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Geologists agree that Devils Tower began as magma, or molten rock buried beneath the Earth’s surface. What they cannot agree upon are the processes by which the magma cooled to form the Tower, or its relationship to the surrounding geology of the area. Numerous theories have been suggested to explain how Devils Tower formed. Geologists Carpenter and Russell studied Devils Tower in the late 1800s and concluded that the Tower was formed by an igneous intrusion (the forcible entry of magma through other rock layers). Later geologists searched for more detailed explanations.
The simplest explanation is that Devils Tower is a stock—a small intrusive body formed by magma which cooled underground and was later exposed by erosion (Figure 1). In 1907, scientists Darton and O’Hara decided that Devils Tower must be an eroded remnant of a laccolith. A laccolith is 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 (Figure 2). This idea was quite popular in the early 1900s when numerous studies were done on a number of laccoliths in the American southwest.
Other ideas have suggested that Devils Tower is a volcanic plug or that it is the neck of an extinct volcano (Figure 3). The limited evidence of volcanic activity (volcanic ash, lava fows, or volcanic debris) in the area creates doubt that the Tower was part of a volcanic system. It is possible that this material may simply have eroded away. In 2015, geologist Prokop Závada and his colleagues proposed their own hypothesis for the formation of the Tower. They compared it to a similar butte formation in the Czech Republic. Their hypothesis suggests that the Tower is the result of a maar-diatreme volcano (Figure 4). These form when magma encounters groundwater beneath the Earth’s surface. The super-heated water becomes steam. This steam expands explosively creating a crater on the surface. The crater fills with lava which cools and solidifes into a dome structure. Erosion wears away portions of the dome to create the Tower as we see it today.
The concept of erosion exposing the Tower is common to all of its modern formation theories. Ironically, the erosion which exposed the Tower also erased the evidence needed to determine which theory of Devils Tower’s formation is the correct one.
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