Furnas Volcano is one of the three quiescent central volcanoes of S. Miguel Island, in the Azores Archipelago, Portugal. Furnas volcanic activity has been essentially explosive, erupting material of trachytic composition. In the last 5000 years Furnas Volcano erupted at least 10 times. These eruptions were characterised by alternating episodes of magmatic and phreatomagmatic activity of plinian and sub-plinian magnitude. Geomorphologically, in the summit of the volcano it is possible to observe a depression formed by two main calderas. The most recent eruptions (1439-43; 1630) occurred inside the caldera with the formation of tuff rings and domes.
At the present, fumarole grounds, thermal springs, CO2 rich mineral cold waters and several soil diffuse degassing areas are the main secondary volcanic phenomena that can be found at Furnas Volcano.
Radon (222Rn) may be an important indicator of deep processes and its study can be mainly applied to the identification of active faults, seismovolcanic monitoring and geothermal prospection. The present quiescent period of Furnas volcano can be considered as ideal to define the background level of 222Rn degassing contributing to the recognition of future signals of volcanic unrest.
The 222Rn monitoring in Furnas Volcano includes mapping of soil anomalies, soil gas continuous monitoring measurements and regular sampling of some springs.
In what concerns mapping, a soil radon survey was performed inside Furnas caldera and the field work was developed during summer months time between 2005 and 2009. A total of 301 station points were measured with a solid state alpha detector (RAD7). 222Rn measurements oscillated between 0 and 387527,3 Bq/m3 being the average value 11305,9 Bq/m3. The spatial distribution analysis of the data allowed to identify 222Rn anomalous zones that were compared with structural and geological structures.
Continuous monitoring of 222Rn soil degassing started in June 2009. The RAD7 monitoring station was installed inside Furnas Volcano caldera, near Furnas Lake fumarolic field, at the same monitoring ground as the soil CO2 flux permanent station (GFUR2). During this monitoring period 222Rn concentration values oscillated between 0 and 74517,9 Bq/m3. Fluctuations on Radon values were checked with environmental data provided by the meteorological station coupled to GFUR2 to identify external influences. Barometric pressure, rainfall, wind speed, air temperature, air relative humidity, soil temperature, and soil water content were the considered variables. The parameters that seem to influence the 222Rn fluctuations are air temperature and wind speed.
A total of 12 springs (hydrothermal, carbonated and cold waters) were sampled every two months since November 2007 until May 2009. In each spring two samples were taken for laboratory analysis with RAD7 station and the water temperature was simultaneously measured. During the period under analysis the 222Rn concentration and the temperature values of the springs were quite stable, which seems quite promising to identify changes on the volcanic activity.