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1.
The origin of reverse grading in air-fall pyroclastic deposits has been ascribed to: (1) changing conditions at an erupting vent; (2) deposition in water; or (3) rolling of large clasts over smaller clasts on the surface of a steep slope. Structural features in a deposit of air-fall pumice lapilli in the Coso Range, California, indicate that reverse grading there formed by a fourth mechanism during flow of pumice. Reverse-graded beds in this deposit occur where pumice lapilli fell on slopes at or near the angle of repose and formed as parts of the blanket of accumulating pumice became unstable and flowed downslope. The process of size sorting during such flow is probably analogous to that which sorts sand grains in a reverse fashion during avalanching on the slip faces of sand dunes, attributed by Bagnold (1954a) to a grain-dispersive pressure acting on particles subjected to a shear stress. In view of the several ways in which air-fall pyroclastic debris may become reverse graded, caution is advised in interpretation of the origin of this structure both in modern and in ancient deposits.  相似文献   

2.
Several hot-rock avalanches have occurred during the growth of the composite dome of Mount St. Helens, Washington between 1980 and 1987. One of these occurred on 9 May 1986 and produced a fan-shaped avalanche deposit of juvenile dacite debris together with a more extensive pyroclastic-flow deposit. Laterally thinning deposits and abrasion and baking of wooden and plastic objects show that a hot ash-cloud surge swept beyond the limits of the pyroclastic flow. Plumes that rose 2–3 km above the dome and vitric ash that fell downwind of the volcano were also effects of this event, but no explosion occurred. All the facies observed originated from a single avalanche. Erosion and melting of craterfloor snow by the hot debris caused debris flows in the crater, and a small flood that carried juvenile and other clasts north of the crater. A second, broadly similar event occured in October 1986. Larger events of this nature could present a significant volcanic hazard.  相似文献   

3.
This study investigates the types of subaqueous deposits that occur when hot pyroclastic flows turbulently mix with water at the shoreline through field studies of the Znp marine tephra in Japan and flume experiments where hot tephra sample interacted with water. The Znp is a very thick, pumice-rich density current deposit that was sourced from subaerial pyroclastic flows entering the Japan Sea in the Pliocene. Notable characteristics are well-developed grain size and density grading (lithic-rich base, pumice-rich middle, and ash-rich top), preponderance of sedimentary lithic clasts picked up from the seafloor during transport, fine ash depletion in coarse facies, and presence of curviplanar pumice clasts. Flume experiments provide a framework for interpreting the origin and proximity to source of the Znp tephra. On contact of hot tephra sample with water, steam explosions produced a gas-supported pyroclastic density current that advanced over the water while a water-supported density current was produced on the tank floor from the base of a turbulent mixing zone. Experimental deposits comprise proximal lithic breccia, medial pumice breccia, and distal fine ash. Experiments undertaken with cold, water-saturated slurries of tephra sample and water did not produce proximal lithic breccias but a medial basal lithic breccia beneath an upper pumice breccia. Results suggest the characteristics and variations in Znp facies were strongly controlled by turbulent mixing and quenching, proximity to the shoreline, and depositional setting within the basin. Presence of abundant curviplanar pumice clasts in submarine breccias reflects brittle fracture and dismembering that can occur during fragmentation at the vent or during quenching. Subsequent transport in water-supported pumiceous density currents preserves the fragmental textures. Careful study is needed to distinguish the products of subaerial versus subaqueous eruptions.  相似文献   

4.
A devastating pyroclastic surge and resultant lahars at Mount St. Helens on 18 May 1980 produced several catastrophic flowages into tributaries on the northeast volcano flank. The tributaries channeled the flows to Smith Creek valley, which lies within the area devastated by the surge but was unaffected by the great debris avalanche on the north flank. Stratigraphy shows that the pyroclastic surge preceded the lahars; there is no notable “wet” character to the surge deposits. Therefore the lahars must have originated as snowmelt, not as ejected water-saturated debris that segregated from the pyroclastic surge as has been inferred for other flanks of the volcano. In stratigraphic order the Smith Creek valley-floor materials comprise (1) a complex valley-bottom facies of the pyroclastic surge and a related pyroclastic flow, (2) an unusual hummocky diamict caused by complex mixing of lahars with the dry pyroclastic debris, and (3) deposits of secondary pyroclastic flows. These units are capped by silt containing accretionary lapilli, which began falling from a rapidly expanding mushroom-shaped cloud 20 minutes after the eruption's onset. The Smith Creek valley-bottom pyroclastic facies consists of (a) a weakly graded basal bed of fines-poor granular sand, the deposit of a low-concentration lithic pyroclastic surge, and (b) a bed of very poorly sorted pebble to cobble gravel inversely graded near its base, the deposit of a high-concentration lithic pyroclastic flow. The surge apparently segregated while crossing the steep headwater tributaries of Smith Creek; large fragments that settled from the turbulent surge formed a dense pyroclastic flow along the valley floor that lagged behind the front of the overland surge. The unusual hummocky diamict as thick as 15 m contains large lithic clasts supported by a tough, brown muddy sand matrix like that of lahar deposits upvalley. This unit contains irregular friable lenses and pods meters in diameter, blocks incorporated from the underlying dry and hot pyroclastic material that had been deposited only moments earlier. The hummocky unit is the deposit of a high-viscosity debris flow which formed when lahars mingled with the pyroclastic materials on Smith Creek valley floor. Overlying the debris flow are voluminous pyroclastic deposits of pebbly sand cut by fines-poor gas-escape pipes and containing charred wood. The deposits are thickest in topographic lows along margins of the hummocky diamict. Emplaced several minutes after the hot surge had passed, this is the deposit of numerous secondary pyroclastic flows derived from surge material deposited unstably on steep valley sides.  相似文献   

5.
The vesicularity, permeability, and structure of pumice clasts provide insight into conditions of vesiculation and fragmentation during Plinian fall and pyroclastic flow-producing phases of the ~7,700 cal. year B.P. climactic eruption of Mount Mazama (Crater Lake), Oregon. We show that bulk properties (vesicularity and permeability) can be correlated with internal textures and that the clast structure can be related to inferred changes in eruption conditions. The vesicularity of all pumice clasts is 75-88%, with >90% interconnected pore volume. However, pumice clasts from the Plinian fall deposits exhibit a wider vesicularity range and higher volume percentage of interconnected vesicles than do clasts from pyroclastic-flow deposits. Pumice permeabilities also differ between the two clast types, with pumice from the fall deposit having higher minimum permeabilities (~5᎒-13 m2) and a narrower permeability range (5-50᎒-13 m2) than clasts from pyroclastic-flow deposits (0.2-330᎒-13 m2). The observed permeability can be modeled to estimate average vesicle aperture radii of 1-5 µm for the fall deposit clasts and 0.25-1 µm for clasts from the pyroclastic flows. High vesicle number densities (~109 cm-3) in all clasts suggest that bubble nucleation occurred rapidly and at high supersaturations. Post-nucleation modifications to bubble populations include both bubble growth and coalescence. A single stage of bubble nucleation and growth can account for 35-60% of the vesicle population in clasts from the fall deposits, and 65-80% in pumice from pyroclastic flows. Large vesicles form a separate population which defines a power law distribution with fractal dimension D=3.3 (range 3.0-3.5). The large D value, coupled with textural evidence, suggests that the large vesicles formed primarily by coalescence. When viewed together, the bulk properties (vesicularity, permeability) and textural characteristics of all clasts indicate rapid bubble nucleation followed by bubble growth, coalescence and permeability development. This sequence of events is best explained by nucleation in response to a downward-propagating decompression wave, followed by rapid bubble growth and coalescence prior to magma disruption by fragmentation. The heterogeneity of vesicle sizes and shapes, and the absence of differential expansion across individual clasts, suggest that post-fragmentation expansion played a limited role in the development of pumice structure. The higher vesicle number densities and lower permeabilities of pyroclastic-flow clasts indicate limited coalescence and suggest that fragmentation occurred shortly after decompression. Either increased eruption velocities or increased depth of fragmentation accompanying caldera collapse could explain compression of the pre-fragmentation vesiculation interval.  相似文献   

6.
Palaeomagnetic data from lithic clasts collected at 46 sites within layers 1 and 2 of the 1.8-ka Taupo ignimbrite, New Zealand, have been used to determine the palaeotemperatures and thermal structure of the deposit on its emplacement. Equilibrium temperatures from sites less than 30–40 km from vent are 150–300 °C, whereas at greater distances site equilibrium temperatures increase up to 400–500 °C. This variation is seen in both layer 1 and 2 deposits, with values for layer 1 being somewhat cooler, and with its increase in temperature occurring at a greater distance from vent. A temperature maximum at ~50 km from vent coincides with a zone of pink thermal-oxidation colouration of pumices previously inferred to reflect higher emplacement temperatures. Additional palaeomagnetic data collected by us and others from pumice clasts show comparable temperature variations, but these temperature estimates are shown here to be due to a chemical remanence and unreliable for accurate temperature estimates. Cooler temperatures in proximal parts of the ignimbrite are consistent with admixture of >20% cold lithic clasts at source and interaction with the pre-eruption Lake Taupo. The similar, but offset, increases in equilibrium temperatures for medial and distal layers 1 and 2 are consistent with both layers being deposited from the same flow. However, any proximal deposits left by the later, hotter material must have been subsequently eroded, or be so thin that our collection failed to sample them. Radial asymmetries in equilibrium temperatures as well as other physical parameters suggest that the deposit emplacement temperature is primarily determined at source, rather than by interaction with air during transport. These data support previous interpretations that a concentrated basal flow played a dominant role in emplacement and deposition of the Taupo ignimbrite.Editorial responsibility: T. Druitt  相似文献   

7.
Very thick units of massive pumice and lithic clast-rich breccia in the Early Permian Berserker beds at Mount Chalmers, Queensland, are deposits from cold, water-supported, volcaniclastic mass flows emplaced in a below-wave base submarine setting. Adjacent to syn-volcanic andesitic and rhyolitic sills and dykes, the pumice-lithic breccia shows a well-developed eutaxitic texture. The eutaxitic foliation is parallel to intrusive contacts and extends as far as a few metres away from the contact. At these sites, pumice clasts are strongly flattened and tube vesicles within the pumice clasts are compacted and aligned parallel to the direction of flattening. Some lenticular pumice clasts contain small (2 mm), round, quartz-filled amygdales and spherulites. Further away from the sills and dykes, the pumice clasts have randomly oriented, delicate tube vesicle structure and are blocky or lensoid in shape. Round amygdales were generated by re-vesiculation of the glass and the spherulites indicate devitrification of the glass at relatively high temperatures. The eutaxitic texture is therefore attributed to re-heating and welding compaction of glassy pumice-lithic breccia close to contacts with intrusions. In cases involving sills, secondary welding along the contacts formed extensive, conformable, eutaxitic zones in the pumice-lithic breccia that could be mistaken for primary welding compaction in a hot, primary pyroclastic deposit.  相似文献   

8.
Summary The application of the progressive thermal demagnetization procedure of volcanic rock debris has been frequently used to determine the emplacement temperatures of pyroclastic deposits and thus to characterize the nature of these volcanic deposits. This debris consists of a mixture of juvenile fragments derived from the explosive fragmentation of erupting magma and an assortment of lithic clasts derived mainly from the walls of a volcanic conduit, as well as from the ground. The temperature at which the clasts were deposited can be estimated by analyzing its remanent magnetization. To do this, oriented samples of clasts are subjected to progressive thermal demagnetization and the directions of the resulting remanent vectors provide the necessary information. Clasts of basalt, andesite, limestone, pumice and homebricks have previously been used to estimate the emplacement temperatures of pyroclastic deposits. According to our data, clasts of red sandstones also seem to be good carriers of thermoremanent magnetization. We have carried out a paleomagnetic study on a Quaternary, lithic-rich, massive, pyroclastic deposit from the Puig d'Adri volcano (Catalan Volcanic Zone), which contains a large number of red sandstone clasts. It is concluded that the studied deposit cannot be considered as a lahar or as a pyroclastic surge deposit, considering both the emplacement temperature and the morphological features.Presented at 3rd Biennial Meeting on New Trends in Geomagnetism, Smolenice Castle, West Slovakia, June 22–29, 1992  相似文献   

9.
Paleomagnetic data from lithic clasts collected from Mt. St. Helens, USA, Volcán Láscar, Chile, Volcán de Colima, Mexico and Vesuvius, Italy have been used to determine the emplacement temperature of pyroclastic deposits at these localities and to highlight the usefulness of the paleomagnetic method for determining emplacement temperatures. At Mt. St. Helens, the temperature of the deposits (T dep ) at three sites from the June 12, 1980 eruption was found to be ≥532°C, ≥509°C, and 510–570°C, respectively. One site emplaced on July 22, 1980 was emplaced at ≥577°C. These new paleomagnetic temperatures are in good agreement with previously published direct temperature measurements and paleomagnetic estimates. Lithic clasts from pyroclastic deposits from the 1993 eruption of Láscar were fully remagnetized above the respective Curie temperatures, which yielded a minimum T dep of 397°C. Samples were also collected from deposits thought to be pyroclastics from the 1913, 2004 and 2005 eruptions of Colima. At Colima, the sampled clasts were emplaced cold. This is consistent with the sampled clasts being from lahar deposits, which are common in the area, and illustrates the usefulness of the paleomagnetic method for distinguishing different types of deposit. T dep of the lower section of the lithic rich pyroclastic flow (LRPF) from the 472 A.D. deposits of Vesuvius was ~280–340°C. This is in agreement with other, recently published paleomagnetic measurements. In contrast, the upper section of the LRPF was emplaced at higher temperatures, with T dep ~520°C. This temperature difference is inferred to be the result of different sources of lithic clasts between the upper and lower sections, with the upper section containing a greater proportion of vent-derived material that was initially hot. Our studies of four historical pyroclastic deposits demonstrates the usefulness of paleomagnetism for emplacement temperature estimation.  相似文献   

10.
Previous research indicates that Yakushima Island, southwestern Japan, may have been struck by a huge tsunami before or soon after the arrival of the Koya pyroclastic flow during the 7.3 ka caldera‐forming Kikai eruption, but this has not yet been confirmed. This paper describes sedimentological and chronostratigraphic evidence showing that Unit TG, one of three gravel beds exposed on the Koseda coast of northeast Yakushima Island and investigated here, is a tsunami deposit. Unit TG is a poorly sorted, 30 cm thick gravel bed overlying a wave‐cut bench and underlying a Koya pyroclastic flow deposit. Sparse wood fragments in Unit TG were dated at 7 416–7 167 cal year BP. The constituent gravel clasts of Unit TG are similar in composition to those of modern beach and river deposits along the Koseda coast. Unit TG also contains pumice clasts whose chemistry is identical to that of pumice derived from the 7.3 ka eruption at Kikai caldera. The long‐axis orientations and composition of gravel clasts in Unit TG suggest that they were transported by a landward‐travelling high‐particle‐concentration flow, which suggests that Unit TG was deposited by a tsunami run‐up flow during the 7.3 ka Kikai caldera eruption, just before the arrival of the major Koya pyroclastic flow at the Koseda coast. Whether the 7.3 ka tsunami was caused by a volcanic eruption or an earthquake remains unclear, but Unit TG demonstrates that a tsunami arrived immediately before emplacement of a Koya pyroclastic flow.  相似文献   

11.
Thermal remanent magnetization analyses were carried out on ceramic fragments and lithic clasts embedded in the first pumice fall deposits of the Minoan eruption. The aim of this study is to estimate the equilibrium temperature after deposition of these pyroclastic fall deposits and their thermal effect on the pre-Minoan surface. A total of 30 samples from 22 independent ceramic fragments and 20 samples from 14 lithic clasts have been studied. Samples were collected from the Megalochori Quarry, located at the southern part of Santorini island. Stepwise thermal demagnetization reveals that the ceramics were mostly re-heated at temperatures around 140–180°C; in few ceramics a higher temperature component is also present, probably related to the original heating or the use of the ceramics before the eruption. Thermal demagnetization of the lithic clasts shows similar results with slightly higher re-heating temperatures, around 180–240°C. The estimated temperatures represent the equilibrium temperatures obtained after the deposition of the pumice fall and show that the pyroclastic fall deposits at a distance of around 6 km from the eruption vent maintained a temperature high enough to re-heat the buried ceramics at temperatures around 140–180°C.  相似文献   

12.
Stratigraphic studies on the active and potentially active volcanoes of the Lesser Antilles have revealed two main types of andesitic pyroclastic deposit. One with dense clasts in a poorly vesicular ash represents nuée ardente eruptions of Pelean type and the other group of vesicular pumice and ash represent both Plinian airfall and ash-pumice flow eruptions. The pumiceous deposits can be divided into airfall lapilli, airfall ash, crystal-pumice surge, ashpumice flow and ash hurricane types. No pumice eruptions have been witnessed in the Lesser Antilles during the period of written history although the stratigraphy of archaeological sites shows they occurred in pre-Columbian times. Detailed stratigraphic studies of Mt. Pelée, Martinique, and the Quill, St. Eustatius, show that, throughout their history, pumice eruptions have alternated with nuée ardente eruptions with approximately equal frequency. The widespread occurrence of pumiceous deposits on many of the West Indian volcanoes and the frequent alternations in the stratigraphic sections suggest the high probability that they will be witnessed in the future. On Martinique, some on the late prehistoric pumiceous pyroclastic flow deposits (the ash hurricanes) have been traced 20 km from the central vent to the out-skirts of Fort de France, indicating that they are the major hazard in the Lesser Antilles. Measured stratigraphic sections show that the Pelean type nuée ardente deposits are separated from the pumiceous pyroclastic deposits by others of intermediate vesicularity and appearance. The presence of such deposits of intermediate vesicularity could provide a future warning of impending change in pyroclastic style. As no such deposits formed on Mt. Pelée this century the present «safer» episode of nuée ardente (Pelean type) activity is expected to continue.  相似文献   

13.
Two groups of poorly sorted ash-rich beds, previously interpreted as rain-flushed ashes, occur in the ca. AD 180 Hatepe Plinian pumice fall deposit at Taupo volcano, New Zealand. Two ash beds with similar dispersal patterns and an aggregate thickness of up to 13 cm make up the lowermost group (A). Group A beds extend 45 km north-east of the vent and cover 290 km2. In the southern part of the group A distribution area, a coarse ash to lapilli-size Plinian pumice bed (deposit B) separates the two group A beds. The scarcity of lapilli (material seen elsewhere from the still-depositing pumice fall) in group A beds indicates that they were rapidly transported and deposited. However, this rapid transportation and deposition did not produce cross-bedding, nor did it erode the underlying deposits. It is proposed that thick (>600 m) but dilute gravity currents generated from the collapsing outer margin of the otherwise buoyant Hatepe Plinian eruption column deposited the group A beds. The upper ash beds (group C) consist of one to seven layers, attain an aggregate thickness of 35 cm, and vary considerably in thickness and number of beds with respect to distance from vent. Group C beds contain variable amounts of ash mixed with angular Plinian pumices and are genuine rain-flushed ashes. Several recent eruptions at other volcanoes (Ukinrek Maars, Vulcan, Rabaul, La Soufrère de Guadeloupe and Soufrière, St Vincent) have produced gravity currents similar in style, but much smaller than those envisaged for group A deposits. The overloaded margins of otherwise buoyant eruption plumes generated these gravity currents. Laboratory studies have produced experimental gravity current analogues. Hazards from dilute gravity currents are considerable but often overlooked, thus the recognition of gravity current deposits will contribute to more thorough volcanic hazard assessment of prehistoric eruption sequences.  相似文献   

14.
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   

15.
Thermal remanent magnetism provides a method of quantitatively determining the emplacement temperature of individual lithic clasts in a volcaniclastic rock. The technique is reviewed and applied to two types of Quaternary pyroclastic deposit on Santorini. Emplacement-temperature estimates for lithic clasts from two co-ignimbrite lithic breccias (Cape Riva and Middle Pumice eruptions) range from 250°C to 580°C, showing unambiguously that the breccias were emplaced hot. Good precision on temperature estimates (about ±20°C) were obtained from the Cape Riva breccias. Lithics in a Plinian airfall deposit from the Middle Pumice eruption give less precise results because the primary magnetisation has been partly overprinted by chemical (and/or viscous) remanence, and some clasts may have rotated during compaction of the deposit. Temperatures from proximal airfall are consistent with welding of the deposit within 1.5 km from vent. Temperature estimates for lithic clasts further from vent scatter, but a falloff of temperature away from vent can be recognised if an average emplacement temperature for the whole deposit is identified at each location. The study highlights some difficulties in interpreting quantitative temperature estimates for prehistoric pyroclastic deposits.  相似文献   

16.
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   

17.
 Pyroclastic flows generated in the 19–20 April 1993 eruption of Lascar Volcano, Chile, produced spectacular erosion features. Scree and talus were stripped from the channels and steep slopes on the flanks of the volcano. Exposed bedrock and boulders suffered severe abrasion, producing smoothed surfaces on coarse breccias and striations and percussion marks on bedrock and large boulders. Erosional furrows developed with wavelengths of 0.5–2 m and depths of 0.1–0.3 m. Furrows commonly nucleated downstream of large boulders or blocks, which are striated on the upstream side, and thereby produced crag-and-tail structures. Erosive features were produced where flows accelerated through topographic restrictions or where they moved over steep slopes. The pyroclastic flows are inferred to have segregated during movement into lithic-rich and pumice-rich parts. Lithic-rich deposits occur on slopes up to 14°, whereas pumice-rich deposits occur only on slopes less than 4°, and mainly at the margins and distal parts of the 1993 fan. The lithic-rich deposits contain large (up to 1 m) lithic clasts eroded from the substrate and transported from the vent, whereas pumice-rich deposits contain only small (typically <2 cm) lithic clasts. These observations suggest that lithic clasts segregated to the base of the flows and were responsible for much of the erosive phenomena. The erosive features, distribution of lithic clasts and deposit morphology indicate that the 1993 flows were highly concentrated avalanches dominated by particle interactions. In some places the flows slid over the bedrock causing abrasion and long striations which imply that large blocks were locked in fixed positions for periods of about 1 s. However, shorter striae at different angles, impact marks, segregation of the deposits into pumice- and lithic-rich parts, and mixing of bedrock-derived lithic clasts throughout the deposits indicate that clasts often had some freedom of movement and that jostling of particles allowed internal mixing and density segregation to occur within the flows. Received: 15 July 1996 / Accepted: 15 January 1997  相似文献   

18.
The Filakopi Pumice Breccia (FPB) is a very well exposed, Pliocene volcaniclastic unit on Milos, Greece, and has a minimum bulk volume of 1 km3. It consists of three main units: (A) basal lithic breccia (4–8 m) mainly composed of angular to subangular, andesitic and dacitic clasts up to 2.6 m in diameter; (B) very thickly bedded, poorly sorted pumice breccia (16–17 m); and (C) very thick, reversely graded, grain-supported, coarse pumice breccia (6.5–20 m), at the top. The depositional setting is well constrained as shallow marine (up to a few hundred metres) by overlying fossiliferous and bioturbated mudstone. This large volume of fine pumice clasts is interpreted to be the product of an explosive eruption from a submarine vent because: (1) pumice clasts are the dominant component; (2) the coarse pumice clasts (>64 mm) have complete quenched margins; (3) very large (>1 m) pumice clasts are common; (4) overall, the formation shows good hydraulic sorting; and (5) a significant volume of ash was deposited together with the coarsest pyroclasts.The bed forms in units A and B suggest deposition from lithic-rich and pumiceous, respectively, submarine gravity currents. In unit C, the coarse (up to 6.5 m) pumice clasts are set in matrix that grades upwards from diffusely stratified, fine (1–2 cm) pumice clasts at the base to laminated shard rich mud at the top. The coarse pumice clasts in unit C were settled from suspension and the framework was progressively infilled by fine pumice clasts from waning traction currents and then by water-settled ash. The FPB displays important features of the products of submarine explosive eruptions that result from the ambient fluid being seawater, rather than volcanic gas or air. In particular, submarine pyroclastic deposits are characterised by the presence of very coarse juvenile pumice clasts, pumice clasts with complete quenched rims, and good hydraulic sorting.Electronic Supplementary Material Supplementary material is available for this article if you access the article at . A link in the frame on the left on that page takes you directly to the supplementary material.Editorial responsibility: J. Donelly-Nolan  相似文献   

19.
 Coarse, co-ignimbrite lithic breccia, Ebx, occurs at the base of ignimbrite E, the most voluminous and widespread unit of the Kos Plateau Tuff (KPT) in Greece. Similar but generally less coarse-grained basal lithic breccias (Dbx) are also associated with the ignimbrites in the underlying D unit. Ebx shows considerable lateral variations in texture, geometry and contact relationships but is generally less than a few metres thick and comprises lithic clasts that are centimetres to a few metres in diameter in a matrix ranging from fines bearing (F2: 10 wt.%) to fines poor (F2: 0.1 wt.%). Lithic clasts are predominantly vent-derived andesite, although clasts derived locally from the underlying sedimentary formations are also present. There are no proximal exposures of KPT. There is a highly irregular lower erosional contact at the base of ignimbrite E at the closest exposures to the inferred vent, 10–14 km from the centre of the inferred source, but no Ebx was deposited. From 14 to <20 km from source, Ebx is present over a planar erosional contact. At 16 km Ebx is a 3-m-thick, coarse, fines-poor lithic breccia separated from the overlying fines-bearing, pumiceous ignimbrite by a sharp contact. This grades downcurrent into a lithic breccia that comprises a mixture of coarse lithic clasts, pumice and ash, or into a thinner one-clast-thick lithic breccia that grades upward into relatively lithic-poor, pumiceous ignimbrite. Distally, 27 to <36 km from source Ebx is a finer one-clast-thick lithic breccia that overlies a non-erosional base. A downcurrent change from strongly erosional to depositional basal contacts of Ebx dominantly reflects a depletive pyroclastic density current. Initially, the front of the flow was highly energetic and scoured tens of metres into the underlying deposits. Once deposition of the lithic clasts began, local topography influenced the geometry and distribution of Ebx, and in some cases Ebx was deposited only on topographic crests and slopes on the lee-side of ridges. The KPT ignimbrites also contain discontinuous lithic-rich layers within texturally uniform pumiceous ignimbrite. These intra-ignimbrite lithic breccias are finer grained and thinner than the basal lithic breccias and overlie non-erosional basal contacts. The proportion of fine ash within the KPT lithic breccias is heterogeneous and is attributed to a combination of fluidisation within the leading part of the flow, turbulence induced locally by interaction with topography, flushing by steam generated by passage of pyroclastic density currents over and deposition onto wet mud, and to self-fluidisation accompanying the settling of coarse, dense lithic clasts. There are problems in interpreting the KPT lithic breccias as conventional co-ignimbrite lithic breccias. These problems arise in part from the inherent assumption in conventional models that pyroclastic flows are highly concentrated, non-turbulent systems that deposit en masse. The KPT coarse basal lithic breccias are more readily interpreted in terms of aggradation from stratified, waning pyroclastic density currents and from variations in lithic clast supply from source. Received: 21 April 1997 / Accepted: 4 October 1997  相似文献   

20.
Mount St. Helens has been a prolific source of tephra-fall deposits for about 40 000 years. These tephra deposits (1) record numerous explosive eruptions, (2) form important regional time-stratigraphic marker beds, and (3) record repeated changes in composition within and between eruptive periods.Recognized tephra strata record more than 100 explosive eruptive events at Mount St. Helens; those tephra strata are classified as beds, layers, and sets. Tephra sets, each of which consists of a group of beds and layers, define in part the nine eruptive periods recognized at the volcano. Individual tephra sets are distinguished from stratigraphically adjacent sets by differences in composition or by evidence of clapsed time.Several tephra units from Mount St. Helens form important marker beds at distances of hundreds of kilometers downwind from the volcano. Cummingtonite phenocrysts, which are known in ejecta from only Mount St. Helens in the Pacific Northwest, characterize some marker beds and readily identify their source.The tephra sequence also records eruption of the mafic andesites that mark the appearance of the modern Mount St. Helens and numerous changes in composition among dacite, basalt, and andesite since that time.  相似文献   

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