dike intrusion associated with the 2000 eruption of miyakejima volcano japan the model for the 2000 dike intrusion event between kozushima and miyakejima volcano japan was reinvestigated after the sudden earthquake swarm in miyakejima volcano a dike intrusion of large volume was detected by the nationwide gps network geonet the displacements detected with gps stations over an area with a radius of about 200 km shows a distribution that is consistent with the dike source being located near miyakejima volcano

an overview of experimental models to understand a complex volcanic instability application to mount etna italy abstract volcanic edifices are often unable to support their own load triggering the instability of their flanks many analogue models have been aimed especially in the last decade at understanding the processes leading to volcano flank instability general behaviors were defined and the experimental results were compared to nature however available data at well studied unstable volcanoes may allow a deeper understanding of the specific processes leading to instability providing insights also at the local scale etna italy constitutes a suitable example for such a possibility because of its well monitored flank instability for which different triggering factors have been proposed in the last two decades among these factors recent insar data highlight the role played by magmatic intrusions and a weak basement under a differential unbuttressing at the volcano base this study considers original and recently published experimental data to test these factors possibly responsible for flank instability with the final aim to better understand and summarize the conditions leading to flank instability at etna in particular we simulate the following processes a the long term activity of a lithospheric boundary as the malta escarpment separating the ionian oceanic lithosphere from the continental sicilian lithosphere below the most unstable east flank of the volcano b spreading due to a weak basement with different boundary conditions c the pressurization of a magmatic reservoir as that active during the 1994 2001 inflation period d dike emplacement as observed during the major 2001 and 2002 2003 eruptions the experimental results suggest that 1 the long term activity of a lithospheric tectonic boundary may create a topographic slope which provides a differential buttressing at the volcano base a preparing factor to drive longer term 10 5 years instability on the east flank of the volcano 2 volcano spreading 4 years has limited effect on flank instability at etna 3 magmatic intrusions 1 years both in the form of mogi like sources or dikes provide the most important conditions to trigger flank instability on the shorter term

surface stresses associated with arrested dykes in rift zones many theoretical models predict that arrested dykes may generate major grabens at rift zone surfaces arrested dyke tips in eroded rift zones however are normally not associated with major grabens or normal faults that could be generated by dyke induced stresses ahead of the tips and normal faults and grabens tend to be less common in those parts of eroded rift zones where dykes are comparatively abundant similarly there are feeder dykes as well as dykes arrested a few metres below the surface that do not generate faults or grabens at the surface here i propose that this discrepancy between theoretical models and field observations may be explained by the mechanical layering of the crust numerical models presented here show that abrupt changes in young s moduli layers with high dyke normal compressive stresses stress barriers and weak horizontal contacts have large effects on the dyke induced stress fields for the models considered the surface tensile stresses induced by arrested dykes are normally too small to lead to significant fault or graben formation at the rift zone surface the only significant dyke induced surface tensile stresses 2 mpa in these models are for a dyke tip arrested at 1 km depth below the surface of a rift zone with a weak contact at 400 m depth and subject to extension that tensile stress however peaks above the ends of the weak horizontal contact which in the model considered occur at distances of 4 km to either side of the dyke and shows no simple relation to the depth to the dyke tip thus for a layered crust with weak contacts straightforward inversion of surface geodetic data to infer dyke geometries may result in unreliable results

dislocation modelling of the 1989 dike intrusion into the flank of mount etna sicily we have applied a dislocation model consisting of a dipping tabular body with constant dislocation perpendicular to the plane of the body to surface displacements related to an intrusion in the south flank of etna volcano sicily ground deformation was measured by a trilateration array and automatic tilt measurements during the 1989 eruption of the volcano significant line length changes and tilt were measured associated with an episodic 0 8 km propagation of the intrusion into the measurement array we incorporated the orientation and location of measured surface cracking into a bayesian nonlinear inversion of the data to determine the geometry of the intrusion we suggest the intrusion caused a 1 m wide opening of a vertical crack that extended from a depth of 1 km to within a few hundred meters of the surface we relate the distribution of surface cracking and seismicity associated with the fracture to stresses generated in the slip weakening zone associated with the crack edge

segmented lateral dyke growth in a rifting event at barðarbunga volcanic system iceland crust at many divergent plate boundaries forms primarily by the injection of vertical sheet like dykes some tens of kilometres long1 previous models of rifting events indicate either lateral dyke growth away from a feeding source with propagation rates decreasing as the dyke lengthens2 3 4 or magma flowing vertically into dykes from an underlying source5 6 with the role of topography on the evolution of lateral dykes not clear here we show how a recent segmented dyke intrusion in the bar arbunga volcanic system grew laterally for more than 45 kilometres at a variable rate with topography influencing the direction of propagation barriers at the ends of each segment were overcome by the build up of pressure in the dyke end then a new segment formed and dyke lengthening temporarily peaked the dyke evolution which occurred primarily over 14 days was revealed by propagating seismicity ground deformation mapped by global positioning system gps interferometric analysis of satellite radar images insar and graben formation the strike of the dyke segments varies from an initially radial direction away from the bar arbunga caldera towards alignment with that expected from regional stress at the distal end a model minimizing the combined strain and gravitational potential energy explains the propagation path dyke opening and seismicity focused at the most distal segment at any given time and were simultaneous with magma source deflation and slow collapse at the bar arbunga caldera accompanied by a series of magnitude m 5 earthquakes dyke growth was slowed down by an effusive fissure eruption near the end of the dyke lateral dyke growth with segment barrier breaking by pressure build up in the dyke distal end explains how focused upwelling of magma under central volcanoes is effectively redistributed over long distances to create new upper crust at divergent plate boundaries

the role of grain size ratio in the mobility of mixed granular beds abstract the main goal of the study was to understand the effects of grain size distribution on the stability of beds in the sand silt range which is a critical subject for the understanding of geomorphological processes in aquatic environments although theoretical models can explain the mobilization of a mixed bed there is a clear lack in knowledge regarding the stabilizing effect of non cohesive fine material to connect existing findings we analysed bed stability in relation to grain size distribution in laboratory experiments erosion experiments in an annular flume were conducted using beds of different size compositions of spherical glass beads i e a the grain size ratio rd d50 coarse d50 fine the relative size of coarse and fine grains d50 39 367 m and b the amount of fines several glass bead combinations with unimodal and bimodal grain size distributions rd 3 9 5 8 and 9 4 and varying fine fractions 10 40 dry weight were subjected to increasing flow speeds 0 01 0 19 m s 1 using acoustic doppler velocimetry adv and optical backscatter the flow profile in the vicinity of the bed surface the changes in bed morphology and the suspended sediment concentration ssc were measured a new method was developed to evaluate the bed level changes detected by the adv as a proxy for the bed mobility we found different modes of bed mobility depending on the grain size ratio for low grain size ratios an increase in the fine fraction to 40 led to increased bed level changes during the experiment and the mobilization of the mixed bed at the highest flow speed for high ratios an increase in fine fraction to 40 led to a decrease of bed level changes and the beds remained stable i e no bed forms developed even at the highest flow speed therefore increasing the amount of fine particles can lead to different modes of behaviour depending on the grain size ratio for a bimodal sediment bed with spherical grains under unidirectional flow conditions the grain size ratio can be used to estimate the bed mobility i e to evaluate if a bed is likely to mobilize at flow speeds of up to 0 19 m s 1 hence for the stability of a non cohesive mixed bed in the sand silt range the grain size ratio plays a fundamental role while the amount of fine grains is a secondary factor

creep dike intrusion and magma chamber deflation model for the 2000 miyake eruption and the izu islands earthquakes 1 analysis of global positioning system data shows shrinkage of miyake island and the widening between nijima and kozu islands during the period of the miyake island volcanic activity and the ensuing izu islands earthquakes in 2000 the estimated time evolution of a model consisting of a dike creeping faults and a mogi source suggests that a crack opening on miyake island occurred immediately after the start of the seismic activities on 26 june and ended within several days at the west coast of miyake island after the seabed eruption on 27 june magma migrated from miyake island to kozu and nijima islands within several days the estimated volume of intruded magma totals around 1 2 109 m3 associated with the magma intrusion between miyake and kozu islands left lateral and right lateral creep motions occurred in regions off the west coast of miyake island and near kozu island the accumulated moment energy is equivalent to an earthquake of mw 6 6 and 6 6 for right lateral and left lateral creeping faults respectively the estimated magma chamber continued deflation beneath the southwestern part of miyake island from 26 june totaling around 0 12 109 m3 in volume change in addition to the collapse volume of 0 6 109 m3 at the summit of mount oyama on miyake island the volume change on miyake island can be compensated by the migrated magma toward kozu island from the deflation source beneath miyake island the deflation speed of the magma chamber beneath miyake island decreased and increased before and after the eruption of 14 15 july and 18 august suggesting a change in balance of mass influx and draining out rate of the magma chamber

dyke intrusion model for the 1989 seismovolcanic activity off ito central japan in the last 10 years extensive earthquake swarms have occurred intermittently in the region east of izu peninsula central japan in july 1989 an active swarm again took place off ito city and among all past activities it was nearest to land and shallowest this activity eventually became a weak submarine eruption the first of its kind in this region associated with this event a borehole tiltmeter at 92 m depth installed several kilometers from the source area detected distinct continuous tilt change with the following features 1 tilt of 20 rad was observed within 1 week preceding the eruption 2 tilt change showed a good correlation to swarm activity 3 no obvious coeruption tilt change was observed on the basis of the tilt record and swarm distribution we constructed a dislocation model to interpret the seismovolcanic activity the model consists of two tensile faults each of which corresponds to magma intrusions in may and july and a right lateral reverse fault representing the largest earthquake m5 5 at 1109 lt on july 9 this model can simulate not only the above tilt data but also the record of a nearby dilatometer the results from daily laser distance measurements and daily global positioning system monitoring as well as results from other conventional distance surveys and leveling surveys in the surrounding region from this model we conclude that the intersection of the shear fault with the tensile fault caused magma to come up into soft sedimentary layers at the seabottom creating a lava dome later an instability brought about a magma phreatic explosion which did not generate any substantial crustal deformations at deeper levels

laboratory investigations into wave period effects on sand bed erodibility under the combined action of waves and currents laboratory experimental data on sediment threshold conditions for fine to coarse sands under the combined action of waves and currents are presented higher flows are needed to satisfy threshold under short period waves than for longer period waves the combined threshold velocities indicate that under the shorter 5 s period waves the oscillatory and steady flows are independent of each other but interact to a greater extent as the wave period increases the derived threshold flow conditions are converted into threshold shear stress values using existing linear and non linear models the performance of the models for various periods of oscillation is then examined in view of the assumption that a unique threshold value exists for a particular grain size all models suggest that the critical stress of a particular grain is higher under shorter period waves for the range of grain sizes presented using different models for waves of different periods appears to remove the effect of wave period upon the data it is suggested that theories used by the existing models are insufficient to fully describe the interaction of the steady and oscillatory flows during the experiments

oxidative mobilization of cerium and uranium and enhanced release of immobile high field strength elements from igneous rocks in the presence of the biogenic siderophore desferrioxamine b abstract polyvalent trace elements such as the high field strength elements hfse are commonly considered rather immobile during low temperature water rock interaction hence they have become diagnostic tools that are widely applied in geochemical studies we present results of batch leaching experiments focused on the mobilization of certain hfse y zr hf th u and rare earth elements from mafic intermediate and felsic igneous rocks in the presence and absence respectively of the siderophore desferrioxamine b dfob our data show that dfob strongly enhances the mobility of these trace elements during low temperature water rock interaction the presence of dfob produces two distinct features in the rare earths and yttrium rey patterns of leaching solutions regardless of the mineralogical and chemical composition or the texture of the rock type studied bulk rock normalized rey patterns of leaching solutions with dfob show i a very distinct positive ce anomaly and ii depletion of la and other light rey relative to the middle rey with a concave downward pattern between la and sm these features are not observed in experiments with hydrochloric acid acetic acid or deionized water in dfob bearing leaching solutions ce and u are decoupled from and selectively enriched relative to light rey and th respectively due to oxidation to ce iv and u vi oxidation of ce3 and u4 is promoted by the significantly higher stability of the ce iv and u vi dfob complexes as compared to the ce iii and u iv dfob complexes this is similar to the relationship between the ce iv and ce iii pentacarbonate complexes that cause positive ce anomalies in alkaline lakes however while formation of ce iv carbonate complexes is confined to alkaline environments ce iv dfob complexes may produce positive ce anomalies even in mildly acidic and near neutral natural waters siderophore promoted dissolution processes also significantly enhance mobility of other immobile hfse and may not only cause or modify ce anomalies and th u fractionation but also mobilization of and fractionation between zr hf th and redox insensitive rey during weathering pedogenesis diagenesis and incongruent dissolution of particles in seawater and freshwater siderophores may significantly affect the use of hfse as geochemical tools concave downward light rey patterns may be used as a biosignature for water rock interaction in the presence of siderophores enhanced and preferential mobilization of u relative to th in the presence of siderophores may produce th u signals comparable to those indicative of weathering under oxidized conditions which might constrain the use of u concentrations and th u ratios as a paleoredox proxy the enhanced mobilization of zr and especially hf from igneous rocks in the presence of dfob might have implications for the use of the latter as a tracer for the impact of continental weathering on seawater chemistry because siderophore complexes affect the particle reactivity of hf and zr they may prevent effective removal of terrigenous hf and zr during aggregation coagulation of riverine particles in estuaries siderophore promoted solubilization and stabilization might hence be an additional way to transport continental hf and zr to the oceans furthermore siderophore enhanced mobilization may also have implications for the remediation techniques employed to immobilize hfse such as u th and rey at nuclear waste and reprocessing sites and at rey ore processing plants where soils are commonly contaminated with these sometimes radioactive heavy metals

do critical stresses for incipient motion and erosion really exist the concept of critical stress for the initial motion of noncohesive sediment beds under turbulent flow conditions is reviewed observational definitions of incipient motion are many and not entirely compatible some laboratory flume observations of sediment movement suggest that no true threshold exists current understanding of turbulent fluid motion at a sediment bed suggests that some particle movement must occur at all nonzero time mean velocities a combined model for flume flow and sediment transport having no threshold is seen to explain features of data that have been previously used to support the threshold concept

a reinterpretation of seismicity associated with the january 1983 dike intrusion at kilauea volcano hawaii in january 1983 a dike intrusion fissure eruption generated a swarm of 375 magnitude 1 to 3 earthquakes along a 16 km segment of kilauea s middle east rift zone we searched the hawaiian volcano observatory catalog for multiplets of similar events from this region from 1980 through 1985 and obtained precise relative locations by waveform cross correlation over 150 of the intrusion earthquakes could be grouped into 14 multiplets of five or more events with sufficient similarity for accurate relocation some multiplets were active for only a few minutes during the downrift migration phase of the seismic swarm consistent with generation near the propagating dike tip while others were active for several days the two multiplets nearest the origin of the seismic swarm include events from the preceding days and months most multiplets span only 50 to 100 m following relocation are located at about 3 to 4 km depth and appear to deepen downrift the catalog depths of those earthquakes in multiplets and those not in multiplets are similar suggesting that most of the recorded seismicity may have come from a very limited depth interval despite the fact that the dike breached the surface by analogy with a mechanical model used to explain a similar clustering of background seismicity in the upper east rift in 1991 we infer that the earthquakes are generated in regions of high stress concentration immediately above kilauea s deforming deep rift body this conclusion is consistent with the depth of the top of the deep rift body inferred from geodetic data and with numerical calculations suggesting that a significant ambient differential stress is required for dikes to produce earthquakes larger than magnitude 1

deflection of dykes into sills at discontinuities and magma chamber formation abstract many rift zones contain composite volcanoes central volcanoes most of which are supplied with magma from shallow crustal magma chambers volcanotectonic studies of eroded rift zones such as in iceland as well as geophysical studies indicate that many shallow magma chambers evolve from sills here i provide field description of sills and extinct magma chambers in the quaternary and tertiary palaeorift zones of iceland focusing on the felsic pluton of slaufrudalur southeast iceland as a fine example of a magma chamber developed through the injection of sills in this paper i review analyse and develop further two related mechanisms for the deflection of dykes into sills and thus the potential initiation of a magma chamber namely a cook gordon debounding delamination and b stress barriers and propose a third mechanism c favourable material toughness ratios due to elastic mismatch difference in young s moduli or stiffnesses of layers in contact in the cook gordon mechanism a weak contact opens up as a result of dyke induced tensile stress this mechanism is likely to operate primarily at shallow depths as is supported by field observations and numerical models a stress barrier is a layer where the local stress is unfavourable to a particular type of rock fracture here a dyke field observations and numerical models show that on meeting a stress barrier a dyke either changes into a sill or becomes arrested the material toughness mechanism indicates that when the upper layer at a contact has the same or less stiffness than the lower layer hosting the dyke there is little tendency for the dyke to become deflected into the contact however when the upper layer is stiffer such as when a stiff basaltic lava flow is on the top of a soft pyroclastic layer the dyke tends to become deflected into the contact to form a sill field results suggest that all these mechanisms may cause dyke deflection into sills and may operate together particularly where the rock consists of alternating soft and stiff layers since the quaternary lava pile in iceland has many more soft hyaloclastite layers than the tertiary lava pile these mechanisms were probably very efficient in generating sills and by implication shallow magma chambers during the quaternary this may be one reason why in comparison with the tertiary lava pile the quaternary lava pile contains so many sills and extinct plutons and active shallow magma chambers

bed load transport of sediment mixtures experiments were conducted on alluvial beds of four sediment mixtures having different arithmetic mean diameters and standard deviations the bed load transport rates of individual fractions were measured einstein s method proffitt and sutherland s method and misri s method of calculation of the bed load transport of individual fractions for sediment mixtures were checked with carefully collected laboratory data by analyzing the data on lines similar to those of misri et al their method of calculation of the bed load transport rate for individual fraction is modified to make it applicable over a wide range of parameters the relationship between the dimensionless grain shear stress and the dimensionless bed load transport rate developed by misri et al for uniform sediments was made applicable to sediment mixtures by introducing a multiplying correction factor the parameters affecting this correction factor were identified and a relationship obtained for the same by analysis of an extreme amount of laboratory and river data use of this factor along with the transport law for uniform sediments enables determination of the rate of bed load transport of the individual fractions of a mixture

dike propagation energy balance from deformation modeling and seismic release magma is transported in the crust mainly by dike intrusions in volcanic areas dikes can ascend towards the free surface and also move by lateral propagation eventually feeding flank eruptions understanding dike mechanics is a key to forecasting the expected propagation and associated hazard several studies have been conducted on dike mechanisms and propagation however a less in depth investigated aspect is the relation between measured dike induced deformation and the seismicity released during its propagation we individuated a simple equation that can be used as a proxy of the expected mechanical energy released by a propagating dike and is related to its average thickness for several intrusions around the world afar japan mt etna we correlate such mechanical energy to the seismic moment released by the induced earthquakes we obtain an empirical law that quantifies the expected seismic energy released before arrest the proposed approach may be helpful to predict the total seismic moment that will be released by an intrusion and thus to control the energy status during its propagation and the time of dike arrest

the variability of critical shear stress friction angle and grain protrusion in water worked sediments the erodibility of a grain on a rough bed is controlled by among other factors its relative projection above the mean bed its exposure relative to upstream grains and its friction angle here we report direct measurements of friction angles grain projection and exposure and small scale topographic structure on a variety of water worked mixed grain sediment surfaces using a simple analytical model of the force balance on individual grains we calculate the distribution of critical shear stress for idealized spherical grains on the measured bed topography the friction angle projection and exposure of single grain sizes vary widely from point to point within a given bed surface the variability within a single surface often exceeds the difference between the mean values of disparate surfaces as a result the critical shear stress for a given grain size on a sediment surface is characterized by a probability distribution rather than a single value on a given bed the crtitical shear stress distributions of different grain sizes have similar lower bounds but above their lower tails they diverge rapidly with smaller grains having substantially higher median critical shear stresses large numbers of fines trapp ed within pockets on the bed or shielded by upstream grains are effectively lost to the flow our calculations suggest that critical shear stress as conventionally measured is defined by the most erodible grains entrained during transient shear stress excursions associated with the turbulent flow this implies a physical basis for the indeterminacy of initial motion these observations suggest that transport rate shear stress relationships may be controlled in part by the increasing numbers of grains that become available for entrainment as mean shear stress increases they also suggest that bed textures and grain size distributions may be controlled within the constraints of an imposed shear stress and sediment supply regime by the influence of each size fraction on the erodibility of other grain sizes present on the bed

laboratory investigations into wave period effects on sand bed erodibility under the combined action of waves and currents by g voulgaris et al comments

the march 1981 mount etna eruption inferred through ground deformation modelling abstract the march 1981 eruption of mt etna was characterised by the fast propagation of a long eruptive fissure approximately 7 km and by an unusually high eruption rate the continuous tilt recorded by a bore hole station located on the southern flank showed a small inflation 3 4 months before the eruption followed by a rapid deflation during the fissure opening the discrete deformation measurements electroptical distance measurements edm and levelling showed large horizontal changes in the volcano flanks and smaller vertical ones close to the eruptive fissure we modelled the deformation pattern associated with the 1981 eruption using horizontal 57 data and vertical 50 data measurements two geometric constraints were considered for the eruptive fissure the dislocation theory was used to model the intrusion by characterising it in two parts as a double tensile crack which represents the initial deeper intrusion first crack and then the final shallower part associated with the eruptive fissure second crack an analytic inversion was performed obtaining a very good solution and then the robustness of this result was verified through an accurate trial and error search the first deeper crack extends vertically from about 300 to 1700 m above sea level a s l it was widely opened opening 5 m dipping w and radiating from the summit of the volcanic edifice in the nnw direction for approximately 3 km in the shallowest part last 500 m the intrusion propagated radially in the northern sector along the nnw direction second crack matching the surface location of the observed eruptive fissure we interpret that the deeper crack formed during the 3 4 months preceding the eruption

imaging magma transport during the 1997 seismic swarm off the izu peninsula japan the spatio temporal evolution of a propagating magma filled crack was estimated from inversion of global positioning system gps data tiltmeters and leveling the dike opened at a maximum rate of 50 millimeters per day and had a peak magma flux of 2 x 10 6 cubic meters per day although the spatial resolution was limited slow upward propagation was resolved during the 9 day long intrusion in contrast the earthquakes migrated rapidly upward during the first 12 hours of the swarm and nearly all of the seismic energy was released in the first 2 days comparison of inversion results with accurate hypocenter locations will lead to improved understanding of magma transport through the brittle crust and of the causes of volcanic seismicity

evidence that coupling to magma chambers controls the volume history and velocity of laterally propagating intrusions 1 in this paper i present a simple analytical solution for the unsolved problem of a propagating dike coupled to a magma chamber as recognized by segall et al 2001 the flow of the magma from a chamber to a dike is described by an ordinary differential equation for the unknown pressures of chamber and dike the most intuitive assumption on magma exchange is that the volume gained by the intrusion equals the volume lost by the chamber this constraint however implies a chamber so large that its pressure does not drop during the feeding process and an incompressible magma that does not expand or vesiculate as it intrudes models assuming constant driving pressure fail to explain observations here mass conservation constrains pressure as magma flows from the chamber to the dike and the compressibilities of magma and sources control volume changes these assumptions allow me to decouple the equations and solve the system analytically the model predicts that chamber and dike volume change exponentially with time as v t v 1 exp t the asymptotic volume v and the timescale are found to be functions of rock magma chamber and dike parameters and of the initial pressure conditions assuming that the intrusion is shaped as an elongating cuboid its velocity is found to change as v v0exp t where v0 is the initial dike velocity the model is successfully tested against the best observations available for lateral dike propagation events the volume history of chamber and dike during the 2000 miyakejima intrusion japan and dike velocity during the 1978 krafla event iceland and during some intrusions following the 2005 event in afar ethiopia this paper confirms and extends the results of a previous study by rivalta and segall 2008 who found the final ratio between dike volume and the volume withdrawn from the chamber to be rv 1 4 m 3 1 where is the host rock rigidity and m is the magma compressibility here i demonstrate that the formula for rv holds at any time during the intrusion not just at the end this model confirms that some magma chambers behave as stiff magma tanks able to inflate large dikes as balloons and demonstrates that high volume ratios are unlikely if the chamber has a pure sill shape fitting observations with the model allows us to estimate several dike chamber magma and rock parameters

volcanomagnetic evidence of the magmatic intrusion on 13thmay 2008 etna eruption 1 during the onset of 2008 volcanic crisis at mt etna the near real time magnetic data provided a continuous updating of the volcano activity state on the northern flank on the morning of 13th may 2008 significant local magnetic field changes marked the resumption of the eruptive activity characterized by the opening of a fracture field on the northern flank and an eruptive fissure in the valle del bove in agreement with the northward propagation of seismic events magnetic signals at 5 stations in the summit area revealed a nearly nnw sse oriented magmatic intrusion which started at about 9 00 gmt propagated northward for about 2 km and stopped at 14 00 gmt before reaching the north east rift magnetic variations with amplitude ranging between 1 8 nt and 6 5 nt are consistent with those calculated from piezomagnetic models where stress induced changes in rock magnetization are produced by the magmatic intrusion

geometric difference between non feeder and feeder dikes feeder dikes bring magma to the surface non feeder dikes become arrested and never reach the surface the differences if any between these dike types remain largely unexplored because in the field it is normally unknown if a particular dike is a feeder or non feeder here we present measurements of feeder and non feeder dikes exposed from depths of 200 m to the surface in the walls of the a d 2000 caldera collapse of the miyakejima volcano japan a typical feeder thickness reaches a maximum of 2 4 m at the surface decreases rapidly to 1 m at a depth of 20 40 m and then remains constant to the bottom of the exposure by contrast a typical non feeder thickness reaches a maximum of 1 5 2 m at 15 45 m below the tip and then decreases slowly with depth to 0 5 1 m at the bottom of the exposure we propose that free surface effects and magmatic overpressure driving pressure changes during the eruption cause the overall shape of a feeder to differ from that of a non feeder

three dimensional model of the feeder conduit of the 1983 eruption of mt etna volcano from ground deformation measurements a levelling network and a horizontal distance measuring network down mt etna s south flank were measured before during and after the 1983 eruption we analyse the large movements recorded using theoretical models of surface displacement to determine the shape and position of the feeder conduit that supplied magma to the eruption the results indicate that a dyke the top surface extending nearly horizontally connects the eruption site to a point near the southeast crater the top of the dyke lies at an altitude of 2400 m to 2500 m and dips between 75 and 90 to the west more tentative evidence indicates that the top of the active dyke dropped by about 150 m between the 25th and the 95th day of the eruption and that a small branching dyke 1 km long was emplaced to the east of the main dyke but did not attain the surface the implications of these observation are discussed

form and dimensions of dykes in eastern iceland abstract a study was made of about 700 dykes in eastern iceland the majority of these belong to three swarms about 73 dip within 10 of the vertical most strike between 10 and 40 ne the strike of the dykes within the southernmost swarm alftafjordur changes along its trace from almost n at the north end to nne ssw southward along the swarm the average thickness of the dykes is about 4 1 m and the thickness does not change notably along the alftafjordur swarm the thinner dykes tend to have smaller grains than the thicker dykes of five dykes followed laterally the longest is over 22 km the thickness of individual dykes changes irregularly along their length and the dyke is often offset where its thickness changes many dykes appear to be completely discontinuous but some parts are connected by veins where the dykes end in a vertical section most of them simply taper away only about 10 5 of the dykes occupy faults the mechanical and thermal effects of the dykes on the country rock are small many of the dykes appear to be non feeders i e dykes that never reached the surface to feed lava flows using the length width ratio the depth of origin of three dykes has been estimated the maximum depth of origin of these three dykes is 7 5 km 9 km and 10 km below the original surface

constraints on dike propagation from continuous gps measurements the january 1997 east rift zone eruption on kilauea volcano hawaii occurred within a network of continuous global positioning system gps receivers the gps measurements reveal the temporal history of deformation during dike intrusion beginning 8 hours prior to the onset of the eruption the dike volume as a function of time estimated from the gps data using elastic green s functions for a homogeneous half space shows that only two thirds of the final dike volume accumulated prior to the eruption and the rate of volume change decreased with time these observations are inconsistent with simple models of dike propagation which predict accelerating dike volume up to the time of the eruption and little or no change thereafter deflationary tilt changes at kilauea summit mirror the inferred dike volume history suggesting that the rate of dike propagation is limited by flow of magma into the dike a simple lumped parameter model of a coupled dike magma chamber system shows that the tendency for a dike to end in an eruption rather than intrusion is favored by high initial dike pressures compressional stress states large compressible magma reservoirs and highly conductive conduits linking the dike and source reservoirs comparison of model predictions to the observed dike volume history the ratio of erupted to intruded magma and the deflationary history of the summit magma chamber suggest that most of the magma supplied to the growing dike came from sources near to the eruption through highly conductive conduits interpretation is complicated by the presence of multiple source reservoirs magma vesiculation and cooling as well as spatial variations in dike normal stress reinflation of the summit magma chamber following the eruption was measured by gps and accompanied a rise in the level of the pu u o o lava lake for a spheroidal chamber these data imply a summit magma chamber volume of 20 km3 consistent with recent estimates from seismic tomography continuous deformation measurements can be used to image the spatiotemporal evolution of propagating dikes and to reveal quantitative information about the volcanic plumbing systems

toughness and failure of volcanic edifices abstract major volcanic edifices are of two main types stratovolcanoes composite edifices and basaltic edifices shield volcanoes these types of edifices show several remarkable differences for example while some basaltic edifices erupt very frequently many stratovolcanoes erupt very infrequently also most basaltic edifices have gentle slopes commonly 2 12 whereas stratovolcanoes have steeper slopes particularly in their uppermost parts where the slopes may reach 35 42 there have been various attempts to explain these differences but here the focus is on edifice strength or rather toughness toughness of a material is a measure of its resistance to fracture alternatively for a tough material large amounts of energy are needed to cause failure most quoted material toughness critical energy release rate values for rocks are 20 400 j m 2 for extension fractures such as dykes and 0 01 0 03 mj m 2 for shear fractures such as dip slip faults the related property of critical stress intensity or fracture toughness is for most rocks 0 5 3 mpa m 1 2 these values for material and fracture toughness refer to small essentially homogeneous laboratory rock samples here i present equations for calculating the material toughness in volcanic edifices based on the measured or estimated dimensions of associated large structures such as dykes and faults for typical dykes the calculated material toughness is 1 3 47 mj m 2 with a corresponding stress intensity of 114 690 mpa m 1 2 similarly for a 3 8 km long dip slip fault in an edifice the calculated material toughness is 2 3 mj m 2 and the corresponding stress intensity 150 mpa m 1 2 these results compare well with previous material toughness estimates for large strike slip faults and stress intensity estimates for large dykes generalising these results and using field data as well as materials science results i conclude that stratovolcanoes normally have a much higher material toughness than basaltic edifices this is primarily because of different internal structures a basaltic edifice is made of layers and contacts all of which have essentially similar mechanical properties whereas a stratovolcano is made of layers and contacts many of which have widely different properties accordingly it requires much more energy is more difficult for a fracture to propagate through a stratovolcano than through a basaltic edifice thus contrasting toughnesses may be one principal reason why these edifices differ in slope and in failure frequencies including the number of dyke fed eruptions caldera collapses and landslides

injection and arrest of dykes implications for volcanic hazards dykes are the principal channels through which magma reaches the surface in volcanic eruptions for this reason dykes observed in the field are commonly assumed to be feeders to lava flows the actual proportion of dykes reaching the surface is however poorly known in order to develop models for the purpose of estimating volcanic hazard this proportion must be known this follows because such models should not only consider the probability of dykes being injected from magma chambers during periods of unrest in the associated volcanoes but also the probability of the injected dykes being arrested this paper presents field data on several thousand dykes from iceland and tenerife canary islands indicating that many and probably most dykes become arrested at various crustal levels and never reach the surface to feed eruptions using the results of analytical and numerical models it is shown that for common loading conditions the stress field in the vicinity of a magma chamber may favour the injection and propagation of dykes while the stress field at a certain distance from the chamber favours dyke arrest this means that many dykes that are injected from the chamber propagate only for a very limited distance from the chamber to the point where they become arrested the implication is that during periods of unrest in volcanoes the probability of volcanic eruption is only a small fraction of the probability of dyke injection from the source magma chamber

insights of dyke emplacement mechanics from detailed 3d dyke thickness datasets abstract dyke thickness datasets offer new insights into the detailed 3d geometry of dyke swarms and an exceptional opportunity to evaluate theoretical emplacement models the swartruggens kimberlite dyke swarm extends over 7 km along strike and intrudes a dolerite quartzite shale and andesitic lava succession the star kimberlite dykes cut shales and sandstones intersect a large dolerite sill and extend 15 km along strike both dyke swarms comprise anastomosing en echelon segments each several hundred metres long in total 1532 swartruggens dyke thickness measurements were taken to 750 m below the surface over a 250 m depth range and 3354 star dyke thickness measurements were taken over a 520 m depth range the swartruggens dyke thicknesses are 0 05 1 95 m mean 0 64 m whereas the star kimberlites range from

dike induced faulting in rift zones of iceland and afar geodetic data and field observations demonstrate that the emplacement of dikes in volcanic rift zones frequently generates normal faulting and graben subsidence at the earth9s surface elastic modeling of the vertical ground surface displacements above dikes and faults indicates that the extent of graben subsidence can be achieved only if fault slip extends virtually to or beyond the dike plane at depth a mechanical model that includes dikes and frictional faults shows that dike opening tends to compress and lock faults located to either side of the dike therefore slip extending into or beyond the dike cavity must occur either 1 on faults that intersect the dike near its top above the zone of dike induced compression or 2 on faults that slip ahead of the dike as it propagates laterally data from iceland indicate that slip occurred on deep faults that presumably slipped in advance of the laterally propagating dike

the spectrum of basaltic feeder systems from effusive lava eruption to explosive eruption at miyakejima volcano japan basaltic feeder systems exposed in the caldera wall of miyakejima volcano are classified into three groups 1 effusive feeders 2 moderately explosive feeders and 3 highly explosive feeders the surface deposits and feeder systems reveal a wide variation in the explosivity of the eruptions that produced them ranging from non explosive lava effusions to violent explosive eruptions despite the apparent lack of influence of external water effusive feeders are filled with coherent non fragmented intrusive rock indicating no significant fragmentation in the feeder system the other two types of feeder systems consist of a coherent dike in their deeper part and a pyroclastic fill in their uppermost part their uppermost parts show an upward flaring shape the transition from coherent intrusion to pyroclastic fill in the feeder systems suggests underground fragmentation of the rising magma the depth of the coherent pyroclastic transition is deeper 20 150 m in highly explosive feeders than in the moderately explosive feeders 20 m and coincides with the depth at which the system flares upwards presence of lithic fragments derived from the host rock in the products of the highly explosive feeder systems indicates the removal of the wall rock by explosive activity this observation suggests that the fragmentation of rising magma promoted the enlargement of the feeder systems to form their upward flaring shapes by mechanical erosion and wall collapse

physical processes of shallow mafic dike emplacement near the san rafael swell utah some 200 shonkinite dikes sills and breccia bodies on the western colorado plateau of south central utah were intruded from approximately 3 7 to 4 6 ma contemporaneous with mafic volcanism along the nearby plateau margin thicknesses of dikes range to about 6 m the log normal mean thickness is 85 cm despite the excellent exposures of essentially all dikes in strata of the jurassic san rafael group their number is indeterminate from their outcrop and spacing because they are everywhere greatly segmented by our grouping of almost 2000 dike segments most dikes are less than 2 km in outcrop length the longest is 9 km because the san rafael magmas were primitive and probably ascended directly from the mantle dike lengths in outcrop are much less than their heights the present exposures probably lie along the irregular upper peripheries of dikes that lengthen and merge with depth orientations of steps on dike contacts record local directions of dike fracture propagation about half of the measurements plunge less than 30 showing that lateral propagation at dike peripheries is as important as the vertical propagation ultimately responsible for ascent the san rafael dikes now exposed after erosion of about 0 5 1 5 km appear to thicken and shorten upward probably because near surface vesiculation enhanced magmatic driving pressures propagation likely ceased soon after the first dike segments began to feed nearby sills or vented to initiate small volume eruptions most of the dikes are exposed in clastic strata of the jurassic san rafael group they probably acquired their strikes however while ascending along well developed joints in massive sandstones of the underlying glen canyon group rotation of far field stresses during the emplacement interval cannot account for disparate strikes of the dikes which vary through 110 most lying between north and n25 w rather the two regional horizontal principal stresses were probably nearly equal and so the dominant n75 e direction of dike opening was not strongly favored across the center of the swarm about 10 to 15 dikes overlap and produce 15 20 m of dilation many are in sufficient proximity that later dikes should be thinner than earlier ones if neither the magma pressures nor regional stresses were changing during the emplacement interval however dike thicknesses vary systematically neither along the length of the swarm nor in proportion to the number of neighboring dikes it appears that crustal extension during the magmatic interval relieved compressive stresses localized by intrusion

swell transformation across the continental shelf part i attenuation and directional broadening abstract extensive wave measurements were collected on the north carolina virginia continental shelf in the autumn of 1999 comparisons of observations and spectral refraction computations reveal strong cross shelf decay of energetic remotely generated swell with for one particular event a maximum reduction in wave energy of 93 near the virginia coastline where the shelf is widest these dramatic energy losses were observed in light wind conditions when dissipation in the surface boundary layer caused by wave breaking whitecaps was weak and wave propagation directions were onshore with little directional spreading these observations suggest that strong dissipation of wave energy takes place in the bottom boundary layer the inferred dissipation is weaker for smaller amplitude swells for the three swell events described here observations are reproduced well by numerical model hindcasts using a parameterization of wave friction over a movable sandy bed directional spectra that are narrow off the s

initial motion and pivoting characteristics of sand particles in uniform and heterogeneous beds experiments and modelling experiments are described in which the threshold conditions for sediment entrainment are measured for uniform and mixed sand beds beneath both steady and combined steady oscillatory flows derived critical shear stresses are compared with the mixed bed entrainment model of wiberg smith 1987 as predicted by the model coarser grains within a sand mixture are entrained at lower bed shear stresses than progressively finer grains entrainment occurs generally at lower shear stresses than predicted by the model especially under unidirectional flows this may be the result of grains resting in unusually unstable positions during the experiments because the beds are unworked at the start of the experiments the model of wiberg and smith predicts threshold conditions more accurately for the mixed beds if the bed pivoting angle is correctly defined the pivoting angles of the beds used here are measured using a new technique designed specifically for comparison with the threshold data the measured angles repeat the finding that the coarse grains are more mobile than the finer fractions of a mixture the results are poorly described by the pivoting angle model presented by wiberg smith 1987 and are better represented by a model of the form d di d50 after li komar 1986 where and are empirical constants the threshold model is found to be more effective using the improved pivoting relationship the entrainment of grains is found to be easier beneath unidirectional flows than combined flows in accordance with previous authors findings a suggestion that this result is caused by a change in the erosion mechanism beneath wave flows is made wave boundary layers may act as an extended laminar sublayer over bed grains and reduce the erosive efficiency of the overlying current flow the results of the experiment have implications for the natural sorting mechanisms of sediment beds being deposited in near threshold flows

sequential leaching of marine ferromanganese precipitates genetic implications abstract sequential leaching experiments were carried out on twenty one hydrogenetic crust samples from different locations in the central pacific and the results are compared with four crust and nodule samples of different genetic origin fe vernadite mno2 is the most important minor and trace metal bearing mineral phase and shows high concentrations of co ni cd ti ba zn and cu which are present in seawater mainly as hydrated and labile complexed cations elements forming carbonate and hydroxide complexes and oxyanions in seawater like pb mo v and minor proportions of cu and zn are bound in the fe00h fraction the ca carbonate phase and the residual fraction which consists of aluminosilicates and crystalline oxides show only minor heavy metal associations e g cu and zn only a small part of the high ti concentrations in hydrogenetic crusts is of detrital origin ti mainly forms a hydrogenetic phase probably consisting of ti02 2h20 intergrown with the amorphous fe00h phase analyses of different sample types and crust layers show that aging and diagenetic effects can change the phase associations of some elements the most notable change was observed for pb which during phosphatization of crusts is transferred from the mn fe oxide fraction to the apatite fraction a colloid chemical model for the hydrogenetic precipitation of ferromanganese crusts on seamounts is proposed in the first stage mn2 rich water from the oxygen minimum zone is mixed with oxygen rich deep water and oxidized mn iv and other metals like fe ti al and si form oxide and hydroxide colloids these form mixed colloidal phases and scavenge trace metals by sorptive processes which are dominated by coulombic and chemical interactions between colloid surfaces and dissolved metal species in stage two the colloidal phases precipitate on the substrate rocks of the seamounts as ferromanganese oxide encrustations incorporating the sorbed heavy metals into the mineral phases

the mechanics of large volcanic eruptions abstract the mechanical conditions for a volcanic eruption to occur are conceptually simple a magma driven fracture normally a dyke must be able to propagate from the source to the surface the mechanics of small to moderate eruptions eruptive volumes less than 10 km3 is reasonably well understood whereas that of large eruptions eruptive volumes of 10 1000 km3 is poorly understood here i propose that while both large and small eruptions are primarily driven by elastic energy and may come from the same magma chambers and reservoirs the mechanisms by which the elastic energy is transformed or relaxed in these eruptions are different more specifically during small to moderate eruptions the excess pressure in the source the primary pressure driving the eruption falls exponentially until it approaches zero whereby the feeder dyke closes at its contact with the source and the eruption comes to an end under normal conditions the ratio of the eruptive and intrusive material of the eruption to the volume of a totally molten shallow basaltic crustal magma chamber at the common depth below the surface of 1 5 km is about 1400 and that of a partially molten deep seated basaltic magma reservoir in the lower crust or upper mantle is about 5000 many magma chambers are partially molten in which case the ratio could be close to that of reservoirs most magma chambers are estimated to be less than about 500 km3 for which the maximum eruptive volume would normally be about 0 4 km3 an eruptive volume of 1 km3 would require a totally molten chamber of about 1400 km3 while chambers of this size certainly exist witness the volumes of the largest eruptions large eruptions of 10 1000 km3 clearly require a different mechanism namely one whereby the excess pressure is maintained during much of the eruption i suggest that the primary excess pressure maintenance mechanism is through caldera subsidence for shallow magma chambers and graben subsidence for deep seated magma reservoirs in this mechanism it is the subsidence of tectonic origin and associated volume reduction shrinkage of the magma source that drives out an exceptionally large fraction of the magma in the source thereby generating the large eruption most explosive eruptions that exceed volumes of about 25 km3 and many smaller are associated with caldera collapses the data presented suggest that many large effusive basaltic eruptions in iceland in the united states and elsewhere are associated with large graben subsidences in terms of the present mechanism successful forecasting of large eruptions requires understanding and monitoring of the volcanotectonic conditions that trigger large caldera and graben subsidences

effects of mechanical layering of host rocks on dike growth and arrest abstract many dikes intruding the layered host rock of miyakejima and piton de la fournaise are arrested segmented and show variation in thickness these geometric factors can be largely attributed to the mechanical layering of the host rocks that constitute these volcanoes some dikes are arrested at the base of stiff high young s modulus rock layers whereas others are segmented in the layers we use finite element models fem to simulate a typical dike arrest at the base of a stiffer layer the dike may become arrested when it reaches at the base of the stiffer layer for several reasons first the dike induced tensile stress may not be high enough to break the rock second the stiff layer may act as a stress barrier third the material toughness of the layers may be unfavorable for vertical dike propagation and result either in dike arrest or dike deflection when the magmatic overpressure driving pressure is sufficiently high dike can intrude into the overlying layer some deflected dikes resume their vertical propagation thereby generating offset segments the layering of a volcano thus commonly controls the dike propagation paths and if and where the dikes become arrested our measurements show that the dikes in these volcanoes tend to be comparatively thick where they dissect soft pyroclastic layers and thin where they dissect stiff lava flows and sills numerical fem simulations indicate that the variation of dike thickness observed in miyakejima and piton de la fournaise can be broadly explained in terms of layer stiffness differences of one or two orders of magnitude

the influence of clay on the threshold of movement of fine sandy beds the erosion threshold i e surface shear strength of mixed sediment deposits has been tested under the action of unidirectional currents mean critical speeds 4 mm above the bed ranging from 12 4 cm s 1 to 18 2 cm s 1 or simulated waves mean critical periods ranging from 2 0 s to 6 7 s for near bed wave amplitudes of 0 28 0 57 m using an oscillating tray in a rectangular recirculating flume the sediment mixtures investigated consisted of angular fine grained quartz sands d50 152 5 and 215 m and cohesive estuarine mud the investigation has shown that the cohesive material increases generally the erosion threshold of sandy deposits the rate of increase is smaller for mud contents 30 clay mineral content 11 14 by dry weight and is attributed to an increase of the internal angle of friction when the clay mineral content exceeds 11 14 by dry weight then the sand particles are not in contact with each other consequently the clay component controls the erodibility of the sediment mixture finally a prolonged and intense stress history can increase considerably by a factor of 1 3 1 7 the original erosion threshold value of these sediments

a comparison between shields threshold criterion and the movement of loosely packed gravel in a tidal channel observations of the threshold of movement of loosely packed gravel in a tidal current are described for gravel with equivalent spherical diameters d in the range 0 2 d 5 0cm the critical friction velocity u c corresponding to the initiation of sediment transport is given by u c 7 0 d0 2 at large values of d within the quoted range the value u c is significantly lower than would be obtained by a shields experiment u c d0 5 by comparing our values of u c with those obtained under well controlled laboratory conditions the discrepancy with shields is shown to be due to the open spacing between and exposure of individual pebbles on the seabed by comparing our results with those from upland gravel streams and flume experiments it is suggested that shields assumed an excessively large water depth to particle size ratio as a constraint within which the critical sediment entrainment number 0c is valid

calcium phosphate control of rey patterns of siliceous ooze rich deep sea sediments from the central equatorial pacific abstract rare earth elements and yttrium rey are often used as proxies for paleo environmental conditions and for the reconstruction of element sources and transport pathways many geological systems are well described with respect to the behavior of rey but deep sea sediments with their manifold processes impacting the sediment during early diagenesis leave some questions about the origin and development of the shale normalized rey reysn patterns unanswered here we report rey data for sediment solid phase and pore water from the upper 10 m of deep sea sediments from the clarion clipperton zone ccz in the central equatorial pacific the solid phase rey profiles show highest concentrations at depth below 5 8 m the reysn patterns show an enrichment in middle rey mrey lasn gdsn between 0 35 and 0 60 gdsn ybsn between 1 19 and 1 47 and either no or negative cesn and ysn anomalies i e chondritic to sub chondritic y ho ratios between 24 7 and 28 7 based on correlation analyses of bulk sediment element concentrations and sequential extractions we suggest that a ca phosphate phase controls the distribution and the patterns of rey in these silty clay pelagic sediments rich in siliceous ooze the mrey enrichment develops at the sediment water interface and intensifies systematically with depth the negative cesn anomaly intensifies with depth possibly because ce is mostly bound to mn and fe oxyhydr oxides therefore ce concentrations remain relatively constant throughout the sediment core while its trivalent rey neighbors are mostly hosted by the ca phosphate phase that continuously incorporates rey from ambient pore waters the non redox sensitive trivalent rey concentrations increase with depth producing or enhancing a negative cesn anomaly through coupled substitution of rey3 and na for ca2 the solid phase reysn pattern is therefore determined by the pore water reysn pattern and not suitable for paleoceanographic interpretation the similarity of the pore water and solid phase reysn patterns suggests however that only minor fractionation occurs during rey incorporation into the ca phosphate crystal structure

dynamic inversion of ground deformation data for modelling volcanic sources etna 1991 93 the 1991 93 etna eruption that represented the most important recent eruptive event both in terms of duration 472 days and total volume of erupted lava ca 250 106 m caused marked ground deformation measured by using different geodetic techniques such as edm gps levelling and tiltmetry an inversion termed dynamic because it took into account the sequence of events considering both the separate and cumulative effects of two sources a shallow tensile crack and a deeper ellipsoidal deflating source was performed it was based on 163 data collected by the different techniques plus 2 geometrical conditions for constraining the geometry of the crack on the surface the final solution furnished a good fit for all measurements defining a shallow tensile crack located inside the volcanic edifice which represents the effect of the intrusion and a depressuring body with centre at ca 3000 m b s l which could represent the centre of a generalized and more complex deflation of the volcanic edifice during the course of the eruption

incipient motion of sand gravel sediment mixtures experiments on the incipient motion of gravel and sand mixtures of 0 100 10 90 25 75 45 55 and 100 0 percent ratios of gravel and sand respectively were made in a laboratory flume a series of transport experiments was made for each of the five sediment beds and the critical shear stress for motion was calculated for 12 size ranges by interpolating or extrapolating the bed shear stress for a very small transport rate the sand in each of the five bed mixtures began to move at nearly the same bed shear stress all gravel sizes began to move at nearly the same bed shear stress in the 100 gravel experiments but in the sand gravel experiments the gravel showed an increase in critical shear stress with increasing size reasons for this change in the initiation of motion of the gravel from the 100 gravel to the sand gravel mixtures may result from the abundant sand sized sediment inhibiting the formation of a coarse bed surface layer in the sand gravel experiments

imaging the dynamics of dyke propagation prior to the 2000 2003 flank eruptions at piton de la fournaise reunion island 1 the relationship between shallow magma storage and flank injections in volcanic edifices is rarely documented we analyse the time series of geodetic and seismic data collected at piton de la fournaise volcano la reunion during 9 flank eruptions from 2000 to 2003 the data depicts how magma injections supplied flank eruptions at piton de la fournaise volcano and allows to precisely determine the direction and duration of dike intrusions indicating a two phases pattern consistent over all the eruptions 1 fast vertical migration of the dyke 2 m s from the magma chamber to the surface in 10 to 50 minutes and 2 slower lateral migration 0 2 0 8 m s in tens of minutes to hours controlled by the rift zone geometry

conditions for the arrest of a vertical propagating dyke magma ascent towards the earth s surface occurs through dyke propagation in the vast majority of cases we investigate two purely mechanical effects unrelated to cooling or solidification that lead to the arrest of propagation so that no eruption occurs the first is that the input of magma from the source is not maintained continuously such that a fixed volume of magma is released laboratory experiments show that in this case the dyke stops at a finite distance from the source this behaviour is specific to the fracturing process in 3 d we derive a relationship for the minimum magma volume required for an eruption as a function of magma buoyancy and source depth when large magma volumes are available eruption may also be prevented by a thick low density layer in the upper crust numerical studies of dyke propagation show that the dyke continues to rise even though it is negatively buoyant magma accumulates in a swollen nose region at the interface between the low density layer and the dense basement magma overpressure is largest at this interface and increases with increasing penetration into the upper layer it may become large enough to induce horizontal fractures in the dyke walls and lateral intrusion of a sill which prevents eruption this requires that the thickness of the low density layer exceeds a threshold value that depends on the density contrast between magma and host rock if the magma volume is smaller than a threshold value neither sill intrusion nor eruption are possible and magma gets stored in a horizontal blade shaped dyke straddling the interface scaling laws for variations of ascent rate and for the minimum magma volume allow diagnosis of a failed eruption

applications of grain pivoting and sliding analyses to selective entrapment of gravel and to flow competence evaluations models of sediment threshold by grain pivoting or sliding over underlying particles are examined in order to explore their application to evaluations of selective entrainment of gravel by flowing water of special interest is whether such process based models provide satisfactory evaluations of flow competence and the movement of large clasts by floods a detailed derivation is undertaken focusing first on the fluid flow and forces at the particle level the resulting threshold equation for the particle level velocity is then modified to yield the mean entrainment stress for the flow as a whole this approach is appropriate for considerations of selective entrainment of grains of varying sizes within a deposit the sorting being due to their relative projection distances above the bed and the dependence of their pivoting angles on grain size and shape the resulting threshold equations contain a number of coefficients e g drag and lift whose values are poorly known but can be constrained by requiring agreement with the shields curve for the threshold of grains in uniform deposits if pivoting coefficients based on laboratory measurements with tetrahedral arrangements of particles are used in the models smaller degrees of selective sorting are predicted than found in the field measurements of gravel entrainment however if reasonable modifications of those coefficients are made for expected field conditions then the models yield good agreement with the data sliding models where sorting is due entirely to projection distances of the grains above the bed yield somewhat poorer agreement with the field data however the sliding models may have support from laboratory experiments on gravel entrainment in that the data and theoretical curves have similar concave trends the existing measurements lack documentation of the mechanisms of grain movement so it is not possible to conclusively determine the relative importance of grain pivoting versus sliding in spite of such uncertainties the results are encouraging and it is concluded that pivoting and sliding models for grain entrainment do have potential for field computations of selective entrainment and flow competence

attenuation and source parameters of shallow microearthquakes at mt etna volcano italy we estimated the attenuation laws of high frequency seismic waves in the shallow crust depth 5 km and earthquake source parameters by using a se lected data set of 320 shallow events 2 6 md 4 2 recorded at mt etna volcano during the last two flank eruptions occurring in 2001 and 2002 2003 the quality factor q was estimated from spectra of p and s waves for 24 stations of the local permanent network by applying a spectral ratio technique the results show varia tions in both qp and qs as a function of frequency according to the power law q q0f n with n ranging between 0 3 and 1 3 for p waves and between 0 2 and 0 9 for s waves as typical of volcanic environments strong azimuthal variations of qp were also found suggesting the presence of local strong lateral heterogeneities and or of fluid filled cracked volumes after correction for attenuation we estimated the source parameters seismic moment source radius and stress drop of a subset of 66 shallow events under the assumption of a circular dislocation the estimated seismic moments m0 range from 10 13 to 10 15 n m the source radii r are confined between 100 and 1000 m and stress drop dr ranges between 0 2 mpa and about 4 mpa combining the source parameters obtained in this study with those calculated by pataneet al 1997 for an old data set of smaller microearthquakes 10 9 m0 10 14 n m recorded in the same area we re evaluated the scaling relationship between seismic moment m 0 and corner frequency f c for the earthquakes with m 0 ranging between 10 13 and 10 15 n m we confirm that microearthquakes at mt etna seem not to obey a

volcano tectonic earthquakes a new tool for estimating intrusive volumes and forecasting eruptions abstract we present data on 136 high frequency earthquakes and swarms termed volcano tectonic vt seismicity which preceded 111 eruptions at 83 volcanoes plus data on vt swarms that preceded intrusions at 21 other volcanoes we find that vt seismicity is usually the earliest reported seismic precursor for eruptions at volcanoes that have been dormant for decades or more and precedes eruptions of all magma types from basaltic to rhyolitic and all explosivities from vei 0 to ultraplinian vei 6 at such previously long dormant volcanoes because large eruptions occur most commonly during resumption of activity at long dormant volcanoes vt seismicity is an important precursor for the earth s most dangerous eruptions vt seismicity precedes all explosive eruptions of vei 5 and most if not all vei 4 eruptions in our data set surprisingly we find that the vt seismicity originates at distal locations on tectonic fault structures at distances of one or two to tens of kilometers laterally from the site of the eventual eruption and rarely if ever starts beneath the eruption site itself the distal vt swarms generally occur at depths almost equal to the horizontal distance of the swarm from the summit out to about 15 km distance beyond which hypocenter depths level out we summarize several important characteristics of this distal vt seismicity including swarm like nature onset days to years prior to the beginning of magmatic eruptions peaking of activity at the time of the initial eruption whether phreatic or magmatic and large non double couple component to focal mechanisms most importantly we show that the intruded magma volume can be simply estimated from the cumulative seismic moment of the vt seismicity from log10 v 0 77 log moment 5 32 with volume v in cubic meters and seismic moment in newton meters because the cumulative seismic moment can be approximated from the size of just the few largest events and is quite insensitive to precise locations the intruded magma volume can be quickly and easily estimated with few short period seismic stations notable cases in which distal vt events preceded eruptions at long dormant volcanoes include nevado del ruiz 1984 1985 pinatubo 1991 unzen 1989 1995 soufriere hills 1995 shishaldin 1989 1999 tacana 1985 1986 pacaya 1980 1984 rabaul 1994 and cotopaxi 2001 additional cases are recognized at frequently active volcanoes including popocateptl 2001 2003 and mauna loa 1984 we present four case studies pinatubo soufriere hills unzen and tacana in which we demonstrate the above mentioned vt characteristics prior to eruptions using regional data recorded by neic we recognized in near real time that a huge distal vt swarm was occurring deduced that a proportionately huge magmatic intrusion was taking place beneath the long dormant sulu range new britain island papua new guinea that it was likely to lead to eruptive activity and warned rabaul volcano observatory days before a phreatic eruption occurred this confirms the value of this technique for eruption forecasting we also present a counter example where we deduced that a vt swarm at volcan cosiguina nicaragua indicated a small intrusion insufficient to reach the surface and erupt finally we discuss limitations of the method and propose a mechanism by which this distal vt seismicity is triggered by magmatic intrusion

a model for radial dike emplacement in composite cones based on observations from summer coon volcano colorado usa we mapped the geometry of 13 silicic dikes at summer coon an eroded oligocene stratovolcano in southern colorado to investigate various characteristics of radial dike emplacement in composite volcanoes exposed dikes are up to about 7 km in length and have numerous offset segments along their upper peripheries surprisingly most dikes at summer coon increase in thickness with distance from the center of the volcano magma pressure in a dike is expected to lessen away from the pressurized source region which would encourage a blade like dike to decrease in thickness with distance from the center of the volcano we attribute the observed thickness pattern as evidence of a driving pressure gradient which is caused by decreasing host rock shear modulus and horizontal stress both due to decreasing emplacement depths beneath the sloping flanks of the volcano based on data from summer coon we propose that radial dikes originate at depth below the summit of a host volcano and follow steeply inclined paths towards the surface near the interface between volcanic cone and basement which may represent a neutral buoyancy surface or stress barrier magma is transported subhorizontally and radially away from the center of the volcano in blade like dikes the dikes thicken with increasing radial distance and offset segments and fingers form along the upper peripheries of the intrusions eruptions may occur anywhere along the length of the dikes but the erupted volume will generally be greater for dike fed eruptions far from the center of the host volcano owing to the increase in driving pressure with distance from the source observed eruptive volumes vent locations and vent area intrusions from inferred post glacial dike fed eruptions at mount adams washington usa support the proposed model hazards associated with radial dike emplacement are therefore greater for longer dikes that propagate to the outer flanks of a volcano

on the mechanisms governing dike arrest insight from the 2000 miyakejima dike injection abstract magma stored beneath volcanoes is sometimes transported out of the magma chambers by means of laterally propagating dikes which can lead to fissure eruptions if they intersect the earth s surface the driving force for lateral dike propagation can be a lateral tectonic stress gradient the stress gradient due to the topographic loads the overpressure of the magma chamber or a combination of those forces the 2000 dike intrusion at miyakejima volcano izu arc japan propagated laterally for about 30 km and stopped in correspondence of a strike slip system sub perpendicular to the dike plane then the dike continued to inflate without further propagation abundant seismicity was produced including five m 6 earthquakes one of which occurred on the pre existing fault system close to the tip of the dike at approximately the time of arrest it has been proposed that the main cause for the dike arrest was the fault induced stress here we use a boundary element numerical approach to study the interplay between a propagating dike and a pre stressed strike slip fault and check the relative role played by dike fault interaction and topographic loading in arresting the miyakejima dike we calibrate the model parameters according to previous estimates of dike opening and fault displacement based on crustal deformation observations by computing the energy released during the propagation our model indicates whether the dike will stop at a given location we find that the stress gradient induced by the topography is needed for an opening distribution along the dike consistent with the observed seismicity but it cannot explain its arrest at the prescribed location on the other hand the interaction of dike with the fault explains the arrest but not the opening distribution the joint effect of the topographic load and the stress interaction with strike slip fault is consistent with the observations provided the pre existing fault system is pre loaded with a significant stress released gradually during the dike fault interplay our results reveal how the mechanical interaction between dikes and faults may affect the propagation of magmatic intrusions in general this has implications for our understanding of the geometrical arrangement of rift segments and transform faults in mid ocean ridges and for the interplay between dikes and dike induced graben systems

three dimensional deformation mapping of a dike intrusion event in sakurajima in 2015 by exploiting the right and left looking alos 2 insar one of the limitations of the interferometric synthetic aperture radar insar is its one dimensional measurement capability although three dimensional 3 d deformation can be studied if there are three or more measurements with different viewing geometries it has not been executed because almost all sar data are acquired using a right looking geometry for the sakurajima volcanic activity on 15 august 2015 alos 2 conducted sar observations from four different viewing directions ascending descending and right left looking enabling the retrieval of 3 d deformation data only from the insar results we have retrieved 3 d deformation with high precision and resolution by a weighted least squares approach expansive deformation of over 10 cm has been observed with standard errors of 0 8 3 4 and 0 7 cm for east west north south and up down components respectively it is inferred that a dike of 1 7 106m3 volume intruded beneath the showa crater at a depth of 0 4 1 2 km

bed load transport of mixed size sediment fractional transport rates bed forms and the development of a coarse bed surface layer fractional transport rates bed surface texture and bed configuration were measured after a mixed size sediment had reached an equilibrium transport state for seven different flow strengths in a recirculating laboratory flume fractional transport rates were also measured at the beginning of each run when the bed was well mixed and planar the start up observations allow us to describe the variation of fractional transport rates with bed shear stress for a constant bed surface texture and bed configuration the start up and equilibrium observations together allow for the first time an unambiguous description of the mutual adjustment among the transport the bed configuration and the bed surface as the transport system moves toward equilibrium we find that a substantial interaction exists among the transport bed surface and bed configuration bed forms and a coarse surface layer coexist over a range of bed shear stress under some flow conditions the size and shape of the bed forms are controlled by the presence of the coarse surface layer at higher flows the coarse surface layer is eliminated by scour in the lee of the bed forms if the bed surface is defined as that over which the bed forms move a coherent relation between the bed surface texture and the transport grain size distribution may be defined at equilibrium the transport rates of all fractions were not equally mobile defined as identical transport and bed grain size distributions although equal mobility was approached for runs in which the bed shear stress was more than twice that for initial motion of the mixture under some flow conditions the transport was observed to adjust away from equal mobility as the bed adjusted from a well mixed start up condition to an equilibrium state development of a partial static armor wherein some individual grains become essentially immobile even though other grains in the same fraction remain in transport is suggested to explain these adjustments between the transport and bed surface grain size distributions constraints on equilibrium mixed size sediment transport are defined the special conditions for which equal mobility must hold and the relevance to natural conditions of flume results and the equal mobility concept are discussed

a constant influx model for dike propagation implications for magma reservoir dynamics 1 most observations of seismicity rate during dike propagation on basaltic volcanoes show 1 rate stationarity despite possible variations of the dike tip velocity 2 frequent lack of clear and monotonic hypocenter migration following dike propagation and 3 event occurrences located backward with respect to the dike tip position on these bases the origin of the seismicity contemporary to dike intrusion within basaltic volcanoes cannot be solely related to the crack tip propagation seismicity rather appears to be the response of the edifice itself to the volumetric deformation induced by the magma intruding the solid matrix the volume change induced into the volcano edifice over time by the intruding magma is equal to the magma flux injected into the dike from the reservoir the consequence of this is that the stationary seismicity rate observed during the intrusion is a proxy for the magma flux withdrawn from the reservoir we consider a two phase dike propagation model including a first vertical propagation followed by a lateral migration along a lithological discontinuity we explore 1 under which geophysical conditions the vertical dike is fed at constant flow rate of magma and 2 dike propagation patterns implications entailed by constant volumetric flux on the piton de la fournaise volcano case study suggest a minimum size for the magma reservoir of about 1 km3 and a maximum value for the initial magma reservoir overpressure of about 2 2 mpa considering similar magma inflow rates during vertical and lateral dike propagation phases we reproduce independent estimates of propagation velocities rise times and injected volumes when applying the model to the august 2003 piton de la fournaise eruption

geochemistry of the krivoy rog banded iron formation ukraine and the impact of peak episodes of increased global magmatic activity on the trace element composition of precambrian seawater abstract pure superior type banded iron formation bif samples from the krivoy rog supergroup ukraine are excellent archives of ambient early precambrian seawater they show low concentrations of incompatible elements such as zr hf and th and shale normalized rare earths and yttrium rey sn patterns similar to those of modern seawater i e heavy rey sn enriched patterns with positive la sn gd sn and y sn anomalies lack of ce sn and presence of positive eu sn anomalies indicate rey contributions to anoxic ferruginous seawater from high temperature hydrothermal fluids the depositional age of the krivoy rog bif is ill defined but a late archean to paleoproterozoic age has been suggested based on u pb zircon ages for units stratigraphically above and below the bif we determined sm nd isotopic compositions of pure and impure samples from the krivoy rog bif which yield an errorchron with an apparent age of 2406 350 ma mswd 15 that falls within this broad age range all pure bif samples show chondrite normalized subscript cn rey patterns with strong positive eu cn anomalies that are typical for archean but rather rare and much less pronounced in proterozoic bifs associated schists also show archean rather than post archean style rey distributions the rey geochemistry of both chemical and epiclastic sediments therefore is more consistent with a late archean rather than a post archean depositional age of the krivoy rog supergroup initial nd values of impure bifs and of associated schist reveal variable contributions from ttgs less radiogenic in nd and a more radiogenic component possibly comprised of basement amphibolites or mafic volcanics of the stratigraphically underlying new krivoy rog group the purest krivoy rog bif representing local krivoy rog seawater displays an nd 2 60 ga value of 2 3 this value is less radiogenic than impure krivoy rog bifs or other near contemporaneous neoarchean pure chemical sediments to preserve this specific local isotopic fingerprint in anoxic archean seawater the krivoy rog bif must have been deposited in an isolated sea basin with limited exchange with ferruginous deep waters of the open ocean a compilation of rey data for high purity precambrian bifs reveals that eu cn eu cn ratios of precambrian seawater follow a general global evolution curve that shows specific peaks which reflect times of increased high temperature hydrothermal rey input into seawater following declining eu cn eu cn ratios from the eoarchean to the mesoarchean the ratios suddenly rise at 2 7 ga and reach a maximum at 2 6 ga indicating an increased flux of high temperature hydrothermal rey to neoarchean seawater which supports the hypothesis that times of widespread bif deposition coincided with periods of intense submarine hydrothermal activity probably triggered by major mantle plume events this association is supported by a strong increase of the nd t values of pure seawater archives at 2 7 2 6 ga which reflects an increased flux of mantle nd into seawater these results suggest that eu rey systematics and potentially nd systematics are robust tools to indentify episodes of enhanced mantle plume activity

sand grain threshold in relation to bed stress history an experimental study besides particle size density and shape the erodibility of a sediment bed depends also upon the exposure to prethreshold velocities in the overlying flow such flow effectively rearranges the grains at and below the bed surface causing them to become more resistant to subsequent erosion the effects of the stress history leading up to the critical condition for sediment movement are investigated for unidirectional flows generated in a recirculating laboratory flume the sediment beds investigated consisted of cohesionless quartz sand grains with mean grain diameters of 0 194 mm fine sand 0 387 mm medium sand and 0 774 mm coarse sand with narrow particle size distributions the critical threshold shear velocity target value for the three beds was established within 2 5 min of increasing the flow from zero velocity the subsequent experiments were performed under prethreshold velocities at 70 for 5 10 20 40 and 80 min exposure duration 80 for 5 10 20 40 and 80 min exposure duration 90 and 95 for 5 10 20 40 80 and 120 min exposure duration of the target value following exposure to these different prethreshold conditions the flow was increased then to reach actual critical conditions within a period of 2 5 min the critical condition for the initiation of sediment movement was established using visual observation supplemented by video recordings according to the yalin criterion the results show that if the exposure duration to prethreshold velocities remains constant then the critical shear velocity increases with increasing prethreshold velocity likewise if the prethreshold velocity remains constant then the critical shear velocity increases with increasing exposure duration in some circumstances the critical shear velocity was found to increase by as much as 27 an empirical formula is proposed to account for the exposure correction to be applied to the critical shear velocities of sand sized sediment beds this is prior to their inclusion into bedload transport formulae for an improved prediction of the magnitude and nature of transport

graben formation and dike arrest during the 2009 harrat lunayyir dike intrusion in saudi arabia insights from insar stress calculations and analog experiments

crustal extension from exposed sheet intrusions review and method proposal abstract sheet intrusions are probably the most widespread magmatic features on earth they create a space for themselves by dilating the host rock the exposed sheet intrusions allow an estimate of a minimum value of the extension to which the crustal layers and the volcanic edifices are subject the correct evaluation of the contribution of sheet intrusions to the crustal extension is of great importance in various geologic settings the current methods used to evaluate the extension due to exposed sheet intrusions are reviewed in this paper the effect of the dip of sheet intrusions on the evaluation of horizontal crustal extension is analysed if the dip effect is neglected and the intrusions are sub vertical the error due to the dip effect in the extension evaluation is small if the intrusions are shallow dipping the error may be great and varies as a function of the dyke dip for dykes with dip range 45 65 the error due to neglecting the dip effect is 30 to 15 increasing discrepancies of more than 25 are produced on the computed dilation when the intrusions progressively dip less than 50 a simple method based on structural field measurements of exposed sheet intrusions and on trigonometric considerations is presented as an improvement of pre existing methods to estimate the horizontal component of the crustal extension using this method field data of 1154 dykes from different geodynamic contexts are analysed the horizontal component of extension is computed using dyke data from the tertiary massifs of lanzarote and tenerife and from the active stratovolcanoes of monte somma vesuvius and etna since the analysed dykes are steeply dipping the cumulative difference between the extension computed neglecting and taking into account the dip effect remains less than 10 for all the data sets the percentage difference is nearly constant and cumulatively higher for monte somma where the mean dip of the intrusions is smaller in lanzarote 237 dykes emplaced between 15 5 and 3 7 ma give a peak extension of 276 m in direction n135 and a time averaged minimum extension rate of 1 9 10 2 to 2 3 10 2 mm a 1 in tenerife an extension of 562 m is computed in the direction n65 from 563 dykes emplaced from 7 4 to 3 3 ma the time averaged minimum rate of extension for the tertiary massifs of tenerife ranges from 0 14 to 0 25 mm a 1 the peak extension computed for monte somma is 81 7 m in the direction n90 based on 96 exposed sheets very likely most of monte somma sheets intruded within 12 ka giving a time averaged minimum extension rate of 7 mm a 1 from 184 exposed dykes of etna a minimum extension rate of 6 2 mm a 1 is computed peak extension is 212 m in direction n35 the poor resolution of age constraints may affect deeply the reliability of the extension rates however from the examples presented it appears that the extension rates of pre erosional successions from intraplate volcanic islands can be smaller than those of active stratovolcanoes this may suggest a more active dynamics in the stratovolcanoes

diagenetic uptake of rare earth elements by bioapatite with an example from lower triassic conodonts of south china abstract the distribution patterns of rare earth elements rees are frequently used as proxies for ancient seawater chemistry or paleomarine environmental conditions however recent work has shown that diagenesis can lead to remobilization and inter elemental fractionation of rees and that these effects often occur in conjunction with redox reactions in sediment porewaters here we review existing literature on the diagenetic fluxes of rees in marine sediments and porewaters in order to systematize existing knowledge on this subject rees undergo significant redistribution among sediment phases during both early and late diagenesis as a consequence of adsorption and desorption processes remobilization of rees commonly leads to inter elemental fractionation variously leading to enrichment or depletion of the light middle or heavy ree fractions further ree remobilization can be facilitated by redox changes e g through reductive dissolution of host phases in suboxic and anoxic porewaters characteristic ree distribution patterns develop through these processes 1 a flat distribution signifying predominantly terrigenous siliciclastic influence 2 a middle ree bulge probably due to adsorption of light and heavy rees to mn and fe oxyhydroxides respectively and 3 heavy ree enrichment indicative of hydrogenous seawater influence note all patterns in this paper are normalized to the ree composition of average upper continental crust or ucc in the second part of this study we undertake an analysis of the ree distributions in conodonts and whole rock samples from west pingdingshan a permian triassic boundary section in south china using ree th and y ho ratios we show that almost all of the conodont samples have a strong diagenetic overprint and that the hydrogenous ree fraction is small and not isolatable furthermore the conodonts contain two diagenetic ree components one characterized by low ree 100 300 ppm high ree th ratios 1000 strong middle ree enrichment and eu eu ratios of 1 5 2 0 and the second by high ree 300 2000 ppm low ree th ratios 20 30 little or no middle ree enrichment and eu eu ratios of 1 0 the first component exhibits a pronounced middle ree bulge that represents an early diagenetic signature associated with suboxic conditions possibly related to adsorption of rees onto fe mn oxyhydroxides in the shallow subsurface environment the second component shows a flat ree distribution that is similar to both our whole rock samples and average ucc indicating derivation from rees released from detrital siliciclastics e g clay minerals probably at a range of burial depths from shallow to deep failure of the conodont samples to yield an isolatable hydrogenous component demonstrates that bioapatite does not always preserve a primary marine ree signature given that bioapatite rees have been widely used for this purpose often on the assumption of minimal or no diagenetic influence our findings are likely to necessitate a re evaluation of the results of many earlier studies in general we counsel caution in inferring a hydrogenous origin for rees in bioapatite owing to frequent diagenetic alteration of ree distributions

intrusive mechanism of the 2002 ne rift eruption at mt etna italy inferred through continuous microgravity data and volcanological evidences 1 in october 2002 a new eruption began at etna lava flows were issued from two different fissure systems on the ne and s flanks of the volcano a continuous microgravity sequence acquired at a station on the n slope 2800 m a s l shows a marked decrease about 400 gal in less than one hour about 4 hours before lava was first emitted from the eruptive fissures along the ne rift this anomaly reversed soon afterward at a high rate the strong gravity decrease is interpreted as the opening by external forces of a shallow fracture system 1 km w of the gravity station magma from the central conduit entered the new fracture system passively and propagated through it towards lower portions of the ne rift both the arrangement of the new fracture system and the eruptive dynamics are in keeping with the inferred intrusive mechanism

eruption versus intrusion arrest of propagation of constant volume buoyant liquid filled cracks in an elastic brittle host received 9 january 2009 revised 31 march 2009 accepted 3 april 2009 published 5 june 2009 1 when a volume of magma is released from a source at depth one key question is whether or not this will culminate in an eruption or in the emplacement of a shallow intrusion we address some of the physics behind this question by describing and interpreting laboratory experiments on the propagation of cracks filled with fixed volumes of buoyant liquid in a brittle elastic host experiments were isothermal and the liquid was incompressible the cracks propagated vertically because of liquid buoyancy but were then found to come to a halt at a configuration of static mechanical equilibrium a result that is inconsistent with the prediction of the theory of linear elastic fracture mechanics in two dimensions we interpret this result as due to a three dimensional effect at the curved crack front horizontal cracking is necessary in order for vertical propagation to take place as the crack elongates and thins the former becomes progressively harder and in the end impossible to fracture we present a scaling law for the final length and breadth of cracks as a function of a governing dimensionless parameter constructed from the liquid volume the buoyancy and host fracture toughness an important implication of this result is that a minimum volume of magma is required for a volcanic eruption to occur for a given depth of magma reservoir

imaging composite dike propagation etna 2002 case 1 late on the night of 26 october 2002 a dike intrusion started suddenly at mount etna producing intense explosive activity and lava effusion on the southern flank five to six hours afterward a long field of eruptive fractures propagated radially along the northeastern flank of the volcano producing marked variations at the permanent tilt network the dike propagation velocity was inferred by the associated seismicity we modeled the temporal evolution of the continuously recorded tilt data both during the vertical dike propagation on the high south flank on 26 october and during the radial propagation along the northeast flank between 27 and 28 october the reproduction of the recorded tilt signal allowed us to describe the geometry and characteristics of the two dikes in greater detail than the previous static inversion we deduced that the eruption was characterized by an unusual composite mechanism clearly showing a transition from a nearly pure opening mode displacement to a mechanism characterized by an equally strong normal dip slip component and a smaller left lateral strike slip component in this study we demonstrate the interaction between the final segment of the dike and a preexisting structure that was reactivated in response to the intrusion we show that tilt and its modeling represent a powerful tool to verify and constrain dike intrusions in detail

laboratory investigations into the threshold of movement of natural sand sized sediments under unidirectional oscillatory and combined flows the threshold of movement of sediment obtained from sandbanks within the bristol channel uk is investigated under unidirectional oscillatory and combined flows the experiments were undertaken in a recirculating unidirectional laboratory flume containing an oscillating plate to simulate wave action with movement along the same axis as the unidirectional flows the sand samples consisted of cohesionless quartz grains with median grain sizes between 0 315 and 0 513 mm the experiments were performed under flow velocities measured at 2 cm above the bed ranging between 0 and 24 cm s 1 and oscillatory currents wave periods of 5 12 and 15 s ranging from 0 to 28 cm s 1 the critical conditions for the initiation of sediment movement were assessed by visual observation using the yalin criterion the results show that under unidirectional flow there is a slight overestimation of the threshold of naturally graded sediments derived on the basis of empirically derived threshold curves for artificially prepared sediments under similar flow conditions in the case of oscillatory flows the threshold for the natural sands is found to be higher than that predicted by previously derived empirical curves under combined flows wave period is shown to control threshold conditions with the unidirectional and oscillatory flow components combining in a linear fashion for long period 12 s and 15 s waves in contrast in the presence of short period 5 s waves the unidirectional and oscillatory components of the flow appear to decouple for high orbital velocities in both cases the effect of the wave period on threshold diminishes

threshold of coarse sediment transport in broad and narrow natural streams the threshold of coarse sediment transport has been examined in natural streambeds in an upland pennine u k area threshold values of the total boundary shear stress t0 for a given grain size in a narrow natural stream w d 11 efficiency in the entrainment process is related not only to the overall channel geometry but also varies as a function of discharge in channels characterized by compound roughness empirical curves relating t0 and a mean grain size d5 are presented but are limited in application to streams of similar physical and hydraulic characteristics as the ones examined in this investigation considerable divergence is noted between these empirical functions and a summary empirical function for general application obtained from a published source the reasons for this divergence are discussed the influence of grain shape was found not to be important in the initiation of motion criterion this conclusion may reflect the limited range of natural grain shapes in the study streams but might reasonably apply to other field investigations of similar streams modifications of the shields and yalin diagrams are suggested for practical applications in shallow streams with poorly graded bed material the shields parameter may be regarded as an inverse function of the relative protrusion of individual grains in the shallow flow depth d5 d the increased importance of augmented drag forces in the entrainment process in shallow flows is suggested as the physical explanation for the reduced values of the shields parameter however the relationships presented should not be applied to laboratory experiments concerned with well graded sediments therefore beds with little deviation in level in which the shields parameter may be regarded as constant at high reynolds grain numbers consistent estimated field values of a threshold sediment transport parameter might be used to compare field data to threshold values derived from statistical arguments and laboratory experiments reported in the literature

bed load transport of coarse nonuniform sediment experiments involving measurements of the bed load transport rates of different fractions in a mixture are reported a conceptual model for the effect of a particular size of sediment on the transport rates of other sizes of sediment is proposed the experimental data are analyzed with the dual objectives of assessing the accuracy of existing methods of computation of bed load transport and of proposing a new method of computation

laboratory measurements of pivoting angles for applications to selective entrainment of gravel in a current important to grain entrainment by a flowing fluid is the pivoting angle of the grain about its contact point with an underlying grain a series of experiments has been undertaken to determine how this angle depends on grain shape rollability and angularity on the ratio of the size of the pivoting grain to those beneath and on factors such as imbrication the experiments involved gravel sized spheres ball bearings and marbles natural pebbles selected for their approximately triaxial ellipsoid shapes and angular crushed basalt pebbles the pivoting angles for these grains were measured on an apparatus consisting of a board which can be progressively inclined the angle of the board being equal to the pivoting angle at the instant of grain movement the pivoting angles of spheres showed reasonable agreement with a theoretically derived equation showing much better agreement than in previous studies which utilized sand sized spheres a series of measurements with spheres ranging from sand to gravel sizes reveals that the pivoting angles decrease with increasing particle size our results are therefore consistent with the earlier studies limited to sand size spheres the cause of this size dependence is unknown since moisture and electrostatic binding can be ruled out similar size dependencies are also found for the ellipsoidal pebbles and angular gravel the experiments with ellipsoidal pebbles demonstrated a strong shape dependence for the pivoting angle being a function of the ratio of the pebble s smallest to intermediate axial diameters this ratio controls the grain s ability to roll and pivot with small ratios of these diameters the pebbles tended to slide out of position whereas with ratios closer to unity circular cross section true pivoting took place and the angles were smaller experiments with flat pebbles placed in an imbricated arrangement yielded much larger angles than when the pebbles lay in a horizontal position the pivoting angle being increased approximately by the imbrication angle the angular crushed gravel also required high pivoting angles apparently due to interlocking of the grains resulting from their angularity other factors being equal the measurements of pivoting angles demonstrate that the order of increasing difficulty of entrainment is spheres ellipsoidal grains angular grains and imbricated grains the results obtained here make possible the quantitative evaluation of these shape effects on grain threshold as well as evaluation of the selective entrainment of grains from a bed of mixed sizes

a dike intrusion model in and around miyakejima niijima and kozushima in 2000 from june 26 2000 an earthquake swarm started in miyakejima about 50 km south off honshu central japan eruptions of miyakejima and five large earthquakes with magnitudes 6 0 and above occurred for the following 2 months together with a large number of smaller earthquakes over 100 000 in this study we focused on spatio temporal crustal deformation observed at gps stations in this area in order to explain the seismicity and the crustal deformation we considered a dike intrusion model placed between miyakejima and kozushima then we attempted to obtain spatio temporal distributions the amount of the dike intrusion deflation beneath miyakejima and fault slips of the seven large events by geodetic data inversion for this purpose we divided the time series of gps data from june 12 to august 27 into 10 periods which are related to significant events in order to find location depth strike and dip of the dike plane and a depth of deflation beneath miyakejima we used the monte carlo method and tested 10 000 models for each period as a result we estimated that the total amount of the dike intrusion and the deflation beneath miyakejima reached about 1 1 10 9 and 5 4 10 8 m 3 respectively the maximum amount of the dike intrusion and the deflation beneath miyakejima reached over 3 5 10 8 m 3 during the period from july 20 to july 28 2000 and over 1 7 10 8 m 3 during the period from june 15 to june 28 2000 respectively temporal change of the amount of the dike intrusion corresponds well to that of the deflation beneath miyakejima until the middle of july however since large amount of the dike intrusion from july 20 to 28 did not correspond to that of the deflation beneath miyakejima we deduced that the magma source changed from miyakejima to kozushima and the magma might come from sub crustal magma pockets from the middle of july if we assume the open crack associated with the dike intrusion and the deflation beneath miyakejima are filled with magma with a density of 2500 kg m 3 the mass would be about 2 75 10 9 and 1 35 10 9 tons respectively we deduced that at least 1 4 10 9 tons of magma came from sub crustal magma pockets d 2002 elsevier science b v all rights reserved

pre eruption deformation caused by dike intrusion beneath kizimen volcano kamchatka russia observed by insar abstract interferometric synthetic aperture radar insar images reveal a pre eruption deformation signal at kizimen volcano kamchatka russia where an ongoing eruption began in mid november 2010 the previous eruption of this basaltic andesite to dacite stratovolcano occurred in 1927 1928 insar images from both ascending and descending orbital passes of envisat and alos palsar satellites show as much as 6 cm of line of sight shortening from september 2008 to september 2010 in a broad area centered at kizimen about 20 cm of opening of a nearly vertical dike provides an adequate fit to the surface deformation pattern the model dike is approximately 14 km long 10 km high centered 13 km beneath kizimen and strikes ne sw time series analysis of multi temporal interferograms indicates that 1 intrusion started sometime between late 2008 and july 2009 2 continued at a nearly constant rate and 3 resulted in a volume expansion of 3 2 107 m3 by september 2010 i e about two months before the onset of the 2010 eruption earthquakes located above the tip of the dike accompanied the intrusion eventually magma pressure in the dike exceeded the confining strength of the host rock triggering the 2010 eruption our results provide insight into the intrusion process that preceded an explosive eruption at a pacific rim stratovolcano following nearly a century of quiescence and therefore have implications for monitoring and hazards assessment at similar volcanoes elsewhere

sequence of rifting in afar manda hararo rift ethiopia 2005 2009 time space evolution and interactions between dikes from interferometric synthetic aperture radar and static stress change modeling thirteen dike intrusions in the manda hararo rift afar ethiopia from september 2005 to june 2009 studied using an extensive interferometric synthetic aperture radar insar data set provide insight into the mechanics of a major active rift kinematic inversions of insar data reveal that dikes opened by 0 8 3 5 m at an average 5 km depth with volumes of 0 04 0 2 km 3 with up to 12 m opening and a volume greater than 1 km 3 for the september 2005 megadike dikes have their source in a midsegment magma reservoir which induces a local shallowing of the brittle ductile boundary presumably due to thermal weakening of the lithosphere the smaller dikes in 2006 2009 were emplaced in regions of minimum opening of the september 2005 megadike above the central magma reservoir in contrast the most voluminous dike intrusions in 2006 2009 occurred near the locus of the peak of maximum opening of the september 2005 megadike 10 km north of the magma source this may suggest that tension on the plate boundary was highest there both prior to 2005 and possibly also after 2005 evolution and distribution of normal stress on the plate boundary throughout the rifting episode may indicate that tension near to the magma reservoir is lower than toward segment ends average relief of normal stresses of tectonic origin coeval to dike intrusions is comparable with shear stress drops for earthquakes presumably because dikes in the manda hararo rift are intruded at low magma pressure and high tectonic stress time space evolution and interactions between dikes from interferometric synthetic aperture radar and static stress change modeling

a comparison of rift zone tectonics in iceland and hawaii observations of eroded volcanic rift zones indicate that dikes in iceland are typically several times thicker than those in hawaii geodetic and seismic observations of active rifts however suggest that dike heights in the two regions are similar provided the elastic properties of the rift zones are the same this implies that dikes are intruded with higher driving pressures magma pressure minus compressive stress perpendicular to the dike plane in iceland than hawaii a second major difference between the two regions is the greater prevalence of large normal fault scarps in rift zones in iceland from this it can be infered that a lower percentage of dikes breach the surface in iceland than in hawaii thus although dikes in iceland are intruded with higher driving pressures they possess lower absolute magma pressures than in hawaii these differences can be interpreted in terms of the tectonic settings in the two regions in iceland a steady remote extension reduces the horizontal stress perpendicular to the rift zone allowing dikes to be intruded with low absolute pressures but high driving pressures when magma becomes available in hawaii a more continuous magma supply on the timescale over which the dike induced stresses are relaxed and perhaps a greater role for intrusions in driving long term rift extension ensure that the rift compressive stress is not relaxed significantly before the next dike is intruded thus the magma pressure must be nearly sufficient for eruption in order for intrusion to occur if the mechanism for relaxing the rift compressive stress were less efficient still then an even higher percentage of dikes would erupt and at times the rift zone trend could become an unfavorable orientation for dike intrusion such might be the case at mauna loa which lacks large rift zone faults and fissures and possesses numerous radial vents outside its two main rift zones

downstream fining in large sand bed rivers abstract downstream fining of bed sediments is a well known phenomenon in gravel bed rivers but also occurs in large sand bed rivers the underlying processes however are not necessarily the same due to the difference in sediment mobility grain size distribution width and geographical setting downstream fining studies focusing on large sand bed rivers are scarce nevertheless in this paper the processes that affect downstream fining in gravel bed rivers are reviewed and it is evaluated to what extent they are relevant for large sand bed rivers furthermore several new downstream fining processes are discussed ending in a conceptual model of downstream fining in large sand bed rivers in deriving the conceptual model downstream fining processes are divided into three categories abrasion selective transport and sediment addition extraction abrasion rates in large sand bed rivers are small due to the durable lithologies the small grain size the high degree of rounding the dominance of grinding as abrasion process and the dominant suspended transport mode of the bed sediment only in the upstream part of large sand bed rivers abrasion rates may be somewhat higher selective transport rates in large sand bed rivers everywhere overrun abrasion rates but are small compared to selective transport rates in gravel bed rivers because the unimodal sediments in sand bed rivers exhibit stronger hiding exposure effects during threshold of motion conditions but especially because many sand bed rivers though not all are in state of fully mobilised transport making differences in threshold of motion between coarse and fine grains relatively unimportant selective transport in these rivers is probably mainly the result of the presence of suspended load transport in combination with the effects of dune and perhaps bend sorting which are all processes that act on a relatively large scale within sand bed rivers the degree of selective transport probably decreases downstream due to the change in mixture bimodality and sorting efficiency there are indications that coarse grains may sometimes even be more mobile than fine grains in the downstream part of sand bed rivers note that in contrast to abrasion selective transport only produces a stable downstream fining pattern if the river profile is concave generally a stronger profile concavity leads to a higher aggradation rate and a stronger downstream fining trend sediment addition and extraction processes can obscure the effects of abrasion and selective transport in sand bed rivers overbank sedimentation for instance may significantly decrease the downstream fining rate in large sand bed rivers because especially fine grains are removed from the bed dredging can also strongly affect the downstream fining trend depending on the volume and the grain size of the sediment involved meander migration may cause a nett loss of coarse grains from the channel in aggrading circumstances increasing the downstream fining rate river bifurcations can cause a discontinuity in the downstream fining pattern because bend sorting upstream of the bifurcation causes the river branch that originates in the outer bend of the main channel to be much coarser than the other branch all these addition and extraction processes not only have a direct effect on the grain size composition of the river bed but also indirectly influence downstream fining by their effect on the longitudinal bed profile other sediment addition and extraction processes such as tributary confluences non alluvial sediment sources and dumping are less common in large sand bed rivers but can locally have a great effect a full determination of the relative importance of all downstream fining processes in large sand bed rivers requires extensive field measurements the construction of detailed fractionwise sediment balances and the development of numerical downstream fining models that incorporate all processes described above

a quantitative approach to the dike intrusion process inferred from a joint analysis of geodetic and seismological data for the 1998 earthquake swarm off the east coast of izu peninsula central japan 1 a series of earthquake swarms off the east coast of izu peninsula central japan is typically associated with dike intrusion around active volcanoes numerous studies in this region have documented the seismic activity and ground deformations without fully resolving the process of dike intrusion this study involves a joint analysis of geodetic and seismic data from the most recent major activity in this region occurring in 1998 to reveal the process of dike intrusion quantitatively we combine data from a dense gps array including 10 temporary stations with seismic analysis published previously we estimate the location of the dikes from seismic data and the volume of intruded magma from geodetic data combined analysis of both data sets enables us to reveal the kinematic process of dike intrusion associated with the 1998 earthquake swarm the process of dike intrusion is quantitatively modeled using the time evolution of recorded dike volume and the use of seismic hypocenters as an indicator of the site of instantaneous dike intrusion on the basis of these observations we propose the following process of the dike intrusion the magma rose upward through a pathway aligned vertically and created a dike when it lost buoyancy we also evaluate the dike intrusion process quantitatively using a model of a circular dike that grows under a continuous magma supply from a deep magma reservoir from the observational data we infer the time evolution in the dike shape pressure distribution and magma viscosity in the dike the acquired viscosity increases over time and it may be reasonable because magma cools gradually however its value is too large compared to typical basaltic magma expected in this region it may represent magma within the dike that is nearly solidified otherwise the magma is supplied intermittently and the dike growth is mainly controlled by the magma supply rate from a deep reservoir the quantitative description of the dike intrusion process in nature described in this study is very rare and our results provide constraints on several parameters that have not been solved in many previous theoretical studies on the dike intrusion process

observations of wave generated vortex ripples on the north carolina continental shelf 1 sand ripples with wavelengths between 0 5 and 3 m were observed on the bottom across the u s east coast continental shelf off north carolina during three side scan sonar surveys in september and december 1999 ripples were present in about 75 of the survey images in particular in regions with coarser sediments analysis of surficial sediment samples shows that median grain diameters range from 0 1 to 4 7 mm with large variations on the inner shelf over distances 1 km the observed ripple properties are consistent with wave generated vortex ripples analysis of concurrent wave observations indicates that the ripple crests were aligned perpendicular to the average direction of near bottom wave induced motions during preceding events that were sufficiently energetic to mobilize surficial sediments furthermore the ripple wavelengths proportionality to near bottom wave orbital excursions is consistent with wave formed vortex ripples these findings support the hypothesis that the observed strong attenuation of waves across the shelf resulted from form drag over large vortex ripples

seismicity during lateral dike propagation insights from new data in the recent manda hararo dabbahu rifting episode afar ethiopia seismicity released during lateral dike intrusions in the manda hararo dabbahu rift afar ethiopia provides indirect insight into the distribution and evolution of tensile stress along this magma assisted divergent plate boundary in this paper 5 dike intrusions among the 14 that form the 2005 present rifting episode are analyzed with local and regional seismic data during dike intrusions seismicity migrates over distances of 10 15 km at velocities of 0 5 3 0 km h away from a single reservoir in the center of the rift segment confirming the analogy with a slow spreading mid ocean ridge segment comparison with geodetic data shows that the reservoir is located 7 km down rift from the topographic summit of the axial depression dikes emplaced toward the north are observed to migrate faster and to be more voluminous than those migrating southward suggesting an asymmetry of tension in the brittle elastic lithosphere seismicity during dike injections is concentrated near the propagating crack front in contrast faults and fissures in the subsurface appear to slip or open aseismically coeval with the intrusions the seismic energy released during dike intrusions in the manda hararo rift appears to be primarily modulated by the local magnitude of differential tensile stress and marginally by the rate of stress change induced by the intrusion the low level of seismic energy accompanying dike intrusions despite their significant volumes is likely an indicator of an overall low level of tension in the lithosphere of this nascent plate boundary