Devils Tower stands as a striking natural landmark, its distinctive columns rising dramatically from the Wyoming landscape. This geological marvel holds significant cultural importance, recognized as America’s first national monument and revered as a sacred site by several Native American tribes. For many Indigenous peoples, including the Lakota, Cheyenne, and Kiowa, the Tower is known by names such as “Bear Lodge” or “Bear’s Tipi,” reflecting its deep spiritual meaning.
The Core Rock: Phonolite Porphyry
Devils Tower is primarily composed of a unique igneous rock called phonolite porphyry. This crystalline rock formed from magma that solidified beneath the Earth’s surface. Fresh samples appear light to dark-gray or greenish-gray, though weathering can lighten the color.
The term “porphyry” describes its texture, characterized by larger, visible crystals, known as phenocrysts, embedded within a much finer-grained matrix. This texture indicates a two-stage cooling history: an initial slow cooling deep underground allowed the larger crystals to grow, followed by a more rapid cooling phase for the surrounding fine-grained material. Phonolite is uncommon and gets its name, “sounding stone,” from the metallic sound it makes when struck. It is also silica-undersaturated, lacking quartz crystals.
The Building Blocks: Minerals of Devils Tower
The phonolite porphyry of Devils Tower is a complex mixture of various minerals, each contributing to its overall characteristics. Prominent among these are the white, conspicuous crystals of alkali feldspar, such as anorthoclase, sanidine, or orthoclase, which form the larger phenocrysts within the rock.
Darker, smaller pyroxene crystals, including aegirine-augite and aegirine, often give the rock its greenish-gray color. The fine-grained matrix also contains feldspathoids like nepheline, leucite, sodalite, haüyne, and analcime. Minor amounts of other minerals, such as sphene (titanite), apatite, and magnetite, are also present.
From Magma to Monument: The Tower’s Formation
Devils Tower originated from magma that intruded into surrounding sedimentary rock layers deep beneath the Earth’s surface. This occurred approximately 40.5 to 65 million years ago. The molten rock did not erupt onto the surface but instead cooled and solidified while still underground.
As the magma cooled, it contracted, leading to the formation of a distinctive pattern of vertical cracks, known as columnar jointing. The contraction created polygonal columns, predominantly hexagonal, but also including four, five, and seven-sided shapes, extending from the base to the top of the Tower. These columns formed perpendicularly to the cooling surfaces of the magma.
Over millions of years, erosion gradually removed the softer sedimentary rocks that once encased the harder igneous core of Devils Tower. Water and wind continuously wore away these surrounding layers, including the Spearfish, Gypsum Springs, and Sundance Formations. This erosion ultimately exposed the towering phonolite porphyry structure we observe today. Evidence of this ongoing process can be seen in the talus, piles of broken columns and rock fragments, accumulating at the base of the Tower.