Dyke Propagation Research Articles

Volcano load control on dyke propagation and vent distribution: Insights from analogue modeling

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

Volcanoes as elastic inclusions: Their effects on the propagation of dykes, volcanic fissures, and volcanic zones in Iceland

Mechanically, many volcanoes may be regarded as elastic inclusions, either softer (with a lower Young's modulus) or stiffer (with a higher Young's modulus) than the host-rock matrix. For example, many central volcanoes (stratovolcanoes, composite volcanoes) are composed of rocks that are softer than the crustal segments that host them. This is particularly clear in Iceland where central volcanoes are mostly made of soft rocks such as rhyolite, pyroclastics, hyaloclastites, and sediments whereas the host rock is primarily stiff basaltic lava flows. Most active central volcanoes also contain fluid magma chambers, and many have collapse calderas. Fluid magma chambers are best modelled as cavities (in three dimensions) or holes (in two dimensions), entire calderas as holes, and the ring faults themselves, which commonly include soft materials such as breccias, as soft inclusions. Many hyaloclastite (basaltic breccias) mountains partly buried in the basaltic lava pile also function as soft inclusions. Modelling volcanoes as soft inclusions or holes, we present three main numerical results. The first, using the hole model, shows the mechanical interaction between all the active central volcanoes in Iceland and, in particular, those forming the two main clusters at the north and south end of the East Volcanic Zone (EVZ). The strong indication of mechanical interaction through shared dykes and faults in the northern cluster of the EVZ is supported by observations. The second model, using a soft inclusion, shows that the Torfajökull central volcano, which contains the largest active caldera in Iceland, suppresses the spreading-generated tensile stress in its surroundings. We propose that this partly explains why the proper rift zone northeast of Torfajökull has not managed to propagate through the volcano. Apparently, Torfajökull tends to slow down the rate of southwest propagation of the rift-zone part of the EVZ. The third model, again using a soft inclusion, indicates how the lateral propagation of a segment of the 1783 Laki fissure became arrested in the slopes of the hyaloclastite mountain Laki.

A perturbation solution for dyke propagation in an elastic medium with graded density

SUMMARY We present a perturbation method to investigate the steady-state propagation of a dyke from an over pressured source (e.g. a magma chamber) into a semi-infinite elastic solid with graded mass density. The non-linear dyke propagation/magma transport problem is reduced to a series of linear problems using a perturbation technique with the small non-dimensional parameter e = 12ηV /(H 2 �ρ 0g), where η is the magma viscosity, V the propagation velocity, �ρ 0 the difference between the densities of the host rock and magma at the dyke tail, g the gravitational acceleration and H a parameter on the order of maximum dyke thickness. In general, the perturbation method is applicable to mafic dyke propagation at a relatively low propagation velocity wherein e remains small (e.g. less than 0.1). We describe an integral equation approach to obtain the stress intensity factor at the dyke tip and the separation displacement of the two dyke surfaces. Numerical examples are presented to examine the effects of various physical parameters, for example, buoyancy, density gradation, propagation velocity and overpressure on the dyke propagation behaviour. It is found that dyke propagation could reach a steady-state in some specific ranges of growth for a given density gradient of the host rock. The propagation tends to decelerate when the dyke tip approaches the level of neutral buoyancy.

How local stresses control magma-chamber ruptures, dyke injections, and eruptions in composite volcanoes

To assess the probability of a volcanic eruption during an unrest period, we must understand magma-chamber rupture and dyke propagation to the surface, as well as dyke arrest at depth in the volcano. Dyke propagation and arrest depend strongly on the local stresses in the individual mechanical layers which constitute the volcano. The local stresses are primarily determined by the loading conditions (tectonic stress, magmatic pressure, or displacement) and the mechanical properties of the layers. In the absence of stress monitoring of volcanoes, the local stresses must be inferred from models, either analytical or numerical. This paper reviews many analytical and numerical models of local stresses around magma chambers, as well as analytical models and numerical examples of dyke-injection and eruption frequencies. Most analytical models of magma chambers ignore the mechanical properties of the individual layers and their contacts, assume the volcano to behave as a homogeneous, isotropic, elastic half space or a semi-infinite plate, and are of two main types: nuclei of strain and cavities. The best-known nucleus of strain is the point-source Mogi model, used to explain surface deformation as a result of either increase or decrease in magma pressure in a chamber whose depth is also inferred from the surface data. The model explains stresses and displacements far away from the chamber, but neither the stress concentration around the chamber, which determines if and where chamber rupture and dyke injection take place, nor the shape, size, and likely tectonic evolution of the chamber. In the cavity or (two-dimensional) hole model the magma chamber has a finite size. Thus, the local stresses at, and away from, the boundary of a chamber can be calculated. For various loading conditions, an analytical cavity model gives a crude indication of the local stresses in a volcano and its surface deformation. However, variation in mechanical properties, and contacts, between layers are ignored. The analytical cavity model thus cannot be used for detailed analyses of the local stresses in a composite volcano. The numerical models presented here show that the local stresses in a volcano depend strongly on the magma-chamber geometry and the mechanical properties of its layers which are often contrasting, particularly at shallow depths. For example, lava flows, welded pyrolastic units, and intrusions may be very stiff (with a high Young's modulus), whereas young and non-welded pyroclastic and sedimentary units may be very soft (with a low Young's modulus). Consequently, the local stresses may change abruptly from one layer to the next; for example, one layer may favour dyke propagation while an adjacent layer favours dyke arrest. No dyke-fed eruption can occur if there is any layer along the potential path of the dyke to the surface where the stress field is unfavourable to dyke propagation. If such a layer occurs, the dyke normally becomes arrested and an eruption is prevented. The present results indicate that during unrest periods composite volcanoes commonly develop local stresses that arrest dykes and prevent eruptions, in agreement with field observations. These results underline the need for in situ stress monitoring of volcanoes to assess the probability of dyke-fed eruptions.

Dyke nucleation and early growth from pressurized magma chambers: Insights from analogue models

Nucleation and dyke growth from magma chambers are important, yet poorly understood aspects of magma transport in the lithosphere. To tackle this issue, we conducted qualitative experiments using gelatin and silicone to simulate, respectively, country rock and magma. In our experiments the pressurization of the magma-filled reservoir was the main cause of dyke formation rather than the density difference between magma and country rock. Our results show that an overpressurized magma chamber leads to almost simultaneous nucleation of dykes in different parts of the reservoir, especially at places with large angular irregularities where local perturbation of elastic stress occurs. Early propagation of the leading-edge of a dyke results in arc-shape tabular intrusions that are controlled by the stresses surrounding the overpressurized magma chamber. The overall process of early dyke formation reveals three stages: 1) very rapid increase of the fracture length at constant width, 2) slow down (and probably arrest) of the intrusion front and 3) thickening of the intrusion with an almost stationary intrusion front. These stages suggest that dyke propagation to the surface occurs through several steps separated by the time needed for the renewal of magma pressure in the reservoir. In consequence, our experiments suggest that overpressurization of a magma chamber is a necessary condition that must be met to explain why some magmas can reach the surface even when the reservoir is located in a region close to the neutral buoyancy level, but that the distribution of such overpressure is a dynamic variable that requires further examination.

Crustal structure of the Southwest Indian Ridge at 66°E: seismic constraints

The Southwest Indian Ridge represents a slow-spreading end-member of the global mid-ocean ridge system, and as such its structure places important constraints on models of melt supply and delivery from the mantle at ridges. We present results from a wide-angle seismic experiment conducted at the ridge axis at 66°E, in a region that has comprehensive swath bathymetric, gravity and magnetic data coverage and where the full spreading rate is ∼12 mm yr −1 . Based on these data, the experiment traversed four spreading segments. Crustal thickness and velocity structure were determined along three intersecting profiles each ∼100 km long using shots from a 10-gun, 71 L tuned airgun array towed at 15 m depth and fired at 40 s intervals, recorded on three ocean bottom hydrophones on each profile. OBH data show high-amplitude arrivals from oceanic Layer 2, lower-amplitude arrivals from Layer 3 and wide-angle reflections from the Moho. Forward modelling and inversion of traveltime picks from these data show that the crust consists of a 1.5–2.5-km-thick Layer 2 with a high velocity gradient and a 0.5–3.0-km-thick Layer 3 with a low velocity gradient, and a crustal thickness of 2.2–5.4 km. Additional constraints on the models come from 2-D modelling of gravity data along the profiles, corrected for 3-D effects of off-line bathymetry. Along-axis, the thickness of Layer 2 varies little, but Layer 3 is thick at segment centres and very thin at segment boundaries. Along a flowline profile, crustal thickness varies by up to 75 per cent from its minimum value in ∼3 Myr. The reduced crustal thickness is consistent with observations from very slow-spreading ridge axes elsewhere and may be explained by conductive cooling of the upwelling mantle. The large along-axis variations in Layer 3 thickness indicate that magmatic accretion is focused at segment centres and melt is delivered to segment ends perhaps only by lateral dyke propagation. Flowline variations in crustal thickness may result from episodicity of melt supply on timescales of ∼3 Myr and by tectonic extension during amagmatic periods. Velocities at the top of Layer 2 are poorly correlated with crustal age based on magnetic anomalies, suggesting also that episodicity is decoupled between adjacent segments.

Imaging the dynamics of dyke propagation prior to the 2000–2003 flank eruptions at Piton de La Fournaise, Reunion Island

The relationship between shallow magma storage and flank injections in volcanic edifices is rarely documented. We analyse the time series of geodetic and seismic data collected at Piton de La Fournaise volcano (La Reunion) during 9 flank eruptions from 2000 to 2003. The data depicts how magma injections supplied flank eruptions at Piton de la Fournaise volcano, and allows to precisely determine the direction and duration of dike intrusions, indicating a two phases pattern, consistent over all the eruptions: 1) Fast vertical migration of the dyke (∼2 m/s) from the magma chamber to the surface in 10 to 50 minutes, and 2) slower, lateral migration (0.2–0.8 m/s) in tens of minutes to hours controlled by the rift zone geometry.

The effects of layering and local stresses in composite volcanoes on dyke emplacement and volcanic hazards

Abstract Since most volcanic eruptions are fed by dykes, any assessment of volcanic hazards in an area must include an evaluation of the probability of injected dykes either reaching the surface or becoming arrested. Composite volcanoes are normally composed of alternating stiff (high Young's modulus) and soft (low Young's modulus) layers. Numerical models indicate that during unrest periods with magma-chamber inflation, the local stresses in composite volcanoes commonly prevent dyke-fed eruptions: while the stresses in the stiff layers may favour dyke propagation and seismogenic faulting, the local stresses in the soft layer remain seismically quiet and favour dyke arrest. Geodetic and field studies also indicate that most dykes never reach the surface, and that only a small fraction of the magma volume injected from a chamber erupts at the surface. I propose that for a dyke-fed eruption to occur, all the layers along the potential pathway of the dyke must have local stresses that favour magma-driven extension-fracture propagation. Thus, the stress field along the pathway must be homogenised. To cite this article: A. Gudmundsson, C. R. Geoscience 337 (2005).

Rhyodacitic fissure eruption in Southern Andes ( Cordón Caulle; 40.5°S) after the 1960 (Mw:9.5) Chilean earthquake: a structural interpretation

The 1960 rhyodacitic fissure eruption in the Cordón Caulle Volcanic Complex (CCVC), located in Southern Andes (40.5°S) was a unique volcanic episode. The remarkable eruption was triggered by the greatest recorded subduction-zone earthquake, starting 38 h after the main shock, 240 km inland. The structural behaviour, two compound fissures opening along a margin-oblique (NW) structure related to the Quaternary evolution of the CCVC, suggests that the prefractured nature of the upper crust in the Southern Andes was an influential condition for volcanic eruptions. From historical data and morphologic and structural analysis, we suggest that NW structures constitute pathways of steady magmatic ascent. Thus, during the great seismic event, and catalysed by the fluid pressure around a fault, it would have been reactivated allowing, initially, the propagation of a non-Andersonian dyke. Then, the silicic magma would have reached the surface by ‘seismic pumping’. Once the initial activity in the reactivated segments ceased, the local stress field would have changed, favouring the formation of new failures, this time almost parallel to the maximum horizontal stress, and promoting magma transport as Andersonian dykes. Although the rheologic characteristics of the silicic lavas erupted together with the structural behaviour and seismic features of this eruptive cycle constitute rather exceptional conditions in the Southern Andes record, the prefractured nature of the upper crust and the shifting propagation of Andersonian and non-Andersonian dykes provide a theoretical framework to analyse the neotectonics of the volcanic arc in a convergent margin.

Magma storage and horizontal dyke injection beneath a volcanic edifice

A volcanic edifice generates a compressive stress field in the upper crust which affects magma transport. As a central cone builds up, primitive magmas can no longer erupt through the focal area and stall at depth. Within a vertical column terminating below the Earth’s surface, magma is overpressured with respect to the surroundings and may feed a horizontally propagating dyke. We define three regimes for magma transport as a function of magma buoyancy, edifice size and density stratification in the upper crust: (1) eruption through the summit, (2) storage beneath the edifice, (3) horizontal propagation to feed a distal eruptive vent. We study the dynamics of horizontal dyke propagation away from the focal area of a volcanic field. Large horizontal propagation distances can only be achieved if an edifice prevents eruption through the focal area and if magma is negatively buoyant at shallow depth. With no edifice, a horizontal dyke is always tallest at the injection point, implying that, if it is able to breach the surface, it only does so in the focal area. With an edifice, at small distances from the axis, confining stresses due to the edifice load act to impede vertical propagation. In this case, a horizontal dyke does not grow vertically at the injection point and develops a hump at some distance from the focal area, which accounts for distal eruptive centers. During propagation, this hump may be far removed from the dyke tip and migrates at a different velocity. All else being equal, a decreasing supply rate or a decreasing magma viscosity generate eruptive centers at increasing distances from the focal area. This is consistent with the distribution of erupted products in many volcanic fields, such that, with increasing distance from the focal area, magma compositions are less and less evolved.

Preliminary results of a study of magnetic properties in the Foum-Zguid dyke (Morocco)

This work focuses on the study of flow and propagation of magma using rock magnetic analyses along sections across the thick Jurassic dyke of Foum-Zguid (Southern Morocco). Thermomagnetic data show that Ti-poor titanomagnetite is the main magnetic carrier. Petrographic analysis shows that the main Ti phase (ilmenite) occurs either as lamellae within spinel (center of the dyke) or as isolated grains (dyke margin). Bulk magnetic properties display distinct behavior according to the distance to the dyke margin; grain size of the main magnetic carrier decreases towards the center of the dyke, while the natural remanent magnetization and the bulk magnetic susceptibility increase. Only the magnetic susceptibility ellipsoid close to the dyke margin corresponds to that usually found in thin dykes, with the magnetic foliation sub parallel to dyke margins. Maximum principal axis is in most cases either parallel or perpendicular to the intersection between the planes of magnetic foliation and dyke wall. Moreover, when this axis is perpendicular to the intersection it is associated with a more oblate magnetic susceptibility ellipsoid shape, indicating the presence of complex magnetic fabrics. The studied magnetic properties show that, in this 100 m wide thick dyke, flow structures related with dyke propagation are only preserved close to the quickly cooled dyke margins.

An image of a magma intrusion process inferred from precise hypocentral migrations of the earthquake swarm east of the Izu Peninsula

Earthquake swarms in the area east of the Izu Peninsula, Central Japan have been active and have been repeated intermittently since 1978 after 40 years of quiescence. The activities were always accompanied by crustal deformations, which were well modelled by dyke intrusions. To study the process of a dyke intrusion, precise hypocentres of the latest activity occurring in 1998 are obtained in this paper using waveform similarity, by which new images of volcanic processes have been successfully acquired at several volcanoes. The relocated hypocentres are mainly aligned on a thin vertical plane with a circular shape at a depth of 3–7 km, and there is an aseismic area at the centre. The normal direction to the plane coincides well with the direction of tectonic extensional stress around the hypocentral area and matches theoretical models well. At the beginning of the activity, a small fraction of the events occurred at greater depth, where hypocentres align on a vertical line and migrate upward at a rate of 1 km h−1. The migration rate in this stage agrees well with a theoretical model of buoyancy-driven dyke propagation. After 1 day from the beginning, earthquakes began to occur around the aseismic area, and hypocentres spread on the thin circular plane described above. During this activity, the hypocentres seemed to migrate downwards and upwards from the centre. The earthquakes are caused by shear fracture at the tip of the dyke and the migration of hypocentres reflects that the dyke expands downward and upward from the neutrally buoyant layer that is located at the centre of the swarm. In this paper, we also propose a process of magma intrusion based on the precise migration of hypocentres, and show that the 1998 activity is composed of a few simple volcanic processes: magma rising by buoyancy, it staying at a neutral buoyancy point and spreading outward from the neutral point with elastic fracture by an inside excess magma pressure.

Dynamic dyke propagation deduced from tilt variations preceding the March 9, 1998, eruption of the Piton de la Fournaise volcano

During the hour preceding the March 9, 1998, eruption of the Piton de la Fournaise volcano, the deformation network of the Observatory recorded some large deformations in the summit area. A broadband seismic station of the GEOSCOPE global network, RER, is located about 8 km away from the summit of the volcano. Signals from that station may be interpreted as tilt changes. The combination of the above two kinds of signals allows, by using a tensile fault model, to constrain the geometry as well as some characteristics (volume, propagation velocity) of the dyke that intruded the summit area during the hour preceding the beginning of eruptive activity.

Crustal anatexis and its relation to the exhumation of collisional orogenic belts, with particular reference to the Himalaya

AbstractWe review the causes, mechanisms and consequences of crustal anatexis during the exhumation of metamorphic terranes, from a petrological perspective. During both prograde and retrograde metamorphism, limited influx of free hydrous fluids may result in small volumes of very hydrous melts, which cannot ascend far (if at all) before reaching their solidus. If thermal conditions for dehydration melting are attained in fertile micaceous crustal layers, much larger volumes of water-undersaturated granitic magmas may result, especially where limited external fluid influx raises water activities above those that may be buffered by dehydrating hydrous phases. Magmas have specific trace element characteristics depending on the reaction which formed them which, combined with accessory phase thermometry, may enable the (P-T) conditions of melting to be ascertained. Small volume-fraction magmas will typically remain as in situ migmatites unless their extraction is assisted by deformation. In turn, deformation will be focused in weaker partially molten zones, so that water-undersaturated magmas may often be mobilized. Once segregated, their ascent is limited by the rate of dyke propagation, and they may reach shallow levels (<2 kbar) before crystallizing. The complex interplay between deformation and melting is exemplified by the Miocene evolution of the central Himalaya, where thrust and normal faulting, melting and exhumation were all simultaneously active processes which were linked by feedback relations. In the Nanga Parbat Massif of the western Himalaya, rapid post-Miocene denudation and vigorous fluid flux enabled rocks to experience more than one episode of melting simultaneously, at different levels of the same exhuming crustal section.

Heat transfer and melting in subglacial basaltic volcanic eruptions: implications for volcanic deposit morphology and meltwater volumes

Abstract Subglacial volcanic eruptions can generate large volumes of meltwater that is stored and transported beneath glaciers and released catastrophically in jökulhlaups. At typical basaltic dyke propagation speeds, the high strain rate at a dyke tip causes ice to behave as a brittle solid; dykes can overshoot a rock-ice interface to intrude through 20–30% of the thickness of the overlying ice. The very large surface area of the dyke sides causes rapid melting of ice and subsequent collapse of the dyke to form a basal rubble pile. Magma can also be intruded at the substrate-ice interface as a sill, spreading sideways more efficiently than a subaerial flow, and also producing efficient and widespread heat transfer. Both intrusion mechanisms may lead to the early abundance of meltwater sometimes observed in Icelandic subglacial eruptions. If meltwater is retained above a sill, continuous melting of adjacent and overlying ice by hot convecting meltwater occurs. At typical sill pressures under more than 300 m ice thickness, magmatic CO 2 gas bubbles form c. 25 vol% of the pressurized magma. If water drains and contact with the atmosphere is established, the pressure decreases dramatically unless the overlying ice subsides rapidly into the vacated space. If it does not, further CO 2 exsolution plus the onset of H 2 O exsolution has the potential to cause explosive fragmentation, i.e. a fire-fountain that forms at the dyke-sill connection, enhancing melting and creating another candidate pulse of meltwater. The now effectively subaerial magma body becomes thicker, narrower, and flows faster so that marginal meltwater drainage channels become available. If the ice overburden thickness is much less than c. 300m the entire sill injection process may involve explosive magma fragmentation. Thus, there should be major differences between subglacial eruptions under local or alpine glaciers compared with those under continental-scale glaciers.

The effect of edifice load on magma ascent beneath a volcano

A volcanic edifice exerts a large load at Earth's surface and modifies the stress field at depth. We investigate how this affects upward dyke propagation towards the surface. For given edifice dime...

The growth of dykes from magma chambers

Failure of the margins of magma chambers produces dykes which may breach the ground surface and cause an eruption. The pressurization and failure of liquid-filled cavities is investigated with experiments in which gelatine is used as the analogue for a crustal medium. A theoretical analysis is derived which considers dyke nucleation to occur by the failure of a pre-existing magma-filled crack in the chamber wall. Flow of magma from the chamber into the crack gradually pressurizes and widens the crack. Eventually, the stress conditions for failure of the crack-tip are attained and dyke propagation commences. Magma viscosity influences the flux into the crack and hence the rate at which the crack-pressure increases. Higher viscosity magmas consequently require greater chamber overpressures and/or longer time delays before dyke nucleation occurs. The theoretical analysis is in approximate agreement with experimental results and provides estimates of the critical conditions required for a magma chamber to rupture. If a basaltic magma chamber is suddenly pressurized (e.g., by replenishment), the delay before dyke nucleation is predicted to be on the order of hours to days. For rhyolitic magmas, the delays are typically many years. Where chambers are gradually becoming pressurized (e.g., by exsolution during magmatic crystallization), cracks in the walls of relatively silicic magma chambers are likely to be further from pressure equilibration with the chamber, resulting in the development of higher chamber overpressures before dyke nucleation occurs. Solidification in silicic cracks which are slowly pressurizing may inhibit crack-tip failure; greater chamber overpressures are consequently required before dykes eventually form. The implications of higher chamber overpressures during dyke nucleation are that repose periods are longer, dykes propagate further before freezing, which allows dykes to transport silicic magmas over substantial distances, and a more vigorous eruption occurs if the dyke breaches the surface.

Dyke propagation with distributed damage of the host rock

Observations of off-plane inelastic deformation around dykes motivate consideration of models of fluid-driven crack propagation in a solid which can undergo material degradation, or damage. The application to dyke propagation of a recently proposed damage rheology [Lyakhovsky et al., J. Geophys. Res. 102 (1997) 27635–27649] based on thermodynamical principles and experimental measurements is discussed. The rate of accumulation of damage in this rheology is the product of a material-dependent parameter c d and the square of the strain. For geological values, a dimensionless parameter c d η/Δ P characterizing the ratio of a damage timescale to a flow timescale is very small, where η is the magmatic viscosity and Δ P the driving pressure. As a result, significant rates of damage are confined to a small region near the dyke tip, where the strain is large. Consideration of possible singularities in near-tip solutions, shows that the rate of propagation is governed by the viscous fluid mechanics. To a good approximation, the rate has a value equal to that given by the zero-stress-intensity solutions of previous models based on linear elastic fracture mechanics. Predictions from the damage rheology both of a narrow damage zone and of the rate of propagation are in good agreement with observations.

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.

Rheological controls on Grenvillian intrusive suites: implications for tectonic analysis

Studies of plutonic suites in western Grenville, along with the recent advances in understanding magma ascent and emplacement, and interactions between magma and their country rock, provide insights on the use of intrusive rocks to infer the accretionary history of deeply-exposed Precambrian orogens. In the Grenvillian Central Metasedimentary Belt of Québec, terrane juxtaposition was inferred to be late based on the distribution of 1.08 Ga potassic alkaline plutons. This view was questioned, however, upon recognition that magma ascent by dyke propagation can be stalled to enable the formation of plutons upon intersecting rheologically softer units, for example Grenvillian marble. In this context, preferential ponding of magmas in marble rather than tectonics can control regional distribution of plutons. The spatial association of plutons with marble and their contact relationships are, at the local scale, obscured by transformation of marble to skarn and by mechanical excision of marble from the paragneiss wall-rock sequence. In contrast, plutons and dykes of an older, 1.17 Ga, less alkaline magma association are shown by field and remote-sensing studies to be evenly distributed across the various lithotectonic domains of the belt. Their sheet-like emplacement along the belt boundary constrains Grenvillian tectonic assembly in the region to be early. Contrasting loci, type and degree of deformation of intrusive bodies illustrate that host-rock rheological differences influence the final characteristics of an intrusive suite. Consequently, the nature of the host is an important variable in inferring timing relationships and tectonic scenarios from intrusive suites in high-grade terranes.