Devils Tower, a striking geological feature in northeastern Wyoming, is a massive, fluted butte rising hundreds of feet above the surrounding terrain. Designated as the first U.S. National Monument in 1906, its sheer, vertical sides and distinctive column pattern have long captivated observers. While the exact mechanics of its formation are still debated by geologists, the overall story involves a deep subterranean event followed by millions of years of erosion.
The Magma Rises Underground
The geological story of Devils Tower begins deep beneath the surface of the Earth approximately 50 to 60 million years ago. Intense tectonic pressures were at work in western North America, contributing to the uplift of the Rocky Mountains and the nearby Black Hills. Magma, or molten rock, began to well up from the deep crust, forcing its way into existing layers of sedimentary rock.
This event was a subterranean intrusion, not a surface eruption, where the magma pushed into the overlying rock strata. The igneous material solidified underground, likely forming a stock or a laccolith—a mass of rock that bulges the sedimentary layers above it. The tower rock is a light to dark-gray igneous material called phonolite porphyry, which contains large crystals of white feldspar. Radiometric dating established its age at about 40.5 million years, marking when the molten rock cooled and crystallized while encased by much softer sedimentary rocks.
How the Distinctive Columns Formed
The tower’s distinctive, columnar appearance is the result of a physical process called columnar jointing, which occurred as the immense body of magma cooled. Because the molten rock was insulated by surrounding sedimentary material, this cooling process happened extremely slowly. As the hot magma solidified into igneous rock, it underwent significant volumetric contraction, or shrinking.
This contraction created immense tensional stress within the brittle rock mass. To relieve this stress, the rock fractured, with cracks radiating outward from central points. When these fractures met, the resulting pattern created polygonal columns, which are most often six-sided but can also form with four, five, or seven sides.
The sheer size of the columns, which can reach up to 20 feet in width and hundreds of feet in height, is a direct consequence of the slow, uniform cooling environment underground. Slower cooling allows tensional forces to work over a larger area, producing wider, more regular columns than those found in rapidly cooled surface lava flows. These joints formed perpendicular to the primary cooling surfaces, resulting in the mostly vertical orientation observed today.
The Final Stage: Exposure by Erosion
The final act in the formation of Devils Tower was a long-term process of differential erosion that took millions of years to complete. The dense, hard phonolite porphyry core proved far more resistant to weathering than the surrounding sedimentary rocks. The uplift of the Black Hills region increased the gradient of local streams and rivers, significantly accelerating the rate of erosion.
Water, primarily from the Belle Fourche River and its tributaries, acted as the main agent of removal, gradually stripping away the softer rock layers. The sedimentary rocks, including shales and sandstones, were easily broken down and carried away by wind and water flow. This continuous removal of the less-resistant material eventually exposed the harder igneous rock core that had been hidden for tens of millions of years.
Scientists estimate that this process of exposure began between 5 and 10 million years ago, allowing the gray columns to stand out above the surrounding landscape. The tower is essentially an exhumed geological feature, or monadnock, that has resisted erosion and stands above the surrounding plain. The large piles of broken rock fragments, known as scree, at the tower’s base are evidence that the erosion of the harder core is still occurring today.