Activity in the main crater remained stable in August, with weak vapor emission from sediment fans on the N side of the crater lake and active fumaroles in the NE part of the lake. The lake remained green, and its level has varied by only a few centimeters in recent months. Erosion of the crater walls carried sediment to fans on its N, W, and E sides. Temperature and pH of springs monitored around the volcano had not changed from previous months. The number and energy release of local seismic events have decreased considerably in the past 3 months. A UNA seismic station (IRZ2) 5 km WSW of the main crater registered 27 small events in August, down from 33 in July.

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