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1.
The violent August 16–17, 2006 Tungurahua eruption in Ecuador witnessed the emplacement of numerous scoria flows and the deposition of a widespread tephra layer west of the volcano. We assess the size of the eruption by determining a bulk tephra volume in the range 42–57 × 106 m3, which supports a Volcanic Explosivity Index 3 event, consistent with calculated column height of 16–18 km above the vent and making it the strongest eruptive phase since the volcano’s magmatic reactivation in 1999. Isopachs west of the volcano are sub-bilobate in shape, while sieve and laser diffraction grain-size analyses of tephra samples reveal strongly bimodal distributions. Based on a new grain-size deconvolution algorithm and extended sampling area, we propose here a mechanism to account for the bimodal grain-size distribution. The deconvolution procedure allows us to identify two particle subpopulations in the deposit with distinct characteristics that indicate dissimilar transport-depositional processes. The log-normal coarse-grained subpopulation is typical of particles transported downwind by the main volcanic plume. The positively skewed, fine-grained subpopulation in the tephra fall layer shares close similarities with the elutriated co-pyroclastic flow ash cloud layers preserved on top of the scoria flow deposits. The area with the higher fine particle content in the tephra layer coincides with the downwind prolongation of the pyroclastic flow deposits. These results indicate that the bimodal distribution of grain size in the Tungurahua fall deposit results from synchronous deposition of lapilli from the main plume and fine ash elutriated from scoria flows emplaced on the western flank of the volcano. Our study also reveals that inappropriate grain-size data processing may produce misleading determination of eruptive type.  相似文献   

2.
A model for the numerical simulation of tephra fall deposits   总被引：4，自引：2，他引：4  
A simple semianalytical model to simulate ash dispersion and deposition produced by sustained Plinian and sub-Plinian eruption columns based on the 2D advection–dispersion equation was applied. The eruption column acts as a vertical line source with a given mass distribution and neglects the complex dynamics within the eruption column. Thus, the use of the model is limited to areas far from the vent where the dynamics of the eruption column play a minor role. Vertical wind and diffusion components are considered negligible with respect to the horizontal ones. The dispersion and deposition of particles in the model is only governed by gravitational settling, horizontal eddy diffusion, and wind advection. The model accounts for different types and size classes of a user-defined number of particle classes and changing settling velocity with altitude. In as much as wind profiles are considered constant on the entire domain, the model validity is limited to medium-range distances (about 30–200 km away from the source).The model was used to reconstruct the tephra fall deposit from the documented Plinian eruption of Mt. Vesuvius, Italy, in 79 A.D. In this case, the model was able to broadly reproduce the characteristic medium-range tephra deposit. The results support the validity of the model, which has the advantage of being simple and fast to compute. It has the potential to serve as a simple tool for predicting the distribution of ash fall of hypothetical or real eruptions of a given magnitude and a given wind profile. Using a statistical set of frequent wind profiles, it also was used to construct air fall hazard maps of the most likely affected areas around active volcanoes where a large eruption is expected to occur.  相似文献   

3.
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4.
A study of pyroclastic deposits from the 1815 Tambora eruption reveals two distinct phases of activity, i.e., four initial tephra falls followed by generation of pyroclastic flows and the production of major co-ignimbrite ash fall. The first explosive event produced minor ash fall from phreatomagmatic explosions (F-1 layer). The second event was a Plinian eruption (F-2) correlated to the large explosion of 5 April 1815, which produced a column height of 33 km with an eruption rate of 1.1 × 108 kg/s. The third event occurred during the lull in major activity from 5 to 10 April and produced minor ash fall (F-3). The fourth event produced a 43-km-high Plinian eruption column with an eruption rate of 2.8 × 108 kg/s during the climax of activity on 10 April. Although very energetic, the Plinian events were of short duration (2.8 h each) and total erupted volume of the early (F-1 to F-4) fall deposits is only 1.8 km3 (DRE, dense rock equivalent). An abrupt change in style of activity occurred at end of the second Plinian event with onset of pyroclastic flow and surge generation. At least seven pyroclastic flows were generated, which spread over most of the volcano and Sanggar peninsula and entered the ocean. The volume of pyroclastic flow deposits on land is 2.6 km3 DRE. Coastal exposures show that pyroclastic flows entering the sea became highly fines depleted, resulting in mass loss of about 32%, in addition to 8% glass elutriation, as indicated by component fractionation. The subaqueous pyroclastic flows have thus lost about 40% of mass compared to the original erupted mixture. Pyroclastic flows and surges from this phase of the eruption are stratigraphically equivalent to a major ash fall deposit (F-5) present beyond the flow and surge zone at 40 km from the source and in distal areas. The F-5 fall deposit forms a larger proportion of the total tephra fall with increasing distance from source and represents about 80% of the total at a distance of 90 km and 92% of the total tephra fall from the 1815 eruption. The field relations indicate that the 20-km3 (DRE) F-5 deposit is a co-ignimbrite ash fall, generated largely during entrance of pyroclastic flows into the ocean. Based on the observed 40% fines depletion and component fractionation from the flows, the large volume of the F-5 co-ignimbrite ash requires eruption of 50 km3 (DRE, 1.4 × 1014 kg) pyroclastic flows.  相似文献   

5.
We studied the distribution of tephra deposits discharged by the basaltic (52–54% SiO2) explosive eruption of 1973 on Tyatya Volcano (Kunashir I., Kuril Islands). We made maps showing lines of equal tephra thickness (isopachs) and lines of maximum size of pyroclastic particles (isopleths). These data were used to find the parameters of explosive activity using the standard techniques for each of the two phases of this eruption separately. The first, phreatomagmatic, phase discharged 0.008 km3 of tephra during the generation of maars on the volcano’s northern slope. The tephra mostly consisted of fragmented host rocks with admixtures of fragments of low vesiculated juvenile basalt. The phase lasted 20 hours, the rate of pyroclastic discharge was 2 × 105 kg/s; the eruptive plume reached heights of 4–6 km with wind speeds within 10 m/s. The second, magmatic, phase discharged 0.07 km3 of tephra during the generation of the Otvazhnyi scoria cone on the volcano’s southeastern slope. The tephra mostly consisted of juvenile basaltic scoria. The highly explosive Plinian part of this phase lasted 36 hours, the rate of pyroclastic discharge was 8 × 105 kg/s; the eruptive plume reached heights of 6–8 km with wind speeds of 10–20 m/s. The total tephra volume discharged by the eruption was approximately 0.08 km3; the total amount of ejected pyroclastic material (including the resulting monogenic edifices) was 0.11 km3; the volume of erupted magma was 0.05 km3 (the conversion was based on 2800 kg/m3 density); the volcanic explosivity index, or VEI, was 3. The production rate of the Tyatya plumbing system is estimated as 3 × 105 m3 magma per annum.  相似文献   

6.
Cotopaxi, the highest active volcano on earth and one of the most dangerous of Ecuador is constituted by a composite cone made up of lava and tephra erupted from the summit crater. The activity of the present volcano begun with large-volume plinian eruptions followed by a succession of small-volume lava emissions and pyroclastic episodes which led to the edification of a symmetrical cone. The growth of the cone was broken by an episode of slope failure, the scar of which is now obliterated by recent and historical products. Volcanic history, eruptive frequency and characteristics of the activity were investigated by studying the stratigraphy of tephra and carrying out fifteen new 14C dating on paleosols and charcoals. The investigated period is comprised between the slope failure and the present. The deposit of the volcanic landside (dry debris avalanche of Rio Pita), previously believed to be between 13,000 and 25,000 yr B.P., is now considered to have an age slightly older than 5000 yr B.P. The stratigraphy of tephra of the last 2000 years reveals the existence of 22 fallout layers. Seven of them were dated with 14C whereas three were ascribed to the eruptions of 1534, 1768 and 1877 on the basis of comparison with historical information.Maximum clast size distribution (isopleths) of 9 tephra layers points out that the sustained explosive eruptions of Cotopaxi during the last 2000 years are characterized by very high dispersive power (plinian plumes with column heights between 28 and 39 km) and high intensity (peak mass discharges from 1.1 to 4.1 × 108kg/s). The magnitude (mass) of tephra fallout deposits calculated from distribution of thickness (isopaches) are, however, moderate (from 0.8 to 7.2 × 1011 kg). The limited volume of magma erupted during each explosive episode is consistent with the lack of caldera collapses. Small-volume pyroclastic flows and surges virtually accompanied all identified tephra fallouts. During such an activity large scale snow/ice melting of the summit glacier produced devastating mudflows comparable in scale to those of 1877 eruption. By assuming a 1:1 correspondence between fallout episodes and generation of large-scale lahar, we have estimated an average recurrence of one explosive, lahartriggering event every 117 years over the last two millennia. This value compares well with that calculated by considering the period since Spanish Conquest. The probability of having an eruption like this in 100 or 200 years is respectively of 0.57 and 0.82. Such an high probability underscores the need for quick actions aimed at the mitigation of Cotopaxi lahar hazard along all the main valleys which originate from the volcano.  相似文献   

7.
A model for sedimentation from turbulent suspensions predicts that tephra concentration decreases exponentially with time in an ascending volcanic column and in the overlying umbrella cloud. For grain-size distributions typical of plinian eruptions application of the model predicts for thickness variations in good agreement with the exponential thinning observed in tephra fall deposits. The model also predicts a proximal region where fallout from the plume margins results in a more rapid decrease in thickness so that the deposit shows two segments on a thickness versus distance plot. Several examples of deposits with two segments are known. The distance at which the two segments intersect is a measure of eruption column height. The thickness half-distance ( equivalent to the dispersal index of Walker) is strongly correlated with column height, but is also weakly dependent on grain-size distribution of the ejecta. For a dispersal index of 500 km2 (the plinian/subplinian boundary of Walker) column heights between 14 and 18 km are calculated. For ultraplinian deposits with D>50000 km2 column heights of at least 45 km are implied. Model grain-size distributions of the deposits have sorting values comparable to those observed in tephra fall deposits formed from eruption columns in a weak or negligible cross-wind. Median diameter decreases exponentially with distance as is observed. Sorting () improves with distance as is observed in plinian deposits in a weak wind. However, tephra fall deposits formed in strong winds do not show improved sorting with distance and proximal deposits are typically somewhat better sorted than the model calculations. Differences are attributed to the influence of wind which disperses particles further than predicted in our model and which has an increasing influence as particle size decreases.  相似文献   

8.
The morphology, grain size characteristics and composition of ash particles in 30 ka to 150 ka tephra layers from the Byrd ice core were examined to characterize the eruptions which produced them and to test the suggestion that they were erupted from Mt. Takahe, a shield volcano in Marie Byrd Land, West Antarctica. Volcanic deposits at Mt. Takahe were examined for evidence of recent activity which could correlate with the tephra layers in the ice core.Coarse- and fine-ash layers have been recognized in the Byrd ice core. The coarse-ash layers have a higher mass concentration than the fine-ash layers and are characterized by fresh glass shards > 50 μm diameter, many containing elongate pipe vesicles. The fine-ash layers have a lower mass concentration and contain a greater variety of particles, typically < 20 μm diameter. Many of these particles are aggregate grains composed of glass and crystal fragments showing S and Cl surface alteration. The grain-size distributions of the coarse and fine-ash layers overlap, in part because of the aggregate nature of grains in the fine-ash layers. The coarse-ash layers are interpreted as having formed by magmatic eruption whereas the fine-ash layers are believed to be hydrovolcanic in origin.Mt. Takahe is the favored source for the tephra because: (a) chemical analyses of samples from the volcano are distinctive, being peralkaline trachyte, and similar in composition to the analyzed tephra; (b) Mt. Takahe is a young volcano (< 0.3 Ma); (c) pyroclastic deposits on Mt. Takahe indicate styles of eruption similar to that inferred for the ice core tephra; and (d) Mt. Takahe is only about 350 km from the calculated site of tephra deposition.A speculative eruptive history for Mt. Takahe is established by combining observations from Mt. Takahe and the Byrd ice core tephra. Initial eruptions at Mt. Takahe were subglacial and then graded into alternating subaerial and subglacial activity. The tephra suggest alternating subaerial magmatic and hydrovolcanic eruptions from 30 to 20 ka B.P., followed by a sustained period of hydrovolcanic eruptions from 20 to 14 ka B.P., which peaked at 18 ka B.P.  相似文献   

9.
The Katla volcano in Iceland is characterized by subglacial explosive eruptions of Fe–Ti basalt composition. Although the nature and products of historical Katla eruptions (i.e. over the last 1,100 years) at the volcano is well-documented, the long term evolution of Katla’s volcanic activity and magma production is less well known. A study of the tephra stratigraphy from a composite soil section to the east of the volcano has been undertaken with emphasis on the prehistoric deposits. The section records ∼8,400 years of explosive activity at Katla volcano and includes 208 tephra layers of which 126 samples were analysed for major-element composition. The age of individual Katla layers was calculated using soil accumulation rates (SAR) derived from soil thicknesses between 14C-dated marker tephra layers. Temporal variations in major-element compositions of the basaltic tephra divide the ∼8,400-year record into eight intervals with durations of 510–1,750 years. Concentrations of incompatible elements (e.g. K2O) in individual intervals reveal changes that are characterized as constant, irregular, and increasing. These variations in incompatible elements correlate with changes in other major-element concentrations and suggest that the magmatic evolution of the basalts beneath Katla is primarily controlled by fractional crystallisation. In addition, binary mixing between a basaltic component and a silicic melt is inferred for several tephra layers of intermediate composition. Small to moderate eruptions of silicic tephra (SILK) occur throughout the Holocene. However, these events do not appear to exhibit strong influence on the magmatic evolution of the basalts. Nevertheless, peaks in the frequency of basaltic and silicic eruptions are contemporaneous. The observed pattern of change in tephra composition within individual time intervals suggests different conditions in the plumbing system beneath Katla volcano. At present, the cause of change of the magma plumbing system is not clear, but might be related to eruptions of eight known Holocene lavas around the volcano. Two cycles are observed throughout the Holocene, each involving three stages of plumbing system evolution. A cycle begins with an interval characterized by simple plumbing system, as indicated by uniform major element compositions. This is followed by an interval of sill and dyke system, as depicted by irregular temporal variations in major element compositions. This stage eventually leads to a formation of a magma chamber, represented by an interval with increasing concentrations of incompatible elements with time. The eruption frequency within the cycle increases from the stage of a simple plumbing system to the sill and dyke complex stage and then drops again during magma chamber stage. In accordance with this model, Katla volcano is at present in the first interval (i.e. simple plumbing system) of the third cycle because the activity in historical time has been characterized by uniform magma composition and relatively low eruption frequency.  相似文献   

10.
The eruption of Novarupta within the Katmai Volcanic Cluster, south-west Alaska, in June 1912 was the most voluminous eruption of the twentieth century but the distal distribution of tephra deposition is inadequately quantified. We present new syntheses of published tephrostratigraphic studies and a large quantity of previously un-investigated historical records. For the first time, we apply a geostatistical technique, indicator kriging, to integrate and interpolate such data. Our results show evidence for tephra deposition across much of Alaska, Yukon, the northern Pacific, western British Columbia and northwestern Washington. The most distal tephra deposition was observed around 2,500?km downwind from the volcano. Associated with tephra deposition are many accounts of acid deposition and consequent impacts on vegetation and human health. Kriging offers several advantages as a means to integrate and present such data. Future eruptions of a scale similar to the 1912 event have the potential to cause widespread disruption. Historical records of tephra deposition extend far beyond the limit of deposition constrained by tephrostratigraphic records. The distal portion of tephra fallout deposits is rarely adequately mapped by tephrostratigraphy alone; contemporaneous reports of fallout can provide important constraints on the extent of impacts following large explosive eruptions.  相似文献   

11.
长白山天池火山千年大喷发空降碎屑物的数值模拟   总被引：5，自引：1，他引：4  
于红梅  许建东  赵谊 《地震地质》2007,29(3):522-534
文中以Suzuki火山灰扩散数学模型为基础,考虑了空气参数随海拔高度的变化和不同大小的颗粒由于内含气泡数量的不同而造成的密度不同,计算了不同尺寸颗粒的最终沉降速度和沉降时间。并对喷发柱扩散概率浓度的计算公式进行了修正,对长白山天池火山千年大喷发空降碎屑物的空间分布进行了数值模拟。模拟时根据风速随高度的变化应用3个模型:1)固定风速30m/s;2)风速从地球表面线性增加到对流层顶部,在平流层的速度为对流层顶部风速的0.75倍(又称MW1模型);3)风速在对流层与MW1相同,但是从对流层顶部到20km高处风速线性减小,20km高度以上的风速为对流层顶部的10%(又称MW2模型)。通过与前人的结果进行比较,说明了模型的合理性,最后分析了模拟结果与前人结果之间存在差异的原因  相似文献   

12.
The eruption of Toba (75,000 years BP), Sumatra, is the largest magnitude eruption documented from the Quaternary. The eruption produced the largest-known caldera the dimensions of which are 100 × 30 km and which is surrounded by rhyolitic ignimbrite covering an area of over 20,000 km2. The associated deep-sea tephra layer is found in piston cores in the north-eastern Indian Ocean covering a minimum area of 5 × 106 km2. We have investigated the thickness, grain size and texture of the Toba deep-sea tephra layer in order to demonstrate the use of deep-sea tephra layers as a volcanological tool. The exceptional magnitude and intensity of the Toba eruption is demonstrated by comparison of these data with the deep-sea tephra layers associated with the eruptions of the Campanian ignimbrite, Italy and of Santorini, Greece in Minoan time. The volume of ignimbrite and distal tephra fall deposit produced in the Toba eruption are comparable, a total of at least 1000 km3 of dense rhyolitic magma. In contrast the volume of dense magma produced by the Campanian and Santorini eruptions are approximately 70 and 13 km3 respectively. Thickness versus distance data on the three deep-sea tephra layers show that eruptions of smaller magnitude than Santorini are unlikely to be preserved as distinct tephra layers in most deep-sea cores. In proximal cores all three tephra layers show two distinct units: a lower coarse-grained unit and an upper fine-grained unit. We interpret the lower unit as a plinian deposit and the upper unit as a co-ignimbrite ash-fall deposit, indicating two major eruptive phases. The Toba tephra layer is coarser both in maximum and median grain size than the Campanian and Santorini layers at a given distance from source. These data are interpreted to indicate a very high cruption column, estimated to be at least 45 km. We have applied a method for estimating the duration of the Toba eruption from the style of graded-bedding in deep-sea tephra layers. Studies of two cores yield estimates of 9 and 14 days. The eruption column height and duration estimates both indicate an average volume discharge rate of approximately 106 m3/sec. The Toba eruption therefore was not only of exceptional magnitude, but also of exceptional intensity.  相似文献   

13.
A theoretical model of clast fallout from convective eruption columns has been developed which quantifies how the maximum clast size dispersal is determined by column height and wind strength. An eruption column consists of a buoyant convecting region which rises to a heightH B where the column density equals that of the atmosphere. AboveH B the column rises further to a heightH T due to excess momentum. BetweenH T andH B the column is forced laterally into the atmosphere to form an upper umbrella region. Within the eruption column, the vertical and horizontal velocity fields can be calculated from exprimental and theoretical studies and consideration of mass continuity. The centreline vertical velocity falls as a nearly linear function over most of the column's height and the velocity decreases as a gaussian function radially away from the centreline. Both column height and vertical velocity are strong functions of magma discharge rate. From calculations of the velocity field and the terminal fall velocity of clasts, a series of particle support envelopes has been constructed which represents positions where the column vertical velocity and terminal velocity are equal for a clast of specific size and density. The maximum range of a clast is determined in the absence of wind by the maximum width of the clast support envelope.The trajectories of clasts leaving their relevant support envelope at its maximum width have been modelled in columns from 6 to 43 km high with no wind and in a wind field. From these calculations the shapes and areas of maximum grain size contours of the air-fall deposit have been predicted. For the no wind case the theoretical isopleths show good agreement with the Fogo A plinian deposit in the Azores. A diagram has been constructed which plots, for a particular clast size, the maximum range normal to the dispersal axis against the downward range. From the diagram the column height (and hence magma discharge rate) and wind velocity can be determined. Historic plinian eruptions of Santa Maria (1902) and Mount St. Helens (1980) give maximum heights of 34 and 19 km respectively and maximum wind speeds at the tropopause of m/s and 30 m/s respectively. Both estimates are in good agreement with observations. The model has been applied to a number of other plinian deposits, including the ultraplinian phase of theA.D. 180 Taupo eruption in New Zealand which had an estimated column height of 51 km and wind velocity of 27 m/s.  相似文献   

14.
During an explosive volcanic eruption, tephra fall out from the umbrella region of the eruption cloud to the ground surface. We investigated the effect of the intensity of turbulence in the umbrella cloud on dispersion and sedimentation of tephra by performing a series of laboratory experiments and three dimensional (3-D) numerical simulations. In the laboratory experiments, spherical glass-bead particles are mixed in stirred water with various intensities of turbulence, and the spatial distribution and the temporal evolution of the particle concentration are measured. The experimental results show that, when the root-mean-square of velocity fluctuation in the fluid (Wrms) is much greater than the particle terminal velocity (vt), the particles are homogeneously distributed in the fluid, and settle at their terminal velocities at the base of the fluid where turbulence diminishes. On the other hand, when Wrms is as small as or smaller than vt, the particle concentration increases toward the base of the fluid during settling, which substantially increases the rate of particle settling. The results of the 3-D simulations of eruption cloud indicate that Wrms is up to 40 m/s in most of the umbrella cloud even during a large scale plinian eruption with a magma discharge rate of 109 kg/s. These results suggest that relatively coarse pyroclasts (more than a few mm in diameter) tend to concentrate around the base of the umbrella cloud, whereas fine pyroclasts (less than 1/8 mm in diameter) may be distributed homogeneously throughout the umbrella cloud during tephra dispersion. The effect of the gradient of particle concentration in the umbrella cloud explains the granulometric data of the Pinatubo 1991 plinian deposits.  相似文献   

15.
Explosive eruptions associated with tephra deposits that are only exposed in proximal areas are difficult to characterize. In fact, the determination of physical parameters such as column height, mass eruption rate, erupted volume, and eruption duration is mainly based on empirical models and is therefore very sensitive to the quality of the field data collected. We have applied and compared different modeling approaches for the characterization of the two main tephra deposits, the Lower Pumice (LP) and Upper Pumice (UP) of Nisyros volcano, Greece, which are exposed only within 5 km of the probable vent. Isopach and isopleth maps were compiled for two possible vent locations (on the north and on the south rim of the caldera), and different models were applied to calculate the column height, the erupted volume, and the mass eruption rate. We found a column height of about 15 km above sea level and a mass eruption rate of about 2 × 107 kg/s for both eruptions regardless of the vent location considered. In contrast, the associated wind velocity for both UP and LP varied between 0 and 20 m/s for the north and south vent, respectively. The derived erupted volume for the south vent (considered as the best vent location) ranges between 2 and 27 × 108 m3 for the LP and between 1 and 5 × 108 m3 for the UP based on the application of four different methods (integration of exponential fit based on one isopach line, integration of exponential and power-law fit based on two isopach lines, and an inversion technique combined with an advection–diffusion model). The eruption that produced the UP could be classified as subplinian. Discrepancies associated with different vent locations are smaller than the discrepancies associated with the use of different models for the determination of erupted mass, plume height, and mass eruption rate. Proximal outcrops are predominantly coarse grained with ≥90 wt% of the clasts ranging between −6ϕ and 0ϕ. The associated total grainsize distribution is considered to result from a combination of turbulent fallout from both the plume margins and the umbrella region, and as a result, it is fines-depleted. Given that primary deposit thickness observed on Nisyros for both LP and UP is between 1 and 8 m, if an event of similar scale were to happen again, it would have a significant impact on the entire island with major damage to infrastructure, agriculture, and tourism. Neighboring islands and the continent could also be significantly affected.  相似文献   

16.
The last magmatic eruption of Soufrière of Guadeloupe dated at 1530 A.D. (Soufrière eruption) is characterized by an onset with a partial flank-collapse and emplacement of a debris-avalanche that was followed by a sub-plinian VEI 2–3 explosive short-lived eruption (Phase-1) with a column that reached a height between 9 and 12 km producing about 3.9 × 106 m3 DRE (16.3 × 106 m3 bulk) of juvenile products. The column recurrently collapsed generating scoriaceous pyroclastic flows in radiating valleys up to a distance of 5–6 km with a maximum interpolated bulk deposit volume of 11.7 × 106 m3 (5 × 106 m3 DRE). We have used HAZMAP, a numerical simple first-order model of tephra dispersal [Macedonio, G., Costa, A., Longo, A., 2005. A computer model for volcanic ash fallout and assessment of subsequent hazard. Comput. Geosci. 31, 837–845] to reconstruct to a first approximation the potential dispersal of tephra and associated tephra mass loadings generated by the sub-plinian Phase 1 of the 1530 A.D. eruption. We have tested our model on a deterministic average dry season wind profile that best-fits the available data as well as on a set of randomly selected wind profiles over a 5 year interval that allows the elaboration of probabilistic maps for the exceedance of specific tephra mass load thresholds. Results show that in the hypothesis of a future 1530 A.D. scenario, populated areas to a distance of 3–4 km west–southwest of the vent could be subjected to a static load pressure between 2 and 10 kPa in case of wet tephra, susceptible to cause variable degrees of roof damage. Our results provide volcanological input parameters for scenario and event-tree definition, for assessing volcanic risks and evaluating their impact in case of a future sub-plinian eruption which could affect up to 70 000 people in southern Basse-Terre island and the region. They also provide a framework to aid decision-making concerning land management and development. A sub-plinian eruption is the most likely magmatic scenario in case of a future eruption of this volcano which has shown, since 1992, increasing signs of low-energy seismic, thermal, and acid degassing unrest without significant deformation.  相似文献   

17.
In volcanic risk assessment it is necessary to determine the appropriate level of sophistication for a given predictive model within the contexts of multiple sources of uncertainty and coupling between models. A component of volcanic risk assessment for the proposed radioactive waste repository at Yucca Mountain (Nevada, USA) involves prediction of dispersal of contaminated tephra during violent Strombolian eruptions and the subsequent transport of that tephra toward a hypothetical individual via surface processes. We test the suitability of a simplified model for volcanic plume transport and fallout tephra deposition (ASHPLUME) coupled to a surface sediment-transport model (FAR) that calculates the redistribution of tephra, and in light of inherent uncertainties in the system. The study focuses on two simplifying assumptions in the ASHPLUME model: 1) constant eruptive column height and 2) constant wind speed and direction during an eruption. Variations in tephra dispersal resulting from unsteady column height and wind conditions produced variations up to a factor of two in the concentration of tephra in sediment transported to the control population. However, the effects of watershed geometry and terrain, which control local remobilization of tephra, overprint sensitivities to eruption parameters. Because the combination of models used here shows limited sensitivity to the actual details of ash fall, a simple fall model suffices to estimate tephra mass delivered to the hypothetical individual.  相似文献   

18.
Mount Etna volcano was shaken during the summer 2001 by one of the most singular eruptive episodes of the last centuries. For about 3 weeks, several eruptive fractures developed, emitting lava flows and tephra that significantly modified the landscape of the southern flank of the volcano. This event stimulated the attention of the scientific community especially for the simultaneous emission of petrologically distinct magmas, recognized as coming from different segments of the plumbing system. A stratigraphically controlled sampling of tephra layers was performed at the most active vents of the eruption, in particular at the 2,100 m (CAL) and at the 2,550 m (LAG) scoria cones. Detailed scanning electron microscope and energy dispersive x-ray spectrometer (SEM-EDS) analyses performed on glasses found in tephra and comparison with lava whole rock compositions indicate an anomalous increase in Ti, Fe, P, and particularly of K and Cl in the upper layers of the LAG sequence. Mass balance and thermodynamic calculations have shown that this enrichment cannot be accounted for by “classical” differentiation processes, such as crystal fractionation and magma mixing. The analysis of petrological features of the magmas involved in the event, integrated with the volcanological evolution, has evidenced the role played by volatiles in controlling the magmatic evolution within the crustal portion of the plumbing system. Volatiles, constituted of H2O, CO2, and Cl-complexes, originated from a deeply seated magma body (DBM). Their upward migration occurred through a fracture network possibly developed by the seismic swarms during the period preceding the event. In the upper portion of the plumbing system, a shallower residing magma body (ABT) had chemical and physical conditions to receive migrating volatiles, which hence dissolved the mobilized elements producing the observed selective enrichment. This volatile-induced differentiation involved exclusively the lowest erupted portion of the ABT magma due to the low velocity of volatiles diffusion within a crystallizing magma body and/or to the short time between volatiles migration and the onset of the eruption. Furthermore, the increased amount of volatiles in this level of the chamber strongly affected the eruptive behavior. In fact, the emission of these products at the LAG vent, towards the end of the eruption, modified the eruptive style from classical strombolian to strongly explosive.  相似文献   

19.
Tephra, usually produced by explosive eruptions, is deposited rapidly, hence, it can serve as a distinctive and widespread synchronous marker horizon correlating terrestrial, marine and ice core records. The tephra from Changbaishan Millennium eruption, a widely distributed tephra, is an important marker bed across the Japan Sea, Japan Islands and even in the Greenland ice cores 9000km away from volcanic vent. In this study, a discrete tephra was identified in the Quanyang peat~45km northeast to the Changbaishan volcano. Radiocarbon 14 C dating on the plant remains constrains an age of 886-1013calAD(95.4%)to the tephra layer, which can correspond to the Millennium eruption of Changbaishan in time. In addition, there was no similar volcanic eruption in the surrounding areas except Changbaishan at the same time. This tephra shows rhyolitic glass shards major element compositions similar to those rhyolitic tephra from Millennium eruption. This study illustrates that tephra from Millennium eruption has been transported to Quanyang peat~45km northwest to the Changbaishan volcano. Additionally, the diameter of the pumice lapilli is up to 0.3cm, implying that the tephra must be transported more distal away from Quanyang peat and formed a widely distributed isochronic layer. Glass geochemistry of the Quanyang tephra, different from the distal tephra recorded at Sihailongwan, Japan, and Greenland ice, shows a close affinity to the pyroclastic flow deposits of the Millennium eruption while not from fall deposits. This may indicate that distribution of the Millennium eruption of Changbaishanin in different directions may be controlled by different stages of eruption. This layer with well-defined annual results can be used to optimize the chronological framework of the corresponding sedimentary environment, thus facilitating more accurate discussion of corresponding environmental changes, which can achieve the contrast of the ancient climate records in the whole Northeast China-Japan and arctic regions.  相似文献   

20.
Most tephra fallout models rely on the advection–diffusion equation to forecast sedimentation and hence volcanic hazards. Here, we test the application of the advection–diffusion equation to tephra sedimentation using data collected on the proximal (350 to ~1,200?m from the vent) to medial (greater than ~1,200?m from the vent) tephra blanket of a basaltic cinder cone, Cerro Negro volcano, located in Nicaragua. Our understanding of tephra depositional processes at this volcano is significantly improved by combination of sample pit data in the medial zone and high-resolution ground-penetrating radar (GPR) data collected in the near vent and proximal zones. If the advection–diffusion equation applies, then the thickness of individual tephra deposits should have Gaussian crosswind profiles and exponential decay with distance away from the vent. At Cerro Negro, steady trade winds coupled with brief eruptions of relatively low energy (VEI 2–3) create relatively simple deposits. GPR data were collected along three crosswind profiles at distances of 700–1,600?m from the vent; sample pits were used to estimate thickness of the 1992 tephra deposit up to 13?km from the vent. Horizons identified in proximal GPR profiles exhibit Gaussian distributions with a high degree of statistical confidence, with diffusion coefficients of ~500?m2?s?1 estimated for the deposits, confirming that the advection–diffusion equation is capable of modeling sedimentation in the proximal zone. The thinning trend downwind of the vent decreases exponentially from the cone base (350?m) to ~1,200?m from the vent. Beyond this distance, deposit overthickening occurs, identified in both GPR and sample pit datasets. The combined data reveal three depositional regimes: (1) a near-vent region on the cone itself, where fallout remobilizes in granular flows upon deposition; (2) a proximal zone in which particles fall from a height of less than ~2?km; and (3) a medial zone, in which particles fall from ~4 to 7?km and the deposit is thicker than expected based on thinning trends observed in the proximal zone of the deposit. This overthickening of the tephra blanket, defining the transition from proximal to medial depositional facies, is indicative of transition from sedimentation dominated by fallout from plume margins to that dominated by fallout from the buoyant eruption cloud—a feature of deposits previously identified in larger-volume eruptions. We interpret this change to represent a change in diffusion law, occurring at total particle fall times (the fall time threshold of numerical models) of ~400?s. Thus, the detailed GPR profiles and pit data collected at Cerro Negro help to validate current numerical models of tephra sedimentation.  相似文献   

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