Devils Tower in Wyoming is such an extraordinary sight that its creation myth almost seems possible. Giant bears are said to have scratched its surface attempting to climb to the top. But the vertical lines adorning the sides of the almost 390-meter-tall rock are not claw marks. They are actually the edges of roughly hexagonal columns of igneous rock, similar to other geometric landmarks such as Ireland's Giant's Causeway or Devils Postpile in California. The columns form as molten rock cools and contracts, cracking apart. But how did the sky-scraping columns of Devils Tower form—belowground or as part of a violent eruption?
A number of hypotheses for the 49-million-year-old monolith have been put forward over the years but most involve a subterranean explanation. The most popular explanations today are that it was either born as an inflating half dome of magma squeezed in between subsurface layers of rock or within a conduit deep inside a volcano.
Prokop Závada, a geologist at The Czech Academy of Sciences, and three of his colleagues came to this mystery by way of a stout butte known as Bořeň in the Czech Republic that shares some similarities with Devils Tower (although it is more rounded and covered with trees). The researchers concluded that Bořeň is the product of a sudden type of volcano called a maar-diatreme, which blasts a crater in the land surface when a body of magma underground encounters groundwater. After the blast, they believe, a flat dome of lava filled the crater. Erosion ate away at the edges of that dome until only the innermost portion remained, standing as an isolated butte. Given its similarities to Bořeň, Závada and his team turned their attention to Devils Tower to see if the same explanation would fit there.
There were two primary characteristics the researchers used to hone in on the origins of Devils Tower: the shape of its distinctive columns and the alignment of magnetic minerals within them. The researchers were permitted by the National Park Service to collect one new rock sample to analyze its magnetic properties, and added this to existing measurements made over 30 years ago from a few other sections of the tower. Near the base the tiny, needle-shaped magnetic minerals within the rock are generally close to vertical, the result of the direction the magma flowed before it solidified. Closer to the top of the formation, however, the orientation of those minerals becomes horizontal.
With that information in hand, the researchers turned to models, both digital and physical: The physical one involved squeezing soft plaster upward through an inverted cone full of sediment until it formed a mound atop the surface. This approximated the eruption of lava following the explosive phase of a maar-diatreme volcano. When the plaster hardened, the researchers could cut it open to examine the interior structure, highlighted by some colorant that formed stripes distorted by flow. Because they mixed in some magnetic particles as well, they could also measure their orientation just as they had done at Devils Tower. The digital model was a computer simulation of cooling igneous rock. It allowed the researchers to compare columns that would be produced in different scenarios of the tower’s formation. Columns form perpendicular to the surface along which the rock cools, so they can tell you about the original shape of the body of molten rock.
The vertical columns of Devils Tower, which splay outward near the bottom, match the pattern expected if the formation were indeed the plug in the neck of a funnel-shaped crater filled with a dome of erupted lava. In that portion of the plaster models the orientation of the magnetic particles also matches the orientation of the minerals from Devils Tower: closer to vertical around the base and horizontal near the top. In a paper published in Geosphere Závada and his team thus concluded that like Bořeň in the Czech Republic, “Devils Tower is a remnant of a coulee or low lava dome that was emplaced into a broad phreatomagmatic crater at the top of a maar-diatreme volcano.”
Western Washington University’s Bernard Housen, who was not involved in the study, finds the work “interesting and certainly very plausible” but he is not convinced it rules out the other possibilities. There just isn’t very much data from Devils Tower itself, given that so little geologic sampling of the national monument has been allowed. “So, it is likely that the origin of Devils Tower will remain uncertain in part due to the protections we have enabled to make sure it is preserved,” Housen says.
It might be possible to study the similar, if less spectacular, buttes in the vicinity of Devils Tower, however, which likely formed in the same way—with or without the help of giant bears.