natural tracers for identifying the origin of the thermal fluids emerging along the aegean volcanic arc greece evidence of arc type magmatic water atmw participation abstract the aegean volcanic arc is the result of a lithosphere subduction process during the quaternary time starting from the soussaki area from west to east the arc proceeds through the islands of egina methana milos santorini the columbus bank kos and nisyros volcano tectonic activities are still pronounced at santorini and nisyros in form of seismic activity craters of hydrothermal explosions hot fumaroles and thermal springs a significant number of cold water springs emerge in the vicinity of hot waters on these islands chemical and isotopic analyses were applied on water and fumaroles samples collected in different areas of the volcanic arc in order to attempt the assessment of these fluids stable isotopes of water and carbon have been used to evaluate the origin of cold and thermal water and co 2 chemical solute concentrations and isotopic contents of waters show that the fluids emerging in egina soussaki methana and kos areas represent geothermal systems in their waning stage while the fluids from milos santorini and nisyros proceed from active geothermal systems the 2 h 18 o cl relationships suggest that the parent hydrothermal liquids of nisyros and milos are produced through mixing of seawater and arc type magmatic water atmw with negligible to nil contribution of local ground waters and with very high participation of the magmatic component which is close to 70 in both sites a very high magmatic contribution to the deep geothermal system could occur at santorini as well perhaps with a percentage similar to nisyros and milos but it cannot be calculated because of steam condensation heavily affecting the fumarolic fluids of nea kameni before the surface discharge the parent hydrothermal liquid at methana originates through mixing of local groundwaters seawater and atmw with a magmatic participation close to 19 all in all the contribution of atmw is higher in the central eastern part of the aegean volcanic arc than in the western sector this difference which is spotted in the variable isotopic composition of the sampled fluids from west to east along the arc is probably due to several causes including the tectonic regime the depth of the deep reservoir below sea level the age of volcanic activity and in general the geomorphologic state of each island

gravity anomalies and flexure of the lithosphere at ascension island summary ascension island in the northern south atlantic forms the summit of a volcanic edifice 60 km in diameter which places a substantial load on the underlying young oceanic lithosphere an analysis of a combined data set of recent and historical surface gravity and bathymetry measurements on and around the island suggests that the lithosphere responds to this load by flexure equivalent to that of an elastic plate only z3 km thick and that the mean density of the volcanic edifice is 2500 kg m 3 a steep gravity gradient across the island cannot be explained by a simple flexural model and must be attributed to lateral density variations within the volcano itself the effective elastic thickness is considerably less than the expected z 12 km mechanical thickness of the 6 ma lithosphere loaded by the volcano and less even than zero age elastic thicknesses commonly observed at slow spreading ridges with axial rift valleys the unusually small elastic thickness may be attributed to the combined effects of the high curvature beneath the island which produces bending stresses that are limited by the yield stress envelope localized heating of the lithosphere during emplacement of the island and crustal thickening when these factors are taken into account the observed flexure is consistent with rheological models based on experimental rock mechanics

origin of the lethal gas burst from lake monoun cameroun abstract on 15 august 1984 a lethal gas burst issued from a submerged 96 m deep crater in lake monoun in cameroun western africa killing 37 people the event was associated with a landslide from the eastern crater rim which slumped into deep water waters below 50 m are anoxic dominated by high fe 2 600 mg l and hco 3 1900 mg l anoxic and supersaturated with siderite which is a major component of the crater floor sediments the unusually high fe 2 levels are attributed to reduction of laterite derived ferric iron gradually brought into the lake as loess and in river input sulfur compounds are below detection limits in both water and gas gases effervescing from depressurized deep waters are dominantly co 2 with minor ch 4 having 13 c of 7 18 and 54 8 per mil respectively bacterial decomposition of organic matter may account for the methane but 14 c of lake water indicates that only 10 of the carbon is modern giving an apparent age of 18 000 years the dominant source of carbon is therefore attributed to long term emission of co 2 as volcanic exhalation from vents within the crater which led to gradual build up of hco 3 in the lake the density stratification of the lake may have been upset by an earthquake and underwater landslide on 15 august which triggered overturn of the lake and caused nucleation of co 2 in the deep water the resultant ebullition of co 2 from deep lake waters led to a gas burst at the surface and locally generated a water wave up to 5 m high people travelling through the gas cloud were asphyxiated presumably from co 2 and suffered skin discoloration from unidentified components

microbial controls on the origin and evolution of coal seam gases and production waters of the walloon subgroup surat basin australia abstract the walloon subgroup coal seam gas csg play in the surat basin queensland is australia s pre eminent onshore gas field concerted multi disciplinary research is underway investigating the distribution origin and composition of waters and gases in this dominantly microbial csg reservoir to guide both continued production and potential microbially enhanced coal bed methane mecom applications however prior to the present research a detailed study of co produced waters and gases from across the surat basin was not available in the public domain this study tested whether co produced water compositional and stable isotopic data show relationships with production gas stable isotope compositions to elucidate further evidence for microbial co2 reduction and explore the down dip geochemical evolution of walloon coal bed waters and gases a total of 41 wells were sampled with 50 water and 25 gas samples spanning the 3 major production areas of the surat basin detailed isotopic and hydrochemical analysis of these samples revealed distinct spatial trends between the different production locales water compositions were distinct for each of the production regions reflecting the different lithologies of adjacent recharge zones differing fluid rock interactions likely different microbial consortia and the extent of methanogenesis on the western side of the basin near roma waters were the freshest with the lowest median values for alkalinity 861 mg l and cl 588 mg l and a 13cdic of 14 2 on the eastern side of the basin the kogan nose waters were the most saline with the highest median values for na 1955 mg l cl 2280 mg l and 13cdic 20 0 also in the east in the present gas fairway the undulla nose waters had the highest median alkalinity 1841 mg l and were found to have a na excess median 1050 mg l and a lower than expected median 13cdic 14 0 co mingled produced methane carbon isotope values 13c 57 0 to 44 5 from both the upper juandah and lower taroom coal measures plot within the mixed thermogenic microbial genetic field by contrast deuterium isotopic difference 2h h2o ch4 values and cross plots of 2h h2o and 18o h2o suggest that microbially mediated co2 reduction is the dominant methane generation process in situ at a given depth the undulla nose waters in the east are more depleted in 2h and 18o than elsewhere in the surat basin which may suggest these samples have been more heavily impacted by water rock microbial reactions 14c values from the 3 production regions 0 115 to 1 769 pmc age 32 400 to 50 000 years before present b p suggest that walloon coals likely recharged in the last 50 000 years limit of radiocarbon dating consistent with these dates 2h h2o and 18o h2o values for the surat basin 2h 32 to 56 18o 5 9 to 9 0 echo the stable isotopic composition of meteoric waters during the initial part of the last glacial period in southeast queensland based on a strong correlation between 2h ch4 and 2h h2o we suggest that methane was generated since the late pleistocene pca analysis showed a degree of positive correlation between total alkalinity and both the 13cdic median 14 2 and 13c ch4 median 52 1 vectors that is consistent with finite reservoir effects the results inform ongoing studies of gas distribution and origins and mecom potential in the surat basin and underpin a broader study examining aquifer interactions

the bimodal ph distribution of volcanic lake waters abstract volcanic lake waters have a bimodal ph distribution with an acidic mode at ph 0 5 1 5 and a near neutral mode at ph 6 6 5 with relatively few samples having ph 3 5 5 to investigate the reasons for this distribution the irreversible water rock mass exchanges during the neutralization of acid so4 cl waters with andesite under both low and high temperature conditions were simulated by means of the eq3 6 software package version 7 2 reaction path modeling under low temperature and atmospheric pco2 and fo2 suggests that several homogeneous and or heterogeneous ph buffers exist both in the acidic and neutral regions but no buffer is active in the intermediate central ph region again the same titration under high temperature hydrothermal magmatic conditions is expected to produce comparatively infrequent aqueous solutions with ph values in the 3 5 5 range upon their cooling below 100 c substantially different ph values are obtained depending on the cooling paths either through boiling or conductive heat losses these distinct ph values are governed by either hso4 and hcl aq in poorly neutralized aqueous solutions or the co2 aq hco3 couple and the pco2 value as well in neutralized aqueous solutions finally mixing of the acid lake water with the aqueous solutions produced through high temperature titration and cooled below 100 c is unlikely to generate mixtures with ph values higher than 3 unless the fraction of the acidic water originally present in the lake becomes very small which means its virtually complete substitution summing up the evidence gathered through reaction path modeling of the neutralization of acid lake waters with andesite both at low and high temperatures explains the scarcity of volcanic lake waters with measured ph values of 3 5 5

a model for the plate tectonic evolution of the east central pacific based on seasat investigations abstract a complete seasat dataset is used to investigate the small scale bathymetry of a 15 by 15 area of the east central pacific northeast of pitcairn island which was involved in the miocene ridge jump and reorientation two fracture zones fz1 and fz2 are mapped along 450 km at an azimuth of n95 e while the signature of the austral fracture zone disappears east of 135 w several seamounts are discovered or confirmed along the oeno henderson ducie lineament and the degree of isostatic compensation of a number of islands at the eastern end of the tuamotu group is studied islands of the northern branch have practically no signatures in the geoid while the southern ones must have formed on lithosphere at least 10 ma old on the basis of these constraints we present a model of the evolution of spreading since 40 ma b p in which the deactivation of the austral fracture zone and the transfer of its offset to fz1 in the late oligocene is interpreted as due to rift propagation triggered by a northern tuamotu hotspot we propose that the oeno ducie chain is the surface expression of a southern tuamotu hotspot deviated 15 in azimuth by the presence of a young and fresh fracture zone thus our model predicts that hotspots and ridge systems can have a moderate influence on each other s kinematics and surface expressions which does not however extend to permanent trapping

the plinian fallout associated with quilotoa s 800 yr bp eruption ecuadorian andes abstract large volcanic eruptions at dacitic or rhyolitic volcanoes often generate exceptional volumes of fine ash that mantles an area up to a million km 2 these eruptions are characterized by extreme fragmentation of the magma and hence extraordinary dispersal of ash and are categorized as plinian ultraplinian or phreatoplinian events large volume co ignimbrites or co plinian ashes are often produced by such eruptions high fragmentation indices of 90 are attributed to the violent eruption of silicic magma especially if augmented by fuel coolant reactions produced when abundant external water interacts with the magma the present study documents a case where the fine ash 1 mm diameter fall deposit related to the plinian phase of the eruption comprises the overwhelming bulk about 87 wt of the eruptive products this is another example demonstrating the predominance of a widespread fine grained co plinian ash which follows the initial coarser lapilli fall historical eruptions at two other andean volcanoes quizapu chile and huaynaputina peru and at santa maria guatemala and novarupta alaska produced similar ash fall sequences quilotoa s 800 yr bp eruption in the ecuadorian andes is an example of a powerful plinian eruption at a small dacitic volcano it produced a crystal rich fine grained ash layer which mantled an extrapolated 810 000 km 2 with 1 mm thickness and has a bulk magma volume of about 18 3 km 3 stratigraphic and petrographic studies as well as granulometric and density analyses were employed to investigate these eruptive products particularly the distal co plinian fall deposits the eruptive cycle consisted of a phreatomagmatic triggering the establishment of a powerful sustained plinian column and subsequently partial column collapses leading to widespread surge and ash flow activity quilotoa s 800 yr bp eruption has a vei volcano explosivity index 6 and generated a maximum column height of about 35 km and a mass discharge rate of 2 10 8 kg s in comparison pinatubo s 1991 eruption had similar column heights and discharge rates but smaller erupted volumes factors that possibly contributed to the extreme explosiveness and resulting fine grained ash production include inferred volatile overpressures in the crystal dominated dacite magma a rapid eruption onset a forceful and sustained plinian column possibly through a crater lake thus promoting extreme fragmentation of the magma but without leaving evidence of wet phreatoplinian type deposits

water chemistry of lake quilotoa ecuador and assessment of natural hazards a geochemical survey carried out in november 1993 revealed that lake quilotoa was composed by a thin 14 m oxic epilimnion overlying a 200 m thick anoxic hypolimnion dissolved co2 concentrations reached 1000 mg kg in the lower stratum loss of co2 from epilimnetic waters followed by calcite precipitation and a consequent lowering in density was the apparent cause of the stratification the cl so4 and hco3 contents of lake quilotoa are intermediate between those of acid so4 cl crater lakes and those of neutral hco3 crater lakes indicating that lake quilotoa has a memory of the inflow and absorption of hc1 and s bearing volcanic magmatic gases the mg ca ratios of the lake waters are governed by dissolution of local volcanic rocks or magmas but k na ratios were likely modified by precipitation of alunite a typical mineral in acid so 4 cl crater lakes the constant concentrations of several conservative chemical species from lake surface to lake bottom suggest that physical chemical and biological processes did not have enough time after the last overturn to cause significant changes in the contents of these chemical species this lapse of time might be relatively large but it cannot be established on the basis of available data besides the lake may not be close to steady state mixing of lake quilotoa waters could presently be triggered by either cooling epilimnetic waters by 48c or providing heat to hypolimnetic waters or by seismic activity although quilotoa lake contains a huge amount of dissolved co2o 3 10 11 gu at present the risk of a dangerous limnic eruption seems to be nil even though some gas exsolution might occur if deep lake waters were brought to the surface carbon dioxide could build up to higher levels in deep waters than at present without any volcanic re awakening due to either a large inflow of relatively cool co 2 rich gases or possibly a long interval between overturns periodical geochemical surveys of lake quilotoa are therefore recommended q 2000 elsevier science b v all rights reserved

thermal and mechanical constraints on the lithosphere beneath the marquesas swell the marquesas islands like most active or recently active hotspot chains lie on anomalously shallow sea floor and their subsidence history points to a fundamentally thermal origin for the swell1 as do heat flow and subsidence data for the swells beneath bermuda2 and hawaii3 4 here we apply a new linear filtering technique to geoid height and depth anomaly data over the marquesas to constrain the depth to the high temperature low density material responsible for elevating the swell we determine an isostatic compensation depth of 45 5 km for the swell and best fitting values for lithospheric flexural rigidity and island load density of 5 1022 n m and 2 700 kg m 3 respectively the swell s shallow compensation depth and low flexural rigidity call for anomalously high temperatures within the lithosphere5 6

lithospheric flexure and the evolution of sedimentary basins the sediments that have accumulated in sedimentary basins during geological time represent a load on the lithosphere that should respond by flexure simple elastic and viscoelastic maxwell plate models have been used to examine quantitatively the contribution of flexure to basin formation the models have been used to predict the stratigraphy and gravity anomalies associated with basins for different thermal and loading histories the predictions of the models have been compared with observed stratigraphy and free air gravity anomalies from interior and cratonic basins the best overall fit to the observations is for an elastic plate model in which the flexural strength of the lithosphere increases with age a similar model has recently been used successfully to explain observations from continental margin basins oceanic islands and seamounts and deep sea trench outer rise systems this model explains the increase in the overall width of basins during their evolution as well as the stratigraphy of the basin edges the apparent decrease in the widths of some basins through time can be explained by the model if sediment deposition is followed by erosion of the basin and its edges the models results suggest that the flexural properties of continental and oceanic lithosphere are generally similar and that flexure is an important factor to consider in backstripping studies in which the tectonic subsidence of a basin is isolated and stratigraphic studies in which relative changes of sea level are estimated

basalts from mururoa fangataufa and gambier islands french polynesia geochemical dependence on the age of the lithosphere abstract basaltic volcanics from mururoa fangataufa and gambier islands which belong to the pitcairn island chain have been studied for trace elements and sr nd and pb isotopic composition while the trace element ratios remain constant the 87 sr 86 sr ratios range from 0 70292 to 0 70367 143 nd 144 nd ranges from 0 51303 to 0 51288 and the 206 pb 204 pb isotopic composition ranges from 18 99 to 19 62 these values which are clearly distinct from those of pitcairn basalts vary according to distance from the present hot spot and are correlated with the age of the underlying lithosphere it is suggested that the basalts studied were produced by variable degrees of partial melting from an oceanic lithospheric plate metasomatized by prior passage over the easter hot spot

quilotoa volcano ecuador an overview of young dacitic volcanism in a lake filled caldera abstract quilotoa volcano an example of young dacitic volcanism in a lake filled caldera is found at the southwest end of the ecuador s volcanic front it has had a long series of powerful plinian eruptions of moderate to large size vei 4 6 at repetitive intervals of roughly 10 15 thousand years at least eight eruptive cycles labeled q i to q viii with increasing age over the past 200 ka are recognized often beginning with a phreatomagmatic onset and followed by a pumice rich lapilli fall and then a sequence of pumice crystal and lithic rich deposits belonging to surges and ash flows these unwelded pyroclastic flows left veneers on hillsides as well as very thick accumulations in the surrounding valleys the farthest ash flow having traveled about 17 km down the toachi valley the bulk volumes of the youngest flow deposits are on the order of 5 km 3 but that of q i s 800 yr bp ash fall unit is about 18 km 3 in the last two eruption cycles water has had a more important role upon quilotoa s low relief volcanic edifice three calderas are recognized the formation of the oldest one predates the q iv cycle and the others occurred during the q ii and q i cycles dacite lava domes are common along the present caldera rim and most were emplaced at the end of the q ii cycle older domes of dark dacite belong to the q iii and iv cycles the explosive onset of the q i cycle expulsed as much as 250 million m 3 of the lake s water resulting in large debris flows that scoured the eastern flanks of the edifice and descended the toachi river little variation in the mineralogy and chemistry of quilotoa s eruptive products is observed suggesting that the source is a homogeneous magma body at shallow depth both the pyroclastic material and the domes are composed chiefly of gray porphyritic dacites carrying large phenocrysts of plagioclase amphibole biotite and occasionally quartz

poas volcano crater lake acts as a condenser for acid metal rich brine models of formation of volcanic hosted metal deposits hypothesize that subsurface brines transport metals to sites of deposition laguna caliente in the active crater of poas volcano central costa rica consists of acid sulphate chloride brine with extreme ph 0 0 lake water sampled between 9 and 12 january 1987 at eight sites within the 40 m deep 210 m wide lake revealed temperatures between 58 and 64 c and densities measured in the laboratory at 60 c of 1 0575 0 0015g cm 3 the water s extreme acidity reflects the dominating input of acid fumaroles the presence of precipitated silica gypsum and sulphur as sediments reflects lake water equilibrium with those phases mass balance considerations require that dense brines percolate downwards from the lake into the volcano these may act as a source of metal rich brine feeding an underground ore forming hydrothermal system

evolution and strength of pacific fracture zones previous studies have shown that pacific fracture zones are strong in some locations sustaining the stresses associated with differential subsidence across a locked fault while at other locations they show signs of weakness acting as preferential conduits for volcanism or supporting anomalously low shear stresses we find that half of all geosat crossings of central pacific fracture zones are inconsistent with a flexurally maintained scarp we test two hypotheses for the origin of such anomalous crossings 1 anomalous structures formed at the transform fault during times of changes in plate motions and preserved at strong fracture zones and 2 anomalous structures representing a posttransform response of the fracture zone to subsequent tectonic activity we find that three quarters of the anomalous crossings occur over the parts of fracture zones that formed during or immediately subsequent to times of changes in spreading directions with the exception of several locations overprinted by hot spot volcanism these same crossings show no obvious correlation with regional tectonic processes these observations suggest that most anomalous fracture zone topography is inherited from the transform fault and is not a product of subsequent activity consistent with the hypothesis that fracture zones generally remain strong throughout their lifetimes

late cretaceous and paleogene tectonic evolution of the north pacific ocean the late cretaceous history of the northern pacific ocean has not been adequately deciphered largely because a major plate reorganization occurred during the cretaceous magnetic quiet interval using primary data to reconstruct plate motions from fracture zone trends and late cretaceous seafloor spreading magnetic anomalies allows formulation of a reasonable sequence of events that accounts for all the geologic features of that region especially the emperor and chinook troughs the primary event in our reconstruction is the subduction of the old northwest pacific triple junction new relative plate motions imposed by formation of convergent boundaries along both the northern pacific and farallon plates caused the farallon plate to crack this subdivision occurred 82 m y ago and resulted in the formation of the kula and chinook plates the chinook plate was bounded on the north by the chinook kula ridge the western arm of the great magnetic bight on the west by the southern emperor trough a slowly spreading rift valley on the south by the mendocino transform and on the east by the chinook farallon ridge the northern emperor trough formed the initial western boundary of the kula plate from 82 m y ago to 50 m y ago this configuration was stable the chinook plate expanded the chinook kula ridge migrated north and the kula plate was subducted with the subduction of this ridge the chinook plate adhered to the pacific plate and the pacific farallon ridge became the north south trending feature reflected by anomalies 22 and younger this reconstruction obviates the awkward late cretaceous shifting of triple junctions found in other proposed histories and provides reasonable explanations for the several structural features of the region to achieve closure of vector triangles representing the relative late cretaceous plate motions some extension must occur presumably across the eastern mendocino fracture zone between the northern and southern portions of the farallon plate these two plates apparently were independent units between 105 and 50 m y ago the initiation and cessation of rapid north farallon north american convergence 82 and 50 m y ago may have determined the nature and duration of the laramide orogeny

geochemical insight into differences in the physical structures and dynamics of two adjacent maar lakes at mt vulture volcano southern italy 1 we report on the first geochemical investigation of the monticchio maar lakes mt vulture volcano southern italy covering an annual cycle that aimed at understanding the characteristic features of the physical structures and dynamics of the two lakes we provide the first detailed description of the lakes based on high resolution conductivity temperature depth ctd profiles chemical and isotopic h and o compositions of the water and the amounts of dissolved gases e g he ar ch4 and co2 the combined data set reveals that the two lakes which are separated by less than 200 m exhibit different dynamics one is a meromictic lake where the waters are rich in biogenic and mantle derived gases while the other is a monomictic lake which exhibits complete turnover of the water in winter and the release of dissolved gases our data strongly suggest that the differences in the dynamics of the two lakes are due to different density profiles affected by dissolved solutes mainly fe which is strongly enriched in the deep water of the meromictic lake a conceptual model of water balance was constructed based on the correlation between the chemical composition of the water and the hydrogen isotopic signature gas rich groundwaters that feed both of the lakes and evaporation processes subsequently modify the water chemistry of the lakes our data highlight that no further potential hazardous accumulation of lethal gases is expected at the monticchio lakes nevertheless geochemical monitoring is needed to prevent the possibility of vigorous gas releases that have previously occurred in historical time

water and gas chemistry of lake nyos and its bearing on the eruptive process abstract the isotopic and chemical composition of water samples collected from lake nyos some two and eight weeks after the eruption of august 21 1986 point to the existence of three distinct mixing regimes involving three water components an essentially homogeneous unmixed body of water at depths 100 m overlain by water increasingly affected by surface evaporation and a 5 10 m layer containing recent but pre eruption rain water the cationic constituents na k mg ca mn fe of the lake water correspond to the dissolution of around 0 1 g of local rock the waters are close to saturation with respect to siderite the composition of the gas dissolved in the deep lake waters 0 65 b w of co2 pco2 4 4b corresponds in mmol mol 1 to 996 co2 2 0 ch4 2 0 n2 0 05 ar 0 004 he 0 0002 h2 0 0001 ne co 2 13 c 3 4 and of he rair 5 5 suggest deep magmatic origins the 13c and 2h content of ch4 organic sedimentary origin the presence of aromatic hydrocarbons indicates very high temperatures of hydrocarbon formation exsolution of gas will first lead to the precipitation of siderite then iron hydroxide the chemistry of the lake waters points to a loss of some 240 000 t of co2 from the upper 100 m of the lake their isotopic composition is consistent with the assumption that the eruption was triggered by the accumulation of cold rain waters at the lake surface prior to the eruption inducing partial convective overturn there is no need to invoke addition of chemical or isotopic constituents from deeper levels during the eruption

non linear altimetric geoid inversion for lithospheric elastic thickness and crustal density summary the inversion of high resolution geoid anomaly maps derived from satellite altimetry should allow one to retrieve the lithospheric elastic thickness t e and crustal density r c indeed the bending of a lithospheric plate under the load of a seamount depends on both parameters and the associated geoid anomaly is correspondingly dependent on the two parameters the diverence between the observed and modelled geoid signatures is estimated by a cost function j of the two variables t e and r c we show that this cost function forms a valley structure along which many local minima appear the global minimum of j corresponding to the true values of the lithospheric parameters classical gradient methods fail to find this global minimum because they converge to the first local minimum of j encountered so that the final parameter estimate strongly depends on the starting pair of values t e r c we here implement a non linear optimization algorithm to recover these two parameters from altimetry data we demonstrate from the inversion of synthetic data that this approach ensures robust estimates of t e and r c by activating two search phases alternately a gradient phase to find a local minimum of j and a tunnelling phase through high values of the cost function the accuracy of the solution can be improved by a search in an iteratively restricted parameter subspace applying our non linear inversion to the great meteor seamount geoid data we further show that the inverse problem is intrinsically illposed as a consequence minute geoid or gravity data errors can induce large changes in any recovery of lithospheric elastic thickness and crustal density

placing bounds on lithospheric deformation in the central pacific ocean abstract lineations in the gravity field in the central pacific ocean have been explained by models of both small scale convection and lithospheric boudinage the boudinage model requires deformation of the plate and we have investigated bounds on the amount of deformation we calculate separations between a pair of fracture zones on the pacific plate which occur in an area where the lineations are well developed and compare these values to the separation of the corresponding fracture zone pair on the nazca plate where no lineations have been observed satellite and surface ship data was used to locate fracture zones and small circles were fitted to sections of picks which were chosen around their points of intersection with magnetic isochrons great circle distances from these small circles to stage poles were calculated and the difference between these distances gave the separations between the fracture zone pairs we found that between anomalies 13 and 7 the difference in fracture zone spacing on the pacific plate compared to the nazca plate appears to increase as the age of oceanic lithosphere decreases the maximum difference calculated was 97 km about 4 of the distance between the fracture zones and we attribute this difference to the re orientation of the ancient pacific farallon spreading centre between anomalies 7 and 5 the main lithospheric stretching models require the fracture zone separation to increase with age but this is not seen in our results

grim gravity model improvement using lageos grim3 l1 improved modeling of the earth s geopotential is one of the major tasks of satellite geodesy the availability of a new set of precise lageos satellite laser ranging slr data as well as seasat acquired observations enabled us to compute an improved solution in the grim model series the new geopotential grim3 l1 recombined the satellite perturbation normals of the former grim3 solution with new normal equation subsystems derived from an additional 16 months of lageos slr data oceanic 1 1 mean free air anomalies recovered from seasat altimetry an updated set of terrestrial 1 1 mean free air anomalies and several condition equations for zonal and resonant harmonics the combined normal equations were solved for a complete set of spherical harmonics up to degree and order 36 and the coordinates of 109 tracking stations comparisons with altimetry derived quantities terrestrial gravity anomalies from satellite to satellite tracking longitude accelerations of 24 hour satellites and satellite orbit fits show a substantial improvement over previous grim solutions and indicate that the lageos data have contributed considerably to the reliable estimation of the long wavelength portion of the gravity field

hydrochemistry of two adjacent acid saline lakes in the andes of northern chile numerous closed basins in central andes contain saline lakes and salt crusts salars almost all of them are neutral or alkaline lakes only two lakes out of 84 studied in bolivia and chile are acid lakes they are located in two adjacent intravolcanic basins in northern chile the origin of acidity is due to the juxtaposition of two factors the very strong hydrothermal alteration of volcanic rocks in the drainage basins and the high sulfur content in the whole area primary minerals are replaced by amorphous silica limonite chlorite white mica and elemental sulfur the buffer capacity of the volcanic rocks has been considerably lowered and is no longer sufficient to neutralize the sulfuric acid generated by oxidation of native sulfur dissolution modeling of chlorite white mica and sulfur in a neutral inflow water leads to a composition very much like that of the acid inflows the modeled acid solution is oversaturated with respect to alunite by several orders of magnitude exactly as observed in natural acid inflows some of the alunite reported in the altered zones of the drainage basins could be supergene in origin the simulation of evaporation of acid inflows shows a good agreement between salar brines and computed brines indicating that the acidic brines do not originate in the past hydrothermal activity but result from recent supergene processes the leaching of the severely altered volcanic rocks and the direct evaporation of the resulting acidic waters aluminium shows a conservative behavior in both salars reflecting the drastic lowering of the saturation index of al sulfate minerals in evaporating acid brines which is confirmed by the lack of detection of alunite within both salars

rifting of old oceanic lithosphere geophysical data from five regions in the pacific and indian oceans reveal that long distance 400 km spreading center jumps have occurred in the past the present day seafloor morphology is used to develop a scenario for a spreading center jump the major events are 1 thinning and weakening of the lithosphere at the future rifting site 2 rifting of the weakened lithosphere during rifting the crack is filled from above by normal faulting and wedge subsidence viscous upwelling fills the crack from below 3 spreading at the rift site results in a ridge bounded by two troughs spreading ceases at the dying spreading center resulting in a deep central graben surrounded by flexural ridges periods of slow spreading at both spreading centers produce rough topography 4 ageing and cooling that produce a general deepening of the abandoned spreading ridge and also reduce the thermal contrast across the fossil rifting site the new spreading center develops into a normal spreading rift the major topographic expressions apparent in the seafloor today are the deep trough of the abandoned spreading center and the proximal and distal troughs which formed when the emerging spreading center bisected the fossil rifting site the proximal trough nearer the new spreading ridge and the distal trough farther from the new ridge are first order topographic features 100 1000 km long and 300 km wide resembling fracture zones with which they are often confused they share with fracture zones the characteristic of bringing together fragments of lithosphere of different ages but unlike fracture zones they are generally parallel to magnetic lineations

evolution of a vent hosted hydrothermal system beneath ruapehu crater lake new zealand a two year chemical monitoring program of ruapehu crater lake shows that it has evolved considerably since the volcano s more active eruptive periods in the early 1970s the present ph 20 c of 0 6 is about one half unit more acid than the baseline values in the 1970s whereas s cl ratios have increased markedly owing in part to declining hcl inputs into the lake but also to absolute increases in so4 levels which now stand at the highest values ever recorded increases in k mg and na mg ratios over the 20 year period are attributed to hydrothermal reaction processes in the vent which are presently causing dissolution of previously formed alteration phases such as natroalunite these observations combined with results of a recent heat budget analysis of the lake have led to the development of hydrothermal convection model for the upper portion of the vent possible vent lake chemical reaction processes between end member reactants have been modelled with the computer code chiller the results are consistent with the view that variations in lake chemistry which are initiated by the introduction of fresh magmatic material into the vent reflect the extent of dissolution reaction progress on the magmatic material and or its alteration products the results also provide insights into the role of such vent processes in the formation of high sulfidation type ore deposits

monitoring of volcanic eruptions at yugama crater lake by aqueous sulfur oxyanions abstract variations of polythionates sulfane disulfonates and sulfate in the yugama crater lake japan have been monitored for more than 25 years just before the 1982 eruption at the crater lake polythionate ions decreased to zero from the normal level of about 2000 ppm and sulfate ions increased from 2500 to 5000 ppm during the 1982 eruption polythionate and sulfate ions varied inversely in concentration and the variations exactly coincided with the frequency of volcanic earthquakes and subsequent explosions these observations are interpreted in terms of aqueous reactions of fumarolic so2 h2s gases resulting in precipitation of alunite the behavior of polythionate and sulfate ions strongly suggests that they are useful indicator for prediction of impending volcanic hazards from active crater lakes

genetic algorithms for geophysical parameter inversion from altimeter data summary a new approach for inverting several geophysical parameters at the same time from altimeter and marine data by implementing genetic algorithms gas is presented these original techniques of optimization based on non deterministic rules simulate the evolution of a population of candidate solutions for a given objective function to minimize they offer a robust and efficient alternative to gradient techniques for non linear parameter inversion here genetic algorithms are used for solving a discrete gravity problem of data associated with an undersea relief to retrieve seven parameters at the same time the elastic thickness the mean ocean depth the seamount location longitude latitude its amplitude radius and density from its observed gravity geoid signature this approach was also successfully used to adjust lithosphere parameters in the real case of the rarotonga seamount 21 2us 159 8uw in the southern cook islands region where ga simulations provided robust estimates of these seven parameters the ga found very realistic values for the mean ocean depth and the seamount amplitude and the precise geographical location of rarotonga island moreover the values of elastic thickness y14 15 km and seamount density y2850 2870 kg m x3 estimated by the ga are consistent with the ones proposed in earlier studies

geochemical and biochemical evidence of lake overturn and fish kill at lake averno italy abstract lake averno is situated in the homonymous crater in the northwestern sector of the campi flegrei active volcanic system in campania region italy in february 2005 a fish kill event was observed in the lake prompting a geochemical survey to ascertain the possible cause in february 2005 a geochemical survey revealed that the lake water was unstratified chemically and isotopically presumably as a result of lake overturn this fish kill phenomenon was recorded at least two other times in the past in contrast to the february 2005 results data collected in october 2005 shows the lake averno to be stratified with an oxic epilimnion surface to 6 m and an anoxic hypolimnion 6 m to lake bottom at about 33 m chemical and isotopic compositions of lake averno waters suggest an origin by mixing of shallow waters with a na cl hydrothermal component coupled with an active evaporation process the isotopic composition of dissolved inorganic carbon as well as the composition of the non reactive dissolved gas species again supports the occurrence of this mixing process decreasing levels of so 4 and increasing levels of h 2 s and ch 4 contents in lake water with depth strongly suggests anaerobic bacterial processes are occurring through decomposition of organic matter under anoxic conditions in the sediment and in the water column sulfate reduction and methanogenesis processes coexist and play a pivotal role in the anaerobic environment of the lake averno the sulfate reducing bacterial activity has been estimated in the range of 14 22 mol m 2 day 1 total gas pressure of dissolved gases ranges between 800 and 1400 mbar well below the hydrostatic pressure throughout the water column excluding the possibility at least at the survey time of a limnic eruption vertical changes in the density of lake waters indicate that overturn may be triggered by cooling of epilimnetic waters below 7 c this is a possible phenomenon in winter periods if atmospheric temperatures remain frosty for enough time as occurred in february 2005 the bulk of these results strongly support the hypothesis that fish kill was caused by a series of events that began with the cooling of the epilimnetic waters with breaking of the thermal stratification followed by lake overturn and the rise of toxic levels of h 2 s from the reduced waters near the lake bottom

the elastic thickness of the lithosphere in the pacific ocean abstract in this study we present determinations of the effective elastic thickness t e of the oceanic lithosphere along pacific chains or archipelagoes t e is determined by computing the deflection of a continuous elastic plate under the load of volcanoes and constrained by geoid heights provided by seasat in the south central pacific estimates of 14 km for the marquesas and 6 km or less for the pitcairn mururoa gloucester chain are in good agreement with a previous work in this region cook austral and society chains around the line islands chain seasat data reveal that the bathymetry is poorly known preventing fine analysis meanwhile t e looks globally very low 6 km except for three volcanoes but these results may be unreliable the easter chain features low t e values 6 km with no noticeable variation along the chain higher values are found for a samoan island manuae 24 km and along the hawaiian emperor seamounts chain from 32 km at the eastern end of the chain to 21 5 km for the hawaiian volcanoes and from 25 5 to 15 km for the emperor seamounts the large number of t e estimates obtained in this study points out a noticeable difference between north and south pacific results those from the north pacific agree with the general trend increase with the square root of age plate at loading time while those from the south central pacific are much lower according to their plate age these low t e results from the south pacific are only partly explained by taking account of thermal perturbations using the rejuvenation model therefore these results then point out a regional difference in oceanic lithosphere

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

magnetotelluric imaging of the society islands hotspot in april june 1989 seafloor magnetotelluric data across and along the leading edge of the tahiti hotspot were obtained the magnetotelluric response functions were found to be strongly influenced by bathymetric and island effects and a new procedure for modeling and removing this distortion using a thin sheet approach combined with the measured water depths is introduced the corrected response functions are consistent with a two dimensional structure inversion of the data shows a slightly higher conductivity relative to a reference site located away from the hotspot down to 130 km depth beneath the active area southeast of tahiti underlain by a more resistive structure there is a suggestion for a change in conductivity in the 400 450 km depth range which is consistent with elevated temperatures this result is consistent with a mantle plume of limited extent less than 150 km radius located near the leading edge of the tahiti hotspot the magnetotelluric data provide no evidence for lithospheric thinning or for a strong thermal influence over a large area as would be required by a superswell model

the effective elastic lithosphere under the cook austral and society islands abstract we have determined the elastic thicknesste of the oceanic lithosphere along two volcanic chains of the south central pacific cook austral and society islands we used a three dimensional spatial method to model the lithospheric flexure assuming a continuous elastic plate the model was constrained by geoid height data from the seasat satellite along the cook austral chain the elastic thickness increases westward from 2 4 km at mcdonald hot spot to 14 km at rarotonga at mcdonald seamount however the data are better explained by a local compensation model the observed trend shows an increase ofte with age of plate at loading time however the elastic layer under the cook austral appears systematically thinner by several kilometers than expected for normal seafloor suggesting that substantial thermal thinning has taken place in this region considering the apparent thermal age of the plate instead of crustal age improves noticeably the results along the society chainte varies from 20 km under tahiti to 13 km under maupiti which is located 500 km westward when plotting together the society and cook australte results versus age of load we notice that within the first five million years after loading te decreases significantly while tending rapidly to an equilibrium value this may be interpreted as the effect of initial stress relaxation which occurs just after loading inside the lower lithosphere and suggests that the presently measured elastic thickness under the very young tahiti load 0 8 ma is not yet the equilibrium thickness

structural evolution of preexisting oceanic crust through intraplate igneous activities in the marcus wake seamount chain multichannel seismic reflection studies and seismic refraction surveys with ocean bottom seismographs in the marcus wake seamount chain in the northwestern pacific ocean reveal p wave velocity structures of hot spot origin seamounts and adjacent oceanic crust inside the seamounts are central high velocity 6 5 km s structures extending nearly to the top that may indicate intrusive cores thick sediment layers up to 4 km with p wave velocities of 4 5 km s have accumulated on seafloor that predates seamount formation downward crustal thickening of up to 2 km was documented beneath a large seamount cluster but thickening was not confirmed below a small seamount cluster volume ratios of an intrusive core to a seamount body are 15 20 indicating that most of the supplied magma was consumed in forming the thick sedimentary and volcaniclastic layer constituting the seamount flanks underplating and downward crustal thickening may tend to occur when second or later intrusive cores are formed in a seamount p wave velocities in the lowest crust and in the uppermost mantle below the seamount chain are 0 1 0 2 km s higher and 0 3 0 5 km s lower respectively than velocities below oceanic crust we explain this difference as a result of sill like intrusion of magma into the lower crust and uppermost mantle reflected waves observed at offsets 200 km are from mantle reflectors at depths of 30 45 km and 55 70 km the shallower reflectors may indicate structures formed by intraplate igneous activities and the deeper reflectors may correspond to the lithosphere asthenosphere boundary

the marquesas line swell mapping of depth anomalies in the central pacific reveals a broad shallow region herein termed the marquesas line swell it has the morphology and geoid signature typical of a midplate hotspot swell and probably marks the tertiary trace of the marquesas hotspot the swell is elongate in the nw se direction about 800 m high and about 1500 km wide it appears to extend continuously between the marquesas islands and the line cross trend a positive geoid height anomaly over the swell can be separated from the large regional trend the transfer function between depth anomaly and geoid height is not well defined but is consistent with local isostatic compensation with an average root depth about 30 km 40 km 95 confidence interval below the swell s surface direct spatial comparisons between geoid height and depth anomaly suggest that this root depth is constant along the length of the swell and has a value of about 40 km 10 km 95 confidence interval the crest of the swell appears to subside as normal seafloor which is only 25 m y old the prediction of the lithospheric thinning hypothesis a permanent density decrease within the lithosphere can also explain the subsidence and geoid height anomaly roll cell dynamic uplift can explain the subsidence but is difficult to reconcile with the geoid data

possible perturbations to hotspot traces and implications for the origin and structure of the line islands perturbations to hotspot signals recorded on the surface of the earth will produce hotspot traces with characteristics more complex than those of simple time progressive volcanic chains some of these perturbations are a consequence of normal plate tectonics some are observed interactions of hotspot magmatism with lithospheric structure and processes and others are hypothetical the line islands volcanic chain has a complex structure and some characteristics that cannot be reconciled with those expected of a simple time progressive hotspot trace two models for the development of the line islands are presented model 1 assumes that the line islands were formed between 130 and 67 ma and model 2 assumes an age span of 90 to about 15 ma and incorporates the tuamotu islands as an extension of the line islands in model 1 most of the complex structure and subsidence of the chain can be explained in terms of perturbations to the hotspot signal model 2 gives a good geometric fit to the volcanic chain but fails to explain the anomalous subsidence of the line islands

global mean sea surface based upon the seasat altimeter data a mean sea surface map has been computed for the global ocean areas between 70 n latitude and 62 s latitude based upon the 70 day set of seasat altimeter data recorded in 1978 a combination of regional crossover adjustment techniques and a global reference surface using the best available geodetic parameters for seasat has formed the basis for the computation the overall rms value of the crossover residuals in the open ocean areas was 13 cm after the adjustment a contour map of the crossover residuals is very similar in nature to earlier contour maps of the collinear pass results for mesoscale variability the mean sea surface is presented in the form of a global contour map a 0 5 0 5 grid and a computerilluminated image

rigidity of the atlantic oceanic lithosphere beneath new england seamounts abstract investigation of the flexural response of the oceanic plate subject to loading of new england seamounts provides an insight into the rigidity of the lithosphere fourier domain analysis of the bathymetry and gravity anomalies over new england seamounts suggests that internal density perturbations exist in the crust and or uppermost mantle the loads due to the topography of the seamounts and density perturbations must have been supported by the lithosphere we group the seamounts into three representative ages and adopt a 3 d thin elastic plate model over an invicid mantle to assess the rigidity of the lithosphere under these loads the rigidity values are about 3 7 10 23 n m for lithospheric ages ranging from 30 to 60 my the base of the elastic part of the lithosphere follows an isotherm in the range of 750 850 c suggesting a strong lithosphere that makes the north atlantic ocean difficult to initiate subduction however the thermal blanketing by the thick sediments at scotian basin reduces the rigidity by one to two orders of magnitude and may facilitate initiation of subduction of the north atlantic oceanic lithosphere at scotian margin

reappraising elastic thickness variation at oceanic trenches we reassess the variation of elastic thickness as a function of lithospheric plate age using a global database of bathymetric and free air gravity profiles which are perpendicular to oceanic trenches as in many previous studies our starting point is the well known floating elastic plate model in order to remove the influence of short wavelength features not associated with lithospheric bending adjacent profiles from 10 myr bins have been stacked together to construct average profiles with standard deviations each average profile was then inverted in a two stage procedure first singular value decomposition was used to determine two unknown flexural parameters together with a regional slope and offset for any given elastic thickness this procedure was repeated for a range of elastic thicknesses second residual misfit was plotted as a function of elastic thickness and the global minimum was identified this two stage procedure makes no prior assumptions about magnitude of the load size of the bending moment or whether the elastic plate is broken continuous we obtained excellent fits between theory and observation for both bathymetric and gravity profiles from lithosphere with an age range of 0 150 ma the shape of the residual misfit function indicates the degree of confidence we have in our elastic thickness estimates the lower limit of elastic thickness is usually well determined but upper limits are often poorly constrained inverse modeling was carried out using a range of profile lengths 250 300 500 and 700 km in general our estimates show no consistent increase of elastic thickness as a function of plate age this surprising result is consistent with recent reassessments of elastic thickness beneath seamounts and implies either that elastic thickness is independent of plate age or that elastic thickness cannot be measured with sufficient accuracy to reveal such a relationship modeling of short free air gravity profiles 250 300 km does tentatively suggest that elastic thickness increases linearly from 5 to 10 km between 0 and 20 ma and from 10 to 15 km between 20 and 150 ma this variation roughly matches the depth to the 200 c isotherm which corresponds to an homologous temperature of 0 4 for wet peridotite unfortunately for longer profile lengths there is no temporal dependence and elastic thicknesses vary considerably for all plate ages bathymetric profile modeling yields similar results although uncertainties are larger

geochemistry mineralogy and chemical modeling of the acid crater lake of kawah ijen volcano indonesia the kawah ijen volcano with a record of phreatic eruptions has its 1000 m wide crater filled with a lake that has existed for at least one century at present the lake waters are hot t 37 c strongly mineralized tds 105 g l and extremely acidic ph 0 4 by its volume the javanese lake is probably the largest accumulation in the world of such acidic waters mineralogy of the suspended solids within the lake waters suggests that concentrations of si ca ti and ba are controlled by precipitation of silica gypsum anatase and barite lake sediment is composed of chemical precipitates with composition similar to the suspended solids thermodynamic calculations predict that the lake waters have reached equilibrium with respect to cristobalite barite gypsum anglesite celestite and amorphous silica in agreement with the analytical observations significant concentrations of ferric iron suggest that the current lake waters are fairly oxidized sulfides are absent in the water column but are always present in the native s spherules that form porous aggregates which float on the lake the presence of native s provides direct evidence of more reduced conditions at the lake floor where h 2 s is probably being injected into the lake with progressive addition of h 2 s to the acid waters native s pyrite and enargite are theoretically predicted to be saturated reactions between upward streaming h 2 s bearing gases discharged by subaqueous fumaroles and metals dissolved in the acidic waters could initiate precipitation of these sulfides a model of direct absorption of hot magmatic gases into cool water accounts for the extreme acidity of the crater lake results show that strongly acidic sulfate rich solutions are formed under oxidizing conditions at high gas water ratios reactions between the acidic fluids and the ijen andesite were modeled to account for elevated cation concentrations in lake water current concentrations of conservative rockforming elements are produced by dissolution of approximately 60 g of andesite per kg of acid solution complete neutralization of the acid lake waters by reaction with the wallrock produces a theoretical alteration assemblage equivalent to that observed in volcano hosted acid sulfate epithermal ore deposits

transport and sedimentation dynamics of transitional explosive eruption columns the example of the 800 bp quilotoa plinian eruption ecuador abstract impact of large scale explosive eruptions largely depends on the dynamics of transport dispersal and deposition of ash by the convective system in fully convective eruptive columns ejected gases and particles emitted at the vent are vertically injected into the atmosphere by a narrow buoyant column and then dispersed by atmosphere dynamics on a regional scale in fully collapsing explosive eruptions ash partly generated by secondary fragmentation is carried and dispersed by broad co ignimbrite columns ascending above pyroclastic currents in this paper we investigate the transport and dispersion dynamics of ash and lapillis during a transitional plinian eruption in which both plinian and co ignimbrite columns coexisted and interacted the 800 bp eruptive cycle of quilotoa volcano ecuador produced a well exposed tephra sequence our study shows that the sequence was accumulated by a variety of eruptive dynamics ranging from early small phreatic explosions to sustained magmatic plinian eruptions to late phreatomagmatic explosive pulses the eruptive style of the main 800 bp plinian eruption u1 progressively evolved from an early fully convective column plinian fall bed to a late fully collapsing fountain dense density currents passing through an intermediate transitional eruptive phase fall syn plinian dilute density currents in the transitional u1 regime height of the convective plinian column and volume and runout of the contemporaneous pyroclastic density currents generated by partial collapses were inversely correlated the convective system originated from merging of co plinian and co surge contributions this hybrid column dispersed a bimodal lapilli and ash fall bed whose grain size markedly differs from that of classic fall deposits accumulated by fully convective plinian columns sedimentological analysis suggests that ash dispersion during transitional eruptions is affected by early aggregation of dry particle clusters

using crater lake chemistry to predict volcanic activity at poas volcano costa rica monitoring of crater lake chemistry during the recent decline and disappearance of the crater lake of poas volcano revealed that large variations in so4 cl f cl and mg cl ratios were caused by the enhanced release of hcl vapor from the lake surface due to increasing lake temperature and solution acidity variation in the concentration of polythionic acids h2sxo6 x 4 6 was the most reliable predictor of renewed phreatic eruptive activity at the volcano exhibiting sharp decreases three months prior to the initiation of phreatic eruptions in june 1987 polythionic acids may offer a direct indicator of changing subsurface magmatic activity whereas chloride based element ratios may be influenced by surface volatilization of hcl and subsequent recycling of acidic fluids in crater lake volcanoes

lithospheric flexure and thermal anomalies plots of effective elastic thickness te of the oceanic lithosphere as a function of age when elastically deformed show a general increase in te with the square root of age although there is much scatter some of the scatter can be eliminated by correcting for the effect of finite yield strength in rocks which makes te for more sharply bent plates underestimate the true depth tm to the rheological boundary at the base of the high strength mechanical lithosphere after converting effective values te to true thicknesses tm it appears that the mechanical lithosphere is always thinner beneath islands and seamounts compared to lithosphere of similar age flexed at subduction zones from several other lines of evidence such as depth anomalies seamount subsidence and heat flow data it has been suggested that the thermal structure in plates subjected to hot spot type volcanism is reset to values appropriate for much younger lithosphere the reduction in mechanical plate thickness beneath hot spot volcanoes is also consistent with this concept of thermal rejuvenation since the strength of rocks decreases exponentially with rising temperature this dependence implies that variations in mechanical plate thickness can be used to map temperature anomalies in the lithosphere since the mechanical plate thickness depends principally upon the temperature at one depth specifically the depth at which the temperature reaches 550 600 c depending on strain rate flexural data are particularly sensitive indicators of the vertical structure of lithospheric reheating compared to quantities dependent on depth integrals of temperature such as swell height and subsidence the thermal rejuvenation hypothesis predicts that the flexural profile should mimic viscoelastic relaxation in the first 10 ma after loading if initially reheating is confined to depths close to and below the 550 c isotherm the flexural data combined with depth anomaly information for hawaii point to extremely rapid reheating localized in the lower lithosphere rejuvenation in the society islands on the other hand is compatible with slower methods of heat injection involving the entire lithospheric column

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

water chemistry of lake albano italy abstract lake albano was stratified at the time of our survey in december 1997 in agreement with previous observations martini et al geochem j 28 1994 173 184 cioni et al report for the civil protection department 1995 pedreschi accad lucch sci lett arti 1995 39 in the absence of phenomena induced by seismic activity either local or regional lake stratification may be perturbed by cooling of shallow waters below 8 5 c circulation is expected to homogenize lake waters and eventually to trigger gas exsolution when total gas pressure exceeds hydrostatic pressure in december 1997 total gas pressure in lake water was very close to atmospheric pressure 0 9 1 3 bar at all depths possibly due to the occurrence of a recent episode of circulation and presumed gas exsolution the state of saturation of lake albano waters and the similarity of the relative concentrations of na k mg and ca in lake waters local groundwaters and local volcanic rocks indicate that na k mg and ca concentrations in lake albano waters are mainly governed by incongruent dissolution of local volcanic rocks coupled with minor calcite precipitation at shallow depths