Geological Survey Bulletin 1021-I Geology of Devils Tower National Monument, Wyoming

The origin of Devils Tower has been a matter of speculation for many years, and even today after detailed geologic mapping of the area, no conclusive proof of its mode of origin can be presented.

Several theories of the origin have been proposed. One of the more popular of these is that it is the neck of an extinct volcano (Carpenter, 1888; Dutton and Schwartz, 1936). Another theory is that Devils Tower and Missouri Buttes (a mass of the same type of rock about 4 miles northwest of the Tower) are the remnants of a laccolith (a tabular intrusive igneous body, thickest in the middle, and with a relatively level floor), the vent for which was under Missouri Buttes (Jaggar, 1901, p. 264). Darton (1901, p. 69) believed that the Tower is the remnant of a laccolith, smaller than the one proposed by Jagger, the feeding vent for which was underneath the Tower.

Much more detailed geologic work will have to be done in the surrounding area before the mode of origin of Devils Tower may be proved conclusively. The evidence gathered during the present investigation, however, suggests that Devils Tower is a body of intrusive igneous rock, which was never much larger in diameter than the present base of the Tower, and which at depth (1,000 feet or more) is connected to a sill or laccolith type body. The bases for this theory are—

1. The exposed portion of the Tower is the result of recent erosion. At the time of its intrusion it was surrounded and probably covered by several hundred feet of sedimentary rock.

2. The mineral composition and texture are more typical of shallow intrusive rocks, which are formed at depth, than extrusive rocks, which are formed on the surface.

3. No evidence of extrusive igneous activity has been found in the surrounding area.

4. Missouri Buttes, about 4 miles to the northwest, and the Tower have the same composition so it is assumed that they were derived from a common magma; possibly the magma of a large intrusive body, such as a laccolith or sill.

5. In a well drilled about 1-1/2 miles southwest of Missouri Buttes, near the center of a structural dome, rock similar to the Tower and Missouri Buttes was encountered at about 1,400 feet below the base of Missouri Buttes. Inasumuch as the thickness of the sedimentary rocks in this area is normally much greater than this depth, the rock in the drill hole probably represents an intrusive body, rather than the Precambrian igneous rocks upon which the younger sedimentary rocks were deposited.

6. The relatively small amount of talus, slope wash, or terrace gravel derived from the Tower and Missouri Buttes suggests that they have not been extensively eroded, and therefore the original igneous bodies were not much larger than at present.

7. Columnar jointing is common in intrusive bodies formed at comparatively shallow depths.