Roberto Santacroce

The campanian ignimbrite: a major prehistoric eruption in the Neapolitan area (Italy)

A geological, chemical and petrographical study of the Campanian ignimbrite, a pyroclastic flow deposit erupted about 30,000 years ago on the Neapolitan area (Italy), is reported. The ignimbrite covered an area of at least 7,000 km2; it consists of a single flow unit, and the lateral variations in both pumice and lithic fragments indicate that the source was located in the Phlegraean Fields area.Textural features, areal distribution and its morphological constraints suggests that the eruption was of the type of highly expanded low-temperature pyroclastic cloud. The original composition was strongly modified by post-depositional chemical changes involving most of the major and trace elements. No primary differences in the composition of the magma have been recognized. The Campanian ignimbrite is a nearly saturated potassic trachyte, similar to many other trachytes of the Quaternary volcanic province of Campania. Its chemistry indicates an affinity with the so-called «low-K association» of the Roman volcanic province.

Magma mixing and convective compositional layering within the Vesuvius magma chamber

The pumice-fall deposits of the last two Plinian eruptions of Vesuvius-a.d. 79 “Pompei” and 3700 b.p. “Avellino”-show a marked vertical compositional variation from white phonolite at the base to grey tephritic phonolite at the top. In both Avellino and Pompei sequences a compositional gap separates white from grey pumice. Grey and white pumice have distinct Sr and Nd isotopic compositions (grey pumice: 87Sr/36Sr=0.70749-56, 143Nd/144Nd=0.512507 for Pompei; 0.70760-69, 0.512504 for Avellino; white pumice: 0.70757-78 for Pompei; 0.70729-42 for Avellino). K-feldspar separated from both grey and white pumice has, in all cases, a “white” 87Sr/86Sr ratio (0.70766-79 for Pompei, 0.70728-33 for Avellino). The observed variations are interpreted as reflecting a pre-eruptive zonation of the magma chamber. Although mineralogical evidence of interaction between magma and calcareous country rocks exists in both eruptions, crustal contamination has not significantly modified the isotopic signatures of the erupted products. Petrographic and isotopic evidence of syneruptive magma mingling occur in Pompei grey pumice as well as in Avellino white and grey pumice, but they do not fully explain all the observed geochemical and isotopic variations. These variations are related to the complex refilling history of the magmatic system and result by fractional crystallization and mixing processes acting within the magma chamber. Preliminary data from other Plinian and subplinian sruptions of the Somma-Vesuvius point out the repeticive behaviour of 87Sr/86Sr variation in the last 25 000 years, hence suggesting a single magma chamber and continuity of the feeding system.

The somma-vesuvius magma chamber: a petrological and volcanological approach

The volcanic history of Somma-Vesuvius indicates that salic products compatible with an origin by fractionation within a shallow magma chamber have been repeatedly erupted («Plinian» pumice deposits). The last two of these eruptions, (79 A.D. and 3500 B.P.) were carefully studied. Interaction with calcareous country rocks had limited importance, and all data indicate that differentiated magmas were produced by crystal-liquid fractionation within the undersaturated part of petrogeny’s residua system at about 1 kb water pressure. The solid-liquid trend indicates that the derivative magmas originated by fractionation of slightly but significantly different parental liquids. Some lavas of appropriate composition were selected as parental liquids to compute the entity of the fractionation. Results suggest that in both bases a fractionation of about 70 weight % was needed to produce liquids with the composition of the pumice. The combination of all data indicates that the two Plinian eruptions were fed by a magma chamber (3–4 km deep) having a volume of approx. 2.0–2.5 km3. The temperature of the magma that initially entered the chamber was about 1100°C, whereas the temperature of the residual liquids erupted was Plinian pumice was 800° and 850°C respectively. There is no evidence that such a magma chamber existed at Vesuvius after the 79 A.D. eruption. These results have relevant practical implications for volcanic hazard and monitoring and for geothermal energy.

Magmatic and phreatomagmatic phases in explosive eruptions of Vesuvius as deduced by grain-size and component analysis of the pyroclastic deposits

Abstract Grain-size and component analyses have been carried out on pyroclastic deposits of three well-known explosive eruptions of Vesuvius: those of 79 A.D., 1631 and 1906. These eruptions cover a wide energy spectrum, from Plinian to Strombolian, and each includes a transition from a magmatic to a well-documented and clearly distinguishable phreatomagmatic phase of activity. The chronicles of the eruptions are revisited in light of the granulometric and component data obtained from the study of the deposits and interpretation of the dynamics of the main eruptive episodes is provided for each eruption. The phreatomagmatic products (originated from explosive interaction of magma with groundwater) have characteristic grain-size and component distribution patterns distinct from associated purely magmatic pyroclastic products: in any eruption phreatomagmatic products exhibit: (a) a marked increase in both the lithic/juvenile and the crystal/total juvenile ratios, and (b) a preferential fragmentation of the juvenile fraction. Moreover, eruption energy, degree of evolution of erupted magma, lithic content and depth of provenance are all clearly correlated. The abundance of lithics and crystals in phreatomagmatic deposits can be ascribed to (a) preferential fragmentation of the aquifer-hosting rocks due to explosive vaporization of ground water; (b) vent flaring and craterization possibly related to overpressure conditions following the entrance of large quantities of steam in the conduit; (c) indirect enrichment in both crystal and lithic fractions by removal of juvenile fines from the eruptive cloud. In each eruption some of the deposits with “phreatomagmatic” component distribution patterns also exhibit evidence of steam condensation (accretionary lapilli, vesiculated tuff, mud coating, soft deformations) and some do not: these are therefore to be considered sufficient but not necessary diagnostic features for a hydromagmatic origin of the deposit (“wet” in the sense of Sheridan and Wohletz, 1983). “Dry” phreatomagmatic products correspond to higher energy conditions allowing production and maintenance of overheated steam. The magma/water ratio in terms of heat exchange surface (i.e. the degree of primary fragmentation of the magma) is certainly the main factor controlling the efficiency and the energy balance of the interaction. A series of observations strongly suggest that a primarily fragmented magma is a basic requisite for the explosive magma/water interaction: (a) occurrence of short phreatomagmatic episodes interrupting lava fountains activity (1906 eruption) in connection with drawdown of the magma column in the conduit; (b) parallel increase in depth of magma fragmentation level and magma/water interaction level (as deduced by the nature of lithics), and (c) preferential occurrence of phreatomagmatic activity at the end of explosive eruptions (when juvenile gas pressure declines and water from the confining aquifer more easily enters the conduit).

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.

Plio-quaternary volcanism in Ecuador

Extensive sampling, major element chemistry on over 300 samples and K-Ar radiometric dating have been carried out on the Ecuadorian Upper Tertiary–Quaternary volcanoes. The results show important space–time variations of the volcanic activity, between Late Miocene time and the present. In Late Miocene time a continuous volcanic belt, located approximately along the present volcanic front (VF), affected the whole country from the Cuenca basin to the south, up to Colombia to the north. Major changes occurred at about 5 Ma: volcanic activity stopped south of the Guayaquil fault belt and never resumed; to the north the active volcanic axis shifted eastward to the Cordillera Real (CR) area with a simultaneous relative decrease in intensity. Since Early Quaternary time the volcanic belt widened westward to the Western Cordillera where the volcanism resumed at about 1.5–1.0 Ma, giving rise to the very wide active volcanic zone of Ecuador. The Plio-Quaternary products show significant longitudinal and latitudinal chemical and mineralogical changes. Volcanics of the VF and Interandean Depression contain amphibole and define a calc-alkaline trend with a K 2 O content lower than that of the CR products, which are characterized by a mostly anhydrous phenocryst assemblage. In both areas andesites dominate, but extreme compositions (basaltic andesites and rhyolites) are more diffuse in the CR than the VF. No significant transverse zoning has been detected in the northern region (north of the Chota-Mira transverse tectonic line). The observed temporal and spatial variations are interpreted as a result of the subduction of the Carnegie Ridge anomalous oceanic crust, underthrusting of which began approximately 6 Ma ago.

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).

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

Characteristics of May 5–6, 1998 volcaniclastic debris flows in the Sarno area (Campania, southern Italy): relationships to structural damage and hazard zonation

The destructive power of debris flows on alluvial fan surfaces was investigated by studying the event of May 5–6, 1998 in the Sarno area (Campania, southern Italy). We assessed some physical and dynamic properties of debris flows, such as volumes, peak discharge and mobility, in order to obtain velocity profiles along alluvial fan surfaces. Reconstructed peak velocities vary from 20 m s−1 to 14 m s−1, while the runouts from the alluvial fan apexes range between ∼900 m and ∼2000 m. Debris flow velocities exponentially decrease from the alluvial fan apex to the distal depositional zone but abruptly drop to zero after entering the densely inhabited areas. Based on velocity data and estimates of flow density, the values of impact pressure on rigid structures (i.e. buildings) were assessed through the calculation of hydrostatic pressure and dynamic overpressure. The data from the study area show that at flow velocities >4–5 m s−1 the dynamic overpressure accounts for most of the observed damage, whereas at lower velocities the contributions of the hydrostatic and dynamic pressures become similar. The comparisons between the calculated impact pressures and the damage on structures show that for loading >35 kPa (velocity >3 m s−1) most of buildings were destroyed or severely damaged. For values <35 kPa only minor damage occurred to the structures. Finally, an empirical law for the assessment of impact pressure on spreading areas was presented, useful for hazard zonation in similar geomorphologic settings.

The A.D. 472 “Pollena” eruption: volcanological and petrological data for this poorly-known, plinian-type event at vesuvius

Abstract The pyroclastic products of a poorly-known eruption of Vesuvius (ascribed by a combination of historic and radiocarbon data to A.D. 472) have been investigated from both volcanological and petrological points of view. The eruptive sequence starts with pumice-fall deposits (three units can be recognized) that darken upwards where there are sandwave interbeds. Surge deposits cover the pumice-fall bed and thick pyroclastic-flow deposits represent the uppermost levels of the deposit. Isopach maps of both the pumice-fall and pyroclastic-flow deposits led to an estimate of the total volume of tephra of about 0.32 km3. The eruptive sequence and the distribution of lithic ejecta are similar to those of the major Plinian eruptions of Somma-Vesuvius (although the volume involved is significantly lower) and reflect an increase in the hydromagmatic character of the eruption with time. The products range in composition from phonolites (first-erupted) to phonolitic leucitites with gradual changes upwards. Whole rock chemistry and microprobe mineralogy indicate that the Pollena sequence represents a liquid line of descent towards the phonolitic minimum of petrogenys residua system. Fractionation occurred within a shallow magma chamber (PH2O probably slightly higher than 1 kb) and was mainly controlled by leucite and clinopyroxene. The basic parental magma approached the composition of the recent period (A.D. 1631–1944), tephritic leucitites of Vesuvius. The phonolitic magma can be derived from a leucititic parent by fractionating about 50% solid phases. A two-stage fractionation model is suggested: the first stage occurred during the rise of magma from the deep source and the second within the shallow magma chamber. The rate of magma introduction during the 150 to 200 year repose time preceeding the eruption probably averaged 1.2 to 1.7 × 10−3 km3-yr−1. These conditions were probably favorable for the occurrence of magma-mixing within the convecting zone of the magma chamber.