magmatic precursors to the 18 may 1980 eruption of mount st helens usa perhaps the most difficult task facing volcanologists today is that of distinguishing between low level volcanic restlessness and activity that presages a full scale eruption we illustrate these difficulties by reexamining the sequence of events that led to the 18 may 1980 eruption of mount st helens activity that is often presented as a classic example of early phreatic explosions leading to a magmatic eruption this interpretation is problematic however given the known ascent of juvenile magma to within 250 m of the surface prior to the climactic event reexamination of ash samples from several precursory events shows that small amounts of juvenile material were erupted as early as 28 march 1980 just two weeks after the first seismic signals of reawakening and that the juvenile content of the ash had probably increased by early may the magmatic component of these eruptions was not recognized at the time because of the high crystallinity of the juvenile material a signature of extensive degassing induced crystallization during magma ascent identification of juvenile material in the precursory ash from mount st helens has important implications for initiation of eruptive activity

hydrothermal element fluxes from copahue argentina a beehive volcano in turmoil copahue volcano erupted altered rock debris siliceous dust pyroclastic sulfur and rare juvenile fragments between 1992 and 1995 and magmatic eruptions occurred in july october 2000 prior to 2000 the copahue crater lake acid hot springs and rivers carried acid brines with compositions that reflected close to congruent rock dissolution the ratio between rock forming elements and chloride in the central zone of the volcano hydrothermal system has diminished over the past few years reflecting increased water rock ratios as a result of progressive rock dissolution magmatic activity in 2000 provided fresh rocks for the acid fluids resulting in higher ratios between rock forming elements and chloride in the fluids and enhanced mg fluxes the higher mg fluxes started several weeks prior to the eruption model data on the crater lake and river element flux determinations indicate that copahue volcano was hollowed out at a rate of about 20 000 25 000 m 3 yr but that void space was filled with about equal amounts of silica and liquid elemental sulfur the extensive rock dissolution has weakened the internal volcanic structure making flank collapse a volcanic hazard at copahue

a review on phreatic eruptions and their precursors abstract an analysis of the reported information on 132 historical phreatic events indicates that most of these 115 were not followed by a magmatic or phreatomagmatic eruption only in 18 cases precursory phenomena are reportedly lacking whereas they are recognized for the majority 72 of the considered cases these precursors have been analyzed as a function of the kind of phreatic event and of its possible triggering mechanism unfortunately the quality of the available information is poor however data suggest that anomalous seismicity ground deformation changes in the output chemistry and temperature of fumaroles and thermal springs are potential mid term precursors whereas seismic tremor is a common short term precursor

temporal variations in fumarole gas chemistry at poas volcano costa rica we report the chemical and isotopic composition of fumarole gas discharges collected at poas volcano costa rica from 2001 to 2014 covering a period during which the volcano experienced a series of phreatic eruptions march 2006 to october 2014 the relative abundances of poas c s h o gas species are controlled by reactions involving the so2 h2s and so so2 gas buffers indicating magmatic temperatures of up to 800 c although fumarole outlet temperatures are 120 c for most samples so2 is the dominant sulfur gas and hcl contents are relatively high gas compositional changes within the magma lake hydrothermal system likely result from a combination of several processes including 1 the injection of new and undegassed magma in late 2000 early 2001 2 the heating of the hydrothermal system accompanied by gas pressure build up and 3 hydrofracturing through 2006 these processes culminated in the phreatic eruptions of 2006 and 2008 since 2005 the lake level has declined and is now january 2014 at the lowest level 10 m since the last periods that it dried out completely april 1984 and april 1994 the most recent data of 2014 show high level of degassing from the dome fumaroles and the release of hcl rich and co2 poor gases implies that the magma injected in late 2000 continues to supply volatiles our data show that time series sampling of fumarole gases provides important insights to better understand magmatic and hydrothermal processes at active volcanoes and also to potentially forecast phreatic eruptions

remobilization of andesite magma by intrusion of mafic magma at the soufriere hills volcano montserrat west indies the 1995 1999 eruption of the soufriere hills volcano montintroduction serrat has produced a crystal rich andesite containing quenchthe soufriere hills volcano montserrat located in the textured mafic inclusions which show evidence of having been molten lesser antilles island arc in the west indies began to when incorporated into the host magma individual crystals in the erupt on 18 july 1995 after about 350 years of dormancy andesite record diverse histories amphibole phenocrysts vary from young et al 1998b the eruption was preceded by a pristine and unaltered to strongly oxidized and pseudomorphed by period of seismic swarms which began in january 1992 anhydrous reaction products plagioclase phenocrysts are commonly previous earthquake crises have occurred on montserrat reverse zoned often with dusty sieve textures reverse zoned rims are in 1896 1897 1932 1935 and 1966 1967 powell 1938 also common on orthopyroxene phenocrysts pyroxene geothermometry shepherd et al 1971 initial phreatic and phregives an average temperature of 858 20 c for orthopyroxene atomagmatic activity was followed by extrusion of an phenocryst cores whereas reverse zoned rims record temperatures andesitic lava dome in november 1995 pyroclastic flows from about 880 to 1050 c the heterogeneity in mineral rim generated by gravitational collapse of the unstable dome compositions zoning patterns and textures is interpreted as reflecting have been the dominant form of activity throughout the non uniform reheating and remobilization of the resident magma eruption but periods of explosive activity producing body by intrusion of hotter mafic magma convective remobilization pumice flows and falls have also occurred robertson et results in mixing together of phenocrysts that have experienced al 1998 young et al 1998b dome growth ceased different thermal histories depending on proximity to the intruding suddenly in march 1998 and further activity up to may mafic magma the low temperature and high crystallinity are 1999 has mainly involved gravitational collapses of the interpreted as reflecting the presence of a cool highly crystalline dome and periods of ash venting with some small scale magma body beneath the soufriere hills volcano the petrological vulcanian explosive events observations in combination with data on seismicity extrusion rate the soufriere hills volcano is the only active centre and so2 fluxes indicate that the current eruption was triggered by on montserrat the volcano is a composite of at least recent influx of hot mafic magma five andesitic lava domes flanked by pyroclastic deposits rea 1974 identified four other volcanic centres all

forecasting etna eruptions by real time observation of volcanic gas composition it is generally accepted but not experimentally proven that a quantitative prediction of volcanic eruptions is possible from the evaluation of volcanic gas data by discussing the results of two years of real time observation of h2o co2 and so2 in volcanic gases from mount etna volcano we unambiguously demonstrate that increasing co2 so2 ratios can allow detection of the pre eruptive degassing of rising magmas quantitative modeling by the use of a saturation model allows us to relate the pre eruptive increases of the co2 so2 ratio to the refilling of etna s shallow conduits with co2 rich deep reservoir magmas leading to pressurization and triggering of eruption the advent of real time observations of h2o co2 and so2 combined with well constrained models of degassing represents a step forward in eruption forecasting

turmoil at turrialba volcano costa rica degassing and eruptive processes inferred from high frequency gas monitoring eruptive activity at turrialba volcano costa rica has escalated significantly since 2014 causing airport and school closures in the capital city of san jose whether or not new magma is involved in the current unrest seems probable but remains a matter of debate as ash deposits are dominated by hydrothermal material here we use high frequency gas monitoring to track the behavior of the volcano between 2014 and 2015 and to decipher magmatic versus hydrothermal contributions to the eruptions pulses of deeply derived co2 rich gas co2 s total 4 5 precede explosive activity providing a clear precursor to eruptive periods that occurs up to 2weeks before eruptions which are accompanied by shallowly derived sulfur rich magmatic gas emissions degassing modeling suggests that the deep magmatic reservoir is similar to 8 10km deep whereas the shallow magmatic gas source is at similar to 3 5km two cycles of degassing and eruption are observed each attributed to pulses of magma ascending through the deep reservoir to shallow crustal levels the magmatic degassing signals were overprinted by a fluid contribution from the shallow hydrothermal system modifying the gas compositions contributing volatiles to the emissions and reflecting complex processes of scrubbing displacement and volatilization h2s so2 varies over 2 orders of magnitude through the monitoring period and demonstrates that the first eruptive episode involved hydrothermal gases whereas the second did not massive degassing 3000t d so2 and h2s so2 1 followed suggesting boiling off of the hydrothermal system the gas emissions show a remarkable shift to purely magmatic composition h2s so2 0 05 during the second eruptive period reflecting the depletion of the hydrothermal system or the establishment of high temperature conduits bypassing remnant hydrothermal reservoirs and the transition from phreatic to phreatomagmatic eruptive activity

source mechanism of a very long period event at mt ontake central japan response of a hydrothermal system to magma intrusion beneath the summit abstract a very long period vlp volcanic event which occurred on 25 january 2007 beneath mt ontake central japan was recorded by nearby broadband seismometers as well as the japanese nationwide hi net and f net seismic networks the vlp waveforms show a pulse like shape with a duration of approximately 1 min the vlp event was accompanied by a burst sequence of long period lp events we carried out waveform inversion to determine the source mechanism and location of the vlp event our analysis indicates volumetric changes in an inclined crack seismic moment of 10 14 nm at the vlp source which is located at 0 6 km above sea level beneath the summit of mt ontake and above dikes determined from gps data the source time functions of the vlp event showed a pulse like triangular shape indicating inflation followed by deflation of the crack at the source we interpreted the result of the waveform inversion as follows gases released from the intruded magma heated an aquifer system beneath the summit of mt ontake a break of unstable superheated water in a hydrothermal crack in the system caused vaporization of water and associated crack inflation which was followed by discharge of steam from the crack resulting in its collapse these processes caused the vlp event which may have been a precursor of a small phreatic eruption that occurred in late march 2007

long term fluctuations in volcanic activity implications for future environmental impact acidic crater lakes at persistently active volcanoes act as both an index and a moderator of volcanic processes a catastrophic drop in lake level can therefore lead to serious local environ

the chemistry of fumarolic vapor and thermal spring discharges from the nevado del ruiz volcanic magmatic hydrothermal system colombia on the basis of three sets of gas samples taken during the three months preceding the november 13 1985 eruption of nevado del ruiz from fumaroles on the crater floor the composition of the vapors discharged following minor eruptive activity in september corresponded to that of an essentially magmatic gas during the subsequent quiet period in october the proportion of a hydrothermal component in the vapor discharge increased steadily up until the major eruption the gases were found to be exceptionally high in total sulfur and are super saturated with respect to elemental sulfur their thermodynamic equilibration temperatures range from 200 to 600 c gases and waters from fumaroles and thermal springs on the flanks of the volcano are likely to be derived from a two phase vapor brine envelope surrounding the magmatic system the chemistry of the fumarole discharges is compatible with the assumption of an eruptive mechanism involving the build up of pressure in an extensive vapor zone by gas released from an essentially stagnant already extensively degassed body of magmatic material at depth production of such a residual gas over extended periods may account for the large amount of so2 discharged from the volcano in relation to the small amount of ejected solids

coseismic deformation of the 2016 taiwan mw6 3 earthquake using insar data and source slip inversion abstract an earthquake of seismic moment magnitude mw 6 3 occurred in southern taiwan on february 6th 2016 in this paper we use differential interferometric synthetic aperture radar d insar technology and data from the sentinel 1a s 1a iw radar satellite to estimate the coseismic deformation of this event on the basis of ascending data insar analysis reveals a circle like uplift area 45 km in diameter 20 km northwest of the epicenter that has a maximum line of sight los displacement of 12 cm south and east of this uplift area minor subsidence occurred while descending data indicated uplift in the west and sink in the east with maximum values of 8 0 cm and 6 0 cm respectively displacements are continuously distributed across the entire deformed region implying that the seismogenic fault did not pierce the earth s surface based on these insar displacements coupled with gps observations we inverted the distribution of source slip using an elastic half space fault model both separately and jointly four kinds of inversion results consistently show a slip concentration to the northwest of the epicenter with maximum slip in the range 0 35 0 55 m confined to depths of 6 15 km in the crust the source fault is dominated by thrust with left lateral strike slip joint inversion using insar ascending and descending data as well as gps data resulted in maximum slip of 0 44 m within the range determined by separate inversions inversion results show that this was a mw6 25 earthquake consistent with measurements collected by the united states geological survey usgs

volcano tectonic earthquakes a new tool for estimating intrusive volumes and forecasting eruptions abstract we present data on 136 high frequency earthquakes and swarms termed volcano tectonic vt seismicity which preceded 111 eruptions at 83 volcanoes plus data on vt swarms that preceded intrusions at 21 other volcanoes we find that vt seismicity is usually the earliest reported seismic precursor for eruptions at volcanoes that have been dormant for decades or more and precedes eruptions of all magma types from basaltic to rhyolitic and all explosivities from vei 0 to ultraplinian vei 6 at such previously long dormant volcanoes because large eruptions occur most commonly during resumption of activity at long dormant volcanoes vt seismicity is an important precursor for the earth s most dangerous eruptions vt seismicity precedes all explosive eruptions of vei 5 and most if not all vei 4 eruptions in our data set surprisingly we find that the vt seismicity originates at distal locations on tectonic fault structures at distances of one or two to tens of kilometers laterally from the site of the eventual eruption and rarely if ever starts beneath the eruption site itself the distal vt swarms generally occur at depths almost equal to the horizontal distance of the swarm from the summit out to about 15 km distance beyond which hypocenter depths level out we summarize several important characteristics of this distal vt seismicity including swarm like nature onset days to years prior to the beginning of magmatic eruptions peaking of activity at the time of the initial eruption whether phreatic or magmatic and large non double couple component to focal mechanisms most importantly we show that the intruded magma volume can be simply estimated from the cumulative seismic moment of the vt seismicity from log10 v 0 77 log moment 5 32 with volume v in cubic meters and seismic moment in newton meters because the cumulative seismic moment can be approximated from the size of just the few largest events and is quite insensitive to precise locations the intruded magma volume can be quickly and easily estimated with few short period seismic stations notable cases in which distal vt events preceded eruptions at long dormant volcanoes include nevado del ruiz 1984 1985 pinatubo 1991 unzen 1989 1995 soufriere hills 1995 shishaldin 1989 1999 tacana 1985 1986 pacaya 1980 1984 rabaul 1994 and cotopaxi 2001 additional cases are recognized at frequently active volcanoes including popocateptl 2001 2003 and mauna loa 1984 we present four case studies pinatubo soufriere hills unzen and tacana in which we demonstrate the above mentioned vt characteristics prior to eruptions using regional data recorded by neic we recognized in near real time that a huge distal vt swarm was occurring deduced that a proportionately huge magmatic intrusion was taking place beneath the long dormant sulu range new britain island papua new guinea that it was likely to lead to eruptive activity and warned rabaul volcano observatory days before a phreatic eruption occurred this confirms the value of this technique for eruption forecasting we also present a counter example where we deduced that a vt swarm at volcan cosiguina nicaragua indicated a small intrusion insufficient to reach the surface and erupt finally we discuss limitations of the method and propose a mechanism by which this distal vt seismicity is triggered by magmatic intrusion

radiative transfer corrections for accurate spectroscopic measurements of volcanic gas emissions there is widespread use of passive remote sensing techniques to quantify trace gas column densities in volcanic plumes utilizing scattered sunlight as a light source examples include passive doas cospec and the so2 camera in order to calculate trace gas concentrations or volcanic emission fluxes knowledge about the optical path through the plume is necessary in the past a straight photon path through the plume has always been assumed although it was known that this is not always true here we present the results of model studies conducted specifically to quantify the effects of realistic radiative transfer in and around volcanic plumes on ground based remote sensing measurements of so2 the results show that measurements conducted without additional information on average photon paths can be inaccurate under certain conditions with possible errors spanning more than an order of magnitude both over and underestimation of the true column density can occur actual errors depend on parameters such as distance between instrument and plume plume so2 concentration plume aerosol load as well as aerosol conditions in the ambient atmosphere as an example a measurement conducted with an so2 camera is discussed the results of which can only be correctly interpreted if realistic radiative transfer is considered finally a method is presented which for the first time allows the retrieval of actual average photon paths in spectroscopic i e doas measurements of adequate resolution by allowing for a wavelength dependent column density during the evaluation of doas measurements we show how radiative transfer effects can be corrected using information inherently available in the measured spectra thus greatly enhancing the accuracy of doas measurements of volcanic emissions

relationship between surface water temperature and ice sheet expansion during the middle miocene abstract a three fold expansion of the antarctic ice sheet at 13 60 12 82 and 11 60 ma has been inferred from 18o maxima analyzed in planktonic and benthic foraminiferal tests although accompanying changes in sea surface temperature have not been detailed we present estimated changes in middle miocene surface water temperatures based on analysis of 18o in planktonic foraminifera collected at mid latitude deep sea drilling project sites in the north atlantic and south pacific oceans we also identify periods of ice sheet growth based on comparisons of benthic and planktonic foraminiferal 18o values our results indicate 1 a distinct cooling of the sea surface from 13 6 to 13 5 ma immediately following a peak in ice volume at 13 6 ma 2 a cooling of the sea surface during a period of increasing ice volume from 13 2 to 13 0 ma and 3 a development of the antarctic ice sheet during a period of cooling of the sea surface centered at 11 6 ma

explosive tephra emissions at mount st helens 1989 1991 the violent escape of magmatic gas following storms from 24 august 1989 until 18 june 1991 mount st helens produced at least 28 shallow explosion like seismic events with signatures similar to those produced by gas explosions on the dome during the mid 1980s at least six were accompanied by violent emission off non juvenile tephra ejection of blocks of rock nearly 1 km from the vent and avalanching of debris off the north side of the dome four produced no emission of tephra and the remainder occurred when the crater could not be observed although later crater visits found no new tephra deposits all six confirmed emissions and most although not all other seismic events took place hours to days after storms over time periods ranging from 2 to 15 days statistical goodness of fit tests give a probability of 1 x 10 2 to 1 x 10 5 that the precipitation prior to emissions was fortuitously greater than normal similar tests for precipitation prior to all 28 events give probabilities of 3 x 10 2 to 1 x 10 3 the short delay between storms and emissions suggests that the events that follow storms originate at very shallow depth probably within the dome itself although the exact causal mechanism is not known it is speculated that slope instability or accelerated growth of cooling fractures following storms may have released gas trapped within or at the base of the dome of the events that did not follow storms three were located seismically between 1 4 and 2 1 km depth thus the events as a group are not confined to shallow depth and probably record gas transport from a deeper magmatic source a deep source is also suggested by an unprecedented 4 yr long increase in background seismicity at 2 to 9 km depth that peaked in 1989 and 1990 calculations indicate that the energy content of gas exsolved from the crystallizing conduit was probably sufficient to power the emissions

short period volcanic gas precursors to phreatic eruptions insights from poas volcano costa rica abstract volcanic eruptions involving interaction with water are amongst the most violent and unpredictable geologic phenomena on earth phreatic eruptions are exceptionally difficult to forecast by traditional geophysical techniques here we report on short term precursory variations in gas emissions related to phreatic blasts at poas volcano costa rica as measured with an in situ multiple gas analyzer that was deployed at the edge of the erupting lake gas emitted from this hyper acid crater lake approaches magmatic values of so2 co2 1 6 days prior to eruption the so2 flux derived from magmatic degassing through the lake is measureable by differential optical absorption spectrometry sporadic campaign measurements which allows us to constrain lake gas output and input for the major gas species during eruptive and non eruptive periods we can further calculate power supply to the hydrothermal system using volatile mass balance and thermodynamics which indicates that the magmatic heat flux into the shallow hydrothermal system increases from 27 mw during quiescence to 59 mw during periods of phreatic events these transient pulses of gas and heat from the deeper magmatic system generate both phreatic eruptions and the observed short term changes in gas composition because at high gas flux scrubbing of sulfur by the hydrothermal system is both kinetically and thermodynamically inhibited whereas co2 gas is always essentially inert in hyperacid conditions thus the so2 co2 of lake emissions approaches magmatic values as gas and power supply to the sub limnic hydrothermal system increase vaporizing fluids and priming the hydrothermal system for eruption our results suggest that high frequency real time gas monitoring could provide useful short term eruptive precursors at volcanoes prone to phreatic explosions

the origin of volcano tectonic earthquake swarms volcano tectonic vt seismicity is commonly recorded prior to and during eruptions vt seismicity may reflect stresses induced by dike propagation as indicated by propagating hypocenters and fault plane solutions fps reflecting regional stresses or stresses induced by dike inflation indicated by randomly distributed hypocenters and fps with pressure axes rotated 90 to regional maximum compression although numerical models of dike induced stresses indicate that both regimes should occur during dike emplacement this has not yet been observed according to published seismic data instead 90 rotated fps are observed in some cases while propagating hypocenters mark dike formation in other cases we suggest that differences in the seismic expression of upper crustal magma migration may result from differences in the regional tectonic setting and in the nature of magma wall rock interactions ultimately it may be possible to use information contained in vt seismicity to forecast changes in a volcano s behavior by establishing the characteristic stress field response for a given volcano or through a deeper understanding of the complex relationships between vt seismicity local crustal stresses and the physical mechanisms of magma migration

chemical evolution and volcanic activity of the active crater lake of poas volcano costa rica 1993 1997 abstract concentrations of chloride and sulfate and ph in the hot crater lake laguna caliente at poas volcano and in acid rain varied over the period 1993 1997 these parameters are related to changes in lake volume and temperature and changes in summit seismicity and fumarole activity beneath the active crater during this period lake level increased from near zero to its highest level since 1953 lake temperature declined from a maximum value of 70 c to a minimum value of 25 c and ph of the lake water increased from near zero to 1 8 in may 1993 when the lake was nearly dry chloride and sulfate concentrations in the lake water reached 85 400 and 91 000 mg l 1 respectively minimum concentrations of chloride and sulfate after the lake refilled to its maximum volume were 2630 and 4060 mg l 1 respectively between january 1993 and may 1995 most fumarolic activity was focused through the bottom of the lake after may 1995 fumarolic discharge through the bottom of the lake declined and reappeared outside the lake within the main crater area the appearance of new fumaroles on the composite pyroclastic cone coincided with a dramatic decrease in type b seismicity after january 1996 between may 1995 and december 1997 enhanced periods of type a seismicity and episodes of harmonic tremor were associated with an increase in the number of fumaroles and the intensity of degassing on the composite pyroclastic cone adjacent to the crater lake increases in summit seismic activity type a b and harmonic tremor and in the height of eruption plumes through the lake bottom are associated with a period of enhanced volcanic activity during april september 1994 at this time visual observations and remote fumarole temperature measurements suggest an increase in the flux of heat and gases discharged through the bottom of the crater lake possibly related to renewed magma ascent beneath the active crater a similar period of enhanced seismic activity that occurred between august 1995 and january 1996 apparently caused fracturing of sealed fumarole conduits beneath the composite pyroclastic cone allowing the focus of fumarolic degassing to migrate from beneath the lake back to the 1953 1955 cone changes in the chemistry of summit acid rain are correlated changes in volcanic activity regardless of whether fumaroles are discharging into the lake or are discharging directly into the atmosphere

intense magmatic degassing through the lake of copahue volcano 2013 2014 here we report on the first assessment of volatile fluxes from the hyperacid crater lake hosted within the summit crater of copahue a very active volcano on the argentina chile border our observations were performed using a variety of in situ and remote sensing techniques during field campaigns in march 2013 when the crater hosted an active fumarole field and in march 2014 when an acidic volcanic lake covered the fumarole field in the latter campaign we found that 566 to 1373 t d 1 of so2 were being emitted from the lake in a plume that appeared largely invisible this combined with our derived bulk plume composition was converted into flux of other volcanic species h2o 10989 t d 1 co2 638 t d 1 hcl 66 t d 1 h2 3 3 t d 1 and hbr 0 05 t d 1 these levels of degassing comparable to those seen at many open vent degassing arc volcanoes were surprisingly high for a volcano hosting a crater lake copahue s unusual degassing regime was also confirmed by the chemical composition of the plume that although issuing from a hot 65 c lake preserves a close to magmatic signature eq3 6 models of gas water rock interaction in the lake were able to match observed compositions and demonstrated that magmatic gases emitted to the atmosphere were virtually unaffected by scrubbing of soluble s and cl species finally the derived large h2o flux 10 988 t d 1 suggested a mechanism in which magmatic gas stripping drove enhanced lake water evaporation a process likely common to many degassing volcanic lakes worldwide

fluid volcano interaction in an active stratovolcano the crater lake system of poas volcano costa rica rowe g l jr brantley s l fernandez m fernandez j f borgia a and barquero j 1992 fluid volcano interaction in an active stratovolcano the crater lake system of po is volcano costa rica j volcanol geotherm res 49 2351 seismic and geochemical data collected at po is volcano costa rica since 1978 suggest that temperature and chemical variations recorded in subaerial fumaroles and the crater lake are related to episodic release of heat and volatiles associated with hydrofracturing of the upper margin of the shallow magma body power outputs associated with these events approach 600 mw and are superimposed on a baseline energy flux of approximately 200 mw the baseline heat flux suggests a magma solidification rate of 0 012 km3 yr and background volatile release rates of 1000 t d h20 66 t d s 13 t d ci and 0 5 t d f these fluxes are comparable to fluxes of f c1 and s exiting the magmatic hydrothermal system through acidic flank springs and are about a factor of five less than estimated volatile fluxes through summit fumaroles during the high temperature event of 1981 1983 mass balance considerations suggest that heat released at the lake surface is primarily supplied by the ascent of heated brines supplemented by condensation of fumarolic steam calculated average seepage rates out of the lake 450 kg s indicate rapid convection of acidic lake brine through underlying lake sediments and pyroclastic deposits circulation of the extremely corrosive lake brine through the volcanoclastic material beneath the lake may enhance subsurface permeabilities fluxes of rock forming elements observed in the rio agrio basin on the northwest flank of po is suggest that dissolution and removal of volcano elastic material occurs at a rate of approximately 1650 m3 yr however estimated sulfur fluxes from the cooling magma body suggest that porosity created by this dissolution could quickly be filled by the deposition of native sulfur finally analysis of heat and water budgets for the crater lake over the period 1978 1989 indicates that the recent decline and disappearance of the lake was caused by variations in summit rainfall combined with an increase in subsurface heat flow to the lake in 1987 1988 the increase in heat flow is related to continued degassing of high temperature volatiles following a hydrofracturing magma ascent episode that occurred in 1986

cyclic processes and factors leading to phreatic eruption events insights from the 25 september 2007 eruption through ruapehu crater lake new zealand abstract a dangerous block ash eruption occurred without warning from ruapehu crater lake on 25 september 2007 ending a 6 month period of temperature decline in the lake the northerly directed blast sent ballistics of up to 2 m diameter more than 2 km from the source the ejecta comprised a variety of volcano lacustrine lithologies many showing evidence of intense hydrothermal alteration petrographic studies of the ballistics reveal numerous examples of pores vesicles being filled with assemblages of elemental sulphur anhydrite pyrite and natroalunite and many ballistics exuded liquid sulphur from their pores during cooling at atmospheric pressure up to 5 of the ash was comprised of fine grained juvenile glass material confirming the presence of magma in the shallow vent environment a long recognised characteristic of this volcano is the thermal cycling behaviour of the crater lake behaviour which has remained effectively unchanged through two magmatic eruption events in 1945 and in 1995 96 airborne measurements of co2 and so2 flux from the volcano since 2003 show a strong positive correspondence with temperature cycling in the lake pointing to a definitive link between heat and mass discharge through the system low so2 emission rates and c s mole ratios 30 were typical of the period 2003 to early 2007 and appear to reflect vigorous scrubbing of s gases in the hydrothermal system during this time to better understand the chemical and physical processes operating in this very dynamic volcanic hydrothermal system and its propensity to develop potentially dangerous hydrothermal seals we have developed an integrated finite difference tough2 heat mass transport simulation of the hydrothermal environment the model which is constrained by observed heat and mass flux characteristics of the volcano demonstrates the transient nature of the heating cycles on the vent lake system of particular interest is the role of co2 as a relatively inert pressure transmitting medium in the hydrothermal environment seal formation was simulated using the reactive transport code x1t which showed that condensation of magmatic vapour into a highly porous medium of andesitic composition leads to rapid formation of an elemental sulphur anhydrite natroalunite mineral assemblage and a drastic reduction in permeability we suggest that since the 1995 96 vent clearing eruptions a shroud of elemental sulphur and other alteration phases formed along the margins of the 2 phase liquid vapour region enclosing the conduit this effectively seals the upper portion of the vent from the adjacent hydrostatically controlled environment and allows for development of pressurised vapour static gas columns beneath the northern and central vents

poas volcano costa rica geology of the summit region and spatial and temporal variations among the most recent lavas prosser j t and carr m j 1987 po s volcano costa rica geology of the summit region and spatial and temporal variations among the most recent lavas in s n williams and m j carr editors richard e stoiber 75th birthday volume j volcanol geotherm res 33 131 146 the recent eruptive history of po s volcano is described here on the basis of field mapping of the summit area this period which may represent 10 to 20 000 years and less than 10 of the history of the volcano began with caldera formation subsequent events included composite cone construction faulting and flank subsidence flank fissure eruption and multiple crater collapses local and regional structures have controlled the locations of summit vents and flank cinder cones chemical compositions of lavas of known stratigraphic position demonstrate temporal magmatic variation at po s three similar felsic to mafic magmatic sequences occurred at the summit the first and second sequences were separated by flank and summit eruptions of a distinct magma batch enriched in tio and p205 the three felsic to mafic sequences appear to be cyclical and the repeated similar variations are interpreted as progressive tapping of zoned magma bodies developed repeatedly from a common parent by similar crystal fractionation processes the breaks between the three sequences correlate with shifts or major modifications of the eruptive center the chemical composition of the most recent lava erupted in 1954 indicates that the volcano is near the end of the present sequence the lavas are calc alkaline basalts and andesites that are similar to the mafic lavas that comprise the bulk of the central american volcanic front the samples can be divided into 3 spatial temporal geochemical groups a summit group that represents the most recent activity a tio2 rich group that erupted primarily on the south flank about 7500 yr b p and an al203 rich group that erupted primarily from vents north of the presently active main crater geochemical variations and projections into pseudoternary cmas diagrams suggest a moderate pressure for the summit group that is a magma chamber at intracrustal to subcrustal depths the al203 rich group define smaller primary phase volumes for olivine and plagioclase which suggests a deeper and or more water rich magma chamber the tio2rich group appears to be a batch of mixed magma with the summit group magma as one endmember and a less siliceous magma as the other