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1.
The distribution, stratigraphic relationships and fragmental components of the May 8 and 20, 1902, pyroclastic flows from Mt. Pelée, Martinique, together with eyewitness accounts, suggest the following explanation for those eruptions. The eruptions were vertically directed magmatic (perhaps initiated phreatically), and contained abundant juvenile lithics from congealed magma of the dome and neck. This resulted in a two-part eruption column having (1) a dense, lithic-charged part which collapsed into the crater and flowed out of a pre-existing notch in its side, giving rise to pyrochlastic flows, and (2) a magmatically derived column containing gases, juvenile vitric material and crystals which largely by-passed the neck and dome and escaped into the atmosphere. All of the energy of the flows was apparently focused through the notch. They emerged fully turbulent and flowed down Rivière Blanche. Gravity segregation of large and abundant fragments soon resulted in a dense, high-concentration, poorly fluidized block-and-ash flow confined to the valley, leaving above a fully turbulent, high-energy ash-cloud surge. As the ash-cloud surge moved down the mountain, it continued to expand outward. The process of gravity segregation continued as the ash-cloud surge expanded, resulting in secondary block-and-ash underflows. Toward St. Pierre, the secondary block-and-ash flows developed on a gently sloping upland surface 100 m or more above the valley of Rivière Blanche. The turbulent, fragment-depleted surges above the secondary block-and-ash flows maintained sufficient energy to devastate the landscape outward to about 3000 m, including St. Pierre. The surges refracted around obstacles and in one place, moved up a small valley in a direction opposite to the main flows.  相似文献   

2.
A series of pristine block-and-ash flow deposits from the May–June 2006 eruption of Merapi represent an exceptional record of small-volume pyroclastic flows generated by gravitational lava-dome collapses over a period of about two months. The deposits form nine overlapping lobes reaching ~ 7 km from the summit in the Gendol River valley on the volcano's southern flank, which were produced by successive flows generated during and after the major dome-collapse event on June 14. Both, single pulse (post-June 14 events) and multiple-pulse pyroclastic flows generated by sustained dome collapses on June 14 are recognised and three types of deposits, spread over an area of 4.7 km², are distinguished, totalling 13.3 × 106 m3: (1) valley-confined basal avalanche deposits (11.7 × 106 m3) in the Gendol River valley, (2) overbank pyroclastic-flow and associated surge deposits (1.4 × 106 m3), where parts of the basal avalanche spread laterally onto interfluves and were subsequently channeled into the surrounding river valleys and (3) dilute ash-cloud surge deposits (0.2 × 106 m3) along valley margins. Variations in the distribution, surface morphology and lithology of the deposits are related to the source materials involved in individual pyroclastic-flow-forming events and varying modes of transport and deposition of the different flows. Inferred flow velocities of the largest block-and-ash flows generated on June 14 vary from 43.8–13.5 m/s for the basal avalanche and from 62.6–24.2 m/s for the ash-cloud surge. The minimum temperatures range from 400 °C for the basal avalanche to 165 °C for the overlying ash cloud. Due to the potential of being re-channeled into adjacent river valleys and flowing laterally away from the main river channel, the overbank pyroclastic flows are considered the most hazardous part of the block-and-ash flow system. The conditions that lead to their development during flow transport and deposition must be taken into account when assessing future pyroclastic flow hazards at Merapi and similar volcanoes elsewhere.  相似文献   

3.
Pyroclastic flows from the 1991 eruption of Unzen volcano,Japan   总被引：1，自引：0，他引：1  
Pyroclastic flows from Unzen were generated by gravitational collapse of the growing lava dome. As soon as the parental lobe failed at the edge of the dome, spontaneous shattering of lava occurred and induced a gravity flow of blocks and finer debris. The flows had a overhanging, tongue-like head and cone- or rollershaped vortices expanding outward and upward. Most of the flows traveled from 1 to 3 km, but some flows reached more than 4 km, burning houses and killing people in the evacuated zone of Kita-kamikoba on the eastern foot of the volcano. The velocities of the flows ranged from 15 to 25 m/s on the gentle middle flank. Observations of the flows and their deposits suggest that they consisted of a dense basal avalanche and an overlying turbulent ash cloud. The basal avalanche swept down a topographic low and formed to tongue-like lobe having well-defined levees; it is presumed to have moved as a non-Newtonian fluid. The measured velocities and runout distances of the flows can be matched to a Bingham model for the basal avalanche by the addition of turbulent resistance. The rheologic model parameters for the 29 May flow are as follows: the density is 1300 kg/m3, the yield strength is 850 Pa, the viscosity is 90 Pa s, and the thickness of the avalanche is 2 m. The ash cloud is interpreted as a turbulent mixing layer above the basal avalanche. The buoyant portions of the cloud produced ash-fall deposits, whereas the dense portions moved as a surge separated from the parental avalanche. The ash-cloud surges formed a wide devastated zone covered by very thin debris. The initial velocities of the 3 June surges, when they detached from avalanches, are determined by the runout distance and the angle of the energy-line slope. A comparison between the estimated velocities of the 3 June avalanches and the surges indicates that the surges that extended steep slopes along the avalanche path, detached directly from the turbulent heads of the avalanches. The over-running surge that reached Kita-Kamikoba had an estimated velocity higher than that of the avalanche; this farther-travelled surge is presumed to have been generated by collapse of a rising ash-cloud plume.  相似文献   

4.
The chronology of deposits of the 1976 eruption of Augustine volcano, which produced pyroclastic falls, pyroclastic flows, and lava domes, is determined by correlating the stratigraphy with published records of seismicity, plume observations, and distant ash falls. Three thin air-fall ash beds (unit A1, A2 and A3) correlate with events near the beginning of the 1976 eruption on 22 and 23 January. On 24 January a small-volume, ash-cloud-surge deposit (unit S) accumulated over the north half of Augustine Island. A series of pumiceous pyroclastic flows represented by the lobate pumiceous deposits (unit F) occurred on 24 January and locally melted the snowpack to cause small pumice-laden floods. A thin ash bed (unit A4) was deposited on 24 January, and the main plinian eruption (unit P) occurred on 25 January. In middle to late February and again in mid April, lava domes were extruded at the summit accompanied by incandescent block-and-ash flows down the north flank. A hut near the north coast of the island was mechanically and thermally damaged by the small-volume ash-cloud surge of unit S before the eruption of the pumice flow of unit F; the metal roof was then penetrated by lithic fragments of the plinian fall of 25 January. Explosive eruptions in the early stage of an eruption-like that which deposited unit S — are important hazards at Augustine Island, as are infrequent debris avalanches and attendant tsunamis.deceased on 18 May 1980  相似文献   

5.
A study of emplacement temperatures was carried out for the largest of the 22 November 1994 nuée ardente deposits at Merapi Volcano, based mainly on the response of plastic and woody materials subjected to the hot pyroclastic current and the deposits, and to some extent on eyewitness observations. The study emphasizes the Turgo–Kaliurang area in the distal part of the area affected by the nuée ardente, where nearly 100 casualties occurred. The term nuée ardente as used here includes channeled block-and-ash flows, and associated ash-clouds of surge and fallout origins. The emplacement temperature of the 8 m thick channeled block-and-ash deposit was relatively high, 550°C, based mainly on eyewitness reports of visual thermal radiance. Emplacement temperatures for ash-cloud deposits a few cm thick were deduced from polymer objects collected at Turgo and Kaliurang. Most polymers do not display a sharp melting range, but polyethylene terephthalate used in water bottles melts between 245 and 265°C, and parts of the bottles that had been deformed during fabrication molding turn a milky color at 200°C. The experimental evidence suggests that deposits in the Turgo area briefly achieved a maximum temperature near 300°C, whereas those near Kaliurang were <200°C. Maximum ash deposit temperatures occurred in fallout with a local source in the channeled block-and-ash flow of the Boyong river valley; the surge deposit was cooler (180°C) due to entrainment of cool air and soils, and tree singe-zone temperatures were around 100°C.  相似文献   

6.
Several hot-rock avalanches have occurred during the growth of the composite dome of Mount St. Helens, Washington between 1980 and 1987. One of these occurred on 9 May 1986 and produced a fan-shaped avalanche deposit of juvenile dacite debris together with a more extensive pyroclastic-flow deposit. Laterally thinning deposits and abrasion and baking of wooden and plastic objects show that a hot ash-cloud surge swept beyond the limits of the pyroclastic flow. Plumes that rose 2–3 km above the dome and vitric ash that fell downwind of the volcano were also effects of this event, but no explosion occurred. All the facies observed originated from a single avalanche. Erosion and melting of craterfloor snow by the hot debris caused debris flows in the crater, and a small flood that carried juvenile and other clasts north of the crater. A second, broadly similar event occured in October 1986. Larger events of this nature could present a significant volcanic hazard.  相似文献   

7.
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8.
Nuées ardentes associated with dome collapse on 22 November 1994, at Merapi volcano traveled to the south–southwest as far as 6.5 km, and collectively accumulated roughly 2.5–3 million cubic meters of deposits. The damaged area comprises 9.5 km2 and is covered by two nuée ardente facies, a conventional “Merapi-type”, valley-fill block-and-ash flow facies and a pyroclastic surge facies. The proximal deposits reflect the accumulation of dozens of nuées ardentes, with many subsidiary flow units. The distal deposits are more simply organized, as only a few individual events reached to distances >3.5 km. The stratigraphic relationships north of Turgo hill indicate that the surge deposits are a facies of particularly mobile nuées ardentes that also deposited channeled block-and-ash flow facies. They further suggest that the surge facies beyond the channel margins correlate laterally with a finer-grained sublayer locally developed at the base of the block-and-ash flow facies. Eyewitness reports suggest that the emplacement of the block-and-ash flow facies in the distal part of the Boyong river may have followed, by a short time interval, the destruction and deposition of the surge facies at Turgo village. The stratigraphy is in accord with the eyewitness reports. The surge facies was emplaced by a dilute surge current, detached from the same dome-collapse nuée ardente that, as a separate flow unit, subsequently emplaced the distal block-and-ash deposit in the Boyong valley. The detachment occurred at higher elevations, likely at or above the slope break at about 2000 m elevation. This flow separation enabled the surge current to shortcut over the landscape and to emplace its deposit even as the block-and-ash flow continued its tortuous southward movement in the Boyong channel. Dome-collapse nuée ardente activity formed the bulk of the eruption, which was accompanied by virtually no significant vertical summit explosive activity.  相似文献   

9.
The 0.196 Ma, lithic-rich Abrigo Ignimbrite on Tenerife, Canary Islands contains localised massive, coarse pumice-rich ignimbrite lobes (MPRILs). They typically form low ridges up to 2 m high with axes parallel to the flow direction, and, in cross-section, they range from symmetrical to asymmetrical and highly skewed lobate bedforms generally with flat bases. The major components are rounded pebble- to cobble-sized phonolitic pumice clasts within an ignimbritic matrix of ash, fine lithics and minor crystals, which varies from lithic-rich to lithic-poor. Commonly, there is a vertical increase in pumice concentration from matrix-supported texture at the base to clast-supported at the top, accompanied by an increase in pumice clast size. MPRILs often thin and grade laterally perpendicular to current flow into planar pumice concentration zones. They occur at one or more stratigraphic levels as either solitary lobes associated with flat topography or as multiple onlapping lobes or within a laterally complex stratified pumice-rich ignimbrite facies (LCSPIs) near palaeotopographic highs.MPRILs are original depositional features, not erosional in origin and are derived from a larger pyroclastic flow. It is likely that pumice was segregated to the upper and outer regions of the parent flow causing a significant rheological contrast with the lower lithic-rich zone. The more pumice-rich parts are interpreted to have detached from the parent flow as it decelerated onto gentler slopes or interacted with topographic highs and raced ahead as mobile derivative pyroclastic flows. The flow-parallel ridge shape of MPRILs may be a result of fingering within these flows or concentration of pumice within the intermediary clefts. Deposition occurred “en masse” at the termination of the flow front. The resultant lobate deposits were then overridden and mantled by normal ignimbrite facies from either a later flow pulse or the following main part of the parent flow.  相似文献   

10.
The November 1994 eruption at Merapi volcano provided good evidence of decoupling of dome-collapse pyroclastic flows and of large-scale detachment of an ash-cloud surge (ACS) component from the basal block-and-ash flow (BAF). Timing and stratigraphic relationships of the largest 1994 ACS indicate that this escaped from the valleys, travelled well ahead of the BAF, arrived at the termination tens of seconds before it and deposited a discrete ACS deposit beneath the BAF unit. This suggests that the ACS detachment mostly occurred relatively high on the volcano slope, likely at the foot of the proximal cone. Later pyroclastic flow eruptions in January 1997 and July 1998 also showed evidence of ACS detachment, although to a lesser extent, suggesting that ACSs could be a frequent hazard at Merapi volcano. Based on an extensive review of the available literature and on field investigations of historical deposits, we show here that flow decoupling and ACS detachment in the way inferred from the 1994 eruption is a common process at Merapi. The ACS-related destructions outside valleys were frequently reported in the recent past activity of the volcano, i.e. in at least 16 pyroclastic flow eruptions since 1927. Destruction occurred systematically in eruptions where maximum runout of the BAFs was 6.5 km or more, and occurred rarely for BAF runouts of 4.5 km or less. The ACS deposits have been recognized beneath some valley-filling BAF units we attribute to some recent destructive eruptions, i.e. the 1930, 1954, 1961 and 1969 eruptions. Topographic conditions at Merapi volcano favouring ACS detachment include: (a) the high slope (30°) of the proximal cone, leading to high proximal velocities of the pyroclastic flows and thus to the transfer of large amounts of particles into the ash cloud; (b) the strong break in slope at the foot of the proximal cone, where the velocity of the basal BAF is strongly reduced and a major ACS component is thought to form and detach by shearing over the BAF; and (c) the small depth of most valleys in the first kilometres beyond the foot of the cone, which allows minor ACS components to escape from the valleys during travel of the BAF; however, flow decoupling and ACS detachment occur for only some of the numerous pyroclastic flows that follow the same path in a given eruption. This indicates that topography alone cannot lead to flow decoupling. We suggest two factors that control flow decoupling and its extent. The main one is flow volume (and thus flux, as both are correlated in almost instantaneous, dome-collapse events), as suggested by the observed relationship between flow decoupling and the travel distance of the pyroclastic flows. The second factor is the amount of available ash in the flow at its early stage, which influences the mass and thus momentum of the ash cloud. The amount of ash in the pyroclastic flows of Merapi may depend on several factors, among which are (a) the physical and thermal state of the part of the active dome that collapses, and (b) the proportion of older, cold rocks incorporated in the flow, either by undermining of surrounding summit rocks by the current pyroclastic flow activity or by erosion on the upper slopes.  相似文献   

11.
The grainsize characteristics of dune-bedded pyroclastic surge bedsets are surveyed. The variance between coarsest and finest beds ranges from 1 to 6 phi in different surge bedsets, and it increases as the grainsize of the coarsest bed increases, reflecting an increasing velocity of emplacement. Deposits of wet surges, identified as those which contain accretionary and ash-coated lapilli, tend to be finer and show less variance, this partly because wet ash is cohesive, but mainly because wet surges tend to be weaker. Dry surge bedsets are strongly fines-depleted, wet ones less so. The lack of erosion of underlying ash layers shows that the environment is a strongly depositional one. Individual bedsets are demarcated by thin intervening fine ash-fall layers, which are the complementary ash-cloud deposits settled or flushed out after the passage and decay of each turbulent surge. Surge deposits are generally less coarse than the coarsest associated airfall deposits, which shows that they are formed by generally weaker events.This study helps interpret the dune-bedded parts of the landscape-mantling May 18th 1980 “blast” deposit of Mount St. Helens. The blast was a very violent event, but the variance and the grainsize of the coarsest bed are those of a relatively weak surge. This suggests that the dune-bedding was produced by a weak effect, such as minor turbulence in a thin pyroclastic flow coming to rest in a mountainous terrain roughened by tree stumps and fallen logs.  相似文献   

12.
Young pumice deposits on Nisyros,Greece   总被引：1，自引：1，他引：1  
The island of Nisyros (Aegean Sea) consists of a silicic volcanic sequence upon a base of mafic-andesitic hyaloclastites, lava flows, and breccias. We distinguish two young silicic eruptive cycles each consisting of an explosive phase followed by effusions, and an older silicic complex with major pyroclastic deposits. The caldera that formed after the last plinian eruption is partially filled with dacitic domes. Each of the two youngest plinian pumice falls has an approximate DRE volume of 2–3 km3 and calculated eruption column heights of about 15–20 km. The youngest pumice unit is a fall-surge-flow-surge sequence. Laterally transitional fall and surge facies, as well as distinct polymodal grainsize distributions in the basal fall layer, indicate coeval deposition from a maintained plume and surges. Planar-bedded pumice units on top of the fall layer were deposited from high-energy, dry-steam propelled surges and grade laterally into cross-bedded, finegrained surge deposits. The change from a fall-to a surge/flow-dominated depositional regime coincided with a trend from low-temperature argillitic lithics to high-temperature, epidote-and diopside-bearing lithic clasts, indicating the break-up of a high-temperature geothermal reservoir after the plinian phase. The transition from a maintained plume to a surge/ash flow depositional regime occurred most likely during break-up of the high-temperature geothermal reservoir during chaotic caldera collapse. The upper surge units were possibly erupted through the newly formed ringfracture.  相似文献   

13.
El Chichón volcano consists of a 2-km wide Somma crater compound cone 0.2 Ma old with peripheral domes with a central crater reactivated several times during the Holocene. The most recent eruption at El Chichón occurred from March 28 to April 4, 1982, resulting in the worst volcanic disaster during historical times in Mexico, killing more than 2000 people and destroying nine towns and small communities. The volcanic hazard map of El Chichón is based on detailed field work that documented twelve eruptions during the last 8000 years, and computer simulations. To validate the results, computer simulations were first performed over pre-1982 topography mimicking the extent of the actual deposits produced and afterwards run over post-1982 topography. These eruptions have produced pyroclastic fall, surge, flow and lahar deposits. Pyroclastic flows have different volumes and Heim coefficients varying from 0.2 (pumice flows), to 0.15 (block-and-ash flows) and 0.10 (ash flows). Simulations using FLOW3D and TITAN2D indicate that pumice flows and block-and-ash flows can fill the moat area and follow main ravines up to distances of ca. 3 km from the crater, with no effect on populations around the volcano. On the other hand, more mobile ash flows related to column-collapse events can reach up to 4 km from the vent, but will always follow the same paths and still not affect surrounding populations. The energy-cone model was used to simulate the outflow of pyroclastic surges based on the 1982 event (H/L = 0.1 and 0.2), and shows that surges may reach some towns around the volcano.  相似文献   

14.
This paper documents a complex sequence of interbedded lapilli-fall, base-surge, and pyroclastic-flow deposits, here named the Monte Guardia sequence, that erupted from volcanic centers in the southern part of Lipari (Aeolian Island Arc). Radiocarbon data from ash-flow tuffs above and below this sequence bracket its eruption between 22,600 and 16,800 years ago. Geologic evidence, however, suggests that this single eruptive cycle had a more restricted duration of years to tens-of-years. The basis for our interpretations comes from data measured at 38 detailed sections located throughout the island. The Monte Guardia sequence rests on a series of lower rhyolitic endogenous domes in the southern part of Lipari and it covers the oldest lavas, lahars, and pyroclastic flows in the north. Only in the northeast part of the island is it covered by younger deposits which there consist of lapilli tuffs and lavas of the Monte Pilato rhyolitic cycle. The deposit ranges in thickness from more than 60 m surrounding the vents in the south to less than a few decimeters at 10 km distance in the north. Throughout most of the island the Monte Guardia sequence overlies a thin andesitic lapilli-fall layer which is a key bed for correlation. This lapilli tuff probably erupted from a volcanic center on another island of the Aeolian Arc (possibly Salina). The principal activity of the Monte Guardia sequence started with an explosion that formed a continuous breccia blanket covering most of the island. Some pumiceous blocks within this breccia are composed of alternating bands of acidic and andesitic composition suggesting that the initiation of pyroclastic activity could have been triggered by magma mixing. Typical Monte Guardia sequence consists of explosive products that grade from magmatic (pumice-fall) to phreatomagmatic (base-surge) character. The eruptive cycle is characterized by a number of energy decreasing megarhythms that start with a lapilli-fall bed and end with a base-surge set that progresses through sand-wave, massive, and planar beds. Isopach maps of the fall and surge deposits indicate that both types were directed to the northwest by prevailing winds. Existing topographic relief was an additional factor that affected the emplacement of surge products. At the end of the cycle andesitic pyroclastic flows and rhyolitic endogenous domes were emplaced above the Monte Guardia deposits near the vent.  相似文献   

15.
The eruptive history of Kuju volcano on Kyushu, Japan, during the past 15,000 years has been determined by tephrochronology and 14C dating. Kuju volcano comprises isolated lava domes and cones of hornblende andesite together with aprons of pyroclastic-flow deposits on its flanks. Kuju volcano produced tephras at roughly 1000-yr intervals during the past 5000 years and 70% of the domes and cones have formed during the past 15,000 years. The youngest magmatic activity of Kuju volcano was the 1.6 km3 andesite eruption about 1600 years ago which emplaced a lava dome and block-and-ash flow. Kuju volcano shows a nearly constant long-term eruption rate (0.7–0.4 km3 for 1000 years) during the past 15,000 years. This rate is within the range of estimated average eruption rates of late Quaternary volcanoes in the Japanese Arc, but is about one order of magnitude higher than the eruption rate of Unzen volcano. Kuju volcano has been in phreatic eruption since October 1995. The late Quaternary history of Kuju indicates that it poses a significant volcanic hazard, primarily due to block-and-ash flows from collapsing lava domes.  相似文献   

16.
The Sarikavak Tephra from the central Galatean Volcanic Province (Turkey) represents the deposit of a complex multiple phase plinian eruption of Miocene age. The eruptive sequence is subdivided into the Lower-, Middle-, and Upper Sarikavak Tephra (LSKT, MSKT, USKT) which differ in type of deposits, lithology and eruptive mechanisms.The Lower Sarikavak Tephra is characterised by pumice fall deposits with minor interbedded fine-grained ash beds in the lower LSKT-A. Deposits are well stratified and enriched in lithic fragments up to >50 wt% in some layers. The upper LSKT-B is mainly reversely graded pumice fall with minor amounts of lithics. It represents the main plinian phase of the eruption. The LSKT-A and B units are separated from each other by a fine-grained ash fall deposit. The Middle Sarikavak Tephra is predominantly composed of cross-bedded ash-and-pumice surge deposits with minor pumice fall deposits in the lower MSKT-A and major pyroclastic flow deposits in the upper MSKT-B unit. The Upper Sarikavak Tephra shows subaerial laminated surge deposits in USKT-A and subaqueous tephra beds in USKT-B.Isopach maps of the LSKT pumice fall deposits as well as the fine ash at the LSKT-A/B boundary indicate NNE–SSW extending depositional fans with the source area in the western part of the Ovaçik caldera. The MSKT pyroclastic flow and surge deposits form a SW-extending main lobe related to paleotopography where the deposits are thickest.Internal bedding and lithic distribution of the LSKT-A result from intermittent activity due to significant vent wall instabilities. Reductions in eruption power from (partial) plugging of the vent produced fine ash deposits in near-vent locations and subsequent explosive expulsion of wall rock debris was responsible for the high lithic contents of the lapilli fall deposits. A period of vent closure promoted fine ash fall deposition at the end of LSKT-A. The subsequent main plinian phase of the LSKT-B evolved from stable vent conditions after some initial gravitational column collapses during the early ascent of the re-established eruption plume. The ash-and-pumice surges of the MSKT-A are interpreted as deposits from phreatomagmatic activity prior to the main pyroclastic flow formation of the MSKT-B.  相似文献   

17.
火山碎屑流堆积因其巨大的危害性而成为火山学研究的重要课题之一。文中对长白山区天池火山千年大喷发火山碎屑流堆积进行了粒度分析。分析结果表明,火山碎屑流堆积分选差,伴生的灰云浪堆积分选性好。火山碎屑流堆积中的岩屑和浮岩的平均最大粒度随着离开火山口距离的加大而减小,反映火山碎屑流搬运过程中存在重力分选和机械磨损作用。火山碎屑流在搬运过程中发生了流体化作用,离开火山口越远流体化速度越小,反映火山碎屑流在搬运过程中发生了流体化去气作用,这种作用使火山碎屑流的粘度和屈服强度增加而导致沉积。流体化速度是控制搬运距离的因素之一,远源多渠道火山碎屑流的汇合使流体化速度增大,搬运距离更远,造成的灾害范围也增大  相似文献   

18.
Volcanic ash, generated in the long-lived eruption of the Soufrière Hills volcano, Montserrat, is shown to contain respirable (sub-4 μm) particles and cristobalite, a crystalline silica polymorph. Respirable particles of cristobalite can cause silicosis, raising the possibility that volcanic ash is a respiratory health hazard. This study considers some of the main factors which affect human exposure to respirable volcanic ash, namely, the composition and proportions of respirable ash, and the composition and concentrations of airborne suspended particulates. The composition, size distribution and proportion (by weight) of respirable particles in representative samples of the Soufrière Hills tephra (dome-collapse ash-fall deposits, dome-collapse pyroclastic-flow matrix, Vulcanian explosion ash and mixed ash) have been characterized. Dome-collapse ash-fall deposits are significantly richer in respirable particles (12 wt%) than the other tephra samples, in particular the matrices of dome-collapse pyroclastic-flow deposits (3 wt%). Within the respirable fraction, dome-collapse ash contains the highest proportion of crystalline silica particles (20–27 number%, of which 97 wt% is cristobalite), compared with other primary tephra types (0.4–5.6 number%). This enrichment of crystalline silica in the dome-collapse ash is most pronounced in the very fine particle fraction (sub-2 μm). The results are explained as being due to significant size fractionation during fragmentation of pyroclastic flows, resulting in a fines-depleted dome-collapse matrix and a fines-rich dome-collapse ash deposit. For all sample types, the sub-4 μm fraction comprises 45–55 wt% of the sub-10 μm fraction. Aeolian deposit, lahar deposit and airborne samples of suspended ash, collected on filters, were characterized. These samples show enrichment of crystalline silica in the respirable fraction (10–18 number%). The results are consistent with ash in the environment having a mixed origin but originating predominantly from dome-collapse eruptions. The reworked ash, however, contains low proportions of respirable ash (～3 wt%) compared to primary ash samples. The concentration of ash particles re-suspended by road vehicles on Montserrat is found to decrease exponentially with height above the ground, indicating higher exposure for children compared with adults: PM4 concentration at 0.9 m (height of two-year-old child) is 3 times that at 1.8 m (adult height). The composition of the re-suspended road particles is similar to that re-suspended by the wind.  相似文献   

19.
The dynamics and thermodynamics of large ash flows   总被引：6，自引：6，他引：0  
 Ash flow deposits, containing up to 1000 km3 of material, have been produced by some of the largest volcanic eruptions known. Ash flows propagate several tens of kilometres from their source vents, produce extensive blankets of ash and are able to surmount topographic barriers hundreds of metres high. We present and test a new model of the motion of such flows as they propagate over a near horizontal surface from a collapsing fountain above a volcanic vent. The model predicts that for a given eruption rate, either a slow (10–100 m/s) and deep (1000–3000 m) subcritical flow or a fast (100–200 m/s) and shallow (500–1000 m) supercritical flow may develop. Subcritical ash flows propagate with a nearly constant volume flux, whereas supercritical flows entrain air and become progressively more voluminous. The run-out distance of such ash flows is controlled largely by the mass of air mixed into the collapsing fountain, the degree of fragmentation and the associated rate of loss of material into an underlying concentrated depositional system, and the mass eruption rate. However, in supercritical flows, the continued entrainment of air exerts a further important control on the flow evolution. Model predictions show that the run-out distance decreases with the mass of air entrained into the flow. Also, the mass of ash which may ascend from the flow into a buoyant coignimbrite cloud increases as more air is entrained into the flow. As a result, supercritical ash flows typically have shorter runout distances and more ash is elutriated into the associated coignimbrite eruption columns. We also show that one-dimensional, channellized ash flows typically propagate further than their radially spreading counterparts. As a Plinian eruption proceeds, the erupted mass flux often increases, leading to column collapse and the formation of pumiceous ash flows. Near the critical conditions for eruption column collapse, the flows are shed from high fountains which entrain large quantities of air per unit mass. Our model suggests that this will lead to relatively short ash flows with much of the erupted material being elutriated into the coignimbrite column. However, if the mass flux subseqently increases, then less air per unit mass is entrained into the collapsing fountain, and progressively larger flows, which propagate further from the vent, will develop. Our model is consistent with observations of a number of pyroclastic flow deposits, including the 1912 eruption of Katmai and the 1991 eruption of Pinatubo. The model suggests that many extensive flow sheets were emplaced from eruptions with mass fluxes of 109–1010 kg/s over periods of 103–105 s, and that some indicators of flow "mobility" may need to be reinterpreted. Furthermore, in accordance with observations, the model predicts that the coignimbrite eruption columns produced from such ash flows rose between 20 and 40 km. Received: 25 August 1995 / Accepted: 3 April 1996  相似文献   

20.
The initial explosions at Mount St. Helens, Washington, on the moring of 18 May 1980 developed into a huge pyroclastic surge that generated catastrophic floods off the east and west flanks of the volcano. Near-source surge deposits on the east and west were lithic, sorted, lacking in accretionary lapilli and vesiculated ash, not plastered against upright obstacles, and hot enough to char wood — all attributes of dry pyroclastic surge. Material deposited at the surge base on steep slopes near the volcano transformed into high-concentration lithic pyroclastic flows whose deposits contain charred wood and other features indicating that these flows were hot and dry. Stratigraphy shows that even the tail of the surge had passed the east and west volcano flanks before the geomorphically distinct floods (lahars) arrived. This field evidence undermines hypotheses that the turbulent surge was itself wet and that its heavy components segregated out to transform directly into lahars. Nor is there evidence that meters-thick snow-slab avalanches intimately mixed with the surge to form the floods. The floods must have instead originated by swift snowmelt at the base of a hot and relatively dry turbulent surge. Impacting hot pyroclasts probably transferred downslope momentum to the snow surface and churned snow grains into the surge base. Melting snow and accumulating hot surge debris may have moved initially as thousands of small thin slushflows. As these flows removed the surface snow and pyroclasts, newly uncovered snow was partly melted by the turbulent surge base; this and accumulating hot surge debris in turn began flowing, a self-sustaining process feeding the initial flows. The flows thus grew swiftly over tens of seconds and united downslope into great slushy ejecta-laden sheetfloods. Gravity accelerated the floods to more than 100 km/h as they swept down and off the volcano flanks while the snow component melted to form great debris-rich floods (lahars) channeled into valleys.  相似文献   

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