Fumarolic activity in the yellowish-green crater lake continued through October, and mats of sulfur were observed floating on the water's surface. In September, strong bubbling was observed in the N- and NW-central parts of the lake, which was at 76.3°C with a pH <0.5. Lake measurements on 16 October indicated that the lake level had fallen 15 cm since 18 September, despite continued rains; water temperature was 74.4°C. Fumaroles on the 1953-55 dome ranged to 90.6°C in September and 88.4°C in October.

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: R. Barquero and G. Soto, ICE.