Devils Tower, a striking geologic formation in northeastern Wyoming, rises dramatically from the landscape. Designated as America’s first National Monument in 1906 by President Theodore Roosevelt, this prominent feature is instantly recognizable around the world. Understanding the tower requires looking back through deep geologic time, where the forces of heat and water sculpted the earth over millions of years.
The Material: What the Tower is Made Of
The tower is primarily formed of an igneous rock known as phonolite porphyry. Igneous rock forms when molten material, or magma, cools and solidifies. The term phonolite refers to the rock’s composition, which is silica-poor and rich in alkali metals, sometimes producing a ringing sound when struck.
The porphyry texture means larger crystals, called phenocrysts, are embedded within a finer-grained matrix. These visible white feldspar crystals grew while the magma cooled slowly beneath the surface, providing the necessary resistance to withstand the relentless forces of erosion.
The Initial Event: Magma Intrusion and Columnar Jointing
The formation began approximately 40 to 50 million years ago during the significant mountain-building associated with the Black Hills uplift. Magma pushed up from deep within the earth’s crust through sedimentary rock layers. This molten material did not erupt onto the surface but stalled and cooled underground in an intrusive event.
The slow, uniform cooling of this magma created the tower’s most distinctive feature: columnar jointing. As the magma solidified into phonolite porphyry, the material contracted, creating stress fractures that propagated inward from the cooling surfaces. These fractures formed a highly regular pattern of interlocking, polygonal joints.
The majority of the columns are five- or six-sided, though four- and seven-sided columns also exist. The consistent cooling produced columns hundreds of feet tall and up to 10 feet wide. The resulting structure was a hardened plug of igneous rock buried deep beneath the surface.
The Shaping Force: Erosion and Exposure
For millions of years following the intrusion, the phonolite porphyry core remained hidden, buried one or two miles beneath the earth’s surface. The tower was eventually revealed through erosion and weathering.
The surrounding landscape consisted of softer sedimentary rock formations, such as the Triassic-age Spearfish Formation and the Jurassic-age Sundance Formation. These layers of sandstone, shale, and siltstone were easily stripped away by the Belle Fourche River and other erosional agents, lowering the surrounding land surface.
The phonolite porphyry is significantly harder and more resistant to weathering than the surrounding sedimentary rocks. Beginning five to ten million years ago, the differential erosion rate stripped away the soft outer layers, leaving the dense, hardened core standing in stark relief. The immense pile of broken columns and boulders, known as talus, visible at the base is evidence of the tower’s continued, slow erosion.
The Mystery: Competing Theories of Formation
While the general sequence of intrusion, cooling, and erosion is accepted, the precise shape of the subsurface structure remains debated. The disagreement centers on the original form of the magma body before erosion exposed it.
One long-held theory suggests the tower is an eroded laccolith, a mushroom-shaped intrusion that spread laterally between rock layers without reaching the surface. Another prominent hypothesis is the volcanic neck or plug theory, which posits the tower is the solidified conduit of an ancient, extinct volcano. This volcanic model suggests that evidence of an accompanying cone, such as ash or lava flows, has been completely erased by erosion.
Geologists generally agree the structure is a type of stock—a small, intrusive body. However, the very erosion that exposed the tower also removed much of the critical evidence needed to definitively settle the debate. The exact geometry underground remains a compelling scientific mystery.