City Of Rocks State Park in New Mexico is a captivating natural landscape characterized by towering rock columns, or pinnacles, reaching heights of up to 40 feet. These formations, sculpted by millions of years of erosion, resemble city streets, giving the park its distinctive name. This article delves into the geological history, formation, and unique features of this fascinating natural wonder.
The Volcanic Origins of City of Rocks
The story of City of Rocks begins approximately 34.9 million years ago with a massive volcanic eruption from the Emory caldera, located in the southern Black Range. This eruption produced the Kneeling Nun Tuff, an ash-flow tuff composed of pumice, volcanic ash, gas, and coarser materials. The intense heat of the eruption caused the ash and pumice to compact and weld together, forming a solid rock layer.
Figure 1: View of Table Mountain, northeast of City of Rocks State Park, showcasing the surrounding tuff formations.
As the tuff cooled and contracted, vertical cracks known as columnar jointing developed. These joints, visible in the cliffs of Table Mountain (Figure 1), played a crucial role in shaping the rock formations at City of Rocks. Subsequent eruptions deposited additional ash-flow tuffs in the area, contributing to the complex geological layers observed today.
Erosion: Sculpting the Cityscape
While volcanic activity laid the foundation for City of Rocks, erosion was the artist that sculpted its unique landscape. Over millions of years, weathering processes, including freeze-thaw cycles, wind, and water, gradually widened the vertical cracks in the tuff.
Figure 2: White arrows indicate fractures that have guided the erosion of the tuff, shaping the distinct rock formations. Note the person for scale.
Water seeping into cracks expanded upon freezing, further widening them. Vegetation growing in the fissures contributed to the erosion process. Wind removed finer particles, and both surface and subsurface weathering shaped the rocks into the dramatic pinnacles and “streets” we see today. The flared bases of many rocks indicate periods of subsurface weathering.
The Composition of the Rocks
The City of Rocks tuff is primarily rhyolite, a volcanic rock. Close examination reveals phenocrysts, or mineral grains, embedded in a finer groundmass. These crystals include hornblende, biotite, quartz, and feldspar (Figures 3 and 4). The matrix, while too fine to see with the naked eye, consists of pumice, feldspar, quartz, and hornblende. Potassium-rich sanidine crystals within the tuff have allowed for precise radiometric dating, confirming the age of the formations.
Figure 3: Close-up of the Kneeling Nun Tuff, showing pumice (pink) and lithic fragments (brown) within the crystal-rich gray matrix.
Figure 4: A fracture within one of the “streets” at City of Rocks State Park, highlighting the ongoing erosion process.
Exploring City of Rocks Today
City of Rocks State Park offers visitors a unique opportunity to explore this geological wonder. Hiking trails wind through the rock formations, providing breathtaking views and opportunities for rock climbing and photography. The park is also home to diverse plant and animal life, reflecting its location in the Chihuahuan Desert. From its volcanic origins to its sculpted present, City of Rocks stands as a testament to the power of geological processes over vast spans of time.
References
-
Elston, W.E. (1957). Geology and mineral resources of the Dwyer quadrangle, Grant, Luna, and Sierra Counties, New Mexico. New Mexico Bureau of Mines and Mineral Resources, Bulletin 37.
-
McIntosh, W.C., Kedzie, L.L., & Sutter, J.F. (1991). Paleomagnetism and 40Ar/39Ar ages of ignimbrites, Mogollon-Datil volcanic field, southwestern New Mexico. New Mexico Bureau of Mines and Mineral Resources, Bulletin 135.
-
Mueller, J.E., & Twidale, C.R. (1988a). Geomorphic development of City of Rocks, Grant County, New Mexico. New Mexico Geology, 10, 73-79.
-
Mueller, J.E., & Twidale, C.R. (1988b). Landform development of City of Rocks State Park and Giant of the Mimbres. In G.H. Mack, T.F. Lawton, & S.G. Lucas (Eds.), Cretaceous and Laramide tectonic evolution of Southwestern New Mexico (New Mexico Geological Society, Guidebook 39, pp. 185-190).
-
Seager, W.R., Clemons, R.E., Hawley, J.W., & Kelley, R.E. (1982). Geology of the northwest part of Las Cruces 1° x 2° sheet, New Mexico. New Mexico Bureau of Mines and Mineral Resources, Geologic Map GM-53.
