El volcan poas, costa rica : cronologia y caracteristicas de la actividad

The Poas Volcano in Costa Rica is one of the most interesting eruptive centers in the Country due to the variety and peculiarity of its volcanic manifestations. In this paper a short description of the structure of the active caldera and its relationship to the local geology is made. The volcanic activity shows a clear long period cyclic process. A bibliographic survey on the recent activity (from the beginning of the last century) of the volcano is also included. The activity may be classified in four distinct categories. a) fumarolic and/or solphataric. b) mud plumes. c) phreatic explosions (geyser-type). d) phreato-magmatic and lava eruptions. Analysis of the phenomena observed in 1980-81 suggest that the increase in temperature of the fumaroles from 92°C in December, 1980 to 960°C during the period March to November, 1981 was due to the rise and overflow of the fluids concentrated in the shallow magma body involved in the eruption of 1953. Fracturation of the solid part of the magma body had been shown by the local seismic activity beginning on July 27, 1980 and lasting about 10 days. It is finally suggested that all complex manifestations of this volcano are produced by the same eruptive mechanism.