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Geology of the National Parks in Pictures - Devils Tower National Monument

My next post about the Geology of the National Parks Through Pictures is from our move across the country from Utah to New York. Along the way we visited 13 National Parks as well as some other sites. This was the 6th National Park along the way.

You can find more Geology of the National Parks Through Pictures as well as my Geological State Symbols Across America series at my website Dinojim.com.

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Devils Tower National Monument
Obligatory entrance sign shot, however this is probably one of my favorite alignments of the sign to the feature of the park.
While describing the geology of Devils Tower you would probably want to start with the tower first (duh). However, in order to understand how the tower formed, you need to understand the rocks that were here before the tower, or what would become the tower, some of which still remain.
While many of the rocks that Devils Tower formed within have long since eroded away, there are still significant amounts of older rocks that the tower currently resides on. You can see some of these rocks as you drive up to the tower from the main entrance (photo with the entrance sign and directly above).
Geology of the Devils Tower sedimentary rocks. Image courtesy of NPS.
The bright red rocks at the base of the hillside are the Spearfish Formation, which can be seen most easily in the photo above without the entrance sign. These rocks, dark red sandstones and maroon siltstones, formed during the Permian and Triassic Periods (~ 225 to 195 million years ago) when this area was covered by a shallow inland sea. The deep red color is from the oxidation (rusting) of iron within the deposits. After the Triassic Period, during the Middle Jurassic (~170 million years ago), the inland seas periodically withdrew producing periods of evaporation. This evaporation lefts gypsum rich deposits in the form of the Gypsum Springs Formation.
View facing away from the tower. You can see the Spearfish Formation in the distance towards the let half of the photo.
Afterwards, the Middle Jurassic (~165 million years old) Stockade Beaver Shale then represents the deepening of the inland seas, with the shales deposited in deeper waters than the previous Spearfish Formation. The final sedimentary deposits that remain within the park are the Late Jurassic (~155 million years old) coastal dunes and beach deposits of the Hulett Sandstone. Within these rocks that form the cliff edge surrounding the tower, ripples from the former beaches can still be seen.

The formation of the tower itself is uncertain though. There are several theories that have been proposed with none, to date, taking precedence. It is without a doubt that the tower formed from some variety of magmatic intrusion within the surrounding sedimentary rocks one to two miles below the (former) surface that began about 50 million years ago. This magma then cooled and solidified, forming the basis of the tower itself. Then, around 5 to 10 million years ago, the overriding sedimentary rocks started to erode away and expose the tower, leaving behind the much more resistant rocks.

The prevailing theories are that Devils Tower formed as a: Igneous stock. The magma intruded into the sedimentary rock layers forming a body roughly the shape of the modern day tower. The magma then cooled, and eventually the overriding sedimentary rocks eroded away, along the outer edges of the igneous body for the last several million years. Laccolith. This is a larger igneous intrusion with a more mushroom shape to it. Again, the magma solidified and then eroded away once the overlying sediment was removed and eventually eroded down to the tower shape as we know it today. Volcanic plug. Essentially this was the root of a volcano, where magma was rising up through the volcano and eventually as the volcano went extinct, the magma within the volcano solidified. This is deeper within the earth than what is known as a "volcanic neck". The volcanic neck theory is discussed below. Maar-diatreme volcano. This is the most recent suggestion where the magma caused the groundwater to become superheated and eventually explode, creating a crater. The crater was then filled with magma and solidified. Eventually the surrounding rocks eroded away, again, leaving the tower as we see it today.

The rock itself is what is known as a phonolite porphyry. Phonolite is a type of igneous rock that is rich in alkali feldspar and moderate amounts of silica (quartz). As you can see in the diagram below, phonolite has one of the highest percentages of alkali feldspar. The alkali feldspar in this instance is orthoclase, which makes up the second part of the rock name - porphyry. Porphyry, AKA porphyritic rocks, are rocks composed of two principle grain sizes, a background, where the mineral crystals are usually indistinguishable, and the much larger crystals, known as phenocrysts. In the photo above, orthoclase is the mineral that makes up the phenocrysts, the large white crystals on a background that is more grey in color. Igneous rock diagram. Image courtesy of Moayyed et al., 2008.
But one of the most distinguishable factors of the tower, besides its size amongst a fairly featureless plain, is the columnar jointing. Columnar jointing is what produces those vertical lines seen at a distance of the tower. These columns are roughly hexagonal in shape, however they are not consistent and Devils Tower has many that are pentagonal columns as well, as well as ranging in diameter from 10 feet across to 4 feet across. These columns are formed when the igneous body (magma) is cooling and the magma starts to shrink (contract). This contracting produces stress lines which radiate out through the resulting rock, forming this columnar jointing pattern. These joint lines align perpendicularly to the direction of the cooling front. Since the jointing lines are running vertical, it can be assumed that the cooling front was essentially horizontal above the tower. With the fractures running vertically through the height of the tower, boulders breaking off and forming a pile of boulders, the scree, are fairly common, especially through geological history.

One other theory of the tower's formation is that of a volcanic neck. A volcanic neck is related to the volcanic plug, however a neck would be seen as the central portion of a volcano, as opposed to deep within the earth. In this instance, if this were a volcanic neck, what we would see is that Devils Tower is the core of the volcano, with the sloped sides having been eroded away. However, the jointing pattern, which runs vertically and curves towards the bottom of the tower, indicates that this theory of a volcanic neck is incorrect. If this were a former volcanic neck, the jointing pattern would run towards the center of the tower, as opposed to running nearly vertical.
References https://www.nps.gov/deto/learn/nature/geologicformations.htm https://www.nps.gov/deto/learn/nature/tower-formation.htm https://www.nps.gov/places/spearfish-and-inyan-kara.htm https://ngmdb.usgs.gov/Geolex/UnitRefs/HulettRefs_8580.htm https://www.science.org/doi/10.1126/science.134.3487.1373 https://www.britannica.com/science/phonolite https://www.geowyo.com/devils-tower--black-hills.html https://pubs.geoscienceworld.org/aapg/aapgbull/article-abstract/40/1/84/34214/Gypsum-Spring-Formation-Northwestern-Black-Hills https://ngmdb.usgs.gov/Geolex/UnitRefs/StockadeBeaverRefs_10594.html https://npshistory.com/publications/deto/brochures/geology-2017.pdf https://www.sciencedirect.com/science/article/abs/pii/S0009281906000225

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