![]() ![]() The pristine interpretation of Monte Amiata ignimbrites and rheoignimbrites by Rittmann was not universally accepted (e.g., G.P.L. Monte Amiata is that volcano for which the word “rheoignimbrite” was first coined by Alfred Rittmann to indicate a volcanological process explaining the concomitant lava- and pyroclastic-like textural and geological characteristics found in some silicic volcanic rocks. Idealized cross section showing the stratigraphic relationships and the internal architecture of the completely effusive silicic Monte Amiata composite volcano. Monte Amiata is a silicic (mainly trachydacite) middle Pleistocene volcano ( Figure 1) of the Tuscan Magmatic Province (Italy ) that focused on the interest of volcanologists and petrologists for about 300 years since the eighteenth century 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. However, some key questions regarding the eruption and emplacement mechanisms of large volume silicic lavas remain still open. 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. 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). Overall, the best known and described silicic effusive products are rhyolite lava domes and stubby obsidian flows restricted to the near-vent areas. Moreover, silicic lava effusions are poorly constrained because of a paucity of direct observations on historical eruptions (i.e., Colima, Mexico, 1998–1999, Santiaguito, Guatemala, 1922 to present, Chaitén, Chile, 2008–2009, Cordón Caulle, Chile, 2011–2012, ). 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. ![]() In contrast, silicic effusive eruptions are correlated with domes and short and thick flows. 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.ĭue to their relatively high viscosity and volatile content, silicic (SiO 2 > 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., ). 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. The absence of fragmental textures, both at micro- and macro-scale, supports the effusive nature for the SLLFs. Internal shear-bedding and crystals and vesicles lineations define planar to twisted and straightened outflow layering. 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. 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. It is one of the prime volcanoes that have been involved in the volcanological debate on the genetic interpretation of large silicic flows. Let’s explore how the silica (SiO 4) and dissolved gas (like CO 2, SO 2, and H 2O) composition affect the eruption style.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).
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