Report on Irazu (Costa Rica) — December 1991
Bulletin of the Global Volcanism Network, vol. 16, no. 12 (December 1991)
Managing Editor: Lindsay McClelland.
Irazu (Costa Rica) Fumarolic activity; crater lake rises
Please cite this report as:
Global Volcanism Program, 1991. Report on Irazu (Costa Rica) (McClelland, L., ed.). Bulletin of the Global Volcanism Network, 16:12. Smithsonian Institution.
9.979°N, 83.852°W; summit elev. 3432 m
All times are local (unless otherwise noted)
Fieldwork on 19 November showed continued fumarolic activity at the base of the main crater, with temperatures of 90°C, similar to those of previous months. The crater lake had an average temperature of 26.7°C, and a pH of 3.0 (compared to 3.5 in September). Water level had risen 40 cm since October, and the lake radius had grown 5 m. The color of the lake had changed, probably from sediment input. Many subaqueous fumaroles were observed. The summit station (ICR) recorded 36 microearthquakes during November, a decline from the high rates during seismic swarms several months earlier.
Geological Summary. Irazú, one of Costa Rica's most active volcanoes, rises immediately E of the capital city of San José. The massive volcano covers an area of 500 km2 and is vegetated to within a few hundred meters of its broad flat-topped summit crater complex. At least 10 satellitic cones are located on its S flank. No lava flows have been identified since the eruption of the massive Cervantes lava flows from S-flank vents about 14,000 years ago, and all known Holocene eruptions have been explosive. The focus of eruptions at the summit crater complex has migrated to the W towards the historically active crater, which contains a small lake of variable size and color. Although eruptions may have occurred around the time of the Spanish conquest, the first well-documented historical eruption occurred in 1723, and frequent explosive eruptions have occurred since. Ashfall from the last major eruption during 1963-65 caused significant disruption to San José and surrounding areas.
Information Contacts: G. Soto, R. Barquero, and Mario Fernández, ICE.