Raffaello Cioni

Tephrostratigraphy, chronology and climatic events of the Mediterranean basin during the Holocene: An overview:

The identification and characterisation of high-frequency climatic changes during the Holocene requires natural archives with precise and accurate chronological control, which is usually difficult to achieve using only 14C chronologies. The presence of time-spaced tephra beds in Quaternary Mediterranean successions represents an additional, independent tool for dating and correlating different sedimentary archives. These tephra layers are potentially useful for resolving long-standing issues in paleoclimatology and can help towards correlating terrestrial and marine paleoclimate archives. Known major tephras of regional extent derive from central and southern Italy, the Hellenic Arc, and from Anatolia. A striking feature of major Holocene tephra deposition events in the Mediterranean is that they are clustered rather than randomly distributed in time. Several tephra layers occurred at the time of the S1 sapropel formation between c. 8.4 and 9.0 ka BP (Mercato, Gabellotto-Fiumebianco/E1, Cappadocia) and other important tephra layers (Avellino, Agnano Monte Spina, ‘Khabur’ and Santorini/Thera) occurred during the second and third millennia BC, marking an important and complex phase of environmental changes during the mid- to late-Holocene climatic transition. There is great potential in using cryptotephra to overlap geographically Italian volcanic ashes with those originating from the Aegean and Anatolia, in order to connect regional tephrochronologies between the central and eastern Mediterranean.

Thermal and compositional evolution of the shallow magma chambers of Vesuvius: Evidence from pyroxene phenocrysts and melt inclusions

A large number of clinopyroxene (cpx) phenocrysts and hosted melt inclusions (MI) in pyroclasts from several Vesuvius eruptions were analyzed. Assuming that their temperature of homogenization (Thom) reflects that of crystallization of the host mineral and that after homogenization and quenching, composition of the MI represents the melt from which the host crystallized, each cpx-MI pair records equilibrium conditions at Thom. These data were used to discuss the magma evolution within shallow magma chamber. They formed by means of the periodic arrival of mafic batches, recorded by the MI in diopside (Fs4–7) crystallized at 1150°–1200°C. Under open conduit conditions the chambers are small, and each magma pulse induces thermal and compositional variations recorded by oscillatory zoned pyroxenes (Fs4–15). Their MI and Thom summarize the evolution of the chamber resulting from cycles of magma injection, crystal settling, and magma extraction. The deposits of the explosive eruptions which reopen the conduit reflect variable mixing during magma withdrawal from zoned reservoirs, whose layering was deciphered through MI and Thom in salite (Fs15–30). We suggest that the Vesuvius magma chambers evolve from prolate toward subequant, changing their layering with increasing volume and age: (1) initial stage, high aspect ratio chamber, homogeneous mafic melt (T°C∼1100) crystal enriched downward; (2) young stage, medium aspect ratio, continuous gradation from mildly evolved (T°C∼1050) to felsic melt (T°C∼850–900); and (3) mature stage, low aspect ratio, twofold chamber with stepwise gradient separating lower, convective, mildly evolved portion (T°C∼1050) from upper, stratified, felsic portion (T°C∼800–950).

Tephra sedimentation during the 2010 Eyjafjallajökull eruption (Iceland) from deposit, radar, and satellite observations

The April-May 2010 eruption of Eyjafjallajokull volcano (Iceland) was characterized by a nearly continuous injection of tephra in the atmosphere that affected various economic sectors in Iceland and caused a global-wide interruption of air traffic. Eruptive activity during 4-8 May 2010 was characterized based on short-duration physical parameters in order to capture transient eruptive behavior of a long-lasting eruption (i.e., total grainsize distribution, erupted mass and mass eruption rate averaged over 30-minute activity). Resulting 30-minute total grainsize distribution based on both ground and MSG-SEVIRI satellite measurements is characterized by Mdphi of about 2 phi and a fine-ash content of about 30wt%. Accumulation rate varied by two orders of magnitude with an exponential decay away from the vent, whereas Mdphi shows a linear increase until about 18 km from vent reaching a plateau of about 4.5 phi between 20-56 km. Associated mass eruption rate is in between 0.6-1.2 x 10^5 kg s^-1. In-situ sampling showed how fine ash mainly fell as aggregates of various typologies. About 5 to 9 wt% of the erupted mass remained in the cloud up to 1000 km from the vent, suggesting that nearly half of the ash >7 phi settled as aggregates within the first 60 km. Particle sphericity and shape factor varied between 0.4 and 1 with no clear correlation with size and distance from vent. Our experiments also demonstrate how satellite retrievals and Doppler radar grainsize detection can provide real-time description of the source term but for a limited particle-size range.

Assessing pyroclastic fall hazard through field data and numerical simulations: Example from Vesuvius

Received 12 April 2001; revised 2 April 2002; accepted 7 May 2002; published 1 Feburary 2003. [1] A general methodology of pyroclastic fall hazard assessment is proposed on the basis of integrated results of field studies and numerical simulations. These approaches result in two different methods of assessing hazard: (1) the ‘‘field frequency,’’ based on the thickness and distribution of past deposits and (2) the ‘‘simulated probability,’’ based on the numerical modeling of tephra transport and fallout. The proposed methodology mostly applies to volcanoes that, by showing a clear correlation between the repose time and the magnitude of the following eruptions, allows the definition of a reference ‘‘maximum expected event’’ (MEE). The application to Vesuvius is shown in detail. Using the field frequency method, stratigraphic data of 24 explosive events in the 3–6 volcanic explosivity index range in the last 18,000 years of activity are extrapolated to a regular grid in order to obtain the frequency of exceedance in the past of a certain threshold value of mass loading (100, 200, 300, and 400 kg/m 2 ). Using the simulated probability method, the mass loading related to the MEE is calculated based on the expected erupted mass (5 10 11 kg), the wind velocity profiles recorded during 14 years, and various column heights and grain-size populations. The role of these factors was parametrically studied performing 160,000 simulations, and the probability that mass loading exceeded the chosen threshold at each node was evaluated. As a general rule, the field frequency method results are more reliable in proximal regions, provided that an accurate database of field measurements is available. On the other hand, the simulated probability method better describes events in middle distal areas, provided that the MEE magnitude can be reliably assumed. In the Vesuvius case, the integration of the two methods results in a new fallout hazard map, here presented for a mass loading value of 200 kg/m 2 . INDEX TERMS: 8404 Volcanology: Ash deposits; 3210 Mathematical Geophysics: Modeling; 1035 Geochemistry: Geochronology; 5480 Planetology: Solid Surface Planets: Volcanism (8450); 9335 Information Related to Geographic Region: Europe; KEYWORDS: Vesuvius, ash fallout, explosive eruptions, volcanic hazard, numerical modeling, hazard mapping

Petrology and geochemistry of the ultrapotassic rocks from the Sabatini Volcanic District, central Italy: the role of evolutionary processes in the genesis of variably enriched alkaline magmas

Abstract The Sabatini Volcanic District (SVD) is a large volcanic field characterised by the lack of any major volcanic center. Its activity, spread over a wide area, started at 0.6 Ma and developed through five main phases, during which several calderas and the Bracciano lake volcano-tectonic depression were formed. All the volcanic rocks belong to the Roman-type ultrapotassic series (HKS), ranging from leucite tephrites to leucite and hauyne phonolites. Although the major- and compatible-element contents indicate a single series of evolution, there are differences in the incompatible trace-element abundances. A high-Ba series (HBaS) has been distinguished from a low-Ba series (LBaS), with the former also enriched in all other incompatible elements (e.g., REE, Nb, Zr, Th) except Rb. The HBaS rocks are plagioclase-free, leucite-bearing lavas and were abundantly outpoured from the Bracciano volcanoes during the third and fifth phase of activity. Plagioclase- and phlogopite-bearing rocks constitute the LBaS and were erupted during the other phases generally from smaller and eccentric volcanic centers. The initial 87 Sr/ 86 Sr values are higher in the HBaS rocks and do not vary significantly with magma evolution (0.71047–0.71080), but cover a wider range in the LBaS rocks (0.70944–0.71038), with the lowest Sr isotope ratios occurring in the least evolved lavas. The higher Ca content in the olivine and Ti and Al IV in the clinopyroxene, and the lower ulvospinel content of the Ti-magnetites of the HBaS rocks suggest an evolution at lower pressure and higher temperature for this magma. The observed petrologic characteristics suggest that the HBaS magma evolved at lower depths by processes of refilling, tapping, fractionation and probably assimilation (RTFA), where the crystallisation rate of clinopyroxene+leucite±olivine dominates over the input rate of the fresh magma. The LBaS magma evolved at slightly higher pressure, in separate and small magma bodies, by fractional crystallisation of clinopyroxene+plagioclase±phlogopite±olivine that was often associated with crustal assimilation (AFC). It has been suggested that RTFA processes with high input rate/crystallisation rate ratios could also be responsible for the differentiation between the HBaS and LBaS. The different processes of evolution undergone by the HBaS and LBaS could have been related to the different volumes of magma rising from the source.

Upward migration of Vesuvius magma chamber over the past 20,000 years

Forecasting future eruptions of Vesuvius is an important challenge for volcanologists, as its reawakening could threaten the lives of 700,000 people living near the volcano. Critical to the evaluation of hazards associated with the next eruption is the estimation of the depth of the magma reservoir, one of the main parameters controlling magma properties and eruptive style. Petrological studies have indicated that during past activity, magma chambers were at depths between 3 and 16 km (refs 3–7). Geophysical surveys have imaged some levels of seismic attenuation, the shallowest of which lies at 8–9 km depth, and these have been tentatively interpreted as levels of preferential magma accumulation. By using experimental phase equilibria, carried out on material from four main explosive events at Vesuvius, we show here that the reservoirs that fed the eruptive activity migrated from 7–8 km to 3–4 km depth between the ad 79 (Pompeii) and ad 472 (Pollena) events. If data from the Pomici di Base event 18.5 kyr ago and the 1944 Vesuvius eruption are included, the total upward migration of the reservoir amounts to 9–11 km. The change of preferential magma ponding levels in the upper crust can be attributed to differences in the volatile content and buoyancy of ascending magmas, as well as to changes in local stress field following either caldera formation or volcano spreading. Reservoir migration, and the possible influence on feeding rates, should be integrated into the parameters used for defining expected eruptive scenarios at Vesuvius.

Plinian and Subplinian Eruptions

Abstract The term “plinian” encompasses powerful explosive eruptions characterized by the quasi-steady, hours-long, high-speed discharge into the atmosphere of a high-temperature, multiphase mixture (gas, solid, and liquid particles), forming a buoyant vertical column that reaches heights of tens of kilometers and often alternates with phases of column collapse. Subplinian eruptions have lower intensity but dynamics similar to plinian events, mainly distinguishing by the occurrence of high-frequency fluctuations or of temporary breaks in the discharge. In these eruptions, the repeated generation of short-lived convective plumes may alternate with phases of quiescence or of lower intensity, explosive or effusive activity.The complex dynamics of all these events is modulated by the conditions of magma withdrawal from crustal reservoirs, and ascent, fragmentation, and dispersal in the atmosphere. The accurate observation of the sedimentological and compositional features of the deposits is a powerful tool to quantify the main parameters which control these eruptions.

Variability of the eruption dynamics during a Subplinian event: the Greenish Pumice eruption of Somma-Vesuvius (Italy)

Subplinian eruptions are generally characterized by unsteadiness in magma discharge, which is reflected in the formation of strongly oscillating convective columns. As a result, the pyroclastic fall deposits show clear grain size stratification, and are frequently interlayered with small-volume pyroclastic density currents. The causes of this pulsatory behavior are at present not well understood, and represent one of the most interesting topics for future research in explosive volcanism. The Greenish Pumice (16 020±130 yr BP) is one of the higher magnitude Subplinian eruptions of Somma–Vesuvius and shows an alternation of pyroclastic fall and flow deposits. Given their complexity, the deposits were subdivided into seven lapilli beds, five ash beds, and deposits derived from pyroclastic density currents. The stratigraphic sequence records the transition from an initial phase of quasi-steady discharge resulting in a Subplinian convective column to phases of more discontinuous, pulsating activity, with the formation of Vulcanian to Subplinian plumes. Juvenile clasts have low phenocryst contents and exhibit a broad range in color, vesicularity, and groundmass crystallinity throughout the eruptive sequence. The density of juvenile fragments shows a broad range within each single layer, ranging from 0.4 to 2 g/cm3. Maximum lithics in the lapilli layers suggest column heights between 17 and 20 km (mass discharge rate from 1.5 to 3×107 kg/s). Volume estimates based on proximal and medial deposits give a very conservative value of 0.47 km3 that, based on thickness data at two distal sites, can be expanded to a volume up to three times larger, greater than the typical values of mid-intensity eruptions at Somma–Vesuvius. We suggest that the complex dynamics of the Greenish Pumice (GP) eruption are related to syn-eruptive viscosity gradients in the rising magma, induced by an important, degassing related, groundmass crystallization during ascent along the conduit. Phreatomagmatic activity did not play a major role in the eruption dynamics.

MeMoVolc report on classification and dynamics of volcanic explosive eruptions

Classifications of volcanic eruptions were first introduced in the early twentieth century mostly based on qualitative observations of eruptive activity, and over time, they have gradually been developed to incorporate more quantitative descriptions of the eruptive products from both deposits and observations of active volcanoes. Progress in physical volcanology, and increased capability in monitoring, measuring and modelling of explosive eruptions, has highlighted shortcomings in the way we classify eruptions and triggered a debate around the need for eruption classification and the advantages and disadvantages of existing classification schemes. Here, we (i) review and assess existing classification schemes, focussing on subaerial eruptions; (ii) summarize the fundamental processes that drive and parameters that characterize explosive volcanism; (iii) identify and prioritize the main research that will improve the understanding, characterization and classification of volcanic eruptions and (iv) provide a roadmap for producing a rational and comprehensive classification scheme. In particular, classification schemes need to be objective-driven and simple enough to permit scientific exchange and promote transfer of knowledge beyond the scientific community. Schemes should be comprehensive and encompass a variety of products, eruptive styles and processes, including for example, lava flows, pyroclastic density currents, gas emissions and cinder cone or caldera formation. Open questions, processes and parameters that need to be addressed and better characterized in order to develop more comprehensive classification schemes and to advance our understanding of volcanic eruptions include conduit processes and dynamics, abrupt transitions in eruption regime, unsteadiness, eruption energy and energy balance.

Apulian Bronze Age pottery as a long-distance indicator of the Avellino Pumice eruption (Vesuvius, Italy)

Abstract During the Bronze Age, Vesuvius had a Plinian eruption whose deposits are known as the Avellino Pumice. The eruption spread a blanket of white and grey pumice across southern Italy, and there was a severe impact on proximal areas. Assessment of volcanological factors for the Plinian phase gives intensities of 5.7 × 107 kg s−1 for the white pumice phase and 1.7 × 108 kg s−1 for the grey pumice phase, corresponding to column heights of 23 and 31 km, respectively. Volume (magnitude) calculations using the crystal concentration method (CCM) give respectively 0.32 and 1.25 km3 of deposit, in a total minimum period of about 3 h. Archaeometric studies on Bronze Age domestic pottery from several settlements in Apulia (SE Italy) reveal the presence of pumice fragments mixed with the clay, and petrological and chemical criteria suggest that these pumices are from the Avellino eruption. This relationship allows us to fix precise correlations between different archaeological facies of the Italian Bronze Age. To explore the possibility of an extensive use of pumices in these distal regions (about 140 km from Vesuvius), we calculated the possible thickness of the tephra blanket. We propose a method to extrapolate proximal data on the deposit to calculate its minimum distal thickness. Such a method could also be used in volcanic hazard studies to assess the distal impact of large past eruptions.