The interaction of ascending magmas with groundwater at near-surface levels may result in sudden changes in eruptive behaviour, e.g. between strombolian and phreatomagmatic styles. This change is particularly acute at hydraulically-charged ocean island volcanoes where tiny variations in the water/magma ratios may turn relatively mild basaltic volcanism into violent phreatomagmatic activity. Flores is one of the two islands of the Azores that lie west of the Mid-Atlantic Ridge. Since its settlement in the 15
th century the island did not experienced any eruption and therefore its volcanic hazard is considered to be low. However, Flores has a rich Holocene record of phreatomagmatic and magmatic volcanism that created several large maars and tuff rings. Very little is known about these recent eruptions, and therefore the volcanic hazard potential of Flores Island should not be underestimated and the possibility of future highly-explosive eruptions should be properly considered.
Our study aims to assess the volcanic hazard potential of Flores, through a detailed reconstruction of its Holocene eruptive history, using tephrostratigraphy of terrestrial and lacustrine records combined with glass geochemistry and radiocarbon dating. Two main recent volcanic centres have been recognized, both characterized by a basaltic to phonotephritic initial magmatic phase that rapidly shifted to a highly explosive phreatomagmatic phase:
i) Funda Volcanic System (FVS) which is located on the southern central part of the island and comprises three main craters (including Funda and Rasa lakes) aligned along a SE-NW direction. FVS has a complex history and a large variety of volcanic products such as fall deposits of magmatic and phreatomagmatic origin, and dense and dilute pyroclastic current deposits.
ii) Comprida Volcanic System (CVS) is located in the northern central part of the island and includes at least four craters, Comprida, Negra, Seca and Branca.
Our field observations suggest a cyclicity in the style of Holocene volcanic eruptions, invariably resulting in abrupt transitions from strombolian to phreatomagmatic events, demonstrating that small monogenetic eruptions may rapidly turn into a highly hazardous events at hydraulically-charged ocean island volcanoes such as not only the case of Flores Island but also the case of many Islands of the British overseas territories. Future work of this project will involve a paleoclimate analysis of the Atlantic North during the time of these Holocene eruptions, in order to investigate how the hazard potential of small-volume monogenetic eruptions may be amplified by environmental controls such as rainfall variability, which ultimately controls the available groundwater.