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1.
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   

2.
Experiments have been performed to determine the effect of deformation on degassing of bubble-bearing melts. Cylindrical specimens of phonolitic composition, initial water content of 1.5 wt.% and 2 vol.% bubbles, have been deformed in simple-shear (torsional configuration) in an internally heated Paterson-type pressure vessel at temperatures of 798–848 K, 100–180 MPa confining pressure and different final strains. Micro-structural analyses of the samples before and after deformation have been performed in two and three dimensions using optical microscopy, a nanotomography machine and synchrotron tomography. The water content of the glasses before and after deformation has been measured using Fourier Transform Infrared Spectroscopy (FTIR). In samples strained up to a total of γ ∼ 2 the bubbles record accurately the total strain, whereas at higher strains (γ ∼ 10) the bubbles become very flattened and elongate in the direction of shear. The residual water content of the glasses remains constant up to a strain of γ ∼ 2 and then decreases to about 0.2 wt.% at γ ∼ 10. Results show that strain enhances bubble coalescence and degassing even at low bubble volume-fractions. Noticeably, deformation produced a strongly water under-saturated melt. This suggests that degassing may occur at great depths in the volcanic conduit and may force the magma to become super-cooled early during ascent to the Earth’s surface potentially contributing to the genesis of obsidian.  相似文献   

3.
Mechanisms of bubble coalescence in silicic magmas   总被引：1，自引：1，他引：0  
Bubble coalescence is an important process that strongly affects magmatic degassing. Without coalescence, bubbles remain isolated from one another in the melt, severely limiting gas release. Despite this fact, very little has been done to identify coalescence mechanisms from textures of magmatic rocks or to quantify the dynamics of bubble coalescence in melts. In this paper, we present a systematic study of bubble-coalescence mechanisms and dynamics in natural and experimentally produced bubbly rhyolite magma. We have used a combination of natural observations aided by high-resolution X-ray computed tomography, petrological experiments, and physical models to identify different types of bubble?Cbubble interaction that lead to coalescence on the timescales of magma ascent and eruption. Our observations and calculations suggest that bubbles most efficiently coalesce when inter-bubble melt walls thin by stretching rather than by melt drainage from between converging bubble walls. Orders of magnitude are more rapid than melt drainage, bubble wall stretching produces walls thin enough that inter-bubble pressure gradients may cause the melt wall to dimple, further enhancing coalescence. To put these results into volcanogical context, we have identified magma ascent conditions where each coalescence mechanism should act, and discuss the physical conditions for preserving coalescence structures in natural pumice. The timescales we propose could improve volcanic eruption models, which currently do not account for bubble coalescence. Although we do not address the effect of shear strain on bubble coalescence, the processes discussed here may operate in several different eruption regimes, including vesiculation of lava domes, post-fragmentation frothing of vulcanian bombs, and bubbling of pyroclasts in conduits.  相似文献   

4.
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   

5.
In the shallow magma chambers of volcanoes, the CO2 content of most basaltic melts is above the solubility limit. This implies that the chamber contains gas bubbles, which rise through the magma and expand. Thus, the volume of the chamber, its gas volume fraction and the gas flux into the conduit change with time in a systematic manner as a function of the size and number of gas bubbles. Changes in gas flux and gas volume are calculated for a bubble size distribution and related to changes in eruption regimes. Fire fountain activity, only present during the first quarter of the eruption, requires that the bubbles are larger than a certain size, which depends on the gas flux and on the bubble content[1]. As the chamber degasses, it loses its largest gas bubbles and the gas flux decreases, eventually suppressing the fire fountaining activity. Ultimately, an eruption stops when the chamber contains only a few tiny bubbles. More generally, the evolution of basaltic eruptions is governed by a dimensionless number, τ * ≈ τgΔρaO2/(18μhc), where τ = a characteristic time for degassing; a0 = the initial bubble diameter; μ = the magma viscosity; and hc = the thickness of the degassing layer. Two eruptions of the Kilauea volcano, Mauna Ulu (1969–1971) and Puu O'o (1983—present), provide data on erupted gas volume and the inflation rate of the edifice, which help constrain the spatial distribution of bubbles in the magma chamber: bubbles come mainly from the bottom of the reservoir, either by in situ nucleation long before the eruption or within a vesiculated liquid. Although the gas flux at the roof of the chamber takes similar values for both eruptions, the duration of both the fire fountaining activity and the entire eruption was 6 times shorter at Mauna Ulu than during the Puu O'o eruption. The dimensionless analysis explains the difference by a degassing layer 6 times thinner in the former than the latter, due to a 2 year delay in starting the Mauna Ulu eruption compared to the Puu O'o eruption.  相似文献   

6.
Plinian/ignimbrite activity stopped briefly and abruptly 16 and 45 h after commencement of the 1912 Novarupta eruption defining three episodes of explosive volcanism before finally giving way after 60 h to effusion of lava domes. We focus here on the processes leading to the termination of the second and third of these three episodes. Early erupted pumice from both episodes show a very similar range in bulk vesicularity, but the modal values markedly decrease and the vesicularity range widens toward the end of Episode III. Clasts erupted at the end of each episode represent textural extremes; at the end of Episode II, clasts have very thin glass walls and a predominance of large bubbles, whereas at the end of Episode III, clasts have thick interstices and more small bubbles. Quantitatively, all clasts have very similar vesicle size distributions which show a division in the bubble population at 30 μm vesicle diameter and cumulative number densities ranging from 107–109 cm–3. Patterns seen in histograms of volume fraction and the trends in the vesicle size data can be explained by coalescence signatures superimposed on an interval of prolonged nucleation and free growth of bubbles. Compared to experimental data for bubble growth in silicic melts, the high 1912 number densities suggest homogeneous nucleation was a significant if not dominant mechanism of bubble nucleation in the dacitic magma. The most distinct clast populations occurred toward the end of Plinian activity preceding effusive dome growth. Distributions skewed toward small sizes, thick walls, and teardrop vesicle shapes are indicative of bubble wall collapse marking maturation of the melt and onset of processes of outgassing. The data suggest that the superficially similar pauses in the 1912 eruption which marked the ends of episodes II and III had very different causes. Through Episode III, the trend in vesicle size data reflects a progressive shift in the degassing process from rapid magma ascent and coupled gas exsolution to slower ascent with partial open-system outgassing as a precursor to effusive dome growth. No such trend is visible in the Episode II clast assemblages; we suggest that external changes involving failure of the conduit/vent walls are more likely to have effected the break in explosive activity at 45 h.  相似文献   

7.
Vulcanian eruptions are common at many volcanoes around the world. Vulcanian activity occurs as either isolated sequences of eruptions or as precursors to sustained explosive events and is interpreted as clearing of shallow plugs from volcanic conduits. Breadcrust bombs characteristic of Vulcanian eruptions represent samples of different parts of these plugs and preserve information that can be used to infer parameters of pre-eruption magma ascent. The morphology and preserved volatile contents of breadcrust bombs erupted in 1999 from Guagua Pichincha volcano, Ecuador, thus allow us to constrain the physical processes responsible for Vulcanian eruption sequences of this volcano. Morphologically, breadcrust bombs differ in the thickness of glassy surface rinds and in the orientation and density of crack networks. Thick rinds fracture to create deep, widely spaced cracks that form large rectangular domains of surface crust. In contrast, thin rinds form polygonal networks of closely spaced shallow cracks. Rind thickness, in turn, is inversely correlated with matrix glass water content in the rind. Assuming that all rinds cooled at the same rate, this correlation suggests increasing bubble nucleation delay times with decreasing pre-fragmentation water content of the melt. A critical bubble nucleation threshold of 0.4–0.9 wt% water exists, below which bubble nucleation does not occur and resultant bombs are dense. At pre-fragmentation melt H2O contents of >∼0.9 wt%, only glassy rinds are dense and bomb interiors vesiculate after fragmentation. For matrix glass H2O contents of ≥1.4 wt%, rinds are thin and vesicular instead of thick and non-vesicular. A maximum measured H2O content of 3.1 wt% establishes the maximum pressure (63 MPa) and depth (2.5 km) of magma that may have been tapped during a single eruptive event. More common H2O contents of ≤1.5 wt% suggest that most eruptions involved evacuation of ≤1.5 km of the conduit. As we expect that substantial overpressures existed in the conduit prior to eruption, these depth estimates based on magmastatic pressure are maxima. Moreover, the presence of measurable CO2 (≤17 ppm) in quenched glass of highly degassed magma is inconsistent with simple models of either open- or closed-system degassing, and leads us instead to suggest re-equilibration of the melt with gas derived from a deeper magmatic source. Together, these observations suggest a model for the repeated Vulcanian eruptions that includes (1) evacuation of the shallow conduit during an individual eruption, (2) depressurization of magma remaining in the conduit accompanied by open-system degassing through permeable bubble networks, (3) rapid conduit re-filling, and (4) dome formation prior to the subsequent explosion. An important part of this process is densification of upper conduit magma to allow repressurization between explosions. At a critical overpressure, trapped pressurized gas fragments the nascent impermeable cap to repeat the process.  相似文献   

8.
Bubble growth in rhyolitic melts: experimental and numerical investigation   总被引：2，自引：0，他引：2  
 Bubble growth controlled by mass transfer of water from hydrated rhyolitic melts at high pressures and temperatures was studied experimentally and simulated numerically. Rhyolitic melts were hydrated at 150 MPa, 780–850  °C to uniform water content of 5.5–5.3 wt%. The pressure was then dropped and held constant at 15–145 MPa. Upon the drop bubbles nucleated and were allowed to grow for various periods of time before final, rapid quenching of the samples. The size and number density of bubbles in the quenched glasses were recorded. Where number densities were low and run duration short, bubble sizes were in accord with the growth model of Scriven (1959) for solitary bubbles. However, most results did not fit this simple model because of interaction between neighboring bubbles. Hence, the growth model of Proussevitch et al. (1993), which accounts for finite separation between bubbles, was further developed and used to simulate bubble growth. The good agreement between experimental data, numerical simulation, and analytical solutions enables accurate and reliable examination of bubble growth from a limited volume of supersaturated melt. At modest supersaturations bubble growth in hydrated silicic melts (3–6 wt% water, viscosity 104–106 Pa·s) is diffusion controlled. Water diffusion is fast enough to maintain steady-state concentration gradient in the melt. Viscous resistance is important only at the very early stage of growth (t<1 s). Under the above conditions growth is nearly parabolic, R2=2Dtρm(C0–Cf)/ρg until the bubble approaches its final size. In melts with low water content, viscosity is higher and maintains pressure gradients in the melt. Growth may be delayed for longer times, comparable to time scales of melt ascent during eruptions. At high levels of supersaturation, advection of hydrated melt towards the growing bubble becomes significant. Our results indicate that equilibrium degassing is a good approximation for modeling vesiculation in melts with high water concentrations (C0>3 wt%) in the region above the nucleation level. When the melt accelerates and water content decreases, equilibrium can no longer be maintained between bubbles and melt. Supersaturation develops in melt pockets away from bubbles and new bubbles may nucleate. Further acceleration and increase in viscosity cause buildup of internal pressure in the bubbles and may eventually lead to fragmentation of the melt. Received: 19 June 1995 / Accepted: 27 December 1995  相似文献   

9.
Equilibrium and disequilibrium degassing of a volatile phase from a magma of K-phonolitic composition was investigated to assess its behavior upon ascent. Decompression experiments were conducted in Ar-pressurized externally heated pressure vessels at superliquidus temperature (1050 °C), in the pressure range 10–200 MPa using pure water as fluid phase. All experiments were equilibrated at 200 MPa and then decompressed to lower pressures with rates varying from 0.0028 to 4.8 MPa/s. Isobaric saturation experiments were performed at the same temperature and at 900–950 °C to determine the equilibrium water solubility in the pressure range 30–250 MPa. The glasses obtained from decompression experiments were analyzed for their dissolved water content, vesicularity and bubble size distribution. All decompressed samples presented a first event of bubble nucleation at the capsule–melt interface. Homogeneous bubble nucleation in the melt only occurred in fast-decompressed experiments (4.8 and 1.7 MPa/s), for ΔP ≅ 100 MPa. For these decompression rates high water over-saturations were maintained until a rapid exsolution was triggered at ΔP > 150 MPa. For slower rates (0.0028, 0.024, 0.17 MPa/s) the degassing of the melt took place by diffusive growth of the bubbles nucleating at the capsule–melt interface. This process sensibly reduced water over-saturation in the melt, preventing homogeneous nucleation to occur. For decompression rates of 0.024 and 0.17 MPa/s low water over-saturations were attained in the melt, gradually declining toward equilibrium concentrations at low pressures. A near-equilibrium degassing path was observed for a decompression rate of 0.0028 MPa/s. Experimental data combined with natural pumice textures suggest that both homogeneous and heterogeneous bubble nucleations occurred in the phonolitic magma during the AD 79 Vesuvius plinian event. Homogeneous bubble nucleation probably occurred at a depth of ∼ 3 km, in response to a fast decompression of the magma during the ascent.  相似文献   

10.
Degassing through open paths such as bubble and/or fracture networks is considered to be the principal mode of degassing in silicic magmas. However, its detailed mechanisms remain unclear. To investigate the behavior of bubbles in a hypothetical open-system condition, we performed a series of vesiculation experiments on natural rhyolitic obsidian using a newly designed semipermeable cell, which artificially maintains a pressure difference between its inside and outside. The thick-wall cell maintains a constant volume within the sample chamber, while allowing water vapor to escape the cell during the experimental runs. The cells containing obsidian cores with ca. 0.66 wt.% initial water content were externally heated to 1000 °C for a period of 1–288 h. The run charges generally showed a zonal structure composed of two contrasting regions: a central region within which the bubbles were concentrated (bubble-rich core, BC) and a bubble-free melt region surrounding the BC (bubble-free margin, BFM). With increased heating duration, the thickness of the BFM increased via dissolution of the outermost bubbles in the BC. The water content was nearly uniform throughout the BC, whereas an outward-decreasing gradient was observed in the BFM. We found that diffusive dehydration occurred from the sample surface, and the bubbles were resorbed into the melt. Thus, the BFM–BC boundary moved inwards. These processes were modelled numerically, and the calculation results were in good agreement with the experimental data. If a “lifetime” of open paths is approximated as the relaxation time of a melt in a shallow volcanic environment, then the paths have to be pinched off quickly (1.2 h at maximum pressure difference between the open path and the melt) and thus the thickness of the bubble-free layer reaches at most ~ 0.1 mm. For the formation of bubble-free obsidian layers with a width of a few millimeters, which are often observed in natural obsidian flows, open paths should be maintained for at least a few hundred hours.  相似文献   

11.
Pre-eruptive conditions and degassing processes of the AD 79 plinian eruption of Mt. Vesuvius are constrained by systematic F and Cl measurements in melt inclusions and matrix glass of pumice clasts from a complete sequence of the pumice-fallout deposits. The entire ‘white pumice’ (WP) magma and the upper part of the ‘grey pumice’ (GP) magma were saturated relative to sub-critical fluids (a Cl-rich H2O vapour phase and a brine), with a Cl melt content buffered at ~ 5300 ppm, and a mean H2O content of ~ 5%. The majority of the GP magma was not fluid-saturated. From these results it can be estimated that the WP magma chamber had a low vertical extent (< 500 m) and was located at a depth of ~ 7.5 km while the GP magma reservoir was located just beneath the WP one, but its vertical extent cannot be constrained. This is approximately two times deeper than previous estimates. H2O degassing during the WP eruption followed a typical closed-system evolution, whereas GP clasts followed a more complex degassing path. Contrary to H2O, Cl was not efficiently degassed during the plinian phase of the eruption.

This study shows that F and Cl behave as incompatible elements in fluid-undersaturated phonolitic melts. H2O saturation is necessary for a significant partitioning of Cl into the fluid phase. However, Cl cannot be extracted in significant quantity from phonolitic melts during rapid H2O degassing, e.g. during plinian eruptions, due to kinetics effects. Halogen contents are better preserved in volcanic glass (melt inclusions or matrix glass) than H2O, therefore the combined analysis of both volatile species is required for reliable determination of pre-eruptive conditions and syn-eruptive degassing processes in magmas stored at shallow depths.  相似文献   


12.
Magma degassing at Soufrière Hills Volcano (SHV) is characterised by an almost permanent SO2 flux and a HCl production rate which mainly depends on dome growth rate. Degassing processes have been studied through textural, H2O and halogen analyses of clasts collected between 1995 and 2006 on the dome and in pyroclastic flows and vulcanian eruption deposits. Cl, Br and I are strongly depleted in melts during H2O degassing with no significant Cl–Br–I fractionation, whereas F is almost unaffected. All magmas erupted at SHV have followed a multi-step degassing path from the magma chamber up to a shallow depth ( 1 km, P  20 MPa). From that depth, however, effusive and explosive paths are distinct; vulcanian eruptions are the result of closed system degassing (CSD), while effusive dome growth is the result of CSD up to a very shallow depth (≤ 200 m, P  5–2 MPa) followed by open system degassing (OSD). CSD is modelled using the H2O solubility law, the perfect gas law and partition coefficients of halogens between a rhyolitic melt and H2O vapour (dv − li). Gas loss characteristic of OSD is modelled using a Rayleigh law. Degassing induced crystallisation is introduced through the ratio of crystallisation and degassing rates, which ranges from 150–500. dv − lCl for OSD ranges between 50–300, increasing with melt Cl content. For CSD, the lower effective dv − lCl ( 20) is attributed to kinetic effects.

Dome forming activity has a greater impact on atmospheric chemistry than vulcanian eruptions because OSD is much more efficient at extracting halogens. The model shows that HCl flux is a good proxy for the dome forming eruption rate. Comparison between model and measured gas compositions suggests a high HBr–BrO conversion rate (BrO/Total Br  1/3) in the SHV gas plume.

The degassing behaviour of Cl, Br and I implies similar Cl/Br ( 160) and Br/I ( 90) in initial melts, volcanic clasts and high temperature gases. The low Cl/Br at SHV compared to other island arcs ( 250–300) is attributed to a shallow, pre-eruptive Br enrichment. The almost permanent dome extrusion at SHV since 1995 has likely had a significant regional atmospheric impact because of the very efficient effusive degassing and the high conversion rate of halogens into reactive species within the gas plume.  相似文献   


13.
Critical to understanding explosive eruptions is establishing how accurately representative pyroclasts are of processes during magma vesiculation and fragmentation. Here, we present data on densities, and vesicle size and number characteristics, for representative pyroclasts from six silicic eruptions of contrasting size and style from Raoul volcano (Kermadec arc). We use these data to evaluate histories of bubble nucleation, coalescence, and growth in explosive eruptions and to provide comparisons with pumiceous dome carapace material. Density/vesicularity distributions show a scarcity of pyroclasts with ～65–75 % vesicularity; however, pyroclasts closest to this vesicularity range have the highest bubble number density (BND) values regardless of eruptive intensity or style. Clasts with vesicularities greater than this 65–75 % “pivotal” vesicularity range have decreasing BNDs with increasing vesicularities, interpreted to reflect continuing bubble growth and coalescence. Clasts with vesicularities less than the pivotal range have BNDs that decrease with decreasing vesicularity and preserve textures indicative of processes such as stalling and open system degassing prior to vesiculation in a microlite-rich magma, or vesiculation during slow ascent of degassing magma. Bubble size distributions (BSDs) and BNDs show variations consistent with 65–75 % representing the vesicularity at which vesiculating magma is most likely to undergo fragmentation, consistent with the closest packing of spheres. We consider that the observed vesicularity range may reflect the development of permeability in the magma through shearing as it flows through the conduit. These processes can act in concert with multiple nucleation events, generating a situation of heterogeneous bubble populations that permit some regions of the magma to expand and bubbles to coalesce with other regions in which permeable networks are formed. Fragmentation preserves the range in vesicularity seen as well as any post-fragmentation/pre-quenching expansion which may have occurred. We demonstrate that differing density pyroclasts from a single eruption interval can have widely varying BND values corresponding to the degree of bubble maturation that has occurred. The modal density clasts (the usual targets for vesicularity studies) have likely undergone some degree of bubble maturation and are therefore may not be representative of the magma at the onset of fragmentation.  相似文献   

14.
A series of experiments was conducted to test concepts of porous flow degassing of rhyolitic magma during ascent and of the subsequent collapse of vesicles in degassed magma to form obsidian. Dense, synthetically hydrated, natural glasses were pressurized under water-saturated conditions and then decompressed to achieve a range of porosities in the presence of a tracer vapor, D2O. Rapid isotopic exchange indicative of vapor transport rather than of simple diffusion occurred at a porosity >60 vol.%, in accord with earlier gas permeability measurements on cold natural samples. In another series of experiments, natural and synthetic pumices, vesiculated by degassing to atmospheric pressure, rapidly collapsed to dense glass on repressurization to the modest pressures prevailing in lava flows. No relict bubble textures remained. These results support the hypothesis that effusive eruptions result from the syneruptive escape of gas from permeable magmatic foam, and that a process analogous to welding yields dense lavas when such foams are extruded.  相似文献   

15.
An instrument package for simulating basaltic eruptions (IPSBE) with a height of 18 m has been developed for investigating the processes that occur during Strombolian eruptions. The device follows the geometrical ratio between the actual plumbing system of a volcano, with the ratio of conduit diameter to conduit height being 1 to 1000. For the first time in physical modeling studies, we created conditions in which a moving gas-saturated model liquid enters the conduit; this enabled us to study bubble nucleation, expansion, and coalescence, the generation and transformation of gas structures, and the kinetic features shown by the evolution of the gas phase. These experiments revealed a novel (previously unknown) flow pattern of two-phase mixtures in a vertical column, viz., a cluster flow that involves the regular alternation of compact clusters of gas bubbles that are separated by a fluid that does not involve a free gas phase. It is shown that the liquid, bubble, cluster, and slug flow patterns are mutually transformed under certain conditions; they are polymorphous modifications of a gas-saturated liquid moving in a vertical pipe. The data thus acquired suggested a new model for the gas-liquid movement of a magma melt in a conduit: depending on the type of gas-liquid flow behavior at the vent, the crater will exhibit different types of explosive activity, including actual explosions.  相似文献   

16.
We examine the physics of growth of water bubbles in highly viscous melts. During the initial stages, diffusive mass transfer of water into the bubble keeps the internal pressure in the bubbles close to the initial pressure at nucleation. Growth is controlled by melt viscosity and supersaturation pressure and radial growth under constant pressure is approximately exponential. At later stages, internal pressure falls, radial growth decelerates and follows the square-root of time. At this stage it is controlled by diffusion. The time of transition between the two stages is controlled by the decompression, melt viscosity and the Peclet number of the system. The model closely fit experimental data of bubble growth in viscous melts with low water content. Close fit is also obtained for new experiments at high supersaturation, high Peclet numbers, and high, variable viscosity. Near surface, degassed, silicic melts are viscous enough, so that viscosity-controlled growth may last for very long times. Using the model, we demonstrate that bubbles which nucleate shortly before fragmentation cannot grow fast enough to be important during fragmentation. We suggest that tiny bubbles observed in melt pockets between large bubbles in pumice represent a second nucleation event shortly before or after fragmentation. The presence of such bubbles is an indicator of the conditions at fragmentation. The water content of lavas extruded at lava domes is a key factor in their evolution. Melts of low water content (<0.2 wt%) are too viscid and bubbles nucleated in them will not grow to an appreciable size. Bubbles may grow in melts with 0.4 wt% water. The internal pressure in such bubbles may be preserved for days and the energy stored in the bubbles may be important during the disintegration of dome rocks and the formation of pyroclastic flows.  相似文献   

17.
Thermal diffusivity of rhyolite melt and rhyolite foam (70–80% porosity) has been measured using the radial heat transfer method. Cylindrical samples (length 50–55 mm, diameter 22 mm) of rhyolite melt and foam have been derived by heating samples of Little Glass Mountain obsidian. Using available data on heat capacity and density of rhyolite melt, the thermal conductivity of samples has been determined. The difference in thermal conductivity between rhyolite melt and foam at igneous temperatures ( 1000°C) is about one order of magnitude. The effect of thermal insulation of magmas due to vesiculation and foaming of the top layer is discussed in terms of the data obtained using a simple illustrative model of magma chamber convection.  相似文献   

18.
The evolution of bubble size distributions in volcanic eruptions   总被引：1，自引：0，他引：1  
We review observations of bubble size distributions (BSDs) generated during explosive volcanic eruptions and laboratory explosions, as inferred from vesicle size distributions found in the end products. Unimodal, polymodal, exponential and power law BSDs are common, even in the absence of coalescence, and both power law and exponential distributions have been generated in the same eruption. To date theoretical models have proposed incompatible mechanisms for producing the various distributions. We here present a unifying mechanism. Data from our laboratory analogue experiments suggest that power law distributions are associated with highly non-equilibrium degassing. A numerical model is developed in which bubbles nucleate repeatedly and grow in the spaces between those of previous generations, where, in a non-equilibrium degassing scenario, the volatile concentration remains high. This process causes the BSD to evolve from unimodal, through exponential, into a power law. The exponent of the power law is a measure of the number of nucleation events, or the duration of the nucleation period compared with the timescale of bubble growth. The mathematical inevitability of the evolution from unimodal (Poissonian) to power law is discussed. The findings may resolve the apparent contradiction between the equilibrium degassing conduit flow models and the non-equilibrium degassing conditions derived from bubble growth models of explosive volcanic eruptions. The process of ongoing nucleation is the mechanism whereby the volcanic system maintains near-equilibrium in the case of rapid depressurisation and slow volatile diffusion.  相似文献   

19.
The vesicle size distribution (VSD) and rare gas abundances in popping rocks from 14°N on the Mid-Atlantic Ridge provide constraints on the behavior of volatiles during ridge crest volcanism. These popping rocks, which contain 16–18 volume percent vesicles, are rare mid-ocean ridge basalt (MORB) magmas which appear to have retained much of their volatile inventory. The logarithm of vesicle population density displays the same linear correlation with decreasing size in two of the samples studied. This implies that continuous and simultaneous nucleation and bubble growth have occurred during magma ascent, with no significant perturbations due to accumulation, coalescence or loss of bubbles. In contrast, most MORB magmas display low vesicularities and we suggest that they have suffered some degree of pre-eruptive vesicle loss. We tentatively propose that large vesicles are produced by coalescence when MORB melt is at rest in chambers and conduits, and may be lost during early gas-rich episodes. Most MORB would represent residual liquids which erupt after vesicle loss has occurred, whereas popping rocks would represent a rare case where physical sorting of vesicles from melt did not occur, because storage in a magma chamber did not occur.The rare gas concentrations in the studied popping rocks are the highest yet measured in glassy ridge basalts ([He] > 50 μccSTP/g). The rare gas abundance pattern of these popping rocks probably resembles the pattern for non-vesiculated MORB magma and potentially reflects that of the depleted mantle source. This pattern is similar to the “mean MORB” pattern (computed from MORB glasses with40Ar/36Ar > 10,000) although a higher enrichment in He (and possibly Ne) compared to the heavier rare gases is observed in MORB. The overall similarity in abundance patterns for MORB and popping rocks indicates that vesiculation and vesicle loss do not fractionate the ArKrXe relative abundances from those in non-vesiculated magma, and that the modern flux ratios of these gases at ridges are similar to their elemental ratios in the depleted mantle. The degassing flux of He at ridge crests estimated from the MORB He deficit relative to popping rocks is comparable to the flux derived from the3He budget for the abyssal ocean. This suggests that degassing at ridges may be strongly influenced by the dynamics and style of submarine volcanism.  相似文献   

20.
This paper describes unusual rhyolitic deposits at Dalakvísl, Torfajökull, Iceland that were emplaced during a Quaternary subglacial eruption. Despite its small volume (<0.2 km3), the eruption mechanisms were highly variable and involved both explosive and intrusive phases. The explosive phase involved vesiculation-driven magma fragmentation at the glacier base and generated a pumiceous pyroclastic deposit containing deformed sheets of dense obsidian. Textures suggest that the obsidian was generated by the collapse of partly fragmented foam that was intruding the deposit and water contents indicate quenching at elevated pressures. In contrast, the intrusive phase of the eruption generated vesicle-poor quench hyaloclastites associated with a variety of peperitic lava bodies. The presence of juvenile-rich fluvio-lacustrine sediments is the first documented evidence that meltwater may pond close to the vent during subglacial rhyolite eruptions if the bedrock topography is favourable. In order to explain the variable eruption mechanisms, a conceptual model is presented in which the transition from an explosive to an intrusive eruption was controlled by the space available for fragmentation within the subglacial cavity melted above the vent. When the cavity became completely filled by volcanic deposits, the vent became blocked and rising magma was forced to intrude through poorly consolidated debris. This led to arrested fragmentation and welding of foam domains to form vesicle-poor obsidian lava; the transition to an intrusive eruption has taken place. Although this vent-blocking mechanism is particularly relevant to subglacial eruptions, it may also apply to subaerial rhyolitic eruptions, where patterns of explosive and effusive activity cannot be explained by shallow degassing processes alone. Meanwhile, the variable style of a small-volume subglacial rhyolite eruption further highlights the complex processes that mediate volcano-ice interactions.  相似文献   

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