Devils Tower in Wyoming is surrounded by myths and mysteries. To the Sioux people, this site was sacred and some of their stories tell how this mountain formed. A long time ago a giant bear chased a group of children onto the flat top of the mountain. Out of reach of the animal, the bear started to scratch the rocks with its claws, forming the characteristic joints in the rock. Reportedly Devils Tower got its name from this legend, as “bear” was erroneously translated as “bad god”, later becoming the “devil”.
Today this 1,267 feet high pinnacle of phonolite (a silica-poor fine-grained igneous rock) is described in many textbooks as an intrusion of igneous rock that never reached the surface to form a volcano. However, there are a number of issues with this model. If the intrusion formed deep underground, erosion needed to remove an extraordinary amount of sedimentary rocks to expose Devils Tower. The phonolite is dated to 50 million years, not enough time to explain the needed rates of erosion.
Also, the strange geometry of the phonolite columns forming the pinnacle poses a problem. Such columns form when igneous rocks cool and shrink in volume. The columnar joints develop at approximately right angles to the cooling surfaces which are usually the tops or bottoms of lava flows, the walls of a magma chamber or a volcanic conduit. In the case of the Devils Tower, the columns are tilted to the base suggesting also that the cooling surface, the walls of the volcanic plug, were tilted. But why the columns then become almost vertical at the top of the pinnacle, and so a challenge to modern rock-climbers, is a bit of a mystery. In the core of the Devils Tower intrusion cooling should be very slow with no preferred orientation.
A now open access paper, published in 2015 in the journal Geosphere, proposes an alternative model to explain some of the observed geological oddities. The authors suggest that Devils Tower is not the remains of an igneous intrusion, but the eroded remains of a volcanic conduit (or diatreme) connecting a magma chamber to a superficial lava lake. Fifty million years ago uprising magma encountered groundwater, resulting in a phreatomagmatic eruption. The powerful steam explosions caused by water vaporized by the hot lava formed a maar, a broad volcanic crater, filled in part by debris and lava. Finally, after the phreatomagmatic activity eventually stopped, a lava lake formed in the maar. As the lava cools, columnar joints form. The joints are vertical on the flat top of the lava lake, but tilted on the bottom as the cooling surface, the basin of the lava lake, is tilted. Erosion removes the sediments surrounding the former lava lake and also part of the lava is eventually removed, exposing finally Devils Tower.
