Obsidian from the north side of Grants Mesa, a youthful volcanic complex just north of Grants, New Mexico, and from La Jara Mesa to the northeast is dated at 3.2 ± 0.3 m.y. Perlite, now being mined in an open pit along the northeast edge of Grants Mesa, corresponds to the obsidian in age. Other rock units in the complex contain too little potassium to give reliable ages but can be related to the glassy phase by crosscutting and superposition. A sample from Mount Taylor, the major volcanic feature in the area, is dated at about 2.6 m.y.

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