the lamongan volcanic field east java indonesia physical volcanology historic activity and hazards abstract the lamongan volcanic field lvf in east java indonesia consists of up to 61 basaltic cinder or spatter cones a minimum of 29 prehistoric maars and a central compound complex comprising three main vents including the historically active lamongan volcano the field lies on the sunda volcanic arc and abuts onto the currently active tengger caldera semeru complex to the west between 1799 and 1898 lamongan was persistently active and discharged up to 15 lava flows of basaltic to basaltic andesite composition which is probably more than what was erupted by any other indonesian volcano in that period several seismic crises have been recorded in the lvf since lamongan s last eruption there are no existing records of cinder cone or maar forming eruptions in the area despite the possible hazards posed by future activity in the lvf the history and structure of the field have been subject to little scrutiny this contribution presents the results of a detailed morphometric and volumetric investigation of prehistoric vents and historic lava flows in the lvf using field data historic records satellite imagery and modelling of erupted volumes a typical lvf maar is characterised as a crater with a diameter of 450 m generated by an eruption with a model ejecta volume of 0 016 km 3 dense rock equivalent dre and a model juvenile component volume of 0 0096 km 3 dre diameter depth ratios of some maars display a morphological freshness comparable to recently formed phreatomagmatic eruption craters elsewhere the median lvf cinder spatter cone has a basal diameter of 600 m a height of 75 m a volume of 0 009 km 3 and a model total erupted volume of 0 014 km 3 variation in cone basal diameter to height ratios indicates that the age of some prehistoric cones may not exceed a few centuries and morphological classification identifies the youngest prehistoric vents on the northern flanks of the central edifice historic activity from vents on the western flanks of lamongan produced at least 0 05 km 3 of basaltic to basaltic andesite lava resurfacing rates at lamongan in the 19th century imply an age of 900 2100 years for the surface of the volcano the steady state eruptive rate from 1843 1898 is evaluated as 0 025 m 3 s 1 which is low in comparison to estimates from many presently active systems using eruptive rate and volume data the age of the lvf is estimated as between 13 and 40 ka placing it among the youngest dated indonesian volcanoes complex regional tectonics in conjunction with magmatic pressure have probably been the major influences on the distribution of volcanism in the lvf the occurrence of maar forming eruptions may have been controlled by a porous substrate possibly a deposit from the holocene iyang argapura complex to the east volcanic hazards from future eruptions in the lvf include base surges ash flows ashfall and ejected blocks associated with phreatomagmatic activity and ashfall and ejecta from magmatic eruptions

structure of the pliocene camp dels ninots maar diatreme catalan volcanic zone ne spain maar volcanoes expose shallower or deeper levels of their internal structure as a function of the degree of erosion in el camp dels ninots maar diatreme catalan volcanic zone spain the tephra ring has been largely eroded and the remaining volcanic deposits infilling the diatreme are hidden under a lacustrine sedimentary infill of the crater the volcano shows hardly any exposure so its study needs the application of direct e g boreholes and indirect shallow geophysics subsurface exploration techniques additionally this maar diatreme was built astride two different substrates i e mixed setting as a result of its location in a normal fault separating neogene sediments from paleozoic granites in order to characterize the internal structure and post eruption stratigraphy of the maar diatreme we did geological studies mapping continuous core logging and description of the tephra ring outcrops and near surface geophysics including nine transects of electric resistivity tomography and a gravity survey results show that the deeper part of the diatreme is excavated into granites and is relatively steep and symmetrical the uppermost diatreme is asymmetrical because of mechanical contrast between granites and pliocene sands the maar crater contained a lake permanently isolated from the surrounding relief and was deep enough to host anoxic bottom waters while its margins had shallower waters these lake conditions preserved the remarkable pliocene fossil record found in the lacustrine sediments

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

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

the 1245 yr bp asososca maar eruption the youngest event along the nejapa miraflores volcanic fault western managua nicaragua abstract the asososca tephra comprises the asymmetrically distributed well stratified phreatomagmatic products of the 1245 125 120 yr bp asososca maar eruption west of managua nicaragua it is the youngest eruptive unit related to the monogenetic volcanism concentrated along the n s trending nejapa miraflores active fault and intersecting e w to ne sw and nw se minor faults the close relationship between monogenetic volcanism and the structural local tectonic framework is evidenced by the lineament domains and vent alignment patterns identified by remotely sensed images and dems of the research area the asososca maar lake bathymetry suggests that the crater was formed by coalescing vents migrating during the eruptive event the asososca tephra dominantly consists of accidental lithics disrupted from the underlying stratigraphic units observed inside and around the maar crater dry base surge bedsets are dominant throughout the eruptive unit showing facies variation from proximal cross stratified beds to mid distal plane parallel and wavy parallel beds grain size analyses indicate that base surges transported dominantly 1 0 and 1 sized particles at the bedload in form of traction carpets while fine ash material in continuous suspension was minimal juvenile fragments were only identified in the finer fractions varying from vesiculated scoriae to dense sideromelane fragments sem analyses suggest that the asososca eruption resulted from a highly efficient fuel coolant interaction between a tholeiitic basaltic melt and an aquifer hosted in a shallow level between 50 and 200 m of olivine bearing scoria ash and lapilli beds the explosion triggered the country rock comminution and the production of moss like fused shaped and blocky ash shards with stepped surfaces quenching cracks pitting alteration skins and adhered particles all indicative of phreatomagmatic fragmentation the very recent age of the asososca maar eruption confirms that the densely inhabited managua area is volcanically active and that hazard studies should focus on the potential for phreatomagmatic eruptions

holocene glacier fluctuations a global overview of glacier advances and retreats grouped by regions and by millennia for the holocene is compiled from previous studies the reconstructions of glacier fluctuations are based on 1 mapping and dating moraines defined by 14 c tcn osl lichenometry and tree rings discontinuous records time series and 2 sediments from proglacial lakes and speleothems continuous records time series using 189 continuous and discontinuous time series the long term trends and centennial fluctuations of glaciers were compared to trends in the recession of northern and mountain tree lines and with orbital solar and volcanic studies to examine the likely forcing factors that drove the changes recorded a general trend of increasing glacier size from the earlyemid holocene to the late holocene in the extra tropical areas of the northern hemisphere nh is related to overall summer temperature forced by orbitally controlled insolation the glaciers in new zealand and in the tropical andes also appear to follow the orbital trend i e they were decreasing from the early holocene to the present in contrast glacier fluctuations in some monsoonal areas of asia and southern south america generally did not follow the orbital trends but fluctuated at a higher frequency possibly triggered by distinct teleconnections patterns during the neoglacial advances clustered at 4 4e4 2 ka 3 8e3 4 ka 3 3e2 8 ka 2 6 ka 2 3e2 1 ka 1 5e1 4 ka 1 2e1 0 ka 0 7e0 5 ka corresponding to general cooling periods in the north atlantic some of these episodes coincide with multidecadal periods of low solar activity but it is unclear what mechanism might link small changes in irradiance to widespread glacier fluctuations explosive volcanism may have played a role in some periods of glacier advances such as around 1 7e1 6 ka coinciding with the taupo volcanic eruption at 232 5 ce but the record of explosive volcanism is poorly known through the holocene the compilation of ages suggests that there is no single mechanism driving glacier fluctuations on a global scale multidecadal variations of solar and volcanic activity supported by positive feedbacks in the climate system may have played a critical role in holocene glaciation but further research on such linkages is needed the rate and the global character of glacier retreat in the 20th through early 21st centuries appears unusual in the context of holocene glaciation though the retreating glaciers in most parts of the northern hemisphere are still larger today than they were in the early and or mid holocene the current retreat

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

deep bedded ultramafic diatremes in the missouri river breaks volcanic field montana usa 1 km of syn eruptive subsidence the ultramafic eocene missouri river breaks volcanic field mrbvf montana usa includes over 50 diatremes emplaced in a mostly soft substrate the current erosion level is 1 3 1 5 km below the pre eruptive surface exposing the deep part of the diatreme structures and some dikes five representative diatremes are described here they are 200 375 m across and have sub vertical walls their infill consists mostly of 55 90 bedded pyroclastic rocks fine tuffs to coarse lapilli tuffs with concave upward bedding and 45 10 non bedded pyroclastic rocks medium lapilli tuffs to tuff breccias the latter zones form steep columns 15 135 m in horizontal dimension which cross cut the bedded pyroclastic rocks megablocks of the host sedimentary formations are also present in the diatremes some being found 1 km or more below their sources the diatreme infill contains abundant lithic clasts and ash sized particles indicating efficient fragmentation of magma and country rocks the spherical to sub spherical juvenile clasts are non vesicular they are accompanied by minor accretionary lapilli and armored lapilli the deposits of dilute pyroclastic density currents are locally observed our main interpretations are as follows 1 the observations strongly support phreatomagmatic explosions as the energy source for fragmentation and diatreme excavation 2 the bedded pyroclastic rocks were deposited on the crater floor and subsided by 1 0 1 3 km to their current location with subsidence taking place mostly during the eruption 3 the observed non bedded pyroclastic columns were created by debris jets that punched through the bedded pyroclastic material the debris jets did not empty the mature diatreme occupying only a fraction of its width and some debris jets probably did not reach the crater floor 4 the mature diatreme was nearly always filled and buttressed by pyroclastic debris at depth there was never a 1 3 1 5 km deep empty hole with sub vertical walls otherwise the soft substrate would have collapsed inward which it only did near the surface to create the megablocks we infer that syn eruptive subsidence shifted down bedded pyroclastic material and shallow sedimentary megablocks by 0 8 1 1 km or more after which limited post eruptive subsidence occurred this makes the mrbvf diatremes an extreme end member case of syn eruptive subsidence in the spectrum of possibilities for maar diatreme volcanoes worldwide

composite tuff ring cone complexes in jeju island korea possible consequences of substrate collapse and vent migration hydromagmatic volcanoes tuff rings and tuff cones in jeju island korea are divided into holocene volcanoes that were constructed upon rigid plateau forming lavas and early to middle pleistocene volcanoes that underlie the lavas and were constructed upon fragile unconsolidated sediments the older volcanoes are distinguished from the holocene volcanoes in that the former have irregular morphologies the irregular morphologies resulted from superposition or juxtaposition of multiple rim deposits of partial tuff rings and or tuff cones that have contrasting lithofacies characteristics bed attitudes and paleoflow directions the rim deposits are bounded by laterally persistent truncation surfaces which suggest large scale collapse of the substrate together with the overlying tephra piles and or erosion during a break in eruptive activity collapse was probably caused by the instability of the friable sedimentary substrate removal of lateral support because of downward quarrying of volcanic conduits and liquefaction of the water saturated substrate by volcanic seismicity the path of magma supply was diverted in some cases after collapse giving rise to migration of the active vent in other cases the eruption style changed abruptly from cone forming fallout dominated to ring forming surge dominated eruptions after collapse the resultant volcanic edifices thus consist of multiple rim beds of partial tuff rings and or tuff cones commonly with non circular morphologies substrate collapse and lateral vent migration as observed in jeju island appears to be an important phenomenon in hydromagmatic volcanoes because they are commonly constructed upon unstable sedimentary substrates substrate collapse and vent migration needs to be taken into account as an important control on volcano morphology and structures and as a means of lateral enlargement of terrestrial submarine or extraterrestrial volcanoes

emplacement of the cabezo maria lamproite volcano miocene se spain lamproite volcanoes are uncommon in the geological record but are exceptionally well preserved in the betic cordilleras of se spain where they erupted during the late miocene tortonian to messinian stages the parent melts are thought to have been channelled through major lithospheric faults to erupt at or near the faulted margins of neogene sedimentary basins lamproite magmas are thought to be relatively co2 poor 1 wt and are typically characterised by an effusive eruption style and the development of lava lakes and scoria cones cabezo maria is a relatively small 550 m diameter lamproite volcano that was emplaced within the shallow water marine influenced vera basin the lamproites are compositionally similar to those of the roman province and generally less potassic k2o 5 wt than other ultra potassic rocks in se spain e g cancarix fortuna the initial eruption stages were dominated by explosive magma water interactions and the formation of peperites these are characterised by angular fragments of glassy lamproite lavas and isolated lobes incorporated in sediments locally showing the effects of thermal metamorphism further elutriation pipes and jigsaw fit textures are observed in the peperites the lavas and peperites are overlain by outward dipping well stratified scoria deposits defining part of a cinder cone which is inferred to have emerged above sea level steep internal contacts with inward dipping structureless breccias likely represent the inner wall deposits of a central conduit the deposits are cross cut by late stage dykes which supplied fissure eruptions of geochemically similar lavas capping the scoria cone the transition from explosive to effusive behaviour may reflect the decreased availability of water possibly due to downward migration of the feeder conduit below the level of water saturated sediments

a transition from strombolian to phreatomagmatic activity induced by a lava flow damming water in a valley abstract the golan heights is a plio pleistocene volcanic plateau cinder cones of late pleistocene age are very common in the eastern and northern golan while phreatomagmatic deposits are relatively rare and occur just in two structures the maar of birket ram and the tuff ring of mt avital the complex of mt avital includes two large cinder cones a tuff ring with an elongated central depression and several basaltic flows some of them breach the cinder cones the exposed eruptive history of the complex includes 1 an early stage of basaltic lava flows 2 strombolian activity and the buildup of the southern cinder cone 3 a second stage of basaltic flows and the buildup of the northern cinder cone and then a transition to 4 phreatomagmatic explosions the phreatomagmatic deposits include surges lapilli fallout deposits and coarse grained lithic tuff breccias which were found up to 200 m above the central depression basaltic and scoriaceous clasts are the main component of all deposits while juvenile material is usually a minor component almost absent in the lapilli deposits it is suggested that the phreatomagmatic events in mt avital were induced by the infiltration of water from a lake that existed in a nearby topographic low quneitra valley the lake was formed or significantly expanded at about 300 ka due to a lava flow that blocked the drainage of the valley to the west the interlayering of tuff and scoria at the top of the northern cinder cone and the good preservation of a lava flow top breccia under the surges imply that the phreatomagmatic activity immediately followed and even coincided with the last stages of strombolian activity it is suggested that the dry wet transition was triggered by the effusion of the second stage lavas and the buildup of the northern cinder cone which probably caused a reduction of pressure in the magmatic system and allowed the lake water an access to the magmatic system the minimum age of the phreatomagmatic events is determined by a 54 ka musterian site which lies directly on top of the tuff in the quneitra valley

variation of cenozoic extension and volcanism across the southern sierra madre occidental volcanic province mexico the middle to late cenozoic tectonic magmatic evolution of the sierra madre occidental volcanic province south of the tropic of cancer is summarized and analyzed for the first time based on new geologic and structural work and published information in the eastern part of the study region mesa central physiographic province silicic volcanism occurred in a short lived episode culminating at ca 30 ma and was followed by crustal scale extension between 30 and 27 ma in the western part of the study area sierra madre occidental physiographic province a voluminous episode of ignimbrite volcanism at 24 21 ma was succeeded by east west extension that produced regularly spaced grabens affecting only the upper crust in the westernmost part of the study region an andesitic to rhyolitic arc formed between 17 and 12 ma was affected by crustal scale north northwest trending extensional faulting leading to the formation of the gulf of california in the mesa central the maximum extension was oriented approximately east west and amounted to 20 in the eastern sierra madre occidental physiographic province extension was only 8 and oriented approximately east west we observe that trenchward shifting of the climax of subduction volcanism and extension occurred during late oligocene early miocene and late miocene time comparison with the offshore tectonics indicates that the first two tectonic magmatic pulses coincide with periods of fast spreading at the pacific farallon boundary south of the shirley fracture zone we propose that increases in the spreading rate are related to periods of high subduction rate which in turn correspond to episodes of retreating subduction a retreating slab may have generated a flux of hotter asthenospheric material into the mantle wedge producing widespread melting at the base of the crust as well as intraarc extension in the overriding plate boundary conditions i e plate tectonics ultimately determined timing magnitude and orientation of extension whereas volcanic and tectonic styles are controlled by the internal structure of crustal blocks and by the gravitational and thermal effects of magmatism

reconstructing the eruptive history of a monogenetic volcano through a combination of fieldwork and geophysical surveys the example of puig d adri garrotxa volcanic field puig d adri is a complex volcano in the garrotxa volcanic field 0 7 0 01 ma the youngest part of the catalan volcanic zone ne iberian peninsula the construction of this volcano involved the superimposition of three volcanic edifices with five lithostratigraphic units in which several strombolian and phreatomagmatic episodes alternated geological and geophysical surveys performed using electrical resistivity tomography and self potential have contributed to revealing the internal structure of the volcano and the succession of deposits the ring cone morphology of the phreatomagmatic part of the structure shows how a number of variables including the way in which magma and water interacted greatly influenced the eruptive dynamics and the mechanisms of formation a further major issue reflected in the final structure of the edifice is the process of vent migration which was controlled by the characteristics of the substrate and its interaction with the magma as it rose supplementary material granulometric and component histograms of the samples not shown in figure 4 are available at http doi org 10 6084 m9 figshare c 3262498

contrasting mechanisms of magma fragmentation during coeval magmatic and hydromagmatic activity the hverfjall fires fissure eruption iceland growing evidence for significant magmatic vesiculation prior to magma water interaction mwi has brought into question the use of diagnostic features such as low vesicularities and blocky morphologies to identify hydromagmatic pyroclasts we address this question by quantifying co variations in particle size shape and texture in both magmatic and hydromagmatic deposits from the hverfjall fires fissure eruption iceland overlapping vesicularity and bubble number density distributions measured in rapidly quenched magmatic and hydromagmatic pyroclasts indicate a shared initial history of bubble nucleation and growth with substantial vesiculation prior to mwi hydromagmatic fragmentation occurred principally by brittle mechanisms where the length scale and geometry of fracturing was controlled by the bubble population this suggests that the elevated fragmentation efficiency of hydromagmatic deposits is driven at least in part by brittle disintegration of vesicular pyroclasts due to high thermal stress generated during rapid cooling in this way the shape and size distributions of hydromagmatic pyroclasts both critical input parameters for ash dispersion models are strongly influenced by the dynamics of vesiculation prior to mwi this result underlines the need to analyse multiple grain size fractions to characterise the balance between magmatic and hydromagmatic processes during the hverfjall fires eruption the external water supply was sufficient to maintain mwi throughout the eruption with no evidence for progressive exhaustion of a water reservoir we suggest that both the longevity and the spatial distribution of mwi were determined by the pre existing regional hydrology and represent continuous interaction between a propagating dike and a strong groundwater flow system hosted within permeable basalt lavas

non explosive lava water interaction in skaelingar iceland and the formation of subaerial lava pillars hollow cylinders of basalt 2 5 m tall and generally 1 m in diameter were generated by non explosive lava water interactions during the emplacement of the laki lava flow in iceland during 1783 1784 we know of only one location within the laki lava flow where these basalt formations occur a valley called skaelingar located at 64 0 n 18 5 w which contains a tributary stream to the skafta river skaelingar was temporarily filled with laki lava when the main body of the lava flow advancing down the skafta river valley became blocked forcing lava to flow upstream into tributary valleys along the north side of the river after the blockage within the skafta river valley was removed the laki lava mostly drained out of these tributary valleys we refer to the remaining vertical hollow basalt pipes as lava pillars because they morphologically resemble subaerial lava trees and submarine lava pillars that have been observed at mid ocean ridges we propose that the subaerial pillars formed as an inflating lava flow advanced slowly over water saturated ground or perhaps into temporarily ponded water causing heated columns of water to rise between adjacent advancing lava lobes the subaerial pillars continued to grow in height and diameter as the lava flow inflated when the lava drained back out of the valley the lava pillars were left standing thus the icelandic subaerial pillars represent a non explosive interaction between lava and water

volcanology of tuzo pipe gahcho kue cluster root diatreme processes re interpreted abstract the middle cambrian 540 ma gahcho kue kimberlite field is situated about 275 km ene of yellowknife nwt canada the kimberlites were emplaced into 2 6 ga archean granitic rocks of the yellowknife supergroup four larger kimberlite bodies 5034 tesla tuzo and hearne as well as a number of smaller pipes and associated sheets occur in the field in plan view the tuzo pipe has a circular outline at the surface and it widens towards deeper levels the pipe infill consists of several types of coherent and fragmental kimberlite facies coherent or apparent coherent possibly welded kimberlite facies dominate at depth but also occur at shallow levels as dikes intruded late in the eruptive sequence or individual coherent kimberlite clasts the central and shallower portions of the pipe consist of several fragmental kimberlite varieties that are texturally classified as tuffisitic kimberlites the definition geometry and extent of the geological units are complex and zones controlled by vertical elements are most significant the fluidal outlines of some of the coherent kimberlite clasts suggest that at least some are the product of disruption of magma that was in a semi plastic state or even of welded material ragged clasts at low levels are inferred to form part of a complex peperite like system that intrudes the base of the root zone a variable often high abundance of local wall rock xenoliths between and within the kimberlite phases is observed varying in size from sub millimeter to several tens of meters wall rock fragments are common at all locations within the pipe but are especially frequent in a domain with a belt like geometry between 120 and 200 m depth in the pipe steeply outward dipping bedded deposits made up of wall rock fragments occur in deep levels of the pipe and are especially common under the downward widening roof segments the gradational contact relationships of these deposits with the surrounding kimberlite bearing rocks as well as their location suggest that they formed more or less in situ different breccia facies inside the pipe suggest an origin by slumping grain flows rock fall or pyroclastic deposition the shape and facies architecture of the tuzo pipe suggests that the studied section of the pipe lies at a root zone diatreme transitional structural level composite coherent kimberlite clasts imply that recycling processes were active over time while reworked wall rock rich deposits and ductily deformed clasts of welded kimberlite point to the presence of temporary cavities in the root zone the emplacement of the tuzo pipe did not occur in a single violent explosion but involved repetitive volcanic explosions alternating with periods of relative quiescence the observed features are typical of phreatomagmatic processes which may include phases of less explosive magmatic activity

growth of an emergent tuff cone fragmentation and depositional processes recorded in the capelas tuff cone sao miguel azores abstract the capelas tuff cone is an emergent surtseyan type tuff cone that erupted in shallow seawater off the coast of sao miguel azores in this paper we present a detailed stratigraphic study which is used to infer depositional processes and modes of fragmentation for the capelas tuff cone deposits the growth of the tuff cone can be divided into three stages based on variations in depositional processes that are probably related to differences in water magma w m ratios the first stage corresponds well to wet surtseyan type activity where wet fallout is the dominant depositional process with only minor representation of pyroclastic surge deposits the second stage of the eruption is suggested to be the result of alternating wet and slightly drier periods of surtseyan activity with an overall lower w m ratio compared to the first stage the drier surtseyan periods are characterized by the presence of minor grain flow deposits and undulating pyroclastic surge deposits that occasionally display relatively dry structures such as strongly grain segregated layers and brittle behavior when impacted by ballistic ejecta the first deposits of the second stage show an intense activity of pyroclastic surges but fallout commonly modified by surges is still the dominant depositional process during the second stage the third stage represents a final effusive period with the build up of a scoria cone and ponded lava flows inside the tuff cone crater phreatomagmatic fragmentation as seen by studies of the fine ash fraction

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

el pozo volcanic complex evolution of a group of maars central mendoza province argentina abstract the el pozo volcanic complex in the south central mendoza province is formed by three maars the first to be described in the payenia volcanic province and one of the few examples in argentina the maars of el pozo volcanic complex 1 2 and 3 show large craters 0 97 km 1 2 km and 0 96 km respectively and their depth is between 10 and 57 m below the pre eruptive surface their deposits are described from the only outcrops that have been found called el pajarito and el visitante it was interpreted that the first maar to be formed would have been no 2 from which only the deposits corresponding to el pajarito 3 6 m and the lower section of el visitante 28 m are preserved we suggest that its shape is the result of lateral vent migration and coalescence of successive vents then after a short break represented by the angular unconformity in the el visitante section the active vent would have shifted more to the w and the maar no 3 is thought to have been formed the activity of this maar is represented in el visitante upper section with features resulting from the presence of water in the time of deposition maar no 1 was probably active simultaneously with maar no 3 with which coalesces but the lack of outcrops does not guarantee this affirmation towards the end of the eruptive cycle depletion of water produced a change in activity from phreatomagmatic to strombolian and effusive this gave rise to the nested pyroclastic scoria cone el pozo cone which lies in the maars 1 and 3 craters floors el pozo cone is related to lava flows with a tumulus and the deposit of a lapilli bearing tephra plume

origin of rhyolitic lavas in the mesa central mexico by crustal melting related to extension the emplacement of a voluminous sequence of rhyolitic lava flows and domes characterizes oligocene volcanism in the mesa central mc of mexico its dominant effusive style of emplacement contrasts deeply with the predominantly explosive volcanism of the sierra madre occidental volcanic province toward the west whole rock geochemical major and trace elements and sr nd isotopic data of the mc oligocene rhyolitic lavas document a marked change in magma composition at around 30 ma allowing us to distinguish between a lower and an upper sequence lavas from the lower sequence are geochemically similar to the high k rhyolitic rocks of the eastern sierra madre occidental major and trace element variations are characteristic of mantle derived magmas evolving through fractional crystallization the initial 87 sr 86 sr and e nd values are nearly constant 0 70644 0 70770 and 1 2 to 2 1 respectively and indicate some contribution from crustal material lavas from the upper sequence are high silica peraluminous rhyolites with strong enrichment in fluorine and in some incompatible lithophile elements rb la sm yb y th u nb ta and strong depletion in the feldspar compatible elements sr ba eu initial 87 sr 86 sr ratios of the upper sequence lavas are high and variable 0 70812 0 72190 and decrease as silica content increases whereas the e nd values are relatively constant 1 4 to 2 8 the trace element behavior indicates an origin by variable degrees of non modal partial melting of granulitic low crustal rocks and chemical disequilibrium during melting processes the high and variable sr isotopic ratios could also be related to isotope disequilibrium melting processes if the isotopic heterogeneities between individual mineral phases were preserved during heating of the source rocks the changes in geochemical compositions are related to the onset of crustal extension at high strain rates documented for the mc crustal extension promoted crustal melting at high melting rates high melt segregation rates rapid ascent of low viscosity fluorine rich magmas and inhibited melt stagnation in magma chambers such conditions favored the effusive volcanic style and support the possibility of melting under disequilibrium conditions

types of phreatomagmatic volcanoes in the western snake river plain idaho usa abstract the western snake river plain graben in southwestern idaho includes a large hydrovolcanic field which was produced in late miocene to pleistocene time by the interaction of rising basaltic magmas with the waters and water saturated deposits of an enormous freshwater lake lake idaho the phreatomagmatic volcanoes in this field may be grouped into three types emergent subaqueous and subaerial emergent volcanoes which began erupting under water and built up above the lake level are relatively large and symmetrical are dominated by bedded tuffs and late magmatic deposits and are excellent indicators of water depth at the time of the eruption subaqueous volcanoes which never built up above the lake level are relatively small and asymmetrical are dominated by basal massive deposits and are potentially useful in discriminating between deep and very deep water settings subaerial volcanoes which were formed when magmas intercepted buried aquifers and interacted explosively with water are small tuff rings and maars with variable shapes are composed of subequal although variable proportions of basal massive deposits bedded tuffs and late magmatic deposits and are useful in determining the stratigraphic successions underlying them

miocene hydrovolcanism in nw colorado usa fuelled by explosive mixing of basic magma and wet unconsolidated sediment the yampa and elkhead mountains volcanic fields were erupted into sediment filled fault basins during miocene crustal extension in nw colorado post miocene uplift and erosion has exposed alkali basalt lavas pyroclastic deposits volcanic necks and dykes which record hydrovolcanic and strombolian phenomena at different erosion depths the occurrence of these different phenomena was related to the degree of lithification of the rocks through which the magmas rose hydrovolcanic interactions only occurred where rising basaltic magma encountered wet porous non lithified sediments of the 600 m thick miocene brown s park formation the interactions were fuelled by groundwater in these sediments there was probably no standing surface water dykes intruded into the sediments have pillowed sides and local swirled inclusions of sediment that were injected while fluidized in steam from heated pore water volcanic necks in the sediments consist of basaltic tuff sediment blocks and separated grains derived from the sediments lithic blocks mostly derived from a conglomerate forming the local base of the brown s park formation and dykes composed of disaggregated sediment the necks are cut by contemporaneous basalt dykes hydrovolcanic pyroclastic deposits formed tuff cones up to 100 m thick consisting of bedded air fall pyroclastic surge and massive poorly sorted deposits mpsds all these contain sub equal volumes of basaltic tuff and disaggregated sediment grains from the brown s park formation possible explosive and effusive modes of formation for the mpsds are discussed contemporaneous strombolian scoria deposits overlie lithified cretaceous sedimentary rocks or thick basalt lavas volcanic necks intruded into the cretaceous rocks consist of basalt clasts some with spindle shape lithic clasts and megacrysts derived from the magma and are cut by basalt dykes rarely strombolian deposits are interbedded with hydrovolcanic pyroclastic deposits recording changes in eruption behaviour during one eruption the hydrovolcanic eruptions occurred by interaction of magma with groundwater in the brown s park sediments the explosive interactions disaggregated the sediment such direct digestion of sediment by the magma in the vents would probably not have released enough water to maintain a water magma mass ratio sufficient for hydrovolcanic explosions to produce the tuff cones probably additional water perhaps 76 of the total was derived by flow through the permeable sediments especially the basal conglomerate to the formation and into the vents

large phreatomagmatic vent complex at coombs hills antarctica wet explosive initiation of flood basalt volcanism in the ferrar karoo lip the mawson formation and correlatives in the transantarctic mountains and south africa record an early eruption episode related to the onset of ferrar karoo flood basalt volcanism mawson formation rocks at coombs hills comprise mainly 80 vol structureless tuff breccia and coarse lapilli tuff cut by irregular dikes and sills within a large vent complex 30 km2 quenched juvenile fragments of generally low but variable vesicularity accretionary lapilli and country rock clasts within vent fill and pyroclastic density current deposits point to explosive interaction of basalt with groundwater in porous country rock and wet vent filling debris metre scale dikes and pods of coherent basalt in places merge imperceptibly into peperite and then into surrounding breccia steeply dipping to sub vertical depositional contacts juxtapose volcaniclastic rocks of contrasting componentry and grainsize these sub vertical tuff breccia zones are inferred to have formed when jets of debris steam water passed through unconsolidated vent filling deposits these jets of debris may have sometimes breached the surface to form subaerial tephra jets which fed subaerial pyroclastic density currents and fall deposits others however probably died out within vent fill before reaching the surface allowing mixing and recycling of clasts which never reached the atmosphere most of the ejecta that did escape the debris filled vents was rapidly recycled as vents broadened via lateral quarrying of country rock and bedded pyroclastic vent rim deposits which collapsed along the margins into individual vents the unstratified poorly sorted deposits comprising most of the complex are capped by tuff lapilli tuff and tuff breccia beds inferred to have been deposited on the floor of the vent complex by pyroclastic density currents development of the extensive coombs hills vent complex involved interaction of large volumes of magma and water we infer that recycling of water as well as recycling of pyroclasts was important in maintaining water supply for phreatomagmatic interactions even when aquifer rock in the vent walls lay far from eruption sites as a consequence of vent complex widening the proportion of recycled water increased with vent complex size in the same way that the proportion of recycled tephra did though water recycling leaves no direct rock record the volcaniclastic deposits within the vent complex show through their lithofacies structural architecture lithofacies characteristics and particle properties clear evidence for extensive and varied recycling of material as the complex evolved

morphotectonic setting of maar lakes in the campo de calatrava volcanic field central spain sw europe abstract in the campo de calatrava volcanic field ccvf central spain the eruption of pliocene pleistocene maar craters into two clearly distinct types of pre volcanic rocks allows the observation and comparison of hard substrate and soft substrate maar lakes hard substrate maars formed when phreatomagmatic processes affected the jointed paleozoic igneous and metamorphic rocks hard substrate giving rise to funnel like maar lake basins soft substrate maars resulted from phreatomagmatic volcanic processes affecting poorly consolidated pliocene sediments forming bowl like maar lake basins pre volcanic bedrock determined the post eruptive lacustrine architecture in the craters and favored a higher preservation of hard substrate maars in comparison to soft substrate maars this is because the hard substrate maars surrounded by a deep stable crater wall are more capable of collecting sediments in their basins these sediments could be preserved for longer than similar deposits in broad shallow maars with a soft substrate ancient soft substrate maars do not usually preserve their original morphology well and can be identified only by their lacustrine deposits carbonate lacustrine palustrine deposits surrounding a bowl like depression are the remnants of this second type of maar lake and allow reconstruction of the original morphology of ancient soft substrate maar craters geophysical electrical tomography ground surveys and geomorphologic geologic mapping techniques were combined with fieldwork and facies analysis in order to locate and accurately characterize the pliocene pleistocene soft substrate maar volcanic structures of the ccvf

eruptive history of an alkali basaltic diatreme from elie ness fife scotland the elie ness diatreme fife scotland is an ideal place to study the internal architecture and emplacement processes of diatremes elie ness is one of approximately 100 alkali basaltic diatremes and intrusions in the east fife area emplaced during upper carboniferous to early permian times into an extensive rift system in the northern variscan foreland within the diatreme seven lithofacies and three lithofacies associations lfas 1 3 are recognised field petrographic and geochemical studies demonstrate that the diatreme experienced a protracted history of eruption and infill initially driven by volatile expansion and later by magma water interaction massive lapilli tuffs of lfa 1 contain abundant highly vesicular juvenile scoria and magma coated clasts which are best explained by a magmatic origin for the early explosive eruptions on a large scale the tuffs are well mixed and locally exhibit small scale degassing structures attributed to fluidisation processes occurring within the diatreme fill the occurrence of abundant volcaniclastic autoliths and megablocks within lfa 1 can be explained by subsidence of volcaniclastic strata from the maar crater and upper diatreme during emplacement pyroclastic density current deposits of lfa 2 form a series of continuous sheets across the diatreme some of which may have originated from phreatomagmatic explosions in a neighbouring vent we attribute the overall bedding pattern to a combination of primary volcanic processes and post depositional folding related to movement along an adjacent fault minor steeply inclined breccias and tuffs of lfa 3 cross cut the lfa 2 succession and are interpreted as late stage volcaniclastic dykes and conduits signalling the final phase of eruptive activity at elie ness the study offers new insights into the volcanic evolution of diatremes fed by low viscosity alkali rich magmas

the miocene costa giardini diatreme iblean mountains southern italy model for maar diatreme formation on a submerged carbonate platform in this paper we present a model for the growth of a maar diatreme complex in a shallow marine environment the miocene age costa giardini diatreme near sortino in the region of the iblei mountains of southern sicily has an outer tuff ring formed by the accumulation of debris flows and surge deposits during hydromagmatic eruptions vesicular lava clasts accretionary lapilli and bombs in the older ejecta indicate that initial eruptions were of gas rich magma abundant xenoliths in the upper late deposited beds of the ring suggest rapid magma ascent and deepening of the eruptive vent is shown by the change in slope of the country rock the interior of the diatreme contains nonbedded breccia composed of both volcanic and country rock clasts of variable size and amount the occurrence of bedded hyaloclastite breccia in an isolated outcrop in the middle lower part of the diatreme suggests subaqueous effusion at a low rate following the end of explosive activity intrusions of nonvesicular magma forming plugs and dikes occur on the western side of the diatreme and at the margins close to the contact between breccia deposits and country rock they indicate involvement of volatile poor magma possibly during late stages of activity we propose that initial hydromagmatic explosive activity occurred in a shallow marine environment and the ejecta created a rampart that isolated for a short time the inner crater from the surrounding marine environment this allowed explosive activity to draw down the water table in the vicinity of the vent and caused deepening of the explosive center a subsequent decrease in the effusion rate and cessation of explosive eruptions allowed the crater to refill with water at which time the hyaloclastite was deposited emplacement of dikes and plugs occurred nonexplosively while the breccia sediment was mostly still soft and unconsolidated locally forming peperites the sheltered low energy lagoon filled with marine limestones mixed with volcaniclastic material eroded from the surrounding ramparts ultimately lagoonal sediments accumulated in the crater until subsidence or erosion of the tuff ring caused a return to normal shallow marine conditions

geophysical surveys of the joya honda maar mexico and surroundings volcanic implications abstract joya honda jh is a quaternary maar excavated in mesozoic limestone it is located in central mexico and belongs to the ventura volcanic field vvf which is composed by cinder cones and maars made of intraplate type mafic alkalic rocks volcanoes in the region form n20w lineaments roughly parallel to a regional set of normal faults but there is no obvious relation between these faults and vent distribution in the exposed geology around the maar the volcanic rock volume is small in the vvf and most volcanoes and their products are scattered in a region where outcrops are dominated by limestone the near vent tephra associated to the jh maar lies north of the crater this relation suggests that the crater was formed by directed hydromagmatic explosions and may indicate an inclined volcanic conduit near the surface the tephra stratigraphy suggests that the initial explosions were relatively dry and the amount of water increased during the maar forming eruption therefore the existing model of the maar diatreme formation may not be applicable to joya honda as it requires the formation of a cone of depression in the aquifer and deepening of the focii of the explosions as the crater and underlying diatreme grew thus it is unlikely that there is a diatreme below joya honda aeromagnetic data shows a boundary between two regional magnetic domains near the elongated volcanic cluster of the vvf the boundary is straight with a distinct kink from ne to nw trend near jh the limit between the domains is interpreted as fault contacts between mid tertiary volcanic rocks and marine mesozoic sedimentary rocks hence magma ascent in the area may have been facilitated by fractures near the surface magnetic and gravimetric ground surveys show that the anomalies associated with the maar are not centered in the crater which could be consistent with an inclined volcanic conduit a magnetic profile measured on exposed limestone across the volcanic lineament failed to show an anomaly such as that caused by a connecting dike between the volcanoes therefore either the dike does not exist or it is so deep or so thin that it is beyond the limit of detection of the method and or equipment used thus the volcanic conduit immediately below joya honda can be reasonably modeled in the shape of a plug a 2 d model of the crater anomaly is consistent with a roughly tabular deposit formed by fall back pyroclasts and slump deposits near the surface based on this result we propose an alternative model for the formation of maar type volcanoes excavated in hard rock where there is no evidence of a gradual decrease of the water magma ratio as the eruption advanced

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

miocene lamproite volcanoes in south eastern spain an association of phreatomagmatic and magmatic products abstract a series of small miocene 8 3 6 7 ma lamproite rock occurrences as monogenetic volcanoes and or dykes cover a large area in southeastern spain these rocks are associated with extensional basins filled by neogene deposits in the betic and subbetic structural units at cancarix sierra de las cabras calasparra barqueros cerro de monagrillo jumilla and vera eruptions occurred whereas at fortuna mula and zeneta there were only small scale intrusions mainly dykes this paper describes volcanic centers at cancarix calasparra and barqueros which show initial phreatomagmatic eruptions driven by interaction of rising lamproite magma with groundwater tuff ring formed during this volcanic activity subsequent activity consisted of dome extrusion in the vent areas of cancarix and calasparra and by explosive to effusive magmatic activity accompanied by extensive lava flows at barqueros calasparra and cancarix are relatively symmetric monogenetic tuff rings filled by late stage massive vertical plug extruded as degassed crystalline high viscosity magma along the volcanic conduit barqueros was initially a tuff ring whose late stage hawaiian style fountaining generated spatter and clastogenic lavas that built the intra tuff ring cone of cabezo del morron finally extensive lava flows spread from the base of the cone toward the northern part of the edifice variations in the tectonic extensional regime and local hydrogeologic conditions shallow aquifers influenced the occurrence of these lamproite volcanoes late stage magma rise was dependent on the magmatic volatile regime being already degassed at calasparra and cancarix by showing higher viscosity high crystallization rate of intra tuff ring dome extrusions or still rich in volatiles at barqueros displaying lower viscosity lava fountaining and then lava flows

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

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

entrainment of country rock during basaltic eruptions of the lucero volcanic field new mexico as magma rises through the lithosphere it may entrain wall rock debris the entrainment process depends on the local hydrodynamic regime of the magma e g velocity temperature bulk density the extent of interaction of magma with groundwater and the mechanical properties of the wall rocks wall rock entrainment results in local flaring of dikes and conduits which in turn affects the hydrodynamics of magma ascent and eruption we studied upper crustal xenoliths erupted from small volume basaltic volcanoes of the lucero volcanic field west central new mexico in order to assess the relative importance of various entrainment mechanisms during a range of eruptive styles including strongly hydrovolcanic strombolian and effusive processes total xenolith volume fractions ranged from 0 3 0 9 in hydrovolcanic facies to 10 4 10 2 in strombolian facies the volcanoes erupted through a thick well characterized sequence of paleozoic and lower mesozoic sedimentary rocks so that erupted xenolith

the origin of a large 3 km maar volcano by coalescence of multiple shallow craters lake purrumbete maar southeastern australia lake purrumbete maar is located in the intraplate monogenetic newer volcanics province in southeastern australia the extremely large crater of 3000 m in diameter formed on an intersection of two fault lines and comprises at least three coalesced vents the evolution of these vents is controlled by the interaction of the tectonic setting and the properties of both hard and soft rock aquifers lithics in the maar deposits originate from country rock formations less than 300 m deep indicating that the large size of the crater cannot only be the result of the downwards migration of the explosion foci in a single vent vertical crater walls and primary inward dipping beds evidence that the original size of the crater has been largely preserved detailed mapping of the facies distributions the direction of transport of base surges and pyroclastic flows and the distribution of ballistic block fields form the basis for the reconstruction of the complex eruption history which is characterised by alternations of the eruption style between relatively dry and wet phreatomagmatic conditions and migration of the vent location along tectonic structures three temporally separated eruption phases are recognised each starting at the same crater located directly at the intersection of two local fault lines activity then moved quickly to different locations a significant volcanic hiatus between two of the three phases shows that the magmatic system was reactivated the enlargement of especially the main crater by both lateral and vertical growth led to the interception of the individual craters and the formation of the large circular crater lake purrumbete maar is an excellent example of how complicated the evolution of large seemingly simple circular maar volcanoes can be and raises the question if these systems are actually monogenetic

shallow seated explosions in the construction of the motukorea tuff ring auckland new zealand evidence from lithic and sedimentary characteristics at least 52 eruption centres are scattered within the 360 km2 auckland volcanic field avf motukorea now an island in the waitemata harbour is one of 39 avf volcanoes that experienced a phreatomagmatic explosive phase before a magmatic phase the volcano erupted through a 200 300 m thick consolidated mudstone sandstone sequence of the miocene waitemata group which overlies the waipapa terrane greywacke basement detailed field descriptions of the sedimentary characteristics of the early phreatomagmatic deposits were carried out along with examination of lithics the ejecta ring deposit comprises 55 to 60 vol lithics of which waitemata group fragments constitute approximately 90 vol whereas 10 vol are waipapa fragments suggesting a dominance of shallow fragmentation the sedimentary characteristics of the stratigraphic sequence at motukorea suggest a dominance of wet surges at the beginning of the eruption with progression into drier sequences upwards this is reflected in increasing inter bedded juvenile pyroclast dominated fall deposits up sequence these characteristics are attributed to the changing hydrogeological conditions within the diatreme and the host rocks these findings shed light on the eruption dynamics of phreatomagmatic eruptions through consolidated rocks in the avf and enable the depiction of a scenario of future eruptions within the field in similar substrates

counterclockwise rotations in the late eocene oligocene volcanic fields of san luis potosi and sierra de guanajuato eastern mesa central mexico abstract we used paleomagnetic and structural data to investigate the late eocene oligocene tectonic evolution of the mesa central area in mexico the mesa central was affected by nw trending faults tepehuanes san luis fault system coeval with a late eocene oligocene ignimbrite flare up and by post 27 ma nne trending grabens related to the basin and range we obtained reliable paleomagnetic directions from 61 sites within the late eocene oligocene volcanic series 30 to 27 ma of the san luis potosi volcanic field and sierra de guanajuato for each site we also measured the anisotropy of magnetic susceptibility ams tilt corrections were made using ams data for 33 sites where in situ bedding measurements were not available paleomagnetic directions indicate counterclockwise rotations of about 10 with respect to stable north america after 30 25 ma structural data suggest that the volcanic succession was mainly affected by normal faults however we also found evidences for oblique or horizontal striae showing a left lateral component along nw trending faults and a right lateral component along ne trending faults both motions are consistent with a n s extension oblique to the tepehuanes san luis fault system previous paleomagnetic studies in northern and southern mexico show the prevalence of minor left lateral shear components along regional scale transpressional and transtensional lineaments our paleomagnetic data may reflect thus small vertical axis rotations related to a minor shear component coeval with the oligocene intra arc extension in central mexico

overview and plumbing system implications of monogenetic volcanism in the northernmost andes volcanic province abstract monogenetic volcanic fields are commonly related to rifts and or intraplate tectonic settings however they are also less commonly recognised in subduction zones including both front and back arc volcanoes e g monogenetic volcanoes associated with the llaima volcano in chile or the san agustin volcanic field in colombia here we describe monogenetic volcanic fields associated with subduction related polygenetic volcanism in the northernmost part of the andes northern volcanic zone nvz 2 s to 4 30 n these fields are associated with the main axis of the quaternary volcanoes they are linked to the polygenetic san diego cerro machin volcano tectonic province 140 km long scvtp in colombia the chain that hosts the iconic nevado del ruiz volcano presently three monogenetic volcanic fields with a typical calc alkaline signature have been identified on both sides of this province from south to north they are 1 pijaos monogenetic volcanic field pmvf 2 villamaria termales monogenetic volcanic field vtmvf and 3 samana monogenetic volcanic field smvf pmvf is located 25 km south of cerro machin volcano the southernmost active volcano of the scvtp this field was formed by at least two eruptions with both effusive and explosive eruptive styles three cones and a maar are recognised previous work has defined the volcanoes as basaltic andesitic in composition which we recognised as the most mafic expression mgo 10 11 wt in the whole scvtp its source is related to the same but deeper magma that feeds the volcanoes in the scvtp stratigraphic relationships show that the volcanoes are younger than the underlying alluvial and volcaniclastic ibague fan 5 km of the axis of the province this field is made up of at least 14 volcanoes aligned with the active villamaria termales fault system as previous work has recognised the volcanism has been mainly effusive represented by lava domes and some lava flows since the 1980s studies have framed these andesitic to dacitic volcanoes in the context of the history of the nevado del ruiz volcano it is inferred that the magmatic source is a shallow magma reservoir underneath the scvtp stratigraphic and field relationships have shown that the last eruption occurred

pyroclastic deposits of san diego maar central colombia an example of a silicic magma related monogenetic eruption in a hard substrate

morphological comparison of distributed volcanic fields in the main ethiopian rift using high resolution digital elevation models abstract distributed volcanic fields of scoria cones maars tuff cones and lava flows represent a crucial expression of the interplay between magmatism tectonics and surface processes in continental rifts we compare the cone morphology of two fields butajira and east ziway in the main ethiopian rift mer using high resolution digital elevation models dems to investigate volumes surface environment and age of emplacement volumes of individual cones are similar in both fields mean 4 106 m3 and lower than in other distributed volcanic fields located in extensional environments volcanism at butajira has created linear clusters of superimposed cones suggesting a small number of larger 107 108 m3 fissure eruptions compared to the isolated events at east ziway the presence of maars and deep craters within cones at butajira indicates phreatomagmatic activity not seen at east ziway where cones may have been emplaced during a dry climate phase shape parameters including curvature of cone rims and ratios of cone height width and crater depth suggest younger eruption ages at butajira compared to east ziway and the possibility of ongoing activity in this area a broader survey of cone fields across the mer using the aster global digital elevation model fails to identify systematic variation in height width and volume from cone morphology limited by low resolution high resolution topography provides a useful insight into rift environments at the time of emplacement and further surveys across the main ethiopian rift may advance our understanding of distributed volcanism in extensional settings and the hazards they present

late permian early triassic environmental changes recorded by multi isotope re os n hg data and trace metal distribution from the hovea 3 section western australia abstract the temporal coincidence between the late permian mass extinction lpme and the emplacement of siberian trap basalts suggests a causal link between the two events here we discuss stratigraphic changes of organic and inorganic including isotopic geochemical properties of marine sediments across the permian triassic boundary ptb in the hovea 3 core western australia a key ptb section in the southern neo tethys ocean these data are compared with published data from the meishan section southern china and from the opal creek section in western canada providing a view of tethys and panthalassa changes at the ptb trace metal and n isotopic data together with organic matter properties suggest that anoxic conditions were established prior to the lpme intensified close to the lpme and continued with photic zone euxinia into the early triassic for the hovea 3 section re os ages confirm changhsingian 253 5 1 4 ma deposition of the dated interval sampled immediately below the stratigraphic level characterized by major lithological and isotopic changes evaluation of re os n and hg elemental and isotopic data for hovea 3 suggests that anoxic conditions in the latest permian were generally unrelated to direct magmatic contributions a major increase in the initial os isotopic ratio of lower triassic shales suggest an 8 increase in the early triassic continental runoff based on moderately conservative assumptions for end members contributing os to the permian triassic ocean comparison to other ptb sections confirms a global signal of increasing re os ratios in the late permian and major and long lived changes in the isotopic composition of the post extinction ocean a distinct peak in hg concentrations carrying a volcanic isotopic signature also identified in other ptb sections likely represents a major pulse of siberian trap volcanism this hg peak in the hovea 3 section however is detected above the stratigraphic level containing multiple other widely recognized and more permanent geochemical changes therefore direct volcanic inputs to the permian triassic ocean likely post date the lpme in this western australian section

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