Transition from steady to unsteady Plinian eruption column: the VEI 5, 4.6 ka Fogo A Plinian eruption, São Miguel, Azores

Abstract The 4.6 ka Fogo A trachytic Plinian eruption on Sao Miguel Island, Azores, Portugal, is composed by four main phases: 1) Initial phreatomagmatic activity which emplaced thin surge deposits with accretionary lapilli; 2) fully buoyant steady Plinian activity which generated a prominent trachytic basal fall-out deposit; 3) partial collapsing activity, which led to the emplacement of two trachytic–trachydacitic intra-plinian ignimbrites (pink and black ignimbrites) and associated surge deposits, inter-stratified within two fall-out deposits; 4) vent widening and fully collapsing activity which caused the emplacement of the climactic brown trachydacitic ignimbrite. The deposits of these four main phases have been organised into three members FGA 1 , FGA 2 and FGA 3 , sub-divided into 15 stratigraphic or emplacement units. The fluctuations from a steady to unsteady Plinian eruption column parallel the change in composition of the magma from trachytic to trachydacitic. Although minimal, the change in composition resulted in a marked change in colour of the juvenile clasts from white at the base (first phase) to pink and light-dark grey banded and black (second and third phases) and to dark brown (fourth phase). Density analysis of juvenile clasts revealed that the change in composition did not significantly affect the density of the juvenile pumice pyroclasts, the vesicularity of which increases from 70% to 85% up through the sequence. The pink and the black intra-plinian ignimbrites were emplaced only, within the narrow paleo-valleys of the southern flank with maximum thicknesses of 9 m and 5 m respectively, transitioning onto adjacent topographic highs where thin veneer surge layers were deposited. The fall-out deposits inter-bedded between the ignimbrites lack basal reverse grading, indicating that the eruption column continued to be sustained during the partial collapses generating the intra-plinian ignimbrites. The climactic dark brown ignimbrite was emplaced radially with a maximum thickness of > 40 m. The absence of oxidation surfaces or evidence of reworking within the Fogo A deposits, indicates that there were no time breaks during the different phases of the eruption. The on-land bulk volume of the three ignimbrites has been estimated around 3.2 km 3 , while the volume of the intra-plinian fall-out deposits is ~ 0.003 km3. These bulk volume estimations, summed with the basal fall-out volume calculated by Walker and Croasdale (1970), makes a total bulk volume for the Fogo A deposits of 4.4 km 3 indicating a Volcanic Explosively Index of 5, higher than previously estimated. This result is fundamental for the forecasting a possible scenario for a future eruption.