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The Campanian Ignimbrite eruption (39 ka) was the most powerful eruptive event of the Campi Flegrei caldera (southern Italy). This event coincided with the onset of a cold climatic phase and the Paleolithic transition from Neanderthals to modern humans. The eruption started with a sustained column that emplaced a widespread pyroclastic fall deposit covering an area >4000 km2 within the 15 cm isopach. For the first time, we present the complete longitudinal variation from the coarse and 10-m-thick proximal (down to 15 km) sequence, through the well-stratified pumice lapilli deposit in medial areas (30−80 km), to the distal tephra (hundreds to thousands of kilometers distant). The Plinian pumice fall deposit shows a strong lateral heterogeneity due to variations in stratification, grading, and abundance of components. All Campanian Ignimbrite fall layers display Plinian dispersal. Variation of grain size with stratigraphic height suggests that the convective plume was far from steady state. Initially, the plume dispersed 1.3 km3 of tephra toward the ENE (N75°E) from a height of 29 km (layer A). A gradual increase in intensity resulted in inverse grading of layer B. Column height increased from 26 to 37 km at a vertical velocity of 3.6 m/s. It had a main dispersal axis to the east (N97°E) and a secondary lobe to the southeast (N137°E). During this phase, a maximum volume of 1.73 km3 of tephra was emplaced. Accessory lithics concentrated in layer C are possibly due to vent clearing after partial blockage of the vent. The 33-km-high column dispersed ejecta to the east (N95°E), with only 0.2 km3 of tephra erupted during this phase. During the successive gradual decline in eruption intensity (column height decreased from 38 to 32 km at a velocity of 4.2 m/s), a normally graded layer (D) with a volume of 1 km3 accumulated to the east (N95°E). The sustained column phase ended with a pulsating and partially collapsing column that reached 23 km in height and dispersed 1.1 km3 of a stratified and lithic-rich succession (layer E) to the southeast (N112°E). If we include the distal co-Plinian deposit, a total volume of 7.8 km3 (dense rock equivalent [DRE]) of magma was released (containing 0.09 km3 of accessory lithics). Tephra mass for the single layers is on the order of 1011 kg, and the total mass is ∼2 × 1012 kg (2 × 1013 kg including the co-Plinian ash). Mass discharge rates ranged from 0.9 to 6.7 × 108 kg/s. The calculated magnitude of the sustained column phase is 6.3. The duration of the Plinian phase of this eruption, based on the ratio of two parameters, erupted mass divided by discharge rate, is estimated to have been ∼20 h (including co-Plinian ash). This study shows that Plinian deposits are not always homogeneous and, as for pyroclastic current deposits, can show an articulate architecture. Only the complete reconstruction of vertical and lateral variations in components, stratification, and grading might provide insights into the temporal and spatial evolution of the sustained plume.

A large number of people died during an explosive eruption of Kīlauea Volcano in 1790 CE. Detailed study of the upper part of the Keanakāko‘i Tephra has identified the deposits that may have been responsible for the deaths. Three successive units record shifts in eruption style that agree well with accounts of the eruption based on survivor interviews 46 yr later. First, a wet fall of very fine, accretionary-lapilli–bearing ash created a “cloud of darkness.” People walked across the soft deposit, leaving footprints as evidence. While the ash was still unconsolidated, lithic lapilli fell into it from a high eruption column that was seen from 90 km away. Either just after this tephra fall or during its latest stage, pulsing dilute pyroclastic density currents, probably products of a phreatic eruption, swept across the western flank of Kīlauea, embedding lapilli in the muddy ash and crossing the trail along which the footprints occur. The pyroclastic density currents were most likely responsible for the fatalities, as judged from the reported condition and probable location of the bodies. This reconstruction is relevant today, as similar eruptions will probably occur in the future at Kīlauea and represent its most dangerous and least predictable hazard.

Ilchulbong (Sunrise Peak) tuff cone is a United Nations Educational, Scientific, and Cultural Organization (UNESCO) World Heritage site that owes its scientific importance to the outstanding coastal exposures that surround it. It is a type locality for the sedimentary evidence of pyroclastic transport and depositional processes that occur during phreatomagmatic basaltic eruptions. Its classically shaped cone morphology was long considered to result from a single short-lived eruption. Reanalysis of the sedimentary sequence has now revealed two subtle paraconformities in the deposits, indicating that the eruption occurred in three separate phases. The break between phases I to II was "long," i.e., days to weeks, during which erosion and compaction of the lower cone occurred, leading to a volcanowide discontinuity surface and reworked deposits in the lower flanks. Possibly following cooling and solidification of the initial shallow conduit system, the phase II eruption resumed from a new vent location ∼600 m away, before being terminated by a very brief interval and final eruption of phase III from the same general site. The second pause led to a more subtle discontinuity, marked by differences in deposit character above and below it, and it was associated with a period of seismicity-induced slumping and slipping of deposits. Detailed geochemical study of the juvenile clasts through the sequence reveals that three separate alkali basaltic magma pulses were erupted, corresponding directly to the three eruption phases. Magmas 1 and 3 may be genetically related, with the former showing evidence for longer periods of shallow-level fractionation, whereas magma 2 originated from a slightly different mantle domain, although at a similar depth. These results demonstrate that breaks in monogenetic eruption sequences may represent deep magma system processes, and they raise fundamental questions concerning the mechanisms of triggering and eruption of multiple magma pulses in small-volume basaltic volcanoes. This study shows the importance of recognizing sedimentary evidence of time breaks in such eruption sequences as potential indications of shifts in eruption chemistry and vent location. These have important implications for the controls of vent migration at other monogenetic volcanoes and for hazard planning during future similar eruptions.

The most powerful category of explosive volcanic eruptions, called "ultraplinian," was proposed in 1980 on the basis of the distribution of a single pyroclastic fall product of one phase of the 1800a Taupo eruption in New Zealand. Dispersal data, a measure of the "footprint" of the deposit, were used subsequently to estimate eruption plume heights of 50–51 km, more than 10 km higher than observed or estimated for any historical Plinian eruption. Today, this unit remains the only deposit to have met the rigorous ultraplinian dispersal criteria, and it is an important and widely cited exemplar in physical volcanology. The earlier study was based on total thicknesses for the entire bed and grain-size data across that full thickness. We have now subdivided this bed into 26 subunits and measured their individual thicknesses and selected maximum clast sizes. Our data show that the apparent large footprint of this bed is an artifact of a previously unrecognized shift in the wind field during the eruption, rather than extreme eruptive vigor. Our study demonstrates the dangers of applying full-thickness approaches to even seemingly uniform fall deposits. The results throw into some doubt the need for the term ultraplinian, at least for this deposit. With the revision of plume heights for the Taupo ultraplinian, a height range of 35–40 km may be practical for use as an upper limit for source parameters in models of transport and dispersal of volcanic ash.

The size distribution of particles in pyroclastic fall deposits has been interpreted as a strong indication of efficiency of the fragmentation process during the parent explosions. Fall deposits, however, contain two distinct populations of pyroclasts: juvenile, i.e., magmatic clasts fragmented from the magma, and wall-rock particles derived from walls of the vent and volcanic conduit. While sizes and shapes of the former are always sensitive indicators of eruptive processes, properties of the wall-rock population may reflect various external influences. Here we describe how the wall-rock component of a powerful historical eruption largely inherited its size distribution from properties of the host rocks, and how this, in turn, strongly biases the entire deposit grain size distribution of the near-vent deposits. The use of wall-rock particles to infer style of fragmentation and eruption must always be coupled with other independent lines of evidence. © 2010 Geological Society of America.

The Cerro Toledo Rhyolite comprises a group of domes and tephra which were erupted during the interval between two caldera-forming ignimbrites, the Tshirege Member and Otowi Member of the Bandelier Tuff, in the Jemez Volcanic Field, New Mexico. To provide a chronologic framework for geochemical and isotopic studies on these rhyolites, which record the evolution of the Bandelier magma system during this interval, a 40 Ar/ 39 Ar geochronology study was undertaken. Pumice from major pyroclastic fall deposits within the rhyolite tephra and samples from the rhyolite domes were dated as well as the stratigraphically bracketing Bandelier Tuff. Analyzed crystal populations range from being fairly homogeneous juvenile material to very heterogeneous mixed juvenile and xenocrystic assemblages. In most cases dominant groups of juvenile sanidine crystals define 40 Ar/ 39 Ar ages which agree with stratigraphic constraints. Plagioclase analyses are distinctly more scattered and do not typically define reasonable ages. The 40 Ar/ 39 Ar ages for the two members of the Bandelier Tuff yield an interval of 380 ± 20 k.y. between these caldera-forming eruptions. During this interval nine major pyroclastic pumice units were deposited in the sections studied, for which six yield isochron ages, one a weighted mean age, one a maximum age, and one no reliable age due to lack of sanidine. 40 Ar/ 39 Ar dates on pumice fall units within the Cerro Toledo Rhyolite tephra indicate that eruptive activity occurred at >1.59, 1.54, 1.48, 1.37 and 1.22 Ma. 40 Ar/ 39 Ar dating of Cerro Toledo Rhyolite domes indicates these were erupted within the caldera at 1.54, 1.45, 1.38–1.34, and 1.27 Ma. The dates obtained indicate that eruptive activity occurred throughout the 380 k.y. interval between the two members of the Bandelier Tuff, but suggest that eruptions producing both tephra and domes occurred during discrete intervals at ca. 1.54, 1.48, and 1.38–1.34 Ma. The interval from 1.34 to 1.38 Ma was particularly active; 7 of 18 units dated are these ages.

The lower Miocene Peach Springs Tuff exposed in the Newberry Mountains, California, was deposited within a paleovalley trending S65°W. Exposures within the paleovalley contain lithic breccia intercalated with ash-rich ignimbrite. The clast assemblage of the lithic breccias matches the rock types of the paleovalley walls, and therefore the clasts were not derived from a distant eruptive vent. Flow direction, breccia bed thickness, grain-size data, and sedimentary textures indicate that the lithic breccias were deposited from density currents within the pyroclastic flow that moved down tributaries and into the main paleovalley to be intermingled with the ash-rich pyroclastic flow. A model is proposed whereby a turbulent boundary layer at the base of the pyroclastic flow is induced by surface roughness of the substrate and incorporates loose material from the substrate to produce a high-density ground layer that decouples from the lower-density, ash-rich pyroclastic flow. After decoupling has occurred, the high-density, lithic-rich ground layer moves independently from the ash-rich pyroclastic flow. The lithic breccia horizons have many characteristics of proximal (lag) breccias, and caution must be used when inferring distance from vent in ancient ignimbrites based on the occurrence of coarse breccias.

Large-scale fluid dynamical processes during explosive eruptions within calderas are examined numerically by solving the full set of two-phase hydrodynamic equations with a topographic barrier, representing the rim of a caldera. The effect of the caldera rim on eruption dynamics depends on the relative locations of the rim and the impact zone where tephra collapsing from the eruption column strikes the ground. The distance of the impact zone from the vent is proportional to the collapse (fountain) height of the eruption column. Three significantly different eruption patterns have been observed in the simulations: (1) If the impact zone is outside the caldera rim, relatively continuous pyroclastic flow occurs outside the caldera. (2) If the impact zone is on or near the caldera rim, an initial pyroclastic current flow out of the caldera and is followed by a lapse in outflow during which the cladera fills up with ash. (3) If the impact zone is inside the rim, all initial pyroclastic flows are contained within the caldera unless the flows have sufficiently high initial densities and velocities to carry them over the rim. In most cases, recirculation of pyroclasts into the base of the column causes fountain height to decrease dramaticallymore » with time due to the {open_quotes}choking{close_quotes} effect of the ash. This recycling of ash in turn reduces the ability of pyroclastic flows to surmount the rim. The numerical models suggest several processes that cause the formation of multiple cooling and flow units in deposits outside a caldera from a single eruption of steady discharge. Compositional gaps may occur in outflow ignimbrite due entirely to interaction of eruption and emplacement dynamics with topography; sharp compositional gradients within a magma chamber are not necessarily implied by compositional gaps in outflow units. 35 refs., 10 figs., 1 tab.« less

Dacitic pyroclastic flow deposits within the Tecuya Formation of the San Emigdio Mountains, California, exhibit a clear transition from subaerial to submarine deposition across the southeastern margin of the San Joaquin basin. The dacitic pyroclastic deposits are part of an informal volcanic member within the Tecuya Formation, consisting of a lower dacitic unit and an upper basaltic unit. Volcanogenic facies transitions within each of these units can be traced westward, obliquely across paleoshoreline, for 15 km. Depositional environments of the dacitic unit are well constrained by underlying nonvolcanic siliciclastic and overlying basaltic deposits. The lowermost dacitic unit consists of pyroclastic surge and fallout deposits, which overlie nonmarine alluvial-fan deposits and were produced by subaerial, phreatomagmatic eruptions. A second, magmatic eruptive stage resulted in deposition of dacitic pyroclastic flows on top of the subaerial surge and fallout deposits, and within marine, nearshore to outer-shelf settings. These pyroclastic flows, generated from subaerial vents, were high-concentration, poorly fluidized flows which traveled down paleoslope into the marine part of the basin. Upon reaching water, the hot pyroclastic flows continued to flow subaqueously, partially mixed with water, and were quenched. Transitions to cool submarine pyroclastic debris flows may have occurred. Surface transformations of the submarine pyroclastic flows/pyroclastic debris flows generated dilute pyroclastic turbidity flows, which at times became detached and continued to flow independently downslope. Progradation of submarine pyroclastic flow deposits to the west into deeper marine settings likely accompanied the growth of subaerial volcanic vents to the east. A second, probably submarine, vent also produced submarine pyroclastic flow deposits in the western part of the basin. Deposition of reworked tuff and epiclastic debris flows within marine settings accompanied primary pyroclastic surge, fallout, and flow deposition. The Tecuya volcanic member is an important ancient example in which the depositional environments of subaerial to submarine pyroclastic deposits are well constrained by bounding units. The characteristics of the subaerial and submarine pyroclastic flow deposits within the Tecuya volcanic member thus may be useful in other ancient volcaniclastic successions which lack bounding units of obvious paleoenvironmental origin.

Here we describe a two-day fi eld trip to examine Quaternary volcanism in the Canadian Cascade arc, named the Garibaldi volcanic belt. Day 1 of the trip proceeds along the Whistler corridor from Squamish to Pemberton and focuses on Quaternary glaciovolcanic deposits. Interactions between volcanoes and ice in the Garibaldi volcanic belt have been common during the past two million years and this has resulted in a diverse array of landforms, including subglacial domes, tuyas, impounded lava masses, and sinuous lavas that exploited within-ice drainage systems. On Day 2, the trip heads northwest of Pemberton, British Columbia, along logging roads to see deposits from the 2360 yr B.P. eruption of the Mount Meager volcanic complex. This eruption began Plinian-style, generating pyroclastic fall and fl ow deposits and ended with the production of block and ash pyroclastic fl ows by explosive (Vulcanian) collapse of lava domes (e.g., Soufriere Hills). Many of the traits of the deposits seen on this two day trip are a refl ection of, both, the style of eruption and the nature of the surrounding landscape. In this regard, the trip provides a spectacular window into the nature and hazards of effusive and explosive volcanism occurring in mountainous terrains and the role of water and ice.

The ANtarctic geological DRILLing Program (ANDRILL) successfully recovered 1138.54 m of core from drill hole AND-2A in the Ross Sea sediments (Antarctica). The core is composed of terrigenous claystones, siltstones, sandstones, conglomerates, breccias, and diamictites with abundant volcanic material. We present sedimentological, morphoscopic, petrographic, and geochemical data on pyroclasts recovered from core AND-2A that provide insights on eruption styles, volcanic sources, and environments of deposition. One pyroclastic fall deposit, 12 resedimented volcaniclastic deposits and 14 volcanogenic sedimentary deposits record a history of intense explosive volcanic activity in southern Victoria Land during the Early Miocene. Tephra were ejected during Subplinian and Plinian eruptions fed by trachytic to rhyolitic magmas and during Strombolian to Hawaiian eruptions fed by basaltic to mugearitic magmas in submarine and/or subglacial to subaerial environments. The long-lived Mount Morning eruptive center, located ∼80 km south of the drill site, was recognized as the probable volcanic source for these products on the basis of volcanological, geochemical, and age constraints. The study of tephra in the AND-2A core provides important paleoenvironment information by revealing that the deposition of primary and moderately reworked tephra occurred in a proglacial setting under generally open-water marine conditions.

Pyroclastic flows entering the sea may cause tsunamis at coastal volcanoes worldwide, but geophysically monitored field occurrences are rare. We document the process of tsunami generation during a prolonged gigantic collapse of the Soufriere Hills volcano lava dome on Montserrat on 12–13 July 2003. Tsunamis were initiated by large-volume pyroclastic flows entering the ocean. We reconstruct the collapse from seismic records and report unique and remarkable borehole dilatometer observations, which recorded clearly the passage of wave packets at periods of 250–500 s over several hours. Strain signals are consistent in period and amplitude with water loading from passing tsunamis; each wave packet can be correlated with individual pyroclastic flow packages recorded by seismic data, proving that multiple tsunamis were initiated by pyroclastic flows. Any volcano within a few kilometers of water and capable of generating hot pyroclastic flows or cold debris flows with volumes greater than 5 × 10 6 m 3 may generate significant and possibly damaging tsunamis during future eruptions.

The Pliocene Mushono tephra bed, central Japan, is characterized by fluvio-lacustrine volcaniclastic resedimentation at distal locations from the volcanic source. Sedimentary responses to an explosive eruption are well recorded in the resedimented Mushono tephra bed. Immediately after the eruption, large volumes of primary volcaniclastic material were reworked from proximal areas and discharged into the distal basin. Overloading of fluvial systems by pyroclastic debris resulted in the development of low-sinuosity active fluvial channel systems that carried sediment-laden, hyperconcentrated flows. The fluvial system flowed into a lake more than 150 km from the inferred source location, resulting in delta formation and progradation. Primary pyroclastic fall deposits are only a few centimeters thick but are overlain by reworked volcaniclastic deposits more than 5 m thick, which show coarsening-upward successions representing fluvio-lacustrine deltaic progradation. The reworked succession consists of six sedimentary facies, which can be interpreted as prodelta, mouth bar, distributary-channel, hyperconcentrated-flow, inter-distributary-lowland or floodplain, and slump deposits. Rounded, cobble-sized pumice clasts in prodelta and distributary-channel deposits and thick, reworked deposits imply that the majority of resedimented volcaniclastic material was derived from voluminous proximal ignimbrite, rather than thin, fine-grained distal fallout ash material. Vertical facies changes are concordant with a decrease in sediment yields with time. Lateral facies distributions correspond to episodic influxes of volcaniclastic sediments that controlled the morphology of discrete fluvio-lacustrine deltas. The distal sedimentary response to an explosive silicic eruption is thus clearly recorded in vertical and lateral facies distribution of reworked volcaniclastic sediments.

The largest and most intense lava-dome collapse during the eruption of Soufriere Hills volcano, Montserrat, 1995–2004, occurred 12–13 July 2003. The dome collapse involved around 200 x 106 m3 of material and was associated with a phenomenon previously unknown at this volcano. Large pyroclastic flows at the peak of the dome collapse interacted explosively with seawater at the mouth of the Tar River Valley and generated a hot, dry base surge that flowed 4 km inland and 300 m uphill. The surge was destructive to at least 25 m above the ground and it carbonized vegetation. The resulting two-layer deposits were as much as 0.9 m thick. Although the entire collapse lasted 18 h, the base surge greatly increased the land area affected by the dome collapse in a few minutes at the peak of the event, illustrating the complex nature of the interaction between pyroclastic flows and seawater.

Volcanic-ash clouds can be fed by an upward-directed eruption column (Plinian column) or by elutriation from extensive pyroclastic flows (coignimbrite cloud). There is considerable uncertainty about which mechanism is dominant in large-scale eruptions. Here we analyze in a novel way a comprehensive grain-size database for pyroclastic deposits. We demonstrate that the Mount Pinatubo climactic eruption deposits were substantially derived from coignimbrite clouds, and not only by a Plinian cloud, as generally thought. Coignimbrite ash-fall deposits are much richer in breathable ,10 mm ash (5‐25 wt%) than pure Plinian ash at most distances from the source volcano. We also show that coignimbrite ash clouds, as at Pinatubo, are expected to be more water rich than Plinian clouds, leading to removal of more HCl prior to stratospheric injection, thereby reducing their atmospheric impact.

Hybrid fall deposits in the Bishop Tuff show features common to both archetypal fall and surge deposits. Like normal-fall deposits, they have an overall plane-parallel bedding and flat-lying pumice clasts but also, like surge deposits, they show variable development of cross-bedding, some crystal and pumice sorting, and some rounding of pumice clasts. All variations exist from normal-fall deposits, through streaky material with incipient development of cross-bedding, to the hybrid fall deposits with well-developed cross-bedding. The streaky and hybrid deposits are interpreted as fall material contemporaneously redeposited by strong (up to 40 m/s) swirling winds, comparable to firestorm whirlwinds, generated by air currents associated with coeval emplacement of pyroclastic flows. Recognition of hybrid fall deposits is important in interpreting the dynamics of explosive eruptions and correctly assessing volcanic hazards. However, although such deposits may be commonly produced by explosive eruptions, especially where pyroclastic flows accompany fall activity, they are likely to be overlooked, or wrongly interpreted as surge deposits or secondary, reworked material.

Yellow, hydrothermally altered, lithic-dominated pyroclastic breccia deposits, having only a minor juvenile component, at Galeras volcano, Colombia, contain evidence that they were emplaced as hot, fluidized, true pyroclastic flows. The distribution of the flows around the caldera rim, their transport mechanisms inferred from grain-size analyses, and the presence of charcoal and coarse channel and fine overbank facies indicate that these deposits were emplaced as true pyroclastic flows. These deposits are the result of a regularly repeated eruption sequence, involving clearing of vent material as a prelude to more typical pumiceous pyroclastic flows. Recognition of the mode of origin of these deposits is important in terms of understanding Vulcanian-type eruption and flow- emplacement mechanisms. Failure to recognize the explosive eruption origin of these deposits at Galeras and other volcanoes could result in serious underestimation of potential hazards.

The 79 A.D. plinian eruption of Vesuvius ejected ∼4 km3 (ORE) of phonolitic magma over a period of ∼19 hr. A change in magma composition during the eruption is marked by a sharp transition from white, evolved phonolitic pumice to denser, overlying gray pumice, at mid-level within the fall deposit. Deposition of the upper, gray pumice fall was interrupted six times by the emplacement of pyroclastic surges and flows. Reverse size grading is conspicuous in the fall deposit. Measurements of maximum pumice and lithic diameters have been used to construct isopleths for eight chronostratigraphic levels within the fall deposit. The temporal evolution of eruption column height and magma discharge rate have been evaluated from these isopleths, using a theoretical model of pyroclast dispersal from explosive eruptions. During ejection of the white pumice, the column height rose from 14 to 26 km, as the magma discharge rate increased to 7.7 × 107 kg/s. Shortly after onset of the gray pumice fall, the column reached its maximum altitude of 32 km, with a discharge rate of 1.5 × 108 kg/s. Subsequent generation of surges and pyroclastic flows was associated with fluctuations in column height, supporting an origin by column collapse. At the white-gray boundary in the fall deposit, pumice density increases abruptly from 0.60 g/cm3 in the white pumice to 1.10 g/cm3 at the base of the gray pumice. Higher in the gray fall, the density decreases continuously to 0.60 g/cm3. The variation in pumice density is attributed primarily to differences in volatile content of two magmas which were tapped and mixed in varying proportions during ascent and eruption.

The 1815 eruption of Tambora is one of the largest explosive volcanic events of the past 10 000 yr. By a conservative estimate, 175 km3 of nepheline-normative trachyandesitic pyroclastic material (equivalent to about 50 km3 of dense rock) was erupted in 24 h. Major activity began with a brief plinian phase followed by pyroclastic flows; much of the ignimbrite that was produced shows evidence of deposition by turbulent flows. A 6×7-km caldera, about 1 km deep, formed at the end of the pyroclastic flow phase; caldera volume closely matches that of magma ejected. Plinian and co-ignimbrite ash fall >1 cm thick covered >500 000 km2 of the Java Sea and surrounding islands, accounting for more than two-thirds of the magma volume erupted. This distal tephra is known only from 1815 reports, indicating that some great eruptions, especially those from island volcanoes, may be virtually undetectable in the geologic record without supporting information, such as deep-sea core data. Land-based volume estimates may be insufficient to characterize eruption magnitude in many cases.

The far-reaching directed “blast” of Mount St. Helens and the devastating nuees ardentes of Mount Pelee produced deposits that in many places consist of three main layers: a basal gravelly or sandy layer 1, a massive or bedded ash layer 2, and a capping ash-cloud layer 3. These thin and in part landscape-mantling deposits are generally ascribed to pyroclastic surges, and the dune bedding seen in parts of layer 2 has reinforced this interpretation. There are several reasons, however, for preferring a pyroclastic-flow origin: (1) much of layer 2 is unequivocal pyroclastic flow; (2) the tripartite subdivision and landscape mantling habit is similar to that displayed by ignimbrites of the low-aspect-ratio type; (3) the deflation of a pyroclastic surge and the subsequent deposition of particulate material will tend not to conserve the fine ash and dust that are abundant in valley-ponded parts of layer 2 and cannot explain the observed tendency for layer 2 to show stronger fines depletion on ridges than in valley bottoms; and (4) the coarseness and the variance of grain size found within dune-bedded bed sets in Mount St. Helens layer 2 are like those of very weak pyroclastic surges, and the dune bedding most likely resulted from local minor turbulence (as could have been caused by surface roughness in a mountainous terrain littered with tree stumps and fallen trees) in a thin depositing pyroclastic flow. The directed blast and nuees ardentes at Mount St. Helens and Mount Pelee are interpreted to have been violently emplaced pyroclastic flows producing deposits of low-aspect-ratio type, the characteristics of which stem from an exceptionally high flow velocity.

Observations of the panoramic photographs transmitted by Veneras 13 and 14 indicate that many visible rock units are layered and that at least a portion of the fine material could be the result of in situ geomorphic disintegration. Chemical and material property experiments indicate that the chemical composition of the material at the landing sites is essentially basaltic, that the bearing strength is minimal (a few kilograms per square centimetre, or a few hundred kilopascals), that the density is less than 1.5 g/cm 3 , that the porosity is very high (>50%), and that the electrical resistivity is very low ( The discussion on the possible geological nature of the surface of Venus presented in Part I (Florensky and others, 1977a) is extended in light of these new data. Six possible origins of the rock units were discussed: (1) surface lava extrusion; (2) igneous intrusion, later exposed by erosion; (3) pyroclastic fall; (4) impact ejection and lithification; (5) sedimentary deposits lithified at depth and later exposed by erosion; and (6) surface metamorphism due to unique Venusian surface conditions. It is concluded that, although none of the above hypotheses can be conclusively proven or disproven, hypothesis 6 is to be advanced to the forefront. This is suggested by (1) the low density and low bearing strength, (2) the presence of layering, (3) the possible presence of sedimentary structures in the rock units, such as cross-bedding and ripple marks, (4) the low electrical resistivity, which may indicate chemical alterations of surface material, and (5) the albedo of the loose, fine material, which is lower than the albedo of the rock units. This again suggests chemical alterations. Within the context of this hypothesis, the rock units are sedimentary or sedimentary-volcanic. Lithification occurred in the past due to surface chemical effects. The rock units, analogous with a similar terrestrial phenomenon, can be referred to as “duricrusts” (with the understanding that the Venusian process is completely different from the terrestrial process). Presently, the rock units are undergoing disintegration, indicating that changes have occurred in the surface conditions at the landing sites.

Soufriere is a stratovolcano just higher than 1,000 m which dominates the northern part of the island of St. Vincent in the Lesser Antilles. A succession of pyroclastic fall deposits produced by this volcano covers about 55% of the island. Maximum thicknesses away from the crater area range from 50 m in the northeastern area of the island to 2 m in the Kingstown area at the southern tip. These deposits, believed to be of late Pleistocene age, were produced during a unique phase of extremely violent activity which may not have lasted for more than a few hundred years; during this time, the volcano was in nearly constant activity. The eruptions took place from an open summit crater similar to the present one, which may have been periodically blocked by lava domes during short quiescent spells. The deposits were produced by plinian, vulcanian, and strombolian type eruptions, some of which may have been directed blasts. The composition of the pyroclastic falls is distinctly more mafic than that of the average Soufriere lava from previous eruptions. Chemical variation in vertical sections shows that tephra with the highest silica content is ejected at the start of an eruption, usually as pumice, and is followed by more mafic material. High-level fractionation of basaltic magma under the volcano is inferred from this pattern of behavior.

Thin-bedded, dacitic, pumiceous pyroclastic-flow deposits partly cover the steep northern flank of Mount St. Helens volcano, Washington. They are termed proximal bedded pyroclastic-flow (PBPF) deposits and were formed during the eruption of May 18, 1980. These unconsolidated deposits, as much as 20 m thick, are characterized by well-defined, chiefly plane-parallel bedding sets separated by erosion surfaces. Most beds dip generally parallel to the slope of the underlying volcano flank, which averages 15° to the north but locally is as much as 30°. Individual beds range in thickness from 2 mm to >1 m. Cross-bedding, in which bedding sets dip gently to the north or locally to the south, is abundant. Cross-bedding is generally associated with large longitudinal dunes, apparently antidunes. Some cross-bedding sets, however, constitute small longitudinal anti-dunes (chute-and-pool structures) containing stoss-side beds that migrated south and dip steeply south toward the crater source. We suggest that PBPF deposits, base-surge deposits, and ignimbrite-veneer deposits are products of a broad spectrum of high-flow–regime pyroclastic processes. PBPF deposits at Mount St. Helens formed during deposition of high-velocity, partly turbulent, dry, fluidized pyroclastic flows that poured profusely down the volcano flank on May 18. Some beds of the PBPF deposits probably were deposited from typical high-concentration laminar pyroclastic flows. Other beds, especially those that display evidence of high-angle stoss-side accumulation, were probably deposited from low-concentration turbulent flow under high velocity as pyroclastic surges. Most beds in the PBPF deposits, however, may have formed during transitional flow conditions. Pyroclastic-surge and transitional flow-surge conditions may have resulted when the flows exceeded the threshold velocity or threshold minimum particle concentration for pyroclastic flows while lofting over obstacles or depressions or after collapse from Plinian ash columns.

Wave-like bedforms of large amplitude and wavelength (6 m, 60 m) occur in an exposure at the base of the Fish Canyon Tuff, Colorado, an ignimbrite (ash-flow tuff) of great volume (> 3,000 km 3 ). The bedforms are located some 30–40 km from the center of the source caldera. They are interpreted to be deposits from a series of large pyroclastic surges, precursors to the main pyroclastic flows. The pyroclastic surge beds have bigger wave forms than those previously recognized. Such megawaves were perhaps produced by violent explosions related to major unroofing of a magma chamber. The depositional sequence of this great ignimbrite-forming event is similar to that of much smaller eruptions and implies a similar eruptive sequence.

Two pumiceous vitric-crystal tuffs, the Hatton Tuff Lentil and Beavers Bend tuff, occur in the deep-marine Mississippian Stanley Group. These widespread rhyodacitic tuffs range in thickness from 7 to 40 m and are separated by tens of metres of non-tuffaceous quartzose and feldspathic turbidite sandstone and shale. The tuffs consist of varying proportions of ash-sized embayed quartz crystals, plagioclase crystals (oligoclase to andesine), relict shards, volcanic dust, and altered flattened pumice fragments. Each pyroclastic unit consists of two or more tuff lithologies, including a thick lower unstratified pumiceous vitric-crystal tuff with a density-graded crystal-rich base overlain by thin-bedded pumiceous tuff and an upper massive fine-grained siliceous vitric tuff. These tuffs were probably formed by highly explosive eruptions of vesiculating acidic magma from a vent or fissure that produced incandescent avalanches of pyroclastic debris and accompanying ash clouds. The hot turbulent suspensions were rapidly quenched by sea water to form steam-inflated density slurries that flowed into the Ouachita basin. Pyroclastic flows created thick, density-graded, pumiceous vitric-crystal tuff. Numerous smaller density slurries following the main flow in rapid succession deposited the overlying bedded pumiceous tuff. Toward the end of each volcanic eruption, continuous settling of fine ash formed thick, fine-grained upper vitric tuff. Isopach maps of tuff thicknesses, an isopleth map of pumice sizes, logarithmic plots of crystal size versus distance, paleocurrent indicators, and Late Mississippian paleogeography suggest a southern volcanic source that may have been part of a magmatic arc formed at a continental margin during plate convergence between the North American plate and a southern continental(?) plate.

Types of layer 1 deposits in ignimbrite sequences include ground-surge, ground-layer, and jetted deposits. Ground-surge deposits are generally thought to represent deposition from turbulent, low-concentration pyroclastic density currents resulting from “ash hurricane,” initial stages of Plinian eruption column collapse, or blast-wave phenomena. Ground layers form by sedimentation from the fluidized head of a pyroclastic flow, and jetted deposits form by explosive jetting from the flow head as a result of heating of ingested air. Theoretical evidence suggests some layer 1 deposits may result from turbulent boundary layers at the base of pyroclastic flows. Deposits formed from this process will have characteristics that depend on local flow conditions and thus may be largely independent of distance from vent. The turbulenit-boundary-layer model may explain the occurrence of surgelike deposits at distances from sources that appear too far for transportation by a low-concentration, turbulent pyroclastic surge.

The upper Quaternary pyroclastic flow deposits of Laacher See volcano show compositional and structural facies variations on four different scales: (1) eruptive units of pyroclastic flows, composed of many flow units; (2) depositional cycles of as many as five flow units; flow units containing (3) regional intraflow-unit facies; and (4) local intraflow-unit subfacies. These facies can be explained by successively overlapping processes beginning in the magma column and ending with final deposition. The pyroclastic flow deposits thus reflect major aspects of the eruptive history of Laacher See volcano: (a) drastic changes in eruptive mechanism due to increasing access of water to the magma chamber and (b) change in chemical composition and crystal and gas content as evacuation of a compositionally zoned magma column progressed. The four scales of facies result from four successive sets of processes: (1) differentiation in the magma column and external factors governing the mechanism of eruption; (2) temporal variations of factors inducing eruption column collapse; (3) physical conditions in the eruption column and the way in which its collapse proceeds; and (4) interplay of flow-inherent and morphology-induced transport mechanics.

Many ignimbrite flow units show a reverse grading of large pumice clasts and a normal grading of large lithic clasts. Each ignimbrite flow unit has a basal layer finer grained than the body of the ignimbrite, with a ground surge deposit commonly underlying the ignimbrite, and a fine ash-fall deposit commonly overlying it. These two types of deposit, although not an integral part of the ignimbrite, are produced by the same eruptive act, and they and the ignimbrite constitute the several and varied products of a Pelean-phase eruption. Volcanic eruptions in which ignimbrite is generated show the following sequence of events so often that it may be regarded as the normal one: (1) a highly explosive, often Plinian, phase, producing a pumice-fall deposit; (2) a Pelean phase; and (3) an effusive phase, producing a lava flow. This sequence is believed to represent the tapping of progressively deeper levels in the magma chamber and the escape during the eruption of magma of progressively lower gas content.

Subaqueous pyroclastic flows form almost one-half of the 10,000-foot Ohanapecosh Formation (Eocene and Oligocene?) in the eastern part of Mount Rainier National Park, Washington. Most of these flows probably originated by the sloughing of debris from the flanks of active underwater volcanoes during and after pyroclastic eruptions. Some of the pyroclastic flows caused directly by underwater eruptions may not have been completely quenched and could have traveled as steam-inflated slurries of pyroclastic debris and water. Most of them, however, were probably cold or only slightly warm as they flowed into deeper water. The Ohanapecosh subaqueous pyroclastic flows are extensive, nonwelded deposits of lapilli-tuff or fine tuff-breccia ranging in thickness from 10 to more than 200 feet. They are interbedded with thinner and generally finer turbidity-current and ash-fall deposits formed by smaller and more water-rich slumps of pyroclastic debris from the underwater volcanoes and by ash falls that rained into the water. The three main types of flows are thought to be related to three different kinds of volcanic activity. The most common flows—those containing a variety of lithic fragments and variable amounts of pumice—were probably produced by underwater phreatic eruptions. The flows rich in pumice and glass shards were probably caused by underwater eruptions of rapidly vesiculating magma which, on land, would have produced hot ash flows or ash falls. The least common flows—those containing only one or two kinds of lithic fragments—were probably derived from fairly homogeneous bodies, such as domes, spines, and lava flows that were erupted into water and fragmented by steam-blast explosions. Remnants of the underwater Ohanapecosh volcanoes consist chiefly of coarse tuff-breccia piled around filled volcanic vents. Unbrecciated lava flows are subordinate and were probably deposited on land when the volcanoes succeeded in growing into islands.