Report on Pacaya (Guatemala) — March 1981
Scientific Event Alert Network Bulletin, vol. 6, no. 3 (March 1981)
Managing Editor: Lindsay McClelland.
Pacaya (Guatemala) Lava flows and Strombolian activity
Please cite this report as:
Global Volcanism Program, 1981. Report on Pacaya (Guatemala). In: McClelland, L. (ed.), Scientific Event Alert Network Bulletin, 6:3. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198103-342110.
14.382°N, 90.601°W; summit elev. 2569 m
All times are local (unless otherwise noted)
Newspapers reported that vigorous magmatic activity had begun at Pacaya on 9 February. According to newspapers, activity peaked 18 February at 1730 as lava overflowed the N rim of the crater and began to move down Pacaya's N flank.
Michigan Tech. Univ. geologists climbed Pacaya again on 5 March. Since their last visit 14 February, the level of the lava lake in MacKenney Crater had risen considerably, the two coalesced spatter vents had grown, and an additional small spatter vent had formed in the S part of the crater on the lava lake surface. The new S vent continuously extruded two pahoehoe lava flows but was not the source of any Strombolian explosions. A few small pahoehoe flows were also moving across the E side of the crusted lava lake surface. Nearly continuous weak Strombolian activity occurred from the two older vents. The smaller N vent had many small Strombolian explosions at intervals of 10-20 seconds. From the larger vent, activity was cyclical, consisting of a 1-5-second explosion that ejected spatter to 200-300 m above the vent, followed almost instantly by a large increase in gas emission that peaked in about 1 minute, decreased slowly, then dropped sharply about 30 seconds before the next explosion. Gases above the vent had an intense blue color. The alignment of the three vents in MacKenney Crater indicated that the activity may have been from a fissure trending approximately N-S.
The geologists estimated that the lava flow descending the N flank had a volume of about 2 x 104 m3 on 5 March. They estimated the total volume of 1981 lava at about 1 x 106 m3, for an average eruption rate of about 4 x 104 m3/day. The lava was petrologically similar to lavas from eruptions since 1970, consisting of basalt with abundant plagioclase phenocrysts and sparse olivine phenocrysts.
Geologic Background. 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: T. Bornhorst and C. Chesner, Michigan Tech. Univ.