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

electrical resistivity tomography revealing the internal structure of monogenetic volcanoes 1 eruptive activity of individual monogenetic volcanoes usually lasts a few days or weeks however their short lifetime does not always mean that their dynamics and structure are simple monogenetic cones construction is rarely witnessed from the beginning to the end and conditions for observing their internal structure are hardly reached we provide high resolution electrical resistivity sections 10 m electrode spacing of three monogenetic cones from northeastern spain comparing our results to geological observations to interpret their underground continuation the 100 m maximum depth of exploration provides information on almost the entire edifices highlighting the relationships between strombolian and hydromagmatic deposits in two multiphase edifices a main observation is a column of distinct resistivity centered on the puig d adri volcano which we interpret as the eruptive conduit this method can provide valuable information on the past volcanic dynamics of monogenetic volcanic fields which has real implications for the forecast of future activity

terrestrial heat flow in cuba abstract the results of two joint expeditions to cuba to measure terrestrial heat flow in 1983 and 1986 are summarized twenty three new values are presented which confirm the low heat flow in practically all of the island the mean and standard deviation 44 7 13 4 mw m 2 agree well with previous observations

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

discovery of the first quaternary maar in the bohemian massif central europe based on combined geophysical and geological surveys abstract based on results of previous investigations of tephra tuff volcaniclastic deposits and a geophysical survey in the surroundings of the elezna h rka quaternary volcano west bohemia we performed detailed geophysical surveys using gravimetry magnetometry and electrical conductivity techniques striking anomalies were revealed in a morphological depression near m tina west bohemia as a strong evidence of the assumed maar diatreme structure the sharp isometric gravity low of 2 30 mgal as well as the corresponding positive magnetic anomaly of 200 nt with a negative rim on its northern side indicate a steeply dipping geological body of low density and containing magnetic rocks minerals magnetic survey also showed pronounced local anomalies outside the depression that can reflect relicts of the tephra rim of the maar this geophysical evidence was then proven by an exploratory drilling near the centre of the gravity anomaly macroscopic on site evaluation of the core and more detailed sedimentological petrochemical palynological and microbiological laboratory analyses further confirmed the existence of a maar structure filled by 84 m of lake sediments reflecting a succession of several warm and cold climatic periods results of palynological analyses confirm the presence of a continuous palaeoclimate archive with at least three successive warmer periods of most probably interstadial character from the upper quaternary saalian complex therefore the recovered sediment sequence holds strong potential for in depth paleoclimate reconstruction and deep biosphere studies at the bottom of the m tina 1 my 1 borehole 84 85 5 m country rock debris was found containing also volcanic bombs and lapilli the discovered volcanic structure is considered to be the first known quaternary maar diatreme volcano on the territory of the bohemian massif because of hidden active magmatic processes in combination with earthquake swarm seismicity ca 20 30 km north of the m tina maar reconstruction of the palaeovolcanological evolution is important for evaluation of hazard potential of the ne and e part of the cheb basin

tectonically controlled vent migration during maar diatreme formation an example from a miocene half graben basin in se korea abstract understanding the roles of external controls such as hydrology and tectonics is crucial for interpreting the evolution of a phreatomagmatic volcano and its subsurface structure called the diatreme the yangpori diatreme in a miocene terrestrial half graben basin of korea provides an opportunity to assess the roles of synvolcanic fault movement in forming a maar diatreme volcano the exposed part of the diatreme fill consists of two offset stacked sequences with contrasting lithofacies characteristics a lower sequence early diatreme fill of locally stratified or bedded basaltic lapilli tuff and tuff in the northeastern part and an upper sequence later diatreme fill of dacite clast rich and disorganized tuff breccia in the southwestern part the boundary between these sequences is abrupt suggesting eruption of the volcano in two distinct phases the first phase of eruption was driven by phreatomagmatic explosions mainly within the basin fill some of the diatreme fill was deposited in a local pond by debris flows and suspension settling of fines whereas the rest by fallout from eruption plumes and jets the second phase of eruption was driven by phreatomagmatic explosions at a fracture aquifer within the basement accompanying sudden lowering of the explosion locus the later diatreme fill therefore contains abundant basement derived dacite clasts its deposition was accomplished by en masse fallout of volcanic debris from dense collapsing columns a number of faults and shear fractures were generated within and in the vicinity of the diatreme together with penecontemporaneous basaltic intrusions structural and geochemical analyses show that many of these structures are genetically related with the formation of the diatreme the early diatreme fill was also tilted toward the southwest suggesting southwestward shift of the locus of explosion and diatreme excavation the explosion locus of the yangpori volcano is therefore interpreted to have migrated both vertically and laterally the abrupt lithofacies boundary between the early and later diatreme fills and the proximity of the diatreme to the basin margin suggests that the migration was caused by an abrupt tectonic activity near the basin margin most probably the feeder dike was dislocated by movement along the basin margin fault which created abundant fractures deep within the basement that redirected ascending magma and opened a new hydraulically active zone in the basement synvolcanic faulting is therefore interpreted to have played a crucial role in forming the offset stacked architecture of the yangpori diatreme sequences the faulting is also inferred to have influenced significantly the morphology and internal facies architecture of unpreserved ejecta rimbeds

eruptive style of the young high mg basaltic andesite pelagatos scoria cone southeast of mexico city the eruption of the pelagatos scoria cone in the sierra chichinautzin monogenetic field near the southern suburbs of mexico city occurred less than 14 000 years ago the eruption initiated at a fissure with an effusive phase that formed a 7 km long lava flow and continued with a phase of alternating and or simultaneous explosive and effusive activity that built a 50 m high scoria cone on the western end of the fissure and formed a compound lava flow field near the vent the eruption ended with the emplacement of a short lava flow that breached the cone and was accompanied by weak explosions at the crater products consist of a microlite rich high mg basaltic andesite samples were analyzed to determine the magma s initial properties as well as the effects of degassing induced crystallization on eruptive style although distal ash fallout deposits from this eruption are not preserved a recent quarry exposes a large section of the scoria cone detailed study of exposed layers allows us to elucidate the mode of cone building activity petrological and textural data combined with models calibrated by experimental work and melt inclusion analyses of similar magmas elsewhere indicate that the magma was initially hot 1 200 c gas rich up to 5 wt h2o crystal poor 10 vol fo90 olivine phenocrysts and thus poorly viscous 40 80 pa s during the early phase low magma ascent velocity at the fissure vent allowed low viscosity magma to degas and crystallize during ascent producing lava flows with elevated crystal contents at t 1 100 c and blocky surfaces later the closure of the fissure by cooling dikes focused the magma flow at a narrow section of the fissure this led to an increased magma ascent velocity rapid and shallow degassing 3 km deep triggered 40 vol microlite crystallization limited times for gas escape and higher magma viscosity 6 105 4 106 pa s drove strong explosions of highly 60 80 vol and finely vesicular magma coarse clasts broke on landing which implies brittle behavior due to complete solidification this requires sufficient time to cool and in turn implies ejection heights of over 1 km which is much higher than normal strombolian activity hence magma viscosity significantly impacts eruption style at monogenetic volcanoes because it affects the kinetics of shallow degassing the long lasting eruptions of jorullo and paricutin which produced similar magmas in western mexico were more explosive this can be related to higher magma fluxes and total erupted volumes implications of this study are important because basaltic andesites are commonly erupted to form monogenetic scoria cones of the trans mexican volcanic belt

late quaternary paleolimnology of onepoto maar auckland new zealand implications for the drivers of regional paleoclimate abstract a high resolution record of lacustrine environments spanning ca 30 9 cal ka bp was obtained from onepoto maar northern north island new zealand the multi proxy record of environmental change is constrained by tephrochronology and ams 14 c ages and provides evidence for episodes of rapid environmental change during the last glacial coldest period lgcp 28 5 18 cal ka bp and late glacial interglacial transition lgit from northern new zealand the onepoto pollen record indicates that the lgcp was cold dry and windy in the auckland region with vegetation dominated by herbs and grasses in a beech forest mosaic at the same time the diatom record indicates oligotrophic conditions with low lakes levels and turbulence whilst cladocerans indicate low water temperatures the inference of cold dry and windy conditions during the lgcp is supported by geochemical evidence for increased sediment influx charcoal and co 2 limiting conditions for terrestrial macrophytes rapid climate amelioration and forest expansion after ca 18 cal ka bp corresponds with reduced sediment influx diatom and cladoceran inferred higher lake levels indicating increasing moisture availability and temperature diatom flora indicates that an oligotrophic circumneutral lake was becoming established during the lgit but conditions were still variable between ca 13 8 and 12 5 cal ka bp two brief drier and possibly cooler episodes are apparent ca 13 8 13 2 and 13 12 5 cal ka bp followed by a warm phase combined with generally stable high lake levels between ca 12 5 and 10 5 cal ka bp subsequently the lake water chemistry became more alkaline and eutrophic possibly because sea level had risen high enough by this time to influence ground water chemistry

a study of clast size distribution ash deposition and fragmentation in a hawaiian style volcanic eruption ash deposits from hawaiian style eruptions are generally of small volume but are important in understanding the mechanisms of these eruptions an analysis of the ash deposit from the 1959 kilauea iki eruption of kilauea volcano is presented and the whole deposit grain size distribution for the eruption is calculated the grain size distribution is found to be heavily skewed towards the coarsest clast sizes and to differ considerably from size distributions for plinian deposits

gis methods applied to the degradation of monogenetic volcanic fields a case study of the holocene volcanism of gran canaria canary islands spain abstract modeling of volcanic morphometry provides reliable measurements of parameters that assist in the determination of volcanic landform degradation variations of the original morphology enable the understanding of patterns affecting erosion and their development facilitating the assessment of associated hazards a total of 24 volcanic holocene eruptions were identified in the island of gran canaria canary islands spain 87 of these eruptions occurred in a wet environment while the rest happened in a dry environment 45 of holocene eruptions are located along short barrancos s type less than 10 km in length 20 along large barrancos l type 10 17 km in length and 35 along extra large barrancos xl type more than 17 km in length the erosional history of holocene volcanic edifices is in the first stage of degradation with a geomorphic signature characterized by a fresh young cone with a sharp profile and a pristine lava flow after intensive field work a careful palaeo geomorphological reconstruction of the 24 holocene eruptions of gran canaria was conducted in order to obtain the digital terrain models dtms of the pre and post eruption terrains from the difference between these dtms the degradation volume and the incision rate were obtained the denudation of volcanic cones and lava flows is relatively independent both their geographical location and the climatic environment however local factors such as pre eruption topography and ravine type have the greatest influence on the erosion of holocene volcanic materials in gran canaria although age is a key factor to help understand the morphological evolution of monogenetic volcanic fields the gran canaria holocene volcanism presented in this paper demonstrates that local and regional factors may determine the lack of correlation between morphometric parameters and age consequently the degree of transformation of the volcanic edifices evolves in many cases independently of their age

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

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

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

extracting phreatic evaporation from remotely sensed maps of evapotranspiration 1 one of the most important parameters related to soil salinization is the direct evaporation from the groundwater phreatic evaporation if the groundwater table is sufficiently close to the surface groundwater will evaporate through capillary rise in recent years several methods have been suggested to map evapotranspiration et on the basis of remote sensing images these maps represent the sum of both transpiration of vegetation and evaporation from the bare soil however identifying the amount of phreatic evaporation is important as it is the dominant flux in the salt balance of the soil the interpretation of stable isotope profiles at nonirrigated areas in the unsaturated zone allows one to quantify phreatic evaporation independently of the transpiration of the vegetation such measurements were carried out at different locations with a different depth to groundwater the benefit is twofold 1 a relation between phreatic evaporation rates and the depth to groundwater can be established 2 by subtracting the measured values of phreatic evaporation from remotely sensed values of et vadose et consisting of transpiration and excess irrigation water in the unsaturated zone can be estimated at the sampling locations a correlation between the normalized differential vegetation index and the calculated vadose et rates could be established r2 0 89 with this correlation the contribution of phreatic evaporation can be estimated this approach has been tested for the yanqi basin located in western china finally the distribution of phreatic evaporation was compared to a soil salinity map of the project area on a qualitative basis

high resolution temperature measurements in the borehole yaxcopoil 1 mexico within the frame of the international continental deep drilling program icdp and as a part of the chicxulub scientific drilling project csdp high resolution temperature measurements were performed in the borehole yaxcopoil 1 yax 1 the temperature was logged to the depth of 858 m seven times between march 6 19 2002 starting 10 days after the hole was shut in and mud circulation ceased successive logs revealed only small temperature variations in time and space indicating a fast temperature recovery to almost undisturbed conditions prior to the first log from these logs a mean temperature gradient of 37 mk m was determined below the uppermost 250 m another temperature log was recorded on may 24 2003 15 months after the shut in to a depth of 895 m the obtained temperature profile is very similar to the 2002 profile with an insignificantly higher mean gradient below 250 m that may indicate a long term return to the pre drilling temperature the temperature in the uppermost part of the hole bears signs of considerable influence of a convective contribution to the vertical thermal heat transfer the depth extent of the convection seems to have deepened from 150 m in march 2002 to 230 m in may 2003 based on the observed temperature gradient and the rock types encountered in the borehole above 670 m the conducted heat flow is expected to be in the range 65 80 mw m2

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

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

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

reconstruction of the volcanic history of the tacambaro puruaran area michoacan mexico reveals high frequency of holocene monogenetic eruptions the 690 km2 tacambaro puruaran area located at the arc front part of the michoacan guanajuato volcanic field in the trans mexican volcanic belt tmvb records a protracted history of volcanism that culminated with intense monogenetic activity in the holocene geologic mapping 40ar 39ar and 14c radiometric dating and whole rock chemical analyses of volcanic products provide insights to that history eocene volcanics 55 40 ma exposed at uplifted blocks are related to a magmatic arc that preceded the tmvb early tmvb products are represented by poorly exposed pliocene silicic domes 5 2 ma quaternary 30 km may be related to high rates of magma production at depth and a favorable tectonic setting

patterns of intraplate volcanism controlled by asthenospheric shear volcanism observed far from plate boundaries in the interior of oceanic and continental plates may result from flow in the underlying mantle comparison between a numerical model of mantle flow and the spatial distribution of intraplate volcanism indicates that rapid shear motion in the mantle may drive melting that causes intraplate eruptions most of earth s volcanism occurs at plate boundaries in association with subduction or rifting a few high volume volcanic fields are observed both at plate boundaries and within plates fed by plumes upwelling from the deep mantle1 the remaining volcanism is observed away from plate boundaries it is typically basaltic effusive and low volume occurring within continental interiors2 3 4 5 6 7 or creating seamounts on the ocean floor8 9 10 11 this intraplate volcanism has been attributed to various localized processes12 such as cracking of the lithosphere8 13 14 small scale convection in the mantle beneath the lithosphere15 16 17 or shear induced melting of low viscosity pockets of asthenospheric mantle that have become embedded along the base of the lithosphere18 here we compare the locations of observed intraplate volcanism with global patterns of mantle flow from a numerical model we find a correlation between recent continental and oceanic intraplate volcanism and areas of the asthenosphere that are experiencing rapid shear due to mantle convection we detect particularly high correlations in the interior of the continents of western north america eastern australia southern europe and antarctica as well as west of the east pacific rise in the pacific ocean we conclude that intraplate volcanism associated with mantle convection is best explained by melting caused by shear flow within the asthenosphere whereas other localized processes are less important

textural studies of vesicles in volcanic rocks an integrated methodology abstract vesicles in volcanic rocks are frozen records of degassing processes in magmas for this reason their sizes spatial arrangements numbers and shapes can be linked to physical processes that drive magma ascent and eruption although numerous techniques have been derived to describe vesicle textures there is no standard approach for collecting analyzing and interpreting vesicular samples here we describe a methodology for techniques that encompass the entire data acquisition process from sample collection to quantitative analysis of vesicle size and number carefully chosen samples from the lower mean and higher density vesicularity endmembers are characterized using image nesting strategies we show that the texture of even microvesicular samples can be fully described using less than 20 images acquired at several magnifications to cover efficiently the range of existing vesicle sizes a new program foams was designed to perform the quantification stage from vesicle measurement to distribution plots altogether this approach allows substantial reduction of image acquisition and processing time while preserving enough user control to ensure the validity of obtained results we present three cameo investigations on basaltic lava flows scoria deposits and pumice layers to show that this methodology can be used to quantify a wide range of vesicle textures which preserve information on a wide range of eruptive conditions

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

geomorphological reconstruction and morphometric modelling applied to past volcanism accurate and precise morphometric modelling can be extended to past eruptions after a careful palaeogeomorphological reconstruction of volcanic landforms in an oceanic island this reconstruction was used to derive the pre post eruption and present day digital elevation models dem the correct pixel size interpolation method and quality of these dems are discussed the process in a geographical information system framework of the geological information of cone lava flow and tephra fall deposits together and the aforementioned dems allowed the determination of main morphological features of these volcanic landforms and their derivatives the calibration and validation of morphometric modelling were performed on simulated volcanic landforms and verified by a case study a typical holocene monogenetic basaltic eruption at gran canaria island spain

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

detailed investigation of preserved maar structures by combined geophysical surveys the detection of completely preserved maar structures is important not only for underground mapping but also for paleoclimate research because laminated maar lake sediments may contain a very detailed archive of climate history objective evidence for the existence of such structures can only be provided by geophysics and boreholes the combination of gravity and magnetic ground surveys appears to be an excellent tool to detect and identify buried maar structures their prominent properties are an almost circular gravity minimum corresponding to a crater filled with limnic sediments of low density and a magnetic anomaly caused by a pyroclastic or basaltic body in the diatreme which indicates the volcanic character seismic measurements provide the most detailed information about the internal structure of the maar sediments zones of low seismic reflectivity and very low density represent sediments of the late maar lake period the early lake period is indicated by debris flow deposits and turbidites represented by seismic reflectors the seismic sections clearly reveal the bowl like structure of the maar outside this bowl like structure there are only a few reflections which represent the basement taking into account the shape of the gravity anomaly seismic information allows geometrical modelling of the maar structure optimal drilling sites can be selected based on the results of geophysical surveying comparing the results of combined geophysical surveys above two maar structures of different ages yields a marked similarity in their geophysical pattern

a note on maar eruption energetics current models and their application hydromagmatic eruptions convert thermal into mechanical energy via the expansion of ground and or surface water several models address the energetics of these eruptions based on different physical volcanological approaches here we test different models with two case studies in the colli albani volcanic district central italy the monogenetic prata porci and the polygenetic albano maars test results are mutually consistent and show cumulative mechanical energy releases on the order of 1015 1017 j for single maars the fraction of thermal energy turned into mechanical ranges from 0 45 as calculated from the theoretical maximum mechanical energy through 0 1 calculated from country rock fragmentation crater formation and ballistic ejection to 0 03 derived from magma fragmentation by thermohydraulic explosions it appears that the energy released during the most intense hydromagmatic events may account for a dominant fraction of the total mechanical energy released during the whole maar eruptive histories finally we consider the role of magmatic explosive activity intervening during maar eruptions in causing departures from predictions of the models evaluated

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

first heat flow density assessments in cuba abstract the first determinations of heat flow density in cuba are reported precise temperature loggings were carried out in 12 holes in the western and central parts of cuba along the northwestern shore the mean temperature gradient ranges from 14 16 mk m in the pinar del rio province and 18 22 mk m east of habana to 30 mk m in northern matanzas in the central basin it ranges from 23 24 mk m west of ciego de avila to 28 39 mk m east of sancti spiritus rock samples for laboratory determination of thermal conductivity could be collected only from two holes in pinar del rio their mean conductivity amounts to 4 1 w m 1 k 1 from other holes no core samples were available and characteristic rocks were collected from surface outcrops in the vicinity of each hole the measured conductivity ranges from 0 8 to 3 0 w m 1 k 1 heat flow density assessments revealed very low heat flow near habana and south of varadero 30 40 mw m 2 and relatively higher but still rather subnormal values in pinar del rio 60 mw m 2 and in the central basin 50 65 mw m 2 the generally low heat flow density found in western and central parts of the island thus seems to agree well with the results of marine observations in surrounding areas reported by other authors

longevity of a small shield volcano revealed by crypto tephra studies rangitoto volcano new zealand change in eruptive behavior of a basaltic field abstract the life span of small volcanoes in terrestrial basaltic fields commonly considered monogenetic can be difficult to assess due to a paucity of datable materials capable of providing a 102 103 year age resolution we have used microscopic tephra layers crypto tephra in lake sediments to determine the longevity of rangitoto volcano a small shield that represents the most recent volcanism in the auckland volcanic field avf new zealand previous studies suggested construction in a relatively short interval at 550 500 cal yrs bp in contrast the tephra record shows evidence of intermittent activity from 1498 140 to at least 504 6 cal yrs bp a longevity of 1000 years rangitoto volcano is thought to represent about half the magma erupted in the 250 ka history of avf thus the avf has experienced a dramatic shift to prolonged and voluminous central vent volcanism in its most recent history this demonstrates the difficulty in determining time erupted volume relationships in such fields previous avf hazard risk modeling based on isolated short lived

application of fractal fragmentation theory to natural pyroclastic deposits insights into volcanic explosivity of the valentano scoria cone italy abstract the extent of fragmentation triggered by basaltic volcanic eruptions has been studied by analyzing the grain size distribution of natural pyroclastic deposits and applying the fractal fragmentation theory for this purpose 75 samples have been collected from individual layers along horizontal section of the well exposed valentano scoria cone italy the cone is constituted by well defined layers of fall deposits of up to 180 cm in thickness an important feature of the stratigraphic sequence is the occurrence of several intercalated layers called breccias with clasts of different characteristics angular shape low porosity and high crystallinity in respect to the normal layers results indicate that all achieved grain size distributions show fractal behavior and can be clearly correlated with a single fragmentation event namely the magmatic fragmentation during the eruption the fractal dimension of fragmentation d increases as fragmentation efficiency increases values of d are found to vary significantly along the pyroclastic sequence defining two major trends 1 a general long range increase of d during the course of the eruption and 2 a superimposed high frequency oscillating variation of fragmentation efficiency noteworthy is the fact that the highest values of d are measured for pyroclastic deposits following breccia deposits application of a conceptual model of fractal fragmentation as claimed by natural data indicates that larger values of d have to be associated with a higher probability of fragmentation thus suggesting that the d value can be used as a proxy for the fragility i e ability to fragment of samples fractal dimension of fragmentation of single layers is positively correlated with the vesicularity of clasts this indicates that the bubble content in the ascending magma prior to and at fragmentation likely was the key factor modulating the variable fragmentation efficiency observed in the studied pyroclastic sequence we hypothesize that the long range variation of d may be associated with magma fragmentation triggered by expansion of gas bubbles progressively coalescing below solid plugs during later stages of eruptive activity whereas high frequency variations of d are interpreted as due to magma fragmentation during the slug flow regime characterizing the strombolian activity of the scoria cone

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

quantification of vesicle characteristics in some diatreme filling deposits and the explosivity levels of magma water interactions within diatremes abstract vesicles within juvenile fragments in mafic pyroclastic deposits contain important information about the state of the magma at the time of fragmentation there have been few vesicle studies of juvenile pyroclasts from mafic phreatomagmatic deposits however and none we can find from maar diatreme volcanoes in this paper we document the vesicularity and vesicle population characteristics of juvenile fragments sampled from non bedded lithified deposits of the coombs hills diatreme complex part of the ferrar large igneous province antarctica the diatreme filling pyroclastic deposits dominated by lapilli tuffs and tuff breccias contain typically abundant lithic clasts derived mostly from the enclosing sedimentary sequence and several types of juvenile clasts ranging from blocky to fluidal or raggy in the samples measured 77 80 of the juvenile pyroclasts ranging in size from 0 5 mm to fine lapilli is in the non vesicular to incipiently vesicular range fluidal or rag shaped juvenile clasts some circular vesicles and the lack of microlites all suggest that the coombs hills magma had a relatively low viscosity prior to fragmentation despite the basaltic andesite composition this low viscosity allowed parts of the magma to be fragmented in a non brittle fashion during phreatomagmatic explosions and to form fluidal clasts phreatomagmatic explosions in diatremes can therefore produce diverse types of juvenile clasts simultaneously and the proportions of each will depend on the explosivity of the magma water slurry interaction and other factors recycling of fragments is also thought to be an important factor in generating mixtures of different types of juvenile fragments in diatremes

the morphology and formation of flow lobe tumuli on icelandic shield volcanoes three types of tumuli are recognized on monogenetic shield volcanoes in iceland namely 1 lava coated tumuli 2 upper slope tumuli and 3 flow lobe tumuli flow lobe tumuli grade into lava rises which are larger inflation structures but both structures have a similar mode of emplacement the areal distribution of flow lobe tumuli in the flow field differs from that of lava coated tumuli and upper slope tumuli flow lobe tumuli are frequent in the medial and distal parts of pahoehoe flow fields whereas the other two tumulus types are more frequent in the proximal parts of the flow fields the flow lobe tumuli also differ from the two other tumulus types as regards their morphology they are significantly larger have shallower flanks and do not have extensive outflows from the cracks the vesicle distribution within the flow lobe tumuli is similar to that of p type flow lobes a fairly dense zone is situated between the upper and lower vesicular zones each flow lobe tumulus is an individual lobe that inflates and forms tension cracks in the lava crust major lava tubes are mostly absent from the areas where flow lobe tumuli form and therefore the emplacement of flow lobe tumuli must be related to secondary short lived pathways of lava the flow lobe tumuli inside the flow fields of icelandic shield volcanoes are morphologically identical to those flow lobe tumuli that are found in the margins of flow fields of other icelandic volcanoes these observations allow us to model the flow lobe tumuli as morphological variants of p type flow lobes forming along terminating secondary lava paths flow lobe tumuli form at low lava supply rates of the order of 10 2 10 5 m3 s and are generated by inflation of the lava crust as a result of magmatic overpressure in the associated lava core we model the tumulus as a circular elastic plate that becomes uplifted inflated and fractured by a uniform magmatic overpressure at its bottom we conclude that 1 the effective thickness of the lava crust that is subject to bending is 0 2 0 4 m and remains essentially constant during the formation of the tumulus and 2 for flow lobe tumuli that are supplied with magma from tubes that originate in overflow from the lava lake of a shield volcano the elevation difference between the tumuli and the surface of the source lava lake is commonly around 100 m this elevation difference can give rise to a static magmatic overpressure of 2 5 mpa in the tumulus whereas the overpressure needed to form a typical flow lobe tumulus is only 0 2 1 mpa

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

correlation of basaltic tephra from mt wellington volcano implications for the penultimate eruption from the auckland volcanic field abstract mt wellington volcano in the auckland volcanic field is a basaltic complex comprising several vents tuff rings cones and lava flows interstitial glass in shard and lapilli groundmass displays considerable variability 41 48 wt sio 2 anhydrous but within sample variation is relatively narrow mostly 2 allowing two broad compositional affinities to be recognised that relate to the eruptive sequence the tuff ring contains lower sio 2 glasses whereas the main cinder cone comprises higher sio 2 glasses fe ti oxides also display a moderate compositional range ulvospinel usp 40 55 although within sample variation is generally narrower 5 usp allowing tuff ring deposits usp 50 compositional outliers are evident amongst the fe ti oxide crystals in many samples and probably result from slow post depositional cooling and or variable oxidation conditions during syn eruption recycling at the vent although abundant microlites restrict the deposits that can be geochemically fingerprinted randomly selected analyses of glass shards show only limited variation within samples 1 wt sio 2 suggesting that variable microlite formation had little effect on the residue melt the larger compositional differences such as that between the tuff ring and cinder cone also do not appear to be related to microlite formation because both deposits display a similar range in microlite content correlations between the edifice and distal deposits in maar sediments provide insights to the eruption episode they demonstrate that the main cinder cone was a late stage event there was no significant hiatus months to decades within the entire sequence and that the cinder cone was responsible for the wide dispersal of ejecta in the region by placing the mt wellington event within a stratigraphic framework of well dated tephra from other volcanoes it is possible to evaluate radiocarbon chronologies and constrain the eruption age to 10 000 bp

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

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