Bruno Scaillet, in Forecasting and Planning for Volcanic Hazards, Risks, and Disasters, 2021 3.2 Componentry of pyroclastic deposits
The conditions favoring a mixed distribution of facies within the caldera are discussed below. The yellow zeolite-rich NYT also occurs in coastal cliffs beneath Rione Terra ( Fig. 11.2F), the historic center of Pozzuoli. However, both gray and yellow facies are found among the few NYT outcrops within the caldera ( Di Giuseppe et al., 2017) ( Fig. At the larger distances, therefore, it appears that the Pozzolana was able to cool rapidly below the temperature threshold necessary for zeolitization ( Fig. 11.2D Scherillo, 1955 Scherillo and Franco, 1960). Outside the caldera, the NYT is yellow and lithified to distances of about 6 km, but further away, it retains its original loose texture and gray color. Almost continuously over the Pozzolana deposit are the products of c.13 ka Plinian eruption of Pomici Principali (PP), (E) red dashed line separates the unaltered Pozzolana present at the base of Posillipo fault scarp from the upper zeolite-rich NYT, (F) zeolite-rich NYT resurgent dome at Rione Terra cliff in Pozzuoli. (A) Ramping of Neapolitan Yellow Tuff (NYT) products on the Torregaveta volcano (Tg), 7 km from the source toward W, (B) ramping of NYT on the Vomero hill (Naples area), 4 km from the source toward E-NE (C) crystals of zeolites (phillipsite and chabazite), present in the altered NYT secondary electron image (D) deposits of loose nonaltered NYT (Pozzolana = P), outcropping 7 km from the source toward E (Ponti Rossi–Naples). The rate of zeolitization is poorly constrained in volcanic environments, but observations of color changes in tephra deposits from the 1538 CE eruption of Monte Nuovo suggest that the yellow color can be acquired in about 100 years or less ( Parascandola, 1946).įigure 11.2. Where water saturation has prevented oxidation (e.g., below the water table or sea level), the NYT has a pale-green color, but this changes to yellow on exposure to air. The yellow color is produced by the oxidation of iron in the zeolites. Lithification is the result of secondary crystallization of zeolites (acicular crystals of phillipsite and rhombohedral chabazite Fig. 11.2C), which form by the reaction of glass and susceptible mineral phases with saturated alkaline solutions at temperatures of 150–250☌ and moderate to low pressures ( Franco, 1974 De’ Gennaro et al., 1999 Gatta et al., 2010). All the deposits were originally Pozzolana. Pyroclastic deposits from the NYT eruption vary from loose pale gray material (the locally named Pozzolana) to the yellow, lithified tuff that gives the deposit its name ( Fig. 11.2A,B). The distinction can be challenging, particularly after hydrothermal alteration and tectonic deformation, and is best made on the basis of the sedimentary structures. Older literature may use the term “tuffs” for what are really volcaniclastic sedimentary rocks.
Once part of a pyroclastic deposit is reworked and redeposited by wind, water or ice, it is considered to be a volcaniclastic sediment rather than pyroclastic, because reworking modifies the sorting, fabric and bedform of the deposit, and the shape of individual particles (e.g., by abrasion). Because of this, explosive eruptions are associated with catastrophic sedimentary events (e.g., lahars, jökulhlaups and volcaniclastic turbidity currents Smith and Lowe, 1991). Pyroclastic deposits, being initially loose unstable accumulations without stabilizing vegetation, are susceptible to rapid erosion, particularly during and shortly after an eruption. Eliza Calder, in Encyclopedia of Geology (Second Edition), 2021 Sedimentary Reworking
0 kommentar(er)
