sulfur dioxide from nevado del ruiz volcano colombia total flux and isotopic constraints on its origin abstract nevado del ruiz volcano has been releasing extraordinarily large volumes of sulfur dioxide gas from its arenas crater since september 1985 and probably since the renewal of volcanic activity began in late 1984 the combined release from eruptive and passive flux has been approximately 3 4 1 0 10 6 metric tons in the four years after november 1984 this value combined with data of others on pre eruption sulfur content of the magma allows the calculation of a minimum required volume of magma of 0 92 km 3 consideration of the continued high flux and typical ratios of erupted to degassed magma indicate that the true minimum volume of magma involved is probably at least 4 6 9 2 km 3 a systematic study of the various species of sulfur in the volcanic plume and the hydrothermal system has provided a characterization of the geochemistry and sulfur isotopic variation consideration has been given to five potential sources of the sulfur dioxide dissolution of subvolcanic evaporites remobilization and oxidation of native sulfur within summit crater fumaroles large scale release from the hydrothermal system by reduction of sulfate or water rock interaction assimilation and oxidation of a pre existing sulfide deposit and magmatic volatiles the data are most realistically explained by the passage of magmatic gas through the hydrothermal system with some disproportionation of sulfur dioxide to produce the high sulfate content and low ph of the waters this model is also most consistent with the distribution of acid sulfate chloride hot springs high chloride and fluoride content of the hydrothermal waters sulfide deposition within the volcanic pile and the transient increase in sulfate content of the acid sulfate chloride hot springs that reached a maximum approximately one year after the november 1985 eruption the magmatic model is in apparent conflict with the absence of general deformation from the time that data began to be collected about one week before the eruption until the time this manuscript was finalized september 1989 although this conflict cannot be resolved with certainty it is noted that the absence of deformation is only an indication of the lack of magma movement not of the absence of magma an alternative magmatic model considers the available geological data and proposes that the system is related to a large scale caldera in this model the degassing is related to a long term process of exsolution of magmatic volatiles from a large volume of crystallizing magma left from the construction of modern ruiz this model can better explain the combined geological and geophysical data as well as the geochemical and isotopic data the nature of the degassing data at ruiz is interesting because it is an example of an explosive eruption cycle in which the gas flux is actually dominated by passive release the nature of the degassing allows us to hypothesize that some major change in the system occurred in april may 1986 and to predict that the present high flux can be expected to last for another decade

dyking at epr 16 n hypermagmatic ridge segment insights from near seafloor magnetics abstract high resolution near seafloor magnetic data have been acquired over the 16 n hypermagmatic segment of the east pacific rise using an autonomous underwater vehicle this survey proves to be ideal to test the relative efficiency of various inversion methods applied to data acquired at a more or less constant altitude above the seafloor unlike other methods a recently published bayesian inversion preserves the short wavelengths and allows for the resolution of a high resolution reduced to the pole magnetic anomaly this anomaly unveils the presence of several laterally adjacent dykes associated with individually separated axial summit troughs the observation of such anomalies and therefore of shallow dykes confirms the hypermagmatic character of the segment in a location where complex magma chambers have been imaged in seismic reflection studies variable intensity of the magnetic anomalies reflects the depth of the dyke swarms and ultimately the timing and style of eruptive events helping to constrain the spreading axis evolution

potential field inversion for a layer with uneven thickness the tyrrhenian sea density model abstract inversion of large scale potential field anomalies aimed at determining density or magnetization is usually made in the fourier domain the commonly adopted geometry is based on a layer of constant thickness characterized by a bottom surface at a fixed distance from the top surface we propose a new method to overcome this limiting geometry by inverting in the usual iterating scheme using top and bottom surfaces of differing but known shapes randomly generated synthetic models will be analyzed and finally performance of this method will be tested on real gravity data describing the isostatic residual anomaly of the southern tyrrhenian sea in italy the final result is a density model that shows the distribution of the oceanic crust in this region which is delimited by known structural elements and appears strongly correlated with the oceanized abyssal basins of vavilov and marsili

a revised magnitude scale for south australian earthquakes a new local magnitude scale for south australian earthquakes has been developed from first principles the scale is defined by m l s a 0 7 log 10 a w a 1 10 log 10 0 0013 c i where awa is the equivalent wood anderson trace amplitude in millimeters is the epicentral distance in kilometers and ci is a station correction the formula accommodates the local attenuation function for south australia it is significantly less than richter s log a function but agrees closely with the recent results of boore and hutton 1984 for southern california the attenuation constants and the station corrections were determined by detailed statistical analysis of data from hundreds of earthquakes station corrections vary from 0 6 to 0 2 magnitude units and show a broad correlation with the regional geology south australian earthquake magnitudes are obtained from short period vertical component seismographs of dissimilar frequency response to that of the horizontal wood anderson instrument preliminary analysis of three component data from the longer period characteristic adelaide ade seismic station suggests that the departure from standard instrument type makes little difference to actual magnitude determinations

a geomagnetic record over the last 3 5 million years from deep tow magnetic anomaly profiles across the central indian ridge high resolution records of the geomagnetic field intensity over the last 4 myr provided by paleomagnetic analyses of marine sediments have shown the occurrence of short lived low field intensity features associated with excursions or short polarity intervals in order to evaluate the ability of marine magnetic anomalies to record the same geomagnetic events we have collected six deep tow 500 m above the seafloor and several sea surface magnetic anomaly profiles from the central indian ridge across the brunhes matuyama and gauss chrons i e from the ridge axis to anomaly 2a after removal of topography latitude and azimuth effects we converted distances into time sequences using well dated polarity reversal anomalies as tie points we calculated the average signal to test the robustness of the short wavelength anomalies the resulting stacked profile is very similar to stacked sea surface and downward continued profiles from the central indian ridge the east pacific rise and the pacific antarctic ridge our results suggest that in addition to polarity reversals to previously suggested geomagnetic events subchrons or excursions within the brunhes and matuyama chrons a new small scale magnetic anomaly likely generated by several closely spaced geomagnetic field intensity variations represent the major contributor to the detailed shape of recent marine magnetic anomalies in investigated areas we observe a dense succession of microanomalies that are correlated excursions ontong java 1 and 2 and gilsa is found after the olduvai chron the near bottom results support the existence of three geo magnetic features between the gauss matuyama boundary and olduvai they also suggest three geomagnetic events during the c2a i n subchron within the gauss chron this study emphasizes the potential of deep tow magnetic surveys in detecting fluctuations in geo magnetic field intensity and in particular short lived excursions a poorly constrained part of the geomagnetic field temporal variation spectrum

magma at depth a retrospective analysis of the 1975 unrest at mount baker washington usa mount baker volcano displayed a short interval of seismically quiescent thermal unrest in 1975 with high emissions of magmatic gas that slowly waned during the following three decades the area of snow free ground in the active crater has not returned to pre unrest levels and fumarole gas geochemistry shows a decreasing magmatic signature over that same interval a relative microgravity survey revealed a substantial gravity increase in the 30 years since the unrest while deformation measurements suggest slight deflation of the edifice between 1981 83 and 2006 07 the volcano remains seismically quiet with regard to impulsive volcano tectonic events but experiences shallow 10 km long period earthquakes reviewing the observations from the 1975 unrest in combination with geophysical and geochemical data collected in the decades that followed we infer that elevated gas and thermal emissions at mount baker in 1975 resulted from magmatic activity beneath the volcano either the emplacement of magma at mid crustal levels or opening of a conduit to a deep existing source of magmatic volatiles decadal timescale multi parameter observations were essential to this assessment of magmatic activity

electromagnetic induction studies in the eyre peninsula south australia magnetic field fluctuations have been recorded by an array of portable three component magnetometers at 60 sites across the eyre peninsula in south australia between december 1993 and march 1995 an additional 54 magnetometer data records collected prior to 1989 and described by milligan 1989 and milligan white chamalaun 1989 were included in the analysis a major conductive feature in the crust first noted by white milligan 1984 as the eyre peninsula anomaly epa is re examined to assess its continuity to the north of the original arrays and to investigate its relationship with major tectonic features magnetic field time series were converted to induction arrows in the frequency domain these induction arrows were initially inverted using the minimum structure 2 d occam approach to estimate the electrical conductance of the crust following this thin sheet forward modelling was used to examine the relationship between the conductance and the dominant tectonic features the principal results of the modelling are that a narrow conductive feature extends inland from the coast about 160 km before terminating and the conductance is in the range 3000 to 10 000 s which decreases inland a strong correlation exists between the electrical conductance of the eyre peninsula and bouguer gravity anomalies and in particular the epa is coincident with a significant bouguer gravity gradient there is also good agreement between the locations of the foci of earthquakes of magnitude greater than 4 0 and the epa we believe that the anomaly is associated with a geological fracture in the precambrian upper crust as a result of crustal extension prior to the rifting of australia from antarctica in the jurassic 160 ma

direct rate measurements of eruption plumes at augustine volcano a problem of scaling and uncontrolled variables the march april 1986 eruption of augustine volcano alaska provided an opportunity to directly measure the flux of gas aerosol and ash particles during explosive eruption most previous direct measurements of volcanic emission rates are on plumes from fuming volcanoes or on very small eruption clouds direct measurements during explosive activity are needed to understand the scale relationships between passive degassing or small eruption plumes and highly explosive events conditions on april 3 1986 were ideal high winds clear visibility moderate activity three measurements were made 1 an airborne correlation spectrometer cospec provided mass flux rates of so2 2 treated filter samples chemically characterized the plume and 3 a quartz crystal microcascade impactor provided particle size distribution atmospheric conditions on april 3 caused the development of a lee wave plume which allowed us to constrain a model of plume dispersion leading to a forecast map of concentrations of so2 at greater distances from the vent on april 3 1986 the emission rate of so2 at augustine was 24 000 t d one of the largest direct volcanic rate measurements yet recorded with a cospec the results coupled with analytical results from samples simultaneously collected on filters allow us to estimate hcl emissions at 10 000 t d and ash eruption rate at 1 5 106 t d based on other data this ash eruption rate is about 1 50 of the maximum rate during the march april activity filter samples show that the gas aerosol proportions for sulfur and chlorine are about 10 1 and 4 1 respectively by contrast measurements of augustine s plume together with ground based gas sampling in july 1986 when the volcano was in a posteruptive fuming state are 380 t d so2 and approximately 8000 t d hcl with no ash emission the observations of large cl releases at augustine support the cl abundance conclusions of johnston 1980 based on study of melt inclusions in the 1976 lavas the results reinforce the need for more measurements during eruptions and for better understanding of scaling of volcanic emissions of various eruptive components

evaluation of sulfur dioxide emissions from explosive volcanism the 1982 1983 eruptions of galunggung java indonesia abstract galunggung volcano java awoke from a 63 year quiescence in april 1982 and erupted sporadically through january 1983 during its most violent period from april to october the cikasasah volcano observatory reported 32 large and 56 moderate to small eruptions from april 5 through september 19 the total ozone mapping spectrometer toms carried on nasa s nimbus 7 satellite detected and measured 24 different sulfur dioxide clouds an estimated 1730 kilotons kt of so2 were outgassed by these explosive eruptions the trajectories and rapid dispersion rates of the so2 clouds were consistent with injection altitudes below the tropopause an additional 300 kt of so2 were estimated to have come from 64 smaller explosive eruptions based on the detection limit of the toms instrument for the first time an extended period of volcanic activity was monitored by remote sensing techniques which enabled observations of both the entire so2 clouds produced by large explosive eruptions using toms and the relatively lower levels of so2 emissions during non explosive outgassing using the correlation spectrometer or cospec based on cospec measurements from august 1982 to january 1983 and on the relationship between explosive and non explosive degassing approximately 400 kt of so2 were emitted during non explosive activity the total sulfur dioxide outgassed from galunggung volcano from april 1982 to january 1983 is calculated to be 2500 kt 30 from both explosive and non explosive activity while galunggung added large quantities of sulfur dioxide to the atmosphere its sporadic emissions occurred in relatively small events distributed over several months and reached relatively low altitudes and are unlikely to have significantly affected aerosol loading of the stratosphere in 1982 by volcanic activity

fluid evolution and graphite genesis in the deep continental crust metamorphism in deep continental crust is often accompanied by carbon rich h2o poor fluids1 5 and is characterized by the development of granulite facies mineral assemblages3 5 graphite a minor phase in many high grade metamorphic rocks6 17 has also been recognized in recent xenoliths of deep crustal material18 demonstrating its presence in modern deep continental crust although graphite is most common in biotite schists it has also been recorded in pyroxene gneisses two feldspar quartz pegmatites marbles and quartzo feldspathic gneisses11 15 19 the presence of supracrustal sequences in granulite facies regions6 19 suggests that the latter developed through progressive metamorphism and dehydration of shallow level crustal material much of this material must have experienced a metamorphic recrystallization in the amphibolite facies accompanied by the development of hornblende although graphite in the high grade rock may develop from oxidation of organic material this may not be the most common means for graphite genesis in much of west greenland14 and in southern norway12 the low grade hornblende bearing material does not contain significant quantities of organic compounds although graphite is found in higher grade metamorphic equivalents this strongly suggests that introducing co2 into deep crustal material leads to fluid evolution and graphite formation i outline here the evolution of fluid composition in the c o h system which is in equilibrium with common crustal mineral assemblages thermodynamic data for co2 h2o mixtures20 and mineral components21 are used to model the chemical changes in fluid composition which accompany introduction of co2 into various types of rock the results demonstrate that graphite genesis is a direct consequence of deep crustal metamorphism in the presence of a co2 rich fluid phase in addition the calculations demonstrate that the deep continental crust will not be a reservoir for large volumes of methane rich gas

gas emissions from failed and actual eruptions from cook inlet volcanoes alaska 1989 2006 cook inlet volcanoes that experienced an eruption between 1989 and 2006 had mean gas emission rates that were roughly an order of magnitude higher than at volcanoes where unrest stalled for the six events studied mean emission rates for eruptions were 13 000 t d co2 and 5200 t d so2 but only 1200 t d co2 and 500 t d so2 for non eruptive events failed eruptions statistical analysis suggests degassing thresholds for eruption on the order of 1500 and 1000 t d for co2 and so2 respectively emission rates greater than 4000 and 2000 t d for co2 and so2 respectively almost exclusively resulted during eruptive events the only exception being two measurements at fourpeaked while this analysis could suggest that unerupted magmas have lower pre eruptive volatile contents we favor the explanations that either the amount of magma feeding actual eruptions is larger than that driving failed eruptions or that magmas from failed eruptions experience less decompression such that the majority of h2o remains dissolved and thus insufficient permeability is produced to release the trapped volatile phase or both in the majority of unrest and eruption sequences increases in co2 emission relative to so2 emission were observed early in the sequence with time all events converged to a common molar value of c s between 0 5 and 2 these geochemical trends argue for roughly similar decompression histories until shallow levels are reached beneath the edifice i e from 20 35 to 4 6 km and perhaps roughly similar initial volatile contents in all cases early elevated co2 levels that we find at these high latitude andesitic arc volcanoes have also been observed at mid latitude relatively snow free basaltic volcanoes such as stromboli and etna typically such patterns are attributed to injection and decompression of deep co2 rich magma into a shallower chamber and open system degassing prior to eruption here we argue that the c s trends probably represent tapping of vapor saturated regions with high c s and then gradual degassing of remaining dissolved volatiles as the magma progresses toward the surface at these volcanoes however c s is often accentuated due to early preferential scrubbing of sulfur gases the range of equilibrium degassing is consistent with the bulk degassing of a magma with initial co2 and s of 0 6 and 0 2 wt respectively similar to what has been suggested for primitive redoubt magmas

grain boundary graphite in rocks and implications for high electrical conductivity in the lower crust the origin of zones of high electrical conductivity in the lower continental crust is a long standing mystery possible explanations include the presence of brines1 2 partial melt3 serpentine4 and graphite5 when discussing the occurrence of graphite in the crust many petrologists have considered phase relations as they would have existed at the peak of metamorphism or during igneous emplacement6 10 here we show that a fine film of graphite is present on the grain boundaries in three rocks from the laramie anorthosite complex in two of these rocks graphite was stable during igneous crystallization but in the other it was not we maintain that in all of the rocks the grain boundary graphite precipitated from a co2 rich fluid during cooling the chemical processes that produced the grain boundary graphite in these rocks are likely to operate in many lower crustal rocks we therefore contend that because the films we observe are capable of producing the high conductivity that is seen in the lower crust grain boundary graphite should be considered as a possible cause for some conductivity anomalies

monitoring the 2004 andesitic block lava extrusion at volcan de colima mexico from seismic activity and so2 emission the 2004 andesitic block lava extrusion at volcan de colima mexico was accompanied and followed by numerous seismic signals associated with rockfalls pyroclastic flows and explosive events we analyze temporal variations in the number of rockfalls and explosions the seismic signal duration of rockfalls and the energy of the explosion and compare this with both the rate of magma discharge and so2 emission characteristics of seismic signals and so2 emission are compared with those observed during the 1998 1999 volcan de colima block lava extrusion for both eruptions the explosive activity was low during the lava extrusion and increased after its termination the variation in the daily number and the total duration of rockfall seismic signals gives a good reflection of the development of the lava emission process an increase in magmatic degassing so2 flux was observed some days before the onset of lava extrusion the degassing strongly decreased some days before the peak in the rate of the 1998 1999 lava emission but reached its peak together with the peak in the rate of the 2004 lava emission these features of seismic activity and so2 emission demonstrate that they are good tools for monitoring the extrusion process

electrical conductivity anomalies and their relationship with the tectonics of south australia summary geomagnetic variation studies have been conducted in the gawler craton and adelaide geosyncline of south australia the magnetometer stations extend from the coast up to the southern edge of the 1970 array of gough mcelhinny lilley the coast effect is the dominant feature of the data but use is made of the hypothetical event technique to identify two zones of telluric current concentration both of them appear to be associated with linear zones of enhanced electrical conductivity within the crust the southern eyre peninsula anomaly lies within the gawler craton and may identify a major fracture or shear in the upper crust the conductivity anomaly within the adelaide geosyncline appears to be the continuation of the flinders anomaly discovered by the 1970 array study it correlates well with the arcuate fold pattern of the southern flinders zone of the

magnetic anomalies at the puna ridge a submarine extension of kilauea volcano implications for lava deposition high resolution magnetic data collected along the axis and the south flank of the puna ridge the submarine extension of the east rift zone of kilauea volcano hawaii aid in the interpretation of magmatic processes crustal magnetization varies along the crest of the puna ridge three prominent magnetization highs are located between the depths of 1500 and 2500 m each of the magnetization highs is associated with distinct volcanic features a flow field on top of fissured terrain a lava plateau and a staircase of lava terraces the crustal magnetization highs can be explained by the presence of these volcanic features taking into account water depth and the size of the volcanic features and making simple assumptions about the rock magnetization no measurements of rock magnetization have been made on puna ridge samples we assume two values 50 and 10 a m based on values from mid ocean ridge basalts and the estimated ages of puna ridge rocks clague et al 1995 the two shallowest magnetization highs are adjacent to a landslide scarp lava flows overtop the scarp suggesting that a dike has been emplaced and eruptions have occurred since the landslide formed a possible scenario is that the collapse of the flank led to dike intrusion and eruption at this location because of a reduction in the horizontal stress within the edifice our observations suggest that flows are starting to fill in the gap caused by the landslide to rebuild this section of the north flank

character mass distribution and origin of tephra fall deposits of the 1989 1990 eruption of redoubt volcano south central alaska abstract the 1989 1990 eruption of redoubt volcano spawned about 20 areally significant tephra fall deposits between december 14 1989 and april 26 1990 tephra plumes rose to altitudes of 7 to more than 10 km and were carried mainly northward and eastward by prevailing winds where they substantially impacted air travel commerce and other activities in comparison to notable eruptions of the recent past the redoubt events produced a modest amount of tephra fall deposits 6 10 7 to 5 10 10 kg for individual events and a total volume dense rock equivalent of about 3 5 10 7 m 3 of andesite and dacite two contrasting tephra types were generated by these events pumiceous tephra fall deposits of december 14 and 15 were followed on december 16 and all later events by fine grained lithic crystal tephra deposits much of which fell as particle aggregates the change in the character of the tephra fall deposits reflects their fundamentally different modes of origin the pumiceous deposits were produced by magmatically driven explosions the finegrained lithic crystal deposits were generated by two processes hydrovolcanic vent explosions generated tephrafall deposits of december 16 and 19 such explosions continued as a tephra source but apparently with diminishing importance during events of january and february ash clouds of lithic pyroclastic flows generated by collapse of actively growing lava domes probably contributed to tephra fall deposits of all events from january 2 to april 26 and were the sole source of tephra fall for at least the last 4 deposits

character mass distribution and origin of tephra fall deposits from the 2009 eruption of redoubt volcano alaska highlighting the significance of particle aggregation abstract the 2009 eruption of redoubt volcano included 20 tephra producing explosions between march 15 2009 and april 4 2009 utc next generation radar nexrad data show that plumes reached heights between 4 6 km and 19 km asl and were distributed downwind along nearly all azimuths of the volcano explosions lasted between 0 8 mm thick including communities along the kenai peninsula 80 100 km and the city of anchorage 170 km trace ash on march 15 a small 4 6 km asl phreatic explosion containing minor non juvenile ash erupted through the summit ice cap the first five magmatic explosions events 1 5 occurred within a 6 hour period on march 23 plumes rose to heights between 5 5 km and 14 9 km asl during 2 to 20 minute duration explosions and were dispersed mainly along a nne trajectory trace ash fall was reported as far as fairbanks owing to a shift in wind direction and heavy snowfall during these events field discrimination among many of these layers was possible all deposits comprise a volumetrically significant amount of particle aggregates yet only event 5 deposits contain coarse clasts including glacier ice the most voluminous tephra fall was deposited on march 24 event 6 from a 15 minute explosion that sent a plume to 18 3 km asl and dispersed tephra to the wnw within 10 km of the vent this deposit contains 1 11 cm pumice clasts in a matrix of 1 2 mm aggregate lapilli a small dome was presumably emplaced between march 23 and march 26 and was subsequently destroyed during 1 14 minute magmatic explosions of events 7 8 march 26 that sent plumes between 8 2 km and 19 km asl ash fell along a broad swath to the ese covering communities along the kenai peninsula with up to 1 mm of ash proximal deposits are largely composed of aggregate lapilli of 1 2 mm with very little coarse juvenile material events 9 18 march 27 sent plumes between 5 2 km and 15 5 km asl during particle size data showing a preponderance of fine ash even in the most proximal locations along with the abundance of aggregate lapilli documented in most samples confirms that particle aggregation played a significant role in the 2009 eruption and induced premature fallout of fine ash

time varying effects in magnetic mapping amphidromes doldrums and induction hazard a magnetic amphidrome is defined as a place where changes of the magnetic field over time as measured by a total field magnetometer are reduced to the point of being negligible the reduction is caused by destructive interference between the vertical and horizontal components of the time varying field at an ideal amphidrome variations with time are suppressed completely and the total magnetic field magnitude is steady such a phenomenon may be expected to depend on the frequency content of the time variations in the vertical and horizontal components the subject is treated first in terms of the quiet daily variation sq which is studied on a global basis it is seen that there are magnetic latitude bands north and south of the equator where the quiet daily variation is minimal these zones are called the diurnal doldrums in addition to this global pattern the magnetic daily variation is modified by earth s conductivity structure locally and sq amphidrome behavior may be aided or obstructed lo

dynamics of carbon dioxide emissions crystallization and magma ascent hypotheses theory and applications to volcano monitoring at mount st helens measurements of co2 fluxes from open vent volcanos are rare yet may offer special capabilities for monitoring volcanos and forecasting activity the measured fluxes of co2 and so2 from mount st helens decreased from july through november 1980 but the record includes variations of co2 so2 in the emitted gas and episodes of greatly increased fluxes of co2 we propose that the co2 flux variations reflect two gas components a a component whose flux decreased in proportion to 1 t with a co2 so2 mass ratio of 1 7 and b a residual flux of co2 consisting of short lived large peaks with a co2 so2 mass ratio of 15 we propose two hypotheses a the 1 t dependence was generated by crystallization in a deep magma body at rates governed by diffusion limited heat transfer and b the gas component with the higher co2 so2 was released from ascending magma which replenished the same magma body the separation of the total co2 flux into contributions from known processes permits quantitative inferences about the replenishment and crystallization rates of open system magma bodies beneath volcanos the flux separations obtained by using two gas sources with distinct co2 so2 ratios and a peak minus background approach to obtain the co2 contributions from an intermittent source and a continuously emitting source are similar the flux separation results support the hypothesis that the second component was generated by episodic magma ascent and replenishment of the magma body the diffusion limited crystallization hypothesis is supported by the decay of minimum co2 and so2 fluxes with 1 t after 1 july 1980 we infer that the magma body at mount st helens was replenished at an average rate 2 8 106 m3 d 1 which varied by less than 5 during july august and september 1980 the magma body volume 2 4 3 0 km3 in early 1982 was estimated by integrating a crystallization rate function inferred from co2 fluxes to maximum times 20 4 years estimated from the increase of sample crystallinity with time these new volcanic gas flux separation methods and the existence of relations among the co2 flux crystallization rates and magma body replenishment rates yield new information about the dynamics of an open vent replenished magma body

australia s southern margin a product of oblique extension abstract recently developed detachment models of continental margin formation interpret the southern margin of australia to have formed when the lower plate australian margin was pulled out from beneath the upper plate antarctic margin data now available and summarised in this paper point very strongly to a generally nw se direction of initial continental extension for the southern margin in contrast to the widely held picture of simple n s rifting the evidence for this extension direction comes from the analysis of deep seismic data acquired by the bureau of mineral resources in 1986 in the central great australian bight gab the gravity field of the gab seismic and magnetic basement structures in the eyre sub basin polda trough ceduna depocentre and duntroon basin and from the analysis of the magnetic seafloor spreading anomalies produced during the slow first phase of drifting between australia and antarctica further it is now believed that the formation of the southern margin of australia can be described in terms of three phases of continental extension el to e3 and two phases of drifting d1 and d2 in summary these phases were as follows 1 e1 approximately 300 km of late jurassic or older to early cretaceous nw se oriented extension in the gab with strike slip motion in the nascent otway basin and along the tasmanian margin 2 e2 120 km of early cretaceous nne ssw oriented extension which formed the basins of southeastern australia otway bass gippsland and which probably produced a structural overprinting in the gab basin 3 e3 d1 minor continental extension and the first 500 km of slow drift between australia and antarctica on an azimuth of 165 wrenching on the tasmanian margin 4 d2 2600 km of fast drifting between australia and antarctica on a n s azimuth

extremely asymmetric magmatic accretion of oceanic crust at the ends of slow spreading ridge segments we report the results of a deep towed magnetic survey of part of a mid atlantic ridge spreading segment analysis of the magnetic reversals indicates that for the past 0 7 m y magmatic accretion at the end of the segment has been effectively one sided with new crust being added only to the outside corner of the ridge offset eastern flank and not to the inside corner western flank spreading on the inside corner was accommodated by significant displacement on a single large fault the area between the fault and the axial volcanic ridge was effectively a thin static sliver at the plate boundary during this process in the short term asymmetric magmatic accretion was probably accomplished by progressively shifting the axial volcanic ridge to a new location at the inside corner western edge of the old one asymmetric spreading is unlikely to be sustainable as a steady state process the termination of a period of asymmetric spreading may be achieved either by establishing a new axial volcanic ridge to a position on the outside corner eastern plate thus isolating the old axial volcanic ridge on the inside corner plate or by simply arresting movement on the large fault and reverting to symmetric spreading at the axial volcanic ridge highly asymmetric accretion may be a common process at slow spreading segments particularly near discontinuities this asymmetry cannot be maintained for long periods and may be directly linked to intervals of spreading by tectonic extension

locating the spreading axis along 80 km of the mid atlantic ridge south of the atlantis transform the zone of active eruptive fissuring the spreading axis is investigated at an accretionary segment of the mid atlantic ridge high resolution side scan images are used to produce a detailed geologic map of the median valley floor of segment 18 located immediately south of the atlantis transform 30 n the spreading axis is defined by distinguishing between primary eruptive vents seamounts and hummocky ridges fed by underlying dikes and secondary vents seamounts and terraces fed from lava tubes or channels and is found to be about 2 km wide or less along the entire segment it is impossible to narrow the width further with the data we have in hand for comparison at faster spreading ridges such as the east pacific rise the spreading axis is generally equated to the width of the axial summit trough typically 50 100 m wide within the brunhes anomaly the high resolution structure of the central anomaly magnetic high camh obtained from several cross axis deep tow tracks is used to define regions of recent major volcanism the locations of the camh peaks generally conform to our pick of the spreading axis in some places the peaks are located several kilometers away from the spreading axis possibly indicating that large volumes of lava have been deposited there our interpretation of many volcanic structures as secondary vents at segment 18 leads to the conclusion that lava channels and tubes commonly develop to transport lava to these off axis deposition sites

photogrammetric monitoring of lava dome growth during the 2009 eruption of redoubt volcano abstract the 2009 eruption of redoubt volcano alaska began with a phreatic explosion on 15 march followed by a series of at least 19 explosive events and growth and destruction of at least two and likely three lava domes between 22 march and 4 april on 4 april explosive activity gave way to continuous lava effusion within the summit crater we present an analysis of post 4 april lava dome growth using an oblique photogrammetry approach that provides a safe rapid and accurate means of measuring dome growth photogrammetric analyses of oblique digital images acquired during helicopter observation flights and fixed wing volcanic gas surveys produced a series of digital elevation models dems of the lava dome from 16 april to 23 september the dems were used to calculate estimates of volume and time averaged extrusion rates and to quantify morphological changes during dome growth effusion rates ranged from a maximum of 35 m 3 s 1 during the initial two weeks to a low of 2 2 m 3 s 1 in early summer 2009 the average effusion rate from april to july was 9 5 m 3 s 1 early rapid dome growth was characterized by extrusion of blocky lava that spread laterally within the summit crater in mid to late april the volume of the dome had reached 36 10 6 m 3 roughly half of the total volume and dome growth within the summit crater began to be limited by confining crater walls to the south east and west once the dome reached the steep north sloping gorge that breaches the crater growth decreased to the south but the dome continued to inflate and extend northward down the gorge effusion slowed during 16 april 1 may but in early may the rate increased again this rate increase was accompanied by a transition to exogenous dome growth from mid may to july the effusion rate consistently declined the decrease is consistent with observations of reduced seismicity gas emission and thermal anomalies as well as declining rates of geodetic deflation or inflation these trends suggest dome growth ceased by july 2009 the volume of the dome at the end of the 2009 eruption was about 72 10 6 m 3 more than twice the estimated volume of the largest dome extruded during the 1989 1990 eruption in total the 2009 dome extends over 400 m down the glacial gorge on the north end of the crater with a total length of 1 km width of 500 m and an average thickness of 200 m

geomagnetic field variability during the cretaceous normal superchron during the cretaceous normal superchron 121 83 million years ago the polarity of the earth s geomagnetic field remained stable for an unusually long time deep tow magnetic data suggest that despite the stability of the polarity the field varied greatly throughout the interval

electrical resistivities of crustal fluids results of calculations on the electrical resistivities of crustal fluids as a function of temperature pressure and fluid composition indicate that crustal fluids can range in resistivity from approximately 1 m the dominant contribution to decreasing the resistivities of crustal fluids comes from increased concentrations of chloride and bicarbonate salts and increased temperature and density of the fluids calculations indicate that the sharpest decrease in fluid resistivity occurs at shallow depths 10 km and temperatures 300 c can not be readily explained as a product of variations in resistivities of crustal fluids these results indicate that a transition in mineralogy or pore shape geometry is more likely responsible for the observed decrease in electrical resistivity of the middle crust than a major change in fluid resistivity

the 1989 1990 eruptions of redoubt volcano an introduction abstract redoubt volcano located on the west side of cook inlet in south central alaska erupted explosively on over 20 separate occasions between december 14 1989 and april 21 1990 fourteen lava domes were emplaced in the summit area thirteen of which were subsequently destroyed the eruption caused economic losses estimated at over 160 000 000 making this the second most costly eruption in u s history this economic impact provided the impetus for a integrated comprehensive account of an erupting volcano using both modern and classical research and modern techniques which in turn led to advances in eruption monitoring and interpretation research on such topics as dome formation and collapse and the resulting pyroclastic flows elutriated ash lightning tephra and flooding was blended with the rapid communication of associated hazards to a large user group the seismology successes in predicting and monitoring eruption dynamics were due in part to 1 the recognition of long period seismic events as indicators of the readiness of the volcano to erupt and 2 to the development of new tools that allowed the seismicity to be assessed instantaneously integrated studies of the petrology of erupted products and volatile content over time gave clues as to the progress of the eruption towards completion

the 1989 1990 eruption of redoubt volcano alaska impacts on aircraft operations abstract the december 1989 june 1990 eruption of redoubt volcano affected commercial and military air operations in the vicinity of anchorage alaska these effects were due to the direct impact of volcanic ash on jet aircraft as well as to the rerouting and cancellations of flight operations owing to eruptive activity between december and february five commercial jetliners were damaged from ash encounters the most serious incident took place on december 15 1989 when a boeing 747 400 aircraft temporarily lost power of all four engines after encountering an ash cloud as the airplane descended for a landing in anchorage while there were no injuries to passengers the damage to engines avionics and aircraft structure from this encounter is estimated at 80 million four additional encounters between jet aircraft and redoubt ash clouds occurred in the anchorage area on december 15 and 16 1989 and february 21 1990 none resulted in engine failure two additional encounters took place on december 17 1989 when jet airliners encountered the redoubt cloud over west texas at the time of these encounters the cloud was up to 55 hours old and had traveled in excess of 2 900 nautical miles 5 300 km following the december 15 encounters anchorage international airport remained open however most airline companies canceled operations for up to several days as communications between federal agencies and airlines improved and as a better understanding of the nature and behavior of ash rich eruption clouds was achieved most airlines resumed normal service by early january 1990 the resulting loss of revenue at anchorage international airport during several months following the eruption is estimated to total 2 6 million the impact on general aviation and military operations consisted mostly of cancellation and rerouting of flights

crustal magnetization of brothers volcano new zealand measured by autonomous underwater vehicles geophysical expression of a submarine hydrothermal system near bottom magnetic anomaly data have been acquired using autonomous underwater vehicles at brothers volcano southern kermadec arc new zealand crustal magnetization for the study area was obtained by inverting the magnetic data and shows a strong correlation between areas of low magnetization and four hydrothermal fields one of which was unknown prior to our surveys the magnetization pattern is consistent with a model of discrete individual zones of fluid upflow focused along caldera ring faults that provide preferred pathways for the ascent of the hydrothermal fluids differences in the amplitude of the magnetization over the vent fields appear to correlate with age and temperature variations of the hydrothermal fields

an application of thin sheet electromagnetic modelling to the tasman sea abstract the method of thin sheet approximation is invoked for the ocean layer to model and interpret magnetotelluric mt data observed on the floor of the tasman sea between australia and new zealand a technique to remove 3d distortion from the observed seafloor mt data is developed from the thin sheet model mt responses the de distorted data then appear isotropic and are reinverted using 1d procedures this approach thus assists interpretation through a combination of 3d forward modelling procedures and 1d inversion a seafloor sediment layer is shown to have a strong effect on observed seafloor mt data also an exercise is carried out to compare the results given by the vertical gradient sounding vgs method with seafloor mt data the electrical conductivity structure beneath the tasman sea is analysed in terms of an upper mantle lithosphere and asthenosphere and a highly conducting lower mantle the observed anisotropy in seafloor mt data is diagnostic of a lithospheric conductivity of less than 10 4 s m 1 conductivity rises by two orders of magnitude below 80 km to greater than 10 2 s m 1 and is probably related to an asthenospheric layer in the upper mantle seismic interpretations similarly place a low velocity zone below a depth of 70 km at a depth of approximately 400 km the conductivity is approximately 1 s m 1 consistent with global estimates for the lower mantle there is little evidence for major change with age in the structure across the tasman sea

variations in layer 2a thickness and the origin of the central anomaly magnetic high the seismically determined off axis thickening of the extrusive layer is apparently at odds with the magnetic anomaly high t ypic ally associated with the ridge crest the positive magnenzauon contrast at the ridge crest is most likely caused by rapid alteration of the extrusive source layer which occurs over spatial scales 2 3 km comparable to that of the proposed layer 2a thickening we present magnetic rema nence data from basalts dredged on and near the east pacific rise axis at 12on which are compatible with a rapid magnetization reduction 20 k y to decay to l e together with near bottom magnetic profiles from the ultra fast spreading east pacific rise at 19 5os these data suggest that previous estimates of the time constant of alteration inferred from slow spreading ridges 0 5 m y may be more than an order of magnitude too high seismic studies of the fast spreading east pacific rise epr indicate a rapid thickening of seismic layer 2a within a few kilometers of the ridge axis vera et al 1990 toomey et ai 1990 detrick et al 1993 these seismic data have been interpreted to reflect an approximate doubling of the extrusive layer thickness from 100 200 m to 300 500 m through addition of flows whi ch may extend 1 2 km off axis christeson et al 992 vera and diebold in press such thickening of the extrusive source layer had been argued on the basis of the widths of magnetic reversal transitions e g macdonald et al 1983 however this geometry for a uniformly magnetized source layer should produce a marked magnetic anomaly minimum over the ridge axis and is therefore difficult to reconcile with the pronounced anomaly high typically associated with the ridge crest klitgord 1976 sea surface magnetic profiles from the ultra fast spreading 77 mrn yr half rate epr near 19 5os figure 1 illustrate the short wavelength axial anomaly high within the 120 km wide central anomaly which has been termed the central anomaly magnetic high camh klitgord 1976 the camh typically has a wavelength of 8 10 km in sea surface profiles at in termediate spreading ridges 15 45 mm yr the camh occupies a larger proportion of the central anomaly and at slow spreading ridges it is not readily distinguishable in sea surface profiles from the central anomaly because the wavelengths become similar near bottom magnetic profiles figure 1 typically have higher amplitude and shorter wavelength 2 4 kin axial anomalies as expected from the closer proximity to the source klitg6rd et al 1975 miller 1977 perram et al 1990 and even allow distinction of the camh from the central anomaly at some slow spreading ridges macdonald 1977 the higher temporal resolution of faster spreading ridges therefore provides the best opportunity to evaluate the processes responsible for the camh thickness variations in the extraslye layer at fast spreading fi dges have a profound effect on the axial magnetic anomaly since the presence of an axial magma chamber and the low re rnanent intensity of intrusive rocks e g smith and banerice 1986 pariso et al 1991 minimize deeper source contributions in the near ridge environment based on seismic

magnetic structure of a slow spreading ridge segment insights from near bottom magnetic measurements on board a submersible 1 near bottom magnetic measurements on board submersible nautile were carried out on the mid atlantic ridge 21 40 n segment and deep sea geomagnetic vector anomalies along 19 dive tracks were obtained by applying the processing method for shipboard three component magnetometer data a forward modeling technique using short wavelength components of the anomalies arising from local topography and vertical motion of the submersible was designed to estimate the absolute magnetization intensity of the seafloor in the vicinity of the spreading axis a considerable number of magnetization estimations are reliably confirmed by the high correlation between observed and modeled anomalies whereas less reliable estimations are obtained off axis probably because the sediment buries the basement topography the natural remanent magnetization nrm measured on basalt samples collected during these dives is compared with the magnetization from anomalies though both results give a similar range of magnetization intensity no correlation is confirmed between them possibly because the magnetization from anomalies represents laterally averaged seafloor magnetization whereas the nrm has variations at the scale of individual pillow or lava pile equivalent magnetization inverted from the sea surface magnetic anomalies shows axial magnetization increases significantly from the segment center to the segment ends however the results of eight dives conducted near the spreading axis at different locations along the segment show much less variation in magnetization intensity along the axis we ascribe the high equivalent magnetization at segment ends to preferential serpentinization of peridotite near the segment ends and the associated formation of magnetite the results of three across axis transects composed of 15 dives running in the spreading direction can be consistently interpreted as recording geomagnetic paleointensity variations during the brunhes epoch although magnetization lows are generally correspondent to periods of low paleointensity they show deeper drop than predicted from the paleointensity variation

flux measurements of explosive degassing using a yearlong hydroacoustic record at an erupting submarine volcano at nw rota 1 are primarily h2o so2 and co2 instantaneous fluxes varied by a factor of 100 over the deployment using melt inclusion information to estimate the concentration of co2 in the explosive gases as 6 9 0 7 wt we calculate an annual co2 eruption flux of 0 4 0 1 tg a 1 this result is within the range of measured co2 fluxes at continuously erupting subaerial volcanoes and represents 0 2 0 6 of the annual estimated output of co2 from all subaerial arc volcanoes and 0 4 0 6 of the mid ocean ridge flux the multiyear eruptive history of nw rota 1 demonstrates that submarine volcanoes can be significant and sustained sources of co2 to the shallow ocean

airborne measurements of particle and gas emissions from the december 1994 january 1995 eruption of popocatepetl volcano mexico abstract airborne and ground based correlation spectrometer cascade impactor and photoelectric counter together with intake filter probes measurements are described for the volcanic emissions from popocatepetl volcano mexico from december 23 1994 to january 28 1995 measurements of so 2 restarted 48 h after the eruption onset of december 21 1994 maximum sulfur dioxide 4560 t d 1 plus 3 8 10 4 t d 1 of particulate matter were ejected on december 24 1994 the maximum rate of ejection occurred coincidentally with the maximum amplitude of harmonic tremor and the maximum number of seismic type b events sulfur dioxide emission rates ranged from 1790 to 2070 t d 1 december 23 24 1994 afterwards sulfur dioxide emission rates clearly indicated a consistent decline however frequent gas and ash emission puffs exhibited so 2 fluxes reaching values as high as 3060 t d 1 the emission so 2 baseline for the period of study february 1994 january 1995 was about 1000 t d 1 ejection velocity of particulate matter was approximately 270 m s 1 reaching a height of about 2 5 km over the summit the immediate aerosol dispersion area was estimated at 6 0 10 4 km 2 maximum the microscopic structure of particles aerosol and tephra showed a fragile material probably coming from weathered crustal layers x ray fluorescence and neutron activation analysis from the impactor samples found the following elements si al ca s p cl k ni fe ti sc cu zn mn sr cr co y br se ga rb hg and pb morphological analysis shows that ash samples might be from pulverized basaltic rock indicating that the popocatepetl eruption of december 21 1994 was at low temperature the microscopic structure of puff material showed substance aggregates consisted of fragile rock water and adsorbed so 2 these aggregates were observed within water droplets of approximately 1 mm and even larger sulfur transformations in the droplets occurred intensively volcanic ash contained 5 6 of sulfur during the first expulsion hours elemental relative concentrations with respect to al show that both si and s have relative concentrations 1 i e 13 73 and 2 17 respectively in agreement with the photoelectric counter and cospec measurements

a crustal conductor on eyre peninsula south australia short period variations in the geomagnetic field components were recorded in a closely spaced network of sites on southern eyre peninsula south australia they reveal the existence of a linear zone of very good electrical conductivity within the late archaean early proterozoic gawler craton there is strong enhancement of the horizontal component variations transverse to the zone and an abrupt phase change in the vertical component particularly for periods near 2 103s these features are consistent with a line or narrow thin sheet of current flowing in a conductor at a depth 10 km this almost certainly requires a major fracture or shear zone within the crystalline basement for which there is supporting geophysical evidence superficial cenozoic deposits blanket much of the area and consequently the basement geology is not known in detail although there are structures parallel to the conductor 20 km to the east identification of a major linear crustal feature in such an area demonstrates the usefulness of geomagnetic variation studies for understanding deep geology

airborne filter pack measurements of s and cl in the plume of redoubt volcano alaska february may 2009 abstract filter pack data from six airborne campaigns at redoubt volcano alaska are reported here these measurements provide a rare constraint on cl output from an andesitic eruption at high emission rate 104 t d 1 so2 four s cl ratios measured during a period of lava dome growth indicate a depth of last magma equilibration of 2 5 km the s cl ratios in combination with cospec so2 emission rate measurements indicate hcl emission rates of 1500 3600 t d 1 during dome growth so2 and hcl emission rates at redoubt volcano correlate with each other and were low prior to the eruption high during the eruption and low after the eruption s cl ratios measured by filter pack at andesitic volcanoes have a small range of variance with no clear trends seen for eruptive versus passive activity the very few s cl ratio measurements by filter pack at andesitic volcanoes are not as predictive of future volcanic activity as has been demonstrated for basaltic volcanoes this may be because there are so few of these measurements we have demonstrated it is possible to collect these samples by air between explosions during lava dome building eruptions we recommend more filter pack sampling be performed at andesitic volcanoes to determine the technique s utility for volcano monitoring filter pack data has been demonstrated to be useful for calculating the depth of magma equilibration at volcanoes including redoubt volcano

near bottom magnetic signatures of submarine hydrothermal systems at marsili and palinuro volcanoes southern tyrrhenian sea italy near bottom magnetic field data were collected using a towed magnetometer over selected parts of palinuro and marsili submarine volcanoes in the southern tyrrhenian sea italy we obtained equivalent magnetizations maps at these sites by inverting the corresponding magnetic anomalies highlighting the seafloor expression of hydrothermal alteration zones of reduced magnetization are interpreted as evidence for alteration related to hydrothermal processes they are associated with water column and seafloor observations of hydrothermal activity and altered host rocks at marsili volcano a large elliptical area of reduced magnetization is located south of the summit cone and perpendicular to the trend of marsili volcano itself this confirms the presence of a large hydrothermal system centered over the more recent eastern volcanic ridge and its associated magma chamber similarly areas of reduced magnetization over palinuro caldera are consistent with hydrothermal venting occurring along the caldera walls consistent with permeability structures related to caldera ring faults providing preferred pathways for the upflow of hydrothermal fluids

reduction of potential field data to a horizontal plane most existing techniques for potential field data enhancement and interpretation require data on a horizontal plane hence when observations are made on an irregular surface reduction to a horizontal plane is necessary to effect this reduction an equivalent source distribution that models the observed field is computed on a mirror image of the observation surface this irregular mirror image surface is then replaced by a horizontal plane and the effect of the equivalent sources is computed on the required horizontal level this calculated field approximates the field reduced to a horizontal plane the good quality of this approximation is demonstrated by two dimensional synthetic data examples in which the maximum errors occur in areas of steep topographic gradients and increased magnetic field intensity the approach is also applied to a portion of a helicopter borne aeromagnetic survey from the gaspe region in quebec canada where the results are a horizontal shifting of anomaly maxima of up to 150

long term changes in quiescent degassing at mount baker volcano washington usa evidence for a stalled intrusion in 1975 and connection to a deep magma source abstract long term changes have occurred in the chemistry isotopic ratios and emission rates of gas at mount baker volcano following a major thermal perturbation in 1975 in mid 1975 a large pulse in sulfur and carbon dioxide output was observed both in emission rates and in fumarole samples emission rates of co 2 and h 2 s were 950 and 112 t d respectively in 1975 these decreased to 150 and 2 ch 4 ratio since 1975 suggest a long steady trend back toward a more hydrothermal gas signature the helium isotope ratio is very high 7 r c r a but has declined slightly since the mid 1970s and 13 c co 2 has decreased by 1 over time both trends are expected from a gradually crystallizing magma while other scenarios are investigated we conclude that magma intruded the mid to shallow crust beneath mount baker during the thermal awakening of 1975 since that time evidence for fresh magma has waned but the continued emission of co 2 and the presence of a long term hydrothermal system leads us to suspect some continuing connection between the surface and deep convecting magma

quantifying tectonic strain and magmatic accretion at a slow spreading ridge segment mid atlantic ridge 29 n high resolution deep towed side scan sonar data are used to characterize faulting and variations in tectonic strain along a segment of the slow spreading mid atlantic ridge near 29 n sonar data allow us to identify individual fault scarps to measure fault widths and spacing and to calculate horizontal fault displacements heave and tectonic strain we find that over long periods of time 1 myr on average tectonic strain is 10 on average and does not vary significantly along axis there is a marked asymmetry in tectonic strain that appears to be linked to asymmetric accretion along the whole segment indicated by 50 lower tectonic strain on the east flank than on the west flank these variations in tectonic strain do not correlate directly with changes in fault spacing and heave fault spacing and heave increase from the center of the segment toward the end inside corner on the west flank and from the outside to the inside corner across the axis these parameters remain relatively constant along the segment on the east flank and across the axis at the segment center tectonic strain appears to be decoupled from magmatic accretion at timescales 1 myr as the decrease in magma supply from the segment center toward the end inferred from variations in crustal thickness along the axis is not correlated with a complementary increase in tectonic strain instead tectonic strain remains relatively constant along the axis at 7 on the east flank and at 15 on the west flank these results indicate that variations in fault development and geometry may reflect spatial differences in the rheology of the lithosphere and not changes in tectonic strain or magma supply along axis

electrical conductivity of dry lower crustal rocks this study provides values of electrical conductivity of possible lower crustal materials to assist interpretation of lower crustal magnetotelluric soundings we present mean values of conductivity measurements collected from the literature for dry mafic and silicic rocks in the temperature range of 500 c to 1000 c we observe statistically significant differences between rock types mafic rocks are better conductors than granites by about half an order of magnitude and within the mafic group aphanitic fine grained rocks have higher conductivity than phaneritic coarse grained ones best fitting curves of log conductivity versus temperature are presented for each rock type to show mean log conductivity values together with standard deviations so that most probable temperature ranges can be inferred from conductivity because the laboratory rocks are dry their conductivities are lower at a given temperature than they would be if fluids or volatiles were present hence any temperatures inferred from

evaluation of redoubt volcano s sulfur dioxide emissions by the ozone monitoring instrument abstract the 2009 eruption of redoubt volcano alaska provided a rare opportunity to compare satellite measurements of sulfur dioxide so2 by the ozone monitoring instrument omi with airborne so2 measurements by the alaska volcano observatory avo herein we 1 compare omi and airborne so2 column density values for redoubt s tropospheric plume 2 calculate daily so2 masses from mount redoubt for the first three months of the eruption 3 develop simple methods to convert daily measured so2 masses into emission rates to allow satellite data to be directly integrated with the airborne so2 emissions dataset 4 calculate cumulative so2 emissions from the eruption and 5 evaluate omi as a monitoring tool for high latitude degassing volcanoes a linear correlation r2 0 75 is observed between omi and airborne so2 column densities omi daily so2 masses for the sample period ranged from 60 1 kt on 24 march to below detection limit with an average daily so2 mass of 6 7 kt the highest so2 emissions were observed during the initial part of the explosive phase and the emissions exhibited an overall decreasing trend with time omi so2 emission rates were derived using three methods and compared to airborne measurements this comparison yields a linear correlation r2 0 82 with omi derived emission rates consistently lower than airborne measurements the comparison results suggest that omi s detection limit for high latitude springtime conditions varies from 2000 to 4000 t d cumulative so2 masses calculated from daily omi data for the sample period are estimated to range from 542 to 615 kt with approximately half of this so2 produced during the explosive phase of the eruption these cumulative masses are similar in magnitude to those estimated for the 1989 90 redoubt eruption strong correlations between daily omi so2 mass and both tephra mass and acoustic energy during the explosive phase of the eruption suggest that omi data may be used to infer relative eruption size and explosivity further when used in conjunction with complementary datasets omi daily so2 masses may be used to help distinguish explosive from effusive activity and identify changes in lava extrusion rates the results of this study suggest that omi is a useful volcano monitoring tool to complement airborne measurements capture explosive so2 emissions and provide high temporal resolution so2 emissions data that can be used with interdisciplinary datasets to illuminate volcanic processes

detachment faulting and the evolution of passive continental margins major detachment faults play a key role in the lithospheric extension process in the basin and range province and may also be important in other continental extension terranes such detachment faulting leads to an inherent asymmetry of extensional structure and of uplift subsidence patterns detachment models developed for the formation of metamorphic core complexes can also be applied to the formation of passive continental margins we therefore suggest the existence of upper plate and lower plate passive margins these give rise to a complementary asymmetry of opposing margins after continental breakup transfer faults offset marginal features and allow margins to switch from upper plate to lower plate characteristics along strike

anhydrite in the 1989 1990 lavas and xenoliths from redoubt volcano alaska abstract the eruption of redoubt volcano in alaska produced a moderate sulfur emission estimated at 1 10 tons so 2 but relatively small volume of lava 0 11 km with pre eruption estimates of 840 950 c and f o 2 1 5 to 2 0 log units above nno swanson s e nye c j miller t p avery v f 1994 magma mixing in the 1989 1990 eruption of redoubt volcano part ii evidence from mineral and glass chemistry journal of volcanology and geothermal research 62 453 468 petrologic estimates of sulfur production sigurdsson h devine j d davis a n 1985 the petrologic estimation of volcanic degassing jokull 35 1 8 from this eruption gerlach t westrich h r casadevall t j finnegan d l 1994 vapor saturation and accumulation in magmas of the 1989 1990 eruption of redoubt volcano alaska journal of volcanology and geothermal research 62 317 337 are considerably less than the measured sulfur emission leading workers to propose the existence of a pre eruption vapor phase to explain the excess sulfur initial examination of the 1989 1990 redoubt eruptive products reported anhydrite nye c j swanson s e avery v f miller t p 1994 geochemistry of the 1989 1990 eruption of redoubt volcano part i whole rock major and trace element chemistry journal of volcanology and geothermal research 62 429 452 in interstitial glass from some cognate gabbroic xenoliths but anhydrite was not noted in any of the andesites a boeing 747 encountered the ash plume from the initial eruptive phase on december 15 1989 and provided ash samples that reportedly contained gypsum bayhurst g k wohletz k h mason a s 1994 a method for characterizing volcanic ash from the december 15 1989 eruption of redoubt volcano alaska u s geological survey bulletin 2047 13 17 however the identification was based on eds analyses on a sem and the mineral could have been anhydrite reexamination of the 1989 1990 redoubt lavas and xenoliths revealed the presence of rare fine grained 10s of microns anhydrite crystals in the lavas and larger 10s to 100s of microns crystals in the xenoliths the anhydrite forms elongate crystals with sharp contacts against the groundmass glass grains show rounded terminations suggesting breakdown of the anhydrite perhaps during eruption the anhydrite is a sparse phase in the lavas but occurs in lavas and cognate xenoliths erupted throughout the eruption the presence and breakdown of anhydrite may explain the excess sulfur associated with the redoubt eruption a careful search of other freshly erupted andesitic lavas solidified from hydrous oxidizing magmas should reveal other examples of anhydrite and cause a rethinking of the source of sulfur in volcanic emissions

absolute magnetization of the seafloor at a basalt hosted hydrothermal site insights from a deep sea submersible survey the analysis of high resolution vector magnetic data acquired by deep sea submersibles dsss requires the development of specific approaches adapted to their uneven tracks we present a method that takes advantage of 1 the varying altitude of the dss above the seafloor and 2 high resolution multibeam bathymetric data acquired separately at higher altitude by an autonomous underwater vehicle to estimate the absolute magnetization intensity and the magnetic polarity of the shallow subseafloor along the dss path we apply this method to data collected by dss nautile on a small active basalt hosted hydrothermal site the site is associated with a lack of magnetization in agreement with previous findings at the same kind of sites the contrast between nonmagnetic sulfide deposits stockwork zone and strongly magnetized basalt is sufficient to explain the magnetic signal observed at such a low altitude both normal and reversed polarities are observed in the lava flows surrounding the site suggesting complex history of accumulating volcanic flows

satellite measurement of sulfur dioxide from the redoubt eruptions of 1989 1990 abstract the mass of so 2 emitted by the 16 major explosive eruptions of redoubt volcano between december 1989 and april 1990 have been examined by the total ozone mapping spectrometer toms carried on the nimbus 7 satellite because of low light levels during the winter months toms could not detect so 2 at high northern latitudes thus the major eruptions from december through february could not be monitored unless winds brought the clouds to latitudes lower than about 58 n only two so 2 clouds were observed in the satellite data an approximately 100 kiloton kt cloud on december 16 over nevada and eastern california and a 10 kt cloud on march 9 directly over the volcano we speculate that the major eruption on december 15 at 1015 hour produced the 100 kt cloud seen on december 16 and the mass of so 2 injected into the atmosphere at that time was 175 50 kt

volatile emissions from the crater and flank of oldoinyo lengai volcano tanzania as a comparison to airborne infrared ir flux measurements ground based sampling of fumarole and soil gases was used to characterize the quiescent degassing of co2 from oldoinyo lengai volcano aerial and ground based measurements are in good agreement 75 of the aerially measured co2 flux at lengai 0 05 0 06 1012 mol yr 1 or 6000 7200 tonnes co2 d 1 can be attributed to seven large crater vents in contrast to etna and vulcano island where 15 50 of the total co2 flux emanates diffusely through the volcanic flanks diffuse emissions were measured only within 500 m of the crater rim at lengai contributing 2 of the total flux the lack of extensive flank emissions may reflect the dimensions of the magma chamber and or the lack of a shallow fluid flow system thermodynamic restoration of fumarole analyses shows that gases are the most co2 rich and h2o poor reported for any volcano containing 64 74 co2 24 34 h2o 0 88 1 0 h2 0 1 0 4 co and 0 1 h2s hcl hf and ch4 volatile emissions of s cl and f at oldoiyno lengai are estimated as 4 5 1 5 and 1 0 107 mol yr 1 respectively accuracy of the airborne technique was also assessed by measuring the c emission rate from a coal burning power plant co2 fluxes were measured within 10 near the plant however poor resolution at increased distances caused an underestimation of the flux by a factor of 2 the relatively large co2 fluxes measured for alkaline volcanoes such as oldoinyo lengai or etna may indicate that midplate volcanoes represent a large yet relatively unknown natural source of co2

near bottom magnetic survey of the mid atlantic ridge axis 24 24 40 n implications for crustal accretion at slow spreading ridges near bottom magnetic field measurements provide increased spatial resolution over sea surface magnetic data and allow a detailed analysis of the accretionary and tectonic processes at work in a slow spreading environment through the use of magnetic inversion methods and forward modeling we investigate the fine scale magnetic structure of young oceanic crust along four near bottom profiles that cross the two bathymetric segments immediately north of the kane transform on the mid atlantic ridge our results show the presence of a narrow peaked central anomaly magnetic high camh located over the zone of most recent volcanism we hypothesize that the pattern of high magnetization at segment ends and relatively low values at their centers is largely the result of increased iron and titanium content away from segment centers magnetization lows which are not observed in the sea surface data flank the camh and are associated with the axial valley walls these lows may locate areas of intense magnetic source layer disruption blocks of relatively high magnetization found outside the valley walls suggest a cyclic faulting process whereby discrete blocks of relatively unaltered crust are transported out of the rift valley by adjacent zones of faulting these observations of fine scale magnetic structure show that while the emplacement of crust at slow spreading ridges is highly focused subsequent tectonic disruption and alteration at the rift valley walls degrades the crustal magnetization signal and may be the primary reason for atlantic magnetic anomaly variability

resistivity structure of unzen volcano derived from time domain electromagnetic tdem survey abstract time domain electromagnetic tdem surveys were carried out around unzen volcano shimabara peninsula south west japan in 2001 and 2002 in the eastern part of the peninsula the surveys were a completion of the previously interpreted tdem data collected from surveys in 1995 in the western part of the peninsula interpretation of the recorded transient magnetic fields resulted in a general feature of the resistivity structure which are composed of three main layers a resistive surface layer a conductive layer below 10 m from about sea level to 2 5 km below the surface and a third resistive layer the conductive layer is considered to be a complex of water saturated and altered layers which spreads widely beneath shimabara peninsula the spatial conductance distribution shows a w e trending pattern of high conductance from tachibana bay to the summit of unzen fugen dake this evidence indicates that volcanic gas was supplied to shallow layer in the western part of the peninsula which is in accordance with the path of magma migration derived from seismic and deformation studies another high conductance zone was also found near mayu yama in the eastern part of shimabara peninsula indicating magma degassing process due to the possibility of the presence of a magma system beneath it

fault structure and detailed evolution of a slow spreading ridge segment the mid atlantic ridge at 29 n we present preliminary results of a detailed near bottom study of the morphology and tectonics of the 29 n broken spur segment on the slow spreading mid atlantic ridge using principally the tobi deep towed instrument the survey covered two thirds of the segment length including all of its southern non transform boundary and extended off axis of 40 km 3 3 ma on either side we obtained nearly complete near bottom sidescan sonar coverage and deep towed three component magnetic observations along 2 km spaced e w tracks sidescan data reveal new details of fault structure and evolution faults grow by along axis linkage in the inside corner they also link in the axis normal direction by curving to meet the next outer older fault this leads to wider spaced faults compared to segment centre or outside corner outward facing faults exist but are rare the non transform offset is characterised by faults that are highly oblique not parallel to the spreading direction and show cross cutting relations with ridge parallel faults to the north suggesting along axis migration of the offset almost all volcanic activity occurs within 5 km of the axis most fault growth is complete within 15 km of the axis 1 2 ma though large scarps continue to be degraded by mass wasting beyond there crustal magnetisation is strongly three dimensional the current neovolcanic zone is slightly oblique to earlier reversal boundaries and its magnetisation rises to a maximum of 30 a m 1 near its southern tip the central magnetisation high tapers southwards and is asymmetric with a sharp western but gradual eastern boundary we infer a highly asymmetric accretion of layer 2 near the segment end older magnetic anomalies are kinked and sometimes missing we interpret these observations as evidence of a rapid 18 km southward migration of the segment boundary during the past 1 8 ma and present a series of reconstructions illustrating this tectonic history

induction arrows from offshore floating magnetometers using land reference data summary induction arrows are a traditional output of magnetovariational experiments and represent transfer functions used quantitatively in the inversion of geomagnetic depth sounding data to give earth electrical conductivity structure in this paper a technique is tested in which total eld variations are combined with horizontal eld data recorded simultaneously at remote stations in order to derive induction arrows the method is rst demonstrated using total eld data recorded on land by an aeromagnetic base station and then applied to data obtained from magnetometers oated oishore on the sea surface the oating magnetometers were deployed in two con gurations one tethered to the sea oor the second free oating

the electrical conductivity of the oceanic upper mantle summary previous inversions of sea floor magnetotelluric mt sounding data have predicted upper mantle electrical conductivities which are more than an order of magnitude higher than laboratory measurements of the conductivity of olivine would suggest and controlled source field electromagnetic csem soundings require a lithospheric mantle conductivity of 3 x lop5 sm which is so low that the electromagnetic em coast effect would produce more anisotropy in mt soundings than is observed we address these issues by constructing an olivine mantle model for conductivity and examining the inversion of mt data from three sea floor sites and show that the incompatibilities can be largely resolved if the effects of oceans and coastlines are considered our mantle model is based on recent measurements of olivine conductivity the conductivity of tholeiite melt a thermal model for the upper mantle based on lithospheric cooling and the temperature of a a 3 olivine transition the pyrolite model of mantle petrology and conductivities derived from csem sounding we propose archie s law with exponent 2 and interconnected tubes as realistic lower and upper bounds for the effect of partial melt on rock conductivity and archie s law with exponent 1 5 as the preferred estimate the 1 d forward response of this model is not compatible with observed sea floor mt data three data sets presented by oldenburg 1981 pass a test for one dimensionality based on the size of the residuals when fit with parker s d algorithm but two of the three soundings fail a test for independence of residuals we also find that the presence of the upper mantle high conductivity zone previously inferred from these data is highly dependent on data misfit and not required when the misfit criterion is relaxed a reasonable amount re inversions of the mt data produce models which are incompatible with our petrological model of mantle conductivity however by adding an ocean with various coastlines of simple geometry to our petrological model and solving the forward 3 d problem using thin sheet analysis we predict mt responses which are distorted in a manner that is remarkably similar to the observed data

evidence for dike emplacement beneath iliamna volcano alaska in 1996 two earthquake swarms comprising 88 and 2833 locatable events occurred beneath iliamna volcano alaska in may and august of 1996 swarm earthquakes ranged in magnitude from 0 9 to 3 3 increases in so2 and co2 emissions detected during the fall of 1996 were coincident with the second swarm no other physical changes were observed in or around the volcano during this time period no eruption occurred and seismicity and measured gas emissions have remained at background levels since mid 1997 earthquake hypocenters recorded during the swarms form a cluster in a previously aseismic volume of crust located to the south of iliamna s summit at a depth of 1 to 4 km below sea level this cluster is elongated to the nnw sse parallel to the trend of the summit and southern vents at iliamna and to the regional axis of maximum compressive stress determined through inversion of fault plane solutions for regional earthquakes fault plane solutions calculated for 24 swarm earthquakes located at the top of the new cluster suggest a heterogeneous stress field acting during the second swarm characterized by normal faulting and strike slip faulting with p axes parallel to the axis of regional maximum compressive stress the increase in earthquake rates the appearance of a new seismic volume and the elevated gas emissions at iliamna volcano indicate that new magma intruded beneath the volcano in 1996 the elongation of the 1996 1997 earthquake cluster parallel to the direction of regional maximum compressive stress and the accelerated occurrence of both normal and strike slip faulting in a small volume of crust at the top of the new seismic volume may be explained by the emplacement and inflation of a subvertical planar dike beneath the summit of iliamna and its southern satellite vents

seismic evolution of the 1989 1990 eruption sequence of redoubt volcano alaska abstract redoubt volcano in south central alaska erupted between december 1989 and june 1990 in a sequence of events characterized by large tephra eruptions pyroclastic flows lahars and debris flows and episodes of dome growth the eruption was monitored by a network of five to nine seismic stations located 1 to 22 km from the summit crater notable features of the eruption seismicity include 1 small long period events beginning in september 1989 which increased slowly in number during november and early december 2 an intense swarm of long period events which preceded the initial eruptions on december 14 by 23 hours 3 shallow swarms 0 to 3 km of volcano tectonic events following each eruption on december 15 4 a persistent cluster of deep 6 to 10 km volcano tectonic earthquakes initiated by the eruptions on december 15 which continued throughout and beyond the eruption 5 an intense swarm of long period events which preceded the eruptions on january 2 and 6 nine additional intervals of increased long period seismicity each of which preceded a tephra eruption hypocenters of volcano tectonic earthquakes suggest the presence of a magma source region at 6 10 km depth earthquakes at these depths were initiated by the tephra eruptions on december 15 and likely represent the readjustment of stresses in the country rock associated with the removal of magma from these depths the locations and time history of these earthquakes coupled with the eruptive behavior of the volcano suggest this region was the source of most of the erupted material during the 1989 1990 eruption this source region appears to be connected to the surface by a narrow pipe like conduit as inferred from the hypocenters of volcano tectonic earthquakes concentrations of shallow volcano tectonic earthquakes followed each of the tephra eruptions on december 15 these shocks may represent stress readjustment in the wall rock related to the removal of magma and volatiles at these depths this shallow zone was the source area of the majority of long period seismicity through the remainder of the eruption the long period seismicity likely reflects the pressurization of the shallow portions of the magmatic system

long term geochemical surveillance of fumaroles at showa shinzan dome usu volcano japan this study investigates 31 years of fumarole gas and condensate trace elements data from showa shinzan a dacitic dome cryptodome complex that formed during the 1943 1945 eruption of usu volcano forty two gas samples were collected from the highest temperature fumarole named a 1 from 1954 800 c to 1985 336 c and from lower temperature vents condensates were collected contemporaneously with the gas samples and we reanalyzed ten of these samples mostly from the a 1 vent for 32 cations and three anions modeling using the thermochemical equilibrium program solvgas shows that the gas samples are mild disequilibrium mixtures because they a contain unequilibrated sedimentary ch4 and nh3 b have unequilibrated meteoric water or c lost co either by air oxidation or by absorption by the sodium hydroxide sampling solution solvgas also enabled us to restore the samples by removing these disequilibrium effects and to estimate their equilibrium oxygen fugacities and amounts of s2 and ch4 the restored compositions contain 98 h2o with minor to trace amounts of co2 h2 hcl so2 hf h2s co s2 and ch4 we used the restored gas and condensate data to test the hypotheses that these time series compositional data from the dome s fumaroles provide 1 sufficient major gas data to analyze long term degassing trends of the dome s magma hydrothermal system without the influence of sampling or contamination effects 2 independent oxygen fugacity versus temperature estimates of the showa shinzan dacite 3 the order of release of trace elements especially metals from magma and 4 useful information for assessing volcanic hazards the 1954 1985 restored a 1 gas compositions confirm the first hypothesis because they are sufficient to reveal three long term degassing trends 1 they became increasingly h2o rich with time due to the progressive influx of meteoric water into the dome 2 their csandscl ratios decreased dramatically while their clf ratios stayed roughly constant indicating the progressive outgassing of less soluble components f cl s c from the magma reservoir and 3 their h2oh2 co2coandh2sso2 ratios increased significantly in concert with equilibrium changes expected for the 500 c temperature drop when plotted against reciprocal temperature the restored gas log oxygen fugacities follow a tight linear trend from 800 c to nno 2 5 at 400 c this trend largely disproves the second hypothesis because the oxygen fugacities for the 800 c restored gases can only be explained by mixing of hot magmatic gases with 350 c steam from superheated meteoric water but above 800 c this trend intersects the opposing linear trend for other usu eruptive products implying a log oxygen fugacity of 11 45 at 902 c for the showa shinzan magma the time series trace element data also disprove the third hypothesis because rock and incrustation particle contaminants in the condensates account for most of the trace element variation nonetheless highly volatile elements like b and as are relatively unaffected by this particle contamination and they show similar time series trends as cl and f finally except for infrequent sampling around the 1977 usu eruption the results generally confirm the fourth hypothesis since the time series trends for the major gases and selected trace elements indicate that with time the system cooled degassed and was infiltrated by meteoric water all of which are positive signs that volcanic activity declined over the 31 year history this study also suggests that second boiling of shallow magma within and possibly beneath the cryptodome sustained magmatic degassing for at least 20 years after emplacement

geomagnetic deep sounding experiment in the central flinders ranges of south australia abstract a closely spaced geomagnetic deep sounding array study was conducted in the central flinders ranges of south australia the results indicate an anomaly which follows the main fold trend to the east rather than the north south trend suggested by earlier studies although the coast effect obscures some of the detailed characteristics of the anomaly the data particularly at high frequencies favour an interpretation in terms of a crustal anomaly probably associated with the lower members of the adelaide geosyncline the torrens hinge zone which is a major tectonic boundary at the western margin of the adelaide geosyncline has no associated induction anomaly

geochemistry of the 1989 1990 eruption of redoubt volcano part i whole rock major and trace element chemistry the 1989 1990 eruption of redoubt volcano produced medium k calc alkaline andesite and dacite of limited compositional range 58 2 63 4 sio2 and entrained quenched andesitic inclusions 55 sio2 which bear chemical similarities to the rest of the ejecta the earliest december 15 magmas are pumiceous often compositionally banded and the majority is relatively mafic 59 sio2 the most silicic magmas of the eruption are the late december to early january domes up to 63 4 sio2 subsequent magmas formed domes and rare pumices which converge on 60 sio2 chemical variations among ejecta comprise tight linear two component arrays for all elements for which the analytical uncertainty is much less than the compositional range the two component arrays are interpreted as mixing arrays between unrelated magmas because several of the arrays are at steep angles to the normal liquid line of descent additionally the felsic endmember cannot be easily related to the mafic endmember by normal high temperature igneous processes e g the silicic endmember has higher zr yet lower hf than the mafic endmember also relative enrichments of highly incompatible elements are dramatically different across the arrays the mixing event must have preceded eruption by a significant yet unspecified amount of time because groundmass glass compositions are homogeneous for all post december samples swanson et al 1994 this volume in spite of the whole rock chemical diversity this implies time for additional crystallization after the mixing event swanson et al 1994 this volume discuss evidence for a potentially different mixing event recorded only in december 15 magmas cognate cumulate xenoliths composed of pl cpx opx hb mt melt were recovered from january and april deposits these blocks differ from local batholithic country rock in their low concentrations of incompatible elements e g rb 60 wt they have mg cr ni sc and v contents higher than the andesites but lower than redoubt basalts and basaltic andesites thus they may be crystallization products of andesites but do not represent the cumulate residue of basalt fractionation the xenoliths were probably derived from a shallow or intermediate crustal chamber

proximal pyroclastic deposits from the 1989 1990 eruption of redoubt volcano alaska stratigraphy distribution and physical characteristics abstract more than 20 eruptive events during the 1989 1990 eruption of redoubt volcano emplaced a complex sequence of lithic pyroclastic flow surge fall ice diamict and lahar deposits mainly on the north side of the volcano the deposits record the changing eruption dynamics from initial gas rich vent clearing explosions to episodic gas poor lava dome extrusions and failures the repeated dome failures produced lithic pyroclastic flows that mixed with snow and glacial ice to generate lahars that were channelled off drift glacier into the drift river valley some of the dome failures occurred without precursory seismic warning and appeared to result solely from gravitational instability material from the disrupted lava domes avalanched down a steep partly ice filled canyon incised on the north flank of the volcano and came to rest on the heavily crevassed surface of the piedmont lobe of drift glacier most dome collapse events resulted in single monolithologic massive to reversely graded medium to coarse grained sandy pyroclastic flow deposits containing abundant dense dome clasts these deposits vary in thickness grain size and texture depending on distance from the vent and local topography deposits are finer and better sorted down flow thinner and finer on hummocks and thicker and coarser where ponded in channels cut through the glacial ice the initial vent clearing explosions emplaced unusual deposits of glacial ice snow and rock in a frozen matrix on the north and south flanks of the volcano similar deposits were described at nevado del ruiz columbia and have probably been emplaced at other snow and ice clad volcanoes but poor preservation makes them difficult to recognize in the geologic record in a like fashion most deposits from the 1989 1990 eruption of redoubt volcano may be difficult to recognize and interpret in the future because they were emplaced in an environment where glacio fluvial processes dominate and quickly obscure the primary depositional record

long period seismicity at redoubt volcano alaska 1989 1990 related to magma degassing abstract the mass of exsolved magmatic h 2 o is estimated and compared to the mass of superheated steam 25 50 mtons released through the resonating crack producing the december 13 14 1989 swarm of long period seismic events at redoubt volcano results indicate degassing of a h 2 o co 2 so 2 saturated magma upon ascending from at least 12 km to 3 4 km beneath the crater as the source of the superheated steam the mass of exsolved h 2 o 3 2 250 mtons is estimated from solubility diagrams of h 2 o co 2 saturated silicate melts for the ascent history of the redoubt magmas crystal size distribution seismological petrological and geochemical data are used to constrain the ascent history of the two andesitic magmas prior to the eruption two stages of crystallization are inferred from crystal size distributions of plagioclase crystals in andesites erupted in december 1989 the first stage occurred 30 150 years before the eruption in both magmas and the second stage occurred at least 8 years and 15 years before the eruption in the dacitic andesite and rhyolitic andesite respectively the depths of crystallization are constrained from the spatial and temporal variations of volcano tectonic earthquakes locations lahr et al 1994 and from the p wave and s wave velocity structures benz et al 1996 these data suggest that the rhyolitic andesite magma ascended to a depth of 7 8 km within at least 15 years of the eruption within at least 8 years of the eruption the dacitic andesite magma migrated to a depth just below the other magma body where it resided until hours to days of the eruption at this time the dacitic andesite magma mixed with the rhyolitic andesite magma and established the reservoir for the eruption near the top of the reservoir some of the mixed magma was displaced into fractures which extended 4 5 km toward the surface this displaced magma created the eruption conduit and released the fluids related to the resonating crack this scenario is consistent with the trends in major and trace element chemistry and the stability of hornblende in the pre eruption redoubt magmas it also provides a source for the so 2 and co 2 emissions measured during the eruption

crustal structure of the mid atlantic ridge south of the kane fracture zone from seafloor and sea surface gravity data seafloor and sea surface gravity data are inverted together to construct a model for the near axis crustal structure of a slow spreading ridge the seafloor data set offers two main advantages it allows us to recover shorter wavelengths signal and to constrain the value of a potential field at two different levels the model we propose here would not have been derived from sea surface data alone it is based on a dense sea surface gravity coverage and on 121 sea bottom gravity measurements collected in the mid atlantic ridge at kane mark area during the hydrosnake 1988 and gravinaute 1993 cruises the primary goal of the seafloor surveys was to test for the presence of a magma reservoir beneath the axial neovolcanic ridge first a forward two dimensional 2 d model of the crustal structure across the axis is fit to observed gravity anomalies using constraints from geological and structural observations bouguer anomalies computed from sea bottom measurements and downward continuation of sea surface measurements both constrain the forward modeling this forward model is the starting point of a 2 d monte carlo inversion of seafloor and sea surface data in addition to the crustal thickness variations along axis our data document the amplitude variations of the crustal thickness and or its density in the across axis direction the model resulting from our inversion exhibits several features of the crustal structure in the mark area 1 the presence of a low density ap 300 50 kg m 3 body beneath the neovolcanic ridge is suggested and could correspond to a magma chamber or more probably to a highly hydrothermally fissured zone 2 both long and short wavelength gravity signals exhibit a difference between the western and eastern sides of the axial domain the mean value and the amplitude of bouguer anomalies are higher on the western part this difference suggests that axial processes in this area are very asymmetric 3 abyssal hills are not associated with a single gravity signature for instance on the west side of the axis one of the explored hills has no bouguer anomaly and is interpreted as a neovolcanic ridge whereas the others are associated with a shifted bouguer anomaly high and are interpreted as having thinner magmatic crust 4 the last feature of the crustal fabric we document here is the asymmetric emplacement of some deep rocks outcrops in the mark area we find that pink hill a topographic high where serpentinized peridotites are outcropping is much more serpentinized on its east flank toward the axial valley than on its west flank alteration occurring mainly by fluid circulation through faulted zones the asymmetric serpentinization suggests that deep origin rocks have outcropped by means of a main fault zone and are not emplaced by diapirism

origin of magnetic highs at ultramafic hosted hydrothermal systems insights from the yokoniwa site of central indian ridge high resolution vector magnetic measurements were performed on an inactive ultramafic hosted hydrothermal vent field called yokoniwa hydrothermal field yhf using a deep sea manned submersible shinkai6500 and an autonomous underwater vehicle r2d4 the yhf has developed at a non transform offset massif of the central indian ridge dead chimneys were widely observed around the yhf along with a very weak venting of low temperature fluids so that hydrothermal activity of the yhf was almost finished the distribution of crustal magnetization from the magnetic anomaly revealed that the yhf is associated with enhanced magnetization as seen at the ultramafic hosted rainbow and ashadze 1 hydrothermal sites of the mid atlantic ridge the results of rock magnetic analysis on seafloor rock samples including basalt dolerite gabbro serpentinized peridotite and hydrothermal sulfide showed that only highly serpentinized peridotite carries high magnetic susceptibility and that the natural remanent magnetization intensity can explain the high magnetization of yokoniwa these observations reflect abundant and strongly magnetized magnetite grains within the highly serpentinized peridotite comparisons with the rainbow and ashadze 1 suggest that in ultramafic hosted hydrothermal systems strongly magnetized magnetite and pyrrhotite form during the progression of hydrothermal alteration of peridotite after the completion of serpentinization and production of hydrogen pyrrhotites convert into pyrite or nonmagnetic iron sulfides which considerably reduces their levels of magnetization our results revealed origins of the magnetic high and the development of subsurface chemical processes in ultramafic hosted hydrothermal systems furthermore the results highlight the use of near seafloor magnetic field measurements as a powerful tool for detecting and characterizing seafloor hydrothermal systems

magnetic structure of an oceanic core complex at the southernmost central indian ridge analysis of shipboard and deep sea three component magnetometer data 1 we investigated the magnetic structure of an oceanic core complex occ at the southernmost central indian ridge cir near 25 s on the basis of newly collected magnetic and bathymetric data and submersible dives the occ is located off axis on the western flank of the cir and is characterized by typical megamullion morphology with flow line parallel corrugations the occ is similar in size to those found along the mid atlantic ridge mar but the cir occ formed in an estimated 0 8 ma which is slightly shorter than the average time for the formation of occs along the mar the magnetization intensity over the cir occ is quite weak compared to that over adjacent seafloor of the same age mostly gabbroic rocks were recovered from outcrops during submersible dives fault rocks were also recovered from the corrugated surface suggesting that the occ surface represents the plane of a detachment fault although the magnetization intensity is rather low we were able to identify the brunhes matuyama boundary and the jaramillo and olduvai subchrons on and around the occ using vector magnetic analysis over the occ the jaramillo subchron which cannot be recognized from magnetic data acquired at the sea surface was identified using data from a magnetometer attached to a submersible the locus of this subchron is consistent with that observed over the seafloor adjacent to the occ combining the results of magnetic survey and rock sampling we would like to point out the possibility that the gabbroic rocks constituted the occ and preserved seafloor spreading history although there are several interpretations of the low magnetization

emission rates of sulfur dioxide and carbon dioxide from redoubt volcano alaska during the 1989 1990 eruptions airborne measurements of sulfur dioxide emission rates in the gas plume emitted from fumaroles in the summit crater of redoubt volcano were started on march 20 1990 using the cospec method during the latter half of the period of intermittent dome growth and destruction between march 20 and mid june 1990 sulfur dioxide emission rates ranged from approximately 1250 to 5850 t d rates notably higher than for other convergent plate boundary volcanoes during periods of active dome growth emission rates following the end of dome growth from late june 1990 through may 1991 decreased steadily to less than 75 t d the largest mass of sulfur dioxide was released during the period of explosive vent clearing when explosive degassing on december 14 15 injected at least 175 000 50 000 tonnes of so2 into the atmosphere following the explosive eruptions of december 1989 redoubt volcano entered a period of intermittent dome growth from late december 1989 to mid june 1990 during which redoubt emitted a total mass of so2 ranging from 572 000 90 000 tonnes to 680 000 90 000 tonnes from mid june 1990 through may 1991 the volcano was in a state of posteruption degassing into the troposphere producing approximately 183 000 50 000 tonnes of so2 we estimate that redoubt volcano released a minimum mass of sulfur dioxide of approximately 930 000 tonnes while cospec data were not obtained frequently enough to enable their use in eruption prediction so2 emission rates clearly indicated a consistent decline in emission rates between march through october 1990 and a continued low level of emission rates through the first half of 1991 values from consecutive daily measurements of sulfur dioxide emission rates spanning the march 23 1990 eruption decreased in the three days prior to eruption that decrease was coincident with a several fold increase in the frequency of shallow seismic events suggesting partial sealing of the magma conduit to gas loss that resulted in pressurization of the shallow magma system and an increase in earthquake activity unlike the short term so2 decrease in march 1990 the long term decrease of sulfur dioxide emission rates from march 1990 through may 1991 was coincident with low rates of seismic energy release and was interpreted to reflect gradual depressurization of the shallow magma reservoir the long term declines in seismic energy release and in so2 emission rates led avo scientists to conclude on april 19 1991 that the potential for further eruptive activity from redoubt volcano had diminished and on this basis the level of concern color code for the volcano was changed from code yellow volcano is restless earthquake activity is elevated activity may include extrusion of lava to code green volcano is in its normal dormant state

dome growth and destruction during the 1989 1990 eruption of redoubt volcano abstract much of the six month long 1989 1990 eruption of redoubt volcano consisted of a dome growth and destructive phase in which 14 short lived viscous silicic andesite domes were emplaced and 13 subsequently destroyed the life span of an individual dome ranged from 3 to 21 days and volumes are estimated at 1 10 6 to 30 10 6 m 3 magma supply rates to the vent area averaged about 5 10 5 m 3 day for most of the dome building phase and ranged from a high of 2 2 10 6 m 3 per day initially to a low of 1 8 10 5 m 3 per day at the waning stages of the eruption the total volume of all domes is estimated to be about 90 10 6 m 3 and may represent as much as 60 70 of the volume for the entire eruption the site of 1989 1990 dome emplacement like that in 1966 was on the margin of a north facing amphitheatre like summit crater the domes were confined on the east and west by steep cliffs of pre eruption cone building volcanic rocks and thus were constrained to grow vertically rapid upward growth in a precarious site caused each dome to spread preferentially to the north resulting in eventual gravitational collapse as long as the present conduit remains active at redoubt volcano any dome formed in a new eruption will be confined to a narrow steeply sloping gorge leading to rapid vertical growth and a tendency to collapse gravitationally repetitive cycles of dome formation and failure similar to those seen in 1989 1990 are probably the norm and must be considered in future hazard analyses of redoubt volcano

tephra fall deposits from the 1992 eruption of crater peak alaska implications of clast textures for eruptive processes the 1992 eruption of crater peak mount spurr alaska involved three subplinian tephra producing events of similar volume and duration the tephra consists of two dense juvenile clast types that are identified by color one tan and one gray of similar chemistry mineral assemblage and glass composition in two of the eruptive events the clast types are strongly stratified with tan clasts dominating the basal two thirds of the deposits and gray clasts the upper one third tan clasts have average densities between 1 5 and 1 7 g cc and vesicularities phenocryst free of approximately 42 gray clasts have average densities between 2 1 and 2 3 g cc and vesicularities of approximately 20 both contain abundant microlites average maximum plagioclase microlite lengths 13 15 m in gray clasts in the upper layer are similar regardless of eruptive event and therefore the repose time between them and are larger than average maximum plagioclase microlite lengths 9 11 m in the tan clasts in the lower layer this suggests that microlite growth is a response to eruptive processes and not to magma reservoir heterogeneity or dynamics furthermore we suggest that the low vesicularities of the clasts are due to syneruptive magmatic degassing resulting in microlitic growth prior to fragmentation and not to quenching of clasts by external groundwater

thirty years of seismic network recording in south australia selected results abstract the year 1993 marks 30 years of seismic network recording of earthquakes in south australia the network currently comprises 17 short period instruments of which six use digital recording and five employ triaxial sensors approximately 350 earthquakes are located within the state each year using a computerised seismic analysis system developed by the phillip institute of technology a duration based magnitude scale equivalent to the richter m l scale has been developed for most stations the pre network historical record of earthquake activity in south australia dates back to 1837 epicentres are available for just 45 earthquakes of these ten are of magnitude five or greater the instrumental data over the last 30 years show a similar trend in epicentral pattern to the historical earthquakes with the major zones being the flinders ranges eyre peninsula and the southeast there have been 40 earthquakes of magnitude four or greater since 1963 the largest earthquake being of magnitude six in the musgrave ranges during 1986 mine and quarry blasts within the state have been useful in calibration of the network both in terms of hypocentral location and amplitude studies the explosions have also furnished valuable information on the crustal velocity distribution epicentral co ordinates of mine blasts are generally good to within 3 5 km the explosions yield richter magnitudes of one charge size 1 or 2 tonnes to 2 5 charge size 50 tonnes the crustal s waves used in earthquake magnitude determinations exhibit an amplitude distance decay coefficient of 1 2 the p wave velocity function of the crust increases non linearly with depth from about 5 0 km s 1 near the surface to 6 4 km s 1 at 20 km depth where the gradient reduces with the velocity reaching a value of about 7 4 km s 1 at a depth of 40 km

geochemical monitoring system ii prototype gms ii installation at the acqua difesa well within the etna region first data during the 1999 volcanic crisis abstract the geochemical monitoring system ii gms ii prototype was designed assembled tested and installed at the acqua difesa test site near belpasso ct within the etna region in the frame of an ec funded program named automatic geochemical monitoring of volcanoes the easily changeable and versatile configuration of the remote station provides actual data every 10 min recording groundwater temperature electrical conductivity ph redox potential dissolved co 2 he atmospheric pressure and air temperature the radon sensor was assembled despite no jet added to the remote station an automatic sampler collects every 72 h 3 days two bottles of water acidified and not for further laboratory analyses the choice of the acqua difesa test site arises from preliminary volcanological studies deserving special interest i e for the high co 2 degassing for the high he isotopic values exhibiting a direct mantle signature of the volcanic gases and for the peculiar sensitivity to volcanic activity as tested by discrete monitoring in the past and confirmed by the present study during the discussed monitoring period october 1998 june 1999 an enhanced volcanic activity of etna was observed it started on july 1998 with enhanced plume degassing and since september 1998 with periodical lava fountains as well as with episodic low energy earthquakes and seismic swarms m max 3 1 till june 1999 the activity culminated with a summit crater fracture involving a sub terminal eruption on 04 02 1999 the summit crater fracture was not accompanied by enhanced seismicity despite moderate seismic energy release occurring in the succeeding months in that monitoring period the acqua difesa gms ii station recorded a few geochemical anomalies mostly in temperature ph co 2 electrical conductivity and eh as well as in the concentration of some major elements trend mostly in mg and hco 3 the geochemical continuous and discrete data set is discussed in comparison with the available information on the ongoing volcanic and seismic activity tentatively enhancing the comprehension of the aquifers mass heat transport properties of the volcano during crises the experimental observations may be defined at present the forerunners of the etna eruptions affecting the single out acqua difesa well it is nowadays a self consistent site with its own response to the pre eruptive processes defined sensitive to the etna aquifer modifications induced by the volcanic activity in a recurrent way thus we defined the typical answer of this specific test site to the pre eruptive processes and to the associated seismicity further data may modify this statement the station prototype configuration and the results may give hints for future volcanic eruption and seismic activity forecasting

the tamar conductivity anomaly magneto variational measurements in eastern tasmania have shown the presence of a conductivity anomaly coinciding with the tamar lineament magneto telluric observations indicate a shallow body 2 5 km deep with a high conductivity 0 2 1 s m 1 analogue model measurements made at the university of victoria have been used to correct for the effect of the oceans surrounding tasmania rather than model the form of the conducting body the results have been interpreted in terms of current flowing in a nnw sse direction in a channel 40 km wide with a sharp eastern edge and a more gradual tapering to the west the most likely explanation of the high conductivity is fractured rock saturated with conducting pore fluids

evaluation of gases condensates and so2 emissions from augustine volcano alaska the degassing of a cl rich volcanic system after the march april 1986 explosive eruption a comprehensive gas study at augustine was undertaken in the summers of 1986 and 1987 airborne cospec measurements indicate that passive so2 emission rates declined exponentially during this period from 380 45 metric tons day t d on 7 24 86 to 27 6 t d on 8 24 87 these data are consistent with the hypothesis that the augustine magma reservoir has become more degassed as volcanic activity decreased after the spring 1986 eruption gas samples collected in 1987 from an 870 c fumarole on the andesitic lava dome show various degrees of disequilibrium due to oxidation of reduced gas species and condensation and loss of h2o in the intake tube of the sampling apparatus thermochemical restoration of the data permits removal of these effects to infer an equilibrium composition of the gases although not conclusive this restoration is consistent with the idea that the gases were in equilibrium at 870 c with an oxygen fugacity near the ni nio buffer these restored gas compositions show that relative to other convergent plate volcanoes the augustine gases are very hcl rich 5 3 6 0 mol hcl s rich 7 1 mol total s and h2o poor 83 9 84 8 mol h2o values of d and 18o suggest that the h2o in the dome gases is a mixture of primary magmatic water pmw and local seawater part of the cl in the augustine volcanic gases probably comes from this shallow seawater source additional cl may come from subducted oceanic crust because data by johnston 1978 show that cl rich glass inclusions in olivine crystals contain hornblende which is evidence for a deep source 25km for part of the cl gas samples collected in 1986 from 390 642 c fumaroles on a ramp surrounding the inner summit crater have been oxidized so severely that restoration to an equilibrium composition is not possible h and o isotope data suggest that these gases are variable mixtures of seawater fmw and meteoric steam these samples are much more h2o rich 92 97 h2o than the dome gases possibly due to a larger meteoric steam component the 1986 samples also have higher cl s s c and f cl ratios which imply that the magmatic component in these gases is from the more degassed 1976 magma thus the 1987 samples from the lava dome are better indicators than the 1986 samples of degassing within the augustine magma reservoir even though they were collected a year later and contain a significant seawater component future gas studies at augustine should emphasize fumaroles on active lava domes condensates collected from the same lava dome fumarole have enrichments ot 107 102 in cl br f b cd as s bi pb sb mo zn cu k li na si and ni lower temperature 200 650 c fumaroles around the volcano are generally less enriched in highly volatile elements however these lower termperature fumaroles have higher concentration of rock forming elements probably derived from the wall rock

direct inversion of potential fields from an uneven track with application to the mid atlantic ridge current methods of potential field inversion require measurements to be reduced onto a level plane resulting in a loss of resolution this is especially true for draped surveys in areas of extreme topography we examine a method that bypasses this limiting step by making use of an approximately equivalent geometry from which data are directly inverted corrections are applied to an initial solution to account for errors generated by the transformation of geometries a numerical model and an example data set are used to explore this method s benefits and drawbacks in relation to the conventional approach of data reduction onto a level plane prior to inversion we show that this direct inversion method is particularly better than the conventional approach at resolving fine scale features such as the narrow zone of crustal emplacement at the mid atlantic ridge and adjacent areas of tectonic disruption and magnetic source alteration

proposed relation between anomalous geomagnetic variations and the tectonic history of south india abstract anomalous geomagnetic variations have been observed at all geomagnetic stations in south india and they have earlier been attributed to ocean and crustal conductors the spectral character of induced variations indicate that in addition to coastal effects and channelling of currents through the palk straits there is a concentration of currents in a conducting body at deeper levels in the crust or mantle current thinking on crustal evolution and the influence of precambrian lineaments on tectonism and mineralisation up to recent times provides support for the conjecture that zones of high electrical conductivity exist in the crust which are related to tectonic and mineral influences the eastern ghats and granulite terrain of south india and sri lanka are probably associated with such a zone of fundamental weakness in the crust extended graphite mineralisation along the eastern ghats occurrence of economic graphite deposits near trivandrum and in sri lanka and the association of alkaline and ultrabasic metallic minerals with deep faults in south india are indications of the possible chemical causes of high electrical conductivity in some regions of the crust similar relations between induced anomalous variations and tectonism and mineralisation in the precambrian crystalline crust have been found in north america and scotland however the regions of high electrical conductivity have not yet been mapped in detail in south india to confirm such a relationship