|Craig working on some XRF analysis of basalt in the PGM lab.|
The Hohokam preferred to manufacture groundstone tools from vesicular basalt. This stone is a volcanic rock that has cavities throughout and in many cases large-grain mineral inclusions. This irregular texture provides an ideal surface for shearing large-grained seeds, such as corn. Additionally, the stone maintains an effective grinding surface after repeated use, whereas other material types, such as granite, must be regularly roughened to maintain grinding efficiency. These favorable performance characteristics were well-known to the Hohokam, as evidence by the abundance of vesicular basalt groundstone tools at their settlements.
|Mano & Metate grinding exhibit at PGM|
My research seeks to determine the method or methods by which Hohokam villagers acquired vesicular basalt for groundstone tool production. To accomplish this goal, I first collected hundreds of rock samples from nearly two dozen vesicular basalt outcrops in the Hohokam region. These source materials were then analyzed using Energy-Dispersive X-ray Fluorescence Spectroscopy (EDXRF), a nondestructive analytical technique that characterizes the chemical composition of the sample. The analysis found that the majority of vesicular basalt outcrops used by the Hohokam are chemically distinct. This observation is important because it allows us to analyze vesicular basalt groundstone artifacts from archaeological contexts and match them to the source from which they were derived.
|Example of an XRF analysis graph|
Posted by Craig Fertelmes, Gila River Indian Community - Cultural Resource Management Program Archaeologist & Arizona State University Doctoral Candidate in Archaeology/Anthropology