An Intrusion of Molten Rock
Devils Tower is an igneous intrusion, a mass of molten rock that pushed its way into the existing sedimentary rock layers approximately 40.5 to 50 million years ago. This magma cooled and solidified one to two miles below the Earth's surface, never erupting as a volcano. The specific rock type is a rare phonolite porphyry, a light to dark gray or greenish-gray rock with distinct white feldspar crystals. The porphyritic texture, which consists of large crystals embedded in a fine-grained matrix, indicates a two-stage cooling process: a slow initial cooling deep underground, followed by more rapid cooling as the magma moved closer to the surface.
The vertical columns are the result of a process called columnar jointing. As the molten phonolite cooled, it contracted and shrank, creating stress fractures. These fractures propagated deep into the rock mass as it solidified. To relieve stress equally, the cracks ideally form at 120-degree angles to each other, resulting in near-perfect hexagonal columns. At Devils Tower, the columns also appear with four, five, and seven sides. These are the largest examples of columnar jointing in the world. The columns measure up to 2 meters (6.5 feet) wide and reach heights of 180 meters (600 feet).
Unveiled by Deep Time
For millions of years, the hardened igneous rock of Devils Tower remained buried. The process of its exposure began between 5 and 10 million years ago. The surrounding landscape consisted of softer sedimentary rocks, such as the red sandstone and maroon siltstone of the Spearfish Formation, which were deposited in a shallow sea during the Triassic period.
The relentless forces of erosion, primarily from water carried by the Belle Fourche River, began to strip away these less-resistant sedimentary layers. The phonolite porphyry, being much harder, resisted this erosion. Over millions of years, the surrounding land was carved away, gradually revealing the tower. This process continues today; as the sedimentary rock at the tower's base erodes, more of the formation becomes exposed. The large boulder field at the base of the tower is evidence of the slow erosion of the columns themselves. The tower rises 264 meters (867 feet) from its base to the summit and stands 386 meters (1,267 feet) above the Belle Fourche River.