Devils Tower, a soaring monolith in northeastern Wyoming, is one of the most recognizable geological features in the United States. Rising dramatically from the surrounding plains, its striking appearance, characterized by immense vertical columns, prompts a frequent question: is this remarkable national landmark the remnant of an ancient volcano? Understanding the true origin of this structure requires looking beneath the surface to uncover the history of its formation.
Addressing the Volcano Misconception
Devils Tower is definitively not a volcano. It was never a mountain that erupted lava, ash, and gases onto the surface. There is no evidence of the volcanic debris, such as ash beds or lava flows, that would be expected if a massive eruption had occurred in the surrounding area. The common misconception arises because the Tower is composed of igneous rock, which is formed from cooled magma. Its isolated, imposing shape further contributes to the idea of a volcanic mountain whose softer outer cone was removed. This unique structure is a product of molten rock, but the process happened entirely underground.
Defining Devils Tower’s True Geological Origin
The scientific classification for Devils Tower is an igneous intrusion, a mass of magma that solidified beneath the Earth’s surface. Geologists primarily debate whether the intrusion was a laccolith or a volcanic neck. A laccolith is a mushroom-shaped body of magma that forces its way between sedimentary rock layers, causing overlying layers to bulge upward. The alternative model suggests the Tower is the remnant of a volcanic neck, the solidified magma that plugged the main vent of an ancient, extinct volcano whose cone has vanished.
The rock itself is called phonolite porphyry. This is a fine-grained igneous rock, typically light to dark-gray or greenish-gray. The “porphyry” part indicates the rock contains conspicuous, larger crystals of white feldspar embedded within the finer material. This specific composition gives the Tower its resistance to weathering and erosion.
The Process of Intrusion and Exposure
The formation of Devils Tower began deep beneath the surface approximately 50 to 60 million years ago. Molten magma welled up, intruding into existing layers of sedimentary rock. The magma cooled and crystallized slowly, insulated by the considerable weight and thickness of the surrounding soft rock layers. This slow, underground cooling process is responsible for the unique crystalline structure and the massive size of the columns that define the Tower.
The exposure of this igneous core is a testament to differential erosion. For millions of years, water, wind, and ice relentlessly wore away the softer sedimentary rocks around the intrusion. These sedimentary layers, composed of sandstone, shale, and gypsum, were carried away by the Belle Fourche River and its tributaries. The phonolite porphyry of the Tower is significantly harder and more resistant to erosion than the surrounding sedimentary rocks. Beginning perhaps 5 to 10 million years ago, the harder igneous rock was gradually revealed as the landscape was stripped away.
The Mystery of the Columns
The Tower’s most visually striking feature is the pattern of vertical columns, a phenomenon known as columnar jointing. These columns were not carved by erosion but formed naturally as the magma cooled and solidified. As the molten rock cooled, it began to contract, creating immense shrinkage stress throughout the mass. To relieve this stress, a pattern of cracks, or joints, formed and propagated through the cooling rock.
The most efficient way for a uniformly contracting material to relieve stress is by forming a network of cracks that intersect at 120-degree angles, naturally creating six-sided or hexagonal columns. While the columns are predominantly hexagonal, the Tower also features columns with four, five, and seven sides. These columns are enormous, often measuring up to 10 to 20 feet wide and soaring hundreds of feet from the base to the summit, making the Tower the world’s largest example of this geological structure.