petrogenesis of peralkaline rhyolites in an intra plate setting glass house mountains southeast queensland australia we report petrological and geochemical data on coeval trachybasalts syenites with enclaves trachytes peralkaline rhyolites and peraluminous rhyolites from the glass house mountains maleny noosa area southeast queensland australia this rock association and the unique characteristics of the peralkaline rhyolites offer convincing lines of evidence that the petrogenesis of the peralkaline rhyolites is a straightforward consequence of protracted fractional crystallization from basaltic melts of alkali rich composition compared to the common peraluminous rhyolites elsewhere the peralkaline rhyolites here are characterized by elevated abundances of most incompatible elements especially the very high nb vs th and ta vs u the very low ba sr and eu and the extremely high sr 87 sr 86 ratio the high nb and ta are inherited from the parental alkali basaltic melts the low ba sr and eu result from removal of plagioclase during the protracted fractional crystallization these rocks altogether define a rb sr isochron of similar to 28 ma which is similar to ar ar age data on these rocks in the literature the extremely high sr 87 sr 86 ratio of the peralkaline rhyolites up to 1 88 is actually characteristic of peralkaline rhyolites because of extreme sr also eu and ba depletion and thus the very high rb sr ratio that is the sr in these rocks is essentially radiogenic sr 87 accumulated from the rb 87 decay since the volcanism we suggest that the petrogenesis of the peralkaline rhyolites from the glass house mountain area may be of general significance globally

volatile characteristics of peralkaline rhyolites from kenya an ion microprobe infrared spectroscopic and hydrogen isotope study peralkaline rhyolites of the greater olkaria volcanic complex kenya rift valley were derived from separated though closely related magma chambers ion microprobe analyses of glass inclusions in quartz phenocrysts show pre eruptive water contents of up to 3 4 wt contrasting with previous estimates that the magmas were anhydrous the values approach predicted solubility levels corresponding to water saturation at low crustal pressures 1 kbar the glass matrices of the rhyolites have low water contents ranging from 0 07 to 0 46 wt suggesting significant degassing during or prior to eruption infrared measurements of the matrix glasses show variation in the relative proportions of the two hydrous species dissolved in the glasses the amount of molecular water determined semi quantitatively apparently increases with increased fluorine content and peralkalinity this suggests a competition between hydroxyl groups and fluoride ions for similar sites within the melt structure the mechanism of degassing has been investigated using hydrogen isotopes the range of d values in most rocks can be produced by varied degrees of open system degassing of rhyolite melt initially in equilibrium with water of a fixed or limited d value there is evidence to suggest that closed system degassing may also have been a significant component in some rhyolites the exact mechanisms of degassing remain uncertain particular problems include the relative contribution of open and closed system degassing during eruption and the initial vapour compositions and solubility relationships

phase equilibria of pantelleria trachytes italy constraints on pre eruptive conditions and on the metaluminous to peralkaline transition in silicic magmas the relationships between trachytes and peralkaline rhyolites i e pantellerites and comendites which occur in many continental rift systems oceanic islands and continental intraplate settings is unclear to fill this gap we have performed phase equilibrium experiments on two representative metaluminous trachytes from pantelleria to determine both their pre eruptive equilibration conditions pressure temperature h2o content and redox state and liquid lines of descent experiments were performed in the temperature range 750 950 c pressure 0 5 1 5 kbar and fluid saturation conditions with xh2o h2o h2o co2 ranging between zero and unity redox conditions were fixed below the nickel nickel oxide buffer nno the results show that at 950 c and melt water contents h2omelt close to saturation trachytes are at liquidus conditions at all pressures clinopyroxene is the liquidus phase being followed by iron rich olivine and alkali feldspar comparison of experimental and natural phases abundances and compositions yields the following pre eruptive conditions p 160 5 kbar t 925 25 c h2omelt 261 wt and fo2 between nno 0 5 and nno 2 a decrease in temperature from 950 c to 750 c as well as of h2omelt promotes a massive crystallization of alkali feldspar to over 80 wt iron bearing minerals show gradual iron enrichment when t and fo2 decrease trending towards the compositions of the phenocrysts of natural pantellerites despite the metaluminous character of the bulk rock compositions residual glasses obtained after 80 wt crystallization evolve toward comenditic compositions owing to profuse alkali feldspar crystallization which decreases the al2o3 of the melt leading to a consequent increase in the peralkalinity index pi molar na2o k2 o al2 o3 this is the first experimental demonstration that peralkaline felsic derivatives can be produced by low pressure fractional crystallization of metaluminous mafic magmas our results show that the pantelleritic magmas of basalt trachyte rhyolite igneous suites require at least 95 wt of parental basalt crystallization consistent with trace element evidence redox conditions through their effect on fe ti oxide stabilities control the final iron content of the evolving melt

volatile emission during the eruption of baitoushan volcano china north korea ca 969 ad magma volume dre 24 5 km3 the main phase ca 95 vol is represented by comenditic tephra deposited dominantly as widespread fallout blankets and proximal ignimbrites the eruption column is estimated to have reached ca 25 km and thus entered the stratosphere a late phase 5 vol is represented by trachyte emplaced chiefly as moderately welded ignimbrites the comendites contain 3 and the trachytes 10 20 vol phenocrysts mainly anorthoclase hedenbergite and fayalite primary glassy melt inclusions with no signs of leakage were found only in phenocrysts in the comenditic tephra whereas those in phenocrysts in the trachytes are devitrified the comendite magma is interpreted to have been generated by fractional crystallization from a trachyte magma represented by melt inclusions in the phenocrysts in the comendite tephra the mass of volatiles emitted to the atmosphere during the eruption was estimated using the petrologic method the average h2o concentration of the comenditic matrix glass is 1 5 wt probably largely secondary and of the corresponding melt inclusions 5 2 wt melt inclusions in feldspar and quartz present the highest halogen concentrations with a calculated average for chlorine of 4762 ppm and for fluorine of 4294 ppm the comenditic matrix glasses are represented by a fluorine rich 3992 ppm f and fluorine poor group 2431 ppm f averaging 3853 ppm for chlorine only 20 of all sulfur analyses of the comenditic matrix glasses and melt inclusions are above the detection limit of 250 ppm s the difference between pre and post eruptive concentration of h2o is at least 3 7 0 6 wt h2o taking into consideration re hydration of the matrix glass and possible leakage of melt inclusions the difference between pre and post eruptive concentrations of the halogens amounts to 909 90 ppm cl and 1863 280 ppm and 302 40 ppm f the difference for s was estimated based on the average of the maximum s concentrations in the melt inclusions 455 ppm s and the detection limit resulting in 205 40 ppm s the calculated mass of volatiles injected into the atmosphere based on the erupted magma volume and volatile data is 1796 453 megatons for h2o 45 10 megatons for chlorine 42 11 megatons for fluorine and 2 0 6 megatons for sulfur the 969 20 ad eruption of baitoushan volcano one of the largest eruptions of the past 2000 years is thought to have had a substantial but possibly short lived effect on climate

pre eruptive melt composition and constraints on degassing of a water rich pantellerite magma fantale volcano ethiopia to determine the pre eruptive composition of peralkaline magma at frantale volcano ethiopia we have studied glass inclusions in phenocrysts from a lateceupting glassy pantelleritic lava flow matrix glass and crystal free glass inclusions in quartz were analyzed for all major and most minor elements by electron microprobe and for h2o and 15 lithophile trace elements by ion microprobe sims compositions of inclusions may have been slightly modified by post trapping quartz crystallization the average concentrations of all constituents but silica may be artificially high by 10 relative glass inclusions contain extreme enrichments in h2o mean of 4 6 to 4 9 wt and several lithophile trace elements which suggest that the lava erupted from a highly evolved water rich fraction of magma the pre eruptive concentration of water was much higher than that generally considered to occur in pantellerite magmas trends observed for lithophile elements in whole rock samples from pre syn and post caldera eruptive units are mimicked in glass inclusions from the studied pantellerite lava concentrations of rb y zr nb and ce cl increase with progressive differentiation with the exception of cl and h2o contents the composition of matrix glass is similar to that of glass inclusions suggesting that few constituents exsolved from magma or cooling glass eruption and quench of the lava occurred rapidly and the matrix glass is largely compositionally representative of melt higher average abundances of cl and h2o in glass inclusions suggest that these volatiles exsolved after melt entrapment degassing could have occurred as either an equilibrium or disequilibrium process

growth of megaspherulites in a rhyolitic vitrophyre megaspherulites occur in a rhyolitic vitrophyre near the base of a thick rhyolitic vitrophyre that occupies a late eocene to early oligocene volcanic tectonic basin near silver cliff colorado diameters of the megaspherulites range from 0 3 to over 3 66 m including an alteration layer the megaspherulites are compound spherulites they are holocrystalline very fine to fine grained composed of sanidine and quartz and surrounded by a thin sanidine quartz rind and an alteration layer containing mordenite and 15 a montmorillonite megaspherulite crystallization began soon after the vitrophyre was emplaced as the result of sparse heterogeneous nucleation under highly nonequilibrium conditions spherulite growth proceeded in a diffusion controlled manner under lowered viscosity as a result of a high water content 5 to 7 wt which also contributed to sparse nucleation when nucleation did occur it was at a large t 245 350 c on near critical sized nuclei once the spherulites began to grow continued nucleation was on existing growth cones and new independent spherulites did not form sanidine crystals grew with a fibrous habit which is favored by a large t restricting fibril lengths and diameters between 10 30 and 3 7 m respectively during crystallization these growth cones impinged upon each other resulting in fibril cone free areas these cone free interconal areas consist of coarser fine grained phases dominated by quartz which crystallized from the melt as it accumulated between the crystallizing sanidine fibrils of the cones the anhydrous nature of the sanidine and quartz suggests that their crystallization concentrated a water rich residual fluid enriched in ca mg and fe and depleted in si k and na at the megaspherulite vitrophyre interface this fluid phase enclosed the megaspherulites resulting in the hydrothermal alteration of the vitrophyre forming a hydrothermal alteration layer as indicated by the presence of the minerals mordenite and montmorillonite

warm core vortex amplification over land abstract a convectively generated mesoscale vortex that was instrumental in initiating and organizing five successive mesoscale convective systems over a period of three days is documented two of these convective systems were especially intense and resulted in widespread heavy rain with localized flooding based upon radar and satellite data the detectable size of the vortex became much larger following the strong convective developments nearly tripling its initial diameter over its three day life cycle during nighttime when convection typically intensified within the vortex movement of the system tended to slow following dissipation of the convection in the morning the daytime movement accelerated cross sections of potential vorticity taken through the vortex center clearly show a maximum at midlevels and a well defined minimum directly above the vortex and the potential vorticity maximum were essentially colocated and the system was nearly axisymmetric in the vertical over the three day life

case studies of developing east pacific easterly waves this paper contains a study of developing east pacific easterly waves observed during the summer of 1991 with aircraft in situ and radar data and geosynchronous infrared satellite imagery the circulation and divergence profiles surface momentum and entropy fluxes as well as 700 mb and 975 mb saturated and actual equivalent potential temperatures were measured repeatedly for three disturbances that evolved into east pacific hurricanes as the disturbances develop the level of maximum updraught mass flux decreases which leads to increased convergence at low levels this process appears to be coupled to an increase in the mid level relative humidity which in turn suppresses the development of convective downdraughts comparison of the respective tendencies of convergence and surface friction to spin up and spin down a developing disturbance suggests that friction is unimportant in the early stages of development however as the surface circulation develops the two effects become comparable in magnitude if surface friction is deposited through a fairly deep layer of the troposphere the observations appear to rule out boundary layer ekman balance in these developing systems though more accurate observations of the eddy fluxes of absolute vorticity into such systems are needed to answer this question with certainty

compositional and thermodynamic variability in a stratified magma chamber evidence from the green tuff ignimbrite pantelleria italy the green tuff ignimbrite pantelleria is compositionally zoned from pantellerite at the base to comenditic trachyte at the top the variation apparently representing an inverted vertical zonation in the pre eruptive reservoir the main phenocryst assemblages are alkali feldspar olivine clinopyroxene ilmenite apatite in the trachytes and alkali feldspar aenigmatite clinopyroxene quartz in the rhyolites thermodynamic modelling indicates that the temperature range was 900 700 c fo2 fmq 1 5 to fmq 0 5 where fmq is fayalite magnetite quartz buffer and asio2 relative to quartz saturation 0 74 1 00 melt water contents ranged from 1 wt in the trachytes to 4 wt in the pantellerites matrix glass analyses in the more evolved rocks are highly variable showing that compositional layers in the upper parts of the reservoir formed by fractional crystallization were mixed during eruption the proportion of rhyolitic to trachytic melts increasing towards the top of the reservoir some areas of glass have low al2o3 contents 5 16 5 46 wt and high feo contents 9 66 10 02 wt making them the most evolved melts yet reported from pantelleria the new glass data reveal how whole rock analyses do not truly reflect the complete range of melt compositions in the pre eruptive reservoir the trachytes contain 40 modal phenocrysts which with relatively high ba contents and positive eu anomalies are considered to have originated in a feldspar accumulitic layer phenocrysts in the trachytes are commonly heavily resorbed inferred to be a result of heating by influxes of intermediate composition magma which however were not erupted it is argued that magmas of intermediate composition were present in the green tuff reservoir but were efficiently trapped in a crystal rich layer below the green tuff magmas which was eventually erupted during a resurgent phase as the montagna grande trachyte

the diversion capacity of capillary barriers an arrangement of unsaturated fine grained soil overlying unsaturated coarse grained soil along a sloping contact can under appropriate circumstances divert infiltrating water away from the coarser material such an arrangement is called a capillary barrier the water diverted by a capillary barrier flows downdip above the contact the volume of water moving laterally increases in the downdip direction as additional infiltration is diverted by the barrier sufficiently far downdip the laterally moving water wets the contact to the point that an amount of water equal to the infiltration flows downward through the coarse soil the lateral flow at such a point represents the diversion capacity of the capillary barrier because this flow will not increase farther downdip if the width measured in the direction of dip of the system is large enough that total infiltration exceeds the diversion capacity the downdip portion of the barrier will not be effective the diversion capacity can be calculated exactly in the quasi linear approximation where the relationship between relative permeability krel and pressure potential takes the form krel e this calculation shows that an upper bound on the width of capillary barriers is ks tan o q where ks is the saturated hydraulic conductivity of the fine soil o is the dip angle of the contact and q is the infiltration rate

the pre eruptive volatile contents of recent basaltic and pantelleritic magmas at pantelleria italy abstract pantelleria island located in the sicily channel rift zone italy is the type locality for the peralkaline rhyolitic rocks called pantellerites in the last 50 ka after the large green tuff caldera forming eruption volcanic activity at pantelleria has consisted of effusive and explosive eruptions mostly vented inside and along the rim of the caldera and producing silicic lava flows lava domes and poorly dispersed pantelleritic pumice fall deposits basaltic cinder cones and lava flows are only present outside the caldera in the nw sector of the island the most recent basaltic cuddie rosse 20 ka and pantelleritic cuddia randazzo and cuddia del gallo 6 ka pyroclastic products were sampled to investigate magmatic volatile contents through the study of melt inclusions the melt inclusions in pyroxene and olivine phenocrysts of cuddie rosse scoriae have an alkali basalt composition the dissolved volatiles comprise 0 9 1 6 wt h2o several hundred ppm of co2 1600 2000 ppm of sulphur and 500 900 ppm of chlorine the water carbon dioxide couple gives a confining pressure 2 kbar prior to the eruption this result indicates that episodes of magma ponding and crystallization occurred in the upper crust prior to eruption the melt inclusions in feldspar fayalite and aenigmatite phenocrysts of cuddia del gallo and cuddia randazzo pumice have a pantelleritic composition agpaitic indices 1 3 2 1 up to 4 4 wt h2o 8700 ppm cl 6000 ppm f and co2 below the detection limit sulphur averaging 420 ppm has been measured in cuddia randazzo melt inclusions these data indicate relatively high volatile contents for these low energy strombolian type eruptions melt inclusions in cuddia del gallo pumice show the most evolved composition agpaitic indices 2 2 1 and the highest volatile content in agreement with fluid saturation conditions in the magma chamber prior to the eruption this implies a confining pressure of 1 kbar for the top of the pantelleritic reservoir the composition of melt inclusions and mineralogical assemblage of cuddia randazzo pumice indicate that it has a lower evolutionary degree agpaitic indices 1 3 1 8 and lower pre eruptive cl and h2o contents than cuddia del gallo pumice an increase in pressure due to the exsolution of volatiles in the upper part of the pantelleritic reservoir may have triggered the cuddia del gallo explosive eruption evidence of widespread pre eruptive mingling between trachytes and pantellerites suggests that the intrusion of trachytic magma into the pantelleritic reservoir likely played a major role in destabilizing the magma system just prior to the cuddia randazzo event

capillary barrier effect from underlying coarser soil layer infiltration tests were conducted on soil columns of silty sand over pea gravel concrete sand over pea gravel and silty sand over concrete sand to investigate the capillary barrier effect of an underlying coarser soil layer water movement across the interface occurred when the suction head at the interface reached the breakthrough head of the coarser lower soil layer defined as the suction head at which the coarser layer first became conductive regardless of infiltration rate or the properties of the overlying finer soil layer thus the coarser lower soil layer controlled breakthrough in this study after infiltration was terminated the suction head near the interface increased above the breakthrough head and the barrier was restored the breakthrough head did not change substantially after eight test cycles of breakthrough and restoration for a capillary barrier with a pea gravel as the coarser lower soil layer the barrier formed with the concrete sand as the coarser layer permitted breakthrough at a greater suction head than did the barrier with the pea gravel indicating that the more uniform and coarse the lower soil layer is the more effective the capillary barrier

volatiles in pantellerite magmas a case study of the green tuff plinian eruption island of pantelleria italy abstract the green tuff gt plinian eruption the largest in magnitude at pantelleria erupted 3 to 7 km3 dre of pantellerite magma and a small volume of trachyte fifty nine anorthoclase hosted melt inclusions from the two basal pumice members were analyzed by ft ir spectroscopy in order to assess the pre eruptive h2o content in the pantellerite melt microanalytical methods were used to determine major element cl f and s contents melt inclusions and glassy groundmasses have a nearly homogeneous pantelleritic composition peralkaline index 1 9 2 2 and variable water contents ranging from 1 4 to as high as 4 2 wt i e much higher than the 1 4 wt of earlier published studies the chlorine content is constant at about 1 wt combined cl and h2o data were used to estimate a confining pressure of about 50 mpa depth around 2 3 km for the gt magma chamber the chamber was characterized by a compositional zoning with a dominant pantellerite overlying a trachyte magma soon after the gt eruption intra caldera volcanism was dominated by the eruption of voluminous trachyte lava flows while pantellerite melt production resumed after about 20 ka with numerous low volume mildly explosive strombolian to effusive eruptions comparison with data from the literature reveals that despite the different explosivity the post caldera strombolian eruptions and the gt plinian eruption were fed by pantelleritic magmas with similar water contents chlorine and co2 contents suggest that the young magma reservoirs feeding the strombolian to effusive activity were deeper h 4 5 km than the much larger based on erupted volumes magma chamber which fed the gt eruption

pre eruptive volatile content and degassing history of an evolving peralkaline volcano abstract the volatile contents of melt inclusions trapped within phenocrysts of quartz and feldspar in peralkaline rhyolites from mayor island new zealand have been studied using fourier transform infrared spectroscopy and ion microprobe analyses the glass inclusions analyzed span the 130 ka subaerial eruptive history of the island and come from volcanic deposits representing a wide range of eruptive styles explosive vs effusive magmatic vs phreatomagmatic low vs high discharge rates the water content of all inclusions analysed is uniformly high at 4 4 wt h2o whereas co2 contents are below the limits of detection 50 ppm chlorine in melt inclusions ranges from 2070 to 5200 ppm while coexisting matrix glasses have generally lower cl concentrations of 1700 4200 ppm the apparent bulk distribution coefficient describing cl partitioning between melt and vapor phase cl fluid cl melt during degassing is 5 15 fluorine appears to be less affected by eruptive degassing than is cl and melt inclusion and matrix glass f concentrations show significant overlap 1400 2520 ppm f in inclusions 1550 2890 ppm f in matrix glass the observed invariance of melt inclusion water content with sample age contrasts with ion microprobe data on incompatible trace elements e g zr nb which suggest 35 fractionation between the oldest and youngest samples this along with the cl partitioning behaviour suggests that at least the upper erupting portion of the mayor island magma chamber was water saturated throughout the volcano s 130 ka eruptive history furthermore the large range of eruptive styles observed on mayor island are not due to differences in water concentration of the erupting magma but instead must reflect differences in rates of magma ascent and supply which control the efficiency of non explosive degassing

evolution of peralkaline silicic complexes lessons from the extrusive rocks abstract young 4 wt and the rhyolitic varieties evolve to temperatures 4 10 6 pa s result in rapid growth of volcanic edifices highly efficient crystal melt separation and the ubiquitous development of compositionally zoned caps to reservoirs no peralkaline equivalent of the monotonous intermediate ignimbrites found in some calc alkaline systems has yet been found

measuring the degree of nanotilization of volcanic glasses understanding syn eruptive processes recorded in melt inclusions iron and water content substantially affect the physical properties of natural silicate melts and may therefore influence magmatic and volcanic processes such as crystallization degassing flow behaviour and eruptive style here we present raman spectroscopic data for a set of synthetic and natural multicomponent glasses with varying iron oxidation state and water content we systematically study the effect of different raman excitation sources on the spectral response of both crystal free and magnetite nanolite bearing glasses spanning basaltic to calc and per alkaline rhyolitic compositions based on these data we document changes in raman spectra resulting from the formation of nano scale crystals we show that the peak located at similar to 970 cm 1 is directly related to the presence of fe2o3 dissolved in the quenched melt structure and that this feature is present regardless of the chemical composition of the sample and the raman excitation source we further show that a peak between 670 and 690 cm 1 which is not related to any other spectral feature of the glass structure reveals the presence of nanolites based on systematic spectral investigations of this feature we present a new index that allows to identify if iron is present in the nanocrystalline state and or bound in the glass structure since the melt structural and physical effects of the formation of nanolites can heavily affect the flow behaviour of melts and the eruptive style of volcanoes the results presented in this study significantly broaden the application of raman spectroscopy for investigations of nano heterogeneity in synthetic and natural glasses we apply this method to study both the degree of nanolitization as well as the h2o content and iron oxidation state of groundmass glasses as well as melt inclusions and glass embayments in explosive products from pantelleria island italy we observe that the process of nanotilization is not purely restricted to magnetite nanolites but that raman spectroscopy may also identify the incipient crystalization of pyroxene and feldspar at sub micron scale the data document that nanolite formation correlates well with the observed intensity of the respective eruptions suggesting that structural changes in the melt caused by incipient crystallization play an important role in defining the eruptive style of relatively low viscosity magmas

spherulites and thundereggs from pitchstones of the deccan traps geology petrochemistry and emplacement environments spherulites and thundereggs are rounded typically spherical polycrystalline objects found in glassy silicic rocks spherulites are dominantly made up of radiating microscopic fibers of alkali feldspar and a silica mineral commonly quartz they form due to heterogeneous nucleation in highly supercooled rhyolitic melts or by devitrification of glass associated features are lithophysae stone bubbles which have an exterior sometimes concentric shells of fine quartz and feldspar and internal cavities left by escaping gas when filled by secondary silica these are termed thundereggs here we describe four distinct occurrences of spherulites and thundereggs in pitchstones mostly rhyolitic some trachytic of the deccan traps india the thundereggs at one locality were previously misidentified as rhyolitic lava bombs and products of pyroclastic extrusive activity we have character ized the thundereggs petrographically and geochemically and have determined low contents of magmatic water 0 21 0 38 wt in them using fourier transform infrared spectros copy we consider that the spherulite bearing outcrops at one of the localities are of lava flows but the other three represent subvolcanic intrusions based on the structural disposition of the deccan sheet intrusions studied here and considerations of regional geology we suggest that they are cone sheets emplaced from a plutonic center now submerged beneath the

mesoscale interactions in tropical cyclone genesis with the multitude of cloud clusters over tropical oceans it has been perplexing that so few develop into tropical cyclones the authors postulate that a major obstacle has been the complexity of scale interactions particularly those on the mesoscale which have only recently been observable while there are well known climatological requirements these are by no means sufficient a major reason for this rarity is the essentially stochastic nature of the mesoscale interactions that precede and contribute to cyclone development observations exist for only a few forming cases in these the moist convection in the preformation environment is organized into mesoscale convective systems each of which have associated mesoscale potential vortices in the midlevels interactions between these systems may lead to merger growth to the surface and development of both the nascent eye and inner rainbands of a tropical cyclone the process is essentially stochastic but the degree of stochasticity can be reduced by the continued interaction of the mesoscale systems or by environmental influences for example a monsoon trough provides a region of reduced deformation radius which substantially improves the efficiency of mesoscale vortex interactions and the amplitude of the merged vortices further a strong monsoon trough provides a vertical wind shear that enables long lived midlevel mesoscale vortices that are able to maintain or even redevelop the associated convective system the authors develop this hypothesis by use of a detailed case study of the formation of tropical cyclone oliver observed during toga coare 1993 in this case two dominant mesoscale vortices interacted with a monsoon trough to separately produce a nascent eye and a major rainband the eye developed on the edge of the major convective system and the associated atmospheric warming was provided almost entirely by moist processes in the upper atmosphere and by a combination of latent heating and adiabatic subsidence in the lower and middle atmosphere the importance of mesoscale interactions is illustrated further by brief reference to the development of two typhoons in the western north pacific

the role of vortical hot towers in the formation of tropical cyclone diana 1984 abstract a high resolution 3 km horizontal grid spacing near cloud resolving numerical simulation of the formation of hurricane diana 1984 is used to examine the contribution of deep convective processes to tropical cyclone formation this study is focused on the 3 km horizontal grid spacing simulation because this simulation was previously found to furnish an accurate forecast of the later stages of the observed storm life cycle the numerical simulation reveals the presence of vortical hot towers or cores of deep cumulonimbus convection possessing strong vertical vorticity that arise from buoyancy induced stretching of local absolute vertical vorticity in a vorticity rich prehurricane environment at near cloud resolving scales these vortical hot towers are the preferred mode of convection they are demonstrated to be the most important influence to the formation of the tropical storm via a two stage evolutionary process i preconditioning of the local environment via diabatic production of mul

the geochemical signature of fluid saturated magma determined from silicate melt inclusions in ascension island granite xenoliths abstract silicate glasses in crystal free to crystal poor melt inclusions from two coarse grained granite xenoliths in alkaline volcanic rocks of ascension island were analyzed for 29 major minor and trace elements for most constituents the glass compositions are similar to those of the volcanic whole rocks and the xenoliths we interpret the glasses to be chemically representative of granite melt the melt inclusions are silicic alkaline contain low s and p abundances and are enriched in h 2 o cl f and na relative to k inclusions from one xenolith contain 1 3 wt f on average whereas those from the other xenolith contain half that amount the melt inclusion compositions allow investigation of the means and extent of granite magma evolution the presence of magmatic fluid inclusions in close proximity to melt inclusions in the phenocrysts indicates that the granite melt was saturated in one or more volatile phases roedder and coombs 1967 the cl and h 2 o contents of the melt inclusions are consistent with the exsolution of volatile phase s at pressures of 3000 to 2000 bars the glasses also show trends involving the cl h 2 o ratio and the incompatible trace elements in melt that are indicative of melt evolution via crystal fractionation of volatile phase saturated melt these trends should be useful for recognizing volatile phase saturation in other cl enriched melts

differentiation of tholeiitic basalt to a type granite in the sept iles layered intrusion canada the undeformed 564 ma sept iles layered intrusion quebec canada is a large igneous body of c 20 000km3 from the base to the top it consists of a layered series dominated by troctolite and gabbro an anorthositic upper border series and a dominantly granitic upper series the parent magma of the layered series is inferred to be an iron rich tholeiitic basalt 48 wt sio2 15wt feot whole rock compositions from the chilled margin dykes cross cutting the layered series and silicic rocks from the upper series display continuous major and trace element geochemical trends ranging from basalts to ferroan metaluminous a type granites 77 wt sio2 initial 143nd 144nd 0 51201 0 51207 and 87sr 86sr 0 70353 0 70548 indicate a juvenile mantle source and minimal contamination by old crust 1 2 during crystallization geochemical modeling using the melts thermodynamic calculator combined with equations predicting mineral melt equilibria from experiments on tholeiitic basalts indicate that basaltic to monzonitic melt compositions are in equilibrium with the troctolites and gabbros of the layered series fe ti oxides saturate early in the layered series after 14 fractionation of plagioclase olivine cumulates further fractionation of fe ti oxide bearing gabbros drives the residual liquids toward silica enrichment and iron depletion major and trace element modeling indicates that the a type granites from the upper series were produced by protracted fractional crystallization of an iron rich basaltic parent magma at a fraction of residual liquid of only 8 the observed relative volumes of mafic cumulates and silicic rocks in the intrusion are in agreement with the calculations most of the intermediate compositions correspond to magmatic mafic enclave bearing granitoids and display geochemical evidence of hybridization intermediate compositions produced by fractional crystallization are scarce and a daly gap occurs from 57 to 67 wt sio2 this gap could result either from the fractional crystallization process or from silicate liquid immiscibility during that compositional interval

physical volcanology and eruption dynamics of peralkaline agglutinates from pantelleria peralkaline rhyolitic lava like lobes showing direct evidence for a spatter fed origin are associated with eruptive centres that document a transition from explosive to effusive fountain fed activity owing to the preservation of spatter clasts clast dimensions can be used to model the eruptive dynamics of these deposits we conducted an integrated field experimental and theoretical study of the cala di tramontana centre pantelleria italy with the aim to better understand its eruptive behaviour this centre is composed of four units and a close association of fall lapilli tack welded agglutinate and spatter fed lava like lobes these units were fed initially from a fissure vent and later from a circular vent for the last erupted lobe field evidence indicates that fall out accumulated rapidly coalescing into a 10 m thick deposit that flowed downslope and became lava like high eruption rates and high heat flows are interpreted to promote welding of spatter proximal to the vent overburden pressures and thermal insulation from the accumulating spatter caused complete homogenisation of spatter within the interior of the flow laboratory vesiculation experiments show that peralkaline obsidian can vesiculate at temperatures as low as 620 c over a 4 hour period and indicate that airborne agglutinate can deform and weld rapidly within minutes of landing we measured viscosity on a natural pantellerian sample using the parallel plate method over a temperature range of 560 600 c viscosities of 1010 1011 pa s and an activation energy of 311 kj mol were obtained based on undeformed clast sizes and densities eruption dynamics modelling indicates that gas eruption velocities ranged from 90 to 130 m s 1 erupted mass fluxes were 200 kg s 1 m 1 and 2 104 kg s 1 from the fissure and localised circular vents respectively magmatic viscosities of between 105 and 107 pa s for t 750 850 c precluded intermittent strombolian activity due to gas bubble coalescence in addition the amount of magmatic water available to drive the eruption fountains ranged from 0 4 to 0 6 wt significantly less than the estimated 1 5 wt of exsolved volatiles implying that significant passive degassing occurred

the effect of vertical shear on tropical cyclone intensity change abstract the effect of vertical shear on tropical cyclone intensity change is usually explained in terms of ventilation where heat and moisture at upper levels are advected away from the low level circulation which inhibits development a simple two layer diagnostic balance model is used to provide an alternate explanation of the effect of shear when the upper layer wind in the vortex environment differs from that in the lower layer the potential vorticity pv pattern associated with the vortex circulation becomes tilted in the vertical the balanced mass field associated with the tilted pv pattern requires an increased midlevel temperature perturbation near the vortex center it is hypothesized that this midlevel warming reduces the convective activity and inhibits the storm development previous studies have shown that diabatic heating near the storm center acts to reduce the vertical tilt of the vortex circulation these studies have also shown that there is an adiabatic process that acts to redu

the occurrence and distribution of mo and molybdenite in unaltered peralkaline rhyolites from pantelleria italy small hexagonal and triangular platelets of molybdenite mos2 5 to 25 m in diameter were identified in phenocrysts and matrix glass of unaltered felsic volcanic rocks from pantelleria italy the mos2 occurs commonly in pantellerites peralkaline rhyolites rarely in pantelleritic trachytes and never in trachytes the occurrence of euhedral mos2 platelets in all phenocryst phases in matrix glass and even in some melt inclusions indicates that mos2 precipitated directly from the peralkaline melt despite mos2 saturation the melt glass contains greater than 95 of the mo in pantellerian rocks x ray fluorescence analyses of 20 whole rocks and separated glasses show that whole rocks consistently contain less mo than corresponding matrix glasses the differences being in proportion to phenocryst abundances the mo contents increase with differentiation from trachytes 2 12 ppm to pantellerites 15 25 ppm and correlate positively with incompatible elements such as th y and nb the mo concentrations as determined by secondary ion mass spectrometry are essentially the same in matrix glasses and melt inclusions showing that mo did not partition strongly into a volatile fluid phase during outgassing the high mo contents of the pantellerites relative to metaluminous magmas with 1 5 ppm may be due to several factors 1 the enhanced stability of highly charged cations such as mo6 u4 and zr4 in peralkaline melts 2 the rarity of fe ti oxides and litanite into which mo might normally partition 3 reduced volatility of mo in low fo2 h2o poor 1 2 wt peralkaline magmas geochemical modeling indicates that the precipitation of mos2 can be explained simply by the drop in temperature during magmatic differentiation the occurrence of mos2 in pantellerites may result from their high mo concentrations and low redox state ni nio 2 5 relative to metaluminous magmas causing them to reach mos2 saturation at magmatic temperatures the apparent absence of mos2 microphenocrysts in more oxidized metaluminous rhyolites may indicate that mo is dissolved primarily as a hexavalent ion in those magmas

ion microprobe analyses of trace elements in anorthoclase hedenbergite aenigmatite quartz apatite and glass in pantellerite evidence for high water contents in pantellerite melt

on capillary barrier effects and debris slide triggering in unsaturated layered covers abstract the present work is focused on investigating the potential role of capillary barriers on the stability of layered soil deposits laying upon high permeability steep slopes in presence of capillary barrier effects often possible on a soil stratigraphy characterized by contrasting hydraulic properties pyroclastics deposit over pumices silt upon fractured bedrock silt over sand failure might originate in less permeable layers where unsaturated conditions might provide a crucial contribution to stability basic ideas and considerations have been drawn from a peculiar case study the catastrophic landslide occurred in campania southern italy on may 1998 which is discussed in this paper from a different perspective the areal extent of failures distribution their dense concentration at the steeper and higher slopes the extremely high number of initial failures occurred within a relatively short time span suggest that the intimate cause of the event is related at some extent to the soil state characteristic parameters rather than positive pore pressure pulses from water circulation in coarser layers or groundwater level rising from bedrock and springs a widespread loss of soil strength related to saturation can contribute to explain such phenomena capillary barrier effects favor accumulation and lateral distribution of percolating rainfall and possibly divert flowlines downslope thus leading to localized increases in water content and loss of strength diversion of percolating waters facilitates lateral redistribution of soil moisture along the slope with possible further localized saturation because more permeable layers may act temporarily as barriers and not as drains as one could expect numerical 2 d infiltration models with parameters calibrated versus laboratory data show a relevant influence of capillary barrier effects on infiltration in layered slopes with vertically stratified variations of hydraulic properties and point out significant localized moisture accumulation with possible consequences on slope stability landslide initiation phenomena in layered covers might thus be conditioned by the increase in saturation within the least permeable soil layers due to capillary barrier phenomena

the role of environmental shear and thermodynamic conditions in determining the structure and evolution of mesoscale convective systems during toga coare abstract a collection of case studies is used to elucidate the influence of environmental soundings on the structure and evolution of the convection in the mesoscale convective systems sampled by the turboprop aircraft in the tropical ocean global atmosphere toga coupled ocean atmosphere response experiment coare the soundings were constructed primarily from aircraft data below 5 6 km and primarily from radiosonde data at higher altitudes the well documented role of the vertical shear of the horizontal wind in determining the mesoscale structure of tropical convection is confirmed and extended as noted by earlier investigators nearly all convective bands occurring in environments with appreciable shear below a low level wind maximum are oriented nearly normal to the shear beneath the wind maximum and propagate in the direction of the low level shear at a speed close to the wind maximum when there is appreciable shear at middle levels 800 400 mb convective bands form parallel to the shear with

the effect of vertical wind shear on radiative convective equilibrium states abstract using a three dimensional cloud ensemble model a systematic exploration is undertaken of radiative convective equilibrium states as a function of the structure and magnitude of an imposed background flow with vertical shear in such simulations mesoscale organization appears naturally independent of the particulars of the initial condition as the magnitude of an imposed low level shear increases the convection becomes increasingly organized in lines or arcs propagating broadly downshear as predicted by earlier work when the shear is very strong the convection tends to organize into lines at an angle to the shear such that the line normal component is not far from its theoretical optimal value midlevel shear favors shear parallel lines but if it occurs in conjunction with sufficiently strong low level shear the convection can become very strongly organized into lines or arcs generally orthogonal to the low level shear optimal organization occurs when the depth of the shear layer is c

phase equilibrium constraints on pre eruptive conditions of recent felsic explosive volcanism at pantelleria island italy we present experimental phase equilibria carried out on a pantelleritic bulk rock composition with a peralkalinity index pi molar na2o k2o al2o3 1 4 which is representative of the most energetic pumice fall eruption of the resurgent post caldera volcanism on pantelleria island for the explored conditions p 25 150 mpa t 680 800 c h2omelt up to 6 wt fo2 nno where nno is nickel nickel oxide buffer clinopyroxene is the liquidus phase followed by alkali feldspar and then quartz the crystallization of amphibole is limited to temperatures below 700 c aenigmatite crystallizes near the liquidus for p 100 mpa when clinopyroxene is the sole liquidus phase the coexisting melt is characterized by a decrease in peralkalinity index to 1 2 if compared with the starting material approaching comendite composition in contrast when alkali feldspar joins the liquidus phase assemblage residual melts become extremely peralkaline pi 3 2 with feo contents in excess of 14 wt comparison of our experiments with natural phase abundances and compositions constrains the depth of the fastuca pantellerite reservoir to be around 5 1 km or p 120 20 mpa at t 730 10 c with h2omelt around 4 wt and fo2 1 5 log unit below nno such conditions correspond to the dominant phase assemblage of feldspar clinopyroxene and aenigmatite amphibole occurrence requires both higher melt water contents and lower temperatures up to 6 5 wt at 680 c and may reflect provenance from a deeper and wetter portion of the tapped reservoir in contrast quartz is more probably related to crystallization during magma ascent which explains the lower melt water content of its melt inclusions overall the results confirm that pantellerites may be both water rich and reduced magmas these characteristics imply that any coexisting fluid phase in the reservoir will be dominated by water and hydrogen species the latter amounting to a few mole per cent magma unrest at pantelleria may thus be heralded by emissions of hydrous and reduced gases i e co2 poor and h2 rich if the present day reservoir is felsic in composition

the behavior of inclined covers used as oxygen barriers covers with capillary barrier effects ccbe have been recently proposed as a viable option for gas migration barriers however the effect of geometry on ccbe performance has not been clearly demonstrated in this paper the results of a laboratory study performed with an original apparatus called the inclined box are presented the results obtained show that the hydraulic behavior of layered covers is influenced by the inclination of the slope generally the upper part of the slope contains less water than the lower part this means that the upper part is less efficient than the lower part for limiting gas migration the authors have also studied an existing site where a ccbe was built on a sloping surface after validation of the numerical model with in situ measured data the model was used to perform a parametric study to quantify the influence of the main ccbe parameters on its performance the results obtained confirm those measured in the lab and clearly show how geometry influences the performan

prolonged history of silicic peralkaline volcanism in the eastern pacific ocean socorro island mexico is an alkaline and peralkaline volcanic island located in the eastern pacific ocean on a mid ocean ridge spreading center that was abandoned at 3 5 ma silicic peralkaline rocks comprise up to 80 of the surface of the island rendering socorro virtually unique in the pacific ocean precise replicate 40ar 39ar ages of 21 peralkaline trachytes and rhyolites reveal a history of episodic volcanic activity from 540 to 370 ka that may have culminated with caldera formation repose periods between these episodes may have had maximum duration of 30 kyr after up to 200 kyr of quiescence 40ar 39ar ages indicate that postcaldera silicic peralkaline activity commenced by 180 ka forming the cerro evermann formation postcaldera mafic alkaline lavas of the lomas coloradas formation erupted dominantly between 70 and 150 ka based upon relative age relations the dominant lithology of precaldera and syncaldera silicic peralkaline deposits on socorro is nonfragmental and nonvesicular and lacks lithic fragments and fiamme despite this numerous lines of evidence including welding zonation presence of a proximal ignimbrite or co ignimbrite deposit association with a caldera and compositional heterogeneity within eruptive units suggest that they are dominantly ash flow tuffs a change in eruptive style from predominantly explosive to predominantly effusive followed caldera formation and suggests that a change in the efficacy of magma degassing may be linked to caldera formation on the basis of the presence of a caldera the magma chamber associated with socorro island is shallow and probably resides within the upper oceanic crust or the edifice this together with a prolonged history of silicic magmatism indicates that intrusion of mafic magma maintained thermal viability of the magmatic plumbing system the minimum calculated growth rate for the entire volcanic edifice 7 10 4 km3 yr exceeds those of nonhotspot off axis volcanoes in the pacific by almost an order of magnitude eruption rates for subaerial phases on socorro may be several orders of magnitude smaller than this growth rate and are comparable to subaerial eruption rates of isolated ocean islands related to mantle plumes

the origin of trachyte and pantellerite from pantelleria italy insights from major element trace element and thermodynamic modelling abstract trachyte and peralkaline rhyolite pantellerite and comendite frequently comprise the felsic end member in bimodal suites in continental rift and oceanic island settings in these settings the relationship between the mafic mildly alkaline or transitional basalt and felsic lavas is ambiguous major and trace element models and isotopic data are often consistent with an origin for felsic lavas from either fractional crystallization of transitional basalt or partial melting of alkali gabbro followed by fractional crystallization in this paper we present representative mineral analyses and whole rock analyses from forty samples of a basalt trachyte pantellerite suite collected at pantelleria italy in the strait of sicily rift zone and compare the results of major and trace element modelling with the results of thermodynamic melts modelling from these results we conclude that metaluminous trachyte formed as a result of 70 to 75 low pressure 0 1 gpa fractional crystallization of an assemblage of plagioclase clinopyroxene olivine magnetite and apatite from a hydrous 1 0 1 5 wt h2o transitional basalt magma at relative oxygen fugacities approximately one log unit below the fayalite magnetite quartz buffer fmq 1 the daly gap a lack of intermediate 49 62 wt sio2 volcanic rocks at pantelleria is concluded to be primarily the result of rapid differentiation through that interval relatively rapid crystallization at low pressure may have effected the partial degassing of water saturated 4 wt h2o metaluminous trachyte magma some metaluminous trachyte lavas have positive eu anomalies high k rb ratios high concentrations of ba and low concentrations of incompatible trace elements these are interpreted to be the result of up to 40 accumulation of alkali feldspar comenditic trachyte pantelleritic trachyte and pantellerite formed after an additional 20 to 80 fractional crystallization of an assemblage dominated by alkali feldspar from metaluminous trachyte magma the most evolved pantellerite lavas and tuffs are the result of a total of 95 fractional crystallization of transitional basalt with phenocrysts that equilibrated at low temperatures 4 wt water contents

design and laboratory verification of a physical model of sloping capillary barrier a physical model was designed and constructed to study the mechanisms associated with capillary barriers for slope stabilization purposes prior to construction of the model various numerical anal

tropical storm formation in a baroclinic environment abstract an analysis is presented of the large scale conditions associated with the initial development of tropical storm diana september 1984 in a baroclinic environment ordinary extratropical wave cyclogenesis began along an old frontal boundary east of florida after 0000 utc 7 september and culminated in tropical cyclogenesis 48 h later water vapor satellite imagery showed that the initial cyclogenesis and incipient tropical storm formation was nearly indistinguishable from a classical midlatitude development cyclogenesis occurred in three stages a large scale cold trough and associated frontal system crossed the atlantic coast while a small potential vorticity maximum aloft fractured off the main trough and stalled over central florida in the first stage as the main trough sheared off eastward cyclogenesis began along the southwestern end of the stalled frontal zone east of florida anticyclogenesis to the north in the wake of the shearing trough allowed a surge of cooler and drier air to flo

long lived mesoconvective vortices and their environment part ii induced thermodynamic destabilization in idealized simulations abstract idealized numerical simulations are used to quantify the effect of quasi balanced lifting arising from the interaction of the ambient vertical shear with midtropospheric cyclonic vortices mcvs generated by mesoscale convective systems on thermodynamic destabilization over a range of ambient vertical shear strengths and vortex characteristics observed in part i maximum upward displacements occur beneath the midtropospheric potential vorticity anomaly near the radius of maximum tangential vortex winds the location of the region of upward displacements relative to the ambient vertical shear vector depends on the relative strength of the vortex tangential flow and the ambient vertical shear and ranges from downshear for vortices of moderate strength in strong ambient vertical shear to 90 to the left of downshear for strong vortices in weak ambient vertical shear although significant upward displacements occur most rapidly with small vortices in strong ambient vertical shear maximum upward di

unravelling contamination signals in biogenic silica oxygen isotope composition the role of major and trace element geochemistry the oxygen isotope composition of diatom silica 18odiatom provides valuable information for palaeoclimate studies where carbonate proxies are either rare or absent in many lakes and ocean sediments unfortunately mineral and rock fragments found alongside diatoms in most sediments can be problematic as the method used will liberate oxygen from all components within the sediment producing both high frequency noise and low frequency excursions that can resemble climate signals removal of mineral contamination to date has largely relied upon the combination of chemical leaching and physical separation techniques e g sieving density this combination can be inefficient and often significant proportions of contaminants are present in the purified diatom sample using electron optical imaging and whole rock geochemistry on previously purified diatom samples a mass balance approach has been developed whereby the types and proportions of residual contaminants are identified by integrating this information with measured oxygen isotope ratios of the contaminants it is then possible to remove contamination effects from the 18odiatom record contamination effects relating to carbonates tephra and silt are modelled for cores from lake tilo ethiopia and lake baikal siberia in both lakes the new modelled 18odiatom curves show less high frequency noise thus enabling better resolution of low frequency climate signals

the role of synoptic scale flow during tropical cyclogenesis over the north atlantic ocean abstract the synoptic scale flow during tropical cyclogenesis and cyclolysis over the north atlantic ocean is investigated using compositing methods genesis and lysis are defined using the national hurricane center nhc now known as the tropical prediction center best track data genesis lysis occurs when nhc first last identifies and tracks a tropical depression in the final best track dataset storm centered composites are created with the analysis of the tropical oceanic lower level atoll 900 hpa and 200 hpa winds for june november produced by nhc for the years 1975 93 results show that significant regional differences exist in 200 hpa flow during genesis across the atlantic basin composites of genesis in the western part of the basin show a 200 hpa trough ridge located to the west east of the atoll disturbance in the eastern half of the basin composites of genesis show a sprawling 200 hpa ridge centered northeast of the atoll disturbance the major axis of this elliptically shaped 20

water flow in unsaturated soil in layered slopes abstract the water flow in unsaturated soil in layered slopes sandwiching coarse materials was measured using inclined soil containers placed under a rain simulator the refraction law of water flow in saturated soils was extended to the flow in unsaturated soils the extended refraction law of water flow in saturated and unsaturated soils was applied to the flow at the interface between the masa sandy loam and either the plant or gravel layer using a jump condition and particular assumptions of hydraulic conductivity of materials the data obtained on infiltration steady percolation and drainage in the soil containers could be well explained by the proposed new concept of saturated and unsaturated refraction flow so far as we know the work reported here is the first specifically designed to elucidate the effects of abrupt change of hydraulic conductivity on the flow of water in unsaturated soil slopes

petrogenesis of basalt comendite and basalt pantellerite suites terceira azores and some implications for the origin of ocean island rhyolites petrogenetic modeling of the recent lava succession of santa barbara and pico alto volcanoes and associated basaltic lavas indicates that there are two discrete lava series present one erupted from the axial rift linking the two central volcanoes and one associated with monogenetic cones scattered around the flanks of santa barbara the felsic lavas of both volcanoes are peralkaline and appear to be derived from associated basalts by fractional crystallization of an assemblage including essential amphibole trace element abundances in the felsic lavas particularly those of sr and ree cannot be reconciled with an origin through partial melting of basaltic material at the base of the volcanic pile the difference between the comenditic and pantelleritic differentiation trends of santa barbara and pico alto is attributed primarily to fo2 control of the crystallizing assemblage probably related to thermal dissociation of magmatic water in the santa barbara magma chamber this effect may be augmented by minor differences in parent basaltic compositions the pico alto pantellerites being derived from the rift basalts whereas the santa barbara comendites are derived from the off rift basalts a compositional gap between 54 and 64 silica content in the lavas is not present if the suite is extended to include co magmatic hypabyssal xenoliths leading to the inference that the gap in this and other bimodal suites results solely from a relative inability of magma of intermediate composition to erupt

the formation of moist vortices and tropical cyclones in idealized simulations abstractthe upscale aggregation of convection is used to understand the emergence of rotating coherent midtropospheric structures and the subsequent process of tropical cyclone formation the cloud model version 1 cm1 is integrated on an f plane with uniform sea surface temperature sst and prescribed uniform background flow deep convection is maintained by surface fluxes from an ocean with uniform surface temperature convection begins to organize simultaneously into moist and dry midtropospheric patches after 10 days after 20 days the patches begin to rotate on relatively small scales moist cyclonic vortices merge eventually forming a single dominant vortex that subsequently forms a tropical cyclone on a realistic time scale of about 5 days radiation that interacts with clouds and water vapor aids in forming coherent rotating structures using the path to genesis provided by the aggregated solution the relationship between thermodynamic changes within the vortex and changes in the character

on the formation of amex tropical cyclones irma and jason abstract the amex observational dataset with its high temporal and spatial resolution has been used to study the formation and structure of tropical cyclones irma and jason these systems developed and evolved entirely within the experiment s special observing network the study is mostly based upon six hourly numerical analyses of the mass and wind fields on 11 vertical levels over a 1 25 lat long grid the systems are traced from prior to the formation of a resolvable closed surface circulation to when they were operationally classified as tropical cyclones the discussion focuses on the synoptic to cyclone scale changes during formation time sections of various kinematic variables together with an index of deep convection obtained from digital satellite cloud imagery are used to trace the development both systems developed during active phases of the monsoon and initially were of maximum intensity in the middle troposphere low level spinup occurred in three stages the first stage was associate

experimental constraints on pre eruption conditions of pantelleritic magmas evidence from the eburru complex kenya rift the phase relationships and compositions of a pantellerite from the eburru complex in the kenya rift valley have been determined at 150 mpa and under reducing conditions 2 log units below the ni nio solid buffer the effects of temperature and melt water content on phase relationships have been explored alkali feldspar and quartz crystallise alone at temperatures above 700 c irrespective of melt water content below 700 c sodic amphibole and clinopyroxene also crystallise the amphibole being the liquidus phase under water rich conditions the coexistence of amphibole phenocrysts with alkali feldspar and quartz in a crystal poor pantellerite implies temperatures below 700 c and melt water contents higher than 4 wt possibly up to 5 6 wt pantellerites have lower liquidus temperatures than associated comendites which supports a parent daughter relationship between the two magma types the melts produced in the experiments extend the compositional trend displayed by the natural rock series and reproduce some extreme compositions occasionally observed in alkaline volcanic series with feo contents above 12 wt and na2o contents approaching 10 wt pantellerites are therefore the true near minimum melt compositions of alkaline oversaturated magma series

boundary effect on the hydraulic characteristics of gravelly soil slopes in physical model tests abstract the stability of a residual gravelly soil slope during heavy rainfall is closely related to the seepage field which is directly affected by the seepage boundaries of the slope to examine the boundary effect a novel model flume that can implement permeable and impermeable boundary conditions was designed with this flume hydraulic model tests of a gravelly soil slope considering those two seepage boundaries were carried out the results show that distinct spatial morphology of seepage fields is reflected by the hydraulic characteristics of the slope models with different seepage boundaries the slope with permeable boundaries generally exhibits a larger average infiltration rate as well as a relatively smaller moving speed of wetting front in the lower part compared to the slope with impermeable boundaries the residual moisture content in the deep lower part of the slope with permeable boundaries is 6 6 cm3 cm3 smaller than that of the slope with impermeable boundaries nevertheless the maximum pore water pressure in the middle part of the slope with permeable boundaries is 1 2 kpa higher than that in the corresponding part of the slope with impermeable boundaries it is noted that regional pore water pressures in the shallow lower and shallow middle parts of the slope with impermeable boundaries exhibit large fluctuations with an amplitude over 0 5 kpa the seepage field in the slope with permeable boundaries presents a multi dimensional development which may lead to a local failure of the slope the results are of great significance in studying the similarity of seepage boundary condition in the field of similarity theory on landslide model tests and provide an experimental basis for establishing rainfall induced landslide theory

performance of an instrumented slope covered by a capillary barrier system abstracta capillary barrier as a cover system is a two layer system of distinct hydraulic properties to prevent water infiltration into the underlying soil by utilizing unsaturated soil mechanics principles this paper illustrates the application of the capillary barrier system on a slope that experienced shallow slip failures to prevent future rainfall induced slope failures in this study the capillary barrier system was designed as a cover system for residual soil slopes with a steep slope angle under heavy rainfall conditions of the tropics the capillary barrier system was constructed using fine sand as the fine grained layer and granite chips as the coarse grained layer both layers were contained in a cellular confinement system the slope was instrumented with tensiometers and piezometers the tensiometers were installed at different depths from about 0 5 m to 2 0 m below the slope surface an adjacent original slope without the capillary barrier system was also instrumented using tensiometers to

the influence of convectively generated thermal forcing on the mesoscale circulation around squall lines abstract the dynamical processes that determine the kinematic and thermodynamic structure of the mesoscale region around 2d squall lines are examined using a series of numerical simulations the features that develop in a realistic reference simulation of a squall line with trailing stratiform precipitation are compared to the features generated by a steady thermal forcing in a dry simulation with no microphysical parameterization the thermal forcing in the dry simulation is a scaled and smoothed time average of the latent heat released and absorbed in and near the leading convective line in the reference simulation the mesoscale circulation in the dry simulation resembles the mesoscale circulation in the reference simulation and around real squall lines it includes an ascending front to rear flow a midlevel rear inflow a mesoscale up and downdraft an upper level rear to front flow ahead of the thermal forcing and an upper level cold anomaly to the rear of the thermal forcing it is also shown t

eruption and emplacement of a basaltic welded ignimbrite during caldera formation on gran canaria the 14 1 ma old composite ignimbrite cooling unit p1 45 km3 on gran canaria comprises a lower mixed rhyolite trachyte tuff a central rhyolite basalt mixed tuff and a slightly rhyolite contaminated basaltic tuff at the top the basaltic tuff is compositionally zoned with a an upward change in basalt composition to higher mgo content 4 3 5 2 wt b variably admixed rhyolite or trachyte commonly 5 wt and c an upward increasing abundance of basaltic and plutonic lithic fragments and cognate cumulate fragments the basaltic tuff is divided into three structural units i the welded basaltic ignimbrite which forms the thickest part c 95 vol and is the main subject of the present paper ii poorly consolidated massive bomb and block rich beds interpreted as phreatomagmatic pyroclastic flow deposits and iii various facies of reworked basaltic tuff tuff unit i is a basaltic ignimbrite rather than a lava flow because of the absence of top and bottom breccias radial sheet like distribution around the central tejeda caldera thickening in valleys but also covering higher ground and local erosion of the underlying p1 ash a gradual transition from dense rock in the interior to ash at the top of the basaltic ignimbrite reflects a decrease in welding the shape of the welding profile is typical for emplacement temperatures well above the minimum welding temperature a similar transition occurs at the base where the ignimbrite was emplaced on cold ground in distal sections in proximal sections the base is dense where it was emplaced on hot felsic p1 tuff the intensity of welding especially at the base and the presence of spherical particles and of mantled and composite particles formed by accretion and coalescence in a viscous state imply that the flow was a suspension of hot magma droplets the flow most likely had to be density stratified and highly turbulent to prevent massive coalescence and collapse model calculations suggest eruption through low pyroclastic fountains 1000 m high with limited cooling during eruption and turbulent flow from an initial temperature of 1160 c the large volume of 26 km3 of erupted basalt compared with only 16 km3 of the evolved p1 magmas and the extremely high discharge rates inferred from model calculations are unusual for a basaltic eruption it is suggested that the basaltic magma was erupted and emplaced in a fashion commonly only attributed to felsic magmas because it utilized the felsic p1 magma chamber and its ring fissure conduits evolution of the entire p1 eruption was controlled by withdrawal dynamics involving magmas differing in viscosity by more than four orders of magnitude the basaltic eruption phase was initially driven by buoyancy of the basaltic magma at chamber depth and continued degassing of felsic magma but most of the large volume of basalt magma was driven out of the reservoir by subsidence of a c 10 km diameter roof block which followed a decrease in magma chamber pressure during low viscosity basaltic outflow

a vortical hot tower route to tropical cyclogenesis abstract a nonhydrostatic cloud model is used to examine the thermomechanics of tropical cyclogenesis under realistic meteorological conditions observations motivate the focus on the problem of how a midtropospheric cyclonic vortex a frequent by product of mesoscale convective systems during summertime conditions over tropical oceans may be transformed into a surface concentrated warm core tropical depression as a first step the vortex transformation is studied in the absence of vertical wind shear or zonal flow within the cyclonic vorticity rich environment of the mesoscale convective vortex mcv embryo the simulations demonstrate that small scale cumulonimbus towers possessing intense cyclonic vorticity in their cores vortical hot towers vhts emerge as the preferred coherent structures the vhts acquire their vertical vorticity through a combination of tilting of mcv horizontal vorticity and stretching of mcv and vht generated vertical vorticity horizontally localized and exhibiting conve

new data on magmatic h 2 o contents of pantellerites with implications for petrogenesis and eruptive dynamics at pantelleria infrared spectroscopic analyses of melt inclusions in quartz phenocrysts from pantellerites erupted at pantelleria italy show that the magmas contained moderate pre eruptive h2o contents ranging from 1 4 to 2 1 wt melt h2o concentrations increase linearly with incompatible elements demonstrating that h2o contents were not buffered significantly during fractionation by any crystalline or vapor phase the relatively low h2o contents of pantellerites are consistent with an origin by partial melting of alkali gabbros rather than fractional crystallization of basalt preeruptive h2o concentrations do not correlate with the volume or explosivity of pantellerite eruptions decompression history is critical in determining the style of pantellerite and other eruptions

the formation of hurricane humberto 2001 the importance of extra tropical precursors the authors examine the formation of hurricane humberto 2001 in the atlantic basin precursor disturbances originating in the extratropics appear critical for inducing tropical cyclogenesis one precursor near the tropopause arose during the development of hurricane erin on 7 september and could be tracked continuously for 12 days until 1200 utc 19 september a fully explicit numerical simulation with the psu ncar mesoscale model produces a generally realistic evolution of the developing storm during a 4 day period beginning 0000 utc 19 september diagnostics of the simulations reveal that moderate vertical shear was crucial for organizing convection and this shear was enhanced on the periphery of an upper level vortex within a lower to mid tropospheric baroclinic zone downdraughts and evaporative cooling produced a rearward propagating bore that organized new convection in a notably moistened environment with small cape vorticity generation maximized near 1 km above mean sea level and the vortex built upward with time this evolution contrasts markedly with downward development conceptual models of tropical cyclogenesis and points to the crucial need for observations in the lower troposphere before depression formation convective scale dynamics were governed by intense convective cores which produced most of the diabatic heating and lower tropospheric vertical vorticity they were constrained by vertical shear to populate the downshear left quadrant of the vortex and therefore their collective behaviour was organized despite being quasi random within the favoured region ultimately the shear that helped organize the convection in the developing depression hampered humberto s subsequent intensification copyright 2006 royal meteorological society

explosive eruptive history of pantelleria italy repeated caldera collapse and ignimbrite emplacement at a peralkaline volcano a new pre green tuff 46 ka volcanic stratigraphy is presented for the peralkaline pantelleria volcano italy new 40ar 39ar and paleomagnetic data are combined with detailed field studies to develop a comprehensive stratigraphic reconstruction of the island we find that the pre 46 ka succession is characterised by eight silica rich peralkaline trachyte to pantellerite ignimbrites many of which blanketed the entire island the ignimbrites are typically welded to rheomorphic and are often associated with lithic breccias and or pumice deposits they record sustained radial pyroclastic density currents fed by low pyroclastic fountains the onset of ignimbrite emplacement is typically preceded more rarely followed by pumice fallout with limited dispersal and some eruptions lack any associated pumice fall deposit suggesting the absence of tall eruption columns particular attention is given to the correlation of well developed lithic breccias in the ignimbrites interpreted as probable tracers of caldera collapses they record as many as five caldera collapse events in contrast to the two events reported to date inter ignimbrite periods are characterised by explosive and effusive eruptions with limited dispersal such as small pumice cones as well as pedogenesis these periods have similar characteristics as the current post green tuff activity on the island and while not imminent it is reasonable to postulate the occurrence of another ignimbrite forming eruption sometime in the future

parametric study of unsaturated drainage layers in a capillary barrier unsaturated drainage layers udls have been demonstrated to greatly increase the lateral diversion capacity of capillary barriers the inclusion of a udl allows native soils suitable for vegetation growth to be used as the finer soil as lateral drainage properties of the layer no longer need to be considered a comprehensive numerical study was conducted to investigate the influence of the interface slope and the udl material on the system s ability to laterally divert downward moving moisture a capillary barrier system with and without a udl was simulated for 10 years using daily varying climatic data for three locations in the us three different sands were simulated as the udl and were modeled at slopes of 5 10 and 20 the numerical results confirm that the inclusion of an unsaturated drainage layer at the fine coarse interface of a capillary barrier can provide significant improvements in the performance of the cover system by laterally draining water this improvement in performance may allow the system to be successfully implemented in climates wetter than previously were thought suitable the diversion length the distance water is diverted laterally with no downward flow through the fine coarse interface of a capillary barrier with a udl wasmore found to be proportional to the slope of the fine coarse interface in addition a relationship between lateral diversion lengths in a capillary barrier and the udl material was developed and found to be dependent on the unsaturated flow characteristics of the udl these relationships allow the performance of a variety capillary barrier udl designs to be calculated knowing the behavior of one system for a given location less

magma systems of the kutcharo and mashu volcanoes ne hokkaido japan petrogenesis of the medium k trend and the excess volatile problem abstract despite the existence of a growing database of the amount of h 2 o in melt inclusions it is difficult to calculate the bulk h 2 o content for vapor saturated magma because the observable h 2 o concentration in melt is limited by its solubility to overcome this problem we focus on k 2 o which shows similar geochemical behavior to h 2 o but is largely retained within melt in the case of volatile saturation and magma degassing as an initial trial of this approach we document the evolution of k 2 o content for the kutcharo and mashu volcanoes in northeastern hokkaido japan which are located close to each other and have similar timing of eruptive activity but are chemically distinct with kutcharo being medium k in composition and mashu being low k despite the contrasting bulk rock compositions of the two volcanoes the melt inclusions show a smooth compositional trend that is continuous from low k basalt to andesite rhyolite medium k rhyolite and finally high k rhyolitic melt the observed medium k series probably is made by mixing of low k basalt and medium k rhyolite which derive from solidified low k basalt thermodynamic computations using melts show that the observed chemical variation in melt inclusions can be explained by fractional crystallization and subsequent re melting of low k basalt in addition the similar sr nd and pb isotopic compositions of the basalt and rhyolite combined with the progressive enrichment of trace elements from basalt to rhyolite support the interpretation that the two rock types are genetically related the observed evolution of k 2 o contents suggests that the total volume of basalt supplied intermittently beneath the volcanoes was approximately 10 times the volume of erupted rhyolite magma the solidified basalt which contains h 2 o 5 wt s 0 3 wt and cl 0 01 wt supplied a much larger amount of magmatic volatiles to the overlying felsic magma than could be dissolved

the acoculco caldera complex magmas genesis evolution and relation with the acoculco geothermal system abstract the acoculco caldera complex acc is located in the eastern part of the trans mexican volcanic belt mexico the acc was formed 2 7 ma and since then volcanic activity has persisted until 0 06 ma through the emission of domes cinder cones fissure lava flows and two ignimbrite eruptions dated at 1 2 and 0 65 ma the pervasive hydrothermal alteration in the central part of the acc has motivated considerable geothermal exploration however none of it assessed the relation between the magmatic heat supply and the evolution of magmas including their reservoir depth magmatic processes were investigated with petrography major oxides trace element geochemistry mineral chemistry and isotopic analysis in addition we performed a series of hydrothermal experiments in order to constrain the storage depth for the magma tapped 1 2 ma during a reactivation of the caldera we investigated the origin of magmas tapped during the eruptive history of the acc and the magmatic processes magma mixing assimilation and crystallization that modified these magmas after the caldera collapse 2 7 ma the local stress field was probably modified and allowed the ascent of peralkaline magmas through new plumbing systems such magmas mixed with calc alkaline magmas and formed the post caldera volcanism influx of new magma is considered as the probable heat supply that maintains the active magmatic system whereas assimilation has been negligible but yields insights about the depth at which processes have occurred our work suggests that there are either shallow storage zones 200 500 bars where magmas crystallize and either erupt or stall and cool or there are intrusive magmas unrelated to the acc suite that have accumulated nearby and heat the reservoir magmas

high resolution 40ar 39ar chronostratigraphy of the post caldera 20 ka volcanic activity at pantelleria sicily strait abstract the island of pantelleria sicily strait the type locality for pantellerite has been the locus of major caldera forming eruptions that culminated ca 50 ka ago in the formation of the cinque denti caldera produced by the green tuff eruption the post caldera silicic activity since that time has been mostly confined inside the caldera and consists of smaller energy eruptions represented by more than twenty coalescing pantelleritic centers structurally controlled by resurgence and trapdoor faulting of the caldera floor a high resolution 40 ar 39 ar study was conducted on key units spanning the recent post 20 ka intracaldera activity to better characterize the present day status and forecast the short term behavior of the system based on the temporal evolution of the latest eruptions the new 40 ar 39 ar data capture a long term 15 ka decline in eruption frequency with a shift in eruptive pace from 3 5 ka 1 to 0 8 ka 1 associated with a prominent paleosol horizon marking the only recognizable volcanic stasis around 12 14 ka this shift in extraction frequency occurs without major changes in eruptive style and is paralleled by a subtle trend of decreasing melt differentiation index we speculate that this decline probably occurred i without short term variations in melt production differentiation rate in a steady state compositionally zoned silicic reservoir progressively tapped deeper through the sequence and ii that it was possibly modulated by outboard eustatic forcing due to the 140 m sea level rise over the past 21 ka the intracaldera system is experiencing a protracted stasis since 7 ka coupled with recent geodetic evidence of deflation and subsidence of the caldera floor the system appears today to be on a wane with no temporal evidence for a short term silicic eruption

baroclinically induced tropical cyclogenesis abstract the authors investigate the transition of numerous subtropical cyclones into late season tropical storms and hurricanes during the 2000 and 2001 atlantic tropical cyclone seasons in all transitioning cases 10 the 900 200 hpa wind shear was initially near or in excess of the upper limit of vertical shear deemed suitable for tropical cyclogenesis in many of these cases the vertical shear decreased markedly near or prior to the time of tropical cyclone formation in cases that did not become named tropical cyclones either the tropospheric vertical shear remained in excess of 15 20 m s 1 or the underlying sea surface temperature sst dropped below about 26 c prior to or during the weakening of the shear cases in which the shear remained large featured multiple short wave upper tropospheric troughs interacting with the developing lower tropospheric disturbance such that classical occlusion did not occur through detailed analysis and simulation of the development of hurricane michael in 2000

evolution of multiscale vortices in the development of hurricane dolly 2008 abstract as a follow up to a previously published article on the initial development and genesis of hurricane dolly 2008 this study further examines the evolution of and interactions among multiscale vortices ranging from the system scale main vortex l 150 km to the intermediate scale cloud clusters 50 km l 150 km and individual vorticity rich convective cells l 50 km it is found that there are apparent self similarities among these vortices at different scales each of which may undergo several cycles of alternating accumulation and release of convective available potential energy enhanced surface fluxes below individual cyclonic vortices at each scale contribute to the sustainment and reinvigoration of moist convection that in turn contributes to the maintenance and upscale growth of these vortices spectral analysis of horizontal divergence and relative vorticity further suggests that the cloud cluster scale and system scale vortices are predominantly balanced while the individual

evidence for a copper bearing fluid in magma erupted at the valley of ten thousand smokes alaska several lines of evidence point to the existence of a pre eruptive fluid phase in the magmas erupted in 1912 at the valley of ten thousand smokes alaska the discovery of rare cu rich bubbles in some melt inclusions is best explained by the random entrapment of a low density fluid phase within growing phenocrysts the h2o s and cl concentrations of the high silica rhyolite are also consistent with fluid saturation in the rhyolite water contents as determined by infrared spectroscopy on doubly polished melt inclusions cluster between 3 5 and 4 5 wt h2o without any apparent differences between magmas vented explosively and effusively these concentrations would be sufficient for h2o saturation at a pressure of 100 mpa though the presence of other volatiles such as co2 and so2 could allow saturation with respect to a fluid phase at higher pressures the s cu and cl contents of the phenocryst assemblages as determined by xrf analyses are too low to account for the decrease in the concentrations of s and cu in the melt and only modest increase in cl with differentiation therefore the behavior of these volatile elements was controlled either by eryptic fractionation if phenocryst phasese or more likely by partitioning into a coexisting fluid phase the presence of this low density phase apparently provided metals such as cu with a volatile phase into which they could partition the concentration of cu in this fluid reached tens to hundreds of times that of the coexisting silicate melt and may have been as rich as 0 05 wt cu the distribution of cu in the magma was also controlled by sulfides such as intermediate solid solution and pyrrhotite which cyrstallized directly from the silicate melt

trace element partitioning in pantellerites and trachytes abstract in order to investigate the effect of increasing melt peralkalinity on partitioning partition coefficients have been determined using neutron activation analyses of coexisting phenocrysts and glass of five samples from pantelleria spanning the range trachyte to pantellerite alkali feldspar partition coefficients for fe rb ba sr and eu vary with melt peralkalinity due to changes in melt polymerization and to the systematic increase in xor and decrease in xan of the feldspar in going from trachyte to pantellerite fe partition coefficients increase from 0 04 to 0 10 presumably because fe 3 increasingly substitutes in the feldspar tetrahedral site as melt activity of al declines and fe concentrations increase partition coefficients for trivalent light rees decrease and the partitioning pattern becomes flatter the most evolved samples having some of the lowest published values for feldspar the hundredfold decline in eu partition coefficients 2 5 to 0 024 and the decrease in the size of the positive partitioning anomaly are attributed to increasing eu3 eu2 in the melt as it becomes more peralkaline as well as to concomitant decrease in the ca content of feldspar as a result the behavior of eu during fractional crystallization of peralkaline suites is fundamentally different from that in metaluminous suites absolute abundances rise and the size of the negative eu anomaly changes little with fractionation beyond pantelleritic trachyte barium and strontium do not decline to extremely low concentrations in pantellerites as they do in highly evolved metaluminous rhyolites because the feldspar partition coefficients drop precipitously with peralkalinity similarly rb contents do not get large because the partition coefficient increases slightly 0 11 to 0 25 with peralkalinity and is sufficient to make rb only weakly incompatible in the feldspar dominated assemblage partition coefficients calculated for sr and ba from natural phenocryst matrix pairs can be gross over estimates in these feldspar rich felsic rocks if phenocrysts are zoned this effect is severe for assemblages with bulk distribution coefficients d greater than 4 even at very small crystal contents and for 4 d 2 it is significant if crystal contents exceed about 25 some of the very high values and much of the variability reported in the literature for ba and sr feldspar partition coefficients may be a result of this effect in contrast to feldspar partition coefficients for apatite and mafic phases change little with peralkalinity partition coefficients for rees and the compatible elements sc co and cr are all less than half those in fe rich clinopyroxene and olivine of similar composition in high silica rhyolite although absolute levels of partition coefficients are a function of melt polymerization the shapes of the ree partitioning patterns appear to be related to the composition of the clinopyroxene scandium is most strongly concentrated in clinopyroxene partition coefficient 27 60 having larger values in more fe rich compositions apatite shows a wide range in composition within the suite the concentrations of si fe rees and y increasing and mg ca and p decreasing in more evolved samples these trends however parallel those in coexisting glasses so that partition coefficients are essentially constant in peralkaline suites cs is the most incompatible of commonly analyzed elements but its great mobility during alteration and weathering limits its use as a differentiation index next in order of incompatibility are th u hf zr and ta which have small but finite values of d due to the partitioning of th and u into apatite 2 the accommodation of zr and hf by clinopyroxene 0 4 and titanomagnetite 0 2 and relatively high concentrations of ta in cossyrite and fe ti oxides the next most incompatible are the lightest and the heaviest rees and finally the middle trivalent rees and rb plots of pairs of relatively incompatible elements form straight line trends over large compositional ranges in peralkaline suites because the replacement of olivine titanomagnetite by cossyrite as the melt becomes more peralkaline causes little change in d the ree patterns of titanomagnetite and cossyrite are both approximately flat at values less than 0 1 and the relatively high titanomagnetite partition coefficients for ta 2 zr and hf when combined with the very low partition coefficients for these elements for olivine approximate the moderate partition coefficients for cossyrite

the genesis of hurricane guillermo texmex analyses and a modeling study abstract the transformation of a mesoscale convective system into hurricane guillermo was captured by aircraft and doppler wind data during the tropical experiment in mexico the early phase of the system evolves in a way very similar to previously documented mesoscale convective systems with a midlevel mesocyclone developing in the stratiform precipitation region more unusually the cyclone extends to low altitudes a weak cyclone is discernable even in the 300 m altitude wind field after another day of evolution a small surface based warm core cyclone is observed to develop within the relatively cold air associated with the mesocyclone aloft this mesocyclone develops into a hurricane over the subsequent day a nonhydrostatic axisymmetric numerical model is used to explore the evolution of the initially cold core midlevel vortex into a tropical cyclone a mesoscale midlevel showerhead is switched on in an initially quiescent tropical atmosphere overlying a warm ocean surface evaporation of t

silicic magmas from the continental cameroon volcanic line oku bambouto and ngaoundere 40 ar 39 ar dates petrology sr nd o isotopes and their petrogenetic significance the intraplate cameroon volcanic line cvl straddles the african south atlantic continent ocean boundary and is composed mainly of alkaline basic volcanic rocks voluminous silicic volcanics characterize the continental sector of the cvl we present here new geochemical isotopic sr nd o and 40ar 39ar geochronological data on the main silicic volcanic centres of the western mt oku sabga and mt bambouto and eastern ngaoundere plateau cameroon highlands the silicic volcanism of mt oku sabga and mt bambouto occurred between 25 and 15 ma and is represented by voluminous quartz normative trachytes and minor rhyolitic ignimbrites at mt bambouto central volcano about 700 m of silicic volcanics erupted in less than 2 7 million years these silicic volcanics are associated with slightly to moderately alkaline basalts and minor basanites in general onset of the silicic volcanism migrated from ne oku 25 ma to sw sabga 23 ma bambouto 18 ma and mt manengouba 12 ma the silicic volcanism of the ngaoundere plateau eastern branch of the cvl is instead dominated by nepheline normative trachytes which are associated with strongly alkaline basalts and basanitic rocks these ne trachytes are younger 11 9 ma than the q trachytes of the western highlands the least differentiated silicic volcanics are isotopically similar 87sr 86sr 0 51278 to the associated alkaline basalts suggesting differentiation processes without appreciable interaction with crustal materials such interactions may however have played some role in the genesis of the most evolved silicic volcanics which have 87sr 86sr as high as 0 705 0 714 fractional crystallization is the preferred mechanism for genesis of the silicic melts of both western and eastern highlands as shown by modeling major and trace element variations the genesis of the least evolved q trachytes from the western highlands starting from slightly to moderately alkaline basalts is compatible with fractionation of dominantly plagioclase clinopyroxene and magnetite crystal fractionation may have occurred at low pressure and at qfm buffer f o2conditions parental magmas of the ngaoundere ne trachytes are likely instead to have been strongly alkaline basalts which evolved through crystal fractionation at higher p 6 2 kbar and f o2 qfm 2 the migration 25 to 12 ma of the silicic volcanism from ne to sw in the continental sector of the cvl is reminiscent of that 31 5 ma of the onset of the basic volcanism in the oceanic sector principe to pagalu islands of the cvl these ages and that 11 9 ma of the silicic volcanism of the ngaoundere plateau indicate that the cameroon volcanic line as a whole may not be easily interpreted as the surface expression of hot spot magmatism

analytical models for bubble growth during decompression of high viscosity magmas analytical models for decompressional bubble growth in a viscous magma are developed to establish the influence of high magma viscosity on vesiculation and to assess the time scales on which bubbles respond to decompression instantaneous decompression of individual bubbles analogous to a sudden release of pressure e g sector collapse is considered for two end member cases the infinite melt model considers the growth of an isolated bubble before significant bubble interaction occurs the shell model considers the growth of a bubble surrounded by a thin shell and is analogous to bubble growth in a highly vesicular magmatic foam results from the shell model show that magmas less viscous than 109 pa s can freely expand without developing strong overpressures the timescales for pressure re equilibration are shortened by increased ratios of bubble radius to shell thickness and by larger decompression time scales for isolated bubbles in rhyolitic melts infinite melt model are significantly longer implying that such bubbles could experience internal pressures greater than the ambient pressure for at least a few hours following a sudden release of pressure the shell model is developed to assess bubble growth during the linear decompression of a magma body of constant viscosity for the range of decompression rates and viscosities associated with actual volcanic eruptions bubble growth continues at approximately the equilibrium rate with no attendant excess of internal pressure the results imply that viscosity does not have any significant role in preventing the explosive expansion of high viscosity foams however for viscosities of 109 pa s there is the potential for a viscosity quench under the extreme decompression rates of an explosive eruption it is proposed that the typical vesicularities of pumice of 0 7 0 8 are a consequence of the viscosity of the degassing magmas becoming sufficiently high to inhibit bubble expansion over the characteristic time scale of eruption for fully degassed silicic lavas with viscosities in the range 1010 to 1012 pa s time scales for decompression of isolated bubbles can be hours to many months

evidence of magma mixing in the daly gap of alkaline suites a case study from the enclaves of pantelleria italy the island of pantelleria consists of trachytes pantellerites and minor mildly alkaline basalts rocks of intermediate composition falling in the so called daly gap such as mugearites benmoreites and mafic trachytes occur only in the form of enclaves in trachytes and pantellerites inside the main caldera of the island caldera cinque denti which collapsed during the green tuff ignimbrite eruption at 50 ka the enclaves include volcanic subvolcanic and intrusive rock types the enclaves in host trachyte contain traces of glass devitrified glass occurs within enclaves in host pantellerites minerals in the enclaves show regular compositional variations with whole rock silica content glass present in the medium grained samples is interpreted to be the result of incipient melting the major and trace element compositions of the enclaves show regular and linear variations between an evolved mafic magma hawaiite and a felsic end member similar to the green tuff trachyte fractional crystallization modelling of compatible and incompatible trace elements v ni zr la sm lu nb y th does not reproduce the observed trends rocks of intermediate composition within the daly gap can be explained only by magma mixing between an already differentiated mafic magma hawaiite and an anorthoclase rich trachytic melt in the lower and higher parts respectively of a stratified magmatic chamber medium grained enclaves are interpreted as the result of fragmentation of solidified mixing layers in the roof of the magma chamber during the eruption of the green tuff when the collapse of the caldera took place diffusion calculations suggest a residence time of 5 days for the enclaves in their host magmas

numerical simulations of the genesis of hurricane diana 1984 part i control simulation abstract the complete transformation of a weak baroclinic disturbance into hurricane diana is reproduced by numerical simulations using the fifth generation pennsylvania state university national center for atmospheric research mesoscale model three distinct phases of the evolution are evident first baroclinic and barotropic development strongly modified by the effects of latent heating occurs during the latter part of this phase the low level circulation is strengthened through the axisymmetrization of remote potential vorticity anomalies that are generated by condensational heating and then advected toward the incipient storm the axisymmetrization process evinces properties of both nonlinear discrete vortex merger and vortex rossby wave dynamics the transformation from cold core to warm core vortex occurs in this development stage in the second phase lasting 10 12 h little deepening occurs spiral bands of convection begin to form and the core of the storm moistens eventually reaching 95

the influence of trace amounts of water on the viscosity of rhyolites as a major volatile in volcanic systems water has a significant influence on the rheological properties of silicic magmas this is especially so at minor water contents relevant to the emplacement of silicic lavas to investigate the influence of water on the viscosity of natural rhyolitic obsidians a novel strategy has been adopted employing parallel plate and micropenetration techniques viscosities have been determined on three types of material a raw water bearing obsidians b remelted 1650 c 1 atm degassed glasses of the obsidians and c hydrothermally hydrated 1300 c 3 kbar obsidians ten natural rhyolitic obsidians peraluminous calc alkaline and peralkaline were employed seven originated from lava flows and contained 0 2 wt h2o two samples were f rich from pyroclastic successions and one was an obsidian cobble with 1 5 wt water also associated with pyroclastic units melt compositions and water contents were stable during viscometry the measured decreases in activation energies of viscous flow and viscosity with small amounts of water are much greater than the shaw calculation scheme predicts in addition a marked non linear decrease in exists with increasing water content in contrast to the case for peralkaline rhyolites 0 1 0 2 wt water decreases activation energies significantly up to 30 for calc alkaline compositions these results have important implications for the ease of near surface degassing of silicic magmas during emplacement and permit the testing of calculational models for viscosity largely based on synthetic systems

gregarious tropical convection abstract a beat source with a vertical profile like that of observed tropical mesoscale convective systems mcss is shown to cause through inviscid gravity wave dynamics upward displacement at low levels in a mesoscale region surrounding the heating typical values are 10 30 area contraction at the surface everywhere within 270 km of the heating 6 h after it starts as a result conditions near an existing mcs but beyond the area of mcs outflow become more favorable for the development of additional convection this theory predicts that cloud clusters should be gregarious infrared satellite imagery confirms that almost half of the cold cloudiness observed in a month over the oceanic warm pool region was contributed by just 14 objectively defined multiday supercluters

western north pacific tropical cyclone characteristics stratified by genesis environment abstractthe characteristics of tropical cyclones tcs in the summer and autumn seasons over the western north pacific that are associated with different environmental factors that influence tc genesis tcg were studied we objectively categorized factors into the five tcg factors classified by ritchie and holland 1999 monsoon shear line sl monsoon confluence region cr monsoon gyre gy easterly wave ew and the rossby wave energy dispersion from a preexisting tc ptc the gy tcs tended to develop slowly and the highest rates of occurrence of rapid intensification ri were found for the cr tcs whereas the gy tcs rarely experienced ri the average storm size of the gy tcs at the time of formation was the largest of the averages of the other tc types while the ew and ptc tcs were smaller although these differences disappeared at the mature time there were no significant differences in the sea surface temperature sst beneath the tcs but the tropical cyclone heat potential tchp of t

scale interactions during the formation of typhoon irving the development of typhoon irving is investigated using a variety of data including special research aircraft data from the tropical cyclone motion tcm 92 experiment objective analyses satellite data and traditional surface and sounding data the development process is treated as a dry adiabatic vortex dynamics problem and it is found that environmental and mesoscale dynamics mutually enhance each other in a cooperative interaction during cyclone formation synoptic scale interactions result in the evolution of the hostile environment toward more favorable conditions for storm development mesoscale interactions with the low level large scale circulations and with other midlevel mesoscale features result in development of vorticity in the midlevels and enhancement of the low level vorticity associated with the developing surface cyclone multiple developments of mesoscale convective systems after the storm reaches tropical depression strength suggests both an increase in low level confluence and a tendency toward recurrent development of associated mesoscale convective vortices this is observed in both aircraft data and satellite imagery where subsequent interactions including mergers with the low level tropical depression vortex are observed a contour dynamics experiment suggests that the movement of mesoscale convective systems in satellite imagery corresponds well to the movement of their associated midlevel vortices results from a simple baroclinic experiment show that the midlevel vortices affect the large scale low level circulation in two ways 1 initially interactions between midlevel vortices produce a combined vortex of greater depth 2 interaction between midlevel vortices and the low level circulation produces a development downward of the midlevel vorticity this strengthens the surface vortex and develops a more cohesive vortex that extends from the surface through the midtroposphere

numerical simulations of the genesis of hurricane diana 1984 part ii sensitivity of track and intensity prediction abstract the authors examine numerous simulations that probe the dynamics governing the intensification and track of tropical cyclone diana 1984 simulated in part i the development process is fundamentally dependent on a preexisting upper tropospheric trough ridge couplet this couplet focuses mesoscale vertical motion that in turn produces lower tropospheric potential vorticity pv anomalies which form the seed for the tropical storm removal of this trough ridge couplet from the initial conditions eliminates cyclogenesis the simulated rate of development of diana in the prehurricane phase depends principally on choices of cumulus parameterization boundary layer treatment sea surface temperature and grid spacing simulations with cumulus schemes that allow more grid scale precipitation on the 9 km grid exhibit unrealistic grid scale overturning and slower intensification primarily due to production of cyclonic vorticity anomalies at large radii use of an innermost nest with 3 km grid spacing

pre eruptive water content of rhyolitic magmas as determined by ion microprobe analyses of melt inclusions in phenocrysts abstract the ion microprobe was used to analyze trapped melt inclusions in phenocrysts from two rhyolitic eruptions for h2o f and incompatible trace elements eleven melt inclusions from phenocrysts in air fall tephra from obsidian dome near long valley usa gave an average of 4 1 1 2 wt h2o and showed large variations in f and incompatible trace elements reflecting a complex history reported to involve mixing of at least two magma types eight inclusions in phenocrysts from the taupo ultraplinian event new zealand gave 4 5 0 8 wt h2o with small variations in the other elements analyzed measured water contents are similar to earlier estimates of the pre eruptive water contents of these and similar rhyolites and major and trace element analyses of inclusions compare closely with bulk analyses suggesting that the analyses of inclusions represent the chemistry of the magma at the time of entrapment

geology of the peralkaline volcano at pantelleria strait of sicily situated in a submerged continental rift pantelleria is a volcanic island with a subaerial eruptive history longer than 300 ka its eruptive behavior edifice morphologies and complex multiunit geologic history are representative of strongly peralkaline centers it is dominated by the 6 km wide cinque denti caldera which formed ca 45 ka ago during eruption of the green tuff a strongly rheomorphic unit zoned from pantellerite to trachyte and consisting of falls surges and pyroclastic flows soon after collapse trachyte lava flows from an intracaldera central vent built a broad cone that compensated isostatically for the volume of the caldera and nearly filled it progressive chemical evolution of the chamber between 45 and 18 ka ago is recorded in the increasing peralkalinity of the youngest lava of the intracaldera trachyte cone and the few lavas erupted northwest of the caldera beginning about 18 ka ago inflation of the chamber opened old ring fractures and new radial fractures along which recently differentiated pantellerite constructed more than 25 pumice cones and shields continued uplift raised the northwest half of the intracaldera trachyte cone 275 m creating the island s present summit montagna grande by trapdoor uplift pantellerite erupted along the trapdoor faults and their hingeline forming numerous pumice cones and agglutinate sheets as well as five lava domes degassing and drawdown of the upper pantelleritic part of a compositionally and thermally stratified magma chamber during this 18 3 ka episode led to entrainment of subjacent crystal rich pantelleritic trachyte magma as crenulate inclusions progressive mixing between host and inclusions resulted in a secular decrease in the degree of evolution of the 0 82 km3 of magma erupted during the episode

numerical study of convection observed during the winter monsoon experiment using a mesoscale two dimensional model abstract a two dimensional version of the pennsylvania state university mesoscale model has been applied to winter monsoon experiment data in order to simulate the diurnally occurring convection observed over the south china sea the domain includes a representation of part of borneo as well as the sea so that the model can simulate the initiation of convection also included in the model are parameterizations of mesoscale ice phase and moisture processes and longwave and shortwave radiation with a diurnal cycle this allows use of the model to test the relative importance of various heating mechanisms to the stratiform cloud deck which typically occupies several hundred kilometers of the domain frank and cohen s cumulus parameterization scheme is employed to represent vital unresolved vertical transports in the convective area the major conclusions are ice phase processes are important in determining the level of maximum large scale heating and vertical motion because there is a strong anvil componen

what percentage of western north pacific tropical cyclones form within the monsoon trough abstractit is frequently stated that 70 80 of western north pacific tropical cyclones form within the monsoon trough but without an objective definition of the term several definitions are tested here when the monsoon trough mt is defined as the contiguous region where long term 1988 2010 mean july november 850 hpa relative vorticity is positive 73 of all july november tropical cyclones form within the mt this percentage varies interannually however from as low as 50 to nearly 100 the percentage correlates with the nino 3 4 index with more storms forming within the mt during warm periods when the mt is defined instead using long term monthly mean 850 more than 80 of tropical cyclones form within the mt in all months except july and august when more than 30 of storms form poleward of the mt it is hypothesized that the known peak in the frequency of upper tropospheric midlatitude wave breaking in july and august is responsible it is argued that any long term mean provides a suita

evolution and emplacement of high fluorine rhyolites in the mesoproterozoic gawler silicic large igneous province south australia the gawler range volcanics grv and the hiltaba suite hs of south australia form a silicic dominated large igneous province the gawler slip emplaced in an intracontinental setting during the mesoproterozoic emplacement of the grv lasted for a short period of time 2 ma and can be separated into two main phases the first phase lower grv is composed of thick 3 km sequences erupted from distinct centres and includes small to moderate volume up to 150 km3 felsic lavas ignimbrites and minor mafic and intermediate lavas the upper grv include extensive felsic lavas that are up to 1000 of km3 in volume and 200 km across using well preserved quartz hosted melt inclusions we investigated the composition of the lower grv including major trace and volatile elements the results indicate high concentrations of k2o 7 8 wt rare earth and high field strength elements and low concentrations of ca mg ni cr sr and ba in comparison with felsic continental crust overall melt inclusion compositions match whole rock geochemical characteristics we demonstrate that the grv magma was f rich 1 3 wt and had high temperature for a silicic magma high f concentrations and high temperature would have resulted in lower than usual polymerisation of the melt and relatively low viscosity these characteristics help explain how very voluminous felsic magma was erupted effusively and emplaced as lavas other intracontinental slip contain extensive felsic lavas and ignimbrites which appear to share similar geochemical characteristics we also show that selective alteration caused depletion of whole rock compositions in some trace elements namely pb u and sn