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1.
Following an intersection of rising magma with drifts of the potential Yucca Mountain nuclear waste repository, a pathway
is likely to be established to the surface with magma flowing for days to weeks and affecting the performance of engineered
structures located along or near the flow path. In particular, convective circulation could occur within magma-filled drifts
due to the exsolution and segregation of magmatic gas. We investigate gas segregation in a magma-filled drift intersected
by a vertical dyke by means of analogue experiments, focusing on the conditions of sustained magma flow. Degassing is simulated
by electrolysis, producing micrometric bubbles in viscous mixtures of water and golden syrup, or by aerating golden syrup,
producing polydisperse bubbly mixtures with 40% of gas by volume. The presence of exsolved bubbles induces a buoyancy-driven
exchange flow between the dyke and the drift that leads to gas segregation. Bubbles segregate from the magma by rising and
accumulating as a foam at the top of the drift, coupled with the accumulation of denser degassed magma at the base of the
drift. Steady-state influx of bubbly magma from the dyke into the drift is balanced by outward flux of lighter foam and denser
degassed magma. The length and time scales of this gas segregation are controlled by the rise of bubbles in the horizontal
drift. Steady-state gas segregation would be accomplished within hours to hundreds of years depending on the viscosity of
the degassed magma and the average size of exsolved gas bubbles, and the resulting foam would only be a few cm thick. The
exchange flux of bubbly magma between the dyke and the drift that is induced by gas segregation ranges from 1 m3 s−1, for the less viscous magmas, to 10−8 m3 s−1, for the most viscous degassed magmas, with associated velocities ranging from 10−1 to 10−9 m s−1 for the same viscosity range. This model of gas segregation also predicts that the relative proportion of erupted degassed
magma, that could potentially carry and entrain nuclear waste material towards the surface, would depend on the value of the
dyke magma supply rate relative to the value of the gas segregation flux, with violent eruption of gassy as well as degassed
magmas at relatively high magma supply rates, and eruption of mainly degassed magma by milder episodic Strombolian explosions
at relatively lower supply rates. 相似文献
2.
Many basaltic volcanoes emit a substantial amount of gas over long periods of time while erupting relatively little degassed lava, implying that gas segregation must have occurred in the magmatic system. The geometry and degree of connectivity of the plumbing system of a volcano control the movement of magma in that system and could therefore provide an important control on gas segregation in basaltic magmas. We investigate gas segregation by means of analogue experiments and analytical modelling in a simple geometry consisting of a vertical conduit connected to a horizontal intrusion. In the experiments, degassing is simulated by electrolysis, producing micrometric bubbles in viscous mixtures of water and golden syrup. The presence of exsolved bubbles induces a buoyancy-driven exchange flow between the conduit and the intrusion that leads to gas segregation. Bubbles segregate from the fluid by rising and accumulating as foam at the top of the intrusion, coupled with the accumulation of denser degassed fluid at the base of the intrusion. Steady-state influx of bubbly fluid from the conduit into the intrusion is balanced by outward flux of lighter foam and denser degassed fluid. The length and time scales of this gas segregation are controlled by the rise of bubbles in the horizontal intrusion. Comparison of the gas segregation time scale with that of the cooling and solidification of the intrusion suggests that gas segregation is more efficient in sills (intrusions in a horizontal plane with typical width:length aspect ratio 1:100) than in horizontally-propagating dykes (intrusions in a vertical plane with typical aspect ratio 1:1000), and that this process could be efficient in intermediate as well as basaltic magmas. Our investigation shows that non-vertical elements of the plumbing systems act as strong gas segregators. Gas segregation has also implications for the generation of gas-rich and gas-poor magmas at persistently active basaltic volcanoes. For low magma supply rates, very efficient gas segregation is expected, which induces episodic degassing activity that erupts relatively gas-poor magmas. For higher magma supply rates, gas segregation is expected to be less effective, which leads to stronger explosions that erupt gas-rich as well as gas-poor magmas. These general physical principles can be applied to Stromboli volcano and are shown to be consistent with independent field data. Gas segregation at Stromboli is thought likely to occur in a shallow reservoir of sill-like geometry at 3.5 km depth with exsolved gas bubbles 0.1–1 mm in diameter. Transition between eruptions of gas-poor, high crystallinity magmas and violent explosions that erupt gas-rich, low crystallinity magmas are calculated to occur at a critical magma supply rate of 0.1–1 m3 s− 1. 相似文献
3.
M. A. Alidibirov 《Bulletin of Volcanology》1994,56(6-7):459-465
Fragmentation of magma containing gas bubbles is of great interest in connection with developing models for the formation of pyroclastics and for volcanic blasts (explosions). This paper considers the problem of fragmentation of highly viscous (>108 Pa s) or solidified magma containing bubbles with excess gas pressure. It is suggested that the fragmentation of magma be considered on the basis of the fragmentation wave theory proposed by Nikolsky and Khristianovich, which is generally applicable to gas-dynamic phenomena occurring in mines. Then it becomes possible to derive the equations of conservation for the fragmentation wave front which moves into a body of magma from its free surface. As a result, the velocity, N, of magma fragmentation, and the velocity, u, of the movement of the gas-pyroclastic mixture behind the fragmentation wave front, are determined. Calculations show that N can reach 5 m/s. Therefore the duration of the fragmentation of the magma body (blast duration) proves to be long. The suggested model explains the possibility of several explosions during the blast as a result of the fragmentation wave stopping, and accounts for the angular shape of pyroclasts by the brittle disruption of interbubble partitions during fragmentation wave propagation through the porous magma body. The initiation and cessation of fragmentation are defined by magma porosity, magma tensile strength, and the pressure differential between gas pressure in pores and the atmospheric pressure. The physical model of magma fragmentation developed explains the mechanism of energy release during volcanic blasts of the Vulcanian or Pelean types. 相似文献
4.
Fragmentation, or the "coming apart" of magma during a plinian eruption, remains one of the least understood processes in
volcanology, although assumptions about the timing and mechanisms of fragmentation are key parameters in all existing eruption
models. Despite evidence to the contrary, most models assume that fragmentation occurs at a critical vesicularity (volume
percent vesicles) of 75–83%. We propose instead that the degree to which magma is fragmented is determined by factors controlling
bubble coalescence: magma viscosity, temperature, bubble size distribution, bubble shapes, and time. Bubble coalescence in
vesiculating magmas creates permeability which serves to connect the dispersed gas phase. When sufficiently developed, permeability
allows subsequent exsolved and expanded gas to escape, thus preserving a sufficiently interconnected region of vesicular magma
as a pumice clast, rather than fully fragmenting it to ash. For this reason pumice is likely to preserve information about
(a) how permeability develops and (b) the critical permeability needed to insure clast preservation. We present measurements
and calculations that constrain the conditions (vesicularity, bubble size distribution, time, pressure difference, viscosity)
necessary for adequate permeability to develop. We suggest that magma fragments explosively to ash when and where, in a heterogeneously
vesiculating magma, these conditions are not met. Both the development of permeability by bubble wall thinning and rupture
and the loss of gas through a permeable network of bubbles require time, consistent with the observation that degree of fragmentation
(i.e., amount of ash) increases with increasing eruption rate.
Received: 5 July 1995 / Accepted: 27 December 1995 相似文献
5.
In explosive magma eruptions, magma ascends through a conduit as a Poiseuille flow at depth, and gas exsolves gradually and expands as the pressure decreases (bubbly flow regime). When the volume fraction of gas becomes sufficiently large, liquid or solid parts of magma fragment into droplets or ashes, and the flow dynamics becomes governed by the gas phase (gas–ash flow regime). We propose a new flow regime, which we call fractured-turbulent flow regime, between the bubbly flow regime and the gas–ash flow regime. In the new regime, both liquid magma and gas are continuous phases. The high connectivity of the two phases allows the relative velocity between them to increase significantly. We present one sample calculation, which displays basically explosive characteristics, but has three features distinct from previous models. The explosive characteristics are manifested as the fragmentation of the magma and the high speed jet that issues from the vent. The first distinct feature is a nearly lithostatic pressure distribution, which results from the increase of the height of the fragmentation surface. The second one is the atmospheric pressure at the vent; the flow is not choked. The third one is that the relative velocity between the gas and the ash is large at the vent despite the large interaction force between the two phases. The large relative velocity is established in the fractured-turbulent regime, and is maintained in the subsequent gas–ash flow regime. 相似文献
6.
James E. Gardner 《Bulletin of Volcanology》2009,71(8):835-844
Most, if not all, magmas contain gas bubbles at depth before they erupt. Those bubbles play a crucial role in eruption dynamics,
by allowing magma to degas, which causes the magma to accelerate as it ascends towards the surface. There must be a limit
to that acceleration, however, because gas bubbles cannot grow infinitely fast. To explore that limit, a series of experiments
was undertaken to determine the maximum rate at which bubbly high-silica rhyolite can decompress. Rhyolite melt that was hydrated
at 150 MPa with ~5.3 wt.% dissolved water and contained 7 to 18 vol.% bubbles can degas in equilibrium at 875°C when decompressed
at rates up to 1.2 MPa s−1 from 150 to 78 MPa, and up to 1.8 MPa s−1 when decompressed further to 42 MPa. In contrast, that same rhyolite cannot degas in equilibrium at 750°C if decompressed
faster than 0.015–0.025 MPa s−1. When combined with other published experiments, the maximum rate of decompression for equilibrium degassing is found to
increase by a factor of ten for every 50–75°C increase in temperature. When compared to predictions from conduit flow models
that assume equilibrium degassing, it is found that such models greatly over-estimate the rate at which relatively cold rhyolite
can decompress, whereas that assumption is largely correct for hot rhyolite, and thus for most other magmas, all of which
are less viscous than rhyolite. In addition, most bubbles that were 20–30 μm in size at high pressure were lost from the population
at low pressure. That absence suggests that only relatively large vesicles seen in volcanic pumice may be relics of pre-eruptive
bubbles, even if small bubbles were originally present at depth. 相似文献
7.
We present a visco-elastic bubble growth model, accounting for viscous and elastic deformations and for volatile mass transfer
between bubbles and melt. We define the borders between previous bubble growth models accounting for incompressible viscous
melt, and our new model accounting also for elastic deformation; this is done by a set of end-member analytical solutions
and numerical simulations. Elastic deformation is most prominent for magma of small vesicularity, where the growth regime
depends on the shear modulus. For high shear modulus, bubble growth is slow and follows an exponential law in a viscous growth
regime, while for low shear modulus bubbles quickly follow a square-root diffusive solution. Our model provides all the elastic
components (stresses, strains and strain rates) required for defining criteria for failure and magma fragmentation. We suggest
two failure criteria, a stress related one based on the internal friction and the Mohr-Coulomb failure theory, and a strain
related one based on fibre elongation experiments. We argue that both criteria are equivalent if we consider their shear modulus
dependency and its effect on magma rheology. Last, we apply our model to the process of bubble nucleation. In the incompressible
case, following nucleation, growth is slow and leads to long incubation times during which bubbles may be dissolved back into
the melt. The elastic response in magmas with low shear modulus results in a short incubation time, increasing the probability
of survival. The above effects emphasize the significance of visco-elasticity for the dynamic processes occurring in magmas
during volcanic activity. 相似文献
8.
Influence of pre-eruptive storage conditions and volatile contents on explosive Plinian style eruptions of basic magma 总被引：2，自引：2，他引：0
Sub-Plinian to Plinian eruptions of basic magma present a challenge to modeling volcanic behavior because many models rely
on magma becoming viscous enough during ascent to behave brittlely and cause fragmentation. Such models are unable, however,
to strain low viscosity magma fast enough for it to behave brittlely. That assumes that such magmas actually have low viscosities,
but the rare Plinian eruptions of basic magma may in fact result from them being anomalously viscous. Here, we examine two
such eruptions, the 122 B.C. eruption of hawaiitic basalt from Mt. Etna and the late Pleistocene eruption of basaltic andesite
from Masaya Caldera, to test whether they were anomalously viscous. We carried out hydrothermal experiments on both magmas
and analyzed glass inclusions in plagioclase phenocrysts from each to determine their most likely pre-eruptive temperatures
and water contents. We find that the hawaiite was last stored at 1,000–1,020°C, whereas the basaltic andesite was last stored
at 1,010–1,060°C, and that both were water saturated with ∼3.0 wt.% water dissolved in them. Such water contents are not high
enough to trigger Plinian explosive behavior, as much more hydrous basic magmas erupt less violently. In addition, despite
being relatively cool, the viscosities of both magmas would range from ∼102.2–2.5 Pa s before erupting to ∼104 Pa s when essentially degassed, all of which are too fluid to cause brittle disruption. Without invoking special external
forces to explain all such eruptions, one of the more plausible explanations is that when the bubble content reaches some
critical value the fragile foam-like magma disrupts. The rarity of Plinian eruptions of basic magma may be because such magmas
must ascend fast enough to retain their bubbles. 相似文献
9.
Fragmentation of magma during Plinian volcanic eruptions 总被引：2，自引：0，他引：2
James E. Gardner Richard M. E. Thomas Claude Jaupart Steve Tait 《Bulletin of Volcanology》1996,58(2-3):144-162
The ratio of the volume of vesicles (gas) to that of glass (liquid) in pumice clasts (V
G
/V
L
) reflects the degassing and dynamic history experienced by a magma during an explosive eruption. V
G
/V
L
in pumices from a large number of Plinian eruption deposits is shown here to vary by two orders of magnitude, even between
pumices at a given level in a deposit. These variations in V
G
/V
L
do not correlate with crystallinity or initial water content of the magmas or their eruptive intensities, despite large ranges
in these variables. Gas volume ratios of pumices do, however, vary systematically with magma viscosity estimated at the point
of fragmentation, and we infer that pumices do not quench at the level of fragmentation but undergo some post-fragmentary
evolution. On the timescale of Plinian eruptions, pumices with viscosities <109 Pa s can expand after fragmentation, as long as their bubbles retain gas, at a rate inversely proportional to their viscosity.
Once the bubbles connect to form a permeable network and lose their gas, expansion halts and pumices with viscosities <105 Pa s can collapse under the action of surface tension. Textural evidence from bubble sizes and shapes in pumices indicates
that both expansion and collapse have taken place. The magnitudes of expansion and collapse, therefore, depend critically
on the timing of bubble connectivity relative to the final moment of quenching. We propose that bubbles in different pumices
become connected at different times throughout the time span between fragmentation and quenching. After accounting for these
effects, we derive new information on the fragmentation process from two characteristics of pumices. The most important is
a relatively constant minimum value of V
G
/V
L
of ∼1.78 (64 vol.% vesicularity) in all samples with viscosities >105 Pa s. This value is independent of magma composition and thus reflects a property of the eruptive mechanism. The other characteristic
is that highly expanded pumices (>85 vol.% vesicularities) are common, which argues against overpressure in bubbles as a mechanism
for fragmenting magma. We suggest that magma fragments when it reaches a vesicularity of ∼64 vol.%, but only if sheared sufficiently
strongly. The intensity of shear varies as a function of velocity in the conduit, which is related to overpressure in the
chamber, so that changes in overpressure with time are important in controlling the common progression from explosive to effusive
activity at volcanoes.
Received: 19 April 1995 / Accepted: 3 April 1996 相似文献
10.
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
11.
12.
This study assesses the effect of decompression rate on two processes that directly influence the behavior of volcanic eruptions: degassing and permeability in magmas. We studied the degassing of magma with experiments on hydrated natural rhyolitic glass at high pressure and temperature. From the data collected, we defined and characterized one degassing regime in equilibrium and two regimes in disequilibrium. Equilibrium bubble growth occurs when the decompression rate is slower than 0.1 MPa s–1, while higher rates cause porosity to deviate rapidly from equilibrium, defining the first disequilibrium regime of degassing. If the deviation is large enough, a critical threshold of super-saturation is reached and bubble growth accelerates, defining the second disequilibrium regime. We studied permeability and bubble coalescence in magma with experiments using the same rhyolitic melt in open degassing conditions. Under these open conditions, we observed that bubbles start to coalesce at ~43 vol% porosity, regardless of decompression rate. Coalescence profoundly affects bubble texture and size distributions, and induces the melt to become permeable. We determined coalescence to occur on a time scale (~180 s) independent of decompression rate. We parameterized and incorporated our experimental results into a 1D conduit flow model to explore the implications of our findings on eruptive behavior of rhyolitic melts with low crystal contents stored in the upper crust. Compared to previous models that assume equilibrium degassing of the melt during ascent, the introduction of disequilibrium degassing reduces the deviation from lithostatic pressure by ~25%, the acceleration at high porosities (>50 vol%) by a factor 5, and the associated decompression rate by an order of magnitude. The integration of the time scale of coalescence to the model shows that the transition between explosive and effusive eruptive regimes is sensitive to small variations of the initial magma ascent speed, and that flow conditions near fragmentation may significantly be affected by bubble coalescence and gas escape.Editorial responsibility: D. Dingwell 相似文献
13.
We investigate the effects of vertical relative motion between gas and liquid on eruption styles by formulating a model for 1-dimensional steady flow in volcanic conduits. As magma ascends and decompresses, volatiles exsolve and volume fraction of gas increases. As a result, magma fragmentation occurs and the flow changes from bubbly flow to gas-pyroclast flow. In our model, a transitional region (‘permeable flow region’) is introduced between the bubbly flow region and the gas-pyroclast flow region. In this region, both the gas and the liquid are continuous phases, allowing the efficient vertical escape of gas through the permeable structure. We describe the features of conduit flow with relative motion of gas and liquid using non-dimensional numbers α, γ and ε. The parameter α represents the ratio of effects of wall friction to gravitational load, and is proportional to magma flow rate. The parameter γ represents the degree of decompression for the gas-pyroclast flow to reach the sound velocity at α = 1, and is proportional to rc2/μ for given magma temperature and initial volatile content, where rc is conduit radius and μ is liquid viscosity. The parameter ε is defined as the ratio of liquid–wall friction force to liquid–gas interaction force in the permeable flow region, and represents the efficiency of gas escape from magma. The values of γ and ε are determined only by magma properties and geological conditions such as liquid viscosity, magma permeability and conduit radius. We formulate a 1-dimensional steady-state conduit flow model to find non-dimensional magma flow rate α as a function of magma properties and geological conditions (e.g., γ and ε) under given boundary conditions. When the relative motion is taken into account with the assumption that magma fragmentation occurs when the gas volume fraction reaches some critical values, the pressure at the fragmentation level (Pf) decreases as the magma flow rate (α) decreases or the efficiency of gas escape (ε) increases, because gas escape suppresses the increase in the gas volume fraction accompanying magma ascent. When ε is so large that Pf is below the atmospheric pressure (Pa), the flow reaches the vent before fragmentation at low α. On the other hand, when ε is so small that Pf is greater than Pa, the flow reaches the vent after fragmentation at high α. These steady-state solutions of the flow at low and high α correspond to effusive and explosive eruptions, respectively. We present a graphical method to systematically find α. On the basis of the graphical method, a simple regime map showing the relationship between the assemblage of the solutions of conduit flow and the magma properties or the geological conditions is obtained. 相似文献
14.
Igneous enclaves, chilled bodies of magma with compositions contrasting with those of their hosts, have long been recognized in felsic plutonic rocks. Similar enclaves occur in felsic pyroclastic rocks despite the apparent difficulty of their survival of the explosive eruption process without fragmentation. The occurrence of andesitic ignimbrites with textural evidence of generation by mechanical mixing of felsic and mafic ash indicates that in some instances basaltic enclaves in felsic magmas that erupted explosively do indeed undergo fragmentation and homogenization with their host. Two exposures of rhyolitic ignimbrite that hosts basaltic enclaves, and of andesitic ignimbrite, in coastal Maine demonstrate the set of conditions necessary for survival of basaltic enclaves during catastrophic explosive eruptions. Relatively lower viscosity of basaltic enclaves compared to the rhyolitic host magma permits vesicle networks to develop as volatiles exsolve from the melt and form bubbles. The vesicle networks provide sufficient permeability for exsolving gases to escape the basaltic magma bodies, hence sparing the basaltic enclaves from fragmentation. If adequate permeability for volatile escape does not develop, the expanding bubbles are trapped within the basaltic enclave and ultimately, with depressurization during rise of the magma to the surface, cause fragmentation of the basaltic magma. In this case, the basaltic ash and the host rhyolitic ash homogenize, producing a hybrid ignimbrite, while the surrounding viscous rhyolitic magma behaves typically, with a small volume of the rhyolitic magma retaining its coherence as pumice bodies while most of the magma fragments shortly after vesiculation to become ash. These observations suggest a distinction between the voluminous andesites associated with subduction zones, for which attainment of intermediate composition occurred as a result of petrologic processes unique to subduction zones, and hybrid andesitic ignimbrites, which are spatially associated with bimodal magmatic systems in a variety of tectonic settings and are the result of mechanical mixing of ash during pyroclastic flow. 相似文献
15.
J. Stewart Turner Herbert E. Huppert R.Stephen J. Sparks 《Journal of Volcanology and Geothermal Research》1983,16(3-4)
Previous laboratory experiments investigating the fluid dynamics of replenished magma chambers have been extended to model effects resulting from the release of gas. Turbulent transfer of heat between a layer of dense, hot and volatile-rich mafic magma overlying cooler more evolved magma can lead to crystallization and exsolution of volatiles in the lower layer. Small gas bubbles can cause the bulk density to decrease to that of the upper layer and thus produce sudden overturning and initiate mixing, followed by further exsolution of gas and explosive eruption. These processes have been modelled in the laboratory using a chemical reaction between sodium or potassium carbonate and nitric acid to release small bubbles of CO2. We have investigated both the initial overturning produced by gas release in the lower layer, and the subsequent evolution of gas due to intimate mixing of the two layers. The latter experiments, in which the reactants remained isolated in the two layers until overturning occurred, demonstrated unambiguously that the fluxes of chemical components across the interfaces between convecting layers are very slow compared to the flux of heat. This shows that the evolution of layers of magma of different origins and composition can take place nearly independently of each other. The magmas can coexist in the same stratified chamber, until their bulk densities become equal and they mix together. The processes illustrated in these experiments could occur in H2O-bearing magmas such as in the calcalkaline association and in CO2-bearing mafic magmas such as in silica undersaturated suites. 相似文献
16.
Fred Witham 《Earth and Planetary Science Letters》2011,301(1-2):345-352
Magmas progressively exsolve volatiles as they ascend towards the Earth's surface, such that their volatile content is a function of pressure. Water and carbon dioxide concentrations measured in melt inclusions from degassing volcanoes rarely coincide with modelled degassing trends. I show that observed melt inclusion trends can be reproduced through mixing of magmas, either during convection within the volcanic conduit, or within a subterranean magma reservoir. No fluxing gas phase or post-entrapment loss of water need be invoked. A permeable network allowing gas transport is still required to avoid fragmentation of magma at shallow depths. 相似文献
17.
本文利用有限元及有限差分的方法,对壳内岩浆房或岩浆囊中的岩浆在构造应力及由于围岩与岩浆的密度差产生的浮力作用下,沿已有断层向上运移的动力学过程进行了数值模拟.在岩浆囊顶部与上覆岩层接触处,沿着已有微小破裂,岩浆在一定超压力条件下使已有断层张开并继续向上延伸,从而形成岩浆向上运移的通道.研究了岩浆黏度、密度差、模型深度对最小超压力（岩浆运移到地表所需的最小岩浆房超压力）的影响.在10 km深度的地壳中,若岩浆黏度为0.1~103 Pa·s,当超压力达到17~20 MPa时,岩浆压力可以驱动岩浆运移到地表层;同时,岩浆动力黏度越大,使岩脉运移到地表需要的超压力就越大.当密度差为300~700 kg·m-3,其变化对超压力的影响比较小.本文亦对比了三维应力条件和二维平面应变条件下不同结果,比较了不同条件下岩浆运移造成的地表垂直位移变化.结合长白山天池火山地区的区域地质环境,对长白山天池火山岩浆运移条件进行了参数试验性计算分析,估算了在给定长白山天池火山模型条件下地下可能存在的岩浆囊的大小,其结果对认识长白山天池火山地区岩浆活动及相关的预测和监控有参考意义. 相似文献
18.
Erwan Hallot Philippe Davy Jean de Bremond d'Ars Bernard Auvray Hervé Martin Henri Van Damme 《Journal of Volcanology and Geothermal Research》1996,71(1):31-44
Injection of Newtonian crystal-free magmas into a partially crystallised host which may exhibit non-Newtonian properties produces magmatic structures such as pipes, syn-plutonic dikes or dendritic structures. Field relationships between the structure and the host rock commonly indicate what the rheological contrasts during the injection were. The manner in which a magma deforms in response to injection is mainly linked to crystal content and strain rate (i.e., injection rate). Three kinds of behaviour can be distinguished: (1) Newtonian at low crystal contents; (2) Non-Newtonian at intermediate (40–60%) crystal contents, or at high crystal contents if the strain rate is small; and (3) brittle failure at high crystal content or strain rates.Petrologic observations indicate that injection can take place when the host magma still behaves as a fluid. To investigate the physics of the injection process we review the results of injection experiments in non-Newtonian fluids. These experiments were performed to study viscous fingering in 2-D Hele Shaw cells. They provide the first step to establishing the main non-Newtonian effects during the formation of interfacial instabilities arising when a Newtonian fluid is injected into a more viscous fluid or paste. The qualitative comparison of the morphological features of the interfaces between the fluids in the experiments with those in nature suggests that, in magmas, irregularities of the interfaces (dikes and dendrites) result from non-Newtonian properties of the host. We conclude that fluid-like deformation, rather than brittle behaviour of the host, during injection is likely to produce the general features observed on the field. Cooling effects might be responsible for the widespread phenomenon of fragmentation. We emphasise that the main effect of non-Newtonian properties in partially crystallised magmas is to generate strongly heterogeneous media producing discontinuities which could explain the main morphological features of syn-plutonic injection structures. 相似文献
19.
Caroline Bouvet de Maisonneuve Olivier Bachmann Alain Burgisser 《Bulletin of Volcanology》2009,71(6):643-658
Silicic pumices formed during explosive volcanic eruptions are faithful recorders of the state of the magma in the conduit,
close to or at the fragmentation level. We have characterized four types of pumices from the non-welded rhyolitic Kos Plateau
Tuff, which erupted 161,000 years ago in the East Aegean Arc, Greece. The dominant type of pumice (>90 vol.%) shows highly
elongated tubular vesicles. These tube pumices occur throughout the eruption. Less common pumice types include: (1) “frothy”
pumice (highly porous with large, sub-rounded vesicles), which form 5–10 vol.% of the coarsest pyroclastic flow deposits,
(2) dominantly “microvesicular” and systematically crystal-poor pumices, which are found in early erupted, fine-grained pyroclastic
flow units, and are characterized by many small (<50 μm in diameter) vesicles and few mm-sized, irregular voids, (3) grey
or banded pumices, indicating the interaction between the rhyolite and a more mafic magma, which are found throughout the
eruption sequence and display highly irregular bubble shapes. Except for the grey-banded pumices, all three other types are
compositionally identical and were generated synchronously as they are found in the same pyroclastic units. They, therefore,
record different conditions in the volcanic conduit leading to variable bubble nucleation, growth and coalescence. A total
of 74 pumice samples have been characterized using thin section observation, SEM imagery, porosimetry, and permeametry. We
show that the four pumice types have distinct total and connected porosity, tortuosity and permeability. Grey-banded pumices
show large variations in petrophysical characteristics as a response to mingling of two different magmas. The microvesicular,
crystal-poor, pumices have a bimodal bubble size distribution, interpreted as reflecting an early heterogeneous bubble nucleation
event followed by homogeneous bubble nucleation close to fragmentation. Finally, the significant differences in porosity,
tortuosity and permeability in compositionally identical tube and frothy pumices are the result of variable shear rates in
different parts of the conduit. Differential shear rates may be the result of either: (1) pure shear, inducing a vertical
progression from frothy to tube and implying a relatively thick fragmentation zone to produce both types of pumices at the
same time or (2) localized simple shear, inducing strongly tubular vesicles along the wall and near-spherical bubbles in the
centre of the conduit and not necessarily requiring a thick fragmentation zone. 相似文献
20.
David A. Clague Jennifer B. Paduan Alice S. Davis 《Journal of Volcanology and Geothermal Research》2009,180(2-4):171
Glassy lava fragments were collected in pushcores or using a small suction-sampler from over 450 sites along the Juan de Fuca Ridge, Blanco Transform Fault, Gorda Ridge, northern East Pacific Rise, southern East Pacific Rise, Fiji back-arc basin, and near-ridge seamounts in the Vance, President Jackson, Taney, and a seamount off southern California. The samples consist of angular glass fragments, limu o Pele, Pele's hair, and other fluidal fragments formed during pyroclastic eruptions. Since many of the sites are deeper than the critical point of seawater, fragmentation cannot be hydrovolcanic and caused by expansion of seawater to steam. The glass fragments have a wide range of MORB compositions, ranging from fractionated to primitive and from depleted to enriched. Enriched magmas, which have higher volatile contents, may form more abundant pyroclasts than depleted magmas. Eruptions with high effusion rates produce sheet flows and abundant pyroclasts whereas those with low effusion rates produce pillow ridges and few pyroclasts. This relation suggests that high effusion and conduit rise rates are coupled to high magmatic gas contents. The eruptions are mainly effusive with a minor strombolian bubble burst component. We propose that the gas phase is an added component of variable amounts of magmatic foam from the top of the magma reservoir. As the mixture of resident magma and foam rises in the conduit, the larger bubbles in the foam rise more quickly and sweep up the smaller bubbles nucleating and growing from the resident magma. On eruption, the process of bubble coalescence is more complete for the slower rising, gas-poor lavas that erupt as pillow lavas whereas the limu o Pele associated with sheet flow eruptions commonly contain several percent vesicles that avoided coalescence during ascent. The spatter erupted at the vent is quench granulated in seawater above the vent, reducing the pyroclast grainsize. The granulated spatter and limu o Pele fragments are then entrained in a rising plume of seawater heated by the eruption, which disperses them to distances as great as 5 km from the vent. 相似文献