Devils Tower, a striking monolith rising from the plains of Wyoming, is one of North America’s most recognizable geological features. Its unique vertical columns often lead people to assume it is made of basalt. The answer is straightforward: it is not basalt, but rather a different type of igneous rock formed deep beneath the Earth’s surface. This landmark, known to the Lakota as Mateo Tepee or Bear Lodge, showcases a rare material and a spectacular cooling process. The confusion is understandable because the geometric columns resemble those found in basalt formations, but the rock’s chemical makeup is distinctly different.
The Actual Composition of Devils Tower
The massive rock that forms Devils Tower is classified by geologists as phonolite porphyry. This is an intrusive igneous rock, meaning the molten material solidified before reaching the Earth’s surface. The “phonolite” designation refers to the rock’s chemical composition, which is rich in alkali minerals, specifically potassium and sodium feldspars, and feldspathoids like nepheline.
This composition separates it from basalt, which is rich in magnesium and iron (mafic). Basalt is poor in silica, while phonolite is a silica-undersaturated rock. Although both rock types can exhibit the column structure, their distinct elemental makeup means they form under different conditions.
The second part of the classification, “porphyry,” describes the rock’s texture, which is a mixture of crystal sizes. The rock is characterized by large, visible crystals, called phenocrysts, embedded within a very fine-grained matrix. These phenocrysts, often white feldspar crystals, formed during an initial period of slow cooling deep underground. The surrounding groundmass then solidified more quickly, locking the larger crystals in place and creating the porphyritic texture.
Understanding Columnar Jointing
The Tower’s signature appearance comes from columnar jointing, which creates the impression of colossal, vertical planks fitted together. This physical reaction occurs in many types of igneous rock when a large body of magma cools and contracts. As the rock cools from high temperatures, it shrinks in volume, generating immense tensional stress.
To relieve this stress, fractures develop perpendicular to the cooling surface. These stress points ideally radiate three cracks at 120-degree angles, a geometry that most efficiently releases the tension. When these fractures propagate and intersect, they naturally form a pattern of polygonal columns.
While the hexagonal column is the most common shape, the columns at Devils Tower exhibit variations, including four, five, and seven sides. The enormous size of the columns, some reaching ten feet in width, suggests a very slow and uniform cooling process.
The Geological Formation Process
The story of Devils Tower began approximately 50 to 60 million years ago during a time of significant tectonic activity. A body of molten phonolite magma welled up from deep within the Earth’s crust and was forcefully injected into overlying layers of sedimentary rock. Geologists agree that the magma solidified well beneath the surface, though the exact shape of the intrusion remains debated.
It may have formed as a laccolith, where the magma spread out horizontally, or as a volcanic neck, the solidified conduit of a defunct volcano. Regardless of the initial shape, the magma cooled slowly underground, forming the hard, crystalline phonolite porphyry.
The tower was gradually exposed by a process called differential erosion, not thrust up from the ground. The surrounding sedimentary rocks, such as red sandstone and shale, were relatively soft and easily eroded by wind and water over millions of years. The igneous phonolite rock, being much harder and more resistant to weathering, remained in place.
Beginning between five and ten million years ago, this erosion stripped away the softer material, leaving the resistant, column-jointed core standing hundreds of feet above the surrounding terrain. Continuous weathering is still visible today in the massive apron of talus, or fallen rock fragments, that collects at the base of the Tower.