estimating thermal inflow to el chichon crater lake using the energy budget chemical and isotope balance approaches abstract el chichon crater lake appeared immediately after the 1982 catastrophic eruption in a newly formed 1 km wide explosive crater during the first 2 years after the eruption the lake transformed from hot and ultra acidic caused by dissolution of magmatic gases to a warm and less acidic lake due to a rapid magmatic to hydrothermal transition input of hydrothermal fluids and oxidation of h 2 s to sulfate chemical composition of the lake water and other thermal fluids discharging in the crater stable isotope composition d and 18 o of lake water gas condensates and thermal waters collected in 1995 2006 were used for the mass balance calculations cl so 4 and isotopic composition of the thermal flux from the crater floor the calculated fluxes of thermal fluid by different mass balance approaches become of the same order of magnitude as those derived from the energy budget model if values of 1 9 and 2 mmol mol are taken for the catchment coefficient and the average h 2 s concentration in the hydrothermal vapors respectively the total heat power from the crater is estimated to be between 35 and 60 mw and the co 2 flux is not higher than 150 t day or 200 gm 2 day 1

hydrogeochemical surveillance at el chichon volcano crater lake chiapas mexico abstract el chichon volcano has an eruptive record of at least 12 major eruptions in the holocene the latest one in march april 1982 causing the worst volcanic disaster in the history of mexico after about 6 centuries of quiescence this eruption destroyed a large dome and opened a 1 km wide crater a lake formed within the crater shortly after the eruption has been an important source of information about the evolution of the post eruptive processes the fluctuations of the crater lake water physicochemical parameters observed since 1983 have allowed in identifying hydrothermal waters and h 2 s rich gases influenced by tectonic and meteorological effects as the main contributors to its composition here we propose some methods to help in assessing the state of the volcano derived from the relative contribution of these factors as an easy to implement volcanic surveillance method in potentially active volcanoes with crater lakes or other volcano influenced water sources

major and trace element geochemistry of neutral and acidic thermal springs at el chichon volcano mexico implications for monitoring of the volcanic activity abstract four groups of thermal springs with temperatures from 50 to 80 c are located on the s sw w slopes of el chichon volcano a composite dome tephra edifice which exploded in 1982 with a 1 km wide 160 m deep crater left very dynamic thermal activity inside the crater variations in chemistry and migration of pools and fumaroles drastic changes in the crater lake volume and chemistry contrasts with the stable behavior of the flank hot springs during the time of observations 1974 2005 all known groups of hot springs are located on the contact of the basement and volcanic edifice and only on the w sw s slopes of the volcano at almost same elevations 600 650 m asl and less than 3 km of direct distance from the crater three groups of near neutral ph 6 springs at sw s slopes have the total thermal water outflow rate higher than 300 l s and are similar in composition the fourth and farthest group on the western slope discharges acidic ph 2 saline 10 g kg of cl water with a much lower outflow rate water rock interaction modeling of main types of the el chichon thermal waters using regular log q k graphs saturation indices vs temperature showed maximum equilibrium temperature slightly higher than 200 c acidic waters are equilibrated with some clay minerals at about 120 c three main sources of the salinity of thermal water are suggested on the basis of mixing plots and isotopic data a magmatic source for co2 boron sulfur and a limited part of cl volcanic rock source for the major cations and trace elements the oil bearing evaporitic basement source oil field brine for nacl br a part of ca and some trace elements all flank thermal springs end up in the river rio magdalena that has a variable seasonal flow rates from 4 to 20 m3 s any changes in the chemistry of springs must notably change the composition of the streams draining hot springs and eventually rio magdalena a monthly geochemical monitoring of rio magdalena and streams draining main hot springs would be a useful tool for surveying the activity of the volcano

explosion quakes at stromboli italy abstract this paper reports the results of two seismic experiments aimed at determining the wave field of explosion quakes at stromboli island mediterranean sea southern italy the typical strombolian activity mostly consists of explosive phenomena causing pyroclastic materials to be emitted together with jets of volcanic gases from one or more craters stromboli is an active volcano characterized by persistent seismic activity consisting of explosion quakes that are seismic events associated with the explosive volcanic phenomena explosion quakes are short lived seismic events occurring intermittently whose amplitude tends to decrease with distance from the vent a distinctive feature of explosion quakes is the presence on seismograms of two often clearly distinct seismic phases the first low frequency seismic phase 3 4 hz after one second or more the first seismic phase of explosion quakes has been shown to be characterized by a nearly radial linear polarization and by an apparent propagation velocity estimated at 600 800 m s the second phase is characterized by a more chaotic motion and a lower apparent propagation velocity of 150 450 m s the wavefield associated with the first low frequency seismic phase appears to be generated by a resonating p wave seismic source accompanying gas explosion and emission of pyroclastic materials the wavefield associated with the second high frequency seismic phase of explosion quakes appears to be mainly composed of scattered and converted waves due to the critical topography of the volcano

hydrochemical dynamics of the lake spring system in the crater of el chichon volcano chiapas mexico abstract el chichon volcano chiapas mexico erupted violently in march april 1982 breaching through the former volcano hydrothermal system since then the 1982 crater has hosted a shallow 1 3 3 m acidic ph 2 2 and warm 30 c crater lake with a strongly varying chemistry cl so 4 0 79 molar ratio the changes in crater lake chemistry and volume are not systematically related to the seasonal variation of rainfall but rather to the activity of near neutral geyser like springs in the crater soap pool these soap pool springs are the only sources of cl for the lake their geyser like behaviour with a long term months to years periodicity is due to a specific geometry of the shallow boiling aquifer beneath the lake which is the remnant of the 1983 cl rich 24 000 mg l crater lake water the soap pool springs decreased in cl content over time the zero time extrapolation 1982 year of the eruption approaches the cl content in the initial crater lake meanwhile the extrapolation towards the future indicates a zero cl content by 2009 1 this particular situation offers the opportunity to calculate mass balance and cl budget to quantify the lake spring system in the el chichon crater these calculations show that the water balance without the input of sp springs is negative implying that the lake should disappear during the dry season the isotopic composition of lake waters d and 18 o coincide with this crater lake sp dynamics reflecting evaporation processes and mixing with sp geyser and meteoric water future dome growth not observed yet in the post 1982 el chichon crater may be anticipated by changes in lake chemistry and dynamics

a 550 year old plinian eruption at el chichon volcano chiapas mexico explosive volcanism linked to reheating of the magma reservoir 1 some 550 years ago 1320 1433 a d a powerful plinian eruption at el chichon volcano in southern mexico produced a widespread pumice fall deposit we subdivided the deposit into three parts on the basis of structural and textural characteristics pumice lithology and density granulometry and petrologic geochemical attributes the deposit covers an area of 1500 km2 within the 1 cm isopach and has a minimum estimated bulk volume of 2 8 km3 1 1 km3 dense rock equivalent dre its eruptive column reached an altitude of 31 km consideration of field evidence the presence and nature of mafic enclaves and chemical data strongly suggest that the 550 year b p eruption is linked with the intrusion of a high temperature basaltic magma into preexisting but stagnated trachyandesitic magma beneath el chichon thorough mixing of the two magmas produced a compositionally uniform hybrid trachyandesite magma average sio2 55 3 wt which subsequently underwent crystal growth and gas exsolution ultimately overpressurizing the zoned magmatic system to erupt explosively on the basis of el chichon s known eruptive history the intrusion mixing event occurred sometime after the 900 year b p eruption the hybrid magma had a preeruption temperature of 820 830 c and was water undersaturated 5 6 wt h2o at pressures of 2 2 5 kbar

chemical characteristics of the crater lakes of popocatetetl el chichon and nevado de toluca volcanoes mexico abstract three crater lakes from mexican volcanoes were sampled and analyzed at various dates to determine their chemical characteristics strong differences were observed in the chemistry among the three lakes nevado de toluca considered as dormant el chichon at a post eruptive stage and popocatepetl at a pre eruptive stage not surprisingly no influence of volcanic activity was found at the nevado de toluca volcano while the other volcanoes showed a correlation between the changing level of activity and the evolution of chemical trends low phs so 4 2 and f decreased irregularly at el chichon from 1983 until 1997 major cations concentrations also diminished at el chichon a 100 increase in the so 4 2 content was measured at popocatepetl between 1985 and 1994 an increase in the mg cl ratio between 1992 mg cl 0 085 and 1994 mg cl 0 177 was observed at popocatepetl before the disappearance of the crater lake in 1994 it is concluded that chemical analysis of crater lakes may provide a useful additional tool for active volcano monitoring

strike slip faults and k alkaline volcanism at el chichon volcano southeastern mexico abstract el chichon volcano is located in the northern part of the state of chiapas in southeastern mexico the volcano is situated within the transcurrent fault province characterized by a series of rising and sinking blocks bounded by sinistral strike slip faults the basement around el chichon is a sequence of jurassic evaporates cretaceous limestones and tertiary terrigenous rocks folded into open and an echelon structures the main structures in the area are the la union and caimba anticlines and the buena vista syncline the region is cross cut by two conjugate fault systems a dextral strike slip n s trending set and a sinistral strike slip e w set the most significant fault of the latter system is the san juan fault which controlled the emplacement of a basaltic dike near the village of chapultenango some 1 1 ma ago and the activity of el chichon during the last 0 2 ma additionally a series of n45 e trending faults chapultengo fault system have produced a half graben geometry of blocks on top of which el chichon has been emplaced microstructures such as slickensides tension fractures layer parallel slip stylolites and macro and meso folds of the sedimentary rocks indicate that during the late miocene the el chichon area was affected by a maximum principal stress 1 oriented n70 e a minimum principal stress 3 oriented n20 w and a vertical intermediate principal stress this stress pattern indicates that the area underwent a strike slip motion that produced widespread deformation the occurrence of crustal earthquakes

reappraisal of the 1982 eruptions of el chichon volcano chiapas mexico new data from proximal deposits additional data from proximal areas enable a reconstruction of the stratigraphy and the eruptive chronology of phases iii and iv of the 1982 eruption of el chichon volcano phase iii began on 4 april at 0135 gmt with a powerful hydromagmatic explosion that generated radially fast moving 100 ms 1 pyroclastic clouds that produced a surge deposit s1 due to the sudden reduction in the confining pressure the process continued by tapping of magma from a deeper source causing a new explosion the ejected juvenile material mixed with large amounts of fragmented dome and wall rock which were dispersed laterally in several pulses as lithic rich block and ash flow f1 partial evacuation of juvenile material from the magmatic system prompted the entrance of external water to generate a series of hydromagmatic explosions that dispersed moisture rich surge clouds and small volume block and ash flows iu up to distances of 3 km from the crater the eruption continued by further decompression of the magmatic system with the ensuing emission of smaller amounts of gas rich magma which with the strong erosion of the volcanic conduit formed a lithic rich plinian column that deposited fallout layer b associated with the widening of the vent an increase in the effective density of the uprising column took place causing its collapse block and ash flows arising from the column collapse traveled along valleys as a dense laminar flow f2 in some places flow regime changes due to topographic obstacles promoted transformation into a turbulent surge s2 which attained minimum velocities of approximately 77 ms 1 near the volcano the process continued with the formation of a new column on 4 april at 1135 gmt phase iv that emplaced fall deposit c and was followed by hydromagmatic explosions which produced pyroclastic surges s3

morphology of ash aggregates from wet pyroclastic surges of the 1982 eruption of el chichon volcano mexico the detailed stratigraphic study of the pyroclastic surge units s1 iu and s3 produced during the most violent phases of the 1982 eruption of el chichon volcano contains a complex succession of hydromagmatic events triggered by the interaction of different proportions of magma and external water component analyses of the horizons within single units reveal that almost all wet and cohesive horizons contain ash aggregates based on their morphology and internal structure four different types of aggregates were distinguished a accretionary lapilli b armored lapilli c irregular aggregates and d cylindrical aggregates the first three types have been described in the volcanological literature field and experimental studies cylindrical forms are reported here for the first time these hollow cylindrical aggregates consist of concentric layers of crystals and glass fragments set in a finer grained matrix they formed around millimeter size foliage fragments that are locally preserved in the interior of the aggregates as scorched or completely carbonized vestiges sem analyses suggest different mechanisms of formation for the four types of aggregates irregular aggregates and armored lapilli formed nearly instantaneously whereas accretionary lapilli and cylindrical aggregates resulted from progressive aggregation of ash in different regions of the eruptive cloud

thermal evolution of the crater lake of copahue volcano with aster during the last quiescence period between 2000 and 2012 eruptions abstract active crater lakes are the surface expression of volcanic activity at the top of complex magmatic hydrothermal systems the volcanic and hydrothermal system copahue is one of the most important active systems in argentina it lies at the border of argentina and chile and is very important since the copahue and caviahue towns are very close and constitute important touristic resources including a ski center the aim of this work is to improve the temperature register of the volcano and contribute towards a better understanding of the thermal behavior of the system during the quiescence period between the 2000 and 2012 eruptions the surface temperature of the crater lake was estimated by using the normalized emissivity method nem on 70 images acquired by the advanced spaceborne thermal emission and reflection radiometer aster and we combined them with field measurements in order to capture the fundamental variations of the system the analysis allowed the identification of two different thermal periods i january 2001 august 2003 and ii december 2004 november 2011 characterized by temperature ranges of 43 50 c and 30 45 c respectively beyond these ranges temperature anomalies or moments of high temperature variability occurred which corresponded to surface expressions of volcanic activity in august 2004 the lake froze almost completely whereas in march 2012 peak temperatures were recorded however a significant drop in temperature prior to the eruption of december 22 2012 was observed which might constitute a characteristic feature of the system furthermore the crater lake of copahue volcano is classified as a lake of high activity the observations and interpretations presented constitute a useful baseline and a significant contribution towards a better understanding of the functioning of the volcano the mitigation of volcanic hazards and the identification of volcanic unrest

the 1982 eruptions of el chichon volcano mexico stratigraphy of pyroclastic deposits abstract following a repose period of 500 to 600 years el chichon volcano in se mexico erupted explosively during the months of march and april 1982 three major eruptions involving trachyandesite magma injected ash into the stratosphere on 29 march at 0515 gmt and on 4 april at 0139 and 1110 gmt the first major eruption generated only tephra fall which was dispersed ene in the troposphere over the yucatan peninsula and wsw in the stratosphere towards the pacific ocean the second eruption produced tephra fall and pyroelastic surges and pyroclastic flows by repeated collapse of a lithicrich eruption column the surge and flow activity devastated a roughly circular area 153 km 2 around el chichon to a distance of about 6 km from the vent nine villages within the zone were either partially or completely destroyed and loss of human life was perhaps as high as 2000 pyroclastic flows and surges were also generated during the third eruption although ash fall was the primary activity as a result of both the second and third eruptions tephra was transported ene in the troposphere and wsw in the stratosphere the total volume of material ejected during the three major eruptions 0 50 km 3 d r e is represented by tephra fall 0 37 km 3 pyroclastic surges 0 09 km 3 and pyroclastic flows and debris flows 0 04 km 3 as a result of the eruptions a new 1 km wide and 300 m deep crater was formed by the disruption and excavation of a pre existing summit dome

extreme alteration in an acid sulphate geothermal field sulphur springs saint lucia abstract sulphur springs is a vigorous geothermal field associated with the soufriere volcanic centre in southern saint lucia intensely altered rocks occur over an area of at least 200 400 m together with bubbling hot pools and fumaroles the pools are sodium calcium sulphate type with phs of 3 7 and temperatures of 41 97 c fumaroles have temperatures up to and at times above 100 c gases collected from both fumaroles and bubbling pools have high contents of co2 601 993 mmol mol and commonly high h2s 3 190 mmol mol least altered host rocks surrounding sulphur springs comprise calc alkaline feldspar quartz porphyritic dacites of near uniform chemical composition these rocks which form massive domes and volcaniclastic units were emplaced 13 30 ka ago lindsay et al 2013 within the geothermal field the dacites have been moderately to highly altered to acid sulphate assemblages of kaolinite illite smectite alunite natroalunite cristobalite opaline silica native sulphur jarosite and amorphous compounds muddy sediments from grey to blackish thermal pools additionally contain iron sulphides despite intense alteration of the volcanic rocks and derived volcaniclastic sediments zr and ti have remained essentially immobile which allows identification of the precursors dacites and calculation of mass changes many of the altered rocks and sediments show major depletions of fe mg ca na extremely altered samples additionally show notable losses of al p k ree and y implying leaching by highly acidic waters a few of the highly altered rocks however have gained al together with p presumably due to neutralization or evaporation of acidic thermal waters also present within the geothermal field are metre scale zones with very high contents of silica and native sulphur and surface thermal waters define a linear trend in a plot of 18o vs d and reach high levels of isotopic enrichment the enrichment trend is ascribed to progressive evaporation in the thermal pools the low slope of the isotopic trend 1 4 is interpreted as being due to surficial mixing between meteoric waters and steam condensates derived at about 230 c from the deep brine that underlies the geothermal field the whole rock o h isotopic composition of four highly altered samples kaolinite smectite opal sulphates can be accounted for if the alteration fluid was a 50 50 mixture of steam and meteoric water and alteration occurred at 50 125 c whole rock 34s values of samples containing variable amounts of sulphates sulphides and native s are all notably negative 5 to 16 consistent with derivation of sulphur from volcanic gases such as so2 and h2s altered rocks and thermal pool sediments in the geothermal field are commonly enriched in hg and se up to 50 mg kg at sulphur springs primary dacitic rocks have been chemically and mineralogically transformed by reaction with acidic waters that formed when uprising volcanic gases condensed into neutral meteoric waters in many cases this has led to substantial mass losses or gains in al p and rees regardless of the degree of alteration however the precursor rocks can still be identified using ti zr relations

d h and 18 o 16 o ratios of water in the crater lake at kusatsu shirane volcano japan abstract the d h and 18o 16o ratios of water in the active crater lake situated on the kusatsu shirane volcano japan are about 20 and 6 respectively higher than local meteoric water the ratios show seasonal variations superimposed on a gradual change over nine years the isotopic ratios started to increase in early 1990 and decrease in the spring of 1995 the seasonal variation which is high in winter and low in summer correlates with the temperature difference between lake water and ambient air the large temperature difference in winter enhances the evaporation of lake water and produces the enriched isotopic ratios relative to the ratios in summer the accumulation of snow and the decrease in the flux of meteoric water into the lake strengthens the winter time isotopic enrichment the enriched isotopic ratios of the lake water over a long time result from the addition of an end member with heavy isotopic ratios contained in a thermal fluid supplied to the lake considering the water balance in the lake the isotopic ratios of the thermal fluid were found to be close to the lake water itself suggesting the circulation of the lake water seeping through lake floor based on the correlation between cl concentration and the isotopic ratios the contribution by the heavy end member was estimated to be 25 36 relative to the enrichment by evaporation the heavy end member could be a liquid phase evolved from a parental fluid which is a mixture of local meteoric water and a magmatic fluid as found in high temperature volcanic gases

dynamic fluid recycling at laguna caliente poas costa rica before and during the 2006 ongoing phreatic eruption cycle 2005 10

co2 flux and chemistry of el chichon crater lake mexico in the period 2013 2015 evidence for the enhanced volcano activity the co2 flux from el chichon crater in the period from 2014 to 2015 increased by a factor of 3 from 260 t d 1 to 800 t d 1 compared to the 2007 2008 data the mechanism triggering the co2 flux from the lake surface has changed from being dominantly diffusive to mainly advective bubbling the cl concentration was steadily decreasing during the last two decades in the na cl boiling springs feeding the lake since 2013 for both lake and sp springs the cl concentration has increased up to the 2005 cl concentration levels a large degassing spot in the middle of the lake was found in april 2015 with a co2 flux of more than 10 000 g m 2 d 1 these observations are the evidence of the onset of a new stage of activity within the volcano hydrothermal system of el chichon volcano associated most probably with changes in the magmatic activity at depth

long term avhrr observations of surface radiative flux from el chichon crater lake 1996 2006 abstract it has been shown that due to the small surface of crater lakes temperature surveillance is a problem using meteorological satellites this is particularly true for el chichon surface lake because it s about one tenth of an avhrr pixel at nadir in order to guarantee at least one unmixed pixel in avhrr data it is necessary to use only avhrr data from noaa satellite passes as close as possible to the nadir for the period 1996 2006 therefore avhrr data of el chichon s crater lake were only used it they were cloudless and had scan angles close to nadir the analysis of the time series data shows that lake surface temperature had annual maximum values 35 c during 1996 and 1997 then surface temperature decay with a negative exponential trend reaching a steady state of about 30 c in the last years 2004 2006 a seasonal temperature variation between the dry december to may and the wet june to november seasons is also observed differences between nocturnal and midday temperatures indicate the influence of lake energy emission including reflectance at midday under a strong short wave solar radiation water surface radiative flux under these conditions reaches an average of 77 8 w m 2 and a maximum of 187 1 w m 2 whereas nocturnal heat output from el chichon crater lake has an average surface radiative flux of 20 4 w m 2 and a maximum of 74 3 w m 2 the results show that radiant flux avhrr data can still be considered a useful tool for detecting thermal changes in small crater lakes over time that would warrant a real time warning monitoring system

tracing thermal aquifers of el chichon volcano hydrothermal system mexico with 87sr 86sr ca sr and ree abstract the volcano hydrothermal system of el chichon volcano chiapas mexico is characterized by numerous thermal manifestations including an acid lake steam vents and boiling springs in the crater and acid and neutral hot springs and steaming ground on the flanks previous research on major element chemistry reveals that thermal waters of el chichon can be divided in two groups 1 neutral waters discharging in the crater and southern slopes of the volcano with chloride content ranging from 1500 to 2200 mg l and 2 acid to neutral waters with cl up to 12 000 mg l discharging at the western slopes our work supports the concept that each group of waters is derived from a separate aquifer aq 1 and aq 2 in this study we apply sr isotopes ca sr ratios and ree abundances along with the major and trace element water chemistry in order to discriminate and characterize these two aquifers waters derived from aq 1 are characterized by 87 sr 86 sr ratios ranging from 0 70407 to 0 70419 while sr concentrations range from 0 1 to 4 mg l and ca sr weight ratios from 90 to 180 close to average values for the erupted rocks waters derived from aq 2 have 87 sr 86 sr between 0 70531 and 0 70542 high sr concentrations up to 80 mg l and ca sr ratio of 17 28 aquifer 1 is most probably shallow composed of volcanic rocks and situated beneath the crater within the volcano edifice aquifer 2 may be situated at greater depth in sedimentary rocks and by some way connected to the regional oil gas field brines the relative water output l s from both aquifers can be estimated as aq 1 aq 2 30 both aquifers are not distinguishable by their ree patterns the total concentration of ree however strongly depends on the acidity all neutral waters including high salinity waters from aq 2 have very low total ree concentrations 4 ground waters draining fresh and old pyroclastic deposits rich in magmatic anhydrite it is suggested that the main mechanism controlling the concentration of ree in waters of el chichon is the acidity as low ph results from the shallow oxidation of h 2 s contained in hydrothermal vapors ree distribution in thermal waters reflects the dissolution of volcanic rocks close to the surface or lake sediments as is the case for the crater lake

update on the fluid geochemistry monitoring time series for geothermal systems in dominica lesser antilles island arc 2009 2017 abstract with updated geochemical and isotopic compositions obtained over the period 2009 to 2017 this study presents revised fluid characterizations for the active volcanic hydrothermal systems on the island of dominica in the lesser antilles which were first reported in 2011 hydrothermal waters of dominica cover a wide spectrum of ph temperatures and chemical composition the ph of the thermal waters ranges from acidic to neutral ph values of 1 7 8 and the waters are predominantly na so4 in character na 14 2127 mg l so4 1 1725 mg l likely formed as a result of dilution of acidic h2s rich gases in near surface oxygenated groundwater and have experienced limited water rock interaction the geochemical composition of the waters for most of the hydrothermal systems studied indicate no significant changes with the exception of the boiling lake which experienced a short 6 week episode of instability in november 2016 which appeared to be associated with a small mud rich explosion unlike the last such event in december 2004 which was reported to be earthquake triggered this event is possibly the consequence of a moderate sized landslide into the lake the lake draining episodes have been accompanied by changes in composition between na so4 and na cl which is attributed to hydrothermal fluid contributions from two different aquifers a shallower acid sulphate hydrothermal aquifer and a deeply sourced brine aquifer reservoir temperatures determined by quartz geothermometers have not changed significantly over the monitoring period suggesting steady state degassing of the magma chambers in two areas temperatures have increased watten waven from 83 90 c to 89 139 c and sulphur springs from 145 152 c to 93 243 c the elevated reservoir temperatures have affected the isotopic composition of the waters 18o 5 7 to 9 1 and d 8 to 20 5 that reflect a dominantly meteoric source with boiling degassing and evaporation also playing an important role the time series data suggests that some hydrothermal areas are experiencing increased steam evaporation over time whereas other waters are becoming more meteoric the 13cdic is decoupled from the deuterium and oxygen 18 isotopes and shows very little variation over time but a broad range in values from 11 to 5 the dominant process contributing to 13cdic is degassing of primarily magmatic co2 as exhibited by the bubbling pools hydrothermal streams have experienced mixing with biogenic co2 sources including plant respiration and methanogenesis the slight variations observed from site to site are likely a consequence of fractional degassing of the magma chamber during exsolution of co2 over the sampling period 2014 2017 the temperatures and 13c values do not change which suggests a current steady state of degassing

the 1982 eruptions of el chichon volcano mexico 2 observations and numerical modelling of tephra fall distribution tephra fallout from the a 1 march 29 0532 ut b april 4 0135 ut and c april 4 1122 ut 1982 explosive eruptions of el chichon produced three tephra fall deposits over southeastern mexico bidirectional spreading of eruption plumes as documented by satellite images was due to a combination of tropospheric and stratospheric transport with heaviest deposition of tephra from the ene tropospheric lobes maximum column heights for the eruptions of 27 32 and 29 km respectively have been determined by comparing maximum lithic clast dispersal in the deposits with predicted lithic isopleths based on a theoretical model of pyroclast fallout from eruption columns these column heights suggest peak mass eruption rates of 1 1 108 1 9 108 and 1 3 108 kg s maximum column heights and mass eruption rates occured early in each event based on the normal size grading of the fall deposits sequential satellite images of plume transport and the production of a large stratospheric aerosol plume indicate that the eruption columns were sustained at stratospheric altitudes for a significant portion of their duration new estimates of tephra fall volume based on integration of isopach area and thickness yield a total volume of 2 19 km3 1 09 km3 dre dense rock equivalent or roughly twice the amount of the deposit mapped on the ground up to one half of the erupted mass was therefore deposited elsewhere as highly dispersed tephra

geological structure composition and evolution of crustal layers of the japan sea abstract seismic refraction data have shown that the crustal thickness in the japan sea basins japan basin and yamato basin is 12 16 km but the crustal thickness under the yamato rise has not been determined although it seems to be considerably larger the crustal thickness in the asian continental slope and shelf reaches 26 km all crustal structure studied has a similar seismic layering a sedimentary layer a layer 2 or intermediate layer and a layer 3 or the lower crustal layer the velocities and ranges of velocity variations in the layers are almost similar the greatest difference in crustal thickness is between layer 2 and layer 3 the thickness of layer 2 in the deep sea basins is 1 5 5 3 km beneath the yamato rise it is 6 0 7 5 km and beneath the shelf and continental slope it is 9 8 11 7 km the composition of layer 2 beneath the topographic highs and the continental slope has been studied by bottom sampling proterozoic to paleogene granitic rocks and archean proterozoic metamorphic rocks are the most common rock types these rocks were formed at great and medium crustal depths presumably in crust with a thickness of 30 40 km containing a layer 2 with a thickness of 10 15 km the discrepancy between original and present crustal thickness suggests that the continental crust was thinned during the formation of the japan sea basin magmatic erosion of the intermediate layer during the ascent of mantle masses is proposed as a probable mechanism of this process the result of this process could be a replacement of the sialic layer of the crust by basaltic rocks which resulted in the formation of acid and alkahne melts erupted onto the surface during oligocene early miocene time

volcan el chichon mexico pre 1982 s rich eruptive activity abstract mapping and sampling of the interior of el chichon volcano was made possible by the exposures of the 1982 explosion crater the 1 km diameter 200 m deep crater exposes rocks produced during several eruptive episodes volcanic domes talus breccias plinian airfall deposits and pyroclastic flow deposits of compositionally similar alkalic hornblende trachyandesites with phenocrysts of anhydrite all of the rocks except those leached of anhydrite have extremely high s contents el chichon consists of a volcanic dome complex airfall debris and pyroclastic outflow sheets prior to 1982 its last major activity was about 650 100 yr b p

geochemistry of the acid kawah putih lake patuha volcano west java indonesia abstract kawah putih is a summit crater of patuha volcano west java indonesia which contains a shallow 300 m wide lake with strongly mineralized acid sulfate chloride water the lake water has a temperature of 26 34 c ph s tot 2500 4600 ppm and cl 5300 12 600 ppm and floating sulfur globules with sulfide inclusions are common sulfur oxyanion concentrations are unusually high with s 4 o 6 2 s 5 o 6 2 s 6 o 6 2 2400 4200 ppm subaerial fumaroles thermodynamic modeling of the lake fluids indicates that the lake water is saturated with silica phases barite pyrite and various pb sb cu as bi bearing sulfides when sulfur saturation is assumed precipitating phases predicted by the model calculations are consistent with the bulk chemistry of the sulfur rich bottom sediments and their identified mineral phases much of the lake water chemistry can be explained by congruent rock dissolution in combination with preferential enrichments from entering fumarolic gases or brines and element removal by precipitating mineral phases as indicated by a comparison of the fluids volcanic rocks and lake bed sediment flank springs on the mountain at different elevations vary in composition and are consistent with local rock dissolution as a dominant factor and ph dependent element mobility discharges of warm sulfate and chloride rich water at the highest elevation and a near neutral spring at lower level may contain a small contribution of crater lake water the acid fluid induced processes at patuha have led to the accumulation of elements that are commonly associated with volcano hosted epithermal ore deposits the dispersal of heavy metals and other potentially toxic elements from the volcano via the local drainage system is a matter of serious environmental concern

some hydro geochemical fluctuations observed in mexico related to volcanic activity abstract some experiences obtained in mexico pursuing the identification of geochemical precursors and changes associated to different kinds of volcanic activity are discussed three recent events of volcanic unrest are described in the context of an active geological environment the identification of some geochemical precursors helped to evaluate the eruptive potential during two volcanic crises tacana 1986 colima 1991 the significant compositional fluctuations observed after a volcanic catastrophic eruption el chichon 1982 are also discussed it is concluded that even simple geochemical methods may provide a useful tool for the evaluation of the level of activity and the recognition of some eruption precursors

the 1982 eruptions of el chichon volcano mexico 3 physical properties of pyroclastic surges two major pyroclastic surges generated during the 4 april 1982 eruption of el chichon devastated an area of 153 km2 with a quasi radial distribution around the volcano the hot surge clouds carbonized wood throughout their extent and were too hot to allow accretionary lapilli formation by vapor condensation field evidence indicates voidage fraction of 0 99 in the surge cloud with extensive entrainment of air thermal calculations indicate that heat content of pyroclasts can heat entrained air and maintain high temperatures in the surge cloud the dominant bed form of the surge deposits are sand waves shaped in dune forms with vertical form index of 10 20 characterized by stoss side erosion and lee side deposition of 1 10 cm reversely graded laminae a systematic decrease in maximum lithic diameter with distance from source is accompanied by decrease in wavelength and amplitude modal analysis indicates fractionation of glass and pumice from the surge cloud relative to crystals resulting in loss of at least 10 25 of the cloud mass due to winnowing out of fines during surge emplacement greatest fractionation from the 1 0 0 0 grain sizes reflects relatively lower pumice particle density in this range and segregation in the formative stages of the surge cloud extensive pumice rounding indicates abrasion during bed load transport flow of pyroclastic debris in the turbulent surge cloud was by combination of bed load and suspended load transport the surges are viewed as expanding pyroclastic gravity flows which entrain and mix with air during transport the balance between sedimentation at the base of the surge cloud and expansion due to entrainment of air contributed to low cloud density and internal turbulence which persisted to the distal edge of the surge zone

high frequency field auto sampling of volcanic waters discharged near craters of active volcanoes temporal changes in volcanic water chemistry are reliable indicators of changes in volcanic activity conventionally volcanic water discharges are monitored through repetitive sampling and subsequent laboratory analysis samples are usually taken at intervals ranging between weekly and annually due to the campaign style of observations however samples collected in this way to track chemical changes were insufficient in terms of number frequency and temporal coverage completeness to allow quantitative tracking of volcanic activity in this study we thus developed a field auto sampling tool for collecting volcanic waters discharged near craters to improve the frequency of water sampling the basis of the developed tool is a commercially available xy plotter a drawing robot which can be controlled by a computer numerical control cnc operator code called g code to collect the water in the arranged sampling bottles the drawing pen of the xy plotter was replaced with a plastic drip pipe connected to a peristaltic pump a field test using the g code operated field auto sampling tool gofat was conducted at a hot spring at a crater area of hakone volcano in japan samples were collected at three hour intervals for 40 consecutive days the gofat successfully operated without stopping and collected a total of 332 water samples using the high temporal resolution samples obtained by the gofat we observed short term significant positive so4 peaks that endured for approximately a day which had not been detected by a repetitive sampling carried out in previous studies at this volcano our results clearly show the importance of high temporal resolution sampling even during quiescent periods to clarify the characteristics of the observed hot spring

crater lake and post eruption hydrothermal activity el chichon volcano mexico abstract explosive eruptions of volcan el chichon in chiapas mexico on march 28 and april 3 4 1982 removed 0 2 km3 of rock to form a 1 km wide 300 m deep summit crater by late april 1982 a lake had begun to form on the crater floor and by november 1982 it attained a maximum surface area of 1 4 105 m2 and a volume of 5 106 m3 accumulation of 4 5 m of rainfall between july and october 1982 largely formed the lake in january 1983 temperatures of fumaroles on the crater floor and lower crater walls ranged from 98 to 115 c by october 1983 the maximum temperature of fumarole emissions was 99 c in january 1983 fumarole gas emissions were greater than 99 vol h2o with traces of co2 so2 and h2s the water of the lake was a hot t 52 58 c acidic ph 0 5 dilute solution 34 046 mg l 1 dissolved solids cl s 20 5 sediment from the lake contains the same silicate minerals as the rocks of the 1982 pyroclastic deposits together with less than 1 of elemental sulfur the composition and temperature of the lake water is attributed to 1 solution of fumarole emissions 2 reaction of lake water with hot rocks beneath the lake level 3 sediments washed into the lake from the crater walls 4 hydrothermal fluids leaching sediments and formational waters in sedimentary rocks of the basement 5 evaporation and 6 precipitation

co2 and he degassing at el chichon volcano chiapas mexico gas flux origin and relationship with local and regional tectonics during 2007 2008 three co2 flux surveys were performed on el chichon volcanic lake chiapas mexico with an additional survey in april 2008 covering the entire crater floor including the lake the mean co2 flux calculated by sequential gaussian simulation from the lake was 1 190 march 2007 730 december 2007 and 1 134 g m 2 day 1 april 2008 with total emission rates of 164 9 5 march 2007 59 2 5 december 2007 and 109 6 6 t day 1 april 2008 the mean co2 flux estimated from the entire crater floor area was 1 102 g m 2 day 1 for april 2008 with a total emission rate of 144 5 9 t day 1 significant change in co2 flux was not detected during the period of survey and the mapping of the co2 flux highlighted lineaments reflecting the main local and regional tectonic patterns the 3he 4he ratio as high as 8 1 ra for gases in the el chichon crater is generally higher than those observed at the neighbouring transmexican volcanic belt and the central american volcanic arc the co2 3he ratios for the high 3he 4he gases tend to have the morb like values 1 41 109 and the co2 3he ratios for the lower 3he 4he gases fall within the range for the arc type gases the high 3he 4he ratios the morb like co2 3he ratios for the high 3he 4he gases and high proportion of morb co2 m 25 15 at el chichon indicate a greater depth for the generation of magma when compared to typical arc volcanoes

sulfur as a binding agent of aggregates in explosive eruptions understanding the mechanisms that govern ash aggregation is of critical importance in volcanology aggregation reduces the residence time of ash 2 mm in the atmosphere strongly by enhancing the sedimentation of finer grained material generated during explosive eruptions to date experimental studies have focused on the expectation that water provides the strongest bonds between particles to form spherical to oblate aggregates typically less than or equal to a few mm occasionally up to several cm preserved in pyroclastic deposits under water rich conditions individual accreted particles rarely exceed 1 mm in pyroclastic density current deposits produced during the 1982 eruption of el chichon mexico which emitted 7 5 tg of so2 in the atmosphere aggregate particles one to several millimeters strongly cemented by a s rich film are common they exhibit similarities with aggregates found in sulfur cones at poas volcano costa rica we propose that sulfur is the binder between the silicate grains such a binding capacity implies a relatively fluid behavior of sulfur such as might be expected in the low viscosity temperature regime just above its melting point if so then the explosive ejection of sulfur during eruptions combined with its ability to act as a cement for particles 2 mm would imply that size fractions of lapilli can be efficiently removed from eruptive clouds a few kilometers from the vent such an aggregation mechanism would have important implications for pyroclast dispersal models and hazard assessment

large debris avalanches and associated eruptions in the holocene eruptive history of shiveluch volcano kamchatka russia shiveluch volcano located in the central kamchatka depression has experienced multiple flank failures during its lifetime most recently in 1964 the overlapping deposits of at least 13 large holocene debris avalanches cover an area of approximately 200 km2 of the southern sector of the volcano deposits of two debris avalanches associated with flank extrusive domes are in addition located on its western slope the maximum travel distance of individual holocene avalanches exceeds 20 km and their volumes reach 3 km3 the deposits of most avalanches typically have a hummocky surface are poorly sorted and graded and contain angular heterogeneous rock fragments of various sizes surrounded by coarse to fine matrix the deposits differ in color indicating different sources on the edifice tephrochronological and radiocarbon dating of the avalanches shows that the first large holocene avalanches were emplaced approximately 4530 4350 bc from 2490 bc at least 13 avalanches occurred after intervals of 30 900 years six large avalanches were emplaced between 120 and 970 ad with recurrence intervals of 30 340 years all the debris avalanches were followed by eruptions that produced various types of pyroclastic deposits features of some surge deposits suggest that they might have originated as a result of directed blasts triggered by rockslides most avalanche deposits are composed of fresh andesitic rocks of extrusive domes so the avalanches might have resulted from the high magma supply rate and the repetitive formation of the domes no trace of the 1854 summit failure mentioned in historical records has been found beyond 8 km from the crater perhaps witnesses exaggerated or misinterpreted the events

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

the heat source of ruapehu crater lake deductions from the energy and mass balances abstract regular observations of temperature outflow rates and water chemistry of crater lake mt ruapehu new zealand have been made for the last 25 years these data have been used to derive a model of the dynamics of the lake and determine the input of energy mass and chloride from the volcano to the crater lake the recent relatively quiescent state of the volcano when virtually no heat has been input to the lake has also enabled an assessment to be made of the surface heat loss characteristics which play an important role in the model of the lake the modelling suggests that since about 1982 the ratio of the volcanic heat to mass added to the base of the lake is about 6 mj kg which is not compatible with heating of the lake by magmatic steam alone thus only about 50 of the heating has been by magmatic steam it is suggested that heat could be transferred from a magmatic source to the region below the lake by a heat pipe mechanism commonly associated with geothermal systems the simultaneous upward movement of vapour phase and downward movement of liquid phase from condensed vapour allows efficient heat transfer without overall mass transfer the permeability necessary to supply the required heat is of the order of 10 darcy and is consistent with a rubble filled vent for at least the last five years there has been a characteristic pattern in the crater lake temperature record with alternate heating and cooling phases the heating phase generally lasts for one or two months while the cooling phase lasts for six months to a year a possible explanation for this cyclic behaviour is the presence of a layer of liquid sulphur under crater lake acting as a partial barrier between the heat pipe and the lake the unusual variations of the viscosity of liquid sulphur with temperature will mean that at temperatures greater than 160 c the layer of sulphur becomes highly viscous and would block any upwards steam flow and hence stop the heat input to crater lake so producing a cooling phase this blockage would last until the heating from below raised the temperature of the sulphur beyond the high viscosity region so gases could again pass through the sulphur

development of lithic breccias in the 1982 pyroclastic flow deposits of el chichon volcano mexico abstract pyroclastic flow deposit f1 volume 0 02 km 3 produced during phase iii of the 1982 eruption of el chichon volcano mexico contains basal lithic breccias the breccia layers are well exposed in el platanar gully between 2 and 4 km east of the volcano crater the lithic breccias are inversely graded as defined by lithics dense juvenile blocks and pumice supported in a coarse sandy matrix composed of the same constituents the contact between the main body of the pyroclastic flow deposit and the lithic breccias is generally sharp and planar but not erosive in some outcrops it is gradational and is only shown by an alignment of lithic clasts the origin of these beds is interpreted to be due to a hydraulic jump in the moving pyroclastic flow formed after a pronounced slope break from 11 to 3 at the site where the flow began to be funnelled into the el platanar gully we have investigated the possible modes of formation of the lithic breccias with analog laboratory experiments the experiments show that coarse localized segregations could form through a number of mechanisms the field observations interpreted with the assistance of the laboratory results suggest that pyroclastic flow 1 which produced deposit f1 moved as a kind of density stratified flow with a basal lithic rich zone transporting larger particles and an upper less dense zone transporting smaller particles in suspension at the slope break flow 1 lost competence and dumped the largest particles forming a piled up breccia downstream somewhat smaller lithic particles may have been deposited as ballistics from a low angle jetlike structure comprising a hydraulic jump this deposit thins with distance downstream once the deposit was sufficiently thick on its upstream end particles may have been re entrained into the jet to be deposited further along the flow in a low angle downstream prograding bouldery dune reverse grading in the dune beds may result from kinetic sieving we suggest that planar contacts between the lithic breccias and the associated pyroclastic flow deposit may be the result of strong density and velocity gradients between the two parts of the flow

volcanic lake systematics i physical constraints volcanic lakes have a wide range of characteristics and we make an attempt to delineate the limiting physical conditions for several lake classes the ratio between heat input and heat dissipation capacity of a lake constrains the temperature for perfectly mixed steady state volcanic lakes poorly mixed lakes are also conditioned by this ratio but their temperature structure is also strongly influenced by the hydrodynamics resulting from different mechanisms of heat transfer the steady state temperatures of volcanic lakes are largely determined by the magnitude of the volcanic heat influx relative to the surface area of the lake small lakes have only a small capacity for heat dissipation and their temperature rises quickly with only small heat inputs large lakes are buffered against variations in heat input both the heat dissipation and meteoric water input into a lake are functions of lake surface area and therefore each lake water temperature demands a certain precipitation rate for mass conservation independent of lake size the results of energy mass balance modeling shows that under common atmospheric conditions most steady state volcanic lakes are unlikely to maintain a temperature in excess of 45 50 c validation of the volcanic lake model was performed using published data from yugama lake japan and the keli mutu lakes indonesia also the model was applied to 24 natural systems to provide a baseline assessment of energy fluxes under the model assumptions so future work on those systems can identify nuances in individual systems that deviate from the simple model conditions we recommend the model for use in assessing temperature variations and volcanic lake stability in settings with known physical and atmospheric conditions application of the energy mass balance calculations of model lakes provides a genetic classification scheme largely based on physical process parameters

geochemistry of the volcano hydrothermal system of el chichon volcano chiapas mexico the 1982 eruption of el chichon volcano ejected more than 1 km3 of anhydrite bearing trachyandesite pyroclastic material to form a new 1 km wide and 300 m deep crater and uncovered the upper 500 m of an active volcano hydrothermal system instead of the weak boiling point temperature fumaroles of the former lava dome a vigorously boiling crater spring now discharges 20 kg s of cl rich 15 000 mg kg and sulphur poor 200 mg kg of so4 almost neutral ph up to 6 7 water with an isotopic composition close to that of subduction type magmatic water d 15 18o 6 5 this spring as well as numerous cl free boiling springs discharging a mixture of meteoric water with fumarolic condensates feed the crater lake which compared with values in 1983 is now much more diluted 3000 mg kg of cl vs 24 030 mg kg less acidic ph 2 6 vs 0 56 and contains much lower amounts of s 200 mg kg of so4 vs 3550 mg kg with 34s 0 5 4 2 17 in 1983 agua caliente thermal waters on the southeast slope of the volcano have an outflow rate of approximately 100 kg s of 71 c na ca cl water and are five times more concentrated than before the eruption b r molina unpublished data relative n2 ar and he gas concentrations suggest extensional tectonics for the el chichon volcanic centre the 3he 4he and 4he 20ne ratios in gases from the crater fumaroles 7 3ra 2560 and agua caliente hot springs 5 3ra 44 indicate a strong magmatic contribution however relative concentrations of reactive species are typical of equilibrium in a two phase boiling aquifer sulphur and c isotopic data indicate highly reducing conditions within the system probably associated with the presence of buried vegetation resulting from the 1982 eruption all cl rich waters at el chichon have a common source this water has the appearence of a partially matured magmatic fluid condensed magmatic vapour neutralized by interaction with fresh volcaniclastic deposits and depleted in s due to anhydrite precipitation shallow ground waters emerging around the volcano from the thick cover of fresh pumice deposits red waters are ca so4 rich and have a negative oxygen isotopic shift probably due to ongoing formation of clay at low temperatures

the crater lake and hydrothermal system of mount pinatubo philippines evolution in the decade after eruption the june 1991 eruption of mount pinatubo philippines breached a significant pre eruptive magmatic hydrothermal system consisting of a hot 300 c core at two phase conditions and surrounding cooler 260 c liquid outflows to the n and s the eruption created a large closed crater that accumulated hydrothermal upwellings near surface aquifer and meteoric inflows a shallow lake formed by early september 1991 and showed a long term increase in level of 1 m month until an artificial drainage was created in september 2001 comparison of the temporal trends in lake chemistry to pre and post eruptive springs distinguishes processes important in lake evolution the lake was initially near neutral ph and dominated by meteoric influx and cl so4 and cl hco3 hydrothermal waters with peaks in so4 and ca concentrations resulting from leaching of anhydrite and aerosol laden tephra magmatic discharge acidity ph 2 and rock dissolution peaked in late 1992 during and immediately after eruption of a lava dome on the crater floor since cessation of dome growth trends in lake ph increase from 3 to 5 5 temperature decline from 40 to 26 c and chemical and isotopic composition indicate that magmatic degassing and rock dissolution have declined significantly relative to the input of meteoric water and immature hydrothermal brine higher concentrations of cl na k li and b and lower concentrations of mg ca fe so4 and f up to 1999 highlight the importance of a dilute hydrothermal contribution as do stable isotope and tritium compositions of the various fluids however samples taken since that time indicate further dilution and steeper trends of increasing ph and declining temperature present gas and brine compositions from crater fumaroles and hot springs indicate boiling of an immature cl so4 geothermal fluid of near neutral ph at approximately 200 c rather than direct discharge from magma it appears that remnants of the pre eruptive hydrothermal system invaded the magma conduit shortly after the end of dome emplacement blocking the direct degassing path this along with the large catchment area 5 km2 and the high precipitation rate of the area led to a rapid transition from a small and hot acid lake to a large lake with near ambient temperature and ph this behavior contrasts with that of peak activity lakes that have more sustained volcanic gas influx e g kawah ijen indonesia poas and rincon de la vieja costa rica

chemical composition of fumarolic gases and spring discharges from el chichon volcano mexico causes and implications of the changes detected over the period 1998 2000 abstract since the march april 1982 eruption of el chichon volcano intense hydrothermal activity has characterised the 1 km wide summit crater this mainly consists of mud and boiling pools fumaroles which are mainly located in the northwestern bank of the crater lake during the period 1998 2000 hot springs and fumaroles discharging inside the crater and from the southeastern outer flank agua caliente were collected for chemical analyses the observed chemical fluctuations suggest that the physico chemical boundary conditions regulating the thermodynamic equilibria of the deep rock fluid interactions have changed with time the chemical composition of the lake water characterised in the period 1983 1997 by high na cl ca 2 and so 4 2 contents experienced a dramatic change in 1998 1999 turning from a na cl to a ca 2 so 4 2 rich composition in june 2000 a relatively sharp increase in na and cl contents was observed at the same time so 2 h 2 s ratios and h 2 and co contents in most gas discharges increased with respect to the previous two years of observations suggesting either a new input of deep seated fluids or local variations of the more surficial hydrothermal system migration of gas manifestations enhanced number of emission spots and variations in both gas discharge flux and outlet temperatures of the main fluid manifestations were also recorded the magmatic hydrothermal system of el chichon is probably related to interaction processes between a deep magmatic source and a surficial cold aquifer an important role may also be played by the interaction of the deep fluids with the volcanic rocks and the sedimentary limestone and evaporites basement the chemical and physical changes recorded in 1998 2000 were possibly due to variations in the permeability of the conduit system feeding the fluid discharges at surface as testified by the migration of gas and water emanations two different scenarios can be put forward for the volcanic evolution of el chichon 1 build up of an infra crater dome that may imply a future eruption in terms of tens to hundreds of years 2 minor phreatic phreatomagmatic events whose prediction and timing is more difficult to constrain this suggests that unlike the diminished volcanic activity at el chichon after the 1982 paroxistic event the volcano hydrothermal fluid discharges need to be more constantly monitored with regular and more frequent geochemical sampling and at the same time a permanent network of seismic stations should be installed