petrology of the great abitibi dyke superior province canada the great abitibi dyke gad which can be traced northeast for 700 km across the abitibi belt in the southeastern superior province of the canadian shield is composed of olivine gabbro to monzodiorite weakly saturated to undersaturate d in silica all rocks of the gad can be derived by mainly plagioclase and olivine fractionation from a parental magma corresponding in composition to chilled margin samples two units can be distinguished a marginal unit unit 1 representing 0 50 crystallization and a central unit unit 2 found over about half of the dyke length representing 50 70 crystallization modelling using pearce elemental ratio analysis quantifies the fractionation history and allows mass balance calculations over the present exposure level of the dyke the approximate balance between the amounts of cumulate and fractionated rocks suggests that unit 1 differentiated essentially in situ as a closed system in contrast unit 2 rocks were formed by loss of substantial plagioclase and olivine from the parental magma this fractionate must have either been lost to depth or left behind in an external chamber unit 2 rocks show depletion in plagioclase and enrichment in mafic minerals along strike towards the southwest deeper exposure level a trend explained by density stratification regional variation in unit 1 chemistry is interpreted in terms of lateral magma injection towards the northeast from the locus of keweenawan rift volcanism feldspar olivine and augite compositions are linearly correlated with equilibrium temperature and extent of magma evolution the parent magma had a trace element chemistry corresponding to a within plate setting and was probably derived from an incompatible element enriched mantle similar to the source for ocean island basalts oibs

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

controlling factors on earthquake swarms associated with magmatic intrusions constraints from iceland abstract the currently two most important surveying tools in volcano monitoring are observations of seismicity and surface deformation magma migration within the earth s crust is frequently associated with seismic activity often occurring as distinct earthquake swarms understanding the exact nature and significance of these swarms is important when evaluating volcanic crisis situations there seems however to be no general correlation between the amount of seismic energy release and the rate and volume of magma on the move which may complicate an immediate risk evaluation it has previously been shown that stressing rate appears as the controlling factor on the occurrence of seismicity during intrusion however in this paper we emphasize that this is only true under specific circumstances other factors influence the evolution and resulting seismic energy release and each intrusion scenario needs to be evaluated separately three icelandic cases where seismic swarms have been confirmed through deformation measurements to be related to magmatic movements were selected we show how the relationship between volume change and resulting seismicity varies greatly between cases and suggest that in a general approach the most important factor governing the level of seismic energy release is the background stress state tectonic setting regional stresses and tectonic history are therefore all of great importance when evaluating a magmatically induced seismic crisis the stressing rate only plays the dominant role when either a stress in the intruded area is close to the yield strength before intrusion or possibly b when the intrusion is extraordinarily voluminous and rapid

volcanic spreading at kilauea 1976 1996 the rift system traversing about 80 km of the subaerial surface of kilauea volcano has extended continuously since the m 7 2 flank earthquake of november 1975 widening across the summit has amounted to more than 250 cm decelerating after 1975 from about 25 to 4 cm yr 1 since 1983 concurrently the summit has subsided more than 200 cm even as the adjacent south flank has risen more than 50 cm the axes of the upper zones about 10 km from the summit subsided before 1983 at average rates of 9 and 4 cm yr 1 respectively and at rates of 4 and 3 cm yr 1 since the middle southwest rift zone is also subsiding and at the other end of kilauea s subaerial rift system subsidence along the lower east rift zone has averaged 1 2 cm yr 1 deformation of kilauea s south flank has been continuous although subject as well to displacements caused by major rift zone seismic swarms whereas horizontal strains across the subaerial south flank seem to have been generally compressive after 1975 they have been extensional since about 1980 or 1981 interrupted only by the east rift zone dike intrusion of 1983 because the magnitudes of these contractions and extensions are much less than the extension across the rift system the subaerial south flank is apparently sliding seaward on its basal decollement more than it is accumulating horizontal strains within the overlying volcanic pile kilauea suffers from gravitational spreading made even more unstable by accumulation of magma along the rift system at depths in excess of about 4 5 km in the presence hot rock incapable of withstanding deviatoric stresses this seismicly quiescent zone decouples the south flank from the rest of hawaii s volcanic edifice the rift zones at lesser depths exhibit a more brittle and therefore sporadic extensional behavior judging from the modern extension record of the summit which both predates the m 7 2 earthquake of 1975 and has outlived its 10 year period of aftershocks kilauea will continue to spread along its rift system as its south flank slips seaward to accommodate the accretion of magma and its relatively dense olivine rich differentiate

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

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

timing of emplacement of the muskox intrusion constraints from coppermine homocline cover strata the muskox intrusion is a middle proterozoic shallow level mafic ultramafic layered intrusion in the northwest corner of the bear province of the canadian shield the intrusion was emplaced into the lower proterozoic akaitcho group and hepburn metamorphic plutonic complex of the wopmay orogen and overlying middle proterozoic coppermine homocline strata to date radiometric paleomagnetic and geochemical evidence has produced only equivocal results concerning the age of the muskox intrusion relative to overlying coppermine river group basalts and mackenzie diabase dykes stratigraphic redefinition of and structural relationships to the cover sequence of the intrusion provide the best available evidence for the age of the muskox intrusion although previously inferred to be a pre dismal lakes group structure the canoe lake fault is here shown to cross cut dismal lakes strata with west side down displacement equivalent to that documented to the south where it offsets the intrusion this relationship suppo

time dependent dike propagation from joint inversion of seismicity and deformation data dike intrusions both deform earth s surface and induce propagating earthquake swarms we develop methods to utilize both deformation and seismicity from brittle volcano tectonic earthquakes to image time dependent dike propagation dieterich s 1994 seismicity rate theory is used to relate dike induced stress changes to seismicity rate and is combined with elastic green s functions relating dike opening to deformation different space time patterns of seismicity develop if earthquakes occur at the same depth as the dike compared to above below the dike in the former seismicity initiates near the dike s leading edges but shuts off as the dike tips pass and seismogenic volumes fall into stress shadows in the latter seismicity continues at a decaying rate after the tips pass we focus on lateral propagation and develop a nonlinear inversion method that estimates dike length and pressure as a function of time the method is applied to the 2007 father s day intrusion in kilauea volcano seismicity is concentrated at 3 km depth comparable to geodetic estimates of dike depth and decays rapidly in time with lateral propagation only and a vertical dike tip line it is difficult to fit both gps data and the rapid down rift jump in seismicity suggesting significant vertical propagation for the events to have occurred below the dike requires a very short aftershock decay time hence unreasonably high background stressing rate the rapid decay is better explained if the dike extends somewhat below the seismicity we suggest that joint inversion is useful for studying the diking process and may allow for improved short term eruption forecasts

spongy pahoehoe in hawaii a study of vesicle distribution patterns in basalt and their significance a well defined pahoehoe lava type that is very common medially and distally in hawaii is characterized by a high concentration and fairly uniform distribution of spherical or near spherical vesicles measurements of vesicle sizes and concentrations have been made on ten of these spongy pahoehoe lava flow units the vesicles increase in size toward the middle accompanied by a moderate increase in lava porosity the close approach to bilateral symmetry on either side of the horizontal median plane and the common occurrence of a median gas blister shows that no significant upward movement of vesicles occurred suggesting that the lava possessed a yield strength and was more or less static olivine phenocrysts when present are however concentrated in the lower half of the same flow units showing that the lava previously lacked a significant yield strength the vesicles are regarded as early inherited from the vent but the size characteristics of the vesicle population are a late formed feature vesicles grew in static lava mostly by coalescence and it is postulated that coalescence was promoted by the presence of abundant diktytaxitic voids which punctured the walls of contiguous vesicles zones in which the vesicle concentration is lower and the vesicles are larger and strongly deformed interrupt the symmetry of some spongy pahoehoe units and gas blisters higher than the median plane occur in many examples these zones are interpreted to result from late stage shearing and point to a mechanism by which vesicles may be eliminated from a lava

evolution of dike opening during the march 2011 kamoamoa fissure eruption kīlauea volcano hawai i 1 the 5 9 march 2011 kamoamoa fissure eruption along the east rift zone of k lauea volcano hawai i followed months of pronounced inflation at k lauea summit we examine dike opening during and after the eruption using a comprehensive interferometric synthetic aperture radar insar data set in combination with continuous gps data we solve for distributed dike displacements using a whole k lauea model with dilating rift zones and possibly a deep decollement modeled surface dike opening increased from nearly 1 5 m to over 2 8 m from the first day to the end of the eruption in agreement with field observations of surface fracturing surface dike opening ceased following the eruption but subsurface opening in the dike continued into may 2011 dike volumes increased from 15 to 16 to 21 million cubic meters mcm after the first day eruption end and 2 months following respectively dike shape is distinctive with a main limb plunging from the surface to 2 3 km depth in the up rift direction toward k lauea s summit and a lesser projection extending in the down rift direction toward pu u at 2 km depth volume losses beneath k lauea summit 1 7 mcm and pu u 5 6 mcm crater relative to dike plus erupted volume 18 3 mcm yield a dike to source volume ratio of 2 5 that is in the range expected for compressible magma without requiring additional sources inflation of k lauea s summit in the months before the march 2011 eruption suggests that the kamoamoa eruption resulted from overpressure of the volcano s magmatic system

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

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

mechanical models for dikes a third school of thought abstract geological and geophysical data from continental volcanic centers and giant radial swarms and from oceanic shield volcanoes and rift zones indicate that dikes propagate laterally for distances that can be 10 to over 100 times their height what traps dikes within the shallow lithosphere and promotes these highly eccentric shapes gravity induced stress gradients in the surrounding rock and pressure gradients in the magma are the primary loading mechanisms pressure gradients due to magma flow are secondary to insignificant because the flow direction is dominantly horizontal this configuration of vertical blade shaped dikes with horizontal dike propagation and magma flow is fundamentally different from the two dike model configurations described in a recent review paper as two schools of thought for mechanical models of dikes in school i a dike is disconnected from its source and ascends under the influence of buoyancy in school ii a dike is connected to a magma reservoir and is driven upward by magma flux from the source we review the geological and geophysical data supporting the vertical dike horizontal flow propagation configuration and suggest the abundance and veracity of these data in many different geological settings and the modeling results that address this physical process warrant adding this as a third school of thought a new analytical solution for the boundary value problem of a homogeneous isotropic and linear elastic solid with a vertical fluid filled crack is used to investigate the effects of gravitationally induced stress and pressure gradients on the aperture distribution dike tip stress intensity and stable height model results indicate that in a homogeneous crust dikes can achieve stable heights greater than a kilometer only if the host rock fracture toughness k ic 100 mpa m 1 2 however density stratification of the crust is an effective mechanism for trapping kilometer scale dikes even if the host rock is very weak k ic 0 this analysis may explain why vertical dikes propagate laterally for great distances but reside within a narrow range of depths in the crust

neutral buoyancy and the structure of mid ocean ridge magma reservoirs the mechanical structure of the regions for shallow storage and lateral intrusion of mid ocean ridge basalt morb is explored in part by examining the density range appropriate for picritic to tholeiitic liquids and liquid crystal mixtures and its relationship to the nonlinear density depth structure of the ridge magma density spans the range 2 6 to at least 2 82 mg m 3 and incorporates the effects of fractional crystallization on the density evolution the incorporation of olivine phenocrysts and the contributions of water in melt phase density reductions the melt and melt crystal density band crosses the in situ rock density curve over the 1 to 3 km depth interval beneath the volcanic surface within that averaged interval picritic to tholeiitic melt and melt crystal mixtures are in approximate mechanical equilibrium with their surroundings and the interval is referred to as the morb horizon of neutral buoyancy hnb the detailed locations of the transition regions between the negative and neutral buoyancy and between the neutral and positive buoyancy regimes are dependent on melt phase composition as well as the presence of suspended mineral phases and their identity in the east pacific rise the hnb has a 1 1 correspondence with the inferred location of the sheeted dike complex and the seismically detected minimum compressional wave velocity region in ophiolite complexes the region corresponds to the sheeted dike complex and the uppermost isotropic gabbros above the hnb is the region of negative buoyancy seafloor to an averaged 1 km depth within which negative buoyancy forces may induce the descent of nonvesiculated melt this region corresponds to the pillow basalt sediment sequence the region of positive buoyancy extends from 3 km to the depth of melt generation and picritic melt ascends in dikes and veins of finite height 500 m to 11 km and relatively narrow width 20 mm to 100 cm the finite strengths of high temperature fluid weakened rocks mandate finite height melt parcels along the ascent pathway the magmatic source region is therefore never hydraulically connected with the surface nor with the near surface reservoir thus relatively locally integrated buoyancy has a natural role in the ascent of melt from the source region to the region of neutral buoyancy during the final stages of magma ascent hydrothermal fluid circulation enshrouds the hnb in a three dimensional envelope as suggested by the depth extent of alteration mineralogy in ophiolite complexes while hydrothermal cooling may play an important role in arresting the ascent of magma in very low spreading rate environments gravitational equilibrium represents the effectively dominant theme in intermediate and high spreading rate regions such as the east pacific rise elastic crack penetration of rheological transition zones near the hnb identify modes of crack tip morphologic changes as a function of young s modulus ratios e2 e1 above and beneath the transition and affect the style of intrusion while melt buoyancy determines the ultimate equilibrium resting place elastic crack stability considerations relate the vertical magma pressure gradients for tholeiitic pt and picritic pp magmas to the vertically varying gradient in the horizontal stress component h and suggest further subdivision into two horizons of neutral buoyancy one for tholeiitic magma hnbt and one for the picritic liquid olivine mixtures hnbp these intervals are very approximate but are estimated as 600 m to 1400 m and 1400 to 3000 m depth respectively beneath the east pacific rise axis as oceanic lithosphere is created at the rise axis and spreads laterally the horizons of negative buoyancy and neutral buoyancy ride with it and may continue to modulate the subsurface magma dynamics that influence off axis volcanism

a hotspot model for iceland and the voring plateau the proximity of a hotspot to a spreading center may result in the channeling of atmosphere to the section of rise crest closest to the hotspot this produces more basalt and thicker crust at these locations thus forming a plateau over time the geometric constraints of such a model predict a unique orientation location and age progression for a plateau formed by this mechanism the hotspot will channel material to the closest part of the rise therefore the orientation of the plateau will differ from that of a hotspot track by the component of absolute motion perpendicular to the rise axis the plateau will be symmetric with respect to the location of the rise axis at the time of formation also the age progression of the plateau will be contemporaneous with the age of formation of the seafloor on either side because the plateau is seafloor just with thicker crust a set of reconstructions based upon magnetic isochrones and a fixed hotspot reference frame is presented for the norwegian greenland sea as a means of evaluating the model s predictions by locating the iceland hotspot reconstructing the relative positions of the greenland and european plates and then assuming material would be channeled from the hotspot to the closest section of the rise crest we can trace the tectonic evolution of the greenland faeroe and voring plateaus the model is able to locate the plateaus explain their orientations and predict an age progression that satisfies observed age determinations the analysis demonstrates that both plateaus could have been formed by the iceland hotspot with the greenland faeroe plateau being in effect a continuation of the voring plateau which was cut off due to transform motion between the northern and southern spreading centers

rapid deformation of kilauea volcano global positioning system measurements between 1990 and 1996 campaign global positioning system gps measurements from 1990 to 1996 are used to calculate surface displacement rates on kilauea volcano hawaii the gps data show that the south flank of the volcano which has generated several large earthquakes in the past 3 decades is displacing at up to 8 cm yr to the south southeast the summit and rift zones are subsiding with maximum subsidence rates of 8 cm yr observed a few kilometers south of the summit caldera elastic dislocation modeling of the gps data suggests that the active sources of deformation include deep rift opening along the upper east and east rift zone fault slip along a subhorizontal fault near the base of the volcano and deflation near the summit caldera a nonlinear optimization algorithm was used to explore the parameter space and to find the best fitting source geometry there is a broad range of model geometries that fit the data reasonably well however certain models can be ruled out including those that have shallow rift opening or shallow fault slip some offshore aseismic slip on a fault plane that dips between 258 north northwest and 88 south southeast is required best fitting slip and rift opening rates are 23 28 cm yr although rates as low as 10 cm yr are permitted by the data

the nature of mis 3 stadial interstadial transitions in europe new insights from model data comparisons 15 abrupt warming transitions perturbed glacial climate in greenland during marine isotope stage 3 mis 3 60 27 ka bp one hypothesis states that the 8 16 degrees c warming between greenland stad

multiple melt injection along a spreading segment at askja iceland lower crustal earthquakes 12 25 km depth have been detected since august 2005 in the askja volcanic system along the north iceland rift in the normally ductile part of the crust the earthquakes occur in three clusters which have stable dimensions and locations through time and are interpreted as positions of repeated melt supply from the mantle to the lower crust seismic velocity vp vs ratios are consistent with the presence of partial melt in the lower crust at askja the spatial separation of the clusters shows that there are multiple positions of melt injection within this one magmatic segment and all three positions are currently active this pattern of melt supply is more like that observed on fast spreading ridges than slow spreading ridges and is probably a consequence of the increased melt production beneath iceland compared to the rest of the mid atlantic ridge however the relative number of earthquakes in each cluster shows that two thirds of the melt is supplied to the central volcano askja i e segment center during the last major rifting episode shallow lateral melt migration occurred from the magma chamber beneath the volcano therefore on long time scales melt supply is probably greater at the segment center with melt redistribution in the upper crust even though there are multiple points of lower crustal injection along the segment

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

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

getting granite dikes out of the source region whether a dike can propagate far from a magma reservoir depends upon the competition between the rate at which propagation widens the dike and the rate at which freezing constricts the aperture available for magma flow various formulations are developed for a viscous fluid at temperature t sub m intruding a growing crack in an elastic solid the initial solid temperature equals t sub m at the source and decreases linearly with distance from the source if t sub m is the unique freezing temperature of the fluid dike growth is initially self similar and an essentially exact solution is obtained if t sub m is above the solidus temperature the solution is approximate but is designed to overestimate the distance the dike may propagate the ability of a dike to survive thermally depends primarily upon a single parameter that is a measure of the ratio of the dike frozen margin thickness to elastic thickness perhaps more intuitively one may define a minimum distance from the essentially solid reservoir wall to the point at which the host rock temperature drops below the solidus necessary for dikes to propagate far into subsolidus rock it is concluded that for reasonable material properties and source conditions most basalt dikes will have little difficulty leaving the source region but most rhyolite dikes will be halted by freezing soon after the magma encounters rock at temperatures below the magma solidus while these results can explain why granitic dikes are common near granitic plutons but rare elsewhere the potentially large variation in magmatic systems makes it premature to rule out the possibility that most granites are transported through the crust in dikes nonetheless these results highlight difficulties with such proposals and suggest that it may also be premature to rule out the possibility that most granite plutons ascend as more equidimensional bodies

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

triggered earthquakes suppressed by an evolving stress shadow from a propagating dyke the role of static versus dynamic stresses in earthquake clusters is unclear analysis of earthquakes triggered by a dyke intrusion at an icelandic volcano unambiguously demonstrates that static stresses are important for earthquake clustering large earthquakes can generate small changes in static stress increases that trigger aftershock swarms or reductions that create a region of reduced seismicity a stress shadow1 2 however seismic waves from large earthquakes also cause transient dynamic stresses that may trigger seismicity3 4 this makes it difficult to separate the relative influence of static and dynamic stress changes on aftershocks dyke intrusions do not generate dynamic stresses so provide an unambiguous test of the stress shadow hypothesis here we use gps and seismic data to reconstruct the intrusion of an igneous dyke that is 46 km long and 5 m wide beneath bar arbunga volcano central iceland in august 2014 we find that during dyke emplacement bursts of seismicity at a distance of 5 to 15 km were first triggered and then abruptly switched off as the dyke tip propagated away from the volcano we calculate the evolving static stress changes during dyke propagation and show that the stressing rate controls both the triggering and then suppression of earthquake rates in three separate areas adjacent to the dyke our results imply that static stress changes help control earthquake clustering similar small static stress changes may be important for triggering seismicity near geothermal areas regions being hydrofractured and deflating oil and gas fields

geochemistry and petrogenesis of basaltic rocks from coppermine river area northwest territories proterozoic basaltic flows 2000 m thick and associated dykes and sills from the coppermine river area northwest territories have chemical compositions typical of continental tholeiites the low mg fe ratio and abundances of ni and cr indicate that the lavas were extensively fractionated prior to extrusion the variations of incompatible elements such as k rb ree y zr nb and th suggest that the rocks were affected by interaction with continental crust the samples least affected by contamination have trace element compositions very similar to those of p type mid ocean ridge basalts it is suggested that continental tholeiites have been generated from the same source as p type oceanic tholeiites and geochemical features such as the enrichment of some lithophile elements in many of these rocks may be related to crustal contamination the variations within the volcanic pile of the coppermine river area are related to those of an exposed part of the muskox layered intrusion

spatial and temporal variations of the surface albedo and other factors influencing urumqi glacier no 1 in tien shan china glacier albedo controls the surface energy budget the variability of which affects the glacier surface melt rate and in turn impacts the mass balance of the glacier during 2013 and 2014 spatial and temporal variations of albedo were investigated using 18 landsat images of urumqi glacier no 1 factors influencing these spatiotemporal profiles were analyzed an established retrieval process including geolocation radiometric calibration atmospheric topographic and anisotropic correction and narrow to broadband conversion was applied for the first time to landsat 8 images differences between landsat image derived albedo values and albedo values measured using a handheld spectroradiometer ranged from 0 024 to 0 049 spatial and temporal variations of surface albedo were significant especially in the ablation area the variability of the values of ice albedo ranged from 0 06 to 0 44 due to topographic effects and light absorbing impurities the results suggest that this retrieval method can be used to investigate the spatial and temporal variability of surface albedo from landsat 8 images on mountain glaciers moreover as constant albedo values for ice and snow cannot be assumed the distribution of albedo was not completely dependent on altitude under conditions of more intense ablation and by reason of light absorbing impurities during the melt season

aspect ratios and magma overpressures of non feeder dikes observed in the miyake jima volcano japan and fracture toughness of its upper part 1 we present a new method for estimating the length and maximum thickness aperture of a dike from the observed opening at one dike tip we apply the method to 15 arrested non feeder dikes where the upper tip is known the lower tip unknown in the caldera walls of miyake jima japan to estimate the length thickness ratio as well as the magma overpressure and fracture toughness the calculated length thickness ratio ranges from 61 to 246 with an average of 136 the ratios are low because the dikes are emplaced close to the surface in comparatively compliant soft rocks using these ratios and the appropriate elastic constants the calculated magmatic overpressures of the dikes are between 2 3 and 8 9 mpa and the stress intensity factors between 38 and 117 mpa m1 2 all these values are within the range of typical in situ estimates supporting the validity of this new method

correction to modeling the growth of laccoliths and large mafic sills role of magma body forces in the paper modeling the growth of laccoliths andlarge mafic sills role of magma body forces by andrew p bunger and alexander r cruden journal of geophysicalresearch 116 b02203 doi 10 1029 2010jb007648 thetwo sentences preceding equation 14 should be replacedwith finally equation 10 shows that the quantity p

what controls the along strike slopes of volcanic rift zones we investigate the dynamics of viscous pressure losses associated with lateral magma transport in volcanic rift zones by performing 1 coupled elastic hydrodynamic simulations of downrift magma flow in dikes and 2 analog experiments mimicking lateral dike propagation in the presence of an along rift topographic slope it is found that near source eruptions are likely to be favored by shallow slopes while distant downrift eruptions may be encouraged by steeper slopes provided that along rift variations in the tectonic stress are negligible or uncorrelated on the timescale of multiple dike intrusions this implies the existence of a critical slope to which a volcanic rift zone would naturally evolve such behavior is produced by three dimensional 3 d elastic effects and is controlled by the ratio of the driving pressure gradient due to the along strike topographic slope to the vertical gradient in the excess magma pressure in the dike this model may be viewed as complementary to commonly cited mechanisms that appeal to magma viscosity and the dynamics of freezing of lava flows at the surface to explain the low profiles of basaltic shield volcanoes our estimated values of the critical slopes are in general agreement with observations in hawaiian rift zones but further development of fully 3 d models is required for more accurate predictions

the krafla fissure swarm iceland and its formation by rifting events fissure swarms at divergent plate boundaries are activated in rifting events during which intense fracturing occurs in the fissure swarm accompanied by intrusion of magma to form dikes that sometimes lead to eruptions to study the evolution of fissure swarms and the behaviour of rifting events detailed mapping was carried out on fractures and eruptive fissures within the krafla fissure swarm kfs fracture densities of dated lava flows ranging from 10 000 years bp to 30 years old were studied and the fracture pattern was compared with data on the historical m vatn rifting episode 1724 1729 and the instrumentally recorded krafla rifting episode 1975 1984 additionally the interaction of transform faults and fissure swarms was studied by analysing the influence of the husavik transform faults on the kfs during the historical rifting episodes eruptions on the fissure swarm occurred within 7 km from the krafla central volcano although faults and fractures were formed or activated at up to 60 70 km distance this is consistent with earlier rifting patterns as holocene eruptive fissures within the kfs are most common closer to the central volcano most fractures within the central krafla caldera are parallel to the overall orientation of the fissure swarm this suggests that the regional stress field is governing in the krafla central volcano while the local stress field of the volcano is generally weak a sudden widening of the graben in the northern kfs and a local maximum of fracture density at the junction of the kfs and the extrapolation of the husavik transform fault zone indicates possible buried continuation of the husavik transform fault zone which extends to the kfs eruptive fissures are found farther away from the krafla central volcano in the southern kfs than in the northern kfs this is either due to an additional magma source in the southern kfs the hei arspor ur volcanic system or caused by the husavik transform faults transferring some of the plate extension in the northern part fracture density within particular lava flow fields increases with field age indicating that repeated rifting events have occurred in the fissure swarm during the last 10 000 years bp the fracture density in the kfs is also generally higher closer to the krafla central volcano than at the ends of the fissure swarm this suggests that rifting events are more common in the parts of the fissure swarm closer to the krafla central volcano

jointing around magmatic dikes as a precursor to the development of volcanic plugs basaltic volcanic eruptions typically initiate as fissures fed by dikes in some cases eruptions become localized through discrete vents until the flow is sustained through a single vent underlain by a volcanic plug at which point the eruption is capable of delivering much greater volumes of magma to the surface existing theories to explain flow localization focus on the relationship between non uniform dike thickness and variations in magma flow rates here we propose a mechanism for conduit widening that precedes flow localization and we present geologic evidence from an oligocene aged dike plug system at ship rock new mexico we use techniques of structure from motion to map 621 joints in the sedimentary rock adjacent to the intrusions and find that these joints are part of a systematic set oriented perpendicular to the dikes and localized to within a meter of the dike contacts this joint set along with a set of dike parallel joints and the bedding planes divides the strata into rectangular blocks that subsequently are entrained in the flowing magma to widen the dikes using field and lab data and a stress analysis we show that the dike perpendicular joints are intimately associated with the emplacement of magma and thermal pressurization of pore fluids in the host rock this fracturing process is a precursor to entrainment of the host rock and sustained flow through discrete vents

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

numerical modelling of dyke interaction and its influence on oceanic crust formation abstract dykes are an essential element in building oceanic crust most prominent in sheeted dyke complexes in the upper crust since dykes alter the magnitude and orientation of the local stress field they cannot be treated as passive infillings of extensional fractures we use a quasi static iterative 2 d boundary element method allowing for a wholesale movement of fluid filled fractures effects of stress and pressure gradients buoyancy and enclosed fluid mass are considered the implications of the dyke induced stress field are analysed combining the simulation of fracture propagation with computation of dyke interaction dyke interaction occurs by the adaptation of ascending dykes to the stress field caused by previous fractures arrested in the crust and leads to focussing and crossing of dykes examples for applications are introduced concerning e g the generation of a magma chamber and the formation of the sheeted dyke complex our main results are that the interaction between dykes can be considerable and that the most important controlling factor is stress the interaction is small when the horizontal tensional stress is large compared to the pressure in the dyke head otherwise dykes tend to attract each other and to form centres of high dyke density or sill layers

dynamics of dikes versus cone sheets in volcanic systems igneous sheet intrusions of various shapes such as dikes and cone sheets coexist as parts of complex volcanic plumbing systems likely fed by common sources how they form is fundamental regarding volcanic hazards yet no dynamic model simulates and predicts satisfactorily their diversity here we present scaled laboratory experiments that reproduced dikes and cone sheets under controlled conditions our models show that their formation is governed by a dimensionless ratio 1 which describes the geometry of the magma source and a dynamic dimensionless ratio 2 which compares the viscous stresses in the flowing magma to the host rock strength plotting our experiments against these two numbers results in a phase diagram evidencing a dike and a cone sheet field separated by a sharp transition that fits a power law this result shows that dikes and cone sheets correspond to distinct physical regimes of magma emplacement in the crust for a given host rock strength cone sheets preferentially form when the source is shallow relative to its lateral extent orwhen the magma influx velocity or viscosity is high conversely dikes form when the source is deep compared to its size or when magma influx rate or viscosity is low both dikes and cone sheets may form fromthe same source the shift fromone regime to the other being then controlled by magma dynamics i e different values of 2 the extrapolated empirical dike to cone sheet transition is in good agreement with the occurrence of dikes and cone sheets in various natural volcanic settings

dike emplacement at bardarbunga iceland induces unusual stress changes caldera deformation and earthquakes a 45 km long regional dike was emplaced over a period of 2 weeks in august 2014 at the boundary between the east and north volcanic zones in iceland this is the first regional dike emplacement in iceland monitored with modern geophysical networks the importance of which relates to regional dikes feeding most of the large fissure e g eldgja 934 and laki 1783 and lava shield e g early holocene skjaldbreidur and trolladyngja eruptions during this time the dike generated some 17 000 earthquakes more than produced in iceland as a whole over a normal year the dike initiated close to the bardarbunga volcano but gradually extended to the northeast until it crossed the boundary between the east volcanic zone evz and the north volcanic zone nvz we infer that the strike of the dike changes abruptly at a point from about n45 e coinciding with the trend of the evz to n15 e coinciding with the trend of the nvz this change in strike occurs at latitude 64 7 exactly the same latitude at which about 10 ma dikes in east iceland change strike in a similar way this suggests that the change in the regional stress field from the southern to the northern part of iceland has been maintained at this latitude for 10 million years analytical and numerical models indicate that the dike induced stress field results in stress concentration around faults and particularly shallow magma chambers and calderas in its vicinity such as tungnafellsjokull kverkfjoll and askja in particular the dike has induced high compressive shear and tensile stresses at the location of the bardarbunga shallow chamber and caldera ring fault where numerous earthquakes occurred during the dike emplacement many of which have exceeded m5 the largest m5 7 the first segment of the dike induced high tensile stresses in the nearby part of the bardarbunga magma chamber ring fault resulting in radially outward injection of a dike from the chamber at a high angle to the strike of the regional dike the location of maximum stress at bardarbunga fluctuates along the chamber ring fault boundary in harmony with dike size and or pressure changes and encourages ring dike formation and associated magma flow within the chamber caldera collapse and or eruption in some of these volcanoes is possible most likely in bardarbunga but depends largely on the future development of the regional dike

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

spatiotemporal evolution of dike opening and decollement slip at kīlauea volcano hawai i 1 rapid changes in ground tilt and gps positions on k lauea volcano hawai i are interpreted as resulting from a shallow two segment dike intrusion into the east rift zone that began at 1217 utc 0217 hst on 17 june 2007 and lasted almost 3 days as a result of the intrusion a very small volume of basalt about 1500 m3 erupted on 19 june northward tilt at a coastal tiltmeter subsidence of south flank gps sites southeastward displacements at southwestern flank gps sites and a swarm of flank earthquakes suggest that a slow slip event occurred on the decollement beneath k lauea s south flank concurrent with the rift intrusion we use 4 min gps positions that include estimates of time dependent tropospheric gradients and ground tilt data to study the spatial and temporal relationships between the two inferred shallow steeply dipping dike segments extending from the surface to about 2 km depth and decollement slip at 8 km depth we invert for the temporal evolution of distributed dike opening and decollement slip in independent inversions at each time step using a nonnegative least squares algorithm on the basis of these inversions the intrusion occurred in two stages that correspond spatially and temporally with concentrated rift zone seismicity the dike opening began on the western of the two segments before jumping to the eastern segment where the majority of opening accumulated dike opening preceded the start of decollement slip at an 84 confidence level the latter is indicated by the onset of northward tilt of a coastal tiltmeter displacements at southwest flank gps sites began about 18 h later and are interpreted as resulting from slow slip on the southwestern flank additional constraints on the evolution of the intrusion and decollement slip come from inversion of an envisat interferogram that spans the intrusion until 0822 utc on 18 june 2007 combined with gps and tilt data this inversion shows that up to 0822 utc on 18 june decollement slip is only required in a limited region offshore of ka ena point a similar inversion of the complete event which includes gps and tilt data up to 21 june and a second envisat interferogram spanning the complete intrusion until 21 june shows decollement slip spread westward across the south flank this may suggest westward migration of the decollement slip as the event progressed

dyke emplacement in a layered and faulted rift zone abstract in rift zone eruptions magma is normally transported to the surface through dykes most injected dykes however do not make it to the surface but rather become arrested at certain depths in the rift zone surface deformation studies are routinely used to infer the depths to the tips as well as the geometries of arrested dykes most such studies model the rift zone as a homogeneous isotropic elastic half space all rift zones however contain normal faults and are composed of rock layers with different mechanical properties the numerical models presented here indicate that when the upper tip of a vertically propagating dyke reaches the same crustal level as the bottom tips of the boundary faults of an active graben the faults close as the tip continues its propagation up into the graben the overpressured dyke encourages reverse slip on the boundary faults the present numerical models also indicate that mechanical layering has strong effects on dyke emplacement and the surface deformation associated with arrested dykes in rift zones in particular soft low young s modulus layers and weak contacts at shallow depths contribute to offset arrest and aperture thickness variations of the dykes and suppress the dyke induced surface stresses and associated brittle deformation when the magmatic overpressure of a dyke is the only loading the dyke tends to be thicker inside soft layers than inside stiff high young s modulus layers soft layers and weak contacts discontinuities common at shallow depths in rift zones suppress dyke tip tensile stresses thereby encouraging dyke arrest and may result in little if any surface stresses and brittle deformation being induced by an arrested dyke weak contacts also encourage transfer of the dyke induced surface tensile stresses to the regions above the lateral ends of the contacts then the tensile stresses at the rift zone surface in a large area above the dyke itself become small or negligible all the tensile stress occur above the lateral ends of the contact which may be many kilometres from the dyke tip for a dyke arrested in a layered rift zone straightforward inversion of surface deformation data may therefore yield geometric results that have little relation to the actual geometry of the arrested dyke the results of the numerical models presented here are generally supported by field observations of dykes in rift zones