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1.
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   

2.
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   

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.
Large-scale collapse of a dacite dome in the late afternoon of 15 September 1991 generated a series of pyroclastic-flow events at Unzen Volcano. Pyroclastic flows with a volume of 1×106 m3 (as DRE) descended the northeastern slope of the volcano, changing their courses to the southeast due to topographic control. After they exited a narrow gorge, an ash-cloud surge rushed straight ahead, detaching the main body of the flow that turned and followed the topographic lows to the east. The surge swept the Kita-Kamikoba area, which had been devastated by the previous pyroclastic-flow events, and transported a car as far as 120 m. Following detachment, the surge lost its force after it moved several hundred meters, but maintained a high temperature. The deposits consist of a bottom layer of better-sorted ash (unit 1), a thick layer of block and ash (unit 2), and a thin top layer of fall-out ash (unit 3). Unit 2 overlies unit 1 with an erosional contact. The upper part of unit 2 grades into better-sorted ash. At distal block-and-ash flow deposits, the bottom part of unit 2 also consists of better-sorted ash, and the contact with the unit 1 deposits becomes ambiguous. Video footage of cascading pyroclastic flows during the 1991–1995 eruption, traveling over surfaces without any topographic barriers, revealed that lobes of ash cloud protruded intermittently from the moving head and sides, and that these lobes surged ahead on the ground surface. This fact, together with the inspection by helicopter shortly after the events, suggests that the protruded lobes consisted of better-sorted ash, and resulted in the deposits of unit 1. The highest ash-cloud plume at the Oshigadani valley exit, and the thickest deposition of fall-out ash over Kita-Kamikoba and Ohnokoba, indicate that abundant ash was also produced when the flow passed through a narrow gorge. In the model presented here, the ash clouds from the pyroclastic flows were composed of a basal turbulent current of high concentration (main body), an overriding and intermediate fluidization zone, and an overlying dilute cloud. Release of pressurized gas in lava block pores, due to collisions among blocks and the resulting upward current, caused a zone of fluidization just above the main body. The mixture of gas and ash sorted in the fluidization zone moved ahead and to the side of the main body as a gravitational current, where the ash was deposited as surge deposits. The main body, which had high internal friction and shear near its base, then overran the surge deposits, partially eroding them. When the upward current of gas (fluidization) waned, better-sorted ash suspended in the fluidization zone was deposited on block-and-ash deposits. In the distal places of block-and-ash deposits, unit 2 probably was deposited in non-turbulent fashion without any erosion of the underlying layer (unit 1).  相似文献   

5.
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   

6.
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   

7.
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   

8.
The 18–24 January 1913 eruption of Colima Volcano consisted of three eruptive phases that produced a complex sequence of tephra fall, pyroclastic surges and pyroclastic flows, with a total volume of 1.1 km3 (0.31 km3 DRE). Among these events, the pyroclastic flows are most interesting because their generation mechanisms changed with time. They started with gravitanional dome collapse (block-and-ash flow deposits, Merapi-type), changed to dome collapse triggered by a Vulcanian explosion (block-and-ash flow deposits, Soufrière-type), then ended with the partial collapse of a Plinian column (ash-flow deposits rich in pumice or scoria,). The best exposures of these deposits occur in the southern gullies of the volcano where Heim Coefficients (H/L) were obtained for the various types of flows. Average H/L values of these deposits varied from 0.40 for the Merapi-type (similar to the block-and-ash flow deposits produced during the 1991 and 1994 eruptions), 0.26 for the Soufrière-type events, and 0.17–0.26 for the column collapse ash flows. Additionally, the information of 1991, 1994 and 1998–1999 pyroclastic flow events was used to delimit hazard zones. In order to reconstruct the paths, velocities, and extents of the 20th Century pyroclastic flows, a series of computer simulations were conducted using the program FLOW3D with appropriate Heim coefficients and apparent viscosities. The model results provide a basis for estimating the areas and levels of hazard that could be associated with the next probable worst-case scenario eruption of the volcano. Three areas were traced according to the degree of hazard and pyroclastic flow type recurrence through time. Zone 1 has the largest probability to be reached by short runout (<5 km) Merapi and Soufrière pyroclastic flows, that have occurred every 3 years during the last decade. Zone 2 might be affected by Soufriere-type pyroclastic flows (∼9 km long) similar to those produced during phase II of the 1913 eruption. Zone 3 will only be affected by pyroclastic flows (∼15 km long) formed by the collapse of a Plinian eruptive column, like that of the 1913 climactic eruption. Today, an eruption of the same magnitude as that of 1913 would affect about 15,000 inhabitants of small villages, ranches and towns located within 15 km south of the volcano. Such towns include Yerbabuena, and Becerrera in the State of Colima, and Tonila, San Marcos, Cofradia, and Juan Barragán in the State of Jalisco.  相似文献   

9.
In 2006 Merapi volcano, Indonesia, erupted for a few months, producing several block-and-ash flows reaching a maximum distance of 7.5 km from the main vent. During the eruption, we conducted a survey on those flow deposits in the Gendol Valley at Kaliadem village, about 4.5 km from the Merapi submit, using a Ground Penetrating Radar (GPR). The upper deposit was studied in its distal reaches, whereas the one below was studied in its medial reaches. The field study was carried out with a commercial RAMAC® GPR coupled with 100 MHz antennas, and the data treatment conducted with Reflex™ software. From this survey, we determined both deposits' local (1) thickness – reaching a maximum of 15 m – and (2) internal architecture. This last one is governed by long reflecting horizons extending over 20 to 30 m that delimit layers showing progradation patterns in their distal reaches. Within these layers we could also observe an internal architecture of still unknown origin. The layers are interpreted as the result of the flow pulses that progressively deposited downstream-ward by progradation. However the interpretation of those GPR profiles is a bit hazardous, because of the absence of outcrops, and we can only proceed by analogy with other studies. Nevertheless, despite numerous limitations, GPR is a helpful tool to understand pyroclastic deposits' structure when no visual observations are available.  相似文献   

10.
We present Titan2D simulations of two well-characterized block-and-ash flow (BAF) events of the 2006 eruption of Merapi (Java, Indonesia) that affected the Gendol valley on the volcano’s southern flank and adjacent, densely populated interfluve (non-valley) areas: (1) a single dome-collapse event to the south that generated one of the smaller, post-June 14 flows and (2) a sustained, multiple dome-collapse event, also directed to the south, that produced the largest flows of the 2006 eruption emplaced in the afternoon of June 14. Using spatially varying bed friction angles, Titan2D is capable of reproducing the paths, velocities, runout distance, areas covered and deposited volumes of these flows over highly complex topography. The model results provide the basis for estimating the areas and levels of hazards associated with BAFs generated during relatively short as well as prolonged dome-collapse periods and guidance during future eruptive crises at Merapi.  相似文献   

11.
A devastating pyroclastic surge and resultant lahars at Mount St. Helens on 18 May 1980 produced several catastrophic flowages into tributaries on the northeast volcano flank. The tributaries channeled the flows to Smith Creek valley, which lies within the area devastated by the surge but was unaffected by the great debris avalanche on the north flank. Stratigraphy shows that the pyroclastic surge preceded the lahars; there is no notable “wet” character to the surge deposits. Therefore the lahars must have originated as snowmelt, not as ejected water-saturated debris that segregated from the pyroclastic surge as has been inferred for other flanks of the volcano. In stratigraphic order the Smith Creek valley-floor materials comprise (1) a complex valley-bottom facies of the pyroclastic surge and a related pyroclastic flow, (2) an unusual hummocky diamict caused by complex mixing of lahars with the dry pyroclastic debris, and (3) deposits of secondary pyroclastic flows. These units are capped by silt containing accretionary lapilli, which began falling from a rapidly expanding mushroom-shaped cloud 20 minutes after the eruption's onset. The Smith Creek valley-bottom pyroclastic facies consists of (a) a weakly graded basal bed of fines-poor granular sand, the deposit of a low-concentration lithic pyroclastic surge, and (b) a bed of very poorly sorted pebble to cobble gravel inversely graded near its base, the deposit of a high-concentration lithic pyroclastic flow. The surge apparently segregated while crossing the steep headwater tributaries of Smith Creek; large fragments that settled from the turbulent surge formed a dense pyroclastic flow along the valley floor that lagged behind the front of the overland surge. The unusual hummocky diamict as thick as 15 m contains large lithic clasts supported by a tough, brown muddy sand matrix like that of lahar deposits upvalley. This unit contains irregular friable lenses and pods meters in diameter, blocks incorporated from the underlying dry and hot pyroclastic material that had been deposited only moments earlier. The hummocky unit is the deposit of a high-viscosity debris flow which formed when lahars mingled with the pyroclastic materials on Smith Creek valley floor. Overlying the debris flow are voluminous pyroclastic deposits of pebbly sand cut by fines-poor gas-escape pipes and containing charred wood. The deposits are thickest in topographic lows along margins of the hummocky diamict. Emplaced several minutes after the hot surge had passed, this is the deposit of numerous secondary pyroclastic flows derived from surge material deposited unstably on steep valley sides.  相似文献   

12.
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   

13.
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   

14.
Quilotoa volcano, an example of young dacitic volcanism in a lake-filled caldera, is found at the southwest end of the Ecuador's volcanic front. It has had a long series of powerful plinian eruptions of moderate to large size (VEI = 4–6), at repetitive intervals of roughly 10–15 thousand years. At least eight eruptive cycles (labeled Q-I to Q-VIII with increasing age) over the past 200 ka are recognized, often beginning with a phreatomagmatic onset and followed by a pumice-rich lapilli fall, and then a sequence of pumice, crystal, and lithic-rich deposits belonging to surges and ash flows. These unwelded pyroclastic flows left veneers on hillsides as well as very thick accumulations in the surrounding valleys, the farthest ash flow having traveled about 17 km down the Toachi valley. The bulk volumes of the youngest flow deposits are on the order of 5 km3, but that of Q-I's 800 yr BP ash-fall unit is about 18 km3. In the last two eruption cycles water has had a more important role.  相似文献   

15.
We use a well-monitored eruption of Tungurahua volcano to test the validity of the frictional behaviour, also called Mohr–Coulomb, which is generally used in geophysical flow modelling. We show that the frictional law is not appropriate for the simulation of pyroclastic flows at Tungurahua. With this law, the longitudinal shape of the simulated flows is a thin wedge of material progressively passing, over several hundreds of metres, from an unrealistic thickness at the front (<<1 mm) to some tens of centimetres. Simulated deposits form piles which accumulate at the foot of the volcano and are more similar to sand piles than natural pyroclastic deposits. Finally, flows simulated with a frictional rheology are not channelised by the drainage system, but affect all the flanks of the volcano. In addition, their velocity can exceed 150 m s−1, allowing pyroclastic flows to cross interfluves at bends in the valley, affecting areas that would not have been affected in reality and leaving clear downstream areas that would be covered in reality. Instead, a simple empirical law, a constant retarding stress (i.e. a yield strength), involving only one free parameter, appears to be much better adapted for modelling pyroclastic flows. A similar conclusion was drawn for the Socompa debris avalanche simulation (Kelfoun and Druitt, J Geophys Res 110:B12202, 2005).  相似文献   

16.
Of 1.1 million people living on the flanks of the active Merapi volcano, 440,000 are at relatively high risk in areas prone to pyroclastic flows, surges, and lahars. For the last two centuries, the activity of Merapi has alternated regularly between long periods of viscous lava dome extrusion, and brief explosive episodes at 8–15 year intervals, which generated dome-collapse pyroclastic flows and destroyed part of the pre-existing domes. Violent explosive episodes on an average recurrence of 26–54 years have generated pyroclastic flows, surges, tephra-falls, and subsequent lahars. The 61 reported eruptions since the mid-1500s killed about 7000 people. The current hazard-zone map of Merapi (Pardyanto et al., 1978) portrays three areas, termed ‘forbidden zone’, ‘first danger zone’ and ‘second danger zone’, based on successively declining hazards. Revision of the hazard map is desirable, because it lacks details necessary to outline hazard zones with accuracy, in particular the valleys likely to be swept by lahars, and excludes some areas likely to be devastated by pyroclastic gravity-currents such as the 22 November 1994 surge. In addition, risk maps should be developed to incorporate social, technical, and economic factors of vulnerability.Eruptive hazard assessment at Merapi is based on reconstructed eruptive history, on eruptive behavior and scenarios, and on existing models and preliminary numerical modeling. Firstly, the reconstructed eruptive activity, in particular for the past 7000 years and from historical accounts of eruptions, helps to define the extent and recurrence frequency of the most hazardous phenomena (Newhall et al., 2000; Camus et al., 2000). Pyroclastic flows traveled as far as 9–15 km from the source, pyroclastic surges swept the flanks as far as 9–20 km away from the vent, thick tephra fall buried temples in the vicinity of Yogyakarta 25 km to the south, and subsequent lahars spilled down the radial valleys as far as 30 km to the west and south. At least one large edifice collapse has occurred in the past 7000 years (Newhall et al., 2000; Camus et al., 2000). Secondly, four eruption scenarios are portrayed as hazardous zones on two maps and derived from the past eruptive behavior of Merapi and from the most affected areas in the past. Thirdly, simple numerical simulation, based on a Digital Elevation Model, a stereo-pair of SPOT satellite images, and one 2D-orthoimage helps to simulate pyroclastic and lahar flowage on the flanks and in radial valley channels, and to outline areas likely to be devastated.Three major threats are identified: (1) a collapse of the summit dome in the short-to mid-term, that can release large-volume pyroclastic flows and high-energy surges towards the south–southwest sector of the volcano; (2) an explosive eruption, much larger than any since 1930, may sweep all the flanks of Merapi at least once every century; (3) a potential collapse of the summit area, involving the fumarolic field of Gendol and part of the southern flank, which can contribute to moderate-scale debris avalanches and debris flows.  相似文献   

17.
High-resolution satellite radar observations of erupting volcanoes can yield valuable information on rapidly changing deposits and geomorphology. Using the TerraSAR-X (TSX) radar with a spatial resolution of about 2 m and a repeat interval of 11 days, we show how a variety of techniques were used to record some of the eruptive history of the Soufriere Hills Volcano, Montserrat between July 2008 and February 2010. After a 15-month pause in lava dome growth, a vulcanian explosion occurred on 28 July 2008 from a vent that was hidden by dense cloud. We were able to show the civil authorities using TSX difference images of surface roughness change that this explosion had not disrupted the dome sufficiently to warrant continuation of a previous, precautionary evacuation. Change difference images also proved to be valuable in mapping new pyroclastic flow deposits: the valley-occupying block-and-ash component tended to increase backscatter and the marginal surge deposits to reduce it, with the pattern reversing after the event due to erosion and deposition. By comparing east- and west-looking images acquired 12 h apart, the deposition of some individual pyroclastic flows can be inferred from change differences. Some of the narrow upper sections of valleys draining the volcano received many tens of metres of rockfall and pyroclastic flow deposits over periods of a few weeks. By measuring the changing radar shadows cast by these valleys in TSX images the changing depth of infill by deposits could be estimated. In addition to using the amplitude data from the radar images we also used their phase information within the InSAR technique to calculate the topography during a period of no surface activity. This enabled areas of transient topography, crucial for directing future flows, to be captured.  相似文献   

18.
The geological evolution of Merapi volcano, Central Java, Indonesia   总被引：1，自引：0，他引：1  
Merapi is an almost persistently active basalt to basaltic andesite volcanic complex in Central Java (Indonesia) and often referred to as the type volcano for small-volume pyroclastic flows generated by gravitational lava dome failures (Merapi-type nuées ardentes). Stratigraphic field data, published and new radiocarbon ages in conjunction with a new set of 40K–40Ar and 40Ar–39Ar ages, and whole-rock geochemical data allow a reassessment of the geological and geochemical evolution of the volcanic complex. An adapted version of the published geological map of Merapi [(Wirakusumah et al. 1989), Peta Geologi Gunungapi Merapi, Jawa Tengah (Geologic map of Merapi volcano, Central Java), 1:50,000] is presented, in which eight main volcano stratigraphic units are distinguished, linked to three main evolutionary stages of the volcanic complex—Proto-Merapi, Old Merapi and New Merapi. Construction of the Merapi volcanic complex began after 170?ka. The two earliest (Proto-Merapi) volcanic edifices, Gunung Bibi (109?±?60?ka), a small basaltic andesite volcanic structure on Merapi’s north-east flank, and Gunung Turgo and Gunung Plawangan (138?±?3?ka; 135?±?3?ka), two basaltic hills in the southern sector of the volcano, predate the Merapi cone sensu stricto. Old Merapi started to grow at ~30?ka, building a stratovolcano of basaltic andesite lavas and intercalated pyroclastic rocks. This older Merapi edifice was destroyed by one or, possibly, several flank failures, the latest of which occurred after 4.8?±?1.5?ka and marks the end of the Old Merapi stage. The construction of the recent Merapi cone (New Merapi) began afterwards. Mostly basaltic andesite pyroclastic and epiclastic deposits of both Old and New Merapi (<11,792?±?90 14C years BP) cover the lower flanks of the edifice. A shift from medium-K to high-K character of the eruptive products occurred at ~1,900 14C years BP, with all younger products having high-K affinity. The radiocarbon record points towards an almost continuous activity of Merapi since this time, with periods of high eruption frequency interrupted by shorter intervals of apparently lower eruption rates, which is reflected in the geochemical composition of the eruptive products. The Holocene stratigraphic record reveals that fountain collapse pyroclastic flows are a common phenomenon at Merapi. The distribution and run-out distances of these flows have frequently exceeded those of the classic Merapi-type nuées ardentes of the recent activity. Widespread pumiceous fallout deposits testify the occurrence of moderate to large (subplinian) eruptions (VEI 3–4) during the mid to late Holocene. VEI 4 eruptions, as identified in the stratigraphic record, are an order of magnitude larger than any recorded historical eruption of Merapi, except for the 1872?AD and, possibly, the October–November 2010 events. Both types of eruptive and volcanic phenomena require careful consideration in long-term hazard assessment at Merapi.  相似文献   

19.
The Pucón eruption was the largest Holocene explosive outburst of Volcán Villarrica, Chile. It discharged >1.0 km3 of basaltic-andesite magma and >0.8 km3 of pre-existing rock, forming a thin scoria-fall deposit overlain by voluminous ignimbrite intercalated with pyroclastic surge beds. The deposits are up to 70 m thick and are preserved up to 21 km from the present-day summit, post-eruptive lahar deposits extending farther. Two ignimbrite units are distinguished: a lower one (P1) in which all accidental lithic clasts are of volcanic origin and an upper unit (P2) in which basement granitoids also occur, both as free clasts and as xenoliths in scoria. P2 accounts for ∼80% of the erupted products. Following the initial scoria fallout phase, P1 pyroclastic flows swept down the northern and western flanks of the volcano, magma fragmentation during this phase being confined to within the volcanic edifice. Following a pause of at least a couple of days sufficient for wood devolatilization, eruption recommenced, the fragmentation level dropped to within the granitoid basement, and the pyroclastic flows of P2 were erupted. The first P2 flow had a highly turbulent front, laid down ignimbrite with large-scale cross-stratification and regressive bedforms, and sheared the ground; flow then waned and became confined to the southeastern flank. Following emplacement of pyroclastic surge deposits all across the volcano, the eruption terminated with pyroclastic flows down the northern flank. Multiple lahars were generated prior to the onset of a new eruptive cycle. Charcoal samples yield a probable eruption age of 3,510 ± 60 14C years BP.  相似文献   

20.
Assessments of pyroclastic flow (PF) hazards are commonly based on mapping of PF and surge deposits and estimations of inundation limits, and/or computer models of varying degrees of sophistication. In volcanic crises a PF hazard map may be sorely needed, but limited time, exposures, or safety aspects may preclude fieldwork, and insufficient time or baseline data may be available for reliable dynamic simulations. We have developed a statistically constrained simulation model for block-and-ash type PFs to estimate potential areas of inundation by adapting methodology from Iverson et al. (Geol Soc America Bull 110:972–984, 1998) for lahars. The predictive equations for block-and-ash PFs are calibrated with data from several volcanoes and given by A = (0.05 to 0.1)V 2/3, B = (35 to 40)V 2/3, where A is cross-sectional area of inundation, B is planimetric area and V is deposit volume. The proportionality coefficients were obtained from regression analyses and comparison of simulations to mapped deposits. The method embeds the predictive equations in a GIS program coupled with DEM topography, using the LAHARZ program of Schilling (1998). Although the method is objective and reproducible, any PF hazard zone so computed should be considered as an approximate guide only, due to uncertainties on the coefficients applicable to individual PFs, the authenticity of DEM details, and the volume of future collapses. The statistical uncertainty of the predictive equations, which imply a factor of two or more in predicting A or B for a specified V, is superposed on the uncertainty of forecasting V for the next PF to descend a particular valley. Multiple inundation zones, produced by simulations using a selected range of volumes, partly accommodate these uncertainties. The resulting maps show graphically that PF inundation potentials are highest nearest volcano sources and along valley thalwegs, and diminish with distance from source and lateral distance from thalweg. The model does not explicitly consider dynamic behavior, which can be important. Ash-cloud surge impact limits must be extended beyond PF hazard zones and we provide several approaches to do this. The method has been used to supply PF and surge hazard maps in two crises: Merapi 2006; and Montserrat 2006–2007.  相似文献   

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