long term ground displacement observations using insar and gnss at piton de la fournaise volcano between 2009 and 2014 abstract monitoring ground surface displacement of volcanoes over a long period of time helps improve understanding of the volcano structure dynamics and mechanisms in this paper we investigate the spatio temporal behavior of the long term displacement field at piton de la fournaise volcano la reunion island by means of x band insar and gnss time series analysis from 2009 to 2014 we propose a new correction approach based on principal component analysis to mitigate the long wavelength artifact in the interferograms over the study area our results show that most of the volcanic edifice including the central cone the eastern flank and recent lava flow fields was affected by varying degrees of ground motion more importantly a widespread sector 20 km2 affected by time dependent downward and eastward motion was observed on the eastern flank of piton de la fournaise the combined analysis of insar and gnss time series and previous studies allows us to confirm that this large mobile sector on the eastern flank underwent deformation for at least 7 years following the end of the large march april 2007 eruption including a stage of transient strain from the end of eruption to february 2011 followed by a stage of steady state strain until late 2014 several possible origins including a pre existing structural discontinuity summit stress associated with magmatism and activated fault movement are explored to explain the observed widespread long term seaward motion that is considered to be direct evidence of potential flank instability

influence of cone morphology on dykes stromboli italy abstract the island of stromboli represents the emerged part of a 2 6 km high composite volcano elongated in a ne direction over the period 100 13 ka bp the morphological evolution of the volcano was characterised by a series of summit caldera collapses between major building phases from 13 ka bp onward a series of large lateral collapses occurred northwestwards in one sector of the cone the results of the field study of the total 94 dyke population is discussed in this paper they show that during the last 100 ka the majority of dykes were injected with a nnegssw to ene wsw strike along a ne sw weakness zone crossing the volcano summit this is interpreted as a volcanic rift the geometry of which is controlled by regional tectonic stresses dykes injected prior to 13 ka bp at lower altitudes are generally steeper than those injected at higher elevations after 13 ka bp dykes were injected along the ne sw weakness zone and parallel and close to the margins of the earliest sector collapse at dips of 20 80 inclined towards the collapse depression i suggest that this is linked to unbuttressing of the collapse depression or to seaward gravity instability of the depression infill deposits extensive emplacement of long dykes along the ne sw weakness zone occurred only when the volcano regained a conical shape some of the dykes emplaced close to the original surface of the cone bent to assume a dip direction parallel to the local slope i propose that this local change of dyke geometry can be explained by the direct influence of the free surface of the cone leading to a rotation of the least principal stress which assumes an orientation perpendicular to the slope

time space modeling of the dynamics of santorini volcano greece during the 2011 2012 unrest the 2011 2012 unrest of santorini thera volcano aegean sea greece was associated with microseismicity confined to the kameni line kl a major tectonovolcanic lineament and has been regarded as a single magmatic episode produced by a spherical source derived from inversion of gps data however such a source is a few kilometers away from the kl and cannot explain observed microseismicity for this reason we divided the unrest episode into five periods based on the fluctuations of seismicity and deformation rates and investigated the connection between seismicity and two spherical magmatic point sources for each period based on a new inversion algorithm and consistent gps data we recognized during the volcano unrest episode an unstable pattern of intrusions correlating with both the kl and columbo line cl a second major tectonovolcanic lineament intrusions correlating with cl appear relatively persistent aseismic small and shallow which is consistent with marine geophysical evidence for arrested shallow dykes and geodetic evidence from a previous inflation episode during the two periods of intense seismicity sources close to the kl explaining seismicity were obtained this unstable pattern of intrusions explains both the well observed location and timing of seismicity as well as ground deformation and is consistent with results of an okada type inversion for a sill and a dyke the stress interactions between the two sources agree with coulomb failure stress models santorini appears to be affected by concurrent offset magma pulses and only recent activity from a magma pulse below the kl produced microseismic swarms

relationship between petrologic processes in the plumbing system of mt etna and the dynamics of the eastern flank from 1995 to 2005 abstract mt etna volcano is located in a complex tectonic setting with large sectors of its eastern flank sliding seaward at different rates according to recent petrologic studies the etna s plumbing system has a multifaceted geometry variable in space and time and consisting of storage zones at different depths where magma ascending to the surface undergoes various processes mainly fractional crystallization and mixing in this framework we investigated a possible cause effect relationship between the flank displacement and pre eruptive magmatic processes in the shallow plumbing system furthermore a comparison between petrologic data and deformative patterns evidences that from 1995 to july 2001 the aforementioned pre eruptive magmatic processes did not significantly influence the volcano eastern flank dislocation which stayed slow and fairly regular by contrast the onset of the 2001 flank eruption leads to an acceleration of the movement as a consequence of the ascent of a primitive volatile rich sub aphyric basaltic magma from a deeper about 10 km b s l reservoir during the 2001 and 2002 03 activity at least for the decade 1995 2005 pre eruptive magmatic processes in the shallow portion

the long term growth of volcanic edifices numerical modelling of the role of dyke intrusion and lava flow emplacement abstract the contribution of intrusive complexes to volcano growth is attested by field observations and by the monitoring of active volcanoes we used numerical simulations to quantitatively estimate the relative contributions to volcano growth of elastic dislocations related to dyke intrusions and of the accumulation of lava flows the ground uplift induced by dyke intrusions was calculated with the equations of okada bull seismol soc am 75 1985 1135 the spreading of lava flows was simulated as the flow of a bingham fluid with realistic parameters for dyke statistics and lava flow rheology we find the contribution of dyke intrusions to the growth of a basaltic shield archetype to be about 13 in terms of volume and 30 in terms of height the result is strongly dependent on the proportion of dykes reaching the surface to feed a lava flow systematic testing of the model indicates that edifices tend to be high and steep if dykes are thick and high issued from a small and shallow magma chamber and if they feed lava flows of high yield strength the simulation was applied to ko olau o ahu is hawai i and piton de la fournaise reunion is volcanoes the simulation of ko olau with dyke parameters as described by walker geology 14 1986 310 u s geol surv prof pap 1350 1987 961 and with lava flow characteristics collected at kilauea volcano hawai i is results in an edifice morphology very close to that of the real volcano the best fit model of the piton de la fournaise central cone with its steep slope and e w elongation is obtained by the intrusion of 10 000 short and thick dykes issued from a very small and shallow magma chamber and feeding only 700 low volume lava flows the same method may be applied to the growth of basaltic shields and other volcano types in different environments including non terrestrial volcanism

morphometry of terrestrial shield volcanoes abstract shield volcanoes are described as low angle edifices built primarily by the accumulation of successive lava flows this generic view of shield volcano morphology is based on a limited number of monogenetic shields from iceland and mexico and a small set of large oceanic islands hawaii galapagos here the morphometry of 158 monogenetic and polygenetic shield volcanoes is analyzed quantitatively from 90 meter resolution srtm dems using the morvolc algorithm an additional set of 24 lava dominated shield like volcanoes considered so far as stratovolcanoes are documented for comparison results show that there is a large variation in shield size volumes from 0 1 to 1000 km3 profile shape height basal width h wb ratios mostly from 0 01 to 0 1 flank slope gradients average slopes mostly from 1 to 15 elongation and summit truncation although there is no clear cut morphometric difference between shield volcanoes and stratovolcanoes an approximate threshold can be drawn at 12 average slope and 0 10 h wb ratio principal component analysis of the obtained database enables to identify four key morphometric descriptors size steepness plan shape and truncation hierarchical cluster analysis of these descriptors results in 12 end member shield types with intermediate cases defining a continuum of morphologies the shield types can be linked in terms of growth stages and shape evolution related to 1 magma composition and rheology effusion rate and lava pyroclast ratio which will condition edifice steepness 2 spatial distribution of vents in turn related to the magmatic feeding system and the tectonic framework which will control edifice plan shape and 3 caldera formation which will condition edifice truncation

the tectonically controlled emplacement of a vertically sheeted gabbro pyroxenite intrusion feeder zone of an ocean island volcano fuerteventura canary islands abstract the miocene px1 gabbro pyroxenite pluton fuerteventura canary islands is a 3 5 5 5 km shallow level intrusion 0 15 0 2 gpa and 1100 1120 c interpreted as the feeder zone to an ocean island volcano it displays a vertical magmatic banding expressed in five 50 to 100 metre wide nne ssw trending alkaline gabbro sequences alternating with pyroxenites this emplacement geometry was controlled by brittle to ductile shear zones generated by a regional e w extensional tectonic setting that affected fuerteventura during the miocene at a smaller scale the px1 gabbro and pyroxenite bands consist of metre thick differentiation units which suggest emplacement by periodic injection of magma pulses as vertical dykes that amalgamated similarly to a sub volcanic sheeted dyke complex individual dykes underwent internal differentiation following a solidification front parallel to the dyke edges this solidification front may have been favoured by a significant lateral horizontal thermal gradient expressed by the vertical banding in the gabbros the fractionation asymmetry within individual dykes and the migmatisation of the wall rocks pyroxenitic layers result from the fractionation and accumulation of clinopyroxene olivine plagioclase crystals from a mildly alkaline basaltic liquid they are interpreted as truncated differentiation sequences from which residual melts were extracted at various stages of their chemical evolution by subsequent dyke intrusions either next to or within the crystallising unit compaction and squeezing of the crystal mush is ascribed to the incoming and inflating magma pulses the expelled interstitial liquid was likely collected and erupted along with the magma flowing through the newly injected dykes clinopyroxene mineral orientation as evidenced by ebsd and micro x ray tomography investigations displays a marked pure shear component supporting the interpretation of the role of compaction in the generation of the pyroxenites conversely gabbro sequences underwent minor melt extraction and are believed to represent crystallised coalesced magma batches emplaced at lower rates at the end of eruptive cycles clinopyroxene orientations in gabbros record a simple shear component suggesting syn magmatic deformation parallel to observed nne ssw trending shear zones induced by the regional tensional stress field this emplacement model implies a crystallisation time of 1 to 5 years for individual dykes consistent with px1 emplacement over less than 0 5 my a minimum amount of approximately 150 km3 of magma is needed to generate the pluton part of it having been erupted through the central volcanic centre of fuerteventura if the regional extensional tectonic regime controls the px1 feeder zone initiation and overall geometry rates and volumes of magma depend on other source related factors high injection rates are likely to induce intrusion growth rates larger than could be accommodated by the regional extension in this case dyke intrusion by propagation of a weak tip combined with the inability of magma to circulate through previously emplaced and crystallised dykes could result in an increase of non lithostatic pressure on previously emplaced mushy dyke walls thus generating strong pure shear compaction within the pluton feeder zone and interstitial melt expulsion these compaction dominated processes are recorded by the cumulitic pyroxenite bands

multiple reinjections and crystal mush compaction in the solidification evolution of the karaj dam basement sill northern iran abstract the karaj dam basement sill is a 460 m thick saucer shaped sill situated in the alborz magmatic belt northern iran the results of geochemical textural and field relations reveal characteristics of a sill with a well developed s shaped compositional profile which could be subdivided into distinct parts and suggest that the sill was repeatedly split and reinjected with fresh magma in the upper half of the previous emplacements over to partly intra accretion whole rock and mineral compositional profiles have recorded five to six discrete injections of magma each of which individually show an upward increase in terms of primitivity which represents partial crystallization in feeder conduits the first three small successive pulses of magma emplaced in the basal 150 m of the floor sequence were followed by voluminous fourth and fifth pulses in the upper portion of the sill during final two pulses the system acts as a closed system for each independently and evolves through compositional convection or compaction aided in situ crystallization theoretical models for convection and compaction shows the significance of both processes considering the final porosity of 0 1 v c v a 1 and 0 1 m year when the crystal mush varies between 1 to 0 01 m with the liquid viscosity ranging from 85 to 15 pa s our calculations further indicate that compaction driven velocity of liquid expulsion w hardly exceeds the v a in the basal 150 m of the floor sequence the highest velocity is reached w 1 m year only if the crystal mush thickness is no less than 240 m with the porosity of 0 6 and the liquid viscosity of no more than 15 pa s on the other hand compaction is highly effective in the upper portion of the sill transfer of residual liquid from the compacting lower solidification front to the dilating upper solidification front resulted in characteristic chemical and mineralogical effects such as the depletion of the lower half of the sill and the enrichment of the sandwich zone in incompatible elements and modal granophyre crystallization of the fourth pulse of magma produced a peak in incompatible clement concentrations at a sandwich horizon located at the 410 m level where the floor and roof of the sill appeared to converge at this stage subsequent to the time of this enriched zone crystallizing the fifth pulse of magma was emplaced near this level and inflated the chamber vertically for 200 m cumulus material containing interstitial melt and subsequent buoyancy driven upward transport of interstitial melt in this pulse concentrated incompatible elements at 47 m below the main peak at the stratigraphic height of 360 370 m the boundaries between the successive pulses were cryptic and represent a gradational contact in terms of grain size chemical composition and crystallization sequence this implies a short time interval between the emplacements of the magma this sill is comparable with antarctica beacon sill and hudson river palisades sill and the recognition of evidence for reinjection and compaction in these macroscopically uniform sills as well as the karaj dam basement sill suggests that these processes may be common in the construction of sills with a thickness of more than 100 m

field evidence for flank instability basal spreading and volcano tectonic interactions at mt cameroon west africa the mt cameroon volcano is the highest and most active volcano of the cameroon volcanic line little geological information is available for improving the understanding of the structure of this large volcanic system and its relationship to regional tectonics after reviewing the tectonic evolution of the region the analysis of a digital elevation model and results from a field campaign dedicated to mapping geological structures in the summit area and at the se base of mt cameroon are presented mt cameroon is a lava dominated volcano with long steep over 30 flanks it is elongate parallel to its well defined rift zone the summit plateau is bordered by 10 m high cliffs formed by summit subsidence along normal faults geological profiles were measured along rivers cutting through a topographic step at the se base of mt cameroon this step is associated with deformed miocene sediments from the douala basin that are overlain by volcanic products weak sediments of this area are deformed by 050 060 and 020 030 trending asymmetrical folds verging toward the se and thrusts faults related to the spreading of the volcano over its mechanically weak substratum combined remote sensing and field observations suggest that spreading is accommodated by summit subsidence and flanks sliding both slow spreading movements and catastrophic collapses of the steep flanks are interpreted to result from complex interactions between the growing edifice repeated dyke intrusions the weak sedimentary substratum and tectonic structures

revised magnetostratigraphy of the plio pleistocene sedimentary sequence of the olduvai formation tanzania abstract the magnetic stratigraphy of the plio pleistocene sedimentary formation of the olduvai gorge tanzania which contains a well known series of prehistoric pebble cultures and hominid remains has been reinvestigated sixty one sites were sampled along 90 metres of stratigraphic section extending from the base of bed i into the masek beds palaeo and rock magnetic analyses reveal that magnetizations are stable carried by low coercivity magnetic minerals titano magnetites and are likely to be of depositional origin a revised magnetostratigraphic scale is proposed the base of the major normal magnetozone n1 interpreted as the olduvai sub chron lies ca 6 m below tuff ia i e significantly below the previously defined position this revision associated to 40ar 39ar ages recently determined on the tuffs of bed i supports the validity of the astronomical calibration of the geomagnetic polarity time scale for the olduvai sub chron the correlation of the normal magnetozone n2 with jaramillo or jaramillo cobb mountain is suggested by the discovery of a reverse magnetozone r3 in the uppermost part of the pleistocene sequence masek beds and implies that the fossiliferous sequence of the olduvai formation was entirely deposited before the brunhes matuyama boundary

analysing the topographic form of stratovolcanoes abstract the goal of this study is to explore ideal analytic functional forms that best fit the topographic shape of n 190 isolated stratovolcanoes from around the world using a stochastic model for the piling of lava flows to demonstrate one set of physical processes that give rise to a stratovolcano s topography we find that although the ideal form fits well for many stratovolcanoes there exist deviations from this shape particularly where the edifice is more protuberant than expected we explore sub surface factors such as the emplacement of magma beneath a volcano edifice that may be responsible for these deviations and estimate the relative contribution of dyke intrusions on the height and shape of a volcanic edifice versus the constructional piling of lava from this comparison we are able to gain more insight into the role of important subsurface physical factors such as chamber depth and total intruded volume we find that low basal ellipticity is a characteristic of nearly all volcanoes that fit the ideal stratovolcano form by fitting along each quadrant bisected by either a semi major or semi minor ellipse axis we find that the volcanoes chosen in this study fall into four groups 1 volcanoes that are axisymmetric and fit the ideal form well 2 volcanoes that are not axisymmetric but may be fit to the ideal form using different fitting parameters in each quadrant 3 volcanoes that have axisymmetric protuberant deviations in all quadrants and 4 volcanoes that have non axisymmetric protuberant deviations that are only present in a couple of quadrants we show that in some cases the non axisymmetry of volcanoes in category 2 may be understood as the result of the piling of eruptive products along a regional planar trend for edifices in categories 3 and 4 we model protuberant deviations as the result of a statistical distribution of elastic tensile dislocations representing dykes which emanate from a magma reservoir and cause uplift at the surface we support this interpretation of the excess topography in a few case studies where independent geologic and geophysical data corroborate our model results

lava flow hazard and risk at mt cameroon volcano mt cameroon is one of the most active effusive volcanoes in africa about 500 000 people living or working around its fertile flanks are subject to significant threat from lava flow inundation lava flow hazard and risk were assessed by simulating probable lava flow paths using the downflow code the vent opening probability density function and lava flow length distribution were determined on the basis of available data from past eruptions at mt cameroon volcano code calibration was performed through comparison with real lava flow paths the topographic basis for simulations was the 90 m resolution srtm dem simulated lava flows from about 80 000 possible vents were used to produce a detailed lava flow hazard map the lava flow risk in the area was mapped by combining the hazard map with digitized infrastructures i e human settlements and roads results show that the risk of lava flow inundation is greatest in the most inhabited coastal areas comprising the town of limbe which constitutes the center of cameroon s oil industry and an important commercial port buea the second most important town in the area has a much lower risk although it is significantly closer to the summit of the volcano non negligible risk characterizes many villages and most roads in the area surrounding the volcano in addition to the conventional risk mapping described above we also present 1 two reversed risk maps one for buildings and one for roads where each point on the volcano is classified according to the total damage expected as a consequence of vent opening at that point 2 maps of the lava catchments for the two main towns of limbe and buea illustrating the expected damage upon venting at any point in the catchment basin the hazard and risk maps provided here represent valuable tools for both medium long term land use planning and real time volcanic risk management and decision making

thermochronology and the three dimensional cooling pattern of a granitic pluton an example from the toki granite central japan the three dimensional spatial variations in the cooling pattern of the toki granitic body a zoned pluton in central japan have been evaluated quantitatively by thermochronology using cooling age determination based on the different closure temperatures for target mineral species the toki granite has hornblende k ar ages of about 74 3 3 7 ma n 2 closure temperature of 510 25 c biotite k ar ages of 78 5 3 9 to 59 7 1 5 ma n 33 300 50 c and zircon fission track ages of 75 6 3 3 to 52 8 2 6 ma n 44 240 50 c the spatial variation in the biotite k ar age is similar to that in the zircon fission track age in samples collected from 11 boreholes and seven outcrop sites in the toki granite indicating that cooling was effectively from the roof and also from the northwest margin this cooling pattern shows a strong correlation with the alumina saturation index asi distribution of the body larger asi values correspond to earlier and more rapid cooling after emplacement and smaller value to slower cooling toki granite was effectively cooled from the peraluminous regions where assimilation of country sedimentary rock was most extensive

experiments on rift zone evolution in unstable volcanic edifices large ocean island volcanoes frequently develop productive rift zones located close to unstable flanks and sites of older major sector collapses flank deformation is often caused by slip along a decollement within or underneath the volcanic edifice we studied how such a stressed volcanic flank may bias the rift zone development the influence of basal lubrication and lateral flank creep on rift development and rift migration is still poorly constrained by field evidence here our analog experiments provide new insights we injected colored water into gelatin cones and found systematic orientations of hydro fractures dikes propagating through the cones at the base of the cone diverse friction conditions were simulated by variation of the basal creep conditions we modeled radial dike swarms collinear rift zones and three armed rift systems it is illustrated that a single outward creeping flank is sufficient to modify the entire rift architecture of a volcano the experiments highlight the general unsteadiness of dike swarms and that the distribution and alteration of weak substratum may become a major player in shaping a volcano s architecture

postrifting anelastic deformation around the spreading plate boundary north iceland 2 implications of the model derived from the 1987 1992 deformation field a decade long rifting episode that began in the krafla volcanic system north iceland in 1975 caused substantial regional postevent anelastic deformation this was modeled as viscous relaxation in an elastic viscoelastic structure by hofton and foulger this issue the results from modeling the deformation detected in north iceland have far reaching implications both for local and regional processes and for the fundamental behavior of deformation around spreading plate boundaries in general tilt in the vicinity of the krafla volcano fits the model well after 1988 1989 which suggests that the volcano magma chamber stopped inflating deflating in 1988 a viscosity of 0 8 1018 pa s was required to match the local tilt data less than that predicted for north iceland as a whole vertical motion measured using the global positioning system gps 1987 1992 around the ice cap vatnajokull is inconsistent with isostatic uplift using the elastic viscoelastic model to predict motion in other regions of iceland suggests that the deformation effects of the krafla episode are significant in many parts of iceland and should be taken into account when modeling deformation there though not a realistic plate boundary model interesting complexities of the elastic viscoelastic model are highlighted by deformation modeling of an infinitely long dike this predicts that the amount of horizontal displacement close to the dike may exceed the amount of initial dike opening early in the spreading cycle a more realistic approximation to the plate boundary in north iceland involving five overlapping segments that experience dike emplacements at discrete intervals suggests that the width of the zone within which transient time dependent deformation occurs may be several hundred kilometers wide considerably wider than the neovolcanic zone a kinematic approach to describing plate motions is not appropriate close to spreading plate boundaries and elsewhere where the viscosity of the earth is low

how do deep water volcanoes grow abstract deep water volcanoes are emplaced in water depths 1 0 km and are widespread along continental margins and in ocean basins whilst the external morphology of deep water volcanoes can be mapped using bathymetric surveys their internal structure and true volume remain enigmatic it is thus difficult to determine how deep water volcanoes grow we investigate 13 late miocene to quaternary deep water volcanoes that are imaged in 3d by seismic reflection data from the northern south china sea which allow us to quantify their external morphology and examine their internal structure these deep water volcanoes were emplaced in water depths 1 5 km are relatively small 15 mpa which are typical of water depths 1 5 km inhibited degassing and fragmentation of ascending magma and thus erupted lava this lack of degassing and fragmentation permitted effusive eruptions during the latter stages of volcanism our models for volcano growth in the deep submarine realm demonstrate the power of using 3d seismic data when investigating the internal structure and total volume of deep water volcanoes

local and regional controls on the lateral emplacement of the ben hiant dolerite intrusion ardnamurchan nw scotland abstract sub volcanic and related satellite intrusions record the delivery storage and accommodation of magma before eruption however when several volcanic centres are in close proximity the relationship between centre and satellites may be ambiguous here we examine the structure of the ben hiant dolerite satellite intrusion discuss its relationship with the ardnamurchan central complex located 2 km to the northwest and explore the possibility of a genetic connection with the next nearest centre the mull central complex 35 km to the southeast structural field observations and anisotropy of magnetic susceptibility ams fabric analyses reveal that the ben hiant dolerite was emplaced as a series of lobes in a sequence of stacked sheets the ams fabric data further indicate that the ben hiant dolerite intruded laterally and may have been sourced from either the ardnamurchan central complex or the mull central complex testing either model and discriminating between these two possible source reservoirs remains an outstanding challenge however the potential implications of the lateral movement of magma over many 10 s of kilometres between upper crustal magmatic centres suggests hidden complexities associated with volcanic plumbing systems that must be tested structurally as well as geochemically

corona classification by evolutionary stage abstract the formation of coronae abundant volcano tectonic features on the surface of venus has been modeled as the interaction of mantle diapirs with the lithosphere however the applicability of this model to features of widely different sizes may not be evident from sar images due to the lack of a common scale we use the radius as a scaling parameter for both the distance from the center and relief obviating this problem and mapping the effects from coronae of different radii onto a single scale normalizing profiles for 394 features we find many features classified by e r stoffanet al 1992 j geophys res 97 13 347 13 378 as different types have very similar profiles and relief coronae distribute into three shapes domal circular calderic two shapes not related to coronae radial and volcanic remain variation between corona shapes is gradational and mirrors the evolution of diapirs from initial domal upwellings to calderas we test the implied age progression using impact crater populations within three corona radii despite the inherent uncertainties of this method crater statistics for a size restricted 275 325 km corona set suggest an age progression from domal through circular to calderic the calderic coronae have an associated impact crater population very similar to the global mean though with slightly too few tectonized craters implying that they approximate the average surface age and are no longer active the circular coronae have a low impact crater density and a slightly elevated proportion of tectonized craters and thus are currently active the domal coronae have fewer than the expected number of impact craters of which more are tectonized than expected suggesting that surface modification has begun

subaerial submarine evidence of structures feeding magma to stromboli volcano italy and relations with edifice flank failure and creep abstract structural field data including parameters of 109 sheets strike mostly n10 55 e average dip angle mostly 70 90 average thickness two peaks at 1 2 m and 4 2 m and of 14 eruptive fissures and vents as well as bathymetric and numerical data suggest that stromboli experienced the development of a volcanic rift zone across the cone summit with a tendency to widen towards the lower submarine flanks during the last 100 ka inside this rift zone intrusive sheets mostly strike ne and become more abundant more steeply dipping and thicker on approaching the rift axis this rift persistence reflects the edifice s state of stress with a nw se horizontal least principal stress on the magma intrusion paths after 13 ka ago sheets mainly intruded with a horseshoe shaped geometry in plan view along the boundaries of the northwestern unstable volcano flank where four main lateral collapses occurred during the last 13 ka they also intruded with a ne strike across the volcano summit and the northeastern volcano flank the horseshoe shaped geometry is influenced by the lateral creep of the volcano flank which provides space for intrusion along the transtensional boundaries of the unstable sector where thinner sheets were emplaced when the collapse depression was filled up by new deposits ne striking thicker dykes emplaced across the northwestern volcano flank indicating a rapid re establishment of regional tectonic forces across the cone unbuttressing due to the present sciara del fuoco depression and ongoing flank creeping can account for intrusions also inside the sciara del fuoco the latter in 2007

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

core stratigraphy constrains bed iv archaeological record at heb site olduvai gorge tanzania abstract the olduvai gorge deposits contain a rich archaeological record documenting the evolution of hominin behavior over the last 2 million years while archaeological assemblages in the lower sedimentary layers beds i ii are well preserved in relatively secure chronostratigraphic contexts the age of overlying beds is poorly constrained due to discontinuous exposures and lack of marker horizons abundant erosional contacts and lack of unweathered dateable material as a result beds iii iv and the masek beds have yet received little attention compared to the underlying beds recently the olduvai gorge coring project ogcp acquired high resolution stratigraphically continuous sedimentary cores of palaeolake olduvai through scientific drilling in order to improve the geological and palaeoenvironmental contexts of the hominin record while other studies reported in this volume have used palaeoenvironmental datasets from the cores to refine the beds i ii records this study combines outcrop and core stratigraphic methods to improve the chronology of bed iv archaeological sites our recent excavation at heb site demonstrates the importance of this approach to improve the stratigraphic positioning and relative age of bed iv archaeological materials our results suggest that the heb site is younger than previously thought because it was formed after depositional fill of an incised valley which substantially removed much of lowermost bed iv in the heb area the lithofacies at this site indicate lake expansion and development of ngorongoro highland sourced braided streams near the lakeshore at this time which coincided with rich assemblages of artifacts fossil fauna and bones bearing butchery marks this setting likely afforded a variety of resources to hominins including potable water raw materials refuge trees and animal food making heb an important site for inferring the cultural and feeding behaviors of homo erectus

subvolcanic structure of the central dike swarm associated with the ring complexes in the shitara district central japan the subvolcanic structure of the central dike swarm associated with the miocene otoge ring complex and the shitara igneous complex central japan has been reconstructed the central dike swarm was supplied from several aligned magma reservoirs flow lineations observed at the margin of the dikes converge towards a region that is regarded as a magma reservoir about 1 2 km below present sea level the minimum diameter of the magma reservoir corresponds to the width of the central dike swarm estimated to be about 3 4 km the inferred magma reservoir of the otoge ring complex may have a zoned structure as suggested by the flow lineations of dikes and the arrangement of cone sheets felsic magma occupied the upper part about 1 2 km below present sea level and basic magma the lower part deeper than 2 km the centre of the shitara igneous complex is interpreted to be composed of several other shallow magma reservoirs the distribution pattern in plan view of the central dike swarm is summarized from the frequency of dikes defined by the number of dikes per kilometre in the direction normal to the trend of the dike swarm and the variations of the different properties of individual dikes along the dike swarm it has a plane of symmetry normal to the dike swarm above the magma reservoir the patterns critical to a general understanding of the dike formation are 1 a region of low dike frequency is present above the magma reservoir and a radial dike pattern occurs around the magma reservoir 2 from both sides of the magma reservoir the axes of high dike frequency extend symmetrically along the central zone of the dike swarm 3 the number as well as the individual and total thickness of felsic dikes increases towards the magma reservoir 4 the number of basic dikes increases towards both sides of the magma reservoir while the individual thicknesses of basic dikes increase with distance from the magma reservoir

bright carbonate surfaces on ceres as remnants of salt rich water fountains abstract vinalia and cerealia faculae are bright and salt rich localized areas in occator crater on ceres the predominance of the near infrared signature of sodium carbonate on these surfaces suggests their original material was a brine here we analyze dawn framing camera s images and characterize the surfaces as composed of a central structure either a possible depression vinalia or a central dome cerealia and a discontinuous mantling we consider three materials enabling the ascent and formation of the faculae ice ascent with sublimation and carbonate particle lofting pure gas emission entraining carbonate particles and brine extrusion we find that a mechanism explaining the entire range of morphologies topographies as well as the common composition of the deposits is brine fountaining this process consists of briny liquid extrusion followed by flash freezing of carbonate and ice particles particle fallback and sublimation subsequent increase in briny liquid viscosity leads to doming dawn observations did not detect currently active water plumes indicating the frequency of such extrusions is longer than years

influence of extrusion rate and magma rheology on the growth of lava domes insights from particle dynamics modeling abstract lava domes are structures that grow by the extrusion of viscous silicic or intermediate composition magma from a central volcanic conduit repeated cycles of growth are punctuated by collapse as the structure becomes oversized for the strength of the composite magma that rheologically stiffens and strengthens at its surface here we explore lava dome growth and failure mechanics using a two dimensional particle dynamics model the model follows the evolution of fractured lava with solidification driven by degassing induced crystallization of magma the particle dynamics model emulates the natural development of dome growth and rearrangement of the lava dome which is difficult in mesh based analyses due to mesh entanglement effects the deformable talus evolves naturally as a frictional carapace that caps a ductile magma core extrusion rate and magma rheology together with crystallization temperature and volatile content govern the distribution of strength in the composite structure this new model is calibrated against existing observational models of lava dome growth results show that the shape and extent of the ductile core and the overall structure of the lava dome are strongly controlled by the infusion rate the effects of extrusion rate on magma rheology are sensitive to material stiffness which in turn is a function of volatile content and crystallinity material stiffness and material strength are key model parameters which govern magma rheology and subsequently the morphological character of the lava dome and in turn stability degassing induced crystallization causes material stiffening and enhances material strength reflected in non newtonian magma behavior the increase in stiffness and strength of the injected magma causes a transition in the style of dome growth from endogenous expansion of a ductile core to stiffer and stronger intruding material capable of punching through the overlying material and resulting in the development of a spine or possibly inducing dome collapse simulation results mimic development of a megaspine upon the influx of fresh magma which leads to the re direction of magma flow creating a new shear zone and the switching of dome growth from one side to the other our model shows similar dome growth dynamics as observed at soufriere hills volcano montserrat indicating a strong correlation between extrusion rate and its subsequent effect on mechanical properties and variations in magma rheology

deformation at stromboli volcano italy revealed by rock mechanics and structural geology abstract we approach the reconstruction of the recent structural evolution of stromboli volcano italy and the analysis of the interplay between tectonics gravity and volcanic deformation by tying together structural lithostratigraphic and rock mechanics data we establish that since 100 ka bp the edifice has faulted and jointed mainly along ne striking planes faults mostly dip to the nw with normal displacement taking also into account the presence of a nw trending regional least principal stress and of tectonic earthquake hypocenters inside the cone we suggest that this fracturing can be related to the transmission of tectonic forces from the basement to the cone dyking concentrated along a main ne trending weakness zone nez across the volcano summit resembling a volcanic rift whose geometry is governed by the tectonic field in the past 13 ka stromboli experienced a reorganisation of the strain field which was linked with the development of four sector collapses affecting the nw flank alternating with growth phases the tectonic strain field interplayed with dyking and fracturing related to unbuttressing along the collapse shoulders we propose that tectonics control the geometry of dykes inside the cone and that these in turn contribute to destabilise the cone flanks

seismic imaging of the olduvai basin tanzania abstract a 5 6 km long line of refraction and reflection seismic data spanning the pliocene pleistocene fill of the olduvai basin tanzania is presented the line is oriented along a northwest southeast profile through the position of olduvai gorge coring project ogcp borehole 2a our aims are to 1 delineate the geometry of the basin floor by tracing bedrock topography of the metaquartzitic and gneissic basement 2 map synsedimentary normal faults and trace individual strata at depth and 3 provide context for the sequence observed in ogcp cores results with refraction tomography and poststack migration show that the maximum basin depth is around 405 m 25 m in the deepest portion which quadruples the thickness of the basin fill previously known from outcrops variations in seismic velocities show the positions of lower density lake claystones and higher density well cemented sedimentary sequences the bed i basalt lava is a prominent marker in the refraction seismic results bottom most sediments are dated to 2 2 ma near where borehole 2a bottoms out at the depth of 245 m however the seismic line shows that the basin fill reaches a maximum stratigraphic thickness of around 380 m deep at borehole 2a in the western basin where the subsidence was greatest this further suggests that potential hominin palaeoenvironments were available and preserved within the basin fill possibly as far back as around 4 ma applying a temporal extrapolation using the average sediment accretion rate

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

aquatic biomarkers record pleistocene environmental changes at paleolake olduvai tanzania abstract several hypotheses invoke climatic variability as a driving force for hominin evolution thus high resolution records of climate and environmental variability from anthropologically significant locations can help test these hypotheses sedimentary sequences recovered by the olduvai gorge coring project ogcp help evaluate climatic and environmental changes at olduvai gorge tanzania through the analyses of various biogeochemical proxies the stratigraphic sequence of ogcp core 2a can be correlated with horizons associated with hominins and is chronologically constrained by distinctive dated horizons such as the bed i basalt and tuff ib the lacustrine interval from 76 6 to 86 9 m depth is ideally suited for high resolution analyses of biogeochemical proxies as it is rich in organic carbon 1 toc the hydrogen isotopic composition of nc31 in this interval of ogcp core 2a records the effects of precession driven wet dry cycles on the terrestrial environment that led to alternations between woodland and grassland ecosystems comparable to those documented by previous investigations of outcrop analogues at olduvai here we examine stratigraphic variations in the abundance of biomarkers c28 steradienes c17 and c23 n alkanes c27 and c28 a norsteranes fern 8 ene and chromans derived from aquatic organisms algae cyanobacteria sponges macrophytes etc to determine the response of paleolake olduvai to climate variability in general these aquatic biomarkers reflect the productivity of the lake environment and exhibit the same precession driven wet dry cycles recorded by terrestrial biogeochemical signatures however they also provide evidence of abrupt

magma budgets and steady state activity of vulcano and stromboli we present three models for magma budget during steady state volcanic activity by which non erupted magma is emplaced as dykes or cumulates or crystallises in place using gas and thermal data we apply our models at vulcano to calculate degassing cooling and crystallisation of magma at a rate of 40 375 kg s 1 within a magma body with an upper surface at a 2 km depth at stromboli we calculate a steady magma supply of 300 1300 kg s 1 to shallow 1 km depths

dike complexes in american samoa abstract coherent dike complexes in which dikes make up more than 50 of the total rock occur in american samoa in two volcanoes on tutuila and in ofu olosega volcano in the manu a islands the dikes are remarkably narrow median width 0 24 0 36 m average width 0 46 0 56 m based on 860 measured dikes significantly narrower than dikes in the koolau volcano oahu hawaii the most closely comparable volcano for which quantitative data are available the vesicularity of these narrow dikes indicates that they were injected at shallow depths below the land surface depths of 200 900 m are inferred from the topography many dikes have prominent bands of vesicles parallel with their margins they tend to form clusters where the weakness of the vesicular rock favored center injection of one dike up the center of another examples are found of center injection repeated up to eight times local breccias often composed largely of dike fragments evidently brecciated in situ occur where irregular dikes were accommodated by deforming the host rocks we see evidence for two dike facies 1 narrow and vesicle banded early formed and shallow injected clustered dikes in the dike complexes which are cut by 2 scattered broad and almost nonvesicular deep injected solitary dikes surface eruptions were fed by solitary dikes the ene trending dikes and elongation of tutuila island contrast with the wnw trend of rift zones and island elongation elsewhere in samoa tutuila is nearly in line with the north fiji transform fracture zone suggesting that at 1 5 1 0 ma when tutuila was formed the volcanism of tutuila was guided by a temporary extension of this fault zone into the pacific plate fault movement offset the samoan hotspot trace by 45 km the en echelon arrangement of dike exposures on tutuila is consistent with left lateral transcurrent motion above this concealed fracture zone

magma supply and storage at kilauea volcano hawaii 1956 1983 abstract shallow crustal magma reservoirs beneath the summit of kilauea volcano and within its rift zones are linked in such a way that the magma supply to each can be estimated from the rate of ground deformation at the volcano s summit our model builds on the well documented pattern of summit inflation as magma accumulates in a shallow summit reservoir followed by deflation as magma is discharged to the surface or into the rift zones magma supply to the summit reservoir is thus proportional to summit uplift and supply to the rift zones is proportional to summit subsidence the average proportionality constant is 0 33 10 6 m 3 rad this model yields minimum supply estimates because it does not account for magma which escapes detection by moving passively through the summit reservoir or directly into the rift zones calculations suggest that magma was supplied to kilauea during july 1956 april 1983 at a minimum average rate of 7 2 10 6 m3 month roughly 35 of the net supply was extruded the rest remains stored within the volcano s east rift zone 55 and southwest rift zone 10 periods of relatively rapid supply were associated with the large kapoho eruption in 1960 and the sustained mauna ulu eruptions in 1969 1971 and 1972 1974 bursts of harmonic tremor from the mantle beneath kilauea were also unusually energetic during 1968 1975 suggesting a close link between kilauea s deep magma supply region and shallow storage reservoirs it remains unclear whether pulses in magma supply from depth give rise to corresponding increases in shallow supply or if instead unloading of a delicately balanced magma transport system during large eruptions or intrusions triggers more rapid ascent from a relatively constant mantle source

thermomechanics of shallow magma chamber pressurization implications for the assessment of ground deformation data at active volcanoes abstract in this study we utilize thermomechanical models to investigate how magma chambers overpressurize as the result of either magmatic recharge or volatile exsolution by implementing an adaptive reservoir boundary condition we are able to track how overpressure dissipates as the magma chamber expands to accommodate internal volume changes we find that the size of the reservoir greatly impacts the resultant magma chamber overpressure in particular overpressure estimates for small to moderate sized reservoirs 1 10 km 3 are up to 70 lower than previous analytical predictions we apply our models to santorini volcano in greece where recent seismic activity and ground deformation observations suggested the potential for eruption the incorporation of an adaptive boundary condition reproduces mogi flux estimates and suggests that the magma reservoir present at santorini may be quite large furthermore model results suggest that if the magma chamber is 100 km 3 overpressures generated due to the high magma flux may not exceed the strength of the host rock thus requiring an additional triggering mechanism for eruption although the adaptive boundary condition approach does not calculate the internal evolution of the magma reservoir it represents a fundamental step forward from elastic mogi models and fixed boundary solutions on which future investigations of the evolution of the magma can be built

an archean megacaldera complex the blake river group abitibi greenstone belt abstract the archean blake river group of the southern abitibi greenstone belt is defined as a subaqueous megacaldera compelling evidence include 1 a radial and concentric organization of synvolcanic mafic to intermediate dykes 2 an overall dome geometry defined by the volcanic strata 3 a peripheral distribution of subaqueous volcaniclastic units 4 a zonal distribution of carbonate alteration and 5 a distinct annular synvolcanic inner and outer ring fault pattern three caldera forming events have been identified 1 the early misema caldera 2 the new senator caldera and 3 the well known noranda caldera the multi vent brg mafic volcanic complex developed on a monotonous sequence of tholeiitic basalts forming a submarine plain and experienced a first major collapse that created the giant misema caldera 80 km in diameter an endogenic dyke swarm intruded the synvolcanic fractures and an underlying magma chamber developed major volcaniclastic units were generated by local volcanic centers and summit calderas formed along the outer and inner ring faults this fault system was used as a conduit for co2 rich hydrothermal activity renewed volcanic activity was associated with a resurgent central dome inside the misema caldera this second collapse event created the 35 km by 14 km nw trending new senator caldera this caldera was produced by multi step sagging after the underlying magmatic chamber migrated to the se and formed the flavrian powel plutons the final collapse resulted in the formation of the noranda caldera that generated a well developed 070 trending fracture pattern associated with several vms deposits the multiple caldera setting provides an effective model to explain the presence of vms mineralization along the synvolcanic fractures associated with the three collapse episodes however the importance of the new senator caldera fracture pattern is emphasized because of its role in the positioning of the giant horne mine deposit

sub milankovitch paleoclimatic and paleoenvironmental variability in east africa recorded by pleistocene lacustrine sediments from olduvai gorge tanzania abstract over the past century numerous discoveries throughout east africa have advanced our understanding of hominin evolution and provided substantive evidence that climatic and environmental variability played a critical role in evolutionary developments stratigraphic records with high temporal resolution aid in testing evolutionary hypotheses that invoke changes in climate and environment at various timescales as drivers of hominin evolution olduvai gorge tanzania has a rich history of hominin fossil discoveries from its 2 0 ma sedimentary sequence which includes multiple deep lake intervals in 2014 the olduvai gorge coring project ogcp recovered a sequence of sediment cores that provide an extensive record of the pleistocene sedimentary history including paleolacustrine systems in this area with 94 core recovery these cores are ideally suited for high resolution analyses of organic geochemical proxies and provide an exceptional opportunity to build on previously published outcrop based paleoenvironmental data from olduvai the ogcp core 2a section between 76 6 and 86 9 m depth is considered as a time stratigraphic equivalent of strata that host the hominin fossils oh24 and oh56 at the olduvai dk archaeological site this depositional interval age bracketed between 1 88 ma bed i basalt and 1 85 ma tuff ib was sampled for organic geochemical analyses the carbon isotopic composition of organic matter 13ctoc from this section varies between values representative of more forested and open grassland ecosystems over 21 kyr this observation is consistent with the milankovitch precession cycle driving the availability of water which was previously observed in lower resolution studies of outcrop samples complementary organic geochemical proxies provide further evidence of these shifts in environmental conditions and record sub milankovitch scale changes superimposed on the precession cycle this suggests the occurrence of short term fluctuations in the environments inhabited by hominins which opens new lines of investigation on how environmental scenarios may affect particular evolutionary mechanisms that drove various evolutionary responses

a new mode of inner volcano growth the flower intrusive structure abstract a multiple sill laccolith nested within a centrally dipping sheet swarm is a recently discovered sub volcanic structure resembling a flower in section view which we have found in some eroded volcanoes in iceland suggesting that this structure can have some general relevance in deforming rock successions in volcanic areas after a brief summary of the main characteristics of these examples with details on stardalur volcano we present an evolutionary model to explain the whole structure both sills and sheets are fed by multiple pulses of magma following radial paths from a shallow chamber the earlier vertical set of the radial sheets above the middle of the chamber are abruptly deflected into sills by the overlying lava succession a proto laccolith starts developing composed of different vertically stacked sills originating from the central vertical feeder zone each sill contributes to the deformation of the overburden room for intrusions is provided by the higher strain that can be attained by the widespread hyaloclastites and breccias which have a softer behaviour with a lower young s modulus than the stiffer higher young s modulus lava flows combined with the doming of the overlying lavas later dikes get deflected outwards along the contact with the proto laccolith and acquire a convex upward shape in cross section

identifying rift zones on volcanoes an example from la reunion island indian ocean we describe a methodology for identifying complex rift zones on recent or active volcanoes where structures hidden by recent deposits and logistical conditions might prevent carrying out detailed fieldwork la reunion island was chosen as a test site we used georeferenced topographic maps aerial photos and digital terrain models to perform a statistical analysis of several morphometric parameters of pyroclastic cones this provides a great deal of geometric information that can help in distinguishing the localisation and orientation of buried magma feeding fractures which constitute the surface expression of rift zones it also allowed the construction of a complete gis database of the pyroclastic cones la reunion is a perfect example where past and active volcanic rift zones are mostly expressed by clusters of monogenic centres the data has been validated in the field and compared and integrated with the distribution and geometry of dyke swarms results show the presence of several main and secondary rift segments of different ages locations and orientations whose origin is discussed considering regional tectonics local geomorphology and volcano deformation

effect of host rock rheology on dyke shape thickness and magma overpressure the size and thickness of dykes is of fundamental importance for volcano dynamics because dykes are the primary path for magma transport and because large numbers of dykes often comprise a major p

long term mass transfer at piton de la fournaise volcano evidenced by strain distribution derived from gnss network basaltic volcanoes are among the largest volcanic edifices on the earth these huge volcanoes exhibit rift zones and mobile flanks revealing specific stress field conditions in this paper we present new deformation data issued from the global navigation satellite systems gnss network installed on piton de la fournaise density of the gnss stations allowed us to reach a sufficient resolution to perform a spatially significant analysis of strain at the scale of the active part of the volcano since 2007 summit inflation during preeruptive eruptive sequences summit extension cone flanks contraction alternates with summit deflation during posteruptive rest periods summit contraction cone flanks extension and generates a pulsation of the volcano this volcano pulsation increases rock fracturing and damage decreases the rock stiffness and increases the medium permeability the deformation regime of the mobile eastern flank evidences mass transfer in depth from the summit to the east during the long term summit deflation recorded between 2011 and 2014 the upper eastern flank extended steadily eastward whereas the lower eastern flank contracted simultaneous extension and eastward displacement of the upper eastern flank and eastward contraction of the middle and lower eastern flank contributes to build the grandes pentes relief steeping the topographic slope we relate the eastern flank topographic slope spatial variations to rock or basal friction angle changes the lower flank contraction process is an evidence of its progressive loading by the upper eastern flank which brings this flank closer to an eventual instability

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

great lava flow fields on venus abstract we have analyzed the morphology dimensions radar backscatter topography sources and tectonic settings of lava flow fields and assemblages of fields on venus with total areas greater than 50 000 km 2 and we term these great flow fields great flow fields are typically several hundred thousand square kilometers in extent with lengths of several hundred kilometers and estimated volumes of 10 3 to 10 4 km 3 we identify five morphological types of flow field and draw a basic distinction between sheet like and digitate morphologies the sheet flow fields represent single massive eruptions from laterally extensive stretches of fissure onto near horizontal terrain while the digitate flows were formed by repeated eruptions we divide digitate fields into aprons fans and subparallel types and identify fields transitional between sheet and digitate morphologies an analysis of the backscatter coefficient of flow units within several of the great flow fields compared to airsar data of terrestrial lava flows suggests that most units have the surface roughness of smooth pavements to pahoehoe however many channel fed flow units with higher backscatter in the digitate fields have a surface roughness comparable to terrestrial aa the great flow fields may have been erupted with effusion rates of between 10 5 and 10 6 m 3 sec 1 most of the great flows were erupted from sources within zones of lithospheric extension and thinning although the set of great flows are younger than the background plains large regions of plains may have been resurfaced by sheet type great flow fields which are no longer visible due to homogenization of surface properties in terms of their dimensions estimated eruption rates possible surface textures and tectonic settings we consider the great flow fields to be venusian analogues of terrestrial flood basalt provinces

relationships between volcano gravitational spreading and magma intrusion volcano spreading with its characteristic sector grabens is caused by outward flow of weak substrata due to gravitational loading this process is now known to affect many present day edifices a volcano intrusive complex can form an important component of an edifice and may induce deformation while it develops such intrusions are clearly observed in ancient eroded volcanoes like the scottish palaeocene centres or in geophysical studies such as in la reunion or inferred from large calderas such as in hawaii the canaries or galapagos volcanoes volcano gravitational spreading and intrusive complex emplacement may act simultaneously within an edifice we explore the coupling and interactions between these two processes we use scaled analogue models where an intrusive complex made of golden syrup is emplaced within a granular model volcano based on a substratum of a ductile silicone layer overlain by a brittle granular layer we model specifically the large intrusive complex growth and do not model small scale and short lived events such as dyke intrusion that develop above the intrusive complex the models show that the intrusive complex develops in continual competition between upward bulging and lateral gravity spreading the brittle substratum strongly controls the deformation style the intrusion shape and also controls the balance between intrusive complex spreading and ductile layer related gravitational spreading in the models intrusive complex emplacement and spreading produce similar structures to those formed during volcano gravitational spreading alone i e grabens folds en echelon fractures therefore simple analysis of fault geometry and fault kinetic indicators is not sufficient to distinguish gravitational from intrusive complex spreading except when the intrusive complex is eccentric from the volcano centre however the displacement fields obtained for 1 a solely gravitational spreading volcano and for 2 a gravitational spreading volcano with a growing and spreading intrusive complex are very different consequently deformation fields like those obtained from geodetic monitoring can give a strong indication of the presence of a spreading intrusive complex we compare the models with field observations and geophysical evidence on active volcanoes such as la reunion island indian ocean ometepe island nicaragua and eroded volcanic remnants such as ardnamurchan scotland and suggest that a combination between gravitational and intrusive complex spreading has been active

tuff fingerprinting and correlations between ogcp cores and outcrops for pre bed i and beds i ii at olduvai gorge tanzania abstract sediment cores retrieved from the pleistocene olduvai basin by the olduvai gorge coring project ogcp provide a high resolution record of tuffs and other volcaniclastic deposits together with a lacustrine sedimentary record full of paleoenvironmental indicators correlating tuffs between the cores and outcrops at olduvai where these tuffs are identified at paleoanthropologically important sites is critical for applying the new paleoenvironmental data to the conditions under which hominins lived tuffs and other volcaniclastic deposits from three cores were analyzed for mineral assemblages and glass and mineral major element compositions feldspar augite hornblende titanomagnetite and glass where possible to compare to published geochemical fingerprint data based on marker tuffs from outcrop equivalents at olduvai gorge in combination with stratigraphic position these mineralogical and geochemical data were used to correlate between the cores and outcrops providing direct temporal tie lines between the cores and sites of paleoanthropological interest direct correlations are most certain for olduvai bed i where all major tuff markers from outcrop are identified for one or more of the three core sites and for the upper part of the underlying ngorongoro formation which includes the coarse feldspar crystal tuff cfct and naabi ignimbrites exposed in the oldest pleistocene exposures of the western gorge also characterized were the mineral and glass compositions of tuffs and ignimbrites pre dating the oldest exposed outcrop units extending our record of explosive events from the ngorongoro volcano while no specific correlations can be confirmed between individual bed ii tuffs in the cores and in outcrops correlations are possible between the cores themselves using newly identified tuff compositions and some potential correlations non unique based on individual mineral phases between core and outcrop can be used in conjunction with other stratigraphic tools to help constrain the intervals in question

stratovolcano growth by co eruptive intrusion the 2008 eruption of tungurahua ecuador 1 volcanic edifices are constructed by a combination of erupted material and internal growth we use the l band satellite alos to produce insar measurements at tungurahua volcano ecuador we find a maximum of 17 5 cm of uplift on the upper western flank between december 2007 and march 2008 coincident with an eruption in february 2008 the deformation can be modeled using an ellipsoidal or sill like source within the edifice the models require an elongated aspect ratio with a length of 4 6 km the intruded volume of 1 2 106 m3 is roughly equivalent to the bulk erupted volume of 1 5 106 m3 and together the values are roughly equal to the long term magma flux the question remains whether this uplift is permanent thus contributing to the internal growth of the edifice or temporary in which case the magma will be released in a future eruption

geochemical fingerprints for olduvai gorge bed ii tuffs and implications for the oldowan acheulean transition bed ii is a critical part of early pleistocene olduvai gorge tanzania its deposits include transitions from humid to more arid conditions with associated faunal changes from homo habilis to erectus and from oldowan to acheulean technology bed ii 1 8 1 2 ma is stratigraphically and environmentally complex with facies changes faulting and unconformities making site to site correlation over the 20 km of exposure difficult bed ii tuffs are thinner less evenly preserved and more reworked than those of bed i five marker tuffs tuffs iia iid bird print tuff bpt plus local tephra were collected from multiple sites and characterized using stratigraphic position mineral assemblage and electron probe microanalysis of phenocryst feldspar hornblende augite titanomagnetite and glass where available composition lowermost bed ii tuffs are dominantly nephelinitic middle bed ii tuffs bpt tuff iic have basaltic components and upper bed ii tuff iid is trachytic the bpt and tuff iid are identified widely using phenocryst compositions high ca plagioclase and high ti hornblende respectively though iid was originally hay 1976 misidentified as tuff iic at loc 91 shk annexe in the side gorge this work helps establish a high resolution basin wide paleolandscape context for the oldowan acheulean transition and helps link hominin faunal and archaeological records

volcanism and tectonics on venus we review recent developments in the study of volcanism and tectonics on venus venus s crust is basaltic dry and probably about 30 km thick the mantle convects giving rise to plumes and has a similar composition and mean temperature 1300 c but a higher viscosity 10 20 pa s than that of the earth inferred melt generation rates constrain the lithospheric thickness to between 80 and 200 km the elastic thickness of the lithosphere is about 30 km on average the present day lack of plate tectonics may be due to strong faults and the high viscosity of the mantle most of the differences between earth and venus processes can be explained by the absence of water venus underwent a global resurfacing event 300 600 ma ago the cause and nature of which remains uncertain the present day surface heat flux on venus is about half the likely radiogenic heat generation rate which suggests that venus has been heating up since the resurfacing event

biogeochemical evidence for environmental changes of pleistocene lake olduvai during the transitional sequence of ogcp core 2a that encompasses tuff ib 1 848 ma abstract olduvai gorge is renowned for discoveries of hominin fossils and tools in a well resolved sedimentary context representing one of the foremost sites in east africa that has afforded critical evidence of hominin evolution in 2014 the olduvai gorge coring project ogcp recovered the first deep sediment cores from this location these cores provide the first opportunity to examine an extensive stratigraphic record from one location and independent of the weathering and related degradation typical of outcrop samples samples from these cores have been correlated to bed i tuffs and basalt marker beds that are well characterized from outcrops within the gorge this study focuses on the biogeochemical investigation of a 10 m segment from ogcp sediment core 2a where age constraints indicate deposition occurred over 50 000 years from 1 8055 0 003 ma to 1 8551 0 013 ma the segment includes tuff ib and if which were deposited at a time that prior research indicates represents marked changes in the paleoenvironment at olduvai with indications that paleolake olduvai may have been completely desiccated by the time tuff if was deposited this environmental change is recorded by the composition of sedimentary organic matter om in terms of bulk organic geochemical properties observed as a decrease in toc and 13corg in addition temporal variations in source specific biomarkers n alkanes phytane steradienes a norsteranes and alkenones reflect a shift from a deeper lake setting with a prevalence of aquatic sourced om combined with input from terrestrial plant waxes to a more shallow perennial lake where evidence of burrows and erosional surfaces is associated with microbially degraded om and intermittent aquatic biomarkers thus variations in biogeochemical proxies complement the sedimentological evidence confirming increased periods of prolonged desiccation of paleolake olduvai beginning around the time of tuff ib deposition

magma chambers modeled as cavities explain the formation of rift zone central volcanoes and their eruption and intrusion statistics the volcanic rift zone in iceland is characterized by elongate volcanic systems consisting of tension fractures normal faults and volcanic fissures where most of the volcanotectonic activity takes place most volcanic systems develop central volcanoes the formation of which is still poorly understood there is great difference in the eruption and intrusion statistics of the volcanic systems inside and outside the central volcanoes in the central volcanoes eruptions are frequent typically one every several hundred years of small volume normally less than 0 1 km3 and fed mainly by thin average thickness of 0 5 m inclined sheets outside central volcanoes the eruptions are rare typically one every several thousand years but of large volume typically more than 1 km3 and fed mainly by thick average thickness of 4 5 m subvertical dikes using the results of a boundary element study of magma chambers modeled as a cavities or holes in a semi infinite plate these empirical relations as well as the formation of specific central volcanoes in the volcanic systems can be explained once a cavity like magma chamber has formed its existence in the rift zone concentrates tensile stress this stress concentration causes the segment containing the magma chamber to rupture much more frequently than the other parts of the volcanic system which partly explains the formation of central volcanoes and their high eruption frequencies this concentration also gives rise to a local stress field that encourages injection of small volume sheets in all directions and at various dips from their source magma chamber the magma chamber acts as a trap for upward propagating dikes from mantle reservoirs and channels magma through inclined sheets toward a limited area at the surface where the central volcano gradually forms the inverse relationship between eruption frequency and eruption volume when the central volcano is compared with other parts of the volcanic system is also partly due to the trap like nature of the magma chamber the chamber is normally much smaller than its source mantle reservoir so that a single magma flow through a dike from the reservoir lasting perhaps many years may trigger tens of magma flows through sheets from the chamber many of which would reach the surface in the central volcano

strike slip fault tectonics and the emplacement of sheet laccolith systems the thverfell case study sw iceland fault geometry and kinematics indicate that two different tectonic regimes affected the late pliocene volcanic succession around the thverfell magmatic complex esja peninsula sw iceland the older phase is characterized by sets of left lateral strike slip e w to ese striking faults and right lateral strike slip n s to ne striking faults the younger phase produced normal dip slip nne striking faults stress tensor calculation for the older regime provides a horizontal ne to e w trending greatest principal stress 1 and a horizontal nw to n s trending least principal stress 3 followed by a stress regime change with a vertical 1 and a wnw trending 3 structural stratigraphic analyses of the eroded thverfell magmatic complex indicate three main systems namely i a centrally dipping sheet swarm e w elongated in plan view ii a sill composed laccolith fed by e w striking dikes and iii nne to ne striking dikes offsetting the previous intrusions these data allow unravelling of a multiphase history of magma tectonics interaction first an excess magma pressure from an underlying magma chamber induced the emplacement of the centrally dipping sheets this was accompanied by magma upwelling along dikes that bent to propagate as sills under the influence of stress and rheological barriers induced by the lava overburden finally regional dikes linked to the wnw trending rift extension were emplaced

shape rheology and emplacement times of small martian shield volcanoes abstract this paper focuses on the shape of 31 shield volcanoes of small to intermediate size a few tens of kilometers in diameter in 5 regions of mars tempe terra syria planum pavonis mons arsia mons and central elysium planitia a model for the shape of these shield volcanoes is applied based upon the concept of porous flow of an unconfined aquifer proposed by lacey et al lacey a ockendon j r turcotte d l 1981 on the geometrical form of volcanoes earth planet sci lett 54 1 139 143 and developed by turcotte and schubert turcotte d schubert g 2002 geodynamics cambridge university press second edition the agreement between the topographic profiles and the theoretical shape suggests that the model is appropriate and can be used to derive the product of flow rate and viscosity q estimates of q are found to be homogeneous within a given volcanic region but show large differences from one region to another values generally decreasing with age from oldest to youngest intrinsically the method cannot separate the flow rate from the viscosity but independent observations on individual lava flows in syria planum and central elysium planitia suggest that this evolution may be explained by decreasing magma viscosity with age independent constraints on the viscosity are used to interpret q variations between volcanic regions except for tempe terra and within error bars relative effusion rates are essentially independent of volcano volumes suggesting that the size of a shield volcano is principally controlled by the duration of volcanic activity the specificity of central elysium planitia is emphasized with highest flow rates associated with lowest lava viscosity however the lower viscosities at central elysium planitia cannot totally explain low q values considering the equation of magma flow through a dike from a magma chamber at depth to the shield we suggest that lower volatile contents and or deeper magma chambers may contribute to the decrease of q values with time finally the emplacement times of the 31 shield volcanoes were calculated using a calibration based upon terrestrial volcanoes mauna loa eldborgir and skjalbrei ur emplacement times of martian volcanoes calculated in this way range from a few hundred thousand years tempe terra to a few tens of thousands of years central elysium planitia despite the simplicity of the model the quality of the fits and the internal consistency of the results are reasonable encouraging development of more advanced models linking shield volcano shape to rheological properties of lavas and their characteristic time scales of emplacement

three hawaiian calderas an origin through loading by shallow intrusions the calderas of kilauea and mauna loa are highly dynamic structures and in the 200 year historic period have varied in volume by a factor of 2 and gained or lost 1 km3 per century the deeply eroded caldera of the extinct koolau volcano in oahu is wider than active hawaiian calderas and its lavas have a strong centripetal dip and funnel structure not evident at kilauea or mauna loa the differences can be attributed to the different erosion depths and the time integrated subsidence profile of kilauea is also a stepped funnel having its apex at halemaumau koolau caldera is the focus of an extraordinarily intense dike complex and an intriguing feature is the great diminution in dike concentration into the caldera it is thought that dike injection in any part of the complex generally continued until it reached 50 to 65 in outer parts of the caldera the complex was maintained at or rebuilt to this value despite subsidence in the center of the caldera where the positive bouguer anomaly is centered subsidence evidently greatly outpaced the capacity of dike injections to rebuild the complex assuming the same dike injection rate as kilauea and mauna loa yields a volumetric subsidence rate in koolau caldera exceeding 1 km3 per century hawaiian calderas are much more dynamic than calderas of silicic volcanoes shaped by frequent small events instead of a few great ones the temporal and volumetric correspondence of historical subsidence events with eruptions is poor and this and the high subsidence rates argue for a caldera forming mechanism that consumes the subsided rocks it is suggested that subsidence is caused by the great localized excess load of intrusive rocks carrying the center of the volcano into the thermally weakened lithosphere above the hawaiian hot spot it is envisaged that under steady state conditions the magma chamber rises as the injection of intrusions causes the level of neutral buoyancy at which the chamber is located to ascend and keeps pace with subsidence

hominin evolution in settings of strong environmental variability abstract investigations into how climate change shaped human evolution have begun to focus on environmental dynamics i e the nature and tempo of climate and landscape variability an approach that de emphasizes static reconstructions of early hominin habitats the interaction among insolation cycles is especially apparent in the paleoenvironmental records of the east african rift system where the longest records of human evolution are preserved however environmental indicators such as deep sea oxygen isotopes terrestrial dust flux paleosol carbon isotopes and lake sediments do not point consistently to any simple trend or climate driver of evolutionary change comparison of environmental indicators cautions against an exclusive focus on any given end member of environmental fluctuation driest or wettest warmest or coolest and argues for the impact of the entire range of variability in shaping evolutionary change a model of alternating high and low climate variability for tropical africa further implies that specific environmental indicators reflect different aspects of east african environmental dynamics the model may thus help reconcile some of the conflicting interpretations about the environmental drivers of hominin evolution first and last appearances of hominin lineages benchmark biogeographic events and the emergence of key adaptations and capacities to alter the surroundings are consistently concentrated in the predicted longest intervals of high climate variability the view that emerges is that important changes in stone technology sociality and other aspects of hominin behavior can now be understood as adaptive responses to heightened habitat instability

palaeosalinity and palaeoclimatic geochemical proxies elements ti mg al vary with milankovitch cyclicity 1 3 to 2 0 ma ogcp cores palaeolake olduvai tanzania abstract the analysis of geochemical palaeoclimate and palaeosalinity proxy elements ti mg and al derived from x ray fluorescence xrf scans of olduvai beds i and ii from olduvai gorge coring project ogcp borehole cores 2a and 3a provides a record of cyclic variation between 1 3 ma and 2 0 ma the boreholes were drilled into the depocentre of the olduvai basin between fifth and flk faults where palaeolake olduvai was most persistent and deepest during most of bed i the lake was particularly deep and probably meromictic preserving high toc contents and commonly preserving fine lamination due to lack of bioturbation accretion rates were also high during bed i when rates of basinal subsidence were maximal due to crustal stretching associated with basaltic volcanism towards the end of bimodal ngorongoro volcanism basaltic magma effusive activity is manifested as tuffs scoriaceous layers and the bed i basalt complex lava flows a magnesium anomaly is recorded in the claystone geochemistry at this time and deposition of dolomite and limestone beds are restricted to this syn volcanic phase of basin history during bed i deposition accretion rates 0 23 mm yr were high enough to permit recognition of cycles with an average periodicity of 22 3 kyr corresponding to the earth s precession only the high values during the mg anomaly are adequate for the application of the palaeosalinity proxy element ratio mg al but ti counts provide a cyclic record in both bed i and bed ii during bed ii deposition the accretion rate was much slower 0 058 mm yr and cyclicity averaged 40 4 kyr corresponding to earth s orbital obliquity the bed ii interval corresponds to mis stages 40 to 64 coincidence of precessional and obliquity minima at 1 8 ma explains the superdrought that affected the basin at the time of emplacement of tuff if when the lake was dried out the aridity of the sequence containing tuff ia is also associated with a precessional minimum the cyclic record suggests that three bed i basalt flows were extruded at 1 94 ma during a time span lasting between 6 kyr and 15 kyr

magma emplacement into the lemptegy scoria cone chaine des puys france explored with structural anisotropy of magnetic susceptibility and paleomagnetic data the lemptegy volcano is a small monogenetic scoria cone located in the chaine des puys auvergne france which erupted about 32 000 years ago a first edifice lemptegy 1 formed during a trachybasalt eruption as a group of satellite vents of the puy de gouttes scoria cone a second trachyandesitic edifice lemptegy 2 formed soon after and completely covered lemptegy 1 with an 80 m high breached cone since 1946 the lemptegy volcano has been quarried for scoria and today offers unprecedented three dimensional exposure of the subvolcanic plumbing system to map the internal flow architecture of the plumbing system and to study the subvolcanic deformation of lemptegy 2 structural mapping petrographic observations anisotropy of magnetic susceptibility ams rock magnetic and paleomagnetic data were collected field structural mapping and thin section study of tension gashes riedel shears striations as well as ductile shear zones and bubbles allow the direction and sense of the magma flow to be determined twenty ams sites were established in ten dikes one to four sites in each dike with 504 specimens analyzed and 479 specimens used to infer magma flow patterns structural data the maximum susceptibility axis k 1 and the imbrication of the magnetic foliation k 1 k 2 planes indicate both upward and downward sense of flow as well as flow toward and away from the central vent rock magnetic experiments reveal that a cubic fe ti oxide phase likely low ti titanomagnetite is the principal magnetic phase carrying both the remanence and anisotropy paleomagnetic data from some sites yield statistically distinct at the 95 confidence level remanence directions while at other sites the data are indistinguishable at the 95 confidence level the paleomagnetic results observed steeply tilted scoria layers internal unconformities and faults show that as each dike was emplaced it displaced earlier dikes evidencing subvolcanic deformation the lemptegy 2 volcano shares similarities in terms of inferred eruption style and structures with other scoria cones such as cerro negro nicaragua and thus provides an excellent field laboratory to investigate active scoria cones world wide

construction and destruction of mont pelee volcano volumes and rates constrained from a geomorphological model of evolution this study presents long term volumes and construction rates for the mont conil mont pelee volcano and rate estimates at which volcanic activity creates relief an algorithm shapevolc is used to numerically model topographic surfaces volcano morphology is analyzed using current digital elevation model in combination with mapped geology to produce 10 paleotopographies at the end of four constructional stages and three destructional events volumes of each constructional stage were estimated at about 35 2 km3 26 2 km3 8 3 km3 and 2 5 km3 for a total cumulative erupted volume of 72 2 km3 we estimate that mont pelee accounted for about 10 of the lesser antilles arc production in the last 100 kyr the volcano has been built at an average rate of 0 13 km3 kyr during the last 550 kyr during that time construction rates varied by a factor of 15 from 0 04 km3 kyr in early stages up to 0 52 km3 kyr after the second flank collapse volumes displaced by each flank collapse were estimated at 14 7 km3 8 8 km3 and 3 5 km3 thus about 37 of the total constructed volume integrated over the volcano s lifetime the rate at which flank collapses removed material off the island is 0 15 km3 kyr in contrast long term erosion rates outside collapsed areas are estimated at about 0 05 0 7 km3 kyr or 11 km3 of material removed this latter rate is not negligible which strengthens the importance of taking into account recurrent small erosional events on the geomorphological evolution of a volcanic island in a tropical context

koolau dike complex oahu intensity and origin of a sheeted dike complex high in a hawaiian volcanic edifice the dike complex in the eroded koolau tholeiitic shield volcano is unique among described dike swarms for its coherence high dike injection intensity relatively uniform and non gaussian 50 to 65 intensity level and lack of dike divergence parts are like a sheeted dike complex though high in an intraplate volcanic edifice a model is proposed in which highly mobile tholeiitic magma having considerable freedom to move through the strongly disjointed volcano seeks zones in which it is gravitationally most stable these zones are marginal parts of the dike complex separating lavas having a lower than magmatic bulk density from a 50 dike complex having a higher than magmatic bulk density this model also explains the strong propensity for the magma of kilauea volcano to remain underground in a gravitationally stable zone magma erupts only when it vesiculates sufficiently often when it has degassed some of it promptly plunges back toward the stable zone again

infrastructure and mechanics of volcanic systems in iceland the divergent plate boundary in iceland consists of more than two dozen systems where most of the volcano tectonic activity takes place at the surface the volcanic systems are characterised by 5 20 km wide and 40 100 km long swarms of tension fractures 102 m long normal faults 103 m long and volcanic fissures the holocene fissure swarms are confined to less than 10 000 year old basaltic lava flows mostly pahoehoe occurring near the centre of the active rift zone in any particular swarm the number of tension fractures exceeds that of normal faults all tension fractures and normal faults are vertical at the surface indicating that the surface parts were generated by an absolute tension in addition to a fissure swarm many volcanic systems have a central volcano some of which have developed collapse calderas in the late tertiary and pleistocene lava pile of iceland extinct volcanic systems are represented by local sheet swarms and regional dyke swarms the sheet swarms are normally circular or slightly elliptical several kilometres in radius and are confined to the extinct central volcanoes many swarms are associated with large plutons exposed at 1 2 km depth beneath the initial top of the rift zone and presumably the uppermost parts of extinct crustal magma chambers and in short traverses up to 90 of the rock may consist of sheets the sheets have a very variable strike dip on average 45 65 mostly towards the centre of the associated volcano and have an average thickness of about 1 m the regional dykes occur outside central volcanoes in swarms that are commonly 50 km long and 5 10 km wide in several kilometre long traverses commonly 1 5 of the rock consist of dykes but occasionally as much as 15 28 most regional dykes are subparallel and subvertical the average dyke thickness in the pleistocene swarms is less than 2 m but 4 6 m in the tertiary swarms while active the volcanic systems in the rift zone are supplied with magma from reservoirs located at the depth of 8 12 km at the boundary between the crust and upper mantle the reservoirs are partially molten with totally molten top regions and of cross sectional areas similar to those of the volcanic systems that they feed some active volcanic systems especially those that develop central volcanoes high temperature areas and calderas have in addition to the deep seated magma reservoirs shallow crustal magma chambers located at 1 3 km depth which in turn are fed by the deeper magma reservoirs it is proposed that the regional dyke swarms are supplied with magma largely from the deep seated reservoirs whereas the local sheet swarms are mainly fed from the associated crustal magma chambers because the volume of a crustal chamber is less than that of its deeper source reservoir a single magma flow dyke intrusion from the reservoir may trigger tens of magma flows sheet intrusions from the chamber which is one explanation for the enormous number of sheets associated with many central volcanoes the sheets follow the stress trajectories of the local stress fields around the source chambers whereas the regional stress field associated with the divergent plate movements controls the emplacement of the regional dykes it is suggested that many dykes develop as self affine some as self similar structures when applied to the krafla volcanic system in northern iceland model calculations suggest that a magma flow from the krafla reservoir with regional dyke formation should occur on average once every several hundred years and that tens of sheets might be injected from the shallow krafla chamber and triggered by a single magma flow from the krafla reservoir these results are in broad agreement with the available data

seafloor expression of sediment extrusion and intrusion at the el arraiche mud volcano field gulf of cadiz 1 the el arraiche mud volcano field consists of eight mud volcanoes up to 255 m high and 5 4 km wide located in the moroccan margin of the gulf of cadiz at water depths between 700 and 200 m available data include detailed swath bathymetry over the entire area dense grids of high resolution seismic data very high resolution deep tow subbottom profiles side scan sonar mosaics over the major structures selected underwater video lines and sediment samples the main morphological aspects of the mud volcanoes are from the margin toward the center a subsidence rim or moat the mud volcano slope in some cases a deep crater and a recent central mud dome at the top the slope is characterized by radial outward sediment flow deposits or by a concentric pattern of terraces and steps the sediment flow deposits can be divided into elongate outflows that accumulate at the base of the slope and short bulky outflow deposits that freeze on the steep slope the crater hosts extruded sediment ranging from fluidized sand to mud breccia with centimeter to meter sized rock clasts issued from several vents within the crater the concentric slope terraces and the central mud dome are interpreted to result from several phases of uplift caused by sediment intrusion or shallow diapirism the mud volcano growth is thus interpreted to result from a combination of extrusive and intrusive processes

post rifting stress relaxation at the divergent plate boundary in northeast iceland interaction of the elastic lithosphere with the underlying anelastic asthenosphere causes strain to propagate along the earth s surface in a diffusion like manner following tectonism at plate boundaries this process transfers stress between adjacent tectonic segments and influences the temporal tectonic pattern along a plate boundary observations of such strain transients have been rare and have hitherto been confined to strike slip and underthrusting plate boundaries1 here we report the observation of a strain transient at the divergent spreading plate boundary in iceland a global positioning system survey undertaken a decade after an episode of dyke intrusion accompanying several metres of crustal spreading reveals a spatially varying strain field with the expected diffusion pulse shape and an amplitude three times greater than the 5 7 cm that would be expected from the average spreading rate2 a simple one dimensional model with a thin elastic layer overlying a viscous layer fits the data well and yields a stress diffusivity of 1 1 a 0 3 m2s 1 combined with struc tural information from magnetotelluric measurements this implies a viscosity of 0 3 2 1019 pa s a value comparable to that derived for iceland from post glacial rebound23 but low compared with estimates for mantle viscosity obtained elsewhere3

geometry and emplacement of the late cretaceous mafic dyke swarms on the islands in zhejiang province southeast china insights from high resolution satellite images abstract the geology of zhejiang coastal area in southeast china is characterized by numerous late mesozoic intrusive rocks and widespread mafic dyke swarms which indicate continental lithospheric extensional events during the jurassic and cretaceous this work is focused on using multisource high resolution remote sensing images worldview2 geoeye1 and quickbird2 to identify the geometry morphology and location of previously undocumented and poorly understood dyke swarms exposed on the islands in zhejiang province the geometry of each dyke is described by its strike length and thickness the spatial distribution of the dyke density and crustal dilation are obtained based on the statistics of 774 extracted mafic dykes field surveys are performed in some islands in order to analyze the detailed geometric features and assess the interpretative accuracy the spectral measurement and analysis of mafic dykes are performed as well for remote sensing imagery processing and lithological interpretation the results show that the frequency distributions of the length and thickness of dykes follow power law curves the maximum and mean dyke thicknesses are 11 2 m and 1 43 m respectively the crustal dilation of the islands ranges from 0 09 to 7 4 from the north to the south islands the dilation decreases gradually the dyke frequency and density have the same distribution as the dilation according to 40 ar 39 ar age zhongshanjie archipelago and u pb zircon age sijiao island of the mafic dyke the dyke swarms on the eastern zhejiang islands emplaced at around 87 97 ma the cross cutting relationships of dykes and host rocks show that the mafic dyke swarms have close spatial correlations with granite the dykes stretch in various directions whereas the ne trending dykes dominate our research reveals four intrusive events that imply the crustal extension and intermittent variation of the regional stress field in the coastal area of southeastern china in the late cretaceous

coherent intrusion complexes in large basaltic volcanoes a new structural model abstract highly concentrated coherent intrusion complexes consisting of thousands of small mafic intrusions occur in probably all major basaltic volcanoes and play an important role in volcano development magma excursions from the high level chamber travel laterally along a surface of neutral buoyancy at the margin of a complex and cause the complex to grow the limited distance however that narrow intrusions can propagate before becoming blocked causes complexes to be wedge like intrusive dike complexes underlie rift zones and asymmetric growth of the dike wedge causes rift zones of shield volcanoes to become non collinear and may initiate a third rift zone in the obtuse angle downbowing of stress trajectories across complexes causes dikes to be non vertical and results in axial subsidence intrusive sheet complexes form instead of dike complexes in volcanic systems that have a restricted ability to expand laterally and accommodate intrusions by expanding vertically instead downbowing of stress trajectories causes sheets to be non horizontal and this combined with subsidence increasing toward the thicker part of the sheet wedge produces the inward and inwardly increasing dip that characterizes cone sheet complexes this mechanism for cone sheets differs considerably from previously proposed mechanisms successive injections of sheets at the top of a sheet complex probably offers the most efficient means of powering a high intensity geothermal field such as the 5000 mw grimsvotn system in iceland it is inferred that similar mechanisms to those in major basaltic edifices operate in spreading ridges study of basaltic edifices has the potential to contribute significantly to the understanding of spreading ridges

near field modifications of stress induced by dyke injection at shallow depth summary the deformation and stress fields accompanying dyke injection in rift zones are described in terms of a vertical crack opening in response to internal overpressure in plane strain configuration previous numerical computations of the displacement field induced by shallow dykes are found to be affected in some cases by significant distortion mainly due to the limited extension of the computational domain difficulties in handling the singularities in the proximity of the free surface and an incorrect choice of the reference frame in particular it is found that no subsidence can be ascribed to the mere opening of a tensile crack in a homogeneous elastic half space if analytical constant dislocation models are employed surface displacements compare reasonably well with crack model solutions if the upper tip is not too shallow however constant dislocation solutions present unphysical singularities along the tips which distort significantly the near field stress pattern with respect to crack solutions if the principal stress axes are computed from crack models a broad region is found on both sides of the dyke where the induced stress has a nearly vertical intermediate axis while the compressive axis is normal to the dyke plane strike slip earthquakes are expected to prevail in this region above the upper crack tip a small region is present where the tensile stress is dominant and the intermediate stress is still vertical in the proximity of the free surface typically within a few tens of metres of it induced tensile stresses are greater than the lithostatic pressure open fissures might then develop in cohesionless soil or pre faulted rock the induced pressure in the host rock is found to be negative suction in the proximity of the ground surface and positive at greater depth fluid flow within the aquifers can be significantly altered by this induced overpressure and by the anisotropic modification of the pre existing permeability according to the modified coulomb failure criterion in the short term the shallower region characterized by suction is strengthened while the deeper pressurized region is weakened these results can explain in a straightforward way the abundance of strike slip focal mechanisms in volcanic areas the switch between tensional and compressional axes inferred from focal mechanisms of earthquakes in connection with a dyke injection episode on mount etna en echelon fracture systems observed at piton de la fournaise above the feeding dyke and precursory geochemical anomalies

surface deformation in volcanic rift zones abstract the principal conduits for magma transport within rift zones of basaltic volcanoes are steeply dipping dikes some of which feed fissure eruptions elastic displacements accompanying a single dike emplacement elevate the flanks of the rift relative to a central depression concomitant normal faulting may transform the depression into a graben thus accentuating the topographic features of the rift if eruption occurs the characteristic ridge trough ridge displacement profile changes to a single ridge centered at the fissure and the erupted lava alters the local topography a well developed rift zone owes its structure and topography to the integrated effects of many magmatic rifting events to investigate this process we compute the elastic displacements and stresses in a homogeneous two dimensional half space driven by a pressurized crack that may breach the surface a derivative graphical method permits one to estimate the three geometric parameters of the dike height inclination and depth to center and the mechanical parameter driving pressure rock stiffness from a smoothly varying displacement profile direct comparison of measured and theoretical profiles may be used to estimate these parameters even if inelastic deformation notably normal faulting creates discontinuities in the profile geological structures open cracks normal faults buckles and thrust faults form because of stresses induced by dike emplacement and fissure eruption theoretical stress states associated with dilation of a pressurized crack are used to interpret the distribution and orientation of these structures and their role in rift formation

morphology of piton de la fournaise basaltic shield volcano la reunion island characterization and implication in the volcano evolution the topography of piton de la fournaise volcano pdf differs from the classic view of basaltic shield volcanoes as it is characterized by 1 several steep slope zones on its flanks and 2 a large u shaped caldera the enclos grand brule structure egbs most of these structures were previously interpreted as the scars of lateral landslides the deposits of which cover the submarine flanks of pdf we carried out a detailed analysis of the morphology of pdf which reveals that the steep slope zones form two independent circumferential structures that continue into the caldera the development of circumferential steep slopes on volcano flanks may have several origins constructive destructive and deformation processes we interpret those processes acting on pdf as caused by the spreading of the volcanic edifice above a weak hydrothermal core leading to outward displacements and a summit extensive stress field the continuity of the steep slope on both sides of the egbs escarpments suggests that this structure was not caused by a 4 5 ka old giant landslide as it is usually proposed but is due to a mainly vertical collapse the recent debris avalanche deposits east of the island indicate that this event likely destabilized part of the submarine flank we propose that the collapse of the grand brule the lower half of the egbs was due to the downward drag related to the dense intrusive complex of the alizes volcano which is located 1 km below the grand brule the collapse of the enclos is interpreted as the consequence of the deformation of the hydrothermal system of the pre enclos volcano although the continuity of the geological and morphological structures between the enclos and the grand brule suggests a narrow link between these two collapse events their chronology and relationship are still uncertain finally we hypothesize that the persistence of the ne and se rift zones during the last 150 ka despite the large changes of the topography related to the recurrent flank destabilizations is linked to a deep sources which can be either underlying crustal faults or the continuous downward subsidence of the alizes intrusive complex

volcano growth mechanisms and the role of sub volcanic intrusions insights from 2d seismic reflection data abstract temporal and spatial changes in volcano morphology and internal architecture can determine eruption style and location however the relationship between the external and internal characteristics of volcanoes and sub volcanic intrusions is often difficult to observe at outcrop or interpret uniquely from geophysical and geodetic data we use high quality 2d seismic reflection data from the ceduna sub basin offshore southern australia to quantitatively analyse 56 pristinely preserved eocene age volcanogenic mounds and a genetically related network of sub volcanic sills and laccoliths detailed seismic mapping has allowed the 3d geometry of each mound to be reconstructed and distinct seismic facies within them to be recognised forty six continental basaltic shield volcanoes have been identified that have average flank dips of 3 parallel seismic reflections within the shield volcanoes are interpreted to represent interbedded volcanic and clastic material suggesting that a series of temporally separate eruptions emanated from a central vent the shield volcanoes typically overlie the lateral tips of sills and we suggest that the intermittent eruption phases correspond to the incremental emplacement of discrete magma pulses within the laterally extensive sill complex eight volcanogenic hydrothermal vents which are also associated with the lateral tips of sills were also recognised and these appear to have formed from the seepage of intrusion related hydrothermal fluids onto the seafloor via emplacement induced fractures this work highlights that deformation patterns preceding volcanic eruptions may i be offset from the eruption site ii attributed to intrusions with complex morphologies and or iii reflect magma movement along pre existing fracture systems these complexities should therefore be considered in eruption forecasting models that link pre eruption ground deformation to subterranean magma emplacement depth and volume more generally our study highlights the key role that seismic reflection data can play in understanding the geometry distribution and evolution of ancient and modern volcanic systems

volcano load control on dyke propagation and vent distribution insights from analogue modeling 1 the spatial distribution of eruptive vents around volcanoes can be complex and evolve as a volcano grows observations of vent distribution at contrasting volcanoes from scoria cones to large shields show that peripheral eruptive vents concentrate close to the volcano base we use analogue experiments to explore the control of volcano load on magma ascent and on vent location results show that the local loading stress field favors eruption of rising magma away from the volcano summit if a central conduit is not established or is blocked two sets of scaled experiments are developed with contrasting rheological properties to analyze similarities and differences in simulated magma rise below a volcano 1 golden syrup magma analogue is injected into a sand plaster mixed layer crust analogue under a cone 2 water or air magma analogues is injected into gelatin under a sand cone rising dykes approaching the cone stress field are stopped by the load compressive stress with continued intrusion dyke overpressure builds up dykes extend laterally until their tips are able to rise vertically again and to erupt in the flank or at the base of the volcano lateral offset of the extrusion point relative to the edifice summit depends on substratum thickness volcano slope and dyke overpressure the 3d geometry of golden syrup intrusions varies with experimental parameters from cylindrical conduits to dyke and sill complexes experimental results are compared with illustrative field cases and with previously published numerical models this comparison enables applications and limitations of the analogue models to be highlighted and allows us to propose a conceptual model for the evolution of vent distribution with volcano growth

postrifting anelastic deformation around the spreading plate boundary north iceland 1 modeling of the 1987 1992 deformation field using a viscoelastic earth structure a third global positioning system gps survey of a regional network surrounding the krafla volcanic system north iceland was conducted in 1992 following a major crustal spreading episode which began in this system in 1975 differencing the 1992 results with those from 1987 and 1990 reveals a regional deformation field with a maximum rift normal expansion rate of 4 5 cm year near the rift decreasing to 3 cm year at large distances the time averaged spreading rate in north iceland 1 8 cm year cannot account for this deformation the vertical deformation field reveals regional uplift throughout the network area at its maximum closest to the rift and decreasing with distance three different models are applied to study the postdike injection ground deformation 1 stress redistribution in an elastic layer over a viscoelastic half space 2 stress redistribution in an elastic viscous layered medium and 3 continued opening at depth on the dike plane in an elastic half space using model 1 the effects of historical episodes in the region are subtracted from the observed displacement fields and the remaining motion is modeled as relaxation following the recent krafla rifting episode the best fit model involves a half space viscosity of 1 1 x 10 18 pa s a relaxation time of 1 7 years and an elastic layer thickness for northeast iceland of 10 km the vertical field indicates that the krafla dike complex rifted the entire elastic layer using model 2 the motion 1987 1990 and 1990 1992 can be simulated adequately given the survey errors but the 1987 1992 deformation is poorly fitted suggesting that a more realistic geophysical model is required using model 3 a range of dikes will fit the deformation field