double flood basalts and plume head separation at the 660 kilometer discontinuity several of the world s flood basalt provinces display two distinct times of major eruptions separated by between 20 million and 90 million years these double flood basalts may occur because a starting mantle plume head can separate from its trailing conduit upon passing the interface between the upper mantle and the lower mantle this detached plume head eventually triggers the first flood basalt event the remaining conduit forms a new plume head which causes the second eruptive event the second plume head is predicted to arrive at the lithosphere at least 10 million years after the first plume head in general agreement with observations regarding double flood basalts

inversion of complex body waves we have developed a method that inverts seismic body waves to determine the mechanism and rupture pattern of earthquakes the rupture pattern is represented as a sequence of subevents distributed on the fault plane this method is an extension of our earlier method in which the subevent mechanisms were fixed in the new method the subevent mechanisms are determined from the data and are allowed to vary during the sequence when subevent mechanisms are allowed to vary however the inversion often becomes unstable because of the complex trade offs between the mechanism the timing and the location of the subevents many different subevent sequences can explain the same data equally well and it is important to determine the range of allowable solutions some constraints must be imposed on the solution to stabilize the inversion we have developed a procedure to explore the range of allowable solutions and appropriate constraints in this procedure a network of grid points is constructed on the i plane where and i are respectively the onset time and the distance from the epicenter of a subevent the best fit subevent is determined at all grid points then the correlation is computed between the synthetic waveform for each subevent and the observed waveform the correlation as a function of and i and the best fit mechanisms computed at each i grid point depict the character of allowable solutions and facilitate a decision on the appropriate constraints to be imposed on the solution the method is illustrated using the data for the 1976 guatemala earthquake

possible electrokinetic origin of large magnetic variations at la fournaise volcano abnormal time variations of the geomagnetic field related to volcanic activity have been reported for several decades1 10 two main candidates have been considered for the source mechanism of these volcanomagnetic variations piezomagnetism and electro kinetic effects we considered both of these mechanisms in our attempts to interpret the most spectacular volcanomagnetic variations observed on piton de la fournaise volcano reunion island 11 but now new analysis of the latest data which covers a time span of more than three years leads us to think that water circulation is the main agent of the strong magnetic signals observed on the volcano in addition to the signals that we correlated with the volcano activity11 12 longer period components also exist as shown here with amplitudes and characteristics that have not been reported before we propose that these magnetic variations are generated by electrokinetic effects in which case the water circulation system into this massif would be a sensor for detecting stress changes linked to the volcano activity

geomagnetic variation anomalies and deflection of telluric currents summary we show that most of the abnormal variations of the transient magnetic field have deflection and canalization of telluric currents as main sources in order to do so we develop a formalism which is valid in a limited frequency range which satisfies the following hypotheses a the thickness of the laterally inhomogeneous upper layer is small with respect to the penetration depth of the transient field b self induction of the heterogeneities of conductivity can be neglected theoretical results and observations performed at mid latitudes are in good agreement the main result we establish is that deviation and canalization of the current responsible for the transient abnormal field do not affect their temporal variations the study of temporal variations of the abnormal field thus provides us with information about the structure of the crust in the region where the deviated and canalized currents are induced

jurassic cretaceous boundary age and mid ocean ridge type mantle source for shatsky rise basalt sills cored in lowermost berriasian sediments on shatsky rise in the northwest pacific yield a mean 4 0 ar 3 9 ar incremental heating age of 144 6 0 8 ma providing a minimum estimate on the age of the jurassic cretaceous boundary the largest part of the rise was emplaced at rates similar to those of several continental flood basalts the rise formed at an oceanic triple junction and the sills as well as lavas from two dredge sites exhibit an ocean ridge type nd pb sr isotope signature this signature is provisionally consistent with either a perisphere or meteorite impact model but is not predicted by the plume head model of plateau formation however no model can yet be rejected given the very sparse sampling of shatsky rise

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

numerical investigations of the mantle plume initiation model for flood basalt events recent studies have linked both the eruption of continental flood basalts and the emplacement of large oceanic plateaus to the initial phase of mantle plume activity i e the plume head model here we develop this concept further through numerical models of a large thermal diapir rising through the mantle and impinging upon the base of the lithosphere a finite element analysis in axisymmetric geometry is used to model the dynamics of solid state convective flow and heat transport in the mantle and an anhydrous batch melting model is used to estimate melt volumes and compositions obtained from partial melting in the shallow mantle we explore the effects of a number of model variables including mantle viscosity structure plume temperature and diameter and the mechanical response of the lithosphere our study emphasizes cases in which the lithosphere does not undergo significant extension prior to large scale melting a standard model is found to be consistent with some observed characteristics of flood basalt plume initiation events this model includes an initial diapir of radius 400 km and initial excess temperature 350oc rising through a mantle of viscosity 102 pa s and impinging beneath a model oceanic lithosphere 100 ma in age if lithospheric extension does not occur melting is almost entirely sublithospheric the melt volume produced is 3 5x106 km 3 and its composition has high mgo content 15 20 wt a time sequence of precursory central axial uplift of several kilometers followed by major melting volcanism is a robust feature of all the models regardless of mantle viscosity structure or lithospheric response low viscosity in the upper mantle compresses the timescale for these events and results in a constriction of the plume head initially impinging on the lithosphere the radial extent of the melting region about 500 km at the time of maximum melt production is only slightly affected by the upper mantle viscosity for nonrifting lithosphere no significant melt generation occurs for initial plume excess temperatures less than about 300oc the initial plume radius has only a modest effect upon the volume of melt obtained the most important aspect of lithospheric response is whether or not the uppermost crust and lithosphere above 40 km depth undergo extension otherwise the ductile strength of the lower lithosphere has little effect if the lithosphere is allowed to extend freely the melt volume increases by about an order of magnitude and it is comparable with some of the largest oceanic plateaus such as ontong java if continental or oceanic flood basalt events occur on old nonrifting lithosphere the primary melts are likely to be much more mgo rich than the basalts erupted at the surface this is consistent with abundant evidence for olivine fractionation in many flood basalt provinces along with the occurrence of picrite lavas and we hypothesize that fractionation occurs at a density trap at the crust mantle boundary moho at this stage and following it is likely that significant lower crustal and mantle lithospheric contamination of mantle plume derived magmas may occur both isotopically and in the trace elements the sequence and timing of both precursory and post magmatic events in the vast well exposed oceanic flood basalt terrane known as wrangellia se alaska british columbia are consistent with the predictions of our model the uplift predicted by our models is of order 2 4 km if we require that mantle plumes are hot enough to melt beneath unrifted lithosphere effects not modeled here such as small scale convection and hydrous melting will tend to reduce the model excess temperature and uplift in order to better understand the relation between mantle plumes and large igneous provinces several important advances are required implementation of a fractional melting model some realistic assessment of melt transport through the mantle and crust and a fully three dimensional convection model

rapid formation of the shatsky rise oceanic plateau inferred from its magnetic anomaly shatsky rise in the northwest pacific ocean is probably the oldest extant oceanic plateau and as with most such features its origin is uncertain both oceanic plateaus and continental flood basalts are thought to be formed by rapid voluminous eruptions that occur when the head of a newly born mantle plume ascends to the base of the lithosphere1 3 high eruption rates have been estimated for flood basalts for example 1 5km3 yr 1 for the deccan traps2 from dating of lava flows but the inaccessibility of oceanic plateaus makes it necessary to extrapolate dating information from a small number of samples and sites4 here we estimate the eruption rate of shatsky rise by a method that is indirect but has the virtue of sampling the entire volume of the plateau above the surrounding sea floor the main southern part of the plateau has a positive magnetic anomaly corresponding to a reversed geomagnetic polarity at the time of eruption using age constraints to identify the longest period of reversed polarity during which the plateau could have formed we estimate that 2 106 km3 of material erupted at a minimum rate of 1 7km3 yr 1 this issomewhat less than the rate of 8 22 km3 yr 1 estimated for the ontong java plateau4 but still represents a massive eruption consistent with the plume head hypothesis

zn isotope compositions of the thermal spring waters of la soufriere volcano guadeloupe island abstract to trace the sources and pathways of zn in hydrothermal systems the zn isotope compositions of seventeen water samples from eight thermal springs and six gas samples from two fumaroles from la soufriere an active volcano on guadeloupe island french west indies fwi were analyzed using a method adapted for purifying zn from fe and so4 enriched thermal solutions the fumaroles are enriched in zn 100 to 8000 times compared to the local bedrock and have isotopic compositions 66zn values from 0 21 to 0 35 similar to or slightly higher than fresh andesite 0 21 the enrichment of zn in the thermal springs compared with the surface waters shows that zn behaves as a soluble element during hydrothermal alteration but is significantly less mobile than na the 66zn values of most of the spring waters are relatively constant approximately 0 70 indicating that the thermal springs from la soufriere are enriched in heavy isotopes i e 66zn compared to the host rocks from 0 14 to 0 42 only three thermal springs have lower 66zn values as low as 0 43 while the zn in the fumaroles is essentially derived from magma degassing which is consistent with a previous study on merapi volcano toutain et al 2008 we show that the zn in the thermal springs is mainly derived from water rock interactions the 66zn enriched isotopic signature in most of the spring waters can be explained qualitatively by the precipitation at depth of sulfide minerals that preferentially incorporate the light isotopes this agrees with the isotopic fractionation that was recently calculated for aqueous complexes of zn the few thermal springs with lower 66zn values also have low zn concentrations indicating the preferential scavenging of heavy zn isotopes in the hydrothermal conduits this study shows that unlike chemical weathering under surface conditions hydrothermal alteration at high temperatures significantly fractionates zn isotopes and enriches thermal waters in heavy zn isotopes e g 66zn continental hydrothermal systems therefore constitute a source of heavy zn isotopes to the oceans this should be taken into account in the global oceanic budget of zn

directed blasts and blast generated pyroclastic density currents a comparison of the bezymianny 1956 mount st helens 1980 and soufriere hills montserrat 1997 eruptions and deposits we compare eruptive dynamics effects and deposits of the bezymianny 1956 bz mount st helens 1980 msh and soufriere hills volcano montserrat 1997 shv eruptions the key events of which included powerful directed blasts each blast subsequently generated a high energy stratified pyroclastic density current pdc with a high speed at onset the blasts were triggered by rapid unloading of an extruding or intruding shallow magma body lava dome and or cryptodome of andesitic or dacitic composition the unloading was caused by sector failures of the volcanic edifices with respective volumes for bz msh and shv c 0 5 2 5 and 0 05 km3 the blasts devastated approximately elliptical areas axial directions of which coincided with the directions of sector failures we separate the transient directed blast phenomenon into three main parts the burst phase the collapse phase and the pdc phase in the burst phase the pressurized mixture is driven by initial kinetic energy and expands rapidly into the atmosphere with much of the expansion having an initially lateral component the erupted material fails to mix with sufficient air to form a buoyant column but in the collapse phase falls beyond the source as an inclined fountain and thereafter generates a pdc moving parallel to the ground surface it is possible for the burst phase to comprise an overpressured jet which requires injection of momentum from an orifice however some exploding sources may have different geometry and a jet is not necessarily formed a major unresolved question is whether the preponderance of strong damage observed in the volcanic blasts should be attributed to shock waves within an overpressured jet or alternatively to dynamic pressures and shocks within the energetic collapse and pdc phases internal shock structures related to unsteady flow and compressibility effects can occur in each phase we withhold judgment about published shock models as a primary explanation for the damage sustained at msh until modern 3d numerical modeling is accomplished but argue that much of the damage observed in directed blasts can be reasonably interpreted to have been caused by high dynamic pressures and clast impact loading by an inclined collapsing fountain and stratified pdc this view is reinforced by recent modeling cited for shv in distal and peripheral regions solids concentration maximum particle size current speed and dynamic pressure are diminished resulting in lesser damage and enhanced influence by local topography on the pdc despite the different scales of the blasts devastated areas were respectively 500 600 and 10 km2 for bz msh and shv and some complexity involving retrogressive slide blocks and clusters of explosions their pyroclastic deposits demonstrate strong similarity juvenile material composes 50 of the deposits implying for the blasts a dominantly magmatic mechanism although hydrothermal explosions also occurred the character of the magma fragmented by explosions highly viscous phenocryst rich variable microlite content determined the bimodal distributions of juvenile clast density and vesicularity thickness of the deposits fluctuates in proximal areas but in general decreases with distance from the crater and laterally from the axial region the proximal stratigraphy of the blast deposits comprises four layers named a b c d from bottom to top layer a is represented by very poorly sorted debris with admixtures of vegetation and soil with a strongly erosive ground contact its appearance varies at different sites due to different ground conditions at the time of the blasts the layer reflects intense turbulent boundary shear between the basal part of the energetic head of the pdc and the substrate layer b exhibits relatively well sorted fines depleted debris with some charred plant fragments its deposition occurred by rapid suspension sedimentation in rapidly waning high concentration conditions layer c is mainly a poorly sorted massive layer enriched by fines with its uppermost part laminated created by rapid sedimentation under moderate concentration weakly tractive conditions with the uppermost laminated part reflecting a dilute depositional regime with grain by grain traction deposition by analogy to laboratory experiments mixing at the flow head of the pdc created a turbulent dilute wake above the body of a gravity current with layer b deposited by the flow body and layer c by the wake the uppermost layer d of fines and accretionary lapilli is an ash fallout deposit of the finest particles from the high rising buoyant thermal plume derived from the sediment depleted pyroclastic density current the strong similarity among these eruptions and their deposits suggests that these cases represent similar source transport and depositional phenomena

palaeomagnetism of cretaceous pacific seamounts summary the palaeomagnetism of cretaceous pacific seamounts as deduced from their magnetic anomalies is reviewed results from 35 seamounts are reported here for the first time of these 35 results 15 are of high enough quality to be useful this has resulted in a considerable increase in the available information concerning the palaeomagnetism of the pacific plate using only the most reliable results we calculate a palaeomagnetic pole for the upper cretaceous at 58 north 350 east with a 95 per cent cone of confidence of only 5 this pole is considerably more accurate than the one previously reported and is farther to the west the previously reported difference between the mean pole position from seamounts close to japan and the mean pole position from seamounts close to hawaii is not real although one group of seamounts close to japan gives an anomalous pole position several mechanisms have been suggested whereby errors might be introduced in the determinations of pole positions from seamounts no error is introduced in the pole position because of the restricted extent of surveys and the consequent inaccuracy in the determination of the regional field the demagnetization effect which can produce differences between the observed direction of magnetization and the direction of the magnetizing field in an elongated body is unimportant for seamounts the new pole position is sufficiently accurate for use in many types of calculations in which it is desired to locate the pacific plate with respect to the earth s spin axis

magnetization of the pacific ocean lithosphere deduced from magsat data magsat anomalies over the pacific ocean are related to volcanic plateaus and the edge effect of the pacific cretaceous quiet zone pcqz four prominent plateaus shatsky rise the mid pacific mountains hess rise and manihiki plateau are considered in this study the major lateral magnetization contrasts associated with these plateaus are considered to arise from the topographic swells and the crustal roots with root thicknesses estimated from topography using an airy isostatic model shatsky rise and the mid pacific mountains are older than the cretaceous quiet period and their natural remanent magnetization nrm is not a significant component of the magnetization contrasts that cause the magsat anomalies associated with these plateaus their magnetic properties are characterized by effective magnetic susceptibility contrasts of 0 07 0 01 s i per unit volume averaged over their topographic swells and crustal roots equivalent to 0 04 if averaged over the entire crustal thicknesses swell plus root plus intervening layer this value agrees well with the estimates of previous investigators for these plateaus and for similar plateaus in other oceans two plateaus of cretaceous quiet period age within the pcqz hess rise and manihiki plateau have origins similar to those of shatsky rise and the mid pacific mountains and they probably have similar effective susceptibility contrasts but they may also carry cretaceous quiet period nrm magnetic anomalies due to hess rise and manihiki plateau calculated from the effective susceptibility contrast deduced for shatsky rise and the mid pacific mountains are substantially larger than the observed magsat anomalies of these plateaus there are also magsat anomalies over parts of the pacific where no plateaus exist some of these anomalies are demonstrated as probably due to a contrast in nrm between the pcqz and its surroundings the total remanent magnetization contrast magnetization contrast in amperes per meter times thickness in meters equivalent to the dipole moment per unit area of the pcqz is 14 000 20 000 amperes 30 which cannot be simply explained by the absolute values of remanent magnetization predicted by conventional models of oceanic crustal magnetic properties modelling of the cretaceous quiet zones in the northern atlantic indicates a similar total remanent magnetization contrast these results may be explained by including uppermost mantle remanent magnetization with an intensity of about 1 2 a m 1 if magnetization persists to a depth of 7 below the seismic moho i e down to the depth of the crustal roots of the plateaus including nrm in the uppermost mantle of the pcqz also serves to reduce the calculated amplitude of the hess rise and manihiki plateau magsat anomalies if the nrm of the crustal roots is less than the nrm of the surrounding uppermost mantle on this basis the estimated intensity of nrm in the uppermost 7 km of the pcqz mantle is 1 3 a m 1 in very good agreement with the intensity estimated from the pcqz edge effect anomalies these estimates of uppermost mantle nrm are probably a maximum limit recognizing that other sources may also contribute to the total remanent magnetization contrasts of the cretaceous quiet zones

the use of velocity spectrum for stacking receiver functions and imaging upper mantle discontinuities summary in order to improve the signal to noise ratio of receiver function data it is typical to stack receiver functions calculated from events at similar distances and back azimuths we have adapted the velocity spectrum stacking vss technique used extensively in reflection seismology to the receiver function method in order to stack data with different ray parameters thereby improving further the signal to noise ratio perhaps more importantly by producing the velocity spectrum stacks we take advantage of the differences in the shapes of the moveout curves of converted phases and reverberations to identify and separate the various phases and to infer velocity structure by comparison of velocity spectrum stacks produced from the observed data at the iris ida broad band station at obninsk russia obn with those produced from prem synthetics we have identified ps phases from the 400 and 670 km discontinuities the p to s ps converted phase from the 400 km discontinuity observed at obn is much larger compared with that of the 670 km discontinuity than is predicted by prem this suggests a higher velocity contrast at 400 km than in prem by adapting a bootstrap method to assess the depth and velocity estimates determined by vss we find that the ps phase from the 670 km discontinuity is best imaged with higher than prem upper mantle p and s velocities and at a depth of 663 3 km we find no evidence of a 220 km discontinuity beneath obn in these data

the relationship between eruptive activity flank collapse and sea level at volcanic islands a long term 1 ma record offshore montserrat lesser antilles hole u1395b drilled southeast of montserrat during integrated ocean drilling program expedition 340 provides a long 1 ma and detailed record of eruptive and mass wasting events 130 discrete events this record can be used to explore the temporal evolution in volcanic activity and landslides at an arc volcano analysis of tephra fall and volcaniclastic turbidite deposits in the drill cores reveals three heightened periods of volcanic activity on the island of montserrat 930 ka to 900 ka 810 ka to 760 ka and 190 ka to 120 ka that coincide with periods of increased volcano instability and mass wasting the youngest of these periods marks the peak in activity at the soufriere hills volcano the largest flank collapse of this volcano 130 ka occurred towards the end of this period and two younger landslides also occurred during a period of relatively elevated volcanism these three landslides represent the only large 0 3 km3 flank collapses of the soufriere hills edifice and their timing also coincides with periods of rapid sea level rise 5 m ka available age data from other island arc volcanoes suggests a general correlation between the timing of large landslides and periods of rapid sea level rise but this is not observed for volcanoes in intra plate ocean settings we thus infer that rapid sea level rise may modulate the timing of collapse at island arc volcanoes but not in larger ocean island settings

structure and hydrogeological functioning of an insular tropical humid andesitic volcanic watershed a multi disciplinary experimental approach summary the aim of the study is to establish an hydrogeological scheme of the structure and the functioning of andesitic volcanic systems in the insular and humid tropical context a watershed has been instrumented at fefe in guadeloupe french west indies in the lesser antilles to measure rainfall runoff and piezometric fluctuations between 2003 and 2006 given the heterogeneity of the volcanic deposits a multi disciplinary approach has been used a geological and an hydrogeological survey an hydrodynamic approach hydraulic tests and signal processing of hydrological time series and finally an hydrogeochemical approach major elements and some trace elements two main superposed aquifer respectively in aerial deposits and in recent nuees ardentes glowing ash and lava flows as well as the underlying aquiclude the highly weathered older substratum were identified at a local scale some hydrogeological heterogeneities internal to the aquifers were highlighted and related to a polyphased building up of the geological formations hydraulic relationships between the multi layered aquifers such as the confinement of the deep aquifer upstream of the basin were identified groundwater stream exchanges were characterised detailing various contributions of the shallow and the deep aquifers to stream and spring waters significant river losses were also observed at a global scale within such relatively permeable volcanic formations the aquifer recharge is about 85 of the annual effective rainfall for an average rainfall year the hydrogeological scheme appears to be coherent with a geological structure of cut and fill paleovalleys frequently observed in andesitic areas consequently groundwater drainage axis are mainly localised in recent nuees ardentes deposits whereas the inversion of relief characteristic of volcanic areas explains the non superposition of surface water and groundwater divides and both surface water losses and springs finally this work shows the highly heterogeneous hydrogeological processes occurring in volcanic formations of andesitic type it also shows the relationships between the geological structure and the hydrogeological functioning of such basins and proposes an affordable methodological approach to investigate such a hydrogeological context in depth

space and ground based measurements of sulphur dioxide emissions from turrialba volcano costa rica remotely sensed measurements of sulphur dioxide so2 emitted by turrialba volcano costa rica are reported for the period september 2009 january 2011 these measurements were obtained using images from advanced spaceborne thermal emission and reflexion radiometer ozone monitoring instrument and a ground based uv camera these three very different instruments provide flux measurements in good agreement with each other which demonstrate that they can be integrated for monitoring so2 fluxes fluxes from turrialba increased fourfold in january 2010 following a phreatic explosion that formed a degassing vent in the w crater of turrialba since then the so2 flux has remained high 30 50 kg s but seems to be showing a slowly decreasing trend we interpret this evolution as the start of open vent degassing from a recently intruded magma body the opening of the degassing vent decreased the confining pressure of the magma body and allowed the gases to bypass the hydrothermal system

geoid height versus topography for oceanic plateaus and swells oceanic plateaus and swells are a major component of the seafloor topography yet they remain among the most poorly understood features this is especially true of the oceanic plateaus which show large variations in crustal thickness to determine the depth and mode of compensation for 53 of the largest plateaus and swells we analyzed the relationship between geoid height and topography in polygonal areas containing each feature both geoid height and topography were first band pass filtered 400 km i 4000 km to isolate the signal associated with local compensation from flexural and deep mantle signals the ratio of geoid height to topography was then determined by fitting a straight line to the data except for nine of the smaller features there is a high correlation between geoid height and topography that is positive in accordance with airy and thermal compensation models eighteen features have high geoid topography ratios that cannot be explained by the airy compensation model of crustal thickening these features thermal swells are partially supported by thermal buoyancy forces in the lower half of the lithosphere the ratios are highest for active hot spot swells and decay with the thermal age of the swell to values consistent with airy compensation of the enduring volcanic edifice the remaining features plateaus have lower geoid topography ratios in agreement with the airy compensation model those plateaus with average height greater than 4 km are thought to be continental fragments the shorter plateaus tend to be volcanic features modified continental plateaus presumably small fragments of extended and intruded continental margin crust cluster around heights of n3 km overlapping the range associated with oceanic plateaus since the origin of many plateaus is poorly understood this global geoid topography analysis provides a new technique for comparing the deep structure of oceanic plateaus and swells

s wave velocity structures of the taipei basin taiwan using microtremor array measurements abstract the s wave velocity structures of the taipei basin in taiwan are investigated using the array records of microtremors at 15 sites dispersion curves at these sites are calculated using the frequency wavenumber f k spectrum method the s wave velocity structures in the taipei basin are then estimated by employing surface wave inversion technique harder strata sites have higher phase velocities than softer sites if the s wave velocity of the tertiary basement is assumed to be 1000 m s then the quaternary alluvial thicknesses in the taipei basin are between about 100 m and 650 m the thickness of the alluvium gradually increases from the southeast to the northwest the inversion results are also in good agreement with well logging data and seismic reflection studies of the taipei basin the study concludes that microtremor array measurement is a useful tool for estimating s wave velocity structure

magnetic lineations within shatsky rise northwest pacific ocean implications for hot spot triple junction interaction and oceanic plateau formation oceanic plateaus are major ocean features yet their origins and development are poorly understood many are huge piles of basalt and it is widely accepted that mantle plumes are their source perhaps from eruptions of the voluminous head of an emerging plume shatsky rise is a basaltic plateau that formed during a period of geomagnetic reversals unlike many mid cretaceous plateaus so magnetic data can help us understand its tectonic history in this study we analyzed magnetic anomaly data from 131 cruises over and around shatsky rise and constructed a magnetic lineation chart for tectonic interpretation a significant finding is that magnetic lineations are traceable through low parts of the rise between volcanic massifs indicating nearly normal lithosphere and between large volcanic edifices many lineations form bights near the rise axis and show former locations of the pacific izanagi farallon triple junction they indicate that the junction was in a ridge ridge ridge rrr configuration and closely followed the rise axis from chron m20 146 ma to chron m4 127 ma lineation and bight geometries indicate that the triple junction jumped northeastward at least nine times often apparently in response to volcanic activity from the mantle plume some jumps however appear related to ridge reorientations numerous features within the rise are parallel to lineations or lineation offsets implying that the plateau formed near the triple junction and was modified by ridge faulting most of our observations support the hypothesis that shatsky rise was formed by a mantle plume that captured a triple junction

seismic evidence for an ancient rift beneath the cumberland plateau tennessee a detailed analysis of broadband teleseismic p waveforms broadband receiver functions developed from teleseismic p waveforms recorded on the midperiod passband of regional seismic test network station rscp are inverted for vertical velocity structure beneath the cumberland plateau tennessee the detailed broadband receiver functions are obtained by stacking source equalizd horizontal components of teleseismic p waveforms the resulting receiver functions are most sensitive to the shear velocity structure near the station a time domain inversion routine utilizes the radial receiver function to determine this structure assuming a crustal model parameterized by many thin flat lying homogeneous layers lateral changes in structure are identified by examining azimuthal variations in the vertical structure the results reveal significant rapid lateral changes in the midcrustal structure beneath the station that are interpreted in relation to the origin of the east continent gravity high located northeast of rscp the results from events arriving from the northeast show a high velocity midcrustal layer not present in results from the southeast azimuth this velocity structure can be shown to support the idea that this feature is part of a keweenawan rift system another interesting feature of the derived velocity models is the indication that the crust mantle boundary beneath the cumberland plateau is a thick probably laminated transition zonemore between the depths of 40 and 55 km a result consistent with interpretations of early refraction work in the area less

the seismic moho structure of shatsky rise oceanic plateau northwest pacific ocean abstract oceanic plateaus are large igneous provinces formed by extraordinary eruptions that create thick oceanic crust whose structure is poorly known owing to the lack of deep penetration seismic data multichannel seismic mcs reflection and wide angle refraction data allow us to show moho structure beneath a large part of the shatsky rise oceanic plateau in the northwest pacific ocean moho reflectors in the two data sets can be connected to trace the interface from the adjacent abyssal plain across much of the interior the reflectors display varied character in continuity shape and amplitude similar to characteristics reported in other locations beneath normal crust the moho is observed at 13 km depth 7 km below the seafloor in mcs data and disappears at 20 km depth 17 km below the seafloor beneath the high plateau moho at the distal flanks dips downward towards the center with slopes of 0 5 1 increasing to 3 5 at the middle flanks seismic moho topography is consistent with airy isostasy confirming this widely applied assumption data from this study show that crustal thickness between the massifs in the interior of the plateau is nearly twice normal crustal thickness despite the fact that this crust records apparently normal seafloor spreading magnetic lineations the moho model allows improved estimates of plateau area 5 33 10 5 km 2 and volume 6 90 10 6 km 3 the latter assuming that the entire crust was formed by shatsky rise volcanism because the massifs formed at spreading ridges this study is unique in showing moho depth and structure over an extraordinarily large area beneath an oceanic plateau giving insight to plateau structure and formation

arc parallel extension and localization of volcanic complexes in guadeloupe lesser antilles 1 subduction of atlantic seafloor under the caribbean plate causes shallow earthquakes within the lesser antilles volcanic arc such earthquakes above the subduction interface show strike slip or normal fault plane solutions the latter with e w striking nodal planes to better assess seismic hazard and the coupling between volcanism and tectonics we investigated faulting related to overriding plate deformation in the guadeloupe archipelago using aerial photographs satellite spot images and topographic maps 1 25000 scale we mapped active and middle to late pleistocene fissures and normal fault systems that cut the uplifted coral platforms grande terre and marie galante and the volcanic rocks of basse terre the available marine geophysical data show that the faults extend offshore to bound submarine rifts the e w striking 1500 m deep v shaped marie galante rift separates the two islands of marie galante and grande terre normal faults in the north of grande terre appear to mark the similarly v shaped western termination of the 5000 m deep n 50e to n130 e striking desirade graben three shallow m 5 5 earthquakes 6 may 1851 29 april 1897 3 august 1992 appear to have ruptured segments of the marie galante rift boundary faults the young la grande decouverte volcanic complex of basse terre including the 1440 a d soufriere dome lies within the western termination of the marie galante rift the ancient volcanic shoulders of the rift buttress the active dome to the north and south which may explain why major prehistoric sector collapses and pyroclastic avalanches have been directed southwestward into the caribbean sea or southeastward into the atlantic ocean the marie galante rift is typical of other troughs transverse to the northeastern edge of the caribbean plate we interpret such troughs which are roughly orthogonal to the arc to result from slip partitioning and extension perpendicular to plate convergence that they disappear southward implies that they result from interaction between the caribbean and north american plates

timing of geothermal activity in an active island arc volcanic setting first 40ar 39ar dating from bouillante geothermal field guadeloupe french west indies mineral separates of adularia have been extracted from three samples of highly silicified hydrothermal breccias newly discovered in the active bouillante geothermal field guadeloupe archipelago and investigated by 40ar 39ar geochronology in order to constrain the timing of geothermal activity in this part of the active lesser antilles island arc the inverse isochron diagram indicates an age of 248 50 ka 2 for all adularia from one breccia sample n 8 with an initial 40ar 36ar ratio of atmospheric composition 309 12 2 attesting that this age is valid this age is concordant with the weighted mean age of 290 40 ka for the same sample adularia from other samples yields concordant ages the obtained 40ar 39ar ages can be related either to the magmatic activity of the bouillante volcanic chain c 850 250 ka ago or to the initiation of the volcanic activity of the active grande decouverte soufriere system 200 ka ago present day our results demonstrate that the bouillante hydrothermal event is coeval with change in the volcanic pulses previously recognized in the magmatic history of the studied area the possible duration calculated for this hydrothermal activity requires at least two superposed volcanic pulses to be developed

hotspots mantle plumes flood basalts and true polar wander persistent long lived stationary sites of excessive mantle melting are called hotspots hotspots leave volcanic trails on lithospheric plates passing across them the global constellation of fixed hotspots thus forms a convenient frame of reference for plate motions through the orientations and age distributions of volcanic trails left by these melting anomalies hotspots appear to be maintained by whole mantle convection in the form of upward flow through narrow plumes evidence suggests that plumes are deflected little by horizontal flow of the upper mantle mantle plumes are largely thermal features and arise from a thermal boundary layer most likely the mantle layer just above the core mantle boundary experiments and theory show that gravitational instability drives flow beginning with the formation of diapirs such a diapir will grow as it rises fed by flow through the trailing conduit and entrainment of surrounding mantle the structure thus develops a large spherical plume head and a long narrow tail on arrival at the base of the lithosphere the plume head flattens and melts by decompression producing enormous quantities of magma which erupt in a short period these are flood basalt events that have occurred on continents and in ocean basins and that signal the beginning of major hotspot tracks the plume supported hotspot reference frame is fixed in the steady state convective flow of the mantle and is independent of the core generated axial dipole paleomagnetic reference frame comparison of plate motions measured in the two frames reveals small but systematic differences that indicate whole mantle motion relative to the earth s spin axis this is termed true polar wander and has amounted to some 12 since early tertiary time the direction and magnitude of true polar wander have varied sporadically through the mesozoic probably in response to major changes in plate motions particularly subduction zone location that change the planet s moments of inertia

copper nickel rich amalgamated ferromanganese crust nodule deposits from shatsky rise nw pacific 1 a unique set of ferromanganese crusts and nodules collected from shatsky rise sr nw pacific were analyzed for mineralogical and chemical compositions and dated using be isotopes and cobalt chronometry the composition of these midlatitude deep water deposits is markedly different from northwest equatorial pacific pcz crusts where most studies have been conducted crusts and nodules on sr formed in close proximity and some nodule deposits were cemented and overgrown by crusts forming amalgamated deposits the deep water sr crusts are high in cu li and th and low in co te and tl concentrations compared to pcz crusts thorium concentrations ppm are especially striking with a high of 152 mean 56 compared to pcz crusts mean 11 the deep water sr crusts show a diagenetic chemical signal but not a diagenetic mineralogy which together constrain the redox conditions to early oxic diagenesis diagenetic input to crusts is rare but unequivocal in these deep water crusts copper ni and li are strongly enriched in sr deep water deposits but only in layers older than about 3 4 ma diagenetic reactions in the sediment and dissolution of biogenic calcite in the water column are the likely sources of these metals the highest concentrations of li are in crust layers that formed near the calcite compensation depth the onset of ni cu and li enrichment in the middle miocene and cessation at about 3 4 ma were accompanied by changes in the deep water environment especially composition and flow rates of water masses and location of the carbonate compensation depth

a numerical model of earthquake induced piezomagnetic anomalies a numerical model for evaluating the order of magnitude of earthquake induced piezomagnetic anomalies is proposed the stress field associated with normal or reverse faulting with a 10 mpa mean stress drop is determined by mean of displacement discontinuity method the induced piezomagnetic tensor is then used to predict the associated magnetic field variations observed at ground surface the model shows that for earthquakes occurring in homogeneous magnetic masses with piezomagnetic properties of andesite rocks the associated anomalies are of the order of 0 2 nt for faults with a 5 km dip slip and 2 nt when the dip slip is 20 km long the variation of magnetic field for the case of a piezomagnetic dyke intruding a nonmagnetic body is found to be much greater for a 4 km thick dyke parallel to the fault and located 4 km from it the observed ground surface magnetic field variation is of the order of 10 nt these results outline that induced piezomagnetic effects strongly depend on the stress drop and stress distribution along and outside the fault the location of the earthquake source with respect to the piezomagnetic body and the piezomagnetic properties of the rock

noble gas isotopes in hydrothermal volcanic fluids of la soufriere volcano guadeloupe lesser antilles arc article the ascending magmatic component in la soufriere volcano guadeloupe lesser antilles was investigated by measuring noble gas concentrations and isotopic ratios in thermal springs and fumaroles a clear magmat ic helium signal 4 he 3 he 89 000 r ra 8 equivalent to the morb value is recorded in most fluids 3 he fluxes in both fumarole and springs have been estimated and related to 3 he content in the magma chamber in order to explain the 3 he flux measured at the surface we conclude that the magma chamber must be reg ularly fed by fresh magma batches using our new results and data from literature we propose that the his torical activity of la soufriere volcano can be explained by both abnormal energy inputs from new batches of magma in the chamber and cycles of clogging opening of the hydrothermal system we propose a new sce nario for the origin of the 1976 1977 crisis whereby a fresh batch of magma could have been emplaced prior to 1968 possibly between 1959 and 1962 in the magma chamber the resulting heat flux is not stored in the different aquifers but preferentially evacuated through fractures reactivated or created during the 1956 phre atic eruption only when the self sealing of the hydrothermal system is sufficiently developed can pressure and temperature within the aquifers rapidly increase to trigger a crisis

bathymetry of shatsky rise northwest pacific ocean implications for ocean plateau development at a triple junction oceanic plateaus are large igneous edifices thought to have been created by nascent mantle plumes but owing to sparse data their origins remain uncertain understanding plateau evolution is important because they are significant ocean features and may provide clues about mantle plume dynamics we constructed a bathymetry map of shatsky rise a large pacific plateau combining multibeam and wide beam echosounder data from 87 cruises and u s navy multibeam contours the rise consists of three large isolated volcanic edifices massifs surrounded by nearly normal lithosphere a linear volcanic ridge and a group of about 80 scattered seamounts massif flank slopes are typically gentle 1 5 and often parallel magnetic lineations or fracture zones the slope angles imply effusive volcanism similar to flood basalts whereas the rise shape suggests formation near the pacific izanagi farallon triple junction with modification of volcano flanks by spreading ridge tectonics edifice sizes and inferred ages imply a trend of decreasing volume and age from southwest to northeast furthermore gaps between massifs suggest episodic volcanism existing data are broadly consistent with the plume head hypothesis wherein the largest edifice formed by massive plume head eruptions the ridge formed from the plume tail and the two massifs in between represent a transition seamounts east of the rise are morphologically distinct which suggests that they may have been formed by a different source or mechanism than shatsky rise proper

crustal structure in eastern kazakh u s s r from teleseismic receiver functions broadband receiver functions determined from teleseismic p waveforms recorded at two seismic stations in eastern kazakh u s s r are inverted for the vertical velocity structure beneath the stations the detailed broadband receiver functions are obtained by stacking source equalized radial components of teleseismic p waveforms a time domain inversion of the radial receiver function is used to determine the structure assuming a crustal model parameterized by flat lying homogeneous layers the general features of the inversion results are a complex shallow crust velocities less than 6 km s in the upper crust a high velocity 6 9 7 5 km s lower crust and a moho that varies between 47 and 57 km depth these results compare favorably with a composite velocity model from deep seismic sounding data but show lower velocities in the upper crust and higher velocities in the lower crust the results indicate that the crust in this region is relatively uniform however the nature and depth of the moho changes significantly across the region the receiver function inversion structure for the kazakh sites is similar to a published receiver function structure for a site on the canadian shield

near surface shear wave velocity structure of the chiayi area taiwan this paper uses microtremor array measurements to obtain an overall view of s wave velocity structure in the chiayi area taiwan the rayleigh wave dispersion curves are calculated using the frequency wavenumber graphic 1 spectrum method and then the s wave velocity structure of the area is estimated by surface wave inversion according to the inversion results the s wave velocities decrease from east to west if the s wave velocity of the bedrock is assumed to be 1500 m s the depths of the alluvium are between 560 and 1400 m gradually increasing from east to west in order to understand the variations of the shallow s wave velocity in the area we sketch 2d and 3d maps using the imaging techniques based on interpolation algorithms from the 3d results the thickness of the sediments having an s wave velocity in the range 270 1500 m s increases from east to west whereas the thickness of the layers having an s wave velocity in the range 1500 2370 m s decreases from east to west the results are also in good agreement with the geological and geophysical information of the chiayi area therefore microtremor array measurement provides a good alternate to estimate shallow s wave velocity structure online material figures showing observed and calculated phase velocities f k spectra dispersion curves inverted velocity models sensitivity kernels and v s contour maps and electric resistivity maps 1 embed inline graphic 1 gif

degassing at la soufriere de guadeloupe volcano lesser antilles since the last eruptive crisis in 1975 77 result of a shallow magma intrusion in the 15 years following the last crisis of phreatic type in 1975 77 the hydrothermal and seismic activity of la soufriere de guadeloupe has displayed an episodic behavior superimposed on a long term decay as inferred from seismicity and degassing monitored through fumarolic emissions and cl content in thermal springs in particular the evolution of the cl content can be explained by a succession of magmatic gas injections into the hydrothermal system this episodic supply of gas may be associated to the thermodynamic evolution of a batch of magma intruded at shallow depth around 1976 progressive magma cooling crystallization gas exsolution and pressure buildup lead to cyclic failure of host rock around the intrusion and episodic release of gas with a decreasing frequency as cooling proceeds this physical model allows to reproduce the pattern of degassing activity of la soufriere including the decreasing frequency of cl pulses with time our preferred forward model of cl pulses involves a magma intrusion volume in the range 0 01 0 52 km 3 and a tensile strength of the surrounding rock in the range 0 1 0 6 mpa these results reinforce the hypothesis of a shallow magma intrusion triggering the 1975 77 crisis and provide an explanation for the degassing and seismic activities observed until 1992

the volcanic activity of la soufriere of guadeloupe lesser antilles structural and tectonic implications abstract among observations made on la soufriere volcano guadeloupe to monitor its its activity are temperature measurements temperature of fumaroles hot springs ground surface and water inside two boreholes 80 and 90 m deep those measurements proved to be of prime importance in understanding mechanisms working in the volcano and governing its activity they lead with other observations such as self potential measurements and geochemical analysis of fumaroles and hot springs to a possible interpretation of phreatic activity rather different from the one retained earlier which was essentially based on seismic data in this model the magma reservoir is only in a stable state and in particular supplies heat at a constant rate to the overlying geological structures the cycle of volcanic activity and period of quiescence is interpreted as due to a cycle of clogging and reopening of the system of fractures transferring heat from a lower aquifer to an upper one and eventually to the atmosphere the model is discussed in regard of other geophysical observations in particular seismicity some inferences are made concerning a new phase of activity and suitable observations to predicit it

tectonic evolution of the northern pacific plate and pacific farallon izanagi triple junction in the late jurassic and early cretaceous m21 m10 abstract magnetic lineations in the northwestern pacific near the shatsky plateau were interpreted to trace the tectonic history of the northern pacific plate from the time of anomalies m21 to m10 at m22 time the pacific farallon izanagi p f i triple junction was located southwest of the plateau and migrating north northwest with respect to the pacific plate shortly thereafter the stress field acting on the three plates changed significantly and the pacific lzanagi ridge reacted by pivoting 24 clockwise from m21 to m19 time several ridge propagation events occurred while the ridge adjusted to the new spreading regime to the east of the m22 location of the junction it appears that this reorganization triggered a series of at least three ridge jumps along the pacific farallon ridge the geometry of magnetic isochron m21 suggests that several triple junctions existed simultaneously during the reorganization bordering one or more microplates differences in the spreading rate of the japanese magnetic lineations north of the plateau imply that one microplate which we call the shatsky microplate existed until about m15 m13 time it was probably captured by either the farallon or the izanagi plate and subsequently destroyed by subduction after m15 the p f i triple junction was reformed on seafloor that is now beneath the shatsky plateau the junction drifted generally northeast either in a ridge ridge ridge or in a ridge fault fault configuration perhaps alternating between the two from m11 m10 time the stress field on the three plates changed again and the pacific farallon ridge southeast of the plateau reoriented 15 clockwise at this time the p f i junction assumed a ridge ridge ridge geometry at the northeast end of the plateau

plateaux ancient and modern geochemical and sedimentological perspectives on archaean oceanic magmatism unequivocal examples of oceanic plateaux older than about 2 1 ga have yet to be identified from the geological record the most likely explanation for this is the partial dismemberment of ancient plateaux at convergent plate margins and their subsequent juxtaposition in accretionary complexes with arc volcanic sedimentary and plutonic rocks a review of cretaceous plateaux in the eastern indian ocean suggests that certain geochemical isotopic criteria e g positive nb anomalies relative to primitive mantle employed by petrologists to identify modern oceanic plateau lavas are of little practical use in distinguishing ancient plateau lavas from arc and volcanic rifted margin basalts the extreme chemical and isotopic heterogeneity of indian ocean plateaux together with large variations in area and volume suggest that these edifices represent one extreme of a spectrum of oceanic plateau compositions the other extreme represented by iceland and ontong java is characterised by relatively homogeneous melt compositions reflecting steady state melting conditions and the absence of a continental lithosphere contaminant to plateau magmas crustal thickness and compositional estimates for archaean oceanic plateaux using the mckenzie bickle method give a mean thickness of 43 km and mean mgo content of 19 wt archaean plateaux were thus compositionally distinct from surrounding ocean floor basaltic komatiite and notably more magnesian than plateaux such as iceland mean for tertiary basalts 6 3 wt mgo and ontong java mean for core samples 6 9 wt mgo by analogy with cretaceous tertiary ultramafic complexes in western colombia the deeper portions of these plateaux may have consisted of noritic rocks underlain by iherzolite pyroxenite gabbronorite and dunite the general absence of such rocks in archaean terrains suggests that these portions of plateaux are only rarely preserved the search for remnants of ancient oceanic plateaux has identified examples of archaean submarine lavas in the canadian shield 2 7 ga southern abitibi belt superior province and kaapvaal craton of southern africa 3 5 3 3 ga barberton and pietersberg belts chemical and isotopic data suggest that komatiite tholeiite units from these areas were derived by partial melting of a plume or asthenospheric mantle source but do not distinguish adequately the tectonic environment rifted continental margin ocean floor oceanic plateau in which these rocks were erupted however the uplift histories of these putative plateau fragments as recorded in sediments bounding and intercalated with the komatiite basalt sequences appear to be broadly comparable to those of the best documented phanerozoic oceanic plateaux although further stratigraphical investigations of presumed plateaux ancient and modern are required to test this hypothesis we believe that the sedimentary record offers the best evidence yet for the formation of oceanic plateaux as far back as the mid archaean 3 5 ga

equator crossing of shatsky rise new insights on shatsky rise tectonic motion from the downhole magnetic architecture of the uppermost lava sequences at tamu massif 1 shatsky rise is a large igneous province lip currently located in the northwestern pacific new downhole magnetic logging data from integrated ocean drilling program iodp hole u1347a at tamu massif of shatsky rise captured the magnetic architecture in the uppermost lava sequence providing a rare opportunity to investigate a time series of the intra plate volcanism in conjunction with the pacific plate construction history centered at the triple junction logging data results indicate that tamu massif was formed during normal polarity periods south of the paleoequator and crossed the equator at some point in the m19 m17 period combining these new observations with previous interpretations of the massif s tectonic history a time series of the latitudinal tectonic motion of a lip and the underlying pacific plate during the plateau formation is postulated

detection of seismic bedrock using radial receiver function the receiver function rf method has been widely applied to estimate velocity structures of earth s crust and mantle using teleseismic data in this study we perform a rf iterative deconvolution method to detect the depth variations of seismic bedrock in the taipei basin and chiayi area we use strong motion data recorded by five and seven stations in the taipei basin and chiayi area respectively the ps p times appear at about 0 235 0 93 s for the taipei basin and 1 015 1 685 s for the chiayi area the time differences imply gradually increases of the bedrock depth from southeast to northwest in the taipei basin and from east to west in the chiayi area our results show that the method can efficiently detect depth variations of seismic bedrock which are consistent with those from other geophysical observations as well

origin of intraplate volcanoes from guyot heights and oceanic paleodepth the height of a guyot as measured from the surrounding regional sea floor to the vol cano s slope break records the water depth at the time the guyot submerged thus guyot heights may be used as indicators of the paleodepth of the surrounding ocean floor we compile data on the heights of 68 intraplate guyots and atolls in the pacific ocean as well as 46 volcanic islands in the pacific atlantic and indian oceans we find that guyot heights generally increase with the age of the lithosphere upon which they werc cmplaced although therc is a large amount of scattcr in nearly all cases seamount height and thus seafloor paleodepth is less than expzcted of normal seafloor these results suggest that most of the volcanoes in this study formed on anomalously shallow seafloor consistent with formation at hotspots to characterize thermal anomalies associ ated with these hotspot swells we model guyot heights by calculating the isostatic uplift predicted for normal lithosphere that has been partly reheated and is underlain by anomalously hot mantle this model is able to explain the anomalous water depth at most of the seamounts with hotspot thermal anomalies of 100 30o0c the heights of a few volcanic chains however are not anomalously low suggesting that these volcanoes are not associated with hotspots in addition the observed trend of hawaiian emperor guyot heights as well as the subdued morphology and gravity signature of the oldest emperor seamounts supports our hypothesis that cretaceous age meiji seamount may have formed on or near a spreading center

the 3100 bp eruption of the soufriere of guadeloupe a transmission electron microscopy study of the cryptodome andesite fragments of unaltered andesite found at all levels in the deposits of the catastrophic flank failure directed blast eruption of the soufriere of guadeloupe in 3100 bp are thought to be remnants of the cryptodome they were observed in analytical transmission electron microscopy for clues to the evolution of the intrusion prior to the eruption several features that could potentially be used as temperature markers were identified among which the angle between microexsolutions of magnetite in augite phenocrysts was used to find an upper boundary of the temperature of the intrusion before the eruption 600 700 c calculation of the time a dyke or sphere shaped intrusion may have taken to cool from the emplacement temperature down to the temperature of exsolution of the magnetite leads to an estimate of the time between emplacement of the intrusion and the eruption which could not have been less than a few tens of years it therefore seems probable that the emplacement of the magmatic intrusion was not the immediate cause of the flank destabilization and catastrophic eruption of the soufriere in 3100 bp

carbon and helium isotopes in thermal springs of la soufriere volcano guadeloupe lesser antilles implications for volcanological monitoring abstract la soufriere guadeloupe lesser antilles is one of the most active and dangerous volcanoes of the lesser antilles arc the renewal of the volcanic activity at la soufriere requires replenishment of the volatile poor andesitic stored magma with more primitive volatile rich basaltic magma this event will trigger early release of large quantities of highly volatile species like co 2 and helium in the hydrothermal system to use these precursors first their baseline abundances and their behaviour in the hydrothermal system must be known during the current phase of dormancy in this study we have combined a previous systematic investigation of noble gases with carbon data of hydrothermal fluids flowing out of la soufriere the results for dissolved gases reveal that an intense interaction between rising magmatic volatiles and groundwaters currently takes place at la soufriere we show that the pristine magmatic composition with a 13 c co2 value of 3 19 0 03 a 3 he 4 he ratio of 8 1 0 2 r a and a co 2 3 he ratio of 1 03 0 18 10 10 may vary during the ascent of magmatic fluids due to either fractionation or mixing processes with meteoric water and or an organic component here we provide a baseline for co 2 and helium isotopes at each monitored spring and describe the superficial processes affecting both elemental and isotopic ratios

a volcanomagnetic observation on mount st helens washington total magnetic fields were recorded at five sites on mt st helens between 23 october and 11 november 1981 an interval which included an extrusive dome building eruption of the volcano two of the magnetometers located in the volcano s crater measured reversible magnetic changes which correspond to fluctuations in tilt measured nearby however the relationship is highly nonlinear electric fields were measured on the east flank of the volcano near its summit to search for electrokinetic effects they show no correlation with the magnetic changes and in the long term are uncorrelated with eruptive activity our favored interpretation of the magnetic changes is that they result from stress induced changes in the magnetization of the volcano magnetic field values returned to pre anomaly values this reversibility rules out pressure induced magnetization as the dominant mechanism and places an upper limit of 300 bars on the stress changes the limited spatial extent of the magnetic anomaly field places the source of stress at shallow depth beneath the crater floor consistent with models based on strain data

thermodynamics of gas and steam blast eruptions eruptions of gas or steam and non juvenile debris are common in volcanic and hydrothermal areas from reports of non juvenile eruptions or eruptive sequences world wide at least three types or end members can be identified 1 those involving rock and liquid water initially at boiling point temperatures boiling point eruptions 2 those powered by gas primarily water vapor at initial temperatures approaching magmatic gas eruptions and 3 those caused by rapid mixing of hot rock and ground or surface water mixing eruptions for these eruption types the mechanical energy released final temperatures liquid water contents and maximum theoretical velocities are compared by assuming that the erupting mixtures of rock and fluid thermally equilibrate then decompress isentropically from initial near surface pressure 10 mpa to atmospheric pressure maximum mechanical energy release is by far greatest for gas eruptions 1 3 mj kg of fluid rock mixture about one half that of an equivalent mass of gunpowder and one fourth that of tnt it is somewhat less for mixing eruptions 0 4 mj kg and least for boiling point eruptions 0 25 mj kg the final water contents of crupted boiling point mixtures are usually high producing wet sloppy deposits final erupted mixtures from gas eruptions are nearly always dry whereas those from mixing eruptions vary from wet to dry if all the enthalpy released in the eruptions were converted to kinetic energy the final velocity v max of these mixtures could range up to 670 m s for boiling point eruptions and 1820 m s for gas eruptions highest for high initial pressure and mass fractions of rock m r near zero for mixing eruptions v max ranges up to 1150 m s all observed eruption velocities are less than 400 m s largely because 1 most solid material is expelled when m r is high hence v max is low 2 observations are made of large blocks the velocities of which may be less than the average for the mixture 3 heat from solid particles is not efficiently transferred to the fluid during the eruptions and 4 maximum velocities are reduced by choked flow or friction in the conduit

distortion of the geomagnetic field in volcanic terrains an experimental study of the mount etna stratovolcano abstract the inclination i declination d and total intensity f of the geomagnetic field were measured on mount etna in 1989 1991 at a dozen sites previously sampled for archeomagnetic studies the purpose of the work was to determine the variations of these parameters at 30 cm above ground level and how the distortion from the main field can affect the archeomagnetic record of volcanic rocks ten measurements were usually performed at each site with a three component flux gate magnetometer whose estimated precision is 0 2 on direction and 50 nt on intensity this was considered sufficient on volcanic areas with highly magnetized rocks and where the geomagnetic gradient may be in excess of 1000 nt m results averaged for each site generally show small variations in intensity 3 of the total field and direction 1 5 the averaged values of the 12 sites i 52 6 d 0 3 f 44010 nt are very close to those measured in sedimentary terrain away from the volcano i 52 9 d 0 35 f 44110 nt themselves consistent with the interpolated igrf in eastern sicily the largest deviations of the geomagnetic direction have been observed on four sites three of them located on the south flank between 1900 and 700 m elevation it is suggested that these anomalies are mainly related to dyke swarms which are common within the south rift zone of mount etna our findings show that reliable archeomagnetic results can be obtained from volcanic rocks provided that lavas of the same eruption are sampled on several sites distributed over the largest possible area

contribution of induced magnetization to magnetization of seamounts a fundamental assumption in modelling seamount magnetic anomalies is that the contribution of induced magnetization is negligible the general consistency of seamount and non seamount palaeopoles scarcity of poles skewed toward the present field direction and the high ratio of remanent to induced magnetization koenigsberger ratio of many oceanic basalts have been cited as evidence supporting this assumption1 2 recent discussions concerning the dominance of normally magnetized seamounts have focused attention on the possible role of viscous and induced magnetization in seamount anomalies3 6 here we determine natural remanent magnetization initial volume susceptibility and the koenigsberger ratio for more than 2 000 samples from a subaerially exposed seamount section on la palma canary islands table 1 by contrast to results from the oceanic crust and ophiolites these data indicate that a variety of rock types are potential magnetic sources the significant induced component of intrusives underscores the importance of the lithological distribution in determining the character of seamount magnetic anomalies the la palma data together with a plausible lithological distribution indicate that induced magnetization may account for one sixth of seamount magnetization

the 1975 1977 crisis of la soufriere de guadeloupe f w i a still born magmatic eruption on july 8 1976 eruptive activity broke out at la soufriere de guadeloupe f w i after about one year of increasing seismic activity seismic activity continued to increase until august 1976 reaching more than 1500 events a 200 fold increase over the preceding quiet period of a few years and an energy output of about 1017 ergs in a day a total of 26 major phreatic eruptions similar to the july 8 outburst took place during an eight months period the steam blasts that characterized the eruptions gave rise to particle and sometimes block charged plumes that deposited an estimated 106 m3 of solids the h2o rich gases emitted during the blasts presumably contained other gases h2s so2 co2 that were partly adsorbed on solid particles all material was erupted at temperatures of the order of 100 to 200 c the observation of vertical migration of earthquake foci in less than a few hours and over about 6 km depth and of abnormal variations of the geomagnetic field indicate a deep energy source for the phreatic eruptions a small proportion of the gases adsorbed on solid particles had a magmatic origin however most of the steam and the tephra seemed to originate from superficial levels of a hydrothermal system similar phreatic eruptions have occurred several times in recorded history in the case of la soufriere the origin of the phreatic eruptions is best described by an abnormal energy input versus steady state from a crustal magma chamber the occurrence of truly magmatic eruptions is presumably inhibited by an extensive hydrothermal system the abrupt release of more power from the magma chamber could have resulted in an explosive pyroclastic eruption substantial improvement of the guadeloupe volcano observatory has followed the 1975 1977 crisis permanent telemetered geophysical networks and regular geochemical observations have provided a five year data base of the volcano behavior in its noneruptive state which can be compared to crisis situations

evidence for a new shallow magma intrusion at la soufriere of guadeloupe lesser antilles insights from long term geochemical monitoring of halogen rich hydrothermal fluids more than three decades of geochemical monitoring of hot springs and fumaroles of la soufriere of guadeloupe allows the construction of a working model of the shallow hydrothermal system this system is delimited by the nested caldera structures inherited from the repeated flank collapse events and the present dome built during the last magmatic eruption 1530 ad and which has been highly fractured by the subsequent phreatic or phreatomagmatic eruptions because it is confined into the low volume highly compartmented and partially sealed upper edifice structure the hydrothermal system is highly reactive to perturbations in the volcanic activity input of deep magmatic fluids the edifice structure sealing and fracturing and meteorology wet tropical regime the current unrest which began with a mild reactivation of fumarolic activity in 1990 increased markedly in 1992 with seismic swarms and an increase of degassing from the summit of the dome in 1997 seismic activity increased further and was accompanied by a sudden high flux hcl rich gas from summit fumaroles we focus on the interpretation of the time series of the chemistry and temperature of fumarolic gases and hot springs as well as the relative behaviours of halogens f cl br and i this extensive geochemical time series shows that the deep magmatic fluids have undergone large changes in composition due to condensation and chemical interaction with shallow groundwater scrubbing it is possible to trace back these processes and the potential contribution of a deep magmatic source using a limited set of geochemical time series t co2 and total s content in fumaroles t and cl in hot springs and the relative fractionations between f cl br and i in both fluids coupling 35 years of geochemical data with meteorological rainfall data and models of ion transport in the hydrothermal aquifers has allowed us to identify a series of magmatic gas pulses into the hydrothermal system since the 1976 1977 crisis the contrasting behaviours of s and cl bearing species in fumarolic gas and in thermal springs suggests that the current activity is the result of a new magma intrusion which was progressively emplaced at shallow depth since 1992 although it might still be evolving the characteristics of this new intrusion indicate that it hasalready reached a magnitude similar to the intrusion that was emplaced during the 1976 1977 eruptive crisis the assessment of potential hazards associated with evolution of the current unrest must consider the implications of recurrent intrusion and further pressurization of the hydrothermal system on the likelihood of renewed phreatic explosive activity moreover the role of hydrothermal pressurization on the basal friction along low strength layers within the upper part of the edifice must be evaluated with regards to partial flank collapse at this stage enhanced monitoring research and data analysis is required to quantify the uncertainties related to future scenarios of renewed eruptive activity and magmatic evolution

relative paleointensity stack during the last 250 kyr in the northwest pacific 1 we have conducted a paleomagnetic study of 10 sediment cores recovered from the northwest pacific the purpose is to establish a regional paleointensity stack during the last 250 kyrs nopapis 250 for global coverage of paleointensity data as well as a regional reference for paleointensity assisted stratigraphy in the studied cores reductive dissolution of magnetite appears to have occurred mainly in glacial periods such horizons accompany large decreases in s ratio s 0 3t 1 irm 0 3t sirm 2 as low as 0 90 the sediments are also intercalated by frequent volcanic ash layers prerequisites for reliable relative paleointensity estimation are not satisfied at these horizons we rejected unsuitable data by quantitative criteria based on s ratio and saturation isothermal remanent magnetization sirm and constructed the nopapis 250 curve from the remaining data in the construction of the curve we chose irm as a normalizer because acquisitions of anhysteretic remanent magnetization arm are considered to be influenced by magnetic interactions among magnetic grains in the northwest pacific sediments relative intensity variation of the nopapis 250 gives a standard deviation that is 33 of the average if the normal distribution is assumed the nopapis 250 curve generally shows similar variation patterns compared with other high resolution records reported from other regions many paleointensity lows are recognized in common six out of the nine lows can be correlated with reported excursions in literature

variation of the electrical resistivity of large rock samples with stress many experimental and theoretical efforts have been devoted to the study of the electrical properties of rocks e g brace et al 1965 madden 1982 parkhomenko 1982 yamazaki 1966 special attention has been paid to possible relationships between variations of mechanical stresses inside the rock and variations of its electrical properties in reference to a possible technique of predicting earthquakes raleigh et al 1977 these studies were generally performed on small decimeter sized samples but the complexity of the physical processes involved and the heterogeneity of rocks in the field make it necessary to try to get results from larger blocks with dimensions of several decimeters sobolev et al 1983 or of meters this study it also appears quite desirable to look for information on the electrical properties of rocks at different scales from a centimeter to a kilometer say to see if some scaling laws could be experimentally established these scaling laws would allow inference of electrical properties at a given scale from the knowledge of the electrical properties at a smaller scale madden 1976 scaling law methods seem to be promising in seismogenesis studies e g allegre et al 1984 turcotte 1986

radiometric dating of three large volume flank collapses in the lesser antilles arc abstract it is now recognised that flank collapses are a recurrent process in the evolution of the lesser antilles arc volcanoes large magnitude debris avalanche deposits have been identified off the coast of dominica martinique and st lucia with associated volumes up to 20 km 3 deplus c le friant a boudon g komorowski j c villemant b harford c segoufin j cheminee j l 2001 submarine evidence for large scale debris avalanches in the lesser antilles arc earth planet sci lett 192 145 157 we present new radiometric dating of three major events using the k ar cassignol gillot technique in the qualibou depression of st lucia a collapse has been constrained by dome emplacement prior to 95 2 ka in dominica where repetitive flank collapse events have occurred le friant a boudon g komorowski j c deplus c 2002 l ile de la dominique a l origine des avalanches de debris les plus volumineuses de l arc des petites antilles c r geoscience 334 235 243 the plat pays event probably occurred after 96 2 ka inside the depression caused by this event scotts head which is interpreted as a proximal megabloc from the subsequent soufriere avalanche event has been dated at 14 1 ka providing an older bound for this event on martinique three different domes within the carbets structure dated at 337 5 ka constrain the age of this high magnitude event finally these results obtained from three of the most voluminous flank collapses provide constraints to estimate the recurrence of these events which represent one of the major hazards associated with volcanoes of the lesser antilles arc

electrical tomography of la soufriere of guadeloupe volcano field experiments 1d inversion and qualitative interpretation the soufriere of guadeloupe lava dome consists of several 3d domains of relatively unaltered massive andesite separated by major radial fractures that reach at least up to about half of the dome s height more than 20 000 geo electrical measurements made on the top and the flanks of the lava dome are used to construct the first geophysical image of the internal structure of this active volcano the main features of the apparent conductivity structure of the lava dome are high conductivity regions associated with the strongly altered material of present and fossil hydrothermal zones local 1d inversions of data subsets belonging to regions where the geological structure is supposed laterally invariant confirm the structures observed in the pseudo sections the relatively stable units lie above a basal inclined layer of highly conductive geological materials interpreted to be more altered and thus characterised by reduced internal friction and probably increased pore pressure past and current hydrothermal fluid circulation through these fractures has promoted rock alteration and particularly along the base of the dome that likely contributes to the instability of parts of the edifice

palaeointensity determinations and rock magnetic properties on basalts from shatsky rise new evidence for a mesozoic dipole low iodp expedition 324 cored igneous rocks from shatsky rise an oceanic plateau in the northwest pacific ocean that formed mainly during late jurassic and early cretaceous times we selected 60 samples from 3 different holes for thellier thellier palaeointensity determinations induced and remanent magnetization curves measured at low and high temperature suggest a diverse and complex magnetic mineralogy with large variations in ti content and oxidation state hysteresis and forc measurements show that most samples contain singledomain magnetic grains after carrying out the palaeointensity determinations only 9 samples satisfied all reliability criteria these gave palaeointensity values between 16 5 and 21 5 t which correspond to average vdm values of 4 9 0 2 1022 am2 for an estimated age of 140 142 ma this value is lower than that for the recent field which agrees with the hypothesis of a mesozoic dipole low

el nino induced onshore transport in the california current during 1982 1983 persistent 9 month large scale positive temperature 3 4 c negative salinity 0 1 0 3 and positive dissolved oxygen 0 5 1 ml l subsurface anomalies characterize the el nino induced onshore transport in the california current during 1982 83 these anomalies characteristic diagrams and sign reversals in the salinity and oxygen anomalies are consistent only with enhanced onshore transport of subarctic water from the offshore california current onshore transport excludes poleward propagating kelvin waves as a generation mechanism for the 1982 83 californian el nino the data however support the conclusion simpson 1983 that an anomalous basinwide atmospheric circulation produced the californian el nino

magnetic measurements on la soufriere volcano guadeloupe lesser antilles 1976 1984 a re examination of the volcanomagnetic effects observed during the volcanic crisis of 1976 1977 abstract during the seismovolcanic crisis of 1976 1977 at la soufriere on guadeloupe a magnetic network of 12 reference markers was set up measurements of the intensity of the earth s magnetic field carried out up to once a day at each marker showed volcano magnetic variations of several nanoteslas nt the variations at certain markers were more or less concealed by transient magnetic variations due to anomalies in conductivity as early as 1978 measurements were resumed and a telemetering network was coupled with the network of reference markers to which 4 new markers were added a detailed study of conductivity anomalies was carried out on the entire volcano contrasts in conductivity linked to the existence of a superficial conducting surface on a ssw nne axis located south of the volcano caused a great lack of homogeneity in the field variations measured at the surface variations greater than about 10 nt appeared in the difference in intensity of the earth s magnetic field between two stations no long term magnetic variation was observed between 1978 and 1984 on the network of markers the accuracy of measurements of volcanic effects was at best 2 nt measurements carried out on the telemetering network during the night refined these results since their accuracy was 1 nt the only significant volcanic crisis between 1978 and 1984 5 7 january 1981 seems to be observed by telemetering stations all the measurements carried out in periods of volcanic inactivity make it possible to re examine the crisis of 1976 1977 though volcanomagnetic effects over short periods cannot be accurately determined variations with a time constant of several weeks were present over the entire volcano these variations were as high as 7 8 nt in remote stations and they can be linked to the three major phases of eruptive activity at la soufriere during the crisis of 1976 1977

flank instability assessment at kick em jenny submarine volcano grenada lesser antilles a multidisciplinary approach using experiments and modeling kick em jenny kej located ca 8 km north of the island of grenada is the only active submarine volcano of the lesser antilles volcanic arc previous investigations of kej revealed that it lies within a collapse scar inherited from a past flank instability episode to assess the likelihood of future collapse we employ here a combined laboratory and modeling approach lavas collected using a remotely operated vehicle rov provided samples to perform the first rock physical property measurements for the materials comprising the kej edifice uniaxial and triaxial deformation experiments showed that the dominant failure mode within the edifice host rock is brittle edifice fractures such as those at champagne vent will therefore assist the outgassing of the nearby magma filled conduit favoring effusive behavior these laboratory data were then used as input parameters in models of slope stability first relative slope stability analysis revealed that the sw to n sector of the volcano displays a deficit of mass volume with respect to a volcanoid ideal 3d surface slope stability analysis using a limit equilibrium method lem showed that kej is currently stable since all values of stability factor or factor of safety fs are greater than unity the lowest values of fs were found for the sw nw sector of the volcano the sector displaying a mass volume deficit although currently stable kej may become unstable in the future instability severe reductions in fs could result for example from overpressurization due to the growth of a cryptodome our modeling has shown that instability induced flank collapse will most likely initiate from the sw nw sector of kej therefore mobilizing a volume of at least ca 0 7 km3 the mobilization of ca 0 7 km3 of material is certainly capable of generating a tsunami that poses a significant hazard to the southern islands of the west indies

geological and geomechanical models of the pre landslide volcanic edifice of guimar and la orotava mega landslides tenerife abstract the geological and geomechanical characterizations of the volcanic rock mass succession affected by the guimar and la orotava mega landslides tenerife canary island are presented here for the first time the estimated subaerial volume of rocks mobilized by each of these landslides is in the range of 30 50 km 3 one of the largest known in the world field data gallery surveys and borehole drilling have allowed the main types of materials and their structural arrangement to be identified based on these information a geological model of the pre landslide volcanic edifice is proposed a palaeo morphological reconstruction has been produced and possible slope angles and heights of the pre landslide volcanic edifice are presented five main lithological units have been identified in the emerged part of the edifice three of them forming the flanks and two forming the structural axis of the island on the flanks lava flows predominate with different degrees of alteration and proportion of dikes increasing near the structural axis the predominant materials on the structural axis are pyroclastic deposits lava flows and dikes in the submarine edifice four main lithological units have been distinguished formed by hyaloclastites pillow lavas dikes and gravitational deposits slope and basin facies the geomechanical characterization of these materials has been obtained from field data boreholes laboratory tests and literature review the geological and geomechanical models obtained provide the fundamental basis for the explanation of the instability processes that generated the guimar and la orotava mega landslides

a new scenario for the last magmatic eruption of la soufriere of guadeloupe lesser antilles in 1530 a d evidence from stratigraphy radiocarbon dating and magmatic evolution of erupted products la soufriere of guadeloupe is a dangerous volcano characterized over the last decade by moderate seismic and fumarolic unrest in the last 15 000 years it has experienced phreatic and magmatic eruptions and unusually numerous flank collapse events sometimes associated with a magmatic eruption we propose a new age of 1530 a d and a new eruptive scenario for the last magmatic eruption on the basis of a novel statistical analysis of radiocarbon age dates and new field and geochemical data this eruption is the only magmatic eruption likely to have occurred in guadeloupe during the last 1400 years the eruption mainly involved an andesitic magma which in the first phase of the eruption partially mixed with a slightly more differentiated magma stored in a small and shallow magma chamber ascent of magma to the surface generated a partial collapse of the hydrothermally altered edifice that increased the magma discharge and led to a sub plinian phase with scoria fallout and column collapse pyroclastic flows followed by near vent pyroclastic scoria fountains the eruption ended with growth of a lava dome our revised interpretation of the last magmatic eruption of la soufriere constitutes the most likely key to a future magmatic eruption scenario for this volcano which displays strong evidence of unrest since 1992

a new method for the determination of the setting temperature of pyroclastic deposits example of guadeloupe french west indies abstract when pyroclastic deposits settle at high temperature in a terrestrial magnetic field they memorize their setting temperature this temperature varies from one type of deposit to the other from ambient temperature for some airfall ashes up to hundreds of degrees centigrade for nuees ardentes or pumice flows these temperatures also vary significantly with distance from the eruptive vent at old events it is often very difficult first to establish the nature of the deposit because of crystallizations or cements created by diagenesis and then to locate the eroded eruptive centers the exact assessment of the setting temperature allows one on the one hand to find both the nature and the emplacement process of the deposit and on the other hand to find the location of the eruptive centers by working out a temperature map the methods used up to now are all based on the analysis of objects buried in the flow or on that of the surrounding rocks where burnt wood or heated objects are concerned this analysis may be chemical or based on fossil thermoremanent magnetization in the case of potteries or rocks such objects are not always to be found so the method proposed here uses samples drawn from the deposit itself the principle derived from thellier s method compares the natural magnetization lost during heating to the loss of total thermoremanent magnetization acquired during cooling in a known field this method essentially differs from that of thellier by a direct reading of the magnetization left at any high temperature this process which requires only two heatings enables one to avoid any difficulty linked to magnetic interactions and to lessen the effect of mineralogical transformations

volcanic hazards from bezymianny and bandai type eruptions major slope failures are a significant degradational process at volcanoes slope failures and associated explosive eruptions have resulted in more than 20 000 fatalities in the past 400 years the historic record provides evidence for at least six of these events in the past century several historic debris avalanches exceed 1 km3 in volume holocene avalanches an order of magnitude larger have traveled 50 100 km from the source volcano and affected areas of 500 1500 km2 historic eruptions associated with major slope failures include those with a magmatic component bezymianny type and those solely phreatic bandai type the associated gravitational failures remove major segments of the volcanoes creating massive horseshoe shaped depressions commonly of caldera size the paroxysmal phase of a bezymianny type eruption may include powerful lateral explosions and pumiceous pyroclastic flows it is often followed by construction of lava dome or pyroclastic cone in the new crater bandai type eruptions begin and end with the paroxysmal phase during which slope failure removes a portion of the edifice massive volcanic landslides can also occur without related explosive eruptions as at the unzen volcano in 1792 the main potential hazards from these events derive from lateral blasts the debris avalanche itself and avalanche induced tsunamis lateral blasts produced by sudden decompression of hydrothermal and or magmatic systems can devastate areas in excess of 500km2 at velocities exceeding 100 m s 1 the ratio of area covered to distance traveled for the mount st helens and bezymianny lateral blasts exceeds that of many pyroclastic flows or surges of comparable volume the potential for large scale lateral blasts is likely related to the location of magma at the time of slope failure and appears highest when magma has intruded into the upper edifice as at mount st helens and bezymianny debris avalanches can move faster than 100 ms 1 and travel tens of kilometers when not confined by valley walls avalanches can affect wide areas beyond the volcano s flanks tsunamis from debris avalanches at coastal volcanoes have caused more fatalities than have the landslides themselves or associated eruptions the probable travel distance l of avalanches can be estimated by considering the potential vertical drop h data from a catalog of around 200 debris avalanches indicates that the h l rations for avalanches with volumes of 0 1 1 km3 average 0 13 and range 0 09 0 18 for avalanches exceeding 1 km3 h l ratios average 0 09 and range 0 5 0 13 large scale deformation of the volcanic edefice and intense local seismicity precede many slope failures and can indicate the likely failure direction and orientation of potential lateral blasts the nature and duration of precursory activity vary widely and the timing of slope faliure greatly affects the type of associated eruption bandai type eruptions are particularly difficult to anticipate because they typically climax suddenly without precursory eruptions and may be preceded by only short periods of seismicity

oceanic plateau formation by seafloor spreading implied by tamu massif magnetic anomalies tamu massif is an immense mesozoic submarine volcano the main edifice of the shatsky rise oceanic plateau it is located at a spreading ridge triple junction but considered to be a shield volcano formed by effusive volcanism from an emerging mantle plume however it is unclear how tamu massif eruptions interacted with the spreading ridges which are enormous linear volcanoes themselves here we create a magnetic anomaly map for tamu massif which can provide clues about crustal formation for tamu massif we find dominantly linear magnetic field anomalies caused by crustal blocks of opposite magnetic polarity this pattern suggests that tamu massif is not a shield volcano but was emplaced by voluminous focused ridge volcanism if the magma source at the shatsky rise was a plume it was closely connected to and controlled by seafloor spreading by implication even the largest oceanic plateau edifices can be formed by seafloor spreading we suggest that the widely accepted analogy between continental flood basalts and oceanic plateaus requires reconsideration the shatsky rise oceanic plateau formed by spreading ridge volcanism according to analyses of linear magnetic anomalies over the tamu massif submarine volcano

ontong java and kerguelen plateaux cretaceous icelands together with iceland the two giant oceanic plateaux ontong java in the western pacific and kerguelen broken ridge in the indian ocean are accumulations of mafic igneous rock which were not formed by normal seafloor spreading we compare published geochronological crustal structure and subsidence results with tectonic fabric highlighted in new satellite derived free air gravity data from the three igneous provinces and conclude that existing evidence weighs lightly against the ontong java and kerguelen plateaux originating at a seafloor spreading centre

combining seismic and electric methods for predicting bedrock depth along a mediterranean soil toposequence abstract bedrock depth provides important information for many environmental and agricultural applications such as shallow groundwater monitoring the determination of soil water availability and the estimation of crop production potential direct estimates of bedrock depth from destructive soil observations are discontinuous and too expensive to be used in large areas geophysical methods are often cited as possible alternatives however their ability to provide reliable estimations of bedrock depth is known to depend greatly on local site characteristics therefore combining geophysical methods based on different physical parameters may help to provide better predictions this study examines the ability of the spectral analysis of surface waves sasw method combined with the classical high resolution electrical resistivity tomography ert method to predict soil depths in a 500 m ranged mediterranean hillslope southern france with increasing soil depths along the slope sasw was performed using the data measured in the field with classical seismic equipment impulse source and geophones distributed along a line in the same place eight transects of ert wenner schlumberger array 1 m electrode spaced were measured under wet and dry conditions to calibrate the geophysical measurements 81 boreholes from two to 5 m deep were interpreted to determine the bedrock depth which was defined as the occurrence in the depth of heterogeneous marine miocene loose sandstone with centimetric laminations ert and sasw were found to have highly variable performances for predicting separately the bedrock depth along the hillslope sasw correctly predicted the bedrock depth in the lower part of the hillslope whereas the data from ert were disrupted by shallow permanent groundwater conversely ert correctly predicted bedrock depth within the upper part of the hillslope whereas a high variability of sasw data near the topsoil caused difficulties for bedrock depth prediction from these results it was possible to define an estimator of bedrock depth according to the presence of shallow groundwater which varies along the slope such that more importance is given to ert estimates in the upper part of the hillslope and more importance is given to sasw in the lower part this study shows the usefulness of such a sensor combination to estimate soil properties when the uncertainties of making predictions vary according to the geophysical methods

volcanomagnetic effects observed on piton de la fournaise volcano reunion island 1985 1987 a permanent network of four to six telemetered magnetic stations has been installed on piton de la fournaise volcano in october 1985 and 1986 eight eruptive sequences have occurred since then some of which affected only the summit whereas others affected the whole massif volcanomagnetic variations are observed with each of these volcanic events the geomagnetic field intensity is measured simultaneously every minute at each station with a precision of 0 25 nt nanotesla the main results are the following volcanomagnetic variations observed with eruptions have amplitudes of several nanoteslas to more than 10 nt with characteristic time durations ranging from l min or less to several months a good correlation between the magnetic variations recorded at the different stations is observed spatial and temporal properties of the volcanomagnetic field appear to be controlled by the features of the eruptions several days before the beginning of the december 1985 march july november december 1986 and january 1987 eruptions changes in the intensity differences are observed with amplitudes of a few nanoteslas up to 5 nt at stations located close to the active part of the rift zone variations are also observed in the most remote stations for larger eruptions in march 1986 november december 1986 and january 1987 correlations between magnetic variations ground deformations and seismic activity have been observed for the december 1985 march 1986 november december 1986 and january 1987 events three main mechanisms could account for the observed volcanomagnetic variations piezomagnetic effects electrokinetic effects and electrical resistivity changes since variations in the stress field in the surroundings can modify the induced and natural magnetizations of magnetized rocks the circulation of water through the interconnected pore network microcracks faults aquifers and also the electrical resistivity of geological structures through the modification of the properties of the pore network

magnetic anomaly map of ori massif and its implications for oceanic plateau formation abstract many oceanic plateaus have been emplaced at or adjacent to mid ocean ridges to explain plateau volume and thickened crust compared to normal oceanic crust hotspot ridge interaction is commonly assumed but the manner of interaction remains unclear the shatsky rise oceanic plateau is a large volcanic mountain that formed at a triple junction during late jurassic and early cretaceous time recent drilling and seismic investigations suggest that the intermediate edifice in the rise ori massif is a central volcano paradoxically magnetic lineations were traced across parts of ori massif implying formation at a spreading ridge in this study we examined magnetic anomalies over and around ori massif to obtain insights about the formation of this volcanic edifice magnetic data from 21 cruises were corrected combined and gridded to construct a magnetic anomaly map forward and inverse magnetic modeling was done to investigate the magnetic structure of ori massif the results imply that this large volcanic edifice is predominantly characterized by linear magnetic anomalies resulting from alternating normal and reversed polarity magnetization blocks analogous to magnetic anomalies recorded by spreading ridges this magnetic structure is not expected for a central volcano that was built by long runout lava flows implying that ori massif eruptions must have been constrained near the ridge axis magnetic bights on the north and south boundaries of ori massif imply that it was bracketed by triple junctions indicating complex ridge tectonics during the formation of shatsky rise the surprising finding that ori massif is traversed by coherent linear magnetic anomalies indicates that oceanic plateaus can record seafloor spreading magnetic anomalies despite large crustal thickness other oceanic plateaus also record linear magnetic anomalies implying a link between divergent plate boundaries and oceanic plateau volcanism

large scale flank collapse events during the activity of montagne pelee martinique lesser antilles 1 a horseshoe shaped structure already identified on the southwestern flank of montagne pelee martinique lesser antilles arc was previously interpreted as resulting of a flank collapse event but no debris avalanche deposits were observed at the time new offshore high resolution bathymetry and geophysical data aguadomar cruise december 1998 to january 1999 r v l atalante lead us to identify three debris avalanche deposits on the submarine western flank of montagne pelee extending down to the grenada basin they display morphological fronts and hummocky morphology on bathymetric data speckled pattern on backscatter data and hyperbolic facies on 3 5 khz and seismic profiles new on land geological studies lead us to identify two other horseshoe shaped structures on the same flank of the volcano the three submarine deposits have been traced back to the structures identified on land which confirms the occurrence of repeated flank collapse events during the evolution of montagne pelee the ages of the last two events are estimated at 9 ka and 25 ka on the basis of 14c and 238u 230th dates every flank collapse produced debris avalanches which flowed down to the caribbean sea we propose that the repeated instabilities are due to the large asymmetry of the island with western aerial and submarine slopes steeper than the eastern slopes the asymmetry results from progressive loading by accumulation of volcanic products on the western slopes of the volcano and development of long term gravitational instabilities meteoric and hydrothermal fluid circulation on the floor of the second flank collapse structure also creates a weakened hydrothermalized area which favors the recurrence of flank collapses

joint inversion of p wave velocity and density application to la soufriere of guadeloupe hydrothermal system we present the result of a 3 d gravity and p wave traveltime joint inversion applied to the hydrothermal system of la soufri ere of guadeloupe the joint inversion process is used here to overcome the different resolution limitations attached to the two data sets p wave traveltimes were obtained from three active seismic surveys that were conducted from 2001 to 2007 gravity data collected during a microgravity campaign is described in a companion paper we use a joint inversion process based on a bayesian formulation and a deterministic iterative approach the coupling between slowness and density is introduced through a supplementary constraint in the misfit function that tries to minimize the distance between parameter values and a theoretical relationship this relationship is derived from measurements on samples representative of mt pel ee of martinique and la soufri ere volcanoes we chose a grid discretization that leads to an under determined problem that we regularize using spatial exponential covariance between the nodes parameters our results are compared to geophysical electromagnetic results obtained using resistivity and vlf surveys they confirm the presence of highly contrasted dense fast and light slow zones in la soufri ere dome and crater basement our images suggest however that some non conductive zones may be massive andesite bodies rather than argilized zones and that these bodies may have deeper roots than hypothesized

flank failure directed blast eruption at soufriere guadeloupe french west indies a 3 000 yr old mt st helens an extensive apparently monogenic debris flow deposit in the basse terre st claude area of southern guadeloupe is contemporaneous with pyroclastic surge deposits that cover an area of at least 60 km2 these 3 000 yr old deposits were emplaced by the avalanching of a sector of soufriere volcano and an associated directed blast similar to the 1956 bezymianni and the may 18 1980 mt st helens eruptions

influence of rainfalls on heat and steam fluxes of fumarolic zones six months records along the ty fault soufriere of guadeloupe lesser antilles fumarolic zones are permeable areas where both steam and heat are expelled to the atmosphere surface fluxes and flows which are representative of the intensity of the hydrothermal circulation in depth can be monitored by thermometers thermal infrared cameras spectrometers or condensers however the superficial activity of fumarolic zones can be modified by the meteorological conditions in particular the rainfalls which might result in erroneous estimations from this perspective we developed a set of physical equations to quantify the effects of rainfalls on the thermal behavior of fumarolic zones results were faced to continuous measurements achieved at the ty fault fumarolic zone la soufriere volcano guadeloupe lesser antilles during six months in 2010 using six vertical series of thermometers measuring the heat transfer in the ground and one condenser measuring the rising steam flux results demonstrate that in the absence of rainfalls heat and steam flux reach an equilibrium that is representa tive of the geothermal flux in depth conversely after the rainfalls the cooling of the ground provokes a deepening of the condensation level the related soil temperature drop can be estimated by computing the heat required to warm the infiltrated water up to boiling temperature while the recovery rate is directly linked to the geothermal flux our observations allow defining in which conditions flux are at steady state but also to build a first order numer ical model allowing estimating both the physical parameters of the ground thermal conductivity precipitation efficiency coefficient and surface flux constant and the long term thermal behavior of the hydrothermal system in particular our results predict that the hydrothermal activity must vanish on the zones where the geothermal flux drops under a certain threshold 60 w m 2 at la soufriere the existence of this limit may have strong implications for the precipitation rate of minerals and the possible reactivation of the fumarolic zones during volcanic crises

volcanomagnetic effects at vulcano island aeolian archipelago italy summary the installation of two continuously operating proton magnetometers 2 km apart and sampling simultaneously every 5 min on vulcano island from 1990 november to 1991 december allowed the detection of some transient anomalies thought to result from internal variations in the physical state of the volcano the diverence of mean values in the magnetic field at the two stations underwent a slow variation of 2 nt during the period of measurements the mean values in the total intensity at one station were inversely correlated to the diverence between the mean values of the two stations this suggests that the contribution to the signal generated by the transitory magnetic field may be broken down into two parts one of external origin which is identical for both stations and another of internal origin proportional to the former but dependent on the site following on from this simple description of the magnetic field a method of analysis was developed to distinguish between transients of volcanomagnetic origin and transients generated by strong variations in the external transitory magnetic field an accurate analysis which takes into account the correlation between the measurements at the two stations leads to the singling out of three magnetic anomalies on 1991 april 2 july 18 and december 23 comparison of the experimental data with the list of earthquakes occurring at vulcano during 1991 showed the close temporal correspondence between the magnetic anomalies and earthquakes the amplitude between 2 and 4 nt spatial extent approximately 2 km and polarity of anomalies point to the piezomagnetic evect as the primary physical mechanism driving these transient changes on the other hand the anomaly with respect to the regional field observed over one year is explicable in terms of a thermomagnetic evect detected at both stations the magnetic static anomaly produced by a completely demagnetized spherical source of 250 m radius located under the la fossa crater at a depth of 1300 m is in agreement with the slow changes observed at vulcano

plateaus from seafloor spreading ocean floor plateaus are not voluminous lava flows from central volcanoes as thought but anomalously thick oceanic crust suggest magnetic anomaly patterns from the shatsky rise in the northwestern pacific ocean

nonuniform magnetization of seamounts a least squares approach residual total magnetic field anomalies relative to the best fitting uniform magnetization models are large and spatially coherent over three small tertiary seamounts on the cocos plate furthermore the paleomagnetic parameters determined from the uniform magnetization models in some cases disagree significantly with those predicted from models of absolute motion for the cocos plate inversion of the sea surface magnetic anomaly using a least squares procedure which allows magnetization to vary among different sections of each seamount shows clear polarity reversals in two of the seamounts studied the reversed zones constitute about 10 of the seamount volume for the third seamount the nonuniform magnetization solution more correctly determines the paleomagnetic inclination and declination for the only section of the dipolar anomaly not obscured by a magnetic lineation traversing the area the polarities indicated in the least squares solutions were confirmed by magnetic gradiometer measurements from the submersible alvin for all three seamounts the paleomagnetic latitudes and or rotation angles in the nonuniform magnetization solutions changed appreciably by as much as 20 to agree better with plate motion models the nonuniform magnetization models reveal that growth rates for such volcanoes approach 1000 km3 ma with the final phase of volcanism consisting of summit as opposed to flank eruptions the largest of the three volcanoes formed off ridge on lithosphere at least 0 5 ma old and may have remained active for 1 ma hydrothermal activity apparently persists at least 0 7 ma after the last major phase of volcanism

localized geomagnetic field changes near active faults in california 1974 1980 currents in the ionosphere and magnetosphere induce changes in the difference fields due to contrasting impedances at magnetometer sites these changes are identified and removed by multichannel predictive filtering using boulder magnetic observatory component fields as inputs to the prediction operator the remaining fluctuations up to several nt are attributed to crustal magnetization changes evidence that these changes are stress related includes the variation prior to the busch fault earthquake 1974 seen at the two stations closest to within 10 km the epicenter also stations within 50 km of the 1979 coyote earthquake undergo localized field change up to a year before that event if truly precursive these variations are on a broader spatial and temporal scale than those for the busch fault event during the period 1975 1978 the geomagnetic variation is approximately uniform across the array without the development of any significant anomalies this period roughly coincides with the 1975 7 1978 7 seismic lull in nearly all of california it is followed by a marked increase in crustal secular variation long range coherence between anomalous variation at different stations is not seen other than the variation before the busch fault earthquake suggesting that localized variation occurs on a scale of less than 10 km coseismic tectonomagnetic events are not observed at the times of the busch and coyote earthquakes but model calculations suggest that observable events are not expected if the observed changes are due to stress they indicate that changes of aseismic stress are significantly larger than coseismic stress release

subsidence and growth of pacific cretaceous plateaus abstract the ontong java manihiki and shatsky oceanic plateaus are among the earth s largest igneous provinces and are commonly believed to have erupted rapidly during the surfacing of giant heads of initiating mantle plumes we investigate this hypothesis by using sediment descriptions of deep sea drilling project dsdp and ocean drilling program odp drill cores to constrain plateau subsidence histories which reflect mantle thermal and crustal accretionary processes we find that total plateau subsidence is comparable to that expected of normal seafloor but less than predictions of thermal models of hotspot affected lithosphere if crustal emplacement was rapid then uncertainties in paleo water depths allow for the anomalous subsidence predicted for plumes with only moderate temperature anomalies and volumes comparable to the sources of modern day hotspots such as hawaii and iceland rapid emplacement over a plume head of high temperature and volume however is difficult to reconcile with the subsidence reconstructions an alternative possibility that reconciles low subsidence over a high temperature high volume plume source is a scenario in which plateau subsidence is the superposition of 1 subsidence due to the cooling of the plume source and 2 uplift due to prolonged crustal growth in the form of magmatic underplating this prolonged crustal growth and uplift scenario may explain the low and thus submarine relief during plume initiation the late stage eruptions found on ontong java 90 ma and manihiki 70 ma a large portion of the high seismic velocity lower crust and the widespread normal faults observed throughout and along the margins of the three plateaus such late stage underplating may have occurred continuously or in discrete stages over 30 m y and implies lower magmatic fluxes than previously estimated

global gravity bathymetry and the distribution of submarine volcanism through space and time 1 the seafloor is characterized by numerous seamounts and oceanic islands which are mainly volcanic in origin relatively few of these features 0 1 however have been dated and so little is known about their tectonic setting one parameter that is sensitive to whether a seamount formed on near or far from a mid ocean ridge is the elastic thickness te which is a proxy for the long term strength of the lithosphere most previous studies are based on using the bathymetry to calculate the gravity anomaly for different values of te and then comparing the calculated and observed gravity anomaly the problem with such an approach is that bathymetry data are usually limited to single beam echo sounder data acquired along a ship track and these data are too sparse to define seamount shape we therefore use the satellite derived gravity anomaly to predict the bathymetry for different values of te by comparing the predicted bathymetry to actual shipboard soundings in the vicinity of each locality in the wessel global seamount database we have obtained 9758 te estimates from a wide range of submarine volcanic features in the pacific indian and atlantic oceans comparisons where there are previous estimates show that bathymetric prediction is a robust way to estimate te and its upper and lower bounds te at sites where there is both a sample and crustal age show considerable scatter however and there is no simple relationship between te and age nevertheless we are able to tentatively assign a tectonic setting to each te estimate the most striking results are in the pacific ocean where a broad swath of on ridge volcanism extends from the foundation seamounts and ducie island easter island ridge in the southeast across the equator to the shatsky and hess rises in the northwest interspersed among the on ridge volcanism are flank ridge and off ridge features the indian and atlantic oceans also show a mix of tectonic settings off ridge volcanism dominates in the eastern north atlantic and northeast indian oceans while flank ridge volcanism dominates the northeastern indian and western south atlantic oceans we have been unable to assign the flank ridge and off ridge estimates an age but the on ridge estimates generally reflect we believe the age of the underlying oceanic crust we estimate the volume of on ridge volcanism to be 1 1 106 km3 which implies a mean seamount addition rate of 0 007 km3 yr 1 rates appear to have varied through geological time reaching their peak during the late early cretaceous and then declining to the present day

a model for episodic degassing of an andesitic magma intrusion 1 episodic magmatic degassing has been observed at numerous volcanoes especially those of intermediate composition it can span timescales from years to decades here we propose a physical model for the degassing of a shallow magma intrusion to explain this phenomenon the magma cools by convection which leads to melt crystallization volatile exsolution and magma overpressure when the pressure reaches a critical value wall rocks fracture and the exsolved gas escapes the intrusion then returns to the initial lithostatic pressure and a new cooling crystallization degassing cycle occurs a series of such cycles leads to episodic degassing the trend and timescale of the degassing process are mainly governed by magma cooling two degassing regimes are exhibited an early phase with a high frequency of gas pulses and a later phase with a lower gas pulse frequency the transition between these two regimes is caused by the viscosity increase when the magma crystallinity exceeds the crystal percolation threshold we find that the time to this transition is dependent on magma volume to a first approximation where observations are available from sustained geochemical surveillance the model provides constraints on key aspects of the subsurface magmatic system with estimation of the volume of an intrusion and tensile strength of the surrounding rocks it therefore represents a relevant tool for volcanic surveillance and hazard assessment

mercury gas emissions from la soufriere volcano guadeloupe island lesser antilles abstract quantifying mercury hg emissions from active volcanoes is of particular interest for better constraining the global cycle and environmental impact of this highly toxic element here we report on the abundance of total gaseous tgm hg 0 g hg ii g and particulate hg p mercury in the summit gas emissions of la soufriere andesitic volcano guadeloupe island lesser antilles where enhanced degassing of mixed hydrothermal magmatic volatiles has been occurring since 1992 from the southern summit crater we demonstrate that hg in volcanic plume occurs predominantly as gaseous mercury with a mean tgm hg p mass ratio of 63 combining the mean tgm h 2 s mass ratio of the volcanic plume 3 2 10 6 measured close to the source vent with the h 2 s plume flux 0 7 t d 1 determined simultaneously allows us to estimate a gaseous mercury emission rate of 0 8 kg yr 1 from la soufriere summit dome somewhat lower tgm s tot mass ratio in fumarolic gases from the source vent 4 4 10 7 suggests that plume chemical composition is not well represented by the emission source fumaroles due to chemical processes prior to or upon discharge current mercury emission from la soufriere volcano represents a very small contribution to the estimated global volcanic budget for this element

steam and gas emission rate from la soufriere volcano guadeloupe lesser antilles implications for the magmatic supply during degassing unrest abstract since its last magmatic eruption in 1530 ad la soufriere andesitic volcano in guadeloupe has displayed intense hydrothermal activity and six phreatic eruptive crises here we report on the first direct quantification of gas plume emissions from its summit vents which gradually intensified during the past 20 years gas fluxes were determined in march 2006 and march 2012 by measuring the horizontal and vertical distributions of volcanic gas concentrations in the air diluted plume and scaling to the speed of plume transport fluxes in 2006 combine real time measurements of volcanic h2s concentrations and plume parameters with the composition of the hot 108 5 c fumarolic fluid at exit fluxes in 2012 result from multigas analysis of h2s h2o co2 so2 and h2 concentrations combined with thermal imaging of the plume geometry and dynamics measurements were not only focused on the most active south crater sc vent but also targeted tarissan crater and other reactivating vents we first demonstrate that all vents are fed by a common h2o rich 97 98 mol fluid end member emitted almost unmodified at sc but affected by secondary shallow alterations at other vents daily fluxes in 2012 averaged 200 tons of h2o 15 tons of co2 4 tons of h2s and 1 ton of hcl increased by a factor 3 compared to 2006 even though modest such fluxes make la soufriere the second most important volcanic gas emitter in the lesser antilles arc after soufriere hills of montserrat taking account of other hydrothermal manifestations hot springs and diffuse soil degassing the summit fumarolic activity is shown to contribute most of the bulk volatile and heat budget of the volcano the hydrothermal heat output 8 mw exceeds by orders of magnitude the contemporaneous seismic energy release isotopic evidences support that la soufriere hydrothermal emissions are sustained by a variable but continuous heat and gas supply from a magma reservoir confined at 6 7 km depth by using petro geochemical data for la soufriere magma s and their dissolved volatile content and assuming a magmatic derivation of sulfur we estimate that the volcanic gas fluxes measured in 2012 could result from the underground release of magmatic gas exsolved from 1400 m3 d 1 of basaltic melt feeding the system at depth we recommend that fumarolic gas flux at la soufriere becomes regularly measured in the future in order to carefully monitor the temporal evolution of that magmatic supply

mass and heat flux balance of la soufriere volcano guadeloupe from aerial infrared thermal imaging abstract la soufriere of guadeloupe is an active volcano of lesser antilles that is closely monitored due to a high eruptive hazard potential since 1992 it exhibits a medium level but sustained background hydrothermal activity with low energy and shallow seismicity hot springs temperature increase and high flux acidic gas fumaroles at the summit the problem of estimating the heat balance and quantifying the evolution of hydrothermal activity has become a key challenge for surveillance this work is the first attempt of a global mapping and quantification of la soufriere thermal activity performed in february 2010 using aerial thermal infrared imagery after instrument calibration and data processing we present a global map of thermal anomalies allowing to spot the main active sites the summit area including the fumaroles of tarissan pit and south crater the ty fault fumarolic zone and the hot springs located at the vicinity of the dome in a second step we deduce the mass and the energy fluxes released by the volcano in particular we propose a simple model of energy balance to estimate the mass flux of the summit fumaroles from their brightness temperature and size in february 2010 tarissan pit had a 22 8 8 1 kg s 1 flux 1970 704 tons day 1 while south crater vents had a total of 19 5 4 0 kg s 1 1687 348 tons day 1 once converted into energy flux summit fumaroles represent 98 of the 106 30 mw released by the volcano the 2 remaining being split between the hot springs and the thermal anomalies at the summit and at the ty fault fumarolic zone these values are in the high range of the previous estimations highlighting the short term variability of the expelled fluxes such a heat flux requires the cooling of 1500 m 3 of magma per day in good agreement with previous geochemical studies

high frequency seismo electromagnetic effects abstract electromagnetic perturbations caused by natural geophysical activity such as earthquakes and volcanic eruptions have been studied since the great alaskan earthquake in 1964 in recent years interest has been increasing because when a precursor event occurs it normally does so less than a few hours before the shock this paper presents a review of the high frequency seismo electromagnetic phenomena already observed and occurring before as well as after the shock the term high frequency is to be taken in the seismic sense i e larger than a few hertz mechanisms that might produce these seismo electromagnetic emissions will be described it is shown that at the moment a causal relationship between electromagnetic signals and seismic activity still remains to be demonstrated theoretical as well as intensive experimental studies are necessary to understand the physical processes responsible for seismo or volcano electric variations electrical resistivity variations electromagnetic effects associated with dilatance as well as electromagnetic fields generated during laboratory experiments or ionospheric disturbances

volcano flank instability in the lesser antilles arc diversity of scale processes and temporal recurrence 1 the 1997 boxing day collapse a remarkable feature of the ongoing eruption of soufriere hills on montserrat has prompted new interest in the study of volcano stability in the lesser antilles building on a few cases documented in the literature we have now identified at least 47 flank collapse events on volcanoes of the caribbean arc where this type of behavior is characteristic and repetitive about 15 events occurred on active volcanoes within the last 12 000 years in the northern part of the arc flank collapses are repetitive do not exceed 1 km3 in volume occur in all directions and are promoted by intense hydrothermal alteration and well developed fracturing of the summit part of the edifices in contrast infrequent but large sector collapses with volumes up to tens of km3 are typical of the southern volcanoes they are always directed to the west as a result of the high overall slopes of the islands toward the deep back arc grenada basin because caribbean islands are small a large part of the resulting debris avalanches have flowed into the sea thus contributing voluminous and sudden inputs of volcaniclastic sediments to the grenada basin deposits from such submarine flows have been identified during the recent aguadomar and caraval oceanographic cruises and traced to their source structures on land edifice collapses have a major influence on subsequent volcanic activity but also are of high concern because of their tsunamigenic potential