Styles of volcanism and volcanic hazards on Furnas volcano, Sao Miguel, Azores

Explosive volcanic activity has been dominant throughout the history of Furnas volcano. Eruptions that switched between magmatic and phreatomagmatic activity have become typical in the more recent history of the volcano with 70% of the eruptions in the last 5000 years being of this style. It is thus likely that future eruptions will be of this type. Magmatic activity gives rise to lapilli-fall whereas phreatomagmatic activity produces tephra-fall, pyroclastic surges, and flows. Strong winds, rainstorms, and/or strongly heterogeneous fallout may have contributed to locally variable internal stratigraphy of tephra from single eruptions. Proximal exposures indicate reworking and erosion of lapilli layers by pyroclastic surges and possible contemporaneity of surge and lapilli fall. Evidence indicates that pulsatory magma supply may play an important role in the switching between magmatic and phreatomagmatic activity. Hazard mapping for two end member eruptions (1630 AD and Furnas 'C' similar to 2000 BP) using a technique with a moving vent location has provided worst case scenario tephra thicknesses in the region around Furnas volcano. Anywhere inside the caldera of Furnas volcano is under serious threat, in the event of an explosive eruption, from destruction by pyroclastic surges. The effect of pyroclastic surges outside the caldera, such as for the villages of Ponta Garca and Povoacao, is Likely to depend largely on vent location. The considerable topographic relief of the region will control areas affected by pyroclastic flows and also lahars. Towns located at the mouths of rivers, e.g., Ribeira Quente and Povoacao, are particularly vulnerable. Lahars present a major hazard at Furnas Volcano. (C) 1999 Elsevier Science B.V. All rights reserved.