Search

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).

Transient volcanic plumes are time-dependent features generated by unsteady eruptive sources, having similar eruption duration and plume development timescales. Their morphological evolution reflects both the discharge history at the vent and air entrainment, crucial parameters controlling volcanic ash dispersal and impact on the environment and human activities. However, despite its importance, transient plume morphology has been scarcely quantified, due to both observational and analytical hindrances. In this study, we introduce new tools to quantify the initial morphological evolution of transient volcanic plumes by applying fractal analysis and plume's perimeter measurements to thermal high-speed and visible-light high-resolution videos of eruptions. Eruptive plumes from Sakurajima (Japan), Stromboli (Italy), and Fuego (Guatemala) volcanoes were recorded during several field campaigns in 2012–2016. The eruption dataset has been complemented by the fractal analysis of three 2D numerical gas-jet simulations at different Reynolds number (2 × 103, 5 × 103 and 10 × 103) in order to provide reference configurations to compare with the natural cases. The two shape analysis methods used show different sensitivities. The ratio between plume and bounding box perimeters appears to be more perceptive of punctual dynamical variations, while fractal analysis reflects the overall plume evolution. Both methods highlight that plume shape complexity increases over time and is related to the formation and development of smaller scale vortexes. The variability of the rate of fractal dimension increase over time (αD) effectively captures plume evolution. It also appears that αD correlates with the ash eruption rate (AER) evolution and the instability of the source. This study shows that discharge history and intensity at the vent are the first order control on plume's shape evolution and, by inference, on its air entrainment ability.

Quaternary carbonate-seated maar-diatremes in the Volsci Range are one of the most intriguing products of the west-directed subduction of the Adriatic slab that drove the development of the Apennine mountain belt in Central Italy. The Volsci Volcanic Field is characterized by phreatomagmatic surge deposits, rich in accidental carbonate lithics, and subordinate Strombolian scoria fall deposits and lava flows, locally sourced from some tens of monogenetic eruptive centers (at least fifty tuff rings and scoria cones). We investigate the subsurface maar-diatreme processes in terms of relationships between faulting and explosive magma-water interaction, as well as the distribution pattern of the eruptive centers. With this aim, we present the following new data: i) description of the fold-and-thrust belt structure and associated eruptive centers, ii) componentry of volcanic rock-types, iii) determination of grain-size, degrees of whiteness and roundness of carbonate lithic inclusions, iv) micropaleontological analysis of carbonate lithics. We show that the clustering of eruptive centers is controlled by tectonic features. A first order control is tentatively related to crustal laceration and deep magma injection along a ENE-trending Quaternary lateral tear in the slab and to Mesozoic rift-related normal faults. A second-order control is provided by orogenic structures (mainly thrust and extensional faults). In particular, magma-water explosive interaction occurred at multiple levels (

Volcanic processes occur in a wide range of temporal and spatial scales. However, a key step of magma ascent is recognizable in the dynamics of gas and magma in the shallow plumbing system, where volatiles play a fundamental role in controlling the eruptive style. With the aim of investigating shallow degassing processes, an experimental setup was deployed at Mt. Yasur, an active volcano located in Tanna Island (Vanuatu arc), from 9th to 12th July 2011. The setup comprised high-speed and thermal cameras, as well as a microphone, capable of recording both in the infrasonic and audible range. The analysis of acoustic signals, validated by observing images from the high-speed and thermal cameras, has enabled characterizing the explosive activity during the investigated period. Two types of explosions, distinct for spectral features and waveforms, were observed: (i) minor events, corresponding to small overpressurized bursts, occurring almost continuously; (ii) major events, characterizing the Strombolian activity at Mt. Yasur. By investigating variation in the occurrence rate of the minor events, we found that, on a short timescale, the dynamics responsible for the two types of explosions are decoupled. These results, together with previous literature data, bring additional evidence of the existence of distinct sources of degassing. Finally, major events can be distinguished as emergent events, i.e. long-lasting signals, corresponding to ash-rich explosions, and impulsive events, featuring shorter duration and larger amplitude.

We present new 40Ar/39Ar data which allow us to refine the recurrence time for the most recent eruptive activity occurred at Colli Albani Volcanic District (CAVD) and constrain its geographic area. Time elapsed since the last eruption (36 kyr) overruns the recurrence time (31 kyr) in the last 100 kyr. New interferometric synthetic aperture radar data, covering the years 1993–2010, reveal ongoing inflation with maximum uplift rates (>2 mm/yr) in the area hosting the most recent (

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.

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.

[1] Relationships between frequency and intensity of volcanic eruptions are actively sought by geophysicists for both monitoring and research purposes. By analyzing surveillance videos of persistent volcanic activity at Stromboli (Italy), we derived the frequency and jet height of >4000 explosions that occurred in 72 h-long time windows sampled yearly from 2005 to 2009. We found a positive relationship linking explosion frequency and jet height (linked to eruption intensity) when averaging the two parameters over time intervals from hours to days, with a stronger correlation for longer intervals. We interpret this behavior as the response of the magmatic system to variable influx of magma and gas at depth, increased flux at depth causing more frequent and stronger explosions at the surface. This relationship entails concurrent control of source processes over explosion frequency and intensity, directly impacting modeling of explosion sources at persistently active volcanoes in general and hazard assessment at Stromboli in particular.

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.

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.

We have refined the clinopyroxene-based hygrometer published by Armienti et al. (2013) for a better quantitative understanding of the role of H2O in the differentiation of Etnean magmas. The original calibration data set has been significantly improved by including several experimental clinopyroxene compositions that closely reproduce those found in natural Etnean products. To verify the accuracy of the model, some randomly selected experimental clinopyroxene compositions external to the calibration data set have been used as test data. Through a statistic algorithm based on the Mallows’ CP criterion, we also check that all model parameters do not cause data overfitting, or systematic error. The application of the refined hygrometer to the Mt. Etna 2011–2013 lava fountains indicates that most of the decreases in H2O content occur at P < 100 MPa, in agreement with melt inclusion data suggesting abundant H2O degassing at shallow crustal levels during magma ascent in the conduit and eruption to the surface.

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.

The solubility of H2O- and CO2-bearing fluids in trachytic and trachybasaltic melts from erupted magmas of the Campi Flegrei Volcanic District has been investigated experimentally at 1100 and 1200 °C, respectively, and at 100, 200, 300, 400, and 500 MPa. The solubility of H2O in the investigated melts varies between 3.48 ± 0.07 wt% at 100 MPa to 10.76 ± 0.12 wt% at 500 MPa in trachytic melts and from 3.49 ± 0.07 wt% at 100 MPa to 9.10 ± 0.11 wt% at 500 MPa in trachybasaltic melts. The content of dissolved CO2 in melts coexisting with the most CO2-rich fluid phase increases from 281 ± 24 ppm at 100 MPa to 2710 ± 99 ppm at 500 MPa in trachyte, and from 727 ± 102 ppm at 100 MPa to 3565 ± 111 ppm at 500 MPa in trachybasalt. Natural samples from the Campanian Ignimbrite eruption (trachyte) and from the Solchiaro eruption (trachybasalt) were collected around the city of Naples and on Procida Island. Deuterium/hydrogen (D/H) ratios were analyzed in natural pumices pre-heated at different temperatures to remove water adsorbed and/or imprinted by glass alteration processes. It has been determined that heating of the glass to 350 °C efficiently removes most of secondary water and the remaining concentrations represent primary magmatic water preserved in the erupted material. Hydrogen isotope composition (with δD values ranging between −70‰ and −110‰) and its correlation with bulk water content in selected pumice samples of the Campanian Ignimbrite eruption are consistent with isotopic fractionation between magmatic fluid and melt during degassing of erupting magma. Hence, the H2O and CO2 contents in natural glasses from pumice samples are considered as minimum estimates on volatile concentrations in the melt just prior to the eruption or at the fragmentation event. The water contents in natural glasses vary from 0.83 ± 0.07 to 3.74 ± 0.06 wt% for trachytes from the Campanian Ignimbrite eruption and from 1.96 ± 0.06 to 3.47 ± 0.07 wt% for trachybasalts from the Solchiaro eruption. The CO2 contents vary from 78 ± 120 ppm CO2 to 1743 ± 274 ppm for trachytes from the Campanian Ignimbrite eruption and from 240 ± 293 to 1213 ± 250 ppm for trachybasalts from the Solchiaro eruption. A combination of natural and experimental data provides minimum pressure estimates for the storage and ascent conditions of magmas. The Campanian Ignimbrite magma could have been stored or ponded during its rising path at two different levels: a deeper one corresponding to depth of about 8 to 15 km and a shallower one at about 1 to 8 km. Trachybasalts from Solchiaro erupted from the deepest level of about 11 km with a storage or ponding level at around 2 to 8 km depth. Although an uncertainty of at least a kilometer has to be considered in estimating storage or ponding depths, these estimates point to significantly deeper magmatic sources for both eruptions as those considered previously.

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.

The angular velocity of Earth's rotation shows decadal oscillations due to the lunisolar gravitational torque, as well as inter- or intra-annual changes arising from the angular momentum exchange between the atmosphere and the solid Earth. The energies involved in the Length of Day (LOD) variations may affect the crustal deformation rate and seismic energy release on a global scale. We found significant correlation between the occurrences of major volcanic eruptions and the LOD pattern since AD 1750. On a multiyear scale, eruption frequency worldwide increases with LOD changes. Moreover, the injection of sulphur gases into the atmosphere during major eruptions is accompanied by significant inter-annual LOD variations. This provides evidence of complex mutual cause-and-effect interactions: stress changes induced by multiyear variations in Earth's spin may affect climactic volcanic activity; also, the atmosphere's dynamic response to volcanic plumes may result in global changes of wind circulation and climate, with consequent LOD variations. © 2013 John Wiley & Sons Ltd.

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.

[1] The identification and evaluation of trigger mechanisms for volcano flank instabilities and/or collapse represent a key issue for risk assessment in densely populated volcanic areas, as well as in long-distance settings, particularly in case of island or coastal volcanoes. Here we address quantitatively the effects of external (seismic) and inner (magmatic) forcing on the stress-strain state associated to flank instabilities at Mount Etna (Sicily, southern Italy) by means of a 2-D finite difference method numerical modeling. Modeled seismic actions include strong near-field, strong far-field, and low-magnitude near-field earthquakes. Magmatic actions consider the inner pressure changes induced by energetic lava fountains in the summit crater area and subvertical and oblique dike ascent below the summit area. Model results are validated in light of available monitoring data and recent eruptive activity. Numerical results show that the main strain effects are produced by high-magnitude near-field earthquakes (expected return time of ~103 yrs) and by vertical uprise of a magma dike below the volcano summit area. Maximum displacements in the order of tens of centimeters may involve the summit area, up to some 106 m3/m over some kilometers laterally. Stress releases up to 107 Pa may affect a limited portion of the magmatic conduit, thus favoring major effusive flank eruptions. Major catastrophic events, such as volcano flank collapse, should not be expected by applying, either individually or combined, the aforementioned actions.

[1] Explosive volcanic activity on Earth is typically discontinuous in space and time. The occurrence of spatial/temporal eruption clusters due to mutual cause-and-effect relationships or external triggers (e.g., tectonic and/or tidal forcing) is still debated. To detect possible clustering of major explosive eruptions, we test the distribution of eruptions with Volcanic Explosivity Index (VEI) ≥ 4 from 1750 to present. The 143 documented VEI ≥ 4 events display a markedly non-uniform frequency distribution, with the highest relative probability of eruption recurrence within 500 km distance from the preceding event. The analogous frequency pattern obtained from randomized data series of the same catalogue suggests that the observed eruption pattern is primarily imparted by the geodynamic distribution of volcanoes (mostly located along tectonically active linear belts), with no evidence of mutual or external influence. Our results highlight a counter-intuitive array of major eruption loci as a consequence of the intrinsic worldwide asymmetry of explosive volcanism, with implications on hazard assessment.

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.

Cored bombs, a kind of pyroclast consisting of a lithic core surrounded by a chilled shell of juvenile material, record the thermal interaction of magma with wall rocks. We performed textural analysis of cored bombs, solid–melt heat-transfer theoretical modelling, and high-temperature coating experiments to put temporal and intensity constraints on the thermal interaction of potassic magma feeder systems with carbonate wall rocks during explosive eruptions in the Quaternary, Colli Albani Volcanic District (Roman Province). It appears that the degree of thermal alteration of lithic cores records the duration of magma-core heat transfer, whereas the core/shell size ratio records the initial entrainment temperature of the lithic fragment. Both parameters appear to vary significantly with the eruptive style, magnitude and vent location. Specifically, small-scale (~ 0.1–1 km 3 DRE) hydromagmatic eruptions show magma-core heat-transfer durations of 0.1–10 s and entrainment temperatures in the range of 100–300 °C in the case of a monogenetic maar located in the Colli Albani peripheral area, while entrainment temperature is as high as to 800 °C for a polygenetic maar in a high-enthalpy geothermal system at the margins of the main Colli Albani magma chamber. A large-scale (~ 30 km 3 DRE) caldera-forming explosive event shows magma-core heat-transfer duration in the order of 10 2 –10 3 s and temperature of 100–500 °C at the initial magma–wall rock contact. On these grounds, we derived the cooling rate of magmas as a function of the initial temperature, mass and size distribution of lithic clasts entrained. Magma cooling by lithic entrainment may have occurred on the same time-scale as that of eruptive pulses (seconds to hours), implying that lithic entrainment may effect changes in magma physico-chemical properties on a short time-scale and, consequently, affect eruptive conduit dynamics.

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

[1] Here we model quantitatively the stress and strain induced by tidal forces as possible triggers of Mount Etna eruptions during 1989, 2000 and 2001, by means of 2D numerical analysis by finite difference method applied to the volcano edifice. Spectral and vectorial analysis of tidal acceleration indicates that the quasi-diurnal, semi-diurnal and ter-diurnal constituents may have induced decompressions up to some kPa in the shallow (i.e., 1,500 m) gas-saturated magma reservoir, which controlled magma vesiculation and the timing of lava fountain activity from the South East Crater (SEC). We consider the geological heterogeneities in the volcanic edifice (i.e., rock geometries and properties, feeder conduit and vent systems, volcanic landforms), the magma properties (i.e., Bulk modulus, volatile content) and the ongoing volcano-tectonic stresses (i.e., volcano flank deformations), which strongly influence the response of eruptive activity to tidal forces. Indeed, the tidal factor may act on flank deformation concomitant to shallow magma inflation. The implications of our model may help understand the eruptive behaviour and the disequilibrium state of the volcano and may be useful for hazard assessment at Mount Etna.

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.

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.