*Physical Volcanology and Facies Analysis of Silicic Lavas: Monte Amiata Volcano (Italy) DOI: http://dx.doi.org/10.5772/intechopen.108348*

Among the BAS trachydacite, we have identified two stratigraphically distinct groups of SLLFs. The oldest SLLFs group crops out at the base of the volcanic stratigraphic sequence (BSS; **Figure 1**) and comprises the Sorgente del Fiora, Piancastagnaio, Abbadia San Salvatore, and Vivo d'Orcia lithostratigraphic and eruptive units (**Figure 2**). They spread NW and SE over a wide area unconformably overlying the pre-volcanic sedimentary substratum and are extensively exposed along the external perimeter of the volcano. They reach distances up to 5–8 km from the probable vent area, with widths up to 1.5–2 km and thickness of 30–90 m (**Table 1**). A second BAS series of SLLFs (MSS; **Figure 1**) is in the NW and S sides of the edifice and comprises the Castel del Piano and Quaranta lithostratigraphic and eruptive units (**Figure 2**). They reached a distance of about 7 km from the probable emission centers, with thicknesses of 50–80 m (**Table 1**). After an interval of weathering (saprolite) and faulting, the resumed MAS volcanic activity (GSS; **Figure 1**) produced extensive SLLFs along the N and S slopes forming Leccio and Pozzaroni lithostratigraphic and eruptive units (**Figure 2**) with length of 5–6 km and thickness ranging 50–70 m (**Table 1**).

The two younger SLLFs (GSS; Leccio and Pozzaroni) show about the same area (5– 6 km2 ) that is the smallest of the Monte Amiata SLLFs. The greatest extent (10– 11 km<sup>2</sup> ) is reached by the SLLFs of MSS (Quaranta and Castel del Piano). Intermediate areas (7–9 km<sup>2</sup> ) are typical of SLLFs effused during BSS, considering in this case the incertitude in vent positioning and the related error in the area calculations (**Table 1**).

The calculated average volume of the SLLFs spans between 0.30 and 0.72 km<sup>3</sup> . To be considered that the error associated with these volumes is quite high, with the exception of Pozzaroni and Leccio SLLFs (**Figure 3a**), due to the incertitude on the thickness distribution along the lava flow's path. There is a faint linear correlation between the calculated volumes and the flows lengths (R: 0.423) (**Table 1** and **Figure 3a**). Moreover, two different alignments are recognizable in both area/length and volume/length graphs (**Figure 3a** and **b**). In particular, (i) there is a very good linear correlation (R: 0.994) between both volume and area and the length of the three


*Slope is average from the inferred vent to the terminus of the flow. S: synthem, SS: subsynthem, and labels of eruptive units as in Figure 2.*

#### **Table 1.**

*Morphometric parameters for the sheet-like lava flow (SLLF) of Monte Amiata.*

#### **Figure 3.**

*Plots (a) length vs. volume and (b) length vs. area for the Monte Amiata SLLFs. R is the linear correlation coefficient. See text for explanation.*

valley-controlled lava flows of Leccio, Vivo d'Orcia, and Piancastagnaio (in its terminal portion), and (ii) a quite good linear correlation (R: 0.894) for the other five lava flows that distributed in larger lava bodies. In this last case, the poor correlation can be attributed to the exposure condition of the lava flows that influence the accuracy of the area calculation. No correlation is observed between area and slope values, suggesting that the different areal distribution of SLLFs can be independent of the underlying morphology [66], but depending probably on the physicochemical properties of the erupted magma (see discussion in Section 5.2).

The present topographic surface of Monte Amiata volcano represents the primary volcanic surface of the subsequent constructional bodies that concurred to build the volcanic edifice, with the little effects of exogenous erosion and tectonic deformation. All SLLFs are characterized by low- or moderate-relief surfaces forming large tabular areas on the slopes of Monte Amiata. Different lava plateaus developed at different altitudes and formed a stepped relief consisting of overlapping plains separated by breaks of slope [45, 62]. This morphology suggests that flows were not confined by channels and spread along the low-relief sides of the volcano. In the case of Vivo d'Orcia (BAS) and Leccio (MAS) eruptive units (**Figure 2**), lavas moved down the outer flank of the volcano following tectonically controlled preexisting drainage, entering and filling river furrows where they stopped.

The source areas of SLLFs are mainly buried by younger volcanic units and probably correspond to the present elongated summit crest of the volcano (**Figure 2**). Exposures of the flows are largely their surficial part; abrupt but irregular scarps identify the flow fronts and margins. Due to the relatively old age of the volcano, the outcrops are subject to possible surface erosion and front collapses (for this reason, the measured lava length and area have to be considered as minimum values) and the exposed logs are often discontinuous.

The opportunity to access to the complete succession of a SLLF was a continuously cored deep borehole (David Lazzaretti borehole; [67–69]) that intersected top-tobottom the Sorgente del Fiora eruptive unit. This drilling is located on the southern slope of the volcano about 2 km NNE of the lava flow front (DL in **Figure 2**) and displays well-preserved lithofacies and internal textures. The Sorgente del Fiora lava was recovered between 147 m and 265 m of depth from the ground level (i.e., between 939 and 821 m a.s.l.) and shows a thickness of 118 m (**Figure 4a**). It overlies a lower

*Physical Volcanology and Facies Analysis of Silicic Lavas: Monte Amiata Volcano (Italy) DOI: http://dx.doi.org/10.5772/intechopen.108348*

volcanic sequence of trachydacite predating the exposed units and is overlain by trachydacite belonging to FSS.
