ANDRADE, M., RAMALHO, R.S., PIMENTEL, A., HERNÁNDEZ, A., KUTTEROLF, S., SÁEZ, A., BENAVENTE, M., RAPOSEIRO, P.M., GIRALT, S. - The Holocene maar-forming eruptions of Flores Island (Azores): insights from subaerial deposits and lacustrine sedimentary records. IAVCEI Commission on Volcanic Lakes 11th Workshop, São Miguel, Azores, 28 agosto - 5 setembro.

​Flores is the westernmost island of the Azores Archipelago, and its central uplands (above 500 m altitude) host several maars and tuff rings, most of them currently occupied by lakes. These phreatomagmatic craters were formed during the last period of volcanism on the island, some 3,000 years ago, with eruptions clustering in two centres of volcanic activity: Funda (FVS) and Comprida (CVS) Volcanic Systems. Critically, an older crater lake – Lagoa da Lomba – is also located in this central area and holds a tephra-rich sedimentary record that extends back to ca. 23,500 cal yr BP, allowing a uniquely detailed analysis of the recent eruptive history of the island, namely the volcanic events that led to the formation of FVS and CVS. With the aim of characterizing the Holocene volcanism of Flores Island and establishing the stratigraphy and eruptive history of the FVS and CVS maar-forming events, we studied the volcano-sedimentary record of Lagoa da Lomba and conducted a detailed characterization of the volcanic sequences cropping out in the central part of the island, work that was complemented by geochemical analysis of glass shards and radiocarbon dating. Our observations show that six volcanic eruptions occurred in the central uplands during the Holocene. The two older ones (6,280 and 4,490 cal yr BP) are only preserved at Lagoa da Lomba, and their source is unknown to date. The three following eruptions (3,430, 3,330 and 3,250 cal yr BP) were sourced from FVS and the most recent one (3,180 cal yr BP) was sourced from CVS. The first and the third FVS eruptions are only preserved subaerially, whilst the CVS and the 3,330 cal yr BP FVS events are preserved both subaerially and at the sedimentary record of Lagoa da Lomba. Evidence of phreatomagmatism is only found for the three most recent eruptions, which suddenly shifted eruptive style from Hawaiian/Strombolian to phreatomagmatic, turning relatively mild basaltic eruptions into violent explosive events. The high superficial and ground-water availability together with the decrease in eruption rates are considered the driving factors controlling this sudden shift to phreatomagmatism at Flores, with clear hazard implications.