Fumarolic activity was limited to the NW flank of the volcano. Temperatures oscillated between 78°C and 85°C. No activity was observed in either the principal crater or Diego de la Haya, just to the SE.

Activity on a local fault caused an earthquake swarm 4 June. The swarm generated landslides in the wall that divides the principal crater from Diego de la Haya. Another stronger swarm occurred 23-24 September affecting the area between Irazú and Turrialba volcano.

Geological Summary. The massive Irazú volcano in Costa Rica, immediately E of the capital city of San José, covers an area of 500 km2 and is vegetated to within a few hundred meters of its broad summit crater complex. At least 10 satellitic cones are located on its S flank. No lava effusion is known since the eruption of the 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 main crater, which contains a small lake. The first well-documented 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. Phreatic activity reported in 1994 may have been a landslide event from the fumarolic area on the NW summit (Fallas et al., 2018).

Information Contacts: R. Stoiber, S. Williams, R. Naslund, C. Connor, J. Prosser, and J. Gemmell, Dartmouth College; E. Fernández S. and J. Barquero H., Univ. Nacional, Heredia.