Luigi La Volpe

Volcanological and petrological evolution of Vulcano island (Aeolian Arc, southern Tyrrhenian Sea)

Petrological and geochemical data are reported for volcanic rocks from Vulcano island. The subaerial volcanism (120 ka to present) built up a NW-SE elongated composite structure, affected by two intersecting multistage calderas. Volcanics older than 20 ka consist mostly of high-K calc-alkaline (HKCA) to shoshonitic (SHO) mafic rocks. These magmas interacted significantly with the continental crust, which generated variable Sr isotopic ratios (0.70412–0.70520). However, a major role was also played by input of parental liquids into the magma chamber, which prevented further evolution of the magmas. HKCA, SHO, and potassic (KS) rocks formed from 20 to 8 ka, display a much larger range of SiO2 (from shoshonites to rhyolites) and higher concentrations of incompatible elements with respect to the previous stage. Sr isotopic ratios show small variations (0.70448–0.70486). Mixing of silicic and mafic liquids and fractional crystallization processes (FC) were the main evolutionary processes during this stage. Volcanics younger than 8 ka consist of SHO and leucite-bearing KS mafic rocks, with abundant intermediate and silicic products. Mafic and intermediate rocks display similar incompatible element abundances and Sr isotopic ratios as the previous stage volcanics, whereas higher 87Sr/86Sr (0.70494–0.70583) are observed in some rhyolites. These products originated from a complex interplay of FC, crustal assimilation, and magma mixing processes. The most mafic rocks show increasing incompatible element abundances, Rb/Sr, Rb/Ba, Mg/Al, Mg/Ca, and a decrease in large ion lithophile to high field strength element ratios, passing from older HKCA-SHO to the younger SHO-KS volcanics. These variations suggest a shifting of magma sources from a slightly metasomatized asthenosphere (fertile peridotite) to a more strongly metasomatized lithospheric mantle (residual peridotite). Time-related petrological and geochemical variations have been used to develop a model for the evolution of the Vulcano plumbing system.

Evolution of the Fossa Cone, Vulcano

Abstract In decreasing order of abundance, the principal emplacement mechanisms for deposits of the Fossa edifice of Vulcano include: dry-surge, wet-surge, pyroclastic-fall, lahar, lava-flow, pyroclastic-flow and epiclastic processes. Most samples from pyroclastic beds on Fossa contain two grain-size populations. The coarser mode at 0.5 - 1.5 φ resulted from modified ballistic transport whereas the finer mode at 2.5 – 3.5 φ is related to surge transport. Reconstruction of the entire stratigraphy of the cone permits the beds to be grouped into five principal eruptive cycles, all of which postdate the lower Pilato ash of Lipari (dated at 11,000 to 8,500 y.b.p): Pte. Nere cycle, Palizzi cycle, Commenda, cycle, Pietre Cotte cycle, and the modern cycle. Additional cycles may be present, but exposures on the cone are insufficient for their adequate definition. All cycles follow a similar stochastic pattern starting with surge eruptions and ending with effusion of lava from the crater rim. The frequency of pyroclastic-fall events increases with time throughout each cycle. This progressive decrease in the efficiency of water/melt interaction through the duration of each cycle may be due to a gradual rise of each new magma column coupled with an increase in magmatic volatiles. Tectonism and magma mixing both play a role in triggering eruptions on Fossa. The present crater lies at the intersection of two prominent tectonic trends. A NNW fracture system parallels the main eruptive centers and an ENE set aligns with the migration of minor vents. The silicic lavas of Fossa contain a mixture of two components. Xenocrysts of plagioclase, augite, and olivine, surrounded by a dark glass are intimately dispersed into an evolved rhyolitic matrix.

The analysis of the influence of pumice shape on its terminal velocity

[1]xa0Pumice particles represent the basic “ingredient” of many large explosive eruptions and form as a result of magma fragmentation inside the conduit. At the onset of eruption, fragmental pumices are expelled at high velocity from the crater by the overpressure of gas liberated on explosion. The resulting multiphase flow is forced through atmosphere by a variety of transportation mechanisms and pumices eventually decouple from the gas flow, settling down to form pyroclastic deposits. Here we propose new experimental data of terminal velocity together with a quantitative shape analysis of a wide range of pumice particles. The resulting model allows predicting the terminal velocity of pumice by means of easily measured particle characteristics, with an average error of 12%, which compares favourably with previous models.

A method for the calculation of the impact parameters of dilute pyroclastic density currents based on deposit particle characteristics

[1]xa0A new method for the reconstruction of the physical characteristics of dilute pyroclastic density currents is introduced. The velocity, density and particle volumetric concentration profiles of the stratified current are calculated, together with the profile of dynamic pressure, which is a useful parameter for checking resistance of buildings. The model links turbulent boundary layer shear flow theory with particle coupling to gas turbulence. The working procedure starts with the recognition in the field of the fining upward sequence of layers formed during the time integrated depositional history of an individual current. Distinct processes of particle transportation and deposition are associated with the different particle modes composing the bedset. The system of equations for the solution of the fluid-dynamic parameters is implemented in two alternative ways. The first one uses data of particles coming from both the basal coarse layer and the overlying laminated layer of the bedset. The second uses features of two distinct components of the laminated layer. A statistical test is performed for checking model results against experimental data of actual particles. Model calculations give the average solution, as well as solutions corresponding to a range of 68% of probability around the average value. The maximum solution can be considered as a safety value for impact parameters. The methodology is applied to deposits of recent eruptions of Vesuvius and Campi Flegrei in Italy, and results are discussed in terms of hazard and expected damage.

K-ar ages of the yemen plateau

Abstract The temporal evolution of the Yemen Trap series is examined in the light of K-Ar radiometric data. Rifting and volcanic activity characterize the Yemen plateau between 30 and 20 m.y. confirming a fairly common history of the Afro-Arabian plate on both sides of the Red Sea.

Large‐scale experiments on the mechanics of pyroclastic flows: Design, engineering, and first results

[1]xa0A newly designed apparatus for experimental studies of pyroclastic flows consists of a cylindrical conduit that is filled with samples of natural volcanic products (tephra). Blowing nozzles in the base plate of the conduit are connected to a volume of highly pressurized gas. Opening of fast solenoid valves results in impulse-like coupling of the released gas to the sample. The system was designed so that the range of mechanical energy transferred to the particle mass in the conduit reflects the mechanical energy observed and measured during fragmentation experiments with melts of similar composition. Depending on the specific mechanical energy (SME) of the system, which results from ΔPV/m, where ΔP is gas overpressure (i.e., pressure > atmospheric), V is gas volume, and m is sample mass, different behaviors are observed. If SME > 2.6 kJ/kg, a dilute plume develops, and particles are sedimented by fallout exclusively. If SME 106, implying that flows are fully turbulent and that particle coupling to gas turbulence of natural pyroclastic flows is replicated by the experiments. The measured shear current velocities are proportional to the impact mass flow rate, i.e., the product of mixture density and impact velocity. Experimental data and grain-size analysis of the produced particle deposits suggest that the scale of the experiment is large enough to reproduce the transport dynamics of natural pyroclastic flows.

Structures and grain size distribution in surge deposits as a tool for modelling the dynamics of dilute pyroclastic density currents at La Fossa di Vulcano (Aeolian Islands, Italy)

Abstract Turbulent, dilute pyroclastic density currents (surges) represent the most common and hazardous type of eruptive event at La Fossa di Vulcano. During the volcanic history these currents have frequently passed over the topographic barrier of La Fossa caldera, maintaining a high velocity. We interpret lateral facies variation in terms of fluid-dynamic behaviour by analyses of structures, thickness and grain size of correlated surge layers from the Palizzi succession. Using these data we validate a sedimentation model of traction fed by a low concentration, turbulent and stratified current. By the use of a sedimentological model, we have then reconstructed the fluid-dynamic parameters of the current and calculated the maximum horizontal distance that solid particles travelled before deposition. Results have been checked against actual deposit features. Consistency of data indicates that the method used in this study, even if it is approximate, and needs further refinement by specific experimental approaches and numerical simulations, gives a reasonable indication of surge characters such as velocity and density, which are relevant for hazard calculation and risk assessment.

Interaction between particles transported by fallout and surge in the deposits of the Agnano-Monte Spina eruption (Campi Flegrei, Southern Italy)

Abstract The Agnano–Monte Spina Tephra (AMST) is a complex sequence of beds generated by contrasting fragmentation and transportation dynamics. The 4.1 ka eruption was accompanied by a volcano-tectonic collapse, part of which is the present Agnano plain. The pyroclastic sequence is subdivided into members and submembers, each characterized by different lithological and sedimentological features. Plinian/subplinian fallout deposits frequently alternate with base-surge beds of phreatomagmatic origin. Analysis of lateral facies variations and vertical facies associations of correlated layers of submembers B2, D2 and E2 reveals that during some eruption phases the contrasting eruptive dynamics were almost contemporaneous. Base-surge deposits of submember B2 formed during the declining stage of a plinian column. They resulted from highly energetic and steady pyroclastic density currents that traveled long distances from the vent area and surmounted topographic obstacles such as the Posillipo hill (∼150 m) and the northern sector of the Camaldoli hill (∼250 m). Submember D2 pyroclastic density currents formed when contemporaneous fallout from an eruptive column was present. Coarse particles from the column settled throughout the pyroclastic density current, determining a significant increase of the solid load of the base surge. The consequent increase of supply rate from the transportation to the depositional zone of the base surge led to the formation of unsteady flows that could not efficiently transport the solid load and did not have the ability to travel long distances. Base-surge deposits of submember E2 were fed by a pulsating phreatomagmatic activity, which was punctuated by a short-lived fallout phase. Fallout material, which was incorporated as an additional load to the base surges, was partially transported by low-energy, steady pyroclastic density currents that traveled over the Agnano plain but did not surmount either the Camaldoli or the Posillipo hills. Only the very fine material, in continuous suspension in the upper, no-shearing part of the base-surge cloud, was dispersed higher in the atmosphere and quietly settled over a large area outside the caldera rim. The phreatomagmatic origin of base surges, which contrasts with the magmatic origin of fallout activity, suggests that the pyroclastic density currents of the Agnano–Monte Spina eruption did not result from eruption column collapses. They were most likely related to radially spreading clouds which were contemporaneous with fallout activity but issued from distinct zones in the vent area. The turbulent nature and the high expansion of base surges made them capable, under certain conditions, of passing over high topographic obstacles, with hazardous effects in distal areas.

Magmatism of the eastern Red Sea margin in the northern part of Yemen from Oligocene to present

The northern part of Yemen during the last 30 Ma has been the site of an intense magmatic activity which can be divided into three main episodes. The first episode lasted from 30 to 18 Ma and was characterized by two climaxes. Huge flood basalt eruptions between 30 to 26 Ma were succeeded by massive ignimbrite sheet deposits and intrusion of silicic subvolcanic masses 22 to 20 Ma ago. Contemporaneously, along the present-day western margin of the Yemen Plateau a basaltic dike swarm was emplaced. Additionally, alkaline and peralkaline granites were intruded at the end of the first climax and throughout the second. After a long quiescence, magmatic activity occurred again in Late Miocene time (about 10 Ma) with extrusion of lava flows in scattered areas (e.g. Al Harf, Wadi As Sirr, Maswar Al Hada, Jabal An Nar). The third magmatic episode has been concentrated in three well defined areas. It began about 6.5 Ma ago in the Dhamar Rada area and later in the Sana Amram and Sirwah Marib areas. New chemical and isotopic data on the magmatic products of North Yemen indicate that extensive crustal contamination, together with simple crystal fractionation processes, have been operating during the magmatic evolution. The most primitive products of each of the three episodes share isotopic and petrological features suggesting a similar mantle source. Isotope geochemistry indicates an important asthenospheric component in the mantle source for the magmas of all three episodes. Finally, a comparison between the coeval igneous rocks of the Yemen and Ethiopia plateaus is attempted.

Geochemical trends in the alkali basaltic suite of the Assab range (Ethiopia)

Abstract Two groups of rocks are distinguished in the Assab volcanic range (Afar) on the basis of their petrology. The first one is a typical alkaline suite with high Li, Ba, Rb, Sr, Th and U concentrations. A second subalkaline group may be related to a parental basalt characterized by low trace-element concentrations and intermediate in character between alkali and tholeiitic basalt. U and Th concentrations of the analyzed Assab basalts, as well the reported values for the alkali and transitional basalts of the East African Rift and the Afar depression, are higher than values reported for similar basalts from oceanic and continental environments.