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

time scales of hybridisation of magmatic enclaves in regular and chaotic flow fields petrologic and volcanologic implications this paper describes numerical models of advection diffusion between enclaves and host magmas applied with the aim of estimating time scales during which enclaves can be homogenised in particular advection was simulated using a numerical system consisting of regular and chaotic regions results indicate that the homogenisation time of enclaves in chaotic regions is several orders of magnitude faster than in regular regions for instance an enclave with a diameter of 100 cm may be homogenised in the chaotic region in 380 years assuming an advection velocity of 10 cm year whereas in the regular region it would require 6 5 105 years for complete homogenisation this implies that in the same magmatic system large differences in the degree of homogenisation may co exist generating magmatic masses with large spatial and temporal inhomogeneities

high spatial resolution electron probe microanalysis of tephras and melt inclusions without beam induced chemical modification wavelength dispersive analysis of tephra grains and melt inclusions by epma has been carried out using a focused beam of 3 m diameter without detected loss of sodium or potassium in standard glasses including anhydrous basalts a slightly hydrated rhyolite and a sodium rich intermediate composition the ability to make analyses without chemical modification is strongly dependent upon current density at the analysis site analysis with narrow beams requires extremely low beam currents that are normally associated with energy dispersive analysis experiments indicate that a value of 0 1 na m2 must not be exceeded at least for moderately hydrated samples if sodium loss is to be avoided high resolution analysis without beam induced analytical artefacts enables fully quantitative analysis of very distal and or highly vesicular tephras and very small melt inclusions without the need to use post analysis corrections this development has enabled high quality analyses from crypto tephra layers that were pre

eruption processes and deposit characteristics at the monogenetic mt gambier volcanic complex se australia implications for alternating magmatic and phreatomagmatic activity the 5 ka mt gambier volcanic complex in the newer volcanics province australia is an extremely complex monogenetic volcanic system that preserves at least 14 eruption points aligned along a fissure system the complex stratigraphy can be subdivided into six main facies that record alternations between magmatic and phreatomagmatic eruption styles in a random manner the facies are 1 coherent to vesicular fragmental alkali basalt effusive hawaiian spatter and lava flows 2 massive scoriaceous fine lapilli with coarse ash strombolian fallout 3 bedded scoriaceous fine lapilli tuff violent strombolian fallout 4 thin medium bedded undulating very fine lapilli in coarse ash dry phreatomagmatic surge modified fallout 5 palagonite altered cross bedded medium lapilli to fine ash wet phreatomagmatic base surges and 6 massive palagonite altered very poorly sorted tuff breccia and lapilli tuff phreato vulcanian pyroclastic flows since most deposits are lithified to quantify the grain size distributions gsds image analysis was performed the facies are distinct based on their gsds and the fine ash to coarse fine ash ratios these provide insights into the fragmentation intensities and water magma interaction efficiencies for each facies the eruption chronology indicates a random spatial and temporal sequence of occurrence of eruption styles except for a magmatic horizon of effusive activity occurring at both ends of the volcanic complex simultaneously the eruption foci are located along nw se trending lineaments indicating that the complex was fed by multiple dykes following the subsurface structures related to the tartwaup fault system possible factors causing vent migration along these dykes and changes in eruption styles include differences in magma ascent rates viscosity crystallinity degassing and magma discharge rate as well as hydrological parameters

mechanisms of hydrovolcanic pyroclast formation grain size scanning electron microscopy and experimental studies pyroclasts produced by explosive magma water interactions are of various sizes and shapes data from analysis of over 200 samples of hydrovolcanic ash are interpreted by comparison with experimentally produced ash grain size and scanning electron microscopy sem reveal information on the formation of hydrovolcanic pyroclasts strombolian explosions result from limited water interaction with magma and the pyroclasts produced are dominantly centimeter sized with increasing water interaction hydrovolcanism increases in explosivity to surtseyan and vulcanian activity these eruptions produce millimeter to micron sized pyroclasts the abundance of fine ash 63 m diameter increases from 5 to over 30 percent as water interaction reaches an explosive maximum this maximum occurs with interactions of virtually equal volumes of melt and water five dominant pyroclast shape types determined by sem result from hydrovolcanic fragmentation 1 blocky and equant 2 vesicular and irregular with smooth surfaces 3 moss like and convoluted 4 spherical or drop like and 5 plate like types 1 and 2 dominate pyroclasts greater than 100 m in diameter types 3 and 4 are typical of fine ash type 5 pyroclasts characterize ash less than 100 m in diameter resulting from hydrovolcanic fragmentation after strong vesiculation fragmentation mechanisms observed in experimental melt water interactions result from vapor film generation expansion and collapse fragments of congealed melt are products of several alternative mechanisms including stress wave cavitation detonation waves and fluid instability mixing all result in rapid heat transfer these mechanisms can explain the five observed pyroclast shapes stress wave fracturing cavitation of the melt results from high pressure and temperature gradients at the melt water interface simultaneous brittle fracture and quenching produces type 1 pyroclasts type 2 develops smooth fused surfaces due to turbulent mixing with water after fracture and before quenching fluid instabilities promote turbulent mixing of melt and water and produce fine ash this kind of fragmentation occurs during high energy explosions the increased melt surface area due to fine fragmentation promotes high efficiency heat exchange between the melt and water shapes of resulting pyroclasts are determined by maximum surface area type 3 and surface tension effects type 4 type 5 pyroclasts result from nearly simultaneous vesicle burst and melt water fragmentation

melting at the base of the greenland ice sheet explained by iceland hotspot history basal melting is widespread in the north central greenland ice sheet geophysical data and numerical modelling suggest a geothermal anomaly in this region resulting from the earlier passage of greenland over the iceland mantle plume

the 1732 surtseyan eruption of eggoya jan mayen north atlantic deposits distribution chemistry and chronology the island of jan mayen in the north atlantic ocean is home to the world s northernmost active subaerial volcano beerenberg of the five known historical eruptions on the island the locations of two eruptions 1732 and 1818 have not been accurately located it is known that the 1732 and 1818 eruptions occurred on the south flank of beerenberg and several eruption sites have been proposed for these events here we show that the tuff cone of eggoya on the sw flank of beerenberg was the site of the 1732 eruption based on interpretation of the deposits field relations and historical sources we further describe the deposits from the eruption and show that eggoya is the largest explosive eruption described from jan mayen emplacing at least at least 0 3 0 4 km3 vei 4 of basanitic tephra up to distances of at least 111 km from jan mayen and covering a minimum area of around 500 km2 within the 2 cm isopach we also present our eruption scenario and show that this was an emergent surtseyan eruption with activity shifting between tephra jetting continuous uprush and more magmatic phases

hydrovolcanic explosions the systematics of water pyroclast equilibration the initial contact of external water with erupting magma and the mass ratio of water to magma govern the development of hydrovolcanic phenomena the phase relations of water within the runout system and the separation of vapor or liquid from the pyroclasts explain gradational transitions between some pyroclastic flows base surges mud hurricanes mudflows and sheetfloods

hybridization of mingling magmas with different densities abstract it is usually assumed that chemical exchange between mafic enclaves considered as blobs of mafic liquids and their host silicic melt proceeds through static diffusion however the chemical boundary layer should keep the interior of magmatic enclaves nearly pristine a feature which is not observed the surface layer of a mafic blob falling in a more silicic melt is likely to be continuously folded in with the inner part a numerical model shows that buoyancy driven forced convection is an efficient way of enhancing the rate at which melt inclusions become hydrous and hybridized by the host melt

modeling tephra dispersal in absence of wind insights from the climactic phase of the 2450 bp plinian eruption of pululagua volcano ecuador the determination of eruptive parameters is crucial in volcanology not only to document past eruptions but also for tephra fallout hazard assessments in most tephra fallout studies eruptive parameters have been determined either by empirical techniques or analytical models but the uncertainty of such parameters is usually not well described we have applied both empirical and analytical models to characterize the climactic phase of the 2450 bp plinian eruption of pululagua bf2 layer and explore the variations in the total erupted mass column height and total grain size distribution both approaches yield comparable results in the total mass of tephra erupted 4 5 1 5 1011 kg while they show some discrepancies for the determination of the column height 36 20 km from empirical techniques and 30 20 km from analytical techniques the total grain size distribution of the bf2 layers varies with the different techniques used for the calculation and significantly affects the outputs of analytical models furthermore the determination of the total grain size distribution depends strongly on the number and spatial distribution of the sample location inverting tephra fallout deposits on the total accumulation or thickness gives a good constraint on the total mass erupted but not on the column height however inverting on individual grain size classes better constrains the possible range of column heights but cannot resolve particle release height results from the inversion on individual grain size classes show that large diffusion coefficients are necessary to model the bf2 layer and might be required to model proximal tephra deposits in order not to overestimate the total erupted mass additionally we used a statistical method smoothed bootstrap approach to quantify the uncertainty in eruptive parameters such as column height and total erupted mass our uncertainty analysis yields a mean total erupted mass of 4 5 0 3 1011 kg and a mean column height of 30 3 km results from the uncertainty analysis compare well with results from other approaches finally although the climactic phase of the 2450 bp plinian eruption of pululagua occurred in relatively calm atmospheric conditions our results show that the dispersion of the bf2 layer was influenced by slight northeasterly wind conditions

depositional processes of the suwolbong tuff ring cheju island korea the suwolbong pyroclastic sequence in the western part of cheju island korea comprises partly preserved rim beds of a quaternary basaltic tuff ring whose vent lies about 1 km seaward of the present shoreline the sequence consists of breccia lapillistone lapilli tuff and tuff eighteen sedimentary facies are established and organized into six lateral facies sequences lfs and seven vertical facies sequences vfs the lfs 1 4 and 5 begin with massive lapilli tuff which transforms downcurrent into either planar bedded lfs 1 undulatory bedded lfs 4 or climbing dune bedded lfs 5 lapilli tuff units they are representative of relatively dry base surge whose particle concentration decreases downcurrent with a progressive increase in both tractional processes and sorting the lfs 2 begins with disorganized and massive lapilli tuff and transforms into crudely stratified units downcurrent it results from relatively wet base surge in which sorting is poor due to the cohesion of damp ash the lfs 3 comprises well sorted lapilli tuff and stratified tuff further downcurrent suggestive of deposition from combined fall and surge of relatively dry hydroclastic eruption all seven vertical facies sequences generally comprise two facies units of coarse grained fines depleted lapilli tuff and an overlying fine grained tuff these sequences are suggestive of deposition from base surge that consists of a turbulent head and a low concentration tail depositional processes in the suwolbong tuff ring were dominated by a relatively dry base surge the base surge comprises turbulent and high concentration suspension near the vent whose deposits are generally unstratified due to the lack of tractional transport as the base surge becomes diluted downcurrent through fallout of clasts and mixing of ambient air it develops large scale turbulent eddies and is segregated into coarse grained bedload and overlying fine grained suspension forming thinly stratified units further downcurrent the base surge may be either cooled and deflated or pushed up into the air depending on its temperature the suwolbong tuff ring comprises an overall wet to dry cycle with several dry to wet cycles in it suggestive of overall decrease in abundance of external water and fluctuation in the rate of magma rise

geochemical variation with time in the cenozoic high k volcanic rocks of the island of lesbos greece significance for shoshonite petrogenesis shoshonitic and calc alkaline volcanic rocks alternate in the miocene volcanic sequence of the island of lesbos in the eastern aegean sea the sequence and chronology of the lavas is well known which has allowed a study of both spatial and temporal geochemical variation early calc alkaline volcanism 21 5 ma was followed by a hiatus the main shoshonitic phase occurred from 17 to 17 7 ma and was followed by minor late calc alkaline volcanism the earliest shoshonitic lavas skoutaros formation were relatively anhydrous with f as an important volatile phase but later lavas sykaminea formation have abundant phenocrysts of hydrous minerals and show ree patterns indicating amphibole fractionation there is petrographic and mineralogic evidence for magma mixing throughout the volcanic sequence shoshonitic lavas enriched in k nb th and 206pb 204pb interbed with more calc alkaline lavas in both more hydrous and less hydrous stratigraphic units these variations in k nb and th cannot be accounted for by fractionation or crustal assimilation rather they reflect different mantle sources with different pb isotope signatures the k rich source is similar to source mantle for many alkaline ultrapotassic rocks the lower k source resembles normal calc alkaline source mantle both sources are unusually high in 207pb and 208pb suggesting strong th and u enrichment similar to that in roman province and sunda arc rocks the observed volcanic sequence is consistent with partial melting caused by a rising volatile front that initially triggered partial melting at higher levels in the mantle but as volatile content increased allowed partial melting at greater depths in less enriched mantle

geochronology and geochemistry of the chuanwulu complex in the south tianshan western xinjiang nw china implications for petrogenesis and phanerozoic continental growth abstract the south tianshan collisional belt stcb formed by the collision between the tarim and central tianshan blocks is pivotal for understanding the complex and prolonged tectonic evolution of the central asian orogenic belt caob the chuanwulu complex is located in the eastern part of the stcb xinjiang province nw china it consists of a gabbro diorite unit unit i and a monzonite syenite unit unit ii which are both cut by some granite dykes unit iii based on la icp ms u pb zircon dating the units i and ii were emplaced at 287 8 4 3 ma and 286 4 2 5 ma respectively the sio 2 contents of the samples from the complex display a wide range from 50 52 wt to 70 64 wt and most samples are of alkaline affinities the magma mixing process during the formation of the complex is suggested by considerable petrographic and geochemical evidence such as mafic microgranular enclaves mees that occur within unit ii disequilibrium textures linear negative correlations between sio 2 and some major elements and a wide range of 87 sr 86 sr t ratios and 18 o smow values unit i is characterized by relatively low sio 2 contents 50 52 55 05 wt high mgo contents 3 29 5 43 wt with mg 0 50 0 56 high sr contents 1646 3101 ppm pronounced negative nb ta anomalies and light rare earth element lree enrichment in combination with their isotopic compositions 87 sr 86 sr t 0 70543 0 70751 e nd t 2 3 to 1 8 18 o smow 5 7 9 4 e hf t 0 2 4 9 these features indicate that they are derived from an incompatible element enriched lithospheric mantle the sio 2 contents of units ii and iii range from 58 88 wt to 65 91 wt and 67 10 wt to 70 64 wt respectively they exhibit pronounced positive zr hf anomalies and negative p and ti anomalies and relatively elevated sr and o isotopic compositions 87 sr 86 sr t 0 70543 0 70751 18 o smow 8 9 9 1 in addition samples of unit iii have high sr 600 ppm to 1201 ppm contents and sr yb 373 to 1905 and low y yb 8 to 12 ratios the felsic rocks units ii and iii have a c type adakite like geochemical signature and are comparable with those of rocks derived from an ancient garnet bearing amphibolite facies lower crust our study complied with other geological evidence indicate that the collision between the central tianshan and tarim blocks and the final amalgamation of the caob should have occurred during late carboniferous on the regional scale we propose that the recycling of ancient lithosphere is the predominant mechanism which is able to account for phanerozoic continental evolution in stcb differing from other tectonic units in caob

identifying magma water interaction from the surface features of ash particles the deposits from explosive volcanic eruptions those eruptions that release mechanical energy over a short time span1 are characterized by an abundance of volcanic ash2 3 this ash is produced by fragmentation of the magma driving the eruption and by fragmenting and ejecting parts of the pre existing crust host rocks interactions between rising magma and the hydrosphere oceans lakes and ground water play an important role in explosive volcanism4 5 because of the unique thermodynamic properties of water that allow it to very effectively convert thermal into mechanical energy although the relative proportion of magma to host rock fragments is well preserved in the pyroclastic rocks deposited by such eruptions it has remained difficult to quantitatively assess the interaction of magma with liquid water from the analysis of pyroclastic deposits2 3 4 5 here we report the results of a study of natural pyroclastic sequences combined with scaled laboratory experiments we find that surface features of ash grains can be used to identify the dynamic contact of magma with liquid water the abundance of such ash grains can then be related to the water magma mass ratios during their interaction

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

morphochemistry of patterns produced by mixing of rhyolitic and basaltic melts abstract in this work we present the results of time series experiments performed by mixing basaltic and rhyolitic melts at high temperature using a device recently developed to trigger chaotic dynamics in a mixing system the morphology of mixing patterns is quantified at different times by measuring their fractal dimension and a linear relationship is derived between mixing time and morphological complexity the complexity of mixing patterns is also compared to the degree of homogenization of chemical elements during mixing and empirical relationships are established between the fractal dimension and the temporal variation of concentration variance of elements new concepts and tools to study the magma mixing process unfold from the experimental results presented in this work the first one is that the mixing patterns are fractals and they can be quantified by measuring their fractal dimension this represents a further step in the quantification of the magma mixing process the second outcome is that the relationship between the fractal dimension of the mixing patterns and mixing time is linear this has important volcanological implications as the analyses of the morphology of mixing patterns in volcanic rocks can be complemented by experiments to build a new chronometer to estimate the mixing to eruption time a further result from this work is the relationship between the fractal dimension of mixing patterns and concentration variance of chemical elements this represents the first morphochemical study in igneous petrology bringing with it the potential to infer the relative mobility of chemical elements during the time progression of mixing by analyzing the morphology of mixing patterns in the rocks

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

analysis and numerical simulation of chaotic advection and chemical diffusion during magma mixing petrological implications abstract structures generated by magma mixing in a lava flow are studied with the aim to understand the interplay between chemical diffusion and the dynamics of the mixing process mesoscopic analysis of mixing structures indicates that magmas mixed intimately generating the contemporaneous occurrence of filament like and globular regions within the same system the extent of chemical exchange between interacting magmas has been measured by epma and lam icp ms analysis on a transect crossing filaments results indicate that elements with similar values of diffusion coefficients display linear correlations in interelemental plots whereas elements with different diffusion coefficients do not show any correlation the mixing process has been simulated by coupling a chaotic advection and a chemical diffusion numerical scheme for several elements simulations show that the occurrence of chaotic flow fields is essential to explain the decoupling of the correlation at a short length scale between elements with similar and different diffusion coefficients as observed in natural samples in particular the sensitivity to initial conditions of chaotic systems induces elements having similar values of diffusion coefficients to be linearly correlated in interelemental plots whereas at the same time the correlation between elements having different diffusion coefficients is lost the degree of correlation of the different elements in the simulations and natural data has been utilized to estimate the intensity of mixing and results indicate intermediate mixing intensities for the natural samples according to field evidence it is shown that chemical diffusion processes coupled with chaotic mixing dynamics can generate strongly dishomogeneous batches of magmas coexisting at a very short length scale of the order of a few microns in which elements display very different degrees of correlation depending on the magnitude of their diffusion coefficients these results open a key question about the suitability of using melt inclusions for petrogenetic purposes in magma mixing systems because they may be affected by the small scale dishomogeneity of the magmatic system on the contrary it is shown that whole rock analysis is a more suitable technique to understand rock petrogenesis because it is poorly influenced by this dishomogeneity

pre emergent construction of a lacustrine basaltic volcano pahvant butte utah usa the subaqueous phases of an eruption initiated approximately 85 m beneath the surface of pleistocene lake bonneville produced a broad mound of tephra a variety of distinctive lithofacies allows reconstruction of the eruptive and depositional processes active prior to emergence of the volcano above lake level at the base of the volcano and very near inferred vent sites are fines poor well bedded broadly scoured beds of sideromelane tephra having local very low angle cross stratification m1 lithofacies these beds grade upward into lithofacies m3 which shows progressively better developed dunes and cross stratification upsection to its uppermost exposure approximately 10 m below syneruptive lake level both lithofacies were emplaced largely by traction from relatively dilute sediment gravity flows generated during eruption intercalated lithofacies are weakly bedded tuff and breccia m2 and nearly structureless units with coarse basal layers above strongly erosional contacts m4 the former combines products of deposition from direct fall and moderate concentration sediment gravity flows and the latter from progressively aggrading high concentration sediment gravity flows early in the eruption subaqueous tephra jetting from phreatomagmatic explosions discontinuously fed inhomogeneous unsteady dilute density currents which produced the m1 lithofacies near the vent dunes and crossbeds which are better developed upward in m3 resulted from interaction between sediment gravity flows and surface waves triggered as the explosion generated pressure waves and eruption jets impinged upon and occasionally breached the surface intermingling of a tephra emplaced after brief transport by tephra jets within a gaseous milieu and b laterally flowing tephra formed lithofacies m2 along vent margins during parts of the eruption in which episodes of continuous uprush produced localized water exclusion zones above a vent m4 comprises mass flow deposits formed by disruption and remobilization of mound tephra intermittent explosive magma water interactions occurred from the outset of the pahvant eruption with condensation entrainment of water and lateral flow marking the transformation from eruptive to sedimentary processes leading to deposition of the mound lithofacies

evidence in archaean alkali feldspar megacrysts for high temperature interaction with mantle fluids growth and regrowth textures trace element and oxygen isotope compositions and water content or species have been studied in alkali feldspars from the late archaean closepet igneous bodies feldspar crystals grew from mixed magmas that are characterized by a high degree of homogenization the 3d depiction of trace element distribution indicates that the crystallization process was followed by interaction with fluids the magmatic system involved in feldspar formation shows non linear dynamics the interaction with fluids is also deterministic but in contrast to magmatic crystallization it shows an increasing persistency in element behaviour the degree of persistency of the element activities in both processes has been calculated using the hurst exponent the recrystallization regrowth process induced by fluids proceeded along crystal fractures and cleavages causing selective enrichment in large ion lithophile elements light rare earth elements pb y and in various water species it did not change the feldspar oxygen isotope signature in turn the incorporation of hydrogen species into feldspar domains reduced al o al defect densities in the structure decreasing their luminescence water speciation shows persistent behaviour during heating the process being reversible at least up to 600 c carbonate crystals with a mantle isotope signature are associated with the re equilibrated feldspar domains the feldspar compositions the abundance of water species in them and the refractory nature of the residuum after heating the unchanged oxygen isotopes and the mantle signature of co precipitated carbonates testify that the observed recrystallization has taken place at temperatures above 600 c with h 2 o co 2 fertile mantle derived fluids the paper draws special attention to some methodological aspects of the problem the multi method approach used here major element trace element and isotope geochemistry infra red cathodoluminescence 3d depiction of geochemical data and fractal statistics may help to recognize and separate the various processes throughout the alteration history of the pluton

trace element diffusion in jadeite and diopside melts at high pressures and its geochemical implication abstract diffusivities of geochemically important trace elements eleven rare earth elements ree rb sr ba and y in jadeite and diopside melts and those of zr nb th and u in jadeite melt have been measured at pressures between 7 5 and 20 kbar and at temperatures 50 200 above the liquidus using diffusion couples and ion microprobe analysis the concentrations of these elements in the experimental charges are close to those in natural igneous rocks in the jadeite melt which is nearly fully polymerized nbo t 0 1 diffusivities of ree increase with increasing ionic radii 2 diffusivities of tri and tetravalent elements increase with increasing pressure at constant temperature whereas those of mono and divalent elements do not change significantly with pressure and 3 diffusivities of these elements decrease with increasing their ionic charge at constant pressure and temperature in the diopside melt which is considerably less polymerized nbo t 2 1 diffusivities of these trace elements depend mainly on their ionic radii rather than ionic charges the diffusivities of mono and divalent ions decrease with increasing ionic radii at constant pressure and temperature and 2 diffusivities of ree are nearly the same as that of ca and decrease with increasing pressure at constant temperature the behavior of these trace elements is correlated with that of major elements in the jadeite melt tri and tetravalent elements behave similarly to network forming cations al and si whereas mono and divalent elements behave as network modifying cations similarly to na in the diopside melt the diffusion behavior of all these trace elements is similar to that of network modifying cations ca and mg the results of the present experiments suggest that the abundance of some trace elements in igneous rocks may have been affected by diffusion process at the magmatic stage in the case of ree for example if two different magmas with high and low ree concentrations become in contact one another by multiple intrusion and a zoned magma chamber is formed diffusion begins to take place between them and near the interface the ree enriched magma will become more light ree depleted whereas ree depleted magma will become more light ree enriched and in addition if magmas are reduced the former will show a negative eu anomaly whereas the latter will show a positive eu anomaly

the volcanic eruption on jan mayen january 1985 interaction between a volcanic island and a fracture zone abstract an eruption started on jan mayen on 6 january 1985 it lasted only 35 40 hours and discharged about 7 million m3 of magma an earthquake swarm accompanied the eruption with individual quakes reaching magnitude 5 the volcanic fissure that opened strikes about midway between the strike of the jan mayen fracture zone 25 n of w and the general trend of the island and its volcanic features sw ne this fact the strong earthquakes and compositional features all suggest that it was an eruption from a leaky fracture zone rather than from the jan mayen magma system proper

geochemistry of lavas from mohns ridge norwegian greenland sea implications for melting conditions and magma sources near jan mayen mohns ridge lavas between 71 and 72 30 n 360 km have heterogeneous compositions varying between alkali basalts and incompatible element depleted tholeiites on a large scale there is a continuity of incompatible element and isotopic compositions between the alkali basalts from the island jan mayen and mohns ridge tholeiites the variation in isotopes suggests a heterogeneous mantle which appears to be tapped preferentially by low degree melts 5 close to jan mayen but also shows its signature much further north on mohns ridge three lava types with different incompatible element compositions e g chondrite normalized la sm n 2 occur in the area at 72 n and were generated from this heterogeneous mantle the relatively depleted tholeiitic melts were mixed with a small degree melt from an enriched source the elements ba rb and k of the enriched melt were probably buffered in the mantle by residual amphibole or phlogopite that such a residual phase is stable in this region of oceanic mantle suggests both high water contents and low mantle temperatures at odds with a hotspot origin for jan mayen instead we suggest that the melting may be induced by the lowered solidus temperature of a wet mantle mohns morb mid ocean ridge basalt and jan mayen area alkali basalts have high contents of ba and rb compared to other incompatible elements e g ba la 10 these ratios reflect the signature of the mantle source ratios of ce pb and rb cs are normal morb mantle ratios of 25 and 80 respectively thus the enrichments of ba and rb are not indicative of a sedimentary component added to the mantle source but were probably generated by the influence of a metasomatizing fluid as supported by the presence of hydrous phases during the petrogenesis of the alkali basalts geophysical and petrological models suggest that jan mayen is not the product of hotspot activity above a mantle plume and suggest instead that it owes its existence to the unique juxtaposition of a continental fragment a fracture zone and a spreading axis in this part of the north atlantic

experimental generation of volcanic pseudotachylytes constraining rheology volcanic systems are highly dynamic environments that comprise rocks and magmas which in the process of strain localisation e g in catastrophic flank collapse or dome building events are candidate materials for the occurrence of frictional melting and the formation of pseudotachylytes we evaluate the frictional behaviour of a plagioclase two pyroxene and glass bearing andesite and introduce an approach to constrain the rheology of frictional melts frictional slip at a rate of 1 3 m s under an axial stress of 1 5 mpa induces heating at a rate approximating 130 c per meter of slip and frictional melting of the andesite occurs at 1000 c which corresponds to a peak in shear stress with continuing displacement the shear stress decreases by 25 from the peak strength and stabilises while the temperature equilibrates to 1230 1290 c the shear stress for the system is congruent with a non arrhenian temperature dependent frictional melt rheology exhibiting a non newtonian viscosity evolving from approximately 104 2 to 103 4 pa s post experiment analysis show a gradation from 1 the host rock to 2 an extraordinary zone of unmelted yet viscously deformed material to 3 a thin outer region of chemically heterogeneous protomelts and to 4 an inner region of chemically homogeneous frictionally generated modified melt in the core of the slip zone we discuss the role and identification of frictional melting in volcanic systems and the implication of viscous remobilisation of magmatic plugs during frictional slip of glass bearing volcanic materials

the dimensions of magmatic inclusions as a constraint on the physical mechanism of mixing abstract silicic lavas often bear inclusions of more mafic lava several physical processes have been proposed by which these magmas may become mixed measurements of the sizes of mafic inclusions in lavas of two calc alkaline volcanoes are used here to obtain a characteristic length scale for this type of mixing and provide a means to discriminate between different mixing processes we quantitatively assess the proposal that in a stratified reservoir dynamic interactions of rising gas bubbles with the interface between a lower layer of mafic magma and an upper layer of silicic magma lead to the formation of vesiculated blobs of the lower magma in the upper one this is due to gravitational instability of a foam layer growing at the interface between the two magmas our theoretical calculations of this instability predict both the correct order of magnitude for the characteristic size of inclusions and the way in which inclusion size varies with the physical properties of the magmas this fluid dynamic framework is used to map out different possible flow regimes and place constraints on the fraction of gas in these systems

time evolution of chemical exchanges during mixing of rhyolitic and basaltic melts we present the first set of chaotic mixing experiments performed using natural basaltic and rhyolitic melts the mixing process is triggered by a recently developed apparatus that generates chaotic streamlines in the melts mimicking the development of magma mixing in nature the study of the interplay of physical dynamics and chemical exchanges between melts is carried out performing time series mixing experiments under controlled chaotic dynamic conditions the variation of major and trace elements is studied in detail by electron microprobe and laser ablation icp ms the mobility of each element during mixing is estimated by calculating the decrease in the concentration variance in time both major and trace element variances decay exponentially with the value of exponent of the exponential function quantifying the element mobility our results confirm and quantify how different chemical elements homogenize in the melt at differing rates the differential mobility of elements in the mixing system is considered to be responsible for the highly variable degree of correlation linear nonlinear or scattered of chemical elements in many published inter elemental plots elements with similar mobility tend to be linearly correlated whereas as the difference in mobility increases the plots become progressively more nonlinear and or scattered the results from this study indicate that the decay of concentration variance is in fact a robust tool for obtaining new insights into chemical exchanges during mixing of silicate melts concentration variance is in a single measure an expression of the influence of all possible factors e g viscosity composition and fluid dynamic regime controlling the mobility of chemical elements and thus can be an additional petrologic tool to address the great complexity characterizing magma mixing processes

chemical composition distribution and origin of silicic volcanic ash layers in the greenland iceland norwegian sea explosive volcanism from 10 to 300 ka as recorded in deep sea sediments explosive ocean island volcanism in the greenland iceland norwegian sea gin sea is indicated by marine tephra layers at 10 300 ka peaks of explosive volcanism occurred in oxygen isotope stages 8 7 5 and 1 the depositional age of the tephra was estimated using the oxygen isotope stratigraphy and dating of marine records geochemical analyses of the tephra layers show that all originate from iceland here we report the characteristics of tephra from these major icelandic events in 30 deep sea cores from the gin sea our findings provide constraints on the distribution of tephra from the eruption source for the vedde ash oxygen isotope stage 1 we estimate a minimum fallout area of 2 105 km2 stretching from central greenland in the west and southern sweden in the east to 71 n in the gin sea the magnitude of the eruption and the regional wind conditions controlled the extent and concentrations of these ash fallout events oceanic circulation and differential settling may have affected the distribution and final deposition of ash particles such as bubble wall shards

insights into magma chamber processes from the analysis of size distribution of enclaves in lava flows a case study from vulcano island southern italy the size distribution of latitic enclaves dispersed in a rhyolitic lava flow is studied enclave size distribution is self similar a feature that can be explained by a fractal fragmentation process and estimated fractal dimension of fragmentation is 2 50 fragment size distribution of enclaves generated by disruption of viscous fingering structures in granitoid rocks is analyzed it is shown that this distribution is fractal with a value of fractal dimension of fragmentation of 2 55 in close agreement with the value estimated for enclaves in the lava flow it is suggested that the fragmentation process producing the size distribution of enclaves in the lava flow may have occurred in response to the disruption of viscous fingering morphologies generated by the injection of the more mafic magma into the felsic one accordingly the size distribution of enclaves dispersed in the lava flow is considered a feature inherited from deep magma chamber processes and gives insights into magma chamber dynamics

grain shape of basaltic ash populations implications for fragmentation here we introduce a new quantitative method to produce grain shape data of bulk samples of volcanic ash and we correlate the bulk average grain shape with magma fragmentation mechanisms the method is based on automatic shape analysis of 2d projection ash grains in the size range 125 63 m loose bulk samples from the deposits of six different basaltic eruptions were analyzed and 20 000 shape measurements for each were obtained within 45 min using the particle insight dynamic shape analyzer pidsa we used principal component analysis on a reference grain dataset to show that circularity rectangularity form factor and elongation best discriminate between the grain shapes when combined the grain population data show that the studied eruptive environments produce nearly the same range of grain shapes although to different extents our new shape index the regularity index ri places an eruption on a spectrum between phreatomagmatic and dry magmatic fragmentation almost vesicle free surtseyan ash has an ri of 0 207 0 002 2 whereas vesiculated hawaiian ash has an ri of 0 134 0 001 2 these two samples define the end member ri while two subglacial one lacustrine and another submarine ash sample show intermediate ris of 0 168 0 002 2 0 175 0 002 2 0 187 0 002 2 and 0 191 0 002 2 respectively the systematic change in ri between wet and dry eruptions suggests that the ri can be used to assess the relative roles of magmatic vs phreatomagmatic fragmentation we infer that both magmatic and phreatomagmatic fragmentation processes played a role in the subglacial eruptions

pyroclast textures of the ilchulbong wet tuff cone jeju island south korea abstract jeju island is a quaternary shield volcano situated approximately 95 km south of the korean peninsula comprising holocene late pleistocene phreatomagmatic deposits and shield forming basaltic to trachytic lavas that overlie early mid pleistocene marine sedimentary and hydrovolcanic deposits ilchulbong tuff cone is among the youngest hydrovolcanic centres on the island formed along the shoreline from a surtseyan style eruption involving interaction of magma with external water we have analysed lapilli from selected sites on the volcanic edifice in terms of vesicle population texture shape and size and quantitatively with reference to bulk vesicularity vesicle number density vesicle dimensions and distributions the lapilli range in total vesicularity from low 15 35 to high 50 80 values and show a modest range of vesicle number densities n v number of vesicles per unit rock volume between 2 26 10 3 and 6 97 10 3 vesicles mm 3 such vesicle number densities are higher than those typical of strombolian n v between 9 3 10 1 and 1 3 10 3 vesicles mm 3 stromboli 2002 lautze and houghton 2007 and hawaiian eruptions and more similar to those of plinian n v between 1 5 and 2 5 10 3 vesicles mm 3 tarawera sable et al 2009 eruptions the lapilli studied were not strongly quenched and we infer that the high vesicle number densities reflect a late nucleated bubble population related in origin and formed in response to high levels of microlite growth in the ilchulbong magma prior to final ascent

statistical analysis of textural data from complex pyroclastic sequences implications for fragmentation processes of the agnano monte spina tephra 4 1 ka phlegraean fields southern italy the agnano monte spina eruption 4 1 ka generated pyroclastic deposits with complex internal stratigraphy that reflects variations in eruptive style to obtain constraints on the fragmentation processes we analysed grain size and scanning electron microscopy sem data by means of multivariate statistical techniques of the cluster and factor analysis types the results allow the identification of end member deposits related to either pure magmatic activity or pure phreatomagmatic activity in addition some deposits show evidence of the contemporaneous operation of both end member fragmentation modes the presence of the two contrasting fragmentation processes during the various phases of explosive activity is tentatively interpreted to be a function in inhomogeneities in gas exsolution processes within the magma column that feeds the eruption

growth and evolution of an emergent tuff cone considerations from structural geology geomorphology and facies analysis of sao roque volcano sao miguel azores abstract the syn eruptive and post eruptive history of sao roque tuff cone its geological setting and volcanological features were studied in detail to understand the role played by the different factors that contributed to the morphological evolution of this relatively simple and small volcanic edifice in addition attention was also focused on the series of natural changes that affected the tuff cone during the course of the years and that finally led to its structural disassembly a novel model is proposed to explain this process the sao roque volcanic centre located on the island of sao miguel azores consists of two well consolidated bodies and numerous small islets that formed more than 4700 years ago during the hydromagmatic activity that took place along an intruding dyke whose nnw sse trend is in agreement with the regional tectonic pattern the eruptive vents probably migrated progressively from sse to nnw forming small edifices through the rapid accumulation of sediments during alternating phases of dry and wet magmatic emissions syn eruptive partial collapses greatly modified the original morphological structure of these edifices probably allowing sea water to continuously flow into the vents the complex interaction of these factors controlled the depth of magma fragmentation producing different types of deposits in which the ash lapilli ratio varies considerably the high water saturation degree of these deposits caused syn eruptive and post eruptive remobilization which resulted in collapses and some small scale landslides post eruptive wnw ese trending transtensional and extensional tectonic activities operated during the initial dissection of the cone generating instability furthermore the rapid accumulation of wet tephra and its following consolidation caused selective collapses that favoured the fragmentation of the deposit and caused the formation of numerous islets separated by radially arranged channels collapses also involved the lava units emplaced in more recent times around the tuff cone which show that brittle deformation has been significant in the area for a prolonged period

estimating the volume of tephra deposits a new simple strategy volume determination of tephra deposits is necessary for the assessment of the dynamics and hazards of explosive volcanoes several methods have been proposed during the past 40 years that include the analysis of crystal concentration of large pumices integrations of various thinning relationships and the inversion of field observations using analytical and computational models regardless of their strong dependence on tephra deposit exposure and distribution of isomass isopach contours empirical integrations of deposit thinning trends still represent the most widely adopted strategy due to their practical and fast application the most recent methods involve the best fitting of thinning data using various exponential segments or a power law curve on semilog plots of thickness or mass area versus square root of isopach area the exponential method is mainly sensitive to the number and the choice of straight segments whereas the power law method can better reproduce the natural thinning of tephra deposits but is strongly sensitive to the proximal or distal extreme of integration we analyze a large data set of tephra deposits and propose a new empirical method for the determination of tephra deposit volumes that is based on the integration of the weibull function the new method shows a better agreement with observed data reconciling the debate on the use of the exponential versus power law method in fact the weibull best fitting only depends on three free parameters can well reproduce the gradual thinning of tephra deposits and does not depend on the choice of arbitrary segments or of arbitrary extremes of integration

initial results of the searise numerical experiments with the models sicopolis and icies for the greenland ice sheet searise sea level response to ice sheet evolution is a us led multi model community effort to predict the likely range of the contribution of the greenland and antarctic ice sheets to sea level rise over the next few hundred years under global warming conditions the japanese ice sheet modelling community is contributing to searise with two large scale dynamic thermodynamic models sicopolis and icies here we discuss results for the greenland ice sheet obtained using both models under the forcings surface temperature and precipitation scenarios defined by the searise effort a crucial point for meaningful simulations into the future is to obtain initial conditions that are close to the observed state of the present day ice sheet this is achieved by proper tuning during model spin up from the last glacial interglacial cycle to today experiments over 500 years indicate that both models are more sensitive exhibit a larger rate of ice sheet mass loss to future climate warming based on the a1b emission scenario than to a doubling in the basal sliding speed ice sheet mass loss varies between the two models by a factor of 2 for sliding experiments and a factor of 3 for climate warming experiments highlighting the importance of improved constraints on the parameterization of basal sliding and surface mass balance in ice sheet models

flow and fragmentation patterns in the silicic feeder system and related deposits in the parana etendeka magmatic province sao marcos south brazil voluminous silicic volcanic products from the paran etendeka magmatic province are exposed in the southern brazil a regional synformal structure torres syncline controls a tick sequence of these units in the region of sao marcos rio grande do sul parallel to the axis of torres syncline a nw se trending zone constituted by the alignment of complex structures was interpreted as the feeding system of silicic volcanism characterized as fissural through a combination of field mapping and determination of fractal dimension fd we have identified and described distinct intermingled morphological domains one related to flow processes and one related to fragmentation processes vertical flow directions in this zone exhibit two preferential directions nw ranging from n272 degrees to n 355 degrees and ne varying from n20 degrees to n85 degrees the systematic repetition and crosscutting relations of domains point towards a cydicity of flow and fragmentation processes in this zone this feeder system intrudes previous basaltic and silicic volcanic rocks the later being composed by banded deposits interpreted as rheoignimbrites and proximal volcanic breccias this set is overlapped by hybrid deposits lava flows and finally massive deposits the silicic volcanism can be summarized at an early pyrodastic stage characterized by rheoignimbrites and proximal volcanic breccias a transition between explosive and effusive events is suggested by the occurrence of hybrid deposits which gradually give place to lobated lavas local alternation with new possibly rheoignimbritic deposits suggests oscillatory fragmentation processes at the conduit related to decompression and volatiles ex solution in an effective permeable system

total grain size distribution and volume of tephra fall deposits on 17 june 1996 ruapehu volcano new zealand produced a sustained andesitic sub plinian eruption which generated a narrow tephra fall deposit extending more than 200 km from the volcano the extremely detailed data set from this eruption allowed methods for the determination of total grain size distribution and volume of tephra fall deposits to be critically investigated calculated total grain size distributions of tephra fall deposits depend strongly on the method used and on the availability of data across the entire dispersal area the voronoi tessellation method was tested for the ruapehu deposit and gave the best results when applied to a data set extending out to isomass values of 1 g m 2 the total grain size distribution of a deposit is also strongly influenced by the very proximal samples and this can be shown by artificially constructing subsets from the ruapehu database unless the available data set is large all existing techniques for calculations of total grain size distribution give only apparent distributions the tephra fall deposit from ruapehu does not show a simple exponential thinning but can be approximated well by at least three straight line segments or by a power law fit on semi log plots of thickness vs area 1 2 integrations of both fits give similar volumes of about 4 106 m3 integration of at least three exponential segments and of a power law fit with at least ten isopach contours available can be considered as a good estimate of the actual volume of tephra fall integrations of smaller data sets are more problematic

the physical volcanology of the 1600 eruption of huaynaputina southern peru volcan huaynaputina is a group of four vents located at 16 36 s 70 51 w in southern peru that produced one of the largest eruptions of historical times when 11 km3 of magma was erupted during the period 19 february to 6 march 1600 the main eruptive vents are located at 4200 m within an erosion modified amphitheater of a significantly older stratovolcano the eruption proceeded in three stages stage i was an 20 h sustained plinian eruption on 19 20 february that produced an extensive dacite pumice fall deposit magma volume 2 6 km3 throughout medial distal and distal parts of the dispersal area a fine grained plinian ashfall unit overlies the pumice fall deposit this very widespread ash magma volume 6 2 km3 has been recognized in antarctic ice cores a short period of quiescence allowed local erosion of the uppermost stage i deposits and was followed by renewed but intermittent explosive activity between 22 and 26 february stage ii this activity resulted in intercalated pyroclastic flow and pumice fall deposits 1 km3 the flow deposits are valley confined whereas associated co ignimbrite ash fall is found overlying the plinian ash deposit following another period of quiescence vulcanian type explosions of stage iii commenced on 28 february and produced crudely bedded ash lapilli and bombs of dense dacite 1 km3 activity ceased on 6 march compositions erupted are predominantly high k dacites with a phenocryst assemblage of plagioclase hornblende biotite fe ti oxides apatite major elements are broadly similar in all three stages but there are a few important differences stage i pumice has less evolved glass compositions 73 sio2 lower crystal contents 17 20 lower density 1 0 1 3 g cm3 and phase equilibria suggest higher temperature and volatile contents stage ii and stage iii juvenile clasts have more evolved glass 76 sio2 compositions higher crystal contents 25 35 higher densities up to 2 2 g cm3 and lower temperature and volatile contents all juvenile clasts show mineralogical evidence for thermal disequilibrium inflections on a plot of log thickness vs area1 2 for the fall deposits suggest that the pumice fall and the plinian ash fall were dispersed under different conditions and may have been derived from different parts of the eruption column system the ash appears to have been dispersed mainly from the uppermost parts of the umbrella cloud by upper level winds whereas the pumice fall may have been derived from the lower parts of the umbrella cloud and vertical part of the eruption column and transported by a lower altitude wind field thickness half distances and clast half distances for the pumice fall deposit suggests a column neutral buoyancy height of 24 32 km and a total column height of 34 46 km the estimated mass discharge rate for the 20 h long stage i eruption is 2 4 108 kg s and the volumetric discharge rate is 3 6 105 m3 s the pumice fall deposit has a dispersal index hildreth and drake 1992 of 4 4 and its index of fragmentation is at least 89 reflecting the dominant volume of fines produced of the 11 km3 total volume of dacite magma erupted in 1600 approximately 85 was evacuated during stage 1 the three main vents range in size from 70 to 400 m alignment of these vents and a late stage dyke parallel to the nnw sse trend defined by older volcanics suggest that the eruption initiated along a fissure that developed along pre existing weaknesses during stage i this fissure evolved into a large flared vent vent 2 with a diameter of approximately 400 m this vent was active throughout stage ii at the end of which a dome was emplaced within it during stage iii this dome was eviscerated forming the youngest vent in the group vent 3 a minor extra amphitheater vent was produced during the final event of the eruptive sequence recharge may have induced magma to rise away from a deep zone of magma generation and storage subsequently vesiculation in the rising magma batch possibly enhanced by interaction with an ancient hydrothermal system triggered and fueled the sustained plinian eruption of stage i a lower volatile content in the stage ii and stage iii magma led to transitional column behavior and pyroclastic flow generation in stage ii continued magma uprise led to emplacement of a dome which was subsequently destroyed during stage iii no caldera collapse occurred because no shallow magma chamber developed beneath this volcano

chaotic dynamics and fractals in magmatic interaction processes a different approach to the interpretation of mafic microgranular enclaves abstract fractals have been used for describing a wide range of natural processes however applications of fractal geometry to the study of magma mingling mixing processes is almost completely lacking in spite of the fact that such an approach could give new insights into the nature of magmatic interactions fractal analyses of two geochemically linked groups of mafic microgranular enclaves mme from the sithonia plutonic complex northern greece have been performed because of the difficulties involved in using contours of enclaves to estimate their fractal dimensions a different approach is used in this paper fractal dimension is estimated using distribution maps of chemical elements inside enclaves the results obtained indicate that a clear correlation between fractal dimensions of enclaves and geochemical evidences of interaction exists and hence the two groups of enclaves reflect different degrees of interaction from a dynamical point of view stretching and folding processes between two magmas with different rheologies are considered representative of magmatic interaction this dynamic system can easily generate chaotic patterns recognized by the presence of fractal structures the contemporaneous occurrence of well mixed regions together with poorly mixed regions in the same system is thought to be similar to the host enclave system found in many granitoid plutons in particular mme are believed to represent islands of partially mixed mafic fluid subjected to different degrees of interaction with the host granitoid whereas host rocks are believed to represent mixed regions where the mixing process acted more efficiently producing almost completely hybrids it is shown that fractal geometry and chaotic dynamics can be considered suitable techniques in studying complex petrological phenomena and they can represent helpful methods in the development of petrogenetic models

pyroclast characteristics of a subaqueous to emergent surtseyan eruption black point volcano california abstract we present the first ever quantitative analysis of pyroclast textures from subaqueous deposits of a surtseyan emergent volcano the volcano black point formed in mono lake s pleistocene precursor lake russell at 13 300 500 bp from an explosive basaltic eruption involving the interaction of shallow lake water 105 m with vesiculating magma the bulk of the edifice comprises unconsolidated tephra and represents submerged stages of eruption thin summit deposits formed during a final phase of emergent eruption and produced a group of low tuff rings lapillus clasts 16 mm to 32 mm and fine ash grains 4 0 0625 mm from the subaqueous deposits have textures that reveal a conduit history of bubble nucleation growth collapse intense microlite crystallisation and magma water interaction lapilli vesicularity ranges broadly to high values up to 89 while vesicle number densities 5 4 102 4 8 104 vesicles per mm3 are comparable to those published for strombolian and plinian basaltic eruptions 1 104 vesicles per mm3 vesicle size and volume distributions suggest continuous bubble nucleation but the suite of data as a whole indicates that a late stage nucleation event played a significant role in shaping the eruption ash grain morphology and surface features indicative of magma water interactions attest to water s role in fragmentation overall the pyroclast textures are consistent with water modification of a rather intense basaltic eruption which might have been of a violent strombolian or plinian style if the eruption had been dry and occurred subaerially

multi stage chaotic magma mixing at turrialba volcano abstract after a century of quiescence turrialba a quaternary stratovolcano in the costa rican volcanic front started a new eruptive cycle in 2010 that is ongoing to this date july 3rd 2019 violent eruptions began in 2014 producing columns of ash that reached 4 km above the summit of the volcano here we provide new major and trace element data from fresh glassy fragments basaltic to rhyolitic and mineralogical data for feldspars olivines and pyroxenes from ash fallout collected from 2016 and 2017 eruptions we investigate the composition of the magma replenishing the system as well as the different juvenile components involved in the latter phase of this renewed eruption cycle we find a range of mafic to felsic compositions in every eruptive event sampled our new geochemical data coupled with monte carlo binary mixing simulations indicate that magmas at turrialba volcano are influenced by the input of a back arc type strongly high field strength element enriched 20 60 ppm component generated by decompression melting from subduction driven corner flow thermodynamic modeling using melts software suggests that fractional crystallization alone fails to explain the compositional range of turrialba s eruptive products we instead propose that a complex multi stage magma mixing process produced renewed activity at turrialba based on the trace element variability the dominant intermediate magmatic compositions observed and the results of our multi element mixing simulations this mixing process occurs upon injection of new mantle derived melts into turrialba s fractionated magmatic reservoirs and is governed by chaotic dynamics induced by gradients in temperature and viscosity

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

textural variation in juvenile pyroclasts from an emergent surtseyan type volcanic eruption the capelas tuff cone sao miguel azores abstract here i present textural data i e vesicularity vesicle size distributions vsd plagioclase crystallinity crystal size distributions csd combined with fractal analyses of particle outlines from a natural succession of alternating fall and surge deposits in the emergent capelas tuff cone azores the textural variation in the capelas succession is surprisingly small considering the wide variety of fragmentation processes vent activity and emplacement mechanisms that are characteristic of emergent eruptions the plagioclase crystal content varies between 24 and 33 vol csd analyses of plagioclase show near linear trends with a slight increase in time for the smallest crystal sizes with surge deposits having more groundmass plagioclase when compared with fall deposits this is consistent with crystallization induced by degassing and decompression at lower eruption rates the vesicularities of the capelas pyroclasts are more variable 18 to 59 vol with vsds displaying kinked trends characteristic of coalescence this is especially evident in the fall deposits and consistent with being formed in continuous uprush jetting with an overall shallow fragmentation level within the conduit bubble coalescence can also be identified in the surge deposits although to a much lesser extent the amount of bubble coalescence is negatively correlated with the amount of groundmass crystallization i e plagioclase in the capelas deposits a relatively broad range of fractal dimensions with average dbox 1 744 and 0 032 for the outlines of pyroclastic fragments emplaced by fall or as surges indicate that there is little difference in the fragmentation process itself at capelas in addition to this the fact that the fractal dimensions for both the fall and surge end members completely overlap suggests that shape modification due to abrasion and chipping of grain edges was minor during emplacement of base surges these results are consistent with emergent eruptions building tuff cones to be a relatively low energy phreatomagmatic landform e g at least when compared with more energetic phreatomagmatic eruptions producing tuff rings and maar volcanoes

melt induced speed up of greenland ice sheet offset by efficient subglacial drainage fluctuations in surface melting are known to affect the speed of glaciers and ice sheets but their impact on the greenland ice sheet in a warming climate remains uncertain although some studies suggest that greater melting produces greater ice sheet acceleration others have identified a long term decrease in greenland s flow despite increased melting here we use satellite observations of ice motion recorded in a land terminating sector of southwest greenland to investigate the manner in which ice flow develops during years of markedly different melting although peak rates of ice speed up are positively correlated with the degree of melting mean summer flow rates are not because glacier slowdown occurs on average when a critical run off threshold of about 1 4 centimetres a day is exceeded in contrast to the first half of summer when flow is similar in all years speed up during the latter half is 62 16 per cent less in warmer years consequently in warmer years the period of fast ice flow is three times shorter and overall summer ice flow is slower this behaviour is at odds with that expected from basal lubrication alone instead it mirrors that of mountain glaciers where melt induced acceleration of flow ceases during years of high melting once subglacial drainage becomes efficient a model of ice sheet flow that captures switching between cavity and channel drainage modes is consistent with the run off threshold fast flow periods and later summer speeds we have observed simulations of the greenland ice sheet flow under climate warming scenarios should account for the dynamic evolution of subglacial drainage a simple model of basal lubrication alone misses key aspects of the ice sheet s response to climate warming

strange attractors in magmas evidence from lava flows abstract magma mixing structures from three different lava flows salina vulcano and lesbos are studied in order to assess the possible chaotic origin of magma mixing processes structures are analysed using a new technique based on image analysis procedures that extract time series that are representative of the relative change in composition through the structures these time series are then used to reconstruct the attractors underlying the magma mixing process and to calculate the fractal dimension of the attractors results show that attractors exist and possess fractional dimensions this evidence suggests that the mixing of magmas is a chaotic process governed by a low number of degrees of freedom in addition fractal dimension analyses allows us to discriminate between different regimes of mixing in the three lava flows in particular our analyses suggest that the lava flow of salina underwent more turbulent mixing than the lava flows of lesbos and vulcano

petrology of the shoshonitic cambasi pluton in ne turkey and implications for the closure of the neo tethys ocean insights from geochemistry geochronology and sr nd isotopes abstract the eastern pontide orogenic belt epob is one of the well preserved continental magmatic arcs in the alpine himalayan belt and consists of paleozoic to miocene plutonic bodies this paper presents new geological bulk rock major and trace element analyses biotite ar ar ages and sr nd isotope data on representative samples from the cambasi granitic body exposed in the northwestern part of the epob although the majority of the plutons in the epob show calc alkaline affinity and i type character the cambasi pluton has a shoshonitic affinity and an i type metaluminous signature the pluton is composed of mainly monzonite quartz monzonite and granite bodies including monzo dioritic mafic microgranular enclaves mmes excluding the mmes these rocks have 57 7 70 2 wt sio2 13 1 17 5 wt al2o3 3 06 3 64 wt na2o 3 66 5 92 wt k2o 0 33 3 3 wt mgo and mg

black point pyroclasts of a surtseyan eruption show no change during edifice growth to the surface from 100 m water depth abstract surtseyan eruptions have long been studied from their edifice deposits emplaced subaqueously or subaerially here we characterize a surtseyan eruption from both edifice and ash sheet deposits using a combination of techniques the volcano studied is black point which erupted into lake russell about 13 000 years ago from a water depth of 105 m producing a total of 0 8 km3 of tephra the techniques applied to samples acquired by targeted sampling based on geological observations in the field include geochemical characterization of the eruption products grain size analysis of the ash size fraction at proximal and medial sites three dimensional characterization of vesicles in lapilli and particle shape analysis for ash grains using the recently introduced freeware partisan the aim of this combined approach is to determine how the eruption evolved through its course from before magma fragmentation to the mechanisms of formation transport and deposition of the tephra field observations are consistent with the ash sheet deposits forming from eruption fed density currents and or fallout through the water column from a subaerial plume umbrella cloud geochemical correlation links the mound deposits as well as the tuff rings with a stratigraphic sequence at about 2 km west from the edifice both show non linear but broadly correlative geochemical variations in deposits from the subaqueous stage of the eruption through the subaerial one pyroclast vesicle textures indicate that volatile expansion played only a minor role in magma fragmentation and that there was a high level of bubble coalescence and hence outgassing during the eruption we infer that fragmentation was mostly phreatomagmatic resulting from explosive interactions between rising magma and water saturated sediments two dimensional particle shape analysis of ash grains shows a large variability in the shape parameters of convexity and solidity this tells us little about fragmentation style but serves as a proxy for magma vesicularity the most interesting aspect of this eruption is that none of the shape parameters or qualitative observations show any sort of systematic change during the course of the eruption despite the eruption having begun in 100 m of water and ended up subaerial

estimating greenland surface melt is hampered by melt induced dampening of temperature variability he positive degree day pdd model provides a particularly simple approach to estimate surface melt from land ice based solely on air temperature here we use a climate and snow pack simulation of the greenland ice sheet modele atmospherique regional mar as a reference to analyze this scheme in three realizations that incorporate the sub monthly temperature variability differently i by local values ii by local values that systematically overestimate the dampened variability associated with intense melting or iii by one constant value local calibrations reveal that incorporating local temperature variability particularly resolving the dampened variability of melt areas renders model parameters more temperature dependent this indicates that the negative feedback between surface melt and temperature variability introduces a non linearity into the temperature melt relation to assess the skill of the individual realizations we hindcast melt rates from mar temperatures for each realization for this purpose we globally calibrate greenland wide constant parameters realization i exhibits shortcomings in the spatial representation of surface melt unless temperature dependent instead of constant parameters are calibrated the other realizations perform comparatively well with constant parametrizations the skill of the pdd model primarily depends however on the consistent calibration rather than on the specific representation of variability

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

chaotic three dimensional distribution of ba rb and sr in feldspar megacrysts grown in an open magmatic system as has been demonstrated in recent years the heterogeneities of coeval magmas can be more successfully revealed by zoned megacrysts rather than by analysis of the whole rocks hosting them here the geochemical heterogeneities of feldspar megacrysts from the karkonosze granite poland are investigated by la icp ms the crystals are the product of migration and growth in regions of poorly mixed magmas 3d modeling of the ba sr and rb distributions emphasizes the importance of micro domain growth morphologies two models of element behavior a relative concentration model and a composition gradient model provide a potentially effective tool for tracking the mixing process on a microscale measured concentrations of elements of different mobilities do not agree with what might be expected from the mixing of two end member magmas if mixing was the only process occurring linear correlations between the concentrations of any two elements should be observed this however is not the case for combinations of any two of the three elements modeling reveals differing non linear correlations between concentrations the megacryst heterogeneities provide an insight into how mixing magmas are chaotically advected to growing crystals and the degree of inter magma element exchange between the magmas

improving surface boundary conditions with focus on coupling snow densification and meltwater retention in large scale ice sheet models of greenland snowpack changes during the melt season are often not incorporated in modelling studies of the surface mass balance of the greenland ice sheet densification of snow accelerates when meltwater is present due to percolation and subsequent refreezing and needs to be incorporated in ice sheet models for ablation calculations in this study simple parameterizations to calculate surface melt snow densification and meltwater retention are included as surface boundary conditions in a large scale ice sheet model of greenland coupling the snow densification and meltwater retention processes achieves a separation of volume and mass changes of the surface layer in order to determine the surface melt contribution to runoff experiments for present day conditions show that snow depth at the onset of melt mean annual near surface air temperature and the mean density of the annual snow layer are key factors controlling the quantity and spatial distribution of meltwater runoff above the equilibrium line on the greenland ice sheet

a new present day temperature parameterization for greenland near surface air temperature 2 m over the greenland ice sheet gris is parameterized using data from automatic weather stations located on land and on the ice sheet the parameterization is expressed in terms of mean annual temperatures and mean july temperatures both depending linearly on altitude latitude and longitude the temperature parameterization is compared to a previous study and is shown to be in better agreement with observations the temperature parameterization is tested in a positive degree day model to simulate the present 1996 2006 mean melt area extent of the gris the model accounts for firn warming rainfall and refreezing of meltwater with different degree day factors for ice and snow under warm and cold climate conditions the simulated melt area extent is found to have reasonable agreement with satellite derived observations

contemporaneous phreatomagmatic and effusive activity along the hverfjall eruptive fissure north iceland eruption chronology and resulting deposits abstract the 2500 bp hverfjall eruption in northern iceland produced pyroclastic and effusive deposits of widely different characteristics along the length of the eruptive fissure the southern half of the fissure was located in a shallow lake whereas the northern part of the fissure extended onto dry land this specific setting with overlapping periods of activity at the different vents resulted in various mingling features between fine grained phreatomagmatic deposits and lava flows here we reconstruct the course of events during this eruption based on field observations and granulometric analyses of the resulting deposits the eruption can be divided into three main phases depending on the main depositional characteristics and vent locations the initial phase is dominated by phreatomagmatic fall deposits which are attributed to an overall high eruption rate in a shallow lacustrine setting the second phases involved opening of two new vents on dry ground and deposition of scoria and lava flows the third and final phase of the eruption is associated with a lowering of the eruption rate in the southernmost vent with a shift in the activity from continuous uprush and fall deposits to discrete explosions and emplacement of base surges these surges display features consistent with drying up with increasing distance from the vent suggesting that their dynamics changed during emplacement most wet surges were channelized within a preexisting graben structure close to the vent but some more dilute i e dry surges were able to flow over this obstacle and continued to flow for more than 5 km away from the vent and 100 m uphill before stopping

holocene explosive volcanism of the jan mayen island volcanic province north atlantic abstract the volcanic island jan mayen located in the norwegian greenland sea hosts the active stratovolcano of beerenberg the northernmost active subaerial volcano in the world at least five eruptions are known from the island following its discovery in the 17th century but its eruptive history prior to this is basically unknown in this paper two sediment cores retrieved close to jan mayen have been studied in detail to shed light on the holocene history of explosive volcanism from the jan mayen volcanic province horizons with elevated tephra concentrations were identified and tephra from these was analysed to determine major element chemistry of the tephra the tephra chemistry was used to provide a link between the two cores and the land based tephra records from jan mayen island we managed to link two well developed tephra peaks in the cores by their geochemical composition and age to jan mayen one of these peaks represents the 1732 ad eruption of eggoya while the other peak represents a previously undescribed eruption dated to around 10 3 ka bp two less prominent tephra peaks one in each core dated to approximately 2 3 and 3 0 ka bp also have a distinct geochemical character linking them to jan mayen volcanism however the most prominent tephra layer in the cores located close to jan mayen and numerous other cores along the jan mayen ridge is the 12 1 ka bp vedde ash originating from the iceland volcanic province we find that the holocene volcanism on jan mayen is much less explosive than volcanism in iceland and propose that either low amounts of explosive volcanic activity from the summit region of beerenberg or small to absent glacier cover on beerenberg is responsible for this

characteristics of dune bedded pyroclastic surge bedsets abstract the grainsize characteristics of dune bedded pyroclastic surge bedsets are surveyed the variance between coarsest and finest beds ranges from 1 to 6 phi in different surge bedsets and it increases as the grainsize of the coarsest bed increases reflecting an increasing velocity of emplacement deposits of wet surges identified as those which contain accretionary and ash coated lapilli tend to be finer and show less variance this partly because wet ash is cohesive but mainly because wet surges tend to be weaker dry surge bedsets are strongly fines depleted wet ones less so the lack of erosion of underlying ash layers shows that the environment is a strongly depositional one individual bedsets are demarcated by thin intervening fine ash fall layers which are the complementary ash cloud deposits settled or flushed out after the passage and decay of each turbulent surge surge deposits are generally less coarse than the coarsest associated airfall deposits which shows that they are formed by generally weaker events this study helps interpret the dune bedded parts of the landscape mantling may 18th 1980 blast deposit of mount st helens the blast was a very violent event but the variance and the grainsize of the coarsest bed are those of a relatively weak surge this suggests that the dune bedding was produced by a weak effect such as minor turbulence in a thin pyroclastic flow coming to rest in a mountainous terrain roughened by tree stumps and fallen logs

on the deformation and freezing of enclaves during magma mixing abstract mixing of an enclave of hot mafic magma into cooler silicic magma involves the competing effects of heat transfer acting to rigidify the enclave and viscous shearing imposed by a flowing host magma acting to deform and disperse the enclave we model the time required to grow a rigid chilled margin and compare this with the time required to deform the initially hot enclave whether an enclave will deform before it freezes depends on the ratio of the thermal and deformation timescales pe app 3 a 2 s r 2 and the dimensionless rigidification temperature t r t s t m t s where app is the applied shear stress a is the enclave radius is the thermal diffusivity s is the viscosity of the host r is the strength of the chilled rind t r is the temperature at which the enclave attains strength r and t m and t s are the initial temperatures of the mafic enclave and silicic host according to the model small values of pe and large values of promote rapid rigidification the model is verified by laboratory experiments on the flow and freezing of polyglycol wax droplets in cold water geological observations show that correlations between enclave size degree of deformation and local shear stress match the model s predictions studies of enclave size and shape as functions of eruption rate and position within lava flows and minor intrusions offer a new technique for studying magma flow processes during eruptions

mingling in mafic magma chambers replenished by light felsic inputs fluid dynamical experiments abstract we investigate by means of laboratory analogues the fluid dynamics of magma hybridisation caused by the intrusion of felsic magma into a mafic magma chamber the input felsic magma is modelled by a cold low density fluid and the resident mafic magma by a layer of fluid wax close to its solidification point the cold rising input drives convection and solidification in the resident fluid and the solidification causes significant perturbations to an otherwise simple input flow greatly enhancing mixing and mingling between the two model magmas the dimensionless parameters controlling the evolution of the system are determined and different morphologies of the solid mass fit into clearly defined fields in the parameter space the most important implication of the experiments is that the freezing of mafic chambers due to felsic intrusion may give rise to intense hybridization of large volumes of magma this potentially important mechanism for hybridization has to date received little attention in the literature

analysis and simulation of magma mixing processes in 3d abstract magma mixing structures from the lava flow of lesbos greece are analyzed in three dimensions using a technique that starting from the serial sections of rock cubes allows the reconstruction of the spatial distribution of magmas inside rocks two main kinds of coexisting structures are observed i active regions ar in which magmas mix intimately generating wide contact surfaces and ii coherent regions cr of more mafic magma that have a globular shape and do not show large deformations the intensity of mingling is quantified by calculating both the interfacial area ia between interacting magmas and the fractal dimension of the reconstructed structures results show that the fractal dimension is linearly correlated with the logarithm of interfacial area allowing discrimination among different intensities of mingling the process of mingling of magmas is simulated using a three dimensional chaotic dynamical system consisting of stretching and folding processes the intensity of mingling is measured by calculating the interfacial area between interacting magmas and the fractal dimension as for natural magma mixing structures results suggest that as in the natural case the fractal dimension is linearly correlated with the logarithm of the interfacial area allowing to conclude that magma mixing can be regarded as a chaotic process since chemical exchange and physical dispersion of one magma inside another by stretching and folding are closely related we performed coupled numerical simulations of chaotic advection and chemical diffusion in three dimensions our analysis reveals the occurrence in the same system of active mixing regions and coherent regions analogous to those observed in nature we will show that the dynamic processes are able to generate magmas with wide spatial heterogeneity related to the occurrence of magmatic enclaves inside host rocks in both plutonic and volcanic environments

the interplay between crystallization replenishment and hybridization in large felsic magma chambers abstract while hybridized granitoid magmas are readily identifiable the mechanisms of hybridization in large crustal magma chambers are so not clearly understood characteristic features of hybrid granitoids are 1 both the granitoid and included enclaves are commonly hybrids as shown by mineralogy geochemistry and isotopes 2 mixing seen in zoned plutons and synplutonic dykes and enclaves occurred early 3 zoned plagioclase phenocrysts commonly show very complex life histories of growth and dissolution 4 mafic end members in hybrids are commonly fractionated magmas and 5 stratification in subvolcanic granitoid magma chambers is not uncommon and stratification has been identified in some deeper level plutons hybridization must overcome the tendency to form a stable stratification of dense mafic magma underlying less dense felsic magma experimental work with magma analogues and theoretical considerations reveal very severe thermal rheological and dynamical limitations on mixing only very similar composition temperature magmas are likely to mix to homogeneity and only moderately silicic hybrids are likely to be produced however impossibly silicic hybrids do exist synchronous interactive fractional crystallization and hybridization may provide a mechanism for hybridization of magmas in the following manner a mafic magma intrudes into the base of a stratified felsic magma and is cooled against it crystallization of the upper boundary layer of the mafic magma yields an eventually buoyant residual melt that overturns and mixes with an adjacent stratum of the felsic magma chamber subsequently melt released by crystallization pf this now hybrid zone mixes with adjacent more felsic zones thus a suite of hybrid magmas are progressively formed density inhibitions are overcome by the generation of relatively low density residual melts as crystallization proceeds later injections are preserved as dykes and enclaves composed of hybrid magma in this process only physically adjacent and dynamically thermally similar magmas directly interact and so may mix to homogeneity finally not simply felsic and mafic endmembers mix but a whole suite of intermediate endmembers participate ranging from relatively mafic through to felsic pairs of magmas direct mixing between the primary magmas only occurs at the beginning

chaotic advection fractals and diffusion during mixing of magmas evidence from lava flows structures of magma mixing from three different lava flows have been analyzed and the degree of mingling has been quantified by measuring the contact perimeter between magmas and the fractal dimension of structures in each lava flow the values of these parameters suggest that the magma mixing structures were produced by chaotic dynamics induced by stretching and folding processes between the interacting magmas the mingling of magmas has been simulated using a chaotic dynamical system consisting of repeated stretching and folding processes the simulation shows the same patterns of variation of contact perimeter and fractal dimension as those observed in natural structures and indicates that magma interaction processes acted with different intensities in the three lava flows in response to different magmatic interaction regimes since physical dispersion of one magma inside another through stretching and folding processes and chemical exchanges are closely related we performed coupled numerical simulations of chaotic advection and chemical diffusion the results show a good agreement between the computed and natural structures in particular the occurrence in the same system of well and poorly mixed regions it is shown that magma interaction processes are able to generate magmatic masses having wide spatial heterogenity at many length scales this occurrence can account for the presence of magmatic enclaves inside host rocks showing a variable degree of hybridization in both plutonic and volcanic environments

stratigraphy sedimentology and eruptive mechanisms in the tuff cone of el golfo lanzarote canary islands the tuff cone of el golfo on the western coast of lanzarote canary islands is a typical hydrovolcanic edifice along with other edifices of the same age it was constructed along a fracture oriented nee sww that coincides with the main structural trend of recent volcanism in this part of the island we conducted a detailed stratigraphic study of the succession of deposits present in this tuff cone and here interpret them in light of the depositional processes and eruptive dynamics that we were able to infer the eruptive sequence is represented by a succession of pyroclastic deposits most of which were emplaced by flow plus a number of air fall deposits and ballistic blocks and bombs we distinguished five different eruptive depositional stages on the basis of differences in inferred current flow regimes and fragmentation efficiencies represented by the resulting deposits the different stages may be related to variations in the explosive energy eight lithofacies were identified based on sedimentary discontinuities grain size components variations in primary laminations and bedforms the volcanic edifice was constructed very rapidly around the vent and this is inferred to have controlled the amount of water that was able to enter the eruption conduit the sedimentological characteristics of the deposits and the nature and distribution of palagonitic alteration suggest that most of the pyroclastic succession in el golfo was deposited in a subaerial environment this type of hydrovolcanic explosive activity is common in the coastal zones of lanzarote and the other canary islands and is one of the main potential hazards that could threaten the human population of this archipelago detailed studies of these hydrovolcanic eruptions such as the one we present here can help volcanologists understand the hazards that this type of eruption can generate and provide essential information for undertaking risk assessment in similar volcanic environments

assessment of current methods of positive degree day calculation using in situ observations from glaciated regions the continued use of the positive degree day pdd method to predict ice sheet melt is generally favoured over surface energy balance methods partly due to the computational efficiency of the algorithm and the requirement of only one input variable temperature in this paper we revisit some of the assumptions governing the application of the pdd method using hourly temperature data from the gc net network we test the assumption that monthly pdd total pddm can be represented by a gaussian distribution with fixed standard deviation of monthly temperature m the results presented here show that the common assumption of fixed m does not hold and that m may be represented more accurately as a quadratic function of average monthly temperature for greenland the mean absolute error in predicting pddm using our methodology is 3 9 c day representing a significant improvement on current methods 7 8 c day when m 4 5 c over a range of glaciated settings our method reproduces pddm on average to within 1 5 8 5 c day compared to 4 4 15 7 c day when m 4 5 the improvement arises because we capture the systematic reduction in temperature variance that is observed over melting snow and ice when surface temperatures cannot warm above o c