Devils Tower, a striking geological feature in northeastern Wyoming, is composed of a unique, intrusive igneous rock called phonolite porphyry. Rising hundreds of feet above the surrounding plains, the tower represents a long-standing geological mystery regarding its precise formation process. It has been a focal point for scientific inquiry and a federally protected natural wonder since 1906.
Composition of the Igneous Core
The rock is classified as a phonolite porphyry, an intrusive igneous rock that solidified deep underground. The term “phonolite” refers to a fine-grained, light-colored rock characterized by a high content of alkali minerals, primarily feldspar. This composition contrasts with the more common basalt, making the tower’s material relatively rare.
The descriptor “porphyry” indicates the rock has two distinct sizes of crystals, resulting from a two-stage cooling history. Conspicuous white crystals of feldspar, measuring up to half an inch in diameter, are visible within a much finer-grained, or aphanitic, groundmass. This suggests an initial phase of slow cooling deep within the Earth, allowing large crystals to grow, followed by a second, more rapid cooling phase. The groundmass is typically light to dark-gray or greenish-gray and contains smaller, dark-green crystals of pyroxene, alongside other alkali minerals.
The Underground Intrusion
The material that became the tower originated as magma that intruded into the Earth’s crust approximately 50 to 60 million years ago. This molten rock welled up from beneath the surface but did not erupt as a volcano, instead forcing its way into existing layers of sedimentary rock. Geologists agree the tower is an intrusive body, meaning it cooled and solidified beneath the surface, but the exact shape of that intrusion remains a point of debate.
Leading theories propose the magma formed either a large, mushroom-shaped mass known as a laccolith, or the solidified conduit, or volcanic neck, of an ancient volcano. In either scenario, the magma slowly cooled and crystallized while entirely buried beneath miles of overlying rock layers. The surrounding tectonic activity of the Laramide Orogeny, which raised the nearby Rocky Mountains and Black Hills, provided pathways for the magma to ascend. This slow, insulated cooling process beneath the surface was responsible for the rock’s porphyritic texture and physical properties.
Formation of the Distinctive Columns
The tower’s most visually striking feature is its arrangement of massive, vertical pillars, which formed through a process called columnar jointing. This fracturing occurs when a large, homogenous mass of igneous rock cools and contracts, similar to how mud cracks when it dries. The contraction creates internal stress, causing fractures to propagate inward from the cooling surfaces.
These fractures form perpendicular to the cooling surfaces, which were the sides and top of the underground magma body. The resulting cracks intersect at angles that minimize the strain, leading to the formation of polygonal columns. While many examples of columnar jointing feature perfect hexagons, the columns at Devils Tower are irregularly shaped, often displaying four, five, six, or seven sides. The immense size of these columns, some reaching up to ten feet in width, results from the intrusion’s extremely slow cooling rate, insulated by the dense sedimentary rock above it.
Exposure Through Differential Erosion
The tower is visible today due to the differential erosion of the surrounding landscape. When the phonolite porphyry intrusion solidified, it was encased by layers of softer, less resistant sedimentary rocks, including red sandstone, shale, and siltstone. Over millions of years, the forces of water and wind, particularly the action of the Belle Fourche River and its tributaries, began to wear away the land.
The sedimentary strata eroded at a significantly faster rate than the hard, erosion-resistant igneous core. This gradual removal of the surrounding rock slowly exposed the tower, a process that began perhaps five to ten million years ago. As the sedimentary layers were stripped away, the gray columns of the phonolite porphyry were left standing tall as a lone, isolated landform.