Geologists Carpenter and Russell studied Devils Tower in the late 19th century and came to the conclusion that it was formed by an igneous intrusion. Modern geologists agree that it was formed by the intrusion of igneous material, but not on exactly how that process took place. Several believe the molten rock comprising the Tower might not have surfaced; others are convinced the tower is all that remains of what once was a large explosive volcano.
In 1907, scientists Darton and O'Harra decided that Devils Tower must be an eroded remnant of a laccolith. A laccolith is a large mass of igneous rock which is intruded through sedimentary rock beds without reaching the surface, but makes a rounded bulge in the sedimentary layers above. This theory was quite popular in the early 20th century since numerous studies had earlier been done on laccoliths in the Southwest.
Other theories have suggested that Devils Tower is a volcanic plug or that it is the neck of an extinct volcano. Presumably, if Devils Tower was a volcanic plug, any volcanics created by it — volcanic ash, lava flows, volcanic debris — would have been eroded away long ago. Some pyroclastic material of the same age as Devils Tower has been identified elsewhere in Wyoming.
The igneous material that forms the Tower is a phonolite porphyry intruded about 40.5 million years ago,[14] a light to dark-gray or greenish-gray igneous rock with conspicuous crystals of white feldspar.[15] As the magma cooled, hexagonal (and sometimes 4-, 5-, and 7-sided) columns formed. As the rock continued to cool, the vertical columns shrank in cross-section (horizontally) and cracks began to occur at 120 degree angles, generally forming compact 6-sided columns. The nearby Missouri Buttes, 3.5 miles (5.6 km) to the northwest of Devils Tower, are also composed of columnar phonolite of the same age. Superficially similar, but with typically 2 feet (0.61 m) diameter columns, Devils Postpile National Monument and Giant's Causeway are columnar basalt.
Devils Tower did not visibly protrude out of the landscape until the overlying sedimentary rocks eroded away. As the elements wore down the softer sandstones and shales, the more resistant igneous rock making up the tower survived the erosional forces. As a result, the gray columns of Devils Tower began to appear as an isolated mass above the landscape.
As rain and snow continue to erode the sedimentary rocks surrounding the Tower's base, more of Devils Tower will be exposed. Nonetheless, the exposed portions of the Tower still experience certain amounts of erosion. Cracks along the columns are subject to water and ice erosion. Erosion due to the expansion of ice along cracks and fractures within rock formations is common in colder climates — a prime example being the featured formations at Bryce Canyon National Park. Portions, or even entire columns, of rock at Devils Tower are continually breaking off and falling. Piles of broken columns, boulders, small rocks, and stones — or scree — lie at the base of the tower, indicating that it was once wider than it is today.[13]
