Devils Tower, a striking natural landmark, rises prominently from northeastern Wyoming. Also known by its Native American name Mato Tipila or Bear Lodge, its unique, fluted appearance sparks curiosity about the forces that shaped it. Understanding its formation involves delving into deep geological time and the processes that sculpt Earth’s surface.
The Underground Origin
Millions of years ago, between 40.5 and 65 million years, magma pushed upward through sedimentary rock layers deep beneath the Earth’s surface. This molten rock solidified underground, forming an intrusive igneous body rather than erupting as a volcano.
The rock of Devils Tower is phonolite porphyry, an igneous rock containing conspicuous white feldspar crystals. These larger crystals, called phenocrysts, indicate a two-stage cooling process. The magma initially cooled slowly at depth, allowing the crystals to grow.
As the magma ascended, it cooled more rapidly at a shallower depth, forming a fine-grained matrix around the crystals. This underground solidification created a mass of hard, resistant igneous rock, contrasting with the softer surrounding sedimentary rocks. This established the resistant core that would become the visible Devils Tower.
The Formation of Distinctive Columns
As the igneous rock cooled and solidified underground, it contracted in volume. This natural process created stresses within the rock mass, leading to vertical cracks or fractures.
These cracks typically formed in a polygonal pattern, often resulting in hexagonal shapes, though four, five, and seven-sided columns are also present. This geological phenomenon is known as columnar jointing, a common feature in igneous rock formations worldwide. The columns at Devils Tower are exceptionally large, some reaching up to 20 feet wide and 600 feet tall.
The joints developed at right angles to the magma body’s cooling surfaces. The size and regularity of the columns are influenced by the rate of cooling, with slower cooling generally producing larger columns. This process gave Devils Tower its distinctive fluted appearance, making it a prominent example of columnar jointing.
Emergence from the Earth
Devils Tower was not thrust from the ground, but gradually revealed over vast geological time. When the igneous intrusion formed, it was buried one to two miles beneath the Earth’s surface, surrounded by softer sedimentary rocks. Over millions of years, these overlying and surrounding sedimentary layers, composed of sandstone, shale, and gypsum, were slowly worn away.
Erosion, driven by wind, water, and ice, carried away the less resistant sedimentary material. The Belle Fourche River and its tributaries played a significant role in transporting this eroded debris. Because the phonolite porphyry of Devils Tower is much harder and more resistant to weathering, it remained standing as the softer surrounding rocks were removed.
This differential erosion, where softer rocks erode faster than harder ones, exposed the resistant igneous core. Evidence of this ongoing process is seen in the large boulder field at the Tower’s base, composed of broken columns that have fallen over time. The exposure of Devils Tower began between 5 and 10 million years ago, revealing the structure visible today.
Ongoing Scientific Discussions
While the general understanding of Devils Tower’s formation involves the intrusion of magma and subsequent erosion, scientific discussions continue regarding the precise nature of the initial magma body. Geologists largely agree that it is an igneous intrusion that cooled underground. However, the exact configuration of this intrusion beneath the surface remains a topic of study.
Two primary hypotheses are frequently discussed. One suggests Devils Tower is the remnant of a laccolith, a mushroom-shaped mass of igneous rock that intrudes between sedimentary layers and pushes them upward without reaching the surface. Another theory proposes it is a volcanic neck, the solidified conduit of an ancient volcano. However, the limited evidence of volcanic ash or lava flows in the area raises questions about it being a direct volcanic vent.
A more recent hypothesis suggests it could be the result of a maar-diatreme volcano, where magma interacts with groundwater, causing explosive steam eruptions and later infilling with lava. These ongoing debates highlight the complexities of interpreting geological features over vast timescales, as the very erosion that exposed Devils Tower may also have removed crucial evidence. Research continues to refine our understanding of this geological marvel.