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Journal of Volcanology and Geothermal Research 104 (2000) 45±64

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Age and archaeological implications of Xitle volcano, southwestern Basin of Mexico-City
C. Siebe*
    Instituto de Geofõsica, Universidad Nacional Autonoma de Mexico, Coyoacan, C.P. 04510, Mexico, D.F., Mexico Received 23 November 1999; revised 6 April 2000; accepted 11 April 2000Abstract 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 2 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. Initialstrombolian 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 beenproposed 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 originatedby 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.
Keywords: Xitle volcano;Basin of Mexico; scoria-and-cinder; Cuicuilco

1. Introduction Xitle Volcano, located at the southwestern limits of the Basin of Mexico produced the ªEl Pedregalº lava ¯ows, which engulfed and covered several Prehispanic settlements, including famous Cuicuilco pyramid (Figs. 1±4). Prior to the eruption, Cuicuilco was situated on a deltaic plain of a stream draining the slopes
* Correspondingauthor. Tel.: 152-5-622-4119/4146; fax: 1525-550-2486. E-mail address: csiebe@tonatiuh.igeofcu.unam.mx (C. Siebe).

of Ajusco stratovolcano. The eruption forced many people to abandon their villages and represents a documentable example of a volcanic disaster in this region. The youthful appearance of Xitle volcanic cone in conjunction with the discovery of archaeological material underneath Xitlelava ¯ows prompted many attempts to determine the numerical age of eruption. Due to the lack of written accounts describing Xitle's eruption, the age of the volcano can only be determined using the radiocarbon dating method. After the initial date of 2422 ^ 250 years bp (C-200) reported by Arnold and Libby (1951), many

0377-0273/00/$ - see front matter q 2000 Elsevier Science B.V. All rightsreserved. PII: S 0377-027 3(00)00199-2

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C. Siebe / Journal of Volcanology and Geothermal Research 104 (2000) 45±64

additional ages have been obtained from organic material collected several cm below the lava (Table 1). However, these ages vary by more than 1000 years and thus do not pinpoint the age of the eruption. 2. Geologic setting The Trans-Mexican Volcanic Belt (TMVB) is anE±W-trending zone located between 19 and 208N latitude that extends ca. 1000 km from the Paci®c to the Gulf of Mexico (Fig. 1). Its origin is related to the subduction of the Cocos Plate beneath the North American Plate. Xitle monogenetic basaltic scoriaand-cinder cone is located within the Sierra del Chichinautzin Volcanic Field (SCVF) in the central part of the TMVB. The SCVF is a volcanic highland...