Devils Tower, located in northeastern Wyoming, stands as a striking geological monument. This imposing monolith is recognized for its steep, fluted sides. Its unique columnar structure fuels curiosity and scientific inquiry. Established as the nation’s first national monument in 1906, Devils Tower remains a subject of geological fascination.
The Foundation: Magma and Intrusion
Devils Tower originated from an igneous intrusion, a process where molten rock, known as magma, pushed its way into existing sedimentary rock layers deep beneath the Earth’s surface. This event occurred 40.5 to 65 million years ago, during the Tertiary period, within the Black Hills region. Unlike a volcano that erupts on the surface, this magma cooled and solidified underground.
The rock forming Devils Tower is phonolite porphyry. Unweathered samples appear light to dark-gray or greenish-gray. It is characterized by a fine-grained matrix embedded with white feldspar crystals (quarter to half-inch in diameter) and smaller dark-green pyroxene crystals. The surrounding landscape consists predominantly of softer sedimentary rocks, which were present before the intrusion.
The Unique Cooling and Shaping
Devils Tower’s distinctive vertical columns are known as columnar jointing. This structure formed as the molten rock cooled and contracted. As the rock cooled, it experienced stress, leading to the development of fractures that radiated from stress points.
These fractures intersect to form polygonal shapes, with many columns exhibit six sides, though four, five, or seven-sided columns also occur. The columns measure 6 to 8 feet in diameter at their base, tapering to about 4 feet near the top. Devils Tower’s columnar jointing is a prominent global example.
Erosion’s Masterpiece
Devils Tower became visible through a process of differential erosion. For millions of years, the softer sedimentary rocks that once surrounded and covered the harder phonolite porphyry were worn away by wind and water. This exposed the more resistant igneous core.
The Tower was initially buried one to two miles beneath the Earth’s surface. Erosional forces began to unveil the Tower between 5 and 10 million years ago. The Belle Fourche River and its tributaries have played a role in transporting away the eroded debris. The large apron of talus, or broken rock fragments, at the base of the Tower demonstrates ongoing erosion.