Devils Tower stands as a remarkable natural landmark, its distinctive form rising dramatically from the landscape. This striking geological feature, with its fluted sides and imposing height, naturally prompts questions about its origins and the powerful forces that shaped it over millions of years. Unraveling the story of Devils Tower reveals a complex interplay of deep Earth processes and surface transformations.
The Underground Genesis
The formation of Devils Tower began deep beneath the Earth’s surface approximately 50 to 60 million years ago, during the Tertiary period. At this time, tectonic pressures caused the uplift of the Rocky Mountains and the Black Hills. Molten rock, known as magma, ascended through the Earth’s crust and intruded into existing layers of sedimentary rock. This magma did not erupt onto the surface as a volcano but instead cooled and solidified underground.
The material that forms Devils Tower is a type of igneous rock called phonolite porphyry, typically light to dark-gray or greenish-gray, characterized by visible crystals of white feldspar within a finer-grained matrix. Geologists agree that Devils Tower is an igneous intrusion, meaning the magma forced its way into and between other rock formations before cooling. Initially, the solidified mass was located one to two miles below the Earth’s surface.
The Formation of Distinctive Columns
As the intruded magma began to cool deep underground, a process known as columnar jointing commenced. Molten rock, when cooling and solidifying, contracts in volume, creating stress within the cooling rock mass. These stresses lead to the development of fractures that typically propagate inward from the cooling surfaces. The fractures tend to form in a polygonal pattern, most commonly resulting in the characteristic hexagonal columns seen at Devils Tower. While hexagons are prevalent, columns can also be four, five, or seven-sided, some reaching up to 20 feet wide and 600 feet tall. This distinctive columnar structure is a direct result of the physical properties of cooling igneous rock.
The Unveiling by Erosion
For millions of years following its underground formation, Devils Tower remained hidden beneath layers of softer sedimentary rock. The tower’s exposure is primarily due to the relentless work of erosion. Wind, water, and ice gradually wore away the less resistant sandstone, shale, and gypsum that surrounded and covered the harder igneous rock. This process, known as differential erosion, demonstrates the greater resistance of the phonolite porphyry to weathering compared to the surrounding sedimentary formations. The Belle Fourche River also played a role, carrying away eroded debris, slowly revealing the towering structure over one to two million years.
Scientific Consensus on Its Origin
Geologists have long studied Devils Tower, and while certain details about its exact original shape are still debated, a general scientific consensus has emerged regarding its formation. Early interpretations in the late 1800s suggested an igneous intrusion. In the early 1900s, theories such as the laccolith model, proposing a mushroom-shaped intrusion that bulged overlying sedimentary layers, gained popularity. Other ideas included a volcanic neck or plug. Current research supports that Devils Tower is an eroded remnant of a small intrusive body, likely a stock or a laccolith, that cooled underground, as indicated by the lack of volcanic ash or lava flows.