequently occupied by a lava lake that interacted with unconsolidated sediments in the maar basin and formed both blocky and globular peperite similar peperite developed in kissomlyo a small tuff ring remnant where dykes invaded lake sediments that formed within a tuff ring lava foot peperite from both hajagos hegy and kissomlyo were formed when small lava flows travelled over wet sediments in craters of phreatomagmatic volcanoes at sag hegy a large phreatomagmatic volcanic complex peperite formed along the margin of a coherent intrusion all peperite in this study could be described as globular or blocky peperite globular and blocky types in the studied fields occur together regardless of the host sediment

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

40ar 39ar geochronology of neogene phreatomagmatic volcanism in the western pannonian basin hungary abstract neogene alkaline basaltic volcanic fields in the western pannonian basin hungary including the bakony balaton highland and the little hungarian plain volcanic fields are the erosional remnants of clusters of small volume possibly monogenetic volcanoes moderately to strongly eroded maars tuff rings scoria cones and associated lava flows span an age range of ca 6 myr as previously determined by the k ar method high resolution 40 ar 39 ar plateau ages on 18 samples have been obtained to determine the age range for the western pannonian basin neogene intracontinental volcanic province the new 40 ar 39 ar age determinations confirm the previously obtained k ar ages in the sense that no systematic biases were found between the two data sets however our study also serves to illustrate the inherent advantages of the 40 ar 39 ar technique greater analytical precision and internal tests for reliability of the obtained results provide more stringent constraints on reconstructions of the magmatic evolution of the volcanic field periods of increased activity with multiple eruptions occurred at ca 7 95 ma 4 10 ma 3 80 ma and 3 00 ma these new results more precisely date remnants of lava lakes or flows that define geomorphological marker horizons for which the age is significant for interpreting the erosion history of the landscape the results also demonstrate that during short periods of more intense activity not only were new centers formed but pre existing centers were rejuvenated

palaeointensities of the auckland geomagnetic excursions by the ltd dht shaw method we report new palaeointensity results concerning the auckland geomagnetic excursions using the double heating technique of the shaw method with low temperature demagnetisation ltd dht shaw method the excursional palaeodirections recorded in six volcanoes of the auckland volcanic field new zealand have been classified into three groups north down nd west up wu and south up su directions in the present study five to six consistent palaeointensities have been obtained from each of five volcanoes recording the auckland geomagnetic excursions the wiri 27 ka crater hill and puketutu volcanoes nd group yielded mean palaeointensities of 10 6 1 2 1 11 8 2 8 and 11 1 0 4t respectively the hampton park volcano 55 ka wu group gave 9 5 1 2t while the mclennan hills volcano su group gave 2 5 0 5t it is notable that consistent palaeointensities have been obtained from the three different volcanoes which have almost the same palaeodirections nd group possibly supporting the reliability of the palaeointensity data these five palaeointensities for the auckland geomagnetic excursions correspond to virtual dipole moments vdms of 0 6 2 1 10 22 am 2 whereas three mean palaeointensities obtained from the auckland volcanoes having non excursional palaeodirections are 13 1 40 0 t giving stronger vdms of 2 1 6 9 10 22 am 2 these results suggest that the dipole component of the geomagnetic field reduced to about 2 10 22 am 2 or less during the auckland geomagnetic excursions

the auckland volcanic field new zealand geophysical evidence for structural and spatio temporal relationships abstract geophysical data from the monogenetic auckland volcanic field reveal complex structural and spatio temporal relationships at different scales the volcanic field is coincident with regional magnetic and gravity anomalies that mark a major crustal suture and with a discontinuity marking a significant structural asperity here the linear regional magnetic anomaly splays into a wide band of nnw trending lineaments arising from serpentinised shear zones in the upper crust that matches the extent of the volcanic field and that may reflect a region of crustal weakness creating preferential permeability however there appears to be no simple correlation between the locations of individual vents and these lineaments that might delineate more shallow structural controls with this orientation probably as a consequence of other structural influences high resolution aeromagnetic data over the volcanic field show that the volcanoes have a wide range of magnetic signatures indicating a variability of subsurface structure scoria cone volcanoes typically have strong anomalies up to several 100 nt whilst tuff ring volcanoes typically have weak anomalies less than 50 nt though the surface geology is not always an indicator of the nature and extent of the subsurface deposits both cone and tuff ring volcanoes in the auckland field appear to be underlain by subsurface bowl shaped bodies of basalt implying that their eruption histories commonly involve lava ponding into early excavated craters the present geophysical data give no evidence for subsurface dyke like structures or for substantial near surface volumes of basaltic rocks where there are no known eruption centres or buried flows aeromagnetic and palaeomagnetic data suggest that a number of adjacent vents with an implied structural linkage may be contemporaneous though other examples occur where vents of clearly different ages exploit the same apparent structure a unique feature of the auckland field is that at least 5 widely separated and structurally unrelated volcanoes are contemporaneous these observations highlight the spatial and temporal heterogeneities that can occur in monogenetic fields and have an important bearing on recurrence rates and temporally linked eruptions which are probably more common than is generally supposed and are key factors in statistically based hazard assessments

distal record of multi sourced tephra in onepoto basin auckland new zealand implications for volcanic chronology frequency and hazards we have documented 80 tephra beds dating from ca 9 5 to 50 ka contained within continuously deposited palaeolake sediments from onepoto basin a volcanic explosion crater in auckland new zealand the known sources for distal 190 km from vent tephra include the rhyolitic taupo volcanic centre 4 and okataina volcanic centre 14 and the andesitic taranaki volcano 40 and tongariro volcanic centre 3 the record provides evidence for four new events between ca 50 and 28 ka mangaone subgroup suggesting okataina was more active than previously known the tephra record also greatly extends the known northern dispersal of other mangaone subgroup tephra ten rhyolitic tephra pre date the rotoehu eruption ca 50 ka and some are chemically dissimilar to post 50 ka rhyolites some of these older tephra were produced by large magnitude events however their source remains uncertain eight tephra from the local basaltic auckland volcanic field avf are also identified interpolation of sedimentation rates allow us to estimate the timing of 12 major explosive eruptions from taranaki volcano in the 27 5 9 5 ka period in addition 28 older events are recognised the tephra are trachytic to rhyolitic in composition all have high k2o contents 3 wt and there are no temporal trends this contrasts with the proximal lava record that shows a trend of increasing k2o with time by combining the onepoto tephra record with that of the previously documented pukaki crater 15 avf basaltic fall events are constrained at 34 6 30 9 29 6 29 6 25 7 25 2 24 2 23 8 19 4 19 4 15 8 and 14 5 ka and three pre 50 ka events this provides some of the best age constraints for the avf and the only reliable data for hazard recurrence calculations the minimum event frequency of both distal and local fall events can be estimated and demonstrates the auckland city region is frequently impacted by ash fall from many volcanoes

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

geomagnetic excursion captured by multiple volcanoes in a monogenetic field 1 five monogenetic volcanoes within the quaternary auckland volcanic field are shown to have recorded a virtually identical but anomalous paleomagnetic direction mean inclination and declination of 61 7 and 351 0 respectively consistent with the capture of a geomagnetic excursion based on documented rates of change of paleomagnetic field direction during excursions this implies that the volcanoes may have all formed within a period of only 50 100 years or less these temporally linked volcanoes are widespread throughout the field and appear not to be structurally related however the general paradigm for the reawakening of monogenetic fields is that only a single new volcano or group of closely spaced vents is created typically at intervals of several hundred years or more therefore the results presented show that for any monogenetic field the impact of renewed eruptive activity may be significantly under estimated especially for potentially affected population centres and the siting of sensitive facilities

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

morphology of a crater filling lava lake margin the peninsula tuff cone tule lake national wildlife refuge california implications for formation of peperite textures abstract a crater filling lava lake basalt at the peninsula tuff cone in northeastern california has features on its base formed by interaction of the basalt with wet hydrovolcanic tuff a spectrum of features including elongated streamlined flute shaped lobes and irregular corrugations are exposed on the base of the lava lake flute shaped lobes range in amplitude from several centimeters to 0 5 m plunge downdip with respect to the subjacent layered tuff toward the center of the crater and are better defined at depths more than 3 m below the lava lake surface these may have been formed by growth of rayleigh taylor instability waves caused by the density contrast between the wet tuff and basalt combined with directed stress from flow of the lava lake radially outward from the crater formation of an insulating vapor film at the wet tuff basalt interface and downward pressure exerted by the lava lake allowed the edge of the lava lake to quench gradually and prevented formation of peperite at greater depths in the lake corrugations exposed on the upper 1 5 m of the lava lake margin increase in size from several millimeters to 10 cm in amplitude toward the top of the lava lake and approximately parallel the surface corrugations may be the remnant of fluidal peperite preserved on the edge of the lava lake where the peperite was eroded or may be deformation features formed within an insulating vapor film due to oscillation of the vapor film these were formed higher in the lava lake where the lava lake basalt was thinner wet tuff deposits confining the lava lake were thinner and temperature differences between the wet tuff and lava were greater if corrugations represent deformation of the lava edge within the vapor film rather than peperite formation they preserve an intermediate process that is involved in the formation of peperite this study offers a unique opportunity to evaluate some of the initial mechanisms involved in the formation of peperite based on features preserved in the lava where the host rock in this case hydrovolcanic tuff has been eroded but the properties of the host rock are known from adjacent exposures

gravimetric study of the shallow basaltic plumbing system of tenerife canary islands abstract recent studies on intraplate volcanic islands reveal that magma ascends sub vertically through these volcanic edifices following well defined zones in the lithospheric mantle and oceanic crust and occasionally suffering lateral propagation along rift zones when it reaches very shallow depths visualizing the internal structure of the shallowest part of such magma plumbing systems is crucial to understanding magma transport and the location of basaltic volcanism in these volcanic environments tenerife canary islands is a very good example of an intraplate volcanic island in which the distribution of basaltic volcanism includes well defined rift systems and wider monogenetic fields where cinder cones form clusters along different tectonic alignments in order to characterize the structure of the uppermost part of the basaltic plumbing system of tenerife we conducted a detailed gravimetric study of the north western central and southern sectors of the island we developed a 3d density contrast model of the subsoil based on the application of a genetic algorithm for the inversion of gravity data the results of our inverse modelling suggest that the observed gravimetric anomalies below the northwestern rift zone are shallow and aligned parallel to this structure this is consistent with a shallow character for the rift system and a lateral migration of magma along it that comes from a deeper zone located towards the east of this rift zone conversely the anomalies located in the south are also shallow but are distributed along more than one alignment and are connected to much deeper sub vertical similar density anomalies these results support the existence of several main channels at the north and south of the island that allow the ascent of deep basaltic magma to shallower zones of tenerife there magma may be either caught by the rifts systems being distributed laterally and occasionally erupting or may continue its ascent to the surface using a secondary system of shallow fractures that dominates the southern part of tenerife the results of our inverse model also offer new insights on others aspects of the internal structure of tenerife such as the distribution of the basaltic shield or that of las canadas caldera

absolute palaeointensity study of the mono lake excursion recorded by new zealand basalts abstract one of the geomagnetic excursions recorded in basalts of the auckland volcanic field new zealand has recently been correlated unequivocally with the mono lake excursion making it the only confirmed record from the southern hemisphere this record is also exceptional in occurring in five separate monogenetic basaltic volcanoes absolute palaeointensity determinations using the microwave technique based on a comprehensive suite of samples of the auckland basalts recording the excursion show that the geomagnetic field in new zealand at the time of the mono lake excursion was reduced to about 14 t i e to 30 of its normal value this result confirms previous estimates based on more limited sampling in addition it provides a comparison of the microwave and ldt dht shaw methods of measuring palaeointensity which give results that are statistically indistinguishable the palaeointensities determined from the five different volcanoes are also indistinguishable though the palaeodirection data suggest that a very small segment of the vgp path may have been recorded this study confirms the reliability of palaeointensity and palaeodirection determinations from these particular new zealand basalts which together with their definitive 40ar 39ar ages establishes this record of the mono lake excursion as one of the best documented consequently there is significant potential in searching for records of this excursion elsewhere in the pacific region for use as a stratigraphic marker in studies of recent volcanism and palaeoclimate reconstructions

detecting and mapping buried buildings with ground penetrating radar at an ancient village in northwestern argentina abstract we describe an archaeo geophysical investigation performed near the palo blanco archaeological site catamarca argentina a large area beyond the northern limit of the site was explored with the ground penetrating radar gpr method in order to detect new buildings the exploration showed signals of mud walls in a sector that was located relatively far from the previously known buildings a detailed survey was performed in this sector and the results showed that the walls belonged to a large dwelling with several rooms the discovery of this dwelling has considerably extended the size of the site showing that the dwellings occupied at least twice the originally assumed area high density gpr surveys were acquired at different parts of the discovered building in order to resolve complex structures interpreted maps of the building were obtained different characteristics of the walls were satisfactory determined in spite of the low contrast of the dielectric constant at their interfaces and the noticeable spatial fluctuations of the signals due to wall collapses systematic excavations confirmed the gpr maps providing further relevant information about the characteristics of the walls and the occupational floor as well as material for radiocarbon analyses these analyses indicated that the discovered building was occupied until about ad 880 this fact extends 180 years the previously known period for the village and also for the settlements in the region formerly ad 200 to ad 700 the performed investigations moreover confirmed different architectural characteristics assumed for palo blanco related to the internal and external layouts interconnection between the rooms without intermediary courtyards seems to indicate a better use of the internal spaces and a more integrated spatial conception in palo blanco than in other villages of the region

decreasing magmatic footprints of individual volcanoes in a waning basaltic field the distribution and characteristics of individual basaltic volcanoes in the waning southwestern nevada volcanic field provide insight into the changing physical nature of magmatism and the controls on volcano location during pliocene pleistocene times the volumes of individual volcanoes have decreased by more than one order of magnitude as have fissure lengths and inferred lava effusion rates eruptions evolved from hawaiian style eruptions with extensive lavas to eruptions characterized by small pulses of lava and strombolian to violent strombolian mechanisms these trends indicate progressively decreasing partial melting and length scales or magmatic footprints of mantle source zones for individual volcanoes the location of each volcano is determined by the location of its magmatic footprint at depth and only by shallow structural and topographic features that are within that footprint the locations of future volcanoes in a waning system are less likely to be determined by large scale topography or structures than were older larger volume volcanoes

elliptical boundary of an intraplate volcanic field auckland new zealand abstract the outer vents in the auckland volcanic field lie within 19 to 559 m mean 224 75 m of a best fit ellipse with a 28 9 km long major axis trending almost north south and a minor axis 16 5 km long the ellipse has formed the outer boundary of the field since the inception of volcanism 140 000 years ago we present the following testable hypotheses as an explanation of this pattern the boundary is the expression of a corresponding elliptical source area at depth in the lithospheric mantle possibly asthenosphere material trapped at this level the ellipse may represent a depth contour on a very small upper mantle dome or of a lens intruding into a neck of an extensional structure alternatively it could be the boundary of a flat elliptical area where tensional stresses allow decompressional melting the elliptical tensional region may either have developed in a releasing bend during strike slip faulting along a fundamental lithosphere structure inherited from mesozoic tectonics associated with the nnw trending dun mountain ophiolite belt or may represent the tip of a fracture along which the auckland volcanic province is propagating northward

evolution and hydrological conditions of a maar volcano atexcac crater eastern mexico abstract the atexcac maar is located in the central part of the serdan oriental lacustrine playa basin in the eastern mexican volcanic belt it is part of a dispersed and isolated monogenetic field consisting of maar volcanoes basaltic cinder cones and rhyolitic domes atexac is a maar volcano excavated into pyroclastic deposits basaltic lava flows and the flanks of a cinder cone cluster which itself was built on a topographic high consisting of limestone it has an ene trending elliptical shape with beds mostly unconsolidated deposits that dip outward at 16 22 the atexcac crater was formed from vigorous phreatomagmatic explosions in which fluctuations in the availability of external water temporal migration of the locus of the explosion and periodic injection of new magma were important controls on the evolution of the maar crater variations in grain sizes and component proportions of correlated deposits from the different sections suggest a migration of the locus of explosions producing different eruptive conditions with fluctuating water magma interactions deposits rich in large intrusive and limestone blocks are associated with a matrix enriched in small andesitic lapilli this could suggest differential degrees of fragmentation due to inherited previously acquired fragmentation and or relative distance to the locus of explosions initial short lived phreatic explosions started at the southwest part of the crater and were followed by an ephemeral vertical column and the influx of external water that led to relatively shallow explosive interactions with the ascending basaltic magma drier explosions progressed downward and or laterally northward sampling subsurface rock types particularly intrusive limestone and andesitic zones as well as localized altered zones n ne caused by repetitive injection of basaltic magma a final explosive phase involved a new injection of magma and a new influx of external water producing wetter conditions at the end of the maar formation we infer the aquifer was formed by fractured rocks predominantly andesitic lava flows and limestone rocks andesitic accessory clasts dominate in all stratigraphic levels but these rocks are not exposed in the nearby area these local hydrogeological conditions contrast with those at nearby maar volcanoes where the water for the magma water interactions apparently mostly came from a dominantly unconsolidated tuffaceous aquifer producing tuff rings with a much lower profile than atexcac

subsurface structure of a maar diatreme and associated tuff ring from a high resolution geophysical survey rattlesnake crater arizona abstract geophysical survey techniques including gravity magnetics and ground penetrating radar were utilized to study the diatreme and tuff ring at rattlesnake crater a maar in the san francisco volcanic field of northern arizona significant magnetic anomalies 1600 nt and a positive gravity anomaly 1 4 mgal are associated with the maar joint modeling of magnetic and gravity data indicate that the diatreme that underlies rattlesnake crater has volume of 0 8 1 km 3 and extends to at least 800 m depth the modeled diatreme comprises at least two zones of variable density and magnetization including a low density highly magnetized unit near the center of the diatreme interpreted to be a pyroclastic unit emplaced at sufficiently high temperature and containing sufficient juvenile fraction to acquire thermal remanent magnetization magnetic anomalies and ground penetrating radar gpr imaging demonstrate that the bedded pyroclastic deposits of the tuff ring also carry high magnetization likely produced by energetic emplacement of hot pyroclastic density currents gpr profiles on the tuff ring reveal long 100 m wavelength undulations in bedding planes elsewhere comparable bedforms have been interpreted as base surge deposits inflated by air entrainment from eruption column collapse interpretation of these geophysical data suggests that rattlesnake crater produced highly energetic phreatomagmatic activity that gave way to less explosive activity as the eruption progressed the positive gravity anomaly associated with the maar crater is interpreted to be caused by coherent bodies within the diatreme and possibly lava ponding on the crater floor these dense magnetized bodies have excess mass of 2 4 10 10 kg and occupy approximately 5 of the diatreme by volume magnetic anomalies on the crater floor are elongate nw se suggesting that the eruption may have been triggered by the interaction of ascending magma with water in fractures of this orientation gpr imaging of the tuff ring also suggests that substantial land slip may have occurred on the western rim perhaps causing part of the tuff ring to collapse into the crater strong radar reflections indicative of well developed weathering horizons are present as well the techniques employed at rattlesnake crater demonstrate the value of combining multiple geophysical techniques in areas where exposures are limited and invasive exploration is not an option

sub surface dynamics and eruptive styles of maars in the colli albani volcanic district central italy protezione civile italy 2004 2006 agreement with istituto nazionale di geofisica e vulcanologia ingv project v3 1

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

aeromagnetic imaging of the dun mountain ophiolite belt in northern new zealand insight into the fine structure of a major sw pacific terrane suture a new aeromagnetic survey of the junction magnetic anomaly in northern new zealand reveals that it is associated with up to 10 subparallel lineaments the junction magnetic anomaly delineates the dun mountain ophiolite belt a remarkably linear though sigmoidally bent major boundary zone traceable through most of new zealand which separates terranes sutured onto the new zealand gondwana margin in late mesozoic time modelling of the lineaments in conjunction with a new review of structures along the dun mountain ophiolite belt throughout new zealand indicates that the dun mountain ophiolite belt in auckland consists of a number of imbricate slices most of which are likely to dip steeply to the east i e overturned away from the former trench and extend to considerable depth in the crust the source bodies for the central high amplitude magnetic lineaments are interpreted to be serpentinite shear or melange zones cutting a large lensoid body of ultramafic rocks similar to those occurring elsewhere along the length of the dun mountain ophiolite belt our data have revealed that a formerly postulated offset of the dun mountain ophiolite belt in the region instead resembles a duplex or thrust imbricate like bundling of shear zones these shear zones were probably initiated as accretion thrusts but complexities in their pattern indicate subsequent shearing including backfolding to an overturned attitude and possibly strike slip to produce the linear nature of the dun mountain ophiolite belt the outer lineaments mostly mark accretion thrusts in the adjacent terranes this study demonstrates that detailed aeromagnetic surveys can delineate the fine structure in ophiolite complexes and thereby give valuable insights into the effects of micro continental collision on an accretionary margin

eruptive styles and inferences about plumbing systems at hidden cone and little black peak scoria cone volcanoes nevada u s a we describe two small scoria cone volcanoes hidden cone and little black peak ages between 320 390 ka in the southwestern nevada volcanic field and discuss their eruption mechanisms and inferences about their plumbing systems cone forming pyroclastic deposits are consistent with eruptive styles ranging from strombolian to violent strombolian and lavas emanated from near the bases of the cones the volcanoes are monogenetic rather than polycyclic as allowed by previous geomorphic interpretations vents at each volcano appear to coincide with pre existing normal faults consistent with observations at older deeply eroded volcanoes in the region the existence of these two volcanoes on a topographically high area particularly hidden cone provides evidence for short feeder dike lengths 500 m at the surface we infer that this short length reflects the small length scale of the mantle source region that was tapped to feed each volcano

shallow plumbing systems for small volume basaltic volcanoes eruptive dynamics in basaltic volcanoes are controlled in part by the conduit geometry however uncertainties in conduit shape and dike to conduit transition geometry have limited our predictive capability for hazards assessments we characterize the subvolcanic geometry of small volume basaltic volcanoes magmatic volatile driven eruptions 0 1 to 0 5 km3 based on a synthesis of field studies of five basaltic volcanoes exposed to varying degrees by erosion and exhibiting feeder dikes conduits and vent areas 250 m depth study areas include east grants ridge new mexico usa basalt ridge east basalt ridge paiute ridge and southeast crater flat nevada usa basaltic feeder dikes 250 to 100 m deep have typical widths of 4 12 m with smooth host rock contacts rhyolite tuff at depths less than 100 m heterogeneities in the host rock form preferential pathways for small dike splays and sills resulting in a 30 m effective width at 50 m depth the development of a complex conduit at depths less than 70 m is reflected in bifurcating dikes and brecciation and incorporation of the country rock the overall zone of effect at depths less than 50 m is 110 m wide 220 m elongated along the feeder dike based on comparisons with theoretical conduit flow models the width of the feeder dike at depths from 250 to 500 m is expected to range from 1 to 10 m and is expected to decrease to about 1 2 m at depths greater than 500 m the flaring shape of the observed feeder systems is similar to results of theoretical modeling using lithostatic pressure balanced flow conditions sizes of observed conduits differ from modeled dimensions by up to a factor of 10 in the shallow subsurface 50 m depth but at depths greater than 100 m the difference is a factor of 2 to 4 this difference is primarily due to the fact that observed eroded conduits record the superimposed effects of multiple eruptive events while theoretical model results define dimensions necessary for a single steady eruption phase the complex details of magma host rock interactions observed at the study areas contact welding brecciation bifurcating dikes and sills and stoping represent the mechanisms by which the lithostatic pressure balanced geometry is attained the similarity in the normalized shapes of theoretical and observed conduits demonstrates the appropriateness of the pressure balanced modeling approach consistent with the conclusions of wilson and head j geophys res 86 2971 3001 1981 for this type of volcano

eruption recurrence rates in a basaltic volcanic field based on tephra layers in maar sediments implications for hazards in the auckland volcanic field long term eruption recurrence rates in monogenetic basaltic volcanic fields are difficult to assess because of low eruption frequencies but they are important because of the spread of human infrastructure into such fields auckland city new zealand is built on the auckland volcanic field a young 250 ka basaltic volcanic field in the absence of abundant material for radiocarbon and isotopic dating an eruptive chronology based on basalt tephra layers deposited in maar lakes was developed interbedded well dated tephra layers from silicic volcanoes some 220 270 km to the south were used as age constraints the basalt tephra layers reveal a pattern of activity not evident from the temporal spatial distribution of volcanic landforms twenty four basalt tephra layers over the last 80 k y represent an average frequency of one per 3 5 k y recurrence times vary from 0 5 k y to 20 k y and show no temporal trend the tephra record shows a major flare up in explosive activity at 32 2 ka during heightened activity at 34 24 ka this was related to a period of simultaneous eruptions from several volcanoes across the field revealed by paleomagnetic and isotopic ages in contrast the field has been relative quiet during the last 20 k y punctuated by the construction of a shield volcano at 0 7 ka thus the surface manifestation of magmatism varies greatly with time complicating long term volcanic hazard forecasting despite the high risk from future basalt activity because of proximity ash fall from distant andesite volcanoes is the most frequently occurring volcanic hazard in auckland city

spatial distribution and alignments of volcanic centers clues to the formation of monogenetic volcanic fields monogenetic basaltic volcanic fields occur worldwide in tectonic environments ranging from extensional to convergent understanding similarities and differences between these fields may help to characterize key controls on their generation such volcanic fields consist of numerous volcanic centers each of which represents a pathway of magma from its source to the surface we analyzed the spatial distribution of volcanic centers in 37 monogenetic volcanic fields and assuming that the distribution of volcanic centers relative to each other is matched by a similar source pattern within the mantle applied the following methods for each 1 the poisson nearest neighbor pnn analysis representing the degree to which the distribution of the volcanic centers departs from a predicted poisson distribution and 2 a volcanic alignment analysis to ascertain the preferential pathways if any used by the magma to reach the surface this is the first comprehensive global comparison of such analyses magma pathways within the brittle upper crust are influenced to various degrees by two end member situations 1 formation of new extension fractures perpendicular to the least compressive stress 3 and 2 re activation of pre existing fractures that are near parallel to the maximum principal stress 1 the results of the pnn analysis show that independently of the tectonic environment most volcanic fields display a clustered distribution of their volcanic centers alignment analysis shows that either the ambient tectonic environment exerts a strong influence on the preferential orientations of the volcanic alignments or that it is in competition with other factors e g pre existing structures local stress changes due to older intrusions overall these results indicate that the propagation of the magma and therefore the spatial distribution of the volcanic centers within volcanic fields is the product of an interplay between deep level influences i e nature of the magma source and shallow level influences i e stress field pre existing crustal structures of the lithosphere

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

dyke emplacement in a layered and faulted rift zone abstract in rift zone eruptions magma is normally transported to the surface through dykes most injected dykes however do not make it to the surface but rather become arrested at certain depths in the rift zone surface deformation studies are routinely used to infer the depths to the tips as well as the geometries of arrested dykes most such studies model the rift zone as a homogeneous isotropic elastic half space all rift zones however contain normal faults and are composed of rock layers with different mechanical properties the numerical models presented here indicate that when the upper tip of a vertically propagating dyke reaches the same crustal level as the bottom tips of the boundary faults of an active graben the faults close as the tip continues its propagation up into the graben the overpressured dyke encourages reverse slip on the boundary faults the present numerical models also indicate that mechanical layering has strong effects on dyke emplacement and the surface deformation associated with arrested dykes in rift zones in particular soft low young s modulus layers and weak contacts at shallow depths contribute to offset arrest and aperture thickness variations of the dykes and suppress the dyke induced surface stresses and associated brittle deformation when the magmatic overpressure of a dyke is the only loading the dyke tends to be thicker inside soft layers than inside stiff high young s modulus layers soft layers and weak contacts discontinuities common at shallow depths in rift zones suppress dyke tip tensile stresses thereby encouraging dyke arrest and may result in little if any surface stresses and brittle deformation being induced by an arrested dyke weak contacts also encourage transfer of the dyke induced surface tensile stresses to the regions above the lateral ends of the contacts then the tensile stresses at the rift zone surface in a large area above the dyke itself become small or negligible all the tensile stress occur above the lateral ends of the contact which may be many kilometres from the dyke tip for a dyke arrested in a layered rift zone straightforward inversion of surface deformation data may therefore yield geometric results that have little relation to the actual geometry of the arrested dyke the results of the numerical models presented here are generally supported by field observations of dykes in rift zones