**2.2 Previous studies and open questions**

Two fundamental geological problems on Monte Amiata volcano emerged from previous studies carried out during the last 60 years including the issues associated with the correct definition of the stratigraphic units and, consequently, of the interpretation of the geological evolution in light of the revised volcanological interpretation of the eruption dynamics and the emplacement processes.

In all previous studies, the reconstruction of the stratigraphic framework was founded mainly on lithological and petrographic characteristics rather than on objective geological criteria applying the principal approach of volcano geology. Consequently, based on supposedly homogeneous and indistinguishable textures, geochemical composition, and mineral paragenesis of rocks, the resulting volcanic history was represented by only three units [39, 40, 43]. This tri-fold division of the volcanic activity of Monte Amiata includes (1) an initial and single extended sheet of vitrophyric flows (Basal Complex Auctorum), (2) a cluster of lava domes and coulées extruded from the axial summit crest of the volcano, and (3) two final trachyandesite (olivine-phyric latite) small lava flows.

In fact, in all previous studies on Monte Amiata, the SLLFs of the lower portion of the stratigraphic succession (Basal Complex Auctorum) were all assigned to a single eruptive event (in turn explosive, effusive, or mixed), which would have marked the beginning of the activity of the volcano. Differently, the stratigraphic reconstruction presented in our works ([45, 59–62], this paper) demonstrates that the SLLFs are not a large indistinct unit produced by a single eruptive event, but that they consist of numerous eruptive units that are distinct in the source area, areal distribution, and volcanological and petrographic features. Moreover, these single eruptive units until now included in the former Basal Complex Auctorum are actually individual lithostratigraphic units found at different levels of the stratigraphic succession (**Figures 1** and **2**).

After the initial definition as ignimbrites and rheoignimbrites [30, 37–39], the genetic interpretation of the extensive sheet-like trachydacite flows of Monte Amiata has been the subject of various conjectures, which, however, was not supported by stratigraphic and structural field data and by an accurate volcanological definition of the depositional facies. Ref. [40] proposed a mixed complex eruption, with an initial explosive phase followed by an effusive phase. The effusive nature of these flows has been generically supposed by various authors [41, 42], albeit in contexts focused on other topics and without any objective observation. Refs. [43, 44] proposed a highly speculative mechanism of the collapse of an endogenous summit mega-dome generating a single predominantly gravitational flow of fragmented and still hot material that was distributed all around the volcano and evolved in a rheomorphic sheet, which flowed like lava after emplacement.

On the basis of the various physical and compositional indicators, stratigraphic relations, and our geologic mapping, we do not agree with these previous interpretations for any of the Monte Amiata sheet-like trachydacite flows.
