airglow observations and modeling of f region depletion zonal velocities over christmas island 1 we report image measurements of plasma depletions in the equatorial f region ionosphere over christmas island 2 1 n 157 4 w dip latitude 2 8 n in the central pacific ocean the observations were made during the equinox period september october 1995 using a utah state university ccd imaging system filtered to observe thermospheric o i 630 0 nm airglow emissions centered at 280 km altitude well defined magnetic field aligned depletions were observed on 18 nights during the campaign including strong postmidnight fossilized structures enabling detailed measurements of their morphology and dynamics the number of depletions was influenced by their initial onset times and their persistence the separations between adjacent depletions ranged from 150 to 250 km in good agreement with prior observations from other sites however measurements of their eastward zonal drift speeds indicated normal behavior peaking around 90 100 m s prior to local midnight with exceptionally high velocities 80 m s during the postmidnight period that persisted until dawn these results differ markedly from optical measurements at similar equatorial latitudes but at different longitude sectors suggesting that the zonal drift velocities can have a significant longitudinal dependence model drift velocities calculated using a simple electric field model with winds defined by the horizontal wind model hwm 07 produced an eastward drift throughout the night but their postmidnight magnitudes were much smaller than observed using a modified hwm 07 wind field a basic nighttime trend similar to the christmas island trend was successfully obtained

interferometer studies of equatorial f region irregularities and drifts a radar interferometer technique developed at jicamarca peru and first used to study electrojet irregularities has now been used successfully to study plasma turbulence in the equatorial f region our first results have shown that the most turbulent echoes appear to come from a region that extends for tens of kilometers in altitude but for only a kilometer or less in the east west direction this slab may very well be the wall of a depleted region a plasma bubble sometimes the irregularities can be tracked as they move eastward or westward velocity profiles for the evening period obtained in this way show a strong shear with westward velocities at the lowest altitudes observed and eastward velocities above a plausible explanation for this shear is that the westward drifts are driven by electric fields produced by westward e region winds and mapped up along magnetic field lines while at higher heights where the electron density is greater the drifts are controlled by the f region dynamo driven by eastward winds

did magma ascent rate control the explosive effusive transition at the inyo volcanic chain california rhyolitic eruptions often begin explosively and then shift to extrusions of lava it is widely believed that the style of eruption is dependent on magma flow rate within the conduit fast ascent leading to explosive eruption and slow ascent favoring effusive behavior currently the velocity of rhyolite magma flowing in a conduit is unknown and therefore eruption models remain untested against geologic evidence of magma ascent we show that explosively erupted rhyolite magma from the inyo volcanic chain ascended slowly centimeters per second at same rates as the dome building phases we propose that the explosive effusive transition sub plinian rhyolite eruptions is governed by the alleviation of exsolved volatile pressure in the shallow conduit rather than the magma flow rate

storm time electric fields in the equatorial ionosphere observed near the dusk meridian 1 we compare the dynamics of plasma drifts vp and ionospheric electric fields ei measured near the dusk meridian at the magnetic equator by the defense meteorological satellite program dmsp f13 satellite with those of the dawn to dusk component of the interplanetary electric field ief ey during four large magnetic storms presented data indicate that during the main phase of the magnetic storms when the interplanetary magnetic field imf had a southward component vp had upward and westward components that ranged up to 120 and 210 m s 1 the corresponding eastward and upward components of ei were approximately 5 and 10 of ey the persistence of eastward ei for many hours indicates that electric field shielding of the inner magnetosphere is ineffective during prolonged periods of southward imf during periods of northward imf ey 0 ionospheric responses depended on prior history no ionospheric response is apparent after strong northward imf turnings preceded by geomagnetic quiet if they occurred after a long southward interval ei often had westward and downward components the polarity of the zonal component of ei remained well correlated with that of ey for up to 21 h during periods of slowly varying imf phenomena associated with southward imf periods are consistent with models that allow for prolonged penetration electric field from high latitudes superposed on those of the sq current system however it is not understood why the remarkable correspondence between the polarities of the zonal component of ei and the ief ey occurs during northward imf in some ey 0 periods the vertical component of ei remained upward dmsp detections of upward ei westward vp during periods of ey 0 are not predicted by existing models

on estimating crystal shape for crystal size distribution analysis abstract the accurate determination of three dimensional crystal habit long intermediate short axis is a requirement to enable correct stereological conversion of 2d crystal size measurements to true 3d crystal size distributions csds in this contribution we introduce a database and spreadsheet program which provides an objective estimate of true crystal habit from raw 2d measurements the database compares the sample s 2d measurements with 2d shape curves for random sections through 703 different habits the output gives the five best match curves and corresponding crystal habits based on a least squares fit between sample and database the minimum sample size required to give good shape estimates from 2d data is tested using random runs of sections through known shapes with different population densities at minimum 75 crystal sections are required to robustly determine crystal habit for csd measurements if crystals are tabular in shape with more acicular shapes requiring a minimum of 250 sections suggesting a sample size of 250 sections to be used for shape determination in natural examples where the true 3d shape is unknown the crystal habit of the best match shape can then be used to provide a more robust csd analysis which in turn can be used to investigate magmatic processes with more certainty

average equatorial zonal and vertical ion drifts determined from san marco d electric field measurements san marco d electric field measurements have been averaged in terms of their equivalent ion drift to produce an average pattern of equatorial zonal and vertical ion drifts variations with season solar activity kp lunar phase and longitude have been analyzed similarities and some differences from previous jicamarca de 2 and ae e results are seen confirmation is given of the dominance of the f region dynamo in the 1900 2100 local time region the daytime zonal ion drift is larger for high f10 7 values than that for low values there is little variation between high and low values of kp superrotation is evident in this data set and is larger at equinox compared to solstice at the june solstice there are significant differences between the average ion drifts in the longitude sector where the geomagnetic equator is north of the geographic equator indian sector and the sector where the geomagnetic equator is south of the geographic equator peruvian sector the daytime upward velocity is larger in the indian sector than in the peruvian sector and it reverses later in the evening in the indian sector daytime westward zonal velocities are larger and the nighttime eastward velocities are smaller in the indian sector a presunrise enhancement is seen in the downward velocity in the indian sector but not in the peruvian sector significant variations are also seen with the phase of the moon in light of current theory the lunar variations suggest a complex interaction of e and f region dynamo sources with conductivity changing in phase and character with latitude

groundmass crystallization of mount st helens dacite 1980 1986 a tool for interpreting shallow magmatic processes the 1980 1986 eruption of mount st helens volcano provides an unprecedented opportunity to observe the evolution of a silicic magma system over a short time scale groundmass plagioclase size measurements are coupled with measured changes in matrix glass plagioclase and fe ti oxide chemistry to document increasing groundmass crystallinity and thus to better constrain proposed physical models of the post may 18 1980 magmatic reservoir measurements of plagioclase microlite and microphenocryst sizes demonstrate that relatively rapid growth approximately 10 9 cm s of groundmass plagioclase occurred immediately subsequent to may 18 relatively rapid plagioclase growth continued through the end of 1980 at an average rate of 3x10 11 cm s plagioclase growth rates then decreased to 1x10 11 cm s through 1986 changes in groundmass crystallinity are reflected in changes in both matrix glass and plagioclase microphenocryst rim chemistry although the matrix glass composition appears to have remained approximately constant from 1981 1986 after a rapid compositional change from may 18 until the end of 1980 plagioclase microphenocrysts show increasingly more complex zoning patterns with time microphenocryst core compositions are commonly positively correlated with crystal size both of these observations indicate continuous groundmass plagioclase growth through 1986 magmatic temperatures estimated from fe ti oxide pairs are approximately constant through 1981 at eruption temperatures of 930 c and at log fo2 of 10 8 by 1985 1986 oxide temperatures decreased to 870 c chemical and textural changes can be explained by 1 rapid degassing and crystallization in response to the intrusion of magma into a shallow 4 5 km reservoir toward the end of the may 18 1980 eruption 2 continued crystallization at a much reduced rate through 1986 due to slow cooling of the shallow magma reservoir growth rates and consequent chemical changes appear to decrease at the end of 1980 this is coincident with the change in eruption style from explosive eruptions sometimes followed by dome growth to solely extrusive dome building events and can be explained by the expected viscosity increase of both degassing and increasing crystallinity the model of twostage crystallization of magma in a shallow reservoir is consistent with conclusions from gas studies casadevall et al 1983 gerlach and casadevall 1986 a b patterns of crater deformation chadwkck et al 1988 and post 1980 seismicity endo et al 1990 although it does not explain the experimental data of hill and rutherford 1989 on the growth rate of amphibole reaction rims textural measurements on mount st helens dacite can also be used to evaluate crystallization kinetics in silicic magmas systems for which experimental data is almost non existent plagioclase growth rates are 5 10 times slower than estimated plagioclase growth rates in basaltic systems a result consistent with the higher viscosity of a more silicic melt furthermore patterns of textural change both average crystal size and number density are similar to those observed during the 1984 mauna loa eruption by lipman and banks 1987 suggesting that the only modification to the crystallization behavior of plagioclase required in extrapolation from basaltic systems is a moderate decrease in rates such that the rate of crystallization scales with the melt viscosity

electrodynamics of the equatorial evening ionosphere 1 importance of winds in different regions the importance of winds at different altitudes and latitudes for the electrodynamics of the low latitude evening ionosphere is examined with a model of the global coupled ionosphere thermosphere system the model reproduces the main observed features of the evening equatorial plasma vortex and the prereversal enhancement pre of the vertical drift the electrodynamics is driven primarily by the zonal wind forced by the diurnally varying zonal pressure gradient force the zonal wind lags the zonal pressure gradient force owing to inertia when ion drag is important the time lag of the wind behind the pressure gradient force is shortened and the high altitude evening wind turns eastward earlier than the wind at lower altitudes where ion drag is less important therefore a vertical shear of the zonal wind tends to develop at altitudes around the transition between small and large ion drag at the bottom of the f region this wind shear is closely associated with the vertical shear in the zonal convection velocity that is part of the evening plasma vortex unlike previous studies we find that the winds driving the pre lie mainly on field lines with apexes above the peak of the equatorial f layer field lines that extend in magnetic latitude out to nearly 30 and encompass the entire evening equatorial ionization anomaly region contrary to previous suggestions the westward convection in the bottomside of the evening plasma vortex is found to weaken rather than strengthen the pre daytime winds have relatively little influence on the low latitude evening electrodynamics

the association of lava dome growth with major explosive activity vei 4 domehaz a global dataset investigation of the global eruptive records of particular types of volcanoes is a fundamental and valuable method of understanding what style of activity can be anticipated in the future and can highlight what might be expected or unusual in particular settings this paper investigates the relationship between large explosions volcanic explosivity index vei 4 and lava dome growth from 1000 ad to present and develops the domehaz database domehaz contains information from 397 dome forming episodes including duration of dome growth duration of pauses in extrusion extrusion rates and the timing and magnitude vei of associated large explosions major explosive activity when associated with dome growth is more likely to occur before dome growth rather than during or at the end of dome forming eruptions in most cases where major explosive activity has been associated with dome growth the eruptions occurred at basaltic andesite to andesitic volcanoes the most common type of dome forming volcano but a greater proportion of dacitic and rhyolitic dome growth episodes were associated with large explosions high extrusion rates 10 m3 s 1 seem to be associated with large explosions and may inhibit degassing or destabilize existing domes leading to explosive decompression large explosions may alternatively be followed by dome growth which represents the clearing of residual magma from the conduit relationships extracted from the global record can be used to construct probability trees for new and ongoing dome forming eruptions or can be used in conjunction with other types of event trees to aid in forecasting volcanic hazards during a crisis especially for volcanoes where data are sparse

crystallization of mount st helens 1980 1986 dacite a quantitative textural approach quantitative measurements of crystal size distributions csds have been used to obtain kinetic information on crystallization of industrial compounds randolph and larson 1971 and more recently on hawaiian basalts cashman and marsh 1988 the technique is based on a population balance resulting in a differential equation relating the population densityn of crystals to crystal sizel i e at steady staten no exp l itg whereno is nucleation density g is the average crystal growth rate is the average growth time and the nucleation ratej nog csd inn vsl plots of plagioclase phenocrysts in 12 samples of mount st helens blast dacite and 14 samples of dacite from the 1980 1986 mount st helens dome are similar and averageg 9 6 1 1 10 3 cm andno 1 2 106 cm 4 reproducibility of the measurements was tested by measuring csds of 12 sections cut from a single sample in three mutually perpendicular directions precision of the size distributions is good in terms of relative but not necessarily absolute values 10 growth and nucleation rates for plagioclase have been calculated from these measurements using time brackets of 30 150 years growth ratesg are 3 10 10 12cm s and nucleation ratesj are 5 21 10 6 cm3 s g andj for fe ti oxides calculated from csd data areg 2 13 10 13 cm sec andj 7 33 10 5 cm3 s respectively the higher nucleation rate and lower growth rate of oxides resulted in a smaller average crystal size than for plagioclase sizes of plagioclase microlites 0 01 mm in the blast dacite groundmass have been measured from backscatter sem photographs nucleation of these microlites was probably triggered by intrusion of material into the cone of mount st helens in spring 1980 this residence time of 52 days gives minimum crystallization estimates ofg 1 3 10 11 cm s andj 9 16 1o3 cm3 s the skeletal form of the microlites provides evidence for nucleation and growth at high values of undercooling t relative to the phenocrysts a comparison of nucleation and growth rates for the two crystal populations phenocrysts vs microlites suggests that while growth rate seems to be only slightly affected by changes in t nucleation rate is a very strong function of undercooling a comparison of plagioclase nucleation and growth rates measured in the mount st helens dacite and in basalt from makaopuhi lava lake in hawaii suggests that plagioclase nucleation rates are not as dependent on composition groundmass textures suggest that plagioclase microphenocrysts crystallized at depth rather than in the conduit in the dome or after extrusion onto the surface most of this crystallization appears to be in the form of crystal growth coarsening of groundmass microphenocrysts at small degrees of undercooling rather than extensive nucleation of new crystals this continuous crystallization in a shallow magmatic reservoir may provide the overpressurization needed to drive the continuing periodic domebuilding extrusions which have been the pattern of activity at mount st helens since december 1980

syneruptive mixing degassing and crystallization at redoubt volcano eruption of december 1989 to may 1990 abstract the 1989 1990 eruption of redoubt volcano included initial energetic vertical venting of pumiceous tephra followed by shedding of dense pyroclastic flows from successive unstable domes the suite of quenched magmatic materials thus produced coupled with detailed seismic observations provides a record of establishment and subsequent evolution of an active magma column in the upper crust the initial eruptive event involved a dacite magma with rhyolite melt and an andesite magma with dacite melt the dacite magma contains reaction rim free hornblende high cl concentrations 0 12 0 02 wt of melt and is microlite free all evidence for rapid ascent from depth in contrast the andesite component is microlite rich though this characteristic may have arisen by mixing between the two magmas rather than by slow ascent abundant dense juvenile lava like clasts with low h2o 0 1 0 3 wt were ejected in the initial eruption event implying that the magma column had reached the near surface and its upper portion had undergone degassing and bubble collapse prior to explosive disruption later eruptive events contain evidence of progressive blending of the two magmatic components as well as mixing between fresh reservoir derived and old conduit resident magma magma apparently ascended to the surface from the 6 10 km deep source chamber in a series of pulses rather than by steady flow or gradual emptying of a shallowly intruded body increased shallow long period seismicity which may indicate subsurface degassing from the flowing magma column occurred 5 days before each major eruptive event petrologic evidence indicates that each eruption contained some magma that ascended quickly from the chamber with time the proportion of new magma in eruptive batches may have decreased however even the final dome contains magma that left the chamber no more than a few days before extrusion together the pulses apparently record passage of an andesite plume through redoubt s dacite magma reservoir

overview of the 2006 eruption of mt merapi abstract in the last part of the 20th century and the beginning of the 21st century mt merapi in central java indonesia erupted about every 2 5 years most of the eruptions were low in explosivity with vei 3 or less eruptions usually involve the formation of a lava dome either in the beginning or in the end of the eruptive episode for the 2006 eruption the precursory signal was first observed in the middle of the year 2005 with a decrease in edm slope distances to points on the rim an increase of seismicity and a possible increase of so 2 emissions those early events marked the beginning of a more continuous period of inflation which led to the eruption in total the pre eruption displacement of the southern rim reached at least 2 4 m toward the measuring station in kaliurang kal from late april until june 2006 a lava dome grew on the summit with a volume that gradually increased until it reached about 4 1 million m 3 in 38 days the total of erupted magma was about 5 3 million m 3 dense rock equivalent dre the dome subsequently collapsed in three steps from june 4 to june 14 leaving an open scar on its southeast side in this paper we detail the changes of dome morphology that were monitored by taking successive photographs from similar positions the eruption in 2006 marked a significant change in summit morphology from west southwestward opening during the 20th century to the currently southeast orientation also an mw 6 4 earthquake occurred on 26 may midway through the eruption which adds interesting questions about the relationship of the eruption and the earthquake edm data from 2006 and previous eruptions show that the summit remains inflated after each eruption i e no significant deflation occurs following eruptions the lack of post eruption deflation suggests that magma remains in the shallow parts of the edifice after the eruption as a result the complex of summit lava domes and their intrusive roots grow with time and merapi s rim and summit become progressively more unstable and prone to collapse

insights into processes and deposits of hazardous vulcanian explosions at soufriere hills volcano during 2008 and 2009 montserrat west indies during the soufriere hills eruption vulcanian explosions have generally occurred 1 in episodic cycles 2 isolated during pauses in extrusion and 3 after major collapses of the dome in a different eruptive context significant vulcanian explosions occurred on 29 july 2008 3 december 2008 and 3 january 2009 deposits are pumiceous except for the 3 december event we reconstructed the dispersal pattern of the deposits and their textural characteristics to evaluate erupted volume and vesicularity of the magma at fragmentation we discuss the implications of these explosions in terms of eruptive processes and chronology and the hazards posed by their sudden and often unheralded occurrence we suggest that overpressurization of the conduit can develop over time scales of months to weeks by a process of self sealing of conduit walls and or the cooling dome by silica polymorphs this work provides new insights for understanding the generation of hazardous vulcanian explosions at andesitic volcanoes

controls on conduit magma flow dynamics during lava dome building eruptions 1 lava dome eruptions commonly display fairly regular alternations between periods of high and low or no activity with timescales typically of weeks to years and sudden transitions between effusive and explosive activity we develop a transient model of the magma flow in a volcanic conduit from an open system magma chamber with continuous replenishment the model incorporates gas exsolution bubble growth gas escape through the magma and decompression induced crystallization and considers variations in magma temperature water content conduit diameter phenocryst size chamber volume and magma rheology calculations show the presence of periodic variations in discharge rate due to the transition from a stable regime when discharge rate is low and crystals grow efficiently leading to high magma viscosity to another stable state when discharge rate is high and crystallization is negligible the difference in discharge rates between these regimes can be several orders of magnitude periods are similar to the observed timescales and mainly depend on the chamber volume the system shows strongly nonlinear responses to the variation of governing parameters if magma has a bingham rheology pauses in discharge rate occur between peaks of discharge and the peaks are much higher than for the case of newtonian rheology large changes in discharge rate and eruptive behavior can occur as the consequence of small changes in magma temperature water content phenocryst size distribution or conduit diameter the system can fluctuate between low and high discharge rates with transitions to explosive activity

analysis of the seismic activity associated with the 2010 eruption of merapi volcano java abstract the 2010 eruption of merapi is the first large explosive eruption of the volcano that has been instrumentally observed the main characteristics of the seismic activity during the pre eruptive period and the crisis are presented and interpreted in this paper the first seismic precursors were a series of four shallow swarms during the period between 12 and 4 months before the eruption these swarms are interpreted as the result of perturbations of the hydrothermal system by increasing heat flow shorter term and more continuous precursory seismic activity started about 6 weeks before the initial explosion on 26 october 2010 during this period the rate of seismicity increased almost constantly yielding a cumulative seismic energy release for volcano tectonic vt and multiphase events mp of 7 5 10 10 j this value is 3 times the maximum energy release preceding previous effusive eruptions of merapi the high level reached and the accelerated behavior of both the deformation of the summit and the seismic activity are distinct features of the 2010 eruption the hypocenters of vt events in 2010 occur in two clusters at of 2 5 to 5 km and less than 1 5 km depths below the summit an aseismic zone was detected at 1 5 2 5 km depth consistent with studies of previous eruptions and indicating that this is a robust feature of merapi s subsurface structure our analysis suggests that the aseismic zone is a poorly consolidated layer of altered material within the volcano deep vt events occurred mainly before 17 october 2010 subsequent to that time shallow activity strongly increased the deep seismic activity is interpreted as associated with the enlargement of a narrow conduit by an unusually large volume of rapidly ascending magma the shallow seismicity is interpreted as recording the final magma ascent and the rupture of a summit dome plug which triggered the eruption on 26 october 2010 hindsight forecasting of the occurrence time of the eruption is performed by applying the material failure forecast method ffm using cumulative real time seismic amplitude rsam calculated both from raw records and on signals classified according to their dominant frequency stable estimates of eruption time with errors as small as 4 h are obtained within a 6 day lapse time before the eruption this approach could therefore be useful to support decision making in the case of future large explosive episodes at merapi

study of the evening plasma drift vortex in the low latitude ionosphere using san marco electric field measurements san marco d electric field measurements have been averaged in terms of the equivalent plasma drifts for days near the 1988 autumn equinox the observed plasma drifts provide a satellite based composite in altitude and local time of the evening enhancements in the zonal and vertical drifts and the shear in the zonal drift the satellite data reveal details of the postsunset plasma drift vortex which have been studied in radar and chemical release experiments the averaged drift data show a proportional relationship between the prereversal enhancement of the vertical drift and the zonal derivative of the zonal drift this relationship suggests that the vertical drift enhancement is caused by a curl free response to the rapid increase of the zonal drift which in turn is tied to the f region zonal winds and the e region sunset the data also show a curl free drop off in altitude of the vertical plasma drift above and below the peak drift at 450 km altitude during the prereversal enhancement comparisons of the averaged data with results of coupled ionosphere electrodynamics models are used to examine the underlying causes of the evening plasma drifts the evening plasma drift vortex is shown to be a single drift structure with a single source the source of the evening drift vortex resides on the bottomside of the integrated f region just as originally supposed by rishbeth 1971 this dominant single source which is produced by the divergence of the wind driven current of the f region causes the evening enhancement of the eastward plasma drift the shear in the eastward drift and the prereversal enhancement of the vertical drift

insights into the october november 2010 gunung merapi eruption central java indonesia from the stratigraphy volume and characteristics of its pyroclastic deposits abstract the 2010 eruption of merapi was the second most deadly in the historic record of this volcano claiming over 380 lives by relating the observations of this eruption with detailed examination of deposit distribution stratigraphy and sedimentology a reconstruction of the properties of the pyroclastic density currents pdcs is presented including the valley controlled block and ash flows bafs and widespread energetic pyroclastic surges the distribution volume and mobility characteristics of all types of pdc during the eruption sequence show evidence for levels of intensity unseen since the large scale 1872 and 1930 eruption phases especially during the climactic events of october 26 and november 5 many tephra falls interbedded with pdc units show that most dome collapse events occurred along with and between explosive vulcanian eruptions the 2010 eruption produced very long runout bafs reaching 16 1 km in the kali gendol on november 5 this runout could be explained by its large volume 20 million m 3 around 10 times that of previous merapi bafs during the last 130 years major avulsion of these dense bafs to form overbank deposits became more common through the eruptive sequence as the valley was progressively filled with successive pdc deposits spreading avulsed bafs were a particular hazard downstream of 10 km where the landscape is less dissected less clear however is why pyroclastic surges extended up to 10 km from the vent on november 5 and 6 4 km on october 26 these expanded much farther from baf margins 2 km than ever seen before at merapi in one location they were decoupled from valley centered bafs with high momentum traveling initially laterally across steep valley systems before draining downslope at this site on the western side of the upper gendol at around 3 km from source surge decoupling was apparently exacerbated by upstream collision and deflection of high flux hot and gas rich bafs against the cliffs of gunung kendil the 1 4 km long cliff face was impacted directly for the first time in 2010 events and may have been responsible for the formation of larger than normal turbulent ash rich clouds above bafs these results imply that future eruption events under the present summit and upper flow path configuration are also highly likely to generate wide dispersal pyroclastic surges and extreme hazard especially now that dense forest has been destroyed on the upper southern slopes of the volcano

transitions between explosive and effusive phases during the cataclysmic 2010 eruption of merapi volcano java indonesia transitions between explosive and effusive activity are commonly observed during dome forming eruptions and may be linked to factors such as magma influx ascent rate and degassing however the interplay between these factors is complex and the resulting eruptive behaviour often unpredictable this paper focuses on the driving forces behind the explosive and effusive activity during the well documented 2010 eruption of merapi the volcano s largest eruption since 1872 time controlled samples were collected from the 2010 deposits linked to eruption stage and style of activity these include scoria and pumice from the initial explosions dense and scoriaceous dome samples formed via effusive activity as well as scoria and pumice samples deposited during subplinian column collapse quantitative textural analysis of groundmass feldspar microlites including measurements of areal number density mean microlite size crystal aspect ratio groundmass crystallinity and crystal size distribution analysis reveal that shallow pre and syn eruptive magmatic processes acted to govern the changing behaviour during the eruption high an up to 80 mol an microlites from early erupted samples reveal that the eruption was likely preceded by an influx of hotter or more mafic magma transitions between explosive and effusive activity in 2010 were driven primarily by the dynamics of magma ascent in the conduit with degassing and crystallisation acting via feedback mechanisms resulting in cycles of effusive and explosive activity explosivity during the 2010 eruption was enhanced by the presence of a plug of cooled magma within the shallow magma plumbing system which acted to hinder degassing leading to overpressure prior to initial explosive activity

topographic and stochastic influences on pahoehoe lava lobe emplacement a detailed understanding of p hoehoe emplacement is necessary for developing accurate models of flow field development assessing hazards and interpreting the significance of lava morphology on earth and other planetary surfaces active p hoehoe lobes on k lauea volcano hawai i were examined on 21 26 february 2006 using oblique time series stereo photogrammetry and differential global positioning system measurements during this time the local discharge rate for peripheral lava lobes was generally constant at 0 0061 0 0019 m3 s but the areal coverage rate of the lobes exhibited a periodic increase every 4 13 0 64 min this periodicity is attributed to the time required for the pressure within the liquid lava core to exceed the cooling induced strength of its margins the p hoehoe flow advanced through a series of down slope and cross slope breakouts which began as 0 2 m thick units i e toes that coalesced and inflated to become approximately meter thick lobes the lobes were thickest above the lowest points of the initial topography and above shallow to reverse facing slopes defined relative to the local flow direction the flow path was typically controlled by high standing topography with the zone directly adjacent to the final lobe margin having an average relief that was a few centimeters higher than the lava inundated region this suggests that toe scale topography can at least temporarily exert strong controls on p hoehoe flow paths by impeding the lateral spreading of the lobe observed cycles of enhanced areal spreading and inflated lobe morphology are also explored using a model that considers the statistical likelihood of sequential breakouts from active flow margins and the effects of topographic barriers

mineral chemistry of hydrothermal biotite from the kahang porphyry copper deposit ne isfahan central province of iran abstract the kahang porphyry cu deposit located northeast of isfahan city in central of iran is associated with a composite miocene stock and ranges in composition from diorite through granodiorite to quartz monzonite field observations and petrographic studies show that the emplacement of the kahang stock occurred in several pulses each associated with its related hydrothermal activity early hydrothermal alteration started with a potassic style in the central part of the system and produced a secondary biotite k feldspar magnetite assemblage accompanied by chalcopyrite and pyrite mineralization propylitic alteration that took place at the same time as the potassic alteration occurred in the peripheral portions of the stock subsequent phyllic alteration overprinted earlier potassic and propylitic alterations biotite grains from the potassic and phyllic zones show distinct chemical compositions the feo tio2 mno k2o and na2o concentrations in biotite from the phyllic alteration zone are lower than those from the potassic alteration zone the f and cl contents of biotite from the potassic alteration zone display relatively high positive correlation with the xmg the fluorine intercept values iv f from the potassic and phyllic alteration zones are strongly correlated with the fluorine chlorine intercept values iv f cl biotite geothermometry for the potassic and phyllic alteration zones based on the biotite geothermometer of beane 1974 yields a temperature range of 422 to 437 c mean 430 c and 329 to 336 c mean 333 c respectively the position of data in log xf xoh ratio vs xmg and xfe diagram suggests that biotite formed under dissimilar composition and temperature conditions in the potassic and phyllic alteration zones calculated log fugacity ratios of fh2o fhf fh2o fhcl and fhf fhcl show that hydrothermal fluids associated with the potassic alteration were distinctively different from those fluids associated with the phyllic alteration zone at kahang porphyry cu deposit the results of this research indicate that the chemistry of biotite is related to the chemical composition of the magma and the prevailing physical conditions during crystallization

the prereversal enhancement of the zonal electric field in the equatorial ionosphere the electric fields in the ionospheric e and f regions near the magnetic equator often show a strong eastward enhancement shortly after sunset and before the eastward normally daytime field reverses to westward several theoretical models of the low latitude fields suggest that this enhancement is caused mainly or entirely by f region winds the f region dynamo but some authors have suggested that it could be produced solely by e region tidal winds we give here additional calculations and arguments in support of the f region source the enhancement of the eastward field for an eastward f region wind turns out to be a simple direct consequence of the fact that after sunset the ionospheric conductivity decreases far more rapidly in the e region than in the f region

cause of different local time distribution in the postsunset equatorial ionospheric irregularity occurrences between june and december solstices 1 global averaged postsunset equatorial ionospheric density irregularity occurrences observed by rocsat during the moderate to high solar activity years of 1999 to 2004 indicate a different local time distribution between june and december solstices the irregularity occurrences during the december solstice show a faster increase rate to peak at 2100 2200 local time while the irregularity occurrences during the june solstice have a slower increase rate and peak one hour later in local time than that in the december solstice the cause of such different local time distributions is attributed to a large contrast in the time of zonal drift reversal and the magnitude of postsunset vertical drift observed by rocsat at longitudes of large magnetic declination in the two solstices that is a delay in the zonal drift reversal in association with a smaller postsunset vertical drift observed at longitudes of positive magnetic declination has greatly inhibited the irregularity occurrences during the june solstice in contrast to an earlier zonal drift reversal together with a large vertical drift occurring at longitudes of negative magnetic declination to accelerate the irregularity occurrences during the december solstice we think that the different geomagnetic field strengths that existed between the longitudes of positive and negative magnetic declinations have played a crucial role in determining the different local time distributions of irregularity occurrences for the two solstices

a model of degassing at galeras volcano colombia 1988 1993 galeras volcano reactivated in early 1988 as evidenced by increased seismicity gas emissions and fumarole temperatures and had a series of explosive eruptions in may 1989 during 1989 1990 the volcano was characterized by high so 2 flux reaching 5000 t metric tons day this period of strong degassing was followed by upward movement of magma july september 1991 and the emplacement of a lava dome october november 1991 since november 1991 long period seismicity and so 2 flux have decreased indicating that the conduit has partially sealed itself probably by crystallization and solidification of the magma on july 16 1992 the dome was destroyed by an explosion consistent with pressurization due to sealing and degassing in a nonporous medium continued pressurization beneath the volcano is indicated by eruptions on january 14 march 23 april 4 april 13 and june 7 1993 glass inclusion and matrix glass analyses support a hypothesis of significant magma degassing at depth in the magma chamber and in the conduit this degassing has reduced the h 2 o s and cl contents of the magma but the f content appears to have increased due to crystallization of anhydrous mineral phases and to the low fluid melt partition coefficient for f despite its degassed state galeras is currently a dangerous volcano because the sealing process forces gas saturated magma to release volatile components into confined spaces thereby causing pressurization and explosive eruptions

textural and micro petrological variations in the eruptive products of the 2006 dome forming eruption of merapi volcano indonesia implications for sub surface processes the interplay between magma ascent degassing and changing magmatic properties are widely recognized as critical factors controlling the style of silicic volcanic eruptions microlite textures in samples from the prolonged dome forming eruption of merapi in 2006 provide a record of changing magmatic ascent conditions and shallow conduit processes throughout the eruption analysis of microlite textural parameters including measurements of areal number density na mean microlite size crystal aspect ratio and groundmass crystallinity combined with the monitoring record and field observations indicate that magma ascent paths change between continuous ascent at varying rates from a deeper magma storage region to ascent being temporarily stalled at shallow depths in the latter stages of the eruption supporting the idea of an ephemeral shallow magma storage region at merapi plagioclase microlite compositions show evidence of decompression induced degassing often displaying rims of anorthoclase and more k rich alkali feldspar sanidine anorthite contents also support the textural data of later erupted magma being temporarily stalled at shallow depths crystal size distributions csds are interpreted to show that both growth dominated and nucleation dominated crystallisation regimes existed during the 2006 eruption resulting from changing conditions of undercooling t during variable magma ascent paths by contrast microlite textural analysis and feldspar microlite compositions of samples from the fast growing lava dome of the second phase of the 2010 eruption prior to the cataclysmic events on 5 november indicate faster ascent rates a crystallisation regime more strongly dominated by nucleation due to high t and interaction of the 2010 magma with more hotter magma from depth

equatorial plasma bubble zonal velocity using 630 0 nm airglow observations and plasma drift modeling over ascension island 1 we present oi 630 0 nm airglow image data from ascension island geographic 7 9 s 14 4 w dip latitude 16 s in the southern atlantic ocean taken with the utah state university all sky ccd camera during 20 march to 7 april 1997 in order to study plasma bubbles occurring in the low latitude nighttime ionosphere the initial plasma bubble onset occurs in the early evening hours at 19 15 20 00 lst and is followed by eastward propagation with an average speed of 90 120 m s prior to local midnight rapidly decreasing around the midnight and postmidnight periods the ascension results are compared with similar observations from christmas island in order to examine the longitudinal variations of epb development and propagation the observed epb velocities from ascension island are also compared with the results of a plasma drift model in a case study during the night of 4 5 april the velocity reveals unusual latitudinal shear up to 0 12 m s km with a reversal to westward flow at low latitudes while eastward flow is maintained at higher latitudes consequently the bubble rotates counterclockwise and tilts eastward significantly away from alignment with the geomagnetic field lines the westward reversal of the drift motion near the geomagnetic equator is most likely the result of a reversal in the f region dynamo or from a large increase in the altitude of the shear node in the f region plasma drift at the geomagnetic equator

the magmatic feeding system of el reventador volcano sub andean zone ecuador constrained by texture mineralogy and thermobarometry of the 2002 erupted products abstract after a 26 years long quiescence el reventador an active volcano of the rear arc zone of ecuador entered a new eruptive cycle which lasted from 3 november to mid december 2002 the initial sub plinian activity vei 4 with andesite pyroclastic falls and flows shifted on 6 and 21 november to an effusive stage characterized by the emission of two lava flows andesite to low silica andesite lava 1 and basaltic andesite lava 2 containing abundant gabbro cumulates the erupted products are medium to high k calc alkaline and were investigated with respect to major element oxides mineral chemistry texture and thermobarometry inferred pre eruptive magmatic processes are dominated by the intrusion of a high t mafic magma possibly up to 1165 15 c into an andesite reservoir acting as magma mixing and trigger for the eruption before this refilling the andesite magma chamber was characterized by water content of 5 3 1 0 high oxygen fugacity nno 2 and temperatures in the upper and lower part of the reservoir of 850 and 952 65 c respectively accurate amphibole based barometry constrains the magma chamber depth between 8 2 and 11 3 km 2 2 km the 6 october 2002 seismic swarm hypocenters from 10 to 11 km preceding el reventador eruption supports the intrusion of magmas at these depths the widespread occurrence of disequilibrium features in most of the andesites e g complex mineral zoning and phase overgrowths indicates that convective self mixing have been operating together with fractional crystallization inferred from the cognate gabbro cumulates before the injection of the basic magma which then gave rise to basaltic andesite and low silica andesite hybrid layers magma mixing in the shallow chamber is inferred from the anomalous sio 2 al 2 o 3 whole rock pattern and strong olivine disequilibria both lavas show three types of amphibole breakdown rims mainly due to heating mixing processes and or relatively slow syn eruptive ascent rate decompression of the magmas the lack of any disequilibrium textures in the pumices of the 3 november fall deposit suggest that pre eruptive mixing did not occur in the roof zone of the chamber a model of the subvolcanic feeding system of el reventador consistent with the intrusion of a low al 2 o 3 crystal rich basic magma into an already self mixed andesite shallow reservoir is here proposed it is also inferred that before entering the shallow chamber the basaltic magma underwent a polybaric crystallization at deeper crustal levels

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

on monthly seasonal longitudinal variations of equatorial irregularity occurrences and their relationship with the postsunset vertical drift velocities 1 monthly variation of global equatorial density irregularity distribution has been obtained with the data taken by rocsat 1 at the 600 km topside ionosphere from march 1999 to june 2004 during high to moderate solar activity years this global distribution of monthly irregularity occurrence rate not only provides the best spatial temporal distribution in existence so far but also serves to fill the gap of irregularity distribution missing over some eastern pacific region where no ground observation is available the 5 1 2 year result of the monthly occurrence pattern indicates a smooth variation across the longitudes contrary to some beliefs that a drastic change in irregularity occurrence pattern has occurred across the eastern pacific longitudes excellent agreement is noted for the current results with aarons 1993 conjectured sketch of the global scintillation distribution furthermore the seasonal longitudinal s l variations of quiet time postsunset vertical drift velocities are found to track closely with the s l variations of irregularity occurrences linear regression analysis between the vertical drift velocity and the irregularity occurrence rate indicates that the vertical drift velocities at three different longitude zones of magnetic declination have good correlations with the irregularity occurrences for all seasons this implies that the averaged postsunset vertical drift velocity is a good indicator for the occurrences of equatorial density irregularities in a longitude region of similar magnetic declination the smooth variation of monthly seasonal longitudinal distributions of the vertical drift velocities that results in the smooth variation of topside density irregularity occurrences implies that the global occurrence pattern of either distribution is controlled by the smooth global variation of the postsunset ionospheric condition related to the magnetic declination effect and the seasonal variation of the ionospheric density level at the dip equator located with respect to the geographic equator there seems little need to include the global distribution of instability perturbation seeds from atmospheric disturbances to complete the global distribution of topside irregularity occurrences

revised global model of thermosphere winds using satellite and ground based observations thermospheric wind data obtained from the atmosphere explorer e and dynamics explorer 2 satellites have been combined with wind data for the lower and upper thermosphere from ground based incoherent scatter radar and fabry perot optical interferometers to generate a revision hwm90 of the hwm87 empirical model and extend its applicability to 100 km comparison of the various data sets with the aid of the model shows in general remarkable agreement particularly at mid and low latitudes the ground based data allow modeling of seasonal diurnal variations which are most distinct at mid latitudes while solar activity variations are now included they are found to be small and not always very clearly delineated by the current data they are most obvious at the higher latitudes the model describes the transition from predominately diurnal variations in the upper thermosphere to semidiurnal variations in the lower thermosphere and a transition from summer to winter flow above 140 km to winter to summer flow below significant altitude gradients in the wind are found to extend to 300 km at some local times and pose complications for interpretation of fabry perot observations

reconstruction of short term palaeoceanographic changes during the formation of the late albian niveau breistroffer black shales oceanic anoxic event 1d se france the niveau breistroffer black shale succession in the vocontian basin se france is the regional equivalent of the widely distributed late albian oceanic anoxic event 1d the studied black shale rich interval at the col de palluel section is 6 28 m thick and comprises four black shale units with up to 2 5 wt total organic carbon toc intercalated with marlstones calcareous nannofossil palynomorph planktic foraminifera and stable isotopic data from the niveau breistroffer succession suggest that short term climate changes influenced its deposition with relatively warm and humid climate during black shale formation in comparison with relatively cool and dry climatic conditions during marlstone deposition an increase in the terrigenous marine ratio of palynomorphs indicates enhanced humidity and higher runoff during black shale formation a nutrient index based on calcareous nannofossils and the abundance pattern of small 63 125 m hedbergellid foraminifera show short term changes in the productivity of the surface water surface water productivity was reduced during black shale formation and increased during marlstone deposition a calcareous nannofossil temperature index and bulk rock oxygen isotope data indicate relative temperature changes with warmer surface waters for black shale samples at these times warm humid climate and reduced surface water productivity were accompanied by greater abundances of subsurface dwelling calcareous nannofossils nannoconids and planktic foraminifera rotaliporids these taxa presumably indicate more stratified surface water conditions we suggest that the formation of the niveau breistroffer black shales occurred during orbitally induced increase in monsoonal activity that led to increasing humidity during periods of black shale formation this in turn caused a decrease in low latitude deep water formation and probably an increase in surface water stratification the combination of these two mechanisms caused depleted o 2 concentrations in the bottom water that increased the preservation potential of organic matter