The following is from Boletín de Vulcanología no. 7.

By January, the small phreatic explosions that began in September 1979 had ended. Activity in January and February was limited to emission of gases with a strong sulfur odor from the N wall of the central "dome." Fumarole temperatures ranged from 56 to 95°C. Mean lake water temperature dropped from 65°C in November 1979 to 55°C in January and 50°C in February. The lake was gray with green patches in January, but had turned to a turquoise color in February. The water level dropped about 30 cm between the January and February surveys.

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. Barquero H., Univ. Nacional, Heredia.