Pacaya displayed weak Strombolian activity during a visit by Michigan Tech. Univ. geologists on 14 February, the first Strombolian activity observed at Pacaya since 1975. Gas emissions have characterized the activity since late 1977.

Lava was fountaining to 200 m at 10-second to 1-minute intervals from two coalesced spatter vents in the center of MacKenney Crater, high on the WNW flank. Four subsidiary vents, two N of the spatter vents and two W of them (in the direction of the volcano summit), also ejected lava. New pahoehoe lava flows, some of which were moving, had filled the N half of the crater floor to the rim. The fountaining was interspersed with intense, pulsating gas emission from the spatter vents.

By 20 February, when Robert Hodder climbed Pacaya, one lava flow had traveled a quarter of the way (about 200 m) down the N flank of MacKenney Crater cone, over one of the September 1969 flows. Within the crater, cracks and pressure ridges in the lava crust indicated continued lava movement. Strombolian activity was occurring at about 30-minute intervals. Patches of sublimate were visible on the SE crater wall.

During a second climb on 28 February Hodder observed that aa lava had flowed about 750 m from the crater rim to the base of MacKenney Crater cone, into the trough between it and the rim of the older Pacaya edifice. The level of lava in the crater had risen. The 2 vents observed on 14 February had totally coalesced and had built cones about 15 m high. The lava crust seemed solid, but incandescence showed through surface cracks at night. Strombolian activity occurred about every 20 minutes. Large cow-dung bombs, hurled as high as 100 m, fell onto the cones and the lava crust. Bomb ejection was sometimes preceded by a puffy steam cloud at least 300 m high. Sublimate solidly coated the SE crater wall. Hodder noted that this eruption seemed similar to that of 1969.

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: W. Rose, Jr., T. Bornhorst, and C. Chesner, Michigan Tech. Univ.; R. Hodder, Univ. of Western Ontario.