The postglacial Papayo lava dome, constructed along the crest of the Sierra Nevada range, is dwarfed by its massive neighbor Iztaccíhuatl, 15 km SW. Papayo has been the source of voluminous dacitic lava flows that have traveled about 10 km from the vent both to the WSW and ENE. The steep-sided Cerro Papayo dacitic lava dome, rising 230 m above the surrounding lava flows, marks the vent. The 84 km2 compound lava field has a volume of about 21 km3 and includes flows that traveled long distances in opposite directions into the Valley of Mexico and toward the Puebla basin. Lavas overlie glacial moraines about 12,000 years old (Nixon, 1989). On the western side the flows surround the glaciated late-Pleistocene lava and pyroclastic cone of Iztaltetlac.