Pii: S0377-0273(00)00199-2

The Pedregal lavas are fresh, well-exposed basaltic ̄ows erupted from the Xitle scoria-and-cinder cone in the southwestern part of the Basin of Mexico. These lavas cover an area of 70 km and were emplaced over pyramids and other buildings (e.g. Cuicuilco and Copilco archaeological sites). Today, a part of Mexico-City (including the National University) is built on the ̄ows. Initial strombolian activity produced an ash fallout layer, which was immediately followed by effusive emplacement of lava ̄ows. The Xitle cone grew on the north-facing slope of Ajusco volcano, and lava ̄owed down to the N±NE until it reached the basin ̄oor. More than 30 radiocarbon dates have been obtained by several workers on charcoal samples from beneath the lava, and several ages for the eruption have been proposed from these dates. Most dated samples were not directly produced by Xitle's eruption but instead are artifacts of human activity that predates the eruption. Thus, these ages (mostly about 2000 bp) are older than the eruption. A new age of 1670 ^ 35 years bp (AD 245±315) obtained on charcoal samples collected just beneath the lavas is favored for the Xitle eruption. These samples originated by ignition of vegetation during the emplacement of hot scoriaceous tephra. The new age is within the Classic period of Mesoamerican archaeology, whereas the earlier reported ages are at the end of the Preclassic. The new age carries important implications for the timing of population shifts within the Basin of Mexico. q 2000 Elsevier Science B.V. All rights reserved.