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We use PyBox, a new numerical implementation of the box-model approach, to reproduce pyroclastic density current (PDC) deposits from the Somma–Vesuvius volcano (Italy). Our simplified model assumes inertial flow front dynamics and mass deposition equations and axisymmetric conditions inside circular sectors. Tephra volume and density and total grain size distribution of EU3pf and EU4b/c, two well-studied PDC units from different phases of the 79 CE Pompeii eruption, are used as input parameters. Such units correspond to the deposits from variably dilute, turbulent PDCs. We perform a quantitative comparison and uncertainty quantification of numerical model outputs with respect to the observed data of unit thickness, inundation areas and grain size distribution as a function of the radial distance to the source. The simulations consider (i) polydisperse conditions, given by the total grain size distribution of the deposit, or monodisperse conditions, given by the mean Sauter diameter of the deposit; (ii) axisymmetric collapses either covering the whole 360∘ (round angle) or divided into two circular sectors. We obtain a range of plausible initial volume concentrations of solid particles from 2.5 % to 6 %, depending on the unit and the circular sector. Optimal modelling results of flow extent and deposit thickness are reached on the EU4b/c unit in a polydisperse and sectorialized situation, indicating that using total grain size distribution and particle densities as close as possible to the real conditions significantly improves the performance of the PyBox code. The study findings suggest that the simplified box-model approach has promise for applications in constraining the plausible range of the input parameters of more computationally expensive models. This could be done due to the relatively fast computational time of the PyBox code, which allows the exploration of the physical space of the input parameters.

A crucial feature to manage a volcanic crisis is the ability of volcanologists to promptly detect an impending eruption. This is often affected by significant uncertainty, mainly for the difficulty in interpreting the monitoring signals in terms of the exact timing of a possible eruption. Here we contribute to this problem, focusing on the states of active volcanoes with closed conduit, as deduced from monitoring data. Four main states can be identified. In the quiescence state 1) the monitoring data lie on a baseline, suggesting the lack of shallow magma/fluid movement. The unrest state is highlighted by minor 2) to major 3) variations in the intensity and rate of monitoring data; in both cases, radial ground deformation pattern and non-migrating seismicity imply shallow magma and/or fluid accumulation. The state of impending eruption 4) is characterised by non-radial, asymmetric ground deformation pattern and migrating seismicity, which suggest that magma approaches the surface through a propagating dyke. As early recognition of this distinctive state is crucial for timely eruption forecast, monitoring activity should be aimed at its prompt detection. The application of this rationale to two types of active volcanoes in densely inhabited areas, a restless caldera (Campi Flegrei) and a quiescent stratovolcano (Vesuvio), highlights its feasibility and importance in eruption forecasting. This rationale may foster a general reference framework to be adopted in case of unrest, supporting in interpreting the monitoring data, as well as more effective: 1) operationally-oriented, monitoring system; 2) probabilistic forecast; 3) use of volcanic alert levels.

Continental rift systems form by propagation of isolated rift segments that interact, and eventually evolve into continuous zones of deformation. This process impacts many aspects of rifting including rift morphology at breakup, and eventual ocean-ridge segmentation. Yet, rift segment growth and interaction remain enigmatic. Here we present geological data from the poorly documented Ririba rift (South Ethiopia) that reveals how two major sectors of the East African rift, the Kenyan and Ethiopian rifts, interact. We show that the Ririba rift formed from the southward propagation of the Ethiopian rift during the Pliocene but this propagation was short-lived and aborted close to the Pliocene-Pleistocene boundary. Seismicity data support the abandonment of laterally offset, overlapping tips of the Ethiopian and Kenyan rifts. Integration with new numerical models indicates that rift abandonment resulted from progressive focusing of the tectonic and magmatic activity into an oblique, throughgoing rift zone of near pure extension directly connecting the rift sectors., Continuous continental rift zones evolve from enigmatic interactions between individual propagating rift segments. Here, the authors document progressive focusing of tectonic and magmatic activity caused by interactions between the Kenyan and Ethiopian rift segments of the East African Rift.

Dissertação (mestrado) — Universidade de Brasília, Instituto de Geociências, Programa de Pós-Graduação em Geologia, 2018. Este trabalho relata o estudo de sequências deposicionais da Formação Onano Superior, referente às fases finais da erupção Onano, na Caldeira de Latera, Província Romana, Distrito Vulcânico Vulsini (0,6-0,13 Ma), Itália. A atividade vulcânica é caracterizada pela alternância entre os estilos eruptivos estrombolianos e freatomagmáticos, diagnosticados pelos depósitos piroclásticos de queda de escórias, púmices, líticos, spatter e depósitos do tipo surge. Os fragmentos juvenis são caracterizados por fenocristais de clinopiroxênio (Mg# = 40,99-89,66), sanidina (Or = 60,74-81,29 mol%), plagioclásio (An = 85,00-94,20 mol%), flogopita e leucita. As fases acessórias incluem magnetita e microcristais de apatita. Dados geoquímicos de rocha total classificam os componentes vulcânicos como alcalinos potássicos e ultrapotássicos insaturados em sílica, indicando um magmatismo bimodal de composição traquibasáltica e tefrítica a fonolítica. Os valores incluem SiO2 = 46,18 a 55,99 wt%, (Na2O + K2O) = 7,21 a 13,21 wt%, Al2O3 = 14,8 a 19,27 wt%, CaO = 4 a 11,87 wt%, Fe2O3 = 4,37 a 8,77 wt% e MgO = 0,89 a 6,66 wt%. A geoquímica dos elementos traço e terras raras é indica composição semelhante às composições de magmatismo em arco vulcânico, com enriquecimento em LILEs e ETR’s leves, juntamente com o empobrecimento em HFSEs e ETR’s pesados. Os padrões multielementares apresentam anomalias positivas em Th, U, K, Pb, Nd e Dy e anomalias negativas em Ba, Nb, Ta, P e Ti. Exibem forte fracionamento nos ETR’s leves em relação aos ETR’s pesados ([La/Yb]N = 21,96 a 31,94) e discreta anomalia negativa em Eu (Eu/EuN = 0,69 a 0,77). As razões 143Nd/144Nd estão entre 0.512106 e 0.512148, valores que geram εNd(0) entre - 9.56 a -10.37, e as razões 87Sr/86Sr estão entre 0.71016 a 0.71036, indicando fonte mantélica enriquecida com processo de contaminação crustal. Os valores de TDM (Ga) variam de 1.09 a 1.29. Com base nos dados geoquímicos e petrológicos sugere-se que os magmas derivam de fusões de uma fonte metassomatizada relacionada à subducção e que a evolução deste magmatismo foi dominada por processos de fracionamento mineral, envolvendo principalmente clinopiroxênio, feldspato e leucita. This work reports the study of depositional sequences of the Onano Superior Formation, referring to the final phases of the Onano eruption, in Latera Caldera, Roman Province, Vulsini Volcanic District (0,6-0,13 Ma), Italy. The volcanic activity is characterized by the alternation between the eruptive strombolian and phreatomagmatic styles, diagnosed by pyroclastic deposits of scoriae, pumice, lithic, spatter and surge deposits. The juvenile fragments are characterized by clinopyroxene (Mg# = 40.99 to 89.66), sanidine (Or = 60.74 to 81.29 mol%), plagioclase (An = 85.00 to 94.20 mol%), phlogopite and leucite phenocrysts. The accessory phases include magnetite and apatite microcrystals. Total rock geochemical data classify the volcanic components as alkaline potassic and ultrapotassic unsaturated in silica, indicating a bimodal magmatism of traquibasaltic and thephritic to phonolitics compositions. Values include SiO2 = 46.18 to 55.99 wt%, (Na2O + K2O) = 7.21 to 13.21 wt%, Al2O3 = 14.8 to 19.27 wt%, CaO = 4 to 11.87 wt%, Fe2O3 = 4.37 to 8.77 wt% and MgO = 0.89 to 6.66 wt%. The geochemistry of the trace and rare earths elements indicate compositions similar to the compositions of volcanic arc magmatism with enrichment in LILEs and LREEs, and depletion in HFSEs and HREEs. The multielement pattern shows positive anomalies in Th, U, K, Pb, Nd and Dy and negative anomalies in Ba, Nb, Ta, P and Ti. They exhibit strong fractionation in the LREEs in relation to the HREEs ([La/Yb]N = 21.96 to 31.94) and discrete negative anomaly in Eu (Eu/EuN = 0.69 to 0.77). The 143Nd/ 144Nd ratios are between 0.512106 and 0.512148, values that generate εNd(0) are between -9.56 and -10.37, and the 87Sr/86Sr ratios are between 0.71016 and 0.71036, indicating mantle source enriched with crustal contamination process. TDM values (Ga) range from 1.09 to 1.29. Based on the geochemical and petrological data it is suggested that the magmas derive from melts of a subduction-related metassomatized source, and that the evolution of this magmatism was dominated by mineral fractionation processes, mainly involving clinopyroxene, feldspar and leucite.

The Baia–Fondi di Baia eruption is one of the sporadic events that have occurred in the western sector of the Campi Flegrei caldera. It dates back to 9525–9696 bp and opened Epoch 2 of the caldera activity after a 1000-year-long period of quiescence. Although relatively small in terms of erupted volume with respect to most of the events of the past 15 ka, the Baia–Fondi di Baia eruption was characterized by a complex series of events, which have led to different interpretations in the literature. We present a detailed stratigraphic study of 40 outcrops in a sector of about 90 km2, coupled with sedimentological (grain size, componentry), physical (density, vesicularity), textural, and compositional analyses of the erupted deposits. Based on these data, we interpret the stratigraphic succession as being related to two distinct eruptive episodes (Baia and Fondi di Baia). These were separated by a short time interval, and each was characterized by different eruptive phases. The Baia eruptive episode started in a shallow-water environment with an explosive vent-opening phase that formed a breccia deposit (Unit I), rapidly followed by alternating fallout activity and dense, pyroclastic density current deposits generation (Unit II). Sedimentological features and pumice textural analyses suggest that deposition of Unit II coincided with the intensity peak of the eruption, with the fallout deposit being characterized by a volume of 0.06 ± 0.008 km3 (corresponding to a total erupted mass of 4.06 ± 0.5 × 1010 kg), a column height of 17 km, and a corresponding mass flow rate of 1.8 × 107 kg s−1. The associated tephra also shows the highest vesicularity (up to 81 vol.%) the highest vesicle number density (1.01 × 108 cm−3) and decompression rate (0.69 MPa s−1). This peak phase waned to turbulent, surge-like activity possibly associated with Vulcanian explosions and characterized by progressively lower intensity, as shown by density/vesicularity and textural properties of the erupted juvenile material (Unit III). This first eruptive episode was followed by a short quiescence, interrupted by the onset of a second eruptive episode (Fondi di Baia) whose vent opening deposited a breccia bed (Unit IV) which at some key outcrops directly overlies the fallout deposit of Unit II. The final phase of the Fondi di Baia episode strongly resembles Unit II, although sedimentological (presence of obsidian clasts which are absent in the Baia deposits) and textural (lower vesicularity, vesicle number density, and decompression rate values) features, together with a more limited dispersal, suggest that this phase of the eruption had a lower intensity. The large range of groundmass glass compositions, associated with variable proportions of highly (phonolitic–trachytic) and mildly (tephriphonolitic–latitic) evolved end-members in the erupted products, also suggests that these eruptive episodes were fed by at least two different magma batches that interacted during the different phases, with an increase of tephriphonolitic–latitic magma occurring during the Fondi di Baia stage.