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1.
The influence of magma expansion due to volatile exsolution and gas dilation on dyke propagation is studied using a new numerical code. Many natural magmas contain sufficient amounts of volatiles for fragmentation to occur well below Earth's surface. Magma fragmentation has been studied for volcanic flows through open conduits but it should also occur within dykes that rise towards Earth's surface. The characteristics of volatile-rich magma flow within a hydraulic fracture are studied numerically. The mixture of melt and gas is treated as a compressible viscous fluid below the fragmentation level and as a gas phase carrying melt droplets above it. The numerical code solves for elastic deformation of host rocks, the flow of the magmatic mixture and fracturing at the dyke tip. With volatile-free magma, a dyke fed at a constant rate in a uniform medium adopts a constant shape and width and rises at a constant velocity. With volatiles involved, magma expands and hence the volume flux of magma increases. With no fragmentation, this enhanced flux leads to acceleration and thinning of the dyke. Simple scaling laws allow accurate predictions of dyke width and ascent rate for a wide range of conditions. With fragmentation, dyke behaviour is markedly different. Due to the sharp drop of head loss that occurs in gas-rich fragmented material, large internal overpressures develop below the dyke tip and induce swelling of the nose region, leading to deceleration of the dyke. These results are applied to the two-month long period of volcanic unrest that preceded the May 1980 eruption of Mount St Helens. An initial phase of rapid earthquake migration from the 7–8 km deep reservoir to shallow levels was followed by very slow progression of magma within the edifice. Such behaviour can be accounted for by magma fragmentation at the top of a dyke.  相似文献   

2.
We present reults from simultaneous precise levelling and gravity surveys on Mount Etna covering the period August 1980–August 1981. The flank eruption of March 1981 erupted 18–35 × 105m3 of lava. Following it, upward movements of more than 17 cm were observed close to the new fissure and a broad, apparently independent, uplift of 5 cm was observed 4 km to the west. A zone of about 2 cm depression to the east of the fissure is insufficient to account for the volume of magma erupted. Gravity results show positive changes of up to 63 microgal, and display good positive correlation with elevation changes. Both sets of measurements appear to be due to new intrusion of magma rather than subsurface magma drainage. Ground deformation close to the new fissure is well modelled by intrusion of a dyke in the zone 100–500 m below the surface, striking along the fissure and of dip between 75–90°. The gravity changes are modelled as due to a deeper intrusion of magma, along the same line but some 1500 m below the surface. The changes were not present immediately after the eruption but occurred during the ensuing 5 months. It is proposed that this introduction of matter occurred by crack propagation along the fissure in the aftermath of the eruption. Towards the west of the fissure, and some 4 km west of the summit, ground deformation is modelled by intrusion of a dyke in the zone 300–1500 m below the surface and dipping at 80–85°. Again, gravity changes appear to be due to magma intrusion at greater depth, close to sea level. In this case gravity changes are interpreted as due to magma density changes, as a result of pressure increase in a larger scale fissure zone. This same pressure increase may be forcing the new intrusion close to the surface, and makes this part of the volcano a region of especially high risk.  相似文献   

3.
Eruptions from the top of a dyke containing two layers of magma can selectively withdraw the upper layer, leaving the dense lower layer undisturbed. Alternatively, if the upper layer is thinner than some critical depth, d, then both layers will be tapped simultaneously. Laboratory experiments yield an equation giving the draw-up depth, d, as a function of dyke geometry, eruption rate, and magma properties. This equation is valid for low to moderate Reynolds numbers and applies to dykes which are much longer than the draw-up depth. Short dykes will yield larger draw-up depths than are predicted by the equation. A large draw-up depth is favoured when the eruption rate, upper layer magma viscosity, or dyke length/breadth ratio is large or the density difference is small. Calculations show that rhyolite-capped dykes can contain several hundred metres thickness of rhyolite when a lower layer is first tapped. Draw-up depths in a dyke are as much as an order of magnitude greater than those for an identical eruption from a large cylindrical chamber tapped by a central vent. Nonetheless, for low effusion rate eruptions from small dykes, as at Inyo Domes, California, relatively small draw-up heights are calculated (e.g. 70 m). This is compatible with the small amounts of mixed magmas found at the transition between the two rhyolite magmas erupted there [11].  相似文献   

4.
The 2002–03 Mt Etna flank eruption began on 26 October 2002 and finished on 28 January 2003, after three months of continuous explosive activity and discontinuous lava flow output. The eruption involved the opening of eruptive fissures on the NE and S flanks of the volcano, with lava flow output and fire fountaining until 5 November. After this date, the eruption continued exclusively on the S flank, with continuous explosive activity and lava flows active between 13 November and 28 January 2003. Multi-disciplinary data collected during the eruption (petrology, analyses of ash components, gas geochemistry, field surveys, thermal mapping and structural surveys) allowed us to analyse the dynamics of the eruption. The eruption was triggered either by (i) accumulation and eventual ascent of magma from depth or (ii) depressurisation of the edifice due to spreading of the eastern flank of the volcano. The extraordinary explosivity makes the 2002–03 eruption a unique event in the last 300 years, comparable only with La Montagnola 1763 and the 2001 Lower Vents eruptions. A notable feature of the eruption was also the simultaneous effusion of lavas with different composition and emplacement features. Magma erupted from the NE fissure represented the partially degassed magma fraction normally residing within the central conduits and the shallow plumbing system. The magma that erupted from the S fissure was the relatively undegassed, volatile-rich, buoyant fraction which drained the deep feeding system, bypassing the central conduits. This is typical of most Etnean eccentric eruptions. We believe that there is a high probability that Mount Etna has entered a new eruptive phase, with magma being supplied to a deep reservoir independent from the central conduit, that could periodically produce sufficient overpressure to propagate a dyke to the surface and generate further flank eruptions.Editorial responsibility: J. Donnelly-Nolan  相似文献   

5.
Failed magmatic eruptions: late-stage cessation of magma ascent   总被引：4，自引：2，他引：2  
When a volcano becomes restless, a primary question is whether the unrest will lead to an eruption. Here we recognize four possible outcomes of a magmatic intrusion: “deep intrusion”, “shallow intrusion”, “sluggish/viscous magmatic eruption”, and “rapid, often explosive magmatic eruption”. We define “failed eruptions” as instances in which magma reaches but does not pass the “shallow intrusion” stage, i.e., when magma gets close to, but does not reach, the surface. Competing factors act to promote or hinder the eventual eruption of a magma intrusion. Fresh intrusion from depth, high magma gas content, rapid ascent rates that leave little time for enroute degassing, opening of pathways, and sudden decompression near the surface all act to promote eruption, whereas decreased magma supply from depth, slow ascent, significant enroute degassing and associated increases in viscosity, and impingement on structural barriers all act to hinder eruption. All of these factors interact in complex ways with variable results, but often cause magma to stall at some depth before reaching the surface. Although certain precursory phenomena, such as rapidly escalating seismic swarms or rates of degassing or deformation, are good indicators that an eruption is likely, such phenomena have also been observed in association with intrusions that have ultimately failed to erupt. A perpetual difficulty with quantifying the probability of eruption is a lack of data, particularly on instances of failed eruptions. This difficulty is being addressed in part through the WOVOdat database. Papers in this volume will be an additional resource for scientists grappling with the issue of whether or not an episode of unrest will lead to a magmatic eruption.  相似文献   

6.
During the July–August 2001 eruption of Mt. Etna development of extensional fractures/faults and grabens accompanied magma intrusion and subsequent volcanic activity. During the first days of the eruption, we performed an analysis of attitude, displacement and propagation of fractures and faults exposed on the ground surface in two sites, Torre del Filosofo and Valle del Leone, located along the same fracture system in the region surrounding the Valle del Bove depression on the eastern flank of Mt. Etna. Fractures and faults formed as the consequence of a shallow intruding dyke system that fed the several volcanic centres developed along the fracture system. The investigated sites differ in slope attitude and in geometrical relationships between fractures and slopes. In particular, the fracture system propagated parallel to the gentle slope (<7° dip) in the Torre del Filosofo area, and perpendicular to the steep slope (25° dip) in the Valle del Leone area. In the Torre del Filosofo area, slight graben subsidence and horizontal extension of the ground surface by about 3 m were recorded. In the Valle del Leone area, extensional faulting forming a larger and deeper graben with horizontal extension of the ground surface by about 10 m was recorded. For the Valle del Leone area, we assessed a downhill dip of 14° for the graben master fault at the structural level beneath the graben where the fault dip shallows. These results suggest that dyke intrusion at Mount Etna, and particularly in the region surrounding the Valle del Bove depression, may be at the origin of slope failure and subsequent slumps where boundary conditions, i.e. geometry of dyke, slope dip and initial shear stress, amongst others, favour incipient failures.  相似文献   

7.
The 1991–1993 eruption was probably the largest on Mt. Etna for 300 years. Since then the volcano has entered an unusually quiescent period. A comprehensive record of gravity and ground deformation changes presented here bracket this eruption and give valuable insight into magma movements before, during and after the eruption. The gravity and deformation changes observed before the eruption (1990–1991) record the intrusion of magma into the summit feeder and the SSE-trending fracture system which had recently been active in 1978, 1979, 1983 and 1989, creating the feeder dyke for the 1991–1993 eruption. In the summit region gravity changes between 1992 and 1993 (spanning the end of the eruption) reflect the withdrawal of magma from the conduit followed more recently (1993–1994) by the re-filling of magma in the conduit up to pre-eruption levels. In contrast, in the vicinity of the fracture zone, gravity has remained at the 1991–1992 level, indicating that no withdrawal has occurred here. Rather, magma has solidified in the fracture system and sealed it such that the 1993–1994 increase in magma level in the conduit was not accompanied by further intrusion into the flanks. Mass calculations suggest that a volume of at least 107 m3 of magma has solidified within the southeastern flank of the volcano.  相似文献   

8.
Kimberlite in certain dykes and in the deepest parts of some diatremes show textural and other features which contrast with those in the breccia diatremes. Some hypabyssal kimberlite intrusions show relatively high-temperature contact phenomena including baking of country-rock sediments and sedimentary xenoliths, and contrasting with the brecciated texture of most diatreme-facies kimberlites, in the hypabyssal kimberlites are numerous examples of preferred orientation of inequidimensional minerals (? trachytic flow texture), and rapid mineralogical gradients from the contact towards the dyke centres that may be attributable to flowage differentiation. In the Benfontein sill (Kimberley area) there is well-developed horizontal banding due to gravitational settling, and pseudo-sedimentary structures are also present. The accumulated evidence indicates that kimberlite existed as a relatively hot fluid up to depths of 2–3 km below the land-surface at the time of intrusion; above this level, gas release caused diatreme formation, brecciation and adiabatic cooling. These views are contrary to those of geologists who postulate eruption of kimberlite as a cold breccia directly from the mantle or deep within the crust, but accords with the views of many Russian geologsts who accept the existence of kimberlite magma, the extrusive equivalent of which is the ultrabasic lava meimechite.  相似文献   

9.
Three-dimensional seismic data from the Faeroe-Shetland Basin provides detailed information on the relationships between sills, dykes, laccoliths and contemporaneous volcanic activity. The data shows that sills are predominantly concave upwards, being complete or partial versions of radially or bilaterally symmetrical forms that possess flat inner saucers connected to a flat outer rim by a steeply inclined sheet. Such morphologies are only partially modified by pre-existing faults. Sills can be sourced from dykes or the steep climbing portions of deeper sills. Both sills and dykes can provide magma to overlying volcanic fissures and sills can be shown to feed shallow laccoliths. Magma flow patterns, as revealed by opacity rendering, suggest that sills propagate upwards and outwards away from the magma feeder. As an individual sill can consist of several leaves emplaced at different stratigraphic levels, and as a sill or dyke can provide magma to volcanic fissures, other sills and laccoliths, the data suggests that neutral buoyancy concepts may not provide a complete explanation for the mechanism and level of sill emplacement. Instead, the data suggests that the presence of lithological contrasts, particularly ductile horizons such as overpressured shales may permit sill formation at any level below the neutrally buoyant level. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. Ken Thomson–deceased, April 2007  相似文献   

10.
The May 2005 eruption of Fernandina volcano, Galápagos, occurred along circumferential fissures parallel to the caldera rim and fed lava flows down the steep southwestern slope of the volcano for several weeks. This was the first circumferential dike intrusion ever observed by both InSAR and GPS measurements and thus provides an opportunity to determine the subsurface geometry of these enigmatic structures that are common on Galápagos volcanoes but are rare elsewhere. Pre- and post- eruption ground deformation between 2002 and 2006 can be modeled by the inflation of two separate magma reservoirs beneath the caldera: a shallow sill at ~1 km depth and a deeper point-source at ~5 km depth, and we infer that this system also existed at the time of the 2005 eruption. The co-eruption deformation is dominated by uplift near the 2005 eruptive fissures, superimposed on a broad subsidence centered on the caldera. Modeling of the co-eruption deformation was performed by including various combinations of planar dislocations to simulate the 2005 circumferential dike intrusion. We found that a single planar dike could not match both the InSAR and GPS data. Our best-fit model includes three planar dikes connected along hinge lines to simulate a curved concave shell that is steeply dipping (~45–60°) toward the caldera at the surface and more gently dipping (~12–14°) at depth where it connects to the horizontal sub-caldera sill. The shallow sill is underlain by the deep point source. The geometry of this modeled magmatic system is consistent with the petrology of Fernandina lavas, which suggest that circumferential eruptions tap the shallowest parts of the system, whereas radial eruptions are fed from deeper levels. The recent history of eruptions at Fernandina is also consistent with the idea that circumferential and radial intrusions are sometimes in a stress-feedback relationship and alternate in time with one another.  相似文献   

11.
The relatively low rates of magma production in island arcs and continental extensional settings require that the volume of silicic magma involved in large catastrophic caldera-forming (CCF) eruptions must accumulate over periods of 10 5 to 10 6 years. We address the question of why buoyant and otherwise eruptible high-silica magma should accumulate for long times in shallow chambers rather than erupt more continuously as magma is supplied from greater depths. Our hypothesis is that the viscoelastic behavior of magma chamber wall rocks may prevent an accumulation of overpressure sufficient to generate rhyolite dikes that can propagate to the surface and cause an eruption. The critical overpressure required for eruption is based on the model of Rubin (1995a). An approximate analytical model is used to evaluate the controls on magma overpressure for a continuously or episodically replenished spherical magma chamber contained in wall rocks with a Maxwell viscoelastic rheology. The governing parameters are the long-term magma supply, the magma chamber volume, and the effective viscosity of the wall rocks. The long-term magma supply, a parameter that is not typically incorporated into dike formation models, can be constrained from observations and melt generation models. For effective wall-rock viscosities in the range 10 18 to 10 20 Pa s –1, dynamical regimes are identified that lead to the suppression of dikes capable of propagating to the surface. Frequent small eruptions that relieve magma chamber overpressure are favored when the chamber volume is small relative to the magma supply and when the wall rocks are cool. Magma storage, leading to conditions suitable for a CCF eruption, is favored for larger magma chambers (>10 2 km 3) with warm wall rocks that have a low effective viscosity. Magma storage is further enhanced by regional tectonic extension, high magma crystal contents, and if the effective wall-rock viscosity is lowered by microfracturing, fluid infiltration, or metamorphic reactions. The long-term magma supply rate and chamber volume are important controls on eruption frequency for all magma chamber sizes. The model can explain certain aspects of the frequency, volume, and spatial distribution of small-volume silicic eruptions in caldera systems, and helps account for the large size of granitic plutons, their association with extensional settings and high thermal gradients, and the fact that they usually post-date associated volcanic deposits.  相似文献   

12.
The Monte Guardia rhyolitic eruption (~22 ka, Lipari, Aeolian Islands, Italy) produced a sequence of pyroclastic deposits followed by the emplacement of lava domes. The total volume of dense magma erupted was nearly 0.5 km3. The juvenile clasts in the pyroclastic deposits display a variety of magma mixing evidence (mafic magmatic enclaves, streaky pumices, mineral disequilibria and heterogeneous glass composition). Petrographic, mineralogical and geochemical investigations and melt inclusion studies were carried out on the juvenile clasts in order to reconstruct the mixing process and to assess the pre-eruptive chemico-physical magmatic conditions. The results suggest that the different mingling and mixing textures were generated during a single mixing event between a latitic and a rhyolitic end member. A denser, mixed magma was first erupted, followed by a larger volume of an unmixed, lighter rhyolitic one. This compositional sequence is the reverse of what would be expected from the tapping of a zoned magma chamber. The Monte Guardia rhyolitic magma, stored below 200 MPa, was volatile-rich and fluid-saturated, or very close to this, despite its relatively low explosivity. In contrast to previous interpretations, there exists the possibility that the rhyolite could rise and erupt without the trigger of a mafic input. The entire data collected are compatible with two possible mechanisms that would generate a reversely zoned sequence: (1) the occurrence of thermal instabilities in a density stratified, salic to mafic magma chamber and (2) the intrusion of rising rhyolite into a shallower mafic sill/dike.  相似文献   

13.
Recent stratigraphic studies at Vesuvius have revealed that, during the past 4,000 years, long lasting, moderate to low-intensity eruptions, associated with continuous or pulsating ash emission, have repeatedly occurred. The present work focuses on the AS1a eruption, the first of a series of ash-dominated explosive episodes which characterized the period between the two Subplinian eruptions of 472 AD and 1631 AD. The deposits of this eruption consist of an alternation of massive and thinly laminated ash layers and minor well sorted lapilli beds, reflecting the pulsatory injection into the atmosphere of variably concentrated ash-plumes alternating with Violent Strombolian stages. Despite its nearly constant chemical composition, the juvenile material shows variable external clast morphologies and groundmass textures, reflecting the fragmentation of a magma body with lateral and/or vertical gradients in both vesicularity and crystal content. Glass compositions and mineralogical assemblages indicate that the eruption was fed by rather homogeneous phonotephritic magma batches rising from a reservoir located at ~ 4 km (100 MPa) depth, with fluctuations between magma delivery and magma discharge. Using crystal size distribution (CSD) analyses of plagioclase and leucite microlites, we estimate that the transit time of the magma in the conduit was on the order of ~ 2 days, corresponding to an ascent rate of around 2 × 10−2 ms−1. Accordingly, assuming a typical conduit diameter for this type of eruption, the minimum duration of the AS1a event is between about 1.5 and 6 years. Magma fragmentation occurred in an inertially driven regime that, in a magma with low viscosity and surface tension, can act also under conditions of slow ascent.  相似文献   

14.
A levelling network and a horizontal distance measuring network down Mt. Etna's south flank were measured before, during and after the 1983 eruption. We analyse the large movements recorded, using theoretical models of surface displacement, to determine the shape and position of the feeder conduit that supplied magma to the eruption. The results indicate that a dyke, the top surface extending nearly horizontally, connects the eruption site to a point near the Southeast Crater. The top of the dyke lies at an altitude of 2400 m to 2500 m, and dips between 75° and 90° to the west.More tentative evidence indicates that the top of the active dyke dropped by about 150 m between the 25th and the 95th day of the eruption, and that a small branching dyke 1 km long was emplaced to the east of the main dyke, but did not attain the surface. The implications of these observation are discussed.  相似文献   

15.
The 2007 effusive eruption of Stromboli followed a similar pattern to the previous 2002–2003 episode. In both cases, magma ascent led to breaching of the uppermost part of the conduit forming an eruptive fissure that discharged lava down the Sciara del Fuoco depression. Both eruptions also displayed a ‘paroxysmal’ explosive event during lava flow output. From daily effusion rate measurements retrieved from helicopter- and satellite-based infrared imaging, we deduce that the cumulative volume of lava erupted before each of the two paroxysms was similar. Based on this finding, we propose a conceptual model to explain why both paroxysms occurred after this ‘threshold’ cumulative volume of magma was erupted. The gradual decompression of the deep plumbing system induced by magma withdrawal and eruption, drew deeper volatile-rich magma into the conduit, leading to the paroxysms. The proposed model might provide a basis for forecasting paroxysmal explosions during future effusive eruptions of Stromboli.  相似文献   

16.
Magma mixing: petrological process and volcanological tool   总被引：4，自引：0，他引：4  
Magma mixing is a widespread, if not universal igneous phenomenon of variable importance. The evidence for magma mixing is found primarily in glassy tephra; the consolidation of lava obscures the evidence. Inclusions of glass in big crystals in tephra, because of their greater range in composition compared to the whole rock and the residual glass, indicate that the big crystals were derived from separate systems which mixed together prior to and during eruption. The observed or reconstructed concentration of K2O in inclusions of glass in large crystals represent the composition of the contaminant and host systems. Selective enrichment in K2O during entrapment of melt by growing crystals is shown to be negligible. The weight percents of K2O in host, contaminant and residual glass and bulk rock determine the proportions of contaminant and host required to yield either the residual glass or bulk rock. In several cases the proportion of contaminant required is substantially larger than the proportion of crystals in the hybrid magma; therefore, by heat budget argument, the contaminant was partly liquid when contamination began. In some tephra individual phenocrysts contain glasses which are more silicic toward the center of the crystal indicating that the crystal grew from a melt whose composition changed in the opposite sense to that expected for progressive solidification of a closed system. Space time associations of compositionally distinct glassy tephra with contaminated magmas suggest coexistence of basaltic and silicic melts within magma systems. Evidence of contamination is present in most tephra studied so far. Magma mixing appears to be the prevalent process whereby contamination occurs. Magma mixing seems to be particularly evident in systems where there is independent evidence for a vapor-saturated magma reservoir. Probably vapor saturation promotes mixing in magma systems. Magma mixing probably is an important mechanism of compositional diversification (differentiation) of volcanic rocks from continental margin and possibly other environments.Textural evidence of the onset of magma mixing can be related to disturbance of a complex reservoir immediately before ascent and eruption. Thus, conditions before mixing can be ascribed to the reservoir. In this way it is possible to learn about the reservoir: its composition, its diversity, its depth, its walls. It is also possible to learn about the causes of eruption: whether by increase in gas pressure due to either progressive consolidation, or heating from below by an injection of hot magma, or by encounter with ground water; whether by buoyant rise. Evaluation of these problems requires also a thorough knowledge of the chronology of particular eruptions. Thus, magma mixing is a useful volcanological tool.  相似文献   

17.
The behaviour of a magma plumbing system during a cycle of volcanic edifice growth is investigated with a simple physical model. Loading by an edifice at Earth's surface changes stresses in the upper crust and pressures in a magma reservoir. In turn, these changes affect magma ascent from a deep source to the reservoir and from reservoir to Earth's surface. The model plumbing system is such that a hydraulic connection is maintained at all times between the reservoir and a deep magma source at constant pressure. Consequently the input rate of magma into the reservoir is predicted by the model rather than imposed as an input parameter. The open hydraulic connection model is consistent with short-term measurements of deformation and seismicity at several active volcanoes. Threshold values for the reservoir pressure at the beginning and end of eruption evolve as the edifice grows and lead to long-term changes of eruption rate. Depending on the dimensions and depth of the reservoir, the eruption rate follows different trends as a function of time. For small reservoirs, the eruption rate initially increases as the edifice builds up and peaks at some value before going down. The edifice size at the peak eruption rate provides a constraint on the reservoir shape and depth. Edifice decay or destruction leads to resumption of eruptive activity and a new eruption cycle. A simple elastic model for country rock deformation is valid over a whole eruptive cycle extending to the cessation of eruptive activity. For large reservoirs, an elastic model is only valid over part of an eruptive cycle. Long-term stress changes eventually lead to reservoir instability in the form of either roof collapse and caldera formation or reservoir enlargement in the horizontal direction.  相似文献   

18.
The AD 79 eruption of Vesuvius is certainly one of the most investigated explosive eruptions in the world. This makes it particularly suitable for the application of numerical models since we can be quite confident about input data, and the model predictions can be compared with field-based reconstruction of the eruption dynamics. Magma ascent along the volcanic conduit and the dispersal of pyroclasts in the atmosphere were simulated. The conduit and atmospheric domain were coupled through the flow conditions computed at the conduit exit. We simulated two different peak phases of the eruption which correspond to the emplacement of the white and gray magma types that produced Plinian fallout deposits with interlayered pyroclastic flow units during the gray phase. The input data, independently constrained and representative of each of the two eruptive phases, consist of liquid magma composition, crystal and water content, mass flow rate, and pressure–temperature–depth of the magma at the conduit entrance. A parametric study was performed on the less constrained variables such as microlite content of magma, pressure at the conduit entrance, and particle size representative of the eruptive mixture. Numerical results are substantially consistent with the reconstructed eruptive dynamics. In particular, the white eruption phase is found to lead to a fully buoyant eruption plume in all cases investigated, whereas the gray phase shows a more transitional character, i.e. the simultaneous production of a buoyant convective plume and pyroclastic surges, with a significant influence of the microlite content of magma in determining the partition of pyroclast mass between convective plumes and pyroclastic flows.  相似文献   

19.
Nearly all eruptions in stratovolcanoes (composite volcanoes, central volcanoes) are supplied with magma through fractures. Consequently, a primary physical condition for an eruption to occur in a stratovolcano is that a magma-driven fracture is able to propagate to the surface. Magma-filled fractures, frozen or fluid, are referred to as sheet intrusions. More specifically, they are named dykes when subvertical, and inclined (or cone) sheets when inclined. Field observations indicate that most sheet intrusions do not reach the surface to feed eruptions but rather become arrested at various crustal depths. For this reason periods of volcanic unrest with sheet injections are much more common than volcanic eruptions. Whether a sheet intrusion becomes arrested or, alternatively, propagates to the surface depends primarily on the stress field in the stratovolcano. A stratovolcano normally consists of layers of contrasting mechanical properties, such as soft (low Youngs modulus) pyroclastic units and stiff (high Youngs modulus) lava flows. We present numerical models indicating that volcanoes composed of such layers commonly develop stress fields encouraging sheet and dyke arrest. The models indicate that a necessary condition for a sheet intrusion to reach the surface and feed a volcanic eruption is that the stress field along the sheet pathway becomes homogenised. We propose that much of the activity in a stratovolcano during a volcanic cycle encourages stress-field homogenisation. Field studies show that the sheet intrusions in individual stratovolcanoes have various dips: some are vertical dykes, others inclined sheets, and still others horizontal sills. Analytical models indicate that the dip of a sheet reaching the surface can have great effects on the magma transport during an eruption. This effect is normally greater for a flat volcano such as a collapse caldera than for a stratovolcano that forms a topographic high. We conclude that the shallower the dip of a sheet intrusion, the less will be its volumetric magma transport to the surface of a stratovolcano.Editorial responsibility: D Dingwell  相似文献   

20.
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