"The volcano has cooled over the past year, with maximum fumarole temperatures on the cone of 575°C. Only very dull, brown incandescence could be occasionally seen, whereas one year ago bright red was seen easily. Water temperature was 41°C and the pH approximately 0. COSPEC measurements of SO₂ flux suggest levels very far below the approximately 800 metric tons/day measured in February 1982 by Casadevall and others (1984). A portable seismograph indicated very little seismic activity in two days of recording in November."

Reference. Casadevall, T., Rose, W.I. Jr., Fuller, W., Hunt, W., Hart, M., Moyers, J., Woods, D., Chuan, R., and Friend, J., 1984, Sulfur dioxide and particles in quiescent volcanic plumes from Poás, Arenal, and Colima volcanoes, Costa Rica and México: JGR, v. 89, no. D6, p. 9633-9641.

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. Stoiber, S. Williams, H.R. Naslund, C. Connor, J. Prosser, and J.B. Gemmell, Dartmouth College; E. Malavassi R. and J. Barquero H., Univ. Nacional, Heredia.