The fluted sides of Devils Tower resemble a bundle of enormous stone pillars. Many rise for hundreds of feet, and some are wide enough to dwarf a person. They were not built upward one at a time, however. The columns are pieces of a once-continuous mass of molten rock that fractured as it lost heat and contracted. Erosion later stripped away the softer rocks around that mass, turning an underground cooling pattern into a prominent Wyoming landform.
The broad sequence is well established, but an important part of the story remains unsettled. Geologists agree on the rock type, the origin of the joints, and the role of erosion. They still debate the original shape and setting of the magma body. The visible tower preserves the interior of an igneous feature while much of the evidence needed to identify that feature has disappeared.
Molten Rock Entered an Older Landscape
Long before Devils Tower formed, shallow seas, coastal environments, and rivers deposited layers of sand, silt, clay, limestone, and gypsum across the region. These sedimentary beds span parts of the Triassic and Jurassic periods and remain visible around the monument. Their colors and fossils record environments that existed tens of millions of years before the tower’s magma arrived.
About 50 million years ago, molten rock pushed into this layered crust during a period of igneous activity associated with uplift in western North America. It cooled into phonolite porphyry, an uncommon, fine-grained igneous rock containing conspicuous larger crystals. Because the magma solidified as one connected body, the columns are not separate lava pipes or individual crystals. Their boundaries are fractures called joints.
Cooling Created a Polygonal Crack Network
Molten rock occupies slightly more volume than the solid rock produced from it. As the magma at Devils Tower cooled and crystallized, it contracted. Once the partly solid mass could no longer deform enough to absorb that shrinkage, tensile stress accumulated. Cracks opened to relieve the stress and divided the rock into polygons.
A cooling fracture tends to advance inward from an exposed or colder surface. The cracks grow broadly perpendicular to the moving boundary between cooler and hotter rock. As several fractures propagate and interact, they adjust toward a pattern that distributes stress efficiently. Six-sided polygons are common in columnar jointing, but nature does not produce a perfect tiled floor. At Devils Tower, many columns have five sides, while others have four, six, or seven.
The exceptional width of the pillars points to the thermal history of the magma. Joint spacing generally increases when a large body loses heat slowly and the cracking front advances gradually. Devils Tower’s columns can approach 10 feet across, far larger than the narrow prisms found in many basalt flows. Small variations in cooling rate, rock composition, water availability, and the geometry of the magma body probably contributed to their irregular shapes and changing orientations.
Horizontal fractures later cut across the long columns. Together with the steep lengthwise joints, these breaks divided the rock into segments. The result is the ribbed wall seen today, not because erosion carved every groove from solid stone, but because weathering widened preexisting cooling fractures and loosened blocks along them.
What Shape Did the Magma Body Have?
One straightforward interpretation treats Devils Tower as a stock: a relatively small intrusion that cooled underground. Other proposals identify it as the remnant of a laccolith, a mushroom-shaped intrusion that spread between sedimentary layers and arched them upward. A familiar older idea calls it a volcanic plug or the neck of an extinct volcano.
That last description is possible but not demonstrated. The surrounding area lacks an unambiguous set of preserved lava flows, ash beds, and volcanic debris connecting the tower to a surface volcano. Those materials could have been removed, but their absence prevents a confident reconstruction. A newer hypothesis proposes that magma interacted explosively with groundwater to form a maar-diatreme crater, which later filled with lava that cooled into a dome.
Every model must explain both the structure of the igneous rock and its relationship to rocks that are no longer present. None changes the physical explanation for the columns themselves: contraction during cooling made the joints. The disagreement concerns where the magma sat and what lay above it when the cracking occurred.
A Hidden Structure Became a Tower
Devils Tower did not rise by being pushed bodily out of the ground. It became prominent because the land around it was lowered. Water, temperature changes, and other weathering processes broke down the surrounding sedimentary formations. Streams, including the Belle Fourche River, transported the debris away. The phonolite resisted erosion better, so progressively more of it stood above the changing surface.
Exposure also began dismantling the tower. Water enters its joints and expands when it freezes, roots exert pressure within openings, and daily or seasonal temperature changes stress the surface. Gravity moves loosened pieces downslope. A boulder field covering roughly 13 acres surrounds the base, where fallen fragments still preserve portions of the original polygonal forms.
The process is slow enough that no major column has been observed falling in recorded history, although smaller fragments break away. Meanwhile, erosion continues to remove softer beds from around the base. Devils Tower is simultaneously being revealed and worn down—the same long process producing both its prominence and its eventual destruction.
Its columns are spectacular because two geological stages align so clearly. Cooling organized the igneous rock into a polygonal fracture network underground. Differential erosion then exposed that internal geometry at landscape scale. The uncertainty surrounding the magma body’s original form does not weaken this account; it shows how a landform can preserve a process with remarkable clarity while losing much of the setting in which that process occurred.