Devils Tower, a prominent geological formation, stands in northeastern Wyoming. This striking monolith rises dramatically from the surrounding landscape with its unique form. Designated as the first U.S. National Monument in 1906, it draws visitors curious about its origins and distinctive appearance. Its sheer size and isolation contribute to its visual impact.
The Geological Puzzle
Devils Tower is characterized by its towering height, rising 867 feet from its base and 1,267 feet above the Belle Fourche River. Its sides are sheer, composed of numerous vertical columns. These columns, predominantly hexagonal, can also exhibit four, five, or seven sides. Some of these columns are large, reaching up to 20 feet in width and hundreds of feet in height.
This distinctive columnar structure, combined with the Tower’s isolated position, has long presented a geological question. The precise way such a formation developed has prompted scientific investigation. The contrast between the Tower and the surrounding softer rock layers highlights its unusual formation.
Magma’s Role: The Intrusion Theory
The most widely accepted scientific explanation for Devils Tower’s formation involves an igneous intrusion. This process occurs when magma pushes upward from deep within the Earth’s crust into existing rock layers. For Devils Tower, this magma solidified underground rather than erupting onto the surface.
The rock that forms Devils Tower is classified as phonolite porphyry. This rock is light to dark-gray or greenish-gray and contains white feldspar crystals. The porphyry texture indicates a two-stage cooling process: slow initial cooling allowed larger crystals to form, followed by more rapid cooling at a shallower depth, creating finer-grained material. Geologists agree that the Tower is the remnant of an intrusive body, such as a “stock” or “laccolith,” types of magma chambers that cool beneath the surface.
Sculpting the Monument: Cooling and Erosion
The vertical columns of Devils Tower are a result of a process called columnar jointing. As the magma cooled and solidified deep within the Earth, it contracted. This contraction caused stress within the rock, forming fractures. These cracks propagate perpendicular to the cooling surfaces, resulting in the characteristic polygonal shapes, most commonly hexagons. The slow cooling process, insulated by the surrounding rock, allowed for the formation of the large columns seen today.
After the igneous rock solidified, the Tower remained buried beneath sedimentary rock for millions of years. The exposure of Devils Tower began through a process known as differential erosion. Softer surrounding sedimentary rocks, such as the Spearfish and Sundance formations, were worn away by wind and water over vast spans of time. Because the phonolite porphyry of the Tower is much harder and more resistant to erosion, it remained standing as softer materials were removed.
The Age and Enduring Questions
The magma intrusion that formed Devils Tower occurred 50 to 60 million years ago, during the Tertiary period. This geological event happened when pressures within western North America led to regional uplift. Following its subterranean formation, erosional forces began to expose the Tower between 5 and 10 million years ago. Rain and snow continue to wear away the surrounding sedimentary rocks, gradually revealing more of the Tower’s base, while also slowly eroding the Tower itself.
While scientists agree on the overall model of intrusion and subsequent erosion, some aspects of Devils Tower’s formation remain subjects of discussion. Questions persist regarding whether the magma ever reached the surface or if it remained entirely underground. The precise mechanisms of cooling and the exact initial shape of the intrusive body also remain topics of scientific inquiry.