Devils Tower, a soaring rock formation in the Black Hills of Wyoming, is one of America’s most recognizable natural landmarks. Observing its steep sides and flat summit, many people wonder if this massive feature is a butte, a mesa, or something else entirely. While its outward appearance resembles these common Western landforms, its deep geological history places it in a different scientific category. Understanding its true nature requires examining its composition and unique origin.
Understanding Mesa and Butte Classifications
Buttes and mesas are isolated hills defined by their shape and rock type. Both feature a distinctive caprock layer, typically hard, flat-lying sedimentary rock like limestone or sandstone, which protects softer rock underneath from erosion. The primary difference between them is the ratio of height to width. A mesa, derived from the Spanish word for “table,” is wider than it is tall, possessing a broad summit. As erosion shrinks the summit area, it eventually transforms into a butte, which is defined as being taller than it is wide.
The Technical Classification of Devils Tower
Devils Tower is technically not a butte, a mesa, or a sedimentary monolith because of its rock composition and subsurface origin. The Tower is composed of a crystalline igneous rock called phonolite porphyry, which formed from magma that cooled underground, not from layers of sediment deposited by water or wind. Geologists classify the Tower as an igneous intrusion, which may be more specifically termed an erosional remnant of a stock or a volcanic neck. A stock is a small, intrusive body of igneous rock that cooled beneath the surface, while a volcanic neck represents the hardened magma conduit of an ancient volcano. Its current form is the result of the surrounding, softer sedimentary rocks eroding away over millions of years, leaving the much harder igneous core exposed.
The Unique Geological Origin
The process that created Devils Tower began approximately 40.5 to 50 million years ago when magma was forcefully injected into the overlying layers of sedimentary rock. This molten material, which was rich in alkaline minerals, never reached the surface but instead solidified deep beneath the Earth. One prevailing theory suggests the magma formed a laccolith, a mushroom-shaped intrusion that pushed the overlying sedimentary layers upward. As the magma cooled over a long period, it contracted and fractured in a highly regular pattern, a phenomenon known as columnar jointing. This contraction created the immense, vertical, polygonal columns that characterize the Tower, with most exhibiting five or six sides.
Exposure Through Erosion
The exposure of the Tower is a relatively recent event in geological time, occurring within the last 5 to 10 million years. Differential erosion is the mechanism responsible for its current appearance, as water and wind gradually wore down the surrounding, less-resistant sedimentary rocks of sandstone and shale. As erosion continues today, the softer rocks at the base are removed, occasionally causing sections of the vertical columns to break free and accumulate in the massive talus slope surrounding the base of the formation.