Devils Tower is made of phonolite porphyry, a type of igneous rock that cooled from magma rich in alkali minerals but relatively low in silica. When freshly exposed, the rock is gray, but surfaces that have been exposed to weather for long periods turn green or brown.1U.S. Geological Survey Bulletin. Geology of Devils Tower National Monument, Wyoming That single rock type accounts for the entire towering monolith, from its famous columnar walls to the rubble field at its base, and understanding it helps explain why the formation looks the way it does and why it still stands while the landscape around it has worn away.
What Phonolite Porphyry Actually Is
Phonolite is an uncommon igneous rock. Most people are familiar with granite or basalt, but phonolite sits in a different chemical neighborhood. It forms from magma that is relatively low in silica (compared to granite) yet rich in sodium and potassium. That alkali-heavy chemistry gives the rock a distinctive mineral makeup and, historically, its name: thin slabs of phonolite ring when struck with a hammer, so early geologists named it after the Greek word for “sound stone.” You will not hear that ringing at Devils Tower unless you break off a thin piece, but the name stuck for the entire rock family.
The “porphyry” part of the name describes the rock’s texture rather than its chemistry. A porphyritic rock is one that cooled in two stages. During the first stage, the magma sat deep underground or in a slowly cooling body, and certain minerals had time to grow into large, visible crystals. During the second stage, the remaining melt cooled more quickly, forming a finer-grained matrix around those earlier crystals. The result is a rock with conspicuous crystals embedded in a denser groundmass, somewhat like raisins in bread dough. At Devils Tower, the large crystals are primarily orthoclase, a potassium-rich feldspar that can be recognized as pale, blocky grains set against the darker surrounding matrix.2PubMed. Potassium-Argon Age of Devils Tower, Wyoming
This combination of alkali-rich chemistry and two-stage cooling is what makes Devils Tower’s rock phonolite porphyry specifically, rather than just “phonolite” or just “porphyry.” It is a precise classification: a fine-grained, alkali-dominated igneous rock carrying large feldspar phenocrysts.
What the Rock Looks and Feels Like
If you could hold a fresh chunk of Devils Tower in your hand, you would see a medium-gray rock with a slightly granular feel, interrupted by the pale orthoclase crystals. The groundmass is dense enough that the rock feels heavy for its size. Laboratory measurements put the density of unweathered phonolite porphyry from the tower at about 2,600 kilograms per cubic meter, which is roughly on par with granite and noticeably heavier than most sedimentary rocks.3GSA Today. Tower in Motion: Resonance Mode Analysis of Devils Tower, Wyoming, USA
Weathered surfaces tell a different story. Exposure to moisture and air over millions of years alters the iron-bearing minerals, producing green and brown coatings that dominate the tower’s visual appearance from a distance.1U.S. Geological Survey Bulletin. Geology of Devils Tower National Monument, Wyoming Visitors sometimes describe the tower as looking greenish-gray in certain light, and that color comes from weathering rinds on the surface rather than from the fresh rock itself. Climb or break into any column, and the interior is still that original gray.
The orthoclase phenocrysts are large enough to see without a magnifying lens. On weathered surfaces they sometimes stand out in slight relief because feldspar resists certain kinds of chemical weathering a bit differently than the surrounding fine-grained matrix. The contrast between these coarse crystals and the finer groundmass is exactly the textural signature that classifies the rock as a porphyry.
How Strong the Rock Is
Phonolite porphyry is a mechanically tough rock, which matters both for the tower’s survival over geologic time and for the rock climbers who depend on its integrity. Researchers have extracted core samples from boulders in the scree field at the base of Devils Tower and tested them in laboratory compression rigs. The testing followed standardized methods, loading cylindrical cores until failure to measure how much stress the rock can withstand and how stiffly it resists deformation.3GSA Today. Tower in Motion: Resonance Mode Analysis of Devils Tower, Wyoming, USA
In practical terms, this means the rock does not crumble easily. The columns of Devils Tower hold together well enough that the formation vibrates as a coherent structure when excited by ambient seismic energy or wind. Researchers have used resonance measurements to study how the entire tower sways, and the fact that it behaves as one rigid body rather than a loose stack of blocks speaks to the mechanical integrity of the phonolite porphyry and the tight interlocking of the columns.
The Famous Columns and Why They Formed
The most striking visual feature of Devils Tower is its columnar jointing: the near-vertical columns, roughly hexagonal in cross-section, that make the tower look like a bundle of enormous pencils. These columns are not a quirk of phonolite specifically; columnar jointing occurs in many types of igneous rock when a hot body of magma cools and contracts. As the rock shrinks, stress fractures propagate inward from the cooling surfaces, and the most energy-efficient pattern for those fractures to relieve the thermal stress is a network of roughly hexagonal cracks, much like the pattern that forms in drying mud, only in three dimensions and on a grand scale.
What makes Devils Tower’s columns remarkable is their size. Individual columns at the tower range from about 1.2 to 2.4 meters (roughly four to eight feet) across, and some extend the full height of the formation. The size of columns in any cooling igneous body depends mainly on how slowly the rock cooled: slower cooling produces larger columns because the thermal gradients are gentler and the crack spacing is wider. The fact that Devils Tower’s columns are so large suggests the magma body cooled gradually, insulated by the surrounding rock.
The columns also vary in orientation. Near the base of the tower, many columns flare outward or even curve, while higher up they are more nearly vertical. This variation has given geologists clues about the original shape of the magma body and how heat escaped from it, since columns grow perpendicular to the cooling surface. If the cooling surface was curved, the columns curve too.
The Sedimentary Rocks at the Base
One reason Devils Tower looks so dramatic is the contrast between its dark igneous phonolite and the brightly colored sedimentary layers that form the hills and slopes around its base. These surrounding rocks are sandstones, shales, and gypsum beds in shades of red, yellow, green, and gray, and they total roughly 120 meters (about 400 feet) in thickness. From oldest to youngest, they include parts of the Spearfish formation (Triassic age), the Gypsum Spring formation (Middle Jurassic), and the Sundance formation (Late Jurassic).4U.S. Geological Survey. Geology of Devils Tower National Monument, Wyoming
These layered rocks are far softer and more easily eroded than the phonolite porphyry. The tower’s very existence as a landmark owes everything to this mismatch. Over millions of years, streams and weather have stripped away the sedimentary cover, lowering the surrounding landscape while the tough igneous core remained. The somber column of phonolite standing above bands of brightly colored sedimentary rock is what gives Devils Tower its visual punch.4U.S. Geological Survey. Geology of Devils Tower National Monument, Wyoming
Visitors hiking around the base can see where the igneous rock meets the sedimentary layers, though the exact contact zone is often covered by scree. The rubble field of fallen column segments at the base of the tower is itself a testament to the rock’s hardness: the boulders survive largely intact after tumbling from heights because phonolite porphyry does not shatter easily on impact the way a limestone or shale would.
How the Magma Got There in the First Place
The rock type answers “what,” but most visitors also wonder “how.” How did a mass of phonolite porphyry end up sticking out of the plains of northeastern Wyoming? This question has genuinely puzzled geologists for over a century, and the debate is not fully settled.
The oldest and most popular explanation, still repeated in many park brochures, is the volcanic neck hypothesis. In this model, Devils Tower is the solidified plug of magma that once filled the throat of a volcano. After the volcano went extinct, erosion stripped away the outer cone and the surrounding softer rock, leaving only the resistant igneous core. It is a clean story, and it fits the general appearance of the tower, but the geologic details around the site have caused some researchers to question it.
An alternative hypothesis treats the tower as the remnant of a laccolith or stock, meaning a blob of magma that intruded between sedimentary layers or into a void underground but never erupted at the surface. In this model, the magma cooled entirely underground, and the tower was later exposed by erosion from above.
A more recent proposal, based on detailed fieldwork and mapping, suggests that Devils Tower is actually the remnant of a coulée, a type of low, thick lava flow, or a low lava dome that was emplaced into a broad crater formed by explosive interactions between magma and groundwater (a maar-diatreme volcano).5GeoScienceWorld (Geosphere). Devils Tower (Wyoming, USA): A lava coulée emplaced into a maar-diatreme volcano? In this scenario, the magma did reach the surface, but it filled a pre-existing explosion crater rather than building a tall volcanic cone. The column orientations and the geometry of the surrounding rock contacts are among the evidence cited in favor of this model.
All three hypotheses agree on the rock type. Whatever the emplacement mechanism, the magma that produced Devils Tower was phonolite porphyry with the same alkali-rich chemistry and the same large orthoclase crystals. The debate is about the plumbing and the surface expression, not about what the rock is.
Why Phonolite and Not Basalt or Granite
Visitors familiar with other famous igneous landmarks sometimes wonder why Devils Tower is not made of a more common rock. Basalt forms most of the world’s columnar-jointed features, and granite makes up much of the exposed igneous rock in mountain ranges. Phonolite is far less common globally, so what made this particular magma different?
The answer lies in the tectonic setting. Devils Tower sits on the northwestern margin of the Black Hills, a region where deep-seated magmatic activity during the Eocene epoch (roughly 40 to 50 million years ago) produced a suite of igneous intrusions that are unusually rich in alkali elements. The magma sources in this region generated rocks ranging from phonolite to nepheline syenite, compositions that reflect partial melting of mantle rock under specific pressure and temperature conditions that favor sodium and potassium enrichment over the silica-rich compositions that produce granite, or the low-alkali compositions that produce basalt.
Devils Tower is not the only alkali-rich intrusion in the area. The Missouri Buttes, a cluster of smaller igneous knobs just a few miles to the northwest, are made of similar rock. So are several other intrusions scattered through the northern Black Hills. The region was a hotspot for this unusual magma chemistry, and Devils Tower is simply the most spectacular survivor of that episode.
Common Misconceptions About the Rock
Several misunderstandings circulate about Devils Tower’s geology, partly because the site is so visually unusual that people reach for dramatic explanations.
One persistent claim is that Devils Tower is a petrified tree stump. This idea circulates widely on social media and draws on the tower’s columnar appearance, which can superficially resemble a bundle of wood fibers. There is no geologic basis for this. The columns are a well-understood product of thermal contraction in cooling magma, and the rock’s mineral composition is unambiguously igneous. Phonolite porphyry contains feldspar, nepheline, and other minerals that form only from molten rock at high temperatures, not from biological material.
Another common error is calling the rock basalt. Because columnar jointing is most famously associated with basalt formations around the world, and because Devils Tower’s columns look similar at a distance, many casual descriptions label it basalt. Chemically, phonolite and basalt are quite different. Basalt is low in both silica and alkalis, while phonolite is low in silica but high in alkalis. The mineral assemblages reflect this: basalt is dominated by pyroxene and calcium-rich plagioclase, while Devils Tower’s phonolite contains orthoclase (a potassium feldspar) and alkali-rich feldspathoids. The distinction is not just academic pedantry; it tells geologists that the magma source and melting conditions were fundamentally different from those that produce the basalt plateaus and sea-floor crust that make up most of the world’s igneous rock by volume.
A third misconception is that the tower is volcanic in the sense of having erupted explosively. While the emplacement debate discussed earlier includes models where magma reached the surface, none of them involve the kind of explosive eruption people picture when they hear “volcano.” The phonolite magma was viscous enough to form a dome or thick flow, not to blast ash into the atmosphere. The tower’s quiet chemistry is actually one reason it survived: the magma pooled and solidified as a coherent mass rather than fragmenting into loose volcanic debris that would have eroded away long ago.
What Climbers and Visitors Notice About the Rock
Rock climbers have an intimate relationship with Devils Tower’s phonolite porphyry because they spend hours gripping its surface. The climbing community has developed its own informal vocabulary for the rock’s features. The columns provide natural crack systems that climbers use as hand jams and foot placements, and the phonolite’s hardness means these cracks tend to have clean, sharp edges rather than the crumbly, rounded textures you might find in sandstone or weathered granite.
The orthoclase phenocrysts sometimes protrude slightly from weathered surfaces, creating small knobs that can serve as handholds. On less weathered faces, the rock is smoother and more uniform, which changes the climbing style from crack climbing to face climbing. The practical upshot is that the rock type directly shapes the climbing experience: phonolite porphyry is hard enough to hold a climber’s weight on small features, rough enough to provide friction, and jointed enough to offer crack systems, but smooth enough in places to demand technical skill.
Visitors who simply walk the paved trail around the base encounter the rock mainly in the form of the massive fallen column segments in the talus field. These boulders are sometimes several meters long and weigh many tons, yet they sit remarkably intact, not crumbled into gravel. Picking up a smaller piece from the scree and turning it in your hand, you can see the gray interior, the pale feldspar crystals, and the thin weathering rind that grades from gray to green or brown. It is one of the few places where a casual visitor can hold a piece of a national monument and see, up close, the rock that built one of the most recognizable landforms in North America.