Tephrochronology, modelling and dynamics of large-magnitude eruptions in the central Mediterranean area

Explosive volcanic eruptions produce large amounts of pyroclastic deposits and inject them in the atmosphere, reaching hundreds or thousands of km from the vent and potentially affecting distal areas. Thus, ‘tephra’ deposits, defined as pyroclastic materials produced by explosive eruptions, are the traces of volcanic events and of the effects they had on the environment and populations. Like detectives, who look for clues to find the solution to a crime, volcanologists investigate tephra deposits’ features to explain the processes that happened in the past. With this regard, tephrostratigraphy and tephrochronology are the primary tools used for the study of tephra deposits, which are recorded in subaerial, marine, lacustrine and glacial environments. Combining stratigraphic, geochemical, geochronological and physical characteristics of the deposits, the source vent can be inferred, the age can be calculated, and the dispersal areas and dynamics of explosive eruptions can be reconstructed. Hence, these techniques are applied for each eruptive unit to describe and reconstruct explosive eruptive histories, which is crucial to quantify the recurrence times and to assess the hazard posed by a volcano, especially for the ones located in highly urbanized areas. Several Quaternary volcanoes are located in the central Mediterranean area, most of which are hosted in central-southern Italy, and several are active (e.g., Somma-Vesuvius, Campi Flegrei, Stromboli, Etna). Specifically, Campi Flegrei (CF) volcanic field, a caldera located west of the urban area of Naples, is the focus of the present dissertation. It is one of the most productive volcanoes of the Mediterranean area, producing more than ~70 eruptions since the last ~15 ka and responsible for one of the highest magnitude eruptions occurred during the Late Pleistocene, the so called ‘Campanian Ignimbrite’ (CI) caldera-forming eruption. The eruption was dated at ~40 ka and produced a Dense Rock Equivalent (DRE) volume of 181–265 km3, thus corresponding to a Volcanic Explosivity Index (VEI) of 7. Later on, two other caldera-forming eruptions occurred, namely the Masseria del Monte Tuff (MdMT, ~29 ka) and the Neapolitan Yellow Tuff (NYT, ~14 ka). Moreover, since the last decades several volcanic events, ascribable to the Campi Flegrei volcanic activity and occurred before the Campanian Ignimbrite eruption (~40 ka) and back to at least ~290 ka, have been recorded. Among them, some very widespread marker tephra layers have been described, i.e., the X-6, X-5 and C-22 eruptions. However, the system is very complex, and the activity preceding the Campanian Ignimbrite has not been recognized in very proximal areas, thus preventing a detailed reconstruction of the eruptive history of the CF. With this regard, distal records have been fundamental to shed light into the CF earliest phase of activity. Since the Campi Flegrei caldera is active and located in a highly populated area, a proper hazard assessment is pivotal, thus the past activity of the volcano requires a detailed reconstruction. In this framework, this thesis aims to refine the Campi Flegrei eruptive history older than ~90 ka, and to investigate the magnitude of the large-magnitude explosive events occurred before the Campanian Ignimbrite eruption. Specifically, the present research provides a multidisciplinary study focused on the Campi Flegrei volcanic activity between ~90 and 160 ka, which is summarized thoroughly in the main body of the thesis. The lithological, geochemical (EMPA, LA-ICP-MS) and geochronological (40Ar/39Ar dating) analysis on tephra layers from mid-distal and distal (e.g., Campania plain, Tyrrhenian Sea, Lower Danube plain) settings helped ascribing new tephra layers to the CF volcanic activity between ~90 and 110 ka and at ~160 ka, and providing new and more accurate eruption ages, in order to refine the spatial and temporal occurrences of several eruptions at CF. Hence, this study allowed to make a further step to understand the pre-CI volcanic activity at Campi Flegrei. Moreover, semi-analytical and empirical models have been applied to the pre-CI large-magnitude eruptions, in order to reconstruct the dispersal areas and to quantify the main eruption source parameters (ESP). Specifically, a detailed study was carried out on the largest eruption occurred between ~90 and ~110 ka, namely the Maddaloni/X-6 eruption, occurred ~109 ka ago. Two distinct phases have been recognized, i.e., a Plinian and a co-ignimbrite, and a semi-analytical model has been applied to reconstruct the tephra dispersal. The results allowed to provide one of the first estimates of the magnitude, eruptive column height and volume of the eruption, which may likely be the second largest eruption from Campi Flegrei. The lack of tephra occurrences in the field prevented the use of the same method for the other eruptions occurred during Middle-Late Pleistocene. Thus, a less sophisticated and empirical method has been used to quantify the order of magnitude and volumes of the other eruptions of the cluster, i.e., the Triflisco, Santa Lucia, Cancello, Montemaoro/X-5 and Taurano Ignimbrite eruptions. The magnitudes mostly range from ~5 to 6, whereas in some cases they reach values >6. Therefore, the new results highlight the presence of several pre-CI large-magnitude eruptions with very widespread dispersal areas. The geochemical and geochronological analyses helped consolidate the correlations among deposits found in mid-distal and distal settings and to deeply understand the Middle-Late Pleistocene volcanic history of Campi Flegrei. In addition, both analytical and empirical methods have been employed to quantify the eruption source parameters of the eruptions. The new data enabled to reduce the recurrence times and to refine the magnitude-frequency relationships of large-magnitude eruptions at CF. The explosive history of Campi Flegrei remains partially incomplete and further investigation is required. Nevertheless, the results and implications of the present research may be pivotal to assess the future hazard in the Neapolitan and in the Mediterranean area, and to understand the magmatic and volcanic evolution of the CF system with a long-term perspective.