Geologic Features of Devils Tower National Monument

Devils Tower really has to be experienced in person to fully feel the majesty of this place. In this post, I'll share a few shots from my trips there and describe some of the geology of it.

Devils Tower

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There's no question that the rocks forming Devils Tower originated as a shallow intrusive body of igneous rock. The rock here is a phonolite, which is primarily a fine-grained igneous rock indicating it cooled from magma fairly quickly. The structure of the hexagonal columns also indicates its origin as a quickly cooling magma. Other than a shallow intrusive body, however, geologists don't know what this structure was. It could have been the core of an ancient volcano, which has been eroded away. It could have been a type of intrusion called a laccolith, which is mushroom-shaped, of another kind called a stock, which is kind of thumb-shaped. Each of these models is plausible, but we may never know exactly how it originated.

The sedimentary rocks surrounding Devils Tower are a big reason for why the place is so beautiful. They add wonderful red and yellow colors to the blue of the sky and the green of the vegetation and the grayish white of the tower itself, creating a delightful palette. The oldest layer at the bottom is the red Triassic Spearfish formation. These rocks surround the tower and are abundantly visible on the Red Rocks trail. Above that are a couple of layers of Jurassic rocks, including the white layers of the Gypsum Springs Fm., an evaporite deposit. There is a disconformity between the Spearfish and the Gypsum Springs, indicating that erosion occurred in between the deposition of the two formations. My personally favorite sedimentary rock here, however, is the yellow Hulett sandstone at the top. It is also Jurassic, and part of the Sundance Formation. While the Spearfish was deposited in an arid environment, the Hulett sands were deposited in shallow marine water near the shore of an ancient beach on the coast of the Sundance Sea. Ripple marks are common on its bedding planes as waves rolled back and forth. At that time, the magma that would form Devils Tower hadn't yet melted out of the deep crust, and wouldn't arrive on the scene for another hundred million years or more.

Devils Tower Phonolite Porphyry

Devils Tower Columns

The rock of Devils Tower itself is a phonolite porphyry with orthoclase phenocrysts. Much of the rock is a very fine-grained gray matrix of feldspar, pyroxene, apatite, and magnetite, which indicates that the magma cooled quickly and the crystals didn't have time to grow. A good number of the lighter gray to white grains here, however, are plenty large enough to see easily (phenocrysts). Those grains are larger, so they had more time to grow. Since they had more time to grow, they must have started growing earlier than the matrix, and the magma must have been cooling more slowly. Since the magma was cooling slowly, it most likely would have been at greater depth in the Earth's crust. The porphyritic texture suggests the magma went through 1) a slow cooling stage where the orthoclase crystals were growing, and 2) a fast cooling stage when the rest of the matrix grew. Stage 2 was likely associated with the emplacement of the magma into the place it now occupies, since it wouldn't move any more once solid. Stage 1 would likely have occurred when the magma was deeper in the crust. This magma crystallized to solid rock about 49 million years ago according to Duke et al. (2002).

The columnar joints are long cracks break up the rock into large roughly hexagonal columns (although they can have 4, 5, 6, or 7 sides). They also indicate an igneous rock that cooled quickly. Even once a magma solidifies, it is still somewhere between 700-1000 °C, much hotter than any rocks around it. As it continues to cool, the rock body shrinks in volume. Instead of shrinking uniformly, however, the solid rock cracks. This cracking accommodates the contraction. Often these cracks form at the top and move downward, forming the vertical columns. Although this is most common, columnar joints can form at any angle. Some of the columns here curve as they approach the base, merging into the more massive rock below. This feature only forms in igneous rocks that crystallize near the Earth's surface; at greater depths they don't cool quickly enough to split, and they are under higher pressure to help keep them together.

The top of Devils Tower is much more weathered than the bottom section. Why is this & what does it mean? The tower sticks up above the surrounding rocks by about 1000' because it is more resistant to erosion than the surrounding sedimentary rocks. When it formed, it was likely covered by several hundred feet of sedimentary rocks that are now worn away. It is easy to assume that erosion happened at a slow, steady pace as the area was uplifted. But if that were the case, the relative amount of weathering affecting the tower should gradually change from intensely weathered at the top to less intense at the bottom. But instead we see a clear distinction in the amount of weathering that has affected the tower. The upper portion has many more fractures and the blocks of rock are more rounded. This suggests that there was a period of time in the past where the upper part of the tower was exposed and being weathered while the lower part was still buried. A subsequent uplift event kicked off a new period of erosion, removing more of the surrounding sedimentary rocks, and exposing the remainder of the tower as we see it today. This model might be tested by modern geochronology techniques that use cosmogenic isotopes to date the amount of time rocks have been exposed to the rays of the Sun. If this model is correct, the rocks at the top of the tower would record much older dates than the material below the weathered section.

Here's a map of the locations where these photos were taken; I hope you use this to explore the geology of this amazing place! More images of the tower are available in my Geophotography Portfolio.

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