Monte Amiata volcano (Tuscany, Italy) in the history of volcanology: 2--its role in the definition of "ignimbrite" concepts and in the development of the "rheoignimbrite" model of Alfred Rittmann

The explosive eruptions that occurred between nineteenth and twentieth centuries produced a fundamental cultural impact on the development of Volcanology. Pyroclastic products and ignimbrites features start to be at the base of an international debate. Various descriptions of explosive eruptions, and a new terminology of their products, such as nuée ardente and ignimbrite, were presented and extensively discussed in the framework of the International Association of Volcanology conferences held in 1961 at Catania and in 1963 at Tokyo. Ignimbrite deposits are first assimilated to welded tuffs. That attention to explosive volcanism of the first half of the twentieth century was the context in which has matured the Alfred Rittmann model of rheoignimbrite as welded ignimbrite showing secondary flowage structures. This term introduced by Rittmann in 1958, and shared by Giorgio Marinelli in 1961, was intended to describe the extensive sheet of acidic vitrophyric volcanic rocks of Monte Amiata volcano, interpreted as lava flows by all previous authors. Rheomorphic ignimbrites, in the Rittmann model, have features that strongly differentiate them from normal ignimbrites and that are very similar to what shown by acidic lava flows, as fluidal structures and wrinkles. The concept of rheomorphic ignimbrite is still in use into the volcanological literature, even if not for the Monte Amiata volcanics, nowadays definitively considered to be lava flows and domes. However, the Rittmann and Marinelli authoritative assumptions inhibited, up to present times, new volcanological interpretation of Monte Amiata acidic lavas.