Report on Pacaya (Guatemala) — November 1991
Bulletin of the Global Volcanism Network, vol. 16, no. 11 (November 1991)
Managing Editor: Lindsay McClelland.
Pacaya (Guatemala) Small explosions and lava flow
Please cite this report as:
Global Volcanism Program, 1991. Report on Pacaya (Guatemala) (McClelland, L., ed.). Bulletin of the Global Volcanism Network, 16:11. Smithsonian Institution.
14.382°N, 90.601°W; summit elev. 2569 m
All times are local (unless otherwise noted)
The following, from Michael Conway, describes 11 November fieldwork by geologists from INSIVUMEH and MTU. On 10 November, a small-volume lava flow began to erupt from the summit of MacKenney cone at about 1600. By the evening of the following day, the flow had traveled 50-100 m down the cone's S flank. This was the first time in several years that a flow was erupted onto the S flank, indicating that the powerful explosive eruptions of July and August resulted in major changes in the vent geometry of MacKenney cone.
Several small eruptions observed on the afternoon of 11 November produced black plumes that rose as much as several hundred meters above the vent, generally accompanied by a jetting sound. Widely spaced (50-125 m) en echelon fractures were identified on the W flank of MacKenney cone. The fractures began 10-30 m below the summit and continued down the flank for roughly 50-100 m. A series of point-source fumaroles marked the fractures, which may have originated as the W part of the cone sagged into an evacuated shallow magma chamber following the powerful pyroclastic eruptions of July-August.
Geological Summary. Eruptions from Pacaya, one of Guatemala's most active volcanoes, are frequently visible from Guatemala City, the nation's capital. This complex basaltic volcano was constructed just outside the southern topographic rim of the 14 x 16 km Pleistocene Amatitlán caldera. A cluster of dacitic lava domes occupies the southern caldera floor. The post-caldera Pacaya massif includes the ancestral Pacaya Viejo and Cerro Grande stratovolcanoes and the currently active Mackenney stratovolcano. Collapse of Pacaya Viejo between 600 and 1500 years ago produced a debris-avalanche deposit that extends 25 km onto the Pacific coastal plain and left an arcuate somma rim inside which the modern Pacaya volcano (Mackenney cone) grew. A subsidiary crater, Cerro Chino, was constructed on the NW somma rim and was last active in the 19th century. During the past several decades, activity has consisted of frequent strombolian eruptions with intermittent lava flow extrusion that has partially filled in the caldera moat and armored the flanks of Mackenney cone, punctuated by occasional larger explosive eruptions that partially destroy the summit of the growing young stratovolcano.
Information Contacts: Rodolfo Morales and Otoniel Matías, Sección de Vulcanología, INSIVUMEH; Michael Conway, Michigan Technological Univ, USA.