seismicity of an andesitic volcano during block lava effusion volcan de colima mexico november 1998 january 1999 the block lava effusion at volcan de colima mexico began on november 20 1998 after 12 months of seismic activity and ended about 80 days later three types of seismic events were observed during the lava effusion volcano tectonic earthquakes occurred mainly at the very beginning and after the termination of lava effusion explosion earthquakes occurred frequently during the period of the maximum rate in lava effusion the remainder of the seismic signals were associated with pyroclastic flows and rockfalls from the lava dome these latter signals increased sharply in number at the onset of lava effusion the rate of occurrence remained high when the lava discharge rate decreased but gradually decreased after the termination of lava effusion maximum daily durations of seismic signals are proportional to the daily volumetric output of lava indicating the dependence of the number of pyroclastic flows on the rate of lava output a log log plot of seismic signal duration vs number of events with this duration displays a linear relationship the short period seismic signals can be divided into three categories based on duration short events with durations less than 100 s intermediate events with durations between 100 and 250 s and long events with durations longer than 250 s we infer that long events correspond to pyroclastic flows with mean deposit volume 2 105 m3 and intermediate events represent pyroclastic flows with mean deposit volume 1 103 m3

gas and aerosol emissions from lascar volcano northern chile insights into the origin of gases and their links with the volcanic activity abstract this study focuses on the chemical compositions of volcanic gases and aerosols emitted by lascar volcano northern chile the sustained volcanic plume was sampled in april 2009 april 2011 and november 2012 through filter packs and analyzed for major gaseous species so 2 hcl and hf and trace elements during fieldwork so 2 flux measurements were also performed by uv spectrometry doas the lascar volcano is a significant and sustained emitter of so 2 between 150 and 940 t d hcl between 170 t d and 210 t d and hf up to 100 t d combining the so 2 normalized trace element concentrations and the so 2 emission rates we evaluate that the quiescent degassing of lascar is an important local source of trace metals to the atmosphere with contributions to global volcanic fluxes generally less than 2 our data were used to infer the origin of the gas emitted at lascar two major degassing sources were identified a deep magmatic reservoir and a shallow hydrothermal system the variable interaction between these two sources is the most likely scenario for explaining the compositional ranges in acid gases but also in a volatile and fluid mobile trace element such as b these variations are related to changes in volcanic activity an extensive interaction between the hot ascending magmatic gases and the shallow aquifer occurred in 2009 possibly due to a long period of quiescence before magmatic activity renewed in 2011 and 2012 our study highlights therefore that filter pack measurements may be used to study changes in subsurface processes that probably play a key role in triggering volcanic eruptions

storm influences on a tidal sandbank s surface middelkerke bank southern north sea the results are presented of two types of pre storm and post storm surveys carried out on the middelkerke bank southern north sea the first type of survey was planned to detect detailed morphological changes in two selected areas of the bank totalling 3 km2 for the survey in these shallow waters to be completed in a fast and accurate way use was made of a special hovercraft based hydrographic system the other type of survey concerned grab samplings covering the sandbank in order to study spatial variations in surficial grain size the pre storm and post storm morphological surveys showed that small and medium dunes disappear under stormy conditions a 5 m westerly displacement of most of the large and very large dunes has been interpreted as the result of bedform migration under normal fair weather locally dominant ebb flow the lowering of the large dunes crests by up to 1 2 m is considered an effect of the storms the accretion of the lower part of the northwestern flank is attributed to the deposition of sand derived from shallow parts of the sandbank during storms the grain size surveys showed that especially the central and northern parts of the sandbank were affected by grain size changes the northwestern flank showing a clear coarsening and the landward southeastern flank a fining with respect to the fair weather situation it is put forward that the waves approaching from the north cause an extra winnowing on the exposed flank of the bank on the southeastern flank of the bank which is better protected against the wave action fine particles are then deposited

seismic quantification of the explosions that destroyed the dome of volcan de colima mexico in july august 2003 in july august 2003 the andesitic lava dome at volcan de colima mexico was destroyed by a sequence of explosions that replaced the 2 106 m3 dome with a crater 200 m across and 30 m deep the two strongest explosions occurred on july 17 and august 28 the initial low frequency impulses that they produced which were recorded on broadband seismic records allowed an estimation of the counter forces of the initiating process as being equal to 0 3 1011 n and 1 1011 n for the july and august events respectively the seismic characteristics follow the nishimura hamaguchi scaling law for volcanic explosions reflecting self similarity in the processes initiating explosive events the results also show that counter forces can discriminate between the sizes of explosive eruptions that are assigned the same magnitude by conventional methods of classification such as the volcanic explosivity index the increasing use of broadband seismometers may therefore provide the basis for using counter forces to determine the magnitude of explosive eruptions

petrology of pumices of april 1993 eruption of lascar atacama chile lascar volcano atacama chile erupted on 18 20 april 1993 several sub plinian explosions occurred and some were mushroom shaped the highest column rose up to 23 km ash clouds crossed south america eastwards dacite pumice falls made of blocks and ashes were deposited on the flanks of the volcano as a result of collapsed columns the pumice contains phenocrysts of plagioclase enstatite augite biotite magnetite and ilmenite and small crystals of apatite the 1992 previous andesite dome inside the crater was destroyed banded blocks resulting from mingling of the dacitic pumice and andesite from the dome are found in the pumice flow both the lava dome and the pumice are representative of the lascar high k magma unit dacitic pumice is a product of crystal fractionation of the andesitic magma

lascar volcano northern chile evidence for steady state disequilibrium lascar volcano located in the central volcanic zone of northern chile is a medium to high k calc alkaline stratovolcano three stages are recognized in the construction of the edifice with activity ranging from andesitic fissure eruptions to a large volume dacitic pumice flow the soncor flow the eruption products are predominantly two pyroxene andesites and dacites with some hornblende dacites occurring in the soncor flow disequilibrium textures resulting from mixing between mafic and acid end members

generation of pyroclastic flows and surges by hot rock avalanches from the dome of mount st helens volcano usa several hot rock avalanches have occurred during the growth of the composite dome of mount st helens washington between 1980 and 1987 one of these occurred on 9 may 1986 and produced a fan shaped avalanche deposit of juvenile dacite debris together with a more extensive pyroclastic flow deposit laterally thinning deposits and abrasion and baking of wooden and plastic objects show that a hot ash cloud surge swept beyond the limits of the pyroclastic flow plumes that rose 2 3 km above the dome and vitric ash that fell downwind of the volcano were also effects of this event but no explosion occurred all the facies observed originated from a single avalanche erosion and melting of craterfloor snow by the hot debris caused debris flows in the crater and a small flood that carried juvenile and other clasts north of the crater a second broadly similar event occured in october 1986 larger events of this nature could present a significant volcanic hazard

source mechanisms of explosions at stromboli volcano italy determined from moment tensor inversions of very long period data 1 seismic data recorded in the 2 30 s band at stromboli volcano italy are analyzed to quantify the source mechanisms of strombolian explosions during september 1997 to determine the source centroid location and source mechanism we minimize the residual error between data and synthetics calculated by the finite difference method for a point source embedded in a homogeneous elastic medium that takes topography into account two source centroids are identified each representative of the distinct event types associated with explosive eruptions from two different vents the observed waveforms are well reproduced by our inversion and the two source centroids that best fit the data are offset 220 and 260 m beneath and 160 m northwest of the active vents the source mechanisms include both moment tensor and single force components the principal axes of the moment tensor have amplitude ratios 1 1 2 which can be interpreted as representative of a crack if one assumes the rock matrix at the source to have a poisson ratio n 1 3 a value appropriate for hot rock both imaged cracks dip 60 to the northwest and strike northeast southwest along a direction parallel to the elongation of the volcanic edifice and a prominent zone of structural weakness as expressed by lineaments dikes and brittle structures for our data set the volume changes estimated from the moments are 200 m 3 for the largest explosion from each vent together with the volumetric source is a dominantly vertical force with a magnitude of 10 8 n consistent with the inferred movement of the magma column perched above the source centroid in response to the piston like rise of a slug of gas in the conduit index terms 7215 seismology earthquake parameters 7280 seismology volcano seismology 8419 8414 volcanology eruption mechanisms keywords very long period seismicity moment tensor inversions eruption mehanics

the arc lavas of the shirahama group japan sr and nd isotopic data indicate mantle derived bimodal magmatism new sr and nd isotopic data are presented and integrated with previous data for the shirahama group mio pliocene mediumk volcanic arc suite of south central honshu japan main results are 1 the shirahama lavas range in sr sr from 0 70315 to 0 70337 and in mnd 144nd from 0 51298 to 0 51306 the sr and nd isotopic data cluster tightly within the mantle array and all lie within an overlapping field of midocean ridge basalt and ocean island basalt 2 small differences exist among the shirahama tholeiitic series calc alkaline series and mixed lavas the present isotopic data are consistent with a previously published model which proposes that chemical variations in magmas of coexisting tholeiitic and calc alkaline series are produced through crystal fractionation from mantlederived magmas of basalt and magnesian andesite respectively moreover the tholeiitic series and the calc alkaline series are isotopically identical thus both magma series can be derived from a source mantle with the same isotopic composition supporting the hypothesis of simultaneous generation of basalt and magnesian andesite magmas from a single diapir rising through the mantle wedge above the subduction zone the differences of water content and temperature within the diapir are again thought to have been produced through dehydration and heating of an isotopically homogeneous hydrous diapir the isotopic data show that the high sio lavas have the same isotopic compositions as more mafic lavas these data and liquid lines of descent of the shirahama magmas suggest that even rhyolites can be produced by differentiation from mantle derived magmas without crustal contamination analyses from 38 other arc volcanoes have been compiled to investigate the intravolcano variability of sr sr twelve of these display no intravolcano strontium isotopic variability as is the case with the shirahama group but others show greater variation of sr 86sr from individual volcanic centers presumably reflecting crustal contamination most of the latter volcanoes are underlain by thick continental crust it is noteworthy however that the greater variations of sr sr correlate with sio2 content andesites or dacites not basalts from the same volcano have the lowest sr sr and these rocks are calc alkaline in terms of feo mgo and sio theoretically assimilation of continental crust by the isotopically uniform shirahama magmas could produce these relationships given that mantle derived basalt and magnesian andesite both encounter continental crust on their ascent to the surface the hotter basalt magma would assimilate more crustal wallrocks than the cooler andesite resulting in the basalt being more radiogenic fractional crystallization magma mixing and or assimilation fractional crystallization of these magmas in crustal magma chambers could produce large compositional variations but the derivatives of the hotter basaltic magmas tholeiitic series in the broad sense would display greater contamination than those derived from the cooler andesitic magmas calc alkaline series

evolution of lascar volcano northern chile lascar volcano in the central andes is an ese wnw elongate composite stratocone stage i 3 at 26 45 ka producing the soncor ignimbrite no caldera formed laharic and alluvial fan deposits and a debris avalanche formed from 26 45 to 19 2 ka indicating a humid climatic period the dacitic and andesitic western stratocone stage iii developed over the soncor vent stage iii culminated in the 9 2 ka tumbres eruption which formed andesitic scoria flow fans and a summit collapse crater stage iv activity switched to the east forming an andesite lava three nested summit craters which repeatedly filled with lava and collapsed absence of significant erosion since 9 2 ka is due to the much drier holocene climate activity since 1984 displays cycles of lava dome formation in the summit crater lava dome subsidence with crater collapse vulcanian to plinian explosive eruptions culminating in the major explosive eruption of 19 20 april 1993 the total volume of magma erupted from lascar over a 43 ka period is estimated at 30 to 40 km 3

surficial sedimentology of the middelkerke bank southern north sea detailed surficial investigations over the middelkerke bank a tidal sand bank in the southern north sea revealed the relationship between morphology surficial structures and grain size parameters data from 85 grab samples all over the bank show that on a bank normal profile the coarser caco3 rich and badly sorted sediments are generally located near the highest point of the bank seaward at the northern end and landward at the southern end sedimentary structures were studied from 239 boxcores sampled on all the morphological units of the bank crest flanks and adjacent channels in the shallower parts foreset beds are preserved while in the deeper zones intense bioturbation arises and destroys any structure the combination between these data and the virtual absence of wave induced structures indicates that the main agents responsible for the bank shaping are the tidal currents

a seismically constrained mass discharge rate for the initiation of the may 18 1980 mount st helens eruption we calculate the vertical mass discharge rate from mount st helens for the first few minutes of the may 18 1980 cataclysmic eruption using a new method based on seismic constraints the observed seismic waves indicate that the seismic source is a series of single forces we model these forces as thrusts due to a combination of the momentum flux of the erupted products and the pressure of the eruptive jet the momentum discharge rate is converted to a mass discharge rate based on estimates of the velocity and jet pressure as constrained by a simple fluid dynamical model only two parameters are necessary for the calculation mach number and sound velocity the calculated mass ejected in the first 100 s is 1 6 10 11 4 6 10 11 kg since the total blast deposit is 3 2 10 11 4 1 10 11 kg one possible interpretation is that the directed blast had a significant 30 vertical component

models of tremor and low frequency earthquake swarms on montserrat recent observations from soufriere hills volcano in montserrat reveal a wide variety of low frequency seismic signals we discuss similarities and differences between hybrid earthquakes and long period events and their role in explosions and rockfall events these events occur usually in swarms and occasionally merge into tremor an observation that can shed further light on the generation and composition of harmonic tremor we use a 2d finite difference method to model major features of low frequency seismic signatures and compare them with the observations a depth dependent velocity model for a fluid filled conduit is introduced which accounts for the varying gas content in the magma and the impact on the seismic signals is discussed we carefully analyse episodes of tremor that show shifting spectral lines and model those in terms of changes in the gas content of the magma as well as in terms of a time dependent triggering mechanism of low frequency resonances in this way we explain the simultaneous occurrence of low frequency events and tremor with a spectral content comprising integer harmonics

tidal and residual currents around a norfolk sandbank currents on either side of well bank were recorded for 41 days in may june 1981 in order to distinguish locally tidally generated asymmetries pertinent to maintenance of the bank from larger scale and wind driven flows semi diurnal currents were dominant nearly rectilinear and slightly inclined to the bank axis fourth diurnal currents were consistent with previously observed and computed fourth diurnal tides for the area overall amplitudes varied as the square of the semi diurnal amplitude hence they appear to be generated by tides but only a small part locally residual currents average over a tidal cycle were always in the sense of clock wise circulation around the bank consistent with simple models and giving strong evidence of local generation by tides this circulation combines with the semidiurnal currents to maintain the bank the slow increase of the residuals with the semi diurnal amplitude is interpreted as an effect of strong semi diurnal currents a persistent component onto the bank 7 m above the bottom at one position may indicate distortion of the frictional layer by the sloping bottom or by bank curvature

buffering of melt oxygen fugacity by sulphur redox reactions in calc alkaline magmas calculated oxygen fugacities of magmas from lascar volcano northern chile and the 1991 eruption of mount pinatubo phillipines are seen to increase with decreasing temperature relative to the fayalite magnetite quartz buffer isopleths of constant h2s so2 at moderate pressures 2 4 kbar lie parallel to the data we conclude that magma mixing prior to eruption liberated large quantities of so2 gas by oxidation of dissolved sulphide in the mafic end member oxygen fugacity of the magmas was then buffered by reduction of this gas to h2s leading to increasingly oxidized magmas such a process might be used to explain the highly oxidized nature of other sulphur rich evolved calc alkaline magmas notably the el chichon trachyandesite

potential gain and loss of sand by some sand banks in the southern bight of the north sea abstract a detailed echo sounding and side scan sonar survey has shown that sand banks in the southern bight of the north sea have rounded heads in the approximate upstream direction and tapered tails in the approximate downstream direction of net regional sand transport thus the plan view of sand banks can be used as an indicator of the approximate direction of net sand transport sand wave aprons occur in front of the heads and downstream of the tails of the sand banks the asymmetry of the sand waves implies that sand is being added to sand wave aprons at the heads and to the upstream flanks of these sand banks but is shed from the downstream sand wave aprons there is no evidence for a closed sand circulation on the banks

centroid single force inversion of seismic waves generated by landslides the centroid moment tensor cmt waveform inversion method of dziewonski et al 1981 is modified to analyze long period seismic waves generated by single force events such as the gigantic landslides associated with the 1980 eruption of mount st helens we refer to the method as centroid single force csf inversion as the result of the inversion we obtain the spatio temporal centroid and three components of a vector we call the csf vector which is the seismic point source representation of a landslide and whose direction is the opposite of the direction of sliding the sealer magnitude of the csf vector measures the overall size of the landslide and is given by mcsf md where m is the mass of the sliding object and d is the sliding distance we apply csf inversion to the long period surface waves generated by the 1980 mount st helens event the 1975 kalapana earthquake and the 1974 mantaro landslide using seismograms from the gdsn ida and hglp networks the csf solution for the st helens event is remarkably consistent with the actual geological observations and proves the efficacy of csf inversion the csf solution for the kalapana event does not fit the overall waveforms better than the cmt solution does however because the cmt solution does not explain the observed love wave radiation pattern it may be necessary to invoke a combination of both types of mechanism for this event although the geometry of the csf solution the direction of the force for the mantaro event is consistent with the actual landslide mcsf is about 5 times smaller than that expected from the mass and travel distance of the landslide estimated by geological observations this discrepancy may suggest either the relatively aseismic nature of this landslide or that the total volume of the slide was overestimated by analogy to single station cmt inversion single station csf inversion also appears to be stable and useful

tidal sand wave formation influence of graded suspended sediment transport 1 the influence of graded suspended sediment transport on the formation of tidal sand waves is investigated hereto in the framework of a linear stability analysis we examine the time development of small amplitude perturbations superimposed to a flat heterogeneous seabed in a shallow tidal sea dominated by one tidal constituent the heterogeneous bottom material is modeled by a bimodal sediment mixture and the transport of sediment close to the bottom as well as in suspension is considered the active layer concept is employed and hiding exposure effects are taken into account the mechanisms giving rise to the sediment sorting process along the bed forms are found to be unaffected by the fractional transport of sediment in suspension on the other hand the formation of sand waves is quantitatively substantially affected by the graded suspended sediment transport and the wavelength of the most amplified mode is found to depend strongly on the characteristics of the sediment mixture in particular the graded suspended sediment transport largely affects the contribution to the amplification rate related to the sorting process this leads to the growth of bed forms characterized by a smaller wavelength compared with that predicted for a uniform sediment when the mean grain size of the sediment mixture is relatively coarse on the other hand longer bed forms appear when the sediment mixture is characterized by a relatively fine mean grain size moreover a destabilizing effect is identified in the former case while the flat bed configuration is stabilized in the latter case a comparison of the theoretical predictions with field observations seems to support the model even though it cannot be considered conclusive

analysis of long period seismic waves excited by the may 18 1980 eruption of mount st helens a terrestrial monopole long period 100 to 260 s love and rayleigh waves excited by the eruption of mount st helens on may 18 1980 and recorded by ida sro and asro stations were analyzed to determine the mechanism of the eruption the amplitude radiation patterns of both rayleigh and love waves are two lobed with a nodal direction in e5 s for rayleigh waves and in n5 e for love waves these radiation patterns preclude any double couple mechanism the radiation pattern the initial phase the relatively large amplitude ratio of love to rayleigh waves and the existence of clear nodes in the radiation patterns of fundamental mode and higher mode rayleigh waves suggest that the source is represented by an almost horizontal less than 15 from the horizontal single force pointed toward s5 w the surface wave spectra fall off very rapidly at periods shorter than 75 s suggesting a very slow source process although the details of the source time history could not be determined a smooth bell shaped time function f o s t 1 2 f o l cos t for 0 t 2 and f o s t 0 for t 2 with 75 s is considered appropriate on the basis of comparison between synthetic and observed seismograms and of the shape of the source spectrum the peak value of the force f 0 is about 10 18 dynes the tailing end of the source time function could not be resolved and some overshoot may be added the magnitude and the time history of the force can be explained by a northward landslide followed by a lateral blast observed at the time of the eruption two distinct events about 110 s apart can be identified on body wave and short period surface wave records the first event may correspond to the earthquake which triggered the landslide and the lateral blast the second event appears to correspond to a second large earthquake and explosion which took place about 2 minutes after the first earthquake

the negra muerta volcanic complex southern central andes geochemical characteristics and magmatic evolution of an episodically active volcanic centre abstract this petrologic analysis of the negra muerta volcanic complex nmvc contributes to understanding the magmatic evolution of eruptive centres associated with prominent nw striking fault zones in the southern central andes specifically the geochemical characteristics and magmatic evolution of the two eruptive episodes of this complex are analysed the first one occurred as an explosive eruption at 9 ma and is represented by a strongly welded fiamme rich andesitic to dacitic ignimbrite deposit the second commenced with an eruption of a rhyolitic ignimbrite at 7 6 ma followed by effusive discharge of hybrid lavas at 7 3 ma and by emplacement of andesitic to rhyodacitic dykes and domes both explosive and effusive eruptions of the second episode occurred within a short time span but geochemical interpretations permit consideration of the existence of different magmas interacting in the same magma chamber our model involves an andesitic recharge into a partially cooled rhyolitic magma chamber pressurising the magmatic system and triggering explosive eruption of rhyolitic magma chemical or mechanical evidence for interaction between the rhyolitic and andesitic magma in the initial stages are not obvious because of their difference in composition which could have been strong enough to inhibit the interaction between the two magmas after the initial explosive stages of the eruption at 7 6 ma the magma chamber become more depressurised and the most mafic magma settled in compositional layers by fractional crystallisation restricted hybridisation occurred and was effective between adjacent and thermally equivalent layers close to the top of the magma chamber at 7 3 ma increments of caldera formation were accompanied by effusive discharge of hybrid lavas through radially disposed dykes whereby andesitic magma gained in importance toward the end of this effusive episode in the central portion of the caldera assimilation during turbulent ascent ata is invoked to explain a conspicuous reversed isotopic signature 87sr 86sr and 143nd 144nd in the entire volcanic series therefore the 7 6 to 7 3 ma volcanic rocks of the nmvc resulted from synchronous and mutually interacting petrological processes such as recharge fractional crystallization hybridisation and assimilation during turbulent ascent ata geochemical characteristics of both volcanic episodes show diverse type and or depth in the sources and variable influence of upper crustal processes and indicate a recurrence in the magma forming conditions similarly other minor volcanic centres in the transversal volcanic belts of the central andes repeated their geochemical signatures throughout the miocene

analysis of long period seismic waves excited by the november 1987 eruption of izu oshima volcano long period seismic waves which were excited by two eruptions of izu oshima volcano in november 1987 and recorded at an abbreviated seismic research observatory station and at several volumetric strain meter stations are analyzed to determine kinematic seismic source models associated with the eruptions the seismic waves have unusual features in the radiation pattern and waveforms of rayleigh waves which cannot be explained in terms of a shear dislocation model waveform inversion is applied to obtain a kinematic source model which is parameterized by a single force and a seismic moment tensor many geophysical and field observations make clear that the two eruptions occurred in a lava drain back stage of the volcanic activity the source model of the seismic event associated with the eruption on november 16 1987 is mainly composed of a vertical single force with a magnitude of 4 1011 n and suggests a rapid collapse of overloading lava into the deeper vent the long period seismic waves excited by the eruption on november 17 1987 can be explained by a force system composed of two vector dipoles with nearly vertical orientation which are separated in time by 81 s the sum of the two dipoles is 1 7 1017 n m this source model suggests that tension crack openings or volume expansions of horizontally extended magma reservoirs or of aquifers occurred because of lava drain back

seismicity of block and ash flows occurring during the 2006 eruption of augustine volcano alaska abstract in january 2006 augustine volcano began erupting following an increase in seismicity that was first noted in late april 2005 thirteen large explosive eruptions of augustine occurred from january 11 to 28 2006 followed by a continuously erupting phase and then by a dome growth phase in which numerous pyroclastic flows and block and ash flows occurred as a new steep sided and unstable dome grew in spring 2006 rockfalls and related events likely block and ash flows dominated the seismic record relative amplitudes at pairs of seismic stations for 68 block and ash flow events were examined to constrain locations of the flow events higher amplitudes were associated with events closer to a given station these relations were confirmed by images collected on a low light camera captured images show a correlation between flow direction and seismic amplitude ratios from nearby stations aue and auw seismic amplitudes and energies of the flow signals measured in several different ways were found to correlate with the surface areas and run out distances of the flows the m l range of rockfalls was 0 1 to 1 1 and seismic efficiencies were estimated to be much less than 1 particle motion analyses showed that the seismic waves contained both body waves and surface waves and demonstrate that the flows were acting as moving sources with velocities of 30 93 m s

modelling the lava dome extruded at soufriere hills volcano montserrat august 2005 may 2006 part ii rockfall activity and talus deformation abstract during many lava dome forming eruptions persistent rockfalls and the concurrent development of a substantial talus apron around the foot of the dome are important aspects of the observed activity an improved understanding of internal dome structure including the shape and internal boundaries of the talus apron is critical for determining when a lava dome is poised for a major collapse and how this collapse might ensue we consider a period of lava dome growth at the soufriere hills volcano montserrat from august 2005 to may 2006 during which a 100 10 6 m 3 lava dome developed that culminated in a major dome collapse event on 20 may 2006 we use an axi symmetrical finite element method model to simulate the growth and evolution of the lava dome including the development of the talus apron we first test the generic behaviour of this continuum model which has core lava and carapace talus components our model describes the generation rate of talus including its spatial and temporal variation as well as its post generation deformation which is important for an improved understanding of the internal configuration and structure of the dome we then use our model to simulate the 2005 to 2006 soufriere hills dome growth using measured dome volumes and extrusion rates to drive the model and generate the evolving configuration of the dome core and carapace talus domains the evolution of the model is compared with the observed rockfall seismicity using event counts and seismic energy parameters which are used here as a measure of rockfall intensity and hence a first order proxy for volumes the range of model derived volume increments of talus aggraded to the talus slope per recorded rockfall event approximately 3 10 3 13 10 3 m 3 per rockfall is high with respect to estimates based on observed events from this it is inferred that some of the volumetric growth of the talus apron perhaps up to 60 70 might have occurred in the form of aseismic deformation of the talus forced by an internal laterally spreading core talus apron growth by this mechanism has not previously been identified and this suggests that the core hosting hot gas rich lava could have a greater lateral extent than previously considered

the rupture process of the 1989 offshore ito earthquakes preceding a submarine volcanic eruption an earthquake swarm at the eastern offshore region of ito the izu peninsula started on june 30 1989 and became more active on and after july 4 the largest earthquake among the swarm mjma 5 5 occurred at 1109 jst 0209 ut on july 9 1989 and a second significant event followed 45 s later a remarkable volcanic tremor started on july 11 and a submarine eruption occurred just above the source region of the largest earthquake on july 13 in this paper we present a new inversion method to derive the source time function of rupture process the source time function is divided into several sections following an automatic algorithm and is interpolated with a cubic spline function the parameters which are the values and first derivatives at knots of the spline function are determined by minimizing a squared error between the observed and synthetic seismograms to demonstrate the performance of this method some numerical experiments simulations are carried out on near field seismic records applying this method to analysis of the two large earthquakes the detailed rupture processes are obtained a heterogeneous crustal structure near the volcanic area affects the rupture processes of these earthquakes the large stress drop due to the largest earthquake just beneath the submarine eruption point would assist a magma ascending and may have triggered the eruption 4 days after these earthquakes

crumbling of dacite dome lava and generation of pyroclastic flows at unzen volcano recent modelling of volcanic eruptions has shown that the efficiency of subsurface degassing of magmas determines whether magma erupts explosively or effuses quietly1 2 slow uprise of magma is often accompanied by effective degassing leading to the extrusion of lava flows and domes although lava dome extrusion is one of the less explosive modes of eruption it is often accompanied by explosive pyroclastic activities3 5 the 1991 eruption of unzen volcano provided an opportunity to observe at close range several types of small scale pyroclastic flow glowing avalanches originating from lava domes most of the pyroclastic flows are of merapi type caused by blocks falling from a collapsing dome others are of pelean type originating in an explosion from the side of a dome the lavas apparently show variable degrees of self explosivity we suggest that variable degrees of degassing of the magma produced a wide range of excess pore pressures in the extruded lava domes resulting in both merapi type and pelean type pyroclastic flows from the domes

the upper crustal magma plumbing system of the pleistocene apacheta aguilucho volcanic complex area altiplano puna northern chile as inferred from the erupted lavas and their enclaves abstract the evolution of the magma plumbing system of the pleistocene apacheta aguilucho volcanic complex area altiplano puna volcanic complex northern chile was investigated through petrographic geochemical and isotopic studies of representative lavas and related enclaves updated available dates of these products both from the apacheta and aguilucho stratovolcanoes and nearby domes allow us to define the activity during the last 1 ma this investigation shows that the andesitic magmas were affected by processes of assimilation plus fractional crystallization afc with a significant role played by amphibole fractionation during their ascent through the upper crust presumably by the interaction with the altiplano puna magma body 15 20 km these andesitic magmas were erupted with no or minor additional contamination at shallower levels or experienced plagioclase dominated fractional crystallization fc to dacite within shallower crustal magma chambers 4 8 km depth the constructional phase of the apacheta and aguilucho stratovolcanoes 1 to ca 0 6 ma reflect a transition from high flux i e flare ups to steady state magmatism as also documented in other pleistocene volcanic complexes of the altiplano puna volcanic complex during this stage the mafic magma recharge was high enough to permit a large spectrum of hybridization of the resident magmas in the upper crust to form the abundant andesites and dacites lavas in contrast at 150 100 ka the magmatism turned to a new stage of recharge waning stage and the episodic intrusion of small volumes of andesitic magmas permitted the remobilization of the crystal rich dacites triggering the extrusions of the chanka chac inca and cerro pabellon domes the andesitic enclaves in the domes studied here represent a snapshot of the magmatic processes of interaction that occurred in the shallower reservoir at the interface of the resident dacite with the ascending andesitic magma nevertheless as there are no dated volcanic products from the area between 0 6 and 0 1 ma the youngest dacitic domes could also be interpreted as the beginning of a new magmatic pulse of the altiplano puna volcanic complex independently from the significance of the mafic recharge at 150 100 ka waning stage vs new pulse the youngest investigated domes share similar geochemical features and crustal depth constraints 4 8 km with the ignimbrites of the altiplano puna volcanic complex therefore suggesting that the remobilized magmas erupted as the domes are possibly remains of older plumbing systems left over from the last magmatic flare up of the altiplano puna volcanic complex

heat and mass flux measurements using landsat images from the 2000 2004 period lascar volcano northern chile a qualitative and quantitative analysis of 13 landsat tm and etm images of lascar volcano for the 2000 2004 period was performed by applying the three bands and three components method to determine heat and mass flux and understand the magma circulation process in a passive degassing volcano related to permanent fumarolic activity the behavior and evolution of spectral radiance during the study period suggest that prior to low to moderate magnitude eruptions these values reach their localized temporal minimum levels corresponding to 1 9 4 38 mw cm2sr m in july 2000 and 4 38 7 11 mw cm2sr m in december 2003 eruptions respectively similar behavior is observed for anomaly area heat and mass fluxes during the 2000 2004 period the heat flux was estimated to vary from 75 46 and 10 527 mw while mass flux ranged between 131 and 18 469 kg s 1 a magma circulation model is proposed to explain these variations where the thermal anomaly is related to the presence of a fumarolic field and fluids movement from a magma chamber located at 10 17 km depth

europa s crust and ocean origin composition and the prospects for life abstract we have considered a wide array of scenarios for europa s chemical evolution in an attempt to explain the presence of ice and hydrated materials on its surface and to understand the physical and chemical nature of any ocean that may lie below we postulate that following formation of the jovian system the europan evolutionary sequence has as its major links a initial carbonaceous chondrite rock b global primordial aqueous differentiation and formation of an impure primordial hydrous crust c brine evolution and intracrustal differentiation d degassing of europa s mantle and gas venting e hydrothermal processes and f chemical surface alteration our models were developed in the context of constraints provided by galileo imaging near infrared reflectance spectroscopy and gravity and magnetometer data low temperature aqueous differentiation from a carbonaceous ci or cm chondrite precursor without further chemical processing would result in a crust ocean enriched in magnesium sulfate and sodium sulfate consistent with galileo spectroscopy within the bounds of this simple model a wide range of possible layered structures may result the final state depends on the details of intracrustal differentiation devolatilization of the rocky mantle and hydrothermal brine reactions could have produced very different ocean crust compositions e g an ocean crust of sodium carbonate or sulfuric acid or a crust containing abundant clathrate hydrates realistic chemical physical evolution scenarios differ greatly in detailed predictions but they generally call for a highly impure and chemically layered crust some of these models could lead also to lateral chemical heterogeneities by diapiric upwellings and or cryovolcanism we describe some plausible geological consequences of the physical chemical structures predicted from these scenarios these predicted consequences and observed aspects of europa s geology may serve as a basis for further analys is and discrimination among several alternative scenarios most chemical pathways could support viable ecosystems based on analogy with the metabolic and physiological versatility of terrestrial microorganisms

the characteristics of seismic signals produced by lahars and pyroclastic flows volcan de colima mexico abstract the general characteristics of seismic signals produced by pyroclastic flows generated by either the collapse of a lava dome or an eruptive column and lahars at volcan de colima mexico are discussed the paper concentrates on the 2004 2006 activity associated with and following the extrusion of andesitic block lava in october november 2004 it is shown that the duration of the broad band seismic records of pyroclastic flows lasts a few minutes while the duration of seismic records of lahars continues for tens of minutes or hours the spectra of seismic records produced by pyroclastic flows are characterized by lower peak frequencies around 3 4 hz than for lahars around 6 8 hz this difference in the frequency content together with the difference in the duration of seismic signals allows early diagnostic of the events in real time

a review of the discrete wavenumber method we present a review of the discrete wavenumber dwn method the method introduced by bouchon and art 1977 allows the simple and accurate calculation of the complete green s functions for many problems in elastodynamics

catastrophic lava dome failure at soufriere hills volcano montserrat 12 13 july 2003 the lava dome collapse of 12 13 july 2003 was the largest of the soufriere hills volcano eruption thus far 1995 2005 and the largest recorded in historical times from any volcano 210 million m3 of dome material collapsed over 18 h and formed large pyroclastic flows which reached the sea the evolution of the collapse can be interpreted with reference to the complex structure of the lava dome which comprised discrete spines and shear lobes and an apron of talus progressive slumping of talus for 10 h at the beginning of the collapse generated low volume pyroclastic flows it undermined the massive part of the lava dome and eventually prompted catastrophic failure from 02 00 to 04 40 13 july 2003 large pyroclastic flows were generated these reached their largest magnitude at 03 35 when the volume flux of material lost from the lava dome probably approached 16 million m3 over two minutes the high flux of pyroclastic flows into the sea caused a tsunami and a hydrovolcanic explosion with an associated pyroclastic surge which flowed inland a vulcanian explosion occurred during or immediately after the largest pyroclastic flows at 03 35 13 july and four further explosions occurred at progressively longer intervals during 13 15 july 2003 the dome collapse lasted approximately 18 h but 170 of the total 210 million m3 was removed in only 2 6 h during the most intense stage of the collapse

seismic and acoustic signatures of surficial mass movements at volcanoes abstract surficial mass movements such as debris avalanches rock falls lahars pyroclastic flows and outburst floods are a dominant hazard at many volcanoes worldwide understanding these processes cataloging their spatio temporal occurrence and detecting tracking and characterizing these events would advance the science of volcano monitoring and help mitigate hazards seismic and acoustic methods show promise for achieving these objectives many surficial mass movements generate observable seismic and acoustic signals and many volcanoes are already monitored significant progress has been made toward understanding modeling and extracting quantitative information from seismic and infrasonic signals generated by surficial mass movements however much work remains in this paper we review the state of the art of the topic covering a range of scales and event types from individual rock falls to sector collapses we consider a full variety of volcanic settings from submarine to subaerial shield volcano to stratovolcano finally we discuss future directions toward operational seismo acoustic monitoring of surficial mass movements at volcanoes

hydrogen isotope evidence for extrusion mechanisms of the mount st helens lava dome from october 1980 to october 1986 the dacite lava dome at mount st helens grew through the accumulation of more than 15 separate lobes during extrusive episodes that lasted from 1 to 24 days except for an anomalous period of continuous growth throughout most of 19831 over these six years the growth rate was generally volume predictable that is the amount of lava added to the dome in a given eruption was roughly proportional to the time since the preceding extrusion2 these well documented eruptions were separated by repose intervals lasting from several months to more than a year2 in an attempt to identify the triggering mechanisms for these periodic eruptions we have used hydrogen isotope analyses to determine water content and deuterium content for 18 samples of the mount st helens dome dacite these isotope data the first ever collected from an active lava dome suggest a steady state process of magma evolution combining crystallization induced volatile production in the chamber with three different degassing mechanisms closed system volatile loss in the magma chamber open system volatile release during ascent and kinetically controlled degassing upon eruption at the surface the data suggest that future dome building eruptions may require a new influx of volatile rich magma into the chamber

sediment transport under wave and current combined flows on a tide dominated shoreface northern coast of france abstract hydrodynamic circulation and sediment transport patterns were investigated on a macrotidal shoreface off the southern north sea coast of france from in situ directional wave and current measurements and through numerical modeling of sediment flux current meters equipped with wave gauges were deployed at two sites on the mid upper shoreface in approximately 5 6 m water depth during 6 and 15 days sediment flux was estimated using the sedtrans96 numerical model li m z amos c l 2001 sedtrans96 the upgraded and better calibrated sediment transport model for continental shelves comp geosc 27 619 645 based on grant and madsen s grant w d madsen o s 1986 the continental shelf bottom boundary layer ann rev fluid mech 18 265 305 combined flow boundary layer theory the hydrodynamic measurements and modeling results show that sediment on the shoreface is essentially transported alongshore in response to shore parallel tidal currents the direction of the net sediment transport is largely determined by the asymmetry of the tidal flows which can be flood or ebb dominated but during important wind speed events winds blowing in the same direction as tidal flow can significantly reinforce tidal currents or conversely can limit tidal current speeds when acting in the opposite direction although sediment transport on this macrotidal shoreface is strongly controlled by tidal processes waves can significantly enhance bed shear stresses resulting in increased sediment remobilization and hence higher sediment flux wave induced onshore sediment transport may occasionally occur but is several orders of magnitude lower than longshore transport this macrotidal shoreface is thus characterized by a hydraulic regime strongly dominated by shore parallel tidal flows that favour longshore sediment dispersal rather than shore normal transport our results imply that the water depths from which sediment may be transported shoreward to beaches is certainly much more restricted on a macrotidal shoreface such as the one bordering the northern coast of france than on micro and mesotidal shoreface environments where weaker tidal currents do not significantly limit cross shore sediment movements

a laboratory scale study on the role of mechanical vibrations in pore pressure generation in pyroclastic materials implications for pyroclastic flows the results of laboratory scale experiments with ignimbrite material are reported and discussed the ignimbrite material contained in a rotating drum is subjected to mechanical vibrations simulating particle agitation at the base of a pyroclastic flow propagating on an irregular substrate it is observed that vibrations lead to a severe decrease of internal friction evidenced by a drastic drop of the avalanche angle of the material in the rotating drum we propose that acoustic streaming a well known phenomenon in engineering applications operates in the ignimbrite material vibrations cause the development of rotational air flow around the particles and in consequence the enhancement of hydrodynamic viscous stresses promotes pore fluid pressure which leads to dynamical weakening through fluidization a fluid mechanical theoretical model accounts for the decrease of the friction coefficient of the ignimbrite material under a wide range of experimental conditions it can be reasonably inferred that sustained high gas pore pressure resulting from mechanical agitation could be present in pyroclastic flows and might contribute to their high mobility

real time and in situ assessment of conduit permeability through diverse long period tremors beneath aso volcano japan abstract long period signal lps 0 2 2 s and very long period signal vlp 2 100 s observed in the shallow volcanic plumbing system are typically repetitive and time invariant in their location and source mechanism offering in situ probes of hot fluid transport over the eruption cycles while the amplitude and activity of volcanic tectonic earthquakes and lp events have been commonly used to infer overpressure within their source region one missing link is an observable that may permit inference on the change in the permeability of the conduit plug wall which can regulate the degree of pressurization affect the mechanical strength of the surrounding rock and consequently the likelihood of an upcoming eruption here we show that during the 2011 2016 eruption cycle at aso volcano in japan long period tremor events a vlp of 15 s period with opposite waveform polarity can be systematically detected and categorized as pressurization and depressurization events in the same crack like conduit we suggest that depending on the strength of the surrounding rock and the permeability of the crack like conduit wall plug pressurization due to magmatic heat and vaporization is more likely to occur when a less permeable conduit plug wall can effectively keep the gas inside the crack like conduit on the other hand depressurization is prone to occur if the conduit wall plug permeability is sufficiently high to allow gas to escape from the conduit considering the amplitude of lpt proportional to the conduit overpressure contrasting energetics of these diverse lpt events allows us to define whether the conduit is prone to pressurization or depressurization providing a framework to infer how the permeability of the conduit wall plug may evolve over an eruption cycle

locating pyroclastic flows on soufriere hills volcano montserrat west indies using amplitude signals from high dynamic range instruments pyroclastic flows are located using amplitude signals from a seven station high dynamic range seismograph array located 1 9 6 1 km from soufriere hills volcano in montserrat west indies locations are determined by measuring the seismograph signal amplitude for an event recorded at several stations in a moving time window analysis for a given window the measured amplitudes are corrected to a trial source location by removing the effect of the surface wave geometric spreading instrument gain and the attenuation at calculated travel times the trial source location is then compared to other trial locations via an iterative localised grid search where the root mean squared amplitude residual a is minimised the process is repeated for subsequent time steps resulting in a best fit event location and size through time the method has been tested on four small events occurring on april 8 1999 august 12 1999 february 25 2001 and july 4 2001 when visual observations of pyroclastic flows coincided with good seismograph station coverage number of stations 5 azimuthal gap 160 based on the location results the four events propagated 0 5 1 4 1 3 and 1 0 km from the dome and had maximum attenuation corrected reduced displacements drq of 9 0 2 8 6 9 and 2 3 cm2 and maximum pyroclastic flow velocities of 7 30 20 and 8 ms 1 respectively a time lapse video of the event of august 12 1999 shows that amplitude based location through time closely matches the observed run out distance and velocity in contrast amplitude based locations for the events of april 8 1999 and july 4 2001 underestimated the actual flow run out by 1 5 km underestimation of the true run out distance is probably due to both the increased distribution of sources as coherent dome material disaggregates into many blocks and signal contamination from other sources results indicate that pyroclastic flows and rockfalls can be located using amplitude signals from high dynamic range seismograph stations yielding estimates of size and trajectory regardless of visibility conditions on the volcano this new method is being tested as a hazard mitigation and research tool on montserrat

environmental setting morphology and volumetric evolution of the middelkerke bank southern north sea abstract the middelkerke bank a tidal sandbank located in the southern north sea on the belgian continental platform has been the object of a multidisciplinary and international research project the morphological and hydrodynamical setting of the study area is presented a complete side scan sonar coverage of the bank allowed a detailed analysis of the morphology of the bank and of its superimposed bedforms large dunes cover the flanks and summit of the middelkerke bank they have a constant orientation a height ranging from 0 5 to 5 m a wavelength from 75 to 150 m and in most cases an asymmetrical profile the slopes of both lee and stoss flanks are very low small and medium dune fields occur on the entire bank and in the adjacent swales they are 2 d and present a wavelength ranging from 1 to 15 m flood and ebb oriented small dunes cover respectively the western and eastern flank of the bank bathymetric surveying of the middelkerke bank was carried out to assess the importance of volumetric variations of the bank mass caused by seasonal factors results show that changes in the bank volume can be quite important the bank s volume decreases in periods of heavy weather after which the process of sand uppiling acting during long periods of fair weather conditions causes the bank to restore itself volumetric variations of the total bank volume in a winter summer period can range along certain sections of the bank from 21 to 26 despite these large seasonal volumetric variations the bank shows a long term dynamic equilibrium as proven by the obtained evolution trend

monitoring and modeling ice rock avalanches from ice capped volcanoes a case study of frequent large avalanches on iliamna volcano alaska abstract iliamna is an andesitic stratovolcano of the aleutian arc with regular gas and steam emissions and mantled by several large glaciers iliamna volcano exhibits an unusual combination of frequent and large ice rock avalanches in the order of 1 10 6 m 3 to 3 10 7 m 3 with recent return periods of 2 4 years we have reconstructed an avalanche event record for the past 45 years that indicates iliamna avalanches occur at higher frequency at a given magnitude than other mass failures in volcanic and alpine environments iliamna volcano is thus an ideal site to study such mass failures and its relation to volcanic activity in this study we present different methods that fit into a concept of 1 long term monitoring 2 early warning and 3 event documentation and analysis of ice rock avalanches on ice capped active volcanoes long term monitoring methods include seismic signal analysis and space and airborne observations landsat and aster satellite data was used to study the extent of hydrothermally altered rocks and surface thermal anomalies at the summit region of iliamna subpixel heat source calculation for the summit regions where avalanches initiate yielded temperatures of 307 to 613 k assuming heat source areas of 1000 to 25 m 2 respectively indicating strong convective heat flux processes such heat flow causes ice melting conditions and is thus likely to reduce the strength at the base of the glacier we furthermore demonstrate typical seismic records of iliamna avalanches with rarely observed precursory signals up to two hours prior to failure and show how such signals could be used for a multi stage avalanche warning system in the future for event analysis and documentation space and airborne observations and seismic records in combination with srtm and aster derived terrain data allowed us to reconstruct avalanche dynamics and to identify remarkably similar failure and propagation mechanisms of iliamna avalanches for the past 45 years simple avalanche flow modeling was able to reasonably replicate iliamna avalanches and can thus be applied for hazard assessments hazards at iliamna volcano are low due to its remote location however we emphasize the transfer potential of the methods presented here to other ice capped volcanoes with much higher hazards such as those in the cascades or the andes

anhydrite in the 1989 1990 lavas and xenoliths from redoubt volcano alaska abstract the eruption of redoubt volcano in alaska produced a moderate sulfur emission estimated at 1 10 tons so 2 but relatively small volume of lava 0 11 km with pre eruption estimates of 840 950 c and f o 2 1 5 to 2 0 log units above nno swanson s e nye c j miller t p avery v f 1994 magma mixing in the 1989 1990 eruption of redoubt volcano part ii evidence from mineral and glass chemistry journal of volcanology and geothermal research 62 453 468 petrologic estimates of sulfur production sigurdsson h devine j d davis a n 1985 the petrologic estimation of volcanic degassing jokull 35 1 8 from this eruption gerlach t westrich h r casadevall t j finnegan d l 1994 vapor saturation and accumulation in magmas of the 1989 1990 eruption of redoubt volcano alaska journal of volcanology and geothermal research 62 317 337 are considerably less than the measured sulfur emission leading workers to propose the existence of a pre eruption vapor phase to explain the excess sulfur initial examination of the 1989 1990 redoubt eruptive products reported anhydrite nye c j swanson s e avery v f miller t p 1994 geochemistry of the 1989 1990 eruption of redoubt volcano part i whole rock major and trace element chemistry journal of volcanology and geothermal research 62 429 452 in interstitial glass from some cognate gabbroic xenoliths but anhydrite was not noted in any of the andesites a boeing 747 encountered the ash plume from the initial eruptive phase on december 15 1989 and provided ash samples that reportedly contained gypsum bayhurst g k wohletz k h mason a s 1994 a method for characterizing volcanic ash from the december 15 1989 eruption of redoubt volcano alaska u s geological survey bulletin 2047 13 17 however the identification was based on eds analyses on a sem and the mineral could have been anhydrite reexamination of the 1989 1990 redoubt lavas and xenoliths revealed the presence of rare fine grained 10s of microns anhydrite crystals in the lavas and larger 10s to 100s of microns crystals in the xenoliths the anhydrite forms elongate crystals with sharp contacts against the groundmass glass grains show rounded terminations suggesting breakdown of the anhydrite perhaps during eruption the anhydrite is a sparse phase in the lavas but occurs in lavas and cognate xenoliths erupted throughout the eruption the presence and breakdown of anhydrite may explain the excess sulfur associated with the redoubt eruption a careful search of other freshly erupted andesitic lavas solidified from hydrous oxidizing magmas should reveal other examples of anhydrite and cause a rethinking of the source of sulfur in volcanic emissions

an interpretation of trends in grain size measures abstract the mean grain size sorting and skewness of a sedimentary deposit are dependent on the sediment grain size distribution of its source and the sedimentary processes of i winnowing erosion ii selective deposition of the grain size distribution in transport and iii total deposition of the sediment in transport if a source sediment undergoes erosion and the resultant sediment in transport is deposited completely the deposit must be finer better sorted and more negatively skewed than the source this trend is referred to as case i the lag remaining after erosion on the other hand must be coarser better sorted and more positively skewed case ii if sediment in transport undergoes selective deposition the resultant deposit can either be finer case iiia or coarser cas iiib than the source but the sorting will be better and the skew more positive although exceptions to these trends may occur they suggest that comparison of one sediment must be made with another for the proper identification of the sedimentary process and therefore it is not possible for a single grain size distribution to identify the depositional environment the trends also suggest that the skewness of a grain size distribution has been widely misinterpreted and implies neither the truncation of one of the tails nor the mixing of more than one mode rather a skewed sediment is the natural result of the sedimentary process in a system of related environments these trends can be used to identify both the probable source and the probable deposit and by inference the net sediment transport paths among sedimentary deposits such an analysis provides a rapid understanding of the sedimentary processes identifies patterns of erosion and accretion and may suggest transport processes

factors controlling sulfur concentrations in volcanic apatite apatite crystals from two types of samples were analyzed by electron microprobe for 15 major and trace elements 1 apatite in h2o and s saturated experimental charges of the 1982 el chichon trachyandesite and 2 apatite in volcanic rocks erupted from 20 volcanoes the so3 contents of the experimental apatite increase with increasing oxygen fugacity from 0 04 wt in reduced charges buffered by fayalite magnetite quartz o2 fmq to 1 0 2 6 wt in oxidized charges buffered by manganosite hausmanite mnh or magnetite hematite mth the so3 contents of mnh and mth buffered apatite also generally increase with increasing pressure from 2 to 4 kbar and decreasing temperature from 950 to 800 8c the partition coefficient for so 3 between apatite and oxidized melt increases with decreasing temperature but appears to be independent of pressure apatites in volcanic rocks show a wide range of so3 contents 0 04 to 0 63 wt our sample set includes one group known to contain primary anhydrite and a second group inferred to have been free of primary anhydrite no systematic differences in apatite s contents are observed between these two groups our study was initiated to define the factors controlling s contents in apatite and to evaluate the hypothesis that high s contents in apatite could be characteristic of s rich anhydrite bearing magmas such as those erupted from el chichon in 1982 and pinatubo in 1991 this hypothesis is shown to be invalid probably chiefly a consequence of the slow intra crystalline diffusion that limits re equil ibration between early formed apatite and the evolving silicate melt contributing factors include early crystallization of most apatite over a relatively small temperature interval common late stage magmatic enrichment of s progressive oxidation during magmatic evolution and strong controls on s contents in apatite exerted by temperature and o2 pressure

influence of shoreface morphological changes since the 19th century on nearshore hydrodynamics and shoreline evolution in wissant bay northern france abstract located at the southern approaches to the north sea the wissant bay has had the most rapidly eroding shoreline in mainland france in recent decades using historical bathymetries spanning the second half of the 19th century to the present an analysis of the long term coastal morphological changes and evolution of a prominent sand bank line bank extending across the bay was carried out a period of overall seabed erosion with a width reduction of the line bank a deepening of the channel between the line bank and the coast and a lowering of the foreshore have been ongoing since the middle of the 20th century this phase followed a period of channel infill in the western part of wissant bay in the early 20th century numerical modelling of wave propagation and tidal circulation was performed using the telemac suite of models to get some insights into the influence of these bathymetry changes on coastal hydrodynamics two seabed configurations were identified revealing a contrasting hydrodynamic circulation an erosional one induces an acceleration of tidal currents and an increase in wave height conversely accumulation leads to an hydrodynamic regime weakening with a decrease in current velocity and to more wave energy dissipation in the nearshore this study highlights the role of morphological feedbacks between nearshore morphology and coastal hydrodynamics the identification of these feedbacks mechanisms at a historical time scale is important to assess potential drivers of coastal changes

on the behaviour of tidal current directions due to the presence of submarine sand waves abstract side scan sonar records reveal that the crestlines of megaripples in the troughs as well as on both sides of the slopes of sand waves form an angle with the crestline of the sand waves itself the existence of such an angle between the crestline of megaripples and sand waves indicates that changes of the tidal current direction across sand waves can be expected direct evidence that such tidal current direction changes occur above sand waves is derived from measurements of the air sea interaction drift buoy asib system at the sea surface and from side scan sonar records of the sea bed both systems were operated from on board research vessels during two c star field experiments in the hoek van holland study area off the dutch coast in april 1996 and in april 1997 measurements performed by electrical resistance wires on board the asib system indicate that variations of the short and moderate wave direction and the wave directional spread are associated with changes of the tidal current direction at the sea surface according to the results of the measurements derived from the asib system and the side scan sonar records it can be summarised that the variation of the direction of short and moderate period water waves as well as changes of the tidal current speed and direction across large sand waves must be considered for correct modelling of the wave current interaction mechanism

grain sorting effects on the formation of tidal sand waves a model is developed to investigate the process which leads to the formation of sand waves in shallow tidal seas characterized by a heterogeneous sea bed composition the main goal of the analysis is the evaluation of the effects that a graded sediment has on the formation of the bottom forms and the investigation of the sorting process induced by the growth of the bottom forms the analysis is based on the study of the stability of the flat bed configuration i e small amplitude perturbations are added to the flat bottom and a linear analysis of their time development is made for an oscillatory tidal current dominated by one tidal constituent the results show that the graded sediment can stabilize or destabilize the flat bottom configuration with respect to the uniform sediment case depending on the standard deviation sigma of the grain size distribution and on the ratio r over cap between the horizontal tidal excursion and the water depth for moderate values of r over cap i e values just larger than the critical value for which the sediment is moved and sand waves appear the presence of a sand mixture stabilizes the flat bed on the other hand for large values or r over cap the mixture has a destabilizing effect in both cases the effect that a sand mixture has on the stability of the flat bed configuration is relatively small moreover for moderate values of r over cap the fine fraction of the mixture tends to pile up at the crests of the bottom forms while the coarse fraction moves towards the troughs for large values or r over cap the grain size distribution depends on the value of sigma the results are physically interpreted and provide a possible explanation of the apparently conflicting field observations of the grain size distribution along the sand wave profile carried out in the north sea

sedimentology and stratigraphy of a tidal sand bank in the southern north sea abstract a set of 125 vibrocores collected on the basis of a dense grid of high and very high resolution seismic coverage over a tidal sand bank the middelkerke bank provides a unique opportunity to reconstruct the stratigraphy and sedimentary facies succession of such a large tidal sand body five sedimentary facies are distinguished they range from clay to pebble and display various types of bedding and lithologies corresponding to different depositional environments a succession of four late quaternary units are defined on the basis of the stratigraphic framework given by the seismic grid the first unit represents the infill of a palaeovalley system by tidally influenced sedimentary facies it is followed by extensive and thick lagoonal or sub tidal deposits the third unit reveals the development of a landward prograding storm dominated sand bar this unit is dominated by clean fine sand the last unit corresponds to the present day tidal sand bank it shows the typical offshore tidal sand bank facies cross bedded sand prograding in an offshore direction like other banks located in a more offshore position the middelkerke bank is a composite bank incorporating facies which do not occur in classic tidal sand banks

volcanic evolution of volcan aucanquilcha a long lived dacite volcano in the central andes of northern chile volcan aucanquilcha northern chile has produced 37 km3 of dacite 63 66 wt silica mainly as lavas with ubiquitous magmatic inclusions 59 62 wt silica over the last 1 million years a pyroclastic flow deposit related to dome collapse occurs on the western side of the edifice and a debris avalanche deposit occurs on the eastern side the 6 000 m high edifice defines a 9 km e w ridge and lies at the center of a cluster of more than 15 volcanoes the aucanquilcha volcanic cluster that has been active for at least the past 11 million years the e w alignment of vents is nearly orthogonal to the arc axis a majority of volcan aucanquilcha was constructed during the first 200 000 years of eruption whereas the last 800 000 years have added little additional volume the peak eruptive rate during the edifice building phases was 0 16 km3 ka and the later eruptive rate was 0 02 km3 ka comparable dacite volcanoes elsewhere show a similar pattern of high volcanic productivity during the early stages and punctuated rather than continuous activity volcan aucanquilcha lavas are dominated by phenocrysts of plagioclase accompanied by two populations of amphibole biotite clinopyroxene fe ti oxides and or orthopyroxene accessory phases include zircon apatite and rare quartz and sanidine one amphibole population is pargasite and the other is hornblende the homogeneity of dacite lava from volcan aucanquilcha contrasts with the heterogeneity 52 66 wt silica at nearby volcan ollague which has been active over roughly the same period of time we attribute this homogeneity at aucanquilcha to the thermal development of the crust underneath the volcano resulting from protracted magmatism there whereas volcan ollague lacks this magmatic legacy

source mechanism of the magnitude 7 2 grand banks earthquake of november 1929 double couple or submarine landslide we have examined p s and surface waves derived from seismograms that we collected for the 1929 grand banks canada earthquake this event is noteworthy for the sediment slide and turbidity current that broke the trans atlantic cables and for its destructive tsunami both the surface wave magnitude m s and the body wave magnitude m b calculated from these seismograms are 7 2 fault mechanisms previously suggested for this event include a nw se striking strike slip mechanism and an approximately e w striking thrust mechanism in addition because of the presence of an extensive area of slump and turbidity current there exists the possibility that sediment slumping could also be a primary causative factor of this event we tested these fault models and a horizontal single force oriented n5 w model representing a sediment slide against our data among these models only the single force model is consistent with the p s and surface wave data our data however do not preclude fault models which were not tested from the spectral data of love waves at a 50 sec period we estimated the magnitude of the single force to be about 1 4 10 20 dynes from this value we estimated the total volume of sedimentary slumping to be about 5 5 10 11 m 3 which is approximately 5 times larger than a recent estimate of volume from in situ measurements the difference in estimates of overall volume is likely due to a combination of the inherent difficulty in estimating accurately the displaced sediments from in situ measurements and of inadequacy of the seismic model or perhaps because not only the slump but also a tectonic earthquake could have been the cause of this event and contributed significantly to the waveforms studied

an insar based survey of volcanic deformation in the central andes we extend an earlier interferometric synthetic aperture radar insar survey covering about 900 remote volcanos of the central andes 14 27 s between the years 1992 and 2002 our survey reveals broad 10s of km roughly axisymmetric deformation at 4 volcanic centers two stratovolcanoes are inflating uturuncu bolivia and hualca hualca peru another source of inflation on the border between chile and argentina is not obviously associated with a volcanic edifice here called lazufre and a caldera cerro blanco also called robledo in northwest argentina is subsiding we explore the range of source depths and volumes allowed by our observations using spherical ellipsoidal and crack like source geometries we further examine the effects of local topography upon the deformation field and invert for a spherical point source in both elastic half space and layered space crustal models we use a global search algorithm with gradient search methods used to further constrain best fitting models inferred source depths are model dependent with differences in the assumed source geometry generating a larger range of accepted depths than variations in elastic structure source depths relative to sea level are 8 18 km at hualca hualca 12 25 km for uturuncu 5 13 km for lazufre and 5 10 km at cerro blanco deformation at all four volcanoes seems to be time dependent and only uturuncu and cerro blanco were deforming during the entire time period of observation inflation at hualca hualca stopped in 1997 perhaps related to a large eruption of nearby sabancaya volcano in may 1997 although there is no obvious relation between the rate of deformation and the eruptions of sabancaya we do not observe any deformation associated with eruptions of lascar chile at 16 other volcanoes that had recent small eruptions or fumarolic activity or associated with a short lived thermal anomaly at chiliques volcano we posit a hydrothermal system at cerro blanco to explain the rate of subsidence there for the last decade we calculate the ratio of the volume of magma intruded to extruded is between 1 10 and that the combined rate of intrusion and extrusion is within an order of magnitude of the inferred geologic rate

sedimentary processes bedforms and facies associated with a coastal headland portland bill southern uk this investigation presents new findings on the sedimentary processes and deposits associated with a coastal headland in a mixed tide wave setting using data compilation bathymetric and sediment distribution maps collected field data side scan sonar and seabed sampling and sand transport modelling sand transport pathways are described a on the basis of coupled system seabed morphology sand distribution and b from bed shear stress and medium grained sand transport rate simulations the presence of a sequence of sedimentary deposits associated with a complex suite of bedforms and sedimentary facies within an overall framework of limited sediment supply represents a gradient in shear stress and sand transport towards the headland these sequences are observed on both sides of portland bill tending towards a symmetrical distribution the sedimentary facies distribution is combined with sand transport rates and maximum bed shear stress distribution to suggest a conceptual model for sand dispersal and deposit formation around headlands sand dispersal can be explained in terms of a headland associated eddy bedload convergent zone concept maximum bed shear stress is observed at the tip of the headland and is associated with bedrock exposed on the seabed the sequence of sedimentary deposits away from the headland is sandbanks shambles and portland banks sand gravel flats sand shoals adamant and west shoals and rippled sand sheets the banks lie in an area of very high bed shear stress u 0 08 m s 1 this is greater than that for other tidal sandbanks not associated with headlands bank formation is the result of the strong convergent transport component enhanced by the development of transient headland eddies the sandbanks are to some extent also morphologically controlled i e the portland bank is affected by the deep bathymetry to the west side of portland bill the principal characteristic of headland associated eddy bedload convergent zones is the development of bed shear stress convergent zones on both sides of the headland two conceptually distinct regions of sedimentary processes associated with the coastal headlands are recognised a an inner zone with increasing gradients in sand transport and bed shear stress towards the headland and b an outer zone in which sand transport is away from the headland associated with a decrease in the bed shear stress these zones merge into a bed shear stress sand bedload transport convergent zone which enhances the formation of sandbanks around headlands thus headland associated sedimentary deposits are a complex system compared with the occurrence of isolated sandbanks described elsewhere the identification of a sequence of deposits related to and extending much farther from the headland than previously supposed demonstrates that the tidal stirring concept of headland associated sandbanks may explain the formation of headland associated sandbanks however it ignores the presence of a suite of sedimentary deposits around such headlands the conceptual model proposed here provides a sedimentological perspective on the formation and occurrence of headland associated deposits including sandbanks

precursory swarms of long period events at redoubt volcano 1989 1990 alaska their origin and use as a forecasting tool abstract during the eruption of redoubt volcano from december 1989 through april 1990 the alaska volcano observatory issued advance warnings of several tephra eruptions based on changes in seismic activity related to the occurrence of precursory swarms of long period lp seismic events dominant period of about 0 5 s the initial eruption on december 14 occurred after 23 years of quiescence and was heralded by a 23 hour swarm of lp events that ended abruptly with the eruption after a series of vent clearing explosions over the next few days dome growth began on december 21 another swarm with lp events similar to those of the first began on the 26th and ended in a major tephra eruption on january 2 eruptions continued over the next two weeks and then ceased until february 15 when a large eruption initiated a long phase of repetitive dome building and dome destroying episodes that continued into april warnings were issued before the major events on december 14 and january 2 but as the eruptive sequence continued after january 2 the energy of the swarms decreased and forecasting became more difficult a significant but less intense swarm preceded the february 15 eruption which was not forecast this eruption destroyed the only seismograph on the volcanic edifice and stymied forecasting until march 4 when the first of three new stations was installed within 3 km of the active vent from march 4 to the end of the sequence on april 21 there were eight eruptions six of which were preceded by detectable swarms of lp events although weak these swarms provided the basis for warnings issued before the eruptions on march 23 and april 6 the initial swarm on december 13 had the following features 1 short duration 23 hours 2 a rapidly accelerating rate of seismic energy release over the first 18 hours of the swarm followed by a decline of activity during the 5 hours preceding the eruption 3 a magnitude range from 0 4 to 1 6 4 nearly identical lp signatures with a dominant period near 0 5 s 5 dilatational first motions everywhere and 6 a stationary source location at a depth of 1 4 km beneath the crater this occurrence of long period events suggests a model involving the interaction of magma with groundwater in which magmatic gases steam and water drive a fixed conduit at a stationary point throughout the swarm the initiation of that sequence of events is analogous to the failure of a pressure relief valve connecting a lower supercharged magma dominated reservoir to a shallow hydrothermal system a three dimensional model of a vibrating fluid filled crack recently developed by chouet is found to be compatible with the seismic data and yields the following parameters for the lp source crack length 280 380 m crack width 140 190 m crack thickness 0 05 0 20 m crack stiffness 100 200 sound speed of fluid 0 8 1 3 km s compressional wave speed of rock 5 1 km s density ratio of fluid to rock 0 4 and ratio of bulk modulus of fluid to rigidity of rock 0 03 0 07 the fluid filled crack is excited intermittently by an impulsive pressure drop that varies in magnitude within the range of 0 4 to 40 bar such disturbance appears to be consistent with a triggering mechanism associated with choked flow conditions in the crack

monitoring volcanic thermal anomalies from space size matters measuring temperatures on volcanoes from space provides important constraints on the transfer of mass and heat to the earth s surface time series of multispectral infrared images acquired by the advanced spaceborne thermal emission and reflection radiometer aster between 2000 and 2009 were inspected to investigate fluctuations in thermal anomalies at both mount erebus antarctica and lascar volcano chile several thermal metrics were explored i maximum pixel temperatures above background ii the spatial extent of low moderate and high temperature anomalies and iii the spatial extent of short wave infrared anomalies the maximum pixel temperature metric correlated to eruptive events at lascar volcano yet displayed significant scatter at erebus the spatial extent of both temperature and short wave infrared anomalies correlates well with eruptive activity at both volcanoes limited variation in the size of thermal anomalies was observed at erebus throughout the time series due to the stability of a long lived lava lake with the exception of a seasonal expansion in low temperature anomalies associated with localized snow melt at the peak this finding has implications for the interpretation of low temperature anomalies at other volcanoes at least two different types of precursory signals are identified at lascar i a gradual increase and ii a dip in the size and intensity of thermal anomalies these thermal precursors appear to be associated with different eruptive styles the former precedes a relatively shallow short lived eruption the later a prolonged eruptive period such thermal precursors could therefore help to constrain not only the timing but also the style and duration of an imminent eruptive episode

morphology and sedimentation of sand bodies in the tidal shelf sea of eastern taiwan strait abstract changyun ridge located in the central eastern part of the taiwan strait consists of sand bodies the eastern changyun sand ridge 65 km long and 13 km wide a sand shoal 53 km long and 22 km wide and a sand sheet 60 km long 40 km wide and 22 m in height the eastern changyun sand ridge and the sand sheet are oriented parallel to the coastline but the sand shoal is perpendicular to the coast the eastern changyun sand ridge is oriented oblique to the narrow reciprocal tidal ellipses the sand shoal and sand sheet are normal and oblique to the broad rotary tidal ellipses respectively surface sediments of the changyun ridge are composed of very fine to coarse sands in a progressive seaward decrease in grain size the configuration and spatial distribution of these three sand bodies with corresponding decrease in tidal speed and in grain size together suggest that the sand bodies developed sequentially from the near shore to the offshore area presumably in response to westward shifting of the north flowing tidal currents sand waves occur widespreadly on the sand shoal and seldom appear on the eastern changyun sand ridge occurrences of sand waves suggest that the sand shoal is considered to be active and the sand ridge to be moribund chirp sonar profiles reveal the internal structures of the sand shoal showing a relatively flat basal reflector underlying the sand body the thickness of the sand accumulation is about 22 25 m and it is firstly determined by the depth difference between the modern sea bed and the basal reflector to better describe morphology and document dimensions of these sand bodies the presence of several short inclined reflectors dipping northeast and down lapping the basal flat reflector indicate that sands are most likely transported from south to north for the sand shoal the sand bodies of the changyun ridge rest upon a relative flat basal reflector suggesting a mainly hydrodynamically controlled origin the variations of the hydrodynamic conditions mainly tidal currents on the central eastern taiwan strait shelf seem to be the primary control on sedimentary facies shift morphological changes and sand ridge growth for the changyun ridge the changyun ridge is ongoing to receive sands from the south in a transitional development stage as long as the sand supply continues and over a longer time period the sand bodies eventually will develop into three individual linear sand ridges separated by swales oriented parallel to the peak currents

immiscible iron and silica rich melts and magnetite geochemistry at the el laco volcano northern chile evidence for a magmatic origin for the magnetite deposits abstract the pliocene el laco volcano northern chile hosts a large iron oxide deposit that occurs as stratiform bodies with underlying feeder zones dykes hosted by hydrothermally altered andesite lava flows phenocrysts in the andesite hosting the mineralization have abundant melt inclusions displaying strong evidence for melt immiscibility most of the pyroxene and the plagioclase phenocrysts contain abundant fe rich melt inclusions with low ti magnetite pyroxene apatite and minor anhydrite the glassy groundmass of these melt inclusions is chemically similar to sub alkaline rhyolitic rocks e g 70 sio 2 si al k na rich large resorbed plagioclase phenocrysts also host another group of melt inclusions with a similar silica rich groundmass but hosting evenly distributed immiscible and complex fe rich spheroidal globules that are enriched in ti p mg ca and s these immiscible globules and the ti poor magnetite daughter crystals in the melt inclusions in pyroxene phenocrysts are thought to be the parental iron oxide melt liquid after which their coalescence leads to the formation of magnetite melts from which the magnetite that formed the el laco iron oxide deposits crystallized and were emplaced the mineralogy and chemistry of these melt inclusions tracks the chemical evolution of the fe ox melts and supports a magmatic origin for the el laco magnetite this is mainly recognized by the continuum of compositions between volcanic separate immiscible fe oxide melts and magnetite ores the trace element composition of the fe rich melt inclusions and of the magnetite in the host andesite and the ore bodies reveals a systematic depletion in most of the elements that are compatible in magnetite the ti content of the magnetite also records the separation and crystallization of the iron rich melt typically the magnetite in the orebodies and the melt inclusions is ti poor compared with titanomagnetite found in the andesite groundmass and that hosted by the hydrothermally altered andesite that is interpreted as being attributed to its crystallization under oxidizing and sulfur rich conditions the cause of this unusual chemical behaviour is probably due to the mechanism s governing the crystallization of the iron rich melt including the temperature changes in chemistry prior to eruption and an increase in the redox conditions of the silicate melt magma oxidation facilitates the prevalence of ferric iron and formation of ti poor magnetite preventing the incorporation of ti in the exsolving iron rich melt the likely reason for the formation of these large ti poor magnetite deposits is likely the contamination of the parental andesitic magma by oxidized crustal rocks and concomitant crystallization of magnetite under oxidizing conditions something that had a significant impact on the elemental distribution

the eruptive chronology of the ampato sabancaya volcanic complex southern peru abstract we have reconstructed the eruptive chronology of the ampato sabancaya volcanic complex southern peru on the basis of extensive fieldwork and a large dataset of geochronological 40 k 40 ar 14 c and 3 he and geochemical major and trace element data this volcanic complex is composed of two successive edifices that have experienced discontinuous volcanic activity from middle pleistocene to holocene times the ampato compound volcano consists of a basal edifice constructed over at least two cone building stages dated at 450 400 ka and 230 200 ka after a period of quiescence the ampato upper edifice was constructed firstly during an effusive stage 80 70 ka and then by the formation of three successive peaks the northern southern 40 20 ka and central cones 20 10 ka the southern peak which is the biggest experienced large explosive phases resulting in deposits such as the corinta plinian fallout during the holocene eruptive activity migrated to the ne and constructed the mostly effusive sabancaya edifice this cone comprised many andesitic and dacitic blocky lava flows and a young terminal cone mostly composed of pyroclastic material most samples from the ampato sabancaya define a broad high k magmatic trend composed of andesites and dacites with a mineral assemblage of plagioclase amphibole biotite ortho and clino pyroxene and fe ti oxides a secondary trend also exists corresponding to rare dacitic explosive eruptions i e corinta fallout and flow deposits both magmatic trends are derived by fractional crystallisation involving an amphibole rich cumulate with variable amounts of upper crustal assimilation a marked change in the overall eruptive rate has been identified between ampato 0 1 km 3 ka and sabancaya 0 6 1 7 km 3 ka this abrupt change demonstrates that eruptive rates have not been homogeneous throughout the volcano s history based on tephrochronologic studies the late holocene sabancaya activity is characterised by strong vulcanian events although its erupted volume remained low and only produced a local impact through ash fallout we have identified at least 6 eruptions during the last 4 5 ka including the historical ad 1750 1784 and 1987 1998 events on the basis of this recurrent low to moderate explosive activity sabancaya must be considered active and a potentially threatening volcano

fe ti p oxide melts generated through magma mixing in the antauta subvolcanic center peru implications for the origin of nelsonite and iron oxide dominated hydrothermal deposits commingling and mixing of k rich basaltic absarokitic and partially crystallized strongly peraluminous s type monzogranitic rhyodacitic magmas have been documented in the 23 8 ma antauta hypabyssal complex of the picotani group puno southeastern peru magma mixing generated intermediate andesitic rocks hosting small volumes of sio2 poor 8 4 wt fe oxide rich melt with tio2 30 wt and p2o5 23 wt contents corresponding to those of the genetically problematic nelsonite ore type microscopic spheres of this composition are embedded in rhyolitic glass surrounding both extensively melted sieve textured plagioclase xenocrysts derived from the rhyodacitic magma and cognate orthopyroxene phenocrysts exhibiting replacive fe rich mantles the andesites also contain up to 15 modal percent of strongly resorbed granite derived quartz xenocrysts with glass orthopyroxene clinopyroxene coronas intense quasipervasive silicification of the fractionating mafic magma is inferred to have displaced the hybridized melt composition into the stable liquid immiscibility region of the na2o k2o mgo al2o3 feo tio2 mno cao p2o5 sio2 h2o system some fe ti p rich bodies have 30 to ca 55 wt percent sio2 and exhibit broadly linear trends of major oxides versus sio2 but the more sio2 deficient spheres exhibit extreme compositional scatter implying the existence of complex immiscibility fields within the parental protonelsonitic melt domain in contrast the predominant apparently conjugate silicate glass 70 78 wt sio2 is relatively consistent in composition and equivalent to a syenogranite and or rhyolite the extreme local variability in the composition of the oxide rich melt bodies may directly reflect their formation in a magma mixing environment in which temperature varied over at least 200 to 300 c such a compositional range also apparent in large scale nelsonite occurrences indicates that strong fractionation of mafic melts under conditions precluding ferroan spinel crystallization may not be a prerequisite for oxide silicate immiscibility and hence nelsonite formation specular hematite is widely disseminated in the glassy andesites commonly clustered around resorbed quartz xenocrysts and microscopic bodies of illite montmorillonite chlorite occur in contact with unaltered glass in unveined andesites implying endogenous sources of respectively iron and hydrothermal fluid moreover the upper paleozoic clastic sedimentary envelope of the antauta center at pucacancha hosts hydrothermal breccias and veins rich in hematite barite quartz chalcedony and epidote with lesser anhydrite and traces of u th ree rich allanite chalcopyrite and molybdenian scheelite this unambiguously hydrothermal mineralization is analogous to that of the iron oxide copper gold clan and we propose that the development of some deposits of this type may have been triggered by the formation and subsequent vesiculation of immiscible fe rich oxide melt during the mixing of mafic and felsic magmas

a single force model for the 1975 kalapana hawaii earthquake a single force mechanism is investigated as the source of long period seismic radiation from the 1975 kalapana hawaii earthquake m s 7 1 the observed love wave radiation pattern determined from the spectra of world wide standard seismograph network and high gain long period records at 100 s is two lobed with azimuth consistent with a near horizontal single force acting opposite strike 330 to the observed displacement direction of the earthquake this pattern is inconsistent with the expected double couple pattern assuming a form of the force time history of a one cycle sinusoid the total duration of the event estimated from rayleigh waves at two international deployment of accelerometers stations is approximately 180 s the peak amplitude f o of the time function is 1 10 15 n from amplitudes of love and rayleigh waves the interpretation is that the bulk of the seismic radiation was produced by large scale slumping of a large area of the south flank of kilauea volcano the single force is a crude representation of the effect on the earth of the motion of a partially decoupled large slide mass using the mass estimated from the tsunami generation area 10 15 10 16 kg the peak acceleration of the slide block 0 1 1 m s 2 inferred from the seismic force is comparable with the acceleration due to gravity on a gently inclined plane the slump model for the kalapana earthquake is also more qualitatively consistent with the large horizontal deformation 8 m on land and tsunami associated with the earthquake which are difficult to explain with the conventional double couple source model the single force source has been used previously to model the long period seismic waves from the landslide accompanying the eruption of mount st helens volcano and may explain other anomalous seismic events as being due to massive slumping of sediments or unconsolidated material and not to elastic dislocation

source mechanism of seismic waves excited by pyroclastic flows observed at unzen volcano japan a number of pyroclastic flows have occurred at unzen volcano unzendake in kyushu japan since the end of may 1991 seismic waves which are excited by the pyroclastic flows contain low frequency components in their initial phases they were recorded by two long period seismographs in the seismic network deployed around the volcano this paper is an attempt to clarify the source mechanism of the seismic waves triggered by the pyroclastic flows visual data show that the pyroclastic flow which excites these seismic waves occurs in three successive steps first fall of a lava dome front dome collapse then its collision with the mountain slope and its fragmentation into small blocks and finally descent of the blocks along the slope we can build a simple mechanical model which gives theoretical time histories of vertical and horizontal components of a force acting on the spot of dome collapse on the mountain slope the model of a mechanism generating these seismic waves was constructed through analysis of the low frequency components with the aid of inversion methods which can estimate source time function of the force acting on the source the estimated time histories are close to those predicted by the simple mechanical model we conclude that the three steps mentioned above are essential in exciting the low frequency seismic signals this idea is supported by the fact that the volume of fallen material estimated from the visual data is proportional to the maximum vertical amplitude of seismic waves triggered by the pyroclastic flow this relation holds for the two pyroclastic flows of june 22 1991 and may 28 1992 which were studied in this paper these findings seismically confirm a visual observation that the pyroclastic flow is a result of dome collapse

the magmatic to magmatic hydrothermal evolution of the el laco deposit chile and its implications for the genesis of magnetite apatite deposits

vibration induced dynamical weakening of pyroclastic flows insights from rotating drum experiments pyroclastic flows are characterized by their high mobility which is often attributed to gas fluidization of the usually fine and or low density particles however the physical mechanism that might drive sustained fluidization of pyroclastic flows over extraordinarily long runout distances is elusive in this letter it is proposed that a powerful mechanism to weaken the frictional resistance of pyroclastic flows would arise from the prolonged and intense mechanical vibrations that commonly accompany these dense gravitational fluid particle flows the behavior of fine powders in a slowly rotating drum subjected to vibrations suggests that fluid particle relative oscillations in granular beds can effectively promote the pore gas pressure at reduced shear rates dynamical weakening as caused by the enhancement of pore fluid pressure may be an important mechanism in any geophysical process that involves vibrations of granular beds in a viscous fluid this is particularly relevant for granular flows involving large amounts of fine and or light particles such as pyroclastic density currents

architecture and long term evolution of a tidal sandbank the middelkerke bank southern north sea abstract the internal structure of the middelkerke bank one of the flemish banks located in the southern north sea off the coast of oostende belgium has been studied in the framework of the marine science and technology mast program co funded by the european community a dense grid of high and very high resolution seismic profiles has been used as well as several vibrocorings seven major seismic units can be identified in the quaternary sediments bounded by major discontinuities correlated across the whole study area the lower units clearly appear as being deposited during periods of relative low sea level channel infillings shoreface estuarine and or ebb tidal delta deposits the present shape of the bank results partly from recent erosional processes reworking the underlying deposits thus the lower part of the bank as a morphological feature does not consist of offshore tidal sands the master bedding of the upper part of the bank consists of inclined reflectors dipping at an angle of about 5 in the same direction as the bank s steep face these reflectors very similar to those described by houbolt 1968 are interpreted as being the result of alternating periods of deposition and erosion related to the episodic combination of tidal currents and storms

the 2007 caldera collapse of piton de la fournaise volcano source process from very long period seismic signals abstract in april 2007 piton de la fournaise volcano experienced its largest caldera collapse in at least 300 yr this event consisted of a series of 48 subsidence increments characterized by very long period vlp seismic signals equivalent to m w 4 4 to 5 4 source analysis of vlp events suggests a piston like mechanism with a collapsing crack source corresponding to the contraction of the magma reservoir and a single force representing the collapse of the above rock column we show that the collapse dynamics is primarily controlled by magma withdrawal from the reservoir which was likely in a bubbly state at the time of the event similar to the 2018 kilauea collapse we observe a reduction of the time interval between successive subsidence increments which results from the acceleration of magma withdrawal and a progressive weakening of the edifice at the beginning of the sequence

petrogenetic and metallogenetic responses to miocene slab flattening new constraints from the el indio pascua au ag cu belt chile argentina the eocene miocene volcanic and hypabyssal rocks of the el indio pascua au ag cu belt in the southern central andean flat slab region are medium high k calc alkaline arc suites ranging in composition from andesite to rhyolite a significant transition in magmatic trace element chemistry coinciding with a pronounced reduction in magma output occurred in the late middle miocene as documented by 40ar 39ar geochronology the upper eocene lower middle miocene rocks exhibit low sr y ratios 50 minor heavy ree fractionation with sm yb ratios not exceeding 3 5 and in some cases minor negative eu anomalies in contrast the largely dacitic rocks erupted after ca 13 ma are depleted in y 10 ppm have generally high but variable sr y ratios 30 200 exhibit moderate middle and heavy ree fractionation sm yb 3 7 5 9 and lack negative eu anomalies the latter features are characteristic of adakitic suites i e slab melts but the regional temporal and spatial distribution of arc magmatism precludes a major magma source in the downgoing slab this evolution is interpreted as reflecting a progressive increase in pressure and the availability of water in the lower crustal site of magma generation establishing both garnet and hornblende as major stable phases in the residuum the pressure in the lower crust increased in response to episodic crustal thickening related to the shallowing of the slab a process recorded by the incision of three regional pediplains over the period 17 6 ma elimination of the subarc asthenospheric mantle and much of the lithospheric mantle by ca 10 ma permitted direct incursion of slab derived highly oxidised metal and volatile rich supercritical fluids into the lower crust stimulating melting of mafic garnet amphibolitic and eclogitic assemblages

sand suspension and transport on the middelkerke bank southern north sea by storms and tidal currents abstract estimates have been made of the suspended sand transport at two sites on the middelkerke bank in the southern north sea from suspended sand profiles and current meter measurements over a period of approximately 40 days sand resuspension was mainly due to waves while transport was dominated by a few hours when large waves coincided with peak flood currents soulsby s relationships for the stress under combined currents and waves were found to be poor predictors for the observed near bed concentrations the mean stress m predicting 45 of the variance while the maximum stress max predicted just 15 and overestimate the effects of the waves when the influence of the stress due to the waves is reduced the variance explained increases to 67 the sand transport rate on the steep slope of the bank was 10 times that of the southern end and was up slope at 25 to the bank axis in the direction of the major axis of the tidal ellipse the transport on the steep slope was mainly in the size range 100 140 m which did not occur in any significant proportion in samples of the sea bed at that site but was advected from deeper water to the southeast excluding this finer component the transport rates of coarser sand 200 m at the two sites were similar over the 40 day period the up slope transport during storms suggests that waves play an important part in the bank maintenance and are not simply the mechanism which prevents the continual growth of the sand bank due to asymmetrical transport by the tidal currents alone the transport rates are consistent with a time scale of 10 2 10 3 years for the formation of the middelkerke bank

sand wave migration predictor based on shape information 1 migration of offshore seabed waves which endangers the stability of pipelines and communication cables is hard to measure the migration rates are small compared to the measurement errors here sandwave migration rates are determined from the change in the crest position deduced from long time series of bathymetric echo sounding data the crests are identified as local extremes in a bathymetric profile after low pass filtering this approach is applied to both two dimensional data and to profiles along pipelines a consistent migration rate of several meters per year is found a strong correlation between the sandwave shape and the migration rate is translated in a migration predictor the predictor assumes that the sandwaves migrate in the direction of the steepest slope following a quadratic relation with the asymmetry furthermore it is included that longer waves travel faster but higher waves travel slower the predictor is calibrated against data from nine areas and validated using three additional areas an error analysis using markers shows that the error of the predictor is small compared to the noise in the individual crest position observations

the 2001 2004 dome forming eruption of shiveluch volcano kamchatka observation petrological investigation and numerical modelling there have been three episodes of lava dome growth at shiveluch volcano kamchatka since the plinian explosive eruption in 1964 the episodes in 1980 1981 1993 1995 and 2001 2004 have discharged at least 0 27km 3 of silicic andesite magma a time averaged mean extrusion rate of 0 2m 3 s is thus estimated for the last 40years here the 2001 2004 activity is described and compared with the earlier episodes the recent activity involved three pulses in extrusion rate and a transition to ongoing lava extrusion estimated magma temperatures are in the range 830 to 900 c with 850 c as the best estimate using the plagioclase amphibole phenocryst assemblage and fe ti oxides melt inclusions in amphibole and plagioclase have maximum water contents of 5 1wt implying a minimum pressure of 155mpa for water saturated conditions the magma chamber depth is estimated to be about 5 6km or more a result consistent with geophysical data the thicknesses of opx mt amph reaction rims on olivine xenocrysts are used to estimate the residence time of olivine crystals in the shallow chamber in the range 2 months to 4 years suggesting replenishment of deeper magma into the shallow chamber contemporaneous with eruption the absence of decompression driven breakdown rims around amphiboles indicates ascent times of less than 7days volcanological observations of the start of the 2001 2004 episode suggest approximately 16 days for the ascent time and a conduit equivalent to a cylinder of diameter approximately 53 71m application of a conduit flow model indicates that the magma chamber was replenished during the 2001 2004 eruption consistent with the results of olivine reaction rims and that the chamber has an estimated volume of order 7km 3

sand waves and sediment transport around the end of a tidal sand bank two detailed surveys have been made of the north western end of the haisborough sand off the norfolk coast using echosounder 3 5 khz reflection profiler and side scan sonar asymmetrical sand waves indicate north westerly sand movement on the southern side and south easterly sand movement on the steeper northern side of the bank secondary superimposed megaripples which are probably better indicators of sediment movement give evidence of a cross bank component between the north westerly and south easterly facing sand waves on the tip of the bank there is a zone of symmetrical sand waves these are usually taken to indicate zero net transport but in this case the oblique orientation of megaripples in their troughs indicates transport parallel to the sand wave crests this suggests the route by which sand travels around the end of the bank to form a roughly closed circulation sediment textural parameters support the notion that sand is winnowed from the foot of the bank on both sides and is transported to the middle with an overall net transport from the south to the north analysis of charts dating back to 1886 shows that the bank is stable within the error limits of position fixing though that could allow more than 0 25 km shift to the north east in 100 years to pass undetected a box model is drawn up for the estimated sediment fluxes around the end of the bank and implications for residence times and circulation rates are drawn from it