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

development of a subvolcanic structure by the interaction of liquid filled cracks abstract the interaction of liquid filled cracks may be an important process for magma accumulation and the development of subvolcanic structures the horizontal interaction was studied through the stress analyses around pressured cracks in various regional stress fields the vertical interaction was studied using gelatin experiments buoyaney driven cracks coalesce more easily than pressure cracks the theory of the three dimensional crack interaction was discussed there are two types of crack interaction the interaction between liquid filled cracks may cause coalescence of cracks the interaction between a liquid filled crack and a solid filled crack never causes coalescence the stress relaxation by ductile or brittle behavior or by the driving out of magma decreases the local stress field generated by the accumulation of magma filled cracks as the regional differential stress increases and the minimum compressive principal stress becomes tensile magma filled cracks become parallel to each other as the supply rate of magma filled cracks becomes relatively large or as the regional differential stress decreases and the minimum compressive stress increases the arrangement of magma filled cracks varies from an offset pattern to a radial one because the local compressive stress field cannot be decreased this process leads to the development of subparallel or radial dikes in two dimensions or a dike sheet complex in three dimensions a large magma chamber such as a sphere or a cylinder is not always necessary for the development of a basaltic volcano in case of the interaction between liquid filled cracks magma filled cracks coalesce easily into one large crack and diminish the local compressive stress field last the theory of the crack interaction is applied to developments of various volcanoes the fissure pattern of hekla volcano iceland is explained in terms of the variation of the stress relaxation the difference in fissure pattern between izu oshima and miyakejima volcanoes japan is interpreted by the variation of the differential stress the local stress balance including the stress relaxation is kept in the developments of etna volcano italy and kilauea volcano hawaii

basaltic magma reservoirs factors controlling their rupture characteristics and evolution the manner in which magma enters and leaves a magma reservoir is a fundamental aspect of magmatic activity a quantitative model is developed to evaluate the variation of stress acting on the wall of a magma reservoir as a function of depth factors assessed include the size of the reservoir its depth of burial the strength of the country rocks and the variation with depth of the densities of the magma and country rocks it is shown that small magma reservoirs with halfheights less than 1 km centered at levels of neutral buoyancy can grow in all directions with nearly equal ease by injecting dikes into their surroundings at the sites of wall ruptures in larger reservoirs with half heights greater than 2 km however the stress required to cause wall failure is much less at the depth corresponding to the center of the reservoir than it is near the top or bottom thus larger reservoirs are likely to grow predominantly sideways by lateral dike injection in these cases vertical growth will not be important unless significant accumulation of low density material can occur in the upper part of the reservoir the formation of a low density layer due to gas exsolution or chemical differentiation of the magma can facilitate failure of the roof and upward dike migration from a mature reservoir as the reservoir grows and evolves through time the increasing magma pressure head at the depth corresponding to the center of the reservoir means that the amount of gas exsolution or chemical differentiation required to allow the occurrence of vertical dike emplacement also increases ultimately a vertical reservoir size is reached beyond which the excess stress required to cause lateral dike injection is so much less than that required to cause vertical injection that essentially no further vertical growth of the reservoir will occur the reservoir will thus develop a more laterally elongate shape with time the final aspect ratio will then depend mainly on the supply rate of magma to the reservoir from the mantle

temperatures of entering magma formation and dimensions of magma chambers of volcanoes a mechanism of formation of magma chambers that feed volcanoes is discussed heat conditions and dimensions of magma chambers which have existed for more than several thousand years may become stable the approximate equations of heat balance of these chambers are derived by calculating the temperaturet1 of the magma entering chambers and the radiia of chambers calculations show that the radius of the shallow peripheral chambers of the avachinsky volcano is less than 3 3 5 km possible maximum radii of peripheral magma chambers were estimated for the kamchatkan volcanoes of medial size the temperature difference in their chambers may reach 100 200 c this method can be applied to the calculations of roots of central type volcanoes

on the effect of crustal layering on ring fault initiation and the formation of collapse calderas collapse calderas can be attributed to subsidence of the magma chamber roof along bounding sub vertical normal faults ring faults after a decompression of the magma chamber it has previously been shown that for ring faults to initiate and thus facilitate collapse the stress field both at the surface and around the magma chamber must satisfy specific critical conditions here we present new numerical models that use a finite element method to investigate the effects of crustal layering on local stress field distribution results are compared with existing criteria for ring fault initiation different subsurface scenarios were simulated by varying the stiffness young s modulus of layers placed above the magma chamber and the host rock in which the chamber is seated we consider depressurisation of a magma chamber so as to simulate magma withdrawal results indicate that mechanical layering is a further first order variable in the rare achievement of stress conditions required for ring fault formation and may be influential in facilitating or inhibiting caldera collapse we show that for a given geometrical set up the magnitude and position of maximum tensional stress at the earth s surface are influenced by the occurrence and relative distribution of mechanically stiff or soft lithologies above the magma chamber for example tensional stress at surface may be reduced by the presence of stiff layers e g lavas or increased by soft layers e g pyroclastic units compared to generic simulations using a homogeneous background medium overall we find that the presence of mechanically soft material promotes surface fracture initiation in addition we suggest that the position of peak tensional stress at surface derived by the numerical models does not represent that related to the position of the bounding ring fault but is instead related to the position of initial tensional fractures appearing prior to the collapse faults

structure of tertiary and pleistocene normal faults in iceland normal faults were measured in three selected areas of the late tertiary and pleistocene lava pile in iceland the arithmetic mean throw of 315 faults is 8 2 m the minimum being 0 5 m and the maximum 150 m about 93 of the faults have throws of less than 20 m the mean strike of the faults is n31 e but the mode is n45 e the faults dip from 42 to 89 nearly 80 dip between 65 and 79 and the mean dip is 73 fault breccia is common and varies in thickness from 0 05 m to 2 m there is a positive linear relation between breccia thickness and throw the linear correlation coefficient being 0 78 it is proposed that the normal faults in iceland form by two basic mechanisms one is the development of faults from sets of inclined joints in the lava pile the initially vertical columnar joints in the lavas gradually decrease in dip with depth in the crust because the regional dip of the lavas increases by about 1 for every 150 170 m depth the inclined joints form sets of en echelon cracks that may link up through tension cracks that propagate from near the ends of the joints the other mechanism involves formation and growth of normal faults by coalescence of smaller tension fractures the great majority of fractures within the active rift zone are tension fractures but when they attain the length of several hundred meters they normally change into normal faults it is concluded that normal fault development in iceland commonly starts at depths of about 1 km in the crust

formation and mechanics of magma reservoirs in iceland summary provided magma flow in a partially molten layer follows darcy s law of flow in porous media it can be demonstrated that magma in the proposed magma layer beneath iceland flows towards regions of reduced crustal thickness in these regions magma accumulates to form partially molten magma reservoirs which are thus located in the magma layer these reservoirs either give rise to surface eruptions or feed totally molten shallow magma chambers located in the crust that may erupt it is argued that the melt fraction varies throughout a reservoir being highest near the top where the reservoir may be totally molten but that the average is 0 25 it is concluded that for individual eruptions the reservoirs behave on the whole as a poroelastic material it is suggested that formation of crustal magma chambers is facilitated by the occurrence of stress barriers that lead to the formation of thick sills while liquid such sills absorb the magma of all dykes that enter them and may evolve into magma chambers ideal sites for stress barriers and thus for magma chambers are formations where individual layers have different elastic properties i e where the crust behaves as a multilayer denoting the bulk volume of the reservoirs by vb the volume of the crustal chambers by v and the volume of lava and dyke material in individual eruptions by ve it is found that vb 5365ve and v 1850ve using the estimated volumes of exposed plutons as a basis it is found that provided the feeder reservoir does not participate in the eruption the volume of individual eruptions from elastic crustal magma chambers in iceland is less than 0 25 km3 usually less than 0 1 km3 and that typical central volcano eruptions should be about 0 02 km3 for eruptions of the order of 1 km3 or larger the reservoirs must supply magma during the eruption it is concluded that from the point of view of volume historical eruptions of typical size could have come from elastic crustal magma chambers

mid ocean ridges fine scale tectonic volcanic and hydrothermal processes within the plate boundary zone a first order model of spreading centers as idealized linear boundaries of crustal and lithospheric generation provides only a gross understanding of global scale plate kinematics as we attempt to understand the complexity of crustal and lithospheric structure of two thirds of the earth s surface it is becoming increasingly necessary to study the tectonic volcanic and hydro thermal processes within the spreading center plate boundary zone all ocean ic crust bears the imprint of these processes this review focuses on a few selected topics concernining the fine scale tectonics and geophysics of the active axial zone of mid ocean ridges with reference to associated volcanic and hydrothermal processes it draws heavily on recent studies that use deeply towed instrument packages multi beam bathymetric mapping ocean bottom instruments and alvin e g the famous amar rise and galapagos expeditions we begin with a review of the large scale structure of spreading centers we then take a close look at the axial neovolcanic zone and progress away from the axis through the active tectonic zones next we consider the characteristics of the axial magma chamber and associated hydrothermal actvity as well as the generation of magnetic anomaly stripes and their implications for crustal generation one of our findings is that the initial two dimensional model of volcanic and tectonic zones must be expanded upon to allow for variations

axial summit trough of the east pacific rice 9 10 n geological characteristics and evolution of the axial zone on fast spreading mid ocean ridge the nature and morphological characteristics of axial summit troughs on fast 90 130 mm yr 1 full spreading rate and superfast spreading 130 mm yr 1 mid ocean ridge crests reflect the time integrated effects of long term magmatic cycles short term volcanic episodicity and the tensional stress regime imposed on young ocean crust two principal types of axial trough morphology have been identified and associated with distinct volcanic and tectonic processes occurring at fast and superfast spreading mid ocean ridge crests 1 narrow axial troughs 300 2000 m wide and 30 100 m deep on the east pacific rise crest are classified as axial summit graben the dimensions of axial summit graben as well as the morphological and structural character of their walls and floors suggest a primary tectonic origin an axial summit graben may contain a nested axial summit collapse trough implying that processes responsible for these endemic features may be linked near bottom side looking sonar and observational data collected using the towed vehicle argo i and submersible alvin have been used to characterize the axial summit trough of the fast spreading east pacific rise between 9 and 10 n a four stage model is presented for the evolution of this axial summit collapse trough as well as for other well studied portions of the east pacific rise crest from 21 n to 20 s we propose that the transition from a narrow surface collapse dominated axial trough to a broader fault bounded graben is controlled by the relative importance of diking volcanism hydrothermal cooling and tectonism along a ridge segment over time periods 104 years

modelling of surface ground deformations in the phlegraean fields volcanic area italy a preliminary finite elements model of the ground deformations observed at phlegraean fields is proposed the model assumes an oblate spheroid magma chamber at the depth of 5 4 km with major semiaxis of 1 5 km and minor semiaxis of 0 75 kin the dimensions of the magma chamber have been evaluated by using a thermal model based on the assumptions that a progressively cooling huge magmatic body is responsible for the volcanic activity at phlegraean fields in the last 35 000 years surface deformations caused by an overpressure of 30 mpa in the magma chamber have been calculated constant and temperature dependent elastic parameters of the surrounding medium have been considered vertical displacements of the order of those presently observed at phlegraean fields can be obtained only with temperature dependent elastic properties of the medium

possible effect of aspect ratios of magma chambers on eruption frequency the eruption frequency of volcanoes even within the same region is highly variable in volcanic regions associated with stress fields that are essentially uniform but anisotropic as at plate boundaries the explanation for the range in eruption frequency must lie within the volcanic systems themselves i propose that differences in eruption frequency of volcanoes within the same volcanic region may be due to different aspect ratios of the magma chambers that feed the volcanoes for a magma chamber of elliptical cross section with half height h and half width w the higher the h w ratio the more frequent will be the eruptions

off rift and rift zone palaeostresses in northwest iceland abstract the vestfirdir peninsula of northwest iceland consists mainly of gently dipping basaltic lava flows 8 14 ma old detailed studies of its principal tectonic elements basaltic dykes normal faults and minor faults and observations of the trends of the major fjords and valleys make it possible to reconstruct the evolution of the associated stress field the results show that a kink shaped curved rift zone existed in this area until about 5 ma ago in the northern part of the peninsula the rift zone trended nne but ene in the southern part it is proposed that the tectonic elements of the vestfirdir peninsula were generated in two stress fields one operating inside the rift zone the other operating outside it the rift zone stress field was responsible for the formation of most normal faults dykes and the main extension direction inferred from the minor faults conversely the off rift stress field was responsible for the formation of several holocene normal faults in the southern part of the peninsula the direction of the secondary extension inferred from the minor faults as well as for most major fjords and valleys in the area boundary element studies suggest that the off rift stress field generated on the concave side of a curved rift zone is different from that generated on its convex side absolute tensile stresses that encourage the formation of valleys and fjords concentrate very near to or at the convex margin but at a considerable distance from the concave margin applying these model results to iceland as a whole it is concluded that the valley formation in northwest iceland occurred at a large distance form the associated rift zone whereas in east iceland the valleys formed near to the margin of the rift zone furthermore the model predicts that zones of relative tension may propagate radially from the convex side of the rift zone and act as magma channels for volcanoes the location and orientation of major fjords and valleys in east and northwest iceland as well as the existence of several major off rift holocene volcanoes support the model predictions

propagation pathways and fluid transport of hydrofractures in jointed and layered rocks in geothermal fields abstract palaeofluid transporting systems observed as networks of mineral filled veins in deeply eroded parts of extinct geothermal fields indicate that hydrofractures commonly supply fluids to geothermal fields here we examine well exposed vein networks that occur at crustal depths of around 1 5 km below the initial surface of the tertiary lava pile in north iceland the veins are located in the damage zone of a major fault zone that dissects basaltic lava flows the most common host rocks of geothermal fields in iceland the lava flows contain numerous weaknesses particularly columnar cooling joints and contacts for hydrofractures to supply fluids to geothermal fields the fractures must be able to propagate and transport fluids to the surface we explore hydrofracture pathway formation using boundary element models of hydrofractures with fluid overpressure varying linearly from 10 mpa at the fracture centre to 0 mpa at the fracture tip or the fluid front the hydrofractures propagate through a vertically jointed and horizontally layered pile of lava flows with a general rock matrix young s modulus of 1 10 10 pa and a poisson s ratio of 0 25 the joints and contacts between layers are modelled as internal springs each with a stiffness strength of 6 mpa m the location and sizes of discontinuities as well as the location of the hydrofracture tip vary between the models the results indicate that tensile stresses generated at the tip of an overpressured hydrofracture can open up horizontal and vertical discontinuities out to a considerable distance from the tip and that these discontinuities eventually link up to form the hydrofracture pathway analytical models indicate that for a hot spring of a given yield associated with a fault the dimensions of the fluid transporting part of the fault are likely to be similar for a typical normal fault and a strike slip fault also a hot spring of yield 180 l s the maximum in the low temperature fields of iceland can be supplied through a hydrofracture of aperture 3 mm and trace length 1 2 m these dimensions are very similar to those of typical veins in the studied networks buoyancy rather than excess pressure in the fluid source appears to be the primary driving force of hydrofractures in the geothermal fields of iceland

kinematic analysis of radial structures around irnini mons venus abstract radial structures to the north east of irnini mons are discernable as ridges at the full magellan image resolution of 75 m pixel using fmap tiles while a magma source modeled as a pressurized hole in an elastic plate usually allows for only extensional radial structures the perturbation of regional stresses around the hole can explain the presence of radial ridges sixteen numerical models were run to see what effect changes in magma pressure and regional stresses had on the magnitude of the maximum principal stresses and the orientation of potential features around a hole in a plate these models were arranged into four groups based on model input and setup 1 uniaxial compressive stress 2 uniaxial tensile stress 3 biaxial stress and 4 multiple holes nine of the sixteen models result in radial ridges in the orientation observed around irnini mons all of these models incorporate a regional n s compression flows from irnini mons are superimposed on an older regional plains material deformed by a regional set of east west trending wrinkle ridges implying a regional north south compression affected the area the existence of radial ridges on the irnini flows implies that the regional n s compression that caused the e w trending wrinkle ridges was still active during the formation of irnini mons however the magnitude of the regional compressive stress required for radial ridge formation could be 1 5 mpa less than the compressive strength of the material indicating that radial ridges could form after wrinkle ridge formation has ceased

numerical modelling of stress fields and fracture around magma chambers we investigate the formation of cracks around magma chambers and their relation to caldera collapse we discuss basic background physics and the causes of caldera collapse events then examine the suitability of the smoothed particle hydrodynamics sph method for these applications by studying simple two dimensional systems the sph method and equations for solid body materials are discussed and tested for the stress concentration around a circular hole the effect of altering the depth of an underground magma chamber is investigated the results are compared to similar calculations using boundary element techniques a fracture model is introduced and tested for the brittle fracture of plates in tension we apply our fracture model to investigate the fractures that develop around magma chambers we find that our results are in general agreement with experiments in particular we find that in the absence of tension a sudden rise in the chamber pressure can initiate fractures which propagate towards but do not reach the surface at the same time fractures can propagate down from the surface towards the chamber under tension and pressure increase fractures can develop and reach the surface in a form that could lead to piston collapse we find that the fractures depend in a complex way on the geometry of the chamber together with the history of country rock tension and sudden jumps in the chamber pressure

origin of the sheeted dike complex at superfast spread east pacific rise revealed by deep ocean crust drilling at ocean drilling program hole 1256d 1 superfast spread 22 cm a upper oceanic crust drilled at ocean drilling program site 1256 comprises a thick sequence of extrusive lavas underlain by a thin region of sheeted dikes the sheeted dikes at this site are characterized by the intimate association of in situ hyaloclastic fragmentation and hydrothermal alteration lithostatic and magmastatic pressure estimates reveal that there is no effective level of neutral buoyancy within the extrusive layer to trap dikes in the crust this results in a situation that favors the extrusion rather than intrusion of magma fractures created by magmas rising in dikes were the loci of intense hydrothermal circulation and provided the pathways for subsequent dike intrusions magma rich conditions expected for fast spreading ridge segments give rise to a rapid increase in magmatic pressure which can result in dike intrusion even under only small deviatoric stress this allows for the emplacement of narrow dikes in the upper crust with magmas more likely to extrude and hence the high extrusive intrusive ratio for site 1256

insar observations and models of crustal deformation due to a glacial surge in iceland surges are common at all the major ice caps in iceland ice masses of gigatons may shift from the upper part of the outlet glacier towards the terminus in a few months advancing the glacier front by up to several kilometres the advancing ice front may be up to 100 m thick increasing the load on crustal rocks correspondingly we use the observed change in crustal loading during a surge of the western part of the vatnajokull ice cap iceland during 1993 1995 and the corresponding elastic crustal deformation surveyed with interferometric synthetic aperture radar to investigate the material properties of the solid earth in this region crustal subsidence due to the surge reaches 75mm at the edge of the si ujokull outlet glacier this signal is mixed with a broad uplift signal of 12 mm yr relative to our reference area caused by the ongoing retreat of vatnajokull in response to climate change we disentangle the two signals by linear inversion finite element modelling is used to investigate the elastic earth response of the surge as well as to confirm that no significant viscoelastic deformation occurred as a consequence of the surge the modelling leads to estimates of the young s modulus and poisson s ratio of the underlying earth comparison between the observed and modelled deformation fields is made using a bayesian approach that yields the estimate of a probability distribution for each of the free parameters residuals indicate a good agreement between models and observations one layer elastic models result in a young s modulus of 43 2 49 7 gpa 95 per cent confidence and poisson s ratio of 0 0 27 after removal of outliers our preferred model with two elastic layers provides a better fit to the whole surge signal this model consists of a 1 km thick upper layer with an average young s modulus of 12 9 15 3 gpa and poisson s ratio of 0 17 overlying a layer with an average young s modulus of 67 3 81 9 gpa and poisson s ratio of 0 25

fracture analysis hydrothermal mineralization and fluid pathways in the neogene geitafell central volcano insights for the krafla active geothermal system iceland abstract studies on hydrothermal fluids circulation and tectonic structures improve the geothermal exploration and exploitation in iceland in this paper we present the results of an integrated study structural kinematic fluid inclusions analyses carried out on a fossil and exhumed geothermal system at geitafell se iceland considered an analogue of the active krafla geothermal system ne iceland our work is addressed to the relationships between the main structures and the paleo geothermal fluids circulation at the boundary between the magma chamber with magma of gabbroic composition and host rocks with the aim to get information on those factors having controlled the hydrothermal fluids flow its storage and the rocks fluid interaction at 1 2 km crustal depth the structural study is mostly based on defining the architecture of mineralized brittle shear zones where by measuring length and width of mineralized fractures permeability was computed in 1 3 10 14 m2 as regards the hydrothermal paleo fluids fluid inclusions in hydrothermal quartz and calcite usually associated with epidote and chlorite i e the typical alteration minerals found in icelandic geothermal fields recorded a fluid circulation with temperature ranging from about 255 c to 320 c and boiling at the highest temperature transient pressure drop likely related to fracture opening and propagation during tectonic activity triggered local boiling process testified by fluid inclusions in a calcite sample during a late evolution the system progressively cooled down to 60 90 c paleo geothermal fluids were characterized by low salinity meteoric water although higher salinity fluids up to 10 6 wt nacl equiv entered in the system and mixed with meteoric fluid in the end based on results from fluid inclusions derived fluid density permeability values and modeling of viscosity hydraulic conductivity between 2 8 10 8 to 1 8 10 7 m s was also computed within the fluids channelling brittle shear zones we finally encourage the key study of fossil geothermal systems as a tool for getting parameters commonly obtained after drilling in active systems

how hydrofractures become arrested fluids in the earth s crust are commonly transported by hydrofractures such as dykes and mineral veins many of which become arrested at various crustal depths hydrofractures are commonly arrested some showing blunt tips at contacts between soft low young s modulus and stiff high young s modulus layers for example many dyke tips are arrested at contacts between soft pyroclastic rocks and stiff basaltic lava flows and vein tips at contacts between soft marl and stiff limestone theoretical models indicate that overpressured buoyant hydrofractures in homogeneous isotropic host rocks should normally reach the surface in layered host rocks however abrupt changes in young s moduli horizontal discontinuities and layers with unusually high fracture perpendicular stresses encourage hydrofracture arrest it is proposed that for layer parallel loading stiff layers favour hydrofracture arrest during active compression but soft layers during extension it is concluded that for hydrofracture propagation to occur the stress field along its potential pathway must be essentially homogenous

effect of local and regional stress fields on sheet emplacement in west iceland abstract at the tertiary reykjadalur central volcano in west iceland there is a remarkable change in strike dip thickness and percentage of sheets at a distance of about 9 km from the centre of the volcano at distances of less than 9 km the sheets strike in several main directions their average arithmetic mean dip is 45 and the average thickness 1 m locally the sheets make up nearly 90 of the rock but in 1 1 5 km long profiles they are 5 8 7 9 one third of the sheets are porphyritic containing as much as 50 of plagioclase phenocrysts within and in the vicinity of the central volcano there is thus a clear swarm of inclined cone sheets at distances greater than 9 km the sheets suddenly dip much more steeply average 81 become thicker average 3 15 m strike parallel to the nearby volcanic zones and have all the characteristics of regional dyke swarms in 2 5 10 5 km long profiles these dykes are only 0 83 1 84 of the rock one fourth of the dykes are porphyritic but the percentage of plagioclase phenocrysts is normally less than 10 we interpret this abrupt change from a local swarm of inclined sheets to swarms of regional dykes as reflecting changes in the stress field we propose that a local stress field controlled the strike dip and partly thickness of the sheets to a distance of 9 km from the centre of the volcano this local stress field which is also thought to have given rise to the collapse caldera associated with the volcano is attributed to a crustal magma chamber at a shallow depth below the volcano this crustal magma chamber is regarded as the source of most of the inclined sheets the regional stress fields associated with the volcanic zones is thought to have controlled dyke emplacement at greater distances from the volcano we suggest that many of the regional dykes were injected from a deep seated magma reservoir located at boundary between the crust and upper mantle

influence of angularities on magma tapping processes abstract basic principles controlling tensional failure of a solid are applied in this work to investigate the role played by angularities the conditions explored apply equally to the initial rupture of sections of the walls of a magma chamber or reservoir and to the propagation of the leading edge of a three dimensional intrusion the results show that even when the internal pressure inside a cavity might be uniform the resulting stress on the material surrounding it can have contrastingly different values depending on its position relative to any angularities such variability explains why similar volumes of magma trapped under similar conditions at depth and therefore having equivalent buoyancy sometimes may feed a tapping event and fail to do so at other times in addition it can be appreciated why sometimes veins and dykelets only a few millimeters thick are formed whereas on other occasions it is possible to create a dyke several meters wide filled with magma of similar composition and intruding very similar country rock the approach followed here also provides an alternative mechanism for the formation of blunt ended dykes and outlines the form in which those intrusions are likely to evolve under certain circumstances

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

rock mass strength and slope stability of the hilina slump kīlauea volcano hawai i abstract stability of the hilina slump on the south flank of k lauea volcano hawai i is investigated using slope stability analyses based on rock mass mechanics the hilina slump is an active example of the giant landslides that are observed around submarine flanks of ocean island volcanoes based on edifice topology along with derived rock mass strength and deformability parameters slope stability analyses predict that the geometries of present day failure surfaces of the hilina slump are shallow located at less than 3 5 km depth below the upper surface of the edifice the failure surfaces at the base of the hilina slump are predicted to be structurally independent from a previously interpreted subjacent detachment at the contact between the k lauea edifice and the oceanic crust based on these model results the hilina slump is envisioned to ride atop the south flank of k lauea as it spreads seaward along this slipping basal detachment under present day slope and sea level configurations local horizontal ground accelerations greater than 0 4 0 6 g are predicted to cause slip along failure surfaces within hilina slump therefore dynamic stress changes due to slip along the subjacent detachment may potentially trigger slip along failure surfaces of the hilina slump model derived stability and failure surface geometries can account for the distinct observed distribution of slip along specific hilina faults following the m fa 7 9 1868 ka u and m s 7 2 1974 kalapana earthquakes further the spatial distribution of south flank aftershocks m 1 5 recorded between 1950 and 1976 are consistent with the predicted distribution of potential earthquake induced slip of the hilina faults

gravity and magnetic studies of the subglacial grimsvotn volcano iceland implications for crustal and thermal structure the highly active subglacial grimsvotn volcano is associated with a 20 mgal bouguer gravity high and a broad magnetic low the magnetic low roughly coincides with the gravity high but short wavelength magnetic highs are superimposed on the low especially along caldera rims forward models of the gravity and magnetic fields show variable magnetization 0 12 a m 1 in the uppermost 1 2 km of the crust probably reflecting differences in the proportions of crystalline rocks and hyaloclastites and variations in hydrothermal alteration the models are consistent with but do not require a 10 km3 molten magma chamber in the shallow crust beneath the main caldera between 1 5 and 4 km depth a dense body 2 95 3 0 mg m 3 with a volume of 400 km3 is located probably composed of gabbro modeling suggests that this intrusive body is nonmagnetic which may imply that its temperature is at or above the curie point the distribution of geothermal activity at the glacier bed is consistent with heat extraction from this dense hot intrusive body it may be an important source for the presently observed geothermal heat output of 2000 4000 mw at grimsvotn this high heat transfer rate is difficult to reconcile with estimates of long term heat production in an icelandic rift zone volcano the heat flux may be a transient phenomenon caused by enhanced vertical permeability due to a recent caldera collapse and subsequent heat mining of the intrusive body

geochemical comparison of core extracts and oil samples in reservoir abstract a geochemical study was undertaken to compare the geochemical parameters obtained from the analysis of different sample types of oils collected from the same horizons in particular produced oil samples and oil extracted directly from reservoir cores in the laboratory and analysed by slightly different methods it is not easy to carry out a major comparison because of the scarcity of horizons from which both dst oil samples and cored reservoir samples are available bp was only able to locate such samples from three wells kirklington 2 onshore england qusahwira 1 middle east and 26 28 1 offshore ireland the composition and concentration petroleum rock percentage of the oil in these three wells varied widely some molecular maturity parameters e g methylphenanthrenes and sterane isomerisation ratios suggest that the extraction process mobilises lower maturity material from the cores in some cases the parameters contradict one another possibly indicating measurement of different components of the petroleum or inherent uncertainties in the analysis source type indicators are observed for some of the oils and their respective core extract samples from the same intervals in the same wells which suggest derivation from different types of organic material the results show that it is not possible to characterise an oil definitively by examination of the core extract that the differences may not be systematic and that comparison of oil and extract data from different wells or horizons may be misleading this study demonstrates some of the differences that need to be taken into account during interpretation

the nature and significance of magma chamber margins in ophiolites examples from the norwegian caledonides abstract large scale intrusive contacts with associated marginal series have been encountered within norwegian ophiolite complexes at karmoy solund and leka the contacts limit individual magma chambers and are found at different structural levels of the plutonic suites examples of magma chamber margins adjacent to interlayered ultramafic and gabbroic rocks modally layered gabbros high level gabbros and sheeted dykes are described the nature of the intrusive boundaries and the presence of partially resorbed xenoliths in the vicinity of the intrusive margins suggest that stoping and assimilation have been important mechanisms during the development of the magma chambers characteristic marginal series are developed along the intrusive boundaries the thicknesses and appearance of these series vary with depth in the complexes whereas the marginal series are well developed within the uppermost levels of the plutonic complexes exhibiting rock types such as microgabbro massive gabbro and magnetite gabbro the marginal series observed at lower levels are thinner and also devoid of chilled facies rocks and magnetite gabbros the marginal series may be subdivided into border and roof series the latter are characterized by an intimate relationship with sheeted dykes which comprise dyke swarms formed both prior to during and subsequent to crystallization of the roof series based on these relationships the dykes can be subdivided into rooted and rootless dykes a multiple magma chamber model with magma chambers migrating from a low to a high level within the oceanic crust is proposed on the basis of the observed features

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

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

mechanisms of normal fault development at mid ocean ridges 1 slow spreading ridge segments are characterized not only by small closely spaced faults that develop near the segment center but also by large widely spaced faults that develop near the segment ends typically at the inside corner of a ridge offset intersection in this study we investigate the competing effects of stress accumulation in the lithosphere and the yield strength of the lithosphere in controlling the location of normal fault formation and direction of propagation seismic velocity models from the mid atlantic ridge in the oceanographer hayes region and 29 n and the east pacific rise at 9 n were used to estimate the along axis change in dynamic young s modulus corresponding thermal and rheologic models were calculated to estimate the along axis variation in yield strength we then develop a thin plate model to calculate the predicted location of fault initiation or reactivation and the subsequent propagation direction for different combinations of linear along axis gradients in young s modulus and yield strength on the basis of this model we define two modes of normal fault development at slow spreading segments mode c center faults which develop at the segment center and propagate outward and mode e end faults which develop at the segment ends and propagate inward mode c faults are predicted to form at ridges where the along axis variation in yield strength dominates the along axis accumulation of stress conversely mode e faults are predicted to develop at ridges where stress accumulation toward segment ends overcomes the high yield strength in these locations in addition to the accumulation of stress caused by along axis gradients in young s modulus we illustrate that shear stresses resisting relative plate motion along a transform fault will generate higher effective stress at inside corners possibly concentrating mode e faulting in these locations at fast spreading ridges where along axis gradients in stress and lithospheric strength are relatively small more uniform patterns of faulting are predicted the results of this study quantify how the interplay between the along axis variations in stress state and the mechanical properties of the lithosphere controls the style of fault development at mid ocean ridge segments

an analysis of the nonlinear magma edifice coupling at grimsvotn volcano iceland continuous monitoring of seismicity and surface displacement of active volcanoes can reveal important features of the eruptive cycle here high quality gps and earthquake data recorded at grimsvotn volcano by the icelandic meteorological office during the 2004 2011 inter eruptive period are analyzed these showed a characteristic pattern with an initial 2 year long exponential decay followed by 3 year long constant surface displacement inflation rate we model it by using a one magma reservoir model in an elastic damaging edifice with incompressible magma and constant pressure at the base of the magma conduit seismicity rate and damage were first modeled and simple analytical expressions were derived for the magma reservoir overpressure and surface displacement as functions of time very good fits of the seismicity and surface displacement data were obtained by fitting only three phenomenological parameters characteristic time and power strain show maxima from which reference times were inferred that split the inter eruptive period into five periods after the pressurization periods damage occurring in the third period induced weakly nonlinear variations in magma overpressure and flow and surface displacement during the fourth period the damage dominated and variations became more strongly nonlinear the reservoir overpressure decreased and magma flow increased this process lasted until the power strain reached its second maximum where instability was generalized this maximum is a physical limit the occurrence of which shortly precedes rupture and eventually eruption this analysis allows characterization of the state of the volcanic edifice during the inter eruptive period and supports medium term prediction of rupture and eruption

effusion rate trends at etna and krafla and their implications for eruptive mechanisms abstract using effusion rates obtained from ground and satellite based data we build a data set of 381 effusion rate measurements during effusive activity at etna and krafla between 1980 and 1999 this allows us to construct detailed effusion rate curves for six fissure fed eruptions at etna and krafla and four summit fed eruptions at etna these define two trends type i and ii type i trends have effusion rates that rise rapidly to an initial peak before declining more slowly resulting in an exponential decrease in eruption rate and declining growth in cumulative volume type ii trends are characterised by steady effusion and eruption rates and hence a linear increase in cumulative volume the former is typical of fissure eruptions and can be explained by tapping of an enclosed pressurised system the latter are typical of persistent etnean summit eruptions plus one persistent effusive eruption at stromboli 1985 1986 examined here and can be explained by overflow of the time averaged magma supply we use our effusion rate data to assess the magma balance at etna 1980 1995 and krafla 1975 1984 between 1980 and 1995 etna was supplied at a time averaged rate of 6 8 2 3 m 3 s 1 of which 13 was erupted at krafla 817 30 10 6 m 3 was erupted and intruded during 1975 1984 and the ratio of erupted to intruded volume was 0 3 at etna there is evidence for intrusion of the unerupted magma within and beneath the edifice as well as storage in the central magma column at krafla unerupted magma was intruded into a rift zone but an increasing proportion of the supply was erupted from 1980 onwards a result of the rift zone capacity being reached magma intruded prior to an eruptive event may also be entrained and or pushed out during eruption to contribute to the initial high effusion rate phases of type i events the detail in our effusion rate curves was only possible using a thermal approach which estimates effusion rates using satellite data we look forward to analysing satellite derived effusion rate trends in real time using data from current and soon to be launched sensors

on the mechanics of caldera resurgence the mechanisms suggested to cause caldera resurgence can be categorized as 1 viscous rebound of magma 2 regional detumescence and 3 pressure against the base of the caldera block due to magmatic vesiculation and convection or addition of new magma the critical observation that resurgence takes of the order of 103 105 years insists that the causative mechanism be regulated by a highly viscous e g 1022 pa s structural member of the crust the relatively low viscosity magma and a purely elastic crust cannot be important in achieving this time scale the most likely mechanism is the coupled effect of regional detumescence and magmatic vesiculation detumescence compresses the magma chamber squeezing magma upward against and through the caldera block vesiculation also compresses the chamber and tends to dome the caldera block the times of both detumescence and doming of the caldera block by magmatic pressure are probably inversely proportional to the size of the magma chamber but the rate of cooling of the chamber is proportional to its size because solidification spreads viscous relaxation over a wide region of the crust and curtails resurgence there is likely to be a minimum volume of magma that can undergo resurgent doming it is difficult to be exact about this critical size until more precise models have been developed

stress estimates from the length width ratios of fractures the hypothesis is advanced that provided young s modulus and poisson s ratio of the rock are known the length width ratios of tension fractures can be used to estimate the tensile stress assumed constant along the length of each fracture at the time of fracture formation the hypothesis is tested on a fissure swarm in a 10 000 year old basaltic lava in iceland the length width ratios of the fissures give the average tensile stress as of the order of a few mpa

structure of the sheeted dike complex of the samail ophiolite near ibra oman the sheeted dike complex is a regionally mappable geologic unit with a consistent position in the samail ophiolite stratigraphy discontinuous exposures of the complex in the ibra area of the southeast oman mountains provide structural data that allow reconstruction of a paleo spreading ridge axis of 347 for the region and suggests a maximum spreading width of 275 km for the ophiolite contact relations within the well exposed al ahmadi section indicate that the complex was formed by multiple parallel intrusion over a narrow zone at the paleo ridge axis split dikes are abundant facing directions indicate a spreading ridge southwest of the present outcrop area in conflict with other spreading axis direction indicators

petrogenesis of basalt comendite and basalt pantellerite suites terceira azores and some implications for the origin of ocean island rhyolites petrogenetic modeling of the recent lava succession of santa barbara and pico alto volcanoes and associated basaltic lavas indicates that there are two discrete lava series present one erupted from the axial rift linking the two central volcanoes and one associated with monogenetic cones scattered around the flanks of santa barbara the felsic lavas of both volcanoes are peralkaline and appear to be derived from associated basalts by fractional crystallization of an assemblage including essential amphibole trace element abundances in the felsic lavas particularly those of sr and ree cannot be reconciled with an origin through partial melting of basaltic material at the base of the volcanic pile the difference between the comenditic and pantelleritic differentiation trends of santa barbara and pico alto is attributed primarily to fo2 control of the crystallizing assemblage probably related to thermal dissociation of magmatic water in the santa barbara magma chamber this effect may be augmented by minor differences in parent basaltic compositions the pico alto pantellerites being derived from the rift basalts whereas the santa barbara comendites are derived from the off rift basalts a compositional gap between 54 and 64 silica content in the lavas is not present if the suite is extended to include co magmatic hypabyssal xenoliths leading to the inference that the gap in this and other bimodal suites results solely from a relative inability of magma of intermediate composition to erupt

fluid overpressure and stress drop in fault zones using the average aspect length maximum thickness ratios of 379 mineral filled extension mode i veins from an active fault zone the fluid overpressure during their development with reference to the minimum compressive principal stress 3 is estimated at 20 mpa emplacement of such veins increases 3 and can generate a temporary stress barrier to the propagation of subsequent hydrofractures on meeting a subhorizontal stress barrier vertically propagating hydrofractures may change into water sills where the fluid pressure is at or above lithostatic in this model stress barriers and thus water sills can form at any depth in and in any type of fault zones for such a high fluid pressure the product of the coefficient of sliding friction and the normal stress in the modified griffith criterion becomes essentially zero and the driving stress associated with faulting equal to twice the in situ tensile strength of the host rock for typical in situ tensile strengths of 2 3 mpa the driving stresses for slip on overpressured fault planes is 4 6 mpa these results are in good agreement with the commonly measured average static stress drops of 3 6 mpa during earthquakes

geology and seismic structure of the northern section of the oman ophiolite in the north oman mountains a continuous ophiolite succession is exposed from tectonized harzburgites and dunites at the base through layered gabbros and peridotites high level gabbros and plagiogranite to a dike swarm and pillowed volcanics overlain by pelagic shales the upper part of this sequence possesses a static metamorphic overprint which passes downward from greenschist facies in the lowermost volcanics and most of the dike swarm to amphibolite facies in the lowermost dike swarm and in the high level intrusives these pervasively altered rocks are underlain by layered gabbros and peridotites where hydration is restricted to fractures compressional and shear wave velocities have been measured to confining pressures of 6 kbar for 139 water saturated cores from this sequence the samples were field oriented and collected from known stratigraphic levels which allows the construction of velocity depth profiles and an examination of anisotropy within the ophiolite compressional wave velocities measured for the basalt section are quite variable ranging from 4 5 to 6 0 km s at in situ pressures within the sheeted dike section compressional and shear wave velocities vary from 5 4 to 6 3 km s and from 2 9 to 3 6 km s respectively from the basalt sediment contact through the dike section velocities increase rapidly with depth this gradient is related to increasing metamorphic grade and a decrease in grain boundary porosity the top of seismic layer 3 is located near the lower boundary of the sheeted dike swarm and marks the transition from greenschist facies to amphibolite facies metamorphics the dikes decrease in abundance downward into the high level intrusives coarse grained gabbroic and granitoid rocks which often contain abundant hornblende with isotropic textures and lacking compositional layering mean compressional and shear wave velocities increase uniformly from 6 7 and 3 6 km s respectively in the highlevel intrusives to 7 5 and 3 9 km s respectively at the base of the layered sequence measured velocities of the ultramafic tectonites are highly variable because of the anisotropy and serpentinization petrofabrics of relic olivine in the wadi ragmi region show strong preferred fabrics for the tectonites and calculated compressional wave velocities for the serpentine free rocks range from 7 8 to 8 5 km s with the fast direction parallel to the direction of spreading inferred from dike orientations in excellent agreement with observed upper oceanic mantle seismic anisotropy the crust mantle boundary which is defined by the abrupt increase in seismic velocities coincides with the petrological contact between layered gabbros and peridotites and tectonized harzburgite and dunites

analytical models of magma chamber stability an abridged critical review of key concepts abstract key aspects that need to be taken into consideration when attempting to establish an elastic model of magma chamber stability and the various forms in which those parameters have been introduced in analytical models of magma chamber stability are examined in critical form these aspects include the geometry of the chamber the relative distance and geometry of an outer boundary the nature of the surface forces that act on each of those two physical boundaries the role played by a body force related to the gravitational attraction and the fact that such force acts on any imaginary surface within the body of interest as well as some assumptions concerning the description of the lithostatic stress and the criteria used to determine the conditions of tensile failure of the rock the examination made here reveals that very often the analytical models include one or more sources of internal inconsistencies that are rooted on the disregard of conditions used by the original elastic models after reviewing key aspects of elastic models the reader is expected to acquire the tools required to fully appreciate the advantages and limitations of specific chamber models and to decide which of those models is better suited to solve the particular problem at hand

seamount formation and associated caldera complex and hydrothermal mineralization in ancient oceanic crust troodos ophiolite cyprus abstract the inter graben zone between the solea and mitsero grabens in the northern part of the troodos ophiolite cyprus displays igneous tectonic and physiographic features that are characteristic of seamount volcanism at modern oceanic spreading centers in this zone the 200 m high alestos hill volcanic edifice consisting of pillow lavas with preserved lava tubes and cone sheet intrusions is interpreted to have developed as a seamount fed by a late stage off axis shallow level gabbroic pluton that was intrusive into young oceanic crust concentric cone sheet intrusions and associated faults and shear zones in and around the alestos hill seamount collectively form a seafloor caldera complex which probably formed during the extrusion of lavas and subsequent cauldron subsidence over the gabbroic pluton the ene trending elongated massive sulfide ore deposit formation in the alestos memi area represents a focused upflow zone for hydrothermal fluids driven by the same off axis gabbroic intrusion responsible for formation of the alestos hill seamount the ene trending and up to 150 m wide alestos memi fault zone dissects the elongated ore body in a dextral sense and might have provided a high permeability conduit in the upper crust for upwelling hydrothermal flow at the time of magmatism the geometry of this fault zone and its kinematic relations with a n s striking alestos forest normal fault in the west are suggestive of its transfer fault origin reminiscent of mineralized transfer faults found at the mid atlantic ridge the active tag hydrothermal mound with massive sulfide deposits that is located on a small volcanic dome 3 km off axis and along a transfer fault in the trans atlantic geotraverse tag area at 26 n provides a modern analogue for the alestos hill seamount and associated faulting and mineralization in the ancient oceanic crust of the troodos ophiolite these findings indicate that off axis magmatism played a major role in crustal accretion hydrothermal alteration and ore mineralization during the late stage evolution of the troodos ophiolite

numerical simulation of caldera formation due to collapse of a magma chamber 1 the caldera structure after collapse of the magma chamber was estimated by numerical simulation in the simulation the collapse of the magma chamber was approximated by the contraction of a small sphere in the elastic medium and the distribution of plastic and or rupturing area was calculated using the coulomb failure criterion under the assumption of an elastic perfectly plastic material given an undefined or isotropic regional stress field the plastic area caldera was found to develop as a circular depression on the surface appearing funnelform in cross section under an anisotropic regional stress field the caldera developed in the direction of the maximum compression or minimum extension axis in all simulations the collapse of the magma chamber resulted in the formation of an outward dipping reverse ring fault around the area above the chamber and an inward dipping normal ring fault at the periphery of the caldera

width change of a planar magma path implication for the evolution and style of volcanic eruptions abstract the width change of a planar path filled with magma is analyzed assuming that the process is controlled by laminar flow of an incompressible viscous magma between movable parallel walls of country rock the analysis shows that the width of a planar path changes at a rate is proportional to the effective opening pressure which is defined as the pressure of the penetrating magma plus the tensile stress component stretching the fluid interface when the opening pressure is a positive constant the fissure widens at an accelerating rate and finally diverges at a critical time constant which depends on the initial size of the fissure and the viscosity of magma as well as on the opening pressure in contrast the fissure gradually narrows under a negative opening pressure this result is applicable to a variety of magmatic processes first the orientation of a volcanic fissure or a dike is such that the tensile stress component is maximized because a fissure with this orientation becomes wider than those at other orientations as it has the minimum value of critical time constant second local irregularities in the geometrical and mechanical conditions of the magma sheet lead to significant variations in fissure width in the widening process this nonuniform increase in width is further exaggerated by mechanical and thermal erosion of the wall rock which makes the irregular surfaces irrecoverable in this manner a planar magma path tends to be split into isolated cylindrical conduits when the magma pressure is reduced and the fissure closes as a whole some actual fissure eruptions such as those at kilauea and izu oshima show evolutionary processes consistent with this prediction another application of the analysis is to explain how the style of a volcanic eruption is determined by the tectonics specifically extensional tectonics which gives a positive opening pressure for any magma pressure allows magma to ascend dispersively along various paths and produces a group of small monogenetic volcanoes in contrast contractional tectonics in which new fissures can be made only for a very high magma pressure concentrates magma into a restricted number of vents and leads to the growth of large polygenetic volcanoes

stress fields generating ring faults in volcanoes ring faults in volcanoes have been recognized for a long time but their mechanics of formation is still poorly understood while the subsidence on a ring fault during a large eruption from the associated chamber is easily understood the initiation of the fault itself has been difficult to explain for a ring fault to form the tensile and shear stresses at the surface of the volcano must peak at a certain radial distance from the surface point above the center of the chamber empirical evidence however shows that during most periods of unrest in active volcanoes the stress field is not favorable for the initiation of ring faults we made several tens of boundary element models of magma chambers of various geometries and subject to different loading conditions the results indicate that overpressure or underpressure in the chamber as the only loading is unlikely to initiate ring faults for a spherical chamber subject to horizontal extension or doming and a sill like chamber subject to horizontal extension the tensile and shear stresses at the surface of the volcano peak at a certain radial distance from the surface point above the center of the chamber then however the maximum stresses occur at the boundary of the chamber itself which would normally lead to sheet injection rather than to ring fault formation by contrast a sill like magma chamber subject to doming gives rise to a stress field suitable for the initiation of a ring fault

hydrofracturing stress measurements in the iceland research drilling project drill hole at reydarfjordur iceland two independent suites of hydrofracturing stress measurements were conducted in the top 600 m of the iceland research drilling project deep hole at reydarfjordur east iceland as indicated by the continuously extracted drill core the tested section consists of tertiary subaerial tholeiitic lava flows cut by many basaltic dikes the density of the basalt was used to estimate the vertical stress v in megapascals at depth d in meters v 0 027d in the 33 to 270 m depth range the horizontal principal stresses hmax and hmin as calculated from hydrofracturing results increase gradually with depth but at a lower rate than the vertical stress hmin 2 1 0 016d hmax 3 3 0 016d in the 300 to 550 m depth range a major perturbation occurs with both horizontal stress magnitudes rapidly rising between 300 and 400 m and then uniformly decreasing between 400 and 550 m to a level apparently predictable by the relationships obtained in the top 270 m conditions favoring normal faulting v hmax hmin were established in the 540 to 580 m range and are conjectured to persist at greater depths since the vertical stress gradient is higher than that of hmax and hmin this would be in accord with the predominant fault type in east iceland the relative low horizontal stress values and the small difference between them are supportive of ocean floor spreading theories and contradict near surface high compressive stresses previously determined by hast 1973 in east iceland the large increase in horizontal stress in the range of 300 to 500 m depth is not understood a correlation appears to exist between the zone of almost unintruded thick lava flows 200 500 m and that of high stresses an intuitive model suggesting locked in stresses in this zone is proposed the hydrofracturing determined overall mean direction of hmax is n50 e while a more selective sampling of results yields n40 e the measured stress directions appear to be reasonably compatible with the local structure such as dike direction nne and lava dip wsw significantly hmax direction is subparallel to the n20 e trend of the nearest fissure swarms in the axial rift zone this almost coincidence of directions like the measured stress magnitudes is also in agreement with the model of spreading lithospheric plates being pulled apart

crustal spreading due to dikes and faults in southwest iceland abstract a 30 km long profile extending from 3 million year old pleistocene rocks to the holocene rocks of the rift zone in southwest iceland is dissected by 443 dikes and 156 normal faults nearly all the dikes are basaltic with an average strike of n26 e which is subparallel to the trend of fractures in the nearby rift zone their average dip is 69 and average thickness 1 37 m both values much lower than averages for regional tertiary dike swarms in iceland many dikes are seen to change into sills the great majority of the faults are pure dip slip with an average throw of 10 2 m their average strike n37 e is different from the average strike of the dikes in this area and the fractures in the nearby rift zone their average dip is 75 2 which is somewhat steeper than in the tertiary areas but the dips are less in the interbedded sedimentary layers than in the lava flows growth faults are very rare in the area and most of the faults appear to have completed their growth in less than 10 000 years in subsections crustal dilation due to dikes is 0 4 3 7 that due to faults is 0 6 5 3 and that due to both dikes and faults is 1 04 6 44 in these subsections average fault spacing is one fault per 170 563 m spacing decreases toward the active rift zone it is concluded that in typical fissure swarms of the rift zone the dilation at the surface is almost entirely due to extensional fractures and gaping vertical normal faults in the depth range of a couple of hundred metres to perhaps 1 km the non vertical faults contribute most to the dilation at deeper crustal levels however dilation due to dikes becomes dominant

towards more realistic values of elastic moduli for volcano modelling abstract the accuracy of elastic analytical solutions and numerical models widely used in volcanology to interpret surface ground deformation depends heavily on the young s modulus chosen to represent the medium the paucity of laboratory studies that provide young s moduli for volcanic rocks and studies that tackle the topic of upscaling these values to the relevant lengthscale has left volcano modellers ill equipped to select appropriate young s moduli for their models here we present a wealth of laboratory data and suggest tools widely used in geotechnics but adapted here to better suit volcanic rocks to upscale these values to the scale of a volcanic rock mass we provide the means to estimate upscaled values of young s modulus poisson s ratio shear modulus and bulk modulus for a volcanic rock mass that can be improved with laboratory measurements and or structural assessments of the studied area but do not rely on them in the absence of information we estimate upscaled values of young s modulus poisson s ratio shear modulus and bulk modulus for volcanic rock with an average porosity and an average fracture density quality to be 5 4 gpa 0 3 2 1 gpa and 4 5 gpa respectively the proposed young s modulus for a typical volcanic rock mass of 5 4 gpa is much lower than the values typically used in volcano modelling we also offer two methods to estimate depth dependent rock mass young s moduli and provide two examples using published data from boreholes within k lauea volcano usa and mt unzen japan to demonstrate how to apply our approach to real datasets it is our hope that the data and analysis presented herein will assist in the selection of elastic moduli for volcano modelling to this end we provide a microsoft excel spreadsheet containing the data and necessary equations to calculate rock mass elastic moduli that can be updated when new data become available the selection of the most appropriate elastic moduli will provide the most accurate model predictions and therefore the most reliable information regarding the unrest of a particular volcano or volcanic terrain

evolution of stress fields and faulting in seismic zones measurements indicate that stress magnitudes in the crust are normally limited by the frictional equilibrium on pre existing optimally oriented faults fault zones where these limitations are frequently reached are referred to as seismic zones fault zones in the crust concentrate stresses because their material properties are different from those of the host rock most fault zones are spatially relatively stable structures however the associated seismicity in these zones is quite variable in space and time here we propose that this variability is attributable to stress concentration zones that migrate and expand through the fault zone we suggest that following a large earthquake and the associated stress relaxation shear stresses of a magnitude sufficient to produce earthquakes occur only in those small parts of the seismic zone that because of material properties and boundary conditions encourage concentration of shear stress during the earthquake cycle the conditions for seismogenic fault slip migrate from these stress concentration regions throughout the entire seismic zone thus while the stress concentration regions continue to produce small slips and small earthquakes throughout the seismic cycle the conditions for slip and earthquakes are gradually reached in larger parts of and eventually the whole seismogenic layer of the seismic zone prior to the propagation of an earthquake fracture that gives rise to a large earthquake the stress conditions in the zone along the whole potential rupture plane must be essentially similar this follows because if they were not then on entering crustal parts where the state of stress was unfavourable to this type of faulting the fault propagation would be arrested the proposed necessary homogenisation of the stress field in a seismic zone as a precursor to large earthquakes implies that by monitoring the state of stress in a seismic zone its large earthquakes may possibly be forecasted we test the model on data from iceland and demonstrate that it broadly explains the historical as well as the current patterns of seismogenic faulting in the south iceland seismic zone

a stress controlled mechanism for the intensity of very large magnitude explosive eruptions abstract large magnitude explosive eruptions are the result of the rapid and large scale transport of silicic magma stored in the earth s crust but the mechanics of erupting teratonnes of silicic magma remain poorly understood here we demonstrate that the combined effect of local crustal extension and magma chamber overpressure can sustain linear dyke fed explosive eruptions with mass fluxes in excess of 10 10 kg s from shallow seated 4 6 km depth chambers during moderate extensional stresses early eruption column collapse is facilitated with eruption duration of the order of few days with an intensity of at least one order of magnitude greater than the largest eruptions in the 20th century the conditions explored in this study are one way in which high mass eruption rates can be achieved to feed large explosive eruptions our results corroborate geological and volcanological evidences from volcano tectonic complexes such as the sierra madre occidental mexico and the taupo volcanic zone new zealand

rock mechanics observations pertinent to the rheology of the continental lithosphere and the localization of strain along shear zones emphasized in this paper are the deformation processes and rheologies of rocks at high temperatures and high effective pressures conditions that are presumably appropriate to the lower crust and upper mantle in continental collision zones much recent progress has been made in understanding the flexure of the oceanic lithosphere using rock mechanics based yield criteria for the inelastic deformations at the top and base at mid plate depths stresses are likely to be supported elastically because bending strains and elastic stresses are low the collisional tectonic regime however is far more complex because very large permanent strains are sustained at mid plate depths and this requires us to include the broad transition between brittle and ductile flow moreover important changes in the ductile flow mechanisms occur at the intermediate temperatures found at mid plate depths two specific contributions of laboratory rock rheology research are considered in this paper first the high temperature steady state flow mechanisms and rheology of mafic and ultramafic rocks are reviewed with special emphasis on olivine and crystalline rocks rock strength decreases very markedly with increases in temperature and it is the onset of flow by high temperature ductile mechanisms that defines the base of the lithosphere the thickness of the continental lithosphere can therefore be defined by the depth to a particular isotherm tc above which at geologic strain rates the high temperature ductile strength falls below some arbitrary strength isobar e g 100 mpa for olivine tc is about 700 800 c but for other crustal silicates tc may be as low as 400 600 c suggesting that substantial decoupling may take place within thick continental crust and that strength may increase with depth at the moho as suggested by a number of workers on independent grounds put another way the moho is a rheological discontinuity a second class of laboratory observations pertains to the general phenomenon of ductile faulting in which ductile strains are localized into shear zones ductile faults have been produced in experiments of five different rock types and is generally expressed as strain softening in constant strain rate tests or as an accelerating creep rate stage at constant differential stress a number of physical mechanisms have been identified that may be responsible for ductile faulting including the onset of dynamic recrystallization phase changes hydrothermal alteration and hydrolytic weakening microscopic evidence for these processes as well as larger scale geological and geophysical observations suggest that ductile faulting in the middle to lower crust and upper mantle may greatly influence the distribution and magnitudes of differential stresses and the style of deformation in the overlying upper continental lithosphere

emplacement of dikes sills and crustal magma chambers at divergent plate boundaries abstract a model of dike emplacement at divergent plate boundaries is developed which predicts that below the uppermost 1 3 km of the oceanic crust the number and lengths of dikes in any particular swarm should increase but that the thickness should decrease with depth in the crust these predictions are supported by field observations it is proposed that rapid injections of dikes at divergent plate boundaries may temporarily alter the normal stress field in such a way that at a certain level in the crust subsequent dikes change into sills such sills may develop into crustal magma chambers either when many nearby sills combine into a large one or when an initial sill absorbs the magma of many dikes that enter it in a rapid succession while the sill is liquid for the fastest spreading ridges the initial thickness of a sill that is to develop into a crustal chamber need only attain 20 m whereas at the slowest spreading ridges the initial thickness must exceed 170 m because thin sills are much more likely to form than thick sills the probability of sills developing into crustal chambers increases with dike intrusion frequency hence with spreading rate furthermore the model predicts first that with increasing spreading rate the length of the chamber along the ridge axis should increase and second that the most likely site for magma chambers is at depths of 2 4 km both these predictions are supported by available data

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

central vs flank eruptions at teide pico viejo twin stratovolcanoes tenerife canary islands abstract teide pico viejo stratovolcanoes on the island of tenerife canary islands have undergone several flank and central vent lava flow and explosive phonolitic eruptions during the last 35 ka without any apparent structural or petrological control on such a random location of eruption foci the morphology and size of the volcanic edifices have been changing during that period but this seems to have not affected the location of new eruptions in order to investigate what determines the occurrence of either central or flank eruptions at teide pico viejo stratovolcanoes we have performed several numerical experiments considering a wide range of situations in which the main physical conditions topography size depth and shape of the magma chambers presence of deviatoric stresses internal structure of the volcanic system have been changed the results obtained show that the main control on the pathway that magma will follow to leave the chamber and reach the surface is exerted by the stress field distribution around the chamber which is a function of the shape of the chamber this suggests that in the case of teide pico viejo where there is not a preferential location for new vents each new phonolitic eruption is preceded by the modification of the existing magma chamber or the emplacement of a new batch of magma that does not necessarily matches the structure of the previous one the presence of multiple shallow reservoirs containing eruptible magma also influences the stresses distribution around the erupting magma chamber the numerical results also show that the existence of an extensional stress field plays a significant role in determining the position of the phonolitic feeder dykes

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

formation of dykes feeder dykes and the intrusion of dykes from magma chambers the paper discussed the formation of dykes and applies the results to iceland it is postulated that dykes follow the pathway of least work and of least tensile strength and thereby intrude the subvertical joints in lava flows it is suggested that dykes form in magma pulses where each dyke is split in two by the next magma pulse and so on in iceland the estimated time between successive magma pulses is of the order of several hundred days statistical considerations indicate that in iceland the probability of seeing dykes end upwards is only 1 2 which agrees with observations it is concluded that many and probably the majority of dykes are non feeders this together with the low probability of finding the connection between feeder and lava flow explains the scarcity of observed feeder dykes it is concluded that overpressure in shallow magma chambers needed to drive magma through crustal fractures in iceland is usually smaller than the tensile strength of the host rock several mpa which thereby is the critical factor in dyke intrusion

the difference between oceanic and continental tholeiites a fluid dynamic explanation continental tholeiites have higher sio2 k2o and light rare earth element contents and more evolved isotopic characteristics than their oceanic counterparts these differences can be explained if the compositions of the parent magmas to both types of tholeiites are similar but if continental magmas assimilate significant amounts of continental crust en route to the surface although there is little doubt that most continental tholeiites have assimilated crustal material the lcoation and mechanism of assimilation remain uncertain longhi 1981 has argued that magmas derived directly from the mantle should crystallize little orthopyroxene the abundance of orthopyroxene in most continental layered intrusions suggests that they have crystallized from magmas which have assimilated continental crust since orthopyroxene is an early crystallizing phase in layered intrusions this assimilation must occur early before the magma enters the chamber assimilation can occur at the margins of the dykes which feed magma chambers depending on the nature of the flow if the flow is turbulent the high temperatures at the centre of the dyke will extend to the margins and the magma will erode the dyke walls if the flow is laminar a conductive profile develops at the margin and the flowing magma chills against the walls protecting them from thermal erosion the nature of flow in a dyke can be predicted from the reynolds number the criteria for turbulence reynolds number calculations suggest that the flow of primitive magmas in continental dykes will be fully turbulent and if this is the case assimilation of low melting point components in the walls of the dyke is inevitable it is therefore suggested that many of the geochemical characteristics of continental tholeiites result from melting at the walls of dykes as primitive magmas ascend through the crust

time dependent chamber and vent conditions during explosive caldera forming eruptions abstract we use a modified version of the cpiuc model macedonio g neri a marti j folch a 2005 temporal evolution of flow conditions in sustained explosive eruptions journal of volcanology and geothermal research 143 153 172 to simulate chamber and vent conditions during the different phases of a piston like caldera forming eruption our idealized caldera forming scenario assumes an initial central vent conduit that after critical chamber decompression migrates to a fissure vent peripheral conduit s further decompression leads to final piston like subsidence which stops only after the virtual destruction of the magmatic reservoir the simulations find that the pressure at the conduit entrance drops during the decompression phases at a rate depending on the conduit geometry chamber volatile zonation and fragmentation threshold the higher the volume contrast between the initial central vent and the final peripheral fissure vent conduits the higher the pressure drop and the jump in the mass eruption rate pressure increases back to lithostatic during piston subsidence while some compressible magma remains within the chamber finally during the later phase pressure experiments a gentle increase or decrease depending on the balance between deposition of intra caldera material and decrease in the contents of volatiles as deeper chamber levels are tapped

formation and growth of normal faults at the divergent plate boundary in iceland the divergent plate boundary in iceland is characterized by 40 80 km long and 5 10 km wide swarms of tension fractures 102 m long and normal faults 103 m long the upper part of the crust is mainly composed of lava flows with abundant columnar joints that are mostly perpendicular to the lava contacts the lava flows are horizontal at the surface of the rift zone but become tilted at the rate of 1 for every 150 m depth in the crust at the surface of the rift zone the joints are vertical and parallel to the vertical principal stress because of tilting of the lava pile the columnar joints become oblique to this stress hence becoming potential shear fractures and form echelon sets at greater depths in the crust theoretical considerations suggest that normal faults start to nucleate on sets of en echelon columnar joints and or large scale tension fractures at crustal depths of 0 5 1 5 km the width depth must be the smallest controlling dimension of many faults nevertheless there is a positive linear relation r 0 91 between maximum throw and length of the holocene faults if the faults grow as self similar structures the throw length relationship can be explained by a similar relation between fault length and width

the 1991 eruption of hekla iceland the eruption that started in the hekla volcano in south iceland on 17 january 1991 and came to an end on 11 march produced mainly andesitic lava this lava covers 23 km2 and has an estimated volume of 0 15 km3 this is the third eruption in only 20 years whereas the average repose period since 1104 is 55 years earthquakes as well as a strain pulse recorded by borehole strainmeters occurred less than half an hour before the start of the eruption the initial plinian phase was very short lived producing a total of only 0 02 km3 of tephra the eruption cloud attained 11 5 km in height in only 10 min but it became detached from the volcano a few hours later several fissures were active during the first day of the eruption including a part of the summit fissure by the second day however the activity was already essentially limited to that segment of the principal fissure where the main crater subsequently formed the average effusion rate during the first two days of the eruption was about 800 m3 s 1 after this peak the effusion rate declined rapidly to 10 20 m3 s 1 then more slowly to 1 m3 s 1 and remained at 1 12 m3 s 1 until the end of the eruption site observations near the main crater suggest that the intensity of the volcanic tremor varied directly with the force of the eruption a notable rise in the fluorine concentration of riverwater in the vicinity of the eruptive fissures occurred on the 5th day of the eruption but it levelled off on the 6th day and then remained essentially constant the volume and initial silica content of the lava and tephra the explosivity and effusion rate during the earliest stage of the eruption as well as the magnitude attained by the associated earthquakes support earlier suggestions that these parameters are positively related to the length of the preceeding repose period the chemical difference between the eruptive material of hekla itself and the lavas erupted in its vicinity can be explained in terms of a density stratified magma reservoir located at the bottom of the crust we propose that the shape of this reservoir its location at the west margin of a propagating rift and its association with a crustal weakness all contribute to the high eruption frequency of hekla

evaluation of the origin hypotheses of pantheon fossae central caloris basin mercury the origin of pantheon fossae a complex structure consisting of radial graben in the center of the caloris basin mercury has been debated since the structure was first imaged by the messenger spacecraft three different formation hypotheses have been suggested i e an origin associated with the apollodorus impact into a previously domed caloris basin floor graben formation as surface expressions of dike intrusions and basin interior uplift alone in order to test the scenarios detailed observations from the currently available imagery were compared to the proposed formation mechanisms we evaluate these origin hypotheses by means of detailed interpretations of the graben characteristics and patterns by comparing to radial structures from earth and venus and by mechanical analyses for each formation hypothesis results indicate that the formation of pantheon fossae as the result of doming in the central part of the caloris basin is more likely than it having formed in association with a radially symmetric stress field centered at or near the apollodorus crater that would have been created by a magma chamber or been superimposed on a pre existing dome due to impact mechanics

microearthquakes beneath the median valley of the mid atlantic ridge near 23 n hypocenters and focal mechanisms we report hypocenters and focal mechanisms of microearthquakes located by an ocean bottom seismic network deployed in the median valley of the mid atlantic ridge near 23 n during a 3 week period in early 1982 the network consisted of seven ocean bottom hydrophones and three three component ocean bottom seismometers the instrument coordinates were acoustically determined to within 25 m at the 1 standard deviation level of confidence the hypocentral parameters of the 26 largest microearthquakes are reported 18 of these events have epicenters arid focal depths which are resolvable to within 1 km formal error at the 95 confidence level microearthquakes occur beneath the inner floor of the median valley and have focal depths generally between 5 and 8 km beneath the seafloor composite fault plane solutions for two spatially related groups of microearthquakes beneath the inner floor indicate normal faulting along fault planes that dip at angles of 30 or more these solutions are similar to the mechanisms of nearby large earthquakes microearthquakes also occur beneath the steep eastern inner rift mountains the rift mountain earthquakes have nominal focal depths of 5 7 km and epicenters as distant as 10 15 km from the center of the median valley but these hypocenters have larger uncertainties because of the possible effects of large topographic relief and associated lateral heterogeneity in velocity structure we interpret the depth distribution and source mechanisms of these microearthquakes to indicate that this segment of ridge axis is undergoing brittle failure under extension to a depth of at least 7 8 km we infer that the entire crustal column has been cooled to temperatures within the brittle field of behavior and that significant time has elapsed since the most recent episode of volcanic or shallow magmatic activity along this ridge segment

dynamic and static moduli static and dynamic bulk moduli ks and kd are measured as continuous functions of pressure from zero to 2 3 kilobars for two sandstones a tuff limestone granite and oil shale results for the sandstones and granite are in good agreement with previously reported data with ks kd varying from about 0 5 at atmospheric pressure to close to unity at pressures 2 kilobars and above for rocks behaving elastically under static loading the ks kd ratio is inversely related to the microcrack density for the limestone time dependent deformation associated with pore collapse results in ks kd ratios approaching 0 1 at high pressure upon unloading while initially high 1 0 at high pressures ks kd becomes lower than values obtained during loading at low pressures 1 kilobar due to opening of microcracks generated during pore collapse for the oil shale in which few microcracks exist ks kd remains relatively constant with pressure at a value of about 0 7

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

structure of a sheet laccolith system revealing the interplay between tectonic and magma stresses at stardalur volcano iceland abstract this paper provides detailed structural data on the faults and intrusions of the eroded stardalur volcano in the esja peninsula at the sw margin of the icelandic rift and new lithostratigraphic data on the host rocks a swarm of centrally dipping sheets emplaced with a fan shaped geometry in section view depicts an ellipse in plan view with an about e w major axis sheet intensity reaches 80 in one 100 m transect near the central portion of the swarm where sheets strike e w with sub vertical dip a 200 m thick multiply intrusive body forms a laccolith in the middle of the centrally dipping sheet swarm at a higher altitude individual intrusive units within this laccolith are sub vertical and strike e w in the center acquiring a sill like geometry outwards and were emplaced in a stress field locally dominated by a n s directed least principal stress 3 within the inclined sheet swarm the stress tensor rotated in response to an excess magma pressure probably linked to an underlying magma chamber which interacted with the tectonic stresses crosscutting relationships between the various intrusions and with the host rock indicate that the activity in the central zone of the stardalur volcano ended with the emplacement of the laccolith later eruptions if they occurred at all were fed by centrally dipping sheets in the final stage nne striking regional vertical dykes and normal faults cut the previous structures in response to dominant regional plate tectonic stresses with a wnw ese directed 3 and a nne ssw directed intermediate principal stress 2

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

a review of the origins of subparallel ridges and troughs generalized morphological predictions from terrestrial models the morphologies of ridges and troughs on earth and the processes involved in their formation are reviewed for application to sets of subparallel ridges and troughs on outer planet satellites we consider tension fracturing normal faulting thrust faulting buckle folding strike slip tectonism fissure eruption wall diapirism surface folding of a flow and near surface laccolithic intrusion the landform characteristics and geologic associations predicted of each process are generalized to be independent of planet specific parameters we find that the combinations of landforms derived from each process are unique making each distinguishable in the idealized case morphological parameters most diagnostic of formational process are ridge and trough cross sectional shape termination style and incipient character the characteristics described are useful in constraining the origin of ridge and trough terrains observed in voyager images and will aid in analysis of high resolution images from the galileo and cassini spacecraft

formation of crystal magma chambers in iceland formation of crustal magma chambers in iceland may be facilitated by the occurrence of stress barriers that lead to formation of thick sills such sills absorb the magma of all dikes that enter them and may evolve into magma chambers ideal sites for stress barriers and hence for magma chambers are rock formations where individual layers have different elastic properties the rocks formed during the pleistocene have notably different elastic properties and when buried in the volcanic zones they form more promising sites for magma chambers than the tertiary rocks this may explain why the number of magma chambers indicated by the number of corresponding central volcanoes during the late pleistocene i e during the past 0 7 m y appears to be proportionally greater than the number of chambers i e central volcanoes active during tertiary time

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

a new felsic cone sheet swarm in the central atlantic islands the cone sheet swarm of boa vista cape verde abstract the island of boa vista is one of the oceanic islands with the largest amount of felsic rocks reported in the world 50 of the total outcropping igneous rocks of boa vista the felsic rocks are trachytes and phonolites generated within the second main stage of the volcanic activity that lasted about four million years three consecutive felsic episodes are distinguished the first occurred between 14 5 ma and 13 5 ma the second between 13 6 ma and 12 0 ma and the third and last between 12 0 ma and 10 4 ma their geochemical composition reveals the presence of crystal fractionation processes with participation of feldspars and amphibole as well as different accessory phases involved in each episode the boa vista felsic cone sheet swarm the youngest third episode is formed by several hundreds of peralkaline ne phonolitic sheets distributed within a circular surface about 11 km in radius the cone sheets conform a single general pattern characterized by a decreasing dip outward the structure together with a constant dip of each individual sheet the average inclination of sheets is around 40 in the sectors next to the centre of the structure whereas it is only about 30 next to its periphery the magmatic focus of the sheets is located over 3 km deep almost coinciding with the convergence centre of a contemporary basic radial dike swarm

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

extensional tectonics in the vicinity of a transform fault in north iceland at the junction between an oceanic transform fault and the adjacent ridge segments there is normally a curved fabric the trollaskagi peninsula is located just south of the junction between the kolbeinsey ridge and the tjornes fracture zone and offers a rare opportunity for studying the three dimensional tectonics of the associated curved fabric for the purpose of elucidating its infrastructure we measured the attitudes of tens of lava flows and several hundred dykes faults and mineral veins in 28 profiles in the northern half of the trollaskagi peninsula at sea level the general dip of the lava pile is 6 14 but in the northernmost part of the peninsula the dip is 10 36 with an average of 22 most dykes are basaltic and strike north or nne they are exceptional as regards thickness frequency of multiple dykes and crustal dilation the average thickness 5 9 m and a crustal dilation of 28 in a 4 5 km long profile are the highest figures reported from regional dyke swarms in iceland most faults are normal faults the curved fabric is limited to the northernmost part of the trollaskagi peninsula and consists of two main sets of fractures one set includes north and nne striking faults mineral veins and dykes the other set consists of nw striking oblique faults mineral veins and tilted lava flows a boundary element model indicates that the abnormal lava dip and the nw striking oblique extensional structures in the northernmost part of the peninsula are attributable to the stress field associated with the junction between the kolbeinsey ridge and the tjornes fracture zone

stress fields associated with oceanic transform faults abstract although their formation is commonly attributed to shear stresses oceanic transform faults contain many structural elements indicating extension across them examples include transform parallel tension fractures normal faults grabens dykes and small scale spreading centres at the junctions between transform faults and adjacent ridge segments there are commonly sets of oblique fractures a detailed field study of oblique fractures in the vicinity of a junction between a mid ocean ridge and an oceanic fracture zone in north iceland shows that they are mostly tension fractures and normal faults this paper presents the results of a boundary element study of the stress field associated with the junction between an oceanic transform fault and its ridge segments the results show that uniaxial tensile loading plate pull parallel with the transform fault tends to lock it but also gives rise to shear stress concentration in a zone that coincides with the fault this shear stress is primarily responsible for the strike slip faulting and associated earthquakes in the transform fault to generate the transform parallel fracture zone graben normal faults tensile fractures dykes and spreading centres however requires tensile loading parallel with the ridge axis the results suggest that biaxial ridge parallel and ridge perpendicular tensile loading generates a stress field that 1 unlocks and makes slip easier on the transform faults and 2 explains the oblique fractures at the ridge transform junctions

effect of tensile stress concentration around magma chambers on intrusion and extrusion frequencies what controls the intrusion and extrusion frequencies associated with volcanoes is still poorly understood i propose that for volcanoes at divergent plate boundaries these frequencies may be largely determined by the tensile stress concentration around the magma chambers that feed them this stress concentration is mainly a function of the applied tensile stress associated with spreading and the aspect height width ratios of the chambers high spreading rates and or aspect ratios lead to high rates of tensile stress concentration around the chambers and to an increase in their intrusion frequencies it is found that for chambers at the same depth in a volcanic zone the one of the highest aspect ratio will normally intrude magma most frequently also if the chambers are of equal aspect ratios the one at the greatest depth will intrude magma most frequently because the extrusion frequency of a volcano is a fraction of its intrusion frequency the extrusion frequency may also be largely determined by the rate of tensile stress concentration around the magma chamber that feeds the volcano these results are applied to the divergent plate boundary in iceland where many of the volcanoes appear to be fed by double chambers that is shallow chambers fed by deep seated chambers it is found that except when the aspect ratio of the deep seated chamber is much less than that of the shallow chamber the intrusion frequency of the shallow chamber is normally largely controlled by that of the deep seated chamber it is concluded that whereas the short term i e 103 yrs extrusion frequencies of volcanoes at the plate boundary in iceland may be similar to the dike intrusion frequencies of the source chambers the long term i e 104 yrs extrusion frequencies may be about ten times lower than the intrusion frequencies

seismicity characteristics of a potentially active quaternary volcano the tatun volcano group northern taiwan the tatun volcano group tvg is located at the northern tip of taiwan near the capital taipei and close to two nuclear power plants because of lack of any activity in historical times it has been classified as an extinct volcano even though more recent studies suggestthattvgmighthavebeenactiveduringthelast20ka inmay2003aseismicmonitoringprojectatthetvgareawasinitiated by deploying eight three component seismic stations some of them equipped with both short period and broadband sensors during the 18 months observation period local seismicity mainly consisted of high frequency earthquakes either occurring as isolated events or as a continuous sequence in the form of spasmodic bursts mixed and low frequency events were also present during the same period eventhoughtheyoccurredonlyrarely arrivaltimesfromeventswithclearp s wavephaseswereinvertedinordertoobtaina minimum 1dvelocity modelwithstation corrections probabilistic nonlinear earthquakelocationswere calculated for allthese events using the newly derived velocity model most high frequency seismicity appeared to be concentrated near the areas of hydrothermal activity forming tight clusters at depths shallower than 4 km relative locations calculated using the double difference method and utilising catalogue and cross correlation differential traveltimes showed insignificant differences when compared to the nonlinear probabilistic locations in general seismicity in the tvg area seems to be primarily driven by circulation of hydrothermal fluids as indicated by the occurrence of spasmodic bursts mixed low frequency events and a b value 1 17 0 1 higher than in any other part of taiwan these observations that are similar to those reported in other dormant quaternary volcanoes indicate that a magma chamber may still exist beneath tvg and that a future eruption or period of unrest should not be considered unlikely

laboratory simulations of tensile fracture development in a volcanic conduit via cyclic magma pressurisation during volcanic unrest high magma pressure induces cracking and faulting of the country rock providing conduits for the transport of magma and other fluids these conduits known as dykes are fundamental structures for the transport of magma to the surface in volcanically active regions the mechanics of dyke propagation is not yet fully understood but is crucial to better model dyke emplacement and eruption in volcanoes central to this need is a greater understanding of the mechanical properties of the magma country rock interaction as a function of known magmatic pressure temperature and stress here we report data from a series of experiments in which we cyclically compress viscoelastic rhyolitic magma at 828 c 892 c and 918 c inside a cylindrical conduit like shell of basalt from mt etna italy until fracture occurs the compression is performed under strain rates cyclically varying between 5 10 6 and 5 10 5 s 1 the resultant monitored axial loading and relaxation illustrates how the presence of a visco elastic fluid magma controls the stress induced at the conduit margin boundary this is achieved by analysing the viscoelastic relaxation through time to calculate an apparent modulus which is found to decrease with both increasing temperature and time in the 4 cycles before failure we find that the apparent modulus decreases from 180 to 40 gpa 80 to 20 gpa and 8 to 1 gpa for imposed stress cycles at 828 c 892 c and 918 c respectively we theoretically estimate a tensile strength at failure of approximately 7 11 mpa consistent with recent field data and in agreement with a model derived from the sample geometry and basic material parameters post experimental neutron computed tomography and microscopic analyses further reveal the fragmentation of the melt and generation of tuffisite veins inside the conduit due to spontaneous crack nucleation associated with conduit wall fracture the geometry of the rupture area inside the melt is akin to a mach cone associated with supershear fractures we discuss our findings in terms of magma rock interaction leading to dykes tuffisite veins and magma fragmentation

effects of an oceanic anoxic event on the stable carbon isotopic composition of early toarcian carbon the early toarcian transgression is marked by the occurrence of organic carbon rich shales in large parts of western europe and in other parts of the world as well based on the positive carbon isotope excursion of pelagic limestones in the middle to upper part of the falciferum zone in several of the tethyan sections the widespread occurrence of the early toarcian shales was explained by an oceanic anoxic event oae jenkyns 1988 jenkyns and clayton 1997 the rapid burial of large amounts of organic carbon which is rich in 12 c would have led to a relative enrichment in 13 c of the global carbon reservoir and hence to an increase in delta 13 c of the limestones in southwest germany and a number of other locations both organic and inorganic carbon display a negative isotope excursion in the upper part of the tenuicostatum zone and the lower part of the falciferum zone synchronous to black shale deposition and predating the positive isotopic excursion kuspert 1982 1983 it is presently unclear what the exact cause for this widespread negative isotopic excursion is jenkyns and clayton 1997 the objective of our study was to examine if the negative carbon isotope excursion of organic matter om in the southwest german toarcian shales can be attributed to compositional changes of the om or if the excursion is related to variations in delta 13 c sub co2 aq in the photic zone to this end we analyzed the molecular and stable carbon isotope composition of om in ten samples spanning the early toarcian toc 2 5 10 5 percent hi 350 700 biomarker distributions and kerogen pyrolysates differ only slightly among the samples and delta 13 c values of primary production markers follow closely those of associated carbonate and kerogen delta 13 c sub carb toc this strongly suggests that differences in delta 13 c toc are related to differences in delta 13 c of co 2 aq in the photic zone supporting kuspert9s model of advection of isotopically light organic matter derived co 2 from anoxic bottom waters evidence for euxinic conditions extending into the photic zone was found by the presence of derivatives of carotenoids such as isorenieratane from anoxygenic phototrophic sulfur bacteria their concentrations are extremely low in black shales from the tenuicostatum zone but are significant in the falciferum and bifrons zone with a maximum in the exaratum subzone suggesting that photic zone euxinia pze was a common feature of the early toarcian sea in southwest germany our data therefore support the kuspert model of advection of isotopically light organic matter derived co 2 from anoxic bottom waters to the upper part of the photic zone resulting in a negative isotope spike the enrichment in 13 c in the middle of the falciferum zone which followed the negative spike may be attributed to the burial of isotopically light organic matter through worldwide black shale deposition