a heating process of kuchi erabu jima volcano japan as inferred from geomagnetic field variations and electrical structure since august 2000 we have recorded the total intensity of the geomagnetic field at the summit area of kuchi erabu jima volcano where phreatic eruptions have repeatedly occurred a time series analysis has shown that the variations in the geomagnetic field since 2001 have a strong relationship to an increase in volcanic activity these variations indicate thermal demagnetization of the subsurface around the presently active crater the demagnetization source for the early variations until summer 2002 was estimated at about 200 m below sea level for the variations since 2003 the source was modeled on the basis of the expansion of a uniformly magnetized ellipsoid the modeling result showed that the source is located at 300 m above sea level beneath the crater we carried out an audio frequency magnetotelluric survey with the aim of obtaining a relation between the demagnetization source and the shallow structure of the volcano a two dimensional inversion applied to the data detected two good conductors a shallow thin one which is restricted to a region around the summit area while the other extends over the edifice at depths between 200 and 800 m these conductors are regarded as clay rich layers with low permeability which were assumed to be generated through hydrothermal alteration the demagnetization source for the early variations was possibly located at the lower part of the deep conductor and the source after 2003 lies between the two conductors where groundwater is considered to be abundant based on these results as well as on seismological geodetic and geochemical information we propose a heating process of the kuchierabu jima volcano in the initial stage high temperature volcanic gases supplied from the deep seated magma remained temporarily at the level around the lower part of the less permeable deep conductor since the ascent path had not yet been established then when the pathway developed as a result of repeated earthquakes it became possible for a massive flux of volcanic gases to ascend through the conductor the high temperature gases reached the aquifer located above the conductor and the heat was efficiently transported to the surrounding rocks through the groundwater as a consequence an abrupt increase of the gas flux and di ffusion of the heat through the aquifer occurred and the high temperature zone expanded since the high temperature zone is located beneath another conductor which acts as caprock we assume that the energy of the phreatic explosion is accumulated there

eruption mechanism as inferred from geomagnetic changes with special attention to the 1989 1990 activity of aso volcano abstract geomagnetic changes associated with the volcanic activity of aso volcano were detected with a dense network of continuously recording proton precession magnetometers during the period from june 1989 to june 1990 magnetic date clearly indicate that changes in the magnetization within the volcano are most probably caused by temperature changes this activity can be divided into five stages which are characterized by magnetization and demagnetization of the volcano these magnetic changes with durations of a few months are definitely correlated with some typical volcanic events at the crater as well as the volcanic tremor activity the demagnetization stage appears when the vent is covered by a water pool or the volcanic activity is enhanced the magnetization stage follows the opening of a vent and several large explosions which make the vent permeable the source of magnetic changes lies at a depth of about 200 m below the crater rim in the southwestern part of the active crater magnetic moments responsible for observed magnetic changes at stages 1 2 and 4 are 3 4 5 2 and 2 2 wbm respectively the corresponding source volume is estimated as a single sphere of radius 40 50 m or a spherical shell of 100 m or so an effective mechanism of rapid heating cooling exists which is ascribed to the interaction of groundwater and superheated vapor i e a shallow hydrothermal system this hydrothermal system driven by the surface cap of the vent controls every feature of the eruptions at the final outlet of aso volcano

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

unrest signals after 46 years of quiescence at cumbre vieja la palma canary islands abstract monogenetic eruptions are the most common volcanic activity in the world however unrest monitoring data are scarce due to the long intervening quiescence periods this study analyzes unrest signals recorded in one of the largest monogenetic fields in the canary islands cumbre vieja la palma two seismic swarms were registered in october 2017 and february 2018 with b values of 1 6 0 1 and 2 3 0 2 respectively suggesting an intense magmatic fluids contribution gas and or magma both swarms were linked to changes in gas emissions increases in hydrogen concentration and r ra c up to 7 52 0 05 were recorded before the first swarm at the sampling point closest to where seismicity was located indicating a deep gas input after the second swarm increases in r ra c and thoron soil concentration were recorded at two locations this dataset is compatible with a stalled magmatic intrusion at ca 25 km depth with an estimated volume between 5 5 10 4 km3 and 3 10 2 km3

combined electromagnetic geochemical and thermal surveys of taal volcano philippines during the period 2005 2006 since 1572 33 phreatic to phreatomagmatic eruptions have occurred on taal volcano philippines some of them causing several hundred casualties considering the time delay between two consecutive eruptions there is an 88 probability that taal volcano should have already erupted since 1992 several phases of seismic activity have been recorded accompanied by ground deformation opening of fissures and surface activity the volcanic activity of taal appears to be controlled by dike injections and magma supply buffered by a hydrothermal system that releases fluids and heat through boiling and subsequent steaming in early 2005 a multidisciplinary project was launched for studying the hydrothermal activity to map the hydrothermal system combined surveys were carried out to investigate self potential total magnetic field ground temperature and carbon dioxide soil degassing along with satellite thermal imaging of the main crater lake the elevated temperatures and high concentrations of carbon dioxide as well as electromagnetic anomalies indicate large scale hydrothermal degassing this process is enhanced along the tectonic features e g crater rim and faults of the volcano while active fissures opened along the e w northern flank during the 1992 1994 seismic activity heat and fluids from the hydrothermal system are essentially released in the northern part of the crater which is bounded to the south by a suspected nw se fault along which seismicity seems to take place and dikes are thought to be intruded during the january 2005 surveys a new seismic crisis started and the felt earthquakes prompted spontaneous evacuation of hundreds of inhabitants living on the volcano repeated surveys show changes of self potential total magnetic field and ground temperature with time without any noticeable spatial enlargement these observations suggest that the northern flank located between the crater rim and the 1992 1994 fissures is connected with a deep thermal source in main crater and is reactivated during seismic crises this sector could be subjected to flank failure

relationship between volcanic activity and shallow hydrothermal system at meakandake volcano japan inferred from geomagnetic and audio frequency magnetotelluric measurements abstract hydrothermal activity at meakandake volcano japan from 2004 to 2014 was investigated by using long term geomagnetic field observations and audio frequency magnetotelluric amt surveys the total intensity of the geomagnetic field has been measured around the summit crater ponmachineshiri since 1992 by kakioka magnetic observatory we reanalyzed an 11 year dataset of the geomagnetic total intensity distribution and used it to estimate the thermomagnetic source models responsible for the surface geomagnetic changes during four time periods 2004 2006 2006 2008 2008 2009 and 2013 2014 the modeled sources suggest that the first two periods correspond to a cooling phase after a phreatic eruption in 1998 the third one to a heating phase associated with a phreatic eruption in 2008 and the last one to a heating phase accompanying minor internal activity in 2013 all of the thermomagnetic sources were beneath a location on the south side of ponmachineshiri crater in addition we conducted amt surveys in 2013 and 2014 at meakandake and constructed a two dimensional model of the electrical resistivity structure across the volcano combined the resistivity information and thermomagnetic models revealed that the demagnetization source associated with the 2008 eruptive activity causing a change in magnetic moment about 30 to 50 times greater than the other sources was located about 1000 m beneath ponmachineshiri crater within or below a zone of high conductivity a few ohm meters whereas the other three sources were near each other and above this zone we interpret the conductive zone as either a hydrothermal reservoir or an impermeable clay rich layer acting as a seal above the hydrothermal reservoir along with other geophysical observations our models suggest that the 2008 phreatic eruption was triggered by a rapid influx of heat into the hydrothermal reservoir through fluid rich fractures developed during recent seismic swarms the hydrothermal reservoir remained hot after the 2008 eruption and heat was sporadically transported upward through its low permeability ceiling

the stratigraphy and structure of the chitradurga region an illustration of cover basement interaction in the late archaean evolution of the karnataka craton southern india abstract the succession in the late archaean supracrustal belts in the chitradurga region begins with basal quartz pebble conglomerates and quartzites which were deposited on a basement of tonalitic granitic gneisses ca 3000 ma containing tracts and enclaves of older supracrustal rocks the basal beds are overlain by basic metalavas and amphibolites interbedded with cross bedded quartzites and siliceous phyllites this series passes up into siliceous phyllites locally containing a large lenticular mass of polymict conglomerate talya conglomerate overlain by banded iron formations and siliceous phyllites including sporadic chert pebble conglomerates greater subsidence in part of the basin accommodated greater thicknesses of basic volcanic and acid pyroclastic rocks the latter being more abundant at higher levels with progressive deepening of the whole basin fine grained graded greywackes and local polymict conglomerates aimangala conglomerate were deposited the turbidite sequence forms the youngest sedimentary rocks exposed estimated total stratigraphic thickness is at least 10 km in deeper parts of the basin deformation led to a series of upright anticlines and synclines and related ls fabrics with extreme variations in plunge of the coaxial fold axes and l fabrics within axial surfaces which maintain a steep orientation low grade metamorphism outlasted the deformation sedimentation and volcanism in the chitradurga supracrustal belt is linked to variable subsidence of the basement foundation probably as a result of rifting the structural evolution of the belt followed as a direct continuation of events that controlled the depositional and volcanic history variable displacement of the basement during these late archaean events in the karnataka craton took place on shear zones and related fractures that acted as channels for water and co 2 as indicated by vein minerals and alteration products uplift of basement segments and their distension by granitic diapirs led to deformation of the supracrustal cover made ductile by its high water content

geometric analysis and significance of mesoscopic shear zones in the precambrian gneisses around the kolar schist belt south india mesoscopic shear zones are well preserved in the gneisses around the kolar schist belt located in the eastern part of the northern granite greenstone association of the south indian precambrian terrain the majority of these shear zones are of ductile to brittle ductile type with both dextral and sinistral senses of movement the detailed geometry and large scale mapping show that sinistral shear zones strike dominantly nw whereas dextral shear zones strike dominantly ne and form a conjugate pair the bisectors of statistically preferred orientations of these two sets of shear zones indicate that they developed in response to an e w subhorizontal compression which can be correlated to the late stage of the f2 folding seen inside the schist belt displacement vectors along these shear zones are invariably subhorizontal precluding the possibility of overthrusting or underthrusting relationship between the schist belt and surrounding gneissic terrains if the schist belt is a major shear zone the displacement across it can not be more than 15 km

new geological and hydrogeological implications of the resistivity distribution inferred from audiomagnetotellurics over la fournaise young shield volcano reunion island abstract an audiomagnetotelluric survey has been performed along an inactive flank of la fournaise volcano massif in reunion island to study the subsurface resistivity structure one dimensional modelling of the amt data at each site revealed an extensive low resistivity less than 10 m zone at a few hundred meters below the surface the significance of this unexpected conductive substratum is discussed in relation with the proposed impact of volcano tectonic processes caldera and landslides collapses and a new interpretation of the geological structure is proposed moreover it is likely that these conductors are poorly permeable argilaceous materials then they coincide with a limit in permeability and determine groundwater behavior

locally distributed ground deformation in an area of potential phreatic eruption midagahara volcano japan detected by single look based insar time series analysis abstract although it is difficult to monitor the spatial extent and temporal evolution of local and small magnitude ground inflation this information is vital to assess the potential for phreatic eruption herein we demonstrate the detection of locally distributed ground deformation preceding the enhancement of geothermal activity in the midagahara volcano japan through the application of single look based interferometric synthetic aperture radar analysis in the jigoku dani geothermal area the ground deformation proceeded at a low speed of 4 cm year at most with a spatial extent of 500 m in the east west direction and 250 m in the north south direction the deformation can be recognized to progress from 2007 at the latest to 2010 after which the geothermal activity increased with the collapse of sulfur towers and the appearance of active fumaroles and boiling water on the ground surface the most deformed area corresponds to the geothermal area with the highest activity observed on the ground surface assuming a sill opening model the deformation source is estimated to be located at a depth of 50 m from the surface with a speed of 7 cm year at most which is consistent with the depth of the highly conductive medium inferred from magnetotelluric analyses this may suggest that volcanic fluid and or heat was injected into the fluid rich medium from depth and caused the ground inflation our results demonstrate that high spatial resolution deformation data can be an effective tool to monitor subsurface pressure conditions with pinpoint spatial accuracy during the build up to phreatic eruptions

precambrian continental crust of india and its evolution the indian precambrian continental crust exhibits a variety of geological features fashioned at different times by different geotectonic processes the bulk of this crust was formed prior to 2600 m y ago and remobilized at least twice between 2600 2000 m y ago early proterozoic mobile belt epmb and 2000 1500 m y ago middle proterozoic mobile belt mpmb three early precambrian nucleii karnataka kn jeypore bastar jbn and singhbhum sn appear to have survived in the craton and are characterized by low grade supracrustals and tonalitic trondhjemite gneisses formed 3800 2600 m y ago the epmb event involved sedimentation amphibolite granulite facies metamorphism and co 2 k metasomatism and produced amphibolite facies rocks and k granites in the north and charnockite and other granulite facies rocks in the south gold sporadically distributed in the supracrustal rocks of the craton was remobilized during the epmb event k granites form a garland around the central dharwar craton sugg

three dimensional resistivity structure of iwo yama volcano kirishima volcanic complex japan relationship to shallow seismicity surface uplift and a small phreatic eruption

volcanomagnetic changes accompanying the onset of the 2002 2003 eruption of mt etna italy abstract remarkable changes in the local magnetic field were associated with the onset of the 2002 2003 flank eruption at mt etna after differential magnetic field measurements were filtered from the external noise by using adaptive filters we recognized two stages in the total intensity changes which are closely related to different volcanic events a rapid variations of about 4 5 nt associated with october 26 seismic swarm recorded beneath the summit craters b step like variations of 9 10 nt coincident with october 27 eruptive fissures opening up in the north flank these observations are generally consistent with those calculated from simple magnetic models of these volcanic processes in which the magnetic changes are generated by stress redistribution due to magmatic intrusions at different depth the magnetic data not only allow the timing of the intrusive event to be described in greater detail but also together with other volcanological and geophysical evidences permit some constraints to be set on the characteristics of propagation of a shallow dike firstly at around midnight on 26 october magma was rapidly injected to a depth of 3 4 km just below the summit craters secondly after 1 00 on 27 october continued intrusion magma occurred upward and culminated a few hundred meters below the free surface fractured along a n e direction thirdly at about 2 28 magma gave rise to an explosive fissural vent at the northern base of the ne crater near 3000 m a s l finally at about 5 00 the first eruptive fissure became active along the eastern border of the ne rift at 2500 m a s l the rate of growth of the magnetic anomalies moreover leads to the interpretation that the magmatic intrusion travelled northward from base of the ne crater to the ne rift at approximately 14 m min

the influence of recent lead loss on the interpretation of disturbed upb systems in zircons from igneous rocks in east greenland abstract a series of caledonian igneous rocks from east greenland have been dated by means of u pb isotope determinations on zircons all discordias obtained give too low intercept ages in one case more than 200 ma younger than the rb sr mineral isochron age from the same sample leach experiments show that recent or subrecent loss of lead is responsible for the geologically meaningless ages obtained further it is shown that the recent lead loss is not caused by laboratory treatment but must be related to natural processes x ray investigations combined with transmission electron microscope analyses show a correlation between recent lead loss and the chemistry of the investigated zircons

first detection of precursory ground inflation of a small phreatic eruption by insar abstract phreatic eruptions are caused by pressurization of geothermal fluid sources at shallow levels they are relatively small compared to typical magmatic eruptions but can be very hazardous however owing to their small magnitudes their occurrences are difficult to predict here we show the detection of locally distributed ground inflation preceding a small phreatic eruption at the hakone volcano japan through the application of interferometric synthetic aperture radar analysis the ground inflation proceeded the eruption at slow speed of 5 mm month with a spatial size of 200 m in the early stage and then it accelerated 2 months before the eruption that occurred for the first time in 800 900 yrs the ground uplift reached 30 cm and the eruption occurred nearby the most deformed part the deformation speed correlated well with inflation of spherical source located at 4 8 km below sea level thus suggesting that heat and or volcanic fluid supply from the spherical source maybe magma reservoir directly drove the subsurface hydrothermal activity our results demonstrate that high spatial resolution deformation data can be a good indicator of subsurface pressure conditions with pinpoint spatial accuracy during the preparatory process of phreatic eruptions

three dimensional resistivity structure and magma plumbing system of the kirishima volcanoes as inferred from broadband magnetotelluric data broadband magnetotelluric mt measurements were conducted in 2010 and 2011 in the vicinity of shinmoe dake volcano in the kirishima volcano group japan where sub plinian eruptions took place 3 times during 26 27 january 2011 by combining the new observations with previous mt data it is found that an anomalous phase in excess of 90 is commonly observed in the northern sector of the kirishima volcano group because the anomalous phase is not explained by 1 d or 2 d structure with isotropic resistivity media 3 d inversions were performed by applying small errors to the anomalous phase we successfully estimated a 3 d resistivity structure that explains not only the normal data but also the anomalous phase data the final model shows a vertical conductor that is located between a deep seated conductive body at a depth greater than 10 km and a shallow conductive layer by applying the findings of geophysical and petrological studies of the 2011 sub plinian eruptions we infer that the subvertical conductor represents a zone of hydrothermal aqueous fluids at temperatures over 400 c in which a magma pathway interconnected melt is partially and occasionally formed before magmatic eruptions to the north of the deep conductor earthquake swarms occurred from 1968 to 1969 suggesting that these earthquakes were caused by volcanic fluids

precambrian tectonics and crustal evolution in south india about 3 4 ga ago voluminous calc alkaline magmas represented by the granitoid gneisses of southern india were emplaced into a now poorly preserved non continental crust unstable ensialic basins initiated at about 3 0 ga were filled with volcanic and sedimentary rocks up to about 2 6 ga this basement cover sequence was deformed at the close of the archaean first by northward accretion and thickening of several crustal slabs formerly separated by prisms of stable shelf sediments structures produced by this episode were refolded and dislocated by large n s strike slip shear belts to impart intense steep planar fabrics to large volumes of the crust fluids rich in co possibly derived from sedimentary material driven beneath the crustal slabs by the thickening mechanism purged the deep crust of h o to form granulites upward migration of hot h o rich fluids encouraged partial melting at intermediate crustal levels these late archaean tectonic and thermal events began soon after the close of basin f

petrography chemistry and isotopic ages of peninsular gneiss dharwar acid volcanic rocks and the chitradurga granite with special reference to the late archean evolution of the karnataka craton southern india abstract new petrographic major and trace element and isotopic age data are presented for four suites of rocks from the low to medium grade part of the south indian archaean craton in central karnataka two suites are from the peninsular gneiss basement to the late archaean dharwar supergroup the third is a suite of dharwar potassic rhyolites and the fourth is from the chitradurga granite which has intrusive contacts with the dharwar supergroup the chikmagalur granite granodiorite which with its host gneisses forms part of the basement peninsular gneiss to the dharwar supergroup in the bababudan basin yields rb sr and pb pb whole rock isochron dates of 3080 110 ma and 3175 45 ma respectively an initial 87 sr 86 sr ratio of 0 7013 0 0009 and a 1 value 238 u 204 pb of 7 98 host tonalitic gneisses to the chikmagalur granite give an rb sr whole rock isochron date of 3060 160 ma with an initial 87 sr 86 sr ratio of 0 7015 0 0004 and a pb pb whole rock isochron date of 3190 100 ma with a model 1 value of 8 00 a suite of granites and granodioritic tonalitic gneisses chitradurga granitic rocks and gneisses forming part of the peninsular gneiss basement to the dharwar supergroup west of the chitradurga belt yields rb sr and pb pb whole rock isochron dates of 2970 100 ma and 3028 28 ma respectively with an initial 87 sr 86 sr ratio of 0 7035 0 0013 and a model 1 value of 7 62 the suite of dharwar acid volcanic rocks from north of honnali defines a poorly fitted 207 pb 206 pb linear array mswd 15 8 which gives an apparent age of 2565 28 ma and a model 1 value of 7 46 the poor fit of this isochron shows that a closed system was not maintained a conclusion supported by high k abundances which are attributed to a syn or post magmatic ion exchange process the chitradurga granite which intrudes the older rocks of the dharwar supergroup in the chitradurga belt yields a well fitted pb pb whole rock isochron date of 2605 18 ma with a model 1 value of 7 68 the u pb relations in these suites from karnataka are in marked contrast with the commonly severe u depletion and consequently unradiogenic pb isotopic compositions observed in deeply eroded high grade archaean gneiss terranes such as those bordering the north atlantic this contrast indicates that the cratonic rocks in central karnataka represent a relatively high level in the original archaean continental crust

evolution of volcanic gas composition during repeated culmination of volcanic activity at kuchinoerabujima volcano japan abstract chemical and isotopic compositions of low to medium temperature fumarolic gasses were measured at various fumaroles of kuchinoerabujima volcano from 1993 to 2009 by the combination of fumarolic gas sampling and multi gas measurement of volcanic plumes repeated culmination of the volcanic activity was observed as contemporaneous occurrence of seismic swarms summit inflation and demagnetization almost every two years after 1999 fumarolic activity also increased parallel with these activities new fumarolic fields of low temperature fumaroles at boiling point formed at the southern and western rims of the summit crater in 2003 and intense degassing activity started at medium temperature fumaroles inside the summit crater in 2008 the low temperature fumarolic gasses have peculiar composition with high h2 h2o co co2 and so2 h2s ratios typical features of high temperature volcanic gasses but also with low hcl and total s contents typical features of low temperature gasses these features indicate that the gasses are formed by low temperature differentiation of high temperature gasses variation of h and o isotopic ratios of the low temperature gasses indicates that the gasses are formed by isoenthalpic processes of vapor liquid separation at 100 c of a high temperature gas and meteoric water mixture implying that the high temperature gas was injected into a shallow aquifer without cooling temperatures of the high temperature gasses were estimated as 550 700 c based on the equilibrium temperature calculation with considering the low temperature differentiation the medium temperature fumarolic gasses from the summit crater have typical compositions of high temperature volcanic gasses and are estimated as the source gas of the boiling point fumaroles the h2 h2o and co2 st ratios of the low temperature fumaroles increased from 2004 to 2009 which is interpreted to be caused by the evolution of thermal structure of a shallow gas storage region of a few hundred meters deep increase of magmatic gas supply caused enlargement of a high temperature zone of the gas storage region widening the surrounding temperature transit zone with smaller temperature gradient the enlargement of the high temperature zone also reduces the overlying low temperature aquifer which caused the decrease of co2 st ratio by reduction of sulfur removal reaction the enlargement of the high temperature zone resulted in the leakage of the high temperature gasses directly to the surface as the medium temperature gasses from the summit crater

geochemistry and petrogenesis of granitoids around the kolar schist belt south india constraints for the evolution of the crust in the kolar area the peninsular gneiss surrounding the n s trending kolar schist belt in the dharwar craton includes several components of granitic rocks that range in composition from tonalite to granite on the west the different phases are referred to as the dod the dosa and the banded gneiss and the patna granite the gneisses on the east are dominantly of one type and are referred to as the kambha gneiss the champion gneiss occurring within the eastern part of the belt is compositionally similar to the plutonic dod gneiss and probably represents a near surface intrusion late aplites are present on both sides of the belt the banded gneiss was formed by partial melting of tonalitic rocks and the aplites were formed by the action of aqueous fluids during amphibolite grade metamorphism excepting these two rocks all other granitoids seem to have been formed from silica oversaturated parent magmas derived from lile enriched mantle sources by fractionation processes including liquid immiscibility and fractional cry

u pb ages of zircon and sphene for two gneiss terranes adjacent to the kolar schist belt south india evidence for separate crustal evolution histories orthogneisses adjacent to the kolar schist belt were emplaced as tonalitic to granodioritic magmas between 2630 and 2530 ma an orthogneiss unit east of the kolar schist belt apparently was a homogeneous granodioritic intrusion at about 2532 ma cooling to the blocking temperature of the u pb system in sphene by 2521 2517 ma gneisses on the west side of the kolar schist belt were emplaced as at least three separate intrusions dated at 2631 2610 and 2552 ma evidence from an older basement into which these rocks were intruded comes from inherited zircons with an age of 2800 ma rocks with zircons of the appropriate age to be part of this basement occur as tectonic fragments and inclusions in the area gneisses west of the kolar schist belt were last cooled through the blocking temperature of the u pb system in sphene at about 2552 ma synchronous with the last major intrusive event late felsic dikes and plutons apparently postdate deformation and yield sphene pb pb ages of 2480 ma a sample of muscov

volcanomagnetic signals associated with the quasi continuous activity of the andesitic merapi volcano indonesia 1990 1995 abstract merapi volcano in java island indonesia is an andesitic stratovolcano which presents long periods of effusive activity during which an endogeneous dome is continuously growing the viscous lava dome gives rise to unstable blocks which collapse or turn into pyroclastic flows when the volcano does not exhibit any surface activity the overpressure within the volcano slowly increases depending on the quietness duration the unrest of the volcano can start with an explosive phase during which the former dome is partly destroyed magnetic variations of different time constant are observed during the 1990 1995 period which includes one gas plume emission on august 26 1990 and two eruptions on january 20 1992 and on november 22 1994 compared with other types of active volcanoes the observed volcanomagnetic variations are very small at the most a few nanoteslas nt to discriminate the variations associated with the global activity from the signals correlated with each unrest phase one has to dissociate the different time constant variations over the six year time span when long term trends are removed from the magnetic field in each station of the network an outstanding correlation between all the magnetic differences is emphasised the midterm variations point out 2 cycles of activity which fit the stress field evolution within the edifice leading to the 1992 and 1994 eruptions a new cycle has started in may 1995 in every identified cycle rapid volcanomagnetic signals are well associated with stress field changes may 1991 september 1991 february 1993 december 1993 some of the volcanomagnetic variations are short term precursory signals as the three months decrease up to 1 3 nt preceding the 1992 eruption the comparison between magnetic data seismicity and surface phenomena implies that the midterm volcanomagnetic variations associated with the cycles of merapi activity are of piezomagnetic origin

stratigraphy pollen history and geochronology of tidal marshes in a gulf of maine estuarine system climatic and relative sea level impacts abstract sedimentologic and stratigraphic characteristics of five tidal marshes in great bay estuary new hampshire which is located on the western boundary of the gulf of maine were assessed from 20 vibracores detailed descriptions of surficial environments pollen analyses and radiocarbon dating modern marsh sequences in great bay estuary initiated with a time transgressive basal peat that formed at the upland brackish marsh boundary the oldest basal peat deposit sampled during this study was dated at 4560 cal yr b p based on an age of 4060 40 14 c yr b p the original tidal marshes that formed in great bay estuary apparently were unable to accrete at a high enough rate to allow seaward expansion resulting in a transgressive sequence of low marsh or mudflat sediments overlying the basal peat the transgressive tidal marsh sequence is capped by high marsh sediments that corresponded to a slowing of relative sea level rsl rise in the region and regressive seaward expansion of the tidal marshes however significant variations from these transgressive regressive sequences occur in the great bay tidal marshes as a result of more recent marsh expansion and tidal channel migrations detailed pollen analyses of a vibracore taken in a tidal marsh along squamscott river with a sedimentary record spanning the last 3900 cal yr b p exhibit five well documented marker horizons including 1 an increase of tsuga hemlock 2 the appearance of picea spruce 3 tsuga and fagus beech declines 4 an increase in ambrosia ragweed coupled with a sudden decrease in quercus oak and 5 the castanea chestnut decline using published ages for these pollen horizons and a calibrated 14 c age obtained during this study for a basal peat accretion rates for this marsh system over five time intervals were calculated 1 0 4 to 0 5 mm yr 1 from 3900 to 2850 cal yr b p 2 0 6 mm yr 1 from 2850 to 1960 cal yr b p 3 0 8 mm yr 1 from 1960 to 580 cal yr b p 4 0 7 to 0 8 mm yr 1 from 580 to 210 cal yr b p and 5 0 8 to 0 9 mm yr 1 over the last 210 years the long term accretion rate for the entire sequence based on the calibrated 14 c age of the basal peat was 0 6 to 0 7 mm yr 1 this result agrees with accretion rates determined from calibrated 14 c ages from two other tidal marsh systems in great bay estuary with accretion rates between 0 6 to 0 8 mm yr 1 higher accretion rates were obtained at a nearby tidal marsh with 1 2 mm yr 1 for just the marsh and 1 3 mm yr 1 for the marsh and underlying subtidal sediment the results of this study indicate that the sedimentology and stratigraphy of moderate size tidal marshes in rocky glaciated coasts are highly variable and are strongly influenced by complex interactions among rsl climatic variations and anthropogenic influences for instance the early development of many of the marshes in the great bay estuary area were likely driven by changes in rates of rsl resulting in transgressive onlap boundaries subsequently overlain by regressive intertidal marsh sequences however more recent changes in tidal marsh sediment composition deposited during a relatively slow steady rsl rise likely result from other forcings such as changes in minerogenic and orogenic sediment inputs due to anthropogenic effects tidal channel migrations major storms causing erosion of the marsh or climatic changes causing shifts in sediment delivery or vegetation

equilibrium temperature and redox state of volcanic gas at unzen volcano japan abstract high temperature volcanic gases were collected from a growing dacitic dome of unzen volcano japan apparent equilibrium temperatures aet were calculated for the following two reactions so2 3h2 h2s 2h2o and co2 h2 co h2o both the aet s are higher than the outlet temperature to two hypotheses are examined as a cause of this disagreement one is a partial advancement in the reaction so2 3h2 h2s 2h2o the other is an addition of external water after a final equilibrium attainment based on these hypotheses the equilibrium compositions temperatures and redox potentials were estimated resulting in similar values the estimated temperatures range from 850 to 911 c which is consistent with a petrological estimation made with the lava however the estimated redox potential is one unit less in logfo2 scale than the petrological estimation

magnetic field changes on white island new zealand and the value of magnetic changes for eruption forecasting for about 15 years after their commencement in 1968 magnetic measurements in the crater of white island gave a good indication of the state of activity and the likelihood of an eruption episode commencing during this period magnetic thermal ground level and chemical measurements were all generally consistent with a model in which these surface changes were caused by increasing temperatures under the donald mound area in the central sub crater in more recent times there has not been such a relationship between the changes around the remnants of donald mound and the volcanic activity which has become localised in the west sub crater it appears that changes under the crater especially lowering of the ground surface and the water table at the west end of the crater have meant that eruptions are no longer preceded by heating under donald mound the west sub crater is not easily accessible for repeat surface surveys one way in which we might be able to monitor future volcanic activity at white island would be by repeated aeromagnetic surveys as the magnetic changes are so large that changes of hundreds of nanoteslas are likely to be observed at practical flying heights

long term thermal activity revealed by magnetic measurements at kusatsu shirane volcano japan abstract repeated geomagnetic measurements commenced around the three summit crater lakes of kusatsu shirane volcano yugama mizugama and karegama lakes in 1976 and continuous measurements commenced in 1990 we reviewed and analyzed these geomagnetic data acquired over the 34 years starting in 1978 changes in the geomagnetic field related to eruptions during 1982 1983 were recorded in the repeated geomagnetic measurements from 1982 to 1985 a thermal demagnetization source was estimated to be 400 m below mizugama crater lake during this period although there were no eruptions from 1988 to 1991 there were numerous volcanic earthquakes thermomagnetic signals due to demagnetization of the material beneath the crater lakes were recorded by the repeated magnetic measurements during this period the demagnetized body was estimated to be 600 m below mizugama crater lake at this time previous seismological and geochemical studies attributed these demagnetization events to the ascent of hydrothermal water and volcanic gas the difference between the depths of demagnetized bodies during these two periods of demagnetization may provide important information about the mechanism of the 1982 1983 eruptions in contrast magnetization associated with cooling of rocks beneath the crater lakes was recorded from 1996 to 2012 according to our thermomagnetic modeling of this period the source of the magnetization was 400 to 700 m below an area immediately northeast of yugama crater lake and the cooling migrated gradually to shallower depths during this period based on our modeling seismological data and geochemical monitoring of yugama lake water we consider that the flux of hydrothermal fluid from depth decreased after 1992 and rock magnetization due to cooling began in 1996

a preparation zone for volcanic explosions beneath naka dake crater aso volcano as inferred from magnetotelluric surveys abstract the 1st crater of naka dake aso volcano is one of the most active craters in japan and known to have a characteristic cycle of activity that consists of the formation of a crater lake drying up of the lake water and finally a strombolian type eruption recent observations indicate an increase in eruptive activity including a decrease in the level of the lake water mud eruptions and red hot glows on the crater wall temporal variations in the geomagnetic field observed around the craters of naka dake also indicate that thermal demagnetization of the subsurface rocks has been occurring in shallow subsurface areas around the 1st crater volcanic explosions act to release the energy transferred from magma or volcanic fluids measurement of the subsurface electrical resistivity is a promising method in investigating the shallow structure of the volcanic edifices where energy from various sources accumulates and in investigating the behaviors of magma and volcanic fluids we carried out audio frequency magnetotelluric surveys around the craters of naka dake in 2004 and 2005 to determine the detailed electrical structure down to a depth of around 1 km the main objective of this study is to identify the specific subsurface structure that acts to store energy as a preparation zone for volcanic eruption two dimensional inversions were applied to four profiles across the craters revealing a strongly conductive zone at several hundred meters depth beneath the 1st crater and surrounding area in contrast we found no such remarkable conductor at shallow depths beneath the 4th crater which has been inactive for 70 years finding instead a relatively resistive body the distribution of the rotational invariant of the magnetotelluric impedance tensor is consistent with the inversion results this unusual shallow structure probably reflects the existence of a supply path of high temperature volcanic gases to the crater bottom we propose that the upper part of the conductor identified beneath the 1st crater is mainly composed of hydrothermally altered zone that acts both as a cap to upwelling fluids supplied from deep level magma and as a floor to infiltrating fluid from the crater lake the relatively resistive body found beneath the 4th crater represents consolidated magma these results suggest that the shallow conductor beneath the active crater is closely related to a component of the mechanism that controls volcanic activity within naka dake

temporal change of characteristics of shallow volcano tectonic earthquakes associated with increase in volcanic activity at kuchinoerabujima volcano japan abstract shallow volcano tectonic vt earthquakes recorded at the kuchinoerabujima island volcano in southwest japan are analyzed in order to clarify the role of hydrothermal activity in the development of volcanic seismicity from analysis of shallow vt earthquakes in 2006 two specific episodes of elevated seismicity are observed in april and november 2006 the vt earthquakes have hypocenters at depths of 0 0 4 km beneath the summit crater and normal fault focal mechanisms with wnw ese extension consistent with the tensional stress field indicated by the alignment of craters and fissures although the hypocenters and focal mechanisms are found to be largely invariant during these episodes the corner frequencies of the vt earthquakes underwent a pronounced increase and decrease accompanying the changes in seismicity rates the corner frequencies increased to 20 25 hz approximately one month prior to the onset of elevated seismicity and then decreased to 10 15 hz in the period of peak seismicity the rupture length also decreased at the onset of seismicity thereafter increasing as the seismicity continued the peak seismicity in terms of the daily number of vt events was accompanied by inflation around the crater suggestive of a pressure increase in the volcanic system it is inferred that the increase in shallow vt seismicity and rupture length is related to the development of a fractured zone the pressure increase in the volcanic system is attributed to the intrusion of hydrothermal fluids which is supported by an observed increase in fumarolic temperature and activity the preceding monochromatic events are thus considered to be generated by the effect of fluid filled cracks the shortening of rupture length is then inferred to be related to the closing of non fluid filled cracks in the fracture zone under the increasing pressure field leading to a transition from monochromatic events to low frequency and shallow vt seismicity