During the past 20 years, many different archaeological studies have been carried out in the vicinity of the Coso sources (see Skinner and Tremaine 1993). Recorded archaeological sites within the Coso Volcanic Field include approximately 150 quarry, 300 off-quarry, and 100 segregated reduction sites (Gilreath and Hildebrandt 1997:1). The results of archaeological investigations of Coso sources and prehistoric quarry sites are summarized and discussed by Gilreath and Hildebrandt (1996, 1997),Eerkens and Rosenthal (2004), and Ericson and Glascocok (2004). Trace element studies of artifacts demonstrate extensive prehistoric use throughout southern California of obsidian from this source area.

In early obsidian characterization investigations, the obsidian from this volcanic complex was considered to belong to a single chemical source. Later investigations by Hughes (1988) resulted in the identification of four geochemically distinguishable sources (West Sugarloaf, Sugarloaf Mountain, West Cactus Peak, and Joshua Ridge) within the volcanic field. Among these four subsources, deposits from the West Sugarloaf and Sugarloaf Mounain sources were the most heavily utilized during the prehistoric period. Most recently, neutron activation studies of Coso obsidian suggest that two additional subsources (Site 6 and Site 2A) may be identified (Ericson and Glascock 2004). Thanks to recent interest in both the archaeological and geothermal potential of this area, the geologic setting, age, and chemical composition of obsidian flows from the Coso Volcanic Field are well documented (Bacon et al. 1980, 1981; Duffield et al. 1980; Ericson 1981:140-143; Hughes 1988; Lanphere et al. 1975).

Obsidian Hydration: Although Hughes (1988) cautions that each Coso subgroup may hydrate at a different rate, Gilreath and Hildebrandt (1997:11,13) present archaeological data that suggest that the differences in rates among sources are probably not significant. The reader is referred to Ericson (1981, 1989), Stevenson and Scheetz (1989), Stevenson et al. (1993), Gilreath and Hildebrandt (1996, 1997), King (2004), and Gold (2005) for further discussion of the numerous hydration rates proposed for sources at the Coso Volcanic Field. Most recently, however, induced hydration experiments by Fredrickson et al. (2006) suggest that there are significant differences in hydration rates between at least two of the Coso source localities.

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