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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.

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 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 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.

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.

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.