Stratigrafia ed evoluzione geologica del vulcano di Monte Amiata

Although the Monte Amiata volcano has been the subject of numerous studies, including the production of geological maps at various scales and different details, there is still not a geological model on which to base a reliable reconstruction of its geological structure and volcanological and magmatological evolution. One of the features that previously restricted the distinction of geological units within the volcano was the apparent lithological uniformity, that produced the distinction of only two lithological types of rocks and the consequent aggregation of all the outcrops in a few stratigraphic units.Here, we describe a geological evolution of the Monte Amiata volcano as was possible to define applying stratigraphic and volcano-geological methodologies into the framework of a robust field-based work. The stratigraphic analysis has taken into account lithological, petrochemical, volcanological, structural, geometric and morphological characteristics of the volcanic rocks and their stratigraphic relationships. The surficial volcanic units were identified as lithostratigraphic units (more than 30 formations) and grouped into UBSU units (2 Synthem and 5 Subsynthem). The proposed geological evolution comprises two main period of activity that correspond to the older Bágnore Synthem and the younger Monte Amiata Synthem. They are separated by a major unconformity corresponding to a surface of erosion, saprolithic weathering and tectonic deformation. Into the Bágnore Synthem, there are two Subsynthem. The lower, the Bagnólo Subsynthem comprises several great lava flows characterized by a vitrophyric perlitic groundmass. They flowed for very long distances (up to 8 km) in all the directions (e.g. Sorgente del Fiora Fm., Marroneto Fm., Piancastagnaio fm., Abbadia San Salvatore fm., Vivo d'Orcia fm.). A surface of erosion and unconformity separates the Bagnólo Subsynthem from the overlying Montearioso Subsynthem. It is composed of more confined vitrophyric lava flows (e.g. Tre Case Fm., Quaranta Fm., Castel del Piano fm.) and one exogenous lava dome and coulée (Poggio Pinzi fm.). All the rocks belonging to the Bágnore Synthem show a well-developed weathering that transformed their uppermost portion in a sandy deposit, yellow and reddish in color. The Monte Amiata Synthem comprises three Subsynthems. The lower Valle Gelata Subsynthem is characterized by exogenous lava domes with thick "coulées" (e.g. Poggio Lombardo Fm., Pozzaroni Fm.) and by long channelized lava flows (e.g. Leccio fm., Coderino fm.). The intermediate Madonna degli Scout Subsynthem is composed of several exogenous lava domes with short "coulées" (e.g. Poggio della Pescina Fm., Poggio Falco Fm., Corno di Bellaria Fm., La Vetta fm., Rifugio Cantore fm.), and minor lava flows. All these units are emplaced upon a morphology that has been moulded by tectonic deformations. The final Prato della Contessa Subsynthem comprises several exogenous lava domes (e.g. La Montagnola fm., Pianello fm.) and lava flows (E.g. Ermeta fm., Le Macinaie fm., Cancelle fm, Fosso La Cocca fm.) from the volcano summit area, and the lateral exogenous lava dome and coulée of the Trauzzolo Fm..