Between 5 and 20 December 1982, a team from PIRPSEV (CNRS) and a volcanological team from the Universidad Nacional de Costa Rica sampled gases from Poás (table 2). The gas temperatures have been variable but generally decreasing: 940°C in June 1981, about 870°C between January and April 1982, about 790°C between April and November 1982, and 731°C on 17 December 1982. Since June 1981, twenty measurements have been collected from the control fissure with the aid of a silica rod of new design: the decrease in temperature at the end of the rod being only 10%. Since December 1981, the ratio S/C appeared to have stabilized at approximately 3, a higher value than in other available data.

Table 2. Average values of gases sampled at Poás, 1981-82. Late-1982 sampling was by a team from PIRPSEV (CNRS) and a volcanological team from the Universidad Nacional de Costa Rica.

Geological Summary. The broad vegetated edifice of Poás, one of the most active volcanoes of Costa Rica, contains three craters along a N-S line. The frequently visited multi-hued summit crater lakes of the basaltic-to-dacitic volcano are easily accessible by vehicle from the nearby capital city of San José. A N-S-trending fissure cutting the complex stratovolcano extends to the lower N flank, where it has produced the Congo stratovolcano and several lake-filled maars. The southernmost of the two summit crater lakes, Botos, last erupted about 7,500 years ago. The more prominent geothermally heated northern lake, Laguna Caliente, is one of the world's most acidic natural lakes, with a pH of near zero. It has been the site of frequent phreatic and phreatomagmatic eruptions since an eruption was reported in 1828. Eruptions often include geyser-like ejections of crater-lake water.

Information Contacts: J. Cheminée, IPG, Paris, M. Javoy, H. Delorme, Univ. de Paris.