In March 1983, small Strombolian explosions began in MacKenney Crater, 50 m in diameter and 30 m deep. Similar activity continued through June. Activity increased on 17 July when explosions were heard and glow seen from nearby towns. By 31 July, the crater was completely filled with blocks and ash. A pyramidal cone 30 m in diameter and 20 m high, with a small upper crater, was present within the summit crater on 21 August. On 4 September, activity increased again as lava flowed from a hornito on the upper S flank of the newly formed cone. On 11 September, the cone was 25 m high and 50 m in diameter with three active vents trending N and two lava flows emerging from its W flank. The two upper vents produced continuous explosions on 15 September, and lava flowed about 200 m downslope from a hornito on the N flank of the summit crater.

Geological Summary. Eruptions from Pacaya 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 older Pacaya Viejo and Cerro Grande stratovolcanoes and the currently active Mackenney stratovolcano. Collapse of Pacaya Viejo between 600 and 1,500 years ago produced a debris-avalanche deposit that extends 25 km onto the Pacific coastal plain and left an arcuate scarp inside which the modern Pacaya volcano (Mackenney cone) grew. The NW-flank Cerro Chino crater 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 covered the flanks of Mackenney cone, punctuated by occasional larger explosive eruptions that partially destroy the summit.

Information Contacts: A. MacKenney, Guatemala City.