Geology of The City of Rocks
The rocks forming City of Rocks are predominantly ash-flow tuffs or ignimbrites that formed by a violent volcanic eruption of pumice, volcanic ash, gas, and coarser material. The ash-flow tuff at City of Rocks is part of the Kneeling Nun Tuff, which erupted 34.9 million years ago from the Emory caldera. The ash-flow tuff that forms City of Rocks is of rhyolite composition.
The Kneeling Nun Tuff represents only one of dozens of huge ash-flow tuff eruptions that occurred between 36 and 24 million years ago in southwest New Mexico. Many of these eruptions were larger than any eruptions known from recorded human history; the Kneeling Nun Tuff eruption was more than one thousand times larger than the 1980 eruption of Mt. St. Helens. The source calderas for some of these ash-flow tuffs were south of Lordsburg, N.M.