Gas emission continued from crater fumaroles (60-95°C) in June. The crater lake temperature was 73°C, similar to December-February values, while lake depth increased to 3 m, coinciding with heavy rainfall. Medium-frequency (2.5 Hz) harmonic tremor was recorded 6-17 June, occurring up to 24 hours/day (figure 38). Seismicity was dominantly low-frequency.

Figure 38. Hours/day of tremor recorded at Poás by the Univ Nacional, June 1991.

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, V. Barboza, R. Van der Laat, and E. Malavassi, OVSICORI; R. Barquero, G. Alvarado, M. Fernández, H. Flores, and S. Paniagua, ICE.