Geology of Devil’s Tower
Geologists agree that Devils Tower was formed by the intrusion (the forcible entry of magma into or between other rock formations) of igneous material. What they cannot agree upon is how that process took place and whether or not the magma reached the land surface.
An older theory is that it is a laccolith – a large, mushroom–shaped mass of igneous rock which intrudes between the layers of sedimentary rocks but does not reach the surface. This produces a rounded bulge in the sedimentary layers above the intrusion. The sediment that surrounds it is more easily eroded away – leaving the standing tower.
Another theory is that it is a volcanic plug. However, there is no evidence of volcanic activity – volcanic ash, lava flows, or volcanic debris – anywhere in the surrounding countryside.
A more modern theory is that is a stock – a small intrusive body formed by magma which cooled underground and was later exposed by erosion.
The tower is made of phonolite porphyry, which is similar in composition to granite but lacks quartz. Phonolite refers to the ringing of the rock when a small slab is struck, and its ability to reflect sound. Porphyry refers to its texture, large crystals of feldspar embedded in a mass of smaller crystals.