Sidrys et al. (1976) stated that an obsidian source area includes several outcroppings of obsidian, limited in spatial extent, which may or may not have common chemical features and may or may not have been used by ancient humans. Michael D. Glascock, of the University of Missouri Research Reactor Center at the University of Missouri (which performs neutron activation analysis), has divided Mesoamerica into nine sub-regions with one or more obsidian sources in each. These subdivisions, while effective at systemizing the source characteristics and allowing for a more easily visualized distribution of sources, are still tentative. They are as follows: