Description and method of exploitation of the Alca obsidian source, Peru

Despite three decades of obsidian studies in the Andes, the extraction zones for this raw material remain poorly understood. The extent and geology of the sources, their intra-source chemical compositional variation. and the means by which the sources were exploited prehistorically, have largely escaped study. This report describes the results of a geoarchaeological survey of the Alca obsidian source in the Cotahuasi Valley, Peru. The survey documented 16 discrete outcrops of obsidian across an area of 50 km2. The obsidian in these outcrops can be found both as volcaniclastic flows and nodules in poorly consolidated volcanic tuff. Neutron activation analyses on samples from these outcrops suggest that all obsidian came from the same parent magma. Five small quarries were found during the course of the survey. No evidence of control over the resource zone was found and we suggest that tight control over obsidian extraction in the valley would have been logistically difficult due to the size and nature of the deposits.