centrifuge modelling of laccolith intrusion abstract laccolith intrusion has been investigated by centrifuge modelling silicone putty representing magma with bingham rheology is intruded by overburden pressure through a circular conduit into a stack of paraffin wax layers which represent sedimentary strata the model intrusions evolve through a sequence of geometric forms similar to those exhibited by natural laccoliths the results are also in accord with published mathematical analyses of laccolith formation the model magma first flows laterally between overburden layers to form a sill as the sill increases in area the cover strata are arched upwards into a sinusoidal bending form subsequently the overburden fails abruptly by kinking above the periphery of the intrusion and the form changes to a cupola separated from surrounding horizontal strata by a sharp inflection in the layering finally strata on the flanks of the cupola undergo a second episode of localized failure the top of the dome becomes flat and the dipping flanks become a monoclinal flexure surrounding this kink form a number of variables influence this evolution increased overburden strength thickness of overburden strata and total thickness of overburden all prolong the bending form to larger intrusion size delay the localized failure which marks the cupola and kink stages and reduce the aspect ratio of the intrusion at all stages effectively bonded overburden interfaces have a similar effect but interfaces with sharply defined yield strengths enhance development of the monoclinal flexure magmatic driving pressure affects the rate of intrusion but not the evolution of its form intrusion rate is also influenced by magma flow properties and probably by the form of the feeder conduit the range of final forms preserved by natural laccoliths probably results from competition between rate of intrusion controlled by driving pressure and conduit geometry and rate of chilling of magma in the laccoliths the model laccoliths form in times that scale to between one and a few tens of years in agreement with available estimates for natural laccoliths

experimental modelling of ground deformation associated with shallow magma intrusions abstract active volcanoes experience ground deformation as a response to the dynamics of underground magmatic systems the analysis of ground deformation patterns may provide important constraints on the dynamics and shape of the underlying volcanic plumbing systems nevertheless these analyses usually take into account simplistic shapes sphere dykes sills and the results cannot be verified as the modelled systems are buried in this paper i present new results from experimental models of magma intrusion in which both the evolution of ground deformation during intrusion and the shape of the underlying intrusion are monitored the models consisted of a molten vegetable oil simulating low viscosity magma injected into cohesive fine grained silica flour simulating the brittle upper crust oil injection resulted is sheet intrusions dykes sills and cone sheets the initial topography in the models was flat while the oil was intruding the surface of the models slightly lifted up to form a smooth relief which was mapped through time after an initial symmetrical development the uplifted area developed asymmetrically at the end of the experiments the oil always erupted at the steepest edge of the uplifted area after the experiment the oil solidified the intrusion was excavated and the shape of its top surface mapped the comparison between the uplifted zone and the underlying intrusions showed that 1 the complex shapes of the uplifted areas reflected the complex shapes of the underlying intrusions 2 the time evolution of the uplifted zone was correlated with the evolution of the underlying intrusion and 3 the early asymmetrical evolution of the uplifted areas can be used to predict the location of the eruption of the oil the experimental results also suggest that complex intrusion shapes inclined sheet cone sheet complex sill may have to be considered more systematically in the analyses of ground deformation patterns on volcanoes

intrusion induced uplift and mass wasting of the palaeogene volcanic landscape of ardnamurchan nw scotland fragmental rocks associated with the exhumed hearths of the palaeogene volcanoes of nw scotland have previously been interpreted as classic examples of vent filling agglomerates of pyroclastic origin mapping and logging of these lithologies at the ardnamurchan central complex indicate that they comprise conglomerates of debris flow origin interbedded with fluvial lacustrine units which developed on a dissected landscape during the volcanic and intrusive activity clast matrix analysis allows the palaeo topography and drainage systems to be inferred and palynological data are used to develop a palaeogeographical model for the volcanic landscape a source region uplift model links the catastrophic mass wasting events to the shallow emplacement of the central complex intrusions

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

numerical modelling of dykes deflected into sills to form a magma chamber abstract most shallow magma chambers are thought to evolve from sills for this to happen several conditions must be met 1 there must be a discontinuity normally a contact that deflects a dyke or an inclined sheet into a sill 2 the initial sill must have a considerable thickness normally depending on dyke injection rates not less than some tens of metres 3 the resulting sill must receive magma through dykes frequently enough so as to stay liquid and expand into a chamber 4 the resulting magma chamber must remain at least partially molten and receive multiple magma injections over a given period of time to build up a volcano on the surface above in this paper we present numerical models based upon field data and geophysical data as to how sills are emplaced and may subsequently evolve into shallow magma chambers we suggest that most sills form when dykes meet contacts particularly weak ones which are unfavourable to dyke propagation a contact may halt arrest a dyke altogether or alternatively deflect the dyke into the contact the three main mechanisms for dyke deflection into a contact are 1 the cook gordon debonding or delamination 2 rotation of the principal stresses generating a stress barrier and 3 an elastic mismatch across a contact between adjacent layers elastic mismatch means that the layers have contrasting young s moduli and varying material toughness once a sill is initiated the developing magma chamber may take various forms many shallow magma chambers however tend to maintain a straight sill like or somewhat flat oblate ellipsoidal geometry during their lifetimes for a sill to evolve into a magma chamber there must be elastic plastic deformation of the overburden and to some extent of the underburden so long as the sill stays liquid subsequent dyke injections become arrested on meeting the sill some magma chambers develop from sill complexes for the sill complex to remain partially molten it must receive a constant replenishment of magma implying a high dyke injection rate alternatively an initial comparatively thick sill may absorb much of the magma of the dykes that meet it and evolve into a single shallow magma chamber

upper silurian and lower devonian graptolites and graptolite biostratigraphy northern yukon canada upper silurian and lower devonian graptolite biostratigraphy of the northern canadian cordillera is revised in the light of new data the zones listed in order of oldest to youngest are lobograptus progenitor saetograptus fritschi linearis and bohemograptus bohemicus tenuis ludlow monograptus formosus pristiograptus ultimus pristiograptus chelmiensis monograptus bouceki and pristiograptus transgrediens praecipuus pridoli monograptus uniformis and m hercynicus lochkov and monograptus falcarius m fanicus m thomasi and m yukonensis prag the following taxa inclusive of new species are described pristiograptus separabilis teller pseudomonoclimacis tetlitensis n sp p richardsonensis n sp p bispinosus n sp monograptus ceratus n sp m aequabilis bardoensis porebska m cf anerosus koren m fanicus koren m hercynicus hercynicus perner m uniformis parangustidens jackson and lenz saetograptus fritschi linearis boucek and abiesgraptus sp

analogue models of laccolith formation abstract this paper describes dynamically scaled analogue models of laccolithic intrusions experiments consisted of the injection of a newtonian fluid low viscosity silicone putty into a sandpack with or without an interbedded ductile layer of silicone putty acting as a potential decollement level boundary conditions were chosen to analyze the influence of the thickness of the brittle cover and of the decollement layer on the pattern of intrusion further experiments were made to examine the effects of an extensional regime during intrusion experiments showed that 1 laccolith formation requires the occurrence of a decollement layer between two competent units 2 the critical thickness of the decollement layer necessary for laccolith formation decreases with increasing depth 3 laccoliths change from lenses to bell shaped with decreasing overburden 4 for a constant thickness of the decollement layer the largest diameter of laccoliths is proportional to the thickness of the overburden and 5 a syn injection gravitational sliding regime results in an asymmetric laccolith with amplification of the bell shape

3 d surface deformation of the 2000 usu eruption measured by matching of sar images a subpixel level offset estimation technique was applied to sar synthetic aperture radar images of usu volcano japan using radarsat sar amplitude images acquired from ascending and descending orbits we generated a map of three dimensional displacement vectors showing ground deformation associated with criptdome formation in april 2000 moreover displacement velocities and volume changes were estimated from april 3 29 inflation was observed with vertical and horizontal displacements each exceeding 20m during the period of major activity from april 3 5 uplift and n s spreading velocities reached 3 3m day and 2 7m day respectively while volume change was 3 4 106 m day although this technique is less precise than other methods and is not very sensitive to local deformations it provides several key advantages including 1 personnel are not required to enter hazardous areas 2 it provides wide spatial coverage and 3 it does not depend on coherence making it more widely applicable than insar

catastrophic collapse at stratovolcanoes induced by gradual volcano spreading unlike ordinary mountains which are formed by slow uplift and erosion volcanoes are constructed rapidly as a consequence many are liable to massive flank failures leading to debris avalanches for example at mount st helens in 1980 such failures occur worldwide about once every 25 years ref 1 and even small ones can present a major hazard in particular if far reaching tsunamis are generated as at mayu yama in 1792 ref 2 previous work has tended to emphasize differences in eruption style associated with flank failure2 but here we focus on the fundamental structural causes of failure most volcanic failures are generated by magmatic intrusion and flank spreading3 we present evidence however that mombacho volcano in nicaragua experienced a previously unrecognized type of failure triggered by sub volcanic basement spreading notably collapses related to basement spreading do not require that the volcano be magma tically active and thus flank failure may pose a significant risk even at inactive volcanoes which are rarely monitored

the volcano tectonic evolution of concepcion nicaragua we describe the evolution of concepcion volcano by integrating regional geology eruptive activity morphology stratigraphy petrology structure and active deformation data this nicaraguan volcano is set close to the back limb of the northwest trending tertiary rivas anticline a regional structure that bounds the southwest side of lake nicaragua concepcion rises 1 600 m above a 1 km thick sequence of quaternary lacustrine mud stones there is no record of volcanism in the lake prior to concepcion in addition the only nearby volcano maderas volcano has not deposited material on concepcion because of the trade winds thus concepcion and made ras too can be considered as pristine volcanic environments unaffected by other centres a topographic rise forms an annulus 20 km in diameter around the cone the rise is created by thrust related folds at the western base where the trade winds have accumulated a thick sequence of tephra and by mud diapirs at the eastern base where only lake mudstones are present four magmatic eruptive episodes exist in the stratigraphic record the first begins with primitive low alumina basalt and subsequently evolves to dacitic compositions the following three episodes begin with high alumina basalts and evolve only to silicic andesites the occurrence of the high alumina basalt after the first episode is indicative of crystal fractionation at lower crustal depths the first episode may be associated with a compressive phase of volcano evolution in this phase the edifice load compresses substrata allowing a longer magma residence time and differentiation in a shallow reservoir possibly located at the density contrast between the lake sediments and the tertiary flysch during the next three episodes the weak sediments below the volcano started to rupture and yield under its increasing load beginning a thrusting diapiring phase of volcano evolution because of outward thrusting vertical and horizontal stresses above the chamber were reduced allowing magma to erupt more easily and to reach a lesser degree of evolution if we consider the future evolution of concepcion the differentiation in the shallow reservoir has probably generated a cumulitic complex which eventually will start to deform and spread beginning another this time plutonic spreading phase this phase which may be beginning now could allow less evolved magmas to be erupted again four components influence the phases of volcano evolution 1 the regional geology that is the boundary condition of the environment 2 the substrata rheology that controls deformation 3 the load of the volcanic edifice and 4 the magma which provides the input of mass and energy our model of volcanic evolution suggests that concepcion is a complex geologic environment the volcanic activity tectonics and hazards can only be constrained through a complete knowledge of the many components of this environment

eruptive and structural history of teide volcano and rift zones of tenerife canary islands the teide and pico viejo stratocones and the northwest and northeast rifts are products of the latest eruptive phase of the island of tenerife initiated with the lateral collapse of its northern flank that formed the las canadas caldera and the icod la guancha valley ca 200 ka the eruptive and structural evolution of this volcanic complex has been reconstructed after detailed geological mapping and radioisotopic dating of the significant eruptive events a set of 54 new 14 c and k ar ages provides precise age control of the recent eruptive history of tenerife particularly teide volcano the third highest volcanic feature on earth 3718 m above sea level 7 km high and unique in terms of its intraplate setting the development of the teide pico viejo volcanoes may be related to the activity of the northwest and northeast rifts volcanic and intrusive activity along both rift zones may have played an important role in activating the gravitational landslide and in the subsequent growth nested within the collapse embayment of an increasingly higher central volcano with progressively differentiated magmas the coeval growth of the central volcano with sustained activity along the rifts led to a clear bimodal distribution in composition of eruptive products with the basaltic eruptions in the distal part of the rifts and phonolitic and more explosive eruptions in the central area where the differentiated stratocones developed current volcanic hazard in tenerife is considered to be moderate because eruptive frequency is low explosivity is modest and the eruptive activity of the teide stratocone seems to have declined over the past 30 k y with only one eruption in this period 1150 yr b p

a global database of composite volcano morphometry we present a global database on the subaerial morphometry of composite volcanoes data was extracted from the 90 m resolution shuttle radar topography mission srtm digital elevation model dem the 759 volcanoes included in the database are the composite i e polygenetic volcanoes listed in the smithsonian institution global volcanism program gvp database that are covered by the srtm dem have a constructional topography and a basal width larger than 2 km the extent of each volcano edifice was defined using the netvolc algorithm which computes outlines by minimizing a cost function based on breaks in slope around the edifices morphometric parameters were then calculated using the morvolc algorithm the parameters characterize and quantify volcano size basal width summit width height and volume profile shape height basal width and summit width basal width ratios plan shape ellipticity and irregularity indexes and slopes in addition 104 well defined and relatively large summit craters calderas were manually delineated and specific parameters were computed most parameters show large variation without clear separations indicating a continuum of volcano morphologies large overlap between the main gvp morphologic types highlights the need for a more rigorous quantitative classification of volcano morphology the database will be maintained and updated through a website under construction

analytical model of surface uplift above axisymmetric flat lying magma intrusions implications for sill emplacement and geodesy abstract in this paper we develop a new axisymmetric analytic model of surface uplift upon sills and laccoliths based on the formulation of a thin bending plate lying on an elastic foundation in contrast to most former models also based on thin bending plate formulation our model accounts for i axi symmetrical uplift ii both upon and outside the intrusion the model accounts for shallow intrusions i e the ratio a h 5 where a and h are the radius and depth of the intrusion respectively the main parameter of the model is the elastic length l which is a function of the elastic properties of the bending plate and of the elastic foundation the model exhibits two regimes depending on the ratio a l when a l a l 5 the uplift is mostly restricted upon the intrusion when the elastic foundation is very stiff our model converges towards that of a clamped plate we provide as supplementary material a matlab function that calculates the surface uplift from the set of system and control parameters we discuss three possible applications of our model i the model can be used to describe sill propagation by introducing a propagation criterion for realistic values our model reproduces well the behavior of horizontal intrusions simulated in experiments ii the model can also be used to compute the critical size of saucer shaped sills it shows for instance that a soft elastic foundation favors the horizontal spreading of sills before they form inclined sheets iii we show that the classical mogi point source model cannot be used to constrain sill properties from the surface uplift we thus propose that our model can be used as a valuable alternative to both simple analytical models like mogi s and more complex numerical models used to analyze ground deformation resulting from sill intrusions in active volcanoes

mechanisms and duration of non tectonically assisted magma emplacement in the upper crust the black mesa pluton henry mountains utah abstract a new study of black mesa pluton henry mountains colorado plateau utah usa indicates that it is a classic example of a small upper crustal pluton assembled over a few years by incremental amalgamation of discrete magma pulses the results of our petrostructural study of the pluton interior allow us to constrain the geometry kinematics and timing of the processes the symmetric internal fabric is interpreted as an evidence for a feeding by below and not laterally the observed rotation of the lineation from wnw ese on the very top to nne ssw below lead us to propose that the fabric at the base of the pluton is a record of magma infilling process and the fabric at the very top is a record of the strain due to the relative movement between magma and wallrocks a consequence is that except at the contact between pluton and wallrocks top and margins the stretching direction recorded by the lineation is not parallel to the flow direction of the magma i e displacement the black mesa pluton is a sheeted laccolith on its western edge and a bysmalith on its eastern edge this e w asymmetry in pluton geometry construction and the symmetrical internal fabric indicates that the apparently different west and east growth histories could have occurred simultaneously our field data indicate pluton growth through an asymmetric vertical stacking of sill like horizontal magma sheets one dimensional thermal models of the pluton provide maximum limits on the duration of its growth we have constrained the number the thickness and the frequency of magma pulses with our structural observations including 1 the emplacement of the pluton by under accretion of successive magma pulses 2 the absence of solid state deformation textures at internal contacts and 3 the apparent absence of significant recrystallization in the wallrocks our results suggest that the emplacement of the black mesa pluton was an extremely rapid event with a maximum duration on the order of 100 years which requires a minimum vertical displacement rate of the wallrocks immediately above the pluton greater than 2 m yr finally our data show that the rates of plutonic and volcanic processes could be similar a significant result for interpretation of magma transfer in arc systems

the three dimensional geometry and growth of forced folds above saucer shaped igneous sills forced folds formed at the seabed immediately overlying shallow 1 km saucer shaped sills along the ne atlantic margin during the early paleogene examples of this sill fold relationship are exceptionally well imaged by high resolution 3d seismic datasets from the ne rockall basin the forced folds are domal in shape 2 4 km in diameter exhibit a structural relief of up to 350 m and comprise sediment volumes of ca 1 km3 a comparison of the thickness distribution across and volume of a saucer shaped sill with a high intrusion diameter to depth ratio and the structural relief and volume of its associated forced fold shows a remarkable equivalence this has the important implication that the structural relief on intrusion related forced folds can be used as an estimate of the thickness of the underlying sill the analysed forced folds are interpreted to have formed through three continual growth stages that are directly linked to the mechanical emplacement of the underlying saucer shaped sills their growth was associated with an increase in faulting of the overlying strata and influenced coeval or subsequent development of polygonal fault systems within the overburden these structures represent a new type of four way dip closed hydrocarbon trap

lithological controls on igneous intrusion induced ground deformation ground deformation commonly precedes volcanic eruptions although its relationship to underlying intrusion networks is complex we use 3d seismic reflection data to examine the link between a saucer shaped sill and an overlying forced fold formed at the contemporaneous palaeosurface our results highlight a disparity in size between the sill and fold which we attribute to accommodation of magma by overburden uplift and fluid expulsion from the host rock sill transgression occurred in response to plastic deformation of the host rock and did not produce seismically resolvable uplift inversion models of ground deformation should therefore acknowledge host rock behaviour during intrusion supplementary material uninterpreted seismic section and sill thickness fold amplitude data are available at www geolsoc org uk sup18663

the mechanics and dynamics of sills in layered elastic rocks and their implications for the growth of laccoliths and other igneous complexes abstract the recent experimental work by kavanagh et al kavanagh j l menand t and sparks r s j 2006 an experimental investigation of sill formation and propagation in layered elastic media earth planet sci lett 245 799 813 shows that lithological discontinuities and rigidity contrasts can control the formation and dynamics of sills at interfaces separating upper rigid strata from lower weaker strata the present paper extends this work and focuses on its implications in terms of the length and time scales associated with the development of laccoliths and other igneous complexes sill formation controlled by rigidity contrasts is shown to provide a growth mechanism for laccoliths the formation of a sill provides favourable rigidity anisotropy for the emplacement of subsequent sills so that laccoliths grow by over accretion under accretion or even mid accretion of successive sills in accord with field data this model predicts that laccoliths grow mainly by vertical expansion representing the cumulative thickness of their internal sills while maintaining a comparatively constant lateral extend the model also predicts that the time scale over which laccoliths form is essentially determined by the cumulative time between successive sill intrusions also sill dynamics are controlled by viscous dissipation of the fluid along their length which have consequences for the size and shape of sills viscously controlled dynamics would enable sills to propagate further and thus to grow thicker than dykes of similar magmas these dynamics would also enable sills to propagate faster and thus to induce non elastic deformations in surrounding rocks that could deviate them from the interface they originally follow this would allow them to feed new sills along other interfaces and could assist in the formation of the step structures and saucer shapes that are commonly observed in sill complexes

flexure and faulting of sedimentary host rocks during growth of igneous domes henry mountains utah abstract a sequence of sedimentary rocks about 4 km thick was bent stretched and uplifted during the growth of three igneous domes in the southern henry mountains mount holmes mount ellsworth and mount hillers are all about 12 km in diameter but the amplitudes of their domes are about 1 2 1 85 and 3 0 km respectively these mountains record successive stages in the inflation of near surface diorite intrusions that are probably laccolithic in origin the host rocks deformed along networks of outcrop scale faults or deformation bands marked by crushed grains consolidation of the porous sandstone and small displacements of sedimentary beds zones of deformation bands oriented parallel to the beds and formation contacts subdivided the overburden into thin mechanical layers that slipped over one another during doming measurements of outcrop scale fault populations at the three mountains reveal a network of faults that strikes at high angles to sedimentary beds which themselves strike tangentially about the domes these faults have normal and reverse components of slip that accommodated bending and stretching strains within the strata an early stage of this deformation is displayed at mount holmes where states of stress computed from three fault samples correlate with the theoretical distribution of stresses resulting from bending of thin circular elastic plates field observations and analysis of frictional driving stresses acting on horizontal planes above an opening mode dislocation as well as the paleostress analysis of faulting indicate that bedding plane slip and layer flexure were important components of the early deformation as the amplitude of doming increased radial and circumferential stretching of the strata and rotation of the older faults in the steepening limbs of the domes increased the complexity of the fault patterns steeply dipping map scale faults with dip slip displacements indicate a late stage jostling of major blocks over the central magma chamber radial dikes pierced the dome and accommodated some of the circumferential stretching

monoclinal bending of strata over laccolithic intrusions abstract sedimentary strata on top of some laccolithic intrusions are nearly horizontal and little deformed but are bent into steeply dipping monoclinal flexures over the peripheries of these intrusions this form of bending is not explained by previous theories of laccolithic intrusion which predict either horizontal undeformed strata over the center and faulted strata around the periphery or strata bent continuously into a dome however a slight generalization of these theories accomodates the observed form and contains the previous forms as special cases a critical assumption is that the strength of contacts within a multilayered overburden is overcome locally by layer parallel shear if this strength is less than the strength of the layers themselves then layers over the center remain bonded together and display negligible bending whereas layers over the periphery slip over one another and are readily bent into a monoclinal flexure

volcanism in a compressional andean setting a structural and geochronological study of tromen volcano neuquen province argentina we document evidence for growth of an active volcano in a compressional andean setting our data are surface structures and 39ar 40ar ages of volcanic products on tromen volcano tromen is an active back arc volcano in the andean foothills of neuquen province argentina its volcanic products are unconformable upon mesozoic strata of the neuquen basin the volcano straddles a n s trending pop up which formed during e w shortening the main underlying structures are eastward verging thrusts their traces curve around the eastern foot of the volcano minor folds and faults also occur in the volcanic cover of tromen as a result of e w shortening new 39ar 40ar ages for these volcanic rocks are younger than 2 27 0 10 ma and show that tromen has been active almost continuously from the late pliocene to the holocene we conclude that volcanism and thrusting have been coeval and that magma must have reached the surface in a tectonic setting of horizontal compression our results have wider implications for magmatic processes in such settings

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

modeling the growth of laccoliths and large mafic sills role of magma body forces 1 the use of elastic plate theory to model the emplacement of laccoliths and large mafic sills has been debated for nearly 40 years these intrusions typically attain a horizontal width that is large relative to the emplacement depth provided that large scale plasticity and or heterogeneity is not observed in the overlying host rock it should then be valid to approximate its deformation based on analysis of a thin elastic plate with effective properties that are the consequence of interaction among heterogeneities that are small relative to the size of the intrusion but the predictions that are usually cited from elastic plate theory are characterized by bell shaped geometry in contrast to the flat topped steep sided geometry typical of many laccoliths or the nearly uniform thickness typical of large mafic sills this fact has motivated several alternate explanations of laccolith and large mafic sill emplacement nonetheless elastic plate theory should be revisited in light of the fact that previous elastic plate based predictions have in general not taken into account an appropriate fracture propagation condition fluid flow in the growing intrusion and importantly the influence of the weight of the magma on intrusion growth we present a model for the growth of circular intrusions that accounts for all of these factors the model predicts the appropriate geometry for both laccoliths and large mafic sills the predicted thickness to length relationships are also consistent with field data hence while it may sometimes be appropriate there is in general no fundamental need to appeal to large scale rock plasticity in order to explain observed intrusion geometries and it may in fact be appropriate to understand the growth of laccoliths and large sills in light of a single underlying mechanical model

sector collapse forming at casita volcano nicaragua catastrophic sector collapse occurs when a volcano becomes structurally unable to support its own load one process particularly capable of weakening the edifice is hydrothermal activity it can produce high pore pressures and alter strong rock to clays alteration can extend progressively over long periods 100 yr allowing deformation to develop slowly before collapse an important finding is that structures produced by such deformation are recognizable and could permit collapse prediction we present the case of casita nicaragua where hydrothermal activity has been weakening the edifice core causing flank spreading altering original constructional shape and steepening flank slopes one side is slumping outward producing a crescentic scar with a basal bulge we identify this feature as the site of a potential sector collapse with conditions ripe for failure

importance of gravitational spreading in the tectonic and volcanic evolution of mount etna the interaction between gravity and thermal effects largely determines the structural and magmatic evolution of volcanic constructs at scales spanning several orders of magnitude from small cones to the oceanic crust1 3 although gravitational spreading is a direct consequence of this interaction and a fundamental process in volcano growth it is rarely recognized as such here we describe a striking example of this process at etna volcano in italy the volcano and its clay rich substratum are slowly spreading towards the east and south driven by gravity spreading produces extensional structures in the summit region and contpressional structures at the base of the volcano eastward movement of the volcanic edifice over a stationary magma supply may be the cause of an apparent westward migration of volcanic activity as gravitational spreading seems to control the location and magnitude of shallow seismicity and flank eruptions an appreciation of its effects could become an essential element of future volcanic hazard evaluation our model proposed here for etna may also be relevant to a reinterpretation of the geological history of a number of other volcanoes

decadal to monthly timescales of magma transfer and reservoir growth at a caldera volcano caldera forming volcanic eruptions are low frequency highimpact events capable of discharging tens to thousands of cubic kilometres of magma explosively on timescales of hours to days with devastating effects on local and global scales1 because no such eruption has been monitored during its long build up phase the precursor phenomena are not well understood geophysical signals obtained during recent episodes of unrest at calderas such as yellowstone usa and campi flegrei italy are difficult to interpret and the conditions necessary for large eruptions are poorly constrained2 3 here we present a study of pre eruptive magmatic processes and their timescales using chemically zoned crystals from the minoan caldera formingeruption of santorini volcano greece4 which occurred in the late 1600s bc the results provide insights into how rapidly large silicic systems may pass from a quiescent state to one on the edge of eruption5 6 despite the large volume of erupted magma4 40 60 cubic kilometres and the 18 000 year gestation period between the minoan eruption and the previous major eruption most crystals in the minoan magma record processes that occurred less than about 100 years before the eruption recharge of the magma reservoir by large volumes of silicic magma and some mafic magma occurred during the century before eruption and mixing between different silicicmagmabatches was still taking place during the final months final assembly of large silicic magma reservoirs may occur on timescales that are geologically very short by comparison with the preceding repose period with major growth phases immediately before eruption these observations have implications for the monitoring of long dormant but potentially active caldera systems

fracture mode analysis and related surface deformation during dyke intrusion results from 2d experimental modelling surface deformation analysis in volcanic edifices in response to shallow magma intrusion is crucial for assessing volcanic hazards in this paper we discuss the effect of dyke propagation mode on surface deformation through 2d laboratory models our experimental setup consists of a hele shaw cell in which a model magma is injected into a cohesive model crust using an optical image correlation technique particle imaging velocimetry we measured the surface deformation the displacements and the strain field induced by magma emplacement within the country rock we identify two types of intrusion morphologies types a and b which exhibit two evolutional stages during the first stage both types resulted in a vertical dyke at depth its propagation was controlled by both shear deformation and tensile opening the model surface lifted up to form a smooth symmetrical dome resulting in tensile cracks during the second stage types a and b experiments differ when the dyke reaches a critical depth in type a the intrusion gradually rotates forming an inclined sheet dipping between 451 and 651 this rotation results in asymmetrical surface uplift and shear failure upon the tip of the dyke in type b the dyke tip interacts with tensile cracks formed during the first stage this fracture controls the subsequent propagation of the dyke toward the surface in both types of experiments intrusions result in surface uplift which can be accommodated by reverse faults our study suggests that dykes propagate as viscous indenters rather than linear elastic fracturing

new technique for dating igneous sills three dimensional seismic interpretation of tertiary sills within the northeast atlantic margin demonstrates that shallow level 20 km long the fill of the minibasin is dated biostratigraphically as 54 6 55 ma which fits well with radiometric dates for the timing of intrusion of sills around this basin ca 53 55 ma the recognition and dating of onlap fill seismic reflector packages that are delimited by forced folds form a new and useful method for dating shallow level igneous intrusions in sedimentary basins a method that provides independent constraints on radiometric dating techniques similar forced folds are recognized from other basins that have shallow level sills suggesting that the technique presented here may be similarly applicable in comparable geologic settings

dykes cups saucers and sills analogue experiments on magma intrusion into brittle rocks magma is transported in the crust by blade like intrusions such as dykes sills saucers and also collects in thicker laccoliths lopoliths and plutons recently the importance and great number o

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

stratigraphy structure and volcanic evolution of the pico teide pico viejo formation tenerife canary islands abstract the structure and volcanic stratigraphy of the pico teide pico viejo pt pv formation deriving from the basanite phonolite stratovolcanoes pt and pv and numerous flank vent systems are documented in detail based on new field and photogeologic mapping geomorphologic analysis borehole data and petrological and geochemical findings results provide insight into the structure and evolution of the pt pv magma system and the long term cyclic evolution of tenerife s post shield volcanic complex the pt pv formation comprises products of central volcanism mainly emplaced into the las canadas caldera lcc and contemporaneous products from adjacent rifts pt pv central volcanic products become more differentiated up section with felsic lavas dominating the recent output of the system this is attributed to the evolution of a shallow magma reservoir beneath pt that was emplaced early in the pt pv cycle on the intra caldera segment of tenerife s post shield rift system the rift axis has been the focus of pt pv intrusive and eruptive activity and has controlled the location of the stratocones the current geometry of the rifts reflects a major structural reorganisation defining the start of the pt pv cycle at 0 18 ma namely the truncation of the north side of the lcc lce by the giant icod landslide the internal stratigraphy of the pt pv formation suggests that pt developed early with pv developing as a satellite vent activity has since alternated between pt and pv due to episodes of vent blockage or chamber sealing these processes have allowed significant volumes of phonolitic magmas to develop and accumulate within the pt chamber which have vented through radial dike systems during tumescence episodes and from the rift system which has permitted lateral magma transport the pt pv magma system is a potentially hazardous source of future felsic eruptive activity on tenerife

implications of simple flexure theory for the formation of saucer shaped sills saucer shaped dolerite sills have been identified in several sedimentary basins they are characteristically sub circular in plan with an inner sill connected to a steeply inclined arcuate sheet extending upwards from the edge of the inner sill and in some cases have raggedly developed flat outer rims by applying elastic plate theory to the overburden we obtain an expression for the maximum radius of a circular sill in terms of emplacement depth excess magma pressure elastic constants of the overburden and the tensile strain at failure this expression predicts that the radius of the flat base of a saucer shaped sill increases with emplacement depth as observed in the western karoo basin south africa some saucer shaped sills display an elliptical morphology from consideration of the flexural strains around the periphery of a horizontal elliptical sill we infer that saucer shaped elliptical sills have been fed from below by feeder dykes oriented along the major axis of the ellipse flat outer rims would have developed at the final stage of each intrusive episode when the excess magma pressure was decreasing towards equilibrium with the overburden load consequently flexural strains would also have decreased so continued intrusion might preferentially occur along a bedding plane

dynamic basis of volcanic spreading the established models of the structure and dynamics of a typical hawaiian volcano lead inevitably to a paradox finite element viscoelastic calculations using tecton melosh and raefsky 1980 1981 predict that a compressional stress field should characterize their upper flanks instead extension is observed the paradox is solved by postulating that volcanic evolution is determined by feedback processes related to growth and spreading of the volcanic edifice many examples of the process of spreading are documented in the geologic literature on volcanoes of a very broad size range and very different geologic settings the process is similar across many orders of magnitude in size the deformation of the substratum related to magma intrusion during lava dome building events is at the lower end of the scale the spreading of seafloor at oceanic ridges is at the higher end of the scale volcanoes may evolve from small constructs into huge shield volcanoes and perhaps into ocean plates key elements for volcanic spreading are 1 the existence of a weak basal layer and 2 a sufficiently high mass and magma influx to drive the process as the mass of a volcano grows it passes through five phases which may overlap repeat or be omitted building compressing thrusting intruding and spreading during the building phase the mass of a volcano does not significantly affect the stress field of the substratum primitive magmas are likely to be erupted as the mass increases the volcano subsides and becomes characterized by a compressional stress field which inhibits any further intrusion of magma into the volcanic edifice magma may differentiate in the crust allowing for large caldera forming eruptions to occur in the next phase thrusting begins on a decollement usually located at the foot of the edifice while the stress field on the upper slopes of the volcano remains compressional the stress at the base close to the center becomes extensional allowing the magma to intrude and differentiate at the base of the edifice a basal intrusive complex begins to grow explosive eruptions may be expected finally spreading of this complex creates an extensional stress field on the upper flanks of the volcano confining compression to the lower slopes of the edifice the extension allows primitive magma to erupt again at the summit the process of spreading seems to have a fundamental influence on the structural and magmatic evolution of volcanoes at every scale this process results from coupling and feedback between the gravitational and thermal fields thus studying this process may improve our understanding of the relation between structural dynamics and magma evolution and of the origin and evolution of volcanic seismicity and eruptions

k ar dating extended into the last millennium application to the youngest effusive episode of the teide volcano spain less than one thousand years dating of the youngest volcanic episode of the teide volcano canary islands spain has been performed using the k ar cassignol technique analyses of about 20 g of pure alkali feldspar yielded a weighted mean age of 800 300 years stratigraphic historic and archeomagnetic dating validate this age and rule out the hypothesis of the presence of significant excess argon contamination for this flow our result shows that the last effusive activity of the teide volcano took place shortly before european settlement in tenerife island this study confirms that the cassignol technique can be applied with success for historic dating and demonstrates that it can now even be extended to the last millennium

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

styles of volcano induced deformation numerical models of substratum flexure spreading and extrusion the gravitational deformation of volcanoes is largely controlled by ductile layers of substrata using numerical finite element modelling we investigate the role of ductile layer thickness and viscosity on such deformation to characterise the deformation we introduce two dimensionless ratios a volcano radius ductile layer thickness and b viscosity of ductile substratum failure strength of volcano we find that the volcanic edifice spreads laterally when underlain by thin ductile layers a 1 while thicker ductile layers lead to inward flexure a 1 the deformation style is related to the switch from predominantly horizontal to vertical flow in the ductile layer with increasing thickness increasing a structures produced by lateral spreading include concentric thrust belts around the volcano base and radial normal faulting in the cone itself in contrast flexure on thick ductile substrata leads to concentric normal faults around the base and compression in the cone in addition we show that lower viscosities in the ductile layer low b lead to faster rates of movement and also affect the deformation style considering a thin ductile layer if viscosity is high compared to the failure strength of the volcano high b then deformation is coupled and spreading is produced however if the viscosity is low low b substratum is effectively decoupled from the volcano and extrudes from underneath it in this latter case evidence is likely to be found for basement compression but corresponding spreading features in the volcano will be absent as the cone is subject to a compressive stress regime similar to that produced by flexure at volcanoes where basement extrusion is operating high volcano stresses and outward substratum movement may combine to produce catastrophic sector collapse an analysis of deformation features at a volcano can provide information about the type of basement below it a useful tool for remote sensing and planetary geology also knowledge of substratum geology can be used to predict styles of deformation operating at volcanoes where features have not yet become well developed or are obscured

experiments on the indentation process during cryptodome intrusions new insights into mount st helens deformation scaled experiments were carried out to study the deformation of a volcanic edifice by forcible intrusion of a cryptodome as the magma analogue is injected vertically into a sand cone asymmetric deformation is caused by the formation of a curved major shear fault which dips inward from one side of the cone to the opposite edge of the intrusion the path of the ascending silicone deviates to follow the trajectory of the fault and a lateral bulge grows slowly from the footwall of the fault the oblique push makes the bulge migrate outward causing extension upslope to form an asymmetric graben in the hanging wall of the major shear fault we suggest that the pattern of internal deformation within mount st helens prior to the may 18 1980 eruption was similar to that observed in our scaled models

a structural outline of the yenkahe volcanic resurgent dome tanna island vanuatu arc south pacific abstract a structural study has been conducted on the resurgent yenkahe dome 5 km long by 3 km wide located in the heart of the siwi caldera of tanna island vanuatu arc south pacific this spectacular resurgent dome hosts a small caldera and a very active strombolian cinder cone the yasur volcano in the west and exhibits an intriguing graben in its central part detailed mapping and structural observations make it possible to unravel the volcano tectonic history of the dome it is shown that following the early formation of a resurgent dome in the west a complex collapse caldera plus graben occurred and this was associated with the recent uplift of the eastern part of the present dome eastward migration of the underlying magma related to regional tectonics is proposed to explain this evolution

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

the history of intrusive activity on the island of la palma canary islands abstract the island of la palma located in the northwest of the canary archipelago consists of two main geological units 1 the subaerial extrusive coberta series covers most of the island and consists of alkali basaltic lavas phonolitic plugs and pyroclastic rocks erupted between 1 61 ma and the recent 2 the basement complex is exposed in the barranco de las angustias and the caldera de taburiente and consists of the submarine intrusive and extrusive seamount series and a basal plutonic complex alkali basaltic dikes of different ages and orientations intruded in both units three groups of dikes and sills can be distinguished based on their field relationships petrography geochemistry and ages the oldest group i represent the feeder dikes to the seamount series intruded between 2 9 and 4 0 ma b p they were originally steep with a n05 w strike and perpendicular to the originally nearly flat bedding of the layered seamount series subsequent to emplacement group i dikes and the extrusives of the seamount series were rotated into their present orientation group ii sills intruded parallel to the bedding of the seamount series 45 230 are coeval with or younger than group i but older than the erosional unconformity separating the coberta and the basement complex i e 1 6 ma 50 vol of the volcanic edifice of the la palma seamount and highlight the importance of intrusive processes for island growth intrusions into central oceanic volcanoes like la palma may result in an uplift and rotation of more than 30 km 3 by up to 45

two stage growth of laccoliths at elba island italy at elba island italy nine shallow level late miocene granite porphyry layers connected by feeder dikes built up three nested christmas tree laccoliths detailed mapping and reconstruction of tectonic history led to restoration of the original 5 km thick sequence and determination of the dimensional parameters of each intrusive layer the laccolith layers were emplaced at depths between 1 9 and 3 7 km exploiting physical discontinuities that served as crustal magma traps inside a stack of nappes the intrusive layers are 50 700 m thick with diameters between 1 6 and 10 km length to thickness relationships for individual laccolith layers show a power law correlation that does not fit the known dimensional distribution for laccoliths but instead fits a line with a slope typical of the theoretical vertical inflation stage of laccolith development this is interpreted as the first reported natural example of the occurrence of a vertical inflation stage during laccolith growth the dimensional data for elba intrusive layers also suggest that laccoliths and plutons commonly form by amalgamation of smaller sheet like bodies while multilayer laccoliths form when coalescence fails possibly owing to the large availability of crustal magma traps

dike induced reverse faulting in a graben normal fault slip of the boundary faults of a graben is commonly attributed to dike induced stresses here however we report for the first time clear field evidence of a large dike induced reverse fault slip on a fault associated with a volcano tectonic graben the measured reverse faulting of at least 5 m occurs in the holocene rift zone of northern iceland the reverse slip has apparently occurred on an existing normal fault as a result of over pressure of a nearby 6 13 m thick 8 ka feeder dike a numerical model of the dike fault interaction supports our interpretation the results indicate that dikes particularly potential feeder dikes may cause large reverse slips on nearby normal faults this conclusion should improve the general understanding of geodetic deformation during volcanic unrest periods and may help forecast dike fed eruptions

extension induced by volcanic loading in regional strike slip zones volcanoes and strike slip pull apart structures are commonly found together we use theoretical predictions and analogue modeling to evaluate the effect of volcanic loading in strike slip zones and the converse effect of regional strike slip faults on volcano structure theoretical predictions indicate that regional stress is reoriented by volcano loading to cause extension analogue models indicate that volcanoes in strike slip zones develop extensional pull apart structures a graben typically forms normal to the minimum regional stress axis 3 at a cone cones emplaced onto preexisting faults increase that fault9s tensional component if the substratum is weak enough to permit volcano spreading thrusts form normal to the maximum stress axis 1 a feedback situation can arise i e loading related extension allows more magma influx more eruption and thus increased loading this process can create large volcano tectonic depressions volcano structures may also be used as indicators of regional strike slip movements

emplacement of multiple magma sheets and wall rock deformation trachyte mesa intrusion henry mountains utah a detailed structural and rock magnetic study of the trachyte mesa intrusion and deformed sedimentary wall rocks henry mountains utah indicates that the intrusion grew vertically and horizontally by the accumulation of multiple horizontal magma sheets 2e3 cm thick shear zones recognized by intensely cataclasized plagioclase phenocrysts define the contact between sheets sheets have bulbous and or steep frontal terminations and are flat on top the foliation within the interior of the sheets near the frontal termination is subvertical this steep foliation rotates into the subhorizontal shear zones near the top and bottom contacts and provides a magma flow direction indicator away from the frontal termination the interior foliation rotates to become subhorizontal similar to the fabric in recent analog experimental studies sheets are interpreted as being emplaced as plug flows both the field fabric and the rock magnetic data collected from 103 locations on the top of the intrusion and from 73 locations along a vertical cross section exposed in a stream gorge support a multi stage model of intrusion growth emplacement begins as narrow magma channels and magma spreads radially outward from the channels to form sheets sheets are stacked upon one another and stop at the same lateral termination the deformation of the sandstones at the margin of the intrusion which are rotated upward from the margin to become the roof is partitioned into layer parallel extension shearing and layer parallel shortening components bulk strain within the thickest sandstone layer indicates w20 thinning and microstructures indicate that the thinning was accommodated by grain scale fractureinduced porosity collapse extension occurred as the layer was stretched over the margin of the rising intrusion shearing and layer parallel shortening are a result of coupling with the underlying sheets as they advanced and accommodated through numerous faults parallel to bedding and at low angles to bedding the deformation of thinner sedimentary layers is consistent with the sedimentary layering immediately in front of an advancing sheet being translated upward and over the top of the sheet as the hinge zone migrates with the front edge of the advancing sheet 2008 elsevier ltd all rights reserved

emplacement of magma pulses and growth of magma bodies magmatism is responsible for the formation of continental and oceanic crusts it is the main agent of mass and heat transfers from the mantle towards the crust the hydrosphere oceanic and continental hydrothermalism and the atmosphere emissions of volcanic gas and ashes its expressions are the crystallization of intrusive rocks and the eruption of volcanic products however with a ratio between the volumes of extrusive and intrusive magmatic rocks estimated to be of the order of 1 5 for most magmatic systems white et al 2006 one of the main characteristics of magmatism is that magmas rarely reach the earth s surface this suggests that the prevailing conditions in the crust are not favourable to the arrival of magma on the earth s surface but instead lean heavily towards the formation of intrusive bodies in the crust these intrusive bodies referred to here as plutons constitute the elementary building brick of the continental crust an increasing number of geophyscial and geochronological data as well as geological observations is currently modifying our understanding of pluton construction when plutons were initially envisaged as quasi spherical bodies growing slowly and essentially by an overall inflation they are now recognized as growing incrementally by the accretion of successive and relatively small magma pulses over variable periods of time from hundred to millions of years depending on geodynamic setting and source fertility this brings new challenges and has far reaching implications according to this new model of plutonism the evolution of magma bodies is related to the processes that control the timescale and the spatial distribution of the successive pulses depending on their emplacement rate and on their ability to amalgamate repeated magma pulses can either rapidly solidify or ultimately build up an active magma chamber thus understanding how magma bodies grow has fundamental implications for the link between volcanism and plutonism as well as for magma differentiation and ultimately for our understanding of the growth and evolution of the earth crusts the concept of pluton incremental growth challenges our understanding as well as our field interpretations of the processes involved during pluton construction indeed these processes and how they operate in governing the emplacement and growth of plutons both in space and time are still debated a state of the art session on these very issues was held at the 2008 general assembly of the european geosciences union as a follow up of this session this special volume emplacement of magma pulses and growth of magma bodies brings together both theoretical models and field studies that cover most aspects of the emplacement and growth of plutons

dynamics of magmatic intrusions in the upper crust theory and applications to laccoliths on earth and the moon 1 to understand the dynamics of shallow magmatic intrusions i propose a theoretical model of magma spreading laterally below an elastic crust nondimensionalization of the flow equation leads to the identification of characteristic scales for the intrusion while the characteristic intrusion length is controlled by the elastic response of the crust its characteristic thickness primarily depends on magma properties and injection rate three spreading regimes are identified and characterized by different morphologies as well as by scaling laws for thickness versus length and thickness versus time the first spreading regime is controlled by the elastic response of the crust and the shape of the flow is self similar when the time length and thickness become larger than an elastic time length and thickness scale the edges of the flow become steeper and the system transitions to a gravity current regime when the intrusion is thick enough to accommodate the pressure head the flow enters a regime of lateral propagation and keeps a constant thickness the intrusion shape in the elastic regime fits the observed shape of terrestrial laccoliths the elastic scaling law for intrusion thickness versus length fits observations of laccoliths at elba island italy and provides for a physical explanation for the observed relationship between length and thickness on terrestrial laccoliths laccoliths are predicted to form over short time scales depending on magma viscosity that vary between approximately a month to several years for felsic magmas on earth on the moon several elongated low slope domes have recently been identified as possibly formed by laccolith intrusions at depth although they are much larger than terrestrial laccoliths because the moon has a smaller gravity than on the earth a deeper magma source and a more mafic magma composition than for terrestrial laccoliths implying smaller pressure gradient and dike width lunar intrusions have a larger characteristic length and a smaller characteristic thickness after nondimensionalization the morphologies length versus thickness of terrestrial and inferred lunar laccoliths follow the same curve and are well fitted by the elastic scaling law this model thus explains the size discrepancy between terrestrial laccoliths and lunar low slope domes therefore low slope domes identified on the moon are good candidates for laccolith type intrusions at depth

edifice and substrata deformation induced by intrusive complexes and gravitational loading in the mull volcano scotland it is likely that the structure of a volcanic edifice can be significantly modified by deformation caused by large shallow intrusions such deformation may interact with that caused by volcano loading we explore such intrusion related and loading related deformation with field evidence and analogue models to do this we have chosen the eroded palaeogene mull volcano scotland that had a major edifice has well exposed intrusions and significant deformation there are thin mesozoic sedimentary rocks forming ductile layers below the volcano but their thickness is insufficient to allow the gravitational spreading of the volcanic edifice especially when considering that a thick lava pile covers them thus intrusive push may have been the driving force for deformation the mull activity migrated toward the northwest forming three successive intrusive complexes centres 1 2 and 3 our detailed fieldwork reveals that deformation due to these was accommodated on three levels along thrust planes in lava sequences along a decollement located in a thin clay rich sediment succession and in basement schists a relative chronology has been established between different groups of structures using dyke and sill cross cutting relationships centre 1 is surrounded by a fold and thrust belt leading to radial expansion in contrast centre 2 and 3 are connected to thrusts located to the south and east bounded by strike slip faults leading to expansion to the southeast the migration of centres and the directed sliding of the edifice may be related to the presence to the southeast of low resistance dalradian basement that failed significantly during growth of centres 2 and 3 to study the observed relationships we have carried out scaled analogue models models are made with fine powder intruded by a viscous magma analogue the models show an intimate relationship between intrusion growth uplift of the volcano and subsequent flank sliding the structures produced can be compared with mull and suggest that the centre 1 thrust belt probably formed following edifice gravitational sliding as a consequence of the uplift associated with centre 1 formation centre 2 and 3 are responsible for the sector sliding of the edifice flank toward the southeast as the magmatic complex became more asymmetric the features observed at mull and in the models are similar to those seen on active volcanoes such as etna providing a structural framework for their deformation and evolution

constraining the depth and timing of large scale conical sandstone intrusions kilometer scale sandstone intrusions a recently recognized end member of the spectrum of soft sediment deformation structures are important components of many deep water sandstone reservoirs they result from the remobilization of considerable volumes of sand during early stages of burial this paper uses three dimensional seismic data calibrated to nearby wells to describe an extensive system of large downward tapering conical sandstone intrusions from the faeroe shetland basin on the uk atlantic margin the intrusions are derived from a middle eocene submarine fan and are injected vertically upward through overlying highly faulted claystones to form conical bodies that interconnect across the 700 m thick claystone interval to feed a set of shallow intrusions located within a few hundred meters of the paleoseabed the forcible intrusion of these shallow sandstone bodies produced complementary folding of the contemporaneous seabed the subsequent onlap of these seabed domal folds took place immediately after the intrusion event and by correlation to biostratigraphic data from nearby wells we date these intrusions as late miocene this is the first time that both the age and emplacement depth range of a system of large scale sandstone intrusions have been constrained

pits rifts and slumps the summit structure of piton de la fournaise a clear model of structures and associated stress fields of a volcano can provide a framework in which to study and monitor activity we propose a volcano tectonic model for the dynamics of the summit of piton de la fournaise la reunion island indian ocean the summit contains two main pit crater structures dolomieu and bory two active rift zones and a slumping eastern sector all of which contribute to the actual fracture system dolomieu has developed over 100 years by sudden large collapse events and subsequent smaller drops that include terrace formation small intra pit collapse scars and eruptive fissures are located along the southern floor of dolomieu the western pit wall of dolomieu has a superficial inward dipping normal fault boundary connected to a deeper ring fault system outside dolomieu an oval extension zone containing sub parallel pit related fractures extends to a maximum distance of 225 m from the pit at the summit the main trend for eruptive fissures is n80 normal to the north south rift zone the terraced structure of dolomieu has been reproduced by analogue models with a roof to width ratio of approximately 1 suggesting an original magma chamber depth of about 1 km such a chamber may continue to act as a storage location today the east flank has a convex concave profile and is bounded by strike slip fractures that define a gravity slump this zone is bound to the north by strike slip fractures that may delineate a shear zone the southern reciprocal shear zone is probably marked by an alignment of large scoria cones and is hidden by recent aa lavas the slump head intersects dolomieu pit and may slide on a hydrothermally altered layer known to be located at a depth of around 300 m our model has the summit activity controlled by the pit crater collapse structure not the rifts the rifts become important on the mid flanks of the cone away from pit related fractures on the east flank the superficial structures are controlled by the slump we suggest that during pit subsidence intra pit eruptions may occur during tumescence however the pit system may become blocked and a flank eruption is more likely intrusions along the rift may cause deformation that subsequently increases the slump s potential to deform conversely slumping may influence the east flank stress distribution and locally control intrusion direction these predictions can be tested with monitoring data to validate the model and eventually improve monitoring

experimental modelling of shallow magma emplacement application to saucer shaped intrusions abstract saucer shaped dolerite and sandstone intrusions are common in sedimentary basins world wide we have conducted a series of scaled experiments simulating the process of magma emplacement in sedimentary basins with particular attention on the formation of saucer shaped sills the model materials were 1 cohesive fine grained silica flour representing brittle crust and 2 molten low viscosity oil representing magma the experiments were performed in both homogeneous and layered models in all the experiments oil injection resulted in doming of the surface in the homogeneous models the injected oil formed cone sheets and sub vertical dykes cone sheets formed for shallow injection 1 3 cm and vertical dykes formed for deeper injection 4 5 cm in layered models the injected oil always formed saucer shaped intrusions our experimental results show that 1 sill intrusion results in the formation of a dome with melt erupting at the rim 2 layering controls the formation of sills and saucer shaped sills 3 saucer shaped sills are fed from the bottom and the fluid flows upward and outward and 4 the diameter of saucer shaped sills increase with increasing emplacement depth the systematic relation between domes and sills and the depth dependence of sill diameters show that saucer shaped intrusions result from the interaction between a growing flat lying shallow sill and doming of the free surface we conclude that saucer shaped intrusions represent fundamental geometries formed by shallow magma intrusion in stratified basins

space accommodation by roof lifting during pluton emplacement at amiata italy the amiata area is characterized by an uplift attributed to pluton emplacement during the pliocene epoch the shape of the buried pluton is constrained by gravity data fieldwork was carried out to identify possible deformations within the overburden in order to evaluate the mechanism of emplacement the attitudes of lower pliocene marine sediments deposited immediately before the uplift were measured also the overall pattern of the deposits defines a dome approximately 2 5 km high and 35 50 km wide consistency between these field data and the existing geophysical data suggests that the dome results from the emplacement of a laccolith like intrusion which was responsible for lifting and flexuring the overburden the amiata case shows that roof lifting is a viable process by which space in the upper crust to a depth of 7 km can be accommodated during pluton emplacement

craters of elevation revisited forced folds bulging and uplift of volcanoes the eighteenth nineteenth century craters of elevation theory required magma to uplift strata doming the surface and creating a central down fallen crater or graben exponents of craters of elevation attempted to apply it to explain the origin of all volcanoes and rapidly the contemporary competing craters of eruption theory replaced it as the paradigm for volcano construction several historic examples have shown that intrusions can cause uplift termed bulges and can create features like those proposed for craters of elevation e g at usu 1944 bezymianny 1955 and mt st helens 1980 work on sedimentary basins that have had igneous activity has shown that intrusions create forced folds that uplift and deform strata in a similar way to that originally proposed for craters of elevation in view of the above we investigate large scale intrusion related topographic changes at two sites where the craters of elevation theory was developed the monogenetic volcanoes of the chaine des puys france and the teide stratovolcano tenerife we combine observations of such features with examples of forced folding to integrate the two fields of research our observations in the chaine des puys show that 1 the petit puy de dome has a bulge of up to 150 m uplift the uplift has a central depressed area a graben a dense network of normal faults basal thrusts and an aborted landslide 2 the grosmanaux volcano is a forced fold created by uplift of a previously flat lying area and has dense faulting and a graben on the resultant topographic bulge it was the site also of a major vulcanian eruption from the associated kilian crater 3 the gouttes volcano was uplifted by an intrusion like the petit puy de dome but then collapsed to generate a landslide and lateral blast 4 excavation in the lemptegy volcano exposes intra eruption intrusions with associated uplift providing examples in cross section of the internal deformation likely to be found inside other chaine des puys uplifted bulges on teide a bulge near the summit shows similar structures and surface tilting as seen on the petit puy de dome and this bulging may have formed during the eruption of the lavas negras the most recent activity on the summit area fault scarps on teide also expose small cryptodomes like those seen at lemptegy these examples coupled with field studies on eroded intrusions data on forced folds in basins and analogue models show how large scale topographic remodelling and structural change can be created by intrusions these can rapidly and significantly change the volcanic edifice a crater of elevation bulge or forced fold that is stabilised by the cooling of the intrusion will remain an important structural element in a volcano this process starts even at the small scale of monogenetic volcanoes and could occur through the lifetime of any growing stratovolcano such activity may be commonplace but may be masked by concomitant eruption or removed by subsequent collapse monitoring and hazard strategies should be ready to deal with such large scale events that will seriously modify the eruptive activity and stability of a volcano within days or weeks

domes pits and small chaos on europa produced by water sills pits domes and small chaos on europa s surface are quasi circular features a few to a few tens of kilometers in diameter we examine if injection of water sills into europa s ice shell and their subsequent evolution can induce successive surface deformations similar to the morphologies of these features we study the dynamics of water spreading within the elastic part of the ice shell and show that the mechanical properties of ice exert a strong control on the lateral extent of the sill at shallow depths water makes room for itself by lifting the overlying ice layer and water weight promotes lateral spreading of the sill in contrast a deep sill bends the underlying elastic layer and its weight does not affect its spreading in that case the sill lateral extent is limited by the fracture toughness of ice and the sill can thicken substantially after emplacement cooling of the sill warms the surrounding ice and thins the overlying elastic ice layer as a result preexisting stresses in the elastic part of the ice shell increase locally to the point that they may disrupt the ice above the sill small chaos disruption of the surface also allows for partial isostatic compensation of water weight leading to a topographic depression at the surface pit of the order of 10 2 m complete water solidification finally causes expansion of the initial sill volume and results in an uplifted topography dome of 10 2 m

an experimental investigation of sill formation and propagation in layered elastic media a series of experiments are described where dyed water a magma analogue was intruded into solid gelatine a crustal analogue to investigate the formation of sills we considered a layered gelatine system with contrasting adjacent layers by varying the density and rigidity of the gelatine we found that experimental sills form when the upper layer is more rigid than the lower layer with intrusion occurring in a plane directly below the interface experimental dykes were observed to propagate to the surface when the young s modulus ratio of upper to lower gelatine layers was less than one experimental dyke arrest occurred when the upper layer was more rigid and the interface was strong two varieties of experimental sill formed when the upper layer was more rigid than the lower layer and the interface was sufficiently weak the form of the intrusion depends on the balance of driving pressures and the young s modulus ratio of contrasting adjacent layers when the rigidity ratio is high and there is a large driving pressure the experimental feeder dyke completely converts to propagate as a sill however when the rigidity ratio and driving pressure are both close to one a dyke sill hybrid forms under these conditions the experimental sill formation is accompanied by contemporaneous dyke intrusion into the overlying more rigid layer during sill propagation deformation structures such as faults and en echelon fractures are formed into the lower layer experimental sill propagation dynamics are controlled by viscous dissipation along the length of the sill causing acceleration with increasing length our study suggests that rigidity contrasts may play a major role in the location of sills and development of igneous complexes in ancient cratonic areas the moho is a suitable site for the preferential formation of sills with higher rigidity continental crust overlying weaker mantle mantle plumes impacting ancient continents provide a situation in which large sills can form to fractionate prior to eruption of flood basalts the boundary between the upper and lower crust conrad discontinuity may provide a preferential focus for the emplacement of sheets of silicic magma at continental arcs where the lower crust is weakened by prolonged heating and possible hydration

spreading and potential instability of teide volcano tenerife canary islands 1 we analyze the role of low strength materials in the potential structural instability related to volcano spreading at the active teide stratovolcano tenerife canary islands to study the low strength materials we took advantage of a network of large tunnels within the volcano excavated for water supply which allows the in situ inspection and reconstruction of both volcano and substratum structure we identified two factors which are potentially important to teide s instability 1 a dipping low strength substratum breccia layer and 2 a hydrothermally altered weak core inside the volcano despite we do not find clear structural evidence for volcano substratum spreading both teide topographic shape and the summit faulting are features similar to those accepted as examples of volcano flank spreading over a weak core the detected asymmetric deformation of the edifice makes the north flank of teide a strong candidate for a potential failure

saucer shaped intrusions occurrences emplacement and implications abstract magmatic intrusions and fluidized injected sands represent the two main types of sheet intrusions in the earth s crust in this paper we show that both intrusion types often display a saucer like geometry as revealed by 3d seismic imaging and field observations saucer shaped intrusions are fairly common in sedimentary basins as for example offshore in the norwegian and north sea basins and onshore in the karoo basin of south africa the formation of the saucer geometry is controlled by the low viscosity of the injected fluid and by the interaction between a growing shallow hydraulic fracture and overburden deformations statistics gathered from observations and modelling show a linear relationship between the depth of emplacement and the size of the saucer shaped intrusions we anticipate future cross disciplinary studies aiming to discover other occurrences of saucer shaped intrusions and to identify the physical processes controlling the development of this fundamental geometry

cyclic caldera collapse piston or piecemeal subsidence field and experimental evidence many multicycle caldera volcanoes display a complex extracaldera fault system genetically linked to caldera evolution on gran canaria canary islands the miocene tejeda caldera was filled by ignimbrites and epiclastic sediments and intruded by syenites and a major cone sheet swarm obscuring intracaldera structural relations extracaldera radial and circumferential faults and dikes are well exposed we used field data and experiments scaled to the island of gran canaria to clarify the poorly understood structural temporal and genetic relationship of peripheral faults to the central caldera the experimental setup comprises a cone of medium grained sand in which a rubber balloon was repeatedly inflated and deflated balloon inflation resulted in updoming and a dominantly radial fault pattern whereas balloon deflation caused piecemeal caldera collapse and a concentric peripheral fault arrangement on the edifice flanks a complex interplay of repeated inflation and deflation cycles of the experimental magma chamber is found to explain the peripheral fault system of the tejeda caldera on gran canaria and offers insight into the structure of the caldera floor and the mechanisms acting in multicycle caldera volcanoes

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

aluminium mobilization in soils and waters the toxicity of dissolved aluminium to many plant and animal species is one of the major deleterious effects of the acidification of the environment al mobilization from insoluble forms in minerals and rocks is governed mainly by the ph of the solution surrounding weathering minerals to a first approximation the solubility product of gibbsite gives an indication of al concentrations expected in solution secondary silicate minerals such as kaolinite imogolite and allophane and sulphate minerals such as alunite maintain lower al concentrations in solution than does gibbsite particularly at ph 5 the cation exchange system of acid soils provides a large reserve of ionic al which can be brought into solution when soluble salts percolate through soils ligands such as fluoride and organic anions which form soluble al complexes combine with much of the al in solution when they are present in suitable concentrations and therefore maintain higher concentrations of al than might be expected from mineral solubility equilibria particularly at ph 5 7 slow reactions involving solid phases and uncertain data for possible al species present limit the rigorous application of equilibrium thermodynamics to these systems but the factors mentioned provide a reasonable understanding of the major features of aluminium mobilization