Devils Tower stands above the surrounding grasslands and ponderosa pine forests like a rocky guardian. Geologists have been studying its formation since the late 1800s and still contemplate how it came to be. While there is broad agreement on much of the Tower’s geological history, theories differ regarding certain specifics. Designated as a protected site in 1906 due to its scientific significance, Devils Tower remains a location for scientific inquiry and public fascination.

It is known that the Tower is made of a rare igneous rock called phonolite porphyry and is the largest example of columnar jointing in the world. To gain a better understanding of the processes that shaped the Tower, we look back into Earth’s history to a time long before this unique feature emerged.

Geological Foundations

Most of the landscape surrounding Devils Tower consists of sedimentary rocks. These rocks form from the solidification of minerals or organic material, typically deposited by water or wind. This process, known as deposition, is common in river deltas and coastal regions. Many fossils found in sedimentary rocks offer insights into ancient ecosystems.

The oldest rocks visible in Devils Tower National Monument were deposited in a shallow inland sea that covered much of the central and western United States during the Triassic period, approximately 225 to 195 million years ago. Dark red sandstone and maroon siltstone, interspersed with shale, can be observed along the Belle Fourche River. The red coloration of these rocks results from the oxidation of iron-rich minerals. This rock layer is referred to as the Spearfish Formation.

Above the Spearfish Formation lies the Gypsum Springs Formation. As mineral-rich water evaporated, deposits of gypsum remained. During the Jurassic period (195 to 136 million years ago), seas would periodically retreat and return.

Offshore clay deposits in deep marine environments transformed into gray-green shales interbedded with sandstones, limestones, and thin layers of red mudstone. These rock layers, known as the Stockade Beaver Member, are part of the Sundance Formation—also from the Jurassic period.
The Hulett Sandstone and Lak Members, also part of the Sundance Formation, consist of yellow, fine-grained sandstones that originated from sand deposited on an ancient beach, with many outcrops displaying preserved symmetrical ripples. Resistant to weathering, these formations create the nearly vertical cliffs that surround the Tower itself.

Seas advanced and retreated; landforms developed and eroded away. New sediments were deposited over time. Approximately 50 to 60 million years ago, during the Tertiary period, tectonic pressures within western North America peaked, uplifting the Rocky Mountains and the Black Hills. At this time or shortly thereafter, magma (molten rock) surged toward Earth’s surface, intruding into the existing sedimentary rock layers.

Theories of Formation

Geologists agree that Devils Tower originated from magma or molten rock buried beneath Earth’s surface. However, they disagree on the processes by which this magma cooled to form the Tower and its relationship to the surrounding geology. Numerous theories have been proposed to explain how Devils Tower came into existence. In the late 1800s, geologists Carpenter and Russell studied Devils Tower and concluded that it was formed by an igneous intrusion (the forcible entry of magma through other rock layers). Subsequently, geologists sought more detailed explanations.

The simplest explanation is that Devils Tower is a stock—a small intrusive body formed by magma that cooled underground and was later revealed by erosion (Figure 1). In 1907, scientists Darton and O’Hara concluded that Devils Tower must be an eroded remnant of a laccolith. A laccolith is a large, mushroom-shaped mass of igneous rock that intrudes between layers of sedimentary rock without reaching the surface, causing a rounded bulge in the overlying sedimentary layers (Figure 2). This idea was quite popular in the early 1900s when numerous studies were conducted on various laccoliths in the American Southwest.

Other theories have suggested that Devils Tower may be a volcanic plug or the neck of an extinct volcano (Figure 3). The scant evidence of volcanic activity (such as volcanic ash, lava flows, or volcanic debris) in the area raises doubts about whether the Tower was part of a volcanic system; it is possible that such material has simply eroded away. In 2015, geologist Prokop Závada and his colleagues proposed their own hypothesis for the Tower’s formation.

They compared it to a similar butte formation in the Czech Republic, suggesting that the Tower resulted from a maar-diatreme volcano (Figure 4). These occur when magma interacts with groundwater beneath Earth’s surface; superheated water turns into steam, which expands explosively to create a crater on the surface. This crater fills with lava that cools and solidifies into a dome structure. Erosion then wears away parts of this dome to create the Tower as we see it today.

The concept of erosion exposing the Tower is common across all modern formation theories. Ironically, while erosion has revealed the Tower, it has also erased much of the evidence needed to determine which theory about its formation is correct.

Columnar Jointing

The columns of Devils Tower are its most impressive characteristic. This appearance, known as columnar jointing, is not exclusive to Devils Tower; however, its columns are unmatched in size. Rising hundreds of feet into the air and measuring up to 10 feet in width, the columns at Devils Tower are truly spectacular.

Columnar formations occur only in igneous rocks. Igneous rocks originate from lava (on Earth’s surface) or magma (beneath Earth’s surface). As molten rock cools from a liquid state to solid form, it begins to contract. This contraction places stress on the cooling rock, causing it to crack. Cracks radiate from stress points, forming hexagonal (six-sided) shapes.

Many examples of columnar jointing form perfect hexagons; however, the columns at Devils Tower are irregularly shaped, possibly due to their large size. Additionally, there are many five-sided or pentagonal columns present on the Tower. Scientists remain uncertain about what causes this variation in shape.

The columns at Devils Tower are similar to those found elsewhere in the world, such as at Devils Postpile National Monument in California. However, Devils Tower’s columnar jointing is the largest and most spectacular example of this fascinating geological phenomenon.

Revealing the Tower

Erosion has dramatically altered the area surrounding Devils Tower. When it formed around 50 million years ago, it was located one to two miles below Earth’s surface. Between 5 and 10 million years ago, erosive forces began to expose the Tower. These forces, particularly water, removed sedimentary rocks above and around it. The harder igneous rock of the Tower is more resistant to erosion, allowing its gray columns to rise prominently above the surrounding landscape.

As rain and snow continue to erode the sedimentary rocks at the base of the Tower while debris is carried away by the Belle Fourche River, more of Devils Tower will be revealed over time. Simultaneously, erosion continues to affect the Tower itself. Evidence of this process can be seen in large boulder fields composed of broken columns at its base as well as many other rocks on nearby hillsides. Although small rocks regularly fall from the Tower, no significant column collapse has been witnessed in recorded history.

The geology of Devils Tower retains an element of mystery for both casual visitors and scientific expertsalike. While many theories surrounding its formation have been understood, its wondrous nature continues to captivate people for thousands of years and remains enchanting today.