Mundula, F.

Preceded by 14 days of intense seismic activity, a new eruption started on the south flank of Mt Etna, Sicily (Italy) early in the morning of 11 March 1669 opening up a series of NS eruptive fissures. The eruption is one of the most destructive flank eruptions of Etna in historical times; it lasted until 11 July, and was characterized by simultaneous explosive and effusive activity during the first three months, while only lava flow output in the last month. The activity built up the large composite cone of the ``Monti Rossi{'' at the lower end of the eruptive fissures, and caused severe damage to the nearby inhabited areas. The prolonged effusive activity generated lava flows for >15 km, which destroyed several villages and the western part of the town of Catania before reaching the coastline and entering the sea. In this paper, we examine the tephro-stratigraphy of the products of the explosie activity. An in-depth analysis of historical accounts was used to define the chronology of the main eruptive phases (precursors, explosive activity and initial effusive phenomena). The geology of the cone and of the fallout deposits were defined through a field survey over a distance of 5 km from the Monti Rossi. Textural (grain-size, morphological, componentry), density and petrological analyses of tephra samples provided a sedimentological, physical and geochemical characterization of erupted products. Integrating ground and historical data enabled defining the evolution of the cone, identifying and correlating four main cone-forming units. By tracing the dispersal map of the main distal tephra beds (the finer ash being dispersed mainly to the NE as far as Calabria and to the south of Sicily and the 10-cm isopach of the total deposit covering an area up to 53 km(2)), we estimated a total tephra fallout volume, including the Monti Rossi cone, of about 6.6 x 10(7) m(3) (about 32 x 10(7) m(3) DRE). The 1669 event can be considered an archetype of the most hazardous expected eruption on the densely populated flanks of Etna. Reconstructing the eruptive chronology and styles of the 1669 eruption therefore, represents the basic data to assess volcanic hazard from eventual similar flank events in the future. (C) 2016 Elsevier B.V. All rights reserved.}

Plinian eruptions are characterized by high intensity and an overall steady character, and result in a stable convective column. The main processes controlling the dynamics of such steady and stable plume systems have been extensively investigated. Conversely, sub-Plinian eruptions are unsteady, as recorded by the large variability of the products and deposits. Our knowledge of the processes creating this unsteadiness on various timescales remains limited, and still requires more observations as well as theoretical and experimental investigation. Here, we focus on the sub-Plinian eruption of the Greenish Pumice (GP, 19,265 ± 105 BP), Mt. Somma-Vesuvius (Italy). On the basis of coupled geochemical and textural analyses of samples from the wellestablished stratigraphy of the GP deposits, we investigate volatiles (H2O, CO2, F, Cl) to better constrain the unsteady sub-Plinian eruptive style. This allows us to carry out a detailed study of the degassing processes in relation to the eruption dynamics. We find that degassing by open-systemprocesses generally dominates throughout the entire eruption, but alternates with episodes of closed-system degassing. The fluctuating degassing regimes, responsible for the variable magma ascent rate within the conduit, are also responsible for the eruptive column instability. Volatile behavior is well correlated with textural heterogeneities of the eruptive products. Both reflect higher conduit heterogeneity than for Plinian eruptions, where we find a higher horizontal gradient in magma ascent velocity due to a smaller conduit diameter.

We describe the products of the hitherto poorly known 512 AD eruption at Vesuvius, Italy. The deposit records a complex sequence of eruptive events, and it has been subdivided into eight main units, composed of stratified scoria lapilli or thin subordinate ash-rich layers. All the units formed by deposition from tephra fallout, pyroclastic density currents of limited extent being restricted to the initial stages of the eruption (U2). The main part of the deposit (U3 and U5) is characterized by a striking grain size alternation of fine to coarse lapilli, similar to that often described for mid-intensity, explosive eruptions. The erupted products have a phonotephritic composition, with progressively less evolved composition from the base to the top of the stratigraphic sequence. Based on different dispersal, sedimentological and textural features of the products, we identify five phases related to different eruptive styles: opening phase (U1, U2), subplinian phase (U3 to U5), pulsatory phreatomagmatic phase (U6), violent strombolian phase (U7) and final ash-dominated phase (U8). A DRE volume of 0.025 km3 has been calculated for the total fallout deposit. Most of the magma was erupted during the subplinian phase; lithic dispersal data indicate peak column heights of between 10 and 15 km, which correspond to a mass discharge rate (MDR) of 5×106 kg s1. The lower intensity, violent strombolian phase coincided with the eruption of the least evolved magma; a peak column height of 69 km, corresponding to an MDR of 1×106 kg s1, is estimated from field data. Phreatomagmatic activity played a minor role in the eruption, only contributing to the ash-rich deposits of U1, U4, U6 and U8. The two most striking features of the 512 AD eruption are the recurrent shifting of the eruption style and the pulsatory nature of the subplinian phase. Basing on a large set of observational data, we propose a model to explain this complex dynamics, also observed in other eruptions of similar scale from Vesuvius and elsewhere. The inbalance between the rates of magma supply and magma eruption may have caused the frequent changes in the eruptive style. Conversely, the high frequency oscillations of magma discharge recorded by the deposits of the subplinian phase were possibly related to cyclic instabilities in the permeability of the low viscosity magma column, which modulated magma fragmentation and discharge.