Phreatic eruptions continued in August from four sites in the crater lake. The strongest eruptions were from the center of the lake, ejecting plumes that were typically ~50 m high. Some were larger, as on 12 August, when a plume rose 200 m. The average lake temperature was 65°C, reaching 100°C near the eruption sites. Chemical analysis of the lake water indicated an increase in its sulfate and chloride content; pH remained at 0.5. The level of the lake continued to descend. Temperatures of fumaroles on the remnants of the 1953-55 [dome] had declined to 379°C from as much as 587°C in April. Measurements on 17 August show 34 µrad of inflation on the S flank of the crater since 28 June. A N-S line across the crater lengthened 84 mm between 6 July and 18 August.

In July, the acidity of the air around the crater caused eye and respiratory discomfort to tourists. Plants were also affected and the impact of acid rain, from Poás fume, has been noted as far as 8 km SW of the crater.

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, E. Fernández, and R. Van der Laat, OVSICORI.