Between 5 and 13 February graduate students from Northern Illinois University in collaboration with INSIVUMEH scientists conducted studies at Pacaya volcano. Focused fumarolic activity was observed at the summit, whereas diffuse gas emissions occurred around the SW flanks. Numerous micro-seismic earthquakes were recorded daily during this period.

Lava samples were collected from the 11 November 1996 flow. Analysis showed that the lava was a highly- vesicular, plagioclase-phyric basalt that resembles basaltic flows from previous eruptions.

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: Otoniel Matías, Seccion Vulcanología, INSIVUMEH (Instituto Nacional de Sismología, Vulcanología, Meteorología e Hydrología of the Ministerío de Communicacíones, Transporte y Obras Publicas), 7A Avenida 14-57, Zona 13, Guatemala City, Guatemala; Barry Cameron and Shane Rundle, Department of Geology, Northern Illinois University, USA.