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1.
The Mawson Formation and correlatives in the Transantarctic Mountains and South Africa record an early eruption episode related to the onset of Ferrar-Karoo flood basalt volcanism. Mawson Formation rocks at Coombs Hills comprise mainly (≥80% vol) structureless tuff breccia and coarse lapilli tuff cut by irregular dikes and sills, within a large vent complex (>30 km2). Quenched juvenile fragments of generally low but variable vesicularity, accretionary lapilli and country rock clasts within vent-fill, and pyroclastic density current deposits point to explosive interaction of basalt with groundwater in porous country rock and wet vent filling debris. Metre-scale dikes and pods of coherent basalt in places merge imperceptibly into peperite and then into surrounding breccia. Steeply dipping to sub-vertical depositional contacts juxtapose volcaniclastic rocks of contrasting componentry and grainsize. These sub-vertical tuff breccia zones are inferred to have formed when jets of debris + steam + water passed through unconsolidated vent-filling deposits. These jets of debris may have sometimes breached the surface to form subaerial tephra jets which fed subaerial pyroclastic density currents and fall deposits. Others, however, probably died out within vent fill before reaching the surface, allowing mixing and recycling of clasts which never reached the atmosphere. Most of the ejecta that did escape the debris-filled vents was rapidly recycled as vents broadened via lateral quarrying of country rock and bedded pyroclastic vent-rim deposits, which collapsed along the margins into individual vents. The unstratified, poorly sorted deposits comprising most of the complex are capped by tuff, lapilli tuff and tuff breccia beds inferred to have been deposited on the floor of the vent complex by pyroclastic density currents. Development of the extensive Coombs Hills vent-complex involved interaction of large volumes of magma and water. We infer that recycling of water, as well as recycling of pyroclasts, was important in maintaining water supply for phreatomagmatic interactions even when aquifer rock in the vent walls lay far from eruption sites as a consequence of vent-complex widening. The proportion of recycled water increased with vent-complex size in the same way that the proportion of recycled tephra did. Though water recycling leaves no direct rock record, the volcaniclastic deposits within the vent complex show through their lithofacies/structural architecture, lithofacies characteristics, and particle properties clear evidence for extensive and varied recycling of material as the complex evolved. Editorial responsibility: J. Donnelly-Nolan  相似文献   

2.
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   

3.
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   

4.
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   

5.
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   

6.
The edifice of Stromboli volcano gravitationally collapsed several times during its volcanic history (>100 ka–present). The largest Holocene event occurred during the final stage of the Neostromboli activity (∼13–5 ka), and was accompanied by the emplacement of phreatomagmatic and lahar deposits, known as the Secche di Lazzaro succession. A stratigraphic and paleomagnetic study of the Secche di Lazzaro deposits allows the interpretation of the emplacement and the eruptive processes. We identify three main units within the succession that correspond to changing eruption conditions. The lower unit (UA) consists of accretionary lapilli-rich, thinly bedded, parallel- to cross-stratified ash deposits, interpreted to indicate the early stages of the eruption and emplacement of dilute pyroclastic density currents. Upward, the second unit (UB) of the deposit is more massive and the beds thicker, indicating an increase in the sedimentation rate from pyroclastic density currents. The upper unit (UC) caps the succession with thick, immediately post-eruptive lahars, which reworked ash deposited on the volcano’s slope. Flow directions obtained by Anisotropy of Magnetic Susceptibility (AMS) analysis of the basal bed of UA at the type locality suggest a provenance of pyroclastic currents from the sea. This is interpreted to be related to the initial base-surges associated with water–magma interaction that occurred immediately after the lateral collapse, which wrapped around the shoulder of the sector collapse scar. Upward in the stratigraphy (upper beds of UA and UB) paleoflow directions change and show a provenance from the summit vent, probably related to the multiple collapses of a vertical, pulsatory eruptive column.  相似文献   

7.
The 1957–1958 eruption of Capelinhos, Faial island, Azores, involved three periods of surtseyan, hydromagmatic activity: two in 1957 and one in 1958. Deposits from this eruption are exposed both in sea cliffs cut into the flanks of the tuff cone and more distally >1 km from the vent. Five lithofacies are identified: lithofacies I is composed of even thickness beds with laterally continuous internal stratigraphy and is interpreted to have been formed by fallout. Lithofacies II consists of beds with internally discontinuous lenses, and has sand-wave structures that increase in abundance toward the outer margins of the tuff cone. This lithofacies is interpreted as having been deposited from pyroclastic surges. Lithofacies III is composed of mantle-bedded deposits with laterally discontinuous internal stratigraphy. This lithofacies is interpreted to have been formed by hybrid processes where fallout of tephra occurred simultaneously with pyroclastic surges. In the outer flanks of the tuff cone, lithofacies III grades laterally into fallout beds of lithofacies I. Lithofacies IV consists of alternating beds of coarse ash aggregates and non-aggregated fine ash, and is particularly well developed in distal regions. Some of this facies was formed by fallout. Alternating beds also occur plastered against obstacles up to 2 km from the vent, indicating an origin from wet pyroclastic surges. The orientation of plastered tephra indicates that the surges were deflected by topography as they decelerated. The distinction between surge and fallout in distal regions is uncertain because wind-drifted fallout and decelerating surge clouds can generate similar deposits. Lithofacies V consists of scoria lapilli beds interpreted to be fallout from hawaiian-style fire-fountaining in the later stages of the eruption. Juvenile pyroclasts within hydromagmatic deposits are predominantly poorly vesicular (25–60% of clasts <30% vesicles). However, on both micro- and macroscopic scales, there is a wide range in clast vesicularity (up to 70% vesicles) indicating that, although fragmentation was predominantly hydromagmatic, vesiculation and magmatic-volatile-driven fragmentation operated simultaneously.  相似文献   

8.
The complex eruption sequence from the ∼1000 A.D. caldera-forming eruption of Volcán Ceboruco, known as the Jala Pumice, offers an exceptional opportunity to examine how pyroclastic material is transported and deposited from pyroclastic density currents over variable topography. Three main pyroclastic surge deposits (S1, S2, and S3) and two pyroclastic flow deposits (Marquesado and North-Flank PFDs) were emplaced during this eruption. Pyroclastic surge deposits are massive, planar, or cross-bedded, poor-to-well sorted, and display fluctuations in thickness, median diameter, sorting, and lithology as a function of distance, topography, and flow dynamics. Marquesado pyroclastic flow deposits reveal lateral variations from massive, poorly sorted deposits located within 5 km of Ceboruco to planar bedded, moderately well sorted deposits located >15 km away over the nearly horizontal topography to the south of Ceboruco. North-Flank pyroclastic flow deposits also reveal lateral variations from massive, poorly sorted deposits located within 4 km of Ceboruco to planar bedded, moderately well sorted deposits located 8 km away atop an escarpment that steeply rises 230 m from the northern valley floor. Field observations, granulometric analyses, component analyses, and crystal sedimentation calculations along flow-parallel sampling transects all suggest that both surges and flows were density stratified currents, where deposition occurred from a basal region of higher particle concentration that was supplied from an overlying dilute layer that transports particles in suspension. This supports the idea of a transition between “flow” and “surge” end members with variations in particle concentration. Topography greatly affects the transport and depositional capacity of the pyroclastic density currents as a result of “blocking”, either by topographic obstacles or by abrupt breaks at the base of volcano slopes, whereas the origin of Jala Pumice surge deposits (phreatomagmatic versus magmatic) appears to have little impact on their flow dynamics. Editorial responsibility: A.W. Woods This revised version was published in February 2005 with corrections to the title. An erratum to this article is available at .  相似文献   

9.
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   

10.
The Koala kimberlite, Northwest Territories, Canada, is a small pipe-like body that was emplaced into the Archean Koala granodiorite batholith and the overlying Cretaceous to Tertiary sediments at ~53 Ma. Koala is predominantly in-filled by a series of six distinct clastic deposits, the lowermost of which has been intruded by a late stage coherent kimberlite body. The clastic facies are easily distinguished from each other by variations in texture, and in the abundance and distribution of the dominant components. From facies analysis, we infer that the pipe was initially partially filled by a massive, poorly sorted, matrix-supported, olivine-rich lapilli tuff formed from a collapsing eruption column during the waning stage of the pipe-forming eruption. This unit is overlain by a granodiorite cobble-boulder breccia and a massive, poorly sorted, mud-rich pebbly-sandstone. These deposits represent post-eruptive gravitational collapse of the unstable pipe walls and mass wasting of tephra forming the crater rim. The crater then filled with water within which ~20 m of non-kimberlitic, wood-rich, silty sand accumulated, representing up to 47,000 years of quiescence. The upper two units in the Koala pipe are both olivine rich and show distinct grain-size grading. These units are interpreted to have been deposited sub-aqueously, from pyroclastic flows sourced from one or more other kimberlite volcanoes. The uppermost units in the Koala pipe highlight the likelihood that some kimberlite pipes may be only partially filled by their own eruptive products at the cessation of volcanic activity, enabling them to act as depocentres for pyroclastic and sedimentary deposits from the surrounding volcanic landscape. Recognition of these exotic kimberlite deposits has implications for kimberlite eruption and emplacement processes.  相似文献   

11.
The tuff ring of Averno (3700 years BP) is a wide maar-type, lake-filled volcano which formed during one of the most recent explosive eruptions inside the Campi Flegrei caldera.The eruptive products consist of (a) a basal coarse unit, intercalated ballistic fallout breccia, subplinian pumice deposits and pyroclastic surge bedsets and (b) an upper fine-grained, stratified, pyroclastic surge sequence.During the deposition of the lower unit both purely magmatic (lapilli breccia) and hydromagmatic episodes (wavy and planar bedded, fine ash pyroclastic surge bedsets) coexisted. The hydromagmatic deposits exhibit both erosive and depositional features. The upper unit mostly comprises fine grained, wet pyroclastic surge deposits. The pyroclastic surges were controlled by a highly irregular pre-existing topography, produced by volcano-tectonic dislocation of older tuff rings and cones.Both the upper and lower units show decreasing depletion of fines with increasing distance from the vent. The ballistic fallout layers, however, exhibit only a weak increase in fines with distance from the vent, in spite of marked fining of the lapilli and blocks. The deposits consist dominantly of moderately to highly vesicular juvenile material, generated by primary magmatic volatile driven fragmentation followed by episodes of near-surface magma-water interaction.The evolution of the eruption toward increased fragmentation and a more hydromagmatic character may reflect that the progressive depletion in magmatic volatiles and a decrease in conduit pressure during the last stage of the eruption, possibly associated with a widening of the vent at sea level.  相似文献   

12.
Observations on phreatomagmatic ash deposits of Phlegraean Fields and Vesuvius supply evidence for the origin of vesiculated tuff in a cool environment. Early deposition by fallout of a matrix-free bed of damp accretionary lapilli is followed by deposition of cohesive mud or a mud rain. The lapilli bed becomes partly or completely transformed into a vesiculated tuff by mud percolation and eventual coalescence of accretionary lapilli with consequent trapping of air originally contained in the interstices. The proposed mechanism accounts for vesiculated tuff formation in distal deposits beyond limits commonly attained by pyroclastic surges. This same mechanism may, nevertheless, also operate in proximal tuff-ring and cone deposits during fallout of phreatomagmatic ash separating bed sets in surge-dominated successions. The sequence of events in the proposed model fits well with the evolution of a cooling phreatomagmatic ash cloud in which early ash aggregation (accretionary lapilli fallout) is followed closely by steam condensation (mud or muddy rainfall). This new model invoking a cool-temperature origin is intended to be complementary to previously proposed theories. Although difficult to assess because of the often complete obliteration of original lapilli, the process is believed to be relatively common in the generasion of vesiculated tuffs within phreatomagmatic deposits.  相似文献   

13.
The late Pleistocene San Venanzo maar and nearby Pian di Celle tuff ring in the San Venanzo area of Umbria, central Italy, appear to represent different aspects of an eruptive cycle accompanied by diatreme formation. Approximately 6x106 m3 of mostly lapillisized, juvenile ejecta with lesser amounts of lithics and 1x106 m3 of lava were erupted. The stratigraphy indicates intense explosive activity followed by lava flows and subvolcanic intrusions. The pyroclastic material includes lithic breccia derived from vent and diatreme wall erosion, roughly stratified lapilli tuff deposited by concentrated pyroclastic surge, chaotic scoriaceous pyroclastic flow and inverse graded grain-flow deposits. The key feature of the pyroclastics is the presence of concentric-shelled lapilli generated by accretion around the lithics during magma ascent in the diatreme conduits. The rock types range from kalsilite leucite olivine melilitite lavas and subvolcanic intrusions to carbonatite, phonolite and calcitic melilitite pyroclasts. Juvenile ejecta contain essential calcite whose composition and texture indicate a magmatic origin. Pyroclastic carbonatite activity is also indicated by the presence of carbonatite ash beds. The San Venanzo maar-forming event is believed to have been trigered by fluid-rich carbonatite-phonolite magma. The eruptive centre the moved to the Pian di Celle tuff ring, where the eruption of degassed olivine melilititic magma and late intrusions ended magmatic activity in the area. In both volcanoes the absence of phreatomagmatic features together with the presence of large amounts of primary calcite suggests carbonatite segregation and violent exsolution of CO2 which, flowing through the diatremes, produced the peculiar intrusive pyroclastic facies and triggered explosions.  相似文献   

14.
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   

15.
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   

16.
This paper documents a complex sequence of interbedded lapilli-fall, base-surge, and pyroclastic-flow deposits, here named the Monte Guardia sequence, that erupted from volcanic centers in the southern part of Lipari (Aeolian Island Arc). Radiocarbon data from ash-flow tuffs above and below this sequence bracket its eruption between 22,600 and 16,800 years ago. Geologic evidence, however, suggests that this single eruptive cycle had a more restricted duration of years to tens-of-years. The basis for our interpretations comes from data measured at 38 detailed sections located throughout the island. The Monte Guardia sequence rests on a series of lower rhyolitic endogenous domes in the southern part of Lipari and it covers the oldest lavas, lahars, and pyroclastic flows in the north. Only in the northeast part of the island is it covered by younger deposits which there consist of lapilli tuffs and lavas of the Monte Pilato rhyolitic cycle. The deposit ranges in thickness from more than 60 m surrounding the vents in the south to less than a few decimeters at 10 km distance in the north. Throughout most of the island the Monte Guardia sequence overlies a thin andesitic lapilli-fall layer which is a key bed for correlation. This lapilli tuff probably erupted from a volcanic center on another island of the Aeolian Arc (possibly Salina). The principal activity of the Monte Guardia sequence started with an explosion that formed a continuous breccia blanket covering most of the island. Some pumiceous blocks within this breccia are composed of alternating bands of acidic and andesitic composition suggesting that the initiation of pyroclastic activity could have been triggered by magma mixing. Typical Monte Guardia sequence consists of explosive products that grade from magmatic (pumice-fall) to phreatomagmatic (base-surge) character. The eruptive cycle is characterized by a number of energy decreasing megarhythms that start with a lapilli-fall bed and end with a base-surge set that progresses through sand-wave, massive, and planar beds. Isopach maps of the fall and surge deposits indicate that both types were directed to the northwest by prevailing winds. Existing topographic relief was an additional factor that affected the emplacement of surge products. At the end of the cycle andesitic pyroclastic flows and rhyolitic endogenous domes were emplaced above the Monte Guardia deposits near the vent.  相似文献   

17.
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   

18.
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   

19.
We present the stratigraphy, lithology, volcanology, and age of the Acahualinca section in Managua, including a famous footprint layer exposed in two museum pits. The ca. 4-m-high walls of the main northern pit (Pit I) expose excellent cross sections of Late Holocene volcaniclastic deposits in northern Managua. We have subdivided the section into six lithostratigraphic units, some of which we correlate to Late Holocene eruptions. Unit I (1.2 m thick), chiefly of hydroclastic origin, begins with the footprint layer. The bulk is dominated by mostly massive basaltic-andesitic tephra layers, interpreted to represent separate pulses of a basically phreatomagmatic eruptive episode. We correlate these deposits based on compositional and stratigraphic evidence to the Masaya Triple Layer erupted at Masaya volcano ca. 2,120 ± 120 a B.P.. The eruption occurred during the dry season. A major erosional channel unconformity up to 1 m deep in the western half of Pit I separates Units II and I. Unit II begins with basal dacitic pumice lapilli up to 10 cm thick overlain by a massive to bedded fine-grained dacitic tuff including a layer of accretionary lapilli and pockets of well-rounded pumice lapilli. Angular nonvesicular glass shards are interpreted to represent hydroclastic fragmentation. The dacitic tephra is correlated unequivocally with the ca. 1.9-ka-Plinian dacitic Chiltepe eruption. Unit III, a lithified basaltic-andesitic deposit up to 50 cm thick and extremely rich in branch molds and excellent leaf impressions, is correlated with the Masaya Tuff erupted ca. 1.8 ka ago. Unit IV, a reworked massive basaltic-andesitic deposit, rich in brown tuff clasts and well bedded and cross bedded in the northwestern corner of Pit I, cuts erosionally down as far as Unit I. A poorly defined, pale brown mass flow deposit up to 1 m thick (Unit V) is overlain by 1–1.5 m of dominantly reworked, chiefly basaltic tephra topped by soil (Unit VI). A major erosional channel carved chiefly between deposition of Units II and I may have existed as a shallow drainage channel even prior to deposition of the footprint layer. The swath of the footprints is oriented NNW, roughly parallel to, and just east of, the axis of the channel. The interpretation of the footprint layer as the initial product of a powerful eruption at Masaya volcano followed without erosional breaks by additional layers of the same eruptive phase is strong evidence that the group of 15 or 16 people tried to escape from an eruption.  相似文献   

20.
Holocene explosive activity of Hudson Volcano, southern Andes   总被引：3，自引：1，他引：2  
 Fallout deposits in the vicinity of the southern Andean Hudson Volcano record at least 12 explosive Holocene eruptions, including that of August 1991 which produced ≥4 km3 of pyroclastic material. Medial isopachs of compacted fallout deposits for two of the prehistoric Hudson eruptions, dated at approximately 3600 and 6700 BP, enclose areas at least twice that of equivalent isopachs for both the 1991 Hudson and the 1932 Quizapu eruptions, the two largest in the Andes this century. However, lack of information for either the proximal or distal tephra deposits from these two prehistoric eruptions of Hudson precludes accurate volume estimates. Andesitic pyroclastic material produced by the 6700-BP event, including a  1 10-cm-thick layer of compacted tephra that constitutes a secondary thickness maximum over 900 km to the south in Tierra del Fuego, was dispersed in a more southerly direction than that of the 1991 Hudson eruption. The products of the 6700-BP event consist of a large proportion of fine pumiceous ash and accretionary lapilli, indicating a violent phreatomagmatic eruption. This eruption, which is considered to be the largest for Hudson and possibly for any volcano in the southern Andes during the Holocene, may have created Hudson's 10-km-diameter summit caldera, but the age of the caldera has not been dated independently. Received: 31 January 1997 / Accepted: 29 October 1997  相似文献   

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