As in the recent past, fumaroles remained active during April-June. Temperatures in the active turquoise-green-colored lake measured 36-37°C, continuing near the low for a 6-year period.

Temperatures measured at the accessible part of the pyroclastic cone remained at 94°C during April-June. These fumaroles provided the main sources of degassing, and noise from escaping gases was audible at the overlook (Mirador). Also, during April-June, the temperatures of weak fumaroles on the S and SW crater walls were 93-94°C. Some new points of weak fumarolic emission appeared during April on the cone's upper NE wall; other weak fumaroles with temperatures of 93°C appeared during May on the lake's N terrace. During June, at a crack in this terrace new fumaroles developed, as they also did on the pyroclastic cone, appearing at the same time that several high- and medium-frequency earthquakes took place.

During April-June gas plumes rose 400-500 m above the crater floor; winds predominantly towards the W and SW flanks occasionally also carried strong, irritating sulfurous odors over the S flank where they were noted by tourists and park rangers.

In the May OVSICORI-UNA report, attention was called to continued bubbling along the lake's S and SW shorelines, a process referred to in earlier reports. Also noted was mass wasting of a terraced area towards the crater lake. During June an upper portion of the pyroclastic cone continued cracking and collapsing into the crater lake.

Although both the earthquake counts and tremor durations for March-May were generally moderate to low, the tremor duration was significant because it had remained absent for all but one month in 1997 (November, when it prevailed for 22 hours). In comparison, looking at 1998 tremor durations for January through June, tremor prevailed for between 0.46 and 55 hours, but during each of the latter 3 months it occurred forBGVN23:03).

Table 8. Poás earthquakes and tremor registered at station POA2 (2.7 km SW of the active crater), April-June 1998. Courtesy of OVSICORI-UNA.

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. Fernandez, V. Barboza, E. Duarte, R. Saenz, E. Malavassi, M. Martinez, and Rodolfo Van der Laat, Observatorio Vulcanologico y Sismologico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica.