**1. Introduction**

Due to their relatively high viscosity and volatile content, silicic (SiO2 > 63 wt%) magmas are mainly erupted explosively producing voluminous fallout and ignimbrite deposits, which can reach areal extension of thousands of square kilometers and thickness of hundreds of meters (e.g., [1]). In contrast, silicic effusive eruptions are correlated with domes and short and thick flows. The emplacement of silicic lavas is commonly governed by physical variables such as the high viscosity, low temperature and volatile content, and low eruption rates. Moreover, silicic lava effusions are poorly constrained because of a paucity of direct observations on historical eruptions (i.e., Colima, Mexico, 1998–1999, [2]; Santiaguito, Guatemala, 1922 to present, [3]; Chaitén, Chile, 2008–2009, [4]; Cordón Caulle, Chile, 2011–2012, [5–7]). Overall, the best known and described silicic effusive products are rhyolite lava domes [8] and stubby obsidian flows restricted to the near-vent areas [9, 10]. Since the 1960s, more extensive silicic volcanic units have been the object of controversy over their origin because of volcanological characteristics typical of either lava flows and welded or rheomorphic pyroclastic rocks (ignimbrite). In more recent times, several of these extensive sheet-like silicic volcanic units were interpreted as lava flows and distinguished from rheomorphic ignimbrites in well-documented geological records worldwide [11–21]. However, some key questions regarding the eruption and emplacement mechanisms of large volume silicic lavas remain still open [22–25].

Monte Amiata is a silicic (mainly trachydacite) middle Pleistocene volcano (**Figure 1**) of the Tuscan Magmatic Province (Italy; [26]) that focused on the interest of volcanologists and petrologists for about 300 years since the eighteenth century [27] and was one of the prime volcanoes that have been involved in the volcanological debate on the genetic interpretation of the enigmatic sheet-like silicic volcanic rocks [28–33].

Monte Amiata is that volcano for which the word "rheoignimbrite" was first coined by Alfred Rittmann [30] to indicate a volcanological process explaining the concomitant lava- and pyroclastic-like textural and geological characteristics found in some silicic volcanic rocks [33]. The pristine interpretation of Monte Amiata ignimbrites and rheoignimbrites by Rittmann was not universally accepted (e.g., [34]; G.P. L. Walker in [32]). However, the concept of rheomorphism, considered such as a post-depositional gravitational flow and deformation process, has been subsequently recognized and applied in volcanology to ignimbrites (e.g., rheomorphic ignimbrites; [35, 36]), pyroclastic fall deposits [32], and lavas. After the Rittmann assumption, the various studies present in the literature [37–44] have in fact suggested not conclusive data and interpretations about the eruption processes of Monte Amiata silicic rocks, mainly because of the lack of exhaustive physical volcanological observations and their interpretations in a modern volcanological framework.

Nowadays, we accept that Monte Amiata is a completely effusive silicic volcano whose activity was dominated by the emplacement of silicic lava flows, exogenous lava domes, and coulées [45]. To support our interpretation, we have performed

#### **Figure 1.**

*Idealized cross section showing the stratigraphic relationships and the internal architecture of the completely effusive silicic Monte Amiata composite volcano. Not in scale.*

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

detailed and systematic field-based investigations on the stratigraphy, structure, physical features, volcanic facies, and macroscopic and microscopic structures and textures of Monte Amiata deposits.

Specifically, this chapter focuses on several sheet-like lava flows (SLLFs) that constitute the main part of the Monte Amiata volcanic edifice and offer an excellent opportunity to learn more about extensive and voluminous silicic lavas. The goals of our study are (1) to discuss some criteria to distinguish silicic lava flows from welded tuffs and rheoignimbrites, (2) to interpret ascent, eruption, and emplacement mechanisms for Monte Amiata trachydacite through physical and observational data, (3) to produce a model of emplacement for the SLLFs, and (4) to suggest reading keys for the interpretation of the eruptive activity, which originates for such kind of magmatic, structural, and volcano-tectonic environments. We are convinced that the conclusions obtained for Monte Amiata can be applied to improve the general understanding of the volcanic processes at the origin of the emplacement of large silicic effusive bodies and to enhance the assessment of their associated volcanic hazard.
