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 Jaggar, 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 1½ 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. Inasmuch 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.

The following new material has been added to this booklet by the National Park Service (Devils Tower National Monument, 1985)

The most recent in depth, geologic study of Devils Tower was done by Don L. Halverson (1980) and presented in a dissertation, to the Graduate Faculty of the University of North Dakota.

He stated that, “The Missouri Buttes and Devils Tower, however, are necks of extinct volcanoes which have been exposed by erosion. This theory was first proposed by Carpenter (1888) and later expanded by Dutton and Schwartz (1936). The material which fed these volcanoes came from a minimum depth of 18 km. Evidence for this conclusion is listed in the following statements:

1. The alloclastic breccia in the vicinity of Devils Tower and the Missouri Buttes is definitely igneous in origin and probably represents periods of violent eruption.

2. A very definite similarity exists between these two features and the volcanic necks in the Taylor Mountain area of New Mexico.

3. The distinctive columns with basal flare are also found in the volcanic necks of the Taylor Mountains (Dutton and Schwartz 1936), but have not been reported in columnar-jointed laccoliths.

4. The Missouri Buttes and Devils Tower were intruded directly through horizontal sediments without disrupting them, even in the immediate vicinity of the igneous bodies.

5. Recent research indicates that many of the laccolithic intrusions in the Black Hills region may have been less passive than previously considered. Sundance Mountain may be a mixed volcanic cone consisting of welded ash fall, massive quartz latite, and ash flow tuffs. Nearby Sugarloaf Mountain is composed of layered tuffs (Fashbough 1979).

6. Collapse of materials into partly evacuated reservoir chambers accounts for the depressions surrounding the Missouri Buttes and Devils Tower. The 90 m of depression at the southern end of the Buttes is difficult to explain with a laccolithic model.

7. Flow directions deduced from oriented thin-sections and field observations indicate mostly vertical flow. It must be noted that in both igneous bodies orientation of some grains is horizontal; this could, however, simply indicate turbulent flow.

8. The stability field for the analcime-liquid system is 5 kbar minimum (Roux and Hamilton 1976), which indicates that the original melt of Devils Tower and Missouri Buttes rock had to originate at a minimum depth of 18 km.

9. It is unlikely that magma which had ascended from great depths and had just penetrated the resistant Hulett Member of the Sundance Formation, as well as the Lakota and Fall River Formations, would be stopped abruptly by the less resistant shales above. When the magma reached the shale beds, the weight of the column of igneous rock could have exceeded the strength of the shale, causing the magma to flow horizontally. No indication of horizontal spread, however, is observed. The continuously cylindrical shape of the intrusions indicates that the magma moved steadily upward and probably reached the surface.

10. Carbonatites have been found, and formally reported, in the nearby Bear Lodge Mountains, and also as fragments in the alloclastic breccias of the Missouri Buttes. Their presence suggest a high volatile content for the magma and the possibility of explosive volcanism.”