"Dartmouth geologists visited Poás on 6 and 10 December. Extensive fumarolic activity continued. Fumaroles had lower temperatures than measured on previous trips, not exceeding 525°C. Lake water sampled 100 m NW of the fumarole bank had a temperature of 36°C and a pH of 0.22 at the surface, and a temperature of 38°C and a pH of 0.08 at 4 m depth."

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: B. Barreiro, H.R. Naslund, R. Stoiber, P. Turner, Dartmouth College.