Furnas is one of three active volcanoes on the island of São Miguel in the Azores. Volcanic activity at Furnas ranges over about 90,000 years and almost all of this activity was explosive. Three different types of tephra fall deposit have been identified: Ash on1y layers, representing predominantly hydromagmatic activity; lapilli on1y layers, reflecting magmatic activity and deposits composed of alternating lapilli and ash formed by eruptions that switched between hydromagmatic activity and magmatic activity a number of times. Pyroclastic deposits composed of alternating layers of lapilli and ash become more abundant in the recent volcanic activity, such that nearly 70% of eruptions in the last 5,000 years were of this type. This style of activity can be considered typical of Furnas. 

Detailed studies have revealed that the magmatic (lapilli forming) phases were associated solely with fall-out processes, whereas hydromagmatic (ash producing) phases of these eruptions were associated both with tephra fall and pyroclastic flows and surges. Such pyroc1astic surges would cause total devastation within the caldera, however the area affected outside the caldera would be c1osely related to vent location. The strong relief of the area would channel pyroclastic flows and lahars down major valleys. The town of Ribeira Quente is under considerable threat from pyroc1astic flows and lahars due to its location at the end of a valley leading direct1y from the caldera. The larger but more distant town of Povoação sitting at the confluence of several river valleys is under major threat from lahars. 

Hazard maps were constructed for the tephra fall of the two end-member eruptions (largest and smallest) that have occurred in the last 5,000 years. These are the 1630 AD eruption and the Furnas ‘C’ eruption (-2,000 BP). The maps were constructed using a moving vent location within the inner caldera from which future eruptions could occur. Worst case scenario conditions were considered such that if the vent is located on edge of the caldera and the wind blowing radially out from this caldera margin. The resulting isopleths maps are therefore the maximum possible thicknesses of tephra that could be produced in any one area by this type of eruption.