Geyser-like phreatic eruptions, the first since September 1980, resumed in June. The crater lake had been relatively quiet and the eroded 1953-55 [dome] in the lake had been gradually cooling for the past several years. The eruptions, from the W part of the crater lake, ejected columns of water and sediments that typically rose to about 30 m height, then fell back into the lake, generating a wave surge. The eruptions were followed by vapor column emission. Sporadic activity continued in July and August. On 1 July, three phreatic eruptions were observed, with ejecta from the last, at 1130, reaching 75 m above the lake. Three more phreatic episodes were seen on 14 August (at 0845, 1120, and 1225), with the second only producing a small bubble. On 19 August, three explosions and another small bubble were observed (at 0958, 1133, 1210, and 1358). Witnesses of the second and third explosions reported that a mud and vapor column rose suddenly from the center of the lake without any apparent premonitory activity. The column ejection was accompanied by a loud noise that increased as the ejecta fell back, causing 1-m waves that eroded sediments at the shore.

During the first 8 months of 1987, fumarole temperatures averaged 540°C at the top of the 1953-55 [dome] and 800°C on its W flank. Temperatures had dropped from more than 800°C in late 1983 to around 300°C in mid-1985. Fumaroles were less vigorous than last year. Crater lake temperature varied, but increased from 58°C in January to 70°C in June; pH was 0-0.5. The lake level has continued to fall, exposing three terraces of gray, water-saturated sediment since January, and dropping about 2 m in the past 6 months. Concentric fractures have developed along the shore, particularly on the E and NE sides (striking N-S and NW-SE respectively). Landslides have occurred on the E wall of the crater and on the 1953-55 [dome].

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, OVSICORI; R. Barquero and Guillermo Alvarado, ICE; G. Soto and Luis D. Morales, Sección de Sismología, Vulcanología, y Exploración Geofísica, Escuela Centroamericana de Geología, Univ de Costa Rica, San José, Costa Rica.