Fumarolic activity continued in the N and NW parts of the crater lake, with loud gas emissions sounding like a jet engine audible from an observation site 1 km S. Gas columns rose >500 m above the lake. Lake level has dropped 1.5 m since May, and the lake temperature was measured as 65°C. Lake color varied from gray to emerald-green or turquoise-green. Sporadic bubbles formed in the center of the lake, and there were frequent phreatic eruptions to 2 m in the NW part of the lake. Gas emissions from the intracrater cone had decreased to a temperature of 89°C, and small landslides had traveled toward the lake from the N side. Fumaroles along terraces on the N side of the lake had temperatures of 113-142°C; fumaroles on the dome were <80°C.

The seismic station 2 km SW of the crater registered 3,844 low-frequency (<2.5 Hz) events in June, an average of 129 events/day; the highest daily total was on 30 June (212). There were also three medium-frequency (2.5-3 Hz) events registered in June.

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: E. Fernández, J. Barquero, and V. Barboza, OVSICORI; G. Soto, ICE.