noble gases and nitrogen mixing and temporal evolution in the fumarolic fluids of vulcano italy abstract since 1979 noble gases he ne ar and n 2 in the medium temperature fumaroles of vulcano crater have been analyzed repeatedly during these 10 years of surveillance variations have been noticed in fluid composition until 1986 40 ar 36 ar ratios up to 2000 were found n 2 ar was 1600 and he ar was 1 9 indicating the presence of a deep component as the result of a mixing between mantle and crustal fluids since 1987 the atmospheric component has increased and the fluid composition has evolved towards an air saturated groundwater asw this change is considered to be the result of mixing between magmatic fluids and meteoric waters that release their atmospheric gases on entering the volcanic system these gases thus mask the magmatic component which has remained more or less constant in the last decade

18o exchange between steam and carbon dioxide in volcanic and hydrothermal gases implications for the source of water abstract the oxygen isotopic compositions of h 2 o and co 2 from 31 fumaroles from different volcanoes vesuvio campi flegrei vulcano and etna in italy soufriere of guadeloupe in lesser antilles prove that 18 o exchange between co 2 and steam is fast enough in natural gas phases to allow rapid isotopic reequilibration within a wide temperature range 100 1000 c the isotopic composition of h 2 o discharged by volcanic hydrothermal fumaroles is affected by isotopic reequilibration along with cooling of the gas phase and this secondary process must be taken into account when interpreting the origin of the water from isotopic data the use of corrected 18 o values computed considering the overall oxygen isotopic composition of the h 2 o co 2 system is therefore compulsory to overcome interpretative biases as an example we apply this method to 100 600 c fumaroles at vulcano crater whose isotopic composition is reinterpreted considering the effects of 18 o exchange between co 2 and h 2 o in contrast to most previous interpretations the corrected isotopic values for h 2 o clearly show that the vulcano system is fed by a two component mixture of magmatic water with a typical andesitic composition and either 18 o shifted seawater or magmatic steam partly condensed within the volcanic ground with minor contribution of local meteoric water the possible additional effects of isotopic reequilibration involving other o and h bearing gas species in volcanic fluids such as so 2 h 2 s hcl hf and h 2 have been also considered the influence of so 2 upon the isotopic composition of h 2 o is subordinate with respect to that of co 2 whereas significant shifts in deuterium composition are expected from hydrogen isotopic exchanges in the h 2 o h 2 s hcl hf h 2 gas system

genesis of fumarolic emissions as inferred by isotope mass balances co2 and water at vulcano island italy we have developed a quantitative model of co2 and h2o isotopic mixing between magmatic and hydrothermal gases for the fumarolic emissions of the la fossa crater vulcano island italy on the basis of isotope balance equations the model takes into account the isotope equilibrium between h2o and co2 and extends the recent model of chemical and energy two end member mixing by nuccio et al 1999 as a result the h2o and co2 content and the d 18o and 13c isotope compositions for both magmatic and hydrothermal end members have been assessed low contributions of meteoric steam added at a shallow depth have been also recognized and quantified in the fumaroles throughout the period from 1988 to 1998 nonequilibrium oxygen isotope exchange also seems to be occurring between ascending gases and wall rocks along some fumarolic conduits the 13cco2 of the magmatic gases varies around 3 to 1 vs peedee belemnite pdb following a perfect synchronism with the variations of the co2 concentration in the magmatic gases this suggests a process of isotope fractionation because of vapor exsolution caused by magma depressurization the hydrogen isotopes in the magmatic gases 1 to vs standard mean ocean water smow as well as the above 13cco2 value are coherent with a convergent tectonic setting of magma generation where the local mantle is widely contaminated by fluids released from the subducted slab magma contamination in the crust probably amplifies this effect the computed isotope composition of carbon and hydrogen in the hydrothermal vapors has been used to calculate the d and 13c of the entire hydrothermal system including mixed h2o co2 vapor liquid water and dissolved carbon we have computed values of about 10 vs smow for water and 2 to 6 5 vs pdb for co2 on these grounds we think that mediterranean marine water dh2o 10 feeds the hydrothermal system it infiltrates at depth throughout the local rocks reaching oxygen isotope equilibrium at high temperatures interaction processes between magmatic gases and the evolving seawater also seem to occur causing the dissolution of isotopically fractionated aqueous co2 and providing the source for hydrothermal carbon these results have important implications concerning fluid circulation beneath vulcano and address the more convenient routine of geochemical surveillance

interaction between the deep fluids and the shallow groundwaters on vulcano island italy the aim of this work is to study the interactions processes between the fluids of deep origin and the shallow groundwaters of the vulcano porto area during 1995 13 well waters were sampled three times may july and november and analysed for major and some minor elements b br and nh4 and for dissolved gases the close relationship of these waters with the deep magmatic source is highlighted by the composition of the dissolved gases furthermore the areal distribution of dissolved species is controlled mainly by the gas fluxes from depth and by the presence of a deeper thermal aquifer the distribution of major anomalies in the parameters measured in the groundwaters in fact overlaps the soil gas fluxes distribution that are related to the existence of preferential upflow routes of fumarolic fluids through weak structural lines in the volcanic edifice

genesis and evolution of the fumaroles of vulcano aeolian islands italy a geochemical model a geochemical model explaining the presence of fumaroles having different gas composition and temperature at the top of the crater and along the northeastern coast of vulcano island is proposed a pressurized biphase liquid vapor reservoir at the depth of about 2 km is hypothesized energy and mass balance sheets controlp t conditions in the system p t must vary along a boiling curve of brine as liquid is present the co2 content in the steam is governed by the thermodynamic properties of the fluids in the h2 nacl co2 system on the assumption that oxygen fugacity in the system is between the hm fmq oxygen buffers observed so2 h2s co2 co co ch4 ratios in the fumarolic gases at the fossa crater appear in equilibrium with a temperature higher than that observed such as may exist at depth the more reduced gas phases present on the sea side may result from re equilibrium processes in shallower aquifers the suggested model would help in monitoring changes in volcanic activity by analyzing fumarolic gases

thermal characterization of the vulcano fumarole field ground based thermal infrared surveys can contribute to complete heat budget inventories for fumarole fields however variations in atmospheric conditions plume condensation and mixed pixel effects can complicate vent area and temperature measurements analysis of vent temperature frequency distributions can be used however to characterise and quantify thermal regions within a field we examine this using four thermal infrared thermometer and thermal image surveys of the vulcano fossa fumarole field italy during june 2004 and july 2005 these surveys show that regions occupied by low temperature vents are characterised by distributions that are tightly clustered around the mean i e the standard deviation is low highly peaked positive kurtosis and skewed in the low temperature direction negative skewness this population is associated with wet fumaroles where boiling controls maximum temperature to cause a narrow distribution with a mode at 90 100 c in contrast high temperature vent regions have distributions that are widely spread about the mean i e the standard deviation is high relatively flat negative kurtosis and skewed in the high temperature direction positive skewness in this dry case fumaroles are water free so that maximum temperatures are not fixed by boiling as a result greater temperature variation is possible we use these results to define two vent types at vulcano on the basis of their thermal characteristics 1 concentrated localized regions of high temperature vents and 2 dispersed low temperature vents these occur within a much larger region of diffuse heat emission across which surfaces are heated by steam condensation the heat from which causes elevated surface temperatures for vulcano s lower fumarole zone high and low temperature vents occupied total areas of 3 and 6 m2 respectively and occurred within a larger 430 m2 vent free zone of diffuse heat emission for this lower zone we estimate that 21 43 103 w of heat was lost by diffuse heat emission a further 4 5 103 w was lost by radiation from high temperature vents and 6 5 103 w from low temperature vents thus radiative heat losses from high and low temperature vents within vulcano s lower fumarole zone respectively account for 10 and 15 of the total heat lost from this zone this shows that radiation from open vents can account for a non trivial portion of the total fumarole field heat budget

trace elements in the thermal groundwaters of vulcano island sicily abstract the chemical concentrations of many trace elements that have never before been examined in vulcano island groundwaters are reported chemical data indicate that rock composition is not the only factor determining the metal content in the aqueous system the observed concentrations depend mainly on 1 the chemical composition of the host rocks 2 the chemical physical conditions of the weathering solution temperature acidity redox conditions 3 input of magmatic gases 4 adsorption of metal ions on oxide particles formation of soluble complexes and or precipitation of solid phases chloride complexes in solution are of minimal significance for all elements with the exception of cd whereas carbonate complexes are important for pb uo 2 zn and ni temperature and trace element distribution maps are also presented

seismic attenuation in faroe islands basalts we analysed vertical seismic profiling vsp data from two boreholes at glyvursnes and vestmanna on the island of streymoy faroe islands to determine the magnitude and causes of seismic attenuation in sequences of basalt flows the work is part of seifaba a major project integrating data from vertical and offset vsp surface seismic surveys core samples and wireline log data from the two boreholes values of effective seismic quality factor q obtained at glyvursnes and vestmanna are sufficiently low to significantly degrade the quality of a surface reflection seismic image this observation is consistent with results from other vsp experiments in the north atlantic region we demonstrate that the most likely cause of the low values of effective q at glyvursnes and vestmanna is a combination of 1d scattering and intrinsic attenuation due to seismic wave induced fluid flow within pores and microcracks tests involving 3d elastic wave numerical modelling with a hypothetical basalt model based on field observations indicate that little scattering attenuation is caused by lateral variations in basalt structure

seismic velocities in fractured rocks an experimental verification of hudson s theory flow of fluids in many hydrocarbon reservoirs aquifers is enhanced by the presence of cracks and fractures these cracks could be detected by their effects on propagation of compressional and shear waves through the reservoir several theories including hudson s claim to predict the seismic effects of cracks although hudson s theory has already been used to calculate crack densities from seismic survey s the predictions of the theory have not yet been tested experimentally on rocks containing a known crack distribution this paper describes an experimental verification of the theory the rock used carrara marble was chosen for its uniformity and low porosity so that the effect of cracks would not be obscured by other influences cracks were induced by loading of laboratory specimens velocities of compressional and shear waves were measured by ultrasound at 0 85 mhz in dry and water saturated specimens at high and low effective pressures the cracks were then counted in polished sections of the specimens in dry specimens with both dry and saturated cracks hudson s theory overpredicted observed crack densities by a constant amount that is attributed to the observed value being systematically underestimated the theory made poor predictions for fully saturated specimens shear wave splitting caused by anisotropy duemore to both crystal and crack alignment was observed cracks were seen to follow grain boundaries rather than the direction of maximum compression due to loading the results demonstrate that hudson s theory may be used in some cases to determine crack and fracture densities from compressional and shear wave velocity data less

seismic attenuation in upper ocean crust at hole 504b seismic attenuation and its relationship to borehole stratigraphy in the upper 1 8 km of ocean basement at hole 504b are determined from analysis of vertical seismic profile vsp data vsp data provide unambiguous measurements of seismic amplitude decay along a vertical propagation path through the crust and ancillary borehole measurements enable detailed modeling of the relative contributions from geometrical spreading scattering and intrinsic loss mechanisms to this decay about 60 of the total observed amplitude decay occurs in the pillow basalt section and is due mostly to geometrical spreading and scattering from impedance contrasts the remaining amplitude decrease is concentrated in two layers at 500 650 and 800 900 meters below seafloor mbsf 225 375 and 525 625 m below basement across which amplitude rapidly decays and the frequency characteristics of the downgoing wave field are significantly and permanently modified attenuation in these layers is not due to scattering but rather to an intrinsic mechanism that can be characterized by q of 10 and 8 respectively it is likely that the q structure of both of these intervals is formed with the crust near the ridge and thus related to fundamental ocean crust forming processes the shallow interval coincides with a change in alteration mineralogy deposited by late stage fluid flow and may separate lower lavas that were emplaced within the rift zone from upper lavas that were emplaced by off axis flow through large lava tubes intrinsic attenuation in the deeper horizon is probably due to an increase in porosity and cracking associated with either intracrustal deformation or subhorizontal faulting there is negligible attenuation of seismic frequencies in the dikes below 1000 mbsf 725 m subbasement

signs of potential renewal of eruptive activity at la fossa vulcano aeolian islands since the end of the last magmatic eruption 1890 activity of la fossa southern tyrrhenian sea italy has consisted of fumarolic emissions of fluctuating intensity fluids are discharged principally at two fumarolic fields located in the northern rim of the active crater and at the beach sited at its northern foot increased thermal seismic and geochemical activity has been recorded since 1978 when an earthquake of m 5 5 occurred in the region this paper combines available geophysical and geochemical information in order to develop a tentative interpretation of two episodes of apparent unrest which occurred in 1985 and 1987 1988 enhancing the risk of renewal of the eruptive activity the 1985 unrest consisted essentially of a sharp build up of the internal pressure in the shallow hydrothermal system which was induced by the injection of hot gases of magmatic origin the crater fumaroles displayed significant increases in co2 and other acid species but their outlet temperature did not change conversely the 1987 1988 episode was characterized by appreciable modifications at the crater fumaroles with only secondary effects at the fumarole system of the beach the sliding of part of the eastern flank of the la fossa cone into the sea occurred on 20 april 1988 when the region was affected by crustal dilatation producing a seismic sequence of relatively high intensity both episodes of unrest were accompanied by increases of local microseismic activity which affected the nothern sector of the island in 1985 and the southern one in 1988 finally a phase of appreciable areal contraction was detected in 1990 probably due to the effect of the cooling and crystallization of magma at relatively shallow depths accompanying the increased thermal activity at the crater fumaroles regional tectonic stress seems to play an important role in the transition of the volcanic system from a phase of relative stability to a phase of apparent unrest inducing the heating and the expansion of shallow hydrothermal fluids available information is insufficient to indicate whether or not the volcano is building towards the renewal of a magmatic eruption and there is no evidence to hypothesize episodes of significant magma migration the frequency of measurements of many parameters needs to be increased in order to learn more about the temporal relationships between geochemical and geophysical variations preceding and accompanying periods of increased thermal activity this will probably be a valid tool for recognizing short term precursors of a future eruption reducing the risk of false alarms

physical chemical conditions of the teide volcanic system tenerife canary islands the teide volcano 3717 m is the central structure of the island of tenerife and at present its morphology is that of a stratovolcano which has grown on a large caldera with a collapse 17 km in diameter which was generated some 0 6 million years ago the different studies that have been carried out seem to indicate that in a oversimplified model there is an intermediate magma chamber with an approximate volume of 30 km3 and located 2 3 km below the actual base of the caldera i e almost at sea level with a temperature of 430 50 c and a pressure of 400 100 bar the summit fumarole emissions are 85 c and are formed mainly of co2 with small amounts of sulphur species h2 ch4 and he the water vapor 68 82 emitted with the gases comes from the vaporization of a perched aquifer in the upper cone as shown by the isotopic analyses

seismic attenuation of atlantic margin basalts observations and modeling paleogene basalts are present over much of the northeastern atlantic european margin in regions containing significant thicknesses of layered basalt flows conducting seismic imaging within and beneath the volcanic section has proven difficult largely because the basalts severely attenuate and scatter seismic energy we use data from a vertical seismic profile vsp from well 164 07 1 that penetrated 1 2 km of basalt in the northern rockall trough west of britain to measure the seismic attenuation caused by the in situ basalts the effective quality factor qeff of the basalt layer is found from the vsp to be 15 35 which is considerably lower more attenuative than the intrinsic attenuation measured on basalt samples in the laboratory we then run synthetic seismogram models to investigate the likely cause of the attenuation full waveform 1d modeling of stacked sequences of lava flows based on rock properties from the same well indicates that much of the seismic attenuation observed from the vsp can b

chemical features and isotopic composition of gaseous manifestations on vulcano island aeolian islands italy an interpretative model of fluid circulation abstract between 1987 and 1995 gases were sampled from various manifestations on vulcano island including various fumaroles at la fossa crater and baia di levante and diffuse soil gas emissions in several areas which are characterized by high c02 fluxes since the end of 1987 volcanic activity has increased considerably showing a marked rise in temperature 300 c in 1987 to 670 c in december 1992 and in output of steam from crater fumaroles in addition variations have been observed in the chemical and isotopic compositions of gases and thermal waters which can be attributed to an increased inflow of deeper fluids the isotopic composition of fumarolec steam seems to be the result of mixing processes between a positive endmember of deep origin d 10 15 18o 6 8 and two endmembers of meteoric origin with different oxygen shifts variations observed in the c02 isotopic composition suggest that all sampled manifestations are fed by a single deep source which may be magmatic such a source would be characterized by the 613cco2 values of high temperature fumaroles 13cco2 0 these values are noticeably more positive than those generally attributed to magmatic gases 13cco2 5 8 implying crustal contamination processes of the magma the observed decrease in 18oco2 values of the crater fumaroles from 20 in 1978 to between 12 and 15 in 1993 1994 was probably caused by two different processes the first is a reduction of the enrichment factor between c02 and h2o which is related to the increased temperature of the gases and the second is a mixing with more negative waters of meteoric origin finally c02 addition or removal processes caused by interactions between deep gases and shallow hydrothermal waters are likely to be responsible for the different chemical and isotopic compositions of gaseous emissions in the vulcano porto area and the crater fumaroles

genesis of chlorine and sulphur in fumarolic emissions at vulcano island italy assessment of ph and redox conditions in the hydrothermal system chlorine and sulphur bearing compounds in fumarole discharges of the la fossa crater at vulcano island italy can be modelled by a mixing process between magmatic gases and vapour from a boiling hydrothermal system this allows estimating the compounds in both endmembers magma degassing cannot explain the time variation of sulphur and hcl concentrations in the deep endmember which are more probably linked to reactions of solid phases at depth before mixing with the hydrothermal vapours based on the p t conditions and speciation of the boiling hydrothermal system below la fossa the hcl and stot contents in the hydrothermal vapours were used to compute the redox conditions and ph of the aqueous solution the results suggest that the haematite magnetite buffer controls the hydrothermal fo2 values while the ph has increased since the end of the 1970s the main processes affecting ph values may be linked to na ca exchanges between evolved seawater feeding the boiling hydrothermal system and local rocks while na is removed from water calcium enters the solution undergoes hydrolysis and produces hcl lowering the ph of the water the increasing water rock ratio within the hydrothermal system lowers the ca availability so the aqueous solution becomes less acidic seawater flowing towards the boiling hydrothermal brine dissolves a large quantity of pyrite along its path in the boiling hydrothermal system dissolved sulphur precipitates as pyrite and anhydrite and becomes partitioned in vapour phase as h2s and so2 these results are in agreement with the paragenesis of hydrothermal alteration minerals recovered in drilled wells at vulcano and are also in agreement with the isotopic composition of sulphur emitted by the crater fumaroles

a 3d discrete numerical elastic lattice method for seismic wave propagation in heterogeneous media with topography 1 a three dimensional elastic lattice method for the simulation of seismic waves is presented the model consists of particles arranged on a cubic lattice which interact through a central force term and a bond bending force particle disturbances are followed through space by numerically solving their equations of motion a vacuum free surface boundary condition is implicit in the method we demonstrate that a numerical implementation of the method is capable of modelling seismic wave propagation with complex topography this is achieved by comparing the scheme against a finite difference solution to the elastodynamic wave equation the results indicate that the scheme offers an alternative 3d method for modelling wave propagation in the presence of strong topography and subsurface heterogeneity we apply the method to seismic wave propagation on mount etna to illustrate its applicability in modelling a physical system

the measurement of velocity dispersion and frequency dependent intrinsic attenuation in sedimentary rocks a series of experiments to determine the elastic properties of a sequence of saturated sedimentary rocks over as wide a frequency range as possible was carried out at the imperial college borehole test site these experiments fall into four categories vertical seismic profiles vsps within the frequency range 30 280 hz crosshole surveys 0 2 2 3 khz sonic logging 8 24 khz and laboratory measurements 300 900 khz the intrinsic attenuation and velocity of compressional and shear waves were measured whenever possible velocity dispersion is observed for both compressional and shear waves the intrinsic attenuation of compressional waves is frequency dependent with a peak in the attenuation in the sonic frequency band the data were modeled assuming the attenuation is caused by local fluid flow in pores of small aspect ratio the modeling indicates that the intrinsic attenuation may be dominated by cracks with aspect ratios of around 10 super 3 to 10 super 4

the evolution of a dynamic geological system the support of a gis for geochemical measurements at the fumarole field of vulcano italy abstract the island of vulcano aeolian arc southern italy has displayed several periods of volcanic unrest since the end of the last eruption 1890 we report here results obtained from a long term survey concerning variations of both the steam output and the exhaling surface area at the summit crater fumarolic field of la fossa the field measurements analysed in a geographical information system gis show a highly dynamic volcanic system in which deep variations in the geochemistry and the temperature of the released fluids were accompanied by fluctuations in the mass output of steam and the topography of the crater field the use of a gis facilitated digitized reconstructions of maps of the crater field in addition to analysis of the steam flux data the fumarolic field expanded its surface area from 50 m 2 in 1983 to more than 2400 m 2 in 1995 accompanied by an increase in steam output from 152 to about 1400 tonnes per day the possibility that the observed phenomena are related to volcano tectonic activity and to magma uprising is taken into consideration

volcanic hazards at fossa of vulcano data from the last 6 000 years stratigraphic reconstruction of the complete sequence of deposits that formed the fossa cone of vulcano has distinguished four principal eruptive cycles punte nere palizzi commenda and pietre cotte at least three additional eruptive cycles one of which ends with the campo sportivo lava occur between deposits of the punte nere and palizzi cycles however exposure is inadequate for their characterization the assignment of the modern deposits that follow the pietre cotte lava is uncertain

isotopic composition of natural thermal discharges on vulcano island southern italy this paper reviews the isotopic 18o16o and d h ratios reported in the literature for phreatic waters in the plain of vulcano island off sicily and condensates from high temperature fumaroles at the top of the summit crater the vulcano phreatic waters have a rather high average annual temperature greater than 30 dgc but do not seem to be heated directly by mixture with condensates of fumarolic steam they seem to originate from rainwater and to undergo high evaporation particularly in summer owing to high conductive heat flow in the island by comparing the 18o16o and d h ratios with cl as hcl content it is estimated that 75 of the steam from the high temperature fumaroles comes from seawater and 25 from low salinity waters the latter however do not seem to come directly from the shallow aquifer of vulcano because the steam from the fumaroles does not contain radiogenic tritium this mixing model and the lack of steam condensation in the vulcano aquifer implies indirectly that there are no steam losses during the rise of deep fluids and that some observed variations in the outlet temperatures without accompanying variations in the composition of the associated gas phase may be due to increased or decreased extent of fracturation in the volcanic edifice as a result for instance of tectonic movements which cause more or less vaporization of low salinity fresh water into the volcanic ducts as well as faster or slower outflow of deep gases

the episodic and abrupt geochemical changes at la fossa fumaroles vulcano island italy and related constraints on the dynamics structure and compositions of the magmatic system abstract herein we report on the chemical and isotopic c h o and he compositions of the fluids from la fossa crater fumaroles of vulcano from 1999 to 2010 consistent with records obtained since the end of the 1980s our data show that the geochemical features of the fumarole system have experienced several episodes of remarkable change each lasting no more than a few months typical signatures of these short term anomalies are large increments in co 2 n 2 and he concentrations coupled to increased 13 c 12 c isotopic ratios but their meaning remains widely debated within a model of fumarolic fluids based on mixing between hydrothermal and magmatic endmembers we have developed a novel approach to constrain chemical he co 2 and n 2 he and isotopic 13 c 12 c d h and 3 he 4 he ratios of the magmatic endmember during the short term anomalies although much of the geochemical variability in fumaroles results from changes in mixing proportions the magmatic fluid unquestionably shows significant variations in time the magmatic he co 2 n 2 he 13 c 12 c and 3 he 4 he values throughout 1988 1996 differed from those feeding the anomaly at the end of 2004 early clues of the new magmatic fluid appeared in 1998 1999 far from any short term anomaly whereas new and old magmatic fluids coexisted after 2004 we quantitatively prove that the detected geochemical changes are consistent with the degassing path of a magma having a latitic composition and suggest the presence of two magma ponding levels at slightly different pressures where bubble melt decoupling can occur the different he isotope compositions at these levels suggest low hydraulic connectivity typical of a complex reservoir with dike and sill structures in this framework the short term geochemical anomalies are probably due to gas accumulation at the top of magma bodies followed by massive escape or activation of new degassing levels in the reservoir for which the stress field almost certainly plays a key role such a scenario explains the observed increases in both fumarole output and shallow high frequency seismicity due to increased pore pressure during the anomalies while being consistent with the concomitant absence of any deep seismicity or ground deformation eventually related to magma movement

review of the evolution of geochemical monitoring networks and methodologies applied to the volcanoes of the aeolian arc italy abstract fluids discharged from volcanic systems are the direct surface manifestation of magma degassing at depth and provide primary insights for evaluating the state of volcanic activity we review the geochemical best practice in volcanic surveillance based to a huge amount of monitoring data collected at different active volcanoes using both continuous and discontinuous approaches the targeted volcanoes belong to the aeolian arc located in the tyrrhenian sea italy and they have exhibited different activity states during the monitoring activities reported here la fossa cone on vulcano island has been in an uninterrupted quiescent stage characterized by variable solfataric activity in contrast stromboli island has shown a persistent mild explosive activity episodically interrupted by effusive eruptions in 1985 2002 2007 and 2014 panarea island which is the summit of a seamount rising from the seafloor of the southern tyrrhenian sea showed only undersea fluid release the only observable clues of active volcanism at panarea island have been impulsive changes in the undersea fluid release with the last submarine gas burst event being observed in november 2002 the geochemical monitoring and observations at each of these volcanoes has directly involved the volcanic plume and or the fumarole vents thermal waters and diffuse soil degassing depending on the type of manifestations and the level of activity encountered through direct access to the magmatic samples when possible and the collection of as much observable data related to the fluid release as possible the aim has been i to verify the thermodynamic equilibrium condition ii to discern among the possible hydrothermal magmatic marine and meteoric sources in the fluid mixtures iii to develop models of the fluid circulation supported by data iv to follow the evolution of these natural systems by long term monitoring and v to support surveillance actions related to defining the volcanic risk and the evaluation and possible mitigation of related hazards the examples provided in this review article show the close relationships among data analysis interpretation and modeling we particularly focus on describing the fieldwork procedures since any theoretical approach must always be verified and supported by field data rather than just by experiments controlled in laboratory indeed the natural systems involve many variables producing effects that cannot be neglected the monitored volcanic systems have been regarded as natural laboratories and all of the activities have focused on both volcanological research and surveillance purposes in order to ensure that these two goals have overlapped an appendix is also included that explains the scientific approach to the systematic activities regarding geochemical monitoring of volcanic activity

interface scattering versus body scattering in subbasalt imaging and application of prestack wave equation datuming the presence of high velocity highly heterogeneous layers such as basalt have a detrimental effect on imaging below those structures internal characteristics of the basalt itself can also degrade the subbasalt image quality in many cases the boundary surfaces of a basalt sequence are not smooth but contain some roughness at a scale similar to the seismic wavelength interface scattering resulting from interface roughness can be a serious problem in this paper body and interface scattering problems are addressed through acoustic finite difference modeling interface scattering seems to have the most detrimental effect on imaging at depth a wave equation prestack datuming procedure is applied to synthetic acoustic data to remove the effects of rough interfaces this computational process transforms seismic data to a new datum plane in the subsurface eliminating propagation effects between the surface and the new datum plane the technique leads to a much improved image of below the basalt with the g

isotopic variation of the hydrothermal system on vulcano island italy the isotopic and chemical composition of thermal waters and the isotopic composition of high temperature fumarolic steam at vulcano island indicate an evolution of the hydrothermal system from its magmatic origin through interaction with rocks and mixing with meteoric water seawater is not a component in this system the isotopic values of the fumaroles indicate a local magmatic composition of 7 to 8 18o and 5 to 15 d the high d values of this vapor may reflect contamination of the magma by connate water the isotopic and chemical composition of water from well w2 indicates the existence of geothermal fluid formed by mixing of hcl and so2 bearing condensates of volcanic origin with groundwater with subsequent evolution via water rock interaction the estimated isotopic composition of the geothermal water is 18o 1 and d 5 local shallow seismic events result in a temporary increase in fracture permeability enabling the mixing of magmatic vapor with groundwater or geothermal water tectonic activity associated with deep structures appears to increase the contribution of magmatic vapor to the fumaroles

geochemical surveillance of the solfatara of pozzuoli phlegraean fields during 1983 geochemical surveillance of the phlegraean fields area has been intensified since 1983 in response to the increased uplift rate brady seismic activity fumarolic gases from solfatara pozzuoli were sampled and analyzed monthly a reducing capacity rc monitoring unit was installed at soffione the most active fumarole in the solfatara system the preliminary analysis of the rc temporal variations suggest they are consistent with the rate of the seismic energy release the composition of fumarolic gases indicates that the equilibrium temperature and pressure are higher than those of sampling the observed variations in ch4 content are explained as an increase of pressure from 1982 to the end of 1983 of about 70 at an extimated depth of 3 3 5 km finally during 1983 there were no geochemical indications of both rising magma and significant accumulation of energy at shallow depth

steam output from fumaroles of an active volcano tectonic and magmatic hydrothermal controls on the degassing system at vulcano aeolian arc the results from measurements of extensive parameters steam output and exhaling surface area taken at the crater fumarolic field of vulcano island aeolian arc southern italy are reported along with a detailed description of the measuring method the steam emission rate increased by 1 order of magnitude during the observation time 1983 1995 while its concentration in the released gases was only slightly changed during the same period the total exhaling surface expanded from 50 m2 to more than 2400 m2 and evolved following preferential trends that coincide with the main tectonic structures of the island the observed peaks in steam output show a positive correlation with episodes of volcanic activity unrest the temporal increase of steam output and the observed development in the exhaling surface areas are consistent with a volcano tectonic triggering of the exhaling activity particularly the peaks in the steam output are interpreted as a consequence of degassing from an active magma slowly moving toward the surface

geogenic and atmospheric sources for volatile organic compounds in fumarolic emissions from mt etna and vulcano island sicily italy 1 in this paper fluid source s and processes controlling the chemical composition of volatile organic compounds vocs in gas discharges from mt etna and vulcano island sicily italy were investigated the main composition of the etnean and volcano gas emissions is produced by mixing to various degrees of magmatic and hydrothermal components vocs are dominated by alkanes alkenes and aromatics with minor though significant concentrations of o s and cl f substituted compounds the main mechanism for the production of alkanes is likely related to pyrolysis of organic matter bearing sediments that interact with the ascending magmatic fluids alkanes are then converted to alkene and aromatic compounds via catalytic reactions dehydrogenation and dehydroaromatization respectively nevertheless an abiogenic origin for the light hydrocarbons cannot be ruled out oxidative processes of hydrocarbons at relatively high temperatures and oxidizing conditions typical of these volcanic hydrothermal fluids may explain the production of alcohols esters aldehydes as well as o and s bearing heterocycles by comparing the concentrations of hydrochlorofluorocarbons hcfcs in the fumarolic discharges with respect to those of background air it is possible to highlight that they have a geogenic origin likely due to halogenation of both methane and alkenes finally chlorofluorocarbon cfc abundances appear to be consistent with background air although the strong air contamination that affects the mt etna fumaroles may mask a possible geogenic contribution for these compounds on the other hand no cfcs were detected in the vulcano gases which are characterized by low air contribution nevertheless a geogenic source for these compounds cannot be excluded on the basis of the present data

crustal structure of the southern margin of the african continent results from geophysical experiments 1 a number of geophysical onshore and offshore experiments were carried out along a profile across the southern margin of the african plate in the framework of the inkaba yeafrica project refraction seismic experiments show that moho depth decreases rapidly from over 40 km inland to around 30 km at the present coast before gently thinning out toward the agulhas falkland fracture zone which marks the transition zone between the continental and oceanic crust in the region of the abruptly decreasing moho depth in the vicinity of the boundary between the namaqua natal mobile belt and the cape fold belt lower crustal p wave velocities up to 7 4 km s are observed this is interpreted as metabasic lithologies of precambrian age in the namaqua natal mobile belt or mafic intrusions added to the base of the crust by younger magmatism the velocity model for the upper crust has excellent resolution and is consistent with the known geological record a joint interpretation of the velocity model with an electrical conductivity model obtained from magnetotelluric studies makes it possible to correlate a high velocity anomaly north of the center of the beattie magnetic anomaly with a highly resistive body

multiparametric approach in investigating volcano hydrothermal systems the case study of vulcano aeolian islands italy seismic activity ground deformation and soil and fumarole temperatures acquired during 2004 2007 at vulcano aeolian islands are analysed and the time relations among the different time series are discussed changes in temperature of fumarolic gases took place during four anomalous periods november 2004 march 2005 october 2005 february 2006 august october 2006 july december 2007 at the same time as an increasing number of volcano seismic events in particular the temperatures at high temperature vents and at steam heated soil ranged in time from 180 to 440 c and from 20 to 90 c respectively the maximum daily number of volcano seismic events was 57 reached during the second anomalous period this seismicity characterised by focal depth generally lower than 1 km below sea level b s l and composed of different kinds of events associated to both resonance and shear failure processes is related to the shallow dynamics of the hydrothermal system during the analysed period very few volcano tectonic earthquakes took place and tilt recordings showed no sharp or important changes in light of such observations the increases in both temperature and volcano seismic events number were associated to increases in the release of gas from a deep and stable magma body without magma intrusions within the shallow hydrothermal system indeed a greater release of gas from depth leads to increased fluid circulation that can promote increases in volcano seismic events number by both fracturing processes and resonance and vibration in cracks and conduits the different trends observed in the measured geochemical and geophysical series during the anomalous periods can be due to either time changes in the medium permeability or a changing speed of gas release from a deep magma body finally all the observed variations together with the changing temporal distribution of the different seismic event kinds suggest that the hydrothermal system at vulcano can be considered unsteady and dynamic

chemical variations in fumarolic gases at vulcano island southern italy seasonal and volcanic effects abstract because of increasing in fumarolic activity at vulcano since september 1987 regular sampling and analysis is conducted on the well known f5 crater fumarole t 330 5 c significant chemical variations have been recorded over the period 1987 1989 variations of water vapour in the fumarolic fluid follow seasonal pattern and can be related to both external seasonal and internal volcanic influences suggesting a control by a shallow water aquifer significant variations of h 2 o and of some species in the anhydrous gas phase h 2 so 2 hcl hf n 2 he probably result from chemical or dynamic changes in the feeding system at depth several mechanism are considered for explaining the chemical trends results in relationship to the recent increase in activity a growing input of deep magmatic fluids is not supported by the low co content and the apparent constancy of the 3 he 4 he ratio apparent equilibrium temperatures close to the fumarolic outlet temperature 350 400 c were calculated suggesting isothermal expansion of the gas from a shallow equilibration zone comparison with the fumaroles at sea level porto di levante suggests that these latter may be fed a different hydrothermal systems variable interaction between both systems may account for some of the chemical variations observed at f5 fumarole

fluid geochemistry at vulcano island a change in the volcanic regime or continuous fluctuations in the mixing of different systems increasing fumarolic activity and the rise of outlet fumarole temperatures has occurred at vulcano island southern italy since september 1987 regular sampling and analysis have been conducted on the well known f5 fumarolic and later on f5ht and fa f umaroles on the fossa crater plus other gas emanations from different sites on the island significant chemical and isotopical oscillations have been recorded over the period 1987 1991 variations of water vapor in the fumarolic fluid follow a seasonal pattern and can be related to both external seasonal and magmatic influences but they suggest a control by a source other than magmatic significant variations of h2o and of some species in the anhydrous gas phase h2 co2 so2 n2 and he have been related in the past to chemical or dynamic changes in the feeding system at depth oscillations of the isotopic composition of helium and carbon suggest possible mixing with a nonsurficial source possibly crustal in origin argon and neon isotopic data support this hypothesis several mechanisms are considered for explaining the chemical and isotopic data in relation to the recent increase in activity not all of the chemical and isotopic variations recorded are related to geophysical parameters such as seismicity and deformation the low co content of the f5 fumarolic and the apparent constancy of the 3he 4he ratio do not suggest an increase in the input of deep magmatic fluids the high co content of the high temperature 650 c fa fumarole located at the interior of the crater along with helium and carbon isotopic ratios similar to other fumarolics is probably due to an easier ascent of these fluids to the surface apparent equilibrium temperatures close to the fumarolic outlet temperature indicate isothermal expansion of the gas from a shallow equilibration zone for both high and lower temperature gases in particular the chemical and isotopic features of fluids belonging to both crater and beach fumarole fields are consistent with the existence of two intermediate system independently feeding the two fields high 3he 4he ratios for the beach fluids similar to those of the crater fluids suggest a large magmatic reservoir at depth feeding both the crater and the beach intermediate reservoirs

multiple hazards and paths to eruptions a review of the volcanic system of vulcano aeolian islands italy abstract vulcano is one of the 7 volcanic islands and 6 seamounts forming the aeolian volcanic district italy vulcano has a long eruptive record and its last eruption 1888 90 originated the definition of the vulcanian eruptive style like most volcanic islands vulcano generates many potentially interconnected hazards determining a potentially high risk here we review the state of knowledge on its geology eruptive activity historical accounts structural setting geophysical and geochemical surveillance and available hazard quantifications in order to have an updated picture of the state knowledge on volcanic hazard we follow a prototypal reviewing scheme based on three standardized steps i review of the volcanic system ii review of available eruptive and non eruptive hazard quantifications iii development of a conceptual interpretative model we find that while a rather vast literature is dedicated to the volcanic system of vulcano and the reconstruction of past events few quantitative hazard assessments exist in addition the range of natural variability considered for each hazard is potentially underestimated e g limited range of considered eruption magnitude and style and of vent position as it is the potential effect of multi hazard impact the developed conceptual model for the feeding system provides a synthetic picture of the present knowledge about the system as emerged from the review in addition it allows for the identification of potential paths to eruption and provides a first order link among the main hazards this review provides an up to date snapshot of existing knowledge on volcanic hazard at vulcano on which to build future hazard quantifications as well as to support present and future decision making

the fracture criticality of crustal rocks summary the shear wave splitting observed along almost all shear wave ray paths in the earth s crust is interpreted as the effects of stress aligned fluid filled cracks microcracks and preferentially oriented pore space once away from the free surface where open joints and fractures may lead to strong anisotropy of 10 per cent or greater intact ostensibly unfractured crustal rock exhibits a limited range of shear wave splitting from about 1 5 to 4 5 per cent differential shear wave velocity anisotropy interpreting this velocity anisotropy as normalized crack densities a factor of less than two in crack radius covers the range from the minimum 1 5 per cent anisotropy observed in intact rock to the 10 per cent observed in heavily cracked almost disaggregated near surface rocks this narrow range of crack dimensions and the pronounced effect on rock cohesion suggests that there is a state of fracture criticality at some level of anisotropy between 4 5 and 10 per cent marking the boundary between essentially intact and heavily fractured rock when the level of fracture criticality is exceeded cracking is so severe that there is a breakdown in shear strength the likelihood of progressive fracturing and the dispersal of pore fluids through enhanced permeability the range of normalized crack dimensions below fracture criticality is so small in intact rock that any modification to the crack geometry by even minor changes of conditions or minor deformation particularly in the presence of high pore fluid pressures may change rock from being essentially intact below fracture criticality to heavily fractured above fracture criticality this recognition of the essential compliance of most crustal rocks and its effect on shear wave splitting has implications for monitoring changes in any conditions affecting the rock mass these include monitoring changes in reservoir evolution during hydrocarbon production and enhanced oil recovery and in monitoring changes before and after earthquakes amongst others

interpretation of a vertical seismic profile conducted in the columbia plateau basalts seismic reflection data are often of poor quality when recorded in areas where volcanic rocks are present at or near the surface in order to investigate this phenomenon a vertical seismic profiling vsp experiment was conducted in the columbia plateau basalts so that the behavior of seismic energy in subsurface volcanic rocks could be observed directly thus giving insight into data acquisition in volcanic terrains the lithologic section at the vsp site consists of low velocity 400 m s to 900 m s alluvium in the uppermost 50 m beneath which are layers of high velocity about 5800 m s high density basalts interbedded with clay layers with much lower velocities about 1700 m s and densities these large velocity and density contrasts dramatically influence wave generation and propagation in spite of the small source borehole offset 61 m large amplitude s waves are generated by the downgoing p waves when they reach a shallow 250 m clay basalt boundary these s waves in turn generate strong

nanometre scale spheroids on sands vulcano sicily possible nannobacterial alteration spheroids in the size range of 30 200 nm coat sand grains on sediments around the island of vulcano they occur as single individuals as ball shaped clumps resembling biological colonies with radiating chains of bodies and eventually merge into sheets in which the individual nannobacteria are no longer evident rarely they appear to bore into the glass grains older rocks are composed mainly of closely packed nanospheroids these spheroids are believed to be nannobacterial cells based on their resemblance to nannobacteria found in other surficial environments and their occurrence in colony like groups if so nannobacteria play a very important role in the destruction of volcanic materials on vulcano

elastic anisotropy due to aligned cracks in porous rock all theoretical expressions which relate the characteristics of saturated aligned cracks to the associated elastic anisotropy are restricted in some important way for example to the case of stiff pore fluids or of the absence of equant porosity or of a moderately high frequency band because of these restrictions previous theory is not suitable for application to the upper crust where the pore fluid is brine kf k820 the equant porosity is often substantial p 0 1 and the frequency band is sonic to seismic this work removes these particular restrictions recognizing in the process an important mechanism of dispersion a notable feature of these more general expressions is their insensitivity at low frequency to the aspect ratio of the cracks only the crack density is critical an important conclusion of this more general model is that many insights previously achieved concerning the shear wave splitting due to vertical aligned saturated cracks are sustained however conclusions on crack orientation or crack aspect ratio which were derived from p wave data or from shear wave critical angles may need to be reconsidered further the non linear coupling between pores and cracks due to pressure equalization effects means that the linear schoenberg muir calculus may not be applied to such systems the theory receives strong support from recent data by rathore et al on artificial samples with controlled crack geometry

the importance of pahoehoe abstract pahoehoe lava flows are common in every basaltic province and their submarine variants pillow lavas and sheet flows cover the bulk of the earth pahoehoe flows are emplaced by inflation the injection of molten lava underneath a solidified crust only in the past few years has an understanding of the inflation process and the ability to recognize ancient inflated lava flows been achieved all large terrestrial basaltic flow fields studied to date including flood basalts were emplaced as thermally efficient inflated compound pahoehoe sheet flows this leads us to propose that this is the standard way of emplacing large lavas the swell hypothesis the atmospheric impact of such flood basalt eruptions could have been protracted and severe providing a plausible link between flood basalt eruptions and mass extinctions

q estimation from vertical seismic profile data and anomalous variations in the central north sea four different methods of estimating q from vertical seismic profile vsp data based on measurements of spectral ratios pulse amplitude pulse width and zeroth lag autocorrelation of the attenuated impulse are described the last procedure is referred to as the pulse power method practical problems concerning nonlinearity in the estimating procedures uncertainties in the gain setting of the recording equipment and the influence of structure are considered in detail vsp data recorded in a well in the central north sea were processed to obtain estimates of seismic attenuation these data revealed a zone of high attenuation from approximately 4 900 ft to 5600 ft with a value of 25 3 results of the spectral ratio analysis show that the data conform to a linear constant q model in addition since the pulse width measurement is dependent upon the dispersive model adopted it is shown that a nondispersive model cannot possibly provide a match to the real data no unambiguous evidence is presented that e

helium isotopic ratio in vulcano island fumaroles temporal variations in shallow level mixing and deep magmatic supply intensive gas emanations occur throughout the island of vulcano italy sharp fluctuations recorded in the crater gas composition suggest the presence of two separate volcanic reservoirs and continuous mixing with another source crustal waters this mixing differs between the beach and crater fumaroles gas samples from three crater fumaroles with temperatures ranging from 200 to 550 c were sampled repeatedly over a one year period during the same interval of time six samples from submarine and subaerial beach fumaroles and water well gases were also sampled gases from one crater fumarole f5 showed variations of 3he4he fumarole to 3he4he air between 5 and 6 correlated with variations of several chemical species high 3he4he ratios for the beach fluids similar to those of crater fluids suggest the existence of a unique large magmatic reservoir at depth feeding both the crater and beach intermediate reservoirs however temporal changes clearly indicate variable degrees of fluids mixing and the geographic distribution of the 3he4he ratios as well as the chemical composition of the fluids suggest the existence between the magma reservoir and the surface of two intermediate different reservoirs independently related to crater and to beach fumaroles

mapping fumarolic fields in volcanic areas a methodological approach based on the case study of la fossa cone vulcano island italy abstract changes in the activity state of a volcano can be inferred by monitoring the steam flux from fumarolic fields in terms of 4d x y z time variations in temperature and extension of the zone during the last decades several studies in this field have been conducted worldwide and at vulcano island italy in particular both direct and remotely sensed measurements have been used for identifying thermally anomalous areas but the possible role of the hydrothermal alteration of volcanic products producing a sealing effect that obscures the surface thermal evidence of fumarolic activity have never been explored the novelty of the present study carried out at la fossa cone vulcano island was the integration of direct and remotely sensed temperature measurements with the evaluation of soil permeability for the precise mapping of areas where shallow hydrothermal circulation could occur even in the absence of surface evidence the main results of this study concern the role of a coating found on rock surfaces and regolith in introducing mapping errors especially during diachronic temperature surveys based on remotely sensed measurements

thermal surveys of the vulcano fossa fumarole field 1994 1999 evidence for fumarole migration and sealing abstract between 1994 and 1999 we carried out five thermal surveys of the vulcano fossa fumarole field each consisted of 485 1213 vent temperature measurements distributed across the field these show that the field has been cooling and expanding where mean and maximum vent temperatures decreased from 216 to180 c and 541 to 428 c respectively between 1994 and 1997 however expansion of the field resulted in increased heat loss from 45 to 67 mw we suggest that cooling and expansion has been caused by increased gas flux and or permeability where increased flow of fluids of magmatic origin during 1996 is implied by chemical and isotopic data capasso g favara r francofonte s inguaggiato s 1999 chemical and isotopic variations in fumarolic discharge and thermal waters at vulcano island aeolian islands italy during 1996 evidence of resumed volcanic activity j volcanol geotherm res 88 1999 167 175 we propose that the size of the conduit has increased causing the fumarole field to expand and vent temperatures to generally decline as gas and heat losses become distributed over a wider area the increase in exhaling area has outweighed the modulating effect of cooling on total heat losses so that heat loss has increased due to an increased input from the magmatic source and or an increase in system permeability within these field wide trends the rim and then the middle zones have been cooling conversely the lower zone showed evidence of warming during 1994 98 before cooling in 1999 throughout the measurement period a se s thermal migration has been apparent and a new hot zone has developed on the field s southern edge we suggest that sealing of fumaroles by complete boiling of marine fluids on the northern edge of the system and higher permeability on the southern edge may have influenced this expansion direction if vent temperatures drop below 100 c then shallow sulfur sealing will quickly diminish fumarole life expectancy and contribute to the abandonment of vents in cooling zones in our model increased gas flux and or permeability e g due to increased magmatic degassing and or fracturing is the primary mechanism driving observed changes in the field temperature size and location deep and shallow conduit sealing are secondary mechanisms influencing the location of new zones once increased gas and heat fluxes have been triggered

seismic signals in geothermal areas of active volcanism a case study from la fossa vulcano italy since the last eruption 1888 1890 volcanism at vulcano aeolian archipelago southern tyrrhenian sea has taken the form of persistent fumarolic activity the gas vapour phases of the geothermal systems are mainly discharged within two restricted areas about 1 km apart from each other in the northern part of the island these areas are la fossa crater and the beach fumaroles of the baia di levante fluids released at the two main fumarolic fields display quite different chemical and temperature characteristics implying different origins the local seismicity essentially takes the form of discrete shocks of shallow origin depth 1 km at la fossa usually with energy 1013ergs they are thought to be related to the uprise of pressurized hot gases and vapours discharged at the crater fumaroles the present investigation points to the existence of two principal categories of seismic events called m shocks and n shocks these are short events normally 10 s m type shocks are thought to be due to resonance vibrations within the interior of the volcano probably driven by the excitation of shock waves within cavities deeply affected by deposition and alteration of self sealant hydrothermal minerals n type events display features that resemble those of volcano tectonic earthquakes but have no recognizable s phases here they are tentatively attributed to microfracturing of rocks which have been extensively hydrothermally altered results of the present study permit a preliminary conceptual model of the local shallow seismic processes in the framework of geochemical modelling of fumarolic activity and geological inferences from geothermal drilling

physical models of fumarolic flow abstract transport of gas from subsurface magma to the atmosphere is modelled as flow through a narrow rough pipe and through a wider conduit filled with spheres representing flow in a porous pipe macroscopic mass momentum and energy balances are written for a control volume representing a short vertical section of the pipe the balances are solved for a perfect gas with the approximate thermodynamic properties of steam losing heat by steady state radial conduction through the pipe walls with boundary conditions of magmatic temperature at the base of the pipe atmospheric pressure at the surface and an ambient geotherm at a specified distance from the conduit results yield the variation of exit temperature with magmatic source depth conduit radius mass flow rate per unit area distance to ambient temperatures vent porosity and porous particle size two ambient geotherms are used 1 a linear gradient from atmospheric to magmatic temperatures and 2 a hydrothermal geotherm based on water saturation temperatures assuming a hydrostatic pressure gradient because individual vents commonly cluster the porous pipe model is considered to be more realistic than the rough pipe model mainly because the conductive cooling approximation is more likely to remain valid exit temperatures fall as conductive losses increase i e as the source depth increases the conduit narrows the mass flow decreases the distance to ambient temperatures reduces or the ambient geotherm cools the porous pipe model is applied to data from two fumarole fields and indicates magmatic source depths of

upper crustal seismic velocity heterogeneity as derived from a variety of p wave sonic logs sonic log measurements provide detailed 1 d information on the distribution of elastic properties within the upper crystalline crust at scales from about one metre to several kilometres 10 p wave sonic logs from six upper crustal drill sites in europe and north america have been analysed for their second order statistics the penetrated lithological sequences comprise archean volcanic sequences proterozoic mafic layered intrusions and precambrian to phanerozoic gneisses and granites despite this variability in geological setting tectonic history and petrological composition there are notable similarities between the various data sets after removing a large scale deterministic component from the observed velocity depth function the residual velocity fluctuations of all data sets can be described by autocovariance functions corresponding to band limited self affine stochastic processes with quasi gaussian probability density functions depending on the maximum spatial wavelength present in the stochastic part of the data the deterministic trend can be approximated either by a low order polynomial best fit or by a moving average of the original sonic log data the choice of the trend has a significant impact on the correlation length and on the standard deviation of the residual stochastic component but does not affect the hurst number for trends defined by low order polynomial best fits correlation lengths were found to range from 60 to 160 m whereas for trends defined by a moving average the correlation lengths are dominated by the upper cut off wavenumber of the corresponding filter regardless of the trend removed the autocovariance functions of all data sets are characterised by low hurst numbers of around 0 1 0 2 or equivalently by power spectra decaying as 1 k a possible explanation of this statistical uniformity is that sonic log fluctuations are more sensitive to the physical state in particular to the distribution of cracks than to the petrological composition of the probed rocks

origin of fumarolic fluids at vulcano italy insights from isotope data and numerical modeling of hydrothermal circulation abstract hydrothermal activity is often associated with active volcanic systems during quiescent times geochemical monitoring of discharged fluids commonly is carried out to gain insights on the state of evolution of the whole volcanic system the interest in geochemical monitoring derives from the observation that compositional variations of discharged fluids are commonly observed as a new eruptive phase is approaching in particular an increase in the gas components of direct magmatic origin may indicate a higher magma degassing rate at depth potentially related to a renewal of explosive activity surveillance programs devoted to hazard mitigation in active volcanic areas generally include periodic analyses of discharged fluids at various locations on the volcanic edifice unfortunately when significant changes are observed in gas composition their correct interpretation in terms of system evolution is not always clear and straightforward several mechanisms may in fact be responsible for differences in the proportion of magmatic gases and shallower fluid components and it is not always possible to recognize the magmatic gas fraction discrimination among fluids of different origin ideally is accomplished based on the isotope composition that each fluid acquires at the time of its generation however this isotope signature can be altered before the fluids reach the surface because of mixing between fluids of different origin or due to reactions that modify the original isotope composition thus the interpretation of isotope data sometimes is quite complex and it can be highly misleading in this work the origin of the steam discharged at the hydrothermal system of vulcano italy was investigated by the means of a dual approach first the available isotope data on the discharged steam were analyzed on the basis of on these and other data a conceptual model of the hydrothermal system was developed and numerical simulations of a multiphase multicomponent flow through a heterogeneous porous medium were carried out to evaluate the effects of various parameters on the system evolution results suggest that the hydrothermal system of vulcano is controlled by the entrance of large quantities of seawater whose original isotope composition has been altered by water rock interaction rock permeability and its distribution at depth were shown to be a primary parameter controlling the overall system evolution the fluid composition and fluid discharge rate at the surface

seismic scattering in the upper crystalline crust based on evidence from sonic logs summary evidence from sonic logs indicates that velocity fluctuations in the upper crystalline crust are remarkably uniform this motivates a generic approach to classifying uppercrustal seismic heterogeneity and to studying implications for seismic wave propagation the resulting canonical model of upper crustal seismic structure is characterized by a spatially isotropic von karman autocovariance function with a z 100 m 0 15 and cj 300 m s small angle scattering theory is used to predict the transition from weak to strong scattering as well as phase fluctuations and scattering attenuation compared with exponential random media v 0 50 the high fractal dimension i e small values of v of upper crustal heterogeneity causes smaller phase fluctuations and transition from weak to strong scattering at lower frequencies and shorter path lengths acoustic finite difference modelling shows that seismic reflections from deterministic features surrounded by heterogeneities are severely degraded when they fall into the strong scattering regime conversely traveltime fluctuations of transmitted waves are found to be relatively insensitive to the transition from weak to strong scattering upper crustal scattering q is predicted to lie between 600 and 1500 which is one to two orders of magnitude higher than q values inferred from seismic data this suggests that seismic attenuation in the upper crystalline crust is dominated by anelastic effects rather than by scattering

spatial domain analysis of carbon dioxide from soils on vulcano island implications for co2 output evaluation abstract the carbon dioxide emissions of volcanoes have been targeted as effective contributors of co 2 to the atmosphere however different sources can be effective and active at the same time in the generation and release of co 2 in volcanic zones since isotopic fingerprinting of co 2 allows the precise identification of different sources coupling carbon isotope and co 2 flux measurements enables the evaluation of the mass contribution of each source to the carbon dioxide emissions this paper accounts for the first extensive spatial analysis of coupled measurements of carbon isotopologues of co 2 in the soil gases and co 2 fluxes discharged by soils on vulcano island an innovative method has been designed tested and fine tuned in the laboratory to measure 13 c co 2 values directly in field using a new type of laser based isotopologues analyzer namely a deltaray thermo fisher scientific the method can be used to determine the carbon isotope composition across the full range of co 2 concentrations in the soil gases 0 100 vol these data have been combined with measurements of the co 2 contents in the soil gases to distinguish co 2 from deep origins from co 2 of biogenic origin in the inhabited area of vulcano porto the method of evaluating the amount of deep origin co 2 in the soil gases is widely applicable in volcanic and geothermal zones for evaluation and monitoring purposes for both gas and volcanic hazards

seismic wave attenuation in volcanic rocks from vsp experiments seismic wave attenuation in the columbia plateau basalts and snake river plain volcanics was analyzed using vertical seismic profiling vsp data the computation of attenuation coefficients is based on fitting a straight line to the logarithm of amplitude ratios computed for fixed values of frequency and variable depth this approach does not require any assumptions on the dependence of q on frequency for the columbia plateau basalts the attenuation coefficients obtained from the field data are smaller than those computed from the synthetic vsp generated using the sonic and density logs indicating that the observed attenuation is related to scattering effects and is substantially larger than the intrinsic attenuation of basalt therefore it is concluded that only a lower bound for q can be established in agreement with recent findings by other authors the effective attenuation of seismic energy in basalts about 1 1 10 4dbs m for the peak frequency is comparable to the effective attenuation observ

vertical ground deformation on vulcano aeolian islands southern italy observations and interpretations 1976 1986 abstract vertical ground deformation studies have continued on vulcano southern italy since 1976 as part of a broader geophysical and geochemical program to monitor activity on the island following a magnitude 5 5 earthquake on 15th april 1978 located a few kilometers south of vulcano the northern part of the island was found to have subsided relative to its southern part two further episodes of displacement the first of inflation and the second of deflation were recorded before september 1981 these displacements have been interpreted using a model that considers the effect of pressure change at the point source located at a depth centered at 6 5 km in the northern part of the island such pressure change according to other geophysical and geochemical data is due to increasing and subsequent decreasing pressure connected with the migration of high temperature fluids through fractures and channels beneath the fossa area opened as a result of the april 15 earthquake subsequent measurements until november 1986 detected only one additional significant movement a small inflation between may 1984 and may 1985 this uplift appears to be related to the shallow seismicity recorded beneath the fossa in this period

permeability dependence of seismic amplitudes can permeability be determined from seismic data this question has been around since maurice biot working for shell in the 1950s introduced the idea that seismic waves induce fluid flow in saturated rocks due to fluid pressure equilibration between the peaks and troughs of a compressional wave or due to grain accelerations in the case of a shear wave biot 1956 established a frequency dependent analytical relation between permeability and seismic attenuation however laboratory sonic log crosswell vsp and surface seismic have all demonstrated that biot s predictions often greatly underestimate the measured levels of attenuation dramatically so for the lower frequency measurements yet if an unresolved link truly exists between seismic amplitudes and permeability the potential benefit to the oil industry is enormous for this reason the department of energy doe brought together 15 participants from industry national laboratories and universities to concentrate for two days on whether permeability information is conceivably contained in and retrievable from seismic data the present article represents much of the workshop discussion which took place 5 6 december 2001 in berkeley california but is not strictly limited to it not all connections between hydrological and seismic properties are considered three dimensional seismic images and time lapse seismic monitoring are routinely used by reservoir engineers in constructing and constraining their reservoir model such imaging applications of seismic surveys to hydrological modeling are not discussed furthermore in fractured reservoirs it is reasonable to postulate that any locally determined seismic anisotropy defines a symmetry class for the geologic material that must also be satisfied by the permeability tensor neither are such material symmetry constraints discussed the focus here is only on whether the permeability of the rocks through which seismic waves propagate directly influences the decay of the wave amplitudes with distance key to addressing this question is an up to date discussion of the likely attenuation

vent temperature trends at the vulcano fossa fumarole field the role of permeability between 1994 and 2010 we completed 16 thermal surveys of vulcano s fossa fumarole field aeolian islands italy in each survey between 400 and 1 200 vent temperatures were collected using a thermal infrared thermometer from distances of 1 m the results show a general decrease in average vent temperature during 1994 2003 with the average for the entire field falling from 220 c in 1994 to 150 c by 2003 however between 2004 and 2010 we witnessed heating with the average increasing to 190 c by 2010 alongside these annual scale field wide trends we record a spatial re organisation of the fumarole field characterised by shut down of vent zones towards the crater floor matched by rejuvenation of zones located towards the crater rim heating may be expected to be associated with deflation because increased amounts of vaporisation will remove volume from the hydrothermal system gambino and guglielmino j geophys res 113 b07402 2008 however over the 2004 2010 heating period no ground deformation was observed instead the number of seismic events increased from a typical rate of 37 events per month during 1994 2000 to 195 events per month during 2004 2010 as part of this increase we noticed a much greater number of high frequency events associated with rock fracturing we thus suggest that the heating event of 2004 2010 was the result of changed permeability conditions rather than change in the heat supply from the deeper magmatic source within this scenario cooling causes shut down of lower sectors and re establishment of pathways located towards the crater rim causing fracturing increased seismicity and heat flow in these regions this is consistent with the zone of rejuvenation which lies towards and at the rim being the most favourable location for fracturing given the stress field of the fossa cone schopa et al j volcanol geotherm res 203 133 145 2011 it is also the most established zone having been active at least since the early twentieth century our data show the value of deploying multi disciplinary geophysical campaigns at degassing fumarolic hydrothermal systems this allows more complete and constrained understanding of the true heat loss dynamics of the system in the case study presented here it allows us to distinguish true heating from apparent heating phases while the former are triggered from the bottom up i e they are driven by increases in heat supply from the magmatic source the latter are triggered from the top down i e by changing permeability conditions in the uppermost portion of the system to allow more efficient heat flow over zones predisposed to fracturing

fluid effects on velocity and attenuation in sandstones measurements of ultrasonic velocity and attenuation in sandstones with a variety of saturating fluids are compared with the predictions of biot s theory of porous media acoustics the data show systematic deviations from biot theory as a function of pore fluid viscosity two non biot attenuation mechanisms are needed to explain the data local flow in microcracks and enhanced attenuation from pore wall roughness a model calculation of attenuation with pore wall roughness shows that attenuation in rocks can be many times larger than biot predictions this attenuation enhancement explains the inability to detect a biot slow wave in fluid saturated rocks and the observed correlation between ultrasonic attenuation and clay content in sandstones the non biot attenuation mechanisms are negligible when extrapolated to the lower frequencies of geophysical field measurements so ultrasonic attenuation in rocks provides no information about attenuation at the field scale

isotope composition of rain water well water and fumarole steam on the island of vulcano and their implications for volcanic surveillance abstract water samples from eight wells in the vulcano porto area and fumarole steam from the crater of vulcano southern italy were collected at monthly intervals over a two year period from february 1987 to january 1989 samples of bulk precipitation were also collected in three points at vulcano porto and vulcano piano island of vulcano and castroreale tyrrhenian coast of sicily again at monthly intervals the weighted mean isotopic composition of rain water on vulcano is d 32 and d 18 o 6 4 results for well waters tend to support the existence of at least two water bodies one of purely meteoric water and the other of brackish thermal water an intermediate water body probably the result of mixing of the other two was also recognized the isotope composition of fumarolic steam from the crater suggests the mixed contribution of both sea and rain waters although mixing is not the only process occurring the existence of a deep marine brine body is inferred characterized by d and 18o values more positive than those of mediterranean sea water a close relationship indicating common source effects is shown by temporal isotopic variations at some wells and the fumarolic steam wich may be used for volcano survey at distance from the active crater

boron isotopic variations in fumarolic condensates and thermal waters from vulcano island italy implications for evolution of volcanic fluids abstract temporal variation in the isotopic composition of boron has been monitored in fumarolic condensates collected over an extended time period 1970 1996 from la fossa crater volcano island we also report comparative boron isotopic data for representative vulcano lavas and for shallow hydrologic samples seawater wells thermal springs from the north flank of la fossa combined with concurrent chemical and isotopic 18o d data for the fumaroles these results indicate that the fumarolic fluids record mixing relations between three distinct fluid end members 1 a dominantly magmatic fluid em1 2 a mixture of modified seawater with magmatic fluid em2 and 3 an aqueous fluid produced from seawater by extensive wall rock reaction evaporation and boiling af differences between the latter two end members are most clearly accentuated on the basis of the boron isotopic data long term compositional variations for crater fumaroles were dominated by em1 af mixing between 1979 88 with progressive decrease in em1 contribution with time and by em2 af mixing between 1988 96 the exact spatial distribution of these fluid reservoirs remains unclear but all must have been present throughout the monitoring period to account for the observed variations moreover the combined b o h data seem to preclude important contributions from shallow meteoric reservoirs marked short term variations in 11b closely coincide with episodes of local seismicity which presumably triggered reorganization of hydrothermal circulation patterns gradual variations over periods up to 3 4 years are associated with relatively low seismicity during which fluid circulation was likely influenced by effects of mineral precipitation on permeability of the hydrologic system

seismic wave propagation in cracked porous media summary the movement of interstitial fluids within a cracked solid can have a significant effect on the properties of seismic waves of long wavelength propagating through the solid we consider three distinct mechanisms of wave induced fluid flow flow through connections between cracks in an otherwise non porous material fluid movement within partially saturated cracks and diffusion from the cracks into a porous matrix material in each case the cracks may be aligned or randomly oriented leading respectively to anisotropic or isotropic wave speeds and attenuation factors in general seismic velocities exhibit behaviour that is intermediate between that of empty cracks and that of isolated liquid filled cracks if fluid flow is significant in the range of frequencies for which considerable fluid flow occurs there is high attenuation and dispersion of seismic waves fluid flow may be on either a wavelength scale or a local scale depending on the model and whether the cracks are aligned or randomly oriented resulting in completely different effects on seismic wave propagation a numerical analysis shows that all models can have an effect over the exploration seismic frequency range

light hydrocarbons in hydrothermal and magmatic fumaroles hints of catalytic and thermal reactions volcanic gaseous mixtures emitted from active volcanoes frequently show variable amounts of saturated alkanes unsaturated alkenes and aromatic volatile hydrocarbons three major patterns of distributions can be recognized apparently related to the chemical physical environment of formation of the gas exhalations alkane rich low temperature gas emissions from recently active volcanic areas aromatic rich hydrothermal manifestations and alkene rich magmatic fumaroles on active volcanoes thermodynamic data together with theoretical and practical findings from the petroleum industry point to two main types of reactions occurring in these volcanic environments cracking and reforming cracking processes mainly caused by thermal effects occur when hydrocarbon bearing hydrothermal fluids enter and mix with a hot and dry rapidly rising magmatic gas phase the most probable products are light alkenes with carbon numbers decreasing with increasing reaction temperatures the presence of aromatic species in hydrothermal fluids can be linked to reforming processes catalysed by several possible agents such as smectites and zeolites generally present in the hydrothermally altered volcanic terranes and facilitated by long residence times in a hydrothermal envelope

squirt flow in fully saturated rocks we estimate velocity frequency dispersion and attenuation in fully saturated rocks by employing the squirt flow mechanism of solid fluid interaction in this model pore fluid is squeezed from thin soft cracks into the surrounding large pores information about the compliance of these soft cracks at low confining pressures is extracted from high pressure velocity data the frequency dependence of squirt induced pressure in the soft cracks is linked with the porosity and permeability of the soft pore space and the characteristic squirt flow length these unknown parameters are combined into one expression that is assumed to be a fundamental rock property that does not depend on frequency the appropriate value of this expression for a given rock can be found by matching our theoretical predictions with the experimental measurements of attenuation or velocity the low frequency velocity limits as given by our model are identical to those predicted by gassmann s formula the high frequency limits may sign

the accuracy of estimating q from seismic data a major aim of seismic interpretation is the inference of petrophysical properties of reservoir rocks because the inversion from seismic to petrophysical characteristics is far from unique this task requires a range of seismic parameters prominent among which are seismic velocity impedance and poisson s ratio the inclusion of seismic absorption in this list could add desirable complementary information for example absorption may be more sensitive to clay content than seismic velocity klimento and mccann 1990 however seismic absorption is difficult to measure particularly over depth intervals as short as most reservoir intervals

earthquake induced variations in the composition of the water in the geothermal reservoir at vulcano island italy abstract during 1979 1989 variations were observed in the oxygen composition of the water contained in the geothermal reservoir at vulcano island italy the reservoir water that has a magmatic origin showed an oxygen composition of 1 0 0 5 18o during periods without local tectonic earthquakes and an oxygen composition of 3 4 0 5 18o after the highest energy seismic activity that occurred recently near the island a slight increase of the 18o value in the reservoir water was also observed after a low energy sequence of tectonic earthquakes that occurred at very shallow depth just beneath vulcano island these 18o variations in the reservoir water are consistent with earthquake induced increases in the contribution from high temperature 18o rich magmatic condensate to the geothermal reservoir and with subsequent decreases in the 18o value due to 18o exchanges at the temporarily increased reservoir temperature during reactions between the highly reactive magmatic condensate and the local rocks only minor changes in the deuterium composition of the reservoir water occurred with time as the d value in the magmatic condensate released from the magma after major local earthquakes quickly approached the d value of the water contained in the geothermal reservoir also the chloride concentration in the reservoir water appears to be linked to the contribution from the magmatic fluid this chloride content seems not to have undergone major changes with time as it may be buffered by temporary increases in the reservoir temperature up to values 300 c induced by major local earthquakes this mechanism may possibly occur also in other magmatic hydrothermal systems

evolution of br cl ratios in fumarolic salammoniac from vulcano aeolian islands italy bromine chlorine ratios in salammoniac samples collected from 1983 to 1994 at the fossa crater fumaroles of vulcano aeolian islands were determined the data obtained in this work have been compared to those obtained by coradossi et al 1985 in order to carry out a systematic study of the br cl ratio variations since 1977 br cl ratios range from 0 04 to 0 29 the br cl ratio variations observed have been interpreted as the result of different contributions among deep and shallow components feeding the fumarolic fluids of vulcano the br cl increases which occurred from 1977 to 1979 in 1983 1984 and in 1992 1993 have been connected to an increased input of magmatic gases

organic and inorganic geochemistry of low temperature gas discharges at the baia di levante beach vulcano island italy abstract discharge from subaereal and submarine gas vents of the baia di levante beach gases from the vulcano island were sampled for major and trace gas components in may and november 1995 chemical compositions and equilibrium calculations suggest three different groups of co 2 rich gas emissions depending on their distance from the la fossa crater 1 gas vents close to the faraglione area are characterised by high h 2 s contents high calculated equilibrium temperatures based on inorganic species and relatively high proportion of alkene compounds 2 gas vents close to vulcanello are characterised by low calculated equilibrium temperatures and low amounts of alkenes and 3 pontile sample has the highest equilibrium co 2 pressure up to 68 bars which may account for the observed absence of benzene the relative large variability of h 2 s in the baia di levante beach gas discharge may be attributed to either different interactions between iron sulphides and weakly acid waters or catalytic effect of elemental sulphur on the de hydrogenation of cyclo hexane thermodynamic calculations suggest that the main inorganic species and ch 4 may have re equilibrated at relatively shallow depth 10 200 m b s l and 30 600 m b s l for a lithostatic and hydrostatic pressure respectively the slow kinetics of reactions in the c n h 2 n c n h 2 n 2 systems with respect to that of ch 4 co co 2 may explain the observed propene propane ratios which can only be reached at reaction temperatures of 300 350 c this low speed of reactions can also explain the observed disequilibrium of c1 c4 alkanes

u series disequilibrium radionuclides in sulphur incrustations from the fumarolic field of vulcano island abstract u series disequilibrium radionuclides 238 u 234 u 210 pb 210 po in sulphur incrustations deposited at la fossa cone vulcano island italy were measured over the period 1990 1995 to collect information about changes in the feeding system of the fumarolic field the 210 po 210 pb and 210 pb pb ratios of the sulphur incrustations allowed the identification of a fumarolic vent which is free from magmatic contribution and remobilized sublimate contamination in spite of the low temperature of the selected fumarole 280 350 c kinetic data show that 210 po is almost fully volatilized due to the relatively high velocity of the gas as a result the 210 po 222 rn activity ratio of the gas can be used to estimate fluid rising times about 100 days 234 u 238 u activity ratios of sulphur incrustations deposited in 1991 1993 showed a shift toward high values typical of nonmagmatic fluids both short lived and long lived u series disequilibrium radionuclides contribute to volcanic forecasting and give information on fluid sources and rising times as well as the residence times of the aquifers involved in the degassing process of the fumarolic field

geochemical modeling of mixing between magmatic and hydrothermal gases the case of vulcano island italy abstract in this study we present a geochemical approach to model the mixing of magmatic and hydrothermal gases our model is based on a rigorous formulation of a perfectly dimensioned system of mass and energy balance equations inputs of the equation set are the h2o and co2 content and gas emission temperature at the surface together with some constraints gained from the chemical physical features of the hydrothermal system the output data give the composition of magmatic gas mixing fractions and temperatures of gas mixtures in the mixing zone the comparison of the emission temperature of fumarolic gases with their calculated mixing temperatures provides valuable indications on the expansion process that gases undergo during their ascent towards the surface our mixing model is exemplified by the fumarolic system of vulcano island italy we calculate the thermodynamic parameters of the hydrothermal system by means of an h2o co2 nacl equation of state while the thermo baric conditions of the magma have been assessed by previous geophysical and petrologic investigations the obtained results show that during volcanic crises at vulcano island and in agreement with magma degassing processes both the co2 and the he contents of magmatic gas display peaks with a progressively declining amplitude suggesting a progressive magmatic impoverishment of volatiles strongly partitioned in the gas phase mixing fractions of various fumaroles are normally variable while they display almost homogeneous values during periods of increased volcanic activity on the basis of results gained some implications regarding the hazard of phreato magmatic explosions are also discussed the high temperature reached by fumarolic gases strongly constrains their probable expansion process during the ascent towards the surface a comparison between the temperature of the fumarolic gas and the calculated mixing temperature at depth indicates that the expansion process can be reasonably described as almost iso enthalpic although field evidences suggest that the fumarolic emission temperature could be affected by the sharp permeability changes in the last few hundred meters therefore our results and field evidences clearly indicate that variations in gas emission temperature cannot be considered a good indicator of volcanic activity as it is strongly dependent on expansion process and ultimately on rock permeability changes the model greatly improves the knowledge of the volcanic system of vulcano island and provides an important tool for the interpretation and evaluation of volcanic activity moreover a similar approach can be considered for other volcanic systems having analogous characteristics

microcracks in rocks a review abstract in the past decade the number of studies about microcracks in rocks has rapidly increased this review of recent work concentrates on microcracks in rock as separate entities emphasizing microcrack morphogenesis kinematics dynamics population statistics and observational techniques cracks are produced when the local stress exceeds the local strength the local stress may be augmented by twin lamellae interactions kink bands and deformation lamellae stress concentrations at grain boundary contacts and around intracrystalline cavities local strength may be reduced along cleavage planes along grain boundaries and along any internal surface as a result of corrosion by chemically active fluids dislocations appear not to be a significant factor for crack nucleation below about 500 c in silicates spatial and temporal changes in temperature can also induce microcracking as a result of differential thermal expansion between grains with different thermoelastic moduli and thermal conductivities the amount of quartz in the rock has a significant effect on thermally induced microcracks because of its large and variable thermal expansivity the application of hydrostatic pressure between 100 and 200 mpa effectively closes most cracks but the closure may not be uniform if crack wall asperities exist hydrostatic pressure appears to stabilize cracks and make crack growth more difficult the number and average size of mechanically induced microcracks is greater in rock deformed at higher pressures the application of a deviatoric stress field on the boundaries of a rock mass results on a microscopic scale in a very complex stress system which greatly affects nucleation and propagation paths the relative amount of intragranular and intergranular cracking appears to depend upon mineralogy rock type and stress state the vast majority of stress induced microcracks in rocks appear to be extensional statistically they are predominantly oriented within 30 of the macroscopic maximum stress direction crack densities increase as macroscopic deviatoric stress increases above a threshold level crack size distributions may be either lognormal or exponential fracture in rock under compressive boundary loads is a result of the coalescence of many microcracks not the growth of a single crack some crack configurations are more favorable for coalescence than others as deviatoric stress increases and rock failure is approached the microcrack population changes spatially from random to locally intense zones of cracking away from the fault the crack density dies off rapidly to the background level a few grains away under lesser deviatoric stresses slow subcritical microcrack growth can occur as a result of stress aided corrosion at the crack tip the rate governing mechanism may be either the chemical reaction rate or the rate at which water can get to the crack tip important details still remain to be worked out