Volcanic geology of Furnas Volcano, Sao Miguel, Azores

Furnas is the easternmost of the three active central volcanoes on the isla nd of Sao Miguel in the Azores. Unlike the other two central volcanoes, Set e Cidades and Fogo, Furnas does not have a well-developed edifice, but cons ists of a steep-sided caldera complex 8 x 5 km across. It is built on the o uter flanks of the Povoacao/Nordeste lava complex that forms the eastern en d of Sao Miguel. Constructive flanks to the volcano exist on the southern s ide where they form the coastal cliffs, and to the west. The caldera margin s tend to reflect the regional/local tectonic pattern which has also contro lled the distribution of vents within the caldera and areas of thermal spri ngs. Activity at Furnas has been essentially explosive, erupting materials of trachytic composition. Products associated with the volcano include plin ian and sub-plinian pumice deposits, ignimbrites and surge deposits, phreat omagmatic ashes, block and ash deposits and dome materials. Most of the act ivity has occurred from vents within the caldera, or on the caldera margin, although strombolian eruptions with aa flows of ankaramite and hawaiite ha ve occurred outside the caldera. The eruptive history consists of at least two major caldera collapses, followed by caldera infilling. Based on C-14 d ates, it appears that the youngest major collapse occurred about 12,000-10, 000 years BP. New C-14 dates for a densely welded ignimbrite suggest that a potential caldera-forming eruption occurred at about 30,000 years BP. Rece nt eruptions ( < 5000 years old) were mainly characterised by alternating e pisodes of magmatic and phreatomagmatic activity of plinian and sub-plinian magnitude, forming deposits of interbedded ash and lapilli. An historical eruption is documented in 1630 AD; new evidence suggests that another occur red during the early occupation of the area at about 1440 AD. (C) 1999 Else vier Science B.V. All rights reserved.