One of the first images that come to mind when thinking of Wyoming is the picturesque Devils Tower in northeastern Wyoming. The eroded igneous pillar stands more than 1,200 feet tall and was declared America’s first national monument in 1906. The tower is a climber’s dream with a summit at more than 5,100 feet above sea level. The rocky top of the tower is an oblong 1.5 acres of native grasses, cactus, and sagebrush, with small critters and an occasional snake as residents.
Devils Tower is on the western edge of the Black Hills uplift and is a diatreme composed of 50–46-million-year-old trachyte of Eocene age, an alkalic igneous rock related to phonolite. Similar rocks occur in the central part of the Bear Lodge Mountains, and in Missouri Buttes, just northwest of Devils Tower. These rocks are porphyritic, were emplaced close to the surface, had rapid cooling rates, and exhibit flow banding. Extrusive flows and pyroclastic deposits occur in limited areas in the Bear Lodge Mountains where magma reached the surface. As the magma cooled and crystallized, it contracted, forming hexagonal columns separated by hundreds of vertical cracks.
There are various theories on how Devils Tower might have formed. According to the National Park Service, geologists agree the tower formed by intrusion of igneous material. However, they do not agree on how the process occurred.
Additional information about the theories, as well as the geology of Devils Tower, can be found on the National Park Service website.
National Park Service, [n.d.], Devils Tower National Monument, Wyoming–Frequently asked questions, accessed February 2017, at https://www.nps.gov/deto/faqs.htm.
National Park Service, [n.d.], Devils Tower National Monument, Wyoming–Geologic formations, accessed February 2017, at https://www.nps.gov/deto/learn/nature/geologicformations.htm.
Lageson, D.R., and Spearing, D.R., 1988, Roadside geology of Wyoming (2d ed.): Missoula, Mont., Mountain Press Publishing Company, 253 p.
Sutherland, W.M., 2008, Geologic map of the Devils Tower 30' x 60' quadrangle, Crook County, Wyoming, Butte and Lawrence Counties, South Dakota, and Carter County, Montana: Wyoming State Geological Survey Map Series 81, scale 1:100,000.
Contact: Christina George, christina.george@wyo.gov
