Devils Tower National Monument in northeastern Wyoming is a dramatic geological feature and a prominent part of the Black Hills uplift. The immense, flat-topped monolith has captivated people for centuries with its striking appearance and distinctive vertical columns. Understanding how this towering formation came to be requires examining the material that forms the tower and the process of its exposure.
The Bedrock: Composition of Devils Tower and Surrounding Geology
The material composing Devils Tower is an intrusive igneous rock classified as phonolite porphyry. This rock is significantly harder and more resistant to weathering than the surrounding rocks. It is a light-to-dark-gray material characterized by white feldspar crystals embedded within a fine-grained matrix.
The bedrock layers surrounding the Tower consist primarily of softer sedimentary rocks laid down in ancient seas. These include the Spearfish Formation (sandstones, siltstones, and shales), the Gypsum Springs Formation, and the Sundance Formation (gypsum and limestones). This contrast in rock type and durability is the key reason behind the Tower’s modern appearance. These surrounding sedimentary rocks were deposited between 225 and 195 million years ago, long before the Tower’s material was introduced.
The Primary Event: Magmatic Intrusion and Cooling
The formation of Devils Tower began approximately 50 to 60 million years ago, during a period of tectonic uplift. Molten rock, or magma, was forced upward from deep within the Earth’s crust through zones of weakness in the overlying sedimentary layers. This magma stalled and pooled underground, forming an intrusive igneous body rather than erupting as a volcano.
The slow cooling of this magma beneath the surface caused the Tower’s most recognizable feature: the vertical columns. As the rock mass cooled uniformly and contracted, immense tensional stress built up. This stress caused the rock to fracture in a predictable pattern, typically forming six-sided or hexagonal cracks known as columnar joints. These fractures extended inward and downward as the body solidified, creating the long, polygonal columns visible today, some of which are hundreds of feet tall.
Sculpting the Monolith: Exposure Through Erosion
Even after the magma solidified into hard phonolite porphyry, the rock remained buried beneath hundreds of feet of sedimentary rock for millions of years. The monolith was sculpted by differential erosion, which began with the uplift of the Black Hills. This regional uplift increased the slope and the cutting power of streams and rivers.
Water and wind gradually stripped away the softer, less resistant sedimentary rock layers surrounding the buried intrusion. The Belle Fourche River and its tributaries actively carried away immense amounts of eroded material. Because the hard phonolite porphyry is far more resistant than the surrounding sandstones and shales, it was left standing as the softer material was systematically removed.
The distinctive columnar joints also facilitated the exposure process. As water penetrated the surrounding sedimentary rock, pieces of the softer material were more easily broken away from the columnar structure. This differential weathering gradually exposed the sides of the ancient intrusion, leaving the 1,267-foot-tall Tower standing above the river valley. Evidence of this ongoing process can be seen in the large talus apron of broken columns and rock fragments accumulated at the base of the formation.
Competing Theories of Formation
Geologists agree on the fundamental two-step process of magmatic intrusion followed by differential erosion, but the exact shape of the igneous body before it was exposed remains debated. Competing hypotheses classify the geometry of the intrusion that cooled underground.
One theory suggests the Tower is the remnant of a laccolith, a mushroom-shaped body of magma that pushed up and deformed the overlying sedimentary layers into a dome. Another idea proposes that the Tower is a volcanic neck or plug, the solidified conduit of an ancient volcano. However, the lack of nearby volcanic ash or lava flows casts doubt on the volcanic neck hypothesis.
A simpler interpretation suggests the Tower is a stock, a small, relatively uniform intrusive body that cooled underground. The erosion that exposed the Tower has also, ironically, removed much of the evidence needed to definitively prove which theory is correct. The debate focuses on the initial dimensions and connection of the underground magma chamber.