共查询到20条相似文献，搜索用时 31 毫秒
1.
Apoyo caldera, near Granada, Nicaragua, was formed by two phases of collapse following explosive eruptions of dacite pumice about 23,000 yr B.P. The caldera sits atop an older volcanic center consisting of lava flows, domes, and ignimbrite (ash-flow tuff). The earliest lavas erupted were compositionally homogeneous basalt flows, which were later intruded by small andesite and dacite flows along a well defined set of N—S-trending regional faults. Collapse of the roof of the magma chamber occurred along near-vertical ring faults during two widely separated eruptions. Field evidence suggests that the climactic eruption sequence opened with a powerful plinian blast, followed by eruption column collapse, which generated a complex sequence of pyroclastic surge and ignimbrite deposits and initiated caldera collapse. A period of quiescence was marked by the eruption of scoria-bearing tuff from the nearby Masaya caldera and the development of a soil horizon. Violent plinian eruptions then resumed from a vent located within the caldera. A second phase of caldera collapse followed, accompanied by the effusion of late-stage andesitic lavas, indicating the presence of an underlying zoned magma chamber. Detailed isopach and isopleth maps of the plinian deposits indicate moderate to great column heights and muzzle velocities compared to other eruptions of similar volume. Mapping of the Apoyo airfall and ignimbrite deposits gives a volume of 17.2 km3 within the 1-mm isopach. Crystal concentration studies show that the true erupted volume was 30.5 km3 (10.7 km3 Dense Rock Equivalent), approximately the volume necessary to fill the caldera. A vent area located in the northeast quadrant of the present caldera lake is deduced for all the silicic pyroclastic eruptions. This vent area is controlled by N—S-trending precaldera faults related to left-lateral motion along the adjacent volcanic segment break. Fractional crystallization of calc-alkaline basaltic magma was the primary differentiation process which led to the intermediate to silicic products erupted at Apoyo. Prior to caldera collapse, highly atypical tholeiitic magmas resembling low-K, high-Ca oceanic ridge basalts were erupted along tension faults peripheral to the magma chamber. The injection of tholeiitic magmas may have contributed to the paroxysmal caldera-forming eruptions. 相似文献
2.
The Tiribí Tuff covered much of the Valle Central of Costa Rica, currently the most densely populated area in the country (∼2.4 million inhabitants). Underlying the tuff, there is a related well-sorted pumice deposit, the Tibás Pumice Layer. Based on macroscopic characteristics of the rocks, we distinguish two main facies in the Tiribí Tuff in correlation to the differences in welding, devitrification, grain size, and abundance of pumice and lithic fragments. The Valle Central facies consists of an ignimbritic plateau of non-welded to welded deposits within the Valle Central basin and the Orotina facies is a gray to light-bluish gray, densely to partially welded rock, with yellowish and black pumice fragments cropping out mainly at the Grande de Tárcoles River Gorge and Orotina plain. This high-aspect ratio ignimbrite (1:920 or 1.1×10−3) covered an area of at least 820 km2 with a long runout of 80 km and a minimum volume outflow of 25 km3 (15 km3 DRE). Geochemically, the tuff shows a wide range of compositions from basaltic-andesites to rhyolites, but trachyandesites are predominant. Replicate new 40Ar/39Ar age determinations indicate that widespread exposures of this tuff represent a single ignimbrite that was erupted 322±2 ka. The inferred source is the Barva Caldera, as interpreted from isopach and isopleth maps, contours of the ignimbrite top and geochemical correlation (∼10 km in diameter). The Tiribí Tuff caldera-forming eruption is interpreted as having evolved from a plinian eruption, during which the widespread basal pumice fall was deposited, followed by fountaining pyroclastic flows. In the SW part of the Valle Central, the ignimbrite flowed into a narrow canyon, which might have acted as a pseudo-barrier, reflecting the flow back towards the source and thus thickening the deposits that were filling the Valle Central depression. The variable welding patterns are interpreted to be a result of the lithostatic load and the influence of the content and size of lithic fragments. 相似文献
3.
R.S.J. Sparks P.W. Francis R.D. Hamer R.J. Pankhurst L.O. O'Callaghan R.S. Thorpe R. Page 《Journal of Volcanology and Geothermal Research》1985,24(3-4)
The 35 × 20 km Cerro Galán resurgent caldera is the largest post-Miocene caldera so far identified in the Andes. The Cerro Galán complex developed on a late pre-Cambrian to late Palaeozoic basement of gneisses, amphibolites, mica schists and deformed phyllites and quartzites. The basement was uplifted in the early Miocene along large north-south reverse faults, producing a horst-and-graben topography. Volcanism began in the area prior to 15 Ma with the formation of several andesite to dacite composite volcanoes. The Cerro Galán complex developed along two prominent north-south regional faults about 20 km apart. Dacitic to rhyodacitic magma ascended along these faults and caused at least nine ignimbrite eruptions in the period 7-4 Ma (K-Ar determinations). These ignimbrites are named the Toconquis Ignimbrite Formation. They are characterised by the presence of basal plinian deposits, many individual flow units and proximal co-ignimbrite lag breccias. The ignimbrites also have moderate to high macroscopic pumice and lithic contents and moderate to low crystal contents. Compositionally banded pumice occurs near the top of some units. Many of the Toconquis eruptions occurred from vents along a north-south line on the western rim of the young caldera. However, two of the ignimbrites erupted from vents on the eastern margin. Lava extrusions occurred contemporaneously along these north-south lines. The total D.R.E. volume of Toconquis ignimbrite exceeds 500 km3.Following a 2-Ma dormant period a single major eruption of rhyodacitic magma formed the 1000-km3 Cerro Galán ignimbrite and the caldera. The ignimbrite (age 2.1 Ma on Rb-Sr determination) forms a 30–200-m-thick outflow sheet extending up to 100 km in all directions from the caldera rim. At least 1.4 km of welded intracaldera ignimbrite also accumulated. The ignimbrite is a pumice-poor, crystal-rich deposit which contains few lithic clasts. No basal plinian deposit has been identified and proximal lag breccias are absent. The composition of pumice clasts is a very uniform rhyodacite which has a higher SiO2 content but a lower K2O content than the Toconquis ignimbrites. Preliminary data indicate no evidence for compositional zonation in the magma chamber. The eruption is considered to have been caused by the catastrophic foundering of a cauldron block into the magma chamber.Post-caldera extrusions occurred shortly after eruption along both the northern extension of the eastern boundary fault and the western caldera margin. Resurgence also occurred, doming up the intracaldera ignimbrite and sedimentary fill to form the central mountain range. Resurgent doming was centred along the eastern fault and resulted in radial tilting of the ignimbrite and overlying lake sediments. 相似文献
4.
The three-day eruption at Novarupta in 1912 consisted of three discrete episodes. Episode I began with plinian dispersal of rhyolitic fallout (Layer A) and contemporaneous emplacement of rhyolitic ignimbrites and associated proximal veneers. The plinian column was sustained throughout most of the interval of ash flow generation, in spite of progressive increases in the proportions of dacitic and andesitic ejecta at the expense of rhyolite. Accordingly, plinian Layer B, which fell in unbroken continuity with purely rhyolitic Layer A, is zoned from >99% to 15% rhyolite and accumulated synchronously with emplacement of the correspondingly zoned ash flow sequence in Mageik Creek and the Valley of Ten Thousand Smokes (VTTS). Only the andesiterichest flow units that cap the flow sequence lack a widespread fallout equivalent, indicating that ignimbrite emplacement barely outlasted the plinian phase. On near-vent ridges, the passing ash flows left proximal ignimbrite veneers that share the compositional zonation of their valley-filling equivalents but exhibit evidence for turbulent deposition and recurrent scour. Episode II began after a break of a few hours and was dominated by plinian dispersal of dacitic Layers C and D, punctuated by minor proximal intraplinian flows and surges. After another break, dacitic Layers F and G resulted from a third plinian episode (III); intercalated with these proximally are thin intraplinian ignimbrites and several andesite-rich fall/flow layers. Both CD and FG were ejected from an inner vent <400 m wide (nested within that of Episode I), into which the rhyolitic lava dome (Novarupta) was still later extruded. Two finer-grained ash layers settled from composite regional dust clouds: Layer E, which accumulated during the D-F hiatus, includes a contribution from small contemporaneous ash flows; and Layer H settled after the main eruption was over. Both are distinct layers in and near the VTTS, but distally they merge with CD and FG, respectively; they are largely dacitic but include rhyolitic shards that erupted during Episode I and were kept aloft by atmospheric turbulence. Published models yield column heights of 23–26 km for A, 22–25 km for CD, and 17–23 km for FG; and peak mass eruption rates of 0.7–1x108, 0.6–2x108, and 0.2–0.4x108 kg s-1, respectively. Fallout volumes, adjusted to reflect calculated redistribution of rhyolitic glass shards, are 8.8 km3, 4.8 km3, and 3.4 km3 for Episodes I, II, and III. Microprobe analyses of glass show that as much as 0.4 km3 of rhyolitic glass shards from eruptive Episode I fell with CDE and 1.1 km3 with FGH. Most of the rhyolitic ash in the dacitic fallout layers fell far downwind (SE of the vent); near the rhyolite-dominated ignimbrite, however, nearly all of Layers E and H are dacitic, showing that the downwind rhyolitic ash is of co-plinian rather than co-ignimbrite origin. 相似文献
5.
The Campanian Ignimbrite (36000 years B.P.) was produced by the explosive eruption of at least 80 km3 DRE of trachytic ash and pumice which covered most of the southern Italian peninsula and the eastern Mediterranean region. The eruption has been related to the 12-x15-km-diameter caldera located in the Phlegraean Fields, west of Naples. Proximal deposits on the periphery of the Phlegraean Fields comprise the following pyroclastic sequence from base to top: densely welded ignimbrite and lithic-rich breccias (unit A); sintered ignimbrite, low-grade ignimbrite and lithic-rich breccia (unit B); lithic-rich breccia and spatter agglutinate (unit C); and low-grade ignimbrite (unit D). Stratigraphic and componentry data, as well as distribution of accidental lithic types and the composition of pumice clasts of different units, indicate that coarse, lithic-rich breccias were emplaced at different stages during the eruption. Lower breccias are associated with fines-rich ignimbrites and are interpreted as co-ignimbrite lag breccia deposits. The main breccia unit (C) does not grade into a fines-rich ignimbrite, and therefore is interpreted as formed from a distinct lithic-rich flow. Units A and B exhibit a similar pattern of accidental lithic types, indicating that they were erupted from the same area, probably in the E of the caldera. Units C and D display a distinct pattern of lithics indicating expulsion from vent(s) that cut different areas. We suggest that unit C was ejected from several vents during the main stage of caldera collapse. Field relationships between spatter agglutinate and the breccia support the possibility that these deposits were erupted contemporaneously from vents with different eruptive style. The breccia may have resulted from a combination of magmatic and hydrothermal explosive activity that accompanied extensive fracturing and subsidence of the magma-chamber roof. The spatter rags probably derived from sustained and vigorous pyroclastic fountains. We propose that the association lithic-rich breccia and spatter agglutinate records the occurrence of catastrophic piecemeal collapse. 相似文献
6.
B N Turbeville 《Bulletin of Volcanology》1992,55(1-2):110-118
The Latera caldera is a well-exposed volcano where more than 8 km3 of mafic silica-undersaturated potassic lavas, scoria and felsic ignimbrites were emplaced between 380 and 150 ka. Isotopic ages obtained by 40Ar/39Ar analysis of single sanidine crystals indicate at least four periods of explosive eruptions from the caldera. The initial period of caldera eruptions began at 232 ka with emplacement of trachytic pumice fallout and ignimbrite. They were closely followed by eruption of evolved phonolitic magma. After roughly 25 ky, several phonolitic ignimbrites were deposited, and they were followed by phreatomagmatic eruptions that produced trachytic ignimbrites and several smaller ash-flow units at 191 ka. Compositionally zoned magma then erupted from the northern caldera rim to produce widespread phonolitic tuffs, tephriphonolitic spatter, and scoria-bearing ignimbrites. After 40 ky of mafic surge deposit and scoria cone development around the caldera rim, a compositionally zoned pumice sequence was emplaced around a vent immediately northwest of the Latera caldera. This activity marks the end of large-scale explosive eruptions from the Latera volcano at 156 ka. 相似文献
7.
The series of eruptions of June 15, 1991 at Mt. Pinatubo, Philippines were observed hourly by satellite. A giant discshaped cloud covering an area of 60,000 km2 appeared in the satellite images at 14:40, Philippine time. The cloud expanded radially against wind of 20 m/s and spread to an area of more than 120,000 km2 within an hour. According to eyewitness accounts there was heavy fine-ash fall after 14:00, intermittent lapilli fall started at about 14:20, and heavy and continuous lapilli fall widely started at about 15:00. The occurrence of the giant cloud roughly corresponded to the initiation of the intermittent lapilli fall.The air-fall deposits of the major eruption are widely distributed, including upwind from the vent. They are composed of 3 units; a silt-size fine-ash layer (Layer B), a lapilli layer commonly including pumice grains of > 1 cm in diameter (Layer C), a lapilli bearing volcanic sand layer (Layer D). Judging from its wide distribution and depletion of coarse, grains, most of the fine ash of Layer B is not distal deposits of a small eruption, but is originated from a large co-ignimbrite cloud. It is suggested that the major eruption started with the generation of a pyroclastic flow, which was subsequently followed by a plinian eruption resulting in the formation of the giant cloud and the lapilli fall.The results of calculations on the dynamics of eruption cloud indicate that the dimension and dynamics of the giant eruption cloud is accounted for by a plinian eruption with a magma discharge rate of the order of 109 kg/s. 相似文献
8.
The Cana Creek Tuff is one of four rhyolitic ignimbrite members of the Late Carboniferous Currabubula Formation, a volcanogenic conglomeratic braidplain sequence exposed along the western margin of the New England Orogen in northeastern New South Wales. The source is not exposed but was probably located tens of kilometres to the west of existing outcrops. The medial to distal parts of the tuff average about 70 m in thickness, are widespread (minimum present area 1400 km2), and comprise a primary pyroclastic facies (ignimbrite, ash-fall tuff) and a redeposited volcaniclastic facies (sandstone, conglomerate). Both facies are composed of differing proportions of crystal fragments (quartz, plagioclase, K-feldspar), pumiceous clasts (pumice, shards, fine ash), and accidental lithics. The eruption responsible for this unit was explosive and of large magnitude (dense rock equivalent volume about 100 km3). That it was also phreatomagmatic in character is proposed on the basis of: the intimate association of primary and redeposited facies; the presence of accretionary lapilli both in ignimbrite and in ash-fall tuff; the fine grain size of juvenile pyroclasts; the low grade of the ignimbrite; and the close similarity in facies, composition and magnitude to the deposits from the 20,000y. B.P. phreatomagmatic eruption at Taupo, New Zealand (the Wairakei and parts of the Hinuera Formations). The eruption began and ended from a vent with excess water available, possibly submersed in a caldera lake, and generated volcaniclastic sheet floods and debris flows. The emplacement of the primary pyroclastic facies is correlated with an intervening stage when the water:magma mass ratio was lower. The deposits from a large-magnitude, phreatomagmatic eruption are predicted to show systematic lateral variations in facies. Primary pyroclastic facies predominate near the source although the preserved stratigraphy is an incomplete record because of widespread contemporaneous erosion. Volcaniclastic facies, redeposited from proximal sites by floods, dominate at medial and distal locations. In areas hundreds of kilometres from the source, the eruption is registered by thin layers of fine-grained airfall ash. 相似文献
9.
The Scafell caldera-lake volcaniclastic succession is exceptionally well exposed. At the eastern margin of the caldera, a
large andesitic explosive eruption (>5 km3) generated a high-mass-flux pyroclastic density current that flowed into the caldera lake for several hours and deposited
the extensive Pavey Ark ignimbrite. The ignimbrite comprises a thick (≤125 m), proximal, spatter- and scoria-rich breccia
that grades laterally and upwards into massive lapilli-tuff, which, in turn, is gradationally overlain by massive and normal-graded
tuff showing evidence of soft-state disruption. The subaqueous pyroclastic current carried juvenile clasts ranging from fine
ash to metre-scale blocks and from dense andesite through variably vesicular scoria to pumice (<103 kg m−3). Extreme ignimbrite lithofacies diversity resulted via particle segregation and selective deposition from the current. The
lacustrine proximal ignimbrite breccia mainly comprises clast- to matrix-supported blocks and lapilli of vesicular andesite,
but includes several layers rich in spatter (≤1.7 m diameter) that was emplaced in a ductile, hot state. In proximal locations,
rapid deposition of the large and dense clasts caused displacement of interstitial fluid with elutriation of low-density lapilli
and ash upwards, so that these particles were retained in the current and thus overpassed to medial and distal reaches. Medially,
the lithofacies architecture records partial blocking, channelling and reflection of the depletive current by substantial
basin-floor topography that included a lava dome and developing fault scarps. Diffuse layers reflect surging of the sustained
current, and the overall normal grading reflects gradually waning flow with, finally, a transition to suspension sedimentation
from an ash-choked water column. Fine to extremely fine tuff overlying the ignimbrite forms ∼25% of the whole and is the water-settled
equivalent of co-ignimbrite ash; its great thickness (≤55 m) formed because the suspended ash was trapped within an enclosed
basin and could not drift away. The ignimbrite architecture records widespread caldera subsidence during the eruption, involving
volcanotectonic faulting of the lake floor. The eruption was partly driven by explosive disruption of a groundwater-hydrothermal
system adjacent to the magma reservoir. 相似文献
10.
G. P. L. Walker 《Bulletin of Volcanology》1981,44(3):223-240
Plinian eruptions are amongst the most powerful of explosive volcanic events, and the extensive pumice deposits which they produce have an exceptionally wide dispersal because of the great eruptive plume height. Historical data on 12 plinian eruptions, and available quantitative data on the deposits of these and 37 other plinian eruptions are collated in this review to characterise further the plinian eruptive style and its products and to establish the known limits of their variation. The deposit volumes have been recomputed according to a standard procedure to provide a better basis for comparison, and they vary over 4 orders of magnitude to reach a maximum of about 100 km3. Almost all volcanic magma compositions apart from the most mafic are represented among the juvenile products; rhyolitic and dacitic deposits account for 80% of the total volume and basaltic ones less than 1%. Compositional zoning is very common. Plinian eruptions are of open vent type and produce deposits which tend to be homogeneous in grain size and constitution through their thickness. Considerable departures from homogeneity often however exist. Finer grained beds which often interrupt the continuity can be produced by a number of different mechanisms, the features of which are summarised. In a significant proportion of plinian deposits the finer beds are the deposits of intraplinian pyroclastic flows, or are related to such flows; pyroclastic flows such as may be attributable to column collapse thus do not form exclusively at the end of the plinian phase. The most recent work indicates that major phreatoplinian eruptions dominated by the copious inflow of water into the vent can produce deposits quite as widely dispersed and as voluminous as the biggest plinian eruptions, and it appears that the characteristics of the grain size populations of the two types tend to converge in the most powerful eruptions. 相似文献
11.
The October, 1902, eruption of Santa Maria Volcano, Guatemala, was one of the largest this century. It was preceded by a great earthquake on April 19 centered at the volcano, as well as numerous other major earthquakes. The 18–20 hour-long plinian eruption on October 25 produced a column at least 28 km high, reaching well into the stratosphere.The airfall pumice deposit covered more than 1.2 million km2 with a trace of ash and was only two meters thick at the vent. White dacitic pumice, dark gray scoriaceous basalt (with physically and chemically mixed intermediate pumice) and loose crystals of plagioclase, hornblende, hypersthene, biotite and magnetite make up the juvenile components of the deposit. Lithic fragments are of volcanic, plutonic, and metamorphic origin. The plinian deposit is a fine-grained, crystal-rich, single pumice fall unit and shows inverse grading. Mapping of the deposit gives a volume of 8.3 km3 within the one mm isopach. Crystal concentration studies show that the true volume erupted was at least 20 km3 (equivalent to 8.5 km3 of dense dacite) and that 90% of the ejecta was less than 2 mm in diameter.The plinian volume eruption rate averaged 1.2 × 105 m3s−1 and the average gas muzzle velocity of the column exceeded 270 ms−1. A total of 8.3 × 1018 J of energy were released by the eruption. A knowledge of both theoretically derived eruption parameters and contemporary information allows a detailed analysis of eruption mechanisms.This eruption was the major stratospheric aerosol injection in the 1902–1903 period. However, mid- to low- latitude northern hemisphere temperature deviation data for the years following the eruption show no significant temperature decrease. This may be explained by the sulfur-poor nature of dacite magmas, suggesting that volatile composition, rather than mass of volatiles, is the controlling parameter in climatic response to explosive eruptions. 相似文献
12.
The 4.3-m.y.-old medium-volume low-aspect-ratio Kizilkaya ignimbrite (50–100 km3 DRE) is one of the most widespread in the Cappadocian Volcanic Province covering about 8500–10,600 km2. The ignimbrite rests on a relatively fine-grained fan of Plinian pumice-fall deposit (Md of 1.0–1.80 mm in proximal locations). The eruptive center was located in the Misli plain northeast of Nigde, as deduced from thickness and grain-size variations of the fall deposit, flow direction indicators, welding patterns of the ignimbrite and the distribution of certain types ofxenoliths. The massive ignimbrite, generally about 15 m thick, covers a paleoplain throughout at least two thirds of its areal extent. It comprizes two flow units, identified by local pumice enrichment in the upper part of the lower unit. The ignimbrite is completely welded in many places. In other places, the lower flow unit is non-welded, particularly where the initial pumice-fall deposit was eroded, a fine-grained ground layer was deposited, and undulating or cross-laminations with antidunes were developed. The ground layer was derived from the ignimbrite ground-mass by loss of fines < 250–500 μm.Depositional characteristics indicate that the ignimbrite was emplaced as high-concentration flows with relatively low velocity and low heat loss during runout. Local development of a ground layer and internal bedding structures indicate local increased turbulence only within individual flow portions due to agitated fluidization from engulfed air. The degree of welding of the lower flow unit was controlled by this turbulence and is not related to thickness variations. 相似文献
13.
Rolf Schumacher Ulrike Mues-Schumacher Vedat Toprak 《Journal of Volcanology and Geothermal Research》2001,112(1-4)
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
14.
A. Di Muro M. Rosi E. Aguilera R. Barbieri G. Massa F. Mundula F. Pieri 《Journal of Volcanology and Geothermal Research》2008,174(4):307-324
Impact of large-scale explosive eruptions largely depends on the dynamics of transport, dispersal and deposition of ash by the convective system. In fully convective eruptive columns, ejected gases and particles emitted at the vent are vertically injected into the atmosphere by a narrow, buoyant column and then dispersed by atmosphere dynamics on a regional scale. In fully collapsing explosive eruptions, ash partly generated by secondary fragmentation is carried and dispersed by broad co-ignimbrite columns ascending above pyroclastic currents. In this paper, we investigate the transport and dispersion dynamics of ash and lapillis during a transitional plinian eruption in which both plinian and co-ignimbrite columns coexisted and interacted. The 800 BP eruptive cycle of Quilotoa volcano (Ecuador) produced a well-exposed tephra sequence. Our study shows that the sequence was accumulated by a variety of eruptive dynamics, ranging from early small phreatic explosions, to sustained magmatic plinian eruptions, to late phreatomagmatic explosive pulses. The eruptive style of the main 800 BP plinian eruption (U1) progressively evolved from an early fully convective column (plinian fall bed), to a late fully collapsing fountain (dense density currents) passing through an intermediate transitional eruptive phase (fall + syn-plinian dilute density currents). In the transitional U1 regime, height of the convective plinian column and volume and runout of the contemporaneous pyroclastic density currents generated by partial collapses were inversely correlated. The convective system originated from merging of co-plinian and co-surge contributions. This hybrid column dispersed a bimodal lapilli and ash-fall bed whose grain size markedly differs from that of classic fall deposits accumulated by fully convective plinian columns. Sedimentological analysis suggests that ash dispersion during transitional eruptions is affected by early aggregation of dry particle clusters. 相似文献
15.
The eruption of Toba (75,000 years BP), Sumatra, is the largest magnitude eruption documented from the Quaternary. The eruption produced the largest-known caldera the dimensions of which are 100 × 30 km and which is surrounded by rhyolitic ignimbrite covering an area of over 20,000 km2. The associated deep-sea tephra layer is found in piston cores in the north-eastern Indian Ocean covering a minimum area of 5 × 106 km2. We have investigated the thickness, grain size and texture of the Toba deep-sea tephra layer in order to demonstrate the use of deep-sea tephra layers as a volcanological tool. The exceptional magnitude and intensity of the Toba eruption is demonstrated by comparison of these data with the deep-sea tephra layers associated with the eruptions of the Campanian ignimbrite, Italy and of Santorini, Greece in Minoan time. The volume of ignimbrite and distal tephra fall deposit produced in the Toba eruption are comparable, a total of at least 1000 km3 of dense rhyolitic magma. In contrast the volume of dense magma produced by the Campanian and Santorini eruptions are approximately 70 and 13 km3 respectively. Thickness versus distance data on the three deep-sea tephra layers show that eruptions of smaller magnitude than Santorini are unlikely to be preserved as distinct tephra layers in most deep-sea cores. In proximal cores all three tephra layers show two distinct units: a lower coarse-grained unit and an upper fine-grained unit. We interpret the lower unit as a plinian deposit and the upper unit as a co-ignimbrite ash-fall deposit, indicating two major eruptive phases. The Toba tephra layer is coarser both in maximum and median grain size than the Campanian and Santorini layers at a given distance from source. These data are interpreted to indicate a very high cruption column, estimated to be at least 45 km. We have applied a method for estimating the duration of the Toba eruption from the style of graded-bedding in deep-sea tephra layers. Studies of two cores yield estimates of 9 and 14 days. The eruption column height and duration estimates both indicate an average volume discharge rate of approximately 106 m3/sec. The Toba eruption therefore was not only of exceptional magnitude, but also of exceptional intensity. 相似文献
16.
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
17.
Nancy K. Adams Shanaka L. de Silva Stephen Self Guido Salas Steven Schubring Jason L. Permenter Kendra Arbesman 《Bulletin of Volcanology》2001,62(8):493-518
Volcán Huaynaputina is a group of four vents located at 16°36'S, 70°51'W in southern Peru that produced one of the largest eruptions of historical times when ~11 km3 of magma was erupted during the period 19 February to 6 March 1600. The main eruptive vents are located at 4200 m within an erosion-modified amphitheater of a significantly older stratovolcano. The eruption proceeded in three stages. Stage I was an ~20-h sustained plinian eruption on 19-20 February that produced an extensive dacite pumice fall deposit (magma volume ~2.6 km3). Throughout medial-distal and distal parts of the dispersal area, a fine-grained plinian ashfall unit overlies the pumice fall deposit. This very widespread ash (magma volume ~6.2 km3) has been recognized in Antarctic ice cores. A short period of quiescence allowed local erosion of the uppermost stage-I deposits and was followed by renewed but intermittent explosive activity between 22 and 26 February (stage II). This activity resulted in intercalated pyroclastic flow and pumice fall deposits (~1 km3). The flow deposits are valley confined, whereas associated co-ignimbrite ash fall is found overlying the plinian ash deposit. Following another period of quiescence, vulcanian-type explosions of stage III commenced on 28 February and produced crudely bedded ash, lapilli, and bombs of dense dacite (~1 km3). Activity ceased on 6 March. Compositions erupted are predominantly high-K dacites with a phenocryst assemblage of plagioclase>hornblende>biotite>Fe-Ti oxides-apatite. Major elements are broadly similar in all three stages, but there are a few important differences. Stage-I pumice has less evolved glass compositions (~73% SiO2), lower crystal contents (17-20%), lower density (1.0-1.3 g/cm3), and phase equilibria suggest higher temperature and volatile contents. Stage-II and stage-III juvenile clasts have more evolved glass (~76% SiO2) compositions, higher crystal contents (25-35%), higher densities (up to 2.2 g/cm3), and lower temperature and volatile contents. All juvenile clasts show mineralogical evidence for thermal disequilibrium. Inflections on a plot of log thickness vs area1/2 for the fall deposits suggest that the pumice fall and the plinian ash fall were dispersed under different conditions and may have been derived from different parts of the eruption column system. The ash appears to have been dispersed mainly from the uppermost parts of the umbrella cloud by upper-level winds, whereas the pumice fall may have been derived from the lower parts of the umbrella cloud and vertical part of the eruption column and transported by a lower-altitude wind field. Thickness half distances and clast half distances for the pumice fall deposit suggests a column neutral buoyancy height of 24-32 km and a total column height of 34-46 km. The estimated mass discharge rate for the ~20-h-long stage-I eruption is 2.4᎒8 kg/s and the volumetric discharge rate is ~3.6᎒5 m3/s. The pumice fall deposit has a dispersal index (Hildreth and Drake 1992) of 4.4, and its index of fragmentation is at least 89%, reflecting the dominant volume of fines produced. Of the 11 km3 total volume of dacite magma erupted in 1600, approximately 85% was evacuated during stage 1. The three main vents range in size from ~70 to ~400 m. Alignment of these vents and a late-stage dyke parallel to the NNW-SSE trend defined by older volcanics suggest that the eruption initiated along a fissure that developed along pre-existing weaknesses. During stage I this fissure evolved into a large flared vent, vent 2, with a diameter of approximately 400 m. This vent was active throughout stage II, at the end of which a dome was emplaced within it. During stage III this dome was eviscerated forming the youngest vent in the group, vent 3. A minor extra-amphitheater vent was produced during the final event of the eruptive sequence. Recharge may have induced magma to rise away from a deep zone of magma generation and storage. Subsequently, vesiculation in the rising magma batch, possibly enhanced by interaction with an ancient hydrothermal system, triggered and fueled the sustained Plinian eruption of stage I. A lower volatile content in the stage-II and stage-III magma led to transitional column behavior and pyroclastic flow generation in stage II. Continued magma uprise led to emplacement of a dome which was subsequently destroyed during stage III. No caldera collapse occurred because no shallow magma chamber developed beneath this volcano. 相似文献
18.
The young non-welded Taupo ignimbrite shows remarkable lateral variations which are documented by granulometric and component analyses, and studies of maximum clast size and density. The grain size spans practically the entire known ignimbrite field, the coarser proximal ignimbrite having a median diameter 100 times greater than the finest distal ignimbrite. The content and maximum size of lithic fragments decrease also by a factor of 100 between proximal and distal parts. The content of free crystals first rises to reach a peak, but thereafter decreases to attain a very low value in far-distal exposures. The pumice maximum size decreases by a factor of about 10, and the most conspicuously coarse pumice rocks occur in a girdle nearly halfway out from vent to distal limit. The pumice in each grain size class decreases in density to half of its near-source value in distal ignimbrite. The overall outward trend is towards an ignimbrite which consists wholly of fine vitric ash; some distal exposures closely approach this condition.These variations are accounted for by a combination of processes operating in the moving ash flow. One is a continuous fragmentation of pumice leading to a rounding of the clasts, a progressive decrease in maximum size, the generation of much vitric dust, and the liberation of crystals. Another is a continuous sedimentation of heavy constituents (lithics and crystals), and an antipathetic rise of lighter coarse pumice towards the top of the flow. These processes operated in a moving flow whose upper layers travelled progressively farther from source; it is the topmost layers, strongly depleted in heavy constituents and enriched in light pumice, which have travelled the farthest and constitute the far-distal parts of the ignimbrite.A number of ignimbrite facies are characterized: the ignimbrite proper, with its proximal, distal, and pumice concentration zone facies; the deposits which form in the head and are then over-ridden by the body of the flow, including the fines-depleted ignimbrite variant and the heavies-enriched ground layer; and the ignimbrite veneer deposits which are left behind by the flow, which differ little from the ignimbrite except in their landscape-mantling form and the occurrence in them of lee-side coarse pumice lenses. 相似文献
19.
Susan L. Donoghue Alan S. Palmer Elizabeth McClelland Kate Hobson Robert B. Stewart Vincent E. Neall Jèrôme Lecointre Richard Price 《Bulletin of Volcanology》1999,61(4):223-240
The ca. 10,500 years B.P. eruptions at Ruapehu volcano deposited 0.2–0.3 km3 of tephra on the flanks of Ruapehu and the surrounding ring plain and generated the only known pyroclastic flows from this
volcano in the late Quaternary. Evidence of the eruptions is recorded in the stratigraphy of the volcanic ring plain and cone,
where pyroclastic flow deposits and several lithologically similar tephra deposits are identified. These deposits are grouped
into the newly defined Taurewa Formation and two members, Okupata Member (tephra-fall deposits) and Pourahu Member (pyroclastic
flow deposits). These eruptions identify a brief (<ca. 2000-year) but explosive period of volcanism at Ruapehu, which we define
as the Taurewa Eruptive Episode. This Episode represents the largest event within Ruapehu's ca. 22,500-year eruptive history
and also marks its culmination in activity ca. 10,000 years B.P. Following this episode, Ruapehu volcano entered a ca. 8000-year
period of relative quiescence. We propose that the episode began with the eruption of small-volume pyroclastic flows triggered
by a magma-mingling event. Flows from this event travelled down valleys east and west of Ruapehu onto the upper volcanic ring
plain, where their distal remnants are preserved. The genesis of these deposits is inferred from the remanent magnetisation
of pumice and lithic clasts. We envisage contemporaneous eruption and emplacement of distal pumice-rich tephras and proximal
welded tuff deposits. The potential for generation of pyroclastic flows during plinian eruptions at Ruapehu has not been previously
considered in hazard assessments at this volcano. Recognition of these events in the volcanological record is thus an important
new factor in future risk assessments and mitigation of volcanic risk at Tongariro Volcanic Centre.
Received: 5 July 1998 / Accepted: 12 March 1999 相似文献
20.
The 161 ka explosive eruption of the Kos Plateau Tuff (KPT) ejected a minimum of 60 km3 of rhyolitic magma, a minor amount of andesitic magma and incorporated more than 3 km3 of vent- and conduit-derived lithic debris. The source formed a caldera south of Kos, in the Aegean Sea, Greece. Textural and lithofacies characteristics of the KPT units are used to infer eruption dynamics and magma chamber processes, including the timing for the onset of catastrophic caldera collapse.The KPT consists of six units: (A) phreatoplinian fallout at the base; (B, C) stratified pyroclastic-density-current deposits; (D, E) volumetrically dominant, massive, non-welded ignimbrites; and (F) stratified pyroclastic-density-current deposits and ash fallout at the top. The ignimbrite units show increases in mass, grain size, abundance of vent- and conduit-derived lithic clasts, and runout of the pyroclastic density currents from source. Ignimbrite formation also corresponds to a change from phreatomagmatic to dry explosive activity. Textural and lithofacies characteristics of the KPT imply that the mass flux (i.e. eruption intensity) increased to the climax when major caldera collapse was initiated and the most voluminous, widespread, lithic-rich and coarsest ignimbrite was produced, followed by a waning period. During the eruption climax, deep basement lithic clasts were ejected, along with andesitic pumice and variably melted and vesiculated co-magmatic granitoid clasts from the magma chamber. Stratigraphic variations in pumice vesicularity and crystal content, provide evidence for variations in the distribution of crystal components and a subsidiary andesitic magma within the KPT magma chamber. The eruption climax culminated in tapping more coarsely crystal-rich magma. Increases in mass flux during the waxing phase is consistent with theoretical models for moderate-volume explosive eruptions that lead to caldera collapse. 相似文献