Geologic Age: A calculated K-Ar age of 0.60 +/- 0.10 Ma is reported by Fiebelkorn et al. (1983; sample M-4-16) for obsidian from East McKay Butte.

Prehistoric Use: Glass from McKay Butte is prominent in the lithic assemblages from Deschutes County sites, particularly at sites located in the Paulina Creek area, and was extensively used prior to the eruption of Newberry Caldera obsidian flows during the mid- to early Holocene (Skinner 1995a:4.33-4.34). McKay Butte glass is also occasionally reported to the west of the source in the Western Cascades, particularly in the Upper Middle Fork of the Willamette River drainage (Skinner and Winkler 1991, 1994). Obsidian studies carried out in conjunction with the PGT-PG&E Pipeline Expansion Project point to a dramatic diachronic shift in source use of McKay Butte obsidian. The obsidian hydration rim distribution frequency of McKay Butte artifacts from local sites decreases significantly at values of less than about 4.5 microns, a rim width that corresponds very approximately to the period of the Mazama ashfall (6,850 ¹⁴C yrs. B.P.; Bacon 1983). After this period, glass from the Newberry Volcano chemical type, available only during the post-Mazama period, is dominant in the Newberry Volcano region. A possible explanation for this temporal shift in McKay Butte use may have been due to burial of the source by Mazama ash. Today, the source is poorly-exposed and glass is apparent mainly in areas of recent logging or road building (Skinner 1995a:4.33-4.34, 1995b:5.32).

Source Assignment Notes: Glass from McKay Butte is chemically similar to that of Quartz Mountain, a complex of rhyolite domes and flows located east of McKay Butte on the lower eastern flank of Newberry Volcano. Titanium (Ti) has proven to be a key element in distinguishing among the two sources; some early attempts at source assignments in this region may have confused McKay Butte with Quartz Mountain and vice-versa. When analytical uncertainties are considered, there is some overlap in the range of titanium content between the two sources and occasionally artifacts must be assigned to the combined Quartz Mountain/McKay Butte chemical group. Although visual characterization methods usually prove to be unreliable, the bluish-gray glass of McKay Butte combined with the presence of small spherulites is quite distinct among other regional sources. Cautiously applied, these megascopic attributes may prove useful in regional obsidian procurement studies.

Obsidian Hydration: Hydration analysis of McKay Butte artifacts recovered from pre- and post-Mazama contexts at sites in the vicinity of the source suggests that the Mazama ash horizon lies at approximately 4.4 microns (Skinner 1995b). Due to the microcrystalline nature of the toolstone from this source, a large proportion of attempted hydration rim measurements are typically unsuccessful.