Understanding unsteady Plinian eruptions: the deposits and processes associated with the complex 4.6 ka Fogo A Plinian eruption sequence, São Miguel Island, Azores
thesisposted on 2017-02-27, 06:08 authored by Pensa, Alessandra
This research has detailed for the first time the eruptive products and of whole 4.6ka Fogo A eruption sequence, São Miguel Island, Azores, Portugal, made famous by Walker and Croasdale (1970). The study of the depositional relationships between each stratigraphic unit has been fundamental for the reconstruction of the eruption column dynamics. Although previous authors published studies on the physical and compositional characteristics of the significant basal fall-out deposit studied by Walker and Croasdale (1970), no one has previously published on the rest of the stratigraphy of the Fogo A sequence. Key findings of this research, include a complex inter-stratification of fall-out and ignimbrite deposits; the delineation of the areal dispersal of the deposits; thermal remanent magnetization, reflectance analysis and heat loss were used to estimate the emplacement temperature of ignimbrites; the discovery of anti-rapakivi texture of feldspar phenocrysts and the finding of an inverse compositional evolution from trachytic (63.38%), toward terms more trachydacitic (59.38%) magma composition during the eruption; assessment of the possible causes of the instability of the eruption column and to better understand how this is connected with the dynamic evolution of the magma source. Although the shift from steady to unsteady eruption column behaviour during explosive Plinian eruptions is a common characteristic of many Plinian eruptions, there is still a considerable discussion in the volcanology community of the causes. The Fogo A eruption sequence was a perfect study case for better understanding the factors causing a shift from steady to unsteady eruption column behaviour. Furthermore the comparison between pTRM analysis on lithic clasts and Reflectance analysis (Ro%) on carbonised organic material, for estimating the emplacement temperature of ignimbrite deposits, reveals, for the first time, excellent comparability of the two methods, and corroborates the reliability of the Ro% data. This will greatly improve the accuracy of the evaluations of emplacement temperature of ignimbrites, allowing the estimation of narrower temperature ranges and the distinction of different temperatures at different stratigraphic horizons within single ignimbrite deposits.