Owing to the existence of a population of several thousand people living within and around its caldera it is considered one of the most dangerous volcanoes in archipelago. Throughout its history, volcanic activity at Furnas has been essentially explosive but eruptive styles have varied from mild effusive activity to at least two caldera-forming eruptions 30,000 and 12,000-10,000 years ago. It is considered very active, since ten sub-Plinian eruptions have occurred within the past 5,000 years, producing a total area of 0.9 kilometres square of fairly homogeneous trachytic magma. The first known historical eruption occurred in 1440, just after early settlers started populating the coasts of São Miguel. The latest eruption occurred after the settlement of the Azores archipelago in 1630 A.D., and caused the deaths of 200 people. Eruptions of the Upper Furnas Group were mainly characterised by alternating episodes of magmatic and phreatomagmatic activity producing deposits of inter-bedded volcanic ash and lapilli that overlie the widespread main deposits from the adjacent Fogo volcano. At least three of these eruptions were accompanied by trachyte dome extrusions in the final phases of the eruption.