Amatitlan, An actively resurging cauldron 10 km south of Guatemala City

A 14×16 km diameter collapse caldera has been recognized 10 km south of Guatemala City, Guatemala. The caldera is north of the presently active volcano Pacaya and west of Agua, a large stratovolcano. The caldera was not previously recognized because its eastern and western margins coincide with faults that outline the Guatemala City graben and because the northern margin of the caldera is buried by pyroclastic rocks. The existence of the northern caldera margin is now established by gravity data and a variety of geological observations including circumferential faults, hot springs, well-log data, and lithological changes in sedimentary rocks. A sequence of nine silicic pyroclastic deposits, totaling a volume of more than 70 km 3 dense rock were erupted from the caldera. The ages of these eruptions are mainly between about 300,000 years B.P. to less than 23,000 years B.P. The rocks erupted at the caldera and its associated vents consist of domes and nonwelded pyroclastic flow, surge, and fall deposits, mainly of rhyolitic to dacitic composition. Successive pyroclastic eruptions have generally become smaller in volume and more silicic with time. Major and minor element chemistry distinguish Amatitlan pyroclastics from those of other nearby calderas. The caldera lies at the intersection of an offset of the volcanic chain (the Palin Shear) and the faults along the volcanic front (Jalpatagua fault zone). The caldera has a heavily faulted resurgent dome crosscut by an impressive longitudinal graben. The graben\u27s alignment with the Jalpatagua fault zone suggests a genetic relationship. The longitudinal graben and resurgent dome are morphologically youthful and are the sites of many young silicic vents. Available seismic data show a heavy concentration of epicenters over the northern part of the resurgent dome, near a young silicic intrusion. The caldera is active and will probably erupt again. Over 1 million people live within 20 km and would be threatened in the event of a moderate eruption. Suggestions for future research focus on hazard assessment and forecastin