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.”
[Illustration: 1.]
[Illustration: 2.]
[Illustration: 3.]