ON THE GEOLOGICAL ORIGIN OF DEVILS TOWER, WYOMING - A NEW HYPOTHESIS CONSTRAINED BY FIELD RESEARCH, ANALOGUE AND THERMAL MODELING DATA, AND GRAVIMETRIC SURVEY

The scientific debate on the origin of Devils Tower, WY, has existed for more than 100 years. Beginning with the early work of Effinger in the 1930s, most hypotheses consider only intrusive scenarios: a remnant volcanic plug, a laccolith, or a volcanic conduit. In our contribution, we conclude that the Devils Tower represents a remnant of an eroded lava lake that was emplaced into a broad crater of a phreatomagmatic volcano. Our hypothesis is based on the occurrence of phreatomagmatic units surrounding the phonolite bodies, analogue modeling results of magma intrusion into phreatomagmatic volcanoes, thermal mathematical modeling of cooling of volcanic bodies, and a detailed gravimetric survey of the area around Devils Tower (DT) and Missouri Buttes (MB). Phreatomagmatic breccia in the vicinity of DT and MB suggests that these phonolite bodies were emplaced into phreatomagmatic volcanoes that are typical with a deep diatreme propagating downwards with each phreatomagmatic explosion and a broad maar crater surrounded by a tephra ring. A new gravimetric survey revealed a prominent low encompassing the MB and a local low on the SW side of DT both matching with outcrops of the phreatomagmatic breccias that are less dense (1540-1910 kg/m³) relative to surrounding rocks in the area (2300-2600 kg/m³). Finite element numerical modeling of cooling reveals a similarity of the DT columnar structure pattern with the thermal structure calculated for a lava lake analogue experiment, and thermophysical properties measured from samples. The columnar structure on DT is interpreted to reflect cooling from two thermal fronts originally representing the bottom and flat top of the original phonolite lava lake, respectively. The MB, in contrast to DT, represent a group of cryptodomes emplaced on the periphery of the phreatomagmatic crater that formed due to branching of the central conduit in the diatreme, as seen in one of our analogue models.