Rheomorphic diapirs in densely welded ignimbrites: The Serra di Paringianu ignimbrite of Sardinia, Italy

Timescale of Emplacement and Rheomorphism of the Green Tuff Ignimbrite (Pantelleria, Italy)

“…Evaluating the time a deposit remained above the glass transition temperature is pivotal to constrain the time‐window during which viscous deformation (both welding and rheomorphism) occurred (e.g., Quane & Russell, 2005). Different studies have been carried out to constrain limits on welding (Heap et al., 2014; Kolzenburg & Russell, 2014; Lavallée et al., 2015; Quane & Russell, 2005; Quane et al., 2009; Russell & Quane, 2005) and rheomorphism (Andrews & Branney, 2011; Bachmann et al., 2000; Branney & Kokelaar, 1992; Mundula et al., 2013; Robert et al., 2013) of ignimbrites. In particular, the rheological model proposed by Russell and Quane (2005) provides a method for investigating the welding of pyroclastic deposits by monitoring viscosity changes due to porosity loss.…”

The Volcanic and Mining Geoheritage of San Pietro Island (Sulcis, Sardinia, Italy): the Potential for Geosite Valorization

“…Welding is a major cause of porosity and permeability decrease of the deposit; for this reason, deflation and gas loss from the deposit are hampered by the viscosity of the material, resulting in general flattening of the coarser particles and retardation in vertical gas migration. Gas released during deflation and compaction so can accumulate locally inside the deposits, forming large, dome-shaped cavities that can slowly migrate by buoyancy toward the upper part of the deposit until it remains plastic; expansion of these gas cavities occur by progressive gas accumulation and decompression related to their vertical migration (Mundula et al 2013). Expansion also causes local compression in the host ignimbrite, and results in strong deflection and deformation of the deposit around these cavities.…”

“…The ignimbrite constituted by a twin sequence of densely welded and partially welded flow units. In particular, they are confined to the upper half of the deposit, that here presents a total thickness up to about 30 meters (Mundula et al 2013). Blisters are here represented by round cavities of variable diameter (from 5 up to about 20 meters) variably eroded (Fig.…”

“…The lateral portions of the blisters are intensely altered and show a strong argillification of the matrix glass, while foliation is deformed, reproducing the final domal shape of the blister. At least 30 blisters are present in the area of La Punta in about 1 km 2 wide area (Mundula et al 2013). The dimension of blisters does not show any relation with their position along the vertical thickness of the deposit.…”

“…Since geodiversity, meant as the range of geological feature of a site (Gray 2013), has been recognised as an important parameter contributing to rank the value of a geosite (Brilha 2016), we point out here the occurrence of diapiric structures at La Punta, at the same stratigraphic level of the blisters. These decametric, mushroom and pillow-shaped structures, have been interpreted as the result of the intrusion of an intermediate, partially welded, flow unit into an upper, densely welded, unit (Mundula et al 2013). Although diapirs are not well exposed as the blisters, they contribute to the scientific interest of this geosite.…”