Physical Volcanology and Facies Analysis of Silicic Lavas: Monte Amiata Volcano (Italy)

Monte Amiata (Italy) is a middle Pleistocene silicic volcano characterized by the extrusion of extensive (5-8 km long and 60 m thick on average) sheet-like lava flows (SLLFs). It is one of the prime volcanoes that have been involved in the volcanological debate on the genetic interpretation of large silicic flows. We performed integrated stratigraphic, volcanological, and structural field survey and petrochemical study of Monte Amiata SLLFs to describe their volcanic facies characteristics and to elucidate their eruptive and emplacement processes. Individual flow units exhibit basal autoclastic breccia beds or shear zones, frontal ramp structures, massive cores with subvertical cooling columnar jointing, coherent non-vesicular upper parts, and plain surfaces with pressure ridges. Internal shear-bedding and crystals and vesicles lineations define planar to twisted and straightened outflow layering. The absence of fragmental textures, both at micro- and macro-scale, supports the effusive nature for the SLLFs. The most common lithology is a vitrophyric trachydacite of whitish to light-gray color, showing a homogeneous porphyritic texture of K-feldspar, plagioclase, pyroxene, and biotite, in a glassy perlitic or microcrystalline poorly vesicular groundmass. Morphological features, facies characteristics, internal structure, and petrographic textures of these silicic sheet-like and long-lasting flows suggest that their effusive emplacement was governed by peculiar physicochemical and structural conditions.