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1.
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   

2.
The Ohakune Craters form one of several parasitic centres surrounding Ruapehu volcano, at the southern end of the Taupo Volcanic Zone. An inner scoria cone and an outer, probably older, tuff ring are the principal structures in a nested cluster of four vents.The scoria cone consists of alternating lava flows and coarse, welded and unwelded, strombolian block and bomb beds. The strombolian beds consist of principally two discrete types of essential clast, vesicular bombs and dense angular blocks. Rare finer-grained beds are unusually block-rich. The tuff ring consists of alternating strombolian and phreatomagmatic units. Strombolian beds have similar grain size characteristics to scoria cone units, but contain more highly vesicular unoxidised bombs and few blocks. Phreatomagmatic deposits, which contain clasts with variable degrees of palagonitisation, consist of less well-sorted airfall deposits and very poorly sorted, crystal-rich pyroclastic surge deposits.Disruption by expanding magmatic gas bubbles was a major but relatively constant influence on both strombolian and phreatomagmatic eruptions at Ohakune. Instead, the nature of deposits was principally controlled by two other variables, vent geometry and the relative influence of external water during volcanism. During tuff-ring construction, magma is considered to have risen rapidly to the surface, and to have been ejected without sufficient residence time in the vent for non-explosive degassing. Availability of external water principally governed the eruption mechanism and hence the nature of the deposits. Essentials clasts of the scoria cone are, by comparison, dense, degassed and oxidised. It is suggested that a change in vent geometry, possibly the construction of the tuff ring itself, permitted lava ponding and degassing during scoria cone growth. During strombolian eruptions, magma remaining in the vent probably became depleted in gas, leading to the formation of an inert zone, or crust, above actively degassing magma. Subsequent explosions had therefore to disrupt both this passive crust and underlying, vesiculating magma “driving” the eruption. Cycles of strombolian eruption are thought to have stopped when the thickness of the inert crust precluded explosive eruption and only recommenced when some of this material was removed, either as a lava flow or during phreatomagmatic explosions when external water entered the vent. Such explosions probably formed the unusually fine-grained and block-rich beds in the strombolian sequence.The Ohakune deposits are an excellent example of the products of explosive eruption of fluid, gas-rich basic magma vesiculating under very near-surface conditions. A complex interplay of rate of magma rise, rate and depth of formation of gas bubbles, vent geometry, abundance of shallow external water, wind velocity and accumulation rate of ejecta determines the nature of deposits of such eruptions.  相似文献   

3.
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   

4.
The Quaternary Herchenberg composite tephra cone (East Eifel, FR Germany) with an original bulk volume of 1.17·107 m3 (DRE of 8.2·106 m3) and dimensions of ca. 900·600·90 m (length·width·height) erupted in three main stages: (a) Initial eruptions along a NW-trending, 500-m-long fissure were dominantly Vulcanian in the northwest and Strombolian in the southeast. Removal of the unstable, underlying 20-m-thick Tertiary clays resulted in major collapse and repeated lateral caving of the crater. The northwestern Lower Cone 1 (LC1) was constructed by alternating Vulcanian and Strombolian eruptions. (b) Cone-building, mainly Strombolian eruptions resulted in two major scoria cones beginning initially in the northwest (Cone 1) and terminating in the southeast (Cones 2 and 3) following a period of simultaneous activity of cones 1 and 2. Lapilli deposits are subdivided by thin phreatomagmatic marker beds rich in Tertiary clays in the early stages and Devonian clasts in the later stages. Three dikes intruded radially into the flanks of cone 1. (c) The eruption and deposition of fine-grained uppermost layers (phreatomagmatic tuffs, accretionary lapilli, and Strombolian fallout lapilli) presumably from the northwestern center (cone 1) terminated the activity of Herchenberg volcano. The Herchenberg volcano is distinguished from most Strombolian scoria cones in the Eifel by (1) small volume of agglutinates in central craters, (2) scarcity of scoria bomb breccias, (3) well-bedded tephra deposits even in the proximal facies, (4) moderate fragmentation of tephra (small proportions of both ash and coarse lapilli/bomb-size fraction), (5) abundance of dense ellipsoidal juvenile lapilli, and (6) characteristic depositional cycles in the early eruptive stages beginning with laterally emplaced, fine-grained, xenolith-rich tephra and ending with fallout scoria lapilli. Herchenberg tephra is distinguished from maar deposits by (1) paucity of xenoliths, (2) higher depositional temperatures, (3) coarser grain size and thicker bedding, (4) absence of glassy quenched clasts except in the initial stages and late phreatomagmatic marker beds, and (5) predominance of Strombolian, cone-building activity. The characteristics of Herchenberg deposits are interpreted as due to a high proportion of magmatic volatiles (dominantly CO2) relative to low-viscosity magma during most of the eruptive activity.  相似文献   

5.
The Croscat pyroclastic succession has been analysed to investigate the transition between different eruptive styles in basaltic monogenetic volcanoes, with particular emphasis on the role of phreatomagmatism in triggering Violent Strombolian eruptions. Croscat volcano, an 11 ka basaltic complex scoria cone in the Quaternary Garrotxa Volcanic Field (GVF) shows pyroclastic deposits related both to magmatic and phreatomagmatic explosions.Lithofacies analysis, grain size distribution, chemical composition, glass shard morphologies, vesicularity, bubble-number density and crystallinity of the Croscat pyroclastic succession have been used to characterize the different eruptive styles. Eruptions at Croscat began with fissural Hawaiian-type fountaining that rapidly changed to eruption types transitional between Hawaiian and Strombolian from a central vent. A first phreatomagmatic phase occurred by the interaction between magma and water from a shallow aquifer system at the waning of the Hawaiian- and Strombolian-types stage. A Violent Strombolian explosion then occurred, producing a widespread (8 km2), voluminous tephra blanket. The related deposits are characterized by the presence of wood-shaped, highly vesicular scoriae. Glass-bearing xenoliths (buchites) are also present within the deposit. At the waning of the Violent Strombolian phase a second phreatomagmatic phase occurred, producing a second voluminous deposit dispersed over 8.4 km2. The eruption ended with a lava flow emission and consequent breaching of the western-side of the volcano. Our data suggest that the Croscat Violent Strombolian phase was related to the ascent of deeper, crystal-poor, highly vesicular magma under fast decompression rate. Particles and vesicles elongation and brittle failure observed in the wood-shaped clasts indicate that fragmentation during Violent Strombolian phase was enhanced by high strain-rate of the magma within the conduit.  相似文献   

6.
Phreatomagmatic activity results from the interaction of magma and external water during a volcanic eruption and is a frequent eruptive phenomenon worldwide. Such ‘fuel-coolant’ reactions change the eruptive dynamics, thus generating particles that reflect the degree of explosivity. Different eruptive phases may thus be identified from deposits, allowing us to reconstruct conditions that prevailed in the past and use these to predict the level of explosivity in a given geological setting in the future.A detailed study of the deposits from Montaña Los Erales, a 70 m-high Quaternary cinder cone belonging to a rift-related chain of vents in the Bandas del Sur region, in Southeast Tenerife, was undertaken. Field observations on excavated sections and SEM analysis of tephra suggest that the eruption style changed progressively from an initial phreatomagmatic phase, through a transitional stage, to one that was entirely Strombolian. To investigate the causes and the nature of these changes in eruptive style, products from each major unit were analysed for their morphology using hand specimen observations, secondary electron microscopy, backscatter electron microscopy, and reflected light microscopy to examine fragment size variation, fragment morphologies, vesicularity, and the level of secondary hydrous alteration (e.g. palagonitisation and zeolitisation). Study results demonstrate that the initial phase of activity was largely driven by magma–water interaction, where magma may have interacted with a lens of fresh ground- or surface water. With proceeding eruptive activity the water became exhausted, giving rise to an entirely Strombolian eruptive style.Examples of phreatomagmatic activity that occur on typical rift-related basaltic vent alignments are not infrequent in the Canary Islands. These vent systems usually erupt in Strombolian fashion, producing scoria and lava flows that do not generally extend far beyond the vent area. However, aligned feeders may intersect different strata, structural features, and different hydrological situations, thus giving rise to activity that is less predictable in setting, intensity, and duration. The occurrence of phreatomagmatism in an otherwise low-explosivity basaltic eruptive environment increases the need for awareness of the geo-hydrological situation in volcanically active areas. Studying the past eruptive history is therefore essential to derive realistic scenarios for future vulnerability evaluation and risk assessment, especially in densely populated areas like the Canary Islands.  相似文献   

7.
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   

8.
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   

9.
The 1886 Plinian eruption of Tarawera, New Zealand, is a unique basaltic fissure-fed eruption with exceptionally well preserved fall deposits to within 200 meters of the source vents. These proximal deposits form a series of spatter/cinder half-cones along the northeastern 8-km-long segment of the 1886 fissure. Here we examine these deposits using grain size and clast componentry techniques. We contrast the products of the phreatomagmatic (phases I and III) and Plinian (Phase II) stages of the eruption and examine deposit variability as a function of contrasting eruptive intensity within the climactic phase (II) of the eruption. The opening phreatomagmatic phase I of the eruption involved gas-rich magma interacting with water and fragmenting at least 300 meters below the surface. The deposits of the climactic phase that followed have relatively uniform grain size but marked contrasts in the relative abundance of juvenile and wall rock (lithic) clasts. Deposits linked to vents associated with the high Plinian plume are more uniform than those characterized by a weaker cone-forming eruption style. During the third, and closing, phase of the eruption, magma withdrawal accompanied the onset of decoupling of the exsolved gas phase, leading to fragmentation at increasingly greater depths and significant wall rock collapse into the erupting vents. Variability in eruptive style during phase II along the fissure appears to be a function of shallow seated controls, in particular the variable extent of incorporation of lithic wall rock into the erupting jet, as a consequence of vent wall collapse. Widely dispersed beds centralized around Plinian sources along the fissure have very low lithic content; cone-forming beds at other craters that contain very high lithic contents. This incorporation led to a significant reduction of the velocity and stability of the jet at these latter steep-walled craters, and induced episodicity in the form of vent-clearing explosions. The result is a large reduction of the physical and thermal ability of these vents to contribute to a stable high eruptive plume. Instead large volumes of ejecta were sedimented prematurely from shallow heights at rates an order of magnitude greater than for historical Strombolian, Hawaiian and subPlinian eruptions. This study illustrates that sustained powerful Plinian eruptions can be accompanied by heterogeneities and instabilities of the eruptive jet. At Tarawera, the record of complex proximal transport and deposition processes in the eruptive jet cannot be inferred from the eruption products at distances greater than 400 m from the eruptive fissure. We suggest that study of ultraproximal deposits, as seen at Mt Tarawera, provides the only opportunity to document the complex, dynamic behavior of the jet region of Plinian eruptions.  相似文献   

10.
The Igwisi Hills volcanoes (IHV), Tanzania, are unique and important in preserving extra-crater lavas and pyroclastic edifices. They provide critical insights into the eruptive behaviour of kimberlite magmas that are not available at other known kimberlite volcanoes. Cosmogenic 3He dating of olivine crystals from IHV lavas and palaeomagnetic analyses indicates that they are Upper Pleistocene to Holocene in age. This makes them the youngest known kimberlite bodies on Earth by >30?Ma and may indicate a new phase of kimberlite volcanism on the Tanzania craton. Geological mapping, Global Positioning System surveying and field investigations reveal that each volcano comprises partially eroded pyroclastic edifices, craters and lavas. The volcanoes stand <40?m above the surrounding ground and are comparable in size to small monogenetic basaltic volcanoes. Pyroclastic cones consist of diffusely layered pyroclastic fall deposits comprising scoriaceous, pelletal and dense juvenile pyroclasts. Pyroclasts are similar to those documented in many ancient kimberlite pipes, indicating overlap in magma fragmentation dynamics between the Igwisi eruptions and other kimberlite eruptions. Characteristics of the pyroclastic cone deposits, including an absence of ballistic clasts and dominantly poorly vesicular scoria lapillistones and lapilli tuffs, indicate relatively weak explosive activity. Lava flow features indicate unexpectedly high viscosities (estimated at >102 to 106?Pa?s) for kimberlite, attributed to degassing and in-vent cooling. Each volcano is inferred to be the result of a small-volume, short-lived (days to weeks) monogenetic eruption. The eruptive processes of each Igwisi volcano were broadly similar and developed through three phases: (1) fallout of lithic-bearing pyroclastic rocks during explosive excavation of craters and conduits; (2) fallout of juvenile lapilli from unsteady eruption columns and the construction of pyroclastic edifices around the vent; and (3) effusion of degassed viscous magma as lava flows. These processes are similar to those observed for other small-volume monogenetic eruptions (e.g. of basaltic magma).  相似文献   

11.
Sinker Butte is the erosional remnant of a very large basaltic tuff cone of middle Pleistocene age located at the southern edge of the western Snake River Plain. Phreatomagmatic tephras are exposed in complete sections up to 100 m thick in the walls of the Snake River Canyon, creating an unusual opportunity to study the deposits produced by this volcano through its entire sequence of explosive eruptions. The main objectives of the study were to determine the overall evolution of the Sinker Butte volcano while focusing particularly on the tephras produced by its phreatomagmatic eruptions. Toward this end, twenty-three detailed stratigraphic sections ranging from 20 to 100 m thick were examined and measured in canyon walls exposing tephras deposited around 180° of the circumference of the volcano.Three main rock units are recognized in canyon walls at Sinker Butte: a lower sequence composed of numerous thin basaltic lava flows, an intermediate sequence of phreatomagmatic tephras, and a capping sequence of welded basaltic spatter and more lava flows. We subdivide the phreatomagmatic deposits into two main parts, a series of reworked, mostly subaqueously deposited tephras and a more voluminous sequence of overlying subaerial surge and fall deposits. Most of the reworked deposits are gray in color and exhibit features such as channel scour and fill, planar-stratification, high and low angle cross-stratification, trough cross-stratification, and Bouma-turbidite sequences consistent with their being deposited in shallow standing water or in braided streams. The overlying subaerial deposits are commonly brown or orange in color due to palagonitization. They display a wide variety of bedding types and sedimentary structures consistent with deposition by base surges, wet to dry pyroclastic fall events, and water saturated debris flows.Proximal sections through the subaerial tephras exhibit large regressive cross-strata, planar bedding, and bomb sags suggesting deposition by wet base surges and tephra fallout. Medial and distal deposits consist of a thick sequence of well-bedded tephras; however, the cross-stratified base-surge deposits are thinner and interbedded within the fallout deposits. The average wavelength and amplitude of the cross strata continue to decrease with distance from the vent. These bedded surge and fall deposits grade upward into dominantly fall deposits containing 75–95% juvenile vesiculated clasts and localized layers of welded spatter, indicating a greatly reduced water-melt ratio. Overlying these “dryer” deposits are massive tuff breccias that were probably deposited as water saturated debris flows (lahars). The first appearance of rounded river gravels in these massive tuff breccias indicates downward coring of the diatreme and entrainment of country rock from lower in the stratigraphic section. The “wetter” nature of these deposits suggests a renewed source of external water. The massive deposits grade upward into wet fallout tephras and the phreatomagmatic sequence ends with a dry scoria fall deposit overlain by welded spatter and lava flows.Field observations and two new 40Ar–39Ar incremental heating dates suggest the succession of lavas and tephra deposits exposed in this part of the Snake River canyon may all have been erupted from a closely related complex of vents at Sinker Butte. We propose that initial eruptions of lava flows built a small shield edifice that dammed or disrupted the flow of the ancestral Snake River. The shift from effusive to explosive eruptions occurred when the surface water or rising ground water gained access to the vent. As the river cut a new channel around the lava dam, water levels dropped and the volcano returned to an effusive style of eruption.  相似文献   

12.
Monogenetic basaltic volcanoes are the most common volcanic landforms on the continents. They encompass a range of morphologies from small pyroclastic constructs to larger shields and reflect a wide range of eruptive processes. This paper reviews physical volcanological aspects of continental basaltic eruptions that are driven primarily by magmatic volatiles. Explosive eruption styles include Hawaiian and Strombolian (sensu stricto) and violent Strombolian end members, and a full spectrum of styles that are transitional between these end members. The end-member explosive styles generate characteristic facies within the resulting pyroclastic constructs (proximal) and beyond in tephra fall deposits (medial to distal). Explosive and effusive behavior can be simultaneous from the same conduit system and is a complex function of composition, ascent rate, degassing, and multiphase processes. Lavas are produced by direct effusion from central vents and fissures or from breakouts (boccas, located along cone slopes or at the base of a cone or rampart) that are controlled by varying combinations of cone structure, feeder dike processes, local effusion rate and topography. Clastogenic lavas are also produced by rapid accumulation of hot material from a pyroclastic column, or by more gradual welding and collapse of a pyroclastic edifice shortly after eruptions. Lava flows interact with — and counteract — cone building through the process of rafting. Eruption processes are closely coupled to shallow magma ascent dynamics, which in turn are variably controlled by pre-existing structures and interaction of the rising magmatic mixture with wall rocks. Locations and length scales of shallow intrusive features can be related to deeper length scales within the magma source zone in the mantle. Coupling between tectonic forces, magma mass flux, and heat flow range from weak (low magma flux basaltic fields) to sufficiently strong that some basaltic fields produce polygenetic composite volcanoes with more evolved compositions. Throughout the paper we identify key problems where additional research will help to advance our overall understanding of this important type of volcanism.  相似文献   

13.
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   

14.
In this paper we present a model for the growth of a maar-diatreme complex in a shallow marine environment. The Miocene-age Costa Giardini diatreme near Sortino, in the region of the Iblei Mountains of southern Sicily, has an outer tuff ring formed by the accumulation of debris flows and surge deposits during hydromagmatic eruptions. Vesicular lava clasts, accretionary lapilli and bombs in the older ejecta indicate that initial eruptions were of gas-rich magma. Abundant xenoliths in the upper, late-deposited beds of the ring suggest rapid magma ascent, and deepening of the eruptive vent is shown by the change in slope of the country rock. The interior of the diatreme contains nonbedded breccia composed of both volcanic and country rock clasts of variable size and amount. The occurrence of bedded hyaloclastite breccia in an isolated outcrop in the middle-lower part of the diatreme suggests subaqueous effusion at a low rate following the end of explosive activity. Intrusions of nonvesicular magma, forming plugs and dikes, occur on the western side of the diatreme, and at the margins, close to the contact between breccia deposits and country rock; they indicate involvement of volatile-poor magma, possibly during late stages of activity. We propose that initial hydromagmatic explosive activity occurred in a shallow marine environment and the ejecta created a rampart that isolated for a short time the inner crater from the surrounding marine environment. This allowed explosive activity to draw down the water table in the vicinity of the vent and caused deepening of the explosive center. A subsequent decrease in the effusion rate and cessation of explosive eruptions allowed the crater to refill with water, at which time the hyaloclastite was deposited. Emplacement of dikes and plugs occurred nonexplosively while the breccia sediment was mostly still soft and unconsolidated, locally forming peperites. The sheltered, low-energy lagoon filled with marine limestones mixed with volcaniclastic material eroded from the surrounding ramparts. Ultimately, lagoonal sediments accumulated in the crater until subsidence or erosion of the tuff ring caused a return to normal shallow marine conditions.  相似文献   

15.
The 2002–03 flank eruption of Etna was characterized by two months of explosive activity that produced copious ash fallout, constituting a major source of hazard and damage over all eastern Sicily. Most of the tephra were erupted from vents at 2750 and 2800 m elevation on the S flank of the volcano, where different eruptive styles alternated. The dominant style of explosive activity consisted of discrete to pulsing magma jets mounted by wide ash plumes, which we refer to as ash-rich jets and plumes. Similarly, ash-rich explosive activity was also briefly observed during the 2001 flank eruption of Etna, but is otherwise fairly uncommon in the recent history of Etna. Here, we describe the features of the 2002–03 explosive activity and compare it with the 2001 eruption in order to characterize ash-rich jets and plumes and their transition with other eruptive styles, including Strombolian and ash explosions, mainly through chemical, componentry and morphology investigations of erupted ash. Past models explain the transition between different styles of basaltic explosive activity only in terms of flow conditions of gas and liquid. Our findings suggest that the abundant presence of a solid phase (microlites) may also control vent degassing and consequent magma fragmentation and eruptive style. In fact, in contrast with the Strombolian or Hawaiian microlite-poor, fluidal, sideromelane clasts, ash-rich jets and plumes produce crystal-rich tachylite clasts with evidence of brittle fragmentation, suggesting that high groundmass crystallinity of the very top part of the magma column may reduce bubble movement while increasing fragmentation efficiency.  相似文献   

16.
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17.
The ～5 ka Mt. Gambier Volcanic Complex in the Newer Volcanics Province, Australia is an extremely complex monogenetic, volcanic system that preserves at least 14 eruption points aligned along a fissure system. The complex stratigraphy can be subdivided into six main facies that record alternations between magmatic and phreatomagmatic eruption styles in a random manner. The facies are (1) coherent to vesicular fragmental alkali basalt (effusive/Hawaiian spatter and lava flows); (2) massive scoriaceous fine lapilli with coarse ash (Strombolian fallout); (3) bedded scoriaceous fine lapilli tuff (violent Strombolian fallout); (4) thin–medium bedded, undulating very fine lapilli in coarse ash (dry phreatomagmatic surge-modified fallout); (5) palagonite-altered, cross-bedded, medium lapilli to fine ash (wet phreatomagmatic base surges); and (6) massive, palagonite-altered, very poorly sorted tuff breccia and lapilli tuff (phreato-Vulcanian pyroclastic flows). Since most deposits are lithified, to quantify the grain size distributions (GSDs), image analysis was performed. The facies are distinct based on their GSDs and the fine ash to coarse+fine ash ratios. These provide insights into the fragmentation intensities and water–magma interaction efficiencies for each facies. The eruption chronology indicates a random spatial and temporal sequence of occurrence of eruption styles, except for a “magmatic horizon” of effusive activity occurring at both ends of the volcanic complex simultaneously. The eruption foci are located along NW–SE trending lineaments, indicating that the complex was fed by multiple dykes following the subsurface structures related to the Tartwaup Fault System. Possible factors causing vent migration along these dykes and changes in eruption styles include differences in magma ascent rates, viscosity, crystallinity, degassing and magma discharge rate, as well as hydrological parameters.  相似文献   

18.
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   

19.
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   

20.
In the mid-fifteenth century, one of the largest eruptions of the last 10 000 years occurred in the Central New Hebrides arc, forming the Kuwae caldera (12x6 km). This eruption followed a late maar phase in the pre-caldera edifice, responsible for a series of alternating hydromagmatic deposits and airfall lapilli layers. Tuffs related to caldera formation ( 120 m of deposits on a composite section from the caldera wall) were emitted during two main ignimbritic phases associated with two additional hydromagmatic episodes. The lower hydromagmatic tuffs from the precaldera maar phase are mainly basaltic andesite in composition, but clasts show compositions ranging from 48 to 60% SiO2. The unwelded and welded ashflow deposits from the ignimbritic phases and the associated intermediate and upper hydromagmatic deposits also show a wide compositional range (60–73% SiO2), but are dominantly dacitic. This broad compositional range is thought to be due to crystal fractionation. The striking evolution from one eruptive style (hydromagmatic) to the other (magmatic with emission of a large volume of ignimbrites) which occurred either over the tuff series as a whole, or at the beginning of each ignimbritic phase, is the most impressive characteristic of the caldera-forming event. This strongly suggests triggering of the main eruptive phases by magma-water interaction. A three-step model of caldera formation is presented: (1) moderate hydromagmatic (sequences HD 1–4) and magmatic (fallout deposits) activity from a central vent, probably over a period of months or years, affected an area slightly wider than the present caldera. At the end of this stage, intense seismic activity and extrusion of differentiated magma outside the caldera area occurred; (2) unhomogenized dacite was released during a hydromagmatic episode (HD 5). This was immediately followed by two major pyroclastic flows (PFD 1 and 2). The vents spread and intense magma-water interaction at the beginning of this stage decreased rapidly as magma discharge increased. Subsequent collapse of the caldera probably commenced in the southeastern sector of the caldera; (3) dacitic welded tuffs were emplaced during a second main phase (WFD 1–5). At the beginning of this phase, magma-water interaction continued, producing typical hydromagmatic deposits (HD 6). Caldera collapse extended to the northern part of the caldera. Previous C14 dates and records of explosive volcanism in ice from the south Pole show that the climactic phase of this event occurred in 1452 A.D.  相似文献   

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