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

wet surge deposits at la fossa di vulcano depositional and eruptive mechanisms abstract wet surge deposits of different volcanic cycles of the recent fossa activity at vulcano have been measured on a bed by bed basis with data recorded to millimeter detail the wet surge layers are varicoloured with variable thickness with the most recurrent thickness being about 1 cm the beds consist of fine ash without internal structures textural features include a accretionary lapilli of maximum size of 0 5 cm dispersed thoroughout the layer or forming continuous layers of submillimeter size b vesiculated layers which represent 10 to 65 of the total deposit vesicles have different shapes and smooth walls varying in volume from 1 to 15 20 c soft sediment types of bedding deformation such as gravity flowage ripples load cast and slumps the slope angle has not influenced either the concentration and size of the accretionary lapilli or the shape size and distribution of vesicles only the thickness of the layers decreases with distance from the vent sem investigations show features indicating the hydromagmatic origin of the deposits and stressing the role of the fluid phase noteworthy is the presence of vesiculated grains produced by magmatic exsolution which show chilling effects on the internal walls of the broken bubbles grain size analyses reveal that the layers are not graded and most of the samples have a median size finer than 50 m the grain size distributions are frequently polymodal suggesting several closely timed explosions as all the beds exhibit the same textures grain size and particle morphology a single mechanism can explain their deposition the depositional unit formed at the base of the cloud through the lateral expansion on the ground of a sticky muddy medium consisting of ash and fluid in general each layer lost its plasticity before the deposition of the next layer the deposition occurred in a nearly continuous sequence without periods of rest long enough to permit erosional discontinuity the eruptions are hydromagmatic and occur where magma at least partially fragmented comes into effective contact with subsurface water the process follows a model suggested by wohletz 1983b 1986 developing in more than one fragmentation event in the turbulent surge cloud both severe hydration and alteration of glassy grains and the formation of accretionary lapilli occur

drivers of explosivity and elevated hazard in basaltic fissure eruptions the 1913 eruption of ambrym volcano vanuatu sw pacific abstract fissure eruptions along linear structures can extend for several tens of kilometres with distinct separate manifestations of volcanism along their length they typically involve low viscosity mafic magmas forming long lava flows and cinder cones eruptions in 1894 and 1913 on ambrym volcano vanuatu showed how these mildly explosive eruptions can rapidly transform into violent explosive events producing significant hazard and widespread volcanic ash clouds during the 1913 episode a fissure began in the central caldera and basaltic magmas broke out in a series of locations down the island s western flank in all sites over 100 m in elevation fissure outbreaks produced vigorous lava fountains and highly fluid lava flows that travelled rapidly to the shoreline when the outbreaks propagated along the island s axis into coastal plain areas a climactic series of explosive eruptions occurred producing a 1 2 km long by 600 m wide maar and tephra ring a further small tuff ring was formed later creating a temporary island 400 m offshore the onshore tephra ring destroyed a hospital and associated buildings its last evacuating occupants were close witnesses to the eruption processes deposits exposed in the lower portion of the tephra ring show that this part of the eruption began with a mild phreatomagmatic explosive eruption from a narrow vent followed by a magmatic scoria producing phase subsequently a complex sequence of highly explosive phreatomagmatic eruptions occurred producing pyroclastic surges along with repeated distinctive breccia horizons rich in coral and lava country rock these features tally with eye witness accounts to indicate that the main eruption phase was produced by a periodically shifting locus of phreatomagmatic fragmentation and eruption along a single e w fissure the glassy and vesicle poor pyroclasts produced during this eruption phase were dominantly fragmented in a brittle manner by magma water interaction low volatile content of the magma upon fragmentation is confirmed by ftir analysis showing

control of strike slip fault on dyke emplacement and morphology strike slip faults are commonly assumed to influence magma transport and emplacement in the earth9s crust however direct observations of magma conduits within strike slip faults are lacking here we provide some of the first detailed field observations of dykes emplaced within strike slip faults in the neuquen basin argentina we show how fault planes within strike slip fault zones affect the emplacement of dyke offshoots resulting in complex dyke morphology our study also emphasizes the importance of pre existing strike slip fault array on the development of dyke swarms showing that orientations of dyke swarms may not systematically relate to the principal tectonic stress axes

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

quantitative experiments on phreatomagmatic explosions abstract an experimental set up for controlled generation of thermal explosions fuel coolant interactions has been built by an interdisciplinary research group of volcanologists physicists and engineering scientists the tee haus thermal explosion experiment during the last 3 years more than 500 experimental runs were performed aim of the project was not to build a mini volcano but to produce igniteable mixtures of water and hot melt on a small scale in order to investigate basic physical aspects such small scale mixtures however most probably also occur during phreatomagmatic events in nature mixtures of water and magma on a large scale may be modelled by combination of small elements earlier experimental studies of thermal explosions were carried out with the use of metal melts nearly exclusively therefore a carbonate melt approx 0 3 kg of 1 1 mass ratio na2co3 k2co3 was used for initial experimental series because of easy handling and more magma similarity by injection of water into this melt explosive mixtures were produced and ignited self triggering explosive interactions occurred in a wide melt temperature range 720 to 1040 c at melt temperatures exceeding 1000 c co2 increasingly degassed presence of non condensible gas bubbles in the melt strongly reduced the explosivity the intensity of explosions only slightly increased with higher melt temperatures but strongly increased with higher water injection velocities because of more intensive mixing a minimum injection velocity was found to be approx 0 7 m s 1 experiments with silicate melts derived from diverse remelted volcanic rocks were carried out with a slightly modified set up explosions were ignited by an additional trigger a shock wave of approx 8 j in volcanic systems shock waves of this magnitude are abundant volcanic tremor etc strong explosions with silicate melts were produced at melt temperatures between 1350 and 1750 c the produced explosions exerted repulsion forces of up to 24 000 n with a typical pulse duration of 1 ms carbonate melt resp 1 5 ms silicate melts the ejection velocity was found to be in a range between 200 and 400 m s 1 explosion energy values up to 500 j were calculated the maximum explosion pressure was calculated to be in the range between 10 and 100 mpa only part of the melt so called interactive melt reacts explosively with injected water most of the melt is passively ejected by and after the thermal explosion different fragmentation processes result in specific grain size and grain shape populations that can be distinguished from each other particles resulting from the fine fragmentation process that precedes and causes the thermal explosion are characterized by angular to subrounded shapes and grain sizes ranging from 20 m to 180 m the amount of interactive melt is proportional to the intensity of the explosion the maximum amount of water vapourized at the explosive interaction so called interactive water was estimated thus the water melt mass ratio for explosive interactions could be calculated the values range between 1 25 and 1 35 carbonate melt resp between 1 6 and 1 25 silicate melts

the caldera forming eruption of the quaternary payun matru volcano andean back arc of the southern volcanic zone abstract the payun matru caldera is a quaternary andean back arc caldera 36 25 s 69 12 30 w and represents the main volcanic edifice of the homonym payun matru volcanic field located within the payenia basaltic province the payun matru volcanic field is also characterized by the payun liso volcano and 220 monogenetic basaltic cones and associated lava flows the pleistocene holocene payun matru caldera presents a long lived pre caldera stage which built a multi vent and shield like volcanic edifice mainly trachytic in composition the syn caldera stage is represented by the trachytic portezuelo ignimbrite the caldera forming eruption deposits constrained between 148 and 82 ka and the resulting caldera depression of 8 5 km in diameter afterwards the caldera event volcanism continued mostly along the caldera rim stratigraphic sections of the portezuelo ignimbrite and its correlations are presented in addition to mineral compositions and previous whole rock chemical analyses only the extra caldera deposits can be observed while the intra caldera ignimbrite is hidden by post caldera volcanic products and unconsolidated modern sediments different facies within the ignimbrite were recognized being most of them massive or eutaxitic and massive lapilli tuff facies deposited by dense pyroclastic density currents fall deposits are found only proximally and restrained to the caldera margin the absence of a widespread fall deposit suggests the lack of a sustained eruptive column at the onset of eruption massive tuff facies at the topmost deposits indicates the development of a co ignimbritic plume as the pyroclastic density currents wanned on the basis of color size and shape vesicularity and crystallinity four distinct juvenile clasts types were defined gray fiamme black juvenile dark gray and light gray pumice available chemical analysis of gray fiamme and black juvenile clasts indicate a similar trachytic composition although their mineralogical composition presents some differences several eruptive units for the northern and southern outer slopes of the caldera were defined on the basis of the juvenile types present and discontinuity surfaces the onset of caldera collapse is not marked by the presence of a lithic breccia but it is suggested by the appearance of dark gray and light gray pumice along with black juveniles in the topmost eruptive units together with an increase in lithic content relative to previous units the sequential appearance of different juvenile clasts types suggests a zoned magma chamber in terms of crystal content and composition

an approach for investigating the depositional mechanism of fine grained surge deposits the example of the dry surge deposits at la fossa di vulcano abstract fine grained thinly laminated surge deposits have been studied in order to understand the physical properties of pyroelastic deposits with a large tractional component and to investigate on type and number of laminae deposited from a single surge cloud the examples are taken from the dry surge deposits of the fossa cone vulcano island where there is a good knowledge of the stratigraphy good exposures of the deposits and an absence of secondary structures analysis of texture and structure allows all the bed facies to be grouped into 4 principal types type 1 is made up of a loose coarse and medium ash in dark grey gently undulating laminae sometimes with cross laminae that alternate with pale grey laminae type 2 consists of grey ash in beds characterized by limited lateral continuity normal grading and a flat non erosive base type 3 is made up of submillimeter ochreous laminae plane parallel or gently undulating type 4 consists of massive beds of fine ash ochre in colour characterized by notably lateral continuity and constant thickness determination of the nature morphology and size of the grains shows that a close relationship exists between the relative abundance and shape of components and the textural and grain size characteristics of the bed facies the results suggest that a single surge event could deposit either a set of plane parallel to transverse laminae as type 1 and type 3 or a pair of laminated beds as type 2 and 4 depending on the flow velocity the effective grain size and density of the solid phase and the particle concentration in the eruptive cloud the almost complete lack of lithics suggests that magma water interaction occurred in sub surface conditions across a fragmented magma surface the efficiency of the interaction was primarily controlled by the ascent velocity of the magma in the conduit and its physical properties

contrasting types of surtseyan tuff cones on marion and prince edward islands southwest indian ocean ten surtseyan tuff cones on marion island 46 54 s 37 46 e and seven on prince edward island 46 38 s 37 57 e were erupted on shallow submerged coastal plains related to normal faulting they range from pleistocene to holocene in age and exhibit a variable degree of erosion by the sea fundamental differences irrespective of age exist between two types type i cones have diameters of 1 1 5 km rim heights of about 200 m and steep 27 outer slopes deposits are plastered against nearby cliffs beds are thin including layers of accretionary lapilli and less than 10 lithic clasts numerous bomb sags soft sediment deformation structures and gravity slides occur on one of these cones mudflows formed small tunnels which resemble lava tubes associated with channels sometimes having oversteepened walls these cones reflect comparatively low energy conditions and probably resulted from extremely wet surges interspersed with mudflows and ballistic falls type ii cones have smaller diameters 0 5 km but widespread fallout surge aprons rim heights are about 100 m and average slope angles are 18 bedding is massive with variable lapilli matrix ratio and more than 10 lithic clasts without bomb sags these cones formed under drier perhaps hotter and more violently explosive conditions than type i though not as energetic as the phreatomagmatic eruptions of terrestrial tuff rings the two types of surtseyan eruptions are explained by invoking not only different water magma ratios in the conduit but also different mechanisms of water magma interaction the slurry model of kokelaar is favoured for type i and a fuel coolant model for type ii the decisive factor is considered to be rate of effusion with rim closure and exclusion of sea water playing a secondary role

growth destruction and facies architecture of effusive and explosive volcanics in the miocene shama basin southwest of saudi arabia subaqueous subaerial volcanism in a lacustrine setting abstract this paper presents for the first time detailed stratigraphical and petrographical analyses of the harrat shama volcanic basin hsvb of miocene age in sw saudi arabia this basin is filled by abundant volcanics intercalated with fluvio lacustrine sedimentary successions these successions are characterized by a syneruptive intereruptive cycle which results from the intercalation periods of volcanics and volcaniclastic sedimentation along with periods of fluvial incision two depositional sequences showing an overall fining upward trend are preserved within the hsvb the first comprises hyaloclastites zeolite bearing bedded tuffs and bedded accretionary lapilli tuffs interpreted as deep lake whereas the second is made up of extensive pumiceous pyroclastic flows peperitic breccias and basaltic lava flows which are in turn capped by siliciclastic deposits of lacustrine deposits the passage between two sequences is defined by lateral variations of the lithofacies and textural features reflecting a change in paleogeography the fining upward volcaniclastic deposits of the hsvb sequence are interpreted to represent the sedimentation pattern induced by tectonic subsidence with the influence of volcaniclastic sediment input indicating well established proximal distal relationship hsvb was evolved through three stages including pre syn and post caldera stages the first two stages correspond to proximal aggradational syn eruptive event whereas the latter stage refers to distal degradational inter eruptive event these stages overlap in time as shown by the alternation of explosive effusive and their resedimented deposits reflecting diverse eruptive sequences formed by different fragmentation processes the difference in the latter coupled with the variability in the magma water interaction and the nature of substrate are responsible for such diversity resulting in progressive environmental changes from subaqueous to subaerial regime from bottom to top the development of such contrast in the depositional sequences reflects variation in the sediment flux and subsidence rate during the evolution of the hsvb

morphometry of scoria cones and their relation to geodynamic setting a dem based analysis abstract the morphometry of a great number of scoria cones belonging to volcanic fields of various geodynamic settings has been measured and analyzed addressing the question whether there is a relation between the prevalent cone shape in a given field and the geodynamic setting of the field itself morphometric analysis was carried out on freely downloadable digital elevation models dems the accuracy of the used dems and the associated error in scoria cone morphometry were determined by cross comparing high resolution lidar derived dems usgs ned tinitaly dem and aster gdem the 10 m tinitaly 01 and usgs ned dems are proven to be suitable for scoria cone morphometry whereas aster gdem can be used reliably for cones with volume greater than 30 10 6 m 3 according to a detailed morphometry of all scoria cones we propose that the cones related to subductional setting show relatively higher values of h c o w co and lower values of w cr w co than the cones related to extensional setting the detected differences can be imputable to peculiar eruption dynamics resulting in slight but systematic changes in shape and differences in lithological and sedimentological characteristics that govern post eruptive erosion to constrain the pathway of scoria cone erosion the detected morphometric changes were also interpreted using a simple linear degradation model utilizing the obtained simulation results the inferred initial cone base and the age of scoria cones we calculated a diffusion coefficient k for several dated cones which are related to the prevalent climate our results despite the high error associated allow to assess the median k for all volcanic fields due to the complexity of the factors behind it is not easy to understand if the prevalent shape characterizing a certain volcanic field is due mainly to sin eruptive or post eruptive mechanisms however our distinction between the two main geodynamic settings may be the first step to decipher these factors

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

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

comparative mass balance of volcanic edifices at the southern volcanic zone of the andes between 33 s and 46 s volcano edifice volume calculations are presented for 65 volcanoes of the 1400 km long chilean southern volcanic zone svz as a basic step in subduction zone mass budgets volume calculations are performed in a geographical information system that integrates digital elevation models based on of shuttle radar topography mission as well as aster gdem topographic data landsat satellite images and geological maps the method of volume calculation is straightforward for isolated morphologically well defined stratovolcanoes uncertainties increase for volcanic edifices that formed on pre existing rugged terrain for multi phase eruptive centers as well as for eroded edifices a revised segmentation of the arc is used to describe the spatial volume distribution of extruded magma along the svz and to discuss controlling tectonic factors peak volumes between arc and back arc are offset by 400 km the total volcanic extrusion is in the range of 10 13 km3 km ma major differences between the svz and the central american subduction system are notable with regard to volcano density and maximum volumes

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

maar diatreme phreatomagmatism at hopi buttes navajo nation arizona usa remnants of an extensive maar diatreme volcanic field are magnificently exposed at various depths of erosion in the hopi buttes volcanic field of northeastern arizona field and petrographic studies of both the maar and diatreme elements of a selection of volcanoes within the field show that 1 lower sections of the maar rim sequences are typically rich in sandy mudrock derived from the pre eruptive mio pliocene bidahochi formation and the muddy bidahochi sediment was soft and wet at the time of maar eruptions 2 beds higher within the rims contain generally increased proportions of sandstone clasts from the triassic wingate formation in the diatremes late emplaced breccia has deeper seated lithics than more marginal breccia emplaced earlier and 3 many vents are topped by megacryst enriched scoria and spatter and deep seated xenoliths are known only from upper diatreme and craterfilling tephra these observations show that 1 eruptions at hopi buttes involved interaction of magma with unconsolidated mudrock at shallow levels and the phreatomagmatic processes that provided the bulk of the energy involved in the violent eruptions were driven by the interaction of magma and wet sediment 2 the locus of explosive activity migrated downward as eruptions progressed and 3 the closing stage of many eruptions was characterized by rapid magma rise and relative depletion of water

depositional mechanisms and alteration processes in different types of pyroclastic deposits from campi flegrei volcanic field southern italy the campi flegrei volcanic field southern italy is composed of a large variety of pyroclastic deposits related to both magmatic and hydromagmatic activity in spite of similarities of composition grain size age and pre eruptive environment the various deposits exhibit great differences in terms of degree and type of alteration of the juvenile components in particular detailed analyses of pyroclasts collected in the numerous monogenetic volcanic cones show the broad range of alteration from fresh glass to complete replacement of the glass by the zeolites the comparative analysis of 17 different deposits has allowed us to conclude that the eruptive and emplacement conditions were the major controls on the zeolitization processes the relationships between volcanological features of deposits and secondary mineralization are consistent with the glass fluid phase reaction in a near closed post depositional system eruptions characterised by separation of water vapour from the pyroclastic particles produce unaltered or poorly altered deposits in contrast eruptions characterised by mechanisms that permit the capture and retention of pore water and water vapour produce zeolitized deposits according to these lines of evidence the zeolitization of pyroclastic products of campi flegrei took place immediately after emplacement during the cooling of wet deposits

seismic rheological model and reflection coefficients of the brittle ductile transition it is well established that the upper cooler part of the crust is brittle while deeper zones present ductile behaviour in some cases this brittle ductile transition is a single seismic reflector with an associated reflection coefficient we first develop a stress strain relation including the effects of crust anisotropy seismic attenuation and ductility in which deformation takes place by shear plastic flow viscoelastic anisotropy is based on the eigenstrain model and the zener and burgers mechanical models are used to model the effects of seismic attenuation velocity dispersion and steady state creep flow respectively the stiffness components of the brittle and ductile media depend on stress and temperature through the shear viscosity which is obtained by the arrhenius equation and the octahedral stress criterion the p and s wave velocities decrease as depth and temperature increase due to the geothermal gradient an effect which is more pronounced for shear waves we then obtain the reflection and transmission coefficients of a single brittle ductile interface and of a ductile thin layer the pp scattering coefficient has a brewster angle a sign change in both cases and there is substantial ps conversion at intermediate angles the pp coefficient is sensitive to the layer thickness unlike the ss coefficient thick layers have a well defined brewster angle and show higher reflection amplitudes finally we compute synthetic seismograms in a homogeneous medium as a function of temperature

the influence of conduit processes on changes in style of basaltic plinian eruptions tarawera 1886 and etna 122 bc basaltic volcanism is most typically thought to produce effusion of lava with the most explosive manifestations ranging from mild strombolian activity to more energetic fire fountain eruptions however some basaltic eruptions are now recognized as extremely violent i e generating widespread phreatomagmatic subplinian and plinian fall deposits we focus here on the influence of conduit processes especially partial open system degassing in triggering abrupt changes in style and intensity that occurred during two examples of basaltic plinian volcanism we use the 1886 eruption of tarawera new zealand the youngest known basaltic plinian eruption and the only one for which there are detailed written eyewitness accounts and the well documented 122 bc eruption of mount etna italy and present new grain size and vesicularity data from the proximal deposits these data show that even during extremely powerful basaltic eruptions conduit processes play a critical role in modifying the form of the eruptions even with very high discharge and presumably ascent rates partial open system behaviour of basaltic melts becomes a critical factor that leads to development of domains of largely stagnant and outgassed melt that restricts the effective radius of the conduit the exact path taken in the waning stages of the eruptions varied in response to factors which included conduit geometry efficiency and extent of outgassing and availability of ground water but a relatively abrupt cessation to sustained high intensity discharge was an inevitable consequence of the degassing processes

buoyancy reversal decoupling and the transition from stratocumulus to shallow cumulus topped marine boundary layers the transition in a marine boundary layer mbl from stratocumulus topped to shallow cumulus topped is investigated by using a large eddy simulation les model the experiments performed aim to examine the influence on the transition of 1 the probability of buoyancy reversal at the mbl top i e situations in which the mixture of two air parcels becomes denser than either of the original parcels due to phase change or other nonlinear processes involved in the mixing and 2 the degree of decoupling in the mbl i e the strength of a shallow stably stratified layer near cloud base our results suggest that a stratocumulus topped mbl is most likely to transit to a cumulus topped one when 1 there exists high probability of buoyancy reversal at the mbl top and 2 the mbl is decoupled due to large surface evaporation we argue that a parameterization that includes representation of those two effects combined has the potential to provide a simple way of predicting the mbl transition in climate models

varicolored and vesiculated tuffs from la fossa volcano vulcano island aeolian archipelago italy evidence of syndepositional alteration processes chemical and mineralogical data for samples collected from a surge sequence from la fossa cone vulcano island italy show a wide variety of alteration states between adjoining beds the macroscopic features of which are expressed by various degrees of reddening the effects of the alteration processes on pyroclastic rocks are as follows hydration and oxidation of each component of the pyroclastic rocks to varying degrees formation of authigenic smectite precipitation of a large variety of soluble salts and corrosion on the surface of glass fragments pitting dry surge beds emplaced from a two phase dry steam solid suspension do not show significant alteration by contrast wet surge deposits suggesting an emplacement from free water bearing turbulent flows show an increasing degree of alteration passing from grey to red coloured beds the strict relationship between the present alteration state and the depositional unit rules out any post depositional processes the occurrence of alteration in wet surge beds and the lack of alteration in dry beds shows that the main controlling agent was water condensed from the eruptive cloud and suggests a syn depositional character to the alteration itself these observed differences can be ascribed to the different chemical reactivities of the water probably related to the amounts of acidic species carried by the eruptive cloud and or by the efficiency of their capture during the condensation of the water vapour

capelinhos 1957 1958 faial azores deposits formed by an emergent surtseyan eruption the 1957 1958 eruption of capelinhos faial island azores involved three periods of surtseyan hydromagmatic activity two in 1957 and one in 1958 deposits from this eruption are exposed both in sea cliffs cut into the flanks of the tuff cone and more distally 1 km from the vent five lithofacies are identified lithofacies i is composed of even thickness beds with laterally continuous internal stratigraphy and is interpreted to have been formed by fallout lithofacies ii consists of beds with internally discontinuous lenses and has sand wave structures that increase in abundance toward the outer margins of the tuff cone this lithofacies is interpreted as having been deposited from pyroclastic surges lithofacies iii is composed of mantle bedded deposits with laterally discontinuous internal stratigraphy this lithofacies is interpreted to have been formed by hybrid processes where fallout of tephra occurred simultaneously with pyroclastic surges in the outer flanks of the tuff cone lithofacies iii grades laterally into fallout beds of lithofacies i lithofacies iv consists of alternating beds of coarse ash aggregates and non aggregated fine ash and is particularly well developed in distal regions some of this facies was formed by fallout alternating beds also occur plastered against obstacles up to 2 km from the vent indicating an origin from wet pyroclastic surges the orientation of plastered tephra indicates that the surges were deflected by topography as they decelerated the distinction between surge and fallout in distal regions is uncertain because wind drifted fallout and decelerating surge clouds can generate similar deposits lithofacies v consists of scoria lapilli beds interpreted to be fallout from hawaiian style fire fountaining in the later stages of the eruption juvenile pyroclasts within hydromagmatic deposits are predominantly poorly vesicular 25 60 of clasts 30 vesicles however on both micro and macroscopic scales there is a wide range in clast vesicularity up to 70 vesicles indicating that although fragmentation was predominantly hydromagmatic vesiculation and magmatic volatile driven fragmentation operated simultaneously

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

geomorphometric variability of monogenetic volcanic cones evidence from mauna kea lanzarote and experimental cones abstract volcanic cones are the most common volcanic constructs on earth their shape can be quantified using two morphometric ratios the crater cone base ratio wcr wco and the cone height width ratio hco wco the average values for these ratios obtained over entire cone fields have been explained by the repose angle of loose granular material i e scoria controlling cone slopes the observed variability in these ratios between individual cones has been attributed to the effect of erosional processes or contrasting eruptive conditions on cone morphometry using a gis based approach high spatial resolution digital elevation models and airphotos two new geomorphometry datasets for cone fields at mauna kea hawaii usa and lanzarote canary islands spain are extracted and analyzed here the key observation in these datasets is the great variability in morphometric ratios even for simple shape and well preserved cones simple analog experiments are presented to analyze factors influencing the morphometric ratios the formation of a crater is simulated within an analog cone i e a sand pile by opening a drainage conduit at the cone base results from experiments show that variability in the morphometric ratios can be attributed to variations in the width height and horizontal offset of the drainage point relative to the cone symmetry axis to the dip of the underlying slope or to the influence of a small proportion of fine cohesive material gis analysis and analog experiments together with specific examples of cones documented in the field suggest that the morphometric ratios for well preserved volcanic cones are controlled by a combination of 1 the intrinsic cone material properties 2 time dependent eruption conditions 3 the local setting and 4 the method used to estimate the cone height implications for interpreting cone morphometry solely as either an age or as an eruption condition indicator are highlighted

stratigraphy petrochemistry and quaternary depositional record of the songaksan tuff ring jeju island korea abstract the songaksan tuff ring str is one of several recent hydrovolcanic centers on jeju island korea which provides an excellent example of proximal to distal facies changes in wet pyroclastic surge deposits a multidisciplinary study has been carried out on the str and adjacent lithostratigraphic units to constrain absolute age geochemical characteristics and quaternary depositional history a number of rock units were identified inside the crater of the str including scoria deposit i trachybasalt lava scoria deposit ii and a late stage basaltic tuff indicative of a rather complex sequence of magmatic and phreatomagmatic eruptions after the construction of the tuff ring petrochemical analysis shows that the str was generated from different magma batches that fractionated from a homogenous magma chamber and the early erupted magma was more evolved and volatile rich reworking of the str commenced shortly after the hydromagmatic eruption in a high energy nearshore environment resulting in deposition of the hamori formation the formation is composed of planar stratified and low to high angle cross stratified tuffaceous pebbly sandstones and occurs up to an altitude of about 4 m above present sea level 14c dating of molluscan shells beneath the formation indicates that it began to be deposited after about 4000 yr bp detailed sedimentary logging reveals that the formation consists of several stratal packages bounded by laterally persistent and distinct lithologic boundaries probably formed by millennial scale sea level fluctuations occurrence of another hydrovolcanic sequence the sinyangri formation on the opposite side of jeju island having similar sedimentary characteristics and ages suggests that the sea level fluctuations as seen in the hamori formation have affected a wide area of jeju island probably related to the high frequency sea level oscillations during the post 6 ka bp regression period in the east asian region it can be concluded that the formation of the str was possible because of the holocene transgression which made the present coastal areas water saturated and adequate for hydrovolcanic eruptions the str in turn contributed to record high frequency sea level fluctuations during the holocene via acting as a local and short lived but affluent source of loose sediment

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

emplacement history and inflation evidence of a long basaltic lava flow located in southern payenia volcanic province argentina abstract the el corcovo lava flow from the huanul shield volcano in the southern mendoza province central western argentina traveled a distance of 70 km and covered a minimum area of 415 km 2 the flow emplacement was controlled both by extrinsic e g topography and intrinsic e g lava supply rate lava physicochemical characteristics factors the distal portion of the lava flow reached the colorado river valley in la pampa province where it spread and then was confined by earlier river channels cross sections through the flow surveyed at several localities show two vesicular layers surrounding a dense central section where vesicles are absent or clustered in sheet shaped and cylindrical shaped structures lavas of the el corcovo flow are alkaline basalts with low values of viscosity the morphological and structural characteristics of the flow and the presence of landforms associated with lava accumulation are the evidence of inflation this process involved the formation of a tabular sheet flow up to 4 m of thick with a large areal extent in the proximal sectors while at terminal sectors frontal lobes reached inflation values up to 10 m the numerous swelling structures present at these portions of the flow suggest the movement of lava in lava tubes we propose that this aspect and the low viscosity of the lava allowed the flow travel to a great distance on a gentle slope relief

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

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

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