Strong fumarolic activity continued at the base of the crater. During August, the level of the crater lake continued to drop despite heavy rainfall. By 13 September the lake was almost dry, with the exception of small puddles of bubbling mud. The lake temperature remained near 85°C in August and September. At least 30 fumarolic vents were visible, emitting steam, sulfur spray, and SO₂; several emitted flames, and others produced a jet engine sound. A pool of boiling mud (137°C in August) remained in the SE part of the lake. Intense rainfall on 13-14 September caused a temporary recharge of the crater lake and the destruction of some small mud/sulfur cones. Subsequent heavy evaporation produced white steam columns that were seen as far away as San José (30 km SSW). During September, small slumps were observed on the E wall of the inner crater, in the vicinity of a large number of cold, acid springs. Fumaroles on the top of the 1953-55 dome had temperatures of up to 93.9°C.

Acid rain continued on the SW flank, damaging crops and causing health problems. During a change in wind direction to the S and SE on 27 August, discomfort and strong sulfur odors were reported to 30 km from the crater. The ambient pH was 4.2 at the crater overlook and 4.75 at Cerro Pelón (2 km SW of the crater).

Seismicity at Poás was dominated by low-frequency earthquakes. The Univ Nacional recorded [daily averages of] 260 low-frequency earthquakes in June, 299 in July, and 222 in August (figure 33). A total of 185 high-frequency earthquakes, with magnitudes oscillating from 1.7 to 2.9 and depths <8 km, were recorded in August. On 19 July a swarm of 15 high-frequency earthquakes occurred. The events had magnitudes ranging from 2.3 to 2.9 (some felt by local residents) and were centered 5-8 km from the crater at depths of 4.6-7.3 km. Medium- and high-frequency tremor increased in August, with episodes of continuous tremor up to 48 hours long.

Figure 33. Number of low-frequency (top) and high-frequency (bottom) earthquakes recorded at Poás by the Universidad Nacional, June-August 1990.

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