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Instituto de Investigação
em Vulcanologia e Avaliação de Riscos
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Referência Bibliográfica

GASPAR, J.L., FERREIRA, T., QUEIROZ, G., BAUBRON, J.C., BAXTER, P. (1998) - High levels of CO2 in the atmosphere of Furna do Enxofre lava cave (Graciosa island, Azores): a case of public health risk. EC Advanced Study Course on "Volcanic Hazard, Monitoring and Risk Mitigation". S. Miguel, Furnas, Junho (Comunicação Oral).


Furna do Enxofre is a magnificent lava cave located below the surface of Graciosa caldera and is believed to be part of the central volcano conduit. It communicates with the exterior through two small NE-SW fissures in one of which a tower with 183 steps was built to facilitate the access of visitants.
The lava cave has a diameter of approximately 150 meters and is about 80 meters high. The roof presents an arched shape composed by perfect lava prismatic blocks and the floor is either rocky or muddy and is dipping towards SE. Near the entrance exists a small fumarolic field while on the bottom of the cave there is a lake of cold water.
On the 6th of August 1992 two people died at Furna do Enxofre and four others were assisted at the Graciosa island health centre. The victims were part of a group from the Portuguese Navy that visited the lava cave during that afternoon. According to witnesses the deaths occurred in a few seconds when two of the men reached a small boat that was in the lake. The four people hospitalised were the ones that tried without success to save their friends. The symptoms reported by all this survivors comprise shortness of breath, muscular weakness and headache, while other members less affected entered in panic and stayed hysterical for several hours. Air samples collected in the same day inside Furna do Enxofre showed CO2 levels of 15% near the lake.
A volcanological survey developed in Furna do Enxofre demonstrated that despite the strong fumarolic activity that exists near the entrance, cold soil degassing along NW-
-SE and NE-SW faults is the main permanent process responsible for the high levels of CO2 in the atmosphere of the lava cave. The environmental conditions are mainly controlled by the CO2 flux and by the efficiency of ventilation and can change drastically in a short period of time. This observations clearly demonstrate that Furna do Enxofre, being one of the most attractive points of Graciosa island, encloses a high public health risk if severe preventive measures remain to be implemented.