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Pyroclastic currents are described as gravity currents, and the classic conceptual model gives a first-order importance to the density of such currents. This directs quantitative models to assume specific flow structures (shallow water or equilibrium turbulent boundary layer), which may apply to restricted volcanic areas independently of source dynamics or may correspond to source dynamics separate from topographic interaction. The recent introduction of two end-members of pyroclastic currents, inertial and forced, is further developed here, leading to a global conceptual model in which source dynamics and topographic interaction are both taken into account. The concept of energy facies is defined here as the ensemble of the first-order indicators of pyroclastic currents (topological aspect ratio, competence ratio and emplacement temperature) that are proxies of the energy of such currents. Nine energy facies are introduced with general applicability and with the goal to globally characterize pyroclastic currents from vent to deposit.

Pyroclastic currents (PCs) represent the most hazardous manifestations of explosive volcanic activity. The evaluation of their mobility, as related to the eruptive source parameters and the interactions with the surroundings along their path, is crucial for hazard assessment at active volcanoes in order to define the areas exposed to their impacts. Here we report on two case studies from the Latera Volcanic Complex (Vulsini Volcanic District, central Italy) that may approach the two end-members of the classical spectrum of PC dynamics in terms of particle concentration and dominant flow regime (i.e., dilute, turbulent vs. concentrated, laminar PCs). For the two examples, we document specific (and contrasting) variation trends of deposit thickness and maximum lithic and pumice clast sizes with distance from vent, which provide a further characterization in terms of forced vs. inertial flow behavior. In the first case, thickness and maximum lithic and pumice sizes show a mild, linear decrease with distance, implying a sort of balance between forced and inertial regimes. Flow pressure and mass progressively drop via deposition, along with flow competence. For given source conditions (i.e., mass eruption rate, fountaining height, erupting mixture, etc.) and slope angle of topography, the PC runout depends primarily on the waning density contrast between the flow and the surrounding atmosphere (density current), ending with the loft of the current. The second case, representative of moderate- to large-volume, dense PCs derived by continuous feeding with high mass discharge rate (i.e., from collapsing Plinian columns or caldera-forming events), shows an increase of deposit thickness with distance, which is accompanied by opposite lithic and pumice clast size-distance trends (i.e., decreasing lithic sizes and increasing pumice sizes downcurrent), compatible with high particle concentration and forced, non-turbulent, granular flow regime. By analogy with the mobility of dry debris flows, we suggest that the sliding component of transport, which prevails in proximal to intermediate settings, may account for the prevailing tendency of the PC to transport than to deposit, which leaves behind relatively thin sheets with coarse lithics, while the dominant spreading component downcurrent would result in increasing accumulation and the stacking of coarse pumice-rich flow lobes in more distal settings. The spreading component of transport would become more and more important with increasing material supply at PC source, resulting in additional runout. These findings cast new light on the first-order controlling factors of flow mobility for the two PC end-members, laying the groundwork for determining the most relevant PC hazard parameters, i.e. the maximum runout and the flow properties upon impact (dynamic pressure, burial potential, temperature).

Through a systematic integrated approach, which combined lithostratigraphic, geochronological and geochemical analyses of tephra from near-source sections of the peri-Tyrrhenian volcanoes and mid to distal settings, here we provide an improved tephrochronological framework for the Marine Isotope Stage 11c interglacial (MIS 11c, ∼425–395 ka) in the Central Mediterranean area. Specifically, we present the complete geochemical dataset and new high-precision 40Ar/39Ar ages of the previously poorly characterized earliest pyroclastic products of the Vico volcano (420–400 ka), including the Plinian eruptions of Vico α and Vico β and the immediately post-dating lower magnitude explosive events. Furthermore, we also provide new geochronological and geochemical data for the distal tephra layers preserved in the aggradational succession of the Tiber delta (San Paolo Formation), Roman area, which records sea level rise relating to the MIS 12 (glacial) to MIS 11 (interglacial) transition. Five pyroclastic units were recognized in Vico volcanic area, four out of which, Vico α, Vico β, Vico βtop (a minor eruption immediately following Vico β and temporally very close to it) and Vico δ were directly dated at 414.8 ± 2.2 ka, 406.5 ± 2.4 ka, 406.4 ± 2.0 ka and 399.7 ± 3.2 ka respectively (2σ analytical uncertainties). These new data allow a critical reappraisal of the previously claimed identifications of Vico tephra from mid-distal to ultra-distal successions (i.e., Vico-Sabatini volcanic districts, Roman San Paolo Formation and Castel di Guido archaeological site, Sulmona Basin, Valdarno and Lake Ohrid), which were unavoidably biased by the poor and incomplete geochemical and geochronological reference datasets previously available. Such an improvement of the tephrochronological framework brings great benefits to any future investigations (e.g., paleoclimatology, archaeology, active tectonic, volcanology) in the dispersal areas of the studied eruptions at the key point in time that is MIS 11.

We present a multidisciplinary study of a fossiliferous site located in the Vulsini Volcanic District, on the western side of the Tiber River Valley north of Rome, highlighting the peculiar geologic factors that contributed to the origin and preservation of an outstanding archaeological record testifying of the early human frequentation in this region. Mighty explosive eruptions since at least 500 ka affected the investigated area eventually culminating in the formation of the huge Bolsena caldera. Tectonic deformation accompanying volcanic activity caused large fault displacements, shaping the ground surface and contributing to route the path, and possibly to trigger, the catastrophic emplacement of volcaniclastic flows. A sedimentary trap originated by fault scarp cutting through a streambed was likely the cause for the large accumulation of bones and stone artifacts ripped up and carried by a volcaniclastic flow at 322 ka. The analysis of the fossil assemblage reveals both gnawing traces by carnivores and cut-marks from the percussion tools employed by humans to butch the carcasses. However, the occurrence of retouched and unretouched blanks within the lithic assemblage also testifies for provenance from a wider area of human activity, which included hunting and scavenging, probably at a nearby butchering site.

The Sabatini Volcanic District (SVD), active between 0.8 and 0.07 Ma, is a volcanic field in the Roman province (Central Italy) located along the Tyrrhenian margin of the Italian peninsula. In this volcanic region, high-K magmas originated from a metasomatised phlogopite-bearing peridotite mantle recording subduction-related fluids and/or melting processes. Here, we investigate magma evolution during the six main eruptive phases of the SVD by means of chemical and isotopic (Sr and Nd) analyses. Specifically, we analyzed clinopyroxene crystals from juvenile pumice and scoria clasts and lavas, from 40 major SVD eruptive units chronologically well constrained by 40Ar/39Ar dating. 87Sr/86Sr and 144Nd/143Nd ratios in SVD clinopyroxene range 0.7095–0.7115 and 0.51210-0.51214, respectively. The mean Sr and Nd isotope compositions of each eruptive phases show a gradual, long-term decrease over the entire SVD eruptive history. However, when considering the distinct temporal windows of the individual eruptive phases, a significant variability of the Sr-Nd isotope ratios emerges, thus highlighting a more complex, time-dependent geochemical trend for the erupted magmas, with respect to a previously described trend at the nearby Colli Albani Volcanic District (0.6–0.04 Ma). Geochemical features of clinopyroxene in lavas and juvenile pyroclasts suggest that magma differentiation occurred in an open system due to assimilation of siliciclastic sedimentary rocks. Moreover, a critical review of the available geochemical data, in light of 40Ar/39Ar ages, allows the recognition of the SVD as the source of widespread tephra markers recorded in the central Mediterranean area by previous works.

Conceptual models explaining the characteristics of ignimbrites and their basal deposits have mainly focused on variations in particle concentration and speed of the parent pyroclastic currents. Here, we focus on the effects of relative proportions of clasts that are well coupled to the carrier gas phase and those that are poorly coupled, i.e., the proportions of fine and/or low-density clasts compared to coarse and/or dense clasts. We document facies and macroscopic clast populations of two ignimbrite-producing eruptive sequences of similar volume and composition, both erupted from Latera caldera (central Italy). The Sovana ignimbrite has 1–15 vol% dense lithic clasts and locally up to 70 vol% in lithic-rich domains. It preserves evidence of emplacement of multiple overlapping flow units with local internal channeling. The ignimbrite is underlain by two basal deposits: (1) a radially distributed, 20–80-cm-thick vitric tuff referred to as BUS (basal unit Sovana) that thins gradually with distance and shows evidence of emplacement by lateral currents; and (2) a fines-depleted, lenticular, massive lapilli tuff, up to 20 cm thick, with abundant lithic clasts. The Sorano ignimbrite, in contrast, has

High-speed imaging of Strombolian explosions brings into view the motion of pyroclasts upon leaving the volcanic vent. The erupted gas-pyroclast mixtures form jets with well-defined leading vortex rings that rise at almost constant velocity proportional to the time-averaged velocity of pyroclasts. The ejection velocity of pyroclasts decreases over time and defines a conical profile centered on the jet central streamline. Pyroclast deceleration patterns are related to their velocity and compatible with drag force but are also strongly controlled by jet dynamics. These patterns include constant, decreasing, or abruptly increasing decelerations up to 104 m s−2. Nonuniform deceleration focuses at the jet sides and, mostly, in a narrow zone across the vortex ring. This deceleration zone is trailed by a reduced drag zone, where deceleration is drastically reduced. In these highly transient eruptions, both zones move upward and attenuate over time. Our results provide the first quantitative mapping of reduced drag zones.

The Campi Flegrei volcanic field (Italy) poses very high risk to the highly urbanized Neapolitan area. Eruptive history was dominated by explosive activity producing pyroclastic currents (PDCs; (Proclastic Density Currents) ranging in scale from localized base surges to regional flows. Here we apply probabilistic numerical simulation approaches to produce PDC hazard maps, based on a comprehensive spectrum of flow properties and vent locations. These maps and provide all probable Volcanic Explosivity Index (VEI) scenarios from different source vents in the caldera, relevant for risk management planning. For each VEI scenario, we report the conditional probability for PDCs (i.e., the probability for a given area to be affected by the passage of PDCs) and related dynamic pressure. Model results indicate that PDCs from VEI, 43 pages, 9 figures

Gravity-driven, ground-hugging gas-pyroclast mixtures produced during explosive volcanic eruptions define a full spectrum of particle concentration, flow regime and particle support mechanisms. To describe these phenomena, the term “pyroclastic density current” (PDC) has become increasingly popular in the last few tens of years. Here, I question the general application of the term PDC to the whole flow spectrum and, instead, I propose the simpler term “pyroclastic current”.

A new image processing technique—Pyroclast Tracking Velocimetry—was used to analyze a set of 30 high-speed videos of Strombolian explosions from different vents at Stromboli (Italy) and Yasur (Vanuatu) volcanoes. The studied explosions invariably appear to result from the concatenation of up to a hundred individual pyroclast ejection pulses. All these pulses share a common evolution over time, including (1) a non-linear decrease of the pyroclast ejection velocity, (2) an increasing spread of ejection angle, and (3) an increasing size of the ejected pyroclasts. These features reflect the dynamic burst of short-lived gas pockets, in which the rupture area enlarges while pressure differential decreases. We estimated depth of pyroclast release to be approximately 1 and 8 m below the surface at Stromboli and Yasur, respectively. In addition, explosions featuring more frequent pulses also have higher average ejection velocities and larger total masses of pyroclasts. These explosions release a larger overall amount of energy stored in the pressurized gas by a combination of more frequent and stronger ejection pulses. In this context, the associated kinetic energy per explosion, ranging 103–109 J appears to be a good proxy for the explosion magnitude. Differences in the pulse-defining parameters among the different vents suggest that this general process is modulated by geometrical factors in the shallow conduit, as well as magma-specific rheology. Indeed, the more viscous melt of Yasur, compared to Stromboli, is associated with larger vents producing fewer pulses but larger pyroclasts.

Two main end-members of eruptive regimes are identified from analyses of high-speed videos collected at Stromboli volcano (Italy), based on vent conditions: one where the vent is completely clogged by debris, and a second where the vent is open, without any cover. By detailing the vent processes for each regime, we provide the first account of how the presence of a cover affects eruptive dynamics compared to open-vent explosions. For clogged vents, explosion dynamics are controlled by the amount and grain size of the debris. Fine-grained covers are entirely removed by explosions, favouring the generation of fine ash plumes, while coarse-grained covers are only partially removed by the explosions, involving minor amounts of ash. In both fine- and coarse-grained cases, in-vent ground deformation of the debris reflect variations in the volumetric expansion of gas in the conduit, with rates of change of the deformation comparable to ground inflation related to pre-burst conduit pressurization. Eruptions involve the ejection of relatively slow and cold bombs and lapilli, and debris is observed to both fall back into the vent after each explosion and to gravitationally accumulate between explosions by rolling down the inner crater flanks to produce the cover itself. Part of this material may also contribute to the formation of a more degassed, crystallized and viscous magma layer at the top of the conduit. Conversely, open-vent explosions erupt with hotter pyroclasts, with higher exit velocity and with minor or no ash phase involved.

The presence of young, highly explosive volcanoes in the densely populated Roman area (central Italy), makes the evaluation of the long-term volcanic hazard a crucial point. The Sabatini Volcanic District (SVD), part of the potassic Quaternary Roman Province, extends over an area of 1800 km2 including the city of Rome. Here we report twenty-six new 40Ar/39Ar age analyses of pyroclastic and lava units in order to put time constraints on the most recent SVD eruptive activity (i.e. post-dating 400 ka), as a groundwork for a detailed reconstruction of the whole volcano-tectonic evolution. Integrated geochronological and field evidence shows that SVD volcanism maintained a typical areal character through the whole history: both effusive and explosive activity phases from different source areas and volcano-tectonic settings overlapped in time and space, while climactic phases migrated repeatedly through time. In particular, new geochronological data better constrain a climactic phase of activity at around 300 ka, related to the Bracciano volcano-tectonic depression and the Sacrofano caldera, and point out that the most recent eruptive activity in the SVD took place from time–space-clustered hydromagmatic centres at least up to 90 ka. In addition, we remark some parallelism among major activity phases in the SVD and the nearby districts of the Roman Province, with possible implications on Quaternary regional tectonics. Finally, new age data put constraints on tephrostratigraphic correlations through the Apennine Chain and the Adriatic Sea.

Textural and chemical variations of juvenile clasts are widely observed in pyroclastic deposits. In particular, the co-existence of whitish, pumiceous, and dark grey, scoriaceous, juvenile clasts has been observed in many eruptive units of well-known volcanoes (i.e., Somma-Vesuvius, Vulsini, Colli Albani, Stromboli). Here we report the example of the Tufo Giallo della Via Tiberina (TGVT) pyroclastic succession, which comprises two eruptive units emplaced at ca. 561 and 548ka, during the early explosive activity of the Sabatini Volcanic District (SVD; Roman Province, central Italy). TGVT deposits, as well as underlying pyroclastic products (FAD, ca. 582ka), are characterized by coexisting whitish pumice and black-grey scoria clasts showing common phonolitic composition but different textural features: white pumice is highly vesicular, vitrophyric, and contains scarce, >50μm-sized, feldspar and clinopyroxene crystals, while black-grey scoria is poorly vesicular, highly crystallized, and contains diffuse leucite phenocrysts. The latter records crystallization under H2O-undersaturated conditions, as opposed to the vitrophyric texture of white pumice indicating higher temperature and H2O concentration. On these grounds, a thermally and H2O-zoned pre-eruptive system has been modelled for the phonolitic magma chambers feeding the early SVD events, in which white pumice and black-grey scoria represent the inner and peripheral portions of the reservoirs, respectively. Extensive leucite+clinopyroxene crystallization in the H2O-undersaturated, peripheral portions of the reservoirs, resulted in water flux toward the inner zones, where the higher temperature and increasing H2O content acted to delay crystallization in the white pumice-feeder magma. The withdrawal of white pumice at the eruption onset produced decompression of the peripheral magma, triggering black-grey scoria eruption during the late phases of explosive events. © 2010 Elsevier B.V.

Large silicic explosive eruptions are the most catastrophic volcanic events. Yet, the intratelluric mechanisms underlying are not fully understood. Here we report a field and laboratory study of the Kos Plateau Tuff (KPT, 161 ka, Aegean Volcanic Arc), which provides an excellent geological example of conduit processes that control magma vesiculation and fragmentation during intermediate- to large-scale caldera-forming eruptions. A prominent feature of the KPT is the occurrence of quite unusual platy-shaped tube pumice clasts in pyroclastic fall and current deposits from the early eruption phases preceding caldera collapse. On macroscopic and SEM observations, flat clast faces are elongated parallel to tube vesicles, while transverse surfaces often occur at ~ 45° to vesicle elongation. This peculiar pumice texture provides evidence of high shear stresses related to strong velocity gradients normal to conduit walls, which induced vesiculation and fragmentation of the ascending magma. Either an increasing mass discharge rate without adequate enlargement of a narrow central feeder conduit or a developing fissure-like feeder system related to incipient caldera collapse provided suitable conditions for the generation of plate tube pumice within magma volumes under high shear during the pre-climactic KPT eruption phases. This mechanism implies that the closer to the conduit walls (where the stronger are the velocity gradients) the larger was the proportion of plate vs. conventional (lensoid) juvenile fragments in the ascending gas–pyroclast mixture. Consequently, plate pumice clasts were mainly entrained in the outer portions of the jet and convecting regions of a sustained, Plinian-type, eruption column, as well as in occasional lateral blast currents generated at the vent. As a whole, plate pumice clasts in the peripheral portions of the column were transported at lower altitudes and deposited by fallout or partial collapse closer to the vent relative to lensoid ones that dominated in the inner column portions. The plate tube pumice proportion decreased abruptly up to disappearance during the emplacement of the main pyroclastic currents and lithic-rich breccias related to extensive caldera collapse at the eruption climax, as a consequence of an overall widening of the magma feeder system through the opening of multiple conduits and eruptive vents, along with fissure erosion, concomitant to the disruption of the collapsing block.

The early activity of the Sabatini Volcanic District (SVD; central Italy) was characterised by highly explosive eruptions that produced widespread subplinian and plinian fall deposits. In this study, four major eruptive units—informally named as units A, B, C and D—were recognised in the 514–449 ka age interval. In particular, unit D was emplaced during the early phase of the 449 ka Tufo Rosso a Scorie Nere pyroclastic flow-forming eruption, the most important event in the whole SVD activity history. Estimates of relevant eruptive parameters indicate tephra fall volumes up to 4 km3 for individual units, peak eruption column heights in the range of 14–29 km and corresponding mass eruption rates of 7.8×106–1.3×108 kg/s. Isopach and isopleth maps of fallout deposits—as well as the distribution of the proximal lag-breccia of the Tufo Rosso a Scorie Nere—consistently indicate a common vent area, which does not correspond to any volcanic centre identified up to now in the SVD. This was located along NE–SW-trending tectonic lineaments that also controlled the location of the other major volcanic centres of the SVD. The characterisation by means of field aspects, grain size, componentry and density and chemical composition of juvenile clasts, renders the studied fall deposits as valuable stratigraphic markers for the SVD and well beyond it. In fact, their wide areal dispersals toward the E and SE may allow correlations on a regional scale for the Quaternary successions of intermountain basins of the Central Apennine and the Adriatic Sea basin successions. Finally, the correct identification of distal tephra from plinian and co-ignimbrite plumes and their attribution to specific explosive eruptions of the SVD and the other volcanic districts of the Roman Province—rather than to local intra-Apennine centres—provides crucial implications for geodynamic reconstructions.

Catastrophic sedimentary processes associated with explosive eruptions represent a significant geologic hazard in volcanic areas. Here we report a striking historic example of an intermediate-scale explosive event whose environmental effects were strongly amplified by secondary rapid mass flows and hydrogeologic disasters. The 472 AD Pollena eruption of Somma-Vesuvius (Campania, Italy) took place in the critical period of the fall of the Western Roman Empire. On the basis of an integrated geologic–archaeologic study we point out evidence of human habitation at the time of the eruption, effects induced and recovery time in a wide territory of Campania, and how the eruption significantly accelerated the deterioration of the local society during the Late Ancient age. The eruption began with a pulsating, sustained eruption column, followed by pyroclastic surges and scoria flows. Hydromagmatism acted early in the event, different from the typical Plinian eruptions of Somma-Vesuvius. Specific facies associations of primary and secondary volcaniclastic deposits characterize three depositional domains, including the volcano slopes, the surrounding alluvial plains and the distal mountains of the Apennine Range. Both volcano slopes and distal mountain slopes supplied loose pyroclastic material to the hyperconcentrated floods and debris flows that spread across the alluvial plains. The great impact of secondary volcaniclastic processes arose from: (1) the high vulnerability of the territory due to its geomorphic context; (2) the humid climatic conditions; (3) the hydromagmatic character of the eruption; (4) the decline of land management at the end of the Roman Empire.

Most studies of volcaniclastic facies and aggradation cycles have been conducted in regions with humid climates, and on regional scales. Here we document a volcaniclastic succession that formed in a semiarid climate characterized by rare, heavy rains onto a relatively barren volcanic landscape on Vulcano Island, Italy. The deposits, which we informally call the Cuesta succession, occur in a small valley between two rhyolite domes, and consist of a sequence of pyroclastic surge and fallout deposits interbedded with their reworked equivalents. A phase of eruptive activity characterized by sporadic hydrovolcanic explosions supplied ash to the valley and its flanks. Runoff events during the eruptive phase continually washed ash down to the valley floor in the form of hyperconcentrated flows. The inferred transport mechanism and depositional facies of these beds reflect the control of primary volcanic fragmentation processes on sedimentation; the abundant, medium-to-coarse ash supplied by eruption was incorporated sufficiently easily into the runoff to hyperconcentrate the flows, but not sufficiently fine grained to make the flows cohesive. These flows moved out onto the floor, merged, and transported ash down the valley axis, depositing the ash as a monotonous succession of massive to faintly laminated beds. The combination of primary deposits and the deposits washed off the valley flanks led to aggradation of the valley floor. As eruptions waned, ash was eroded off the flanks faster than it was replenished, leading to a stratigraphic upward increase in lithic clasts derived from the underlying lavas and a wider range of sedimentary facies as the ash load became more variable. After eruptions ceased and all remaining ash was removed from the flanks, aggradation gave way to degradation to form small canyons that expose the succession.

Textural and compositional features of pyroclastic products erupted during caldera-forming events often reveal the tapping of different portions of variably zoned magma chambers due to changing geometries of the conduit/vent systems. Here we report on ultrapotassic trachytic–phonolitic explosive eruptions of the Roman Province (central Italy), which show remarkable changes of textural features and glass compositions in the juvenile material, even if the bulk chemical composition is essentially constant. In each example, the lower eruption sequence contains whitish, crystal-poor (leucite-free), highly vesicular pumice, emplaced by early Plinian fallout and/or pyroclastic currents; upsection, the eruption sequence contains black, low porphyritic (sanidine + leucite-bearing), moderately vesicular, scoria or spatter, emplaced by major pyroclastic flows (red tuff with black scoria) and associated co-ignimbrite, coarse lithic-rich breccias. This suggests a shift from a central feeder conduit, tapping the central part of the magma chamber, to a ring fracture vent system, tapping the peripheral portions of the magma chamber, during caldera collapse. Key features of these evacuating magma chambers are the thermal and volatile concentration (Xvol) gradients that produce the observed textural and compositional spectrum of trachy-phonolitic rock types. In particular, the degrees of freedom during the crystallization of these ultrapotassic magmas are increased by the variation of the leucite stability field at different PH2O conditions. Both leucite-free and leucite-bearing differentiated ultrapotassic rock types can be produced in the course of individual eruptions, as a result of pre-eruptive conditions in the feeder magma, with no need to invoke different differentiation suites related to mantle source heterogeneities of parental magmas.

Widely dispersed pyroclastic fall deposits are prominent among the volcanic products of the western part of the Vulsini Volcanic District (central Italy). In this paper we describe major pumice fall deposits in the area, that correlate to the early activity of the Latera Volcanic Complex (0.3-0.2 Ma). They represent excellent stratigraphie markers for a wide area mostly extending west and south of the present Latera caldera. Pumice fall deposits lower B and upper B are closely associated with major pyroclastic flow units of the Canino Eruptive Unit; pumice fall C caps the Canino products without relevant time hyatus. Pyroclastic falls D and E, the latter part of the minor Stenzano explosive eruption, belong to the stratigraphic interval between Canino and Farnese eruptive units. Pumice fall F is part of the initial products of the Farnese eruptive sequence. We present detailed stratigraphy, depositional features, vertical and lateral variations of grain size, componentry and chemical composition in individual fallout beds. Source areas, as inferred from thickness and maximum pumice and lithic clast size distributions, are located in the area of the present-day Latera caldera. Tentative estimates of volume, column height and magma discharge rate indicate moderate-large to large Plinian eruptive events.

Explosive volcanic eruptions are defined as the violent ejection of gas and hot fragments from a vent in the Earth's crust. Knowledge of ejection velocity is crucial for understanding and modeling relevant physical processes of an eruption, and yet direct measurements are still a difficult task with largely variable results. Here we apply pioneering high-speed imaging to measure the ejection velocity of pyroclasts from Strombolian explosive eruptions with an unparalleled temporal resolution. Measured supersonic velocities, up to 405 m/s, are twice higher than previously reported for such eruptions. Individual Strombolian explosions include multiple, sub-second-lasting ejection pulses characterized by an exponential decay of velocity. When fitted with an empirical model from shock-tube experiments literature, this decay allows constraining the length of the pressurized gas pockets responsible for the ejection pulses. These results directly impact eruption modeling and related hazard assessment, as well as the interpretation of geophysical signals from monitoring networks.

Modern stratigraphic studies and fi eld mapping of Quaternary volcanic terrains require a multidisciplinary approach for the reconstruction of the time-space history of a volcanic area (e.g., activity phases, eruptive sources, compositional changes, volcano build-up and collapse events) within the framework of concomitant and/or genetically related geological events acting on regional to global scales (e.g., tectonism, climate changes, glacio-eustatism). Here, we report recent results on the stratigraphy, structure, and evolution of the southern part of the Vulsini volcanic district (0.6‐0.1 Ma), Roman Province, in the light of a new geological survey for the 1:50,000 map of Italy (CARG Project). We focus on the integration of different kinds of stratigraphic units, including lithostratigraphic, lithosomatic, and unconformitybounded stratigraphic (UBS) units, to defi ne and group mappable volcanic bodies. Lithostratigraphic units are characterized in terms of textural features, indicative of eruptive and emplacement scenarios, and rock-type compositions. The intervening stratigraphic discontinuities are defi ned in terms of nature, position, and areal extent, and they are attributed to local depositional processes versus signifi cant temporal hiatuses in the eruptive activity or regional and interregional geological events. The mapped volcanic units are thus correlated to lithosomatic units, corresponding to volcanic source edifi ces, as well as to the local and regional UBS unit settings recently defi ned for the Tyrrhenian coast nearby. On these grounds, we illustrate the geological evolution of the study area as the interplay of constructional and destructional volcanic activity, erosion, pedogenesis, secondary volcaniclastic sedimentation during intereruptive periods, and sea-level fl uctuations.

Protezione Civile, Italy 2004–2006 Agreement with Istituto Nazionale di Geofisica e Vulcanologia— INGV, project V3_1

The Late Pleistocene Albano Maar hosted the most recent volcanic activity of the Colli Albani Volcanic District, represented at near-vent sections by a thick pyroclastic succession of seven units clustered in two main eruptive cycles dated at around 70-68 and 41-36 ka B.P., respectively. Recent stratigraphic investigations allowed us to recognise a pyroclastic succession comprising four eruptive units widely spread in the northeastern sectors of the Colli Albani volcano, up to 15 km eastward from the Albano Maar. Integrated tephrostratigraphic, morpho-pedostratigraphic, archaeological, petrological and geochemical analyses enable us to recognise them as distal deposits of the first, third, fifth and seventh Albano Maar eruptions, enlarging significantly their previously supposed dispersion area. Further tephrostratigraphic studies in central Apennine area, allowed us to identify the Albano Maar products in Late Pleistocene deposits of several intermountain basins, extending still further the dispersion area of distal ash fallout as far as 100-120 km from the vent. On the basis of the identification and the study of these previously unrecognised mid-distal Albano Maar deposits, a reappraisal of the eruptive scenarios and related energetic parameters is proposed. © 2006 Elsevier Ltd and INQUA.

The Onano explosive eruption of the Latera Volcanic Complex (Vulsini Volcanoes, Quaternary potassic Roman Comagmatic Region, Italy) provides an interesting example of multiple changes of eruptive style that were concomitant with a late phase of collapse of the polygenetic Latera Caldera. This paper reports a reconstruction of the event based on field analysis, laboratory studies of grain size and density of juvenile clasts, and re-interpretation of available subsurface geology data. The Onano eruption took place in a structurally weak area, corresponding to a carbonate substrate high bordered by the pre-existing Latera caldera and Bolsena volcano-tectonic depression, which controlled the ascent and eruption of a shoshonitic-phonotephritic magma through intersecting rim fault systems. Temporal changes of magma vesiculation, fragmentation and discharge rate, and consequent eruptive dynamics, were strongly controlled by pressure evolution in the magma chamber and changing vent geometry. Initially, pumice-rich pyroclastic flows were emplaced, followed by spatter- and lithic-rich flows and fallout from energetic fire-fountaining. The decline of magma pressure due to the partial evacuation of the magma chamber induced trapdoor collapse of the chamber roof, which involved part of the pre-existing caldera and external volcano slopes and eventually led to the present-day caldera. The widening of the vent system and the emplacement of the main pyroclastic flow and associated co-ignimbrite lag breccia marked the eruption climax. A sudden drop of the confining pressure, which is attributed to a pseudo-rigid behaviour of the magma chamber wall rocks during a phase of rapid magma drainage, led to extensive magma vesiculation and fragmentation. The disruption of the magma chamber roof and waning magma pressure in the late eruption stage favoured the explosive interaction of residual magma with groundwater from the confined carbonate aquifer. Pulsating hydrostatic and magma pressures produced alternating hydromagmatic pyroclastic surges, strombolian fallout and spatter flows.

In moving pyroclastic systems, particles are sorted as a function of their sizes, densities and shapes. The analysis of the distribution of these characteristics in particle populations of pyroclastic deposits is a major tool in evaluating properties and regimes of parent transport systems. This paper reports a study of size-density relationships of component subpopulations from selected Quaternary pyroclastic deposits of the Latera Volcanic Complex (Vulsini, Italy), by means of integrated field and laboratory investigations, including grain-size and component analyses, density measurements and SEM crystal morphoscopy. Three kinds of particle size-density relationships characterize the studied deposits, as follows: (1) a significant correlation of decreasing sizes with increasing component densities exists for direct suspension sedimentation products, including Plinian fall and pyroclastic surge deposits; (2) a wide data dispersion with poor correlation normally occurs for the main zones of deposits from (high-concentration) pyroclastic flows; and (3) component subpopulations occur in a narrow size range irrespective of density in granular systems dominated by grain collisions, such as traction zones beneath (low-concentration) pyroclastic surges and sheared basal layers of pyroclastic flows. The regression coefficients of size-density relationships appear as a comparative measure of particle concentration in the parent systems, consistent with that inferred from the degree of sorting and abrasion of crystal subpopulations from sequential fragmentation/transport analysis of grain-size data, and quantitative morphoscopy. The crystal component becomes less sorted and more abraded as the fit to the size-density relationship decreases from Plinian fall to pyroclastic surge to pyroclastic flow, consistent with increasing particle concentration in the transporting current.

This paper deals with ground-hugging, gas–pyroclast currents from explosive volcanic eruptions and their deposits. Key field observations and laboratory determinations are proposed to relate specific deposit types with flow regimes and particle concentration in the transport and depositional systems. Three relevant flow scenarios and corresponding deposit types have been recognized from a survey of pyroclastic successions of the Vulsini Volcanic District (central Italy): (1) dilute, turbulent, pyroclastic currents producing normally or multiply graded beds by direct suspension sedimentation; (2) concentrated bedload regions beneath suspension currents, depositing inversely graded beds by traction carpet sedimentation; (3) self-sustained, high particle concentration, laminar, mass flows developing massive, poorly sorted bodies, with opposite grading of coarse lithic and pumice clasts, overlying fine-grained, inversely graded, basal layers. Main distinguishing criteria include the occurrence and pattern of clast grading, clast–thickness relationships, grain size, ash matrix componentry and pyroclast size–density relationships. Downcurrent and temporal transitions among identified flow scenarios are likely to occur for changing energy conditions and gas–pyroclast ratio both on regional and local scales. The nature and efficiency of magma fragmentation, volatile content, conduit geometry (which determine the characteristics of the erupted mixture and possible lateral blast component at the vent), and the angle of incidence of the column collapse, are suggested as the main factors controlling the generation of one type over the other at flow inception. Dilute, fine-grained, overpressured eruption clouds are thought to favor the formation of low particle concentration turbulent currents. Column collapse over slightly inclined volcano slopes, causing a high degree of compression of the collapsing mixture and of gas expulsion, would favor the generation of high particle concentration pyroclastic currents.

In this paper we discuss the uncommon case of an energetic, pyroclastic-flow-forming eruption with a SiO2-poor (42–45 wt.%), K-foiditic magma composition. The Trigoria-Tor de' Cenci Tuff (TTC; 561 ka) is the product of the first large-scale explosive event (of the order of 1–10 km3 of erupted products) in the Alban Hills Volcanic District, near the city of Rome, Italy. After an initial Plinian phase that produced a scoria fall horizon, pyroclastic current activity emplaced ash deposits with leucite-bearing juvenile scoria lapilli. The abundance of accretionary lapilli, the most distinctive feature of these deposits, together with the high degree of fragmentation, the abundance of minute lithic inclusions and the morphology of ash particles, indicates a hydromagmatic character for the most part of the eruption. The absence of vent-derived carbonate lithic clasts from the deep regional aquifer and the abundance of cognate lithic fragments suggest that the interaction with external water involved a surficial aquifer in the older Alban Hills volcanic terrains. Perhaps the most striking aspect of the TTC is the K-foiditic composition of the pre-eruptive melt, which, to our knowledge, is unique among explosive events of comparable size elsewhere in the world. The pre-eruptive magma system feeding the TTC was controlled mainly by leucite+clinopyroxene fractionation under aH2O

The Villa Senni Eruption Unit (VSEU) belongs to the Tuscolano-Artemisio phase of volcanic activity in the Alban Hills Volcanic District, the closest to Rome of the recent or active volcanoes of central Italy. The most important products of this eruption are represented by pyroclastic flow deposits, named lower and upper flow unit (LFU and UFU, respectively). Three main rock types form VSEU as follows: (1) juvenile K-foiditic scoria clasts of the LFU; (2) juvenile phonotephritic scoria clasts of the UFU; and (3) holocrystalline phonotephritic lithic inclusions (Italites) in the UFU. On the basis of the chemistry, mineralogy and petrography of the three studied rock types their phase relations have been discussed. Other petrologic constraints from laboratory melting experiments are presented and used to investigate the role of volatiles on the evolution of the magma chamber system. Some broad implications on withdrawal pattern are also presented. It has been verified the LFU rock type can be obtained from a parental melt of UFU composition by a CO2-controlled crystal-liquid fractionation of a solid assemblage close in composition to that of Italites. Because it can be proved that the storage of magma occurred at shallow depth within the Mesozoic carbonate country rocks, it is proposed that CO2 diffusion, originating from thermal decomposition of wall-rock carbonates, controlled the evolution trend of the melt at the periphery of the magma chamber, whereas the inner part of the magma body retained the volatile component (essentially H2O) of the original melt. The corresponding eruption model is therefore comprehensive of an early eruptive phase (LFU rock type) involving the more differentiated central magma bulb, whereas the late eruptive phase (UFU rock type) tapped the more mafic peripheral magma. The Italite xenoliths are believed to represent the chilled margins of the magma chamber.