Andean Fine-grained Volcanics


Field Museum curator Ryan Williams and colleagues have conducted an intensive survey of basalt, rhyolite, and andesite sources in the western Titicaca basin to examine the potential for compositionally matching raw materials from these sources to archaeological artifacts, monuments, and prehistoric architecture on the Altiplano, including archaeological collections from middle Horizon archaeological sites at Taraco and Isla Esteves in the north basin and the sites of Tiwanaku, Lukurmata, and Iwawe in the south.  Basalt and other fine-grained volcanic rocks were a key resource in the ancient Titicaca Basin.  Such rocks have been used since the Archaic period in the manufacture of lithic manuports, monolithic sculpture, and architectural construction.  However, volcanic rocks are distributed non-uniformly across the Titicaca Basin, with geological exposures prominent in the west and east basin, but rare or absent in the north and south. 

Sampling in 32 source localities yielded hundreds of individual rock samples.  These samples were chemically characterized at the Museum's Elemental Analysis Facility using portable X-ray Fluorescence analysis.  The PXRF analysis resulted in the identification of 16 distinct regional groupings of chemically distinct volcanic rocks.  A total of 518 archaeological samples were also analyzed by PXRF, yielding a total of fourteen compositionally distinct "signatures," suggesting a wide network of procurement and exchange for artifacts. 

Map of the Titicaca Basin, showing sampling locations (red circles), archaeological sites (black dots), and patterns of procurement identified by compositional analysis (red and yellow lines).

Only three raw material sources were determined to have been economically important in the archaeological samples from the Taraco and Tiwanaku regions analyzed to date, with the rest comprising only a handful of archaeological samples.  The principal archaeological compositional group from the sites of Taraco and Isla Esteves compositionally matches a source group from the Puno Bay region, including the sources at Chucuito and Cutimbo.  The principal basalt group from Tiwanaku and all material from the sites of Iwawe and Lukurmata matches a source group from the Ccapia volcano across the lake from these sites.  A second group of artifacts from the site of Taraco chemically match outcrops near San Antonio de Esquilache, while a handful of artifacts appear to be associated with outcrops near Huacullani.  Ten minor archaeological compositonal groups remain to be associated with source material, which will require further source prospection in coming field seasons.