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The occurrence of pyroclastic flow deposits is not restricted to polygenetic and/or felsic volcanic systems, but can also occur at mafic monogenetic volcanic centres. At the c. 5 ka Mt Gambier Volcanic Complex, Australia, two small-volume pyroclastic flow deposits resulted from phreatomagmatic eruptive phases of the Blue Lake East and the Valley Lake maar craters. The facies descriptions of the two massive, poorly sorted lapilli tuff and tuff breccia deposits are given. Low-grade carbonized wood fragments in the deposits mark low emplacement temperatures (180–270°C) for these deposits with only c. 12–23% of the total thermal energy of the original magma preserved at the time of deposition. The Blue Lake East vent erupted by forming a short-lived vertical eruption column, which could not be sustained and collapsed. The Valley Lake pyroclastic flow deposit has a highly asymmetric dispersal to the west, indicating that this deposit was formed during an eruptive phase with a large westward-directed lateral component. Such massive, poorly sorted deposits outside the maar crater indicate that some similar deposits in diatremes may have originated from surface eruptions and not just from subsurface debris jets.

In 2010, Gunung Merapi (Central Java, Indonesia) generated two violent eruption sequences on 26 October and 5 November culminating in widespread pyroclastic density currents (PDCs) associated with the destruction of lava domes. Tephra from PDC deposits were analysed to examine pre-dome collapse volatile behaviour and evidence of carbonate assimilation. Secondary-ion mass spectroscopy (SIMS) depth profiles of plagioclase phenocrysts reveal that the 7 Li/ 30 Si ratios in 26 October products are higher in the glass compared to the crystal, indicating a build-up of Li in the groundmass not observed in the 5 November samples. Higher Li in the groundmass suggests gas accumulation and rapid development of conduit overpressure in the shallow plumbing system prior to the initial 26 October explosion, which was only captured through the behaviour of quickly diffusing Li and not H 2 O. Following the explosion-induced decompression, juvenile magma rapidly ascended in great volume to generate extremely destructive PDCs following subsequent dome collapses, particularly on 5 November. Additionally, 26 October tephras contain carbonate grains in the ash component and abundant CO 2 within the lava lapilli groundmass glass, which supports previous studies indicating assimilation of calc-silicate lithologies by the Merapi magma at depth in the plumbing system prior to the onset of 2010 activity.

The Sgurr of Eigg Pitchstone on the Isle of Eigg, NW Scotland, is a crystal-rich, trachydacitic, partially vitrophyric rock, which has previously been interpreted both as a lava and as a sill. We interpret this rock as a chemically zoned, rheomorphic, lava-like ignimbrite that formed during a sustained pyroclastic eruption. The Sgurr of Eigg Pitchstone can be subdivided into discrete emplacement units distinguished by the following features: (1) their present-day weathering characteristics; (2) the orientation, spacing and morphology of the columnar joints; (3) sharp, undulating boundaries with marked topographic breaks. The absence of weathered surfaces, palaeosols, pyroclastic fall deposits or sedimentary rocks at emplacement unit boundaries suggests deposition from a single eruption. The emplacement units, some of which display upper and basal vitrophyres, represent distinct depositional packages that record several rapidly emplaced ignimbrites, which welded, cooled and devitrified as a simple, essentially single, cooling unit, during eruption from a sustained, low pyroclastic column. The Sgurr of Eigg Pitchstone displays a pervasive base-parallel flow banding, which is folded into intrafolial recumbent isoclinal folds. The flow banding and folds indicate that rheomorphism occurred throughout deposition. The Sgurr of Eigg Pitchstone is interpreted as an erosional remnant of an extensive ignimbrite sheet, the first such unit recorded within the North Atlantic Igneous Province.

The Lattari Mountains (a limestone ridge about 20 km south of Vesuvius) received 1–2.5 m of fallout from the famous Plinian eruption of ad 79. As demonstrated by many residual outcrops of thick volcanoclastic debris-flow and alluvial deposits (referred to here as the Durece unit), the pyroclastic fall was soon followed by rapid erosion and landsliding that produced (1) decametre-scale aggradation of some narrow valley floors; (2) reactivation of alluvial fans; and (3) growth of new fan-deltas (extending as far as 500 m) at the coast. This response was primarily due to the steep topography of the area and the high erodability of the pyroclastic materials (light and cohesionless pumice fragments). Several geo-archaeological data indicate that the accelerated sedimentation had a duration of the order of decades and was followed by rapid dissection of the Durece unit deposits and fast dismantling by wave action of the newly created fan-deltas. This case highlights the need to consider the possibly catastrophic reaction of fluvial and coastal systems to large explosive eruptions, even in non-volcanic terrains at some distance from the volcano.

The ad 79 Plinian eruption of Vesuvius that buried Pompeii and Herculaneum began with pumice falls deposited towards the SE of the volcano followed by pyroclastic currents directed towards the SW. These currents reached Herculaneum and rapidly entered the sea, forming a fan. The interpretation of seismic reflection profiles and gravity cores collected off Herculaneum documents a submarine fan-shaped pyroclastic body at 10–140 m water depth that we interpret as the submarine counterpart of the onshore pyroclastic current deposits. This fan, c . 0.3 km 3 in volume, displays a chaotic seismic facies that changes seaward to parallel reflectors and then to wavy reflectors. Gravity cores reveal a succession consisting of centimetre-thick sand- or silt-sized ash couplets followed by a graded gravelly sand-sized bed up to 180 cm thick, containing shell fragments and beach-derived pebbles, overlain by centimetre-thick graded and laminated sandy ash layers. The depositional textures and sedimentary structures of the submarine pyroclastic fan have been interpreted as the product of the interactions between pyroclasts, water waves and tsunamis induced by the ad 79 pyroclastic density currents into the Bay of Naples.

Primary basaltic pyroclastic deposits compose volumetrically c . 35–50% of the Lower Palaeogene Vandfaldsdalen, Mikis and Haengefjeldet Formations within the lower volcanic series of the East Greenland flood basalt province. The eruption and emplacement mechanisms of these deposits can be used to evaluate the volcanotectonic evolution of continental break-up and initiation of flood basalt volcanism. We present a revised stratigraphy for the pre-volcanic sedimentary rocks and synvolcanic pyroclastic and epiclastic deposits and lavas from the Kangerlussuaq Basin, East Greenland. Primary basaltic pyroclastic deposits range from airfall tuffs to bomb beds, surge deposits and vent sites with cinder cones. Early volcanism was apparently restricted to localized basins and formed shield volcano structures 30–40 km in diameter that are characterized by a heterogeneous sequence of intercalated hyaloclastites, sediments, pyroclastic deposits and lavas. The lateral variation of deposit morphology reflects distance from source and indicates a migrating locus of volcanism along the axis of rifting towards the NE, paired with continued down-dropping of inactive volcanic centres. The complex stratigraphic architecture of the initial volcanic deposits reflects local tectonic and sedimentary processes interacting with regional flood basalt volcanism and continental rifting.

Gravitational collapses of the lava dome at Soufriere Hills Volcano on 25 June and 26 December 1997 generated pyroclastic surges that spread out over broad sectors of the landscape and laid down thin, bipartite deposits. In each case, part of the settling material continued to move upon reaching the ground and drained into valleys as high-concentration granular flows of hot (120-410°C) ash and lapilli. These surge-derived pyroclastic flows travelled at no more than 10 m s -1 but extended significantly beyond the limits of the parent surge clouds (by 3 km on 25 June and by 1 km on 26 December). The front of the 25 June flow terminated in a valley about 50 m below a small town that was occupied at the time. Despite their small deposit volumes (5-9 x 10 4 m 3 ), the surge-derived pyroclastic flows travelled as far as many of the Soufriere Hills block-and-ash flows on slopes as low as a few degrees, reflecting a high degree of mobility. An analysis of the deposits from 26 December suggests that sediment accumulation rates of at least several millimetres per second were sufficient to generate pyroclastic flows by suspended-load fallout from pyroclastic surges on Montserrat. Surge-derived pyroclastic flows are an important, and hitherto underestimated, hazard around active lava domes. At Montserrat they formed by sedimentation over large catchment areas and drained into valleys different from those affected by the primary block-and-ash flows and pyroclastic surges, thereby impacting areas not anticipated to be vulnerable in prior hazards analyses. The deposits are finer-grained than those of other types of pyroclastic flow at Soufriere Hills Volcano; this may aid their recognition in ancient volcanic successions but, along with valley-bottom confinement, reduces the preservation potential.

Pyroclastic flows were formed at Soufriere Hills Volcano by lava-dome collapse and by fountain collapse associated with Vulcanian explosions. Major episodes of dome collapse, lasting tens of minutes to a few hours, followed escalating patterns of progressively larger flows with longer runouts. Block-and-ash flow deposit volumes range from 6 m 3 with runouts of 1-7 km. The flows formed coarse-grained block-and-ash flow deposits, with associated fine-grained pyroclastic surge deposits and ashfall deposits. Small flows commonly formed lobate channelized deposits. Large block-and-ash flows in unconfined areas produced sheet-like deposits with tapering margins. the development of pyroclastic surges was variable depending on topography and dome pore pressure. Pyroclastic surge deposits commonly had a lower layer poor in fine ash that was formed at the current front and an upper layer rich in fine ash. Block-and-ash flows were erosive, producing striated and scoured bedrock surfaces and forming channels, many metres deep, in earlier deposits. Abundant accidental material was incorporated. Pyroclastic flow deposits formed by fountain collapse were pumiceous, with narrow sinuous, lobate morphologies and well developed levees and snouts. Two coastal fans formed where pyroclastic flows entered the sea. Their seaward extent was limited by a submarine slope break.

The mid to late Miocene Misaki Formation sediments of the Miura and Boso peninsulas, south central Japan, were deposited in the Pacific-facing forearc region of the proto Izu-Bonin arc at bathyal depths. The hemipelagic background facies, composed mainly of calcareous biogenic and pumiceous voicaniclastic material, are interbedded with thin to thick scoriaceous beds of turbiditic and pyroclastic fall origin. Careful study in the field and in the laboratory of these fine-grained background sediments has revealed the marked influence of bottom currents at certain horizons in producing intervals with distinct muddy contourite characteristics. These include: a general absence of primary structures due to intense bioturbation, some diffuse layering, irregular concentrations of coarser-grained material, sharp and erosive top and bottom lamina contacts, rare micro-cross-lamination disrupted by bioturbation that was continuous with deposition, and a mixed pelagic biogenic (commonly fragmented), volcaniclastic and terrigenous composition. Small-scale cyclicity of variations in grain-size and structural features can be related in part to episodic volcaniclastic input and in part to fluctuation in bottom current strength. Evidence for bottom current reworking of the tops of thin-bedded sandy turbidites is equivocal, and further work is required to resolve this debate. The recognition of Miocene-age contourites from the NW Pacific provides further evidence for the existence of active deep-ocean circulation in the Pacific at this time. However, it is not possible to determine which current system, Antarctic Bottom Water or deep Kuroshio Current, was responsible for these outcrop examples of fossil contourites.

The eruption of Soufriere Hills Volcano, Montserrat (1995-1999) has displayed a wide range of volcanic phenomena: growth of an andesitic lava dome, generation of pyroclastic flows by lava dome collapse and by fountain collapse in explosive eruptions, Vulcanian and sub-Plinian explosivity with accompanying tephra fall, entrance of pyroclastic flows into the sea, sector collapse with formation of a debris avalanche and a high-velocity pyroclastic density current, and generation of lahars. New phenomena include: cyclic patterns of ground deformation linked with shallow seismicity and eruptive activity; pyroclastic flows formed by rapid sedimentation from pyroclastic surges; and an unprecedented slow escalation of eruption intensity. Magma pulsations with timescales of hours to years have been recognized. Transitions from extrusive to explosive activity were triggered by major dome collapses. Relationships between magma ascent dynamics and geophysical signals have been elucidated. Ascending water-rich andesitic magma becomes Theologically stiffened by degassing and groundmass crystallization. Large magma overpressures are consequently developed in the upper conduit, causing shallow seismicity, radial patterns of ground deformation, and cyclic pulsations of eruptive activity. Lava dome growth involved heterogeneous deformation with formation of spines and lobes along shear zones. Collapse of the pressurized dome resulted in substantial pyroclastic surges forming above pyroclastic flows. Influx of hydrous mafic magma remobilized hot crystalline igneous rocks at depths of 5 to 6 km to form crystal-rich andesitic magma and triggered the eruption.

Synopsis This paper describes the first recorded occurrence of a welded lapilli-tuff in the Dinantian, Clyde Plateau Volcanic Formation. The pyroclastic succession which is exposed near Eaglesham, in the East Renfrewshire Hills, comprises three principal units. The lowest, a coarse volcaniclastic breccia formed by mass movement, is overlain by two fining-upward sequences which grade from a basal pyroclastic breccia into lapilli-tuff. The fining-upward and bedded nature of these upper units, coupled with their lack of diagnostic features indicative of pyroclastic ash flows, has led to the conclusion that they are ashfall deposits. Each deposit probably represents an eruptive unit, with the basal breccia indicating a vent-clearing episode during the initial stages of eruption. The heterogeneously welded lapilli-tuffs from the lower of the two cooling units may, therefore, be interpreted as primary, welded ashfall tuff.

We have used a digital model of the topography of Montserrat, a simple mathematical model of gravitational flow and some assumptions of the way in which the next eruption will develop to create a map of the volcanic hazards from Soufriere Hills Volcano. This has been done using an image processing computer to simulate the deposits produced by pyroclastic flows. This technique has the advantages over more traditional cartographic methods of spatial precision, rapid computation of multiple eruption models and the explicit nature of the physical model used. Soufriere Hills Volcano is a small andesitic volcano characterized by a cluster of summit domes and an apron of pyroclastic flow deposits and mudflows upon which several thousand people now live. Most of the flanks were covered by deposits from a series of eruptions from 24000 to 16000 a BP, though there is some evidence that dome growth and small pyroclastic flows have occurred since. The modelling is constrained by field evidence from the deposits of previous eruptions. Although the evidence is not good enough to model individual flow units, the cumulative deposits can be used. From the eruption deposit models we have created a new type of map specifically for emergency planning. This sequential hazard zone map attempts to portray the regions that would be at hazard from pyroclastic flows during successive stages from the start of an eruption whose energy release was increasing with time.

New radiocarbon dates reveal that unlithified pyroclastic deposits erupted from Mt Misery, St. Kitts (principally the Mansion ‘Series’) range from older than 42 000 years to younger than 1750 years B.P. Previously published accounts of Mt. Misery have only described air-fall deposits. However, a newly recognized sequence of pyroclastic flow deposits within the Mansion ‘Series’ is dated as 2000 to 3000 years B.P. The presence of these young deposits indicates that Mt Misery,. like other Lesser Antillean volcanoes, also has a potential pyroclastic flow volcanic hazard.

Summary Ignimbrites at Kidsty Pike, St Sunday Crag and Coniston are thinner than many within the Borrowdale Volcanic Group and occur in distinctive layered sequences which consist mostly of basic to intermediate pyroclastic fall deposits. The ignimbrites from these areas show particulate structure, eutaxitic texture, columnar jointing and the rare development of lithophysae (siliceous nodules). The latter is indicative of the important role of volatiles in the formation and later history of the deposits. Field relations and comparison with modern counterparts suggest eruption of the St Sunday Crag and Coniston ignimbrites from the same centres as the ashfall deposits with which they are interbedded.

The Minoan eruption of Santorini produced the following sequence of deposits: a plinian pumice fall deposit, interbedded surtseyan-type ash fall and base surge deposits, mud-flow deposits and ignimbrite interbedded with very coarse, well-sorted flood deposits. The variation of thickness and grain size in the plinian deposit indicates a vent 1 km west of Thera town. The base surges and surtseyan-type activity is interpreted as the result of sea water entering the magma chamber. The poorly sorted mud-flow deposits and ignimbrite are distinguished on their grain size, temperature and morphological characteristics, which indicate substantial rheo-logical contrasts in the mass flows which produced them. Grain size analyses show wide ranges in the lithic contents of the different types of deposit: ignimbrite (35–60%), mud-flows (20–30%) and the pyroclastic fall and base surge deposits (4–15%). The ignimbrite is enriched in crystals, complemented by depletion in fine air-fall ash beds that interstratify with the ignimbrite. The gas velocity of the plinian phase is estimated as 550 m/s, the eruption column height as greater than 20 km and it is shown that only particles of 2 mm could have reached Minoan Crete.

Thick accumulations of pyroclastic deposits have emanated from the Mt Misery volcano on St Kitts, Lesser Antilles, during two major eruptive episodes (2-3000 and 41000 a BP). Systematic logging of coastal sections in the northern part of the island has revealed a variety of deposits, including pyroclastic flows, surges and intermittent air fall. Palaeosols and one laterally persistant surge deposit provide important marker horizons, permitting correlation of the intervening and less persistent flow deposits. Block and ash flows (traditionally termed nuee ardentes) can be subdivided into andesitic Pelean-type and basaltic andesite St Vincent-type deposits, closely corresponding to the products of historic pyroclastic eruptions in the Lesser Antilles. Finer grained ash flows are equally voluminous and comprise scoria, semi-vesicular andesite and mixed magma and ash deposits. A significant pyroclastic surge component has been recognized with characteristic bedforms. Field evidence suggests that there is a continuous gradation between flow and surge deposits. Collectively the pyroclastic flow deposits are all of the small volume type and are commonly stratified, with poor to moderate sorting and coarse tail grading. Where thickly developed they show an internal depositional sequence compatible with the standard ignimbrite flow unit. Well developed bedforms include steeply incised U-shaped gullies eroded by the infilling flows. The depositional characteristics of these pyroclastic flows appears to be a function of the physical properties of the magma on eruption, initial flow magnitude, potential energy and degree of topographic channel formation. In general grain-size and the erosive potential of the flow increases with increasing channel formation, whilst sorting and the degree of stratification decrease. The development of graded bedding is, however, apparently unaffected.