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1.
A study of pyroclastic deposits from the 1815 Tambora eruption reveals two distinct phases of activity, i.e., four initial tephra falls followed by generation of pyroclastic flows and the production of major co-ignimbrite ash fall. The first explosive event produced minor ash fall from phreatomagmatic explosions (F-1 layer). The second event was a Plinian eruption (F-2) correlated to the large explosion of 5 April 1815, which produced a column height of 33 km with an eruption rate of 1.1 × 108 kg/s. The third event occurred during the lull in major activity from 5 to 10 April and produced minor ash fall (F-3). The fourth event produced a 43-km-high Plinian eruption column with an eruption rate of 2.8 × 108 kg/s during the climax of activity on 10 April. Although very energetic, the Plinian events were of short duration (2.8 h each) and total erupted volume of the early (F-1 to F-4) fall deposits is only 1.8 km3 (DRE, dense rock equivalent). An abrupt change in style of activity occurred at end of the second Plinian event with onset of pyroclastic flow and surge generation. At least seven pyroclastic flows were generated, which spread over most of the volcano and Sanggar peninsula and entered the ocean. The volume of pyroclastic flow deposits on land is 2.6 km3 DRE. Coastal exposures show that pyroclastic flows entering the sea became highly fines depleted, resulting in mass loss of about 32%, in addition to 8% glass elutriation, as indicated by component fractionation. The subaqueous pyroclastic flows have thus lost about 40% of mass compared to the original erupted mixture. Pyroclastic flows and surges from this phase of the eruption are stratigraphically equivalent to a major ash fall deposit (F-5) present beyond the flow and surge zone at 40 km from the source and in distal areas. The F-5 fall deposit forms a larger proportion of the total tephra fall with increasing distance from source and represents about 80% of the total at a distance of 90 km and 92% of the total tephra fall from the 1815 eruption. The field relations indicate that the 20-km3 (DRE) F-5 deposit is a co-ignimbrite ash fall, generated largely during entrance of pyroclastic flows into the ocean. Based on the observed 40% fines depletion and component fractionation from the flows, the large volume of the F-5 co-ignimbrite ash requires eruption of 50 km3 (DRE, 1.4 × 1014 kg) pyroclastic flows.  相似文献   

2.
The largest Plinian eruption of our era and the latest caldera-forming eruption in the Kuril-Kamchatka region occurred about cal. A.D. 240 from the Ksudach volcano. This catastrophic explosive eruption was similar in type and characteristics to the 1883 Krakatau event. The volume of material ejected was 18–19 km3 (8 km3 DRE), including 15 km3 of tephra fall and 3–4 km3 of pyroclastic flows. The estimated height of eruptive column is 22–30 km. A collapse caldera resulting from this eruption was 4 × 6.5 km in size with a cavity volume of 6.5–7 km3. Tephra fall was deposited to the north of the volcano and reached more than 1000 km. Pyroclastic flows accompanied by ash-cloud pyroclastic surges extended out to 20 km. The eruption was initially phreatomagmatic and then became rhythmic, with each pulse evolving from pumice falls to pyroclastic flows. Erupted products were dominantly rhyodacite throughout the eruption. During the post-caldera stage, when the Shtyubel cone started to form within the caldera, basaltic-andesite and andesite magma began to effuse. The trigger for the eruption may have been an intrusion of mafic magma into the rhyodacite reservoir. The eruption had substantial environmental impact and may have produced a large acidity peak in the Greenland ice sheet.  相似文献   

3.
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   

4.
Lithic-rich breccias are described from within a sequence of young (2000–3000 yrs B.P.) scoria and ash flow deposits erupted from Mount Misery and an older pumice and ash flow deposit (ignimbrite) on St. Kitts. Cross sections constructed through pyroclastic flow fans in well-exposed sea cliffs 4–6 km from the vent show that the lithic breccias are lensoid deposits which seem to occur as channel-shaped accumulations (up to > 20 m thick and > 150 m wide) within flow units. The best-developed example infills a deeply incised channel cut into older flow units. The coarsest lithic breccias are clast supported and fines depleted and grade laterally and vertically through finer-grained, matrix-supported breccias into scoria and ash flow deposits. Coarse scoria-concentration zones mainly occur at the tops of scoria and ash flow units but also at the bases, and gas-segregation pipes are common. The lithic breccias are a type of body-concentration deposit as they pass laterally into normal scoria and ash flow deposits and, where best developed, clearly occur above a reversely graded basal shear zone or layer. Grain-size studies indicate the lithic breccias and parent flows are strongly fines depleted and were highly fluidized. We suggest this may be a feature of many Lesser Antillean pyroclastic flows because of increased turbulence-induced fluidization resulting from a high degree of surface roughness caused by the steep (up to 40 °) irregular slopes, densely vegetated sinuous gullies of the tropical volcanoes, and ingestion and ignition of large amounts of lush vegetation. Accumulation of batches of lithics concentrated in the highly fluidized flows began at the break in slope where flows moved from gullies across hydraulic jumps onto the outer coastal flanks. The accumulations of breccias continued to move and be channelled down the central parts of the flows. Initially, on crossing onto the lower slopes, some of these flows seem to have had very powerfully erosive, nondepositional heads, and in the extreme example a deep channel as long as 1–2 km may have cut through underlying flow units at least as far as the present coastline. Much of the overriding remainder of the flow then drained away laterally. Thin, fine-grained ash flow deposits may form a marginal overbank facies to the pyroclastic flow fans.  相似文献   

5.
The 18–24 January 1913 eruption of Colima Volcano consisted of three eruptive phases that produced a complex sequence of tephra fall, pyroclastic surges and pyroclastic flows, with a total volume of 1.1 km3 (0.31 km3 DRE). Among these events, the pyroclastic flows are most interesting because their generation mechanisms changed with time. They started with gravitanional dome collapse (block-and-ash flow deposits, Merapi-type), changed to dome collapse triggered by a Vulcanian explosion (block-and-ash flow deposits, Soufrière-type), then ended with the partial collapse of a Plinian column (ash-flow deposits rich in pumice or scoria,). The best exposures of these deposits occur in the southern gullies of the volcano where Heim Coefficients (H/L) were obtained for the various types of flows. Average H/L values of these deposits varied from 0.40 for the Merapi-type (similar to the block-and-ash flow deposits produced during the 1991 and 1994 eruptions), 0.26 for the Soufrière-type events, and 0.17–0.26 for the column collapse ash flows. Additionally, the information of 1991, 1994 and 1998–1999 pyroclastic flow events was used to delimit hazard zones. In order to reconstruct the paths, velocities, and extents of the 20th Century pyroclastic flows, a series of computer simulations were conducted using the program FLOW3D with appropriate Heim coefficients and apparent viscosities. The model results provide a basis for estimating the areas and levels of hazard that could be associated with the next probable worst-case scenario eruption of the volcano. Three areas were traced according to the degree of hazard and pyroclastic flow type recurrence through time. Zone 1 has the largest probability to be reached by short runout (<5 km) Merapi and Soufrière pyroclastic flows, that have occurred every 3 years during the last decade. Zone 2 might be affected by Soufriere-type pyroclastic flows (∼9 km long) similar to those produced during phase II of the 1913 eruption. Zone 3 will only be affected by pyroclastic flows (∼15 km long) formed by the collapse of a Plinian eruptive column, like that of the 1913 climactic eruption. Today, an eruption of the same magnitude as that of 1913 would affect about 15,000 inhabitants of small villages, ranches and towns located within 15 km south of the volcano. Such towns include Yerbabuena, and Becerrera in the State of Colima, and Tonila, San Marcos, Cofradia, and Juan Barragán in the State of Jalisco.  相似文献   

6.
An extremely large magnitude eruption of the Ebisutoge-Fukuda tephra, close to the Plio-Pleistocene boundary, central Japan, spread volcanic materials widely more than 290,000 km2 reaching more than 300 km from the probable source. Characteristics of the distal air-fall ash (>150 km away from the vent) and proximal pyroclastic deposits are clarified to constrain the eruptive style, history, and magnitude of the Ebisutoge-Fukuda eruption.Eruptive history had five phases. Phase 1 is phreatoplinian eruption producing >105 km3 of volcanic materials. Phases 2 and 3 are plinian eruption and transition to pyroclastic flow. Plinian activity also occurred in phase 4, which ejected conspicuous obsidian fragments to the distal locations. In phase 5, collapse of eruption column triggered by phase 4, generated large pyroclastic flow in all directions and resulted in more than 250–350 km3 of deposits. Thus, the total volume of this tephra amounts over 380–490 km3. This indicates that the Volcanic Explosivity Index (VEI) of the Ebisutoge-Fukuda tephra is greater than 7. The huge thickness of reworked volcaniclastic deposits overlying the fall units also attests to the tremendous volume of eruptive materials of this tephra.Numerous ancient tephra layers with large volume have been reported worldwide, but sources and eruptive history are often unknown and difficult to determine. Comparison of distal air-fall ashes with proximal pyroclastic deposits revealed eruption style, history and magnitude of the Ebisutoge-Fukuda tephra. Hence, recognition of the Ebisutoge-Fukuda tephra, is useful for understanding the volcanic activity during the Pliocene to Pleistocene, is important as a boundary marker bed, and can be used to interpret the global environmental and climatic impact of large magnitude eruptions in the past.  相似文献   

7.
The Pucón eruption was the largest Holocene explosive outburst of Volcán Villarrica, Chile. It discharged >1.0 km3 of basaltic-andesite magma and >0.8 km3 of pre-existing rock, forming a thin scoria-fall deposit overlain by voluminous ignimbrite intercalated with pyroclastic surge beds. The deposits are up to 70 m thick and are preserved up to 21 km from the present-day summit, post-eruptive lahar deposits extending farther. Two ignimbrite units are distinguished: a lower one (P1) in which all accidental lithic clasts are of volcanic origin and an upper unit (P2) in which basement granitoids also occur, both as free clasts and as xenoliths in scoria. P2 accounts for ∼80% of the erupted products. Following the initial scoria fallout phase, P1 pyroclastic flows swept down the northern and western flanks of the volcano, magma fragmentation during this phase being confined to within the volcanic edifice. Following a pause of at least a couple of days sufficient for wood devolatilization, eruption recommenced, the fragmentation level dropped to within the granitoid basement, and the pyroclastic flows of P2 were erupted. The first P2 flow had a highly turbulent front, laid down ignimbrite with large-scale cross-stratification and regressive bedforms, and sheared the ground; flow then waned and became confined to the southeastern flank. Following emplacement of pyroclastic surge deposits all across the volcano, the eruption terminated with pyroclastic flows down the northern flank. Multiple lahars were generated prior to the onset of a new eruptive cycle. Charcoal samples yield a probable eruption age of 3,510 ± 60 14C years BP.  相似文献   

8.
Peak eruption column heights for the B1, B2, B3 and B4 units of the May 18, 1980 fall deposit from Mount St. Helens have been determined from pumice and lithic clast sizes and models of tephra dispersal. Column heights determined from the fall deposit agree well with those determined by radar measurements. B1 and B2 units were derived from plinian activity between 0900 and about 1215 hrs. B3 was formed by fallout of tephra from plumes that rose off pyroclastic flows from about 1215 to 1630 hrs. A brief return to plinian activity between 1630 and 1715 hrs was marked by a maximum in column height (19 km) during deposition of B4.Variations in magma discharge during the eruption have been reconstructed from modelling of column height during plinian discharge and mass-balance calculations based on the volume of pyroclastic flows and coignimbrite ash. Peak magma discharge occurred during the period 1215–1630 hrs, when pyroclastic flows were generated by collapse of low fountains through the crater breach. Pyroclastic flow deposits and the widely dispersed co-ignimbrite ash account for 77% of the total erupted mass, with only 23% derived from plinian discharge.A shift in eruptive style at noon on May 18 may have been associated with increase in magma discharge and the eruption of silicic andesite mingled with the dominant mafic dacite. Increasing abundance of the silicic andesite during the period of highest magma discharge is consistent with the draw-up and tapping of deeper levels in the magma reservoir, as predicted by theoretical models of magma withdrawal. Return to plinian activity late in the afternoon, when magma discharge decreased, is consistent with theoretical predictions of eruption column behavior. The dominant generation of pyroclastic flows during the May 18 eruption can be attributed to the low bulk volatile content of the magma and the increasing magma discharge that resulted in the transition from a stable, convective eruption column to a collapsing one.  相似文献   

9.
The Pagosa Peak Dacite is an unusual pyroclastic deposit that immediately predated eruption of the enormous Fish Canyon Tuff (5000 km3) from the La Garita caldera at 28 Ma. The Pagosa Peak Dacite is thick (to 1 km), voluminous (>200 km3), and has a high aspect ratio (1:50) similar to those of silicic lava flows. It contains a high proportion (40–60%) of juvenile clasts (to 3–4 m) emplaced as viscous magma that was less vesiculated than typical pumice. Accidental lithic fragments are absent above the basal 5–10% of the unit. Thick densely welded proximal deposits flowed rheomorphically due to gravitational spreading, despite the very high viscosity of the crystal-rich magma, resulting in a macroscopic appearance similar to flow-layered silicic lava. Although it is a separate depositional unit, the Pagosa Peak Dacite is indistinguishable from the overlying Fish Canyon Tuff in bulk-rock chemistry, phenocryst compositions, and 40Ar/39Ar age.The unusual characteristics of this deposit are interpreted as consequences of eruption by low-column pyroclastic fountaining and lateral transport as dense, poorly inflated pyroclastic flows. The inferred eruptive style may be in part related to synchronous disruption of the southern margin of the Fish Canyon magma chamber by block faulting. The Pagosa Peak eruptive sources are apparently buried in the southern La Garita caldera, where northerly extensions of observed syneruptive faults served as fissure vents. Cumulative vent cross-sections were large, leading to relatively low emission velocities for a given discharge rate. Many successive pyroclastic flows accumulated sufficiently rapidly to weld densely as a cooling unit up to 1000 m thick and to retain heat adequately to permit rheomorphic flow. Explosive potential of the magma may have been reduced by degassing during ascent through fissure conduits, leading to fracture-dominated magma fragmentation at low vesicularity. Subsequent collapse of the 75×35 km2 La Garita caldera and eruption of the Fish Canyon Tuff were probably triggered by destabilization of the chamber roof as magma was withdrawn during the Pagosa Peak eruption.  相似文献   

10.
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   

11.
 Non-welded, lithic-rich ignimbrites, hereintermed the Roque Nublo ignimbrites, are the most distinctive deposits of the Pliocene Roque Nublo group, which forms the products of second magmatic cycle on Gran Canaria. They are very heterogeneous, with 35–55% volume lithic fragments, 15-30% mildly vesiculated pumice, 5–7% crystals and 20–30% ash matrix. The vitric components (pumice fragments and ash matrix) are largely altered and transformed into zeolites and subordinate smectites. The Roque Nublo ignimbrites originated from hydrovolcanic eruptions that caused rapid and significant erosion of vents thus incorporating a high proportion of lithic clasts into the eruption columns. These columns rapidly became too dense to be sustained as vertical eruption columns and were transformed into tephra fountains which fed high-density pyroclastic flows. The deposits from these flows were mainly confined to palaeovalleys and topographic depressions. In distal areas close to the coast line, where these palaeovalleys widened, most of the pyroclastic flows expanded laterally and formed numerous thin flow units. The combined effect of the magma–water interaction and the high content of lithic fragments is sufficient to explain the characteristic low emplacement temperature of the Roque Nublo ignimbrites. This fact also explains the transition from pyroclastic flows into lahar deposits observed in distal facies of the Roque Nublo ignimbrites. The existence of hydrovolcanic eruptions generating high-density pyroclastic flows, unable to efficiently separate the water vapour from the vitric components during transport, also accounts for the intense zeolitic alteration in these deposits. Received: 5 November 1996 / Accepted: 3 March 1997  相似文献   

12.
Young pumice deposits on Nisyros,Greece   总被引：1，自引：1，他引：1  
The island of Nisyros (Aegean Sea) consists of a silicic volcanic sequence upon a base of mafic-andesitic hyaloclastites, lava flows, and breccias. We distinguish two young silicic eruptive cycles each consisting of an explosive phase followed by effusions, and an older silicic complex with major pyroclastic deposits. The caldera that formed after the last plinian eruption is partially filled with dacitic domes. Each of the two youngest plinian pumice falls has an approximate DRE volume of 2–3 km3 and calculated eruption column heights of about 15–20 km. The youngest pumice unit is a fall-surge-flow-surge sequence. Laterally transitional fall and surge facies, as well as distinct polymodal grainsize distributions in the basal fall layer, indicate coeval deposition from a maintained plume and surges. Planar-bedded pumice units on top of the fall layer were deposited from high-energy, dry-steam propelled surges and grade laterally into cross-bedded, finegrained surge deposits. The change from a fall-to a surge/flow-dominated depositional regime coincided with a trend from low-temperature argillitic lithics to high-temperature, epidote-and diopside-bearing lithic clasts, indicating the break-up of a high-temperature geothermal reservoir after the plinian phase. The transition from a maintained plume to a surge/ash flow depositional regime occurred most likely during break-up of the high-temperature geothermal reservoir during chaotic caldera collapse. The upper surge units were possibly erupted through the newly formed ringfracture.  相似文献   

13.
The explosive rhyolitic eruption of Öræfajökull volcano, Iceland, in AD 1362 is described and interpreted based on the sequence of pyroclastic fall and flow deposits at 10 proximal locations around the south side of the volcano. Öræfajökull is an ice-clad stratovolcano in south central Iceland which has an ice-filled caldera (4–5 km diameter) of uncertain origin. The main phase of the eruption took place over a few days in June and proceeded in three main phases that produced widely dispersed fallout deposits and a pyroclastic flow deposit. An initial phase of phreatomagmatic eruptive activity produced a volumetrically minor, coarse ash fall deposit (unit A) with a bi-lobate dispersal. This was followed by a second phreatomagmatic, possibly phreatoplinian, phase that deposited more fine ash beds (unit B), dispersed to the SSE. Phases A and B were followed by an intense, climactic Plinian phase that lasted ∼ 8–12 h and produced unit C, a coarse-lapilli, pumice-clast-dominated fall deposit in the proximal region. At the end of Plinian activity, pyroclastic flows formed a poorly-sorted deposit, unit D, presently of very limited thickness and exposed distribution. Much of Eastern Iceland is covered with a very fine distal ash layer, dispersed to the NE. This was probably deposited from an umbrella cloud and is the distal representation of the Plinian fallout. A total bulk fall deposit volume of ∼ 2.3 km3 is calculated (∼ 1.2 km3 DRE). Pyroclastic flow deposit volumes have been crudely estimated to be < 0.1 km3. Maximum clast size data interpreted by 1-D models suggests an eruption column ∼ 30 km high and mass discharge rates of ∼ 108 kg s− 1. Ash fall may have taken place from heights around 15 km, above the local tropopause (∼ 10 km), with coarser clasts dispersed below that under a different wind regime. Analyses of glass inclusions and matrix glasses suggest that the syn-eruptive SO2 release was only ∼ 1 Mt. This result is supported by published Greenland ice-core acidity peak data that also suggest very minor sulphate deposition and thus SO2 release. The small sulphur release reflects the low sulphur solubility in the 1362 rhyolitic melt. The low tropopause over Iceland and the 30-km-high eruption column certainly led to stratospheric injection of gas and ash but little sulphate aerosol was generated. Moreover, pre-eruptive and degassed halogen concentrations (Cl, F) indicate that these volatiles were not efficiently released during the eruption. Besides the local pyroclastic flow (and related lahar) hazard, the impact of the Öræfajökull 1362 eruption was perhaps restricted to widespread ash fall across Eastern Iceland and parts of northern Europe.  相似文献   

14.
Pyroclastic deposits from the 1883 eruption of Krakatau are described from areas northeast of the volcano on the islands of Sebesi, Sebuku, and Lagoendi, and the southeast coast of Sumatra. Massive and poorly stratified units formed predominantly from pyroclastic flows and surges that traveled over the sea for distances up to 80 km. Granulometric and lithologic characteristics of the deposits indicate that they represent the complement of proximal subaerial and submarine pyroclastic flow deposits laid down on and close to the Krakatau islands. The distal deposits exhibit a decrease in sorting coefficient, median grain size, and thickness with increasing distance from Krakatau. Crystal fractionation is consistent with the distal facies being derived from the upper part of gravitationally segregated pyroclastic flows in which the relative amount of crystal enrichment and abundance of dense lithic clasts diminished upwards. The deposits are correlated to a major pyroclastic flow phase that occurred on the morning of 27 August at approximately 10 a.m. Energetic flows spread out away from the volcano at speeds in excess of 100 km/h and traveled up to 80 km from source. The flows retained temperatures high enough to burn victims on the SW coast of Sumatra. Historical accounts from ships in the Sunda Straits constrain the area affected by the flows to a minimum of 4x103 km2. At the distal edge of this area the flows were relatively dilute and turbulent, yet carried enough material to deposit several tens of centimeters of tephra. The great mobility of the Krakatau flows from the 10 a.m. activity may be the result of enhanced runout over the sea. It is proposed that the generation of steam at the flow/seawater interface may have led to a reduction in the sedimentation of particles and consequently a delay in the time before the flows ceased lateral motion and became buoyantly convective. The buoyant distal edge of these ash-and steam-laden clouds lifted off into the atmosphere, leading to cooling, condensation, and mud rain.  相似文献   

15.
Basal layered deposits of the large-volume Peach Springs Tuff occur beneath the main pyroclastic flow deposit over a minimum lateral distance of 70 km in northwestern Arizona (USA). The basal deposits are interpreted to record initial blasting and pyroclastic surge events at the beginning of the eruption; the pyroclastic surges traveled a minimum of 100 km from the (as yet unknown) source. Changes in bedding structures with increasing flow distance are related to the decreasing sediment load of the surges. Some bed forms in the most proximal part of the study area (Kingman, Arizona) can be interpreted as being shock induced, reflecting a blast origin for the surges. Component analyses support a hydrovolcanic origin for some of the blasting and subsequent pyroclastic surges. The eruption apparently began with magmatic blasts, which were replaced by hydrovolcanic blasts. Hydrovolcanic activity may be partially related to failure of the conduit walls that temporarily plugged the vent. A single large-volume pyroclastic flow immediately followed the blast phase, and no evidence has been observed for a Plinian eruption column. The stratigraphic sequence indicates that powerful hydrovolcanic blasting rapidly widened the vent, thus bypassing a Plinian fallout phase and causing rapid evolution to a collapsing eruption column. Similar processes may occur in other large-volume ignimbrite eruptions, which commonly lack significant Plinian fallout deposits.  相似文献   

16.
 Additional data from proximal areas enable a reconstruction of the stratigraphy and the eruptive chronology of phases III and IV of the 1982 eruption of El Chichón Volcano. Phase III began on 4 April at 0135 GMT with a powerful hydromagmatic explosion that generated radially fast-moving (∼100 ms–1) pyroclastic clouds that produced a surge deposit (S1). Due to the sudden reduction in the confining pressure the process continued by tapping of magma from a deeper source, causing a new explosion. The ejected juvenile material mixed with large amounts of fragmented dome and wall rock, which were dispersed laterally in several pulses as lithic-rich block-and-ash flow (F1). Partial evacuation of juvenile material from the magmatic system prompted the entrance of external water to generate a series of hydromagmatic explosions that dispersed moisture-rich surge clouds and small-volume block-and-ash flows (IU) up to distances of 3 km from the crater. The eruption continued by further decompression of the magmatic system, with the ensuing emission of smaller amounts of gas-rich magma which, with the strong erosion of the volcanic conduit, formed a lithic-rich Plinian column that deposited fallout layer B. Associated with the widening of the vent, an increase in the effective density of the uprising column took place, causing its collapse. Block-and-ash flows arising from the column collapse traveled along valleys as a dense laminar flow (F2). In some places, flow regime changes due to topographic obstacles promoted transformation into a turbulent surge (S2) which attained minimum velocities of approximately 77 ms–1 near the volcano. The process continued with the formation of a new column on 4 April at 1135 GMT (phase IV) that emplaced fall deposit C and was followed by hydromagmatic explosions which produced pyroclastic surges (S3). Received: 13 May 1996 / Accepted: 12 November 1996  相似文献   

17.
The submarine counterparts of late Quaternary subaerial pyroclastic flow deposits off the western flanks of Dominica, Lesser Antilles, have been investigated by 3.5 kHz seismic profiling and dredging (cruise EN20 of R/V “Endeavor”). Block-and-ash flow deposits formed by dome collapse and a welded ignimbrite from a prominent fan at Grande Savanne, Dominica. This fan can be traced underwater as a major constructional ridge (2–4 km wide and 200–400 m thick) to over 13 km offshore at a water depth of 1800 m. The submarine ridge has a volume of 14 km3 and has the characteristic morphology of a debris flow apron composed of several individual units. The evidence suggests that pyroclastic flows can move underwater without losing their essential character.  相似文献   

18.
Two extensive marine tephra layers recovered by piston coring in the western equatorial Atlantic and eastern Caribbean have been correlated by electron microprobe analyses of glass shards and mineral phases to the Pleistocene Roseau tuff on Dominica in the Lesser Antilles arc. Tephra deposition and transport to the deep sea was primarily controlled by two processes related to two different styles of eruptive activity: a plinian airfall phase and a pyroclastic flow phase. A plinian phase produced a relatively thin (1–8 cm) airfall ash layer in the western Atlantic, covering an area of 3.0 × 105 km2 with a volume of 13 km3 (tephra). The majority of the airfall tephra was transported by antitrade winds at altitudes of 6–17 km. Aeolian fractionation of crystals and glass occurred during transport resulting in an airfall deposit enriched in crystals relative to the source. Mass balance calculation based on crystal/glass fractionation indicates an additional 12 km3 of airfall tephra was deposited outside the observed fall-out envelope as dispersed ash.Discharge of pyroclastic flows into the sea along the west coast of Dominica initiated subaqueous pyroclastic debris flows which descended the steep western submarine flanks of the island. 30 km3 of tephra were deposited by this process on the floor of the Grenada Basin up to 250 km from source. The Roseau event represents the largest explosive eruption in the Lesser Antilles in the last 200,000 years and illustrates the complexity of primary volcanogenic sedimentation associated with a major explosive eruption within an island arc environment.  相似文献   

19.
The Ottaviano eruption occurred in the late neolithic (8000 y B.P.). 2.40 km3 of phonolitic pyroclastic material (0.61 km3 DRE) were emplaced as pyroclastic flow, surge and fall deposits. The eruption began with a fall phase, with a model column height of 14 km, producing a pumice fall deposit (LA). This phase ended with short-lived weak explosive activity, giving rise to a fine-grained deposit (L1), passing to pumice fall deposits as the result of an increasing column height and mass discharge rate. The subsequent two fall phases (producing LB and LC deposits), had model column heights of 20 and 22 km with eruption rates of 2.5 × 107 and 2.81 × 107 kg/s, respectively. These phases ended with the deposition of ash layers (L2 and L3), related to a decreasing, pulsing explosive activity. The values of dynamic parameters calculated for the eruption classify it as a sub-plinian event. Each fall phase was characterized by variations in the eruptive intensity, and several pyroclastic flows were emplaced (F1 to F3). Alternating pumice and ash fall beds record the waning of the eruption. Finally, owing to the collapse of a eruptive column of low gas content, the last pyroclastic flow (F4) was emplaced.  相似文献   

20.
The 227 ka Yellow Trachytic Tuff (YTT) of the Roccamonfina volcano is a multiunit ash-, pumice-, scoria- and lithic-ignimbrite with a proximal sandwave surge deposit. The YTT has an estimated volume of 0.42 km3. It erupted in the northern, subsided sector of the volcano from Gli Stagli caldera, and was channelled down ravines northward between the limestone range of M.Cesima and M. Camino that bounds the depression. Up to 5 YTT units occur close to the outer part of the northern rim of Gli Stagli. The basal four units are separated by lithic-rich marker layers which are inferred to result from gravity segregation followed by shearing. The first three units are consolidated by chabazite cementation, the fourth one is not consolidated. The uppermost unit is altered. One or two units characterize the YTT deposits in medial to distal zones. Here, the unconsolidated unit underlies the consolidated one. Absence of markers precludes correlation with proximal stratigraphy. The YTT is poorly sorted and, except the surge deposit and the altered faciés which are very fine-grained, has moderate median diameter typical of pyroclastic flows. Matrix, pumice, and scoria clasts are poorly vesicular. Matrix shards are equant, blocky-shaped, hydrated, and range from non-vesicular to vesicular. These features suggest that magma-water interaction played a role in the YTT eruption process, with some magmatic fragmentation.The complex near-Gli Stagli-rim YTT sequence could record the arrival of successive flows from the source vent, or also form by interaction of one or two flows with the caldera rim. In both cases, the absence of basal Plinian deposits in YTT units suggests that the eruptions were low pyroclastic fountains. The YTT distribution was controlled by interaction with the northern rim of Gli Stagli caldera and with the limestone range that bounds the northern depression. The near-rim stratigraphy shows the complete record of the eruption, whereas the medial to distal sequences provide only the initial pyroclastic flow possibly with the final flow spilling over the caldera rim. The proximal surge episode probably resulted from higher velocity of a later pyroclastic flow due to steeper slope of the volcano in that locality.  相似文献   

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