*4.2.1 Macroscopic lithofacies and facies association*

Based on observations of outcrops, hand specimens, and drill cores in the Monte Amiata SLLFs trachydacite (**Figure 4**), we have identified four main coherent lithofacies based on variations in groundmass texture, color, and vesicularity. In addition, three fragmental lithofacies have also been recognized based on differences in texture and composition of clasts and matrix. In all these lithofacies, trachydacite is characterized by nearly identical mineral paragenesis (K-feldspar, plagioclase, biotite, and subordinate pyroxene) and phenocrysts size. The characteristics of these lithofacies and their facies association are well exemplified by the lithostratigraphic log of the Sorgente del Fiora eruptive unit in the David Lazzaretti borehole compared with the correlate outcropping sequence (**Figure 4**).

#### **Figure 4.**

*The Sorgente del Fiora eruptive unit is representative of the trachydacite SLLFs of Monte Amiata, showing the vertical distribution and association of the coherent and fragmental lithofacies identified. (a) Lithostratigraphy of the drill core David Lazzaretti (DL in Figure 2) that intersected top-to-bottom the Sorgente del Fiora unit. (b) Examples of the main lithofacies identified and described in the DL core. (c) Generalized lithostratigraphic log through the Sorgente del Fiora unit from outcrops along flow margins and front. (d) Interflow scoria agglomerate (lithofacies g). Hammer for scale. (e) Vesicle layering. Some beds show tubular vesicles a few cm high. In the upper right part of the image, flow bands have been deflected around a lens of coarse vesiculation, implying that the vesicles formed while the lava was still ductile, probably during flowage. (f) Interflow monogenetic breccia with welded vitrophyric matrix (br; lithofacies f) overlain by thin-laminated lava beds (fl). (g) Flow bedding composed of massive porphyritic lava beds separated by sheet joints. (h) Thin flow lamination alternating massive porphyritic, vesicular, and glassy lava layers at mm to cm scale.*

#### **Figure 5.**

*Outcrop photo of lithofacies and physical volcanological features of the trachydacite SLLFs of Monte Amiata. (a) Bands of black obsidianaceous vitrophyric trachydacite (lithofacies c) in white perlitic vitrophyric trachydacite (lithofacies a; Quaranta unit). (b) Deformed mingling texture between black-to-red obsidianaceous vitrophyric trachydacite (lithofacies c) and white perlitic vitrophyric trachydacite (lithofacies a; Abbadia unit). (c) Basal monogenetic breccia with clastic matrix (lithofacies e) separating two flow units (Piancastagnaio unit). Note the inverse gradation with the larger angular clasts at the top. (d) Interflow monogenetic breccia with laminated trachydacite clasts welded in vitrophyric matrix (lithofacies f; Piancastagnaio unit). (e) Coarse vesicularity with tubular-shaped gas cavities confined in layers (Sorgente del Fiora unit). Inset shows the tridimensional geometry of these cavities that are elongated parallel to the flow direction (arrow). (f) An irregular elongate cavity developed around a meta-sedimentary xenolith (Sorgente del Fiora unit). Hammer for scale where not indicated.*

The four coherent lithofacies identified (**Figures 4** and **5**; **Table 2**) are as follows: (a) whitish coherent vitrophyric perlitic trachydacite; (b) gray coherent porphyritic microcrystalline trachydacite; (c) black-to-red coherent vitrophyric obsidianaceous trachydacite; and (d) cream-colored coherent vitrophyric microvesiculated trachydacite. Each of these coherent lithofacies shows internal subtle textural variations that include dimensions and abundance in phenocrysts (from fine-grained to coarse-grained porphyricity), vesicles and crystal concentration zones, vesicles layers, *Physical Volcanology and Facies Analysis of Silicic Lavas: Monte Amiata Volcano (Italy) DOI: http://dx.doi.org/10.5772/intechopen.108348*

and crystals preferential alignment in fluidal texture. Moreover, lithofacies are arranged in variable vertical and horizontal facies association. At the outcrop scale, the contact between texturally distinct lithofacies is usually sharp.

The three fragmental lithofacies (**Figures 4** and **5**; **Table 2**) are as follows: (e) monogenetic, clast-supported to matrix-supported breccia with fine-grained clastic matrix; (f) monogenetic matrix-supported breccia with clasts welded in a vitrophyric matrix; and (g) monogenetic clast-supported scoria agglomerate. All these fragmental lithofacies have to be considered primary volcanic and autoclastic deposits. In particular:

