Impactos de la actividad volcánica en época prehispánica: Estudios edáficos en los sitios arqueológicos de Cerro del Teúl y El Pitayo, Tres Mezquites en el occidente de México

Abstract

Volcanic activity in ancient Mesoamerica was decisive for many populations since it influenced daily life and the way of understanding the world. Eruptive events can provoke the total or partial abandonment of the sites, changes in the social organization, and, inclusively, changes in the pedogenesis with alteration of the natural resources. The impact of volcanic activity can be detected easily. However, it can also be “hidden” among the soil and archaeological materials; consequently, its identification and analysis are essential to understanding archaeological stratigraphy. To document the above, this work presents the results obtained at two sites in western Mexico: Cerro del Teúl in Zacatecas (ca. 100-1531 d.C.) and El Pitayo in Michoacán (200- 600 d.C.). In the case of Cerro de Teúl, the construction fills were analyzed in two stratigraphic profiles: the Patio Hundido (PH21) and the Main Plaza (P2M80). In El Pitayo, construction fills and soil horizons were analyzed in two profiles located in a trench with an E-W orientation (East Trench and South Trench). The main objective was to identify the impact of volcanic activity on the archaeological stratigraphy and the transformations of volcanic materials as a consequence of natural and anthropogenic processes, using colorimetric, granulometric, and micromorphological analyses. In addition, the chemical composition (by X-ray fluorescence) and the petrography of the volcanic ash found in the sites were determined and compared with that from surrounding volcanoes. According to the results, the ash observed in Cerro Teúl originated from the Jala eruption of the Ceboruco volcano, dated 990-1020 AD. This eruption did not produce the site abandonment, as the site was rapidly re-occupied, and the ash was left between the construction fills. In the case of El Pitayo, according to archaeological evidence (the ages of the abandoned artifacts and the absence of volcanic glass on the lower floors of the construction), it is probable that the landslide was emitted during the first half of the 6th century and the site contin- ued developing after the event. The geochemical and mineralogical composition of the volcanic materials found in the excavations has an intermediate-acid affinity, which contrasts with the monogenetic volcanism in the area; however, as the material is reworked, it is probable that the original mineralogical composition has been modified by transport and erosional processes, or diluted by the mixture with more acid sources, as evidenced by the domain of volcanic glass with heterogeneous textures and morphologies. Until now, there is a possible source of emission of the volcanic material associated with that found in the “black earth” at the site of El Pitayo, which is the El infiernillo volcanic event, located south of Michoacán and which has a date of 1500- 1370 BC (Mahgoub et al., 2017). However, the “black earth” that covered the site is after this dating, so chronologically it would not correspond to an eruption that occurred during the occupation of the site, but it allows us to associate it as a product of a landslide from Cerro El Arco, since that the geochemical characterization of the volcanic glass shows a very close affinity to this El Infiernillo event.