comparison of topex sea surface heights and tide gauge sea levels topex sea surface height data from the first 300 days of the mission are compared to sea level data from 71 tide gauges the initial comparison uses sea surface height data processed according to standard procedures as defined in the users handbook it is found that the median correlations for island and for coastal tide gauges are 0 53 and 0 42 respectively the analogous rms differences between the two data sets are 7 9 and 10 4 cm the comparisons improve significantly when a 60 day harmonic is fit to the differences and removed this period captures aliased m2 and s2 tidal energy that is not removed by the tide model making this correction and smoothing the sea surface height data over 25 km along track segments results in median correlations of 0 58 and 0 46 for the islands and coastal stations and median rms differences of 5 8 and 7 7 cm respectively removing once per revolution signals from the sea surface heights results in degraded comparisons with the sea levels it is also found that a number of stations have poor comparisons due to propagating signals that introduce temporal lags between the altimeter and tide gauge time series a final comparison is made by eliminating stations where this propagation effect is large discarding two stations that are suspected to have problems with the sea level data smoothing over 10 day intervals and restricting attention to island gauges this results in a set of 552 data pairs that have a correlation of 0 66 and a rms difference of 4 3 cm the conclusion is that on timescales longer than about 10 days the rms sea surface height errors are less than or of the order of several centimeters

an experimental study of the kinetics of decompression induced crystallization in silicic melt 1 experiments were conducted to study the temporal evolution of feldspar crystallization kinetics during isothermal decompression pinatubo dacite was held at 780 c 220 mpa fo2 nno 2 h2o saturated conditions for an equilibration period decompressed to final pressures pf ranging from 175 to 5 mpa and then held for 0 3 931 hours according to the plagioclase liquidus curve in ph2o t space for the relevant melt composition these decompressions impose effective undercoolings teff of 34 266 c growth of preexisting phenocrysts and newly formed sparse microlites dominate crystallization at 75 pf 150 mpa teff 34 93 c and equilibrium crystal modes are achieved in 168 hours microlite nucleation is the dominant transformation process for 10 pf 50 mpa teff 125 241 c and chemical equilibrium is not attained by 168 hours under these conditions slow steady decompressions typically produced normally zoned euhedral and planar faceted feldspar crystals although anhedral morphologies were produced at very low pf contrary to expectation slowly decompressed samples were usually further from chemical equilibrium than rapidly decompressed samples after similar durations below the initial pressure although counterintuitive these trends are consistent with new constraints on the relative rates of feldspar nucleation and growth controlled by teff and melt viscosity experienced during each decompression path analysis of liquid to solid transformation kinetics using ttt style diagrams shows that crystallization occurs most rapidly at 100 mpa by a crystal growth mechanism the next most efficient crystallization conditions are at 25 mpa in a crystal nucleation dominated regime

repeated fracture and healing of silicic magma generate flow banding and earthquakes textures in an exceptionally preserved effusive rhyolite conduit at torfajokull iceland indicate that rising magma repeatedly fractured and healed at shallow levels in the conduit rfh process anastomosing tuffisite veins filled by fine grained juvenile clasts were generated by shear fracture of highly viscous magma in the glass transition interval welding of the particulate material during subsequent deformation led to thorough healing of veins allowing repeated fracture of the same body of magma we propose that the rfh process is a rechargeable trigger mechanism for hybrid seismicity and show that the time scale of the process and the fractures formed by it are consistent with the repeat time and magnitude of hybrid earthquakes during silicic eruptions the rfh process may also form the flow banding that is nearly ubiquitous in obsidian

energy levels phase and amplitude modulation of the baroclinic tide off hawaii abstract inverted echo sounder data from station aloha north of oahu hawaii are used to determine the absolute energy levels and time varying nature of the first mode baroclinic tide north of hawaii the semidiurnal tide amplitude and phase are modulated with 60 120 day periodicity which indicates interaction between the baroclinic tide and intra annual mesoscale variability seen off hawaii close to the presumed generation site there is less phase modulation and amplitude modulation is uncorrelated with the far field variability these results are consistent with a model of phase locked generation together with phase and amplitude distortion during propagation the sense of the amplitude modulation is opposite to that previously observed at decadal timescales semidiurnal energy flux at station aloha ranges from 2 2 to 8 0 kw m 1 with a mean value of 4 4 kw m 1

correlations of volcanic ash texture with explosion earthquakes from vulcanian eruptions at sakurajima volcano japan abstract we compare the texture of volcanic ash with the maximum amplitude of explosion earthquakes a eq for vulcanian eruptions from sakurajima volcano we analyze the volcanic ash emitted by 17 vulcanian eruptions from 1974 to 1987 using a stereoscopic microscope we classify the glassy particles into smooth surface particles s type particles and non smooth surface particles ns type particles according to their surface conditions gloss or non gloss appearance as an indicator of the freshness of the particles s type particles are further classified into v type particles those including vesicles and nv type particles those without vesicles by means of examinations under a polarized microscopic of polished thin sections cross correlated examinations against seismological data show that 1 the number fraction of s type particles s fraction has a positive correlation with a eq 2 the number ratio of nv type particles to v type particles the n v number ratio has a positive correlation with a eq and 3 for explosions accompanied with bl type earthquake swarms the n v number ratio has a negative correlation with the duration of the bl swarms bl swarms refer to the phenomenon of numerous bl type earthquakes occurring within a few days prior to an increase in explosive activity kamo k 1978 some phenomena before the summit crater eruptions at sakura zima volcano bull volcanol soc japan 23 53 64 the positive correlation between the n v number ratio and a eq could indicate that a large amount of separated gas from fresh magma results in a large a eq plagioclase microlite textual analysis of nv type particles from five explosive events without bl swarms shows that the plagioclase microlite number density mnd and the l w length width ratio have a positive correlation with a eq a comparison between textural data mnd l w ratio crystallinity and the result of a decompression induced crystallization experiment couch s sparks r s j carroll m r 2003 the kinetics of degassing induced crystallization at soufriere hills volcano montserrat j petrol 44 1477 1502 suggests that a plagioclase microlite texture of volcanic ash from eruptions without bl swarms could be generated by a decompression of 100 160 mpa if the mnd is controlled by the water exsolution rate from melt the positive correlation between the mnd and a eq may suggest that a eq becomes large when the effective decompression is large and the water exsolution rate is high from 6 2 10 5 to 1 9 10 4 wt s the estimated magma ascent rate ranges from 0 11 to 0 35 m s which is one order of magnitude faster than that of an effusive eruption and one to three orders slower than those for a sub plinian eruption this suggests that the ascent rate of magma plays an important role in the occurrence of vulcanian eruptions we propose a simple model for vulcanian eruptions at sakurajima volcano that takes into account the correlation between the s fraction and a eq

tephrochronology and absolute centennial scale synchronisation of european and greenland records for the last glacial to interglacial transition a case study of soppensee and ngrip the tephrostratigraphy of soppensee switzerland is presented here for the first time with new geochemical data for four tephra layers derived from three volcanic centres the best available age estimates for the laacher see tephra and vedde ash layers are incorporated into a revision of the radiocarbon based bayesian age model for the last glacial to interglacial transition at soppensee which is used to explore synchronicity of climatic events between europe and the north atlantic local temperature changes recorded by biotic and abiotic proxies in soppensee appear to shift in phase with the oxygen isotope signal in greenland at the onset of the younger dryas however with the clarity of improved precision an apparent offset that likely results from a dating artefact is seen between records at the termination of the stadial event from a methodological standpoint the results clearly show that only by establishing detailed and independent chronological frameworks can abrupt climatic variability be compared between different regions with sufficiently high temporal precision to addresses the notion of hemispheric climatic synchronicity at the centennial scale in the late quaternary continuing to develop such approaches is therefore essential in order to understand how far in time and space the climate has behaved in such a way

interactions of synoptic and planetary waves scale dependent forcing of a gcm abstract in order to better understand planetary wave synoptic wave interactions in the atmosphere and to develop a tool for further studies the authors have applied a wavenumber dependent external forcing to a general circulation model gcm the forcing constrains various length scales of the gcm to be close to those in the evolving analyses of the european centre for medium range weather forecasts and the national centers for environmental prediction the forcing acts either on the planetary waves pw defined as zonal wavenumbers 0 5 the synoptic waves sw defined as zonal wavenumbers 6 20 the synoptic waves plus the zonal mean sw0 or the synoptic waves plus the zonal mean and wavenumber 1 sw01 the form of the forcing is a linear relaxation to the evolving analyses with a time constant of 8 h this forcing is applied only to the temperature and vorticity equations of the gcm which has a spectral truncation of t42 control integrations of length 30 days have been run starting on 15 dece

explosive origin of silicic lava textural and δd h2o evidence for pyroclastic degassing during rhyolite effusion a long standing challenge in volcanology is to explain why explosive eruptions of silicic magma give way to lava a widely cited idea is that the explosive to effusive transition manifests a two stage degassing history whereby lava is the product of non explosive open system gas release following initial explosive closed system degassing direct observations of rhyolite eruptions indicate that effusive rhyolites are in fact highly explosive as they erupt simultaneously with violent volcanic blasts and pyroclastic fountains for months from a common vent this explosive and effusive overlap suggests that pyroclastic processes play a key role in rendering silicic magma sufficiently degassed to generate lava here we use precise h isotope and magmatic h2o measurements and textural evidence to demonstrate that effusion results from explosion s lavas are the direct product of brittle deformation that fosters batched degassing into transient pyroclastic channels tuffisites that repetitively and explosively vent from effusing lava our measurements show specifically that d h ratios and h2o contents of a broad suite of explosive and effusive samples from chaiten volcano hydrous bombs plinian pyroclasts tuffisite veins and lava define a single and continuous degassing trend that links wet explosive pyroclasts 1 6 wt h2o d 76 4 to dry obsidian lavas 0 13 wt h2o d 145 7 this geochemical pattern is best fit with batched degassing model that comprises small repeated closed system degassing steps followed by pulses of vapour extraction this degassing mechanism is made possible by the action of tuffisite veins which by tapping already vesicular or brecciated magma allow batches of exsolved gas to rapidly and explosively escape from relatively isolated closed system domains and large tracts of conduit magma by giving them long range connectivity even though tuffisite veins render magma degassed and capable of effusing they are nonetheless the avenues of violent gas and particle transport and thus have the potential to drive explosions when they become blocked or welded shut thus the effusion of silicic lava traditionally thought to be relatively benign process presents a particularly hazardous form of explosive volcanism

conduit to localized scale degassing during plinian eruptions insights from major element and volatile cl and h2o analyses within vesuvius ad 79 pumice textural investigations of the ad 79 vesuvius pumice emphasize the complexity of magma degassing and crystallization during the eruption which emitted two types of pumice white and gray associated with different magma bodies of phonolitic and tephriphonolitic compositions respectively these studies proposed that velocity gradients caused spatial variations in degassing within the ascending magma column at both the conduit and the localized scale to validate this hypothesis analyses of volatiles cl h2o and major elements in pumice glasses and melt inclusions were performed using high spatial resolution tools microraman spectrometry and electron microprobe and combined with major element and volatile concentration profiles and maps the results indicate that the melt phase differentiated through degassing induced crystallization of leucite and that the gray pumice magma was efficiently homogenized prior to degassing because cl diffuses more slowly than h2o during fast ascent it behaves as an incompatible element and can be used as a tracer of crystallization and h2o degassing we emphasize the importance of strain localization in generating zones of preferential exsolution and permeable pathways for gases and establish degassing scenarios that incorporate the effects of shear zones

magmatic architecture of dome building eruptions at volcan de colima mexico changes in the physical chemical and rheological properties of ascending magma regulate the style of volcanic eruptions volcan de colima s eruptive cycles of lava dome growth and explosions have been thoroughly monitored during the period 1998 2010 and provide a remarkable opportunity for deepening our understanding of the underlying processes responsible for the evolution of magma properties here we integrate direct observation with analytical and experimental data to 1 constrain the configuration of the shallow plumbing system and its influence on eruptive activity 2 describe the rheological behaviour of the magma and 3 assess the conditions that lead to fragmentation and ultimately to explosive eruptions the configuration of the shallow plumbing system was inferred from direct observation of extrusion sites and porosity of the erupted products during the ongoing eruptive phase magma was never extruded from a central vent both explosive and effusive activities were restricted to discrete vents inside the crater extensive field based density measurements on 500 blocks in pyroclastic flow deposits reveal a bimodality of porosity at values of 12 and 26 vol the least porous rocks tend to be altered whereas the more porous rocks are pristine this bimodal distribution combined with the lack of a central vent suggests the presence of a central dense altered plug the fragments of which are entrained during explosive eruptions during effusive periods the plug appears to deflect the ascent of magma at a shallow depth and consequently the site of lava extrusion the rheological properties and deformation induced seismogenic behaviour of the magmas were investigated using a uniaxial deformation apparatus instrumented with acoustic sensors the homogeneity in the physicochemical properties of the erupted magma permits the description of a flow law at eruptive temperature and strain rate conditions the crystal rich magma of volcan de colima exhibits a shear thinning rheology and becomes increasingly brittle at higher strain rates complete failure of magma can be predicted using the material failure forecast method which integrates the acceleration of released acoustic energy throughout the deformation rapid decompression experiments of samples pressurised with argon were performed to assess the fragmentation conditions under which explosive eruptions progress in the absence of gas loss due to permeable flow the pore pressure required to fragment volcanic products is inversely proportional to the porosity at volcan de colima a rapid decompression of 6 mpa is required to fragment magma averaging 26 vol pores and to thereby instigate an explosive eruption analysis of ballistic impacts 4 6 km away from the vent from recent explosive eruptions further requires decompression as great as 20 mpa which is sufficient to disrupt the more porous material as well as a fraction of the denser plug the continuing growth of a lava dome and of a larger more impermeable plug could have consequences for the stability of the edifice and development of stronger and more erratic explosive activity at volcan de colima

the february 2010 arctic oscillation index and its stratospheric connection theatmosphericdynamicsduringtheoccurrenceoftheextremenegativevalueofthe arcticoscillationindexinfebruary2010areinvestigatedusingmeteorologicalfields from era interim and era 40 reanalysis data the study focuses on the possible causes of this anomalous value finding that it was forced by a geopotential anomaly that propagated downward from the stratosphere to the troposphere in association with a major sudden stratospheric warming event which occurred at the end of january an analysis of the dynamics of this warming is also developed together with a comparison with past similar events synoptic and spectral properties of the geopotentialfieldsareanalyzed andthetimeseriesofthe baroclinicactivityindex are computed finding an abrupt increase of the middle latitude baroclinic activity immediately after the central date of the warming event a possible interpretation of this feature is proposed copyright c

petrologic constraints on the decompression history of magma prior to vulcanian explosions at the soufriere hills volcano montserrat abstract a series of 88 vulcanian explosions occurred at the soufriere hills volcano montserrat between august and october 1997 conduit conditions conducive to creating these and other vulcanian explosions were explored via analysis of eruptive products and one dimensional numerical modeling of magma ascent through a cylindrical conduit the number densities and textures of plagioclase microlites were documented for twenty three samples from the events the natural samples all show very high number densities of microlites and 50 by number of microlites have areas 2 pre explosion conduit conditions and decompression history have been inferred from these data by comparison with experimental decompressions of similar groundmass compositions our comparisons suggest quench pressures 13 75 mpa each during ascent from chamber to surface values are consistent with field studies of the same events and statistical analysis of explosion time series data the microlite volume number density trend with depth reveals an apparent transition from growth dominated crystallization to nucleation dominated crystallization at pressures of 7 mpa and lower a concurrent sharp increase in bulk density marks the onset of significant open system degassing apparently due to a large increase in system permeability above 70 vesicularity this open system degassing results in a dense plug which eventually seals the conduit and forms conditions favorable to vulcanian explosions the corresponding inferred depth of overpressure at 250 700 m near the base of the dense plug is consistent with depth to center of pressure estimated from deformation measurements here we also illustrate that one dimensional models representing ascent of a degassing crystal rich magma are broadly consistent with conduit profiles constructed via our petrologic analysis the comparison between models and petrologic data suggests that the dense conduit plug forms as a result of high overpressure and open system degassing through conduit walls

intra annual oscillations at station aloha north of oahu hawaii inverted echo sounders have been used to make continuous measurements of dynamic height at a site chosen to be representative of the central pacific subtropical gyre these show that the dominant features in dynamic height are fluctuations having peak to peak amplitudes of up to 27 dyn cm and timescales of about 100 days empirical orthogonal function analyses of temperature and salinity profiles suggest that if the oscillations are due to vertical motion within the water column the motion is not that of first baroclinic mode but has much higher vertical wavenumber structure zonal wavenumbers associated with the oscillations indicate westward phase propagation and meridional wavenumbers are close to zero the dominant periods are too short for freely propagating rossby waves two explanations of these observations are that the waves could be doppler shifted by the mean flow or that the region could be an area of active forcing

texture and composition of pumices and scoriae from the campi flegrei caldera italy implications on the dynamics of explosive eruptions 1 the campi flegrei cf caldera is one of the most dangerous quiescent volcanic systems in the world its activity mostly resulted in low magnitude explosive eruptions such as that of the monte nuovo tuff cone that represents the last eruptive event within the caldera a d 1538 however there have been more energetic plinian events e g the agnano monte spina eruption 4 1 ka and very highly explosive caldera forming eruptions e g the campanian ignimbrite eruption 39 ka here we integrate new and literature data on the groundmass texture and composition of pyroclastic products from the three above eruptions with the aim of unraveling how volatiles content degassing mechanisms and crystallization processes influence magma explosivity and eruption dynamics at cf previous studies indicate that the investigated rocks share similar major element bulk and phenocryst chemistry also similar is the water content of their trapped melt inclusions these observations suggest that the magmas feeding these eruptions had comparable physicochemical properties during storage in the shallow crust however our investigations indicate that the studied rocks differ in texture and composition of the groundmass and viscosity of the related magmas we ascribe such differences to the variable style of volatile exsolution and outgassing from the melt primarily in response to changes of the rate of magma ascent to the surface we conclude that the magma ascent rate was the key parameter in driving explosive eruptions at cf and we suggest that this parameter may be influenced by magma water interaction and or magma chamber geometry and replenishment

surface manifestation of internal tides in the deep ocean observations from altimetry and island gauges the sea surface height signatures of internal tides in the deep ocean amounting to a few centimeters or less are studied using two complementary measurement types satellite altimetry and island tide gauges altimetry can detect internal tides that maintain coherence with the astronomical forcing island gauges can monitor temporal variability which in some circumstances is due to internal tides varying in response to changes in the oceanic medium this latter mechanism is at work at hilo and other stations on the northern coasts of the hawaiian islands by detecting spatially coherent low frequency internal tide modulations the tide gauges along with inverted echo sounders at sea suggest that the mean internal tide is also spatially coherent satellite altimetry confirms this at hawaii and in many other places topex poseidon altimetry detects mean surface waves spatially coherent and propagating great distances 1000 km before decaying below background noise when temporal variability is small the altimetry plus information on ocean density sets useful constraints on energy fluxes into internal tides at the hawaiian ridge 15 gw of tidal power is being converted from barotropic to first mode baroclinic motion examples elsewhere warn that a simplistic interpretation of the altimetry without regard to variability noise or in situ information may be highly misleading with such uncertainties extension of the hawaiian results into a usefully realistic estimate of the global internal tide energy balance appears premature at this time

annual variations in sea surface height northeast of the hawaiian islands 1 a decade long satellite altimeter data set is used to analyze the annual variations in the sea surface height ssh field northeast of the hawaiian islands the annual harmonic amplitudes for the ssh and the along island geostrophic flow are 2 4 cm and 2 4 cm s 1 respectively although these values are an order of magnitude smaller than the regional mesoscale eddy signals the annual ssh signals have a robust spatial structure that can be expressed as a function of distance northeastward away from the hawaiian islands the annual ssh signals are then described by using trapped annual waves and a basin scale non propagating signal using the linearized vorticity equation we suggest that the trapped annual waves are baroclinic rossby waves that decay from the hawaiian islands

rheological change and degassing during a trachytic vulcanian eruption at kilian volcano chaine des puys france magma ascent during silicic dome forming eruptions is characterized by significant changes in magma viscosity permeability and gas overpressure in the conduit these changes depend on a set of parameters such as ascent rate outgassing and crystallization efficiency and magma viscosity which in turn may influence the prevailing conditions for effusive versus explosive activity here we combine chemical and textural analyses of tephra with viscosity models to provide a better understanding of the effusive explosive transitions during vulcanian phases of the 9 4 ka eruption of kilian volcano chaine des puys france our results suggest that effusive activity at the onset of vulcanian episodes at kilian volcano was promoted by i rapid ascent of initially crystal poor and volatile rich trachytic magma ii a substantial bulk and melt viscosity increase driven by extensive volatile loss and crystallization and iii efficient degassing outgassing in a crystal rich magma at shallow depths trachytic magma repeatedly replenished the upper conduit and variations in the amount of decompression and cooling caused vertical textural stratification leading to variable degrees of crystallization and outgassing outgassing promoted effusive dome growth and occurred via gas percolation through large interconnected vesicles fractures and tuffisite veins fostering the formation of cristobalite in the carapace and talus regions build up of overpressure was likely caused by closing of pore space bubbles and fractures in the dome through a combination of pore collapse cristobalite formation sintering in tuffisite veins and limited pre fragmentation coalescence in the dome or underlying hot vesicular magma sealing of the carapace may have caused a transition from open to closed system degassing and to renewed explosive activity we generalize our findings to propose that the broad spectrum of eruptive styles for trachytic magmas may be inherited from a combination of characteristics of trachytic melts that include high water solubility and diffusivity rapid microlite growth and low melt viscosity compared to their more evolved subalkaline dacitic and rhyolitic equivalents we show that trachytes may erupt with a similar style e g vulcanian but at significantly higher ascent rates than their andesitic dacitic and rhyolitic counterparts this suggests that the periodicity of effusive explosive transitions at trachytic volcanoes may differ from that observed at the well monitored andesitic dacitic and rhyolitic volcanoes which has implications for hazard assessment associated with trachytic eruptions

crystallization kinetics of alkali feldspars in cooling and decompression induced crystallization experiments in trachytic melt cooling and decompression experiments have been carried out on trachytic melts in order to investigate crystallization kinetics of alkali feldspar the effect of the degree of undercooling t t liquidus t experimental and time on nucleation and crystal growth process this experimental work gives us new data about crystallization kinetics of trachytic melts and it that will be useful to better understand the natural system of campi flegrei volcanoes experiments have been conducted using cold seal pressure vessel apparatus at pressure between 30 and 200 mpa temperature between 750 and 855 c time between 7 200 and 57 600 s and redox condition close to the nno 0 8 buffer these conditions are ideal to reproducing pre and syn eruptive conditions of the campi flegrei volcanoes where the conditions pertain to the complete range of pressures temperatures and time at which the experiments were performed alkali feldspar is the main phase present in this trachyte and its abundance can strongly vary with small changes in pressure temperature and water content in the melt implying appreciable variations in the textures and in the crystallization kinetics the obtained results show that crystallization kinetics are strictly related to t time final pressure superheating t and water content in the melt t is the driving force of the crystallization and it has a strong influence on nucleation and growth processes in fact the growth process dominates crystallization at small t whereas the nucleation dominates crystallization at large t time also is an important variable during crystallization process because long experiment durations involve more nucleation events of alkali feldspar than short experiment durations this is an important aspect to understand magma evolution in the magma chamber and in the conduit which in turn has strong effects on magma rheology

the nature and formation of cristobalite at the soufriere hills volcano montserrat implications for the petrology and stability of silicic lava domes cristobalite is commonly found in the dome lava of silicic volcanoes but is not a primary magmatic phase its presence indicates that the composition and micro structure of dome lavas evolve during and after emplacement nine temporally and mineralogically diverse dome samples from the soufriere hills volcano shv montserrat are analysed to provide the first detailed assessment of the nature and mode of cristobalite formation in a volcanic dome the dome rocks contain up to 11 wt cristobalite as defined by x ray dif fraction prismatic and platy forms of cristobalite identified by scanning electron microscopy sem are commonly found in pores and fractures suggesting that they have precipitated from a vapour phase feathery crystallites and micro crystals of cristobalite and quartz associated with vol canic glass identified using sem raman are interpreted to have formed by varying amounts of devitrification we dis cussmechanismsof silicatransport and cristobaliteformation and their implications for petrological interpretations and domestability we conclude 1 that silica maybetransported in the vapour phase locally or from one part of the magmatic system to another 2 that the potential for transport of silica into the dome should not be neglected in petrological and geochemical studies because the addition of non magmatic phases may affect whole rock composition and 3 that the extent of cristobalite mineralisation in the dome at shv is sufficient to reduce porosity hence permeability and may impact on the mechanical strength of the dome rock thereby potentially affecting dome stability

discriminating secondary from magmatic water in rhyolitic matrix glass of volcanic pyroclasts using thermogravimetric analysis abstract pyroclasts from explosive eruptions such as the 1060 ce explosive glass mountain eruption of medicine lake volcano california contain large amounts of water this may be the consequence of diffusive rehydration of the volcanic glass by meteoric secondary water after the eruption discriminating between magmatic and secondary water in the matrix glass of pyroclasts is important because the degassing of magmatic water affects the intensity of volcanic eruptions such discrimination has remained a challenging problem especially because some aspects of water diffusion in silicate glasses at low temperatures and atmospheric pressure remain poorly constrained we used thermogravimetry to analyze the loss of water from natural volcanic glasses and glasses that were hydrated in the laboratory at magmatic temperatures and pressures numerical modeling of diffusive water loss during thermogravimetric analyses accounted for the interconversion of molecular water h 2 o m and hydroxyls groups oh and indicates that glass mountain pumices contain 0 2 0 5 wt primary water but gained 1 2 wt of meteoric water by diffusive rehydration during the past 950 years these results confirm that the majority of magmatic water is lost from the magma during explosive eruptions furthermore the integration of thermogravimetric analysis and numerical modeling facilitates discrimination between the magmatic and secondary water content of volcanic glasses

breadcrust bombs as indicators of vulcanian eruption dynamics at guagua pichincha volcano ecuador vulcanian eruptions are common at many volcanoes around the world vulcanian activity occurs as either isolated sequences of eruptions or as precursors to sustained explosive events and is interpreted as clearing of shallow plugs from volcanic conduits breadcrust bombs characteristic of vulcanian eruptions represent samples of different parts of these plugs and preserve information that can be used to infer parameters of pre eruption magma ascent the morphology and preserved volatile contents of breadcrust bombs erupted in 1999 from guagua pichincha volcano ecuador thus allow us to constrain the physical processes responsible for vulcanian eruption sequences of this volcano morphologically breadcrust bombs differ in the thickness of glassy surface rinds and in the orientation and density of crack networks thick rinds fracture to create deep widely spaced cracks that form large rectangular domains of surface crust in contrast thin rinds form polygonal networks of closely spaced shallow cracks rind thickness in turn is inversely correlated with matrix glass water content in the rind assuming that all rinds cooled at the same rate this correlation suggests increasing bubble nucleation delay times with decreasing pre fragmentation water content of the melt a critical bubble nucleation threshold of 0 4 0 9 wt water exists below which bubble nucleation does not occur and resultant bombs are dense at pre fragmentation melt h2o contents of 0 9 wt only glassy rinds are dense and bomb interiors vesiculate after fragmentation for matrix glass h2o contents of 1 4 wt rinds are thin and vesicular instead of thick and non vesicular a maximum measured h2o content of 3 1 wt establishes the maxi mum pressure 63 mpa and depth 2 5 km of magma that may have been tapped during a single eruptive event more common h2o contents of 1 5 wt suggest that most eruptions involved evacuation of 1 5 km of the conduit as we expect that substantial overpressures existed in the conduit prior to eruption these depth estimates based on magmastatic pressure are maxima moreover the presence of measurable co2 17 ppm in quenched glass of highly degassed magma is inconsistent with simple models of either open or closed system degassing and leads us instead to suggest re equilibration of the melt with gas derived from a deeper magmatic source together these observations suggest a model for the repeated vulcanian eruptions that includes 1 evacuation of the shallow conduit during an individual eruption 2 depressurization of magma remaining in the conduit accompanied by open system degassing through permeable bubble networks 3 rapid conduit re filling and 4 dome formation prior to the subsequent explosion an important part of this process is densification of upper conduit magma to allow repressur ization between explosions at a critical overpressure trapped pressurized gas fragments the nascent impermeable cap to repeat the process

multiple regimes and low frequency oscillations in the southern hemisphere s zonal mean flow abstract this paper studies multiple regimes and low frequency oscillations in the northern hemisphere zonal mean zonal flow in winter using 55 yr of daily observational data the probability density function estimated in the phase space spanned by the two leading empirical orthogonal functions exhibits two distinct statistically significant maxima the two regimes associated with these maxima describe persistent zonal flow states that are characterized by meridional displacements of the midlatitude jet poleward and equatorward of its time mean position the geopotential height anomalies of either regime have a pronounced zonally symmetric component but largest amplitude anomalies are located over the atlantic and pacific oceans high frequency synoptic transients participate in the maintenance of and transitions between these regimes significant oscillatory components with periods of 147 and 72 days are identified by spectral analysis of the zonal flow time series these oscillations are described b

plinian pumice fall deposit of the campanian ignimbrite eruption phlegraean fields italy abstract a plinian pumice fall deposit associated with the campanian ignimbrite eruption 36 ka phlegraean fields caldera italy occurs at the base of the distal grey ignimbrite in 15 localities spread over an area exceeding 1500 km2 between benevento and the sorrentina peninsula in the thickest stratigraphic section at voscone 130 cm 45 km east of the phlegraean caldera centre pozzuoli the deposit consists of two units the lower fall unit lfu is well sorted exhibits reverse size grading and is composed of equidimensional light grey pumice clasts with very subordinate accidental lithics the upper fall unit ufu is from well to poorly sorted crudely stratified richer in lithics and composed of both equidimensional and prolate pumice clasts the two fall units show slightly different dispersal axis n90 for the lfu and n95 for the ufu volumes calculated with the method of pyle 1989 are about 8 km3 for the lfu and 7 km3 for the ufu the maximum height of the eruptive columns are estimated using the model of the maximum lithic clasts dispersal at 44 km for the lfu and 40 km for the ufu classifying both fall units as ultraplinian in character reverse size grading within the lfu suggests an increase of the height of the column and magma discharge rate with time moderate sorting and crude stratification of the ufu are consistent with short period oscillation of the column possibly associated with repeated partial column collapses sharp increases in lithic content at the transition to ufu and within the ufu suggest that changes in the eruptive behaviour was produced by a dramatic increase in conduit vent erosion the phase of column instability preceded the emplacement of widely dispersed pyroclastic flow the ultraplinian nature of the fall fits well with the wide dispersal of the campanian ignimbrite with an estimated aspect ratio of 3 4 10 4 lari

the plinian phase of the campanian ignimbrite eruption phlegrean fields italy evidence from density measurements and textural characterization of pumice textural characterization of pumice clasts from explosive volcanic eruptions provides constraints on magmatic processes through the quantification of crystal and vesicle content size shape vesicle wall thickness and the degree of interconnectivity the plinian fallout deposit directly underlying the campanian ignimbrite ci eruption represents a suitable case to investigate pumice products with different textural characteristics and to link the findings to processes accompanying conduit magma ascent to the crater the deposit consists of a lower lfu and upper ufu pumice lapilli bed generated by the sub steady eruption of trachytic magma with 0 90 with vesicle number density ranging from 107 108 cm 3 the degree of vesicle coalescence is high for all pumice types with interconnected vesicles generally representing more than 90 of the bulk vesicle population the results show a high degree of heterogeneous textures among pumice clasts from both phases of the eruption and within each eruption phase the different pumice types and also within each single pumice type fragment the origin of pumice clasts with different textural characteristics is ascribed to the development of conduit regions marked by different rheological behavior the conclusions of this study are that vesicle deformation degree of coalescence and intense shear at the conduit walls play a major role on the degassing process hence affecting the entire conduit dynamics

geosat and advanced very high resolution radiometer observations of oceanic planetary waves adjacent to the hawaiian islands geosat observations of the central north pacific ocean show westward propagating mesoscale features with length scales 500 km time scales 105 155 days and observed zonal phase speeds commensurate with those of nearly nondispersive baroclinic rossby waves advanced very high resolution radiometer derived sea surface temperatures observed during april 1988 showed a cusp shaped pattern north of the hawaiian islands congruent with concurrent geosat derived sea level height anomalies it is believed that the cusp shaped pattern is the surface manifestation of a baroclinic rossby wave the observed wavelength of the cusp shaped pattern was 560 km it had an observed meridional extent horizontal amplitude of 200 300 km and an inferred zonal propagation speed of 0 052 m s 1 the westward propagating sea level height structure was observed to amplify as it approached 150 w reaching amplitudes of 0 06 0 08 m in the vicinity of the cusp shaped feature these waves are believed to be locally forced by the topography either as the result of eastward flow over the submerged island ridge or as a result of baroclinic instability during periods of westward flow

magma degassing and crystallization processes during eruptions of high risk neapolitan volcanoes evidence of common equilibrium rising processes in alkaline magmas abstract compositional textural and experimental data on products from explosive and effusive eruptions of neapolitan volcanoes campi flegrei and somma vesuvio allow us to constrain degassing and fragmentation conditions during eruptions of alkaline magmas significant differences in compositional and textural features have been recognized between lavas scoria and pumice resulting respectively from effusive moderately and extremely explosive eruptions pumice samples have highly vesicular glassy matrix low microlite number density and moderate to high water content crystal size distributions csd are steep with high intercept values the narrow microlite size range indicates single nucleation event scoria products are characterized by moderate vesicularity and water content they have high number density of microlites which are bimodal in size csd show distinct inflections that are explained as two crystal populations growing in distinct time lava samples generally have low vesicularities moderate to high microcrystalline groundmass and low glass water content the comparison between textural and compositional features of natural rocks with samples obtained by decompression experiments allows us to conclude that degassing processes during magma ascent occurs in near equilibrium conditions even at high decompression rate moderate to long magma rise times calculated in the order of a few days produce open degassing responsible for moderately explosive to effusive activity short magma rise times calculated in the order of a few hours result in closed system degassing that allow explosive fragmentation when the volume of growing bubble reaches a fixed threshold vesicularity and water content measured on matrix glass of pumice indicate that this process occurs at pressure of 10 30 mpa in these conditions degassing fragmentation and in turn the eruptive style is strongly influenced by initial conditions in the magma chamber volatile content temperature pressure instead of decompression rate in contrast with that observed for rhyolitic melts these differences have important consequences in terms of volcanic hazards and risk the low viscosity alkaline magma is able to maintain efficient degassing even during the final stage of magma ascent favoring in the case of closed system fragmentation and explosive activity

injection of vesicular magma into an andesitic dome at the effusive explosive transition article i nfo a single vesicular pyroclast collected from a major 11 10 6 m 3 dome collapse eruption of the soufriere hills volcano montserrat 21st september 1997 was found to contain textural and raman evidence for multiple decompression events and magma injection into the dome just prior to the onset of dome collapse quartz plagioclase and hornblende phenocrysts contain closed and burst melt inclusions mis the latter in the form of either foam or cylindrical melt extrusions from cavities on fracture surfaces fractures within ruptured quartz phenocrysts were infilled with glass whereas those within plagioclase are entirely free of glass indicatingrelativelyearlyfracturing decompression ofthequartz thisinterpretationissupportedbyslightly higher measured h2otin burst mis in quartz 0 44 wt 0 07 1 s d compared with plagioclase 0 31 wt 0 05 1 s d indicating equilibration on rupturing of the quartz at higher ph2o h2ot levels in glass from burst mis in plagioclase are consistent with plagioclase fragmentation at h2o saturation pressures b5 mpa at an estimated depth of b210 m within the dome the fragmentation occurred whilst the groundmass glass was still plastic as evident from the presence of stretched vesicles in glass at fracture terminations the fractures in plagioclase are empty of vapour phase precipitates mainly cristobalite that infills matrix vesicles which constrains the fracturing of plagioclase to the last depressurisation event recorded in the pyroclast which was the dome collapse which led to the pyroclast being ejected from the dome we suggest that our study provides the first direct i e non seismic evidence for the injection of gas charged magma into the dome at the onset of dome collapse marking the transition from effusive to explosive eruption and that the magma injection could have provided the triggering mechanism for dome collapse

analysis of ordinary chondrites using powder x ray diffraction 1 modal mineral abundances abstract powder x ray diffraction xrd is used to quantify the modal abundances in wt of 18 h 17 l and 13 ll unbrecciated ordinary chondrite falls which represents the complete petrologic range of equilibrated ordinary chondrites types 4 6 the xrd technique presents an effective alternative to traditional methods for determining modal abundances such as optical point counting and electron microprobe phase emp mapping the majority of chondrite powders in this study were previously prepared for chemical characterization from 8 to 20 g of material which is consistent with the suggested mass 10 g necessary to provide representative sampling of ordinary chondrites olivine and low ca pyroxene are the most abundant phases present comprising one half to three fourths of total abundances while plagioclase high ca pyroxene troilite and metal comprise the remaining xrd measured mineralogy pigeonite may also be present in some samples but it is fitted using a high ca pyroxene standard so exact abundances cannot be measured directly using xrd comparison of xrd measured abundances with calculated cross iddings pirsson washington cipw normative abundances indicates that systematic discrepancies exist between these two data sets particularly in olivine and high ca pyroxene this discrepancy is attributed to the absence of pigeonite as a possible phase in the cipw normative mineralogy oxides associated with pigeonite are improperly allocated resulting in overestimated normative olivine abundances and underestimated normative high ca pyroxene abundances this suggests that the cipw norm is poorly suited for determining mineralogical modal abundances of ordinary chondrites

the source of 90 day oscillations at wake island energetic 90 day oscillations of sea level have been intermittently observed at wake island in the western tropical pacific during the past 2 decades the oscillations tend to occur about 1 5 years after el nino southern oscillation events to have amplitudes of 10 15 cm and to persist for about 1 year sea surface heights from the geosat altimeter are used to establish that these signals take the form of rossby waves and have an energy source near the big island of hawaii which lies 40 of longitude to the east sea level and upper layer currents from an eddy resolving numerical model are examined and suggest that the energy source is eddies generated off the big island of hawaii these eddies appear to be associated with westward currents that intermittently impinge on the island several alternate hypotheses are also discussed and rejected

influence of pre eruptive degassing and crystallization on the juvenile products of laterally directed volcanic explosions abstract strikingly similar examples of edifice collapse and directed blast are the 18 may 1980 eruption of mount st helens msh washington usa and the 30 march 1956 eruption of bezymianny volcano bz kamchatka russia in these cases flank failures led to near instantaneous decompression and fragmentation of intra edifice cryptodome magma which produced catastrophic laterally directed blasts in both instances the blast products consisted of juvenile material with bimodal density vesicularity distributions low and high density modes at 1900 and 2400 kg m 3 for bz 1600 and 2300 kg m 3 for msh although the proportion of high density material is greater at bz blast materials also exhibit striking variety in groundmass crystallinity 90 vol despite having fairly uniform pheno crystallinities suggesting that degassing driven groundmass crystallization occurred to varying extents within cryptodome magma at both volcanoes new bulk rock h2o and d measurements confirm that progressive open system outgassing occurred prior to both blasts the correlations between crystallinity clast density and bulk h2o contents suggest that syn blast magma expansion was modulated both by non uniform volatile distribution within the cryptodome and rheological controls associated with non uniform crystal content spatial heterogeneities in volatiles and crystallinity within a given cryptodome are attributed to distance from the wallrock margin which probably correlates with timing of magma injection the greater proportion of high density material at bz is speculatively related to lower blast energy compared with msh

bubble nucleation growth and coalescence during the 1997 vulcanian explosions of soufriere hills volcano montserrat abstract soufriere hills volcano had two periods of repetitive vulcanian activity in 1997 each explosion discharged the contents of the upper 0 5 2 km of the conduit as pyroclastic flows and fallout frothy pumices from a deep gas rich zone lava and breadcrust bombs from a degassed lava plug and dense pumices from a transition zone vesicles constitute 1 66 vol of breadcrust bombs and 24 79 of pumices all those larger than a few tens of m being interconnected small vesicles few hundreds of m in pumices are interpreted as pre dating explosion implying pre explosive conduit porosities up to 55 about a sixth of large vesicles in pumices and all those in breadcrust bombs are angular voids formed by syn explosive fracturing of amphibole phenocrysts an intermediate sized vesicle population formed by coalescence of the small syn explosive bubbles bubble nucleation took place heterogeneously on titanomagnetite number densities of which greatly exceed those of vesicles and growth took place mainly by decompression development of pyroclast vesicle textures was controlled by the time interval between the onset of explosion decompression and surface quench in contact with air lava plug fragments entered the air quickly after fragmentation 10 s so the interiors continued to vesiculate once the rinds had quenched forming breadcrust bombs deeper gas rich magma took longer 50 s to reach the surface and vesiculation of resulting pumice clasts was essentially complete prior to surface quench this accounts for the absence of breadcrusting on pumice clasts and for the textural similarity between pyroclastic flow and fallout pumices despite different thermal histories after leaving the vent it also allowed syn explosive coalescence to proceed further in the pumices than in the breadcrust bombs uniaxial boudinage of amphibole phenocrysts in pumices implies significant syn explosive vesiculation even prior to magma fragmentation probably in a zone of steep pressure gradient beneath the descending fragmentation front syn explosive decompression rates estimated from vesicle number densities 0 3 6 5 mpa s 1 are consistent with those predicted by previously published numerical models

an error covariance model for sea surface topography and velocity derived from topex poseidon altimetry in order to facilitate the use of satellite altimeter derived sea surface topography and velocity in oceanographic models a methodology is presented for deriving the total error covariance and its geographic distribution from topex poseidon measurements the model is formulated using a parametric model fit to the altimeter range observations the topography and velocity are modeled with spherical harmonic expansions whose coefficients are found through optimal adjustment to the altimeter range residuals using bayesian statistics all other parameters including the orbit geoid surface models and range corrections are provided as unadjusted parameters the maximum likelihood estimates and errors are derived from the probability density function of the altimeter range residuals conditioned with a priori information estimates of model errors for the unadjusted parameters are obtained from the topex poseidon postlaunch verification results and the error covariances for the orbit and the geoid except for the ocean tides the error in the ocean tides is modeled first as the difference between two global tide models and second as the correction to the present tide model the correction derived from topex poseidon data a formal error covariance propagation scheme is used to derive the total error our global total error estimate for the topex poseidon topography relative to the geoid for one 10 day period is found to be 11 cm rms when the error in the geoid is removed thereby providing an estimate of the time dependent error the uncertainty in the topography is 3 5 cm rms this level of accuracy is consistent with direct comparisons of topex poseidon altimeter heights with tide gauge measurements at 28 stations in addition the error correlation length scales are derived globally in both east west and north south directions which should prove useful for data assimilation the largest error correlation length scales are found in the tropics errors in the velocity field are smallest in midlatitude regions they are less than 2 cm s over most of the southern ocean for both variables the largest errors are caused by uncertainty in the geoid more accurate representations of the geoid await a dedicated geopotential satellite mission substantial improvements in the accuracy of ocean tide models are expected in the very near future from research with topex poseidon data consequently estimates of the time dependent sea surface topography from topex poseidon could achieve a total uncertainty of less than 3 cm rms this level of accuracy from a spaceborne sensor represents a remarkable engineering achievement and a major contribution to ocean science

transitions between blocked and zonal flows in a rotating annulus with topography the mid latitude atmosphere is dominated by westerly nearly zonal flow occasionally this flow is deflected poleward by blocking anticyclones that persist for 10 days or longer experiments in a rotating annulus used radial pumping to generate a zonal jet under the action of the coriolis force in the presence of two symmetric ridges at the bottom of the annulus the resulting flows were nearly zonal at high forcing or blocked at low forcing intermittent switching between blocked and zonal patterns occurs because of the jet s interaction with the topography these results shed further light on previous atmospheric observations and numerical simulations

statistical properties of low frequency variability in the northern hemisphere abstract we discuss some statistical properties of the observed tropospheric circulation in northern hemisphere mid latitudes the data used consist of the twice daily 500 mb height nmc analyses for the period 1966 77 our analysis is performed in the context of the quasi unidimensional theory estimates of the probability distribution of wave amplitude in the low frequency range 10 40 days seem to reveal a bimodal character while similar estimates for the zonal wind fail to show stable multiple peaks in the occupation frequency although the process involved seems to be more complex than gaussian possible dynamical interpretations of such statistical properties are discussed in the light of other properties resulting from different types of analysis the emerging physical picture reveals an intermittent process operating on planetary scales through a predominantly baroclinic conversion in agreement with the theoretical considerations of benzi et al

degassing and microlite crystallization of basaltic andesite magma erupting at arenal volcano costa rica abstract volcanoes can erupt explosively in plinian style or effusively as lava flows most models for such ranges in activity are based on silicic magma which may not be appropriate for less viscous basic magma although basic magma erupting at arenal volcano has not varied significantly in bulk composition the volcano has exhibited a full range in eruptive style from plinian activity in 1968 to strombolian bursts to lava flows we examined groundmass textures of samples erupted over that range of activity to investigate the controls on the variability microlite textures in lavas collected both hot rapid quenched and cold show that most samples have textures that are overprinted by crystallization as a result of cooling despite that overprint microlites in the plinian sample have unique crystal morphologies and vesicles that are much smaller and more spherical than those in the other samples we interpret those differences as recording a change in degassing style as a result of changing ascent rate in the conduit to constrain the potential changes in ascent rate a limited number of decompression experiments were run at rates from 0 0013 to 0 25 mpa s crystal textures and morphologies vary greatly as decompression rates change and compared to our observed differences in the natural groundmass it appears that magma erupted in the plinian event decompressed between 0 0013 and 0 025 mpa s whereas magma erupted in non plinian events decompressed slower than 0 0013 mpa s the change in eruptive style from explosive plinian to lava effusion thus resulted from an order of magnitude decrease in magma ascent rate plinian magma probably rose too quickly to allow bubbles to coalesce and allow the magma to degas efficiently whereas at other times magma rose more slowly which allowed bubbles to coalesce and gas to escape leading to less explosive activity

temporal variation in volcanic ash texture during a vulcanian eruption at the sakurajima volcano japan abstract we discuss the process of vulcanian eruptions on the basis of temporal variation of the texture of andesitic volcanic ash ejected from the sakurajima volcano japan falling ash samples were collected every 2 50 min for petrological analysis these ash samples contained juvenile particles altered rock fragments and fragments of crystals here we subdivide the juvenile particles into blocky vesicular and fluidal particles because the crystallinities of the vesicular and fluidal particles are lower than those of the blocky particles we refer to the vesicular and fluidal particles as low crystallinity particles lcps and the blocky particles as high crystallinity particles hcps the temporal variation observed in ash texture reveals that the ratio of lcps to hcps lcp hcp was small at the initiation of the eruption and increased as the eruption progressed the higher microlite crystallinity in the hcps indicates crystallization under lower pressures for longer durations relative to the lcp magma the increase of the lcp hcp ratio during a single vulcanian eruption suggests that the emission of less viscous gas rich magma under a cap rock increases as the eruption progresses such temporal variation in ash texture reflects the transition from a sudden explosion to continuous ash emission during a vulcanian eruption

on using satellite altimetric heights to provide a spatial context for the hawaii ocean time series measurements abstract it is assumed that the hawaiian ocean time series hot measurements would be usefully complemented by datasets capable of providing a spatial context for the variability observed at the hot site the potential usefulness of high quality sea surface heights from satellite altimeters is evaluated by analyzing data from the topex poseidon t p mission three signals are examined an intra annual oscillation the annual variability and an interannual sea level drop observed at the hot site during 1993 the intra annual signal observed at the hot site is shown to be part of a propagating disturbance that moves energy to the northwest but propagates phase to the southwest farther to the northwest however the energy and phase propagate nearly due west at the annual period the small amplitude observed at the hot site is confirmed but this is not true of the larger scale where annual amplitudes reach 5 10 cm the annual phase is largely consistent with the sea surface response to annual heating although the interannual event is barely resolved by the t p series the drop of dynamic height observed at the hot site is shown to be part of a larger scale event this event is stationary in space and is initially characterized by positive height anomlies near the ridge and negative anomalies further offshore the signs of these two anomalies reverse in just over 1 year the observed pattern gives rise to significant meridional sea surface height gradients and the associated mass transport anomalies are estimated to be 8 sv in all three cases the t p time series although less than 2 years in duration provide information about the spatial structure of the signals that extends the descriptions that would be possible using the hot data alone

pre explosive conduit conditions of the 1997 vulcanian explosions at soufriere hills volcano montserrat i pressure and vesicularity distributions an authoritative case of vulcanian eruptive dynamics is the series of 88 explosions that occurred between august and october 1997 at soufriere hills volcano on montserrat island the state of the magmatic column just before a vulcanian explosion is still poorly understood but conditions the eruptive style this study establishes such a pre explosive stratigraphy by 1 documenting the textures covering the range of the 1997 products 2 quantitative analysis of h2o content in interstitial glass measured by karl fischer titration and 3 combining these data with a simple model linking pre and post explosive vesicularities the model shows that syn explosive degassing affects greatly the way porosity evolves by decompression during an explosion the stratigraphy reconstruction shows a three part vertical layering of the conduit prior to explosion with overall denser values than those previously suggested a dense and strongly degassed plug caps the column it is underlain by a shallow transition zone featuring complex mingling between vesicular and dense magma up to 10 mpa at higher pressure up to 80 mpa lies a more homogeneous zone of relatively dense 10 20 vol magma which was emplaced under partly open system degassing this conduit stratigraphy gives the vision of a strongly heterogeneous magma column immediately prior to its disruption our analysis suggests that fragmenting such a composite magma cannot happen in a single coherent pulse but rather as stages the transition zone contains heterogeneous amounts of exsolved gas that could explain the pulsatory nature of the vulcanian jets at the beginning of the explosions this contrasts with the nearly constant vesicularities of the deeper part of the pre explosive magma column which are propitious to a general short lived disruption

degassing at the soufriere hills volcano montserrat recorded in matrix glass compositions compositions of matrix glasses from the current eruption of soufriere hills volcano indicate that decompression driven crystallization results in 20 70 wt groundmass crystallization during eruption and variable degassing variations in crystallinity and volatile contents water and chlorine of matrix glasses are attributed to variations in extrusion rates and residence times in the lava dome residual water contents in pumice clasts 0 2 0 6 wt indicate minimum pressures of 1 1 3 7 mpa in 1997 vulcanian explosions residual water contents of 1 6 wt in a ballistic block ejected in sub plinian explosive activity on 17 september 1996 imply larger pressure drops similar to20 mpa variable residual water contents in dome samples are consistent with pressure variations of up to 9 mpa in the lava dome interior large variation in chlorine contents between lava blocks compared with explosion pumice clasts indicates that shallow level processes dominate degassing low melt chlorine contents of dome samples cannot be explained by open or closed system degassing especially when crystallization is taken into account instead heterogeneous chlorine leaching by circulation of groundwater vapour in the dome is proposed variable and elevated matrix glass deltad values can also be attributed to interaction with isotopically heavy surface waters hcl emission during the current eruption can be accounted for by cl loss from the melt consistent with the melt being undersaturated in chlorine

the transition from explosive to effusive eruptive regime the example of the 1912 novarupta eruption alaska the shift from explosive to effusive silicic volcanism seen in many historical eruptions reflects a change in the style of degassing of erupted magma this paper focuses on such a transition during the largest eruption of the twentieth century the 1912 eruption of novarupta the transition is recorded in a dacite block bed which covers an elliptical area of 4 km 2 around the vent approximately 700 studied blocks fall into four main lithologic categories 1 pumiceous 2 dense 3 flow banded dacites and 4 welded breccias textural analyses of the blocks indicate portions of the melt underwent highly variable degrees of outgassing vesicle populations show features characteristic of bubble coalescence and collapse a decrease in measured vesicularity and increased evidence for bubble collapse compared with pumice from earlier plinian episodes mark the transition from closed to open system degassing block morphology and textures strongly suggest the magma was first erupted as a relatively gas rich lava dome plug but incomplete out gassing led to explosive disruption heterogeneous degassing of ascending magma began in plinian episode iii and resulted in instability during episode iv dome growth and a series of vulcanian explosion s modeling of the dynamics of explosion initiation and ejecta dispersal indicates that a significant concentration in gas is required to produce the explosions responsible for the observed block field dispersal the amount of gas available in the hot pumiceous dome material appears to have been inadequate to drive the explosion s therefore external water most likely contributed to the destruction

monthly mean sea level fluctuations at honolulu and san francisco and the intervening geostrophic currents abstract monthly mean dynamic topography 0 500 db along the great circle section between honolulu and san francisco reveals four principal current regimes a northerly flow immediately adjacent to the hawaiian islands a broad southerly interior flow incorporating the california front and the california and davidson currents monthly mean sea level values at honolulu and san francisco corrected for sea surface pressure show fair correlations with the surface transport of the individual currents and with the surface transport of the total intervening flow sea level values at honolulu correlate as well with the transports as does the difference in sea levels honolulu minus san francisco values the sea level at san francisco correlates the least with the transports energy spectra computed from the time sequences of transport current width and sea level show in addition to the dominant annual signal spectral peaks at periods of 3 6 months the spectrum of the transport of the broad interior flow

topex poseidon the 2 cm solution the unprecedented accuracy of topex poseidon t p altimeter data warrants a new evaluation of the methods typically used to form time series of sea level change whereas explicit removal of orbit error has always been required as a first step in altimeter data processing the t p analysis presented here is based simply on unadjusted monthly averages this approach has the advantage of retaining the large scale ocean signal which would be distorted by orbit adjustment using 16 months of data we have evaluated the t p monthly means on spatial scales ranging from mesoscale to global in the tropical pacific comparisons with 17 island tide gauge records and dynamic height derived from 36 thermistor moorings show that the altimeter data have an accuracy of approximately 2 cm when averaged over spatial scales of a few hundred kilometers on basin scales in the northern hemisphere similar agreement is found between the t p data and the dynamic height climatology of levitus 1982 these new altimeter observations are thus providing the first reliable view of global sea level changes on seasonal to interannual timescales

saline intrusions in the intermediate waters north of oahu hawaii abstract the hawaii ocean time series hot hydrographic observations at station aloha 22 45 n 158 w reveal that the region is sometimes the site of intrusive interleaving of different water types near the salinity minimum of the north pacific intermediate waters this interleaving is associated with saline laminae or lenses 5 30 km long submesoscale and 5 100 m thick finestructure the temporal variability of these intermediate waters is investigated statistically and a background water type is defined against which to measure the strength of intrusions the salinity minimum shows a relative maximum in variability caused mainly by the episodic appearance of the saline finestructure intrusions which are correlated with low oxygen and silicate concentrations the intermediate salinity minimum has its southern boundary near the latitude of the hawaiian islands where salinity increases towards the south and oxygen gradually decreases thus stirring processes local to the hawaiian ridge may explain the observations possible mechanisms are suggested including fluctuations of the subtropical gyre southern boundary and a hydrodynamically unstable flow at intermediate depths along the ridge either process could transport relatively saline water into the aloha region and would lead to interleaving driven by double diffusion on scales of the order of 10 m in the vertical

the statistical properties of general atmospheric circulation observational evidence and a minimal theory of bimodality in this paper we address the problem of constructing a minimal model of atmospheric circulation able to describe the bulk of the statistical properties of the general circulation in middle latitudes after a brief reexamination of the charney devore theory of multiple equilibria we compare the statistical properties that can be deduced from it with those resulting from analysis of observations of the northern hemisphere middle latitude circulation at 500 mb while the prediction concerning the amplitude of the waves seems to be confirmed by the presence of a clear bimodality of the statistical distribution we find no trace of bimodality in the zonal wind we show how the theory can be brought into agreement with observations by introducing nonlinearity into the wave equation in such a way that the charney devore resonance curve is bent producing different states corresponding to the same value of the zonal wind we finally address ourselves to the question of what physical processes determine the observed unimodal statistics of zonal wind a subject of future work

coupled effect of magma degassing and rheology on silicic volcanism abstract explosive volcanism such as the 1991 mt pinatubo philippines and the 2008 mt chaiten chile eruptions is caused by violent vesiculation of hydrous magma however gas may efficiently separate from magma owing to the enhancement of gas permeability by shear deformation of magma flowing in a volcanic conduit this makes it difficult to maintain the driving force of explosive volcanism although explosive volcanism is actually common here we propose that shear localization in a volcanic conduit controls the eruption style and explosivity based on deformation experiments of vesicular magma linked with synchrotron radiation x ray radiography and computed tomography we observed for the first time in situ that the shear localization caused magma fracturing and formed a slip plane and thus inhibited deformation and outgassing elsewhere we also observed the compaction of vesicular magma into a dense lava as a result of outgassing when shear localization did not occur in a natural setting shear localizes along the edges of a volcanic conduit where the strain rate is high causing a highly permeable fracturing layer to form at the conduit s edge and leaving less sheared and less outgassed magma at its center the less outgassed magma in the center may ascend rapidly and cause explosive volcanism non explosive lava effusion may occur only when shear localization does not occur effectively this new view explains the rapid ascent of viscous magma and the formation of pyroclasts with contrasting vesicularity pyroclastic obsidian and highly vesiculated pumice

timescales of bubble coalescence outgassing and foam collapse in decompressed rhyolitic melts abstract the timescale of degassing and outgassing in hydrous rhyolitic melts is investigated in a wide range of conditions by means of decompression experiments the evolution of vesicularity bubble diameter and number density is characterized as a function of time either of decompression or spent at final pressure in order to determine the effect of final pressure temperature syn versus post decompression degassing melt composition and microlites on the timescale of bubble growth coalescence and outgassing the result suggests that different bubble evolution and degassing outgassing timescale corresponding to explosive and effusive eruption regimes can be cast in bulk viscosity melt bubbles bulk versus decompression time rather than path space the bulk time relationship defines three domains of i bubble nucleation and growth restricted to short durations and high bulk 0 03 h for bulk 10 5 6 pa s ii equilibrium degassing with coalescence increasing from negligible permeability 10 13 m 2 to extensive permeability 10 11 12 m 2 and iii outgassing restricted to long durations and low bulk 10 h for bulk 10 6 pa s permeability 10 10 m 2 that eventually leads to foam collapse these findings are applied to the case studies of mt pelee and mt pinatubo to infer the transition from pumice to dense pyroclasts in volcanic eruptions and the possibility of evolving from an explosive plinian eruption to an effusive dome growth event by giving the vesicular magma enough time to outgas and collapse i e hundreds to tens of hours for bulk 10 5 to 10 4 pa s respectively we also show the drastic effect of microlites on re arranging preexistent bubbles and potentially triggering a late nucleation event

non newtonian rheological law for highly crystalline dome lavas volcanic eruption models are hampered by the lack of multiphase magmatic flow laws most rheological models estimate the viscosity of multiphase lavas via the einstein roscoe equation but this simplification cannot be used for high crystallinity and it does not consider the non newtonian strain rate dependence of viscosity we carried out parallel plate experiments on natural samples to simulate multiphase lava deformation under various stresses and strain rates multiphase lavas exhibit an important component of shear thinning and appear to invalidate the adequacy of einstein roscoe based formulations for highly crystalline lava rheology the remarkable singular dependence of viscosity on strain rate yields a novel universal rheology law at eruptive temperatures t i e log 0 993 8974 t 0 543 log our work reveals the importance of considering microcracking and viscous dissipation at very high strain rate 10 3 s 1 explaining the occurrence of seismic swarms along the conduit margins and consequently supporting plug like magma ascent models

the magmatic plumbing system beneath santiaguito volcano guatemala abstract the silicic dome complex of santiaguito guatemala has exhibited continuous extrusive activity for 90 years despite its longevity remarkably little is known about the magmatic plumbing system beneath santiaguito here we use petrological analyses of lava samples to define this plumbing system from storage in the lower to mid crust through to extrusion onto the surface magmatic storage conditions are constrained using amphibole and plagioclase phenocrysts ascent processes are examined using the breakdown rims of amphibole phenocrysts and the texture and composition of groundmass while shallow processes are revealed by the alteration of titanomagnetites and matrix glass santiaguito magmas contain amphiboles that formed from 24 km to 12 km beneath the surface with temperatures of 940 to 980 c and f o2 of nno 0 4 to nno 1 2 amphibole breakdown rims suggest that during the final phases of ascent magma may rise from 12 km the limit of amphibole stability relatively rapidly 27 to 84 m h 1 we infer from the texture of the groundmass that melt rigidifies prior to extrusion a finding that may have important consequences for conduit dynamics

reconstructing magma failure and the degassing network of dome building eruptions volcanic eruptions are regulated by the rheology of magmas and their ability to degas both detail the evolution of stresses within ascending subvolcanic magma but as magma is forced through the ductile brittle transition new pathways emerge as cracks nucleate propagate and coalesce constructing a permeable network current analyses of magma dynamics center on models of the glass transition neglecting important aspects such as incremental strain accommodation and the key monitoring tool of seismicity here in a combined methods study we report the first high resolution 20 m neutron computed tomography and microseismic monitoring of magma failure under controlled experimental conditions the data reconstruction reveals that a competition between extensional and shear fracturing modes controls the total magnitude of strain to failure and importantly the geometry and efficiency of the permeable fracture network that regulates degassing events extrapolation of our findings yields magma ascent via strain localization along conduit margins thereby providing an explanation for gas and ash explosions along arcuate fractures at active lava domes we conclude that a coupled deformation seismicity analysis holds a derivation of fracture mechanisms and network and thus holds potential application in forecasting technologies

generation of pyroclastic flows by explosive interaction of lava flows with ice water saturated substrate abstract we describe a new type of secondary rootless phreatomagmatic explosions observed at active lava flows at volcanoes klyuchevskoy russia and etna italy the explosions occurred at considerable up to 5 km distances from primary volcanic vents generally at steep 15 35 slopes and in places where incandescent basaltic or basaltic andesitic lava propagated over ice water saturated substrate the explosions produced high up to 7 km vertical ash steam laden clouds as well as pyroclastic flows that traveled up to 2 km downslope individual lobes of the pyroclastic flow deposits were up to 2 m thick had steep lateral margins and were composed of angular to subrounded bomb size clasts in a poorly sorted ash lapilli matrix character of the juvenile rock clasts in the pyroclastic flows poorly vesiculated with chilled and fractured cauliflower outer surfaces indicated their origin by explosive fragmentation of lava due to contact with external water non juvenile rocks derived from the substrate of the lava flows comprised up to 75 in some of the pyroclastic flow deposits we suggest a model where gradual heating of a water saturated substrate under the advancing lava flow elevates pore pressure and thus reduces basal friction in the case of frozen substrate water is initially formed by thawing of the substrate along the contact with lava on steep slope this leads to gravitational instability and sliding of a part of the active lava flow and water saturated substrate the sliding lava and substrate disintegrate and intermix triggering explosive fuel coolant type interaction that produces large volume of fine grained clastic material relatively cold steam laden cloud of the phreatomagmatic explosion has limited capacity to transport upward the produced clastic material thus part of it descends downslope in the form of pyroclastic flow similar explosive events were described for active lava flows of llaima chile pavlof alaska and hekla iceland indicating that this type of explosions and related hazard is common at snow ice clad volcanoes and sometimes happens also on fluid saturated hydrothermally altered slopes

multiple planetary flow regimes and the eddy forcing in northern hemisphere wintertime variability 1 a cluster algorithm is applied to the zonal mean zonal wind u from winter monthly means of the reanalyses of ncep ncar four reproducible and robust clusters are found two regimes represent the opposite phases the northern hemisphere annular mode nam and two regimes represent the opposite phases of a pattern which can be identified with the cold ocean warm land cowl pattern this identification is accomplished by regressing the u on the cowl index derived from 500 hpa height data by wu and straus 2002 transformed eulerian or eliassen palm eddy forcing analysis is used to examine the maintenance of these clusters the results show that barotropic forcing by the eddy momentum fluxes appears to maintain the wind anomalies in the upper troposphere and balances the coriolis acceleration associated with the anomalous transformed mean meridional circulation while the coriolis acceleration maintains the wind anomalies against friction in the lower troposphere this mechanism holds for both nam like and cowl like regimes

kinetics of heterogeneous bubble nucleation in rhyolitic melts implications for the number density of bubbles in volcanic conduits and for pumice textures we performed decompression experiments to simulate the ascent of a phenocryst bearing rhyolitic magma in a volcanic conduit the starting materials were bubble free rhyolites water saturated at 200 mpa 800 c under oxidizing conditions they contained 6 0 wt dissolved h2o and a dense population of hematite crystals 8 7 2 105 mm 3 pressure was decreased from the saturation value to a final value ranging from 99 to 20 mpa at constant temperature 800 c the rate of decompression was either 1 000 or 27 8 kpa s in all experiments we observed a single event of heterogeneous bubble nucleation beginning at a pressure pn equal to 63 3 mpa in the 1 000 kpa s series and to 69 1 mpa in the 27 8 kpa s series below pn the degree of water supersaturation in the liquid rapidly decreased to a few 0 1 wt the nucleation rate dropped and the bubble number density bnd stabilized to a value strongly sensitive to decompression rate 80 mm 3 at 27 8 kpa s vs 5 900 mm 3 at 1 000 kpa s this behaviour is like the behavior formerly described in the case of homogeneous bubble nucleation in the rhyolite h2o system and in numerical simulations of vesiculation in ascending magmas similar degrees of water supersaturation were measured at 27 8 and 1 000 kpa s implying that a faster decompression rate does not result in a larger departure from equilibrium our experimental results imply that bnds in acid to intermediate magmas ascending in volcanic conduits will depend on both the decompression rate left left text d p text d t right right and the number density of phenocrysts especially the number density of magnetite microphenocrysts 1 100 mm 3 which is the only mineral species able to reduce significantly the degree of water supersaturation required for bubble nucleation very low bnds 1 mm 3 are predicted in the case of effusive eruptions left left text d p text d t right right 0 1 kpa s high bnds up to 107 mm 3 and bimodal bubble size distributions are expected in the case of explosive eruptions 1 a relatively small number density of bubbles 1 100 mm 3 will first nucleate in the lower part of the conduit left left text d p text d t right right 10 kpa s either at high pressure on magnetite or at lower pressure on quartz and feldspar or by homogeneous nucleation in the liquid and 2 then extreme decompression rates near the fragmentation level left left text d p text d t right right 103 kpa s will trigger a major nucleation event leading to the multitude of small bubbles typically a few micrometers to a few tens of micrometers in diameter which characterizes most silicic pumices

vesicle connectivity in pyroclasts and implications for the fluidisation of fountain collapse pyroclastic flows montserrat west indies abstract pyroclastic flows are noted for their highly mobile behaviour being able to flow on slopes as low as a few degrees and this has been attributed to fluidisation or partial fluidisation by escaping gases the three types of pyroclastic flows generated during the eruption of soufriere hills volcano on montserrat dome collapse fountain collapse and surge derived differ in their apparent mobility suggesting different degrees of fluidisation and friction reduction in the flowing granular materials several possible sources of fluidising gas operate in pyroclastic flows the purpose of this paper is to assess the feasibility of one mechanism the release during flow of pressurised gases trapped in the vesicles of juvenile clasts measurements with a helium pycnometer show that all vesicles in dense lava blocks from dome collapse pyroclastic flows are connected so that the mechanism is probably not viable for such flows on the other hand pumices from fountain collapse pyroclastic flows from soufriere hills and other volcanoes contain up to 10 vol of isolated vesicles this is confirmed by sem imagery which shows that soufriere hills pumices contain two vesicle populations with a bimodal size distribution the large vesicles are mostly interconnected whereas about two thirds of the smaller population

cristobalite in the 2011 2012 cordon caulle eruption chile cristobalite is a low pressure high temperature polymorph of sio2 found in many volcanic rocks its volcanogenic formation has received attention because 1 pure particulate cristobalite can be toxic when inhaled and its dispersal in volcanic ash is therefore a potential hazard and 2 its nominal stability field is at temperatures higher than those of magmatic systems making it an interesting example of metastable crystallization we present analyses by xrd sem epma laser raman and synchrotron ct of representative rhyolitic pyroclasts and of samples from different facies of the compound lava flow from the 2011 2012 eruption of cordon caulle chile cristobalite was not detected in pyroclasts negating any concern for respiratory hazards but it makes up 0 23 wt of lava samples occurring as prismatic vapour deposited crystals in vesicles and or as a groundmass phase in microcrystalline samples textures of lava collected near the vent which best represent those generated in the conduit indicate that pore isolation promotes vapour deposition of cristobalite mass balance shows that the sio2 deposited in isolated pore space can have originated from corrosion of the adjacent groundmass textures of lava collected down flow were modified during transport in the insulated interior of the flow where protracted cooling additional vesiculation events and shearing overprint original textures in the most slowly cooled and intensely sheared samples from the core of the flow nearly all original pore space is lost and vapour deposited cristobalite crystals are crushed and incorporated into the groundmass as the vesicles in which they formed collapse by strain and compaction of the surrounding matrix holocrystalline lava from the core of the flow achieves high mass concentrations of cristobalite as slow cooling allows extensive microlite crystallization and devitrification to form groundmass cristobalite vapour deposition and devitrification act concurrently but semi independently both are promoted by slow cooling and it is ultimately devitrification that most strongly contributes to total cristobalite content in a given flow facies our findings provide a new field context in which to address questions that have arisen from the study of cristobalite in dome eruptions with insight afforded by the fundamentally different emplacement geometries of flows and domes

coherence of internal tide modulations along the hawaiian ridge long time series of sea level from tide gauges along the north side of the hawaiian ridge and shorter series of dynamic heights inferred from inverted echo sounders moored just north of the main hawaiian islands are examined for evidence of internal tides at the m2 frequency we find that the amplitudes and phases of the m2 tidal components have low frequency variability which is consistent with a superposition of an internal tide with the larger barotropic tide further the low frequency variability is correlated with low frequency changes in the depth of the pycnocline which suggests a simple physical mechanism to account for the low frequency modulations in the internal tidal amplitude these modulations are coherent for long distances along the hawaiian ridge indicating a coherent generation of the internal tide that is consistent with acoustic observations in the north pacific and with recent analyses of sea surface heights from satellite altimetry

mesoscale eddies northeast of the hawaiian archipelago from satellite altimeter observations 1 enhanced mesoscale eddy activity northeast of the hawaiian archipelago was investigated by using the high pass filtered satellite altimetric sea surface height ssh data of the past 16 years the level of eddy kinetic energy eke shows significant interannual variations which appear well correlated to the large scale atmospheric wind forcing associated with the pacific decadal oscillation index the eke levels in the area along the subtropical front zone in 1997 1998 and 2003 2005 for example were three times as large as those in the low eke years spectral analyses in the 3 latitude by 5 longitude subregions revealed the dominant mesoscale periods ranging from 90 days near 18 n to 180 days near 36 n a 3 layer quasi geostrophic model constrained by world ocean atlas 2005 climatologies was used to explore instability of the regional circulation the analysis shows a geographical match between baroclinic instability strength and the observed rms ssh variability and a temporal match between seasonally varying instability strength and the observed seasonal eke modulation and suggests that baroclinic instability by the vertically sheared regional mean circulation provides the energy source for the enhanced eddy variability northeast of the hawaiian archipelago

craters of elevation revisited forced folds bulging and uplift of volcanoes the eighteenth nineteenth century craters of elevation theory required magma to uplift strata doming the surface and creating a central down fallen crater or graben exponents of craters of elevation attempted to apply it to explain the origin of all volcanoes and rapidly the contemporary competing craters of eruption theory replaced it as the paradigm for volcano construction several historic examples have shown that intrusions can cause uplift termed bulges and can create features like those proposed for craters of elevation e g at usu 1944 bezymianny 1955 and mt st helens 1980 work on sedimentary basins that have had igneous activity has shown that intrusions create forced folds that uplift and deform strata in a similar way to that originally proposed for craters of elevation in view of the above we investigate large scale intrusion related topographic changes at two sites where the craters of elevation theory was developed the monogenetic volcanoes of the chaine des puys france and the teide stratovolcano tenerife we combine observations of such features with examples of forced folding to integrate the two fields of research our observations in the chaine des puys show that 1 the petit puy de dome has a bulge of up to 150 m uplift the uplift has a central depressed area a graben a dense network of normal faults basal thrusts and an aborted landslide 2 the grosmanaux volcano is a forced fold created by uplift of a previously flat lying area and has dense faulting and a graben on the resultant topographic bulge it was the site also of a major vulcanian eruption from the associated kilian crater 3 the gouttes volcano was uplifted by an intrusion like the petit puy de dome but then collapsed to generate a landslide and lateral blast 4 excavation in the lemptegy volcano exposes intra eruption intrusions with associated uplift providing examples in cross section of the internal deformation likely to be found inside other chaine des puys uplifted bulges on teide a bulge near the summit shows similar structures and surface tilting as seen on the petit puy de dome and this bulging may have formed during the eruption of the lavas negras the most recent activity on the summit area fault scarps on teide also expose small cryptodomes like those seen at lemptegy these examples coupled with field studies on eroded intrusions data on forced folds in basins and analogue models show how large scale topographic remodelling and structural change can be created by intrusions these can rapidly and significantly change the volcanic edifice a crater of elevation bulge or forced fold that is stabilised by the cooling of the intrusion will remain an important structural element in a volcano this process starts even at the small scale of monogenetic volcanoes and could occur through the lifetime of any growing stratovolcano such activity may be commonplace but may be masked by concomitant eruption or removed by subsequent collapse monitoring and hazard strategies should be ready to deal with such large scale events that will seriously modify the eruptive activity and stability of a volcano within days or weeks

decompression induced crystallization in hydrated silica rich melts empirical models of experimental plagioclase nucleation and growth kinetics isothermal and isobaric crystallization of plagioclase in a water saturated synthetic rhyolitic melt is investigated through time series of decompression experiments the experimental variables are the rate at which samples are initially decompressed 30 150 and 1200 mpa h from 200 mpa and 875 c final pressure 25 to 160 mpa and holding time at final pressure up to 17 days through textural measurements of the crystals plagioclase crystallization kinetics has been characterized in terms of nucleation lag and rates of nucleation and growth plagioclase crystallization is markedly dependent on effective undercooling teff and holding time at crystallization pressure with teff increasing from 55 to 110 c i nucleation lag decreases from 1 2 days to 15 min ii maximum nucleation rates increase from 10 3 to 10 2 mm 2 s 1 and iii maximum growth rates decrease from 10 6 to 5x10 7 mm s 1 the initial decompression rate 30 150 and 1200 mpa h has no systematic control on crystallization at final pressure except for the 1200 mpa h series in which samples show nucleation difficulties from the experimental data of teff constrained plagioclase number density proportion and morphology we provide means to assess conditions of nucleation and growth of natural plagioclase microlites from rapidly ascended rhyolitic melts through the determination of plagioclase liquidus curve and teff prevailing during crystallization

bubble development in explosive silicic eruptions insights from pyroclast vesicularity textures from raoul volcano kermadec arc critical to understanding explosive eruptions is establishing how accurately representative pyroclasts are of processes during magma vesiculation and fragmentation here we present data on densities and vesicle size and number characteristics for representative pyroclasts from six silicic eruptions of contrasting size and style from raoul volcano kermadec arc we use these data to evaluate histories of bubble nucleation coalescence and growth in explosive eruptions and to provide comparisons with pumiceous dome carapace material density vesicularity distributions show a scarcity of pyroclasts with 65 75 vesicularity however pyroclasts closest to this vesicularity range have the highest bubble number density bnd values regardless of eruptive intensity or style clasts with vesicularities greater than this 65 75 pivotal vesicularity range have decreasing bnds with increasing vesicularities interpreted to reflect continuing bubble growth and coalescence clasts with vesicularities less than the pivotal range have bnds that decrease with decreasing vesicularity and preserve textures indicative of processes such as stalling and open system degassing prior to vesiculation in a microlite rich magma or vesiculation during slow ascent of degassing magma bubble size distributions bsds and bnds show variations consistent with 65 75 representing the vesicularity at which vesiculating magma is most likely to undergo fragmentation consistent with the closest packing of spheres we consider that the observed vesicularity range may reflect the development of permeability in the magma through shearing as it flows through the conduit these processes can act in concert with multiple nucleation events generating a situation of heterogeneous bubble populations that permit some regions of the magma to expand and bubbles to coalesce with other regions in which permeable networks are formed fragmentation preserves the range in vesicularity seen as well as any post fragmentation pre quenching expansion which may have occurred we demonstrate that differing density pyroclasts from a single eruption interval can have widely varying bnd values corresponding to the degree of bubble maturation that has occurred the modal density clasts the usual targets for vesicularity studies have likely undergone some degree of bubble maturation and are therefore may not be representative of the magma at the onset of fragmentation

cluster analysis of northern hemisphere wintertime 500 hpa flow regimes during 1920 2014 abstractclusters in the northern hemisphere wintertime 10 day low pass filtered 500 hpa height field are identified using the method of self organizing maps soms results are based on 1 a 57 winter record of era and 2 a 93 winter record of the noaa twentieth century reanalysis 20cr the clusters derived from soms appear to be more robust and more linearly independent than their counterparts derived from ward s method and clusters with comparable numbers of member days are more distinctive in terms of the standardized euclidean distances of their centroids from the centroid of the dataset the reproducible som clusters in the hemispheric domain are 1 the negative polarity of the north atlantic oscillation nao 2 a pattern suggestive of alaska blocking with a downstream wave train extending over north america and the north atlantic 3 an enhancement of the climatological mean stationary wave pattern in the western hemisphere that projects positively upon the pacific north america pna pattern

ascent driven crystallisation of dacite magmas at mount st helens 1980 1986 we introduce a novel scheme that enables natural silicic glasses to be projected into the synthetic system qz ab or h2o in order to relate variations in volcanic glass chemistry to changing pressure p and temperature t conditions in the sub volcanic magma system by this means an important distinction can be made between ascent driven and cooling driven crystallisation under water saturated or undersaturated conditions in samples containing feldspar and a silica phase quartz or tridymite quantitative p t estimates of the conditions of last equilibrium between crystals and melt can be made formation of highly silicic melts i e 77 wt sio2 is a simple consequence of the contraction of the silica phase volume with decreasing pressure such that high silica glasses can only form by crystallisation at low pressure resorption of quartz crystals appears to be a further diagnostic feature of decompression crystallisation groundmass and inclusion glasses in dacites from the 1980 1986 eruption of mount st helens volcano wa span a wide range in sio2 68 80 wt anhydrous the compositions of the least evolved sio2 poor inclusions in amphibole phenocrysts record entrapment of silicic liquids with 5 4 wt water corresponding to a water saturation pressure of 200 mpa at 900 c the compositions of more evolved higher sio2 plagioclase hosted inclusions and groundmass glasses are consistent with extensive ascent driven fractional crystallisation of plagioclase oxide and orthopyroxene phenocrysts and microlites to low pressures during this polybaric crystallisation plagioclase phenocrysts trapped melts with a wide range of dissolved water contents 3 5 5 7 wt magmas erupted during the plinian phase of the 18 may 1980 eruption were derived from a large reservoir at depths of 6 km subsequent magmas ascended to varying depths within the sub volcanic system prior to extraction from glass chemistry and groundmass texture two arrest levels have been identified at depths of 0 5 1 and 2 4 km a single dome sample from february 1983 contains groundmass plagioclase tridymite and quartz testifying to temperatures of at least 885 c at 11 mpa these shallow storage conditions are comparable to those in the cryptodome formed during spring 1980 the corresponding thermal gradient 0 2 c mpa 1 is consistent with near adiabatic magma ascent from 8 km we argue that the crystallisation history of mount st helens dacite magma was largely a consequence of decompression crystallisation of hot magma beyond the point of water saturation this challenges the conventional view that phenocryst crystallisation occurred by cooling in a large magma chamber prior to the 1980 1986 eruption because the crystallisation process is both polybaric and fractional it cannot be simulated directly using isobaric equilibrium crystallisation experiments however calculation of the phase proportions in water saturated 910 15 c experiments by rutherford et al 1985 over the pressure range 220 125 mpa reproduces the crystallisation sequence and phenocryst modes of mount st helens dacites from 18 may 1980 by allowing for the effects of fractional versus equilibrium crystallisation entrained residual source material and small temperature differences between nature and experiment phase compositions can also be matched to the natural samples we conclude that decompression of water saturated magma may be the dominant driving force for crystallisation at many other silicic volcanic centres

factors controlling permeability porosity relationships in magma a model has been developed to investigate the sensitivity of magma permeability k to various parameters power law relationships between k and porosity are revealed in agreement with previous experimental and theoretical studies these relationships take the form mathtype mtef 2 1 feaaeaart1ev0aaatcvaufkttlearuavtnhis1mbaexatlxbi9gbae rbd9wdylwzybwexlmbbxgbcf2cpn2qvrwzqf2zlnharyavp1wzzbit ldhis9wbh5garqqtubsr4rnchbgeagqivci8ffyjh8sipiydhavhbb f9v8qqaqfr0xc9pk0xbba9q8wqffeay bilkvclq jhqpepeea0 as 0fb9pgeayrxxe9vr0 vr0 vqpwqaaeaabigaciaacaqabeaadaabau aaaoqaaiqbdugarzaajagaeyypa0jaem4aasmaei4la8iaemocai3a awbaasqabeaacqaiyagmaagccqgh9aqpcqwghbqycqggoaakcqahgp gzcqghsislcqahgpgzdawgaawcbagaem4yammaemocaihabeaakiab cmcapmaacaaaleqabagaemoyaigaaaaa 4ce4 hat k k r 2 a phi phi cr b where r is the mean bubble radius cr is the percolation threshold below which permeability is zero and a and b are constants it is discovered that mathtype mtef 2 1 feaaeaart1ev0aaatcvaufkttlearuavtnhis1mbaexatlxbi9gbae rbd9wdylwzybwexlmbbxgbcf2cpn2qvrwzqf2zlnharyavp1wzzbit ldhis9wbh5garqqtubsr4rnchbgeagqivci8ffyjh8sipiydhavhbb f9v8qqaqfr0xc9pk0xbba9q8wqffeay bilkvclq jhqpepeea0 as 0fb9pgeayrxxe9vr0 vr0 vqpwqaaeaabigaciaacaqabeaadaabau aaaoqaaiqbdugarzaajagaeyoei0iaeqoxdygaaa 3cdb hat k phi relationships are independent of bubble size the percolation threshold was found to lie at around 30 porosity polydisperse bubble size distributions bsds give permeabilities around an order of magnitude greater than monodisperse distributions at the same porosity if bubbles are elongated in a preferred direction then permeability in this direction is increased but perpendicular to this direction permeability is unaffected in crystal free melts the greatest control on permeability is the ease of bubble coalescence in viscous magmas or when the cooling time scale is short bubble coalescence is impeded and permeability is much reduced this last effect can cause variations in permeability of several orders of magnitude

the pre eruption conditions for explosive eruptions at merapi volcano as revealed by crystal texture and mineralogy abstract we analyze the textures and mineralogy of merapi tephra generated during explosive vei 3 4 eruptions over the past 2000 years and compare these data with those observed for merapi dome and flow lavas we find that the merapi pumiceous tephra and lava textures differ significantly with respect to small size crystal populations but that phenocryst textures are generally similar a similar initial phase of crystallization is indicated for tephras and lavas in mid crustal 10 km depth reservoirs subsequent textural differences are mainly affected by ascent rate and degassing during ascent and for dome lavas with temporary storage in shallower reservoirs these differences also correspond to different eruptive styles our analyses include study of pumices lava and breadcrust bomb samples including samples from some of the most recent explosive episodes prior to the 2010 eruption textural analyses of youthful breadcrust bomb samples yield insights on one type of transition between effusive and explosive eruptive styles involving pressure build up under a degassed crystalline shallow conduit plug in general comparison of the crystal size distributions and calculated residence times among the effusive and explosive eruptive styles suggests that the two main magma product types resided for similar lengths of time in a mid crustal reservoir before ascending toward the surface and either erupting explosively tephra or stagnating in a shallow magma chamber prior to extrusion lava the interpretation is supported by the occurrences of amphibole in pristine condition in tephra and in altered state in lava finally the 1872 and 2010 explosive eruptions are examined and compared with others over the past three millennia the 2010 bulk rock compositions overlap with products of other major explosive merapi eruptions such as the ngrangkah tegalsruni temusari and kepuharjo tephras the 2010 products show a gradational late stage mafic enrichment dissimilar to 1872 which showed little

dynamical interpolation of mesoscale flows in the topex poseidon diamond surrounding the u s joint global ocean flux study bermuda atlantic time series study site an open boundary ocean model is configured in a domain bounded by the four topex poseidon t p ground tracks surrounding the u s joint global ocean flux study bermuda atlantic time series study bats site this implementation facilitates prescription of model boundary conditions directly from altimetric measurements both t p and ers 2 the expected error characteristics for a domain of this size with periodically updated boundary conditions are established with idealized numerical experiments using simulated data a hindcast simulation is then constructed using actual altimetric observations during the period october 1992 through september 1998 quantitative evaluation of the simulation suggests significant skill the correlation coefficient between predicted sea level anomaly and ers observations in the model interior is 0 89 that for predicted versus observed dynamic height anomaly based on hydrography at the bats site is 0 73 comparison with the idealized experiments suggests that the main source of error in the hindcast is temporal undersampling of the boundary conditions the hindcast simulation described herein provides a basis for retrospective analysis of bats observations in the context of the mesoscale eddy field

multiple regimes and low frequency oscillations in the northern hemisphere s zonal mean flow this paper studies multiple regimes and low frequency oscillations in the northern hemisphere zonal mean zonal flow in winter using 55 yr of daily observational data the probability density function estimated in the phase space spanned by the two leading empirical orthogonal functions exhibits two distinct statistically significant maxima the two regimes associated with these maxima describe persistent zonal flow states that are characterized by meridional displacements of the midlatitude jet poleward and equatorward of its time mean position the geopotential height anomalies of either regime have a pronounced zonally symmetric component but largest amplitude anomalies are located over the atlantic and pacific oceans high frequency synoptic transients participate in the maintenance of and transitions between these regimes significant oscillatory components with periods of 147 and 72 days are identified by spectral analysis of the zonal flow time series these oscillations are described by singular spectrum analysis and the multitaper method the 147 day oscillation involves zonal flow anomalies that propagate poleward while the 72 day oscillation only manifests northward propagation in the atlantic sector both modes mainly describe changes in the midlatitude jet position and intensity in the horizontal plane though the two modes exhibit synchronous centers of action located over the atlantic and pacific oceans the two persistent flow regimes are associated with slow phases of either oscillation