The Azores archipelago (Portugal) is located on the triple junction of the American, Eurasian and Nubian plates and is formed by nine volcanic islands. Furnas Volcano is a quiescent central volcano located on the eastern part of S. Miguel Island. This volcano erupted at least 10 times in the last 5000 years, with plinian and sub-plinian eruptions characterized by alternating episodes of magmatic and phreatomagmatic activity. Geomorphologically, in the summit of Furnas Volcano two main nested calderas, several craters and various domes are identified. The most recent eruptions (1439-43 AD and 1630 AD) were characterized by initial explosive episodes responsible by the formation of pumice rings and ended with more effusive episodes marked by the emplacement of inner crater trachytic domes.
In nowadays, Furnas Volcano presents a moderate seismic activity and shows several secondary volcanic phenomena such as fumarole grounds, thermal springs, cold CO2 rich mineral waters and soil diffuse degassing areas.
222Rn and CO2 gases may give important information about deep processes. Soil diffuse degassing studies may be applied on the identification of active faults, seismovolcanic monitoring and geothermal prospection. The quiescent period of Furnas Volcano is ideal to define the background level of 222Rn and CO2 degassing and may contribute to the recognition of future signals of volcanic unrest.
In what concerns 222Rn mapping, a soil radon survey was performed during summer months from 2005 to 2010. A total of 489 station points were measured with a solid state alpha detector (RAD7). 222Rn measurements oscillated between 0 and 387527 Bq/m3 being the average value 11733 Bq/m3. During this survey soil temperature measurements were also performed in the same station points at about 20 cm depth. The temperature ranged between 16,5 and 100 ºC with an average of 23,7 ºC.
Soil CO2 concentration measurements were performed with a GA 2000 gas analyzer, which operates by a dual wave-length infra-red cell. Data sampled during 2001 and 2002 (Sousa, 2003), as well as the measurements performed between 2005 and 2010 were integrated to produce a final CO2 map, based on 1535 sampled points. Soil CO2 concentration values oscillated between 0 and 99,9 %vol. with an average value of 6,3 %vol.
The spatial distribution analysis of the data allowed to identify 222Rn and CO2 anomalous zones that were compared with structural and geological structures. Anomalous zones, for both gases, seem to be associated mainly with NW-SE fault/fracture zones, the presence of craters and the caldera rims. On the top of the domes of the last eruptions these gases are practically absent. The temperature anomalous zones, as well as the CO2 degassing anomalies, are mainly associated with the presence of the main fumarolic fields of Furnas Volcano.