Report on Pacaya (Guatemala) — December 1990
Bulletin of the Global Volcanism Network, vol. 15, no. 12 (December 1990)
Managing Editor: Lindsay McClelland.
Pacaya (Guatemala) Strombolian activity and lava flows
Please cite this report as:
Global Volcanism Program, 1990. Report on Pacaya (Guatemala) (McClelland, L., ed.). Bulletin of the Global Volcanism Network, 15:12. Smithsonian Institution.
14.382°N, 90.601°W; summit elev. 2569 m
All times are local (unless otherwise noted)
Vigorous Strombolian activity continued during December and early January. Explosions occurred at a rate of about 1-2/minute in one 38-minute period of detailed observation during a 5 January summit visit. The explosions hurled incandescent globs of magma to 15-100 m above the crater. Four different lava flows were observed moving down the N slope of MacKenney Crater, before turning W at the break in slope with older lavas. The oldest of the moving flows was aa to blocky lava, which had apparently been active for several days before the visit. This flow was 450 m long, 35 m wide, and 4 m thick, with well-developed levees, and had essentially been separated from its source. Its front was at the base of the steep slopes of MacKenney Crater, where thick and extensive deposits of post-1987 lava created a nearly level flow field. At that point, it was actively collapsing and moved at about 1 m/hour during 5 hours of observation. One incandescent block that spalled off had a temperature of 967°C and a somewhat plastic character when struck by a heavy hammer.
A fast-moving, new flow was observed traveling 15 m/minute in the same channel as the oldest flow, but had not yet advanced to more than half the length of that flow. Lava flows and Strombolian explosions were again visible from Guatemala City in the early morning of 14 January.
The inactive crater and the small crusted-over lava lake, on the N side of MacKenney Crater, were also visited on 5 January. Two smaller lava flows were being emitted from the lava lake, just E of the second, fast-moving flow. These flows had reached lengths of 150 and 300 m, and one was readily accessible. Its temperature was 1086°C and it flowed from its vent at a rate of 6 m/minute. A 2-m-tall, sublimate-encrusted hornito, located near the lava lake, emitted 957°C gases rich in HCl.
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: Stanley N. Williams, Marta Lucía Calvache, Stephen J. Schaefer, Timothy Ross, and other field trip participants, Louisiana State Univ, USA; Otoniel Matías, INSIVUMEH.