Thermal state and implications for eruptive styles of the intra-Plinian and climactic ignimbrites of the 4.6 ka Fogo A eruption sequence, São Miguel, Azores

Abstract

The 4.6 ka Fogo A Plinian eruption was a caldera-forming volcanic event on São Miguel Island, Azores. The deposit succession is very complex, composed of a thick trachytic Plinian fallout deposit interstratified with two intra-Plinian ignimbrites (named “pink ignimbrite” and “black ignimbrite” sequentially). The succession ends with a main ignimbrite (named “dark brown ignimbrite”), which represents the deposit of complete collapse of the eruption column and the end of the eruption. In this work, emplacement temperatures of the three ignimbrites are estimated by study of partial thermal remanent magnetization (pTRM) of lithic clasts. A total of 140 oriented lithic clasts were collected from 15 localities distributed along the northern and southern flanks of Fogo volcano. The paleomagnetic data reveal different emplacement temperatures and thermal histories that were experienced by each ignimbrite. The results indicate the presence of five different paleomagnetic behaviours that suggest emplacement temperatures of 350–400 °C for the first (pink) intra-Plinian ignimbrite, temperatures higher than 580–600 °C for the second (black) intra-Plinian ignimbrite and 250–370 °C for the last (dark brown) climactic ignimbrite. The thermal history experienced by each pyroclastic flow and its ignimbrite deposit was also assessed by the use of the magnetite-ilmenite geothermometer to determine the pre-eruptive magma temperature (estimated to be around 900 °C). We interpret the different emplacement temperatures of the Fogo A ignimbrites as being due to a combination of factors. These include (i) collapse from different heights of the eruption column and the resultant different amounts of air entrainment into the gas-particle mixture, (ii) variable content of lithic clasts and (iii) different types of juvenile clasts in the ignimbrites

Thermal interactions of the AD79 Vesuvius pyroclastic density currents and their deposits at Villa dei Papiri (Herculaneum archaeological site, Italy)

“…We therefore trust the values obtained (cf. Pensa et al, 2015b) that need, though, to be addressed for their variations in such a very limited area. These variations partly reflect the stratigraphy, where the three sites sampled in lower unit EU2/3pf, VP4, VP6 and VP8, show the lowest T dep values (Fig.…”

Temporal evolution of a post-caldera, mildly peralkaline magmatic system: Furnas volcano, São Miguel, Azores

“…(e.g. Moore and Rubin 1991;Wallenstein 1999;Pensa et al 2015), but its age is generally reported as ~5 ka. Although the deposit is not genetically related to Furnas, it is well known and easily identified and, in the absence of well-exposed deposits relating to the second caldera-forming event at Furnas (~12,000 14 C y B.P.…”

Calibrating Carbonization Temperatures of Wood Fragments Embedded within Pyroclastic Density Currents through Raman Spectroscopy

“…The second set of charcoal samples comes from PDC deposits of the 4.6 ka Plinian eruption of Fogo volcano (Azores), known as Fogo A eruption [21,45]. Fogo volcano, located at the center of São Miguel Island, is one and the largest of the three active Quaternary volcanoes composing the Azorean islands.…”

“…In this work, for the first time, we aim to investigate the evolution of Raman spectra at relatively low temperatures (mainly between 250 and 600 • C), more relevant for PDCs flow and emplacement conditions [21,45]. With this purpose, based on already existing charcoal reflectance datasets, we selected charred wood samples from three different eruptions, (Figure 1a): the 2015 Volcan de Colima eruption, (Mexico), the 2015 Calbuco Volcano eruption (Chile) and the 4.6 ka Fogo Volcano eruption (Azores, Portugal).…”