A multi-methodological re-evaluation of the volcanic events during the 1580 CE and 1808 eruptions at São Jorge Island (Azores Archipelago, Portugal)

Abstract

Among all eruptions that took place on the Azores Archipelago since the Portuguese settlement in the XVI century, those of 1580 and 1808 in the island of São Jorge were unusual. The magma erupted in 1580 ascended from at least 23 km and ponded at a minimum depth of 16 km. Here residual magma fractionated mafic phases and plagioclase during 228 years before erupted again in 1808. Magma emission occurred from numerous fissures active at the same time and opening for several kilometres, following a local tectonic trend. Hydro-volcanic outbursts and hot rock fall over steep slopes caused casualties and injuries, and lava flows and heavy tephra fallout affected the local economy. Despite of all these dramatic consequences, the memory of these eruptions is almost vanished. Representative geosites and morphological landforms if not hindered by lush vegetation, have been reshaped by diffuse quarries and reworked to recover croplands and pastures. Aiming at a better comprehension of the multiple hazards associated with volcanic eruptions at São Jorge Island, we retraced the volcanic events of these two eruptions by matching our re-interpretation of historic reports with field data and photogrammetry, and with fluid inclusions, petrography and geochemical data of erupted products.