Lava flows from the saddle vent that began erupting on 5 August 1984 remained active until 6 February 1985 and built a cone approximately 50 m high.

On 1 January, minor pyroclastic activity, which could only be seen from the crater rim, resumed at MacKenney Crater. By 10 February, a steep-sided lava cone about 6 m high had developed within the crater. Pyroclastic material was ejected from the top of the small cone, a lava flow emerged from its E flank, and a lava spine had grown on its W flank. On 17 March, the cone was approximately 20 m high and another explosion crater had formed on its E flank. By mid-March, the activity had begun to be visible from Guatemala City (about 40 km E) on clear days.

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.