共查询到20条相似文献，搜索用时 31 毫秒
1.
Volcan Pico de Orizaba, which marks the eastern end of the Trans-Mexican Volcanic Belt, is one of the largest andesitic composite volcanoes in America. It is located above a series of crustal distensive faults making the boundary of the Coast Plains of the Gulf of Mexico from theAltiplano. For this reason, the volcano shows an asymmetry: from the west, its elevation is about 3,000 m whereas on the eastern side it reaches 4,000 to 4,500 m from its base. The Pico de Orizaba is composed of a primitive stratovolcano raised by a recent summit cone. It has been built by three very distinct volcanic and magmatic phases.
- The first one, probably discontinuous effusive activity, lasted more than one million years. It is mainly composed of two pyroxenes-andesites with scarce associated basaltic and dacitic lava-flows. Amphibole is an accessory mineral in most differentiated lavas. On the eastern flank, numerous massive and autobrecciated lava-flows pass outward into thick conglomeratic formations. This effusive phase has built a primitive central volcano and a parasitic cone: the Sierra Negra.
- The second phase is of short duration — about 100,000 years or less — in comparison with the first period. It seems that this period began with the formation of a caldera followed by the extrusion of amphibole dacite domes and the overflow of viscous silica-rich (andesite to dacite) lava flows on the northern flank. An intense explosive activity develops:pelean nuées ardentes are associated with extrusion of the domes; numerous plinian eruptions leading to widespread dacitic pumiceous air-falls are produced by both the central and the adventive volcanoes. This sequence of events is interpreted as the progressive emptying of a superficial chamber containing differenciated magma. A rhyolite flow erupted during this phase.
- The age of the recent phase is better defined. It started 13,000 years B.P. with the eruption of a dacitic ash-flow containing pumice and scoria-bombs. This was such an intense event that products were found 30 km S.E. of the summit, erasing the top of the former volcano and creating a large crater (4–5 km wide). The present cone, of 1,400–1,500 m elevation, grew in this crater. During a period of 7,000 to 8,000 years, the new stratovolcano experienced various important pyroclastic eruptions with a cycle of the order of 1,000 to 1,500 years. The pyroclastic flows (ash, pumice, and bombs) associated with air-fall deposits are of Saint-Vincent type. They present an heterogeneous dacitic and andesitic magma. The dacitic component is similar to previous differenciated materials. On the other hand, the andesitic magma appears somewhat similar to lava-flows from morphologically young cones erupted outside the central vent system. This eruptive cycle can be interpreted as the result of reoccurring injections of deep basic magma within the crustal chamber. For the last 5,000 years the activity of the modern Pico de Orizaba has again been essentially effusive (andesites) with periodic plinian eruptions.
2.
3.
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
4.
At Cotopaxi volcano, Ecuador, rhyolitic and andesitic bimodal magmatism has occurred periodically during the past 0.5 Ma.
The sequential eruption of rhyolitic (70–75% SiO2) and andesitic (56–62% SiO2) magmas from the same volcanic vent over short time spans and without significant intermingling is characteristic of Cotopaxi’s
Holocene behavior. This study documents the eruptive history of Cotopaxi volcano, presenting its stratigraphy and geologic
field relations, along with the relevant mineralogical and chemical nature of the eruptive products, in order to determine
the temporal and spatial relations of this bimodal alternation. Cotopaxi’s history begins with the Barrancas rhyolite series,
dominated by pumiceous ash flows and regional ash falls between 0.4 and 0.5 Ma, which was followed by occasional andesitic
activity, the most important being the ample andesitic lava flows (∼4.1 km3) that descended the N and NW sides of the edifice. Following a ∼400 ka long repose without silicic activity, Cotopaxi began
a new eruptive phase about 13 ka ago that consisted of seven rhyolitic episodes belonging to the Holocene F and Colorado Canyon
series; the onset of each episode occurred at intervals of 300–3,600 years and each produced ash flows and regional tephra
falls with DRE volumes of 0.2–3.6 km3. Andesitic tephras and lavas are interbedded in the rhyolite sequence. The Colorado Canyon episode (4,500 years BP) also
witnessed dome and sector collapses on Cotopaxi’s NE flank which, with associated ash flows, generated one of the largest
cohesive debris flows on record, the Chillos Valley lahar. A thin pumice lapilli fall represents the final rhyolitic outburst
which occurred at 2,100 years BP. The pumices of these Holocene rhyolitic eruptions are chemically similar to those of older
rhyolites of the Barrancas series, with the exception of the initial eruptive products of the Colorado Canyon series whose
chemistry is similar to that of the 211 ka ignimbrite of neighboring Chalupas volcano. Since the Colorado Canyon episode,
andesitic magmatism has dominated Cotopaxi’s last 4,400 years, characterized by scoria bomb and lithic-rich pyroclastic flows,
infrequent lava flows that reached the base of the cone, andesitic lapilli and ash falls that were carried chiefly to the
W, and large debris flows. Andesitic magma emission rates are estimated at 1.65 km3 (DRE)/ka for the period from 4,200 to 2,100 years BP and 1.85 km3 (DRE)/ka for the past 2,100 years, resulting in the present large stratocone. 相似文献
5.
Recent studies on Teide–Pico Viejo (TPV) complex have revealed that explosive activity of phonolitic and basaltic magmas, including plinian and subplinian eruptions, and the generation of a wide range of pyroclastic density currents (PDCs) have also been significant. We perform a statistical analysis of the time series of past eruptions and the spatial extent of their erupted products, including lava flows, fallout and PDCs. We use an extreme value theory statistical method to calculate eruption recurrence. The analysis of past activity and extent of some well-identified deposits is used to calculate the eruption recurrence probabilities of various sizes and for different time periods. With this information, we compute several significant scenarios using the GIS-based VORIS 2 software (Felpeto et al., J Volcanol Geotherm Res 166:106–116, 2007) in order to evaluate the potential extent of the main eruption hazards that could be expected from TPV. The simulated hazard scenarios show that the southern flank of Tenerife is protected by Las Cañadas caldera wall against lava flows and pyroclastic density currents, but not against ash fallout. The Icod Valley, and to a minor extent also the La Orotava valley, is directly exposed to most of TPV hazards, in particular to the gravity driven flows. This study represents a step forward in the evaluation of volcanic hazard at TPV with regard to previous studies, and the results obtained should be useful for intermediate and long-term land-use and emergency planning. 相似文献
6.
El Chichonal Volcano, within the Sierra of Chiapas (Mexico), became active on March 28, 1982. The summit dome was blasted away, leaving a 1000 m wide and 290 m deep crater in its place. Plinian eruption of March 28–29 produced air fall deposits. More vigorous activity on April 3–4 produced air falls, surges and pyroclastic flows. Villages within 6 km radius were entirely destroyed. The explosive eruptions of Chichonal produced large amounts of K-rich andesitic ash and pumice. The juvenile magma shows little compositional variability. Much of eruptive products is highly porphyritic pumice containing andesine, hornblende, clinopyroxene, biotite and anhydrite phenocrysts. 相似文献
7.
Claude Robin Philippe Mossand Guy Camus Jean-Marie Cantagrel Alain Gourgaud Pierre M. Vincent 《Journal of Volcanology and Geothermal Research》1987,31(1-2)
The evolution of the Colima volcanic complex can be divided into successive periods characterized by different dynamic and magmatic processes: emission of andesitic to dacitic lava flows, acid-ash and pumice-flow deposits, fallback nuées ardentes leading to pyroclastic flows with heterogeneous magma, plinian air-fall deposits, scoriae cones of alkaline and calc-alkaline nature. Four caldera-forming events, resulting either from major ignimbrite outbursts or Mount St. Helens-type eruptions, separate the main stages of development of the complex from the building of an ancient shield volcano (25 × 30 km wide) up to two summit cones, Nevado and Fuego.The oldest caldera, C1 (7–8 km wide), related to the pouring out of dacitic ash flows, marks the transition between two periods of activity in the primitive edifice called Nevado I: the first one, which is at least 0.6 m.y. old, was mainly andesitic and effusive, whereas the second one was characterized by extrusion of domes and related pyroclastic products. A small summit caldera, C2 (3–3.5 km wide), ended the evolution of Nevado I.Two modern volcanoes then began to grow. The building of the Nevado II started about 200,000 y. ago. It settled into the C2 caldera and partially overflowed it. The other volcano, here called Paleofuego, was progressively built on the southern side of the former Nevado I. Some of its flows are 50,000 y. old, but the age of its first outbursts is not known. However, it is younger than Nevado II. These two modern volcanoes had similar evolutions. Each of them was affected by a huge Mount St. Helens-type (or Bezymianny-type) event, 10,000 y. ago for the Paleofuego, and hardly older for the Nevado II. The landslides were responsible for two horseshoe-shaped avalanche calderas, C3 (Nevado) and C4 (Paleofuego), each 4–5 km wide, opening towards the east and the south. In both cases, the activity following these events was highly explosive and produced thick air-fall deposits around the summit craters.The Nevado III, formed by thick andesitic flows, is located close to the southwestern rim of the C3 caldera. It was a small and short-lived cone. Volcan de Fuego, located at the center of the C4 caldera, is nearly 1500 m high. Its activity is characterized by an alternation of long stages of growth by flows and short destructive episodes related to violent outbursts producing pyroclastic flows with heterogeneous magma and plinian air falls.The evolution of the primitive volcano followed a similar pattern leading to formation of C1 and then C2. The analogy between the evolutions of the two modern volcanoes (Nevado II–III; Paleofuego-Fuego) is described. Their vicinity and their contemporaneous growth pose the problem of the existence of a single reservoir, or two independent magmatic chambers, after the evolution of a common structure represented by the primitive volcano. 相似文献
8.
Historical eruptions have produced lahars and floods by perturbing snow and ice at more than 40 volcanoes worldwide. Most of these volcanoes are located at latitudes higher than 35°; those at lower latitudes reach altitudes generally above 4000 m. Volcanic events can perturb mantles of snow and ice in at least five ways: (1) scouring and melting by flowing pyroclastic debris or blasts of hot gases and pyroclastic debris, (2) surficial melting by lava flows, (3) basal melting of glacial ice or snow by subglacial eruptions or geothermal activity, (4) ejection of water by eruptions through a crater lake, and (5) deposition of tephra fall. Historical records of volcanic eruptions at snow-clad volcanoes show the following: (1) Flowing pyroclastic debris (pyroclastic flows and surges) and blasts of hot gases and pyroclastic debris are the most common volcanic events that generate lahars and floods; (2) Surficial lava flows generally cannot melt snow and ice rapidly enough to form large lahars or floods; (3) Heating the base of a glacier or snowpack by subglacial eruptions or by geothermal activity can induce basal melting that may result in ponding of water and lead to sudden outpourings of water or sediment-rich debris flows; (4) Tephra falls usually alter ablation rates of snow and ice but generally produce little meltwater that results in the formation of lahars and floods; (5) Lahars and floods generated by flowing pyroclastic debris, blasts of hot gases and pyroclastic debris, or basal melting of snow and ice commonly have volumes that exceed 105 m3.The glowing lava (pyroclastic flow) which flowed with force over ravines and ridges...gathered in the basin quickly and then forced downwards. As a result, tremendously wide and deep pathways in the ice and snow were made and produced great streams of water (Wolf 1878). 相似文献
9.
A compilation of observations of volcanic eruptions since 1870 and ash stratigraphy shows that Katmai National Monument on the Alaska Peninsula has had a long history of volcanic activity. Six of the recently active vents lie in a gently curved arc, but two lie to the north of this arc and show no obvious structural relationship to it. Recent volcanic events have consisted of fumarolic activity, steaming from main vents, ash eruptions, extrusion of viscous lava flows, and pyroclastic eruptions. The observed activity shows no obvious correlation with a compilation of seismic events recorded teleseismically since 1912 and relocated by the authors using a digital computer. The eruption attributed to Mt. Katmai in 1912 has left many unanswered questions including the thickness of the ash flow tuff in the Valley of Ten Thousand Smokes. Seismic refraction results show that this tuff has a compressional velocity of about 0.6 km/sec and that considerable morainal debris may underlie it at the northern end of the Valley. 相似文献
10.
Pyroclastic deposits exposed in the caldera walls of Santorini Volcano (Greece), contain several prominent horizons of coarse-grained andesitic spatter and cauliform volcanic bombs. These deposits can be traced around most of the caldera wall. They thicken in depressions and are intimately associated with ignimbrite and co-ignimbrite lithic lag breccias. They are interpreted as a proximal facies of pyroclastic flow deposits. Evidence for a flow origin includes the presence of a fine-grained pumiceous matrix, flow deformation of ductile spatter clasts, exceedingly coarse grain sizes several kilometres from any plausible vent, imbrication of flattened spatter clasts, intimate interbedding with normal pyroclastic flow deposits and the presence of inversely graded basal layers. The deposits contain hydrothermally altered, rounded lithic ejecta including gabbro nodules. The andesitic ejecta and the fine matrix are typically moderately to poorly vesicular indicating that magmatic gas had a subordinate role in the eruptive process. The andesitic clasts contain abundant angular lithic inclusions and some clasts are themselves formed of pre-existing agglutinate. We propose that these eruptions occurred when external water gained access to the vents, causing large-scale explosions which formed pyroclastic flows rich in coarse, semifluid but poorly vesicular ejecta. We postulate that large volumes of coarse pyroclastic ejecta and degassed lava accumulated in a deep crater prior to being disrupted by these large explosions to form pyroclastic flows. 相似文献
11.
The 3-month long eruption of Asama volcano in 1783 produced andesitic pumice falls, pyroclastic flows, lava flows, and constructed a cone. It is divided into six episodes on the basis of waxing and waning inferred from records made during the eruption. Episodes 1 to 4 were intermittent Vulcanian or Plinian eruptions, which generated several pumice fall deposits. The frequency and intensity of the eruption increased dramatically in episode 5, which started on 2 August, and culminated in a final phase that began on the night of 4 August, lasting for 15 h. This climactic phase is further divided into two subphases. The first subphase is characterized by generation of a pumice fall, whereas the second one is characterized by abundant pyroclastic flows. Stratigraphic relationships suggest that rapid growth of a cone and the generation of lava flows occurred simultaneously with the generation of both pumice falls and pyroclastic flows. The volumes of the ejecta during the first and second subphases are 0.21 km3 (DRE) and 0.27 km3 (DRE), respectively. The proportions of the different eruptive products are lava: cone: pumice fall=84:11:5 in the first subphase and lava: cone: pyroclastic flow=42:2:56 in the second subphase. The lava flows in this eruption consist of three flow units (L1, L2, and L3) and they characteristically possess abundant broken phenocrysts, and show extensive "welding" texture. These features, as well as ghost pyroclastic textures on the surface, indicate that the lava was a fountain-fed clastogenic lava. A high discharge rate for the lava flow (up to 106 kg/s) may also suggest that the lava was initially explosively ejected from the conduit. The petrology of the juvenile materials indicates binary mixing of an andesitic magma and a crystal-rich dacitic magma. The mixing ratio changed with time; the dacitic component is dominant in the pyroclasts of the first subphase of the climactic phase, while the proportion of the andesitic component increases in the pyroclasts of the second subphase. The compositions of the lava flows vary from one flow unit to another; L1 and L3 have almost identical compositions to those of pyroclasts of the first and second subphases, respectively, while L2 has an intermediate composition, suggesting that the pyroclasts of the first and second subphases were the source of the lava flows, and were partly homogenized during flow. The complex features of this eruption can be explained by rapid deposition of coarse pyroclasts near the vent and the subsequent flowage of clastogenic lavas which were accompanied by a high eruption plume generating pumice falls and/or pyroclastic flows.Editorial responsibility: T. Druitt 相似文献
12.
Vyacheslav M. Zobin Imelda Plascencia Gabriel Reyes Carlos Navarro 《Journal of Volcanology and Geothermal Research》2009,179(1-2):157-167
The general characteristics of seismic signals produced by pyroclastic flows (generated by either the collapse of a lava dome or an eruptive column) and lahars at Volcán de Colima, México are discussed. The paper concentrates on the 2004–2006 activity associated with and following the extrusion of andesitic block-lava in October–November 2004. It is shown that the duration of the broad-band seismic records of pyroclastic flows lasts a few minutes while the duration of seismic records of lahars continues for tens of minutes or hours. The spectra of seismic records produced by pyroclastic flows are characterized by lower peak frequencies (around 3–4 Hz) than for lahars (around 6–8 Hz). This difference in the frequency content together with the difference in the duration of seismic signals allows early diagnostic of the events in real time. 相似文献
13.
James W. Head III Lionel Wilson 《Journal of Volcanology and Geothermal Research》2003,121(3-4):155-193
Submarine pyroclastic eruptions at depths greater than a few hundred meters are generally considered to be rare or absent because the pressure of the overlying water column is sufficient to suppress juvenile gas exsolution so that magmatic disruption and pyroclastic activity do not occur. Consideration of detailed models of the ascent and eruption of magma in a range of sea floor environments shows, however, that significant pyroclastic activity can occur even at depths in excess of 3000 m. In order to document and illustrate the full range of submarine eruption styles, we model several possible scenarios for the ascent and eruption of magma feeding submarine eruptions: (1) no gas exsolution; (2) gas exsolution but no magma disruption; (3) gas exsolution, magma disruption, and hawaiian-style fountaining; (4) volatile content builds up in the magma reservoir leading to hawaiian eruptions resulting from foam collapse; (5) magma volatile content insufficient to cause fragmentation normally but low rise speed results in strombolian activity; and (6) volatile content builds up in the top of a dike leading to vulcanian eruptions. We also examine the role of bulk-interaction steam explosivity and contact-surface steam explosivity as processes contributing to volcaniclastic formation in these environments. We concur with most earlier workers that for magma compositions typical of spreading centers and their vicinities, the most likely circumstance is the quiet effusion of magma with minor gas exsolution, and the production of somewhat vesicular pillow lavas or sheet flows, depending on effusion rate. The amounts by which magma would overshoot the vent in these types of eruptions would be insufficient to cause any magma disruption. The most likely mechanism of production of pyroclastic deposits in this environment is strombolian activity, due to the localized concentration of volatiles in magma that has a low rise rate; magmatic gas collects by bubble coalescence, and ascends in large isolated bubbles which disrupt the magma surface in the vent, producing localized blocks, bombs, and pyroclastic deposits. Another possible mode of occurrence of pyroclastic deposits results from vulcanian eruptions; these deposits, being characterized by the dominance of angular blocks of country rocks deposited in the vicinity of a crater, should be easily distinguishable from strombolian and hawaiian eruptions. However, we stress that a special case of the hawaiian eruption style is likely to occur in the submarine environment if magmatic gas buildup occurs in a magma reservoir by the upward drift of gas bubbles. In this case, a layer of foam will build up at the top of the reservoir in a sufficient concentration to exceed the volatile content necessary for disruption and hawaiian-style activity; the deposits and landforms are predicted to be somewhat different from those of a typical primary magmatic volatile-induced hawaiian eruption. Specifically, typical pyroclast sizes might be smaller; fountain heights may exceed those expected for the purely magmatic hawaiian case; cooling of descending pyroclasts would be more efficient, leading to different types of proximal deposits; and runout distances for density flows would be greater, potentially leading to submarine pyroclastic deposits surrounding vents out to distances of tens of meters to a kilometer. In addition, flows emerging after the evacuation of the foam layer would tend to be very depleted in volatiles, and thus extremely poor in vesicles relative to typical flows associated with hawaiian-style eruptions in the primary magmatic gas case. We examine several cases of reported submarine volcaniclastic deposits found at depths as great as 3000 m and conclude that submarine hawaiian and strombolian eruptions are much more common than previously suspected at mid-ocean ridges. Furthermore, the latter stages of development of volcanic edifices (seamounts) formed in submarine environments are excellent candidates for a wide range of submarine pyroclastic activity due not just to the effects of decreasing water depth, but also to: (1) the presence of a summit magma reservoir, which favors the buildup of magmatic foams (enhancing hawaiian-style activity) and episodic dike emplacement (which favors strombolian-style eruptions); and (2) the common occurrence of alkalic basalts, the CO2 contents of which favor submarine explosive eruptions at depths greater than tholeiitic basalts. These models and predictions can be tested with future sampling and analysis programs and we provide a checklist of key observations to help distinguish among the eruption styles. 相似文献
14.
Stratigraphic studies on the active and potentially active volcanoes of the Lesser Antilles have revealed two main types of andesitic pyroclastic deposit. One with dense clasts in a poorly vesicular ash represents nuée ardente eruptions of Pelean type and the other group of vesicular pumice and ash represent both Plinian airfall and ash-pumice flow eruptions. The pumiceous deposits can be divided into airfall lapilli, airfall ash, crystal-pumice surge, ashpumice flow and ash hurricane types. No pumice eruptions have been witnessed in the Lesser Antilles during the period of written history although the stratigraphy of archaeological sites shows they occurred in pre-Columbian times. Detailed stratigraphic studies of Mt. Pelée, Martinique, and the Quill, St. Eustatius, show that, throughout their history, pumice eruptions have alternated with nuée ardente eruptions with approximately equal frequency. The widespread occurrence of pumiceous deposits on many of the West Indian volcanoes and the frequent alternations in the stratigraphic sections suggest the high probability that they will be witnessed in the future. On Martinique, some on the late prehistoric pumiceous pyroclastic flow deposits (the ash hurricanes) have been traced 20 km from the central vent to the out-skirts of Fort de France, indicating that they are the major hazard in the Lesser Antilles. Measured stratigraphic sections show that the Pelean type nuée ardente deposits are separated from the pumiceous pyroclastic deposits by others of intermediate vesicularity and appearance. The presence of such deposits of intermediate vesicularity could provide a future warning of impending change in pyroclastic style. As no such deposits formed on Mt. Pelée this century the present «safer» episode of nuée ardente (Pelean type) activity is expected to continue. 相似文献
15.
Fuji volcano is the largest active volcano in Japan, and consists of Ko-Fuji and Shin-Fuji volcanoes. Although basaltic in composition, small-volume pyroclastic flows have been repeatedly generated during the Younger stage of Shin-Fuji volcano. Deposits of those pyroclastic flows have been identified along multiple drainage valleys on the western flanks between 1,300 and 2,000 m a.s.l., and have been stratigraphically divided into the Shin-Fuji Younger pyroclastic flows (SYP) 1 to 4. Downstream debris flow deposits are found which contain abundant material derived from the pyroclastic flow deposits. The new14C ages for SYP1 to SYP4 are 3.2, 3.0, 2.9, and 2.5 ka, respectively, and correspond to a period where explosive summit eruptions generated many scoria fall deposits mostly toward the east. The SYP1 to SYP4 deposits consist of two facies: the massive facies is about 2 m thick and contains basaltic bombs of less than 50 cm in size, scoria lapilli, and fresh lithic basalt fragments supported in an ash matrix; the surge facies is represented by beds 1 to 15 cm thick, consisting mainly of ash with minor amount of fine lapilli. The bombs and scoria are 15 to 30% in volume within the massive facies. The ashes within the SYP deposits consist largely of comminuted basalt lithics and crystals that are derived from the Middle-stage lava flows exposed at the western flanks. SYP1 to SYP4 were only dispersed down the western flanks. The reason for this one-sided distribution is the asymmetric topography of the edifice; the western slopes of the volcano are the steepest (over 34 degrees). Most pyroclastic materials cannot rest stably on the slopes steeper than 33 degrees. Therefore, ejecta from the explosive summit eruptions that fell on the steep slopes tumbled down the slopes and were remobilized as high-temperature granular flows. These flows consisted of large pyroclastics and moved as granular avalanches along the valley bottom. Furthermore, the avalanching flows increased in volume by abrasion from the edifice and generated abundant ashes by the collision of clasts. The large amount of the fine material was presumably available within the transport system as the basal avalanches propagated below the angle of repose. Taking the typical kinetic friction coefficient of small pyroclastic flows, such flows could descend the western flanks where scattered houses are below 1,000 m a.s.l. A similar type of pyroclastic flow could result if explosive summit eruptions occur in the future.Editorial responsibility: R Cioni 相似文献
16.
George P. L. Walker 《Journal of Volcanology and Geothermal Research》1981,11(1):81-92
The volcanic eruptions which generate the greatest quantities of fine ash and dust are those of ignimbrite-forming, plinian, vulcanian and phreatomagmatic types; these are also the eruptions which produce the widest dispersal of this material, attributed to the superior height attained by their eruptive columns. However, much of the fine ash and dust may be rapidly flushed out of the eruptive plume by water, particularly in phreatomagmatic eruptions. Recent studies made on the dispersal and grain-size of pyroclastic deposits produced by examples of plinian and phreatomagmatic types, have yielded estimates of the quantities of material generated in each grain-size class, besides the extent of their dispersal. Not all of the fine volcanic particles are produced by fragmentation at the eruptive vent; in ignimbrite eruptions, there is good evidence for their large-scale generation in and loss from the moving ash flows. 相似文献
17.
Matthieu Kervyn Gerald G. J. Ernst Jörg Keller R. Greg Vaughan Jurgis Klaudius Evelyne Pradal Frederic Belton Hannes B. Mattsson Evelyne Mbede Patric Jacobs 《Bulletin of Volcanology》2010,72(8):913-931
On September 4, 2007, after 25 years of effusive natrocarbonatite eruptions, the eruptive activity of Oldoinyo Lengai (OL),
N Tanzania, changed abruptly to episodic explosive eruptions. This transition was preceded by a voluminous lava eruption in
March 2006, a year of quiescence, resumption of natrocarbonatite eruptions in June 2007, and a volcano-tectonic earthquake
swarm in July 2007. Despite the lack of ground-based monitoring, the evolution in OL eruption dynamics is documented based
on the available field observations, ASTER and MODIS satellite images, and almost-daily photos provided by local pilots. Satellite
data enabled identification of a phase of voluminous lava effusion in the 2 weeks prior to the onset of explosive eruptions.
After the onset, the activity varied from 100 m high ash jets to 2–15 km high violent, steady or unsteady, eruption columns
dispersing ash to 100 km distance. The explosive eruptions built up a ∼400 m wide, ∼75 m high intra-crater pyroclastic cone.
Time series data for eruption column height show distinct peaks at the end of September 2007 and February 2008, the latter
being associated with the first pyroclastic flows to be documented at OL. Chemical analyses of the erupted products, presented
in a companion paper (Keller et al. 2010), show that the 2007–2008 explosive eruptions are associated with an undersaturated carbonated silicate melt. This new phase
of explosive eruptions provides constraints on the factors causing the transition from natrocarbonatite effusive eruptions
to explosive eruptions of carbonated nephelinite magma, observed repetitively in the last 100 years at OL. 相似文献
18.
Fine ash content of explosive eruptions 总被引：1，自引：0，他引：1
In explosive eruptions, the mass proportion of ash that is aerodynamically fine enough to cause problems with jet aircraft or human lungs (< 30 to 60 μm in diameter) is in the range of a few percent to more than 50%. The proportions are higher for silicic explosive eruptions, probably because vesicle size in the pre-eruptive magma is smaller than those in mafic magmas. There is good evidence that pyroclastic flows produce high proportions of fine ash by communition and it is likely that this process also occurs inside volcanic conduits and would be most efficient when the magma fragmentation surface is well below the summit crater. Reconstructed total grain size distributions for several recent explosive eruptions indicate that basaltic eruptions have small proportions of very fine ash (~ 1 to 4%) while tephra generated during silicic eruptions contains large proportions (30 to > 50%). 相似文献
19.
Stratigraphic mapping in the lower 3km of the Vazcún Valley on the NE flank of Volcán Tungurahua (Ecuador) provides insight into the effects of long-term geomorphic changes on pyroclastic flow behavior. Exposures of deposits in the Vazcún Valley record activity over the last 2000years, during which time significant changes in the geomorphology of the valley have occurred. Two sets of terraces are present in the lower 2–3km of the valley, the older of which grades into a small debris fan at the mouth of the valley. Each terrace formed during a period of frequent activity that was separated by a long period of quiescence during which the Río Vazcún eroded a channel as deep as 40m reaching the previous base level. The pyroclastic flows from historical eruptions appear to have been largely contained within the channel that is cut through the higher terraces and debris fan. Their surface forms the lower terrace located upstream from the head of the debris fan. Thin pyroclastic deposits exposed within the city of Baños are mostly related to ash cloud surges that detached from the main flows as they slowed down within the channel. The lower reach of the present channel of the Rio Vazcún is very sinuous and deeply incised into the two sets of terraces. The winding channel would severely impede the mobility of future pyroclastic flows resulting in the deposition of thick deposits. Detachment of an overriding ash cloud surge could also occur in this region. Such a surge could be more likely to surmount the channel banks and travel over the surface of the terraces and debris for up to several kilometers from the channel. 相似文献
20.
I. A. Nairn C. O. Mckee B. Talai C. P. Wood 《Journal of Volcanology and Geothermal Research》1995,69(3-4)
Rabaul Caldera is the most recently active (1937–1943) of four adjoining volcanic centres aligned north-south through the northern extremity of eastern New Britain. Geological mapping after the 1983–1985 Rabaul seismic and deformation crisis has partially revealed a long and complex eruption history dominated by numerous explosive eruptions, the largest accompanied by caldera collapse. The oldest exposed eruptives are the basaltic pre-caldera cone Tovanumbatir Lavas K/Ar dated at 0.5 Ma. The dacitic Rabaul Quarry Lavas exposed in the caldera wall and K/Ar dated at 0.19 Ma, are overlain by a sequence of dacitic and andesitic pyroclastic flow and fall deposits. Uplifted coral reef limestones, interbedded within the pyroclastic sequence on the northeast coast, suggest that explosive eruptions in the Rabaul area had commenced prior to the 0.125 Ma last interglacial high sea level stand. The pyroclastic sequence includes the large Boroi Ignimbrites and Malaguna Pyroclastics both 40Ar/39Ar dated at about 0.1 Ma, and the Barge Tunnel Ignimbrite 40Ar/39Ar dated at around 0.04 Ma. Few reliable ages exist for the many younger eruptives. These include Holocene ignimbrites of the latest caldera-forming eruptions—the Raluan Pyroclastics variously dated (14C) at either about 3500 or 7000 yr B.P., and the ca. 1400 yr B.P. Rabaul Pyroclastics. At least eight intracaldera eruptions have occurred since the 1400 yr B.P. collapse, building small pyroclastic and lava cones within the caldera.A major erosional episode is evident as a widespread unconformity in the upper pyroclastic stratigraphy at Rabaul. Lacking relevant radiometric ages, this episode is assumed to have occurred during last glaciation low sea levels and is here arbitarily dated at ca. ?20 ka. At least five, possibly nine, significant ignimbrite eruptions have occurred at Rabaul during the last ?20 ka. The new eruptive history differs considerably from that previously published, which considered ignimbrite eruption and caldera collapse to have first occurred at 3500 yr B.P.Rabaul volcanism has been dominated by two main types: (a) basaltic and basaltic andesite cone building eruptions; and (b) dacitic, and rarely andesitic or rhyolitic, plinian/ignimbrite eruptions of both high- and low-aspect ratio types. The 1400 yr B.P. Rabaul Ignimbrite is a type example of a low-aspect ratio, high-energy, and potentially very damaging eruption. Fine vitric ash deposits, common in the Rabaul pyroclastic sequence, demonstrate the frequent modification of eruptions by external water probably related to early caldera lakes or bays. Interbedding of these fine ashes with plinian pumice lapilli beds suggests that many early eruptions occurred from multiple vents, located in both wet and dry areas. 相似文献