Report on Irazu (Costa Rica) — June 1995
Bulletin of the Global Volcanism Network, vol. 20, no. 6 (June 1995)
Managing Editor: Richard Wunderman.
Irazu (Costa Rica) Heavy rains trigger landslides and lahars
Please cite this report as:
Global Volcanism Program, 1995. Report on Irazu (Costa Rica). In: Wunderman, R. (ed.), Bulletin of the Global Volcanism Network, 20:6. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199506-345060.
9.979°N, 83.852°W; summit elev. 3432 m
All times are local (unless otherwise noted)
In June, the dark yellow, weakly bubbling lake rose to cover the entire crater floor at Irazú. Crater walls continued to slump into the lake on the N, E, and SE sides. At the site of the 9 December 1994 phreatic eruption (on the NW flank), the established fumaroles remained both near the collapsed wall and in the inner vent area. On the NE sector of the 9 December deposit, some fumaroles have ceased, while on the SW sector some new fumaroles have emerged. Accessible fumaroles had temperatures in the 80-90°C range.
The NE flank remained unstable and continued producing small landslides. Heavy rains have triggered lahars that have traveled down the upper to middle reaches of the Sucio river.
On 25 June, 3 earthquakes took place along local faults with epicenters 9-10 km NE of the main crater. The earthquake magnitudes were 2.5, 3.1, and 3.3; depths were 8, 6, and 8 km.
Geologic Background. 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: E. Fernandez, R. Van der Laat, F. de Obaldia, T. Marino, V. Barboza, W. Jimenez, and R. Saenz, Observatorio Vulcanologico y Sismologico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica; Mauricio Mora, Escuela Centroamericana de Geologia, Universidad de Costa Rica; G.J. Soto, Oficina de Sismologia y Vulcanologia del Arenal y Miravalles (OSIVAM), Instituto Costarricense de Electricidad (ICE), Apartado 10032-1000, San Jose, Costa Rica.