Devils Tower is not made of basalt. The iconic rock formation in northeastern Wyoming is composed of phonolite porphyry, a type of igneous rock that differs from basalt in its chemistry, mineral makeup, and the conditions under which it formed.1U.S. Geological Survey Bulletin. Geology of Devils Tower National Monument, Wyoming The confusion is understandable, though, because Devils Tower’s dramatic columnar joints look almost identical to the columns found at famous basalt formations around the world. That visual similarity, combined with how often textbooks and tourist materials use the word “basalt” when discussing columnar rock, has cemented a misconception that persists even among people who have visited the monument in person.
Why So Many People Assume It Is Basalt
Columnar jointing and basalt are so tightly linked in popular imagination that they have become almost synonymous. When most people picture columns of rock splitting into hexagonal pillars, they picture basalt. That association is not wrong in a general sense. Basalt is the most common rock type on Earth’s surface, and basalt flows produce some of the most spectacular columnar formations anywhere, from the Giant’s Causeway in Northern Ireland to the basalt cliffs of Iceland. But the process that creates columnar joints is not exclusive to basalt. Columns form whenever a body of molten or very hot material cools and contracts, and the contraction stresses crack the rock into roughly polygonal pillars. This happens in basalts, yes, but also in other volcanic rocks like tuffs and ignimbrites, and even in some sedimentary rocks like sandstone and clayey mud.2ScienceDirect (Journal of Structural Geology). Reappraising columnar joints in different rock types and settings
Devils Tower’s columns are among the most visually striking examples of this phenomenon on the planet, rising in some cases hundreds of feet. It is easy to see why a visitor, knowing that “basalt makes columns,” would assume that the columns at Devils Tower must be basalt. But the rock itself tells a different story when you examine it closely.
What Phonolite Porphyry Is and How It Differs from Basalt
Phonolite porphyry is an igneous rock, meaning it solidified from magma, just like basalt. But the two rocks sit in very different categories. Basalt is a dark, fine-grained rock that forms from low-silica magma erupting at the surface and cooling quickly. It is dense and heavy, dominated by minerals like pyroxene and plagioclase feldspar. Phonolite, by contrast, forms from magma that is richer in alkali elements like sodium and potassium and contains less silica than granite but more than basalt. The “porphyry” part of the name refers to its texture: large, visible crystals (phenocrysts) embedded in a finer-grained groundmass, indicating that some crystals had time to grow slowly at depth before the remaining liquid cooled more rapidly.
When fresh, the phonolite porphyry at Devils Tower is gray. Weathered surfaces turn green or brown, giving the monument the muted earth tones that visitors see from the trail.1U.S. Geological Survey Bulletin. Geology of Devils Tower National Monument, Wyoming The name “phonolite” comes from the Greek for “sounding stone” because thin slabs of the rock ring when struck, a property related to its mineral composition and density. You would not get the same resonant quality from a piece of basalt.
The chemical distinction matters because it reflects fundamentally different magma sources and conditions. Basalt typically comes from partial melting of the upper mantle and erupts at very high temperatures. Phonolite magma is more evolved, meaning it has undergone more chemical differentiation as it rises through the crust. The alkaline-rich composition of phonolite places it in a family of igneous rocks associated with specific tectonic settings, often areas of crustal extension or hotspot activity rather than the mid-ocean ridges and subduction zones where basalt dominates.
A Hundred-Year Argument Over How It Got There
The rock type was settled relatively early. Geologists recognized it as phonolite porphyry by the late 1800s. What they could not agree on was how that phonolite ended up forming a monolith towering roughly 265 meters above the surrounding landscape. The debate has lasted more than a century and is still not fully resolved.
Early hypotheses proposed that Devils Tower was an intrusive body, meaning it formed underground when magma pushed into surrounding rock and solidified without ever reaching the surface. Specific suggestions included a magmatic stock (a roughly cylindrical plug of cooled magma), a laccolith (a mushroom-shaped intrusion that lifts the rock above it), or a volcanic conduit (the solidified throat of a volcano whose outer cone has eroded away).3Geosphere. Devils Tower (Wyoming, USA): A lava coulée emplaced into a maar-diatreme volcano? The volcanic conduit interpretation became especially popular and found its way into many Earth science textbooks, where it is sometimes still repeated today.
A more recent hypothesis, based on detailed field mapping, analogue modeling, and comparisons with similar phonolite formations elsewhere, argues that Devils Tower is actually the remnant of a coulée or low lava dome. In this scenario, phonolite magma did reach the surface, but it was emplaced into a broad crater at the top of a maar-diatreme volcano, a type of volcanic structure created by explosive interactions between magma and groundwater.3Geosphere. Devils Tower (Wyoming, USA): A lava coulée emplaced into a maar-diatreme volcano? According to this model, the phonolite filled the crater like batter poured into a bowl, then solidified. Over millions of years, erosion stripped away the softer surrounding rock, leaving only the resistant phonolite monolith standing.
Supporting evidence for this hypothesis comes partly from studying a similar phonolite landmark in the Czech Republic called Bořen, along with gravity surveys of the area around Devils Tower that help map what lies beneath the surface.4American Geophysical Union. On the geological origin of Devils Tower (WY, USA) The subsurface gravity data helps geologists estimate the shape of the igneous body below the visible tower, which in turn constrains which emplacement model fits best.
The distinction between “intruded underground” and “erupted into a crater” might sound academic, but it changes the picture meaningfully. If Devils Tower formed underground, the entire visible surface was once buried beneath other rock. If it erupted into a crater, the upper surface may have been exposed to the atmosphere from the start, which affects how the columns formed and how the rock cooled.
Why the Columns Have Two Distinct Zones
One of the most striking features of Devils Tower, and a clue to its cooling history, is that its columns are not uniform from bottom to top. The tower has two visible zones, sometimes called colonnades. The lower colonnade features columns that thin as they go upward, while the upper colonnade has columns of more consistent width. The boundary between these two zones sits at roughly two-thirds of the tower’s height.
This pattern tells geologists something about how fast different parts of the body cooled. The thinning columns in the lower portion suggest that cooling accelerated as the rock solidified. One explanation is that gases escaping from the crystallizing magma circulated through the fracture system of the lower colonnade, carrying heat away more efficiently through convective cooling. Another possibility is that water released from wet sediments at the base of the original crater contributed additional cooling fluid. Either way, faster cooling produces thinner columns because the contraction stresses operate over shorter distances before new cracks form.5Geosphere. Devils Tower (Wyoming, USA): A lava coulée emplaced into a maar-diatreme volcano? – Section: Discussion
The upper colonnade, by contrast, had lower fluid content because gases at those lower pressures could escape directly into the atmosphere. Without the extra convective cooling, the upper portion solidified at a steadier rate, producing columns of more uniform width.5Geosphere. Devils Tower (Wyoming, USA): A lava coulée emplaced into a maar-diatreme volcano? – Section: Discussion
How Columnar Joints Actually Form
The process behind columnar jointing applies regardless of whether the rock is basalt, phonolite, or something else entirely. As a body of hot rock cools, it contracts. Once the temperature drops below the solidus (the point where the rock is completely solid), continued cooling builds up tensile stress. When those stresses exceed the rock’s strength, cracks propagate inward from the cooling surfaces.
Research on Icelandic basalts has pinpointed the temperature window where these cracks first appear. In those basalts, stress builds below about 980 °C, and the first macroscopic fractures open between 890 and 840 °C, well within the solid state of the rock.6PubMed Central. Disclosing the temperature of columnar jointing in lavas The cracks organize into roughly polygonal patterns because that geometry is the most efficient way to relieve contraction stress uniformly across a surface. Energy-minimization models show that the ideal cross-section is a regular hexagon, which is why so many columns approximate a six-sided shape, though in practice you see columns with anywhere from four to eight sides depending on local conditions.7Mathematical Models and Methods in Applied Sciences. A MODEL OF COLUMNAR JOINTING
The specific temperatures at which phonolite porphyry would joint differ from basalt because of its different mineral composition and solidus temperature. But the underlying physics is the same: cool a big slab of rock slowly and uniformly enough, and columns emerge. This is precisely why people confuse Devils Tower with basalt. The visual signature of columnar jointing is the same no matter what rock produces it.
How Old Devils Tower Is
Potassium-argon dating of the orthoclase feldspar crystals in Devils Tower’s phonolite porphyry gives an age of about 40.5 million years, placing it firmly in the Eocene epoch of the Tertiary period.8PubMed. Potassium-Argon Age of Devils Tower, Wyoming At that time, the Black Hills region was volcanically active, producing a cluster of alkaline igneous intrusions and extrusions. Devils Tower is the most famous of these, but it is not alone. A number of smaller igneous bodies dot the surrounding area, all dating to roughly the same period of magmatic activity.
The 40-million-year age is the age of the rock itself, not the age of the tower as a freestanding monolith. Devils Tower became the towering landmark it is today through erosion that stripped away the softer sedimentary rocks around it over millions of years. The Belle Fourche River and its tributaries gradually carved away the surrounding mudstones and sandstones, which are far less resistant to weathering than the hard phonolite porphyry. The tower you see today is essentially a remnant, the most durable piece of a much larger geological system.
What Keeps the Tower Standing and What Threatens It
For a 265-meter column of fractured rock, Devils Tower is remarkably stable, but it is not invulnerable. The same columnar joints that give it its beauty also represent planes of weakness. Blocks and columns periodically detach and fall, building up the talus slopes of broken rock that surround the tower’s base. This is ordinary erosion on geological timescales, but occasionally specific features raise concern.
One such feature is a “leaning column” near a popular climbing route. A USGS investigation found evidence of a recent tensile spalling failure on the pedestal surface directly beneath this column. The spalling, likely caused by the concentrated weight of the leaning column pressing along a narrow line of contact, is what appears to be driving the column’s gradual tilt. While it is unlikely that the weight of climbers alone would dislodge the column, additional spalling from the pedestal could reduce its support further and eventually cause a topple onto the climbing route or the trail section below. The pedestal surface was coated with paint so that any new fractures would be immediately visible during monitoring.9U.S. Geological Survey Open-File Report. Stability of leaning column at Devils Tower National Monument, Wyoming
More recently, researchers have used seismic sensors to study how the entire tower vibrates in response to ambient forces like wind and distant earthquakes. A seismometer placed on the summit in October 2024, alongside two sensors at the base, detected a prominent resonance peak at around 1.1 Hz on horizontal components. The calibrated model derived from these measurements estimated a global Young’s modulus for the tower of about 8 GPa, roughly seven times lower than what intact phonolite porphyry shows in laboratory rock testing.10GSA Today. Tower in Motion: Resonance Mode Analysis of Devils Tower, Wyoming, USA That dramatic reduction is entirely due to the joints. The columnar fractures that define the tower’s appearance also make it far more compliant than a solid block of the same rock would be. In practical terms, Devils Tower sways slightly in the wind and flexes in response to passing seismic waves, behaving less like a rigid monolith and more like a bundle of loosely connected columns.
Phonolite Landmarks Beyond Wyoming
Devils Tower is not the only phonolite formation that gets misidentified as basalt by casual observers. The comparison with the Czech formation called Bořen, used by researchers to help explain Devils Tower’s origin, highlights that phonolite landmarks exist in several parts of the world.4American Geophysical Union. On the geological origin of Devils Tower (WY, USA) The České středohoří (Bohemian Highlands) in the Czech Republic contain numerous phonolite hills and peaks, many of which exhibit columnar jointing that mirrors what is seen in Wyoming. These formations are geologically much younger than Devils Tower and formed in a different tectonic context, but they share the same rock type and the same tendency to confuse visitors who associate all columnar rock with basalt.
The broader point is that phonolite, while far less common than basalt globally, is not rare. It crops up in continental rift zones, oceanic islands, and areas of intraplate volcanism. What makes Devils Tower exceptional is not its rock type alone but the combination of its size, its state of preservation, its dramatic exposure above the surrounding plain, and the sheer regularity of its columnar joints. These factors made it impressive enough to earn designation as the first U.S. national monument in 1906, and they continue to draw roughly half a million visitors per year. Most of those visitors will hear the word “basalt” at some point during their visit, whether from a fellow tourist, a half-remembered geology class, or an outdated guidebook. Now you know to correct them: it is phonolite porphyry, and the difference is real.