tectonic and structural evolution of gough volcano a volcanological model abstract morphostructural stratigraphic and tectonic data indicate that the evolution of gough volcano is similar to other oceanic intraplate volcanoes is older than 1 ma and is related to a transform fault at least six evolutionary stages can be distinguished within two major magmatostructural periods dominated by basaltic and trachytic magmas respectively the basaltic shield volcano is characterized by a curved elongated shape in plan and a rift zone with a high density of dykes combined with a radial intrusive system the latter is interpreted as being fed by a magma chamber some 4 km below the surface the activity of the volcano became more centralized at the end of the basaltic period and its slopes became steeper this corresponds to the development of a shallower and narrower central conduit in the edifice the basaltic period was terminated by formation of a shield caldera related to the 4 km deep magma chamber the term shield caldera is used for a collapse structure that is postmagmatic large in comparison with the diameter of the volcano and delimited by normal faults that do not show a closed circular pattern but rather a series of arcs in contrast summit calderas are defined as smaller circular shaped centrally situated synmagmatic features related to a central shallow column during the basaltic period landslides were generated on the flanks of the edifice as a result of slope stability factors which are not easy to determine at present and dynamic factors among which the intrusion of magma along a curved zone certainly played a major role the trachytic period is characterized by comparatively rare pyroclastic deposits and a large volume of thick flows extruded from domes these extrusions as well as plugs formed from vertical cylindrical columns of magma rising from shallow individual magma pockets fed by the main reservoir

gravity anomalies and crustal signature of volcano tectonic structures of pico island azores pico island is the youngest 300 ka island of the azores archipelago dominated on its western sector by the stratovolcano of pico mountain which rises about 3 5 km above the surrounding ocean floor the other two major morpho tectonic units of the island include the shield volcano of topo and the achada plateau a 29 km long volcanic ridge with a wnw ese trend a gravimetric study on pico island coupled with geological and volcanological data was aimed towards a better understanding of its deep volcano tectonic structures from a total of 407 stations a regional model of density contrasts was developed in order to reproduce the crustal structure of the island in addition a local high resolution model of the sub surface structure of the westernmost part of the island is presented both models were obtained by means of an inversion global adjustment on fixed density contrasts in order to determine the three dimensional geometry of the sources of the anomalous gravity field the most prominent anomalous gravity structure found in pico island is a body with positive density contrast located on the se slopes of topo volcano this anomaly extends to depth of approximately 8 km mean depth of 4 5 km and has a volume of 310 km 3 this anomaly maybe best interpreted by the presence of a major intrusive system beneath topo volcano and can be also correlated to gravitational collapses at the shield volcano a second body of positive density contrast which appears to extend to 4 5 km depth is located near the town of madalena this anomaly may be correlated to a high density anomaly recently detected in the ne sector of faial island smaller sized high density bodies at shallow depths of around 1 3 km are located beneath pico mountain volcano and all together define a semi circular arc with an average diameter of 8 km we propose that this arcuate structure might outline a ring fracture associated with the collapse of the central sector of pico mountain volcano early in its growth low density bodies are located on the nw flanks of pico mountain along a linear ene trending structure these bodies extend to depths of 4 km and have a total volume of 69 km 3 they are interpreted as being associated with an old extensional fracture zone previous studies suggested the existence of a shallow and large magma reservoir with volumes up to 1000 km 3 beneath pico mountain volcano in this study we have not found any evidence of the existence of such huge magma body on the contrary we propose that those discrete low density bodies correspond to complex intersections of different sets of tectonic lineaments along which magma tends to intrude as shallow 2 5 km and small 10 20 km 3 magma pockets c 2006 elsevier b v all rights reserved

insights on the march 1998 eruption at piton de la fournaise volcano la reunion from microgravity monitoring we investigate the temporal gravity changes associated with one of the major recent eruptions at piton de la fournaise volcano march 1998 that occurred after an unusual five year quiet period and initiated a new eruptive cycle repeated microgravity surveys allowed us to measure residual gravity changes up to 100 gal within the enclos fouque caldera for four months before the start of the eruption we first analyzed the temporal gravity changes and the height changes also measured at the gravity benchmarks on the basis of an intrusive dyke model previously proposed for this eruption from insar and tilt data this analysis reveals that such simple model finite rectangular shaped tensile dislocation cannot fit both gravity and geodetic data and leads us to propose a dual source gravity model for this eruption the gravity data inversion using a genetic algorithm search method allows us to quantify the mass change produced by the intrusive dyke 3 9 to 8 7 109kg and to suggest an additional mass increase at sea level depth 4 6 to 7 2 1010kg we interpret the latter as the effect of magma ascent into a reservoir that occurred within the four months before the eruption we suggest that high concentrations of gas content in the magma reservoir account for a higher compressibility of the intruded magma and produce significant mass change with moderate surface deformation the 1996 seismic crisis interpreted as an upward migration of magmatic fluids into a reservoir located at sea level might support such a hypothesis

dynamics and emplacement mechanisms of the successive baige landslides on the upper reaches of the jinsha river china abstract two successive baige landslides are presented in this study to investigate the dynamics and emplacement mechanisms field survey shows that the deposit characteristics of the first landslide are affected by the extremely fractured rock masses and the complex spatial distribution of lithologies in the source area this deposit consists of abundant angular megablocks on the surface supported by clasts with a relatively abundant content of rubble ranging from micrometre to decimetre sizes and rarely well preserved original stratigraphic sequence diapiric structure and convoluted structure these sedimentological characteristics imply that a simple differential shear process induced by compression is distributed through the entire avalanche masses with low internal disturbance during the emplacement of the first landslide the three dimensional discrete element simulations show that i the first landslide reaches its maximum average velocity of 41 m s in 28 s and eventually is bulldozed to a halt in the river within 80 s ii the second landslide is controlled by entrainment which impedes the avalanche from further accelerating and simultaneously imparts kinematic energy to the substrate eventually becoming entirely mobilized and reaching a maximum average velocity of 35 m s in 36 s in the dynamic process of the second landslide the significant entrainment induced enlarged volume its entrainment ratio reaches 3 3 is associated with the steep concave terrain water bearing substrate and lower roughness substrate basal boundary and overloading impact induced erosion is proposed to be the most significant contribution to this entrainment

age and nature of deposits on the submarine flanks of piton de la fournaise reunion island abstract debris avalanche deposits are by far the most extensive and voluminous formations in the submarine domain around reunion island 21 sites sampled offshore of piton de la fournaise were studied all the dredged rocks have a volcanic origin and often consist of blocks buried in a finer grained matrix both massive and vesiculated samples were found but only rare fragments of pillow lavas were encountered all the lavas are basalts with variable amounts of olivine a few plagioclase bearing basalts were also found these lithologies are known among piton de la fournaise subaerial lavas a set of fresh samples representing the various rock types was further studied the microscopic characteristics of these lavas confirm their great similarity with lavas from the subaerial volcano major elements show that all the submarine lavas are transitional basalts with compositions very similar to those of piton de la fournaise subaerial lavas the petrological and geochemical similarity of the dredged samples with piton de la fournaise subaerial lavas indicates that they belong to this volcano the sulfur contents of the dredged samples and of various subaerial lavas from piton de la fournaise were determined on whole rocks the results suggest that massive or slightly vesiculated dredged lavas 0 005 0 010 wt s probably correspond to the internal parts of slowly cooled and subaerially degassed lava flows the dredged pillow lavas display sulfur contents 0 013 0 020 wt s similar to those of the external parts of recent subaerial lavas this indicates that they were primarily degassed in the atmosphere as also suggested by noble gases data or in shallow water only one dredged sample has a relatively high s content 0 034 wt suggesting that it may have been an early lava of a subaerial eruption or a lava directly emplaced under water though at shallow depth thus both the nature of the dredged samples and their sulfur contents indicate that these lavas were subaerially emitted these results thus give strong evidence for landslide deposits having their source areas on land k ar and 230 th 238 u dating were also performed on some samples the dredged lavas involved in the landslides of the volcanic edifice are mostly in the range of 110 ka to

a dynamic interpretation of tristan da cunha volcano south atlantic ocean abstract new observations on the morphostructure eruptive characteristics and intrusive system of tristan da cunha an active intraplate volcano of predominantly trachybasaltic composition are combined with previously published data to arrive at a model of its internal structure volcanic activity on the island of tristan da cunha is characterized by a dual central and lateral feeding system the central activity occurred at the summit presumably on top of an axial magma column the eruptions produced small flows as well as a high percentage of pyroclastic deposits partly of hydrovolcanic origin the basaltic magma probably encountered a shallow water table or ephemeral crater lakes these pyroclastic products contributed in large part to the building up of a well developed central cone from the column a dense swarm of radial dykes were generated by hydraulic fracturing in contrast the lateral activity occurred on a much lower level on the flanks of the volcano the style of eruption was strombolian resulting in scoria cones and flows there is a concentration of strombolian vents around the base of the central cone this concentric pattern is ascribed to conical shear planes in depth created by uplift of the roof of the magma chamber the uplift may have contributed to the elevation of the central cone in addition a number of lateral vents occur along radial fractures that appear to be concentrated along two tectonic axes oriented n170 e and n80 e these directions are parallel to the compressive axes of the stress field in the ocean floor the most prominent axis n170 e connects the two most recent eruptions of rather unusual trachyandesitic entirely effusive character segmented en echelon dykes and fissures observed in plan and in vertical cliffs are related to slope instability when a dyke propagates to the surface it enters a zone of bimodal stress extension and shearing as a result of downslope movement under such conditions en echelon segmentation of the dyke takes place

size and depth of ancient magma reservoirs under atolls and islands of french polynesia using gravity data new insights into the structural and tectonic evolution of islands and atolls in french polynesia are derived from the analysis of gravity data free air anomaly maps were constructed using gravity data from land surveys of 25 islands combined with free air anomaly data derived from satellite altimetry and shipborne gravimeters residual isostatic anomalies were calculated using a three dimensional 3 d four layer crustal model taking into account the bathymetry of the seafloor the topography of the islands and the deflection of the lithosphere under the load of the volcanoes twenty of the 25 islands yield a positive residual anomaly ranging between 9 and 60 mgal negative residual anomalies over four islands probably correspond either to limestone deposits or to fragmented material from aerial volcanism the occurrence of a positive anomaly provides some evidence that there is a solidified magma chamber at depth beneath each of several islands in french polynesia a linear relation between the amplitude of the positive gravity anomaly and island volume is also observed geological features which generate these positive anomalies are described by simple geometric models 3 d ellipsoidal dense bodies which lead to a rough estimate of the size and depth of the magma chambers we then propose a simple linear relation between the volume of such magma chambers and the volume of islands that can be used for other extinct intraplate basaltic volcanoes the diameter of the inferred magma chamber and its projection on the surface of the islands imply that calderas correspond mostly to collapses of the flanks of the edifices rather than to caldera vertical subsidence

three dimensional shear velocity anisotropic model of piton de la fournaise volcano la reunion island from ambient seismic noise we cross correlate 4 years of seismic noise from the seismic network of piton de la fournaise volcano la reunion island to measure the group velocity dispersion curves of rayleigh and love waves we average measurements from vertical and radial components to obtain 577 rayleigh wave dispersion curves the transverse components provided 395 love wave dispersion curves we regionalize the group velocities measurements into 2 d velocity maps between 0 4 and 8 s finally we locally inverted these maps for a pseudo 3 d anisotropic shear velocity model down to 3 km below the sea level using a neighborhood algorithm the 3 d isotropic shear wave model shows three distinct high velocity anomalies surrounded by a low velocity ring the anomaly located below the present plaine des sables could be related to an old intrusive body at the location of the former volcanic center before it migrated toward its present location the second high velocity body located below the summit of the volcano likely corresponds to the actual preferential dyke intrusion zone as highlighted by the seismicity the third high velocity anomaly located below the grandes pentes and the grand brule areas and is an imprint of the solidified magma chamber of the dismantled les alizes volcano radial anisotropy shows two main anomalies positive anisotropy above sea level highlighting the recent edifice of piton de la fournaise with an accumulation of horizontal lava flows and the second one below the sea level with a negative anisotropy corresponding to the ancient edifice of piton de la fournaise dominated by intrusions of vertical dykes

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

evolution of magma conduits during the 1998 2000 eruptions of piton de la fournaise volcano reunion island 1 at basaltic volcanoes magma is transported to the surface through dikes magma filled fractures but the evolution of these dikes as eruptions proceed is rarely documented in march 1998 after five and a half years of quiescence piton de la fournaise volcano reunion island entered into a new eruptive phase characterized by intense eruptive activity coeruptive displacements recorded by interferometric synthetic aperture radar insar for the first five eruptions of the cycle are analyzed using 3 d boundary element models combined with a monte carlo inversion method we show that the eruptions are associated with the emplacement of lateral dikes rooted at depths of less than about 1000 m except for the first march 1998 event where an additional deeper source is required the dikes are located above preeruptive seismic swarms this is consistent with nearly isotropic stress caused by repeated dike intrusions and low confining pressure enhanced by the presence of pores in the shallowest 1000 m of the edifice the volumes of the modeled dikes represent 17 of the volume of emitted lava showing that exogenous growth plays a major role in building the volcano by taking into account the preeruptive seismicity and tilt data together with the results of insar data modeling we find that dikes first propagate vertically from a source region below sea level before being injected laterally at shallow depth this behavior is consistent with the presence of levels of neutral buoyancy at shallow depth in the edifice

the origin of hotspot traces evidence from eastern australia interpretation of available isotopic ages and of published geologic maps of igneous rocks in eastern australia indicates a north south time transgressive pattern of cessation of igneous activity along a curvilinear trace beginning about 35 ma at 20 sup 0 s and extending to the present near 37 sup 0 s however volcanic activity began 70 60 ma over the length of the highlands and persisted through the early cenozoic until progressive cessation began the latitudinal rate of termination is compatible with that predicted by plate reconstructions relative to the hawaiian hotspot but the trend of the trace is incompatible with that predicted unless the hawaiian emperor bend is somewhat greater than 50 m y in age relative to the labrecque et al 1977 time scale the volcanic pattern appears to be incompatible with a hotspot or plume hypothesis paleostress and contemporary stress indicators as well as the volcanic pattern support a model in which the trace forms due to intraplate extension normal to the trend of the trace resulting in pressure release and melting at the base of the lithosphere beginning in the late cretaceous this stress field persisted until 35 ma when progressive reorientation of the stress field occurred cessation of igneous activity reflectedmore onset of compression normal to the trend of the trace as a consequence a migrating stress node is recorded in the progressive pattern of extinction of volcanic activity along the trace evidence from other hotspot traces suggests that they too represent intraplate stress controls it is not necessary to postulate deep sources mantle plumes for the origin of hotspot traces intraplate stress due to drag plate margin forces thermal contraction and nonsphericity of the earth can also reflect absolute motion less

hybrid collapse mechanism at piton de la fournaise volcano reunion island indian ocean 1 recent studies have shown that hydrothermal alteration in the core of volcanoes may weaken the edifice and trigger sector collapse lopez and williams 1993 day 1996 voight and elsworth 1997 van wyk de vries et al 2000 reid et al 2001 alteration of volcanic products must take place over long periods of times before collapse can occur so this process is considered to be less frequent than volcanic failure generated by magmatic intrusion and flank spreading however acceleration of this slow process may occur along one direction as a result of a weak lateral confinement following outward gravitational sliding of a flank of the volcano we present here evidence that the enclos fouque depression of piton de la fournaise volcano results from such a hybrid mechanism seaward sliding of the grand brule trough on top of an ancient hypovolcanic complex led to a lateral unconfined boundary along which collapse of the altered core of the edifice took place the result is a caldera like structure breached on the grand brule slide scaled experiments to simulate this process of pseudocaldera formation reveal tectonic features remarkably similar to those encountered in the field and demonstrate the mechanical consistency of the model similar processes may have played a role in the evolution of other volcanoes

internal structure of tenerife canary islands based on gravity aeromagnetic and volcanological data abstract gravity and magnetic methods have been applied to the tenerife island to provide new information about its internal structure for this study 365 gravity stations covering the central part of the island have been selected the anomalous density maps at different depths were obtained by means of an inversion global adjustment on fixed density contrast to describe the three dimensional 3d geometry of the anomalous bodies on the other hand several analysis techniques such as reduction to the pole spectral analysis low pass filtering terrain correction and forward modelling were applied to process the high resolution data obtained in an aeromagnetic survey completed with marine and terrestrial data the joint analysis of gravity and magnetic anomalies has shown tectonic and volcanic features that define some fundamental aspects of the structural framework and volcanic evolution of the island a strong gravity anomaly produced by a large and deep source has been associated with an uplifted block of the basement beneath the southern part of tenerife the sources of the observed gravity highs from 8 km b s l may be associated with the growth of the submarine shield stage that was clearly controlled by regional tectonic the long wavelength magnetic anomalies reveal highly magnetic sources interpreted as gabbro ultramafic cumulates associated with the root zone of a large dyke swarm this intrusive body could be topped by the emplacement zone of magma chambers that correlate with a magnetic horizon at 5 7 0 8 km depth rooted in this highly magnetic zone two dike like structures can be associated with the magmatic feeding system of large recent basaltic volcanoes a shallow magnetic horizon 1 4 km a s l can be correlated with the bottom phonolites of the las canadas edifice in the central part of the island the coincidence of some gravity and magnetic lows is consistent with the presence of low density and low magnetic materials that infill a collapsed caldera system the structures close to the surface are characterised by low density areas connected with the recent volcanism in particular the minimum over the teide volcano hydrothermal alteration is assumed to be the cause of a short wavelength magnetic low over the teide volcano

the off shore continuation of an active basaltic volcano piton de la fournaise reunion island indian ocean structural and geomorphological interpretation from sea beam mapping abstract in 1984 a sea beam survey of the submarine east flank of piton de la fournaise was performed with r v jean charcot three main types of volcanic or volcano tectonic features have been identified 1 1 the subaerial ne and se volcanic rift zones of piton de la fournaise do not extend more than about five kilometers offshore unlike typical hawaiian rift zones which form narrow 2 4 km ridges extending tens of kilometers from the summit the active rift zones of piton de la fournaise widen downslope attaining more than 10 km at their front 2 2 the submarine extension of the grand brule slide is larger than the subaerial portion the entire slide forms a 7 24 km scar bounded by two ramparts to the north and south the slumped material may have moved as a debris flow forming a large talus downslope of the slide however the submarine counterpart of the south area of the grand brule seems to be composed by a slumped block whose structure is apparently not disturbed 3 3 another prominent feature is a conspicuous topographic high that occupies nearly all the center of the surveyed zone this cast flank submarine plateau cannot be associated with any active structure of piton de la fournaise its surface generally dips gently 2 3 and its northern and southern flanks are extensively cut by landslides cones of variable dimensions are observed on the plateau and farther to the east three hypotheses are examined to account for the origin of this morphology a remnant flank of an ancestral fournaise volcano associated with a large buried intrusion found by drilling beneath the grand brule b distinct volcanic massif or c material of a huge and ancient landslide the geophysical data show that the western part of this submarine plateau is reversely magnetized and associated with a moderate positive gravity anomaly this confirms that as suggested by the bathymetric analysis this part of the plateau is relatively coherent conversely the eastern portion of the plateau appears to be poorly magnetized and composed of low density material probably chaotic materials derived from landslides the reversed magnetization of the western part shows that the whole structure is older than 0 7 ma these results show that the volcanic history of piton de la fournaise is more complex than previously thought

granular mass flows and coulomb s friction in shear cell experiments implications for geophysical flows 1 granular mass flows of rock fragments are studied in the lab by means of a high speed video camera at 2000 frames per second these granular flows are generated using beds of pumice fragments positioned on a rough rotating disk whose angular velocity is controlled by a motor the experimental apparatus allows an understanding of the arrangement of the particles in granular mass flows with relatively small and relatively large values of the savage number the savage number represents the ratio between grain collision stresses and gravitational grain contact stresses in particular these flows develop a basal layer of agitated and colliding particles underneath a relatively rigid upper layer our experimental results suggest the validity on average of the coulomb s relationship between shear and normal forces at the base of granular mass flows irrespective of their savage number value in coulomb s equation the shear stresses do not depend on the shear rate we expect the coulomb friction law to be valid also in moving pyroclastic flows our experiments suggest that the collisions and subsequent comminution of pumice fragments in moving pyroclastic flows could provide ash for the overriding ash clouds in our experiments the amount of ash generated by particle particle and particle boundary interactions increases as the value of the savage number increases in nature part of this ash may also simply move toward the base of the flows because of kinetic sieving

the effects of rockfall volume on runout distance the main goal of this paper is to clarify the effects of rockfall volume on its fluidization for this purpose outdoor rockfall experiments were carried out to analyze runout distances and individual movements of rockfall blocks and numerical simulations were conducted for these experiments to learn more about the mechanism of rockfall fluidization the rockfall experiments were conducted using an artificial slope on which granite slabs were surfaced and overlaid with cubiformed granite blocks the size and number of blocks were varied in this series of experiments further numerical simulations were carried out in which the coordinates of individual rockfall blocks were traced in three dimensional space from rockfall initiation to final deposition it became clear from the experiments and simulations that the runout distance had a positive correlation with the rockfall volume number of rockfall blocks and the runout distance of the gravity center of deposited rockfall mass had a negative correlation with the rockfall volume to clarify the mechanism of these two phenomena the positions of individual blocks from initial arrangements to final depositions were traced in the experimental and numerical simulations this revealed that the relative positions of each block along the slope direction were not changed during rockfall movement the reason for the block sequence preservation was that front facing blocks in the initial cube arrangement accelerated and rear facing blocks decelerated along the slope by internal collision between blocks further as the rockfall volume increased the opportunities for impact among the rockfall blocks increased with the front facing blocks pushed farther whereas the gravity center of the deposited rockfall mass had an inclination to travel shorter distances as rockfall blocks increased according to the increase of kinetic energy dispersed by the collision of blocks

role of olivine cumulates in destabilizing the flanks of hawaiian volcanoes the south flank of kilauea volcano is unstable and has the structure of a huge landslide it is one of at least 17 enormous catastrophic landslides shed from the hawaiian islands mechanisms previously proposed for movement of the south flank invoke slip of the volcanic pile over seafloor sediments slip on a low friction decollement alone cannot explain why the thickest and widest sector of the flank moves more rapidly than the rest or why this section contains a 300 km3 aseismic volume above the seismically defined decollement it is proposed that this aseismic volume adjacent to the caldera in the direction of flank slip consists of olivine cumulates that creep outward pushing the south flank seawards average primary kilauea tholeiitic magma contains about 16 5 wt mgo compared with an average 10 wt mgo for erupted subaerial and submarine basalts this difference requires fractionation of 17 wt 14 vol olivine phenocrysts that accumulate near the base of the magma reservoir where they form cumulates submarine erupted kilauea lavas contain abundant deformed olivine xenocrysts derived from these cumulates deformed dunite formed during the tholeiitic shield stage is also erupted as xenoliths in subsequent alkalic lavas the deformation structures in olivine xenocrysts suggest that the cumulus olivine was densely packed probably with as little as 5 10 vol intercumulus liquid before entrainment of the xenocrysts the olivine cumulates were at magmatic temperatures 1100 c when the xenocrysts were entrained olivine at 1100 c has a rheology similar to ice and the olivine cumulates should flow down and away from the summit of the volcano flow of the olivine cumulates places constant pressure on the unbuttressed seaward flank leading to an extensional region that localizes deep intrusions behind the flank these intrusions add to the seaward push this mechanism ties the source of gravitational instability to the caldera complex and deep rift systems and therefore limits catastrophic sector failure of hawaiian volcanoes to their active growth phase when the core of olivine cumulates is still hot enough to flow

structure of piton de la fournaise volcano la reunion island indian ocean from magnetic investigations an illustration of the analysis of magnetic data in a volcanic area abstract the active piton de la fournaise volcano la reunion island western indian ocean is a basaltic shield built on the flank of the larger and older piton des neiges volcano whose early activity was basaltic but ended with eruption of differentiated alkaline lavas previously few data were available to define the internal and deep structure of piton de la fournaise this report contains interpretation of magnetic data that take the first steps into this domain the data consist of an aeromagnetic survey at a constant elevation of 3048 m 10 000 ft piton de la fournaise rises to a height of 2637 m and a shipborne magnetic survey in the eastern coastal area of the island a first model shows that the island does not behave as a uniformly magnetized body in order to enhance the effects of the internal magnetic contrasts a correction of the topographic effects has been attempted to this purpose it has been possible to make a good estimation of the superficial terrain magnetization in the following way main features of the relief such as caldera ramparts and deep valleys induce short wavelength anomalies analysis of which provides the apparent magnetization of the whole formations exposed in the cliffs these determinations are used to compute a realistic magnetic model of the topography the effect of which is removed from the observed data to obtain a map of the residual anomalies related to internal magnetic variations the main magnetic contrasts are attributed to the differences in intensity of magnetization between the dikes and the surrounding lava flows under volcanic vent areas such as rift zones dikes cluster at depth in complexes and are the sources of high amplitude magnetic anomalies in the piton de la fournaise area dike complexes can be located in the central and eastern parts of the summit caldera and on northeastern and southeastern rift zones a third northwestern rift zone oriented in the direction of piton des neiges is well developed south and southeast of the palmistes plain graben in the northeast part of the caldera the circular depression of the osmondes plain corresponds to the convergence zone of the structural trends of the northeastern rift the caldera dike complex and the northwestern rift the attenuation of anomalies observed above the summit area of piton de la fournaise may be related to the presence of rocks at a temperature above the curie point the orientation of the caldera dike complex may possibly be controlled by the gravitational stresses of the island the intrusion of dikes in that highly active part of the volcano will then displace the unbuttressed east flank seaward much in the manner of kilauea volcano swanson et al 1976 thus explaining the origin of the broad landslide of grand brule further less active northeast and southeast rift zones may control the southward and northward extension of the grand brule structure the formation of the palmistes plain graben may be related to similar phenomena

gravity structure of piton de la fournaise volcano and inferred mass transfer during the 2007 crisis abstract the structure and mass transfers at piton de la fournaise volcano are investigated using new on land and offshore measurements the structures are defined using 2 and 3d modelling short wavelength positive anomalies to the w and nw of the central area are interpreted as shallow piles of thick dense lava flows which have filled volcano tectonic depressions several hundreds of meters deep this finding provides a new insight into the evolution of the volcano conversely the currently active central zone is associated with a negative short wavelength anomaly explained by the presence of thin highly vesiculated and fractured lava flows building the central cone low density formations underlying the rivieres des remparts and langevin areas are tentatively interpreted as breccias related to erosion and or to southward directed mass wasting topographic offshore highs in the continuation of the se and ne rift zones of piton de la fournaise are characterized by negative gravity anomalies and reversed magnetic anomalies their interpretation as hyaloclastites and pillow lavas provides a coherent explanation for both types of signals if we admit that they were primarily constructed during the alizes stage two main deeper dense structures are also found the grand brule complex and a complex beneath the plaine des sables and the enclos fouque both are identified as hypovolcanic intrusive complexes the grand brule complex 10 18 km in plan view shows a marked ns elongated geometry unlike the other comparable complexes on the island the plaine des sables enclos fouque complex a more or less circular structure 5 km in diameter is attributed to the ancient shield stage it is centered on the enclos fault suggesting that the paleo center of the ancient shield was located about 1 5 km eastward of the previously proposed location an apparent contradiction was noted between the seismic and gravity images of the internal structure in particular a high velocity cylinder beneath the summit was not detected in the gravity models this contradiction was resolved by considering the relative sensitivity of each method finally the gravity dataset has allowed us to explore the gravity changes that accompanied the major eruptive and volcano tectonic crisis of april 2007 350 m collapse of the summit and emission of 120 106 m3 of lava the change in the mass distribution of the edifice was large enough to be detected in the survey data collected in the central area before and after the crisis however the accuracy was too low for the precise assessment of the mass transfers

volcano structural evolution of piton des neiges reunion island indian ocean four volcano structural stages have accompanied the building of piton des neiges 1 emergent growth stage of the island the major eruptive system is a rift zone trending n 120 associated with dextral strike slip faults trending n 30 and en echelon extensional fissures trending n 70 breccias and lava tubes produced by aerial and phreatomagmatic activity are injected with outward dipping dike swarms along ring fractures suggesting a mechanism analogous to cauldron subsidence 2 shield building stages of growth are related to fissures along the main rift zone and three minor rifts trending n 160 n 45 and n 10 the summit of the basaltic shield volcano is stretched and collapsed in a graben like caldera depression along normal and antithetic faults 3 differentiated lavas are erupted during two stages separated by the opening of a new caldera corresponding to an explosive activity a silicic cone sheet system and a collapse structure 4 younger volcanic activity restricted to the inside caldera has presumably emptied the underlying magma reservoir building a central volcano collapsed along ring internal dip fractures

modelling of crustal anomalies of lanzarote canary islands in light of gravity data the application of a gravity inversion method enables us to obtain a 3 d density contrast model of the upper crustal anomalies of the volcanic island of lanzarote canary islands for this we use a network of 296 gravity stations distributed over the whole island and a digital terrain model of about 45 000 terrestrial and oceanic data to determine the corresponding terrain correction a density value of 2480 kg m 3 is chosen for this correction by means of a new approach the resulting bouguer anomaly is analysed by means of a least squares prediction which gives us a mean level of uncorrelated observational noise of about 1 2 mgal this anomaly is considered in order to obtain independent information about the inner anomalous mass density distribution by means of a 3 d gravity inversion based on a systematic exploration on a prismatic partition of the subsoil volume and adopting et priori values of the density contrast positive and negative to determine the geometry of the anomalous bodies the problem of non uniqueness of the solution is avoided by using a minimization mix condition on the weighted residuals and the weighted whole anomalous mass the structural solution is finally presented by means of horizontal sections and vertical profiles a main intrusive body is located under the central eastern area and could correspond to a dilated volcanic activity of shield formation it shows a prismatic form of more than 15 km depth subducted with only the ridges remaining as horst blocks moreover the sw and ne extreme areas of the island show smaller and shallower positive bodies interpreted as less developed magmatic intrusions conversely several density lows offer interesting shallow alignments 45 degreesn ene wsw and 125 degreesn wnw ese which could be associated with a fracture system corresponding to structural stress and also correlate with historic eruptions such as for instance the timanfaya eruption the monitoring of several geophysical parameters at two underground geodynamic stations in the ne zone of the island and timanfaya shows characteristic differences between the two zones which confirm crustal anomalies in the second station

structure of reunion island indian ocean inferred from the interpretation of gravity anomalies abstract reunion is a volcanic edifice whose origin is related to a hot spot in the indian ocean only 3 of its volume is emergent many geological and geophysical studies were carried out on reunion island during the 1980 s but few of them allow study of the internal structure of the edifice several gravity surveys have been carried out on the island since 1976 and we have compiled the available data set the lack of data on the western side of the island led us to conduct a regional survey in 1993 to obtain a more homogeneous distribution of the stations computation of bouguer anomalies for different correction densities accounts for the variable density of the rocks constituting the edifice and provides a distribution of gravity anomalies interpreted as dense bodies of intrusive rocks inside the edifice two very large intrusive complexes can be unambiguously recognised one beneath piton des neiges and one beneath the grand brule area both have been penetrated by geothermal exploration drill holes and the first is also known from outcrop observations 2 5d simple models were constructed to reveal the geometry and extent of the buried intrusives they are deeply rooted extending several kilometres below sea level and extensive 20 25 km long and 10 13 km wide for the piton des neiges complex 12 15 km long and some kilometres wide for the grand brule complex the development of such complexes implies that the activity of the two volcanic centres was long lasting and remained stable while the volcanoes were growing the grand brule complex has been interpreted as relics of an old volcano named alizes volcano the interpretation of the gravity maps suggests the presence of a ridge of dense rocks to the north of the axis joining the centres of piton des neiges and piton de la fournaise volcanoes by analogy with the other structures 2 5d models show that this structure would culminate between 0 and 1 km below sea level and be 15 km wide this complex induces a maximum anomaly in takamaka valley and we thus propose to name it takamaka volcano no geological evidence of the nature of these dense rocks is available but the ridge coincides with structures revealed by magnetic and seismic data interpretation of the bouguer anomaly maps suggests that the inner gravity structure of piton de la fournaise is not characterised by the presence of a voluminous dense body but probably by more restricted concentrations of dense rocks some structures can be recognised along the present ne and se rift zones and in the previous central part of piton de la fournaise to the west of the present summit the recent eastward migration of the centre of activity of piton de la fournaise accounts for the lack of a large positive anomaly beneath the active craters

gravity constraints on the structure and volcanic evolution of tenerife canary islands a detailed gravity study of tenerife constrains the internal structure of the island and its active magma system the maximum bouguer anomaly 2636 gu 1 gu 10 6 m s 2 0 1 mgal occurs over the sw part of the postshield las canadas edifice lce while smaller highs occur over the eroded shield volcanoes the main high extends ne and nw over the adjacent dorsal and santiago rift zones a deep low occurs over the northern lce and teide volcano the power spectrum of the anomalies suggests shallow causative structures 3 km inferred to belong to the island s dense core a model computed from the anomalies uses core and carapace densities of 2900 and 2360 kg m 3 respectively except beneath the main high where the core requires a higher density 3210 kg m 3 this part of the body is interpreted as a buried mafic volcano of substantial size the boca tauce volcano containing an old mafic plutonic complex the enigmatic las canadas caldera that truncates the lce shows strong asymmetry dense materials occupy the southern part whereas low density materials occupy the northern part to 2 3 km depth consistent with a vertical collapse origin high density materials in the south are related to the intracaldera extension of the dorsal rift zone the boundary between subcaldera domains is interpreted as the buried head of the giant icod landslide valley the icod landslide appears to have modified the caldera truncating its northern side and to have subsequently controlled the location of the active pico teide pico viejo vent systems voluminous chaotic deposits lying off northern tenerife previously associated with the formation of the caldera and icod valley are better interpreted as being derived from older mass wasting events the southern lce has been buttressed against lateral failure by the buried boca tauce volcano

boundary stresses due to impacts from dry granular flows 1 field data and laboratory experiments suggest that bedrock wear from debris flows is largely due to particle bed impacts rather than solely due to abrasion by sliding and that the associated bedrock erosion rates are dependent on the particle size distribution in the debris flow here we use discrete element method dem simulations with an established contact mechanics model to explore grain size influences on contact forces associated with particle bed impacts in sheared granular mixtures we first compare dem simulations with experimental observations obtained from shallow granular flows in rotating drums of diameters 0 56 m and 4 0 m our simulations reproduce without parameter tuning experimentally measured segregation boundary pressures and height profiles we perform additional simulations systematically varying particle size distributions in binary mixtures we show that local time averaged boundary pressures in thin flows are essentially the normal component of the weight of the flow independent of particle size distribution however other statistical measures of boundary forces scale with mass averaged particle size we demonstrate that this is because individual particle bed impacts rather than impacts from multiple particle collisions dominate the largest contact forces we show that these largest impact forces vary as the square of grain size and the 1 2 power of impact velocity as predicted from the contact mechanics model underlying the dem these results support the particle size dependence of a recently proposed bedrock incision model and suggest that next steps for a predictive bedrock incision model require the statistics of the largest impact velocities

the role of acids in electrical conduction through ice 1 electrical conduction through meteoric polar ice is controlled by soluble impurities that originate mostly from sea salt biomass burning and volcanic eruptions the strongest conductivity response is to acids yet the mechanism causing this response has been unclear here we elucidate conduction mechanisms in ice using broadband dielectric spectroscopy of meteoric polar ice cores we find that conduction through polycrystalline polar ice is consistent with jaccard theory for migration of charged protonic point defects through single ice crystals except that bulk dc conduction is impeded by grain boundaries neither our observations nor modeling using archie s law support the hypothesis that grain boundary networks of unfrozen acids cause significant electrolytic conduction common electrical logs of ice cores by electrical conductivity measurement ecm or dielectric profiling dep and the attenuation of radio waves in ice sheets thus respond to protonic point defects only this response implies that joint interpretation of electrical and chemical logs can determine impurity partitioning between the lattice and grain boundaries or inclusions for example in the greenland ice core project grip ice core from central greenland on average more than half of the available lattice soluble impurities h cl nh4 create defects understanding this partitioning could help further resolve the nature of past changes in atmospheric chemistry

breaching parameters for earth and rockfill dams dam risk analysis is at the heart of dam failure prevention and mitigation in order to assess dam risk it is essential to conduct a quantitative analysis of the process of a dam breach which can be described by such parameters as breach geometry breaching duration and peak outflow rate the main objective of this paper is to develop robust empirical formulas with physical meaning for predicting dam breaching parameters based on past dam failure data a database of 182 earth and rockfill dam failure cases has been compiled among these cases nearly one half are for large dams higher than 15 m a multiparameter nonlinear regression model is recommended to develop empirical relationships between five breaching parameters breach depth breach top width average breach width peak outflow rate and failure time and five selected dam and reservoir control variables dam height reservoir shape coefficient dam type failure mode and dam erodibility the relative importance of each control variable is evaluated the dam erodibility is found to be the most important factor influencing all five breaching parameters the reservoir shape coefficient and the failure mode also play an important role in the prediction models two case studies are presented to show the application of the empirical models developed in this paper

stresses at the base of dry and dense flows of angular rock fragments in 3 d discrete element modeling scaling of basal stress fluctuations versus grain size flow volume and channel width abstract we simulate granular flows of angular rock fragments by means of a three dimensional discrete element modeling to study the basal stresses that these flows exert on the subsurface these granular flows have different grain sizes and different flow volumes and they model dry rock avalanches and dense pyroclastic flows these flows travel on four different concave upward chutes that represent channels on a mountainside or on the flank of a volcano each chute has a different width the stress data demonstrate the validity of a linear relation between two scaling parameters d and parameter d is a scaled basal stress deviation that is equivalent to a scaled particle agitation particle agitation is ultimately responsible for the energy dissipation that governs the mobility of dense geophysical flows in nature parameter contains grain size flow volume and channel width this second parameter is equal to the product of the reciprocal of characteristic numbers of fragments in granular flows since these numbers of particles are dimensionless the linear relation is valid at any scale either in the laboratory or in nature

fault propagation folds induced by gravitational failure and slumping of the central costa rica volcanic range implications for large terrestrial and martian volcanic edifices long sublinear ridges and related scarps located at the base of large volcanic structures are frequently interpreted as normal faults associated with extensional regional stress in contrast the ridges bordering the central costa rica volcanic range ccrvr are the topographic expression of hanging wall asymmetric angular anticlines overlying low angle thrust faults at the base of the range these faults formed by gravitational failure and slumping of the flanks of the range due to the weight of the volcanic edifices and were perhaps triggered by the intrusion of magma over the past 20 000 years the thrust fault underlying alajuela ridge rises from a depth of 700 m at a dip of 15 over a distance of a few kilometers it ramps up at 600 m depth on a 40 slope and terminates in the axis of the forward syncline 400 m below the surface the hanging wall of this fault has been thrusted about 200 m away from the volcanic range axis thrusting on the lower flanks of the ccrvr is compensated by extension on the range axis with formation of summit grabens shallow seismicity in the volcanic range suggests that thrusting is still active we postulate that at kilauea volcano hawaii the gravity and magma induced slumping of the southern flank on low angle thrust faults generates hanging wall anticlines similar to those observed in costa rica these anticlines are hypothesized to occur along the base of the volcano where the thrust faults ramp up toward the sea bottom ridges and scarps between 2000 and 5000 m below sea level are interpreted as the topographic expression of these folds we further suggest that the scarps of the ccrvr and kilauea are valid scaled terrestrial analogs of the perimeter scarp of the martian volcano olympus mons we suggest that the crust below olympus mons has failed under the load of the volcano triggering the radial slumping of the flanks of the volcano on basal thrusts the thrusting would have in turn formed the anticlinal ridges and scarps that surround the edifice the thrusting seems to be preferential toward the northwest and southeast and a northeast southwest rift zone seems to have developed perpendicular to the regional topographic gradient the thrust faults may extend all the way to the base of the martian crust about 40 km and they may have been active until almost the end of the volcanic activity we suggest that gravitational failure and slumping of the flanks of volcanoes is a process common to most large volcanic edifices in the ccrvr and at kilauea this slumping of the flanks is a slow intermittent process but it could evolve to rapid massive avalanching leading to catastrophic eruptions thus monitoring of uplift and displacement of the folds related to the slump tectonics could become an additional effective method for mitigating volcanic hazards

using a genetic algorithm for 3 d inversion of gravity data in fuerteventura canary islands the use of genetic algorithms in geophysical inverse problems is a relatively recent development and offers many advantages in dealing with the non linearity inherent in such applications we have implemented a genetic algorithm to efficiently invert a set of gravity data employing several fixed density contrasts this algorithm determines the geometry of the sources of the anomaly gravity field in a 3 d context the genetic algorithms based on darwin s theory of evolution seek the optimum solution from an initial population of models working with a set of parameters by means of modifications in successive iterations or generations this searching method traditionally consists of three operators selection crossover and mutation acting on each generation but we have added a further one which smoothes the obtained models in this way we have designed an efficient inversion gravity method confirmed by both a synthetic example and a real data set from the island of fuerteventura in the latter case we identify crustal structures related to the origin and evolution of the island the results show a clear correlation between the sources of gravity field in the model and the three volcanic complexes recognized in fuerteventura by other geological studies

vertical segregations in flows of angular rock fragments experimental simulations of the agitation gradient within dense geophysical flows abstract in this paper we illustrate laboratory experiments whose purpose is to study the vertical segregations that are commonly observed in deposits of dense geophysical flows such as pyroclastic flows and rock avalanches in these experiments we use rock cuboids with 5 mm long edges as matrix and rock cuboids with 2 cm long edges as segregating clasts a rotating disk is used to apply frictional stresses at the base of the granular masses in our experiments segregating cuboids with density smaller than or equal to that of the matrix particles rise whereas segregating cuboids with density larger than that of the matrix particles sink the granular flows are imaged through the glass container of the experimental apparatus by a high speed video camera at 2000 fps by means of particle image velocimetry analysis of the movies we study the vertical gradient of particle agitation that exists within the granular flows where agitation increases downward because of the interaction with the subsurface asperities the high speed movies show that it is the particle agitation within the flows that exerts an upward force and that when this force is larger than the weight of the segregating clast the clast rises whereas when it is smaller the clast sinks the most important result in our set of experiments is that the threshold which separates the values of density of the segregating clasts that segregate upward and the values of density of the segregating clasts that segregate downward is larger than the density of the matrix particles this explains the upward segregation of dense lithics that is frequently observed in deposits of geophysical flows this upward segregation is due to the fact that the resultant of the collisions exerted by the matrix particles is a force strong enough to push upward also dense and heavy fragments

a new method for dynamically estimating long term seepage failure frequency for high concrete faced rockfill dams failure frequency of a dam varies greatly with reservoir water level and time it is important but difficult to dynamically estimate failure frequency of high concrete faced rockfill dams cfrds merely based on the probabilistic approach due to limited failure cases of high cfrds in this study a new method is developed for dynamically estimating the long term revolution of seepage failure frequency of high cfrds by combining an analogical approach with the bayesian approach a simple and precise seepage generalized method and a parameter named defect parameter are introduced to provide the basis of comparison between the dam of interest and the accumulated database of high cfrds using the cumulative probability distribution from the database the correlation between annual seepage failure frequency and defect parameter is established a high cfrd case in china with an operation history of slab breakage and reinforcement is selected as an example to illustrate the applicability of the new method it is verified that the variation tendency of safety level of high cfrd is well simulated through the annual seepage failure frequency calculated by the newly proposed method

seismic monitoring and modeling of an active volcano for prediction we describe seismic monitoring and modeling of an active volcano for the purpose of predicting the magma transport process in real time we selected the piton de la fournaise which has been monitored by an observatory since the early 1980s and is active with 28 eruptions from 1980 until august 1992 it erupted on march 9 1998 after an unusually long quiet period the eruption lasted until september 21 1998 before the march eruption we completed the initial construction of a model of a magma system based primarily on long period lp events and coda localization we found that lp events are associated with the lateral movement of magma to the rift zone and not with the vertical movement to the summit we also found that the source of 2 hz lp events is located below that of 1 hz lp events the coda localization is a newly discovered phenomenon that helped to locate a magma body this model was used for interpreting the incoming information from the precursory seismic crisis and the eruption tremor during the 6 months of the eruption the simultaneous monitoring and modeling of an active volcano led us to a quantitative simulation of the eruption process computing excess pressure and flow rate in a system of reservoirs connected by channels

the central european graben system rifting imitated by clay modelling abstract central europe is traversed by an active graben belt between the north sea and the mediterranean along with a lateral rift rift offset between some segments along these offsets connecting the individual segments specific patterns have been observed which may be physiographically compared with transform faults a series of clay experiments were carried out to understand rhinegraben type graben formation especially the transform like features were modelled to learn more about the kinematics of transform faulting in the continental domain it has been attempted to approach natural structural and lithological boundary conditions of graben formation by varying the experimental conditions in such a manner that experimental and in situ conditions resemble in an optimal way the experiments are related to the paleogene extensional stage of the evolution of the rift system and do not consider the pliocene to recent kinematics of shear rifting

incremental collapse and stratocone growth within a funnel shaped caldera guayabo costa rica abstract correlations between the surface geology and detailed borehole stratigraphy density logs particularly in the geothermally active area illustrate that the present morphology and structure of the guayabo caldera is the product of at least four subsidence events much of the sub caldera tuff lava sequence also exposed in the topographic caldera walls is the product of several ca 2 ma old overlapping stratocones formerly present in the area but later subjected to subsidence and edifice collapse the v shaped gravity anomaly across the caldera is due to a funnel shaped cross section developed as follows the earliest and innermost now concealed down faulted zone was filled by a low density pumice unit which extended southwest along an early arc normal graben at least two subsequent subsidence events produced the present scarps and outflow tuff breccias but the erupted volume is markedly lower than the subsidence volume the missing magma may have been diverted to the east where at the same time further stratocone buildup was occurring the gravity signature over this eastern caldera margin suggests the presence of a large dense sub volcanic intrusive complex further edifice collapse and graben formation due to a combination of gravitational failure and arc normal strain are relatively recent and continuing processes the evolution of the guayabo caldera therefore involved a series of subsidence events punctuated by stratocone buildup and collapse the resolution of concentric ring faulted block structures does not support a chaotic collapse model but instead indicates the incremental development by widening of a funnel shaped caldera

gravity fields and the interpretation of volcanic structures geological discrimination and temporal evolution abstract a review of gravity data reflecting shallow concealed structures on volcanoes demonstrates that valuable information may be derived on the geological development of such structures and on the dynamic evolution of active volcanoes to a first approximation all recorded examples of anomalous gravity fields associated with volcanoes are concentric with and have sources that lie within the volcanic edifice positive anomalies with wavelengths less than 20 km and amplitudes up to ca 30 mgal characterise mainly basaltic volcanoes from various tectonic settings and are caused by a relatively dense intrusive complex magma body which contrasts with its surroundings either because the body is more mafic than average or more likely because near surface previously erupted materials are uncompacted negative anomalies with wavelengths up to 100 km and amplitudes up to ca 60 mgal occur over much larger volcanic calderas many of which have erupted highly silicic pyroclastic ash and pumice uncompacted silicic caldera infill with a possible contribution from low density magma bodies is responsible for the observed anomalies there is some evidence for a continuum of gravity anomaly types that corresponds to the geological evolution of volcanic systems generally from primitive rift or subduction related basaltic andesite to mature high level silicic calderas repeated microgravity observations over active volcanoes enable magma movements variations in magma input and changes in magma density to be monitored closely dynamic modelling of volcanic systems is providing new evidence on the behaviour of concealed magma bodies and could have considerable potential for predicting eruptive events

internal structure of piton de la fournaise volcano from seismic wave propagation and earthquake distribution abstract arrival times of seismic waves from local earthquakes are inverted for both locating the source and defining the 3 d velocity heterogeneity of piton de la fournaise the lateral heterogeneity of the 2632 m high edifice is resolved as a high velocity plug 1 5 km in diameter surrounded by a low velocity ring which may be interpreted as due to the construction of fournaise on the flank of the older volcano piton des neiges wave mode conversion detected on three component seismograms provides evidence for boundaries of contrasted velocities pre eruptive swarm earthquakes cluster in the high velocity zone under the dolomieu summit crater low strength and cohesion of the surrounding material account for the lack of seismicity for the final 1 3 km radial flow of magma to the vents in enclos fouque beneath the high velocity plug the existence of a body with low velocity for p and even for s waves is well constrained however the walls and base are poorly defined because of the lack of deep earthquakes for sampling the few earthquakes that are located in this depth region usually occur at a depth of around 1 5 km below sea level in the region of the cone this can be considered providing the upper constraint on the lower limit of the aseismic part of the low velocity body the coincidence in time of their occurrence with the swarms above sea level and the eruptions suggests magmatic activation of the low velocity aseismic volume 1 5 km below sea level under the high velocity plug of the cone further down the concentration of seismicity in two swarms between 2 and 4 km under the eastern flank does not allow the structure to be sampled effectively

anatomy of piton de la fournaise volcano la reunion indian ocean the aim of this work is to propose a general model of piton de la fournaise volcano using information from geological and geophysical studies firstly we make a graphical compilation of all available geophysical information along a w e profile secondly we construct a geological section that integrates both the geophysical information and the geological information the lithosphere beneath piton de la fournaise is not significantly flexed and the crust is underlain by an underplating body which might represent the deep magma reservoir for la reunion volcanism piton de la fournaise is a relatively thin volcano lying on a huge volcanic construction attributed mostly to les alizes volcano indeed if the differentiated rocks observed at the bottom of the riviere des remparts are the top of les alizes volcano the interface with piton de la fournaise may be located at about sea level beneath the summit area the endogenous constructions intrusive complexes related to les alizes and piton de la fournaise volcanoes represent a large volume the huge intrusive complex of les alizes volcano probably rests on the top of the oceanic crust and appears to have a buttressing effect for the present eastern volcano tectonic activity of piton de la fournaise the early piton de la fournaise edifice was built around a focus located beneath the plaine des sables area the center subsequently moved 5 6 km eastward to its current location the dense high velocity body beneath the plaines des sables and the western part of the enclos probably corresponds to the hypovolcanic intrusive complex that developed before the volcanic center shifted to its present day position magma reservoirs may have existed and may still exist as illustrated by the march 1998 crisis at the mechanical and density interface between the oceanic crust and the les alizes edifice strong evidence also exists for the presence of a shallower magma reservoir located near sea level beneath the summit the march 1998 pre eruptive seismic pattern location and upward migration seems to be evidence for a transfer of magma between the two reservoirs the dominant structural feature of the central zone is a collapse structure beneath the summit craters above the inferred magma reservoir near sea level the collapsed column constitutes a major mechanical heterogeneity and concentrates most of the seismic intrusive and hydrothermal activity because of its higher permeability and weaker mechanical strength

grain size and flow volume effects on granular flow mobility in numerical simulations 3 d discrete element modeling of flows of angular rock fragments the results of three dimensional discrete element modeling presented in this paper confirm the grain size and flow volume effects on granular flow mobility that were observed in laboratory experiments where batches of granular material traveled down a curved chute our numerical simulations are able to predict the correct relative mobility of the granular flows because they take into account particle interactions and thus the energy dissipated by the flows the results illustrated here are obtained without prior fine tuning of the parameter values to get the desired output the grain size and flow volume effects can be expressed by a linear relationship between scaling parameters where the finer the grain size or the smaller the flow volume the more mobile the center of mass of the granular flows the numerical simulations reveal also the effect of the initial compaction of the granular masses before release the larger the initial compaction the more mobile the center of mass of the granular flows both grain size effect and compaction effect are explained by different particle agitations per unit of flow mass that cause different energy dissipations per unit of travel distance the volume effect is explained by the backward accretion of the deposits that occurs wherever there is a change of slope either gradual or abrupt our results are relevant for the understanding of the travel and deposition mechanisms of geophysical flows such as rock avalanches and pyroclastic flows

a new model for the evolution of the volcanic island of reunion indian ocean the island of reunion has been studied using data from airborne and shipborne magnetic surveys the subaerial history of reunion spans the last 2 1 million years the brunhes matuyama geomagnetic reversal enables differentiation of volcanic rocks older and younger than 0 78 ma the lower submarine flanks are poorly magnetized and are interpreted as landslide deposits the core of the island is composed of highly magnetized rocks piton des neiges volcano is composed mostly of rocks older than 0 78 ma only its western flank and central area include thick piles of younger rocks piton de la fournaise is a highly and normally magnetized edifice but its northern and eastern flanks are underlain at shallow depth by reversely magnetized formations the latter are regarded as remnants of les alizes volcano associated with the grand brule hypovolcanic complex at the matuyama brunhes transition the island was composed of at least two main volcanoes piton des neiges and les alizes and perhaps also of a third old volcano takamaka at the center north of the island piton de la fournaise grows on the flank of les alizes and is a relatively young focus of volcanism these volcanoes have had successive phases of construction and destruction the analysis of magnetic anomalies over reunion was decisive in defining a new coherent model for the evolution of the island

kar chronology of the ultimate activity of piton des neiges volcano reunion island indian ocean abstract potassium argon measurements were made on 17 samples from the ultimate stage of activity of the piton des neiges volcano which corresponds to a differentiated series with lavas varying from basalt to quartz trachyte samples were selected as a function of their stratigraphic position and their representativity of magmatic evolution the technology used to date young volcanic rocks pleistocene by k ar is described and discussed because of its reliability in the analysis of modern lavas and because of the agreement of the results with the available stratigraphic control the data together with those previously obtained by mcdougall yield ages ranging between 330 000 and 30 000 years with two mainly effusive periods separated by a phase of instability of chiefly explosive volcanism around 190 000 to 150 000 years ago a rough correlation between the chemical evolution of the lavas and time is established the kudo and weil geothermometer when cross checked with buddington and lindsley s geothermometer allows us to establish that the lavas of the two effusive periods correspond to two different depths of the magmatic chamber in the oceanic crust 17 and 10 km respectively the phase of instability may correspond to a sudden variation in these magmatic conditions the differentiated series the ultimate stage of activity of the piton des neiges volcano 330 000 and 30 000 years can be explained as the evolution of a magmatic body isolated when the focus of basaltic eruption migrated southeast from piton des neiges to piton de la fournaise

edifice growth deformation and rift zone development in basaltic setting insights from piton de la fournaise shield volcano reunion island abstract the overall morphology of basaltic volcanoes mainly depends on their eruptive activity effusive vs explosive the geometry of the rift zones and the characteristics of both endogenous and exogenous growth processes the origin of the steep geometry of the central cone of piton de la fournaise volcano which is unusual for a basaltic effusive volcano and its deformation are examined with a combination of a detailed morphological analysis field observations gps data from the piton de la fournaise volcano observatory and numerical models the new caldera walls formed during the april 2007 summit collapse reveal that the steep cone is composed of a pyroclastic core inherited from an earlier explosive phase overlapped by a pile of thin lava flows this suggests that exogenous processes played a major role in the building of the steep central cone magma injections into the cone which mainly occur along the n25 30 and n120 rift zones lead to an asymmetric outward inflation concentrated in the cone s eastern half this endogenous growth progressively tilts the southeast and east flanks of the cone and induces the development of a dense network of flank fractures finally it is proposed that intrusions along the n120 rift zone are encouraged by stresses induced by magma injections along the n25 30 rift zone

three dimensional analysis of water infiltration into the gouhou rockfill dam using saturated unsaturated seepage theory the gouhou dam was a concrete faced rockfill dam built in a steep canyon that collapsed in 1993 due to internal erosion during the initial reservoir filling in this paper the process of water infiltration into the originally unsaturated rockfill dam is studied using three dimensional saturated unsaturated seepage theory the three dimensional characteristics of seepage through the dam bounded by steep abutments the effect of material anisotropy and the effect of rockfill stratifications are studied the three dimensional results are compared with those from two dimensional analyses the three dimensional simulations show that seepage water flows faster and the hydraulic gradients are greater near the abutment boundary in the dam as such the evolution of the seepage failure in the three dimensional cases is faster than that in the two dimensional analyses and the two dimensional analyses will underestimate the risk of seepage failure particularly near the abutment boundary if the materials in the

effect of grain size on mobility of dry granular flows of angular rock fragments an experimental determination abstract rapid mass movements of rock fragments are among the most hazardous natural phenomena they are too dangerous to be approached and studied directly and for this reason experiments are necessary to understand their flow mechanisms here we present the results of experiments on granular flow mobility obtained releasing batches of angular rock fragments down a curved chute our experiments demonstrate that the finer the grain size of the flow the larger its mobility this is accompanied by a decrease of fragments agitation when grain size decreases as shown by particle image velocimetry measurements of high speed video camera images the smaller the agitation the smaller is also the energy dissipated by flows per unit of travel distance this explains the effect of finer grain size on flow mobility these results are an important step towards the prediction of runout distances as a function of the features that characterize the different geophysical flows

internal architecture of la reunion indian ocean inferred from geophysical data the internal structure of la reunion indian ocean is investigated using gravity magnetic and electromagnetic data a general model of the island is derived providing new information on the individual volcanoes as well as on the growth of the island as a whole the reconstructed topography of the island at 780 ka brunhes matuyama magnetic reversal shows a paleo island organized around a huge piton des neiges volcano and a smaller les alizes volcano both having large offshore extensions and dense hypovolcanic complexes during the brunhes period the piton des neiges has growth predominantly in its central and west southwest areas and to a less extent to the north the construction of piton de la fournaise at the east has caused the other major change in the topography of the island this latter volcano the youngest and the smallest of the three identified volcanoes of la reunion only achieves a large thickness beneath its central and southern zones the resistivity and magnetic patterns of its central area support the presence of a hydrothermal system in its western older part shallow highly resistive and dense layers are interpreted as piles of thick lava flows filling paleo depressions in the same area a deep dense body may represent the hypovolcanic intrusive complex from the early stages of piton de la fournaise piton des neiges is by far the largest volcano of la reunion the resistivity data show that its products are more altered than those of piton de la fournaise a hydrothermal alteration is particularly developed around the dense central hypovolcanic intrusive complex this latter characterized by resistive and dense terrains has a lateral extent that coincides with that of the overlying topographic depressions the three central cirques the bebour belouve depression and at its eastern tip the plaine des palmistes a genetic link between the morphology of the volcano and a possible subsidence of the dense complex is investigated but no definitive conclusion is reached the offshore part and the hypovolcanic complex of les alizes volcano have been described by previous works here we identified the reversely magnetized terrains beneath piton de la fournaise with at least part of the construction of les alizes volcano but no geophysical contrast marking at depth the boundary between piton de la fournaise and les alizes is recognized suggesting that the products of both volcanoes are lithologically similar at the scale of our work

a 3 d gravity model for a volcanic crater in terceira island azores the seismic response of the sao sebastiao volcanic crater located at the se end of terceira island azores is characterized by an amplification of ground movements inside the crater with respect to the surrounding area this variability of ground motion is also observed inside the volcanic depression itself to gain insight into this phenomenon a gravimetric survey with dense coverage of the region where the depression is located was carried out data corrections including the calculation of terrain density by the nettleton method and filtering of non correlated anomalies noise by least squares prediction led to a bouguer anomaly map with a standard deviation of 0 213 mgal the analysis of this map and its interpretation is carried out by means of a stabilized non linear inversion methodology resulting in a 3 d model of density contrasts which identifies the geometry of the causative sources in the area the gravity inversion outlines a correlation between the presence of scoria cones eruptive centres and the zones of negative density contrast in the calculated model at sao sebastiao village another important structure with negative density contrast is located in places where a more destructive and strong anomalous behaviour has been observed during several seismic events furthermore the gravity inversion seems to confirm a collapse model for sao sebastiao crater and to indicate the presence of a deep conduit system compatible with the fissural nature of the volcanism in this area

field measurement of basal forces generated by erosive debris flows 1 it has been proposed that debris flows cut bedrock valleys in steeplands worldwide but field measurements needed to constrain mechanistic models of this process remain sparse due to the difficulty of instrumenting natural flows here we present and analyze measurements made using an automated sensor network erosion bolts and a 15 24 cm by 15 24 cm force plate installed in the bedrock channel floor of a steep catchment these measurements allow us to quantify the distribution of basal forces from natural debris flow events that incised bedrock over the 4 year monitoring period 11 debris flow events scoured the bedrock channel floor no clear water flows were observed measurements of erosion bolts at the beginning and end of the study indicated that the bedrock channel floor was lowered by 36 to 64 mm the basal force during these erosive debris flow events had a large magnitude up to 21 kn which was approximately 50 times larger than the concurrent time averaged mean force high frequency greater than 1 hz fluctuating component we interpret these fluctuations as flow particles impacting the bed the resulting variability in force magnitude increased linearly with the time averaged mean basal force probability density functions of basal normal forces were consistent with a generalized pareto distribution rather than the exponential distribution that is commonly found in experimental and simulated monodispersed granular flows and which has a lower probability of large forces when the bed sediment thickness covering the force plate was greater than 20 times the median bed sediment grain size no significant fluctuations about the time averaged mean force were measured indicating that a thin layer of sediment 5 cm in the monitored cases can effectively shield the subjacent bed from erosive impacts coarse grained granular surges and water rich intersurge flow had very similar basal force distributions despite differences in appearance and bulk flow density these results demonstrate that debris flows can have strong control on rates of steepland evolution and contribute to a foundation needed for modeling debris flow incision stochastically

structure and morphology of submarine volcanism in the hotspot region around reunion island western indian ocean the form of the deccan maldives mascarene reunion hotspot trace suggests that it has at least in part been strongly controlled by crustal structures especially fracture zones this makes it difficult to assess the present day or past location of the hotspot and thus complicates the interpretation of african plate motion reconstruction we present here results of a cruise to the reunion area of which the aims were a to determine the extent of present day volcanism associated with the reunion hotspot in the region and b to examine the role of pre existing oceanic crustal structures in controlling the location of present day volcanism additionally we examined the morphology and geology of the important extinct spreading centre southwest of reunion abandoned when spreading jumped to separate seychelles from india during the deccan flood basalt episode some 60 65 ma ago the extensive bathymetrie seismic and geological investigation shows that significant present day hotspot volcanism is confined to the piton de la fournaise edifice on reunion island itself apparently the location of recent reunion volcanism has not been controlled by a crustal fracture and the major fracture zones on both sides of the island are not acting as magma conduits for plate motion reconstruction and plume flux calculation purposes piton de la fournaise must be taken as the present location of the reunion hotspot accretion at the extinct spreading centre progressively ceased at the time of anomaly a27 63 ma and was associated with marked propagation of the rift tips

growth structure instability and collapse of canarian volcanoes and comparisons with hawaiian volcanoes recent onshore and offshore investigations in the canarian archipelago especially in the western islands of la palma and el hierro have greatly improved the understanding of the genesis and evolution of these islands and allow interesting comparisons with other hotspot induced oceanic island volcanoes especially the hawaiian archipelago genesis induced by hotspot activity and consequently shield and post erosional stages of growth allow the definition of similar main stratigraphic units the canarian and hawaiian volcanoes show common constructional and structural features such as rift zones progressive volcano instability and multiple gravitational collapses however the canaries present some important geological differences from the prototypical hotspot volcanoes of the hawaiian islands particularly the absence of significant subsidence the canarian volcanoes remain emergent until completely mass wasted by gravitational collapses and erosion volcanic formations over 20 million years are observable in outcrop in the canaries including the seamount stages of growth in several of the islands

recurrence of major flank landslides during the last 2 ma history of reunion island new detailed swath bathymetry and backscatter data corroborate the existence of four large bulges on the submarine flanks of reunion island these fan shaped promontories are 20 25 km wide at the coastline and 70 150 km across the seafloor 40 50 km offshore their surfaces are characterized by a speckle sonar pattern indicating the presence of large blocks up to several hundred meters across each bulge results from the superposition of multiple landslide deposits whose older ones are dissected and delimited by erosive channels as much as 200 m deep and 20 km long the submarine flanks of reunion island are thus mostly built by accumulation of debris avalanche fans morphologic and geologic evidence define large subaerial source areas for these mass wasting events in particular inferred headwalls of most landslides having affected the piton des neiges massif generally coincide with the boundaries of its cirques mafate salazie and cilaos whereas recurrent landslides have resulted in the formation of large concentric amphitheatre structures through the piton de la fournaise massif thus about 15 slide events accompanied growth of the reunion island shield since 2 ma

detailed gravity study of the offshore structure of piton de la fournaise volcano reunion island we present an interpretation of gravity data acquired in 1984 by the french r v jean charcot on the submarine part of the eastern flank of piton de la fournaise volcano we comment on the bouguer anomaly map and give a quantitative interpretation of three gravity profiles the main results are that a gravity high over grand brule the lower subaerial part of the eastern flank does not extend far offshore and that an anomalous topographic feature discovered in 1982 on the submarine eastern flank is characterized by a large negative anomaly we propose three hypotheses to explain the origin of this anomaly i e it marks the site of a new volcano or it is a consequence of lateral volcanism from a volcano older than piton de la fournaise or more probably it represents a great landslide deposit

influence of particle density on flow behavior and deposit architecture of concentrated pyroclastic density currents over a break in slope insights from laboratory experiments abstract geological granular flows are highly complex gravity driven phenomena whose different behaviors depend on the mechanical properties density and granulometric distributions of the constituent materials years of research have produced significant advances in understanding transport and deposition processes in granular flows however the role and effects of clast densities and density contrast in a granular flow are still not fully understood in this paper we show the effect that pumice has on dry granular flows specifically on flow velocity and longitudinal segregation of the deposits our work confirms by experimental results field observations on pumice lithic segregation and longer pumice runout we report results of velocity decay and deposit architecture for a granular flow passing over a break in slope from 38 to 4 inclination the 30 experimental runs were carried out in a five meter long laboratory flume equipped with a series of sensors that include laser gates and high speed cameras 400 fps we used two polydisperse mixtures of dacitic lithics and rhyolitic pumice in varying amounts with weibull and gaussian particle size distributions the pumice lithic ratio changes the flow response passing over a break in slope this effect is particularly evident starting from 10 of pumice volume into the flow mixture independently of its granulometric distribution runout relates to mass following a power law with an exponent close 0 2 the experiments confirm that pumice segregation affects polydispersed mixtures similarly to what has been observed in real field deposits where density decoupling produces lithic enriched proximal areas and pumice enriched distal areas the results obtained prove that the presence of low density materials in a dense granular flow has a strong influence on its behavior

granular pressure at the base of dry flows of angular rock fragments as a function of grain size and flow volume a relationship from laboratory experiments 1 experiments are carried out by releasing angular rock fragments down a curved chute and by measuring the basal pressures that are exerted by the granular flows on the basal containing surface the substrate the purpose of these experiments is to understand the mechanisms of energy dissipation and interaction with the ground of rock avalanches and dense pyroclastic flows our data show that collisions due to particle agitation affect significantly the basal interaction of granular flows in particular our experiments reveal that particle agitation per unit of flow mass increases as grain size increases or as flow volume decreases with all the other features the same this is so because as grain size increases or as flow volume decreases with all the other features the same there are fewer particles in the flows and the agitation due to the interaction with the rough containing boundary surfaces penetrates relatively more inside the flows the analysis of the experimental data generates a linear relationship between particle agitation expressed as a dimensionless basal pressure deviation and a parameter which is directly proportional to the square of grain size and inversely proportional to the cube root of flow volume this relationship shows the intrinsic ability of the granular flows to dissipate more energy larger particle agitation per unit of flow mass or less energy smaller particle agitation per unit of flow mass as a function of flow volume and grain size

calderas landslides and paleo canyons on piton de la fournaise volcano la reunion island indian ocean based upon a re interpretation of previous data and a new field campaign a structural evolution is proposed for the early history of piton de la fournaise volcano from 500 000 to 50 000 years conceptually it is shown that the formation of a caldera in which lava flows are contained inside the caldera depression gives time for erosion to excavate deep canyons on the external slopes of the volcano for example the riviere des remparts the riviere langevin and the riviere de l est canyons on piton de la fournaise volcano these canyons are infilled when lavas filling the caldera and overflowing its rim are able again to flow on the external slopes of the volcano in the past this excavating infilling process has occurred twice following the formation of the riviere des remparts and morne langevin calderas the formation of the third caldera the plaine des sables caldera was followed by the excavation of the current canyons in addition to this process two large landslides have been documented in the field the first which happened about 300 000 years ago is apparently the first episode of the break up of piton de la fournaise volcano predating the formation of the four large calderas the second landslide which occurred 150 000 years ago and is considered to be less extensive has carried away the entire southern flank of the riviere des remparts caldera

cumulate clasts in the bellecombe ash member piton de la fournaise reunion island and their bearing on cumulative processes in the petrogenesis of the reunion lavas abstract among the ejecta from a phreatomagmatic eruption on piton de la fournaise volcano are wehrlitic and gabbroic cumulate clasts these fresh samples comprise olivine fo85 0 to fo69 0 augite en47 1wo44 6wo44 6 to en41 1fs15 4wo43 5 and plagioclase an82 8ab16 8or0 4 to an53 8ab44 8or1 4 with spinel ti rich ferrichromites to ti magnetite and some ilmenite a few percent of highly vesicular interstitial glass is commonly present textures indicate that the cumulates were not wholly crystalline at the time of eruption and that the glass represents a residual intercumulus melt that varies from basaltic to mugearitic sulphide globules typically spherical and up to 40 m diameter represent immiscible melts within the glasses the sulphides contain relatively high contents of cu and ni and appear to be high temperature homogeneous pyrrhotite rich compositions rank order of element abundance in the sulphides is fe s cu ni co and zn wt s although in one sample ni is more abundant than cu lack of correlation between the sulphide globule compositions and their host glasses is probably due to complex partitioning between the silicate melts silicate and oxide crystals sulphide melt and separating volatile phase whereas the cu cu ni at values of the sulphides are similar to those of the sulphide ore bodies from major gabbroic complexes the ni ni fe values are notably high with regard to the low mgo content of their host glasses oxide pairs regarded as reflecting equilibria prior to eruption indicate temperatures of 1017 897 c and log fo2 values of 9 6 12 7 compaction of analogous cumulate mushes with extraction of relatively low density basaltic to mugearitic and ultimately trachytic residual melts is inferred to have yielded the differentiated series lavas and ash flows of the older volcano on reunion piton des neiges the sequence of crystallisation with augite as the second silicate phase rather than plagioclase as is normal in the lava sequences suggests crystallisation at depths 5 km the bellecombe ash cumulate clasts show more evolved assemblages than most other xenolith suites on the volcano in which dunites predominate and are probably products of stagnated magma batches possibly arrested in sill like intrusions in the deeper crust the cumulates are recent components in what is probably a large ca 70 km across layered complex beneath the volcanic edifices accreted during the build up of reunion

the crustal structure of el hierro canary islands from 3 d gravity inversion el hierro is the youngest and westernmost island of the canarian archipelago due to its singular and interesting characteristics with extensive landsliding studying its crustal structure can provide very useful information about its origin and evolution with this aim we use the gravity inversion methodology obtaining a crustal model of mass distribution we performed a gravity survey of the entire island completing the gravity anomaly map of this area with marine data from the united states geological survey the calculated bouguer gravity anomaly map was studied using a covariance analysis and the least squares technique to obtain crustal information from these data we used a 3d gravity inversion based on genetic algorithms ga thus our inversion technique aims to determine the geometry of the sources of the observed gravity field upon a prismatic partition of the subsoil volume and adopting a priori density contrast values the results of this gravity study of el hierro island show the correlation of several volcanic structures with the distribution of the gravity field sources the characteristic triple rift system of the island is associated with low density areas and the older volcanic stages with high density structures we also found differences among the structures related to the several landslides which helped the island takes shape

the effects of rock heterogeneities on dyke paths and asymmetric ground deformation the example of piton de la fournaise reunion island during pre eruptive periods and eruptions piton de la fournaise volcano shows an asymmetric magma induced pattern of deformation the origin of this asymmetry is not well constrained and the pre eruptive deformation does not have a satisfactory explanation here we present data on the past history and complex structure of the volcano we also provide new field data on exposed dykes in the volcano the field data confirm that the eruptive centre has migrated through time our field data indicate average dyke dips of about 80 by contrast inferred dips of recent feeder dykes using geodetic data are 53 75 we explain this difference as being partly due to the models used to infer the dyke dips from geodetic data which assume the host rock to be homogeneous and isotropic when fractures and the other heterogeneities are taken into account in the models the inferred dyke dips become similar to those of the exposed dykes new numerical models indicate that when the complex internal structure of the volcano is taken into account part of the asymmetry of the ground deformation can be explained part of the deformation asymmetry however is due to the curved structure of the rift system resulting in the injection of curved dykes and the dyke dips 2008 elsevier b v all rights reserved

structural results for la palma island using 3 d gravity inversion 1 a recent gravity survey composed of 317 bench marks all over the island of la palma canary islands is used in combination with satellite data for regional aspects to obtain results about structural properties of the island connected with the tectonic environment and local volcanism to that end a nonlinear three dimensional gravity inversion approach is considered the inversion scheme provides in a nonsubjective form the geometry of the anomalous bodies constructed in a random growth process results from the inversion can be interpreted in the framework of the geologic evolution of this ocean island volcano as a complex composite volcano with a large central body with high density corresponding to the older intrusive part of the basalt complex new unexpected features are enlightened such as large thermal anomalies in the upper mantle southward of la palma as well as fracture en echelon zones associable to a slow active process of dislocation related to the recent volcanism in the southern half of the island the results obtained for la palma as a test site testify to the usefulness of the developed gravity inversion methodology for structural studies on islands in general

three dimensional modeling of the 1983 1984 eruption at piton de la fournaise volcano reunion island displacement vectors on 125 points over a 6 km 2 area were calculated zlotnicki et al 1990 for piton de la fournaise volcano reunion island from photogrammetric surveys conducted in september 1981 and again in september 1984 an eruptive crisis with two episodes occurred during the 3 years between the surveys we modeled the displacements from the photogrammetric surveys using a three dimensional mixed boundary element method the model shows that both eruptive episodes were fed by a single dike oriented 165 from north as the dike approached the ground surface it divided into segments that rotated into an en echelon pattern the model that best fits all observations suggests that the stress state within the volcano is quasi isotropic such that horizontal stresses equal the overburden these high horizontal stresses are incompatible with an elastic edifice loaded merely by gravitational forces however they may be attributed to the cumulative effect of repeated dike intrusions

grain size segregation and levee formation in geophysical mass flows 1 data from large scale debris flow experiments are combined with modeling of particle size segregation to explain the formation of lateral levees enriched in coarse grains the experimental flows consisted of 10 m3 of water saturated sand and gravel which traveled 80 m down a steeply inclined flume before forming an elongated leveed deposit 10 m long on a nearly horizontal runout surface we measured the surface velocity field and observed the sequence of deposition by seeding tracers onto the flow surface and tracking them in video footage levees formed by progressive downslope accretion approximately 3 5 m behind the flow front which advanced steadily at 2 m s 1 during most of the runout segregation was measured by placing 600 coarse tracer pebbles on the bed which when entrained into the flow segregated upwards at 6 7 5 cm s 1 when excavated from the deposit these were distributed in a horseshoe shaped pattern that became increasingly elevated closer to the deposit termination although there was clear evidence for inverse grading during the flow transect sampling revealed that the resulting leveed deposit was strongly graded laterally with only weak vertical grading we construct an empirical three dimensional velocity field resembling the experimental observations and use this with a particle size segregation model to predict the segregation and transport of material through the flow we infer that coarse material segregates to the flow surface and is transported to the flow front by shear within the flow head coarse material is overridden then recirculates in spiral trajectories due to size segregation before being advected to the flow edges and deposited to form coarse particle enriched levees

a simple approach to evaluating leakage through thin impervious element of high embankment dams internal erosion is the most common reason which induces failure of embankment dams besides overtopping relatively large leakage is frequently concentrated at defects of impervious element and this will lead to eventual failure the amount of leakage depends not only on integrity of impervious element but also on dam height shape of valley shape of impervious element and water level in reservoir the integrity of impervious element which represents the relative level of seepage safety is not easy to be determined quantitatively a simple method for generalization of steady seepage state of embankment dams with thin impervious element is proposed in this paper the apparent overall value of permeability coefficient for impervious element can be obtained by this method with reasonable accuracy and efficiency a defect parameter of impervious element is defined as an index to characterize seepage safety of embankment dams it equals the ratio of the apparent overall value of permeability coefficient to the measured value in laboratory for intact materials subsequently seepage safety of three dams is evaluated and the evolution of defect level of impervious element of dams is investigated it is proved that the newly proposed method in this paper is feasible in the evaluation of relative seepage safety level of embankment dams with thin impervious element

from steep slope volcano to flat caldera floor 1 most laboratory experiments of caldera collapse have dealt with reservoir emptying below a flat lying overburden without an overlying analogue volcanic edifice on top the overload and the role of topography are then neglected so that the final flat floor within the caldera is directly linked to the initial one in addition caldera subsidence is commonly attributed to the collapse of the top of a magma chamber linked to eruptions delivering large amounts of volcanic products analogue experiments show that the deformation of a weak clay rich core resulting from the hydrothermal alteration in a volcanic edifice can in certain conditions reproduce the structures of a caldera in particular it is a way to explain the flat floor of a caldera when resurfacing resulting from new eruptions or destructive processes seems unlikely

complete gravity study of piton de la fournaise volcano reunion island abstract we present an interpretation of all available gravity data on piton de la fournaise volcano including a new set collected in the summit area using a linear programming inversion method we determine the extension of the grand brule ultramafic body drilled in 1985 and we propose to interpret it as a frozen magma chamber related to a disappeared volcano we also present evidence for superficial mass excess under the enclos fouque and plaine des sables calderas that we link to the present piton de la fournaise volcanism the detailed gravity survey of the summit area allows us to emphasize the great importance of dyke complexes associated with the ne and se rift zones and with the shallow magma chamber a negative anomaly in the summit area is interpreted as a relative minimum between the positive anomaly of grand brule and those of the western part of the enclos fouque and plaine des sables calderas

the 2009 jiweishan rock avalanche wulong china precursor conditions and factors leading to failure abstract this study investigates the predisposing geological and anthropogenic factors and indicative precursors preceding the fatal 5 june 2009 jiweishan rock avalanche it began as a rockslide and transformed into a rock avalanche that killed 74 people and caused major property loss due to misdiagnosis of the failure mode and underestimation of volume and travel distance besides topography structure and stratigraphy that favoured failure mining below the failed rock mass had changed the state of stress and altered groundwater flow the anthropogenic factors caused deformation and accelerated failure of the slope and therefore were important contributions cracks and a karstic zone had developed along regional joints and defined several boundaries of the failed rock mass furthermore a 2 m wide crack developed since 1958 revealed a potential basal failure surface at the contact between karstic limestone and an underlying pre sheared thin shale aquiclude increasing magnitude and frequency of rockfalls and tension cracks at the front end of the sliding block appeared three days before the event and implied the likely movement direction of the sliding rock mass these could have warned of a developing giant failure topography included a large elevation difference between the toe of the failure surface and the cliff base a narrow valley to channel debris and the steep gradient of the downstream tiekuang creek could have suggested that failure might lead to a rapid and long runout rock avalanche however access to the densely vegetated source area before the failure was difficult and local investigators did not have all of the evidence that might have led them to recognize that potential danger was more than continuing rockfalls

particle breakage of argillaceous siltstone subjected to stresses and weathering abstract particle breakage is an important phenomenon that is observed in high rockfill dams as the dam material is subjected to both weathering and stresses this paper presents a study of the particle breakage of an argillaceous siltstone used in a rockfill dam in southwest china that is 261 5 m in height the region is characterized by abundant rainfall and a wide variation in temperature 10 60 c a series of oedometer tests were performed with specimens subjected to different types of weathering the study aims to examine the development of particle breakage of an argillaceous siltstone and the manner in which the particle size distribution of the weathered material changes over time the results highlight that the combined effect of water and thermal weathering is a major factor controlling the disintegration of soft rocks particle breakage is characterized by three mechanisms angular fracture particle cracking and particle breakup the weathering rate defined as the rate of particle breakage over a weathering cycle declines as the number of cycles increases the ultimate particle breakage induced by weathering approaches a constant despite different axial stresses