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1.
The 1886 eruption of Tarawera, New Zealand, was unusual for a Plinian eruption because it involved entirely basaltic magma, originated in a 17-km-long fissure, and produced extremely overthickened proximal deposits with a complex geometry. This study focuses on an 8-km-long segment cutting across Mount Tarawera where over 50 point-source vents were active during the 5.5-h eruption. A detailed characterization of the proximal deposits is developed and used to interpret the range of styles and intensities of the vents, including changes with time. We identify the four vents that contributed most heavily to the Plinian fall and evaluate the extent to which current volcanic plume models are compatible with the depositional patterns at Tarawera. Three proximal units are mapped that have phreatomagmatic, magmatic, and phreatomagmatic characteristics, respectively. Within the magmatic proximal unit, beds of like character are grouped into packages and delineated on scaled cross sections. Package dispersal is quantified by measuring the linear thickness half-distance (t1/2) in the planes of the fissure walls. Most packages have localized dispersals (low t1/2), indicating that Strombolian-style activity dominated most vents. The more widely dispersed packages (high t1/2) reflect contributions from additional transport regimes that were more vigorous but still contributed considerable material to the proximal region. We conclude that the geometry of the observed proximal deposits requires three modes of fall transport: (1) fallout from the upper portions of the Plinian plumes produced principally by vents in four craters; (2) sedimentation from the margins of the lower portions of the Plinian plumes including the jets and possibly the lower convective regions; and (3) ejection by weak Strombolian-style explosions from vents that did not contribute significant volumes of particles to the high plume. We suggest that the curvature of the velocity profile across the jet region of each plume (1–4 km height) was important, and that the lower velocity at the margins allowed proximal deposition of a large volume of material with a wide grain-size range.  相似文献   

2.
Rothenberg scoria cone Eifel formed by an alternation of three Strombolian and three phreatomagmatic eruptive phases. Eruptions took place from up to six vents on a 600 m-long fissure, building an early tuff ring and then two coalescing scoria cones. Strombolian volcanism dominated volumetrically, as the supply of external water was severely limited. Magma/water interaction only occurred during the opening stages of eruption at any vent, when discharge rates were low and the fragmentation surface was below the water table. The phreatomagmatic deposits consist of relatively well-sorted fall beds and only minor surge deposits. They contain juvenile clasts with a wide range of vesicularity and grain size, implying considerable heterogeneity in the assemblage of material ejected by the phreatomagmatic explosions. the transition from phreatomagmatic to Strombolian eruption at any vent was rapid and irreversible, and Strombolian volcanism persisted even when eruption rates are inferred to have waned at the close of each eruptive phase as, by then, the fragmentation surfaces were high in the growing cones and water was denied access to the magma. The Strombolian deposits are relatively homogenous, consisting of alternating coarser- and finer-grained, well-sorted fall beds erupted during periods of open-vent eruption and partial blockage of the vent respectively. The intervals of Strombolian eruption were always a delicate balance between discharge of freely degassing magma and processes such as ponding of degassed magma in the vent, collapse of the growing cones, and repeated recycling of clasts through the vent. Clear evidence of the instability of the Rothenberg cones is preserved in numerous unconformities within deposits of the inner walls of the cones. The close of Strombolian phases was probably marked by a decreasing supply of magma to the vents accompanied by ponding and stagnation of lava in the craters.  相似文献   

3.
A new model is presented which simulates the dispersal and deposition of material from a Hawaiian eruption column. The model treats the Hawaiian column as a coarse-grained Plinian column and uses a modified version of the Wilson and Walker [Wilson, L., Walker, G.P.L., 1987. Explosive volcanic eruptions: VI. Ejecta dispersal in Plinian eruptions: the control of eruption conditions and atmospheric properties. Geophys. J. R. Astron. Soc. 89, 657–679.] Plinian pyroclast dispersal model to simulate the fall out of material during a Hawaiian eruption. The model results are found to be in good agreement with independent estimates of various parameters made for the 1959 Kilauea Iki eruption of Kilauea volcano. The close agreement between the model results and these independent estimates shows that, dynamically, Hawaiian eruptions are indistinguishable from Plinian eruptions. The major differences in the styles and deposits of these two types of eruptions are accounted for by differences in the mass fluxes and gas contents of the erupting magmas and, most fundamentally, by differences in the grainsize distribution of the erupted clasts. Plume heights predicted by the model are greater than those found for previous models of Hawaiian eruptions. This is because previous models did not allow for the progressive fall out of particles from the plume and, more importantly, made no correction for the velocity disequilibrium between gas and clasts when the grainsize distribution is coarse.  相似文献   

4.
An integrated approach involving volcanology, geochemistry and numerical modelling has enabled the reconstruction of the volcanic history of the Fox kimberlite pipe. The observed deposits within the vent include a basal massive, poorly sorted, matrix supported, lithic fragment rich, eruption column collapse lapilli tuff. Extensive vent widening during the climactic magmatic phase of the eruption led to overloading of the eruption column with cold dense country rock lithic fragments, dense juvenile pyroclasts and olivine crystals, triggering column collapse. > 40% dilution of the kimberlite by granodiorite country rock lithic fragments is observed both in the physical componentry of the rocks and in the geochemical signature, where enrichment in Al2O3 and Na2O compared to average values for coherent kimberlite is seen. The wide, deep, open vent provided a trap for a significant proportion of the collapsing column material, preventing large scale run-away in the form of pyroclastic flow onto the ground surface, although minor flows probably also occurred. A massive to diffusely bedded, poorly sorted, matrix supported, accretionary-lapilli bearing, lithic fragment rich, lapilli tuff overlies the column collapse deposit providing evidence for a late phreatomagmatic eruption stage, caused by the explosive interaction of external water with residual magma. Correlation of pipe morphology and internal stratigraphy indicate that widening of the pipe occurred during this latter stage and a thick granodiorite cobble-boulder breccia was deposited. Ash- and accretionary lapilli-rich tephra, deposited on the crater rim during the late phreatomagmatic stage, was subsequently resedimented into the vent. Incompatible elements such as Nb are used as indicators of the proportion of the melt fraction, or kimberlite ash, retained or removed by eruptive processes. When compared to average coherent kimberlite the ash-rich deposits exhibit ~ 30% loss of fines whereas the column collapse deposit exhibits ~ 50% loss. This shows that despite the poorly sorted nature of the column collapse deposit significant elutriation has occurred during the eruption, indicating the existence of a high sustained eruption column. The deposits within Fox record a complex eruption sequence showing a transition from a probable violent sub-plinian style eruption, driven by instantaneous exsolution of magmatic volatiles, to a late phreatomagmatic eruption phase. Mass eruption rate and duration of the sub-plinian phase of the eruption have been determined based on the dimensions of milled country-rock boulders found within the intra-vent deposits. Calculations show a short lived eruption of one to eleven days for the sub-plinian magmatic phase, which is similar in duration to small volume basaltic eruptions. This is in general agreement with durations of kimberlite eruptions calculated using entirely different approaches and parameters, such as predictions of magma ascent rates in kimberlite dykes.  相似文献   

5.
The 273 ka Poris Formation in the Bandas del Sur Group records a complex, compositionally zoned explosive eruption at Las Cañadas caldera on Tenerife, Canary Islands. The eruption produced widespread pyroclastic density currents that devastated much of the SE of Tenerife, and deposited one of the most extensive ignimbrite sheets on the island. The sheet reaches ~ 40-m thick, and includes Plinian pumice fall layers, massive and diffuse-stratified pumiceous ignimbrite, widespread lithic breccias, and co-ignimbrite ashfall deposits. Several facies are fines-rich, and contain ash pellets and accretionary lapilli. Eight brief eruptive phases are represented within its lithostratigraphy. Phase 1 comprised a fluctuating Plinian eruption, in which column height increased and then stabilized with time and dispersed tephra over much of the southeastern part of the island. Phase 2 emplaced three geographically restricted ignimbrite flow-units and associated extensive thin co-ignimbrite ashfall layers, which contain abundant accretionary lapilli from moist co-ignimbrite ash plumes. A brief Plinian phase (Phase 3), again dispersing pumice lapilli over southeastern Tenerife, marked the onset of a large sustained pyroclastic density current (Phase 4), which then waxed (Phase 5), covering increasingly larger areas of the island, as vents widened and/or migrated along opening caldera faults. The climax of the Poris eruption (Phase 6) was marked by widespread emplacement of coarse lithic breccias, thought to record caldera subsidence. This is inferred to have disturbed the magma chamber, causing mingling and eruption of tephriphonolite magma, and it changed the proximal topography diverting the pyroclastic density current(s) down the Güimar valley (Phase 7). Phase 8 involved post-eruption erosion and sedimentary reworking, accompanied by minor down-slope sliding of ignimbrite. This was followed by slope stabilization and pedogenesis. The fines-rich lithofacies with abundant ash pellets and accretionary lapilli record agglomeration of ash in moist ash plumes. They resemble phreatomagmatic deposits, but a phreatomagmatic origin is difficult to establish because shards are of bubble-wall type, and the moisture may have arisen by condensation within ascending thermal co-ignimbrite ash plumes that contained atmospheric moisture enhanced by that derived from the evaporation of seawater where the hot pyroclastic currents crossed the coast. Ash pellets formed in co-ignimbrite ash-clouds and then fell through turbulent pyroclastic density currents where they accreted rims and evolved into accretionary lapilli.Editorial Responsibility: J. Stix  相似文献   

6.
White Island is an active andesitic-dacitic composite volcano surrounded by sea, yet isolated from sea water by chemically sealed zones that confine a long-lived acidic hydrothermal system, within a thick sequence of fine-grained volcaniclastic sediment and ash. The rise of at least 106 m3 of basic andesite magma to shallow levels and its interaction with the hydrothermal system resulted in the longest historical eruption sequence at White Island in 1976–1982. About 107 m3 of mixed lithic and juvenile ejecta was erupted, accompanied by collapse to form two coalescing maar-like craters. Vent position within the craters changed 5 times during the eruption, but the vents were repeatedly re-established along a line linking pre-1976 vents. The eruption sequence consisted of seven alternating phases of phreatomagmatic and Strombolian volcanism. Strombolian eruptions were preceded and followed by mildly explosive degassing and production of incandescent, blocky juvenile ash from the margins of the magma body. Phreatomagmatic phases contained two styles of activity: (a) near-continuous emission of gas and ash and (b) discrete explosions followed by prolonged quiescence. The near-continuous activity reculted from streaming of magmatic volatiles and phreatic steam through open conduits, frittering juvennile shards from the margins of the magma and eroding loose lithic particles from the unconsolidated wall rock. The larger discrete explosions produced ballistic block aprons, downwind lobes of fall tephra, and cohesive wet surge deposits confined to the main crater. The key features of the larger explosions were their shallow focus, random occurrence and lack of precursors, and the thermal heterogeneity of the ejecta. This White Island eruption was unusual because of the low discharge rate of magma over an extended time period and because of the influence of a unique physical and hydrological setting. The low rate of magma rise led to very effective separation of magmatic volatiles and high fluxes of magmatic gas even during phreatic phases of the eruption. While true Strombolian phases did occur, more frequently the decoupled magmatic gas rose to interact with the conduit walls and hydrothermal system, producing phreatomagmatic eruptions. The form of these wet explosions was governed by a delicate balance between erosion and collapse of the weak conduit walls. If the walls were relatively stable, fine ash was slowly eroded and erupted in weak, near-continous phreatomagmatic events. When the walls were unstable, wall collapse triggered larger discrete phreatomagmatic explosions.  相似文献   

7.
Abstract Tyatya Volcano, situated in Kunashir Island at the southwestern end of Kuril Islands, is a large composite stratovolcano and one of the most active volcanoes in the Kuril arc. The volcanic edifice can be divided into the old and the young ones, which are composed of rocks of distinct magma types, low‐ and medium‐K series, respectively. The young volcano has a summit caldera with a central cone. Recent eruptions have occurred at the central cone and at the flank vents of the young volcano. We found several distal ash layers at the volcano and identified their ages and sources, that is, tephras of ad 1856, ad 1739, ad 1694 and ca 1 Ka derived from three volcanoes of Hokkaido, Japan, and caad 969 from Baitoushan Volcano of China/North Korea. These could provide good time markers to reveal the eruptive history of the central cone, which had continued intermittently with Strombolian eruptions and lava flow effusions since before 1 Ka. Relatively explosive eruptions have occurred three times at the cone during the past 1000 years. We revealed that, topographically, the youngest lava flows from the cone are covered not by the tephra of ad 1739 but by that of ad 1856. This evidence, together with a report of dense smoke rising from the summit in ad 1812, suggests that the latest major eruption with lava effusion from the central cone occurred in this year. In 1973, after a long period of dormancy, short‐lived phreatomagmatic eruptions began to occur from fissure vents at the northern flank of the young volcano. This was followed by large eruptions of Strombolian to sub‐Plinian types occurring from several craters at the southern flank. The 1973 activity is evaluated as Volcanic Explosivity Index = 4 (approximately 0.2 km3), the largest eruption during the 20th century in the southwestern Kuril arc. The rocks of the central cone are strongly porphyritic basalt and basaltic andesite, whereas the 1973 scoria is aphyric basalt, suggesting that magma feeding systems are definitely different between the summit and flank eruptions.  相似文献   

8.
 The Quaternary White Trachytic Tuffs Formation from Roccamonfina Volcano (southern Italy) comprises four non-welded, trachytic, pyroclastic sequences bounded by paleosols, each of which corresponds to small- to intermediate-volume explosive eruptions from central vents. From oldest to youngest they are: White Trachytic Tuff (WTT) Cupa, WTT Aulpi, WTT S. Clemente, and WTT Galluccio. The WTT Galluccio eruption was the largest and emplaced ∼ 4 km3 of magma. The internal stratigraphy of all four WTT eruptive units is a complex association of fallout, surge, and pyroclastic flow deposits. Each eruptive unit is organized into two facies associations, Facies Association A below Facies Association B. The emplacement of the two facies associations may have been separated by short time breaks allowing for limited reworking and erosion. Facies Association A consists of interbedded fallout deposits, surge deposits, and subordinate ignimbrites. This facies association involved the eruption of the most evolved trachytic magma, and pumice clasts are white and well vesiculated. The grain size coarsens upward in Facies Association A, with upward increases of dune bedform wavelengths and a decrease in the proportion of fine ash. These trends could reflect an increase in eruption column height from the onset of the eruption and possibly also in mass eruption rate. Facies Association B comprises massive ignimbrites that are progressively richer in lithic clast content. This association involved the eruption of more mafic magma, and pumice clasts are gray and poorly vesiculated. Facies Association B is interpreted to record the climax of the eruption. Phreatomagmatic deposits occur at different stratigraphic levels in the four WTT and have different facies characteristics. The deposits reflect the style and degree of magma–water interaction and the local hydrogeology. Very fine-grained, lithic-poor phreatomagmatic surge deposits found at the base of WTT Cupa and WTT Galluccio could record the interaction of the erupting magma with a lake that occupied the Roccamonfina summit depression. Renewed magma–water interaction later in the WTT Galluccio eruption is indicated by fine grained, lithic-bearing phreatomagmatic fall and surge deposits occurring at the top of Facies Association A. They could be interpreted to reflect shifts of the magma fragmentation level to highly transmissive, regional aquifers located beneath the Roccamonfina edifice, possibly heralding a caldera collapse event. Received: 26 August 1996 / Accepted: 27 February 1998  相似文献   

9.
The stratigraphic succession of the Pomici di Avellino Plinian eruption from Somma-Vesuvius has been studied through field and laboratory data in order to reconstruct the eruption dynamics. This eruption is particularly important in the Somma-Vesuvius eruptive history because (1) its vent was offset with respect to the present day Vesuvius cone; (2) it was characterised by a distinct opening phase; (3) breccia-like very proximal fall deposits are preserved close to the vent and (4) the pyroclastic density currents generated during the final phreatomagmatic phase are among the most widespread and voluminous in the entire history of the volcano. The stratigraphic succession is, here, divided into deposits of three main eruptive phases (opening, magmatic Plinian and phreatomagmatic), which contain five eruption units. Short-lived sustained columns occurred twice during the opening phase (Ht of 13 and 21.5 km, respectively) and dispersed thin fall deposits and small pyroclastic density currents onto the volcano slopes. The magmatic Plinian phase produced the main volume of erupted deposits, emplacing white and grey fall deposits which were dispersed to the northeast. Peak column heights reached 23 and 31 km during the withdrawal of the white and the grey magmas, respectively. Only one small pyroclastic density current was emplaced during the main Plinian phase. In contrast, the final phreatomagmatic phase was characterised by extensive generation of pyroclastic density currents, with fallout deposits very subordinate and limited to the volcano slopes. Assessed bulk erupted volumes are 21 × 106 m3 for the opening phase, 1.3–1.5 km3 for the main Plinian phase and about 1 km3 for the final phreatomagmatic phase, yielding a total volume of about 2.5 km3. Pumice fragments are porphyritic with sanidine and clinopyroxene as the main mineral phases but also contain peculiar mineral phases like scapolite, nepheline and garnet. Bulk composition varies from phonolite (white magma) to tephri-phonolite (grey magma).  相似文献   

10.
The Fontana Lapilli deposit was erupted in the late Pleistocene from a vent, or multiple vents, located near Masaya volcano (Nicaragua) and is the product of one of the largest basaltic Plinian eruptions studied so far. This eruption evolved from an initial sequence of fluctuating fountain-like events and moderately explosive pulses to a sustained Plinian episode depositing fall beds of highly vesicular basaltic-andesite scoria (SiO2 > 53 wt%). Samples show unimodal grain size distribution and a moderate sorting that are uniform in time. The juvenile component predominates (> 96 wt%) and consists of vesicular clasts with both sub-angular and fluidal, elongated shapes. We obtain a maximum plume height of 32 km and an associated mass eruption rate of 1.4 × 108 kg s−1 for the Plinian phase. Estimates of erupted volume are strongly sensitive to the technique used for the calculation and to the distribution of field data. Our best estimate for the erupted volume of the majority of the climactic Plinian phase is between 2.9 and 3.8 km3 and was obtained by applying a power-law fitting technique with different integration limits. The estimated eruption duration varies between 4 and 6 h. Marine-core data confirm that the tephra thinning is better fitted by a power-law than by an exponential trend.  相似文献   

11.
Intense explosive activity occurred repeatedly at Vesuvius during the nearly 1,600-year period between the two Plinian eruptions of Avellino (3.5 ka) and Pompeii (79 A.D.). By correlating stratigraphic sections from more than 40 sites around the volcano, we identify the deposits of six main eruptions (AP1-AP6) and of some minor intervening events. Several deposits can be traced up to 20 km from the vent. Their stratigraphic and dispersal features suggest the prevalence of two main contrasting eruptive styles, each involving a complex relationship between magmatic and phreatomagmatic phases. The two main eruption styles are (1) sub-Plinian to phreato-Plinian events (AP1 and AP2 members), where deposits consist of pumice and scoria fall layers alternating with fine-grained, vesiculated, accretionary lapilli-bearing ashes; and (2) mixed, violent Strombolian to Vulcanian events (AP3-AP6 members), which deposited a complex sequence of fallout, massive to thinly stratified, scoria-bearing lapilli layers and fine ash beds. Morphology and density variations of the juvenile fragments confirm the important role played by magma-water interaction in the eruptive dynamics. The mean composition of the ejected material changes with time, and shows a strong correlation with vent position and eruption style. The ranges of intensity and magnitude of these events, derived by estimations of peak column height and volume of the ejecta, are significantly smaller than the values for the better known Plinian and sub-Plinian eruptions of Vesuvius, enlarging the spectrum of the possible eruptive scenarios at Vesuvius, useful in the assessment of its potential hazard.  相似文献   

12.
New investigations of the geology of Crater Lake National Park necessitate a reinterpretation of the eruptive history of Mount Mazama and of the formation of Crater Lake caldera. Mount Mazama consisted of a glaciated complex of overlapping shields and stratovolcanoes, each of which was probably active for a comparatively short interval. All the Mazama magmas apparently evolved within thermally and compositionally zoned crustal magma reservoirs, which reached their maximum volume and degree of differentiation in the climactic magma chamber 7000 yr B.P.The history displayed in the caldera walls begins with construction of the andesitic Phantom Cone 400,000 yr B.P. Subsequently, at least 6 major centers erupted combinations of mafic andesite, andesite, or dacite before initiation of the Wisconsin Glaciation 75,000 yr B.P. Eruption of andesitic and dacitic lavas from 5 or more discrete centers, as well as an episode of dacitic pyroclastic activity, occurred until 50,000 yr B.P.; by that time, intermediate lava had been erupted at several short-lived vents. Concurrently, and probably during much of the Pleistocene, basaltic to mafic andesitic monogenetic vents built cinder cones and erupted local lava flows low on the flanks of Mount Mazama. Basaltic magma from one of these vents, Forgotten Crater, intercepted the margin of the zoned intermediate to silicic magmatic system and caused eruption of commingled andesitic and dacitic lava along a radial trend sometime between 22,000 and 30,000 yr B.P. Dacitic deposits between 22,000 and 50,000 yr old appear to record emplacement of domes high on the south slope. A line of silicic domes that may be between 22,000 and 30,000 yr old, northeast of and radial to the caldera, and a single dome on the north wall were probably fed by the same developing magma chamber as the dacitic lavas of the Forgotten Crater complex. The dacitic Palisade flow on the northeast wall is 25,000 yr old. These relatively silicic lavas commonly contain traces of hornblende and record early stages in the development of the climatic magma chamber.Some 15,000 to 40,000 yr were apparently needed for development of the climactic magma chamber, which had begun to leak rhyodacitic magma by 7015 ± 45 yr B.P. Four rhyodacitic lava flows and associated tephras were emplaced from an arcuate array of vents north of the summit of Mount Mazama, during a period of 200 yr before the climactic eruption. The climactic eruption began 6845 ± 50 yr B.P. with voluminous airfall deposition from a high column, perhaps because ejection of 4−12 km3 of magma to form the lava flows and tephras depressurized the top of the system to the point where vesiculation at depth could sustain a Plinian column. Ejecta of this phase issued from a single vent north of the main Mazama edifice but within the area in which the caldera later formed. The Wineglass Welded Tuff of Williams (1942) is the proximal featheredge of thicker ash-flow deposits downslope to the north, northeast, and east of Mount Mazama and was deposited during the single-vent phase, after collapse of the high column, by ash flows that followed topographic depressions. Approximately 30 km3 of rhyodacitic magma were expelled before collapse of the roof of the magma chamber and inception of caldera formation ended the single-vent phase. Ash flows of the ensuing ring-vent phase erupted from multiple vents as the caldera collapsed. These ash flows surmounted virtually all topographic barriers, caused significant erosion, and produced voluminous deposits zoned from rhyodacite to mafic andesite. The entire climactic eruption and caldera formation were over before the youngest rhyodacitic lava flow had cooled completely, because all the climactic deposits are cut by fumaroles that originated within the underlying lava, and part of the flow oozed down the caldera wall.A total of 51−59 km3 of magma was ejected in the precursory and climactic eruptions, and 40−52 km3 of Mount Mazama was lost by caldera formation. The spectacular compositional zonation shown by the climactic ejecta — rhyodacite followed by subordinate andesite and mafic andesite — reflects partial emptying of a zoned system, halted when the crystal-rich magma became too viscous for explosive fragmentation. This zonation was probably brought about by convective separation of low-density, evolved magma from underlying mafic magma. Confinement of postclimactic eruptive activity to the caldera attests to continuing existence of the Mazama magmatic system.  相似文献   

13.
 The evolution of the Somma-Vesuvius caldera has been reconstructed based on geomorphic observations, detailed stratigraphic studies, and the distribution and facies variations of pyroclastic and epiclastic deposits produced by the past 20,000 years of volcanic activity. The present caldera is a multicyclic, nested structure related to the emptying of large, shallow reservoirs during Plinian eruptions. The caldera cuts a stratovolcano whose original summit was at 1600–1900 m elevation, approximately 500 m north of the present crater. Four caldera-forming events have been recognized, each occurring during major Plinian eruptions (18,300 BP "Pomici di Base", 8000 BP "Mercato Pumice", 3400 BP "Avellino Pumice" and AD 79 "Pompeii Pumice"). The timing of each caldera collapse is defined by peculiar "collapse-marking" deposits, characterized by large amounts of lithic clasts from the outer margins of the magma chamber and its apophysis as well as from the shallow volcanic and sedimentary units. In proximal sites the deposits consist of coarse breccias resulting from emplacement of either dense pyroclastic flows (Pomici di Base and Pompeii eruptions) or fall layers (Avellino eruption). During each caldera collapse, the destabilization of the shallow magmatic system induced decompression of hydrothermal–magmatic and hydrothermal fluids hosted in the wall rocks. This process, and the magma–ground water interaction triggered by the fracturing of the thick Mesozoic carbonate basement hosting the aquifer system, strongly enhanced the explosivity of the eruptions. Received: 24 November 1997 / Accepted: 23 March 1999  相似文献   

14.
The pyroclastic deposits of many basaltic volcanic centres show abrupt transitions between contrasting eruptive styles, e.g., Hawaiian versus Strombolian, or `dry' magmatic versus `wet' phreatomagmatic. These transitions are controlled dominantly by variations in degassing patterns, magma ascent rates and degrees of interaction with external water. We use Crater Hill, a 29 ka explosive/effusive monogenetic centre in the Auckland volcanic field, New Zealand, as a case study of the transitions between these end-member eruptive styles. The Crater Hill eruption took place from at least 4 vents spaced along a NNE-trending, 600-m-long fissure that is contained entirely within a tuff ring generated during the earliest eruption phases. Early explosive phases at Crater Hill were characterised by eruption from multiple unstable and short-lived vents; later, dominantly extrusive, volcanism took place from a more stable point source. Most of the Crater Hill pyroclastic deposits were formed in 3 phreatomagmatic (P) and 4 `dry' magmatic (M) episodes, forming in turn the outer tuff ring and maar crater (P1, M1, P2) and scoria cone 1 (M2–M4). This activity was followed by formation of a lava shield and scoria cone 2. Purely `wet' activity is represented by the bulk of P1 and P2, and purely `dry' activity by much of M2–M4. However, M1 and parts of M2 and M4 show evidence for simultaneous eruptions of differing style from adjacent vents and rapid variations in the extent and timing of magma:water interaction at each vent. The nature of the wall-rock lithics, and these rapid variations in inferred water/magma ratios imply interaction was occurring mostly at depths of ≤80 m, and the vesicularity patterns in juvenile clasts from these and the P beds imply that rapid degassing occurred at these shallow levels. We suggest that abrupt transitions between eruptive styles, in time and space, at Crater Hill were linked to changes in the local magma supply rate and patterns and vigour of degassing during the final metres of ascent.  相似文献   

15.
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   

16.
Since the eruption which affected Quito in AD 1660, Guagua Pichincha has been considered a hazardous volcano. Based on field studies and twenty 14C dates, this paper discusses the eruptive activity of this volcano, especially that of the last 2000 years. Three major Plinian eruptions with substantial pumice discharge occurred in the 1st century, the 10th century, and in AD 1660. The ages of organic paleosols and charcoal from block-and-ash flow and fallout deposits indicate that these eruptions occurred near the end of 100 to 200 year-long cycles of discontinuous activity which was comprised of dome growth episodes and minor pumice fallouts. The first cycle took place from ~ AD 1 to 140. The second one developed during the 9th and 10th centuries, lasted 150–180 yr, and included the largest Plinian event, with a VEI of 5. The third, historic cycle, about 200 yr in duration, includes pyroclastic episodes around AD 1450 and AD 1500, explosive activity between AD 1566 and AD 1582, possible precursors of the 1660 eruption in the early decades of the 17th century, and finally the 1660 eruption (VEI 4). A fourth event probably occurred around AD 500, but its authenticity requires confirmation. The Plinian events occurred at the end of these cycles which were separated by repose periods of at least 300 yr. Older volcanic activity of similar type occurred between ~ 4000 and ~ 3000 yr BP.  相似文献   

17.
A series of large hydrothermal eruptions occurred across the Waiotapu geothermal field at about the same (prehistoric) time as the ~AD1315 Kaharoa rhyolite magmatic eruptions from Tarawera volcano vents, 10–20 km distant. Triggering of the Waiotapu hydrothermal eruptions was previously attributed to displacement of the adjacent Ngapouri Fault. The Kaharoa rhyolite eruptions are now recognised as primed and triggered by multiple basalt intrusions beneath the Tarawera volcano. A ~1000 t/day pulse of CO2 gas is recorded by alteration mineralogy and fluid inclusions in drill core samples from Waiotapu geothermal wells. This CO2 pulse is most readily sourced from basalt intruded at depth, and although not precisely dated, it appears to be associated with the Waiotapu hydrothermal eruptions. We infer that the hydrothermal eruptions at Waiotapu were primed by intrusion of the same arrested basalt dike system that drove the rhyolite eruptions at Tarawera. This dike system was likely similar at depth to the dike that generated basalt eruptions from a 17 km-long fissure that formed across the Tarawera region in AD1886. Fault ruptures that occurred in the Waiotapu area in association with both the AD1886 and ~AD1315 eruptions are considered to be a result, rather than a cause, of the dike intrusion processes.Editorial responsibility: J. Donnelly-Nolan  相似文献   

18.
A tephrostratigraphy for Erebus volcano is presented, including tephra composition, stratigraphy, and eruption mechanism. Tephra from Erebus were collected from glacial ice and firn. Scanning electron microscope images of the ash morphologies help determine their eruption mechanisms The tephra resulted mainly from phreatomagmatic eruptions with fewer from Strombolian eruptions. Tephra having mixed phreatomagmatic–Strombolian origins are common. Two tephra deposited on the East Antarctic ice sheet, ~ 200 km from Erebus, resulted from Plinian and phreatomagmatic eruptions. Glass droplets in some tephra indicate that these shards were produced in both phreatomagmatic and Strombolian eruptions. A budding ash morphology results from small spheres quenched during the process of hydrodynamically splitting off from a parent melt globule. Clustered and rare single xenocrystic analcime crystals, undifferentiated zeolites, and clay are likely accidental clasts entrained from a hydrothermal system present prior to eruption. The phonolite compositions of glass shards confirm Erebus volcano as the eruptive source. The glasses show subtle trends in composition, which correlate with stratigraphic position. Trace element analyses of bulk tephra samples show slight differences that reflect varying feldspar contents.  相似文献   

19.
Geological surveys, tephrostratigraphic study, and 40Ar/39Ar age determinations have allowed us to chronologically constrain the geological evolution of the lower NW flank of Etna volcano and to reconstruct the eruptive style of the Mt Barca flank eruption. This peripheral sector of the Mt Etna edifice, corresponding to the upper Simeto valley, was invaded by the Ellittico volcano lava flows between 41 and 29 ka ago when the Mt Barca eruption occurred. The vent of this flank eruption is located at about 15 km away from the summit craters, close to the town of Bronte. The Mt Barca eruption was characterized by a vigorous explosive activity that produced pyroclastic deposits dispersed eastward and minor effusive activity with the emission of a 1.1-km-long lava flow. Explosive activity was characterized by a phreatomagmatic phase followed by a magmatic one. The geological setting of this peripheral sector of the volcano favors the interaction between the rising magma and the shallow groundwater hosted in the volcanic pile resting on the impermeable sedimentary basement. This process produced phreatomagmatic activity in the first phase of the eruption, forming a pyroclastic fall deposit made of high-density, poorly vesicular scoria lapilli and lithic clasts. Conversely, during the second phase, a typical strombolian fall deposit formed. In terms of hazard assessment, the possible occurrence of this type of highly explosive flank eruption, at lower elevation in the densely inhabited areas, increases the volcanic risk in the Etnean region and widens the already known hazard scenario.  相似文献   

20.
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   

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