Devil’s Tower is a giant butte in northeastern Wyoming that stands approximately 867 feet (264 m) from summit to base. I visited while I was on a 6 week geology field camp in South Dakota and Wyoming back in 2011. Geology field camp is kind of a "boot camp" for geologist. But with (presumably) more beer. Generally you're dropped off in the mountains alone or in a small group and forced to apply everything you learned in the classroom. Very much a "sink or swim" scenario. But a fun one. Luckily, Devil's Tower was a "tourist stop" on the trip, so it was not particularly stressful.
The Tower has cultural importance for multiple Native American groups. Oral histories of the Cheyenne suggest that the Tower grew while some were praying to the Great Spirit to escape a bear. The Arapahoe, Crow, Kiowa and Lakota also have oral histories the Tower that involves a bear as well (National Park Service, 2022b).
But how did it form geologically?
Geology
This tower is a dark grey to greenish-grey rock referred to as phonolite porphyry.
What does this mean?
A phonolite is an extrusive igneous rock. An igneous rock is a rock that develops from the cooling and solidification of either lava or magma. Extrusive is generally a term used to refer to igneous rocks that form after they erupt on the surface of the Earth, however here I would personally refer to Devil’s Tower as a “intrusion” rather than an extrusive rock (see below).
A porphyry is a rock that has large crystals surrounded by fine-grained minerals. The large crystals here are 0.6-1.3 cm (0.25-0.50 in) white feldspar. The groundmass (fine-grained minerals) are dark-green pyroxenes. Weathering can change the color slightly, to a light grey or brownish grey (National Park Service, 2022a).
The Tower itself is composed of massive columns referred to as columnar jointing. This jointing style has been seen elsewhere, however not at this size (10 ft width). This jointing only forms with igneous rocks and is the result of contraction stresses that the rock experiences. This causes the rock to crack, which radiates out from stress points, which forms a hexagonal shape with 6-sides (National Park Service, 2019).
Some of the columns have also collapsed around the Tower. They are absolutely massive boulders, some larger than a bus.
How did it form?
Step 1
Well, before Devil’s Tower appeared, layers of sedimentary rocks formed on the landscape.
What is a sedimentary rock?
In the simplest sense, these form through the solidification of individual sediments and organic material over time. Sediments are minerals that have broken off other rocks (i.e., igneous, metamorphic or even other sedimentary rocks) through erosion. They can range in size from clay to boulders. These minerals are carried by some means (water, wind, gravity, etc.), deposited somewhere where they are eventually buried and become lithified (i.e., forming a sedimentary rock).
Sediments can be found throughout the world today and give us clues to what the ancient landscape looked like. Examples of this are the sand that you can find at a beach or a desert (rock equivalent: sandstone). Or mud that can be found in deep seas or river deltas (rock equivalent: shale).
To reconstruct the paleoenvironment for you a little, the oldest rocks that would be found at the base of Devil’s Tower are sandstones and siltstones that were deposited during the Triassic. This region, as well as much of central and western US was a shallow inland sea at the time. Above this (or after this) gypsum deposits were found, indicating a mineral rich water that evaporated. Above this (after this) clays appear in the form of shale, which indicates a deep marine environment. This is followed by a yellow sandstone, which would have been an ancient beach (National Park Service, 2019).
Step 2: Building the Tower
During the Tertiary time period, 50 to 60 million years ago, tectonic pressure caused uplifting in the Rocky Mountains and the Black Hills. During this time, or shortly after, magma pushed up and intruded into the existing sedimentary layers (National Park Service, 2019).
There are a number of theories as to how Devil’s Tower formed. Here are a few:
Stock Theory
In this theory magma intruded into the sedimentary rocks and cooled underground. Later it was exposed through erosion (step 3).
Figure 1. This image is based on figures found on National Park Service (2019). The partially transparent image of Devil's Tower in Steps 1-3 is only the "modern day" position for reference.
Laccolithic Theory
A laccolithic is a mushroom shaped igneous intrusion. Here it is believed that it formed a round bulge in the sediment that was above the tower.
Volcanic Plug Theory
This theory suggests that the Tower is a volcanic plug or the neck of an extinct volcano. There is limited evidence of this being a volcano, however it is possible that the volcanic material just eroded away.
Maar-Diatreme Theory
A maar-diatreme volcano forms when magma encounters groundwater beneath the Earth’s surface. This causes the water to become steam which expands explosively above ground and forms a crater. This crater then fills with lava that solidifies and forms a dome structure. Erosion then wears down portions of the dome, eventually creating the Tower that we see today (National Park Service, 2019).
Figure 2. This image is based on figures from National Park Service (2019).
Step 3: Erosion
As it has previously been mentioned, after the igneous rock was emplaced, the sedimentary rocks surrounding Devil’s Tower began to erode. Between 5 to 10 million years ago the Tower was first exposed (National Park Service, 2019). Because the igneous rocks that the tower are composed of are more resistant than the surrounding sediments, the Tower remained.
Conclusion
I highly recommend that you check out this very interesting and unique geologic formation when you get a change. At the very least go there to see the largest known hexagonal columns!
References
National Park Service. (2019, December 5). How the Tower Formed. Retrieved July 26, 2023, from https://www.nps.gov/deto/learn/nature/tower-formation.htm
National Park Service. (2022a, February 9). NPS Geodiversity Atlas—Devils Tower National Monument, Wyoming. Retrieved July 26, 2023, from https://www.nps.gov/articles/nps-geodiversity-atlas-devils-tower-national-monument-wyoming.htm
National Park Service. (2022b, November 4). First Stories. Retrieved July 26, 2023, from https://www.nps.gov/deto/learn/historyculture/first-stories.htm