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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.
 The rates of passive degassing from volcanoes are investigated by modelling the convective overturn of dense degassed and less dense gas-rich magmas in a vertical conduit linking a shallow degassing zone with a deep magma chamber. Laboratory experiments are used to constrain our theoretical model of the overturn rate and to elaborate on the model of this process presented by Kazahaya et al. (1994). We also introduce the effects of a CO2–saturated deep chamber and adiabatic cooling of ascending magma. We find that overturn occurs by concentric flow of the magmas along the conduit, although the details of the flow depend on the magmas' viscosity ratio. Where convective overturn limits the supply of gas-rich magma, then the gas emission rate is proportional to the flow rate of the overturning magmas (proportional to the density difference driving convection, the conduit radius to the fourth power, and inversely proportional to the degassed magma viscosity) and the mass fraction of water that is degassed. Efficient degassing enhances the density difference but increases the magma viscosity, and this dampens convection. Two degassing volcanoes were modelled. At Stromboli, assuming a 2 km deep, 30% crystalline basaltic chamber, containing 0.5 wt.% dissolved water, the ∼700 kg s–1 magmatic water flux can be modelled with a 4–10 m radius conduit, degassing 20–100% of the available water and all of the 1 to 4 vol.% CO2 chamber gas. At Mount St. Helens in June 1980, assuming a 7 km deep, 39% crystalline dacitic chamber, containing 4.6 wt.% dissolved water, the ∼500 kg s–1 magmatic water flux can be modelled with a 22–60 m radius conduit, degassing ∼2–90% of the available water and all of the 0.1 to 3 vol.% CO2 chamber gas. The range of these results is consistent with previous models and observations. Convection driven by degassing provides a plausible mechanism for transferring volatiles from deep magma chambers to the atmosphere, and it can explain the gas fluxes measured at many persistently active volcanoes. Received: 26 September 1997 / Accepted: 11 July 1998  相似文献   

3.
Intrusive degassing and recycling of degassed and dense magma at depth have been proposed for a long time at Stromboli. The brief explosive event that occurred at the summit craters on 9 January 2005 threw out bombs and lapilli that could be good candidates to illustrate recycling of shallow degassed magma at depth. We present an extensive data set on both the textures and the mineral, bulk rock and glassy matrix chemistry of the “9 Jan” products. The latter have the common shoshonitic–basaltic bulk composition of lavas and scoriae issued from typical strombolian activity. In contrast they differ by the heterogeneous chemistry of their matrix glasses and their crystal textures that testify to crystal dissolution event(s) just prior magma crystallization upon ascent and eruption. Comparison between mineral paragenesis of the natural products and experimental phase equilibria suggest water-induced magma re-equilibration. We propose that mineral dissolution is related to water enrichment of the recycled degassed magma, via differential gas bubble transfer and to some extents its physical mixing with volatile-rich magma blobs. However, all these features illustrate transient processes. Even though evidence of mineral dissolution is ubiquitous at Stromboli, its effect on the bulk magma chemistry is minor because of the subtle interplay between mineral dissolution and crystallization in magmas having comparable bulk chemistry.  相似文献   

4.
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   

5.
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   

6.
We report here the most complete dataset for major and trace elements, as well as Sr isotopic compositions, of magmas erupted by Stromboli since the onset of present-day activity 1,800 years ago. Our data relate to both porphyritic scoria and lava originating in the uppermost parts of the feeding system, plus crystal-poor pumice produced by paroxysmal explosive eruption of deep-seated, fast ascending, magma. The geochemical variations recorded by Stromboli’s products allow us to identify changes in magma dynamics affecting the entire plumbing system. Deep-seated magmas vary in composition between two end-members having different key ratios in strongly incompatible trace elements and Sr isotopes. These features may be ascribed to mantle source processes (fluid/melt enrichment, variable degrees of melting) and occasional contamination by deep, mafic, cumulates. Temporal trends reveal three phases during which magmas with distinct geochemical signatures were erupted. The first phase occurred between the third and fourteenth centuries AD and was characterised by the eruption of evolved magmas sharing geochemical and Sr isotopic compositions similar to those of earlier periods of activity (<12 ka—Neostromboli and San Bartolo). The second phase, which began in the sixteenth century and lasted until the first half of the twentieth century, produced more primitive, less radiogenic, magmas with the lowest Ba/La and Rb/Th ratios of our dataset. The last phase is ongoing and is marked by a magma having the lowest Sr isotopic composition and highest Rb/Th ratio of the dataset. While this new magma can be clearly identified in the pumice erupted during the last two paroxysmal eruptions of 2003 and 2007, shallow degassed magma extruded during this time span records significant geochemical and isotopic heterogeneities. We thus suggest that the shallow reservoir has been only partially homogenised by this new magma influx. We conclude that compositional variations within the shallow magma system of a persistently active volcano provide only a biassed signal of ongoing geochemical changes induced by deep magma refilling. We argue that source changes can only be identified by interpreting the geochemistry of pumice, because it reliably represents magma transferred directly from deep portions of the plumbing system to the surface.  相似文献   

7.
It is commonly assumed that the greater explosivity of andesitic volcanoes is due to higher gas contents, but there is no evidence that they are more gas-rich than basaltic volcanoes of oceanic regions. Their higher explosivity results from greater pressures in upper levels of the eruptive vents. The high viscosity of andesitic magmas retards the expansion of gases exsolving from rising magma and results in higher pressures when the magma approaches the surface. Two basic types of explosive mechanisms can be distinguished. One, which is analogous to a fire hose, carries fragments in a high-velocity, low-pressure gas stream. The ejection velocity of individual fragments is the resultant of the gas-stream velocity and the settling velocity of the fragment of given size in a fluid of appropriate density. The size of ejecta diminishes in a regular fashion outward from the vent. In the second type, which is more like a cannon, blocks are suddenly accelerated by high-pressure gas that is contained in cavities and fractures within a slowly rising magma and tend to have a distribution pattern in which large blocks have been projected farther than small ones. There is no theoretical basis for pressures of more than a few hundred bars if gas is exsolved from a rising magma. Higher pressures can be attained by heating meteoric water under conditions that permit little volumetric expansion.  相似文献   

8.
Ash fallout collected during 4 days of sampling at Stromboli confirms that a crystal-rich (HP) degassed magma erupts during the Strombolian explosions that are characteristic of the normal activity of this volcano. We identified 3 different types of juvenile ash fragments (fluidal, spongy and dense), which formed through different mechanisms of fragmentation of the low-viscosity, physically heterogeneous (in terms of the size and spatial distribution of bubbles) shoshonitic magma. A small amount (less than 3 vol%) of volatile-rich magma with low porphyricity (LP), erupted as highly vesicular ash fragments, has been collected, together with the HP magma, during normal strombolian explosions. Laboratory experiments and the morphological, textural and compositional investigations of ash fragments reveal that the LP ash is fresh and not recycled from the last paroxysm (15 March 2007). We suggest that small droplets of LP magma are dragged to the surface by the time-variable but persistent supply of deep derived CO2-rich gas bubbles. This coupled ascent of bubbles and LP melts is transient and does not perturb the dynamics of the HP magma within the shallow reservoir. This finding provides a new perspective on how the Stromboli volcano works and has important implications for monitoring strategies.  相似文献   

9.
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   

10.
Large vesicles record pathways of degassing at basaltic volcanoes   总被引：2，自引：2，他引：0  
Volcanic degassing is directly linked to magma dynamics and controls the style of eruptive activity. To better understand how gas is transported within basaltic magma we perform a 3D investigation of vesicles preserved in scoria from the 2005 activity at Stromboli volcano (Italy). We find that clasts are characterized by the ubiquitous occurrence of one to a few large vesicles, exhibiting mostly irregular, tortuous, channel-like textures, orders of magnitude greater in volume than all the other vesicles in the sample. We compare observations on natural samples with results from numerical simulations and experimental investigations of vesicle size distributions and demonstrate that this type of vesicle invariably forms in magmas with vesicularities > 0.30 (and possibly > 0.10). We suggest that large vesicles represent pathways used by gas to flow non-explosively to the surface and that they indicate the development of an efficient system that sustains persistent degassing in basaltic systems.  相似文献   

11.
We describe the mineralogy, geochemistry, and mesomicrostructure of fresh subvolcanic blocks erupted during the 5 April 2003 paroxysm of Stromboli (Aeolian Islands, Italy). These blocks represent ∼50 vol.% of the total erupted ejecta and consist of fine- to medium-grained basaltic lithotypes ranging from relatively homogeneous dolerites to strongly or poorly welded magmatic breccias. The breccia components are represented by angular fragments of dolerites entrapped in a matrix of vesiculated (lava-like to scoriae) crystal-rich (CR) basalt. All of the studied blocks are cognates with the CR basalt of the normal Strombolian activity or lavas and they are often coated by a few-centimeter thick layer of crystal-poor (CP) basaltic pumice erupted during the paroxysm. We suggest that they result from the rapid increase of pressure and related subvolcanic rock failure that occurred shortly before the 5 April 2003 explosion, when the uppermost portion of the edifice inflated and suffered brecciation as the result of the sudden rise of the gas-rich CP basalt that triggered the eruption. Dolerites and magmatic matrix of the breccias show major and trace element compositions that match those of the CR basalts erupted during normal Strombolian activity and effusive events at Stromboli volcano. Dolerites consist of (a) phenocrysts normally found in the CR basalts and (b) late-stage magmatic minerals such as sanidine, An60-28 plagioclase, Fe–Mn-rich olivines (Fo68-48), phlogopite, apatite, and opaque mineral pairs (magnetite and ilmenite), most of which are never found both in lava flows and scoriae erupted during the persistent explosive activity that characterizes typical Strombolian behavior. Subvolcanic crystallization of the Stromboli CR magma, leading to slowly cooled equivalents of basalts, could result from transient drainage of the magma from the summit craters to lower levels. Fingering and engulfing of the material that collapsed from the summit crater floor into the shallow basaltic system during the late evening of 28 December 2002 coupled with the short break in the summit persistent explosions between December 2002 and March 2003 permitted the CR magma pockets to solidify as dolerites, which were confined to the uppermost portion of the system and thus not involved in the ongoing flank effusive activity. Crystal size distribution of the basaltic blocks and crystallization of the finer-grained (<0.1 mm) mafic minerals of the dolerites over a time interval of ∼100 days closely agrees with the above interpretation. Vesicle filling (miarolitic cavities) locally found in some dolerites, with minerals deposited as vapor-phase crystallization is a result of continuous gas percolation through the rocks of the uppermost portion of the volcanic system. Poorly welded magmatic breccias formed during syn-eruptive processes of 5 April 2003, when the paroxysm strongly shattered the shallow subvolcanic system and many dolerite fragments were entrapped in the CR magma. In contrast, the high degree of welding between the dolerite clasts and the CR basaltic matrix in the strongly welded magmatic breccias provides a snapshot of subvolcanic intrusions of the CR basalt into the dolerite when, after a 2-month break in activity, CR magmas started to rise again to the summit craters. Blocks similar to these subvolcanic ejecta of 5 April 2003 were also erupted during previous paroxysms (e.g., 1930) suggesting that changes in the usual Strombolian activity (e.g., short breaks in the persistent mild explosions and/or flank effusive activity) lead to transient crystallization of dolerites in the shallow plumbing system.  相似文献   

12.
Magma plumbing system of the 2000 eruption of Miyakejima Volcano, Japan   总被引：1，自引：0，他引：1  
During the 2000 eruption at Miyakejima Volcano, two magmas with different compositions erupted successively from different craters. Magma erupted as spatter from the submarine craters on 27 June is aphyric basaltic andesite (<5 vol% phenocrysts, 51.4–52.2 wt% SiO2), whereas magma issued as volcanic bombs from the summit caldera on 18 August is plagioclase-phyric basalt (20 vol% phenocrysts, 50.8–51.3 wt% SiO2). The submarine spatter contains two types of crystal-clots, A-type and A-type (andesitic type). The phenocryst assemblages (plagioclase, pyroxenes and magnetite) and compositions of clinopyroxene in these clots are nearly the same, but only A-type clots contain Ca-poor plagioclase (An < 70). We consider that the A-type clots could have crystallized from a more differentiated andesitic magma than the A-type clots, because FeO*/MgO is not strongly influenced during shallow andesitic differentiation. The summit bombs contain only B-type (basaltic type) crystal-clots of Ca-rich plagioclase, olivine and clinopyroxene. The A-type and B-type clots have often coexisted in Miyakejima lavas of the period 1469–1983, suggesting that the magma storage system consists of independent batches of andesitic and basaltic magmas. According to the temporal variations of mineral compositions in crystal-clots, the andesitic magma became less evolved, and the basaltic magma more evolved, over the past 500 years. We conclude that gradually differentiating basaltic magma has been repeatedly injected into the shallower andesitic magma over this period, causing the andesitic magma to become less evolved with time. The mineral chemistries in crystal-clots of the submarine spatter and 18 August summit bombs of the 2000 eruption fall on the evolution trends of the A-type and B-type clots respectively, suggesting that the shallow andesitic and deeper basaltic magmas existing since 1469 had successively erupted from different craters. The 2000 summit collapse occurred due to drainage of the andesitic magma from the shallower chamber; as the collapse occurred, it may have caused disruption of crustal cumulates which then contaminated the ascending, deeper basalt. Thus, porphyritic basaltic magma could erupt alone without mixing with the andesitic magma from the summit caldera. The historical magma plumbing system of Miyakejima was probably destroyed during the 2000 eruption, and a new one may now form.Editorial responsibility: S Nakada, T Druitt  相似文献   

13.
Microtextural characteristics of fresh ejecta from Stromboli volcano were examined from three periods of differing eruption style and intensity in 2002. Activity shifted from relatively weak and infrequent ash-charged explosions during January through May into two broad cycles of waxing activity in June through late September, and late September through December, followed by the onset on 28 December of the 2002/2003 effusive eruption. Analyzed sets of lapilli from May, September/October, and 28 December show contrasts in the physical properties of magma resident in the shallow conduit during this range of activity. Three distinct textures are observed among the analyzed pyroclasts: low density (LD) with an abundance of subspherical bubbles, the presence of large, irregularly shaped bubbles, and a light-to-transparent glass matrix; transitional texture (TT) with an intermediate number of subspherical bubbles, a high frequency of large, irregularly-shaped bubbles, and a honey colored glass matrix; and high density (HD) with sparse relatively small bubbles, conspicuous large irregular bubbles, and a dark glass matrix. Observational and quantitative data (density, vesicle size) indicate that these textures are linked through variable residence time in Stromboli’s shallow conduit, with an ongoing evolution from LD to HD magma. Calculations suggest that residual LD magma will evolve to HD texture in a period of hours to days. Contrasting amounts of the LD, TT, and HD magmas are present in each sample, with the most TT in May, the most LD in September/October, and the most HD in December. This implies that the shallow magma had a different rheology at each collection period. The viscosity of LD and HD magmas are calculated to be in the range of 2,000 to 2,600 and 3,000 to 5,000 Pa s, respectively, which, with their changing proportions, must have implications for rates of bubble slug ascent and processes of fragmentation. This study suggests that an increasing maturity of magma in Stromboli’s shallow conduit (with resultant increase in viscosity) feeds back to reduce the intensity of explosions, whereas a steady flux of LD magma favors more powerful explosions.  相似文献   

14.
During June 1999, we measured the amplitude and rate (number of events per second) at which gas exited the vent at Stromboli volcano as discrete gas bursts or puffs. This allowed us to identify two styles of gas burst (puffing) activity. The first is characterized by frequent, rapidly rising puffs, the second by less frequent, slowly rising puffs. Each style persisted over 5–40-min-long durations and was associated with a high and low number of strombolian explosions per hour, respectively. Each period was also associated with characteristic delay times between the arrival of the infrasonic and thermal signals during strombolian explosions; the delays were longer during vigorous puffing periods. To explain our observations, we propose a model in which the degassing process cycles between vigorous and weak degassing phases. During vigorous degassing phases, bubble layers ascend the conduit at a frequency of 0.5–1.0 s−1. This high degassing level reflects a gas-rich magma column and leads to an increased rate in the formation of shallow foams and, hence, an increase in puffing and explosive activity, as well as a higher free surface level and/or gas jet velocity. During weak phases, bubble layers ascend the conduit at a reduced frequency of 0.2–0.3 s−1. During such times the magma column is poor in gas. This leads to a decreased rate of foam layer formation and hence a reduction in puffing and explosive activity, as well as a lower free surface level and/or gas jet velocity. Variations in puffing activity can thus be used to track changes in the rate at which the shallow system is supplied by fresh, gas-rich magma. Our observations indicate that the two degassing styles last from 5 to 40 min and that the switch from one to the other occurs over a matter of minutes.  相似文献   

15.
Chemical analyses of 30 melt inclusions from Satsuma-Iwojima volcano, Japan, were carried out to investigate volatile evolution in a magma chamber beneath the volcano from about 6300 yr BP to the present. Large variations in volatile concentrations of melts were observed. (1) Water concentration of rhyolitic melts decreases with time; 3–4.6 wt.% at the time of latest caldera-forming eruption of Takeshima pyroclastic flow deposit (ca. 6300 yr BP), 3 wt.% for small pyroclastic flow (ca. 1300 yr BP) of Iwodake, post-caldera rhyolitic dome, and 0.7–1.4 wt.% for submarine lava eruption (Showa-Iwojima) in 1934. (2) Rhyolitic melts of the Takeshima and Iwodake eruptions contained CO2 of less than 40 ppm, while the Showa-Iwojima melt has higher CO2 concentration of up to 140 ppm. (3) Water and CO2 concentrations of basaltic to andesitic melt of Inamuradake, a post-caldera basaltic scoria cone, are 1.2–2.8 wt.% and ≤290 ppm, respectively.Volatile evolution in the magma chamber is interpreted as follows: (1) the rhyolitic magma at the time of the latest caldera-forming eruption (ca. 6300 yr BP) was gas-saturated due to pressure variation in the magma chamber because the large variation in water concentration of the melt was attributed to exsolution of volatile in the magma prior to the eruption. Iwodake eruption (ca. 1300 yr BP) was caused by a remnant of the caldera-forming rhyolitic magma, suggested from the similarity of major element composition between these magmas. (2) Volatile composition of the Showa-Iwojima rhyolitic melt agrees with that of magmatic gases presently discharging from a summit of Iwodake, indicating the low pressure degassing condition. (3) The degassing of the magma chamber by magma convection in a conduit of Iwodake during non-eruptive but active degassing period for longer than 800 years decreased water concentration of the rhyolitic magma. (4) Geological and petrological observations indicate that a stratified magma chamber, which consists of a lower basaltic layer and an upper rhyolitic layer, might have existed during the post-caldera stage. Addition of CO2 from the underlying basaltic magma to the upper gas-undersaturated (degassed) rhyolitic magma increased CO2 concentration of the rhyolitic magma.  相似文献   

16.
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   

17.
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   

18.
An understanding of the mechanisms responsible for persistent volcanism can be acquired through the integration of geophysical and geochemical data sets. By interpreting data on micro-gravity, ground deformation and SO2 flux collected at Masaya Volcano since 1993, it is now clear that the characteristically cyclical nature of the activity is not driven by intrusion of additional magma into the system. Rather, it may be due in large part to the blocking and accumulation of gas by restrictions in the volcano substructure. The history of crater collapse and formation of caverns beneath the crater floor would greatly facilitate the trapping and storage of gas in a zone immediately beneath San Pedro and the other craters. Another mechanism that may explain the observed gravity and gas flux variations is the convective overturn of shallow, pre-existing, degassed, cooled, dense magma that is replaced periodically by lower density, hot, gas-rich magma from depth. Buoyant gas-rich magma rises from depth and is emplaced near the surface, resulting in the formation and fluctuation of a low-density gas-rich layer centred beneath Nindirí and Santiago craters. As this magma vigorously degasses, it must cool, increase in density and eventually sink. Five stages of activity have been identified at Masaya since 1853 and the most recent data suggest that the system may have been entering another period of reduced degassing in 2000. This type of analysis has important implications for hazard mitigation because periods of intense degassing are associated with poor agricultural yields and reduced quality of life. A better understanding of persistent cyclically active volcanoes will allow for more effective planning of urban development and agricultural land use.  相似文献   

19.
The ascent of magma during the A.D. 79 eruption of Vesuvius was studied by a steady-state, one-dimensional, and nonequilibrium two-phase flow model. The gas exsolution process was modeled by assuming a chemical equilibrium between the exsolved and dissolved gas, whereas the magma density and viscosity were modeled by accounting for the crystal content in magma. The exsolution, density, and viscosity models consider the effect of different compositions of the white and gray magmas. By specifying the conduit geometry and magma composition, and employing the model to search for the maximum discharge rate of magma which is consistent with the specified geometry and magma composition, the model was then used to establish the two-phase flow parameters along the conduit. It was found that for all considered conditions the magma pressure in the conduit decreases below the lithostatic pressure near the magma fragmentation level, and that in the deep regions of the conduit the white magma pressure is larger and the gray magma pressure is lower than the lithostatic one. The exsolution and fragmentation levels were found to be deeper for the white than for the gray magma, and the changing composition during the eruption causes an increase of the exit pressure and decrease of the exit gas volumetric fraction. The model also predicted a minimum conduit diameter which is consistent with the white and gray magma compositions and mass flow-rates. The predictions of the model were shown to be consistent with column collapses during the gray eruption phase, large presence of carbonate lithics in the gray pumice fall deposit, and magma-water interaction during a late stage of the eruption.  相似文献   

20.
 Lava drainback has been observed during many eruptions at Kilauea Volcano: magma erupts, degasses in lava fountains, collects in surface ponds, and then drains back beneath the surface. Time series data for melt inclusions from the 1959 Kilauea Iki picrite provide important evidence concerning the effects of drainback on the H2O contents of basaltic magmas at Kilauea. Melt inclusions in olivine from the first eruptive episode, before any drainback occurred, have an average H2O content of 0.7±0.2 wt.%. In contrast, many inclusions from the later episodes, erupted after substantial amounts of surface degassed lava had drained back down the vent, have H2O contents that are much lower (≥0.24 wt.% H2O). Water contents in melt inclusions from magmas erupted at Pu'u 'O'o on the east rift zone vary from 0.39–0.51 wt.% H2O in tephra from high fountains to 0.10–0.28 wt.% H2O in spatter from low fountains. The low H2O contents of many melt inclusions from Pu'u 'O'o and post-drainback episodes of Kilauea Iki reveal that prior to crystallization of the enclosing olivine host, the melts must have exsolved H2O at pressures substantially less than those in Kilauea's summit magma reservoir. Such low-pressure H2O exsolution probably occurred as surface degassed magma was recycled by drainback and mixing with less degassed magma at depth. Recognition of the effects of low-pressure degassing and drainback leads to an estimate of 0.7 wt.% H2O for differentiated tholeiitic magma in Kilauea's summit magma storage reservoir. Data for MgO-rich submarine glasses (Clague et al. 1995) and melt inclusions from Kilauea Iki demonstrate that primary Kilauean tholeiitic magma has an H2O/K2O mass ratio of ∼1.3. At transition zone and upper mantle depths in the Hawaiian plume source, H2O probably resides partly in a small amount of hydrous silicate melt. Received: 31 March 1997 / Accepted: 17 November 1997  相似文献   

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